Electric Double Layer Capacitors (EDLCs) have received increasing attention during the past two decades since they fill the gap between batteries and conventional capacitors in the Ragone plot. During the charging process of EDLCs, owning to the electrostatic attraction, the electrolyte ions and the charges at the surface of electrode materials will form the electric double layer (EDL). The main challenge today for EDLCs is to increase their energy density without sacrificing their high power nature. Huge amount of effort has been devoted into developing room temperature ionic liquids (RTILs) as EDLC electrolyte materials since they provide a wide electrochemical window (> 5V) thus an efficient energy density enhancement. Understanding the EDL structure at the electrode/electrolyte interface and its dynamics holds the key to improve EDLCs. Of special interest is information on nanometer length scales which would allow us to find correlations between EDL structures, electrode microstructure and the overall electrochemical performance. Scanning Probe Microscopy (SPM) techniques are well suited to characterize electrode/electrolyte interfacial properties due to their high lateral and vertical resolution.
In this work, out-of-plane and in-plane double layer structure of ionic liquid electrolytes on the highly oriented pyrolytic graphite (HOPG) electrode can be directly visualized by performing force-distance curves and topographic imaging. The effect of electrolyte ion size/shape and electrode bias on the ionic liquid EDL structure can be observed. The possibility of applying this methodology to aqueous electrolytes for other applications or moving further to more advanced techniques (ex. band excitation) to study electrode/electrolyte interface will be discussed.
The work was supported by the Fluid Interface Reactions, Structures and Transport (FIRST), an Energy Frontier Research Center funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences. Measurements were performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences.