The absence of such peak shift in the acidic regime may be a consequence of the acidic H-upd kinetics being too fast, overwhelming the effect of molecular hydrogen. An alternative explanation stems from examining Fig. . The local molecular hydrogen activity is mass transport controlled at the onset of the HOR when the acidic H-upd features appear, meaning that changing the bulk activity would only have an indirect effect on the local molecular hydrogen activity. While it makes intuitive sense that the peak potential would shift to higher potentials with increasing molecular hydrogen activities when the H-upd is an intermediate of the HOR/HER, a mathematical analysis is needed. Using the assumption that the coverage at the equilibrium peak potential and the symmetry coefficient have the value of 0.5 (which leads to slightly incorrect mathematical results and is only intended to give a qualitative view), we find that for both paths the equilibrium peak potential is affected by the presence of molecular hydrogen (Eqs. 53b-c). Interestingly, even in the absence of molecular hydrogen in the Volmer step, the peak position is still dependent on the rate constant of the step that involves hydrogen, either through 𝑘 𝑏 in the T-V path or 𝑘 𝐻 in the H-V path.