Pressure-driven electrokinetic energy conversion in conical nanochannels with ion concentration polarization

Abstract

Electrokinetic energy conversion (EEC) in nanochannels subject to a large pressure gradient (or difference) is affected appreciably by the ion concentration polarization (ICP). The current literature only addresses the ICP effects on EEC in nanochannels with uniform cross-sections, such as cylindrical nanochannels. Here, we adopt a numerical approach which couples the Poisson, Nernst–Planck, and Navier–Stokes equations to perform a detailed analysis of the pressure-driven EEC in nanochannels with varying cross-sections (i.e., conical nanochannels) under the ICP effect. The results reveal that two pressure gradients of the same magnitude but opposite directions can produce regulation effects on the EEC and ICP: under large values of the pressure gradient and surface charge density, a forward pressure gradient (e.g., the pressure gradient driving the fluid flow from the tip to the base of the nanochannel) leads to more nonlinear current-voltage (I-V) curves than a reverse pressure gradient (e.g., the pressure gradient driving the fluid flow from the base to the tip of the nanochannel); however, a forward pressure gradient produces weaker ICP than a reverse pressure gradient. These two opposite regulation behaviors suggest that the nonlinearity of I-V curves and the ICP is negatively interrelated, in contrast to the case of cylindrical nanochannels. Moreover, increasing nanochannel conicity enhances the nonlinearity of I-V curves but weakens the ICP effects, further confirming the negative interrelationship between the ICP and nonlinear I-V curves. This study clarifies the interrelationship between ICP and nonlinear I-V curves and is of practical significance for EEC device design and operation.