The electric potential of counter-ions (protons) in an infinite cylindrical channel is presented as a solution of the Poisson-Boltzmann equation, involving a constant ion charge density along the wall. The distribution of protons is derived and used subsequently to compute the velocity profile and mass flow rate of the corresponding electro-osmotic flow, driven by an electric field. Analytical expressions are derived for all quantities, including the conductivity and water drag coefficient. This analysis relates especially to cylindrical nano-channels of polymer electrolyte membranes such as Nafion and addresses the validity of continuum models for these materials.

A three-dimensional continuum model is explored to investigate the effects of radially dependent system parameters, such as relative permittivity and viscosity, on the transport of proton and water in nanoscale cylindrical pores of a fully hydrated polymer electrolyte membrane (PEM). The model employs Poisson, Nernst-Planck, and Stokes equations. Based on evidence from the literature for the presence of a stagnant water layer near the pore surface, we assume that a no-slip surface is located inside the pore, a few Angstroms from the pore wall. To solve the system numerically, the steady-state solution for the transport of protons and water is considered to be a perturbation around the equilibrium solution. Our results indicate that a radial variation of relative permittivity has the greatest influence on pore conductivity, reducing it by about 50% when compared to that of constant permittivity. On the other hand, viscosity plays the dominant role when the effective water drag within such pores is considered. We conclude that a continuum approach, including constant viscosity, is applicable in nanoscale models provided that the location of the no-slip surface is properly specified and the radial variation of the relative permittivity is taken into consideration.

No abstract

No abstract

No abstract