Evaluation of the dynamic electrochemical stability of ionic liquid–metal interfaces against reactive oxygen species using an in situ electrochemical quartz crystal microbalance

Abstract

The dynamic interactions between the electrochemically generated superoxide radical (O₂˙⁻) and three structurally different ionic liquids (ILs) were characterized using an electrochemical quartz crystal microbalance (EQCM). We established the long-term stability of the interface (on the scale of hours and weeks) against the most ubiquitous interferent in electrochemical energy devices and sensors: oxygen. Oxygen potentially limits the application of IL electrolytes in these devices. In particular, the electrochemical behavior of the O₂/O₂˙⁻ couple and the ion pair formed between the cation of an IL and O₂˙⁻ were evaluated. O₂˙⁻ tends to form ion pair complexes with the cation of an IL, subsequently abstracting a proton to form different products depending on the cationic structure of the IL used. The reversibility of the O₂/O₂˙⁻ electrode reaction depends on the subsequent chemical reactions between O₂˙⁻ and the IL, which are more pronounced at slow scan rates. It was found that O₂˙⁻ was significantly more stable in the IL with the [BMPY] cation than in ILs with imidazolium salts. The stability of the ILs towards O₂˙⁻ attack follows the order [BMPY][NTf₂] > [BdMIM][NTf₂] > [BMIM][NTf₂] as evaluated on a time-scale of a few seconds to minutes and up to 3 weeks. It was found that the formation of the [Cation]⋯O₂˙⁻ ion pair complex lowers the local viscosity of the IL near the electrode, reflected in the change in the oscillating frequency of the quartz crystal electrode. As much as is the feasibility for the formation of this ion pair, that much is the tendency of the IL to lose its electrochemical stability. A combined analysis can provide a quick indication of the dynamic stability of the electrode–electrolyte interface for any IL in the presence of certain electrochemical reactions using the EQCM technique.