Anomalous Capacitance Maximum of the Glassy Carbon−Ionic Liquid Interface through Dilution with Organic Solvents

ABSTRACT: We use electrochemical impedance spectroscopy to measure the effect of diluting a hydrophobic room temperature ionic liquid with miscible organic solvents on the differential capacitance of the glassy carbon−electrolyte interface. We show that the minimum differential capacitance increases with dilution and reaches a maximum value at ionic liquid contents near 5− 10 mol % (i.e., ∼1 M). We provide evidence that mixtures with 1,2-dichloroethane, a low-dielectric constant solvent, yield the largest gains in capacitance near the open circuit potential when compared against two traditional solvents, acetonitrile and propylene carbonate. To provide a fundamental basis for these observations, we use a coarse-grained model to relate structural variations at the double layer to the occurrence of the maximum. Our results reveal the potential for the enhancement of double-layer capacitance through dilution. Room temperature ionic liquids (RTILs) are regarded asthe next-generation electrolytes for electrochemical double-layer capacitors (EDLCs), as their low volatility and wide electrochemical windows (>4 V) can lead to safer devices with greater energy density.1 However, the low conductivity of RTILs limits the rate performance of EDLCs and yields device