Ruddlesden–Popper-Type Oxide La₂NiO_4+δ with Oxygen Interstitials as Efficient Electrode for Proton Ceramic Electrolysis Cells

Water electrolysis using proton ceramic electrolysis cells (PCECs) is considered as an effective technique for green hydrogen production. While Ruddlesden–Popper (RP)-type oxides have drawn great interest owing to their interesting oxygen defect chemistry, their applications in PCECs have not yet been thoroughly investigated. In this work, we synthesize RP-type oxide La2NiO4+δ (LNO) and the well-studied PCEC electrode PrBa0.5Sr0.5Co1.5Fe0.5O6–δ (PBSCF) as the model materials and systematically compare their oxygen defect chemistry and water electrolysis performance. Rietveld refinement of X-ray diffraction patterns and X-ray absorption spectroscopy measurements confirm that LNO loses oxygen in the form of oxygen interstitials upon thermal reduction, leading to a lattice expansion along the ab plane (the perovskite and rock salt layers) and a shrinkage along the c direction (the direction perpendicular to those layers). By contrast, PBSCF loses oxygen from lattice sites and forms oxygen vacancies, showing lattice expansion along all three directions. The electrochemical measurements indicate that the cells with LNO as oxygen electrodes exhibit outstanding water electrolysis performance, with a maximum Faradaic efficiency of 99.01%, noticeably surpassing 77.48% obtained for PBSCF. Our results highlight the potential of oxides with oxygen interstitials as a highly effective oxygen electrode material for PCECs.