MO₂C/C Nanocomposites Prepared by a Solid-Flame Combustion Synthesis Process for the Hydrogen Evolution Reaction

In this study, we present two distinct solid-flame combustion synthesis (SF-CS) methods for the fabrication of two-dimensional (2D) nanosheets of carbon and nanocrystals of molybdenum carbide (Mo₂C). To synthesize 2D carbon nanosheets, poly­(vinyl chloride) polymer powder is combusted in conjunction with magnesium (Mg). In contrast, Mo₂C is generated through the combustion–reduction of MoO₃ using an excess of Mg. It is observed that the crystallization of Mo₂C within liquid Mg yields well-defined nanocrystals with {101} facets. Combining these nanocrystals with 2D carbon nanosheets results in hydrogen evolution reaction (HER) catalysts with a significantly improved performance. A comprehensive comprehension of the underlying mechanisms responsible for the heightened HER performance is elucidated, particularly with regard to the synergistic effects arising from the combination of Mo₂C and graphene-like 2D carbon nanosheets. Furthermore, a density functional theory (DFT) analysis of Mo₂C/interface energies indicates that the presence of a few layers of graphene on the Mo₂C facet surface can enhance the catalytic properties. This is attributed to the fact that carbon layers do not form bonds with hydrogen atoms, which gradually reduces the adsorption energies of hydrogen and facilitates the formation of H₂ molecules.