The driving force behind these reactions is the surface overpotential: the difference between the phase potential of the solid and the phase potential of the electrolyte/membrane. Therefore, two potential equations are solved for in the Fuel Cell and Electrolysis Model: one potential equation (Equation 1.7 (p. 4)) accounts for the electron transport − Ö through the solid conductive materials (that is, the current collectors and solid grids of the porous media); the other potential equation (Equation 1.8 (p. 4)) represents the protonic (that is, ionic) transport of + or − . The two potential equations are as follows: