Size and composition regulated sodium vanadium fluorophosphate wrapped in rGO as an efficient cathode for brackish and seawater desalination

The discovery of novel electrode materials with durable ion removal functionality for a designed capacitive deionization architecture is critical to enable seawater desalination. NASICON-based material, although having high theoretical capacity and fast sodium conductivity and thus a promising electrode material for sodium-ion removal, its desalination performance is limited by the poor electrical conductivity and aqueous electrolyte susceptibility. Herein, nano-sized sodium vanadium fluorophosphate Na₃V₂(PO₄)₂F₃ at reduced graphene oxide (NVPF@rGO) is synthesized to improve electroconductivity and shorten the ion diffusion path, while fluorine content in its composition is regulated to lower the ions' diffusion barrier as a result of larger volume of the crystal. Moreover, the electrolyte concentration-controlled strategy is thoroughly investigated to minimize the dissolubility of the cathode over time. A proposed dual-ion electrochemical deionization system (DEDI) consisting of NVPF@rGO cathode and AgCl@rGO anode shows superior desalination stability in a highly concentrated synthetic seawater environment with ~98% capacity retention in over 500 cycles. Moreover, the salt removal capacity of 60 mg/g and desalination rate of 3.7 mg/g/min were achieved at a current density of 200 mA/g.