where, 𝐶 𝑂 𝑠𝑢𝑟𝑓 and 𝐶 𝑂,𝑟𝑒𝑓 are the concentrations of the oxidized species at the surface and in the bulk solution, while 𝐶 𝑅,𝑠𝑢𝑟𝑓 and 𝐶 𝑅,𝑟𝑒𝑓 are the concentrations of the reduced species at the surface and in the bulk solution. ∝ 𝑎 and ∝ 𝑐 are the transfer coefficients of anodic and cathodic reactions.