[ Interestingly, while in the case of perchlorate the nature of the cation has no effect, it has a noticeable effect in chlorides where the peak positions are shifted to more negative potentials in the order of lithium, sodium, and potassium -in a first approximation associated to the size of the cation, as has been suggested elsewhere . If one would ignore energetic changes to the proton donor and acceptor species one may conclude that higher ionic strengths and bigger cations reduce the H-upd binding energy in chloride containing electrolytes. In any case, both effects pale compared to the effect of the anions and are at best secondary effects. Curiously, the shift in H-upd features in chloride containing electrolytes and the absence of the O-upd peak at around 800 mV in lithium containing electrolytes are the only conclusive cation-effects we have found in our exhaustive study on the HOR/HER and H-upd on Pt(pc). In this context, the CVs obtained at pH 3.5 allow a well-defined determination of surface charge due to the proton mass-transport limitations at negative potentials (Fig. ). These mass transport limitations also cause a substantial uncertainty at higher scan rates as the formation of the H-upd consumes protons which cannot be supplied fast enough at scan rates > 200 mV/s. Overall, we find that the surface charge differs substantially between the presence of chloride and perchlorate. This discrepancy is likely due to the different binding energy patterns of the H-upd while the negative "cut-off" potential caused by the M-T limitation is the same. The same effect would be responsible for the minor differences in surface charges in chloride containing electrolytes where we have just documented different peak positions, which then combine with the same "cut-off" potential. In both cases, the surface charge is significantly higher than one would expect for one ML of hydrogen atoms, supporting our conclusion regarding an unlimited formation of H-upd. Lastly, due to the absence of electrochemical reactions in the M-T limited region, we can accurately determine the double layer capacitance of the hydrogen covered platinum surface (Fig. ): Here we can confirm previous observations on metal surfaces, e.g. gold, that the capacitance appears to approach a minimum value at high scan rates (around 50 µF/cm 2 ) , suggesting that various capacitive processes with different time constants are responsible for the double layer capacitance, which are dominated by the electrolyte composition at small scan rates, and converge to one value at high scan rates. A reasonable interpretation would be that the double layer capacitance at low scan rates includes rearrangements of the anions and cations, while at high scan rates, only the contribution of the reorganization of the interfacial water layer remainscomplementing our interpretation on the presence of anions and cations at the interface: Anions interact directly with the intermediate of the HOR/HER and the H-upd, with hydroxyl anions interacting most strongly, while cations are present but have only secondary effects on this interaction.