Electrochemical and viscoelastic evolution of dodecyl sulfate doped polypyrrole films during electrochemical cycling

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Polyaniline (PANI)

PANI, which can be synthesized with aniline monomer via chemical or electrochemical methods, is another promising electrode material for SC due to its relatively facile synthesis, environmental stability and promising electrochemical behaviors132. However, it should be noted that proton type electrolytes are required for PANI to be properly charged and discharged, therefore, a protic solvent, an acidic solution or a protic ionic liquid is required47, 133.
The electrochemical performance of PANI is intimately related with its morphology, which relies on the synthesis strategies. A better electrochemical behavior can be expected through tuning the PANI morphology during the synthesis process. In general, chemical synthesis can provide a better flexibility for controlling the nucleation and growth during the polymerization, while electrochemical synthesis can generate the PANI directly onto different substrates132. Therefore, chemical synthesis is preferably adopted for preparing a PANI-based nanocomposite and electrochemical counterpart presents its unique advantage for processing binder-free capacitive electrode.
PANI is often composited with other active nanomaterials (especially CNTs) for the practical development of PANI-based SCs. For example, the addition of sulfonated CNTs in PANI (76.4 wt%) exhibited a superior cycling stability, with only 5.4% loss from their initial specific capacitance (515.2 F g-1) after 1000 cycles. it was attributed to the exceptional mechanical support of CNT and the formation of the charge-transfer complex between the PANI and CNTs134. It has also been reported that the combination of a vertically aligned CNT array framework and PANI can provide a hierarchical porous structure, large surface area, and superior conductivity for the composite electrode. This tube-covering-tube nanostructured PANI/CNT composite electrode exhibit a specific capacitance of 1030 F g-1 and a high stability (5.5% capacity loss after 5000 cycles)47.

Polythiophene (PTh)

PTh and its derivatives are another promising electrode materials for SCs and have raised enormous attractions owing to their flexibility, facile synthesis, favorable cyclability and environmental stability128, 135. Unlike PPy and PANI, PTh and its derivatives are both n- and p-dopable. Table I.5 presents an example of the differences between n- and p-dopable PTh derivatives. It is found that the gravimetric specific capacitance in the n-doped form generally presents a lower value than that in the p-doped one. Additionally, the n-doped PTh-based CP exhibits inferior conductivity, which limits their use in the n-doped from as an anode material. Consequently, they are often employed as the positive electrode (p-doped) in SC with a negative electrode made from another materials such as carbon35.
It should be mentioned that poly(3,4-ethylenedioxythiophene) (PEDOT), one of the PTh derivatives has recently gained a lot of research attention. It is highly conductive (300-500 Scm-1)126 and has higher potential window (see Table I.4). Due to its high surface area coupled with high conductivity, this polymer exhibits high charge mobility, resulting in fast electrochemical kinetics35. Additionally, it possesses superior thermal and chemical stability as well as good film-forming properties. However, its large molecular weight leads to a relatively low specific capacitance compared with other CPs (see Table I.4).

Transition metal oxides

Transition metal oxides have been intensively studied as electrode materials due to their fast and reversible redox reactions occurring at the electrode surface14, which can offer additional pseudocapacitances during electrochemical performance. Generally, metal oxides can offer higher energy density than conventional carbon materials and better electrochemical stability than CPs3. The metal oxides for SC electrode materials are expected to be sufficiently conductive and to present superior phase stability with intercalation/deintercalation of ions during redox reactions. To date, ruthenium oxide (RuO2), manganese dioxide (MnO2) and zinc oxide (ZnO) are amongst the most commonly studied metal oxides, which will be discussed in detail in this section.

