High voltage, asymmetric EDLCs based on xerogel carbon and hydrophobic IL electrolytes

The preparation procedure, morphology and the surface chemistry of xerogel carbons, which exhibited up to 120 F g−1 in ionic liquid (IL) at 60 °C, are reported. Performance results of double-layer supercapacitors (EDLCs) featuring an asymmetric configuration with mesoporous xerogel carbon electrodes and hydrophobic ionic liquid electrolyte operating at 60 °C are also shown. The investigated ILs are 1-ethyl-3-methyl-imidazolium-bis(trifluoromethanesulfonyl)imide (EMITFSI) and N-butyl-N-methylpyrrolidinium-bis(trifluoromethanesulfonyl)imide (PYR14TFSI). While the PYR14TFSI features lower conductivity and determines slightly lower xerogel carbon capacitive response than EMITFSI, it enables a wider potential excursion for the negative electrode charging process. This is beneficial for high maximum cell voltages (Vmax) and, thus, for both the specific energy and power of asymmetric EDLCs. The asymmetric configuration we developed, with higher carbon loading at the positive electrode than that at the negative, makes it possible to tune the potential excursion of each electrode so that the EDLCs with EMITFSI and PYR14TFSI attain the safe Vmax of 3.4 V and 3.7 V, respectively, and high specific energy of ca. 30 Wh kg−1 between Vmax and 0.5Vmax over several thousand cycles.