General Carbon-Supporting Strategy to Boost the Oxygen Reduction Activity of Zeolitic-Imidazolate-Framework-Derived Fe/N/Carbon Catalysts in Proton Exchange Membrane Fuel Cells

The oxygen reduction reaction (ORR) activity of the Fe/N/Carbon catalysts derived from the pyrolysis of zeolitic-imidazolate-framework-8 (ZIF-8) has been still lower than that of commercial Pt-based catalysts utilized in the proton exchange membrane fuel cells (PEMFCs) due to low density of accessible active sites. In this study, an efficient carbon-supporting strategy is developed to enhance the ORR efficiency of the ZIF-derived Fe/N/Carbon catalysts by increasing the accessible active site density. The enhancement lies in (i) improving the accessibility of active sites via converting dodecahedral particles to graphene-like layered materials and (ii) enhancing the density of FeN_x active sites via suppressing the formation of nanoparticles as well as providing extra spaces to host active sites. The optimized and efficient Fe/N/Carbon catalyst shows a half-wave potential (E_1/2) of 0.834 V versus reversible hydrogen electrode in acidic media and produces a peak power density of 0.66 W cm^-2 in an air-fed PEMFC at 2 bar backpressure, outperforming most previously reported Pt-free ORR catalysts. Finally, the general applicability of the carbon-supporting strategy is confirmed using five different commercial carbon blacks. This work provides an effective route to derive Fe/N/Carbon catalysts exhibiting a higher power density in PEMFCs.