Transition of Ion Diffusion Mechanism in BaZr _0.1 Ce _0.7 Y _0.1 Yb _0.1 O _3‐δ Electrolyte Under Real Operating Conditions

Protonic ceramic fuel cells (PCFCs) are promising electrochemical power generation devices, yet the ion diffusion behavior within their electrolyte bulk under real operating conditions remains poorly understood, hindering further development from both materials design and operation parameters optimization. This study tackles this issue of the benchmark protonic BaZr_0.1Ce_0.7Y_0.1Yb_0.1O_3-δ (BZCYYb) electrolyte using a combined tool of electrochemical impedance spectroscopy (EIS), single cell testing under varying conditions, H₂O-temperature-programmed desorption coupled with mass spectrometry, time-of-flight secondary ion mass spectrometry characterization, and theoretical calculations. Before the hydration, EIS test confirms BZCYYb is an excellent oxygen-ion conductor at intermediate temperatures. Upon exposure to humidified air, it transitions to a mixed proton and oxygen-ion conductor due to water uptake. Under dry hydrogen atmosphere, protonation proceeds via a newly identified mechanism, hydrogenation of oxygen at grain boundaries, along with hydration from in situ water generation at the cathode during polarization, eliminating the need for pre-humidified fuel gas when operating on hydrogen. At temperatures above 600°C, dehydration dominates, even in humidified conditions, further shifting the electrolyte to a mixed proton and oxygen-ion conductor. These findings offer critical insights for the ion diffusion in protonic perovskites and facilitate the rational design of next-generation PCFCs.