Pseudocapacitive Desalination of Brackish Water and Seawater with Vanadium-Pentoxide-Decorated Multiwalled Carbon Nanotubes

Unraveling the Ion Uptake Capacitive Deionization of Sea- and Highly Saline-Water by Sulfur and Nitrogen Co-Doped Porous Carbon Modified with Molybdenum Sulfide

In parallel to the depletion of potable water reservoirs, novel technologies have been developed for seawater softening, as it is the most abundant source for generating deionized water. Although salt removal at subosmotic pressures and ambient temperatures by applying low-operating potentials with high energy efficiency made capacitive deionization (CDI) an advantageous water-softening process, its practical application is limited by insufficient ion removal capacity and low concentration influent. The performance of a CDI system is in progress with engineering the electrode active materials, also facilitating the advance design in highly saline- and seawater study. Herein, an innovative strategy was developed to provide high-performance CDI systems based on efficient and electrochemical ion-uptake active materials with a simple initial preparation. Nitrogen-doped porous carbons (N-pCs) received benefits from a high specific surface area and good surface wettability. The N-pCs were modified with molybdenum oxide/sulfide intercalative array and developed as CDI electrode active materials for desalination of both low/medium saline- and seawater. The MoS2/S,N-pC electrode materials exhibited perfect optimized salt adsorption capacity (SACs) of 47.9 mg g-1 when compared to N-pC (37.9 mg g-1) and MoO3/N-pC (39.6 mg g-1) counterparts at 1.4 V in a 750 ppm NaCl solution. In addition, the assembled CDI cells exhibited reasonable cycle stability and retained 96.7% of their initial SAC in continuous CDI cycles for 128,000 s. The fabricated CDI cell rendered an excellent salt removal efficiency (SRE, %) of 13.34% from the real seawater sample at 1.2 V. In detail, the SRE % of the NaCl, KCl, MgCl2, and CaCl2 soluble salts with respect to seawater sample exhibited a remarkable SRE % of 30.8%, 36%, 32.6%, and 19.3%, respectively. These SRE % values (>13.34%) provide convincing evidence on the reasonable ion uptake capability of the fabricated CDI cells for removing Na+, K+, Mg2+, and Ca2+ ions compared to other soluble component. The advanced cell design parallel to the promising outcomes provided herein makes these CDI systems immensely propitious for efficient water softening.

Soft-hard interface design in super-elastic conductive polymer hydrogel containing Prussian blue analogues to enable highly efficient electrochemical deionization

The poor cycling stability of faradaic materials owing to volume expansion and stress concentration during faradaic processes limits their use in large-scale electrochemical deionization (ECDI) applications. Herein, we developed a "soft-hard" interface by introducing conducting polymer hydrogels (CPHs), that is, polyvinyl alcohol/polypyrrole (PVA/PPy), to support the uniform distribution of Prussian blue analogues (e.g., copper hexacyanoferrate (CuHCF)). In this design, the soft buffer layer of the hydrogel effectively alleviates the stress concentration of CuHCF during the ion-intercalation process, and the conductive skeleton of the hydrogel provides charge-transfer pathways for the electrochemical process. Notably, the engineered CuHCF@PVA/PPy demonstrates an excellent salt-adsorption capacity of 22.7 mg g-1 at 10 mA g-1, fast salt-removal rate of 1.68 mg g-1 min-1 at 100 mA g-1, and low energy consumption of 0.49 kW h kg-1. More importantly, the material could maintain cycling stability with 90% capacity retention after 100 cycles, which is in good agreement with in situ X-ray diffraction tests and finite element simulations. This study provides a simple strategy to construct three-dimensional conductive polymer hydrogel structures to improve the desalination capacity and cycling stability of faradaic materials with universality and scalability, which promotes the development of high-performance electrodes for ECDI.

Electrode Materials for Desalination of Water via Capacitive Deionization

Recent years have seen the emergence of capacitive deionization (CDI) as a promising desalination technique for converting sea and wastewater into potable water, due to its energy efficiency and eco-friendly nature. However, its low salt removal capacity and parasitic reactions have limited its effectiveness. As a result, the development of porous carbon nanomaterials as electrode materials have been explored, while taking into account of material characteristics such as morphology, wettability, high conductivity, chemical robustness, cyclic stability, specific surface area, and ease of production. To tackle the parasitic reaction issue, membrane capacitive deionization (mCDI) was proposed which utilizes ion-exchange membranes coupled to the electrode. Fabrication techniques along with the experimental parameters used to evaluate the desalination performance of different materials are discussed in this review to provide an overview of improvements made for CDI and mCDI desalination purposes.

