The third part of the thesis focuses on the capacitance properties of the different structures of PEDOT obtained previously. Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and EIS were used to characterize the capacitance properties of the PEDOT films. The cycling stability of the PEDOT films was also studied. Among the various structures of PEDOT film obtained, nanowires array, nanowires network and "bottle-like" structure, the "bottle-like" structure shows the best capacitance and cycling stability. Moreover, the capacitance maintains a high value at high scan rate during CV and high current density during GCD. Besides, "bottle-like" PEDOT@C films were synthesized on carbon disk using glucose as C source. The capacitance behavior and cycling stability of these films were investigated, too.

Furthermore, several studies with another monomer, pyrrole (Py), were conducted: i) oxidation of Py monomer in the presence of methyl orange to obtain nanotubes by an electrochemical route; ii) synthesis of PPy/polydopamine nanocomposite; iii) oxidation of Py monomer in the presence of Keggin-type polyoxometalates (POM), H4SiMo12O40, by a combination of chemical and electrochemical routes.

General Introduction

Over recent decades, electrically conductive polymers such as polypyrrole (PPy), polythiophene (PTh), polyaniline (PANI) and their derivatives have attracted great attention across academia and industry. Among all the conductive polymers, poly (3,4-ethylenedioxythiophene) (PEDOT) has developed into one of the most popular materials due to its high and stable electrical conductivity in oxidized state. It has been widely applied for supercapacitors, electrochemical sensors, thermoelectric devices, electrochromism, and so forth. Despite the huge attraction of PEDOT for scientists from different fields, the template-free electrochemical polymerization of PEDOT nanostructures are still under investigation.

In this thesis, based on previous results obtained for PPy, we mainly focused on template-free electrosynthesis of overoxidized PEDOT (O-PEDOT) film and nanostructured PEDOT in an aqueous medium. The influence of the applied potential, the pH of the monomer solution containing phosphate anions and other influencing factors were studied. Various nanostructures of PEDOT were obtained, their capacitance properties were evaluated.

The first part of the thesis presents the electrosynthesis of an O-PEDOT film under potentiostatic conditions. In an aqueous electrolyte under an anodic polarization, most of the conductive polymers electrogenerated could undergo an overoxidation due to the hydroxyl radicals formed during a side reaction that is water oxidation. We developed a facile and rapid strategy to prepare an ultrathin (15 nm), pinhole-free and non-conductive O-PEDOT film by electropolymerization of EDOT under potentiostatic conditions in the presence of solely weak-acid ions (phosphate). The influence of the pH of the monomer solution as well as the applied potential were studied notably by electrochemical impedance spectroscopy (EIS) and X-ray photoelectron spectroscopy (XPS).

In the second part of the thesis, PEDOT films having various nanostructures have been electrogenerated. The influence of different parameters such as the applied potential, the pH and the anion concentration has been studied and the mechanism of the formation of the nanostructures was discussed. For the first time, we obtained a novel "bottle-like" nanostructure of PEDOT by adding a small amount of LiClO4 in the monomer phosphate solution.

State of the art 1.1 Introduction

In the last few decades, conductive polymers (CPs) have attracted more and more attention from scientists of different fields across academia and industry. In the late 1970s, Alan J. Heeger, Alan G. MacDiarmid and Hideki Shirakawa discovered that silvery films of doping polyacetylene (PA, (CH)x) can achieve high conductivity, over seven orders of magnitude, when exposed to iodine vapor [1,2]. This process is also known as "doping". In 2000, the Nobel Prize in Chemistry was awarded jointly to them for their discovery and development of conductive polymers. Conductive polymers stand out because of the unique combination of their mechanical, electrical, optoelectronic, thermoelectric, photovoltaic and lighting properties. As shown in Figure 1.1, the publications related to the topic of conductive polymers increased year by year over the last 30 years. Among all the CPs, the number of publications focused on PEDOT shows a significant growth trend due to its high stability and excellent chemical and physical properties over various fields. At first, CP was not well accepted as a useful and promising material because the low stability of PA in air strongly limited its application. However, numerous other CPs were synthesized and studied and their stabilities were substantially enhanced. Afterwards, Heywang and his colleague studied and compared the properties of polypyrrole (PPy) [3], polythiophene (PTh) [4] and PA.

They believed that nitrogen or sulfur plays a role in stabilizing the positive charges of polymers.

And it should be possible to create even more stable polymers by incorporating more substituents to stabilize the positive charges in the doped polymers. Monomeric alkylene dioxythiophenes were chosen to help polymers to achieve the best orientation of the unsaturated backbone and to reduce the steric effects at the same time. Finally, Heywang and his colleague successfully synthesized poly(3,4-ethylenedioxythiophene) (PEDOT), which shows great long-term stability, for the first time at the Bayer research laboratory [5,6]. The structure of PEDOT as well as some other popular CPs structures including polyphenylene (PP), polyaniline (PANI), etc. are shown in Figure 1.2 [7]. 4) polyaniline (PANI); (5) polycarbazole; (6) polyazulene; (7) poly(isothionaphthalene); ( 8) poly(dithienothiophene); ( 9) poly(dithienylbenzene); (10) poly(ethylenedioxythiophene) (PEDOT); (11) poly (3-alkylthiophene); (12) poly(phenylenevinylene)(PPV); (13) poly(thienylenevinylene); (14) poly(bipyrrole), poly(bithiophene). [7] Chapter This structure is usually called "edge-on" texture because well-ordered lamellar stacks in the direction normal to the substrate when cast onto thin films. By contrast, the stacking along the chain backbone and π-π direction are more disordered and lie preferentially along the plane of the substrate. Charge transfer is fast along the chain backbone, moderate between the chains and slow between the lamellar planes. Besides, π-stacks are the most important for intermolecular charge transfer. [9,10] Figure 1. 4 Fast, moderate and slow transport along the chain backbone, the π-π stacking and the lamellar stacking respectively in conductive polymers. [9] The conductivity of conductive polymers, including doped forms of PANI, PPy, PEDOT, electroactive polymeric composites/blends and a selection of metals, semiconductors and insulators are shown in Figure 1.5. Electrical properties of undoped CPs are similar to semiconductors, yet the conductivities are not high enough. To achieve high conductivity, a doping process is necessary.

CPs in the doped state have good intrinsic conductivity from a few S cm -1 to 10 5 S cm -1 [8].

The electrical conductivity σ (S cm -1 ) of a material is expressed by

𝜎 = 𝑛𝑞𝜇 (1.1)
where n represents the charge carrier density (in cm -3 ), q is the electronic charge (C) and µ is the mobility of the charges (cm 2 V -1 s -1 ). and insulators. [11,12] Conductive polymers can be p-doped with (counter) anions when oxidized or n-doped with (counter) cations when reduced [13]. Typical p-dopants are iodine, bromine, ferric chloride and arsenic pentafluoride, while the alkali metals are typical n-dopants [14]. The charging processes can be simplified as 𝐶𝑃 → 𝐶𝑃 𝑛+ (𝐴 -) 𝑛 + 𝑛 𝑒 - (p-doping) (

𝐶𝑃 + 𝑛 𝑒 -→ (𝐶 + ) 𝑛 𝐶𝑃 𝑛- (n-doping) (1.3) Some thiophene-based polymers are p-and n-dopable. In contrast, many polymers, such as PPy and PANI, can only be p-doped because the very negative potentials required for n-doping exceeds the reduction potential limit of molecular solvent-based electrolytes [15]. Thus, they are only employed as cathode materials. What's more, PEDOT is mainly present in its more stable oxidized p-doped form and is hard to de-dope due to the stabilizing effect of the electron donating dialkoxy groups.

Chapter 1 State of the art 8

The evolution of the electronic characteristic of PEDOT at different doping levels is shown in Figure 1.6. [16] The optical features of PEDOT at various oxidation levels are measured by ultraviolet-visible-near infrared (UV/Vis-NIR) spectroscopy. Neutral PEDOT is dark purple/blue and has an optical absorption gap around 1.5 eV. [17] After doped, new energy states are introduced in the band gap and the conductivity of the film increased.

Figure 1.6 Electronic structures of conjugated polymer at different oxidation states (doping levels).

Top, from left to right: (i) a neutral chain, (ii) a chain carrying a polaron, (iii) a chain carrying a bipolaron, (iv) a chain or assembly of chains with a high concentration of polarons (intrachain or interchain polaron network), (v) a chain or assembly of chains with a high concentration of bipolarons (intrachain or interchain bipolaron bands). The optical spectra of these species are illustrated at the bottom. [16] The mechanism of electrical conductivity of doped conductive polymers is shown in Figure 1.7.

The dopant introduces a charge carrier into the system by removing or adding electrons from/ to the polymer chain and relocalizing them as polarons or bipolarons thanks to a surrounded crystal lattice distortion. And the dopants move in or out of the polymers, disrupting the stable backbone and allowing charge to be passed through a polymer in the form of polarons or bipolarons when a potential is applied [18]. it to conduct electricity. [8] However, with the advance of CPs investigation, more mechanisms were developed to explain the charge transfer properties of different kinds of CPs. The variable range hopping (VRH) theory [19,20] and the tunneling theory [21,22] are widely used for disordered CPs. And for certain CPs that have higher conductivities, their charge transport is more similar to disorder metals or "quasi1Dmetals" [23] which can be explained by other mechanisms and equations [24,25].

As for PEDOT, when it is synthesized by oxidative polymerization, some electrons are removed and the crosslink behavior is formed between the generated polarons and dopant anions. Therefore, the p-doped PEDOT film is expanded due to the insertion of anions. What's more, swelling or shrinkage of the film can be further observed during the doping/dedoping process when a negative potential is applied. As shown in Figure 1.8, two circumstances could happen during the reduction process, which involves either anion migration or cation migration depending on the size of the neutralized anions. When the dopant anions are small and highly mobile, the anions can be easily expelled to maintain the neutralization of the PEDOT film, leading to a shrinkage of the film during the reduction process. And during the oxidation process, some anions move into the film from the electrolyte again and cause the film to expand. In a word, the redox process is dominated by anion migration in this condition. On the contrary, when doping anions are large and not easily mobile, such as polystyrene sulfonate (PSS), they are wrapped in the polymer matrix. In this case, the swelling and shrinkage process is driven by cations instead of anions. Cations in the solutions migrate into the polymer to neutralize anions and negative charges and cause the film to expand in the reduced state.

Synthesis of conductive polymers

Conductive polymers can be synthesized by chemical and electrochemical methods.

With respect to PEDOT, three main synthesis methods are reported, as shown in The methods of transition metal mediated coupling of dihalogeno derivatives of EDOT is not widely adopted for PEDOT synthesis because the neutral PEDOT product is not soluble or conductive. Non-doped PEDOT was synthesized by Ni-promoted dehalogenation polymerization of the corresponding dichloro-monomer [27] as described in reaction (1.4):

(1.4)

1.2.1 Chemical synthesis

In situ oxidative chemical polymerization

In situ oxidative chemical polymerization can be divided into three ways, including solution-cast polymerization (SCP), vapor phase polymerization (VPP) and oxidative chemical vapor deposition (oCVD). Different from PEDOT dispersion polymerization, PEDOT synthesized by in situ methods can be used directly after polymerization without further processing.

When PEDOT was successfully synthesized for the first time, the method employed by Bayer AG was solution-cast polymerization (SCP). This procedure contains three steps as shown in Figure 1. 10 (a). Firstly, casting the mixture of EDOT monomers and oxidant (for example, Fe(III)) solution, usually in alcohol solvent, on substrate. And then annealing the film to assist polymerization. Lastly, rinsing the film for purification. This method is the simplest way to synthesize PEDOT. The obtained PEDOT film is dark and insoluble [6].

Another chemical method is vapor phase polymerization (VPP). PEDOT with a conductivity exceeding 1000 S cm -1 is synthesized by this method [28]. As shown in Figure 1. 10 (b), three steps are involved in this process. At first, an oxidant layer which contains Fe 3+ (X -)3 and/or other additives is deposited on the substrate by casting or coating solution. Then, the substrate is placed in a VPP chamber, as shown in Figure 1. 10 (c), to be exposed to EDOT vapors. Finally, the PEDOT films are cleaned with ethanol or water to remove excess reagents. The oxidation occurs at the oxidant/vapor interface. And the "bottom up" growth mechanism where the viscous oxidant layer migrates by means of capillary movement through the forming polymer layer has been proved [29].

The polymerization mechanism is described as in Figure 1.11. The reactions can be divided into [10]: (1) Fe(III) oxidizes EDOT and itself is reduced to Fe(II); (2) 2-EDOT dimers are produced by two oxidized EDOT monomers and get further deprotonated by surrounding water molecules;

(3) polymer chains are formed by repeating step (1) and (2). Tosylate (Tos -) ions are inserted as counter-anions to stabilize the oxidized PEDOT.

In the VPP method, the oxidant source, additives and the processing conditions [30], such as temperature, pressure and humidity levels in the reaction chamber have a big influence on the properties of PEDOT film. For example, some oxidant salts, such as FeCl3 [31] and Fe(Tos)3 [32],

usually lead to the formation of hydrate crystallites PEDOT due to their high water affinity. Besides, pyridine can be added as a base inhibitor to improve the PEDOT conductivity from 70 to over 1000 S cm -1 [28].

Oxidative chemical vapor deposition (oCVD) is a single-step vapor synthesis slightly different from VPP. The chamber for oCVD is shown in Figure 1. 10 (d). No pre-treatment is required in this method. The vapors of both monomer and oxidant meet and react at the target surface directly in the reaction chamber for polymerization. The process is simpler but the options of oxidant are limited because they must be volatile. Therefore, metal halogen salts such as FeCl3 [33], CuCl2 [34] are commonly used in oCVD while Fe(Tos)3 is widely used in VPP. polymerization method [35]; (b) vapor phase polymerization [10]; (c) VPP chamber [36]; (d) oxidative chemical vapor deposition (oCVD) chamber [37]. (3) PEDOT polymer is doped with tosylate ions in the film which act as counter ion.

[38]

Dispersion polymerization

The availability of dispersion is an interesting characteristic of PEDOT which allows it to be synthesized in large quantities in solution. PEDOT:PSS, a complex of poly (3,4ethylenedioxythiophene) and poly(4-styrenesulfonate) (PSS, [C8H7SO3 -]n), is the most popular product.

As shown in Figure 1.12, the synthesis of PEDOT: polyelectrolyte is performed in an aqueous solution with peroxodisulfates (e.g. K2S2O8, Na2S2O8) and Fe(III) salts (e.g. FeCl3, Fe2(SO4)3) as the oxidizing agents. The formation is due to electrostatic interaction between the positively charged PEDOT and large-molecular-weight polyanions [35]. The processing conditions (i.e., adding oxidant one-time or step-by-step), acidity of solutions, temperature and mechanical stirring rate all influence the physicochemical properties of the PEDOT:PSS. One-time addition of the oxidant and/or increasing the acidity and temperature could accelerate the polymerization rate, resulting in large size of PEDOT:PSS micelles. This induces poor film formation with high roughness. Although PPS doesn't have an oxidation effect, it has two important impacts [35]. One is the sulfonate groups (SO3 -) of the PSS which could neutralize the positive charges in the oxidized PEDOT. The other is the excessive PSS which plays a role as a surfactant to enhance the stabilization of insoluble PEDOT in aqueous solution. Now, PEDOT:PSS is a commercially available product that has been utilized for various applications. shown in Figure 1.13 [39].

Figure 1.13 Schematic for the electropolymerization of PEDOT doped with ClO4

-on the surface of the metallic active tip of an electrode. [39] The electropolymerization of CPs can be performed by three main techniques: potentiodynamic, potentiostatic and galvanostatic methods. The polymer prepared by potentiodynamic method is deposited in layers because the applied potential is swept between a low and high potential limit in cycles [40]. Moreover, polymers are charged and discharged during experiments. As a result, the growth processes can be interrupted by this method [7]. On the other hand, the applied potential is constant during the potentiostatic method while current varies. The fixed applied potential ensures the integrity of the electrode to be coated [8]. What's more, the current is constant during galvanostatic polymerization. This method is usually applied in industry because two-electrode cells which do not contain a RE are commonly used. Various morphologies of the polymers can be obtained by these methods.

The concentrations of monomer and electrolyte, the applied potential, the pH of the solution, the nature of the working electrode and the temperature have an influence on the electropolymerization process. For example, thiophene monomers adsorbs more efficiently on platinum [4]. Welloriented PPy nanowire arrays can be obtained on gold-coated platinum electrodes while nanowire bundles are found among the nanowires on Pt electrodes under the same condition [START_REF] Huang | Conducting polymer nanowire arrays with enhanced electrochemical performance[END_REF].

The electrolyte to be used can be divided into organic solvents such as acetonitrile and propylene carbonate [4,[START_REF] Hillman | Nucleation and growth of polythiophene films on gold electrodes[END_REF][START_REF] Tsakova | Anodic polymerization of 3,4-ethylenedioxythiophene from aqueous microemulsions[END_REF], aqueous solutions [START_REF] Du | Effects of polymerization potential on the properties of electrosynthesized PEDOT films[END_REF][START_REF] Zhang | Electrochemically controlled formation and growth of hydrogen nanobubbles[END_REF][START_REF] Hui | Electrochemical fabrication of nanoporous polypyrrole film on HOPG using nanobubbles as templates[END_REF] and ionic liquids [START_REF] Pang | Electropolymerization of high quality electrochromic poly(3-alkyl-thiophene)s via a room temperature ionic liquid[END_REF][START_REF] Beaujuge | Color Control in pi-Conjugated Organic Polymers for Use in Electrochromic Devices[END_REF][START_REF] Lu | Use of ionic liquids for piconjugated polymer electrochemical devices[END_REF][START_REF] Liu | Ionic liquids in surface electrochemistry[END_REF][START_REF] Li | A novel electrolyte 1-ethylimidazolium trifluoroacetate used for electropolymerization of aniline[END_REF]. The medium has big effects on the CPs growth mechanism [START_REF] Cao | Effect of solvents and cosolvents on the processibility or polyaniline. 1. solubility and conductivity studies[END_REF][START_REF] Cordova | Effect of anions on the nucleation and growth-mechanism of polyaniline[END_REF][START_REF] Alabdullah | Evidence supporting an emulsion polymerisation mechanism for the formation of polyaniline[END_REF]. In non-aqueous solutions, the nucleophilic attack of OH• on the radical cation is inhibited. The formation of CPs cannot occur in nucleophilic solvents like dimethylformamide or dimethyl sulfoxide, unless a protic acid, like ptoluenesulfonic acid, is added [START_REF] Sadki | The mechanisms of pyrrole electropolymerization[END_REF].

By contrast, considering the operation costs, safety and friendly environment, water is a better medium for electrosynthesis [START_REF] Asami | Electropolymerization of an immiscible monomer in aqueous electrolytes using acoustic emulsification[END_REF], especially for CPs such as PANI and PPy. Because their monomers have a high solubility in water. In terms of EDOT, the solubility is low and the polymerization potential is high. Therefore, it is less easy to obtain PEDOT by an electrochemical route in aqueous solutions.

To enhance the solubility of monomers, surfactant molecules, for example, sodium dodecyl sulfate (SDS) was used. Homogeneous adherent, deep blue PEDOT films were synthesized in this anionic micellar medium which contains 0.05 M EDOT, 0.07 M SDS and 0.1 M LiClO4 by electrochemical way [START_REF] Sakmeche | Anionic micelles; A new aqueous medium for electropolymerization of poly(3,4-ethylenedioxythiophene) films on Pt electrodes[END_REF]. SDS not only increases the solubility of EDOT, but also decreases the oxidation potential and induces an electrocatalytic effect. Besides SDS, polyoxyethylene-10-lauryl ether (C12E10) [START_REF] Tsakova | Anodic polymerization of 3,4-ethylenedioxythiophene from aqueous microemulsions[END_REF],

sodium dodecyl benzene sulfonate (SDBS) [START_REF] Zhang | O/W microemulsion as electrolyte for electro-polymerization of 3,4-ethylenedioxyselenophene[END_REF] and other surfactants can also be used for CPs polymerization. The mechanism of polymerization in a micelle medium is shown in in micelles. [START_REF] Tsakova | Anodic polymerization of 3,4-ethylenedioxythiophene from aqueous microemulsions[END_REF] However, the presence of surfactants may have an effect on the polymerization and produce wastes.

Therefore, another technique called acoustic emulsification was used to allow the formation of CPs in the absence of surfactants. [START_REF] Asami | Electropolymerization of an immiscible monomer in aqueous electrolytes using acoustic emulsification[END_REF] In general, the emulsions have a white appearance. When the emulsions are well dissolved, the solutions become transparent. As shown in

Polymerization mechanisms

Diaz et al. [START_REF] Diaz | Electrochemistry of conducting polypyrrole films[END_REF][START_REF] Genies | Spectroelectrochemical study of polypyrrole films[END_REF] presented the very first mechanistic concept for electropolymerization of conductive polymers. They suggested that the pyrrole polymerization is similar to radical or ionic polymerization processes. Monomers dimerized at the α-position after oxidation and the elimination of protons occurs from a doubly charged σ-dimer to form an aromatic neutral dimer.

Due to stronger conjugation, the dimer is easier to be oxidized than the monomer. It is oxidized Chapter 1 State of the art 20 immediately to its cationic form, undergoing the next step of coupling with the cation of the monomeric radical, and then protons are eliminated again from the resulting charged trimer. This process is summarized in Figure 1.16 [7]. Waltman and Bargon [START_REF] Waltman | Electrically conducting polymers -a review of the electropolymerization reaction, of the effects of chemical-structure on polymer film properties, and of applications towards technology[END_REF][START_REF] Waltman | Reactivity structure correlations for the electropolymerization of pyrrole -an indo cndo study of the reactive sites of oligomeric radical cations[END_REF] have confirmed this mechanism by theoretical studies based on the correlation between the reactivity and the unpaired electron density of the radical cations.

Figure 1.16 Classical formation mechanism of conductive polymers. [7]

Despite the fact that the mechanism proposed by Diaz and his colleagues is the most mentioned and accepted one, the mechanism of polymerization is still under debate. A variety of other mechanisms were proposed by K. J. Kim et al. [START_REF] Kim | Mechanism of electropolymerization of pyrrole in acidic aqueous-solutions[END_REF], Pletcher et al. [START_REF] Asavapiriyanont | The electrodeposition of polypyrrole films from aqueous-solutions[END_REF], Reynolds et al. [START_REF] Qiu | Electrochemically intiated chain polymerization of aqueous-media[END_REF] and other groups.

Polymerization mechanism for PEDOT doped with chloride (PEDOT + /Cl -) potentiostatically has been summarized as follows [START_REF] Yang | Polymer films on electrodes: Investigation of ion transport at poly(3,4ethylenedioxythiophene) films by scanning electrochemical microscopy[END_REF]:

(1.5)

Synthesis of nanostructures

Various CPs nanostructures such as nanowires, nanotubes, nano-networks, nanosheets and nanospheres or microcontainers have been reported [START_REF] Zhang | Synthesis and applications of one-dimensional nano-structured polyaniline: An overview[END_REF]. The common methods can be divided into hard-template methods [START_REF] Martin | Template synthesis of electronically conductive polymer nanostructures[END_REF][START_REF] Kline | Template-grown metal nanowires[END_REF][START_REF] Martin | Nanomaterials -a membrane-based synthetic approach[END_REF], soft-template methods [START_REF] Zhang | Synthesis and applications of one-dimensional nano-structured polyaniline: An overview[END_REF][START_REF] Jang | Conducting polymer nanomaterials and their applications[END_REF] and templateless methods [START_REF] Huang | Nanostructured polyaniline sensors[END_REF].

It's not easy to obtain PEDOT having nanostructures [START_REF] Le | Charge Storage Properties of Nanostructured Poly (3,4-ethylenedioxythiophene) Electrodes Revealed by Advanced Electrogravimetry[END_REF], PEDOT films usually have a cauliflower-like structure [START_REF] Cysewska | Influence of electropolymerization conditions on the morphological and electrical properties of PEDOT film[END_REF][START_REF] Groenendaal | Electrochemistry of poly(3,4-alkylenedioxythiophene) derivatives[END_REF]. PEDOT nanowires are commonly obtained by using silicon or metal oxide template [START_REF] Li | Hierarchical doubleshelled poly(3,4-ethylenedioxythiophene) and MnO2 decorated Ni nanotube arrays for durable and enhanced energy storage in supercapacitors[END_REF][START_REF] Shiu | Morphology Dependence of Silicon Nanowire/Poly(3,4-ethylenedioxythiophene):Poly(styrenesulfonate) Heterojunction Solar Cells[END_REF][START_REF] Chen | Electrochemical grafting passivation of silicon via electron transfer at polymer/silicon hybrid interface[END_REF][START_REF] Le | Understanding the energy storage mechanisms of poly(3,4-ethylenedioxythiophene)-coated silicon nanowires by electrochemical quartz crystal microbalance[END_REF]. To better control the morphology of PEDOT, organic solvents [START_REF] Poverenov | Major Effect of Electropolymerization Solvent on Morphology and Electrochromic Properties of PEDOT Films[END_REF],

surfactants [START_REF] Xiao | Pulse potentiostatic electropolymerization of high performance PEDOT counter electrodes for Pt-free dye-sensitized solar cells[END_REF] and emulsions [START_REF] Lee | Electropolymerization in a confined nanospace: synthesis of PEDOT nanoparticles in emulsion droplet reactors[END_REF] have been used.

Hard-template methods

The hard template is usually a thin porous film of aluminum oxide or polycarbonate [START_REF] Martin | Nanomaterials -a membrane-based synthetic approach[END_REF]. Materials have been deposited in the cylindrical pores of this film to form nanotubes or nanowires. This has been performed by methods such as pressure injection, vapor deposition, chemical deposition, and electrodeposition [START_REF] Cho | Fast electrochemistry of conductive polymer nanotubes: Synthesis, mechanism, and application[END_REF].

