Mme Ozlem Sel

Father Mohamed Halim

My Mother

Fatima Ratmaoui

Jalal, Tarik My Brothers Adil

Nabil My

Z Habibou

T Taeik

M Osfouri

S Houbairi

I Lakkab

J Al-Akhal

; H Japoni

M A Houssa

L Massaoudi

Pr Saad Alami Aicha Younssi

Dr Pierre Guessous

Cyrille Bazin

Dr M Elbasri

Dr F E Salih

Dr L Oularbi

N Achargui

S Chemchoub

A El Attar

T Ez-Zine;

H Filali

A El

B Ochiou

M Breida

F Anouar

; S Oussif

A Aitcheikh

A Fathi

G Achargri

Keywords:

The environment pollution and the ever-increase in demand for energy are the main obstacles in our modern society. To overcome these problems, an innovative and creative approach to pave the way for a rapid, simple and easy growing source of renewable, and clean energy is necessary.

Exploitation of the attractive and the magical properties on the nanometric scale of nanostructured materials is very effective way to boost the performance of energy storage and conversion systems.

In this context, the objective of this Ph.D. thesis is to study new binary and ternary nanocomposites based on carbon nanomaterials, conducting polymer and metal particles for energy storage and conversion. The morphology and the composition of different nanocomposites are characterized by various spectroscopic and microscopic techniques such as FEG-SEM, TEM, XPS, XRD, FTIR and EDX. Thereafter, the nanocomposites are used as electrode materials for supercapacitors and direct methanol fuel cells. For the supercapacitors aspect, electrodes based on Poly(Ortho-Phenylenediamine) (PoPD) and on SWCNT/PoPD were synthesized on gold-patterned Quartz Crystal Microbalance (QCM) and graphite plates. Their redox transformations and the ionic flux mechanisms were investigated in aqueous NaCl electrolyte via Electrochemical Quartz Crystal Microbalance (EQCM) and acelectrogravimetry. Several parameters were considered to explain the electrochemical behaviour of PoPD and SWCNT/PoPD films, such as the nature and the dynamic of the ions transfer at the film/electrolyte interface. In order to correlate between the fundamental understandings and the performance of the electrode, the capacitive performance of PoPD and SWCNT/PopD films are tested in both 3-electrode cell a 2-electrode Swagelok cell. By determining the interfacial flux dynamics and as well as the relative proportions of species transferred at the electrode/electrolyte interface, our results contribute to the understanding of the charge-discharge process of PoPD polymer and SWCNT/PoPD nanocomposite, which have the desirable attributes for energy storage applications. For the direct methanol fuel cell aspect, a nanocomposites based on Carbon Nanofibers (CNF), Poly(Para-Phenylenediamine) (PpPD) and Nickel Particles (NiPs) or Copper (Cu) dendrites was prepared on Carbon Paste Electrode (CPE) using a simple and rapid method. The effects of various parameters on the catalytic performance are studied in order to obtain an optimized composition of the nanocomposites. Then, their electrocatalytic effect for the methanol oxidation reaction (MOR) was studied in alkaline solution. The nanocomposites exhibit a good stability and a high catalytic effect towards MOR.

General Introduction

Since the 70s, the geopolitical problems and the environmental consequences caused by oil, have led to greater interest in renewable energy. Moreover, in the beginning of the 21 st century, attention has been paid to studying the impact of energy factors on the environment and the world's population. The massive growth of population around the world accompanied by a tremendous industrial development result in an enormous demand for global energy consumption. Therefore, when addressing energy issues and meeting the world's energy needs, it is extremely important to consider the serious consequences of this development on the environment, especially the problem of global warming.

Until today, the majority of energy consumption in the world comes from fossil fuels, which is fuelling homes, transportation and factories. The ever-increase demand of energy leads to the exhaustion of these non-renewable ressources, hence, more efforts have been consecrated to developing a cleaner, renewable and environmentally friendly sources of energy. To overcome these problems, an innovative and creative approach to pave the way for a rapid, simple and easy growing source of renewable, and clean energy is necessary. Exploitation of the attractive and enhanced properties on the nanometric scale of nanostructured materials is a very effective way to boost the performance of energy storage and conversion systems. Recently, a large number of researchs have been dedicated to the development and the study of nanomaterials for energy storage and conversion.

Supercapacitor is an energy storage device similar to batteries in design and manufacturing.

They consist of an electrolyte sandwiched between two electrodes where a separator is used between the latters to ensure an electrical isolation. One of the most important components in a supercapacitor is the electrode material. Supercapacitors are high power density and highly stable energy storage solutions based on the electro-adsorption of ions at the surface of the electrode. Indeed, understanding the interfacial ion flux dynamics constitute an important part of the operating principle of supercapacitors. Therefore, the correlation of this fundamental understanding to the charge storage properties of the electrode material is rather significant but requires specific characterization tools.

In addition, fuel cells produce electrical energy by directly converting chemical energy without any combustion reaction. Fuel cells have been considered as a promising technology to overcome the current and future crisis of energy. Direct alcohol fuel cells (DAFCs) present a remarkable advantage over the other types of cells, due to their excellent volumetric energy density, high operating efficiency and ease of fuel transportation/storage. The active components of fuel cells consist of an electrolyte and two electrodes (cathode and anode), the oxidation of fuels occuring at the anode using a catalyst layer. Platinum catalyst is the most commonly used metal for DAFC systems. Although it exhibits a high efficiency for alcohol electrooxidation reaction the limited natural resources of platinum and its high price limit the practical application of DAFC. Moreover, nanostructuration of Pt is considered to increase the real active surface. But it is prone to segregation during the long-term activity, which reduces the active sites of the catalyst resulting in a decrease of its efficiency for electrooxidation of alcohol reaction. Hence, one of the main challenges of DAFC is the development of low-cost and highly efficient catalysts with good resistance to the poisoning species. Throughout this Ph.D. thesis, is focused on: (i) understanding the ions exchange mechanism in nanostructured materials and examining the correlation of these fundamental understanding with the capacitive performance of these materials for supercapacitors; (ii) development of new ternary nanocomposite materials and study their catalytic performances for direct methanol fuel cell.

In this context, this manuscript is divided into six chapters:

Chapter I presents a comprehensive literature review about the fundamentals of energy storage (supercapacitors) and conversion (direct methanol fuel cells) including their history, principle of operation, and applications. Different electrode materials used in supercapacitor and direct methanol fuel cell have been detailed and discussed with their impact on the performance of each technology. At the end of this chapter, the scope and objectives of the present Ph.D. thesis are introduced.

Chapter II presents different microscopic and spectroscopic techniques used for the morphological and structural characterization of our samples, particularly, the principle of operation of each method, type of instruments and experimental conditions used in this Ph.D. thesis. Moreover, the electrochemical and electrogravimetric techniques applied were presented.

Chapter III aims to study the interfacial ion transfer phenomena of the poly(orthophenylenediamine) (PoPD) thin film electrodes by electrochemical quartz crystal microbalance (EQCM) and ac-electrogravimetry techniques. After that, the capacitive performance of PoPD films as electrode for supercapacitors, was tested in a 3 electrode cell and a 2 electrode Swagelok cell. The correlation between the fundamental electrogravimetric results and the capacitive performance of the PoPD electrode were discussed.

First part of chapter IV is dedicated to understand the mechanisms of the charge storage into SWCNT/PoPD nanocomposite via an electrogravimetric study. Then, the second part is focused on impact of the synergistic effect between SWCNT and PoPD: the capacitive charge storage performance was studied in 2-electrode and 3-electrode configuration. The electrogravimetric study was correlated to the performance of SWCNT/PoPD nanocomposite in order to understand the charge-discharge process, which has the desirable attributes for the energy storage applications.

In chapter V the synthesis of new catalyst based on nickel particles (NiPs), carbon nanofibers (CNF) and poly(para-phenylenediamine) (PpPD) are reported. The morphology and structure of the nanocomposite electrodes were characterized. The electrocatalytic effect of the nanocomposite for methanol electroxidation were tested in alkaline solution.

Chapter VI reports the synthesis of copper dendritic nanostructures on carbon nanofibers/poly(para-phenylenediamine) hybrid support. A detailed characterization about the structure and the morphology of the nanocomposite formed was discussed. The performance of the nanocomposite as catalyst for electrooxidation of methanol was tested in alkaline media.

Chapter I: State of the art

Chapter I presents a comprehensive literature review about the fundamentals of energy storage (supercapacitors) and conversion (direct methanol fuel cells) including their history, principle of operation, and applications. Different electrode materials used in supercapacitor and direct methanol fuel cell have been detailed and discussed with their impact on the performance of each technology. At the end of this chapter, we introduce the scope and objectives of the present Ph.D. thesis.

I. Energy storage: Supercapacitors 1. History: discovery and evolution of supercapacitors

In recent years, supercapacitors have attracted a lot of attention because of their high power density, long service life, and fast charge / discharge rate 1 . The first supercapacitor patent was filed in 1957 by Howard Becker of the American General Electric Group 2 . They were composed of porous carbon electrodes and an aqueous electrolyte based on sulfuric acid. However the storage principle was not very well explained. After a few years, researchers at SOHO (Standard Oil Company of Ohio) explained the principle of charge storage in supercapacitors by the "electrochemical double layer" discussed by the physicist Hermann Von Helmholtz. Thereafter, they have filled several patents for supercapacitors of high energy density operating at higher voltages, composed here of graphite electrodes in contact with organic electrolytes 3 . These patents were used by the Japanese company NEC (Nippon Electric Company) which commercialized the first supercapacitors working in aqueous electrolyte in 1971 under the name of "supercapacitors" 4 .

These supercapacitors with small slides and low capacitive performance (energy density (0.5 Wh.kg -1 )), were used in the computer field, mainly to save the memory 5 . Between 1975 and 1980, Brian Evans Conway developed electrode materials with high capacitance based on RuO 2 6 , the charge was here stored by reversible faradic process 7 , opening a new branch of supercapacitors called Pseudo-capacitors. In the late 1970s, many companies started to commercialize supercapacitor devices. In 1978, Panasonic developed its ''Gold Capacitors'' using an organic electrolyte for memory backup. ELNA produced its ''Dynacaps'' in 1987 for low power applications 8 .

Since, many studies have been focused on the development of high power supercapacitors using organic electrolytes. In 1982, PRI (Pinnacle Research Institute) developed a first high power double layer capacitors called ''PRI Ultracapacitor'' for military applications. This work was used by the Maxwell laboratories in 1992 for new application in hybrid vehicles 8 . It was only the beginning of a process, the supercapacitor technology began to attract attention for practical applications in the field of hybrid electric vehicles 9 . Supercapacitors have been used to strengthen the fuel cell in order to provide the power needed for acceleration, and to recover the braking energy 10 . These supercapacitors were developed by Maxwell as part of a program called the ''DOE Ultracapacitor Development Program'' funded by DOE (US Department Of Energy). In 1994, David A. Evans developed a new kind of supercapacitors called "Electrolytic-Hybrid Electrochemical Capacitor", which combine electrostatic and electrochemical interactions 11 .

More recently, FDK Corporation announced its EneCapTen lithium-ion capacitor in 2007.

Today, many companies commercialize supercapacitors such as Maxwell Technologies, Panasonic, NessCap, LS Mtron, and Bolloré group. At the same time, studies are focused on the new supercapacitors materials with exhibits higher performances. Moreover, the disadvantages of supercapacitors, in particular the low energy density and the high cost of production, were identified as major challenges for enhance supercapacitor technology.

Supercapacitor fundamentals

Supercapacitor is an energy storage device similar to batteries in design and manufacturing but with a way of working a little bit different. Supercapacitors consist of two electrodes in contact with an electrolyte and a separator which allowed electrical isolation between the electrodes. One of the most important components in a supercapacitor is the electrode material. Generally, the electrode materials are made from a porous nanomaterial with a high specific surface area. During the charging process, the charges are stored at the electrode/electrolyte interfaces due to mainly an electroadsorption process. During the discharge process, the stored charges cause a movement of the electrons in the external circuit providing electrical energy (Figure I.1). According to charge storage mechanism, two types of supercapacitors can be distinguished: (i) electrical double layer capacitors (ELDC), in which the charges are stored by electrostatic adsorption of ions on the electrode/electrolyte interfaces, (ii) pseudocapacitors, in which the charges are additionally stored by reversible and fast redox reactions in or on the surface of the electrodes.

In EDLC, the usually electrode material based on carbon, is not electrochemically active. In other words, there is no redox reaction on the electrode material during charge and discharge processes and a pure physical charge build-up occurs at the electrode/electrolyte interface [11][12][13] . While, in pseudocapacitors, the electrode material is additionally electrochemically active, based on transition metal oxide or conducting polymer, which can directly store charges during the charge and discharge process [14][15][16] .

Electric double layer capacitors (EDLCs)

EDLCs are physical energy storage devices in which the charge is stored electrostatically due to reversible adsorption of ions onto a charged electrode surface. The operating principle is based on a double layer capacitor, which is formed through charge separation at the electrode-electrolyte interface 17 . The charge storage occurs directly across the double layer of the interface without any charge transfer. Therefore, the EDLCs have very long-term stability, because there is no electrochemical reaction onto the material electrodes and no composition changes during the charge/discharge processes. When applying a potential difference across the electrodes, a charged layer created at the electrodes surface leads to the creation of oppositely charged layer in the electrolyte formed by accumulation of ions near the electrode/electrolyte interface to keep electro-neutrality. The thickness of the created double layer is of the order of 5-10Å in concentrated electrolytes, which is dependent on the electrolyte kind and concentration 18 .

During discharge process, the charge accumulated at the interfaces causes a parallel movement of the electrons in the external circuit, thus, generating electric energy.

Hermann Von Helmholtz reported a model to explain the mechanism of charge storage in EDL, in which, two layers of opposite charges are simultaneously formed maintaining separation equal to their atomic distance at the electrode/electrolyte interface 19,20 be continuous along the layer in the fluid bulk 22,23 . The limitation of Gouy-Chapman model is a higher estimation of capacitance in EDL due to inverse correlation of capacitance with the separation distance. In 1924, Stern and Grahame merged the Helmholtz model and the Gouy-Chapman model. The excess of ions accumulated on the surface of the electrode is distributed between (i) a compact layer close to the surface (called Helmholtz) and (ii) a diffuse layer (called Gouy-Chapman). The first ions of the diffuse layer are not directly on the surface but at a distance of about 0.5 -2 nm (depending on the nature of the electrolyte; aqueous or organic) 12,24 .

In The EDL models allow the physic-chemical phenomena that occur at the electrode/electrolyte interface to be explained.

Pseudocapacitors

Pseudocapacitor is another type of supercapacitors that stores charges by additionally fast and reversible oxidation-reduction reactions called faradic reactions which take place on the surface of the electrode material. This type of storage is also called faradic storage and is complementary to the capacitive storage. It was identified by Conway 6 .

Pseudo-supercapacitors can store much higher energy but a lower power than EDLCs. On the other hand, their cycle life is generally shorter because the electrode materials degrade more quickly due to the redox reactions involved during cycling. The most common used electrode materials are (i) conducting polymers (CPs) such as poly(3,4-ethylenedioxythiophene) (PEDOT) 25- 27 , polypyrrole (PPy) 28,29 , polyaniline (PANI) 25,28 and polythiophene (PTh) 30,31 , and (ii) transition metal oxide such as RuO 2 32,33 , ZnO 16,34,35 , NiO 34,36,37 , and MnO 2 32,35 .

In the case of CPs, the charge/discharge process is related to ion doping and dedoping (intercalation/deintercalation), generally accompanied with free solvent molecules transferred and volumetric swelling-shrinking of the film 28,38 . These processes are likely to lead to a wide variety of unpredictable mechanical defects such as the fatigue of CP electrode, which decreases quickly the capacitance of the CP electrodes 39 . Furthermore, a short potential window is required for CP-based supercapacitors, when the potential window is large, the CP can be degraded at more positive potential 40 , or become insulating at too negative potential. Therefore, the potential window used plays an essential role in the performance of CP-based SCs.

The capacitance, C, is directly related to the amount of charges which can be exchanged through the electrode material and the voltage applied across the device, which can be calculated using the Equation I.3.

𝑪 = 𝒅𝒒 𝒅𝑽 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰. 𝟑)

where dq is the exchanged charge and dV is the voltage change.

The capacitance of the CP material depends on the ion adsorption, the surface and structure of the electrode material, and the pore size distribution.

Hybrid supercapacitors

In order to enhance the energy density of EDLCs and pseudocapacitors, another type of supercapacitors was proposed called ''hybrid supercapacitors'' 41 . The concept of hybrid supercapacitors is based on the combination between different EDLC and pseudocapacitor materials [42][43][44] . The limiting property of EDLC is not present in pseudocapacitor and vice versa, therefore, this combination led to obtain combined performances between EDLCs and pseudocapacitors.

The assembly of two similar electrodes made from the same EDLC and pseudocapacitive components corresponds to a symmetric hybrid supercapacitor [45][46][47] . Whereas, the assembly of two different electrodes made from different materials forms an asymmetric hybrid supercapacitor 48- 50 . All the commercially available hybrid supercapacitors are asymmetric and those with conducting polymer electrodes are of prime interest 51 . Electrical double layer electrodes can be combined with battery electrodes to obtain intermediate capacitance and stability performances between supercapacitors and batteries 52,53 . Amatucci et al. 54 Actually, hybrid capacitors attract more attention to create the future breakthrough supercapacitors with high power and high energy along with long cycling stability 55,56 . supercapacitor-type cathode (AC) and battery-type anode (HTO) 57 .

Capacitance, energy density and power density

The capacitance, energy density and power density are the most important parameters to characterize supercapacitors, these parameters can be measured easily using the following Supercapacitor assembly principle and equivalent electrical circuit 58 .

In the case of symmetric supercapacitors, the two electrodes are made from the same material and the capacitance of the positive electrode C p is equal to the capacitance of the negative electrode C n . The overall capacitance C T would be half of either one's capacitance. While, in the case of asymmetric supercapacitors, the two electrodes have two different materials, the overall capacitance C T is mainly dominated by the one with smaller capacitance.

In a supercapacitors evaluation, energy density (E) and power density (P) are also two important variables determining the supercapacitor performances, which can be calculated by using equations: where R s denotes the Equivalent Series Resistance (ESR, in Ω) of the system. It corresponds to the sum of the various types of resistances related to the intrinsic resistivity of the electrodes, the ionic resistance of the electrolyte and the interface resistances between the current collector and the electrode.

The energy density depends strongly on the following intrinsic parameters of the storage system:

(i) different electrodes materials have different specific area, porosities and particle sizes, which influence the capacitance of the supercapacitor; (ii) different electrolytes have different voltage windows and affect cell voltage. In the case of electrode material made from carbon and the electrolyte is the aqueous solution, the supercapacitor voltage window is about 1 V, while in organic electrolytes the supercapacitor voltage is in the range of 3-3.5 V.

Power density demonstrates how quickly the energy stored in the device can be delivered to an external load. Equations I. 5 shows the ESR has a much stronger effect on the power density.

Therefore, to achieve a high power density of supercapacitor, the ESR should be reduced.

Generally, the power density of a supercapacitors is much higher than that of the batteries, because, the ESR in supercapacitors is much smaller.

Furthermore, as indicated by the Equation I.4, the energy density is proportional to the capacitance of supercapacitor, means that increase the capacitance leads to increase the energy density of the supercapacitor. Therefore, to improve the overall cell capacitance, The specific capacitance of each electrode sould be improved.

To express the capacitance of one electrode, a specific capacitance (C s , expressed in F.g -1 ) is defined as:

𝑪 𝒔 = 𝑪 𝒊 𝑾 (𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝐈. 𝟔)
where W is the weight of the electrode material, and C i is the intrinsic electrode capacitance (anode or cathode). This specific capacitance depends strongly on the electrode structure and the charge adsorption within the electrode material. Therefore, a higher specific capacitance obtained does not mean that the material will be better as supercapacitor electrode. For example, a very thin material film deposited can exhibit a high specific capacitance due to its low weight, while, when we use a thicker film with high capacitance, the specific capacitance of this thicker film may not be as high as expected. Nonetheless, the concept of specific capacitance has been adopted as an important parameter in evaluating SC material performance.

Applications and challenges of Supercapacitor

Applications of supercapacitors

With the increase of demands for renewable, clean and sustainable energy, the advantages of large amount of power in a relatively short time, along with a very high number of charge/discharge cycles and a longer lifetime than batteries have made supercapacitors one of the more important devices for energy storage and power supply. These versatile attributes led to their incorporation in a wide range of applications. Figure I. 5 shows some applications of supercapacitors.

Transport applications

With a rapid charge/discharge associated to high power density, supercapacitors are used to deliver high bursts of power to electric and hybrid cars, and to store energy during the braking 63 .

Moreover, they can be coupled to batteries or fuel cell devices, stop-start to fuel-saving systems or to start motors in big vehicles. In Shanghai, supercapacitors have been used to power electric buses for an autonomy from 3 to 6 km. The supercapacitors were recharged at each bus stop using a pantograph: 30 seconds are enough to charge them to 50% and 80 seconds charge them to 100%. The same principle is used in some tramways to power them during short sections with no overhead line.

Figure I.5. Some application of supercapacitors 18,[59][60][61][62] Military applications Supercapacitors are already used in many military devices such as radio transmitters, standby power supplies and autonomous sensors, control of water levels in submarine ballasts, detonators, pulsed lasers, steering systems, radar for tracking ballistic missiles and satellites. They are also used in emerging technologies such as hybrid vehicles or electric guns for projectile propulsion at very high speeds 18 .

Computers and memory backup chips

Supercapacitors are widely used in consumer electronics as backup energy sources for system memories, microcomputers, system boards and clocks, toys, and mobile phones. They serve as backup power supply in solid state drives (SSD's). Supercapacitors are also used to protect memory against any accidental drop regarding the primary power source. For some portable electronic devices with moderate energy demands, SCs may act as rechargeable stand-alone power sources. Currently, batteries are the most convenient power supplies. However, they require long recharge time. SCs can be recharged quickly, and the repeated charging and discharging can proceed without significant losses in efficiency 64 .

Challenges of supercapacitors

Supercapacitors have many advantages compared to other technologies like batteries. However, they still face some challenges at the contemporary technological stage.

• Low energy density: In comparison with batteries, supercapacitors have a low energy density which is considered as a major challenge for supercapacitors applications.

• High self-discharging rate.

• High cost: Raw material costs are significantly high and play important role in pricing of supercapacitor.

• Properties of electrode materials: Controlling the properties of electrode materials such as surface area and pore size distribution is very important to improve the performance of supercapacitors

• Thickness of electrode materials: The thickness of an electrode also plays a vital role in the performance of a supercapacitor. Therefore, it is essential to consider the thickness of the electrode in relevance to its applicability 18 .

• The connection between the current collector and electrode material: It is important to ensure a high adhesion between the current collector and electrode material.

Electrode materials

The large number of research done on electrode materials for supercapacitors shows that the choice of the active material plays an important role in the performance of the supercapacitors device. Therefore, the majority of SC research are focused on electrode materials in order to further improve the capacitive performance relative to existing electrode materials 1,65 .

The capacitance of supercapacitor highly depends on the specific surface area of the electrode materials. Since during the charge process, not all the specific surface area of the electrode material is electrochemically accessible by ions when the material is in contact with an electrolyte, the capacitance of several electrode materials does not linearly increase with increasing specific surface area. Therefore, the electrochemically accessible surface area is called the electrochemical active surface area. Consequently, the structure of the electrode material plays an important role in the electrochemical active surface area. The relationship between the capacitance to the surface area is theoretically defined as:

𝑪 = 𝜺 𝟎 • 𝜺 𝒓 • 𝑨 𝒅 (𝐄𝐪𝐮𝐚𝐭𝐢𝐨𝐧 𝐈. 𝟕)
where 𝜺 𝟎 (in Fm -1 ) is the vacuum permittivity, 𝜺 𝒓 (in Fm -1 ) is the relative permittivity of the electrolyte, 𝑨 (in m 2 ) is the surface area of the electrode accessible to the electrolyte ions, and 𝒅 (in m) is the effective thickness of the electrical double layer (EDL).

In general, there are three major types of electrode materials of supercapacitor, carbon materials 12,45,[66][67][68] , conducting polymers 25,28,69,70 and metal oxide [71][72][73][START_REF] Zhou | Ultrahigh-Performance Pseudocapacitor Electrodes Based on Transition Metal Phosphide Nanosheets Array via Phosphorization: A General and Effective Approach[END_REF]

Carbon materials

Carbon materials are the most used electrode material for EDLC. Indeed, the low cost of the carbon materials, its high conductivity (up to 50 S.cm -1 ), its high thermic stability and resistance to corrosion offer several advantages. Furthermore, carbon materials present many different forms as shown in Table I.1. ➢ The variety of its allotropes and the excellent control over the various structures which can be achieved.

➢ The low mass density of these various allotropes.

➢ Its relatively high electrical conductivity (from 0.5 to 10 S.cm -1 for active carbon and up to 1000 S.cm -1 for carbon fibers).

➢ The ease of fabrication, along with low prices depending on the allotropes.

➢ Its low chemical reactivity in contact with common electrolytes.

a. Activated carbons (CAs)

AC is the most used in commercial supercapacitors because of its low cost and good performances. Generally, carbon materials are deposited on the electrode surface as film using a binder, such as polyvinylidene fluoride (PVDF), poly (tetrafluoroethylene) (PTFE) or carboxymethyl cellulose (CMC). Its specific surface area, porosity, purity, and electronic conductivity are the parameters influencing supercapacitor performances.

ACs are prepared by creating a three-dimensional porous network in the bulk of the carbon material by an activation process [START_REF] Zhu | Carbon-Based Supercapacitors Produced by Activation of Graphene[END_REF] . This step consists in controlling the oxidation of carbon powders or carbonized organic precursors (wood, nutshells, fruit pits, anthracite, coal, petroleum coke, pitch, peat, sucrose, corn grain, leaves, and straw) [START_REF] Wei | Nanostructured activated carbons from natural precursors for electrical double layer capacitors[END_REF][START_REF] Titirici | Black perspectives for a green future: hydrothermal carbons for environment protection and energy storage[END_REF][START_REF] Wang | Promising porous carbon derived from celtuce leaves with outstanding supercapacitance and CO2 capture performance[END_REF] in their volume using various activation techniques such as oxidation in water vapor, KOH or CO 2 at about 700 ° C. [START_REF] Zhu | Carbon-Based Supercapacitors Produced by Activation of Graphene[END_REF] Activated carbons offer a large surface area (up to  2700 m 2 .g -1 ) due to the micro-and meso-pores created during activation. Compared to CNTs and graphene, ACs present a high mass density ( 0.5 g.cm-3) and a volume capacity around 50 to 80 F.cm -3 . Recently, the specific capacitance of ACs has been significantly improved up to 200 F.g -1 in organic electrolyte [START_REF] Obreja | On the performance of supercapacitors with electrodes based on carbon nanotubes and carbon activated material-a review[END_REF] . The porosity of an activated carbon greatly influences its performance. C. Largeot et al. [START_REF] Largeot | Relation between the ion size and pore size for an electric double-layer capacitor[END_REF] reported an hypothesis where pore size should be higher than the size of solvated ions, around 0.7 nm in a solvent-free ethyl-methylimmidazolium-bis(trifluoro-methane-sulfonyl)imide (EMI-TFSI) ionic liquid.

However, the active surface of microporous carbons (<1 nm) can be saturated with ions, limiting their specific capacitance [START_REF] Mysyk | Confinement of symmetric tetraalkylammonium ions in nanoporous carbon electrodes of electric double-layer capacitors[END_REF] .

b. Graphene (Gr)

Graphene has attracted a great interest over the last years in the field of supercapacitors 57,[START_REF] Wang | Advanced asymmetrical supercapacitors based on graphene hybrid materials[END_REF][START_REF]Interlayer distance controlled graphene[END_REF] .

Due to a two-dimensional structure, graphene can potentially combine a large accessible surface area with a high conductivity. The specific surface area of a graphene monolayer can theoretically reach 2630 m 2 .g -1 . Graphene's properties vary strongly as a function of its production method, which plays an important role in determining the performances of final products [START_REF] Raccichini | The role of graphene for electrochemical energy storage[END_REF] . The common methods adopted for graphene production include reduction of graphene oxide (GO) [START_REF] Chen | High performance supercapacitors based on reduced graphene oxide in aqueous and ionic liquid electrolytes[END_REF][START_REF] Xu | What is the choice for supercapacitors: graphene or graphene oxide?[END_REF][START_REF] Yang | Electrochemically reduced graphene oxide sheets for use in high performance supercapacitors[END_REF] , liquidphase exfoliation [START_REF] Jung | High-yield synthesis of multi-walled carbon nanotubes by arc discharge in liquid nitrogen[END_REF] , mechanical exfoliation [START_REF] Yi | A review on mechanical exfoliation for scalable production of graphene[END_REF] , synthesis on SiC 92 , bottom-up synthesis [START_REF] Cai | Atomically precise bottom-up fabrication of graphene nanoribbons[END_REF] and chemical vapour deposition [START_REF] Chen | Three-dimensional flexible and conductive interconnected graphene networks grown by chemical vapour deposition[END_REF] . The comparison among the aforementioned techniques is presented in Figure I.7. Although many different methods continue to be explored, the largescale production of graphene still remains the key challenge before its widespread application [START_REF] Raccichini | The role of graphene for electrochemical energy storage[END_REF] .

Although capacities of up to 100 F.g -1 have been reported for graphene electrodes in an organic medium [START_REF] Titirici | Black perspectives for a green future: hydrothermal carbons for environment protection and energy storage[END_REF] , the preparation of thick electrodes (100-200 μm) and the assembly of cells of several hundred Farads remains difficult because of the low gravimetric density of exfoliated graphene [START_REF] Wang | Promising porous carbon derived from celtuce leaves with outstanding supercapacitance and CO2 capture performance[END_REF] .

The energy density of the graphene-based electrodes can be improved by functionalization with redox materials [START_REF] Couly | Asymmetric Flexible MXene-Reduced Graphene Oxide Micro-Supercapacitor[END_REF][START_REF] Cho | Ternary composite based on homogeneous Ni(OH)2 on graphene with Ag nanoparticles as nanospacers for efficient supercapacitor[END_REF][START_REF] Wang | Three-Dimensional Printing of Polyaniline/Reduced Graphene Oxide Composite for High-Performance Planar Supercapacitor[END_REF] . Specific capacitance of 150 F.g -1 were obtained for graphene sheets in an organic electrolyte [START_REF] Wu | Physical and electrochemical characterization of activated carbons prepared from firwoods for supercapacitors[END_REF] . Even if this value is close to that of activated carbons, these electrode materials remain only for manufacturing a small systems with very high power (microsupercapacitors) due to the small amount of material used per cm 2 of electrode [START_REF] Balathanigaimani | Highly porous electrodes from novel corn grains-based activated carbons for electrical double layer capacitors[END_REF] . They are mainly synthesized by catalytic vapor phase deposition (CCVD) 105 , electric arc discharge [START_REF] Jung | High-yield synthesis of multi-walled carbon nanotubes by arc discharge in liquid nitrogen[END_REF] and laser ablation 106 . Each of these methods presents advantages and disadvantages resulting in different growth results (Table I.2). The conductivity of perfect MWCNTs is rather comparable to that of perfect SWCNTs because the coupling between nanotubes is weak 104 .

