Rosa ESPINOSA-MARZAL1, Qianlu ZHENG1
1University of Illinois at Urbana-Champaign, Urbana, United States
Supercapacitors are new-generation energy storage devices with high power density and long cycle life. Among electrolytes for supercapacitors, Ionic liquids (ILs) are promising candidates due to low volatility, high thermal stability, and wide electrochemical windows. Since a supercapacitor stores energy by forming an electrical double layer (EDL) at the electrode-electrolyte interface, it is important to understand the EDL properties. Here, the influence of the anion of three ILs with EMIM+ (ethyl-3-methyl imidazolium ethyl sulfate, 1-ethyl-3-methyl imidazolium tris(perfluoroalkyl)trifluoro phosphate, 1-ethyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl)imide), and the influence of the cation of three ILs with TFSI- (1-ethyl-3-methyl imidazolium bis(trifluoromethyl sulfonyl)imide, butyl trimethylammonium bis(trifluoromethyl sulfonyl)imide, ethyl dimethyl propylamine bis(trifluoromethyl sulfonyl)imide) on the EDL properties was investigated. The interfacial structure of the ILs on charged graphene surfaces was investigated by atomic force microscopy (AFM), the electrochemical windows were determined by cyclic voltammetry and the differential capacitance was characterized by electrochemical impedance spectroscopy (EIS). The studies were carried out in equilibrium with dry nitrogen and with 33% RH. Adsorption of dry ILs induced significant n-type doping of graphene, a phenomenon mainly dictated by the IL anion; n-type doping did not happen with the ethylsulphate anion but it was most prominent with TFSI-. The presence of water traces reduced n-type doping and magnified the influence of the IL cation on the capacitance. The analysis of the AFM results allowed us to understand the influence of IL cation, IL anion and water on the differential capacitance at the graphene/IL interface. This work attempts to scrutinize the subtle correlation between the differential capacitance and the nanoscale structures in the EDL. We are also interested in the textured graphene substrates to investigate the effect of strain and doping on the EDL. The knowledge will contribute to the optimization of IL-graphene-based supercapacitors and other electrochemical devices.