One finds that the reactions, recalling that 𝐸 𝑝,𝑓 = 𝐸 𝑝,∞ + 𝜀 𝑠𝑒𝑝,𝑓 and 𝐸 𝑝,𝑏 = 𝐸 𝑝,∞ -|𝜀 𝑠𝑒𝑝,𝑏 |, corresponding to the constant peak positions, do not exhibit a pH dependency as water is the respective proton donor and acceptor (Eqs. 47a, 47d). In contrast, the other two reactions exhibit a pH dependency: The acidic formation of the H-upd consumes protons (Eq. 47b), but the local pH due to the simultaneous HOR causes a deficiency in hydronium ions causing the peak separation to get so large that it reaches potentials where the alkaline formation of the H-upd, utilizing water as the proton donor, becomes favorable. The reverse situation is true for the alkaline case (Eq. 47c). Examining the peak positions at higher scan rates (Fig. ), Eqs. 47a-d tell us that the peak separations should get larger from the contribution of the dimensionless scan rate. This is precisely what we see, as indicated by the arrows in the vertical direction in Fig. . The greater change in the alkaline regime is a first sign, through the contribution of the rate constant to Eq. 44b, of the H-upd being slower at high pH than at low pH values. One also finds that the hydronium and hydroxyl deficient region increase, indicated by the horizontal arrows. We interpret this as the effect of the increased H-upd current at higher scan rates, leading to the local reduction in available hydroxyl/hydronium ions in the forward/backward reaction by the recombination of the produced hydronium/hydroxyl ions with hydroxyl/hydronium ions, leading to a delayed onset of the alkaline oxidation/acid formation of the H-upd. Recalling the acidic CV shape (Fig. ), we notice how the H-upd capacitance sharply drops at potentials slightly negative from the OCP of the HOR/HER, in accordance with aforementioned potential step experiments. At these potentials and pH values below the anion switch potential (pH 9.5-10.5), the local pH is mass transport controlled. This sharp drop is not associated with the Pt surface being fully covered; instead, the local pH increases steeply in a negative potential sweep when approaching the proton transport limiting current, causing a substantial delay in the formation of the H-upd (Eq. 47b). Once the potential is sufficiently negative, further H-upd charge is transferred with water as the proton donor (Eq. 47d). This means two things: First, the idea of one surface hydrogen atom per surface Pt atom is unsubstantiated, as it is derived from the insufficient probing of the H-upd at low potentials. Secondly, it suggests that there may not even be a strict upper bound on the amount of atomic hydrogen that can be formed on the Pt surface. The same has been observed for the O-upd which has been probed to up to three equivalent monolayers without reaching an upper bound of its associated charge. In such case, the rate equations of the H-upd and the HOR/HER using the term (1 -𝜃) are inappropriate and would need to be replaced with alternative terms (Eqs. 49a-b) describing a potentially unbounded coverage with the likely negative electronic effects at increasing coverage on the binding energy and hence the activation barrier, comparable to Frumkin isotherms without a strict upper bound. One example for the Volmer step is given in Eq. 49c with the corresponding equilibrium peak potential for the H-upd given by Eq. 49d.