Synthesis, Structural and Phase Characterisation of proton conducting Oxides
Gaita, Samuel Mwaniki
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Generation of energy and supply to meet the ever-increasing demand without damaging the environmental have contributed to research motivation of fuel cells. This research is an effort in searching for efficient and environmentally friendly alternative energy sources to replace burning of fossil fuels; which has adverse implications in human health and climate change. The current technology of solid electrolytes used in fuel cells allows operation at lower temperature, < 200 0C, and temperatures > 700 0C. The former requires expensive platinum catalysts and the latter needs considerable thermal insulation and longer start-up times. The present challenge is therefore to identify and synthesize electrolytes, which can efficiently operate at intermediate temperatures. Proton conductors composed of perovskite oxides doped with trivalent cations such as yttrium, have shown promising conductivity results in this region. However, the effect of doping levels as well as macro and micro-structural influences on proton mobility is not fully understood. The aim of this study was to use solgel and solid state reaction processes to synthesize alkaline-earths zirconates doped with yttrium at varying percentage levels. It involved characterization of their phase purity, crystal structures, lattice parameters and proton conductivity at different to doping levels. The synthesised samples were analysed using powder X-ray diffraction, electron microscopy and impedance spectroscopy. In addition, complementary techniques such as thermal gravimetric analysis and scanning electron microscopy were to be used. Synthesis of yttrium doped barium zirconate series, BaZr1-xYxO3-ı (BZYO), was possible by use of modified Pechini process up to 50 % doping level (x = 0.5). However, synthesis of yttrium doped strontium and calcium zirconates above 5 % doping level was not possible. Density of the sintered BZYO pellets was 60 % to 74 % of the theoretical density. Rietveld analysis of BZYO was used to prove unit cell expansion with increase in yttrium doping level thus confirming that yttrium was being incorporated into the BZYO structure. The cell parameter of the 20 % Y doped sample however showed an unexpected contraction.