Interface Engineering to Operate Reversible Protonic Ceramic Electrochemical Cells Below 500 °C

The low‐temperature (<500 °C) operation of reversible protonic ceramic electrochemical cells (PCECs) is desirable in achieving efficient and sustainable electricity generation, as well as green hydrogen production. However, significant interfacial resistance, which contributes to both ohmic and polarization resistance, remains a hurdle in lowering the operating temperature. In this study, PrBa 0.5 Sr 0.5 Co 1.5 Fe 0.5 O 5+δ (PBSCF) and BaZr 0.4 Ce 0.4 Y 0.1 Yb 0.1 O 3‐δ (BZCYYb) mono‐grain composite interlayers are introduced, which significantly extend the electrode/electrolyte interface and increase the concentration of vertically aligned oxygen vacancies along the heterointerface. This unique design achieves the lowest ohmic and polarization resistances among previously reported values in solid electrolyte‐based electrochemical cells. As a result, the PCEC can operate at extremely low temperature of 350 °C with an exceptional peak power density of 0.50 W cm −2 in fuel cell mode and current density of 0.25 A cm −2 at 1.3 V in electrolysis cell mode. Furthermore, it demonstrates high energy conversion efficiency and excellent stability under static and dynamic operating conditions.