Electrochemical Water Desalination Research Articles

Best practice for electrochemical water desalination data generation and analysis

Best practice for electrochemical water desalination data generation and analysis

Compressible sponge electrodes by oxidative molecular layer deposition (oMLD) of polyethylenedioxythiophene (PEDOT) onto open-cell polyurethane sponges

The formation of compressible porous sponge electrodes is appealing to overcome diffusion limitations in porous electrodes for applications including electrochemical energy storage, electrochemical water desalination, and electrocatalysis. Previous work has employed wet chemical synthesis to deliver conductive materials into porous polymer sponge supports, but these approaches struggle to produce functional electrodes due to (1) poor electrical connectivity of the conductive network and (2) mechanical rigidity of the foam after coating. In this work we employ oxidative molecular layer deposition (oMLD) via sequential gas-phase exposures of 3,4 ethylenedioxythiophene (EDOT) and molybdenum pentachloride (MoCl5) oxidant to imbibe polyurethane (PU) sponges with electrically-conductive and redox-active poly(3,4 ethylenedioxythiophene) (PEDOT) coatings. We analyze the oMLD deposition on compressive PU sponges and modify the reaction conditions to obtain mechanically compressible and electrically conductive sponge electrodes. We specifically identify the importance MoCl5 dose time to enhance the conductivity of the sponges and the importance of EDOT purge time to preserve the mechanical properties of the sponges. Controlling these variables produces an electrically conductive PEDOT network within the sponge support with reduced impact on the sponge’s mechanical properties, offering advantages over wet-chemical synthesis approaches. The compressible, conductive sponges we generate have the potential to be used as compressible electrodes for water desalination, energy storage, and electrocatalysis.

Structural/Compositional-Tailoring of Nickel Hexacyanoferrate Electrodes for Highly Efficient Capacitive Deionization.

Prussian blue analogs (PBAs) represent a crucial class of intercalation electrode materials for electrochemical water desalination. It is shown here that structural/compositional tailoring of PBAs, the nickel hexacyanoferrate (NiHCF) electrodes in particular, can efficiently modulate their capacitive deionization (CDI) performance (e.g., desalination capacity, cyclability, selectivity, etc.). Both the desalination capacity and the cyclability of NiHCF electrodes are highly dependent on their structural/compositional features such as crystallinity, morphology, hierarchy, and coatings. It is demonstrated that the CDI cell with hierarchically structured NiHCF nanoframe (NiHCF-NF) electrode exhibits a superior desalination capacity of 121.38mg g-1 , a high charge efficiency of up to 82%, and a large capacity retention of 88% after 40 cycles intercalation/deintercalation. In addition, it is discovered that coating of carbon (C) film over NiHCF can lower its desalination capacity owing to the partial blockage of diffusion openings by the coated C film. Moreover, the hierarchical NiHCF-NF electrode also demonstrates a superior selectivity toward monovalent sodiumions (Na+ ) over divalent calcium (Ca2+ ) and magnesim (Mg2+ ) ions, allowing it to be a promising platform for preferential capturing Na+ ions from brines. Overall, the structural/compositional tailoring strategies would offer a viable option for the rational design of other intercalation electrode materials applied in CDI techniques.

Influence of particle size distribution on carbon-based flowable electrode viscosity and desalination efficiency in flow electrode capacitive deionization

Flow-electrode capacitive deionization (FCDI) is a potential energy-efficient electrochemical water desalination technology that combines ion-exchange membranes and flowable electrodes (FE). The performance of activated carbon (AC)-based FEs in FCDI systems is influenced by various factors, including particle size. Here, particle size of commercial AC was reduced by dry ball milling to produce FEs for FCDI. Physical analysis by scanning electron microscopy and particle size analyzer confirmed the particle size reduction after dry ball milling. Then, the effect of particle size distribution on the rheological behavior and desalination performances of AC-based FEs was investigated by desalination performance indicators, such as desalination efficiency, average salt adsorption rate and charge efficiency. FE samples were prepared using different mixtures of fine-size range (0.65–0.92 µm; fine AC, FAC) and large-size range (1.5–2.3 µm; AC) particles. The AC to FAC particle ratio influenced the rheological properties of AC-based FEs in FCDI. Pure FAC and bimodal mixture of 0.75 :0.25 FAC: AC particles displayed the highest desalination efficiency compared with pure AC FEs (71% and 72% versus 34%, respectively). More interestingly, in our FCDI set-up, 0.75:0.25 FAC:AC bimodal mixtures showed better flowing properties and lower pressure drop than the pure FAC FEs characterized by high viscosity.