Metallic nanowires were first synthesized by hard-template methods in the 1970s [START_REF] Xia | One-dimensional nanostructures: Synthesis, characterization, and applications[END_REF][START_REF] Possin | A method for forming very small diameter wires[END_REF][START_REF] Whitney | Fabrication and magnetic-properties of arrays of metallic nanowires[END_REF].

Afterwards, a similar route was applied for CPs. Moreover, micro/nanotubes of CPs, including PPy, PANI, PEDOT, etc., were fabricated in template pores by chemical and electrochemical methods [START_REF] Martin | Template synthesis of oranic microtubules[END_REF][START_REF] Parthasarathy | Template-synthesized polyaniline microtubules[END_REF][START_REF] Demoustier-Champagne | Electrochemically synthesized polypyrrole nanotubules: Effects of different experimental conditions[END_REF][START_REF] Demoustier-Champagne | Effect of electrolyte concentration and nature on the morphology and the electrical properties of electropolymerized polypyrrole nanotubules[END_REF].

Compared to chemical methods, electrodeposition offers a better control of the dimension of the materials. What's more, different structures from hollow nanotubes to solid nanowires can be obtained by adjusting the applied potential, the monomer concentration, and the shape of electrode [START_REF] Cho | Fast electrochemistry of conductive polymer nanotubes: Synthesis, mechanism, and application[END_REF]. Figure 1.17 (a) presents the fabrication process of the template (base electrode) for PEDOT nanowires/ nanotubes. A layer of aluminum, with thickness of 1-2 μm, is sputtered onto an ITOcoated glass. Then the resulting aluminum film is anodized at a constant voltage to generate highly ordered porous alumina structures. The diameters of the pores of the alumina template can be adjusted by controlling the anodization potential and time [START_REF] Crouse | Self-ordered pore structure of anodized aluminum on silicon and pattern transfer[END_REF][START_REF] Chu | Formation and microstructures of anodic alumina films from aluminum sputtered on glass substrate[END_REF].

Electropolymerization of PEDOT was carried out potentiostatically at various monomer concentrations (10-100 mM) and applied potentials (1.0-1.8 V/ Ag/ AgCl) using 0.l M LiClO4 in acetonitrile as electrolyte. At high oxidation potential, two conditions (Figure 1.17 (b) and (c))

involving different oxidation potentials and monomer concentrations are studied. In Figure 1.17 (b), the monomer concentration is high and the applied potential is moderate (< 1.4 V). In this condition, EDOT monomers have enough time to diffuse upto the pore bottom due to the slow reaction rate. As a result, the polymerization reaction occurs on the whole electrode surface without notable preference, which leads to the formation of rigid, dense nanowires. In Figure 1.17 (c), the applied potential is higher (1.8 V) and the monomer concentration is low. In this case, the EDOT polymerization rate is high so the monomers don't have enough time to fill the pores entirely. The polymer prolongs the chains and grows along the template. Therefore, long and thin-walled nanotubes are synthesized. Other parameters including electrolyte concentration, pore diameter, template thickness, electrolyte stirring, temperature have also influenced on the morphology of the nanotubes [START_REF] Xiao | Controlled electrochemical synthesis of conductive polymer nanotube structures[END_REF]. The influence of the shape of the template was investigated at low oxidation potentials (<1.4 V). Considering the very slow polymerization reaction caused by the low applied potential, the mechanism of the polymer growth is different from the one at high applied potential.

Thus, morphology of base electrodes at the pore bottom is essential for the initial polymerization in this condition. The annular-shape electrode at the bottom of the pores is shown in Figure 1.17 (d). Because the sharp tips have a higher charge density, the sharp top surface has more electrochemically active sites than the smooth lateral surface. Therefore, the top end of the conductive polymers becomes active sites for further elongation. By contrast, nanowires are obtained on the flat-top electrode as shown in Figure 1.17 (e). In conclusion, diffusion and reaction kinetics play critical roles at high applied potential while the shape of base electrode has a stronger influence on the growth process at low applied potential.

Apart from the tunnel-shaped templates, some other types of hard template have also been studied.

For example, V2O5 nanofibers [START_REF] Zhang | Synthesis of polyaniline nanofibers by "nanofiber seeding[END_REF][START_REF] Zhang | Narrow pore-diameter polypyrrole nanotubes[END_REF] or MnO2 nanowires [START_REF] Pan | Synthesis of polyaniline nanotubes with a reactive template of manganese oxide[END_REF], poly(lactide-co-glycolide) (PLGA) nanoparticles [START_REF] Antensteiner | Conducting Polymer Microcups for Organic Bioelectronics and Drug Delivery Applications[END_REF] can be served as seeds for CPs growth. What's more, a variety of biological templates such as DNA [START_REF] Houlton | DNA-based routes to semiconducting nanomaterials[END_REF][START_REF] Li | The role of DNA in PANI-DNA hybrid: Template and dopant[END_REF] and tobacco mosaic virus [START_REF] Rong | Tobacco mosaic virus templated synthesis of one dimensional inorganic-polymer hybrid fibres[END_REF] have also been used

to fabricate conducting polymer nanofibers.

Soft-template methods

The soft-template method is another powerful and popular technique to produce CPs nanomaterials.

Surfactants [START_REF] Jang | Facile fabrication of photochromic dye-conducting polymer core-shell nanomaterials and their photoluminescence[END_REF], liquid crystalline phases, structure-directing molecules are common templates.

Compared to hard-template methods, the post-synthesis process for removal of the template is not necessary.

Different micro-/nano-containers of PPy have been synthesized by using pyrrole monomer-filled micelles as soft template [START_REF] Huang | Conducting polypyrrole conical nanocontainers: Formation mechanism and voltage switchable property[END_REF][START_REF] Gao | Electrosynthesis of small polypyrrole microcontainers[END_REF]. As shown in nucleus has an overlapping diffusion zone, which would join together to form a film in the polymerization process. [START_REF] Huang | Conducting polypyrrole conical nanocontainers: Formation mechanism and voltage switchable property[END_REF] In addition, gas bubbles are another type of soft template for electrosynthesization of CPs. The process of nanoporous PPy electrodeposition by using H2 bubbles as template is presented in V) for a short time. The hydrogen bubbles can be observed even after a long time (minutes to hours).

The diameter of bubbles can be controlled by the applied potential value. Then PPy film was deposited on the electrode where no bubbles were present, by cyclic voltammetric method in the range of 0.5-0.7 V at 100 mV s -1 . In Except porous structures, polypyrrole microcontainers with morphology like bowls, cups and bottles have been electrochemically generated by direct oxidation of pyrrole in the aqueous solution of β-naphthalenesulfonic acid (β-NSA), as shown in Figure 1.20. [START_REF] Qu | Electrochemical growth of polypyrrole microcontainers[END_REF] β-NSA is a typical surfactant forming spherical micelles, with an average diameter of 354 nm, which is too small to be the template of the microcontainers whose diameters are about 100 μm. The containers' shape depends on O2 gas bubbles which are released from the surface of the working electrode, produced by water oxidation. The bubble size can be controlled by the electrolysis conditions. What's more, H2 bubbles also can be stabilized by the anionic surfactant molecules and act as template [START_REF] Bajpai | Conducting-polymer microcontainers: Controlled syntheses and potential applications[END_REF].

The bubbles were observed via in situ tapping mode atomic force microscopy [START_REF] Zhang | Electrochemically controlled formation and growth of hydrogen nanobubbles[END_REF]. Moreover, the morphologies of polymers are controllable by adjusting the synthesis conditions (i.e. pH, the concentration of surfactant and monomer, and temperature) in the presence of β-NSA [START_REF] Tavandashti | Controlled growth of hollow polyaniline structures: From nanotubes to microspheres[END_REF]. The typical electrolyte was the aqueous solution of 0.53 mol L -1 pyrrole and 0.57 mol L -1 β-NSA.

The gourdlike (D) and bottlelike (E) microstructures were generated in the media containing 0.28 and 0.15 mol L -1 β-NSA, respectively. Scale bars: 100 µm. [START_REF] Qu | Electrochemical growth of polypyrrole microcontainers[END_REF] O2 or H2 bubbles generated at the working electrode can not only be employed as soft template in monomer aqueous solution. Darmanin's group [START_REF] Darmanin | Exceptionally Strong Effect of Small Structural Variations in Functionalized 3,4-Phenylenedioxythiophenes on the Surface Nanostructure and Parahydrophobic Properties of Their Electropolymerized Films[END_REF][START_REF] Sane | Nanotubular structures through templateless electropolymerization using thieno 3,4-b thiophene derivatives with different substituents and water content[END_REF][START_REF] Fradin | Templateless Electrodeposition of Conducting Polymer Nanotubes on Mesh Substrates[END_REF][START_REF] Fradin | A soft template approach to various porous nanostructures from conjugated carbazole-based monomers[END_REF][START_REF] Thiam | Designing Nanoporous Membranes through Templateless Electropolymerization of Thieno 3,4-b thiophene Derivatives with High Water Content[END_REF] provided a new angle for gas bubbles by adding water in organic solutions. In their investigations, the electropolymerization is surfactantfree because organic solvents (e.g. CH2Cl2) can stabilize the bubbles in place of surfactants. After the electrodeposition process of 1,3-bis(N-carbazolyl)benzene (mCP), a kind of conjugated carbazoles (Figure 1.21 (a)), a smooth polymer (Figure 1.21 (b)) is obtained in anhydrous dichloromethane (CH2Cl2) solution while various porous nanostructures are obtained in dichloromethane saturated with water (CH2Cl2 + H2O) [START_REF] Fradin | A soft template approach to various porous nanostructures from conjugated carbazole-based monomers[END_REF]. The number of potential scans and the anodic charges have an obvious influence on the morphologies of mCP. In Figure 1.21 (c), flower-like structures are formed on the top of nanotubes. In the beginning, nanotubes are formed due to bubbles of nanometric size and the growth is one-dimensional. And then H2 bubbles are released from H2O trace and from H + formed from monomer oxidation and polymerization. The huge amount of H2 bubbles transfers the growth from one-dimensional to two-dimensional and three-dimensional which assists the formation of nanosheets and nanospheres. Moreover, when a positive potential is applied, O2 bubbles play a role as a soft-template. As shown in [START_REF] Fradin | A soft template approach to various porous nanostructures from conjugated carbazole-based monomers[END_REF] In addition, the influence of the amount of water also has been studied during electrodeposition of thieno [3,4-b]thiophene derivatives (Figure 1.22 (a)) [START_REF] Thiam | Designing Nanoporous Membranes through Templateless Electropolymerization of Thieno 3,4-b thiophene Derivatives with High Water Content[END_REF]. As shown in Figure 1. 22 (b) and (c), nanoporous membranes are obtained in dichloromethane saturated with water while vertically aligned nanotubes are observed in high water content solutions. A sharp increase of the number of nanotubes was observed when water content increases from 35% to 65%. However, the diameter and the height of the nanotubes stayed the same.

To be noted, using gas bubbles as template during electrosynthesis can be a soft-template method or templateless method, depending on the bubbles generation process. When the bubbles are electrogenerated during the deposition of polymers, it is a template-free process [START_REF] Thiam | Designing Nanoporous Membranes through Templateless Electropolymerization of Thieno 3,4-b thiophene Derivatives with High Water Content[END_REF]. When the bubbles are generated as a template as a first step, it is a soft-template method [START_REF] Hui | Electrochemical fabrication of nanoporous polypyrrole film on HOPG using nanobubbles as templates[END_REF][START_REF] Mazur | Preparation of three-dimensional polymeric structures using gas bubbles as templates[END_REF].

Templateless methods for PPy nanostructure electrosynthesis

Sometimes, the structures synthesized by using O2 or H2 bubbles as templates are between nanoscale and microscale because the bubbles are generated at the electrode/electrolyte interface.

It is difficult to control the size of the bubbles because there are no restrictions to bubbles coalescence. However, smaller sizes of nanostructures were obtained due to the less coalescence of the bubbles when they are generated at the electrode/film interface [START_REF] Debiemme-Chouvy | Template-free one-step electrochemical formation of polypyrrole nanowire array[END_REF][START_REF] Fakhry | Mechanism of formation of templateless electrogenerated polypyrrole nanostructures[END_REF][START_REF] Fakhry | Templateless electrogeneration of polypyrrole nanostructures: impact of the anionic composition and pH of the monomer solution[END_REF][START_REF] Debiemme-Chouvy | Electrosynthesis of polypyrrole nano/micro structures using an electrogenerated oriented polypyrrole nanowire array as framework[END_REF].

It has been reported in our lab's previous works [START_REF] Debiemme-Chouvy | One-step electrochemical synthesis of a very thin overoxidized polypyrrole film[END_REF][START_REF] Debiemme-Chouvy | An insight into the overoxidation of polypyrrole materials[END_REF][START_REF] Fakhry | Electrochemical Characterisations of Ultra Thin Overoxidized Polypyrrole Films Obtained by One-Step Electrosynthesis[END_REF] that the presence of solely weak acid anions, such as monohydrogenophosphate or carbonate, which confer a relatively high pH to the Py solution, allows one-step electrogeneration of an ultrathin overoxidized polypyrrole (O-PPy) film under potentiostatic and potentiodynamic conditions. Actually, weak acid anions capture the protons which are released during monomer oxidation and lead to a situation that no anions are available at the electrode/solution interface. Due to the high applied potential at the electrode, water oxidation takes place and hydroxyl radicals are generated, these radicals react with the polymer film initially electrogenerated resulting in its overoxidation. The resistance of O-PPy film prepared by one-step is about ten times higher than the one prepared by a two-step process [START_REF] Fakhry | Electrochemical Characterisations of Ultra Thin Overoxidized Polypyrrole Films Obtained by One-Step Electrosynthesis[END_REF]. One can notice that these O-PPy films can be applied to develop biosensors implying H2O2 detection [START_REF] Debiemme-Chouvy | A very thin overoxidized polypyrrole membrane as coating for fast time response and selective H2O2 amperometric sensor[END_REF][START_REF] Debiemme-Chouvy | Characterization of a very thin overoxidized polypyrrole membrane: application to H2O2 determination[END_REF].

In fact, the formation of a thin non-conductive OPPy film is the very first step of the electropolymerization of PPy nanowires which are obtained when no weak acid anions are added to the previous monomer solutions [START_REF] Debiemme-Chouvy | Template-free one-step electrochemical formation of polypyrrole nanowire array[END_REF][START_REF] Fakhry | Mechanism of formation of templateless electrogenerated polypyrrole nanostructures[END_REF][START_REF] Fakhry | Templateless electrogeneration of polypyrrole nanostructures: impact of the anionic composition and pH of the monomer solution[END_REF][START_REF] Debiemme-Chouvy | Electrosynthesis of polypyrrole nano/micro structures using an electrogenerated oriented polypyrrole nanowire array as framework[END_REF]. As shown in Steps ①, ②, ③ last around a few seconds. [START_REF] Debiemme-Chouvy | Electrosynthesis of polypyrrole nano/micro structures using an electrogenerated oriented polypyrrole nanowire array as framework[END_REF] Furthermore, the influence of the concentration of LiClO4 and pH of the solution was investigated.

As shown in Figure 1. 24 (a) and (b), the anodic charge increases with the increase of the perchlorate concentration. When the perchlorate concentration is lower than 5 10 -3 M, thin overoxidized PPy films are produced. Besides, PPy nanowire arrays are obtained when the concentration is around 10 -3 M while nanowire networks are obtained at higher concentration, around 10 -2 M. To be mentioned, PPy with nanostructures are superhydrophilic. As shown in Figure 1.24 (c), superhydrophilic films can only be obtained in solutions whose pH is in the range of 5-9 in the presence of 10 -3 M LiClO4. Cauliflower-like film is observed at lower pH values and very thin film is observed at higher pH. pH has such a significant influence on the PPy film formation due to the relevance of pH value and water oxidation. Water oxidation could not occur at the potential applied when pH is low and PPy keeps growing in this condition which leads to a thick film. On the contrary, too many water molecules are oxidized at high pH and inhibit the film generation. Moreover, the impact of pH varies with different kinds of electrolyte, as shown in

Polymer overoxidation

As mentioned previously, CPs in their doped state show better conductivity than in non-doped state.

However, the stability of CPs varies. Many external causes such as heat, light [START_REF] Crispin | Conductivity, morphology, interfacial chemistry, and stability of poly(3,4-ethylene dioxythiophene)-poly(styrene sulfonate): A photoelectron spectroscopy study[END_REF][START_REF] Marciniak | Light induced damage in poly(3,4-ethylenedioxythiophene) and its derivatives studied by photoelectron spectroscopy[END_REF], oxygen, mechanical stress, etc. can lead to the degradation of CPs [START_REF] Mohammad | FT-IR studies on thermal-degradation of electrically conducting polymers[END_REF].

Compared to PPy, PEDOT is harder to be de-doped due to the stabilizing effect of the electron donating dialkoxy groups. However, a very thin pinhole-free O-PEDOT film could be an essential step of the formation of nanostructured PEDOT films, taking into account the studies performed on polypyrrole [START_REF] Debiemme-Chouvy | Template-free one-step electrochemical formation of polypyrrole nanowire array[END_REF][START_REF] Fakhry | Mechanism of formation of templateless electrogenerated polypyrrole nanostructures[END_REF][START_REF] Al-Mashat | Electropolymerized Polypyrrole Nanowires for Hydrogen Gas Sensing[END_REF]. In addition, overoxidized PEDOT (O-PEDOT) films exhibit some unique properties and can be applied for sensing with determination of dopamine [START_REF] Lin | Strong adsorption characteristics of a novel overoxidized poly(3,4-ethylenedioxythiophene) film and application for dopamine sensing[END_REF], uric acid [START_REF] Ozcan | Preparation of poly(3,4-ethylenedioxythiophene) nanofibers modified pencil graphite electrode and investigation of over-oxidation conditions for the selective and sensitive determination of uric acid in body fluids[END_REF] and lead ion [START_REF] Hui | Comparison of Two Types of Overoxidized PEDOT Films and Their Application in Sensor Fabrication[END_REF]. Therefore, it is essential to have a good understanding of the overoxidation mechanism of PEDOT.

Electropolymerization of EDOT can be carried out in organic and aqueous solutions by cyclic voltammetry or chronoamperometry in a certain range of applied potential. In an aqueous electrolyte, when the applied potential is high, a side reaction that is water oxidation can occur.

Therefore, EDOT electropolymerization could be accompanied by the polymer overoxidation which results from the OH  radicals produced during water oxidation. Du and Wang have shown that in an aqueous solution containing 0.01 M EDOT and 0.1 M LiClO4, the anodic current decreased with time under potentiostatic conditions in the range of 1.1 -1.5 V/SCE, due to the overoxidation of the polymer which becomes non-conductive, as shown in Figure 1.25 (a) [START_REF] Du | Effects of polymerization potential on the properties of electrosynthesized PEDOT films[END_REF].

Overoxidation is avoided in most studies in order to not disturb the EDOT polymerization process.

In general, the most common way to obtain an O-PEDOT film is first preparing a PEDOT film by an electrochemical method and then applying a relatively high potential to overoxidize it in an aqueous solution which doesn't contain any monomer. During the process of overoxidation, the doping anions are expelled from the film, the conjugation length of the polymer is decreased [START_REF] Zykwinska | Electrochemical overoxidation of poly(3,4-ethylenedioxythiophene)-PEDOT studied by means of in situ ESR spectroelectrochemistry[END_REF] and the morphology of the film could be changed [START_REF] Lang | In situ monitoring of the electrochemical degradation of polymer films on metals using the bending beam method and impedance spectroscopy[END_REF][START_REF] Ujvari | Morphological changes in electrochemically deposited poly(3,4-ethylenedioxythiophene) films during overoxidation[END_REF][START_REF] Ujvari | Structural changes during the overoxidation of electrochemically deposited poly(3,4-ethylenedioxythiophene) films[END_REF]. The influence of the overoxidation conditions, such as applied potential [START_REF] Cysewska | Influence of electropolymerization conditions on the morphological and electrical properties of PEDOT film[END_REF][START_REF] Sekli-Belaidi | Voltammetric microsensor using PEDOTmodified gold electrode for the simultaneous assay of ascorbic and uric acids[END_REF] and pH [START_REF] Tehrani | The effect of pH on the electrochemical over-oxidation in PEDOT : PSS films[END_REF], and the electrochemical properties of O-PEDOT have been studied. However, O-PEDOT films prepared by this two-step process are still partially conductive, as shown in Figure 1. 25 (b) and (c). If the film is not conductive, the oxidation/ reduction of the redox species present in solution cannot occur. Therefore, the CV curve should be a line without current detection and the diameter of the semicircle in EIS is very high.

The conductivity of two-step formed O-PEDOT is not so low because the first step leads to a thick PEDOT film and the anions inside of the film are hard to be completely expelled. Sometimes some thin cracks or crevices are observed because of the internal stress changes during the second step, as shown in Figure 1. 25 (d) and (e) [START_REF] Lang | In situ monitoring of the electrochemical degradation of polymer films on metals using the bending beam method and impedance spectroscopy[END_REF]. The mechanism of PEDOT overoxidation was proposed according to FTIR and XPS results, as shown in Figure 1.26. Hydroxyl (I) and carbonyl (II) were likely formed. In this case, the film is still conductive. In other words, the PEDOT is not totally overoxidized.

Applications of conductive polymers in supercapacitors

Conductive polymers have various applications due to their good chemical, electrochemical, physical properties and excellent stability. They can be applied as electrode materials for supercapacitors or other energy storage devices [13,32,[141][START_REF] Bocchetta | Electrochemical Deposition of Polypyrrole Nanostructures for Energy Applications: A Review[END_REF][START_REF] Zhai | 1D Supercapacitors for Emerging Electronics: Current Status and Future Directions[END_REF], biosensors [START_REF] Wallace | Conducting electroactive polymer-based biosensors[END_REF][START_REF] Debiemme-Chouvy | A very thin overoxidized polypyrrole membrane as coating for fast time response and selective H2O2 amperometric sensor[END_REF][START_REF] Kros | Poly(3,4ethylenedioxythiophene)-based glucose biosensors[END_REF][START_REF] Lin | Performing enzyme-free H2O2 biosensor and simultaneous determination for AA, DA, and UA by MWCNT-PEDOT film[END_REF][START_REF] Sehit | Significance of nanomaterials in electrochemical glucose sensors: An updated review (2016-2020[END_REF][START_REF] Hui | Synthesis and electrochemical sensing application of poly(3,4-ethylenedioxythiophene)-based materials: A review[END_REF],

tissue engineering [8], optoelectronic devices [10,35], etc. Here, we mainly focus on the application of CPs as electrode materials in supercapacitors.

Mechanisms

Due to global warming and decreasing availability of fossil fuels, renewable energy production from sun and wind, as well as new generation of energy storage devices are highly required. Figure 1.27 presents the plot of power against energy density, named Ragone plot [START_REF] Service | Materials science -New 'supercapacitor' promises to pack more electrical punch[END_REF], for various energy storage systems. Batteries have good energy densities, however, the power delivery or uptake is slow. Therefore, there is a huge demand in higher-power energy storage systems. And electrochemical capacitors (ECs) meet the need for high-power as a complement to rechargeable batteries thanks to the fast charge and discharge process (in seconds). [149]

The energy density of ECs (about 5 Wh kg -1 ) is lower than batteries due to the restriction of charge storage on or near the surface. But a much higher power delivery or uptake (about 10 kW kg -1 ) can be achieved for shorter times. Electrochemical capacitors, also called supercapacitors, can be divided into electrochemical double layer capacitors (EDLCs) and pseudocapacitors [START_REF] Simon | Materials for electrochemical capacitors[END_REF]. The charge storage mechanisms of these two types of capacitors and batteries are shown in Figure 1.28 [START_REF] Mathis | Energy Storage Data Reporting in Perspective-Guidelines for Interpreting the Performance of Electrochemical Energy Storage Systems[END_REF]. The capacitance of EDCL supercapacitors mainly produces from surface-controlled reactions of electrode materials, which is a non-faradaic process based on ion adsorption. On the contrary, battery-type charge storage is pure diffusion-controlled, which is a faradaic process based on fast surface redox reactions. What's more, a pseudocapacitive process is a combination of faradaic and non-faradaic processes. For EDLCs, the double layer capacitance can be described in a crude approach by [START_REF] Lin | Electrochemical double layer capacitors: What is next beyond the corner?[END_REF]:

𝐶 = 𝜀 𝑟 𝜀 0 𝐴 𝑑 (1.8)
where εr is the electrolyte dielectric constant, ε0 is the dielectric constant of the vacuum, d is the effective thickness of the double layer (charge separation distance) and A is the electrode surface area. Thus the higher the surface of the electrode, the higher the capacitance.

Obviously, the difference of these charge storage mechanisms is if it is surface controlled or diffusion controlled. Trasatti and co-workers proposed a method to estimate the contribution of "inner" (diffusion-controlled) and "outer" (surface-controlled) surfaces to the total charge (qt) measured using cyclic voltammetry [START_REF] Ardizzone | Inner and outer active surface of RuO2 electrodes[END_REF] 𝑞 𝑡 = 𝑞 𝑖 + 𝑞 𝑜 (1.9

)
where qi is the charge at the inner surface and qo is the charge at the outer surface. The foundation of the method is that the instant storage of charged species (ions) at the outer electrode surface is independent of the potential scan rate (non-diffusion-controlled) (v), while charge storage at the inner surfaces is a diffusion-controlled process. [START_REF] Mathis | Energy Storage Data Reporting in Perspective-Guidelines for Interpreting the Performance of Electrochemical Energy Storage Systems[END_REF] Thus, the total measured voltammetric charge (q(v)) could be expressed as a function of scan rate (v) through the following equation:

𝑞(𝑣) = 𝑞 ∞ + 𝑘𝑣 -1 2 ⁄ (1.10)
where 𝑘𝑣 -1 2

⁄ represents the charge storage related to semi-infinite diffusion, k is a constant, and q∞is the charge stored at a high scan rate (v →∞). The extrapolation of q to v 0 gives the total charge (qt) while extrapolation of q to v →∞can provide the charge stored at the outer surfaces (qo).