Ruthenium oxide (RuO2)

RuO2 is widely studied because of its excellent electronic conductivity and multiple oxidation states accessible within 1.2 V. Its pseudocapacitive behavior in acidic solutions has been widely studied over the past years and can be described as a fast and reversible electron transfer coupled with an electro-adsorption of protons on the surface of RuO2 particles, which can be expressed by the following equation3, 14:
parallelly, the redox pseudocapacitance mechanism can occur3. Since the pseudocapacitance of RuO2 originates from the surface redox reactions, where the SSA plays an important role, thus increasing the surface area of the RuO2 becomes one of the most effective methods to enhance the specific capacitance of RuO2-based electrodes. Figure I.12 offers a strategy to synthesize the high SSA SC electrode materials to increase energy and power densities14. The nano-sized RuO2 pseudocapacitive active materials can be deposited onto the high SSA carbon supports, such as carbon grains and CNTs.
Figure I.12. Possible strategies to improve both energy and power densities for SCs14.
RuO2 electrodes, with faradaic nature in charge storage, present an almost rectangular shape as that of EDL in cyclic voltammogram137-138. However, it is not a consequence of pure EDL charging, but of a consequence of fast, reversible successive surface redox reactions74. Although very high specific capacitance exceeding 700 F g-1 was reported in hydrated RuO2 electrodes26, the active material itself turned out to be too expensive and environmentally unfriendly, which greatly limits its widespread application. Therefore, other transition metal oxides such as MnO2 and ZnO have been proposed as alternatives for SC electrode materials.

Manganese dioxide (MnO2)

MnO2 has gained significant attentions as an alternative SC electrode material due to its low cost, low toxicity, abundant resource, environmental friendliness, as well as high specific capacitance139-140. The capacitance of MnO2 mainly comes from pseudocapacitance. Two charge storage mechanisms have been proposed in MnO2 electrodes. The first one implies the intercalation of protons (H+) or alkali metal cations (C+) such as Na+ in the electrode during reduction and their deintercalation during oxidation141-144:
It should be noted that, like RuO2, MnO2 can also exhibit a rectangular cyclic voltammetry shape (typically for EDL), though it possesses a redox nature for charge storage14, 138, 145.
While MnO2 is a promising material for pseudocapacitor applications, they suffer from low electrical and ionic conductivities, which are significantly related to its crystallinity and crystal structure. MnO2 can crystallize into several crystallographic structures, as shown in Figure I.1357. High crystallinity brings about high conductivity whereas loss of surface area available for electrolyte. Contrarily, although low crystallinity leads to a highly porous microstructure of MnO2, the resulting electrical conductivity is low3. Therefore, a trade-off between electrical conductivity of MnO2 and its porous structure for ionic transportation would be reached for specific purposes depending on applications.
Relatively high conductivity of MnO2 can be obtained by increasing the content of crystallinity and tuning the crystal structure, but its conductivity is still poor (ranging from 10-7 to 10-3 S cm-1 based on different crystal structures) compared with bulk single crystal of RuO2 (104 S cm-1)26. Consequently, charge storage of MnO2-based electrodes confines in a very thin layer of the MnO2 surface, translating into significantly lower specific capacitance values for thick MnO2-based electrodes. Nanostructuring is believed to be a highly effective method for accessing all of the MnO2 storage sites26, which might be a strategy to develop the high-loading MnO2 electrodes for SCs.
ZnO is another promising electrode material for SCs due to its low cost, natural abundance, superior electrochemical performance and environmental friendliness9, 78, 146. ZnO nanomaterials can be solely deposited on the substrate38 or composited with other metal oxides147, conducting polymers148 or carbon-based materials149 to serve as energy storage electrodes. However, its poor electrical conductivity remains a major challenge and limits rate capability for high power performance, thus hindering its wide application in energy storage150.
Therefore, one dimensional (1D) ZnO nanostructures have been widely studied because they can provide short diffusion path for ions and efficient mechanical support for other electroactive materials. A high specific capacitance up to 405 F g-1 at 10 mVs-1 was achieved by coating a layer of MnO2 shell onto the 1D single-crystal ZnO nanorod147. Besides, The hybridization of carbon materials with ZnO offers the benefits of both the EDL capacitance of the carbon materials with large SSA and faradaic capacitance of the ZnO, thereby optimizing the electrochemical performance of the ZnO-based SCs. For example, ZnO/reduced graphene oxide/ZnO sandwich-structured composite presented a specific capacitance of 275 F g-1 at scan rate of 5 mVs-1 in 1.0 M Na2SO4 as well as high cycling stability150. Similarly, a high specific capacitance of 314 F g-1 can be delivered by decorating reduced graphene oxide with ZnO nanoparticles without obvious capacitance decay after 1000 cycles151. The electrode composed of 1D ZnO nanostructures sheltered by a thin electrochemically reduced graphene oxide (ERGO) film will be de described in Chapter VI.

