Nitrato-Functionalized Task-Specific Ionic Liquids as Attractive Hypergolic Rocket Fuels

Hypergolic ionic liquids (HILs) as potential replacements for hydrazine derivatives have attracted increasing interest over the last decade. Previous studies on HILs have mostly concentrated on the anionic innovations of ionic liquids to shorten the ignition delay (ID) time, but little attention has been paid to cationic modifications and their structure–property relationships. In this work, we present a new strategy of cationic functionalization by introducing the energetic nitrato group into the cationic units of HILs. Interestingly, the introduction of oxygen-rich nitrato groups into the cationic structure significantly improved the combustion performance of HILs with larger flame diameters and duration times. The density-specific impulse (ρI_sp) of these novel HILs are all above 279.0 s g cm⁻³, much higher than that of UDMH (215.7 s g cm⁻³). In addition, the densities of these HILs are in the range of 1.22–1.39 g cm⁻³, which is much higher than that of UDMH (0.79 g cm⁻³), showing their higher loading capacity than hydrazine-derived fuels in a propellant tank. This promising strategy of introducing nitrato groups into the cationic structures has provided a new platform for developing high-performing HILs with improved combustion properties.