"It is our choices that show what we truly are, far more than our abilities"

-J.K. Rowling-First, I would like to express my gratitude to my supervisors. I am very grateful to Dr. Conchi Ania for this opportunity, her generous guidance, and her constant encouragement. I want to thank Dr. Alicia Gomis Berenguer for her patience and help during my work. I would like to thank Dr. M. Dolores Calzada for her support in this thesis.

Thanks to the jury, the referees and examiners of this thesis. I am honoured that you have accepted these roles.

I would like to acknowledge the University of Orléans (France) and University of Córdoba (Spain) to give me the opportunity to develop my PhD thesis in their doctoral programs under co-supervised regime. Thank to European Research Council (ERC-CoG-648161-PHOROSOL) for the financial support and University of Córdoba for the international mobility fellowship (BOUCO, 2019/00213).

I would also like to express my gratitude to the CEMHTI-HT laboratory for the welcome during these three years. Thanks to Dr. Aurélien Canizarès for his unconditional help with Raman experiments, Dr. Cécile Genevois for her help with the TEM images and Dr. Emmanuel Véron for his guidance with the XRD. Thanks to Dr. Jimmy Nicolle from ICMN for his help with the conductivity measurements.

I would like to say a very special thanks to all the members of POR2E group, thank you for all the scientific and personal support and help during these years.

Thank you for the whole LIPs research group in University of Córdoba for the welcome during my stay in their laboratory.

Finally, I would like to thank all my family, Éride and friends for their constant understanding, love and care.

v

LIST OF FIGURES Figure I.1

Classification of carbon nanostructures upon their different dimensionality: 0D (fullerenes), 1D (single-walled carbon nanotubes), 2D (graphene) and 3D (graphite). Adapted from [START_REF] Bezzon | Carbon nanostructure-based sensors: A brief review on recent advances[END_REF].

Figure I.2

Scheme of the type of pores and external surface in nanoporous materials.

Figure I.3

Scheme of the chemical reactions occurring during the basic catalysed polycondensation of resorcinol and formaldehyde.

Figure I.4

Schematic representation of the relationship between the structure of the carbon materials and their electrical conductivity.

Figure I.5

Schematic representation of most plausible O-containing surface groups of graphene oxide: hydroxyl (pink), lactone (blue), carbonyl (purple), carboxyl (orange), epoxide (green), 5 and 6 membered rings lactols (yellow). (f) graphite, Gr; (g) graphene oxide, GO; (h) reduced graphene oxide, rGO and (i) graphene, G-I. Similar size glass vessels contain ca. 30 mg of each additive.

Figure III.2

Schematic representation of the structure of graphite, graphene oxide and reduced graphene oxide.

Figure III.3

Schematic drawing of the experimental atmospheric pressure microwave plasma set up used to synthesise the graphene materials using ethanol as carbon source (not to scale).

Figure III.4

Schematic process of the static synthetic protocol used for the preparation of the carbon gels and their composites with the additives.

Figure III.5

Schematic process of the dynamic synthetic protocol used for the preparation of the carbon gels and their composites with vi List of figures the additives.

Figure III.6

IUPAC classification of the gas adsorption isotherms. Adapted from [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF].

Figure III.7

IUPAC classification of the hysteresis loops. Adapted from [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF].

Figure III.8

Schematic representation of thermal desorption of the oxygen-functional groups as a function of the temperature.

Figure III.9

Schematic representation of graphitic layers in a crystal.

Figure III.10

Deconvolution of a first and second order Raman spectrum of a carbon black oxide (CB-IIIox).

Figure III.11

Schematic Raman mapping for a heterogeneous material constituted by three phases (blue, green and orange).

Figure IV.43

TEM images of the graphite (Gr), graphene oxide (GO) and reduced graphene oxide (rGO) used in this study.

Figure IV.44

Raman spectra of (a) graphite (Gr), (b) graphene oxide (GO) and (c) reduced graphene oxide (rGO).

Figure IV.45

X-ray diffraction patterns of (a) graphite (Gr), (b) graphene oxide (GO) and (c) reduced graphene oxide (rGO).

Figure IV.46

Relationship between I D /I G ratio (from Raman spectroscopy) and L c (green line) and L a (brown line) (from X-ray diffraction) for all studied materials.

Figure IV.64

Correlation between the atomic carbon content in the reactant, EtOH (green line), and in that estimated from the composition of the carbon-based by-products considering two scenarios: CO, C 2 H 2 , CH 4 , C (s) and C 2 H 4 (blue line) and CO, C 2 H 2 and C (s) (brown line).

Figure IV.77

Mass evolution of samples CB-III, GO and rGO obtained between 20 and 900 °C at 10 °C/min under Ar flow (100 mL/min).

Figure IV.78

Images of (a) the entire monolith of HS-PG and a cross section of (b) HS-PG-CB-III4 and (c) HS-PG-rGO4 monoliths.

Figure VI.1

Cyclic voltammograms (equilibrium response is shown, typically after ca. 5-6 scans); after equilibration of selected carbon gel/CB-III composites in 0.05 M Na 2 SO 4 using Hg/Hg 2 SO 4 (SME) as reference electrode and a graphite rod as counter electrode. Scan rate 0.5 mV/s. xv

LIST OF TABLES Table IV.1

Main textural parameters obtained from the N 2 and CO 2 adsorption isotherms at -196 °C and 0 °C, respectively, for the polymeric gel (sample PG) and the carbon gel (sample CPG).

Table IV.2

Main structural parameters of the studied carbon blacks obtained from the analysis of the deconvoluted Raman spectra.

Table IV.3

Main structural parameters of the studied carbon blacks obtained from X-ray diffraction patterns.

Table IV.4

Main textural parameters of the carbon blacks obtained from the analysis of the N 2 adsorption isotherms at -196 °C.

Table IV.5

Main textural parameters of the materials synthesised with different amounts of CB additive obtained from the analysis of the N 2 adsorption isotherms at -196 °C.

Table IV.6

Main structural parameters of the series of carbon gel composites prepared with different carbon blacks, obtained from the analysis of the deconvoluted Raman spectra.

Table IV.7

Mass losses obtained from the thermogravimetric profiles and during the carbonization process of the series of materials prepared with different amount of CB-III additive. The theoretical mass loss for each composite is also included.

Table IV.8

Main structural parameters of the series of materials with different amount of CB-III additive, obtained from the analysis of deconvoluted Raman spectra.

Table IV.9

Main textural parameters obtained from the N 2 at -196 °C and CO 2 at 0 °C adsorption isotherms for the materials synthesised with different amounts of CB-III as additive.

Table IV.10

Elemental analysis, surface pH and acidity of samples CB-III and CB-IIIox.

Table IV.11

Main structural parameters of samples CB-III and CB-IIIox obtained from the analysis of deconvoluted Raman spectra.

Table IV.12

Main textural parameters obtained from the N 2 adsorption isotherms and intensity ratios obtained from Raman spectra for the series of materials prepared using CB-III and CB-IIIox as additives.

Table IV.13

Mass losses obtained from the thermogravimetric profiles and during the carbonization process of the series of materials prepared using CB-III and CB-IIIox as additives. The theoretical mass loss for each composite is also included.

Table IV.14

Pellet's thickness and conductivity values obtained by 4-point probe method for the carbon gel/additive composites prepared with CB-III and CB-IIIox as additives.

Table IV.15

Main structural parameters of samples Gr, GO and rGO obtained from the analysis of deconvoluted Raman spectra.

Table IV.16

Main structural parameters of samples Gr, GO and rGO obtained from X-ray diffraction patterns.

Table IV.17 Elemental analysis and speciation of surface groups obtained from potentiometric titration for samples Gr, GO and rGO.

Table IV.18

Main textural parameters for sample Gr, GO and rGO obtained from the N 2 and CO 2 adsorption isotherms at -196 °C and 0 °C, respectively.

Table IV.19 pH of the reactants' mixture and main textural parameters obtained from the N 2 adsorption isotherms at -196 °C for Gr, GO and rGO additives and the series of materials synthesised with different amounts of additive.

Table IV.20

Main structural parameters of the series of samples prepared using Gr, GO and rGO additives obtained from the analysis of deconvoluted Raman spectra.

Table IV.21

Molar production rate of CO, C 2 H 2 , CH 4 , C 2 H 4 and C (s) as function of the flow of the carbon precursor, EtOH.

Table IV.22

Main structural parameters of the graphenes prepared using different ethanol flows obtained from X-ray diffraction patterns.

Table IV.23

Main structural parameters of the graphenes prepared using different ethanol flows obtained from the analysis of deconvoluted Raman spectra.

Table IV.24

Data obtained from XPS analysis for the graphenes prepared using different ethanol flows.

Table IV.25

Main textural parameters obtained from the N 2 adsorption isotherms at -196 °C for the series of materials synthesised with different amounts of graphene additive.

Table IV.26

Main structural parameters for the series of materials synthesised with different amounts of graphene additive obtained from the analysis of deconvoluted Raman spectra.

Table IV.27

Main textural parameters obtained from the N 2 adsorption isotherms at -196 °C for the pristine additives and their corresponding samples after the hydrothermal treatment.

Table IV.28

Main textural parameters obtained from the N 2 adsorption isotherms at -196 °C for samples PG and HS-PG.

Table VI.1

Main textural parameters obtained from the N 2 adsorption isotherms at -196 °C for the carbon materials prepared upon photoassisted polycondensation of the reactants in the presence and absence of CB-III as conductive additive.

Table A.I.1

Raman parameters obtained from the deconvoluted spectra for the carbon blacks used as additives.

Table A.I.2

Raman parameters obtained from the deconvoluted spectra for the pristine carbon gel.

Table A.I.3

Raman parameters obtained from the deconvoluted spectra for the carbon gel/CB composites prepared with 12 wt.%.

Table A.I.4

Thickness, diameter and mass of the pellets of carbon blacks, carbon gel and their corresponding carbon gel/CB composites with 12 wt.% additive used for the conductivity measurements. Conductivity values obtained by 4-point probe method.

Table A.I.5

Raman parameters obtained from the deconvoluted spectra for the carbon gel/CB-III composites.

Table A.I.6

Thickness, diameter and mass of the pellets of carbon gel/CB-III composites with various amounts of additive used for the conductivity measurements. Conductivity values obtained by 4-point probe method.

Table A.I.7

Raman parameters obtained from the deconvoluted spectra for CB-III and CB-IIIox.

Table A.I.8

Raman parameters obtained from the deconvoluted spectra for the carbon gel/CB-IIIox composites.

Table A.I.9

Raman parameters obtained from the deconvoluted spectra for graphite, graphene oxide and reduced graphene oxide.

Table A.I.10

Raman parameters obtained from the deconvoluted spectra for the composites prepared with 12 wt.% of graphite, graphene oxide and reduced graphene oxide.

Table A.I.11

Thickness, diameter and mass of the pellets of composites prepared with 4 and 12 wt.% of graphite, graphene oxide and reduced graphene oxide used for the conductivity measurements. Conductivity values obtained by 4-point probe method. Data of their corresponding controls is also shown.

Table A.I.12

Structure, fragmentation peaks and peaks' relative intensity of selected molecules.

Table A.I.13

Raman parameters obtained from the deconvoluted spectra of the graphene materials synthesised by plasma decomposition of ethanol.

Table A.I.14

Raman parameters obtained from the deconvoluted the spectra for the carbon gel/G-I composites.

Table A.I.15

Thickness, diameter and mass of the pellets of the composites prepared with sample G-I as additive. Conductivity values obtained by 4-point probe method. The corresponding controls are also shown for comparison.

LAYOUT

The research described in this thesis has been performed at CEMHTI (Conditions Extremes Materiaux: Haute Temperature et Irradiation) UPR 3079 of CNRS (Centre National de la Recherche Scientifique) in Orléans (France), as associated unit to University of Orléans, and at the group Laboratorio de Innovación en Plasmas (LIPs) of Universidad de Córdoba in Córdoba (Spain), in the framework of an international joint PhD agreement (convention de cotutelle internationale) between both universities. According to this agreement, and as a mean of broadening its diffusion in the scientific field, the English language is used throughout the text, and the defense will also be carried out in English. In addition, to facilitate the diffusion of the PhD thesis in the official languages of both Universities, abstracts summarizing the content of the manuscript will also be provided in Spanish and French.

The present PhD thesis explores the synthesis of different nanoporous carbon materials with improved electrical conductivity. The first chapter corresponds to the introduction that aims to provide a general overview of the synthesis methods employed to obtain the desired materials ranging from the conventional methods to the most advanced. The second chapter is focused to explain the general goals and the specific objectives pursued in this doctoral thesis.

The third chapter is devoted to the description of the materials' origin and its synthesis procedures employed; additionally, the experimental techniques, protocols and equipment used for the development of the research activities, and a description of the fundamentals of such techniques are presented. The results are collected and discussed in Chapter IV, divided in 5 sections.

The first one (Section IV.A) is devoted to the analysis of the synthetic methodology followed for the preparation of the gel and gel/additive composites. The second section (Section IV.B) is focused on the research of the use of carbon black as additive; in this section an exhaustive study of the effect of amount and the nature of the carbon black incorporated is made, discussing the impact in the porosity, structure and electrical conductivity of the obtained materials. The Section IV.C is devoted to the study on the use of graphite, graphene oxide and reduced graphene oxide as additives. The following section (Section IV.D) is dedicated to the use of graphene as additive, as well as the synthesis of graphene by microwave plasma at atmospheric pressure employing different flows of ethanol as a carbon source. The last section (Section IV.E) describes the hydrothermal process applied for selected nanostructured materials and as well as the hydrothermal approach applied to obtain composite gels materials.

xxii

Layout

The main conclusions of the PhD thesis are summarized in Chapter V. A summary of the perspectives and future work employing several innovative techniques is included in Chapter VI.

The research developed in this thesis has been mainly performed at CEMHTI in Orléans, as associated unit to University of Orléans. According to the agreement regulating this joint PhD thesis, a scientific internship of 9 months was carried out in LIPs group of Universidad de Córdoba (2018Córdoba ( -2019)).

The core of the results discussed in this PhD thesis has been published in the following peer-reviewed scientific journals:

SUMMARY

This PhD thesis has been developed in the framework of an international co-supervision between University of Orléans (France) and University of Córdoba (Spain). According to the agreement regulating the joint supervision, the thesis will be written and presented in English;

the oral defence will take place at University of Orléans.

Nanoporous carbons are key materials in many electrochemical applications over a wide variety of competitors (such as noble metals, non-noble metals or metal oxides) due to the diversity of materials with controlled pore architectures combined with adequate bulk and surface properties; particularly, chemical and mechanical stability, biocompatibility, rich surface chemistry and, most importantly, relatively high electronic conductivity.

Although some carbons (e.g., graphite, graphene, carbon nanotubes) present electronic properties close to those of metallic electrodes, this feature depends strongly on the spatial arrangement of the carbon atoms. Indeed, most nanoporous carbons are non-polycrystalline materials with a low degree of structural order, as a result of a high density of defects introduced in the twisted graphitic layers upon the development of a nanopore network. As a result, the electron mobility pathway characteristic of the graphenic sheets is greatly reduced, limiting the conductivity of nanoporous carbons (typically 4-5 orders of magnitude lower than graphite). Thus, efforts are yet needed to prepare nanoporous carbons with high and well-defined pore architectures combining high electrical conductivity without compromising the porous structure.

To increase the conductivity of nanoporous carbon electrodes without compromising the porosity, several strategies have been explored such as: i) doping the carbon material with heteroatoms; ii) coating with a conductive phase such as metallic nanoparticles, metals, other conductive carbon nanostructures; iii) synthesising 3D nanoporous graphene-like architectures, and iv) incorporating a conductive additive in the formulation of carbon electrodes inks. The latter is the common practice for the manufacturing of the electrodes at commercial scale in most electrochemical applications, being carbon black the most popular conductive additive.

Based on the previous studies carried out in the group, the objective of this PhD thesis consisted in exploring various synthetic approaches to obtain nanoporous carbons with high porous networks and enhanced conductivity upon incorporating various carbon nanostructures as conductive additives. In a first approach, the additive was incorporated during the synthesis of the nanoporous carbon material itself, rather than as percolator in the ink formulation typically applied for the preparation of electrodes (e.g., carbon material, binder and percolator). In a second approach, 3D nanoporous graphene-like architectures were obtained by hydrothermal approaches.

xxiv Summary Nanostructured carbon materials were selected to be employed as additives or as starting materials for the synthesis of nanoporous structures. They were chosen upon their composition, morphology and structural features: carbon black (commercial), graphite (commercial), graphene, graphene oxide and reduced graphene oxide (commercial). In this sense, the optimization and analysis of the experimental conditions to synthesise graphene by microwave plasma was carried out. A TIAGO torch microwave discharge at atmospheric pressure was employed allowing the synthesis of graphene by the decomposition of ethanol.

The ethanol flow was varied (from 2.00 to 4.00 g/h) showing differences on the gaseous by-products generated during the synthesis, under lower gas flow (from 2.00 to 3.4 g/h) the main gases detected were C 2 H 2 , CO and C (s), while for higher gas flows (ca. 4.00 g/h) other gases are generated as CH 4 and C 2 H 4 reducing the formation of C (s). Consequently, variations in the graphene production rate were observed being the maximum production 1.55 mg/min for ethanol flow of 3.4 g/h. Despite these differences, the features of the graphene material were slightly influenced by the employed ethanol flow.

Graphene oxide was also synthesised applying a modified Hummers method to graphite.

This method allowed the exfoliation of the graphite and increased its hydrophilic character by the incorporation of O-containing groups (oxygen content ca. 40 %).

After the synthesis and characterisation of the nanostructured materials, the first approach of the thesis was developed carrying out the synthesis of the nanoporous carbons by a modification of the polycondensation of resorcinol and formaldehyde mixtures well reported in the literature for the preparation of carbon gels. This approach allows obtaining highly porous materials with tuneable properties compared to conventional activation methods (e.g., physical and chemical activation of a carbon precursor) that usually render materials with broad pore size distributions in both the micropore and the mesopore range. The well-controlled pore architectures of carbon gels makes this kind of materials excellent ones for electrochemical purposes. To overcome the drawback of their low electrical conductivity, various carbon nanostructures were incorporated. The protocol for the synthesis of carbon gels was adjusted to allow a homogeneous dispersion of the additive in the carbon/additive composites. Furthermore, the effect of various parameters such as the type, amount; hydrophobic/hydrophilic character and composition of the additive was explored. The study revealed the important role of the nature of the additive not only on the conductivity of the carbon gel/additive composite, but also on the development of the porosity during the synthesis. Carbon additives of hydrophobic nature act as a porogen, favouring the development of multimodal pore architectures, with predominance of large mesopores.

Hydrophilic additives hinder the polycondensation reaction, rendering materials with lower porosity. Regarding the conductivity, the composites showed enhanced values, with Summary xxv percolation thresholds of ca. 8 wt.% for carbon black. Comparatively, higher conductivities were obtained for the use of graphene oxide at 12 wt.% (ca. 0.48 S/cm) and carbon black at 24 and 40 wt.% (ca. 0.22 and 0.28 S/cm, respectively). Overall, the results show the preparation of highly porous carbons with controlled mesopore architectures and enhanced conductivity. This facilitates the preparation of conductive carbon electrodes (either in monolithic and powder form), which are materials of interest for multiple processes (e.g., energy storage, supercapacitors, electrochemical sensors).

On the second approach, carbon black, graphene oxide and reduced graphene oxide were selected to be used for hydrothermal synthesis (moderate pressure, ca. 200 bar) of 3D structures. Firstly, aqueous suspensions of the three materials were treated ad 150 °C in an autoclave for 12 h. This hydrothermal treatment revealed an increase in the porosity for the graphite oxide and reduced graphite oxide; however, the carbon black was not affected by the hydrothermal treatment. These variations of the material's behaviour are related with the different nature of the materials. The expansion of the graphene oxide and reduced graphene oxide structures, as well as the partial reduction of the graphene layers allowed the increase of the electrical properties and suggest the possibility of the introduction of linkers between layers to stabilize the porous structure.

When the sol-gel synthesis was carried out under hydrothermal conditions, important differences were observed on the porosity of the pristine gels (without additive), mainly in the mesopores range. On the other hand, the synthesis of gel/additive composites under hydrothermal conditions revealed the formation of two-phases monoliths due to the sedimentation of the additive. Further research in this topic is carried out in the research group with the aim to improve the synthesis of nanoporous materials with enhanced conductivity by various approaches. These include photo-assisted protocols to accelerate the cross-linking of the sol-gel reactants in ca. 30-60 minutes (thereby preventing sedimentation), and self-assembly of graphene oxide and organic linkers in pillared structures.

RESUMEN

Esta tesis doctoral se ha desarrollado en el marco de una cotutela internacional entre la Universidad de Orléans (Francia) y la Universidad de Córdoba (España). De acuerdo con la convención de cotutela que rige el acuerdo entre ambas instituciones, la tesis será escrita y presentada en inglés, y la defensa tendrá lugar en la Universidad de Orléans.

Hoy en día existe un gran número de materiales para diferentes aplicaciones electroquímicas, como son los metales, metales nobles o óxidos metálicos. Sin embargo, los materiales de carbono nanoporoso se han convertido en uno de los más utilizados debido a su gran diversidad ya que permiten combinar su estructura porosa con una adecuada funcionalización. Además, presentan una elevada estabilidad química y mecánica, son biocompatibles y tienen relativamente alta conductividad eléctrica.

Algunos materiales de carbono (como son el grafito, el grafeno o los nanotubos de carbono) presentan propiedades eléctricas similares a los metales, ligadas principalmente a la distribución de los átomos de carbono en su estructura. No obstante, la mayoría de los materiales de carbono con bajo orden estructural y porosidad muestran un comportamiento diferente. Debido a que su estructura está formada por láminas grafénicas curvadas y con defectos, la movilidad de los electrones a través de las láminas grafíticas se ve seriamente reducida. Por esta razón, los materiales de carbono nanoporosos presentan conductividades eléctricas entre 4 y 5 órdenes de magnitud inferiores a la del grafito, este hecho hace que los materiales de carbono nanoporosos no sean buenos candidatos para determinadas aplicaciones, especialmente en el campo de la electroquímica.

Con el objetivo de obtener materiales de carbono con una elevada conductividad eléctrica, diferentes estrategias han sido estudiadas en la literatura, entre las que destacan: i) dopado del material de carbono con heteroátomos; ii) incorporación de una fase conductora (ej., nanopartículas metálicas, aditivos conductores); iii) síntesis de estructuras 3D basadas en grafeno; iv) incorporación post-sintética de un aditivo conductor en la tinta utilizada para la fabricación de electrodos. Esta última opción es la más utilizada en la mayoría de las aplicaciones electroquímicas de los materiales de carbono, siendo el negro de carbono el aditivo más empleado.

El principal objetivo de esta tesis doctoral es la preparación de materiales de carbono nanoporosos que combinen estructura porosa y conductividad eléctrica mejorada. Para ello, y teniendo en cuenta la experiencia previa del grupo de investigación, se utilizaron dos enfoques: i) la utilización de diferentes materiales de carbono nanoestructurados como aditivos de geles de carbono obtenidos mediante la policondensación de mezclas resorcinol y formaldehído, y ii) la preparación de materiales grafénicos 3D con porosidad moderada y conductividad elevada mediante técnicas hidrotermales.

xxviii Resumen Previamente a la síntesis de los materiales porosos, diferentes materiales de carbono nanoestructurados fueron seleccionados (para su utilización bien como aditivos o bien como materiales de partida en función de la ruta sintética) según su composición, morfología y propiedades estructurales: negro de carbono, grafito, grafeno, óxido de grafeno y óxido de grafeno reducido. El negro de carbono, grafito y óxido de grafeno reducido se adquirieron comercialmente; sin embargo, el grafeno y el óxido de grafeno se prepararon en el laboratorio.

En este sentido, la síntesis de grafeno se llevó a cabo a través de plasma de microondas a presión atmosférica utilizando un flujo de etanol como fuente de carbono. Para ello, se optimizaron las condiciones de flujo de etanol, evaluando su efecto en la tasa de producción y en calidad del material obtenido. El análisis de los subproductos gaseosos derivados de la descomposición de etanol en el plasma de microondas (flujo de alimentación entre 2.00 y 4.00 g/h) muestra fundamentalmente la presencia de especies tales como C 2 H 2 , CO y C (s) para flujo bajos (de 2.00 a 3.40 g/h), mientras que cuando se aumentó el flujo de etanol (ca. 4.00 g/h), lo que implica que hay más átomos de carbono disponibles en el medio, otras especies fueron generadas, CH 4 y C 2 H 4 , en detrimento de la generación de C (s). Así, se encontró que la máxima tasa de producción de grafeno fue obtenida para el flujo de etanol de 3.4 g/h, siendo de 1.55 mg/min. Por su parte, la caracterización morfológica de los grafenos sintetizados reveló un orden estructural muy similar en los materiales con independencia del flujo de etanol introducido en el plasma de microondas.

Por otro lado, el óxido de grafeno se sintetizó a partir de grafito mediante una modificación del método de Hummers. Este método mejorado provocó la exfoliación laminar del grafito a la vez que incrementó significativamente el carácter hidrófilo del óxido de grafeno incorporando una gran cantidad de grupos funcionales oxigenados en la superficie y los bordes de las láminas de grafeno (contenido en oxígeno ca. 40 %).

Una vez preparados los materiales nanoestructurados, el primer enfoque se llevó a cabo siguiendo una modificación de la reacción de policondensación de mezclas resorcinol y formaldehído de la síntesis descrita en la literatura, para la incorporación homogénea de los aditivos de carbono. De esta forma, se obtuvieron geles y geles de carbono con un gran desarrollo textural. Se estudió el efecto de varios parámetros como la naturaleza y la cantidad del aditivo en las propiedades (textura, composición, conductividad) de los geles de carbono.

Los resultados obtenidos mostraron un efecto del aditivo de carbono, no solo en la conductividad eléctrica del composite final, sino también en la porosidad de los materiales, favoreciendo la formación de mesoporos. Los aditivos de naturaleza hidrófoba favorecieron un desarrollo multimodal de la porosidad de los composites, con la formación de mesoporos grandes. Por el contrario, los aditivos de naturaleza hidrófila dieron lugar a materiales con mucho menor desarrollo textural. Respecto a la conductividad, en general todos los Resumen xxix composites obtenidos mostraron valores de conductividad eléctrica más elevados comparados con la conductividad obtenida para el gel de carbono sin aditivo. Se obtuvieron límites de percolación para el negro de carbono de un 8 % en masa. A modo de comparación, los mayores valores de conductividad eléctrica se obtuvieron para los composites sintetizados con un 12 % en masa de grafeno oxidado (ca. 0.48 S/cm), seguido por los geles obtenidos tras la adición de negro de carbono en porcentajes de 24 y 40 % en masa (ca. 0.22 y 0.28 S/cm, respectivamente).

Los resultados obtenidos muestran la posibilidad de sintetizar materiales de carbono con elevada conductividad eléctrica y con un desarrollo de la porosidad controlado, especialmente en el rango de los mesoporos. Estos materiales resultan interesantes para su uso en aplicaciones electroquímicas ya que pueden ser utilizados tanto en forma de monolito como en polvo y pueden ser empleados como electrodos, por ejemplo, en aplicaciones de almacenamiento electroquímico de energía o sensores.

Por otro lado, se seleccionaron tres de los materiales de carbono nanoestructurados (negro de carbono, óxido de grafeno y óxido de grafeno reducido) para la síntesis de materiales nanoporosos 3D por vía hidrotermal (presión moderada, ca. 200 bar). En primer lugar, suspensiones acuosas de cada uno de los tres materiales seleccionados fueron tratados a 150 °C en una autoclave durante 12 h. Este tratamiento reveló un incremento de la porosidad del material obtenido para el óxido de grafeno y óxido de grafeno reducido, sin embargo, el desarrollo textural del negro de carbono no se vio afectado por el tratamiento hidrotermal.

Estas diferencias se asocian a la diferente naturaleza de los tres materiales estudiados. El hecho de producir la expansión de la porosidad del óxido de grafeno y el óxido de grafeno reducido, junto con la reducción parcial de las láminas de grafeno permite recuperar la conductividad eléctrica del material, y sugiere que es posible explorar rutas alternativas para la incorporación de pilares entre las láminas grafénicas para estabilizar la porosidad obtenida.

Asimismo, se llevó a cabo la síntesis sol-gel de los composites en condiciones hidrotermales, observándose notables diferencias en la estructura porosa del gel (sin aditivo), especialmente en el rango de mesoporos. Por último, estudios preliminares de las síntesis de los composites mediante tratamiento hidrotermal, mostraron la sedimentación del aditivo durante la misma, no siendo posible la obtención de materiales homogéneos. Actualmente, en el grupo de investigación se sigue trabajando en esta línea con el objetivo de obtener materiales de carbono con porosidad controlada y elevada conductividad. Entre las distintas alternativas, se está explorando la polimerización fotoasistida de los precursores en presencia del aditivo, ya que permite disminuir el tiempo de reacción a 30-60 minutos, mejorando la dispersión del aditivo al evitar la sedimentación. Nowadays there are a large number of carbon materials with a wide variety of features and nanostructures that becomes them in a key material for many applications such as, catalysis, adsorption, wastewater treatment, energy storage and so on. These applications are mainly a consequence of the variety of coordination chemistry of carbon atoms and their unique ability Adapted from [START_REF] Bezzon | Carbon nanostructure-based sensors: A brief review on recent advances[END_REF].

The International Union of Pure and Applied Chemistry (IUPAC) defines a porous material as a solid matrix composed of an interconnected network of pores (voids) filled with a fluid (liquid or gas) [IUPAC, 2019]. The structure of porous carbons is composed by distorted graphene-like layers of sp 2 domains of carbon atoms, with heteroatoms and sp 3 carbon atoms arranged in graphene layers with varied degree of planarity and dimensionality. The different degrees of the staked layers define the graphitization extent of the material. Thus, the structure of the carbon skeleton in porous carbons can be considered as a non-organized carbon phase composed of complex aromatic-aliphatic forms and twisted/distorted graphitic sheets. The crystallites are randomly oriented and interconnected, with voids, defects and interstices between the crystallites, ultimately generating pores of nanometric dimensions. Such structure is strongly dependent on the synthesis conditions [START_REF] Gogotsi | Carbon Nanomaterials[END_REF]Marsh, 2006;Tascón, 2012].

The pores in carbon materials are scattered over a wide range of shapes and sizes (Figure I.2). According to their shape it is possible to classify them in open pores (with access to the surface), closed pores (without access to the surface), blind pores (with a single connection to the surface) and interconnected pores (connected with other pores). Based on the sizes, according to IUPAC [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF], the pores can be classified as micropores (pore widths lower than 2 nm), mesopores (widths between 2 and 50 nm) and macropores (pore widths above 50 nm). In the last decades, the term nanopores was also included to refer to the pores with widths up to 100 nm, accordingly the term nanoporous carbons address to the micro-mesoporous carbon materials with porous widths lower than 100 nm [START_REF] Rouquerol | In: Adsorption by powders and porous solids. Principles, methodology and applications[END_REF].

The size and the accessibility of the pores govern the performance of porous carbons for an intended application such as adsorption of molecules for wastewater treatment, heterogeneous catalysts support, filters and energy storage [START_REF] Bandosz | Surface chemistry of activated carbons and its characterization, In: Activated carbon surfaces in environmental remediation[END_REF]Tascón, 2012]. The chemical activity of the nanoporous carbons also depends on their dimensionality and electronic hybridization of the carbon atoms. For instance, in graphite (carbons in sp 2 hybridization) no interaction in plane is produced with other chemical species in the surrounding environment. However, when defects (free edge sites) or dopants are present, the reactivity of the carbon increases, but their electronic properties may be reduced depending on the nature of the dopant. In this sense, chemical doping with N-groups is reported to increase the electronic conductivity of carbons, whereas the incorporation of electron withdrawal groups (e.g., oxygen, halogens) can drastically decrease the electron mobility.

Defect generation can also alter the local properties of carbon materials.

External surface Micropores Mesopores

Macropores

A wide variety of carbon materials characterised by a high conductivity (such as, graphite, graphene, carbon nanotubes, carbon blacks or carbon nanofibers) have been employed in electrochemical fields (e.g., electrocatalysis, electrochemical sensors). The use of porous electrodes is interesting in many applications, since adsorption and diffusion processes may be crucial, for instance, to enhance the electrode sensitivity (improving contact between the electrode surface and the target molecule) [START_REF] Banks | Current distribution in the three-dimensional random resistor network at the percolation threshold[END_REF]. In this sense, nanoporous carbons are interesting electrode materials due to the possibility to design the nanopore structure, and to tune the surface functionalization to favour electrode/molecule interactions thereby controlling the electrochemical response. However, the electrical conductivity of nanoporous carbons is rather limited (typically 4-5 orders of magnitude lower than graphite) due to the high porosity. Thus, efforts are yet needed to prepare nanoporous carbons with high and well-defined pore architectures combining high electrical conductivity without compromising the porous structure.

I.2. Synthesis of nanoporous carbon materials.

The preparation of nanoporous carbons, also known as activated carbons from charcoals is an ancient artwork, and extensive work has been done to enhance the porosity of the resulting carbon adsorbents. There are conventional synthesis methods that are widely used in different industries and basic research. Additionally, in the last decades some alternatives to these conventional methods have been emerged with the aim to control the porous structure and optimize the resources employed. In the next subsections a brief description of the conventional methods is presented following by the description of the novel synthesis processes. Among the different novel approaches, an exhaustive description of the sol-gel method has been included, since was the selected one to synthesise the nanoporous carbon materials described in this PhD thesis.

I.2.1. Conventional methods.

Activated carbons are produced at industrial scale by so-called conventional activation methods (i.e., physical and chemical activation). Most processes start with a carbonization (pyrolysis) or an oxidation step of the precursor, followed by the exposure of the carbonized material to a reactive atmosphere in the presence of an activating agent that allows the development of a porous structure.

Extensive work has been done to enhance the porosity of the resulting carbon adsorbents by the use of different activating agents, the choice of precursors, the optimization of the activation conditions (temperature, heating rate, gas flow), and the usage of various impregnation methods (soaking, stirring) [Marsh, 2006;Tascón, 2012].

I.2.1.a. Physical activation.

The term thermal or physical activation refers to the development porosity in the gas phase, usually by reaction of a char with oxygen air, CO 2 , steam or their mixtures. The reaction is generally at elevated temperatures (between 750-1100 ᵒC) and proceeds through the formation of surface oxygen complexes. A detailed review on the understanding of the reactions of the physical activation with these gases can be found in [Marsh, 2006]. Physical activation is commonly carried out as a two-stage process, starting by the carbonization of the precursor and further activation of the obtained char. The carbonization or pyrolysis of the precursor involves its thermal transformation to moderate/high temperatures in an inert atmosphere, aiming to eliminate most of the volatile matter. The solid obtained is a carbonaceous char, composed of carbon atoms arrangement in a crosslinked and disorganized structure that has an incipient porosity (interstices between the aromatic layers). A subsequent activation of the char enlarges the incipient porosity and creates new pores. This is carried out under a reactive atmosphere in the presence of oxidants such as oxygen (or air), steam, and carbon dioxide. Many experimental conditions influence the final textural development of the final activated carbon (e.g., the precursor's features, the heating rate, the gas flow rate, the temperature and dwelling time and the activating gas).

I.2.1.b. Chemical activation.

Chemical activation is considered as a reaction between a solid precursor and a chemical activating agent during the heating of the mixture in an inert atmosphere. As opposed to physical activation, this is usually carried out in a single carbonization stage. The most common activating agents are ZnCl 2 , H 3 PO 4 and alkali hydroxides (e.g., KOH and NaOH), and most studied precursors are lignocellulosic materials, coals and chars. Chemical activation has recognized advantages for the production of activated carbons with high developed porosity, as it leads to higher yields and allows the use of lower temperatures than physical activation.

The main parameters that affect the porous texture of the chemically activated carbons are the impregnation ratio (ratio between the raw precursor and activating agent weights), activation temperature and heating rate, dwelling time and activation atmosphere. The activation mechanisms of physical and chemical activation methods are different, and so is the porosity of the resulting carbons. In general, physical activation yields predominantly microporous carbons with a small contribution of mesoporosity, while chemical activation allows the formation of wider micropore size distributions -upon the activating agent usedand favours the development of mesoporosity.

I.2.2. Synthetic approaches for control the porous development.

None of the conventional activation procedures for the synthesis of nanoporous carbons allow a fine control of the porosity of the resulting material. Thus, synthetic alternatives are needed to prepare nanoporous carbons with high and well-defined pore architectures [Mestre, 2017]. Among most relevant methodologies to control the porosity of carbons at nanometric scale, several strategies can be considered:

I.2.2.a. Hydrothermal carbonization.

A hydrothermal carbonization is a thermochemical process in subcritical water for the conversion of wet/dry biomass into carbon materials. During this process, hydrolysis reactions take place boosting the decomposition of the biomass leading to the formation of a carbonaceous solid (known as hydrochar). The conditions of the hydrothermal carbonization conditions (temperature, residence time, pressure and so forth) define the characteristics of the hydrochar (composition, stability, reactivity, porosity). Most hydrochars display poor textural parameters, and very often they are subjected to further post-synthetic activation to improve the porosity. As a result, the control in the final pore structure is also governed by the second activation method. However, hydrothermal carbonization is considered as a cost effective and eco-friendly process, since uses water as a solvent, mild temperatures and self-generated pressure [START_REF] Berl | The development of carbon[END_REF][START_REF] Titirici | Chemistry and materials options of sustainable carbon materials made by hydrothermal carbonization[END_REF]Demir-Cakan, 2019].

I.2.2.b. Nanocasting.

The synthesis of nanoporous carbons by nanocasting requires the use of a template.

Endo-templating or soft-templating methods refer to the use of a template that is self-assembled in a solvent in the presence of a polymerizable carbon precursor. The carbon material is obtained after a carbonization process, where the template is destroyed [START_REF] Lu | Nanocasting: a versatile strategy for creating nanostructured porous materials[END_REF][START_REF] Chuenchom | Recent progress in soft-templating of porous carbon materials[END_REF]. It has been mainly used for obtaining mesoporous carbons through the self-assembly of block copolymers and precursors such as phenolic resins or resorcinol. On the other hand, hard-templating or exo-templating approaches use a porous solid as sacrificial scaffold, and the porous carbon obtained is an inverse replica of the scaffold. It has been mainly used with zeolites of varied topology, and mesoporous silica/oxides and opals or colloidal silica templates to obtain micro-, meso-and macroporous materials, respectively [START_REF] Ryoo | Synthesis of highly ordered carbonmolecular sieves via template-mediated structuraltransformation[END_REF]. Despite hard-templating has marked a milestone in controlling the pore structure of carbons, the process is still quite complex, and presents the disadvantage of the removal of the template.

I.2.2.c. Sol-gel approaches.