A Reverse‐Defect‐Engineering Strategy toward High Edge‐Nitrogen‐Doped Nanotube‐Like Carbon for High‐Capacity and Stable Sodium Ion Capture

AbstractDeveloping high‐performance defect‐rich carbon materials with abundant accessible active sites is exceedingly vital for electrochemical water desalination, but this still remains a significant challenge. Herein, a reverse‐defect‐engineering strategy is reported to synthesize high edge‐nitrogen‐doped nanotube‐like carbon through the annealing process of protonated g‐C3N4 under H2 atmosphere. The hydrogen bonds interaction between the proton and nitrogen atoms performs a crucial role in regulating nitrogen configurations. The nitrogen‐doped carbon obtained from HCl pretreatment (HCl‐NC) reduces the proportion of graphitic N and exhibits a high ratio of pyrrolic N to pyridinic N. Thus, the resulting synergetic structure of high edge‐type N and small graphitic carbon nanodomains ensures more accessible active sites and fast charge‐transfer kinetics simultaneously, contributing to high desalination capacity (100.3 mg g−1 at 1.2 V), fast time‐average specific adsorption rate (1.7 mg g−1 min−1), low energy consumption (82.9 kJ molNaCl−1), and superior cyclic stability (no signs of performance decay after long‐term cycling). The Na+‐intercalation mechanism and structure‐response relationship of HCl‐NC are revealed by the electrochemical quartz crystal microbalance with dissipation monitoring and density functional theory calculations, respectively. This study provides a novel idea to modulate the nanotube‐like, nitrogen‐containing configurations for engineering carbon nanomaterials for advanced electrochemical applications.

Performance and Lifetime of Battery Desalination Cells Based on Nickel Hexacyanoferrate

