The Differential Capacitance of Ionic Liquid / Metal Electrode Interfaces – A Critical Comparison of Experimental Results with Theoretical Predictions

Abstract

Results of potential-dependent differential capacitance measurements on the interface between six different ionic liquids and the (111) surface of single-crystalline gold are presented. The measurements were done by means of broadband impedance spectroscopy in a frequency range from 10 mHz to 1 MHz. We discuss the influence of the IL cation, the IL anion and the cations' alkyl chain length on the interfacial capacitance. Our results suggest that (i) there is no simple relationship between the cation size and the value of the differential capacitance, (ii) the general shape of the potentialdependent differential capacitance curve is more strongly influenced by the IL anion, and (iii) experimental differential capacitance curves do not exhibit a simple "camel-" or "bell-shaped" curvature as predicted by mean-field theories. Furthermore, the broadband measurements show that two capacitive processes can be distinguished, which take place on millisecond and second time scales, respectively. While a millisecond time scale is expected for double-layer charging governed by the bulk conductivity of the IL, the existence of a slow process points to additional barriers for charge transport at the interface. The capacitance contribution of the slow process is most pronounced for ILs based on the N-butyl-N-methyl-pyrrolidinium ([Pyr 1,4 ]) cation. A comparison of capacitance data with insitu STM data from previous studies suggests that the slow process is connected to herringbone-type structures at the interface. While the herringbone superstructure of the Au(111) surface is well known in aqueous electrochemistry, a herringbone-type structure of adsorbed ions was described in a recent MD simulation paper by Federov and coworkers (K. Kirchner, T. Kirchner, V. Ivaništšev, M. V. Fedorov, Electrochim. Acta 2013, in press:

The metal–ionic liquid interface as characterized by impedance spectroscopy andin situscanning tunneling microscopy

“…(i) Roling and coworkers published EIS and in situ STM measurements on Au(111) electrodes in various (imidazoliumand pyrrolidium-based) ILs. [55][56][57][58][59] Their methods, materials, results and conclusions are rather similar to those described above, even if due to differences of representation it is somewhat difficult to compare their capacitance spectra to ours. They could model the frequency dependence of the capacitance spectra by a Cole-Cole function comprising three parameters of capacitances -which modelling is somewhat similar to ours, (cf.…”

Nanostructure of the Ionic Liquid–Graphite Stern Layer

“…The samples were 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 held at each potential for 8 minutes prior to data acquisition to ensure electrochemical equilibrium. In situ cyclic voltammagrams were obtained over a voltage window of ±1 V for all systems investigated and were in accordance with those obtained previously; 70 no electrochemical processes were observed over this voltage range. The OCP for the pure EMIm TFSI, EMIm TFSI + Li TFSI 0.1 wt/wt% and EMIm TFSI + EMIm Cl 0.1 wt/wt% systems were 0.26 V vs. Pt, 0.30 V vs. Pt and 0.42 V vs. Pt, respectively.…”

Minimizing Artifacts in Three-electrode Double Layer Capacitance Measurements Caused by Stray Capacitances

“…[11][12][13][14][15] Several groups used miniaturized cells, since only small amounts of new electrolytes had been synthesized or since purchasable ionic liquids were expensive. [12,[14][15][16] We compare the results obtained in the framework of the equivalent network to experimental double layer capacitance measurements using three different electrochemical cells: (i) Cell with a favorable positioning of the RE, but with an unfavorable ratio of the double layer capacitance to the stray capacitances; (ii) Cell with an unfavorable positioning of the RE, but a favorable ratio of the double layer capacitance to the stray capacitances; and (iii) Cell with both a favorable positioning of the RE and a favorable ratio of the double layer capacitance to the stray capacitances. On basis of these results we give qualitative and quantitative recommendations on how to minimize and/or correct three-electrode artifacts in double layer capacitance measurements, so that precise values for both double layer capacitance and electrolyte resistance can be obtained.…”