Reaction modelling of hydrogen evolution on nickel phosphide catalysts: density functional investigation

Nickel phosphides (Ni_xP_y), particularly Ni₂P, are promising catalysts for the acidic hydrogen evolution reaction (HER). Using density functional theory (DFT), we model HER at the potential of zero charge (PZC), incorporating solvation effects via an explicit water cluster and implicit surrounding solvent. Comparing the Volmer, Tafel, and Heyrovsky steps under saturated hydrogen coverage on Ni₂P(0001) terminations, we find that the Ni₃P₂ (pristine) surface termination prefers the Volmer-Volmer-Tafel (VVT) pathway with activation energy (E_a) of 0.57 eV. Conversely, the Ni₃P₂ + 4P (reconstructed) surface favors the Volmer-Heyrovsky (VH) pathway with E_a = 0.60 eV. For the pristine surface termination, the differential gas-phase hydrogen adsorption free energies (ΔG_diff) correlate with the Volmer and Tafel step reaction energies, and a linear Bell-Evans-Polanyi relationship for the calculated activation and reaction energies validates the usefulness of the ΔG_diff descriptor for the Volmer step under PZC conditions. Nickel atoms play a crucial role in H₂ production on both pristine and reconstructed surfaces, suggesting that modifications of the Ni sites can be used for catalyst design. Our findings highlight the importance of considering surface reconstruction and solvation effects on the HER catalytic performance.