CNTs are usually regarded as a promising candidate for high-power electrode material because of the aforementioned properties. Although the surface area of CNTs (500 m 2 g -1 ) is small as compared to ACs (2000-3000 m 2 g -1 ) 12,107 . The specific capacitance of CNT electrode material ranges between 15 and 200 Fg -1 , depending on the morphology and purity obtained after the production procedure 64,107 . CNTs can also be used as a support in composite electrodes, due to their high mechanical properties and open tubular network, the CNTs are covered with electroactive materials to achieve high capacitance values [108][109][110][111] .

Conducting polymer

In In electrochemistry, conducting polymers are used as a tool for signal amplification (optical, electrical, electrochemical ...) which improves the characteristics and performance of sensors in terms of sensitivity, stability and the limit of detection 124,125 . Electrochemical synthesis can be used both in organic electrolytes 126 , aqueous 127 and recently in ionic liquid medium 128 . Several electrochemical methods can be used for the electropolymerization of conducting polymers: galvanostatic constant current method, potentiostatic controlled potential method or cyclic voltammetry. Due to their superior conductivity, CPs have been widely used as electrode materials for supercapacitors 38 . However, they suffer from the poor cycling stability, which presents the major obstacle hindering the practical application of CPs in supercapacitors 28 . This disadvantage results from the continuous swelling/shrinkage of CP-based electrode during charge/discharge process, leading to a failure of the mechanical integrity and to an eventual cyclability fading.

Therefore, synthesis of binary or ternary composites such as CPs/metal oxides 129 , CPs/carbon materials 130 ➢ Synthetizing conductive polymer nanostructures using a template.

➢ Performing templateless growth of conductive polymer nanowires.

Table I. 4 summarizes the specific capacitance of different CP electrode materials in aqueous and non-aqueous electrolyte 18 .

Table I.4. Different CPs-based pseudocapacitive electrode. 18 Electrode materials Specific capacitance Electrolyte reversible electron transfer coupled with an electroadsorption of protons on the surface of RuO 2 .

Reaction I.3 explains the charge storage mechanism of RuO 2 in acidic solution 65 :

𝑅𝑢𝑂 2 + 𝑥𝐻 + + 𝑥𝑒 -↔ 𝑅𝑢𝑂 2-𝑥 (𝑂𝐻) 𝑥 (0 ≤ 𝑥 ≤ 2) (𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟑)
Other transition metal-oxides such as PbO 2 138 , NiO 36 and MnO 2 35 have shown similar trends as supercapacitor electrode materials in aqueous and acidic electrolytes. The redox behavior arises in such metal oxides due to proton insertion/de-insertion processes 139 . The specific capacitance of these materials was comparatively lower to that of RuO 2 in respective electrolytes 140 , which can be explained by the limited surface layering and difficulty in cationic or protonic diffusion in the bulk material. However, to improve the specific capacitance of such metal-oxides, different strategies can be used such as increasing the surface layering.

In the

Composite Materials

One of the main challenges of supercapacitors is their low energy density, which has limited their application in the energy storage field. The development of hybrid materials could be an appropriate solution to overcome this challenge. Generally, hybrid or composite materials are composed by two or more materials in order to combine theirs different properties. The combination between different materials can provide a positive synergetic effect which lead to: (i) increase of specific surface area, (ii) facilitate the electron conduction, (iii) increase the number of electroactive sites, (iiii) prevent the agglomeration of particles and increase the porosity 65 .

Carbon materials have attracted extensive interest as electrode materials for supercapacitors due to their high electronic conductivity, specific surface area, mechanical properties and cyclic stability. However, their low capacitance is the major obstacle for their application in the field of energy storage. Several researchers have demonstrated that the combination of carbon materials with conducting polymer shows better electrochemical properties and performance.

Carbon have also been used as reinforcement materials for conducting polymer supercapacitors.

Various conducting polymer /carbon material composite have been studied such as PPy/CNT In the Table I.6, the specific capacitance of some composite-based electrode in different electrolytes are reported. Aromatic polymers like poly-o-phenylenediamine (PoPD) have also been used to dope carbon nanomaterials in order to increase their capacitive performance. Deng et al. of the initial capacitance after 5000 cycles.

Conclusions

Supercapacitor is one of the most promising technologies for energy storage. By using carbon materials, conducting polymer or metal oxide as electrode materials, supercapacitors can assure high capacitance, good cyclic stability and excellent rate capability. However, more research should be focused on development of nanocomposites materials based on carbon material, CPs and MOs, to further increase the performances of supercapacitors. Continuous research efforts are necessary to allow these materials and novel devices to meet the appropriate energy demands.

Improving the energy density of supercapacitors can broaden thier area of applications. For that, researchers should be focused on the development of novel techniques to prepare highly porous materials in order to store a large amount of charge leading to an increase of energy density.

Furthermore, electrolytes with stable and wide potential windows associated with an excellent ionic conductivity are necessary, as well as the compatibility between ion size and pore size of electrode materials.

Up to now, industrial development of new devices such as battery-supercapacitor hybrid device, Electrochemical flow capacitor, micro-supercapacitor, electrochromic supercapacitor, photosupercapacitor, thermally chargeable supercapacitor, self-healing supercapacitor, shape memory supercapacitor and piezoelectric supercapacitor continue to be area of intensive research.

In this Ph.D. thesis, an aromatic polymer (PoPD) will be studied (Chapter III) as pseudocapacitive electrode, which can solve the cycling stability problem of the classical CPs (such as PPy, PANI, Pth). After that, the CNT will be combined with PoPD (Chapter VI) in order to further improve its capacitive performance.

II. Energy conversion:

Electrochemical reactions in DMFC

Methanol and water are poured in the fuel reservoir of DMFC, then, the methanol is oxidized on ACL. This methanol oxidation reaction (MOR) generates protons, carbon dioxide and electrons according to the Reaction I.4:

𝐶𝐻 3 𝑂𝐻 + 𝐻 2 𝑂 → 𝐶𝑂 2 + 6𝐻 + + 6𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟒)
The carbon dioxide generated moves towards the fuel reservoir and vented out of the cell. While the electrons move through an external circuit and the protons produced flow through PEM to reach the cathode side. On the cathode site, the atmospheric oxygen is transported through the CCC and CDL and reaches the CCL where it reacts with protons and electrons to produce water according to the Reaction I.5:

Water produced at the cathode leaves the cell mainly by diffusion and natural convection. The released heat is dissipated to the methanol solution on the anode side and to the ambient air on the cathode side. The overall reaction in DMCF can be presented as the reaction I.6:

𝐶𝐻 3 𝑂𝐻 + 3 2 𝑂 2 → 𝐶𝑂 2 + 2𝐻 2 𝑂 + ℎ𝑒𝑎𝑡 (𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟔)
The detailed mechanism of methanol oxidation was proposed by Bagotzky et al. 169 . As shown in

Advantages and disadvantages of DMFC

The direct methanol fuel cell is a low-temperature fuel cells, which can operates at temperatures similar to PEMFC. Nevertheless, it is operated at slightly higher temperatures in order to improve the power density. Methanol is an attractive fuel option because it can be produced from natural gas, although it can be replaced by crude oil, coal or renewable biomass resources. It has the advantage of a high specific energy density (since it is liquid at operating conditions) and it can be operated with methanol/water mixtures.

The main problems related to DMFC technology are:

(i) Slow electro-oxidation kinetics: During electrooxidation reaction, methanol is decomposed to gives intermediates species, some of these species absorb on the catalyst surface and prevent methanol molecules from undergoing further reaction.

Thus, electrooxidation of intermediates is the rate limiting step, which causes the poisoning of the catalyst surface and slows down the methanol oxidation reaction.

(ii) Methanol Crossover: One of the objectives of the PEM membrane is to stop the flow of fuel and oxygen from one electrode to the other. However, in DMFC, the methanol diffuses through Nafion membrane from anode side to cathode site, then, reacts directly with oxygen. The movement of electrons from anode to cathode and with the presence of methanol causes an internal short circuiting resulting in a current loss. -Directly fed with methanol.

-Quick start (without high pressure or high/low temperatures).

-Perspective of progress.

-Methanol is easily more storable than hydrogen.

-Cost of some catalyst like Pt, Pd, etc.

-Low efficiency.

-Technology not demonstrated beyond 1kW (cell degradation).

-Poisoning of catalyst materials.

Anode catalyst

DMFC is an electrochemical device which provides electricity from direct methanol oxidation.

Methanol undergoes an electrochemical reaction at the anode, whereas the oxygen is reduced at the cathode. Recently, the development of high efficient anode catalysts has attracted the attention of researchers, the main factors when developing a new electrocatalyst are the performance and the cost. The performance of electrocatalyst depends on its size, shape and support materials used. Pt catalyst is the most used metal for methanol oxidation reaction.

However, platinum metal is high-cost and suffers from the self-poisoning caused by CO-like species. Therefore, developing an efficient electrocatalyst materials with low-cost metal is mandatory for different catalytic applications and especially for DMFC. In this context, many studies have been conducted and much efforts are in the underway to replace Pt-based catalyst by other cheaper catalysts such as nickel, copper, cobalt etc, under the condition of exhibited a high electrocatalytic performances.

Pt based catalyst

Pt is the most commonly used metal for DMFC systems in both acidic and alkaline media.

Indeed, Pt shows a very good catalystic properties and almost requisite to enhance the efficiency of MOR Nonetheless, the stability of multimetallic catalysts is a crucial problem. Especially, with dissolution of one metal leads to a modification in the structure of the catalyst resulting in the modification of the catalyst properties 193 . Moreover, limited resources and high cost of Pt are viewed as significant obstacles to wide spread commercialization of fuel cells. Therefore, many researchers are focusing on non-Pt based catalysts to overcome these problems.

Non-Pt based catalyst

In order to reduce the cost of the catalyst, various studies have been focused on non-Pt metal as altenative catalyst to replace the Pt. However, in acidic media there is little work on non-Pt based catalysts compared to alkaline environment due to their low stability in acidic media. In the recent years, alkaline media was highly used as electrolyte in DMFCs for two reasons: (i)

effective MOR on the anode catalyst, (ii) the poisoning effect is very weak and (iii) various nonnoble metals could be used as anode catalyst for MOR such as Ni, Cu, Pd, Rh…etc. which are cheaper compared to Pt and exhibit greater catalytic performance in alkaline media. Hence, the alkaline electrolyte can give the best electrochemical performance and pave the way for the use of cheaper catalysts.

Ni-based anode catalyst

Nickel has an atomic number of 28 and is a first row transition element. Its particular properties allows it to be widely used for various applications, including sensors, electrocatalysts, and batteries. In DMFCs, nickel-based electrode has considered as a promising alternative for Pt-based catalyst for methanol electrooxidation 189 . It is well known for its good catalytic performance and the electrochemical oxidation of methanol over Ni(II) and Ni(III) has been widely explained 196,197 .

Generally Additionally, the redox peaks shift towards lower potential during cycling, which is related to the oxygen evolution reaction and increase in the Ni(III) sites 199 .

Cu-based anode catalyst

Copper-based electrocatalyst, is known by its lower affinity for CO compared to Pt, Ru, Rd, therefore, it is very suitable for methanol electrooxidation in alkaline media 203 . The deposition of copper particles on gold electrodes and glassy carbon has been studied in the literature, and is

often carried out at constant potential or using cyclic voltammetry. Cu-based catalyst has received a lot of attention to their highly regarded performance as electrocatalyst 189

Catalyst support materials

The main objective in fuel cell technologies is to develop cheaper materials with excellent catalytic performance and durability. Different approaches have been used to increase the performance and reduce the cost of fuel cell technologies, based on the development of nanostructured supports. Generally, the support materials used for catalyst metal are carbon, conducting polymer, silica, Titania, tungsten oxide, alumina, zeolite and zirconia.

Carbon support

Carbon nanomaterials (especially carbon black) are the most used support for Pt and Pt-Ru catalyst for commercial fuel cells, due to their good electronic properties and high surface area. CNTs with r-GO as highly conductive and synergistic support for PtPd alloy nanoparticles. A good catalytic performances for MOR were archived due to the strong interaction between r-GO-CNTs support and PtPd alloy which facilitated the fast chemisorption of methanol molecules by following the direct or active intermediates pathways.

Conducting polymers support (CPs)

Conducting polymers are conducting supports for immobilizing the metal catalyst to improve their electroctalytic performances, they can improve the dispersion and morphologies of metal catalyst for better performances. Many CP have been used as catalyst support such as polyaniline 215 , polypyrrole 216,217 , PEDOT 218 , poly(N-methylpyrrole) 219 , which give rise to the metal species nearly mimicking the situation in homogeneous systems. Thanks to their gaz/water permeability and conductivity, CPs can be used as ideal support for fuel cells.

Polypyrrole (PPy)

PPy has been widely used as electrode material in various fields of applications. Its electrochemical properties have been attributed to many factors such as steric interactions, changes in the planarity of the polymer backbone, effective orbital overlap, and the nature of the substituents.

PPy presents many advantages like low cost, aqueous solubility, high mechanical properties, and excellent environmental stability. In electrctalytic field PPy has been used as support material for Pt catalyst, the electrode showed high electroactivity performance and increased resistance to CO poisoning. Khalafallah et al. 220 developed a metallic cobalt oxide core wrapped with a polypyrrole nanoshell (PPy/Co 3 O 4 ) for electrocatalytic oxidation of methanol. The results shows that this catalyst has enhanced catalytic properties due to the incorporated PPy shell which improves the number of the electroactive sites, the electrical conductivity and the surface area.

Polyaniline (PANI)

PANI can be synthesized using chemical and electrochemical methods. Microscopic studies show that this polymer possesses a rough surface morphology, but its conductivity is less than that of

Challenges and Applications of Direct Methanol Fuel Cells

Applications of DMFCs

Fuel Cells are considered as environmental friendly because they do not produce toxic byproducts. However, they are not fully emission-free. They still produce carbon dioxide which is a green-house gas. This technology can replace lithium ion batteries and fossil fuels. DMFC is considered as most promising type of direct liquid fuel cell (DLFC) 227 .

DoCoMo proposed a DMFCs with 5.4 V at 700 mA, with 30% methanol for cell phone charge, then it collaborated with Fujitsu to improve the percentage of methanol to 99%. Toshiba developed a DMFCs with an average power output of 12 V for laptop computer, which can work during 5h.

LG Chem Ltd invented a DMFCs for laptop computer. This DMFCs provide a lifespan of about 4000h and can feed a 25 W laptop for 10h. Panasonic invented a DMFCs with high power with power output of 20 W that can operate for 5000h.

In the military field, several smart DMFCs have been developed. Jenny 600, a DMFC system with a fuel cartridge that can store 350 ml of fuel, provides 350 Wh of power and can provide energy more than conventional power sources with 80% less weight. Another DMFC system named Jenny 1200, gives double of power compared to the previous system with a lighter weight. DMCF generator for electrical and electronic devices of military vehicles gives long operational time and a lighter weight compared to conventional power sources.

For medical applications, DMFC systems have been used in ambulances in the UK. They are used to charge equipment and to ensure their good functioning. Moreover, hearing aids where developed in Denmark. Thanks to DMFC, this hearing aid operates for 24h and has a long lifetime of about 6000h. A summary of DMFC applications is shown in Table I.9.

Challenges of DMFCs

DMFC technology presents great possibilities for future generations. However, it still has some problems and limitations that prevent its commercialization. Table I. 10 shows some DMFC challenges and suggested solutions proposed in the literature. • Develop a new catalyst support materials to improve the performance of the catalyst.

• Improve catalyst preparation methods.

Reduce the cost of the catalyst.

• Usage of non-Pt catalyst.

• Reduce catalyst loading.

• Inexpensive membrane.

Improve the durability and stability.

• Improve the performance of catalyst and membrane.

• Optimizing operation conductions.

• Reduce the corrosion of current collectors by deposing resistant coating.

Reduce methanol crossover.

• Use of Low methanol concentration.

• Use of methanol barrier layer.

• Improve the performance of Nafion membrane.

• Use a highly active catalyst.

Enhance water removal at cathode side.

• Provide a high-hydrophilic coating on the cathode side.

• Use of parallel channels on cathode.

• Operating the cell at higher temperature.

Improve the distribution of oxygen at cathode side.

• Reduce water flooding on the cathode.

• Use of multi-layer catalyst.

• Use air filters.

Conclusions

DMFC is one of the most promising technologies for energy conversion. Platinum and Platinumbased catalysts are the most commonly used for DMFC systems. Although they exhibit a high efficiency for methanol electrooxidation reaction, the limited natural resources of platinum and its high price limit the practical application of DMFC. Hence, one of the main challenges of DMFC is the development of low-cost and highly efficient catalysts with good resistance to poisoning species. In this context, many studies have been conducted and much effort is underway to replace Pt-based catalysts by cheaper alternatives such as nickel, copper, cobalt, etc., with the criteria of exhibiting a high electrocatalytic performance. In addition the devlopement of nanocomposite support for these metals based on carbon and conducting polymer is a promosing way to further enhance their efficiency, stability and durability.

III. Objectves of Ph.D. thesis.

The nanocomposites based on conductive polymers and carbon nanomaterials have attracted a lot of attention in different field of research because of their electronic, chemical and physicochemical properties, which open many innovative applications in various areas such as environment, energy, safety, process control, rapid medical diagnostics, or chemical sensors.

In the energy fields, especially in energy storage, understanding electrochemical charge storage behaviour, becomes deservedly necessary. It is well known that interfacial ion flux dynamics constitute an important part of the operating principle of super-and pseudo-capacitors.

Therefore, the correlation of this fundamental understanding to the charge storage properties of nanocomposite thin film electrodes is rather significant but requires specific characterization tools.

In present Ph.D. thesis, two different parts have been studied:

i. In the first part of this thesis, a correlation between the interfacial ion dynamics and charge storage properties of PoPD and SWCNT/PoPD composite has been reported using electrogravimetric techniques (EQCM and ac-electrogravimetry). Using the EQCM and acelectrogravimetry, highly relevant information has been 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, (iii) identification of species transferred in opposite flux directions provided that their kinetics are sufficiently different and (iv) variation of the relative concentrations of the species inside the examined material. After that, a detailed study about the capacitive performances of PoPD and SWCNT/PoPD composite were reported using two-electrode and three-electrode configuration.

ii. In the second part of this thesis new nanocomposites have been developed for direct methanol fuel cell (DMFC), which is considered as promising energy source for portable devices due to their small size, light weight and high energy density. In this context, the aim of this part is studying the oxidation of methanol on nanocomposite materials based on conductive polymer, carbon nanomaterials and metallic nanoparticles. These nanocomposites are synthesized electrochemically using CNF, PpPD and NiPs or Cu densrites. The electrocatalytic effect of the prepared nanocomposites for methanol oxidation reaction was studied in alkaline solution and was compared with other nanocomposite materials in the literature.

Chapter II. Description of classical and advanced methods

Chapter II presents different microscopic and spectroscopic techniques used for the morphological and structural characterization of samples, particularly, the principle of operation of each method, the type of instruments and experimental conditions used in this Ph.D. thesis. At the end, the electrochemical and electrogravimetric techniques applied will be presented.

I. Structural and morphological characterization techniques 1. Scanning electron microscopy (SEM)

SEM is a technique based on electron-matter interactions, which performs a qualitative analysis of the surface morphology of sample. The electron source 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. As a consequence of the primary beam's interaction with the sample, secondary electrons are emitted 1,2 . These electrons are selectively collected in the secondary electron detector through a grid held at a low positive potential with respect to the sample. There, the secondary electrons are counted (number of electrons arrived/unit time) and the count rate is translated into an electrical signal, which is then used to obta 1 in a visualization of the sample 1 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 emitted electrons reach the detector, resulting in darker areas, as opposite to edges and convex surfaces where more electrons escape and reach the detector resulting in brighter areas. Charge builds up on the sample surface due to the electron bombardment of the primary beam and in the case of nonconductive samples this charge cannot be dissipated. This results in images that are too bright throughout the sample. To avoid this phenomena, samples can be sputtered with a very thin layer of metals such as gold or gold/palladium 3 , however In our case, this step was not necessary because all the samples present good electrical contact with the current collector.

In this Ph.D. thesis, the thickness and/or the surface morphology of different electrode materials were investigated with a FEG-SEM (Zeiss, Ultra 55) at an accelerating voltage of 10 kV.

Transmission electron microscopy (TEM)

TEM is microscopic technique based on electron-matter interactions, which can perform the internal microstructure of sample. The electron gun generates a primary electron beam either by thermoionic emission (using a heated filament as the cathode, e.g. LaB 6 ) 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. 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. 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 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 4 .

In this Ph.D. thesis, TEM images were performed using a JEOL JEM-1011 electron 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.

Energy dispersive X-ray spectroscopy (EDX)

EDX is an analytic technique used determine the elemental analysis or chemical characterization In this Ph.D. thesis, the elemental analysis of different samples was performed with an energy dispersive X-ray (EDX) detector coupled to the FEG-SEM.

X-ray diffraction (XRD)

XRD is non-destructive method allowing the identification of crystalline structure of materials.

X-rays are high-energy electromagnetic radiations. They have energies ranging from about 200 eV to 1 MeV, locating between γ-rays and ultraviolet (UV) radiation in the electromagnetic spectrum 6 . XRD is an analytical technique where X-rays are generated in a cathode ray tube by heating a filament to produce electrodes, after that the sample is bombarded with electrons to analyze the transmitted and diffracted beams, which can be used for characterizing and identifying crystalline structures. As shown in Figure II.4 7 , X-rays are generated in an X-ray tube consisting of two electrodes enclosed in a vacuum chamber. The cathode (usually a tungsten filament) is heated to produce the electron emission. They are accelerated by a high potential towards the anode (usually a copper), which is grounded. The electrons with high velocity can collide with the anode and thus induce the emission of x-ray radiations 6 . Each quantum of electromagnetic radiation, or photon, has an energy, E, which is proportional to its frequency, ν:

𝐸 = ℎ𝜈 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰𝑰. 𝟏)
The wavelength (λ) of X-rays related to its frequency is characteristic of the anode material and can be expressed by:

𝜆 = ℎ𝑐 𝐸 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰𝑰. 𝟐)
where h is the Planck's constant and c is the speed of light.

XRD can be effectively used to characterize and identify the crystal structures of materials. It is possible to determine the average spacings between layers or rows of atoms, where Bragg's law plays a fundamental role 8 :

𝑛𝜆 = 2𝑑 𝑠𝑖𝑛𝜃 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰𝑰. 𝟑)
where d presents the interplanar spacing (the difference in path length for the ray scattered from the top plane and the ray scattered from the bottom plane), θ is the scattering angle and n is an integer representing the order of the diffraction peak, shown in Figure II.5 9 . It demonstrates that the constructive wave interference (strong diffraction) occurs when the difference in path length for the top and bottom rays is equal to an integer multiple of the wavelength. In this Ph.D. these, the XRD measurements were performed at ambient conditions using Phillips PANalytical X'Pert Pro diffractometer with Cu Kα radiation (λ = 1.54184 Å).

X-ray photoelectron spectroscopy (XPS)

XPS is one of the most powerful and common chemical analysis techniques to analyze the composition of sample surface, in order to provide the elements constituting the sample, surface Based on the photoelectric effect, the binding energy (E B ) of a core-level electron is overcome by the energy (hν) of the incident X-ray photon, resulting in the core-level electron excited and ejected from the sample. The kinetic energies (E K ) of the ejected photoelectrons are measured by an electron spectrometer whose work function is ϕ. Therefore, the (E B ) of each emitted electron can be described by the following relationship 12 :

𝐸 𝐵 = ℎ𝜈 -𝐸 𝐾 -𝜙 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰𝑰. 𝟒)
XPS spectra are usually plotted by intensity (counts per second) as a function of the E B of the electrons detected, which quantify both peak intensity and peak position. The former measures how much of a material is at the sample surface, while the latter indicate the elemental and chemical composition 10 .

In this Ph.D. thesis, XPS analysis were performed using a using a K-alpha spectrometer (Thermo Scientific) equipped with a monochromatized Al anode (1486.6 eV) and using a pass energy of 100 eV and 20 eV, for acquisition of the survey and high-resolution spectra, respectively. The atomic concentration of the surface (≈ 10 nm) of the samples was calculated after subtraction of the background using the Shirley method 13 .

Fourier-transform infrared spectroscopy (FTIR)

FTIR is an attractive analytical technique used for the identification of the nature of the chemical bonds in a sample and for determining functional groups to obtain information about the possible interactions [14][15][16] . In principle, when a beam of infrared (IR) light passes through a sample, some of the IR light is absorbed by the material and some is transmitted through it. The absorbed photons can excite molecules into a higher vibrational energy state. The energy absorbed corresponds to the vibrational frequency of the sample molecules' functional groups and thus permits its identification 15 .

At the heart of every FTIR is an optical device named an interferometer. The oldest and perhaps the most common type of interferometer in use today is the Michelson interferometer, which consists of four arms (Figure II.7) 17 . The top arm comprises an infrared source and a collimating mirror, which is used to collect the light from the source and parallelize its rays; a fixed mirror is contained in the bottom arm, which is contrary to the moving mirror capable of moving left and right (right arm); the sample and the detector are located in the left arm. In the center of the interferometer is an optical device called a beam splitter, which can transmit and reflect some of the light incident upon it. The light transmitted and reflected by the beam splitter travels towards the fixed mirror and moving mirror, respectively. Then, the light will be reflected from these two mirrors and travel back to the beam splitter, where they are recombined into a single light beam that leaves the interferometer, interacts with the sample, and strikes the detector 14 .

In this Ph.D. thesis, the Fourier-transform infrared spectroscopy (FTIR) were performed using an ATR-FTIR instrument (Perkin Elmer Spectrum 400 FT-IR/FT-NIR Spectrometer).

Raman spectroscopy

Raman is very important technique for characterization of the material structure. Like the FTIR spectroscopy, Raman measures the vibration motions of a molecule. The principal of Raman is based on inelastic scattering of photons, a monochromic light is used, which is generated from a laser in the visible, near ultraviolet, or near infrared. The laser beam can be considered as an oscillating electromagnetic wave with electrical vector. Upon interaction with the sample, it induces electric dipole moment which deforms molecules. Because of periodical deformation, molecules start vibrating with characteristic frequency.

In this Ph.D. thesis, Raman analysis were provided using a Renishaw RM 1000 microraman spectrometer.

II. Electrochemical and electrogravimetric techniques 1. Cyclic voltammetry (CV)

The CV is a potentiodynamic electrochemical measurement technique that measures current versus potential. The principle is to impose 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 18,19 . 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.

The CV can 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 19

Electrochemical impedance spectroscopy (EIS)

The EIS measurements, based on frequency modulation, provide qualitative and quantitative information on the studied electrochemical system (capacitive, resistive, diffusive behavior etc.).

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 about tens of mV). The frequency modulation allows the separation of the different reactions kinetics: fast phenomena occur at high frequencies while slower phenomena are visible at lower frequencies 21 . In general, electrochemical systems have a nonlinear currentvoltage characteristic curves I=f(V). However, working only on a small portion of this curve, the relationship between ΔV (low amplitude sinusoidal perturbation imposed around the stationary potential) and ΔI (low amplitude sinusoidal response in current) may be considered as linear (Figure II.9) 22 .

Figure II.9. Linear current response to a low amplitude sinusoidal perturbation potential around a stationary value 22 .

Therefore, by applying a low amplitude sinusoidal perturbation (ΔV) to the system around a stationary potential (Vs) the current response (ΔI) will also be sinusoidal in shape with a small amplitude around a stationary value Is. The concept of impedance is a generalization of the concept of resistance, applicable to sinusoidal functions with the application of Fourier transforms to the current and potential time-dependent functions. The electrochemical impedance Z(ω) can then be defined as the following transfer function:

𝑍(𝜔) = ∆𝑉 ∆𝐼 (𝜔) (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰𝑰. 𝟓)
where ω = 2πf corresponds to the pulsation and f to the frequency.

𝐶 = - 1 2𝜋𝑓 × 𝐼𝑚(𝑍) (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰𝑰. 𝟔)
In this Ph.D. thesis (chapter V and VI), the EIS analysis were carried out using Autolab (Metrohm-Autolab, Utrecht, Netherlands) potentiostat/galvanostat, using a three-electrode system, in which a saturated calomel electrode (SCE) was used as a reference electrode, a modified CPE as a working electrode and a platinum (Pt) rod as a counter electrode.

Chronoamperometry.

Chronoamperometry is an electrochemical technique used to characterize an electrode surface in order to study reaction mechanisms. The principal of Chronoamperometry consists in imposing on an electrochemical system a fixed potential and measuring the current as function of time.

In this Ph.D. thesis, Chronoamperometry was used to study the long-term stability of different electrode in chapter V and VI using Autolab (Metrohm-Autolab, Utrecht, Netherlands) potentiostat/galvanostat, using a three-electrode system.

Galvanostatic Charge-Discharge (GCD)

GCD is an alternative method to measure the capacitance, resistance and cyclability of the electrode materials. This technique consists in performing charge/discharge cycles. A constant current density is applied and the potential response is measured over time. As shown in Figure II.11 25 , a linear response of potential (in V) with respect to charge/discharge time (in s) is anticipated for EDLC, whereas the non-linearity of charge/discharge curves is normally obtained from the pseudocapacitors, which is distinct from the triangular shape from EDLC response. where I is the discharge current and dV/dt the slope of the discharge curve. The specific capacitance Cs is related to the capacitance of the cell C by:

𝐶 𝑠 = 2𝐶 𝑚 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰𝑰. 𝟖)
where m is the mass of the active material per electrode.

Electrochemical quartz microbalance (EQCM)

EQCM, also called cyclic electrogravimetry, is a technique coupling CV and Quartz Crystal Microbalance (QCM), which cannot only track the current response but also the simultaneous mass change occurring in the working electrode during an electrochemical process [26][27][28] . The overall set-up used for these experiments is depicted in figure II.12.

Ac-electrogravimetry

Ac-electrogravimetry is a pertinent method to study the mechanism of transfer of different species directly or indirectly involved in the charge compensation process associated to an electrochemical process. In this experimental method, these transferred species at different kinetics can be separated and their identification can also be clearly achieved.

