Catalytic Role of Minority Species and Minority Sites for Electrochemical Hydrogen Evolution on Metals: Surface Charging, Coverage, and Tafel Kinetics

Hydrogen evolution reaction (HER: H+ + e(-) -> (1)/H-2(2)) on metals exhibits the characteristic kinetics of electrocatalytic process. Here a theoretical method based on the constant-charge first principles periodic continuum solvation model is proposed to resolve the potential-dependent reaction kinetics on Pt and Au surfaces, and the quantitative linkage is established between the Tafel kinetics (current vs potential) and the electrochemical condition, including the surface structure, the surface charging, and the coverage. The theoretical Tafel slopes for HER are determined to be 83 mV on Pt(111) and 70 mV on Pt(100), which are generally associated with the reactions involving the minority weakly adsorbed H, i.e. the atop H above 1 ML on Pt(111) and the bridging H above 1.5 ML on Pt(100). The mechanism and the contribution of each pathway (Volmer, Tafel, and Heyrovsky pathways) are determined quantitatively. It is revealed that HER at the minority surface steps has a much higher activity than at terraces, which is responsible for the overall activity on a typical Pt electrode. The theoretical model here paved the way toward the large-scale computational screening for both active and economic hydrogen electrode.