In order to be self-sufficient with relatively constant energy output, renewable energy sources, such as solar and wind, require that energy be stored during periods of high energy production so that it can be available during periods of low or zero energy production. Among the many choices for energy storage devices, electrical double layer capacitors (EDLCs), also called supercapacitors, are attracting considerable attention. Supercapacitors store electrical energy via ion electrosorption directly in the EDLs at the electrolyte-electrode interface, suggesting that such liquid-solid interfaces play a dominant role in the underlying energy storage mechanism and the resulting device performance. Because electrical energy in supercapacitors is stored based on physical phenomena rather than chemical reaction (as in batteries), supercapacitors have fast rates of charge/discharge and a virtually limitless number of charge cycles (unlike batteries, which are often limited to 104 or less cycles). Much of the goal of supercapacitor research is aimed at increasing the amount of energy stored (energy density is the strong point in favor of batteries), which in turn focuses attention on the electrolyte, the nature of the electrode, and the electrode-electrolyte interactions.
To date, ionic liquids (ILs) have become emerging candidates for electrolytes used in supercapacitors, due to their exceptionally wide electrochemical window, excellent thermal stability, nonvolatility, and relatively inert nature; meanwhile carbons are the most widely used electrode materials in supercapacitors, due to their high specific surface area, good electrical conductivity, chemical stability in a variety of electrolytes, and relatively low cost. To improve the energy density and the transport properties of the charge carriers in supercapacitors, carbons have been developed in diverse forms such as activated carbons, carbon nanotubes (CNTs), onion-like carbons (OLCs), graphene, and so on. From the molecular view of the electrolyte-electrode interface, based on the geometry carbon electrodes and the way electrolyte ions interact with the electrode surface, in this talk supercapacitors are classified into three categories: (1) the term “endohedral supercapacitor” is used to denote supercapacitors with porous carbon electrodes showing a zero or negative surface-curved pores where ions can enter inside (i.e., the charge stored in the pore, e.g., ions inside CNTs), (2) the term “exohedral supercapacitor” is for supercapactiors in which ions reside on the outer surface of carbon particles (i.e., the charge stored on the positively curved surfaces, e.g., ions on outside surfaces of OLCs and end-capped CNTs), and (3) the term “planar supercapacitor” is for supercapacitors with zero/negligible curvature of electrode (e.g., flat graphene sheets). Using molecular modeling combined with molecular experimental probes, such as SAXS, SANS, NMR, AFM, etc., we have investigated the interfacial phenomena occurring between the IL electrolytes and the “planar”, “exohedral”, and “endohedral” electrodes to understand the energy storage mechanism of supercapacitors that rely on EDLs established at IL-electrode interfaces.