Spin-regulated Ni sites with optimal d-orbital occupancy unlocking unprecedented oxygen evolution activity

Nickel-based layered double hydroxides (LDHs) are widely recognized as promising substitutes for noble metal catalysts in the oxygen evolution reaction (OER). However, conventional Ni²⁺ sites exhibit a high-spin configuration ( ${\text{d}}_{xz}^2{\text{d}}_{yz}^2{\text{d}}_{xy}^2{\text{d}}_{x^2-y^2}^1{\text{d}}_{z^2}^1$) with excessive frontier-orbital occupancy, resulting in weak binding strength toward oxygen intermediates, which dramatically limits their OER performance. Herein, we first report the successful construction of low-spin state Ni²⁺ ( ${\text{d}}_{xz}^2{\text{d}}_{yz}^2{\text{d}}_{xy}^2{\text{d}}_{x^2-y^2}^2{\text{d}}_{z^2}^0$) in NiCoFe-LDH (LS-NCF) through oxygen defect engineering. LS-NCF exhibits a splendid OER activity with an ultra-low overpotential of 241 mV at the current density of 1 A·cm⁻², which is 79 mV lower than that of the conventional NiCoFe-LDH with high-spin Ni²⁺ (HS-NCF), significantly outperforming previously reported transition metal-based catalysts. Comprehensive studies reveal that LS Ni²⁺ with reduced ${\text{d}}_{z^2}$ orbital occupancy effectively enhances oxygen intermediates adsorption through reinforcing the orbital hybridization between Ni 3d and O 2p. Moreover, the d-band center of LS Ni²⁺ is closer to the Fermi level compared to that of HS Ni²⁺, thus accelerating electron transfer. Consequently, the strengthened adsorption of *O intermediate and accelerated electron transfer in LS-NCF efficiently lower the reaction energy barrier of the rate-determining step (*O → *OOH), thereby greatly boosting its OER performance. This work provides valuable insights into designing high-performance Ni-based electrocatalysts via spintronic-level engineering.