Electrochemical characteristics pyrolytic graphite|mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylimidazolium iodide interface

• A mixture of ionic liquids as an electrolyte for electrical double layer capacitors.
• Pyrolytic graphite as a working electrode.
• Capacitance increases at low ac frequencies and more positive electrode potentials.
• Adsorption kinetics of ions depends on addition of EMImI in EMImBF4.
• The system is stable in time.

The interface between pyrolytic graphite (PG) and the mixture of 1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) and 1-ethyl-3-methylimidazolium iodide (EMImI) were electrochemically characterized by using cyclic voltammetry and electrochemical impedance spectroscopy. The addition of EMImI in EMImBF4 influences the region of ideal polarizability and formation/rearrangement kinetics of the electrical double layer. The values of series and parallel capacitances, adsorption kinetics of ions and adsorption capacitance of the PG|EMImI + EMImBF4 interface were obtained. The PG electrode in EMImBF4 is ideally polarizable from −1.1 V to 1.3 V vs. Ag|AgCl. The mixture of ionic liquids was tested within wider potential region to see what happens at more positive potentials considering the possible specific adsorption of I− ions. The shape of the differential capacitance vs. potential curves depends strongly on the addition of EMImI, electrode potential and ac frequency. There are two capacitance maxima in the differential capacitance vs. potential curve. Based on the analysis of impedance data, the first capacitance maximum is mainly caused by the specific adsorption of iodide ions with the partial charge transfer from ions to the electrode surface and vice versa, while the second capacitance maximum has been assumed to be initiated by the faradic pseudocapacitive behavior at the PG|EMImI + EMImBF4 interface.