Two-Dimensional Porous Electrode Model for Capacitive Deionization

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.