Strategic Design of “Three‐in‐One” Cathode Toward Optimal Performance of Proton‐Conducting Solid Oxide Fuel Cell: The Temperature Matters

ABSTRACT “Three‐in‐one” cathode, achieved via B‐site heavy‐doping of transition elements (typically Co, Fe) into proton‐conductive perovskite, holds promise for enhancing the performance of proton‐conducting solid oxide fuel cell (H‐SOFC) operated below 650°C for electricity generation. However, its electrochemical behavior above 650°C, essential for improving the efficiency of H‐SOFC for fuel conversion, remains insufficiently explored. It is still challenging to propose guidance for the design of “three‐in‐one” cathode toward optimal H‐SOFC performance below and above 650°C, with the prerequisite of gaining a comprehensive understanding of the roles of Co and Fe in determining the H‐SOFC performance. This work is to address this challenge. Through theoretical/experimental studies, Co is identified to play a role in improving the oxygen reduction reaction (ORR) activity while Fe plays a role in facilitating the cathode/electrolyte interfacial proton conduction. Therefore, if the operating temperature is above 650°C, lowering the Co/Fe ratio in “three‐in‐one” cathode becomes crucial since the limiting factor shifts from ORR activity to proton conduction. Implementing this strategy, the SOFC using BaCo 0.15 Fe 0.55 Zr 0.1 Y 0.1 Yb 0.1 O 3− δ cathode achieves peak power densities of 1.67 W cm −2 under H‐SOFC mode at 700°C and 2.32 W cm −2 under dual ion‐conducting SOFC mode at 750°C, which are the highest reported values so far.