Fluorine Domains Induced Ultrahigh Nitrogen Solubility in Ionic Liquids
Kun Li, Yanlei Wang, Chenlu Wang, Feng Huo, Suojiang Zhang, Hongyan He
Abstract
Fluorinated ionic liquids (ILs) are well-known as electrolytes in the nitrogen (N 2 ) electroreduction reaction due to their exceptional gas solubility. However, the influence of fluorinated functional group on N 2 solvation and solubility enhancement remains unclear. Massive molecular dynamics simulations and free energy perturbation methods are conducted to investigate the N 2 solubility in 11 traditional and 9 fluorinated ILs. It shows that the fluorinated IL of 1-Ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate ([Emim]FAP) exhibits ultrahigh solubility, 4.844 × 10 –3, approximately 118 times higher than that of traditional IL 1-Ethyl-3-methylimidazolium nitrate ([Emim]NO 3 ). Moreover, fluorinated ILs with more than 10 C–F bonds possess higher N 2 solubility than others and show an exothermic nature during solvation. As the C–F bonds number in ILs decreases, the N 2 solubility decreases significantly and displays the opposite endothermic behavior. To understand the ultrahigh N 2 solubility in fluorinated ILs, we propose a concept of fluorine densification energy (FDE), referring to the average strength of interaction between atoms per unit volume in ILs with fluorine domains, demonstrating a linear relationship with C–F bonds. Physically, lower FDE results in lower N 2 –anion pair dissociation energy and higher free volume, finally enhancing the N 2 solubility. Consequently, medium to long alkyl fluorine tails within a polar environment defines a distinct fluorine domain, emphasizing FDE’s role in enhancing N 2 solubility. Overall, these quantitative results will not only deepen the understanding of N 2 solvation in ILs but may also shed light on the rational design of IL-based high-performance N 2 capture and conversion technologies.