Tandem Desalination/Salination Strategies Enabling the Use of Redox Couples for Efficient and Sustainable Electrochemical Desalination.

As access to fresh water becomes an increasingly serious global issue, developing desalination methods that can reduce not only the cost but also the carbon footprint of desalination has become of utmost importance. In this study, we demonstrate the use of the oxidation and reduction of the same redox couple with fast redox kinetics as the anode and cathode reactions of an electrodialysis (ED) cell. This reduces the thermodynamic equilibrium cell potential to 0 V while also significantly reducing the kinetic overpotentials required for cell operation. As a result, the overall operating voltage of our ED cell is remarkably reduced, making it possible to use ED for seawater desalination and to operate the ED cell by using inexpensive portable power generators that provide a limited voltage. The sustainable use of the redox couple in the ED cell was enabled by a new strategy, where a desalination ED cell and a salination ED cell were operated in tandem. In this tandem system, the electrolytes in the anode and cathode compartments of the two cells were circulated such that the compositional changes of the electrolytes made in the desalination cell could be reversed in the salination cell. As a result, the feedwater (0.6 M NaCl) could be converted to 0 and 1.2 M NaCl solutions in the desalination cell and salination cell, respectively, without the accumulation of salt ions in the anode and cathode compartments. The operating principles and performance of a proof-of-concept tandem desalination/salination system are demonstrated.