Prussian blue analogs, e.g., nickel hexacyanoferrate, NiFe(CN)6 or NiHCF, are promising candidates as low-cost and high-rate intercalation materials for secondary batteries.1–4 Recently, this material class has been shown to possess tremendous potential for a novel energy-efficient water desalination approach.5–8 Rising water demands are exacerbating water scarcity in many world regions. It is estimated that 60% of the global population will face severe water scarcity by 2025.9 The growing water demand necessitates new desalination technologies with high energy efficiency, low capital and operating cost and high freshwater output. In this work, we assess the performance and lifetime of electrochemical water desalination cells based on sodium intercalation into nickel hexacyanoferrate.10–12 The battery desalination cells feature a symmetric design, with two NiHCF electrodes at opposite state-of-charge (SOC), capable of intercalating Na+-ions into their crystal structure. The electrodes are separated by an anion exchange membrane, a porous functionalized polyether ether ketone (PEEK) membrane, that only permits negatively charged ions, e.g., Cl--ions, to pass. Two feed water streams with 20 mM NaCl enter the symmetric cell on either side (see Figure 1a). During charge of the symmetric cell, incoming Na+-ions are removed from one water stream and intercalated into the NiHCF electrode at low SOC. Simultaneously, Na+-ions are deintercalated from the opposite NiHCF electrode at high SOC. In order to maintain charge neutrality, Cl--ions cross the anion exchange membrane. Thus, during every charge/discharge cycle, one water stream is desalinated forming a freshwater stream, while the other is enriched in NaCl forming a brine waste stream (see Figure 1b).In order to quantify performance and lifetime of the novel battery-type water desalination cells, we define and measure objective metrics. We see that energy consumption (Wh/l) and productivity (l/h/m2) of NiHCF/NiHCF cells are superior to cells based on membrane capacity deionization (mCDI). Stable charge/discharge cycling of NiHCF/NiHCF cells can be achieved for over 500 cycles with NaCl feed water, but rapid aging is observed with CaCl2 feeds. Synchrotron-based characterization of NiHCF/NiHCF cells is used to elucidate the reason for capacity fade. X-ray absorption spectroscopy and X-ray fluorescence spectroscopy reveal Fe dissolution from the NiHCF active material as a primary aging mode with CaCl2 water feeds. References Wessells, C. D., Peddada, S. V., Huggins, R. A. & Cui, Y. Nickel hexacyanoferrate nanoparticle electrodes for aqueous sodium and potassium ion batteries. Nano Lett. 11, 5421–5425 (2011).Wessells, C. D. et al. Tunable reaction potentials in open framework nanoparticle battery electrodes for grid-scale energy storage. ACS Nano 6, 1688–1694 (2012).Pasta, M. et al. Full open-framework batteries for stationary energy storage. Nat. Commun. 5, 1–9 (2014).Firouzi, A. et al. Monovalent manganese based anodes and co-solvent electrolyte for stable low-cost high-rate sodium-ion batteries. Nat. Commun. 9, (2018).Pasta, M., Wessells, C. D., Cui, Y. & La Mantia, F. A desalination battery. Nano Lett. 12, 839–843 (2012).Lee, J., Kim, S. & Yoon, J. Rocking Chair Desalination Battery Based on Prussian Blue Electrodes. ACS Omega 2, 1653–1659 (2017).Kim, T., Gorski, C. A. & Logan, B. E. Low Energy Desalination Using Battery Electrode Deionization. Environ. Sci. Technol. Lett. 4, 444–449 (2017).Porada, S., Shrivastava, A., Bukowska, P., Biesheuvel, P. M. & Smith, K. C. Nickel Hexacyanoferrate Electrodes for Continuous Cation Intercalation Desalination of Brackish Water. Electrochim. Acta 255, 369–378 (2017).Jones, E., Qadir, M., van Vliet, M. T. H., Smakhtin, V. & Kang, S. mu. The state of desalination and brine production: A global outlook. Sci. Total Environ. 657, 1343–1356 (2019).Metzger, M. et al. Techno-economic analysis of capacitive and intercalative water deionization. Energy Environ. Sci. 13, 1544–1560 (2020).Sebti, E. et al. Removal of Na+ and Ca2+ with Prussian blue analogue electrodes for brackish water desalination. Desalination 487, (2020).Besli, M. M. et al. Performance and lifetime of intercalative water deionization cells for mono- and divalent ion removal. Desalination 517, 115218 (2021). Figure 1. (a) Battery-type water desalination approach in symmetric NiHCF/NiHCF cells with two salt water streams entering the cell and a brine stream and freshwater stream exiting the cell. (b) During galvanostatic charge/discharge cycling the salt concentrations of brine and freshwater stream can be monitored with microfluidic operando conductivity probes to determine important performance metrics. Figure 1

Spatial Variations of pH in Electrodialysis Stacks: Theory

Electrodialysis (ED) is a well-known electrochemical water desalination technology investigated since the 1950s. In an ED stack, desalination is driven by an applied voltage, which results in selective salt ion electromigration through alternating cation and anion exchange membranes. Transport of hydronium and hydroxide ions during water treatment, together with water dissociation, can lead to unfavorable product acidity or alkalinity, compromise the membrane charge, or enhance scaling. Conversely, pH deviations can also be leveraged to tune the speciation of weak acid/base electrolytes to enhance chlorine disinfectant efficiency, or facilitate electrostatic removal of contaminants with pH-dependent properties. Thus, it is important to understand the effect of varying feedwater salinity and other system parameters on spatial pH deviations in the vicinity of the membrane, and on the pH of the product water.In this work, we extend ED theory to include pH effects in a repeating unit operated in the underlimiting current regime. Different from the single ion exchange membrane repeating unit considered by Sonin and Probstein [1], which is not applicable for systems including pH effects, we solve the concentration profiles of both the salt and water ions in a full repeating unit comprised of a cation exchange membrane and anion exchange membrane pair. To the first time to our knowledge, our model domain encompasses the entire ED repeating unit without assuming prescribed stagnant layer thickness in which the water dissociation reaction occurs. We solve the Nernst-Planck and electroneutrality set of equations in the channels and non-ideal IEMs, following the approach utilized for capturing pH effects in membrane capacitive deionization [2,3] and reverse osmosis [4,5]. We presented results showing fundamental features, including the concentration and pH distribution in the diluate channel, local flux density across the IEMs, and effluent salinity and pH. Our model predicts that including or excluding pH effects lead to essentially identical predictions for salt fluxes across the IEMs, and thus previous models neglecting pH effects are likely accurately predicting desalination. We also show that reducing salinity augments pH perturbations, but that the effluent pH, which consists of mixed acid and alkaline boundary layers at the diluate channel outlet, does not deviate from inlet neutral pH significantly. In the future, this model framework here can be extended to include multi-ionic solution and species with pH-dependent properties, and validated with a dedicated set of experimental results.

