The performance of electrochemical hypochlorite cells as a function of hydrodynamics and electrode material has been studied over a range of conditions. In this, the electrolytic generation of Sodium Hypochlorite (NaOC1) using an undivided cell in a closed-loop experimental equipment was carried out under galvanostatic conditions (ie at constant current). Variations in the concentration of the products (NaOC1, NaC103,02' H2) as well 'as total volume of gas produced were monitored as a function of time. The variables studied include electrolyte (NaCl) flow rate,current density,electrolyte concentration, spacing, and electrolyte temperature. inter-electrode The electrochemical hypochlorite production reaction was then modelled (on the basis of the ~drodynamic conditions within the cell) by generating mathematical equations to describe the behaviour of the hypochlorite cell. Testing of the model equation by matching theoretical plots against experimental values (with the aid of computer), gave results which showed good agreement. The model equation can therefore be used in designing and predicting the behaviour of simple hypochlorite cells. The use of Lead Dioxide (Pb02) as an anode material (in place of Platinum based metals) for ~pochlorite generation was also studied. In general, it was found that the rate of corrosion of the Pb02 anode was higher in dilute brines than in stronger solutions and also higher at lower temperatures.

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In this thesis, the results of an investigation into the template-assisted synthesis of the YSZ/LSM composite for solid oxide fuel cells (SOFCs) are presented. This project is motivated by the prospective application of this technique in SOFCs. Extensive characterization of all samples formed has been carried out using a range of spectroscopic and microscopic techniques. The fabrication of the three-dimensionally ordered macroporous (3-DOM) composite mainly involves the creation of a periodic lattice of colloidal crystals via the self-assembly approach, the synthesis of 3-DOM yttria-stabilized zirconia (YSZ) frameworks using the sol-gel route and the infiltration of the lanthanum strontium manganite (LSM) precursor into the YSZ template. One objective of this project was to obtain a greater understanding of the self-assembly growth process. Since the quality of self-assembled template plays an important role in determining the quality of resulting products, different synthesis conditions that affect the crystallinity have been individually investigated. Based on the results the optimal parameters were proposed. Although cracking phenomena in the template films were observed, and they were intrinsically inevitable in the fabrication of multi-layer colloidal crystal. By carefully control the synthesis parameters, the number of cracks can be minimized. The consequently obtained films exhibited good long-range ordering. All the characterization results confirmed that the obtained polystyrene films were suitable for using as templates in the synthesis of 3-DOM materials. Another objective was the use of colloidal crystal templates for the formation of 3-DOM YSZ and LSM thin films, which have potential applications in SOFCs. The synthesis of 3-DOM materials using the template-assisted approach has been proven as a complex process. A large variety of parameters showed apparent influences on the ultimate quality of 3-DOM films. All these parameters can be classified as four types, namely the template, precursor chemistry, infiltration process and calcination. The effect of each type was investigated and is discussed in this thesis. 3-DOM films exhibited large shrinkage which is attributed to the large difference in the density of the precursor and dense ceramics. A systematic study was carried out to investigate the shrinkage mechanism during the synthesis procedure, and a “floating model” was proposed to interpret the attachment of most ceramic domains when a large shrinkage was observed. Finally 3-DOM YSZ/LSM composites were successfully fabricated using template-assisted growth. The obtained film exhibited partially filled microstructure. Since YSZ/LSM composites have potential uses as the cathode material in solid oxide fuel cells, the microstructure and electrical performance of 3-DOM composites were investigated. In order to evaluate the possible performance of the 3-DOM composite as a cathode, the electrical conductivity was assessed using AC impedance spectroscopy. The impedance spectra exhibit high frequency and low frequency arcs attributed to complex electrical responses of YSZ/LSM composites. The activation energy of the composite film was obtained according to the modified Arrenius equation, and the result showed a typical value for YSZ/LSM composites with a LSM volume fraction of 30-40%. The low conductivity of the composite film is ascribed to the poor structural contact of the 3-DOM composite. In this study, large, ultra-fine polystyrene colloidal crystals were successfully fabricated. 3-DOM YSZ and LSM thin films of high quality were prepared using the polystyrene films as templates. Finally YSZ/LSM composite films were synthesized and characterized to investigate its potential uses as the cathode in SOFCs. All the work presented in this thesis provides a better understanding on the key parameters and mechanisms involved in template-assisted growth.

