Combining Eq. 11a with Eq. 8b, we find that 𝑖 0,𝑚𝑖𝑐𝑟𝑜 is associated to the reciprocal sum of the individual exchange current densities (Eq. 11b). We have derived this expression both for a two-step and a three-step reaction (see SI.M2b and SI.M3b) and postulate that this, and the impedance analogue expressions, are true for any number of reaction steps. Through the definition of the elementary exchange current densities and when Eq 10a is valid we find that 𝑖 0,𝑚𝑖𝑐𝑟𝑜 is proportional to the exchange current density of the src step: