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

Carbon-supported andnitrogen-coordinatedsingleironsitematerials (denoted as Fe-N-C) are the most promising platinum group metal(PGM)-free cathode catalysts for the oxygen reduction reaction (ORR) becauseof 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 sufferfrom low accessible Fe sites because the most active sites are buried within bulkcarbon nanoparticles. The morphology limitation significantly mitigates thecritical three-phase interfaces for creating effective active sites, which requiressufficient ionomer coverage for conducting protons therefore inhibiting the masstransfer of reactants (i.e., O2) within electrodes in proton-exchange membranefuel cells. Herein, we report an effective strategy for designing a core-shellcomposite precursor consisting of a polyhedron N-doped porous carbon corefrom ZIF-8 and a shell from an Fe(III) tetraphenylporphyrin chloride-basedconjugated microporous polymer. The resulting core-shell structured Fe-N-C catalyst contains most of the atomic Fe sites at theshell layer with increased density. The unique catalyst design can shorten the diffusion distance of H+and O2and facilitate H2Oproduct removal, promoting the promoted ORR in thick PGM-free cathodes. Hence, the membrane electrode assembly with optimalFe-N-C catalysts achieved encouraging current densities of 32 mA cm-2at 0.9 ViR???free(1.0 bar O2) and 102 mA cm-2at 0.8 V (1.0bar 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-Ccatalysts with easily accessible single metal active sites in surface layers for the ORR and other critical electrocatalytic reactions.