Superoxide reduction, ionic liquid

D. Hardacre, C. Seddon, K. R. Compton, R. G. Voltammetry of oxygen in the room-temperature ionic liquids l-ethyl-3-methylimida-zolium bis(triflyl)imide and HexEt3N+ TfiN one-electron reduction to form superoxide. Steady-state and transient behavior in the same cyclic voltammogram resulting from widely different diffusion coefficients of oxygen and superoxide. J. Phys. Chem. A 2003,... 

Interestingly, AfNashef et al. [10] reported the first evidence of the electrochemical generation of stable superoxide ions in the ionic liquid, l-n-butyl-3-methylimi-dazolium hexafluorophosphate ([C4mim][PF6]) (Scheme 18.2). However, the attachment of a methyl group to the carbon in the 2-position ([CqdmimjPFs]) led to an irreversible reduction process (Fig. 18.1) which in this work was attributed to the presence of impurities in the IL. The activity of the superoxide anions towards imidazolium-based ILs will be addressed below. 

To learn more about the stability of superoxide in the presence of quatemary-alkyl-ammonium cations, Evans et al. [20] performed a comprehensive study of the oxygen reduction reaction in several ILs containing highly fluorinated anions, including the quaternary ammonium ionic liquids [N2226l[N(Tf)2]. 

Despite the electrochemical stability of pyrrolidinium-based ionic liquids and its stability towards superoxide [21], only a few research works on ORR mechanisms have been reported from a fundamental point of view. Different studies on the oxygen reduction in [C4mpyr] [N(Tf)2] demonstrated an electrochemically reversible one-electron process following Eq. 18.7 [20, 22]. 

Electrocatalytic ORR carries out in three pathways the 1-electron transfer pathway, producing superoxide ion the 2-electron transfer pathway, producing hydrogen peroxide and the 4-electron transfer pathway, producing water. In a non-aqueous aprotic solvent system, a room-temperature ionic liquid system, and on specific transition-metal, macrocyclic-compounds-coated graphite electrodes in alkaline solutions, 1-electron reduction can be observed. Carbon materials, quinone and derivatives, mono-nuclear cobalt macrocyclic compounds, and some chalcogenides can only catalyze 2-electron ORR. Noble metal, noble metal alloy materials, iron-macrocyclic complexes, di-nuclear cobalt macrocyclic complexes, some chalcogenides, and transition-metal carbide-promoted Pt catalysts can catalyze 4-electron reduction. 

Zigah, D., Wang, A. F, Lagrost, C., Hapiot, R. Diffusion of molecules in ionic liquids/organic solvent mixtures. Example of the reversible reduction of O2 to superoxide. J. Phys. Chem. B 2009, 113, 2019-2023. 

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