The surface overpotential form of the Butler-Volmer equation An alternative definition of the overpotential is used, e.g. by Newman; rather than through an equilibrium electrode (𝐸 𝑒𝑞 ) the overpotential is defined using a resting electrode in direct vicinity to the surface of the WE (𝐸 𝑆 ), hence the term surface overpotential, 𝜂 𝑆 = 𝐸 -𝐸 𝑆 (Fig. ). This surface overpotential is then a function of the reactant activities in the vicinity of the WE and its difference to the equilibrium overpotential is called the concentration overpotential 𝜂 conc. (Eq. 9a) by Newman. This definition is discussed in detail in Electrochemical Systems and is used in Newman's papers, such as the discussion of the rotating disc electrode . This leads to an alternative form of the B-V equation (Eq. 9b, derivation in SI.M1d) without concentration terms in front of the exponentials. Its exchange current density will also depend on the reactant activities on the surface, subject to changes over the course of a measurement, which is useful when modeling electrode reactions where the bulk composition is of lower interest. We would recommend to only use one well denoted convention and avoid mixing both forms of the B-V equation as it can lead to confusion.