(358f) Ionic Transport in Charged Porous Media

Energy storage in capacitor porous materials, capacitive deionization (CDI) for water desalination, geophysical applications, and removal of heavy ions from waste streams are some examples of processes where understanding of ionic transport processes is very important. Most studies available in literature apply only to symmetric, binary electrolytes. A new model that computes the individual ionic concentration profiles inside porous electrodes is proposed to simulate ionic transport process. A volume averaging methodology has been used to derive the averaged equations from the point equations and appropriate boundary conditions. We have derived individual ionic volume averaged equations starting from the PNP point equations and the appropriate boundary conditions. The transport parameters have been calculated analytically for isotropic porous media. Our calculations show that the effective diffusivity and mobility tensors are not equal as they are in the point equations. We have also derived the macroscopic Poisson-Boltzmann averaged equation. However, at the macroscopic scale this equation is transformed into a global electroneutrality condition. This condition leads to an open problem due to the loss of microscopic information during the averaging process. In order to deal with this problem we used an already derived model for overlapping electrical double layers. Our results suggest that the EDL overlapping model predicts well qualitative behavior experimentally determined.