(351aq) Electrochemical Water Desalination Using an Iron-Based Redox Couple

Water desalination is a promising technological solution to water scarcity. While several technologies are currently utilized to produce freshwater from saline water sources, there are remaining challenges, including intensive energy usage (e.g., thermal distillation), fouling (e.g., reverse osmosis), and high material costs (e.g., electrodialysis). The use of capacitive or battery electrodes allows for electrochemically driven capture and release of ions, efficient water desalination using inexpensive electrode material, and low voltage operation without producing byproducts. However, the separation can only be achieved using materials available on the electrode surface, which leads to undesirable mixing between cycles and limited magnitude of desalination. To overcome this challenge, we demonstrated a new strategy for water desalination using an iron-based redox couple. Water desalination was achieved in a 4-channel electrochemical flow cell separated by an alternative array of anion and cation exchange membranes. The redox couple was recirculated between two channels at each end of the cell. Electrochemical reactions occurring at carbon electrodes created a charge imbalance and enabled the transport of Na⁺ and Cl^− through cation and anion exchange membranes to maintain electroneutrality. Using 20 mM NaCl as a model brackish water, we continuously produced a desalinated water stream (15 mM NaCl) with energy consumption that was less than one third (< 0.02 kWh m^−3) of that required in electrode-based systems (> 0.06 kWh m^−3) at a productivity of ~ 40 L m^−2 h^−1. Continuous production of freshwater at a drinking water level (0.5 g/L) was achieved using 50 mM NaCl as a feed solution. Other aspects of this system, including electrochemical properties, environmental compatibility, and water recovery ratio, will be discussed as well. Collectively, we show that the use of a redox couple can represent an energy-efficient method for electrochemical water desalination.