where the overpotential η = E -E RHE is defined with respect to the actual equilibrium potential for each hydrogen partial pressure. It should be noted that here, the H 2 dependencies of both the HOR and HER are captured by only one effective reaction order, γ, for the exchange current density. Fitted curves are shown in Fig. ,d and the fitted parameter values are summarized in Table . Our obtained values for α a + α c = 1 are in excellent agreement with the values reported by Rheinländer et al. (γ = 0.54, j 0 0 = (0.55 ± 0.02) mA cm -2 Pt , and α a = 0.48 ± 0.03, with α c = 1 -α a ).