Asymmetric and Symmetric Redox Flow Batteries for Energy-Efficient, High-Recovery Water Desalination

Electrochemical separation offers an energy-efficient means to desalinate brackish water, a relatively untapped but increasingly utilized water source for freshwater supply. Several electrochemical techniques are being developed to enable low-energy desalination combined with energy storage. We report a new approach that produced a peak power density of 6.0 mW cm^-2 from the energy stored in iron cyanide (Fe-CN) and iron citrate (Fe-Cit) redox couples during water desalination, using asymmetric redox flow batteries (RFBs). Desalination and the charging of the redox couples occurred in a four-channel RFB cell. The stored energy was extracted in a two-channel RFB cell. Desalination of model brackish water (2.9 g L^-1) to freshwater (0.5 g L^-1) was also studied in a symmetric system using the environmentally benign Fe-Cit. The process was characterized by low energy consumption (0.56 kW h m^-3), high productivity (41.1 L freshwater m^-2 area h^-1, representing practical operating conditions for brackish water desalination), and high water recovery (91% product-to-intake water ratio, addressing the environmental and economic challenges of brine disposal). The low cell voltage (<0.5 V) required in the reported system is ideally suited for developing modular desalination systems powered by renewables, including solar energy. Collectively, water-based RFBs for desalination and power production would lead to sustainable water-energy infrastructure.