MODELS OF THE HYDROGEN EVOLUTION REACTION ON TRANSITION METAL PHOSPHIDE CATALYST SURFACES

The hydrogen evolution reaction (HER), the cathodic half reaction of the water splitting, allows for the production of H2 from sustainable energy resources and reduces the increasing demand for fossil fuels. Transition metal phosphides (TMPs) have emerged as promising HER catalysts made from cheaper and earth abundant materials. Their differential hydrogen adsorption free energy, ∆G, an important parameter for determining catalytic activity, has exhibited strong dependence to the hydrogen coverage. However, the structures of hydrogen adsorbed on TMP surfaces is unclear, which limits the understanding of the surface catalytic properties. In this research, a combination of density functional theory (DFT) calculation, cluster expansion (CE) method and Monte Carlo (MC) simulation has been used in investigating the atomic-scale structure of hydrogen adsorption on four different TMP surfaces: CoP(101), Co2P(101), FeP(011), and Fe2P(100). The energy of adsorbed hydrogen atom on different sites has then been evaluated as a function of temperature, hydrogen chemical potential, and applied electrical potential. The result shows that the active sites change with different applied potentials, and more active sites can be observed on the surface under a more negative applied potential. Platinum (Pt), the best known HER catalyst, has been studied on its reconstructed (110) surface as a reference using the same method. The ground state structures of Pt(110) and the four TMP surfaces at 300K have been simulated by MC. The result indicates that the probability of finding an isolated hydrogen atom on a TMP surface is much higher than on a Pt(110) surface. Moreover, the adsorption energies of different sites on Pt(110) are quite similar to each other, which are all close to a value of 0eV. However, the hydrogen adsorption energies on TMPs show a relative large difference between different hydrogen sites. Therefore, the HER on TMP surfaces is more likely to undergo the Heyrovsky-Volmer mechanism, since Tafel-Volmer mechanism has been proved to dominate the HER on Pt(110)