Abstract
While the iron-chromium redox flow battery (ICRFB) is a promising low-cost large-scale energy storage solution, one of its main challenges, compared to the vanadium redox flow battery (VRFB), is the capacity decay rate caused by an imbalance between the electrolytes. To counter and rectify the imbalance, an ICRFB requires a rebalancing system, which in turn requires an effective electrolyte monitoring system. This study was conducted to develop a method that could be used to monitor the imbalance and rebalancing of an ICRFB by means of the Fe2+/Fe3+ ratio. Using the near-infrared (NIR) light spectrum (UV–Vis spectrophotometry), a function was derived from Beer's law that could be used to approximate Fe2+ concentrations in acidic aqueous solutions in the presence of Fe3+. To attain this, the effect of temperature, HCl concentration, Cr3+, Fe3+ and dissolved gasses on NIR absorbance was determined. The derived equation was validated using inductively coupled plasma - optical emission spectrometry (ICP-OES) as well as a lab-scale redox flow battery (RFB). Finally, it was shown that the developed analytical method could analyse the redox speciation of iron, differentiating between identical elements in solution with different valences, which confirms its suitability for monitoring the imbalance and rebalancing of an ICRFB.
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