Mesoporous carbon for capacitive deionization of saline water

Charged functional groups modified porous spherical hollow carbon material as CDI electrode for salty water desalination

As a new electrochemical technology, capacitive deionization (CDI) has been increasingly applied in environmental water treatment and seawater desalination. In this study, functional groups modified porous hollow carbon (HC) were synthesized as CDI electrode material for removing Na+ and Cl- in salty water. Results showed that the average diameter of HC was approximately 180 nm, and the infrared spectrum showed that its surface was successfully modified with sulfonic and amino groups, respectively. The sulfonic acid functionalized HC (HC-S) showed better electrochemical and desalting performance than the amino-functionalized HC (HCN), with a maximum Faradic capacity of 287.4 F/g and an adsorptive capacity of 112.97 mg/g for NaCl. Additionally, 92.63% capacity retention after 100 adsorption/desorption cycles demonstrates the excellent stability of HC-S. The main findings prove that HC-S is viable as an electrode material for desalination by high-performance CDI applications.

N, O-codoped carbon aerogel electrode improves capacitive deionization performance

Capacitive deionization (CDI) using porous carbon materials provides an environmentally friendly and sustainable solution to produce affordable fresh water. However, low salt adsorption rates significantly limit its practical application. In this study, N, O-codoped carbon aerogel (NOCA) was prepared by a simple sol-gel method using agar as the carbon framework, NaCl as the template, NH4HCO3 as the nitrogen source and self-blowing agent. The electrochemical and electrosorption properties of the NOCA electrode were significantly better than compared to commercial activated carbon (CAC) electrodes. The NOCA material had the following characteristics: (i) During the rapid freeze-drying process, NaCl crystals provide a three-dimensional network structure for effective dispersion of agar, reducing the agglomeration of particles. The volatile gas generated during the thermal decomposition of NH4HCO3 reduces the plugging of pores. (ii) The fluffy interconnecting network structure of the material enhances its electrical conductivity, providing sufficient channels and adsorption sites for the entry and exit of salt ions. (iii) The abundant hydroxyl and ether groups in the agar enhance the hydrophilicity of the material, whereas N doping further improves the electrical conductivity and reduces the ion transport resistance. The electrosorption capacity and adsorption rate of the NOCA material in 500 mg/L NaCl solution were 22.1 mg/g and 4.4 mg/(g·min), respectively. These values correspond to low energy consumption and high energy recovery efficiency in the electrosorption process. The adsorption capacity remained at 95.5 % after 50 adsorption/desorption cycles. These findings show that NOCA is a novel and potential electrode material for CDI applications.

Upcycling Waste Polyethylene Terephthalate to Produce Nitrogen-Doped Porous Carbon for Enhanced Capacitive Deionization

Porous carbon with a high surface area and controllable pore size is needed for energy storage. It is still a significant challenge to produce porous carbon in an economical way. Nitrogen-doped porous carbon (N-PC) was prepared through carbonization of a mixture of waste PET-derived metal-organic frameworks (MOFs) and ammonium. The obtained N-PC exhibits a large surface area and controlled pore size. When utilized as an electrode material for supercapacitors, the N-PC exhibits a specific capacitance of 224 F g-1, significantly surpassing that of commercial activated carbon (AC), which has a capacitance of 111 F g-1. In the subsequent capacitive deionization (CDI) tests, the N-PC demonstrated a maximum salt adsorption capacity of 19.9 mg g-1 at 1.2 V in a NaCl electrolyte (0.5 g L-1), and the salt adsorption capacity increased to 24.7 mg g-1 at 1.4 V. The N-PC electrode also exhibited superior regeneration. The present work not only presents a potential approach to develop cost-effective electrodes for seawater purification but also paves the way for recycling of waste plastics into high value-added products.

