Hypergolic fuels based on water-stable borohydride cluster anions with ultralow ignition delay times

In bipropulsion systems, hypergolic ionic liquids with B–H groups, as promising alternatives to toxic hydrazine-based fuels, can improve propulsion performance. A series of borohydride cluster-based salts with excellent hydrolytic and thermal stabilities (T_d > 200 °C), and the highest density (1.18 g cm⁻³) of any known borohydride hypergolic ionic liquids were prepared. Upon contact with white fuming nitric acid or N₂O₄, these new salts have ultra-fast ignition delay times (as low as 1 ms), which is a superior ignition performance. Theoretical studies were employed in order to understand their high stability and good ignition performance.