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 = E j10, OER – E jhalf, 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.

The microscopic homogeneity of mixed metals in a single-phase oxide is a critical issue in improving material performance. Aqueous alkaline precipitation is the most common approach but it has the limits of microscopic inhomogeneity because of intrinsically different precipitation rates between metal cations. Herein, we demonstrate a new preparation of uniformly structural substituted cobalt iron oxides via acidic redox-assisted precipitation (ARP) upon the interaction of CoII and K2FeO4. This low-pH synthesis features the redox process between Co and Fe, presumably through the formation of inner-sphere complexes such as [(H2O)5CoII–O–FeVIO3]. With the nucleation starting from such complexes, one obtains a product with predominantly mixed-metal Co–O–Fe moieties, which improves the electrical conductivity of the product. This work further analyzes how the properties of the product species evolve during the hydrothermal synthesis step in the ARP process. We see that the Co/Fe ratio slowly increases from about 1:1 to a final value of 2:1, but does not reach the expected redox stoichiometry of 3:1. At the same time, the magnetization also increases, reaching a value of 16.9 emu g–1 for the final superparamagnetic product, which is three times higher than the value of monometallic Co3O4 and Fe2O3. The cobalt iron oxide samples obtained from ARP also possess superior oxygen evolution activity (307 mV overpotential at 10 mA cm–2 μg–1) compared to a mixture of Co3O4 and Fe2O3 (422 mV) or pure cobalt oxide (350 mV), highlighting the structure-induced enhancement of the catalytic activity. The difficult synthesis of evenly blended trinary/quaternary metals in a single-oxide phase may become possible in the future via ARP.