Highly efficient and stable desalination via novel hybrid capacitive deionization with redox-active polyimide cathode

Hybrid capacitive deionization (HCDI), normally consisting of a faradaic electrode, typically inorganic intercalation compounds, and an electric double layer carbon electrode, is an efficient and high performance alternative to the conventional capacitive deionization. In this work, for the first time we introduced a redox-active polyimide, poly[N,N′-(ethane-1,2-diyl)-1,4,5,8-naphthalenetetracarboxiimide] (PNDIE) as a novel electrode material of HCDI. In 1 M NaCl solution, PNDIE exhibits a capacitor-like fast rate performance and a high specific capacity of 89.9 mAh g⁻¹ at 0.2 A g⁻¹ (corresponding to 359.6 F g⁻¹ at 0.9 V voltage), which outperforms those of most reported carbonaceous and faradaic electrode materials for desalination application. When used as cathode of HCDI with activated carbon anode, PNDIE shows a high sodium uptake capacity of 54.2 mg g⁻¹ and excellent electrochemical stability under open-air environment. Moreover, the parasitic side reaction of PNDIE cathode with dissolved oxygen in HCDI was detailedly studied, which should also cause some capacity loss in HCDI with faradaic electrode but was seldom reported in the previous work. The strategy in this work should provide a new insight in exploring novel redox-active and stable HCDI polymer electrode instead of conventional inorganic intercalation compounds for highly efficient and stable desalination application.