In ac-electrogravimetry, one of the most important components of the experimental set-up is the frequency/voltage (F/V) converter, where the analog processing was obtained by means of a phase lock loop circuit, as described in Although the electrochemical impedance and the electrogravimetric TF are simultaneously measured, the frequency range of measurement for impedance is from 65 KHz to 10 mHz but only from 1 KHz to 10 mHz for the electrogravimetric TF, which is ascribed to a distortion of working frequency signal (phase shift and modulus attenuation) occurring in the detection device (F/V converter) 33 . The F/V converter sensitivity in the linear part is around 29mVHz -1 34 . In order to have a higher level of performance of the F/V converter, the carrier frequency is reduced to a few hundred hertz (generally 400 Hz). Herein, a reference frequency synthesizer (Agilent 33250A) is used to obtain a frequency difference of 400 Hz between the microbalance quartz frequency (shown on the frequency counter) and that from the reference synthesizer. From a theoretical point of view, the concentration variation of each species, ΔC i , under a sinusoidal potential perturbation, ΔE, at the film/electrolyte interface can be described as 33,35 :

Compared to

∆𝐶 𝑖 ∆𝐸 (𝜔)| 𝑡ℎ = -𝐺 𝑖 𝑗𝜔𝑑 𝑓 + 𝐾 𝑖 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰𝑰. 𝟏𝟒)
where K i and G i are the partial derivatives of the flux (J i ) with respect to the concentration and the potential, respectively, and d f is the film thickness.

K i represents the transfer kinetics of each species related to the characteristic frequency of one species (f i ) while G i is the reciprocal of the transfer resistance ( 𝑅𝑡 𝑖 = To illustrate what the experimental ac-electrogravimetry data may look like, the

Chapter III. Correlation between the interfacial ion dynamics and charge storage properties of poly(orthophenylenediamine) electrodes exhibiting high cycling stability *

In this chapter, we report an integrated electrogravimetric study based on electrochemical quartz crystal microbalance (EQCM) unravels the interfacial ion transfer phenomena of the poly(ortho-phenylenediamine) (PoPD) thin film electrodes. The redox transformations and the ionic flux mechanisms were investigated in aqueous NaCl electrolyte via EQCM and ac-electrogravimetry approach. Several parameters were considered to explain the redox behaviour of PoPD film, such as the nature and the dynamic of the ion transfer at the PoPD film/electrolyte interface. To correlate these fundamental understandings to the performance of the electrode, the capacitive performance of PoPD films is tested in a 3 electrode cell and a 2 electrode Swagelok cell. By determining the interfacial flux dynamics and as well as the relative proportions of species transferred at the electrode/electrolyte interface, our results contribute to the understanding of the charge-discharge process of PoPD polymer, which has the desirable attributes for the energy storage applications.

I. Introduction and objectives

Conducting polymers (CPs) have been extensively investigated and solicited in various fields of applications, such as in electrocatalysis, biosensors and microelectronics technology. [1][2][3][4] In energy storage, these pseudo-capacitive materials are regarded as prospective electrode component considering their salient features such as high electrical conductivity and high redox activity, as well as their relatively low cost and ease of synthesis. [5][6][7][8] Specifically, polyaniline (PANI), polypyrrole (PPy), polythiophene (Pth) and their derivatives have extendedly been studied as electrode materials due to their high specific capacitance which is substantially higher than those of the conventional carbon-based electrodes and comparable to pseudocapacitive metal oxides. 9-10 However, their cycling stability is a major impediment that limits their practical application. For example, pristine PANI, PPy and Pth based pseudocapacitive electrodes have been reported to lose substantial amount of their capacitance after about 1000 cycles and there is a lack of information about the electrode stability beyond this threshold. 11 Several methods have been used to enhance the cycling stability of conducting polymer based electrodes. It was postulated that the electrochemical performance is affected by various factors, including synthetic method, dopant ion, substrate and morphology of the conducting polymer. 5 Therefore, the strategies to enhance the cycling stability include (i) encapsulation of the conducting polymer in inorganic networks, such as titania nanotubes 12 , (ii) surface coating with Nafion 13 for example and (iii) doping with organic molecules such as para-toluenesulfonate 14 . For instance, the phytic acid (PA) doped polypyrrole (PPy) has been reported to retain its specific capacitance for about 4000 cycles on specific substrates. 15 Regarding the efforts on the morphology, it have been shown that PPy nanowire arrays retain 70 % of the initial capacitance after several hundred cycles and are advantageous over disordered nanowire networks or conventional films 16 However, the fabrication time, the toxicity and the cost of the dopant materials limit the practical application of these synthesis methods. 11 Additionally, despite the multiple efforts, it remains still challenging to reach cycling stability more than 1000 cycles, except from limited number of reports providing information about the long term electrode stability. For example, Li et al. 11 has shown that the deposition of a thin carbonaceous shell (∼5 nm) onto the CP electrode drastically improves the charge/discharge cycling stability i.e., ∼95 and ∼85% capacitance retention after 10000 cycles, for PANI and PPy, respectively. Similarly, the reduced graphene oxide layer has been employed as a protective layer on PANI to tackle with the cycling stability issue and 75.5% of original capacitance has been maintained up to 10000 cycles 17 .

While substantial research efforts evolve in the direction of protecting the CP based electrodes especially to alleviate swelling and shrinking during cycling and thus, to enhance the cycling stability, one other path is to scrutinize the structural effects by investigating the derivatives of the most researched conducting polymers. In this context, aromatic diamine polymers such as para, ortho and meta-phenylenediamine (PD) have received great attention due to the active free amino and imino groups present in their structure, which provide structural modification possibility. 18 They have been the subject of several studies in the past decade in view of their attractive (electro)chemical properties, [19][20] and also have been receiving attention as pseudocapacitive CP electrodes (ortho-, 21 and para-, 22 ). In addition to their direct intervention as electrode materials, poly ortho-phenylenediamimine (PoPD) polymers doped with glycine was carbonized to obtain nitrogen and oxygen doped hallow carbon spheres for supercapacitor applications. Regarding the synthesis, the electropolymerization of ortho-phenylenediamine (oPD) in acidic, alkaline and neutral aqueous solutions has been examined and electrochemical quartz crystal microbalance (EQCM) has been used to investigate the subtleties of polymerization mechanism and the subsequent electrochemical activity of the resulting electrodes 19,[23][24] .

As an extension to the previous studies 19,[23][24] and due to the developing interest in the PoPD type CP as pseudocapacitive electrodes, a thorough investigation of the electrochemical charge storage behaviour becomes deservedly necessary. It is also noted that, to the best of our knowledge, there is no study describing the interfacial ion flux (insertion/deinsertion and/or electroadsorption/desorption of ions) with the exact identification of the intervening (non)ionic species together with their dynamics of transfer at the PoPD film/electrolyte interfaces. It is wellknown that interfacial ion flux dynamics constitute an important part of the operating principle of super-and pseudo-capacitors, therefore the correlation of this fundamental understanding to the charge storage properties of poly(ortho-phenylenediamine) thin film electrodes is rather significant but requires specific characterization tools.

A marked improvement in understanding the charge storage mechanisms has been obtained thanks to recent progress with electrogravimetric methods (EQCM and EQCM with dissipation monitoring) [25][26][27] . Besides, the complex character of the electrode mass changes, the viscoelastic properties of the electrodes and also possibly formed solid electrolyte interface (SEI) layer can be investigated using multiharmonic EQCM with dissipation monitoring (EQCM-D). [28][29] It can offer opportunities to identify the impact of several parameters (nature of the electrolytes, ions, binder…) on the performance change of the electrodes.

In addition to these developments in the use of EQCM based methods in the energy storage domain, here, a non-conventional complementary technique, the so-called ac-electrogravimetry Correlation between the interfacial ion dynamics and charge storage properties of poly(ortho-phenylenediamine) electrodes exhibiting high cycling stability Chapter III 88 has been proposed to unveil the mechanisms of the ionic exchange process. [30][31][32] It has been demonstrated that highly relevant and complementary information to the classical EQCM 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, (iii) identification of species transferred in opposite flux directions provided that their kinetics are sufficiently different and (iv) variation of the relative concentrations of the species inside the examined material [33][34][35][36][37] . The originality of this method is its ability to discriminate between the interfacial transfer of cations, anions and solvent molecules involved (directly or indirectly) in the charge compensation process which is ultimately related to their charge storage performance.

Thus, we report here on the investigation of the ion exchange mechanisms in PoPD pseudocapacitive electrode. The electropolymerization of oPD was carried out in acidic medium.

Thereafter, the redox transformations and the ionic flux mechanisms were investigated in aqueous NaCl electrolyte via EQCM and ac-electrogravimetry. Several parameters were considered to explain the redox behaviour of PoPD film, such as the nature and the dynamic of the ions transfer at the PoPD film/electrolyte interface. To correlate these fundamental understandings to the performance of the electrode, the cycling stability of the thin PoPD film electrode was tested in two-electrode Swagelok-type cell resulting excellent cycling stability for more than 8000 cycles with 110 F/g (11 mF•cm -2 ) capacitance at 30C.

II. Experimental 1. Electrode Preparation.

Electrochemical polymerization of oPD was performed on the gold electrode (0.2 cm 2 ) of quartz crystal resonators (9 MHz-AWS, Valencia, Spain) and on graphite plates (1 cm 2 ) for the electrogravimetric studies and for the tests in two-electrode Swagelok cells, respectively. Specifically, oPD monomer (1,2-diaminobenzene, Sigma-Aldrich) (5 mM) dissolved in H 2 SO 4 solution (0.1 M) was electropolymerized using cyclic voltammetry (CV). This step was also followed through EQCM measurements i.e. when quartz crystal resonators were used as substrates. The mass of the deposited polymer was evaluated by measuring the resonant frequency of the quartz crystal (∆f), before and after the electropolymerization, which was then converted to the mass change (∆m) by the Sauerbrey equation (∆𝑓 𝑚 = -𝑘 𝑠 . ∆𝑚) where k s =16.31x10 +7 Hz g -1 cm +2 is the experimental sensitivity coefficient. [38][39] 2. Electrode Characterization.

The morphology and the thickness of the PoPD films were analysed using a field emission gun scanning electron microscope (FEG-SEM, Zeiss, Supra 55).

III.2.3. Electrochemical Characterization of the electrodes.

Electrochemical quartz crystal microbalance (EQCM) measurements were carried out in aqueous 0.5 M NaCl solution and pH was adjusted to pH = 2 by the addition of HCl. A three-electrode cell was employed, in which, a platinum grid was used as counter electrode, Ag/AgCl (3M KCl saturated with AgCl) as reference electrode and a gold-patterned quartz substrates operating at 9

MHz as working electrode.

The ac-electrogravimetry measurements were performed using a four-channel frequency response analyser (FRA, Solartron 1254) and a lab-made potentiostat (SOTELEM-PGSTAT). The QCM was used under dynamic regime, the working electrode was polarized at a selected potential, and a sinusoidal small amplitude potential perturbation was applied. The frequency range was (derived from impedance)) were fitted through the theoretical expressions given in Equations II.17, II. 15 and II.18 (see chapter II). Providing that all the TFs including cross and partial TFs (detailed in Refs. 30-31, 33, 35-37 ) were fitted with the same set of parameters (where a good agreement between the experimental and the theoretical functions in terms of both the shape and the frequencies are achieved), then the key parameters (M i , K i and G i ) of our model can be considered as unique.

Galvanostatic charge/discharge (GCD) measurements were performed by a classical two-electrode Swagelok-type cell, where zinc metal was used as both reference and counter electrode and PoPD deposited on graphite plate as working electrode. Zn metal (-0.76 V vs SHE) is chosen among the other counterparts to be employed as electrode material in aqueous medium, as its reduction potential falls within the water stability window. The glass fiber separators (Whatman GF/C with areal weight and thickness of 4.7 mg/cm 2 and 185 µm, respectively) are used for the measurements in Swagelok cells. GCD tests were conducted in an aqueous electrolyte containing 1M NaCl (pH=2) cycled at 30 C current density that is equivalent to 0.63 mA•cm -2 .

III. Results and discussion

Electrochemical synthesis and morphological characterization of the PoPD.

Electrochemical synthesis of the PoPD thin film was performed by CV starting from the oPD monomer. It is important to note that this potential range corresponds to the oxidation of the oPD monomer and/or the formation of dimer and oligomers on the gold electrode of the quartz resonator. In the cathodic sweep, there is no significant mass variation until 0.1 V vs Ag/AgCl then, m increases and reaches a maximum at about -0.1V vs Ag/AgCl, followed by a decrease up to -0.4V corresponding to the redox processes of newly formed PoPD film. These observations are in agreement with earlier reports. 19 The value of peak-to-peak separation increases with the increase of the scan rate, which can be explained by a limitation in the process of charge transfer. 42 The scan rate dependence of the electrochemical response is also reflected in the corresponding mass variations. The general appearance of the anions contribution appears as a mass decrease during reduction and a mass increase during oxidation (for cations, it is vice versa). [30][31][32] The scan rate dependant mass response implies the presence of a slower cation(s) contribution which cannot be detected at higher scan rates and this/these cation(s) contribution becomes more dominant at 25 mV•s -1 .

The mass per electron (MPE = F x (Δm/Δq)) calculated from the EQCM data can provide indications of the nature of the transferred species during cycling. If only one species is involved in the charge storage process, the value of MPE would be its molar mass. The cation and anion contributions lead to negative and positive signs of MPE values, respectively. [32][33] Correlation between the interfacial ion dynamics and charge storage properties of poly(ortho-phenylenediamine) electrodes exhibiting high cycling stability works strengthen this idea by describing that the PoPD films can contain solvent molecules and ionic species at all oxidation states. 19,24 Therefore, this deviation between the experimental and theoretical MPE values merits a special analysis. Thus, electrogravimetric impedance spectroscopy (ac-electrogravimetry) was performed to complement the EQCM data and to get a deeper insight into the interfacial charge compensation behavior occurring in the PoPD thin film electrode, from both gravimetric and kinetic point of view.

Electrogravimetric Impedance Study (Ac-electrogravimetry).

( ) are obtained from ac-electrogravimetry, as described earlier. [30][31][32][33] Distinct from Δq/ΔE() which considers only the interfacial transfer of ionic species, the Δm/ΔE() estimates the species nature and also takes the free solvent contribution into account. [30][31] The electrochemical impedance, ΔE/ΔI() and charge/potential TF, Δq/ΔE() ( anion and cation transfer, respectively (details of the methodology can be found in Refs. [30][31]33 ).

Free solvent molecules can appear in the same quadrant either with anions or with cations, depending on their flux direction. 33 For the fitting process of the Δm/ΔE() TF, two parameters (K i and G i ) previously obtained from impedance and charge/potential TF for each ionic species ( G i (mol.s -1 .cm -2 .V -1 ) The other potentials studied in the range of "0.05 V to -0.25 V vs Ag/AgCl" indicate the persistence of multi-species contribution, with slight variations in quantity and in terms of kinetics of transfer. From their K i and G i parameters, the characteristic frequency, f i , and the resistance of transfer, Rt i , can be calculated using the ). However, this cannot be the only contribution, since in the reverse scan during reduction, a mass increase is observed. The water molecules alone cannot explain the specific shape of the gravimetric response during the reduction process. Thus, there should be a contribution from the cationic species in the electrogravimetric response, as well.

Rt i =1/FG i (.cm 2 ) f i =K i /πd f (z)
The behaviour observed at 100 mV•s -1 becomes less and less obvious at lower scan rates (i.e. 25

and 50 mV•s -1 ) and the EQCM curves becomes dominant in a typical cationic response (Figure

III.5 (B))

. This is explained by the ac-electrogravimetry results showing the contribution of the H + and Na + at lower frequencies. Consequently, the contribution of cations is hardly detected at high scan rates, but starts to dominate the electrogravimetric response at lower scan rates. These results are in good agreement with the discussion of Inzelt et al. 24 on the PoPD charge compensation behavior and provide experimental data for the disentangling the redox behavior of PoPD thin films.

(iii) Complementarity of ac-electrogravimetric and EQCM results -Gravimetric contribution of each species: To further shed light on the multi-species contribution to the charge storage and their interfacial transfer, an approach benefiting from the complementarity of the EQCM and ac-electrogravimetry is proposed. Specifically, the relative concentration changes of each species (ΔC i ) with respect to the potential variation (ΔE) can be obtained from ac-electrogravimetry by using equation III.1 which lead to the estimation of the relative concentration changes of each species between the different states-of-polarization through an integration process as shown in Equation III.2:

()  - =  + C G i i E j d K i f   (Equation III.1) ( ) = 0 00 0 - -=   → CG EE ii ii C C dE dE i EE EK i   (Equation III.2)
Correlation between the interfacial ion dynamics and charge storage properties of poly(ortho-phenylenediamine) electrodes exhibiting high cycling stability 3. Pseudo-capacitive charge storage performances.

Thin film PoPD electrodes-half cell performance (3-electrode configuration).

The charge storage performances were measured in NaCl 0.5M at pH=2. The specific capacitance can be calculated from the CV curves using the following relation, Additionally, the C s values obtained in our study, are in the same order of magnitude or even higher than some of the earlier reports on CPs (Table III.2). 11,[44][45][46][47] However, it is important to recognize that the C s values depends on various factors, such as the synthesis route, additives, dopants, morphology, porosity, as well as the measurement configuration. 7,21 Correlation between the interfacial ion dynamics and charge storage properties of poly(ortho-phenylenediamine) electrodes exhibiting high cycling stability behaviour can be related to the mass transport limitation of the ions resulting in higher overpotentials upon cycling within a fixed voltage range. 48 Note that higher loadings are not investigated on the quartz resonators to be able to maintain the gravimetric regime for the analysis. 24 For that reason thicker electrodes are tested in a two electrode Swagelok configuration on graphite electrodes. visible structural distortion can be observed after long cycling, even though that is not significant enough to effect the structural integrity of the separator, thus it is not detrimental to the cell performance, at least for 8000 cycles.

Thicker film PoPD electrodes-full cell performance (2-electrode configuration).

Correlation between the interfacial ion dynamics and charge storage properties of poly(ortho-phenylenediamine) electrodes exhibiting high cycling stability The panel C shows images after immersing the Zn metal in the electrolyte for one week of resting period (that is the time spent to reach 8000 cycles during galvanostatic cycling).

In order to better position the performance of the PoPD based electrodes, they are compared with the limited number of studies exist which report on the capacitive behaviour of pristine PoPD 18, 21 and PoPD composites. 51-53 Wang et al. described the electrochemical synthesis of PoPD in the presence of sodium dodecyl sulphate (SDS) surfactant. 18 The resulting materials were tested by charging/discharging in 1M KNO 3 at the current density of 0.5 A•g -1 in the potential range of -0.2 to 0.6 V vs SCE (saturated calomel electrode) in a 3-electrode configuration. A capacitance value of about 106 F•g -1 were obtained and the electrode retained 82% of its initial capacitance at the 1000 th cycle. 18 Comparing our findings with the results of Wang et al., 18 it is important to note that pseudo-capacitive behaviour of the PoPD is clearly observed under the conditions of our study, as evidenced by the presence of the obvious redox peaks of PoPD ( ) to that reported in Ref. 18 is obtained, but at a higher current density (6. properties of the PoPD electrodes and compared them with the PoPD-PANI copolymer in 1M H 2 SO 4 using a 3 electrode configuration. In this case, bare PoPD presented the lowest C s (8.3 F•g - 1 ), PANI showed a C s of 123 F•g -1 and PoPD-PANI copolymer exhibited the highest C s (138.9 F•g - 1 ) at 0.1 A•g -1 . 21 The low value obtained for the pristine PoPD electrode can be attributed to the fact that GCD measurements were conducted in the potential range of 0 V to 0.6V vs Ag/AgCl, thus only benefiting from the double layer capacitance of the PoPD polymer, but not from its faradaic capacitance which was enhanced in a subsequent step by copolymerization of PoPD with PANI. 21 In our study it is highlighted that, the potential range can be optimized to test both

IV. Conclusions

A thorough electrogravimetric study which employed EQCM and its complementary counterpart electrogravimetric impedance revealed that the charge compensation process of PoPD thin film electrodes in acidified NaCl electrolyte occurs with the participation of multiple species. This combined methodology identified that the Cl -anions contribute at high frequencies (HFs)

indicating a faster process, followed by the transfer of water molecules with an opposite flux direction (excluded by the anions). Lastly, H + and Na + cations were exchanged at lower frequencies (LFs), exhibiting also opposite flux direction to anions.

Then, the pseudo-capacitive charge storage performances of the thin film electrodes were studied where the PoPD is electrodeposited on the gold-coated quartz resonators. The gravimetric capacitance values (C s in F•g -1 ) of the thin film electrodes could be obtained thanks to the exact determination of the electrode loadings by QCM. C s values as high as  434 F•g -1 are obtained (in a half cell configuration) at a loading of 22 g•cm -2 which is followed by a slight decrease ( 400 F•g -1 ) at the loading range of 22-55 g•cm -2 . These significantly high C s values may imply an efficient interfacial charge transfer which may occur in the thicker electrode configuration.

Accordingly, the pseudo-capacitive charge storage performance of a thicker electrode (in a full cell configuration in which Zn is used as both reference and counter electrode) presented relatively high C s values ( 110 F•g -1 ) and a remarkable cycling stability ( 8000 cycles) with high coulombic efficiency ( 100%). Although gravimetrically not significant, the contribution of protons to the charge storage mechanism is highlighted since they are detected as the dominant species in terms of concentration variation in the electrode during cycling. Therefore, high rate capability, the excellent cycling stability of the PoPD electrodes accompanied with high coulombic efficiency for 8000 cycles have been correlated to the electrolyte composition and the significant role of H + to the charge compensation process has been unraveled, which was made possible with coupled electrogravimetric methods of our study. Furthermore, our results are distinguished from the limited number of reports on the charge storage performance of PoPD based electrodes, where in the present study, the experimental conditions were optimized to have the contributions from both the double layer and the faradaic capacitance. The fact that the pristine PoPD polymer has significant cycling stability shows its promise as an electrode or electrode component in energy storage devices.

Chapter IV. Electrogravimetric study of capacitive charge storage behavior of carbone nanotubes/poly(orthophenylenediamine) nanocomposite: application in supercapacitors

In this chapter, the mechanism of the

I. Introduction and objectives

Development of green technologies for energy conversion and storage has attracted significant interest for producing and storing renewable energy with the ultimate goal of tackling the environmental problems caused by the fossil fuels. Among these technologies, supercapacitors represent a class of the most attractive energy storage devices, which received enormous attention due to their excellent power density, long cycle life and a fast charge/discharge rate [1][2][3][4][5] .

Carbon based materials such as active carbon, graphene and carbon nanotubes (CNT) have been largely studied as electrode materials for electric double layer capacitors (EDLC) due to their attractive characteristics, especially their low cost, high surface area and good mechanical and chemical stability [5][6][7][8] . Moreover, the high porosity of the structure allows easy accessibility of electrolyte ions. CNTs have been considered as one of most promising electrode material for the next generation of high power energy storage [9][10][11] , which exhibit a non-faradic energy storage capability while offering high surface area benefits. However, they exhibit a relatively low specific capacitance which limit their applications.

Nevertheless, CNTs present remarkable properties such as good conductivity, strength and stiffness, which maintain the interest for their use as reinforcements for the next generation of multifunctional composites 12 . Therefore, forming a CNT/conducting polymer (CP) composite has been considered not only to improve the specific capacitance of CNTs but also to overcome the drawbacks of CPs, especially, their cycling stability caused by the volume expansion during the repetitive charge-discharge processes. CPs themselves have also been widely employed as pseudocapacitive electrode materials due to their low cost, good electrical conductivity and relatively high specific capacitance 1,[13][14][15] . Polypyrrole (PPy) and polyaniline (PANI) based electrodes are shown to exhibit excellent specific capacitance values reaching 400 F•g -1 , [15][16][17][18][19] which is larger than that of the carbon nanomaterials. However, their cycling instability remains one of the major problem to get the full benefit of these pseudocapacitive CPs. Liu et al. 15 discussed the stability of PPy and PANI nanowire based electrodes for supercapacitors and reported that their capacitance can be retained but the value is less than 50% after 1000 cycles. The research efforts evolve in the direction of protecting the CP based electrodes for example by a carbonaceous shell 15 , especially to alleviate swelling and shrinking during cycling and thus, to enhance the cycling stability.

In this perspective, the combination of carbon nanotubes with conducting polymers has become a promising approach to provide nanocomposite with enhanced electrochemical properties. In addition, this configuration gives a strong synergetic effect, which improves the capacitive properties regarding the specific capacitance and the cycling stability. Various conducting polymer /carbon material composites have been studied such as PPy/CNT [20][21][22] , PANI/CNT [23][24][25] , poly(3,4-ethylenedioxythiophene) (PEDOT)/CNT [26][27][28] , poly(ortho-phenylenediamine) (PoPD)/Gr 29 , PANI/Gr 30,31 , PPy/Gr 32-34 , and PoPD/N-Mesoporous carbon 35 . Cherusseri et al. 36 showed that oxidized-carbon nanotubes/PPy nanocomposite exhibits a specific capacitance of about 80 F•g -1 at a current density of 7 mA•cm -2 . Cong et al. 37 fabricated PANI nanorods by electropolymerisation on graphene (GN) paper, the resulting composite exhibited high electrical conductivity, good flexibility and good synergetic effect between GN and PANI leading to a high specific capacitance of about 760 F•g -1 with a capacity retention of 82% after 1000 cycles.

Recently, Tahir et al. 26 reported a high-performance micro-SCs based on PEDOT-coated MWCNT nanoporous network. This composite micro-electrode exhibited a maximum specific capacitance of 20.6 mF•cm -2 at 0.1 mA•cm -2 and retained 99.9% of its initial capacitance after 20,000 CV cycles in a PVA/H 2 SO 4 gel electrolyte. This enhanced performance was attributed to the fact that the open porous structure of CPs and the CNT network results in an easy diffusion of ions from the surface to the inside of the electrode.

As an extension to the developments achieved on the most researched CPs, a subsequent path is to scrutinize the structural effects by investigating their derivatives. In this respect, aromatic diamine polymers like PoPD has received attention due to the active free amino and imino groups present in their structure, which provide structural modification possibility 38 . Moreover, PoPD shows excellent electrochemical stability for more than 8000 cycles 39 . Additionally, PoPD has been used to dope carbon nanomaterials in order to increase their capacitive performance. Deng et al. 40 report the synthesis of three-dimensional nitrogen-doped graphene (3DNGN) from PoPD by carbonization. The 3DNGN shows an interconnected porous structure, which exhibits a high capacitive performance due to the large electrolyte-accessible surface area. Yuan et al. 41 described the carbonization of poly ortho-phenylenediamine (PoPD) doped with glycine to obtain nitrogen and oxygen doped hallow carbon spheres for supercapacitor applications. In other works, PoPD has been widely combined with graphene as nanocomposite electrode for supercapacitor 29,[42][43][44] , a satisfactory specific capacitances have been obtained. However, the cycling stability of these composites was tested for only 1000 or 1500 cycles, except from a few limited number of reports providing information about the long term electrode stability. Recently, Yang et al. deposited

PoPD on graphene via in situ polymerization, and tested as electrode material for supercapacitors.

A synergistic effect was noticed which resulted in a high specific capacitance of about 381 F•g -1 in 1M H 2 SO 4 electrolyte at 1 A•g -1 and a retention of 90% of the initial capacitance after 5000 cycles 29 .

In The same electropolymerization method was used to synthesize SWCNT/PoPD nanocomposite on graphite plates (1 cm 2 ) (maintaining the optimised mass ratio given by the QCM measurements) for the electrochemical studies and for the tests in two-electrode Swagelok cells (see chapter III, Section III.2.3). As a blank sample, the electrochemical polymerization of oPD without SWCNT addition has been pursued in a similar manner as described above.

Morphological Characterization

The surface morphology of the SWCNT/PoPD films were characterized by a field emission gun scanning electron microscope (FEG-SEM, Zeiss, Supra 55). Transmission electron microscopy analysis was performed by using a JEOL, JEM-1011 electron microscope. X-ray photoelectron spectroscopy analyses were performed using a K-alpha spectrometer (Thermo Scientific) equipped with a monochromatized Al anode (1486.6 eV) and using a pass energy of 100 eV and 20 eV, for acquisition of the survey and high-resolution spectra, respectively. The atomic concentration of the surface (≈ 10 nm) of the samples was calculated after subtraction of the background using Shirley method 50 . Fourier-transform infrared spectroscopy (FTIR) were performed using an ATR-FTIR instrument (Perkin Elmer Spectrum 400 FT-IR/FT-NIR Spectrometer) in the range of 4000-400 cm -1 .

Electrochemical measurements.

The electrochemical and electrogravimetric performances of SWCNT, PoPD and SWCNT/PoPD electrodes were investigated using two-electrode and three-electrode After that, two redox peaks appeared at around -0.2 V (for the 1 st redox peak) and around 0 V (for the 2 nd redox peak). In the subsequent cycles, the current value of these redox peaks increases, while that of the oxidation peak of the monomer decreases indicating that the PoPD film grows on the surface. In comparison with the PoPD grown on bare gold electrode, the PoPD deposited on gold previously modified with SWCNT shows a higher current density. This behaviour is attributed to the increased surface area due to the SWCNT coating, resulting in improved electrical properties of the composite. The evolution of the CV response during electropolymerization of PoPD is in agreement with the earlier reports 51,52 . Considering the curve at 10 mV.s -1 , a particular behavior is observed: during the reduction, the mass decreases from 0.5 V to 0.1 V, followed by an increase of the mass from 0.1 V to -0.4 V and at the end, the mass decreases again from -0.4 V to -0.5 V. With an initial assessment of this results, the decrease of the mass can be attributed to a major contribution of anions, whereas the increase of the mass can be related to a major response of cations. The higher scan rates lead to a change in mass profiles. This behavior can be related to a multi-species transfer phenomena arising with different proportions and kinetics. In order to well understand this phenomena already revealed by EQCM data, the electrogravimetric impedance spectroscopy (ac-electrogravimetry) was performed at different potentials from 0.5 V to -0. The fitting process shows a good agreement between the experimental and the theoretical curves, which revealed the contribution of three different charged species. K i and G i of each species were obtained through the fitting process and were reported in Table IV 54,55 . Free solvent molecules can appear in the same quadrant either with anions or with cations, depending on their flux direction 56 .

III. Results and discussion

Synthesis and characterization of SWCNT/PoPD nanocomposite

The fitting of the experimental ∆𝑚 ∆𝐸 (𝜔) data using the theoretical expression in Equation II.17

and with the parameters given in Table IV.1 results in a good agreement between experimental and theoretical curves and reveals the participation of three charged species chloride, sodium, proton and free H 2 O molecules.

Escobar-Teran et al. 53 have studied the electrogravimetric behavior of SWCNT under the same conditions of our study. Their results show that the response of SWCNT thin film at 0 V vs.

Ag/AgCl presents the participation of only one charged specie (Cl -) and free solvent (H 2 O).

