Fundamental Studies of Iron Redox Flow Battery Electrochemistry in a Deep Eutectic Solvent

Current redox flow battery (RFB) technologies are limited by low energy and power densities; thus, a recent focus has been on non-aqueous electrolytes. Although organic solvents allow for increased voltage windows, they possess a sensitivity to moisture, low active species solubilities, high flammability and toxicity, and limited availability. Another class of non-aqueous solvents, ionic liquids (IL) or more specifically, deep eutectic solvents (DES) offer promise for future non-aqueous RFBs. These “designer solvents” can be created using a number of combinations of bulky asymmetric cations and weakly coordinating anions, allowing for the ability to tailor different solvent properties, presenting an opportunity to generate optimized solvents. DES are advantageous in that they have large electrochemical windows similar to organic solvents, and they have low volatilities, high thermal stabilities, high polarities, and high conductivities without the need for a supporting electrolyte [1-4]. They offer a “greener” alternative to volatile organic solvents and are non-toxic and nonflammable [5, 6]. Two different deep eutectic systems, reline (1:2 molar ratio choline chloride and urea) and ethaline (1:2 molar ratio choline chloride and ethylene glycol), have been investigated as well as an iron eutectic (1:3 molar ratio iron chloride and ethylene glycol). Wide potential windows of 2.6V and 2.0V are achieved using reline and ethaline respectively (compared to 1.23V for aqueous). Electrolyte properties such as conductivities and viscosities as a function of temperature are reported. Iron plating efficiencies are reported; higher coulombic efficiencies compared to the aqueous system are attributed to elimination of the side reaction of hydrogen evolution (due to the expanded voltage window). Characterization of the electrokinetics, diffusion coefficients, and solubility of the iron chemistry at 50°C have been determined using voltammetric measurements and electrochemical impedance spectroscopy at platinum, glassy carbon, and carbon fiber microelectrodes. Iron was chosen as proof of concept chemistry; higher potential couples are currently under investigation. 

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