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

We demonstrate the phenomenon of induced-charge capacitive deionization that occurs around a porous and conducting particle immersed in an electrolyte, under the action of an external electric field. The external electric field induces an electric dipole in the porous particle, leading to its capacitive charging by both cations and anions at opposite poles. This regime is characterized by a long charging time, which results in significant changes in salt concentration in the electrically neutral bulk, on the scale of the particle. We qualitatively demonstrate the effect of advection on the spatiotemporal concentration field, which, through diffusiophoresis, may introduce corrections to the electrophoretic mobility of such particles.

Figure 1

(a) In ICCDI, an external electric field induces an electric dipole on a porous solid which leads to its capacitive charging over long durations of time. (b) In a typical CDI configuration an electrolyte is deionized by a pair of porous conducting electrodes connected directly to a power source, (c) in contrast to ICCDI the EDL of an impermeable and polarizable particle rapidly charges, resulting in nonpenetrating electric field lines.

Figure 2

Numerical simulation results at a nondimensional time 0.5 (scaled by $a^2/D$) [26], showing changes in the initial uniform concentration distribution ($c_0=1$) due to electrosorption invoked by ICCDI around a porous disk. Panel (a) presents the symmetric case, while panel (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.

Figure 3

Numerical simulation results at a nondimensional time 1 (scaled by $a^2/D$) [26], showing the concentration distribution due to the combined effect of ICCDI, uniform flow (either pressure driven or electroosmotic), and ICEO on a porous cylinder. (a) In the absence of advection, depletion regions are formed at $\theta =0$, $\pi$. (b) With strong ICEO, convection shifts the depletion toward the $\theta =\pm \pi /2$. (c) Under uniform flow, the left depletion region diminishes, while the right one is extended. In (b) and (c) the maximum slip velocity is set as $16D/a$. (d) Illustration of the combined effect of ICEO and advection.

Figure 4

Experimental results showing the concentration field due to ICCDI. Images show fluorescence signal of $100\mu M$ sodium fluorescein under an applied potential difference of 20 V between the right and left electrodes, around a 1.2 mm diameter carbon disk at times (a) 6 s, (b) 48 s, and (c) 108 s. Each frame (a)–(c) presents the change in fluorescence relative to the first frame, and is normalized by it for flat-field correction. (d)–(f) Raw fluorescence images showing the combined effect of ICCDI with pressure-driven flow from left to right, at different flow velocities $u$: (d) $600\mu \mathrm{m}/\mathrm{s}$, (e) $1\mathrm{cm}/\mathrm{s}$, (f) $4\mathrm{cm}/\mathrm{s}$.