Synergizing Proton Transfer and Oxygen Activation via Brønsted/Lewis Acid‐Site Regulation in K‐Doped La 0.5 Sr 0.5‐x K x Mn 0.9 Co 0.1 O 3‐δ Air Electrode for Reversible Protonic Ceramic Electrochemical Cells

Efficient operation of reversible protonic ceramic electrochemical cells (R‐PCECs) requires rapid oxygen‐electrode kinetics in both fuel cell and electrolysis modes, which depend on the concerted coupling of oxygen activation and proton transfer at the surface of air electrode. Conventional compositional and defect regulation can enhance oxygen vacancies, electronic transport, and oxygen exchange, but rarely considers surface acid chemistry as a means of coordinating proton transfer with oxygen activation. Here, A‐site K substitution within a Co‐modified La0.5Sr 0.5‐x K x Mn 0.9 Co 0.1 O 3‐δ framework is used to bridge this gap by regulating Brønsted and Lewis acid sites responsible for proton transfer and oxygen activation, respectively. Coupled with modified metal–oxygen bonding and enhanced oxygen‐defect chemistry, this coordinated surface environment facilitates proton‐coupled‐electron‐transfer (PCET) processes and accelerates reversible reaction kinetics. Accordingly, the optimized electrode delivers a polarization resistance of 0.049 Ω cm 2 and a peak power density of 1.436 W cm −2 at 700°C, and reaches −2.19 A cm −2 at 1.3 V and 650°C in electrolysis mode under 20% H 2 O. These findings identify Brønsted‐Lewis acid‐site synergy as a useful basis for designing high‐performance triple‐conducting perovskite air electrodes for R‐PCEC devices.