Revitalizing interface in protonic ceramic cells by acid etch

Protonic ceramic electrochemical cells hold promise for operation below 600 °C (refs. ^1,2). Although the high proton conductivity of the bulk electrolyte has been demonstrated, it cannot be fully used in electrochemical full cells because of unknown causes³. Here we show that these problems arise from poor contacts between the low-temperature processed oxygen electrode-electrolyte interface. We demonstrate that a simple acid treatment can effectively rejuvenate the high-temperature annealed electrolyte surface, resulting in reactive bonding between the oxygen electrode and the electrolyte and improved electrochemical performance and stability. This enables exceptional protonic ceramic fuel-cell performance down to 350 °C, with peak power densities of 1.6 W cm^-2 at 600 °C, 650 mW cm^-2 at 450 °C and 300 mW cm^-2 at 350 °C, as well as stable electrolysis operations with current densities above 3.9 A cm^-2 at 1.4 V and 600 °C. Our work highlights the critical role of interfacial engineering in ceramic electrochemical devices and offers new understanding and practices for sustainable energy infrastructures.