Solid oxide fuel cells are an attractive technology for energy generation, due to fuel flexibility and high efficiency in fuel conversion, which can be greater than 80% in combined heat and power systems. Due to the high costs and performance degradation associated with high temperature SOFCs, further research is required to reduce the operating. In intermediate temperature solid oxide fuel cells (600 – 1000 K), due to the poor ionic conductivity of the cathode, the oxygen reduction reaction is often limited to the interface between the cathode, electrolyte, and the air (the three-phase boundary). The region in which the oxygen reduction reaction can take place, mixed ionic and electronic conductors can be used as cathode materials. Ruddlesden-Popper materials, An+1BnO3n+1, have shown promise as mixed ionic and electronic conducting cathode materials for intermediate temperature solid oxide fuel cells. They consist of n ABO3 perovskite layers, which can accommodate oxygen vacancies, separated by AO rocksalt layers, which can accommodate oxygen interstitials, providing improved ionic conductivity. The main focus has been on the n = 1 series, which possess K2NiF4-based structures. La2NiO4-based materials are of interest due to their good conductivity in the required temperature range, and compatibility with ceria-based electrolytes. The structure and electronic properties of orthorhombic La2NiO4 have been calculated using PBEsol + U. The formation of oxygen vacancies and interstitials, and their dependence on temperature, oxygen partial pressure and chemical potential, in bulk La2NiO4 and at its low index surfaces has been examined. Formation energies of the defects have been calculated under solid oxide fuel cell operating conditions, with oxygen interstitial formation found to be significantly lower in energy than oxygen vacancy formation. Nudged elastic band calculations have been carried out to examine the mechanism of oxygen diffusion within the LaO layers and to determine the likelihood of diffusion across the LaNiO3 layers.

Modelling La2NiO4 for solid oxide fuel cell cathode applications

253rd National Meeting (2017)

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