Influence of Water on the Interface between Hydrophilic Ionic Liquid and Single Crystal Bismuth Electrodes

Abstract

Ionic liquids (ILs) as solvent-free electrolytes are new favorites for interfacial chemistry as well as for many other potential applications in electrochemical systems, such as supercapacitors, solar cells, fuel cells and secondary ion batteries. One of the main advantages of ILs is the good electrochemical stability. Differently from the hygroscopic, bulky ILs, many ILs are completely miscible with water, conventionally defined as hydrophilic. As an inevitable impurity, water has a strong effect on the surface of ILs within the electrochemical stability region where condensation of water molecules is dependent on applied potential [1-2]. The electrochemical faradic processes at the interface affect both charge transfer and energy storage processes. Those prompt us to focus on the interaction of ILs and water at electrified electrode interface, and the formation kinetics of the electrical double layer (EDL). Adsorption of particles, such as molecules or ions, contributes to the variety of EDL structure at ionic liquid│electrode interface. Halide ions, namely chloride, bromide and iodide, have been studied as part of a group of strongly adsorbing anions to form a dense layer at Bi(hkl) electrodes [3-4]. Electrosorption of water on electrode surface depends not only on the water-ion interaction, but also on the water-eletrode interaction [2]. To further explore the interaction mechanism of water molecules at electrified electrode interface, classical electrochemical methods (cyclic voltammetry and electrochemical impedance spectroscopy) as well as in situ scanning tunneling microscopy (STM) method have been applied to study the effect of water on the interfacial structure of ionic liquid 1-ethyl-3-methylimidazolium trifluoromethylsulfonate (EMImOTf)│Bi(hkl) interface. Single crystal bismuth electrodes as working electrodes have variable electronic properties and surface structure. Electrical equivalent circuit analysis derived from impedance data has been applied to characterize the interfacial processes [3]. The fitted data double layer capacitance (Cdl ) has a good agreement with the series capacitance (Cs ) at lower frequency, suggesting relative sluggishness of dried IL system. Compared with dry EMImOTf, systems with up to 0.1wt% water additive have a pronounced peak in Cdl vs. potential curves, observed within less negative potential region, showing that water has a much stronger interaction with IL anions than with the cations. OTf⁻ anions combined with aliphatic imidazolium cations are generally thought specifically adsorb at metallic electrodes due to the presence of sulfonyl groups. The different metallic properties of bismuth single crystal planes also show a difference in capacitive behavior. Other parameter, such as charge transfer resistance (Rct ) reveals how water adsorption process varies with applied electrode potential. The in situ STM results show that there are no surface reconstruction processes at Bi(111) surface within the studied polarization range. It should be noted, however, that for other single crystal planes surface reconstruction process are observed by varying the applied potential value, being affected by both anion adsorption and water content. Acknowledgements This work was supported by the Estonian Ministry of Education and Research (projects no. IUT 20-13, PUT1033, PUT1107 and PSG 249) and Estonian Centres of Excellence project TK141.