Introduction

Introduction

Challenges in advanced energy storage devices such as batteries and supercapacitors have led to many recent studies on nanostructured electrode materials associated to various applications. The physical properties such as large specific surface area and a suitable pore size of these nanostructured materials must be optimized in order to improve the performance of these energy storage devices. Nevertheless, these goals even if they are achieved are not sufficient to obtain perfect devices with high performance. Therefore, it is highly significant to meticulously study and understand the electrochemical mechanism related to the ionic transfer between the electrode and the electrolyte which constitutes an important part of the operating principle.

Electrochemical double layer capacitors (EDLC) store charge at the electrode/electrolyte interface based on a reversible adsorption of ions. The EDLCs have been launched using porous carbon electrodes. A marked improvement in performance has been obtained thanks to recent progress in the understanding of the charge storage mechanisms and the development of nanostructured materials with high specific surface area and sophisticated structures.

Nevertheless, even when sophisticated methods of investigations are used, a fair exploration of the different mechanisms of transfer related to the charge storage in a certain material is not as easy as envisaged. For example, when certain coupled methods are used, basically current and gravimetric measurements under cycling potentials, the determination of the species involved in the electrochemical process can be challenging or even questionable. In this case, unreasonable assumptions must be done such as a large contribution of the solvent or the cancellation of the anion contribution. 1,2 Another legitimate question can be raised on the efficiency of these nanostructured films and the utility of a large surface specific area associated to well-constructed pores and their accessibility to ions.

To answer these key questions, a non-conventional coupled electrochemical and gravimetric method, specifically the electrochemical impedance spectroscopy (EIS) coupled to a fast quartz crystal microbalance (QCM) also called ac-electrogravimetry, will be used to study the capacitive charge storage behavior of nanostructured carbon electrodes. Indeed, ion transfer phenomena will condition largely the performances of these materials for energy storage applications. Ac-electrogravimetry methodology was largely used for characterizing redox materials such as Prussian Blue or conducting polymers such as polypyrrole or polyaniline. [3][4][5][6] It was demonstrated that highly relevant information can be obtained: (i) kinetics and identification of species transferred between the electrode and the electrolyte, (ii) separation of the different contributions related to the charged and non-charged species involved in the electrochemical processes, and (iii) variation of the relative concentrations of the species inside the examined material. Therefore, the ac-electrogravimetry methodology appears to be an attractive tool to study the nature of the ionic fluxes located at the electrode/electrolyte interface. This Ph.D. thesis aims to clarify the relationship between morphology/structure of the prepared carbon films with the motion of ions and free solvent, simply to obtain a better understanding of the mechanisms of ion/solvent transfer and thus, to validate the positive impact of employment of nanostructured electrodes in energy storage devices.

This manuscript is divided into five chapters; experimental results are presented in the last three:

 Chapter I begins with a comprehensive literature review of different energy storage devices, i.e. batteries and supercapacitors, including history, principle of operation and different applications. More details are given on the components of these systems in terms of materials and electrolytes, and their impact on the supercapacitor performances is discussed. Thereafter, a literature review of different techniques used for characterizing and evaluating the performances of carbon based electrodes in energy storage is provided. Finally, the scope and objectives, starting from the state of art of the present Ph.D. thesis, are introduced.

 Chapter II begins with a detailed description of sample preparation. Mainly three kinds of samples were prepared: Carbon nanotubes (CNTs), Carbon Nanocomposites (CNT/PB or PPY) and Electrochemically Reduced Graphene Oxide (ERGO). Thereafter follows a description of the methods used for the structural and morphological characterization of samples, particularly, the principle of operation of the methods, the type of instruments used and under which conditions the experiments were performed. Finally, the electrochemical characterization techniques applied in this Ph.D. thesis will be presented/described.  Chapter III aims to clarify the mechanism of the ion transfer of carbon-based electrodes in order to have a better understanding of the charge storage mechanisms and to validate the positive impact of the use of nanostructured electrodes in energy storage devices. Here, a pertinent example is presented and detailed to demonstrate the power of our approach for single wall CNT (SWCNT) films tested in aqueous NaCl electrolyte. At the end, a key question, which is on the possibility of deconvolution of the classical EQCM responses, was raised by using ac-electrogravimetry methodology.

 Chapter IV is dedicated to the study of the influence of the CNT structure (single wall, double walls and multi walls) and electrolyte properties on ion electroadsorption dynamics at the electrode/electrolyte interfaces. The influence of electrolyte pH on the capacitive charge storage behavior is also studied in detail. For these kinds of studies, morphological and structural characterization methods (FEG-SEM, HR-TEM, XRD, N2 sorption, Raman spectroscopy), and electrochemical methods such as EQCM and ac-electrogravimetry were employed.

 Chapter V is dedicated to the interpretation of the results obtained from the study of SWCNT/Prussian Blue and SWCNT/Polypyrrole composite electrodes and ERGO thin film electrodes. This approach was examined in order to increase the performances of the pure CNT films either by elaboration of composite electrodes comprised of CNT and redox films or by employing alternative graphene like materials such as ERGO structures. Firstly, the morphological and structural analyses results from the FEG-SEM, HRTEM, EDX and XRD are presented and discussed. Thereafter follows the discussion of the results from the EQCM and the acelectrogravimetry studies in order to reach a profound examination of the electrochemical behavior of such layers.

Chapter I. Bibliography

CHAPTER I Bibliography

Chapter 1 begins with a comprehensive literature review of different energy storage devices like batteries and supercapacitors including history, principle of operation and different applications. More details are given about the components of these systems in terms of materials and electrolytes, and their impact on (supercapacitor) performance is discussed. Thereafter, a literature review of different techniques used for characterization and evaluation of performance of carbon based electrodes in energy storage is provided. Finally, the scope and objectives, starting from the state of art of the present Ph.D. thesis, are introduced.

I-1. Energy Storage: Batteries and Supercapacitors

Electrical energy storage can be divided in two different devicesbatteries and supercapacitors. In batteries, the electrochemical processes give rise to redox reactions, which will be discussed in Section I-1.1. Capacitors where in priori no redox reactions take place (electrical double layer capacitors (EDLCs) and the systems involving both capacitive and redox processes (pseudo-capacitors) will be discussed in Section I-1.2.

I-1.1. Batteries

A battery is an electrochemical device that transforms stored electrochemical energy into electric energy through a redox reaction (oxidation/reduction). 7,8 There are two general classes of batteries: primary batteries (the chemical energy stored in the cell can be used only once) and secondary or rechargeable batteries (which can be recharged; i.e. the redox reaction can be reversed by supplying electric energy to the cell, approximately restoring the original composition of the battery/cell). 9,10 Both primary and secondary batteries are composed of two electrodes (one is called the anode and the other is called the cathode), electrolytes, separators and current collectors. 7

I-1.1.1. History

It can be considered that the history of the modern battery started with the scientific studies of electricity in the 17 th and 18 th centuries by scientists such as C. Du Fay, B. Franklin and L. Galvani.

In 1799, the Italian physicist A. Volta invented the first electrochemical cell, the Voltaic Pile.

It consisted of zinc and copper or silver discs, alternately stacked and separated by brine-soaked felt spacers or cardboard. Although there had been other devices that could produce electricity, the Voltaic Pile was the first to generate a stable and continuous current. 11,12 In 1866, G. Leclanché, developed and patented a battery with electrodes based on zinc, carbon and manganese dioxide and with a solution of ammonium chloride as the electrolyte. The original Leclanché cell (a wet cell) was gradually developed into the carbon-zinc cell (a dry cell), which was the most commonly used small battery during the 20 th century until the introduction of the alkaline cells in the late 1950s. 13,14 The first rechargeable battery was invented by G. Planté in 1859. This lead-acid battery could be recharged by reversing the chemical reaction. Developed forms of Planté's lead-acid battery are still in use, especially in automobiles and in industry. However, lead-acid batteries are heavy and their performance is relatively low in comparison to more modern systems. 15,16 Another rechargeable battery, the nickel-cadmium battery, was invented by W. Jungner in 1899. This battery uses nickel oxide hydroxide (NiO(OH)) and cadmium (Cd) as electrodes and aqueous alkaline potassium hydroxide as an electrolyte (KOH). The nickel-cadmium battery can be over-discharged or stay discharged a long time without losing its performance. However, cadmium is highly toxic and, therefore, the nickel-cadmium battery is gradually replaced by nickel-metal hydride (Ni-MH) and lithium-ion batteries. 8,17,18 In the 1970s, the nickel-hydrogen battery was developed for the space industry. Instead of a cadmium electrode, the nickel-hydrogen battery uses a hydrogen gas electrode. The nickelhydrogen battery has a remarkably long lifetime and it has a higher energy density than the nickelcadmium battery, i.e. more Wh/kg. The nickel-hydrogen battery has been widely used in satellite power systems. 9,17 In 1991, Sony and A. Kasei launched the first rechargeable lithium-ion battery (Li-ion battery or LIB) for commercial consumption. The first generation of Li-ion batteries uses a graphite anode and a lithium cobalt oxide cathode. The electrolyte is a lithium salt (LiPF6) dissolved in an organic solvent, for example ethylene carbonate (EC) and dimethylene carbonate (DMC).

I-1.1.2. Types of Batteries

Primary batteries and secondary (rechargeable) batteries include a variety of electrochemical systems for storing chemical energy. However, only a few of these are efficient enough, taking into account cost as well as storage/size, to be used commercially 7 (Table I-1). Batteries convert chemical energy into electric energy through a redox reaction. The energy released during this reaction is derived from the redox potential (E) of the materials involved. This is expressed in the following equation:

    G nF E I-1
where ∆G stands for the change in free energy, n for moles of electrons, F for Faraday's constant

and ∆E for the redox potential difference. 20 A redox couple consists of two species of the same chemical element. These species are referred to as oxidized species and reduced species, since they have different oxidation states. In the following examples, the oxidized species are written to the left in each redox couple: Cu 2+ /Cu; Cl2/Cl -; Fe 3+ /Fe 2+

; Mg 2+ /Mg; Zn 2+ /Zn; Ag + /Ag. A redox reaction is often described as two halfreactions: a reduction half-reaction, which takes place at the cathode when discharging and an oxidation half-reaction, which takes place at the anode. 21,22 X X + + e -(Oxidation half-reaction) e -+ Y +  Y (Reduction half-reaction)

The standard potential of a redox couple is obtained by coupling the electrode in question to a reference electrode (i.e. a standard hydrogen electrode, SHE) under standard conditions (concentration of 1 M, a pressure of 1 bar and a temperature of 298 o K). Thus, different redox potentials may be compared via the reference electrode. The redox potential can be considered as a way to describe how readily an element forms ions in comparison to other elements through an electrochemical process.

Normally, if there is a direct contact between the two species involved in a spontaneous redox reaction, the energy released dissipates in the form of heat and is of little use. Therefore, the two electrodes of an electrochemical battery/cell are separated and the electrons are exchanged via an electric circuit. To obtain this, a separator allowing only ionic conduction between the electrodes is necessary. To ensure conductivity, the separator, usually a membrane of cellulose or polycarbonate, is soaked in the electrolyte. Since the membrane/separator does not allow the electrons to pass, the electrons are forced to pass via the electric circuit instead. 23 In doing so, the electrons produce a current (See Figure 1). The other function of the separator, to ensure that there is no direct contact between the electrodes, prevents short circuits as the electrodes must be electrically conductive.

The electrodes consist of a solid part in contact with the electrolyte. The contact zone between the solid part and the electrolyte, called the electrode/electrolyte interface, is crucial for the performance of the electrode/battery. 23 The electrolyte contains free ions and, therefore, it is conductive. The most commonly used electrolytes consist of a compound that ionizes when molten or dissolved in an adequate medium, although there are also electrolytes in form of gases or solid (e.g. dry polymer or glass). 8 The current collectors are usually metal foils or grids that are inert under the operational chemical and electrochemical conditions. Frequently, they have also the important role of imparting mechanical stability to the electrodes. [START_REF] Gabrielli | Ac-Electrogravimetry Study of Electroactive Thin Films. I. Application to Prussian Blue[END_REF] A typical structure of a commercial LIB consists of a graphite anode, a cathode formed by lithium metal oxide such as LiCoO2, and a separator embedded in an organic electrolyte containing a lithium salt (e.g. LiPF6, LiAsF6). The electrolyte promotes the ionic transport, which is separated from the electronic transport that happens in the external circuit via electron flow to supply power. Na + (aq) + e -→ Na(s) -2.713

Li + (aq) + e -→ Li(s) -3.040

Throughout the charging process, lithium migrates from the cathode (LiCoO2 in this case)

through the electrolyte and is intercalated into the graphite anode (LixC6). During discharge, the movement of lithium is in a reverse process: Li ions are extracted from the anode and intercalated into the cathode. 26,27 I-1.1.4. Battery Performance When discussing the performance of batteries, there are various aspects that need to be considered. In this section, the most important ones will be discussed, i.e. cell potential, capacitance, specific energy density, specific power density and the C-rate.

Cell Potential (Voltage)

The theoretical standard cell potential (E 0 cell) can be defined as the difference between the theoretical standard potentials of the two half cells in an electrochemical cell:

E°(cell) = E°(cathode) -E°(anode) I-4

However, in practice, the cell potential will be lower due to various factors, ranging from polarization during the charge process to the age of the cell. The cell potential is measured in volts (V) and, therefore, it is often referred to as the voltage of the cell. The standard electrode potential, E°, for an electrode reaction is the potential generated by that reaction under the condition that the reactants and the products are in their standard state in relation to a reference electrode. In aqueous systems, the standard hydrogen potential is taken as the universal reference electrode 22 previously mentioned in Section I-1.1.3. A list containing selected standard electrode potentials is given in Table I-2.

Capacitance

Capacitance is the term used for referring to the quantity of electricity involved in the redox reactions. The theoretical capacitance, Q, can be calculated using the following equation:

charge Q = xnF I-5
where x denotes the number of moles of the electrochemical reaction, n the number of electrons transferred per mole of this reaction and F Faraday's constant.

However, capacitance is usually expressed in Ampere-hours per kilogram (Ah/Kg), i.e. in mass rather than in number of moles, and, then, it is often referred to as specific capacitance (Cspecific). Some values of specific capacitance are shown in Table I-3.

In practice, the theoretical capacitance cannot be obtained, partly because of mass contribution from nonreactive components and partly because various factors impede the completion of the electrochemical reactions. 20,28 Specific Energy Density

If the specific capacitance is multiplied by the average operating cell voltage, the specific energy density is obtained, i.e. the energy stored per unit mass (of the cell or of the active electrode material). The specific energy density is expressed in Watt-hours per kilogram (Wh/kg) (Gravimetric energy density).

Specific Power Density

The specific power density states the quantity of power stored per unit mass, which is expressed in Watt per kilogram (W/kg). The relationship between the specific power density and the specific energy can be visualized in the so-called Ragone plot (Figure I-2). As the Ragone plot shows, the specific energy density has a tendency to drop fairly quickly at higher drains. 29 C-rate

The charge and discharge current of a battery is measured in C-rate. 22,30 In fact, a C-rate is a measure of the rate at which a battery is discharged relative to its maximum capacitance. A 1C rate means that the discharge current will discharge the entire battery in 1 hour. For example, a battery with a capacitance of 100 Amp-hrs equates to a discharge current of 100 Amps. 31 I-1.1.5. Battery Applications Secondary batteries are mainly applied in portable electronics, transport and stationary storage. If measured by the number of units produced, portable electronics constitute the largest of these three groups. Presently, the lithium-ion technology has the highest market share, although

Ni/MH batteries are also frequently used in portable electronic devices. In addition, the use of primary alkaline batteries is still significant in these devices. In the transport sector, the demand for batteries is expected to increase rapidly over the next decade due to environmental reasons (Figure I-3). Because of the international commitments to significantly reduce carbon dioxide emissions from the transport section, government incentives (e.g. subsidies, tax reductions and access to bus lanes) for electric vehicles (EV) and hybrid electric vehicles (HEV) have been implemented in many countries. The most common batteries in EVs and HEVs are Ni/MH, lithium-ion and ZEBRA (sodium-nickel chloride) batteries. 32 I-1.2. Supercapacitors Supercapacitors, also known as electrochemical capacitors (EC) or ultracapacitors, 33 are a relatively new energy storage system that has contributed significantly to the scientific and industrial developments in recent years. 34 The principle of storage is purely electric and the global charge transfer kinetics is faster than that in the batteries. Compared to the latter, supercapacitors store less energy, but they are able to deliver it in a short time, thus, providing high power, as shown in the Ragone plot given in Figure I-2. 35

I-1.2.1. History

The first patented supercapacitor was developed by H. Becker of GEC (General Electric Company) in 1957. [36][37][38][39][40] Becker used electrodes composed of porous carbon and an aqueous electrolyte based on sulfuric acid, which formed an electric double-layer at the electrode/electrolyte interface. [36][37][38][39] A few years later, SOHIO (Standard Oil of Ohio) patented a device using graphite. 36- 38 SOHIO experimented with organic electrolytes, which provided a higher operating voltage than those obtained in aqueous electrolytes. 10,36,37,39,40 In 1971, the SOHIO patent was licensed by NEC (Nippon Electric Company). Under the name of "supercapacitors", NEC successfully introduced these new devices on the market as back-up memories for electronics. 36,[38][39][40] Following the success of NEC, several companies started the production and development of supercapacitors. 36,38 For example, in 1982, Pinnacle Research Institute designed the first high-power supercapacitor, mainly for military applications. 10,38 Today, there are several types of supercapacitors applying different charge storing mechanisms to be able to meet different kinds of energy demands. 39 The two main types are the Electric double-layer capacitors (EDLC) and the so-called Pseudo-capacitors. These are nonfaradaic processes (electrostatic interaction) and faradaic processes, respectively (Figure I-4).

In supercapacitors, if both electrodes have the same design and the same mass loading, the supercapacitor is called symmetric. In contrast, if the electrodes are different, the supercapacitor is called asymmetric.

I-1.2.2. Principle of Operation for EDLC

An EDLC is composed of two electrodes immersed in an electrolyte and separated by a dielectric separator 36,[40][41][42]. The two electrodes can be of identical or different materials, usually nanostructured and porous carbons with high specific surface areas. 36 The electrode materials play a key role, since they largely affect the capacitance values 40,41 and the charge stored in the EDLC. As is the case with capacitors, no redox reactions are involved in EDLCs. 40 The operating principle is based on an electrochemical double layer, which is formed through electrostatic attraction at the electrode/electrolyte interface. 10,36,41 When applying a potential difference across the current collectors, electrostatic charge is stored at the two electrode/electrolyte interfaces, which function as two capacitors in series. 36,41,43 During discharge, the charge accumulated causes a parallel movement of the electrons in the external circuit, thus, generating electric energy, as indicated in Figure I-5. 10,36 Due to the high specific surface area of the electrode materials, the amount of energy that may be stored is much larger than in traditional capacitors 42 and the stored charge can be restored with greater efficiency than in batteries. 44 . Furthermore, in theory, the cycle life of an EDLC is infinite, while batteries have a cycle life of approximately 500-2000 cycles. 43 As mentioned above, the two electrode/electrolyte interfaces in an EDLC function as two capacitors in series (C1 and C2). Thus, the total specific capacitance of the cell (C) depends on the capacitance of each electrode as given by the following equation: 43 45 The relationship between the capacitance C (in Farad), the quantity of stored electrical charge Q (in Coulomb) and the rated voltage V (in Volt) is as follows: 33

Q C= V (I-7)
where the charge Q equals the current I (in Ampere) multiplied with time t (in seconds): 36 Q = I.t (I-8) Usually, the charge Q is expressed in Ah/g (1 Ah=3600 C) and the specific capacitance C is given as the specific capacitance in F/ cm 2 or F/g. 36 I-1.2.3. Components of the Supercapacitors: Active Electrode Materials, Electrolytes, Separator and Current Collectors Supercapacitors are composed of (i) active electrode materials, (ii) electrolytes, (iii) separator and (iv) current collectors.

(i) Active Electrode Materials:

The electrodes, which are composed of active materials and additives, store/deliver charges. 36,46 The electrode material plays a key role in determining the capacitance, the energy density and the power density of the supercapacitor. 47 Furthermore, it affects the series resistance and self-discharge characteristics. 40,46 Therefore, the selection of the electrode materials is crucial and a great number of parameters need to be considered: specific surface area, porosity, structure 48 "tunability" of the morphology (i.e. how readily the material allows manipulation of pore size, pore distribution, surface functional groups etc.), 38,49,48 surface wettability, electrical conductivity 38 , electrochemical stability, 40 thermodynamic stability for a wide operational potential range, cycle stability, 38 mechanical resilience 50 and cost. [38][39][40]48 Since pore size is an important factor, the IUPAC (International Union of Pure and Applied Chemistry) has suggested the following classification:

Figure I-6. The IUPAC classification of pore sizes. 37 macropores, mesopores and nanopores, with diameters of> 50 nm, 2-50 nm and < 2 nm, respectively (Figure I-6). 37 Currently, the three main categories of active electrode materials are based on the carbon materials 36 , transition metal oxides, 36,46 and conducting polymers. 36 Of these three classes of materials, carbon is the most frequently employed in EDLCs. 39,48 Carbons may have very diverse structures, which affect their properties. The most commonly used structures in EDLCs are: (1) onion-like carbons (OLCs), (2) carbon nanotubes (CNTs), (3) graphene, (4) carbide-derived carbons (CDCs), (5) activated carbons (ACs), and (6) templated carbons (Table I-4). 39,41,51 (1) Onion-Like Carbons (OLCs)

OLCs are zero-dimensional (0D) materials 39,52,53 consisting of concentric graphitic shells with a specific surface area that ranges from 200 to 980 m 2 /g depending on the synthesis conditions. 39 OLCs are generally prepared by annealing nanodiamond powders in vacuum or under argon. 39,52 They offer high electrical conductivity, 52 high power 39,52 and high energy densities 39 , but a limited capacitance (around 30 F/g). 39,52 (2) Carbon nanotubes (CNTs)

CNTs are regarded as one-dimensional, since the graphitic walls of which they consist are assembled in tubes that are almost one-dimensional 38 and as the transport of charges only occurs along the tube axis. The diameters of these tubes are approximately 1-50 nm. 39 Depending on the number of graphitic layers (walls), CNTs are divided into two subgroups: single-walled nanotubes (SWNT) and multi-walled nanotubes (MWNT). 39,42,50 Both SWNTs and MWNTs are appreciated for their high electrical conductivity, 42,50 unique pore structure, 54 38,54 , chemical reactivity 54 and thermal stability 50 and relatively low equivalent series resistance (ESR) (lower than activated carbon). 51 Although their surface area is relatively moderate compared to activated carbon, it may be used more efficiently due to the fact that the mesopores are interconnected, thus, providing access to nearly all the surface area and enabling a continuous distribution of charges. 50,51,54 Altogether, these properties make CNTs suitable for high power devices. 42,50,51 The specific capacitance of CNT electrode material ranges between 15 and 200 Fg -1 , depending on the morphology and purity obtained at the production procedure and any subsequent treatment. 38,50 CNTs in sizeable quantities can be produced by three major methods: arc discharge, laser ablation and chemical vapor deposition (CVD). [55][56][57] Each of these methods presents advantages and disadvantages resulting in different growth results (Table I-5). Thus, the choice of the method is dependent on the requested properties of the CNTs. Prasek,J. et al. 61 reported that the choice of catalyst is one of the most important parameters when using the CVD method, since it affects the CNT growth. In addition, the production of CNTs (the CNT growth) depends on the processing conditions such as pressure and temperature. Temperatures > 900°C result in SWCNTs, whereas MWCNTs are produced at temperatures in the range of 700-800°C. 62

(3) Graphene

Another active material that is commonly used in energy-storage mechanisms is the graphene.

The usage of graphene can range from hosting ions (such as Li + or Na + in metal-ion batteries) to storing electrostatic charges on the electrode double-layer (as in EDLC applications) 63 Graphene is composed of pure carbon organized in hexagonal structure in the form of a one atom thick transparent sheet. Graphene can be produced with a mechanical exfoliation of graphite (which is composed of intercalated graphene sheets) with a method called the Scotch tape method.

This approach consists of peeling off a chunk of graphite until only one layer remains. However, this technique does not allow a large scale production of graphene and, since the vast majority of chemical and electrochemical energy storage systems require bulk quantities of graphene, many other methods to synthesize graphene-like materials have been proposed. For example, the reduction of graphene oxide (GO) can be used to elaborate graphene-like materials. GO is commonly synthesized with the Hummers method, 64 which is a reaction involving graphite, sulphuric acid and potassium permanganate. Once the GO is synthesized, its reduction can be performed in different manners: chemically, thermally or electrochemically. The reduction process has a decisive influence, because it determines the quality of the reduced graphene-like material. 65 Many approaches can be performed for chemical reduction with the aim to bring GO structures as close as possible to that of the graphene obtained by the Scotch Tape method. The most common method involves hydrazine. This compound is particularly interesting because, unlike other reducing species such as lithium aluminum hydride (LiAlH4), 65 it does not react with water, thus, avoiding a number of side reactions. However, a large scale production involving hydrazine would be difficult, since this toxic compound cannot be used in the context of sustainable development.

Another method to produce graphene-like material is thermal reduction, which can be performed by thermal treatment of graphene oxide at 1050°C. This thermal process causes the release of CO2, which leads to an increase of the pressure within the material. However, thermal treatment of graphene oxide in this way may cause damage to the material, causing surface defects affecting electronic properties. 65 Electrochemical reduction, which will be discussed in Chapter II, seems to be a good alternative to produce large scale graphene-like samples.

This eco-friendly method avoids using dangerous species like hydrazine and allows highquality graphene-like material to be produced. 65 Some examples of reduced graphene oxide used as electrode material are shown in Table I-6. CDCs are produced by high temperature extraction of metals from carbide precursors. 39,52 Porosity is formed by leaching out metal atoms from the crystal structure of the carbide precursor 52 and the pore structure strongly depends on the carbide precursor and the synthesis temperature. 76 Carbon growth can be controlled on the atomic level, leading to a highly controllable pore size with better than angstrom accuracy. The specific surface area ranges from 1000 to 3000 m 2 /g.

(5) Activated Carbons (ACs)

ACs are generally produced by carbonization of carbonaceous organic precursors (e.g. nut shells, wood, peat or coal) 39,52,53,77 and subsequent activation processes. 52,53,77 The activation processes, which can be physical and/or chemical, 39,77 aim to create a three-dimensional porous network in the bulk of the carbon material. 39,52 In physical activation, the carbon precursor is thermally treated at high temperature in the presence of an oxidizing gas, 39,53,77 e.g. air, 53,77 water vapor, CO2, 52,53,77 or KOH. 52 Chemical activation is performed using an activating agent e.g.

phosphoric acid, potassium hydroxide, sodium hydroxide or zinc chloride (H3PO4, KOH, NaOH, ZnCl2). 52,53,77 During the activation processes, micropores and mesopores are created and the specific surface area can exceed 2000 m 2 /g. 52 However, the control over porosity is limited, resulting in a wide pore size distribution, which means that not all pores are accessible to ions. 39 By selecting carbon precursor, activation method and control of synthesis conditions, AC materials with different functional groups on the carbon surface may be obtained, resulting in a variety of physicochemical properties. 53,77 The most common functional groups contain oxygen and/or nitrogen. 53 In the last few years, the capacitive performance of ACs has improved significantly and ACs have a higher density than CNTs and graphene (AC volumetric capacitance reaches 50-80 F/cm 3 ). 52 Scalable manufacturing and reasonable cost also contribute to ACs being widely used in commercial EDLCs. 52,77 (6)

Templated carbons

Templated carbons are produced by carbonization of a carbon precursor in nanochannels of a template inorganic material and the subsequent removal of the template. [52][53][54]76,77 By the selection of carbon precursor and template, and by the control of carbonization parameters, porous carbons with different physical and chemical properties may be obtained. 53 The template method enables the preparation of 1D, 2D and 3D carbons, e.g. carbon nanotubes, graphene and nanoporous carbons, 52 and can form both microporous and mesoporous carbons. 76 Furthermore, it allows the precise control of pore structure; pore volume 76 and pore size 52 can be controlled through tuning the pore size of the template. 54 Depending on the templates used, the template method can be subdivided into hard-template and soft-template methods. The former refers to a replication synthesis with presynthesized hard templates (inorganic materials such as silica nanoparticles, zeolits, mesoporous silica and MgO) and includes infiltration, carbonization and removal of templates, 54 while the latter uses triblock copolymers as the template 52,54 and includes cocondensation and carbonization. The hard-template method is expensive and time-consuming, making it unsuitable for large-scale production. In addition, the removal of the template requires toxic acids. The soft-template method is less expensive, faster and more environmentally friendly. 54 Because of their precise controllability, templated carbons are suitable to study the effects of pore size, pore shape, channel structures and other parameters concerning ion diffusion and charge storage in nano-confined systems. 53 In addition to carbon structures, the porous texture plays an important role in understanding the correlation between the specific surface area and the capacitance, and the properties of the latter.

