Mesoporous amorphous FePO4 nanosphere@Graphene as a faradic electrode in capacitive deionization for high-capacity and fast removal of NaCl from water

Due to its low energy consumption, capacitive deionization (CDI) is regarded as one of the most promising technologies in brackish water desalination. However, CDI has two drawbacks, namely, low deionization capacity and unavoidable parasitic reactions that limit its further development. Unlike the traditional CDI electrodes that store the ions in the electric double layer capacitors, faradic electrodes store ions at crystallographic sites or between the atomic planes, which could effectively address the issues mentioned above. However, obtaining low-cost faradic electrode materials with a high salt removal capacity for practical applications is still challenging. Here, we report a new material, mesoporous amorphous FePO4 nanosphere@Graphene (FePO4@RGO) synthesized via a one-step hydrothermal method. Electrochemical experiments demonstrate that the formation of a mesoporous structure and covered by graphene can address the drawbacks of pure FePO4 for electroconductivity and ion diffusion rates. When used in desalination, the novel design enhances the desalination capacity, and the rates increase by 60% (50.13-85.94 mg/g) and 180% (0.079-0.24 mg/g/s), respectively, while energy consumption decreases by 25%. The production cost of the material was calculated, and implies that this material has potential for use in large-scale desalination.