In situ electrochemical-X-ray Photoelectron Spectroscopy: Rubidium metal deposition from an ionic liquid in competition with solvent breakdown
Graphical abstract
Highlights
► Rb electrodeposition from ionic liquid (IL) monitored by in situ electrochemistry-XPS. ► At low current densities Rb electrodeposition possible without IL decomposition. ► At high current densities IL decomposition competes with Rb electrodeposition. ► IL reductive breakdown characterised by carbonisation and rapid loss of fluorine. ► Results are consistent with the reduction of the [NTf2]− anion.
Introduction
Rubidium is one of the Group I alkali metals, which are characteristically strong reducing agents. The formal potential () of the Rb/Rb+ couple in aqueous solution is −2.94 V (vs. SHE) [1], while the for Rb/Rb+ in the ionic liquid [C4mPyrr][NTf2] has recently been measured as −3.37 V (vs. Fc/Fc+) [2]. This property makes Rb a potential candidate for use in solid batteries [3] as well as energy storage devices [4]. Rubidium cannot be electrodeposited from aqueous solution or many organic solvents as their potential windows are not wide enough.
Room temperature ionic liquids (RTILs or ILs), which are typically comprised entirely of ions, are promising media for the deposition for active metals such as Rubidium [2], [5]. Their intrinsic ionic conductivity, chemical stability as well as wide electrochemical windows makes the electrodeposition of many active metals possible [6]. Moreover, another important property of ILs is that they possess near zero vapour pressure. The combination of these properties has facilitated novel spectroelectrochemical techniques under high vacuum conditions, including in situ electrochemical-X-ray spectroscopy. A number of publications are available regarding the use of in situ electrochemical-XPS measurements in conjunction with ILs, probing the oxidation of Cu to Cu(I) [7], reduction of Fe(III) to Fe(II) [8], the intercalation of [C2mim][BF4] into HOPG [9], and the XPS induced charging [10] and electrochemical shift in binding energy [11] of ionic liquids. The diverse range of other (non-electrochemical) studies relating to combined IL-XPS studies has recently been reviewed [12].
We have reported the electrodeposition of Group I alkali metals from the ionic liquid N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ([C4mpyrr][NTf2]) on Pt and Ni electrodes [2], [13], [14], [15]. The fundamental kinetic and thermodynamic properties have been determined by means of simulation [2], [13], [14], [15]. In addition, recently we have reported the in situ electrochemical-XPS measurements to monitor the electrodeposition of potassium in the ionic liquid of [C4mpyrr][NTf2] using in situ electrochemical-XPS measurements [16]. Unlike potassium, rubidium electrodeposition is close to the reductive breakdown of [C4mpyrr][NTf2]. In this Letter, we report the use of in situ electrochemical-XPS to explore the breakdown of the solvent in competition with the electrodeposition of rubidium.
Section snippets
Experimental
N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, [C4mpyrr][NTf2], was prepared using standard literature methods [17]. The Rb[NTf2] salt was prepared by the reaction of aqueous solutions of rubidium hydroxide with a 5% molar excess of H[NTf2], followed by repeated dissolution in water then drying under vacuum, until pH neutral.
XPS measurements were performed using a VG ESCALAB MkII spectrometer equipped with a monochromatic Al Kα X-ray source (photon energy of 1486.6 eV). Survey
Experimental set-up
The in situ electrochemical-XPS cell (Figure 1a) and holder (Figure 1b) used in this study were fabricated in-house and are modifications of a previously reported electrochemical setup [16]. Briefly, the cell holder was a previously described mobile cell holder [16] that had been fixed permanently onto the end of an extendable and retractable manipulator arm which was inserted into the XPS chamber through a high vacuum flange. The sample-holding cell consisted of two parts, namely two hollow
Conclusions
The electrodeposition of rubidium has been performed by in situ electrodeposition-XPS employing a novel cell design. The in situ electrochemical-XPS technique allows monitoring of the progress of Rb electrodeposition as well as the competing decomposition of the ionic liquid during the electrodeposition. Passing a high cathodic current for the electrodeposition resulted in rapid decomposition of the ionic liquid in addition to Rb electrodeposition. On the other hand, lowering the current for
Acknowledgements
Professor Christopher Hardacre is thanked for the kind donation of the ionic liquid, which was synthesised by N.S.A.M. N.S.A.M. acknowledges the financial support of the Ministry of Higher Education Malaysia and University of Malaya via a SLAI fellowship. R.W. thanks the Directorate of Higher Education, The Ministry of National Education, Republic of Indonesia for funding. Mr J. Freeman is thanked for assistance with Figure 1, and Mr. C. Jones for the construction of the cell and holder.
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