The molecular hydrogen induced H-upd peak shift Recalling our initial motivation on studying the H-upd, we indeed find that the presence of molecular hydrogen has a direct effect on 𝐸 𝑝,∞ in the alkaline regime, but not in the acidic regime, see Fig. . The equilibrium peak potential decreases with decreasing partial pressure of molecular hydrogen. This suggest that the H-upd can be formed from water or hydronium ions, but also from molecular hydrogen and hence should be the intermediate of the HOR/HER.