Sol-gel approaches are a bottom-up technique that allows the control of the chemical composition and porous features of the resulting carbon. The sol-gel process typically starts with the polymerization and condensation of the precursors, typically hydroxylated compounds (e.g., resorcinol, phenol) and aldehydes (e.g., formaldehyde, glyoxal) in the presence of a catalyst. When the reaction starts, polymeric chains grow into colloidal particles, which crosslinking leads to the stiffening of an organic gel with an interconnected structure.

The catalysed sol-gel polycondensation is an endothermic reaction, for which external heating is required (typically 70-100 ᵒC).

Conventional heating furnaces are applied for this purpose, although methods combining microwave heating [START_REF] Rojas-Cervantes | Some strategies to lower the production cost of carbon gels[END_REF] and photoassisted methods [START_REF] Balan | A green and fast approach to nanoporous carbons with tuned porosity: UV-assisted condensation of organic compounds at room temperature[END_REF] have also been reported to reduce the reaction time and the energy consumption. The curing of the polymeric clusters is therefore important to define the crosslinking and shape of the gel.

Resorcinol-Formaldehyde gels.

After the early works of Pekala and co-workers reporting the preparation of carbon gels by resorcinol-formaldehyde (RF) polycondensation reaction [START_REF] Pekala | Organic aerogels from the polycondensation of resorcinol with formaldehyde[END_REF], nanoporous carbons gels have become interesting materials with an outstanding performance in various fields as adsorbents, catalyst supports, energy storage devices, and electrochemistry [Al-Muhtaseb, 2003;[START_REF] Job | Porous carbon xerogels with texture tailored by pH control during sol-gel process[END_REF][START_REF] Elkhatat | Advances in tailoring resorcinol-formaldehyde organic and carbon gels[END_REF]. In this thesis the RF polycondensation in water solution and basic catalysts process has been followed for the synthesis of nanoporous carbon materials with the aim to control the porous network.

The reaction takes place in two steps: addition (1) and condensation (2) [START_REF] Pekala | Structure of organic aerogels. 1. Morphology and scaling[END_REF]Al-Muhtaseb, 2003] (Figure I.3):

(1) Firstly, the resorcinol anions are formed by hydrogen abstraction that is favoured by the presence of OH -(presence of basic catalyst). Then, through a nucleophilic attack the formaldehyde molecules are added to the resorcinol anions (positions 2, 4 and/or 6) forming hydroxymethyl derivatives.

(2) Condensation of hydroxymethyl derivatives (methylene and ether bonds) takes place giving rise the formation of particles namely clusters. The clusters are crosslinked and grown up to gelation occurs yielding a three-dimensional porous structure.

After the gelation, a curing step is needed in which the remaining hydroxyl groups continue the polymerization reaction increasing the bonds between the clusters. Therefore, a drying process is necessary in order to eliminate the solvent and made accessible the porosity of the gel. Three processes of dry can be applied: i) supercritical drying consists in exchanging the water solvent by an organic solvent and then the latter is replaced by CO 2 under supercritical conditions (high temperature and pressure), after that, the CO 2 is removed changing the conditions (i.e., reduced pressure). The gels obtained by this method are named aerogels.

ii) Freeze-drying, as in the last case, previously, the water is replaced by an organic solvent, then, it is frozen and removed by sublimation. Cryogels are the materials obtained by this technique. iii) Subcritical drying, this method consists in the solvent evaporation rendering materials known as xerogels. The selected drying process may have a high influence on the porous features of the final organic gel. In this PhD thesis, according to the easy-operation and eco-friendly method (avoiding the use of organic solvent) a subcritical drying was applied for all the obtained gels.

The characteristics of the organic gel strongly depend on the solution pH, the components molar ratios, the curing temperature, drying conditions, among others. The effect of some critical parameters is briefly addressed below: i) pH of the reaction medium. As above-mention, the addition step (1) depends of the pH of the media, while the condensation step (2) is not affected by this parameter. When the reaction is carried out at basic pH, the ratio OH -/H + is increased enhancing the formation of hydroxymethyl derivatives and consequently, highly branched clusters are obtained. As pH decreases, the number of resorcinol anions decreases reducing the amount of hydroxymethyl derivatives generated, the obtained clusters are weakly branched and the number of particles in the gel decreases (increasing their size). As a general trend, the higher the pH, the lower the mean pore size of the polymer. Additionally, the high degree of crosslinking (higher pH) confers some strength to the polymeric structure resulting in different mechanical properties as a function of the pH [Job, 2014;Arenillas, 2019].

ii) Ratio of the reactants. The molar ratio of the monomers (R/F) is usually fixed at 0.5. However, several studies reported that the ratio R/C has a high influence in the porous development and the mechanical properties of the organic gels. Materials obtained by the high R/C ratios undergo the formation of large pores (mainly mesopores) while low R/C molar ratios favours the formation of microporous carbon gels with low specific surface areas and poor mechanical properties [Al-Muhtaseb, 2003;[START_REF] Macías | Carbon black directed synthesis of ultrahigh mesoporous carbon aerogels[END_REF].

iii) Gelation temperature. As a general trend, as the temperature of the gelation step (from 70 to 85 °C) increases, the textural features of the gels are improved, specifically in the mesopore range, being the effect on the microporosity almost negligible [START_REF] Isaacs-Páez | Fast synthesis of micro/mesoporous xerogels: Textural and energetic assessment[END_REF].

The carbonization of gel is required to eliminate the volatiles (non-crosslinked organic chains, labile oxygen and hydrogen surface groups) obtaining a rich-carbon material, namely carbon gel. Additional activation can also be performed during or after carbonization, aiming to favour the porous development [Gomis-Berenguer, 2017;Arenillas, 2019].

I.3. Methods to enhance the electrical conductivity.

The electrical conductivity (σ), reciprocal of the resistivity (ρ), is defined as the measure of how easily electrons move through a material under the action of an electric field. Therefore, the electrical conductivity is related to the existence of atomic energy levels that allow the mobility of electrons freely throughout the material network. In the case of metallic conductors, this mobility is due to the presence of d orbitals; while in carbon materials the electronic mobility is due to the delocalization of the electrons in the orbitals with sp 2 hybridisation.

As above-mentioned, carbon exists in many allotropic forms. Graphite, graphene and carbon nanotubes are carbon allotropes characterised by a hexagonal structure with predominantly a sp 2 bond network, which gives rise to highly conductive carbon materials. In contrast, other allotropic forms such as diamond present a tetrahedral disposition with carbon atoms in sp 3 hybridization, reducing the electronic mobility and displaying typically insulating properties. Disordered and porous carbons belong to an intermediate group, as these materials present a broad variety of electronic structures due to different sp 2 /sp 3 bond ratios.

Such carbon materials maybe classified upon their electronic arrangements in: i) graphitic carbons, whose microscopic structure is dominated by a random assembly of nanosized graphitic sp 2 carbon domains; and ii) non-graphitic carbons, with a wide distribution of sp 2 /sp 3 carbon bonds through the structure, thus reducing the mobility of the π-electrons in the conjugated sp 2 carbon domains [START_REF] Dresselhaus | Graphite fibers and filaments[END_REF][START_REF] Takai | Structure and electronic properties of a nongraphitic disordered carbon system and its heat-treatment effects[END_REF]. In the case of porous carbon materials, the porosity influences the conductivity as it commonly breaks the accessibility between conjugated sp 2 domains responsible for the mobility of the electrons. Therefore, nanoporous carbons are frequently characterised by limited electronic conductivity; indeed, as a general trend, the higher the surface area, the lower the electrical conductivity (Figure I.4). This is a drawback of this kind of materials, especially for their use as electrodes for electrochemical applications.

Accordingly, ones the different methods of synthesis to control the porosity of carbon materials are described in the above section; in the following subsections several strategies to increase the conductivity of nanoporous carbon without compromising the porosity have been explored.

I.3.1. Doping the carbon material with heteroatoms.

The introduction of heteroatoms (such as O, N, P or B) is a well-known method for enhancing the electrochemical performance of carbon materials. In particular, doping processes with electron-rich nitrogen (e.g., quaternary nitrogen) atoms moieties increase the electrical conductivity due to formation of a new electronic state near the Fermi level. On the contrary, the presence of surface oxygen increases the electrical resistivity (reduce the conductivity) [START_REF] Ewels | Nitrogen Doping in Carbon Nanotubes[END_REF][START_REF] Pandolfo | Carbon properties and their role in supercapacitors[END_REF]Mostazo-López, 2018;[START_REF] Yang | Improvement of a commercial activated carbon for organic electric double-layer capacitors using a consecutive doping method[END_REF].

I.3.2. Coating with a metallic conductive phase.

The electrical conductivity of the nanoporous carbon material is improved by the uniform dispersion of a metallic conductive phase, including metallic nanoparticles (such as Cu, Ag) or metal oxides [START_REF] Zafra | A novel method for metal oxide deposition on carbon aerogels with potential application in capacitive deionization of saline water[END_REF]Macías, 2016_C]. The success of the method depends strongly on the dispersion and the size of the metallic aggregates, which is also linked to the method and the metal used.

I.3.3. Synthesis of 3D nanoporous graphene-like architectures.

Three-dimensional graphene-based architectures materials exhibit a combination of high structural order and outstanding properties of graphene. As a consequence, the materials present low density, stable mechanical properties, high surface area and high mass transport.

Additionally, the interconnected pores in a 3D network provides the material to fast channels, which favours the mobility of the electrons through the structure, producing a high electrical conductivity and improved structural stability [START_REF] Lee | Extremely high electrical conductance of microporous 3D graphene-like zeolite-templated carbon framework[END_REF][START_REF] Zhao | Porous 3D few-layer graphene-like carbon for ultrahigh-power supercapacitors with well-defined structure performance relationship[END_REF].

The preparation of the 3D macroscopic graphene-based materials can be carried out following diverse techniques: i) flow-directed self-assembly to perform graphene oxide (GO) or reduced graphene oxide (rGO) papers applying a vacuum filtration, ii) layer-by-layer assembly by organic and/or metallic linkers to prepare graphene organic framework (GOF) and graphene-metal organic framework (MOF); iii) chemical vapour deposition (CVD) allows the preparation of graphene-based foams using three-dimensional nickel or copper structures as a template; iv) wet-spinning of graphene oxide by gelation or hydrothermal treatment; v) ice-segregation induced self-assembly allows the conformation of 3D graphene porous building-blocks through the unidirectional freezing of a mixture of graphene and a polymeric compound, such as polystyrene or poly(ethylene oxide) and subsequent sublimation [Huang, 2012;Li, 2012].

I.3.4. Incorporation of conductive additives.

The conductivity of carbon electrodes prepared by incorporating a conductive material in the formulation of the inks is still limited [START_REF] Dangles | Role of conducting carbon in electrodes for electric double layer capacitors[END_REF]Rey-Raap, 2014_A] due to the high percolation thresholds to achieve an adequate 'point to point' conductive network with carbon black, today's most popular conductive additive due to its low cost, and reasonably high chemical stability and electrical conductivity [Béguin, 2009;[START_REF] Dangles | Role of conducting carbon in electrodes for electric double layer capacitors[END_REF]. To overcome this limitation, attempts have been made to incorporate carbon nanostructures with high electrical conductivity and different aspect ratio (such as carbon nanotubes, carbon nanofibers and graphene derivatives) as alternatives to replace carbon black as conductive additive [START_REF] Li | A novel network composite cathode of LiFePO 4 /multiwalled carbon nanotubes with high rate capability for lithium ion batteries[END_REF][START_REF] Pico | Understanding carbon-carbon composites as electrodes of supercapacitors: A study by AC and DC measurements[END_REF][START_REF] Raymundo-Piñero | Carbon nanotubes as nanotexturing agents for high power supercapacitors based on seaweed carbon[END_REF][START_REF] Wang | Understanding and recent development of carbon coating on LiFePO 4 cathode materials for lithium-ion batteries[END_REF][START_REF] Zhang | Percolation threshold of graphene nanosheets as conductive additives in Li 4 Ti 5 O 12 anodes of Li-ion batteries[END_REF].

In a previous study of our research group, a different approach was explored, consisting in incorporating the additives (structural and/or conductive) during the synthesis of the nanoporous carbon material itself [START_REF] Macías | Carbon black directed synthesis of ultrahigh mesoporous carbon aerogels[END_REF]Rasines, 2015_A;Rasines, 2015_B;Macías, 2016_A;Macías, 2016_B]. These studies showed that the incorporation of low amounts of additives could be integrated to the polymeric structure, and reinforce the mechanical and electronic properties of the gels. However, obtaining stable and homogeneous suspensions is not a trivial issue for high loadings and hydrophobic additives [START_REF] Macías | Carbon black directed synthesis of ultrahigh mesoporous carbon aerogels[END_REF][START_REF] Guo | Graphene oxide as an anti-shrinkage additive for resorcinol-formaldehyde composite aerogels[END_REF][START_REF] Canal-Rodríguez | Graphene-doped carbon xerogel combining high electrical conductivity and surface area for optimized aqueous supercapacitors[END_REF].

In this PhD thesis, one of the challenges was the incorporation of different additives to gel to synthesised highly porous carbon materials with enhanced electrical conductivity. With this aim several carbonaceous nanostructured additives were employed including carbon black, graphite, graphene oxide, reduce graphene oxide and graphene.

I.3.4.a. Carbon black.

Carbon blacks are formed through incomplete combustion of either gaseous or liquid hydrocarbons. These materials are important industrial products used as colorants in inks and applied in large amount for rubber reinforcement. They are polycrystalline colloidal carbon particles, which are in different sizes and more or less coalesced into aggregates. Due to the high temperature treatment, their structure is highly ordered consisting in parallel graphene layers with a high number of edge-plane like defects. These properties ensure a high reactivity and electronic conductivity. As a function of the synthesis route, the carbon blacks can exhibit a wide range of properties such as variety of chemical composition, porosity or hydrophilic/hydrophobic character [Marsh, 2016].

I.3.4.b. Graphite.

Graphite is one of the allotropic forms of carbon widespread in nature given its resistance and stability against external agents. In addition to being able to find it naturally, the synthesis of graphite is possible by heating amorphous carbons to temperatures above 2500 ᵒC. This thermal process facilitates the orientation of the different carbonaceous layers, resulting in a material with an ordered structure.

The structure of the graphite is based on a layered arrangement of sp 2 carbon atoms with a planar trigonal stereochemistry forming a flat condensed system of aromatic rings. These sp 2 -based sheets (graphene layers) are bound to each other along the z-axis direction by weak dispersive and Van der Waals forces. The layers do not lie immediately above and below each other; they are displaced, giving rise to two crystalline structures, the hexagonal (ABAB) and the rhombohedral (ABCABC) arrangement. Regardless, the distance between carbon atoms in the same plane is 0.1412 nm, whereas the spacing between layers is 0.33545 nm. Graphite is an anisotropic solid, with different structural properties along the axes [Marsh, 2006].

Among its physicochemical properties, it should be noted that the graphite has a thermal and electrical conductivity with values close to those obtained for metallic materials.

Particularly, the high electrical conductivity of graphite is related to the delocalization of the π-electrons through the basal plane.

I.3.4.c. Graphene oxide.

Graphene oxide (GO) consists of a monolayer or few-layers stacked of graphene whose structure has been modified or distorted by the decoration with oxygen-containing functional groups (hydroxyl, epoxy, carboxyl and carbonyl) or by the presence of holes created as a result of a highly aggressive oxidation process. GO is considered as nonconductive hydrophilic carbon material with high-performance structural properties. The first synthesis process of this material was reported for the first time by Brodie and co-workers in 1859 [START_REF] Brodie | On the atomic weight of graphite[END_REF], since that publication, a significant progress was made being the synthesis reported by Hummers in 1958 [START_REF] Hummers | Preparation of graphitic oxide[END_REF] the most commonly method used today. Briefly, the synthesis method consists on the oxidation of graphite by treatment with KMnO 4 and NaNO 3 in concentrated H 2 SO 4 . Several modifications of the Hummers method can be found on the literature (varying the oxidant-graphite proportion, reaction times or acidic solution composition) that show alternative methods to obtain higher degree of oxidation, higher reaction yield, regular structures and less toxic by-products generation [START_REF] Park | Chemical methods for the production of graphenes[END_REF][START_REF] Marcano | Improved synthesis of graphene oxide[END_REF][START_REF] Cruz-Silva | Super-stretchable graphene oxide macroscopic fibers with outstanding knotability fabricated by dry film scrolling[END_REF].

Nowadays, the GO structure is under discussion as a consequence of its non-stoichiometric nature. However, GO is known to contain a hydrophobic part related to pristine graphite and a hydrophilic part due to the presence of O-containing functional groups bonded to its structure, such as hydroxyl, epoxy, carbonyl, carboxyl, and, in less proportion, five and six-membered ring lactols, both at the basal and at the edges (Figure I.5), generated after the oxidation process [Gao, 2009]. The electronic properties of GO are strongly influenced by its degree of oxidation and chemical composition; the presence of oxygenated groups bound to the structure confers an insulating character to this graphene-based material. However, the partial elimination or reduction of O-functionalization favours the restoration of the aromaticity of the carbon atoms, which improves the sp 2 /sp 3 ratio and enhances the electrical conductivity. The remaining oxygen groups contribute to its hydrophilic character and, thus, to its welldispersion of GO in aqueous solution.

I.3.4.d. Reduced graphene oxide.

Reduced graphene oxide (rGO) is obtained from reduction of GO by chemical, thermal or electrochemical treatments (Figure I.6). The chemical reduction is carried out by reductants such as hydrazine, hydroquinone, sodium borohydride (NaBH 4 ) and ascorbic acid. Reduction via thermal treatment at ca. 200 °C is an efficient and low-cost method and electrochemical reduction has been presented as an effective way to remove oxygen functional groups.

Despite the reduction treatment, rGO contains significant amounts of oxygen resulting in a higher content of sp 2 carbon atoms which allows higher electron transfer rates compared with GO [Gao, 2010;[START_REF] Sun | Growth of graphene from solid carbon sources[END_REF].

I.3.4.e. Graphene.

Graphene has become one of the most studied materials since it was first isolated by Geim and Novoselov in 2004 [START_REF] Novoselov | Electric field effect in atomically thin carbon films[END_REF]. The structure of graphene is composed of a monolayer of sp 2 -bonded carbon atoms in a two-dimensional honeycomb arrangement. Furthermore, this layered structure constitutes the basic unit of other allotropic forms of carbon such as graphite or carbon nanotubes [START_REF] Allen | Honeycomb carbon: A review of graphene[END_REF]. In this sense, the existence of monolayer graphene is limited to certain difficulties such as the expensive and hardly scalable synthetic techniques.

Furthermore, in the literature, the term graphene includes materials that comprise from 1 to 10 layers (monolayer, bilayer or few layers) [START_REF] Sun | Growth of graphene from solid carbon sources[END_REF], being especially important its description since the intrinsic properties of the material depend on that.

Regarding the structure, another aspect to take into account is the tendency to wrinkle and bend of the graphene sheets, being more remarkable for the graphene with lower number of layers. In this sense, graphene can also present defects associated with the presence of pentagonal or heptagonal carbon bonds or holes [START_REF] Sun | Growth of graphene from solid carbon sources[END_REF]. The low reactivity, limited to the unsaturated carbon bonds at the edges, makes graphene a chemically anisotropic material.

The structural properties together with the wide electronic π conjugation in the graphene layers are responsible for the extraordinary properties such as large surface area, excellent thermal and electrical conductivity, optical transparency, high mechanical strength and high elasticity. The high surface area (calculated theoretically, ca. 2630 m 2 /g) and the great electronic mobility endow graphene with exceptional properties to be used in electrochemical applications.

Different methods have been developed for the synthesis of graphene, which can be divided into top-down and bottom-up approaches (Figure I.7) [START_REF] Tour | Top-down versus bottom-up fabrication of graphene-based electronics[END_REF]. On the one hand, the strategy of the top-down method is the progressive reduction of the macroscopic structure of the carbon sources, undertaking, the micromechanical exfoliation of graphite by peel-off of the sheets, this method is also known as the 'Scotch tape' [START_REF] Novoselov | Electric field effect in atomically thin carbon films[END_REF]. As alternative, the graphene can be isolated through the expansion of the graphite layers to achieve their exfoliation into graphene sheets following different routes in solid liquid states or, even, by electrochemical technique. In the same way, the use of GO and derivatives as starting materials allows obtaining graphene through the exfoliation and reduction by chemical and/or thermal treatments [START_REF] Hernandez | High-yield production of graphene by liquid-phase exfoliation of graphite[END_REF][START_REF] Park | Chemical methods for the production of graphenes[END_REF]. On the other hand, the bottom-up techniques consist in the self-assembling the nanostructure from their constituent atoms or molecular fragments. In this case, the chemical vapour deposition (CVD) on the surface of a transition metal catalyst such as nickel and copper, or epitaxial growth on electrically insulating surfaces as SiC [START_REF] Gao | Formation of carbon clusters in the initial stage of chemical vapor deposition graphene growth on Ni (111) surface[END_REF][START_REF] Norimatsu | Epitaxial graphene on SiC{0001}: advances and perspectives[END_REF]. The morphological and structural properties directly depend on the fabrication method, which confers a broad variety of number of layers, number of defects, thermal and electrical conductivity and hydrophobic/hydrophilic character [Mahmoodi, 2018]. Among all the graphene synthesis methods, liquid phase exfoliation (LPE), chemical vapour deposition (CVD) and reduction of graphene oxide have shown their capability to be successfully escalated for meeting industrial requirements while showing different drawbacks.

On the one hand, while both LPE and most reduction treatments exhibit the largest production rates (> 10 g/h) [START_REF] Abdolhosseinzadeh | Fast and fully-scalable synthesis of reduced graphene oxide[END_REF], the graphene synthesised with these methods has typically a large number of structural defects [START_REF] Gomez-Navarro | Atomic structure of reduced graphene oxide[END_REF] and may contain significant amounts of oxygen [START_REF] Schniepp | Functionalized single graphene sheets derived from splitting graphite oxide[END_REF][START_REF] Dong | A non-dispersion strategy for large-scale production of ultra-high concentration graphene slurries in water[END_REF]. Hence, the resulting product needs further processing to eliminate the surplus graphene oxide in the case of the reduction methods, or the separation from the liquid solvent in the case of LPE. On the other hand, while CVD allows for the synthesis of large-scale graphene monolayers [START_REF] Xin | A review on high throughput roll-to-roll manufacturing of chemical vapor deposition graphene[END_REF] meeting high structural quality standards [START_REF] Balci | Electrically switchable metadevices via graphene[END_REF], this method needs the use of specific, high-purity metallic substrates, as well as high temperatures and low pressures as processing conditions.

Furthermore, its production rates remain low compared to those obtained with top-down methods. The combination of CVD with plasma technology (plasma enhanced chemical vapour deposition, PECVD) overcomes the dependence of the synthesis process on high temperatures, while the rest of the drawbacks still remain [Kim, 2013;[START_REF] Yamada | Low temperature graphene synthesis using microwave plasma CVD[END_REF].

Atmospheric pressure microwave plasmas have shown their capability to synthesise high-quality graphene from the decomposition of organic precursors in absence of a substrate [START_REF] Dato | Substrate-free gas-phase synthesis of graphene sheets[END_REF][START_REF] Tatarova | Microwave plasmas applied for the synthesis of free standing graphene sheets[END_REF][START_REF] Rincón | Synthesis of multi-layer graphene and multi-wall carbon nanotubes from direct decomposition of ethanol by microwave plasma without using metal catalysts[END_REF]. Typically, an organic precursor (gas, liquid phase) is introduced in an atmospheric pressure microwave plasma sustained in flowing argon and contained in a reactor. As a result, a solid deposit consisting of graphene flakes is formed downstream and collected either from a filter or directly from the walls of the reactor. This process takes place in a single step, without requiring the separation of the product from a substrate or a solvent. Moreover, no additional hazardous chemical is needed, which makes the process more environmentally friendly than LPE and the reduction of graphene oxide.

Ethanol was usually used as a precursor; although other precursors (e.g., methanol, dimethyl ether, isopropanol) have been proposed as an alternative to ethanol to feed a plasma torch [START_REF] Dato | Substrate-free microwave synthesis of graphene: experimental conditions and hydrocarbon precursors[END_REF], only dimethyl ether results in the production of graphene, while isopropanol yields only carbon soot and methanol does not produce any solid deposit. One of the main challenges of this technology is to increase the production rate of graphene to values close to those exhibited by top-down technologies [START_REF] Dato | Graphene synthesized in atmospheric plasmas-A review[END_REF].

Chapter II:

Objectives During the last decades, the design of nanoporous carbon materials has become a largely investigated topic in several multidisciplinary fields such as catalysis, energy conversion and storage, etc. Owing to their versatility, relative low cost and easy manufacturing, nanoporous carbons are high performance materials in many scientific and technological fields, particularly in electrochemical applications. However, efforts are yet needed to prepare nanoporous carbons with high surface areas and tailor-made pore sizes within the full nanometric scale, that retain, at the same time, a high electrical conductivity. Indeed, the preparation of 3D nanoporous carbon architectures capable of showing improved electrical conductivity without compromising the porous structure is still a challenge.

The main objective of this thesis was to explore the use of various carbon nanostructures as conductive additives for the preparation of highly nanoporous carbons, coupling well-developed porosity with a high electrical conductivity. To attain this goal, polymeric gels and carbon gels were prepared from the catalysed polycondensation of resorcinol-formaldehyde mixtures. A broad range of carbon nanostructures were used as conductive additives, including carbon black, graphene, graphite, graphene oxide and reduced graphene oxide. Various methods were explored for the preparation of the materials, optimizing the protocol needed to obtain a good dispersion of the additives in the resulting carbons. An exhaustive characterisation of both the polymeric gels and the carbon gels was performed to evaluate the influence of the characteristics of the additive on the properties (e.g., porosity, structure, composition, conductivity) of the final materials.

The specific objectives planned in order to achieve the main goal of this thesis were:

 Optimization of the synthesis process for the nanoporous carbons with different additives, aiming to obtain a homogeneous dispersion of the different matrices. The physicochemical properties of the selected carbon additives (e.g., composition, hydrophobic/hydrophilic nature) were found to play a key role to avoid segregation of the different carbon matrices. Thus, the conventional protocols for the preparation of polymeric gels and carbon gels from the polycondensation of resorcinol-formaldehyde mixtures were adapted by incorporating a mechanical stirring before the gelation step.

 Exploring the effect of the nature of the carbon black as additive to the gel and carbon gel, in terms of particle size, porosity and physicochemical features. To this end, four commercial carbon blacks with different characteristics were used.

 Investigation of the influence of the amount of carbon black. Different amounts of a carbon black ranging from 4 to 40 wt.% were studied, and a percolation model was applied to explore the impact of the amount of carbon black in the improvement of the conductivity.

 Analysis of the influence of the hydrophobicity of the carbon black. To modify the nature of the additive, a commercial carbon black was oxidised, transforming a hydrophobic material into a hydrophilic one with similar porosity and particle size.

 Evaluation of the effect of different nanostructured carbon additives, namely graphite, graphene oxide and reduced graphene oxide. The impact of their morphology and functionalization on the characteristics of the obtained composites was analysed.

 To use of graphene as additive. First of all, the synthesis of graphene was carried out by microwave plasma at atmospheric pressure through the decomposition of ethanol. The amount of ethanol as carbon source was optimized, using various flows and characterising the structural features of the graphene materials obtained.

 To investigate the use of hydrothermal treatments at moderate pressures for the synthesis of polymeric gel/additive composites. The hydrothermal conditions were also applied to the selected nanostructured additives themselves (namely, carbon black, graphene oxide and reduced graphene oxide), to investigate a potential structural expansion and impact on their porosity.

Chapter III:

Experimental Section III. A:

Materials

III.A.1. Additives.

A selection of nanostructured carbon materials was carried out for their use as conductive additives, covering carbon blacks (CB), graphite (Gr), graphene, graphene oxide (GO), reduced graphene oxide (rGO) and their corresponding modified counterparts.

CB-IV: supplied by Prolabo (currently Merck, Darmstadt, Germany).

CB-IIIox: oxidised CB-III using a modified Hummers method (see below).

III.A.1.2. Graphite.

Graphite is an allotropic form of carbon consisting of a structure of carbon atoms arranged in a hexagonal lattice building a three-dimensional structure layer-by-layer and highly ordered.

(a) (b) (c) (d) (e) (f) (g) (h) (i)

This arrangement allows high mobility of electrons within the layers, conferring the material an excellent electrical conductivity. Commercial graphite powders (sample labelled as Gr) with an average particle size equal or lower than 20 μm were purchased from Sigma Aldrich (currently Merck).

III.A.1.3. Graphene derivatives.

Graphene is an allotropic form of carbon constituted by a single layer of sp 2 -bonded carbon atoms in a two-dimensional honeycomb arrangement. This material can be considered the basic structural unit of other well-known carbon structures such as graphite, nanotubes (CNTs), fullerenes and nanoporous carbons [START_REF] Allen | Honeycomb carbon: A review of graphene[END_REF][START_REF] Dato | Graphene synthesized in atmospheric plasmas-A review[END_REF].

Graphene oxide (GO) consists of a monolayer or few-layers stack of graphene whose structure has been modified or distorted by the incorporation of oxygen-containing functional groups (hydroxyl, epoxy, carboxyl and carbonyl) or by the presence of holes and other structural defects, usually created as a result of a highly aggressive oxidation process. In this thesis, GO was prepared by oxidation of graphite powders using a modified Hummers method (see below).

Reduced graphene oxide (rGO) is obtained as a result of the partial elimination of the functional groups of oxygen existing on the graphene oxide surface, usually through a chemical, thermal or electrochemical reduction treatment. In this PhD thesis, this material was purchased from Graphenea (San Sebastián, Spain).

III.A.2. Synthesis of graphene by microwave plasma.

Atmospheric pressure microwave plasma has been used to synthesise graphene upon the decomposition of ethanol. The plasma is created by using a TIAGO torch that generates a plasma at the end of the tip of a cylindrical hollow metallic rod. In the flame, two regions can be distinguished: a bright plasma column and a broader shell. The discharge is a jet consisting in a conical core after the tip with a mixing layers surrounding atmosphere.

A schematic representation of the device used in this PhD thesis is shown in Figure III.3; a complete characterisation can be found in [START_REF] Moisan | Waveguide-based single and multiple nozzle plasma torches: the TIAGO concept[END_REF]Rincón, 2013]. High purity Ar (99.999 %) gas is used to initiate and feed the plasma discharge with a flow of 1.00 L/min and controlled by a HI-TECH mass flow controller (HI-TECH, Bronkhorst). The discharge is generated at the end of the tip of a cylindrical hollow metallic rod showing a flame morphology. After plasma ignition, ethanol (EtOH) is introduced to the discharge in gas phase through a steel tube heated at 110 ᵒC to prevent ethanol condensation. For ethanol vaporization a gas-phase liquid delivery system (CEM, Bronkhorst), able to introduce ethanol flows ranging from 0.20 to 10.00 g/h, is utilised. Microwave power (300 W) is supplied in a continuous mode by a 2.45 GHz SAIREM microwave generator (GMP KG/D) equipped with a water-cooled circulator to avoid power reflection damage. The reflected power is kept below 5 % of the input power using a short circuit movable plunger and a triple stub as impedance matching means [Rincón, 2013]. A cylindrical glass reactor is concentrically placed around the metallic rod to reduce the contact of the plasma with the atmospheric air, avoiding the flame instabilities by possible air streams. Besides, the reactor is also used as the chamber where the graphene is retained. The plasma occupied the reactor centre bottom part and graphene flakes are synthesised within the reactor volume and deposited on its inner walls, being directly collected without needing the use of any solvent or other complementary processes. The top outflow in the reactor is used to maintain the atmosphere pressure into it and as the evacuation system of the gaseous by-products from ethanol conversion by the plasma. A similar reactor design was used in [START_REF] Melero | Scalable graphene production from ethanol decomposition by microwave argon plasma torch[END_REF]. To analyse the composition of the gaseous by-products, the top outflow is coupled to a quadrupole mass spectrometer (QMS, PTM63 1121, mod. Omnistar, Pfeiffer Vacuum Technology). A filter Iberfluid) is placed between the glass reactor and the spectrometer probe to avoid the entrance of solid material into the mass spectrometer and prevent external contamination. The molar production values of gaseous by-products (H 2 , CO, C 2 H 2 , C 2 H 4 and CH 4 ) is obtained upon previous calibration of the mass spectrometer considering the calibration factors and sensitivity coefficient of each gas. Identification of the gas by-products was carried out using the theoretical mass spectra given by NIST Library from Varian MS Workstation (version 6.6).

III.A.3. Synthesis of graphene oxide.

The synthesis of GO was carried out by an oxidative treatment using a modification of Hummers method widely reported in the literature [START_REF] Hummers | Preparation of graphitic oxide[END_REF][START_REF] Park | Chemical methods for the production of graphenes[END_REF][START_REF] Marcano | Improved synthesis of graphene oxide[END_REF][START_REF] Cruz-Silva | Super-stretchable graphene oxide macroscopic fibers with outstanding knotability fabricated by dry film scrolling[END_REF]. Briefly, about 3.0 g of graphite powders were dispersed in a mixture of concentrated sulfuric acid (98 %, 360 mL) and concentrated orthophosphoric acid (85 %, 40 mL). After 1 hour of soft mechanical stirring (ca. 200 rpm), 25.0 g of potassium permanganate were portion-wise added to the mixture, keeping the reaction beaker into an ice bath to avoid a temperature rise. After 1 hour of oxidation, the green mixture became a thick slurry. At this point, the mechanical agitation was interrupted and the ice bath was removed, allowing the reaction mixture to reach room temperature for overnight (a brown slurry was obtained). Then, the mechanical stirring was restarted and a solution of 40 mL of peroxide hydrogen (30 %) in 600 mL of cold water was slowly poured to quench the excess of oxidant (i.e., reducing KMnO 4 to MnO 2 ). When the evolution of gases finished, the mechanical stirring was stopped allowing the sedimentation of oxidised material. The supernatant solution was removed and the product was centrifuged and washed with abundant water until pH was close that of distilled water (ca. 5). The resulting slurry was dried applying a soft thermal treatment at 60 °C for 12 h in an oven and stored in a desiccator until further use.

III.A.4. Oxidation of CB-III.

The oxidation of CB-III was carried out following the same process above described, in order to incorporate oxygenated functional groups on the surface of carbon black [START_REF] Hummers | Preparation of graphitic oxide[END_REF][START_REF] Marcano | Improved synthesis of graphene oxide[END_REF][START_REF] Cruz-Silva | Super-stretchable graphene oxide macroscopic fibers with outstanding knotability fabricated by dry film scrolling[END_REF]. The increase in the hydrophilic character as a result of the oxidation is expected to facilitate the dispersion of carbon additive in the aqueous solution used as reaction medium for the synthesis of the carbon gels.

III.A.5. Synthesis of polymeric organic gels and carbon gels.

III.A.5.1. Static synthesis.

Firstly, the polymeric gels were prepared following a hydrothermal process in a close reactor, in which the polycondensation reaction takes place under high pressure and temperature conditions (Figure III.4). For this purpose, following the synthesis described by Pekala in 1989 [START_REF] Pekala | Organic aerogels from the polycondensation of resorcinol with formaldehyde[END_REF], resorcinol (R, 99 % purity, Sigma Aldrich) and formaldehyde (F, 37 wt.% in water, stabilized by 10-15 % of methanol, Merck) were dissolved in deionized water (W) in presence of sodium carbonate -basic catalyst-(C, 99 % purity, Sigma Aldrich) and carbon black or reduced graphene oxide -conductive additive-(CB-III, Superior Graphite Co.).

The molar ratios were fixed at R/F 0.5, R/W 0.06 and R/C 200 according to previous optimization studies of the group [START_REF] Macías | Carbon black directed synthesis of ultrahigh mesoporous carbon aerogels[END_REF][START_REF] Isaacs-Páez | Fast synthesis of micro/mesoporous xerogels: Textural and energetic assessment[END_REF]Rasines, 2015_A;Rasines, 2015_B;Macías, 2016_A;Gomis-Berenguer, 2017]. The amount of reactants was adjusted to obtained ca. 20 mL of reactants mixture. The ratio of the additives was 4 wt.% expressed as grams of additive per gram of R+F. The mixture was maintaining under mechanical stirring (ca. 500 rpm) to assure the homogeneity and the resulting suspension was transferred into a 125 mL Teflon-lined stainless-steel autoclave to be heated at 150 °C for 12 h.

Sol

Gel

Dry step 150 ᵒC, 12 h

Precursorsḿ ixture

After cooling to the room temperature, the obtained monolith was dried at 60 °C until constant mass. Then, the dried monoliths were first crushed in a mortar 15 min before grinding them in a ball mill to get fine powder and kept in a desiccator until further use. A control of the gel without conductive additive was synthesised and also a control of each the additives was made following the same hydrothermal treatment without resorcinol, formaldehyde nor catalysts (480 mg of additive in 20 mL of water) with the aim to study the behaviour of the additive under hydrothermal conditions.

III.A.5.2. Dynamic synthesis.

Due to the need of the agitation in the case of the gel/additive composites' synthesis, a simple modification of the polycondensation reaction was applied. The proportion and mixture of all the reactants was made following the same procedure described above (Section III.A.5.1) but the thermal treatment was modified. The reactants mixture was transferred to glass vessels to carry out the gelation and ageing step in an oil bath at 70 °C for 4 h, a stir bar was introduced into each glass vessel and the mechanical stirring (ca. 500 rpm) was maintained during the gelation process. When the gel started to be formed (ca. after 2 h) the viscosity of the reactants' mixture increased and the stirring was naturally stopped. After the gelation, the vessels containing the gels were transferred to the oven to dry at 150 °C for 12 h (Figure III.5).

The amount of additive varied between 4-40 wt.% (i.e., gram of additive per gram of resorcinol + formaldehyde). The monoliths were crushed in a mortar and grinding them in a ball mill obtaining a fine powder. The pyrolysis (or carbonization) process was carried out in a horizontal tubular furnace under a nitrogen atmosphere, using a gas flow rate of 120 mL/min. The heating rate was 2 °C/min with three dwelling steps of 1 h at 200, 400 and 800 °C [Haro, 2011]. According to the International Union of Pure and Applied Chemistry (IUPAC) the texture of a material is defined as the detailed geometry of the void and pore space [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF].

Sol Gel

Porosity is a concept related to texture and is considered as the ratio of the total pore volume to the volume occupied by a solid. However, since the porosity depends of the employed method for its determination and on the nature of the probe molecule, a given porosity value cannot be regarded as characteristic of the material.

The adsorption (in particular, physisorption) of gases at a constant temperature is the most suitable method used for textural characterisation of all types of solids and fine powders.

Adsorption is defined as the enrichment of molecules, atoms or ions in the vicinity of an interface that occurs whenever the surface of a solid (adsorbent) is exposed to a gas or liquid (adsorptive). According to the nature of such interactions a distinction can be made between chemisorption -formation of a chemical bond with the solid-and physisorption -any chemical bond is formed-.

