Concentration‐Gradient Multichannel Flow‐Stream Membrane Capacitive Deionization Cell for High Desalination Capacity of Carbon Electrodes

Abstract

We present a novel multichannel membrane flow-stream capacitive deionization (MC-MCDI) concept with two flow streams to control the environment around the electrodes and a middle channel for water desalination. The introduction of side channels to our new cell design allows operation in a highly saline environment, while the feed water stream in the middle channel (conventional CDI channel) is separated from the electrodes with anion- and cation-exchange membranes. At a high salinity gradient between side (1000 mm) and middle (5 mm) channels, MC-MCDI exhibited an unprecedented salt-adsorption capacity (SAC) of 56 mg g in the middle channel with charge efficiency close to unity and low energy consumption. This excellent performance corresponds to a fourfold increase in desalination performance compared to the state-of-the-art in a conventional CDI cell. The enhancement originates from the enhanced specific capacitance in high-molar saline media in agreement with the Gouy-Chapman-Stern theory and from a double-ion desorption/adsorption process of MC-MCDI through voltage operation from -1.2 to +1.2 V.

Short Review of Multichannel Membrane Capacitive Deionization: Principle, Current Status, and Future Prospect

“…Zero voltage operation is the most commonly used operation for the regeneration of electrodes without consuming extra energy. For the complete regeneration of certain electrodes, reversed current or voltage modes could be used [15,17].…”

“…By reducing co-ion repulsion and ion swapping (Figure 4), additional ions could be captured to balance the charge neutrality in the macropores of electrodes or at the interface of membranes and electrodes [30]. The introduction of ion exchange membranes not only allows a significant enhancement of desalination performance in conventional CDI but also leads to various types of cell architectures such as hybrid CDI (HCDI) [16], cation intercalation desalination (CID) [25], multichannel flow stream membrane CDI (MC-MCDI) [15], and battery desalination combined with the oxidation processes [44]. Due to the permselective characteristics of ion exchange membranes, membraneinvolved CDI reveals high charge efficiency and enhanced salt removal capacity.…”

“…Moreover, the system maintains high charge efficiency (>90%), indicating that the system is operated efficiently with a small amount of side reactions. Additionally, by using concentrated solutions in the side channels, the porous electrodes could form the EDL even in nanopores, therefore, enhancing the ion removal capacities [15].…”

“…Over the last few decades, CDI has become remarkably advanced with various types of electrodes including capacitive electrodes with carbonaceous materials (i.e., activated carbon, carbon aerogel, carbon nanotubes, and graphene) [11][12][13][14][15] and faradaic electrodes with battery and battery-like materials (i.e., sodium manganese oxide, molybdenum disulfide, Mxene, silver/silver chloride, and Prussian blue analogs) [16][17][18][19][20]. These electrode materials have led to novel systematic developments.…”

“…Recently, in order to overcome these limitations, multichannel flow stream membrane CDI (MC-MCDI) was proposed as an innovative concept of cell design in CDI. MC-MCDI utilizes two independent flow streams for electrodes and treating water, resulting in a favorable environment for the formation of EDL [15,[32][33][34][35]. Through several systematic studies, MC-MCDI has not only successfully been demonstrated as an alternative of conventional CDI systems, but also utilized in various applications.…”