Efficient and Stable In Situ Self‐Assembled Air Electrodes for Reversible Protonic Ceramic Electrochemical Cells

Abstract Reversible protonic ceramic electrochemical cells (R‐PCECs) have garnered significant attention owing to their proficiency in efficiently converting and storing energy. The performance of R‐PCECs is largely limited by the activity and durability of oxygen reduction/evolution reactions in air electrodes. Herein, an Nb and Y doped cobalt‐based double perovskite of PrBaCo 1.8 Nb 0.1 Y 0.1 O 5+δ (denoted as PBCNY) is reported. The perovskite is however in situ assembled into a deficient‐PrBa 1−x Co 1.8 Nb 0.1−x Y 0.1−x O 5+δ parental phase and a Ba 2 YNbO 6 secondary phase during operation, accordingly demonstrating a low area‐specific resistance of 0.24 Ω cm 2 at 600 °C. The high activity is attributed to the enhancement in oxygen vacancy concentration, oxygen surface exchange, and bulk diffusion coefficient, confirmed by X‐ray photoelectron spectroscopy and electrical conductivity relaxation. At 700 °C, when the PBCNY cathode is applied as air electrodes for R‐PCECs, a peak power density of 1.99 W cm −2 and a current density of −5.20 A cm −2 (at 1.3 V) are achieved in fuel cell (FC) mode, and electrolysis (EL) mode with appropriate Faradaic efficiencies. Furthermore, R‐PCECs with PBCNY air electrodes display promising operational stability in FC mode (over 100 h), EL mode (over 200 h), and reversible cyclic testing (29 cycles for 120 h).