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

The ion-pairing phenomenon during the dioxygen (O 2 ) 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 (EMIBF 4 ) 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 EMIBF 4 . The first and second cathodic peak potentials obtained in EMIBF 4 were by 0.07 and 0.65 V more positive than those observed for the reductions of O 2 to superoxide (O 2 •- ) and O 2 •- to peroxide (O 2 2- ) 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 EMIBF 4 . A 57 mV positive shift of the midpoint potential of the O 2 /O 2 •- redox couple with a 10-fold increase of EMIBF 4 in DMSO solution was obtained. The association constant and free-energy change (Δ G ) of the ion-pairing were determined to be 13.5 M -1 and −6.4 kJ mol -1, respectively. The value of Δ G for the comproportionation reaction of O 2 2- and O 2 to form O 2 •- in EMIBF 4 solution (−61.7 kJ mol -1 ) was one-half of that in DMSO solution (−117.7 kJ mol -1 ), suggesting a favorable stability of O 2 2- in EMIBF 4 .