Promoting the electrocatalytic oxygen evolution reaction on NiCo2O4 with infrared-thermal effect: A strategy to utilize the infrared solar energy to reduce activation energy during water splitting

Utilization of the infrared (IR) solar energy remains a challenging task for traditional photo(electro)catalysis. Taking advantage of the IR-thermal effect to facilitate sluggish electrocatalytic reactions emerges as a promising way to utilize the IR band of the solar spectrum. In this work, nickel foam (NF) supported NiCo₂O₄ nanoneedles (NF/NiCo₂O₄ NNs) were prepared to promote the oxygen evolution reaction (OER) via the IR-thermal effect, with the NF/NiCo₂O₄ NNs acting as both the IR absorbing antennae and the OER active anode. The potential required to deliver a current density of 200 mA cm^-2 is negatively shifted from 1.618 V in the dark to 1.578 V under IR irradiation, and the Tafel slope is also decreased from 106 to 89 mV dec^-1. We demonstrate that the enhancement of OER activity is due to the localized temperature rise under IR irradiation. We measured the electrochemical activation energy of OER on NF/NiCo₂O₄ with and without IR irradiation, and the results reveal that IR irradiation reduces the kinetic energy barrier of the OER by IR-thermal effect and then facilitates OER kinetics. This work highlights a new approach to utilizing the IR portion of the sunlight to produce renewable hydrogen energy via water splitting.

Electrochemical Methods for Water Purification, Ion Separations, and Energy Conversion

Agricultural development, extensive industrialization, and rapid growth of the global population have inadvertently been accompanied by environmental pollution. Water pollution is exacerbated by the decreasing ability of traditional treatment methods to comply with tightening environmental standards. This review provides a comprehensive description of the principles and applications of electrochemical methods for water purification, ion separations, and energy conversion. Electrochemical methods have attractive features such as compact size, chemical selectivity, broad applicability, and reduced generation of secondary waste. Perhaps the greatest advantage of electrochemical methods, however, is that they remove contaminants directly from the water, while other technologies extract the water from the contaminants, which enables efficient removal of trace pollutants. The review begins with an overview of conventional electrochemical methods, which drive chemical or physical transformations via Faradaic reactions at electrodes, and proceeds to a detailed examination of the two primary mechanisms by which contaminants are separated in nondestructive electrochemical processes, namely electrokinetics and electrosorption. In these sections, special attention is given to emerging methods, such as shock electrodialysis and Faradaic electrosorption. Given the importance of generating clean, renewable energy, which may sometimes be combined with water purification, the review also discusses inverse methods of electrochemical energy conversion based on reverse electrosorption, electrowetting, and electrokinetic phenomena. The review concludes with a discussion of technology comparisons, remaining challenges, and potential innovations for the field such as process intensification and technoeconomic optimization.

Effects of Artificial River Water on PEM Water Electrolysis Performance

Hydrogen, a clean and renewable energy source, is a promising substitute for fossil fuels. Electricity-driven water electrolysis is an attractive pathway for clean hydrogen production. Accordingly, the development of electrolysis cells has drawn researchers’ attention to capital costs related to noble catalyst reduction and membrane degradation by the contaminations. In the literature, polymer electrolyte membranes (PEMs) have been studied on single cations contamination. In this study, we investigated the performance of a PEM on monovalent and divalent cation contamination by feed water. Artificial river water, called soft water, was used to analyze the effect of impurities on the PEM. The results demonstrated that the operating voltage drastically increased and induced cell failure with increasing Mg2+ and Ca2+ concentrations; however, it did not increase for Na+ and K+ after increase in voltage. Therefore, divalent cations have a stronger affinity than monovalent cations to degrade PEM and should be effectively excluded from the feed water.

