Purification and analysis of ferrous chloride for an Fe/Cr flow battery application

Abstract

The inevitable depletion of fossil fuels, concerns about adverse human health effects of air pollution, reduction of greenhouse gas emissions and climate change are globally recognised as some of the main factors that drive the transition to renewable energy sources. Unfortunately, these renewable energy sources are intermittent by nature and consequently are inharmonious with continuous supply required from large scale electrical grids. To bridge this gap, large scale energy storage solutions (LESS) have to be developed and implemented. As a result, research in the field of scalable electrochemical energy storage technology has received significant attention, resulting in promising solutions such as the redox flow battery (RFB). Although the all vanadium RFB (VRFB) has received the most attention to date, the iron-chromium RFB (ICRFB) holds significant potential for South Africa as both the iron and chrome used in the electrolyte are abundantly available in SA. To address some of the challenges facing the commercialisation of ICRFBs, two aspects related to the iron chrome electrolyte used will be investigated in this study. To develop the ICRFB towards viability and market penetration, the next step is to evolve the lab-scale designs and experiments into large-scale demonstration units that have similar performance than the optimised lab-scale setups. Such upscaling requires significant volumes of electrolyte with a similar purity to those bought at a high cost for the laboratory-scale experiments. Hence, the first aim of this study was to develop a purification technique that could reduce the cost of the ICRFB electrolyte without sacrificing battery performance. This was attained by developing a crystallisation process in an open system where N2 was used to inhibit the oxidation of Fe2+. According to the inductively coupled plasma - optical emission spectroscopy (ICP-OES) and ultraviolet-visible spectrophotometry (UV-Vis) analysis, 81.3% Cu, 70.5% Ni and 62.7% Sn was removed from a low-cost industrial grade ferrous chloride (FeCl2) solution during the crystallisation process. After 8 hours of ICRFB operation, the charge/discharge capacity of the purified electrolyte was 124.6% higher than that of the unrefined FeCl2 solution and 5.6% lower than the lab-grade electrolyte. Similarly, it yielded 6.8% less capacity decay than the unrefined FeCl2 and only 0.1% more than the lab-grade electrolyte. The improved performance was likely due to the removal of impurities, which would result in fewer side reactions such as the hydrogen evolution reaction (HER). One of the drawbacks of the ICRFB compared to VRFB is its high rate of capacity decay resulting in imbalance due to the accumulation of Fe3+ in the positive electrolyte. Developing a simple method for monitoring the electrolyte, i.e. the Fe2+/Fe3+ speciation, during operation, would provide information for the required rebalancing process. Furthermore, such electrolyte monitoring could also provide information on the state of charge (SOC) and state of health

(SOH) of the ICRFB. However, quantifying Fe3+ concentrations are difficult when requiring a non-destructive analysis of iron redox species that are present at high concentrations. In the second part of this study, a method using the near infrared (NIR) light absorption of Fe2+ in conjunction with Beer’s law was developed to accurately and non-destructively quantify both Fe2+ and Fe3+ concentrations. Using 920 nm NIR light (UV-Vis) and the absorption constants, a function was developed to analyse the crystallised FeCl2 solution. In addition, the effects of temperature, the HCl concentration, as well as the presence of Cr3+, Bi catalyst and other dissolved gasses on the adsorption was investigated. The mathematical function of absorption that was derived in this study obtained a detection error of 3.33% for [Fe2+] and 3.08% for [Fe3+] during validation, and could be used to monitor the positive electrolyte of an ICRFB and hence the imbalance at varying temperatures and sample path lengths in- situ. The robust and instantaneous analytical method can be used to quantify dissolved iron species with different valences at high concentrations. This method was then also used during the crystallisation study to analyse the purity of the FeCl2 produced, confirming that the proposed N2 blanketing adequately inhibited Fe2+ oxidation during crystallisation.