The Puzzling Processes at Electrode | Ionic Liquid Interface
Authors: Ers Heigo , Siinor Liis , Pikma Piret 
Published: November 2022

J.M. Lehn stated in his Nobel prize lecture in 1988 that supramolecular chemistry is the chemistry of the intermolecular bond, covering the structure and functions of entities formed by the association of two or more chemical species [1]. The adsorption of organic molecules is the net result of the interactions between the molecules and the two phases, the metal and the electrolyte, and the interactions between the latter two.
To this day, the universal understanding that the adsorption of organic molecules results in the formation of an ordered monolayer is based on the phenomenon observed in mainly aqueous solutions. However, the conception might not be straightforwardly transferrable to the organic additive + ionic liquid | electrode interface. For example, the existence of an interfacial multilayer structure of pure IL ions in contrast with an aqueous electrolyte has been previously shown in both experimental and computational studies. Furthermore, it has been shown that in ILs, if the ions form rigid layers on the electrode, it is necessary to apply an overpotential for interfacial restructuring. Therefore, studying the adsorption of organic additives in ionic liquids (IL) should contribute to a better understanding of metal | IL interfaces.
In the given presentation we overview the characteristics of solid-liquid interface characteristics of various additives from ionic liquid media at different electrodes.
Cyclic voltammetry, electrochemical impedance spectroscopy and in situ scanning tunnelling microscopy measurements were conducted to characterize the electrochemical behavior of the self-assembled layers of 4,4’-bipyridine (4,4’-BP) and 2,2’-bipyridine (2,2’-BP) at the Sb(111) | x-BP+1-ethyl-3-methylimidazolium tetrafluoroborate (EMImBF4) interface [2-3]. The specific adsorption of iodide ions was studied at Bi(111), Cd(0001) and pyrolytic graphite electrodes [4-6]. All the experiments were carried out in a three-electrode electrochemical cell in a glovebox.
The halide ions are surface active ions with specific adsorption behavior [4-6]. The strong specific adsorption of halide ions at single crystal electrodes, as well as other electrodes, is a complicated process due to the partial charge transfer between adsorbed ions and the solid electrode surface. The properties of the adsorption layer depend on the electrode potential and the concentration of the surface-active ions in an electrolyte. The theory that the adsorption of organic molecules results in SAM formation is mainly based on the results observed in aqueous solutions. However, these findings may not be as straightforwardly linkable to the organic additive + ionic liquid | electrode interface. The analysis of cyclic voltammetry and impedance results revealed that 2,2′-BP and 4,4′-BP indeed adsorb at the Sb(111) interface, forming a thin dielectric layer at the electrode surface, confirmed by in situ STM measurements, resulting in the differential capacitance values nearly two times lower compared to EMImBF4.
Acknowledgments:
This work was supported by the Estonian Research Council grant PSG249, and by the EU through the European Regional Development Fund under project TK141 (2014-2020.4.01.15-0011).
References:
[1] Lehn, J.-M., Angew. Chem. Int. Ed. Engl. 1988, 27 (1), 89–112.
[2] Pikma et al., Electrochem. Commun. 2015, 61 (Supplement C), 61–65.
[3] H. Ers et al., Electrochim. Acta 2022, 421, 140468.
[4] H. Ers et al., J. Electroanal. Chem. 2021, 903, 115826.
[5] L. Siinor et al., Electrochem. Commun. 2013, 35, 5–7.
[6] L. Siinor et al., J. Electroanal. Chem. 2014, 719, 133–137.