Advancing mesoporous carbon synthesis for supercapacitors: a systematic investigation of cross-linking agent effects on pore structure and functionality

Soft-templating synthesis provides an effective route to prepare ordered mesoporous carbons (MCs) that can be used for supercapacitors. During this process, the cross-linking of carbon precursors is critical to obtain tailored pore structural MCs, thus careful selection of appropriate cross-linking agents is required. Despite the shift from the prevailing cross-linker formaldehyde to its more environmentally friendly alternatives, detailed understanding on the influence of different cross-linking agents on templating synthesis is still lacking. Therefore, it remains challenging to draw a conclusion regarding which cross-linker can effectively enable an ideal cross-linking and a robust templating synthesis of ordered MCs. This work presents a systematic study, by comparing three typical cross-linkers (formaldehyde, glyoxal, and glyoxylic acid), on the pore architecture, surface functionality, and electrochemical performance of resulting MCs. Both the type of cross-linker and its ratio with precursor monomer were found to be crucial for the pore architecture and electrochemical performance of resulting MCs. Glyoxal showed to be a promising cross-linker for easily generating ordered mesopores between 3.3-6.1 nm when the molar ratio between cross-linker and carbon precursor ranged from 1 to 2, whereas glyoxylic acid and formaldehyde induced interrupted or disordered mesopores. When the resulting MCs were used as supercapacitor electrodes, those cross-linked with glyoxal also led to overall higher capacitance in both 6 M KOH aqueous and ionic liquid [N2220][NTf2]/acetonitrile electrolytes thanks to the dominance of ordered mesopore channels, especially MC prepared at glyoxal/precursor molar ratio of 1.5. These findings on the effect of cross-linking on templating synthesis can be used to guide the customisation of MCs for supercapacitors and other applications by smartly choosing a suitable cross-linking agent and its ratio with the precursor.

Turning waste into valuables: In situ deposition of polypyrrole on the obsolete mask for Cr(VI) removal and desalination

The global mask consumption has been exacerbated because of the coronavirus disease 2019 (COVID-19) pandemic. Simultaneously, the traditional mask disposal methods (incineration and landfill) have caused serious environmental pollution and waste of resources. Herein, a simple and green mass-production method has been proposed to recycle carbon protective mask (CPM) into the carbon protective mask/polydopamine/polypyrrole (CPM/PDA/PPy) composite by in situ polymerization of PPy. The CPM/PDA/PPy composite was used for the removal of Cr(VI) and salt ions to produce clean water. The synergistic effect of PPy and the CPM improved the removal capability of Cr(VI). The CPM/PDA/PPy composite provided high adsorption capacity (358.68 mg g^-1) and economic value (811.42 mg $^-1). Consequently, the CPM/PDA/PPy (cathode) was combined with MnO₂ (anode) for desalination in CDI cells, demonstrated excellent desalination capacity (26.65 mg g^-1) and ultrafast salt adsorption rate (6.96 mg g^-1 min^-1), which was higher than conventional CDI cells. Our work proposes a new low-carbon strategy to recycle discarded masks and demonstrates their utilization in Cr(VI) removal and seawater desalination.

Removal of chloride from water and wastewater: Removal mechanisms and recent trends

Increased chloride concentration can cause salinization, which has become a serious and widespread environmental problem nowadays. This review aims at providing comprehensive and state-of-the-art knowledge and insights of technologies for chloride removal. Mechanisms for chloride removal mainly include chemical precipitation, adsorption, oxidation and membrane separation. In chemical precipitation, chloride removal by forming CuCl, AgCl, BiOCl and Friedel's salt. Adsorbents used in chloride removal mainly include ion exchangers, bimetal oxides and carbon-based electrodes. Oxidation for chloride removal contains ozone-based, electrochemical and sulfate radical-based oxidation. Membrane separation for chloride removal consists of diffusion dialysis, nanofiltration, reverse osmosis and electrodialysis. In this review, we specifically proposed the factors that affect chloride removal process and the corresponding strategies for improving removal efficiency. In the last section, the remaining challenges of method explorations and material developments were stated to provide guidelines for future development of chloride removal technologies.

