Breakthrough in Single Atom Fe Catalysts for Acidic Oxygen Reduction
Proton exchange membrane fuel cells (PEMFCs) are a promising sustainable energy conversion technology due to their environmental friendliness and high efficiency. However, the sluggish kinetics of the four-electron oxygen reduction reaction (ORR) necessitate cathode catalysts requiring over five times the amount of precious metal Pt compared to the anode, limiting widespread PEMFC application. The U.S. Department of Energy emphasizes developing non-precious metal-based catalysts as cost-effective alternatives. Transition metal single atoms (Mn, Co, Cu) anchored on nitrogen-doped carbon (M–N–C) have been developed as efficient ORR electrocatalysts, but most exhibit excellent performance only in alkaline media. The typical MN4 planar coordination renders the central metal vulnerable to hydrogen ion attack, challenging activity and durability in acidic media. Recent studies propose that axial-N coordination enhances stability of atomically dispersed Fe sites for acidic ORR by creating a barrier to Fe dissolution. The induced square-pyramidal crystal field diminishes spin polarization in dz2, dxz, and dyz orbitals, enhancing electronic delocalization of the Fe atom, allowing adsorbed O2 to maintain a low-energy triplet ground state, facilitating activation and reduction. Wang and coworkers constructed a novel curved-surface Fe–N–C (CS Fe/N–C) catalyst with FeN4 single atoms distributed within graphitized multilayered nanoprotrusions on 2D carbon layers. The nanoprotrusions have a mean diameter of ~10 nm and protrude ~4 nm. The curved regions exhibit a high Fe site density of ~1.6 No. nm−2, with 97.6% located deep in the fourth layer, contrasting with lower and more random distribution in planar regions and 2D Fe/N–C. This distribution aligns with iron atom diffusion from core to outer layers during pyrolysis.