Similarly, the voltammetric response of an electrode at various scan rates can be summarized according to [START_REF] Brezesinski | Ordered mesoporous alpha-MoO3 with isooriented nanocrystalline walls for thin-film pseudocapacitors[END_REF][START_REF] Lindstrom | Li+ ion insertion in TiO2 (anatase) .2. Voltammetry on nanoporous films[END_REF]:

𝑖 = 𝑖 𝑐𝑎𝑝𝑎𝑐𝑖𝑡𝑖𝑣𝑒 + 𝑖 𝑑𝑖𝑓𝑓𝑢𝑠𝑖𝑜𝑛 = 𝑎𝑣 𝑏 (1.11)
in which the measured current (i) at a fixed potential obeys a power law relationship with the potential scan rate (v), a and b are adjustable parameters. Taking the logarithm of both sides of this equation yields:

log(𝑖) = log(𝑎) + 𝑏 × log(𝑣) (1.12)
b can be determined as the slope of log(i) vs. log(v) for various scan rates. Kinetic limitations can be estimated from the value of b.

When b=0.5, the current response (i) is proportional to the square root of the scan rate

(i ∝ v 1/2 ,
where v is the scan rate in mV s -1 ) due to diffusion-controlled processes, which is usually found in batteries. In this condition, the current follows the Randles-Sevcik equation [START_REF] Fleischmann | Pseudocapacitance: From Fundamental Understanding to High Power Energy Storage Materials[END_REF]:

𝑖 = 0.4958𝑛𝐹𝐴𝐶 * 𝐷 1 2 ⁄ ( 𝛼𝑛𝐹 𝑅𝑇 ) 1 2 ⁄ 𝑣 1 2 ⁄ (1.13)
where C* is the maximum concentration of the reduced species in the structure and α is the transfer coefficient.

When b=1, the current is proportional to the scan rate (i ∝ v) and the process is surface-controlled.

A typical capacitive behavior is observed in this case. Current (i) varies linearly with the scan rate according to [START_REF] Lindstrom | Li+ ion insertion in TiO2 (anatase) .2. Voltammetry on nanoporous films[END_REF][START_REF] Fleischmann | Pseudocapacitance: From Fundamental Understanding to High Power Energy Storage Materials[END_REF] 𝑖 = 𝐶 𝑑 𝐴𝑣 (1.14) where Cd represents the capacitance and A is the surface area of the active materials.

However, an increasing numbers of new electrode materials (e.g., transition-metal oxides, hydroxides, sulfides, carbides, nitrides, conducting polymers, etc.) that display electrochemical characteristics that are neither purely capacitive nor purely Faradaic have been noticed recently [START_REF] Gogotsi | Energy Storage in Nanomaterials -Capacitive Pseudocapacitive, or Battery-like?[END_REF]. These materials can be classified as pseudocapacitive materials. Therefore, it is important to distinguish the capacitive-like behavior and battery-like behavior by using equation (1.9). b values between 0.5 and 1 are a transition area from battery-like to capacitive-like responses. For example, a spherical diffusion process would lead to a b value of 0.75 [START_REF] Bard | Electrochemical Methods: Fundamentals and Applications[END_REF]. Surface reactions play a dominant role over diffusion controlled reactions when the b value is between 0.85 and 1. The b value is highly dependent on a number of factors such as potential, scan rate, and charge storage mechanisms. [START_REF] Mathis | Energy Storage Data Reporting in Perspective-Guidelines for Interpreting the Performance of Electrochemical Energy Storage Systems[END_REF] In this condition, equation (1.9) is not suitable anymore, the current response (i) at a fixed potential (V) can be described as the combination of two separate mechanisms, namely capacitive effects (k1v) and diffusion-controlled insertion (k2v 1/2 ) according to [START_REF] Brezesinski | Ordered mesoporous alpha-MoO3 with isooriented nanocrystalline walls for thin-film pseudocapacitors[END_REF] 𝑖(𝑣) = 𝑘 1 𝑣 + 𝑘 2 𝑣 current and scan rate. The pseudolinearity and hysteresis of galvanostatic discharge curves can also be used to assess pseudocapacitance. EIS can be used to evaluate pseudocapacitive behavior by analyzing the real/imaginary impedance, real/imaginary capacitance, and phase angle. [START_REF] Fleischmann | Pseudocapacitance: From Fundamental Understanding to High Power Energy Storage Materials[END_REF] In case of GCD, the curve of EDLC has a symmetry triangular shape while linear peak current responses versus scan rate can be found in pseudocapacitive materials. The curve of GCD for batteries is not symmetric. In addition, a quasi-rectangular shape of the CV curve can be found in EDLCs. For pseudocapacitive materials, the mirror image criterion is observed and there is a region where there is no change of the peak position with a variation of the scan rate [START_REF] Conway | Transition from supercapacitor to batery behavior in electrochemical energystorage[END_REF]. These two features can be used to distinguish pseudocapacitors from batteries.

True supercapacitors never show a semicircle in the high-frequency region of EIS [START_REF] Mathis | Energy Storage Data Reporting in Perspective-Guidelines for Interpreting the Performance of Electrochemical Energy Storage Systems[END_REF]. In Figure 1.29 (g), for EDLCs, a 45°line starts immediately from the equivalent series resistance (ESR) followed by a vertical line that is parallel to the imaginary impedance axis (Y-axis) in the lower frequency region. However, this is the ideal situation. In fact, many published papers reported the presence of semicircles for EDLCs mainly due to the interfacial impedance occurring at the current collector/active material interface [START_REF] Portet | Modification of Al current collector surface by sol-gel deposit for carbon-carbon supercapacitor applications[END_REF]. In contrast, the semicircle observed for pseudocapacitive materials arises from charge transfer resistance (RCT). And the diameter of the semicircle is much bigger in batteries.

Influencing factors

Many efforts have been done to enhance the mechanical characteristics and electrochemical performances of supercapacitors. There are many options to reach these goals if we focus on the electrode materials. For example, the morphology of the materials has a huge impact on the capacitance. Moreover, nanocomposites usually show better performance than CPs. The addition of another material can increase the capacitance directly or increase the adhesion/ stability of the materials. [START_REF] Liu | Polydopamine and Its Derivative Materials: Synthesis and Promising Applications in Energy[END_REF][START_REF] Liu | Polyaniline and Polypyrrole Pseudocapacitor Electrodes with Excellent Cycling Stability[END_REF] For example, polydopamine (PDA)/PPy composites present good adhesion and electrical conductance at the same time. PDA is a mimic of the specialized adhesive foot protein, Mefp-5

(Mytilus edulis foot protein-5) [START_REF] Liu | Polydopamine and Its Derivative Materials: Synthesis and Promising Applications in Energy[END_REF]. Under alkaline conditions, it has the ability to spontaneously self-polymerize on various inorganic or organic materials surface and form uniform very adherent films which can be functionalized by covalently linking biomolecules [START_REF] Wang | Incorporating catechol into electroactive polypyrrole nanowires on titanium to promote hydroxyapatite formation[END_REF]. PDA is not electrically conductive and has good biocompatibility. Various electrochemical depositions of PPy/PDA composites have been reported [START_REF] Wang | Incorporating catechol into electroactive polypyrrole nanowires on titanium to promote hydroxyapatite formation[END_REF][START_REF] Popescu | Poly(dopamine) assisted deposition of adherent PPy film on Ti substrate[END_REF][START_REF] Kim | Electrochemical deposition of conductive and adhesive polypyrrole-dopamine films[END_REF][START_REF] Tan | Electrochemical synthesis of conductive, superhydrophobic and adhesive polypyrrolepolydopamine nanowires[END_REF][START_REF] Kim | Electrically Conductive Polydopamine-Polypyrrole as High Performance Biomaterials for Cell Stimulation in Vitro and Electrical Signal Recording in Vivo[END_REF].

CPs with nanostructures usually have better electrochemical performances than the one in bulk due to the higher specific surface area. When CPs serve as supercapacitor electrodes, the ion diffusivity, which is much slower compared with the charge transport, determines the charge-discharge rate.

The ion diffusivity of different nanostructures of CPs has been studied [START_REF] Huang | Conducting polymer nanowire arrays with enhanced electrochemical performance[END_REF][START_REF] Cho | Fast electrochemistry of conductive polymer nanotubes: Synthesis, mechanism, and application[END_REF]. As shown in Figure 1.30 (A), classical PPy film has a higher ion transport rate than PPy arrays because of its straight ion diffusion pathway, which would result in a higher capacitance. But if we take mass and volume into account, then the specific capacitance of classical film will not be higher. Furthermore, counter-ions have to penetrate into the polymer and access the internal part through the pores of the nanowire networks. Therefore, the capacitance of nanowire networks is lower than nanowire arrays thanks to the aligned pore structure of nanowire arrays.

In addition, CPs often suffer from a limited long-term stability during cycling because of the swelling and shrinking of the polymers. The polymer volume increases when they are doped with ions in electrolyte (charging) and decreases when these ions diffuse back to electrolyte (discharging) [START_REF] Wang | A review of electrode materials for electrochemical supercapacitors[END_REF]. The morphology variation of electrode materials during cycling does not only occur to CPs.

As another attractive electrode material for batteries, the morphology of Si nanostructures also changes during cycling. Schematic of morphological changes that occur in Si during electrochemical cycling is shown in Figure 1. 30 (B). The variation of bulk film and nanospheres cannot be ignored during cycling, some materials are lost and the morphology is changed. On the contrary, nanowires maintain good stability during cycling. And the connection between each nanowire and the current collector allows for efficient 1D electron transport along the length of every nanowire.

The problem of losing mass and morphology variation can also be overcome by producing nanocomposites. For instance, deposition of a thin carbonaceous shell onto PANI and PPy surfaces can significantly improve their capacitance retention [START_REF] Liu | Polyaniline and Polypyrrole Pseudocapacitor Electrodes with Excellent Cycling Stability[END_REF]. the length of every NW [START_REF] Chan | Highperformance lithium battery anodes using silicon nanowires[END_REF].

Chapter 2 Materials and Experimental Methods

Materials and experimental methods

The experimental protocols and characterization techniques involved in this thesis are provided in this chapter. The chapter is divided into four sections: (i) list of chemicals, (ii) electrochemical setup, (iii) electrochemical techniques for synthesis and characterization and (iv) physical characterization techniques. All the solutions were prepared with bi-distilled water. Py was distilled before utilization.

Chemicals

To be mentioned, EDOT monomers were stored at -25 ℃ and the solution was deaerated by argon.

It is solid in the refrigerator, as shown in

Electrochemical setup

Electrochemical cell and electrodes

A classical three-electrode electrochemical cell was used in this thesis.

Chapter 2 Materials and experimental methods

58

The experimental setup is shown in Figure 2.2. A platinum grid was used as a counter electrode.

A saturated calomel electrode (SCE) was used as the reference electrode during film electrosynthesis. A double junction was used in order to avoid chloride diffusion into the monomer solution. The working electrode was either Au or Pt (S = 0.07 cm 2 ) for electrochemical studies or Au/mica substrate for XPS analyses and SEM observations. The temperature of the EDOT solution was fixed at 30 °C by water bath heating to be sure that the EDOT monomers were totally dissolved without any addition of an organic solution. The solutions were deaerated by argon bubbling. The EDOT polymerization was conducted under either potentiodynamic or potentiostatic conditions.

Potentiostat

Autolab PGSTAT100 (Ecochemie) was used for electrochemical polymerization and

Electrochemical techniques for synthesis and characterization

Cyclic voltammetry (CV)

CV is the most versatile dynamic electrochemical technique for measuring redox events [2] and to perform electrochemical synthesis. CV consists of cycling the potential of the working electrode (as shown in Potential scan rate also has an influence on the "duck" shape of voltammogram, for a reversible process. Higher scan rates lead to a decrease of the diffusion layer and therefore to a higher current.

For diffusion-controlled system, the Randles-Sevcik equation (2.1) describes how the peak current ip (A) increases linearly with the square root of the scan rate v (V s -1 ), where n is the number of electrons transferred in the redox event, A (cm 2 ) is the electrode surface area, D0 (cm 2 s -1 ) is the diffusion coefficient of the redox species, and C 0 (mol cm -3 ) is its bulk concentration.

𝑖 𝑝 = 0.446𝑛𝐹𝐴𝐶 0 ( 𝑛𝐹𝐷 0 𝑣 𝑅𝑇 ) 1 2 ⁄ (2.1)
Therefore, the system is diffusion controlled when ip varies linearly with v 1/2 . This is an important characterization to determine the diffusion or adsorption process during cycling measurements for energy storage devices. During adsorption-controlled process, current varies linearly with scan rate:

𝑖 𝑝 = 𝑛 2 𝐹 2 4𝑅𝑇 𝐴𝑣 (2.2)
where  is the surface coverage of the adsorbed species (mol cm -2 ).

Although CV is an indispensable characterization method, in the case of conductive polymers, no direct information on the nucleation and growth mechanism can be obtained when it is used for electrodeposition. Moreover, during experiments, polymers are charged and discharged and the growth processes are interrupted. [5] Thus, other complementary techniques are necessary for electrochemical deposition. In this thesis, CV is applied for electrosynthesis and characterization.

Chronoamperometry

Chronoamperometry is another common and simple electrochemical technique. The input and output waveforms are shown in Figure 2.5. First the potential is the open circuit potential (ocp)

and then an applied potential is imposed. When the electrode is at the ocp, no current signal is detected because no current passes through it or the sum of anodic current and cathodic current is null (corrosion). When the applied potential is constant, current-time waveform resulting from electrochemical oxidation or reduction is recorded. The shape of the I-t curve can be explained by the changes in concentration distance profiles of the target redox species. [6] For a diffusioncontrolled process, Cottrell equation (2.3) presents the characteristic shape of the chronoamperogram, where n is the number of electrons involved in the reaction, F is the Faraday constant, A is the area of the electrode (cm 2 ), C 0 is the concentration of the analyte in the bulk of the solution (mol cm -3 ), D is the diffusion coefficient (cm 2 s -1 ) and t is the time (s). Consequently, Quartz crystal microbalance with dissipation monitoring (QCM-D) generates surface-acoustic waves in quartz crystal surface which can probe the structure of films [11]. These analytical techniques based on QCM have been widely used for studies of electrodes for supercapacitors and batteries [12], especially for measurements of ionic fluxes [13] and the studies of conducting polymers, such as PPy [14] and PEDOT [15,16].

i is proportional to t -1/2 .

Electrochemical impedance spectroscopy (EIS)

The electrochemical impedance spectroscopy (EIS) experiment is usually measured by applying alternating current (AC) potential to an electrochemical cell and measuring the current through the cell. It shows the ability of a circuit to resist the flow of electrical current. Unlike Ohm's law, impedance can be applied to circuits containing more than just one resistor.

The frequency modulation allows the separation of the different reaction kinetics: fast phenomena occur at high frequencies while slower phenomena are visible at lower frequencies [18]. In general, electrochemical systems have nonlinear current-voltage characteristic curves I=f(V). If we plot the applied sinusoidal voltage signal on the X-axis of a graph and the sinusoidal response signal I(t) on the Y-axis, an oval known as a "Lissajous figure" will appear, as shown in Figure 2.9 [19]. The relationship between ΔV (low amplitude sinusoidal perturbation imposed around the stationary potential) and ΔI (low amplitude sinusoidal response in current) can be considered as linear if we focus on a small portion of this curve.

Figure 2.9 Linear current response to a low amplitude sinusoidal perturbation potential around

a stationary value [20].

The applied excitation signal can be expressed as:

𝐸 𝑡 = 𝐸 0 sin(𝜔𝑡) (2.6)
Et is the potential at time t, E0 is the amplitude of the signal, and ω is the radial frequency. The relationship between radial frequency ω (expressed in radians/second) and frequency f (expressed in Hertz) is:

𝜔 = 2𝜋𝑓 (2.7)
With Eulers relationship,

exp(𝑗𝜑) = cos 𝜑 + jsin 𝜑 (2.8)
Thus, the potential also can be described as

𝐸 𝑡 = 𝐸 0 exp(𝑗𝜔𝑡) (2.9)
And the current response It is shifted in phase (φ),

𝐼 𝑡 = 𝐼 0 sin(𝜔𝑡 + 𝜑) (2.10)
The impedance can be calculated by Ohm's law as In general, EIS data can be analyzed by fitting it to an equivalent electrical circuit model. For the most common electrical elements, the equation for their current versus voltage relationship and their impedance are listed in

Galvanostatic charge discharge (GCD)

Galvanostatic charge discharge (GCD) is a technique that allows the characterization of the electrochemical performance of a supercapacitor or other energy storages under a constant current density. As shown in Figure 2.11, voltage signal is recorded during charge and discharge process and the plot is usually potential (V) vs. time (s). From this plot, it is easy to tell the mechanism type of capacitors. A triangular shape with linear response curve is observed for ideal capacitors, while the response of pseudocapacitors is non-linear [21]. What's more, the capacitance (C in F) can be calculated from GCD plot as described by:

𝑐 = 𝑖 × ∆𝑡 ∆𝑉 (2.13)
where i (A) is the applied discharge current, Δt (s) is the time elapsed for the discharge cycle and ΔV (V) is the voltage interval of the discharge. C can also be determined by CV, in this case C is equal to the ratio of the current to the potential scan rate. beam-specimen in SEM [23].

Physical characterization techniques

In this work, SEM micrographs were obtained from a field emission gun scanning electron microscope (FEG-SEM), Ultra55 Zeiss, operating at 5 kV.

X-ray photoelectron spectroscopy (XPS)

X-ray photoelectron spectroscopy (XPS) is a surface analysis based on the photoelectric effect. It can identify the chemical elements (binding energy) and their chemical bonding (chemical shift) at the surface (10 nm) of the materials. (3) electronic focusing system; (4) hemispherical electron energy analyzer; (5) electron detector (channeltrons) and ( 6) data acquisition.

As shown in

Chapter 3 One-pot electrosynthesis of overoxidized PEDOT films

One-pot electrosynthesis of overoxidized PEDOT films 3.1 Introduction

Over recent decades, electrically conductive polymers such as polypyrrole (PPy) [1][2][3][4],

polythiophene (PTh) [5][6][7], polyaniline (PANI) [8][9][10] and their derivatives have attracted great attention across academia and industry. Poly(3,4-ethylenedioxythiophene) (PEDOT) has developed into one of the most popular materials among all the conductive polymers due to its high and stable electrical conductivity in oxidized state [11][12][13][14][15]. However, only a few reports studied its overoxidation under anodic polarization in an aqueous medium.

Compared to chemical synthesis, it is easier to control the location, thickness and morphology of PEDOT using an electrochemical route. Electropolymerization of EDOT can be carried out in organic and aqueous solutions by cyclic voltammetry or chronoamperometry in a certain range of applied potential. In an aqueous electrolyte, when the applied potential is high, a side reaction that is water oxidation can occur. Therefore, EDOT electropolymerization could be accompanied by the polymer overoxidation which results from the OH  radicals produced during water oxidation.

Du and Wang have shown that in an aqueous solution containing 0.01 M EDOT and 0.1 M LiClO4, the anodic current decreased with time under potentiostatic conditions in the range of 1.1 -1.5

V/SCE, due to the overoxidation of the polymer which becomes non-conductive [16].

Overoxidation is avoided in most studies in order to not disturb the polymerization process.

However, overoxidized PEDOT (O-PEDOT) films exhibit some unique properties and can be applied for sensing with determination of dopamine [17], uric acid [18] and lead ion [19]. Besides, a very thin pinhole-free O-PEDOT film could be an essential step of the formation of nanostructured PEDOT films, taking into account the studies performed on polypyrrole [20][21][22].

PEDOT films with nanostructures have great potential to be used for various applications.

In general, the most common way to obtain an O-PEDOT film is first preparing a PEDOT film by an electrochemical method and then applying a relatively high potential to overoxidize it in an aqueous solution which does not contain any monomer. During the process of overoxidation, the doping anions are expelled from the film, the conjugation length of the polymer is decreased [23] and the morphology of the film could be changed [24][25][26]. The influence of the overoxidation conditions, such as applied potential [27,28] and pH [29], and the electrochemical properties of O-PEDOT have been studied. However, O-PEDOT films prepared by this two-step procedure are still partially conductive. Because the first step leads to a thick PEDOT film and the anions inside of the film are hard to be completely expelled. Sometimes some thin cracks or crevices are observed because of the internal stress changes during the second step [24].

Herein, we report, for the first time, a one-pot electrosynthesis of ultrathin film of O-PEDOT that was carried out in an aqueous solution containing EDOT monomer and phosphate anions (HPO4 2- and PO4 3-). We confirm that the presence of weak acidic anions such as phosphate in the monomer solution can lead to the formation of an overoxidized polymeric film. The influence of pH and applied potential over the electrochemical process has been studied notably by electrochemical impedance spectroscopy (EIS), and a mechanism of PEDOT overoxidation in an aqueous solution has been proposed including the presence of carboxylic groups which have been successfully activated by using N-ethyl-N'(3-dimethylaminopropyl)carbodiimide hydrochloride (EDC) and Nhydroxysulfosuccinimide (NHS) solution [30][31][32].

EDOT electropolymerization under potentiodynamic conditions

Firstly, to determine the best potential for using the potentiostatic method, EDOT monomers were electrochemically oxidized under potentiodynamic conditions.

EDOT electropolymerization under potentiostatic conditions

During potentiodynamic experiments, polymers are charged and discharged and the growth process is interrupted [34]. Hence, the potentiostatic method was applied in the rest of this work, based on Figure 3.1, the potentials tested were in the range from 0.9 to 1.2 V/SCE.

Influence of the pH of the monomer solution

To study the influence of the pH of the monomer solution, electrosynthesis of PEDOT at different pH were performed under potentiostatic conditions. From Figure 3.2 (a), it is obvious that the anodic current after 300 s of polarization depends on the electrolyte pH. The higher the pH, the lower the final anodic current. Since the applied potential is fixed, the resistance of the film is higher when the current is lower. In other words, the current value indicates the level of overoxidation of the PEDOT film. The charges obtained after 300 s of polarization are reported in (i) S2p

Q anodic /mC cm -2 pH 0.9 V 1 V 1.1 V 1.2 V (b)
Intensity/ a. u.

Binding energy (eV) Chapter 3 One-pot electrosynthesis of overoxidized PEDOT films 82 However, it is difficult to discriminate between the presence of carbonyl or carboxyl groups from XPS analysis. It has been reported that ester groups are observed after PEDOT overoxidation [17].

But ester groups are not stable in alkaline solution. Therefore, we believe that the cleavage of ester groups leads to carboxyl groups. In this condition, a hydroxyl group is attached to one end of the alkene double bond which is also not stable. So enol and its isomer ketone both exist (Figure 3 To further confirm the existence of carboxyl groups, EDC and NHS was applied to activate these groups following the reactions that shown in Figure 3.6 [37].

Chapter 3 One-pot electrosynthesis of overoxidized PEDOT films Intensity / a. u.

Binding energy / eV As shown in Figure 3.9, the mass increases slowly and then remains stable. We assume this is because all the carboxyl groups on the film surface have been converted after about 2000 s. What's more, after rinsing the surface with solution A and then adding solution B for a second time, no obvious mass variation can be observed anymore. This result confirms that the carboxyl groups have been totally activated during the first time. In the future, glucose oxidase enzymes will be immobilized onto the O-PEDOT film in order to obtain a high selective amperometric glucose biosensor, via the detection of H2O2 [38].

Influence of the applied potential

As a further step, the influence of the potential imposed during the polymer synthesis was studied.

Experiments were performed under different potentials from 0.9 to 1.2 V/SCE of the three conditions mentioned before, as in Figure 3.3, which is EDOT monomers with LiClO4 and with K2HPO4 at pH 9 and K2HPO4/K3PO4 at pH 10.5. As one can notice from Figure 3.10 (e), the shape of j(t) curves is the same whatever the applied potential. The current densities are high at the very beginning then they decrease rapidly in the first 20 seconds and then remain stable at a very low value. The charges increase slightly with the increase of the applied potential, they are in the range of 14-30 mC cm -2 . In contrast, the current density at pH 9 is higher than at pH 10.5. And the current density is even higher with the presence of perchlorate. Moreover, the value of applied potential has a bigger impact on the shape of j(t) curves obtained in the presence of perchlorate ions in the monomer solution. Electrochemical characterizations of these films were carried out by CV and EIS in 0.1 M KCl solution containing Fe(CN)6 4-/3-redox probe, as shown in (d) Potential vs. SCE /V 0.9 As indicated before, the total anodic charge is due to monomer oxidation and to water oxidation.

