Oxygen reduction reaction catalysts based on precious metals such as platinum or its alloys are routinely used in fuel cells because of their high activity. Carbon-supported materials containing metals such as iron or cobalt as well as nitrogen impurities have been proposed to increase scalability and reduce costs, but these alternatives usually suffer from low activity and/or gradual deactivation during use. Here, we show that few-walled carbon nanotubes, following outer wall exfoliation via oxidation and high-temperature reaction with ammonia, can act as an oxygen reduction reaction electrocatalyst in both acidic and alkaline solutions. Under a unique oxidation condition, the outer walls of the few-walled carbon nanotubes are partially unzipped, creating nanoscale sheets of graphene attached to the inner tubes. The graphene sheets contain extremely small amounts of irons originated from nanotube growth seeds, and nitrogen impurities, which facilitate the formation of catalytic sites and boost the activity of the catalyst, as revealed by atomic-scale microscopy and electron energy loss spectroscopy. Whereas the graphene sheets formed from the unzipped part of the outer wall of the nanotubes are responsible for the catalytic activity, the inner walls remain intact and retain their electrical conductivity, which facilitates charge transport during electrocatalysis.
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This work was supported in part by a Stinehart Grant for Energy Research at Stanford from the Stanford Precourt Institute for Energy, Intel and NCEM at Lawrence Berkeley Laboratory, which was supported by the US DOE (DE-AC02-05CH11231). W.Z. was supported by the NSF (DMR-0938330). J-C.I. was supported by Oak Ridge National Laboratory's Shared Research Equipment (ShaRE) User Facility, which is sponsored by the Office of Basic Energy Sciences, US Department of Energy. S.J.P. was supported by the Basic Energy Sciences programme of the Materials Sciences and Engineering Division of the US DOE.
The authors declare no competing financial interests.
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Li, Y., Zhou, W., Wang, H. et al. An oxygen reduction electrocatalyst based on carbon nanotube–graphene complexes. Nature Nanotech 7, 394–400 (2012). https://doi.org/10.1038/nnano.2012.72
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