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 (EMIBF4) RTIL were carried out, and the obtained results were compared. Two cathodic peaks at -0.71 and -1.35 V versus Ag vertical bar AgCl vertical bar 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 02 to superoxide (O-2(center dot-)) and O-2(center dot-) 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 EMIBF4. A 57 mV positive shift of the midpoint potential of the O-2/O-2(center dot-) redox couple with a 10-fold increase of EMIBF4 in DMSO solution was obtained. The association constant and free-energy change (Delta G) of the ion-pairing were determined to be 13.5 M-1 and -6.4 kJ mol(-1), respectively. The value of Delta G for the comproportionation reaction Of O-2(2-) and O-2 to form O-2(center dot-) 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 O-2(2-) in EMIBF4.