Application of Ionic Liquids to Space Propulsion

These results have been supplemented by a study of the decomposition of ammonium nitrate (AN) (Farhat et al., 2009a). The heating of small amounts of aqueous AN (50-55 wt-%) shows only the quantitative endothermic vaporization of water and AN, with no evidence of thermal decomposition even at high temperature. In the presence of various mono- and bimetallic catalysts, we can observe a true decomposition, but only Pt-based


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The thrust F (unit N) given by the 2 nd Newton's law: F = (dm/dt).V e , where dm/dt (unit kg s -1 ) is the mass flow rate of the exhaust propellant and V e (unit m s -1 ), the average velocity of the exhaust gas; - The specific impulse I sp (unit s) and the volumetric impulse I vol (unit s kg L -1 ) which characterize the propellant, and are defined by the relation: I sp = V e /g = F/(dm/dt).g (g is the gravitational constant, 9.807 m s -2 ) and I vol = I sp * where  is the density of the propellant; - The efficiency η given in percentage and corresponding to the ratio (power associated with propulsive force)/(power input); it depends on the propulsion type and system. From the kinetic theory of gases, the exit velocity is proportional to (T c /M) 1/2 where M is the average molecular weight of gas mixture (in kg mol -1 ) and T c the gas temperature in the chamber (in K). Therefore, to increase the thrust, we have to increase the chamber temperature and/or decrease the average molar weight of the exit gas. This can be obtained using more energetic propellant, adding electric energy through heating elements or using an electric field and charged particles. The simplest and cheapest propulsion system is the cold gas system where a gas under pressure is released through a remote-controlled valve (Fig. 1). Chemical propulsion uses (i) liquid monopropellant system with catalytic decomposition of an energetic liquid (Fig. 1); (ii) bipropellant system that provides higher performances for launchers; and (iii) solid propellant system best represented by the huge boosters of launchers, that contain mainly ammonium perchlorate (Fig. 2). For electric propulsion systems, three families have been developed, depending on the force used: (i) Coulomb force for electrostatic system, i.e. an electric field is applied to accelerate ions or charged droplets, (ii) electromagnetic field to generate plasma for electrothermal systems, and (iii) Lorentz force for electromagnetic systems. Concerning micropropulsion applications, the first family was developed as ion electrospray (Lozano, 2011). The characteristics of the propulsion types are compared in Table 1 (Lozano, 2011).  .

Oxygen balance
When using propellants containing an oxidizer and a fuel in the same chemical (as in NH 4 NO 3 ) or in two independent species, the best performances are obtained when the oxidizer to fuel ratio is close to the value corresponding to a complete combustion giving nitrogen, steam, carbon dioxide and the most stable metal oxides (if metals are present) as sole products. In this case, the oxygen balance is zero. The oxygen balance corresponds to the mass percentage of released or consumed oxygen by the theoretical decomposition of the oxidizer or the fuel. Oxidizers present a positive oxygen balance whereas fuels display negative oxygen balance. Recently, it was proposed to take the formation of CO instead of CO 2 at high temperatures, into account to calculate the oxygen balance (Jones et al., 2006). In the case of monopropellants composed of single energetic species, the oxygen balance is an important parameter linked to the performance.

Replacement of hydrazine and hydrazine derivatives: toward "green propellants"
Liquid hydrazine as monopropellant and its derivatives (MMH and UDMH) for bipropellant systems have been the most used propellants for space propulsion since the fifties. But they display several drawbacks that lead to increasing handling and storage costs: (i) high crystallization point (1.4 °C for hydrazine); (ii) poisonous and carcinogenic properties; (iii) high vapor pressure at room temperature; and (iv) sensitivity to adiabatic compression (risk of detonation). These drawbacks have prompted, in the last years, the search for possible "green monopropellants" which would be cheaper, and less toxic. Different substitutes have been proposed and can be sorted into three main types: energetic ionic liquids, hydrogen peroxide, and nitrous oxide .

