title: Designer Ionic Liquids for Reversible Electrochemical Deposition/Dissolution of Magnesium.
abstract: Chelating ionic liquids (ILs), in which polyether chains are pendent from the organic pyrrolidinium cation of the ILs (PEGylated ILs), were prepared that facilitate reversible electrochemical deposition/dissolution of Mg from a Mg(BH4)2 source. Mg electrodeposition processes in two specific PEGylated-ILs were compared against that in the widely studied N-butyl-N-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquid (BMPyrTFSI). The two chelating IL systems (one with a pendent polyether chain with three ether oxygens, MPEG3PyrTFSI, and the other with a seven-ether chain, MPEG7PyrTFSI) showed substantial improvement over BMPyrTFSI for Mg electrodeposition/dissolution. The best overall electrochemical performance was in MPEG7PyrTFSI. X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy dispersive X-ray spectroscopy (EDS) were used to characterize galvanostatically deposited Mg, revealing production of pure, dendrite-free Mg deposits. Reversible Mg electrodeposition was achieved with high Coulombic efficiency (CE) of 90% and high current density (ca. 2 mA/cm(2) for the stripping peak). Raman spectroscopy was used to characterize Mg(2+) speciation in the PEGylated ILs and BMPyrTFSI containing Mg(BH4)2 by study of Raman modes of the coordinated and free states of borohydride, TFSI(-), and polyether COC groups. Quantitative analysis revealed that the polyether chains can displace both TFSI(-) and BH4(-) from the coordination sphere of Mg(2+). Comparison of the different IL electrolytes suggested that these displacement reactions may play a role in enabling Mg deposition/dissolution with high CE and current density in these PEGylated IL media. These results represent the first demonstration of reversible electrochemical deposition/dissolution of Mg in an ionic liquid specifically designed with this task in mind.
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Tylan
authors.initials: T
authors.lastname: Watkins
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Ashok
authors.initials: A
authors.lastname: Kumar
authors.affiliation: School of Molecular Sciences, Arizona State University , Tempe, Arizona 85287-1604, United States.
authors.forename: Daniel A
authors.initials: DA
authors.lastname: Buttry