Facile preparation of hydroxide ion or proton conductive ionic liquids by mixing tetra-n-butylphosphonium hydroxide and benzimidazole

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Tetra-n-butylphosphonium benzimidazolate ([P4444][BzIm]) was prepared by mixing equimolar amounts of tetra-n-butylphosphonium hydroxide ([P4444][OH]) and neutral benzimidazole (BzIm). This salt had a melting point (Tm) of 55 °C and good thermal stability up to 300 °C. The equimolar mixture had ionic conductivity of 1.3 × 10−4 S cm−1 at 60 °C. Unequimolar mixing of these components gave rise either to hydroxide ions or proton conductive materials, depending on the mixing ratio. Addition of excess [P4444][OH] to benzimidazole (molar ratio of benzimidazole:[P4444][OH] = 1 : 2) led to good ionic conductivity (1.3 × 10−3 S cm−1 at 60 °C); this is attributed not only to [P4444] cations and [BzIm] anions but also hydroxide anions. Mixtures containing excess benzimidazole were expected to be proton conductors. As an example, the mixture with molar ratio benzimidazole : [P4444][OH] of 3 : 2 had ionic conductivity of 4.1 × 10−4 S cm−1 at 60 °C and a decomposition temperature (Td) above 300 °C. This conductivity was higher than the value expected from the Tg and other physicochemical properties of [P4444][BzIm], suggesting the contribution of proton conduction. Proton conduction was further confirmed by chronoamperometric measurement using electrochemical cells equipped with platinum electrodes.