Previous review papers
Energetic ionic liquids have been previously studied in the late seventies as LGP (Liquid Gun Propellants), but due to development problems, they have been finally discarded for this peculiar application (Klingenberg et al., 1997). More recently, they have been proposed as space monopropellants to replace hydrazine and numerous research papers appeared in this field. A general review focusing on the catalytic decomposition of propellants, including www.intechopen.com ionic liquids, is presented in . Two recent reviews concerning new energetic ILs appeared in (Smiglak et al., 2007) and (Singh et al., 2006), whereas (Chiu & Dressler, 2007) presented a review paper entitled Ionic liquids for space propulsion focusing mainly on ion electrospray electric propulsion (vide infra). have been intensively studied in the past, but the results concerning catalytic ignition remain scarce. The major merit of the catalytic decomposition versus the thermal decomposition is to strongly decrease the ignition temperature needed to trigger the decomposition and, at the same time, to increase the decomposition rate. This is obtained by a complete change in the reaction pathway, with lower activation energies due to the surface active centers of the catalyst. However, this introduces limitations coming from the thermal stability of the catalyst bed as the decomposition temperature rises. Electric ignition has also been used with success for HAN-water solution (Meng et al., 2009), HAN-TEAN-water propellant (Risha et al., 2007) and ADN-based monopropellant (Wingborg et al., 2005). The effects of electric current, voltage, volume, temperature and propellant concentration on the ignition delay time were investigated. For the HAN-TEAN-water mixture, the time delay to peak power decayed exponentially from 160 s to 2-3 s with an increase in the input voltage from 7 to 12 V; beyond 12 V, the time delay dependency became less significant and appeared to remain constant. Resonant laser ignition was also demonstrated for HAN and HEHN mixtures which were successfully and reliably ignited with single laser pulses in the IR, whereas UV did not lead to ignition (Alfano et al., 2009).

Results on HAN-based propellants
HAN-based aqueous blends have been investigated mainly by US (Meinhardt et al., 1999;Wucherer et al., 2000), French (Courthéoux et al., 2002) and Japanese groups (Katsumi et al., 2010 (Klingenberg et al., 1997;Hisatsune et al., 2004). The mass composition HAN 60.8 %-TEAN 19.2 %-H 2 O 20 % corresponds to the balanced equation: 7 NH 3 OHNO 3 + NH(C 2 H 4 OH) 3 NO 3  22 H 2 O(g) + 8 N 2 (g) + 6 CO 2 (g) Different catalysts for HAN decomposition have been previously patented (Schmidt & Gavin, 1996): unsupported platinum group metals (wire mesh and sponge) and supported catalysts (32 % Ir, 10 % Pt, 12 % Rh) were proposed to start ignition between 80 and 120 °C. Decomposition of HAN-glycine-water mixtures on Shell 405 (Ir/Al 2 O 3 catalyst developed for the decomposition of hydrazine) and other home made catalysts was studied for application in small thrusters (Meinhardt et al., 1999). Catalysts prepared more recently by a French team are able to decompose HAN catalytically at room temperature with an ignition delay of around 1 s. Successive injections of the propellant (80 wt.-% HAN, 40 °C) decompose easily on such catalysts, thus displaying a very good catalytic activity (Courthéoux et al., 2005a). Fig. 3 shows the mass spectrum of the gas phase during 5 successive injections of 100 μL aliquots on a catalyst preheated at 85 °C (Farhat et al., 2007). The catalysts consist of 10 wt.-% platinum supported on a silicon-doped alumina prepared using sol-gel process (Nguéfack et al., 2003). The impact of drying procedure (xerogel Vs. aerogel) and active phase introduction way (impregnation or one-step addition before sol formation) on catalytic activity are compared (Courthéoux et al., 2004(Courthéoux et al., , 2005, showing that aerogel based catalysts display always a better activity and stability. The thermal and catalytic decomposition using the optimized catalyst were followed for different HAN concentrations (20, 40 and 79 wt.-% HAN) (Courthéoux et al., 2006).  (Farhat et al., 2007).

