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Gas hydrates are solid crystalline mixtures of water and small gas molecules, such as those found in natural gas. They are stable at temperatures and pressures commonly found on the seafloor and in sub-sea pipelines for the transportation of natural gas. It is the propensity of gas hydrates to cause blockages in pipelines that necessitates the addition of inhibitors to prevent hydrate formation. Recent work has focused on developing 'low dosage hydrate inhibitors', LDHls, which act to delay nucleation or prevent growth while present at low concentrations, but identifying new chemicals to provide more active LDHls has been hindered by the absence of a clear molecularlevel understanding of their activity. Molecular Dynamics simulations designed to shed light on the molecular-level processes of gas hydrate nucleation and inhibition are presented in this thesis. Nucleation of a model methane hydrate has been simulated with and without the presence of known LDHls. Nucleation in the absence of any additives has been investigated by a range of methods, including 'local phase assignment'. Hydrate has been shown to form a dynamic cage structure with elements of hydrate structures I and II. It is proposed that the stable bulk hydrate structure arises out of this dynamic structure. Crystal growth has been calculated to be favourable for clusters in excess of 200 hydrate-like water molecules in size, thus providing an estimate for the critical cluster size in these systems. In a system including the kinetic inhibitor polydimethylaminoethylmethacrylate (PDMAEMA) hydrate growth from a seed crystal of 225 water molecules was accelerated compared to the uninhibited system. Repeated simulations and statistical analysis showed the magnitude of the promotion effect to be greater with increased levels of PDMAEMA immersion. Binding of the PDMAEMA to the surface of the hydrate was seen, which is in agreement with current theories of inhibitor activity. The simulations and protocol have also been extended in systems containing other kinetic inhibitors.

Solid-oxide fuel cells (SOFCs) are energy conversion devices that theoretically have the capability of producing electrical- energy for as long as the fuel and oxidant are supplied to the electrodes and perfonnance is expected for at least 40,000 hours. However, it is observed that perfonnance degrades under repeated thennal cycling conditions, which limits the practicaI.operating life of these SOFCs. Therefore, the mechanism of damage to planar and integrated planar SOFCs (IPt' SOFCs) on thennal cycling is the subject of this thesis. A detailed literature review has been carried out and a mechanical and thennal properties database of the key materials used in these SOFCs has been built up. Extensive work has been done on the residual ~tress analysis of anode-supported and inert substrate supported SOFCs. Analytical model, surface profile measurement (Talysurf) and XRD stress analysis were used to detennine t4e residual stresses in the components. From this study, it was found that the difference of thennal expansion coefficients between components in the SOFCs is the dominant source of stress during thennal cycling in the absence of significant temperature gradient. For the integrated planar SOFCs, it was found tha~ the cells degraded due to the failure of the sealing materials during cooling. For anode supported planar SOFCs, the electrolyte (YSZ) is under high compressive stress when cooling from sintering or operating temperature to room temperature and the anode is under very small tensile stress. The results from theoretical analysis, XRD stress measurement and literature were compared and found that they agreed with each other quite well.

The research aims to explore the impact of sulphur on the Ni-based anodes used in solid oxide fuel cells (SOFCs). The work combines thermodynamic calculation, electrochemical measurements, anode microstructure analysis, and in-situ Raman techniques, to explore the interaction of hydrogen sulphide with nickel based anodes under SOFC operating conditions. Thermodynamic calculations have been made to predict the stability of SOFC·anode materials (Ni, Ceria, Zirconia) when exposed to hydrogen sulphide (H2S) in hydrogen/steam mixtures over a range of partial pressures of sulphur (pS2) and oxygen (p02) representative of fuel cell operating conditions. Measurements on a single fuel cells and anode half cells have been carried· out to study the effect of operating conditions (pH2S, pH2 • pH20, temperature) on the degree and nature of sulphur interaction with the anodes, correlating this with the thermodynamic predictions and microstructural analysis. Microstructural analysis used scanning electron microscopy on anode cermets, supported by work on Ni pellets, to explore anode surface structure alteration under the same test conditions as those used for electrochemical measurement. This allowed changes in anode microstructure induced by sulphur to be coupled to changes in anode electrochemical performance. Both ex-situ and in-situ Raman spectroscopy was also used to detect chemical species formed on the anode surface when exposed to sulphur. The work shows a correlation between electrochemical response and thermodynamic calculation. Nickel and ceria show differing behaviour depending on pH2S, pH2, pH20, or temperature. The impedance response of Ni anodes in hydrogen SUlphide atmospheres also shows a link with anode microstructure. Electrodes with relatively lower initial performance degraded at lower H2S concentrations than those with higher initial performance suggesting that the detrimental effect of sulphur on the anode is dominated by its interaction with three phase boundaries. Anode surface alteration induced by sulphur such as 'faceting' on Ni, and Ni agglomeration, was also observed and correlated with the impedance response. Raman spectroscopy offers promise as a probe to monitor surface electrolyte temperature as well as sulphur species on the nickel species.

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