Improving Electrocatalytic Oxygen Evolution through Local Field Distortion in Mg/Fe Dual-site Catalysts

Transition metal single atom electrocatalysts (SACs) with metal-nitrogen-carbon (M-N-C) configuration show great potential in oxygen evolution reaction (OER), whereby the spin-dependent electrons must be allowed to transfer along reactants (OH^- /H₂ O, singlet spin state) and products (O₂ , triplet spin state). Therefore, it is imperative to modulate the spin configuration in M-N-C to enhance the spin-sensitive OER energetics, which however remains a significant challenge. Herein, we report a local field distortion induced intermediate to low spin transition by introducing a main-group element (Mg) into the Fe-N-C architecture, and decode the underlying origin of the enhanced OER activity. We unveil that, the large ionic radii mismatch between Mg²⁺ and Fe²⁺ can cause a FeN₄ in-plane square local field deformation, which triggers a favorable spin transition of Fe²⁺ from intermediate (d_xy ² d_xz ² d_yz ¹ d_z2 ¹ , 2.96 μ_B ) to low spin (d_xy ² d_xz ² d_yz ² , 0.95 μ_B ), and consequently regulate the thermodyna-mics of the elementary step with desired Gibbs free energies. The as-obtained Mg/Fe dual-site catalyst demonstrates a superior OER activity with an overpotential of 224 mV at 10 mA cm^-2 and an electrolysis voltage of only 1.542 V at 10 mA cm^-2 in the overall water splitting, which outperforms those of the state-of-the-art transition metal SACs.