Exceptionally high proton conductivity in Eu₂O₃ by proton-coupled electron transfer mechanism

Proton conductors are typically developed by doping to introduce structural defects such as oxygen vacancies to facilitate ionic transport through structural bulk conduction mechanism. In this study, we present a novel electrochemical proton injection method via an in situ fuel cell process, demonstrating proton conduction in europium oxide (Eu₂O₃) through a surficial conduction mechanism for the first time. By tuning Eu₂O₃ into a protonated form, H-Eu₂O₃, we achieved an exceptionally high proton conductivity of 0.16 S cm⁻¹. Distribution of relaxation time (DRT) analysis was employed to investigate the proton transport behavior and reveal the significant contribution of surface proton transport to the overall conductivity of Eu₂O₃. Remarkably, H-Eu₂O₃ exhibited a low activation energy for ionic transport, comparable to the best ceramic electrolytes available. The proton-coupled electron transfer (PCET) mechanism describes this novel surficial proton conduction mechanism. These findings provide new possibilities for developing advanced proton conductors with improved performance.