High-Desalination Performance via Redox Couple Reaction in the Multichannel Capacitive Deionization System

Capacitive deionization (CDI) has received much attention, which is considered as a promising electrochemical water desalination technology. However, an obstacle to its conventional application is the limited ion removal capacity of carbon electrodes induced by the electrical double layer. Here, a novel CDI system, called multichannel redox CDI (MC-RCDI), was proposed to enhance the desalination performance of CDI by utilizing the redox couple of ferricyanide/ferrocyanide as well as the adsorption capacity of the conventional carbon electrodes. In the MC-RCDI, the feed water in the middle channel was fully separated from the redox couple-containing electrolyte in the side channel by ion exchange membranes. Then, along with the carbon adsorption capacity, the electrochemically reversible redox reaction provided an additional salt removal capacity driven by the charge inequality. Thus, the MC-RCDI exhibited a significantly enhanced salt adsorption capacity (SAC) of 67.8 mg/g (8.59 μmol/J) and a charge efficiency of ∼90%. The results demonstrated as greater than threefold increase in the salt removal capacity, compared to the system without the redox reaction (SAC ≈ 19.9 mg/g, 7.50 μmol/J). Moreover, in the MC-RCDI system, a sustainable redox reaction allowed a continuous desalination without a discharging step, possibly leading to the desalination of concentrated feed solution, up to seawater level standards.