Iron-glycine precipitation-free electrolyte for aqueous redox flow batteries

Iron-based redox flow batteries are cheap, safe, flexible stationary energy storage devices that can alleviate the demand for critical raw materials in Europe. However, several challenges persist in the development of iron-based electrolytes matching operational requirements, among which is hydrolysis-induced precipitation of iron compounds in mild acidity conditions. In this work, we address the problem of iron precipitation by the addition of glycine into an iron-based aqueous electrolyte. To verify and optimize the electrolyte properties for battery applications, the study addresses the very concentrated conditions typically used in an industrially relevant cell, namely 1 M FeCl2, 1 M FeCl3, and glycine ranging from 0 to 4 M. On these electrolytes, ionic conductivity, dynamic viscosity, and pH are measured and electrochemical characterization is performed through cyclic voltammetry scans with an RDE in a three-electrode cell. Given glycine's feature to suppress redox couple reactivity, the exchange current density was determined employing Koutecký-Levich analysis, particularly suited due to the electrolyte's sluggish kinetic, indicative of a quasi-reversible system. The study shows how the precipitation of insoluble iron hydroxides is strongly suppressed with 1 M glycine while preserving much of the attractive characteristics of the original iron chemistry. A maximum of 120 mS/cm of ionic conductivity is reached at concentrations corresponding to 50 % SoC without supporting electrolyte and with a slight increase in viscosity up to 3 mPa s. The exchange current density ranges from 4 to 10 mA/cm2, with an open circuit potential of 0.54 V vs. Ag/AgCl sat. at 90 % SoC, making this system an attractive catholyte for a high-energy density iron-based RFB.