Optimization of electrolyte composition for the all-iron hybrid flow battery

The two main technical hurdles limiting commercial application of the aqueous all-iron hybrid redox flow battery are the parasitic hydrogen evolution reaction and the poor kinetics of the iron plating and dissolution reactions. In this work these issues are addressed by studying the reactions in various electrolytes with voltammetric methods. The results described in this work offer some promising directions to improve the performance of the all-iron redox flow battery, and thereby increase its future commercial viability. Initially, electrochemical tests were conducted on a glassy carbon electrode, but this was found to be a poor substrate due to poor adherence of the plated iron to the substrate, uneven plating morphology, and a higher plating overpotential on the amorphous glassy carbon compared to a polycrystalline gold electrode, which was used instead in subsequent experiments. The standard electrolyte configuration employed in this work used FeCl2 as active material with NH4Cl as inert supporting electrolyte. To choose the best supporting electrolyte, several salts were studied: either chloride salts with various cations, or ammonium salts with various anions. It was found that of the cations Li+, NH4+, Na+, K+, Cs+, Mg2+ and Al3+, the best results were obtained with Na+ and Li+ as supporting cation, as electrolytes containing these cations decrease the iron plating overpotential by up to 80 mV without significantly increasing the hydrogen evolution rate, resulting in higher coulombic efficiencies. Of the anions F-, SO42-, Cl-, Br-, and I-, it was found that chloride-based supporting electrolytes exhibit by far the best performance, owing to the higher iron solubility and the complete absence of halogen gas evolution in the applied potential window. The effect of elevated temperature was studied, were it was found that increasing the electrolyte temperature decreased the plating overpotential, and increased both the iron deposition and hydrogen evolution rate. Increasing the operating temperature from 25 °C to 50 °C decreased the plating potential by approximately 50 mV, and increasing the temperature further to 80 °C resulted in a 150 mV lower overpotential. Further study revealed that both hydrogen evolution and the iron plating reaction are enhanced by the elevated temperature, but this increase was stronger for the latter reaction, leading to higher coulombic efficiencies at lower overpotential. The addition of low concentration (molar Fe:metal ratio = 100:1) of secondary metal chlorides was studied as a potential method to selectively inhibit the hydrogen evolution reaction. Eleven metals were selected that both had the ability to co-plate or alloy with iron, and have a lower exchange current density for hydrogen evolution than iron. Among the metals that were investigated, the most promising candidates were Cu, Tl, Pb and Sn. The best results were obtained with the thallium electrolyte, yielding up to 80% more metal plated (in C/cm2) than the iron-only electrolyte under identical conditions.