Indium-doped BaHfO3 electrolytes with oxygen vacancies engineering for high proton conductivity and low-temperature solid oxide fuel cells

The development of electrolyte materials with high ionic conductivity at low operating temperatures (400–600 °C) remains a critical challenge for solid oxide fuel cells (SOFCs). Proton-conducting ceramics have emerged as promising candidates due to their low activation energy for proton transport, making them suitable for low-temperature SOFC applications. In this study, a series of BaHf_1-xIn_xO_3-δ (x = 0, 0.05, 0.10, 0.15, and 0.20) ceramics were synthesized via solid-state reaction. The dominant proton conductivity, accompanied by oxygen ion conduction, was confirmed through defect equilibrium model, concentration cell studies, and H/D isotope experiments. X-ray photoelectron spectroscopy (XPS) revealed that indium doping significantly enhances oxygen vacancy concentration, thereby facilitating proton conduction. Compared to undoped BaHfO₃, indium doping markedly increased ionic conductivity and reduced the activation energy for grain boundary conduction. The BaHf_0.85In_0.15O_2.925 (BHI15) electrolyte exhibited a low (ASR) area-specific ohmic resistance of 0.20 Ω cm² and achieved an exceptional power output of 956 mW cm⁻² at 550 °C. These findings underscore the effectiveness of indium doping in enhancing proton conductivity, offering a viable strategy for designing high-performance electrolytes for low-temperature SOFCs.