As demands for alternative sources of energy increase over the coming decades, water electrolysis will play a larger role in meeting our needs. The oxygen evolution reaction (OER) component of water electrolysis suffers from slow kinetics. An efficient, inexpensive, alternative electrocatalyst is needed. We present here high-activity, low onset potential, stable catalyst materials for OER based on a hierarchical network architecture consisting of Fe and Ni coated on carbon fiber paper (CFP). Several compositions of Fe-Ni electrodes were grown on CFP using a hydrothermal method, which produced an interconnected nanosheet network morphology. The materials were characterized by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Electrochemical performance of the catalyst was examined by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). The best electrodes showed favorable activity (23 mA/cm², 60 mA/mg), onset potential (1.42 V vs. RHE), and cyclability. / text

The experimental work discussed in this thesis examines the effect of voltammetry of nanoparticle arrays with a specific focus on the effect of nanoparticle size and surface coverage on the substrate electrode. These effects are investigated in relation to the reduction of hydrogen peroxide, 4-nitrophenol and the hydrogen evolution reaction. In each case, the experimental data obtained is subsequently fitted using numerical simulations to extract quantitative kinetic data. Distinct differences are noted between macro – and nano – scale. The reduction of H₂O₂ reveals the absence of autocatalysis at the nano – scale. Furthermore, peak potential shifts positively with surface coverage under Case 3 diffusion conditions. On the other hand, at essentially isolated particles (Case 1) E_p varies logarithmically with nanoparticle radius. By decreasing nanoparticle size, we promote convergent diffusion and enhance irreversibility of the process. The shift in E_p with surface coverage can be accounted for as the diffusion layers begin to overlap heavily, tending towards more macro – disk type behaviour. We also study the hydrogen evolution reaction at an array of AgNPs. By fitting of the experimental data with numerical simulations we demonstrate altered kinetics between the macro – and nano – scale. Voltammetry at AgNP – arrays also display dependence between surface coverage and current. We attribute this to increased electro – active surface area and decrease in irreversibility of the process, with increasing surface coverage. Numerical simulations are also used to fit experimental data obtained for the reduction of 4 – nitrophenol in acidic media. The AgNP – arrays exhibit significantly different electrode kinetics compared to a macro – disk. Examination of the data obtained for AgNP – arrays at two different acid concentrations implies the rate - determining step is likely the electron transfer process. We therefore infer a change in α the mechanism of the rate – determining step between macro – and nano – scale. An unusually low value of α at the NP – array may indicate that adsorption plays some role in the process. Furthermore, we discuss the size – dependent adsorption behaviour exhibited by silver nanoparticle arrays, such that small particles with diameters below ~ 50 nm do not display the typical underpotential deposition characteristics of corresponding bulk materials or larger nanoparticles. This phenomenon is reported for the deposition of heavy metals (thallium, lead and cadmium) at silver nanoparticle arrays. The stripping voltammetry of arrays of silver nanoparticles has also been investigated. The stripping peak potential is dependent on the degree of surface coverage. Modelling of this system has shown that a one – dimensional diffusion model is appropriate for high surface coverage; essentially it is experiencing planar diffusion. However, for an array of widely dispersed particles, the individual nanoparticle size and convergent diffusion begin to dominate behaviour. A detailed overview of the literature is first discussed, relating to the synthesis of a wide variety of metallic nanoparticles and their practical applications, such as the determination of pH using platinum nanoparticles as covered in Chapter 7. We also discuss the fundamental principles governing the voltammetric behaviour of nanoparticles.

The experimental work discussed in this thesis explores the effects of metal modification on the electroanalytical ability of boron doped diamond electrodes. Boron doped diamond (BDD) electrodes have found increased application to electroanalysis in the past two decades, yet relatively little of the literature is focused on metal, nano and microparticle modification of the substrate. In this thesis three metals have been used to modify the BDD electrode; bismuth, antimony and nickel. Bismuth and antimony nanoparticle modified BDD electrodes were directly compared to unmodified BDD and a bulk bismuth electrodes in the determination of trace levels of cadmium and lead using anodic stripping voltammetry. In both instances, the modified electrode allowed for the simultaneous determination of each analyte that was otherwise unattainable at the unmodified BDD electrode. The nickel modified BDD (Ni-BDD) electrode was used in the determination of organic analytes, namely glucose, methanol, ethanol and glycerol. The nickel nano and microparticle electrodes gave the characteristic Ni(OH)₂/NiOOH redox couple in alkali pH, the oxidised form of which (NiOOH) catalysed the oxidation of the organic analytes. The chapter on glucose sensing with the Ni-BDD electrode is preceded by an extensive literature review on the advances of non-enzymatic glucose sensing, and the application of catalytic metals and nanomaterials in this field. Throughout the course of this DPhil, there has been a collaborative project between Asylum Research and myself within the Compton group to develop a commercial electrochemical atomic force microscope (EC-AFM) cell. The aim was to produce an adaptable EC-AFM cell capable of dynamic electrochemical experiments whilst simultaneously or instantaneously acquiring an AFM image of the modified surface, in-situ. This project was successful, and the EC-AFM cell has contributed to a number of chapters in this thesis, and has now been commercialised.

