(42i) Electrochemical Cell Designed for in Situ Examination of Surfactant Ionic Liquid Interface Structure

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 or require long cycle lifetimes. EDLCs are used in electronics, home appliances, 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 interfacial where solvated ions pack on the electrode surface and balance the charge of the electrode. Ionic liquids (ILs) are a relatively new class of electrolytes being considered for EDLCs. ILs are composed only of ions and their interfacial structure is not readily understood. In order to successfully utilize ILs for energy storage and various electrochemical processes, the structure of ion accumulation must be understood from a fundamental level. Probing the interfacial structure in ILs requires that the IL be maintained without contamination of water or any other foreign species, and careful preparation of the electrodes. An electrochemical cell that can be used for in situ spectroscopic measurements has been designed and used to study to interfacial rearrangement of 4 different IL systems: neat 1-ethyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide, [EMIM][TFSI], neat n-propyl-n-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide, [PYR13][TFSI], and two anionic surfactant ILs, synthesized in house, as additives in [PYR13][TFSI].

Herein the benefits of using the in situ electrochemical cell for spectroscopic methods and its impact on understanding the interfacial layer of ILs is presented. The electrochemical cell was designed with the intent of making in situ surface enhanced Raman spectroscopy (SERS) measurements. Primary benefits of the designed cell include the capability of minimizing IL contamination from contact with air and improperly cleaned electrodes, and the ability to purge desired gasses through the cell. Electrochemical SERS measurements were performed with first activating the electrode surface for SERS via electrodeposition-dissolution cycling of metals (e.g., silver). Potentials were chosen for the specific ILs from along the differential capacitance curve in an attempt to add quantitative arguments to the expected formation and collapse of interface structure. Differential capacitance of the ILs was measured by electrochemical impedance spectroscopy (EIS) and verify that formation and collapse of distinct interfacial structure occurs within the chosen ILs. Overlapping the SERS measurements to specific differential capacitance points has helped build the understanding of IL interactions with charged surfaces by providing a quantitative description of interfacial rearrangements.