valuation tools for SC electrode materials

Cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrochemical impedance spectroscopy (EIS) are the most commonly used techniques to evaluate the electrochemical properties of SCs. Additionally, a coupled electrochemical characterization method, electrical quartz crystal microbalance (EQCM) and a non-conventional method derived from EIS and QCM, the so-called ac-electrogravimetry, has been employed to study the electrochemical performances of SCs. Considering the mechanical properties which are also of importance for an efficient and cyclable SC electrode, a method to study the electrode’s viscoelasticity, namely electroacoustic impedance has also been introduced.

Cyclic Voltammetry (CV)

CV is an effective and basic tool to identify the capacitive behavior of SC electrode materials. The experimental procedure involves potential cycling within a voltage window preselected for a given electrolyte1. Specifically, CV test applies a linear change of potential between positive and negative electrodes for two-electrode configurations, and between reference and working electrodes for three-electrode systems152. The range of potential change is designated as potential window, and the speed of the potential change is called scan rate. Generally, a plot of the current vs. potential is the output and used to evaluate the electrochemical processes of the electrode152.
In a CV experiment, the current response to an applied scan rate will vary depending on whether the electrochemical reaction is diffusion-controlled or surface-controlled (capacitive)26, 153-155. The current response stemmed from capacitive process is proportional to the scan rate, while the current limited by semi-infinite diffusion of electrolyte ions varies with the square root of the scan rate. Therefore, the instantaneous current at a certain potential can be expressed as153:
where k1v is the surface-controlled current and k2v1/2 is the diffusion-controlled current. The coefficients k1 and k2 can be obtained through the linear fitting of voltammetric currents at each potential. It leads to the calculation of k1v and k2v1/2, which, therefore, allows for the separation of
capacitive and diffusion currents A CV response originated from the EDL generally exhibits a rectangular shape14, and the charging and discharging voltammograms are almost mirror images of one another. Contrarily, the pseudocapacitors usually present some redox peaks, leading to a deviation from the rectangular shape in CV curves125, 158-159. It should be noted that the fast, reversible successive surface redox reactions may also present a similar shape as that of EDL in CV14, 137-138, as shown in Figure I.1414. The specific capacitance can be calculated by integrating the voltammetric charges from a CV curve based on Equation I.830, 160:
where m is the mass loading of SC electrodes, v is the scan rate, E1 and E2 are the low and high end potentials, and I(E) depicts the response in current.
Figure I.14. (a) Schematic of CV for a MnO2 electrode cell in mild aqueous electrolyte (0.1 M K2SO4) shows the successive multiple surface redox reactions leading to the pseudocapacitive charge storage mechanism14. (b) CV of Fe2O3/nitrogen-doped graphene composite tested in 2 M KOH solution at 10 mV s−1, and the shadowed areas represent the capacitive contribution155.