Recent progress in materials and architectures for capacitive deionization: A comprehensive review.

Capacitive deionization is an emerging and rapidly developing electrochemical technique for water desalination across the globe with exponential growth in publications. There are various architectures and materials being explored to obtain utmost electrosorption performance. The symmetric architectures consist of the same material on both electrodes, while asymmetric architectures have electrodes loaded with different materials. Asymmetric architectures possess higher electrosorption performance as compared with that of symmetric architectures owing to the inclusion of either faradaic materials, redox-active electrolytes, or ion specific pre-intercalation material. With the materials perspective, faradaic materials have higher electrosorption performance than carbon-based materials owing to the occurrence of faradaic reactions for electrosorption. Moreover, the architecture and material may be tailored in order to obtain desired selectivity of the target component and heavy metal present in feed water. In this review, we describe recent developments in architectures and materials for capacitive deionization and summarize the characteristics and salt removal performances. Further, we discuss recently reported architectures and materials for the removal of heavy metals and radioactive materials. The factors that affect the electrosorption performance including the synthesis procedure for electrode materials, incorporation of additives, operational modes, and organic foulants are further illustrated. This review concludes with several perspectives to provide directions for further development in the subject of capacitive deionization. PRACTITIONER POINTS: Capacitive deionization (CDI) is a rapidly developing electrochemical water desalination technique with exponential growth in publications. Faradaic materials have higher salt removal capacity (SAC) because of reversible redox reactions or ion-intercalation processes. Combination of CDI with other techniques exhibits improved selectivity and removal of heavy metals. Operational parameters and materials properties affect SAC. In future, comprehensive experimentation is needed to have better understanding of the performance of CDI architectures and materials.

Spatial variations of pH in electrodialysis stacks: Theory

Electrodialysis (ED) is a well-known electrochemical water desalination technology investigated since the 1950s. In an ED stack, desalination is driven by an applied voltage, which results in selective salt ion electromigration through alternating cation and anion exchange membranes. Transport of hydronium and hydroxide ions during water treatment, together with water dissociation, can lead to unfavorable product acidity or alkalinity, compromise the membrane charge, or enhance scaling. Conversely, pH deviations can also be leveraged to tune the speciation of weak acid/base electrolytes to enhance chlorine disinfectant efficiency, or facilitate electrostatic removal of contaminants with pH-dependent properties. Thus, it is important to understand the effect of varying feedwater salinity and other system parameters on spatial pH deviations in the vicinity of the membrane, and on the pH of the product water. In this work, we propose an ED model including pH effects in a repeating unit operated in the underlimiting current regime. We numerically solve the concentration profiles of both the salt and water ions in a full repeating unit comprised of a cation and anion exchange membrane pair. For the first time to our knowledge, our model domain encompasses the entire ED repeating unit without assuming prescribed stagnant layer thickness in which the water dissociation reaction occurs. This model can be utilized to compare to experimental datasets in the future, and provide insight towards designing ED systems for removal of pH-dependent species.

