Hydrogen is a renewable energy carrier that can replace the use of fossil fuels. The key challenge for hydrogen production through hydrogen evolution reaction (HER) is to find cost effective and efficient catalysts that can substitute for the high-efficient but costly platinum. Molybdenum carbides are promising materials for HER that particularly have high durability in electrochemical environments. Herein, we develop a kinetic approach based on the Butler-Volmer model and the transition state theory to analyze the HER activity of the hexagonal phase of molybdenum carbide (Gamma-MoC). First-principles density functional theory calculations in conjunction with ab initio thermodynamics show that the stable Gamma-MoC surfaces with low-miller-indices are inert to HER. By comparing to the more catalytic Beta-Mo<sub>2</sub>C, we show that Gamma-MoC have low HER activity because the exposed stable surfaces are either Mo or C rich, while as active surfaces on Beta-Mo2C have 1:1 Mo:C stoichiometry. (Yang and Saidi, Nanoscale, 9, 3252, 2017). However, we show that the Gamma-MoC HER activity can be significantly enhanced through coupling to graphene systems. Further, we show that the HER activity can be optimized by tuning the defect chemistry of graphene resulting in an overpotential of -0.1 V vs. SHE for 10 mA/cm<sup>2</sup> current density.

Hydrogen evolution reaction on coupled gamma-Mo C/graphene catalysts: Insights from a new first-principles kinetic approach

2019 Fall National Meeting

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