Phase segregation of a composite air electrode unlocks the high performance of reversible protonic ceramic electrochemical cells

One breakthrough in developing highly efficient air electrodes for reversible protonic ceramic electrochemical cells (R-PCECs) is optimizing the sluggish oxygen reduction and water oxidation reactions. Here, we present a novel composite material with a nominal formula of high-entropy Ce0.2Ba0.2Sr0.2La0.2Ca0.2CoO3-delta (CBSLCC) that spontaneously self-assembles to three-phase electrocatalysts composed of deficient Ce0.2-yBa0.2Sr0.2-xLa0.2-xCa0.2CoO3-delta (CD-CBSLCC), CeO2, and La0.5Sr0.5CoO3-delta (LSC). Mechanistic studies corroborate that oxygen reduction may occur on entire air electrode surfaces, followed by water formation preferentially at or near CD-CBSLCC. The CeO2 phase could provide or consume protons to facilitate the oxygen evolution/reduction kinetics in R-PCECs. The developed electrodes demonstrate a record-high electrochemical performance in dual modes of fuel cells and electrolysis cells, delivering a peak power density of 1.66 W cm-2 at 600 degrees C and a current density of -1.76 A cm-2 at 1.3 V and 600 degrees C. Excellent operational stabilities of the fuel cell (200 h at 600 degrees C), electrolysis cell (200 h at 600 degrees C), and reversible cycling (548 h at 550 degrees C) provide a promising and reliable step towards realizing the commercialization of R-PCECs. We develop a rational-designed composite perovskite-based air electrode through strategies of high-entropy engineering and self-assembly, demonstrating an exceptional oxygen reduction/evolution reaction activity and durability for reversible protonic ceramic electrochemical cells.