Core–Shell Structured Fe–N–C Catalysts with Enriched Iron Sites in Surface Layers for Proton-Exchange Membrane Fuel Cells

Carbon-supported and nitrogen-coordinated single iron site materials (denoted as Fe–N–C) are the most promising platinum group metal (PGM)-free cathode catalysts for the oxygen reduction reaction (ORR) because of their encouraging activity and continuously improved stability. However, current Fe–N–C catalysts derived from zeolitic imidazolate framework-8 (ZIF-8) nanocrystal precursors via thermal activation at high temperatures often suffer from low accessible Fe sites because the most active sites are buried within bulk carbon nanoparticles. The morphology limitation significantly mitigates the critical three-phase interfaces for creating effective active sites, which requires sufficient ionomer coverage for conducting protons therefore inhibiting the mass transfer of reactants (i.e., O2) within electrodes in proton-exchange membrane fuel cells. Herein, we report an effective strategy for designing a core–shell composite precursor consisting of a polyhedron N-doped porous carbon core from ZIF-8 and a shell from an Fe(III) tetraphenylporphyrin chloride-based conjugated microporous polymer. The resulting core–shell structured Fe–N–C catalyst contains most of the atomic Fe sites at the shell layer with increased density. The unique catalyst design can shorten the diffusion distance of H+ and O2 and facilitate H2O product removal, promoting the promoted ORR in thick PGM-free cathodes. Hence, the membrane electrode assembly with optimal Fe–N–C catalysts achieved encouraging current densities of 32 mA cm–2 at 0.9 ViR-free (1.0 bar O2) and 102 mA cm–2 at 0.8 V (1.0 bar air) and a peak power density of 0.43 W cm–2 (1.0 bar air). This work provides an approach to constructing critical M–N–C catalysts with easily accessible single metal active sites in surface layers for the ORR and other critical electrocatalytic reactions.