The effect of common metallic impurities on the performance of a single-cell iron-chromium redox flow battery

• Investigated metallic impurities in ICRFB. • Direct electrolyte production from alloy. • Cu and Ni detrimentally affect performance. • Trace impurities of K, Na, Ca, and Al reduce capacity decay. In this study the impact of metallic impurities on an ICRFB's performance was investigated, focusing on producing an electrolyte directly from a ferrochrome alloy to reduce processing steps and thus production cost. To determine their influence on ICRFB performance, a synthetic electrolyte was spiked with impurities at varying concentrations. ICP-OES analysis identified Cu, Ni, Co, Mn, V, Ti, Mg, K, Na, Ca, and Al as main impurities in the ferrochrome ore. Among these, Cu and Ni detrimentally affected performance. Cu plated on the negative electrode during charging, potentially causing operational issues at an industrial scale. While Ni decreased various performance metrics (CE, VE, EE and discharge capacity), it did lead to an increased ASR and capacity decay probably due to plating (negative electrode) and dissolution mechanisms leading to the consumption of electrons and production of hydrogen respectively. Interestingly, trace amounts ( > 70 ppm) of K, Na, Ca, and Al reduced the capacity decay rate during cycling. Future research should focus on removing Cu and Ni from industrial electrolytes to validate performance on a larger scale.