Hydrogen evolution mitigation in iron-chromium redox flow batteries via electrochemical purification of the electrolyte

The redox flow battery (RFB) is a promising electrochemical energy storage solution that has seen limited deployment due, in part, to the high capital costs of current offerings. While the search for lower-cost chemistries has led to exciting expansions in available material sets, recent advances in RFB science and engineering may revivify older chemistries with suitable property profiles. One such system is the iron-chromium (Fe-Cr) RFB, which utilizes a low-cost, high-abundance chemistry, but the poor Cr redox reaction kinetics and high hydrogen evolution reaction (HER) rates challenge efficient, long-term operation. Of late, renewed efforts have focused on HER mitigation through materials innovation including electrocatalysts and electrolyte additives. Here, we show electrochemical purification, where soluble contaminants are deposited onto a sacrificial electrode prior to cell operation, can lead to a ca. 5× reduction in capacity fade rates. Leveraging data harvested from prior literature, we identify an association between coulombic efficiency and discharge capacity decay rate, finding that electrochemical purification can enable cell performance equivalent to that with new and potentially-expensive materials. We anticipate this method of mitigating HER may reduce capacity maintenance needs and, in combination with other advances, further durational Fe-Cr RFBs.