Models of the Hydrogen Evolution Reaction on Transition Metal Phosphide Catalyst Surface

The hydrogen evolution reaction (HER) constitutes half of the water splitting reaction, which allows for the production of H2 from renewable energy resources, and reducing the demand for fossil fuels. Transition metal phosphides (TMP) have emerged as promising HER catalysts made from earth-abundant materials. TMP surfaces have a variety of symmetrically distinct sites that may be catalytically active. Their hydrogen adsorption energy has been shown to strongly depend on the coverage of adsorbed hydrogen on these surfaces. However, the structure of hydrogen adsorbed on TMP surfaces is largely unknown, limiting our understanding of the catalytic properties of these surfaces. To address this problem, we used cluster expansions to determine the atomic-scale structure of hydrogen on four different TMP surfaces: FeP(011), Fe2P(100), CoP(101), and Co2P(101). For reference, we constructed a cluster expansion for hydrogen adsorption on the widely-studied Pt(111) surface. These cluster expansions allow us to identify the structure and energetics of adsorbed hydrogen as a function of temperature, applied potential, and hydrogen chemical potential. We also present the results of related kinetic models to better understand the catalytic properties of TMP surfaces.