Knowledge and Technology Used in Capacitive Deionization of Water

The demand for water and energy in today’s developing world is enormous and has become the key to the progress of societies. Many methods have been developed to desalinate water, but energy and environmental constraints have slowed or stopped the growth of many. Capacitive Deionization (CDI) is a very new method that uses porous carbon electrodes with significant potential for low energy desalination. This process is known as deionization by applying a very low voltage of 1.2 volts and removing charged ions and molecules. Using capacitive principles in this method, the absorption phenomenon is facilitated, which is known as capacitive deionization. In the capacitive deionization method, unlike other methods in which water is separated from salt, in this technology, salt, which is a smaller part of this compound, is separated from water and salt solution, which in turn causes less energy consumption. With the advancement of science and the introduction of new porous materials, the use of this method of deionization has increased greatly. Due to the limitations of other methods of desalination, this method has been very popular among researchers and the water desalination industry and needs more scientific research to become more commercial.

The effect of redox potential on the removal characteristic of divalent cations during activated carbon-based capacitive deionization

The main objective of the study is to explore the removal characteristics of Cu²⁺ and Zn²⁺ ions in activated carbon-based capacitive deionization (CDI). In this work, CDI experiments were performed to remove divalent ions (e.g., Cu²⁺, Zn²⁺, and Ca²⁺) from single- and multicomponent aqueous solutions. As evidenced, divalent heavy metals could be successfully removed by charging the CDI cell at 1.2 V. Notably, the preferential removal of Cu²⁺ ions over Zn²⁺ and Ca²⁺ ions was observed in the charging step. The removal capacities for Cu²⁺, Zn²⁺, and Ca²⁺ ions in a competitive environment were 29.6, 19.6, and 13.8 μmol/g, respectively. In contrast, the regeneration efficiencies for the removal of Cu²⁺ and Zn²⁺ were much lower than that of Ca²⁺, suggesting the occurrence of irreversible Faradaic reactions on the cathode. X-ray photoelectron spectroscopy analysis demonstrated that Cu²⁺ ions were reduced to Cu(I) and Zn²⁺ ions were transformed to ZnO/Zn(OH)₂ on the cathode. Therefore, there were two major mechanisms for the removal of divalent heavy metal ions: capacitive electrosorption and cathodic electrodeposition. Specifically, the reduction potential played a crucial role in determining the removal characteristics. When regarding divalent cations with similar hydrated sizes, the divalent cation with a higher reduction potential tended to be separated by cathodic electrodeposition rather than double-layer charging, indicating the high removal selectivity of activated carbon-based CDI. This paper constitutes a significant contribution to promoting the application of CDI for contaminant sequestration.

Development of Lignin-Based Mesoporous Carbons for the Adsorption of Humic Acid

There is an increasing urge to make the transition toward biobased materials. Lignin, originating from lignocellulosic biomass, can be potentially valorized as humic acid (HA) adsorbents via lignin-based mesoporous carbon (MC). In this work, these materials were synthesized for the first time starting from modified lignin as the carbon precursor, using the soft-template methodology. The use of a novel synthetic approach, Claisen rearrangement of propargylated lignin, and a variety of surfactant templates (Pluronic, Kraton, and Solsperse) have been demonstrated to tune the properties of the resulting MCs. The obtained materials showed tunable properties (BET surface area: 95-367 m²/g, pore size: 3.3-36.6 nm, V _BJH pore volume: 0.05-0.33 m³/g, and carbon and oxygen content: 55.5-91.1 and 3.0-12.2%, respectively) and good performance in terms of one of the highest HA adsorption capacities reported for carbon adsorbents (up to 175 mg/g).