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The analysis of the products during the decomposition of HAN-based propellants was performed using a dynamic reactor with on-line mass spectrometer analysis of the gas phase and acid-base titration of the trapped aqueous solution, associated with Raman spectroscopy Farhat et al., 2007). For thermal as well as catalytic decomposition, the gaseous primary products are nitrogen N 2 (major, thermodynamic product) and nitrogen oxide NO (medium, kinetic product) (Fig. 4), whereas nitric acid is the major product in the trapped solution, associated with ammonium nitrate (Farhat et al., 2007). The amount of oxygen species (second expected thermodynamic product) is about two orders of magnitude lower than nitrogen and corresponds to oxygen impurities introduced during the injection. Secondary products are nitrous oxide N 2 O (medium) and nitrogen dioxide NO 2 (traces). The formation of NO 2 traces can be the result of NO oxidation by oxygen impurities. The rise of NO 2 follows the drop of oxygen. The concentration difference between oxygen and NO 2 (about one order of magnitude) is related to the solubility of nitrogen dioxide in water. The simultaneous formation of nitrogen, nitrogen oxide and nitric oxide (primary products) are described by parallel competitive redox reactions between hydroxylammonium and nitrate ions; then, the delayed formation of N 2 O is the result of the decomposition of intermediate ammonium nitrate: 6 NH 3 OHNO 3 (aq) = 3 N 2 (g) + 2 NO(g) + 10 H 2 O(g) + 4 HNO 3 (g) 6 NH 3 OHNO 3 (aq) = 2 N 2 (g) + 2 NH 4 NO 3 (s or l) + 6 H 2 O(g) + 4 HNO 3 (g) These results have been supplemented by a study of the decomposition of ammonium nitrate (AN) (Farhat et al., 2009a). The heating of small amounts of aqueous AN (50-55 wt-%) shows only the quantitative endothermic vaporization of water and AN, with no evidence of thermal decomposition even at high temperature. In the presence of various mono-and bimetallic catalysts, we can observe a true decomposition, but only Pt-based www.intechopen.com catalysts were able to trigger the decomposition at low temperature (210 °C). The addition of Cu or Zn to platinum leads to an improvement of the catalytic activity and a reduction of the decomposition temperature (close to the melting point 170 °C). The product analysis reveals the presence of the same gaseous and condensed products as observed with HAN e.g. major nitrogen and nitric acid. The influence of the fuel (methanol and glycerol) added in stoichiometric proportions, or in excess, to HAN, ADN, or HNF was evaluated (Amariei et al., 2005). Fig. 5 displays the thermogravimetric results obtained for binary (no fuel) and ternary HAN-based propellants.
Obviously, the presence of the fuel increases the ignition temperature for both thermal and catalytic decomposition, disclosing an inhibiting behavior.  "Met" = methanol, "Gly" = glycine (Amariei et al., 2005).
To overcome this problem, azides have been proposed as ionic fuels (Farhat et al., 2009b). But the first results were disappointing: for binary ammonium azide (AA)-water mixture, no azide decomposition occurred even in the presence of the catalyst and only endothermic processes corresponding to water and azide vaporization have been recorded. For ternary mixtures HAN-azide-water, no redox reaction has been observed; when using AA, the predominant step is linked to acido-basic reactions. In the case of ammonium nitrate-azidewater ternary mixtures, no reaction between reactants has been observed, but the presence of azide compounds influences strongly the decomposition of ammonium nitrate and the onset temperature decreases enough to enter the solid domain of AN. Using strand burner technique, it was showed that for binary HAN-water solutions (50-95 wt.-%), the combustion mode is very complex as the linear burning rate increases up to 80 % and decreases from 80 to 95 %. The role of the two-phase region is very important and the boiling of water caused by superheat is responsible for the high burning rate (Katsumi et al., 2010). The combustion mechanism of HAN-based propellant containing ammonium nitrate and methanol shows that the instability of the liquid interface may trigger a sudden increase in the burning rate to a violently high region; methanol was found to reduce the bubble growth rate in the solution. S405 catalyst (which replaces Shell 405 catalyst) showed the best performance , Matsuo et al., 2008. Strand burner investigation of HAN 60 %-glycine 14 %-water 26 % showed that the burning rate exhibited four burning rate regimes for pressures ranging from 1.5 to 18.2 MPa. The temperatures of combustion products were found to be near the water boiling point at pressures lower than 8.8 MPa and under at pressures larger than 8.8 MPa. The observed slope breaks in burning rates are shown to be associated with the reaction mechanism changes between adjacent pressure regimes. Major species detected are nitrogen, nitric oxide, carbon dioxide, and formaldehyde (Chang & Kuo, 2002).

Results on ADN-based propellants
Aqueous ammonium dinitramide (ADN) based mixtures, using as a fuel, glycerol (defining the LMP 101 propellant) or methanol (LMP 103), have been developed by Swedish groups Wingborg et al., 2006;Groenland et al. 2004;Anflo & Möllerberg, 2009) (Groenland et al., 2002). Such catalysts have been investigated and developed in combination with the preparation of other ADN-based ternary mixtures for the propulsion of rocket engines. ADN-based mixtures have been also tested using catalysts developed for HAN (Kappenstein et al., 2004;Farhat et al., 2008). Fig. 6 compares thermal and catalytic decomposition of aqueous ADN (50 wt.-%) and we can observe a full decomposition at lower temperature in the presence of the catalyst. The products are similar to the ones observed during the decomposition of HAN and again ammonium nitrate was found as an intermediate product.

Results on hydrazinium-based propellants (HNF, HEHN)
Hydrazinium NitroFormate (HNF), was mainly developed as a substitute for ammonium perchlorate for solid propulsion, but has been also proposed as a more energetic oxidizer than HAN for monopropellant, when combined with reductants (Fick et al., 2001;Slettenhaar et al., 2003). 44 wt.-% HNF in water (maximum solubility is about 50 wt.-% at 20 °C) was catalytically decomposed with the catalyst Pt/Al 2 O 3 Si previously described for HAN (Farhat et al., 2008). The gaseous products formed during the catalytic decomposition are major N 2 , medium NO, N 2 O and CO 2 , minor CO and traces of NO 2 . Raman analysis of the trapped solution evidenced the complete disappearance of hydrazinium cation, the formation of nitric acid and ammonium, whereas nitroformate anion remains in the solution. Pt, Rh and Pd catalysts supported on doped alumina were used for HNF decomposition (Courthéoux et al., 2004b). The use of 2-hydroxyethylhydrazinium nitrate (HEHN) was proposed recently as hydrazine substitute . HEHN is more stable than HAN and displays a glass transition temperature of -57 °C. The catalytic decomposition in contact with iridium supported on alumina (Shell 405) was demonstrated when the catalyst is preheated at 150 °C, but with the formation of soot most probably linked to a negative oxygen balance: [HO-C 2 H 4 -N 2 H 4 ] + [NO 3 ] -+ 2.25 O 2 = 2 CO 2 + 1.5 N 2 + 4.5 H 2 O

