(487b) Role of Anion in the Electrochemical Flammability Modulation of Room Temperature Energetic Ionic Liquids

Room temperature ionic liquids are considered as non-flammable owing to their exceptional properties such as superior thermal stability and low vapor pressure. Thermally insensitive ionic liquids can produce oxidation resistant species upon thermal decomposition. In contrast room temperature ionic liquids (RTILs) have been named ‘energetic ionic liquids’ (EILs) and regarded as a new generation of green high energy density (HED) hypergolic fuels. High energy density fuels having high energy density and net volumetric energy content are desirable because the performance of propulsion engine deeply depends on the above-mentioned properties of fuel.In light to this the HED fuels come in handy. Among several categories of high energy density fuels ionic liquids have caught attention being a potential ‘green’ fuel. While energetic ionic liquids with metastable anions like azide, dinitramide, borohydride, and azole can thermally decompose to reactive flammable species allowing them to be used in explosive formulations, thermally insensitive RITLs without these reactive anions cannot produce reactive species upon decomposition and require other routes to be activated. In our study we demonstrated that most widely used imidazole cation based ionic liquids can be made flammable through electrochemical route. We have shown that upon electrochemical decomposition, the imidazole cation gives rise to reactive gas species which are combustible with the help of an external ignition source .We can modulate their flammability by controlling the extent of volatility through applied voltage bias. We have showed that the aromaticity of the imidazole ring of the apparent thermally stable and nonflammable imidazole based ionic liquids can be broken electrochemically resulting in the volatilization of the RTILs as reactive flammable species and the removal of voltage bias will terminate this electrochemical reaction and consequently the volatile species generation will come to a halt resulting in flame extinction. The heat feedback from the flame is not enough for the thermally insensitive ionic liquids to sustain the flame and as a result application of voltage bias is always required for continuous combustion. Interestingly what we have found that the electrochemical modulation of the flammability of the ionic liquids largely depends on the anions present regardless of sharing the same organic cation. Despite having the same imidazole cation which upon reduction produces flammable gas species, not all the ionic liquids are combustible highlighting the significant contribution of the anions in the flammability. To demonstrate that we have studied five room temperature ionic liquids sharing the same 1-butyl 3-methylimidazolium cation but different anions such as BF₄^-, PF₆^-, ClO4^-, NO₃^- and CH₃COO^-. We determined the electrochemical stability of these ionic liquids through linear sweep voltammetry to estimate how much voltage bias is required to see oxidation and reduction reactions in them. To understand which ionic liquid among these five generates reactive gas species the most, we determined the rate of electrochemical reaction for all the ionic liquids through current density measurement and correlated the current density to the rate of electrochemical reaction at the electrode surface at multiple voltage bias. We demonstrated that at any given voltage bias, the rate of electrochemical decomposition of these five ionic liquids varies significantly owing to their difference in conductivity. Unlike conventional reactions we saw a gradual rise in rates of the electrochemical reactions of ionic liquids up to a certain period and thought that temperature might be a key factor behind this phenomenon. We showed that the rise in temperature in the electrolysis reactions of the ionic liquids in our system is attributed to convective heat transfer facilitated by the movement of ions through thermal imaging. We predicted the temperature dependence of the physical properties such as viscosity and conductivity of the ionic liquids using the Vogel-Fulcher-Tammann-Hesse (VFT) equation. Subsequently, we correlated the predicted conductivity against the current response to explore their relationship. To determine the control mechanism of the electrochemical reaction of the ionic liquids we calculated the activation energy for their respective reaction and found that the calculated activation energies align with those typical of diffusion-controlled reactions. This is suggestive of diffusion to be the dominant mechanism for the electrochemical reactions of the ionic liquids. We would think the ionic liquid that generated the most reactive species will be the most combustible, but we realized if the generated gas species from anion oxidation has properties that do not aid combustion, the ionic liquid will be incombustible. To demonstrate this, we observed the generated gas species in the electrochemical reaction of five ionic liquids through mass spectrometry. We concluded that among the five imidazole ionic liquids, three are combustible upon electrochemical decomposition and two are not. In summary, the apparent thermally insensitive and non flammable room temperature ionic liquids can be made flammable through electrochemical approach but their flammability significantly depends on the anions present despite sharing the same cation that produces flammable gas species upon reduction.