Interconnected Graphene Hollow Shells for High-Performance Capacitive Deionization

Electrochemical capacitive deionization (CDI) is a promising technology for distributed and energy-efficient water desalination. The development of high-performance capacitive electrodes is critical for enhancing CDI properties and scaling up its applications. Herein, a three-dimensional graphene porous architecture with high CDI performance is successfully constructed by assembling intentionally designed incomplete graphene-based spherical hollow shells. Small graphene oxide (GO) sheets are purposely adopted to prepare sphere shells by wrapping the surface of polystyrene sphere templates. Because the small-sized GO sheets cannot enwrap the spherical templates seamlessly, a unique graphene hollow shell structure with integrally interconnected feature forms upon removal of the templates. Compared to control samples with typical isolated pore structure (3DGA-C) prepared with commonly used large-sized GO sheets, such open and interconnected porous architectures (3DGA-OP) greatly increase their accessibility of specific surface area and pore volume, enabling superior electrochemical performance. The optimized CDI capacities of 3DGA-OP electrodes reach up to 14.4 mg·g^-1 in NaCl aqueous of 500 mg·L^-1 at 1.2 V, which is about 2 times the 3DGA-C ones (6.7 mg·g^-1) and exceeds the CDI values of most reported pure graphene electrodes under the same experimental conditions. This strategy of improving the open interconnectivity between pores illuminates new avenues for developing high performance CDI porous electrodes assembled from two-dimensional materials.

A novel graphene oxide-based ceramic composite as an efficient electrode for capacitive deionization

In this study, N-doped and TiO₂-decorated graphene oxides were developed as efficient nonprecious electrodes for capacitive deionization. The activity of this new material was evaluated in situ and in vivo. The performance of the synthesized material was measured in different saline solutions (0.1, 0.5 and 1.0 M NaCl) as an electrolyte. The results showed that the new material exhibits very good performance (157 F/g at 5 mV/s and 1.0 M NaCl compared to 19.5 F/g for pure graphene oxide). In the desalination test, which was performed in batch mode, the salt adsorption capacity and the efficiency of salt removal were 9.2 mg/g and 98%, respectively. To check the stability, the desalination test was repeated several times, and no change in the performance was observed. The results provide evidence that the newly synthesized material is a potential electrode material for CDI water desalination with satisfactory salt removal ability.

Towards Electrochemical Water Desalination Techniques: A Review on Capacitive Deionization, Membrane Capacitive Deionization and Flow Capacitive Deionization

Electrochemical water desalination has been a major research area since the 1960s with the development of capacitive deionization technique. For the latter, its modus operandi lies in temporary salt ion adsorption when a simple potential difference (1.0-1.4 V) of about 1.2 V is supplied to the system to temporarily create an electric field that drives the ions to their different polarized poles and subsequently desorb these solvated ions when potential is switched off. Capacitive deionization targets/extracts the solutes instead of the solvent and thus consumes less energy and is highly effective for brackish water. This paper reviews Capacitive Deionization (mechanism of operation, sustainability, optimization processes, and shortcomings) with extension to its counterparts (Membrane Capacitive Deionization and Flow Capacitive Deionization).

Water Desalination by Flow-Electrode Capacitive Deionization in Overlimiting Current Regimes

Since flow-electrodes do not have a maximum allowable charge capacity, a high salt removal rate in flow-electrode capacitive deionization (FCDI) can be achieved theoretically by simply increasing the applied voltage. However, present attempts to run FCDI at high voltages are unsatisfactory because of the instability of the module occurring in the overlimiting current regimes. To implement FCDI in the overlimiting current regimes (namely, OLC-FCDI), in this work, we analyzed the voltage-current (V-I) characteristics of several FCDI units. We confirmed that a continuous, rapid, and stable desalination performance of OLC-FCDI can be attained when the employed FCDI unit possesses a linear V-I characteristic (only one ohmic regime), which is distinct from the three V-I regimes in electrodialysis (ohmic, limiting current, and water splitting regimes) and the two in membrane capacitive deionization (ohmic and water splitting regimes). Notably, the linearV-I characteristic of FCDI requires continuous charge percolation near the boundaries of ion-exchange membranes. Effective methods include increasing the carbon content in the flow-electrodes and introducing electrical (carbon cloth) or ionic (ion-exchange resins) conductive intermediates in the solution compartment, which result in corresponding upgraded FCDI units exhibiting extremely high salt removal rates (>100 mg m^-2 s^-1), good cycling stability, and rapid seawater desalination performance under typical OLC-FCDI operation condition (27-40 g L^-1 NaCl, 500 mA). This study can guide future research of FCDI in terms of flow-electrode preparation and device configuration optimization.

