Challenges imposed by thermochemical expansion of solid state electrochemical materials

Abstract

Thermal expansion compatibility is usually considered a sine qua non condition for selection of prospective materials for SOFC and other solid state electrochemical systems due to fragile behaviour of ceramic components, and their limited ability to withstand significant strain. However, chemical expansion may also add non-negligible strain contributions on materials with variable oxygen stoichiometry. Thus, one measured the chemical expansion of representative materials and compiled information on a variety of other electronic or mixed conductors proposed for electrode, membrane or interconnector applications. Selected materials were used to identify trends and guidelines for effects of composition and for structural effects. Dependence on working conditions was focused on temperature and oxygen partial pressure. It was also extended to predict effects exerted by overpotential or changes in gas composition, and dependence on fuel conversion when membrane materials are exposed to high chemical potential gradients under fuel/membrane/air conditions. Redox conditions were predicted by thermodynamic analysis of fuel conversion, for hydrogen and methane-based fuels. These thermodynamic calculations were combined with thermochemical expansion data for representative materials, to predict strain induced on: (1) solid electrolytes or mixed conducting membranes exposed to high chemical potential gradients, (2) cathode materials under combined effects of changes from processing to operation temperatures and polarisation, (3) anode materials on changing from high temperature processing in air to reduce fuel atmospheres. Differences between thermochemical effects on anode supported configurations and electrolyte supported cells were analysed, including lab scale conditions based on thick solid electrolyte cells with thin electrodes. © Springer-Verlag London 2013.