Probing Lithium and Alumina Impurities in Air- and Water Stable Ionic Liquids by Cyclic Voltammetry and In Situ Scanning Tunneling Microscopy

• Published in: Zeitschrift für physikalische Chemie (Neue Folge) Vol. 220; no. 10; pp. 1377 - 1394
• Main Authors: Endres, Frank, El Abedin, Sherif Zein, Borissenko, Natalia
• Format: Journal Article
• Language: English
• Published: De Gruyter Oldenbourg 01.10.2006
• Subjects: Cyclic Voltammetry; In Situ STM; Inorganic Impurities; Ionic Liquids
• ISSN: 0942-9352; 2196-7156
• DOI: 10.1524/zpch.2006.220.10.1377

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 from the synthesis) at the interface electrode/electrolyte by scanning tunneling microscopy on Au(111). It will be shown that the restructuring/reconstruction of Au(111) in ultrapure 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide ([Py ]Tf N) leads 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 ]Tf N liquid, made by a metathesis reaction from [Py ]Cl and LiTf N, is not thoroughly washed after the synthesis, the STM pictures show in a 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]Tf N) can show unexpected behaviour at the electrode/electrolyte interface when after synthesis it is subject to an Al treatment with the aim to remove organic impurities. Al 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.