Study On The Mechanism For Ion-exchange Membrane Dependence Of Highly Efficient Hybrid Capacitive Deionization

Capacitive deionization?CDI?,as an emerging desalination technology,has many potential advantages,such as operating at ambient temperature and pressure,simple devices,low cost and ion selectivity,which has drawn more and more attention from researchers in recent years.However,CDI still rely on ion exchange membrane?IEM?,which is vulnerable to membrane fouling and very expensive,to achieve stable desalination performance.Moreover,electrode material,anion exchange membrane?AEM?and cation exchange membrane?CEM?all play important roles for highly efficient CDI.Therefore,how to reduce the use of expensive IEM,take advantage of the low-cost feature of activated carbon?AC?,and improve the long-term stability and efficiency of CDI,is a core issue to achieve practical and low-cost CDI.The parasitic reactions and the co-ion effects on the carbon anode and cathode materials all play different roles in the performance decay and efficiency reduction of a membrane-free CDI.Therefore,the rational introduction of faradaic anode and cathode materials can effectively reduce the electrochemical side reactions,enhance the coulombic efficiency of CDI,and improve the long-term cycle stability of CDI.In this paper,asymmetric IEM assisted CDI was used to study the important role of AEM/CEM in CDI and hybrid CDI?HCDI?.On this basis,we tried to reduce the dependence of long-term stable asymmetric HCDI on IEM by introducing structural-stable polymer negative electrode materials and anion-selective polymer positive electrode materials.The main research contents and conclusions are as follows:1.We systematically studied the electrochemical performance of AC electrode,and compared the p H variation,coulombic efficiency,charge efficiency,cycling performance and specific salt removal capacity of membrane-free CDI,single AEM assisted CDI,single CEM assisted CDI and MCDI.Through that way,we studied the dependence of CDI efficiency and cycling performance on IEM,which provided guidance for the following three chapters to construct stable HCDI with reducing the use of expensive IEM.2.Taking Na Ti₂?PO₄?₃ with relatively stable electrochemical performance and less electrode dissolution in aqueous solution as the research object,the dependence of traditional cathode hybridized HCDI on CEM was studied by comparing its side reactions and cyclic stability under different voltages,deoxidation condition and whether CEM was added or not.3.In order to avoid the electrode dissolution of inorganic faradaic electrode materials and the poor cycling stability of conductive polymer electrode materials affecting the long-term stability of HCDI,we introduced redox-active polyimide with high structural stability to successfully construct CEM-free stable HCDI.When used as cathode of HCDI without assistance of CEM,PNDIE shows a high sodium uptake capacity of 54.2 mg g^-1 and excellent electrochemical stability under open-air environment.4.Radical polymer,poly?2,2,6,6-tetramethylpiperidinyloxy methacrylate?,with good anion-storage capability was used as the anode of the membrane-free HCDI.PTMA restrains the undesirable faradaic reactions that occur on the conventional carbon anodes and suppresses the cation expulsion effect that dominates the charging process of the carbon anode after the long-term oxidation,degrading the membrane-free CDI process.Compared with AC anode,PTMA anode improves the average charge efficiency from 18.7%to 48.6%.Moreover,A-HCDI exhibits a stable salt removal capacity retention ratio of 67.0%for 500 h,while membrane-free CDI only remains effective before 100 h.