equilibrium), the influence of the HOR reverse reaction becomes negligible and the experimentally derived kinetic currents can be taken to be purely representative of the HER. While mass-transport corrections are essential for the analysis of the HOR branch, previous reports suggest that HER currents are not influenced by mass transport effects. Our results agree with these findings, as our measured HER currents show no rotation-rate dependency for any of the investigated hydrogen saturations, see Fig. . This is easily explicable for the HER forward reaction, where water reactant is available in excess. However, the contribution of the reverse reaction (HOR) to the overall rate would be expected to produce a certain rotation-rate dependency at low overpotentials, which is not observed in the RDE currents shown in Fig. for 1600 rpm (solid lines) and 900 rpm (dash-dotted lines). Previous explanations for the lack of an apparent mass-transport effect due to the reverse reaction propose that the hydrogen produced in the HER is not able to form supersaturated solutions due to bubble formation and, as a result, the surface hydrogen concentration will essentially remain constant during HER and equal to the bulk level. A Tafel fit of the kinetic currents at more negative potentials reveals a Tafel slope of 85 mV dec -1 , plotted as a dashed line in Fig. , which corresponds to a cathodic transfer coefficient of