In Situ Construction of a High‐Entropy Perovskite Based Tri‐Phase Composite Electrode toward Efficient Reversible Solid Oxide Cells

Reversible solid oxide cells (R‐SOCs) are promising for energy applications but face limitations due to poor durability and slow oxygen‐reduction/evolution reactions at air electrodes. Here, a high‐entropy perovskite‐based (HEP) tri‐phase composite, (La 0.2 Sr 0.2 Pr 0.2 Ba 0.2 Ce 0.2 ) x CoO 3‐δ , comprising an A‐site deficient La 0.2 Sr 0.2 Pr 0.2 Ba 0.2 Ce 0.2 CoO 3‐δ , doped‐CeO 2 , and Co 3 O 4 phases are presented. The HEP phase provides catalytic sites and robust frameworks, the doped‐CeO 2 phase enhances oxygen‐ion transport; and the Co 3 O 4 nanoparticles offer additional active sites. The optimized (La 0.2 Sr 0.2 Pr 0.2 Ba 0.2 Ce 0.2 ) 0.7 CoO 3‐δ electrode exhibits promising electrochemical performance: a low area‐specific resistance of 0.058 Ω cm 2 at 700 °C and enhance stability (a 2‐fold improvement in ambient air, a 6‐fold enhancement in moisture resistance, and a 3‐fold increase in Cr tolerance compare to (La 0.6 Sr 0.4 ) 0.95 Co 0.2 Fe 0.8 O 3‐𝛿 ). When applied as an air electrode for R‐SOCs, it delivers excellent performance at 800 °C: a peak power density of 1.68 W cm −2 in fuel cell (FC); a current density of 1.5 A cm −2 at 1.2 V in electrolysis cell (EC). Significantly, this work represents the first application of a high‐entropy‐oxide‐based material as an air electrode in large‐area R‐SOCs (10×10 cm 2 ). The assembled R‐SOC achieves an output of 61 W in FC and 66 A at 1.18 V in EC at 800 °C, highlighting its potential for practical applications.