(Invited) Capacitive Deionization of High-Salinity Water Using Ion-Exchange Membranes

Abstract

It is known that capacitive deionization (CDI) is not suitable for direct desalination of seawater because of its high salinity (~ 35 g L−1) and the limit in applied potential to avoid water electrolysis and oxidation of chloride ions. It has recently been reported, however, that the potential window can be expanded with the formation of an electrode-electrolyte interphase when using highly concentrated aqueous electrolyte. Studies also have proven that increasing ion concentration can greatly enhance the ion capacity of the electrodes. In addition, ion-exchange membranes (IEMs) can be used to control the electrode performance and allow application of reverse potential for electrodesorption. Here, we present the possibility to directly desalt high-salinity water and seawater through membrane CDI (MCDI). An MCDI cell was symmetrically assembled with anion and cation exchange membranes attached respectively to the anode and cathode. Our results show that the MCDI cell equipped with mesoporous carbon can remove 66.7 mg sodium chloride per gram of carbon, while the CDI cell alone (without membranes) could only remove 8.5 mg g−1 under the same conditions. Similarly, the MCDI cell equipped with carbon aerogel demonstrated a salt removal capacity of 37.3 mg g−1 in comparison with 12.9 mg g−1 of the CDI cell. These results indicate that the MCDI cell, assisted with IEMs and overpotential, can directly desalt high-salinity water regardless of the electrode material. Moreover, IEMs can prevent coion desorption at the beginning of the adsorption stage, as well as counterion adsorption at the beginning of the desorption stage, which can increase energy efficiency. For high-salinity water desalination, electrode regeneration is challenging. By applying a reversed low potential and controlling the desorption time, electrode regeneration can become effective. The presentation will include a review of CDI approaches, motivation for overpotential CDI supported by neutron imaging of electrodes and basic electrochemical experiments, characterization of electrode materials, a discussion of overpotential CDI and MCDI results including energy requirements, and a discussion of remaining challenges for direct desalting of high-salinity water via MCDI.