Redox Cycling in Nanogap Electrochemical Cells. The Role of Electrostatics in Determining the Cell Response

Ion transport near interfaces is a fundamental phenomenon of importance in electrochemical, biological, and colloidal systems. In particular, electric double layers in highly confined spaces have implications for ion transport in nanoporous energy storage materials. By exploiting redox cycling amplification in lithographically fabricated thin-layer electrochemical cells comprising two platinum electrodes separated by a distance of 150–450 nm, we observed current enhancement during cyclic voltammetry of the hexaamine­ruthenium­(III) chloride redox couple (Ru­(NH3)6 3/2+) at low supporting electrolyte concentrations, resulting from ion enrichment of Ru­(NH3)6 3/2+ in the electrical double layers and an enhanced ion migration contribution to mass transport. The steady-state redox cycling was shown to decrease to predominately diffusion controlled level with increasing supporting electrolyte concentration. Through independent biasing of the potential on the individual Pt electrodes, the voltammetric transport limited current can be controlled without changing the electrochemical nature at the system. Using finite-element simulations based on numerical solutions to the Poisson and Nernst–Planck equations with Butler–Volmer type boundary conditions, we are able to semiquantitatively predict the voltammetric behavior of the nanogap cell that results from coupling of surface electrostatics and ion transport.