Suppressing the oxygen-related parasitic reactions in NaTi₂(PO₄)₃-based hybrid capacitive deionization with cation exchange membrane

The parasitic reactions leading to capacity fading and charge loss remain a serious issue for capacitive deionization (CDI). NaTi₂(PO₄)₃ (NTP) has recently emerged as a promising faradaic cathode in hybrid CDI (HCDI) with high Na⁺ uptake capacity and good Na⁺ selectivity, but it is still challenged by serious parasitic reactions. Although the irreversible faradaic reactions on carbon electrode are raising growing attention in CDI research field, the parasitic reactions on faradaic materials are seldom studied in HCDI by now. In this work, we evaluated the parasitic reactions of NTP-reduced graphene oxide (rGO) electrode in both three-electrode mode and full-cell HCDI mode. By using deaired electrolyte, the coulombic efficiency of NTP-rGO is significantly enhanced from 75.0% to 98.2% in 3rd cycle, and the capacity retention rate is promoted from 37.5% to 80.3% at the low current density of 0.1 mA g⁻¹ in 100 cycles, suggesting that electrochemical reduction of oxygen and its derived reactions are the main parasitic reactions in NTP-based HCDI. In full-cell HCDI desalination tests, by introducing cation exchange membrane to block the penetration of dissolved oxygen, the parasitic reactions and pH fluctuations are successfully suppressed. The study here provides an insight into understanding and suppressing the parasitic reactions in HCDI, and should be of value to the development of efficient and stable HCDI for practical applications.