Understanding Speciation in Ionic-Liquid Electrolytes for Non-Aqueous Redox Flow Batteries

Abstract

Conventional aqueous redox flow batteries (RFB) are limited by low energy and power densities; thus, recent focus has been on development of non-aqueous electrolytes. Although organic solvents allow for increased voltage windows, they possess a several drawbacks, including sensitivity to moisture, low solubility of active species, and high flammabilities and toxicities. Ionic liquids (IL) offer a “greener” alternative to volatile organic electrolytes as they are non-toxic and nonflammable [1, 2]. Iron chloride based IL electrolytes containing up to 6.3 M iron have been synthesized. It was found that electrolyte conductivity and viscosity, as well as the nature of the electroplated iron, are greatly influenced by electrolyte composition. An electrolyte containing iron chloride, choline chloride, and ethylene glycol in a 1:1:4 molar ratio was attractive due to its high plating efficiencies, fast kinetics, and acceptable conductivity [3]. Electrochemical reactivity of the solute ions as well as the physical properties of the electrolyte are controlled by speciation of the metals in solution. To understand what ions are responsible for the observed properties in these iron IL electrolytes, chemical speciation was investigated using x-ray absorption spectroscopy (XAS) as well as x-ray photoelectron spectroscopy (XPS). The two techniques indicate the presence of organometallic compounds which may hinder the electrokinetics as well as the fluid properties. For example, the conductivity is a function of the mole fraction of iron as well as the iron to chloride ratio (Fig 1 below). Complexes such as [FeCl4]-have been shown to exhibit enhanced fluid properties [4], however, when the iron:chloride ratio is less than 1:4, di-iron species have been known to form [5]. The properties of these iron IL electrolytes as a function of composition coupled with results from XAS and XPS to explain observed trends will be presented. [1] M. J. Earle and R. K. Seddon, Pure Appl. Chem., vol. 72, p. 1391, 2000. [2] K. N. Marsh, A. Deev, A. C.-T. Wu, T. E., and A. Klamt, Korean J Chem. Eng. , vol. 19, 2002. [3] J. S. W. M. A. Miller, and R. F. Savinell, "Iron Ionic Liquid Electrolytes for Redox Flow Battery Applications," Journal of The Electrochemical Society, vol. 163, pp. A578-A579, 2016. [4] I. M. H. C. A. Angell, P. A. Cheeseman, "Physico-chemical and computer simulation studies of the role of cation coordination numbers on melt physical properties," Electrochemical Society Proceedings, p. 138, 1976. [5] M. S. Sitze, E. R. Schreiter, E. V. Patterson, and R. G. Freeman, "Ionic Liquids Based on FeCl3 and FeCl2. Raman Scattering and ab Initio Calculations," vol. - 40, pp. - 2304, 2001. Figure 1