(742b) Modeling and Simulation of the Electrosorption Process in Heterogeneous Porous Media

Tsouris, C., Oak Ridge National Laboratory

Porous electrodes are widely used in electrochemical applications such as capacity deionization, high-density supercapacitors, and blue energy storage. Electrodes that present a combination of macro- and micropores are sought because of the high area available for ion storage. Mesoporous carbon materials have received attention recently due to their high surface area and pore size that is significantly larger than the size of hydrated ions. The volume averaging method has been employed to simulate the charging-discharging processes in porous materials that present a bimodal pore size distribution. The pore size distribution consists of macroporosity outside the particles through which the ions are transported and mesoporosity inside the particles, where the electrical double layers form. Ion adsorption occurs in the mesopores, where the solution exchanges ions with a charged, electrical double-layer (EDL) at the solid-liquid interface. To describe the charging process, we have proposed an EDL model that includes a Stern layer of constant geometrical thickness and a variable electrolyte dielectric constant calculated by the Booth equation. The potential and ionic concentration profiles inside the diffuse layer were computed by solving numerically the Poisson-Boltzmann equation for cylindrical coordinates. The thermodynamic equilibrium assumption has been used to derive one-equation models for salt concentration and electrostatic potential distribution. Theoretical calculations of the transport parameters, effective diffusivity, and effective mobility have been carried out. Comparison between experimental data and theoretical calculations is performed to validate the proposed model.