Co9S8 nanoparticles embedded in a N, S co-doped graphene-unzipped carbon nanotube composite as a high performance electrocatalyst for the hydrogen evolution reaction

In this work, we successfully fabricated a three-dimensional (3D) hierarchical ternary composite by embedding Co9S8 nanoparticles in a nitrogen and sulfur co-doped graphene-unzipped carbon nanotube matrix (Co9S8/NSG-UCNTs) through a facile and controllable one-step pyrolysis method using a graphite oxide/oxidized unzipped carbon nanotubes/cobalt nitrate/thiourea composite as the precursor. The asprepared 3D ternary composite displays superior catalytic performance for the hydrogen evolution reaction (HER), which outperforms most binary or ternary carbon-based composites in the literature. The HER overpotentials are 65 mV and 86 mV when the current densities reach 10 mA cm (- 2) and 20 mA cm (-2), respectively, and the exchange current density reaches 0.503 mA cm (-2). Also, it demonstrates good stability reflected from the negligible activity decrease after 1000 consecutive cycles. The excellent electrocatalytic performance of the Co9S8/NSG-UCNT ternary composite is attributed to the co-effect of the abundant HER active sites induced by Co9S8 nanoparticles, structural defects existing in the carbon support caused by N and S co-doping and outstanding conductivity resulting from the 3D structure.