Constructing Membrane-Free Faradaic Deionization Systems with High Capacity and High Rate of Salt Removal/Recovery

Electrochemical deionization (ECDI) systems are attractive desalination devices that can achieve high salt adsorption capacity (SAC) with acceptable energy consumption, which can be employed as a promising complement or alternative method to other desalination techniques. The deionization mechanisms of the ECDI system can be divided into two distinct categories [1]: (1) formation of the electric double-layer (EDL) on the electrode surface to capture ions and (2) Faradaic materials via redox reactions using the charge compensation, ion intercalation, and compound formation, etc. The Faradaic materials including the pseudocapacitive materials and battery materials, based on the redox mechanisms, generally exhibit a high SAC because of the large charges involving the redox couples within the electroactive materials. Accordingly, the ECDI systems utilizing the pseudocapacitive//pseudocapacitive, battery//battery, and hybrid pseudocapacitive//battery combinations are considered to be the next generation ECDI systems, not only their high SACs but also the high salt-removal rate, especially when the electrochemical reversibility of redox couples within the active materials is high. Furthermore, the Faradaic materials are able to exhibit the unique “memory effect” [2] which can be used to concentrate the high-value ions for resource recovery and circular economy without employing any membranes (i.e., membrane-free ECDIs), which are very attractive to the practical applications. Here, we will provide the design guidelines for constructing the electrochemical deionization systems with high capacity and high rate of salt removal but without any membranes. The examples will include MnO 2 //PPy (PPy: polypyrrol) [1,2], CuHCF@CNT//PPy (CuHCF: copper hexacyanoferrate@carbon nanotube) [3], and PPy-p-TS//PPy-ClO 4 (PPy-p-TS: 4-methylbenzene-sulfonic acid-doped PPy) systems [4]. References: C.-C. Hu, et al., “A highly efficient faradaic desalination system utilizing MnO 2 and polypyrrole-coated titanium electrodes”, Desalination 498 (2021) 114807. C.-C. Hu, et al., “Construction of an inverted-capacitive deionization system utilizing pseudocapacitive materials”, Electrochemistry Communications 104 (2019) 106486. C.-C. Hu, et al., “A high-capacity hybrid desalination system using battery type and pseudocapacitive type electrodes”, Desalination 545 (2023) 116160. C.-C. Hu, et al., "Dopant-designed conducting polymers for constructing a high-performance, electrochemical deionization system achieving low energy consumption and long cycle life”, Chemical Engineering Journal 457 (2023) 141373.