Improving Performance of Electric Double Layer Capacitors with a Mixture of Ionic Liquid and Acetonitrile as the Electrolyte by Using Mass-Balancing Carbon Electrodes

In the present study, we demonstrate the beneficial effect of mass-balancing electrodes in electric double layer capacitors (EDLCs) based on an acetonitrile/ionic liquid electrolyte. Cyclic voltammetry of single carbon electrodes in a mixture of acetonitrile and N-butyl-N-methylpyrrolidinium bis(trifluoromethanesulfonyl)imide (PYR 14 TFSI) 50% wt was performed to calculate the optimum mass ratio of electrodes (m + /m − ) that was found to be 2.0. Asymmetric EDLCs were assembled using this mass ratio and long-term cyclability testing was performed by galvanostatic charge/discharge at different cutoff voltages for 10000 cycles. Symmetric EDLCs were also assembled and characterized for comparison purposes. Asymmetric EDLCs showed an improved performance retaining the 99% of initial specific capacitance (C EDLC ), 96% of initial specific power (P average ) and the 86% of the initial real specific energy (E real ) after 10000 cycles at 3.2 V. On the contrary, the maximum operating voltage for the symmetric EDLC was only 2.7 V since a dramatic decrease in performance was observed at 3.0 V and 3.2 V. By using the mass-balancing strategy, it was possible to extend the maximum operating voltage from 2.7 V to 3.2 V. This increment in operating voltage resulted in higher values of real specific energy (E real ) that increased from 25 Wh kg −1 to 33 Wh kg −1 .