ABSTRACT Reversible protonic ceramic cells (R‐PCCs) enable efficient and reversible steam‐to‐hydrogen conversion. However, their air electrodes rely on proton‐mediated reactions in both electrolysis and fuel cell modes, requiring intrinsically hydrophilic oxides to incorporate water as a proton source. Conventional air electrodes suffer from poor water uptake under low water partial pressure ( p H 2 O), necessitating concentrated steam that compromises both system stability and energy efficiency. Here, we develop a BaCo 0.45 Fe 0.45 In 0.1 O 3‐δ air electrode for R‐PCCs through rational screening of BaFeO 3 ‐based oxides. This material exhibits an unprecedented proton concentration of 10.5 mol% at p H 2 O = 0.02 atm at low temperatures. Notably, steam exposure induces in situ exsolution of BaCoO 3‐δ nanocatalysts, enhancing electrochemical activity. The steam‐driven reconstruction process is systematically elucidated using depth‐profiling characterizations combined with DFT calculations. Benefiting from its super‐hydrophilic property and optimized surface electronic structure, R‐PCCs incorporating this air electrode deliver a remarkable current density of 2615 mA cm −2 at 1.3 V and a peak power density of 1.02 W cm −2 at 600°C, and maintain high Faradaic efficiency across a wide p H 2 O range of 0.006–0.47 atm. This work establishes a hydrophilicity‐driven air‐electrode design strategy for efficient hydrogen production under ultra‐low steam conditions, offering a viable pathway for deployment in freshwater‐scarce regions.
Shi et al. (2026) studied this question.