Bismuth oxychloride nanostructure coated carbon sponge as flow-through electrode for highly efficient rocking-chair capacitive deionization

Rocking-chair capacitive deionization (RCDI), as the next generation technique of capacitive deionization, has thrived to be one of the most promising strategies in the desalination community, yet was hindered mostly by its relatively low desalination rate and stability. Motivated by the goal of simultaneously enhancing the desalination rate and structural stability of the electrode, this paper reports an anion-driven flow-through RCDI (AFT-RCDI) system equipped with BiOCl nanostructure coated carbon sponge (CS@BiOCl for short; its backbone is derived from commercially available melamine foam with minimum capital cost) as the flow-through electrode. Owning to the rational design of the composite electrode material with minimum charge transfer resistance and ultrahigh structure stability as well as the superior flow-through cell architecture, the AFT-RCDI displays excellent desalination performance (desalination capacity up to 107.33 mg g^-1; desalination rate up to 0.53 mg g^-1s^-1) with superior long-term stability (91.75% desalination capacity remained after 30 cycles). This work provides a new thought of coupling anion capturing electrode with flow-through cell architecture and employing a low-cost CS@BiOCl electrode with commercially available backbone material, which could shed light on the further development of low-cost electrochemical desalination systems.

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

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^-1, 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, i.e., 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.

Three-Dimensional Cobalt Hydroxide Hollow Cube/Vertical Nanosheets with High Desalination Capacity and Long-Term Performance Stability

Faradaic electrode materials have significantly improved the performance of membrane capacitive deionization, which offers an opportunity to produce freshwater from seawater or brackish water in an energy-efficient way. However, Faradaic materials hold the drawbacks of slow desalination rate due to the intrinsic low ion diffusion kinetics and inferior stability arising from the volume expansion during ion intercalation, impeding the engineering application of capacitive deionization. Herein, a pseudocapacitive material with hollow architecture was prepared via template-etching method, namely, cuboid cobalt hydroxide, with fast desalination rate (3.3 mg (NaCl)·g^-1 (h-Co(OH)₂)·min^-1 at 100 mA·g^-1) and outstanding stability (90% capacity retention after 100 cycles). The hollow structure enables swift ion transport inside the material and keeps the electrode intact by alleviating the stress induced from volume expansion during the ion capture process, which is corroborated well by in situ electrochemical dilatometry and finite element simulation. Additionally, benefiting from the elimination of unreacted bulk material and vertical cobalt hydroxide nanosheets on the exterior surface, the synthesized material provides a high desalination capacity (117 ± 6 mg (NaCl)·g^-1 (h-Co(OH)₂) at 30 mA·g^-1). This work provides a new strategy, constructing microscale hollow faradic configuration, to further boost the desalination performance of Faradaic materials.

Faradaic Electrodes Open a New Era for Capacitive Deionization

Capacitive deionization (CDI) is an emerging desalination technology for effective removal of ionic species from aqueous solutions. Compared to conventional CDI, which is based on carbon electrodes and struggles with high salinity streams due to a limited salt removal capacity by ion electrosorption and excessive co-ion expulsion, the emerging Faradaic electrodes provide unique opportunities to upgrade the CDI performance, i.e., achieving much higher salt removal capacities and energy-efficient desalination for high salinity streams, due to the Faradaic reaction for ion capture. This article presents a comprehensive overview on the current developments of Faradaic electrode materials for CDI. Here, the fundamentals of Faradaic electrode-based CDI are first introduced in detail, including novel CDI cell architectures, key CDI performance metrics, ion capture mechanisms, and the design principles of Faradaic electrode materials. Three main categories of Faradaic electrode materials are summarized and discussed regarding their crystal structure, physicochemical characteristics, and desalination performance. In particular, the ion capture mechanisms in Faradaic electrode materials are highlighted to obtain a better understanding of the CDI process. Moreover, novel tailored applications, including selective ion removal and contaminant removal, are specifically introduced. Finally, the remaining challenges and research directions are also outlined to provide guidelines for future research.

Exploration of Energy Storage Materials for Water Desalination via Next-Generation Capacitive Deionization

Clean energy and environmental protection are critical to the sustainable development of human society. The numerous emerged electrode materials for energy storage devices offer opportunities for the development of capacitive deionization (CDI), which is considered as a promising water treatment technology with advantages of low cost, high energy efficiency, and wide application. Conventional CDI based on porous carbon electrode has low salt removal capacity which limits its application in high salinity brine. Recently, the faradaic electrode materials inspired by the researches of sodium-batteries appear to be attractive candidates for next-generation CDI which capture ions by the intercalation or redox reactions in the bulk of electrode. In this mini review, we summarize the recent advances in the development of various faradaic materials as CDI electrodes with the discussion of possible strategies to address the problems present.

