Seizing gaseous Fe2+ to densify O2-accessible Fe–N4 sites for high-performance proton exchange membrane fuel cells

Abstract

Increasing the density of Fe–N₄ sites in Fe–N–C materials is pivotal for enhancing the kinetics of the oxygen reduction reaction (ORR) in proton exchange membrane fuel cells (PEMFCs). Fe utilization is a vital parameter for the Fe–N–C catalyst evaluation, but it shows a tendency to decrease with increasing density of the Fe–N₄ sites. Herein, dense edge Fe–N₂₊₂ sites are deposited in the outermost and subsurface layers of a surface-rich pyridinic-N carbon substrate (Fe_g–NC/Phen). We have demonstrated that the surface-rich pyridinic-N carbon substrate is more favorable to form surface Fe–N₂₊₂ sites with superior intrinsic activity. The surface Fe–N₄ sites can improve both the site density and Fe utilization, while shortening the transport pathways of protons and O₂ effectively. By means of these structural advantages, Fe_g–NC/Phen can exhibit a high current density of 0.046 A cm⁻²@0.9 V_iR-free and a high peak power density (P_max) of 1.53 W cm⁻² in 2 bar H₂–O₂ PEMFCs, and outperform almost all the reported M–N–C catalysts. This outstanding performance will inspire relevant research in the distribution of active sites. Moreover, it requires particular attention to obtain a viable solution to performance durability in fuel cells.