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The Scope and Objectives of the Ph.D. Thesis
Ionic transfer and transport phenomena largely influence the performance of the electrode materials for energy storage applications. Some studies suggest the use of microbalances as a means to characterize ion fluxes and transport phenomena for electrodes based on carbon. It has been demonstrated that the response of the microbalance allows an in situ estimation of the ionic flux, which strongly depends on the point of zero charge (PZC), of the pore size and of the size of the ions. Unfortunately, subtleties or more fair explanations cannot be reached through this classical technique.
In this Ph. D. thesis, coupled electrochemical and gravimetric methods, resolved at temporal level, specifically the electrochemical impedance spectroscopy (EIS) coupled with fast quartz microbalance (QCM) (ac-electrogravimetry), have been used to study the capacitive behavior of carbon based electrodes.
The use of ac-electrogravimetry provides very relevant information: kinetics and identification of the species transferred to the electrode/electrolyte interface, separation of the contribution of different species and change of the relative concentration of species within the material. More precisely, all the species involved in the charge storing process can be identified by their atomic mass via a reversible adsorption of ions in the thin carbon films. Consequently, acelectrogravimetry appears to be an attractive tool for studying the nature of the ionic flux at the electrode/electrolyte interface.
In this study, carbon nanotubes (CNTs), graphene and CNTs/polypyrrole, CNTs/Prussian blue nanocomposites have been selected due to their unique set of characteristics suitable for electrode materials in electrochemical devices (EDLCs as well as pseudocapacitors and hybrid capacitors). To achieve these objectives, new models, taking into account of non-redox processes and the combination of redox and non-redox processes, have been developed.
Preparation of Carbon Nanotube (CNT) Based Thin Film Electrodes (Single Wall CNT, Double Wall CNT, Multi Wall CNT)
To achieve nanostructured electrodes, three types of carbon nanotubes: Single Wall CNT (755117-1G, length: 300-2300 nm and diameter: 0.7-1.1 nm), Double Wall CNT (755141-1G, length: 3 μm and diameter: 3.5 nm) and Multi Wall CNT (75517-1G, length: 1 μm and diameter: 9.5), acquired in Sigma Aldrich Company were chosen due to their tailorable pore dimensions and variety of specific surface areas. The preparation of CNT films was carried out according to the method described in the literature.2,140 CNTs were deposited by the « drop-casting » method on a gold electrode (effective surface area of 0.20 cm2) of a quartz crystal resonator (9 MHz -AWS, Valencia, Spain), from a solution of carbon containing 90% CNT powder and 10% PVDF-HFP (Poly(vinylidene fluoride-hexafluoropropylene)) polymer binder in N-methyl-2-pyrrolidone.
Around 8 μL of this solution was deposited on the gold electrode of the QCM. Then, the carbon films were subjected to a heat treatment at 120 °C for 30 minutes, with a heating rate of ~ 5 °C min- 1 to evaporate the residual solvent and improve the adhesion properties of the films on gold electrode (Figure II-1). The film thickness was estimated to be around 500 nm (based on FEG-SEM
measurements).
SWCNT/Prussian Blue Thin Film Electrodes
The preparation of SWCNT/PB nanocomposite films was carried out according to the method described in the literature126. The procedure consists of cycling the SWCNT film in a solution containing 1 x 10-3 M K3Fe(CN)6 and 0.1 M KCl, without the addition of Fe3+ ions, at a scan rate of 0.05 V s-1 in a potential range from -0.2 V to 0.8 V vs. Ag/AgCl during 15 cycles. Then, electrochemical studies were realized in an aqueous solution of 0.5 M KCl, at pH 3.
SWCNT/Polypyrrole Thin Film Electrodes
The elaboration of the SWCNT/Polypyrrole nanocomposite films was established following a method described in the literature4. The procedure consists of cycling the SWCNT film, previously prepared in a solution containing 0.1 M pyrrole (previously distilled) and 0.05 M sodium dodecylsulphate (NaDS), at a scan rate of 0.01 V s-1 in a potential range from 0 V to 0.675 V vs. Ag/AgCl during 2 cycles. The chemicals NaDS (436143-25G) and pyrrole were acquired from Sigma Aldrich. EQCM and ac-electogravimetric measurements were realized in a 0.5 M aqueous solution of CsCl.
Elaboration of Electrochemically Reduced Graphene Oxide (ERGO) Thin Film Electrodes
First, a GO thin film was prepared by depositing a GO suspension (GO powder in doubledistilled water) on a gold-patterned quartz resonator as shown in Figure II-1. A 1 mg/ml GO suspension was prepared from GO powder in double-distilled water. This suspension was then placed in an ultrasonic bath for 3 hours. Then, 20 μL of GO suspension was deposited on the gold electrode of the quartz resonator. To do this in a controlled manner, a laboratory-made mask (proving the exposure of the gold electrode only) was used to prevent the solution to spread out on to the whole surface of the quartz resonators. A GO film was formed after drying in an oven at 70 °C for 1h. The film thickness was estimated to be around 50 nm based on FEG-SEM measurements.
