Local structure of proton conducting Y-doped A Z rO3 (A = Ba , Sr, Ca) perovski tes

Abstract

Burning of fossil fuels has disastrous implications on the environment due to emissions of nitrogen oxides, sulphur oxides and a wide range of toxic organic and inorganic compounds. We are also about to deplete the available natural energy sources and therefore research to find new, sustainable and clean energy sources are crucial for the benefit of present and future generations. Hydrogen fuel cells provide one such alternative and therefore research and development of fuel cells (Fuel cells, 2011) is important. The cells convert chemical energy directly into electrical energy and have an advantage over conventional means of power generation due to their high-energy conversion efficiency besides being environmentally friendly. The first fuel cell was demonstrated over 160 years ago but development continued inhibited by lack of attractive materials, which have high conductivity in the intermediate temperature region of 200 to 500 ◦C. This thesis is based on experimental studies carried out on perovskite structured yttrium doped barium zirconate, calcium zirconate and strontium zirconate systems. The main work included synthesis of yttrium doped barium zirconate with different Y concentrations, followed by studying the local structure of the system as a function of Y concentration and the influence of protons on the local structure surrounding the Y ions. The experiments were performed using x-ray powder diffraction (XRD), to study the average structure of the perovskite lattice, and infrared (IR) and luminescence spectroscopy to study the local structure of the material. The results indicated that increasing Y concentration leads to a slight, but clear, distortion of the local structure of the perovskite and we also found a dependence on hydration. Moreover, the results showed the symmetry around the Y atom was more distorted in the dry sample than in the hydrated. For the 20% Y doped BaZrO3 sample in particular, spectral differences could not be resolved with the techniques used, which would motivate a further study of the local ordering of the dopant atoms