This study has been mainly focused on high temperature solid oxide electrolysis cells (HT-SOECs) for steam electrolysis. The compositions, microstructures and metal catalysts for SOEC cathodes based on (La₀.₇₅Sr₀.₂₅)₀.₉₅Mn₀.₅Cr₀.₅O₃ (LSCM) have been investigated. Hydrogen production amounts from SOECs with LSCM cathodes have been detected and current-to-hydrogen efficiencies have been calculated. The effect of humidity on electrochemical performances from SOECs with cathodes based on LSCM has also been studied. LSCM has been applied as the main composite in HT-SOEC cathodes in this study. Cells were measured at temperatures up to 920°C with 3%steam/Ar/4%H₂ or 3%steam/Ar supplied to the steam/hydrogen electrode. SOECs with LSCM cathodes presented better stability and electrochemical performances in both atmospheres compared to cells with traditional Ni cermet cathodes. By mixing materials with higher ionic conductivity such as YSZ(Y₂O₃-stabilized ZrO₂ ) and CGO(Ce₀.₉Gd₀.₁O₁.₉₅ ) into LSCM cathodes, the cell performances have been improved due to the enlarged triple phase boundary (TPB). Metal catalysts such as Pd, Fe, Rh, Ni have been impregnated to LSCM/CGO cathodes in order to improve cell performances. Cells were measured at 900°C using 3%steam/Ar/4%H₂ or 3%steam/Ar and AC impedance data and I-V curves were collected. The addition of metal catalysts has successfully improved electrochemical performances from cells with LSCM/CGO cathodes. Improving SOEC microstructures is an alternative to improve cell performances. Cells with thinner electrolytes and/or better electrode microstructures were fabricated using techniques such as cutting, polishing, tape casting, impregnation, co-pressing and screen printing. Thinner electrolytes gave reduced ohmic resistances, while better electrode microstructures were observed to facilitate electrode processes. Hydrogen production amounts under external potentials from SOECs with LSCM/CGO cathodes were detected by gas chromatograph and current-to-hydrogen efficiencies were calculated according to the law of conservation of charge. Current-to-hydrogen efficiencies from these cells at 900°C were up to 80% in 3%steam/Ar and were close to 100% in 3%steam/Ar/4%H₂. The effect of humidity on SOEC performances with LSCM/CGO cathodes has been studied by testing the cell in cathode atmospheres with different steam contents (3%, 10%, 20% and 50% steam). There was no large influence on cell performances when steam content was increased, indicating that steam diffusion to cathode was not the main limiting process.

This thesis concerns the electrochemical investigation of high-potential laccases. These multicopper oxidases are efficient electrocatalysts for the dioxygen reduction reaction. A method for stabilising laccase on a graphite electrode was established. The method involved modification of the graphite surface by diazonium coupling of a 2-anthracene molecule. A laccase ‘film’ adsorbed on this modified surface remained stable for over two months and, typically, the current density for dioxygen reduction was doubled compared to a laccase ‘film’ on an unmodified surface. Protein film voltammetry was used to investigate thermodynamic and kinetic aspects of the electrochemical behaviour of laccase. The effect of inhibitors on the magnitude of reduction current and the position of the wave (related to the overpotential for the reaction) was also studied. Fluoride, chloride and azide showed different modes of inhibition and inhibition constants ranged from micromolar for azide to millimolar for chloride. In cyclic voltammetry experiments it was only in the presence of high concentrations of the inhibitors fluoride, chloride and azide that a non-turnover signal, corresponding to a one electron transfer process, was revealed. The evidence suggested that the non-turnover signal arose from interfacial electron transfer between the electrode and the type 1 or ‘blue’ copper. Evaluation of the peak areas allowed determination of the catalytic rate constant, kcat, as 300 s–1, and the electroactive surface coverage as four pmol cm–2. The rate of interfacial electron transfer was rapid enough to not limit catalysis at high overpotentials. A spectroelectrochemical cell was designed to investigate the behaviour of the type 1 copper in the presence of inhibitors and at different pH values. The inhibitors fluoride, chloride and azide had little effect on the reduction potential of the type 1 copper, but at higher pH values the reduction potential of the type 1 copper was decreased.