Ion sizes and pore sizes, and the connection between the pores may have a strong impact on capacitance values, particularly under the high operating current densities that are present in supercapacitors. Four important porous characteristics for carbon materials will be presented: with MgSO4 electrolyte, when the average pore size (0.58 nm) of the carbon material is significantly larger than the size of ions.



When the average pore size (0.51 nm) of the carbon material is smaller than the size of ions, the current is negligible in all the potential range.



When the average pore size (0.51 nm) of the carbon materials is larger than one of the electrolyte ions and smaller than the other, as is the case with Li2SO4 and MgCl2 aqueous electrolytes, a triangular shape and narrowing of the two CV curves appear, which confirm the adsorption of the smaller ion into the pores and the absence of adsorption of the larger ion on the other side of the point of zero charge.

There is no direct relationship between the electrosorption and insertion of a certain ion in pores of different sizes and the capacitance. The latter will be controlled by the correlation between the average pore size and the effective size of ions, which can be studied by CV. By slowing down the applied scan rate, it is possible to force the insertion of ions into the pores if the ions are slightly larger than the average pore size. In this case, ion sieving also results in increased, yet reversible swelling of the pore network.

Another parameter that affects electrosorption is the geometry of the ions from aqueous solutions, as shown by Noked et al. They found that planar solvated ions, e.g. NO3 -, are adsorbed in slitshaped pores, while spherical solvated ions, e.g. Cl -, are excluded. In a similar study, the preference of hydrated monovalent ions (Na + ) to hydrated bivalent ions (Ca 2+ and Mg 2+ ) in micropores was shown.

Furthermore, the sieving effect was demonstrated in a study of the capacitance of an active carbon (AC) in a series of ILs of increasing cation size. Since ILs are solvent-free, the capacitance properties can be interpreted by comparing the size of pores and ions calculated by molecular modeling. This study showed an extensive mismatch between the pore size and the effective size of the cations. 48

b. Ion Desolvation

The effective ion size in aqueous and organic electrolytes is larger than the actual ion size itself due to the solvation shell. Solvation shells are formed around the ions in aqueous and organic electrolytes. Therefore, the effective ion size is larger than the actual ion size itself. Although it is clear that pore size and effective ion size need to be matched, it is difficult to identify the optimum pore size, i.e. the pore size leading to the best EDLC performance. For template carbons, Vix-Guterl Furthermore, it was shown that the ions needed to be at least partially desolvated to access into the pores (for a two-electrode set-up). The importance of the sieving effect, associated to the desolvation process, is highest at the anode (negative electrode), which means that the cell capacitance is essentially controlled by the anode according to Equation I-9.

c. Pore Saturation

Mysyk et al. has shown that the porosity of carbons with subnanometer sized pores (APC carbon) can be saturated with electrolyte ions for high voltage values in 1.5 M TEA-BF4/AN electrolyte. It was found that the capacitive current decreased significantly at voltage higher than 1.5 V, despite that the pore size most probably matched the ion size (Figure I-10). The charge was approximately equal to the theoretical maximum charge storable in the pores larger than the desolvated cations. Therefore, the authors proposed that the observed capacitive current decrease was due to the saturation of the pore volume accessible to ions.

d. Distortion

Ania et al. 82 observed an unexpectedly high capacitance (92 F/g) for a microporous carbon in TEA-BF4/AN electrolyte, where 63 % of the pores were smaller than the desolvated ion size. The authors proposed that distorted cations penetrate the pores under the effect of the electric field,

showing slightly smaller dimensions than their computed rigid size. For some TEA + conformations, a size smaller than the average pore size of the carbon has been computed. (ii) Electrolytes:

The role of the electrolyte is to supply and allow the movement of ions to form the electrical double-layer at the electrode/electrolyte interface. 36,48,83 Consequently, electrolytes need to be good ionic conductors. Along with ionic conductivity, which largely affects the specific power density, the electrolyte stability voltage window is the main criterion for the selection of an electrolyte. 36,48 The stability voltage window sets the limits for the voltage that may be applied to the electrodes without causing decomposition of the electrolyte. This explains the importance of this property, since the specific energy density (E) is proportional to the voltage squared as shown in the following equation:

29 2 1 E = -CV 2 (I-9)
Another important parameter to consider when choosing the electrolyte is ion size, 50 which is discussed in the literature. There are three types of electrolytes used in supercapacitors: aqueous, organic and ionic liquids (ILs) (Table I-7).

The most common aqueous electrolytes are H2SO4, KOH and KCl. They have high ionic conductivity, but narrow voltage windows (approximately 1.2 V). 38 Some of their advantages are low cost, easy handling in an open environment and the availability of diverse pH values. 38,48,84 The most commonly used organic electrolytes are solvents based either on propylene carbonate (PC) or acetonitrile (ACN). These electrolytes have wider voltage windows (approximately up to 2.2 -3 V) 38 , but much lower ionic conductivity than aqueous electrolytes, especially PC, which has an ionic conductivity more than four times lower than acetonitrile. Ionic Liquids 3-5 38,40 <15 37,40 On the other hand, acetonitrile is both toxic and flammable, making PC attractive for safety and environmental reasons. 37,38,48,50 ILs are salts that are liquid (molten) at room-temperature. 38,48 Since they do not contain any solvent, their stability voltage windows only depend on the electrochemical stability of the ions. These electrolytes have wider voltage windows than both aqueous and organic electrolytes (approximately up to 3-5 V). 38 However, the ionic conductivity is low at room temperature, so they are preferably used at higher temperatures. With their many advantages, i.e.

having higher energy density and power density than other electrolytes, having a well-defined ion size (no solvation shell to consider), 41 being nontoxic, nonflammable and chemically stable, the ILs are expected to significantly contribute to the improved performance of supercapacitors in the future, at least for high temperature operation. 29,38,48 (iii) Current collectors:

The role of the current collectors is to transport the electric current between the electrodes and the external loads. 36,38 Consequently, current collectors need to present good electrical conductivities, which also contributes to increase the specific power and to reduce the total resistance of the supercapacitor. 36 Furthermore, they are required to be chemically and electrochemically stable to resist corrosion. 36,38,40 Considering these factors, stainless steel is generally used in aqueous electrolytes, while aluminum alloys are preferred in organic electrolytes 36,38,40 due to its low density and good thermal conductivity. 38 Another important factor to consider is the contact between current collectors and the active layers of the electrodes. 29,36,38 It should be the lowest possible as the interface resistance makes a significant contribution to the total resistance. 36 To obtain this, polymeric binding agents (i.e.

Nafion® and polytetrafluoroethylene) are used. 38

(iv) Separators:

The two main roles of the separator are to electrically isolate the two electrodes to prevent short-circuits and to ensure ionic conductivity. 36,40 In general, the separator consists of a porous membrane. 36 In designing or selecting these membranes, there are several properties that need to be considered. The membrane must be electrically nonconducting, while electrolyte ion permeable with a minimum of ionic resistance. Furthermore, it needs to be chemically resistant (i.e. resistant to (iv) hybrid and composite electrodes have been proposed.

(i) Nanostructuration

The study of different carbon structures and pore textures to increase supercapacitor performance has been briefly discussed in Section I-1.2.3. Also, in the literature, 85 it has been discussed that physical and chemical properties of pure CNTs, including size, purity, defect, shape, functionalization and annealing, affect the performance of supercapacitors. Furthermore, it has been discussed that the composites, including CNT/oxide and CNTs/polymer, enhance the capacitance and keep the stability of the supercapacitor by optimally engineering the composition, particle size, and coverage. Lin, R 39 has mentioned that the composition and associated properties of ILs depend on the cation and anion combinations. There are literally millions of different structures that may form an IL; the number of cation-anion combinations can be as high as 10 18 combinations. 88 However, there are three basic main classes that are specific to different types of applications: Aprotic, Protic and Zwinteerionic as shown in Figure I-14. 39,89 Aprotic ILs seem to be the most used for EDLC applications, because there are a huge interest in increasing the cell voltage above 3V. 39 Therefore, increasing the specific capacitance and the energy density. 40,84,91 In these devices, only part of the charge storage is assured by the double layer at the electrode/electrolyte interface, while another amount of transfer and storage is due to faradic mechanisms (redox reactions and intercalation). 33,51 The principal electrode materials used for charge storage in pseudo-capacitors are metal oxides, such as RuO2 92 and MnO2, 93 and conducting polymers. [94][95][96] However, as in batteries, the redox reactions affect cycle stability due to aging. Therefore, pseudo-capacitors are mainly used when the energy density is a larger issue than cycle stability. 29

(iv) Hybrid Electrodes

The hybrid technology aims to obtain an ideal storage device that has the high power density of a supercapacitor as well as the high energy density of a battery. In the literature, some examples of asymmetrically structured hybrid supercapacitors can be found. These combine a carbon electrode However, in practice, these hybrid systems have intermediate characteristics: a charge/discharge speed slower than the classic supercapacitor (due to the speed of the redox reactions at the faradic electrode) and a shorter lifetime (due to the consummation of the active material caused by the chemical reactions). The evaluation of the actual performance of the hybrid supercapacitors, in terms of power density and energy density and/or cyclability and lifetime, has been the objective of numerous studies. 33,51,97 Recently, JSR Micro and JM Energy Corporation proposed two Li-ion hybrid supercapacitors called the 2300F and 3300F prototypes. Their energy density is about 10 Wh/kg. 98 In order to improve the performance of supercapacitors, another type of electrode materials named nanocomposite electrodes has been reported in the literature, 26 which is summarized in Table I-8.

I-2. Diagnostic Tools for Electrodes in Energy Storage

In order to improve the performances of these various storage systems, it is necessary to use pertinent tools to examine the electrochemical behavior of the different materials suggested.

Electrochemical methods, surface structural analysis procedures and electrogravimetric methods will be discussed in this section. The electrode/electrolyte interfaces of the supercapacitors can be investigated and the energy storage properties can be related to the structural characteristics.

I-2.1. Electrochemical Methods

In this section, three electrochemical methods will be detailed. These are specifically (i) cyclic voltammetry, (ii) galvanostatic charge-discharge and (iii) electrochemical impedance spectroscopy.

I-2.1.1. Cyclic Voltammetry (CV)

The CV is a potentiodynamic electrochemical measurement technique that measures current versus potential. The principle is to impose two successive potential scans (cyclic) between the working electrode and the reference electrode while the current is measured across the working electrode and the counter electrode. 108,109 These data are plotted as current (i) vs applied potential (E/V) to give the cyclic voltammogram trace. The potential range is in general limited by the electrolyte stability window.

In the literature, it has been reported that cyclic voltammetry provides quantitative and qualitative information on the capacitive behavior between the electrolyte ions and the active materials. Also, the presence of parasitic faradic reactions has been identified. where m is the mass per electrode of the active material.

I-2.1.3 Electrochemical Impedance Spectroscopy (EIS)

The EIS measurements, which are based on frequency modulation, provide qualitative information on the studied system (capacitive, resistive, diffusive behavior etc.) and allow the calculation of the capacitance of the electrodes. 110 In contrast to cyclic voltammetry, electrochemical impedance is based on the collection of an alternating current (AC) resulting from applying a sinusoidal potential perturbation with a small amplitude (typically 10 mV rms). 109,114 The Nyquist diagram (-ImZ, ReZ Cartesian coordinates) and the Bode diagram (polar coordinates) are the plots normally used for EIS measurements, which will be further detailed in Chapter II.

In the literature, there are a number of works employing EIS to study carbon electrodes in supercapacitors. 52,115 Net4BF4 in AN at 0.5V/Ref.

For these experimental records, the authors observed that the Nyquist plots of the three CDC samples show a similar shape in disregard of the pore size, characteristics of the capacitive behavior of a porous electrode. At high frequency (> 1 KHz), where the plots intercept the real axis, the supercapacitor behaves like a pure resistance. In the low frequency domain, the huge increase of the (i) X-Ray Diffraction (XRD), (ii) Scanning Electron Microscopy (SEM), (iii) Transmission

Electron Microscopy (TEM), (iv) Energy-Dispersive X-ray Spectroscopy (EDX), and (v) nitrogen physisorption (Brunauer Emmett and Teller (BET)).

I-2.2.1. X-Ray Diffraction (XRD)

XRD is a fruitful characterization technique used to obtain structural information such as the crystal size, the crystallographic structures, the interlayer spacing, the structural strain, the crystal orientation etc. [117][118][119] In addition, detailed information of the electron density distribution can be accessed. 120

Dall'Agnese et al. 121 have used XRD to study the mechanism of Na + intercalation in Two-Dimensional Vanadium Carbide (MXene) (V2CTx) for Sodium-Ion Capacitor applications.

I-2.2.2. Scanning Electron Microscopy (SEM) and FEG-SEM

The scanning electron microscopy (SEM) can be used for the characterization and the exploration of the surface topography. 36,122 SEM forms images from signals produced by the complex interactions between a primary electron beam and the sample, for example, a specimen of CNTs.

There are various types of electron guns. In comparison to thermionic electron guns, field emission guns (FEGs) provide enhanced electron brightness (approximately x 100) and longer lifetime. Furthermore, because of the lower electron energy spread (0.3 eV), the chromatic aberration is significantly reduced and probes smaller than 2 nm can be formed, thus, providing much higher resolution for SEM images. 122

I-2.2.5. Brunauer Emmett and Teller (BET)

In the literature, 34,45 it is very often assumed that the specific surface area estimated through the BET approach is equivalent to the specific surface area of the electrode/electrolyte interface.

Therefore, some proportionality should exist between the estimated gravimetric capacitance and the BET specific surface area measured by nitrogen adsorption at 77 o K. In However, the diameter of the central canal is too large in comparison to the size of the solvated ions. On the other hand, this particular behavior could also be due to a very hydrophobic character of these tubes, as suggested by the very small value of oxygen content (Table I-9). 111,128

I-2.3. Classical Electrogravimetric Investigations: Quartz Crystal Microbalance Based Methods

The most used electrogravimetric method for mass analysis and current changes is the EQCM (Electrochemical Quartz Crystal Microbalance), which will be discussed in this section.

Electrochemical Quartz Crystal Microbalance (EQCM) is a method where a Quartz Crystal Microbalance (QCM) is coupled with an electrochemical measurement. 129,130 This method is also called cyclic electrogravimetry. 131,132 The EQCM is capable of quantitatively measuring small mass changes occurring at the electrode surface during an electrochemical process. It is able to detect mass changes in the order of 1 nanogram. 133 This method has been used to investigate diverse phenomena, for example, metal electrodeposition, [134][135][136] electrocrystallization, 137 electrochromic reactions, 138 intercalation, 131,139 and adsorption and mass changes associated with electrolyte-ion movement. [140][141][142] The EQCM allows the investigation of the capacitive properties by analyzing the capacitive current and the resulting mass changes of the film. The operation principle of the EQCM will be discussed in further detail in Chapter II.

Reviewing the literature, only a limited number of works employing EQCM to study carbon materials can be found. Also, Maier et al. have studied the effect of specific adsorption of cations and their size on the charge-compensation mechanism in carbon micropores. This material was tested in 0.5 M NH 4 Cl solution using the EQCM technique. 144 The authors reported that this technique reveals a complicated interplay between the adsorption of NH 4 + cations and desorption of Cl -anions inside carbon micropores at low surface charge densities, which results in failure of their permselectivity.

Higher negative surface charge densities induce complete exclusion (desorption) of the Cl -co-ions, which imparts purely permselective behavior on the carbon micropores (Figure I-30). 144 In order to quantitatively correlate the mass changes of the adsorbed ions and solvent molecules with the potential-induced variation of the electrode charge density, two terms are introduced by the authors: the potential of zero charge (pzc) and the potential of zero mass change (pzcm). The pzcm should coincide with the pzc, since, according to a conventional point of view, adsorption of anions and cations into carbon micropores occurs at Q>0 and Q<0, respectively. However, the pzcm obtained from the EQCM does not necessarily coincide with the pzc of the electrode, since the former may depend on the dynamics of formation of the ionic part of the electric double layer (EDL), that is, on the relation between the fluxes of counterions (anions at Q>0 and cations at Q<0) and co-ions (cations at Q>0 and anions at Q<0).

Recently, Tsai et al. reported that the combination of the CV and EQCM techniques offers a great opportunity for the understanding of ion transport and extent of solvatation in porous materials, thus, enabling the design of optimized porous materials for the next generation of high energy density supercapacitors. 142 The authors obtained EQCM results from a sample of carbide-derived carbon with a pore size of 1 nm (CDC-1 nm) tested in 2 M EMI-TFSI AN solution (Figure I-31). From the frequency responses, the authors observed that the electrode mass increased during cation/anion adsorption and decreased during cation/anion desorption. 142 In fact, according to a classical approach, adsorption of anions is expected when the CDC electrode surface is positively charged, whereas the adsorption of cations should take place when the surface is negatively charged. 139 Regarding these previous studies, EQCM provides only a global response of the carbon film and kinetic information cannot really be obtained. Therefore, in these previous studies, an unambiguous identification of the species transferred and the determination of the free solvent contribution were not possible. To overcome these limitations, possible alternative methods should be sought.

I-3.The Scope and Objectives of the Ph.D. Thesis

Ionic transfer and transport phenomena largely influence the performance of the electrode materials for energy storage applications. Some studies suggest the use of microbalances as a means to characterize ion fluxes and transport phenomena for electrodes based on carbon. It has been demonstrated that the response of the microbalance allows an in situ estimation of the ionic flux, which strongly depends on the point of zero charge (PZC), of the pore size and of the size of the ions. Unfortunately, subtleties or more fair explanations cannot be reached through this classical technique.

In this Ph. D. thesis, coupled electrochemical and gravimetric methods, resolved at temporal level, specifically the electrochemical impedance spectroscopy (EIS) coupled with fast quartz microbalance (QCM) (ac-electrogravimetry), have been used to study the capacitive behavior of carbon based electrodes.

The use of ac-electrogravimetry provides very relevant information: kinetics and identification of the species transferred to the electrode/electrolyte interface, separation of the contribution of different species and change of the relative concentration of species within the material. More precisely, all the species involved in the charge storing process can be identified by their atomic mass via a reversible adsorption of ions in the thin carbon films. Consequently, acelectrogravimetry appears to be an attractive tool for studying the nature of the ionic flux at the electrode/electrolyte interface.

In this study, carbon nanotubes (CNTs), graphene and CNTs/polypyrrole, CNTs/Prussian blue nanocomposites have been selected due to their unique set of characteristics suitable for electrode materials in electrochemical devices (EDLCs as well as pseudocapacitors and hybrid capacitors). To achieve these objectives, new models, taking into account of non-redox processes and the combination of redox and non-redox processes, have been developed.

Chapter II. Experimental Part

CHAPTER II Experimental Part

Chapter 2 begins with a detailed description of sample preparation. Three kinds of samples were prepared: Carbon nanotubes (CNTs), Carbon Nanocomposites and Electrochemically Reduced Graphene Oxides (ERGO). Thereafter follows a description of the methods used for the structural and morphological characterization of samples, particularly, the principle of operation of the methods, the type of instruments used and under which conditions the experiments were performed.

Finally, the electrochemical characterization techniques applied in this Ph.D. thesis will be presented.

II-1. Preparation Procedure of Carbon Based Thin Film Electrodes

Materials such as Carbon Nanotubes (CNTs), Carbon Nanocomposites and Graphene Oxides were used for producing carbon based thin films on gold-patterned quartz substrates of 9 MHz as working electrodes. The reason for using the gold-patterned quartz substrates of 9 MHz as working electrodes (WE) is twofold: (i) it allows the EQCM and ac-electrogravimetric measurements to be realized, and (ii) it permits the recording of the frequency of the quartz resonator before and after the deposition of the CNTs or Graphene Oxide which is related to the deposited film mass using Sauerbrey's equation which will be detailed in the following.

II-1.1. Preparation of Carbon Nanotube (CNT) Based Thin Film Electrodes (Single Wall CNT, Double Wall CNT, Multi Wall CNT)

To achieve nanostructured electrodes, three types of carbon nanotubes: Single Wall CNT (755117-1G, length: 300-2300 nm and diameter: 0.7-1.1 nm), Double Wall CNT (755141-1G, length: 3 μm and diameter: 3.5 nm) and Multi Wall CNT (75517-1G, length: 1 μm and diameter: 9.5), acquired in Sigma Aldrich Company were chosen due to their tailorable pore dimensions and variety of specific surface areas. The preparation of CNT films was carried out according to the method described in the literature. 2,140 CNTs were deposited by the "drop-casting" method on a gold electrode (effective surface area of 0.20 cm 2 ) of a quartz crystal resonator (9 MHz -AWS, Valencia, Spain), from a solution of carbon containing 90% CNT powder and 10% PVDF-HFP (Poly(vinylidene fluoride-hexafluoropropylene)) polymer binder in N-methyl-2-pyrrolidone.

Around 8 μL of this solution was deposited on the gold electrode of the QCM. Then, the carbon films were subjected to a heat treatment at 120 °C for 30 minutes, with a heating rate of ~ 5 °C min - Thereafter, EQCM and ac-electrogravimetric measurements were realized in 0.5 M aqueous solutions containing Li + , Na + , K + and Cl -ions such as LiCl, NaCl and KCl (at different pH values).

Moreover, measurements were realized in propylene carbonate containing 0.5 M LiClO4.

I-1.2. Elaboration of Nanocomposite Structures

Particularly, nanocomposite electrodes of CNTs and conducting polymers have been intensively studied the last few years to improve the specific capacitance of the supercapacitors. 85,145,146 In this Ph.D. thesis, Prussian Blue (PB) and Polypyrrole were chosen to elaborate nanocomposite structures, because of their interesting properties and applications found in the literature. 126,145,147,148 The elaboration of such thin film nanocomposite electrodes is explained below.

II-1.2.1. SWCNT/Prussian Blue Thin Film Electrodes

The preparation of SWCNT/PB nanocomposite films was carried out according to the method described in the literature 126 . The procedure consists of cycling the SWCNT film in a solution containing 1 x 10 -3 M K3Fe(CN)6 and 0.1 M KCl, without the addition of Fe 3+ ions, at a scan rate of 0.05 V s -1 in a potential range from -0.2 V to 0.8 V vs. Ag/AgCl during 15 cycles. Then, electrochemical studies were realized in an aqueous solution of 0.5 M KCl, at pH 3.

II-1.2.2. SWCNT/Polypyrrole Thin Film Electrodes

The elaboration of the SWCNT/Polypyrrole nanocomposite films was established following a method described in the literature 4 . The procedure consists of cycling the SWCNT film, previously prepared in a solution containing 0.1 M pyrrole (previously distilled) and 0.05 M sodium dodecylsulphate (NaDS), at a scan rate of 0.01 V s -1 in a potential range from 0 V to 0.675 V vs.

Ag/AgCl during 2 cycles. The chemicals NaDS (436143-25G) and pyrrole were acquired from Sigma Aldrich. EQCM and ac-electogravimetric measurements were realized in a 0.5 M aqueous solution of CsCl.

II-1.3. Elaboration of Electrochemically Reduced Graphene Oxide (ERGO) Thin Film Electrodes

First, a GO thin film was prepared by depositing a GO suspension (GO powder in doubledistilled water) on a gold-patterned quartz resonator as shown in Figure II-1. A 1 mg/ml GO suspension was prepared from GO powder in double-distilled water. This suspension was then placed in an ultrasonic bath for 3 hours. Then, 20 µL of GO suspension was deposited on the gold electrode of the quartz resonator. To do this in a controlled manner, a laboratory-made mask (proving the exposure of the gold electrode only) was used to prevent the solution to spread out on to the whole surface of the quartz resonators. A GO film was formed after drying in an oven at 70 °C for 1h. The film thickness was estimated to be around 50 nm based on FEG-SEM measurements.

After the GO was successfully deposited on the quartz, the sample was electrochemically reduced to obtain ERGO. The electrochemical reduction was carried out by chronoamperometry at -1.1 V vs. Ag/AgCl during 5 minutes with a Biologic SP-200 potentiostat under the Ec-Lab software.

II-2. Structural and Morphological Investigation Methods

This section is dedicated to the various structural and morphological methods for characterization of carbon based materials. Particularly, the principle of operation of the methods, the type of instruments and the preparation of the samples for the experiments will be explained.

II-2.1. Scanning Electron Microscopy (SEM)

Figure II-2 shows a schematic description of a SEM set-up. The electron gun generates a primary electron beam in a vacuum. The beam is collimated by electromagnetic condenser lenses, focused by an objective lens, and scanned across the surface of the sample by electromagnetic deflection coils. 149 As a consequence of the sample/primary beam interaction, secondary electrons are emitted from the sample. 122 The secondary electrons are selectively attracted towards the secondary electron detector through a grid held at a low positive potential with respect to the sample. 149 There, the secondary electrons are counted (number of electrons arrived/unit time) and the count rate is translated into an electrical signal, 149 which can be used for a visualization of the sample. 122 The resulting image reflects the surface topology, since the number of electrons that are collected from each point is dependent on surface topology. In shallow surfaces, fewer electrons escape the surface and reach the detector resulting in darker areas, while at the edges and at convex surfaces, more electrons escape and reach the detector resulting in brighter areas. However, charge is built up on the sample surface by the electron bombardment of the primary beam and, in the case of non-conductive samples, this charge cannot be dissipated, which results in images that are bright throughout the sample. To avoid this, samples can be sputtered with a very thin layer of metals such as gold or gold/palladium as a part of the sample preparation. 149 There are various types of electron guns. In comparison to thermionic electron guns, field emission guns (FEGs) provide enhanced electron brightness (approximately x 100) and longer lifetime. Furthermore, because of the lower electron energy spread (0.3 eV), the chromatic aberration is significantly reduced and probes smaller than 2 nm can be formed, thus, providing much higher resolution for SEM images. 122

In this Ph.D. thesis, the morphology and the thickness of the films were investigated by FEG-SEM (Zeiss, Supra 55). Before the analysis, the samples were fixed onto an aluminum stub with a conductive carbon tape and sputter-coated with gold ("JEOL JFC-1300 Auto fine coater").

II-2.2. Transmission Electron Microscopy (TEM)

Figure II-3 shows a schematic presentation of a TEM set-up. The electron gun generates a primary electron beam either by thermoionic emission (using a heated filament as the cathode, e.g.

LaB6) or by field emission (using a fine tungsten tip as the cathode). The primary electron beam is focused by lenses into a thin, coherent beam, which hits the sample, resulting in various kinds of interaction between the electrons and the sample, i.e. absorption, diffraction, and elastic or inelastic scattering. The scattering processes are a result of the interaction between electrons and the nuclei of the sample. Therefore, heavy elements lead to higher scattering, which means that fewer electrons reach the image plate. Since the brightness of the image depends on the number of electrons reaching the image plate, sample regions containing heavy elements, i.e. fewer electrons, appear dark. Conversely, areas that contain only light elements, e.g. organic compounds, appear bright due to a limited scattering of electrons.

After hitting the sample, the transmitted portion of the beam is focused by an objective lens.

The beam can be restricted by objective and select area apertures; the former enhances contrasts by blocking out high-angle diffracted electrons, the latter enables the examination of the periodic diffraction of electrons by ordered arrangements of the atoms in the sample. Thereafter, the electron beam passes magnetic lenses for magnification. Finally, the beam strikes a fluorescent screen coated with CdS and ZnS and the resulting irradiation is recorded by a CDD camera. 149 TEM can be operated in two modes: classic mode and high resolution mode (HR-TEM). In comparison to the classic mode, HR-TEM uses a much larger objective aperture (if any at all) to transfer high spatial frequencies. Then, the directly transmitted beam may interfere with one or more diffracted beams causing a contrast across the image (i.e. the bright-field image), which depends on the relative phases of the various beams. In this Ph.D. thesis, High Resolution Transmission Electron Microscopy (HR-TEM) analyses were performed using a JEOL 2010 UHR microscope operating at 200 kV equipped with a TCD camera. The samples were scraped off the substrate and ultrasonicated in ethanol before being placed on copper TEM grids.

