Salt Removal Rate Research Articles

Redox flow desalination based on the temperature difference as a driving force

How to effectively reduce the consumption of electrical energy is a key topic in many studies of electrochemical desalination. In this work, we use the temperature difference to drive a continuous process of dialysis desalination. The system consists of a thermoelectric unit and a desalination unit connected in series. The thermoelectric unit includes a thermoelectric generator (TEG), a heater as heat source and an air-cooled heat sink to generate electricity and for the desalination unit. The desalination unit contains two platinum-coated hydrophobic carbon cloths as current collectors, a mixture of [Fe(CN)6]3-/4- as anolyte and catholyte, concentrated and diluted salt streams with two cationic and one anionic exchange membranes separated configuration (CEM|AEM|CEM). During the charging process driven by temperature difference, chloride and sodium ions in the diluted salt stream move to the concentrated salt stream and cathodic reservoir, respectively. The results show that the concentration of brine drops significantly from 5,000 ppm to 344.3 ppm as the current decreases to 0.06 mA from the initial 1.30 mA when the temperature difference is maintained at 65 K. Concurrently, the average salt removal rate is up to 8.8 μg cm−2 min−1 and average heat consumption is 284.3 MJ mole-1. Moreover, the influences from the temperature difference, salt feeds content and electrolyte concentration are also investigated in detail. This research has the potential application for the freshwater obtainment via the utilization of waste heat, and will be significant in places with the shortage of the electrical energy such as ships, islands and oceans with the temperature differences.

A highly efficient faradaic desalination system utilizing MnO2 and polypyrrole-coated titanium electrodes

A highly efficient pseudocapacitive desalination system is constructed with positive MnO2 and negative polypyrrole electrodes. Both materials deposited by electrochemical methods exhibit faradaic reactions with the “memory effect” and permit the unique multi-function application with salt-removing and concentrating abilities. The quasi-steady state electrode potentials of both electrodes under charged/discharged states and the cell salt removal capacity (SRC) highly depends on the mass loading of active materials. To ensure the quasi-steady state electrode potentials of materials in their working potential windows, the mass loadings of active materials are carefully controlled by the deposition charge density. After the mass loading adjustment, SRC reaches 78 mg g−1 in a 60-min salt-removing process. The salt removal rate (SRR) reaches the highest value of ca. 5 mg g−1 min−1 during the initial 5 min and continuously decreases to near 0 with prolonging the desalination time. In the stability test, this MnO2//PPy cell remains over 90% of its original SRC in a 100-cycle test. Shortening the time of a single operation segment is an efficient way to increase the average SRR which reaches 212.7 mg g−1 in this MnO2//PPy cell transferring NaCl from one solution to another in a 120-min test using the 4/4-min model.

Influence of natural organic matter on membrane capacitive deionization performance

Membrane capacitive deionization (MCDI) is a prospective desalination technology that removes ions using an electric potential difference across charged porous carbon electrodes. Natural organic matter (NOM) in feed water could influence the electrochemical process by leading to pore-blockages or forming a cake layer on the ion-exchange membrane-coated porous carbon electrode, thereby hindering ion removal. In this study, we explored the influence of different types of NOM, namely, humic acid (HA) and tannic acid (TA), on the MCDI desalination process for feed waters with inorganic salts (NaCl and CaCl2). HA significantly interfered with the adsorption process and reduced the salt removal rate by up to 68% in the case of NaCl-based feed water. However, the influence of HA on salt removal in the case of CaCl2-based feed water was marginal owing to the formation of a charge-neutralized complex, which was caused by the egg-box effect between Ca2+ and HA. TA reduced removal rates of salts (NaCl and CaCl2) by 37% and 60%, respectively. This is because of the lower molecular weight and smaller hydrodynamic diameter of TA relative to that of HA, owing to which TA exhibits a stronger adhesion to the electrode pore structure. Furthermore, as TA substantially reduces MCDI performance with regard to the adsorption of inorganic salts, its presence in feed water results in higher electrical resistance and energy consumption.

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.

Low energy consumption flow capacitive deionization with a combination of redox couples and carbon slurry

Flow-electrode capacitive deionization (FCDI) is a new sustainable desalination technology where continuous desalination can be achieved by the electrodialysis coupling method. However, its development is hindered owing to high energy consumption and low salt removal rate. Herein, by combining ferri-/ferrocyanide redox couple with flow activated carbon (AC)/carbon black (CB) slurry, continuous desalination process is achieved with a high salt removal rate of 1.31 μg cm−2 s−1 and low energy consumption of 102.68 kJ mol−1 at the current density 2.38 mA cm−2 (50 mA current for a 21 cm2 active area). The operating voltage plateau can be reduced to 0.69 V when 10 wt% AC/CB (mass ratio of 9:1) is mixed with 20 mM/20 mM ferri-/ferrocyanide as the flow electrodes, compared with more than 3 V for only carbon flow or redox medium alone. The influences of carbon content and current densities are further investigated to so that the performances can be controlled. This work enables the development of energy-saving desalination systems by coupling FCDI with redox desalination technique.

