Axial oxygen-coordinated polyoxometalate/iron polyphthalocyanine heterojunctions for high-performance alkaline oxygen...

Iron polyphthalocyanine (FePPc) is a structurally well-defined FeN₄ molecular catalyst for the oxygen reduction reaction (ORR). However, its planar and relatively symmetric FeN₄ configuration restricts axial O₂ interaction and limits interfacial electron transfer. Herein, a phosphotungstic acid (PTA, H₃PW₁₂O₄₀)-coupled iron polyphthalocyanine catalyst, denoted FePPc-PTA, is constructed via a solid-state grinding-assisted coupling strategy. The Keggin-type PTA is incorporated into the conjugated FePPc framework, forming axial FeO coordination between PTA-derived oxygen species and Fe centers. X-ray photoelectron spectroscopy (XPS) and Fe K-edge X-ray absorption spectroscopy (XAS) reveal a FeO axial coordination environment and a decreased electron density around the Fe center after PTA incorporation. This axial coordination breaks the original planar FeN₄ symmetry, regulates the local electronic environment of the Fe sites and promotes interfacial charge redistribution, thereby strengthening the axial interaction between Fe centers and oxygen species and accelerating ORR kinetics. Consequently, FePPc-PTA exhibits excellent alkaline ORR activity, with a half-wave potential of 0.907 ± 0.002 V versus the reversible hydrogen electrode (RHE), a kinetic current density of 22.37 ± 0.10 mA cm^-2, a Tafel slope of 41.92 mV dec^-1 and a turnover frequency of 4.40 s^-1 at 0.8 V. When employed in rechargeable Zn-air batteries, the FePPc-PTA + ruthenium dioxide (RuO₂) air cathode delivers a peak power density of 170 mW cm^-2, a specific capacity of 817.28 mAh g^-1 and stable cycling for over 500 h. this work demonstrates that polyoxometalate coupling is an effective strategy for regulating FeN₄ active sites in metal polyphthalocyanines toward high-performance non-precious ORR catalysis.