Capacitive deionization (CDI) is an energy-efficient desalination technique. However, the maximum desalination capacity of conventional carbon-based CDI systems is approximately 20 mg g, which is too low for practical applications. Therefore, the focus of research on CDI has shifted to the development of faradic electrochemical deionization systems using electrodes based on faradic materials which have a significantly higher ion-storage capacity than carbon-based electrodes. In addition to the common symmetrical CDI system, there has also been extensive research on innovative systems to maximize the performance of faradic electrode materials. Research has focused primarily on faradic reactions and faradic electrode materials. However, the correlation between faradic electrode materials and the various electrochemical deionization system architectures, , hybrid capacitive deionization, rocking-chair capacitive deionization, and dual-ion intercalation electrochemical desalination, remains relatively unexplored. This has inhibited the design of specific faradic electrode materials based on the characteristics of individual faradic electrochemical desalination systems. In this review, we have characterized faradic electrode materials based on both their material category and the electrochemical desalination system in which they were utilized. We expect that the detailed analysis of the properties, advantages, and challenges of the individual systems will establish a fundamental correlation between CDI systems and electrode materials that will facilitate future developments in this field.

Recent Advances in Faradic Electrochemical Deionization: System Architectures versus Electrode Materials.

Conducting polymers have become the focus of research due to their interesting properties, such as a wide range of conductivity, facile production, mechanical stability, light weight and low cost and due to the ease with which conducting polymers can be nanostructured to meet the specific application. They have become valuable materials for many applications, such as energy storage and generation. Recently, conducting polymers have been studied to be used in supercapacitors, battery electrode and fuel cells. This article is to briefly discuss the background & theory behind their conductivity as well as to highlight the recent contributions of conducting polymers to the field of energy and their significance. Furthermore, the methods of production of the conducting polymers in addition to the different ways utilised to nano-engineer special morphologies are discussed.

Storing energy in plastics : a review on conducting polymers & their role in electrochemical energy storage

MXene pseudocapacitive electrode material for capacitive deionization

Replacing lithium with sodium-ion batteries for energy storage is of enormous interest, especially from practical and economic considerations. However, it has proved difficult to achieve competitive figures of merit for sodium-ion batteries due to the lack of a detailed understanding of the reaction mechanism(s). Herein, we report a sodium electrochemical conversion reaction with Co3O4 nanoparticles decorated on carbon nanotubes (Co3O4/CNTs) utilizing in situ TEM, down to the atomic-scale. We observe synergetic effects of the two nanoscale components, which provide insights into a new sodiation mechanism, facilitated by Na-diffusion along a CNT backbone and CNT–Co3O4 interfaces. A thin layer of amorphous low conductivity Na2O forms on the CNT surfaces at the beginning of sodiation. The conversion reaction results in the formation of ultrafine metallic Co nanoparticles and polycrystalline Na2O, and fast diffusion of the reaction products which might be due to the quick migration of Na2O under an electron beam. In the desodiation process, the dissociation of Na2O and formation of Co3O4 due to the de-conversion reaction are observed.

Synergistic Sodiation of Cobalt Oxide Nanoparticles and Conductive Carbon Nanotubes (CNTs) for Sodium-Ion Battery

Capacitive deionization (CDI) technology is an effective seawater desalination technology due to its high cycle efficiency and good reversibility property, which has been extensively focused. As one of the core...

Dimensional optimization enable high-performance capacitive deionization

Battery materials as emerging capacitive deionization electrodes for desalination have better salt removal capacities than traditional carbon-based materials. LiMn2O4, a widely used cathode material, is difficult to utilize as a deionization electrode due to its structural instability upon cycling and Mn dissolution in aqueous-based electrolytes. Herein, a facile and low-cost ball-milling routine was proposed to prepare a LiMn2O4 material with highly exposed (111) facets. The prepared electrode exhibited relatively low dissolution of Mn during cycling, which shows its long cycle stability. In the hybrid capacitive deionization system, the LiMn2O4/C electrode delivered a high desalination capacity of 117.3 mg g−1 without obvious capacity decay at a voltage of 1.0 V with a 20 mM initial salt concentration. In addition, the exposed (111) facets significantly alleviated Mn ion dissolution, which also enhanced the structural steadiness. 

