Fluorine Domains Induced Ultrahigh Nitrogen Solubility in Ionic Liquids

Area of Science:

• Electrochemistry
• Materials Science
• Chemical Engineering

Background:

• Fluorinated ionic liquids (ILs) are recognized for their excellent gas solubility, particularly for nitrogen (N₂), a key factor in electroreduction reactions.
• The precise impact of fluorinated functional groups on N₂ solvation and solubility within ILs remains incompletely understood.

Purpose of the Study:

• To investigate the influence of fluorinated functional groups on N₂ solubility in various ionic liquids.
• To elucidate the mechanisms behind enhanced N₂ solubility in fluorinated ILs and establish quantitative relationships.

Main Methods:

• Conducted extensive molecular dynamics simulations.
• Employed free energy perturbation methods to analyze N₂ solubility in 11 traditional and 9 fluorinated ILs.
• Introduced and utilized the concept of fluorine densification energy (FDE).

Main Results:

• The fluorinated IL 1-Ethyl-3-methylimidazolium tris(pentafluoroethyl) trifluorophosphate ([Emim]FAP) demonstrated ultrahigh N₂ solubility (4.844 × 10⁻³), 118 times greater than [Emim]NO₃.
• ILs with over 10 C-F bonds exhibited higher N₂ solubility and exothermic solvation, while fewer C-F bonds led to decreased solubility and endothermic behavior.
• A linear correlation was found between FDE and the number of C-F bonds, with lower FDE correlating to lower N₂-anion dissociation energy and higher free volume, thus enhancing N₂ solubility.

Conclusions:

• Fluorinated ILs, particularly those with extensive fluorination, offer superior N₂ solubility crucial for N₂ electroreduction.
• Fluorine densification energy (FDE) is a key parameter for understanding and predicting N₂ solubility in ILs.
• These findings provide a foundation for designing advanced IL-based materials for efficient N₂ capture and conversion technologies.