Enhanced Salt Removal Performance Using Graphene-Modified Sodium Vanadium Fluorophosphate in Flow Electrode Capacitive Deionization

Designing electrode materials with excellent comprehensive properties was of top priority in promoting development of flow electrode capacitive deionization (FCDI). To date, most FCDI studies involved the application and modification of carbon-based materials, which suffered the contradiction between rheological behavior and electrochemical performance. In this study, a Na+ superionic conductor (NASICON) sodium vanadium fluorophosphate@reduced graphene oxide (NVOPF@rGO) was synthesized and applied as a flow electrode in FCDI. Benefiting from the confinement effect of the three-dimensional (3D) reduced graphene oxide (rGO) network, thin and uniform NVOPF nanosheets formed and provided abundant active sites for adsorbing Na+. Moreover, the interconnected rGO network formed a 3D conductive network for Na+ and electron transport. Compared with an activated carbon (AC)-AC system (AC was used as an anode and a cathode), a NVOPF@rGO-AC system (NVOPF@rGO was used as a cathode and AC was used as an anode) exhibited preferable dispersibility and stability of electrode dispersion, lower internal resistance, higher desalination rate, and lower energy consumption. Besides, the average salt adsorption rate (ASAR) reached 5.32 μg·cm-2·min-1 by adjusting the concentration of the electrode (4.73 wt %), the flow rate of the electrode (25 mL·min-1), and the operation voltage (1.6 V). This study demonstrated the potential of faradic flow electrodes for promoting the development and application of FCDI.