New energetic ionic liquids for space propulsion
Energetic ionic liquids (EILs) are now numerous and fully described in several papers (Singh et al., 2006;Smiglak et al., 2007). New EILs have been recently prepared and characterized, mainly by the research groups of R. D. Rogers in Alabama and J. M. Shreeve in Idaho. The required properties of a possible propellant are: melting point < -40 °C, density > 1.4 g cm -3 , surface tension < 100 dyne cm -1 , viscosity as low as possible, long term stability; the development of an IL toolbar is proposed (Smiglak et al., 2007). Imidazolium, triazolium, and tetrazolium families, with different N-and C-substituents (alkyl, amino, azido…) are presented and discussed (Fig. 7). Azolates are surprisingly stable  and the most promising anion for liquid propellant is 3,5dinitro-1,2,4-triazolate (Fig. 7, species 7). But few EILs have been experimentally investigated as monopropellants. The compound 4-amino-1-methyl-1,2,4-triazolium nitrate (2, Fig.7) displays a glass transition point of -55 °C, shows low sensitivity against impact and friction, and a burn rates with low pressure dependence (Schaller et al., 2009). The decomposition of triazolium (2, Fig. 7) (Chowdhury & Thynell, 2010a) and tetrazolium-based EILs (3, Fig. 7) has been followed by confined rapid thermolysis using FTIR spectroscopy and time-of-flight mass spectroscopy to identify the products. The primary formation of nitrogen and methylisocyanide was evidenced (Chowdhury et al., 2009a). For triazolium EILs, the processes governing the decomposition were found to be autocatalytic in nature and the catalytic species were strong acids generated by the initial decomposition step, with an activation energy of 167-188 kJ mol -1 (Chowdhury & Thynell, 2010b). As an oxygen balance close to zero remains a strong requirement for new propellants, the search for such compounds has been fruitful and new EILs have been proposed (Christe & Drake, 2003;Jones et al., 2006;Tao et al., 2008) and some examples are presented in Fig. 8. Table 2 presents the corresponding oxygen balance values for the formation of CO 2 and CO. As an example, the oxygen balance equation for 1 (Fig. 8), based on CO formation is:

New hypergolic IL fuels for bipropellant systems
Hypergolic fuel-oxidizer systems hypergolic systems have been widely used in bipropellants engines, mainly for upper stage of launchers because they afford restart of the engine, leading to a better accuracy of the trajectory and finally the satellite orbit. The most used fuels are hydrazine, monomethylhydrazine (MMH) or dimethylhydrazine (UDMH), with oxidizing agents such as white fuming nitric acid (WFNA, 100 % HNO 3 ), red fuming nitric acid (RFNA, HNO 3 plus NO 2 ) or nitrogen tetroxide (NTO, N 2 O 4 ). The main drawback of these systems is linked to the toxicity of hydrazines associated with high vapor pressures, which increase strongly the handling and storage costs. The need for non-toxic "green" hydrazine substitutes appeared recently and new hypergolic liquid fuels must present low vapor pressures, high energy densities, low viscosities to enhance ease of mixing, as well as short ignition delays to preclude accumulation of fuel and oxidizer that could lead to an explosion. EILs are the candidates of choice for this application and have been proposed by different research groups in the US.
The first family or ILs fuels exhibiting hypergolic activity toward WFNA was reported by the research group at AFRL Edwards. It corresponds to dicyanamide-based ILs (Chambreau et al., 2008); it must be quoted that the same group prepared also azide-based ILs which show high reactivity with nitric acid, but do not ignite (Schneider et al., 2008a). Then other IL families have been more recently proposed, mainly by the Idaho group, like nitrocyanamide-based (He et al., 2010), or dicyanoborate-based fuels (Zhang & Shreeve, 2011).

Composition and properties of hypergolic IL fuels
The composition of the different hypergolic IL families is presented in Fig. 9 and some of the properties in Table 3. The borate and cyanoborate ILs are water sensitive, whereas the DCBbased ILs remain water-stable. From Table 3, we can sort the three hypergolic families for ignition delay and viscosity that decrease in the order: NCA > DCA > DCB. Table 3 shows that the DCB-based ILs present much shorter ID than the DCA or NCA, associated with these ILs meet all the important criteria to be selected as hypergolic hydrazine substitutes, with ignition delay close to the ID of the current MMH-NTO bipropellant (2 ms Fig. 9. Constituents of the new hypergolic IL families. DCA = dicyanamide, NCA = nitrocyanamide, DNA = dinitramide, B = tetrahydroborate, CB = cyanoborate, DCB = dicyanoborate.