0.9 V 1 V 1.1 V 1.2 V j /mA cm -2 E vs. SCE / V 0 50 100
j /mA cm -2 E vs. SCE / V 0.9 V 1 V 1.1 V 1.2 V
V 1 V 1.05 V 1.1 V 1.15 V 1.2 V j /mA cm -2 E vs. SCE /V (f)
To estimate each contribution, we performed the experiments in the same electrolyte without EDOT monomer. Obviously in this case only water oxidation takes place. The anodic charges due to water oxidation after 300 s of polarization are reported curve (a) in Figure 3.11 while curve (b)

shows the charges obtained in the presence of EDOT in the electrolyte. The differences between (a) and (b) at 0.9, 1 and 1.1 V/SCE are 10.4, 9.1 and 8.7 mC cm -2 , respectively. At 1.1 V/SCE, in the presence of EDOT, the charge is higher while the part due to water oxidation seems lower, which is in agreement with the fact that the CV of the film synthesized at 1.1 V/SCE is flatter than at the other potentials. As at 1.2 V/SCE, the charge is lower than water oxidation because strong water oxidation leads to much more OH  radicals (reaction (3. To confirm these findings, electrochemical impedance spectroscopy experiments were performed at open circuit potential in Fe(CN)6 4-/3-solution. Nyquist diagrams (Figure 3.12 (a)) show slightly depressed semicircles, the amplitude of which varies with the potential applied for the film synthesis. The frequency dependence of the phase points out there is a distribution of time constants.

They can be expected to arise from the polymeric film and the charge transfer impedance responses. [39]. The equivalent circuit is shown in Figure 3. 13 * For the film formed at 0.9 V/SCE, it was necessary to add a Warburg term, W, in series with the electrolyte resistance according to the circuit code LRsW(Qf (Rf (CdlRt))). Additional information can be gained from analysing the film impedance. The CPE behaviour has been explained by assuming a gradient of resistivity across the film since the overoxidation is due to OH  radicals produced at the electrode/film interface. According to [39], assuming the dielectric properties to be homogeneous and a power law for the variations of the resistivity across the film, it is possible to estimate the resistivity domain (ρδ, ρ0) covered by the O-PEDOT films between the two interfaces. Taking δ the film thickness (≈15 nm, Figure 3.3 (c)), ε the dielectric permittivity (≈ 8 [40]), S the electrode surface (0.07 cm 2 ), ρδ and ρ0 are given by the following expressions:

.9 V 1 V 1.05 V 1.1 V 1.15 V 1.2 V -Z'' /kΩ Z' /kΩ

Table 3.3 EIS analyzes. Fitting parameters of O-PEDOT films based on model LRs(Qf (Rf (CdlRt))) (ZsimpWin code) and CPE analysis according to a power law

nf n f g S o Q - = 1 ) ( . ) ( δ ρ δ εε and S g R Z n n δ ρ ρ ω δ - = ≈ = 1 0 ) 0 (
where the quantity g is given by the expression : g = 1+2.88.(1-n) 2.375 according to reference [39].

The calculated values of ρδ and ρ0 are given in Table 3.3, showing that the highest ρ0 resistivity value is obtained for the film formed at 1.1 V/SCE.

Conclusions

Non-conductive and ultrathin O-PEDOT films were prepared by one-pot electrosynthesis in an aqueous solution containing EDOT monomer and only phosphates as anions. This method is simple and environmentally friendly. The schematic model of the PEDOT overoxidation process at different pH is shown in Figure 3.15. By potentiostatic method, the film overoxidation occurs at pH 10.5, the doping anions are expelled from the polymer electrogenerated at the very beginning of the process. Some C and S atoms are oxidized to carboxyl and sulfone, respectively, leading to disruption of the chain conjugation and therefore to the non-conductive property of the film. At pH 9, the film is partially overoxidized. The surface is not as smooth as the totally overoxidized film. Some particles can be observed and the film is still conductive.

The influence of the applied potential was also studied. At pH 10.5, all the potentials from 0.9 V to 1.2 V/SCE lead to the synthesis of an overoxidized PEDOT film. Indeed, in the presence of a redox probe, very low currents are detected by CV and slightly depressed semicircles are obtained in Nyquist diagrams. The film prepared at 1.1 V/SCE shows the lowest redox response and the highest R, which means it is the lowest conductivity.

Electrosynthesis of nanostructured PEDOT film 4.1 Introduction

PEDOT films usually have a cauliflower-like structure [1,2]. In order to obtain nanostructured conductive polymers, hard templates [3][4][5][6] or soft templates [7][8][9] are commonly utilized during synthesis. Recently, bubbles, such as O2 or H2 bubbles [10][11][12][13] have been employed to obtain nanostructures of conductive polymers. Without using a template, the synthesis process can be simplified. And the experiments are more economical and environmentally friendly.

As we discussed in section 1.2.3.3, our group already published some work on one step templateless electrosynthesis of PPy nanowires [14][15][16][17]. We believe that this electrosynthesis mechanism without using a template can be applied to other conductive polymers. PEDOT is one of the most stable conductive polymers. However, EDOT monomers have much lower solubility than pyrrole monomers in an aqueous electrolyte and a higher redox potential. In other words, the balance between EDOT polymerization, water oxidation and PEDOT overoxidation by controlling the applied potential and concentration of anions in one-step electrosynthesis is not trivial.

In this chapter, templateless electrosynthesis of nanostructured PEDOT films are presented.

Various morphologies were obtained by adjusting experimental parameters, such as the concentration of EDOT and LiClO4, the applied potential, pH and temperature of the monomer solution, solution stirring, addition of glycerol to increase solution viscosity and therefore to decrease species diffusion into the electrolyte. In order to obtain nanostructured PEDOT, the most concerning problem is the low solubility of the EDOT monomer in aqueous solution (<0.015 mol L -1 ). At room temperature, various structures were obtained but they were not uniform. Then, a heated water bath was used to maintain a relatively high temperature during the experiments. In this condition, slightly higher concentrations of EDOT can be dissolved and the PEDOT structures are more uniform. Besides, the mechanism of synthesis was discussed.

Electrosynthesis of PEDOT at room temperature

10 mM EDOT

At room temperature, nanostructures of PEDOT are not easy to obtain. As shown in Figure 4. 1 (a) and (b), PEDOT synthesized at 1 V for 600 s has the tendency to be nanostructured, nanowires are obtained and the film is thick (around 500 nm). In order to obtain better nanostructures, the applied potential was changed from one potential to a procedure containing two steps. A high potential (1.1 or 1.2 V) was first applied for a very short time to obtain a partially overoxidized film. And then the applied potential was decreased to a relatively low value (0.8 or 0.9 V) for the rest of the polymerization. Low applied potential allows the polymerization to occur slowly and avoid side reactions. As shown in Figure 4.1 (a), (c) and (d), the current curve decreases to a very low value in the first few seconds and then increases slowly. Besides, the anodic charge passed through the electrode using this two-step method is much lower than the one prepared at 1 V. Lower charge leads to thinner film. And small nanowire arrays are obtained in this condition. However, the film is not uniform. Some large wire structures are observed ( Furthermore, the influence of applied potential was investigated using the solution containing 20 mM EDOT + 0.2 M K2HPO4 + 20 mM LiClO4. The applied potential was between 1 V and 1.2 V.

The chronoamperogram recorded during the synthesis is also shown in Figure 4.5 (blue curve).

Compared to the curve of the synthesis performed at 1.1 V and 1.2 V, the shape of the curve is totally different when the applied potential is 1 V. The current decreases first but not to a very low value, which implies that the film is not overoxidized in the beginning. And then the current slowly increases for about 200 s and then keeps decreasing. From the curve of 1 V, we assume that nanowires are not obtained in this condition. 1 V is not high enough for water oxidation. As a result, nanowires cannot be observed. This assumption was confirmed by SEM. As shown in Secondly, the two-step electrosynthesis was also performed in EDOT emulsions. The influence of LiClO4 concentration and the duration of the first step was studied. As shown in Figure 4.8, the shape of I-t curves are slightly different. The charge is high and the current increases more when the concentration of LiClO4 is higher. Moreover, the curve is flatter when the high potential is applied for 20 s. In other words, 10 s is enough for the partial overoxidation and PEDOT nanostructures can be obtained more obviously in this condition.

Chapter At last, the influence of the pH of the monomer solution was investigated. The experiments were conducted in pH 11.5 emulsion using one-step and two-step electrosynthesis. At this pH, nanowires can be obtained even using the one-step synthesis method. As shown in Figure 4.10, the curve obtained at 0.9 V/SCE for 600 s keeps increasing for about 500 s. Usually, nanowires are obtained when the shape of the I-t curve looks like this. However, from Figure 4.11 (a), it is clear that the structure is more conical-like. The diameter of the wires is more than 200 nm at the bottom, and the wires are sharper at the top. Besides, the film under the nanostructures is thick. In other words, the specific surface area is much lower. On the contrary, the specific surface area is much higher when the film is synthesized by a twostep method. As shown in The difference between (c) and (d) is the applied potential during the second step. At 0.9 V, the nanowires are more uniform. The length of the nanowires are similar, probably due to the same concentration of LiClO4.

Moreover, some thin and long nanowires were obtained when the concentration of EDOT was decreased to 15 mM. As shown in Figure 4.11 (e) and (f), nanowire networks were obtained in this condition. The length of the nanowires is influenced by the ratio between the concentrations of EDOT and perchlorate. The wires are longer when the concentration of perchlorate is higher.

At room temperature, the solubility of EDOT in an aqueous solution is 10 mM. And the concentration of EDOT emulsion was 20 mM. To increase the solubility of EDOT, a water bath was used. EDOT can be easily dissolved in water when the temperature is slightly higher than room temperature.

Electrosynthesis of PEDOT at 30 ℃

Water bath was used to increase the temperature during electropolymerization. 11 mM EDOT can be easily dissolved in water at 30 ℃. Moreover, this temperature is not very different from room temperature. So it won't have a great influence on the kinetics of the reactions.

Influence of pH

First of all, the influence of pH was studied again, the results are in good agreement with those reported in Chapter 3. On the contrary, as shown in Figure 4. 13 (d) and (e), "bottle-like" nanostructures are obtained when the pH of the monomer solution is 10.5 or 11. The size and shape of the structures are similar in these two conditions. The nanostructures are more uniform at pH 10.5. And the chronoamperogram obtained in these two conditions are similar, too. As shown by the orange and green curves of Figure 4.12, during the first few seconds the current decreases, then it increases and finally remains stable. To be mentioned, the I-t curve is not exactly the same every time.

However, the curve always decreases to a very low value and then increases and remains stable.

Sometimes the curve slightly increases and sometimes the curve slightly decreases. In all cases, similar morphologies are obtained.

Furthermore, water electrooxidation depends on the pH according to the Nernst equation (E=E0-0.06 pH, at 30 ℃). The higher the pH is, the lower the potential is. At pH 11.5, after adding a small amount of perchlorate, nanostructures are obtained, as shown in Figure 4.13 (f). However, the PEDOT film does not stick well to the gold substrate so the connection between the film and substrate is bad. This can be explained by the fact that strong water oxidation takes place on the gold electrode surface during the synthesis of the film. Besides, the instability of the blue curve in Figure 4.12 is due to the O2 evolution. Therefore, the film formation process is interrupted by the bubbles and the film is not well attached to the substrate.

In a word, pH plays an important role during PEDOT electrosynthesis. Because the nanostructure formation depends on a side reaction which is water oxidation. In our experimental conditions, pH 10.5-11 is the best pH range for PEDOT nanowire growth. At lower pH, the film is difficult to be overoxidized. And at higher pH, more water molecule oxidation may occur and disrupt the formation of the PEDOT film. In the rest of this section, pH of the monomer solution is fixed at 10.5.

Influence of the applied potential

At pH 10.5, in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.75 mM LiClO4, various potentials from 0.9 to 1.2 V/SCE were applied to test its influence on the structure of the PEDOT film synthesized.

The chronoamperograms are shown in Figure 4.14. When the applied potential is 0.9 V, the charge is very low, almost nothing can be observed on the substrate in this condition (Figure 4. 15 (a)).

114

The film prepared at 1 V for 300 s was already discussed in the previous section. As shown in Similar to the film prepared at room temperature at 1.2 V (Figure 4.7 (c) and (d)), at pH 10.5 and 30 ℃, the film synthesized at 1.2 V for 300 s also contains small nanowires array and large wires (Figure 4. 15 (d) and (e)). The size of these wires is bigger than the size of these obtained at room temperature. Therefore, 1.2 V is too high for the growth of regular PEDOT nanostructures.

Influence of LiClO 4 concentration

As discussed in Section 4.3.1, the concentration of perchlorate has a significant influence on the morphologies of the PEDOT films. Here, we performed more detailed experiments to confirm the influence of perchlorate concentration on the PEDOT polymerization process.

Various concentrations of LiClO4 from 0.1 mM to 10 mM were investigated under an applied potential of 1 V and 1.1 V for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 at pH 10.5.

When the applied potential is 1 V, the I-t curves of the films synthesized in solutions containing 0.1-1 mM are similar, as shown in Q / mC cm On the contrary, the I-t curves performed at 1.1 V are irregular (Figure 4.

-2 1 V 1.1 V

19). The influence of

LiClO4 concentration on the value of the anodic charge at 1.1 V is less obvious than at 1 V (Figure

4.18).

The charge increases with the increase of LiClO4 concentration in the range of 0.1-1 mM.

When the concentration of perchlorate is higher than 1 mM, the anodic charge decreases and then remains stable i.e. independent of the perchlorate concentration. As shown in From Figure 4.20, it is clear that nanowires networks are obtained in all conditions except for 1 mM LiClO4. When the concentration of perchlorate is 1 mM, the structure of the PEDOT film is between nanospheres and "bottle-like" nanowires obtained at 1 V with low concentration of perchlorate. Some "tails" can be observed on the top of nanospheres. In the range of 0.5-1 mM LiClO4, the diameter of nanowires slightly increases with the increase of the perchlorate concentration. The nanowire networks are not uniform when the concentration of perchlorate is too high. For example, in In conclusion, the concentration of perchlorate has a significant influence on the morphologies of PEDOT films. Nanowires networks can be obtained at 1 V when the concentration of perchlorate is higher than 1 mM and at 1.1 V when the concentration of perchlorate is higher than 0.1 mM.

"Bottle-like" nanowires can be obtained at 1 V when the concentration of perchlorate is in the range of 0.1-1 mM. What's more, when the concentration of perchlorate is high (close to 10 mM), the film tends to be less uniform, especially at 1.1 V.

Influence of the polarization time

The "bottle-like" structure of PEDOT film is observed for the first time. To better understand the growth mechanism of this kind of structure, the influence of polarization time was studied. The Because O2 is generated inside the polymeric film, the growth of the bubbles is very limited and therefore their size is at the nanoscale. The O2 bubbles protected some parts of the PEDOT film from overoxidation where ClO4 -are present and therefore monomer oxidation occurs and leads to nanostructures after the anions diffuse from electrolyte bulk. In other words, the bubbles are templates for PEDOT formation and the shape and the position of bubbles are variable during this process which leads to different morphologies with polymerization time. As shown in

Influence of stirring of the monomer solution

In order to confirm the existence of O2 bubbles inside the PEDOT structure during the synthesis, a magnetic stirrer was employed to stir the solution at a very low rotation rate during polymerization.

The stirring of the monomer solution increases the diffusion rate of EDOT monomers and anions and leads to a faster formation of PEDOT and helps the evolution of bubbles. As shown in

Influence of the addition of glycerol

Moreover, glycerol was added into the monomer solution to increase the viscosity (η) of the solution and therefore to decrease the diffusion of the species in the electrolyte. The relationship of diffusion coefficient (D) and viscosity is described by Stokes-Einstein Equation [23,24],

D = 𝑘 𝐵 𝑇 6𝜋𝜂𝑟 (4.4)
where kB is Boltzmann's constant, T is absolute temperature, η is the dynamic viscosity, r is the radius of the spherical particle.

It has been reported that the presence of glycerol has an effect on the morphology of polypyrrole nanospheres during chemical synthesis [25]. Glycerol increases the viscosity of the solution, as shown in Table 4.1 [26,27]. Without the presence of glycerol, the anodic charge is low and "bottle-like" nanostructures are observed (Figure 4. 25 (a) and (b)). After glycerol adding, as shown in Figure 4.26, the anodic charge increases when the glycerol is 10% (v/v) and then decreases when the glycerol is 30% (v/v).

In the presence of 50% (v/v) glycerol, the charge is lower than the one without glycerol. glycerol, nanowire networks were obtained, the thickness of the film is more than 3 μm. The small enhancement of viscosity of the solution suppressed the merge of O2 bubbles thanks to the lower diffusion rate, which leads to the smaller diameter and longer length of nanowires. However, the 10% concentration of glycerol does not have an effect on the anion's diffusion. Therefore, with 10 or 30% of glycerol, PEDOT nanowire networks were generated.

Furthermore, when the concentration of glycerol is higher, the viscosity is higher and the diffusion of anions and O2 bubbles is lower. PEDOT could not be synthesized without the presence of anions.

As a result, less and less PEDOT are formed when the concentration of glycerol is too high. As

shown in The nanowires cannot cover the substrate in this condition. In the meanwhile, the density is higher when the film is prepared with 10 mM LiClO4. Well-arranged nanowires array is obtained. Moreover, the influence of applied potential and the influence of one-step and two-step methods were investigated. and using the two-step method (1.2 V for 20 s + 0.8 V for 280 s). To be noted, the structures of the three last conditions are similar. They all contain large and small wires. The difference between one-step and two-step methods is not obvious in this condition. Although the result of the two-step synthesis (1.2 V for 20 s then 0.8 V for 280 s) shows similar morphologies as one-step method. As shown in Figure 4.34, the films prepared by one-step method are thick and not uniform. To obtain uniform PEDOT films with higher specific area, many experiments were performed and we finally found out that these kinds of films can be observed with polarization of the electrode at 1.1 V for 20 s then 0.8 V for 280 s, as shown in The influences of concentration of perchlorate and the duration of polymerization were studied. As shown in Figure 4.36, higher charge is obtained when the concentration of perchlorate is higher.

As shown in

And with longer duration of the second step, the charge significantly increases after 300 s. The mechanism that occurs under stirring conditions is similar to the one discussed previously (section 4.3.5). Due to the higher diffusion rate of EDOT monomers and anions and larger size of O2 bubbles, the structure is larger than the one prepared without stirring and the top of the structure is not closed.

Conclusions

Various structures of PEDOT films, such as nanospheres, nanowire arrays, nanowire networks as shown in Figure 4.38, were obtained by templateless electrosynthesis method. These morphologies can be obtained by tuning the concentration of EDOT and LiClO4, the applied potential, pH and temperature of the solution.

Moreover, "bottle-like" nanostructures of PEDOT were synthesized for the first time. The synthesis mechanism is discussed by using solution stirring, by adding glycerol or radical scavengers in the monomer solution to change the species diffusion and species concentrations at the electrode/solution interface.

Besides, a two-step electrosynthesis method was proposed to obtain more uniform PEDOT films with a high specific surface area. The high applied potential in the first step makes sure that the film is overoxidized in some places (places not protected by O2 bubbles) and the low applied potential in the second step allows the polymer to grow slowly and uniformly.

In the next Chapter, three types of films will be studied for energy application: nanowire arrays of PEDOT that synthesized in a solution containing 11 mM EDOT + 0.2 M K2HPO4 + 0.075 mM LiClO4 at pH 10.5 at 1.1 V for 300 s; nanowire networks of PEDOT that prepared in a solution containing more perchlorate:11 mM EDOT + 0.2 M K2HPO4 + 0.75 mM LiClO4 at pH 10.5 at 1.1 V for 300 s; "bottle-like" nanostructures of PEDOT film that deposited under lower potential in a solution containing 11 mM EDOT + 0.2 M K2HPO4 + 0.75 mM LiClO4 at pH 10.5, at 1 V for 300 s.

Chapter 5 Capacitance properties of nanostructured PEDOT films

Capacitance properties of nanostructured PEDOT films

Introduction

Nanostructured conductive polymers have various applications due to their good chemical, electrochemical, physical properties, excellent stability and very high surface area. They can be applied as electrode materials for energy storage devices such as supercapacitors [1][2][3][4][5], biosensors [6][7][8][9][10][11], tissue engineering [12], optoelectronic devices [13,14], etc.

Due to global warming and decreasing availability of fossil fuels, renewable energy production from sun and wind, as well as new generation of energy storage devices are highly required.

Batteries have good energy densities, however, the power delivery or uptake is slow. Therefore, there is a huge demand in higher-power energy storage systems. And electrochemical capacitors (ECs) meet the need for high-power as a complement to rechargeable batteries thanks to the fast charge and discharge process (in seconds). Electrochemical capacitors, also called supercapacitors, can be divided into electrochemical double layer capacitors (EDLCs) which are based on ion adsorption and pseudocapacitors which are based on fast redox reactions [15]. Conductive polymers are usually seen as a pseudocapacitor electrode material.

Generally, PEDOT is used as a coating without any structures in supercapacitors. PEDOT films usually have lower specific capacitance than PPy films due to a higher molar mass. However, PEDOT has better stability in the air. And the capacitance can be enhanced by increasing specific surface area or preparing nanocomposites. Therefore, not so many reports concern the relationship between the morphology of PEDOT films and corresponding capacitance properties.

In this chapter, we chose to study the capacitance properties of three typical PEDOT nanostructures including nanowire arrays, nanowire networks and "bottle-like" nanostructures. The PEDOT films were electrosynthesized using an electrochemical quartz crystal microbalance (EQCM) to record the mass variation of the electrode. Cyclic voltammetry (CV), galvanostatic charge discharge (GCD) and electrochemical impedance spectroscopy (EIS) were performed to characterize the capacitance properties of the films. And CV was used to measure the cycling stability of the films. In Figure 5.1 (c), the current increases first and then decreases in the first five to ten seconds. Then, the current increases fast in about 50 seconds and then decreases slowly and remains stable, even increasing slightly. Charge, film mass measured in air (quartz frequency measured before and after synthesis) and the mass to charge ratio are reported in Table 5.1. The three structures according to the order of anodic charge from high to low are nanowire networks, "bottle-like" structures and nanowire arrays. Among them, the "bottle-like" structure presents the highest mass/charge ratio.

This implies that the charges are mostly due to PEDOT deposition in this condition. And in the other conditions, water oxidation accounts for more, with the assumption that the polymerization yield is the same for all the samples. To be mentioned, electroacoustic test (Figure 5.2) shows that the "bottle-like" PEDOT film coated Au/quartz response in perchlorate solution (orange circle) is below the blue line whose slope is -0.01 Ohm/Hz. This confirms that the Sauerbrey equation can be applied to our PEDOT films.

Morphologies and capacitance properties

The electrosynthesis conditions of these films were discussed in Chapter 4. To make it easier to read, the morphologies of these three kinds of PEDOT films are shown in Figure 5.3. From the cross section view, three structures according to the order of height from high to low are nanowire networks, "bottle-like" structures and nanowire arrays, which is in good agreement with the results of charge and mass variation. The slope of the "bottle-like" structure is the closest to a vertical line. From the inset, a semicircle is observed in the curve recorded for the film whose structure is nanowire networks (blue line).

The semicircle suggests that the nanowire network PEDOT film tends to have pseudocapacitive behavior while the other two PEDOT films show double layer capacitive behavior. The capacitance values at 10 mHz for these PEDOT films are reported in only rectangular at low scan rates (< 200 mV s -1 ). For conductive polymer based capacitance electrodes, the common applied scan rates are less than 500 mV s -1 . So this does not mean that the nanowire network PEDOT film is not suitable for supercapacitors. On the contrary, the nanowire arrays and "bottle-like" structures of PEDOT remain excellent capacitive behavior at high scan rates.

Capacitance of the films can be calculated from the CV and GCD curves using equations (5.2) and

(5.3), respectively: [16] 𝐶 = 𝑄 2∆𝑉

(5.2)

𝐶 = 𝑖/( 𝑑𝑉 𝑑𝑡 ) (5.3)
where Q is the total charge, ΔV is the scanned potential window, i is the current, and dV/dt is the slope of the discharge curve.

And the specific capacitance is the value of capacitance divided by the mass of the film which is recorded by EQCM. by GCD remain stable from 0.1 mA cm -2 to 2 mA cm -2 . While the capacitance values of nanowire networks at 2 mA cm -2 is about 60% of 0.1 mA cm -2 .

Similar results were found by CV. As shown in Figure 5.6 (c), the specific capacitances of nanowire arrays, nanowire networks and "bottle-like" structures calculated by CV at 10 mV s -1 are 57 F g -1 , 89 F g -1 and 129 F g -1 , respectively. As shown in Figure 5. 6 (d), the ratios between the capacitance at 1000 mV s -1 and 10 mV s -1 for nanowire arrays, nanowire networks and "bottle-like" structures are 63%, 9% and 55%, respectively. Nanowire arrays and "bottle-like" structures show high capacitance even at high scan rates. This suggests that PEDOT films having these structures can be applied as supercapacitor electrodes since supercapacitors are designed for the fast charge and discharge process. For "bottle-like" structures, the capacitance at 200 mV s -1 is about 82% of the one at 10 mV s -1 . By comparison, the capacitance of PEDOT coated diamond/silicon nanowire electrode in a Fc/Fc + solution at 1 mV s -1 , 10 mV s -1 and 200 mV s -1 are about 114 F g -1 , 82 F g -1 and 24 F g -1 , respectively [17]. Therefore the capacitance at 200 mV s -1 is about 30% of the one at 10 mV s -1 . The PEDOT films synthesized in this work show higher capacitance and higher capacitance retention at high scan rates. The higher capacitance at high scan rate indicates that the capacitance test is not a time-dependent process. In other words, the ion insertion-disinsertion during faradaic reactions is not the dominant process. Trasatti analysis was employed in order to better understand the capacitive energy storage behaviors of these three PEDOT films, especially to determine the capacitive and diffusion contributions. Trasatti and his colleagues proposed a method to estimate the contribution of "inner"

(diffusion-controlled, pseudo-capacitance, Ci) and "outer" (surface-controlled, EDLC, Co) surfaces to the total charge (CT) measured using cyclic voltammetry [18] 𝐶 𝑇 = 𝐶 𝑖 + 𝐶 𝑜 (5.4) The foundation of the method is that the instant storage of charged species at the outer electrode surface is independent of the potential scan rate (v) (non-diffusion-controlled), while the charge As previously mentioned, the contribution of capacitive behavior for PEDOT film with "bottlelike" structures is as high as 78.5%. For this film, the mass variations at different scan rates and at different charge/discharge current densities are shown in Figure 5.8. From these graphs it is clear that the charge compensation implies meanly anions (the electrode mass increases when the anodic current increases i.e. the electron loss is compensated by anion sorption). Actually, the global mass per electron (MPE) was calculated at 0.25 mA cm -2 at charge and discharge. The result shows the molar mass of the species which are involved in the charge compensation process. As shown in Figure 5.9, the mass per electron (MPE) is around 100 g mol -1 at the highest potentials which is in good agreement with the molar mass of ClO4 -(99.5 gmol -1 ). However, at negative potentials, the mass is lower than 100, which suggests that other species are involved, either cations and/or water molecules.

