Spin-regulated Ni sites with optimal d-orbital occupancy unlocking unprecedented oxygen evolution activity
Bin Li, Yanming Yu, Yihao Wang, Ming Xu, Guanjie Li, Simin Xu, Wei Wei, Tingting Cui
Abstract
Nickel-based layered double hydroxides (LDHs) are widely recognized as promising substitutes for noble metal catalysts in the oxygen evolution reaction (OER). However, conventional Ni<sup>2+</sup> sites exhibit a high-spin configuration (d<sub>xz</sub><sup>2</sup>d<sub>yz</sub><sup>2</sup>d<sub>xy</sub><sup>2</sup>d<sub>x</sub><sup>2</sup><sub>−y</sub><sup>2</sup><sup>1</sup>d<sub>z</sub><sup>2</sup><sup>1</sup>) with excessive frontier-orbital occupancy, resulting in weak binding strength towards oxygen intermediates, which dramatically limits their OER performance. Herein, we first report the successful construction of low-spin state Ni<sup>2+</sup> (d<sub>xz</sub><sup>2</sup>d<sub>yz</sub><sup>2</sup>d<sub>xy</sub><sup>2</sup>d<sub>x</sub><sup>2</sup><sub>−y</sub><sup>2</sup><sup>2</sup>d<sub>z</sub><sup>2</sup><sup>0</sup>) 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<sup>−2</sup>, which is 79 mV lower than the conventional NiCoFe-LDH with high-spin Ni<sup>2+</sup> (HS-NCF) and significantly outperforming previously reported transition metal-based catalysts. Comprehensive studies reveal that LS Ni<sup>2+</sup> with reduced<em> </em>d<sub>z</sub><sup>2</sup> 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 Ni<sup>2+</sup> is closer to the Fermi level compared to HS Ni<sup>2+</sup>, 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<strong> </strong>engineering.