The accessible area of a solid depends mainly on the porosity of the surface. In an arbitrary way, it is considered that the outer area corresponds to the area of the walls of the pores and slits having a depth greater than the width. Porous solids usually have an internal area that is of a much higher order of magnitude than the external area. The pore size is generally specified as the pore width, defined as the distance between the two opposite walls. Pore sizes are classified by IUPAC as follows [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]: i) Macropores: pore widths, exceeding about 50 nm.

ii) Mesopores: pore widths between 2 nm and 50 nm.

iii) Micropores: pore widths not exceeding about 2 nm.

In the last decade, IUPAC has accepted the term nanopores to include the above three categories of pores, with an upper limit close to 100 nm width [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]. In addition, some other authors proposed other classifications to distinguish ultramicropores or narrow micropores (widths less than 0.7 nm), medium sized micropores (corresponding to a widths between 0.7 and 1.4 nm) and supermicropores (widths between 1.4 and 3.4 nm) [START_REF] Brunauer | Surface Area Determination[END_REF][START_REF] Dubinin | Characterisation of porous solids[END_REF].

The amount of gas adsorbed by a solid, n, per mass of solid depends on the pressure, p, the temperature, T, and the nature of the gas-solid system. For a given gas adsorbed on a particular solid at a constant temperature, and if the gas is below its critical temperature, the number of moles of gas adsorbed may be expressed as:

n = f ( p p 0 ) T,gas,solid equation III.1
where p0 is the gas saturation pressure and p/p0 is the relative pressure.

This equation is a general expression for an adsorption isotherm [START_REF] Gregg | Adsorption, surface and porosity[END_REF] which represents the relation, at constant temperature, between the amount adsorbed by unit mass of solid and the equilibrium pressure (or relative pressure).

Experimental adsorption isotherms for gas-solid systems are usually presented in graphical form (adsorbed gas amount vs. relative pressure), and display various characteristic. The shapes of the isotherms are important since they provide significant preliminary information about the pore structure of the adsorbent and the gas-solid interaction. All experimental adsorption isotherms should fit at least one, or a combination of two or more, of the six recognised types classified according to the extended IUPAC classification proposed in 2015 [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]. Each type of isotherm is associated to a particular mechanism (physical adsorption, chemical adsorption, capillary condensation), consequently, a simple observation of the experimental adsorption isotherm allows to draw some conclusions on the porous texture of the analysed material.

(Figure III.6)
Reversible Type I(a) and I(b) isotherms are concave to the p/p 0 axis and are characterised by a limiting value of the amount adsorbed, which begins to set at relatively low relative pressure and is extended to the saturation pressure. They are given by microporous solids presenting small external surface areas, as is the case for some activated carbons, molecular sieve zeolites and certain porous oxides. Type I(a) isotherms are given by microporous materials having mainly narrow micropores, whereas Type I(b) isotherms indicates the presence of wider pore size distributions on the materials.

Type II isotherms are given by the adsorption of most gases on non-porous or macroporous adsorbents, with unrestricted monolayer-multilayer adsorption up to high relative pressure.

These isotherms are reversible and concave relative to the x-axis up to a certain point normally referred to as Point B, when the middle almost linear section starts. If the knee is sharp, Point B usually corresponds to the completion of monolayer coverage. After this almost straight section, the slope increases and the curve becomes convex to pressure axis.

The Type III isotherms are convex in relation to the axis of the abscissas over to complete range and consequently do not present a Point B, with no monolayer formation. This shape indicates that the adsorbent-adsorbate interactions are relatively weak and the adsorbed molecules are clustered around the most favourable sites on the surface of a non-porous or macroporous solid.

Type IV isotherms are characteristic of mesoporous adsorbents, as many oxide gels, industrial adsorbents and mesoporous molecular sieves. The adsorption behaviour in mesopores is governed by the adsorbent-adsorptive interactions and the interactions between the molecules in the condensed state. The shape of the initial region is closely related to Type II reaching a characteristic saturation plateau at high relative pressures. In Type IV(a)

isotherms present hysteresis loop associated with capillary condensation that occurs when pore exceeds a certain critical width, depending on the adsorption system and temperature.

Completely reversible Type IV(b) are observed for adsorbents having mesopores of smaller width.

Type V isotherms are convex to x-axis and, to a certain pressure value, similar to the isotherms of Type III, which is indicative of relatively weak adsorbent-adsorbate interactions.

Reversible stepped Type VI isotherms are associated with layer-by-layer adsorption on highly uniform non-porous surfaces. Type VI isotherms are typically obtained with argon or krypton at low temperature of graphitized carbon blacks. Hysteresis loops are associated with capillary condensation occurring in the mesopores and can be attributed to thermodynamic effects (adsorption metastability) and/or network effects and various forms of pore blocking (percolation, cavitation). In a complex pore networks, if wide pores have access to the external surface only through narrow necks (known as ink-bottle pore shape), the desorption vapour pressures are dependent on the size and spatial distribution of the necks, if the neck diameters are not too small, the porous structure may empty at a relative pressure corresponding to a characteristic percolation threshold. In addition, hysteresis may also be caused by the effects of connectivity of the pores [START_REF] Choma | Characterization of nanoporous carbons by using gas adsorption isotherms[END_REF]. Then, useful information concerning the neck size and pore network can be obtained from the desorption branch of the isotherm.

Six different forms of loops have been reported by the extended IUPAC classification Type H3 loop: the resemblance of the adsorption branch of the Type II isotherm; the lower limit of the desorption branch is normally located at the cavitation-induced p/p 0 . The adsorption branch of Type H4 loop is now a composite of Types I and II isotherms, the more pronounced uptake at low relative pressure being associated with the filling of micropores.

Type H5 loop has a distinctive form associated with certain pore structures containing open and partially blocked mesopores.

III.B.1.1. Surface area determination using BET method.

The BET model is based on the several assumptions: i) in each layer, the adsorption rate is equal to the desorption rate; ii) the heat of adsorption from the second layer is constant and equal to the vapour condensation heat; iii) when p is equal to p 0 , the vapour condenses as a liquid and the ordinary number of adsorbed layers is infinite. The BET equation (equation III.2) is expressed as:

n(p) = n m C BET p (p -p 0 ) [1 + (C BET -1) (p/p 0 )] equation III.2
where n(p) is the amount of adsorbed gas at pressure p, nm is the adsorbed amount needed to fill the monolayer, p0 corresponds to the vapour saturation pressure and CBET is designated as the BET constant which is related to the adsorbate-adsorbent interactions.

The application of this equation requires to transform an experimental physisorption isotherm into a BET plot, from that, it is possible to determinate the number of adsorbed moles in the monolayer (n m ) and the equivalent surface area (S BET ). BET equation can be linearised in different forms; in this thesis, the one proposed by [START_REF] Parra | Characterization of activated carbons by the BET equation-An alternative approach[END_REF][START_REF] Parra | Characterization of activated carbons by the BET equation-An alternative approach[END_REF] is used:

1 n (1 -x) = 1 n m + 1 n m C BET ( 1 -x x ) equation III.3
where x is the relative pressure (= p/p0).

BET theory has become a standard procedure for the determination of the surface area of a wide range of porous materials [START_REF] Rouquerol | In: Adsorption by powders and porous solids. Principles, methodology and applications[END_REF]. On the other hand, it is now generally recognised that the theory is based on an oversimplified model of multilayer adsorption and that the reliability of the BET method is questionable unless certain conditions are prescribed.

The BET method can be applied to many Type II and Type IV isotherms, in which the range of linearity of the BET plot is typically restricted to the relative pressure range between 0.05 and 0.3; at higher or lower pressures, the BET equation usually provides adsorbed quantities which are, respectively, lower and higher than actual values. However, the BET equation needs to be applied with extreme caution for microporous materials (i.e., Type I isotherms and combinations of Types I and II or Types I and IV isotherms) because in these cases, it may be very difficult to detect the formation of the monolayer from the multilayer adsorption and micropore filling, for which the linear range of the BET plot may be difficult to locate.

The IUPAC has addressed this concern in the latest review in 2015 [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF],

recommending a procedure to overcome this difficulty and to avoid any subjectivity in evaluating the BET monolayer capacity. The procedure is based on various criteria: i) the C BET constant should be positive (i.e., a negative intercept on the ordinate of the BET plot indicates that the selected range of relative pressures for the application of the equation is not the appropriate one); ii) the application of the BET equation should be restricted to the range where the term n(1 -p/p 0 ) continuously increases with p/p 0 ; iii) the p/p 0 value corresponding to n m should be within the selected BET range.

III.B.1.2. Micropores volume: Dubinin-Radushkevich equation.

In 1947, Dubinin and Radushkevich proposed an equation for the characteristic curve in terms of the fractional filling of the micropore volume. This equation is an empirical method that takes into account the distinct adsorption energies and assumes that the micropore size follows a Gaussian distribution. The linear DR equation (equation III.4) is given as follows:

ln W = ln W 0 -( R T β E 0 ) 2 ln 2 ( p 0 p ) equation III.4
where W (cm 3 /g) is the volume occupied by adsorbed phase at temperature T, W0 (cm 3 /g) corresponds to total volume of micropores, β is the affinity factor of adsorbate-adsorbent -which is 0.34 for nitrogen and 0.36 for carbon dioxide [START_REF] Guillot | Reference isotherm for high pressure adsorption of CO 2 by carbons at 273 K[END_REF]-, and E0 (kJ/mol) is the adsorption energy.

III.B.1.3. Density functional theory.

The Density Functional Theory (DFT) is based on the assumption that the experimental isotherm can be expressed as the sum of the isotherms of individual pores forming the porous solid structure [START_REF] Tarazona | Phase equilibria of fluid interfaces and confined fluids. Non-local versus local density functionals[END_REF][START_REF] Lastoskie | Pore size distribution analysis of microporous carbons: A density functional theory approach[END_REF]. From a mathematical approach, the DFT method applied to the calculation of pore size distributions is based on the integral of the individual isotherms of defined given sizes:

n(p) = ∫ f(w)ρ(p, w)dw w max w min equation III.5
where n(p) is the amount of gas adsorbed at pressure p, wmin and wmax are the minimum and maximum pores width, f(w) is the distribution of the pore volume as a function of the pore width and ρ(p,w) is the molar density of the adsorbate at pressure p in a pore with width w.

The first approaches made assumptions concerning the functional form of the size distribution; more recently, a generalisation was accomplished by numerical deconvolution of the isotherm data using a set of pore shape dependent model isotherms calculated from DFT, each member of the set being representative of a unique, narrow range of pore sizes.

Nowadays, more sophisticated methods based on NLDFT have succeeded in determining the pore size distribution in the entire range of pore sizes accessible by the adsorptive molecule, incorporating concepts related to the dimensionality of the pores, the surface energetic heterogeneity and geometrical corrugation in the equation (2D-NLDFT-HS) [Jagiello, 2013_A;Jagiello, 2013_B;[START_REF] Landers | Density functional theory methods for characterization of porous materials. Colloids and Surfaces A: Physicochem[END_REF].

In this thesis, the textural properties of the materials have been characterised by physical adsorption of various gases. Volumetric analysers were used for the measurement of the adsorption isotherms of N 2 at -196 °C and CO 2 at 0 °C (3Flex and TRISTAR 3020, Micromeritics).

All samples were previously outgassed under vacuum (ca. 10 -1 Pa) at 120 °C (heating rate of 2 °C/min) for 17 h to obtain reproducible and accurate data [Figini-Albisetti, 2010]. Each isotherm was recorded at least in duplicate on fresh sample aliquots, to guarantee the accuracy and reproducibility of the experiments (error was below 2 %). The mathematical approach proposed by Parra et al. [START_REF] Parra | Characterization of activated carbons by the BET equation-An alternative approach[END_REF] was used to determine the right range of relative pressures for the correct application of the BET equation. This approach complies with all the criteria recommended by IUPAC. DR equation has been applied to both the N 2 and CO 2 adsorption isotherms, to obtain the total micropore volume (W 0 , N 2 ) and the volume of narrow micropores (W 0 , CO 2 ). The 2D-NLDFT-HS approach was used to obtain the distribution of pore sizes.

III.B.2. Physicochemical characterisation.

III.B.2.1. Elemental analysis.

Elemental analysis is a technique widely used to make chemical characterisation of the carbon materials, determining the total amount (wt.%) of carbon (C), oxygen (O), hydrogen (H), nitrogen (N) and sulphur (S), which provides useful information about the type of functionalities that exist on the material's surface. This analytical technique is based on the complete combustion of a sample applying a thermal treatment at high temperature, about 950 °C, in a stream of oxygen. The combustion products such as CO 2 , H 2 O, N 2 and SO 2 are measured typically using infrared spectroscopy. In the case of the determination of the oxygen content, the sample is heated up to 1070 °C under inert atmosphere provoking the liberation of different gas molecules (CO, CO 2 , H 2 O). This gas mixture is passed through a bed of graphite powder to reduce them to CO, then, CO is oxidised to CO 2 in a CuO catalysts and it is detected by a chatarometric detector. The determination of most common heteroatoms in carbon materials (i.e., C, H, N, S and O) was carried out by an analyser (Thermo Scientific Flash 2000).

For comparative purposes and to eliminate the moisture, all the samples were previously dried under vacuum at 60 °C for 24 h.

III.B.2.2. Infrared spectroscopy.

Infrared spectroscopy allows the identification of the nature of the sample by detecting the different surface functional groups, this detection is based on the study of the interaction of radiation with matter. The infrared region of the electromagnetic spectrum is generally divided into three zones, with respect to the visible region: i) 400-10 cm -1 (far), ii) 4000-400 cm -1 (mid),

iii) 4000-14000 cm -1 (near). Fourier Transformed Infrared Spectroscopy (FTIR) is applied to the intermediate region of the spectrum, and it may be used for a direct and rapid characterisation of solid samples. Traditionally, the dark materials (e.g., carbon-based materials) are diluted with an appropriate transparent medium to allow sufficient signal intensity (usually potassium bromide, KBr) to obtain transmission infrared spectra.

The substance of interest is submitted to an electromagnetic radiation of known incident intensity and frequency, and the transmitted or scattered intensity and frequency are recorded. Compounds with similar chemical groups exhibit characteristic absorption bands in the IR, giving rise to a spectrum (resultant intensity after interaction with matter as a function of wavelength). The assignment of the infrared absorption bands and peaks to different functionalities of the material surface is made by comparison with the spectra for organic compounds containing similar functional groups. The bands and peaks assignment corresponding to most common functionalities in carbon materials are: C-O stretch of ethers (1000-1300 cm -1 ), alcohols (1049-1276 cm -1 ), phenolic groups (1000-1220, 1160-1200 cm -1 ), quinones (1550-1680 cm -1 ) and carboxylic acids (1120-1200, 1665-1760, 2500-3300 cm -1 ) [START_REF] Bandosz | Surface chemistry of activated carbons and its characterization, In: Activated carbon surfaces in environmental remediation[END_REF].

In this thesis, FTIR spectroscopy measurements were carried out with a Bruker Vertex 80 v (Billerica, MA, USA) using materials dispersed and pressed with dry KBr, keeping a 1:100 ratio (material: KBr, wt./wt.). Pellets were prepared from the mixture and stored overnight in a desiccator. Transmission spectra were carried out between 4000-350 cm -1 (64 scans collected, resolution 4 cm -1 ). The spectra were corrected using KBr pellet as background. At least two pellets of each sample were measured in order to attain the consistency of measurements.

III.B.2.3. Point of zero charge.

The surface chemistry depends essentially on the presence of heteroatoms and on the content and nature of oxygen functional groups on the surface. These groups will determine the hydrophobicity of the carbon and the electronic density of graphitic layers. When a material is immersed in an aqueous solution it develops a surface charge resulting from the dissociation of surface groups or the adsorption of ions in solution.

The pH of the point of zero charge (PZC) of a given material is defined as a pH at which the net surface electrical charge (average of positive and negative charges) is zero. When the pH is lower than the PZC value, surface of the material is positively charged (attracting anions), on the contrary, when pH is above PZC, the surface is negatively charged (attracting cations).

Knowing the pH of the PZC of a material it is possible to predict its distribution of surface charges (i.e., if it will be positively or negatively charged) as a function of the environment; for instance, in an aqueous medium.

The PZC of the studied materials in this thesis was determined by a mass titration method.

The experimental procedure consists in the dispersion of a fix amount of material in a suitable volume of distilled water; the suspension is continuously stirring under a nitrogen atmosphere at room temperature until equilibrium is attained (ca. 48 h). After equilibration, the pH value is measured using a glass electrode; then distilled water is added to the suspension in order to obtain a new dispersion with lower solids concentration. Typical dispersions of the materials in water employed were 2, 4, 6, 8, 10 and 12 % (wt./wt.). The representation of the equilibrium pH vs. solid weight percentage is a curve showing a plateau at high solids concentrations, which is taken as the PZC value of the material (typically between 8-12 wt.%).

III.B.2.4. Potentiometric titration.

The potentiometric titration offers a meaningful description of acidic properties of the carbon surfaces, in terms of their proton affinity distribution, proving qualitative and quantitative information on the number and the strength of the acidic sites. The method consists on an acid-base titration of a suspension of a material in an inert electrolyte that allows the evaluation of material's ability to exchanged protons with the media [Contescu, 1993].

The potentiometric titrations were carried on by an automatic titrator (Metrohm, Titrino Plus 848). Briefly, 0.05 g of material were stirred overnight in 75 mL of electrolyte solution (0.01 M NaNO 3 ) under N 2 atmosphere. After the equilibration time, the pH of the dispersion was adjusted (if necessary) to pH 3 by addition of HCl 0.1 M and the titration is made using NaOH 0.1 M as titrant (previously standardised) until reach the pH 11. The acidic functionalities pK a values and amounts were determined using the SAIEUS program applied on the proton affinity vs. pH curves [START_REF] Jagiello | Carbon surface characterization in terms of its acidity constant distribution[END_REF].

III.B.2.5. Thermal analysis.

Thermogravimetry (TG) is based on the mass change of a sample as a function of temperature under a controlled atmosphere. The sample is subjected to a constant heating or cooling program, and mass variations are continuously measured [START_REF] Hill | For better thermal analysis and calorimetry[END_REF].

Thermogravimetric analyser (thermobalance) consists of a furnace and a microbalance with a control device to measure the temperature and mass of the sample during the test. The gas flow within the thermobalance follows a longitudinal path and is introduced into the upper part of the furnace. The entire scale of the mechanism is in an inert atmosphere, with a carrier gas stream of all the gases generated. Thermogravimetry provides the TG profile, obtained for the representation of the weight change vs. time or temperature, and the corresponding curve of its first derivative as a function of time or temperature (DTG).

The information obtained from TG curves is of empirical nature, since the transition temperatures depend on instrumental parameters and sample characteristics, which makes it difficult to make comparisons between data obtained in different equipment. Among these experimental parameters are the heating rate, reaction atmosphere, the material and the geometry of the sample holder, and the furnace size and shape. In this work, the thermogravimetric analysis was carried out in a Labsys Setaram thermobalance. A continuous N 2 flow of 50 mL/min and a heating rate of 10 ᵒC/min up to 900 ᵒC of final temperature were employed for the analysis. For the samples obtained at high temperature (e.g., graphite, graphene-like and carbon black), an air flow was used as reactive atmosphere (100 mL/min). The amount of sample used for each experiment was ca. 20 mg. Additionally, temperature programmed desorption (TPD) measurements were recorded for selected samples in a thermobalance connected to a mass spectrometer for gas analysis.

About 10-20 mg of sample were heated up to 900 ᵒC (10 ᵒC/min) under a constant argon flow (100 mL/min). The analysis of the composition of the evolved gases by the mass spectrometer allows to quantify certain O-groups. In general, carboxylic groups are decomposed as CO 2 and water (decarboxylation reaction), quinones decompose as CO and phenols decomposition generates CO and H 2 O (decarboxylation reaction). The amounts of the different surface groups can be estimated by deconvolution of the TPD spectra [START_REF] Bandosz | Surface chemistry of activated carbons and its characterization, In: Activated carbon surfaces in environmental remediation[END_REF].

III.B.2.6. X-ray photoelectron spectroscopy.

X-ray photoelectron spectroscopy (XPS) is a quantitative technique that allows obtaining information about the atomic composition, chemical environment and electronic state of the elements forming the sample, especially those located in its surface (up to 1 nm depth).

Surface analysis by XPS is accomplished by irradiating a sample with monoenergetic soft X-rays. Electromagnetic radiation interacts with atoms of the solid in the surface region, causing electrons to be emitted by the photoelectric effect and then their kinetic energy is measured.

XPS experiments were recorded in a PHI 5000 VersaProbe II apparatus (Physical Electronics), using monochromatic Al-Kα (1486.6 eV) radiation and a dual beam charge neutralizing system. Spectra of the dried films were recorded using a 100 µm beam size, operating at 25 W and 15 KV, and using a pass energy of 23.5 eV and a 0.2 eV step size for 50 sweeps. Semi-quantitative elemental surface analyses were obtained from the survey spectra with the corresponding sensitivity factors. Processing of the collected spectra was performed using energy values referenced to the C 1s peak of adventitious carbon located at 284.6 eV, and a Shirley-type background function [START_REF] Wagner | Handbook of X-ray photoelectron spectroscopy[END_REF]. After acquisition of the spectra, MultiPak TM software package was used for data analysis. A Shirley-type background was subtracted from the signals and the recorded spectra were fitted using Gaussian-Lorentzian (90:10) curves, in order to determine the core levels of the binding energy of each element.

III.B.2.7. X-ray diffraction spectroscopy.

X-ray is an electromagnetic radiation of the same kind of visible light, strongly energetic due to its short wavelength (10 -2 to 10 nm). Generally, the longer wavelength used for inorganic and organic materials studies varies between 0.05 and 0.25 nm due to the interatomic distances of the materials are located in this interval. The X-ray diffraction (XRD) spectroscopy is the most widely used method to identify crystalline structures as it allows a rapid and non-destructive analysis of multi-component mixtures without the need for extensive sample preparation. Diffractograms correspond to the representation of the diffracted intensity as a function of the incident angle (typically 2θ); their analysis enables to determine the distribution of atoms, ions or molecules in a crystal lattice. The diffractograms are characteristic of each crystal and result from the interference between the waves associated with X-ray and the electronic cloud of the constituents of the crystal lattice.

X-ray phenomenon is described by Bragg law (equation III.6) that predicts the direction in which constructive interferences occur between beam coherently scattered X-rays by a crystal:

nλ = 2d sinθ equation III.6
where λ is the wavelength of the incident radiation, d is the interatomic distance and θ corresponds to the diffracted angle.

The main application of XRD is the qualitative identification of crystallographic phases in a crystalline sample. Furthermore, other applications are found in the quantitative analysis of crystalline compounds, the determination of many parameters like coefficient of thermal expansion, determination of the crystal symmetry, as well as the determination of the crystallite size along the c-axis, L c , and the size of layer planes, L a , using Scherrer equation (equation III.7):

L = Kλ B cosθ equation III.7
where L is the crystal size, B is the full width at half maximum of a certain peak (FWHM), θ is the diffracted angle, λ is the radiation wavelength and K is the crystal shape factor that depends on the lattice dimension, for carbon materials values 0.9 and 1.84 are used for calculation of L c and L a , respectively as was proposed by Warren and Bodenstein [START_REF] Warren | The diffraction pattern of fine carbon blacks[END_REF].

In the case of graphitic structures (Figure III.9), the XRD peak positions depend upon interlayer spacing (d), and the peak width is highly dependent on size of graphitic domains existing inside the macroscopic graphene materials [START_REF] Sharma | Determination of defect density, crystallite size and number of graphene layers in graphene analogues using X-ray diffraction and Raman spectroscopy[END_REF].

The presence of defects on the ideal graphite crystal provokes the XRD peaks widening as a consequence of imperfect stacking of the layers that produce an increase of interlayer space (d), stacking height (L c ) and in-plane crystal size (L a ). For instance, highly oriented pyrolytic graphites (HOPG) typically presents a d value of 0.335 nm and crystal size higher than 100 nm. In this thesis, powder X-ray diffractograms were recorded in a Bruker diffractometer (D8 Advance) operating at 30 kV and 40 mA and using Cu Kα (0.15406 nm) radiation. Data were collected between 5° and 90 ° with a 0.08 ° step size.

III.B.2.8. Transmission electron microscopy.

Transmission electron microscopy (TEM) is a powerful tool that allows the characterisation of the morphology, the crystalline structure and determination of the crystalline defects. It is a microscopic technique in which the transmission/dispersion of a beam of electrons is used for imaging samples. Heavy metals (with a high electronic density) strongly interact with the electron beam, deflecting it. The use of electrons has the major advantage of high resolution, but has implications on the design of this kind of equipment: i) the lenses used to collimate the beam must be magnetic; ii) the interior of the TEM must be kept in vacuum. In a transmission electron microscope, a sufficiently thin sample amount (5 nm to 0.5 μm of maximum thickness) is irradiated with a monochromatic electron beam of uniform current density. The electrons interact strongly with matter through elastic and inelastic dispersions.

In this thesis, a Philips CM20 microscope was used, operating at 200 kV. Powder samples were dispersed in ethanol upon sonication for 10 min, and deposited on a holey carbon film supported by a copper grid.

III.B.2.9. Raman spectroscopy.

Raman spectroscopy is an optical method that analyses scattered photons, which contain information regarding the vibrational modes of the sample. It is a suitable and non-destructive method to determine the degree of structural order of carbon materials. Raman spectrometry has usually been considered as an alternative to X-ray diffraction with respect to which it offers main advantages such as a higher surface selectivity (with a sampling depth estimated to be about 100 nm) and the possibility of sampling reduced areas of the surface. Despite the First-order Raman spectra (1100-1800 cm -1 ) of most carbon forms contain, at least, two characteristic peaks labelled as G band (or graphitic band) at ∼ 1580 cm -1 and D band (or defect band) at ∼ 1360 cm -1 , which are related to the vibrations of the sp 2 building blocks in carbon materials and their defects, providing information on the structural order and disorder.

As D band is associated to structural disorder, it grows up in terms of intensity, with an increment of disorder in the material, as a consequence the ratio of the intensities of the D and G bands is indicative to the structural disorder. The second-order Raman spectra appears at frequencies range between 2300 and 3300 cm -1 and the predominant band is the 2D band that is associated with the structural order.

However, theoretical and experimental studies have indicated that structural details of carbon materials are also carried in other parts of the Raman spectrum, with bands experimentally observed particularly in non-crystalline nanoporous disordered carbons related to various defect-induced phenomena [START_REF] Cançado | Quantifying defects in graphene via Raman spectroscopy at different excitation energies[END_REF][START_REF] López-Díaz | Evolution of the Raman spectrum with the chemical composition of graphene oxide[END_REF]. A broad shoulder can be appeared between the D and G peaks namely the D'' (∼ 1500-1550 cm -1 ) and is related to amorphous phases. Another band (∼ 1150-1200 cm -1 ), D*, is related to disordered graphitic lattices provided by sp 2 -sp 3 bonds. Other defect-related features such as D' and D + D' bands can be observed.

Confocal Raman imaging

The Raman spectroscopy offers the information of the local composition and structure of the materials. However, by the Confocal Raman imaging is possible to obtain a spatial distribution of the different constituent of the material, for that, a zone of the sample is selected by a video camera and this zone is divided by different pixels of determined size. The laser is focalised on the first pixel and the corresponded Raman spectrum is registered. The sample is displaced until the laser is focalised on the second pixel and other Raman spectra is collected, the same process is followed until all sample's pixels are analysed. At the end of the analysis the number of the spectra is the number of the defined pixels. Finally, an image is reconstructed from over 300 Raman spectra in different points of the scanned area.

Owing to its high spatial resolution (ca. 1.3 µm), performing Raman imaging on a non-homogeneous sample surface gives access to local composition and spatial arrangement of various molecular or solid species. In this thesis Raman spectra were recorded in ambient conditions in a Renishaw InVia

Qontor spectrometer equipped with 514.5 nm laser. The spectra were collected under a Leica DM2500 optical microscope with a x50 long working distance objective (ca. 10 mm). The scattered Raman light was dispersed by a holographic grating of 600 grooves/mm, in order to acquire the whole range of interest for carbons (500-5000 cm -1 ). Each spectrum was recorded with an integration time of 5 s; data presented represents the average of three measurements. In this thesis, the electrical resistivity of the samples was measured using a four-point probe method following the general principles of ASTM standard methods D4496-87

S S S t I V a) b)
[ASTM D4496, 2013] (Figure III.12). Briefly, pellets of the samples (ca. 90 wt.% of carbon powders, 10 wt.% polyvinylidene fluoride binder) were prepared by compaction of the powders under 5 tons pressure. The diameter of the pellets was ca. 10 mm and their thickness varied between 0.10-0.30 mm (total weights between 8-12 mg). As all samples were prepared with the same amount of binder, its insulating effect is considered as equal for all the samples and allows the comparison between them. Resistance of the pellets was measured at room temperature and atmospheric pressure using a Lucas Labs four-point probe stand (S-302-6)

with a Signatone four-point probe head (SP4-62-045-TBY) to make electrical contact. A constant current (between 1-10 mA) was applied to the surface of the pellets through the probes, and the voltage drop was recorded.

The bulk resistivity of the samples was calculated following the Van der Pauw method

[ [START_REF] Van Der Pauw | A method of measuring specific resistivity and Hall effect of discs of arbitrary shapes[END_REF] by the application of the equation III.9:

ρ = U I π ln 2 𝑡 equation III.9
where U is the voltage drop (V), I is the current intensity (A), and t is the thickness of the pellets (m).

Chapter IV:

Results and discussion

This section describes the synthesis of the polymeric gel and the carbon gel as well as the gel/additive composites. In the first Subsection IV.A.1, the preparation of the polymeric gels and the carbon gels from the polycondensation of resorcinol/formaldehyde mixtures in the absence of additives is described. Due to the later incorporation of the additives, the experimental protocol was modified and compared to the previous one (Subsection IV.A.2).

Section IV. A:

Experimental protocol for the preparation of the polymeric gels and carbon gels and was associated to the loss of the moisture of the sample ca. 6 %, the next mass loss was of 16 % and took place between 200-400 °C that corresponds to the decomposition of volatile matter and carboxylic and anhydrides groups present in the gel. Finally, between 400-800 °C, 24 % of the gel mass is lost, which is associated to the elimination of carbonyl, phenolic and quinones groups [START_REF] Lin | Effect of synthesis pH on the structure of carbon xerogels[END_REF][START_REF] Contreras | A comparison of physical activation of carbon xerogels with carbon dioxide with chemical activation using hydroxides[END_REF]. After carbonization, the shape of the N 2 isotherm remained unchanged, whereas the total pore volume increased significantly, particularly in the low relative pressure range. This indicates that upon a controlled thermal treatment at slow heating rates, the microporosity of the sample increased while the mesoporosity remained rather unchanged. Indeed, the specific d evaluated by the 2D-NLDFT-HS method applied to N 2 isotherms surface area of the carbon gel was twice larger than that of the polymeric gel, while the total pore volume increased from 0.377 to 0.488 cm 3 /g after the thermal treatment. Also, while carbonization brought about an important increase in the microporous volume, the mesoporosity was quite similar in both samples. This was corroborated by the analysis of the Por.

stirring in the reactants' mixture provokes the sedimentation by gravity during the synthesis -CB-III is a hydrophobic additive, which dispersion in aqueous solutions is not stable after long times-. As seen, the adsorption isotherm of the upper part corresponding to the orange-ish aliquot displayed a similar feature than that of the gel prepared in the absence of carbon additive (PG). On the contrary, the adsorption isotherm of the lower transversal cut where the carbon black sedimented shows a different shape, with a distinctive curvature in the hysteresis loop in the desorption branch. Such curvature will be further analysed in detailed in the next section (Section IV.B), but is in agreement with previous studies from the group on the preparation of gels with carbon black additives [START_REF] Macías | Carbon black directed synthesis of ultrahigh mesoporous carbon aerogels[END_REF]. In any case, it is evident that the uneven distribution of the carbon black in the material brings about important differences in the porosity. Thus, in order to obtain a homogeneous distribution of the carbon black, and homogenous polymeric gels and carbons, the synthetic approach was modified incorporating mechanical stirring to improve the dispersion of the additive during the polycondensation reaction as described in the experimental section (Chapter III). To verify if the stirring has an impact in the characteristics of the prepared materials (e.g., porosity), a control synthesis in the absence of additives was carried out in the Based on the above-described findings, hereinafter in this thesis all the materials (including the gels without additive) will be synthesised with mechanical stirring. The low magnification images show the shape and degree of branching of the CB aggregates, whereas the high magnification micrographs show in detail the shape of the primary particles and the nanostructure (internal to primary particles) of the carbon black nanoparticles. As seen, samples CB-I, CB-II and CB-IV present quite similar degree of aggregation (number of primary particles per aggregate); in contrast, the particles of CB-III seem more packed, showing a higher degree of aggregation. The average particle size was estimated from the analysis of the TEM micrographs, by considering eight images and measuring ca. eighty particles. Average particle sizes between 30-50 nm were estimated for samples CB-I and CB-II; samples CB-III (ca. 50-80 nm) and CB-IV (ca. 80-100 nm) rendered larger nanoparticles. These values are in agreement with those reported in the literature for other carbon blacks [START_REF] Gnanamuthu | Electrochemical properties of Super P carbon black as an anode active material for lithium-ion batteries[END_REF][START_REF] Réjasse | TEM study of the reaction mechanisms involved in the carbothermal reduction of hafnia[END_REF][START_REF] Suryanto | Surface-oxidized carbon black as catalyst for water oxidation and alcohol oxidation reactions[END_REF].

CB-I

Super P Li

CB-III

Super Graphite Co.

CB-II

Super C65

CB-IV

Prolabo 60-80 nm 30-50 nm 30-50 nm 80-100 nm

Regarding nanostructure, defined as the structural arrangement of the graphite-like sheet in terms of length, curvature or undulation, important differences can be distinguished between the selected carbon blacks. Sample CB-III shows the most ordered nanostructure, presenting a larger number of stacked graphitic carbon in its spherical primary particles; however, in the case of samples CB-I and CB-II, the domains with an ordered graphitic structure are shorter, although the particles still display a spheroidal shape with concentric undulations. On the contrary, sample CB-IV displayed a lower order, with a quasi-amorphous arrangement. Through the comparison of the Raman spectra displayed in Figure IV.9 a, the carbon blacks can be categorized into three different grades of structural order [Ferrari, 2006]. In increasing order, CB-IV was the least ordered carbon material, as inferred by the larger width of the bands and the higher height of the valley between the main bands (D and G) of the first-order region and the low definition and intensity of the second-order bands. Samples CB-I and CB-II, showed a slightly improved structural order, as pointed out by the better resolution of the first and second-order bands. On the other hand, CB-III showed a well-defined Raman spectrum.

The narrow and sharp bands in the whole spectral range and the clear separation between the D and G bands in the first-order region (i.e., valley bands almost disappear) confirm the higher structural order of this sample (in agreement with the observations in the TEM images). These structural differences were examined more adequately taking into account the contribution of the different bands contributing to the spectra and their assignment according to literature [Pimenta, 2007;[START_REF] Cançado | Quantifying defects in graphene via Raman spectroscopy at different excitation energies[END_REF]. The first-order region was deconvoluted into the characteristic G (∼ 1580 cm -1 ) and D (∼ 1350 cm -1 ) bands of graphite and derived carbon materials. To better fit this spectral region, different contributions of bands D' (∼ 1600 cm -1 ), D" (∼ 1506 cm -1 ) and D* (∼ 1220 cm -1 ) were introduced [START_REF] Cuesta | Raman microprobe studies on carbon materials[END_REF][START_REF] Jawhari | Raman spectroscopic characterization of some commercially available carbon black materials[END_REF][START_REF] Claramunt | The importance of interbands on the interpretation of the Raman spectrum of graphene oxide[END_REF]. They correspond to disorder in the surface, out of plane defects, and disorder graphitic lattices and impurities, respectively [START_REF] Vollebregt | Influence of the growth temperature on the first and second-order Raman band ratios and widths of carbon nanotubes and fibers[END_REF]. The presence of these bands and their relative positions, widths and intensities provide information about the existence and nature of surface defects in graphene-like materials [START_REF] López-Díaz | Evolution of the Raman spectrum with the chemical composition of graphene oxide[END_REF].

The second-order region was deconvoluted into the 2D band (at ∼ 2700 cm -1 ) and the broad bands from 2300 to 3100 cm -1 assigned to 2D' and combination bands (G*, D + D') [Ferrari, 2000]. For all analysed materials, the spectra deconvolution was carried out by the conventional fitting procedures by Origin program, after using a lineal baseline and Voigt-functions (square root of reduced χ 2 < 3, and coefficient of determinations R 2 > 0.99).

The main parameters are shown in Tables IV.2 and A.I.1 (Annex I). As seen, the frequencies of the bands remained mostly unchanged, whereas the widths and relative intensities (ratio of band intensities) presented significantly changes. The latter are often used as an indicator of the density of defects in carbon materials [START_REF] Díez-Betriu | Raman spectroscopy for the study of reduction mechanisms and optimization of conductivity in graphene oxide thin films[END_REF]].

In the first-order region, a direct relationship can be observed between the I D /I G ratio and the width of the D and G bands, indicating different structural order of the samples. For instance, the width of G band was significantly lower for sample CB-III, indicating a low density of structural defects in the sp 2 network of this material [Cançado, 2007]. The low I D /I G ratio of this sample corroborated this observation. In addition, the I 2D /I G ratio followed the same trend, reflecting the trend of structural order of the carbon blacks as: CB-IV <CB-I <CB-II <CB-III. The disorder induced band broadening in the G band in the rest of the CB is accompanied by prominent D bands (ca. 1350 cm -1 ) and the appearance of D' and D" bands, associated to disorder defects in the carbon basal planes [START_REF] Tuinstra | Raman spectrum of graphite[END_REF]Ferrari, 2000;[START_REF] Kudin | Raman spectra of graphite oxide and functionalized graphene sheets[END_REF][START_REF] Claramunt | The importance of interbands on the interpretation of the Raman spectrum of graphene oxide[END_REF]. The contributions of the combination bands in the second-order region are also higher for samples CB-I, CB-II and CB-IV, indicating a broken symmetry of the graphitic domains, likely as a result of defects. , respectively [START_REF] Warren | experimental and theoretical physics established[END_REF]Franklin, 1951]. Samples CB-I, CB-II and CB-IV presented patterns with broad bands characteristics of carbon materials with randomly oriented stacked graphitic layers, in agreement with a low structural order degree observed by Raman spectroscopy (Figure IV.9, Table IV.2). On the other hand, CB-III displayed sharp and well-defined peaks, characteristic of a more ordered material. Furthermore, this sample displayed a peak at ca. 53 °, ascribed to the second-order reflection (004), corroborated a higher order degree [Franklin, 1950].

