Significantly improved stability of hybrid capacitive deionization using nickel hexacyanoferrate/reduced graphene oxide cathode at low voltage operation

Currently, hybrid capacitive deionization (HCDI) with faradaic material cathode and active carbon (AC) anode has attracted much attention due to its very excellent desalination performance even in highly-concentrated saline water. However, the carbon oxidation reaction occurring at a potential of 0.7–0.9 V in a CDI system still exists at the carbon anode in a HCDI system, which causes the performance degradation during long-term operation. Introducing costly ion exchange membranes has been reported to solve this issue. In this work, we report a more simple and cost-saving approach to improve the desalination performance of membrane-free HCDI cell by using nickel hexacyanoferrate/reduced graphene oxide (NiHCF/rGO) cathode which possesses high theoretic capacity and low Na⁺ intercalation/extraction potential at a low voltage operation (0.6 V). A high desalination capacity of 22.8 mg g⁻¹ is achieved for this membrane-free HCDI cell, higher than those for the conventional AC//AC CDI cell. More importantly, the AC//NiHCF/rGO cell shows a stable cycling performance with a capacity retention of 78% at 0.6 V over 100 desalination-regeneration cycles, significantly improved compared with that at 1.2 V. The strategy should provide a helpful guidance for practical CDI operation.