The influence of addition (5 wt%) of 1-ethyl-3-methylimidazolium iodide (EMImI) in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF₄) on the melting point, bulk conductivity and electrochemical properties of the room temperature ionic liquid (RTIL) mixture as an electrolyte in electrical double layer capacitor (EDLC) has been studied using cyclic voltammetry, electrochemical impedance spectroscopy, constant current charge-discharge and constant power discharge methods. Electrodes were prepared of the D-glucose derived activated carbon powder, which was prepared by hydrothermal carbonization method followed by additional pyrolysis and carbon dioxide activation steps. The specific conductivity reduced from 13.6 mS cm⁻¹ to 12.8 mS cm⁻¹ and the melting point increased from 17°C to 19.5°C by adding 5 wt% of EMImI into EMImBF₄. However, noticeable increase in specific capacitance (245 F g⁻¹ at 1.0 V), specific energy (36.7 Wh kg⁻¹) and specific power and minor increase in characteristic relaxation time constant (from 1.45 s to 1.48 s) values has been established for EDLCs using the mixture of RTIL as an electrolyte. The EDLC based on EMImBF₄ + 5 wt% EMImI demonstrated nearly 50% higher stored charge values than EDLCs without EMImI addition.

Cyclic voltammetry and electrochemical impedance spectroscopy have been applied for investigation of electrochemically polished Bi(111) electrode in 1-ethyl-3-methyl imidazolium tris(pentafluoroethyl)trifluorophosphate (EMImFAP), 1-ethyl-3-methyl imidazolium tetracyanoborate (EMImTCB) and 1-ethyl-3-methyl imidazolium tetrafluoroborate (EMImBF 4 ) ionic liquids. The region of ideal polarizability, series resistance and capacitance, limiting high-frequency and low-frequency capacitances have been calculated. The lower series capacitance values at electrode potential less negative than the potential of the minimum in the capacitance vs. voltage curve for Bi(111)|EMImFAP than that for EMImBF 4 , and especially for EMImTCB, have been explained by the bigger diameter of FAP − , higher cation and anion sizes symmetry, and less expressed surface activity (i.e. lower closest approach of the FAP − mass centre to an electrode surface) compared with BF 4 − and TCB − anions.

The influence of the electrode potential on the <mml:math display="inline"><mml:mrow><mml:mtext>Bi</mml:mtext><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mn>111</mml:mn></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mo form="infix">∣</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:math>Bi(111)∣1 -ethyl-3-methylimidazolium tetrafluoroborate interface has been studied by cyclic voltammetry and electrochemical impedance. The weak deviation of the system from capacitive behavior for the ideally flat electrode has been established within the moderate electrode potential region from −0.9 to −0.1 V vs <mml:math display="inline"><mml:mrow><mml:mtext>Ag</mml:mtext><mml:mo form="infix">∣</mml:mo><mml:mtext>AgCl</mml:mtext></mml:mrow></mml:math>Ag∣AgCl in 1-ethyl-3-methylimidazolium tetrafluoroborate. The parallel faradaic processes are possible outside the potential region from −0.9 to −0.1 V. The results obtained for the <mml:math display="inline"><mml:mrow><mml:mtext>Bi</mml:mtext><mml:mrow><mml:mo>(</mml:mo><mml:mrow><mml:mn>111</mml:mn></mml:mrow><mml:mo>)</mml:mo></mml:mrow><mml:mo form="infix">∣</mml:mo><mml:mn>1</mml:mn></mml:mrow></mml:math>Bi(111)∣1 -ethyl-3-methylimidazolium tetrafluoroborate interface have been compared with those for gold, mercury, and glassy carbon electrodes.