Ionic Liquid Fuels Research Articles

Auto-ignition of ionic liquid fuels with hydrogen peroxide triggered by copper-containing liquid promoter

Research into next-generation propellants with green fuel–oxidizer pairs to replace the currently used highly toxic hydrazine–N2O4 system has attracted widespread attention. Ionic liquids (ILs) and hydrogen peroxide have demonstrated their feasibility as a green fuel and an oxidizer, respectively. However, the realisation of effective auto-ignition is the key problem. In this study, a new strategy to trigger the auto-ignition of ILs fuels with hydrogen peroxide by using a unique copper-containing liquid as the promoter is developed. The copper-containing promoter is designed such that its cationic structure is similar to that of the ILs fuels. Based on the principle of “like dissolves like,” the fuel and promoter can be miscible at any ratio to eventually form a catalytic fuel. In addition, the physicochemical properties (e.g. density, viscosity and decomposition temperature) and performance parameters (e.g. ignition delay time and specific impulse) of the as-prepared catalytic fuel are completely characterised. Owing to their excellent hypergolic performance with a short ignition delay time of 16 ms, in combination with the advantages of simple preparation, perfect solubility and green characteristics, the catalytic fuel–oxidizer pair demonstrates promise as bipropellants for rocket applications.

Electrochemical Modulation of the Flammability of Ionic Liquid Fuels.

Flammability and combustion of high energy density liquid propellants are controlled by their volatility. We demonstrate a new concept through which the volatility of a high energy density ionic liquid propellant can be dynamically manipulated enabling one to (a) store a thermally insensitive oxidation resistant nonflammable fuel, (b) generate flammable vapor phase species electrochemically by applying a direct-current voltage bias, and (c) extinguish its flame by removing the voltage bias, which stops its volatilization. We show that a thermally stable imidazolium-based energy dense ionic liquid can be made flammable or nonflammable simply by application or withdrawal of a direct-current bias. This cycle can be repeated as often as desired. The estimated energy penalty of the electrochemical activation process is only ∼4% of the total energy release. This approach presents a paradigm shift, offering the potential to make a "safe fuel" or alternatively a simple electrochemically driven fuel metering scheme.

Designing Difunctional Promoters for Hypergolic Ignitions of Green Bipropellants Combining Ionic Liquids with H2O2

The emergence of tetrahydroborate (BH4–)/cyanoborohydride (BH3CN–) anion-based ionic liquid fuels in combination with high test peroxide (>90%H2O2, HTP) oxidizers has accelerated the greening of bipropellants. However, most BH4– and BH3CN– anion-based ionic liquids are sensitive to water, making it difficult to store them. Here, novel difunctional promoters are designed for hypergolic ignition of BH4–/BH3CN– anion-free ionic liquids with 90%H2O2. The transition metal in anions of promoters is expected to catalyze the exothermic decomposition of H2O2, and the substituted borohydride in cations of promoters acts as the ignition source. These novel difunctional promoters show good solubility in commercially available 1-allyl-3-methylimidazolium dicyanamide and 1-butyl-3-methylimidazolium dicyanamide ionic liquid fuels, and the composite fuels exhibit high density, acceptable viscosity, and high thermostability. The addition of difunctional promoters ensures the smooth hypergolic ignition of BH4–/BH3CN– anion-free ionic liquid fuels with a minimum ignition delay time of 34.0 ms, and no apparent microexplosion and secondary combustion are observed during the ignition process. With the increase in the amount of the promoter, density specific impulses of the composite fuels improve gradually. This work provides a platform strategy for designing promoters by synergy of cations and anions and makes efforts to seek green bipropellants.

Effects of oxidizing additives on the physical properties and ignition performance of hydrogen peroxide-based hypergolic propellants

Herein, the concept of novel hypergolic pairs for the enhanced ignition performance of hypergolic propellants is proposed. The proposed hypergolic combinations comprised ionic liquid fuels, namely, 1-ethyl-3-methyl-imidazolium-thiocyanate (EMIM SCN) and 1-butyl-3-methyl-imidazolium-thiocyanate (BMIM SCN), and hydrogen peroxide (H2O2) with oxidizing additives. LiNO3 and NH4NO3 were used as oxidizing additives, which were dissolved in 60–95 wt% H2O2 to enhance the physical properties and ignition performance. Adding the oxidizing additives decreased the theoretical performance of the hypergolic pairs, however, the presence of oxidizing additives drastically reduced the freezing point of the mixture with 95 wt% H2O2. Particularly, an abrupt change was observed in the freezing point of the mixture with the LiNO3 additive. At only 5 wt% LiNO3, the freezing point reached −30 °C. The ignition performance of the novel hypergolic pairs was significantly enhanced by the introduction of oxidizing additives. In the drop test, the LiNO3 additive exerted a substantially greater effect on the enhancement of ignition performance than that exerted by NH4NO3. Furthermore, this positive contribution was maximized with the decrease in the hydrogen peroxide concentration. Although the addition of NH4NO3 to 90 and 95 wt% H2O2 adversely affected the ignition performance, the combination of BMIM SCN and 60 wt% H2O2–containing LiNO3 extended the hypergolicity limit. This study is the first attempt to introduce nitrate salts as oxidizing additives in hydrogen peroxide with ionic fuels, including SCN anions. Nitrate salts exhibit potential to serve as additives that can be used with hydrogen peroxide to enhance the properties of oxidizers. The nitrate salt usage helps lower the freezing point of oxidizers. Further, it shortens the ignition delay time of the hypergolic pairs.

Additive-promoted hypergolic ignition of ionic liquid with hydrogen peroxide

The exploration of environmentally friendly hypergolic combinations of ionic liquid fuels and hydrogen peroxide as an oxidizer offers opportunities to replace commonly used conventional toxic, corrosive and carcinogenic hydrazine-based liquid hypergolic combinations. Various research opportunities await regarding the detailed investigations of green hypergolic ionic liquids (HILs) with rocket grade hydrogen peroxide (RGHP, >85% H2O2). In this work, the combustion of HILs 1-ethyl-3-methyl imidazolium cyanoborohydride ([EMIM][BH3CN]) and 1-allyl-3-ethyl imidazolium cyanoborohydride ([AEIM][BH3CN]), and HIL-additive mixtures with 95% H2O2 has been investigated. A new additive, 1,3-dimethyl imidazolium copper iodide ([diMIM]n[Cu2I3]n) was synthesized successfully; a structural investigation was performed through the use of single-crystal x-ray diffraction analysis through which a crystal density of 3.22 g/cm3 was revealed. The physicochemical properties (density, viscosity, and decomposition temperature) as well as performance parameters (ignition delay and specific impulse) of 2 to 15 wt% of [diMIM]n[Cu2I3]n in [EMIM][BH3CN] were determined. A 15 wt% of [diMIM]n[Cu2I3]n exhibited ignition delay time (IDT) values of 13 and 29 ms under fuel-rich and oxidizer-rich conditions, respectively; these IDT values are 100 times lower than those for [EMIM][BH3CN]. Interestingly, [diMIM]n[Cu2I3]n-[EMIM][BH3CN] combinations revealed high density (>0.98 g/cm3), good thermal (>200 °C) and chemical stability, and 5.6–6.0% higher density specific impulse than those found for unsymmetrical dimethyl hydrazine. The additive-promoted hypergolic combustion of HIL with RGHP opens a new avenue to the replacement of conventional toxic hypergolic combinations.

“Tandem-action” ferrocenyl iodocuprates promoting low temperature hypergolic ignitions of “green” EIL–H2O2bipropellants

The fast auto-ignition of energetic ionic liquid fuels with H2O2was achieved under low temperature conditions by using new promoters, paving the way for the development of green bipropellants.

The ignition process measurements and performance evaluations for hypergolic ionic liquid fuels: [EMIm][DCA] and [BMIm][DCA

Bipropellant liquid rocket engines are typically fueled by hydrazines that exhibit extreme toxicity and carcinogenity. Recently hypergolic ionic liquids (HILs) have been emerging as environmentally friendly propellant that holds the potential for hydrazine replacement due to their low volatility, easy and safe storage and handling. The HIL is a novel concept both for chemistry community and for combustion community, and plenty of research waits to be initiated to promote practical propulsion application, especially for comprehensive understanding on their fundamental combustion. In this work, hypergolic ignition of two HILs (1-ethyl-3-methylimidazolium dicyanamide([EMIm][DCA]) and 1-butyl-3-methyl-imidazolium dicyanamide ([BMIm][DCA]) were tested with three oxidizers (white fuming nitric acid (WFNA), red fuming nitric acid(RFNA), and nitrogen tetroxide). A drop test system was designed to control the collision event of a single droplet of fuel with an oxidizer pool. Two independent high speed cameras were spontaneously employed to capture the self-luminous images of the whole hypergolic ignition process as well as the initial impact process. The results showed that hypergolic ignition process of [EMIm][DCA]-WFNA is quite similar to that of monomethyl hydrazine-WFNA. Combined with thermo physical properties of HILs, hypergolic ignition process of [EMIm][DCA]-WFNA is proposed. To qualitatively demonstrate the overall hypergolic ignition and reactivity, the explosion delay time and the ignition delay times of various combinations of the fuel-oxidizer pairs were determined. The effects of the Weber number on the explosion delay time and the ignition delay time of [EMIm][DCA]-WFNA are quantitatively investigated. Hypergolicity of different propellant combinations is tested to investigate the influence of oxidizer’s oxidative capacity and HILs’ cation structure. Additionally, the propulsive performance of the two dicyanamide-based IL fuels was evaluated and compared with traditional hydrazine-based fuels, based on a steady rocket model under the same initial conditions. Results show that using WFNA as the oxidizer, BMIM DCA and EMIM DCA presented a comparable propulsive performance to that of UDMH.

Theoretical performance evaluation of hypergolic ionic liquid fuels with storable oxidizers

The density specific impulse of 14 hypergolic ionic liquids with various oxidizers has been theoretically evaluated and found to be higher than the conventional fuel, UDMH.

