Mechanistic Insights into the Hydrogen Oxidation Reaction on PtNi Alloys in Alkaline Media: A First-Principles Investigation

Abstract

The promising alkaline anion exchange membrane fuel cell suffers from sluggish kinetics of the hydrogen oxidation reaction (HOR). However, the puzzling HOR mechanism hinders the further development of highly active catalysts in alkaline media. In this work, we conducted detailed first-principles calculations to acquire a deep understanding of the alkaline HOR mechanism on PtNi bulk alloys [Pt₃Ni(111), Pt₂Ni₂(111), and PtNi₃(111)] and its surface alloy [PtNi_surf(111)]. The full free energy profiles suggest that the HOR on PtNi alloys proceeds <i>via</i> the Tafel-Volmer mechanism, that is, the direct decomposition of H₂ into two adsorbed H, followed by its reaction with OH^- in the electrolyte, as the rate-determining step, to form H₂O. Therefore, the HOR activity of PtNi alloys is solely impacted by the adsorption of hydrogen, rather than hydroxyl species, though the oxophilicity is also enhanced by alloying Pt with Ni. Thermodynamically, a moderate H adsorption free energy, Δ<i>G</i>_H* ≈ 0.414 eV, is calculated to be an optimal candidate for the HOR at pH = 13. Alloying Pt with Ni can elevate the d-band center (ε_d), push the value of Δ<i>G</i>_H* closer to 0.414 eV, and thus lower the free energy barrier (<i>E</i>_a) of the rate-determining Volmer reaction, leading to the highest HOR activity of PtNi₃(111) among all considered PtNi alloys. This situation is further confirmed by both the microkinetic model and the Tafel plot, where PtNi₃(111) exhibits the highest reaction rate (<i>r</i> = 9.42 × 10³ s^-1 site^-1) and the largest exchange current density (<i>i</i>₀ = 1.42 mA cm^-2) for HOR in alkaline media. This work provides a fundamental understanding of the HOR mechanism and theoretical guidance for rational design of electrocatalysts for HOR in alkaline media.