The Influence of Water on the Double-Layer Capacitance of an Ionic Liquid
Authors: Friedl Jochen , Stimming Ulrich 
Published: February 2020

Solvent-free electrolytes, room temperature ionic liquids (RTILs) have properties in between solid electrolytes and electrolyte solutions with a number of potential applications in electrochemical systems. One of their main advantages is their large potential window, in some cases 4.5-6V 1,2. The presence of impurities, water in particular, may reduce the width of this potential window. The adsorption of water at electrodes has only recently been investigated theoretically3 and experimentally2,4. Feng et al. studied humid imidazolium-based ionic liquids and the adsorption of water at a carbon-like electrode using molecular dynamics simulations3. They found that the adsorption of water increases with potential and that the electrosorption effect is stronger at potentials positive of the potential of zero charge (p.z.c.), due to the stronger interaction of water with the anions. Motobayahsi and Osawa used Surface Enhanced Infrared Absorption Spectroscopy (SEIRAS) as a tool to detect the presence of water in the electrochemical double layer (ECDL) of a humid imidazolium-based RTIL at an Au electrode 4. They confirmed that water accumulates at the electrode even at low concentrations and that the adsorption is more pronounced at potentials positive of the p.z.c..
In this study we employ electrochemical impedance spectroscopy to determine the electrochemical double layer capacity CDL of 1-butyl-1-methy-pyrrolidinium bis[(trifluoromethyl)sulfonyl]imide (PyrTFSI) in contact with an Au(111) electrode. The water content of PyrTFSI is carefully adjusted and monitored from very dry (from 1.4 water molecules for 1000 RTIL anions (1000 : 1.4) to 1000 : 87.4). Recorded Nyquist spectra are fitted to three equivalent circuits and we give a detailed physical explanation for the single circuit elements. Electrochemical Scanning Tunnelling Microscopy is used to monitor the Au(111) surface to ensure that the electrode does not change during the electrochemical measurements.
Our experimentally obtained CDL vs. potential curves are compared to curves calculated by mean-field theory (Fig. 1). Together, experiment and theory yield a conclusive understanding of the influence of water on the electrode-RTIL interface:
1.Our results show potential-dependent agglomeration of water in the ECDL, as found by AFM 4;
2.Theoretical predictions regarding the sensitivity of CDL of RTILs towards water are confirmed 3;
3.Theoretical tools such as mean-field theory calculations with the compacity parameter γ of RTILs 5 are shown to be able to reproduce experimental data. From the parameters used in the calculation properties of RTILs, like the preferred interaction of water with anions, can be deducted.
(1) Buzzeo, M. C.; Hardacre, C.; Compton, R. G. ChemPhysChem
2006, 7 (1), 176–180.
(2) Friedl, J.; Markovits, I. I. E.; Herpich, M.; Feng, G.; Kornyshev, A. A.; Stimming, U. ChemElectroChem
2016, 71 (111), 311–315.
(3) Feng, G.; Jiang, X.; Qiao, R.; Kornyshev, A. A. ACS Nano
2014, 8 (11), 11685–11694.
(4) Motobayashi, K.; Osawa, M. Electrochem. commun.
2016, 65, 14–17.
(5) Kornyshev, A. A. J. Phys. Chem. B
2007, 111 (20), 5545–5557.

Figure 1:
(left)
Normalized double layer capacitance over potential of 1-butyl-1- methyl-pyrrolidinium bis[(trifluoromethyl)sulfonyl]imide with various contents of water obtained from electrochemical impedance spectroscopy. (right) Double layer capacitance over potential calculated from mean field theory with the driest ionic liquid in red and the wettest in teal. with Graph adapted from
2
.

Figure 1