Miniaturized glucose-oxygen biofuel cells are useful to power implantable medical devices such as biosensors. They are small, more biocompatible and run continuously on glucose and oxygen, providing cleaner energy at neutral environment. A typical glucose-oxygen biofuel cell consists of an anode with glucose oxidase (GOx) and a cathode with various oxygen reducing catalysts. This thesis describes experimental investigations of the major issues of such systems, viz.: complex electrode fabrication, enzyme instability and inefficient oxygen reduction. Electrodes were built using the simple and scaleable bulk modification method, where all the material was simply mixed and bound together into composites with epoxy resin. For the anodes, the composite made of 10% GOx with 7:7 TTF-TCNQ was found optimal. The GOx electrodes were modified with various enzyme stabilisers (PEI, DTT, PEG, GLC, FAD and mixture of PEI:DTT and PEI:FAD) and 2% of PEI-DTT (1:1 w/w) was most effective in the presence of O2. Its maximum output current density was 1.8 x 10-2 ± 9.9 x 10-3 A.m-2. It also showed the resistant against O2 electron deprivation and enzyme inhibition. Its KM.was 5 mM. For the cathodes, various oxygen reducing catalysts (metalised carbon, anthroquinone modified carbon, laccase and bilirubin oxidase) were incorporated into graphite composite and the electrodes were pretreated in different media in order to enhance their catalytic activity. None showed four-electron O2 reduction. NaOH-pretreated cobalt (II) salophen composite electrodes showed two-electron O2 reduction and were most catalytic. Its standard catalytic rate constant was 1.2 x 10-5 ± 1.2 x 10-6 m.s-1. Of the catalysts examined, metal complex composites gave the best results for oxygen-reducing cathodes and their pretreatment led to the synergetic effect because it increased the concentration of catalytic surface oxygen groups and enhanced oxygen reduction.

Hydrogen, as a clean and renewable fuel, may play a key role in the near future because of the increasing cost of fossil fuels and the impact of CO2 on the climate. Fuel cells are electrochemical devices which directly convert chemical energy stored in hydrogen into electrical energy at high efficiency with only water and heat as byproducts. A leading candidate fuel cell technology for operation on hydrogen fuels is the proton exchange membrane fuel cell (PEMFC). But today its commercialization remains limited, mainly because of the price of the materials used for electrode manufacture. Catalysts based on precious metals such as platinum, which are currently inherent to PEMFCs, preclude inexpensive mass production. In contrast an alternate fuel cell technology well suited to hydrogen fuels, the alkaline fuel cell (AFC), offers the potential for low cost, mass producible fuel cells, without the need for platinum based catalysts, but has received less attention in recent years. The aim of this work is to develop AFC gas diffusion cathodes using new substrate materials (nickel foam and porous silver membranes) which ally mechanical support, current collection and catalyst support so as to reduce the cost of the electrode. Silver, which is one of the most active materials for the oxygen reduction reaction (ORR) and which is 100 times cheaper than platinum, has been used as the catalyst in this work. The effect of optimising the cathode performance has been monitored using DC polarization curves and electrochemical impedance spectroscopy. Both the nickel foam and porous silver membrane substrates have been successfully developed as the gas diffusion medium in aqueous alkaline media. Silver plated nickel foam showed a decrease in both the Ohmic and charge transfer resistance compared to uncoated nickel foam, leading to improved performance. The porous silver membrane showed good performance in a passive air-breathing cell (50 mW cm-2 at 25 oC) due to its high surface area and optimised hydrophobic properties.

