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内容記述 |
Fe,N-codoped carbon (Fe?N?C) electrocatalysts have been extensively studied for the oxygen reduction reaction (ORR) in acidic media for polymer electrolyte fuel cells (PEFC). Although Fe?N?C electrocatalysts are known to have relatively high ORR activity in non-platinum-group-metal (PGM) electrocatalysts in acidic media, they tend to produce 10?20% of hydrogen peroxide (H2O2), which can lead to the degradation of catalysts and membranes of PEFCs, as a byproduct via two-electron transfer. Doping of another metal into Fe?N?C produces heterometallic active sites and improves the ORR activity and product selectivity to water particularly for Cu,Fe,N-doped carbon electrocatalysts (Cu,Fe)?N?C. Although such bimetallic carbon electrocatalysts, which are also known as dual metal atom catalysts (DACs), have been intensively studied and developed for the ORR, determining their experimental descriptors such as electron transfer rates, active-site utilization factors and redox potentials remains difficult to date. In this work, we prepared Cu,Fe,N-codoped carbon nanotubes, (Cu,Fe)?N?CNT, as an ORR electrocatalyst in acidic media and determined experimental descriptors of turnover frequencies, H2O2 yields, active-site utilization, site densities, electron transfer rates and redox potentials of (Cu,Fe)?N?CNT. To prepare (Cu,Fe)?N?CNT, a silica coating method in pyrolysis suppresses the formation of carbon-coated metal oxide or carbide nanoparticles, which are known to be inactive for the H2O2 reduction, leading to minimizing H2O2 yields <1% for the ORR and maximizing utilization factors of active sites up to 82%. In situ X-ray absorption spectroscopy enables us to determine redox potentials of E(FeIII/FeII)0’ = 0.65 V vs. RHE and E(CuII/CuI)0’ = 0.45 V for the Fe and Cu-based active sites of (Cu,Fe)?N?CNT, respectively. These results suggest that FeN4 sites with more positive redox potential act as the main active site, supported by CuNx sites. Our study can provide insights into designing and developing highly active and selective electrocatalysts with bimetallic active sites from the viewpoint of experimental descriptors including redox potentials. |