Reactivity of hypergolic IL fuels
One example of hypergolic drop test followed by high speed camera is given in Fig. 10 (cation 1 + DCB). The fuel sample is dropped into a beaker containing an excess of WFNA. The ignition delay is defined as the elapsed time between the fuel-oxidizer contact and the appearance of a luminous flame. New azide-functionalized IL associated with DCA or NCA have been demonstrated to be hypergolic, whereas azide-based ILs display non hypergolic behavior . The hypergolic reaction of sodium dicyanamide with nitric acid was followed to investigate the ignition and reaction mechanism (Litzinger & Iyer, 2011). Gas phase products were analyzed by mass spectrometry, and the proposed balanced equation is: Na + N(CN) 2 -+ 4 HNO 3  2 CO 2 + 3 N 2 O + 2 H 2 O + NaNO 3 The major products are CO 2 and N 2 O. Traces of NO 2 , HNO 3 and carbamic acid H 2 NCOOH (i.e. HNCO + H 2 O) have been detected. No nitrogen, ammonia and hydrogen cyanide has been evidenced. Fig. 10. Ignition delay test shown with a series of high-speed camera photos. A droplet of IL contacting WFNA (Zhang & Shreeve, 2011).

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The reaction of 1-ethyl-3-methyl imidazolium dicyanamide (EmimDCA) with WFNA was followed by IR spectroscopy and the presence of CO 2 , N 2 O, H 2 O and isocyanic acid HNCO was evidenced whereas HCN, HONO, NO and CO do not form (Chowdhury at al. 2009b). The use of labeled nitric acid (H 15 NO 3 ) leads to the formation of 14 N 15 NO and H 14 NCO, proving that the NO functionality of nitrous oxide derived from nitric acid and the isocyanic acid nitrogen atom comes from DCA anion only. Propellant gels can bring safety and insensitivity to tactical systems fueled by storable hypergolic propellants. The unique rheological behavior of gel phase can curtail leakage from damaged tanks and can reduce the volatility of the propellants. Drop ignition tests have been followed used gelled fuel -liquid oxidizer or liquid fuel -gelled oxidizer combination. The ignition of liquid and gelled methyl ethyl imidazolium DCA have been demonstrated (Coil, 2010).

Electric propulsion
Among the different propulsion options for small satellites (from micro to nano), ion electrospray propulsion system (iEPS) is a good candidate for high precision applications (Table 1) and with the possibility of miniaturization. Ions are emitted from a conducting liquid in contact with a metallic tip (tungsten, nickel) and accelerated by a static electric field (Fig. 11, left). The tip can be externally wetted in the case of a bulk metal or internally in the case of a porous metal. The liquid is a liquid metal (e.g. indium) or an ionic liquid. The advantages of ILs over liquid metals are: (i) very low vapor pressure, (ii) low surface tension, (iii) possibility to emit positive and negative ions; they are thus described as "a plasma in a bottle" (Lozano, 2011). To scale up the thrust, iEPS can be arranged in the form of arrays of densely packed porous metal emitters; the delivery of the IL propellant uses the passive capillary force (Fig. 11, right). The meniscus of the propellant, when under the influence of an electric field exceeding a critical value will deform into a cone (known as Taylor cone) where the surface tension is balanced by the electrostatic force. An alternating polarity of 1 Hz suppresses the electro-chemical effects. If the electric field extractor Emitter coated with an ionic liquid 1 kV extractor Emitter coated with an ionic liquid 1 kV Fig. 11. Scheme of an ion electrospray propulsion system (left); propellant storage and delivery for an array of emitters (right) (Lozano, 2011).
Ionic liquids used as propellants for iEPS do not need to be energetic as it was the case previously, but they must satisfy the requirement to change the classical operation mode from a droplet dominated emission mode (as it is the case for colloid thrusters) to a pure ion emission regime. ILs classically used for iEPS comprise EMIM-BF4, EMIM-Im and EMIM-Beti (Fig. 12) (Daily, 2008;Ticknor et al., 2010;Legge & Lozano, 2011).