La2Mo2O9, as a new fast ionic conductor, has been investigated widely due to its high ionic conductivity which is comparable to those of the commercialized materials. However, little work has been reported on the oxygen transport and diffusion in this candidate electrolyte material. The main purpose of this project was to investigate oxide ion diffusion in La2Mo2O9 and also the factors which could affect oxygen transport properties. Oxygen isotope exchange followed by Secondary Ion Mass Spectrometry (SIMS) measurements were employed to obtain oxygen diffusion profiles. A correlation between oxygen ion transport and the electrochemical properties such as ionic conductivity was built upon the Nernst Einstein equation relating the diffusivity to electrical conductivity. In-situ neutron diffraction and AC impedance measurements were designed and conducted to investigate the correlation between crystal structure and oxygen transport in the bulk materials. Other techniques, such as synthesis, microstructure studies, and thermal analysis were also adopted to study the electrochemical properties of La2Mo2O9. The results of the study on the effects of microstructure on oxygen diffusion in La2Mo2O9 revealed that the grain boundary component played a significant role in electrochemical performance, although the grain size seemed to have little influence on oxygen transport. The oxygen isotope exchange in 18O2 was successfully carried out by introducing a silver coating on the sample surface, which solved the main difficulty in applying oxygen isotope exchange on pure ionic conductors. The ionic conductivity obtained from the diffusion coefficients was consistent with the result from AC impedance spectroscopy. The number of mobile oxygen ions was estimated to be 5 per unit cell. There was a difference of oxygen self diffusion coefficient when the isotope exchange was conducted in 18O2 and H2 18O. The activation energy of oxygen diffusion in humidified atmosphere was higher than that measured in dry atmosphere. It indicated that the humidified atmosphere had affected oxygen transport in the material. The studies on hydroxyl incorporation and transport explained the decreased oxygen diffusion coefficients in wet atmosphere and also suggested proton conductivity in La2Mo2O9, which leads to further investigation on applications of La2Mo2O9 as a proton conductor. In-situ neutron diffraction and AC impedance measurement revealed a close relationship between crystal structure and ionic conductivity. The successful application of this technique provides a new method to simultaneously investigate crystal structure and electrical properties in electro-ceramics in the future.

Oxygen excess lanthanum nickelate, La2NiO4+δ (LNO), is a candidate cathode material for intermediate temperature solid oxide fuel cells (IT-SOFCs). The aim of this work is to investigate the properties of LNO in the intermediate temperature regime (500 – 700°C). The structure and stability of LNO has been studied by in-situ high resolution synchrotron x-ray diffraction and thermal analysis. A bi-phasic orthorhombic room temperature structure was identified, which undergoes a transition to a tetragonal phase. The phase change occurs over the temperature range 250°C to 450°C and is associated with loss of oxygen on heating. LNO undergoes an oxidation reaction, catalysed by platinum, above 800°C where it begins to form the higher order Ruddlesden-Popper phases, La3Ni2O7-δ and La4Ni3O10-δ. The oxygen ion transport properties of LNO have been studied by determining the oxygen tracer diffusion and surface exchange coefficients (D* and k*, respectively). LNO displays high D* and reasonable k* values and exhibits low activation energies for these processes (0.54eV and 0.63eV, respectively). The low activation energy for diffusion is associated with a high oxygen interstitial concentration between 350°C – 550°C. The compatibility of LNO with IT-SOFC electrolytes was investigated using high resolution x-ray synchrotron diffraction techniques. The stability of composites of LNO with Ce0.9Gd0.1O2-δ was found to be highly dependent on oxygen partial pressure and temperature and no reaction phase was observed in composites exposed to atmospheric oxygen. Studying composites in-situ revealed a series of reaction processes that have not previously been identified from ex-situ diffraction techniques. The performance of LNO as a cathode was studied by AC impedance of symmetrical cells with Ce0.9Gd0.1O2-δ and La0.8Sr0.2Ga0.8Mg0.2O3-δ electrolytes. Significant enhancement of the cathode performance was achieved by the addition of a thin compact layer of LNO at the electrode/electrolyte boundary; an area specific resistance (ASR) of 0.5 Ω.cm2 was measured at 800°C in a symmetrical cell with this layered structure. The decrease in ASR is believed to be a result of improved contact at the electrolyte/cathode boundary enhancing the oxygen ion transfer to the electrolyte, and an increase in the cathode surface area for the oxygen reduction reaction to occur.

