where ℰ is the local electric field strength across the electrode-solution interface and 𝑟̂ is the distance through which the electron travels from the electrode to the acceptor. With the assumption of the directions of ℰ and 𝑟̂ being parallel at large overpotentials, Eq. 5 entails that 𝐻 78 is proportional to the interfacial electric field. With the Gauss' law, ℰ is linearly correlated with the surface charge density (|𝜎|):