Exceptional capacitive deionization rate and capacity by block copolymer-based porous carbon fibers

Capacitive deionization (CDI) is energetically favorable for desalinating low-salinity water. The bottlenecks of current carbon-based CDI materials are their limited desalination capacities and time-consuming cycles, caused by insufficient ion-accessible surfaces and retarded electron/ion transport. Here, we demonstrate porous carbon fibers (PCFs) derived from microphase-separated poly(methyl methacrylate)-block-polyacrylonitrile (PMMA-b-PAN) as an effective CDI material. PCF has abundant and uniform mesopores that are interconnected with micropores. This hierarchical porous structure renders PCF a large ion-accessible surface area and a high desalination capacity. In addition, the continuous carbon fibers and interconnected porous network enable fast electron/ion transport, and hence a high desalination rate. PCF shows desalination capacity of 30 mgNaCl g-1 PCF and maximal time-average desalination rate of 38.0 mgNaCl g-1 PCF min-1, which are about 3 and 40 times, respectively, those of typical porous carbons. Our work underlines the promise of block copolymer-based PCF for mutually high-capacity and high-rate CDI.

Sustainable lignin-derived hierarchically porous carbon for capacitive deionization applications

Cross-linked lignin with glyoxal leads to a support mesopore structure of lignin-based porous carbon with improved capacitive deionization performance.

Fabrication of an antimony doped tin oxide–graphene nanocomposite for highly effective capacitive deionization of saline water

In this study, antimony doped tin oxide loaded reduced graphene oxide (ATO–RGO) nanocomposites were synthesized via a facile hydrothermal approach. As a typical N-type semiconductor, the ATO in the composite can enhance the conductivity between graphene sheets, thus improving the specific capacitance and electrosorption performance. Under the optimal conditions, the largest surface area was 445.2 m² g⁻¹ when the mass content of ATO in the nanocomposite was 20 wt%. The synthesized optimal ATO–RGO electrode displayed excellent specific capacity (158.2 F g⁻¹) and outstanding electrosorptive capacity (8.63 mg g⁻¹) in sodium chloride solution, which were much higher than the corresponding results of pristine graphene (74.3 F g⁻¹ and 3.98 mg g⁻¹). At the same applied voltage, electrosorption capacity and charge efficiency of the ATO–RGO (20 wt%) material were better than those of reported carbon materials in recent years.

Antimony doped tin oxide–graphene nanocomposites synthesized via a facile hydrothermal approach displayed good specific capacity and electrosorptive capacity.

Effect of Hydrophilicity of Activated Carbon Electrodes on Desalination Performance in Membrane Capacitive Deionization

Membrane capacitive deionization (MCDI) is a modification of capacitive deionization (CDI) using ion-exchange membranes (IEM) in front of the electrodes. Electrode properties, especially the specific surface area, are known to be strongly related with desalination performance in CDI, but the effects of other properties in MCDI are not fully understood. The objective of this study was to investigate the effect of hydrophilicity in activated carbon electrodes on desalination performance in MCDI. Two types of activated carbon (P60 and YS-2) whose specific surface areas were similar were used as electrode materials, but they had different hydrophilicity (i.e., P60 was originally hydrophobic and YS-2 was relatively hydrophilic due to its nitrogen-containing surface chemistry). These hydrophilic electrodes (either the electrode itself or modified with polydopamine (PDA)) led to an increase in the salt adsorption capacity (SAC) in MCDI because they facilitated the access of both ions and water molecules into the electrode pores. In particular, the SAC of the P60 electrode displayed a large increase to almost reach that of the YS-2 electrode due to the improved hydrophilicity with PDA modification and the insignificant effects of PDA modification on an already hydrophilic YS-2 electrode. Additionally, PDA-modified IEM in MCDI reduced the SAC as a result of the additional insulating PDA layer with little changes in hydrophilicity.

Enhancement in Desalination Performance of Battery Electrodes via Improved Mass Transport Using a Multichannel Flow System

Desalination technologies have heavily been investigated to utilize the abundant salt water on Earth due to the global freshwater shortage. During recent years, the desalination battery (DB) has attracted attention for its low-cost, eco-friendly, and energy-efficient characteristics. However, the current DB system is subject to inevitable performance degradation because of the mass-transfer limitation at the electrode-electrolyte interface, particularly when the system is used to treat brackish water. Here, we present a novel strategy to overcome the intrinsic mass-transfer limitation of DB in brackish water using an effective cell design based on a multichannel flow system. Compared to the conventional DB that consists of one feed channel, the multichannel desalination battery (MC-DB) is configured using two side channels introducing a highly concentrated solution to the electrodes and one middle feed channel for water desalination. The MC-DB showed a desalination capacity of 52.9 mg g^-1 and a maximum salt removal rate of 0.0576 mg g^-1 s^-1 (production rate of 42.3 g m^-2 h^-1) when a salinity gradient between the feed streams in the middle (10 mM NaCl) and side (1000 mM NaCl) channels was present, which were 3-fold higher than those in the case with no salinity gradient. In addition, the high concentration solution in the side channel significantly enhanced the rate capability of MC-DB, allowing the system to operate under a high current density of 40 A m^-2 with a desalination capacity of 34.1 mg g^-1. Considering the effect of electrolyte concentration on the battery electrode performance through electrochemical characterization, the highly saline medium at the side channel in the MC-DB creates an optimal environment for the battery electrode to fully capitalize the high desalination capacity, salt removal rate, and capacity retention of the battery electrodes.

Rational Design of Carbon Nanomaterials for Electrochemical Sodium Storage and Capture

Electrochemical sodium storage and capture are considered an attractive technology owing to the natural abundance, low cost, safety, and cleanness of sodium, and the higher efficiency of the electrochemical system compared to fossil-fuel-based counterparts. Considering that the sodium-ion chemistry often largely deviates from the lithium-based one despite the physical and chemical similarities, the architecture and chemical structure of electrode materials should be designed for highly efficient sodium storage and capture technologies. Here, the rational design in the structure and chemistry of carbon materials for sodium-ion batteries (SIBs), sodium-ion capacitors (SICs), and capacitive deionization (CDI) applications is comprehensively reviewed. Types and features of carbon materials are classified into ordered and disordered carbons as well as nanodimensional and nanoporous carbons, covering the effect of synthesis parameters on the carbon structure and chemistry. The sodium storage mechanism and performance of these carbon materials are correlated with the key structural/chemical factors, including the interlayer spacing, crystallite size, porous characteristics, micro/nanostructure, morphology, surface chemistry, heteroatom incorporation, and hybridization. Finally, perspectives on current impediment and future research directions into the development of practical SIBs, SICs, and CDI are also provided.

Bimetallic Metal-Organic Framework-Derived Carbon Nanotube-Based Frameworks for Enhanced Capacitive Deionization and Zn-Air Battery

Carbon-based materials have attracted intensive attentions for a wide range of energy and environment-related applications. Energy storage/conversion devices with improved performance have been achieved by utilization of metal-organic-framework (MOF)-derived carbon structures as active materials in recent years. However, the effects of MOF precursors on the performance of derived carbon materials are rarely investigated. Here, we report that the incorporation of small amount of Fe or Ni in Co-based MOFs leads to a significant enhancement for the derived carbon nanotube-based frameworks (CNTFs) in Na+/Cl- ion electrosorption. Further investigation revealed the enhanced performance can be attributed to the improved specific surface area, electrical conductivity, and electrochemical activity. Notably, the CoFe-CNTF derived from bimetallic CoFe-MOFs achieves a high ion adsorption capacity of 37.0 mg g-1, superior to most of recently reported carbon-based materials. Furthermore, the CoFe-CNTF also demonstrates high catalytic activity toward oxygen evolution reaction (OER) with a Tafel slope of 87.7 mV dec-1. After combination with three-dimensional graphene foam (3DG), the resultant CoFe-CNTF-coated 3DG is used as air-cathode to fabricate a flexible all-solid-state Zn-air battery, which exhibits a high open circuit potential of 1.455 V. Importantly, the fabricated flexible battery can light a light-emitting diode (LED) even when it is bent. This work provides new insights into designs of high-performance and flexible electrode based on MOF-derived materials.