II-2.3. Energy Dispersive X-rays (EDX)

Normally, EDX is associated to SEM or TEM. When the electron beam of the SEM is scanned across the sample, it causes excitation of characteristic x-rays from the atoms in the top few microns. The energy of each x-ray is specific to the atom from which it escaped. The EDX system collects the x-rays, sorts them by energy and displays the number of x-rays versus their energy. This qualitative EDX spectrum is captured electronically for display and processing (Figure II-4). In this Ph.D. thesis, the elemental analysis was performed with an energy dispersive X-ray (EDX) detector associated to the FEG-SEM.

II-2.4. X-ray Diffraction

The definition of X-rays is short-wavelength electromagnetic radiation produced either by the deceleration of high-energy electrons or by electronic transitions involving electrons in the inner orbitals of atoms. X-rays are produced in a vacuum tube containing two electrodes: an anode (usually of copper) and a cathode (usually a tungsten filament). First, the cathode is heated to provoke the emission of electrons and then, a high voltage is applied across the electrodes, resulting in streams of electrons (cathode rays) that are accelerated from the cathode to the anode and induce the emission of X-ray radiation from the anode. X-ray diffraction can be used to investigate crystal structures. Ideally, crystal structures consist of the infinite repetition of identical structural units in space, which can be either a single atom (such as in copper, silver or gold) or various atoms or molecules. 149 When investigating crystal structures by X-ray diffraction, Bragg's Law is the most basic and important principle (Figure II-5). Positive interference only occurs if the phase shift of the two rays is a multiple of the wavelength of the X-rays (λ). Since the phase shift is 2 d sinθ, the Bragg equation 120,149,153,154 is:

2 sin nd   (II-1)
In this Ph.D. thesis, the crystal structure of the samples were analyzed by using an Empyrean Panalytical X-ray diffractometer (Cu Kα radiation, λ = 1.541 84 Å) operated at 60 kV. where P and P0 are the equilibrium and the saturation pressure of adsorbates at the temperature of adsorption, n is the adsorbed gas quantity and C is the BET constant,

1 exp      L EE C RT (II-4)
where 1

E is the heat of adsorption for the first layer and L E is the heat of adsorption for the second and higher layers and is equal to the heat of liquefaction.

The specific surface area SBET (m 2 /g) is obtained from the monolayer capacity by applying the following equation:

8 2 1 *10 ( ) BET m m A S n N m g     (II-5)
where NA is the Avogadro constant and m  is the so-called cross-sectional area (the average area occupied by each molecule in a complete monolayer). The BET equation is applicable for surface area analysis of nonporous and mesoporous materials consisting of pores of wide pore diameter (> 4nm), but, in a strict sense, it is not applicable to microporous adsorbents. In addition, it seems that the BET method is inaccurate for estimating the surface area of mesoporous molecular sieves of pore widths < ca. 4 nm, because pore filling is observed at pressures very close to the pressure range where monolayer-multilayer formation on the pore walls occurs, which may lead to a significant overestimation of the monolayer capacity. [155][156][157] The BJH 158 method for calculating pore size distribution (PSD) is based on a model of the adsorbent as a collection of cylindrical pores. The theory accounts for capillary condensation in the pores using the classical Kelvin equation, which in turn assumes a hemispherical liquid-vapor meniscus and a well-defined surface tension,  , of the bulk fluid, the densities of the coexistent liquid ρ I and gas ρ g ( ∆ρ = ρ I ρ g ) and the contact angle θ of the liquid meniscus against the pore wall. For cylindrical pores the modified Kelvin equation is given by:

0 ln( / ) 2 cos / ( )        pc P P RT r t (II-6)
where R is the universal gas constant, P is the Pressure, Po is the saturation vapor pressure at the adsorption temperature, p r the pore radius and c t the thickness of an adsorbed multilayer film;

which is formed prior to pore condensation.

In this Ph.D. thesis, Nitrogen adsorption-desorption isotherms of the samples were recorded at 77 K (temperature of liquid N2) using a BELSORP-max high performance surface area and porosity analysis instrument. Samples were previously degassed at 250 o C and 0.397 µPa. The specific surface area and pore size distribution (PSD) were calculated according to the BET and BJH method. The name Piezoelectricity derives from the Greek words, piezein, which means to press [159][160][161] and elektron 161 or in other words, this term means "electricity by pressure". 160 The piezoelectric effect was first reported by the Curie brothers in 1880 [START_REF] Gabrielli | Ac-Electrogravimetry Study of Electroactive Thin Films. I. Application to Prussian Blue[END_REF]160,[162][163][164][165][166][167][168][169] and, a year later, the name "piezoelectricity" was proposed by Hankel 160,164 . Furthermore, Lippmann predicted the converse effect based on thermodynamic considerations 162,165,167,170 , which was verified by the Curies the same year. The piezoelectric effect is a property of certain materials (without a center of symmetry) that become electrically charged when a mechanical stress is applied (direct effect). Conversely, these materials can be deformed under the action of an external electrical field (converse effect). 109,167 That is to say, the applied potential creates a reorientation of the dipoles in the acentric material causing its shear deformation . This is the basic principle of the microbalance operation.

II

The direct and converse piezoelectric effect can be applied in several areas. The direct effect is used in sensor applications, 109,165,171 while the converse effect is used in ultrasound and communication techniques such as measuring the speed of sound, transmitting signals, remote controlling of equipment etc. Another important application is the field of actuators such as loudspeakers, piezoelectric motors, inkjet printers 109,165,[171][172][173] etc.

Of all the piezoelectric materials existing in nature or made industrially, quartz is one of the most commonly used. This material is insoluble in water and can resist to temperatures up to 573°C without losing its piezoelectric properties 174 . The quartz (empirical formula is SiO2) has trigonaltrapezohedral crystal symmetry resulting from its hexagonal prism structure with six cap faces at each end, which are shown in Figure The Z axis of quartz is called the optic axis. This axis of symmetry of order 3 is in an orthogonal X (electrical axis), Y (mechanical axis) coordinate system (Figure II-7c). 176,177 The cutting of the quartz crystal, with respect to its crystallographic axes, determines the mode of vibration and its frequency domain. The thickness of the quartz determines its resonant frequency. On each side of the disk, a gold electrodeis deposited by vacuum evaporation. Each electrode is in the form of a disk, 5 mm in diameter, extended by a "leg" for the electrical connection. In practice, a sub-layer of chromium is sandwiched between the crystal surface and the gold electrode to have better adhesion. In general, the gold electrode has a thickness of 100 to 200 nm and the sub-layer of chromium is about ten times finer. The piezoelectric effect can take place in the area defined by the two gold electrodes that are placed opposite to each other, the so-called active area.

II-3.1.2. Working Principle of QCM

The quartz crystal can convert a mechanical stress into an electrical signal, which can be used to make a transducer or a sensor. 178 The piezoelectric transducer is a device that transforms an interaction at the modified electrode/liquid interface into a measurable electrical signal. 109 The piezoelectric transducer, also called quartz crystal microbalance 179 (QCM), is an attractive tool for gravimetric measurements. Applications can be found in many research fields such as chemical sensors 179,180 and biosensors. 181 This device permits the measurement of a mass sample deposited on the surface of a piezoelectric crystal through the change of its resonant frequency characteristics. 133,138,163,177,182,183 The QCM is based on a thin quartz crystal linked to an electrical circuit that is strong enough to create vibrational motion of the crystal at its resonant frequency (crystal oscillator). 163,178,184 The microbalance frequency may be followed over time, the precision depending on the measuring parameters and the stability of the microbalance. [185][186][187] The QCM is a high precision instrument which permits the determination of the mass variations of the quartz surfaces by measuring the changes of the resonance frequency of the quartz resonator.

In 1959, Sauerbrey 188 was the first to establish an equation (II-7) showing the relationship between the mass and frequency changes. When a rigid, purely elastic and electroacoustically thin layer is deposited on the quartz, the relationship between the mass of the material and the quartz crystal frequency change is:

2 2 qq f o f k x m m ms Sn             (II-7)
where f m  is the frequency change of the microbalance, is the mass change of the microbalance active surface (g), q

 is the quartz density (2.648 g.cm -3 ), μq is the shear modulus of a shear AT quartz crystal (2.947 x 10 11 g cm -1 s -2 ), fo is the fundamental resonant frequency of the quartz (Hz), S is the active surface on the quartz corresponding to the metal electrode deposited on it (cm 2 ), n is the overtone number and ks is the theoretical sensitivity factor (Hz g -1 cm 2 ).

With a QCM operating at 9 MHz, the theoretical sensitivity factor calculated from the Sauerbrey equation is equal to 18.3x10 7 Hz g -1 cm 2 . Bizet et al. 189 have determined the sensitivity coefficients by electrodeposition of copper for QCMs operating at 6, 9 and 27 MHz. Frequency changes of these three transducers were followed as a function of time during copper electrodeposition at different electrical current values. For resonators operating at 9 MHz, the experimental mass sensitivity factor found is equal to 16.31 (± 0.32)10 7 Hz g -1 cm 2 at an imposed current of 0.5 mA, a value which is in fairly good agreement with the theory. This experimental mass sensitivity factor is used throughout the present Ph.D. thesis.

The experimental procedure for estimating exp s k is as follows: A cathodic current is applied to the gold electrode deposited on the quartz surface in a Cu 2+ containing solution. This results in the reduction of Cu 2+ ions to Cu 0 and, therefore, leads to the electrogeneration of copper on the gold electrode (Cu 2+ + 2e - Cu). The deposit is accumulated on the microbalance electrode, thus, causing a resonant frequency decrease due to the augmentation of mass during the electrodeposition. The current applied is controlled through a galvanostat and the mass change, ∆m, is calculated by assuming that the efficiency of the reduction reaction of copper is equal to 100%.

The experimental mass sensitivity factor is determined according to the following equation (II-8):

exp m s f f t k S S m t m            (II- 8)
where ∆t is the time variation. By plotting the frequency vs time, the ratio

Δf Δt is experimentally determined. The ratio t m  
is calculated using Faraday's law, assuming a 100% efficiency. The charge variation, ∆q, is given by equation (II-9):

q i t     (II-9)
where i is the current.

The number of moles of Cu deposited is calculated from:

Cu e e q i t n n F n F        (II-10)
The mass change of the copper deposited is defined by:

Cu e it m n M nF        (II-11)
Dividing equation (II-11) by ∆t, it comes:

ee m j M j t S t n F n F            (II-12)
where J corresponds to the current density (A cm -2 ), M is the molar mass of copper (63 g mol -1 ), F is Faraday's constant (96500 C mol -1 ) and ne-is the electron number. Therefore, by inserting ∆ƒ/∆t and ∆m/∆t in equation (II-8), can be estimated.

II-3.1.3. Experimental Set-Up

The quartz crystal and its electrodes make up a piezoelectric resonator: surface changes (in

II-3.2. Cyclic Electrogravimetry (EQCM)

As mentioned previously, cyclic electrogravimetry, also commonly called EQCM, is a method based on the coupling between cyclic voltammetry (CV) and gravimetric measurements through a QCM device. Using this method, information on the mass changes of the electrode (calculated from Sauerbrey's equation) during an electrochemical process can be obtained.

II

II-3.2.2. Experimental Set-Up

The experimental set-up is composed of an electrogravimetric cell (AWS, Spain) with three electrodes: a working electrode, WE, (a gold electrode deposited on a quartz resonator, 0.5 cm in diameter and an effective surface area of 0.2 cm 2 ), a reference electrode, RE, in silver/silver chloride (Ag/AgCl) and a counter electrode, CE, a platinum grid. This cell is connected to a potentiostatic system (Biologic SP200 or Autolab PG stat 100), a frequency-meter (Yokogawa), the QCM and a computer (Figure II-12).

In this Ph.D. thesis, EQCM is utilized to analyze (i) the electrodeposition process of electroactive films, and (ii) the insertion and/or electroadsorption process of super-and/or pseudocapacitive electrode materials.

The QCM electrode that operates as the working electrode is in contact with the electrolyte.

For example, metallic cations may be dissolved in an acidic solution. By applying a negative potential to the working electrode, the cations can be deposited onto the electrode surface as metal atoms and the deposited mass is determined using the QCM. Moreover, the current during the deposition is also monitored and, by integration, the charge per surface unit can be measured. If all the charges are used for the deposition, the charge-to-mass ratio can be determined. This relation between the charge transfer and the mass deposited on the WE can be used as an alternate method for evaluating the thickness of a deposited layer. 193 Assuming that the number of moles of the substance deposited on the electrode equals the number of moles of electrons interchanged during the reaction, and knowing the density of the material deposited onto the quartz electrode, the thickness of the layer can be estimated as follows:

1 1 qM h nFS   (II-13)
where S is the surface where the substance is deposited, q is the charge involved in the process, n is the number of electrons interchanged in the reaction, F is Faraday's constant, M is the molecular weight and 1  is the density of the substance.

In the case of supercapacitor studies, the mass change, according to Sauerbrey's equation, will produce a resonant frequency shift of the quartz resonator controlled via a circuit oscillator. 194,195 This frequency is monitored by the frequency meter and, at the same time, the resulting current is measured by the potentiostat. These measurements are registered in a central computer.

II-3.2.3. Calculation of the F(dm/dq) Function

The analysis of EQCM data by means of the F(dm/dq) function gives the mass/charge ratio values at each applied potential during a voltammetric scan, which provides the information on the apparent mass of the ions, M, involved in the charge compensation. 133,177 The instantaneous value of this ratio at each potential needs a prior evaluation of the dm/dt curve. For this calculation, mass, m, and current values, i, need to be previously filtered by a numerical procedure based on the fast Fourier transform and then, smoothed through Savitzky-Golay procedure. Then, Fdm/dq can be evaluated as:

/ / / dm dm dt dm dt M F F F dq dq dt i    (II-14)
where F is the Faraday constant (C. mol -1 ), dm is the mass variation, dq is the charge variation and M is the global molar mass. A proof of this relation is given by considering the insertion reaction of a monovalent cation:

1 2 , k k P C P C            (II-15)
where, <P> is the free site in the film, C + is the monovalent cation and <P,C + > is the site associated with a monovalent cation.

When a potential variation, ∆E, is applied to the electrochemical system, mass and charge variations may occur according to the relation:

  C dq dq F M dn dm (II-16) dq dm F M C    (II-17)
where dq is the charge variation during the insertion of C + and dn the variation of the number of moles of C + .

This proof was made considering only one cation contribution. The calculation is similar when only one anion is involved in the electrochemical process. If there is another species participating, the limitation of this method appears as only a global molar mass that represents the contribution of both species can be estimated. Under such circumstances, it is impossible to discriminate between the contributions of different species, solely based on the cyclic electrogravimetry (EQCM) approach.

II-3.3. Electrochemical Impedance Spectroscopy (EIS)

Electrochemical reactions can be composed of a succession of steps with different kinetics.

To better understand the detailed mechanism of an electrochemical system, an appropriate method of separation of each reaction is necessary. However, conventional techniques do not always allow the distinction between these elementary steps to be done. Therefore, electrochemical impedance spectroscopy (EIS) can be useful as it often provides the information necessary to distinguish one reaction step from the other. In general, electrochemical impedance has been used to study the reaction mechanisms occurring at the electrode/electrolyte interface. 196,197 Also, EIS has been employed in different domains such as: corrosion, 198 energy 39,113,115 and biosensors. 179

II-3.3.1. Principle

EIS measurements are often performed under potentiostatic regulation. The basic principle of EIS is to apply a low amplitude sinusoidal perturbation, at a certain frequency, given around a stationary potential. The electrochemical system is under stable condition and the low level of disturbance associated with frequency modulation should help to separate the different reactions with different kinetics: fast phenomena occur at high frequencies while the slower phenomena are visible at lower frequencies. 37,40,198,199 In general, electrochemical systems have a nonlinear currentvoltage I=f(V) characteristic curve (Figure II-13). However, working only on a small portion of the so-called Lissajous curve, the relationship between ∆V (low amplitude sinusoidal perturbation imposed around the stationary potential) and ∆I (low amplitude sinusoidal response in current) may be considered as linear (Figure II-13).

Therefore, by applying a low amplitude sinusoidal perturbation, ∆V, to the system around a stationary potential, Vs, the current response will also be sinusoidal in shape, and with a small amplitude, ∆I, around a stationary value Is.

The concept of impedance is a generalization of the concept of resistance, applicable to sinusoidal functions (there is a Fourier transform to all the functions).

The transfer function ∆V/∆I(ω), called the electrochemical impedance () Z  , can be defined with ω = 2πf corresponding to the pulsation. The electrochemical impedance () Z  is a complex number that can be represented either in polar coordinates (|Z|,φ), or Cartesian coordinates (Re Z, Im Z) :

( ) exp Re Im     Z Z j Z j Z (II-18)
The relationship between these quantities are:

-on the one hand, When applying a low amplitude sinusoidal perturbation potential to an electrochemical cell, the total current obtained is the sum of Faraday's current, if, and the capacitive current, ic. The faradic process characterizes the transfer of electrons at the electrode/electrolyte interface. The double layer defines the electrical charge at the interface and is characterized by a capacitance, Cd.

Therefore, it is possible to model in a classical manner an electrochemical cell by an equivalent circuit comprising a double layer capacitance in parallel with a transfer resistance, Rt, and introducing in series an electrolyte, Re, (Figure II-14a). 109,199,200 Consequently, the electrochemical impedance of the equivalent circuit is as follows:

1 () 1     e d t ZR jC R (II-23)

II-3.3.2. Experimental Set-Up

The experimental set-up used in this Ph.D. thesis is shown in Figure II-15. It consists of:

-A SOLETEM-PGSTAT100 potentiostat that works under potentiostatic mode, and imposes a given stationary potential, Vs.

-A frequency generator, included in the Frequency Response Analyzer (FRA 1254, Solartron), that delivers the low amplitude sinusoidal perturbation, ∆V.

-A three-electrode cell.

II-3.4. Ac-Electrogravimetry -A Fast Electrogravimetric Method

As mentioned previously, the ac-electrogravimetry consists of coupling EIS measurements with a fast response quartz crystal microbalance (QCM) used in dynamic mode. This technique was first proposed by Gabrielli et al. 201 at the LISE laboratory in France and it can be used to discriminate the activity of the different species involved in the charge transfer during an electrochemical process. More precisely, it provides access to relevant information on the kinetics of species transferred at the electrode/electrolyte interface and their transport in the bulk of the materials, the nature of these species as well as their relative concentration within the material.

Consequently, ac-electrogravimetry appears to be an attractive tool for studying the nature of the ionic flux at the electrode/electrolyte interface.

II-3.4.1. Principle

The principle of ac-electrogravimetry is analogous to the principle of the electrochemical impedance described in the previous section. A low amplitude sinusoidal perturbation, ∆V, applied to an electrochemical system, can lead to changes in the mass of the film placed on the microbalance surface, ∆m, allowing the measurement of the electrogravimetric transfer . 4,131,202 The measurements are made with the same three-electrode cell as described in Section II-3.2.2. When the low amplitude sinusoidal perturbation, ∆V, is applied to the film deposited on the gold electrode of the QCM, some of the charged species in the solution participate in the charge compensation process within the electroactive material. This insertion/removal of species into/from () ()

t e t E V RR I I R                (II-24)
where Rt is the resistance used in the counter electrode circuit, incorporated in the potentiostat for measuring the current, and Re is the electrolyte resistance taken from the high frequency value of (∆V/∆I) (ω).

As mentioned previously, the raw transfer function given by the 1254 FRA,

()    f V V , must
be corrected to obtain the final mass/potential transfer function

()    m E , ( ) ( ) ( ) ( ) f m mf V f V mm E f V V E               (II-25)
Four different transfer functions can be distinguished, as described below: Hz g -1 cm 2 as justified previously. 189

-   m m f    is related
-    f m V f  
is the inverse of the f/v converter sensitivity and can be estimated by using the following equation:

( ) ( ) mm f f ff e V e V        (II-26)
where

    / / m s f e f e       
is the sensitivity of the reference synthesizer by assuming

ms ff    .    f V e   /
is the inverse of the global calibration transfer function, (∆Vƒ/∆e)(ω). It has been observed that the voltage perturbation during a calibration is ∆e = |∆e|.sin(ωt). Furthermore, the sensitivity of the converter is given by ∆Vƒ/∆ƒs. 109

-    V V f  
is the raw experimental transfer function obtained from the 1254 FRA. The useable frequencies, that is, where the system can be used without any correction, ranges from low frequencies to 1 Hz for this particular configuration. If measurements are expected above 1 Hz, a modulus and phase shift correction is necessary by using calibration curves such as those given in From the experimental data, the following transfer functions are calculated and are taken into account in the fittings in II-3.5.2 as follows:

Using the electrochemical impedance and the relation

q j I    
, the charge/potential TF can be estimated:

exp exp 1      qI E j E (II-29)
The disturbance causes a charge compensation within the film: species come and go at the discretion of the disturbance signal. Physically, this corresponds to a variation of mass.

From the experimental electrogravimetric transfer function, it is possible to calculate the experimental partial transfer functions, eliminating the contribution of an anion or a cation, where four species are taken into account: cation1 (c1), cation 2 (c2), anion (a) and solvent (s). For example, if the contribution of the cation 2 is extracted, the contribution of the cation 1, the solvent and the anion is calculated according to the following equation:

1 2 exp exp exp        c as c m m m q E E F E (II-30)
Furthermore, if the contribution of the cation 1 is extracted, it comes:

2 1 exp exp exp        c as c m m m q E E F E (II-31)
and if the contribution of the anion is extracted, it comes :

12 exp exp exp       c c s a m q mm E E F E
(II-32)

II-3.5.2. Fitting from Mathcad Simulation Data

The fitting of the experimental TFs with the theoretical TFs (which will be described in the following) by using Mathcad software (PTC Mathcad 15.0) allows the pertinent information on the charge compensation process to be obtained. where <P> is the free site "in" the film, <P,C + > is the site occupied with a monovalent cation and <P,A -> is the site occupied with a monovalent anion. The ki are related to the transfer kinetic rates.

Then, the instantaneous flux of each species, Ji, can be written as follows:

For a cation:

12 ,max min ( ( )[ ] ( ))           c f c sol c C C C C J d k C C C k C C (II-35)
For an anion:

1 2 ,max min ( ( )[ ] ( ))           a f a sol a A A A A J d k C C A k C C (II-
dt C d d J i f i ) (    (II-37)
where df is the film thickness (m), i J  is the variation of the flux related to the species i crossing the film/electrolyte interface (mol.m -2 .s -1 ) and i C  is the variation of the concentration in the film (mol.m -3 ).

In dynamic regime, the flux change i J  is a function of the potential E and the concentration of the species, i C  , can also be written as:

           ii i i i i i i JJ J C E K C G E CE (II-38) Then, the ( ) i th C E   
transfer function can be calculated using the equations (II-37) and (II-38):

( )        i i i th C G E j d K (II-39)
where ω = 2πƒ is the pulsation, ƒ is the frequency of potential modulation and Ki and Gi are the partial derivatives of the flux with respect to the concentration and the potential, respectively.

E i i i C J K            and i C i i E J G          (II-40)
Then, the "virtual" transfer function given in equation (II-39) allows the calculation of all the other measured transfer functions. Thus, the following theoretical expressions have been used to fit all the experimental transfer functions. From the equation (II-39) and considering four species involved in the electrochemical process: cation 1, c1, cation 2, c2, anion, a, and solvent, s, the following equations can be listed:

  11 1 ()      cc fc th CG E j d K (II-41)   2 2 2 ()       c c fc th C G E j d K (II-42)   ()       a a fa th C G E j d K (II-43)   ()       s s fs th C G E j d K (II-44)

II-3.5.2.1. Charge/Potential Transfer Function

The charge/potential transfer function,

     th q E
, calculated for the adsorption/desorption of two cations, c1 and c2, and an anion, a, using the Faraday number, F can be written as follows :

  12                    c c a f th C C C q dF E E E E (II-45)
Replacing the equations (II-39) with the equation (II-45) gives:

12 12 ( ) ( ) ( )                c c a f th f c f c f a G G G q dF E j d K j d K j d K (II-46) II-3.5.2

.2. Electrochemical Impedance

The theoretical electrochemical impedance can be calculated as follows:

  12 1                    c c a f th C C C qI dF E j E E E E (II-47)
Replacing the equation (II-39) with the equation (II-47) gives:

1 2 1 2 1 ()                      th c c a f f c f c f a E I G G G j d F j d K j d K j d K (II-48)
In theory, if faradic reactions are present, the theoretical Faradaic impedance

()  F th Z
should be taken into account.

II-3.5.2.3. Electrogravimetric Transfer Function

The theoretical electrogravimetric transfer function,

     th m E
, can be calculated, taking into account the charged/uncharged species contribution:

12 12 ()                   c c a s f c c a s th C C C C m d m m m m E E E E E (II-49)
Then, replacing the equation (II-39) with the equation (II-49) results in:

12 12 12 () ( ) ( ) ( ) ( )                        c c a s f c c a s th f c f c f a f s G G G G m d m m m m E j d K j d K j d K j d K (II-50)
where mc1, mc2, ma and ms are the atomic weight of involved species.

From the theoretical overall electrogravimetric transfer function (II-50), it is possible to calculate the theoretical partial transfer functions by removing the c2 contribution, calculating

  1    c as th q E ; or the c1 contribution, calculating;   2    c as th q E or the anion contribution, calculating   12    c c s th q E
; as shown in the following equations:

    1 1 1 2 2 ()                   c as c a s f c c a c s th C C C m d m m m m m E E E E (II-51)     2 1 2 1 1 ()                     c as c a s f c c a c s th C C C m d m m m m m E E E E (II-52)     12 12 1 2 2 1 ()                  c c s c c s f c c c c s th C C C m d m m m m m E E E E (II-53)
Chapter III. Ion Dynamics in SWCNT Based Thin Film Electrodes

CHAPTER III

Ion dynamics in SWCNT Based Thin Film Electrodes

In this chapter the Ac-electrogravimetry is chosen as a characterization methodology due to its ability to give very relevant information such as: kinetics and identification of species transferred at the electrode/electrolyte interface, separation of the contribution of different species and determination of the relative concentration changes of the species in the electrode. The aim of this part is to contribute to the clarification of the mechanism of the ion transfer of carbon based electrodes. It will provide a better understanding of the charge storage mechanisms and validate the positive impact of using nanostructured electrodes in energy storage devices. Here, a pertinent example is presented and detailed to demonstrate the power of our approach for SWCNT films tested in NaCl electrolyte. At the end, a key question about the possibility to deconvolute the classical EQCM responses is raised, and the dynamic and gravimetric aspects of this global response are discussed.

III-1. Structure and Morphology of the SWCNT Powders and Prepared Thin Film Electrodes

The surface morphology of the SWCNT thin films deposited on the gold electrode of the quartz resonators were characterized by FEG-SEM. Prior to the thin film preparation, the SWCNT powders were characterized by HRTEM, XRD and N2 sorption.