This thesis concerns several advances in electroanalytical chemistry which are separated into four parts: the electrochemical investigation of diffusional behaviour, the mechanistic and kinetic study of electrochemistry with room temperature ionic liquids (RTILs), the study of weakly-supported electrochemistry and a comparison of the Butler-Volmer and Marcus-Hush kinetic theories of electron transfer. A study of the diffusional behaviour of electroactive species is essential for further studies, especially in the case when electrochemistry is complicated through ion-pairing interactions between the electroactive species and other electrolytes. In Part II of this thesis, the possibility of the ferricenium ion-paired with perchlorate and hexauorophosphate in acetonitrile was discussed firstly employing chronoamperometric technique. Afterwards, the hexaammineruthenium III/II couple supported by chloride, nitrate and sulfate respectively was studied by a similar method. In order to avoid unwanted ion-pairing effects, room temperature ionic liquids can be applied as solvent, which provide high conductivity by their own ionic nature so that experiments can be conducted without adding additional supporting ions. Because of RTILs have distinctive properties, for example, high viscosity, high conductivity and ionic nature, electrochemistry could be greatly changed compared to those in conventional solvents. Part III of this thesis gives a detailed description of this topic. First, a study of the reduction of 1,4-benzonquinone in 1-ethyl-3-methylimidazolium bis(triuoromethanesulfonyl)imide is presented to show the new mechanistic insight into comproportionation in a electrochemical process. Second, a discussion of the oxidation of hydroquinone in the same RTIL is introduced to suggest a possible ECE scheme which was never reported before. The interest of weakly supported electrochemistry is also well-established, which not only provides another alternative strategy to avert ion-pairings but also offers more physical insights into electrochemical processes. Quantitative methods analysing voltammetries without an excess amount of supporting electrolyte are developed by introducing a migration term into the mass transport equation. In Part IV, new mechanistic insights into the reduction of 2-nitrobromobenzene and the dimerisation of 2,6-diphenylpyrylium in acetonitrile were provide by using weakly-supported cyclic voltammetry. Also, pulse techniques was also adopted to investigate the reduction of cobaltocenium and cobalt(III) sepulchrate, giving an alternative way for electrochemical analysis. A major application of electroanalytical chemistry is investigating electrochemical kinetics. Two kinetic models mostly concerned by electrochemists are Butler-Volmer and Marcus-Hush formalisms. The classic phenomenological model, Butler-Volmer formalism successfully describes most common electron transfer kinetics but shows little reference with nature of the involved species, solution and electrode material, while a more physically insightful theory, the Marcus-Hush formalism, takes species natural properties, for instance, a change of distances or geometry in the solvation or coordination shells of the redox, into account although it requires more complex formulations. Comparative studies of these two theories are presented in Part V in order to improve our understanding of the electron transfer kinetics under different circumstances. First, comparison of cyclic voltammograms of the reduction of europium(III) and 2-methyl-2-nitropropane at mercury microhemispherical electrodes was carried out. Second, square wave and differential pulse voltammetric techniques were also employed to further discriminate the two kinetic models. These studies all find that the symetric Marcus-Hush theory assuming the reactants and products have identical force constant dose not satisfactorily agree with the experimental results. Hence, the introduction of asymmetric Marcus-Hush theory was presented considering different oxidative and reductive reorganization energies, which gives reasonable agreement with experiments and makes this theory more insightful.

