Electrochemical Oxidation of Hydrogen in Bis(trifluoromethylsulfonyl)imide Ionic Liquids under Anaerobic and Aerobic Conditions.

The electrochemical behavior of hydrogen oxidation on a platinum electrode in two aprotic room temperature ionic liquids (RTILs)-1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [Bmim][NTf2] and 1-butyl-1-methyl-pyrrolidinium bis(trifluoromethylsulfonyl)imide [Bmpy][NTf2]-was investigated in both anaerobic and aerobic conditions. At platinum electrode in the ILs, the first step of hydrogen oxidation is the formation of Pt-H(ad) (the Tafel step), which is similar to those observed in the aqueous electrolytes. However, there are differences in the oxidation steps (the Heyrovsky and Volmer steps). In ILs, the oxidation of Pt-H(ad) forms a hydrogen radical and a proton rather than a proton or a water in aqueous acid or alkaline electrolytes, respectively. This difference is significant as it results in a completely different following reaction pathway in the anaerobic vs aerobic conditions. A coupled chemical reaction between oxygen and hydrogen oxidation intermediates was observed in aerobic conditions which has a correlation with hydrogen concentrations. Furthermore, the overall rate of hydrogen oxidation is shown to be much higher in [Bmpy][NTf2] than that of [Bmim][NTf2], which is rationalized as the result of both higher solubility of hydrogen and the unique IL-electrode interface structure which promotes the hydrogen adsorption in [Bmpy][NTf2] than that of [Bmim][NTf2]. This study is the first example showing that hydrogen oxidation mechanism in aprotic ILs follows two different oxidation mechanisms in anaerobic and aerobic conditions.