PEDOT films of nanowire networks

The CV curves of PEDOT films of nanowire networks at various scan rates, from 10 mV s -1 to 1000 mV s -1 , before cycling are shown in After 2500 cycles at 100 mV s -1 , the capacitance of PEDOT nanowire arrays at 10 mV s -1 decreases from 89 F g -1 to 58 F g -1 and the capacitance at

PEDOT film of "bottle-like" structures

Since the capacitance properties of PEDOT film of "bottle-like" structures is better than the other two morphologies, higher scan rate and more cycles were performed for this film.

CV curves of "bottle-like" PEDOT film at different scan rates from 10 mV s -1 to 1000 mV s -1 before and after 10000 cycles at 1000 mV s -1 in 0. Furthermore, the capacitance of "bottle-like" PEDOT at 10 mV s -1 decreases from 129 F g -1 to 111 F g -1 which is about 86% of the film before cycling. CV at 10 mV s -1 , 200 mV s -1 , 1000 mV s -1 before and after 10000 cycles at 1000 mV s -1 are shown in Figure 5.16. At 1000 mV s -1 , the area of the CV curves is similar while the shape of the curves is slightly different. At lower scan rates (10 mV s -1 , 200 mV s -1 ), the quasi-rectangular shape has remained but the area of the curve has decreased. Moreover, EIS was used to characterize the PEDOT nanowire arrays before and after 10000 cycles at 1 V s -1 . As shown in Figure 5.17, the slope of the curves is similar and the capacitances calculated before and after cycling at 0.01 mHz are 93.5 F g -1 and 83.5 F g -1 , respectively. Furthermore, the Trasatti method was also used to characterize the film after cycling. As shown in Figure 5.18, after 10000 cycles at 1 V s -1 , the specific capacitance of the film decreased from 138 F g -1 to 115 F g -1 . While the part due to capacitive behavior decreased from 108 F g -1 to 102 F g -1 .

The retention of total capacitance is about 83% while the retention of the part resulting from capacitive behavior is 94%. So the ratio of Ccapacitive/Ctotal slightly increased from 78.5% to 88.1 % after cycling. This implies that the high cycling stability is resulted from the high capacitive behavior. PEDOT film before and after cycling at 1 V s -1 for 10000 cycles.

Finally, the SEM micrographs of "bottle-like" structures before and after cycling are shown in structures has high capacitance and good cycling stability at 1 V s -1 . The capacitance retention after cycling at 1 mV s -1 for 10000 cycles is about 86%. cycles at 1 V s -1 .

Carbonaceous shell-coated PEDOT film

Liu and his colleagues [21] proposed a method to enhance the cycling stability of polyaniline and polypyrrole films by deposition of a thin carbonaceous shell onto the polymer surface. In their work, carbonaceous shell-coated polyaniline and polypyrrole electrodes achieved good capacitance retention of ∼95 and ∼85%, respectively after 10000 cycles at 100 mV s -1 . Carbon cloth was used as a working electrode. And cycling stability test was conducted in 1 M H2SO4.

In this work, to try to increase stability of the "bottle-like" PEDOT film, a similar deposition strategy was employed. The PEDOT film was deposited at a carbon disk at 1 V/SCE for 300 s in a CV measurements were performed before and after 10000 cycles at 1 V s -1 . As shown in Figure 5.22 (a), capacitance retention is higher than 100% and remains stable during cycling. The capacitance at 10 mV s -1 before and after cycling is 3.8 and 3.38 mF, respectively. The capacitance retention is 89%, which is slightly higher than PEDOT film prepared under the same condition without carbonaceous coating (86%, see section 5.2.3.3). And the capacitance of PEDOT film, C@PEDOT film before and after cycling are shown in Figure 5. 22 (b). After coating, the capacitance slightly decreased. However, the variation of the weight of the film is not measurable in our experimental conditions. Therefore, although the capacitance stability of C@PEDOT film is good as expected. It is not easy to infer the effect of the carbonaceous film for capacitance. This preliminary study should be investigated deeper.

Conclusions

The microstructures have a significant influence on the capacitance and cycling stability of the PEDOT films. Among three types of films, the specific capacitance of PEDOT "bottle-like"

structures is the highest and the specific capacitance of PEDOT nanowire arrays is the lowest. The capacitance maintains a relatively high value at high scan rate when the structure is nanowire arrays and "bottle-like" structures. This is due to the high contribution of capacitive behavior.

Moreover, the "bottle-like" structures show the best cycling stability. The capacitances at 10 mV s -1 before and after 10000 cycles at 1 V s -1 are 129 F g -1 and 111 F g -1 , respectively. The capacitance retention is about 86%.

Carbonaceous shell-coated PEDOT film was prepared. The cycling stability of the film is good, the capacitance remains the same after 10000 cycles at 1 V s -1 . The capacitances at 10 mV s -1 before and after 10000 cycles at 1 V s -1 are 3.8 F and 3.38 mF. The capacitance retention is about 89%, which is slightly higher than PEDOT film prepared under the same condition without carbonaceous coating.

General conclusions and perspectives

In this work, we successfully obtained non-conductive overoxidized PEDOT films and various nanostructured PEDOT films by template-free electrochemical processes in aqueous solution. The nanostructured films have high specific capacitance and an excellent stability at high potential scan rate.

First of all, non-conductive and ultrathin O-PEDOT films were prepared by one-pot electrosynthesis in an aqueous solution containing EDOT monomer and only phosphate ions as anions. This method is simple and environmentally friendly. By potentiostatic method, the best overoxidation of the PEDOT film was obtained at pH 10.5, the doping anions were expelled from the polymer electrogenerated at the very beginning of the process. Some C and S atoms were oxidized to carboxyl and sulfone, respectively, leading to disruption of the chain conjugation and therefore to the non-conductive property of the film. The influence of the applied potential was also studied. At pH 10.5, all the potentials from 0.9 V to 1.2 V/SCE led to the synthesis of an overoxidized PEDOT film. Indeed, in the presence of a redox probe, very low currents were detected by CV and slightly depressed semicircles were obtained in Nyquist diagrams. The film prepared at 1.1 V/SCE has shown the lowest redox response and the highest R, which means it has the lowest conductive. By XPS, the presence of some carboxyl groups have been evidenced, therefore it may be possible to anchor enzymes on the surface of an O-PEDOT film in order to develop biosensors.

Secondly, various morphologies of PEDOT films, such as nanospheres, nanowire arrays and nanowire networks were obtained by templateless electrosynthesis methods. These morphologies can be obtained by adjusting the concentration of EDOT and LiClO4, the applied potential, pH and temperature of the monomer solution, etc. Moreover, "bottle-like" nanowires of PEDOT were synthesized for the first time. The synthesis mechanism was discussed by using stirring of the solution, addition of glycerol and radical scavengers. Besides, a two-step electrosynthesis method was proposed to obtain more uniform PEDOT films with a high specific surface area. The high applied potential in the first step allows to be sure that some areas of the film are well overoxidized (zones where no O2 bubbles are present) and the low applied potential in the second step allows the polymer to grow slowly and uniformly.

Finally, the influence of nanostructures on the capacitance and cycling stability of the PEDOT films was studied. Among nanowire arrays, nanowire networks and "bottle-like" structures, the specific capacitance of PEDOT "bottle-like" structures was the highest and the specific capacitance of PEDOT nanowire arrays was the lowest. The capacitance has remained at a relatively high value at high potential scan rate when the structure was nanowire arrays and "bottle-like" structures.

Moreover, the "bottle-like" structure has shown the best cycling stability. The capacitance at 10 mV s -1 was 129 F g -1 . After cycling at 1 V s -1 for 10,000 cycles, the retention was about 86%.

Carbonaceous shell-coated PEDOT film was prepared. After 10,000 cycles at 1 V s -1 , the capacitance retention was about 89%, which is slightly higher than PEDOT film prepared under the same condition without carbonaceous coating.

However, since the variation of the weight of the film is not measurable in our experimental conditions, it is not easy to really determine the effect of the carbonaceous film on capacitance.

This preliminary study should be investigated deeper. Furthermore, some other composites can be prepared based on nanostructured PEDOT. For example, PEDOT/PDA should have better adherence to substrates than PEDOT.

Except the main body of the thesis, several attempts of conductive polymers and nanocomposites are presented in the Annex. Firstly, PPy nanotubes were obtained by potentiostatic and potentiodynamic methods using MO aggregates as templates in the presence of H + (without Fe 3+ ).

Besides, PPy/PDA nanowires were obtained at 0.8 V/SCE for 300 s in a solution containing 0.15 M Py + 0.05 M DA + 0.2 M KH2PO4/K2HPO4 at pH 6.8. Moreover, polyoxometalate (SiMo12O40 4-) doped PPy films were obtained by potentiostatic method in solution containing Py monomers, weak acid anions, perchlorate and a small amount of H4SiMo12O40.

Annex. Electrosynthesis of nanostructured polypyrrole and polypyrrole based nanocomposites A.1 Introduction

Recently, nanostructures or nanocomposites of conductive polymers were well studied in order to obtain better electrochemical or mechanical properties. Here, we present some ideas about electrosynthesis methods of PPy nanotubes, PPy/polydopamine (PPy/PDA) and PPy/polyoxometlate nanocomposites (PPY/POM).

By chemical methods polypyrrole nanotubes can be synthesized in the presence of FeCl3 as oxidant and azo dye methyl orange (MO). [1,2] The formation of the fibrillar complex of FeCl3 and MO is a self-assembly process. And the aggregation served as a reactive self-degradable seed template leading to the formation of PPy hollow nanotubular structures. Ammonium peroxysulfate (APS) could also be used as the oxidant instead of FeCl3. [3] Moreover, MO aggregation can be formed in the presence of H + and PPy nanotubes obtained by electrochemical methods in this condition.

[4] PEDOT nanotubes can be obtained by similar methods, too. [5] The pH value of the electrolyte has an influence on the morphologies of MO aggregation and PPy nanotubes [4]. When pH is below 4.4, insoluble nanostructures with rectangular or cylindrical shape with diameters ranging from 40 nm to micrometers are formed due to the intramolecular interactions [6]. However, stainless steel mesh was used as a working electrode in the electrosynthesis process. Compared to other metal electrodes, the positive charged Fe ions were dissolved from the working electrode and acted as flocculants which helped to the formation of MO aggregates [7]. In another word, iron ions were involved in the synthesis process although FeCl3 was not added into the solution. To the best of our knowledge, electrosynthesis of PPy nanotubes by using MO as a template are all in the presence of iron ions. Dopamine (3,4-dihydroxyphenethylamine) is a molecule that has good adherence on various substrates, it could undergo an oxidative polymerization process leading to polydopamine (PDA).

By combining PDA and PPy, high conductivity and high adherence film can be achieved. Moreover, these two materials show good biocompatibility. PPy/PDA composites can be prepared by chemical methods [8] and electrochemical methods [9][10][11]. The morphologies of the composites can be controlled by adjusting the ratio between DA and Py by chemical methods [12]. However, by electrochemical methods, it is not obvious to obtain PPy/PDA nanocomposite having nanostructures. PPy/PDA nanowires can be obtained by electrosynthesis in the presence of sodium dodecyl benzene sulfonate (SDBS) [13] or by a two-step method on biomedical titanium [14,15].

During the film synthesis, DA was firstly oxidized to dopaquinone (DQ) and dopaminochrome.

DQ or dopaminochrome may interact with pyrrole monomers through chemical bonds, hydrogen bonds or π-π interactions to reduce conjugation energy of the pyrrole ring, which makes the production of pyrrole radical cation much easier. SDBS is used as a surfactant during the synthesis.

Without SDBS, nanowires cannot be obtained by one step synthesis in the presence of other dopants including Cl -, ClO4 -, sodium p-toluenesulfonate (TosNa) and phosphate-buffered saline (PBS, pH 6). By two-step method, a prenucleation film (PNF) was formed by electrochemical method at first, and then nano-architectured PPy was galvanostatically synthesized on PNF at different currents for 40 minutes. PPy/PDA nanocomposites prepared by one-step electrosynthesis without surfactant have not been reported yet.

Polyoxometalates (POMs), one of nanoscale versatile metal oxide clusters, attracted great attention in catalysis technology, electrochemistry and material science due to their special redox properties [16]. Among all the types of POMs, the Keggin structure is one of the best one known, which has a general formula of [XM12O40] n- [17] . The structure has full tetrahedral symmetry, as shown in H4SiMo12O4 is a commonly used Keggin-type POM. It can lead to the polymerization of pyrrole since it is able to oxidize pyrrole monomers and become trapped in the polymer as a dopant.

Polymer/POM composites are usually prepared by chemical methods [18][19][20]. By electrosynthesis, PPy/POM can be prepared as a coating without nanostructures [21][22][23]. But the nanostructured PPy/POM nanocomposites prepared by an electrochemical method is still a challenge.

A.2 Electrosynthesis of PPy nanotubes by using MO

To find out if an electrochemical method can replace the role of oxidant (FeCl3), Au or Au/mica were used as working electrodes, instead of stainless steel mesh [4].

Without the addition of FeCl3, nanofibers of MO were formed in acid solution, as shown in

A.3 Electrosynthesis of a PPy/PDA composite

Nano-architectured PPy can be obtained by a two-step method on biomedical titanium [14,15].

The first step is to obtain a prenucleation film (PNF), and then nano-architectured PPy was galvanostatically synthesized on PNF at different currents for 40 minutes. To confirm if PPy/PDA can be polymerized on the electrode directly without the formation of a prenucleation film. Taking into account [14,15], the film was synthesized on Au/mica at 1.5 mA cm -2 for 40 min in a solution containing 0.15 Py + 0.1 dopamine (DA) + 0.2 K2HPO4 at pH 6. pH of the solution should be lower than 7 because DA can be 'self-polymerized' at pH higher and PDA is a non-conductive polymer. Since PPy/PDA nanowires are not obtained on gold by similar method reported in [14,15]. The potentiostatic method was employed. Actually, dopamine powder is under formation of the species SiMo VI 10Mo V 2O40 6-, SiMo VI 8Mo V 4O40 8-, SiMo VI 6Mo V 6O40 10-. Each reaction corresponds to the reversible exchange of 2 electrons, which can be written as: In order to obtain more PPy-SiMo12O40 4-nanowires, higher concentration of LiClO4 was used. As shown in Figure A. 16 (a), the anodic charge of film prepared at 0.85 V is slightly higher than the one prepared at 0.8 V. As a result, more nanowires were obtained at 0.85 V, as shown in

As shown in

A.5 Conclusions

In this annex, we presented preliminary results concerning PPy, three different experiments including electrosynthesis of PPy nanotubes by using MO as template, electrosynthesis of PPy/PDA nanowires and electrosynthesis of PPy doped with SiMo12O40 4-nanowires.

In the presence of H + (without Fe 3+ ), rectangular MO aggregates were formed and acted as template Par ailleurs, la stabilité au cyclage de ces trois types de film a été étudiée. Parmi eux, la structure "bottle-like" du PEDOT montre une bonne capacitance et une excellente stabilité en cyclage dans LiClO4. A 10 mV s -1 , leur capacité est de 129 F g -1 et surtout après 10.000 cycles à 1 V s -1 la rétention de capacité est de 86%. La variation de masse du film lors du cyclage et le calcul de la masse par électron (MPE) montrent que ce sont principalement les anions (ClO4 -) qui sont impliqués dans le processus de compensation de charge.

Figure 1 . 1

 11 Figure 1.1 Publications related to conductive polymers (CPs) and PEDOT since 1970. (Data from Web of Science).

Figure 1 . 2

 12 Figure 1.2 Various conductive polymer structures: (1) polyacetylene (PA); (2) polypyrrole (PPy), polythiophene (PTh); (3) poly-p-phenylene (PPP); (4) polyaniline (PANI); (5) polycarbazole;(6)

1 State of the art 5 PEDOT (structure 10 in Figure 1 . 2 ) 1 . 3 .

 51213 has a dioxyalkylene bridging group across the 3-and 4positions of its heterocyclic ring compared to PTh. The unique and high electrical conductivity of CPs results from the conjugated systems consisting of alternating single and double carbon-carbon bonds, as shown in Figure The hybridization of the carbon atoms produces 3 hybrid sp 2 orbitals and one non-hybrid pz-orbital. The sp 2 orbitals form in-plane σ-bonds with adjacent atoms while pz-orbitals of adjacent carbon atoms form out-of-plane π-bonds. The localized single σ-bonds guarantee the strength of the chains, while pz-orbitals in the series of π-bonds overlap each other, allowing the electrons to be more easily delocalized and able to move freely between the atoms.

Figure 1 . 3

 13 Figure 1.3 Simplified schematic of a conjugated backbone: a chain containing alternating single and double bonds.[8]

Figure 1 . 5

 15 Figure 1.5 Log-scale conductivity chart showing conductive polymers, including doped forms of PANI, PPy, PEDOT, conductive polymeric composites and a selection of metals, semiconductors

Figure 1 . 7 A

 17 Figure 1.7 A simplified explanation of the electrical conductivity of conductive polymers: (A) The dopant removes or adds an electron from/to the polymer chain, creating a delocalized charge; (B) It is energetically favourable to localize this charge and surround it with a local distortion of the crystal lattice; (C) A charge surrounded by a distortion is known as a polaron (a radical ion associated with a lattice distortion); (D) The polaron can travel along the polymer chain, allowing

Figure 1 . 8

 18 Figure 1.8 Ionic actuation mechanism of p-doped polymers: (A) anion transport dominated whenthe anions are small and mobile; (B) cation transport dominated when the anions are large and have a low mobility.[26]

Figure 1 . 9 ,

 19 including chemical polymerization, electrochemical polymerization and transition metal mediated coupling polymerization. Chemical polymerization can be further divided into in situ oxidative chemical polymerization and PEDOT dispersion polymerization.

Figure 1 . 9

 19 Figure 1.9 Schematics of synthesis methods of PEDOT.

Figure 1 . 10

 110 Figure 1.10 Schematics of in situ oxidative chemical polymerization of PEDOT. (a) solution-cast

Figure 1 . 11

 111 Figure 1.11 Proposed mechanism for vapor phase polymerization of EDOT. (1) EDOT is oxidized by Fe(III) to a cation radical; (2) EDOT cation radicals form dimers that subsequently get deprotonated; (3) PEDOT polymer is doped with tosylate ions in the film which act as counter ion.

Figure 1 .

 1 Figure 1.12 (a) Schematic of polyelectrolyte complex polymerization of PEDOT; (b) Reaction schematic of the synthesis of PEDOT:PSS.[35]

Figure 1 .

 1 14.

Figure 1 . 14

 114 Figure 1.14 Schematics of the phenomena taking place during polymerization of EDOT (EDT, )

Figure 1 .

 1 15 (A), the EDOT solution treated by mechanical stirring demulsification immediately after emulsifying while the solution treated by ultrasonication remains unchanged for at least 60 minutes. Water-insoluble amines (n-octylamine and n-decylamine) were successfully electrosynthesized by the same method[START_REF] Atobe | Electrochemical Reaction of Water-Insoluble Organic Droplets in Aqueous Electrolytes Using Acoustic Emulsification[END_REF].Furthermore, clear and transparent nanoemulsions can be prepared by tandem acoustic emulsification. As shown in Figure1.15 (B), the nanoemulsion maintains the clear and transparent appearance even after 12 months without any surfactant.[START_REF] Nakabayashi | Highly clear and transparent nanoemulsion preparation under surfactant-free conditions using tandem acoustic emulsification[END_REF][START_REF] Nakabayashi | Tandem acoustic emulsion, an effective tool for the electrosynthesis of highly transparent and conductive polymer films[END_REF]

Figure 1 .

 1 Figure 1.15 (A) Photographic observations of emulsification and demulsification of EDOT (7.5 mmol) in 1.0 M LiClO4 aqueous solution. Emulsifying method: (a) mechanical stirring (1500 rpm), (b) ultrasonication (20 kHz, 22.6 W cm -2 ) for 60 s [56]; (B) Photographs of the tandem acoustic emulsification treatment of EDOT in 1.0 M LiClO4 aqueous solution. (a) Original EDOT/ aqueous solution mixture. Emulsification after (b) 20 kHz, 5 min, (c) 20 kHz, 5 min then 1.6 MHz, 5 min, and (d) 20 kHz, 5 min then 1.6 MHz, 5 min then 2.4 MHz, 5 min treatments.[START_REF] Nakabayashi | Highly clear and transparent nanoemulsion preparation under surfactant-free conditions using tandem acoustic emulsification[END_REF]

Figure 1 .

 1 Figure 1.17 (a) Diagram of the synthesis of PEDOT nanotubes in porous alumina structures on an ITO/glass substrate. (b)(c) Schematics of PEDOT growth mechanism at high applied potential (≥1.4 V) (b) under sufficient monomer supply (100 mM EDOT); (c) under insufficient monomer supply (10 mM EDOT).(d)(e) Growth mechanisms of PEDOT nanostructures at low oxidation potentials (<1.4 V) (d) at annular electrodes; (e) at flat-top electrodes. SEM micrographspresenting the base electrode morphologies, and TEM micrographs showing the PEDOT nanostructures synthesized at the corresponding base electrodes.[START_REF] Cho | Fast electrochemistry of conductive polymer nanotubes: Synthesis, mechanism, and application[END_REF]

Figure 1 . 18 ,

 118 the morphologies in the left and right column are different since the contact angle of a pyrrole droplet on the Pt and Au/Pt surface is different. On the Pt electrode, the contact angle of the pyrrole droplet is more than 90 degree.Then pyrrole monomers are polymerized on the edge of anchored micelles due to the high electric field originating from edge effects. Finally, hollow conical nanostructures are slowly produced. On the contrary, on the Au/Pt surface, each individual nucleus has a diffusion zone. And the growth of different diffusion zones overlaps into a bulk film, leading to a cauliflower-like PPy film.

Figure 1 .

 1 Figure 1.18 a) Contact angle of a pyrrole droplet on a Pt plate in reaction solution containing 10 mM Py + 0.04 M D-CSA + 1mM NaCl at pH 6. b) Contact angle of a pyrrole droplet on a Au/Pt plate in reaction solution. c) Formation mechanism of PPy conical nanocontainers: pyrrole-filled micelles are anchored on the substrate, conical nanocontainers are then formed by a slow

Figure 1 .

 1 Figure 1.19 (a)-(d). The bubbles are produced by applying a highly negative potential (about -1.2

Figure 1 .

 1 19 (e), bubbles and PPy are detected. And porous PPy film is observed in Figure 1.19 (f) after rinsing and drying the film. The density of the porous can be controlled by the applied potential and polymerization time.

Figure 1 . 19

 119 Figure 1.19 Schematic illustration of the procedure for fabrication of nanoporous PPy films: (a) a bare HOPG substrate; (b) the electrochemical formation of hydrogen nanobubbles on the HOPG substrate; (c) pyrrole electropolymerization around the hydrogen nanobubble template; (d)

Figure 1 .

 1 Figure 1.20 SEM micropraphes of polypyrrole microstructures formed by electro-oxidation of pyrrole for 120 s at constant potentials of 0.8 V/SCE (A), 1.0 V/SCE (B), and 1.2 V/SCE (C, D, E).

Figure 1 . 21 (

 121 d), the structures vary from nanotubes to ribbon-like structures with the increase of the charge.Therefore, nanostructures of carbazole-based polymers can be electrosynthesized by cyclic voltammetric and potentiostatic methods with the help of O2 or H2 bubbles depending on different applied conditions.

Figure 1 .

 1 Figure 1.21 (a) Monomer structure of mCP; (b) SEM micrograph of mCP prepared by cyclic voltammetry after 3 scans in CH2Cl2; (c) SEM micrographs of mCP films obtained by cyclic voltammetry after various potential scans number (1, 3 and 5) in CH2Cl2 + H2O; (d) SEM micrographs of mCP deposited at a constant potential and different anodic charges (25, 100 and 400 mC•cm -2 ) in CH2Cl2 + H2O. (Scale bar: 1 µm)[START_REF] Fradin | A soft template approach to various porous nanostructures from conjugated carbazole-based monomers[END_REF]

Figure 1 .

 1 Figure 1.22 (a) Monomer structure of thieno-NaPh; SEM micrographs of thieno-NaPh deposited in (b) CH2Cl2 + H2O (saturated), (c) CH2Cl2 + H2O (35%) and CH2Cl2 + H2O (65%) by cyclic voltammetry after 3 scans. (Scale bar: 1 µm)[START_REF] Thiam | Designing Nanoporous Membranes through Templateless Electropolymerization of Thieno 3,4-b thiophene Derivatives with High Water Content[END_REF]

Figure 1 .

