Cu, Fe, N-doped Carbon Nanotubes Prepared through Silica Coating for Selective Oxygen Reduction to Water in Acidic Media

We report Cu, Fe, N-doped carbon nanotubes, (Cu,Fe)−N−CNT, as electrocatalysts for the oxygen reduction reaction (ORR) in acidic media. (Cu,Fe)−N−CNT was prepared using a silica coating method in pyrolysis to minimize the formation of carbon-coated metal oxide or carbide nanoparticles, which are known to be inactive for the H₂O₂ reduction. (Cu,Fe)−N−CNT shows a turnover frequency of 0.66 e⁻ site⁻¹ s⁻¹ at +0.8 V vs. RHE and H₂O₂ yields of <1 % for the ORR with a utilization factor of active sites of 82 %. Kinetic analysis reveals that 4e⁻ transfer rates for (Cu,Fe)−N−CNT are higher than those of a monometallic counterpart of Fe−N−CNT. In situ X-ray absorption spectroscopy enables us to determine redox potentials: E°’(Fe^III/Fe^II)=0.65 V vs. RHE and E°’(Cu^II/Cu^I)=0.45 V for (Cu,Fe)−N−CNT, and E°’(Fe^III/Fe^II)=0.65 V for Fe−N−CNT. These results indicate that bimetallic doping into carbon nanotubes gives the effect on kinetic parameters but not on thermodynamic ones. In other words, there is no direct electronic interactions between the Cu and Fe active sites for (Cu,Fe)-N-CNT because such interactions should modulate their redox potentials.