A Rotating Ring Disk Electrode Study of the Oxygen Reduction Reaction in Lithium Containing Dimethyl Sulfoxide Electrolyte: Role of Superoxide

Abstract

We have employed the rotating ring disk electrode (RRDE) technique to study the oxygen reduction reaction (ORR) on gold and glassy carbon cathodes in dimethyl sulfoxide (DMSO) electrolytes containing lithium salts. At the gold ring electrode at 3.0 V vs. Li/Li + (0.1 M LiPF 6 ) soluble superoxide radical anion undergoes oxidation to O 2 under convective-diffusion conditions. For both glassy carbon and gold cathodes, typical oxygen reduction current-potential curves are sensitive to rotation speed and undergo a maximum and further electrode passivation by formation of Li 2 O 2 while the Au ring electrode currents follow the same peak shape with detection of soluble superoxide at the ring downstream in the electrolyte solution. Unlike the behavior in acetonitrile-lithium solutions, LiO 2 is more stable in DMSO and can diffuse out in solution and be detected at the ring electrode. While in cyclic voltammetry both time and potential effects are convoluted, we have carried out RRDE chrono-amperometry experiments at the disk electrode with detection of superoxide at the Au ring so that thus potential and time effects were clearly separated. The superoxide oxidation ring currents exhibit a maximum at 2.2 V due to the interplay of O 2 − formation by one-electron O 2 reduction, Li 2 O 2 disproportionation and two-electron O 2 reduction.

A Comprehensive Study on Oxygen Reduction and Evolution from Lithium Containing DMSO Based Electrolytes at Gold Electrodes

“…The shoulder in the current at the disc is paralleled by a peak in the current at the ring. This is most probably due to the formation of superoxide at the disc electrode and its subsequent oxidation at the ring electrode, as has been discussed in literature several times 10,11 However, the transition from the direct to the indirect pathway of peroxide formation has not been observed in an RRDE-measurement before. This is due to the fact that in the current study the electrolyte was not saturated with oxygen, but purged with a mixture of only 10% O 2 and 90% Ar.…”

“…[4][5][6] The process of Li 2 O 2 formation in DMSO seems to be unique as it involves the formation of a superoxide intermediate. [7][8][9][10][11] This has not only been shown by CV and rotating ring disc electrode (RRDE) measurements but also by combining electrochemistry with spin-trap experiments and EPRspectroscopy 12 as well as by DEMS experiments. 7 The remarkable stability of superoxide, despite the presence of Li + -cations, was ascribed to the large donor number of DMSO 8,13 of nearly 125 kJ/mol 14 as compared to acetonitrile with a donor number of 58.9 kJ/mol.…”

“…37 RRDE-measurements in connection with potential step experiments also show that ring currents are observed irrespective to which potential the disc electrode is stepped, whereas the peak height of the ring current depends on the disc potential. 11 It is the prevailing view in literature that the species that reacts at the ring electrode is superoxide. 6,8,10,11 However, it is also conceivable that hindered nucleation results in the formation of dissolved LiO 2 − which could react at the ring electrode.…”

“…11 It is the prevailing view in literature that the species that reacts at the ring electrode is superoxide. 6,8,10,11 However, it is also conceivable that hindered nucleation results in the formation of dissolved LiO 2 − which could react at the ring electrode. Since the value of z, which is below 2 suggestes the formation of superoxide, we deem this interpretation unlikely, but it cannot be ruled out based on the presented data.…”

EQCM study of oxygen cathodes in DMSO LiPF6 electrolyte

“…[12,24,25] Furthermore, re-oxidation of O 2 -can be detected in fast cyclic voltammetry with limited cathodic excursion so that further reduction or disproportion of superoxide is avoided [26]. The preferential solvation of Li + by DMSO molecules is also reflected in the lower electrode potential of Li/Li + in DMSO with respect to low donor number acetonitrile by 0.5 V. [27] In a recent communication of rotating ring disk electrode (RRDE) and electrochemical quartz crystal microbalance (EQCM) study of ORR in LiPF 6 /DMSO under a burst of soluble lithium superoxide oxidation measured by the ring current at short times has been described in coincidence with a time lag in the mass uptake by the EQCM.…”