 1 23 (A), different shapes of curves are obtained in different solutions. When only LiClO4 is present in the solution, the polymerization occurs constantly and the current keeps increasing like curve (a). Cauliflower-like PPy film is observed in this condition. Ultrathin and nonconductive O-PPy film is synthesized in solutions that only contain weak acidic anions. The I-t curve in this case is like curve (A-b). The current keeps decreasing to a minimal value. And its morphology is shown in Figure 1.23 (B). In the presence of phosphate and perchlorate (curve (A-c)), the current decreases then slightly increases, PPy nanowires are obtained. The mechanism proposed to explain this synthesis is shown in Figure 1.23 (C). Actually, the nanowire formation process is composed of three steps. The first one is the classical electrodeposition of a thin PPy film which leads to the release of protons and therefore to a decrease of weak-acidic anion concentration at the electrode/electrolyte interface. The second step concerns the oxidation of water leading in some places to the overoxidation of the polymer due to hydroxyl radicals resulted from water oxidation H2O → OH  + H + + e - (1.6) and due to the formation of O2 nanobubbles at the electrode/film interface (i.e. inside the film formed during the first step) H2O2 → O2 + 2 H + + 2 e - (1.7)that protect some parts of the polymer against the action of OH•, i.e. against overoxidation. Finally, when the anion concentration begins to increase at the film/ solution interface due to the anion diffusion from the electrolyte bulk, PPy nanowires are formed at places where O2 nanobubbles have evolved.

Figure 1 . 2 M

 12 Figure 1.23 (A) Current density versus time of polarization at 0.8 V/SCE in 0.15 M Py + (a) 0.2 M ClO4 -, (b) 0.2 M HPO4 2-, (c) 0.2 M HPO4 2-+ 10 -3 M ClO4 -[119]. (B) SEM micrograph of cross-section (tilt angle 65°) of the polymer deposits obtained in conditions (A-b) [119]. (C) General mechanism of PPy nanowire electrogeneration. Left: Drawing showing the evolution of the polymer deposit with the anodic polarization time, in Py aqueous solution containing a high concentration of K2HPO4 and a low concentration of LiClO4. Right: SEM micrograph (side view, 65°) of PPy nanowires electrosynthesized at 0.75 V/SCE for 200 s in 0.15 M Py + 0.2 M K2HPO4 + 10 -3 M LiClO4 aqueous solution. Steps ①, ③, ④: Py oxidation; step ②: water oxidation leading to OH• and O2 formation. O2 locally protect the PPy film against the action of OH• which overoxidizes PPy. Steps ①, ②, ③ last around a few seconds.[START_REF] Debiemme-Chouvy | Electrosynthesis of polypyrrole nano/micro structures using an electrogenerated oriented polypyrrole nanowire array as framework[END_REF]

Figure 1 .

 1 Figure 1.24 (d).

Figure 1 .

 1 Figure 1.24 (a) Anodic charge vs. perchlorate concentration of the Py solution. Charge passes through a Pt electrode polarized at 0.78 V/ SCE for 600 s. Electrolyte: LiClO4 + 0.2 M K2HPO4 + 0.15 M Py. (OPPy: overoxidized PPy; SH PPy: superhydrophilic PPy electrodeposit); (b) SEM micrographs from PPy films electrogenerated in (A) 10 -4 M LiClO4 + 0.2 M K2HPO4 + 0.15 M Py;

Figure 1 .

 1 Figure 1.25 (a) Chronoamperometric responses of EDOT electropolymerization on Pt electrode in 0.1 M LiClO4 aqueous solution containing 0.0 1 M EDOT, Panel A: The applying potential is (a) 0.88 V/ SCE, (b) 0.95 V/ SCE, (c) 1.0 V/ SCE and (d) 1.1 V/ SCE, Panel B: The applying potential is (a) 1.1 V/ SCE, (b) 1.5 V/ SCE, (c) 1.4 V/ SCE and (d) 1.2 V/ SCE [44]; (b) CV of O-PEDOT film overoxidized at 1.35 V/ SCE for various time in 0.1 M KCl containing 5 mM [Fe(CN)6] 4-/3- [134]; (c) EIS results of O-PEDOT film overoxidized at 1.35 V/ SCE for 45 s in 0.1 M KCl containing 5 mM [Fe(CN)6] 4-/3-[134]; (d) SEM image using secondary electrons and (e) the corresponding backscattered SEM image of O-PEDOT film overoxidized by CV in 0.1 M H2SO4 [138].

Figure 1 . 26

 126 Figure 1.26 The possible structure of O-PEDOT. For clarity, doped electrolyte ions and sulfone groups are omitted [132].

Figure 1 . 27

 127 Figure 1.27 Specific power against specific energy, also called Ragone plot, for various electrical energy storage devices. If a supercapacitor is used in an electric vehicle, the specific power shows how fast one can go, and the specific energy shows how far one can go on a single charge. Times indicated are the time constants of the devices, obtained by dividing the energy density by the power.

Figure 1 . 28

 128 Figure 1.28 Illustration of the electrode processes occurring at (a) electrical double-layer capacitive, (b) pseudocapacitive, and (c) faradaic electrodes.[START_REF] Mathis | Energy Storage Data Reporting in Perspective-Guidelines for Interpreting the Performance of Electrochemical Energy Storage Systems[END_REF]

15 ) 1 . 29 .

 15129 Cyclic voltammetry (CV) responses, galvanostatic charge discharge (GCD) responses and electrochemical impedance spectroscopy (EIS) diagrams of EDLCs, pseudocapacitive materials and batteries are shown in Figure They are essential and typical techniques for capacitive materials characterization. CV can identify the "mirror image" criterion and relationship between

Figure 1 . 29

 129 Figure 1.29 Archetypal electrical output behavior of three main types of electrodes, including (a, d) electrical double layer, (b, e) pseudocapacitive, and (c, f) battery type. (a-c) Schematic of galvanostatic charge-discharge profiles showing linear and nonlinear responses with time and (df) corresponding CV profiles. (g) Typical Nyquist plot representations for an EDLC (green curve),pseudocapacitive materials (blue) and battery (red). ESR, 45° and 90° lines are indicated.[START_REF] Mathis | Energy Storage Data Reporting in Perspective-Guidelines for Interpreting the Performance of Electrochemical Energy Storage Systems[END_REF]

Figure 1 .

 1 Figure 1.30 (A) Ion diffusivity. Cross-sectional view of (a) PPy film, (b) NW networks, and (c) NW arrays. Their thicknesses were all controlled in the range of 2 to 2.5 m. (d) Schematic model comparing the ion diffusion for PPy film, NW networks, and NW arrays [41]; (B) Schematic of morphological changes that occur in Si during electrochemical cycling. (a) The volume of silicon anodes changes by about 400% during cycling. As a result, Si films and particles tend to pulverize during cycling. Much of the material loses contact with the current collector, resulting in poor transport of electrons, as indicated by the arrow; (b) NWs grown directly on the current collector do not pulverize or break into smaller particles after cycling. Rather, facile strain relaxation in the NWs allows them to increase in diameter and length without breaking. This NW anode design has each NW connecting with the current collector, allowing for efficient 1D electron transport down

2. 3 . 5

 35 Galvanostatic charge discharge (GCD) ......................................................................... 2.4 Physical characterization techniques .................................................................................... 2.4.1 Scanning electron microscopy (SEM) ........................................................................... 2.4.2 X-ray photoelectron spectroscopy (XPS) ...................................................................... References ..................................................................................................................................

Figure 2 . 1

 21 (a). At room temperature, it is a transparent solution, as shown in Figure2.1 (b). When it is not totally dissolved in water, oily drops of monomer can be observed, as shown in Figure2.1 (c). It has been reported that EDOT solubility in water at 25 ℃ is 2.1 g L -1 (0.0148 mol L -1 )[1]. However, in our experimental condition, the maximum concentration of EDOT is about 10 mM at room temperature.

Figure 2 . 1

 21 Figure 2.1 (a) Photo of EDOT which was just taken out of the refrigerator; (b) photo of EDOT at room temperature; (c) photo of a solution containing undissolved EDOT monomer.

Figure 2 . 2

 22 Figure 2.2 Schematic of three-electrode electrochemical cell setup for polymerization.

  characterization. GPES software (Ecochemie) was used for chronoamperometry, cyclic voltammetry (CV), galvanostatic charge and discharge (GCD). FRA software (Ecochemie) was used for electrochemical impedance spectroscopy (EIS). Electrochemical quartz crystal microbalance (EQCM) (FLUKE PM6685) was used for deposition on Quartz. Gamry (reference 600+) was used for long cycling. The pictures of the potentiostats, including Autolab, Gamry and EQCM set-up are shown in Figure 2.3.

Figure 2 . 3

 23 Figure 2.3 Pictures of (a) Autolab and (b) Gamry potentiostats and (c) EQCM set-up.

Figure 2 . 4

 24 (a)), which is immersed in an unstirred solution, and measuring the resulting current[3]. A typical voltammogram is shown in Figure2.4 (b). The anodic and cathodic peaks are observed clearly and the peak currents (ip,a and ip,c) are marked. The two peaks are separated due to the diffusion of the analyte to and from the electrode. If the process is chemically and electrochemically reversible, the difference between the anodic and cathodic peak potentials, so called peak-to-peak separation (ΔEp), is 59/n mV (n being the number of electron transferred), at 25℃.[4] ΔEp increases when there is a barrier to electron transfer (electrochemical irreversibility).

Figure 2 . 4

 24 Figure 2.4 (a) Applied potential as a function of time for a generic cycling voltammetry experiment; (b) typical voltammogram in the presence of a redox couple.

Figure 2 . 5 3 )

 253 Figure 2.5 Typical applied potential as a function of time as an input and typical corresponding current-time transient waveform in a generic chronoamperometry experiment.

Figure 2 . 6

 26 Figure 2.6 (a) Photo of a quartz; the schematic of (b) disk face and (c) cross-section carrying the two gold electrodes [7].

Figure 2 . 7 Chapter 2

 272 Figure 2.7 Schematic illustration of a thickness shear mode quartz resonator.[8]

Figure 2 . 8

 28 Figure 2.8Experimental set-up of an EQCM.[17]

  results can be presented by a Bode plot or a Nyquist plot, as shown in Figure2.10. In the Nyquist diagram, each point corresponds to a certain frequency: the high frequency limit (HF) corresponds to the high-frequency resistance, often equal to the resistance of the electrolyte. The low frequency limit (BF), if obtained, is equal to the inverse of the slope of the stationary currentvoltage curve. The Bode plot illustrates separately the module |Z| and the phase φ as a function of the logarithm of the frequency, log ƒ, (where ƒ= ω/2π). The response current is the sum of Faraday's current (if) and capacitive current (ic). The faradaic process characterizes the transfer of electrons at the electrode/electrolyte interface. The capacitive processes characterize the electrical charge at the interface that creates a double layer defined of a certain capacitance Cd. Therefore, the electrochemical cell can be modeled by an equivalent circuit, as shown in Figure2.10 (a), comprising a double layer capacitance (Cd) in parallel with a transfer resistance (Rt), and introducing in series an electrolyte with resistance (Re).[7, 8]

Figure 2 .

 2 Figure 2.10 (a) Example of an equivalent electrical circuit. Plots of impedance of an electrochemical cell in (b) Nyquist diagram and in (c) Bode diagram.[7]

Figure 2 . 11

 211 Figure 2.11 Scheme potential profiles of supercapacitor devices: pseudocapacitor and ideal electrical double-layer capacitor.[21]

2. 4 . 1

 41 Scanning electron microscopy (SEM) Scanning electron microscopy (SEM) is one of the most versatile techniques for the characterization of the morphology of materials. SEM is performed under high vacuum and the surface of samples are conductive. The cross section of SEM column and the diagram of the interaction of beam-specimen in SEM are shown in Figure 2.12 (a) and (b). SEM images are generated by raster-scanning a focused electron beam over a sample and detecting the secondary electrons emitted or the electrons backscattered by the sample. (a) (b)

Figure 2 .

 2 Figure 2.12 (a) Cross section of the column of SEM [22]; (b) Diagram of the interaction of

Figure 2 . 13 ,

 213 XPS spectra are obtained by irradiating the sample with a monochromatic X-ray beam. Core electrons of the sample surface overcome their binding energy (EB) and then escape.[24] XPS spectra are usually plotted by intensity vs. EB. The kinetic energies (EK) of the ejected photoelectrons are measured by an electron spectrometer whose work function is e  . The EB of each emitted electron can be calculated as follow:𝐸 𝐵 = ℎ𝑣 -𝐸 𝐾 -𝑒  (2.14)In this thesis, XPS measurements were performed on a VG ESCALAB 250i-XL spectrometer using monochromatic Al K radiation as the X-ray source. Survey and high-resolution spectra were recorded with a pass energy of 100 eV and 20 eV, respectively, in the constant analyzer energy mode. The photoelectron collect angle was 90 with respect to the plane of the sample.

Figure 2 .

 2 Figure 2.13 (a) XPS principle; (b) Schematic representation of an XPS spectrometer: (1) X-rays tube; (2) sample; (3) electronic focusing system; (4) hemispherical electron energy analyzer;(5)

Figure 3 . 1 Figure 3 . 1

 3131 Figure 3.1 Cyclic voltammograms (CV) at a Pt electrode, at 20 mV s -1 , in solution containing 0.011 M EDOT and (a) 0.2 M K2HPO4, pH 9 ; (b) 0.2 M K2HPO4/K3PO4, pH 10.5; (c) 0.1 M LiClO4, pH 10.5. (d) CV responses at a bare Pt electrode and at PEDOT films obtained after syntheses shown in (a), (b) and (c), in 0.01 M K4Fe(CN)6/ K3Fe(CN)6 + 0.1 M KCl solution.

Figure 3 . 2

 32 Figure 3.2 (b). Notice that these charges are due to EDOT oxidation and/or water oxidation. The lowest charges are obtained for pH around 10.5-11, in this pH range whatever the applied potentials tested, the charges are similar. These results are in good agreement with our previous work [35]. The process contains four main reactions. During EDOT electropolymerization (reaction (3.1)) H + are released, these cations are captured by the phosphate anions (reaction (3.2)). When no anions (A -) are available at the electrode/solution interface, reaction (3.1) can't take place. Water oxidation occurs and hydroxyl radicals are generated (reaction (3.3)) due to the relatively high applied potential and pH of the solution. And hydroxyl radical formation is essential for the overoxidation of the polymer (reaction (3.4)).

Figure 3 . 2

 32 Figure 3.2 (a) Current density vs. time at a Au electrode polarized at 0.9 V/SCE, in electrolyte of pH 3.3, 7, 9 and 11.5; (b) Anodic charge vs. pH after 300 s of polarization of a Au electrode at different potentials in 0.011 M EDOT + 0.2 M H3PO4/KH2PO4 or KH2PO4/K2HPO4 or K2HPO4/K3PO4 solution.

Figure 3 . 3

 33 Figure 3.3 SEM micrographs of the cross section of (a) PEDOT-ClO4 film electrogenerated on Au/mica at 0.9 V/SCE for 150 s in 0.011 M EDOT + 0.1 M LiClO4 aqueous solution; (b) O-PEDOT-9 film electrogenerated on Au/mica at 1.1 V/SCE for 300 s in 0.011 M EDOT + 0.2 M HPO4 2 aqueous solution at pH 9; (c) O-PEDOT-10.5 film electrogenerated on Au/mica at 1.1V/SCE for 300 s in 0.011 M EDOT + 0.2 M HPO4 2-/PO4 3-aqueous solution at pH 10.5. Tilt: 60°.

Figure 3 . 4 1 .

 341 Figure 3.4 (d-i). For the C1s photo-peak a new contribution around 288.5 eV is detected and it is ascribed to carbonyl or carboxyl groups. For the S2p spectra, a new contribution at high binding energy, around 168 eV, is also detected, which is ascribed to sulfone groups. These peaks which are not detected for the PEDOT-ClO4 sample, confirm that the films obtained in phosphate monomer solution are overoxidized, at least partially. The C/S atomic ratios calculated for the three films are reported in Table3.1. The value for PEDOT-ClO4 is higher than that of the other two films, indicating a loss of C during the polymer overoxidation, likely the C of the ethylenedioxy functional group. What's more, among these three films, only the baseline of the survey spectrum of O-PEDOT-10.5 is very high, that is due to the non-conductive electrical property of the film.Therefore, O-PEDOT-9 is partially overoxidized and O-PEDOT-10.5 is totally overoxidized. A non-conductive polymer is obtained at pH 10.5, the doping anions are totally expelled, some C and S atoms are oxidized and the double bonds are no more conjugated.

Figure 3 . 4 Figure 3 . 3 .

 3433 Figure 3.4 (a)(b)(c) XPS survey, (d)(e)(f) C1s spectra and (g)(h)(i) S2p spectra from PEDOT-ClO4, O-PEDOT-9 and O-PEDOT-10.5 samples prepared by the same experimental conditions as in Figure 3.3. (a)(d)(g): PEDOT-ClO4 film; (b)(e)(h): O-PEDOT-9 film; (c)(f)(i): O-PEDOT-10.5 film.

. 5 (Figure 3 . 5

 535 Figure 3.5 Structures of (a) PEDOT; (b) and (c) possible final products after its overoxidation.

Figure 3 . 6 3 . 7 ,

 3637 Figure 3.6 Scheme of EDC/NHS-activated amine coupling reactions.[37]

Figure 3 . 7

 37 Figure 3.7 XPS survey of O-PEDOT film functionalized with EDC/NHS.

Figure 3 . 8

 38 Figure 3.8 Illustration of the EQCM experiment setup.

Figure 3 . 9

 39 Figure 3.9 Mass variation vs. time during activation of carboxyl groups present at the surface of an O-PEDOT film.

Figure 3 .Figure 3 . 2 .

 332 Figure 3.10 (d), it is obvious that the influence of film synthesis potential is high for a monomer solution of pH 9. The lowest response is also obtained at 1.1 V/SCE. And the current response is much lower than in Figure 3.10 (b), which is in good agreement with the XPS analyses which indicate that the film is partially overoxidized in this condition. On the contrary, all the voltammograms depicted in Figure 3.10 (f) are flat. The current densities are lower than 0.3 mA cm -2 and the oxidation and reduction of the redox probe hardly even occur, indicating on the one hand that the films are non-conductive and on the other hand that they are pinhole-free since the redox species could not reach the metal electrode. The lowest current (green line in Figure 3.10 (f)) is obtained for the film synthesized at 1.1 V/SCE (see inset ofFigure 3.10 (f)). All the anodic charges obtained for the three conditions are reported in Table3.2. At pH 10.5, the charge slightly increases with the increase of the applied potential.

4 V

 4 /mA cm-2

Figure 3 . 2 M

 32 Figure 3.10 (a)(c)(e) Current density vs. time during electrodeposition and (b)(d)(f)corresponding CV curves in 0.01 M Fe(CN)6 4-/3-+ 0.1 M KCl solution, at 20 mV s -1 of a Au electrode polarized at different potentials in (a)(b) 0.011 M EDOT + 0.1 M LiClO4 solution; (c)(d) 0.011 M EDOT + 0.2 M K2HPO4 solution at pH 9; (e)(f) 0.011 M EDOT + 0.2 M K2HPO4/K3PO4 solution at pH 10.5. Inset: Current density at 0.4 V vs. SCE in function of the applied potential.

Figure 3 . 11 1 V

 3111 Figure 3.11 Anodic charge after 300 s of polarization at different potentials of (a) Au electrode in 0.2 M K2HPO4/K3PO4 solution, without monomer, at pH 10.5; (b) Au electrode in 0.011 M EDOT +0.2 M K2HPO4/K3PO4 solution at pH 10.5; (c) O-PEDOT coated Au electrode in 0.2 M K2HPO4/K3PO4 solution, without monomer, at pH 10.5. The O-PEDOT film was prepared at 1.1 V for 300 s in 0.011 M EDOT + 0.2 M K2HPO4/K3PO4 at pH 10.5.

Figure 3 .Figure 3 . 13 .

 3313 Figure 3.12 (a) Nyquist plots of PEDOT coated Au electrodes (j(E) curves are shown in Figure 3.10 (f)) in 0.01 M K4Fe(CN)6/K3Fe(CN)6 + 0.1 M KCl solution, (b) Resistance vs. potential imposed during the film synthesis.

Figure 3 . 13 3 . 3 .Figure 3 . 14

 31333314 Figure 3.13 Equivalent electrical circuit model used to fit the impedance response of the O-PEDOT films according to the ZsimpWin circuit code LRs(Qf(Rf(CdlRt))).

  and the Nyquist plots are shown in Figure 3.14.

Figure 4 . 1

 41 (d)). These results are reproducible.

Figure 4 . 1

 41 Figure 4.1 Comparison of one-step and two-step method for PEDOT electrosynthesis at room temperature in a solution containing 10 mM EDOT + 0.1 M K2HPO4 + 10 mM LiClO4. (a) Chronoamperograms at a Pt electrode (S=0.07 cm 2 ) polarized at 1 V/SCE for 600 s and at 1.2 V for 10 s then at 0.8 V for 590 s. (b) SEM micrograph of a PEDOT film synthesized on Pt/mica at 1 V/SCE for 600 s; (c)(d) SEM micrographs of a PEDOT film synthesized on Pt/mica at 1.2 V for 10 s then at 0.8 V for 590 s. Tilted.

Figure 4 . 2

 42 Figure 4.2 Chronoamperograms at a Pt electrode (S=0.07 cm 2 ) polarized in solution containing 10 mM + EDOT + 0.1 M K2HPO4 + 0.5 or 1 or 5 or 10 mM LiClO4 at 1.2 V for 10 s then at 0.8 V for 590 s.

Figure 4 . 3 102 4. 2 . 2

 4310222 Figure 4.3 SEM micrographs of PEDOT films synthesized on Au/mica at 1.2 V for 10 s then at 0.8 V for 590 s in solution containing 10 mM EDOT + 0.1 M K2HPO4 and various concentrations of LiClO4: (a) 0.5 mM; (b) 1 mM; (c) 5 mM; (d) 10 mM. Tilted. Scale bar: 400 nm.

Figure 4 . 4

 44 Figure 4.4 Photo of an emulsion containing 20 mM EDOT.

Figure 4 . 5

 45 Figure 4.5 Chronoamperograms of Au/mica polarized in solutions containing 10 or 20 mM EDOT + 0.2 M K2HPO4 + 10 or 20 mM LiClO4 at various potentials (1 or 1.1 or 1.2 V) for 900 s.

Figure 4 . 6

 46 Figure 4.6 SEM micrographs of PEDOT films synthesized on Au/mica at 1.1 V for 900 s in solutions containing (a) 10 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4; (b) 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4; (c) 20 mM EDOT + 0.2 M K2HPO4 + 20 mM LiClO4. Tilted. Scale bar: 400 nm.

Figure 4 . 7 (

 47 a) and (b), the film is thick (more than 2 μm) and the main structure is nanosphere. On the contrary, the curve of the one prepared at 1.2 V is similar to the curve of the polymer prepared at 1.1 V. I-t curve increases more sharply and therefore the anodic charge during polymerization is higher. As shown in Figure4.7 (c) and (d), nanowires are obtained. But the wires are not uniform. Some bigger wires can be observed in this condition.

Figure 4 . 7

 47 Figure 4.7 SEM micrographs of PEDOT films synthesized on Au/mica at (a)(b) 1 V/SCE for 900 s (b)(c) 1.2 V/SCE for 900 s in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 20 mM LiClO4. Tilted.

Figure 4 . 8 As shown in Figure 4 . 9 ,

 4849 Figure 4.8 Chronoamperograms at a Au electrode polarized at 1.2 V for 10 s then at 0.8 V for 590 s or 1.2 V for 20 s then at 0.8 V for 580 s in solution containing 20 mM EDOT + 0.2 M K2HPO4 + 10 or 20 mM LiClO4..

Figure 4 . 9

 49 Figure 4.9 SEM micrographs of Au/mica polarized (a,b) in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4 at (a) 1.2 V for 10 s then at 0.8 V for 590 s; (b) 1.2 V for 20 s then at 0.8 V for 580 s; (c,d) in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 20 mM LiClO4 at (c) 1.2 V for 10 s then at 0.8 V for 590 s; (d) 1.2 V for 20 s then at 0.8 V for 580 s. Tilted. Scale bar: 400 nm.

Figure 4 . 10

 410 Figure 4.10 Chronoamperograms of a Au/mica polarized at various potentials in solution containing 15 or 20 mM EDOT + 0.1 M K2HPO4 + 1 or 10 mM LiClO4 at pH 11.5.

Figure 4 .

 4 Figure 4.11 SEM micrographs of Au/mica polarized (a) at 0.9 V for 600 s in a solution containing 20 mM EDOT + 0.1 M K2HPO4 + 10 mM LiClO4; (b) at 1.2 V for 20 s then at 0.8 V for 580 s in a solution containing 20 mM EDOT + 0.1 M K2HPO4 + 10 mM LiClO4; (c) at 1.2 V for 20 s then at 0.8 V for 580 s in a solution containing 20 mM EDOT + 0.1 M K2HPO4 + 1 mM LiClO4; (d) at 1.2 V for 20 s then at 0.9 V for 580 s in a solution containing 20 mM EDOT + 0.1 M K2HPO4 + 1 mM LiClO4; (e) and (f) at 1.2 V for 20 s then at 0.8 V for 580 s in a solution containing 15 mM EDOT + 0.1 M K2HPO4 + 10 mM LiClO4. pH = 11.5. Tilted. Scale bar: 400 nm.

Figure 4 . 10 ,

 410 the current decreases to a very low value at the first step and then it increases. The film is thinner compared to the one shown in Figure 4.11 (a). The nanostructures are not uniform and higher than 1 μm when the concentration of LiClO4 is high (10 mM), as shown in Figure 4.11 (b). Similarly, the nanowires prepared with low concentration of LiClO4 (1 mM) are also not uniform, as shown in Figure 4.11 (c). However, the nanowires are smaller in this condition. By contrast, the nanowires shown in Figure 4.11 (d) are more uniform.

