(325d) Modeling of the Electrosorption Process in Heterogeneous Porous Media Using a Volume Averaging Method

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-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 sizes significantly larger than the size of hydrated ions.

The volume averaging method has been used to simulate the charge-discharge process 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 microporosity inside the particles, where the electrical double layers form.

No assumption is made on the size of the ‘small’ pores; however, they should be significantly smaller than the ‘big’ pores outside. Based on this constraint we can say that ion adsorption will occur only in the small pores inside the solid particles. The solution inside the small pore space exchanges ions with a charged, electrical double-layer (EDL) on the solid wall.  An EDL model is necessary to describe the charge process inside the micropores of mesoporous materials. We have proposed an EDL model that includes a Stern layer of constant geometrical thickness and variable electrolyte dielectric constant calculated using the Booth equation. The potential and ionic concentration profiles inside the diffuse layer were computed by using an analytical solution for slit-shaped pores.

The thermodynamic equilibrium assumption has been used to derive one equation models for salt concentration and electrostatic potential distribution. Theoretical calculation of the transport parameters, effective diffusivity and effective mobility, have been carried out.

Neutron imaging has been employed to visualize lithium ions in mesoporous carbon materials, which are used as electrodes in electrosorption processes. Experiments were conducted with a flow-through capacitive deionization cell designed for neutron imaging and with lithium chloride (⁶LiCl) as the electrolyte. Sequences of neutron images have been obtained at a relatively high concentration of lithium chloride (⁶LiCl) solution to provide information on the transport of ions within the electrodes. These experiments provide a valuable way to compare theory and experiments and to validate the proposed model.