Roles of Ion Pairing on Electroreduction of Dioxygen in Imidazolium-Cation-Based Room-Temperature Ionic Liquid

Abstract

The ion-pairing phenomenon during the dioxygen (O2) reduction reaction (ORR) in a room-temperature ionic liquid (RTIL) was recognized and extensively studied using a voltammetric technique. Cyclic voltammetric two one-electron ORRs at a gold electrode in dimethyl sulfoxide (DMSO) solution containing 0.1 M tetraethylammonium perchlorate and in 1-ethyl-3-methylimidazolium tetrafluoroborate (EMIBF4) RTIL were carried out, and the obtained results were compared. Two cathodic peaks at −0.71 and −1.35 V versus Ag|AgCl|NaCl (sat.) and a well-defined anodic peak at −0.71 V and two ill-defined anodic peaks at potentials more positive than −0.2 V were observed in EMIBF4. The first and second cathodic peak potentials obtained in EMIBF4 were by 0.07 and 0.65 V more positive than those observed for the reductions of O2 to superoxide (O2•-) and O2•- to peroxide (O22-) in DMSO solution, respectively. Such a potential shift in the ORR was considered to be due to the ion-paring phenomenon as observed in the redox reaction of 1,4-benzoquinone in DMSO in the presence of EMIBF4. A 57 mV positive shift of the midpoint potential of the O2/O2•- redox couple with a 10-fold increase of EMIBF4 in DMSO solution was obtained. The association constant and free-energy change (ΔG) of the ion-pairing were determined to be 13.5 M-1 and −6.4 kJ mol-1, respectively. The value of ΔG for the comproportionation reaction of O22- and O2 to form O2•- in EMIBF4 solution (−61.7 kJ mol-1) was one-half of that in DMSO solution (−117.7 kJ mol-1), suggesting a favorable stability of O22- in EMIBF4.