Entropy‐Engineered Fluorite Surfaces for Oxygen Catalysis and Humidity Tolerance in Reversible Protonic Ceramic Cells

Reversible protonic ceramic electrochemical cells (R‐PCECs) require air electrodes with fast oxygen reduction/evolution reactions (ORR/OER) and excellent durability under low‐temperature, high‐humidity operating conditions. Herein, we report a rationally designed fluorite catalyst, Pr 0.025 Nd 0.025 La 0.025 Sm 0.025 Ce 0.9 O x (PNLSC), deposited on a PrBa 0.8 Ca 0.2 Co 2 O 5+δ (PBCC) backbone for constructing a hybrid air electrode with enhanced electrocatalytic activity and structural durability. Systematic electrochemical investigations, combined with entropy‐dependent comparisons, demonstrate that the designed PNLSC coating can accelerate surface oxygen exchange kinetics, facilitate ionic transport, and suppress surface degradation in humidified atmospheres. In situ infrared spectroscopy analyses directly reveal the optimized surface hydration kinetics and suppressed hydroxyl‐related surface degradation on the PNLSC hybrid electrode, providing mechanistic evidence for its enhanced oxygen reaction activity and proton transport. Furthermore, a maximum power density of 2.10 W cm −2 and an electrolysis current density of −3.23 A cm −2 at 1.3 V are achieved for the single cells with the hybrid electrode at 700°C, accompanied by excellent durability over 200 h and reversible cycling stability for 140 h. This work establishes an entropy‐enabled fluorite surface engineering that couples accelerated electrocatalysis with intrinsic humidity resistance, offering a universal strategy for designing next‐generation air electrodes in R‐PCECs.