Spin-State Engineering of Single-Atom Nickel Catalysts for Acidic Electrosynthesis of Hydrogen Peroxide

Electrosynthesis of hydrogen peroxide (H₂O₂) via the two-electron oxygen reduction reaction offers a sustainable method for chemical manufacture. However, catalysts in acidic media still face inherent trade-offs between activity and selectivity. Herein, a ligand-field-tailoring strategy is introduced that uses a nickel-porphyrin molecular precursor and a pyrolysis-driven competitive coordination process to precisely program the local environment of single-atom Ni sites. The introduction of Ni─O bonds weakens the ligand field and increases electron pairing, promoting a transition from low-spin to intermediate-spin states at the Ni center. Density functional theory (DFT) calculations reveal a negative correlation between the magnetization and both OOH adsorption energy (ΔG_OOH) and orbital interaction strength. In the intermediate-spin state, incompletely occupied dz² orbitals facilitate moderate OOH binding, facilitating H₂O₂ production. Furthermore, spin polarization accelerates electron transfer to OOH, reducing the energy barrier of the rate-determining step and optimizing reaction kinetics. The catalyst exhibits an exceptional H₂O₂ production rate (6.4 mol g^-1 h^-1) and selectivity (>95%) under acidic conditions, outperforming most transition metal-based catalysts and even many noble metal systems. This study unveils the role of spin-state modulation in optimizing electrocatalysis, opening new avenues for designing high-performance catalysts for H₂O₂ synthesis and other catalytic reactions involving oxygen intermediates.