Heterogeneous CoO/Co₃O₄ Nanostructures as Efficient Anodes for Hydrogen Generation via Acidic Water Electrolysis

The development of nonprecious metal-based catalysts for the acidic oxygen evolution reaction (OER) is essential for large-scale proton exchange membrane water electrolyzer (PEMWE) deployment. In this study, we present a straightforward approach to construct heterogeneous CoO/Co3O4 nanostructures on a carbon fiber paper substrate by simply calcining metallic Co nanoflakes in air. This heterostructure exhibits significantly enhanced OER performance compared to that of the pure Co3O4 material in acidic environments, achieving an impressively low overpotential of 396 mV at the current density of 10 mA cm–2. Moreover, the catalytic performance was well retained for up to 145 and 32 h at 10 and 100 mA cm–2 in the half-cell test, respectively, and for at least 160 h at 100 mA cm–2 in a homemade PEMWE. Detailed characterizations and theoretical calculations reveal that the heterogeneous configuration of the nanostructure plays a critical role in modulating the adsorption of oxygenated species on CoO/Co3O4 by optimizing the valence band center of the material. By this means, an effective alleviation of the overstrong binding of intermediates was achieved on the nanostructures, thereby enhancing the OER activity. Meanwhile, the presence of low-valent Co2+ species allows to alleviate the overoxidation of Co active sites, which promotes the OER stability. Post-OER measurements indicate that the dissolution of the Co element, catalyst detachment from the substrate, and electrooxidation of CoO subparts during the OER process have caused the degradation of CoO/Co3O4.