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.