Chapter 4

 4 Electrosynthesis of nanostructured PEDOT film 111 At pH 9, which is about the pH of a K2HPO4 solution, the addition of a small amount of perchlorate did not lead to the formation of PEDOT nanostructures. As shown in Figure4.12 and Figure4.13 (a)-(c), similar results are obtained at pH 9 and 10. The I-t curve keeps decreasing and the film is not nanostructured. According to our results on PEDOT overoxidation, the film is not enough overoxidized at these pH.

Figure 4 . 12

 412 Figure 4.12 Chronoamperograms at a gold electrode polarized at 1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.75 mM LiClO4 at various pH from 9 to 11.5.

Figure 4 .

 4 Figure 4.13 SEM micrographs of PEDOT films synthesized at 1 V/SCE for 300 s in 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.75 mM LiClO4 solutions at different pH values. (a, b) pH=9; (c) pH=10; (d) pH=10.5; (e) pH=11; (f) pH=11.5. Tilt 60°.

Figure 4 .

 4 Figure 4.15 (b), nanowires were obtained. Moreover, nanowire networks were obtained for the film synthesized at 1.1 V for 300 s. As shown in Figure 4.15 (c), the diameter of the wires is about 70 nm. And the substrate is totally covered by long nanowires.

Figure 4 . 14

 414 Figure 4.14 Chronoamperograms at a gold electrode polarized at various potentials for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.75 mM LiClO4 at pH 10.5.

Figure 4 .

 4 Figure 4.15 SEM micrographs of PEDOT films synthesized in solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.75 mM LiClO4 at pH 10.5 at (a) 0.9 V; (b) 1 V; (c) 1.1 V; (d, e) 1.2 V for 300 s. (a)-(c) and (e) samples are tilted (60°).

Figure 4 . 16 .Figure 4 . 16

 416416 Figure 4.16 Chronoamperograms at a gold electrode polarized at 1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + various concentrations of LiClO4 at pH 10.5.

Figure 4 .Chapter 4 118 As shown in Figure 4 . 18 ,

 44118418 Figure 4.17 SEM micrographs of PEDOT films synthesized at 1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + LiClO4, at pH 10.5. LiClO4 concentration: (a) 0.1 mM; (b) 0.5 mM; (c) 0.75 mM; (d) 1 mM; (e) 5 mM; (f) 10 mM. Samples are tilted (60°).Scale bar: 400 nm.

Figure 4 . 18

 418 Figure 4.18 Variation of the anodic charge in function of LiClO4 concentration of the EDOT solution. The films were prepared at 1 V/ 1.1 V for 300 s in solution containing 11 mM EDOT + 0.2 M K2HPO4/ K3PO4 + LiClO4, at pH 10.5.

Figure 4 .

 4 19, almost all the curves decrease with time, only the rate and duration of the decrease is different.

Figure 4 .

 4 20 (e) and (f), the size of the wires is huge and the network cannot totally cover the substrate.

Figure 4 .

 4 Figure 4.20 SEM micrographs of PEDOT films synthesized at 1.1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + LiClO4 at pH 10.5. LiClO4 concentration: (a) 0.1 mM; (b) 0.5 mM; (c) 0.75 mM; (d) 1 mM; (e) 5 mM; (f) 10 mM. Samples tilted: 60°.

PEDOT film was electrosynthetized at 1 VFigure 4 . 21 4 . 22 .

 1421422 Figure 4.21 Chronoamperogram at a gold electrode polarized at 1 V/SCE for 900 s in 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.5 mM LiClO4 solution at pH 10.5.

Figure 4 . 123 Figure 4 .

 41234 Figure 4.22 SEM micrographs of PEDOT films synthesized at 1 V/SCE for different time in 11 mM EDOT + 0.2 M K2HPO4/ K3PO4 + 0.5 mM LiClO4 solutions at pH 10.5. (a) 25 s ; (b) 50 s; (c) 300 s; (d) 600 s; (e, f) 900s. Tilted 60°. Scale bar: 400 nm.

Figure 4 . 23 (

 423 c), the growth of "bottle-like" PEDOT happens along with the evolution of O2 nanobubbles. In the orange box of Figure4.23 (b), the hollow section structure proves the existence of O2 during polymerization. However, this mechanism can be better confirmed by transmission electron microscopy (TEM).

Figure 4 .

 4 24 (a), the stirring facilitates the polymerization in the first 100 s. From the top of view of SEM micrographs with and without stirring at the same condition (Figure4.24 (b)-(d)), some nanowires are obviously hollow with stirring. For the most part of the film, the morphology is as in Figure4.24 (c). However, some large hollow structures can be observed in some places, as shown in

Figure 4 .

 4 Figure 4.24 (d). Stirring of the solution leads to the collapse of O2 bubbles. And the larger bubbles cause the hollow structure.

Figure 4 .

 4 Figure 4.24 (a) Chronoamperograms and (b)(c)(d) SEM micrographs of PEDOT films synthesized at Au/mica at 1.1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.75 mM LiClO4 at pH 10.5. (b) without stirring; (c) and (d) stirring. Scale bar: 400 nm.

Figure 4 .

 4 Figure 4.25 SEM micrographs of PEDOT films synthesized at 1.1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.075 mM LiClO4 at pH 10.5. (a) and (b) without glycerol; (c) and (d) containing 10% (v/v) glycerol; (e) containing 30% (v/v) glycerol; (f) containing 50% (v/v) glycerol. (b) and (d) are tilted.

Figure 4 .

 4 Figure 4.26 (a) Chronoamperograms at a gold electrode polarized at 1.1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.075 mM LiClO4 + glycerol at pH 10.5. (b) Anodic charges of polymerization process corresponding to the conditions of (a).

Figure 4 .

 4 25 (f), almost none PEDOT nanostructures are observed when the glycerol concentration is 50%.4.3.7 Influence of scavenger of hydroxyl radicalN,N'-dimethylthiourea (DMTU) is known as a radical scavenger. Since OH  plays an important role in the PEDOT nanostructure formation, the addition of radical scavenger can confirm the involvement of OH  in the electrosynthesis of PEDOT nanostructure. In theory, the addition of DMTU can modify PEDOT overoxidation and allow to obtain different morphologies of PEDOT.As shown in Figure4.27, the shape of I(t) curves and anodic charges of PEDOT polymerization in the presence of 0, 10 -7 , 10 -6 M DMTU are similar. While the anodic charge is clearly lower when the concentration of DMTU is 10 -5 M, actually DMTU can also react with EDOT radicals formed during monomer oxidation.

Figure 4 . 27 From Figure 4 . 28 ,

 427428 Figure 4.27 Chronoamperograms of PEDOT films synthesized at gold electrode at 1.1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4/K3PO4 + 0.1 mM LiClO4 at pH 10.5 with 0 / 10 -7 M / 10 -6 M / 10 -5 M DMTU.

Figure 4 . 131 4. 3 . 8

 413138 Figure 4.28 SEM micrographs of PEDOT films synthesized on Au/mica at 1.1 V/SCE for 300 s in solutions containing 11 mM EDOT + 0.2 M K2HPO4/ K3PO4 + 0.1 mM LiClO4 and various concentrations of DMTU at pH 10.5. (a) without DMTU; (b) with 10 -7 M DMTU; (c) with 10 -6 M DMTU and (d) with 10 -5 M DMTU. Tilted. Scale bar: 400 nm.

Figure 4 . 29

 429 Figure 4.29 Electrosynthesis of PEDOT on Pt electrode by cyclic voltammetry at 20 mV/s in solution containing 20 mM EDOT + 0.2 M K2HPO4 + LiClO4. (a)(b) without LiClO4; (c)(d) 1 mM LiClO4; (c)(d) 10 mM LiClO4. (a)(c)(e) cyclic voltammograms; (b)(d)(f) SEM micrographs of the films synthesized on Au/mica under the same conditions corresponding to (a)(c)(e), respectively. Tilted. Scale bar: 2 μm.

Figure 4 . 30

 430 Figure 4.30 Influence of LiClO4 concentration during electrosynthesis of PEDOT at pH 9. (a) Chronoamperogram at Au electrode (S=0.07 cm 2 ) polarized in solutions containing 20 mM EDOT + 0.2 M K2HPO4 + 0 or 1 or 10 mM LiClO4 at 1.1 V/SCE for 300 s. SEM micrographs of PEDOT film synthesized on Au/mica at 1.1 V/SCE for 300 s in a solution containing (b) 20 mM EDOT + 0.2 M K2HPO4; (c) 20 mM EDOT + 0.2 M K2HPO4 + 1 mM LiClO4; (d) 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4. Tilted. Scale bar: 800 nm.

Figure 4 . 31 and

 431 Figure 4.32, various PEDOT structures are obtained by changing the applied potential. Nanospheres-like structures are obtained at 0.8 and 0.9 V. Large nanowires with a diameter around 200 nm are obtained at 1 V. Nanowire arrays are obtained at 1.1 V, 1.2 V

Figure 4 . 31

 431 Figure 4.31 Chronoamperograms of PEDOT films synthesized on Au electrodes at various potentials in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4.

Figure 4 .Chapter 4

 44 Figure 4.32 SEM micrographs of PEDOT films synthesized at various potentials for 300 s in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4 at pH 9, at (a) 0.8 V/SCE; (b) 0.9 V/SCE; (c) 1 V/SCE; (d)1.1 V/SCE; (e) 1.2 V/SCE; (f) 1.2 V/SCE 20 s 0.8 V/SCE 280s. Tilted. Scale bar: 600 nm.

Figure 4 .

 4 Figure 4.33 (a) Chronoamperograms and (b)(c)(d) SEM micrographs of PEDOT films synthesized at 0.8 V/SCE for 900 s in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4. (c) and (d) samples are tilted.

Figure 4 .

 4 35.

Figure 4 .

 4 Figure 4.34 SEM micrographs of PEDOT films synthesized at (a) 1.2 V; (b) 1 V for 300 s in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4. Tilted. Scale bar: 1 μm.

Figure 4 .

 4 Figure 4.35 SEM micrographs of PEDOT films synthesized at 1.1 V/SCE for 10 s then at 0.8 V/SCE for 290 s in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4. (b) sample is tilted. Scale bar: 400 nm.

Figure 4 .

 4 Figure 4.36 (a) Influence of perchlorate concentrations during electrosynthesis of PEDOT films synthesized at 1.1 V/SCE for 10 s then at 0.8 V/SCE for 290 s or 590 s in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 0/ 0.1/ 0.5/ 1/ 5/ 10 mM LiClO4; (b) Variation of anodic charge in function of perchlorate concentration during electrosynthesis of PEDOT films synthesized at 1.1 V/SCE for 10 s then at 0.8 V/SCE for 290 or 590 s in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + LiClO4.

Figure 4 .

 4 Figure 4.37 (a) Chronoamperograms of PEDOT films synthesized at Au electrode at 1.2 V/SCE for 20 s then at 0.8 V/SCE for 280 s in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4 with and without stirring; (b)(c)(d) SEM micrographs of PEDOT films synthesized on Au/mica at 1.2 V/SCE for 20 s then at 0.8 V/SCE for 280 s in a solution containing 20 mM EDOT + 0.2 M K2HPO4 + 10 mM LiClO4. (b) without stirring; (c) and (d) stirring. (d) is tilted. Scale bar: 800 nm.

Figure 4 .

 4 Figure 4.38 SEM micrographs of various morphologies of PEDOT obtained using a templateless electrochemical route. (a) 15 mM EDOT + 0.1 M K2HPO4 + 10 -2 M LiClO4 (pH=11.5) at 1.2 V for 20 s then at 0.8 V for 580 s; (b) 20 mM EDOT + 0.1 M K2HPO4 + 10 -2 M LiClO4 (pH=9) at 1.2 V for 20 s then at 0.8 V for 580 s; (c) 20 mM EDOT + 0.1 M K2HPO4 + 10 -2 M LiClO4 (pH=9) at 0.8 V for 900 s; (d) 20 mM EDOT + 0.1 M K2HPO4 + 10 -2 M LiClO4 (pH=9) at 1.2 V for 900 s. Tilted. Scale bar: 600 nm.

5. 3

 3 Carbonaceous shell-coated PEDOT film ........................................................................... 5.4 Conclusions ........................................................................................................................ References ................................................................................................................................

  The variations of current and mass recorded during the electrosynthesis of these three types of film are shown in Figure 5.1 (a)-(c). The values of synthesis charge and electrode mass variation measured in air are shown in Figure 5.1 (d). From Figure 5.1 (a)-(c), it is obvious that the mass changes rapidly when the current changes fast. In Figure 5.1 (a) and (b), the curve of current is high at first and then decreases rapidly in the first twenty seconds and then decreases very slowly.

Figure 5 . 1

 51 Figure 5.1 Electrosynthesis of PEDOT films on Au/quartz. PEDOT films prepared in a solution containing (a) 11 mM EDOT + 0.2 M K2HPO4 + 0.075 mM LiClO4 at pH 10.5 at 1.1 V for 300 s; (b) 11 mM EDOT + 0.2 M K2HPO4 + 0.75 mM LiClO4 at pH 10.5 at 1.1 V for 300 s; (c) 11 mM EDOT + 0.2 M K2HPO4 + 0.75 mM LiClO4 at pH 10.5 at 1 V for 300 s; (d) Final charge and film mass in air of the polymer film formed in (a)(b)(c).

Figure 5 . 2

 52 Figure 5.2 Motional resistance vs.resonance frequency of the (PEDOT)/quartz in air and in 0.1 M LiClO4 solution.

Figure 5 . 3 Chapter 5 5 . 4 .

 53554 Figure 5.3 SEM micrographs of PEDOT films prepared on Au/mica. PEDOT films prepared in a solution containing (a)(b) 11 mM EDOT + 0.2 M K2HPO4 + 0.075 mM LiClO4 at pH 10.5 at 1.1 V for 300 s; (c)(d) 11 mM EDOT + 0.2 M K2HPO4 + 0.75 mM LiClO4 at pH 10.5 at 1.1 V for 300 s; (e)(f) 11 mM EDOT + 0.2 M K2HPO4 + 0.75 mM LiClO4 at pH 10.5 at 1 V for 300 s. (b)(d)(f) are tilted.

Figure 5 . 4

 54 Figure 5.4 Nyquist plots of three PEDOT films prepared as Figure 5.1 (a)-(c) on quartz. The morphologies of (a)(b)(c) are nanowire arrays, nanowire networks and "bottle-like" structures, respectively. The inset shows the high-frequency region.

Figure 5 . 5 .

 55 and CV experiments were performed in 0.1 M LiClO4 solution, the results are shown in The symmetric triangular shape of the GCD curves and almost rectangular shape of the CV curves imply an electrochemical double layer capacitive response instead of faradaic reactions. The times of one cycle recorded at 0.1 mA cm -2 by GCD methods of Figure 5.5 (c) and (e) are similar, which indicates that the capacitances of these two films at this current density are close. Besides, the CV curves in Figure 5.5 (b) and (f) are close to rectangular shape regardless the scan rates (from 10 mV s -1 to 1000 mV s -1 ) while the CV curve in Figure 5.5 (d) is

Figure 5 . 5 Figure 5 . 6

 5556 Figure 5.5 (a)(c)(e) Galvanostatic charge discharge responses and (b)(d)(f) cyclic voltammograms of three PEDOT films in a solution contains 0.1 M LiClO4. (a)(b) PEDOT film prepared as Figure 5.1 (a); (c)(d) PEDOT film prepared as Figure 5.1 (b); (e)(f) PEDOT film prepared as Figure 5.1 (c).

Figure 5 . 6

 56 Figure 5.6 (a)(b) Specific capacitance and (c)(d) capacitance retention of three PEDOT films prepared as in Figure 5.1 (a)-(c) calculated from (a)(c) GCD responses and (b)(d) CV responses. The film structures are indicated in (b).

Figure 5 . 7

 57 Figure 5.7 (a) Specific gravimetric capacitance of three PEDOT films vs. reciprocal of the square root of potential scan rate; (b) reciprocal of the specific gravimetric capacitance vs. the square root of potential scan rate; (c) specific capacitance of the films showing diffusion and capacitive contributions.

Figure 5 . 8 Figure 5 . 9

 5859 Figure 5.8 (a) Mass variation vs. applied potential of "bottle-like" PEDOT film at different scan rates; (b) Mass variation vs. time of "bottle-like" PEDOT film at different charge/discharge current densities.

Figure 5 . 10

 510 Figure 5.10 Cycling stability of PEDOT nanowire arrays in 0.1 M LiClO4 solution. (a) CV from 10 mV s -1 to 1000 mV s -1 before cycling test; (b) CV at 100 mV s -1 during cycling; (c) CV after 2500 cycles at 100 mV s -1 from 10 mV s -1 to 1000 mV s -1 ; (d) Specific capacitance vs. scan rate before and after cycling.

Figure 5 .

 5 Figure 5.11 SEM micrographs of PEDOT nanowire arrays (a) before cycling; (b) after cycling in 0.1 M LiClO4 at 100 mV s -1 for 2500 cycles. Tilted. Scale bar: 800 nm.

Figure 5 .

 5 12 (a). Several CV curves recorded during the cycling are shown in Figure5.12 (b). The area of the CV curve decreases and the shape of the curve is less rectangular-like during cycling at 100 mV s -1 . After cycling, CV curves obtained at different scan rates are shown in Figure5.12 (c). Specific capacitances at different scan rates before and after cycling are depicted in Figure5.12 (d).

Figure 5 . 12 5 . 13 .

 512513 Figure 5.12 Cycling stability of PEDOT nanowire networks in 0.1 M LiClO4 solution. (a) CV before cycling at scan rate from 10 mV s -1 to 1000 mV s -1 ; (b) CV at 100 mV s -1 during cycling; (c) CV after 2500 cycles at 100 mV s -1 at scan rates from 10 mV s -1 to 1000 mV s -1 ; (d) Specific capacitance vs. scan rate before and after cycling.

Figure 5 .

 5 Figure 5.13 SEM micrographs of PEDOT nanowire networks (a) before cycling; (b) after cycling in 0.1 M LiClO4 at 100 mV s -1 for 2500 cycles. Tilted. Scale bar: 800 nm.

1 M

 1 LiClO4 are shown in Figure 5.14. The shape of the curves and the area of the curves before and after cycling are similar. The cycling performance and specific capacitance value at different scan rates are shown in Figure 5.15. After 10000 cycles at 1000 mV s -1 , the capacitance at 1000 mV s -1 remains the same, about 71 F g -1 .

Figure 5 . 14

 514 Figure 5.14 Cycling stability of "bottle-like" PEDOT in 0.1 M LiClO4 solution. (a) CV at scan rates from 10 mV s -1 to 1000 mV s -1 before cycling; (b) CV after 10000 cycles at 1000 mV s -1 performed at scan rates from 10 mV s -1 to 1000 mV s -1 .

Figure 5 .

 5 Figure 5.15 (a) Capacitance retention during cycling of "bottle-like" PEDOT film at 1000 mV s -1 in 0.1 M LiClO4; (b) Specific capacitance at different scan rates before and after 10000 cycles at 1000 mV s -1 .

Figure 5 .

 5 Figure 5.16 CV of "bottle-like" PEDOT film before and after 10000 cycles at 1000 mV s -1 at (a) 10 mV s -1 ; (b) 200 mV s -1 ; (c) 1000 mV s -1 . Electrolyte: 0.1 M LiClO4.

Figure 5 . 17

 517 Figure 5.17 Nyquist plots of "bottle-like" structure PEDOT film before and after 10000 cycles at 1 V s -1 .

Figure 5 .

 5 Figure 5.18 (a) Gravimetric capacitance of "bottle-like" PEDOT film before and after cycling vs. reciprocal of the square root of potential scan rate. (b) Reciprocal of the gravimetric capacitance vs. the square root of potential scan rate. (c) Capacitive and diffusion contributions of "bottle-like"

Figure 5 . 19 .

 519 Figure 5.19. The structure does not change a lot. Therefore, PEDOT film with "bottle-like"

Figure 5 .

 5 Figure 5.19 SEM micrographs of "bottle-like" PEDOT film (a) before cycling; (b) after 10000

  solution containing 11 mM EDOT + 0.2 M K2HPO4 + 0.75 mM LiClO4 at pH 10.5. The chronoamperogram of the polymerization process is shown in Figure 5.20 (a) and the CV responses of the PEDOT film recorded in 0.1 M LiClO4 are shown in Figure 5.20 (b). The shapeof CV is quasi-rectangular, which is similar to the one prepared on gold. This suggests that the "bottle-like" PEDOT film grows well on gold and carbon. The 2D surface of PEDOT film is about 0.88 cm 2 . Then, the electrode was put in a Teflon lined stainless steel autoclave containing 0.05 M glucose solution (3 mL). The sealed autoclave was heated at 160 ℃ in an oven for 2 h and then cooled down at room temperature. The CV responses of the PEDOT@C film recorded in 0.1 M LiClO4 are shown in Figure5.20 (c). The quasi-rectangular shape remains the same. Since the mass of the PEDOT film is in microgram scale. The mass of the film cannot be tested by microbalance. Therefore, the specific capacitance of these films could not be calculated.

Figure 5 .Figure 5 . 21 .

 5521 Figure 5.20 (a) Chronoamperogram of PEDOT film deposited at a carbon disk at 1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4 + 0.75 mM LiClO4 at pH 10.5; (b) CV of PEDOT film prepared as (a) characterized in 0.1 M LiClO4 at different scan rates; (c) CV of PEDOT@C film characterized in 0.1 M LiClO4 at different scan rates. The coating is prepared by heating at 160 ℃ in a Teflon lined stainless steel autoclave containing 0.05 M glucose solution.

Figure 5 .

 5 Figure 5.21 SEM micrographs of (a) PEDOT film prepared at a carbon disk at 1 V/SCE for 300 s in a solution containing 11 mM EDOT + 0.2 M K2HPO4 + 0.75 mM LiClO4 at pH 10.5; (b) PEDOT@C film whose coating was prepared by heating at 160 ℃ in a Teflon lined stainless steel autoclave containing 0.05 M glucose solution.

Figure 5 .

 5 Figure 5.22 (a) Cycling performance of PEDOT@C film at 1 V s -1 in 0.1 M LiClO4; (b) capacitance of PEDOT film and PEDOT@C film at different scan rates before and after cycling. The results obtained for the initial PEDOT film are also shown for comparison.

Figure A. 1 .

 1 Figure A.1. The heteroatom (in red) is located in the center and caged by 12 octahedral MO6 -units linked to each other by the neighboring oxygen atoms.

Figure A. 1

 1 Figure A.1 Keggin-structure.

Figure A. 2 .

 2 The diameter of the rectangular nanostructures is about 120-180 nm. These nanostructures are the template of PPy nanotubes.

Figure A. 2

 2 Figure A.2 Scheme and SEM micrograph of MO aggregation in acid aqueous solution.

Figure A. 3

 3 Figure A.3 SEM micrographs of PPy films prepared at 0.8 V/SCE for 300 s in solutions at pH 4 containing (a)(b) 100 mM Py + 5 mM MO; (c)(d) 10 mM Py + 5 mM MO; (e)(f) 10 mM Py +10 mM MO +10 mM K2HPO4. (a)-(d) and (f) samples are tilted.

Figure A. 4

 4 Figure A.4 (c) and (d). According to the spectra of Energy-dispersive X-ray spectroscopy (EDX), as shown in Figure A.5. The peak of S which should appear at around 2.3 keV (green line) is not detected. This suggests that PPy grew on the surface of MO during polymerization and after MO was dissolved in water, PPy structure is hollow inside.

Figure A. 4

 4 Figure A.4 (a) Cyclic voltammograms during PPy electrosynthesis at 20 mV s -1 for 5 cycles on Au/mica in a solution containing 10 mM Py + 10 mM MO + 100 mM LiClO4 at pH 3.3. (b) SEM micrograph of the film prepared in (a) condition. (c)(d) SEM micrographs of (b) film after immersing in water for 2 hours to remove the MO aggregation and PPy on the surface. Tilted.

Figure A. 5

 5 Figure A.5 EDX spectra of the PPy film prepared at 20 mV s -1 for 5 cycles on Au/mica in a solution containing 10 mM Py + 10 mM MO + 100 mM LiClO4 at pH 3.3 shown in Figure A.4 (c).The analyze zone is shown in the inset micrograph.

Figure A. 6

 6 (a), under this current density the electrode potential is high. And the curve is not smooth due to the variation of surface area during synthesis. The film is thick and nanospheres instead of nanowires were obtained, as shown in Figure A.6 (b). This result may be due to the lack of prenucleation film or the different nature of working electrode materials.

Figure A. 6

 6 Figure A.6 (a) Galvanostatic polarization curve (b) SEM micrograph of PPy/PDA film synthesized on Au/mica at 1.5 mA cm -2 for 40 min in a solution containing 0.15 M Py + 0.1 M DA + 0.2 M K2HPO4 at pH 6.

A. 8 ,

 8 dopamine.hydrochloride form (C8H11NO2•HCl), thus the monomer solution contains Cl -. To find out the effects of HCl, dopamine and Py, electrosynthesis was performed in different solutions at 0.8 V/ SCE for 300 s in solutions (at pH 6.8) containing (i) 0.1 M DA + 0.2 M KH2PO4/K2HPO4 (no Py), (ii) 0.15 M Py + 0.2 M KH2PO4/K2HPO4 (no DA), (iii) 0.15 M Py + 0.2 M KH2PO4/K2HPO4 + 0.1 M HCl (no DA) and (iv) 0.15 M Py + 0.1 M DA + 0.2 M KH2PO4/K2HPO4. The chronoamperograms obtained with these four conditions are shown in Figure A.7. The current is high at first and then decreases rapidly to a relatively low value and remains stable. The anodic charge is high when the solution contains DA. As expected, SEM micrographs shown in Figure confirm that the morphologies of these films are different. Nanospheres on a thin film were observed in 0.1 M DA + 0.2 M K2HPO4 solution. A thin film without any nanostructures was observed in 0.15 M Py + 0.2 M K2HPO4 solution without DA or HCl (OPPy film). In the presence of HCl without DA, nanowires were obtained. And a thick layer of nanospheres similar to Figure A.6 (b) was obtained with the electrolyte containing 0.15 M Py + 0.1 M DA + 0.2 M K2HPO4 solution. Unfortunately, DA does not exist without HCl. However, these results show that the DA is involved in the electrosynthesis process of PPy/PDA nanocomposites and has an influence on the morphology of the film.

