Physicochemical Properties of Fluoride-Based Eutectic Electrolytes Composed of Tetramethylammonium Fluoride and <i>N</i>-Methyltrifluoroacetamide

Development of F−-containing electrolytes with high electrochemical oxidation stability is crucial for electrochemical fluorination. Although various combinations of fluoride salts and solvents have been investigated for preparing the electrolytes, there have been few studies on eutectic systems using fluoride salts and solid hydrogen bond donors, especially amide compounds. In this study, fluoride-based eutectic electrolytes (F-EEs) composed of tetramethylammonium fluoride ([TMA]F) and N-methyltrifluoroacetamide (MTFAA) as an amide compound are developed, and their physicochemical properties are evaluated including comparison with that composed of [TMA]F and N-methylacetamide (MAA). The F-EEs which are in liquid state at room temperature are successfully obtained for [TMA]F·x[MTFAA] with x = 8.0 and 4.0 owing to melting-point depression by hydrogen bonds between N–H and F−. The F-EEs with MTFAA and MAA exhibits the favorable ion transport properties because they have only one N–H bond per molecule and hydrogen bonds of N–H···F− are moderate. The F-EEs with MTFAA exhibit superior electrochemical oxidation stability due to the CF3 group compared to that with MAA and will be promising as electrolytes for electrochemical fluorination applications.