(220f) Observation of Surface-Charge-Induced Overlimiting Current In Porous Materials

Salt transport in bulk electrolytes occurs by diffusion and convection, but in microfluidic devices and porous media, the presence of charged side walls leads to additional surface transport mechanisms, surface conduction and electro-osmotic flows, which become more important as the bulk salt concentration decreases. As a result, it is possible to exceed the diffusion-limited current to a membrane or electrode. In this work, we present experimental observations of over-limiting current to an ion-exchange membrane through a porous glass frit with submicron pores. The current-voltage relation and approximately constant over-limiting conductance agree well with theoretical predictions, and high current behavior is consistent with the propagation of “desalination shocks”. We also demonstrate the continuous extraction of depleted solution from the frit near the membrane under conditions of over-limiting current. The advantage of working with porous media rather than microfluidic devices is that we can cheaply and easily “scale up” this phenomenon to macroscopic volumes for practical applications to water desalination and purification.