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 Ce_0.2Ba_0.2Sr_0.2La_0.2Ca_0.2CoO_3−δ (CBSLCC) that spontaneously self-assembles to three-phase electrocatalysts composed of deficient Ce_0.2−yBa_0.2Sr_0.2−xLa_0.2−xCa_0.2CoO_3−δ (CD-CBSLCC), CeO₂, and La_0.5Sr_0.5CoO_3−δ (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 °C and a current density of −1.76 A cm⁻² at 1.3 V and 600 °C. Excellent operational stabilities of the fuel cell (200 h at 600 °C), electrolysis cell (200 h at 600 °C), and reversible cycling (548 h at 550 °C) provide a promising and reliable step towards realizing the commercialization of R-PCECs.