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.
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