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. 2021 May 14;23(18):10756-10762.
doi: 10.1039/d0cp04811a. Epub 2021 Mar 26.

Enrichment effects of ionic liquid mixtures at polarized electrode interfaces monitored by potential screening

Sunghwan Shin  1 , Francesco Greco, Florian Maier, Hans-Peter Steinrück
Affiliations

Enrichment effects of ionic liquid mixtures at polarized electrode interfaces monitored by potential screening

Sunghwan Shin et al. Phys Chem Chem Phys. .

Abstract

The behavior of ionic liquids (ILs) at charged interfaces is pivotal for their application in supercapacitors and electrochemical cells. Recently, we demonstrated for neat ILs that potential screening at polarized electrode interfaces shows a characteristic voltage dependence, as determined in situ by X-ray photoelectron spectroscopy. Herein, we use this fingerprint-type behavior to characterize the nature of the IL/electrode interfaces for IL mixtures of [C8C1Im][Tf2N] and [C8C1Im]Cl on Au and Pt electrodes. For Au, the IL/electrode interfaces are dominated by the Cl- anions, even down to a 0.1 mol% [C8C1Im]Cl content. In contrast, [Tf2N]- anions enrich at the IL/Pt electrode interfaces down to 10 mol% [C8C1Im][Tf2N]; only at lower concentrations does a transition to Cl- enrichment occur. These mixture studies demonstrate that even small concentrations of another IL or contamination, e.g. remaining from synthesis, can strongly influence the situation at charged IL interfaces.

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Conflict of interest statement

There are no conflicts to declare.

Figures

Fig. 1
Fig. 1. (a) Schematic sketches of the potentials at the IL electrode interfaces and the structure of the EDL, for the electrochemical cell with two Pt electrodes in contact with neat [C8C1Im][Tf2N]. The capacitances at the cathode and anode interfaces are indicated as Ccathode and Canode, respectively (note that for zero applied cell voltage (center), the numbers of anions and cations at the interface may not be exactly equal). (b) XPS spectra of the F 1s region according to Uapplied. The XPS spectrum at 0 V is shown in gray as a reference in the +2 V and −2 V spectra. The potential of the IL (EIL) and the binding energy of F 1s at each applied voltage are indicated by dashed lines.
Fig. 2
Fig. 2. Cathodic voltage of various [C8C1Im][Tf2N]xCl1−x mixtures versus applied voltage with x = 1, 0.999, 0.99, 0.9, 0.5, 0.1, 0.01 and 0, for (a) Pt/Pt electrodes and (b) Au/Au electrodes. The ideal lines for equal potential drops at the anode and cathode interfaces are indicated as dashed straight lines with a slope of +0.5 V/V. The error bars show standard deviations of the measurements. The data for the neat ILs (x = 1 and 0; open symbols) are taken from ref. 45. At the top of the figure, the structure of the ions of the IL mixture is depicted.
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