(476b) Anomalous Capacitance Maximum of the Carbon-Ionic Liquid Interface through Dilution with Organic Solvents

Electrical double layer capacitors have emerged as a promising means to store electrical energy reversibly and with high power output. At present, optimizing their energy densities, which are typically lower than batteries, while maintaining high conductivity is difficult because basic design principles for double layer capacitors do not exist. By studying the effect of diluting room temperature ionic liquids with organic solvents on the double layer capacitance, we can begin to gain insight into the factors governing optimal device performance. Specifically, we have used electrochemical impedance spectroscopy to measure the double layer capacitance of a standard ionic liquid diluted with a series of miscible organic solvents in contact with a glassy carbon electrode. We find that the double layer capacitance exhibits a maximum as a function of dilution, whose magnitude near the open circuit potential increases with decreasing polarity of the solvent. Because this behavior is not easily explained by existing mean field theories of the electric double layer, we have performed coarse-grained molecular dynamic simulations to better understand the deriving forces and molecular origins of this effect.