After the GO was successfully deposited on the quartz, the sample was electrochemically reduced to obtain ERGO. The electrochemical reduction was carried out by chronoamperometry at – 1.1 V vs. Ag/AgCl during 5 minutes with a Biologic SP-200 potentiostat under the Ec-Lab software.
Structural and Morphological Investigation Methods
This section is dedicated to the various structural and morphological methods for characterization of carbon based materials. Particularly, the principle of operation of the methods, the type of instruments and the preparation of the samples for the experiments will be explained.
Scanning Electron Microscopy (SEM)
Figure II-2 shows a schematic description of a SEM set-up. The electron gun generates a primary electron beam in a vacuum. The beam is collimated by electromagnetic condenser lenses, focused by an objective lens, and scanned across the surface of the sample by electromagnetic deflection coils.149 As a consequence of the sample/primary beam interaction, secondary electrons are emitted from the sample.122 The secondary electrons are selectively attracted towards the secondary electron detector through a grid held at a low positive potential with respect to the sample.149 There, the secondary electrons are counted (number of electrons arrived/unit time) and
the count rate is translated into an electrical signal,149 which can be used for a visualization of the sample.122 The resulting image reflects the surface topology, since the number of electrons that are collected from each point is dependent on surface topology. In shallow surfaces, fewer electrons escape the surface and reach the detector resulting in darker areas, while at the edges and at convex surfaces, more electrons escape and reach the detector resulting in brighter areas. However, charge is built up on the sample surface by the electron bombardment of the primary beam and, in the case of non-conductive samples, this charge cannot be dissipated, which results in images that are bright throughout the sample. To avoid this, samples can be sputtered with a very thin layer of metals such as gold or gold/palladium as a part of the sample preparation.
Transmission Electron Microscopy (TEM)
Figure II-3 shows a schematic presentation of a TEM set-up. The electron gun generates a primary electron beam either by thermoionic emission (using a heated filament as the cathode, e.g. LaB6) or by field emission (using a fine tungsten tip as the cathode). The primary electron beam is focused by lenses into a thin, coherent beam, which hits the sample, resulting in various kinds of interaction between the electrons and the sample, i.e. absorption, diffraction, and elastic or inelastic scattering. The scattering processes are a result of the interaction between electrons and the nuclei
of the sample. Therefore, heavy elements lead to higher scattering, which means that fewer electrons reach the image plate. Since the brightness of the image depends on the number of electrons reaching the image plate, sample regions containing heavy elements, i.e. fewer electrons, appear dark. Conversely, areas that contain only light elements, e.g. organic compounds, appear bright due to a limited scattering of electrons.
After hitting the sample, the transmitted portion of the beam is focused by an objective lens. The beam can be restricted by objective and select area apertures; the former enhances contrasts by blocking out high-angle diffracted electrons, the latter enables the examination of the periodic diffraction of electrons by ordered arrangements of the atoms in the sample. Thereafter, the electron beam passes magnetic lenses for magnification. Finally, the beam strikes a fluorescent screen coated with CdS and ZnS and the resulting irradiation is recorded by a CDD camera.149 TEM can be operated in two modes: classic mode and high resolution mode (HR-TEM). In comparison to the classic mode, HR-TEM uses a much larger objective aperture (if any at all) to transfer high spatial frequencies. Then, the directly transmitted beam may interfere with one or more diffracted beams causing a contrast across the image (i.e. the bright-field image), which depends on the relative phases of the various beams.
Table of contents :
Introduction
CHAPTER I. Bibliography
I-1. Energy Storage: Batteries and Supercapacitors
I-1.1. Batteries
I-1.1.1. History
I-1.1.2. Types of Batteries
I-1.1.3. Principle of Operation
I-1.1.4. Battery Performance
I-1.1.5. Battery Applications
I-1.2. Supercapacitors
I-1.2.1. History
I-1.2.2. Principle of Operation for EDLC
I-1.2.3. Components of the Supercapacitors
I-1.2.4. Proposed Strategies Towards Higher Performance in Supercapacitors
I-1.2.5. Supercapacitor Applications
I-2. Diagnostic Tools for Electrodes in Energy Storage
I-2.1. Electrochemical Methods
I-2.1.1. Cyclic Voltammetry (CV)
I-2.1.2. Galvanostatic Charge-Discharge method (GCD)
I-2.1.3 Electrochemical Impedance Spectroscopy (EIS)
I-2.2. Structural and Morphological Analysis: XRD, SEM, TEM, EDX and BET
I-2.2.1. X-Ray Diffraction (XRD)
I-2.2.2. Scanning Electron Microscopy (SEM) and FEG-SEM
I-2.2.3. Transmission Electron Microscopy (TEM or HRTEM)
I-2.2.4 Energy-Dispersive X-ray Spectroscopy (EDX)
I-2.2.5. Brunauer Emmett and Teller (BET)
I-2.3. Classical Electrogravimetric Investigations: Quartz Crystal Microbalance Based Methods.
