Experimental Identification of Oxygen Dissociation Bottleneck in Proton-Conducting Ceramic Fuel Cells

Proton-conducting ceramic fuel cells (PCFCs) enable efficient electrochemical energy conversion, yet further performance improvements are limited by sluggish cathodic oxygen reduction kinetics and the lack of direct identification of the rate-determining step under operating conditions. Here, we experimentally identified that oxygen dissociation from superoxide intermediates (O22−) limits cathodic polarization in PCFC devices with a PrBa0.5Sr0.5Co1.5Fe0.5O5+δ (PBSCF) cathode. Modulation-excitation infrared spectroscopy directly observes that superoxide is the major intermediate for the oxygen reduction reaction. We also isolate the intrinsic cathodic kinetics of PBSCF using hydrogen–deuterium isotope effects combined with impedance spectroscopy on working PCFC devices, which achieves peak power densities of 517-1077 mWcm−2 at 550-650 °C under a fuel-oxidant combination of H2-air. The cathodic polarization resistance exhibits an oxygen partial pressure dependence with characteristic exponents above 0.5 and shows no measurable dependence on water vapor pressure. This kinetic signature is uniquely consistent with oxygen dissociation via O⁻ formation from O22− intermediates, assisted by oxygen vacancies at the cathode–gas interface. These results provide experimental evidence that oxygen dissociation constitutes the dominant kinetic bottleneck in the PCFC device, establishing a clear design principle: strong electron-donating character coupled with a high density of surface oxygen vacancies is essential to accelerate cathodic kinetics.