Galvanostatic Charge-Discharge (GCD)

GCD is another widely used technique to characterize the electrochemical performance of SCs under a constant current density161. A consecutive charging/discharging of the working electrode is performed at a constant current density with or without a dwelling period (a time period between charging and discharging while the peak voltage V0 remains constant). A linear response of potential (in V) with respect to charge/discharge time (in s) is anticipated for EDLC, whereas the non-linearity of charge/discharge curves is normally obtained from the pseudocapacitors, which is distinct from the triangular shape from EDLC response117, as shown in Figure I.15162. Generally a plot of the potential (E) vs. time (t) is the output. Choosing a proper level of the applied constant current is critical to produce consistent and comparable data from a GCD test.
GCD test is regarded as the most versatile and accurate approach in characterizing SC devices. All three core parameters of SC devices, cell (total) capacitance CT, operating voltage V0, and equivalent series resistance Rs, can be obtained through this methodology and subsequently used to derive most of the other properties, such as power and energy densities, and leakage and peak current152. GCD can also be used to test the cycling performance of SCs. Furthermore, the specific capacitance (Cs) can be calculated via GCD using the following equation:
where I is the applied current, Δt is the discharge time, m is the mass loading of SC electrodes and ΔV is the potential drop during discharge64, 113, 162.

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Electrochemical Impedance Spectroscopy (EIS)

EIS is powerful diagnostic tool that not only enables the equivalent series resistance and potential-dependent faradaic resistance of the device to be separately evaluated, but also allows the calculation of capacitance of the electrode1, 68. More generally, the mechanism of different electrochemical reactions can be discover by the EIS approach when an appropriate model is used. It is conducted by applying a sinusoidal potential perturbation with a small amplitude (typically 10-20 mV) over a range of frequencies, which must not cause the system to shift from its equilibrium state1, 68. The impedance (Z) is defined as Z=Z’ jZ » , where Z´and Z » are the real and imaginary part, respectively. The resulting data are usually expressed graphically either in a Bode plot to visualize the dependence of both the absolute value of impedance and phase angle on the frequency, or in a Nyquist plot to show the imaginary and real parts of the electrochemical impedance on a complex plane66, 163.
EIS has been widely used to characterize the electrochemical properties of SC electrodes. As presented in Figure I.16a and b, it is performed to evaluate the effect of carbonaceous coating on the interfacial charge transfer in PANI and PPy nanowires covered by a thin layer of carbonaceous shell of ~5 nm (i.e., PANI@C and PPy@C electrodes)27. The Rs of PANI@C and PPy@C electrodes are higher than the bare polymer electrodes owing to the presence of additional contact resistance between carbonaceous shell and polymer core. Besides, the EIS study of 2D Ti3C2Tx (MXene), a promising material in electrochemical energy storage applications, has also been reported164. Figure I.16c shows a comparable spectra in the low-frequency domains of the Ti3C2Tx electrode measured in 1 M LiCl and MgCl2 solutions at -0.2 V vs. Ag/AgCl. However, the high-frequency domain of the electrode spectra in MgCl2 solution exhibits a depressed semicircle, which is absent in the impedance spectrum measured in LiCl (inset in Figure I.16c). This semicircle is believed to stem from the ion transfer across the electrode/electrolyte interface and the Mg2+ transfer is much slower than the transfer of singly charged Li+. Furthermore, the effect of cycling on the capacitive behavior of the electrode is investigated in MgCl2 solution (Figure I.16d), demonstrating that an aging process (100 cycles at a rate of 1 mV s−1 ) can result in a further retardation of the interfacial Mg2+ ion transfer in the high-frequency domain (inset in Figure I.16d).

Electrical Quartz crystal microbalance (EQCM)

