  The hydrogen oxidation reaction (HOR) and hydrogen evolution reaction (HER)
play an important role in hydrogen based energy conversion. Recently, the
frustrating performance in alkaline media raised debates on the relevant
mechanism, especially on the role of surface hydroxyl (OH*). We present a full
pH range electrode/electrolyte kinetics simulation for HER/HOR on Pt(111), with
the potential-related rate constants been calculated with density functional
theory methods. The polarization curves agree well with the experimental
observations. The stability of OH* is found to be unlikely an effective
activity descriptor since it is irrelevant to the onset potential of HOR/HER.
Degree of rate control analyses reveal that the alkaline current is controlled
jointly by Tafel and Volmer steps, while the acidic current solely by Tafel
step, which explains the observed pH-dependent kinetics. Therefore, it is also
possible to reduce the overpotential of alkaline HER/HOR by accelerating the
Tafel step besides tuning the hydrogen binding energy.
