Defects evolution of Ca doped La₂NiO_4+δ and its impact on cathode performance in proton-conducting solid oxide fuel cells

As a typical Ruddlesden-Popper oxide, La₂NiO_4+δ draws special attention for its high oxygen ion conducting behavior and special interstitial oxygen defects which enables it a promising electrocatalyst toward oxygen reduction reaction. In this work, Ca-doped La₂NiO_4+δ samples are prepared and their structure and defect evolution are investigated as Ca content. Electrical conductivity and electron conduction relaxation (ECR) investigations suggest that La_1.9Ca_0.1NiO_4+δ has the great electronic conductivity and the highest oxygen surface exchange coefficient and oxygen bulk diffusion coefficient at intermediate temperatures. These results may imply that more gas oxygen has inserted into La_1.9Ca_0.1NiO_4+δ sample, suggesting the native interstitial oxygen defects in it. X-ray photon spectroscopy (XPS) results confirm that La_1.9Ca_0.1NiO_4+δ have more active oxygen species when compared with La₂NiO_4+δ and La_1.8Ca_0.2NiO_4+δ. The great oxygen exchange and bulk diffusion properties along with its great stability in steam involved atmosphere enables it a promising for cathode for H–SOFCs. Compared with that using La₂NiO_4+δ cathode, peak powder density of H–SOFC using La_1.9Ca_0.1NiO_4+δ single phase cathode improves about 30.5% at 700 °C, suggesting that accelerating oxygen reduction reaction can effectively improve cathode performance of H–SOFCs.