SWCNT thin films were prepared directly on the gold electrode surface using the PVDF-HFP as a polymer binder according to the protocol given in the experimental part (See Chapter II Section SWCNTs is ~ 1-2 nm and present bundles of at least 5 to 10 CNTs together. These dimensions are in good agreement with the values for similar structures reported in the literature. 113,115 Figure III-2A and 2B depicts the XRD pattern of the SWCNTs. The strongest diffraction peak related to the SWCNT structure at the angle (2θ) of ~ 25° can be indexed as the (002) reflection of the hexagonal graphite structure similar to the structures reported in the literature. 206 The sharpness of the peak at the angle (2θ) of 25° is related to the graphitic structure and a slight broadening in The slight distortion from a perfect rectangular shape is due to the presence of a slight faradic contribution to the charge storage. This small faradic contribution can be due to several reactions such as (i) the reduction of the protons present in the electrolyte, (ii) the reduction of remaining surface functional groups on SWCNTs and/or (iii) catalyst remaining after purifying carbon nanotubes. A fairly large number of publications related to the carbons with oxygen-containing surface functional groups have been found in the literature. [208][209][210][211][212][213] The corresponding mass variations is followed by simultaneous QCM measurements as a function of potential. In According to electrostatic considerations, ions contributing to the charge compensation should be the cations and anions at a potential below and above the pzc, respectively. Indeed, when a more cathodic potential than pzc is applied to the electrode, the electrons generated at SWCNT film/electrolyte interface are compensated mainly by the cations present in the electrolyte via an electroadsorption process: the global mass of the electrode increases and this effect is determined real time with the sufficiently sensitive EQCM device. On the contrary, when a more anodic potential than pzc is applied to the electrode, the anions are expected to be electroadsorbed to charge compensate the positive charges generated at the SWCNT film/electrolyte interface. To obtain indications on the nature of the transferred species, the / /

m m t FF q Q t       
function was calculated over the whole potential range, as explained previously (Chapter II, Section 3.2.3.). Our findings and the literature results strongly suggest that the EQCM data envelops the subtleties of the supercapacitive charge storage. This global EQCM response, measured at a certain scan rate cannot give an easy access to the details related to the capacitive charge storage mechanism. Therefore, an ac-electrogravimetric study is conducted for the deconvolution of the global EQCM response into distinct contributions with a resolution at the temporal level.

III-2.2. Fdm/dq Function Calculations

Before going into the details of the ac-electrogravimetric study, the specific capacitance of the SWCNT based thin film electrodes are briefly discussed below.

III-2.3. Specific Capacitance Calculations

The specific capacitance values were calculated by using the following relation:

I (Amper) C (F/g)= specific v (volt/second)× electrode mass (g) s
where C is the specific capacitance, I is the measured current and vs is the scan rate used.

By using the data presented in

III-3. Ac-Electrogravimetric Studies of SWCNT Thin Film Electrodes in Aqueous NaCl

Electrolyte

The supercapacitive charge storage behavior was studied by ac-electrogravimetry in aqueous NaCl, at stationary potential values in the range of -0.45 V to 0.45 V vs Ag/AgCl. For the sake of clarity, the methodology of data interpretation at a selected potential will be given which allows the model used here to be justified. Then, an extension to other potentials and pertinent parameters can be obtained from this approach will be examined. At the end, the deconvolution of the classical ∆m-E curves (obtained from EQCM responses) will be our center of focus using the key results given by the ac-electrogravimetry. For the latter, the aim is to reach a deconvolution both at a gravimetric and a kinetic level.

III-3.1. EQCM versus Ac-Electrogravimetry

Ac-electrogravimetric results presented below were performed at -0.4 V vs Ag/AgCl. This part is dedicated to a step-by-step clarification of our ac-electrogravimetry experimental data analysis with different examples.

In Figure III-6A and B, the two main experimental TFs are presented:

() q E    and () m E   
.In the former, two loops can be observed with a reasonable frequency separation, the high frequency loop being twice as high in diameter as the low frequency one.

At this stage, it is impossible to unambiguously attribute these two loops to two different ionic responses. For the second experimental TF,

() m E   
, a large semi-circle was seen in the third quadrant. It indicates the contribution of cations mainly but without a fair separation of the different responses. For that, several configurations were tested using the models described in Chapter II-3.5 with the strict criteria that a good agreement between the experimental and the theoretical functions in terms of shape and frequencies was achieved. For the theoretical functions, the equations presented in Chapter II are adapted based on the number of species considered in the different hypothesis considered (i.e. Eq II-46 for

() q E    , Eq. II-50 for () m E   
and Eqs. II.51-53 for partial mass/potential transfer functions).

(i)

Hypothesis I: Considering Two Species: Solvated Na + and H2O

In Figure III-6, the theoretical transfer functions were calculated taking into account a single cation (Na + .H2O) and free solvent (H2O) molecules. It is evident that the charge/potential TF cannot be fitted with this consideration.

It appears that for low frequencies another charged species is involved as can be seen in the Re(m/ E) C2S /g cm -2 V -1

F

Figure III-6. Experimental and theoretical transfer functions at -0.4 V vs Ag/AgCl in 0.5 M NaCl. The theoretical functions were calculated taking into account only one cation (Na + . H2O) and free solvent (H2O). Theoretical functions were calculated with the following parameters: df = 0.4 µm, Kc2 = 6.91 x 10 -3 cm s -1 , Gc2 = 4.01 x 10 -6 mol s -1 cm -2 V -1 , Ks = 3.14 x 10 -3 cm s -1 , Gs = 4.08 x 10 -6 mol s -1 , mc2 = 23+18 g mol -1 , ms = 18 g mol -1 . The theoretical functions were calculated taking into account of two cations (Na + . H2O) and (H + ). Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 5.40 x 10 -5 cm s -1 , Gc1 = 1.19 x 10 -8 mol s -1 cm -2 V -1 , Kc2 = 6.91 x 10 -3 cm s -1 , Gc2 = 4.08 x 10 -6 mol s -1 , mc2 = 23+18 g mol -1 , mc1 = 1 g mol -1 . Re(m/E) C2S /g cm -2 V -1

F

Figure III-8. Experimental and theoretical transfer functions at -0.4V vs Ag/AgCl in 0.5 M NaCl. The theoretical functions were calculated taking into account two cations (Na + . H2O) and (H + ) and free solvent (H2O). Theoretical functions were calculated with the following parameters: df = 0.4 µm, Kc1 = 5.40 x 10 -5 cm s -1 , Gc1 = 1.19 x 10 -8 mol s -1 cm -2 V -1 , Kc2 = 6.91 x 10 -3 cm s -1 , Gc2 = 4.08 x 10 -6 mol s -1 , , Ks = 3.14 x 10 -3 cm s -1 , Gs = 4.08 x 10 -6 mc2 = 23+18 g mol -1 , mc1 = 1 g mol -1 .

It is important to highlight a delicate point in our ac-electrogravimetric analysis. Figure III-8E shows a small contribution at low frequencies in the fourth quadrant. Characteristically, this should correspond to a response of an anion. 109 In fact, this contribution is that of a cation of which atomic mass is smaller than the fastest cation. This is evident from Equation II-51 in Chapter II where the response is multiplied by mc1-mc2. As c1 corresponds to H + and c2 corresponds to Na + .H2O, mc1-mc2 value is negative which makes it appear as a "virtual anion". These partial electrogravimetric transfer functions (Fig. III-8E andF) provides a crosscheck for validating the hypothesis involving three different species and a better separation of the various contributions.

At this selected potential (-0.4 V vs Ag/AgCl), the hydrated Na + appears at high frequency (fast process) whereas the protons participate at low frequency (slow process). The kinetics of the free solvent molecules is close to that of hydrated Na + species suggesting that polar water molecules are electrodragged together with hydrated Na + . 218 corresponding Ki (kinetics of transfer) and Gi (related to the ease or difficulty of interfacial transfer) parameters were estimated as a function of potential. The Gi parameter was used to estimate the transfer resistance,

1 i i Rt FG 
, of the species at the film/electrolyte interface.

-0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5

1E-5 1E-4 1E-3 0,01 Cl - H 2 O H + K i / s cm -1 E/V vs Ag/AgCl Na + . H 2 O
-0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 Two different cationic species and free water molecules were detected in the potential range from -0.4 V to 0.1 V (no major anions contribution in that region). The hydrated Na + ions are persistently the fastest species, i.e.

    2   K Na .H O K H ii (Fig. III-9A
). The Rti (transfer resistance) parameter which can explain the ease or the difficulty in the transfer at the electrode/electrolyte interface present the following order:

    2    Rt H Rt Na .H O i i
(inverse of the Ki) (Fig. III-9B). Furthermore, the transfer kinetics of free water molecules are somewhat close to the values of the Na + .nH2O. Additionally, these water molecules have the same flux directions as the cations, suggesting that it might be the water molecules that accompany the transfer of Na + .nH2O, most likely due to an electrodragging effect as previously discussed at -0.4V vs Ag/AgCl.. These findings strongly suggest that Na + .H2O, although formally larger than H + , are faster and easier to be electroadsorbed at the SWCNT/electrolyte interfaces. This, a priori anomalous behavior can be discussed in terms of cation concentration or of the relationship between the dehydration energy and the ion size. For the former, the proton concentration is lower compared to the sodium and can affect the kinetics of transfer. , is more informative: two loops are obtained, at high frequency (HF) a small loop in the first quadrant, associated to the cation and at medium and low frequency (LF) loop in the second and third quadrant, correlated to solvent and cation, respectively

(Figure III-12B).
The nature of the ions and uncharged species is estimated by their molar mass thanks to the electrogravimetric transfer function. Li + ions and free solvent (PC) participate at HF and medium frequencies, respectively, while H3O + participates at LF.

It is noted that the free solvent molecules (PC) has the opposite flux direction with respect to the cations, probably providing space to the electroadsorption of the cations at the electrode/electrolyte interfaces.

It is important to note that the HF cation response is very small and not clearly visible. Indeed, the molar mass of Li + is relatively small and leads to this small contribution. Among the several configurations tested, the Li + ions contribution was absolutely necessary to fit the experimental data. Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 9.43 x 10 -5 cm s -1 , Gc1 = 1.08 x 10 -7 mol s -1 cm -2 V -1 Kc2 = 2.07 x 10 -3 cm s -1 , Gc2 = 7.26 x 10 -6 mol s -1 cm -2 V -1 , Ks = 1.86 x 10 -3 cm s - 1 , Gs = -7.25 x 10 -7 mol s -1 ,mc1 = 19 g mol -1 , mc2 = 7 g mol -1 , ms = 102 g mol -1 .

The presence of H3O + along with the Li + species is related to the measurement conditions, which are not performed in a glove-box, leading to a certain amount of water in PC especially in the presence of LiClO4 salt.

The presence of three different species (estimated by fitting the experimental data) was further confirmed by carefully analyzing the partial electrogravimetric transfer functions (Figures III-12C For the more anodic potentials, we have detected the anionic species contribution which was not the case for the results at -0.4 V vs Ag/AgCl. Considering the transfer resistance values of the ions, Rti, the Li + is the easiest species to be transfered while the H3O + is the most difficult to transfered. This is in line with their kinetics and probably due to the lower concentration of the H3O + compared to the Li + species in the electrolyte.

Furthermore, the transfer resistance values of the free solvent are close to the ClO4 -at anodic potentials. At cathodic potentials, the free solvent is first more difficult to be transferred compared to Li + but then approaches to the Rti of the cations when the potential is even more cathodic (note C -C values of the free solvent. Overall, the ClO4 -is the most significant species in terms of relative concentration which is probably related to the pzc which has moved to more cathodic potentials compared to the aqueous media (different dielectric constants and ions present in the electrolyte). The ac-electrogravimerty results presented here showed the possibility to deconvolute the EQCM response into dynamic and gravimetric components even in organic electrolytes, and the viscosity of the media is low enough to allow the microbalance in our system to be functioned correctly.

III-5. Conclusions

SWCNT carbon-based electrodes were tested in aqueous and organic electrolytes. The results of pore morphology/structure and classic EQCM confirms that SWCNTs have interesting properties for energy storage applications.

From the traditional point of view, when the carbon electrode surface is negatively charged, one can expect the adsorption of the cationic species, and when the surface is positively charged, the adsorption of the anions should take place.

In this chapter, we have shown that this nominal or simplified presentation which assumes that one net charge stored on the electrode corresponds to one single ion adsorption does not take into account of several aspects.

Our ac-electrogravimetric study higlights the (i) the ion solvation effect, (ii) the possible presence of more than one ionic species which may favor/disfavor the kinetics of electroadsoprtion/desorption, and (iii) the influence of free electrolyte molecules that can interact, indirectly, with porous carbon/electrolyte interfaces.

Specifically, the chemical nature and the role of each species: anion, cation, solvated cation and free solvent, directly or indirectly involved in the charge storage, have been identified during the cathodic and anodic polarization by ac-electrogravimetric measurements. For example, in contact with an aqueous NaCl electrolyte, evidence was found that there are two types of cations (Na + .H2O and H + ) electroadsorbed with different kinetics for cathodic potentials and the Cl -ions for anodic potentials together with free water molecules. The reconstruction of the total mass response from independent ac-electrogravimetry measurements agrees perfectly well with the global EQCM response. Our findings reveal the unique sensitivity of the ac-electrogravimetry to provide a fair gravimetric and dynamic deconvolution of the global EQCM responses. The inner diameter of the DWCNTs was estimated to be ~4 nm using the HRTEM technique (See Figure IV-1C). Our HRTEM images indicated that there is also a certain distribution of CNTs with more than two walls which is in line with the literature results. Indeed, DWCNTs are particular Multi-Wall CNTs (MWCNTs) since they have the smaller number of concentric walls.

Chapter

For this reason one can expect DWNTs to exhibit an intermediate behavior between SWNTs and

MWNTs with number of walls higher than 2. 113,115 Compared to the MWCNTs which will be introduced later in the chapter, the number of walls of CNTs presented in The pore structure of the DWCNT was determined by nitrogen adsorption measurements. Figure

IV-2B shows the nitrogen adsorption-desorption isotherms of the DWCNT. The shape of the isotherms can be considered as type-IV curves 207 (characteristic for mesoporous adsorbents) with a slight hysteresis loop at high relative pressures (P/P0) between 0.6-0.9. The Brunauer-Emmett-Teller (BET) method was applied for the specific surface area determination of the DWCNTs and was estimated to be ~ 552 m 2 .g -1 . Furthermore, the nitrogen sorption isotherm was used to estimate broad shoulder up to 15-20 nm, probably related to the voids between the nanotube bundles, as suggested by microscopy images.

Such structural and morphological characterizations were also performed for the MWCNTs. The pore structure of the MWCNT was determined by nitrogen adsorption measurements. In the following part of this section, the CNT based thin films deposited on the quartz resonators are investigated by QCM based electrochemical methods. The main objective is to have a better understanding of the effect of structure, morphology and pore dimensions of the electrode as well as electrolyte properties on the charge storage mechanisms. From the three Figures IV-5B, D and F, it should be noted that for anodic potentials, positive

IV-2. Influence of the CNT Types

IV-2.1. EQCM Study of SWCNT, DWCNT and MWCNT in Aqueous NaCl Electrolyte

Fdm/dq values are estimated which corresponds to anion contribution. As for cathodic potentials, negative Fdm/dq values were calculated associated to cations. In all the cases, higher or lower atomic weight compared to Na + or Cl -ions are found which indicates a complex ion transfer behavior. The observed higher values further indicate that the ions are hydrated and/or accompanied by free solvent molecules during their transfer. Particularly in MWCNTs, higher values at anodic potentials are observed which could correspond to anions accompanied by free solvent molecules.

The specific capacitance was calculated using the relation expressed in Chapter III Section 2.3. The measured capacitance values are around 25 F.g -1 , 26 F.g -1 , and 7 F.g -1 for the SWCNTs, DWCNTs and MWCNTs, respectively. These values are slightly lower especially for MWCNTs, but in the same order as the reported values in the literature. 85 The loops in the third quadrant are characteristic for cation contributions or free solvent molecules following the same flux direction. The fitting of the experimental data by Mathcad software, using Equations given in Chapter II Section 3.5, shows the presence of two charged species for the three films: Na + .nH2O at higher frequencies and H + at lower frequencies. The loop at intermediate frequencies is attributed to the free solvent. Without this contribution it is impossible to fit correctly the experimental mass/potential TF.

IV-2.2. Ac-electrogravimetric Study of various CNT Thin Film Electrodes in Aqueous NaCl

Electrolyte

The identification of these species was achieved by the determination of their molar mass Normally, this should match the response of an anion but in fact, this contribution is that of a cation whose atomic mass becomes negative due to the system of correction. This is evident from Equation II-51 that the response is multiplied by mc1-mc2. As c1 corresponds to H + and c2 corresponds to Na + .H2O, the value is negative. These partial electrogravimetric transfer functions provide a crosscheck for validating the hypothesis involving three different species and a better separation of the various contributions.

() E I    , (B) () q E    , (C) () m E    , (D) () cs m E    1 , (E) () cs m E    2
. Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 5.04 x 10 -5 cm s -1 , Gc1 = 1.24 x 10 -8 mol.s -1 .cm -2 V-1, Kc2 = 6.91 x 10 -3 cm.s -1 , Gc2 = 4.01 x 10 -6 mol.s -1 cm -2 .V -1 , Ks = 3.14 x 10 -3 cm.s -1 , Gs = 4.08 x 10 -6 mol.s -1 , mc1 = 1 g.mol -1 , mc2 = 23+18 g.mol -1 and ms = 18 g.mol -1 .

() E I    , (B) () q E    , (C) () m E    , (D) () cs m E    1 , (E) () cs m E    2
.Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 3.64 x 10 -5 cm.s -1 , Gc1 = 1.39 x 10 -8 mol.s -1 .cm -2 .V-1, Kc2 = 2.89 x 10 -3 cm.s -1 , Gc2 = 4.65 x 10 -6 mol.s -1 .cm -2 V -1 , Ks = 6.28 x 10 -4 cm.s -1 , Gs = 9.11 x 10 -7 mol.s -1 , mc1 = 1 g.mol -1 , mc2 = 23+18 g.mol -1 and ms = 18 g.mol -1 .

() E I    , (B) () q E    , (C) () m E    , (D) () cs m E    1 , (E) () cs m E    2
. Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 2.39 x 10 -4 cm.s -1 , Gc1 = 3.58 x 10 -9 mol.s -1 .cm -2 V -1 , Kc2 = 5.66 x 10 -3 cm.s -1 , Gc2 = 1.10 x 10 -6 mol.s -1 .cm -2 .V -1 , Ks = 5.03 x 10 -4 cm.s -1 , Gs = 5.23 x 10 -6 mol.s -1 , mc1 = 1 g.mol -1 , mc2 = 23+18 g.mol -1 and ms = 18 g.mol -1 .

118

-0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 1E-5 1 0,0 0,1 0,2 0,3 0,4 0,5 1E the Cl -ion is as fast as the Na + .nH2O (Fig. IV-9 B and C). Furthermore, the transfer kinetics of free water molecules are somewhat close to the values of the Na + .nH2O ion at cathodic potentials and to that of Cl -ions at anodic potentials. Additionally, these water molecules have the same flux directions as the ions, suggesting that it might be the water molecules that accompany the transfer of Na + .nH2O and Cl -, most likely due to the an electrodragging phenomena. Finally, the H + ion is the slowest species at cathodic potentials for these three types of films (Figures IV-9A, B and C) showing Ki values in the same order of magnitude, this is in line with their substantially lower concentration in the media. If we compare the Na + .nH2O dynamics if three different CNTs, hydrated sodium species are slightly faster to be transferred in SWCNTs compared to the other two types of CNTs. 1 0,0 0,1 0,2 0,3 0,4 0,5 -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0

1E-4 1E-3 0,01 Cl - H 2 O H + K i / s cm -1 E/ V vs. Ag/AgCl Na + .nH 2 O SWCNT A -0,
-5 1E-4 1E-3 0,01 H + Cl - H 2 O Na + .nH 2 O E/ V vs. Ag/AgCl K i /s cm -1 B DWCNT -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 1E-5 1E-4 1E-3 0,01 K i /s cm -1 H + Na + .nH 2 O Cl - H 2 O E/ V vs. Ag/AgCl

119

IV-3.2. Ac-electrogravimetric Study of SWCNT Thin Film Electrode in Aqueous NaCl

Electrolyte at different pH values.

In this part, a comparison and analysis of the ac-electrogravimetric responses of the SWCNT at pH 2, pH 7 and pH 10 will be presented (Figures IV-14, IV-6 and IV-15, respectively). First, the electrochemical impedance,

() E I   
, responses present a distorted straight line indicating that there is a multi-ion transfer contribution for the three pH values. Particularly at pH 2, the distortion is very important, which is probably due to the reduction of H + amplified by the high proton concentration in this case.

The charge/potential transfer functions (TFs),

() q E   
, show one main loop (Figures IV-14B, V-6B and V-15B), which can be attributed to one ionic species. At pH 7 and 10, a small loop appears at low frequencies. The size of the bigger loop is equivalent for all the pH values. Since the low frequency part of the

() q E   
function is pH dependent, it may imply that these part of the response is related to the protons.

In the mass/potential transfer function,

() m E   
, one big loop appears in the third quadrant at high and medium frequencies (Figures IV-14C, IV-6C and IV-15C). This loop is bigger at pH 7 compared to pH 10 and pH 2. This variation in size is related to differences in the time constants (Ki, Gi) of the species transferred and also depends on the mass of the species involved. The loop in the third quadrant is characteristic ofcation and free solvent molecules in the same flux directions.

Here, it represents Na + .H2O (c2) and H + (c1) for cations and free solvent (s) molecules in the same flux direction for all the pH values.

Partial transfer electrogravimetric transfer functions were also calculated and fitted for the corresponding to the free water and a small one at LF in the fourth quadrant. In this latter case, as explained previously, an anion like response is seen and it is due to the correction procedure where a mc1-mc2 is attributed to the proton response. ()

E I    , (B) () q E    , (C) () m E    , (D) () cs m E    1 , (E) () cs m E    2
. Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 6.16 x 10 -5 cm.s -1 , Gc1 = 4.93 x 10 -9 mol.s -1 .cm -2 V -1 , Kc2 = 5.66 x 10 -3 cm.s -1 , Gc2 = 3.45 x 10 -6 mol.s -1 .cm -2 .V -1 , Ks = 1.26 x 10 -3 cm.s -1 , Gs = 6.28 x 10 -8 mol .s -1 , mc1 = 1 g.mol -1 , mc2 = 23+18 g.mol -1 and ms = 18 g mol -1 . ()

E I    , (B) () q E    , (C) () m E    , (D) () cs m E    1 , (E) () cs m E    2
. Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 1.63 x 10 -5 cm.s -1 , Gc1 = 1.96 x 10 -9 mol.s -1 .cm -2 V -1 , Kc2 = 6.91 x 10 -3 cm.s -1 , Gc2 = 4.22 x 10 -6 mol.s -1 .cm -2 .V -1 , Ks = 3.14 x 10 -3 cm.s -1 , Gs = 4.71 x 10 -6 mol.s -1 , mc1 = 1 g.mol -1 , mc2 = 23+18 g.mol -1 and ms = 18 g.mol -1 .

As mc2 is higher than mc1, a negative value is thus found and leads to this atypical response. At pH 2, the water contribution decreases drastically as shown in -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 4x10 A -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 10 -5 10 -4 10 -3 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4

H + pH2 H + pH7 H + ph10 E/V vs. Ag/AgCl K i /s cm -1 B -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 1E-5 1E-4 1E-3 K i /s cm -1 Cl -pH2 Cl -pH7 Cl -ph10 C E/V vs. Ag/AgCl
3 4 5 6 Na + .nH 2 O pH2 Na + .nH 2 O pH7 Na + .nH 2 O ph10 Rt i /1/FG i / ohm cm -2 A E/V vs. Ag/ AgCl
-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4

10 3 2x10 3 3x10 3 4x10 3 5x10 3 H + pH2 H + pH7 H + ph10 E/V vs Ag/ AgCl Rt i /1/FG i / ohm cm -2

B

-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 10 -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5

0,0 0,1 0,2 0,3 pH 2 H + Cl - H 2 O Na.H 2 O E/V vs. Ag/Ag Cl C i -C o ) / mmol cm -3
A -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5

0,0 0,1 0,2 0,3 Cl - H + H 2 O Na + .nH 2 O E/V vs. Ag/Ag Cl C i -C o ) / mmol cm -3 pH 7 B
-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,0 0,1 0,2 0,3 ac-electrogravimetric data at different pH. These correspond to the capacitance attributed to the cations Na + .nH2O and H + and the anion Cl -. The capacitance concerning the cations Na + .nH2O and H + are higher at pH 7 and pH 10 than at pH 2 (Figures IV-19A and 19B).

H + H 2 O Na + .H 2 O E/V vs. Ag/Ag Cl (C i -C o ) / mmol cm
-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 As a conclusion of this part, one can say that the pH value can drive completely the kinetics of transfer and the quantities inside the SWCNT film. Thus, it is possible to favor cations electroadsorption at high pH values and anion electroadsorption at low pH values.

IV-3.3. EQCM Study of SWCNT in Different Aqueous Electrolyte: Effect of the Cation Size

Up to now, the results presented concern only one type of electrolyte composition, i.e. 0.5 M aqueous NaCl at different pH values.

In this part, the influence of the cation size was investigated over the electrogravimetric response of the SWCNT films. values. They are in the same order of magnitude as those reported in the literature. 85,221 This confirms that SWCNT is an interesting candidate as supercapacitor electrode materials but the ions electroadsorption process is not straightforward and different electrolyte compositions may lead to different electrochemical properties.

-0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 This distortion may be attributed to a multi-ion transfer contribution. 22C, IV-6C and IV-23C). These loops in the third quadrant are attributed to cation contributions (Li + .H2O, Na + .H2O and K + ) at high frequencies, to free solvent molecules (H2O) in the same flux direction at medium frequencies and to the H + at low frequencies. Furthermore, in KCl the cation at high frequencies participates freely without hydration, while in LiCl and NaCl the cations are hydrated. It means that three cations are electroadsorbed with different degrees of hydratation. In fact, Li + and Na + can be more easily hydrated because of their small size, whereas the larger cations such as K + has a weaker interaction with water. In other words, Li + and Na + ion species are more tightly bonded to their water molecules in their hydration shell, thus it is possible to have a higher hydration level of the electroadsorbed lithium and sodium species than the potassium species, i.e. dehydration of K + is easier than the other Na + and Li + . 219,222 The presence of three different species were further confirmed by carefully analyzing the partial is due to the difference in molecular weight of the ions, as explained previously. In these partial TFs, the different ions contributions do not necessarily appear more clearly but a good agreement between the theoretical and experimental curves confirms the validity of our hypothesis. 0,0 0,2 0,4 0,6 0,8 1,0 1,2 0,0 0,2 0

-400 0 400 m/µg.cm -2 LiCl i/µA.cm -2 25mv.s -1 50mv/s -1 100mv/s -1 A 0,0 0,4 0,8 E/V vs. Ag/AgCl -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0
-2 V -1 Re(q/E)/mC cm -2 V -1 B -1,5 -1,0 -0,5 0,0 -1,5 -1,0 -0,5 0,0 H + H 2 O L i + . H
V -1 Re(m/E)/g cm -2 V -1 C -1,5 -1,0 -0, 5 0,0 -1,5 -1,0 -0,5 0,0 H
-Im(m/E) C1S /g cm -2 V -1 Re(m/E) C1S /g cm -2 V -1 D -1,5 -1,0 -0,5 0,0 -1,5 -1,0 -0,5 0,0 H 2 O L i + . H 2
() E I    , (B) () q E    , (C) () m E    , (D) () cs m E    1 , (E) () cs m E    2
. Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 5.40 x 10 -4 cm s -1 , Gc1 = 7.03 x 10 -9 mol.s -1 cm -2 V -1 , Kc2 = 5.03x 10 -3 cm.s -1 , Gc2 = 3.02 x 10 -6 mol.s -1 cm -2 V -1 , Ks = 3.14 x 10 -3 cm.s -1 , Gs = 2.99 x 10 -6 mol.s -1 , mc1 = 1 g.mol -1 , mc2 = 7+18 g.mol -1 and ms = 18 g.mol -1 .

-Im(m/E) C1S /g cm -2 V -1 Re(m/E) C1S /g cm -2 V -1 D -2,0 -1,5 -1,0 -0,5 0,0 -2,0 -1,5 -1,0 -0,5 0,0 H 2 O
() E I    , (B) () q E    , (C) () m E    , (D) () cs m E    1 , (E) () cs m E    2
. Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 3.90 x 10 -5 cm.s -1 , Gc1 = 1.91 x 10 -8 mol.s -1 cm -2 V -1 , Kc2 = 4.15x 10 -3 cm.s -1 , Gc2 = 4.69 x 10 -6 mol.s -1 cm -2 .V -1 , Ks = 2.51 x 10 -3 cm.s -1 , Gs = 1.23 x 10 -6 mol.s -1 , mc1 = 1 g.mol -1 , mc2 = 39 g.mol -1 and ms = 18 g.mol -1 .