A successful transition from a vanadium redox flow battery stack to an energy efficient electrochemical desalination module

Vanadium Redox Flow Batteries (VRFB) are considered prime candidates for stationary energy storage due to their unique capability to decouple power and energy. With the aim of making electrochemical water desalination more energy efficient when scaling up the technology, potential benefits from repurposing a commercial VRFB stack as a Capacitive Deionization (CDI) module were studied. Accordingly, a 5-cell CDI Stack equipped with 1250 cm2 electrodes was assembled by revamping a 5kWh-VRFB. Electrosorption experiments showed a great versatility with a maximum salt adsorption capacity of 14.2 mg g−1 and an average salt adsorption rate of 0.321 mg g−1 min−1. Furthermore, a long-term test performed over 100 cycles validated the robustness of the system by achieving salt concentration reductions of 10–12 mM at a constant production rate of 4 L h−1 m−2. Moreover, adapting a VRFB stack specifically designed for energy optimization allowed a substantial reduction of energy consumption of 75% (from 0.84 kW h m−3 to 0.21 kW h m−3) by effectively recovering the energy employed in the desalination step. These results demonstrated the successful transition from VRFB to CDI showcasing the relevance of the Water-Energy Nexus and opening the path for exploring a second life of exhausted VRFB stacks.

Development of an integrated capacitive-electrodialysis process (CapED) for continuous, low-energy electrochemical deionization

A membrane stack configuration with activated carbon electrodes was developed to integrate the capacitive and electrodialysis contributions for continuous, low energy desalination. • A cell was developed by stacking IEMs between capacitive electrodes. • During the charging step, the ions were removed by electrosorption/electrodialysis. • During the discharging step, the ions were removed by dialytic separation. • The continuous desalination was achieved by a cyclic charging/discharging process. • High charge efficiency and low energy consumption were achieved. Significant efforts have been made to develop advanced energy-efficient desalination technologies for delivering freshwater. Capacitive deionization and electrodialysis represent electric field-driven separation technologies for water desalination. In this study, a new design for a membrane stack configuration based on an integrated capacitive-electrodialysis process (CapED) was proposed for continuous and energy-efficient desalination. From the activated carbon anode to the cathode, an anion exchange membrane (AEM), a cation exchange membrane (CEM), an AEM, and a CEM were placed in order (anode/AEM/CEM/AEM/CEM/cathode). Herein, three chambers between two capacitive electrodes were separated by a CEM and an AEM, which enabled simultaneous and continuous generation of desalinated and concentrated solutions. During charging, capacitive electrosorption and electrodialytic separation accounted for desalination. During discharging, the energy stored by capacitive electrosorption was utilized for desalination through dialytic separation. Therefore, continuous desalination could be achieved by a cyclic charging/discharging process. In addition, the salt removal amount was enhanced by increasing the voltage with respect to the different capacitive and electrodialytic contributions. The energy consumption ranged between 0.53 and 1.45 kWh/kg with high charge efficiency, indicating a low energy requirement. Our research showed that CapED was effective when integrating capacitive and electrodialytic contributions for electrochemical water desalination.

Sulfonated Phthalocyanine Redox Flow Cell for Electrochemical Water Desalination

A reliable clean water supply is now one important and challenging issue for the human society. With the facts that only 0.4% of the water on earth is readily drinkable and 98% of the water is saltwater, water desalination for clean water production has become a key strategic solution to satisfy the increasing global water demand. Over the years, different saltwater desalination technologies were researched and developed, including reverse osmosis membrane desalination (RO), electrodialysis (ED), capacitive desalination (CDI) and multi-effect distillation (MED). However, some inherent technical issues, like high operation cost, large capital cost and/or low desalination capacity, associate with these existing technologies that lead to a high clean water production cost and limit their applications. Thus, there is a need of new water desalination technology that can purify saltwater with low cost and high efficiency.In this work we report a new water-soluble nickel phthalocyanine tetrasulfonic acid tetrasodium salt (NiPcTATS)-based redox flow desalination battery (RFDB) for safe, effective and efficient water-salt separation. The fabricated RFDB cell exhibited a high desalination rate at 3.4 gNaCl/(molNiPcTATS∙hr) even with a low 0.5 V cell voltage and required as low as 11.5 kJ/mol salt in energy consumption. The cell showed excellent reversibility and durability for long-term operation, benefiting from the fast-redox kinetics and chemical stability of the redox couple. With no consumption of the redox couple and energy recovery during the salination process, this new method provides a continuous desalination capability with unlimited salt removal capacity. We also demonstrated the use of simple solar panel for effectively driving the RFDB cell, proving it a promising and practical technology for clean water production in remote places like islands and ships.

