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Influence of Zn2+ and Water on the Transport Properties of a Pyrrolidinium Dicyanamide Ionic Liquid

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journal contribution
posted on 08.05.2014, 00:00 by T. J. Simons, P. M. Bayley, Z. Zhang, P. C. Howlett, D. R. MacFarlane, L. A. Madsen, M. Forsyth
In order to expand our understanding of a potential zinc-based battery electrolyte, we have characterized the physical and transport properties of the ionic liquid (IL) 1-butyl-1-methylpyrrolidinium dicyanamide ([C4mpyr]­[dca]) containing various levels of both Zn2+ and H2O. Detailed measurements of density, viscosity, conductivity, and individual anion and cation diffusion coefficients using pulsed-field-gradient (PFG) NMR combined with NMR chemical shifts and spin–lattice relaxation (T1) NMR experiments provide insights into the motion and chemical environment of all molecular species. We find that the various techniques for probing ion transport and dynamics form a coherent picture as a function of electrolyte composition. Zn2+ addition causes a moderate reduction in the self-diffusion of the IL anion and cation, whereas the addition of H2O increases ion mobility by increasing the liquid’s overall fluidity. Temperature-dependent 13C T1 experiments of the dca carbon analyzed using Bloembergen–Purcell–Pound fits show monotonic slowing of anion dynamics with Zn2+ addition, suggesting increased Zn2+/dca association. T1 experiments show minimal change in the spin–lattice relaxation of cation or anion upon H2O addition, suggesting that H2O is playing no significant role in Zn2+ speciation. Finally, we employ a novel electrophoretic NMR technique to directly determine the electrophoretic mobility of the C4mpyr cation, which we discuss in the context of impedance-based conductivity measurements.