Figure A. 7

 7 Figure A.7 Chronoamperograms of films prepared on Au/mica at 0.8 V/ SCE in different solutions at pH 6.8.

Figure A. 8

 8 Figure A.8 SEM micrographs of films prepared on Au/mica at 0.8 V/ SCE for 300 s in solution ofat pH 6.8 containing (a) 0.1 M DA + 0.2 M KH2PO4/K2HPO4; (b) 0.15 M Py + 0.2 M KH2PO4/K2HPO4; (c) 0.15 M Py + 0.2 M KH2PO4/K2HPO4 + 0.1 M HCl; (d) 0.15 M Py + 0.1 M DA + 0.2 M KH2PO4/K2HPO4. Tilted.

Figure A. 9 1 M

 91 Figure A.9 (a) Cyclic voltammograms and (b) Nyquist plot of PPy/PDA film characterized in 0.1 M LiClO4. The film was prepared at 0.8 V/ SCE for 300 s at a gold electrode (S = 0.07 cm 2 ) in solution containing 0.15 M Py + 0.1 M DA + 0.2 M KH2PO4/K2HPO4 at pH 6.8.

Figure A. 10

 10 Figure A.10 Chronoamperograms of PPy/PDA films prepared on Au/mica at 0.8 V/ SCE in different solutions at pH 6.8.

Figure A. 11

 11 Figure A.11 SEM micrographs of PPy/PDA films prepared on Au/mica at 0.8 V/ SCE for 300 s in different solutions at pH 6.8. (a) 30 mM Py + 20 mM DA + 0.2 M KH2PO4/K2HPO4; (b) 30 mM Py + 50 mM DA + 0.2 M KH2PO4/K2HPO4; (c) 50 mM Py + 50 mM DA + 0.2 M KH2PO4/K2HPO4; (d) 75 mM Py + 50 mM DA + 0.2 M KH2PO4/K2HPO4. Tilted.

Figure A. 12

 12 Figure A.12 (a) Chronoamperograms of PPy/PDA films prepared on Au/mica at 0.8 V/ SCE for 300 s in different solutions at pH 6.8. SEM micrographs of PPy/PDA films prepared on Au/mica at 0.8 V/ SCE for 300 s in solutions containing (b) 0.15 M Py + 0.01 M DA + 0.2 M KH2PO4/K2HPO4 at pH 6.8; (c) 0.15 M Py + 0.05 M DA + 0.2 M KH2PO4/K2HPO4 at pH 6.8. Scale bar: 800 nm.

Figure A. 14 2 M

 142 Figure A.14 Cyclic voltammograms of films characterized at various scan rates in 1 M H2SO4 solution. The films were prepared at a gold electrode in a solution containing (a) 0.15 M Py + 0.2 M K2HPO4 + 5 10 -3 M LiClO4 at 0.8 V for 300 s; (b) 0.15 M Py + 0.2 M K2HPO4 + 5 10 -3 M LiClO4 at 0.85 V for 300 s; (c) 0.15 M Py + 0.2 M K2HPO4 + 5 10 -3 M LiClO4 + 10 -4 M H4SiMo12O40 at 0.8 V for 300 s; (d) 0.15 M Py + 0.2 M K2HPO4 + 5 10 -3 M LiClO4 + 10 -4 M H4SiMo12O40 at 0.85 V for 300 s.

Figure A. 15

 15 Figure A.15 SEM micrographs of (a) PPy film prepared at 0.8 V for 600 s on Au/mica in a solution containing 0.15 M Py + 0.2 M K2HPO4 + 5 10 -3 M LiClO4; (b) PPy/H4SiMo12O4 film prepared at 0.8 V for 600 s on Au/mica in a solution containing 0.15 M Py + 0.2 M K2HPO4 + 5 10 -3 M LiClO4 + 10 -4 M H4SiMo12O40. Scale bar: 800 nm.

Figure A. 16

 16 (b) and (c).

Figure A. 16

 16 Figure A.16 (a) Chronoamperograms of PPy doped SiMo12O40 4-films prepared at 0.8 V and 0.85 V for 600 s on Au/mica in a solution containing 0.15 M Py + 0.2 M K2HPO4 + 5 10 -3 M LiClO4 + 10 -4 M H4SiMo12O40; SEM micrograph of PPy/ H4SiMo12O40 films prepared at (b) 0.8 V and (c) 0.85 V for 600 s on Au/mica in a solution containing 0.15 M Py + 0.2 M K2HPO4 + 5 10 -3 M LiClO4 + 10 -4 M H4SiMo12O40. Scale bar: 800 nm. Tilted.

Chapitre 1

 1 for the electrosynthesis of PPy. PPy nanotubes were obtained by potentiostatic and Résumé de la thèse en français Electrosynthèse et caractérisation de polymères conducteurs nanostructurés sans emploi de gabarit Cette thèse 'CSC' préparée par Madame Dantong WANG au Laboratoire Interfaces et Systèmes Electrochimiques (UMR 8235, Sorbonne Université -CNRS) porte sur l'électrosynthèse de polymères conducteurs nanostructurés sans l'emploi de gabarit et leur caractérisation chimique et électrochimique. Deux polymères ont fait l'objet de cette étude le polypyrrole (PPy) et le poly(3,4éthylènedioxythiophène) (PEDOT), ce dernier étant le plus étudié. Au cours des dernières décennies, les polymères conducteurs électriques tels que le polypyrrole (PPy), le polythiophène (PTh), la polyaniline (PANI) et leurs dérivés ont suscité une grande attention aussi bien dans le monde universitaire qu'industriel. Parmi tous les polymères conducteurs, le poly(3,4-éthylènedioxythiophène) (PEDOT) est devenu l'un des matériaux les plus populaires en raison de sa conductivité électrique élevée et de sa stabilité à l'état oxydé. Il a été largement employé pour la mise au point de supercondensateurs, de capteurs électrochimiques, de dispositifs thermoélectriques, pour l'électrochromisme, etc… Malgré l'important attrait du PEDOT pour les scientifiques de différents domaines, la polymérisation électrochimique du monomère sans emploi d'un gabarit pour l'obtention de films de PEDOT nanostructurés est toujours à l'étude. Au cours de cette thèse, en se basant sur les résultats précédemment obtenus pour le polypyrrole, nous nous sommes principalement concentrés sur l'électrosynthèse sans 'template' de film de PEDOT suroxydé (O-PEDOT) et de PEDOT nanostructuré, synthèse conduite dans un électrolyte aqueux. L'influence du potentiel appliqué pour oxyder les monomères, du pH de l'électrolyte renfermant les monomères et des anions phosphate et d'autres facteurs ayant une influence sur la cinétique et le mécanisme réactionnel de croissance du PEDOT ont été étudiés. Différentes nanostructures de PEDOT ont été obtenues, les propriétés de supercapacité des films nanostructurés ont été déterminées par électrochimie classique et par spectroscopie d'impédance électrochimique. Dans le chapitre 1, une vue d'ensemble concernant le contexte de recherche nécessaire à la réalisation de cette thèse a été présentée en détail. Ce chapitre comprend trois parties principales : le contexte des polymères conducteurs, leur synthèse et leurs applications dans les supercondensateurs. En particulier, la méthode de synthèse de polymères conducteurs nanostructurés sans l'emploi de gabarit a été discutée. Les travaux précédemment menés dans notre laboratoire ont montré que la présence d'anions d'acide faible tels que le monohydrogénophosphate ou le carbonate, anions qui confèrent un pH relativement élevé à la solution de monomère de pyrrole, permet l'électrogénération en une étape d'un film ultrafin de polypyrrole suroxydé (O-PPy). En fait, les anions d'acides faibles capturent les protons qui sont libérés pendant l'oxydation des monomères et conduisent à une situation où aucun anion n'est disponible à l'interface électrode/solution. En raison du potentiel élevé appliqué à l'électrode, l'oxydation de l'eau a lieu et des radicaux hydroxyle sont générés, ces radicaux réagissent avec le film polymère initialement électrogénéré, ce qui entraîne sa suroxydation. Par ailleurs, la formation d'un mince film de O-PPy non conducteur est la toute première étape de l'électropolymérisation des nanofils de PPy qui sont obtenus lorsque la solution de Py contient des anions d'acide faible et les ions perchlorate. Comme le montre la Figure 1 (A), différentes courbes I(t) sont obtenues suivant la composition de l'électrolyte. Lorsque les seuls anions présents sont les ions ClO4 -, la polymérisation du Py se produit constamment, le courant d'oxydation est continue et élevé (courbe (A-a)). Un film PPy ayant une structure dite de chou-fleur est. Si les perchlorates sont remplacés par des ions HPO4 2-, un film de O-PPy ultra mince et non-conducteur est obtenu (Figure 1 (B)). La réponse I-t (courbe (A-b)) décroit fortement. En présence de phosphate et de perchlorate (courbe (A-c)), le courant diminue puis augmente légèrement, des nanofils de PPy sont obtenus. Le mécanisme proposé pour expliquer cette synthèse est présenté sur la Figure 1 (C).En fait, le processus de formation des nanofils peut se décomposer en trois étapes. La première est l'électrodéposition classique d'un film mince de PPy qui conduit à la libération de protons et donc à une diminution de la concentration en anions d'acide faible à l'interface électrode/électrolyte. La deuxième étape concerne l'oxydation de l'eau conduisant par endroits à la suroxydation du polymère due aux radicaux hydroxyle résultant de l'oxydation de l'eau et à la formation de nanobulles d'O2 à l'interface électrode/film (c'est-à-dire à l'intérieur du film formé lors de la Résumé de la thèse en français 194 première étape) nanobulles qui protègent certaines parties du polymère de l'action des OH•, c'està-dire de la suroxydation. Enfin, lorsque la concentration en anions réaugmente à l'interface film/solution en raison de la diffusion d'anions présents au sein de l'électrolyte, des nanofils de PPy se forment aux endroits où des nanobulles d'O2 étaient présentes.

Figure 1 (

 1 Figure 1 (A) Densité de courant en fonction du temps de polarisation à 0,8 V/SCE dans 0,15 M Py + (a) 0,2 M ClO4 -, (b) 0,2 M HPO4 2-, (c) 0,2 M HPO4 2-+ 10 -3 M ClO4 -. (B) Micrographie MEB de la section transversale (65°) d'un film obtenu dans les conditions (A-b). (C) Mécanisme général de l'électrogénération de nanofils de PPy. A gauche: Schéma montrant l'évolution du dépôt de polymère avec le temps de polarisation, dans une solution aqueuse de Py contenant une forte concentration de K2HPO4 et une faible concentration de LiClO4. Étapes ①, ③, ④ : Oxydation du Py ; étape ② : oxydation de l'eau conduisant à la formation de OH• et de O2. O2 protège localement le film PPy contre l'action de OH• qui sur-oxyde le PPy. Les étapes ①, ②, ③ durent environ quelques secondes. A droite: Micrographie MEB (vue latérale, 65°) de nanofils de PPy électrosynthétisés à 0,75 V/SCE pendant 200 s dans une solution aqueuse de Py 0,15 M + 0,2 M K2HPO4 + 10 -3 M LiClO4.

Figure 2

 2 Figure 2 Schéma réactionnel de la formation de film de PEDOT suroxydé (O-PEDOT) ou non en fonction du pH de la solution de monomère (EDOT) et de la présence d'ions perchlorate.

Figure 3

 3 Figure 3 Micrographie MEB d'un film de PEDOT suroxydé obtenu par électrooxydation de EDOT dans une solution de pH 10,5 à 1,0 V/ECS et diagrammes d'impédance de films préparés à différents potentiels.

Figure 4

 4 Figure 4 Micrographies MEB de films de PEDOT nanostructurés. Conditions de synthèse : (a) 11,5 mM EDOT + 0,2 M K2HPO4/ K3PO4 0,75 mM LiClO4 + 30% (v/v) glycerol, pH 10,5, polarisation à 1,1 V/ECS pendant 300 s. (b) 11 mM EDOT + 0,2 M K2HPO4/ K3PO4 + 0,75 mM LiClO4, pH 10,5, sous agitation, polarisation à 1,1 V/ECS pendant 300 s (c) 11,5 mM EDOT + 0,2 M K2HPO4/ K3PO4 + 0,75 mM LiClO4, pH 10,5, polarisation à 1 V/SCE pendant 300 s; (d) 11,5 mM EDOT + 0,2 M K2HPO4/ K3PO4 + 0,75 mM LiClO4, pH 10,5, polarisation à 1 V/ECS pendant 600 s. Barre d'échelle : 800 nm.

Figure 5

 5 Figure 5 (a)(c)(e) Réponses charge-décharge galvanostatiques (b)(d)(f) voltammogrammes cycliques de trois films PEDOT dans 0,1 M LiClO4. (a)(b) réseaux de nanofils préparés dans une solution contenant 11 mM EDOT + 0,2 M K2HPO4 + 0,075 mM LiClO4 à pH 10,5 à 1,1 V pendant 300 s ; (c)(d) réseaux de nanofils préparés dans une solution contenant 11 mM EDOT + 0,2 M K2HPO4 + 0,75 mM LiClO4 à pH 10,5 à 1,1 V pendant 300 s ; (e)(f) structures en forme de bouteille préparées dans une solution contenant 11 mM EDOT + 0,2 M K2HPO4 + 0,75 mM LiClO4 à pH 10,5 à 1 V pendant 300 s.

  CE). Au, Pt, glassy carbon, indium doped tin oxide (ITO), etc. are commonly used as WE materials. CPs are deposited onto the WE surface directly during polymerization. RE is used to apply or measure the WE potential and CE is used to close the electrical circuit, which

	1.2.2 Electrochemical synthesis
	1.2.2.1 Setup and influence factors
	Electrochemical synthesis allows the conductive polymers to be deposited onto the substrate under
	more accurate control than chemical methods. In general, electropolymerization is carried out in a
	three-electrode cell, which contains a working electrode (WE), a reference electrode (RE) and a
	counter electrode (

is usually a Pt grid. In addition, the solution contains CP monomers, dopant ions and solvent molecules. The electropolymerization process of ClO4 -doped PEDOT in three-electrode cells is

Table 1 .1.Table 1 . 1

 111 To be noted, in spite of the fact that typical CPs such as PPy and PEDOT can be synthesized by chemical and electrochemical routes, some novel CPs with modified monomers are only amenable to chemical polymerization. Moreover electrochemical synthesis is limited to those systems whose monomers can be oxidized to reactive radical ion intermediates when a potential is applied during polymerization. Some features of chemical and electrochemical polymerization of CPs are summarized in Comparison of chemical and electrochemical polymerization of CPs

		Chemical polymerization	Electrochemical polymerization
	Products	Powders	Films
	Bulk production	Yes	No
	Nanostructures	-	Various morphologies with well-controlled thickness [70, 71]
	Conductivity [72]	Relatively low	Relatively high
	Conditions	High temperature/ pressure is needed sometimes	Relatively simple
	Can be applied to [8]	All the polymers	Some polymers
		The solvent, the oxidant,	Monomer concentration, applied
	Factors	the concentration of the reagents, reaction time, temperature,	potential, electrolyte concentration, solution pH,
		stirring rate, etc.	temperature, etc.

Table of Contents

 of 2 Materials and experimental methods ........................................................................................... .2.2 Potentiostat .................................................................................................................... 2.3 Electrochemical techniques for synthesis and characterization ...........................................

2.1 Chemicals ............................................................................................................................. 2.2 Electrochemical setup .......................................................................................................... 2.2.1 Electrochemical cell and electrodes .............................................................................. 22.3.1 Cyclic voltammetry (CV) .............................................................................................. 2.3.2 Chronoamperometry ...................................................................................................... 2.3.3 Electrochemical quartz crystal microbalance (EQCM) ................................................ 2.3.4 Electrochemical impedance spectroscopy (EIS) ...........................................................

Table 2 .1 Chemicals

 2

		Name	Molecular formula	Assay (%)
		3,4-ethylenedioxythiophene (EDOT)	C6H6O2S	97
	Solutions	Pyrrole (Py) Phosphoric acid	C4H5N H3PO4	98 85
		Ethanol	C2H5OH	96
		Potassium phosphate monobasic	KH2PO4	ReagentPlus
		Potassium phosphate dibasic	K2HPO4	98.0-105
		Potassium phosphate tribasic	K3PO4	98
		Potassium hydroxide	KOH	86.0
		Lithium perchlorate	LiClO4	≥99.0
		Potassium chloride	KCl	100.4
	Salts	Potassium hexacyanoferrate (III) Potassium hexacyanoferrate (II) trihydrate K4Fe(CN)6•3H2O K3Fe(CN)6	100 100.6
		N-ethyl-N'(3-
		dimethylaminopropyl)carbodiimide	C8H17N3	≥98
		hydrochloride (EDC)
		Glucose oxidase from aspergillus niger (GOx)	-	100,000-250,000 units/g

Table 2 .2.Table 2 . 2

 222 Common electrical elementsAnother important circuit element to be considered is the diffusion impedance W due to the transport of the redox species in the electrolyte. For the infinite plane diffusion case (Warburg

	Component	Current vs. Voltage	Impedance
	resistor	E=IR	Z=R
	inductor	E=L di/dt	Z=jωL
	capacitor	I=C dE/dt	Z=1/jωC

Table of contents 3 .

 of3 One-pot electrosynthesis of overoxidized PEDOT films ........................................................... 3.1 Introduction .......................................................................................................................... 3.2 EDOT electropolymerization under potentiodynamic conditions ....................................... 3.3 EDOT electropolymerization under potentiostatic conditions ............................................. 3.3.1 Influence of the pH of the monomer solution ............................................................... 3.3.2 Discussion of the final products of overoxidation ........................................................

3.3.3 Influence of the applied potential .................................................................................. 3.4 Conclusions .......................................................................................................................... References ..................................................................................................................................

Table 3 .

 3

		C	O	S	Cl	P	C/S
	PEDOT-ClO4	62.8	28.7	7.9	0.6	-	7.9
	O-PEDOT-9	60.8	29.7	9.2	-	0.3	6.6
	O-PEDOT-10.5 61.4	29.2	9.4	-	-	6.5

1 Atomic concentration in at. % of C, O, S, Cl and P of the polymer and the atomic ratios of C/S of PEDOT-ClO4, O-PEDOT-9 and O-PEDOT-10.5 films, obtained from XPS data.

Table 3 . 2

 32 Anodic charge from j(t) curves depicted inFigure 3.10.

	Applied potential	0.011 M EDOT + 0.1 M LiClO4	0.011 M EDOT + 0.2 M K2HPO4 at pH 9	0.011 M EDOT + 0.2 M K2HPO4/K3PO4 at pH 10.5
	0.9 V	16.3 mC	3.2 mC	1.0 mC
	1 V	18.3 mC	5.7 mC	1.0 mC
	1.1 V	18.6 mC	3.7 mC	1.5 mC
	1.2 V	13.7 mC	4.0 mC	2.0 mC

Table 4 . 1

 41 Viscosity of glycerol/water at 30 ℃[26, 27].

	Vglycerol / Vwater	Viscosity / N s m -2
	10 / 90	0.0010833
	30 / 70	0.0022268
	50 / 50	0.0056996
	EDOT polymerization was performed in solutions containing 11 mM EDOT + 0.2 M

K2HPO4/K3PO4 + 0.075 mM LiClO4 and different volumes of glycerol at pH 10.5, under 1.1 V for 300 s. The perchlorate concentration is very low because the diffusion of anions is decreased in the presence of glycerol.

Table of contents 5 .

 of5 Capacitance properties of nanostructured PEDOT films ......................................................... 5.1 Introduction ........................................................................................................................ 5.2 Capacitance properties of three typical nanostructured PEDOT films .............................. 5.2.1 Synthesis of nanostructured PEDOT films ................................................................. 5.2.2 Morphologies and capacitance properties ................................................................... 5.2.3 Cycling stability of the PEDOT films ......................................................................... 5.2.3.1 PEDOT films of nanowire arrays ......................................................................... 5.2.3.2 PEDOT films of nanowire networks ....................................................................

5.2.3.3 PEDOT film of "bottle-like" structures ...............................................................

5.2 Capacitance properties of three typical nanostructured PEDOT films 5

  .2.1 Synthesis of nanostructured PEDOT filmsAs discussed in Chapter 4, various morphologies of PEDOT films can be obtained by adjusting the pH of the monomer solution, the concentrations of LiClO4 and the applied potential. Here, we prepared three typical nanostructured PEDOT films including nanowire arrays, nanowire networks and "bottle-like" nanostructures. They were synthesized by the potentiostatic method by EQCM.

	Nanowire arrays of PEDOT were synthesized in a solution containing 11 mM EDOT + 0.2 M
	K2HPO4 + 0.075 mM LiClO4 at pH 10.5 at 1.1 V/SCE for 300 s. Nanowire networks of PEDOT
	were prepared in a solution containing more perchlorate:11 mM EDOT + 0.2 M K2HPO4 + 0.75
	mM LiClO4 at pH 10.5 at 1.1 V/SCE for 300 s. And finally "bottle-like" nanostructures of PEDOT
	film were deposited under lower potential in a solution containing 11 mM EDOT + 0.2 M K2HPO4
	+ 0.75 mM LiClO4 at pH 10.5, at 1 V/SCE for 300 s.

Table 5 . 1

 51 Final charge, film mass and ratio of mass/charge of three kinds of PEDOT film

		Anodic charge mC	Film mass μg	Mass/Charge g mC -1
	(a) Nanowire arrays	33	7	0.21
	(b) Nanowire networks	77	20	0.26
	(c) "Bottle-like" structures	53	17	0.32

Table 5 . 2 .

 52 The "bottlelike" structure PEDOT film has the highest capacitance value and the highest specific capacitance value.

Table 5 . 2

 52 Capacitances of three PEDOT films calculated by EIS

Chapter 4 Electrosynthesis of nanostructured PEDOT film

Table of contents

storage at the inner surface is a diffusion-controlled process [19]. Thus, the total measured charge (C(v)) could be expressed as a function of potential scan rate (v) through the following equation:

where 𝑘𝑣 -1 2

⁄ represents the charge storage related to semi-infinite diffusion, k is a constant, and

C∞is the charge stored at high scan rate (v →∞). The extrapolation of C to v 0 gives the total charge (Ct) while extrapolation of C to v →∞can provide the charge stored at the outer surfaces (Co). The value of Co can be estimated from the linear extrapolation of C against v -1/2 at low scan rates and the value of CT can be estimated by the linear extrapolation of C -1 against v 1/2 [16,20].

The values of Co and CT are calculated by Trasatti method as shown in 138 F g -1 , respectively. Co/CT of these films is 58.3%, 33.9%, 78.5%, which results from the capacitive behavior. These values are in good agreement with the results of CV, GCD and EIS.

The proportion of capacitive behavior is high for nanowire arrays and "bottle-like" structures. The capacitances of these two films show better capacitance at high scan rates because the diffusion contribution which is related to scan rate is low. Therefore, the influence of scan rate is not significant in these cases.

In conclusion, the PEDOT film morphology has a significant influence on the capacitance values and the capacitive energy storage behaviors of the films. The specific capacitance of "bottle-like"

structures is the highest (129 F g -1 , at 10 mV s -1 ) and the specific capacitance of PEDOT nanowire arrays is the lowest (57 F g -1 , at 10 mV s -1 ). The capacitance remains at a relatively high value at high scan rate when the structure is nanowire arrays or "bottle-like" structures. And the ratio of capacitive and diffusion contributions for "bottle-like" structures is the highest among these three structures, which is 78.5%.

Cycling stability of the PEDOT films

Cycling stability is also an important parameter for supercapacitor electrode materials. Therefore the PEDOT films were cycled in 0.1 M LiClO4 by CV at different scan rates.

PEDOT films of nanowire arrays

Firstly, films having nanowire arrays structure were tested. The CV curves at various scan rates from 10 mV s -1 to 1000 mV s -1 before cycling are shown in

A.4 Electrosynthesis of PPy/POM nanowires

PPy nanowires can be synthesized by potentiostatic method in the presence of weak acid anions in aqueous solutions. [24,25] In order to find out if PPy doped with a polyoxometalate can maintain the nanowires structure, a small amount of H4SiMo12O4 (10

Conclusion générale et perspectives

Dans ce travail, nous avons obtenu avec succès des films de PEDOT suroxydés non conducteurs et divers films de PEDOT nanostructurés par voie électrochimique sans emploi de gabarit, dans des solutions aqueuses. Certains de ces films nanostructurés ont une valeur de capacité spécifique élevée et une excellente stabilité en cyclage.

En dehors du sujet principal de cette thèse qui porte sur des études avec l'EDOT, des essais de synthèse de polypyrrole sous différente forme ont été conduits, à savoir (i) oxydation du pyrrole (Py) en présence de méthyl orange pour obtenir des nanotubes de PPy par une voie électrochimique, (ii) synthèse de nanocomposite PPy/polydopamine, (iii) l'oxydation du monomère de Py en présence de polyoxométalates (POM) de type Keggin, SiMo12O40 4-, ce qui conduit à une combinaison de synthèse par voie chimique, les monomères de Py étant oxydés par SiMo12O40 4-, et par voie électrochimique (polarisation de l'électrode), les films de PPy nanostructurés ainsi synthétisés renferment des POMs.