Controlling the Active Sites of Sulfur-Doped Carbon Nanotube-Graphene Nanolobes for Highly Efficient Oxygen Evolution and Reduction Catalysis

Controlling active sites of metal-free catalysts is an important strategy to enhance activity of the oxygen evolution reaction (OER). Many attempts have been made to develop metal-free catalysts, but the lack of understanding of active-sites at the atomic-level has slowed the design of highly active and stable metal-free catalysts. A sequential two-step strategy to dope sulfur into carbon nanotube-graphene nanolobes is developed. This bidoping strategy introduces stable sulfur-carbon active-sites. Fluorescence emission of the sulfur K-edge by X-ray absorption near edge spectroscopy (XANES) and scanning transmission electron microscopy electron energy loss spectroscopy (STEM-EELS) mapping and spectra confirm that increasing the incorporation of heterocyclic sulfur into the carbon ring of CNTs not only enhances OER activity with an overpotential of 350 mV at a current density of 10 mA cm(-2), but also retains 100% of stability after 75 h. The bidoped sulfur carbon nanotube-graphene nanolobes behave like the state-of-the-art catalysts for OER but outperform those systems in terms of turnover frequency (TOF) which is two orders of magnitude greater than (20% Ir/C) at 400 mV overpotential with very high mass activity 1000 mA cm(-2) at 570 mV. Moreover, the sulfur bidoping strategy shows high catalytic activity for the oxygen reduction reaction (ORR). Stable bifunctional (ORR and OER) catalysts are low cost, and light-weight bidoped sulfur carbon nanotubes are potential candidates for next-generation metal-free regenerative fuel cells.