Nanocomposites of transition-metal carbides on reduced graphite oxide as catalysts for the hydrogen evolution reaction

Transition metal carbides (TMC), Mo2C, Fe3C, and WC.W2C, were synthesized as well-dispersed nanoparticles on a reduced graphite oxide (rGO) substrate and each of their hydrogen evolution reaction (HER) activities was compared with that of a bulk TMC powder. Among the TMC/rGO nanocomposites, only Mo2C/rGO exhibited an obvious increase in the capacitance-normalized current density compared to the bulk powder. We performed density functional theory calculations which suggest that the distinctive activity of Mo2C can be attributed to the structure-sensitive binding energies of atomic H, which binds less strongly on the stepped surface than the flat surface of Mo2C, while other TMCs do not exhibit clear structure-sensitivity. The charge transfer resistances were significantly reduced by using rGO as the substrate, but its correlation with the HER activity was not strong. An analysis of the kinetic data suggested that hydrogen evolution on the TMC surface follows the Volmer-Tafel mechanism with the Tafel step (recombination) rate limiting, which is responsible to the weak correlation between the charge transfer properties and HER activity.