Two-electron quasi-reversible reduction of dioxygen at HMDE in ionic liquids: Observation of cathodic maximum and inverted peak

In this study, the cathodic maximum and inverted peak, which are observed for the first time during dioxygen reduction reaction (ORR) at a hanging mercury drop electrode (HMDE) in N,N-dialkylimidazolium-cation-based ionic liquids (ILs), were extensively studied in ILs, aqueous KOH and dimethylsulfoxide (DMSO) solutions using cyclic voltammetric and CCD video measurements. ORR at HMDE in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) and 1-n-butyl-3-methylimidazolium tetrafluoroborate ILs was observed to be a two-electron, quasi-reversible, diffusion-controlled process. Such ORR was considered to take place via the abstraction of proton at the 2-position of N,N-dialkylimidazolium cation (i.e., 1-ethyl-3-methylimidazolium cation (EMI+)) of IL, which was confirmed by studying the ORR in DMSO solution containing EMIBF4. Cyclic voltammetric results revealed that the occurrence of cathodic maximum and inverted peak has a threshold condition and that their magnitude is dependent on potential scan rate. Using a CCD video system, the cathodic peak including the cathodic maximum of the complicated ORR was found to be involved with a series of motions in a switching mode: upward → downward (during cathodic maximum) → upward. Similarly, a change of motion from strong upward to downward mode at HMDE/solution interface occurred when the inverted peak of the complicated ORR was switched to the anodic peak during the anodic potential scan. The adsorption-desorption of the carbene species formed via the de-protonation of EMI+, which was examined in KOH solution containing EMIBF4, was found to be associated with the cathodic maximum and inverted peak. The mechanism of ORR involving the cathodic maximum and inverted peak is discussed.