(Keynote) Oxygen Reduction Activity of MOF-Derived N-C Catalysts: Effect of Iron Traces or True Activity of N-Groups?
In this work, Fe-N-C and N-C catalysts were synthesized by pyrolyzing a commercial ZIF 8 and phenanthroline, with or without addition of iron acetate, respectively. Pyrolysis is performed in Ar, or in Ar and then in NH3. The exact Fe contents in the catalysts were measured from the height of the absorption step at Fe K-edge, using X-ray absorption spectroscopy. Various sources of iron contamination for the N-C samples are investigated, such as quartz tube cleanness, pyrolysis gas (NH3 is a corrosive gas) and C and N precursors (commercial ZIF-8, phenanthroline). Figure 1 shows the polarization curves measured in fuel cell for samples pyrolyzed in Ar (open symbols) or in Ar+NH3 (filled symbols). Two observations can be made: i) The ORR activity enhancement between samples obtained from 0%Fe/0%phen/MOF and 1%Fe/20%phen/80%MOF is circa 30 after Ar-pyrolysis (open triangle vs open diamond), in relative agreement with Fe bulk contents of ca 0.03 and 3.0 wt %, respectively ii) the same factor is only circa 4 after Ar and NH3 pyrolysis (filled triangle vs filled diamond), this time in contradiction with the Fe bulk contents of ca 0.07 wt % and 6 wt %, respectively. The high ORR activity of ZIF-8 pyrolyzed first in Ar and then in NH3 either comes from specific N-groups formed during NH3 pyrolysis, or from Fe traces present in the commercial MOF or unintentionally added during the synthesis. Several possible sources of Fe traces, such as contaminated quartz tube, ballmilling process, types of gas used for pyrolysis, or purity of the MOF precursor were investigated and will be discussed.
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