The development of a rigorous nanocharacterization scheme for electrochemical systems

This thesis reports on a methodology for the nanocharacterization of complex electrochemical systems. A series of powerful techniques have been adapted and applied to studies of two scientifically important electrochemical systems; namely polymer membrane solid-state ion-selective electrodes (ISEs) and electrochemically generated tetracyanoquinodimethane (TCNQ) charge-transfer materials. These studies have mainly encompassed the use of neutron reflectometry (NR), electrochemical impedance spectroscopy (EIS), secondary ion mass spectrometry (SIMS), small angle neutron scattering (SANS), synchrotron radiation / Fourier transform-infrared microspectroscopy (SR / FT-IRM), synchrotron radiation / X-ray photoelectron spectroscopy (SR / XPS) and synchrotron radiation / grazing incidence X-ray diffraction (SR / GIXRD). Significantly, an NR technique has been specially developed to enable simultaneous EIS measurements through the development and refinement of a novel electrochemical / reflectometry cell. Furthermore, the development of a versatile electrochemical cell that is capable of allowing SR / GIXRD measurements to be made in practically any conceivable electrochemical problem has also been of great significance.The investigation of polymer membrane solid-state ISEs focused on the problem of water layer formation at the buried polymer interface after prolonged exposure to an analyte. Initially, a rigorous surface and materials characterization scheme was developed and applied to plasticized poly(vinylchloride) (PVC) coated wire electrodes (CWEs) that are known to be adversely affected by water layer formation. It was determined that water and the associated ions from the sample analyte were transported through the PVC membrane. This resulted in the formation of a water layer (approximately 120 Å thick) at the substrate / ion-selective membrane interface. The results of the study suggested that this event occurred after 3 to 20 hours of constant exposure to solution. Moreover, the water layer at the buried interface was found to contain traces of plasticizer, whilst nanodroplets of water were also found in the membrane. The former is evidence for the exudation of plasticizer from the PVC membrane into the water layer at the buried interface.Further investigations on a solid-state ISE utilizing a hydrophobic poly(methylmethacrylate) / poly(decylmethacrylate) (PMMA / PDMA) copolymer as the ion-selective membrane revealed that water was transported through the membrane at a far slower rate than that of plasticized PVC ISEs. In fact, a regular ISE of this type severely restricted water accumulation at the buried interface, with such an event occurring after 460 hours. In addition, water was restricted to accumulation as droplets at the buried interface, as opposed to continuous water layers. A negligible amount of water was found in the bulk of this hydrophobic polymer membrane.Given CWEs are susceptible to forming water at the buried interface, it is customary to employ solid-contact (SC) underlayers. The primary function of the SC is to provide an appropriate mechanism for ion-to-electron transduction. Certain SCs are also theorized to discourage the formation of water layers. The results of this thesis revealed that a hydrophobic poly(3-octylthiophene-2,5-diyl) (POT) SC can prevent the formation of a water layer in SC ISEs altogether. This is not only achieved through the hydrophobic nature of POT, but also through the fact that the underlayer of POT is able to cover any imperfections at the buried interface, which water can use as a site for accumulation.By contrast, a hydrophilic polymer SC, known as poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS), was found to scavenge available traces of water at the buried interface. Instead of forming a well defined water layer or even water-droplets at the buried interface, the PEDOT:PSS SC system was found to soak up all traces of water transported through the ion-selective membrane to the buried interface. Water was detected in the PEDOT:PSS vii underlayer in a miscible state and not as a separate phase as observed with the CWE systems.The mechanism for ion-to-electron transduction in electroactive polymer SCs was also investigated. The study was performed in order to address the extent to which charger-transfer events occur throughout the underlying polymer SC. By studying the electrochemical doping of POT with [3,5-bis(triflouro-methyl)phenyl]borate (TFPBˉ) ions it was shown that the ion-to-electron transduction process is surface confined. This outcome demonstrates that the performance of various SCs does not depend on the thickness of the polymer film. In fact, it is proposed that the sparing use of the SC material may possibly achieve better charge-transfer performance. Such a hypothesis is based on the reduced electron path through the SC, hence reducing the probability that electrons are hindered by impurities and film imperfections. The suggestion of surface confined charge-transfer events also supports previous notions that the effectiveness of SCs is based on the capacitive nature of the material.The final part of the thesis deals with the characterization of the structure and morphology of TCNQ-based charge-transfer materials. Due to the lack of prior research on the electrochemical syntheses and structures of these materials, Cd(TCNQ)2 and Zn(TCNQ)2 were studied. By using SR / GIXRD together with synchrotron powder diffraction, the electrochemically synthesized Cd(TCNQ)2 was found to be crystallographically similar to the powder sample. Subtle differences between the two materials were evident; however, it was found that the major phase of non-hydrated Cd(TCNQ)2 phase was present in both samples. Notably, this phase was found to have a tetragonal unit cell, with cell parameters: a = 16.78Å and c = 8.83Å.Finally, a potential-dependant voltammetric study was carried out on a Zn(TCNQ)2 system. This was done in order to investigate the effects of electrodepositing Zn(TCNQ)2 under different electrochemical conditions. It was found that the material electrocrystallized prior to, or at, the peak potential for reduction of TCNQ to TCNQˉ comprised two layers. The upper layer was shown to consist of a densely packed and highly amorphous layer of Zn(TCNQ)2, while the lower layer was a crystalline phase of Zn(TCNQ)2. The material deposited at a potential after the peak suggested that only the crystalline phase of Zn(TCNQ)2 was present. This finding is significant for two reasons. First, in electrochemistry, it demonstrates that the in situ SR / GIXRD technique can be used to interrogate electrode reaction products under different voltammetric conditions. Next, it is important in the manufacture of electrocrystallized materials, where it demonstrates that complete control of the morphology and major phases is possible and that SR / GIXRD is a useful research tool to study the process

