Sulfur and molybdenum Co-doped graphitic carbon nitride as a superior water dissociation electrocatalyst for alkaline hydrogen evolution reaction

An efficient structural doping method (thermal condensation) is proposed to promote the process of water dissociation in alkaline medium by doping nonmetal (sulfur) and metal (molybdenum) in graphite carbon nitride (g-C3N4). The original flake morphology remains after doping, where S substituting N, and Mo locating in the N cavity of C-N skeleton. We suggest that the negatively charged N decreases the electron population of the Mo center, which makes it become susceptible to forming sigma bond for Mo-H2O complex. On contrary, the positively charged S enriches the number of electrons in the center of Mo, which leads to pi-back donation from S -> Mo-H2O to the antibonding orbital of H2O, thus resulting in the fracture of H-O bond and facilitating the dissociation of adsorbed water. Therefore, benefited from the ingenious doping design and synergy of Mo-(S, N)-C, the Mo-S-CN nanosheets significantly enhanced the water dissociation performance, which can be represented by a quite smaller overpotential and Tafel slope. Overall, this work provides a generic strategy for comprehending the relationship between heteroatom doping and the performance in alkaline media of g-C3N4 based catalysts.