Two-Dimensional Porous Electrode Model for Capacitive Deionization

Abstract

Ion transport in porous conductive materials is of great importance in a variety of electrochemical systems including batteries and supercapacitors. We here analyze the coupling of flow and charge transport and charge capacitance in capacitive deionization (CDI). In CDI, a pair of porous carbon electrodes is employed to electrostatically retain and remove ionic species from aqueous solutions. We here develop and solve a novel unsteady two-dimensional model for capturing the ion adsorption/desorption dynamics in a flow-between CDI system. We use this model to study the complex, nonlinear coupling between electromigration, diffusion, and advection of ions. We also fabricated a laboratory-scale CDI cell which we use to measure the near-equilibrium, cumulative adsorbed salt, and electric charge as a function of applied external voltage. We use these integral measures to validate and calibrate this model. We further present a detailed computational study of the spatiotemporal adsorption/desorption dynamics under constant voltage and constant flow conditions. We show results for low (20 mM KCl) and relatively high (200 mM KCl) inlet ion concentrations and identify effects of ion starvation on desalination. We show that in both cases electromigrative transport eventually becomes negligible and diffusive ion transport reduces the desalination rate.

Induced-Charge Capacitive Deionization: The Electrokinetic Response of a Porous Particle to an External Electric Field

“…In light of better agreement with some experimental regimes [1,[25][26][27], we here adopt the modified Donnan (mD) model for capacitive charging which assumes no transport in the micropore region, but note that the qualitative results of our study , showing changes in the initial uniform concentration distribution (c0 = 1) due to electrosorption invoked by ICCDI around a porous disk. (a) presents the symmetric case, while (b) shows the asymmetric case where the positive ion has a higher mobility than the negative ion, and a larger depletion region forms around the negative pole.…”

“…Theoretical analysis: We begin by considering ion transport in porous media with a bimodal pore size distribution, characterized by a hierarchical structure having two types of pores. For activated carbon, relevant for our study, these are electro-neutral macropores of a typical scale of 1µm, and electrically charged micropores with overlapping EDLs of a typical scale of 1 nm, which occupy regions of porosities p M and p m , respectively [1,7,[24][25][26][27][28]. In our notation, the subscripts m, M, B indicate a physical quantity within the distinct regions of the micropores (m), macropores (M), and bulk (B), whereas +, − distinguish between cations and anions.…”

A Polyoxometalate‐Based Binder‐Free Capacitive Deionization Electrode for Highly Efficient Sea Water Desalination

“…In CDI, the removal of Na + and Cl À is achieved by an electric double layer between the interface of electrode and sea water. [6][7][8][9][10] During the charge process, Na + and Cl À are absorbed on correspondinge lectrodes by electrostatic attraction under low voltage. [11][12][13][14] The regeneration of the CDI electrode is realizedb yt he discharge process, in which Na + and Cl À are releaseda gain.…”

Construction of Maximum Tortuosity of Single Fluid Path in Grid-based 2-d System (5×20) and 3-d system (5×20×3) for Certain Value of Porosity

“…A two dimensional model of porous medium is considered, which is relatively similar to [10], where upper and lower plate are assumed to have ϕ = 0. Part with blue plates in figure 1 represent the material, while the empty spaces represent the void in the porous medium.…”