(504c) Advancing the Fundamental Understanding of Redox-Driven Separations for Sustainable Water Desalination

Redox-driven water desalination employs electrochemical reactions that take place at the electrode/electrolyte interface as a means to separate salt from water. Using redox-active ions is an emerging trend that integrates the charge/discharge process of redox flow batteries with salt separation, called redox flow desalination, which takes advantage of energy storage and conversion, opening up greater opportunities to effectively manage the water-energy nexus. In addition, the cell potential can be minimized using a single redox couple in a symmetric system, in contrast with conventional electrodialysis-based approaches, wherein the electrolysis of water utilized to drive salt separation requires relatively high equilibrium and overpotential. Thus, small, modular water desalination systems are possible using redox flow desalination. However, there are technical barriers hindering practical applications, including the electrochemical behaviors of redox-active ions under circumneutral aqueous conditions and their interactions with other components of real source waters, which require fundamental research for the rational design and operation of redox-driven separation systems. This talk presents our current strategies for utilizing redox-active ions to drive salt separation. We have explored several redox-active complexed Fe(II)/Fe(III) species showing the solution composition-dependent electrochemical properties, solubility, and stability, all of which greatly limit the choice of a redox couple to mediate salt separation. One successful ligand used to solubilize Fe(II)/Fe(III) species at neutral pH while maintaining electrochemical activity is citrate, serving as a cost-effective and environmentally-sound alternative to broadly used complexing agents such as cyanide. However, a high redox electrolyte concentration used to overcome the limited electrochemical kinetics of the Fe(II)/Fe(III)-citrate complex adversely affected the selectivity of an ion-exchange membrane separating the redox electrolyte from source waters, thus decreasing the separation efficiency. The same issue was identified in a redox-driven solar desalination process developed to enable energy-neutral production of freshwater. Although the use of I^−/I₃^− simultaneously mediated the sunlight-to-electricity conversion and salt separation in a single device, a small amount of the redox couple was released into product water. Therefore, advancing the fundamental understanding of the electrochemical, physicochemical, and transport properties of redox-driven separation processes is of critical importance to move beyond the early-stage research.