The interlayer spacing (d 002 ) was evaluated at the angle of the (002) reflection in the XRD patterns. Likewise, the crystalline dimensions were estimated using Scherrer's equation, with the calculation of the parameter L c (002) and L a (10) [START_REF] Warren | The diffraction pattern of fine carbon blacks[END_REF][START_REF] Pawlyta | Raman microspectroscopy characterization of carbon blacks: Spectral analysis and structural information[END_REF]. Such parameters are compiled in Table IV.3. The calculated values of d 002 for all carbon blacks were higher than the value corresponding to graphite (ca. 0.335 nm), being the closest value that of the sample with higher structural order (sample CB-III). Similarly, the average height of the stacked graphitic layers, L c , is ca. 2 nm for all the samples with the exception of CB-III. In the same way, L a , related with the length of the graphitic domains was higher for the former carbon black. L a parameters can be also calculated from the Tuinstra-Koening equation (TK equation) [START_REF] Tuinstra | Raman spectrum of graphite[END_REF]: L a = 4.4 / (I D /I G ). However, the values calculated by TK equation were ca. twice higher than those obtained from XRD. These differences could be justified by the validity of TK equation since was reported that is not accurate for L a values below 2 nm [Ferrari, 2000]. Consequently, the values obtained by for each carbon black. As it can be seen, both parameters followed an inverse relationship;

those materials with much higher disorder degree (larger I D /I G values) presented lower values of L a , corroborating the same structural order obtaining from both techniques [START_REF] Jawhari | Raman spectroscopic characterization of some commercially available carbon black materials[END_REF]. Figure IV.12 shows the N 2 adsorption/desorption isotherms at -196 °C of the four carbon blacks. All the materials presented similar isotherm shapes, Type II, according to the IUPAC classification [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF], with a low amount of gas adsorbed in the low relative pressure range and a higher contribution at relative pressures above 0.8 for all of them. This is characteristic of materials with a low porosity. Indeed, low surface areas ranging from 30 to 60 m 2 /g (Table IV.4) were obtained, and a low microporosity. Most importantly, the adsorption and desorption branches are reversible in the whole range of relative pressures, indicating the lack of interparticle mesoporosity arising from the aggregation of the CB nanoparticles. As it can be seen, the incorporation of the carbon black additives provoked the significant changes in the shape of the hysteresis loop of the isotherms and in the amount of gas adsorbed respect to the material prepared in absence of carbon black.

As was described in the previous section, the isotherms of the gel prepared in the absence of carbon additive as the corresponding carbon material obtained after the heat treatment displayed a Type IVa character according to IUPAC classification [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]], with a marked hysteresis loop between 0.4 and 0.8 of relative pressures. The position and shape of Regarding the gel/CB composites, the adding of the carbon black additives provoked a change in the position and shape of the hysteresis loop of the isotherms, which shifted towards higher relative pressures (0.5 for PG-CB-IV12 and 0.6 for samples PG-CB-II12 and PG-CB-I12) reaching 0.95 of relative pressure. Additionally, an inflection point (a curvature) is observed in the adsorption and desorption branches about 0.8 of relative pressure for all the composites. This stepped shape of the hysteresis loop was previously reported [START_REF] Macías | Carbon black directed synthesis of ultrahigh mesoporous carbon aerogels[END_REF] and is associated to the presence of multimodal mesopores structure comprised of constricted pore necks and bodies [Rasines, 2015_B]. After the thermal treatment, the gas uptake increase in the whole relative pressure range and the shape of the N 2 isotherm is conserved. As is inferred from the knee of the isotherm at low relative pressures, no changes on the microporous structure was noticed after the incorporation of the carbon blacks, this was also corroborated by the analysis of the microporosity values evaluated by the DR equation.

Table IV.5 collected the textural parameters of gel/CB composites and the data corresponding to the pristine materials prepared are also included for comparison. the amount of carbon black incorporated (ca. 12 wt.%). As seen, the specific surface area (S BET ) and, following the same trend, the micropore volume (W 0 ) of the synthesised materials agree on the theoretical values calculated by the mixture rule. However, the total pore volume (V PORES ) and the volume of mesopores are much larger than theoretically predicted. Thus, the incorporation of carbon black presents a strong impact on the development of the mesoporous network of materials regardless of the morphological properties of the various carbon blacks used as additives. This is closely related to the evolution of the hysteresis loop.

In order to delve into this phenomenon, the hysteresis loop was divided into two regions: the first (noted as V1) corresponds to the volume adsorbed between the beginning of the loop at 0.4 relative pressure to the inflection point (IP) in the adsorption branch; while the second (notes as V2) corresponds to the volume adsorbed from the inflection point to 1 of relative pressure. As for the material without additive, the hysteresis loop is represented in its entirety by V1, since it lacks an inflection point. In the case of the gel/CB composites, similar values of V1 were obtained for PG and CPG, indicating that the V1 is not affected by carbonization; while the values obtained for V2 showed an increase for the carbon gel/CB composites compared with their gel counterparts (PG series). This fact indicates that the carbonization mainly affects to the development of the large mesopores range. For the CB-I and CB-II carbon composites, the inflection point remains constant after thermal treatment; however, for CB-III and CB-IV carbon gel composites IP was observed at higher relative pressure for the PG series. This behaviour is in agreement with the obtained nitrogen isotherms.

The pore size distribution of PG and CPG series are shown in the Figure IV.15, showing a multimodal distribution of all the composites. For the gel composites, as was observed on the nitrogen isotherms at low relative pressures, similar microporous network was revealed. On the contrary, the pore size distribution show differences on the higher pore sizes (mesopores); concretely, CB-III and CB-IV gel composites showed the narrowest distribution. Additionally, after the carbonization process the mesopores were enlarged for all the carbon composites. The surface pH of the carbon blacks was about 6.5 for all of them, close to the pH of the precursors' mixture (RF+catalyst) without additive (pH 6.9) and the individual of each carbon black. Therefore, the role of pH is not a decisive factor in the development of the porosity of different gel/CB composites prepared using different types of carbon black.

CPG-CB-III12 CPG-CB-I12 CPG-CB-II12 CPG-CB-IV12 CPG

In the case for CPG-CB-I12 and CPG-CB-II12, the CB is distributed as smaller aggregates in a discontinued manner. This fact is expected to have a high influence in the final electrical properties of the resulting carbon gel/CB composites, as it will be discussed below.

The Raman spectra of the carbon gel/CB composites were first collected with the conventional point-by-point data collection technique, although this presented a drawback, since as seen previously in the TEM images, the composites present aggregates of different size of carbon black along the matrix of the pristine carbon gel. As alternative, the Raman mapping allowed to recover of 300 Raman spectra taken by an area of ca. 0.02 mm 2 of each carbon gel/CB composites. As a result of this analysis and by means of a mathematical treatment, the average Raman spectra were obtained for each composite, shown in showing the lowest value of I D /I G ratio for CPG-CB-III12 which corresponds to the highest order composite. In the same way, the rest synthesised composites showed the same tendency as the Raman spectra of its corresponding pristine carbon blacks. I D /I G ratio I 2D /I G ratio These differences must be explained considering various aspects such: i) the intrinsic conductivity of the additive; ii) its particle size -as it can influence the percolation path-, and iii) the porosity and/or tortuosity of the carbon gel/CB composites. The highest conductivity was obtained for sample CPG-CB-III12, which may be due to the high conductivity of the additive CB-III itself -compared to the other CB-. Interestingly, sample CPG-CB-IV12 presented a rather high conductivity, despite the low value obtained for the CB-IV additive. This could be attributed to the particle size of the additive (ca. the largest among the CB), that would favour the formation of an interconnected pathway within the nodules of the carbon matrix. This phenomenon is schematized in following the large conductive aggregates (orange lines) formed due to the high size of the CB nanoparticles. This path facilitates the fast-electronic mobility improving the conductivity of the material. On the contrary, for sample CPG-CB-I12 a discontinuous path is observed due to the smaller size of the primary CB-I particles and the lower aggregation between them as a result of the low loading of additive. Hence, the electronic mobility and the final conductivity of the material are hindered. Similar observations have been reported for the addition of conductive additives, such as carbon nanotubes or carbon black, to polymeric blends in which a dual phase continuity is needed to improve the conductivity of the composites [START_REF] Miyasaka | Electrical conductivity of carbon-polymer composites as function of carbon content[END_REF][START_REF] Sumita | Dispersion of fillers and the electrical conductivity of polymer blends filled with carbon black[END_REF]Soares, 1997].

IV.B.2. Amount of carbon black.

To carry out the study on the influence of the amount of additive, carbon black CB-III was selected due to its high structural order and conductivity. For this purpose, carbon black mass ratios between 4 and 40 wt.% (grams of CB-III per gram of reactants) were used. Beyond this amount, the dispersion of the CB in the reactants' mixture became too difficult. The nomenclature of the gels/CB-III composites is PG-CB-IIIZ, where Z accounts for the amount of carbon black; the carbonized gels are denoted as CPG-CB-IIIZ. which corresponds to the removal of moisture (even though the analysis was measured over dried samples). For the organic gel synthesised without additive (sample PG), an overall mass loss of ca. 46 % was obtained, with two broad peaks centred between 400-600 °C. The first one between 200 and 500 °C is associated to the decomposition of volatile matter and carboxylic and anhydrides groups. The second peak spanning between 500 and 800 °C corresponds to the removal of carbonyl, phenolic and quinones groups [START_REF] Lin | Effect of synthesis pH on the structure of carbon xerogels[END_REF][START_REF] Contreras | A comparison of physical activation of carbon xerogels with carbon dioxide with chemical activation using hydroxides[END_REF]. As expected, the DTG profiles of the samples prepared with different amounts of CB-III additive presented a similar shape than the gel without additive, with slightly differences in the overall mass loss. A clear correlation was observed between the mass loss and the amount of carbon black of the samples, which seems reasonable considering that the volatiles released during the thermal treatment arise from the composition of the gel, rather than the carbon black. Indeed, the theoretical mass loss estimated by the general mixing rule, taking into account the composition of the materials and the mass loss corresponding to the individual components (gel and additive), fits with the experimental values (Table IV.7). This also discarded the creation of chemical bonds between the CB-III and matrix gel during the synthesis, which is reasonable considering the hydrophobic nature of the CB-III.

Infrared spectroscopy analysis of the materials also confirmed the lack of chemical bonds between the CB-III and the gels (Figure IV.22). For the gels, the characteristic bands reported for polymeric gels prepared by RF polycondensation mixtures were obtained: C-O-C stretching of methylene ether bridges formed during the polycondensation of RF (1213, 1092 cm -1 ); O-H stretching (3000-3500 cm -1 ); carboxylic acids, lactones, and anhydrides (1720 cm -1 ); conjugated C=O and aromatic ring stretching (bands at 1600-1650 cm -1 ); CH 2 bending (1470 cm -1 ); and CH out of plane deformation in aromatic rings (880 cm -1 ) [START_REF] Pekala | Organic aerogels from the polycondensation of resorcinol with formaldehyde[END_REF]Fathy, 2016]. c)

The intensity of the G band (assigned to ordered graphitic domains) and the contributions of the bands in the second-order range of the spectrum more pronounced for the CB-III. This is in agreement with the disordered structure of the matrix of the carbon gels [Ferrari, 2000;Pimenta, 2007], compared to the order domains in the carbon black. The carbon black pattern presents the two main peaks at ca. 25 ° and 43 ° associated to the (002) and ( 10) reflections, characteristics of the ordered materials. For the non-carbonized samples (series PG), a broad peak between 10 ° -34 ° is observed, indicative of a completely amorphous structure in these materials. For the carbonized samples, the aromatization of the carbon network allowed to distinguish the broad (002) reflection at ca. 22 ° corresponding to disordered carbons. The peaks at ca. 25 ° and 43 °, associated with the ordered graphitic structure provided by the carbon black, are evident in all the samples with additive, and their intensity increased with the amount of CB-III. For the gels, the total pore volume ranged from 0.377 to 0.597 cm 3 /g for PG and PG-CB-III40, respectively. However, the increase in the nitrogen uptake is more pronounced at relative pressures above 0.4, indicating that the incorporation of the CB-III has a dominant impact on the mesoporosity rather than on the microporosity of the gels. The CO 2 adsorption isotherms at 0 °C, typically used to evaluate the narrow microporosity of carbon materials [START_REF] Garrido | Use of N 2 vs. CO 2 in the characterization of activated carbons[END_REF] are shown in Figure IV.25 e-f. All the isotherms display a concave shape with

x-axis, characteristic of materials with narrow micropores. The gas uptake was quite close for all the samples, regardless the content of additive. Table IV.9 compiles the volume of narrow micropores evaluated using the DR equation; as seen, the values ranged from 0.11 to Relative pressure (p/p 0 )

CPG CPG-CB-III4 CPG-CB-III8 CPG-CB-III12 CPG-CB-III16 CPG-CB-III24 CPG-CB-III40 f)
Table IV.9. Main textural parameters obtained from the N 2 at -196 °C and CO 2 at 0 °C adsorption isotherms for the materials synthesised with different amounts of CB-III as additive. The impact of the CB-III in the development of the porosity of the materials was more evident in the volume distribution of the hysteresis loop within the whole range of relative pressures. To facilitate the analysis, we differentiated two regions in the loops: V1 related first part of the hysteresis loop (0.4 and the relative pressure of the inflection point (IP) in the adsorption branch) and a second part, V2 (between relative pressure of IP and 1). In the case of the samples without CB-III additive, V1 accounts for the full loop, as V2 is not detected.

Sample

Interestingly, while the increase in V2 follows a linear correlation with the amount of CB-III additive (for both the organic and the carbon gels), the evolution of V1 is discrete (ca. an increase between 4 and 12 wt.% of CB-III, and a disrupt above this value) and does not follow (cm indicate larger pore sizes, this increment reveals that the CB-III is responsible for the enlargement of the primary mesopore structure (higher IP and V1), and for the creation of a secondary network of mesopores of larger sizes. The materials with lower CB-III amount presented the higher slopes, pointing to higher resistance values, and thus, lower conductivity. For instance, a decrease of ca. 12-15 times resistance was obtained for the carbon gel prepared with the highest amount of carbon black.

A rise in the conductivity of the gel/CB-III materials was expected owing to the high intrinsic conductivity of the CB-III used as additive (e.g., 1.64 S/cm), compared to the poorly conductive carbon matrix of the carbon gels (e.g., 0.022 S/cm measured under the same conditions). Such a rise was, however, somewhat smaller when compared to the values reported for similar carbon electrodes when a CB-III additive was incorporated in the electrode's ink (i.e., after the synthesis of the carbon material was used as electrode) [Rey-Raap, 2014_A]. This suggests a different connectivity between the CB-III and carbon gel particles, depending on the electrode preparation process.

To further clarify this aspect, the dependence of the conductivity of the gel/CB-III samples on the amount of conductive additive was analysed considering a percolation model and a general mixing rule (Figure IV.30). The rule of mixtures would predict the conductivity of materials' pellets as if both components (carbon gel and CB-III) were segregated like a homogeneous mixture [START_REF] Sihvola | Mixing rules and percolation[END_REF]. On the other hand, according to the standard percolation theory in isotropic materials, the bulk conductivity of a gel/CB-III composite with concentration Φ of a conducting phase would behave as a power law of the form [START_REF] Sihvola | Mixing rules and percolation[END_REF]:

σ = σ o (Φ -Φ c ) β (equation IV.1)
where σ is the conductivity of the carbon gel/CB composite (S/cm), σ o is characteristic conductivity of the carbon gel without additive (S/cm), Φ is the fraction of the CB-III additive (wt.%), Φ c is the fraction of the additive at the percolation threshold (wt.%), and β is a critical exponent related to the dimensionality of the material [Batrouni, 1996].

The trend of the conductivity value with the amount of additive incorporated did not follow a linear correlation for all the studied amounts (Figure IV.30 a), it followed a power-like regime characteristic of percolating systems. For the samples with 4 and 8 wt.% of additive, the conductivity remains almost unaltered and close to the conductivity of the carbon gel without additive. However above 8 wt.% an abrupt change is observed and the conductivity increases The percolation threshold for the conductive additive loading was evaluated by plotting the log (σ) vs. log (Φ) as depicted in Figure IV.30 c. The percolation threshold was found at 8-12 wt.%, where the conductivity of the gel/CB-III materials showed a marked increase, seen by the intersection of both straight-line fits. This value is higher than percolation thresholds reported for CB-III and other carbon additives; for instance, 0.19-4 vol.%. for MWCNT and graphene-derived materials [START_REF] Liang | Electromagnetic interference shielding of graphene/epoxy composites[END_REF]Rey-Raap, 2014_A;[START_REF] Ram | Electrical properties of polyvinylidene fluoride (PVDF)/multi-walled carbon nanotube (MWCNT) semi-transparent composites: Modelling of DC conductivity[END_REF][START_REF] Verma | Excellent electromagnetic interference shielding and mechanical properties of high loading carbon-nanotubes/polymer composites designed using melt recirculation equipped twin-screw extruder[END_REF], 2-3 wt.% for polystyrene/graphite and epoxy/graphite composites [START_REF] Chen | Preparation of polystyrene-graphite conducting nanocomposites via intercalation polymerization[END_REF][START_REF] Thongruang | Correlated electrical conductivity and mechanical property analysis of high-density polyethylene filled with graphite and carbon fiber[END_REF][START_REF] Taherian | Electrical conductivity in polymer-based composites: experiments, modelling, and applications[END_REF], and 7.5 wt.% for carbon fiber/polyethylene composites [START_REF] Ezquerra | Alternating-current electrical properties of graphite, carbon-black and carbon-fiber polymeric composites[END_REF][START_REF] Thongruang | Correlated electrical conductivity and mechanical property analysis of high-density polyethylene filled with graphite and carbon fiber[END_REF]. This is a consequence of the synthetic route, since the CB-III was incorporated in the reactants' mixture (before formation of sol-gel and the carbonization) and not in the formulation of the electrodes ink (e.g., post-synthesis of the carbon material), as is usually the case in the literature.

The two well-defined regimes corresponding to different exponent β values (i.e., intersection of lines) observed in the plot (Figure IV.30 c), indicate that the systems follow a tunnelling-percolating regime, rather than a pure percolation model [START_REF] Balberg | Tunneling and nonuniversal conductivity in composite materials[END_REF][START_REF] Stauffer | Introduction to Percolation Theory[END_REF]Park, 2013]. In this case, the conductivity of the composites depends on tunnelling processes occurring between the conducting particles of the CB-III; since these are embedded in a less conductive medium -the matrix of the carbon gel-the distribution function of the conducting particles within the bulk material becomes more important than the bulk composition itself. The existence of percolation-tunnelling systems has been proposed for other specific distributions of conducting and insulating phases involving carbon black additives [START_REF] Vionnet-Menot | Tunneling-percolation origin of nonuniversality: Theory and experiments[END_REF].

IV.B.3. Hydrophobic nature of the carbon black.

IV.B.3.1. Characterisation of the oxidised carbon black.

With the aim to study the effect of the hydrophobic/hydrophilic character of the carbon black used as additive, sample CB-III (of hydrophobic nature) was oxidised to incorporate O-containing surface functionalities. This is expected to modify the dispersion of the additive nanoparticles in the reactants' mixture (e.g., aqueous solution).

The oxidation was carried out by a modified Hummers reaction as described in the experimental section (Chapter III). It is interesting to note that no changes in the physical appearance of the CB-III were observed after the oxidation. TEM images (Figure IV.31) showed similar particle size in the additive (average ca. 50 nm) before and after oxidation. The spherical aggregates were detected in both materials indicating that the structure, morphology and distribution of the graphitic domains is maintained after the oxidation. The amount of oxygen in CB-IIIox was ca. 26.9 wt.% (Table IV.10), confirming a severe oxidation of the carbon black. This increase was accompanied by a sharp fall in the surface pH from 6.2 to 2.6 indicating the presence of the acidic functionalities after the treatment.

Additionally, an increase of the sulphur content was observed from ca. 0 to 6.1 %, this is associated to the oxidation process, that was made in strong acidic media employing H 2 SO 4 , provoking the remaining S-groups on the oxidised sample even after extensive washing [START_REF] Dimiev | Pristine graphite oxide[END_REF]. The functionalization of the CB-III was also analysed by thermogravimetry (Figure IV.32).

The mass loss (dry basis) goes from 0.2 % for the pristine CB-III to 35.0 % to the oxidised material confirming the incorporation of surface functional groups upon oxidation. [ [START_REF] Bandosz | Characterization of the surfaces of activated carbons in terms of their acidity constant distributions[END_REF][START_REF] Contescu | Selection of water-dispersible carbon black for fabrication of uranium oxicarbide microspheres[END_REF]].

CB-III CB-IIIox

This was confirmed by the infrared spectroscopy analysis (Figure IV.34). For CB-IIIox sample the broad band associated to the O-H stretching was obtained (3000-3500 cm -1 ), additionally, two bands at 1650-1600 cm -1 and ca. 1720 cm -1 were observed corresponding to conjugated C=O and aromatic ring stretching, and carboxylic acids, lactones, and anhydrides, respectively.

At lower wavenumber (1213, 1092 cm -1 ) the bands corresponding to C-O-C stretching were also detected [Fathy, 2016]. IV.12). By the comparison of the Raman spectrum corresponding to the oxidised carbon black against the precursor material, important differences were appreciated. As expected, the incorporation of the oxygenated groups to the surface of the material as well as the defects created by the strong oxidative treatment caused a considerable increase in the carbon black graphite structure disorder [START_REF] Osswald | Monitoring oxidation of multiwalled carbon nanotubes by Raman spectroscopy[END_REF][START_REF] López-Díaz | Evolution of the Raman spectrum with the chemical composition of graphene oxide[END_REF]. porosity. In the case of the gels (non-carbonized samples), the presence of CB-IIIox resulted in a Type II isotherm characteristic of materials with a poor development of porosity, this behaviour was observed for the both studied amounts. It should be noted that the addition of a highly oxidised additive to the aqueous reactants' mixture produces a significant variation of the media pH increasing the H + /OH -ratio of the solution. The mixture without additive, due to the presence of base catalysts (Na 2 CO 3 ), has a pH ca. 6.9, after the addition and homogeneous dispersion of the additive the pH of solution was found as 1.8 and 1.2 for the mixtures with 4 and 12 wt.%, respectively. No change on the pH was observed for the samples made with CB-III at any of both amounts. It is well known that the pH of the reaction media has an important effect in the polycondensation reaction having a high influence in the porosity development of the final gels [START_REF] Pekala | Organic aerogels from the polycondensation of resorcinol with formaldehyde[END_REF][START_REF] Lin | Effect of synthesis pH on the structure of carbon xerogels[END_REF]. When pH of the precursors' solution decreases, the mesoporous of the gel become wider and the formation of macroporous are favoured. This is associated to the fact that at low pH, during the first step of polycondensation reaction, the number of resorcinol anions is reduced since they are formed by hydrogen abstraction (hindered by H + concentration), as a consequence also a decrease on the number of hydroxylated derivatives (formed by the formaldehyde addition to the resorcinol anions) is produced. Then, during the second step of the reaction, the condensation is favoured producing branched clusters and high particle sizes avoiding the crosslinking and prevented the porous network development [START_REF] Pekala | Structure of organic aerogels. 1. Morphology and scaling[END_REF][START_REF] Shaheen | Preparation and properties of resorcinol-formaldehyde organic and carbon gels[END_REF][START_REF] Job | Porous carbon xerogels with texture tailored by pH control during sol-gel process[END_REF][START_REF] Rey-Raap | A visual validation of the combined effect of pH and dilution on the porosity of carbon xerogels[END_REF]. Summarizing, the combine effect of the abrupt pH decrease of the reactant media and the well-dispersion of the additive during the polycondensation reaction hinder the polycondensation and crosslinking of the reactants, preventing the development of the porosity of the gels/CB-IIIox structures.

For the carbonized samples, Type I isotherms were obtained regardless the amount of carbon black, indicative of the development of a microporous structure. Indeed, the surface areas and micropore volumes are similar to those of the carbonized gel/CB counterparts (Table IV.12). The mesopore structure is however lost, as seen in the fact that the hysteresis loop was not formed. The development of the microporous structure upon carbonization is a common feature in these materials [START_REF] Zubizarreta | Development of microporous carbon xerogels by controlling synthesis conditions[END_REF], and is associated to the elimination of the oxygen groups.

Table IV.12. Main textural parameters obtained from the N 2 adsorption isotherms and intensity ratios obtained from Raman spectra for the series of materials prepared using CB-III and CB-IIIox as additives. The DTG profiles of the gel/CB composites (Figure IV.39 b) revealed two main peaks, as was discussed in the previous section, centred at 400 and 600 °C for all the materials that correspond to the decomposition of volatile matter, carboxylic and anhydrides (lower temperature) and to removal of carbonyl, phenolic and quinones groups (higher temperature).

Sample

However, for the composites elaborated with the CB-IIIox, the mass loss is delayed to higher temperatures (from ca. 200 °C to 250 °C), as clearly seen in the Figure IV.39 a. This could be associated to the higher strength of the bonds between the carbon and the weaker oxygenated groups for these composites. As was already observed for the different amounts of CB-III incorporated in the above section, a direct correlation was observed between the mass loss and the amount of carbon black of the samples. For the samples with 4 wt.% of additive, a mass loss of ca. 44 % was observed; in contrast, for the samples with 12 wt.% of additive, the mass loss was smaller, as expected due to the lower amount of gel matrix (Table IV.13).

Table IV.13. Mass losses obtained from the thermogravimetric profiles and during the carbonization process of the series of materials prepared using CB-III and CB-IIIox as additives.

The theoretical mass loss for each composite is also included.

Sample

CPG-CB-IIIox4

CPG-CB-III4 CPG-CB-III12 CPG-CB-IIIox12

The well-distribution of the CB-IIIox additive through the RF polymeric matrix was confirmed by the Confocal Raman imaging (Figure IV.41). The variation of the I 2D /I G ratio for sample CPG-CB-IIIox12 was employed to reconstruct the images. IV.14.

After oxidation treatment, CB-III become to insulating material (no electrical contact between the material and the points was reached) due to the presence of oxygenated groups.

On the other hand, the results show almost no differences in the conductivity values calculated for the carbons prepared with an amount of additive (CB-III or CB-IIIox) of 4 wt.%. In contrast, for 12 wt.% of additive, the carbon synthesised with CB-IIIox presented a slightly higher conductivity. It should be reminded that the conductivity is determined on the samples carbonized at 800 °C, thus the amount of oxygen is expected to be low. The higher conductivity of CPG-CB-IIIox12 compared to CPG-CB-III12 must be attributed to either the differences in the porosity and/or the distribution of the carbon black in the amorphous carbon matrix. Since in both cases the amount of additive (ca. 12 wt.%) is above the percolation threshold, data would indicate that the additive is better distributed throughout sample CPG-CB-IIIox12, which also displayed a predominantly microporous structure (as opposed to CPG-CB-III12 with a marked mesoporous character). This effect was not observed in the carbon gels synthesised with lower amount of additive, indicating that the low concentration of the additive (below the percolation threshold) governs the electrical conductivity, regardless the distribution of the particles. Section IV.C:

Graphite, graphene oxide and reduced graphene oxide as additives

IV.C.1. Characterisation of the additives.

Graphite (Gr), graphene oxide (GO) and reduced graphene oxide (rGO) were selected as additives. Commercial graphite powder, from Sigma-Aldrich, was employed as additive and as precursor for the preparation of graphene oxide by the modified Hummers method (see Chapter III). As partially reduced graphene oxide (rGO) commercial sample, purchased from Graphenea, was also used. graphene-like materials, GO and rGO, being more notable for the rGO. The higher magnification images show the nanostructure of the materials allowing to detect the number of graphene layers, as well as its extension and separation. Gr presents numerous layers stacked forming big slabs of large sheets. After oxidation of Gr, the obtained GO showed shorter sheets, indicating the breakage of the slabs. Additionally, the number of stacked layers decreased and the distance between them increased, as expected after the intercalation of water molecules and the incorporation of functionalised groups upon oxidation [START_REF] Sharma | Determination of defect density, crystallite size and number of graphene layers in graphene analogues using X-ray diffraction and Raman spectroscopy[END_REF]. On the other hand, rGO material showed lower separation between layers compared to the GO, associated to the presence of a lower density of functional groups.

GO

Graphene oxide structure as inferred from the well-defined and narrow G band (∼ 1580 cm -1 ), of higher intensity than that of D band (∼ 1350 cm -1 ). The oxidation of Gr to render GO brought about the appearance of a prominent wide D band in the first-order, indicating the incorporation of defects in the graphenic sheets upon oxidation. The second-order spectra showed the same trend; while for Gr a well-defined profile was observed, with a high contribution of the 2D band centred at ∼ 2700 cm -1 , GO displayed a broadening of the 2D band and a sharp decrease in intensity, characteristic of disordered carbon structures. Sample rGO displayed a

Raman spectrum very similar to that of GO, with a large contribution of the D band (higher intensity than G band) in the first-order, and the broad bands in the second-order, although all the bands are narrower and better-defined than in GO. The shapes in the Raman spectra confirmed the observations obtained from the TEM images, showing a higher structural order for graphite followed by the rGO. To obtain more information about the structural arrangement of the additives, all Raman spectra were deconvoluted (Tables IV.15 and A.I.9, Figure A.I.7). The first-order region comprised five contributions, D* (∼ 1220 cm -1 ), D (∼ 1350 cm -1 ), D'' (∼ 1506 cm -1 ), G (∼ 1580 cm-1 ) and D' (∼ 1600 cm -1 ) [Ferrari, 2006;Pimenta, 2007;[START_REF] Jia | Graphene edges: A review of their fabrication and characterization[END_REF]. The second-order region was deconvoluted into four main contributions: 2D (∼ 2700 cm -1 ), G* (∼ 2550 cm -1 ), D + D '(∼ 2900 cm -1 ) and 2D' (∼ 3100 cm -1 ) bands. A slight shift in the position of G band of GO towards longer wavenumbers -compared to that of graphite-confirmed the presence of oxygen-containing functional groups [START_REF] Veerapandian | characterization and electrochemical properties of functionalized graphene oxide[END_REF]. The main difference among Gr and GO was the broadening of D and G bands, indicating the presence of structural defects upon oxidation. These are partially eliminated in rGO that is characterised by narrower bandwidths compared to GO. This is in agreement with data from the literature [START_REF] López-Díaz | Evolution of the Raman spectrum with the chemical composition of graphene oxide[END_REF]. Franklin, 1951]. The contribution of the ( 001) and ( 004) planes at 42 ° and 54 °, respectively, were also detected; these are usually associated to highly ordered graphites with low structural defects [START_REF] Warren | experimental and theoretical physics established[END_REF]Franklin, 1951]. For GO, a sharp peak was detected at ca. 9.7 °, indicating that the material has an interlayer spacing ca. three times higher than that in Gr. This confirmed the expansion of the graphitic layers after incorporation of the O-groups upon oxidation, as was also detected by TEM images. It is important to mention that GO has a highly hydrophilic character (as it will be discussed below); thus, the increase in the interlayer distance is also provoked by the presence of water molecules between the layers [START_REF] Sharma | Determination of defect density, crystallite size and number of graphene layers in graphene analogues using X-ray diffraction and Raman spectroscopy[END_REF]. The XRD pattern of rGO clearly showed a much lower crystallinity than the other two materials; a broad ill-defined band was observed at 24.3 ° along with a small peak at 44 °, indicating a somewhat turbostratic structure and a short-range structural order of this material. The widening of the XRD peaks is related with the presence of structural defects and/or heteroatoms that produce shorter sp 2 domains. The interlayer distance value was

0.366 nm (Table IV.16) indicating the stacking of the layers without recovering the values typical of graphite. This indicates that the removal of the O-groups upon the reduction process did not bring about a restoration of the sp 2 graphitic domains, creating a rather defective/distorted graphenic network. The size of the crystalline graphitic domains can be estimated by the Scherrer's equation as the L c and L a values, which are associated to the crystallite size along the c-axis and size of the graphenic planes, respectively. The values obtained for the three materials are collected in Table IV.16. As seen, L c and L a values decreased for GO and rGO, being the effect more pronounced for the former. This is in agreement with the observations obtained from Raman and TEM, pointing out the presence of surface defects and low range sp 2 domains in both GO and rGO.

L (nm)

out the high structural long-range order of graphite compared to GO and rGO [Ferrari, 2000;Pimenta, 2007]. A similar observation was noticed in the case of carbon blacks in Section IV.B.

The chemical composition of the three additives was analysed by various techniques;

namely FTIR, potentiometric titration and elemental analysis.

The elemental analysis (Table IV.17) confirmed the large oxidation extent of GO compared to the pristine graphite, and the lower amount of O-containing groups in the partially reduced rGO. It should be noted that a small contribution of sulphur was detected in GO, likely associated to the remaining sulphate groups after the oxidation (which is carried out in H 2 SO 4 medium). The nature of the surface groups was investigated by potentiometric titration and FTIR.

Table IV.17 shows the evolution of the surface pH values of the additives, going from neutral pH in the case of Gr to acidic character for GO and rGO, being the GO the material with the lowest pH. The acidic character of these two materials was also confirmed by quantification of the amount of protons released/adsorbed to/from an aqueous solution evaluated by potentiometric titration. As it can be seen in Table IV.17, GO displayed 3 times total amount of protons released to the solution than rGO, which is in agreement with its higher functionalization and acidic character. The analysis of the pK a distribution profiles (Annex I, Figure A.I.8) allowed the identification and quantification of different oxygenated groups in GO. Two main types of acidic functional groups were identified at pK a 9-11 due to phenolic groups, and at pK a 4-6 relative to carboxylic acid groups in different environments (Table IV.17 A.I.9), as with other studies in the literature for GO and rGO materials [START_REF] Konkena | Understanding aqueous dispersibility of graphene oxide and reduced graphene oxide through pKa measurements[END_REF][START_REF] Ederer | Quantitative determination of acidic groups in functionalized graphene by direct titration[END_REF][START_REF] Orth | pKa determination of graphene-like materials: Validating chemical functionalization[END_REF].

The infrared spectrum of graphite only presents a small band of the vibration band of C=C around 1632 cm -1 , characteristic of sp 2 domains (Figure IV.47). In contrast, various bands appeared in GO as a result of the oxidation. In the high frequency region, the broad band at 3587-3370 cm -1 is assigned to  (OH),  (COOH), and  (C-OH); a shoulder at 2770 cm -1 assigned to aldehydes. In the lower frequency region, several bands between 1800 and 1000 cm -1 are observed, that have been ascribed to strongly absorbed H 2 O, epoxides (1279 and 858 cm -1 ), primary/secondary alcohols and phenolic alcohols (1347 and 1048 cm -1 ), carboxylic acids (1727 cm -1 ), conjugated ketone groups (1598, 1565 cm -1 ) and sp 2 hybridized C-C bonding modes (1645 cm -1 ) [START_REF] Coates | Interpretation of infrared spectra, a practical approach[END_REF]. Furthermore, the peaks at 1218 and 648 cm -1 are characteristic of sulphates and sulphonates, and would account for the sulphur detected in the elemental analysis, after decomposition of the sulfuric acid used as a solvent in the modified Hummers method use to synthesise GO. Similarly, the peaks at 1198 and 943 cm -1 indicate a contribution of phosphates, most likely generated as by-product of the decomposition of H 2 SO 4 /H 3 PO 4 solvent in the oxidation. In rGO, the intensities of the peaks associated to O-groups decreased significantly, but some of them are still evident. Along with the contribution of sp 2 domains (1645 cm -1 ), the broad peak of hydroxyl groups at 3350 cm -1 , and those corresponding to conjugated ketones and carboxylates are still clearly seen. The porous structure of the three additives was analysed by the N 2 and CO 2 adsorption/desorption isotherms obtained at -196 °C and 0 °C, respectively (Figure IV.49). Gr and GO presented a negligible pore structure, characterised by a Type III isotherm according to the IUPAC classification [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]. On the other hand, rGO showed a Type IV isotherm with a prominent hysteresis loop above 0.4 of relative pressure that revealed the development of a micro-and mesoporous structure. Table IV.18 collects the textural parameters obtained from the N 2 isotherms, showing a S BET for rGO of 473 m 2 /g with a high volume of mesopores. In agreement with the gas adsorption isotherms, Gr and GO exhibited low surface areas of ca. 10-12 m 2 /g. CO 2 isotherms at 0 °C revealed a low adsorption volume in all relative pressure ranges discarding the presence of the narrow micropores.

IV.C.2. Characterisation of the gel and carbon gel/additive composites.

As described in the experimental section (Chapter III), the additives were incorporated to the RF mixtures under constant stirring, allowing a uniform dispersion of the additives through the resulting monoliths and providing homogeneous materials. Composites with two concentrations of additive were prepared: 4 and 12 wt.% (expressed in grams of additive per gram of R+F). The composites were labelled -following the same criteria stablished in the previous section-as PG-XY, where X refers to the additive used (Gr, GO or rGO) and Y denotes the amount of additive incorporated; in turn, the carbon gels composites obtained after the heat treatment were labelled CPG-XY. The gel composites prepared incorporating graphite as additive showed isotherms similar to that of the gel without additive, regardless of the amount added (PG-Gr4 and PG-Gr12). The gel without additive displayed a Type IVa isotherm (according to IUPAC classification) [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF], with a H2(b) Type hysteresis loop from 0.4-0.8. For PG-Gr4 and PG-Gr12, there is a slight increase in the total pore volume and a shift in the desorption branch of the hysteresis loop towards higher relative pressures as the amount of additive increased.

Interestingly, the desorption branch of the three isotherms was similar, indicating an enlargement in the size on the necks of the mesopores. A similar trend was obtained upon carbonization, with an increase in the microporosity and the maintenance of the mesopore When rGO was used as additive, the shift in the position of the hysteresis loop towards higher relative pressures and the increase in the pore volume of the gel/additive composite samples were more pronounced. For instance, the total pore volume of PG-rGO12 was almost 3 times higher than that of PG. For PG-rGO12, the isotherm displayed a Type IV shape with a H1 hysteresis loop (almost vertical and parallel adsorption/desorption branches), indicating a narrower distribution of mesopores of larger sizes. Indeed, similar microporous volumes (Table IV.19) were obtained for PG-rGO4 and PG-rGO12, with large differences in their mesopore volumes (ca. 0.462 and 1.010 cm 3 /g, respectively).

In the case of materials prepared with 4 and 12 wt.% of GO, the resulting gel/additive composites displayed a sharp reduction in the amount of gas adsorbed in the entire range of relative pressures, indicating a low porosity (accessible to nitrogen at -196 ᵒC). The existence of the narrow microporosity could be analysed by the CO 2 adsorption isotherms at 0 °C. After carbonization, the samples displayed some porosity, being mainly microporous as opposed to the composites without additive or with rGO and graphite additives. The evolution of the porous features upon carbonization of the sample is summarized in the textural parameters in Table IV.19. In the case of the composites prepared with GO as additive, the carbonization resulted in a significant increase in the micro-and mesopore volumes and in the surface area (an increase of ca. 98 % for the latter parameter). The increase in the S BET for the composites prepared with Gr and rGO was much lower, around 49 % and 63 %, respectively. According to these results, it seems that the formation of the porosity during the RF polycondensation would not be straightforwardly correlated to the porosity of the additive; Gr and GO (both non-porous) have contrary effects: GO hinders the textural development, while Gr favours the formation of a mesoporous network. Furthermore, rGO with a mesoporous structure itself, provokes the formation or larger pore volumes and mesopores (and most likely macropores, although evidences of this has not gathered by mercury porosimetry). It seems that the chemical composition and/or the morphology of the additive would be more important than its intrinsic textural features. This would be in agreement with previous observations for the role of the functionalization of the additive, explored upon oxidation of CB-III (Section IV.B.3), In that case, the oxidation of the carbon black resulted in the hindering of the formation of a mesoporous structure, with the final carbon materials displaying a predominantly microporosity.

