2a) The Tafel exchange current density The Tafel exchange current density derived from the constant-coverage-Tafel region can be determined from either elementary step through the steady state condition (Eq. 7a), neglecting cathodic/anodic terms according to Eq. 10 and extrapolating to zero overpotential. We have derived the following expressions for a two-and three-step reaction (see SI.M2c and SI.M3c) and postulate that they can be generalized: