which, again, is formulated on a fixed potential scale. Accordingly, j 0 0 is the exchange current density at a reference pressure of p H2 = p ⊖ = 1 bar, cf. Eq. ( ). We note that, within the rate-determining step (RDS) approximation, the rate expressions of the Heyrovsky-Volmer and Tafel-Volmer mechanisms can be written in such a form, as given in Eqs. (S27)-(S34)