(18) where i 0 is the exchange current density, α is the charge transfer coefficient of anode and cathode for the negative electrode, c s Red and c s Ox indicate the surface concentration of the reductant and oxidant, respectively, and c ref is the reference concentration at which the exchange current density was measured, assumed to be equal for reductant and oxidant. In Eq. 18, the overpotential is defined as η = Φ ele -Φ ion -U 0 , which can be viewed as the difference between the surface potential (Φ ele -Φ ion ) and the equilibrium potential (U 0 ) of the particular halfreaction. In the current study, the overpotential is calculated from the difference in electrical potential on the surface of the current collector and the ionic potential on the membrane :