ReaxFF molecular dynamics simulations of intermediate species in dicyanamide anion and nitric acid hypergolic combustion

Ionic liquids based on the dicyanamide anion (DCA) are of interest as replacements for current hypergolic fuels, which are highly toxic. To better understand the reaction dynamics of these ionic liquid fuels, this study reports the results of molecular dynamics simulations performed for two predicted intermediate compounds in DCA-based ionic liquids/nitric acid (HNO3) combustion, i.e. protonated DCA (DCAH) and nitro-dicyanamide-carbonyl (NDC). Calculations were performed using a ReaxFF reactive force field. Single component simulations show that neat NDC undergo exothermic decomposition and ignition. Simulations with HNO3 were performed at both a low (0.25 g ml−1) and high (1.00 g ml−1) densities, to investigate the reaction in a dense vapor and liquid phase, respectively. Both DCAH and NDC react hypergolically with HNO3, and increased density led to shorter times for the onset of thermal runaway. Contrary to a proposed mechanism for DCA combustion, neither DCAH nor NDC are converted to 1,5-Dinitrobiuret (DNB) before thermal runaway. Details of reaction pathways for these processes are discussed.

Assessment of Imidazole-Based Ionic Liquids as Dual-Mode Spacecraft Propellants

Imidazole-based ionic liquids are investigated in terms of dual-mode chemical monopropellant and electrospray rocket propulsion capabilities. A literature review of ionic liquid physical properties is conducted to determine an initial, representative set of ionic liquids that shows favorable physical properties for both modes, followed by numerical and analytical performance simulations. The ionic liquids 1-butyl-3-methylimidazolium dicyanamide, 1-butyl-3-methylimidazolium nitrate, and 1-ethyl-3-methylimidazolium ethyl sulfate meet or exceed the storability properties of hydrazine, and their electrochemical properties indicate that they may be capable of electrospray emission in the purely ionic regime. These liquids are projected to have 13–23% reduced monopropellant propulsion performance in comparison to hydrazine due to the prediction of solid carbon formation in the exhaust. The use of these ionic liquids as a fuel component in a binary monopropellant mixture with hydroxylammonium nitrate shows a 1–4% improved specific impulse over some “green” monopropellants. Also, this avoids volatility issues and reduces the number of electrospray emitters by 18–27% and the power required by 9–16%, with oxidizing ionic liquid fuels providing the greatest savings. A fully oxygen-balanced ionic liquid will exceed the state-of-the-art performance in both modes but will require advances in hardware technology in both modes to achieve minimum functionality.

Decomposition of Monopropellant Blends of Hydroxylammonium Nitrate and Imidazole-Based Ionic Liquid Fuels

Potential dual-mode monopropellant/electrospray-capable mixtures of hydroxylammonium nitrate with ionic liquid fuels [Bmim][] and [Emim][] are synthesized and tested for catalytic decomposition in a microreactor setup. The setup is benchmarked using a 30% hydrogen peroxide solution decomposed via silver catalyst. Results show similar trends but with variance in the quantitative data obtained in the literature. This was found to be a direct result of the sample-holder geometry. Hydrazine decomposition was conducted on an unsupported iridium catalyst. The same trends in terms of pressure-rise rate during decomposition () are obtained with unsupported catalyst but at 100 °C instead of room temperature for tests conducted on supported catalysts in the literature. For the [Bmim][]/hydroxylammonium nitrate propellant, rhenium catalyst preheated to 160 °C yielded a pressure-rise rate of , compared to for iridium catalyst and for no catalyst at the same temperature. [Emim][]/hydroxylammonium nitrate propellant shows slightly less activity at 160 °C preheat temperature, yielding a pressure-rise rate of 20, 4, and for injection onto rhenium, iridium, and the thermal plate, respectively. Final results indicate that desirable ignition performance may potentially be obtained by using a supported rhenium catalyst.

Ionic Liquid Dual-Mode Spacecraft Propulsion Assessment

Analytical and numerical investigations of the performance of a series of potential dual-mode propulsion systems using ionic liquids are presented. A comparison of the predicted specific impulse of ionic liquids with hydrazine and unsymmetrical dimethylhydrazine shows that ionic liquid fuels have a 3–12% lower specific impulse when paired with a nitrogen tetroxide oxidizer. However, when paired with hydroxylammonium nitrate oxidizer, the specific impulse of the ionic liquids is 1–4% lower than that of hydrazine and unsymmetrical dimethylhydrazine paired with nitrogen tetroxide. Analytical investigation of an electrospray electric propulsion system shows that ion extraction in the pure ion regime provides a very high specific impulse, outside the optimum range for potential missions. Results suggest a deceleration grid, a lower ion fraction, or emission of higher solvated states is required. Analysis of a dualmode ionic-liquid-propelled spacecraft shows that the electric propulsion component determines the overall feasibility comparedwith current technology. Results indicate that the specific power for an ionic liquid electrospray systemmust be at least 15 W=kg in order for a dual-mode ionic liquid system to compete with traditional hydrazine and Hall thruster technology.