Iron Electrodeposition in a Deep Eutectic Solvent for Flow Batteries

Abstract

Iron chloride in a deep eutectic solvent containing high concentrations of iron with choline chloride and ethylene glycol have been synthesized. It was found that physical properties of the electrolytes, as well as the nature of the electroplated iron are greatly influenced by electrolyte composition. This is not surprising in that electrochemical reactivity of the solute ions as well as the physical properties of the electrolyte are controlled by speciation of the metals in solution. When the chloride to iron ratio is ≥4:1, complexes such as [FeCl 4 ] − and [FeCl 4 ] 2− were shown to be the dominant species using X-ray absorption near edge structure measurements coupled with Raman spectroscopy. However, when the chloride to iron ratio falls below 4:1, the ethylene glycol was found to complex the iron; the presence of this complex hinders fluid properties as shown by an order of magnitude decrease in solution conductivity as well as alter the iron deposition mechanism. © The Author(s) 2017. Flow batteries present a potentially low cost energy storage solution that is flexible in design due to external storage of the electrolyte. They offer reversible energy storage along with the load-leveling capabilities needed to facilitating implementation of renewable energy sources. Unlike conventional batteries, dissolved reactive species are stored in tanks and flowed through an electrolytic stack, where the electrochemical reactions occur, allowing for scalability. Redox flow batteries (RFBs) employ a redox chemistry at both electrodes; examples include the all-vanadium, 1 iron-chrome, 2,3 or a metal-free system utilizing quinones. 4 There are also several hybrid configurations, such as the all-iron, 5 all-copper, 6 and zinc halide 7 system which involve metal deposition/dissolution at one electrode. With the hybrid configurations, the storage capacity is related to the amount of metal that can be stored within the stack. The reactions for the all-iron chemistry are shown by Equations 1 and 2 where iron metal is deposited at the negative electrode upon charge. The all-iron chemistry represents a cost effective and environmentally friendly RFB chemistry and was thus chosen for this study.Positive Electrode: FeAqueous based flow battery systems have been widely studied, [8][9][10] however there is a growing interest in non-aqueous electrolytes with larger electrochemical windows (1.5−5.0 V 11 ) as a means to increase power density. Unfortunately, organic solvents have safety hazards associated with them due to their volatile and flammable nature and contamination by trace amounts of moisture or oxygen can have detrimental effects on battery performance. 12 The solubility of active species in these types of electrolytes are often very low, limiting the energy storage capacity. 13,14 Ionic liquids (IL) offer a different approach to increase energy and power densities of current non-aqueous redox flow battery (RFB) technologies. They possess wide electrochemical windows similar to organic solvents but with the advantages o...

Electrodeposition and Growth of Iron from an Ethylene Glycol Solution

“…al. 19 used ethylene glycol as an electrolyte but in the presence of choline chloride (ChCl) and FeCl 2 that gave different behavior and results compared with the present work. They also used different ratios from ChCl:EG: FeCl 2 and obtained different results at each ratio.…”

Electrochemical Characteristics and Transport Properties of V(II)/V(III) Redox Couple in a Deep Eutectic Solvent: Magnetic Field Effect

“…DES can be prepared and used under environmental conditions. In consideration of these appealing features such as a wide electrochemical stability window (Lloyd et al, 2013 ; Miller et al, 2017 ), better biocompatibility (Ding et al, 2018 ; Xu J. et al, 2019 ), potential biodegradability, low cost, simple preparation procedures, and multi-electron transfer reactions (Zhang et al, 2017 ), so it is emerged as promising electroactive materials to build sustainable RFBs (Jhong et al, 2009 ; Zhang et al, 2018 ).…”

Effects of Electrolyte Additives on Nonaqueous Redox Flow Batteries

“…The deep eutectic solvent (DES) can be recognized as a peculiar ionic liquid; it is consisting of a conjunction with a stoichiometric ratio of acceptors of hydrogen bond (like quaternary ammonium salts) and donors of hydrogen bond (like compounds of amides, carboxylic acids, and polyols) into the eutectic mixture. We can prepare and use DES under ambient conditions; low cost is the main advantage of DES, which is cheaper than conventional ionic liquids to an order of magnitude; DES is easy to prepare and its overall biodegradability is an advantage too [17][18][19][20]. People have carried out preliminary works on the DES.…”