The only mechanism matching a value of α a + α c = 1 is the Tafel-Volmer mechanism with a rate-determining Volmer step, as already pointed out by Rheinländer et al. Instead, a low-coverage OPD (overpotential deposition) species 31 is commonly invoked as active H ad of the HOR/HER mechanism and spectroscopic evidence for such a species has been presented. Fitting results for α a + α c = 2, in turn, are less conclusive than for α a + α c = 1. Several of the rate expressions for both the Heyrovsky-Volmer and Tafel-Volmer mechanisms have an H 2 reaction order of one for the HOR branch and zero for the HER, cf. supplementary material, in agreement with our results of m HOR = 0.93 and m HER = 0.00. We therefore do not attempt any mechanistic interpretation of the fitting results for α a + α c = 2.