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. Mossbauer 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-N-4/C moiety compared to PAA-Fe-N, which consists of approximate to 15% low-coordinated Fe-N-2/N-3 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 H-2-air fuel cell. Density functional theory (DFT) calculations suggest that the Fe-N-4/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-N-2/N-3 moieties, thereby strongly stabilizing Fe centers against demetalation.