Electrochemical Desalination Using Intercalating Electrode Materials: A Comparison of Energy Demands

One approach for desalinating brackish water is to use electrode materials that electrochemically remove salt ions from water. Recent studies found that sodium-intercalating electrode materials (i.e., materials that reversibly insert Na⁺ ions into their structures) have higher specific salt storage capacities (mg_salt/g_material) than carbon-based electrode materials over smaller or similar voltage windows. These observations have led to the hypothesis that energy demands of electrochemical desalination systems can be decreased by replacing carbon-based electrodes with intercalating electrodes. To test this hypothesis and directly compare intercalation materials, we examined nine electrode materials thought to be capable of sodium intercalation in an electrochemical flow cell with respect to volumetric energy demands (W·h·L^-1) and thermodynamic efficiencies as a function of productivity (i.e., the rate of water desalination, L·m^-2·h^-1). We also examined how the materials' charge-storage capacities changed over 50 cycles. Intercalation materials desalinated brackish water more efficiently than carbon-based electrodes when we assumed that no energy recovery occurred (i.e., no energy was recovered when the cell produced electrical power during cycling) and exhibited similar efficiencies when we assumed complete energy recovery. Nickel hexacyanoferrate exhibited the lowest energy demand among all of the materials and exhibited the highest stability over 50 cycles.

Effect of conductive additives on the transport properties of porous flow-through electrodes with insulative particles and their optimization for Faradaic deionization

Deionization devices that use intercalation reactions to reversibly store and release cations from solution show promise for energy-efficient desalination of alternative water resources. Intercalation materials often display low electronic conductivity that results in increased energy consumption during desalination. Accordingly, we performed experiments to quantify the impact of the size and mass fraction of conductive additives and insulative active particles on the effective electronic conductivity, ionic conductivity, and hydraulic permeability of porous electrodes. We find that Ketjen black conductive additives with nodules <50 nm in diameter produce superior electronic conductivity at lower mass fractions than the larger carbon blacks commonly used in capacitive deionization. Hydraulic permeability and effective ionic conductivity depend weakly on carbon black content and size, though smaller active particles decrease hydraulic permeability. Based on these results we analyzed the energy consumption and salt removal rate of different electrode formulations by constructing an electrochemical Ashby plot predicting the variation of desalination performance with electrode transport properties. Optimized electrodes containing insulative Prussian blue analogue (PBA) particles were then fabricated and used in an experimental cation intercalation desalination (CID) cell with symmetric electrodes. For 100 mM NaCl influent energy consumption varied from 7 to 33 kJ/mol when current density increased from 1 to 8 mA/cm², approaching ten-fold increased salt removal rate at similar energy consumption levels to past CID demonstrations. Complementary numerical and analytical modeling indicates that further improvements in energy consumption and salt removal rate are attainable by enhancing transport in solution and within PBA agglomerates.

Efficient Sodium-Ion Intercalation into the Freestanding Prussian Blue/Graphene Aerogel Anode in a Hybrid Capacitive Deionization System

In this study, we introduced an efficient hybrid capacitive deionization (HCDI) system for removal of NaCl from brackish water, in which Prussian blue nanocubes embedded in a highly conductive reduced graphene oxide aerogel have been used as a binderfree intercalation anode to remove Na⁺ ions. The combination of redox-active nanocubes and the three-dimensional porous graphene network yielded a high salt removal capacity of 130 mg g^-1 at the current density of 100 mA g^-1. Moreover, energy recovery and energy consumption upon different desorption voltages of the HCDI system were investigated and the result showed a notably low energy consumption of 0.23 Wh g^-1 and a high energy recovery of 39%. Furthermore, the real-time intercalation process was verified by in situ X-ray powder diffraction measurements, which confirmed the intercalation and deintercalation processes during charging and discharging, respectively. Eventually, a perfect stability of the desalination unit was confirmed through the steady performance of 100 cycles. The improved efficiency as well as ease of fabrication opens a shiny horizon for our HCDI system toward commercialization of such technology for brackish water desalination.

