Effects of aluminum, iron, and manganese sulfate impurities on the vanadium redox flow battery

Abstract

The cost of the electrolyte is a major drawback for implementation of vanadium redox flow batteries (VRFBs). Since a small increase in the electrolyte purity higher than 98.5% can have a significant impact on the electrolyte costs, understanding the effects of impurities on VRFB performance is essential. In this work the effect of Al3+, Fe2+, Mn2+ impurities on the electrochemical activity, VRFB performance and durability is studied by cyclic voltammetry, flow battery charge/discharge, in-situ hydrogen evolution measurement, and material characterization. The presence of Mn2+ ions at concentrations of up to 0.1 M is found to have a negligible impact on charge-discharge efficiencies over 200 cycles. With Al3+ or Fe2+ ions in the electrolyte, severe detrimental effects on battery performance and durability are observed. The presence of Fe2+ decreases the VRFB discharge capacity by 36%. In the presence of Al3+ an alumina containing precipitate is formed on the electrodes, severely affecting the battery performance. Although the lowest battery performance occurs in the presence of a mixture of the three impurities, the precipitation and capacity decay are less severe than with Al3+ ions alone. This suggests that the presence of Mn2+ and/or Fe2+ helps to stabilise the electrolyte and mitigate the precipitation.