Novel membrane-free hybrid capacitive deionization with a radical polymer anode for stable desalination

Currently, limited charge efficiency and poor long-term desalination stability remain the major challenges for the practical application of membrane-free capacitive deionization (CDI). Hybrid capacitive deionization (HCDI) is suggested to effectively address these issues. Although cathode-hybridized HCDI has been studied widely, anode-hybridized HCDI has been seldom reported to date. In this work, a radical polymer, poly (2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), with good anion-storage capability was used as the anode of the membrane-free HCDI (denoted as A-HCDI) for the first time. 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% in the first 20 cycles at 1.2 V. Moreover, A-HCDI exhibits a long-term stable salt removal capacity of 13.9 mg g⁻¹ during charging/discharging at 1.2/0 V for 500 h, while membrane-free CDI only remains effective before 70 h. The novel A-HCDI provides a new approach for efficient and stable membrane-free electrochemical desalination.