Evolution of atomic-scale dispersion of FeNx in hierarchically porous 3D air electrode to boost the interfacial electrocatalysis of oxygen reduction in PEMFC

Metal-nitrogen-carbon (M-N-C) materials show great advantages for catalyzing the oxygen reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, both the low density of single atomic (SA) MNx active sites and restricted mass transfer render these M-N-C based air electrodes inferior in cell performance. In this study, a new ZIF8-derived Fe-N-C catalyst/electrode design combining local chemistry tuning and primary morphology tailoring to address the above two critical issues is shown. The introduction of nitrogencarbon defects in ZIF8 host enables a controlled atomic-scale dispersion of FeNx moieties, increasing their content in support materials. Also, the simultaneous structural arrangement of individual ZIF8 nano-grains endows the catalyst with a unique porous micro-spheric morphology. This result in an advanced 3D air electrode featuring dense SA FeNx sites and ample, multiscale macro-sized pore channels, which can significantly increase the intrinsic catalytic activity, facilitate bulk mass transport, and generate more effective triple-phase interfaces for ORR. The present catalyst/electrode design exhibits a record large peak power density of ca. 0.60 W cm-2 under practical air conditions. This approach provides a feasible way for boosting the air cathode interfacial ORR and further enlightens electrode designs for energy devices involving multiphase electrochemical reactions.