Fluorine Domains Induced Ultrahigh Nitrogen Solubility in Ionic Liquids

K Li and YL Wang and CL Wang and F Huo and SJ Zhang and HY He, JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, 146, 25569-25577 (2024).

DOI: 10.1021/jacs.4c06784

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 (EmimFAP) exhibits ultrahigh solubility, 4.844 x 10(-3), approximately 118 times higher than that of traditional IL 1-Ethyl-3-methylimidazolium nitrate (EmimNO3). 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.

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