Iron Electrodeposition in a Deep Eutectic Solvent for Flow Batteries

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 [FeCl4]− and [FeCl4]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.