The poor porous development of the PG-GO series can also be rationalized considering several hypotheses: i) the presence of an adequate high density of oxygenated groups (Table IV.17) could facilitate the rate of the crosslinking of the gel during the synthesis [Liu, 2013;Canal-Rodríguez, 2019]; if the rate and the number of bonds created with the O-groups of the additive are too high, a compact/dense composite could be obtained, characterised by a poor porous development. Similar findings on the impact of an accelerated polycondensation reaction on the porosity of the resulting materials have been reported, for instance, for RF and melamine mixtures [Rasines, 2015_A;Rasines, 2015_B]. ii) The acidic character of the GO additive (Table IV.17) provokes a decrease in the pH of the polycondensation reaction, that would be affecting the mechanism of polycondensation of the reactants [START_REF] Pekala | Organic aerogels from the polycondensation of resorcinol with formaldehyde[END_REF][START_REF] Lin | Effect of synthesis pH on the structure of carbon xerogels[END_REF], as was previously discussed for the composites made with CB-IIIox additive (Section IV.B.3). The reactants' mixture varied from 6.9 (without additive) to ca. 2.0 pH units after incorporating GO (Table IV .19). This is associated to the presence of hydroxyl groups on GO surface (Figures IV.47 and IV.48,Table IV.17) that release H + in the aqueous medium of the reaction, thereby producing a notable decrease of the pH media despite the presence of Na 2 CO 3 as catalyst. Neither Gr nor rGO produce an important variation of the pH (Table IV.19), as expected given their low functionalization.

Figure IV.51 shows the correlation between some textural parameters (e.g., S BET and V PORES ) and the pH of the reaction mixture. It is clearly shown how for the acidic pH (composites made with GO as additive) the S BET and V PORES were the lowest values obtained; however, for the rest of additives, that rendered a pH media between 6.4 and 6.9 shows, as the pH increase the higher BET areas were obtained. Similar trend was observed for the V PORES . These findings corroborate that, at strongly acidic pH, the condensation reaction is favoured producing branched clusters and high particle sizes avoiding the crosslinking and preventing the porous network development [START_REF] Pekala | Structure of organic aerogels. 1. Morphology and scaling[END_REF][START_REF] Shaheen | Preparation and properties of resorcinol-formaldehyde organic and carbon gels[END_REF][START_REF] Job | Porous carbon xerogels with texture tailored by pH control during sol-gel process[END_REF][START_REF] Rey-Raap | A visual validation of the combined effect of pH and dilution on the porosity of carbon xerogels[END_REF], producing composites with lower porosity. Similar effect of the reactants' pH was reported by Rey-Raap et al. [Rey-Raap, 2014_B], the authors showed three types of porous xerogels (microporous, micro-mesoporous and macroporous), synthesised by microwave heating, as a function of the relation of the dilution ratio (solvent's amount/reactants' amount) and solution pH. The authors revealed that below pH of 5.5 the obtained materials were predominantly macroporous with values of V PORES similar to our values obtained for PG-GO composites (close to 0.1 cm 3 /g). The carbonization of the gel/additive composites provokes an increase of the porosity of all the composites, this increase is more notable for the CPG-GO carbon materials that suffered an increase of BET area from 16 and 42 to 663 and 645 m 2 /g for 4 and 12 wt.% of additive, respectively. It should be pointing out that the values of the BET area of all the carbon composites were similar regardless the pH of the reactants' mixture pH. This in agreement with the literature that reported a constant S BET area for RF carbon gels synthesised between pH of 5.5-6.7 [START_REF] Lin | Effect of synthesis pH on the structure of carbon xerogels[END_REF][START_REF] Job | Porous carbon xerogels with texture tailored by pH control during sol-gel process[END_REF][START_REF] Rey-Raap | A visual validation of the combined effect of pH and dilution on the porosity of carbon xerogels[END_REF].

The correlation of the textural parameters obtained from the nitrogen isotherms at -196 ᵒC with the values predicted by the mixing rule is shown in Figure IV.52. The specific surface area and the volume of micropores calculated by DR formulism were somewhat fitted with the theoretical values, with the exception of the predicted values for the composites prepared with GO and rGO as additives. This can be justified by the lower pH obtained in the GO, as discussed above. For all the composites, the most significant differences were found for the total pore volume and the volume of mesopores (Figure IV.52 b and d), where the composites prepared with Gr and rGO presented a higher mesoporosity than that predicted by a mixing rule. The opposite situation was obtained for the carbon gel/GO composites. It is clear that the use of the additives favoured the formation of a mesopore structure, as it had also been observed for the series prepared with various amounts of carbon black (see Section IV.B.2).

FTIR analysis was performed to monitor the chemical composition of the different composites. As seen in Figure IV.53, the spectra displayed several peaks associated to different oxygen functional groups and carbon structure of the gel/graphitic additive composites. The main bands observed were a broad peak from 3650 to 2900 cm -1 (O-H stretching), a shoulder at 2769 cm -1 (aldehydes), a sharp peak at 1730 cm -1 (C=O of carboxylic acids, lactones and anhydrides), a peak between 1600-1550 cm -1 (conjugated C=O and aromatic ring stretching), two contributions at 1280 and 860 cm -1 (epoxides and peroxides) and, finally, two peak centred at 1470 and 880 cm -1 (CH 2 bending and CH out of the plane deformation in aromatic rings) [START_REF] Pekala | Organic aerogels from the polycondensation of resorcinol with formaldehyde[END_REF]Fathy, 2016;[START_REF] Parviz | Tailored network formation in graphene oxide gels[END_REF]. Comparing the profiles of the composites obtained with the three graphitic materials and the pristine gel, no notable differences were observed between samples. However, the measurements of the pH PZC revealed variations between them showing a pH of 4.7, 2.2 and 3.0 for PG-Gr12, PG-GO12 and PG-rGO12, respectively, pointing out the oxygen-containing groups of the additive have an effect on the acidic character of the final composite. As seen in the previous section (Section IV.B), the difference in the charge of functional groups on the surface of the materials has an impact on the behaviour of the additives and, therefore, on their interaction with the precursors of the polymer reaction in an aqueous medium. As was observed in the TEM images (Figure IV.55), the aggregation of the different additives depends on their hydrophobic/hydrophilic character. Thus, in the I 2D /I G mapping of the CPG-Gr12 sample, a colour gradation can be observed allowing to distinguish the graphite aggregates (dark areas) through the amorphous RF matrix. In the same way as the carbon gel/CB composites of the previous section (Section IV.B), the lack of functional groups on the surface confers hydrophobic properties to the material and makes it difficult to interact with the monomers of the polymeric reaction that crosslinking the monomers of resorcinol and formaldehyde that give rise to the matrix. On the other hand, in the images obtained from the mapping of the CPG-GO12 and CPG-rGO12 composites, the colour distribution is uniform, confirming that the functionality on the surface facilitates the homogeneous dispersion of the additive promoted by a possible interaction between the oxygen functional groups bounded in the layers of these graphene materials and precursor reagents of the polymer matrix. In order to obtain the most accurate results, the analysis by Confocal Raman spectroscopy was performed with the materials with the highest content of graphitic additive, with respect to the two percentages tested, 4 and 12 wt.%.

The deconvoluted Raman spectra shown in Figure IV.56 (right column) correspond to the average of the spectra taken the surface of each carbon gel/additive composites by Confocal Raman spectroscopy, (ca. 300 spectra of each sample). As it can be observed, the three composites present a quite similar profile, where the contribution of the D and G bands are centred, at ∼ 1350 and ∼ 1590 cm -1 , respectively, and was accompanied by several contributions from the rest of the bands of the first-order region. Regarding the second-order region, CPG-Gr12 displayed a well-defined contribution of the 2D band, located at ∼ 2700 cm -1 , respect to the sum of the bands in the second order region. From the data extracted from the deconvolution of the spectra, collected in Tables IV.20 and A.I.10, it is seen how the position of the main bands, D, G and 2D were not presented significant differences between the synthesised composites using the three graphitic additives. However, regarding the ratio of intensities between these bands, CPG-Gr12 presents the lowest value for the I D /I G ratio and the highest for the I 2D /I G ratio, which means, this carbon gel/Gr composite presented the highest structural order. Thus, a correlation between the individual order of graphite and its corresponding composite exists, since the pristine graphite has an I 2D /I G ratio of 0.55 compared to 0.19 that graphene oxide and its reduced material presented, respectively. The composites prepared with these two materials, CPG-GO12 and CPG-rGO12, present similar values for the ratios between the intensities of the bands, confirming the addition of both additives derived from graphite provokes a degree of structural distortion very similar. In conclusion, the effect of the hydrophobic/hydrophilic character and the oxygenated functional groups of the additives led the development of different porosity. The modification of the pH of the reaction medium, after the incorporation of the additive, resulted as critical parameter, since lower pH (obtained for the mixtures with GO) hindered the development of mesoporosity. Therefore, the interaction of the additives with different content in functional oxygen groups with the polymeric precursors of the matrix determined the size of the pores, synthesising carbon/additive composites with a variable contribution of micro-, meso-and macropores. Further, the order and the aggregation of the graphitic additives have a strong influence on the resulting structural properties of each carbon/additive composite. In order to relate the structural and textural properties of the synthesised composites with the impact on electrical conductivity, disc-shaped pellets (90 wt.% material and 10 wt.% PTFE binder) were prepared from the carbon composites made with 4 and 12 wt.% of each graphitic additive (Table A.I.11). The U-I curves obtained by the four-probe method are represented in were electrically conductive being their conductivity range from ca. 0.048 to 0.480 S/cm. As was observed for the carbon black in the Section IV.B, after the addition of the additive all the composites showed higher conductivity compared to the pristine carbon sample, CPG, revealing the improvement of the electron mobility when the additive is incorporated in the matrix structure. In all cases an increase of the amount of additive from 4 wt.% to 12 wt.% produced an increase of the final conductivity. Interesting results were obtained for the case of the materials synthesised with GO as additive. It is well-known that graphene oxide is an insulator due to the important presence of oxygenated groups [START_REF] Stankovich | Graphene-based composite materials[END_REF][START_REF] Si | Synthesis of water soluble graphene[END_REF], indeed was not possible to measure a value of resistivity by the four-probe method since the pellet of this material was not established an electrical contact with the tips. However, the carbon gel/GO composites showed an increase on the conductivity value in comparison with the CPG, in fact the material CPG-GO12 yielded the higher electrical conductivity (ca. 0.480 S/cm). The increase of the conductivity of the carbon gel/GO composites, despite of the insulating character of GO, is associated to the thermal treatment of the gel composite that allowed the removal of the functional groups of the additive provoking the loss of the insulating character of the additive and, as a consequence, increasing the conductivity of the composite.

On the other hand, the composites made with rGO as additive showed the lower conductivity, with values very close to those obtained for the carbon composites made with the same amount of CB-III (0.058 and 0.104 S/cm for 4 and 12 wt.% of CB-III, respectively (Section IV.B)). This is associated to the higher presence of wider pores (Figure IV.50 and Table IV.19) that difficult the movement of the electrons through the composite material reducing its conductive character. In the case of the use of graphite as additive, in spite of the high conductivity of the pristine graphite (ca. 19.75 S/cm), the carbon gel/Gr composites revealed similar conductivity values than those obtained for the carbon gel/GO composites.

This conductivity was lower than that expected and probably is associated to the presence of the graphite aggregates observed in the Raman mapping that produce the formation of isolated islands that avoids the formation of conductive paths reducing notably the movement of the electrons and as a consequence its conductive value.

This section is devoted to the synthesis of graphene and its use as additive to polymeric gel and carbon gel. The first Subsection, IV.D.1, is focused on the synthesis of graphene by microwave plasma through the ethanol decomposition; the optimization of the experimental method is discussed. In IV.D.2. Subsection, the structural and chemical characterisation of the synthesised materials as a function of the experimental conditions is described. In the third Subsection, IV.D.3, the use of graphene as additive to polymeric and carbon gels is investigated. The study includes the analysis of the effect of the amount of graphene to the porosity, structural order and electrical conductivity of the gel/graphene and carbon gel/graphene composites.

Section IV.D:

Graphene as additive

IV.D.1. Synthesis of graphene by microwave plasma.

Since it was first discovered in 2004 [START_REF] Novoselov | Electric field effect in atomically thin carbon films[END_REF] and owing to its outstanding mechanical, thermal, electrical and optical properties [START_REF] Mittal | A review on carbon nanotubes and graphene as fillers in reinforced polymer nanocomposites[END_REF], graphene has been in the spotlight of scientific and technological research. Among the different processes found in the literature for the synthesis of this coveted carbon material, atmospheric microwave plasma is presented as an excellent alternative, as bottom-up techniques, since it allows obtaining high-quality graphene through the decomposition of carbon precursors of low cost in the absence of catalysts and substrates.

The plasma was created by using a TIAGO torch whose experimental set up is detailed in Chapter III, and ethanol was used as carbon source. Previous studies carried out at Laboratorio de Innovación en Plasmas (LIPs) of Universidad de Córdoba in Córdoba (Spain) have established and optimized the main operating parameters of the plasma device -e.g., input power of the microwave generator, flow of Ar as plasmonic gas-for the synthesis of gaseous or solid materials [START_REF] Rincón | Hydrogen production from ethanol decomposition by microwave plasma TIAGO torch[END_REF][START_REF] Rincón | Experimental research on ethanol-chemistry decomposition routes in a microwave plasma torch for hydrogen production[END_REF][START_REF] Melero | Scalable graphene production from ethanol decomposition by microwave argon plasma torch[END_REF]. For a given input power, the amount of ethanol decomposed in the Ar/EtOH plasma increases with the Ar flow, keeping the optimal conditions to avoid the extinction of the discharge (reaching an efficient energy coupling between Ar/EtOH plasma, i.e., reflected power below 5 %). On the other hand, as the power of the microwave generator increases, the plasma is capable of decomposing higher flows of EtOH. However, the stability of the Ar/EtOH plasma is significantly reduced when conditions reach their maximum. Additionally, the presence of environmental air inside the reactor strongly affects the kinetics and the ethanol decomposition pathways in a TIAGO torch plasma, due to the formation of different byproducts as a result of the interaction between air and the active species generated in the Ar/EtOH plasma [START_REF] Rincón | Experimental research on ethanol-chemistry decomposition routes in a microwave plasma torch for hydrogen production[END_REF]. This behaviour affects plasma colour, being violet for low The mass spectrum revealed that, as the flow rates of Ar and EtOH increase, the presence of air becomes less significant, and the energy coupling of the Ar/EtOH plasma improves, giving rise mainly to H 2 , CO, C 2 H 2 as gaseous by-products and a solid carbon, C (s) (Figure IV.60 b). The detection of these light gases and the formation of a carbonaceous deposit is in agreement with the theoretical study carried out by Tsyganov et al. [START_REF] Tsyganov | On the plasma-based growth of 'flowing' graphene sheets at atmospheric pressure conditions[END_REF]. On the contrary, by-products such as H 2 , H 2 O, CO and CO 2 are produced (mass spectrum is not shown) at low flows due to the high concentration of remaining air inside the reactor, avoiding the formation of the carbonaceous solid material. Additionally, the signal m/z 31 a.m.u. was detected in both cases, which is ascribed to the base peak of ethanol corresponding to the hydroxymethyl cation ( + CH 2 OH); the low intensity of this peak after 160 min allowed to determine a conversion rate higher than 98 %, assuming that it is mainly transformed into hydrogen (m/z 2 a.m.u.) and carbon monoxide (m/z 28 a.m.u.) according to follow equation [START_REF] Rincón | Hydrogen production from ethanol decomposition by microwave plasma TIAGO torch[END_REF]:

C 2 H 5 OH → 3 H 2 + CO + C (s) (equation IV.2)
Considering all the above, and based on previous studies of the group [START_REF] Rincón | Hydrogen production from ethanol decomposition by microwave plasma TIAGO torch[END_REF][START_REF] Rincón | Experimental research on ethanol-chemistry decomposition routes in a microwave plasma torch for hydrogen production[END_REF][START_REF] Melero | Scalable graphene production from ethanol decomposition by microwave argon plasma torch[END_REF], moderate operational conditions -an input power of 300 W and an argon flow of 1.00 L/min-were chosen to study the formation of graphene under varied flows of ethanol as carbon source. The amount of ethanol added to the plasma discharge ranged from 2.00 to 4.00 g/h; these values were below the maximum amount of ethanol withstood by the plasma. The obtained materials were labelled as a function of the five ethanol flows investigated (in g/h), hence 2.00 (sample G-I), 2.90 (sample G-II),

G-III

G-IV G-V

cases. On the other hand, no significant differences were observed on the evolution of the gases as a function of time for the different flows of EtOH. However, slight differences appeared in the amounts of the generated gases. The production of solid carbonaceous deposit followed a parabolic trend, reaching the maximum value (1.55 mg/min) at 3.40 g/h of EtOH flow. The slight difference of this value respect to that reported by [START_REF] Melero | Scalable graphene production from ethanol decomposition by microwave argon plasma torch[END_REF] is mainly associated to the different geometry of the reactors used in both studies, as it is known that this parameter has a strong influence on the plasma reactions, kinetics and stability [START_REF] Dato | Graphene synthesized in atmospheric plasmas-A review[END_REF]. To understand the decreasing trend in the production of carbonaceous solids at high ethanol flows, despite increasing the amount of ethanol fed to the plasma discharge, the molar production rate (mmol/min) of each carbon-based chemical (i.e, CO, C 2 H 2 , CH 4 and C 2 H 4 ) was calculated (Table IV.21).

Unlike the trend observed for solid carbon, the molar production of CO, C 2 H 2 , CH 4 and C 2 H 4 increases with the flow of ethanol. The trend is more pronounced for CO and C 2 H 2 , especially at higher ethanol flows. Since higher amounts of ethanol fed to the discharge promotes higher number of C atoms available in the discharge, the molar production of carbon-based by-products and H 2 were also evaluated per molar flow rate of ethanol (Figure IV.63). As seen, the amount of produced H 2 , CO and C 2 H 2 was rather similar (with variations lower than 10 %) for all the ethanol flows. On the contrary, significant variations were observed in the production of CH 4 , C 2 H 4 and C (s), with higher amounts of the gases formed for the largest flows of ethanol, accompanied by a low production of solid carbon. This is in agreement with other studies reporting the optimum conditions for the plasma production of hydrogen under a constant ethanol flow and varying the argon flow [START_REF] Jiménez | Hydrogen production from ethanol decomposition by a microwave plasma: Influence of the plasma gas flow[END_REF][START_REF] Tsyganov | On the plasma-based growth of 'flowing' graphene sheets at atmospheric pressure conditions[END_REF], indicating the reversed trends for the production of methane compared to hydrogen, acetylene and carbon deposits. Interestingly, when only the contribution of CO, C 2 H 2 and C (s) is considered (scenario ii), the mismatch in the calculation is more noticeable at larger ethanol flows. This highlights that large EtOH flows favour the formation of CH 4 and C 2 H 4 over C (s). This finding is most remarkable since it demonstrates that the modification of the operational conditions of Ar/EtOH TIAGO torch discharge (in particular, the amount of ethanol added to the plasma gas) can direct the decomposition of ethanol towards a preferential formation of either gaseous or solid by-products. Regardless of the ethanol amount fed to the plasma discharge, similar structures are detected for all materials. Furthermore, some amorphous carbonaceous particles were identified throughout the synthesised material with the highest flow of ethanol (G-V).

Likewise, the high-magnification images reveal the layer-by-layer structure of the graphene sheets, being detected high-quality graphene structures with ranges between 2 and 10 layers (denoted by arrows in the images) as well as curved graphene layers (framed areas) for all the materials. This could indicate that this synthesis process allows the formation of pure graphene without the presence of other carbon nanostructures [START_REF] Melero | Scalable graphene production from ethanol decomposition by microwave argon plasma torch[END_REF]. According to the literature [START_REF] Moisan | Waveguide-based single and multiple nozzle plasma torches: the TIAGO concept[END_REF][START_REF] Jiménez | Hydrogen production from ethanol decomposition by a microwave plasma: Influence of the plasma gas flow[END_REF][START_REF] Melero | Scalable graphene production from ethanol decomposition by microwave argon plasma torch[END_REF], the operating conditions imposed on the ethanol decomposition by plasma highly affect the structural and As shown in Table IV.22, the interlayer spacing average (d 002 ) calculated by Bragg's equation for the graphenes obtained was turned out to be 0.344 ± 0.001 nm for all the materials, being slightly higher than the characteristic value of graphite (0.335 nm) [START_REF] Chung | Review graphite[END_REF][START_REF] Iwashita | Specification for a standard procedure of X-ray diffraction measurements on carbon materials[END_REF] and which is related to the downshifted at peak position (002). Furthermore, thermogravimetric analysis in the presence of air (Annex I, Figure A.I.10) confirmed the stability of the materials up to temperatures above 550 °C, with no mass losses at lower temperatures that would correspond to either oxygen groups or amorphous carbon [START_REF] Shtein | Characterization of graphene-nanoplatelets structure via thermogravimetry[END_REF]. At ~ 600-800 °C, total combustion was observed for all the samples. The different temperatures observed for the samples can be attributed to the presence of structural defects in the graphene materials that affect their reactivity in air.

Raman spectra of the obtained materials are shown in Figure IV.67 a. Regardless the ethanol flow, all the samples present the characteristic bands of the carbon materials. In the first-order region, the main contribution corresponds with the G band (1580 cm -1 ), associated with ordered sp 2 structure. The position of this band confirms that the presence of graphene oxide can be discarded (its G band is located at 1594 cm -1 ) [START_REF] Stankovich | Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide[END_REF][START_REF] Kudin | Raman spectra of graphite oxide and functionalized graphene sheets[END_REF].

The minor contribution of the D band (1350 cm -1 ) evidences a slightly amount of defects in the structure of the materials. Regarding the second-order spectra, the shape, width, and symmetry of the 2D band provide useful insight about the quality of the produced graphene materials.

First-order and second-order Raman spectra were deconvoluted (Annex I, Figure A.I.11) into 9 bands, allowing a more adequately estimation of the structural order of graphene [START_REF] Malard | Raman spectroscopy in graphene[END_REF][START_REF] Childres | Raman spectroscopy of graphene and related materials new developments in photon and materials research[END_REF][START_REF] López-Díaz | Evolution of the Raman spectrum with the chemical composition of graphene oxide[END_REF]. From these data, the ratio between the intensities of D and G bands (I D /I G ratio) provides important information about the contribution of possible defects that cause the distortion of graphene sheets, while I 2D /I G ratio can be related with the number of layers in the graphene sheets [Sun, 2010;[START_REF] Lavin-López | Influence of the total gas flow at different reaction times for CVD-graphene synthesis on polycrystalline nickel[END_REF].

/I G ratio

For all the samples, the I 2D /I G ratio are closely to 1 (from 0.91 to 1.03), indicating that the graphene sheets are composed for few layers. The lowest value (0.91) was obtained for the sample synthesised through the decomposition of 4.00 g/h of ethanol (G-V), pointing out that this material presents the highest number of graphene layers. This observation is in agreement with the TEM images. IV.22). As the I D /I G ratio increased, L a -length of the planar units that constitute graphene layers-values decreased. The maximum value of L a corresponds to the sample obtained by plasma sustained with 2.90 g/h of ethanol flow, which is close to L a values reported in graphene synthesised by CVD [START_REF] Terasawa | Growth of graphene on Cu by plasma enhanced chemical vapor deposition[END_REF]. On the other hand, the values obtained for L c -average height of the stacked graphenic layers-remains almost constant, pointing out similar stacking for the five materials [Ferrari, 2000;Pimenta, 2007]. In all the cases, the intensity of the peak assigned to sp 3 carbon was significantly lower than those assigned to sp 2 and to any kind of C-O bonds. In addition, the presence of the signal associated to the π-π* plasmon/shake-up satellite peak evidences the presence of conjugated aromatic domains and also confirms the absence of graphene oxide [START_REF] Malesevic | Synthesis of few-layer graphene via microwave plasma enhanced chemical vapour deposition[END_REF]Gao, 2009;[START_REF] Ganguly | Probing the thermal deoxygenation of graphene oxide using high-resolution in situ X-ray-based spectroscopies[END_REF]. As seen, the shape of the nitrogen isotherms and the volume adsorbed of the gel/additive and carbon gel/additive samples are different from that of the material without additive. The isotherms of the composites displayed a Type IVa character according to the IUPAC classification [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF], with a prominent hysteresis loop in the desorption branch at relative pressures above 0.4. The loop shifted towards higher relative pressures compared to the material without additive. These characteristics are related to materials with a Relative pressure (p/p 0 )

PG PG-G-I4 PG-G-I12 G-I a) b) Relative pressure (p/p 0 ) CPG CPG-G-I4 CPG-G-I12
considerable contribution of microporosity and a large developed in the mesopores range. In contrast, the graphene itself presents a Type II isotherm characteristic of a non-porous or macroporous material, with a high pore volume at relative pressure close to unity, likely due to interparticle condensation. As above-described for other additives, the shape of the isotherm was retained after pyrolysis at 800 °C, with a pronounced increase in the micropore volume.

From the comparison of the isotherms as well as the textural parameters collected in Table IV.25, the most significant difference was found in the total pore volume, being greater as the amount of additive increases. This is especially noticeable for the carbon gel composites, going to 0.675 and 0.876 cm 3 /g for composites prepared with 4 and 12 wt.% of G-I, respectively; whereas a value of 0.488 cm 3 /g was obtained for CPG. It should also be noted that, as observed for other additives, the volume of micropores was almost not affected by the incorporation of G-I additive; thus, the additive influences the development of the mesoporosity, provoking an increase of ca. 52 and 71 % in the volume of mesopores for CPG-G-I4 and CPG-G-I12, respectively. Regarding the specific surface area in the PG series, a decrease was observed as the amount of additive increased (Table IV .25). This is in agreement with the decrease in the volume adsorbed at low relative pressures observed in Figure IV.71. On the other hand, the carbonization of the gel/G-I composites increased the specific surface area (ca. 800 m 2 /g), with similar values to all the carbon composites regardless the amount of additive. When the textural parameters were compared with their corresponding theoretical values obtained through the general rule of mixtures (Figure IV.72), S BET and W 0 followed the same trend as expected. However, for the PG series, the experimentally obtained values were lower than those predicted by the rule of mixtures, while for the CPG series these values are very close to the theoretical ones. On the other hand, the total pore volume and the mesopore volume As was reported for the series prepared using CB-III as additive (Section IV.B), the position and the shape of the hysteresis loop were strongly related to the amount of additive. As it can be seen in the isotherms (Figure IV.71), the hysteresis loop shifted to higher relative pressure when G-I was incorporated, and, spanned through a larger range of relative pressures with the amount of additive. The changes in the shape of the isotherms were analysed by splitting the hysteresis loop in two regions: V1 related to the volume comprised between relative pressures

CPG CPG-G-I12 G-I

The dark areas in the images of CPG-G-I12 correspond to graphene aggregates detected within the matrix of the carbon gel. This suggests that the additive does not hinder the crosslinking of the reactants. It can also be observed that the layer-by-layer arrangement of graphene (denoted with arrows in Figure IV.73) is retained in the composites. The incorporation of a higher percentage of graphene, from 4 to 12 wt.%, brought about an increase in the order of the carbon gel composites, as inferred from the narrowing of the bandwidths for the D and G bands. In conclusion, the use of graphene as additive allowed to obtain carbon gel/graphene composites with multimodal porous network and a well-dispersion of the additive, leading to materials with better average structural order. A remarkable decrease in the resistance was obtained for the composites (ca. 63 and 81 % for CPG-G-I4 and CPG-G-I12, respectively) compared to sample CPG prepared in the absence of additive. The conductivity of the pellets of carbon gel/graphene composites was improved by 3-5 times compared to that of the carbon gel (0.022 S/cm). The values are lower than those recorded for a pellet of pristine graphene in the same conditions (ca. 7.18 S/cm). An increase from 4 to 12 wt.% graphene provokes a remarkable increase in the electrical conductivity; similar results were previously observed for CB-III (Section IV.B). Comparatively, the conductivity values of the series of carbon gel/G-I composites are of the same order than those of the series carbon gel/CB-III with similar amounts of additive, despite the higher conductivity of G-I compared to CB-III. In the previous sections, it was demonstrated that the distribution of the additive within the RF matrix is crucial to obtain materials with enhanced electrical conductivity. This indicates a similar integration of both additives, and that for low amounts of This section is focused on the preparation of 3D-nanoporous gel/additive composites using a hydrothermal route under hydrothermal conditions. The section is divided in two subsections: IV.E.1, in this part a hydrothermal process was applied to selected nanostructured additives, namely carbon black, graphene oxide and reduced graphene oxide, to explore their potential expansion. IV.E.2 Subsection is based on the analysis of the gel/additive composites obtained under hydrothermal treatment.

Section IV.E:

Hydrothermal treatment

IV.E.1. Hydrothermal treatment of the additives CB-III, GO and rGO.

Hydrothermal approaches are quite popular synthetic methods for the preparation of different types of materials, since they offer advantages related to energy savings due to moderate temperatures, good nucleation control, high reaction rates, good particle shape and size control, among most representatives [START_REF] Byrappa | Hydrothermal technology for nanotechnology[END_REF]. In this thesis, the hydrothermal conditions were obtained in a Teflon-lined stainless-steel autoclave capable of operating up to 200 bars.

Three of the used nanostructured carbons were selected for this study: graphene oxide (GO), reduced graphene oxide (rGO) and carbon black (CB-III). They were subjected to a hydrothermal treatment in aqueous medium as detailed in the experimental section (Chapter III), aiming to analyse the impact of the pressure-temperature conditions on the formation of a porous structure. The additives after the hydrothermal treatment were labelled as HS-X, where X corresponds to the acronym of the additive. On the other hand, GO and rGO were modified after the hydrothermal treatment. As seen in Figure IV.76, after the treatment, GO suffers a notable increase in its porous structure, the microporous volume goes from ca. 0 to 0.028 cm 3 /g (Table IV.27) and hysteresis loop is formed above relative pressures of 0.4 indicating the development of mesoporous structure. It could be associated to the expansion of the graphene layers during the hydrothermal treatment under pressure; it seems that the elimination of the high number of oxygen-containing groups (see Section IV.C) produces an elevate evolution of gasses, mainly CO and CO 2 , these gases provoke the elongation of the layers separation transforming a non-porous material (GO) to a micro-/mesoporous carbon (HS-GO) [START_REF] Stankovich | Synthesis of graphene-based nanosheets via chemical reduction of exfoliated graphite oxide[END_REF]Lian, 2010]. The evolution of the gases was confirmed by TG profile of GO (Figure IV.77) that revealed an important mass loss of ca. 35 % up to 225 ᵒC, it should be mention that the thermogravimetric analysis was made under atmospheric pressure, thus the gas evolution under high pressure conditions will produce at lower temperatures. On the contrary, the characteristic hysteresis loop at relative pressures above 0.4 of the rGO material was preserved after the hydrothermal treatment, although the overall gas uptake was reduced indicating the partial loss of the mesoporous structure. This fact indicates that the expanded graphene-like layers of rGO undergo a partial stacking under the moderate temperature and pressure conditions. This could be induced by the removal of the oxygenated groups remaining on the surface of the rGO, in this case, the amount of functional groups is lower -compared to GO-(Section IV.C) and the gas evolution is not enough to produce the expansion of the layers (the overall mass loss up to 900 ᵒC was lower than 20 %), the contrary effect is produce, since the elimination of the groups, located between layers, produce their staking. IV.28). On the contrary, the analysis of the porosity of the different fractions (upper/lower) corresponding to the different coloured phases of sample HS-PG-rGO4 revealed the same gas adsorption isotherm. This finding is quite surprising given the visual differences (colour) between the phases. This phenomenon could be associated to the fact that the additive rGO hindered the RF polycondensation reaction, provoking a widening of the pore structure additive, as discussed above for the materials prepared at atmospheric pressure. However, further analysis, in terms of chemical composition and morphology, are needed to clarify this behaviour.

Based on the obtained results, it seems that the hydrothermal synthesis is a good alternative to obtain nanoporous carbon materials with a 3D nanoporous structure.

Nonetheless, further studies need to be done to improve the dispersion of the carbon additives (e.g., the use of stirred hydrothermal reactors), to avoid the formation of two segregated and heterogeneous phases. Chapter V:

Conclusions

The aim of this PhD thesis has been the employ of various carbon nanostructures as conductive additives for the preparation of highly porous nanoporous carbons, combining controlled-developed porosity with a high electrical conductivity. Different carbon nanostructures were used as conductive additives, including carbon black, graphene, graphite, graphene oxide and reduced graphene oxide. All of them were introduced in the first step of the synthesis of polymeric gels and carbon gels, obtained from the polycondensation reaction of resorcinol-formaldehyde mixture. The synthesis protocol was optimised to attain a good dispersion of the additives in the resulting carbon materials. The effect of the additive's features on the properties (e.g., porosity, morphology, chemical composition, conductivity) of the final materials was explored.

The most significant conclusions that can be inferred from this PhD thesis are summarised below:

 The incorporation of mechanical stirring to the experimental protocol of the resorcinol-formaldehyde polycondensation reaction, allowed an homogeneous dispersion of the carbon additives within the polymeric matrix without hindering the polycondensation of the reactants nor modifying the physicochemical features of the pristine materials (in the absence of additive). In this thesis, the modified synthesis protocol has allowed the synthesis of monophasic monoliths of gel/additive composites with a maximum additive's percentage of 40 wt. %.

 The nature of the carbon black employed as additive to the gel and carbon gel, in terms of particle size (30-100 nm), porosity and physicochemical features, has an important influence on the dispersion of the additive through the polymeric matrix. The presence of carbon black aggregates provokes the formation of an interconnected percolation path, which facilitates the electron's mobility increasing the electrical conductivity. Hence, in general terms, higher particle size of carbon black produced the formation of a continuous percolation path rendering carbon gel composites with higher electrical conductivity. Additionally, the incorporation of the carbon black presented a strong impact on the development of the mesoporous network regardless of the morphological properties of the carbon black incorporated.

 The amount of carbon black as additive (from 4 to 40 wt.%) showed a notable impact on the porous features of the carbon gel composites, mainly on the mesoporous range. The N 2 adsorption/desorption isotherms at -196 °C of the samples showed an inflection point (curvature) in both, adsorption and desorption branches, that was more marked for higher amounts of carbon black. The characterisation of the materials revealed that the incorporation of carbon black provoked the development of an interconnected mesoporous network of constricted necks and bodies. Additionally, the electrical conductivity was also influenced, showing an increase with the amount of additive incorporated following a percolation trend. The amount of 8 wt.% was found as the percolation threshold.  The study of three nanostructured carbon materials as additives, namely graphite, graphene oxide and reduced graphene oxide revealed that the development of the porosity of the composites would not be straightforwardly correlated to the porosity of the additive. The porous network of the final material was mainly related to the physicochemical features of the employed additive. The oxygenated functional groups of the additives and its ability to modify the pH of the reactants' media resulted as critical parameters since they will determine the textural development of the final composite.

Additionally, the order and aggregation of the additives showed an effect of the electrical conductivity of the carbon gel composites. The highest conductivity was obtained for the composite elaborated with 12 wt.% of graphene oxide rendering a value of ca. 0.48 S/cm.

 The study of the synthesis of graphene by microwave plasma showed an impact of the plasma generation conditions (e.g., fed flow of EtOH as carbon source) on the synthesis process, strongly influencing the generation of the diverse by-products, as well as the structural quality of the graphene materials obtained. The gel and carbon gel composites elaborated with graphene as additive revealed similar conductivities of those materials obtained with the incorporation of carbon black, probably associated to the presence of graphene aggregates formed during the composite synthesis.

 Hydrothermal approaches (high temperature at ca. 200 bars) were applied to selected nanostructured carbon additives showing that, depending of the features of the material, an expansion or stack of the structures was produced. Additionally, due to the static synthesis process, the sedimentation of the additive was produced rendering two well-defined phases monoliths of gel/additive composites. Further studies are required to optimise the hydrothermal synthesis method to obtain homogeneous composites.

Chapter VI:

Perspectives and future work

In this PhD thesis two approaches (e.g., sol-gel and hydrothermal) have been mainly

explored to obtain materials with high porosity and electrical conductivity using sol-gel and hydrothermal approaches. The outcome of our investigations has shown that several aspects of the experimental protocols followed should be improved and combined to obtain materials with better features, as well as to complete the electrochemical characterisation of the prepared materials. In this regard, some research activities have been initiated in the framework of this PhD thesis, although they would need to be further completed before extracting conclusion. A summary can be found below:

 Electrochemical characterisation of the samples.

To further confirm the beneficial impact of the incorporation of the carbon additives on the electrical conductivity of the carbon gel composites, electrochemical characterisation of the samples was initiated. It has not been included in the core of the PhD thesis since the experimental work was not finalized for all the prepared materials. Electrodes of the composites were prepared (ca. 1 cm of diameter pellets of 90 wt.% carbon gel and 10 wt.% PTFE as binder) and characterised by cyclic voltammetry in a neutral electrolyte (0.05 M Na 2 SO 4 ) using a 3-electrode cell configuration (Hg/Hg 2 SO 4 as reference electrode and a graphite rod as counter electrode) in a Swagelok-type-cell. As an example, data corresponding to selected samples of the series gel/CB-III composites is shown and discussed below. Before pyrolysis, the gel material behaves as an insulator (extremely low electrochemical response in the cyclic voltammetry curves) (Data not shown). This is in agreement with the measurements of conductivity for this material form the 4-probe technique, and despite the large porosity of the material is due to the high functionalization of the material and the electron withdrawal character of the O-groups.

For the pyrolysed carbons (with and without CB additive), the voltammograms displayed the typical capacitive behaviour of porous materials due to the accumulation of charge at the electrical double layer. Interestingly, the voltammograms displayed a butterfly-shape, with a symmetric increase in the current density during positive and negative polarization. This behaviour has been commonly reported for porous carbons, nanotubes and graphene electrodes in 3-electrode cell configurations [Hahn, 2004;[START_REF] Ruch | Electrochemical characterization of single-walled carbon nanotubes for electrochemical double layer capacitors using non-aqueous electrolyte[END_REF][START_REF] Yamada | Electrochemical behavior of metallic and semiconducting single-wall carbon nanotubes for electric double-layer capacitor[END_REF]Paek, 2013.]. It is associated to point of zero charge of the electrodes, and interpreted as the potential dependence of the ion penetration into nanopores -assumed to be minimal at the point of zero charge- [START_REF] Salitra | Carbon electrodes for double-layer capacitors I. relations between ion and pore dimensions[END_REF].

