Selection and optimization of carbon-based electrode materials for flow-electrode capacitive deionization

Flow-electrode capacitive deionization (FCDI) is an emerging desalination technology for its high desalination efficiency and continuous operation. This study systematically investigated the feasibility of various representative carbon-based materials and their combinations as the flow-electrode with the primary focus on their desalination efficiency and energy efficiency in FCDI. Among individual carbon-based electrode materials, namely activated carbon (AC), carbon fiber (CF), carbon black (CB), multi-walled carbon nanotubes (MWCNTs), and graphene nanoplatelets (GNPs), the nano-scale materials obviously outperformed micron-scale materials. Whereas, the combinations of AC + MWCNTs/GNPs provided superior desalination performance than AC + CB while working as blended flow-electrodes in FCDIs due to the different spatial configurations of three nano-scale particles. Furthermore, CB was tested as the base material in combination with MWCNTs and GNPs respectively. Approximately 110–130% increments in maximum salt adsorption rate were obtained for both blended electrodes compared to individual CB electrodes. The highest SARmax of 1.59 ± 0.02 μg cm−2 s−1 was observed in FCDI driven by the CB + GNPs electrode at a content of 1.5/0.5 wt%. Supported by the results of field emission scanning electron microscopy (FESEM) and electrical impedance spectroscopy (EIS), the 2D spatial morphologies of GNPs were able to build similar but more stable “bridges” between base electrode particles (i.e., AC or CB) as CNTs. Its sheet-like structure avoided agglomeration problems and largely promoted the conductivity of the product flow-electrodes.