Structural and morphological characterization techniques

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Energy storage: Supercapacitors

History: discovery and evolution of supercapacitors

In recent years, supercapacitors have attracted a lot of attention because of their high power density, long service life, and fast charge / discharge rate1. The first supercapacitor patent was filed in 1957 by Howard Becker of the American General Electric Group2. They were composed of porous carbon electrodes and an aqueous electrolyte based on sulfuric acid. However the storage principle was not very well explained. After a few years, researchers at SOHO (Standard Oil Company of Ohio) explained the principle of charge storage in supercapacitors by the « electrochemical double layer » discussed by the physicist Hermann Von Helmholtz. Thereafter, they have filled several patents for supercapacitors of high energy density operating at higher voltages, composed here of graphite electrodes in contact with organic electrolytes3. These patents were used by the Japanese company NEC (Nippon Electric Company) which commercialized the first supercapacitors working in aqueous electrolyte in 1971 under the name of « supercapacitors »4. These supercapacitors with small slides and low capacitive performance (energy density (0.5 Wh.kg-1)), were used in the computer field, mainly to save the memory5. Between 1975 and 1980, Brian Evans Conway developed electrode materials with high capacitance based on RuO26, the charge was here stored by reversible faradic process7, opening a new branch of supercapacitors called Pseudo-capacitors. In the late 1970s, many companies started to commercialize supercapacitor devices. In 1978, Panasonic developed its ‘‘Gold Capacitors’’ using an organic electrolyte for memory backup. ELNA produced its ‘‘Dynacaps’’ in 1987 for low power applications8.
Since, many studies have been focused on the development of high power supercapacitors using organic electrolytes. In 1982, PRI (Pinnacle Research Institute) developed a first high power double layer capacitors called ‘‘PRI Ultracapacitor’’ for military applications. This work was used by the Maxwell laboratories in 1992 for new application in hybrid vehicles8. It was only the beginning of a process, the supercapacitor technology began to attract attention for practical applications in the field of hybrid electric vehicles9. Supercapacitors have been used to strengthen the fuel cell in order to provide the power needed for acceleration, and to recover the braking energy10. These supercapacitors were developed by Maxwell as part of a program called the ‘‘DOE Ultracapacitor Development Program’’ funded by DOE (US Department Of Energy). In 1994, David A. Evans developed a new kind of supercapacitors called « Electrolytic-Hybrid Electrochemical Capacitor », which combine electrostatic and electrochemical interactions11. More recently, FDK Corporation announced its EneCapTen lithium-ion capacitor in 2007.
Today, many companies commercialize supercapacitors such as Maxwell Technologies, Panasonic, NessCap, LS Mtron, and Bolloré group. At the same time, studies are focused on the new supercapacitors materials with exhibits higher performances. Moreover, the disadvantages of supercapacitors, in particular the low energy density and the high cost of production, were identified as major challenges for enhance supercapacitor technology.