Regarding the effect of the carbon black additive, the electrochemical response of the electrodes did not follow the expected trend up the increased conductivity of the materials is necessary to consider that the cyclic voltammetry is direct correlated to the formation of the electrical double layer of the electrodes, which mainly depends upon the microporosity of the samples. Thus, as the main modification of the porous features of the electrodes is the mesopore volume, its contribution would not be accounted for in the voltammetric scans. All this points out that additional electrochemical characterisation of the samples, for instance by impedance spectroscopy, would be necessary to clarify this behaviour.

 Hydrothermal procedures

Further work is needed to avoid the non-homogeneous dispersion of the carbon black additive during the hydrothermal carbonization process, that lead to a certain phase segregation as described in Chapter IV, Section E. One of the possibilities envisaged is to incorporate agitation to the hydrothermal reactors with the aim to incorporate the mechanical agitation during the synthesis, which, in the case of synthesis under atmospheric pressure, resulted the key parameter to avoid the formation of two segregated phases allowing the synthesis of the homogeneous materials.

A second approach is the synthesis of graphene oxide frameworks through the crosslinking between various organic linkers and the moieties of graphene oxide, aiming to render pillared structures with 3D porosity [START_REF] Herrera-Alonso | Intercalation and stitching of graphite oxide with diaminoalkanes[END_REF][START_REF] Srinivas | Porous graphene oxide frameworks: Synthesis and gas sorption properties[END_REF]. Such GOF structures are developed by the covalent bonding of the linkers with the graphene-like sheets, this process provokes an irreversible expansion of the pristine GO. This research line is currently being investigated in the group, using the prepared GO and diboronic acids and amines as linkers [START_REF] Amayuelas | Preparation and characterization of porous graphene-oxide frameworks[END_REF]. So far, the effect of some operating conditions (e.g., loading of the autoclave, ratio linkers to GO, temperature, time) has been carried out. Gas adsorption data has shown that the materials present a constricted nanoporosity, which can be modulated upon optimizing the synthesis conditions. The characterisation of the obtained materials by different techniques such as XRD, FTIR, and TGA also demonstrated the presence of chemical bonds between the linker and the surface groups of the GO. Further research is currently ongoing aiming to increase the surface area and to control the average pore size of these materials.

 Photoassisted methods

One of the alternatives to improve the dispersion of the conductive additive during the polycondensation reactions is to explore the use of photoassisted reactions to accelerate the preparation of the materials. Based on the experience of the group, photoassisted reactions can decrease significantly the reaction time from several hours to 30-60 minutes, which is expected to have an impact in the dispersion of the additive (e.g., avoiding sedimentation and/or segregation). For an initial test, the experimental procedure and precursors were modified so as to match the optimized conditions previously developed in the group (ongoing PhD in the group) for the polycondensation of polyhydroxylated precursors with aldehydes [START_REF] Balan | A green and fast approach to nanoporous carbons with tuned porosity: UV-assisted condensation of organic compounds at room temperature[END_REF]Fernández de Córdoba, 2019]. Briefly, about 0.83 g of the organic precursor (2,4,6-trihydroxybenzoic acid) were dissolved in 40 mL of ethanol, and then the CB-III (ca. 0.18 and 1.8 g corresponding to 1 and 10 wt.%) was dispersed in the solution by stirring for 10 min. A surfactant (1.64 g) was incorporated to the dispersion and stirred for 30 min.

Finally, the aldehyde (0.83 mL of glyoxal, 40 wt.% in aqueous solution) is incorporated to the mixture and allowed to stir for 60 min under dark conditions. The reactants mixture is transferred to glass disses and irradiated under simulated solar light (300 W Xe lamp; aliquots of 8.8 g) for 60 minutes. The agitation was maintained during the illumination, although after 10 min of light exposure the viscosity of mixture increased significantly and the agitation stopped. The black solid recovered after 60 min irradiation was pyrolysed at 600 °C (i.e., 120 mL/min N 2 , 2 °C/min, 2 h). The yield of pyrolysis was ca. 27 and 51 % for the samples prepared with 1 and 10 wt.% of additive, respectively; the values are much higher than that obtained for the control carbon material prepared in the absence of carbon black (ca. 19 wt.%). Most importantly, the prepared carbons presented a well dispersion of the CB-III additive (even the one prepared with 10 wt.%).

Regarding porosity, the N 2 adsorption/desorption isotherms at -196 °C (Figure VI.2) evidenced that, as also observed for the polycondensation of RF mixtures, the incorporation of the carbon black has a strong impact in the mesoporosity of the resulting carbons. Indeed, the hysteresis loop of the isotherms shifted upwards to higher relative pressures in the presence of the carbon black. The shape of the loop also changed, widening for the sample prepared with 1 wt.% of additive and displaying a less prompt fall in the desorption branch (indicating pores with wider access). For the sample prepared with 10 wt.% additive, the hysteresis loop showed almost parallel branches, suggesting a homogenous distribution of mesopores. The specific surface area (S BET ) and the total pore volume (V PORES ) also decreased, although both samples still displayed a notable porous character (Table VI.1). It is worth noticing that the decrease in the pore volumes mainly affected the microporosity, whereas the volume of mesopores remained almost the same. The values are the result of the average of four measurements.

Annex II:

Publications

Figure I. 6

 6 Figure I.6Schematic diagram of the reduction of graphene oxide to obtain reduced graphene oxide.

Figure I. 7

 7 Figure I.7Schematic representation of bottom-up and top-down techniques commonly used for the preparation of graphene.

Figure III. 1

 1 Figure III.1 Images of the carbon additives selected in this study: (a) carbon black, CB-I; (b) carbon black, CB-II; (c) carbon black, CB-III; (d) carbon black, CB-IV; (e) oxidised carbon black, CB-IIIox; (f) graphite, Gr; (g) graphene oxide, GO; (h) reduced graphene oxide, rGO and (i) graphene, G-I. Similar size glass vessels contain ca. 30 mg of each additive.

Figure

  Figure III.12(a) Images of the pellets prepared for the measurements of the resistivity; (b) schematic representation of a 4-probe system -S corresponds to distance between the probes and t is the thickness of the pellet-.

Figure IV. 1

 1 Figure IV.1 (left) Thermogravimetric profile of sample PG in Ar (50 mL/min, 2 °C/min) and (right) temperature profile with three dwelling steps at 200, 400 and 800 °C.

Figure IV. 2

 2 Figure IV.2 (a) N 2 adsorption/desorption isotherms at -196 °C and (b) CO 2 adsorption isotherms at 0 °C of samples PG (light blue) and CPG (dark blue); filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 3

 3 Figure IV.3 (a) Cumulative and (b) incremental pore volumes of samples PG and CPG evaluated from the nitrogen adsorption data at -196 °C, applying the 2D NLDFT HS model.

Figure IV. 4

 4 Figure IV.4 (left) Image of the wet gel composite made with 8 wt.% of CB-III; (right) sketch showing the colour gradation through the gel monolith indicating the upper and lower sections.

Figure

  Figure IV.5 N 2 adsorption/desorption isotherms at -196 °C of (a) two transversal cuts (upper and lower) of PG-CB-III8 composite and sample PG synthesised without (w/o) mechanical stirring; (b) PG synthesised with and without mechanical stirring; filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 6

 6 Figure IV.6 Images of the pristine wet gel (PG) obtained in the (a) absence and (b) presence of mechanical stirring. (c) Dried monolith of pristine gel (PG) obtained under stirring conditions.

Figure

  Figure IV.7 N 2 adsorption/desorption isotherms at -196 °C of two transversal cuts (upper and lower) of PG-CB-III8 composite and sample PG synthesised under mechanical stirring; filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 8

 8 Figure IV.8TEM images of the carbon blacks selected in this study. The average particle size range estimated for each material is indicated in the header.

Figure

  Figure IV.9 (a) Raman spectra of the carbon black additives (shifted for clarify). (b) Example of Raman spectrum deconvolution for sample CB-I.

Figure

  Figure IV.10X-ray diffraction patterns of the carbon black additives.

Figure

  Figure IV.11 (a) Correlation between L a obtained from XRD data and L a evaluated from the Tuinstra-Koening equation applied to Raman spectra; (b) relationship between I D /I G ratio and L a for all the studied carbon blacks.

Figure

  Figure IV.12 N 2 adsorption/desorption isotherms at -196 °C of all the studied carbon blacks; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure IV.13 N 2 adsorption/desorption isotherms at -196 °C of all the series of materials prepared with different carbon blacks: (a) Pristine gel, (b) gel/CB composites, (c) pristine carbon gel and (d) carbon gel/CB composites; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure IV.14 (a-d) Correlation of selected experimental textural parameters (surface area, pore volumes, micro and mesopores volumes) of gel and carbon gel/CB composites with the values predicted by the general rule of mixtures. Dashed lines indicate the expected trend following predictions of the general mixing rule; (e,f) evolution of V1, V2 and IP for the studied samples.

Figure IV. 15

 15 Figure IV.15Cumulative volume of (a) gel and (b) carbon gel/CB composites synthesised using similar amounts of different carbon blacks, evaluated from the adsorption branch of nitrogen adsorption data and applying 2D-NLDFT-HS model.

Figure

  Figure IV.16TEM images of the carbon gel/CB composites synthesised with different carbon blacks.

Figure IV. 17

 17 Figure IV.17Average Raman spectra of the carbon gel/CB composites.

Figure

  Figure IV.18I D /I G and I 2D /I G ratios for the series of carbon gel/CB composites prepared with the four studied carbon blacks.

Figure

  Figure IV.19 U-I curves for the (a) carbon blacks and the (b) series of carbon gel/CB composites; (c) conductivity values obtained for all the studied materials; (d) schematic percolation path followed by the electrons in samples CPG-CB-I12 (up) and CPG-CB-III12 (down).

Figure IV. 20

 20 Figure IV.20Selected TEM images of the carbon black additives, the polymeric gel composites (series PG) and the carbon gel composites (series CPG) prepared with different amounts of carbon black, CB-III. For clarity, images are shown at various magnifications.

Figure

  Figure IV.21 (a) Mass evolution (%) and (b) derivative thermogravimetric profiles (DTG) in Ar (50 mL/min, 20 °C/min) for samples CB-III, PG, PG-CB-III4, PG-CB-III12 and PG-CB-III40.

Figure IV. 22

 22 Figure IV.22FTIR spectra of the polymeric gel/CB-III composites before carbonization.

Figure IV. 23

 23 Figure IV.23 Raman image reconstruction showing the I D /I G ratios for samples (a) CPG, (c) CPG-CB-III4 and (e) CPG-CB-III16; (b) Raman spectra of CPG and CB-III; Raman spectra of different positions in the reconstructed Raman mapping for (d) CPG-CB-III4 and (f) CPG-CB-III16 corresponding to dark and light areas (see arrows in plot c) and the average.

Figure

  Figure IV.24X-ray diffraction patterns of the polymeric gel/CB-III and carbon gel/CB-III composites.

Figure

  Figure IV.25 (a-d) N 2 adsorption/desorption isotherms at -196 °C -(b,d) have been shifted ca. 150 cm 3 /g for clarity-; (e,f) CO 2 isotherms at 0 °C of the gel and carbon gel composites synthesised with different amounts of CB-III as additive. Data corresponding to the carbon black is also shown for comparison; filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 26

 26 Figure IV.26 Correlation of experimental (a) micropore volume, (b) mesopore volumes, (c) surface area and (d) V1 of the hysteresis loop of the series of gel/CB-III and carbon gel/CB-III composites with the values predicted by the general mixing rule. Dashed lines indicate the expected trend following predictions of the general mixing rule.

Figure IV. 27

 27 Figure IV.27Correlation between the mesopore volumes (a) V1 and (b) V2, and (c) the relative pressure of the inflection point in the N 2 adsorption isotherms with the amount of carbon black for series PG and CPG.

Figure

  Figure IV.28 (a) Cumulative and (b) derivative pore volumes of the carbon gel/CB-III composites evaluated from the adsorption branch of nitrogen adsorption data and applying 2D-NLDFT-HS model.

Figure

  Figure IV.29 (a) U-I curves and (b) electrical conductivity values for the series of carbon gel/CB-III composites.

Figure

  Figure IV.30 (a) Correlation of the electrical conductivity with the amount of carbon black; (b) correlation between the experimental conductivity and the values predicted by the general mixing rule; (c) log-log plot of the conductivity as a function of the amount of conductive phase following a percolation model.

Figure

  Figure IV.31TEM images of CB-III and CB-IIIox at different magnifications.

Figure

  Figure IV.32 (a) Mass evolution and (b) derivative thermogravimetric profiles (DTG) of CB-III and CB-IIIox in argon (100 mL/min, 10 °C/min) from 20 to 900 °C.

Figure IV. 33

 33 Figure IV.33Proton binding curves for samples CB-III and CB-IIIox.

Figure IV. 34

 34 Figure IV.34 FTIR spectra of samples (a) CB-III and (b) CB-IIIox.

Figure

  Figure IV.35 N 2 adsorption/desorption isotherms at -196 °C of samples CB-III and CB-IIIox; filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 36

 36 Figure IV.36Raman spectra and deconvoluted peaks of samples (a) CB-III and (b) CB-IIIox.

Figure

  Figure IV.37X-ray diffraction patterns of samples CB-III and CB-IIIox.

Figure

  Figure IV.38 N 2 adsorption/desorption isotherms at -196 °C of the polymeric gel and carbon gel composites synthesised with 4 and 12 wt.% of CB-III and CB-IIIox additives; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure IV.39 (a) Mass evolution and (b) derivative thermogravimetric profile (DTG) of PG, and the series of gel/CB-III and gel/CB-IIIox composites in argon (50 mL/min, 20 °C/min).

Figure

  Figure IV.40 TEM images of samples CPG-CB-III4 (first row), CPG-CB-IIIox4 (second row), CPG-CB-III12 (third row) and CPG-CB-IIIox12

Figure

  Figure IV.41 (a) Raman image reconstruction showing the I 2D /I G ratio for sample CPG-CB-IIIox12; (b) Raman spectra of the pristine carbon gel (CPG), the carbon gel/CB-IIIox composite (12 wt.%) and the additive (CB-IIIox).

Figure

  Figure IV.42 U-I curves for sample CPG and the series of carbon gel/additive composites prepared with 4 and 12 wt.% of CB-III and CB-IIIox additives.

Figure IV. 47

 47 Figure IV.47FTIR of each material: (a) graphite (Gr), (b) graphene oxide (GO) and (c) reduced graphene oxide (rGO). Data is shifted for clarity.

Figure IV. 48

 48 Figure IV.48Thermogravimetric profiles of each additive obtained between 25 and 900 °C at 10 °C/min under Ar flow (100 mL/min).

Figure IV. 49

 49 Figure IV.49Adsorption/desorption isotherms of (a) N 2 at -196 °C and (b) CO 2 at 0 °C of additives; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure IV.50 N 2 adsorption/desorption isotherms at -196 °C corresponding to gel and carbon gel composites synthesised with (a,b) graphite, (c,d) graphene oxide and (e,f) reduced graphene oxide in two amount of additive, 4 and 12 wt.%; filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 51

 51 Figure IV.51Correlation of pH and textural parameters (a) S BET , specific surface area, and (b) V PORES , total pores volume.

Figure IV. 52

 52 Figure IV.52Correlation of selected experimental textural parameters (surface area, pore volumes) of gel and carbon gel/additive composites with the values predicted by the general rule of mixtures. Dashed lines indicate the expected trend following predictions of the general mixing rule.

Figure IV. 53

 53 Figure IV.53FTIR spectra of (a) PG and PG/additive composites using (b) graphite, (c) graphene oxide and (d) reduced graphene oxide as additives.

Figure

  Figure IV.54 (a) TG and (b) DTG profiles in argon (50 mL/min, 20 °C/min) from 20 to 900 °C of PG and gel/additive composites synthesised with 12 wt.% of each additive.

Figure IV. 55

 55 Figure IV.55TEM images of the carbon gel/additive composites synthesised using Gr, GO and rGO additives.

Figure IV. 56

 56 Figure IV.56Raman reconstruction images and the corresponding deconvolution of the average Raman spectrum obtained for samples (a,b) CPG-Gr12, (c,d) CPG-GO12 and (e,f) CPG-rGO12.

Figure

  Figure IV.57 U-I curves for (a) graphite series, (b) graphene oxide series and (c) reduced graphene oxide series.

Figure IV. 58

 58 Figure IV.58Conductivity values obtained for the series of materials prepared with Gr, GO and rGO additives.

Figure IV. 59

 59 Figure IV.59Maximum ethanol inlet withstood by Ar TIAGO plasmas sustained at different Ar flows and input power. (Unpublished data from LIPs group at University of Córdoba, herein included with permission for comparison and clarification purposes).

Figure

  Figure IV.60 (a) Schematic representation of products obtained from both fundamental routes for ethanol decomposition in an Ar TIAGO torch plasma sustained with 300 W. (Unpublished data from LIPs group at University of Córdoba, herein included with permission for comparison and clarification purposes). (b) Mass spectra of an Ar plasma and Ar-EtOH plasma (ethanol flow of 4.00 g/h and argon flow of 1.00 L/min after 160 minutes). For clarity, ion current intensities are normalized to the maximum ion current signal, corresponding to m/z 40 a.m.u. ascribed to Ar + .

Figure IV. 61

 61 Figure IV.61 Time evolution of the ion intensity of mass fragments m/z (left column) 2, 26 and 28 a.m.u., and (right column) 16, 18, 27 and 31 a.m.u. for G-I, G-II, G-III, G-IV andG-V.

Figure

  Figure IV.62 (a) Carbon solid production rate as a function of the EtOH flow. (b) Image of the reactor employed.

Figure IV. 63

 63 Figure IV.63Mol formation of (a) H 2 , CO, (b) C 2 H 2 , CH 4 , C(s) and C 2 H 4 per mol of ethanol feed to the discharge. The percentage indicates the difference between the minimum and maximum value for each

Figure IV. 65

 65 Figure IV.65TEM images of the graphene samples materials prepared at different ethanol flows.

Figure

  Figure IV.66X-ray diffraction patterns of the graphene materials prepared at different ethanol flows (shifted for clarify).

Figure

  Figure IV.67 (a) Raman spectra of the graphenes prepared at different ethanol flows (shifted for clarify); (b) correlation between I D /I G and I 2D /I G ratios with the ethanol flow.

Figure IV. 68

 68 Figure IV.68Correlation of the I D /I G ratio (blue line) from Raman spectroscopy and, L c (green line) and L a (brown line) obtained from XRD for the graphenes prepared at different ethanol flows.

Figure IV. 69

 69 Figure IV.69 Survey XPS spectra of the graphene samples synthesised by plasma decomposition of ethanol using different flows: (a) 2.00 g/h (sample G-I), (b) 2.90 g/h (sample G-II), (c) 3.40 g/h (sample G-III), (d) 3.80 g/h (sample G-IV), and (e) 4.00 g/h (sample G-V).

Figure IV. 70

 70 Figure IV.70 Deconvoluted C 1s spectra of samples (a) G-I, (b) G-II, (c) G-III, (d) G-IV, and (e) G-V; (f) correlation between I D /I G ratio and the area C sp 3 .

Figure

  Figure IV.71 N 2 adsorption/desorption isotherms at -196 °C of (a) polymeric gel and (b) carbon gel composites prepared using G-I as additive. Data corresponding to the additive and the control without additive are also included; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure IV.72 (a-d) Correlation of selected experimental textural parameters (surface area, pore volumes) of gel and carbon gel/graphene composites with the values predicted by the general rule of mixtures. Dashed lines indicate the expected trend following predictions of the general mixing rule; (e,f) evolution of V1, V2 and IP for the gel and carbon gel/graphene composites.

Figure

  Figure IV.73TEM images of samples CPG and G-I, and the carbon gel/G-I composite prepared with 12 wt.% of additive.

Figure

  Figure IV.74 (a,b) Raman spectra of the pristine materials, carbon gel matrix (CPG) and graphene (G-I), respectively; (c,e) Raman image reconstruction showing the I 2D /I G ratios for samples CPG-G-I4 and CPG-G-I12; (d,f) Raman spectra of different positions in the reconstructed Raman mapping for sample each composite corresponding to dark and light areas, and the average.

Figure

  Figure IV.75 (a) U-I curves and (b) conductivity values obtained for samples CPG, G-I and the carbon gel/G-I composites prepared with 4 and 12 wt.% of additive.

Figure

  Figure IV.76 N 2 adsorption/desorption isotherms at -196 °C of samples (a) CB-III and HS-CB-III, (b) GO and HS-GO, (c) rGO and HS-rGO; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure IV.79 N 2 adsorption/desorption isotherms at -196 °C obtained for samples PG and HS-PG; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure IV.80 (a) Cumulative and (b) incremental pore volume of samples PG and HS-PG, obtained applying the 2D-NLDFT-HS method to the N 2 adsorption data.

Figure

  Figure IV.81 N 2 adsorption/desorption isotherms at -196 °C of samples (a) HS-PG-CB-III4 and (b) HS-PG-rGO4 (bulk and upper and lower aliquots). The isotherm of HS-PG is also included; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure VI.2 N 2 adsorption/desorption isotherms at -196 °C of the carbon materials prepared upon photoassisted polycondensation of the reactants in the presence and absence of CB-III as conductive additive; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure A.I.1 Deconvolution of Raman spectra for the carbon blacks used as additives: (a) CB-I, (b) CB-II, (c) CB-III, and (d) CB-IV.

Figure

  Figure A.I.2 Deconvolution of Raman spectrum of pristine carbon gel sample (CPG).

Figure

  Figure A.I.3 Deconvolution of Raman spectra for the carbon gel/CB composites prepared with 12 wt.% of additive: (a) CPG-CB-I12, (b) CPG-CB-II12, (c) CPG-CB-III12, and (d) CPG-CB-IV12.

Figure

  Figure A.I.4 Deconvolution of Raman spectra for the carbon gel/CB-III composites prepared with (a) 4 and (b) 16 wt.% of CB-III as additive.

Figure

  Figure A.I.5 pK a profiles of CB-III and CB-IIIox obtained from potentiometric titration.

Figure A.I. 6

 6 Figure A.I.6 Deconvolution of Raman spectra for carbon gel/CB-IIIox composites prepared with (a) 4 and (b) 12 wt.% of CB-IIIox as additive.

Figure

  Figure A.I.7Deconvolution of Raman spectra of graphite (Gr), graphene oxide (GO) and reduced graphene oxide (rGO).

Figure

  Figure A.I.8 (a) Proton binding curves and (b) pKa profiles of graphite (Gr), graphene oxide (GO) and reduced graphene oxide (rGO).

Figure

  Figure A.I.9TPD-MS profiles corresponding to m/z 18 (H 2 O), 28 (CO) and 44 (CO 2 ) of the synthesised graphene oxide used as additive.

Figure

  Figure A.I.10Mass evolution in air (100 mL/min) of the graphenes prepared at different ethanol flows; heating rate, 20 ᵒC/min.

Figure

  Figure A.I.11 Deconvolution of Raman spectra of the graphene materials synthesised by plasma decomposition of ethanol using various flows: (a) 2.00 g/h (sample G-I), (b) 2.90 g/h (sample G-II), (c) 3.40 g/h (sample G-III), (d) 3.80 g/h (sample G-IV), and (e) 4.00 g/h (sample G-V).

  to react with heteroatoms or other carbon atoms producing their incorporation to the surface or within the carbonaceous structures. Additionally, numerous carbon nanomaterials displayed curved structures, forming a wide spectrum of morphologies. Attending to their dimensionality (Figure I.1), carbon materials can be classified as: 0D (e.g., fullerenes), 1D (e.g., single-walled and multi-walled nanotubes), 2D (e.g., graphene) and 3D (e.g., graphite, diamond, amorphous carbons).

Figure I. 1 .

 1 Figure I.1. Classification of carbon nanostructures upon their different dimensionality: 0D (fullerenes), 1D (single-walled carbon nanotubes), 2D (graphene) and 3D (graphite).Adapted from[START_REF] Bezzon | Carbon nanostructure-based sensors: A brief review on recent advances[END_REF].

Figure I. 2 .

 2 Figure I.2. Scheme on the type of pores and external surface in nanoporous materials.

Figure I. 3 .

 3 Figure I.3. Scheme of the chemical reactions occurring during the basic catalysed polycondensation of resorcinol and formaldehyde.

Figure I. 4 .

 4 Figure I.4. Schematic representation of the relationship between the structure of the carbon materials and their electrical conductivity.

Figure I. 5 .

 5 Figure I.5. Schematic representation of most plausible O-containing surface groups of graphene oxide: hydroxyl (pink), lactone (blue), carbonyl (purple), carboxyl (orange), epoxide (green), 5 and 6 membered rings lactols (yellow).

Figure I. 6 .

 6 Figure I.6. Schematic diagram of the reduction of graphene oxide to obtain reduced graphene oxide.

Figure I. 7 .

 7 Figure I.7. Schematic representation of bottom-up and top-down techniques commonly used for the preparation of graphene.

  Figure III.1 shows images of glass vessels of similar sizes containing ca. 30 mg of each additive. Their origin and main characteristics are described below.

Figure III. 1 .

 1 Figure III.1. Images of the carbon additives selected in this study: (a) carbon black, CB-I; (b) carbon black, CB-II; (c) carbon black, CB-III; (d) carbon black, CB-IV; (e) oxidised carbon black, CB-IIIox; (f) graphite, Gr; (g) graphene oxide, GO; (h) reduced graphene oxide, rGO and (i) graphene, G-I. Similar size glass vessels contain ca. 30 mg of each additive.

Figure III. 2 .

 2 Figure III.2. Schematic representation of the structure of graphite, graphene oxide and reduced graphene oxide.

Figure III. 3 .

 3 Figure III.3. Schematic drawing of the experimental atmospheric pressure microwave plasma set up used to synthesise the graphene materials using ethanol as carbon source (not to scale).

Figure III. 4 .

 4 Figure III.4. Schematic process of the static synthetic protocol used for the preparation of the carbon gels and their composites with the additives.

Figure III. 5 .

 5 Figure III.5. Schematic process of the dynamic synthetic protocol used for the preparation of the carbon gels and their composites with the additives.

Figure III. 6 .

 6 Figure III.6. IUPAC classification of the gas adsorption isotherms. Adapted from[START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF].

[

  [START_REF] Thommes | Physisorption of gases, with special reference to the evaluation of surface area and pore size distribution (IUPAC Technical Report)[END_REF]] associated with certain well-defined pore structure (Figure III.7).

Figure III. 7 .

 7 Figure III.7. IUPAC classification of the hysteresis loops. Adapted from [Thommes, 2015].

Figure III. 8 .

 8 Figure III.8. Schematic representation of thermal desorption of the oxygen-functional groups as a function of the temperature.

Figure III. 9 .

 9 Figure III.9. Schematic representation of graphitic layers in a crystal.

  phenomena involved these two techniques, the structural information provided by XRD and Raman spectrometry has traditionally been assumed as equivalent.

Figure III. 10 .

 10 Figure III.10. Deconvolution of a first and second order Raman spectrum of a carbon black oxide (CB-IIIox).

  An illustrative spectrum of purified carbon black oxide synthesised by modified Hummers method is shown in FigureIII.10.

Figure III. 11 .

 11 Figure III.11. Schematic Raman mapping for a heterogeneous material constituted by three phases (blue, green and orange).

  Raman imaging was performed using the fast Streamline TM mode of the spectrometer setup. A zone of ca. 180 x 115 µm² on the samples was scanned with a step of 1.3 μm in two dimensions (accounting for the spatial resolution), resulting in 12000 acquired spectra recorded over 19.5 h of experiment. The LiveTrack TM mode was used, allowing to maintain the the measurements whatever the surface state, which becomes essential when recording powders. Wire TM software was used to remove cosmic rays, and to perform the spectral curve fitting (line positions are obtained by a classical Gaussian-Lorentzian fitting process).III.B.2.10. Electrical conductivity determination: 4-probe method.The electrical conductivity (σ) of the materials is a property that indicates whether a material allows the flow of current or not; this leads to the classification of the materials in conductors, semiconductors and insulators. Additionally, the electrical resistivity (ρ) is related with the resistance of a material to the flow of electrical charges (electrons, ions and holes) that will vary according to the material itself, as well as to the length, width and thickness of the sample. The conductivity is expressed as the reciprocal of the resistivity: behave as semiconductors, due to the π electrons and the holes present in the graphene layers, which can act as charge carriers. The conductivity of carbons is thus responsible for numerous electrical, electronic and thermal properties of the carbon surfaces. Although the enhancement of conductivity of carbon at high temperatures is related to structural rearrangements rather than to thermal excitation of valence electrons, and the numerous limitations of electrical conductivity measurements apply to porous solids, the conductivity of carbons provides valuable information of the carbon surfaces.

Figure III. 12 .

 12 Figure III.12. (a) Images of the pellets prepared for the measurements of the resistivity; (b) schematic representation of a 4-probe system -S corresponds to distance between the probes and t is the thickness of the pellet-.

IV.A. 1 .

 1 Synthesis of the pristine polymeric gels and carbon gels.The preparation of the polymeric and carbon gels was initially carried out following the experimental protocol based on the previous experience of the group[Macías, , 2017]. Briefly, the reactants' solution is mixed in a vessel and allowed to polymerize at 70 °C as indicated in the experimental chapter. To obtain the carbon gels (samples named as CPG), a thermal treatment was applied to the polymeric gel (PG) to densify it. With the aim to keep the porous network of the gel, a slow pyrolysis with three dwelling steps of 1 h (200, 400 and 800 °C) was applied with a heating rate of 2 °C/min; this carbonization treatment allowed the slow evolution of the generated gases during the decomposition of the gel avoiding the collapse of the porous structure during the densification process[Haro, 2011]. The carbonization profile (Figure IV.1) applied for the pristine gel reveals a total mass loss of 45 % that was divided in three steps. The first loss took place below 200 °C

Figure IV. 1 .

 1 Figure IV.1. (left) Thermogravimetric profile of sample PG in Ar (50 mL/min, 2 °C/min) and (right) temperature profile with three dwelling steps at 200, 400 and 800 °C.

Figure

  Figure IV.2 a and TableIV.1 show the N 2 adsorption isotherms at -196 °C of the polymeric gel and the carbon gel, as well as the main textural parameters therein obtained. Both samples displayed N 2 isotherms belonging to Type IVa according to IUPAC classification

Figure IV. 2 .

 2 Figure IV.2. (a) N 2 adsorption/desorption isotherms at -196 °C and (b) CO 2 adsorption isotherms at 0 °C of samples PG (light blue) and CPG (dark blue); filled symbols: adsorption branch; open symbols: desorption branch.

  Main textural parameters obtained from the N 2 and CO 2 adsorption isotherms at -196 °C and 0 °C, respectively, for the polymeric gel (sample PG) and the carbon gel (sample CPG).

  pore size distribution (PSD) applying the 2D-NLDFT-HS model to the N 2 adsorption data (Figure IV.3). Additionally, the narrow microporosity was analysed from the CO 2 adsorption isotherms at 0 °C (Figure IV.2 b), revealing a concave-shape isotherms for both samples. The amount of CO 2 adsorbed increased after the pyrolysis, indicating the presence of narrow micropores in sample CPG.

Figure IV. 3 .

 3 Figure IV.3. (a) Cumulative and (b) incremental pore volumes of samples PG and CPG evaluated from the nitrogen adsorption data at -196 °C, applying the 2D-NLDFT-HS model.

Figure

  Figure IV.4. (left) Image of the wet gel composite made with 8 wt.% of CB-III; (right) sketch showing the colour gradation through the gel monolith indicating the upper and lower sections.

Figure IV. 5 .

 5 Figure IV.5. N2 adsorption/desorption isotherms at -196 °C of (a) two transversal cuts (upper and lower) of PG-CB-III8 composite and sample PG synthesised without (w/o) mechanical stirring; (b) PG synthesised with and without mechanical stirring; filled symbols: adsorption branch; open symbols: desorption branch.

  -III8 (w/o stirring, lower) PG-CB-III8 (w/o stirring, upper) PG (w/o stirring)

Figure IV. 6 .

 6 Figure IV.6. Images of the pristine wet gel (PG) obtained in the (a) absence and (b) presence of mechanical stirring. (c) Dried monolith of pristine gel (PG) obtained under stirring conditions.

  absence and presence of mechanical stirring. The obtained samples looked alike at first sight (see images in Figure IV.6 a-b), with no significant differences in the synthesis yields, gelation times, or the consistency of the gels. The textural characterisation of the samples confirmed that the mechanical stirring does not affect the formation of the gels, since the porosity of the polymeric gel without additive prepared in the absence and presence of mechanical stirring was similar (Figure IV.5 b). In addition, the N 2 adsorption/desorption isotherms of the upper and lower portions of the 8 wt.% of carbon black additive overimposed in the whole range of relative pressures (Figure IV.7) corroborating the homogeneity of the samples obtained by dynamic conditions.

Figure IV. 7 .

 7 Figure IV.7. N2 adsorption/desorption isotherms at -196 °C of two transversal cuts (upper and lower) of PG-CB-III8 composite and sample PG synthesised under mechanical stirring; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  FigureIV.8 shows the TEM images corresponding to the selected carbon blacks, allowing to visualise useful information about the shape and morphology of the nanoparticle aggregates.

Figure IV. 8 .

 8 Figure IV.8. TEM images of the carbon blacks selected in this study. The average particle size range estimated for each material is indicated in the header.

Figure IV. 9 .

 9 Figure IV.9. (a) Raman spectra of the carbon black additives (shifted for clarify). (b) Example of Raman spectrum deconvolution for sample CB-I.

  Figure IV.9 b presents an example of the deconvolution of the Raman spectrum for sample CB-I. The deconvolution of Raman spectra of all CB is collected in Annex I (Figure A.I.1).

  The X-ray diffraction patterns complemented the information obtained about the nanostructure of carbon blacks. The corresponding diffractograms are shown in Figure IV.10. Three main peaks can be distinguished in the patterns positioned closely to the characteristic reflection (002) and bands (10) and (11) of carbon materials are identified at ca. 25 °, 44 ° and 79 ° (2θ)

Figure IV. 10 .

 10 Figure IV.10. X-ray diffraction patterns of the carbon black additives.

  Figure IV.11 a shows the correlation between L a obtained from XRD patterns (L a -XRD) and from TK equation (L a -TK). A linear trend was observed, indicating that Raman spectroscopy and XRD revealed the same behaviour of this parameter.

  Raman spectroscopy will not be considered. Figure IV.11 b shows I D /I G ratio and L a -XRD values

Figure IV. 11 .

 11 Figure IV.11. (a) Correlation between L a obtained from XRD data and L a evaluated from the Tuinstra-Koening equation applied to Raman spectra; (b) relationship between I D /I G ratio and L a for all the studied carbon blacks.

  Figure IV.12. N 2 adsorption/desorption isotherms at -196 °C of all the studied carbon blacks; filled symbols: adsorption branch; open symbols: desorption branch.

  Figure IV.13. N 2 adsorption/desorption isotherms at -196 °C of all the series of materials prepared with different carbon blacks: (a) Pristine gel, (b) gel/CB composites, (c) pristine carbon gel and (d) carbon gel/CB composites; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure IV.14 a-d shows the correlation between the textural parameters obtained from the experimental characterisation of the materials by nitrogen isotherms with the values predicted by the general rule of the mixtures taking into account the values of the pristine materials and

Figure

  Figure IV.14. (a-d) Correlation of selected experimental textural parameters (surface area, pore volumes, micro and mesopores volumes) of gel and carbon gel/CB composites with the values predicted by the general rule of mixtures. Dashed lines indicate the expected trend following predictions of the general mixing rule; (e,f) evolution of V1, V2 and IP for the studied samples.

Figure IV. 15 .

 15 Figure IV.15. Cumulative volume of (a) gel and (b) carbon gel/CB composites synthesised using similar amounts of different carbon blacks, evaluated from the adsorption branch of nitrogen adsorption data and applying 2D-NLDFT-HS model.

Figure

  FigureIV.16 shows the TEM images of the carbon gels composites synthesised with the four carbon blacks. In all cases it is possible to distinguish the carbon black particles/aggregates

Figure IV. 16 .

 16 Figure IV.16. TEM images of the carbon gel/CB composites synthesised with different carbon blacks.

Figure

  Figure IV.17.

Figure IV. 17 .

 17 Figure IV.17. Average Raman spectra of the carbon gel/CB composites.

Figure IV. 18 .

 18 Figure IV.18. I D /I G and I 2D /I G ratios for the series of carbon gel/CB composites prepared with the four studied carbon blacks.

Figure

  Figure IV.19. U-I curves for the (a) carbon blacks and the (b) series of carbon gel/CB composites; (c) conductivity values obtained for all the studied materials; (d) schematic percolation path followed by the electrons in samples CPG-CB-I12 (up) and CPG-CB-III12 (down).

Figure

  Figure IV.19 d that illustrates a continuous percolation path (blue lines) in sample CPG-CB-III12,

Figure

  FigureIV.20 shows the TEM images of the gels and the carbons prepared in the presence of the different amounts of CB-III additive. The images of the polymeric gel and the carbon gel without additive as well as carbon black are also included as reference. The images show how the incorporation of the CB-III does not affect the morphology of the polymeric matrix compared to the polycondensation reaction without additive. In the presence of CB-III, the spherical-shaped nanometric aggregates of the CB-III were clearly distinguished within the matrix of the polymeric gel and the carbon gels; the continuous conductive network of the CB-III aggregates is more evident in the samples prepared with the higher amounts of carbon black. The average particle size of the CB-III aggregates in the gel/CB-III composites remained unmodified (60-80 nm), indicating that the particles of the additive do not suffer fragmentation or crack during the synthesis (despite the vigorous magnetic stirring) for any amount of additive incorporated. As was previously observed, following the mechanical stirring of the suspensions of carbon black and reactants, guaranteed the uniform distribution of the different amounts of additive during the initial steps of the synthesis, hence, no significant differences were observed in the distribution of the CB-III neither in the organic gels (series PG, before pyrolysis) nor in their corresponding carbon gels (series CPG, after carbonization). This also indicates the carbonization most likely does not bring about strong structural rearrangements between the matrix of the gel and the CB-III additive, which might otherwise favour the aggregation of the CB-III in the carbon gels.

Figure IV. 20 .

 20 Figure IV.20. Selected TEM images of the carbon black additives, the polymeric gel composites (series PG) and the carbon gel composites (series CPG) prepared with different amounts of carbon black, CB-III. For clarity, images are shown at various magnifications.

Figure IV. 21 .

 21 Figure IV.21. (a) Mass evolution (%) and (b) derivative thermogravimetric profiles (DTG) in Ar (50 mL/min, 20 °C/min) for samples CB-III, PG, PG-CB-III4, PG-CB-III12 and PG-CB-III40.

Figure IV. 22 .

