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 [BF4]-, [PF6]-, [NTf2]-, 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 [C2mim][NTf2] and [N4,1,1,1][NTf2] but not in the more hydrophobic [P14,6,6,6][NTf2]. The presence of moisture has a significant impact on ORR currents in [C2mim][NTf2], even at extremely low humidity levels, which was verified by the higher level of water incorporation in [C2mim][NTf2] compared with [N4,1,1,1][NTf2] and [P14,6,6,6][NTf2]. Hydrophobic and large RTIL cations and anions (e.g., [P14,6,6,6]+ and [FAP]-) are recommended for applications where a stable ORR current response is required under humidified conditions.