Supercapacitor fundamentals

Supercapacitor is an energy storage device similar to batteries in design and manufacturing but with a way of working a little bit different. Supercapacitors consist of two electrodes in contact with an electrolyte and a separator which allowed electrical isolation between the electrodes. One of the most important components in a supercapacitor is the electrode material. Generally, the electrode materials are made from a porous nanomaterial with a high specific surface area. During the charging process, the charges are stored at the electrode/electrolyte interfaces due to mainly an electroadsorption process. During the discharge process, the stored charges cause a movement of the electrons in the external circuit providing electrical energy (Figure I.1). According to charge storage mechanism, two types of supercapacitors can be distinguished: (i) electrical double layer capacitors (ELDC), in which the charges are stored by electrostatic adsorption of ions on the electrode/electrolyte interfaces, (ii) pseudocapacitors, in which the charges are additionally stored by reversible and fast redox reactions in or on the surface of the electrodes.
In EDLC, the usually electrode material based on carbon, is not electrochemically active. In other words, there is no redox reaction on the electrode material during charge and discharge processes and a pure physical charge build-up occurs at the electrode/electrolyte interface11–13. While, in pseudocapacitors, the electrode material is additionally electrochemically active, based on transition metal oxide or conducting polymer, which can directly store charges during the charge and discharge process14–16.
EDLCs are physical energy sElectric double layer capacitors (EDLCs)torage devices in which the charge is stored electrostatically due to reversible adsorption of ions onto a charged electrode surface. The operating principle is based on a double layer capacitor, which is formed through charge separation at the electrode-electrolyte interface17. The charge storage occurs directly across the double layer of the interface without any charge transfer. Therefore, the EDLCs have very long-term stability, because there is no electrochemical reaction onto the material electrodes and no composition changes during the charge/discharge processes. When applying a potential difference across the electrodes, a charged layer created at the electrodes surface leads to the creation of oppositely charged layer in the electrolyte formed by accumulation of ions near the electrode/electrolyte interface to keep electro-neutrality. The thickness of the created double layer is of the order of 5-10Å in concentrated electrolytes, which is dependent on the electrolyte kind and concentration18. During discharge process, the charge accumulated at the interfaces causes a parallel movement of the electrons in the external circuit, thus, generating electric energy.
Hermann Von Helmholtz reported a model to explain the mechanism of charge storage in EDL, in which, two layers of opposite charges are simultaneously formed maintaining separation equal to their atomic distance at the electrode/electrolyte interface19,20. The Helmholtz model is shown in Figure I.2 (a). The capacitance can be measured according to the general capacitance Equation I.1: 𝑪=𝑨×Є𝟎𝒅 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰.𝟏) where C denotes the capacitance measured in farads, A denotes the surface area, Є0 denotes the permittivity of free space and d denotes the effectual width of the electric double layer also termed as Debye length.
Figure I.2. Proposed mechanisms for charge storage in electric double layer capacitors: (a) the Helmholtz model, (b) the Gouy-Chapman model and (c) the Stern model, showing the inner Helmholtz plane (IHP) and outer Helmholtz plane (OHP)21.
However, the diffusion of ions has not been taken into account in the Helmholtz EDL model. Consequently, Gouy and Chapman have been modified the simple Helmholtz EDL model considering the existence of diffuse a layer (Figure I.2 (b)), in which the charge is distributed to be continuous along the layer in the fluid bulk22,23. The limitation of Gouy-Chapman model is a higher estimation of capacitance in EDL due to inverse correlation of capacitance with the separation distance. In 1924, Stern and Grahame merged the Helmholtz model and the Gouy-Chapman model. The excess of ions accumulated on the surface of the electrode is distributed between (i) a compact layer close to the surface (called Helmholtz) and (ii) a diffuse layer (called Gouy-Chapman). The first ions of the diffuse layer are not directly on the surface but at a distance of about 0.5 – 2 nm (depending on the nature of the electrolyte; aqueous or organic)12,24.
In 1948, Grahame improved the Stern model by dividing the compact layer into an inner Helmholtz plane (IHP) and an outer Helmholtz plane (OHP)). IHP is formed by solvent molecules or specifically adsorbed ions (non-solvated very close to the interface). OHP is composed of solvated ions closest to the electrode interface (Figure I.2 (c)).
The total capacitance of the electrode can be represented by combination of the capacitance CH due to the Helmholtz layer and the component Cd due to the diffusion layer: 𝟏𝑪=𝟏𝑪𝑯+𝟏𝑪𝒅 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰.𝟐)
The EDL models allow the physic-chemical phenomena that occur at the electrode/electrolyte interface to be explained.

