Effect of Ionic Liquid Structure on the Oxygen Reduction Reaction Under Humidified Conditions

Abstract

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.

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

“…The change in the mechanism results in much larger currents recorded in wet conditions compared to dry conditions. The ORR was studied in a wide range of ILs with different anions and cations, and the IL structure was found to have a very large influence on the ORR currents in humidified conditions . At high humidities, currents increased in ILs according to the trends associated with the bulk solubility of water (up to a 6 times current increase for an imidazolium IL with the hydrophilic [BF 4 ] − anion).…”

“…By keeping the IL anion constant ([TFSI] − ) and changing the cation, it was shown that more hydrophilic cations (e.g., in [C 2 mim] + or [C 4 mpyrr] + ) in the EDL can be easily penetrated by water, which can then react with the electrogenerated superoxide radical. However, for much more hydrophobic cations (e.g., [P 6,6,6,14 ] + ), the dense hydrophobic layer formed at the electrode (due to the presence of long alkyl chains) effectively prevents water molecules from reacting with the electrogenerated superoxide radical in the interfacial region (see Figure a) . This was confirmed by AFM studies that showed that the [C 2 mim] + interfacial structure was obvious in dry conditions (see Figure b) but collapsed at 40% relative humidity .…”

“…The ORR was studied in a wide range of ILs with different anions and cations, and the IL structure was found to have a very large influence on the ORR currents in humidified conditions. 83 At high humidities, currents increased in ILs according to the trends associated with the bulk solubility of water (up to a 6 times current increase for an imidazolium IL with the hydrophilic [BF 4 ] − anion). However, at more moderate humidity levels below 40% relative humidity, the cation was found to have the most influence.…”

“…In very wet, water-saturated environments (e.g., >60% relative humidity), a careful choice of the anion is even more important, with [FAP] − proving to be the best anion for repelling water near the EDL. 83 In contrast, the accessibility of water can be also beneficial for sensing. For example, Chi et al 84 showed that trace water in [C 4 mpyrr][TFSI] can provide a proton source that facilitates the electrooxidation of acetaldehyde.…”

Modified Screen Printed Electrode Suitable for Electrochemical Measurements in Gas Phase

“…Voltammograms recorded at both unmodified and modified SPCE-RTILs showed a single cathodic peak located at about −0.85 V to which a very small return peak was associated in the reverse scan. This behavior, similar to that previously reported for the O 2 reduction in other RTILs, , pointed out that the prevailing reduction pathway is that usually occurring in wet media. On the other hand, Karl Fisher titrations conducted on the [BMIM]­[NTF 2 ] employed by us indicated the presence of a water content of about 0.23% w/w.…”