Organic superbase-mediated synthesis of borohydride ionic liquids as novel composite hypergolic fuels

This study prepared a series of novel hypergolic fluids based on borohydride ionic liquids and organic superbase using an in situ synthetic method. In these hypergolic fluids, ionic liquids in 1,5-diazabicyclo[4.3.0]non-5-ene (DBN) and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) acted as triggers for the self-ignition of DBN and DBU upon contact with high-concentration hydrogen peroxide (H₂O₂). These hypergolic fluids had high densities (>1.000 ​g ​cm⁻³), low viscosities (as low as 34.03 ​cP), and acceptable ignition delay times (IDT). The ignition processes of the hypergolic fluids with 90% H₂O₂ as an oxidizer were first investigated in this study, and they differed from the previously reported ignition phenomena. Different from the case with white fuming acid (WFNA) as an oxidizer, the ignition processes of hypergolic fluids with 90% H₂O₂ as an oxidizer did not exhibit secondary rebound and splashing and formed a homogeneous mixed layer when the droplets were in contact with 90% H₂O₂. The different ignition processes significantly influenced the properties of hypergolic fluids. Compared with the hypergolic fluids with WFNA as an oxidizer, those with 90% H₂O₂ as an oxidizer showed a shorter IDT (IDT_{min[90% H2O2]=}28.3 ​ms, IDT_min[WFNA]=126 ​ms) and formed stable flames without secondary combustion. These results demonstrate that the in-situ synthesized fuels in this study hold great promise as green fuels in hypergolic propulsion systems.