Mixing Ionic Liquids Affects the Kinetics and Thermodynamics of the Oxygen/Superoxide Redox Couple in the Context of Oxygen Sensing

The electrochemical oxygen reduction reaction is vital for applications such as fuel cells, metal air batteries and for oxygen gas sensing. Oxygen undergoes a 1-electron reduction process in dry ionic liquids (ILs) to form the electrogenerated superoxide ion that is solvated and stabilized by IL cations. In this work, the oxygen/superoxide (O₂/O₂^•-) redox couple has been used to understand the effect of mixing ILs with different cations in the context of developing designer electrolytes for oxygen sensing, by employing cyclic voltammetry at both gold and platinum electrodes. Different cations with a range of sizes, geometries and aromatic/aliphatic character were studied with a common bis(trifluoromethylsulfonyl)imide ([NTf₂]⁻) anion. Diethylmethylsulfonium ([S_2,2,1]⁺), N-butyl-N-methylpyrrolidinum ([C₄mpyrr]⁺) and tetradecyltrihexylphosphonium ([P_14,6,6,6]⁺) cations were mixed with a common 1-butyl-3-methylimidazolium ([C₄mim]⁺) cation at mole fractions (x) of [C₄mim]⁺ of 0, 0.2, 0.4, 0.6, 0.8, and 1. Both the redox kinetics and thermodynamics were found to be highly dependent on the cation structure and the electrode material used. Large deviations from “ideal” mixtures were observed for mixtures of [C₄mim][NTf₂] with [C₄mpyrr][NTf₂] on gold electrodes, suggesting a much higher amount of [C₄mim]⁺ ions near the electrode surface despite the large excess of [C₄mpyrr]⁺ in the bulk. The electrical double layer structure was probed for a mixture of [C₄mim]_0.2[C₄mpyrr]_0.8[NTf₂] using atomic force microscopy measurements on Au, revealing that the first layer was more like [C₄mim][NTf₂] than [C₄mpyrr][NTf₂]. Unusually fast kinetics for O₂/O₂^•- in mixtures of [C₄mim]⁺ with [P_14,6,6,6]⁺ were also observed in the electrochemistry results, which warrants further follow-up studies to elucidate this promising behavior. Overall, it is important to understand the effect on the kinetic and thermodynamic properties of electrochemical reactions when mixing solvents, to aid in the creation of designer electrolytes with favorable properties for their intended application.