I-3.The Scope and Objectives of the Ph.D. Thesis
CHAPTER II. Experimental Part
II-1. Preparation Procedure of Carbon Based Thin Film Electrodes
II-1.1. Preparation of Carbon Nanotube (CNT) Based Thin Film Electrodes (Single Wall CNT, Double Wall CNT, Multi Wall CNT)
II-1.2. Elaboration of Nanocomposite Structures
II-1.2.1. SWCNT/Prussian Blue Thin Film Electrodes
II-1.2.2. SWCNT/Polypyrrole Thin Film Electrodes
II-1.3. Elaboration of Electrochemically Reduced Graphene Oxide (ERGO) Thin Film Electrodes
II-2. Structural and Morphological Investigation Methods
II-2.1. Scanning Electron Microscopy (SEM)
II-2.2. Transmission Electron Microscopy (TEM)
II-2.3. Energy Dispersive X-rays (EDX)
II-2.4. X-ray Diffraction
II-2.5. Nitrogen Physisorption and BET Surface Area Determination
II-3. Electrochemical and (Electro)gravimetric Techniques
II-3.1. Quartz Crystal Microbalance (QCM)
II-3.1.1. Piezoelectricity
II-3.1.2. Working Principle of QCM
II-3.1.3. Experimental Set-Up
II-3.2. Cyclic Electrogravimetry(EQCM)
II-3.2.1. Principle
II-3.2.2. Experimental Set-Up
II-3.2.3. Calculation of the F(dm/dq) Function
II-3.3. Electrochemical Impedance Spectroscopy (EIS)
II-3.3.1. Principle
II-3.3.2. Experimental Set-Up
II-3.4. Ac-Electrogravimetry – A Fast Electrogravimetric Method
II-3.4.1. Principle
II-3.4.2. Experimental Method: ΔVf/ΔV
II-3.4.3. Calibration and Corrections of the Ac-Electrogravimetry System
II-3.5. Data Treatment of Ac-Electrogravimetry
II-3.5.1. Experimental data
II-3.5.2. Fitting from Mathcad Simulation Data
CHAPTER III. Ion dynamics in SWCNT Based Thin Film Electrodes
III-1. Structure and Morphology of the SWCNT Powders and Prepared Thin Film Electrodes
III-2. Classical Electrogravimetric Study of SWCNT Thin Film Electrodes in Aqueous NaCl Electrolyte
III-2.1. EQCM measurements on SWCNT Thin Film Electrodes
III-2.2. Fdm/dq Function Calculations
III-2.3. Specific Capacitance Calculations
III-3. Ac-Electrogravimetric Studies of SWCNT Thin Film Electrodes in Aqueous NaCl Electrolyte
III-3.1. EQCM versus Ac-Electrogravimetry
III-4. Ac-Electrogravimetric Study of SWCNT Thin Films in Organic Electrolytes
III-5. Conclusions
Table of Contents
CHAPTER IV. Influence of the CNT Type, Structure and Electrolyte Properties on Ion Dynamics.
IV-1. Structure and Morphology of the DWCNT and MWCNT Powder and Thin Film Electrode
IV-2. Influence of the CNT Types
IV-2.1. EQCM Study of SWCNT, DWCNT and MWCNT in Aqueous NaCl Electrolyte
IV-2.2. Ac-electrogravimetric Study of various CNT Thin Film Electrodes in Aqueous NaCl Electrolyte
IV-3. Influence of the Electrolyte Properties
IV-3.1. EQCM Study of SWCNTs in Aqueous NaCl Electrolyte at different pH
IV-3.2. Ac-electrogravimetric Study of SWCNT Thin Film Electrode in Aqueous NaCl Electrolyte at
different pH values.
IV-3.3. EQCM Study of SWCNT in Different Aqueous Electrolyte: effect of the cation size
IV-3.4. Ac-electrogravimetric Study of various SWCNT Thin Film Electrodes in Different Aqueous
Electrolytes: effect of the cation size
IV-4. Conclusions
CHAPTER V. Composite Thin Film Electrodes and Beyond Carbon Nanotubes
V-1. Composite Thin Film Electrodes
V-1.1. SWCNT/Prussian Blue Thin Film Electrodes
V-1.1.1. Structure and Morphology of the SWCNT/PB Composites
V-1.1.2. EQCM Study of the SWCNT/PB Composites
V-1.1.3. Ac-electrogravimetry Study of the SWCNT/PB Composites
V-1.2. SWCNT/Polypyrole Thin Film Electrodes
V-1.2.1. Structure and Morphology of the SWCNT/PPy Composites
V-1.2.2. EQCM Study of the SWCNT/PPy Composites
V-1.2.3. Ac-electrogravimetry Study of the SWCNT/PPy Composites
V-2. Beyond Carbon Nanotube Based Electrodes
V-2.1. Electrochemically Reduced Graphene Oxide (ERGO) Thin Film Electrodes
V-2.1.1. Structure and Morphology of the ERGO Thin Film Electrodes
V-2.1.2. EQCM Study of the ERGO Thin Film Electrodes
V-2.1.3. Ac-electrogravimetry Study of ERGO Thin Film Electrodes
V-3. Conclusions
General Conclusions
Future work
References
