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

• Published in: Advanced energy materials 2016-03, Vol.6 (5), p.np-n/a
• Main Authors: El-Sawy, Abdelhamid M., Mosa, Islam M., Su, Dong, Guild, Curtis J., Khalid, Syed, Joesten, Raymond, Rusling, James F., Suib, Steven L.
• Format: Article
• Language: English
• Subjects: Active control; bifunctional catalysts; Carbon; Catalysis; Catalysts; Center for Functional Nanomaterials; Fuel cells; MATERIALS SCIENCE; metal-free catalysis; nanolobes; Nanostructure; Nanotubes; Oxygen; oxygen evolution reaction; oxygen reduction; oxygen reduction reaction; Reduction; Spectrum analysis; Strategy; Sulfur; Sulfur-Doped Carbon
• ISSN: 1614-6832; 1614-6840
• DOI: 10.1002/aenm.201501966

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. A sequential two‐step strategy to dope sulfur into carbon nanotube–graphene nanolobes is developed. Advanced characterization techniques confirm that increasing the incorporation of heterocyclic sulfur into the carbon ring of CNTs not only enhances OER activity at an overpotential of 350 mV but also retains 100% of stability for 75 h.