An in-situ Raman spectroscopic study on the cathodic process of EMITFSI ionic liquid on Ag electrodes

Abstract

Ionic liquids (ILs) are novel type of electrolytes and have found various applications in electrochemistry. Understanding their electrochemical stability has been one of fundamental and important subject of researches, which would promote further application of ionic liquids in electrochemistry. Electrochemical surface enhanced Raman spectroscopy (EC-SERS) can provide abundant finger print information of surface-bound species for elucidating reaction mechanism, while normal Raman (NR) spectroscopy of thin layer region near electrode surface can provide more information about products without interference of signals from surface adsorption. Here we report combined EC-SERS and NR studies with DFT calculations on the cathodic process of 1-ethyl-3-methylimidazolium bis (trifluoromethylsulfonyl) imide (EMITFSI) on Ag electrodes, aiming to identify reaction products and to reveal reaction mechanisms. It has been found that during the reduction of EMI cation, N-heterocyclic carbenes (NHCs) as an intermediate product were formed first and then underwent dimerization to form double-bond dimer. Single-bond dimer were also generated through single-electron radicals whose lifetime was too short to be detected directly though. In addition, dealkylation may happen, leading to formation of methylimidazole and ethylimidazole. This work not only deepens our insight into the cathodic process of EMI+, but also provides a guideline for electrochemical Raman study to be employed for tracing and understanding the process of electrode reaction.