These values are self-consistent with the first-order assumption underlying the general KL mass-transport correction employed. The value of α a ≈ 1 corresponds to a Tafel slope of 60 mV dec -1 , which is drastically different from the value of 121 mV dec -1 determined from the Tafel fit at larger HOR overpotentials, cf. Fig. . Results for α a + α c = 2 would thus imply rather constant reaction orders, but an increase in Tafel slope by a factor of two going from small to larger overpotentials for the HOR.