Enhancement of nitrogen and sulfur co-doping on the electrocatalytic properties of carbon nanotubes for VO2+/VO2 + redox reaction

Heteroatom doping on the surface of an electrode and catalyst can impact the surface and electronic properties. Herein, nitrogen and sulfur co-doped multi-walled carbon nanotubes (denoted as MWCNTsNS) prepared via a pyrolysis method, in which thiourea served as both nitrogen and sulfur sources, were investigated as an electrocatalyst for the VO2+/VO2 (+) redox couple in vanadium redox flow battery. It was revealed that the pyrolysis process had no effect on the microstructure of MWCNTs. The VO2+/VO2 (+) redox reaction on MWCNTs-NS exhibited higher electrochemical kinetics when compared with that on pristine and nitrogen-doped MWCNTs. The nitrogen and sulfur co-doping for MWCNTs can decrease the charge transfer resistance of the VO2+/VO2 (+) redox reaction. Static cells using graphite felt modified by the MWCNTs samples were employed to evaluate their electrocatalytic properties for the VO2+/VO2 (+) reaction. The cell using the MWCNTs-NS electrocatalyst showed the smallest electrochemical polarization, resulting in a larger energy density and energy efficiency. The results indicate that MWCNTs-NS is a novel efficient catalyst for the VO2+/VO+2 redox reaction with excellent electrocatalytic properties.