Confined Redox Reactions of Iodide in Carbon Nanopores for Fast and Energy-Efficient Desalination of Brackish Water and Seawater

Faradaic deionization is a promising new seawater desalination technology with low energy consumption. One drawback is the low water production rate as a result of the limited kinetics of the ion intercalation and insertion processes. We introduce the redox activities of iodide confined in carbon nanopores for electrochemical desalination. A fast desalination process was enabled by diffusionless redox kinetics governed by thin-layer electrochemistry. A cell was designed with an activated carbon cloth electrode in NaI aqueous solution, which was separated from the feedwater channel by a cation-exchange membrane. Coupled with an activated carbon counter electrode and an anion-exchange membrane, the half-cell in NaI with a cation-exchange membrane maintained performance even at a high current of 2.5 A g^-1 (21 mA cm^-2 ). The redox activities of iodide allowed a high desalination capacity of 69 mg g^-1 (normalized by the mass of the working electrode) with stable performance over 120 cycles. Additionally, we provide a new analytical method for unique performance evaluation under single-pass flow conditions regarding the water production rate and energy consumption. Our cell concept provides flexible performance for low and high salinity and, thus, enables the desalination of brackish water or seawater. Particularly, we found a low energy consumption (1.63 Wh L^-1 ) for seawater desalination and a high water production rate (25 L m^-2  h^-1 ) for brackish water.

Ion Removal Performance, Structural/Compositional Dynamics, and Electrochemical Stability of Layered Manganese Oxide Electrodes in Hybrid Capacitive Deionization

Hybrid capacitive deionization (HCDI) is a derivative of capacitive deionization (CDI) method for water desalination, in which one carbon electrode is replaced with a redox-active intercalation electrode, resulting in substantial improvements in ion removal capacity over traditional CDI. The search for high-performing intercalation host compounds is ongoing. In this study, two-layered manganese oxides (LMOs), with sodium (Na-birnessite) and magnesium (Mg-buserite) ions stabilizing the interlayer region, were for the first time evaluated as HCDI electrodes for the removal of ions from NaCl and MgCl₂ solutions to understand structural/compositional dynamics and electrochemical stability of LMO electrodes over extended cycling. Both materials demonstrated excellent initial ion removal performance with the highest capacities of 37.2 mg g^-1 (637 μmol g^-1) exhibited by Mg-buserite in NaCl solution and 50.2 mg g^-1 (527 μmol g^-1) exhibited by Na-birnessite in MgCl₂ solution. The performance decay observed over the course of 200 ion adsorption/ion release cycles was attributed to two major phenomena: oxidation of carbon electrode and evolution of the structure/composition of LMO electrodes. The latter involves disorder in stacking of Mn-O layers and changes in the interlayer spacing/interlayer ions reflecting the composition of the solution being desalinated. This work highlights the importance of understanding the interactions between the HCDI electrodes and solutions containing different ions and the structural analysis of redox-active material in intercalation electrodes over the course of operation for gaining insight into the fundamental processes governing desalination performance and developing next-generation HCDI systems with long-term electrochemical stability.

Charge and Potential Balancing for Optimized Capacitive Deionization Using Lignin-Derived, Low-Cost Activated Carbon Electrodes

Lignin-derived carbon is introduced as a promising electrode material for water desalination by using capacitive deionization (CDI). Lignin is a low-cost precursor that is obtained from the cellulose and ethanol industries, and we used carbonization and subsequent KOH activation to obtain highly porous carbon. CDI cells with a pair of lignin-derived carbon electrodes presented an initially high salt adsorption capacity but rapidly lost their beneficial desalination performance. To capitalize on the high porosity of lignin-derived carbon and to stabilize the CDI performance, we then used asymmetric electrode configurations. By using electrodes of the same material but with different thicknesses, the desalination performance was stabilized through reduction of the potential at the positive electrode. To enhance the desalination capacity further, we used cell configurations with different materials for the positive and negative electrodes. The best performance was achieved by a cell with lignin-derived carbon as a negative electrode and commercial activated carbon as a positive electrode. Thereby, a maximum desalination capacity of 18.5 mg g^-1 was obtained with charge efficiency over 80 % and excellent performance retention over 100 cycles. The improvements were related to the difference in the potential of zero charge between the electrodes. Our work shows that an asymmetric cell configuration is a powerful tool to adapt otherwise inappropriate CDI electrode materials.