Examining the Impact of Solution and Surface Composition on Positive Electrode Kinetics for the All-Iron Redox Flow Battery

Abstract

Efforts to reduce the cost of long-duration energy storage systems have drawn increased interest in all-iron redox flow battery systems due to their inexpensive electrolytes. Previous studies have shown that the kinetics of the positive electrode for the Fe(II, III) reaction in these systems are sluggish on carbon surfaces and require significant overpotentials. Here, we identify how changes to the electrode surface and electrolyte composition can influence the electrochemical rate constants to reduce required overpotentials using a rotating disc electrode (RDE) assembly. The impacts of solution composition in terms of pH, chloride content, and cation species were investigated using a range of electrolyte compositions reported for all-iron flow battery systems. The electrode materials studied were glassy carbon, pyrolytic graphite basal plane, and pyrolytic graphite edge plane. Untreated and electrochemically oxidized electrode materials were quantified using X-ray photoelectron spectroscopy (XPS), and the reaction rate constants were quantified using electron impedance spectroscopy (EIS). Rate constants of the Fe(II, III) reaction on untreated electrode materials were largest on platinum and lowest on glassy carbon with pyrolytic materials providing comparable results. For the materials examined, increases electrolyte chloride content generally decreased the obtained rate constant.