Delamination of NiFe layered double hydroxides into perforated monolayers for efficient water splitting

The introduction of vacancies can significantly change the coordination and valence states of the catalytic active sites, thereby modulating the electronic structure to promote the oxygen evolution reaction (OER). However, atomic-level vacancy engineering on low-dimensional layered double hydroxides (LDHs) has not been achieved, which could be due to the significant structural damage and/or carbonate (CO₃^2-) contamination occurring during the vacancy creating process. In this study, atomic-scale cation vacancies were generated in LDHs without apparent structure damage and carbonate contamination. Perforated monolayer nanosheets with an utmost exposure of active sites were successfully obtained through a subsequent exfoliation in formamide. Compared to bulk LDHs, the flocculated vacancy-containing nanosheets exhibit a small overpotential of 245 mV at a current density of 10 mA cm^-2 and maintain excellent stability at a high current density of 500 mA cm^-2. Density functional theory (DFT) calculations indicate that introducing cation vacancies on monolayer NiFe-LDH nanosheets and creating unsaturated Ni-Fe sites can effectively reduce the Gibbs free energy of the OER process. The two-electrode electrolyzer assembled with commercial Pt/C for overall water splitting can operate at a cell voltage as low as 1.50 V to yield a current density of 10 mA cm^-2. It also demonstrates long-term stability of 50 h at a large current density of 500 mA cm^-2. The current strategy of atomic cation vacancy engineering on monolayer LDHs provides important insights into the design of low-cost LDH-based catalysts toward efficient alkaline water electrolysis and other energy-related applications.