Improving the Stability of Non-Noble-Metal M–N–C Catalysts for Proton-Exchange-Membrane Fuel Cells through M–N Bond Length and Coordination Regulation

An effective and universal strategy is developed to enhance the stability of the non-noble-metal M–N_x/C catalyst in proton exchange membrane fuel cells (PEMFCs) by improving the bonding strength between metal ions and chelating polymers, i.e., poly(acrylic acid) (PAA) homopolymer and poly(acrylic acid–maleic acid) (P(AA-MA)) copolymer with different AA/MA ratios. Mössbauer spectroscopy and X-ray absorption spectroscopy (XAS) reveal that the optimal P(AA-MA)–Fe–N catalyst with a higher Fe³⁺–polymer binding constant possesses longer Fe-N bonds and exclusive Fe–N₄/C moiety compared to PAA–Fe–N, which consists of ≈15% low-coordinated Fe–N₂/N₃ structures. The optimized P(AA-MA)–Fe–N catalyst exhibits outstanding ORR activity and stability in both half-cell and PEMFC cathodes, with the retention rate of current density approaching 100% for the first 37 h at 0.55 V in an H₂–air fuel cell. Density functional theory (DFT) calculations suggest that the Fe–N₄/C site could optimize the difference between the adsorption energy of the Fe atoms on the support (E_ad) and the bulk cohesive energy (E_coh) relative to Fe–N₂/N₃ moieties, thereby strongly stabilizing Fe centers against demetalation.