Investigation of Ammonium-and Phosphonium-Based Deep Eutectic Solvents as Electrolytes for a Non-Aqueous All-Vanadium Redox Cell

Abstract

The charge/discharge characteristics for vanadium acetylacetonate in deep eutectic solvents were evaluated using an H-cell with an anion-exchange membrane separator for the first time. Coulombic (CE) and energy efficiencies (EE) of the electrolyte containing V(acac) 3 /0.5 M TEABF 4 in DES3 (a hydrogen bonded eutectic between choline chloride and ethylene glycol) were obtained as 49-52% and 25-31%, respectively, when charging from 0 to 50% of theoretical maximum state-of-charge for 12 cycles. The low CE may be due to the crossover of the active species through the separator, or to the loss of active vanadium due to a parasitic reaction. However, the CE was similar to that for acetonitrile (CH 3 CN) indicating the promise of DESs as suitable electrolytes for future evaluation. Charge and discharge voltages are respectively higher and lower than the formal cell potential obtained by voltammetry. Ohmic drop in the DES results from the low conductivity of the electrolyte and the relatively large distance between the two electrodes in the H-cell. Further studies require investigation in a flow cell with analyses of polarization curves and impedance to determine the loss mechanisms in sufficient detail. Low energy density is often reported as a barrier in the commercialization of redox flow batteries using current aqueous electrolytes. Non-aqueous electrolytic solvents offer a wide potential window of operation and increase the energy capacity of the system. 2-4 In contrast to organic systems which are either scarce or environmentally unfriendly, ionic liquids (ILs) have emerged as a relatively new class of non-aqueous electrolytes for energy storage applications. • C. ILs have many favorable characteristics, e.g., low volatility, high intrinsic conductivity, large electrochemical window, etc. In addition, ILs can be tuned by combining different cations and anions. However, many reports point out the hazardous toxicity and the poor biodegradability of most ILs. 7 ILs with high purity are also required since impurities, even in trace amounts, affect their physical properties. Additionally, their synthesis is not entirely environmentally friendly since it generally requires a large amount of salts and solvents in order to completely exchange the anions. 10 These drawbacks together with the high price of common ILs unfortunately hamper their industrial applications. Such issues may be overcome by using deep eutectic solvents (DESs). 11,12 A DES is a eutectic mixture of an organic salt (ammonium or phosphonium) and a hydrogen bond donor (HBD), that is made up of different components such as amides, metallic salts, alcohols, carboxylic acids and amines that may be used as complexing agents (typically an H-bond donor). 13,14 DESs have a melting point that is far below that of either individual constituent. The mechanism is that the complexing agent interacts with the anion and increases its effective size. This, in turn, decreases the anionic interaction with the cation thereby reducing the salt-HBD lattice energy and causing a depression in the melting point of the mixture. z E-mail: [email protected] they are simple to synthesize on a large scale. 27 Furthermore, their physicochemical and electrochemical properties (viscosity, conductivity, electrochemical stability, diffusion coefficients, etc.) have been evaluated in a similar manner to that for ILs. The synthesized DESs are applied as electrolytes to determine the effects of the electrode and solvent in our electrochemical system. Ferrocene/ferrocenium (Fc/Fc + ) or cobaltocenium/cobaltocene (Cc + /Cc) redox couples have been investigated as candidates of internal references to provide a known and stable reference point in various DESs. Despite the significance of DESs and their remarkable advantages, their applications in redox flow batteries (RFBs) are limited. The experiments reported here are based on commercially sourced raw materials without additional purification (i.e., these are preliminary experiments that enable an informed decision for choosing appropriate raw materials for preparing DESs for practical experiments with non-aqueous RFB prototypes in future work). The purpose is to prove that a redox battery could be charged/discharged successfully without the need for complex synthesis and purification processes that could possibly lower the economics of the entire process