While, the electrogravimetric response of PoPD polymer film (see chapter III, Figure III.9) shows no species involvement at the same potential. Accordingly, the combination between SWCNT pH=2 Species identification

(i) M i (g.mol -1 ) K i (cm.s -1 ) G i (mol.s -1 .cm -2 .V - 1 ) Rt i =1/FG i (.cm 2 ) f i =K i /πd f (z) at 0 V a Cl - 35

Transfer dynamics of different species involved in the charge storage mechanism of SWCNT/PoPD film

In Figure IV.5 and IV.6, ac-electrogravimetry results at 0 V vs Ag/AgCl were described, then these analyses were performed at several potential values in the range of -0.4 V to 0.4 V vs Ag/AgCl. From the ac-electrogravimetric measurements, the transfer kinetics and resistance of each species can be determined. By using K i and G i parameters of each transferred species, the characteristic frequency, f i , and the transfer resistance of transfer, Rt i , were calculated using the following formula: SWCNT/PoPD nanocomposite during the oxidation shows a large mass increase followed by a small mass decrease. It is important to highlight that this EQCM profile described with the participation of anions became more pronounced at higher scan rates. Indeed, the anion contribution is detected at HF (faster process) in the ac-electrogravimetric measurements which strengthens the idea that anion contribution is amplified for the high values at the scan rates.

i i f K f d =  and
1 F i i Rt G = .
While at lower scan rates (from 50 to 10 mV.s -1 ), the behavior observed at 100 mV.s -1 becomes less and less obvious and the mass response ( The (𝑪 𝒊 -𝑪 𝟎 ) evolutions of Na + and H + are higher than the (𝑪 𝒊 -𝑪 𝟎 ) changes of Cl -and H 2 O, especially at cathodic potentials. It is important to note that in spite of lower kinetics of transfer, the contribution of protons in the process is not negligible. Their participation to the charge compensation mechanism in PoPD was described as a proton addition-elimination reaction, occurring due to the electron transfer, together with the motion of electrolyte ions 57 . In comparison with the gravimetric response of SWCNT 53

2 1 2 1 1 () 2 ( ) E s E C I E dE mv E E = -  (Equation IV.1)
where m is the mass loading of the electrodes, v is the scan rate, E 1 and E 2 are the low and high end potentials, and I(E) depicts the response in current. increasing scan rate from 10 to 100 mV•s -1 . The redox peak current density is higher at the higher scan rates. The redox peak separation increases with the scan rate as well, which may indicate a slight increase in the resistance of the electrode and a limitation in the charge-transfer kinetics 43 .

Accordingly, the C s of the electrode is higher at the slower scan rates but  95% of the C s (at 10 mV•s -1 ) can still be obtained at 100 mV•s -1 (inset of The measurements are performed in 0.5M NaCl at pH=2.

Investigation of the charge storage behaviour of the optimized SWCNT/PoPD

nanocomposites in a two-electrode configuration.

In order to further appraise the cell performance, the nanocomposite electrodes with the optimized composition were prepared on graphite substrates and tested in two-electrode configuration by GCD measurements and compared with the responses of the SWCNT and PoPD films alone. nanocomposite is made of bundles with a larger diameter (100-200 nm). This observation suggests that the electropolymerization occurs around the SWCNT bundles and the latter is coated with a PoPD polymer layer. Such morphologies are previously reported where drop-cast SWCNT layers are electrochemically coated with PoPD and they are shown to be advantageous for electrochemical sensing (i.e.; voltammetric detection of colchicine) and electrocatalytic reduction of oxygen 63,64 . PEDOT coated MWCNT porous networks were also prepared by using an EDOT monomer solution containing MWCNTs and this method also resulted in similar morphological features 26 .

The SWCNT/PoPD IV.3).

IV. Conclusions.

We demonstrate the potential of the SWCNT/PoPD nanocomposite materials to enhance the performance of the pristine PoPD electrodes for energy storage applications. Several compositions were prepared ranging from 25 to 83 wt% of PoPD incorporated in the final SWCNT/PoPD (1:5) electrodes were tested as cathodes where Zn metal was used as negative electrode in an aqueous electrolyte. The cell exhibits areal capacitance of 38.5 mF•cm -2 at 0.73 mA•cm -2 and gravimetric capacitance of 350 F•g -1 at a current density of 7.3 A•g -1 , as well as long lifetime of more than 11000 cycles. This device structure with a battery-type anode and a capacitor-type cathode is highly promising, may provide a new concept for developing highperformance micro-supercapacitors and also shows the potential perspective of the conducting polymer based nanocomposite electrodes for miniaturized electronics.

I. Introduction and objectives

In recent years, direct methanol fuel cells (DMFCs) have been considered as one of the most promising energy conversion systems which can be suitable for vehicles, laptops, and mobile phones applications 1 . Indeed, they offer several advantages including low operating temperature, easy transportation and storage of methanol, high energy conversion efficiency, and low emission of pollutants 2,3 . However, it is also widely accepted that there are several disadvantages which prevent the commercialization of DMFCs and many efforts have been put forward to overcome these obstacles. It seems that the main problems encountered with the methanol oxidation reaction (MOR) are related to the fact that its electrochemical conversion into carbon dioxide and proton is slow and incomplete on bare electrodes. The use of high-cost platinum metal (Pt)

is almost requisite to enhance the efficiency of MOR but Pt suffers from the self-poisoning caused by CO-like species [4][5][6] . Therefore, the development of an efficient electrocatalyst material with a low-cost metal is mandatory for different catalytic applications 7,8 and particularly interesting for DMFC 9 . In this context, many studies have been conducted and much efforts are underway to replace Pt-based catalysts by cheaper alternatives such as nickel 10 , copper 11 , cobalt 12,13 , etc., with the criteria of exhibiting a high electrocatalytic performance [14][15][16][17] . Among these alternatives, Ni is a low-cost catalyst which exhibits high electrocatalytic activity towards MOR in alkaline media and a good resistance to poisoning [18][19][20] . However, its stability still needs to be improved to achieve the performance of the noble metals 17 .

Different strategies have been developed to enhance the stability of Ni such as (i) the synthesis of Ni-based alloys [21][22][23] or (ii) the development of active support materials 17 . Zhu et al. 24 reported that 3D porous graphene could be employed as an excellent support for Ni nanoparticles (NiNps)

for methanol electro-oxidation. 3D graphene presents an effective three dimensional passageway for accelerating the charge transport and the electron transfer on the electrocatalyst, leading to an enhancement of the stability and the catalytic activity of NiNps. Wang et al. 25 prepared the NiNps supported on nitrogen-doped honeycomb-like carbon (CNFs) for effective methanol electro-oxidation. It was shown that this catalyst provides enhanced catalytic activity and stability arising from the uniform distribution and anchoring of NiNps on CNFs. The improved electronic interactions between CNFs and NiNps are also considered as an asset for the catalytic activity enhancement. Thamer et al. 26 and Al-Enizi et al. 27 showed that the use of nitrogen doped CNF (N-CNFs) as support for Ni-based catalyst offers low interfacial resistance with strong adhesion on the surface. Additionally, the good interaction between N-CNFs and Ni caused by nitrogen doping results in distinct electrocatalytic performance towards MOR. According to the results of these previous works, direct deposition of metal catalyst on a carbon support especially

Morphological and structural characterization

The surface morphology was analyzed by a Zeiss, Supra 55 FEG-SEM operating at an accelerating voltage of 5 KV. The chemical composition of the nanocomposite was analyzed by EDX coupled to FEG-SEM instrument. The XRD analysis were investigated by a Panalytical Empyrean diffractometer equipped with a Cu-Kα as radiation source (Kα = 1.54 Å). The FTIR analysis was carried out using an ABB Bomem FTLA 2000 spectromer, and the Raman spectra of the catalyst was recorded using a Renishaw RM 1000 microraman spectrometer.

Electrochemical techniques

Cyclic voltammetry, electrochemical impedance spectroscopy and chronoamperometry have been carried out using Autolab (Metrohm-Autolab, Utrecht, Netherlands) potentiostat/galvanostat, controlled by GPES 4.9 software. The electrochemical cell containing a three-electrode system was employed, in which a saturated calomel electrode (SCE) was used as a reference electrode, a modified CPE as a working electrode and a platinum (Pt) rod as a counter electrode. All the experiments were measured at room temperature.

III. Results and discussion

Preparation of CPE/CNF/PpPD/NiPs

Morphological and structural characterization 2.1. SEM and EDX analysis

The surface morphology of the CPE, CPE/CNF and CPE/CNF/PpPD/NiPs was investigated by

XRD analysis

The The FTIR spectrum of CPE/CNF/PpPD/NiPs exhibits several functional groups that appear at 1527 and 1688 cm -1 assigned to C-N and C=C stretching vibration of PpPD structure respectively 48 . The peaks located at 1134, 1744 and 3610 cm -1 are attributed respectively to the carbonyl (C=O), epoxy (C-O) and hydroxyl (-OH), arising from the carboxylic groups which indicates the successful functionalization of CNF 49,50 , while the characteristic adsorption bond of NiPs appears at 663 cm -1 51 . of about 431 mV. After the modification with CNF/PpPD and CNF/PpPD/NiPs nanocomposites, a considerable decrease in the value of  p was observed, which is equal to 110 and 100 mV, respectively. Moreover, the current density of CPE/CNF/PpPD and CPE/CNF/PpPD/NiPs were increased by 210% and 220% respectively. These results can be explained by the large surface area and the enhanced conducting properties of CNF/PpPD, which is combined synergistically with the catalytic properties of NiPs, leading to a significant improvement in the rate of electron transfer reaction at the electrode surface. This phenomenon was reported in previous works 49,53 .

Electrochemical characterization

The surface area of the CPE used in this study is 0.07 cm 2 , this value represents the geometric surface of the electrode, not the surface area involved in the electrochemical processes. To determine exactly the real surface area involved, the electroactive surface of each electrode was calculated according to Randles-Sevcik Equation V.1 55 :

𝑖 𝑝 = 2.69 × 10 5 𝑛 3 2 𝐴𝐷 1 2 𝐶𝑣 1 2 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑽. 𝟏)
where i p is the peak current (A); n is the electron transfer number (𝑛=1), A is the electroactive surface area (cm 2 ), D is the diffusion coefficient (6.7•10 6 cm 2 s -1 ), C is the concentration (mol•cm -3 ), and v is the san rate (V s -1 ). The electroactive surface area was calculated to be 0.017 cm 2 , 0.037

Methanol electrooxidation on the catalysts

To study the performance of the modified electrodes for methanol electrooxidation, the catalytic Methanol is oxidized at a potential more positive than that of the NaOH formation, indicating that the NiOOH acts as an electrocatalyst for MOR 7,18,19 The combination between CNFs and PpPD for supporting NiPs shows great activity for MOR.

The hybrid structure of CNF/PpPD composite increases the active surface area of the catalyst which leads to increase the adsorption rate of methanol molecules on the surface. In addition, NiPs supported on CNF/PpPD exhibits a better catalytic activity for MOR than that of the NiPs deposited on CNFs support 22,23 .

Effect of the PpPD thickness

The effect of the polymer film thickness on the electrooxidation of methanol was investigated using cyclic voltammetry in the potential range between 0 V to 1 V. The Normally, the increase of the nickel content, increase the electrocatalytic activity of the catalyst, but the excess of the NiPs on the electrode surface can affect negatively the electrocatalytic performance by reducing the number of active sites of the material and shortening the electron transfer path 24 . The optimal mass of NiPs is 0.25 µg, which reveals the best electrocatalytic activity of CNF/PpPD/NiPs nanocomposite for methanol electrooxidation, this mass was maintained throughout this study. M. This proves that the catalyst has a high tolerance for adsorption products and saturation of its surface is reached at a higher methanol concentration. In comparison with other catalysts such as Ni/N-CNFs/graphite 26 and NiO/N-CNF 27 , which show a saturation of the catalyst at 1.5 M and 0.5 M of methanol, respectively, the nanocomposite of our study presents an improved performance.

To further evaluate the obtained results in this work, the catalytic performance and the testing conditions of our catalyst for the methanol electrooxidation were compared with other electrodes in the literature (Table V.3). As shown in Table V.3, the performance of our catalyst regarding the current density of methanol electrooxidation peak is much higher compared to a commercial and other nanocomposite catalysts in literature, which highlight its potential use as an anode in DMFCs.

IV. Conclusion

Chapter VI. CNF/PpPD/Cu ternary composite for methanol electrooxidation

Chapter VI reports on a new catalyst consisted of copper based dendritic structures deposited on carbon nanofibers /poly(paraphenylenediamine) (PpPD) nanocomposite support using a facile synthesis approach. The morphology and structure of the prepared catalysts were characterized using XRD, SEM, TEM and EDX. Electrochemical impedance spectroscopy (EIS) is used to evaluate the participation of each component on the electrochemical properties of the composite. The effects of copper content on the electrocatalytic performance of CNF/PpPD/Cu were evaluated by cyclic voltammetry (CV) and chronoamperometry. The electrocatalytic effect of the CNF/PpPD/Cu for methanol electroxidation was determined in alkaline solution. The results highlight the synergetic effect of CNF/PpPD/Cu in improving the electrochemical activity and stability as well as the vast potential of CNF/PpPD/Cu as a low-cost catalyst for DMFCs.

I. Introduction and objectives

In the previous chapter, the performances of nickel particles deposited on CNF/PpPD support for electrooxidation of methanol have been evaluated in alkaline media. The ternary composite showed a high catalytic activity for methanol oxidation reaction (MOR) and exhibited a current density of 38.1 mA cm -2 in 1M of methanol. In this chapter, the catalytic performance of copper dendritic nanostructure deposited on CNF/PpPD support will be in focus to demonstrate the applicability of the CNF/PpPD support to other metals, with the ultimate aim of providing a promising catalyst for DMFCs. Copper exhibits high electrochemical and thermal stability, thus attracted a great attention for many electrochemical processes especially in alkaline media.

Pattanayak et al. 1 fabricated cuprous oxide (Cu 2 O) on a PPy-GO hybrid support for electrooxidation of methanol. The catalyst exhibits a current density of about 0.3 mA cm -2 in 2 M methanol, however its stability was tested for only 15 minutes and it seems to be weak.

Recently, Bastürk et al. 2 reported the fabrication of three-dimensional copper nanodomes as anode material for direct methanol fuel cells. The results show that the catalyst has good stability and high tolerance to CO ads poisoning. The enhanced activity was related to good intrinsic activity of Cu and the large number of available electrochemical active sites compared to bulk Cu. In another work, Long et al. 3 synthesized core-shell structured Cu@Pt/C catalyst for MOR. The results show that alloying Pt with Cu increases the catalytic performance. However, the preparation method of these catalysts is not facile and requires extended synthesis time.

Here, a simple and rapid preparation method for a Pt-free ternary composite consisting of a copper dendritic nanostructure deposited on a carbon nanofibers/poly(para-phenylenediamine) (CNF/PpPD) support is decribed. The electrodeposition of Cu dendrites onto the previously prepared CNF/PpPD support at a fixed potential takes about 45 seconds. The resulting composites were characterized using, XRD, SEM, TEM and EDX. Electrochemical Impedance Spectroscopy (EIS) is used to evaluate the effect of each component on the electrochemical properties of the composite. The catalytic performance of CNF/PpPD/Cu for methanol electrooxidation was tested in alkaline media using EIS, CV and chronoamperometry techniques. For comparison, Cu dendritic nanostructures supported on CNF (CNF/Cu) and PpPD (PpPD/Cu) were also prepared on CPE using the same process described above in order to evaluate the reactivity of each component.

II. Experimental 1. Electrode preparation

Morphological and structural characterization

The surface morphology of the prepared electrodes was analysed by a field emission gun scanning electron microscope (FEG-SEM, Zeiss, Supra 55). Transmission electron microscopy analysis was performed by using a JEOL, JEM-1011 electron microscope. X-ray photoelectron spectroscopy analyses were performed using a K-alpha spectrometer (Thermo Scientific) equipped with a monochromatized Al anode (1486.6 eV) and using a pass energy of 100 eV and 20 eV, for acquisition of the survey and high-resolution spectra, respectively. The atomic concentration of the surface (≈ 10 nm) of the samples was calculated after subtraction of the background using Shirley method 4 . The chemical composition of the nanocomposite was analyzed by EDX coupled to FEG-SEM instrument. The XRD analysis was performed by a Panalytical Empyrean diffractometer equipped with a Cu-Kα as radiation source (Kα = 1.54 Å).

Electrochemical measurements

The electrochemical explorations of the modified electrodes were carried out using a threeelectrode configuration, in which a saturated calomel electrode (SCE) was used as a reference electrode, a modified CPE as a working electrode and a platinum (Pt) rod as a counter electrode. In addition, oxygen has a similar distribution as Cu, indicating the formation of a copper oxide and/or copper hydroxide phases. The distribution of nitrogen element presents some wellproportioned zones, which are similar to the Cu distribution. This observation could be explained by the presence of amine groups on the surface, offered by the PpPD polymer, which have a direct impact on the nucleation and growth of the copper dendrites, as well as improving their electrocatalytic performance [11][12][13][14][15] . In order to determine the phase of the copper deposited, further characterization was performed by X-ray diffraction (XRD).

XRD and XPS analysis

EIS measurements

The EIS was used to evaluate the electrochemical properties of the modified electrodes after

Effect of Cu content and methanol concentration.

The effect of Cu content on the electrocatalytic activity of CNF/PpPD/Cu was studied using CV. CNF/PpPD/Cu shows a higher efficiency for MOR.

Chronoamperometric measurements

In order to evaluate the long-term stability of the catalyst, the chronoamperometric measurements were performed in 0.1 M NaOH solution containing 1.5 M methanol.

IV. Conclusions

In For the PoPD electrode, the charge compensation process in acidified NaCl electrolyte occurs with the participation of multiple species. This combined methodology identified that the Cl - anions contribute at high frequencies (HFs) indicating a faster process, followed by the transfer of water molecules with an opposite flux direction (excluded by the anions). Lastly, H + and Na + cations were exchanged at lower frequencies (LFs), exhibiting also opposite flux direction to anions.

For the PoPD electrode studied in chapter III, the charge compensation process in acidified NaCl electrolyte occurs with the participation of multiple species. This combined methodology identified that the Cl -anions contribute at high frequencies (HFs) indicating a faster process, followed by the transfer of water molecules with an opposite flux direction (excluded by the anions) occurring at intermediate frequencies. Lastly, H + and Na + cations were exchanged at lower frequencies (LFs), exhibiting also opposite flux direction to anions. Then, the pseudo-capacitive charge storage performances of the thin film electrodes were studied where the PoPD is electrodeposited on the gold-coated quartz resonators. The gravimetric capacitance values (C s in F•g -1 ) of the thin film electrodes could be obtained thanks to the exact determination of the electrode loadings by QCM. C s values as high as  434 F•g -1 are obtained (in a half cell configuration) at a loading of 22 g•cm -2 . These significantly high C s values may imply an efficient interfacial charge transfer compared with a thicker electrode configuration. Accordingly, the pseudo-capacitive charge storage performance of a thicker electrode (in a full cell configuration in which Zn is used as both reference and counter electrode) presents relatively high C s values ( 110 F•g -1 ), a remarkable cycling stability ( 8000 cycles) with a high coulombic efficiency ( 100%).

Although gravimetrically not significant, the contribution of protons to the charge storage mechanism is highlighted since they are detected as the dominant species in terms of concentration variation in the electrode during cycling. Therefore, the high rate capability, the excellent cycling stability of the PoPD electrodes accompanied with the high coulombic efficiency General conclusions and perspectives for 8000 cycles have been correlated to the electrolyte composition and the significant role of H + to the charge compensation process has been unravelled, which was made possible with coupled electrogravimetric methods of our study.

In chapter IV, the potential of the SWCNT/PoPD nanocomposite materials in enhancing the performance of the pristine PoPD electrodes for energy storage applications was evaluated.

The charge compensation process in NaCl electrolyte shows the participation of multiple species:

Cl -anions transferred at high frequencies, followed by the transfer of water molecules with the same flux direction (excluded by the anions) at intermediate frequencies. Lastly, Na + and H + cations were exchanged at lower frequencies (LFs) with the opposite flux direction to anions and water. Moreover, the charge transfer process of SWCNT/PoPD exhibits a positive synergetic effect, leading to the activation of other active sites for ions exchanges. Several compositions were tested and a mass ratio of 1:5 (SWCNT/PoPD) was determined resulting in optimized charge storage performances where, precise compositions of SWCNT/PoPD ratios were defined in a QCM study. The experimental conditions optimized with the QCM study were used as foundation for the preparation of electrodes to be tested in two-electrode Swagelok cells. SWCNT/PoPD (1:5) electrodes were tested as cathodes where Zn metal was used as anode in an aqueous electrolyte. The cell exhibits areal capacitance of 38.5 mF•cm -2 at 0.73 mA•cm -2 and gravimetric capacitance of 350 F•g -1 at a current density of 7.3 A•g -1 , as well as long lifetime of more than 11 000 cycles. Thus, this device structure with a battery-type anode and a capacitortype cathode is highly promising, may provide new ideas for developing high-performance microsupercapacitors and also shows the potential perspective of the conducting polymer based nanocomposite electrodes for miniaturized electronics.

In chapter V, the performances of ternary nanocomposite based on CNF, PpPD, and NiPs Pendant le processus de charge, les charges sont stockées aux deux interfaces électrode / électrolyte en fonction de la charge des électrodes. Pendant le processus de décharge, les charges stockées provoquent un mouvement des électrons dans le circuit externe fournissant de l'énergie électrique (Figure 1). Selon le mécanisme de stockage de charge, on peut distinguer deux types de supercondensateurs: (i) les condensateurs électriques à double couche (ELDC), dans lesquels les charges sont stockées par adsorption électrostatique des ions aux interfaces électrode / électrolyte. (ii) Des pseudocondensateurs dans lesquels les charges sont stockées par des réactions redox réversibles et rapides entre électrodes et électrolyte.

Préparation de l'électrode

La microbalance à quartz nous permet de suivre les variations de masse en fonctions du potential appliquée pendant l'électropolymérisation. Les variations de fréquence du résonateur du quartz au cours de l'électropolymérisation ont été converties en variations de masse correspondantes à l'aide de l'équation de Sauerbrey (∆𝑓 𝑚 = -𝑘 𝑠 . ∆𝑚) avec

Ac-électrogravimétrie

Les mesures d'ac-électrogravimétrie ont été effectuées à différents potentiels de -0,05 V à -0,25 V vs Ag/AgCl avec une amplitude de 50 mV. Ces plages de potentiel correspondent à la plus grande pente de la courbe de la variation de masse des données de l'EQCM présentées sur la Cette comparaison apporte un soutien à la contribution multi-espèces révélée par l'acélectrogravimétrie et met en évidence la complémentarité de l'EQCM et de l'acélectrogravimétrie.

On peut souligner que le processus de compensation de charge de PoPD dans des électrolytes aqueux (dans du NaCl acidifié) se produit avec la participation de plusieurs espèces, chacune jouant un rôle à différentes échelles temporelles. Bien que la contribution gravimétrique de H + n'est pas significative, mais sa contribution au mécanisme de stockage de charge est mise en évidence, à savoir (i) plus de dix fois par rapport à la variation de la concentration de Na + (Figure

Les performances pseudo-capacitives de PoPD

Les performances de stockage de charge du film de PoPD ont été mesurées dans une solution de NaCl 0,5 M à pH=2. La capacité spécifique peut être calculée à partir des courbes CV en utilisant l'équation suivante : précédents sur les polymères conducteurs [14][15][16] .

Figure I. 1 .

 1 Figure I.1. Schematic representation of a supercapacitor in a charged and discharged state.

  . The Helmholtz model is shown in Figure I.2 (a). The capacitance can be measured according to the general capacitance Equation I.1: 𝑪 = 𝑨 × Є 𝟎 𝒅 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰. 𝟏)where C denotes the capacitance measured in farads, A denotes the surface area, Є 0 denotes the permittivity of free space and d denotes the effectual width of the electric double layer also termed as Debye length.

Figure I. 2 .

 2 Figure I.2. Proposed mechanisms for charge storage in electric double layer capacitors: (a) the Helmholtz model, (b) the Gouy-Chapman model and (c) the Stern model, showing the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP) 21 . However, the diffusion of ions has not been taken into account in the Helmholtz EDL model. Consequently, Gouy and Chapman have been modified the simple Helmholtz EDL model considering the existence of diffuse a layer (Figure I.2 (b)), in which the charge is distributed to

  1948, Grahame improved the Stern model by dividing the compact layer into an inner Helmholtz plane (IHP) and an outer Helmholtz plane (OHP)). IHP is formed by solvent molecules or specifically adsorbed ions (non-solvated very close to the interface). OHP is composed of solvated ions closest to the electrode interface (Figure I.2 (c)). The total capacitance of the electrode can be represented by combination of the capacitance C H due to the Helmholtz layer and the component C d due to the diffusion layer:

  suggested a basic asymmetric hybrid supercapacitor consisting of the positive electrode of activated carbon (EDLC) and negative electrode of Li 4 Ti 5 O 12 (Faradaic electrode) in an organic electrolyte in which, the EDLC material supplies high power density while the pseudocapacitive material imparts the high energy density. The combination between carbon electrode (of supercapacitor type) and a faradaic electrode (of battery type) is shown in Figure I.3.

Figure I. 3 .

 3 Figure I.3. Concept of hybrid supercapacitors. Hybrid supercapacitors are comprised of

Figure I. 4

 4 Figure I.4 shows that the entire cell is equivalent to two capacitors in series. The overall capacitance (C T ) of the entire cell can be calculated using the Equation I.3,

Figure I. 4 .

 4 Figure I.4. Supercapacitor assembly principle and equivalent electrical circuit 58 .

  𝑽 𝟐 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰. 𝟓)

76 Figure I. 6 .

 766 Figure I.6. Specific capacitance of various electrode materials reported in the literature

Figure I. 7 .

 7 Figure I.7. Common methods for graphene production. Each method has been evaluated in terms of graphene quality (G), cost aspect (C; a low value corresponds to high cost of production), scalability (S), purity (P) and yield (Y) 86 .c. Carbon nanotubes (CNTs)Carbon nanotubes (CNTs), one-dimensional materials, have been widely studied as electrode materials for supercapacitors over the last years 100-103 , due to their high electrical conductivity, excellent mechanical, chemical and thermal stability, and unique tubular structure12 . Based on the number of the graphitic layers, CNTs can be categorized into two subgroups: Single-Walled Carbon Nanotubes (SWCNT) and Multi-Walled Carbon Nanotubes (MWCNT). SWCNTs consist of a single graphite sheet seamlessly wrapped into a cylindrical tube, whereas MWCNTs comprise an array of such nanotubes that are concentrically nested like rings of a tree trunk 104 .

  1974, the chemist Hideki Shirakawa from Tokyo Metropolitan University in Japan discovered accidentally the first intrinsic conductive polymer (ICP). It was a polyacetylene film (PAc). To polymerize its ICP material, Hideki Shirakawa had used Ziegler-Natta catalyst. After this discovery, he collaborated with the chemist Alan G. MacDiarmid and the physicist A. J. Heeger from the University of Pennsylvania in the United States. Together, they have increased the conductivity of the doped PAc from 10 -9 S.cm -1 to 10 2 S.cm -1 . 26 years later (in 2000), H.Shirakawa, A.G. MacDiarmid and A. J. Heeger won a Nobel Prize in Chemistry for the discovery and development of CP 123 . This discovery has attracted the attention of many scientists to synthesis other CPs such as polyacetylene, polyaniline, polypyrrole polythiophenes, which are more stable than polyacetylene. Conducting polymers have very interesting physicochemical and electrochemical properties, new perspectives in the field of materials and modified electrodes. Moreover CPs present many properties such as good mechanical properties, high conductivity, resistance to corrosion, and low cost. The conductivity of CPs is due to the presence of conjugated single and double bonds along the polymer skeleton. CPs can be either p-doped with counter anions upon oxidation or ndoped with counter cations upon reduction. These two charge/discharge processes are expressed as following 38 : 𝐶𝑃 ↔ 𝐶𝑃 𝑛+ + (𝐴 -) 𝑛 + 𝑛𝑒 -(𝑝 -𝑑𝑜𝑝𝑖𝑛𝑔) (𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏) 𝐶𝑃 + 𝑛𝑒 -↔ (𝐶 + ) 𝑛 𝐶𝑃 𝑛-(𝑛 -𝑑𝑜𝑝𝑖𝑛𝑔) (𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟐)

Figure I. 8 .

 8 Figure I.8. Structure of the three main conducting polymers

4 . 3 .

 43 Transition metal oxides (TMOs)Transition metal oxides are promising electrode materials (Figure I.6) used to increase the capacitance stored in supercapacitors and maintain a high power density109,133,134 . TMOs store energy not only electrostatically but also display redox reactions within appropriate potential windows135 . Indeed, they have a large number of oxidation states and can be prepared with a large specific surface area. Their number of oxidation states is in general very important for the charge storage behavior, the high number of oxidation states, resulting in a high number of possible redox reactions which offers a high charge storage capacity 136 . Ruthenium oxide (RuO 2 ) is one of the most commonly studied metal oxides as supercapacitor electrode materials and has a relatively high theoretical specific capacitance of about 1350 F.g -1

Figure I. 9 .

 9 Figure I.9. Electrochemically synthesized and vertically aligned carbon nanotube/polypyrrole film 157 . In other study, Han et al. 142 synthesised PPy on He plasma etched CNT film (HCNTF) by an electrodeposition method, the results shows that the He plasma etching can produce amorphous carbon on the CNT film and improve the hydrophilicity of CNT film which increase the adhesion between PPy and CNT film leading to improve the specific capacitance (414 F/g) and the cycling stability (retention of about 92% of the initial specific capacitance after 5000 cycles). Roy et al. 158 reported a composite based on PANI and MWCNT synthesized by chemical polymerization. The presence of 10% of MWCNT shows a better specific capacitance and the MWCNTs are wrapped with PANI. The role of MWCNTs is to enhance the mechanical stability and to make interlinked porous structure in order to improve the active surface area, while PANI polymer plays the main role in charge storage through a faradic process. Malik et al. 146 deposited PANI on nitrogen doped vertically aligned carbon nanotubes (NCNT) by electrodeposition. The composite was used as electrode for supercapacitor and shows good performances, which are related to many factor: (i) the rapid electron transport between CNT and PANI caused by the formation of a CNT core and PANI shell; (ii) the open porous array structure of the composite allows fast diffusion of ions maintaining high ionic conductivity within the electrode. In microsupercapacitors (MSCs), recently Tahir et al. 110 reported a high-performance MSCs based on PEDOT-coated MWCNT nanoporous network. This composite-based electrode shows a

  167 report a new approach to synthesize three-dimensional nitrogen-doped graphene (3DNGN) from PoPD by carbonization. The 3DNGN shows an interconnected porous structure with large surface area and high content of nitrogen doping, which leads to improve the wettability of the material and large electrolyte-accessible surface area resulting in high capacitive performance in aqueous electrolyte. Yang et al.148 deposited PoPD on graphene via in situ polymerization, and tested the electrode material for supercapacitors. A good synergistic effect was noticed leading to a high specific capacitance of about 381 F/g in 1M H 2 SO 4 electrolyte at 1 A/g and a retention of 90%

Fuel cell 1 . History and generality 1 . 1 .

