Probing lithium and alumina impurities in air- and water stable ionic liquids by cyclic voltammetry and in situ scanning tunneling microscopy

In this paper we report on some results on the electrochemical breakdown of ionic liquids at the cathodic limit of the electrochemical window and on the probing of inorganic impurities in ionic liquids (originating front the synthesis) at the interface electrode/electrolyte by in situ scanning tunneling microscopy on An(111). It will be shown that the restructing/reconstuction of Au(111) in ultrapure 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py-1.4]Tf2N) lead to a transition from a wormlike surface structure to a surface with well defined terraces. Close to the cathodic decomposition of the liquid the quality of the STM pictures gets worse and at the onset of massive ionic liquid breakdown a film forms on the gold surface finally shielding completely the gold terraces. When the [Py-1.4]Tf2N liquid, made by a metathesis reaction from [Py-1.4]Cl and LiTf2N, is not thoroughly washed after the synthesis, the in situ STM pictures show in I wide potential range the same Surface Structures as with the ultrapure liquid, but in the cathodic regime the deposition of lithium in only a few monolayers is observed. In the respective cyclic voltammograms on Au(111) there is clear evidence for lithium deposition. We show furthermore that a spectroscopically ultrapure 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)amide ionic liquid ([EMIm]Tf2N) call show unexpected behaviour at the electrode/electrolytc interface when after synthesis it is subject to an Al2O3 treatment with the aim to remove organic impurities. Al2O3 seems to be dissolved in the ionic liquid in low concentrations and deposited at the electrode Surface. At lower electrode potentials the alumina seems to be reduced to metallic aluminium. Our results show that even ultrapure ionic liquids can contain inorganic impurities which are very difficult to probe with conventional analytical methods. Better synthesis routes will be necessary to make ultrapure ionic liquids for fundamental physicochemical studies.