Axial oxygen-coordinated polyoxometalate/iron polyphthalocyanine heterojunctions for high-performance alkaline oxygen reduction in zinc-air batteries

A phthalocyanine heterojunction oxygen-reduction catalyst formed by axial coordination of oxygen atoms from terminal groups was constructed via a solid-state coupling strategy. • FePPc-PTA exhibits excellent alkaline ORR activity with a half-wave potential of 0.907 V and Zn-air batteries performance with a specific capacity of 817.28 mAh g −1 and stable cycling for over 500 h. • Axial coordination breaks the original planar Fe N 4 symmetry and modulates the local electronic environment of the Fe sites thereby accelerating ORR kinetics. Iron polyphthalocyanine (FePPc) is a structurally well-defined Fe N 4 molecular catalyst for the oxygen reduction reaction (ORR). However, its planar and relatively symmetric Fe N 4 configuration restricts axial O 2 interaction and limits interfacial electron transfer. Herein, a phosphotungstic acid (PTA, H 3 PW 12 O 40 )-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 Fe O 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 Fe O axial coordination environment and a decreased electron density around the Fe center after PTA incorporation. This axial coordination breaks the original planar Fe N 4 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 (RuO 2 ) 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 Fe N 4 active sites in metal polyphthalocyanines toward high-performance non-precious ORR catalysis Download: Download high-res image (121KB) Download: Download full-size image