Critical assessment of superbase-derived protic ionic liquids as electrolytes for electrochemical applications

Non-volatile superbased-derived protic ionic liquids are thermally stable and highly conductive, thus hold great promises for electrochemical applications. However, systematic accounts of their electrochemical properties are yet to be established. In this contribution, five hydrophobic superbase-derived protic ionic liquids (PILs) have been prepared from Bronsted superbases and the salts of strong acids, and their decomposition temperature, density, conductivity and viscosity have been measured. The greatest viscosity was observed with the superbased PILs, [MTBDH][NfO] (2212 cP) and the least with [MTBDH][NTf2] (121 cP). Greatest conductivity was measured for [MTBDH][NTf2] (1.54 mS cm(-1)) and the least for [MTBDH][NfO] (0.089 mS cm(-1)). By combining density, conductivity and viscosity, a Walden plot was set up to demonstrate the degree of ionization, or 'ionicity' of each of the five PILs is greater than 10%. Their electrochemical characteristics were determined using cyclic voltammetry. Two IUPAC-recommended internal potential reference systems, ferrocene/ferrocenium and cobaltocenium/cobaltocene, were assessed for use in the five PILs. Potential windows of the five PILs were established at glassy carbon, gold and platinum electrodes, where the widest potential window was observed with glassy carbon electrodes with no direct correlation found between the Delta pKa values and the potential windows. The widest potential window was measured in [MTBH][beti] (4.3 +/- 0.1 V) and the shortest in [HNC(dma) H][beti] (2.7 +/- 0.1 V). The double layer capacitance was also investigated for potential applications in supercapacitors. (C) 2018 Elsevier Ltd. All rights reserved.