Aprotic-heterocyclic anion (AHA) ionic liquids (ILs) thermophysical properties can be tuned by functionalizing both cation and anion with substituents to acquire desired properties such as enhanced CO2 absorption capacity and thermal stability. To understand how substituents on anion affects AHA ILs thermophysical properties, halide substituted benzimidazole ([5-X-BnIm]-, where X=F, Cl, Br, and I) paired with triethyl-octyl-phosphonium cation ([P2228]+) are characterized. Most thermophysical properties follows the order of electronegativity of halide substituents, with exception of thermal stability. Interestingly, halide substituents significantly improve thermal stability compared to their unsubstituted moiety, [P2228][BnIm], showing minimal sign of degradation up to 513.15 K with one of [P2228][5-X-BnIm]. More characterizations such as X-ray photoelectron spectroscopy and decomposition mechanism studies were conducted to understand observed thermal stability and to develop design strategies to improve thermal stability.
