In situ determination of the nickel oxidation state in La2NiO4+δ and La4Ni3O10-δ using X-ray absorption near-edge structure

Ruddlesden-Popper phases of the general stoichiometry Ln(n+1)Ni(n)O(3n+1) are potential solid oxide fuel cell cathodes displaying attractive electronic and ionic conductivities. To ensure long term performance, understanding of the materials degradation mechanisms occurring during operation is vital. One of the key processes in the cathode is the oxygen reduction and incorporation reaction which involves the redox of the transition metal cation species. In order to understand these processes in situ characterisation techniques are required that probe cation oxidation state directly. In this work two cathode materials, La2NiO4+delta and La4Ni3O10-delta, have been investigated by X-ray absorption spectroscopy of the near-edge region (XANES) of the Ni K-edge at room temperature and 650 degrees C. From these data it has been determined that a polynomial relationship between Ni oxidation state and edge position exists. Further the Ni oxidation state, and hence oxygen non-stoichiometry, was found to reduce on heating under static air for both La2NiO4+delta and La4Ni3O10-delta, implying that ionic conductivity in these materials is significantly affected by Ni valence. This is correlated with previous modelling studies of the defect chemistry of La2NiO4+delta to confirm the nature of the charge compensation mechanism and by extension the mobile species.