Hybrid engines
To improve the performances of a monopropellant engine and reduce the risk linked to the simultaneous presence of an oxidizer and a fuel for the long term storage, the oxidizer can be use alone and after decomposition, the hot gaseous oxygen can burn a solid fuel grain in a hybrid solid/liquid engine (Fig. 13). The proposed IL oxidizers can be concentrated aqueous solution of HAN (hydroxylammonium nitrate) or HAN plus AN (ammonium nitrate) and the fuel polyethylene or HTPB (hydroxy-terminated polybutadiene) (Biddle & Sutton, 1985;Ramohalli & Dowler, 1995 Fig. 13. Scheme of a HAN-based hybrid engine. Despite the interest of such energetic IL-based hybrid engine, the current development replaced EIL by hydrogen peroxide; this is most probably due to the formation of major nitric acid (kinetic product) instead of expected oxygen (thermodynamic product) during thermal or catalytic decomposition of HAN: 5 [NH 3 OH] + [NO 3 ] -(l)  3 N 2 (g) + 8 H 2 O(g) + 4 HNO 3 (g) [NH 3 OH] + [NO 3 ] -(l)  N 2 (g) + 2 H 2 O(g) + O 2 (g)

Solid propellant stabilizers
Double base (DB) propellants are composed primarily of nitroglycerin (NG) and nitrocellulose (NC). The decomposition reaction is complex, depends on the temperature, but also on the storage history when exposed to conditions in excess of the qualified limit; this last point needs the use of inert stabilizers with a performance decrement (specific impulse, burn rate, energy release…). One solution is to introduce energetic stabilizers that do not carry a performance penalty and ILs are proposed for the next stabilizer generation because they offer the possibility to choose ions that provide task-specific performance: one ion has stabilization capability (generally linked to an affinity for NO x species) whereas the counterion displays an energetic nature. The feasibility of creating such ILs has been recently demonstrated from NDPA (nitrodiphenylamine) and choline (Fig. 14) (Forton et al., 2010). The use of simpler tetra-alkyl ammonium cations was not successful.

Conclusion
We have seen that different ionic liquids are proposed for the different propulsion systems (monopropellants, hypergolic bipropellants, ion electrospray, hydrid engines). The current and future challenges that can be drawn from this review are summarized as follows: (i) to find new and safe EILs whose thermal or catalytic decomposition avoid the formation of nitric acid as a primary product; (ii) to develop new catalysts able to decompose EILs at low temperature and stable at high temperature; (iii) to find new fuels associated to IL oxidizers but with no inhibiting properties for the catalysts; and (iv) to develop cheaper and safer synthetic method to prepare ILs. Currently, ionic liquids are part of the GRASP project (GReen Advanced Space Propulsion) of the seven framework program of the European Community (GRASP, FP7). This volume, of a two volume set on ionic liquids, focuses on the applications of ionic liquids in a growing range of areas. Throughout the 1990s, it seemed that most of the attention in the area of ionic liquids applications was directed toward their use as solvents for organic and transition-metal-catalyzed reactions. Certainly, this interest continues on to the present date, but the most innovative uses of ionic liquids span a much more diverse field than just synthesis. Some of the main topics of coverage include the application of RTILs in various electronic applications (batteries, capacitors, and light-emitting materials), polymers (synthesis and functionalization), nanomaterials (synthesis and stabilization), and separations. More unusual applications can be noted in the fields of biomass utilization, spectroscopy, optics, lubricants, fuels, and refrigerants. It is hoped that the diversity of this volume will serve as an inspiration for even further advances in the use of RTILs.