Comparison of Electroreduction of Sulfur Dioxide in Ionic Liquid at Gold and Platinum Electrodes

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The electrochemical reduction of sulfur dioxide (SO2) has been studied in ionic liquid (IL) 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([Bmpy][NTf2]) at gold and platinum electrodes by cyclic voltammetry and in-situ electrochemical quartz crystal microbalance. A mechanism for the electrode reaction was proposed that the main process in the reduction was one electron-transfer process and the product was stable SO2 -• radical anion. While two anodic peaks were observed on the oxidation wave at both working electrodes. The anodic peak which appeared at a less negative potential was assigned to the oxidation of free SO2 -• radicals. However, the peak appearing at more positive potential was assigned to the oxidation of SO2 -• which was solvated into IL electrolyte on the electrode surface. Comparing gold and platinum electrodes, the gold electrode gave a better sensitivity due to the interaction between sulfur and gold surface. Based on the above proposed SO2 electrochemical reduction mechanism, real time SO2 detection by amperometric method was also investigated. The results demonstrated that the detection limit and sensitivity for SO2 was 39.4 ppm and 99 µA/[%SO2], respectively, in [Bmpy][NTf2] at gold electrode. A stable response was able to achieve over 30 days. Because of the attractive physical properties of ILs (e.g., less volatile and high thermal stability), the utilization of IL provides a novel electrochemical approach to develop a robust SO2 sensor.