Current-Induced Membrane Discharge

Abstract

Possible mechanisms for over-limiting current (OLC) through aqueous ion-exchange membranes (exceeding diffusion limitation) have been debated for half a century. Flows consistent with electroosmotic instability (EOI) have recently been observed in microfluidic experiments, but the existing theory neglects chemical effects and remains to be quantitatively tested. Here, we show that charge regulation and water self-ionization can lead to OLC by "current-induced membrane discharge" (CIMD), even in the absence of fluid flow. Salt depletion leads to a large electric field which expels water co-ions, causing the membrane to discharge and lose its selectivity. Since salt co-ions and water ions contribute to OLC, CIMD interferes with electrodialysis (salt counter-ion removal) but could be exploited for current-assisted ion exchange and pH control. CIMD also suppresses the extended space charge that leads to EOI, so it should be reconsidered in both models and experiments on OLC.

Time-dependent ion selectivity in capacitive charging of porous electrodes

“…The complex interplay of these parameters may also explain the contradictory results on ion selectivity in the CDI literature [10,12,13,14,18]. To analyze these experiments in detail a full two-dimensional dynamic multicomponent CDI model is required including also proton transport [9,68] and the pH dependence of the ionic composition in those cases that amphoteric ions such as bicarbonate anions were used.…”

“…In commercial desalination technologies, such as reverse osmosis and multistage flash distillation, freshwater is produced at relatively high energy cost and often requires re-mineralization for human consumption. For brackish water or wastewater it can be advantageous to use a different type of technology, namely techniques where ions are removed from the feed water under the influence of electrical field effects, such as in electrodialysis [8,9], capacitive deionization [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27][28], membrane capacitive deionization [29][30][31][32][33][34], desalination using microchannels [35], batteries [36], microbial desalination cells [37] and wires [38]. Such techniques have the potential to be energy-efficient as they focus on the removal of the (often relatively few) ions in the water to obtain freshwater in this way.…”

Concentration polarization, surface currents, and bulk advection in a microchannel

“…A number of different mechanisms have been suggested as explanation for this overlimiting current, most of which are probably important for some system configuration or another. The suggested mechanisms include bulk conduction through the extended space-charge region [4,5], current induced membrane discharge [6], water-splitting effects [7,8], electroosmotic instability [9,10], and most recently, electro-hydrodynamic chaos [11,12].…”

A Model for the Operation of Perovskite Based Hybrid Solar Cells: Formulation, Analysis, and Comparison to Experiment

“…This is tantamount to assuming that there are sufficient surface recombination sites, so as to ensure that local equilibrium is always maintained, and that tunnelling effects at the contacts (and the diode-like behavior) can be neglected. 1 In order for the model to have a genuine equilibrium in the dark, in which the applied voltage and all current flows are zero, it is required that the global condition (A.11) be satisfied (derived in the appendix).…”