An advanced hydrogen evolution reaction (HER) catalyst was designed by using Co, Mo and S to prepare a CoMoS4/sulfur-doped carbon (SDC) composite. Interestingly, CoMoS4/SDC resulted in an improvement in the HER performance with the use of varying amounts of SDC. Therefore, the fundamental structural changes of CoMoS4/SDC were scrutinized using various physicochemical techniques, which identified Co9S8 as the dominant active phase. The optimized CoMoS4/SDC ratio demonstrated excellent HER activity, with an onset potential of 132 mV and a low Tafel slope of 48 mV/dec. Moreover, the stability of CoMoS4/SDC with respect to the HER was demonstrated over a wide range of pH values from 0 to 14. In addition, the developed catalyst was modified in a proton exchange membrane (PEM) water electrolyzer to investigate its sustainability, and it exhibited good stability for 100 h at a current density of 110 mA/cm(2) and an overpotential of 220 mV. There was no deterioration in the catalytic activity over the 100 h of operation. (C) 2019 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.