Reversible protonic ceramic electrochemical cells (R‐PCECs) are emerging as ideal devices for highly efficient energy conversion (generating electricity) and storage (producing H2) at intermediate temperatures (400–700 °C). However, their commercialization is largely hindered by the development of highly efficient air electrodes for oxygen reduction and water‐splitting reactions. Here, the findings in the design of a highly active and durable air electrode are reported: high‐entropy Pr0.2Ba0.2Sr0.2La0.2Ca0.2CoO3−δ (HE‐PBSLCC), which exhibits impressive activity and stability for oxygen reduction and water‐splitting reactions, as confirmed by electrochemical characterizations and structural analysis. When used as an air electrode of R‐PCEC, the HE‐PBSLCC achieves encouraging performances in dual modes of fuel cells (FCs) and electrolysis cells (ECs) at 650 °C, demonstrating a maximum power density of 1.51 W cm−2 in FC mode, and a current density of −2.68 A cm−2 at 1.3 V in EC mode. Furthermore, the cells display good operational durabilities in FC and EC modes for over 270 and 500 h, respectively, and promising cycling durability for 70 h with reasonable Faradaic efficiencies. This study offers an effective strategy for the design of active and durable air electrodes for efficient oxygen reduction and water splitting.

Solid oxide electrochemical cells (SOECs) have demonstrated the potential to be highly efficient devices for electrochemical CO2 reduction (CO2R) at intermediate temperatures. However, the performance and widespread applications for CO2R largely hinge on the sluggish reaction kinetics and poor durability of the state‐of‐the‐art electrodes. Here, the findings in enhancing the reaction activity and durability of a perovskite‐based electrode are reported, Sr2Fe1.5Mo0.3Cu0.2O6‐δ (SF1.5MC), for electrochemical oxidation of H2 and reduction of CO2. Under typical operating conditions, the SF1.5MC electrode is elegantly reconstructed into three phases of oxygen vacancy‐rich double perovskite (DP), Ruddlesden‐popper (RP), and Cu‐Fe metals, as confirmed by X‐ray diffraction and scanning transmission electron microscopy. When applied as a fuel electrode for an electrolyte‐supported SOEC, decent performances are demonstrated at 800 °C, showing a maximum power density of 1.51 W cm−2 in fuel cell mode (on H2 fuel) and a current density of 1.94 A cm−2 at 1.4 V in electrochemical CO2R to CO with high Faradaic efficiencies of ≈100% and good durability.

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