Enhanced salt removal performance of flow electrode capacitive deionization with high cell operational potential

Capacitive deionization (CDI) is an emerging technology with great potential to be energy efficient and allow cost-effective operation for the removal of salt ions from saline water. The introduction of flow electrode to CDI greatly enhanced the salt removal performance at much higher salt concentration, even at seawater level, due to not needing a discharging process, which is required for conventional CDI desalination. Since the oxidation of carbon electrodes and water electrolysis leads to the decrease in desalination performance and long-term stability of the CDI cell, it has typically been operated at the potential difference < 1.2 V. However, due to the various resistances of CDI components, it is known that the electrode potential is lower than the applied cell potential difference. Based on such knowledge, in this study, we operated FCDI desalination at various operational potential differences from 1.2 to 4.5 V, and studied the carbon oxidation and electrochemical characteristics by FT-IR, Raman spectroscopy, and EIS measurements. Desalination efficiency and salt removal rate increase with higher operational potential difference. Our results confirmed that up to 2.5 V, no noticeable carbon oxidation and gas generation resulting from water electrolysis occurred.

Enabling Superior Sodium Capture for Efficient Water Desalination by a Tubular Polyaniline Decorated with Prussian Blue Nanocrystals.

The application of electrochemical energy storage materials to capacitive deionization (CDI), a low-cost and energy-efficient technology for brackish water desalination, has recently been proven effective in solving problems of traditional CDI electrodes, i.e., low desalination capacity and incompatibility in high salinity water. However, Faradaic electrode materials suffer from slow salt removal rate and short lifetime, which restrict their practical usage. Herein, a simple strategy is demonstrated for a novel tubular-structured electrode, i.e., polyaniline (PANI)-tube-decorated with Prussian blue (PB) nanocrystals (PB/PANI composite). This composite successfully combines characteristics of two traditional Faradaic materials, and achieves high performance for CDI. Benefiting from unique structure and rationally designed composition, the obtained PB/PANI exhibits superior performance with a large desalination capacity (133.3 mg g-1 at 100 mA g-1 ), and ultrahigh salt-removal rate (0.49 mg g-1 s-1 at 2 A g-1 ). The synergistic effect, interfacial enhancement, and desalination mechanism of PB/PANI are also revealed through in situ characterization and theoretical calculations. Particularly, a concept for recovery of the energy applied to CDI process is demonstrated. This work provides a facile strategy for design of PB-based composites, which motivates the development of advanced materials toward high-performance CDI applications.

Facile synthesis of pinecone biomass-derived phosphorus-doping porous carbon electrodes for efficient electrochemical salt removal

Electrochemical salt removal performance from water is greatly affected by the characteristics of porous carbon electrode materials. Nevertheless, facile eco-friendly synthesis of electrode materials with low cost for efficient salt electrosorption performance is still a huge challenge. Here, we readily prepared phosphorus-doped porous carbon derived from pinecones (PPCP), an abundant biomass waste, by using H3PO4 as the activating agent at appropriate activation temperature. The PPCP samples displayed an increase of the specific surface area and pore volume with rising activation temperature (700 − 900 °C). It has been demonstrated that the PPCP800 electrode displayed the highest specific capacitance due to relatively high specific surface area and degree of graphization, superior porous structure, the highest phosphorus content and lowest charge transfer resistance. Besides, the PPCP800 electrode displayed a high deionization capacity of 14.62 mg g−1 at 1.2 V in 1000 mg L−1 NaCl solution with the fastest salt removal rate. Furthermore, the prepared PPCP800 electrode exhibited good reproducibility in long-term consecutive operation. Hence, the phosphorus-doped porous carbons derived from pinecones might be promising candidates for capacitive deionization applications.

