Nitrogen and sulfur co-doped graphene/carbon nanotube as metal-free electrocatalyst for oxygen evolution reaction: the enhanced performance by sulfur doping

• Metal-free 3D architecture N,S co-doped GR/CNT is prepared by a one-step method.
• N,S co-doped GR/CNT exhibits good activity and stability for OER.
• S doping is indicated beneficial for OER performance of metal-free catalysts.
• The catalytic kinetics is highly correlated with the content of C-S-C structure.
• 3D architecture composed of GR and CNT also contributes to the OER activity.

Highly active metal-free electrocatalysts consisting of earth-abundant elements for oxygen evolution reaction (OER) are extremely desired for renewable energy technologies. Here we prepare the nitrogen and sulfur co-doped graphene/carbon nanotube (NS-GR/CNT) with 3D architecture by one-step hydrothermal method, which presents good performance for OER. The as-prepared NS-GR/CNT exhibits more negative onset potential and lower Tafel slope (0.56 V, 103 mV decade−1 vs. S.C.E. in 0.1 M KOH) compared to single N doped graphene/carbon nanotube (0.65 V, 285 mV decade−1), which indicates S doping can significantly enhance the OER performance. The X-ray photoelectron spectroscopy reveals that the thiophene-like S (C-S-C) is the dominant S species in all the S doped samples. NS-GR/CNT with C-S-C content of 0.26% has the Tafel slope of 151 mV decade−1 while the value for NS-GR/CNT with C-S-C content of 1.09% is 103 mV decade−1. The decreased Tafel slope demonstrates the catalytic kinetics are highly correlated with the content of C-S-C. Density functional theory calculations suggest that C-S-C may improve the catalytic kinetics by facilitating the adsorption of the OH− intermediate. Besides, the 3D architecture composed of graphene and CNTs also contributes to the good performance and chronoamperometric measurement demonstrates the good durability of NS-GR/CNTs.

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