Overlimiting Current and Shock Electrodialysis in Porous Media

Abstract

Most electrochemical processes, such as electrodialysis, are limited by diffusion, but in porous media, surface conduction and electroosmotic flow also contribute to ionic flux. In this article, we report experimental evidence for surface-driven overlimiting current (faster than diffusion) and deionization shocks (propagating salt removal) in a porous medium. The apparatus consists of a silica glass frit (1 mm thick with a 500 nm mean pore size) in an aqueous electrolyte (CuSO4 or AgNO3) passing ionic current from a reservoir to a cation-selective membrane (Nafion). The current-voltage relation of the whole system is consistent with a proposed theory based on the electroosmotic flow mechanism over a broad range of reservoir salt concentrations (0.1 mM to 1.0 M) after accounting for (Cu) electrode polarization and pH-regulated silica charge. Above the limiting current, deionized water (≈10 μM) can be continuously extracted from the frit, which implies the existence of a stable shock propagating against the flow, bordering a depleted region that extends more than 0.5 mm across the outlet. The results suggest the feasibility of shock electrodialysis as a new approach to water desalination and other electrochemical separations.

Experimental Verification of Overlimiting Current by Surface Conduction and Electro-Osmotic Flow in Microchannels

“…While previous studies have established the simple power-law scaling of OLC with salt concentration and surface charge in a fixed geometry [35,36], our data reveal a nonmonotonic dependence on the microchannel depth with a minimum conductance at a depth of roughly 8 μm. From previous theory [31,35], the OLC due to SC is proportional to the volume-area ratio, or inverse of depth, d −1 , in nanochannels and thus the OLC decreases with increasing depth. In contrast, the OLC due to electroosmotic surface convection in microchannels is predicted to have the opposite trend, scaling as d 4=5 .…”

“…This new mode of surface convection thus cannot be described by classical Taylor-Aris dispersion [33,34]. The EOF mechanism, extended for "eddy fingers" in a random porous medium, has been confirmed indirectly by experiments measuring the current-voltage relation, scaling with salt concentration or surface charge, and desalination efficiency of "shock electrodialysis" [35]. The SC mechanism has also been confirmed in straight nanopores with controlled surface charge by again predicting the current-voltage relation and by ex situ imaging of metal electrodeposits grown along the pore walls by SC [36].…”

“…According to the theory using the typical surface charge of 50 mC in aqueous solution [31], the dominant mechanism of driving overlimiting current should vary with the microchannel depth among SC for d < 8 μm, electro-osmotic surface convection for 8 μm < d < 400 μm, and electro-osmotic instability on the membrane for d > 400 μm. The quasisteady current-voltage relation is measured by linear sweep voltammetry with a slow sweep rate at 0.2 V=15 sec from 0 to 7 V, which is slow enough to avoid any overshoot of the limiting current signifying transient effects [35,38]. A customized LABVIEW program automatically records the current data at each step.…”

Analysis of electrolyte transport through charged nanopores

“…It is well known that interfaces between charged membranes or nanochannels and unsupported bulk electrolytes lead to ion concentration polarization outside the membrane, e.g., in classical electrodialysis [33][34][35], but complex nonequilibrium electrokinetic phenomena resulting from strong concentration polarization have recently been discovered inside membrane pores or microchannels, such as deionization shock waves [32,[36][37][38][39][40][41] and overlimiting current sustained by surface conduction (electromigration) and electro-osmotic flow [42][43][44][45] with applications to nanotemplated electrodeposition [46] and water desalination by "shock electrodialysis" [47]. In most situations for nanochannels, the ions remain in local quasiequilibrium, since electromigration FIG.…”

Hydrodynamic dispersion in long microchannels under conditions of electroosmotic circulation: II. Electrolytes

“…For example, the concentration "shock waves" that occur in current-polarized sub-micro-channels in contact with nano-channels have been predicted for the first time by ) and further explored in (Mani and Bazant 2011;Dydek et al 2011;Yaroshchuk 2012;Deng et al 2013). In another publication (Zangle et al 2010), the authors considered the implications of depletion and enrichment "shocks" for the analyte pre-concentration via stacking and/or focusing.…”