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Pseudocapacitors

Pseudocapacitor is another type of supercapacitors that stores charges by additionally fast and reversible oxidation-reduction reactions called faradic reactions which take place on the surface of the electrode material. This type of storage is also called faradic storage and is complementary to the capacitive storage. It was identified by Conway6.
Pseudo-supercapacitors can store much higher energy but a lower power than EDLCs. On the other hand, their cycle life is generally shorter because the electrode materials degrade more quickly due to the redox reactions involved during cycling. The most common used electrode materials are (i) conducting polymers (CPs) such as poly(3,4-ethylenedioxythiophene) (PEDOT)25–27, polypyrrole (PPy)28,29, polyaniline (PANI)25,28 and polythiophene (PTh)30,31, and (ii) transition metal oxide such as RuO232,33, ZnO16,34,35, NiO34,36,37, and MnO232,35.
In the case of CPs, the charge/discharge process is related to ion doping and dedoping (intercalation/deintercalation), generally accompanied with free solvent molecules transferred and volumetric swelling-shrinking of the film28,38. These processes are likely to lead to a wide variety of unpredictable mechanical defects such as the fatigue of CP electrode, which decreases quickly the capacitance of the CP electrodes39. Furthermore, a short potential window is required for CP-based supercapacitors, when the potential window is large, the CP can be degraded at more positive potential40, or become insulating at too negative potential. Therefore, the potential window used plays an essential role in the performance of CP-based SCs.
The capacitance, C, is directly related to the amount of charges which can be exchanged through the electrode material and the voltage applied across the device, which can be calculated using the Equation I.3. 𝑪=𝒅𝒒𝒅𝑽 (𝑬𝒒𝒖𝒂𝒕𝒊𝒐𝒏 𝑰.𝟑) where dq is the exchanged charge and dV is the voltage change.
The capacitance of the CP material depends on the ion adsorption, the surface and structure of the electrode material, and the pore size distribution.

Hybrid supercapacitors

In order to enhance the energy density of EDLCs and pseudocapacitors, another type of supercapacitors was proposed called ‘‘hybrid supercapacitors’’41. The concept of hybrid supercapacitors is based on the combination between different EDLC and pseudocapacitor materials42–44. The limiting property of EDLC is not present in pseudocapacitor and vice versa, therefore, this combination led to obtain combined performances between EDLCs and pseudo-capacitors.
The assembly of two similar electrodes made from the same EDLC and pseudocapacitive components corresponds to a symmetric hybrid supercapacitor45–47. Whereas, the assembly of two different electrodes made from different materials forms an asymmetric hybrid supercapacitor48–50. All the commercially available hybrid supercapacitors are asymmetric and those with conducting polymer electrodes are of prime interest51. Electrical double layer electrodes can be combined with battery electrodes to obtain intermediate capacitance and stability performances between supercapacitors and batteries52,53. Amatucci et al.54 suggested a basic asymmetric hybrid supercapacitor consisting of the positive electrode of activated carbon (EDLC) and negative electrode of Li4Ti5O12 (Faradaic electrode) in an organic electrolyte in which, the EDLC material supplies high power density while the pseudocapacitive material imparts the high energy density. The combination between carbon electrode (of supercapacitor type) and a faradaic electrode (of battery type) is shown in Figure I.3.

Table of contents :