EQCM is based on quartz crystal microbalance (QCM) technology. The QCM comprises a thin piezoelectric quartz crystal sandwiched between two metal electrodes, where an alternating electric field across the crystal is established and causes thickness-shear vibrations of the crystal around its resonant frequency165-167. The signal transduction mechanism of the QCM originates from the piezoelectric property of the quartz crystal, which was first discovered in 1880 by Curie brothers168. They discovered that the application of a mechanical stress to the surfaces of various crystals, including quartz, rochelle salt (NaKC4O6·4H2O) and tourmaline, produced a corresponding electrical potential across the crystal whose magnitude was proportional to the applied stress. The name “piezoelectricity” was proposed by Hankel after one year.
Shortly after their initial discovery, the Curie brothers experimentally verified the reverse piezoelectric effect by which the application of an electric field across the crystal afforded a corresponding mechanical strain. As shown in Figure I.17165, the shear strain is induced through the reorientation of the dipoles in a piezoelectric material by an applied potential. Additionally, the motion of the material is proportional to the applied potential. It should be noted that the reverse piezoelectric effect is the basis of the QCM. The application of an alternating electric field across a quartz crystal produces a vibrational (or oscillatory) motion in the quartz crystal parallel to the surface of the crystal. It leads to the establishment of a transverse acoustic wave that propagates across the crystal thickness (dq). An acoustic standing wave can be formed when the acoustic wave length=2 dq165. The quartz oscillator vibrates with minimal energy dissipated at a characteristic resonant frequency, therefore it is deemed as an nearly ideal oscillator169.
Figure I.17. Schematic representation of the converse piezoelectric effect for shear motion. The electric field induces reorientation of the dipoles of the acentric material, resulting in a lattice strain and shear deformation of the material. Direction of shear is dependent upon the applied potential while the extent of shear strain depends on the magnitude of the applied potential165.
The nodes of the acoustic wave are positioned in the quartz interior (in the center of the quartz crystal at the fundamental frequency, i.e., at the overtone order, n = 1), while the antinodes are located on both surfaces. Correspondingly, when a material is deposited on the surface of the quartz, the acoustic wave will span the quartz/film interface and propagate through the deposited film (Figure I.18a). It is implicitly assumed that a continuous displacement (or shear stress) exists across the quartz/film interface, which is referred to as the “no-slip” condition165. In other words, the deposited film could be regarded as an extension the quartz. The film-deposited quartz crystal performs as a composite resonator165, 170. It, in turn, can be affected by the electrolyte solution in which it is immersed170-171. The thickness increase of this composite resonator due to the deposited film is equivalent to the increase of the wavelength (=2 dq) and results in a decrease of the quartz fundamental frequency169. The composite resonator performs in such a situation that can be modeled by an equivalent electrical circuit, of which the most commonly used is the Butterworth-Van-Dyke (BVD) model (Figure I.18b)170. This will be discussed later in the description of the electroacoustic impedance measurements (Section 1.4.6).
Figure I.18. (a) An oscillating mass-loaded quartz crystal immersed in a liquid medium acting as a composite resonator. (b) The corresponding equivalent electrical circuit as per the Butterworth-Van-Dyke Model170.
