Graphene Activation Explains the Enhanced Hydrogen Evolution on Graphene-Coated Molybdenum Carbide Electrocatalysts

Molybdenum carbides (MoxC) have shown high catalytic activities toward hydrogen evolution reaction (HER) when coupled with graphene. Herein, we use density functional theory (DFT) calculations in conjunction with ab initio thermodynamics and electrochemical modeling on gamma-MoC supported graphene to determine the origin of the enhanced HER activities. In addition to previous claims that graphene's main role is to prevent agglomeration of MoxC nanoparticles, we show that the interplay between gamma-MoC coupling and graphene defect chemistry activates graphene for the HER. For gamma-MoC supported graphene systems, the HER mechanism follows the Volmer-Heyrovsky pathway with the Heyrovsky reaction as the rate-determining step. To simulate the electrochemical linear sweep voltammetry at the device level, we develop a computational current model purely from the thermodynamic and kinetics descriptors obtained using DFT. This model shows that gamma-MoC supported graphene with divacancies is optimum for HER with an exchange current density of similar to 1 x 10(-4) A/cm(2) and Tafel slope of similar to 50 mV/dec(-1), which are in good agreement with experimental results.