Comment on “Techno-economic analysis of capacitive and intercalative water deionization” by M. Metzger, M. Besli, S. Kuppan, S. Hellstrom, S. Kim, E. Sebti, C. Subban and J. Christensen, Energy Environ. Sci., 2020, 13, 1544

We discuss a recent publication in Energy & Environmental Science that presented a techno-economic analysis of electrochemical water desalination technologies.

Sulfonated Phthalocyanine Redox Flow Cell for High-Performance Electrochemical Water Desalination

A reliable clean water supply is now one important and challenging issue for the human society. With the facts that only 0.4% of the water on earth is readily drinkable and 98% of the water is saltwater, water desalination for clean water production has become a key strategic solution to satisfy the increasing global water demand. Over the years, different saltwater desalination technologies were researched and developed, including reverse osmosis membrane desalination (RO), electrodialysis (ED), capacitive desalination (CDI) and multi-effect distillation (MED). However, some inherent technical issues, like high operation cost, large capital cost and/or low desalination capacity, associate with these existing technologies that lead to a high clean water production cost and limit their applications. Thus, there is a need of new water desalination technology that can purify saltwater with low cost and high efficiency.In this work we report a new water-soluble nickel phthalocyanine tetrasulfonic acid tetrasodium salt (NiPcTATS)-based redox flow desalination battery (RFDB) for safe, effective and efficient water-salt separation. The fabricated RFDB cell exhibited a high desalination rate at 3.4 gNaCl/(molNiPcTATS∙hr) even with a low 0.5 V cell voltage and required as low as 11.5 kJ/mol salt in energy consumption. The cell showed excellent reversibility and durability for long-term operation, benefiting from the fast-redox kinetics and chemical stability of the redox couple. With no consumption of the redox couple and energy recovery during the salination process, this new method provides a continuous desalination capability with unlimited salt removal capacity. We also demonstrated the use of simple solar panel for effectively driving the RFDB cell, proving it a promising and practical technology for clean water production in remote places like islands and ships.

Sulfonated nickel phthalocyanine redox flow cell for high-performance electrochemical water desalination

With clean water availability raising a great technological and social challenge, saltwater desalination has become a key strategic solution. Herein, we report a nickel phthalocyanine tetrasulfonic acid tetrasodium salt (NiPcTATS)-based redox flow desalination cell (RFDC) that allows safe, efficient and continuous water-salt separation. With the new, water-soluble NiPcTATS/NiPcTATS+ redox couple as electrolyte, the RFDC effectively desalinates saltwater and achieves a high average salt removal rate of 3.11 gNaCl/(molNiPcTATS·h) and a low energy consumption of 15.0 kJ/molNaCl at 0.5 V cell voltage. The cell exhibits excellent reversibility and durability for long-term operation, benefiting from the fast-redox kinetics and chemical stability of the redox couple. Efficient solar-driven RFDC for water desalination is demonstrated by powering the cell with a solar panel. This RFDC method provides a low-cost, efficient and safe strategy to purify saltwater, and the findings with NiPcTATS-based redox couple offer new opportunities of utilizing metal phthalocyanines and derivatives in water desalination research.

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).