where Δf denotes the measured frequency shift, f0 is the frequency of the quartz in air prior to the film deposited, Δm is the corresponding mass change, A is the piezoelectrically active area, µq is the shear modulus andq is the quartz density. When a QCM is used in conjunction with electrochemical measurements, such as CV and GCD, it is frequently referred to as electrochemical quartz crystal microbalance (EQCM).
EQCM has developed into a powerful in situ technique to measure ionic fluxes in different electrochemical systems167, 173-174, such as conducting polymers, carbon materials and metal oxides, Here, not only the current response but also the simultaneous mass variation of the electrode is tracked during an electrochemical process.
For example, a mass increase/decrease upon reduction/oxidation was observed in PPy film by EQCM, which is presumably due to the insertion/expulsion of cations (Figure I.19a)175. Moreover, the mass response, translated from EQCM frequency shift, can separate the ionic fluxes in some basic case for carbon electrode due to adsorption of cations on the negatively charged surface (Q<0) and anions on the positively charged surface (Q>0) (Figure I.19b)176. Furthermore, EQCM has also been used to study the relationship between the pore sizes of the electrode and ion sizes in the electrolyte. The different ion adsorption behaviors of two carbide-derived carbons with average pore sizes of 1 nm (CDC-1 nm) and 0.65 nm (CDC-0.65 nm) in neat and solvated 1-Ethyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide (EMI-TFSI) (2 M EMI-TFSI in acetonitrile) electrolytes have been reported. Figure I.19c167 describes an example for CDC-1 nm electrode, whose pore size is larger than ion size (0.76 and 0.79 nm for the EMI+ cation and TFSI− anion). Both (solvated)cations and anions are able to participate in charge balance mainly depending on the potential of zero charge, i.e., cations at Q<0 and anions at Q>0. However, for CDC-0.65 nm electrode, whose pore size is smaller than ion size, no mass change is detected in neat EMI-TFSI electrolyte, and only solvated cation (EMI+ hydrated with 1.6 acetonitrile (AN) molecule averagely, i.e., EMI+ + 1.6 AN) response is observed in solvated electrolyte (2 M EMI-TFSI/AN) when the CDC-0.65 nm electrode is negatively charged. Additionally, the electrochemical behavior of a transition metal oxide, MnO2, has been characterized in 0.5 M LiClO4 and NaClO4 electrolytes. The current response and the simultaneous mass change are obtained by EQCM measurements (Figure I.19d)177, from which the mass per mole of electrons (MPE) exchanged between the electrode and the electrolyte can be estimated according to MPE = F Δm/Δq.
Figure I.19. EQCM measurements for a polypyrrole film in 0.1 M Na2SO4 (a),175 a carbon electrode in a 0.5 M NH4Cl aqueous solution at a scan rate of 20 mVs-1 (b),176 a carbide-derived carbon with average pore size of 1 nm in neat 1-Ethyl-3-methylimidazolium bis(trifluoromethane-sulfonyl)imide (EMI-TFSI) electrolyte167 (c) and a film of Li-birnessite type MnO2 in 0.5 M LiClO4 and 0.5 M NaClO4 at a scan rate of 25 mVs-1 (d).
As discussed above, classical EQCM response allows for insights into the ionic flux exchanged between the electrode and the electrolyte. But it remains challenging for the deconvolution of the global electrogravimetric response into gravimetric and temporal components since the measurements are limited to scan rates or current densities. To overcome these limitations, the ac-electrogravimetry is suggested as a complementary tool to the EQCM, where the different scenarios of the charge compensation process in different electrodes can be scrutinized.

