Electrochemical hydrogen evolution on Ni₂P: insights from constant-potential DFT and microkinetic modelling

Hydrogen evolution on nickel phosphides (Ni₂P) is strongly influenced by surface structure and applied potential. Density functional simulations in combination with the constant-potential approach and a hybrid solvation model show that on Ni₂P(001), pristine Ni₃P₂ terminations favour Volmer-Volmer-Tafel energy landscapes, while phosphorus-reconstructed surfaces favour Volmer-Heyrovsky pathways. However, steady-state microkinetic modelling reveals that kinetic competition under bias shifts the predominant current response toward Volmer-Heyrovsky cycles on both terminations. The reconstructed surface achieves substantially lower overpotentials (120-180 mV at 10-100 mA cm^-2), in close agreement with experimental measurements, while the pristine surface remains less active (330-370 mV). The rate is limited by Volmer on the pristine surface and Heyrovsky on the reconstructed one, with Tafel steps largely potential-insensitive. Together, these results connect atomistic energetics to steady-state polarisation behaviour, clarifying how bias and hydrogen configurations shape HER activity. The trends further suggest that modifying the local electronic environment through electropositive species may reduce critical barriers and enhance performance.