The grain boundary properties of CGO10 (Ce0.9Gd0.1O1.95) and CGO20 (Ce0.8Gd0.2O1.95) have been investigated by impedance spectroscopy and high resolution scanning transmission electron microscopy-electron energy loss spectroscopy (STEM-EELS). High density polycrystalline ceramic specimens with a range of grain sizes (0.17-2.7μm) have been prepared from high purity (SiO2 ~120-210 ppm) commercially available powders. Impedance measurements obtained for these ionic conducting materials were interpreted using the brick layer model. The specific grain boundary conductivity [delta*GB]was found to be 2-3 orders of magnitude lower than the grain bulk conductivity [delta G]. The grain boundary blocking effect in these materials has been attributed to the formation of grain boundary space charge layers. The brick layer model was used to estimate the space charge layer thickness (δ) from impedance spectroscopy measurements and was found to be in the range 1-2 nm. High resolution STEM-EELS measurements were carried out over a number of grain boundaries in CGO10 and CGO20. These measurements showed an increase in the gadolinium to cerium and oxygen to cerium atomic ratios (Gd:Ce and O:Ce respectively) at the grain boundary region. These findings are in accordance with the space charge layer model. From these measurements the space charge layer thickness (δ) was shown to be in the range 1-2 nm. Analysis of the cerium M4,5-edge fine structure showed the cerium oxidation state to remain unchanged in the space charge layer in all grain boundaries studied.

Electrophoretic deposition (EPD) was used to form a composite layer of mercaptoundeconic acid (MUA) capped CdSe-TiO2 nanoparticle on a fluorine doped indium tin oxide (FTO) substrate. The CdSe-TiO2 layer can be employed to fabricate a quantum dot sensitized solar cells (QDSSC), increased contact between CdSe and TiO2 nanoparticles and leading to improved efficiency of the solar cells. A colloidal suspension of TOPO capped CdSe nanoparticles was prepared by the hot injection method, followed with ligand exchange in order to produce MUA capped CdSe nanoparticles. CdSe particle of diameter in the range of 2.44 nm to 3.26 nm were to be used in this research. The TiO2 nanoparticles were prepared by hydrolysis of titanium isopropoxide in water and produced particles size of 4.66 nm. Both nanoparticles were suspended in ethanolic medium. Electrophoretic deposition parameters were optimized. The results show that an applied voltage of 5 V, was suitable to be used to deposit single layer of MUA capped CdSe, TiO2 nanoparticles and the mixture of MUA capped CdSe-TiO2 nanoparticles. Smooth, uniform and dense layer were produced under this applied voltage. EPD also allows deposition of multilayer structures, in this research two layer structures of MUA capped CdSe on electrophoretically deposited TiO2 on FTO and mixed MUA capped CdSe-TiO2 on electrophoretically deposited TiO2 on FTO were formed. Three layer structures of MUA capped CdSe/MUA capped CdSe-TiO2/ TiO2/FTO were also synthesised. The photocurrent was measured on single layer, two layer and three layers electrodes. The optimum photocurrent parameters for each single layer were studied, in order to measure the photocurrent at the best condition possible. The highest IPCE value recorder was 0.70 % on MUA capped CdSe on FTO, with the MUA capped CdSe size of 2.94 nm. The lowest IPCE, 0.011 %, was obtained from three layer structure of MUA capped CdSe/MUA capped CdSe-TiO2/ TiO2/FTO.