Role of anions in the electrochemical modulation of flammability of ionic liquids

Abstract

The flammability and combustion of liquid fuels is very dependent on volatility. Although room temperature ionic liquids (RTILs) without metastable anions possess high energy density, they are often considered non-flammable due to their low vapor pressure. We have demonstrated that these thermally stable and seemingly non-flammable imidazole cation based RTILs without metastable anions can be made flammable by applying a voltage bias which generates volatile flammable gaseous species. Conversely removing the voltage bias terminates the generation of these gaseous species, rendering the liquid non-flammable again. Utilizing this concept, we extend our study to investigate the effect of different anions BF₄⁻, ClO₄⁻, NO₃⁻, PF₆⁻ and CH₃COO⁻, paired with the same 1-butyl-3-methylimidazolium cation. We find that the rate of gaseous species generation largely depends on the conductivity of the RTILs. RTIL with higher conductivity produces more gaseous species. However, we also found that despite generating reactive gaseous species, some RTILs can still remain non-flammable. Mass spectrometric analysis of the gaseous species generated during the electrochemical decomposition shows that if the species generated from anodic oxidation possess flame inhibition properties, they can interfere with the combustion of the flammable species generated at the cathode, making them non-flammable. Flammability of other RTILs that do not have inhibiting species generated at the anode can be modulated electrochemically.