Rock Salt Oxide Hollow Spheres Achieving Durable Performance in Bifunctional Oxygen Energy Cells

The interchangeable operation of alkaline oxygen evolution and reduction using bifunctional electrocatalysts in devices consolidates the commercialization milestone of energy storage in hydrogen, and yet, it is usually limited by issues of carbon corrosion in electrocatalysts and inhomogeneous electrode fabrication. Here, we demonstrate a synthetic route toward carbon-free ternary rock salt oxide (i.e., NiO/CoO/FeO) hollow spheres with silver decoration on the surface for durable operation in bifunctional cells. These Ag-decorated ternary oxides exhibit an overall bifunctional potential gap (ΔE = Ej10, OER – Ejhalf, ORR) of 0.89 V. Operando Raman studies show that the rock salt structure shows the phenomenon of a highly reversible local environment during the charge–discharge OER cycles, a key characteristic for high durability in bifunctional devices. At equivalent content between Ni and Co, NiOOH is the main OER-active species with CoOOH and/or CoO2 as the cocatalyst, where the presence of the FeO dopant facilitates structural activation and reversibility. With the proof-of-concept performance in anion-exchange membrane (AEM) devices, the catalysts achieve a durable cyclic operation with a high current density (1000 mA cm–2) at smaller potentials (2.03 V) than RuO2 (2.16 V) under the electrolyzer mode, while it can yield two times greater power density (96.98 mW cm–2) than Pt/C (53.58 mW cm–2) in the fuel cell mode.