137

-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4

4x10 -3 5x10 -3 6x10 -3 7x10 -3 8x10 -3 E/ V vs. Ag/AgCl K i /s cm -1 Li + .nH 2 O (LiCl) Na + .nH 2 O (NaCl) K + (KCl)
A -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4

5E-5 6E-5 7E-5 8E-5 9E-5 1E-4 1,1E-4 K i /s cm -1 H + (LiCl) H + (NaCl) H + (KCl) B E/ V vs. Ag/AgCl
-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4

-3 E/ V vs. Ag/AgCl K i /s cm -1 Cl -(LiCl) Cl -(NaCl) Cl - 10 -5 10 -4 10
3 4 5 6 7 8 E/V vs. Ag/ AgCl Rt i / 1/FG i / ohm cm -2 Li + .nH 2 O (LiCl) Na + .nH 2 O (NaCl) K + (KCl)
A -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4

1x10 3 2x10 3 3x10 3 4x10 3 E/V vs. Ag/ AgCl Rt i /1/FG i / ohm cm -2 H + (LiCl) H + (NaCl) H + (KCl)

B

-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 Li + .nH2O ions dynamics is much lower since it is much more difficult for them to get rid of their hydration shell. It is important to note that the n is estimated as 1 in our simulations, for both Na + .nH2O and Li + .nH2O (Figures IV-6 and IV-22) and the bulk hydration numbers of these ions are much higher, i.e. these ions are only partially dehydrated. 139 -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,0 0,1 0,2 0,3

H 2 O Cl - H + H 2 O Li + . H 2 O C i -C 0 ) / mmol cm -3 E/V vs. Ag/AgCl
A -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,0 0,1 0,2 0,3 For the H + ion, the transfer seems more difficult in LiCl and KCl media than in NaCl media, The values of the instantaneous capacitance of the cations (Li + .nH2O, Na + .nH2O and K + ) at cathodic potentials are presented as follows (in terms of the higher capacitance observed):

H 2 O Cl - H + H 2 O Na + .nH 2 O E/V vs. Ag/AgCl (C i -C 0 ) / mmol cm -3 B -0,5 -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,5 0,0 0,1 0,2 0,3 H 2 O Cl - H + H 2 O K + C i -C 0 ) / mmol cm -3 E/V
Na + .nH2O>Li + .nH2O>K + (Figure IV-27A).
This difference in capacitance between the cations is probably related to the phenomena of dehydration, differences of cation sizes adsorbed within the porous material and the accessibility of the porous structure by the ions. For the H + , the instantaneous capacitance values are more significant in NaCl and KCl aqueous electrolyte than in LiCl (Figure 27B) which may indicate a competition between hydrated Li + ions and the protons for similar electroactive sites of the SWCNT film. Finally, at anodic potentials, the relative capacitance values of the Cl -are equivalent in all the three electrolytes (Figure IV-27C) which indicates that the instantaneous capacitance of the anions is not effected by different co-ions such Li + , Na + and K + . It is important to highlight here the advantageous nature of ac-electrogravimety which allows a separation of the global capacitance values to distinct contributions of each species to be obtained.

IV-4. Conclusions

SWCNT, DWCNT and MWCNT carbon-based electrodes were prepared on gold electrodes of the quartz resonators and their electrochemical charge storage behavior evaluated in aqueous electrolytes, i.e. 0.5 M NaCl at pH 2, pH 7 and pH 10 was presented. The cation size of the electrolyte was also varied from Li + , Na + to K + (in LiCl, NaCl and KCl), the results of SWCNTs were discussed in this chapter.

We have conducted a comparative study among different CNTs with different pore dimensions via an ac-electrogravimetric study and related the results to their structural and morphological characteristics. First of all, several species were detected to intervene to the electroadsorption process for all types of CNTs: there are two types of cations (Na + .H2O and H + ) electroadsorbed with different kinetics for cathodic potentials and the Cl -ions for anodic potentials together with free water molecules. When we compare the Na + .nH2O dynamics (can be considered as the major cation detected) in three different CNTs, hydrated sodium species are slightly faster to The cation size dependence of the electrodeposition phenomena was studied by changing the electrolyte cation from Li + , Na + to K + and the results are shown for SWCNTs. The chemical nature and the role of each species, i.e. anion, cation, solvated cation, free solvent, directly or indirectly involved in the charge storage, have been identified during the cathodic and anodic polarization in ac-electrogravimetry measurements. The theoretical transfer functions have been calculated taken into account three different species (Li + .H2O, H + and H2O), (Na + .H2O, H + and H2O), (K + , H + and H2O) for LiCl, NaCl and KCl electrolytes, respectively. Very good agreements between experimental and theoretical data for all functions of estimated transfers were obtained. Based on the Ki values presented in Figure IV-24A, the transfer of Na + .nH2O ion and K + ion is faster compared to that of Li + .nH2O for cathodic potentials. This observation is in agreement with the ions size and the affinity for dehydration. Li + and Na + can be more easily hydrated because of their small sizes, whereas the larger cations such as K + has a weaker interaction with water. In other words, Li + and Na + species are more tightly bonded to their water molecules in their hydration shell compared to the potassium species, i.e. dehydration of K + is easier than the other Na + and Li + . K + ions due their completely dehydrated state, they probably become equivalent to partially dehydrated Na + .nH2O, in terms of kinetics. Li + .nH2O ions dynamics is much lower since it is much more difficult for them to get rid of their hydration shell making their transfer the slowest among the other cations. Our results clearly indicates that the dehydration affinity and the ions size plays a role in the electroadsorption dynamics of the species on the SWCNT based electrodes. However, although Li + .nH2O ions presents slower dynamics and difficulty to be transferred (Figures IV-24A

and 25A), our results indicated that eventually similar amounts of ions are transferred (Na + .nH2O, K + and Li + .nH2O). These findings strengthens the idea that the electroadsorption of these species takes place at different sites of the SWCNT electrodes, probably governed by the species size and dynamics of transfer and the accessibility of these sites to the species.

Chapter V. Composite Thin Film Electrodes and Beyond Carbon Nanotubes

CHAPTER V Composite Thin Film Electrodes and Beyond Carbon Nanotubes

This chapter is dedicated to the interpretation of the results obtained from the study of SWCNT/Prussian Blue (PB) and SWCNT/Polypyrrole (PPy) composite and electrochemically reduced graphene oxide (ERGO) thin film electrodes. This approach was examined in order to increase the performances of the pure CNT films by functionalizing the CNT materials by redox films or by using alternative carbon based materials such as ERGO structures. Firstly, the results from the FEG-SEM, HRTEM, EDX and XRD studies of the structure and the morphology of the samples are presented and discussed. Thereafter follows the discussion of the results from the EQCM and the ac-electrogravimetry studies in order to thoroughly examine the electrochemical behavior of such layers. Three zones are defined.

V-1. Composite Thin Film Electrodes

-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3  Zone I, between -0.4V to 0.1V, the species involved could be the hydrated K + .



Zone II, range -0.1V to 0.15 V, the species involved could be K + and H3O + which is coherent with the PB response.



Zone III, range 0.15V to 0.4V, the species involved could be Cl -but the high values calculated indicate the contribution of free solvent. However, another method for calculation of the capacitance adapted to faradaic process has been proposed using the following relation

1 2 21 () 2(
)

E E i E dE C E E mv   
where C is the specific capacitance of electrode, E 1 and E 2 are the working voltage in cyclic voltammetry, and 1 2 ()

E E i E dE 
is the total voltammetric charge in cyclic voltammetry, m is the mass of the film, and v is the scan rate. The value is estimated to be around 114 F g -1 which is four times higher than the capacitance values obtained from SWCNT film (See Figure III-5). This improvement in capacitance is due to the faradaic process that occurring in the redox zone. ) as a function of potential at a scan rate of 50 mV.s -1 calculated for 0.5M KCl.

V-1.1.3. Ac-electrogravimetry Study of the SWCNT/PB Composites

The measurements are performed each 100 mV in the range from -0.4V to 0.4V vs Ag/AgCl. However, it should be noted that there is no evident part with a slope equal to 45° or below in the electrochemical impedance response, therefore, the rate limiting step is not the mass transport in the films or in the solution, but rather ionic transfer between the solution and the film. 3 The charge/potential transfer functions (TFs),

Δq (ω) ΔE (Figure V-6B)
, permit the separation of the ionic contributions, however, without any possibility to identify the ionic species involved. = 1.51 x 10 -4 cm s -1 , Gc1 = 3.77 x 10 -8 mole s -1 cm -2 V -1 Kc2 = 5.03 x 10 -3 cm s -1 , Gc2 = 3.72 x 10 -6 mole s -1 cm -2 V -1 , Ks = 2.77 x 10 -3 cm s -1 , Gs = 7.47 x 10 -6 mole s -1 ,mc1 = 1 g mole -1 , mc2 = 39 g mol -1 , ms = 18 g mol -1 .

-2 V -1 Re(q/E)/mC cm -2 V -1 B -3 -2 -1 0 -3 -2 -1 0 H + H 2 O K + 1
V -1 Re(m/E)/g cm -2 V -1 C -2 -1 0 -2 -1 0 H + H 2 O 1 Hz 10 Hz 0,1 Hz Experimental Theoretical and : freq. decade -Im(m/E) c1s /g cm -2 V -1 Re(m/E) c1s /g cm -2 V -1 D -3 -2 -1 0 -3 -2 -1 0 K + H 2 O
In the mass/potential transfer function Δm (ω) ΔE

, one big loop appears in the third quadrant at high and medium frequencies (See Figure V-6C). The loops in the third quadrant are characteristic for cation contributions or free solvent molecules in the same flux direction. Another contribution also appears at lower frequencies in the fourth quadrant (anions with opposite flux direction) highlighting the challenge in the exact identification of these two or three loops. The fitting of the experimental data by Mathcad software, using the equations given in Chapter II Section 3.5, showed the presence of two charged species: K + at higher frequencies and Cl -at lower frequencies.

The loop at intermediate frequencies is attributed to the free solvent. The identification of these species was achieved by the determination of their molar mass using Equation II-50 Chapter II. -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 The transfer resistance values shown in Figure V-7B reveal that K + is transferred easier than Cl -, which is corroborated by the kinetic values previously mentioned. For free solvent molecules, the transfer resistance value is smaller for the extreme potential values but in the PB electroactivity domain, this value becomes higher: H2O is more difficult to be transferred in the redox PB domain compared to potassium ions. Indeed, when only PB films are examined, free water molecules were not detected which is in line with the present observations. 3,203 The transfer kinetic rates of H + in the SWCNT/PB are more significant than the transfer kinetic rates of H + in SWCNT, probably due to the difference in pH ( As mentioned previously, in SWCNT/PB the transfer kinetic rates of K + is equivalent to the transfer kinetic rates of K + from the SWCNT (Figure IV-24A), thus advantageously, the presence of PB does not hinder the electroadsorption process of cations.

1E-5 1E-4 1E-3 0,01 K + Cl - H 2 O K i /s cm -1 E/ V vs. Ag/AgCl A -0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 1 10 100 B H + K + H 2 O Cl - Rt i /ohm cm -2 E/V vs Ag/AgCl
-0,4 -0,3 -0,2 -0,1 0,0 0,1 0,2 0,3 0,4 0,0 As a comparison, we have detected that the Ci-C0 values of the species K + , Cl -and free solvent are more significant in the composite SWCNT-PB thin films than in pristine SWCNT thin films.

Furthermore, the Ci-C0 values of the free solvent in the SWCNT-PB are also higher and equivalent, at anodic potentials, and cathodic potentials, respectively; as compared to the pristine SWCNT thin films (Figure IV-26C). These results indicate that the relative concentration of the species is significantly increased in the case of the SWCNT/PB composite structure, which probably is the reason for the significantly higher specific capacitance values obtained in Figure V-5 (114 F g -1 ). It is important to remind that this value is obtained from cyclic voltammetry and corresponds to a certain scan rate of the potential and may not be representative of all the species contributing with different kinetics. Therefore, we have calculated from the instantaneous capacitance values of each ion from ac-electrogravimerty data. (Cl -), respectively and calculated using the relation expressed previously. The capacitance values calculated from the ac-electrogravimetry data differ from that found from the cyclic voltammetry but they are in the same order of magnitude. This difference is due to the kinetics and ease/difficulty of each species to be inserted/electroadsorbed (Ki, Rti). Ac-electrogravimetry takes into account of these aspects and the capacitance contribution of two different species with different kinetic constants are separately calculated.

In comparison, the instantaneous capacitance values of the K + and H + are slightly higher for the SWCNT-PB than those for the SWCNT (Figures IV-27A compared to that of pristine SWCNTs. When the composite film was reduced, a mass increase was observed and when it was oxidized, a mass loss was observed. Indeed, when the polymer film was 2 0,0 0,2 0,4 0, with high number of hydration or Na + accompanied by free solvent. In anodic potentials, the values of the molecular mass vary in the range 0-50 g.mol -1 , which is also difficult to be attributed to a single ion contribution.

-1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 . Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 9.50 x 10 -5 cm s -1 , Gc1 = 7.98 x 10 -9 mol s -1 cm -2 V -1 Kc2 = 3.46 x 10 -3 cm s -1 , Gc2 = 9.3 x 10 -7 mol s -1 cm -2 V -1 , Ks = 1.97 x 10 -3 cm s -1 , Gs = 1.99 x 10 -6 mol s -1 ,mc1 = 1 g mol -1 , mc2 = 23+18 g mol -1 , ms = 18 g mol -1 .

Consequently, it is difficult to extract information from this transfer function. The charge/potential transfer functions (TFs),

Δq (ω) ΔE

(Figure V-14B), permit the separation of the ionic contributions, however, without any possibility to identify the ionic species involved. exhibit a good agreement between the theoretical and experimental data. As mentioned for the other electrode systems presented earlier, these partial electrogravimetric transfer functions provide a crosscheck for validating the hypothesis involving three different species and a better separation of the various contributions. If the atomic masses chosen to identify the species are different than reality, a good agreement cannot be obtained. Given that we have a perfect agreement for all the TFs, we can say that our assumption is valid for modelling all the TFs at this potential (-0.4V vs Ag/AgCl).

For potentials more cathodic than -0.3 V vs Ag/AgCl, the contribution of only three species is found mixing the two cations, Na + .nH2O and H + , and free solvent water following the same transfer direction. For more anodic potential values, the proton contribution disappears and chloride species is found at very low frequencies.

-1,4 -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 10 2 0,0 0,2 0, Additionally, the concentration of the protons in the electrolyte is much lower compared to that of sodium species at this pH. Furthermore, the transfer kinetic rates of free water molecules are somewhat close to the values of the Na + .nH2O ions. Chloride ions appear only at low frequencies and at more anodic potentials.

K i /s cm -1 E/V vs. Ag/Ag Cl H + Na + .nH 2 O H 2 O Cl - A -1,
In SWCNT/PPy composites, the transfer kinetic rates of Na + .nH2O are only slightly lower than the Ki of the Na + .nH2O in pristine SWCNTs and higher than the Ki of the Na + species detected in pristine PPy (a previous work from our group) 224 , measured under similar conditions. These results indicate that the composite structure does not hinder the sodium species transfer.

Additionaly, the contribution of the sodium species to the charge compensation process (both electrostatic and faradaic) is enlarged to the whole potential range, i.e. 0.4V to -1.2V vs Ag/AgCl (larger than that used for SWCNTs alone).

For the other species, the following trends were observed: (i) the transfer kinetic rates of the Chapter III) and that of found in doped PPy electrodes. 224 Taking into consideration of the Ki and Rti values of various species detected, one can suggest that there is no major hinderence of the ions transfer when doped PPy is deposited on the SWCNT electrodes. The species detected are related to the electroactivity of the two components. For example, at the anodic potentials, there is no cation contribution in pristine SWCNTs whereas it exits in the composite structure, due to the PPy.

-1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 -1,4 -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 Also, in the case of SWCNT-PB, we have detected that the Ci-C0 values of the species K + , Cl -and free solvent are more significant than in pristine SWCNT thin films. Consequently, the instantaneous capacitance values of the K + and H + are slightly higher for the SWCNT-PB than those for the SWCNT (Figures IV-27A and B). From these points of view, one can say that SWCNT-PB composites are more performant than the SWCNT-PPy composites. To reach a definite conclusion on this point, further studies for example by changing the SWCNT/PPy or SWCNT/PB ratios are required. Clearly, in both composites, cationic species contribution (sodium in the present study) to the charge compensation process (both electrostatic and faradaic) is enlarged to the whole potential window (larger than that used for SWCNTs alone). This can be considered as an advantage when they are used as capacitive electrodes.

V-2. Beyond Carbon Nanotube Based Electrodes

V-2.1. Electrochemically Reduced Graphene Oxide (ERGO) Thin Film Electrodes

The electrochemical reduction was performed in a 3-electrode electrochemical cell, as described previously in Chapter II. The reduction was achieved in aqueous 0.5M NaNO3 solution.

Nitrogen bubbling was performed for 10 minutes to remove the dissolved oxygen in the solution. A platinum grid and an Ag/AgCl served as the counter electrode and the reference electrode, respectively. This characteristic crumbled aspect remains after electrochemical reduction and is similar to that of GO sheets reduced chemically or thermally. 225 This geometric wrinkling is caused by nanoscale interlocking of GO sheets, bringing good mechanical properties, surface roughness and a larger specific surface area. 225 The cross-section of the GO layers, deposited on the QCM electrode surface, before and after reduction shown in Figure V-19 allows the determination of the thin film thickness (70 nm and 50nm for GO and ERGO, respectively).

V-

The reduced thickness may indicate the removal of oxygen containing functional groups in the GO carbon plane due to the reduction process, which is in agreement with earlier studies. 226 EDX results showed a decrease of the amount of oxygen on the surface.

According to Table V-1, the amount of oxygen declines from 36.1% to 20.6% during the reduction. Although not completely (lost oxygen), this result suggests that a substantial amount of the oxygen containing functional groups are removed during electrochemical reduction which resulting in a graphene-like material. The broad peak appearing at 2=25° corresponds to a d-spacing of 0.38nm. This decrease compared to that of GO (0.70 nm) is attributed to the oxygen removal. 232 The electrochemical reduction of grapheme oxide was followed in situ using Raman spectroscopy. Figure V-22 shows the Raman spectra of pristine GO (t = 0 min) and ERGO (t = 44 min). These Raman spectra exhibit three particular bands: one of the most prominent feature around 1580 cm -1 is the so-called G mode (G as "graphitic") due to the relative motion of sp 2 carbon atoms which appears around 1580 cm -1 ; the other one which appears around 1350 cm -1 is the D band (D as "defect") and is due to the out-of-plane breathing mode of the sp 2 carbons. 233 The small double band around 2700 cm -1 is the 2D mode (sometimes called G' mode). The D mode is usually related to a forbidden transition, however in the presence of disorder the symmetry is broken and the transition becomes allowed. As first reported on amorphous carbon 65 and then, on graphene 234,235 , this band appears for example on the edge of graphite and also on the basal planes if sp 3 defects break the sp 2 lattice in case of covalent modification of graphene In a theoretical view, if only the cation is transferred during the electroadsorption process, the mass variation should be about 5 times higher when cycled in aqueous KCl electrolyte because of the higher molar mass of K + ions compared to Li + (MK + =39 g mol -1 , MLi + =7g mol -1 ). This might indicate that the Li + species are adsorbed together with free solvent molecules, and this phenomenon is more pronounced compared to that in KCl electrolyte.

Another explanation, is that both species are electroadsorbed in their hydrated form, but Li + and K + ions have different degrees of hydration. In fact, Li + can be more easily hydrated because of its small size, whereas the interaction between the bigger K + atoms and water is weaker (electrostatic interaction inversely related to the distance separating the two species).

In other words, Li + ion species are more tightly bonded to their water molecules in their hydration shell, thus it is possible to have higher hydration level of the electroadsorbed lithium species compared to potassium species.

For NaCl medium, the ERGO modified quartz resonators exhibit slightly higher current values and mass response compared to the two other media. This indicates a more complex mechanism of ion electroadsorption than usually described in the literature, where a single negative charge on a capacitive material often is described as being compensated by the adsorption of a single electrolyte cation. It also indicates that the kinetic limitation is due to the ionic transfer rather the electronic transfer as the same film was used to test the three different ions.

-1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 -2000 2 0,0 0,2 0,4 0,6 0, -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6 0,8 This value is slightly lower, but in the same order of magnitude as the reported values in the literature, such as the work of Gunasekaran et. al (175 F.g -1 under similar conditions). 225 This confirms that ERGO can be an interesting candidate to be used as a material for supercapacitors. Now, it seems interesting to understand the capacitive charge storage in these type of electrodes. The fitting of the experimental data by Mathcad software, using the equations given in Chapter II Section 3.5, showed the presence of two charged species: Li + .H2O at higher frequencies and Li + at lower frequencies. A beginning of a loop representing free solvent at very low frequencies, in the electrogravimetric function is observed (Figure V-26C). The identification of these species was achieved by the determination of their molar mass using Equation II-50 Chapter II and their respective kinetics of transfer were determined by the Ki values. Thus, the transfer is faster for hydrated species since they appear at higher frequencies, in spite of their larger size. This finding suggests that size is not the only criterion for the determination of the kinetics of transfer.

The match between the size of the species and the electroadsorption sites also plays an important  

E I    , (B).   q E    , (C).   m E   
, and two partial transfer functions,(E). The cations are hydrated in the aqueous electrolytes and should partially or totally leave their hydration shells before getting electroadsorbed between the ERGO sheets or directly on the surface of the material.

 

Basically, the non-hydrated cations can be considered as the cations that leave all of their hydration shell and hydrated cations are those that leave a part of their hydration shell before electroadsorption between the ERGO sheets and on sites closer to the surface, respectively. The free solvent molecules follow the same flux direction as the cationic species but they are much slower and more difficult to be transferred (Figures V-28E and F) in the three different electrolytes studied.

-1,0 -0,8 -0,6 -0,4 -0,2 0,0 ). This quantity can be estimated through the following relation

1E-6 1E-5 1E-4 K c1 / cm.s -1 E/ V vs. Ag/AgCl K + Li + Na + A -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0 500 1000 1500 2000 B Rt 1 / .cm 2 E/ V vs Ag/AgCl K + Na + Li + -1,0 -0,8 -0,6 -0,4 -0,2 0,0 1E-6 1E-5 1E-4 C K c2 / cm.s -1 E/ V vs. Ag/AgCl K + .nH 2 O Li + .nH 2 O Na+.nH 2 O -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0 500 1000 1500 2000 D Rt 2 / .cm 2 E/ V vs. Ag/AgCl K + .nH 2 O Na + .nH 2 O Li + .nH 2 O -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,0 1,0x10 -5 2,0x10 -5 E K s / s.cm -1 E/ V vs. Ag/AgCl H 2 O(KCl) H 2 O(NaCl) H 2 O(LiCl) -1,0 -0,8 -0,6 -0,4 -0,2 0
0 ii i CG EK       
and then, after integration Ci-C0 values can be estimated.

-1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,0 0,5 28 and V-31). The literature results agree that ions with higher charge density have higher number of tightly bound water molecules. This has also been proposed as the mechanism behind the cellular ion channels being closed to strongly hydrated Na + ions while allowing formally larger, but more easily dehydrated, K + ions to pass through. 219,222 Here, this phenomenon has been shown for the first time to play a role in the capacitive charge storage behavior of ERGO electrodes.

1,0 1,5 2,0 E/ V vs. Ag/AgCl C 1 -C 0 / mmol.cm -3 Li + Na + K + A -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,0 0,5 1,0 1,5 2,0 2,5 B C 1 -C 0 / mmol.cm -3 Li + nH 2 O Na + nH 2 O K + nH 2 O E/ V vs. Ag/AgCl -1,0 -0,8 -0,6 -0,4 -0,2 0,0 -0,5 0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 E/ V vs. Ag/AgCl C 1 -C 0 / mmol.cm -3 H 2 O (LiCl) H 2 O (NaCl) H 2 O (KCl) C -1,0 -0,8 -0,6 -0,4 -0,2 0,0 1,5 2

V-3. Conclusions

SWCNT/PB

The electrochemical characterization was first done by cyclic electrogravimetry (EQCM), which was performed in 0.5M aqueous KCl electrolyte, from 0.45V vs Ag/AgCl to -0.45V vs Ag/AgCl. The capacitance of the film, the mass change and the Fdm/dq function were calculated as a function of the potential. The SWCNT/PB film shows a capacitive behavior in the CNT zone and a redox behavior in the PB zone. Additionally, to be able to characterize the EQCM response in depth, an ac-electrogravimetry study was performed in the same media under a sinusoidal potential perturbation. Experimental data were fitted using a theoretical model to find the kinetic parameters.

It was found that the transfer of K + was faster than the transfer of Cl -in the studied potential window. Probably, this is related to the fact that both the adsorption/desorption and the insertion/expulsion phenomena occur for K+, while only adsorption/desorption occurs for Cl -.

Indeed, for pure PB films, the anion contribution was never seen and for that, is related to only electroadsorption.

SWCNT/PPy

The electrochemical characterization was first done by cyclic electrogravimetry (EQCM), which was performed in 0.5M aqueous NaCl electrolyte, from 0.45V vs Ag/AgCl to -1.2V vs Ag/AgCl. The capacitance of the film, the mass change and the Fdm/dq function were calculated as a function of the potential. The SWCNT/PPy film shows a pseudo-capacitive behavior.

Additionally, to be able to characterize the EQCM response in depth, an ac-electrogravimetric study was performed in the same media under a sinusoidal potential perturbation. Experimental data were fitted using a theoretical model to find the kinetic parameters. It was found that the transfer of Na + .nH2O was faster than the transfer of H + and Cl -in the studied potential window.

On the other hand, in SWCNT/PPy the transfer kinetic rates of Na + .nH2O are equivalent to the transfer kinetic rates of the Na + .nH2O from SWCNT and more significant than the transfer kinetic rates of the Na + mentioned in the literature 224 . The transfer kinetic rates of the H + in the SWCNT/PPy are more significant than the transfer kinetic rates of H + in the SWCNT. The transfer kinetic rates of the Cl -in the SWCNT/PPy are more or less equivalent to the transfer kinetic rates of the Cl -in the SWCNT and more significant than the transfer kinetic rates of the Cl -mentioned in the literature. 224

General Conclusions

Carbon nanotubes (CNTs) of "single-wall" type (SWCNT), "double-walled" (DWCNT), "multi-walled" (MWCNT) and Electrochemically Reduced Graphene Oxide films were prepared on gold electrodes of microbalance and tested in different electrolytes such as LiCl, NaCl and KCl.

The influence of pore morphology/structure over the classical electrochemical responses demonstrates that ERGO film is the best candidate for energy storage applications. To understand in details the electrochemical mechanism correlated to this high level of performances, different sophisticated techniques were performed to characterize the film morphology/structure and the transfer reactions involved in the storage reactions.

In CNTs the chemical nature and the role of each species, anion, cation, solvated cation, free solvent, directly or indirectly involved in the charge storage mechanism, have been clearly identified during the cathodic and anodic polarization through ac-electrogravimetry measurements for the very first time. For that, different experiments were done and the theoretical transfer functions have been calculated taking into account three different species (Li + .H2O, H + and H2O), (Na + .H2O, H + and H2O), (K + , H + and H2O) for LiCl, NaCl and KCl, respectively. Very good agreements between experimental and theoretical data for all the transfer functions were obtained.

From these modelling and data treatment, different subtleties were obtained concerning the transfer of the species between the electrolyte/film as the separation of the different contributions and also some key parameters as their identification, their rate of transfer or their ease of transfer. Of course, these characteristic parameters, correlated to the performances of the energy storage devices, cannot be obtained with classical methods of investigations such as classical EQCM. In the case of CNTs films, the transfer of hydrated lithium, hydrated sodium and potassium occurs at high frequencies (fast motion), the free solvent transfer occurs at medium frequencies and the proton transfer occurs at lower frequencies (slow motion). The transfer of hydrated lithium, hydrated sodium, potassium and free solvent is easier than that of the transfer of H + . These results suggest that the adsorption of these species takes place at different sites within the electrodes of the CNTs.

The deconvolution of the global response of EQCM into distinct contributions (identification of ions, solvation numbers and free solvent contributions) and the kinetics information on the species electroadsorption are the strong assets of the ac-electrogravimetry and cannot be attained via EQCM alone.