Construction of 2D g-C3N4 lateral-like homostructures and their photo- and electro-catalytic activities

g-C3N4 crystalline/amorphous lateral-like homostructures were prepared using crystalline g-C3N4 nanosheets as seeds via sequential edge-epitaxy growth. The homojunction effectively separates photogenerated carriers, resulting in high photo- and electro-catalytic activities

In situ structural characterization of electrochemical systems using synchrotron-radiation techniques

In recent times, in situ synchrotron-radiation techniques have been used extensively in studies of scientifically and technologically important electrochemical systems. In this review, we showcase the power of electrochemical measurement, either active or passive, in combination with in situ synchrotron-radiation techniques, using research reported on battery systems, corroding metals, ion-selective electrodes and fuel cells. Moreover, we review the specialized electrochemical cells and general design principles utilized by electrochemical synchrotron researchers

Self-Recovery Chemistry and Cobalt-Catalyzed Electrochemical Deposition of Cathode for Boosting Performance of Aqueous Zinc-Ion Batteries

Rechargeable Zn-ion batteries working with manganese oxide cathodes and mild aqueous electrolytes suffer from notorious cathode dissolution during galvanostatic cycling. Herein, for the first time we demonstrate the dynamic self-recovery chemistry of manganese compound during charge/discharge processes, which strongly determines the battery performance. A cobalt-modified δ-MnO2 with a redox-active surface shows superior self-recovery capability as a cathode. The cobalt-containing species in the cathode enable efficient self-recovery by continuously catalyzing the electrochemical deposition of active Mn compound, which is confirmed by characterizations of both practical coin-type batteries and a new-design electrolyzer system. Under optimized condition, a high specific capacity over 500 mAh g−1 is achieved, together with a decent cycling performance with a retention rate of 63% over 5,000 cycles. With this cobalt-facilitated deposition effect, the battery with low concentration (0.02 M) of additive Mn2+ in the electrolyte (only 12 atom % to the overall Mn) maintains decent capacity retention

The influence of thermal degradation on the electrodeposition of aluminium from an air- and water-stable ionic liquid

Aluminium electrodeposition is demonstrated from a thermally degraded ionic liquid solution. NMR and voltammetric analyses established that Al3+ reduction was remarkably similar to that in non-degraded IL solutions suggesting that the electroactive metal-containing species was unaffected by heat treatment. Electron microscopy revealed a significant grain refinement of the deposited metal. © the Owner Societies 2013

Favourable surface properties of boron-doped diamond electrodes for aluminium electrodeposition from ionic liquids

Aluminium electrodeposition on a boron-doped diamond (BDD) electrode was studied in the ionic liquid: 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C 4mpyr][NTf 2]. Cyclic voltammograms recorded on BDD electrodes in 1 m solutions of AlCl 3 in [C 4mpyr][NTf 2] were compared with those recorded on Au, Al and glassy carbon (GC) electrodes. The BDD electrode exhibits similar voltammetric responses for Al electrodeposition as on metallic electrodes but without interferences from alloy formation or reaction of the electrode material. Conversely, nucleation and growth of Al is significantly more difficult on freshly abraded GC. The electrochemical stability and activity of BDD over a wide potential range make it a highly favourable surface for studying metal deposition at very negative potentials in ionic liquids. © 2012 Elsevier B.V

