Atomistic Simulations of Hydrogen Production Kinetics at Novel Fuel Cell Electrodes

Electrocatalysts such as platinum (Pt), are used to minimize the overpotential for the hydrogen evolution reaction (HER) during electrochemical water splitting or fuel cell operation. Iron phosphide materials (FexP) were recently identified as one of the more promising transition metal phosphide electrocatalysts for HER that can substitute Pt at a much lower cost. To further understand their activity’s origin, the thermodynamics and kinetics of hydrogen recombination was investigated on six FexP (x=1,2,3) surface facets using density functional theory (DFT). As the differential Gibbs free binding energy of hydrogen (∆G_H) reaches zero, the adsorption of protons on FexP surfaces and the desorption of H2 are both expected to be facile. We analyzed the recombination kinetics of H2 on FexP surfaces for the number of hydrogen coverage that satisfy the necessary criterion ∆GH≈0 kJ/mol. The activation and recombination energies for 2 H* to form H2 were calculated using the Vienna Ab initio Simulation Package (VASP). The energy change of different hydrogen combination from 2 H* to H2 was used to identify the easiest detachable H2 from each surface. The most favourable H2 combination pair for each FexP surface was selected to calculate its activation barrier using the Nudged Elastic Band (NEB) algorithm. Our results show that a stable intermediate, i.e. a surface hydrogen molecule (H2*) is formed on the surface before desorption of H2. Our results also suggest that Fe2P and FeP have better HER kinetics, which we tentatively attribute to their lower metal concentration allowing for higher H mobility.