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.

Ionic liquids (ILs) have become highly popular solvents over the last two decades in a range of fields, especially in electrochemistry. Their intrinsic properties include high chemical and thermal stability, wide electrochemical windows, good conductivity, high polarity, tunability, and good solvation properties, making them ideal as solvents for different electrochemical applications. At charged surfaces such as electrodes, an electrical double layer (EDL) forms when exposed to a fluid. IL ions form denser EDL structures compared to conventional solvent/electrolyte systems, which can cause differences in the behavior for electrochemical applications. This Perspective discusses some recent work (over the last three years) where the structure of the EDL in ILs has been examined and found to influence the behavior of supercapacitors, batteries, sensors, and lubrication systems that employ IL solvents. More fundamental work is expected to continue in this area, which will inform the design of solvents for use in these applications and beyond.

The oxygen reduction reaction (ORR) is widely studied in room-temperature ionic liquids (RTILs) but typically in dry environments. Because water is known to affect diffusion coefficients and reaction outcomes, the influence of water on the ORR is expected to be significant. We have therefore studied the effect of RTIL structure on the ORR at different relative humidity (RH) levels using cyclic voltammetry. A broad range of cations including imidazolium, ammonium, pyrrolidinium, pyridinium, sulfonium, and phosphonium, and anions such as and[FAP] − were employed. The cation was found to have a large effect on the reduction current of oxygen, even at low humidity levels (<40 RH %), whereas the anion mainly influenced the current at higher humidity levels (>65 RH%). Consequently, the choice of cation needs to be carefully considered when selecting a suitable RTIL solvent for oxygen reduction in humidified environments. The size, structure, and hydrophobicity of the ions were found to dictate the degree at which the RTIL is susceptible to changes in humidity. The physical characteristics of the RTIL electric double layer on platinum electrode surfaces were further investigated by atomic force microscopy force-curve studies in three selected RTILs. The results suggest that there is a significant amount of water incorporated at the electrode−RTIL interface in [C 2 mim][NTf 2 ] and [N 4,1,1,1 ][NTf 2 ] but not in the more hydrophobic [P 14,6,6,6 ][NTf 2 ]. The presence of moisture has a significant impact on ORR currents in [C 2 mim][NTf 2 ], even at extremely low humidity levels, which was verified by the higher level of water incorporation in [C 2 mim][NTf 2 ] compared with [N 4,1,1,1 ][NTf 2 ] and [P 14,6,6,6 ][NTf 2 ]. Hydrophobic and large RTIL cations and anions (e.g., [P 14,6,6,6 ] + and [FAP] − ) are recommended for applications where a stable ORR current response is required under humidified conditions.

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