where x is a linear distance parameter, ca p is the maximum concentration of adsorption sites on the 3PB, and ca and cd are respectively the concentration of species adsorbed on the 3PB and the diffusion species on the 3PB into the porous electrode. Assuming negligible chemisorption (i.e. rcs = 0), physically adsorbed species are immobile (i.e. Da = 0 is the surface diffusion coefficient of ions along the 3PB) while the adsorption rate constant ka follows a temperature-dependent Arrhenius law. In the continuum limit, internal configurations are described by an internal variable γ :