High‐Entropy Perovskite Nanofibers for Bifunctional Air Electrodes in Reversible Protonic Ceramic Electrochemical Cells

Abstract Protonic ceramic electrochemical cells (PCECs) offer a promising platform for efficient energy conversion and storage at reduced temperatures (<650 °C). However, the commercial viability of PCECs remains hindered by the limited durability and sluggish oxygen electrocatalysis at the air electrode. Herein, a high‐entropy perovskite oxide (HEPO), Pr 0.2 Ba 0.2 Sr 0.2 La 0.2 Nd 0.2 CoO 3‐δ (PBSLNC), in a nanofiber architecture is presented as a bifunctional air electrode material for reversible PCEC operation. The unique A‐site compositional complexity promotes defect tolerance, abundant oxygen vacancies, rapid surface exchange kinetics while maintaining exceptional phase stability in harsh environments. Density functional theory calculations elucidate the entropically modulated Co environment that enhances hydroperoxide adsorption—a key step for oxygen electrocatalysis, thereby facilitating both oxygen reduction and evolution reactions. When deployed into a PCEC configuration, the PBSNLC nanofiber electrode achieves a peak power density of 1.4 W cm −2 at 650 °C and electrolysis current density of 3.98 A cm −2 at 650 °C, along with outstanding operational stability during continuous operation for over 1000 h. These findings provide valuable guidance for the strategic design of highly efficient and stable bifunctional air electrodes for PCECs.