Influence of experimental conditions on the electrochemical window. Case study on bis(trifluoromethylsulfonyl)imide-based ionic liquids

Ionic liquids (ILs) are often selected as electrolytes in various devices, such as batteries, capacitors and sensors, because of their chemical stability, non-flammability, low volatility, high electrical conductivity, low melting point, high thermal stability, and wide electrochemical window (EW). The width of the EW of an IL depends on the cation type, the water content, the electrode material, and the chosen specific current density used to measure it. In this work, the EWs of four ILs containing the common and stable bis(trifluoromethylsulfonyl)imide anion and triethylsulfonium, 1-(2-hydroxyethyl)-3-methylimidazolium, 1-methyl-1-propylpiperidinium and 1-butyl-1-methylpiperidinium cations were investigated by linear sweep voltammetry in order to analyze their electrochemical stability toward reduction and oxidation at polycrystalline platinum, gold and glassy carbon disc electrodes under anhydrous conditions. Specific current densities of 0.1 mA·cm−2 or 1 mA·cm−2 or the linear fitting method were used for the systematic comparison of the EWs obtained from each dataset. The results indicate the dependence of the EW width of each IL on the electrode material and, more importantly, on the chosen specific current density. In this paper, we present new insight into some of the difficulties encountered during determination of the EW for ILs.