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

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.