Unifying the Cation Effects in Alkaline Hydrogen Evolution Reaction through Atomistic Simulations

Tuning the electrolyte composition provides an effective route to regulate interfacial electrocatalytic kinetics and optimize catalytic performance. Here, we systematically investigate the microscopic mechanisms by which Li+, Na+, and K+ influence alkaline hydrogen evolution reaction (HER) kinetics at the Pt(100)/aqueous interface, with particular focus on the Volmer step and interfacial OH– migration. We evaluated the Volmer step kinetics using constrained ab initio molecular dynamics and free energy perturbation methods to calculate interfacial reaction barriers and water acidity constants. All three cations accelerate the Volmer process, while Li+-hydrated water shows the lowest acidity, thereby facilitating water dissociation within its hydration shell. In comparison, Na+ and K+ enhance HER by forming multication-coordinated interfacial water structures that stabilize the generated OH– species. The kinetics of interfacial OH– migration were further analyzed by monitoring the hydrogen-bond network between the primary and secondary water layers. Molecular dynamics simulations reveal that cations with larger ionic radii and higher concentrations weaken interfacial hydrogen bonding due to steric hindrance, thereby slowing OH– migration, particularly in K+-rich environments. Additionally, the solution pH modulates the potential scale, further influencing the kinetics of alkaline HER. Based on these insights, we establish a unified theoretical model that integrates both Volmer reaction and OH– migration kinetics, successfully rationalizing the experimentally observed dependence of alkaline HER activity on cation species, electrolyte concentration, and solution pH.