Hypergolic coordination compounds as modifiers for ionic liquid propulsion

Ionic liquids (ILs) have been regarded as green hypergolic materials since 2008, and hundreds of hypergolic ionic liquids have been reported thus far. However, only few have been proved to meet practical applications, due to their complex preparation technology and long hypergolic ignition delay (ID). Herein, to improve the properties of hypergolic ignition of ILs, we demonstrate a new design strategy of hypergolic coordination compounds (HCCs). Eight HCCs were synthesized based on two commonly used hypergolic ILs components (imidazolium = IM, dicyanamide = DCA) and four transition metal ions (Mn, Co, Ni, Cu) and were characterized through infrared spectroscopy, X-ray powder diffraction, and single-crystal X-ray diffraction. The experimental results showed that these HCCs exhibited excellent hypergolic ignition properties, and the ignition and combustion properties could be modulated by changing the metal, ligand, and anion. Additionally, the effects of HCCs on the ignition and combustion of organic hypergolic IL were investigated. The Cu-based HCCs showed the best performance among these new hypergolic materials. Compared with the HCCs-free IL, the IL system with 5% of [Cu(AIM)₄](DCA)₂ exhibited a sharp reduction in the ID time of 1-allyl-3-methylimidazolium dicyanamide (AMIMDCA) (6 ms vs. 45 ms).