Irreversible One-Electron Reduction of Dioxygen in Ionic Liquid Containing Olefinic Substituents

• Published in: International journal of electrochemical science 2016, Vol.11 (1), p.792-803
• Main Authors: Islam, Md Mominul, Kojima, Shimpei, Ferdousi, Begum Nadira ferdousi, Ohsaka, Takeo
• Format: Article
• Language: English
• Subjects: Cyclic voltametry; Double-potential step chronoamperometry; Ionic liquids; Irreversible oxygen reduction; Reaction of superoxide ion with olefin
• ISSN: 1452-3981; 1452-3981
• DOI: 10.1016/S1452-3981(23)15884-6

We observed for the first time an irreversible one-electron oxygen (O2) reduction reaction (ORR) in an ionic liquid (IL) containing olefinic substituents, 1,3-diallylimidazolium bis(trifluoromethansulfonyl)imide ([DiAlI+][N(Tf)2-]). Cyclic voltammograms measured for the ORR at glassy carbon, gold and platinum electrodes in the potential range of 0 → –1.3 → 0 V in [DiAlI+][N(Tf)2-] were observed with an irreversible cathodic peak at -0.95 V vs. Ag wire, while a quasi-reversible redox response for the O2/superoxide ion (O2•−) couple at a formal potential of -0.87 V was obtained in an analogous IL, [BMI+][N(Tf)2-] ([BMI+]: 1-butyl-3-methylimidazolium). The plot of current density (j) of the cathodic peak vs. square root of potential scan rate in [DiAlI+][N(Tf)2-] was found to be a straight line passing through the origin, suggesting that the ORR in [DiAlI+][N(Tf)2-] is diffusion-controlled. By examining the effect of water on the ORR, its irreversibility was justified not to result from the protonic impurity in [DiAlI+][N(Tf)2-]. The number of electrons involved in the ORR was confirmed to be one by comparing the experimental chronoamperometric current-time (j-t) curve with that derived theoretically. The saturated concentration and diffusion coefficient of O2 were simultaneously determined to be 1.6 mM and 1.3×10-4 cm2 s-1, respectively, via a computer simulation of the j-t curve. The one-electron reduction product of O2 (i.e., O2•- species) was proposed to undergo a follow-up reaction with the allyl group of [DiAlI+] to form an “intermediate”. Using a double-potential step chronoamperometric technique, the rate constant of this reaction was estimated to be 0.1 M-1 s-1.