Probing Space Charge and Resolving Overlimiting Current Mechanisms at the Micro-Nanochannel Interface Using Electrochemical Impedance Spectroscopy

We present new experimental and theoretical results concerning the nature of the overlimiting current in micro-nanochannel devices. Previous work has focused on the respective roles of a variety including electro-convective instability, shock-propagation, and electro-osmosis. Here we demonstrate a primary mechanism for the overlimiting conductance arises from the structure of the extended space charge developing at the depleted interface during high-current concentration polarization. This results in a maximum of the resistance which is captured well by asymptotic analysis of the extended space charge. Further, we show that this effect is strongly modulated by voltage-dependent changes in selectivity and the field-focusing effect. Lastly, we present experimental results demonstrating an anomalous resistance minimum occurring in devices with shorter nanochannels. Based on results of numerical simulation, this is hypothesized to result from the interaction between surface conduction mechanisms and the space charge structure at the nanochannel entrances.