Overlimiting current in a microchannel

TL;DR

This paper investigates over-limiting current in microchannels by analyzing the roles of surface conduction, electro-osmotic flow, and electro-osmotic instability, revealing mechanisms relevant for applications in separation, desalination, and energy storage.

Contribution

It introduces simple models and simulations to identify and explain three distinct mechanisms driving over-limiting current in microchannels.

Findings

Surface conduction dominates in very thin channels.

Electro-osmotic flow is the main mechanism in thicker channels.

Electro-osmotic instability occurs at the membrane end in very thick channels.

Abstract

We revisit the classical problem of diffusion-limited ion transport to a membrane (or electrode) by considering the effects of charged side walls. Using simple mathematical models and numerical simulations, we identify three basic mechanisms for over-limiting current in a microchannel: (i) {\it surface conduction} carried by excess counterions, which dominates for very thin channels, (ii) convection by {\it electro-osmotic flow} on the side walls, which dominates for thicker channels and transitions to (iii) {\it electro-osmotic instability} on the membrane end in very thick channels. These intriguing electrokinetic phenomena may find applications in biological separations, water desalination, and electrochemical energy storage.