Electroreduction of Oxygen in a Series of Room Temperature Ionic Liquids Composed of Group 15-Centered Cations and Anions

Abstract

The electrochemical reduction of oxygen is reported in four room temperature ionic liquids (RTILs) based on quaternary alkyl -onium cations and heavily fluorinated anions in which the central atom is either nitrogen or phosphorus. Data were collected using cyclic voltammetry and potential step chronoamperometry at gold, platinum, and glassy carbon disk electrodes of micrometer dimension under water-free conditions at a controlled temperature. Analysis via fitting to appropriate theoretical equations was then carried out to obtain kinetic and thermodynamic information pertaining to the electrochemical processes observed. In the quaternary ammonium electrolytes, reduction of oxygen was found to occur reversibly to give stable superoxide, in an analogous manner to that seen in conventional aprotic solvents such as dimethyl sufoxide and acetonitrile. The most significant difference is in the relative rate of diffusion; the diffusion coefficients of oxygen in the RTILs are an order of magnitude lower than in common organic solvents, and for superoxide these values are reduced by a further factor of 10. In the quaternary phosphonium ionic liquids, however, more complex voltammetry is observed, akin to that expected for the reduction of oxygen in acidified organic media. This is shown to be consistent with the occurrence of a proton abstraction reaction between the electrogenerated superoxide and quaternary alkyl phosphonium cations following the initial electron transfer.

Oxygen reduction voltammetry on platinum macrodisk and screen-printed electrodes in ionic liquids: Reaction of the electrogenerated superoxide species with compounds used in the paste of Pt screen-printed electrodes?

“…Voltammetry for O 2 reduction has been widely reported in the literature in some (but not all) of these RTILs on glassy carbon (GC)/Au/Pt macro-, [22,25,26] Pt/Au/GC micro- [15,23,27,28] and Au microarray [29] electrodes, with similar voltammetric wave shapes to those observed in Figure 3. Table 2 shows the peak-to-peak separations (ΔE pp ) for the O 2 /O 2 -redox couple presented in Figure 3.…”

“…In the literature, it has been suggested that both imidazolium and phosphonium cations are weakly acidic [12,27] and may partake in an acid-base reaction in the presence of highly basic species (e.g. superoxide).…”

The electrochemistry of simple inorganic molecules in room temperature ionic liquids

“…Chronoamperometric transients were performed in the same manner as described in previous reports. 16,17 The experimental data were fit to the Shoup and Szabo expression, 20 as employed by Evans et al 21 The microdisk working electrodes were polished on soft lapping pads (Buehler, Illinois) with alumina slurries of size 1 m and 0.3 m respectively. The steady-state voltammetry of a 2 mmol L -1 solution of ferrocene in acetonitrile containing 0.1 mol L -1 tetra-n-butylammonium perchlorate (TBAP) was used to calculate the electrode diameter, adopting a value for the diffusion coefficient of 2.3 10 -9 m 2 s -1 at 298 K. 22 …”

Reactivity of the Ionic Liquid Pyr14TFSI with Superoxide Radicals Generated from KO2or by Contact of O2with Li7Ti5O12

“…In view of this advantage, the suitability of different ionic liquids for metal-air battery application, as measured by their stability toward superoxide radicals, has been studied by voltammetry 13,[22][23][24][25][26] as well as by tests employing KO 2 24,27,28 or in molecular orbital calculations. 25 While imidazolium-based ionic liquids were found to react with O…”