2D metal–organic-framework array-derived hierarchical network architecture of cobalt oxide flakes with tunable oxygen vacancies towards efficient oxygen evolution reaction

The development of highly active dual-functional electrocatalysts, especially for oxygen evolution reaction (OER), is highly desirable to electrocatalyze water splitting for hydrogen production. Herein, a cobalt metal-organic frameworks (Co-MOFs) array is employed as the platform to fabricate oxygen-defect-rich Co3O4 flakes that are vertical grown on nickel foam substrates. This MOFs-derived Co3O4 arrays show a hierarchical interconnected porous flake network structure with tunable oxygen vacancies. The unique structural features obtained from an optimal hydrogenation condition render outstanding catalytic performance toward the oxygen evolution in alkaline media (an ultralow overpotential of 205 mV at 10 mA cm−2; a small Tafel slope of 65.3 mV dec−1 as well as high stability). This OER performance is among the best non-noble metal catalysts reported to date. Meanwhile, this defect-rich Co3O4 array electrode is also efficient for catalyzing hydrogen evolution in the same basic solution (overpotential of ∼108 mV at 10 mA cm−2). The electrolyzer for overall water splitting can deliver a current density of 100 mA cm−2 at a cell voltage as low as 1.84 V. Density functional theory calculation reveals that the enhanced OER performance mainly arises from the oxygen vacancies and consequently the lowered activation energy as well as improved electrical conductivity.