Supercapacitors, or electrical double layer capacitors, are energy storage devices that offer higher power when compared to batteries. EDLCs are ideal for applications that can utilize a fast charge/discharge cycle and require long cycle lifetimes and are used in electronics, home appliances, and in some large-scale energy regeneration for automobiles and industrial equipment. Charge is stored in an EDLC by the accumulation of ions in the electrode-electrolyte interface. Aqueous and organic based systems create a well understood structure of accumulated ions where solvated ions pack into the interface and balance the charge of the electrode. Ionic liquids are a new class of electrolyte being considered for application in EDLCs. Ionic liquids are composed only of ions and do not form a structure at the interface that is readily understood. In order to successfully utilize Ionic Liquids for energy storage such as with supercapacitors, the structure of ion accumulation must be understood at a fundamental level. We investigated the structure of the ionic liquid 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [EMIM][TFSI], on two different electrodes and quantified the interfacial rearrangement thereby opening the door to begin a rational study of the interface through modification of the ionic liquid.

Differential capacitance of [EMIM][TFSI] on glassy carbon and silver electrodes has been measured by electrochemical impedance spectroscopy (EIS). The interfacial rearrangement was further characterized using electrochemical surface enhanced Raman spectroscopy (SERS). Potential dependence of the differential capacitance demonstrates the camel-like structure indicating formation and collapse of ion structure in the interfacial layer. The EIS data suggest that the formation of the structure occurs through multiple processes which change as the capacitance maximum is approached. On the silver electrode distinct shifts in the vibrational energies of the interface are shown via SERS to occur as the cell potential is forced away from open circuit potential which is associated with local changes in polarizability of molecular interactions associated with the interfacial structure. The series of techniques presented in this work open the path for different ionic liquids to be studied.