Atomically Dispersed Fe-N-x/C Electrocatalyst Boosts Oxygen Catalysis via a New Metal-Organic Polymer Supramolecule Strategy

The development of high-performance oxygen reduction reaction (ORR) catalysts derived from non-Pt group metals (non-PGMs) is urgent for the wide applications of proton exchange membrane fuel cells (PEMFCs). In this work, a facile and cost-efficient supramolecular route is developed for making non-PGM ORR catalyst with atomically dispersed Fe-N-x/C sites through pyrolyzing the metal-organic polymer coordinative hydrogel formed between Fe3+ and -L-guluronate blocks of sodium alginate (SA). High-angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and X-ray absorption spectroscopy (XAS) verify that Fe atoms achieve atomic-level dispersion on the obtained SA-Fe-N nanosheets and a possible fourfold coordination with N atoms. The best-performing SA-Fe-N catalyst exhibits excellent ORR activity with half-wave potential (E-1/2) of 0.812 and 0.910 V versus the reversible hydrogen electrode (RHE) in 0.5 (M) H2SO4 and 0.1 m KOH, respectively, along with respectable durability. Such performance surpasses that of most reported non-PGM ORR catalysts. Density functional theory calculations suggest that the relieved passivation effect of OH* on Fe-N-4/C structure leads to its superior ORR activity to Pt/C in alkaline solution. The work demonstrates a novel strategy for developing high-performance non-PGM ORR electrocatalysts with atomically dispersed and stable M-N-x coordination sites in both acidic and alkaline media.