The influence of electrolyte pH on the capacitive behavior of CNT films has also been analyzed by ac-electrogravimetry. The ac-electrogravimetric results confirm that the cations are predominantly electroadsorbed when the surface is negatively charged while the anions are electroadsorbed when the surface is positively charged. Furthermore, the displacement of the PZC is pH-dependent.

Finally, nanocomposite films, namely SWCNT/PB (Prussian Blue) and SWCNT/PPy (Polypyrrole) were electrochemically examined. The main idea was to emphasize the capacitive and faradic behavior of these different films by combining two materials. The chemical nature and the role of each species involved in the pseudo-capacitive and capacitive processes were highlighted by the ac-electrogravimetry.

For ERGO electrodes, the ac-electrogravimetry detected in aqueous electrolytes (LiCl, NaCl and KCl) two types of cationic species: hydrated and dehydrated cations. The hydrated ions are the fastest and the easiest species to be transferred, indicated by their higher Ki (kinetics of transfer) and lower Rti (transfer resistance) values. The free water molecules contribute at the lowest frequencies, making them the slowest species to contribute indirectly to the charge storage mechanism. These experimental results strengthen the idea that the complete dehydration of the ions require higher amount of energy, which renders the completed dehydrated ions slower than partially dehydrated counterparts.

When the aqueous electrolyte cation is replaced (from Li + or Na + or K + , both for the completely dehydrated and partially dehydrated cation populations) the following trend was observed: K + species are faster to be electroadsorbed compared to Na + species, and the Li + species are persistently the slowest species in this comparison. This, a priori anomalous behavior is an experimental proof of the relationship between the dehydration energy and the ion size. Our findings are related to the phenomena that smaller ions are more tightly bound to their water molecules such that an inverse relationship between the dehydration energy and the ion size exists.

Potassium species (completely or partially dehydrated) require less energy to get rid of their water molecules (compared to sodium, and sodium ions compared to lithium ions), which probably confers a rapid (faster transfer kinetics, Ki) and an easy transfer (low transfer resistance, Rti)

characteristics on potassium species. The literature results agree that ions with higher charge density have higher number of tightly bound water molecules. This phenomenon has been shown for the first time to play a role in the capacitive charge storage behavior of carbon based electrodes.

The methodology adapted to characterize carbon based electrodes in this Ph.D. thesis can be suggested as a baseline diagnostic tool to study the pore/ion size relationship, the concentration and the solvent effects, the dynamics of the ions interactions at the interfaces (electroadsorption and/or faradaic process) of the electrode materials which may pave the way towards more performant electrode/electrolyte systems in energy storage devices.

Future work

Carbon nanotube films were tested in Aqueous and Organic electrolytes and characterized by EQCM and ac-electrogravimetric techniques. In future work, these carbon nanotubes could be tested in Ionic Liquids (ILs) or mixtures between ILs/organic solvent due to their commendable properties in terms of energy density and wider voltage window as well as the fact that they consist of only ions. A limited number of work have been found in the literature, employing EQCM to study carbon films in Ionic Liquids. 142 However, as mentioned previously, EQCM alone provides a global response of the film. Ac-electrogravimetry could be employed for a better understanding of the mechanism of ion adsorption at the electrode/electrolyte interface.

The studies of mechanical properties of Carbon Nanotube films, particularly the motional resistance of films, could be monitored by techniques already used at the LISE-Laboratory like network analyzer methods. Gravimetric and mechanic results could be obtained and analyzed during the same experiment. However, the mechanical aspect could have an influence and the gravimetric results. For example, mechanic changes could occur in the film after a large amount of cycles (cyclic voltammetry). Therefore, the measures of electromechanic transfer functions similar to electrogravimetric transfer function techniques could be employed in future studies.

Materials more complex than CNT/PB or CNT/PPy composites will be studied in order to increase the performances of the pure CNT films either by functionalizing the CNT materials in ternary composites or by using CNT Hybrid Composite. Thereafter, the EQCM and acelectrogravimetry could be employed in order to examine deeply the electrochemical behavior of such layers. Les systèmes de stockage électrochimiques de l'énergiesont traditionnellement divisés entre les systèmes de forte densité (batteries) et les systèmes de forte puissance (capacités ou supercapacités).

Résumé de la Thèse en Français

Introduction

Ici, nous nous intéresserons à ces derniers dispositifs. Aujourd'hui, il en existe plusieurs types utilisant différents mécanismes de stockage. 1 Les deux principaux sont les systèmes à double couche électriques (EDCLs) et les pseudocapacités (Voir Figure 1).

Résumé de Thèse

196

(2) nanotubes de carbone (CNTs), (3) graphène, (4) carbone de type carbure (CDCs), (5) carbones actifs (ACs), et (6) carbones structurés (Voir Table 1). Les critères pour un électrolyte idéal repose sur une large fenêtre électrochimique (>4 V), une conductivité spécifique supérieure à 75 mS.cm -1 , une bonne stabilité thermique jusqu'au 300 o C et une faible toxicité. 6 Cependant de nombreux efforts dans la recherche d'électrolytes performants ont été orientés vers des milieux très conducteurs, stables et avec une fenêtre de potentiel la plus large possible (voir Table 2). Une modélisation des réponses d'ac-électrogravimétrie permet de remonter à la masse atomique des différentes espèces impliquées dans les processus électrochimiques. Les cinétiques de transfert et les résistances de transfert de ces mêmes espèces sont aussi estimées de la même manière.

Chapitre III

Le troisième chapitre est dédié à la clarification des mécanismes de transfert des ions à l'interface électrode à base de CNT/électrolyte. Il s'agit aussi de valider l'impact positif de l'utilisation d'électrodes nanostructurés dans des dispositifs de stockage d'énergie. Ici, un exemple détaillé est présenté afin de montrer la puissance de notre approche pour les films de type SWCNT testés en milieu NaCl. À la fin, une déconvolution des réponses gravimétriques via une EQCM classique est montrée grâce à l'apport de la technique d'ac-électrogravimétrie. De plus, une étude des mécanismes en solvant organique est aussi présentée dans ce chapitre.

Mesures de microbalances classiques

La figure 4 (A et B) présente des courbes d'électrogravimétrie cyclique pour un film SWCNTs plongé dans NaCl 0,5M à pH neutre. La figure 4B donne la variation de masse du film accompagnée de la variation du courant dans une gamme de potentiel entre 0,45V et -0,45V vs Ag/AgCl. La voltammétrie cyclique a été effectuée à différentes vitesses de balayage et présente un comportement capacitif typique caractérisé par une forme rectangulaire dans le domaine de potentiel -0,1 V à 0,4 V vs Ag /AgCl. Ce résultat suggère que ce comportement est principalement dû à l'adsorption des ions sur la surface des nanotubes de carbone, c'est à dire sans aucune réaction faradique. L'écart par rapport à un comportement purement capacitif à des valeurs plus cathodiques (Fig. 4A) est probablement dû à la présence d'oxygène ou de groupes fonctionnels de surface ou du catalyseur restant après purification des nanotubes de carbone. [16][17][18][19] En ce qui concerne les réponses en masse, une réponse similaire est observée pour toutes les vitesses de balayage (Fig. 4B). La variation de masse de l'électrode de SWCNT (Fig. 4B) présente un changement de pente, La voltammetrie cyclique présentée Figure 7 présente un comportement capacitif entre -0,1V et -0,45V et entre 0,2V et 0,45V. Ceci est caractéristique de la réponse électrochimique des films de CNTs pures. Dans la gamme entre 0,05V et 0,15V, des pics d'oxydation/réduction sont observés, ce qui est caractéristique de la réponse du PB. Les valeurs de courant sont plus importantes ici comparativement à celles indiquées dans la littérature. 15,23,[START_REF] Gabrielli | Ac-Electrogravimetry Study of Electroactive Thin Films. I. Application to Prussian Blue[END_REF] . Ceci est certainement lié à la plus grande surface spécifique. Compte tenu de la variation de masse de l'électrode, un comportement particulier est observé : la contribution des cations est présente entre -0.45V et -0.15V alors que la contribution des anions est visible entre 0,2V et 0,45V. De plus, les valeurs de variation de masse sont plus importantes que celles trouvées dans la littérature 15,24 et aussi plus importantes que les valeurs de variation de masse pour les films de SWCNT seuls.

Les données expérimentales obtenues par ac-électrogravimétrie ont été ajustées en utilisant les modèles présentés auparavant. Ainsi, il a été observé que le transfert de K + était plus rapide que le transfert de Cl -dans la fenêtre de potentiel étudiée. -1,4 -1,2 -1,0 -0,8 -0,6 -0,4 -0,2 0,0 0,2 0,4 0,6

Figure I- 1 :

 1 Figure I-1:Schematic operating principle of a typical rechargeable LIB.[START_REF] Benmouhoub | Influence of the Incorporation of CeO2 Nanoparticles on the Ion Exchange Behavior of Dodecylsulfate Doped Polypyrrole Films: Ac-Electrogravimetry Investigations[END_REF]

Figure I- 2 :

 2 Figure I-2: Ragone plot for electrochemical energy storage devices. 29

Figure I- 3 .

 3 Figure I-3. CO2 Equivalents Lifecycle Comparison: conventional gasoline vehicle (CV), battery electric vehicle (BEV) and hybrid vehicle.

Figure I- 4 .

 4 Figure I-4. Categorization of supercapacitors.

  Figure I-5. Discharged (right) and charged (left) states of an electrochemical capacitor. 45

  (Figure I-7).

Figure 1 - 7 .

 17 Figure 1-7. Double Layer Energy Storage in Graphene when a potential is applied.

  (a) ion sieving, (b) ion desolvation (c) pore saturation and (d) distortion. a. Ion Sieving Ion sieving was defined by Aurbach et al. and denotes the possibility of selective electrosorption of ions based on size. 78-80 The findings of Aurbach et al. can be summarized as follows (Figure I-8):There is a typical capacitive rectangular-shape of the cyclic voltammogram (CV)

Figure I- 8 .

 8 Figure I-8. Voltammograms of activated carbon electrodes with average pore size of 0.58 nm and 0.51 nm obtained in 0.1 M MgSO4 solution. For comparison, curves are also given for pores of 0.51 nm in 0.1 M Li2SO4 and MgCl2 solutions. The current is normalized per unit of BET SSA. 48

Figure I- 9 .

 9 Figure I-9. Capacitance in two electrolytic media versus the volume of micropores smaller than 0.7-0.8 nm for various template carbons. 48 et al. 81 showed that the capacitance is proportional to the ultra-micropore volume (pores smaller than 0.7-0.8 nm) measured by CO2 gas sorption, in both aqueous and organic media (Figure I-9).

Figure I- 10

 10 Figure I-10. (a) Cyclic voltammograms for EDLCs based on APC carbon (left-hand side yaxis for current) and AVC carbon (right-hand side y-axis for current. 48

  electrolytes and electrode materials) 38 , thermally resistant 40 , flexible (i.e. endure pressure and volume changes) and easily soaked in electrolytes. 38 The most commonly used materials are cellulose 40 and polymers. 38 Polymer separators can be categorized according to structures: fibrous structures and monolithic networks with defined pores 38 (Figure I-11).

Figure I- 11 :

 11 Figure I-11: Scanning electron microscopy (SEM) images of (a) Millipore JVWP separator with fibrous structure and (b) GE Osmotics K50CP01300 separator with monolithic/defined pores. 38

  an ideal EDLC electrolyte such as a wide electrochemical window (>4), specific conductance of > 75 mS.cm -1 at room temperature, thermal stability up to 300°C, low toxicity have been discussed in Section I-1.2.3. However, numerous research efforts regarding the Equation I.9 in different electrolytes have been directed at the design of highly conductive, stable electrolytes with a wider voltage window 29 (Figure I-12).

Figure I- 12 .

 12 Figure I-12. Graph depicting the increase of energy density with ILs compared to other electrolytes 39 According to Figure I-12, ILs seem to be the most important electrolyte in terms of energy density and wider voltage window. However, as mentioned previously, the ionic conductivity is low at room temperature. Another example found in the literature 48 considering the wide voltage window is shown in Figure I-13.

Figure 1 - 13 .

 113 Figure 1-13. Comparison of the EDLC operating voltage achievable with organic electrolyte and ionic liquids based electrolytes. AN (acetonitrile), PC (propylene carbonate), AND (adiponitrile), AlKylat. Cyc. Carb. (alkylated cyclic carbonate), EC (ethylene carbonate), DMC (dimethyl carbonate), LiPF6 (lithium hexafluorophosphate), IL (ionic liquids). 48

Figure I- 14 .

 14 Figure I-14. Design of ILs for specific applications. 89

(

  of supercapacitor type) with a faradaic electrode (of battery type) (Figure I-15 d and e).

Figure I- 15 .

 15 Figure I-15. Schematic illustrations of different storage mechanisms. Faradic reactions are involved in a) pseudocapacitance and c) intercalation; non-Faraday process is represented in EDLCs in b). Combining either a/b or c/d forms an asymmetric supercapacitor d) or a Li-ion supercapacitor e). 91

Figure I- 16 .

 16 Figure I-16. Cyclic voltammogram curves of (a) ideal electrochemical capacitor, (b) EDLC, and (c) pseudocapacitive materials. 38

Figure I- 18 .

 18 Figure I-18. Cyclic voltammetry a MnO2-electrode cell in K2SO4(0.1 M ) shows the successive multiple surface redox reactions leading to the pseudo-capacitive charge storage mechanism. The red (upper) part is related to the oxidation from Mn(III) to Mn(IV) and the blue (lower) part refers to the reduction from Mn(IV) to Mn(III). 29

Figure I- 20 .

 20 Figure I-20. Nyquist plot for 4 cm 2 cells assembled with electrodes constituted with activated carbon in 1.5M Net4BF4. 115 imaginary part of the impedance corresponds to the capacitive behavior linked with the ion adsorption in the whole porous network, in this case BF4 -anion. Finally, the authors considered that this part can be characterized by the extent of deviation from the 90° theoretical vertical line observed for an ideal capacitance.

Figure I- 21 .

 21 Figure I-21. Nyquist plots of the three CDC samples in AN + 1.5 M Net4BF4 electrolyte at an applied voltage of 0.5V/Ref which concerns the anion and cation electro-adsorption, respectively. A zoom at high frequencies is shown in the onsets. 40

  Figure I-22 shows ex. situ XRD patterns of V2CTx recorded at different voltages. The authors observed that the (002) peak shifts continuously and reversibly from 9° to 12° during cycling between 1 and 3.5V versus Na + /Na. In this potential range, the change is perfectly reversible, thus demonstrating that there are no undesired side reactions. During sodiation, the c-lattice parameter increases with the amount of Na + stored. The authors conclude that the V2CTx material stores energy through intercalation of Na ions in between the layers. 121

Figure I- 22 .

 22 Figure I-22. XRD patterns at different potentials. (*) Peak of unreacted V2AIC. 121

Figure I- 23 .

 23 Figure I-23. (a) FEG-SEM image of the raw sample of DWCNTs. Inset (b) shows a higher magnification image of the bundles of DWCNTs. 115

Figure I- 24 .

 24 Figure I-24. SEM images of (a) the as-grown forest and (b) shape-engineered SWCNTs. 85

Figure I- 26 .

 26 Figure I-26. EDX analysis of a CNT film before (a) and after (b) the formation of PB. 126

  Figure I-27, an example for a series of nanoporous carbons obtained by KOH activation of bituminous coal pyrolyzed at 520 o C-1000 o C is shown. 127 The proportionality is observed only for the low values of the BET specific surface area. Above 2000 m 2 g -1 , the observed values may be caused by an overestimation of the values of the specific surface area by the BET model. In another work, Frackowiak et al. 128 have studied the effects of structures and diameters of CNTs, and microtexture and elemental composition of the materials on the capacitance. TableI-9shows that the capacitance increases with increased specific surface area. The smallest value is obtained in CNTs with closed tips and graphitized carbon layers, where the mesopore volume for the diffusion of ions and the active surface area for the formation of the electrical double layer are very limited. The nanotubes with numerous edge planes, either due to herringbone morphology (A900Co/Si) or due to amorphous carbon coating (A700Co/Si), are the most efficient materials for the collection of charges. Quite moderate performance is given by straight and rigid nanotubes of large diameter (P800A1) despite a relatively high specific surface area.

Figure I- 27 .

 27 Figure I-27. Gravimetric capacitance in various electrolytes (1 mol L -1 H2SO4, 6 mol L -1 KOH and 1 mol L -1 TEABF4 in acenitonitrile) vs BET specific surface area for a series of bituminous coal-derived carbons activated by KOH at 800 o C. 127

  2,143 Barisci et al. monitored the mass changes of carbon nanotube films as a function of the applied potential in both aqueous and non-aqueous solutions (Figure I-28). 143 The authors reported that the mass changes as a function of the applied potential are not directly proportional to the cation mass but they are associated with double-layer charging. The mass changes of nanotube films in acetonitrile solutions are similar to those observed for aqueous electrolytes (Figure I-28a) when the cation is tetrabutylammonium (Figure I-28b). However, solutions of Li + cause a substantial increase in the nanotube film mass associated with cation insertion, which is not readily reversed, thus reflecting slow Li + expulsion (Figure I-28c). The authors concluded that the movements of anions, cations, and solvent molecules in the pores of the carbon nanotube films during potential sweeping involve complicated mechanisms. 143

Figure I- 28 .

 28 Figure I-28. Current-potential and mass-potential curves for a CNT film in (a) 0.1 M H2SO4, (b) 0.1M TBATFB/acetonitrile and (c) 0.1M LiClO4/acetonitrile. Scan rate 50mV s -1 (reference electrode Ag/Ag + ). 143

Figure I- 29 .

 29 Figure I-29. (a) Cyclic voltammetry curves measured at a scan rate of 5 mV s -1 Ag/AgCl in aqueous 0.5M CsCl. (b) Simultaneously measured QCM responses. The red straight lines relate to the potential-time dependencies; the black and the blue curves represent the related mass changes of the cations (C + ) and anions (A + ) during potentiodynamic cycling of the electrodes. 2

Figure I- 30 .

 30 Figure I-30. Mass change of the carbon electrode in a 0.5M aqueous solution of NH 4 Cl (scan rate = 20 mVs -1 ) measured simultaneously with the CV of the largest 1.2V amplitude (see inset, broken line). The inset also shows a series of CVs measured with different potential amplitudes. 144

Figure I- 31 .

 31 Figure I-31. CV and QCM frequency responses of CDC-1 nm films in 2 M EMI-TFSI/AN. 142

1

  to evaporate the residual solvent and improve the adhesion properties of the films on gold electrode (Figure II-1). The film thickness was estimated to be around 500 nm (based on FEG-SEM measurements).

Figure II- 1 .

 1 Figure II-1.Schematic representation of CNT Film Preparation.

Figure II- 2 .

 2 Figure II-2. Schematic description of a SEM set-up. 150

Figure II- 3 .

 3 Figure II-3. Schematic description of a TEM/HRTEM set-up. 151

Figure II- 4 .

 4 Figure II-4. Schema showing the working principle of an EDX detector.

Figure II- 5 .C

 5 Figure II-5. Schematic diagram for determining Bragg's Law. 36

Figure II- 6 .

 6 Figure II-6. Schematic representation of the deformation of a polarized quartz material.163,178

Figure II- 8 .

 8 Figure II-8. Quartz disk face (A) and cross-sectional (X) carrying the two gold electrodes 109

  Figure II-9. Schematic illustration of a thickness shear mode quartz resonator.

  mass, mechanical stress etc.) are transformed into electrical signals. The resonant frequency of the quartz resonator varies under the effect of surface changes, 109,178 viscoelastic changes 190 and viscosity of the solution 191 . However, in the literature, it has been reported that at a certain frequency of QCM, the viscosity of the solution is not affected. 192 In this Ph.D. thesis, the set-up used for measuring mass/frequency change is a lab made microbalance developed at the LISE laboratory. A gold-patterned 9 MHz quartz crystal resonator (AWS, Spain or TEMEX, France) is inserted in an electrochemical cell (AWS Company, Spain, used for the first time in this Ph.D. thesis) and connected to a specific electronic circuit, the whole system being called an oscillator (Figure II-10). Thus, it is possible to monitor the evolution of microbalance frequency, fm, in real time through the FRQM software developed at LISE.

Figure II- 10 .

 10 Figure II-10. Set-up of the electrochemical device used in this work.

Figure II- 11 .

 11 Figure II-11. The EQCM results of Single Wall CNT thin films current vs potential (A) and mass variation vs potential (B) profiles measured in aqueous electrolyte 0.5 M NaCl.

Figure II- 12 .

 12 Figure II-12. Experimental set-up of an EQCM 193

Figure II- 13 .

 13 Figure II-13. Linear current response to a low amplitude sinusoidal perturbation potential around a stationary value.

  Figure II-14. (a) Equivalent electrical circuit. Plots of impedance of an electrochemical cell in (b) the Nyquist diagram and in (c) the Bode diagram. 199

Figure

  Figure II-15. Experimental set-up for electrochemical impedance measurements.

.

  Using the gold electrode of the quartz resonator as a working electrode, two distinct transfer functions can be obtained simultaneously: an electrochemical impedance (

  the film, driven by the potential values, is detectable through the microbalance frequency change, ∆ƒm. Using the Sauerbrey equation (Equation II-7), the mass changes of the film, ∆m, can be estimated. The modulation frequency of the perturbation signal provides additional information, i.e. it allows the separation of the kinetics of different species involved in the charge compensation of the film. The measurements are controlled by a computer using FRACOM software, a software developed at the LISE laboratory. Figure II-16 shows the principle of the measurement of the electrogravimetric transfer function, ∆m/∆E(ω).

For

  Figure II-16. Principle of measuring the electrogravimetric transfer function ∆Vf/∆V.

FigureFirstly

  Figure II-18. Ac-electrogravimetric set-up 203The signal from the microbalance, ƒm, is detected by means of a lab made frequency/voltage converter,204,205 which subtracts a reference signal, ƒr, close to the frequency of the microbalance issued by the Marconi 2023 frequency synthesizer. Then, the ƒm-ƒr signal is converted into voltage, Vƒ : if ƒm varies from the ∆ƒm, the converter delivers a voltage variation, ∆Vƒ. The resulting signal, ∆Vƒ, and the current response, ∆I, of the working electrode are simultaneously sent to the FRA-Solartron 1254, which allows the two transfer functions, ∆Vƒ/∆V(ω) and ∆V/Rt∆I(ω), to be determined(Figure II-19). ∆V is the raw potential response, which takes into account the electrolyte resistance, and Rt is the resistance in the counter electrode circuit, which allows the current, ∆I to be measured. Usually, a 10 -3 Hz to 60 kHz frequency range is used to measure these transfer functions.109,203

Figure

  Figure II-19. Scheme of the calibration of the frequency/voltage converter for determining the transfer function ∆Vƒ/∆e (ω).

Figure

  Figure II-20.

Figure II- 20 .

 20 Figure II-20. Modulus and phase of the global calibration of the frequency/voltageconverter. 203

Figure

  Figure II-21. Measurements of the transfer function ∆fs/∆e from the Agilent 33250Asynthesizer for three settings depth modulation (FM): 10 Hz, 100 Hz, 300 Hz.

  Specifically, the fitting of the data (at each potential perturbation applied) is performed via Mathcad software by calculating the electrochemical impedance, the charge/potential transfer function, the global and partial mass/potential transfer functions at different modulation frequencies with different values of the parameters. The fitting procedure respects the strict criteria that a good agreement between the experimental and the theoretical function in terms of shape and frequencies was achieved. This provides relevant information on the ions transfer characterized by the Ki and Gi, parameters and identifies the nature of the species involved by their molar mass, mi.Let us consider the following electroadsorption/desorption mechanism which intervenes in our electrochemical system:

  Figure III-1. FEG-SEM of an SWCNT based thin film deposited on the gold electrode of a quartz resonator (A), cross section (B) and HRTEM images of the SWCNTs (C) and (D).

Fig

  Fig. III-2A and 2B indicates the presence of a certain disorder in their structure in agreement with the HRTEM investigations.

  Figure III-2. XRD spectra (A) and (B) of SWCNTs (the 2 theta region related to SWCNTs are shown in (B) and # and * are related to the sample holder). Nitrogen sorption isotherm (C) and the pore size distribution (PSD) of SWCNTs.

  Figure III-3. EQCM results of SWCNT thin films -the CV curves (A) and the simultaneously obtained mass changes (B) measured in aqueous 0.5 M NaCl electrolyte at pH=7.

Figure

  Figure III-4 shows the Fdm/dq values which are equivalent to the molecular mass of the electroadsorbed species at a given scan rate. 109The values are calculated as a function of the potential in NaCl electrolyte (See Chapter II for details of calculation). The Fdm/dq shows a value between -40 to -23 g.mol -1 for potentials in the range of the 0.05 V to -0.4 V vs Ag/AgCl, particularly at 100 mV.s -1 scan rate. It is important to note here that the negative and positive molar mass values correspond to the cations and anions, respectively. 109 Absolute values of the Fdm/dq function is considered for discussion. The molecular mass at 23 g.mol -1 may correspond to the cation contribution and the molar mass higher than 23 g.mol -1 may indicate the presence of hydrated cations and/or hydrated cations accompanied by free solvent molecules in the same flux direction during the transfer. In the anodic part, higher molar mass values than the respective anion in the electrolyte are observed especially at high scan rates.Therefore, a concomitant transfer of the anions with free solvent can be expected. Additionally, a strong scan rate dependence of the Fdm/dq function is observed at more anodic potentials.The contribution of (i) counter ions solely, (ii) counter ions and solvent or (iii) both counterand co-ions (without solvent) in carbon based electrodes were intensively investigated byLevi and Aurbach et al. 144 , and similar cases were considered in doped conducting polymers and conventional redox systems by

  Figure III-5. Specific capacitance values of SWCNT thin films (calculated from Figure III-3A) as a function of potential, estimated at selected scan rates for 0.5 M NaCl pH=7.

  . III-7A). It should be underlined that in this case the theoretical well the experimental data by using the free solvent contribution (Fig.III-6B). Nevertheless, the fitting of several other functions including of simulation has been realized with the involvement of two cations. No anion contribution is expected at -0.4 V vs Ag/AgCl.

FFigure III- 7 .

 7 Figure III-7. Experimental and theoretical transfer functions at -0.4V vs Ag/AgCl in 0.5 M NaCl. The theoretical functions were calculated taking into account of two cations (Na + . H2O) and (H + ). Theoretical functions were calculated with the following parameters: df = 0.4µm, Kc1 = 5.40 x 10 -5 cm s -1 , Gc1 = 1.19 x 10 -8 mol s -1 cm -2 V -1 , Kc2 = 6.91 x 10 -3 cm s -1 , Gc2 = 4.08 x 10 -6 mol s -1 , mc2 = 23+18 g mol -1 , mc1 = 1 g mol -1 .

  Different Parameters over the Applied Potentials-the Nature and the Kinetics of the Species Above discussion was based on the detailed analysis of the ac-electrogravimetric data at a selected potential. Such measurements and data analysis were performed at different stationary potential values in the range of -0.45 V to 0.45 V vs Ag/AgCl. The nature of the species and the

Figure III- 9 .

 9 Figure III-9. Kinetic transfer rates Ki (A), and transfer resistances Rti (B) for all species estimated from the fitting of the ac-electrogravimetry data measured in 0.5 M NaCl pH=7.

Figure

  FigureIII-9 shows the evolution of kinetic transfer rates, Ki, and the transfer resistances, Rti, of the species as a function of the applied potential. It is easier to discuss this curves in two regions, corresponding to the more cathodic and more anodic potentials with respect to the pzc of the SWCNT based thin films. In agreement with the EQCM data in Fig.III-3, the pzc can be considered to be around 0.1 V vs Ag/AgCl.

Figure III- 10 .Furthermore

 10 Figure III-10. Schematic representation of the interaction of ions with carbon nanotube based electrodes on gold electrodes of the quartz resonator in aqueous 0.5 M NaCl at pH=7.

  Figure III-12. Experimental and theoretical ac-electrogravimetric data of the SWCNT thin film in 0.5 M LiClO4 measured at -0.4V vs Ag/AgCl. (A) Δq (ω) ΔE , (B) Δm (ω) ΔE , (C)

  Figure III-13. The kinetic transfer rates Ki (A), and transfer resistances Rti (B) parameters for all species estimated from the fitting of the ac-electrogravimetric data measured in 0.5M LiClO4/PC.