The work presented in this thesis involves the use of room temperature ionic liquids (RTILs) as solvents in electrochemical experiments for the detection of hydrogen sulfide. The fundamentals of electrochemistry are presented, followed by an overview of RTILs in terms of their properties, applications and their behaviour as electrochemical solvents compared to conventional solvents. This is followed by an outline of electrochemical detection of various gases in aqueous, organic and ionic solvents. The results of 8 original studies are then presented as follows: <ul><li>The study of the electrochemical window of twelve different room temperature ionic liquids using cyclic voltammetry vs. an internal redox couple for two defined current densities, and observation of water uptake of different ionic liquids under different conditions using a Karl Fischer titrator.</li><li>The reduction of hydrogen sulfide in various room temperature ionic liquids at a platinum electrode, measured using cyclic voltammetry. Also, solubilities and diffusion coefficients of hydrogen sulfide determined by potential step chronoamperometry.</li><li>The oxidation of hydrogen sulfide in various room temperature ionic liquids at a platinum electrode measured using cyclic voltammetry and the simulation of the electrochemical signal using experimentally defined parameters.</li><li>The disproportionation of N,N-dimethyl-p-phenylenediamine (DMPD) in room temperature ionic liquids using cyclic voltammetry, and computational simulation of the voltammetry of DMPD using experimentally defined parameters to elucidate kinetic and thermodynamic data. DMPD was examined as a mediating species for hydrogen sulfide detection.</li><li>The oxidation of catechol and dopamine in ionic liquids using cyclic voltammetry and observing adsorption effects when varying solvent anion. Catechol was examined as a mediating species for hydrogen sulfide detection.</li><li>The electrochemical oxidation of NADH in ionic liquids using cyclic voltammetry and observing the ”switching on or off” of the electrochemical signal when varying the solvent anion. NADH was examined as a mediating species for hydrogen sulfide detection.</li><li>The mediated detection of hydrogen sulfide utilizing various mediating species in several ionic liquids using cyclic voltammetry, and the elucidation of the mediating mechanism of hydrogen sulfide in 3,5-tert-butyl-o-benzoquinone.</li><li>The observation of the diffusion of ferrocene in an ionic liquid at ring-recessed disc microelectrode arrays in generator-collector mode using potential step chronoamperometry.</li><ul> The results presented show that room temperature ionic liquids perform well as solvents in gas sensors, and could be viable alternatives to traditional organic solvents. Ionic liquids have also been observed to be tuneable in their reactions with analytes depending on the constituent cations and, in particular, anions. This tuneability is advantageous as specific combinations of cations and anions can be chosen to suit particular experiments.

Hydrogenases are a class of metalloenzymes which catalyse H₂ oxidation and its reverse reaction, H⁺ reduction. There is interest in investigating how H₂ as an energy carrier is cycled in biology. Hydrogenases have also been studied extensively because there are potential applications for them as catalysts for H₂ oxidation in fuel cells or H₂ production via light-driven water splitting. For these applications, the ability for the hydrogenase to work in the presence of O₂ is an important issue. The microorganism Ralstonia eutropha is a well-studied model aerobic H₂ oxidiser: it can adopt H₂ as the sole energy source to grow cells in the presence of O₂. It produces at least three distinct O₂-tolerant NiFe hydrogenases: the membrane-bound hydrogenase (MBH), the NAD⁺-reducing soluble hydrogenase (SH) and the regulatory hydrogenase (RH). This Thesis employs protein film electrochemistry (PFE) to study the SH and RH. It is found that the SH is able to work in both direction (H₂ oxidation and H⁺ reduction) with minimum overpotential, which is critical in coupling 2H⁺/H₂ cycling with the closely spaced NAD⁺/NADH potential. Reactions of the SH with O₂ have been investigated, revealing at least two distinct O₂-inactivated states, but consistent with the requirement for the SH to function in air, it can be reactivated in the presence of O₂ at low potentials which could be provided by the NAD⁺/NADH pool in vivo. The affinity of the RH for H₂ was determined by PFE and found to be slightly higher than that of the SH and MBH. This may provide a way for the microbe to regulate hydrogenase expression in response to the H₂ availability. Carbon monoxide and O₂-inactivated states of the RH have been identified for the first time, confirming that a constricted gas channel is not sufficient to explain its O₂ tolerance. Observation of potential dependent reactions in hydrogenases means that it is important to have spectroscopic methods for characterising states triggered by inhibitors and potential. An Infrared spectroelectrochemical approach suitable for studying metalloenzymes has been developed and preliminary spectra on RH recorded. This method should provide many opportunities for future studies of redox states of hydrogenases.

