Current-induced membrane discharge

TL;DR

This paper introduces a new mechanism called current-induced membrane discharge (CIMD) that explains over-limiting current in ion-exchange membranes through chemical effects, challenging existing theories and impacting electrodialysis and pH control.

Contribution

It demonstrates that chemical effects like charge regulation and water self-ionization can cause over-limiting current without fluid flow, providing a new theoretical framework for membrane discharge.

Findings

CIMD causes membrane discharge and loss of selectivity.

CIMD interferes with electrodialysis by affecting salt co-ions.

CIMD suppresses electro-osmotic instability, altering existing models.

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 electro-osmotic 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.