 111 HistoryThe concept of fuel cell was demonstrated by Sir Humphry Davy in the early nineteenth century. In 1838, C.F. Schönbein discovered the fuel cell principal by using hydrogen and oxygen. Thereafter, W.R. Grave made the first fuel cell at low temperature using sulfuric acid as electrolyte and platinum electrodes. After 1945, Unite States, Germany and URSS have used fuel cell to develop generators for industrial applications. During the 1960s, NASA was focalised on the development of fuel cell generators dedicated for manned space missions. After the outbreak of the Yom Kippur War in October 8, 1973, the OPEC ''Organization of the Petroleum Exporting Countries'' imposed an oil embargo and increased the price of oil from 2.90 dollars/barrel to 11.65 dollars/barrel, which forced governments to embrace the concept of energy efficiency. General Motors was marked one of the earliest road-going electric vehicle powered by fuel cell. During the 1960s, Shell developed direct methanol fuel cell (DMFC) using a liquid fuel, which was considered as a great advantage for vehicles applications. After that, the word's major carmakers, especially from Germany, USA and Japan began to develop hydrogen fuel cell systems for electric vehicles, and the focus had shifted back to hydrogen fuel, which generates no harmful exhaust emissions. In 1990s, fuel cell was used for small stationary applications such as residential micro-CHP and backup power for telecoms sites. Then, California Air Resources Board (CARB) introduced the Zero Emission Vehicle (ZEV), and several carmakers such as General Motors, Toyota and DaimlerChrysler have invested in Proton exchange membrane fuel cell (PEMFC) and conducted a scientific research to further develop this technology. The fuel cells commercialization (PEMFC and DMFC) started in 2007 for different applications such as boats and campervans. Moreover, a large number of micro fuel cells were sold in the portable sector. In past decade, the uptake of fuel cell by consumers was facilitated and PEMFC/DMFC have dominated the portable, stationary and transport sectors.Figure I.10 summarizes the history of fuel cell.

Figure I. 10 .

 10 Figure I.10. History and development of fuel cell

Figure I. 11 .

 11 Figure I.11. Different types of fuel cell The usage of different type of fuel cells depends on their performance, costs, and typical applications. PEMFCs shows the highest power density and fastest start-stop capacity. In Direct Liquid fuel cells (DLFCs), liquid fuel such as methanol, ethanol, formic acid, etc, are used as electrolyte. Compared to H 2 -PEMFC, DLFCs have a high theoretical energy density and easy storage, transportation of liquid fuel. Actually, H2-PEMFC, DMFC and Solid Oxide Fuel Cell (SOFC) are the most common fuel cell types. However, each one shows advantages and disadvantages (Table I.7).

3. Direct methanol fuel cells (DMFCs) 3 . 1 .

 31 DMFC components.The Figure I.12 represents the principle of DMFC. It mainly composed of an anode current collector (ACC), cathode current collector (CCC) and membrane electrode assembly (MEA). The MEA consists of anode diffusion layer (ADL), anode catalyst layer (ACL), polymer electrolyte membrane (PEM), cathode catalyst layer (CCL), and cathode diffusion layer (CDL). Small holes are provided on the anode end plate for fuel injection and carbon dioxide exhaust. To provide a perfect sealing, gaskets are used between the cell components. ACL and CDL are used as a porous surface for adhering the catalyst layers, electron conducting to the current collector, uniform distribution of reactants on the catalyst layer. PEM, separator between anode and cathode, permits the flow of protons from the anode side to the cathode side and restricts the passage of electrons. The most currently used PEM is Nafion, a perfluorosulfonic acid polymer, due to its high proton conductivity and thermal/chemical stability. There are several types of Nafion membranes based on their thickness. However, the passage of methanol through the Nafion membrane is the main problem of DMFC, which reduces its performance.

Figure I. 12 .

 12 Figure I.12. Schematic of DMFC 168 .

Figure I. 13 ,

 13 Figure I.13, the oxidation of methanol can follows several ways to produce carbon dioxide as the final product. The good way of working is to obtain stable compounds, as formaldehyde (CH 2 O) and formic acid (HCOOH). Moreover, the oxidation of methanol may generate carbon monoxide (CO) as an intermediate product, which play an important role in poisoning of Pt catalyst by occupying the active sites.

Figure I. 13 .

 13 Figure I.13. Methanol oxidation mechanism 170

Figure I. 15 .

 15 Figure I.15. TEM images of Pt/C (a), Pt3Ni1/C (b), Pt1Ni1/C (c), Pt1Ni3/C (d), and Acid-Pt1Ni1/C (e), respectively 191 . Recently, Wang et al. 192 reported a ternary PtRuCu aerogels using one-step in situ reduction (Figure I.16). PtRuCu aerogels was tested for the methanol oxidation reaction in alkaline media and a high catalytic effect was obtained. The interconnected porous 3D structure of this catalyst promote the mass and electron transfer and slow Ostwald ripening. The addition of oxophilic metals (Ru and Cu) to Pt could adsorb oxygen-containing species and improve the catalytic performance. In addition, Ru and Cu can control a permanent d-band filling via ligand effect and decrease the d-bond energy of Pt.

Figure I. 16 .

 16 Figure I.16. Schematic illustration of the synthesis of PtRuCu hydrogels 192 .

Figure I. 17 .

 17 Figure I.17. Scanning electron microscopic images of (a) Pt-Ru/C, (b) Ni/PPy and (c) Ni/SPPy 195 .

Figure I. 18 .

 18 Figure I.18. Activation process of Ni in NaOH solution using CV. Wang et al. 200 synthesized Ni nanoparticles supported on nitrogen-doped honeycomb-like carbon framework (CNFs) using a low cost and scalable method. The synthesized catalyst was used for methanol oxidation in alkaline media. A good catalytic performance was obtanied due to the presence of CNFs which provide anchoring sites for active nickel nanoparticles. The incorporation of nitrogen into CFs increases the electronic effect and contributes to improve the activity of the catalyst. In other research, Zhu et al. 201 reported a three-dimensional porous graphene (3D-G) supported Ni nanoparticles for methanol electrooxidation. Good catalytic performance was obtained by 3D-G with abundant pore architecture, which hinder the agglomeration of Ni catalyst and offer readily accessible channels for the diffusion of methanol to the active sites of catalyst surface.

  CNTs and CNFs have been brought several advantages such as an improve the efficiency of the catalyst and present an electronic properties, which are highly desirable for reducing catalyst loadings thereby lowering the overall fuel cell cost 208 . Carbon nanofibers (CNFs) CNFs have been studied from both fundamental and applied points of view. CNFs have been largely used as support for DMFCs. As shown in the Figure I.19, there are three types of CNFs: ribbon-like CNFs, platet CNFs and herringbone CNFs depending on the orientation of the nanofibers with respect to the growth axis 209 .

Figure I. 19 .

 19 Figure I.19. Schematic illustration of : platet CNFs, ribbon-like CNFs, herringbone CNFs 209 . Calderon et al. 210 studied Pt-Ru support on CNFs for DMFC. Pt-Ru/CNF shows a higher catalytic performance compared to the commercial Pt-Ru/C E-TEK catalyst which was found to be the most efficient in the conversion of methanol to CO 2 . They suggested that CNF support exhibits an excellent CO tolerance and high catalytic activity toward MOR. In other work, Mu et al. 211

  polypyrrole. PANI prepared by potential cycling yields to a good-quality film. In DMFC, the use of PANI as support material can reduce the poisoning of Pt by CO. Das et al.221 deposited Ni catalyst on PANI and partially sulfonated Pani (SPANI). In comparison with the commercial Pt-Ru/C catalyst, Ni/Pani and Ni/SPANI shows better catalytic effect for MOR. Especially, SPANI emerged as the best support for higher deposition of Ni catalyst with a better dispersion and distribution and small size. Composite support Various hybrid materials have been used as support for metal catalysts in order to enhance their catalytic performances. The preparation of porous hybrid composites based on carbon and conducting polymer can exhibit a special properties that cannot be attained by the individual components. The carbon support plays an important role to enhance the dispersion and the performance of the metal by providing an underlying framework for facile electron conduction. However, carbon support suffers from the corrosion and dissolution, which decrease its efficiency. The deposition of conducting polymer on a carbon support can improve the electrochemical and mechanical stability of the catalyst and exhibits good tolerance to CO poisoning during the methanol electrooxidation 208 . Recently, Arukula et al. 222 reported the synthesis of rGO/PANI composite support for Pt-Pd alloy catalyst for methanol electrooxidation. PANI deposited on the surface of rGO leads to the uniform scattering of Pt-Pd, which benefits for adsorption and decomposition of methanol molecules. In other study, Eswaran et al. 223 prepared Palladium nanoparticles (PdNPs) on graphitic carbon nitride/PANI composite by onestep preparation (see Figure I.20). The catalyst shows superior electro-oxidation and good catalytic performance towards MOR compared to commercial catalysts.

Figure I. 20 .

 20 Figure I.20. Scheme representation of the preparation of the graphitic carbon nitride/PANI/PdNPs electrode for MOR 223 . PpPD is a conducting polymer having the required chemical attributes and provides a porous network including a high number of amine groups. The latter can improve the deposition conditions and thus, the electronic properties of the metal for catalytic conversion 224 . The potential of PpPD has been recognized and therefore combined with rGO or multiwalled carbon nanotubes (MWCNTs) as an ideal precursor to enhance the catalyst activity and durability for MOR 225,226 . Wu et al. 225 reported Pt-Ru nanoparticles (Pt-RuNps) supported on PpPDfunctionalized multiwall carbon nanotubes (MWCNTs). The results indicated that PpPD helped inhibit the agglomeration and the migration of the Pt-RuNps by coordination with the amine groups (Figure I.21). Besides, PpPD immobilized on MWCNTs by π-π stacking providing a good composite support for an effective attachment and dispersion of PtRuNps.

Figure I. 21 .

 21 Figure I.21. TEM images of Pt-RuNPs/PPDA-MWCNTs (a, b and c) and Pt-RuNPs/MWCNTs: (d, e and f) 225 .

Figure II. 1

 1 shows a schematic description of a SEM set-up.

Figure II. 1 .

 1 Figure II.1. Simplistic view of a scanning electron microscope in operation 2 .

  Figure II.2 shows a schematic presentation of a TEM set-up.

Figure II. 2 .

 2 Figure II.2. Schematic description of a TEM set-up 2 .

  of the sample and usually coupled to SEM or TEM techniques. 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 (Figure II.1). 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 xrays versus their energy. This qualitative EDX spectrum is captured electronically for display and processing (Figure II.3).

Figure II. 3 .

 3 Figure II.3. Simplistic view of an EDX detector in operation 5 .

Figure II. 4 .

 4 Figure II.4. Schematic cross section of an X-ray tube 7 .

Figure II. 5 .

 5 Figure II.5. Illustration for the Bragg's law 9 .

Figure II. 6 .

 6 Figure II.6. Schematic description of a XPS set-up 5 .

Figure II. 7 .

 7 Figure II.7. The FTIR spectrophotometer block diagram 17 .

Figure II. 8 .

 8 Figure II.8. Cyclic voltammogram curves of (a) ideal EDLC, (b) EDLC, and (c) pseudocapacitive materials 20 . As shown in Figure II.8, the rectangular CV represents an ideal supercapacitor response (Figure II.8 (a)). However, in practice, a more or less deformed rectangular shape is obtained due to contact resistance and electrolyte resistance in the pores (Figure II.8 (b)). Two redox peaks are observed in Figure II.8 (c), caused by faradic reactions, which are characteristic for pseudocapacitive behavior.

ZFigure II. 10 .

 10 Figure II.10. Typical equivalent circuit used to fit EIS data and the resulting impedance Nyquist diagram. Adapted from 23 . From the Nyquist diagram, qualitative and quantitative information can be obtained on the electrode electrochemical behavior. An equivalent series resistance (ESR) R e can be extracted from the point of intersection between the impedance curve and the abscise axis (where Im(Z)=0) (Figure II.10). The slope of the low frequency line is linked to the diffusion of a certain ion/molecule in the solution. Diffusion results in a deviation of the experimental data from the vertical line. The imaginary part of the impedance can be used to estimate the frequency dependence of the capacitance with the following equation 24 :

Figure II. 11 .

 11 Figure II.11. Schematic GCD response of EDLC (a) and hybrid supercapacitor (b) 25 . From the slope of the discharge curve shown in Figure II.11, the cell capacitance (C) can be calculated using the following equation:

Figure II. 12 .

 12 Figure II.12. Experimental set-up for EQCM measurements. The gravimetric measurements were performed based on a quartz crystal microbalance (QCM) working with the well known piezoelectric effect.Figure II.13 schematically illustrates the piezoelectric effect through the generating of an electric charge as the result of a force applied on the piezoelectric material 29 . The gravity centers of the negative and positive charges of each molecule coincide before the material subjected to the external force (Figure II.13 (a)), resulting in the reciprocal cancellation of the external effects from negative and positive charges; upon an external force, dipoles can be formed because of the separation of the positive and negative gravity centers of the molecules (Figure II.13 (b)); Then, an electric field appears in the material (FigureII.13 (c)). Contrarily, when a voltage is applied on a piezoelectric material, a corresponding mechanical deformation would be produced, which is called reverse piezoelectric effect and serves as the fundamental principle for the microbalance operation.

Figure II. 13 .

 13 Figure II.13. Simple molecular model for explaining the piezoelectric effect: (a) unperturbed molecule; (b) molecule subjected to an external force, and (c) polarizing effect on the material surfaces 29 . Levi et al. 30 have studied the effect of specific adsorption of cations and their size on the chargecompensation mechanism in carbon micropores in 0.5 M NH 4 Cl solution using the EQCM technique. This study showed that the EQCM technique reveals a complicated interplay between the adsorption of NH4 + cations and desorption of Cl -anions into 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 II.14).

Figure II. 15 .

 15 Figure II.15. The CV curves (A), the simultaneous mass responses (B) and the FΔm/Δq function (C) of the SWCNT based thinfilms in 0.5 M NaCl at pH=7 31 .In the present Ph.D. thesis, a gold (0.2 cm 2 ) coated quartz resonator with a diameter of 14 mm (9 MHz-AWS, Valencia, Spain) is used. The electrochemical performances of different electroactive materials-loaded quartz resonator were characterized by EQCM. In this case, the QCM was coupled with CV measurements to track the simultaneous frequency shift (Δf) of the quartz resonator electrode during the electrochemical process, which can be converted into mass response (Δm) according to Sauerbrey equation 32 :

Figure II. 16 .

 16 More precisely, the microbalance frequency signal is translated into the voltage signal in frequency/voltage converter and sent to the Frequency Response Analyzer (FRA, SOLARTRON 1254), as shown in Figure II.8. The electrogravimetric transfer function (TF), ∆𝑚 ∆𝐸 (𝜔) can be simultaneously obtained with the electrochemical impedance, ∆𝐸 ∆𝐼 (𝜔). It should be noted that these two TFs are not directly obtained. The raw TFs, ∆𝑉 𝑓 ∆𝑉 (𝜔) and ∆𝑉 ∆𝐼 (𝜔) are directly measured by the FRA from which the ∆𝑚 ∆𝐸 (𝜔) and ∆𝐸 ∆𝐼 (𝜔) are obtained after a numerical treatment in Mathcad software.

Figure II. 16 .

 16 Figure II.16. Experimental set-up for ac-electrogravimetry measurements 33 .

  TF, which consider the ionic species transfer, the ∆𝑚 ∆𝐸 (𝜔) TF also takes the free solvent contribution into account. It should be noted that the microbalance frequency change, ∆𝑓 𝑚 , is generally around tens of hertz while the carrier frequency, 𝑓 𝑚 , is around 9 MHz.

Finally

  were measured by the FRA, which leads to the electrogravimetric TF ( ∆𝑚 ∆𝐸 (𝜔)) and the electrical TF ( ∆𝐸 ∆𝐼 (𝜔)) to be obtained simultaneously at a given potential and frequency modulation, f (pulsation 𝜔 = 2𝜋𝑓). Based on the electrochemical impedance ( ∆𝐸 ∆𝐼 (𝜔)), the experimental charge/potential ( ∆𝐸 ∆𝐼 (𝜔)) TF can be calculated in order to have a clearer view on the ionic species involved in the electrochemical process: . 𝟏𝟎)Correspondingly, from the experimental electrogravimetric TF, ∆𝑚 ∆𝐸 (𝜔), several partial mass/potential transfer functions can also be calculated by removing one of the participants in the charge compensation process. For example, if four species are involved in the electrochemical process: cation 1 (c 1 ) with valence Zc 1 , cation 2 (c 2 ) with valence Z c2 , anion (a) with valence Za and solvent (s), the different experimental partial mass/potential TFs can be estimated by extracting contributions from anion, cation 1 and cation 2, respectively, which can be expressed as following equations: 𝑰𝑰. 𝟏𝟑)

𝑡ℎ

  TFs are shown schematically in Figure II.17. For a single ion transfer at the electrode/electrolyte interface, the ∆𝐸 ∆𝐼 (𝜔)| 𝑡ℎ TF (Figure II. 17 (a)) shows a loop at high frequency corresponding to the charge transfer resistance, followed by a straight line at low frequency indicating the transfer of one ion. The ∆𝑞 ∆𝐸 (𝜔)| 𝑡ℎ TF (Figure II. 17 (b)) displays a single loop related to the charge compensation process (cation or anion). It should be noted that this loop is always in the same quadrant. In the case of ∆𝑚 ∆𝐸 (𝜔)| 𝑡ℎ TF (Figure II. 17 (c)), a typical anion and cation contribution appear characteristically on the I and the III quadrant (Cartesian system), respectively.

Figure II. 17

 17 Figure II.17 Simulations of some of the Transfer Functions (TFs) for ∆𝐸 ∆𝐼 ⁄ (𝜔) (a, d), ∆𝑞 ∆𝐸 ⁄ (𝜔) (b, e) and ∆𝑚 ∆𝐸 ⁄ (𝜔) (c, f). The response of one and two charged species was depicted in (a, b, c) and (d, e, f), respectively. Note: H 2 O contribution was not taken into account here. In these representations, the time constants of ionic species are different enough so that they are seen as separate contributions 36 .

  between 63 kHz ad 10 mHz. The mass change, ∆m, of the quartz electrode was measured simultaneously with the ac response, ∆I, of the electrochemical system. The resulting signals were sent to a four-channel FRA, which permitted the electrogravimetric transfer function (TF) and the electrochemical impedance to be simultaneously obtained at a given potential. The main TFs (electrogravimetric TF (

  Figure III.1 (a) shows the CV curves during the formation of the PoPD thin film on the gold electrode of the quartz crystal. This specific substrate was chosen to be able to perform electrogravimetric studies thereafter (Figure III.1 (b)).

Figure III. 1 . 42 Figure III. 2 .

 1422 Figure III.1. PoPD thin film synthesis by electropolymerization on a gold electrode of a quartz resonator in an aqueous solution of H 2 SO 4 (0.1 M) with oPD (5 mM). (a) CV Curves, (b) corresponding mass variations (m) obtained by EQCM at a scan rate of 50 mV•s -1 .

  A continuous mass increase was observed (only the first 5 cycles are shown in Figure III.1 (b)) which resulted in a 22 µg.cm -2 of total deposited mass after typically 200 CV cycles. Figure III.3 shows the FEG-SEM image of a representative PoPD film deposited on the gold electrode of a quartz resonator.The figure reveals the full coverage of the gold surface with the PoPD film which has an average thickness of 150 nm. The electrogravimetric response of the resulting PoPD film will be investigated below at the section 3.2.1. One can consider that a pure gravimetric response can be obtained as the film is acoustically thin enough (150 nm).43

Figure III. 3 .

 3 Figure III.3. FEG-SEM image of a PoPD thin film on the gold electrode of a quartz resonator after 200 CV cycles. 2. Electrogravimetric characterization of charge storage behavior. 2.1. Electrochemical Quartz Crystal Microbalance (EQCM).

Figure III. 4

 4 Figure III.4 shows the EQCM responses of PoPD thin films in 0.5 M NaCl solution at different pH values.

Figure III. 4 .

 4 Figure III.4. (A) CV curves and (B) the corresponding mass variations (m) of a PoPD film electrodeposited on a gold coated quartz resonator. Measurements are performed in 0.5 M NaCl, at three different pH values (pH=2, 7 and 12) and at a scan rate of 50 mV•s -1 .

Figure III. 5 ( 94 Figure III. 5 .

 5945 Figure III.5. (A) CV curves, (B) Corresponding mass variation and (C). 𝐹 ∆𝑚 ∆𝑞 function of PoPD film deposited on the gold coated quartz resonator measured in 0.5 M NaCl, with different scan rates, at pH=2.

Figure III. 5

 5 (C)presents the MPE of the species involved during EQCM at 25 mV•s -1 (calculated for oxidation and reduction), presenting a value ranging from -10 to -5 g•mol -1 , respectively. From these two values and the MPE of Na + , Cl -and H + of -23 g•mol -1 , 35 g•mol -1 and -1 g•mol -1 , respectively, it is more likely that a multi-ion contribution in the charge compensation process is present. Earlier

  i) Identification of the species contributing to the charge compensation: Ac-electrogravimetric measurements were carried out at various states-of-polarization from -0.05 V to -0.25 V vs Ag/AgCl with an interval of 50 mV. These potential range corresponds to the highest slope of the mass-potential curve in the EQCM data shown in Figure III.6 (A). Two important transfer functions (TF), charge/potential TF (Δq/ΔE()) (derived from impedance, ΔE/ΔI()) and mass/potential TF (Δm/ΔE()

Figure 7 (Figure

 7 Figure III.7 (A) and (B)).

Figure III. 6 . 1 . 1 .Figure III. 7 .

 6117 Figure III.6. (A) EQCM curve and (B to E) ac-electrogravimetric data shown at two selected potentials. The ∆𝑞 ∆𝐸 (𝜔) transfer function at (B) -0.05V and (c) -0.25V (C); and ∆𝑚 ∆𝐸 (𝜔) transfer function at (D) -0.05V and (E) -0.25V of the PoPD film measured in 0.5M NaCl solution at pH=2. The fitting parameters are given in Table III.1.

Figure III. 6 (

 6 Figure III.6 (D) and (E) present the experimental and theoretical Δm/ΔE() TFs which show several well defined semi-circles appearing at different quadrants. It is important to note that the contributions, in our study, in the first/ second and the third/fourth quadrant are due to the

Figure III. 6 ( 5 MTable III. 1 : 8 .

 6518 B) and (C)) are used in equation II.17 (see chapter II).Notably, the identification of the participants in the charge compensation process can be achieved thanks to the molar mass (M i ) of the species intervening in electrogravimetric TF (Δm/ΔE()) (Equation II.17). The fitting of the experimental Δm/ΔE() data (measured in 0.NaCl and at pH=2) using theoretical function in Equation II.17 revealed a configuration involving the transfer of anions (Cl -), cations (H + and Na + ) and free H 2 O molecules which leads a good agreement between experimental and theoretical data (Figure III.6 (D) and (E)). Estimated values for K i and G i (the inverse of the transfer resistance), Rt i and f i parameters are extracted from the fitting results of ac-electrogravimetry measurements of the PoPD film at -0.05 V and -0.25 V vs Ag/AgCl, measured in 0.5 M NaCl at pH=2. Correlation between the interfacial ion dynamics and charge storage properties of poly(ortho-phenylenediamine) electrodes exhibiting high cycling stability Chapter III 98 In order to further confirm the presence of the four different species identified in the Figure III.6 at -0.05 V and -0.25 V, the partial electrogravimetric transfer functions have been reported in Figure III.Using the partial electrogravimetric transfer functions (equations II.19, II.20, II.21 in chapter II), the number of different species involved in the charge compensation process can be evaluated by the elimination of the contribution of one charged species. Thus, the proposed model for the ions transfer mechanism of PoPD can be controlled. As shown in Figure III.8 for the both potentials -0.25 V (Figure III.8 (A, C and E)) and -0.05 V (Figure III.8 (B, D and F)), all the transfer functions show a good agreement between the experimental and the theoretical curves, which confirm our hypothesis about the nature, kinetics and concentrations of different identified species.

Figure III. 8 .

 8 Figure III.8. Ac-electrogravimetry data shown at two selected potentials.∆𝑚 ∆𝐸 | 𝑐2𝑎𝑠 (𝜔) TF at (A) -

Figure III. 9 .

 9 Figure III.9. Variation of the (A) transfer resistance (Rt i ) and (B) characteristic frequency (f i ) for PoPD film deposited on the gold coated quartz resonator, measured in 0.5 M NaCl at pH=2. The f i and Rt i values are given in Table III.1.

  III.9 (A) and (B)). In the potential range studied, the tendency of f i (Cl -) > f i (H 2 O) > f i (H + ) > f i (Na + ) is observed (Figure III.9 (A)) presenting the order of the kinetics of transfer of the species which is inversely proportional to their resistance of transfer, Rt i , at most of the potentials shown in Figure III.9 (B). The multi species contribution with different interfacial transfer kinetics revealed in acelectrogravimetry is in good agreement and also strengthens the ideas discussed previously to explain the scan rate dependent mass-potential curves in EQCM (Figure III.5 (A) and (B)). In a simplistic discussion, the redox behaviour of PoPD thin film can be compensated either by insertion of anions or expulsion of cations during oxidation (vice versa during reduction). In the EQCM data in Figure III.5 (B), during oxidation at high scan rates (100 mV•s -1 ), a mass increase Correlation between the interfacial ion dynamics and charge storage properties of poly(ortho-phenylenediamine) electrodes exhibiting high cycling stability Chapter III is followed by a subsequent mass diminution. This observation may correspond to the insertion of anions which is followed by the expulsion of cations and/or water molecules by exclusion. Since ac-electrogravimetry results indicate the opposite flux direction of solvent molecules which appear at the following frequencies right after the anions (Figure III.6 (D) and (E)), it is more likely that anions insertion during oxidation excludes some water molecules which results in this subsequent mass decrease during oxidation (Figure III.5 (B)

Figure III. 10 .C

 10 Figure III.10. The ac-electrogravimetric results of the PoPD film deposited on the gold coated quartz resonator. (A) The variation of the relative concentration (Ci-C 0 ), (B) the relative mass variation of each species measured in NaCl 0.5M, pH=2 and (C) the reconstruction of total mass variation from ac-electrogravimetric results and comparison with the EQCM response at 25 mV•s -1 .

  Figure III.11 (A). For this analysis, the typical loading of 22 g•cm -2 was used, which corresponds to a film thickness of 150 nm estimated through FEG-SEM analysis (Figure III.3). The C s values were increased with a decrease in the scan rate and reached a maximum value of 420 F•g -1 at 50 mV•s -1 (Figure III.11 (A)). However, the C s values at the scan rates of 10 and 25 mV•s -1 are slightly lower and deviates from the usual scan rate dependence trend of the C s (Figure III.11 (A)). The behavior has previously been observed for thin conducting polymer or metal/metal oxide based electrode in aqueous electrolytes and deserves to be investigated.

Figure III. 11 . 1 A

 111 Figure III.11. Variation of the specific capacitance (Cs) of PoPD films deposited on gold-coated QCM and measured in NaCl 0.5M, pH=2. Cs is plotted (A) as a function of the potential scan rate (loading of 22 g•cm -2 ) and (B) as a function of the loading of the QCM resonator at 50 mV•s -1 . Then, in order to check the effect of the film thickness, several films were prepared by increasing the number of CV cycles during their electrodeposition. The specific capacitance (C s ) of the PoPD thin films is evaluated as a function of the loading (proportional to film thickness) at a scan rate of 50 mV•s -1 . As shown in Figure III.11 (B), C s values presents two different trends as a function of the loading (i.e. in the range of 7.5 -22 g•cm -2 and 22 g•cm -2 -55 g•cm -2 ). It starts with a markedly different behaviour of the 7.5 g•cm -2 loading (electrodeposited by 50 CV cycles), which shows the lowest C s value. Probably 50 CV cycles are not enough to form completely the PoPD structure (and therefore, the electrode surface is not fully covered (Figure III.12)), which is in agreement with the absence of well-defined redox peaks in the CV response shown in Figure III.11 (B) inset. Following this value, C s shows a significant increase in the range of 7.5 -22 g•cm -2 , reaching to values as high as  434 F•g -1 at 22 g•cm -2 .

Figure III. 13 Schema III. 1 .

 131 Figure III.13 presents the galvanostatic charge-discharge (GCD) performance of the PoPD electrode deposited on a graphite electrode. The mass loading and the thickness of the electrode is around 100 g•cm -2 and 700 nm, respectively. The mass loading is two times higher than that of the maximum mass deposited on Au coated quartz electrodes (Figure III.13 (B)). The current applied to the electrode is 0.63 mA•cm -2 (6.3 A•g -1 ) that is equivalent to 30 C current density. The test was performed in a 2 electrode cell in where Zn is used as both reference and counter electrodes (Schema III.1).

Figure III. 14 .

 14 Figure III.14. The FEG-SEM images of the fresh (A and B) and after 8000 cycles in Swagelok cells in 0.5M NaCl, pH=2 (C and D). The EDX spectra of the cycled electrode surface indicating the presence of the glass fiber residue originating from the separator used in Swagelok cells.

Figure III. 15 .Figure III. 16 .

 1516 Figure III.15. (Panel A) SEM images of the glass fiber separator (Whatman GF/C), Glass fiber separator (Whatman GF/C) (B) before and (panel C) after cycling.