Efficient treatment of brine wastewater through a flow-through technology integrating desalination and photocatalysis

Many current treatments for brine wastewaters are energy-intensive, chemical-intensive, and involve independent process in the removal of salts and contaminants. We demonstrate that through the integration of capacitive deionization and photocatalysis reactions within carbon nanotubes (CNTs) based membrane system, we are able to realize the purification and desalination of wastewaters via single-step, energy-efficient, and environmentally friendly route. We firstly designed the membrane system consisting of graphitic carbon nitride (g-C₃N₄), CNTs membrane, and poly(vinyl alcohol)-formaldehyde (PVF) foam. Then, two identical membrane systems were used as permeable electrodes and photocatalytic microreactors to construct the flow-through setup. The tests of the setup with a variety of dye solution, antibiotics solution, and actual wastewaters prove that wastewaters passing through the setup promptly turn to clean water with significantly decreased salinity. This is because the setup can use C₃N₄ modified CNTs membrane to adsorb organic contaminants and inorganic ions and decompose contaminants via photocatalysis reactions. In addition, by discharging the setup, its adsorption capacity towards salts is easily recovered. Consequently, the flow-through setup is observed to exhibit stable performance for concurrent removal of organic contaminants and inorganic salts in multiple cycles.

Energy Efficiency of Capacitive Deionization

Capacitive deionization (CDI) as a class of electrochemical desalination has attracted fast-growing research interest in recent years. A significant part of this growing interest is arguably attributable to the premise that CDI is energy efficient and has the potential to outcompete other conventional desalination technologies. In this review, systematic evaluation of literature data reveals that while the absolute energy consumption of CDI is in general low, most existing CDI systems achieve limited energy efficiency from a thermodynamic perspective. We also analyze the causes for the relatively low energy efficiency and discuss factors that may lead to enhanced energy efficiency for CDI.

Free-standing flexible film as a binder-free electrode for an efficient hybrid deionization system

In recent years, capacitive deionization (CDI) has emerged as an energy efficient and cost-effective technology for the desalination of brackish water. However, high energy consumption and poor desalting efficiency at high salinity levels have hampered the application of CDI for seawater (∼35 000 mg L-1). A novel method of CDI termed hybrid capacitive deionization (HCDI) employs the use of a faradaic electrode paired with a capacitive electrode. Doing so increases the salt removal capacity to approximately three times that of conventional activated carbon (AC) materials (∼10 mg g-1). Herein, we report experimental results of our HCDI cell using free-standing Na2Ti3O7-CNT@reduced graphene oxide (NCNT@rGO) film as a binder-free negative and activated carbon@reduced graphene oxide (AC@rGO) film as the positive electrode. The HCDI cell is operated under a constant current mode. During desalination, sodium ions are intercalated into the negative electrode (NCNT@rGO) whereas chloride ions are adsorbed onto the surface of the positive electrode (AC@rGO). We observed a high removal capacity of 129 mg g-1 at the low energy consumption of 0.4 W h g-1 for a salt concentration of ∼3000 mg L-1 at 50 ml min-1 flow rate. The higher performance of our electrodes over conventional ones (∼109 mg g-1, 0.68 W h g-1) is attributed to the absence of binders or conductive additives and the unique nano-architectured sandwich structure of NCNT@rGO. The advantageous features of our electrodes shed new insight into the development of CDI materials and show promise for low-cost, scalable systems.