Influence of anion composition and size on the double layer capacitance for Bi(111)|room temperature ionic liquid interface

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 (EMImBF4) 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 EMImBF4, 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 BF4− and TCB− anions.