Figure III. 11

 11 inset). Similar capacitance value (around 110 F•g -1

  3 A•g -1 ) accompanied with high coulombic efficiency for 8000 cycles, indicating the high rate capability and the excellent cycling stability of the PoPD electrodes of our study (Figure III.13 (C)). Furthermore, a more recent study evaluated the capacitive

  capacitive and pseudo-capacitive phenomena of PoPD (Figure III.13). As it is widely accepted, the cycling performance is one of the bottleneck in the field of conductive polymer based electrodes,5, 7, 18, 54 and has been tackled with, for example by the encapsulation of the CPs under a protective shell.11 Therefore, the performances obtained in our study with the pristine PoPD polymer electrodes are quite significant, especially taking into account its very long cycling stability (Figure III.13).

  candidates for supercapacitors, the capacitive performance of the SWCNT/PoPD nanocomposite were studied in 2-electrode and 3-electrode configurations. By determining the interfacial flux dynamics and as well as the relative proportions of species transferred at the electrode/electrolyte interface, our results contribute to the understanding of the charge-discharge process of SWCNT/PoPD nanocomposite, which has the desirable attributes for the energy storage applications.

  this work, a new electrode material for supercapacitor based on SWCNT/PoPD nanocomposite was fabricated by a simple and rapid strategy through electropolymerization of oPD on an electrode modified by a network composed of SWCNT and poly(vinylidene fluoridehexafluoropropylene) (PVDF-HFP). A dispersion of SWCNT/PVDF-HFP was deposited on both gold (for three-electrode cell tests) and graphite (for two-electrode cell tests) electrodes, followed by electropolymerization of oPD on them using cyclic voltammetry. Thereafter, the energy storage mechanisms were investigated in aqueous NaCl electrolyte via EQCM and ac-electrogravimetry.Several parameters were considered to explain the electrochemical behaviour of SWCNT/PoPD nanocomposite film, such as the nature and the dynamic of the ions transfer at the SWCNT/PoPD film/electrolyte interface. The composition and the morphological characterization of the nanocomposite were carefully investigated prior to the electrochemical tests. The capacitive performances of SWCNT/PoPD nanocomposite networks as electrode materials for supercapacitors were evaluated in aqueous NaCl electrolyte using both three and two-electrode cells. Different mass ratios between SWCNT and PoPD were tested in order to investigate the optimized SWCNT/PoPD composition in terms of capacitive performance. The resulting nanocomposite networks exhibit desirable (electro)chemical attributes for micro-energy storage devices26,45,46 . 90 wt. % of SWCNT powder (length: 300-2300 nm and diameter: 0.7-1.1 nm, Sigma-Aldrich) and 10 wt. % of PVDF-HFP polymer (poly(vinylidene fluoride-hexafluoropropylene), Sigma-Aldrich) were dispersed in NMP (N-methyl-2-pyrrolidone) (0.9 mg SWCNT and 0.1mg PVDF-HFP in 5 ml NMP). A 10µl portion of this dispersion was drop-cast on the gold electrode (0.2 cm 2 ) of quartz crystal resonators (9 MHz-AWS, Valencia, Spain) and on graphite plates (50 µl for 1 cm 2 ) and dried for 2h at 120 °C47 , this SWCNT/PVDF-HFP electrode is called SWCNT modified electrode thereafter. The electrochemical polymerization of oPD was performed on SWCNT modified electrodes (gold or graphite) using three-electrode configuration, in which, a SWCNT modified electrodes as working electrode, Ag/AgCl (3M KCl saturated with AgCl) as reference electrode and a platinum grid as counter electrode were used. The oPD monomer (1,2diaminobenzene, Sigma-Aldrich) (5 mM) was dissolved in H 2 SO 4 solution (0.1 M) and electropolymerized using cyclic voltammetry (CV) from -0.35 to 0.8 V vs Ag/AgCl at a scan rate of 50 mV•s -1 .In order to optimize the composition of SWCNT/PoPD nanocomposite, several mass ratio between SWCNT and PoPD have been studied. The mass ratio has been varied by increasing the number of the electropolymerization cycles of oPD, while the SWCNT mass was kept constant for all the ratios studied. The mass ratio was evaluated by measuring the resonant frequency of the quartz crystal (∆f), before and after the deposition of SWCNT and of PoPD on gold-coated quartz crystal resonators. The ∆f is then converted into the mass change (∆m) by the Sauerbrey equation (∆𝑓 𝑚 = -𝑘 𝑠 . ∆𝑚) where k z =16.31 Hz.g -1 .cm 2 is the experimental sensitivity coefficient48,49 . The mass ratio used for EQCM and ac-electrogravimetry measurements was maintained at 1:5 (SWCNT:PoPD), which also exhibited the best results (among the other ratios studied) in term of charge storage performances.

configurations in 0. 5 M

 5 NaCl salt containing electrolyte and pH was adjusted to 2 by the addition of HCl. For the three-electrode cell configuration, a platinum grid as a counter electrode, Ag/AgCl (3M KCl saturated with AgCl) as reference electrode and modified gold-patterned quartz substrates operating at 9 MHz (AWS, Spain) as working electrode were used. Electrogravimetric study of capacitive charge storage behavior of carbone nanotubes/ poly(ortho-phenylenediamine) nanocomposite: application in supercapacitors Chapter IV 120The ac-electrogravimetry measurements were performed using the same parameters descripted in Chapter III (electrode characterization part). Galvanostatic Charge/Discharge (GCD) measurements were performed in a two-electrode cell configuration, using a classical twoelectrode Swagelok-type cell, as described in Chapter III. Since the mass loading of the SWCNT/PoPD is 0.1 mg•cm 2 , the current applied during electrochemical reduction and oxidation is calculated to be 7.3 A•g -1 or 0.73 mA•cm -2 .

Figure IV. 1 (

 1 Figure IV.1 (a) schematically describes the preparation method of the nanocomposite electrodes. A SWCNT network is formed on a conducting substrate (Au electrode or graphite) and subsequently coated with PoPD by employing an electrochemical synthesis method already described in Chapter III (Figure IV.1 (b)). During their 1 st cycles, both electrodes show the oxidation peak of the oPD monomer around 0.6 V with a current density of about 2 mA.cm -2 .

Figure IV. 1 .

 1 Figure IV.1. (a) Schematic description of the SWCNT/PoPD nanocomposite electrode preparation, (b) polymerization of oPD on gold electrode and gold electrode modified by SWCNT network. The resulting films were characterized using FEG-SEM coupled to an EDX detector. Figure IV.2 shows the FEG-SEM images of (a) SWCNT, (b) PoPD and (c and d) SWCNT/PoPD composite deposited on the gold-patterned quartz substrates (for the EQCM and ac-electrogravimetry studies).Figure IV.2 (a) shows the surface morphology of the SWCNT thin film, displaying a

Figure IV. 2 (

 2 a) shows the surface morphology of the SWCNT thin film, displaying a network composed of bundles of SWCNT with an average size of about 10 nm 53 . The FEG-SEM image of PoPD film on gold electrode (Figure IV.2 (b)) shows a nodular structure on the surface which extends to a compact film with an average thickness of about 150 nm. In the Figures IV.2

(c) and IV. 2 (

 2 d), the SWCNT/PoPD composite images show a part of a network of SWCNTs integrated in the PoPD film. This morphology is expected to provide a high mechanical robustness and continuous pathways for electron transport. As it is evident in the cross-section image (Figure IV.2 (d)), the composite film with an average thickness of about 700 nm presents a highly porous structure and abundant presence of bundles made of SWCNTs, offering a high surface area for the electrochemical reactions and accessibility for the electrolyte species. The association of two components gives the possibility to benefit from both EDLC behaviour of the SWCNT and the faradic reaction of the PoPD.

Figure IV. 2 .

 2 Figure IV.2. FEG-SEM images of SWCNT (a), PoPD (b), SWCNT/PoPD nanocomposite (c and d) on the gold electrode of the quartz resonators. The SWCNT/PoPD composites were analysed by EDX coupled to FEG-SEM in order to determine the chemical composition. As shown in Figure IV.3, the intense C peak in the EDX spectrum indicates the presence of C related to the SWCNTs and PoPD. The presence of N and O confirm the formation of PoPD on the SWCNT film. In addition, the spectrum shows the presence of Au and Si of the gold electrode and the quartz resonator, respectively.

Figure IV. 3 . 2 .

 32 Figure IV.3. EDX spectrum of SWCNT/PoPD nanocomposite deposited on quartz-crystal gold electrode.

Figure IV. 4 (

 4 Figure IV.4 (a) shows the typical redox behavior of PoPD. However, compared to the CV curves of the PoPD (see Chapter III, Figure III.5) the cyclic voltammograms of SWCNT/PoPD present a higher current density caused by the capacitive behavior of SWCNT. The peak-to-peak separation increases with the increase of the scan rate from 10 to 100 mV.s -1 , due to a limitation in the charge transfer process. The simultaneous mass changes were reported as a function of potential and at different scan rates, in Figure IV.4 (b). Considering the curve at 10 mV.s -1 , a

  5 V. The latter complementary method has been shown beneficial to study interfacial charge transfer mechanisms. With these analyses, we expect to identify the nature and kinetics of transfer of different species contributed into the interfacial charge compensation occurring in SWCNT/PoPD composite films.

1 .

 1 Identification of the involved species in the charge storage mechanism of SWCNT/PoPD Ac-electrogravimetric measurements were performed in aqueous NaCl solution at various statesof-polarization from 0.5 V to -0.5 V vs Ag/AgCl with a potential perturbation amplitude of 50 mV. The electrochemical impedance, ∆𝐸 ∆𝐼 (𝜔), charge/potential transfer function, were obtained from ac-electrogravimetry. The charge/potential transfer function, ∆𝑞 ∆𝐸 (𝜔) can determine the interfacial transfer of ionic species, while the mass/potential transfer function, ∆𝑚 ∆𝐸 (𝜔) discriminate the nature of different species transferred and also takes the free solvent contribution into account 54,55 .

Figure IV. 5 5 (

 55 Figure IV.5 shows the results of electrochemical impedance, ∆𝐸 ∆𝐼 (𝜔), charge/potential transfer

. 1 .

 1 The electrochemical impedance and charge/potential transfer function give the contribution of the charged species but do not allow their identification. Therefore, the mass/potential transfer function, tracking the flux of both charged and neutral species by a fair identification and estimation of their kinetics of interfacial transfer.Table IV.1:Estimated values for K i ,, G i , Rt i and f i parameters extracted from the fitting of acelectrogravimetry data of the SWCNT/PoPD film at 0 V vs. Ag/AgCl, measured in 0.5 M NaCl at pH=2. The parameters obtained from the electrochemical impedance, of the charged species are used in the fitting of the mass/potential transfer function using the theoretical expression given in Equation II.17 (see Chapter II). An additional parameter appears in the theoretical mass/potential transfer function (Equation II.17), i.e. the molar mass (M i parameter) of each species. The latter parameter is used to identify the nature of the ionic or non-ionic species contributing to the charge compensation mechanism.

Figure IV. 5 (

 5 Figure IV.5 (c) presents the response of ∆𝑚 ∆𝐸 (𝜔) for SWCNT/PoPD composite. The curve shows two well defined loops emerged in different quadrants. From a theoretical point of view, the contributions in the first and second the quadrant are due to anion transfer, while the contribution in the third and fourth quadrant are related to cation transfer54,55 . Free solvent

Figure IV. 5 .

 5 Figure IV.5. Electrochemical impedance diagram ∆𝐸 ∆𝐼 (𝜔) (a); ∆𝑞 ∆𝐸 (𝜔) transfer function (b); and

Figure IV. 6 .

 6 Figure IV.6. Partial ac-electrogravimetric data shown 0 V vs. Ag/AgCl. ∆𝑚 ∆𝐸 | 𝑐2𝑎𝑠 (𝜔) TF

  Figure IV.7 (a) and (b) present the variation of f i and Rt i of each species at different potential values in the range studied. The results show that the order of the transfer kinetics at different potentials presents the following tendency: f i (Cl -) > f i (H 2 O) > f i (Na + )> f i (H + ). As shown in Figure IV.7 (b), the transfer resistance of the species is inversely proportional to their kinetics of transfer.

Figure IV. 7 .

 7 Figure IV.7. Variation of the characteristic frequency (f i ) (a) and the transfer resistance (Rt i ) (b) of the SWCNT/PoPD film measured in 0.5M NaCl solution at pH=2. The ac-electrogravimetry reveal the contribution of multi-species with different kinetics and resistance of transfer. These results are in good agreement and confirm the ideas discussed previously (Figure IV.4 (b)) to explain the scan rate dependent mass-potential curves in EQCM results. As shown in Figure IV.4 (b), at 100 mV.s -1 , the electrogravimetric behavior of

Figure IV. 4 ( 3 . 3 . 1 ,

 4331 b)) in EQCM becomes dominant by the cation contribution. It means that the cation contribution is hardly detected at high scan rates, but starts to dominate the electrogravimetric response at lower scan rates.At more anodic potentials (in the range of 0V to +0.4V vs Ag/AgCl), the SWCNT/PoPD nanocomposite shows the contribution of Cl -and H 2 O, which can be caused only by the presence of SWCNT53 . Indeed, the PoPD alone does not show any gravimetric response at these potentials.But the contributions of Na+ and H + , seen in the case of the SWCNT/PoPD composite, have not Electrogravimetric study of capacitive charge storage behavior of carbone nanotubes/ poly(ortho-phenylenediamine) nanocomposite: application in supercapacitors Chapter IV 129 been detected neither SWCNT nor PoPD films. This effect can be related to a synergetic effect between SWCNT and PoPD, which creates other active site in the nanocomposite structure. Correlation between ac-electrogravimetric and EQCM results for SWCNT/PoPD film Among the qualities of ac-electrogravimetry, it can give an estimation about the change of the relative concentration of each species with respect to the potential variation. Using the Equation III.the estimation of the relative concentration changes of each species were calculated between the different states-of-polarization through an integration process as shown in Equation III.2 (see Chapter III).

Figure IV. 8 (

 8 Figure IV.8 (a) presents the relative concentration changes (𝑪 𝒊 -𝑪 𝟎 ) of individual species identified from ac-electrogravimetry.

Figure IV. 8 .

 8 Figure IV.8. Ac-electrogravimetric results of the SWCNT/PoPD film deposited on gold coated quartz resonator. Variation of the relative concentration (Ci-C 0 ) (a) and the relative mass variation of each species (b) calculated at different potentials in NaCl 0.5M, pH=2.

  and of PoPD 39 (see Chapter III) under the same conditions, the SWCNT/PoPD nanocomposite present collective effect caused by the redox process of PoPD and the electroadsorption process on SWCNT. Using the relative concentration changes presented in Figure IV.8 (b), the variation of the relative mass of each species at different potentials was calculated and reported in Figure IV.8 (b). The gravimetric contributions of the species involved in the electrochemical process present different Electrogravimetric study of capacitive charge storage behavior of carbone nanotubes/ poly(ortho-phenylenediamine) nanocomposite: application in supercapacitors Chapter IV 130 behaviour, Na + mainly dominates the m response, while the H + has not a significant effect due to its low atomic mass. The gravimetric reconstruction of the global mass variation can be obtained by the addition of individual mass contributions presented in Figure IV.8 (b). A good agreement is achieved between m EQCM measured with the classical EQCM (at 10 mV•s -1 ) (Figure IV.9 (a)) and m ac- electrogravimetry reconstructed from ac-electrogravimetry (Figure IV.9 (b)) in the studied potential range. This comparative study provides a support for the multi-species contribution revealed by ac-electrogravimetry and further evidences the complementarity between EQCM and acelectrogravimetry.

Figure IV. 9 .

 9 Figure IV.9. Ac-electrogravimetric results of SWCNT/PoPD film deposited on a gold coated quartz resonator. EQCM response at 10 mV.s -1 (a) and total mass variation reconstructed from ac-electrogravimetry (b), measurements in NaCl 0.5M, pH=2. The present electrogravimetric study of SWCNT/PoPD nanocomposite provided fundamental insights into the charge storage mechanism in aqueous NaCl electrolyte where the participation of multiple species was detected.4. Supercapacitive charge storage performances 4.1. Electrochemical characterization in 3-electrode configuration (i) Optimization of composition in the SWCNT/PoPD nanocomposite films The SWCNT/PoPD nanocomposite structure is expected to have an enhanced electrical conductivity due to an ameliorated electron percolation benefiting from the SWCNT network which is intimately related to the electrochemical charge storage properties of the electrode. To investigate the optimized SWCNT/PoPD composition in terms of electrochemical properties, several nanocomposites were fabricated with different SWCNT/PoPD mass ratios, (i.e.; 3:1; 1:2; 1:4; 1:5 corresponding to 20; 50; 150; 200 CV cycles of electropolymerization of oPD, respectively). The ratio between SWCNT and PoPD was adjusted by performing the

Figure IV. 10 .

 10 Figure IV.10. (a) CV responses of SWCNT/PoPD nanocomposite thin films as a function of composition (at 50 mV.s -1 ) and (b) variation of the specific capacitance of SWCNT/PoPD nanocomposite electrodes with different SWCNT/PoPD mass ratios. The measurements are performed in 0.5M NaCl.The C s values are quite moderate when the SWCNT content is high which is due to the dominant capacitive response of SWCNT (typically  20 F.g -1 58,59 ) over faradaic contribution of the PoPD.

Figure IV. 11 (

 11 Figure IV.11 (b) shows the C s of all the electrodes calculated from CV measurements at 50 mV•s - 1 and using the equation IV.1. Accordingly, SWCNT/PoPD (1:5) electrode exhibits the highest C s of about 537 F•g -1 compared to those of SWCNT (22 F•g -1 ) and PoPD (420 F•g -1 ). The enhanced electrochemical behaviour of the SWCNT/PoPD (1:5) is further investigated in terms of rate capability and long-term cycling performance (Figure IV.11 (c) and (d)).

Figure IV. 11

 11 Figure IV.11 (c) shows the CV patterns of SWCNT/PoPD (1:5) nanocomposite with an

  Figure IV.11 (d)), indicating that electrolyte ions can move freely within the electrode and have sufficient time to complete the charge compensation process 62 . The cycling stability of SWCNT/PoPD (1:5) electrode (loading of 45 g•cm -2 ) on gold electrode of the quartz resonator was investigated by CV in a three-electrode configuration in 0.5M NaCl at a scan rate of 50 V•s -1 (Figure IV.11 (d)). It is interesting to note that SWCNT/PoPD (1:5) nanocomposite electrode exhibits not only high C s ( 540 F•g -1 corresponds to 24.4 mF.cm -2 ) but also excellent cycling stability (measured up to 1000 cycles) without a noticeable loss of the initial C s .

Figure IV. 11 .

 11 Figure IV.11. (a) CV response of SWCNT/PoPD (1:5) nanocomposite electrode and comparison to pristine PoPD and SWCNT counterpart (at 50 mV•s -1 ) where current density is normalized by the mass loading and (b) specific capacitance of prisitine PoPD, SWCNT and SWCNT/PoPD (1:5) nanocomposite electrode. (c) CV response of SWCNT/PoPD (1:5) nanocomposite electrode at different scan rates and (d) the specific capacitance (C s ) measured at 50 mV•s -1 for 1000 cycles and a function of scan rate using the Eq. 1 (in the inset of the panel).

Figure IV. 12 (

 12 Figure IV.12 (a) shows a representative FEG-SEM image of the SWCNT powder, revealing a significantly high density of the CNT bundles of ranging from 5 to 20 nm. This powder is used to prepare a slurry with PVDF-HFP binder (with 90:10 wt. %) in NMP to drop-cast on graphite substrates. Figure IV.12 (b) presents the TEM image of the SWCNT based thin film which is obtained by scraping the substrate surface and indicates the presence of CNT bundles of 5-20 nm, in agreement with the FEG-SEM image in Figure IV.12 (a).

Figure IV. 12 .

 12 Figure IV.12. (a) FEG-SEM image of SWCNT powder, (b) TEM image of the SWCNT/PVDF-HFP film (90:10 wt%) scrapped from the graphite electrode, (c) SWCNT/PVDF-HFP coating on graphite electrode after 1000 electropolymerization cycles of oPD, leading to SWCNT/PoPD (1:5) electrode on graphite and (d) TEM image of SWCNT/PoPD (1:5) scrapped from the graphite electrode. The electropolymerization of oPD monomer is performed on the SWCNT modified substrates for 1000 cycles and the FEG-SEM image of the resulting nanocomposite is shown in Figure IV.12 (c). Contrary to the SWCNT bundle diameter of 10-20 nm, the SWCNT/PoPD (1:5)

2 .Figure IV. 13 . 2 .

 2132 Figure IV.13. Baseline corrected FTIR spectra of SWCNT, PoPD and SWCNT/PoPD (1:5) nanocomposite electrodes deposited on the graphite substrates.

Figure IV. 14 .Figure IV. 15 .Figure IV. 16

 141516 Figure IV.14. (a) XPS survey spectrum, (b) high resolution C 1s spectrum and (c) high resolution N 1s spectrum of SWCNT/PoPD (1:5) nanocomposite thin films deposited on the graphite electrode. C1 spectrum was fitted with five peaks: typically, two main contributions at 284.4 (46.1 %) and 285.7 (44.0 %) which are assigned to the C=C and C=N, respectively, and three residual contributions at 287.2, 288.8 and 290.5 eV ascribed to C-O, C=O and C = OOH bonds, correspondingly. It is noted that the peak at 285.7 eV assigned to C=N confirms the presence of N groups in the material after the PoPD electropolymerization 29 . Furthermore, Figure IV.14 (c) shows the N 1s spectrum of the nanocomposite electrode resolved into three different types of nitrogen species: : -N = (399.0 eV ; 69.8 %), -NH 2 (400.4 eV ; 25.9 %)), and -NH + = (401.6 eV ; 4.3 %)) 65,68-71 , demonstrating the presence of PoPD on the SWCNT modified substrate. High resolution spectra of C1s for SWCNT and C1s and N1s for PoPD pristine electrodes are given in Figure IV.15 (a) and (b), (c), respectively.

Figure IV. 16 .

 16 Figure IV.16. (a) Schematic description of the 2-electrode test configuration and Galvanostatic charge/discharge (GCD) curves of three different electrode materials (SWCNT, PoPD and SWCNT/PoPD (1:5) electrodes at 7.3 A•g -1 (0.73 mA•cm -2 ) presented as (b) voltage-time graphs and (c) discharge capacitance as a function of cycle number for the cell. The electrochemical behavior of SWCNT/PoPD nanocomposite electrode which exhibits the highest discharge capacitance value among the three electrodes was further analyzed.Figure IV.17 (a) shows the typical pseudo-capacitive behaviour attested by a non-triangular voltage-time graph. The oxidation and reduction peaks appear at around 0.95 and 0.45 V vs Zn, respectively, as depicted in Figure IV.17 (a).

Figure IV. 17 .

 17 Figure IV.17. Galvanostatic charge/discharge (GCD) curves of the SWCNT/PoPD (1:5) electrode at 7.3 A•g -1 (0.73 mA•cm -2 ) presented as (a) voltage-time graphs and (b) discharge capacitance as a function of cycle number for the cell. These peak positions agrees well with the 3-electrode measurements performed with a Ag/AgCl reference electrode (Figure IV.11 (a)) since the reference electrode potentials of Ag/AgCl (vs SHE) is +0.197 V and Zn is -0.760 (vs SHE). The nanocomposite electrode exhibited excellentcycling performance with a discharge capacitance value of around 350 F•g -1 where 82% of its intial C s is maintained after more than 10000 cycles at 7.3 A•g -1 . Furthermore, the columbic efficiency stays close to 100% over the course of 11000 cycles, indicating the excellent reversible electrochemical response. To highlight the improvements brought by this work with respect to previous reports on similar material combinations, the present results are compared with previously published works in the literature (TableIV.3).

  Although several factors including the electrode configuration, thickness of films, mass, cell components including the electrolyte, separator, current collectors, binders and measurement parameters affect the capacitance values42,73 , it is fair to emphasize the remarkable cycling stability of the two-electrode Swagelok cell shown in Figure IV.17. Cycling stability of the electrodes are

  retention, 1000 cycles, at 0.5 A.g -1 , potential window 0.1V to 0.9V vs SCE b , activated carbon as counter electrode, in 1M KNO sodium dodecyl sulphate, b SCE: Saturated calomel electrode, c PpPD: Poly(paraphenylenediamine), d rGO: reduced graphene oxide also verified after electrochemical tests and FEG-SEM images of the SWCNT/PoPD (1:5) electrode on graphite and Zn electrode are provided in Figures IV.18(a), (b) and (d) which were compared with those of the fresh electrodes in figure IV.12 (c) and figure IV.18 (d), respectively.No significant morphological changes were observed even after 11000 cycles (under the conditions of our study), except the presence of residual glass fibers originating from the separator used in the Swagelok tests (identified by the EDX analysis shown in Figures IV.18

  (a), (b) and (d), further attesting of the cycling stability of the SWCNT/PoPD (1:5) and Zn electrode under the experimental conditions. The nanocomposite electrode presents an accessible porous structure and high electrical conductivity due to the presence of the SWCNTs, offering a high surface area for the electrochemical reactions. Therefore, SWCNT/PoPD nanocomposite can be considered as an interconnected porous conductive structure accessible for the electrolyte species and thus provides the benefits of both EDLC behaviour of the SWCNT and faradic contribution of the PoPD.

Figure IV. 18 .

 18 Figure IV.18. The FEG-SEM images of SWCNT/PoPD (1:5) electrode (a) after 11000 cycles in Swagelok cells in 0.5M NaCl, pH=2 and (b) the EDX spectra of the cycled electrode surface indicating the presence of the glass fiber residue originating from the separator used in Swagelok cells. The panels (c) and (d) are the FEG-SEM images of Zn metal before cycling and after cycling, respectively.

  precise compositions were defined by means of a QCM study. The resulting electrodes with various compositions were electrochemically tested among which 1:5 weight ratio (83 wt% of PoPD) (SWCNT/PoPD) delivered an optimum charge storage performance. The compositional characteristics optimized in the first part of the study were used as base for the preparation of electrodes to be tested in two-electrode Swagelok cells.

  PpPD) was carried out using a simple electrochemical method. The morphology and structure of the nanocomposite electrodes are characterized. The effects of various parameters such as the PpPD film thickness and the NiPs content on the electrocatalytic performance of CPE/CNF/PpPD/NiPs are evaluated which lead to the optimized composition. The electrocatalytic effect of the nanocomposite for methanol electroxidation is tested in alkaline solution. These findings truly highlight the synergetic effect of CNF/PpPD in enhancing the electrochemical activity and stability and the vast potential of CPE/CNF/PpPD/NiPs as lowcost catalyst and electrodes for DMFCs. * Halim, E. M. et al. Synthesis of carbon nanofibers/poly(para-phenylenediamine)/nickel particles nanocomposite for enhanced methanol electrooxidation. Int. J. Hydrog. Energy 44, 24534-24545 (2019)

Figure V. 2 (

 2 Figure V.2 (a)shows the CVs of the electropolymerization of pPD on the CPE previously modified with CNFs (CPE/CNF) in 0.1 M H 2 SO 4 solution containing 5 mM pPD, in the potential window between -0.3 V and 0.9 V at a scan rate of 50 mV s -1 . The first cycle shows the appearance of a redox peaks at E a =0.58 V and E c =0.44 V with a large current density, which is caused by the oxidation and the reduction of pPD monomer. During the successive cycles, another redox peaks appears at E a = 0.43 V and E c =0.26 V corresponding to the formation of the electroactive PpPD film[39][40][41] . The current density of the second redox peaks is relatively small and grows during cycling, contrary to those of the first redox peaks, indicating that the growth rate of the PpPD film is considerably slow and a thin film was formed on CPE/CNF. The mechanism of the electropolymerization is represented in the Figure V.2 (b). pPD monomer oxidized at positive potential to produce cationic radical, after that, pPD cationic radicals react together to form PpPD film. Two structures of PpPD can be formed as shown in the Figure V.2 (b) 42,43 . The electrodeposition of NiPs on the CPE previously modified with CNF/PpPD was performed in a 1 M NiCl 2 solution by applying a fixed potential (-0.3 V) for 45 s. This step was followed by the activation of NiPs in a 0.1 M NaOH solution using cyclic voltammetry between 0 V and 0.8 V (Figure V.2 (c)). During the activation process, a pair of well-defined redox peaks appeared at

Figure V. 2 .

 2 Figure V.2. CV curves of the electropolymerization of pPD on the CPE/CNF in 0.1 M H 2 SO 4 c+ 5 mM of pPD monomer, between -0.3 V and 0.9 V for 10 cycles, at a scan rate of =50 mV s - 1 (a), The electropolymerization mechanism of PpPD (b) and cyclic voltammetry of CPE/CNF/PpPD/NiPs electrode measured in 0.1 M NaOH solution, between 0 V and 0.8 V for 15 cycles, at a scan rate of =50 mV s -1 (c).

FEG-

  SEM coupled to an EDX detector. The CPE surface (Figure V.3 (a)) shows irregularly distributed flakes of graphite with a micrometer-size. In the case of CPE/CNF, Figure V.3 (b) shows that the CNFs deposited on CPE electrode are uniformly dispersed and forming a rougher Synthesis of carbon nanofibers/poly(para-phenylenediamine)/nickel particles nanocomposite for enhanced methanol electrooxidation Chapter V 152 surface, which offers a larger surface area than CPE. The image of CPE/CNF/PpPD/NiPs surface (Figure V.3 (c)) displays the formation of a very thin layer of PpPD polymer on the surface of the CPE/CNF, this thin layer of conducting polymer connects the fibers and forms a nanocomposite on the CPE surface. Moreover, the NiPs are well formed on the surface with a particle size of about 300 nm. The EDX analysis was used to determine the chemical composition and elements present in CPE/CNF/PpPD/NiPs. As shown in Figure V.3 (d), carbon, oxygen and nitrogen were detected in the electrode surface, which confirms the successful formation of the PpPD on CNF. Moreover, the nickel was detected with a small quantity, which will be further analyzed with the XRD.

Figure V. 3 . 153 Figure V. 4 .

 31534 Figure V.3. SEM images of bare CPE (a), CPE/CNF (b) and CPE/CNF/PpPD/NiPs (c); EDX spectrum of CPE/CNF/PpPD/NiPs (d). The EDX mapping images of CPE/CNF/PpPD/NiPs are given in Figure V.4. It can be seen that the carbon element is well-proportioned in the electrode surface, due to the presence of CNFs and the graphite flakes of the bare CPE. The oxygen and nitrogen elements originating from PpPD are well dispersed on the surface and certifies the assembly of PpPD thin film on the CNFs. Moreover, a homogeneous dispersion of nickel element confirms the formation of NiPs on the CPE/CNF/PpPD surface, generating more active sites with catalytic properties.

  X-ray diffraction was used to characterize the crystalline phases of the CNF/PpPD/NiPs nanocomposite deposited on ITO substrates and the XRD pattern of the nanocomposite is presented in the Figure V.5. The spectrum presents a peak located at 2θ=26° corresponding to the hexagonal structure of the CNFs. Moreover, the apparition of three peaks attributed to Ni(111), Ni(200) and Ni(220), at about 43°, 50° and 74°, respectively, indicates the formation of a pure phase of nickel 47 . In addition, the crystalline peaks of the ITO substrate are present in the XRD pattern.

Figure V. 5 .154 2 . 3 .

 523 Figure V.5. XRD patterns of ITO, ITO-NiPs and ITO-CNF/PpPD/NiPs. Further characterizations by FTIR and Raman spectroscopy confirm the successful formation of the CNF/PpPD/NiPs nanocomposite on the electrode.

Figure V. 6 . 2 . 4 .Figure V. 7 Figure V. 7 .

 62477 Figure V.6. FTIR spectra of CPE/CNF/PpPD/NiPs. The formation of PpPD/NiPs nanocomposite on CNFs was further clarified by conducting Raman spectroscopy as shown in Figure V.7. From the obtained Raman and FTIR characterization, it can be concluded that the nanocomposite PpPD/NiPs/CNF was successfully synthetized on CPE.2.4. Raman analysis.Figure V.7 shows the Raman spectra of CPE/CNF/PpPD/NiPs. The Raman spectrum of the CPE/CNF/PpPD/NiPs shows the two remarkable peaks located at 1457 cm -1 and 1585 cm -1 , related to the graphitic structure of the CNFs52 . Moreover, the presence of other bands at 1330 cm -1 , 1366 cm -1 and 1080 cm -1 are related to the C-N + stretching and to the C-H of the PpPD quinoid, respectively53 . In addition, the band of Ni-O stretching vibration was observed at 560 cm -1 which is related to the NiPs54 .