Figure

  Figure III-13 shows the evolution of transfer kinetic rates, Ki, and the transfer resistance, Rti, of the species as a function of the applied potential. Based on the Ki values presented in Figure III-13A, the Li + ion is the fastest of the four species present. Furthermore, the transfer rates of the free solvent in the Figure III-13A are somewhat close to the Li + ion and the ClO4 -ion, probably due to the exclusion phenomenon. It is noted that the PC has persistanly the opposite flux direction with the ionic species.

  Figure III-14. Evolution of the relative concentration, 0 i CC of each species over the applied potential measured in 0.5M LiClO4.

  Figure IV-1. FEG-SEM of a DWCNT based thin film deposited on the gold electrode of a quartz resonator (A), cross section (B) and HRTEM images of the DWCNTs (C) and (D).

Figure

  Figure IV-1A shows a representative FEG-SEM image of the DWCNT based thin film electrode surface, revealing the fact that CNT bundles are formed with an average thickness (or width) of 20 nm. The cross-section of the DWCNTs shown in Figure IV-1B permits the estimation of the film thickness. The film thickness deposited on the gold electode of quartz resonators is limited to ~ 1.5 µm, to keep the gravimetric regime.

  Fig. IV-1 is lower, and this sample is called DWCNT, thereafter.

  Figure IV-2. XRD spectra (A) of DWCNTs. Nitrogen sorption isotherm (B) and the pore size distribution (PSD) (C) of DWCNTs.

  Figure IV-3. FEG-SEM of a MWCNT based thin film deposited on the gold electrode of a quartz resonator (A), cross section (B) and HRTEM images of the MWCNTs (C) and (D).

Figure

  Figure IV-3A shows a representative FEG-SEM image of the MWCNT samples, revealing the fact that MWCNT bundles are slightly larger compared to SWCNT and DWCNTs, with a width of 30-40 nm (Figure IV-3A). The cross-section of the MWCNTs shown in Figure IV-3B permits the estimation of the film thickness. Similar to the DWCNTs, the film thickness deposited on the gold electode of quartz resonators is limited to ~ 1.5 µm, to keep the gravimetric regime for futher analysis with QCM based methods. The HRTEM in Figure IV-3C reveals a clear increase of the number of walls compared to DWCNTs and the multi wall structure of the CNTs. The inner diameter of the nanotubes varies and daverage is estimated as ~ 8 nm (Figure IV-3C and 3D). The crystalline structures of the MWCNTs were evaluated by XRD (Figure IV-4A) and similar results to that of DWCNTs were obtained. The strongest diffraction peak related to the DWCNT structure at the angle (2θ) of ~ 25° and the peak at 2θ ~ 42° can be indexed as (002) and(100) reflections and are attributed to the hexagonal graphitic structure.206

Figure

  Figure IV-4B shows the nitrogen adsorption-desorption isotherms of the MWCNT. The increase in the diameter of the nanotubes is reflected as a decrease in the specific surface area which was estimated by BET approach as 300 m 2 g -1 (compared with DWCNT and SWCNT). The PSD of the MWCNTs (Figure IV-4C) is quite broad indicating the presence of several dimensions in the samples. The peak around 15-20 nm can be attributed to the outer diameter of the MWCNTs and the broad distribution centered at 40 nm can be attributed to the inter bundle voids of the MWCNTs, in agreement with the microscopy images in Figure IV-3C and 3D.

FigureFFigure IV- 5 .

 5 FigureIV-5 shows the EQCM responses of CNT films obtained in aqueous electrolytes of 0.5M NaCl in a potential window of 0.45V to -0.45V vs Ag/AgCl. A growing capacitive current is observed when the scan rate increases(Figures IV-5 A, C and E). Furthermore, the capacitive current of the DWCNT is more significant than the capacitive current of the SWCNT and the MWCNT following the order: iDWCNT>iSWCNT>iMWCNT. This difference in current is probably due to the pore structure of the materials and its accessibility to the electrolyte ions. In this study, the different CNT films show quasi-rectangular shaped responses indicating that the charge storage is mainly due to the reversible electroadsorption/electrodesorption of electrolyte ions. The slight distortion from a perfect rectangular shape is due to the presence of a slight faradaic contribution to the charge storage as already mentioned.Concerning the mass changes, there is a difference between the responses of the SWCNTs, DWCNTs and MWCNTs following the same order as the current values, i.e., mDWCNT>mSWCNT>mMWCNT. Furthermore, the reversibility of the mass response is more evident in SWCNTs and DWCNTs than in MWCNTs (Figures IV-5 A, C and E) where a large hysteresis was observed. For the SWCNTs and DWCNTs, the mass change is more significant at cathodic potentials than at anodic potentials. In contrast, in MWCNTs, the mass change is slightly more significant at anodic potentials than at cathodic potentials (it is also noted that the pzc is shifted slightly towards more cathodic potentials (Figures IV-5 E). In other words, transfers of cationic species are more significant in DWCNTs and SWCNTs, while the transfers of anionic species are more important in MWCNTs in this potential range. This difference in behavior is probably related to the different adsorption sites of the species and different surface properties of the CNTs. The Figures IV-5 B, D and F show the Fdm/dq values which can be considered equivalent to the molecular mass of the electroadsorbed species. The values are calculated as a function of the potential in NaCl electrolyte. The values vary in the range of 125 to -50 g.mol -1 , 50 to -35 g.mol -1 and 180 to -70 g.mol -1 , for the SWCNT, DWCNT and MWCNT thin films, respectively.

Figures

  Figures IV-6, IV-7 and IV-8 show an example of the experimental and theoretical transfer functions (TFs) obtained from ac-electrogravimetry of a SWCNT, DWCNT and MWCNT thin film in 0.5 M NaCl electrolyte at -0.4V vs Ag/AgCl. The measurements were performed each 100 mV in the range from 0.4V to -0.4V vs Ag/AgCl. The experimental data were fitted according to the model presented in Chapter II. Figures IV-6-7-8 reveal a good agreement between experimental data and theoretical curves. First, the

Figure

  Figure IV-6B shows two loops with a small one at LF whereas in the case of DWCNTs and MWCNTs (Figures IV-7B and V-8B), only one big loop is observed. In these latter cases, it big loop can also be attributed to two species but with time constants not sufficiently different from each other. The global and partial mass /potential transfer functions are more informative to clarify these possibilities.

  using Equation II-50 in Chapter II and their respective kinetics of transfer were characterized by the Ki values. The presence of three different species estimated by simulating the experimental data was further confirmed by carefully analyzing the partial electrogravimetric transfer function, for example, by removing the cation 1, c1, contribution (Figures IV-6E, IV-7E and IV--52 and 51). In Figures IV-6D and V-7D, a small contribution at low frequency is shown in the fourth quadrant.

  Figure IV-6. Experimental and theoretical ac-electrogravimetric data of the SWCNT thin film in 0.5 M NaCl measured at -0.4V vs Ag/AgCl. (A)

Figure IV- 7 .

 7 Figure IV-7. Experimental and theoretical ac-electrogravimetric data of the DWCNT thin film in 0.5 M NaCl measured at -0.4V vs Ag/AgCl. (A)

Figure IV- 8 .

 8 Figure IV-8. Experimental and theoretical ac-electrogravimetric data of the MWCNT thin film in 0.5 M NaCl measured at -0.4V vs Ag/AgCl. (A)

CMWCNTFigure IV- 9 .

 9 Figure IV-9. Constants of transfer kinetics, Ki, of SWCNT (A), DWCNT (B), MWCNT (C) estimated from the fitting of the ac-electrogravimetric data measured in aqueous 0.5M NaCl at pH7.

Figure

  Figure IV-9 shows the evolution of the constant transfer kinetics, Ki, of the species as a function of the applied potential. Based on the Ki values presented in Figure IV-9, the Na + .nH2O ion is the fastest of the three species in SWCNTs whereas (Fig. IV-9A), in DWCNTs and MWCNTs,

  Figure IV-10. Transfer resistance, Rti, of SWCNT (A), DWCNT (B), MWCNT (C) estimated from the fitting of the ac-electrogravimetric data measured in aqueous 0.5M NaCl at pH 7.

  Figure IV-12. Instantaneous capacitance, C'i, values of SWCNT (A), DWCNT (B) and MWCNT (C) thin films calculated from ac-electrogravimetric data.

Figure

  Figure IV-12 shows the instantaneous capacitance values for each ion (C'i) calculated as a function of the potential. These correspond to the capacitance attributed to each ions and the ability to take identified charges, which are the cations Na + .nH2O and H + and the anion Cl -, following a potential step. The instantaneous capacitance values for the SWCNT and DWCNT are equivalents for the two cations Na + .nH2O and H + at cathodic potentials and for the anion Cl -at anodic potentials (Figures IV-12A and B). Finally, the instantaneous capacitance values are lower in the MWCNT than in the SWCNT and DWCNT for all the species (Figure IV-12 C), which is coherent with the results of the relative concentration variations mentioned previously. This effect can be attributed to the lower specific surface determined in the case of the MWCNTs.

FigureFFigure IV- 13 .

 13 Figure IV-13C, D and F show the variation of the Fdm/dq values, obtained from the reduction branch in the range 0.4V to -0.4V vs Ag/AgCl measured in 0.5M NaCl. The values vary in the range of 90 to -25 g.mol -1 , 140 to -50 g.mol -1 and 70 to -40 g.mol -1 , for pH 2, pH 7 and pH 10, respectively. The observed higher values further indicate that the ions are hydrated and/or accompanied by free solvent molecules during their transfer. Nevertheless, the values obtained at pH 2 at cathodic potentials could correspond to Na + or a low hydratation level of Na + . The EQCM does not provide information on which of the possible scenarios that actually takes place such as hydrated or dehydrated species.

  three pH values in order to crosscheck the different hypothesis considered previously about the ions and solvent species involved. First, c2 contribution was removed from the global fitted as shown in Figures IV-14D, III-6D and IV-15D. At pH 7 and 10, the responses are quite similar with a big loop in the third quadrant at medium frequency (MF)

  Figure IV-14. Experimental and theoretical ac-electrogravimetric data of the SWCNT thin film in 0.5 M NaCl measured at -0.4V vs Ag/AgCl pH 2. (A)

  Figure IV-15. Experimental and theoretical ac-electrogravimetric data of the SWCNT thin film in 0.5 M NaCl measured at -0.4V vs Ag/AgCl pH 10. (A)

Figure

  only the c1 and water contributions are seen as given in Figures IV-14E, III-6E and IV-15E.

Figure IV- 16 .

 16 Figure IV-16. Constants of transfer kinetics, Ki, for ions (A) Na + .nH2O, (B) H + and (C) Cl - estimated from the fitting of the ac-electrogravimetric data measured in 0.5M NaCl at different pH values.

Figure

  Figure IV-16 shows the evolution of the constants of transfer kinetics, Ki, of the species as a function of the applied potential at different pH. Based on the Ki values presented in Figure IV-16A, ion transfer kinetics of the Na + .nH2O at pH 7 and pH 10 are similar whereas the corresponding Ki values are lower at pH 2. The available (or measured) points of Ki for Na + .nH2O at pH 2 are limited compared to the two other pH. This is due to the PZC value which is pH dependent and shifts to the more cathodic potentials at low pH values, therefore the contribution of cations is limited in this

  Figure IV-17. Transfer resistance, Rti, for the different ions (A) Na + .nH2O, (B) H + and (C) Cl - estimated from the fitting of the ac-electrogravimetric data measured in 0.5M NaCl at different pH values.

Figure

  Figure IV-18 shows the relative concentration change for the ionic species. The (Ci-C0) values of the Na + .nH2O at pH 7 and pH 10 are significantly higher than the (Ci-C0) values of the other charged species (Figures IV-18B and C) regarding the same potential windows. In contrast, at pH 2, the (Ci-C0) values of the Cl -are significantly higher than the (Ci-C0) values of the other species (Figure IV-18A) always keeping the same potential range. In other words, low pH values amplify the anion contribution whereas high pH values amplify the cationic response.

  Figure IV-18. Evolution of the relative concentration, Ci-C0, of each species over the applied potential measured in 0.5M NaCl at different pH: (A) pH 2, (B) pH 7and (C) pH 10.

Figure

  Figure IV-19 shows the instantaneous capacitance values of each ion, C'i), calculated from

  Figure IV-19. Instantaneous capacitance, C'i, values of ions (A) Na + .nH2O, (B) H + and (C) Cl - calculated from ac-electrogravimetric data at different pH.

  Figures IV-20 B, D and F show the Fdm/dq values obtained from the reduction branch of the EQCM results measured in different aqueous electrolytes. The values vary in the range of 150 to -25 g.mol -1 , 150 to -55 g.mol -1 and 1500 to -90 g.mol -1 in LiCl, NaCl and KCl, respectively. The observed higher values just confirm that the ions are hydrated and/or accompanied by free solvent molecules during their transfer.

Figure

  Figure IV-21 show the capacitance curves as a function of the applied potential in 0.5M LiCl, NaCl and KCl, respectively. According to Figures IV-21A, B and C, the measured capacitance values are around 25 to 30 F.g -1 for the three different electrolytes with homogeneous

Figure IV- 21 .

 21 Figure IV-20. EQCM responses of SWCNT measured in different aqueous electrolyte 0.5 M LiCl (A), 0.5 M NaCl (C) and 0.5 M KCl (E). The values corresponding to the average molecular weight of the species involved in the charge compensation, Fdm/dq, are shown as a function of the potential and estimated for the three electrolytes (obtained from the reduction branch of EQCM data presented in Figure IV 20 A, C and E).

  show one loop in the first quadrant(Figures IV-22B, IV-6B and IV-23B), which can be attributed to only one species or to two species, the latter is thecase if the time constants of ions are not sufficiently different from each other. Nevertheless, at low frequencies and in all the electrolytes a small loop appears. In NaCl, this response is slightly amplified compared to those occurs in LiCl and KCl electrolytes.The mass/potential transfer function, of LiCl, NaCl, KCl show one big loop in the third quadrant from 100 Hz to 1 Hz (Figures IV-

  -22E, IV-6E and IV-23E). Particularly in NaCl and KCl media, a small contribution is shown in the fourth quadrant at low frequencies (Figures IV-6D and IV-23D), which

  Figure IV-22. Experimental and theoretical ac-electrogravimetric data of the SWCNT thin film in

FFigure IV- 23 .

 23 Figure IV-23. Experimental and theoretical ac-electrogravimetric data of the SWCNT thin film in

CFigure IV- 24 .

 24 Figure IV-24. Constants of transfer kinetics, Ki , of ions (A) Li + .nH2O, Na + .nH2O, K + (B) H + and (C) Cl -estimated from the fitting of the ac-electrogravimetric data measured in 0.5M LiCl, NaCl and KCl.

Figure IV- 24

 24 Figure IV-24 shows the evolution of the constant kinetic transfer, Ki, of the species as a function of the applied potential measured in different aqueous electrolytes. Based on the Ki values presented in Figure IV-24A, the transfer of Na + .nH2O ion and K + ion is faster compared to that of Li + .nH2O for cathodic potentials. For the H + , the Ki values are more or less equivalent in the three different media as shown in Figure IV-24B. This observation is in agreement with the ions size and the affinity for dehydration. K + ions due their completely dehydrated state, they probably become equivalent to partially dehydrated Na + .nH2O, in terms of kinetics.

Figure IV- 25 .

 25 Figure IV-25. Transfer resistance values for three electrolytes (A) Li + .nH2O, Na + .nH2O, K + (B) H + and (C) Cl -estimated from the fitting of the ac-electrogravimetric data measured in 0.5M LiCl, NaCl and KCl.

  Figure IV-24B shows that H + has much lower kinetics than the other cations, which may be related to the fact that they are formally the smallest ions with more difficulty to dehydrate. Since their concentration is low in LiCl, NaCl and KCl electrolytes, they were not involved in the ion size/dehydration relationship discussion.Finally, for the Cl -, the Ki values are higher for LiCl and KCl media than in NaCl at anodic potentials (Figure IV-24C).

Figure IV- 25

 25 Figure IV-25 shows the transfer resistance, Rti, values for charged species measured in the three different aqueous electrolytes: LiCl, NaCl and KCl. As shown in Figure IV-25A, the transfer of the Li + ion is more difficult than the transfer of the Na + .nH2O and K + .

Figure V- 26 .

 26 Figure V-26. Evolution of the concentration, Ci-C0 of each species over the applied potential measured in 0.5M (A) LiCl, (B) NaCl, (C) KCl aqueous electrolytes.

  particularly at potentials closer to PZC (Figure IV-25B). Finally, the transfer of the Cl -ion is more difficult in NaCl media than in KCl and LiCl media (Figure IV-25C). As expected, the transfer resistance of the species follows a reverse order of the kinetics of transfer of species, i.e. species with slower dynamics of transfer are more difficult to be transferred.

Figure

  Figure IV-26 shows the relative concentration change for the ionic species. The Ci-C0 values of the Li + .nH2O, Na + .nH2O and K + are equivalent but significantly higher than the Ci-C0 values of the H + for all the studied electrolytes. (Figures IV-26A, IV-26B and IV-26C) which is agreement with the lower concentrations of protons in the electrolyte at this pH. Earlier it was shown that the Li + .nH2O ions presented slower dynamics and difficulty to be transferred (Figures IV-24A and 25A), but eventually similar amounts of species are transferred. This indicates that the concentration of electroadsorbed species depends majorly to the specific surface area of the electrode, here SWCNT electrodes.

Figure

  Figure IV-27 shows the instantaneous capacitance values for each ion, Ci', calculated as a function of the applied potential. These values correspond to the capacitance attributed to the each cations, Li + .nH2O, Na + .nH2O, K + , H + and the respective anion Cl -of the three different electrolytes.

  Figure IV-4). The effect of the electrolyte pH on the CNTs electroadsorption phenomena was evaluated by changing the pH of the NaCl solution to pH 2, pH 7 and pH 10 (presented for SWCNTs). In the same potential range, low pH values amplify the anion contribution whereas high pH values amplify the cationic response (Figure IV-18). Nevertheless, regarding the concentration changes for exactly the same potential step, for example 0.3V around the PZC (in Figure IV-18A), Ci-C0 values for Na + .nH2O and Cl -are completely equivalent; thus the pH changes shifts the Ci-C0 curves only along the potential axis. This means that it is possible to adjust the pH of the aqueous electrolyte to preferentially electroadsorb cations or anions in the same potential range.

  Figure V-1. Prussian blue was deposited on the SWCNTs via cyclic voltammetry. After 15 cycles, nanosized cubes were formed and distributed along the thin layer of CNT films (Figure V-1A), which corresponds to PB (Prussian Blue).

Figure V- 2 .

 2 Figure V-2. EDX pattern of a SWCNT/PB nanocomposite film.

Figure V- 4

 4 Figure V-4 shows the variation of Fdm/dq values as a function of the potential applied, obtained from the reduction branch, in the range 0.4V to -0.4V vs Ag/AgCl measured in 0.5M KCl.

  Figure V-4. The Fdm/dq values as a function of the potential applied, obtained from the reduction branch of the EQCM results measured in 0.5 M KCl with a scan rate of 50 mV.s -1 .

Figure V- 5

 5 Figure V-5 depicts the variation of capacitance values as a function of the potential applied in the same conditions as mentioned previously. The capacitance values are estimated to be around 25 F g -1 in the zones III and I, which is in good agreement with the values obtained for SWCNTs described earlier in Chapter III. The capacitance value of the oxidation peak (zone PB) is estimated to be around 250 F g -1 .

Figure V- 5 .

 5 Figure V-5. Specific capacitance values (calculated from Figure VI-4) as a function of potential at a scan rate of 50 mV.s -1 calculated for 0.5M KCl.

Figure V- 6

 6 Figure V-6 shows an example of the experimental and theoretical transfer functions (TFs) obtained from ac-electrogravimetry of a SWCNT/PB thin film in 0.5 M KCl electrolyte at -0.4V vs Ag/AgCl. The experimental data were fitted according to the model presented in Chapter III. Figure V-6 reveals a good agreement between experimental data and theoretical curves. First, the

Figure V -

 V Figure V-6B shows two loops, a big one for the high frequencies and a small one for the low frequencies. They can be attributed to any of the species present in the electrolyte and their time constants are sufficiently different from each other. Only the mass/potential TF permits this identification when associated to partial TFs.

  Figure V-6. Experimental and theoretical ac-electrogravimetry data of a SWNT-PB thin film in

  Their respective kinetics of transfer were determined by the Ki values. The presence of three different species estimated by simulating the experimental data was further confirmed on a fair analysis of the partial electrogravimetric transfer function, for example, by removing the c2

  Figure V-7D and E exhibit a good agreement between the theoretical and experimental data. This partial electrogravimetric transfer function provides a crosscheck for validating the hypothesis involving three different species and allowing a better separation of the various contributions to be attained. The same fitting procedure and validation process was used for all the other potentials and the pertinent parameters for the ions transfer are estimated..

Figure V- 7 .

 7 Figure V-7. The kinetic constants Ki (A), and transfer resistances Rti (B) for all species estimated from the fitting of the ac-electrogravimetry data measured in 0.5M KCl.

Figure V- 7

 7 Figure V-7 shows the evolution of transfer kinetics, Ki, and the transfer resistance, Rti, of the species as a function of the potential applied. This potential range corresponds to the conditions where the electroadsorption/desorption and insertion/expulsion process of the composite thin film are observed. Based on the Ki values presented in Figure V-7A, the K + ion is the fastest of the three species. Furthermore, the transfer kinetics of free water molecules are somewhat close to the values of the K + ion. Additionally, these water molecules have the same flux directions as the cations, suggesting that it might be the water molecules that accompany the transfer of K + , most likely due to the electrodragging. Chloride anions appear only for the more anodic potentials which indicates that the capacitive CNTs film behavior is dominant in this potential range. If we compare the K + ions kinetics of the composite film with that observed in pristine SWCNTs, one can say that the presence of PB on the surface of SWCNTs does not slow down the transfer dynamics of K + ions (similar Ki in SWCNTs, Figure IV-24A). Additionally, the contribution of potassium ions in the composite film is enlarged to the whole potential range (Figure V-7A).

  Figure IV-24B). The transfer kinetic rates of Cl -in the SWCNT/PB are less significant than the transfer kinetic rates of Cl -in the SWCNT (Figure IV-24C). Considering the transfer resistance values of Kti ions, the K + from the SWCNT-PB are equivalent to the K + from the SWCNT (Figure IV-25A). The H + from the SWCNT-PB are easier transferred than H + from SWCNT (Figure IV-25B). The Cl -from SWCNT-PB are more difficult transferred than the Cl -from the SWCNT (Figure IV-25C).

Figure V- 8 .

 8 Figure V-8. Evolution of the relative concentration, Ci-C0, of each species over the applied potential measured in 0.5M KCl.

  Figure V-9. Instantaneous capacitance values, Ci', calculated from ac-electrogravimetry data.

Figure V- 9

 9 Figure V-9 shows the instantaneous capacitance values of each ion, Ci', calculated as a function of the potential. These correspond to the capacitance attributed to cation (K + ) and anion

  Figure V-10. SEM-FEG images of SWCNT/PPy nanocomposite film in surface (A) and in cross-section (B).

  Figure V-11. The EQCM results of SWCNT/PPy thin films: (A) current vs. potential and (B) mass variation vs. potential profiles measured in aqueous electrolyte 0.5 M NaCl.

Figure V- 12

 12 Figure V-12 shows the Fdm/dq function calculated from the EQCM data as a function of potential, which is equivalent to the global molecular mass of the species obtained from the reduction branch in the range 0.4V to -1.2V vs Ag/AgCl in 0.5M NaCl. At more cathodic potentials, the values of the molecular mass vary in the range of 50-200 g.mol -1 which could correspond to Na +

Figure

  Figure V-12.The Fdm/dq values as a function of the applied potential, obtained from the reduction branch of the EQCM results measured in 0.5 M NaCl.

Figure

  Figure V-13 depicts the variation of capacitance values as a function of the potential applied under the same conditions as mentioned previously. The capacitance values are estimated to be around 25 to 30 F g -1 in the range from 0.45V to -1V vs. Ag/AgCl. These values of capacitance are in the same order of magnitude that pristine SWCNT thin film electrodes.A small dependence to the scan rate values is observed (more pronounced in the Fdm/dq function in Figure V-12 but also in Figure V-13) which suggest that depending on the scan rate, different species may contribute to the electroadsorption and faradaic processes. Indeed, the information obtained from the EQCM is are insufficient for a thorough explanation of the transfer mechanism of the different species. As ac-electrogravimetry was proven to be a strong complementary tool in the CNT study, the composite films were also analyzed with this methodology.

Figure

  Figure V-14 reveals a good agreement between experimental data and theoretical curves. First, the

  Figure V-14B shows one flattened loop, which can be attributed to at least two species, where their time constants are not sufficiently different from each other to obtain perfectly separated loops. In the mass/potential transfer function Δm (ω) ΔE , one big loop appears in the third quadrant involving contributions at high and medium frequencies (Figure V-14C). The loops in the third quadrant are characteristic for cation contributions or free solvent molecules in the same flux direction. The fitting of the experimental data by Mathcad software, using the equations given in Chapter II, showed the presence of two charged species: Na + .nH2O at higher frequencies and H + at lower frequencies. The loop at intermediate frequencies is attributed to the free solvent. The identification of these species was achieved by the determination of their molar mass using Equation II-50 in Chapter II and their respective kinetics of transfer were illustrated by their Ki values. The presence of three different species estimated by simulating the experimental data was further confirmed by carefully analyzing the partial electrogravimetric transfer function, for example, by removing the cation 1 contribution and calculating -52 and 51). Figures V-14D and E

  Figure V-15.The transfer kinetic constants, Ki (A), and transfer resistance, Rti (B) for all species estimated from the fitting of the ac-electrogravimetry data of SWCNT-PPy composite measured in 0.5M NaCl.

H

  + in the SWCNT/PPy are faster than the transfer kinetic rates of H + in the SWCNT and (ii) the transfer kinetic rates of the Cl -in the SWCNT/PPy are more or less equivalent to the transfer kinetic rates of the Cl -in the SWCNT (See Figure III-9 Chapter III) and more significant than the transfer kinetic rates of the Cl -for pristine doped PPy.224 Considering the transfer resistance values of the Rti ions, the Na + .nH2O species from the SWCNT-PPy are slightly more difficult to be transferred compared with the Na + .nH2O species in the SWCNT (10 ohm.cm -2 versus 2 ohm.cm -2 ) (FigureV-15 and Figure III-9) The Rti of H + in the SWCNT-PPy composite is more or less equivalent to that in pristine SWCNT. The Rti of Cl -in the SWCNT-PPy composite is comparable to that of obtained in pristine SWCNT (See FigureIII-9

Figure V- 16 .

 16 Figure V-16. Evolution of the relative concentration, Ci-C0 of each species over the potential applied measured in 0.5M NaCl SWCNT-PPy.

Figure V- 16

 16 Figure V-16 shows the relative concentration change for the ionic species. The Ci-C0 values for H2O are significantly higher than the Ci-C0 values of Na + .nH2O, H + and Cl -. The relative concentration change of the Na + .nH2O in SWCNT-PPy are equivalent to the Ci-C0 values of the Na + .nH2O in pristine SWCNT. These values are also in good agreement with the Ci-C0 values of Na + detected in doped PPy structures. 224 The same trend was observed for H + and Cl -anions, Ci-C0 values of the H + and Cl -did not change considerably in the SWCNT-PPy composite compared to SWCNTs alone (Figure III-11A Chapter III). The Ci-C0 values of the free solvent in SWCNT-PPy composite are significantly higher than the Ci-C0 values of the free solvent in the SWCNT. These values are rather equivalent to the that obtained for doped PPy thin films (based on earlier work in our group). 224 This indicates that the water transfer is related to the doped PPy of the composite electrode.

  Figure V-17.The instantaneous capacitance values calculated from ac-electrogravimetry data SWCNT-PPY. To evaluate the capacitance contribution of each species, the instantaneous capacitance values of each ion (C'i) calculated as a function of the potential (Figure V-17). These correspond to the capacitance attributed to cation 1 (H + ), cation 2 (Na + .nH2O) and anion (Cl -), respectively. The capacitance values calculated from the ac-electrogravimetry data slightly differ from that found from the cyclic voltammetry because it takes into account of dynamic aspects of ion transfer. These instantaneous capacitance values of the Na + .nH2O, H + and Cl -are lower than those obtained in the previous composite electrode with PB (SWCNT-PB, Figure IV-12A Chapter IV).

