Spin–orbital regulation of iron–nitrogen sites in phthalocyanine catalysts for ultrahigh energy efficient Zn–air/iodide hybrid batteries

Abstract

As a promising energy storage technology, zinc–air/iodide hybrid batteries (ZAIHBs) require efficient bifunctional catalysts capable of simultaneously mediating both the oxygen reduction reaction (ORR) and iodide oxidation reaction (IOR). Herein, we demonstrate a spin–orbital and hybridized energy level strategy by constructing heteroatom-doped carbon nanotube-supported iron phthalocyanine polymers (S/N-CNT@PFePc). Zero-field cooling (ZFC) measurements reveal that both S and N doping at the axial position induces a spin-state transition from low-spin to intermediate-spin in the Fe centers, stemming from the broken symmetry of the crystal field. Moreover, density functional theory (DFT) calculations justify that axial S coordination results in reduced energy gaps between the intermediates (*OH and *I) and the Fe 3d_z² orbital, enabling enhanced electron transfer compared with N-doping. As a result, the spin–orbital regulation in S-CNT@PFePc achieves a record-low voltage gap ORR–IOR potential gap of 0.34 V (ΔE = E^IOR_j=10 − E^ORR_1/2). Impressively, when implemented in ZAIHBs, the S-CNT@PFePc-based cathode delivers an exceptional energy efficiency of 70%, with remarkable stability, retaining 66% efficiency after 500 hours; this is among the best performances reported for hybrid battery systems. Furthermore, flexible solid-state ZAIHBs incorporating this catalyst demonstrate excellent mechanical robustness and cycling stability, underscoring their practical applicability.