Capacitive deionization (CDI) is an energy saving and environmentally friendly technology for water desalination. However, classical CDI is challenged by a low salt removal capacity. To improve the desalination capacity, electrode materials utilizing the battery mechanism for salt ion removal have emerged as a new direction more recently. In this work, we report a study of amorphous iron phosphate (FePO4) as a promising electrode material for pseudocapacitive sodium ion removal. Sodium ions can be effectively, reversibly intercalated and de-intercalated upon its electrochemical reduction and oxidation, with an excellent sodium ion capacity under half-cell testing conditions. By assembling a hybrid CDI (HCDI) system utilizing the FePO4 electrode for pseudocapacitive sodium ion removal and active carbon electrode for capacitive chloride ion removal, the cell exhibited a high salt removal capacity and good reversibility and durability, which was attributed to the advantageous features of amorphous FePO4. The HCDI system achieved a high deionization capacity (82 mg g-1) in 10 mM NaCl, a fast deionization rate (0.046 mg g-1 s-1), and good stability and cyclability.
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