Differential Capacitance Measurements in Solvent‐Free Ionic Liquids at Hg and C Interfaces

Abstract

Series‐stacked, double‐layer carbon capacitors are slated to be used in electric vehicles for power management as well as in consumer electronics for memory backup and burst power. Nonaqueous electrolytes are preferred over aqueous electrolytes, since a wider voltage window can be accessed in the former electrolytes, thereby requiring fewer cells in the series stack. However, it has historically been difficult to assess whether the organic solvent and/or the supporting electrolyte determine the anodic limit. We have eliminated this ambiguity by using solvent‐free ionic liquids where the source of anodic oxidation may be ascribed to the anion alone. Even though the new ionic liquids manifested high oxidation limits, we found that when used in practical capacitors comprising high‐surface‐area carbon cloth electrodes, a much lower capacitance (compared to smooth electrodes) was achieved. To understand whether the observed decrease in capacitance might be due to the microporosity of the carbon cloth electrode or to practical limitation of the device itself, we first measured the differential capacitance ( C dl ) at a Hg/1‐ethyl‐3‐methyl imidazolium imide. The integral capacitance at the Hg interface was then calculated and compared with that of a smooth glassy carbon electrode, a carbon yarn, and a cloth electrode. In addition, the effect of ( CF 3 SO 2 ) 3 C − , ( CF 3 SO 2 ) 2 N − , CF 3 SO 3 − , and BF 4 − on C dl were interpreted based on existing theories of double‐layer structure.