A Fe-NC electrocatalyst boosted by trace bromide ions with high performance in proton exchange membrane fuel cells

Replacement of expensive and rare platinum with metal-nitrogen-carbon catalysts for oxygen reduction reactions in proton exchange membrane fuel cells is hindered by their inferior activity. Herein, we report a highly active iron-nitrogen-carbon catalyst by optimizing the carbon structure and coordination environments of Fe-N₄ sites. A critical high-temperature treatment with ammonium chloride and ammonium bromide not only enhances the intrinsic activity and density of Fe-N₄ sites, but also introduces numerous defects, trace Br ions and creates mesopores in the carbon framework. Notably, surface Br ions significantly improve the interaction between the ionomer and catalyst particles, promoting ionomer infiltration and optimizing the O₂ transport and charge transfer at triple-phase boundary. This catalyst delivers a high peak power density of 1.86 W cm^-2 and 54 mA cm^-2 at 0.9 V_iR-free in a H₂-O₂ fuel cells at 80 °C. Our findings highlight the critical role of interface microenvironment regulation.