Highly Active Nanocomposite Air Electrode with Fast Proton Diffusion Channels via Er Doping‐Induced Phase Separation for Reversible Proton Ceramic Electrochemical Cells

Abstract Highly active and durable air electrodes are crucial for the commercialization of reversible proton ceramic electrochemical cells (R‐PCECs) for large‐scale energy conversion and storage that may be developed by introducing oxygen ion, electron, and proton triple conducting species into the electrode materials. Here, a new triple conducting nanocomposite is reported as a promising air electrode of R‐PCECs, which consists of a dominated cubic perovskite Ba_0.5Sr_0.5Co_0.72Fe_0.18Er_0.09O_3‐δ and a minor Er₂O₃ phase, developed by Er doping induced phase separation of Ba_0.5Sr_0.5(Co_0.8Fe_0.2)_0.9Er_0.1O_3‐δ precursor. The Er doping stimulates the primary perovskite phase to possess excellent hydration capability and oxygen activation ability, while the Er₂O₃ minor phase, as a high‐speed proton transport channel, further cooperates with the perovskite main phase to boost the kinetic rate of the electrode for both oxygen reduction and evolution reactions (ORR/OER). As a result, the corresponding R‐PCEC achieves extraordinary electrochemical performance in fuel cell (1.327 W cm⁻² at 650 °C) and electrolysis modes (−2.227 A cm⁻² at 1.3 V and 650 °C), which exceed the similar cell with a typical Ba_0.5Sr_0.5Co_0.8Fe_0.2O_3‐δ single‐phase perovskite air electrode by 82.3% and 122.7%, respectively. This Er‐doping induced phase separation provides a new way for new bifunctional electrodes development.