Curvature Strain-Induced Electron Spin Leveraging in d Orbitals toward Oxygen Reduction for Zn–Air Batteries

Iron phthalocyanine (FePc), with its well-defined FeN₄ active site, has attracted significant interest as a promising nonprecious catalyst in the oxygen reduction reaction (ORR). However, its rigid square-planar geometry hinders O═O bond activation and cleavage of the O═O bond, resulting in sluggish ORR kinetics. Herein, we introduce nitrogen-doped, defect-rich carbon nano-onions (CNO) (with positive Gaussian curvature) as a support to induce geometric distortion in FePc for enhancing the ORR activity. Magnetic measurements and theoretical calculations reveal that the introduction of CNO effectively promotes a spin state transition of Fe ions from low-spin (LS, t_2g⁶ e_g⁰) to intermediate-spin (IS, t_2g⁵ e_g¹), which enables electron filling of the π* orbitals (via d-p orbital coupling), subsequently weakening the overadsorption of oxygenated intermediates on the FeN₄ sites. The as-fabricated FePc/CNO electrocatalyst exhibits exceptional ORR performance in alkaline media. Furthermore, a configured FePc/CNO-based Zn-air battery exhibits excellent performance, as well, demonstrating its potential for practical energy applications.