Aluminium oxidation at high anodic potentials in an AlCl <inf>3</inf>-containing air- and water-stable ionic liquid solution

The observation of a multi-step stripping process at high positive potentials is reported for aluminium electrodeposition from a solution of AlCl3 in the air- and water-stable ionic liquid 1-butyl-1- methylpyrrolidinium bis(trifluoromethylsulfonyl)imide. Cyclic voltammograms recorded on an aluminium-coated boron-doped diamond electrode revealed complex oxidative behaviour, suggesting that a passive layer forms on the freshly deposited aluminium that hinders electro-oxidation. High positive potentials are required to induce chloride attack of the passive film, composed of Al-NTf 2 compounds, and facilitate the rapid oxidation of the underlying aluminium metal. These results have important implications in bipolar-pulse electroplating and electrorefining applications where electrodissolution of aluminium is required. © 2013 Published by Elsevier B.V. All rights reserved

Comparison of corrosion behaviour and passive film properties of 316L austenitic stainless steel in CO2 and N2 environments

This study investigated the susceptibility to pitting corrosion of 316L in CO2and N2environments at temperatures from 30 to 80°C in 3 wt-% NaCl at pH 4. Results from cyclic polarisation technique confirm greater pitting susceptibility of 316L in the CO2environment. Electronic properties and composition of the passive film were identified by electrochemical impedance spectroscopy, Mott–Schottky, and X-ray photoelectron spectroscopy. Increasing temperature negatively affects the passive film stability, and its influences are amplified in the presence of CO2as compared to N2. In the CO2environment, the passive film becomes porous with the increasing temperature leading to higher defects (donor/acceptor densities)

Recent advances in anion-doped metal oxides for catalytic applications

Metal oxides have been extensively applied as heterogeneous catalysts in various chemical processes, including conventional heterogeneous catalysis, photocatalysis, and membrane catalysis. The catalytic performance of an oxide heterogeneous catalyst can be affected by its lattice structure, electronic structure, surface properties, bulk defects, and metal-oxygen bond strength. As a catalytic membrane, the catalytic performance of an oxide may also strongly depend on its oxygen-ion diffusion properties. Cation doping has been extensively adopted to tailor, both physically and chemically, the properties of oxide materials, such as lattice structure, electronic structure, lattice defects and diffusion behavior, so as to alter their catalytic performance for various redox reactions. Very recently, anion doping into the oxygen site has emerged as a new strategy for tuning the chemical and physical properties of metal oxides, and thus for regulating their catalytic behavior. Here, a timely review of recent progress in the development of advanced oxide catalysts based on oxygen-site anion doping is provided. Emphasis is given to the effect of doping anions into the metal oxide lattice on the physical and chemical properties, and consequently the performance in various catalytic applications, including the oxidative dehydrogenation of ethane (ODE), oxidative coupling of methane (OCM), photocatalytic reduction of dyes, and ceramic membrane-based oxygen separation. The aim of the current review is to offer some insightful perspectives to guide the development of functional oxide materials based on the anion site doping strategy toward application in heterogeneous catalysis. The knowledge gained here may also be useful for other application fields, such as electrochemical energy storage devices and sensors

Tuning the Electrochemical Property of the Ultrafine Metal-oxide Nanoclusters by Iron Phthalocyanine as Efficient Catalysts for Energy Storage and Conversion

Nanoclusters (NCs) have been demonstrated of outstanding performance in electrochemical energy storage and conversion technologies due to their strong quantum confinement effects and strong interaction with supports. Here, we developed a class of ultrafine metal-oxide (MOx, M = Fe, Co and Ni) NCs incorporated with iron phthalocyanine (FePc), MOx/FePc-G, supported on graphene as high-performance catalysts for oxygen reduction reaction (ORR), oxygen evolution reaction (OER), and carbon dioxide reduction (CO2RR). The high activities for ORR and OER are attributed to the electron donation and accepting ability of the highly redox active of FePc-G that could tune the properties of MOx. The FeOx/FePc-G exhibits an extremely positive half-wave potential (E1/2) of 0.888 and 0.610 V for ORR in alkaline and neutral conditions, respectively, which is around 60 mV more positive than that of Pt/C. And NiOx/FePc-G shows similar OER activity with the state-of-the-art catalysts, Ir/C, and better performance than NiFeO NCs supported on graphene. Remarkably, the CoOx/FePc-G and NiOx/FePc-G show high activity and selectivity to reduce CO2 into CO with a low onset potential of −0.22 V (overpotential is 0.11 V)