General Introduction
Chapter I: State of the art
I. Energy storage: Supercapacitors
1. History: discovery and evolution of supercapacitors
2. Supercapacitor fundamentals
2.1. Electric double layer capacitors (EDLCs)
2.2. Pseudocapacitors
2.3. Hybrid supercapacitors
2.4. Capacitance, energy density and power density
3. Applications and challenges of Supercapacitor
3.1. Applications of supercapacitors
3.2. Challenges of supercapacitors
4. Electrode materials
4.1. Carbon materials
4.2. Conducting polymer
4.3. Transition metal oxides (TMOs)
4.4. Composite Materials
5. Conclusions
II. Energy conversion: Fuel cell
1. History and generality
1.1. History
2. Types of fuel cells.
3. Direct methanol fuel cells (DMFCs)
3.1. DMFC components.
3.2. Electrochemical reactions in DMFC
3.3. Advantages and disadvantages of DMFC
4. Anode catalyst
4.1. Pt based catalyst
4.2. Non-Pt based catalyst
4.3. Catalyst support materials
5. Challenges and Applications of Direct Methanol Fuel Cells
5.1. Applications of DMFCs
5.2. Challenges of DMFCs
6. Conclusions
III. Objectves of Ph.D. thesis.
Chapter II. Description of classical and advanced methods
I. Structural and morphological characterization techniques
1. Scanning electron microscopy (SEM)
2. Transmission electron microscopy (TEM)
3. Energy dispersive X-ray spectroscopy (EDX)
4. X-ray diffraction (XRD)
5. X-ray photoelectron spectroscopy (XPS)
6. Fourier-transform infrared spectroscopy (FTIR)
7. Raman spectroscopy
II. Electrochemical and electrogravimetric techniques
1. Cyclic voltammetry (CV)
2. Electrochemical impedance spectroscopy (EIS)
3. Chronoamperometry.
4. Galvanostatic Charge-Discharge (GCD)
5. Electrochemical quartz microbalance (EQCM)
6. Ac-electrogravimetry
Chapter III. Correlation between the interfacial ion dynamics and charge storage properties of poly(ortho-phenylenediamine) electrodes exhibiting high cycling stability
I. Introduction and objectives
II. Experimental
1. Electrode Preparation.
2. Electrode Characterization.
III. Results and discussion
1. Electrochemical synthesis and morphological characterization of the PoPD.
2. Electrogravimetric characterization of charge storage behavior.
2.1. Electrochemical Quartz Crystal Microbalance (EQCM).
2.2. Electrogravimetric Impedance Study (Ac-electrogravimetry).
3. Pseudo-capacitive charge storage performances.
3.1. Thin film PoPD electrodes-half cell performance (3-electrode configuration).
3.2. Thicker film PoPD electrodes-full cell performance (2-electrode configuration).
IV. Conclusions
Chapter IV. Electrogravimetric study of capacitive charge storage behavior of carbone nanotubes/poly(ortho-phenylenediamine) nanocomposite: application in supercapacitors
I. Introduction and objectives
II. Experimental
1. Electrode Preparation
2. Morphological Characterization
3. Electrochemical measurements.
III. Results and discussion
1. Synthesis and characterization of SWCNT/PoPD nanocomposite
2. Electrochemical Quartz Crystal Microbalance (EQCM)
3. Electrogravimetric Impedance Spectroscopy (Ac-electrogravimetry)
3.1. Identification of the involved species in the charge storage mechanism of SWCNT/PoPD
3.2. Transfer dynamics of different species involved in the charge storage mechanism of SWCNT/PoPD film
3.3. Correlation between ac-electrogravimetric and EQCM results for SWCNT/PoPD film
4. Supercapacitive charge storage performances
4.1. Electrochemical characterization in 3-electrode configuration
4.2. Investigation of the charge storage behaviour of the optimized SWCNT/PoPD nanocomposites in a two-electrode configuration.
IV. Conclusions.
Chapter V. Synthesis of carbon nanofibers / poly(para-phenylenediamine) / nickel particles nanocomposite for enhanced methanol electrooxidation
I. Introduction and objectives
II. Experimental
1. Electrode Preparation.
2. Morphological and structural characterization
3. Electrochemical techniques
III. Results and discussion
1. Preparation of CPE/CNF/PpPD/NiPs
2. Morphological and structural characterization
2.1. SEM and EDX analysis
2.2. XRD analysis
2.3. FTIR analysis of CPE/CNF/PpPD/NiPs.
2.4. Raman analysis.
3. Electrochemical characterization
4. Methanol electrooxidation on the catalysts
4.1. Effect of the PpPD thickness
4.2. Effect of NiPs content
4.3. Effect of methanol concentration
4.4. Chronoamperometric measurements
IV. Conclusion
Chapter VI. CNF/PpPD/Cu ternary composite for methanol electrooxidation
I. Introduction and objectives
II. Experimental
1. Electrode preparation
2. Morphological and structural characterization
3. Electrochemical measurements
III. Results and discussion
1. Preparation of CNF/PpPD/Cu on CPE substrate
2. Morphological and structural characterizations
2.1. FEG-SEM, TEM and EDX analysis
2.2. XRD and XPS analysis
2.3. EIS measurements
3. Catalytic performances of the prepared electrodes
3.1. Electrooxidation of methanol
3.2. Effect of Cu content and methanol concentration.
3.3. Chronoamperometric measurements
IV. Conclusions
General conclusions and perspectives
Résumé de la thèse en français

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