(5g) Overlimiting Current In Microchannels

In classical electrochemical systems, the maximum sustainable current to a membrane or electrode is limited by diffusion and convection. In this work, we show that overlimiting currents can be achieved in micro-structures due to the nonlinear effect of surface conduction, even in the absence of convection. While the neutral bulk current remains limited, the surface current through the diffuse double layers allows for higher total currents, even at modest potentials. This phenomenon will generate strong concentration polarization and depletion regions that depend on the surface charge density, initial ion concentration, and channel size. An area-averaged 1D model was developed to further understand and quantify this phenomenon in a capped microchannel. From this model concentration profiles and current-voltage curves were simulated for a variety of system parameters. It was found that under overlimiting conditions, current varies linearly with voltage despite the nonlinearity of the mechanism. While the bulk concentration is depleted, the surface charge remains constant, establishing a constant resistance and linear current profile.