  The electrochemical behavior of CPE, CPE/CNF/PpPD and CPE/CNF/PpPD/NiPs was characterized in KCl solution (0.1 M) containing 5 mM of [Fe(CN) 6 ] 3-/[Fe(CN) 6 ] 4-. The CV curves of three electrodes (Figure V.8 (a)) show a well-defined redox peaks. The bare CPE exhibits a low current density of about 0.35 mA cm -2 with a high value of peak-to-peak separation ( p )

Figure V. 8 .

 8 Figure V.8. Cyclic voltammetry of CPE, CPE/CNF/PpPD and CPE/CNF/PpPD/NiPs between -0.3 V and 0.9 V at a scan rate of =50 mV s -1 (a) and EIS spectra of CPE, CPE/CNF/PpPD and CPE/CNF/PpPD/NiPs measured at E=0.2 V vs. SCE, with amplitude of 10 mV (b) in 0.1 M KCl solution containing 5 mM of [Fe(CN) 6 ] 3-/[Fe(CN) 6 ] 4-.

  effect of CPE/CNF/NiPs and CPE/CNF/PpPD/NiPs was evaluated by cyclic voltammetry in alkaline medium with and without the addition of 1 M of methanol. The responses of both electrodes in 0.1 M NaOH solution in the absence of methanol are shown in Figure V.9 (a). Two well-defined peaks were observed, an anodic peak at 0.52 V and a cathodic peak at 0.3 V corresponding to the redox Ni(OH) 2 /NiOOH transformation. After the addition of 1 M of methanol into the NaOH solution (Figure V.9 (b)), a new anodic peak appears around 0.8 V for the both electrodes, indicating the electrooxidation of methanol at the electrode surfaces.

Figure V. 9 .

 9 Figure V.9. Cyclic voltammetry of the electrochemical responses of CPE/CNF/NiPs and CPE/CNF/PpPD/NiPs in 0.1 M NaOH solution without (a) and with (b) 1 M methanol, between 0 V and 1 V, at a scan rate of =50 mV s -1 .

Figure V. 10

 10 (a) shows the corresponding current density of the methanol electrooxidation peak with different PpPD amount. The film thickness associated to different electrical charge (during electrodeposition by CV) was calculated in table V.1 using the equation V.262 :𝑒 = 𝑀 × 𝑄 𝑛 × 𝐹 × 𝑆 × 𝜌 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑽. 𝟐)where e is the film thickness, M is the molar mass of the polymer (108.14 g mol -1 ), Q is the electrical charge associated with polymer formation, n is the number of electrons involved (n=1), F is the Faraday constant (96485.33 C mol -1 ), S is the area of the working electrode surface (0.07 cm -2 ), and ρ is the density of the polymer (ρ=1). Synthesis of carbon nanofibers/poly(para-phenylenediamine)/nickel particles nanocomposite for enhanced methanol electrooxidation Chapter V 159 As shown in the Figure V.10 (a) and table V.1, the current density increases with the increase of the polymer thickness up to 50 nm. However, a further increase of the thickness of polymer film decreases the current density of CPE/CNF/PpPD/NiPs during the methanol electrooxidation. The increase of the PpPD thickness lead to increase the number of amine groups, which is very important to form NiPs with enhanced electrocatalytic performance 60 . But, a thick PpPD film (more than 50 nm) may suppresses the electrical connexion between CNFs and NiPs 63 , leading to decrease in the electrocatalytic activity of CPE/CNF/PpPD/NiPs. The film thickness of about 50 nm was maintained throughout the study. Here, the NiPs amount was fixed at about 0.25 µg.

Figure V. 10 .Film4. 2 . 3 :

 1023 Figure V.10. Variation of current density of the methanol electrooxidation peak (obtained by cyclic voltammetry) at CPE/CNF/PpPD/NiPs with different PpPD film thickness (a), different electrodeposition time of Nickel (PpPD thickness is fixed to 10 CV cycles) (b) and Cyclic voltammetry of CPE/CNF/PpPD/NiPs in 0.1 M NaOH solution with various concentrations of methanol, between 0 V and 1 V, at a scan rate of =50 mV s -1 (c).Table V.1. Thicknesses (e) of PpPD films as a function of electrical charge (Q) associated with PpPD formation. Number of cycles of the pPD electropolymerization

3 .

 3 nanofibers/poly(para-phenylenediamine)/nickel particles nanocomposite for enhanced methanol electrooxidation Effect of methanol concentration CPE/CNF/PpPD/NiPs revealed its high electrocatalytic activity for methanol electrooxidation and the PpPD film thickness and NiPs amount was optimized, i.e., PpPD of ( 50 nm) 10 CV cycles for electrodeposition and 45s ( 0.25 µg) of NiPs electrodeposition. Here, the electrocatalytic activity of CPE/CNF/PpPD/NiPs is studied in methanol solution with a gradual increase of the concentration (Figure V.10 (c)). As shown in Figure V.10 (c), the current density of the oxidation peak at 0.8 V increases with the increase of the methanol concentration up to 2

  In the practical terms, the long-term stability of the catalyst is necessary for commercially viable DMFC systems. The chronoamperometric analysis was used to study the long-term stability of CPE/CNF/PpPD/NiPs, CPE/CNF/NiPs and Pt in 1.5 M methanol + 0.1 M NaOH solution at 0.8 V for period of 6 h. As shown in the Figure V.11, the current density of CPE/CNF/PpPD/NiPs is much higher than those of CPE/CNF/NiPs and Pt. On the other hand, the prepared electrodes show only a slow decrease in the current density during 6 h.In comparison with previous works, CPE/CNF/PpPD/NiPs exhibits a high catalytic performance with a slow decrease in the current density than other nanocomposite electrodes such as, Ni/SPPy16 , Pt@TiN/rGO65 and Cu 2 O/PPy-GO66 , which exhibit a current density less than 1 mA cm -2 . It is also noteworthy that the long-term stability was tested for less than 1 h, in these studies. These comparisons further point out the improved performance of the optimized composition of CPE/CNF/PpPD/NiPs.

Figure V. 11 . 1 M

 111 Figure V.11. Chronoamperograms of CPE/CNF/PpPD/NiPs, CPE/CNF/NiPs and Pt in 0.1 M NaOH solution containing 1.5 M of methanol at 0.8 V for 6 h.

A

  novel low-cost catalyst based on NiPs supported on CNF/PpPD was synthesized on CPE by a simple and rapid electrochemical method. The electropolymerization of PpPD was carried out in acidic media on CPE surface previously modified with the functionalized CNFs followed by the electrodeposition of NiPs at a fixed potential. The resulting nanocomposite electrodes were characterized using various morphological and structural techniques such as FEG-SEM, EDX, Synthesis of carbon nanofibers/poly(para-phenylenediamine)/nickel particles nanocomposite for enhanced methanol electrooxidation Chapter V 163 XRD, FTIR and Raman spectroscopy. The electrochemical and electrocatalytic behaviour was assessed by CV, EIS and chronoamperometry. Our results demonstrate that the NiPs were successfully deposited on CNF/PpPD surface and strongly suggest that the presence of CNF and PpPD conducting polymer enhances the electronic transfer at the electrode interface. The electrocatalytic tests also indicate the collective contribution and synergistic effect of the electrode components which leads to the enhanced electrocatalytic performance of CPE/CNF/PpPD/NiPs towards methanol oxidation with high stability and durability compared to precious metal based electrode such as Pt. The proposed electrodes are relatively inexpensive to produce, convenient to use and constitute a good alternative to noble metal catalysts. Further studies are underway to exploit these nanocomposites for energy storage applications.

  Graphite powder (1 g) was mixed with of mineral oil (300 µl) for 30 min using a mortar to prepare carbon paste. The homogeneous paste was packed into the cavity (3 mm diameter) of a Teflon tube electrode and polished on a white paper to get a smooth surface. After that, 10 µl of CNFs dispersion (2 mg CNF in 1 mL DMF) was drop-cast on the surface of the CPE and dried for 10 min at 80 °C. Then, PpPD film was formed by immersing the CPE-CNF in a 0.1 M of sulfuric acid solution containing 5 mM of pPD monomer. Then, the electropolymerization was performed by CV for 10 cycles between -0.1 V and 0.9 V vs saturated calomel electrode (SCE) at 50 mV s -1 where a Pt rod was used as a counter electrode. The Cu based dendritic structures were electrodeposited on the modified electrode surface by applying a fixed potential of -0.5 V for 45 s in a 0.1 M of Na 2 SO 4 solution containing 0.1 M CuCl 2 . The prepared CPE-CNF/PpPD/Cu electrode was washed with double distilled water.

Figure VI. 1

 1 presents a schematic description of the preparation process.

FigureVI. 1 .

 1 FigureVI.1. Schematic description of the fabrication strategy of CPE/CNF/PpPD/Cu.

  Figure VI.2. (a) and (b) shows the 1 st and the 10 th cycles of the electropolymerization of pPD, respectively. The results obtained on two types of substrates are shown, i.e. CPE and CPEpreviously modified with CNFs. The voltammograms of both electrodes display the presence of two redox peaks. The first redox peak is attributed to the oxidation and the reduction of pPD monomer, and the second corresponds to the formation of the electroactive PpPD film[5][6][7] . The current density of the second redox peak grows during cycling, contrary to that of the first redox peaks, indicating that the formation of a thin electroactive film on the electrode. The electropolymerization of pPD on CPE-CNF present a higher current density and reduced peakto-peak separation values, which can be explained by the high surface area and the enhanced electrical properties of CNFs.After the formation of the PpPD film, the copper dendritic nanostructures were electrodeposited on the electrode at a fixed potential of -0.5 V vs. Ag/AgCl for 45 s. The Figure VI.2 (c) shows the electrodeposition curve of Cu on CPE-CNF/PpPD electrode. In the beginning of the electrodeposition, the current increases rapidly due to a fast nucleation-growth process 8,9 . After 30 s, the current stabilize at current density of about -2.12 mA.cm -2 , which is related to the depletion of the Cu ion concentration near to the electrode surface8 .

Figure VI. 2 . 2 .

 22 Figure VI.2. Cyclic voltammetry (a) 1 st cycle, (b) 10 th cycle of the electropolymerization of pPD on the CPE and on CPE-CNF in 0.1 M H 2 SO 4 containing 5 mM of pPD monomer, between -0.1 V and 0.9 V for 10 cycles, at a scan rate of =50 mV s -1 and (c) chronoamperometric curve of the electrodeposition of Cu on CPE-CNF/PpPD at a fixed potential of -0.5 V for 45 s in 0.1M Na 2 SO 4 containing 0.1 M CuCl 22. Morphological and structural characterizations 2.1. FEG-SEM, TEM and EDX analysisThe morphology of CPE, CPE-CNF, CPE-CNF/PpPD and CPE-CNF/PpPD/Cu electrodes were analyzed using FEG-SEM. Figure VI.3. (a) displays the morphology of CPE surface which shows irregularly distributed flakes of graphite with a micrometer-size 7 . Figure VI.3. (b) shows that the CNFs (with an average diameter of 100 nm) deposited on CPE electrode are uniformly dispersed, forming a CNF network and offering a larger surface area compared to that of CPE. Regarding the CPE-CNF/PpPD (Figure VI.3. (c)), it is evident that a very thin-film of the PpPD polymer is formed on the CNFs and connects the nanofibers 10 . In the case of CPE-CNF/PpPD/Cu electrode (Figure VI.3. (d-g)) displays the formation of a three-dimensional copper dendritic nanostructure. Further examinations of the electrode surface (Figures VI.3. (e) and (f)) reveal that the dendrites have a hierarchical structure formed by multiple branches attached to a long axis. A zoom on the

Figure VI. 3 .

 3 Figure VI.3. SEM images of CPE (a), CPE-CNF (b), CPE-CNF/PpPD (c) and CPE-CNF/PpPD/Cu (d, e, f, g).

  Figure VI.4 (b and c) display the TEM images of a copper dendrites formed on CNF/PpPD support. The dendrite (Figure VI.4 (d)) shows the presence of a very thin layer on the surface, which may be in agreement with the hypothesis of the formation of two phases of copper.

Figure VI. 4 .

 4 Figure VI.4. TEM images of CNF/PpPD (a), CNF/PpPD/Cu (b and c) and Cu dendrite (d). The surface composition of CNF/PpPD/Cu electrode was analysed by Energy-dispersive X-ray (EDX) detector which is coupled to FEG-SEM. EDX spectrum depicted in Figure VI.5 (a) demonstrates the presence of N, O, C and Cu elements. N element is related to the PpPD polymer, whereas C and O originate principally from CNFs. The EDX mapping was performed to study the elemental distribution on the surface of CNF/PpPD/Cu electrode. As shown in Figure VI.5 (panels b-f), carbon and oxygen elements show a distribution related to CNFs. The nitrogen coming from PpPD polymer is well dispersed on the surface and certifies the assembly of PpPD thin film on the CNFs.

Figure VI. 5 .

 5 Figure VI.5. EDX spectrum of CNF/PpPD/Cu (a), Elemental mapping analysis of CNF/PpPD/Cu (panels b-f).

16 .

 16 The X-ray diffraction (XRD) was used to characterize the crystalline phase of CPE-CNF/PpPD/Cu electrode, and the result was reported in Figure VI.6. The XRD pattern shows the presence of two crystalline phases of copper. The diffraction peaks at 16.7°, 23.8°, 39.8°, 47°, 53.2°, 77.7° and 83.5° correspond respectively to the (022), (021), (130), (112), (150), (153) and (242) planes of Cu(OH) 2 , which match well to the orthorhombic Cu(OH) 2 (JCPDS 01-072-0140) The peaks which appear at 36.4°, 42.3°, 61.3°, 73.5° are related to (111), (200), (220) and (311) planes of the cubic Cu 2 O (JCPDS 01-077-0199)17 . In addition, the XRD spectrum shows the presence of a peak at 2θ=26° related to the hexagonal structure of the CNFs and the crystalline peaks of CPE substrate.

Figure VI. 6 .

 6 Figure VI.6. XRD pattern of CPE-CNF/PpPD/Cu. The surface chemical composition of the CNF/PpPD/Cu nanocomposite was determined by Xray photoelectron spectroscopy (XPS). The survey spectrum (Figure VI. 7(a)) of the nanocomposite shows the presence of C, O, Cu and N elements with atomic percentage of 62.64, 25.62, 6.95 and 4.79, respectively.

Figure VI. 7 .

 7 Figure VI.7. (a) XPS survey spectrum, (b) high resolution Cu 2p spectrum, (c) high resolution Cu LMM spectrum and (d) high resolution O 1s spectrum of CPE-CNF/PpPD/Cu.

Figure VI. 7 (

 7 Figure VI.7 (b) and (d) present the high resolution spectra of Cu and O elements, respectively. Cu2p spectrum resolved into several peaks: two peaks at around 934.7 and 954.5 eV ascribed to Cu2p 3/2 and Cu2p 1/2 , respectively, which are related to Cu(II) (Cu(OH) 2 ). Additionally, the shakeup satellite peaks at around 932.2 and 962.7 eV for Cu 2p 3/2 and Cu 2p 1/2 , respectively, indicating the presence of an unfilled Cu3d 9 shell and confirming the existence of Cu(II) at the surface 18,19 . The two contributions located at 932.4 and 952.5 eV correspond to Cu2p 3/2 and Cu2p 1/2 , respectively, of Cu(I) or Cu(0). The shape of the LMM Auger peak (FigureVI.7 (c)) is located at 570.6 eV, confirming the presence of Cu(I) 20 . The low intensity of peaks of Cu 2 O and the high intensity of peaks of the Cu(OH) 2 indicate the formation of a protection layer of Cu(OH) 2 on Cu 2 O 18 . Figure VI.7 (d) shows the O1s spectrum of the nanocomposite. The spectrum presents a main contribution at 531.3 eV which can be ascribed to oxygen in Cu(OH) 2 , which confirms the formation of Cu(OH) 2 layer 21 .

Figure VI. 8 . 1 .

 81 Figure VI.8. EIS spectra of CPE, CPE-PpPD, CPE-CNF and CPE-CNF/PpPD/Cu measured at E=0.4 V vs. SCE, with amplitude of 10 mV in 0.1 M KCl solution containing 5 mM of [Fe(CN) 6 ] 3-/[Fe(CN) 6 ] 4-. The CPE substrate presents a large semicircle in the region of high to low frequency, followed by sloped line at low frequency. The electropolymerization of PpPD thin film on the CPE surface, improves the electrochemical properties of the electrode by reducing the electron transfer resistance from 20 k to 12 k These values are obtained from an extrapolation of the high frequency loops to the real axis of the diagram. Indeed, the presence of PpPD thin film increases the electron transfer at the electrode surface by improving the electronic properties. The CPE substrate contains the non-conductive paraffin oil which reduces the surface area and then, the electron transfer capability. The deposition of CNFs on CPE substrate decreased the R ct to 7 k owing to its good electronic properties and high surface area. In the case of CPE-CNF/PpPD/Cu film, the R ct is significantly lower as compared to that of CPE-CNF, CPE-PpPD and bare CPE electrodes. According to these results, the electropolymerization of PpPD on CNFs can generate a synergistic effect and presents enhanced electrical properties, which leads to the promotion of the electron transfer at the electrode. In addition, the incorporation of Cu on the CNF/PpPD offers a large number of active sites which can catalyse the reaction of [Fe(CN) 6 ] 3-/[Fe(CN) 6 ] 4-.

Figure VI. 9 .

 9 Figure VI.9. Cyclic voltammetry of (a) CNF/PpPD/Cu in 0.1 M NaOH solution with and without 1.5 M methanol, and (b) PpPD/Cu, CNF/Cu and CNF/PpPD/Cu in 0.1 M NaOH solution with 1.5 M methanol, between -0.2 V and 1.2 V vs SCE, at a scan rate of =50 mV s -1 .

Figure VI. 10

 10 Figure VI.10 (a) shows different CV curves in 0.1 M NaOH solution containing 1.5 M methanol with different Cu mass just by varying the Cu electrodeposition time. The electrocatalytic effect increases with the increase of the deposition time of Cu until 45 s. However, a further increase of the deposition time decreases the electrocatalytic effect of CNF/PpPD/Cu catalyst. Normally,

Figure VI. 10 .

 10 Figure VI.10. Cyclic voltammetry of CNF/PpPD/Cu: (a) in 0.1 M NaOH solution+1.5 M methanol using different electrodeposition time of Cu, and (b) in 0.1 M NaOH solution with various concentrations of methanol, between -0.2 V and 1.2 V vs SCE, at a scan rate of =50 mV s -1 . The catalytic effect of CNF/PpPD/Cu towards MOR was studied with a gradual increase of the concentration of methanol.Figure VI.10 (b) shows the CV curves of CNF/PpPD/Cu in 0.1 M NaOH solution with different methanol concentrations. The current density of the methanol oxidation peak increase with the increase of the methanol concentration until 3 M. In comparison with other catalyst from the literature 1,24-26 and our previous work 10 (see chapter V),

Figure VI. 11

 11 shows the chronoamperometric curves of CNF/Cu, PpPD/Cu, CNF/PpPD/Cu and Pt measured over 6h.

Figure VI. 11 .

 11 Figure VI. 11. Chronoamperograms of CNF/Cu, PpPD/Cu, CNF/PpPD/Cu and Pt in 0.1 MNaOH solution containing 1.5 M of methanol at 0.9 V for 6 h. CNF/PpPD/Cu electrode exhibits a higher current density, which confirms the CV results reported in Figure VI.9 (b). In comparison with CNF/Cu, PpPD/Cu shows a better stability during 6h, confirming that the presence of amine groups offered by the polymer on the electrode surface impacts positively the stability and the durability of the Cu. The current density of PpPD/Cu is lower than that of CNF/Cu; this can be explained by the high surface area and the good electrical properties of the CNF. In the case of CNF/PpPD/Cu, the current density is much higher than that of CNF/Cu and PpPD/Cu and present a good stability, indicating that the combination of CNF and PpPD as support for Cu creates a good synergistic effect which results in a further improvement of the activity, stability and durability of the Cu.

  this work, a new catalyst based on Cu deposited on CNF/PpPD support was prepared using a simple and rapid process. PpPD was electropolymerized on CNF using CV technique, then the Cu was electrodeposited at a fixed potential. Various spectroscopic and microscopic techniques such as XRD, TEM, MEB and EDX were used to characterize the structure and the morphology of the prepared nanocomposites. The results show that the nanocomposite was successfully prepared. The electrochemical characterization by EIS suggests that the deposition of Cu on CNF/PpPD support improves the electronic properties of the nanocomposite. After the preparation and the characterization of CNF/PpPD/Cu nanocomposite, its electrocatalytic performance for methanol oxidation was tested by CV and chronoamperometry in alkaline media. CNF/PpPD/Cu catalyst shows a high catalytic activity and good stability over 6 hours.General conclusions and perspectivesIn energy storage systems, understanding the charge storage mechanisms in the electrode material is very important and becomes deservedly necessary to improve their performance. It is well-known that interfacial ion flux dynamics constitute an important part of the operating principle of super-and pseudo-capacitors. Therefore the correlation of this fundamental understanding to the charge storage properties of the electrode materials is rather significant but requires specific characterization tools. EQCM have been widely used to study the charge transfer properties of the electrode materials for the energy storage domain. Here, a non-conventional complementary technique, the so-called ac-electrogravimetry, has been proposed to unveil the mechanisms of the ionic exchange process of the PoPD and of the SWCNT/PoPD electrods.

  were tested in another aspect related to the energy conversion domain. A low-cost catalyst based on NiPs supported on CNF/PpPD was synthesized on CPE (Carbon Paste Electrode) substrate by a simple and rapid electrochemical method. The electropolymerization of PpPD was carried out in acidic media on CPE surface previously modified with the functionalized CNFs followed by the electrodeposition of NiPs at a fixed potential. The resulting nanocomposite electrodes were characterized using various morphological and structural techniques such as FEG-SEM, EDX, XRD, FTIR and Raman spectroscopy, confirming the nanocomposite structude. The electrochemical and electrocatalytic behaviour was assessed by CV, EIS and chronoamperometry. Our results demonstrate that the NiPs were successfully deposited on CNF/PpPD surface and strongly suggest that the presence of CNF and PpPD conducting polymer enhances the electronic transfer at the electrode interface. The electrocatalytic tests also indicate the collective Les nanocomposites à base de polymères conducteurs électroniques et de nanomatériaux carbonés, reste aujourd'hui un champ disciplinaire très étudié par les différentes communautés scientifiques du fait de leurs nombreuses propriétés électroniques, chimiques et physicochimiques, ce qui leurs ouvrent beaucoup d'applications innovantes dans des domaines aussi variés allant du domaine environnementales, de l'énergie, de la sécurité, du contrôle de procédés, du diagnostic médical rapide, ou encore les capteurs chimiques. Les nanostructures de carbone tels que NTC, NFC, graphène ont largement été utilisés dans plusieurs domaines industriels, notamment dans le stockage et la conversion de l'énergie grâce à leur bonne stabilité, leur surface spécifique élevée et leur capacité pour l'accumulation de charges à l'interface électrode/électrolyte. Les polymères conducteurs ont été aussi largement utilisés comme matériau d'électrode pour les supercondensateurs grâce à leurs capacités spécifiques élevées, mais ils sont limités par leur faible stabilité. La combinaison des nanostructures de carbone avec les polymères conducteurs a mené à la création des nouveaux matériaux composites avec des propriétés améliorées relativement aux matériaux de base. Dans le cadre de cette thèse, des nanocomposites binaire à base de carbone (NTC) et polymère conducteur (PoPD) et ternaire à base de carbone (NFC), de polymère conducteur (PpPD) et de particules (NiPs) ont été élaborés. Après l'élaboration des conditions optimales de synthèse, les caractéristiques morphologique et structurale des films obtenus ont été examinées en utilisant les méthodes microscopiques et spectroscopiques tels que le MEB, le MET, l'EDX, la DRX, l'XPS, le Raman et la FTIR. Dans un premier temps, les performances des films élaborés (de PoPD et de NTC/PoPD) ont été étudiées pour le stockage d'énergie. Deux configurations ont été examinées, la première à trois électrodes qui consiste à étudier principalement les mécanismes de transferts des ions dans un électrolyte aqueux, et la deuxième configuration à deux électrodes consiste à étudier les performances capacitives de ces films. Afin de bien comprendre les mécanismes de stockage d'énergie dans ces matériaux, une étude électrogravimétrique a été effectuée en utilisant des méthodes électrogravimétriques. Notamment la méthode d'ac-électrogravimétrie, qui est une technique analogue à l'impédance électrochimique et qui permet d'identifier les espèces impliquées lors du processus d'échange au sein de l'électrode. Dans un deuxième temps, nous avons élaboré un nanocomposite ternaire à base de NFC, de PpPD et de NiPs pour l'oxydation du méthanol. L'optimisation des conditions expérimentales a été prise en compte et plusieurs paramètres ont été étudiés et optimisés tels que l'épaisseur du film de PpPD, le taux de particules de Nickel, la concentration du méthanol, etc.

Chapitre I : 1 .

 :1 Etat de l'art. Le premier chapitre présente une étude bibliographique fondamentale sur les supercondensateurs et les piles à combustible. Les principes fondamentaux du stockage de l'énergie (supercondensateurs) et de la conversion l'énergie (piles à combustible au méthanol directes) ont été abordés, incluant l'historique, le principe de fonctionnement et les applications possibles. Différents matériaux d'électrodes utilisés dans les supercondensateurs et les piles à combustible à méthanol direct ont été détaillés et discutés. Stockage d'énergie : supercondensateur. Ces dernières années, les supercondensateurs ont beaucoup attiré l'attention en raison de leur densité de puissance élevée, de leur longue durée de vie et de leur taux de charge / décharge rapide. Le premier brevet de supercondensateur a été déposé en 1957 par Howard Becher du groupe American General Electric. Le supercondensateur est un dispositif de stockage d'énergie similaire aux batteries. Les supercondensateurs sont constitués de deux électrodes en contact avec un électrolyte et d'un séparateur permettant une isolation électrique entre les électrodes. Le composant le plus important dans un supercondensateur est le matériau de l'électrode. Généralement, les matériaux d'électrode sont fabriqués à partir d'un nanomatériau poreux ayant une surface spécifique élevée.

Figure 1 .

 1 Figure 1. Représentation schématique d'un supercondensateur à l'état chargé et déchargé. La capacité, la densité d'énergie et la densité de puissance sont les paramètres les plus importants pour caractériser les supercondensateurs. Ces paramètres peuvent être facilement mesurés à l'aide des équations suivantes :

Figure 2 . 2 .

 22 Figure 2. Quelques applications de supercondensateurs Les nombreuses recherches effectuées sur les matériaux d'électrode pour les supercondensateurs montrent que le choix du matériau joue un rôle très important sur les performances capacitives du dispositif. Par conséquent, la majorité des recherches en cours sur les SC portent sur les matériaux d'électrodes afin d'améliorer encore les performances capacitives par rapport aux matériaux d'électrodes existants. Le tableau suivant présente les capacités spécifiques des différents matériaux d'électrodes étudiés dans la littérature.

Figure 4 . 2 𝑂 2 + 2 →

 4222 Figure 4. Représentation schématique d'une pile à combustible a méthanol direct. Le méthanol et l'eau sont versés dans le réservoir de carburant de la DMFC, puis le méthanol est oxydé sur l'ACL. Cette réaction d'oxydation du méthanol génère des protons, du dioxyde de carbone et des électrons selon la réaction suivante :

  La polymérisation électrochimique de l'oPD a été réalisée sur une électrode d'or (0,2 cm 2 ) de résonateurs à quartz (9 MHz-AWS, Valence, Espagne) par la CV dans une gamme de potentiel de -0.35 à + 0.8 V vs. Ag/AgCl, à 50 mV.s -1 (Figure5(a)). Les variations de masse sont enregistrées simultanément durant l'électropolymérisation (Figure 5 (b)). Le monomère oPD (1,2diaminobenzène, Sigma-Aldrich) (5 mM) dissous dans une solution de H 2 SO 4 (0.1 M) a été électropolymérisé en utilisant une voltammétrie cyclique (CV). Cette étape a également été suivie par des mesures EQCM.

Figure 5 .

 5 Figure 5. Courbes de CV de l'électro polymérisation de oPD sur une d'or de quartz dans une solution aqueux de H 2 SO 4 (0.1 M) + oPD (5 mM) (A) et les courbes de la variation de la masse lors de l'électro polymérisation obtenus par EQCM à une vitesse de balayage de 50 mV•s -1 (B). Au cours du premier balayage, un pic d'oxydation avec un courant important apparaît à environ +0,62 V vs Ag/AgCl, indiquant l'oxydation du monomère oPD. Lors des cycles suivants, le courant de ce premier pic diminue de manière significative, alors que le courant d'un nouveau pic redox à E cathodic = -0,16 V et E anodic = -0,12 V vs Ag/AgCl est évident. L'apparition d'autres pics redox est également notée vers 0 V vs Ag/AgCl. Ensuite, ces deux pics redox (dans la plage de 0,2 V à -0,4 V vs Ag/AgCl) sont fusionnés comme observé pour le 200 ème cycle de la courbe CV présentée sur la Figure 5 (A).

Figure 7 .

 7 Figure 7. (A) courbes de CV et (B) les courbes des variations de masse correspondantes (m) du film de PoPD dans une solution de NaCl (0.5 M), à un pH=2 et une vitesse de balayage de 50 mV•s -1 .

Figure 7 (

 7 B).Une fonction de transfert importantes, Δm/ΔE() à obtenue à partir de ac-électrogravimétrie, cette fonction de transfert peut estimer la nature des espèces chargés et prend également en compte la contribution de solvant.

Figure 8 ..

 8 Figure 8. ∆𝑚 ∆𝐸 (𝜔) à (A) -0.05V et (B) -0.25V du film de PoPD mesurée dans une solution de 0.5M NaCl à pH=2. Les Figures 8 (A) et (B) présentent les fonctions de transfert expérimentales et théoriques Δm/ΔE() qui montrent plusieurs demi-cercles bien définis apparaissant à différents quadrants.Il est important de noter que les contributions dans les premier/deuxième et les

Figure 9 .

