The Regulation Mechanism of Oxygen Vacancies in Ruddlesden–Popper Perovskite Ln₂NiO₄ (Ln = La, Pr, Nd) Air Electrode for Reversible Protonic Solid Oxide Cells

Reversible protonic solid oxide cells (R-PSOCs) are promising green energy storage devices for efficient hydrogen/electricity conversion. Due to the complex environment of the air electrode, the microscopic influence mechanism of oxygen vacancies in perovskites on oxygen reduction reaction (ORR) and oxygen evolution reaction (OER) is unclear. In this study, the layered Ruddlesden-Popper perovskite Ln₂NiO₄ (Ln = La, Pr, Nd) air electrodes are constructed to investigate the effect of oxygen vacancies on the water/oxygen coupling in dual mode. The Pr₂NiO_4+δ full cell exhibits the highest peak power density of 0.692 W cm^-2 in fuel cell mode and a maximum current density of -1.2 A cm^-2 in electrolysis cell mode at 700 °C. The changes in electrochemical impedance spectroscopy show that Pr₂NiO_4+δ can absorb a small amount of interfacial water in SOFC mode to promote triple-conductivity. Meanwhile, it can have good electrolytic performance in an atmosphere of 10% H₂O in the SOEC mode. The enriched oxygen vacancies of Pr₂NiO_4+δ can provide a broad platform for both the ORR and OER, while the appropriate hydrophilicity can achieve a better balance state by the competitive adsorption of water/oxygen. These comprehensive characteristics make Pr₂NiO_4+δ suitable to be a potential Ruddlesden-Popper perovskite air electrode material for RSOCs.