Ion Transport within High Electric Fields in Nanogap Electrochemical Cells

Ion transport near an electrically charged electrolyte/electrode interface is a fundamental electrochemical phenomenon that is important in many electrochemical energy systems. We investigated this phenomenon using lithographically fabricated thin-layer electrochemical cells comprising two Pt planar electrodes separated by an electrolyte of nanometer thickness (50-200 nm). By exploiting redox cycling amplification, we observed the influence of the electric double layer on transport of a charged redox couple within the confined electrolyte. Nonclassical steady-state peak shaped voltammograms for redox cycling of the ferrocenylmethyltrimethylammonium redox couple (FcTMA(+/2+)) at low concentrations of supporting electrolyte (≤10 mM) results from electrostatic interactions between the redox ions and the charged Pt electrodes. This behavior contrasts to sigmoidal voltammograms with a diffusion-limited plateau observed in the same electrochemical cells in the presence of sufficient electrolyte to screen the electrode surface charge (200 mM). Moreover, steady-state redox cycling was depressed significantly within the confined electrolyte as the supporting electrolyte concentration was decreased or as the cell thickness was reduced. The experimental results are in excellent agreement with predictions from finite-element simulations coupling the governing equations for ion transport, electric fields, and the redox reactions. Double layer effects on ion transport are generally anticipated in highly confined electrolyte and may have implications for ion transport in thin layer and nanoporous energy storage materials.