Square-pyramidal Fe-N4 with defect-modulated O-coordination: Two-tier electronic structure fine-tuning for enhanced oxygen reduction

Adjusting the electronic and geometric structures of the Fe-N4 active site using axially coordinated ligands has been shown to be effective for improving the performance of the electrocatalytic oxygen reduction reaction (ORR). Any progress beyond that remains extremely challenging, however. Here, we demonstrate a two-tier electronic modulation strategy using defect-modulated O-coordination to further optimize the electronic structure of the Fe center in iron phthalocyanine (FePc), which realizes greatly enhanced ORR performances. Such an atomically dispersed FePc–O–defect catalyst is achieved using a wet ball-milling process. The mechanochemically constructed active site is a square-pyramidal Fe-N4, with the Fe atom located out of the N4-plane toward an axially coordinated O that is singly bonded to graphene at vacancy defects. The FePc–O–defect catalyst delivers a half-wave potential of 0.929 V with mass specific activity of 76.2 A g−1catalyst at 0.9 V in alkaline media. These values are the highest among the reported non-precious metal electrocatalysts and exceed the baseline with bare O-coordination.