Experimental and Theoretical Insights of Anion Regulation in MOF-Derived Ni–Co-Based Nanosheets for Supercapacitors and Anion Exchange Membrane Water Electrolyzers

The anionic components have a significant role in regulating the electrochemical properties of mixed transition-metal (MTM)-based materials. However, the relationship between the anionic components and their inherent electrochemical properties in MTM-based materials is still unclear. Herein, we report the anion-dependent supercapacitive and oxygen evolution reaction (OER) properties of in situ grown binary Ni–Co–selenide (Se)/sulfide (S)/phosphide (P) nanosheet arrays (NAs) over nickel foam starting from MOF-derived Ni–Co layered double hydroxide precursors. Among them, the Ni–Co–Se NAs exhibited the best specific capacity (289.6 mA h g–1 at 4 mA cm–2). Furthermore, a hybrid device constructed with Ni–Co–Se NAs delivered an excellent energy density (74 W h kg–1 at 525 W kg–1) and an ultra-high power density (10 832 W kg–1 at 46 W h kg–1) with outstanding durability (∼94%) for 10 000 cycles. Meanwhile, the Ni–Co–Se NAs showed superior electrocatalytic OER outputs with the lowest overpotential (235 mV at 10 mA cm–2) and Tafel slope. In addition, Ni–Co–Se NAs outperformed IrO2 as an anode in an anion exchange membrane water electrolyzer at a high current density (>1.0 A cm–2) and exhibited a stable performance up to 48 h with a 99% Faraday efficiency. Theoretical analyses validate that the Se promotes OH adsorption and improves the electrochemical activity of the Ni–Co–Se through a strong electronic redistribution/hybridization with an active metal center due to its valence 4p and inner 3d orbital participations. This study will provide in-depth knowledge of bifunctional activities in MTM-based materials with different anionic substitutions.