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Facet-Controlled LiMn2O4/C as Deionization Electrode with Enhanced Stability and High Desalination Performance

The world is suffering from chronic water shortage due to the increasing population, water pollution and industrialization. Desalinating saline water offers a rational choice to produce fresh water thus resolving the crisis. Among various kinds of desalination technologies, capacitive deionization (CDI) is of significant potential owing to the facile process, low energy consumption, mild working conditions, easy regeneration, low cost and the absence of secondary pollution. The electrode material is an essential component for desalination performance. The most used electrode material is carbon-based material, which suffers from low desalination capacity (under 15 mg·g−1). However, the desalination of saline water with the CDI method is usually the charging process of a battery or supercapacitor. The electrochemical capacity of battery electrode material is relatively high because of the larger scale of charge transfer due to the redox reaction, thus leading to a larger desalination capacity in the CDI system. A variety of battery materials have been developed due to the urgent demand for energy storage, which increases the choices of CDI electrode materials largely. Sodium-ion battery materials, lithium-ion battery materials, chloride-ion battery materials, conducting polymers, radical polymers, and flow battery electrode materials have appeared in the literature of CDI research, many of which enhanced the deionization performances of CDI, revealing a bright future of integrating battery materials with CDI technology.

https://www.mdpi.com/2073-4441/12/11/3030/pdf

A Review of Battery Materials as CDI Electrodes for Desalination

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Capacitive deionization (CDI) is a newly developed desalination technology with low energy consumption and environmental friendliness. The surface area restricts the desalination capacities of traditional carbon-based CDI electrodes while battery materials emerge as CDI electrodes with high performances due to the larger electrochemical capacities, but suffer limited production of materials. LiMn2O4 is a massively-produced lithium-ion battery material with a stable spinel structure and a high theoretical specific capacity of 148 mAh·g−1, revealing a promising candidate for CDI electrode. Herein, we employed spinel LiMn2O4 as the cathode and activated carbon as the anode in the CDI cell with an anion exchange membrane to limit the movement of cations, thus, the lithium ions released from LiMn2O4 would attract the chloride ions and trigger the desalination process of the other side of the membrane. An ultrahigh deionization capacity of 159.49 mg·g−1 was obtained at 1.0 V with an initial salinity of 20 mM. The desalination capacity of the CDI cell at 1.0 V with 10 mM initial NaCl concentration was 91.04 mg·g−1, higher than that of the system with only carbon electrodes with and without the ion exchange membrane (39.88 mg·g−1 and 7.84 mg·g−1, respectively). In addition, the desalination results and mechanisms were further verified with the simulation of COMSOL Multiphysics.

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Spinel LiMn2O4 as a Capacitive Deionization Electrode Material with High Desalination Capacity: Experiment and Simulation

Capacitive deionization (CDI) is an emerging eco-friendly desalination technology with mild operation conditions. However, the energy consumption of CDI has not yet been comprehensively summarized, which is closely related to the economic cost. Hence, this study aims to review the energy consumption performances and mechanisms in the literature of CDI, and to reveal a future direction for optimizing the consumed energy. The energy consumption of CDI could be influenced by a variety of internal and external factors. Ion-exchange membrane incorporation, flow-by configuration, constant current charging mode, lower electric field intensity and flowrate, electrode material with a semi-selective surface or high wettability, and redox electrolyte are the preferred elements for low energy consumption. In addition, the consumed energy in CDI could be reduced to be even lower by energy regeneration. By combining the favorable factors, the optimization of energy consumption (down to 0.0089 Wh·gNaCl−1) could be achieved. As redox flow desalination has the benefits of a high energy efficiency and long lifespan (~20,000 cycles), together with the incorporation of energy recovery (over 80%), a robust future tendency of energy-efficient CDI desalination is expected.

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Yuxin Jiang

Papers

Facet-Controlled LiMn2O4/C as Deionization Electrode with Enhanced Stability and High Desalination Performance

A Review of Battery Materials as CDI Electrodes for Desalination

Facet-Controlled LiMn2O4/C as Deionization Electrode with Enhanced Stability and High Desalination Performance

Spinel LiMn2O4 as a Capacitive Deionization Electrode Material with High Desalination Capacity: Experiment and Simulation

Energy Consumption in Capacitive Deionization for Desalination: A Review