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.

Na3(VO)2(PO4)2F nanocuboids/graphene hybrid materials as faradic electrode for extra-high desalination capacity.

Capacitive deionization (CDI) is considered as a promising desalination technology due to its low energy consumption and no two-second pollution. But the development of traditional CDI is limited by its two drawbacks, which are low deionization capacity and unavoidable parasitic reactions. Hybrid capacitive deionization (HCDI), which is composed of a faradic electrode and an electrical-double-layer electrode, effectively solves the above problem. Herein, we report a typical NASICON material Na3(VO)2(PO4)2F and modify it with rGO, then apply it in HCDI firstly and receive a superior desalination performance. Five samples are prepared by adding different contents GO solution and we choose the best one (NVOPF-4) with the lowest resistance for the desalination tests according to electrochemical performance. The result of desalination shows a high desalination capacity of 175.94 mg·g-1, low energy consumption of 0.35 kWh·kg-NaCl-1, and the energy recovery is 20% at a current density of 25 mg·g-1. NVOPF@rGO displays a promising ability for desalination in capacitive deionization, further confirming NASICON be a suitable material type for HCDI electrode materials.