 22 Figure IV.22. FTIR spectra of the polymeric gel/CB-III composites before carbonization.

Figure

  Figure IV.23 a, c and e show the reconstructed images corresponding to the variation of the fitted I D /I G ratio in the scanned area for the CPG, and carbon gel/CB-III composites made with two percentages (4 and 16 wt.%). The images were reconstructed from over 300 Raman spectra in different points of the scanned area (spatial resolution of ca. 1.3 μm), the dark zones correspond to areas with low I D /I G ratios -between 0.6 and 0.7-indicating the higher ordered zones, while the light areas are associated to higher ratios (less ordered regions). For sample CPG a homogeneous distribution of I D /I G was observed (no-colour gradation was distinguished in the image). However, when the CB-III was incorporated the coexistence of CB-III aggregates and a carbon gel matrix in the sample was revealed by the colour gradation. The dark zones in the reconstructed Raman map correspond to the presence of the CB-III additive; local Raman spectra of these dark areas (arrows) confirmed the structural order provided by the CB-III, with the appearance of the characteristic peak in the second-order range of the spectrum. On the other hand, the light (yellow) areas represent high I D /I G ratios, the Raman spectrum of this zone is characteristic of the carbon gel matrix, as also obtained for the CPG sample.With the aim of comparison, the average Raman spectra were shown for all the samples, revealing a lower I D /I G average for the sample with higher amount of conductive additive(Tables IV.8 and A.I.5, Figure A.I.4). This indicates that the structural order increased in the areas with a higher density of CB-III, with no apparent changes in the structure of the carbon gel/CB-III composites, compared to the structure of the individual components in the materials at this length scale.

Figure IV. 24 .

 24 Figure IV.24. X-ray diffraction patterns of the (a) polymeric gels and (b) carbon gel/CB-III composites.

  cm 3 /g. This is in agreement with the observation of the lower impact of the CB-III in the micropores of the materials inferred from the nitrogen adsorption isotherms.

Figure IV. 25 .

 25 Figure IV.25. (a-d) N 2 adsorption/desorption isotherms at -196 °C -(b,d) have been shifted ca. 150 cm 3 /g for clarity-; (e,f) CO 2 isotherms at 0 °C of the gel and carbon gel composites synthesised with different amounts of CB-III as additive. Data corresponding to the carbon black is also shown for comparison; filled symbols: adsorption branch; open symbols: desorption branch.

  99; b evaluated by the DR method; c evaluated by the 2D-NLDFT-HS method It should be noted that the specific surface areas, total pore volumes and micropore volumes increased significantly after the carbonization, whereas the mesopore volumes remained quite similar -especially for the samples with low amount of carbon black-. Indeed, the experimental surface areas and micropore volumes matched the values predicted by a general mixing rule (Figure IV.26) taking into account the textural features corresponding to gel and CB-III. Nevertheless, the mesopores volumes did not follow this trend, confirming the influence of the amount of additive in the development of the mesoporous network. An important feature of the gas adsorption isotherms is the evolution of the shape and the position of the hysteresis loop (Figure IV.25 a-d) with the amount of conductive additive. As was discussed in the previous sections (Section IV.A), the pristine gel (sample PG) exhibited a narrow loop between 0.4 and 0.8 of relative pressures and the incorporation of the additive provokes a loop widening and the formation of an inflection point in both the adsorption and desorption branches. Above 8 wt.% of CB-III, the hysteresis loops become steeper and the adsorption and desorption branches are somewhat parallel over the entire range of relative pressures. This feature is preserved in the carbonized samples.

Figure IV. 26 .

 26 Figure IV.26. Correlation of experimental (a) micropore volume, (b) mesopore volumes, (c) surface area and (d) V 1 of the hysteresis loop of the series of gel/CB-III and carbon gel/CB-III composites with the values predicted by the general mixing rule. Dashed lines indicate the expected trend following predictions of the general mixing rule.

  the expected trend considering a mixing rule(Figure IV.26 d). Both findings confirm that the appearance of a secondary mesopore network is directly connected with the presence of the carbon black additive. The relative pressure of the inflection point as a function of the amount of CB-III follows a similar trend to V1 for the gels (Figure IV.27). Since higher relative pressures

Figure IV. 27 .

 27 Figure IV.27. Correlation between the mesopore volumes (a) V1 and (b) V2, and (c) the relative pressure of the inflection point in the N 2 adsorption isotherms with the amount of carbon black for series PG and CPG.

Figure IV. 28 .

 28 Figure IV.28. (a) Cumulative and (b) derivative pore volumes of the carbon gel/CB-III composites evaluated from the adsorption branch of nitrogen adsorption data and applying 2D-NLDFT-HS model.

Figure

  Figure IV.29 shows the U-I curves obtained for pellets (90 wt.% material and 10 wt.% PTFE binder) of the prepared carbon materials by the four-probe measurements. The conductivity values and the pellets' dimensions are collected in TableA.I.6. The linearity of the response retrieved for all the samples confirmed that sheet resistances can be confidently evaluated obtained from the slope of the U-I curves.

Figure IV. 29 .

 29 Figure IV.29. (a) U-I curves and (b) electrical conductivity values for the series of carbon gel/CB-III composites.

  following a linear trend to 0.06 to 0.021 S/cm for samples CPG-CB-III8 and CPG-CB-III24, respectively, finally the trend suffers a new change for the highest amount of CB-III material (CPG-CB-III40) which presents a similar conductivity of CPG-CB-III24. This suggests that a different regime governs the electronic transport properties of the composites and can be attributed to the presence of a 3D electrically conductive network provided by the CB-III particles within the carbon gel matrix, which would facilitate the electron mobility between the conductive additive particles.

Figure IV. 30 .

 30 Figure IV.30. (a) Correlation of the electrical conductivity with the amount of carbon black; (b) correlation between the experimental conductivity and the values predicted by the general mixing rule; (c) log-log plot of the conductivity as a function of the amount of conductive phase following a percolation model.

Figure

  Figure IV.30 b shows the correlation between the experimental conductivity and the values predicted by the mixing rules, as seen, the experimental conductivity data of the gel/CB-III composites cannot be fitted to the rule of mixtures, being the experimental values ca. 1.5-3 times lower than those predicted. This fact indicates that the poorly-conductive layer of the carbon gel separating the CB-III aggregates plays a dominant role in the conductivity of the bulk gel/CB-III composites.

Figure IV. 31 .

 31 Figure IV.31. TEM images of CB-III (first row) and CB-IIIox (second row) at different magnifications.

Figure

  Figure IV.32. (a) Mass evolution and (b) derivative thermogravimetric profiles (DTG) of CB-III and CB-IIIox in argon (100 mL/min, 10 °C/min) from 20 to 900 °C.

Figure IV. 33 .

 33 Figure IV.33. Proton binding curves for samples CB-III and CB-IIIox.

Figure IV. 34 .

 34 Figure IV.34. FTIR spectra of samples (a) CB-III and (b) CB-IIIox.

  Figure IV.35. N 2 adsorption/desorption isotherms at -196 °C of samples CB-III and CB-IIIox; filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 36 .

 36 Figure IV.36. Raman spectra and deconvoluted peaks of samples (a) CB-III and (b) CB-IIIox.

Figure

  Figure IV.37. X-ray diffraction patterns of samples CB-III and CB-IIIox.

Figure

  Figure IV.38 shows the N 2 adsorption/desorption isotherms obtained at -196 °C for the materials prepared with the oxidised carbon black at 4 and 12 wt.%. Surprisingly, the incorporation of the oxidised additive has a strong influence in the development of the

Figure IV. 38 .

 38 Figure IV.38. N 2 adsorption/desorption isotherms at -196 °C of the polymeric gel and carbon gel composites synthesised with 4 and 12 wt.% of CB-III and CB-IIIox additives; filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 39 .

 39 Figure IV.39. (a) Mass evolution and (b) derivative thermogravimetric profile (DTG) of PG, and the series of gel/CB-III and gel/CB-IIIox composites in argon (50 mL/min, 20 °C/min).

  Regardless the additive employed, the experimental mass loses fits with that obtained by the general rule of mixtures confirming the lack of interaction between the gel matrix and the CB-IIIox in spite of the hydrophilic character of the additive.TEM images of the carbon gels made with 4 wt.% and 12 wt.% of each CB-III and CB-IIIox revealed important differences in the morphology(Figure IV.40). When the CB-III was oxidised, the distribution of the additive through the polymeric matrix was quite uniform, without particles' agglomeration. This contrast with the spherical-shaped nanometric aggregates embedded in the organic matrix observed when hydrophobic CB-III was used. Samples CPG-CB-IIIox4 and CPG-CB-IIIox12 show a homogeneous structure; at high magnification, it is not possible to distinguish the graphitic domains well distributed within the amorphous gel matrix for any of the carbon gel/CB-IIIox composites, assuring the uniform distribution of the oxidised additive regardless the amount incorporated. This fact confirmed that the hydrophilic character of CB-IIIox favours the dispersion in the reactants' mixture (e.g., an aqueous solution) and consequently, avoids the formation of agglomerated additive nanoparticles.

Figure IV. 40 .

 40 Figure IV.40. TEM images of samples CPG-CB-III4 (first row), CPG-CB-IIIox4 (second row), CPG-CB-III12 (third row) and CPG-CB-IIIox12 (fourth row) at different magnifications.

Figure IV. 41 .

 41 Figure IV.41. (a) Raman image reconstruction showing the I 2D /I G ratio for sample CPG-CB-IIIox12; (b) Raman spectra of the pristine carbon gel (CPG), the carbon gel/CB-IIIox composite (12 wt.%) and the additive (CB-IIIox).

Figure IV. 42 .

 42 Figure IV.42. U-I curves for sample CPG and the series of carbon gel/additive composites prepared with 4 and 12 wt.% of CB-III and CB-IIIox additives.

  This section is devoted to the use of graphite, graphite oxide and reduced graphite oxide as additive in the synthesis of carbon and gel/additive composites. The additives were selected based onto their different structural, morphological and chemical features. Firstly, in Subsection IV.C.1 an exhaustive characterisation of the three additives is carried out, with the aim to relate later its properties with the development of the composites. In Subsection IV.C.2 the analysis of the composites obtained employing two amounts of each additive (4 and 12 wt.%) is presented, showing important differences in terms of structural order, porosity and electrical conductivity, mainly associated to the nature of the employed additive.

Figure IV. 43 .

 43 Figure IV.43. TEM images of the graphite (Gr), graphene oxide (GO) and reduced graphene oxide (rGO) used in this study.

Figure

  Figure IV.43 shows the TEM images of the three selected additives at different magnifications illustrating the morphology of each material. The analysis of the low magnification images revealed a fluffy structure characteristic of graphite and their derivate

Figure

  FigureIV.44 shows the Raman spectra of the three additives, pointing out structural differences between them. Concerning the first-order spectra, Gr displayed a high ordered

Figure IV. 44 .

 44 Figure IV.44. Raman spectra of (a) graphite (Gr), (b) graphene oxide (GO) and (c) reduced graphene oxide (rGO).

Figure IV. 45 .

 45 Figure IV.45. X-ray diffraction patterns of (a) graphite (Gr), (b) graphene oxide (GO) and (c) reduced graphene oxide (rGO).

Figure IV. 46 .

 46 Figure IV.46. Relationship between I D /I G ratio (from Raman spectroscopy) and L c (green line) and L a (brown line) (from X-ray diffraction) for all studied materials.

Figure

  Figure IV.46 displays a correlation between the I D /I G ratio and L c (002) and L a (10) both values calculated from XRD analysis. As the I D /I G ratio increased, L c -average height of the stacked graphenic layers-and L a -length of the graphitic domains-values decreased, pointing

  ). These titration results are consistent with those obtained by other characterisation techniques, like infrared spectroscopy (Figure IV.47) and TPD-MS (Figure

Figure IV. 47 .

 47 Figure IV.47. FTIR of each material: (a) graphite (Gr), (b) graphene oxide (GO) and (c) reduced graphene oxide (rGO). Data is shifted for clarity.

Figure IV. 48 .

 48 Figure IV.48. Thermogravimetric profiles of each additive obtained between 25 and 900 °C at 10 °C/min under Ar flow (100 mL/min).

a

  Figure IV.49. Adsorption/desorption isotherms of (a) N 2 at -196 °C and (b) CO 2 at 0 °C of additives; filled symbols: adsorption branch; open symbols: desorption branch.

Figure

  Figure IV.50 shows the nitrogen adsorption/desorption isotherms at -196 ᵒC of the synthesised composites, with important differences observed depending on the nature of the additive employed.

  , the contribution in the microporosity was somewhat lower for the composites (compared to CPG).

Figure IV. 50 .

 50 Figure IV.50. N2 adsorption/desorption isotherms at -196 °C corresponding to gel and carbon gel composites synthesised with (a,b) graphite, (c,d) graphene oxide and (e,f) reduced graphene oxide in two amount of additive, 4 and 12 wt.%; filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 51 .

 51 Figure IV.51. Correlation of pH and textural parameters (a) S BET , specific surface area, and (b) V PORES , total pores volume.

  Figure IV.52. Correlation of selected experimental textural parameters (surface area, pore volumes) of gel and carbon gel/additive composites with the values predicted by the general rule of mixtures. Dashed lines indicate the expected trend following predictions of the general mixing rule.

Figure IV. 53 .

 53 Figure IV.53. FTIR spectra of (a) PG and PG/additive composites using (b) graphite, (c) graphene oxide and (d) reduced graphene oxide as additives.

Figure

  Figure IV.55. TEM images of the carbon gel/additive composites synthesised using Gr, GO and rGO additives.

  Confocal Raman spectroscopy allowed studying the structural modifications that occurred with the incorporation of graphitic additives during the synthesis of gel/additive compounds. Variation of the I 2D /I G ratio of the carbon gel/additive composites prepared with an additive content of 12 wt.% by weight reported the images arranged in the left column of FigureIV.56 (spatial resolution of 1.3 µm). The data analysis related to the light areas with a low structural order (I 2D /I G ratio ca. 0.1) and the dark parts corresponds to more ordered regions, which means, higher content in the graphitic additive.

Figure IV. 56 .

 56 Figure IV.56. Raman reconstruction images and the corresponding deconvolution of the average Raman spectrum obtained for samples (a,b) CPG-Gr12, (c,d) CPG-GO12 and (e,f) CPG-rGO12.

Figure IV. 57 .

 57 Figure IV.57. U-I curves for (a) graphite series, (b) graphene oxide series and (c) reduced graphene oxide series.

Figure

  Figure IV.57, the carbon material (without additive) and the pristine additives are also plotted in order to make a complete comparison. The well-fitting to a linear trend of the composites in the U-I plots confirmed the adequacy of the method for estimating the resistance from the slope. In all cases, this slope decreased as the amount of additive added increased which indicates less resistance and, therefore, greater conductivity.

Figure

  Figure IV.58 shows the evolution of the electrical conductivity values of the materials as a function of the amount of added additive. All the carbon gel/graphitic additive composites

Figure IV. 58 .

 58 Figure IV.58. Conductivity values obtained for the series of materials prepared with Gr, GO and rGO additives.

  As an example, Figure IV.59 shows the dependence of selected parameters that govern the synthesis of graphene with the flow of ethanol. The contribution of this PhD thesis focused on establishing the dependence of the flow of ethanol as carbon source on the quality and the production rate of graphene.

Figure IV. 59 .

 59 Figure IV.59. Maximum ethanol inlet withstood by Ar TIAGO plasmas sustained at different Ar flows and input power. (Unpublished data from LIPs group at University of Córdoba, herein included with permission for comparison and clarification purposes).

  EtOH flows and green for larger EtOH flows, as was indicated in Figure IV.60 a.

Figure IV. 60 .Figure

 60 Figure IV.60. (a) Schematic representation of products obtained from both fundamental routes for ethanol decomposition in an Ar TIAGO torch plasma sustained with 300 W. (Unpublished data from LIPs group at University of Córdoba, herein included with permission for comparison and clarification purposes). (b) Mass spectra of an Ar plasma and Ar-EtOH plasma (ethanol flow of 4.00 g/h and argon flow of 1.00 L/min after 160 minutes). For clarity, ion current intensities are normalized to the maximum ion current signal, corresponding to m/z 40 a.m.u. ascribed to Ar + .

3

  .40 (sample G-III), 3.80 (sample G-IV) and 4.00 (sample G-V).

Figure

  Figure IV.61 shows the evolution of the gas by-products as a function of the reaction time, under high EtOH flow conditions. All ion currents tend to stabilize at ca. 60 minutes after the beginning of the synthesis. The detailed analysis of the temporal behaviour of the m/z signals of the gases was carried out considering two groups: group 1 for m/z 2, 26 and 28 a.m.u.; and group 2 for m/z 16, 18, 27 and 31 a.m.u. It can be observed that the only signals showing a

Figure IV. 61 .

 61 Figure IV.61. Time evolution of the ion intensity of mass fragments m/z (left column) 2, 26 and 28 a.m.u., and (right column) 16, 18, 27 and 31 a.m.u. for G-I, G-II, G-III, G-IV and G-V.

  Figure IV.61 (continued). Time evolution of the ion intensity of mass fragments m/z (left column) 2, 26 and 28 a.m.u., and (right column) 16, 18, 27 and 31 a.m.u. for G-I, G-II, G-III, G-IV and G-V.

Figure

  Figure IV.62. (a) Carbon solid production rate as a function of the EtOH flow. (b) Image of the reactor employed.

Figure

  Figure IV.62 and Table IV.21 show the carbonaceous solid material production rates at each experimental condition. The values correspond to the average of two different batches, where the variation for each batch was found below 5 % assuring the reproducibility of the process.

Figure

  FigureIV.64 shows the correlation between the number of carbon atoms in the reactant (ethanol) and in the carbon-based by-products considering two scenarios according to the routes proposed for the decomposition of ethanol using plasma: i) conversion to CO, C 2 H 2 , CH 4 , C (s) and C 2 H 4 and ii) conversion in C 2 H 2 , CO and C (s). As seen, the number of carbon atoms in ethanol correlates well with the sum of the carbon atoms obtained when the contribution of all carbon-based by-products (scenario i), ca. CO, C 2 H 2 , CH 4 , C (s) and C 2 H 4 is considered.

Figure IV. 63 .

 63 Figure IV.63. Mol formation of (a) H 2 , CO, (b) C 2 H 2 , CH 4 , C (s) and C 2 H 4 per mol of ethanol feed to the discharge. The percentage indicates the difference between the minimum and maximum value for each gas.

Figure

  Figure IV.65 exhibits the TEM images of the carbon materials corresponding to the decomposition of five different ethanol flows (2.00, 2.90, 3.40, 3.80 and 4.00 g/h) by microwave plasma at atmospheric pressure. Low-magnification TEM images show areas with high transparency ascribed to the characteristic structure of extended graphene sheets, while the dark zones correspond to the folded graphene sheets.

Figure

  Figure IV.65. TEM images of the graphene materials

Figure IV. 66 .

 66 Figure IV.66. X-ray diffraction patterns of the graphene materials prepared at different ethanol flows (shifted for clarify).

Figure

  Figure IV.67 b and Table IV.23 show both ratios as a function of the ethanol flow. As seen, I D /I G and I 2D /I G ratios follow inverse trends. On the one hand, the lower values obtained for I D /I G ratio indicate lower amount of defect present in the materials, being the sample G-II the most ordered material.

Figure IV. 67 .

 67 Figure IV.67. (a) Raman spectra of the graphenes prepared at different ethanol flows (shifted for clarify); (b) correlation between I D /I G and I 2D /I G ratios with the ethanol flow.

Figure

  Figure IV.68 shows the correlation between the I D /I G ratio and L c (002) and L a (10) both values calculated from XRD analysis (TableIV.22). As the I D /I G ratio increased, L a -length of the

Figure IV. 68 .

 68 Figure IV.68. Correlation of the I D /I G ratio (blue line) from Raman spectroscopy and, L c (green line) and L a (brown line) obtained from XRD for the graphenes prepared at different ethanol flows.

  Data analysis and fitting were performed with MultiPak TM software, after Shirley background subtraction. The C 1s peak of each material was deconvoluted into seven Voigt-functions (90 % Gaussian-10 % Lorentzian), hence aromatic carbon (C sp 2 , 284.4 eV); aliphatic carbon (C sp 3 , 285.2 eV); hydroxyl (C-OH, 285.7 eV); epoxy (C-O-C, 286.7 eV); carbonyl (C=O, 287.3 eV), carboxyl (O-C=O, 288.5 eV) and plasmon (π-π*, 290.7 eV) [Moulder, 1995; Moreno-Castilla, 2000], shown in Figure IV.70 a-e.

Figure IV. 70 .

 70 Figure IV.70. Deconvoluted C 1s spectra of samples (a) G-I, (b) G-II, (c) G-III, (d) G-IV, and (e) G-V; (f) correlation between I D /I G ratio and the area C sp 3 .

  Figure IV.70 f displays the correlation between I D /I G ratio and the C sp 3 area vs. the ethanol flow and, as seen the similar trends are followed, indicating a good agreement of both techniques regarding the estimation of the number of defects.

Figure IV. 71 .

 71 Figure IV.71. N 2 adsorption/desorption isotherms at -196 °C of (a) polymeric gel and (b) carbon gel composites prepared using G-I as additive. Data corresponding to the additive and the control without additive are also included; filled symbols: adsorption branch; open symbols: desorption branch.

  followed a different trend (Figure IV.72 b and d), with theoretical values lower than the experimental ones for both composites. It can be inferred then that G-I turned out to have a strong influence on the development of the mesopore structure of the composites, which was intensified with the amount of additive.

Figure

  Figure IV.72. (a-d) Correlation of selected experimental textural parameters (surface area, pore volumes) of gel and carbon gel/graphene composites with the values predicted by the general rule of mixtures. Dashed lines indicate the expected trend following predictions of the general mixing rule; (e,f) evolution of V1, V2 and IP for the gel and carbon gel/graphene composites.

  from 0.4 up to the value corresponding to the inflection point (IP) in the adsorption branch, and V2 related to the volume comprised between the relative pressure of IP and 1. For the samples without additive (PG and CPG) only V1 is present and represents the full loop. The second region (V2) is developed with the G-I additive, showing a linear correlation with the amount of G-I (Figure IV.72 e). The evolution of V1 is more discrete, being the value greater for the sample prepared with 4 wt.% additive. The same trend can be observed for the evolution of the relative pressure of the IP (Figure IV.72 f). In sum, G-I as additive provoked the development of a second type of mesoporosity in the carbon gel/G-I composites. TEM images of samples composites CPG and CPG-G-I12 (Figure IV.73) confirmed that the presence of graphene has no effect on the morphology of the RF matrix structure.

Figure

  Figure IV.73. TEM images of samples CPG and G-I, and the carbon gel/G-I composite prepared with 12 wt.% of additive.

Figure

  Figure IV.74 shows the structural analysis using Raman and Confocal Raman image spectroscopy, which allowed the estimation of the degree of dispersion of the conductive additive in the matrix. Due to the structural homogeneity of the pristine materials, CPG and G-I, they were analysed by individual Raman spectroscopy (Figure IV.74 a-b). As discussed in the previous sections, Raman spectra of both materials have two characteristic bands of carbon materials centred at ca. 1350 and 1580 cm -1 , corresponding to the bands D and G, respectively. The contribution of the 2D band (ca. 2700 cm -1 ) in the second-order region is clearly more significant for graphene. For this reason, the I 2D /I G ratio was used to reconstruct the Confocal Raman imaging, as an effective way to differentiate the ordered graphene aggregates from the amorphous carbon matrix. The reconstructed images (Figures IV.74 c and e) show the variation of the fitted I 2D /I G ratio in the scanned area for CPG-G-I4 and CPG-G-I12, respectively. The dark zones are associated with high values of I 2D /I G ratio -between 0.7 and 0.9-indicating high ordered regions; the light zones correspond to lower ratios corresponding to a low structural order.The individual Raman spectra of these differenced parts were extracted to determine if each pristine material retains its initial arrangement in the carbon gel/graphene composites or if a new structure is obtained. As it can be seen in Figures IV.74 d and f, Raman spectra of the light areas correspond to that of the amorphous carbon matrix (CPG), while dark areas show the characteristic profile of graphene (G-I). The comparison of the average spectra shows how as the amount of graphene increased, the contribution of the 2D band also increased (i.e., higher I 2D /I G ratio). The main parameters of the deconvoluted average Raman spectra corresponding to each material are collected in TableIV.26.

Figure IV. 74 .

 74 Figure IV.74. (a,b) Raman spectra of the pristine materials, carbon gel matrix (CPG) and graphene (G-I), respectively; (c,e) Raman image reconstruction showing the I 2D /I G ratios for samples CPG-G-I4 and CPG-G-I12; (d,f) Raman spectra of different positions in the reconstructed Raman mapping for sample each composite corresponding to dark and light areas, and the average.

  The electrical conductivity of pellets of carbon gel/graphene composites containing 90 wt.% of the carbon materials and 10 wt.% PTFE binder was evaluated from the four-probe method. The recorded U-I curves are shown in Figure IV.75 a, showing a linear response for all the materials. This allows a confident estimation of the resistance from the slope of the U-I curves, and later calculation of the resistivity and conductivity values using the Van der Pauw method [Van der Pauw, 1958]. The obtained values are showing in Figure IV.75 b and Table A.I.15 (Annex I).

Figure

  Figure IV.75. (a) U-I curves and (b) conductivity values obtained for samples CPG, G-I and the carbon gel/G-I composites prepared with 4 and 12 wt.% of additive.

  the conductivity of the final composite is dominated by the regions of the poorly conductive amorphous carbon matrix.

Figure

  Figure IV.76 shows the N 2 adsorption/desorption isotherms at -196 °C of the pristine (CB-III, GO and rGO) and hydrothermal treated additives (HS-CB-III, HS-GO and HS-rGO). In the case of CB-III, the hydrothermal treatment does not have a significant influence in the porosity of the material, since the gas adsorption isotherms of both materials presented similar shapes. This is somewhat expected considering that selected carbon black additive is a hydrophobic material with a highly ordered structure (Section IV.B), thus it is expected to be stable under moderate temperature and pressure conditions. This stability was also confirmed by the thermogravimetric profile(Figure IV.77), in which the CB-III follows a flat mass loss close to zero.

Figure IV. 76 .

 76 Figure IV.76. N 2 adsorption/desorption isotherms at -196 °C of samples (a) CB-III and HS-CB-III, (b) GO and HS-GO, (c) rGO and HS-rGO; filled symbols: adsorption branch; open symbols: desorption branch.

Figure IV. 77 .

 77 Figure IV.77. Mass evolution of CB-III, GO and rGO obtained between 20 and 900 °C at 10 °C/min under Ar flow (100 mL/min).

Figure

  Figure IV.79. N 2 adsorption/desorption isotherms at -196 °C obtained for samples PG and HS-PG; filled symbols: adsorption branch; open symbols: desorption branch.

aFigure

  Figure IV.81. N 2 adsorption/desorption isotherms at -196 °C of samples (a) HS-PG-CB-III4 and (b) HS-PG-rGO4 (bulk and upper and lower aliquots). The isotherm of HS-PG is also included; filled symbols: adsorption branch; open symbols: desorption branch.



  The hydrophobic/hydrophilic character of the additive resulted a crucial parameter to the final properties of the material, both porous development and electrical conductivity. The incorporation of a hydrophilic additive (oxidised carbon black) to the reactants' mixture provoked an abrupt pH decrease of the reactants' media, this fact joined to the well-dispersion of the additive (due to its affinity with the reaction solvent) during the polycondensation reaction hindered the polycondensation of the reactants, preventing the development of the porosity of the composite's structures. Respect to the conductivity values, a slightly improvement was observed, compared to the hydrophobic additive, mainly associated to the greater distribution of the additive.

Figure VI. 1 .

 1 Figure VI.1. Cyclic voltammograms (equilibrium response is shown, typically after ca. 5-6 scans); after equilibration of selected carbon gel/CB-III composites in 0.05 M Na 2 SO 4 using Hg/Hg 2 SO 4 (SME) as reference electrode and a graphite rod as counter electrode. Scan rate 0.5 mV/s.

  (e.g., 0.022, 0.104, 0.159, and 0.273 S/cm for CPG, CPG-CB-III12, CPG-CB-III16, and CPG-CB-III40, respectively, Figure IV.29 (Chapter IV)). The similar electrochemical response of CPG-CB-III12 and CPG-CB-III16 (in terms of current density) agrees with the porous features of these samples (Chapter IV, Table IV.9), since both have similar surface areas and micropore volumes (being the development of mesopore volume the main difference between them). On the other hand, and CPG-CB-III40 displayed a lower surface area and micropore volume, which would account for the decreased current density in Figure VI.1. To understand these results, it

Figure VI. 2 .

 2 Figure VI.2. N 2 adsorption/desorption isotherms at -196 °C of the carbon materials prepared upon photoassisted polycondensation of the reactants in the presence and absence of CB-III as conductive additive; filled symbols: adsorption branch; open symbols: desorption branch.

F

  Fathy, N.A.; Rizk, M.S.; Awad, R.M.S. Pore structure and adsorption properties of carbon xerogels derived from carbonization of tannic acid-resorcinol-formaldehyde resin.

Figure A.I. 1 .

 1 Figure A.I.1. Deconvolution of Raman spectra for the carbon blacks used as additives: (a) CB-I, (b) CB-II, (c) CB-III, and (d) CB-IV.

Figure A.I. 3 .

 3 Figure A.I.3. Deconvolution of Raman spectra for the carbon gel/CB composites prepared with 12 wt.% of additive: (a) CPG-CB-I12, (b) CPG-CB-II12, (c) CPG-CB-III12, and (d) CPG-CB-IV12.

Figure A.I. 6 .

 6 Figure A.I.6. Deconvolution of Raman spectra for carbon gel/CB-IIIox composites prepared with (a) 4 and (b) 12 wt.% of CB-IIIox as additive.

  Figure A.I.7. Deconvolution of Raman spectra of graphite (Gr), graphene oxide (GO) and reduced graphene oxide (rGO).

Figure A.I. 8 .

 8 Figure A.I.8. (a) Proton binding curves and (b) pK a profiles of graphite (Gr), graphene oxide (GO) and reduced graphene oxide (rGO).

Figure A.I. 9 .

 9 Figure A.I.9. TPD-MS profiles corresponding to m/z 18 (H 2 O), 28 (CO) and 44 (CO 2 ) of the synthesised graphene oxide used as additive.

Figure A.I. 10 .

 10 Figure A.I.10. Mass evolution in air (100 mL/min) of the graphenes prepared at different ethanol flows; heating rate, 20 ᵒC/min.

Chapter I: Introduction I.1. General aspects of carbon materials.

  Il existe aujourd'hui un grand nombre de matériaux à base de carbone qui suscitent un énorme intérêt grâce à la combinaison de propriétés physiques et chimique uniques, leur permettant d'être utilisés dans un grand nombre de technologies telles que la catalyse, l'adsorption, le traitement des eaux usées, le stockage de l'énergie, etc. Ces applications sont principalement la conséquence de la chimie de coordination unique des atomes de carbone et Cette disposition des couches de graphène est responsable des propriétés physicochimiques des matériaux carbonés. Ainsi, la structure du squelette carboné dans les carbones poreux peut être considérée comme une phase carbonée non organisée composée de formes aromatiques-aliphatiques complexes et de feuilles graphitiques tordues/déformées. Sur la base des études précédentes menées dans le groupe, l'objectif de cette thèse de doctorat consistait à explorer diverses approches synthétiques pour obtenir des carbones nanoporeux à réseau poreux élevé et à conductivité accrue en incorporant diverses nanostructures de carbone comme additifs conducteurs. Une large gamme de nanostructures de carbone a été utilisée comme additifs conducteurs, notamment le noir de carbone, le graphène, le graphite, l'oxyde de graphène et l'oxyde de graphène réduit. Dans une première approche, l'additif a été incorporé lors de la synthèse du matériau de carbone nanoporeux lui-même, plutôt que comme percolateur dans la formulation de l'encre généralement appliquée pour la préparation des électrodes (par exemple, matériau en carbone, liant et percolateur). Dans une deuxième approche, des architectures 3D de type graphène nanoporeux ont été obtenues par voie hydrothermale. Le débit d'éthanol a été varié (de 2.00 à 4.00 g/h), montrant des différences sur les sous-produits gazeux générés pendant la synthèse. Pour un débit de gaz plus faible (de 2.00 à 3.4 g/h), les principaux gaz détectés étaient le C 2 H 2 , le CO et le C(s), tandis que pour des débits

	Résumé xxxvi Résumé	xxxv Résumé xxxvii
	de carbone (12 % en poids, exprimé en grammes de CB par gramme de résorcine + microporosité étroite (narrow microporosity) doit être confirmé par des mesures d'isothermes composites ont été préparées et caractérisées par voltampérométrie cyclique en utilisant une
	formaldéhyde) au mélange RF. La caractérisation des composites de gels de carbone a permis d'adsorption d'autres gaz (e.g., CO 2 à 0 °C), ce résultant diffère des obtenus en absence de l'a configuration de cellule à 3 électrodes.
	de conclure que la nature du noir de carbone utilisé comme additif au gel et au gel de carbone, additif et en présence du même additif de caractère hydrophobe. En ce qui concerne les Dans la seconde approche, le noir de carbone, l'oxyde de graphène et l'oxyde de graphène
	de gaz plus élevés (environ 4.00 g/h), d'autres gaz sont générés comme le CH 4 et le C 2 H 4 , a une influence importante sur la dispersion de l'additif à travers la matrice polymère ; valeurs de conductivité, une légère amélioration a été observée, par rapport à l'additif réduit ont été sélectionnés pour être utilisés pour évaluer l'impact de la synthèse
	réduisant la formation de C (s). En conséquence, des variations dans le taux de production du notamment en termes de taille des particules (30-100 nm), de porosité et de caractéristiques hydrophobe, principalement associée à la plus grande distribution de l'additif. hydrothermale dans leur porosité. Dans un premier temps, les suspensions aqueuses des trois
	Ces feuilles graphitiques (ou cristallites) sont orientées de manière aléatoire et graphène ont été observées, la production maximale étant de 1.55 mg/min pour un débit physico-chimiques. D'autre part, graphite, oxyde de graphène et oxyde de graphène réduit ont été sélectionnés matériaux ont été traitées à 150 °C dans un autoclave pendant 12 heures. Ce traitement
	interconnectées, avec des pores, des défauts de bordure et des interstices entre les cristallites, d'éthanol de 3.4 g/h. Malgré ces différences, les caractéristiques du graphène ont été La présence d'agrégats de noir de carbone provoque la formation d'un chemin de comme additifs en fonction de leurs différentes caractéristiques structurelles, morphologiques hydrothermal a révélé une augmentation de la porosité de l'oxyde de graphite et de l'oxyde de
	générant finalement des pores de dimensions nanométriques. légèrement influencées par le flux d'éthanol utilisé. percolation interconnecté, qui facilite la mobilité de l'électron en augmentant la conductivité et chimiques. L'étude de ces trois additifs a révélé que le développement de la porosité des graphite réduit, mais le noir de carbone n'a pas été affecté par le traitement hydrothermal. Ces
	De manière générale, les propriétés de matériaux carbonés -notamment leur texture électrique. Ainsi, en termes généraux, une taille de particule plus élevée du noir de carbone a composites ne serait pas directement corrélé à la porosité de l'additif. Le réseau poreux du variations du comportement du matériau sont liées à la nature différente des matériaux.
	poreuse et leur composition-dépendent des celles du matériau d'origine, mais aussi des produit la formation d'un chemin de percolation continu rendant les composites de gel de matériau final était principalement lié aux caractéristiques physico-chimiques de l'additif L'expansion de la structure de l'oxyde de graphène et de l'oxyde de graphène réduit, ainsi que
	carbone avec une conductivité électrique plus élevée. En outre, l'incorporation du noir de utilisé. Les groupes fonctionnels oxygénés de l'additif et sa capacité à modifier le pH du milieu la réduction partielle des couches de graphène ont permis l'augmentation des ces propriétés
	carbone a eu un impact important sur le développement du réseau mésoporeux, du réactif sont devenus des paramètres critiques puisqu'ils détermineront le développement texturales et suggèrent la possibilité d'introduire des agents de liaison entre les couches pour
	indépendamment des propriétés morphologiques du noir de carbone incorporé. de la texture du composite final. De plus, l'ordre et l'agrégation des additifs ont montré un stabiliser la structure poreuse.
	Pour réaliser l'étude sur l'influence de la quantité d'additif, l'un des noirs de carbone a été effet de la conductivité électrique des composites de gel de carbone. Lorsque la synthèse sol-gel a été réalisée dans des conditions hydrothermales, des
	sélectionné en raison de son ordre structurel et de sa conductivité élevée. À cette fin, des Enfin, l'un des graphènes synthétisés par plasma a aussi été sélectionné comme additif différences importantes ont été observées sur la porosité des gels vierges (sans additif),
	rapports de masse de noir de carbone compris entre 4 et 40 % en poids (grammes de CB par conducteur. Les composites de gel et de gel de carbone élaborés en incorporant 4 et 12 % en principalement dans le domaine des mésopores. D'autre part, la synthèse des composites
	gramme de réactifs) ont été utilisés. Au-delà de cette quantité, la dispersion du CB dans le poids de graphène comme additif au mélange RF ont révélé des conductivités similaires des gel/additif dans des conditions hydrothermales a révélé la formation de monolithes à deux
	mélange des réactifs devenait trop difficile. matériaux obtenus avec l'incorporation de noir de carbone, probablement associées à la phases due à la sédimentation de l'additif. Sur la base des résultats obtenus, il semble que
	La variabilité de la quantité de noir de carbone comme additif a montré un impact notable présence d'agrégats de graphène formés pendant la synthèse du composite. d'autres études doivent être menées pour améliorer la dispersion des additifs de carbone dans
	sur les caractéristiques poreuses des composites de gel de carbone, principalement dans la En conclusion, les additifs de carbone de nature hydrophobe agissent comme un porogène, les procèdes longs, afin d'éviter la sédimentation et l'obtention de matériaux hétérogènes.
	gamme des mésoporeux. Les isothermes d'adsorption/désorption de N 2 à -196 °C des favorisant le développement d'architectures de pores multimodales, avec une prédominance
	échantillons ont montré un point d'inflexion dans les deux branches d'adsorption et de des grands mésopores. Les additifs hydrophiles entravent la réaction de polycondensation,
	désorption, qui était plus marqué pour des quantités plus importantes de noir de carbone. La rendant les matériaux moins poreux. En ce qui concerne la conductivité, les composites ont
	caractérisation des matériaux a révélé que l'incorporation du noir de carbone a provoqué le montré des valeurs améliorées, avec des seuils de percolation d'environ 8 % en poids pour le
	développement d'un réseau mésoporeux interconnecté de cous et de corps rétrécis. En plus, la noir de carbone. En comparaison, des conductivités plus élevées ont été obtenues pour
	conductivité électrique a également été influencée, montrant une augmentation de la quantité l'utilisation de l'oxyde de graphène à 12 % en poids (ca. 0.48 S/cm) et du noir de carbone à 24
	d'additif incorporée suivant une tendance de percolation. et 40 % en poids (ca. 0.22 et 0.28 S/cm, respectivement). Dans l'ensemble, les résultats
	Dans le but d'étudier l'effet du caractère hydrophobe du noir de carbone utilisé comme montrent la préparation de carbones très poreux avec des architectures de mésopores
	additif, un des additifs de nature hydrophobe a été oxydé par la méthode Hummers afin contrôlées et une conductivité accrue. Cela facilite la préparation d'électrodes de carbone
	d'incorporer des fonctionnalités d'oxygène. Le caractère hydrophobe/hydrophile de l'additif a conductrices (soit sous forme monolithique, soit sous forme de poudre), qui sont des
	l'additif non seulement sur la conductivité du composite gel de carbone/additif, mais aussi sur donné lieu à un paramètre crucial pour les propriétés finales du matériau, tant en ce qui matériaux intéressants pour de multiples processus (par exemple, le stockage d'énergie, les
	le développement de la porosité pendant la synthèse. concerne le développement de la porosité que la conductivité électrique. L'incorporation de supercondensateurs, les capteurs électrochimiques).
	Tout d'abord, quatre noirs de carbone ont été sélectionnés comme additifs conducteurs sur noir de carbone oxydé au mélange de réactifs a provoqué une brusque diminution du pH du Pour confirmer l'impact bénéfique de l'incorporation des additifs au carbone sur la
	la base de leurs différences en termes de morphologie, de taille moyenne des agrégats de milieu réactionnel ; malgré la bonne dispersion de l'additif du caractère hydrophile (en raison conductivité électrique des composites de gel de carbone, la caractérisation électrochimique
	nanoparticules et d'ordre graphitique. Ces additifs de carbone ont été utilisés pour préparer un de son affinité avec le solvant de réaction) le composite obtenu ne présente pas un des échantillons a été lancée. Elle n'a pas été incluse dans le noyau du doctorat car les travaux
	gel et des composites gel de carbone/CB, en incorporant une quantité discrète de chaque noir développement de porosité accessible à l'azote à -196 °C. Même si la présence de expérimentaux n'ont pas été finalisés pour tous les matériaux préparés. Les électrodes des

