Comparison of usual electrode materials in a n,n-trimethylbutylammnium bis (trifluoromethanesulfonyl) imide ionic liquid
Résumé
Ionic liquids (ILs) have been studied for two decades as electrolytes for various electrochemical experiments and more specifically for separation and electrodeposition of metals or semiconductors. Among the numerous anions synthesized to design ILs, the bis(trifluoromethanesulfonyl)imide (NTf$_2^-$) anion is commonly used for its electrochemical stability and hydrophobicity. In particular, the combination of short-chained quaternary ammonium cations and the NTf$_2^-$ anion gives hydrophobic ILs that have large electrochemical windows (5,0 to 6,0 V) and reasonably low viscosities (20 - 150 mPa.s.) Thus, this family of ILs has become a very common one for electrochemical studies. However, fundamental understanding on the phenomena occurring at the electrode/electrolyte interface in these ILs is still needed in order to select the right electrode material for a given application.The use of electrochemical techniques requires electrodes that are inert in the working potential range. Thus, the present work focusses on a study and comparison of the behavior of three common electrode materials (glassy carbon, platinum and gold) in N-Trimethyl-N-butylammonium bis(trifluoromethanesulfonyl)imide ([N$_{4111}$][NTf$_2$]). These three materials are commonly considered as totally inert on the electrochemical window of [N$_{4111}$][NTf$_2$] (-3,0 to +2,7 V vs Fc$^+$/Fc), with current densities in the mA range. However, when trying to observe and interpret signals in the uA range, extensive study of phenomena occurring at the electrode/IL interface has to be completed 4,5. Thanks to transient electrochemistry, and by studying the influences of water concentration and acidity in the IL, we could get a better understanding of the different phenomena occurring at the electrode/electrolyte interface. The relative role of the anion was also investigated.
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