  Figure V-18. Thin layers of GO sheets appear crumpled and folded into a typical wrinkled structure.

  Figure V-18. FEG-SEM images of GO (A and B) and ERGO (C and D) thin films.

  Figure V-20. HR-TEM images of ERGO (A-D) and selected area electron diffraction patterns (SAED) (E and F taken from A and C, respectively).

  Figure V-21. XRD spectra of GO powder (A) and ERGO thin films deposited on quartz resonators (B). (Cu Kα radiation (λ = 1.541 84 Å), main peaks for GO and ERGO are indexed, residual peaks are due to the substrate).

  Figure V-22. Evolution with time of the Raman spectra of a graphene oxide sample upon polarization at -1.1V vs. Ag/AgCl. Spectra were normalized vs. D peak intensity

F

  Figure V-23. EQCM results of ERGO thin films current vs. potential and mass variation vs. potential profiles measured in different aqueous electrolytes: 0.5 M LiCl (A and B), 0.5 M NaCl (C and D) and 0.5 M KCl (E and F).

Figures V- 24

 24 Figures V-24 A, B and C show the capacitance curves as a function of the applied potential. The capacitance was calculated by using the relation expressed in Chapter III. According to Figure V-24, the capacitance values measured are almost the same in the three different electrolytes with a value around 100 F.g -1 .

Figure V- 24 .

 24 Figure V-24. Specific capacitance values (calculated from Figure 16) as a function of selected scan rate calculated for LiCl (A), NaCl (B) and KCl (C) aqueous electrolytes.

Figures V- 25 Figure V- 25 .

 2525 Figures V-25 A, B and C show the Fdm/dq values, which is equivalent to the global molecular mass of the species electroadsorbed. The values are calculated as a function of the potential in the three different electrolytes. The values vary in the range of 20-30 g.mol -1 , 30-50 g.mol -1 and 30-40 g.mol -1 in LiCl, in NaCl and in KCl, respectively. If there is strictly one nonhydrated species involved in the reversible electroadsorption process, this function should be equivalent to the molecular mass of the cation in the electrolyte. The observed higher values further indicate that the cations are hydrated and/or accompanied by free solvent molecules during their transfer. These results are somewhat in line with the previous reports on carbon based electrodes 2,142 but clearly show the limitations of the classical EQCM technique. The EQCM does not provide unambiguous information on which of the possible scenarios that actually takes place.

  Figure V-26. Experimental and theoretical ac-electrogravimetry data of ERGO thin film in 0.5 M aqueous LiCl measured at -0.6V vs Ag/AgCl. (A).

  species, specifically Li + , Li + .nH2O and H2O.

  Figure V-27. Schematic representation of the electroadsorption sites for Li + .nH2O and Li+ in ERGO thin films.

Figure

  Figure V-28 shows the Ki and Rti values for charged and non-charged species measured in the three different aqueous electrolytes, LiCl, NaCl and KCl. According to Figure V-28, the transfer is slower and more difficult for non-hydrated cations. The larger cations are transferred faster (Figures V-28A and B).

Figure V- 28 .

 28 Figure V-28. The transfer kinetic constant, Ki, and transfer resistance, Rti for all species estimated from the fitting of the ac-electrogravimetry data measured in the three different aqueous electrolytes in LiCl, NaCl and KCl.

Figure

  Figure V-29 shows the capacitance values relative to each cation (C'c1 and C'c2) calculated as a function of the potential. The C'c1 and C'c2 correspond to the capacitance attributed to cation 1(Li + , Na + or K + ) and cation 2 (Li + .H2O, Na + .H2O or K + .H2O), respectively. The capacitance values calculated from the ac-electrogravimetry data slightly differ from that obtained from the cyclic voltammetry, but they are in the same order of magnitude. The capacitance values from cyclic voltammetry correspond to relatively faster processes (high scan rate). However, here the capacitance contribution of two different species with different kinetic constants is calculated separately.

  Figure V-30. Relative concentration variations of three different species non-hydrated cations (A), hydrated cations (B) and free solvent molecules (C) in aqueous LiCl, NaCl and KCl electrolytes together with the hydration values (n) estimated for the hydrated cation given by acelectrogravimetric data.

  Figure V-31. Schematic representation of the interfacial phenomena of ionic species at the ERGO/electrolyte interfacescation size dependence.

  Les phénomènes de transfert aux interfaces et de transport ionique influencent grandement les performances des matériaux d'électrode pour des applications dans le domaine du stockage de l'énergie.Certaines études suggèrent l'utilisation de microbalances classiques comme moyen pour caractériser les flux d'ions entre des électrodes à base de carbone et un électrolyte. Il a été démontré que la réponse de la microbalance permet de caractériser ce flux ionique, celui-ci dépendant fortement du point de charge nulle (PZC) du matériau, de la taille des pores et de la taille des ions impliqués dans le processus électrochimique. Malheureusement, les subtilités ou des explications justes ne peuvent pas être atteintes par cette technique classique. De plus, il y a des questions auxquelles on ne peut répondre : pourquoi on obtient des masses molaires élevées qui ne correspondent à aucune espèce présente, qu'elles sont les espèces qui sont transférées rapidement et facilement, quel est le rôle du solvant libre, comment estimer la solvatation des cations etc.Dans cette thèse, des méthodes électrochimiques et gravimétriques, résolues au niveau temporel, comme la spectroscopie d'impédance électrochimique (EIS) couplée à une microbalance à quartz rapide (ac-électrogravimétrie), ont été utilisées pour étudier le comportement capacitif des électrodes à base de carbone. L'utilisation de l'ac-électogravimétrie fournit des informations très pertinentes : la cinétique et l'identification des espèces transférées à l'interface électrode/électrolyte, la séparation de la contribution des différentes espèces et le changement de la concentration relative des espèces au sein du film. Plus précisément, toutes les espèces impliquées dans les processus de compensation de charge peuvent être identifiés par leur masse atomique selon une adsorption réversible d'ions. Par conséquent, l'ac-électrogravimétrie apparaît comme un outil intéressant pour ces études.Dans ce travail, des nanotubes de carbone (CNTs), de l'oxyde de graphène réduit et des nanocomposites CNTs/polypyrrole ou Bleu de Prusse ont été sélectionnés comme matériaux de base pour réaliser des dispositifs pseudo capacitifs. Pour réaliser cette étude, de nouveaux modèles électrogravimétriques qui tiennent compte des processus redox et capacitifs ont été utilisés pour la première fois.Chapitre I.Le premier chapitre de cette thèse présente une étude bibliographique sur les dispositifs de stockage de l'énergie (batteries et super-capacités) selon trois parties. Dans un premier temps, un petit historique, les principes d'opération et les domaines d'applications sont présentés. La deuxième partie est consacrée aux composants de ces systèmes en termes de matériaux et d'électrolytes. La troisième partie de ce chapitre est reliée aux différentes techniques utilisées pour la caractérisation et l'évaluation des performances des électrodes à base de carbone.

Figure 1 .

 1 Figure 1. Différentes catégories des dispositifs de type super-capacité.

  sur la partie expérimentale de ce travail. Les procédures de préparation des films, ainsi que les méthodes utilisées pour la caractérisation structurale/morphologique des films, en particulier, le principe de fonctionnement, le type d'instrument utilisé et les conditions dans lesquelles les expériences ont été effectuées, sont présentées. Les techniques de caractérisation électrochimiques en particulier l'ac-électrogravimetrie sont aussi détaillées dans ce chapitre.Pour réaliser nos électrodes nanostructurés trois types de nanotubes de carbone, SWCNT, DWCNT et MWCNT ainsi qu'un oxyde de graphène ont été choisis en raison de leurs dimensions de pores et de leurs surfaces spécifiques. Les films de carbone ont été préparés selon la méthode de « dropcasting » décrite dans la littérature 10-13 sur une électrode d'or (S= 0,20 cm 2 ) de microbalance à quartz (9MHz -AWS, Valencia, Espagne), à partir d'une solution contenant les nanotubes ou l'oxyde de graphène. Les proportions suivantes ont été utilisées : 90% de Carbone et 10% de PVDF-HFP (Poly(vinylidene fluoride-hexafluoropropylène)) dans du NMP (N-méthylpyrrolidone).Puis, une fois déposés les films de carbone ont été soumis à un traitement thermique à 120 o C pendant 30 minutes, avec une vitesse de montée en température de 5 o C min -1 afin d'évaporer le solvant résiduel et améliorer les propriétés d'adhérence du film sur l'or (Voir Figure2).

Figure 2 .

 2 Figure 2. Représentation schématique de la préparation de films de CNTs ou d'oxyde de graphène.

Figure 3 .

 3 Figure 3. Dispositif expérimental de mesure d'ac-électrogravimétrie. 15

Figure 4 .

 4 , autour de 0,1 V vs Ag/AgCl. Cela correspond au point de charge nulle (pzc) qui est autour de cette valeur. Les valeurs de capacité spécifique ont été estimées en utilisant les courbes I-E de la Fig.4A.Les valeurs de capacités spécifiques atteignent 25 F g -1 pour une vitesse de balayage de 100 mV s -1 (fig.4C). Cette valeur est du même ordre de grandeur que celles qui sont données dans la littératureCourbes d'électrogravimétrie cyclique (A) et (B), de capacité spécifique (C) et de fonction Fdm/dq (D) pour un film de SWCNTs en milieu NaCl 0,5M -pH 7. Balayage de potentiel entre 0,45V et -0,45V vs Ag/AgCl pour 25, 50 et 100 mV s -1 . Pour avoir plus d'indications sur la nature des espèces adsorbées ou désorbées, la fonction Fdm/dq a été calculée; elle correspond à la masse molaire de l'espèce échangée pour une vitesse de balayage donnée. La figure 4D présente la variation de la fonction Fdm/dq en fonction du potentiel entre -0,4 V et 0,4V vs Ag/AgCl. La fonction Fdm/dq, en particulier à haute vitesse de balayage, montre une valeur de Fdm/dq qui varie entre -40 et -23 g mole -1 pour des potentiels entre -0,4V et 0,05V vs Ag/AgCl. Cela correspond à la contribution de cations dont la masse molaire supérieure à 23 g mole -1 trouvée peut être expliquée par la participation soit du solvant libre transféré dans le même sens que les Na + , soit du solvant associé aux ions Na + . Pour répondre de manière plus précise, des mesures d'ac-électrogravimétrie ont été menées pour identifier les différentes espèces et déterminer le rôle de chaque espèce, anion, cation, cation solvaté, solvant libre, directement ou indirectement impliqués dans le processus électrochimique. Mesures d'ac-électrogravimétrie Les mesures d'ac-électrogravimétrie ont été effectuées dans une solution de NaCl 0,5 M à différents potentiels entre -0,4 V et 0,4 V vs. Ag/AgCl. À chaque potentiel imposé, la réponse de la masse ∆m et du courant ∆I à une perturbation sinusoïdale de faible amplitude ∆E (80mV), a été mesurée à différentes fréquences de modulation comprises entre 63 kHz et 10 mHz. Les résultats seront limités à des mesures en solution aqueuse NaCl 0,5M. Par exemple à -0,4 V vs Ag/AgCl, les fonctions de transfert théoriques ont été ajustées aux fonctions de transfert expérimentales en ne tenant compte que de trois espèces (Na + .H2O, H + et H2O). Dans ce cas, nous avons obtenu un très bon accord pour les différentes fonctions de transferts y compris partielles. Le transfert du Na + hydraté intervient aux hautes fréquences (plus rapide) et celui du proton intervient à plus basse fréquence (plus lent). La cinétique de l'eau libre est très proche de celle du Na + hydraté. Les valeurs de résistances de transfert ionique montrent que le transfert du sodium hydraté est beaucoup plus facile que celui du proton. La résistance de transfert du solvant libre est proche de celle de transfert de sodium hydraté. Malgré une cinétique lente et des valeurs de résistance de transfert élevées, la contribution des protons dans le processus de stockage de charge n'est pas négligeable. Ces résultats suggèrent que l'adsorption de ces différentes espèces a certainement lieu sur différents sites au sein des films de SWCNT. L'intérieur des tubes de SWCNT (diamètre ~ 1 nm) est probablement accessible pour les protons et pas pour les Na + plus gros. Les ions Na + hydratés peuvent être adsorbés à l'extérieur des nanotubes et/ou entre les faisceaux des SWCNT. En regardant les valeurs similaires des constantes cinétiques de transfert et des résistances de transfert pour le sodium hydraté et le solvant libre, ce dernier semble accompagner les ions Na + hydratés. Ces molécules d'eau libre peuvent également être des molécules d'eau qui quittent la coquille de solvatation du Na + pour lui permettre d'accéder à des sites au sein des films. Le desolvatation partielle des ions avant d'être adsorbés dans les pores plus petits a été décrite dans la littérature 21,22 (Voir Figure 5).

Figure 5 .Figure 6 .Figure 7 .

 567 Figure 5. Représentation schématique de l'interaction des ions avec des électrodes à base de nanotubes de carbone déposés sur une électrode d'or de microbalance à quartz.

Figure 8 .FFigure 9 .

 89 Figure 8. Réponses électrogravimétriques d'un film de SWCNT/PPy : (A) courant vs potentiel et (B) variation de masse vs potentiel mesurée dans une solution aqueuse de NaCl 0.5M.

Table I - 1 :

 I1 Types of Batteries.19

	I-1.1.3. Principle of Operation
	Battery
	type

  [START_REF] Benmouhoub | Influence of the Incorporation of CeO2 Nanoparticles on the Ion Exchange Behavior of Dodecylsulfate Doped Polypyrrole Films: Ac-Electrogravimetry Investigations[END_REF]

Table I -

 I 2. Standard electrode potentials in an aqueous electrolyte at 298 K.

	Reduction Half-Reaction	E o (V)
	F2(g) + 2e -→ 2 F -(aq)	+2.866
	MnO2(s) + 4H + (aq) +2e -→Mn 2+ (aq) + 2H2O(l)	+1.23
	O2(g) + 4H + (aq) + 4e -→ 2H2O(l)	+1.229
	2IO3 -(aq) + 12H + (aq) + 10e -→ I2(s) + 6H2O(l)	+1.20
	Br2(l) + 2e -→ 2Br -(aq)	+1.065
	NO3 -(aq) + 4H + (aq) + 3e -→ NO(g) + 2 H2O(l)	+0.956
	Ag + (aq) + e -→ Ag(s)	+0.800
	Fe 3+ (aq) + e -→ Fe 2+ (aq)	+0.771
	O2(g) + 2H + (ag) + 2e -→ H2O2(aq)	+0.695
	I2(s) + 2e -→ 2I -(aq)	+0.535
	Cu 2+ (aq) + 2e -→ Cu(s)	+0.340
	S(s) + 2H + (aq) + 2e -→ H2S(g)	+0.14
	2H + (aq) + 2e -→ H2(g)	0

Table I -

 I 3. Characteristics of different battery technologies. 8

	Battery	Anode Cathode	Reaction	Theoretical	Theoretical
	Type				Capacitance	Specific
					(Ah/Kg)	Energy
						(Wh/Kg)
	Lead	Pb	PbO2	Pb+PbO2+2H2SO4 2PbSO4+2H2O	120	252
	Ni-Cd	Cd	Nickel	Cd+2NiOOH+2H2O	181	244
			oxide	2Ni(OH)2+Cd(OH)2
	Ni-MH	Metal	Nickel	MH+NiOOHM+Ni(OH)2	178	240
		hydrid	oxide
		e
	Li-ion	LixC6	LixC6	LixC6+Li1-xCoO2	100	410
				LiCoO2+C6
	Li-S	Li	Li	Lix+SSLix+Li1-x		500-600
	Li-air	Li	Li	2Li+O2Li2O2	3840	11400

Table I -4.

 I Different carbon structures and some of their characteristics.41

	excellent mechanical properties
	Material	Carbon	Carbon	Graphene Activated	Carbide	Templated
		onions	nanotubes		carbon	derived	Carbon
						carbon
	Dimensionality	0-D	1-D	2-D	3-D	3-D	3-D
	Conductivity	High	High	High	Low	Moderate	Low
	Volumetric	Low	Low	Moderate	High	High	Low
	Capacitance
	Cost	High	High	Moderate	Low	Moderate	High
	Structure

Table I - 5 .

 I5 Summary and comparison of synthesis methods of CNTs.

	Method		Arc-discharge		Laser-ablation		CVD
	Pioneer		Iijima 58			Guo et al. 59			Yacaman et al. 60
	How		CNT growth on graphite	Vaporization	of	a	Fixed	bed	method:
			electrodes during direct	mixture	of	carbon	acetylene decomposition
			current	arc-discharge	(graphite) and transition	over graphite-supported
			evaporation of carbon in	metals located on a	iron particles at 700°C
			presence of an inert gas	target to form CNTs
	Yields		< 75%			< 75%			>75%
	SWCNT	or	Both			Only SWCNTs		Both
	MWCNT
	Advantage		Simple, inexpensive	Relatively high purity	Simple, inexpensive,
						CNTs, room-temperature	low temperature, high
						synthesis option with continuous laser	purity and high yields, aligned growth is possible, fluidized bed
									technique for
									large-scale
	Disadvantage Purification	of	crude	Cannot		produce	CNTs are usually
			product	is	required,	MWCNTs, method only	MWNTs, parameters
			method cannot be scaled	adapted to lab-scale,	must closely be
			up, must have high	crude		product	watched to obtain
			temperature			purification required	SWNTs

Table I -

 I

	(4)	Carbide-Derived Carbons (CDCs)
	Year	Electrode material	Electrolyte	Specific	Specific	Ref.
				Capacitance	Current
				F g -1	A g -1
	Thermally exfoliated graphite oxide	1M H2SO4	117	0.12	66
	Chemically modified graphene	5.5M KOH	135	1.3	67
	Low temperature exfoliated graphene	5.5M KOH	264	0.1	68
	Reduced graphene oxide sheets	30% KOH	205	0.1	69
	Graphene nanosheets	30% KOH	150	0.1	70
	Graphene hydrogel	5M KOH	222	1	71
	Graphene paper	1M H2SO4	120	0.12	72
	Multilayered graphene film	1M H2SO4	215	0.1	73
	Reduced graphene oxide	1M H2SO4	348	0.2	74
	Reduced graphene oxide by flame-induced	6M KOH	221	0.1
	reduction of oxide graphite oxide

6. Summary of graphene based EDLCs using aqueous electrolytes.

Table I -

 I 7. Voltage windows and ionic conductivity for different kinds of electrolytes.

	Electrolyte	Voltage Window (V)	Ionic Conductivity (mS.cm -1 )
	Aqueous	≤ 1,2 37,38	>400 37,40
	Organic (NEt4BF4/PC)	3,0 37	13 37
	Organic (NEt4BF4/ACN)	2,7 37	56 37

  85 For example, pyrrole treated-functionalized SWCNTs have reported high values of specific capacitance (350 F/g), power density (4.8 kW/kg) and energy density (3.3 kJ/kg). 86 Also high values of specific capacitance (485 F/g) , specific power (228 Wh/kg) and specific energy (2250 W/kg) were observed for 73 wt% PANI deposited onto SWCNTs.87

Table I -

 I 8. Electrochemical performance of various carbon-metal oxide nanocomposites. 26 UPS), elevators, cranes etc.), electric utility applications (electricity grids) and transportation applications (starting engines, electric and hybrid vehicles, hybrid buses, trolley buses, trams etc.).33 These markets are expected to grow rapidly in the coming years, 33 although supercapacitor technology still needs to overcome or circumvent the problem of low energy density to improve their performance.33,107

	supply (
	Metal oxides-carbon	Amount of	Specific	Specific	Electrolyte
	Nanocomposites	metal	capacitance	capacitance
		oxides loading	based on	based on metal
		(%)	composite	oxides
			electrodes	alone (F g -1 )
			(F g -1 )
	RuO2/carbon black 31	27	221	854	1 M H2SO4
	RuO2/carbon black 31	40	407	863	1 M H2SO4
	RuO2/CNT 99	17	_	1192	0.5 M
					H2SO4
	RuO2/graphene 100	38.3	_	570	1 M H2SO4
	MnO2/CNT/PEDOT-	60	_	129	1M Na2SO4
	PSS 101
	MnO2/carbon 102	70	218	_	1M Na2SO4
	MnO2/graphene oxide 103	90.7	197.2	211.2	1M Na2SO4
	Co3O4/graphene 104	24.4	_	243.2	6 M KOH
	NiO/CNT 105	50	523.37	1037.74	6 M KOH
	NiO/CNT 105	80	326.2	405.5	6 M KOH
	V2O5/CNT 106	80	288	_	1 M LiClO4
	I-1.2.5. Supercapacitor Applications
	Initially, supercapacitors were mainly used for applications requiring low voltage and low
	capacitance, such as back-up memories, security and alarm systems, toys and watches etc. 33 Still,

these kinds of consumer electronics are the main domain for supercapacitors. However, the development of low cost supercapacitors with higher capacitance that also can be used at high voltage has opened up new markets, for example, industrial applications (uninterruptible power

Table I -9. BET

 I specific surface area, mesopore volume, percentage of oxygen, and capacitance of the analyzed nanotubes.

	Type of nanotubes	A700Co/Si	A900Co/Si	A600Co/NaY	P800/Al
	Vmeso (cm 3 STP/g)	435	381	269	643
	SBET (m 2 /g)	411	396	128	311
	Oxygen (mass %)	10.8	4.6	0.8	<0.3
	Capacitance (F/g)	80	62	4	36

  to the Sauerbrey equation (II-6) and is equal to 1/ theor

		k	s	or	exp 1/ s k 	where
	exp s k is the experimental mass/microbalance frequency factor. At 9 MHz, exp s k is equal to 16.3 x 10 -7

  is the concentration in the film of cations or anions associated to the P sites and ,

			36)
	i where ,max C	is the maximum concentration of P sites occupied in the film by cations or anions,
		C	is the
		i min
	minimum concentration of P sites occupied in the film by cations or anions.
	Under the effect of a sinusoidal potential perturbation, E  , imposed on the film/electrolyte
	interfaces, sinusoidal concentration fluctuations, C  and the flux,
		i

i C i J  , are observed (a: anion, c: cation, s: solvent), in the following form:

Table V -

 V 1: EDX results of GO and ERGO thin films: carbon and oxygen amount in GO and ERGO.

		GO	ERGO
	Average Carbon content (%)	63.9±1.5%	79.4±3.6%
	Average Oxygen content (%)	36.1±1.5%	20.6±3.6%

Table 1 .

 1 1,4,5 Différentes structures de carbone et ses principales caractéristiques.4

	Material	Carbon	Carbon	Graphene	Activated	Carbide	Templated
		onions	nanotubes		carbon	derived carbon	Carbon
	Dimensionality	0-D	1-D	2-D	3-D	3-D	3-D
	Conductivity	High	High	High	Low	Moderate	Low
	Volumetric	Low	Low	Moderate	High	High	Low
	Capacitance
	Cost	High	High	Moderate	Low	Moderate	High
	Structure

Table 2 .

 2 Tension admissibles et conductivité ionique pour différents types d'électrolytes.

	Electrolyte	Fenêtre électrochimique (V

) Conductivité ionique (mS.cm -1 )

	Aqueuse	≤ 1,2 7,8	>400 7,9
	Organique	3,0 7	13 7
	(NEt4BF4/PC)
	Organique (NEt4BF4/ACN)	2,7 7	56 7
	Liquides ioniques	3-5 8,9	<15 7,9

Finalement, différentes techniques telles que des méthodes d'imagerie/caractérisation (SEM-FEG, MET, BET, DRX), électrochimiques (voltametrie cyclique, spectroscopie d'impédance électrochimique), de gravimétrie (microbalance à quartz) classique ont été employées pour caractériser la morphologique, la structure et la réponse électrochimique de ces différents films.

  Ceci est probablement lié au fait qu'à la fois des phénomènes d'électro adsorption/désorption et redox sont impliqués pour K + , tandis que Cl -n'est lié qu'aux phénomènes d'adsorption/désorption. En effet, pour les films de PB purs, la contribution d'anions n'a jamais été vue et pour cela, elle ne peut être liée qu'à un mécanisme d'électroadsorption.La cinétique de transfert de K + est équivalente celle observée dans le cas de SWCNT tout seul et celle de H + est plus rapide que la cinétique de transfert de H + dans SWCNT seul. , probablement en raison de la différence de pH. La cinétique de transfert de Cl -pour SWCNT/PB est plus lente que celle de Cl -dans le SWCNT pur. Les valeurs de résistance de transfert de K + sont équivalentes pour les deux types de film, SWCNT/PB et SWCNT tout seul. Le H + pour SWCNT/PB est plus facile à transférer que le H + pour SWCNT tout seul. Finalement, le Cl - de SWCNT/PB est plus facile à transférer que le Cl -de SWCNT tout seul. Résultats SWCNT/PPy Les Figures 8A et B montrent les variations de courant et de masse d'un film SWCNT/ PPy quand il est oxydé et réduit entre -1,2V et 0.45V vs Ag/AgCl dans une solution de NaCl 0,5M à pH 7.

Acknowledgements I would like to thank the research director of LISE, Professor François Huet, for providing the laboratories, and SENESCYT for funding this research work. Special thanks to my manuscript reviewers, Dr. Francisco Vicente and Dr. Michael Holzinger as well as Dr. Christel Laberty, and Dr. Stéphanie Betelu as jury members for my oral defence. Many thanks go to my main Ph.D. supervisor, Dr. Hubert Perrot, for these years of cultivating me as a researcher. Thank you for giving me this opportunity to work under your guidance and to learn from your research expertise. Thank you for your support and available for discussions despite your busy schedules. To co-supervisor, Dr. Ozlem Sel, I would like to express my gratitude for your scientific guidance and encouragement throughout these 4 years. Thank you for the constructive discussions and being supportive at work, with positive and pleasant disposition all the time. I would like to express my appreciation to the colleagues behind the logistic, administrative and technical assistance, Françoise Pillier, Cyril Bazin, Florence Billon, Daniel Rose, Axel Desnoyers, Veronique Martin, Martine Chaduc, Isabelle Lefebvre and all colleagues at LISE. Merci pour vos délicatesse! I would also like to express a special thanks to my wife for her collaboration in the manuscript of the thesis.

ERGO

The electrochemical characterization was first done by cyclic electrogravimetry, which was performed in 0.5M aqueous LiCl, NaCl and KCl electrolytes, from 0.7 V vs Ag/AgCl to -1.1V vs Ag/AgCl. The capacitance of the film, the mass variation and the Fdm/dq function were calculated as a function of the potential. The ERGO films presented relatively high double layer capacitance values with regard to classic systems such as carbon nanotube based electrodes. The capacitance and the mass variation estimated from EQCM response were almost the same in the different electrolytes studied. Due to the different atomic weighs used in this experiment, this proposes that free solvent molecules and/or hydrated species were also involved in the electrochemical process.

There are studies on this aspect of the supercapacitive behavior that use EQCM.

Additionally, to be able to deeply characterize the EQCM response, ac-electro gravimetric study was performed in the same media under a sinusoidal potential perturbation. Experimental data were fitted using a theoretical model to find kinetic parameters to compare the behavior of various species in different electrolytes. The transfer of K + was faster because it was easier for it to remove its hydration shell because of its big size. On the contrary, Li + was very strongly attached with its water molecules, making its transfer slower. This kind of discussion based on the ion size and kinetics exist in the supercapacitive material field. However, to the best of our knowledge, it is the first study showing this behavior experimentally thanks to the coupled electrochemical characterization methods.

Conclusions

Résumé de la Thèse en

Français sont fortement liés aux molécules d'eau ce qui rend le transfert plus lent. Dans un même électrolyte, deux cations sont détectés, l'un rapide et partiellement hydraté et l'autre plus lent, du même cation mais totalement désolvaté. Ainsi, l'électroadsorption de ces deux cations au sein des films d'ERGO pourrait se produire sur deux sites différents ; directement sur la surface (sites les plus accessibles) ou entre les piles des feuillets de graphène (sites moins les moins accessibles) (voir Figure 10).

Conclusion générale

Des