Brackish water is a compelling resource to help mitigate water scarcity, which allows for producing freshwater at a lower energy cost than seawater due to relatively low salt concentrations (1 – 10 g/L). Several electrochemical separation techniques are being explored for brackish water desalination such as electrodialysis (ED) and capacitive deionization (CDI). The use of redox flow batteries (RFBs) for water desalination is an emerging approach to enable low-voltage (versus electrolysis of water in ED) and continuous operation (versus cyclic operation in CDI). However, identifying a non-toxic, affordable, and efficient redox couple remains a major challenge. We demonstrate that the iron complexed with citrate (Fe-Cit) allows for energy-efficient desalination at high water recovery ratios. In a symmetric RFB cell comprising four channels divided by cation and anion exchange membranes, the redox electrolyte was recirculated to outer channels for electrochemical reactions, and a feed solution (2.9 g/L NaCl) was fed to inner channels that continuously produced diluate and concentrate streams. We achieved an energy efficiency of 22% for producing freshwater (0.5 g/L) at a water recovery ratio of 91% and productivity of 41.1 L-freshwater per m<sup>2</sup>-cell pair area per h (LMH). In an asymmetric RFB cell, ferri-/ferrocyanide (Fe-CN) was coupled with Fe-Cit, which stored energy during desalination and produced high power densities in a separate, two-channel RFB cell. Any unintentional release of Fe-CN poses potential environmental concerns, and Fe-Cit represents a non-toxic and affordable alternative. The talk will discuss remaining challenges for practical water desalination applications, including water transport, crossover, and electrochemical stability.

Iron-based redox flow batteries for water desalination

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