Nonadditivity of Faradaic Currents and Modification of Double Layer Capacitance in the Voltammetry of Mixtures of Ferrocene and Ferrocenium Salts in Ionic Liquids

摘要

Electrochemical studies on the Fc + e- / Fc+ (Fc )nferrocene) process have been undertaken via thenoxidation of Fc and reduction of Fc+ as the hexafluorophosphaten(PF6n-) or tetrafluoroborate (BF4n-) saltsnand their mixtures in three ionic liquids (ILs) (1-butyl-n1-methylpyrrolidinium bis[(trifluoromethyl)sulfonyl]imide,n1-butyl-3-methylimidazolium tetrafluoroborate,nand 1-butyl-3-methylimidazolium hexafluorophosphate).nData obtained at macro- and microdisk electrodesnusing conventional dc and Fourier-transformed largeamplitudenac (FT-ac) voltammetry reveal that diffusionncoefficients for Fc and Fc+ differ significantly and arena function of the Fc and Fc+ concentration, in contrastnto findings in molecular solvents with 0.1 M addednsupporting electrolyte media. Thus, the Faradaic currentsnassociated with the oxidation of Fc (Fc0/+) andnreduction of FcPF6 or FcBF4 (Fc+/0) when both Fc andnFc+ are simultaneously present in the ILs differ fromnvalues obtained when individual Fc and Fc+ solutionsnare used. The voltammetry for both the Fc0/+ and Fc+/0nprocesses exhibited near-Nernstian behavior at a glassyncarbon macrodisk electrode and a platinum microdisknelectrode, when each process was studied individuallynin the ILs. As expected, the reversible formal potentialsn(E°′) and diffusion coefficients (D) at 23 ( 1 °C werenindependent of the electrode material and concentration.nHowever, when Fc and FcPF6 or FcBF4 were bothnpresent, alterations to the mass transport processnoccurred and apparent D values calculated for Fc andnFc+ were found to be about 25-39% and 32-42%nlarger, respectively, than those determined from individualnsolutions. The apparent value of the double layerncapacitance determined by FT-ac voltammetry fromnindividual and mixed Fc and Fc+ conditions at the GCnelectrode was also a function of concentration. Doublenlayer capacitance values increased significantly withnthe concentration of Fc and FcPF6 or FcBF4 whennspecies were studied individually or simultaneously,nbut had a larger magnitude under conditions wherenboth species were present. Variation in the structurenof the ILs and hence mobilities of the ionic species,nwhen Fc and FcPF6 or FcBF4 are simultaneouslynpresent, is considered to be the origin of the nonadditivitynof the Faradaic currents and variation inncapacitance.