Electroreduction of dioxygen in 1-n-alkyl-3-methylimidazolium tetrafluoroborate room-temperature ionic liquids

The electrode reaction of the molecular oxygen (O-2)/superoxide ion (O-2(-)) redox couple at glassy carbon (GC), gold (Au), and platinum (Pt) electrodes in three 1-n-alkyl-3-methylimidazolium tetrafluoroborate (AMIBF(4)) room-temperature ionic liquids (RTILs), 1-ethyl-3-methylimidazolium tetrafluoroborate, 1-n-propyl-3-methylimidazolium tetrafluoroborate, and 1-n-butyl-3-methylimidazolium tetrafluoroborate, has been analyzed quantitatively using cyclic voltammetry, normal pulse voltammetry, and hydrodynamic chronocoulometry (HCC). Cyclic voltammetric measurements showed that the redox reaction of the O-2/O-2(-) couple in these RTILs is a quasi-reversible process and that the resulting O-2(-) is stable. The relevant thermodynamic and kinetic parameters [the formal potential (E-0'), the standard rate constant (k(0)), and the cathodic transfer coefficient (alpha(c))] of the O-2/O-2(-) redox couple were evaluated using cyclic and normal pulse voltammetry: (i) the E-0' values are almost the same irrespective of the media and electrode materials examined, i.e., ca. -1.0 V vs. an internal standard potential of the ferrocene (Fc)/ferricinium ion (Fc(+)) redox couple, (ii) the k(0) value is dependent on the electrode material and increases in the order k(0) (Pt), k(0) (Au), k(0) (GC), and (iii) the values of alpha(c) are in the range of 0.35-0.47. The diffusion coefficients (D-O2) and saturated concentrations (C-O2) of O-2 in three AMIBF(4) RTILs were also determined by HCC. (C) 2004 The Electrochemical Society.