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

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.