Asimismo, se están explorando rutas xxx Resumen sintéticas de autoensamblado de moléculas orgánicas y grafeno oxidado, para obtener redes 3D porosas. Résumé xxxi RÉSUMÉ Cette thèse de doctorat a été développée dans le cadre d'une cotutelle internationale entre l'Université d'Orléans (France) et l'Université de Cordoue (Espagne). Conformément à l'accord régissant la supervision conjointe, la thèse a été rédigée et sera présentée en anglais; la soutenance orale aura lieu dans l'Université d'Orléans (aussi en anglais). leur réactivité chimique, qui permettent l'incorporation à la surface d'une grande variété d'hétéroatomes. Parmi eux, les matériaux carbonés poreux (ou nanoporeux) sont très utilisés dans de nombreuses applications électrochimiques dans les domaines des capteurs et du stockage et conversion d'énergie. Ce choix matériaux à base de carbone se porte sur leur compétitivité par rapport à leurs concurrents -les métaux ou les oxydes métalliques-, grâce à une très grande diversité des matériaux avec des réseaux de pores contrôlées combinées à des propriétés de volume et de surface adéquates ; en particulier, la stabilité chimique et mécanique, la biocompatibilité, la richesse de la chimie de surface. Néanmoins, les propriétés des matériaux carbonés à très haute porosité ne sont pas nécessairement optimales pour des applications électrochimiques, due à une conductivité électronique relativement faible. De manière générale, la cette caractéristique dépend fortement de la disposition spatiale des atomes de carbone (et notamment du procédé de fabrication et du traitement thermique), leur structure, composition, et porosité. L'Union Internationale de Chimie Pure et Appliquée (an anglais IUPAC) définit les matériaux poreux comme des matrices solides composées d'un réseau interconnecté de pores (vides) remplis d'un fluide (liquide ou gaz). Les matériaux carbonés peuvent être considérés comme des atomes de carbone regroupés en cycles aromatiques. De ce fait, ils ont une forte teneur en atomes de carbone hybridés sp 2 qui confèrent un ordre bidimensionnel dans les couche carbonées similaire à celles du graphène. Ces couches de carbone sont plus ou moins planes en fonctions de la présence de défauts de bordure dans les feuillets, du dégrée de graphitisation et de la présence des liaisons avec d'autres hétéroatomes (généralement à travers d'atomes de carbone hybridés sp 3 , avec rupture de la double liaison carbone-carbones de type sp 2 ) pour former des groupes fonctionnels de surface stables. Les couches de graphène sont connectées par des interactions de type Van der Waals soit dans une structure ordonnée (comme dans du graphite ou graphène) ou soit dans un xxxii Résumé empilement désorganisé des couches (structure turbostratique) comme dans la plupart des matériaux poreux. conditions de synthèse, avec une grande variabilité des propriétés obtenues pour des matériaux carbonés produits à partir du même type de précurseur initiale. Les pores des matériaux à base de carbone sont généralement dispersés sur un large éventail de formes et de tailles. En fonction de leur taille, les pores peuvent se classer en micropores (largeur de pore inférieure à 2 nm), mésopores (largeur entre 2 et 50 nm) et macropores (largeur de pore supérieure à 50 nm). Concernant les propriétés électroniques, certains matériaux carbonés comme le graphite, le graphène, les nanotubes de carbone) présentent des propriétés électroniques proches de celles des électrodes métalliques. Par contre, la plupart des matériaux carbonés nanoporeux présentent une conductivité très limité généralement de 4 à 5 ordres de grandeur inférieurs à ceux du graphite), en raison de son faible degré d'ordre structurel et d'une forte densité de défauts de bordure introduits dans les couches graphitiques tordues lors du développement du réseau de nanopores. En conséquence, la voie de mobilité des électrons caractéristique des feuilles de graphite est fortement réduite. Des efforts sont donc encore nécessaires pour préparer des carbones nanoporeux avec des architectures de pores élevés et bien définis combinant une conductivité électrique élevée sans compromettre la structure poreuse. Pour augmenter la conductivité des électrodes en carbone nanoporeux sans compromettre la porosité, plusieurs stratégies ont été explorées, telles que: i) le dopage du matériau en carbone avec des hétéroatomes (tels que O, N, P ou B); ii) le revêtement avec une phase conductrice telle que des nanoparticules métalliques, des métaux, d'autres nanostructures de carbone conductrices; iii) la synthèse d'architectures 3D de type graphène nanoporeux, et iv) l'incorporation d'un additif conducteur dans la formulation des encres des électrodes en carbone. Ce dernier point est la pratique courante pour la fabrication des électrodes à l'échelle commerciale dans la plupart des applications électrochimiques, le noir de carbone étant l'additif conducteur le plus populaire.

Résumé xxxiii

Des matériaux nanostructurés à base de carbone ont été sélectionnés comme additifs ou comme matériaux de départ pour la synthèse des matériaux de carbone nanoporeux à conductivité amélioré. Ils ont été choisis en fonction de leur composition, de leur morphologie et de leurs caractéristiques structurelles: noir de carbone (commercial), graphite (commercial), graphène, oxyde de graphène et oxyde de graphène réduit (commercial).

Dans ce sens, l'optimisation et l'analyse des conditions expérimentales pour la synthèse du graphène par plasma micro-ondes ont été réalisées. Comme on le trouve dans la littérature, les plasmas micro-ondes à pression atmosphérique ont montré leur capacité à synthétiser du graphène de haute qualité à partir de la décomposition de précurseurs organiques en l'absence de substrat. Généralement, un précurseur organique (gaz, phase liquide) est introduit dans un plasma micro-ondes à pression atmosphérique maintenu dans de l'argon en écoulement et contenu dans un réacteur. Un dépôt solide constitué de flocons de graphène se forme alors en aval et est collecté soit par un filtre, soit directement sur les parois du réacteur.

Ce processus se déroule en une seule étape, sans nécessiter la séparation du produit d'un substrat ou d'un solvant. En outre, aucun produit chimique dangereux supplémentaire n'est nécessaire, ce qui rend le processus plus respectueux de l'environnement que le LPE et le rGO.

L'éthanol était généralement utilisé comme précurseur ; bien que d'autres précurseurs (par exemple, le méthanol, l'éther diméthylique, l'isopropanol) aient été proposés comme alternative à l'éthanol pour alimenter une torche à plasma, seul l'éther diméthylique permet de produire du graphène, tandis que l'isopropanol ne produit que de la suie de carbone et que le méthanol ne produit aucun dépôt solide. L'un des principaux défis de cette technologie -en tant que technique ascendante-est d'augmenter le taux de production de graphène à des valeurs proches de celles des technologies descendantes. Une Torche à Injection Axiale sur Guide d'Ondes (TIAGO) à décharge micro-onde à pression atmosphérique a été utilisée, permettant la synthèse du graphène par décomposition de xxxiv Résumé l'éthanol. L'oxyde de graphène a également été synthétisé en appliquant une méthode Hummers modifiée. Cette méthode a permis l'exfoliation du graphite et a augmenté son caractère hydrophile par l'incorporation d'une variété de groupes d'oxygène à la surface ave une teneur en oxygène d'environ 40 %. La caractérisation par FTIR et l'analyse thermogravimétrique ont révélé la présence d'anhydrides carbonyles/groupes carboxyliques principalement.

Après la synthèse et la caractérisation des additifs de carbone, la première approche de la thèse a été développée en réalisant la synthèse des carbones nanoporeux par une réaction de polycondensation de mélanges de résorcinol et de formaldéhyde bien connus dans la littérature pour la préparation de gels de carbone. Ce protocole de synthèse a présenté quelques défis lorsque la polymérisation a été effectuée en présence des additifs, en raison de la sédimentation des particules pendant la polycondensation. Afin d'obtenir une bonne distribution des additifs de carbone, et des gels polymères et des gels de carbone homogènes, l'approche synthétique a été modifiée en incorporant une agitation mécanique pour améliorer la dispersion de l'additif pendant la réaction de polycondensation.

Cette approche permet d'obtenir des matériaux très poreux avec des propriétés accordables par rapport aux méthodes d'activation classiques (par exemple, activation physique et chimique d'un précurseur de carbone) qui rendent généralement des matériaux avec de larges distributions de taille de pores à la fois dans la gamme des micropores et des mésopores. Les architectures de pores bien contrôlées des gels de carbone font de ce type de matériaux d'excellents matériaux pour les applications électrochimiques. En outre, l'effet de divers paramètres tels que le type, la quantité, le caractère hydrophobe/hydrophile et la composition de l'additif ont été étudiés. L'étude a révélé le rôle important de la nature de

Table IV .

 IV 2. Main structural parameters of the studied carbon blacks, obtained from the analysis of the deconvoluted Raman spectra.

	Sample	D shift

(cm -1 ) G shift (cm -1 ) 2D shift (cm -1 ) D FWHM (cm -1 ) G FWHM (cm -1 ) 2D FWHM (cm -1 )

								I D /I G	I 2D /I G
	CB-I	1349	1589	2697	112	79	207	1.37	0.22
	CB-II	1350	1591	2689	114	80	191	1.36	0.22
	CB-III	1344	1572	2683	51	40	74	0.45	0.72
	CB-IV	1356	1590	2709	157	81	270	1.53	0.20

Table IV

 IV

.3. Main structural parameters of the studied carbon blacks obtained from X-ray diffraction patterns.

Sample d 002 (nm) L c (002) (nm) L a (10)

				(nm)
	CB-I	0.355	2.066	1.982
	CB-II	0.358	1.998	2.153
	CB-III	0.345	7.290	5.308
	CB-IV	0.356	2.104	1.437

Table IV .

 IV 4. Main textural parameters of the carbon blacks obtained from the analysis of the N 2 adsorption isotherms at -196 °C.

	Sample	S BET (m 2 /g)	V PORES (cm 3 /g) a	W 0 (cm 3 /g) b	V MICRO (cm 3 /g) c	c /g) V MESO (cm 3
	CB-I	62	0.179	0.023	0.012	0.145
	CB-II	62	0.176	0.024	0.011	0.151
	CB-III	40	0.161	0.016	0.008	0.131
	CB-IV	32	0.147	0.014	0.005	0.135

a Evaluated at p/p 0  0.99; b evaluated by the DR method; c evaluated by the 2D-NLDFT-HS method

Table IV .

 IV 5. Main textural parameters of the materials synthesised with different amounts of CB additive obtained from the analysis of the N 2 adsorption isotherms at -196 °C.

	Sample	S BET (m 2 /g)	V PORES (cm 3 /g) a	W 0 (cm 3 /g) b	V MICRO (cm 3 /g) c	V MESO (cm 3 /g) c	S BET (Theoretical) (m 2 /g)	d
	CB-I	62	0.179	0.023	0.012	0.145	--
	CB-II	62	0.176	0.024	0.011	0.151	--
	CB-III	40	0.161	0.016	0.008	0.131	--
	CB-IV	32	0.147	0.014	0.005	0.135	--
	PG	384	0.377	0.120	0.091	0.266	--
	PG-CB-I12	343	0.562	0.124	0.068	0.477	345
	PG-CB-II12	331	0.576	0.118	0.064	0.496	345
	PG-CB-III12	333	0.456	0.089	0.070	0.370	343
	PG-CB-IV12	333	0.495	0.106	0.066	0.419	342
	CPG	829	0.488	0.306	0.289	0.190	--
	CPG-CB-I12	716	0.775	0.257	0.228	0.535	737
	CPG-CB-II12	688	0.788	0.259	0.211	0.565	737
	CPG-CB-III12	726	0.636	0.253	0.229	0.395	734
	CPG-CB-IV12	708	0.654	0.250	0.232	0.404	733
	a Evaluated at p/p 0  0.99;	b evaluated by the DR method;	c evaluated by the 2D-NLDFT-HS method;	d estimated
	by rule of mixtures

Table IV . 6 .

 IV6 Main structural parameters of the series of carbon gel composites prepared with different carbon blacks, obtained from the analysis of the deconvoluted Raman spectra.

	Sample	D shift (cm -1 )	G shift (cm -1 )	2D shift (cm -1 )	D FWHM (cm -1 )	G FWHM (cm -1 )	2D FWHM (cm -1 )	I D /I G I 2D /I G
	CPG	1352	1591	2702	145	59	232	1.48	0.23
	CPG-CB-I12	1347	1597	2699	146	75	269	1.29	0.22
	CPG-CB-II12	1350	1592	2705	147	73	224	1.26	0.21
	CPG-CB-III12	1345	1591	2699	135	71	88	0.90	0.38
	CPG-CB-IV12	1351	1595	2699	157	73	215	1.51	0.17

Table IV .

 IV 7. Mass losses obtained from the thermogravimetric profiles and during the carbonization process of the series of materials prepared with different amount of CB-III additive. The theoretical mass loss for each composite is also included.

	Sample	Total mass loss (%) a	Theoretical mass loss (%) b	Carbonization loss (%) c
	PG	46.1	--	49.2
	PG-CB-III4	42.9	44.3	47.9
	PG-CB-III12	40.6	40.6	45.4
	PG-CB-III40	30.3	27.7	32.6
	CB-III	0.2	--	3.2

a Evaluated from TG analysis; b evaluated by rule of mixture; c experimental loss during the carbonization process

Table IV .8.

 IV Main structural parameters of the series of materials with different amount of CB-III additive, obtained from the analysis of deconvoluted Raman spectra.

	Sample	D shift (cm -1 )	G shift (cm -1 )	2D shift (cm -1 )	D FWHM (cm -1 )	G FWHM (cm -1 )	2D FWHM (cm -1 )	I D /I G	I 2D /I G
	CPG	1352	1591	2702	145	59	232	1.48	0.23
	CPG-CB-III4	1342	1589	2702	129	65	123	1.08	0.25
	CPG-CB-III16	1345	1589	2703	125	66	82	0.82	0.38
	Similar observations were gathered by XRD patterns of the materials, shown in Figure IV.24.

Table IV .

 IV 10. Elemental analysis, surface pH and acidity of samples CB-III and CB-IIIox.

	Sample	C (wt.%)	H (wt.%)	O (wt.%)	S (wt.%)	Surface pH	Acidity (mmol H + /g)
	CB-III	99.4	< 0.05	< 0.1	< 0.1	6.23	0.49
	CB-IIIox	65.4	< 0.05	26.9	6.1	2.64	2.21

Table IV .

 IV 11. Main structural parameters of samples CB-III and CB-IIIox obtained from the analysis of deconvoluted Raman spectra.

	Sample	D shift (cm -1 )	G shift (cm -1 )	2D shift (cm -1 )	D FWHM (cm -1 )	G FWHM (cm -1 )	2D FWHM (cm -1 )	I D /I G	I 2D /I G
	CB-III	1344	1572	2683	51	40	74	0.45	0.72
	CB-IIIox	1350	1580	2692	78	55	94	0.97	0.60

Table IV .

 IV 14. Pellet's thickness and conductivity values obtained by 4-point probe method for the carbon gel/additive composites prepared with CB-III and CB-IIIox as additives.

	Sample	CB-III CB-IIIox CPG CPG-CB-III4 CPG-CB-IIIox4 CPG-CB-III12 CPG-CB-IIIox12
	Pellet's thickness (mm)	0.141	0.137	0.132	0.099	0.093	0.097	0.091
	Conductivity (S/cm)	1.640	b.d.l.	0.022	0.058	0.047	0.104	0.158

b.d.l.: below the detection limit

Table IV .

 IV 15. Main structural parameters of samples Gr, GO and rGO obtained from the analysis of deconvoluted Raman spectra.

	Sample	D shift (cm -1 )	G shift (cm -1 )	2D shift (cm -1 )	D FWHM (cm -1 )	G FWHM (cm -1 )	2D FWHM (cm -1 )	I D /I G	I 2D /I G
	Gr	1355	1582	2712	50	20	69	0.11	0.55
	GO	1359	1593	2718	137	63	212	1.31	0.19
	rGO	1353	1590	2702	92	56	200	1.55	0.19
	These differences were also observed in the XRD patterns of the samples (Figure IV.45). The
	most remarkable profile was that obtained for Gr, presenting a narrow peak centred at 26.2 °
	associated to the (002) plane with an interplanar distance (d 002 ) of 0.339 nm (Table IV.16),
	characteristic of graphites [

Table IV .

 IV 16. Main structural parameters of samples Gr, GO and rGO obtained from X-ray diffraction patterns.

	Sample	Shift (002) (2 θ)	d 002 (nm)	L c (002) (nm)	L a (10) (nm)
	Gr	26.2	0.339	19.0	20.9
	GO	9.7	0.914	10.5	7.6
	rGO	24.3	0.366	0.93	5.1

Table IV .

 IV 17. Elemental analysis and speciation of surface groups obtained from potentiometric titration for samples Gr, GO and rGO.

		Elemental analysis		Potentiometric titration
	Sample	C (wt.%)	O (wt.%)	S (wt.%)	pH (surface)	pKa 4-6 carboxyl (mmol H + /g)	pKa 7-8 lactones (mmol H + /g)	pKa 9-11 phenols (mmol H + /g)	Conc.TOTAL (mmol H + /g)
	Gr	99.5	< 0.1	< 0.1	7.3	--	--	--	--
	GO	51.6	40.3	3.3	2.9	0.5	0.7	2.0	3.2
	rGO	83.0	12.2	< 0.1	4.7	0.1	0.3	0.7	1.1

Table IV .

 IV 18. Main textural parameters for sample Gr, GO and rGO obtained from the N 2 and CO 2 adsorption isotherms at -196 °C and 0 °C, respectively.

Table IV .

 IV 19. pH of the reactants' mixture and main textural parameters obtained from the N 2 adsorption isotherms at -196 °C for Gr, GO and rGO additives and the series of materials synthesised with different amounts of additive.

Table IV .

 IV 20. Main structural parameters of the series of samples prepared using Gr, GO and rGO additives obtained from the analysis of deconvoluted Raman spectra.

	Sample	D shift (cm -1 )	G shift (cm -1 )	2D shift (cm -1 )	I D /I G	I 2D /I G
	CPG-Gr12	1348	1590	2706	1.13	0.25
	CPG-GO12	1347	1592	2700	1.42	0.17
	CPG-rGO12	1353	1591	2701	1.41	0.15

Table IV .

 IV 21. Molar production rate of CO, C 2 H 2 , CH 4 , C 2 H 4 and C (s) as function of the flow of the carbon precursor, EtOH.

					Molar production (x10 -2 mmol/min)
	Sample	Fl (EtOH) (g/h)	Production rate (mg/min)	CO	C 2 H 2	CH 4	C 2 H 4	C (s)
	G-I	2.00	1.08 ± 0.07	68.4 ± 0.2	26.6 ± 0.1	2.53 ± 0.02	3.1 ± 08	8.9 ± 0.6
	G-II	2.90	1.45 ± 0.04	104.4 ± 0.4 39.4 ± 0.2	2.35 ± 0.05	6.6 ± 0.2	12.1 ± 0.4
	G-III	3.40	1.55 ± 0.04	120.2 ± 0.6 48.8 ± 0.3	5.52 ± 0.08	8.5 ± 0.6	12.9 ± 0.4
	G-IV	3.80	1.43 ± 0.07	132.4 ± 0.6 53.2 ± 0.4	8.28 ± 0.02	11.3 ± 0.2 11.9 ± 0.6
	G-V	4.00	1.17 ± 0.03	130.5 ± 0.6 59.2 ± 0.5	11.28 ± 0.05 12.6 ± 0.2	9.8 ± 0.2

  Correlation between the atomic carbon content in the reactant, EtOH (green line), and in that estimated from the composition of the carbon-based by-products considering two scenarios: CO, C 2 H 2 , CH 4 , C (s) and C 2 H 4 (blue line) and CO, C 2 H 2 and C (s) (brown line).

	a)		4.50
			4.00
			3.50
	n i /n EtOH	1.00 1.10 2.50 3.00	15.2 %
			0.90	9.4 %
			0.80			H 2	CO
	Figure IV.64.	2.0	2.5	3.0 Fl(EtOH) (g/h)	3.5	4.0
	b)	0.39 0.42	10.3 %
		0.36
	n i /n EtOH	0.09 0.12	45.6 %
		0.06	71.3 %
							51.0 %
		0.03
		0.00		C 2 H 2	CH 4	C (s)	C 2 H 4
				2.0	2.5	3.0	3.5	4.0
					Fl(EtOH) (g/h)

IV.D.2. Characterisation of the graphenes.

Table IV .

 IV 22.Main structural parameters of the graphenes prepared using different ethanol flows obtained from X-ray diffraction patterns.

	Sample	2θ (002) (degrees)	d 002 (nm)	L c (002) (nm)	L a (100) (nm)
	G-I	25.8	0.345	4.108	15.283
	G-II	26.0	0.343	4.257	17.542
	G-III	25.9	0.344	4.195	14.103
	G-IV	25.9	0.343	4.120	11.217
	G-V	25.9	0.344	4.101	13.661

Table IV .

 IV 23.Main structural parameters of the graphenes prepared using different ethanol flows obtained from the analysis of deconvoluted Raman spectra.

	Sample	D shift (cm -1 )	G shift (cm -1 )	2D shift (cm -1 )	D FWHM (cm -1 )	G FWHM (cm -1 )	2D FWHM (cm -1 )	I D /I G	I 2D /I G
	G-I	1351	1579	2698	44	31	58	0.26	0.93
	G-II	1346	1578	2694	44	29	56	0.20	1.02
	G-III	1348	1576	2691	45	29	61	0.24	1.00
	G-IV	1349	1578	2695	52	29	58	0.27	1.03
	G-V	1345	1574	2689	54	31	64	0.26	0.91

Table IV .

 IV 24. Data obtained from XPS analysis for the graphenes prepared using different ethanol flows.

	Sample	Fl(EtOH) (g/h)	C (at. %)	O (at. %)	C/O ratio	% Area C sp 2	% Area C sp 3	sp ratio 2 /sp	3
	G-I	2.00	96.6	3.4	28.4	58.76	15.55	3.78
	G-II	2.90	98.3	1.7	57.8	63.31	12.45	5.09
	G-III	3.40	98.5	1.5	65.7	63.11	13.52	4.67
	G-IV	3.80	98.0	2.0	49.0	57.74	15.39	3.75
	G-V	4.00	95.8	4.2	22.8	52.36	14.97	3.50
	Data extracted from deconvoluted C 1s spectra.

Table IV .

 IV 25. Main textural parameters obtained from the N 2 adsorption isotherms at -196 °C for the series of materials synthesised with different amounts of graphene additive.

Table IV .

 IV 26. Main structural parameters for the series of materials synthesised with different amounts of graphene additive obtained from the analysis of deconvoluted Raman spectra.

	Sample	D shift (cm -1 )	G shift (cm -1 )	2D shift (cm -1 )	D FWHM (cm -1 )	G FWHM (cm -1 )	2D FWHM (cm -1 )	I D /I G	I 2D /I G
	CPG	1352	1591	2702	145	59	232	1.48	0.23
	CPG-G-I4	1350	1590	2705	126	57	72	0.76	0.43
	CPG-G-I12	1346	1585	2700	120	55	67	0.69	0.60
	G-I	1349	1578	2698	44	31	58	0.23	0.97

Table IV .

 IV 27. Main textural parameters obtained from the N 2 adsorption isotherms at -196 °C for the pristine additives and their corresponding samples after the hydrothermal treatment.

	Sample	S BET (m 2 /g)	V PORES (cm 3 /g) a	W 0 (cm 3 /g) b	V MICRO (cm 3 /g) c	c /g) V MESO (cm 3
	CB-III	40	0.161	0.016	0.008	0.131
	HS-CB-III	44	0.210	0.020	0.025	0.242
	GO	10	0.023	0.001	0.001	0.019
	HS-GO	108	0.143	0.028	0.016	0.118
	rGO	473	1.108	0.165	0.074	0.968
	HS-rGO	355	0.594	0.142	0.075	0.513

a Evaluated at p/p 0  0.99; b evaluated by the DR method; c evaluated by the 2D-NLDFT-HS method

Table IV .

 IV 28. Main textural parameters obtained from the N 2 adsorption isotherms at -196 °C for samples PG and HS-PG.

  J. Anal. Appl. Pyrolysis 2016, 119, 60-68. Fernández de Córdoba, M.C.; Balan, L.; Ania, C.O. Light-induced synthesis of ordered mesoporous carbons. Proceedings of the Int. Carbon Conference 2019, Lexington (USA) 15 th -19 th July, 2019. Ferrari, A.C.; Meyer, J.C.; Scardaci, V.; Casiragui, C.; Lazzeri, M.; Mauri, F.; Piscanec, S.; Jiang, D.; Novoselov, K.S.; Roth, S.; Geim, A.K. Raman spectrum of graphene and graphene layers. PRL 2006, 97, 187401. Ferrari, A.C.; Robertson, J. Interpretation of Raman spectra of disordered and amorphous carbon. Phys. Rev. B. 2000, 61, 14095. Figini-Albisetti, A.; Velasco, L.F.; Parra, J.B.; Ania, C.O. Effect of outgassing temperature on the performance of porous materials. Appl. Surf. Sci. 2010, 256, 5182-5186. Kim, Y.S.; Lee, J.H.; Kim, Y.D.; Jerng, S.-K.; Joo, K.; Kim, E.; Jung, J.; Yoon, E.; Park, Y.D.; Seoab, S.; Chun, S.-H. Methane as an effective hydrogen source for single-layer graphene synthesis on Cu foil by plasma enhanced chemical vapor deposition. Nanoscale 2013, 5, 1221-1226.

Franklin, R.E. The structure of graphitic carbons. Acta Cryst. 1951, 4, 253.

K

Table A .

 A I.1. Raman parameters obtained from the deconvoluted spectra for the carbon blacks used as additives. Deconvolution of Raman spectrum of pristine carbon gel (sample CPG).

			CB-I			CB-II
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1212	662	281	1202	372	255
	D	1349	4688	112	1350	3960	114
	D´´	1524	1687	215	1521	1090	207
	G	1589	3427	79	1591	2185	80
	D´	1605	644	44	1607	325	46
	G´	2507	258	300	2500	149	273
	2D	2696	736	207	2689	487	190
	D+D´ Figure A.I.2.	2916	719	224	2908	453	229
	2D´	3163	134	203	3157	91	204
			CB-III			CB-IV
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1148	96	183	1223	1707	262
	D	1344	3148	50	1356	4462	157
	D´´	1471	152	174	1534	3505	203
	G	1572	7040	40	1590	2912	81
	D´	1606	551	12	1604	873	46
	G´	2431	218	64	2511	302	349
	2D	2683	5035	74	2709	589	270
	D+D´	2916	270	76	2913	638	241
	2D´	3214	241	48	3121	256	251

Table A .

 A I.2. Raman parameters obtained from the deconvoluted spectra for the pristine carbon gel.

	(a.u.)
	Intensity
	800 1100 1400 1700 2000 2300 2600 2900 3200 3500
			Raman shift (cm -1 )
			CPG
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1229	4857	222
	D	1352	13256	145
	D´´	1519	8125	167
	G	1591	8955	59
	D´	1617	8386	54
	G´	2515	976	291
	2D	2702	2073	232
	D+D´	2918	2892	251
	2D´	3158	993	201

Table A .

 A I.3. Raman parameters obtained from the deconvoluted spectra for the carbon gel/CB composites prepared with 12 wt.%.

			CPG-CB-I12			CPG-CB-II12
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1218	14	207	1225	2715	264
	D	1347	48	146	1350	10221	147
	D´´	1520	28	189	1528	5313	202
	G	1597	38	75	1592	8131	73
	D´	1607	12	43	1610	3123	46
	G´	2546	2	271	2539	1022	329
	2D	2699	8	269	2705	1698	224
	D+D´	2918	9	226	2910	2502	232
	2D´	3131	4	210	3138	1053	229
			CPG-CB-III12			CPG-CB-IV12
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1215	5	188	1204	18	197
	D	1345	17	135	1351	62	157
	D´´	1505	9	189	1522	35	167
	G	1591	20	71	1595	41	73
	D´	1609	2	42	1612	23	61
	G´	2556	2	307	2527	5	291
	2D	2699	7	88	2699	7	215
	D+D´	2915	4	259	2916	12	278
	2D´	3167	1	178	3162	4	177

Table A .

 A I.5. Raman parameters obtained from the deconvoluted spectra for the carbon gel/CB-III composites.

			CPG-CB-III4			CPG-CB-III16
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1228	15	208	1235	24	213
	D	1342	37	129	1345	60	126
	D´´	1504	22	189	1499	36	173
	G	1589	34	65	1589	72	67
	D´	1610	16	52	1617	16	67
	G´	2558	4	251	2561	7	283
	2D	2702	9	123	2702	27	82
	D+D´	2917	10	279	2923	15	276
	2D´	3179	3	149	3184	6	164

Table A .

 A I.6. Thickness, diameter and mass of the pellets of carbon gel/CB-III composites with various amounts of additive used for the conductivity measurements. Conductivity values obtained by 4-point probe method.

	Sample

Conductivity (S/cm) Thickness (cm) Diameter (cm) Mass (mg)

	CPG	0.022	0.0132	1.01	10.05
	CPG-CB-III4	0.058	0.0099	1.03	7.1
	CPG-CB-III8	0.064	0.0115	1.03	8.2
	CPG-CB-III12	0.104	0.0097	1.02	7.8
	CPG-CB-III16	0.159	0.0113	1.03	8.5
	CPG-CB-III24	0.220	0.0126	1.02	8.1
	CPG-CB-III40	0.273	0.0162	1.03	8.8
	CB-III	1.639	0.0213	1.03	7.6
	Values represent the average of 4 measurements.

Table A .

 A I.8. Raman parameters obtained from the deconvoluted spectra for the carbon gel/CB-IIIox composites.

			CPG-CB-IIIox4			CPG-CB-IIIox12
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1249	5	232	1257	10	236
	D	1352	9	138	1350	18	123
	D´´	1510	7	172	1506	13	190
	G	1592	8	75	1590	19	73
	D´	1612	5	54	1611	6	43
	G´	2548	1	293	2557	2	279
	2D	2698	1	228	2698	4	131
	D+D´	2924	2	304	2918	4	286
	2D´	3184	1	148	3174	1	150

Table A .

 A I.9. Raman parameters obtained from the deconvoluted spectra for graphite, graphene oxide and reduced graphene oxide

			Gr			GO			rGO
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1232	22	297	1153	1024	181	1167	462	206
	D	1355	1041	50	1359	29086	137	1353	16187	92
	D´´	1492	41	39	1530	8188	133	1537		140
	G	1582	9282	20	1593	22234	63	1590	10415	56
	D´	1613	246	33	1616	13547	43	1619		29
	G´	2456	313	59	2537	1129	221	2492	379	222
	2D	2712	5104	69	2718	4323	212	2701		200
	D+D´	2940	122	83	2941	6372	212	2930		180
	2D´	3244	632	22	3168	2569	153	3156	780	179

Table A .

 A I.10. Raman parameters obtained from the deconvoluted spectra for the composites prepared with 12 wt.% of graphite, graphene oxide and reduced graphene oxide.

			CPG-Gr12			CPG-GO12			CPG-rGO12
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1214	10	199	1223	4	205	1201	9	193
	D	1348	36	150	1347	13	141	1353	38	161
	D´´	1519	20	170	1515	8	180	1504	16	113
	G	1589	32	60	1592	9	71	1591	27	86
	D´	1620	14	53	1610	5	57	1608	20	59
	G´	2549	3	270	2543	1	343	2526	2	296
	2D	2706	8	122	2700	2	229	2701	4	252
	D+D´	2919	8	294	2923	3	255	2932	7	293
	2D´	3176	2	156	3162	1	173	3175	2	151

Table A .

 A I.11. Thickness, diameter and mass of the pellets of composites prepared with 4 and 12 wt.% of graphite, graphene oxide and reduced graphene oxide used for the conductivity measurements. Conductivity values obtained by 4-point probe method. Data of their corresponding controls is also shown.

	Sample

Conductivity (S/cm) Thickness (cm) Diameter (cm) Mass (mg)

	CPG	0.022	0.0132	1.01	10.1
	CPG-Gr4	0.113	0.0099	1.03	10.4
	CPG-Gr12	0.340	0.0124	1.02	9.2
	Gr	19.766	0.0290	1.01	9.5
	CPG-GO4	0.100	0.0077	1.00	6.4
	CPG-GO12	0.480	0.0101	1.03	6.7
	GO	b.d.l.	0.0111	1.02	8.8
	CPG-rGO4	0.048	0.0124	1.01	7.9
	CPG-rGO12	0.131	0.0165	1.02	7.6
	rGO	2.623	0.0041	1.00	8.2

Values represent the average of 4 measurements. (b.d.l.: below the detection limit)

Table A .

 A I.12. Structure, fragmentation peaks and peaks' relative intensity of selected molecules.

	Species	Structure	Fragmentation peaks	Relative intensity
	CH 2 CH 5 OH, ethanol		31	100
			45	51.4
			30	8.2
	H 2 O, water		18	100
			17	21.2
			16	0.9
	CO, carbon monoxide		28	100
			12	4.7
			16	1.7
			29	1.1
	CO 2 , carbon dioxide		44	100
			16	9.6
			28	9.8
	HCN, hydrogen		27	100
	cyanide		26	16.8
	C 2 H 2 , acetylene		26	100
			25	19.2
			24	5.0
	C 2 H 4 , ethylene		28	100
			27	64.8
			26	62.3
			25	11.7
			24	3.7
	C 2 H 6 , ethane		28	100
			27	33.3
			30	26.2
			29	21.7
	C 4 H 2 , diacetylene		50	100
			49	36.0
			48	8.4
	C 6 H 6 , benzene		78	100
			77	28.4

Table A .

 A I.14. Raman parameters obtained from the deconvoluted the spectra for the carbon gel/G-I composites.

			CPG-G-I4			CPG-G-I12
	Band	Shift (cm -1 )	Intensity	FWHM (cm -1 )	Shift (cm -1 )	Intensity	FWHM (cm -1 )
	D*	1259	9	240	1240	11	227
	D	1350	19	126	1346	25	119
	D´´	1512	12	199	1498	15	188
	G	1590	24	57	1585	35	54
	D´	1622	9	42	1620	13	47
	G´	2537	2	290	2546	3	278
	2D	2705	10	72	2700	21	67
	D+D´	2919	5	305	2921	7	265
	2D´	3182	1	144	3186	3	158

Table A .

 A I.15. Thickness, diameter and mass of the pellets of the composites prepared with sample G-I as additive. Conductivity values obtained by 4-point probe method. The corresponding controls are also shown for comparison.

	Sample

Conductivity (S/cm) Thickness (cm) Diameter (cm) Mass (mg)

	CPG	0.022	0.0132	1.01	10.05
	CPG-G-I4	0.060	0.0110	1.01	7.1
	CPG-G-I12	0.116	0.0102	1.00	6.2
	G-I	7.184	0.0091	1.03	3.6

Vol. (cm 3 /g) Pore size (nm)

/g,STP)Relative pressure (p/p 0 ) PG (w/o stirring) PG (stirring)

/g,STP)Relative pressure (p/p 0 )CB-I CB-II CB-III CB-IV

/g), experimenalV 1 (cm 3 /g), rule of mixtures

/g)Pore size (nm)

Annex II: Publications

Acknowledgements

larger than that of PG. The desorption branch of HS-PG displayed a hysteresis loop spanning from 0.7 to 0.9, indicating the enlargement of the mesopores -compared to PG-. Indeed, the mesopore volume obtained applying the 2D-NLDFT-HS method revealed an increase from 0.266 to 0.904 cm 3 /g, with an average mesopore size of 4.5 nm for PG and 13.5 nm for HS-PG as can be inferred from the PSD (Figure IV.80). In sum, these findings revealed that the application of moderate pressure during the polycondensation reaction favoured the development of large mesopores, while the microporosity remained quite unaffected.

For the textural characterisation of the gel/additive composites, the transversal cut sections of the monoliths were analysed separately (Figure IV.81). In the case of materials synthesised with carbon black as additive, clear differences were observed between the isotherms of the upper and lower fractions. Although both fractions displayed similar isotherm shapes (Type IVa), the position and shape of the hysteresis loop is different. For the upper fraction, the hysteresis loops spanned from 0.6 to 0.9 of relative pressures, and shows almost parallel adsorption/desorption branches, characteristic of materials with a narrow distribution of pore sizes. This isotherm shape resulted very similar to that obtained for the pristine gel (HS-PG), however the hysteresis loop is shifted to lower relative pressures and presented lower total pore volume, indicating that the upper section is not only constituted by the pristine gel (as occurred in the case of synthesis without mechanical stirring at atmospheric pressure described in the Section IV.A). For the lower fraction, the hysteresis loop spanned from 0.8 to 1 of relative pressure and showed the characteristic inflection point in the desorption branch, as similarly observed for the gel/additive composites obtained at atmospheric pressure (Section IV.A.2, sample PG-CB-III8 (lower)).

Por. Vol. (cm