Key role of e_g^* band broadening in nickel-based oxyhydroxides on coupled oxygen evolution mechanism

A coupled oxygen evolution mechanism (COM) during oxygen evolution reaction (OER) has been reported in nickel oxyhydroxides (NiOOH)-based materials by realizing e_g^* band (3d electron states with e_g symmetry) broadening and light irradiation. However, the link between the e_g^* band broadening extent and COM-based OER activities remains unclear. Here, Ni_1-xFe_xOOH (x = 0, 0.05, 0,2) are prepared to investigate the underlying mechanism governing COM-based activities. It is revealed that in low potential region, realizing stronger e_g^* band broadening could facilitate the ^*OH deprotonation. Meanwhile, in high potential region where the photon utilization is the rate-determining step, a stronger e_g^* band broadening would widen the non-overlapping region between d_z² and a_1g^* orbitals, thereby enhancing photon utilization efficiency. Consequently, a stronger e_g^* band broadening could effectuate more efficient OER activities. Moreover, we demonstrate the universality of this concept by extending it to reconstruction-derived X-NiOOH (X = NiS₂, NiSe₂, Ni₄P₅) with varying extent of e_g^* band broadening. Such an understanding of the COM would provide valuable guidance for the future development of highly efficient OER electrocatalysts.