A New Strategy of Using Cation Intercalation Electrodes for Water Desalination

Water desalination is a promising technological solution to water scarcity. While several technologies are currently utilized to produce freshwater from saline water sources, there are remaining challenges including intensive energy usage (e.g., thermal distillation), fouling (e.g., reverse osmosis), and high material costs (e.g., electrodialysis). The use of cation intercalation electrodes allows for electrochemically driven capture and release of ions, thus effectively desalinating water using inexpensive electrode materials that require low voltage without producing byproducts. To date, previous approaches using cation intercalation electrodes have focused on intercalation and deintercalation of Na+ in order to directly capture Na+ that is abundant in saline water. This approach has limited the choice of electrode materials and electrochemical properties, which are crucial for efficient water desalination. To overcome this challenge, we demonstrated a new strategy for water desalination using cation intercalation electrodes that interact with K+. Water desalination was achieved in a 4-channel electrochemical flow cell separated by an alternative array of anion and cation exchange membranes with copper hexacyanoferrate (CuHCF) as a model electrode material. Deintercalation of K+ upon oxidation of CuHCF enabled the transport of Cl− through an anion exchange membrane from the adjacent channel that produces desalinated water. Intercalation of K+ upon reduction of CuHCF enabled the transport of Cl− through an anion exchange membrane to the adjacent channel that produces concentrated water. A cation exchange membrane located between desalinated and concentrated water channels allowed for separating Na+. Using the new concept of utilizing K+, several performance parameters were evaluated including electrochemical properties, salt separation, and energy consumption. Collectively, we show that the use of K+ as an intercalant for cation intercalation electrodes can represent a new strategy for electrochemical water desalination.

Charge-transfer materials for electrochemical water desalination, ion separation and the recovery of elements

Reversible electrochemical processes are a promising technology for energy-efficient water treatment. Electrochemical desalination is based on the compensation of electric charge by ionic species, through which the ions are immobilized and, thereby, removed from a feed-water stream flowing through a desalination cell. For decades, electrochemical desalination has focused on the use of carbon electrodes, but their salt-removal ability is limited by the mechanism of ion electrosorption at low molar concentrations and low charge-storage capacity. Recently, charge-transfer materials, often found in batteries, have demonstrated much larger charge-storage capacities and energy-efficient desalination at both low and high molar strengths. In this Review, we assess electrochemical-desalination mechanisms and materials, including ion electrosorption and charge-transfer processes, namely, ion binding with redox-active polymers, ion insertion, conversion reactions and redox-active electrolytes. Furthermore, we discuss performance metrics and cell architectures, which we decouple from the nature of the electrode material and the underlying mechanism to show the versatility of cell-design concepts. These charge-transfer processes enable a wealth of environmental applications, ranging from potable-water generation and industrial-water remediation to lithium recovery and heavy-metal-ion removal. Electrochemical processes enable energy-efficient desalination of water and the separation and recovery of elements. In this Review, we discuss the mechanisms and materials of this emerging generation of water-remediation technology.

Pulsed Electrodeposition of Ultrathin Polyaniline Films and Mechanistic Understanding of their Anion-Mediated Electrochemical Behavior

Electrode materials which undergo anion insertion are of interest for next-generation energy storage devices and electrochemical water desalination. Polyaniline (PANI) is a robust and well-studied electrode material for anion insertion, but to date we have been unable to establish the electrochemical and anion-binding performance of PANI at nanoscale thicknesses because these thicknesses are not obtainable using standard PANI deposition procedures. Here, we report a new pulsed electrodeposition procedure for the controlled growth of PANI thin films with nanometer-scale thickness control. Using this pulsing technique, we deposit nanoscale (<100 nm) thin films of PANI onto stainless steel (SS) disks and gold electrochemical quartz crystal microbalance (EQCM) crystals. We examine the charge rate and anion-binding properties as a function of nanoscale thickness, and reveal previously undescribed insights into the charge storage and anion binding mechanisms of PANI. In particular, we identify that PANI films of <10 nm thickness provide rapid chloride ion uptake for desalination applications, and that thicker ∼100 nm PANI films are able to achieve rapid charge storage in sulfate solutions due to SO42−/HSO4− embedded within the PANI acting as a proton donor/acceptor. The results we report will inform the use of PANI thin films for energy storage and desalination applications.

High-Desalination Performance via Redox Couple Reaction in the Multichannel Capacitive Deionization System

Capacitive deionization (CDI) has received much attention, which is considered as a promising electrochemical water desalination technology. However, an obstacle to its conventional application is ...