Effect of Humidity and Impurities on the Electrochemical Window of Ionic Liquids and Its Implications for Electroanalysis

Abstract

Replacing conventional aqueous-based electrolytes with room-temperature ionic liquids (RTILs) for electrochemical applications is a major research focus. However, in applications where RTILs are exposed to real-world environments, their hygroscopic nature affects their promising physicochemical properties, such as broad electrochemical windows (EWs) and high chemical stability. In this study, the electrochemical windows of nine commercially available RTILs have been determined on platinum thinfilm electrodes in "dry" conditions (4.3−6.5 V) via cyclic voltammetry, and a systematic study over a wide humidity range (relative humidity (RH) between <1 and >95%) has been carried out. A significant reduction in the EW occurs even at low moisture contents (<10 RH%), which is especially evident for the most electrochemically stable ions in the study (i.e., [C 4 mpyrr] + , [FAP] − , and [NTf 2 ] − ). At saturated water levels, the electrochemical windows come close to that of water (approximately 2 V) regardless of the cation or anion structure, where the electrolyte behavior changes from "water-in-RTIL" to "RTIL-in-water." Additionally, the appearance of redox peaks from dissolved impurities inherent to the RTIL becomes more obvious with increasing water content. The effect of moisture on the electrochemical response of two model species where the presence of water does not alter the electrochemical mechanism, i.e., decamethylferrocene and ammonia, was also studied. For ammonia, the increase in current is not only caused by a change in the transport properties of the electrolyte (lower viscosity) but also by a shift in the anodic limit of the electrochemical window. This is believed to be the most detailed study of the effect of water on RTILs over a wide humidity range and emphasizes the importance of understanding the effect of water on voltammetric responses of dissolved species in RTILs under different environmental conditions.

Examining the Electrochemical Nature of an Ionogel Based on the Ionic Liquid [P66614][TFSI] and TiO2: Synthesis, Characterization, and Quantum Chemical Calculations

“…58,59 Because increased water content lowers viscosity, a reduced viscosity over time may be expected due to the absorption of moisture into the IL and the ionogel, leading to an increased diffusion coefficient of dissolved species such as analytes and/or impurities. 60 Additionally, viscosity is sensitive to variation in temperature. Increasing the temperature is typically expected to result in a decreased viscosity.…”

Electrical Double Layer Structure in Ionic Liquids and Its Importance for Supercapacitor, Battery, Sensing, and Lubrication Applications

“…29 The operating potential range or "electrochemical windows" of ILs are also very sensitive to changes in humidity levels in the environment above the IL. 30,31 The electrochemical window (EW) of "dry" ILs is determined by the oxidation and reduction of the anion and cation, respectively; EWs were found to be ca. 4−7 V on a Pt surface, depending on the IL structures.…”

“…However, "wet" RTILs had very narrow EWs on Pt at ca. 1.5−2.5 V, 30 suggesting that the anodic and cathodic limits are determined by the oxidation and reduction of water. Even at very low humidity levels in ILs (as low as 10% relative humidity), a significant amount of water can be present in the interfacial electrode/RTIL region that reduces the EW.…”

“…However, “wet” RTILs had very narrow EWs on Pt at ca. 1.5–2.5 V, suggesting that the anodic and cathodic limits are determined by the oxidation and reduction of water. Even at very low humidity levels in ILs (as low as 10% relative humidity), a significant amount of water can be present in the interfacial electrode/RTIL region that reduces the EW. , Understanding this effect is important because some applications require operation in real environments where moisture will be present at significant levels.…”

“…1.5–2.5 V, suggesting that the anodic and cathodic limits are determined by the oxidation and reduction of water. Even at very low humidity levels in ILs (as low as 10% relative humidity), a significant amount of water can be present in the interfacial electrode/RTIL region that reduces the EW. , Understanding this effect is important because some applications require operation in real environments where moisture will be present at significant levels. In situ atomic force microscopy (AFM) studies reveal that water electrosorption has a profound impact on the double-layer structure near charged and electrified planar surfaces .…”