Graphene, as a recently discovered carbon allotrope, possesses with it many outstanding properties ranging from high electrical conductivity to great mechanical strength. Single layer graphene can be prepared by mechanical cleavage of graphite or by a more sophisticated method, CVD. However, the scale-up process for these preparation techniques is still unconvincing. Solution-processed graphene from exfoliation of graphite oxide on the other hand provides an alternative prospect resulting in the formation of graphene nanoplatelets (GNPs), which can be readily manipulated to tailor-suit various application demands. The main aim of the thesis is to explore the possibility and availability of this versatile method to produce graphene nanoplatelet and its composites with good all-round performance in energy and bioanalytical applications. A range of physical and chemical characterisation techniques were utilised including SEM, TEM, AFM, XPS, XRD, DLS, FTIR, Raman and UV-Vis spectroscopy in order to investigate the structural and chemical information of the graphene-based materials prepared. Functionalisation of graphene oxide with polyelectrolyte polymer could facilitate deposition of platinum nanoparticles in the formation of Pt-GNPs composites. The resultant composite was employed for bioanalytical application in the detection of an important neurotransmitter, glutamate, based on glutamate oxidase enzyme. The performance of Pt-GNPs based glutamate sensor exhibited enhanced sensitivity and prolonged stability compared to the sensors based on Pt decorated diamond or glassy carbon electrodes. The significant interfering effect from concomitant electrochemically active biological compounds associated with Pt-GNPs electrode however could be alleviated via opting for Prussian blue deposited GNPs electrode instead. The oppositely charged Pt-GNPs due to different functionalising polymers were also subject to self-assembly, which was enabled by the electrostatic interaction of the opposite charges of Pt-GNPs. The self-assembled film showed enhanced mechanical stability than the conventional drop-casted film and provided reasonably good activity towards oxidation of hydrogen peroxide. Three-component composite of graphene, nanodiamond and polyaniline was prepared via in-situ polymerisation for usage as an electrode material in electrochemical capacitors ("supercapacitors"). The addition of graphene was shown to significantly enhance specific capacitance while nanodiamond could improve the stability of the electrode by strengthening the polymer core. Another approach to produce a supercapacitor was via electrodeposition of nickel and cobalt hydroxides on graphene oxide film corporated with bicarbonate salt. The film was then subject to thermal reduction of GO and expansion of graphene layers within the film was observed. This leavening process enhanced the surface area of graphene film and thus the higher specific capacitance was obtained. The decoration of nickel and cobalt hydroxides onto the film also boosted the specific capacitance further however the poor cycling stability of the heated film still remained an issue. Graphene nanoplatelets were also used as a support for electrodeposition of Pt nanoparticles for methanol oxidation in acidic media. The preferential phase of the Pt deposited and large surface area of graphene in comparison to other carbon supports studied led to good catalytic activity being observed.

Hydrogenases catalyse the interconversion of H₂ and H⁺. Protein Film Electrochemistry (PFE), a technique in which a redox enzyme is adsorbed directly onto an electrode, enables a detailed description of the catalytic function of these metalloenzymes to be obtained. Unlike small-molecule electrocatalysts, the hydrogenase active site is surrounded by a protein structure ensuring that it is relatively unperturbed by the electrode surface. In this thesis, PFE is used alongside mathematical modelling to explain differences between [NiFe]- and [FeFe]-hydrogenases, highlighting some important considerations for efficient, reversible electrocatalysis. This thesis probes the unusual reaction between [NiFe]-hydrogenases and cyanide. Through a detailed study utilising PFE, Electron Paramagnetic Resonance (EPR) and Attenuated Total Reflection Infrared spectroelectrochemistry (ATR-IR), it is demonstrated that cyanide promotes the formation of the inactive Ni-B state. Preferred formation of the Ni-B state over more slowly reactivating Unready states is considered an important characteristic of the O₂-tolerant class of [NiFe]-hydrogenases. The nature of the Ni-L state, commonly thought to be an artefact formed when a [NiFe]-hydrogenase is exposed to visible light, is probed via EPR and ATR-IR. In this thesis, the Ni-L state is shown to occur in samples of Hydrogenase-1 from Escherichia coli that have not been exposed to visible light, calling into question the true nature of this state. Finally, this thesis details the first study in which PFE is used to investigate the spontaneous incorporation of a synthetic active site mimic complex into apo-hydrogenase. Incorporation into apo-hydrogenase from Chlamydomonas reinhardtii and Clostridium pasteurianum is discussed, in both cases resulting in fully functional [FeFe]-hydrogenase, electrochemically indistinguishable from the native enzyme.