Multisite Coadsorption of the *OOH Intermediate on NiFeOOH Hierarchical Nanosheet Arrays Boost Water Electro-Oxidation at Ultrahigh Current Densities

NiFeOOH nanosheets are promising non-noble metal catalysts for the oxygen evolution reaction (OER). However, their application is hindered by a high formation energy barrier of the *OOH intermediate. Herein, NiFeOOH hierarchical (Hier-NiFeOOH) nanosheet arrays with abundant oxygen vacancies (Ov) are grown in situ on a nickel foam (NF) substrate. The presence of Ov enhances the exposure of Ni active sites, thereby promoting OH– adsorption. More importantly, the small secondary nanosheets (∼50 nm) enable extensive exposure of the hierarchical nanosheet interfaces, which provide three-dimensionally distributed adsorption sites. Such sites enable coadsorption of the *OOH intermediate via Ni–O bonds and hydrogen bonds. The multisite coadsorption greatly reduces the formation energy barrier of the *OOH intermediate during the OER process. The synergistic effect between the hierarchical nanosheet interfaces and Ov boosts the OER intrinsic activity by 9 times compared with the NiFeOOH nanosheet. Additionally, the small size of secondary nanosheets guarantees a high density of interfaces formed on the primary microsheets. Consequently, the Hier-NiFeOOH/NF electrode delivers overpotentials of 240 mV and 300 mV to achieve current densities of 100 mA cm–2 and 1 A cm–2, respectively, while maintaining excellent stability for over 1000 h at 1 A cm–2. Furthermore, a homemade water electrolyzer assembled with the Hier-NiFeOOH/NF electrode exhibits superior durability during repeated start-shutdown cycles.