Faradaic Rectification in Electrochemical Deionization and Its Influence on Cyclic Stability

Capacitive deionization (CDI) is a typical configuration of electrochemical deionization, which suffers from severe desalination capacity degradation derived from uncontrolled parasitic reactions. In this work, Faradaic rectification, the phenomenon by which electrode potentials and side reactions are dynamically regulated due to the asymmetrical anode/cathode Faradaic reactions, was studied under various CDI operation conditions. It was found that the Faradaic rectification in CDI would lead to capacity degradation indirectly by accelerating carbon anode oxidation and would be influenced by the cell voltage, flow rate, and asymmetric electrode construction. We also found an unconventional degradation mechanism in Faradaic cathode hybrid-CDI (HCDI) caused by the dramatic electrode-potential redistribution, which is derived from Faradaic rectification rather than the electrode structure decay. By adding a cation-exchange membrane to block the dissolved oxygen from cathode, the Faradaic rectification was suppressed successfully, and thus, the cyclic performance of CDI and HCDI was significantly increased by 59 and 46%, respectively (in 100 h cycling). This study provides an insight into understanding the Faradaic rectification in electrochemical deionization and its influence on CDI/HCDI cyclic stability, which should be of value to future explore cost-competitive membrane-less electrochemical deionization construction.