The electrochemical reduction of oxygen in room temperature ionic liquids for use in a lithium-air battery

Abstract

Experiments were undertaken to investigate the electrochemical reduction of oxygen (O2) in various room temperature ionic liquids (RTILs) with the aim of using a RTIL as an electrolyte in a lithium-air battery cell. Cyclic voltammetry (CV) in oxygenated RTILs was undertaken at Au and Pt microdisc electrodes. It was confirmed that the reduction of O2 in pyrrolidinium based RTILs occurs in two one-electron reduction steps, from O2 to superoxide (O2•?) and from O2•? to peroxide (O22?). It was found that the reduction of O2 was fully reversible in pyrrolidinium based ionic liquids. It was found that the O2 reduction reaction was unstable in imidazolium based RTILs, due to the reaction of O2•? with the imidazolium cations.
The diffusion coefficient (DO) and solubilities (cO) of O2 in the RTILs were found at various temperatures at Au and Pt microelectrodes using potential step chronoamperometry (PSCA). It was found that DO increases with temperature, due to the decreasing viscosity of the RTIL, while cO in RTILs decreases with temperature. It was found that attaching fluorine groups to the anion of the RTIL increased cO but also increased the viscosity and decreased DO. The most suitable RTIL for use in the lithium-air battery was found to be 1-butyl-1-methyl pyrrolidinium Nonafluorobutylsulfonyl(trifluoromethylsulfonyl)imide.
Attempts were made to measure DO and cO in the RTILs containing significant concentrations of Li+. The presence of Li+ caused the formation of a passivating layer of lithium peroxide (Li2O2) on the working electrode, complicating the interpretation of the PSCA results. It was found that the addition of the Lewis acid tris(pentafluorophenyl)borane (TPFPB) increased the solubility of Li2O2 to a slight degree; however the increase was not sufficient to prevent electrode passivation. TPFPB also increased the viscosity of the RTIL and therefore decrease the value of DO.
Experiments were undertaken to determine accurate values of DO and cO by measuring the pressure drop in a sealed vessel containing the RTIL and O2. This method was also used to investigate the effect of a redox shuttle compound, reduced ethyl viologen ditriflate [EtV]+, on the values of DO and cO. It was also found that the free volume of the RTIL influences DO. The greater amount of free volume present in the RTIL, the faster O2 is able to diffuse through the RTIL. It was also found that the presence of small quantities of [EtV]+ increased the effective value of DO, but large quantities decreased DO due to the increasing viscosity. A novel optical method of measuring DO was examined using the absorbance of [EtV]+ as an oxygen indicator.

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Identifiers

ORCID for Andrew Lodge: orcid.org/0000-0001-5004-208X

ORCID for John R. Owen: orcid.org/0000-0002-4938-3693

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