Nitrogen/sulfur co-doped helical graphene nanoribbons for efficient oxygen reduction in alkaline and acidic electrolytes

Nitrogen-doped multiwalled carbon nanotubes (CNx-MWCNTs) with bamboo structures have been unzipped via a helical mechanism, and nitrogen-doped graphene oxide nanoribbons (CNx-GONRs) with a multifaceted microstructure have been obtained. CNx-GONRs have then been exposed to nitrogen and/or sulfur precursors at high temperatures to obtain doped graphene nanoribbons. The effects of single-doping, two-step-doping, and co-doping time and temperature on the concentration of the dopants and their functional groups have been investigated by X-ray photoelectron spectroscopy (XPS). The oxygen reduction reaction (ORR) activity of the doped helical GNRs have been measured in both alkaline and acidic electrolytes, and compared with the state-of-the-art platinum/carbon (Pt/C) electrocatalysts. The nitrogen/sulfur co-doped graphene nanoribbons (CNx/CSx-GNRs) with helically unzipped structures have shown highly efficient ORR activity that is comparable to Pt/C in onset potential, exchange current density, four electron pathway selectivity, kinetic current density, and methanol tolerance. Such helical CNx/CSx-GNRs will greatly contribute to the development of precious metal-free carbon nanomaterials as electrocatalysts in the cathode of metal-air batteries, and alkaline and proton-exchange membrane fuel-cells. (C) 2016 Elsevier Ltd. All rights reserved.