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 /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 Fe -polymer binding constant possesses longer FeN bonds and exclusive Fe-N /C moiety compared to PAA-Fe-N, which consists of ≈15% low-coordinated Fe-N /N 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 -air fuel cell. Density functional theory (DFT) calculations suggest that the Fe-N /C site could optimize the difference between the adsorption energy of the Fe atoms on the support (E ) and the bulk cohesive energy (E ) relative to Fe-N /N moieties, thereby strongly stabilizing Fe centers against demetalation.