Ac-electrogravimetry

Ac-electrogravimetry is based on a QCM used in dynamic regime and coupled with electrochemical impedance spectroscopy, which was proposed in 1988 by Gabrielli178. When a uniform thin layer of a foreign material is added to the surface of the quartz crystal resonator, it can be used as the working electrode (WE) following a classical electrochemical configuration179. When a sinusoidal potential perturbation is applied to an electroactive film, it induces concentration variation of the species which results from the species transfer for charge compensation purposes. These concentration or mass variations of species can be tracked thanks to the frequency variation of a specific QCM used under dynamic regime, i.e., under frequency potential modulation of the WE. Through this method, the species transferred, if they intervene with different kinetics in an electrochemical process, can be separated and a clear identification of the species with their molar mass can be achieved175, 177, 180-184.

Table of contents :

Chapter I: Introduction 
I.1.Fundamentals of supercapacitors (SCs) 
I.1.1. Electrical double layer capacitors (EDLCs)
I.1.2. Pseudocapacitors
I.1.3. SC capacitance, energy and power density
I.2.Challenges and applications of SCs 
I.3. Electrode materials 
I.3.1. Carbon materials
I.3.2.Conducting polymers (CPs)
I.3.3.Transition metal oxides
I.4.Evaluation tools for SC electrode materials 
I.4.1. Cyclic Voltammetry (CV)
I.4.2. Galvanostatic Charge Discharge (GCD)
I.4.3. Electrochemical Impedance Spectroscopy (EIS)
I.4.4. Electrical Quartz crystal microbalance (EQCM)
I.4.5. Ac electrogravimetry
I.4.6. Electroacoustic measurements
I.5.Objectives and outline of the thesis
Chapter II:Experimental procedures
II.1. Materials characterization techniques 
II.1.1. Fourier transform infrared (FTIR) spectroscopy
II.1.2. Ultraviolet-visible (UV-vis) spectroscopy
II.1.3. Scanning electron microscopy (SEM)
II.1.4. X-ray diffraction (XRD)
II.1.5. X-ray photoelectron spectroscopy (XPS)
II.2. Electrochemical and electrogravimetric characterization 
II.2.1. Electrochemical quartz crystal microbalance (EQCM)
II.2.2. Ac-electrogravimetry
II.3. Electroacoustic impedance measurement 
Chapter III: Electrochemical and viscoelastic evolution of dodecyl sulfate doped polypyrrole films during electrochemical cycling
III.1. Preamble and Objectives
III.2.Experimental Methods and Theoretical Background
III.2.1. Film preparation and characterization
III.2.2. Electrogravimetric measurements
III.2.3. Electroacoustic impedance measurements
III.3. Results and Discussion
III.3.1. Cyclic electrogravimetric behavior
III.3.2. Ac electrogravimetric investigations
III.3.3. Viscoelastic property changes upon film aging
III.4. Conclusions
Chapter IV: Tuning charge storage properties of reduced graphene oxides evidenced by in situ gra vimetric and viscoelastic explorations and viscoelastic explorations
IV.1. Preamble and Objectives
IV.2. Experimental Methods and Theoretical BackgroundExperimental Methods and Theoretical Background
IV.2.1. Synthesis of ERGO electrodes and structural characterization
IV.2.1. Synthesis of ERGO electrodes and structural characterization
IV.2.2. Electroacoustic impedance
IV.2.2. Electroacoustic impedance measurementsmeasurements
IV.2.3. Electrogravimetric measurements
IV.2.3. Electrogravimetric measurements
IV.3. Results and Discussion
IV.3.1. Morphology and structure of ERGO electrodes
IV.3.1. Morphology and structure of ERGO electrodes
IV.3.2. Viscoelasticity of ERGO electrodes and its influence on electrogravimetric performance
IV.3.2. Viscoelasticity of ERGO electrodes and its influence on electrogravimetric performance
IV.3.3. Cyclic electrogravimetric behavior.3.3. Cyclic electrogravimetric behavior
IV.3.4. AcAc–electrogravimetric investigationselectrogravimetric investigations
IV.4. Conclusions
Chapter V: Tracking interfacial charge transfer behavior of hydrothermally synthesized ZnO nanostructures via complementary electrogravinanostructures via complementary electrogravimetric methodsmetric methods
V.1. Preamble and Objectives
V.2. Experimental Methods and Theoretical Background
V.2.1. Electrode preparation and characterization
V.2.2. Theoretical considerations for acac–electrogravimetryelectrogravimetry
V.3. Results and DiscussionDiscussion
V.3.1. Cyclic Electrogravimetry (EQCM) and QCM–coupled GCD:coupled GCD:
V.3.2. QCM–coupled to Electrochemical Impedance Spectroscopy (coupled to Electrochemical Impedance Spectroscopy (AcAc–electrogravimetry)electrogravimetry)
V.3.3. Comparison of the EQCM and AcAc–electrogravimerty mass responseselectrogravimerty mass responses
V.4. ConclusionsConclusions
Chapter VI: Reduced graphene oxide–sheltered ZnO nanosctructures showing enhanced electrochemical sheltered ZnO nanosctructures showing enhanced electrochemical performance revealed by an in situ electrogravimetric studyperformance revealed by an in situ electrogravimetric study
VI.1. Preamble and Objectives
V.2. Experimental Methods and Theoretical BackgroundBackground
VI.2.1. Synthesis of ZnO seed layer
VI.2.2. Synthesis of ZnO nanostructures
VI.2.3. Preparation procedures for ZnO@ERGO electrode
VI.2.4. Morphological observation of the electrode
VI.2.5. Complementary electrogravimetric characterizations (EQCM and acelectrogravimetric characterizations (EQCM and ac–electrogravimetry)electrogravimetry)
VI.3. Results and Discussion

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