Enhanced Desalination Performance of a Flow-Electrode Capacitive Deionization System by Adding Vanadium Redox Couples and Carbon Nanotubes

Flow-electrode capacitive deionization (FCDI) technology can achieve continuous desalination via the electrodialysis coupling method. However, electrical energy is still highly consumed. In this work, the flow carbon nanotubes (CNTs) and vanadium redox couples are utilized as the flow electrode material together with AC to achieve the energy-saving desalination process. The V2+/V3+ ions are oxidized/reduced at the positive/negative electrode chambers under the constant current applied. The ions in salt feed can be continuously removed through the electrodialysis process in a three-membrane configuration (AEM|CEM|AEM). The carbon nanotubes play double roles of both electron transporter and capacitive ion capturer together with activated carbon. Excellent electrochemical desalination can be obtained. In the current sample tests, the desalination rate can be up to 0.253 μg cm-2 s-1, and the energy consumption of 72.62 kJ mol-1 is achievable by adding 1 wt % CNTs and 20 mM/20 mM V2+/V3+ to 6.41 wt % activated carbon flow electrode at the current density 0.43 mA cm-2. This demonstrates the possibility of low-energy desalination with the continuous process. Our study provides an efficient way to promote the FCDI desalination performance, which will contribute to the development of FCDI technology in the future.