 9 Figure 9. Les résultats de l'ac-électrogravimétrie du film de PoPD. (A) La variation de la concentration relative (𝑪 𝒊 -𝑪 𝟎 ), (B) la variation de la masse relative de chaque espèce mesurée dans une solution de NaCl (0.5M), pH=2 et (C) la reconstruction de la variation de la masse totale à partir des résultats de l'ac-électrogravimétrie et la comparaison avec la réponse de masse de l'EQCM à 25 mV•s -1 .

  et (ii) son transfert est plus rapide que celui de Na + (Figure8) dans les conditions de notre étude.

  la masse du film de PoPD (obtenue par QCM), v est la vitesse de balayage, E 1 et E 2 sont les potentiels haut et bas et I(E) représente la réponse en courant.Les électrodes de film mince de PoPD utilisées dans les sections précédentes mesuraient environ 4,4 µg sur 0,2 cm 2 de l'électrode d'or des résonateurs à quartz (22 g•cm -2 ). La masse de PoPD a été déterminée en mesurant les changements de fréquence avant et après la formation du film. Tout d'abord, les valeurs de C s en fonction de la vitesse de balayage utilisé dans l'analyse CV sont présentées sur la Figure10. Pour cette analyse, on a utilisé la charge typique d'environ 22 µg•cm - 2 , ce qui correspond à une épaisseur de film d'environ 150 nm estimée par analyse du MEB (Figure6).La C s ont été mesurées pour une vitesse de balayage varie entre 10 et 200 mV.s -1 (Figure10). Les valeurs de C s augmente quand on diminue la vitesse, elle passe de 370 F•g -1 à 10 mV•s -1 à 420 F•g - 1 à 50 mV•s -1 . A partir de 50 mV•s -1 , les valeurs de C s diminuent et attient une valeur de 260 F•g - 1 . Le maximum de C s est enregistré pour une vitesse de balayage de 50 mV•s -1 . Cependant, les valeurs de C s aux vitesses de balayage de 10 et 25 mV•s -1 sont légèrement inférieures. Ce comportement a déjà été observé pour des électrodes minces à base de polymère conducteur ou de métal/oxyde métallique dans des électrolytes aqueux. En outre, les valeurs de Cs obtenues dans notre étude sont du même ordre de grandeur ou même plus élevées que certains des rapports

Figure 10 .Figure 11

 1011 Figure 10. La variation de la capacité spécifique (C s ) du film de PoPD déposé sur une électrode d'or de QCM mesurée dans une solution de NaCl 0.5M, pH=2 en fonction de la vitesse de balayage (22 g•cm -2 ).

Figure 11 . 1 A 1 .

 1111 Figure 11. Les courbes de charge/décharge galvanostatique de PoPD supercondensateur à 6,3 A•g -1 . (A) et (B) sont des courbes de potentiel-temps à différentes échelles de temps montrant la stabilité à long terme des électrodes. (C) les capacités de décharge en fonction du nombre de cycles.

Figure 12 .

 12 Figure 12. Electropolymérisation d'oPD sur une électrode d'or et une électrode de SWCNT.

2 .

 2 Caractérisation électrogravimétrique par EQCM et ac-électrogravimétrie. 2.1. EQCM La Figure 14 présente l'évolution de la réponse ∆𝑚 ∆𝐸 (𝜔) du composite SWCNT / PoPD. La courbe montre deux boucles bien définies apparues dans différents quadrants. L'ajustement des données expérimentales ∆𝑚 ∆𝐸 (𝜔) montre un bon accord entre les courbes expérimentales et théoriques et révèle la participation de trois espèces chargées (anions: Cl -; cations: Na + et H + ) et le solvant (molécules H 2 O).

Figure 14

 14 Figure 14. ∆𝑚 ∆𝐸 (𝜔) à 0 V du film de SWCNT/PoPD mesurée dans une solution de 0.5M NaCl à pH=2.

3 .Figure 15 . 1 . 2 .

 31512 Figure 15. La capacité de décharge en fonction du nombre de cycle de charge/décharge galvanique (GCD) de l'électrode SWCNT/PoPD à 7,3 A•g-1 (0,73 mA•cm -2 ) Chapitre V : Synthèse du nanocomposite de nanofibres de carbone/poly (para-phénylènediamine)/particules de nickel pour l'électrooxydation du méthanol. Nous rapportons dans ce chapitre la synthèse d'un catalyseur à faible coût basé sur des particules de nickel (NiPs), des nanofibres de carbone (NFC) et de la poly (para-phénylènediamine) (PpPD) par une méthode électrochimique. La morphologie et la structure des nanocomposites sont caractérisées par MEB, EDS, DRX, FTIR et Raman. Les effets de plusieurs paramètres tels que l'épaisseur du film de PpPD et la teneur en NiPs sur les performances électrocatalytiques du nanocomposite NFC/PpPD/NiPs sont évalués, pour obtenir une composition optimale. L'effet électrocatalytique du nanocomposite pour l'électrooxydation du méthanol est testé dans une solution alcaline. Nous résultats mettent en évidence l'effet synergique du NFC/PpPD sur l'amélioration de l'activité et de la stabilité électrochimiques du nanocomposite NFC/PpPD/NiPs en tant que catalyseur et électrodes à faible coût pour les DMFC. 1. Préparation du NFC/PpPD/NiPs.

Figure 16 . 1 .

 161 Figure 16. (a) Image de MEB, (b) spectre EDS ,(c) spectre DRX ,(d) spectre Raman, (e) spectre FTIR ; du nanocomposite NFC /PpPD/NiPs.

Figure 19 .

 19 Figure 19. Courbes de CV de NFC/PpPD/Cu dans une solution de NaOH (0.1 M) avec et sans la presence de 1.5 M de methanol (a), Courbes de CV de PpPD/Cu, NFC/Cu and NFC/PpPD/Cu dans une solution de NaOH (0.1 M) contient 1.5 M de methanol (b), Courbes de CV de NFC/PpPD/Cu dans une solution de NaOH 0,1 M avec différentes concentrations de méthanol, et courbe de chronoampérométrie NFC/PpPD/Cu dans une solution de NaOH (0.1 M) contient 1.5 M de methanol (d).

Table I . 1 .

 I1 Different Carbon Structures Used in EDLCs.

	Material	Carbon	Carbon	Graphene	Activated	Carbide	Templated
		onions	nanotubes		carbon	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

Carbon materials present others advantages such as:

Table I . 2 .

 I2 Comparison of synthesis methods of CNTs 112 .

	Method		Arc-discharge		Laser-ablation	CVD
	Pioneer		Iijima 113			Guo et al. 114	Yacaman et al. 115
	How		CNT	growth	on	Vaporization of a	Fixed bed method:
			graphite	electrodes	mixture of carbon	acetylene decomposition
			during direct current	(graphite)	and	over graphite-supported
			arc-discharge		transition	metals	iron particles at 700°C
			evaporation of carbon	located on a target to
			in presence of an inert	form CNTs
			gas
	Yields		< 75%			< 75%	>75%
	SWCNT	or	Both			Only SWCNTs	Both
	MWCNT
	Advantages		Simple, inexpensive		Relatively high purity	Simple, inexpensive, low
						CNTs,	room-	temperature, high purity
						temperature synthesis	and high yields, aligned
						option	with	growth	is	possible,
						continuous laser	fluidized bed technique
							for large-scale
	Disadvantages Purification of crude	Cannot	produce	CNTs	are	usually
			product is required,	MWCNTs,	method	MWNTs,	parameters
			method cannot be	only adapted to lab-	must closely be watched
			scaled up, must have	scale, crude product	to obtain SWNTs
			high temperature		purification required
	In the Table I.3 we report a summary of the specific capacitance of different types of carbon
	electrode materials.

Table I . 6 .

 I6 Composite-based electrode for supercapacitors.

	Electrode	Specific	Electrolyte	Cycling		Scan Rate or	references
	material	capacitance		stability		Current
						density
	PANI/rGO	409 F.g -1	1M H 2 SO 4	81% after 5000	1 A.g -1	159
				cycles
	PANI/GO	276 F.g -1	1M H 2 SO 4	80% after 500	0.5 A.g -1	160
				cycles
	PANI/CNF	234 F.g -1	1M H 2 SO 4	90% after 1000		161
				cycles
	PPy/CNT	202 F.g -1	H2PO4/PVA	50% after 500	1.8 A.g -1	162
				cycles
	PPy/GN	590 F.g -1	1M KCl	91% after 1000	5 A.g -1	163
				cycles
	GO/PPy	370 F.g -1	1M H 2 SO 4	91% after 4000	0.5 A.g -1	164
				cycles
	PEDOT/rGO	103 F.g -1	6M KOH	60% after 1000	10 mV.s -1	165
				cycles
	N-MC/PoPD	229 F.g -1	6M KOH	90% after 1000	1 A.g -1	154
				cycles
	r-GO-a-PoPD	381 F.g -1	1M H 2 SO 4	90% after 5000	1 A.g -1	148
				cycles
	PPD-C-DCNT	388 F.g -1	1M H 2 SO 4	85,7%	after	1 A.g -1	166
				10000

Table I . 7 :

 I7 Comparison of the most important fuel cell types.

	Type of fuel	DMFC	H2-PFMFC	SOFC
	cell
	Operating	<120	<120	800-1000
	temperature (ºC)
	Type of	Nafion membrane	Nafion membrane	Yttria stabilized
	electrolyte	-Methanol-	-Hydrogen-	zirconia
	Charge carrier	H +	H +	O 2-
	in the electrolyte
	Anode reaction	CH 3 OH + H 2 O → CO 2 + 6H +	H 2 → 2H + + 2e -	H 2 + O 2-
		+ 6e -		→ H 2 O + 2e -
	Cathode reaction	3 2 ⁄ O 2 + 6H + + 6e -→ 3H 2 O	1 2 ⁄ O 2 + 2H + + 2e -→ H 2 O	1 2 ⁄ O 2 + 2e -→ O 2-
	Anode electrode	Pt. PtRu	Pt. PtRu	Ni YSZ
	Cathode	Pt	Pt	Lanthanimstrotium
	electrode			manganite (LSM)
	Released power	< 5 kW	5-250 kW	100-250 kW
	Advantages	Easy fuel storage, simple	High-power density	Diversified fuel, non-
		structure without thermal		noble metal for
		management		catalyst
	Disadvantages	Low-power density	Low energy density of storing	Slow start-up due to
			hydrogen at high pressure or in	high operating
			metal hybrid	temperature
	DMFC shows a simple structure, high energy density and low pollution, all these advantages make
	this technology as promising candidate for application in portable electronic devices. DMFCs
	have theoretical energy density, which is 15 times more than that of Li-ion batteries. For these
	reasons, the second part (Chapters V and VI) of our Ph.D. thesis will focus on the development
	of catalysts for DMFC applications. Therefore, the following paragraph is dedicated to DMFCs.

Table I .8

 I Summarizes other advantages and disadvantages of DMFC.

	Table I.8. Advantages and disadvantages of DMFC.
	Advantages	Disadvantages
	-The energy density of methanol is much
	greater than that of hydrogen.

in Figure I.14 180 . Figure I.14. Schematic

  [171][172][173][174] . Sharma et al.173 used platinum metal supported on reduced graphene oxide illustration of bifunctional mechanism for PtRuNi/MWCNTs electrode surface in the methanol solution180 .Pt catalyst is easily poisoned in acidic media by the adsorption of CO during the MOR, but in alkaline media this poisoning effect is very weak 187 . The electrooxidation of methanol on PtWang et al. 190 studied the catalytic performance of Pt and Pt_Pd catalysts supported on nitrogendoped graphene in alkaline solution. This work shows that during the MOR in alkaline media Pt adsorbs OH species. These species react with the adsorbed species to form CO 2 .Lu et al. 191

	(Pt/RGO) as electrocatalyst for methanol oxidation in acidic solution. In comparison with
	commercial Pt/C catalyst, Pt/RGO showed an unprecedented CO poisoning tolerance and
	exhibits high catalytic mass activity associated to a high electrochemical active surface area was
	noticed. Wang et al. 175 developed a catalyst system based on Pt nanoparticle supported on carbon
	nanofibers (CNFs) (Pt/CNF). This catalyst was applied for methanol oxidation in H 2 SO 4
	solution. The performance of Pt/CNF catalyst was compared to that of the commercial E-TEK
	Pt/C catalyst. The results show that Pt/CNF exhibit a higher electrocatalytic activity toward
	methanol oxidation reaction and CO poisoning tolerance. The Kinetics of MOR in acidic media
	are very slow at low over-potential and Pt metal suffers from CO adsorption, which decreases its
	catalytic activity and stability. Many research have done on Platinum-Ruthenium alloy (Pt-Ru) as
	promising catalyst for DMFC in acidic media 176-178 . Ru is known by its ability to oxidize CO into CO 2 through the bi-functional mechanism and/or a ''ligand effect'' in electrocatalyst forming OH species on the surface of Ru at low potential 177 according to the following reactions: 𝑃𝑡 + 𝐶𝐻 3 𝑂𝐻 → 𝑃𝑡𝐶𝑂 𝑎𝑑𝑠 + 4𝐻 + + 4𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟕) 𝑅𝑢 + 𝐻 2 𝑂 → 𝑅𝑢(𝑂𝐻) 𝑎𝑑𝑠 + 𝐻 + + 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟖) 𝑃𝑡𝐶𝑂 𝑎𝑑𝑠 + 𝑅𝑢(𝑂𝐻) 𝑎𝑑𝑠 → 𝐶𝑂 2 + 𝑃𝑡 + 𝑅𝑢 + 𝐻 + + 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟗) Several approaches have been considered to further improve the activity of Pt-Ru catalyst towards methanol oxidation reaction such as the catalyst structure, morphology, composition and addition of a third metal. Recently, many metals have been introduced to Pt-Ru alloy such as Ni 179,180 , Cu 181 , Mo 182,183 , Fe 184-186 . Zhao et al. 180 electrodeposited PtRuNi alloy on MWCNTs using a full-electrochemical deposition. The electrocatalytic properties of PtRuNi/MWCNTs nanocomposite for methanol oxidation reaction were investigated. The PtRuNi/MWCNTs shows improved resistance to CO poisoning and high catalytic activity, which is attributed to the good synergetic effect between the nanoparticles and MWCNTs and the three dimension flowerlike structure leading to a much higher accessible surface area of Pt. The synergetic effect adsorbed species, which in turn react with OH species to produce CO 2 . The reaction mechanism is showed below 188,189 : 𝑃𝑡 + 𝑂𝐻 -→ 𝑃𝑡 -(𝑂𝐻) 𝑎𝑑𝑠 + 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟎) 𝑃𝑡 + (𝐶𝐻 3 𝑂𝐻) 𝑠𝑜𝑙 → 𝑃𝑡 -(𝐶𝐻 3 𝑂𝐻) 𝑎𝑑𝑠 (𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟏) 𝑃𝑡 -(𝐶𝐻 3 𝑂𝐻) 𝑎𝑑𝑠 + 𝑂𝐻 -→ 𝑃𝑡 -(𝐶𝐻 3 𝑂) 𝑎𝑑𝑠 + 𝐻 2 𝑂 + 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟐) 𝑃𝑡 -(𝐶𝐻 3 𝑂) 𝑎𝑑𝑠 + 𝑂𝐻 -→ 𝑃𝑡 -(𝐶𝐻 2 𝑂) 𝑎𝑑𝑠 + 𝐻 2 𝑂 + 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟑) 𝑃𝑡 -(𝐶𝐻 2 𝑂) 𝑎𝑑𝑠 + 𝑂𝐻 -→ 𝑃𝑡 -(𝐶𝐻𝑂) 𝑎𝑑𝑠 + 𝐻 2 𝑂 + 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟒) 𝑃𝑡 -(𝐶𝐻𝑂) 𝑎𝑑𝑠 + 𝑂𝐻 -→ 𝑃𝑡 -(𝐶𝑂) 𝑎𝑑𝑠 + 𝐻 2 𝑂 + 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟓) 𝑃𝑡 -(𝐶𝐻𝑂) 𝑎𝑑𝑠 + 𝑃𝑡 -(𝑂𝐻) 𝑎𝑑𝑠 + 2 𝑂𝐻 -→ 2 𝑃𝑡 + 𝐶𝑂 2 + 2 𝐻 2 𝑂 + 2 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟔) 𝑃𝑡 -(𝐶𝐻𝑂) 𝑎𝑑𝑠 + 𝑃𝑡 -(𝑂𝐻) 𝑎𝑑𝑠 + 𝑂𝐻 -→ 𝑃𝑡 + 𝑃𝑡 -(𝐶𝑂𝑂𝐻) 𝑎𝑑𝑠 + 𝐻 2 𝑂 + 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟕) 𝑃𝑡 -(𝐶𝑂) 𝑎𝑑𝑠 + 𝑃𝑡 -(𝑂𝐻) 𝑎𝑑𝑠 + 𝑂𝐻 -→ 2 𝑃𝑡 + 𝐶𝑂 2 + 𝐻 2 𝑂 + 𝑒 -(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟖) between the three metals was explained by the bifunctional mechanism showed catalyst in alkaline environment undergoes several successive reactions, these reactions produce 𝑃𝑡 -(𝐶𝑂) 𝑎𝑑𝑠 + 𝑃𝑡 -(𝑂𝐻) 𝑎𝑑𝑠 → 𝑃𝑡 + 𝑃𝑡 -(𝐶𝑂𝑂𝐻) 𝑎𝑑𝑠 (𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟏𝟗)
	𝑃𝑡 -(𝐶𝑂𝑂𝐻)

𝑎𝑑𝑠 + 𝑂𝐻 -→ 𝑃𝑡 -(𝑂𝐻) 𝑎𝑑𝑠 + 𝐻𝐶𝑂𝑂 - (𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟐𝟎) 𝑃𝑡 -(𝐶𝑂𝑂𝐻) 𝑎𝑑𝑠 + 𝑃𝑡 -(𝑂𝐻) 𝑎𝑑𝑠 → 2 𝑃𝑡 -𝐶𝑂 2 + 𝐻 2 𝑂 (𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟐𝟏)

  In contact with alkaline solution, nickel has been shown to become covered with nickel hydroxide layer, which play an important role toward methanol oxidation reaction. Therefore, an activation process in alkaline media is highly recommended. A cyclic voltammogram run at a nickel catalyst in NaOH solution is shown in Figure I.18. During the activation process, a pair of well-defined

	redox peaks appeared, which correspond to the redox Ni(OH) 2 /NiOOH transformation
	(reaction I.24). This process shows the formation a rapid conversion of a surface layer and not
	diffusion or kinetically controlled reaction step 196 .
	𝑁𝑖(𝑂𝐻) 2 + 𝑂𝐻 -↔ 𝑁𝑖𝑂𝑂𝐻 + 𝐻 2 𝑂 + 𝑒 -	(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟐𝟒)
	In the subsequent scans, the current density of the redox peaks increases indicating the
	continuous enhancement of the accessible electroactive species (Ni(II) and Ni(III)) at the
	electrode surface and the NiOOH grown by the entry of OH-ions into Ni(OH) 2 198,199 .
	, during the MOR on Ni-based catalyst, some species are formed such as formate,
	formaldehyde and carbonate. Formaldehyde is an active intermediate specie for methanol
	oxidation. Fleischmann et al. 199 have proposed a general mechanism for primary alcohol
	oxidation:
	𝑁𝑖𝑂𝑂𝐻 + 𝑅𝐶𝐻 2 𝑂𝐻 → 𝑁𝑖(𝑂𝐻) 2 + 𝑅𝐶 ∎ 𝐻𝑂𝐻	(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟐𝟑)
	𝑅𝐶 ∎ 𝐻𝑂𝐻 + 3𝑂𝐻 -→ 𝑅𝐶𝑂𝑂𝐻 + 2𝐻 2 𝑂 + 3𝑒 -	(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟐𝟒)

  Good results were obtained and were attributed to the fact that RGO/CF support has low charge transfer resistance and offers abundant graphitic-N and oxygen functions with strong interaction with PtNPs. Bessel et al.212 shows that 5 wt.% Pt deposited on platelet and ribbon graphite nanofibers present activities comparable to that displayed by about 25 wt% platinum on Vulcan carbon. Moreover, the graphite nanofibers support reduce the CO poisoning than the traditional catalyst support.

	Carbon nanotubes (CNTs)
	CNTs are the most widely explored carbon nanomaterials as catalyst for fuel cell applications.
	They have emerged as a promising material candidate to replace carbon black. The deposition
	of metal catalyst on CNTs is difficult because they are chemically inert, therefore, the
	functionalisation of CNTs surface using mixture of acids is very recommended to improve their
	interaction with metal catalyst. In DMFCs, CNTs have been largely studied and used as support
	for both noble and non-noble metals. Zhao et al. 180 studied PtRu and PtRuNi clusters supported
	on MWCNTs as electrocatalyst for MOR. In this study, MWCNTs were electrochemically
	activated to generate oxygen functional groups, which improve the catalytic performance of the
	electrocatalyst and resistance to CO poisoning. Liang et al. 213 reported the synthesis of nitrogen-
	doped oxidized CNTs as support for Pt catalyst. The obtained nitrogen-doped CNTs improved
	the dispersion of Pt nanoparticles and facilitated the MOR by modifying the electronic structure
	of Pt catalyst, through catalyst-support interaction. In other research, Yousaf et al. 214 combined

211

reported graphene-embedded carbon fiber as support for Pt nanoparticle (PtNPs) for methanol oxidation and oxygen reduction reactions.

Table I . 9 .

 I9 Current application of DMFC 227 .

	Field	Examples of application and company	References
	Education	• ENESSERE Horizon Bio Energy Education Kit
		(ENESSERE)
			229,230
	Medical	Yorkshire Ambulance services (UK)	231,232
		Hearing aid (Denmark)
	Military	Portable military device	233,234
		On board military vehicle generator
	Security	Smoke detector	232,235
		Fire alarm
		Police radio station
	Telecommunication Backup power supply for telecommunication base station
		(IdaTech)
			237

228

Electronic

• Fuel cartridge for laptops (Toshiba, LG, Panasonic, NEC, Fujitsu, Samsung)

• Mobil phone charger (DoCoMo, Fujitsu)

• Power pack (Toshiba, Panasonic) 236 Industrial Material handling equipment (Oorja Protonics)

Table I .

 I 10. DMFC challenges and suggested solutions 168 .

	Challenges	Suggested solutions
	Slow anodic kinetics.

• Develop a new anode catalyst with high catalytic effect.

Table III . 2 :

 III2 Papers about Polyaniline and Polypyrrole.

	Material	Scan Rate	Electrolyte	Specific	Capacity	retention	reference
		or Current		capacitance	(number of cycles)
		density
	PPy-NW	20 mV.s -1	H 2 SO 4 (1M)	110 F.g -1	̴ 25% (5000)		11
	PANI-NW	20 mV.s -1	H 2 SO 4 (1M)	191 F.g -1	̴ 42% (5000)		11
	PPy	50 mV.s -1	H 2 SO 4 (0.5M)	230 F.g -1	87% (1000)		44
	PPy	50 mV.s -1	H 2 SO 4 (0.5M)	250 F.g -1	82% (1000)		44
	PPy	100 mV.s -1	H 2 SO 4	215 F.g -1			45
	PANI	4 mA.cm -2	NaNO 3 (1M)	150 F.g -1			46
	PANI	50 mV.s -1	H 2 SO 4 (0.5M)	451 F.g -1			44
	PPY	3 A.g -1	KCl (1M)	576 F.g -1	82% (1000)		55
	PANI	20mV.s -1	H 2 SO 4 (0.5M)	192 F.g -1	67% (5000)		56
	PoPD	50mV.s -1	NaCl (0.5M)	434 F.g -1			This
		6.3 A•g -1	NaCl (0.5M)	110 F.g -1	100% (8000)		work

Table IV .

 IV

3. Comparison of electrochemical performance of PoPD and PoPD based nanocomposites with literature data.

  . A methanol oxidation mechanism on Ni-based electrode is proposed based on previous works where methanol is oxidized by NaOH in alkaline aqueous solution according to the following reaction 27 : As shown in the Figure V.9, CPE/CNF/PpPD/NiPs exhibited superior electrocatalytic activity towards methanol electrooxidation with a current density of 38.1 mA cm -2 , in comparison with CPE/CNF/NiPs, which displayed a current density of 30 mA cm -2 . A significant increase in the current density by 25.6% was shown in the presence of the PpPD polymer. This improved catalytic activity is attributed to the amine groups present in the polymer chain, which play an important role to improve the activity and durability of the NiPs 60 . Elbasri et al. reported that CPE/NiPs shows current density of 0.57mA cm -2 61 , in comparison with our electrode, the current density increases by 6680% with the addition of CNF/PpPD. Synthesis of carbon nanofibers/poly(para-phenylenediamine)/nickel particles nanocomposite for enhanced methanol electrooxidation

	Chapter V
	𝑁𝑖𝑂𝑂𝐻 + 𝑚𝑒𝑡ℎ𝑎𝑛𝑜𝑙 → 𝑁𝑖(𝑂𝐻) 2 + 𝑝𝑟𝑜𝑑𝑢𝑐𝑡𝑠	(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑽. 𝟐)
	Generally, during the MOR, species such as formate, formaldehyde and carbonate are formed.
	Formaldehyde is an active intermediate species for methanol oxidation. Fleilschmann et al. have
	proposed a general mechanism for primary alcohol oxidation 58,59 :
	𝑁𝑖𝑂𝑂𝐻 + 𝑅𝐶𝐻 2 𝑂𝐻 → 𝑁𝑖(𝑂𝐻) 2 + 𝑅𝐶 ∎ 𝐻𝑂𝐻	(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑽. 𝟑)
	𝑅𝐶 ∎ 𝐻𝑂𝐻 + 3𝑂𝐻 -→ 𝑅𝐶𝑂𝑂𝐻 + 2𝐻 2 𝑂 + 3𝑒 -	(𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑽. 𝟒)
		158

Table V . 2 .

 V2 Mass of NiPs as a function of electrical charge (Q) associated with NiPs formation.

Table V . 3 .

 V3 The comparison of the catalytic performance of the modified electrodes.

	Catalyst

Scan rate (mV s -1 ) Concentration of methanol (M ) Current density of the methanol electrooxidation peak (mA cm -2 ) References

	Rt-NG	20	1	0.15	64
	Pt@TiN/rGO	50	0.5	3.09	65
	Cu 2 O/PPy-Go	100	2	0.3	66
	Pt/PDDA-G	20	1	40	67
	Ni/SPPy	100	2	0.38	16
	Pt-Ru/C
	(Commercial)

. However, this value is exhibited only for very thin oxide films or optimized nanostructured materials. Its capacitive properties have been widely studied and can be described as a fast and

𝑂 2 + 6𝐻 + + 6𝑒 -→

3𝐻 2 𝑂 + ℎ𝑒𝑎𝑡 (𝑹𝒆𝒂𝒄𝒕𝒊𝒐𝒏 𝑰. 𝟓)

Acknowledgements

My heartfelt thanks go to the Ph.D student Yousra Massaoudi for her moral support and for her help during the redaction of my thesis and my papers.

This work was sponsored by MESRSFC (Ministère de l'Enseignment Supérieur et de la my appreciation of the Erasmus + Program.

on CNFs seems to be a crucial strategy to enhance the performance of electrochemical catalysts.

Furthermore, the recent developments in the electrospinning methods make the synthesis of CNFs and metal-decorated CNFs more simple and affordable 28 . Another approach and one of the best ways to prevent poisoning of the electrode and to reduce catalyst loss and cost is to use a combination of supporting materials such as carbon nanomaterials and polymers. Sen et al.

studied the MOR on Pt catalyst deposited on different carbon nanomaterials [29][30][31] and on composite supports formed with reduced graphene oxides/polyaniline or graphene oxide/polyvinylpyrrolidone. The prepared materials exhibit a better catalytic activity than the individual metal-carbon catalysts 32,33 . In a similar approach, the use of conducting polymers (CPs) such as polypyrrole (Ppy) 17 , polyaniline (Pani) 34 , polydiaminonaphthalene 35 with carbon nanomaterials has been suggested as hybrid support for Ni catalyst. Mao et al. 17 compared the catalytic performance of NiS/Ni(OH) 2 @Go and NiS/Ni(OH) 2 @Ppy/Go catalysts toward methanol oxidation. It was found that the NiS/Ni(OH) 2 @Ppy/Go catalyst exhibits better performances for methanol oxidation due to the coordination interaction between N-H segments in Ppy polymer and Ni 2+ . Moreover, an increase in the thermal stability of the composite was noted with the addition of Ppy. Indeed, the electronic states, the nucleation and the growth process of the metal catalyst can strongly be affected by the presence of amine groups on the surface of the electrode. In this regard, Poly (para-phenylenediamine) (PpPD) is a conducting polymer having the required chemical attributes and provides a porous network including a high number of amine groups. The latter can improve the deposition conditions and thus the electronic properties of the metal for catalytic conversion 36 . The potential of PpPD has been recognized and therefore combined with rGO or multiwalled carbon nanotubes (MWCNTs) as an ideal precursor to enhance the catalyst activity and durability for MOR 37,38 .

Based on these previous developments, we developed a new low-cost catalyst for DMFCs, composed of CNFs, PpPD and NiPs using a simple and rapid electrochemical method. Fieldemission gun scanning electron microscopy (FEG-SEM) coupled with an energy dispersive X-ray detector (EDX) were used in order to examine the morphology of the surface of the CPE/CNF/PpPD/NiPs. The structure of the nanocomposite was analyzed by X-ray diffraction For comparison, NiPs supported CNF was also prepared on CPE (CPE/CNF/NiPs) using the same process described above. This study, in addition to demonstrate the applicability of the CNF/PpPD support to other metals (Ni in Chapter V and here Cu), also indicates the good synergistic effect and collective contribution between the catalyst components, which improve the overall electrocatalytic performance for MOR. In the field of energy storage it may be worth considering to study:

General conclusions and perspectives

General conclusions and perspectives

• The ions transfer mechanism of SWCNT/PoPD in organic and ionic liquid electrolytes by EQCM and ac-electrogravimetry methods.

• The performance of SWCNT/PoPD in organic electrolyte and ionic liquid electrolytes for both batteries and supercapacitors.

• The combination of PoPD polymer with metal oxide such as RuO 2 or MnO 2 for supercapacitors could solve the problem of the low energy density.

In the field of energy conversion it may be worth considering to study:

General conclusions and perspectives

• The performance comparison between ortho, meta, and para-phenylenediamine polymers as support for nickel particles.

• Other catalysts such as CNF/PpPD/Co in order to obtain a high catalytic effect and an excellent durability towards methanol oxidation reaction.

• The catalytic performance of SWCNT/PoPD as support for nickel particles for methanol oxidation.

• The performance of CNF/PpPD/NiPs and CNF/PpPD/Cu nanocomposites as an electrode materials for PEMFC.

Résumé de la thèse en français

Dans cette section nous présentons un résumé detaillé en français qui regroupe les informations comportement particulier a été observé. Au cours de la réduction, la masse diminue sur la gamme de potentiel de 0,5 V à 0,1 V, puis la masse augmente de 0,1 V à -0,1 V, après une nouvelle diminution de la masse a été observée de -0,1 V à -0,5 V. Avec une première évaluation de ces résultats, la diminution de la masse peut être attribuée à une contribution majeure des anions et l'augmentation de la masse peut être liée à une réponse importante des cations. L'augmentation du taux de balayage montre un changement sur les variations de masse, qui deviennent plus complexes. Ce comportement peut être lié à un phénomène de transfert de multi-espèces apparaissant avec différentes concentrations et cinétiques. Afin de bien comprendre ce phénomène et d'expliquer les données d'EQCM, des mesures de ac-électrogravimétrie ont été réalisées à différents potentiels de 0,5 V à -0,5 V, ce qui permet d'identifier la nature et le mécanisme de transfert des différentes espèces.

Ac-électrogravimétrie

Des mesures d'ac-électrogravimétrie en ont été effectuées dans une solution aqueuse de NaCl à différents potentiels de 0,5 V à -0,5 V vs Ag / AgCl avec une amplitude de 50 mV.

La composition du nanocomposite a été analysée par l'EDX (Figure 16