Is the mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-butyl-3-methylimidazolium tetrafluoroborate applicable as electrolyte in electrical double layer capacitors?

The influence of small addition (from 0.005 to 1 M) of 1-butyl-3-methylimidazolium tetrafluoroborate (BMImBF4) in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) on the melting point, bulk conductivity and electrochemical properties of ionic liquid mixtures as electrolytes in electrical double layer capacitors (EDLCs) has been studied using cyclic voltammetry and electrochemical impedance spectroscopy methods. The melting point of EMImBF4 can be reduced from 13.5 °C to 5.6 °C by adding small amount of BMImBF4 (0.5 M) without noticeable decrease in capacitance (135 F g− 1 at 3.2 V), energy (49 Wh kg− 1) and power (38 kW kg− 1) densities and minor increase in characteristic relaxation time (~ 16 s) for EDLCs studied. However, the EDLCs based on EMImBF4 with 1 M addition of BMImBF4 (melting point of mixture − 6 °C) demonstrated noticeable decrease in capacitance (~ 122 F g− 1 at 3.2 V), energy (43 Wh kg− 1) and power (30 kW kg− 1) densities and increase in characteristic relaxation time to 25 s.

► A mixture consisting of two ionic liquids (IL) was used as electrolyte in EDLC. ► The melting point of ionic liquid in excess was reduced nearly 20 °C. ► The electrochemical characteristics of EDLC were studied at room temperature. ► Good electrochemical characteristics have been achieved for two IL based EDLCs.