Lowering the operating temperature of protonic ceramic electrochemical cells to

Protonic ceramic electrochemical cells (PCECs) can be employed for power generation and sustainable hydrogen production. Lowering the PCEC operating temperature can facilitate its scale-up and commercialization. However, achieving high energy efficiency and long-term durability at low operating temperatures is a long-standing challenge. Here, we report a simple and scalable approach for fabricating ultrathin, chemically homogeneous, and robust proton-conducting electrolytes and demonstrate an in situ formed composite positive electrode, Ba0.62Sr0.38CoO3−δ–Pr1.44Ba0.11Sr0.45Co1.32Fe0.68O6−δ, which significantly reduces ohmic resistance, positive electrode–electrolyte contact resistance and electrode polarization resistance. The PCECs attain high power densities in fuel-cell mode (~0.75 W cm−2 at 450 °C and ~0.10 W cm−2 at 275 °C) and exceptional current densities in steam electrolysis mode (−1.28 A cm−2 at 1.4 V and 450 °C). At 600 °C, the PCECs achieve a power density of ~2 W cm−2. Additionally, we demonstrate the direct utilization of methane and ammonia for power generation at