While multiple theories for the rate constant exist, notably the transition state theory , the phenomenological Arrhenius relationship (Eq. 2b) is widely employed to derive the Butler-Volmer equation using the preexponential factor 𝔸 𝑓/𝑏 and the activation barrier 𝔼 𝐴,𝑓/𝑏 (Fig. ). To simplify the rate expression, we will use the stoichiometric product of the reactant/product activities 𝑆 moving forward (Eq. 2c and 2d).