Removal of Na+ and Ca2+ with Prussian blue analogue electrodes for brackish water desalination

Desalination of brackish water sources is critical to addressing the growing global freshwater demand. One promising approach is electrically driven desalination using intercalation electrodes. While intercalation electrodes have been widely researched for energy storage applications, only a small subset of those materials is suitable for desalination. Here we report the synthesis, characterization, and in-device testing of three Prussian blue analogue intercalation compounds: copper, manganese, and zinc hexacyanoferrate with formulas KxM[Fe(CN)6]z·nH2O (M = Cu, Mn, Zn). The desalination performance for each of these materials against carbon electrodes is reported for Na+ intercalation and for Ca2+ intercalation using 1000 ppm NaCl and 1000 ppm CaCl2 feed solutions, respectively. While the copper and manganese analogs showed promising performance for Na+ and Ca2+ intercalation, the zinc compound was unstable and underwent rapid dissolution. Manganese hexacyanoferrate showed the best desalination performance in terms of salt removal capacities and salt removal rates with NaCl while copper hexacyanoferrate performed the best with CaCl2. The manganese analog proved to be the most stable intercalation material, retaining 83% and 72% of its salt removal capacity after 280 cycles in NaCl and CaCl2 feed solutions respectively.

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.

Pseudocapacitive Behaviors of Polypyrrole Grafted Activated Carbon and MnO2 Electrodes to Enable Fast and Efficient Membrane-Free Capacitive Deionization.

Capacitive deionization (CDI) has emerged as a promising technique for brackish water desalination. Here, composites of polypyrrole grafted activated carbon (Ppy/AC) were prepared via in situ chemical oxidative polymerization of pyrrole on AC particles. The Ppy/AC cathode was then coupled with a MnO2 anode for desalination in a membrane-free CDI cell. Both the Ppy/AC and MnO2 electrodes exhibited pseudocapacitive behaviors, which can selectively and reversibly intercalate Cl- (Ppy/AC) and Na+ (MnO2) ions. Compared to AC electrodes, the specific capacitances of Ppy/AC electrodes increased concurrently with the pyrrole ratios from 0 to 10%, while the charge transfer and ionic diffusion resistances decreased. As a result, the 10%Ppy/AC-MnO2 cell showed a maximum salt removal capacity of 52.93 mg g-1 (total mass of active materials) and 34.15 mg g-1 (total mass of electrodes), which was higher than those of conventional, membrane, and hybrid CDI cells. More notably, the salt removal rate of the 10%Ppy/AC-MnO2 cell (max 0.46 mg g-1 s-1 to the total mass of active materials and 0.30 mg g-1 s-1 to the total mass of electrodes) was nearly 1 order of magnitude higher than those in most previous CDI studies, and this fast and efficient desalination performance was stabilized over 50 cycles.

Enhanced electrokinetic removal of problematic salts in arid and semi-arid areas

Recently, soil salinization in arid regions has caused considerable damage to ecosystems, agricultural productivity, and civil engineering infrastructure and facilities. This threat has become one of the most hazardous environmental issues and is the main reason for land degradation, desertification, and reduced soil load-bearing capacities. Conventional techniques of mitigating soil salinization such as soil irrigation, soil substitution, planting salt-accumulating plants, and soil amendment are expensive, time-consuming, and ineffective. Thus, the remediation of salt-affected soils represents a new challenge in the mitigation of soil salinization in arid and semi-arid lands. In this work, the feasibility of an electrokinetic treatment that removes salts from saline soil was evaluated. Three experimental electrokinetic tests were conducted with a laboratory-made electrokinetic apparatus using deionized (DI) water as the working solution. The potential gradient was set to either 1.5, 2.0, or 2.4 V/cm. This range of values was chosen to ensure low energy consumption and to suppress heating, which has adverse effects on the treatment. An enhanced electrokinetic approach (EEA) was developed by investigating the effects of diiferent electrical applied voltages (EAVs) on the removal of salts. The results revealed that the salt concentration gradually decreased and the salt removal rate increased as the EAV was increased. The highest salt removal rate occurred with an EAV of 2.4 V/cm, which yielded removal efficiencies of 89% and 88% for calcium (Ca2+) and sulfate (SO42−) ions, respectively. This research shows that the proposed electrokinetic technique has the potential to highly efficiently remediate and reclaim problematic salt-affected soils in arid and semi-arid areas.

Optimal conditions for efficient flow-electrode capacitive deionization

One of the current barriers to achieving fast and stable performance for flow-electrode capacitive deionization (FCDI) is determining optimal operating parameters. To date, however, no consensus has been reached for universal conditions for FCDI. Through experimental and modeling approaches in this study, we systematically evaluated the influence of applied potential (V = 1.2–2.4 V) and electrolyte concentration (C0 = 0.05–0.5 M) on the FCDI and electrodialysis (ED) desalination processes. Evaluation indicators include the concentration decrease in the desalinated solution, salt removal rates, pH fluctuations, charge efficiency, and energy consumption. Results demonstrated that the dynamic curves of concentration decrease at 2.0 V nearly overlapped with the response at 1.6 V at certain electrolyte concentrations, while the salt removal rates at 0.2 M salt concentration were the best among all concentrations tested at a range of applied potential. It was thus concluded that the optimum conditions for FCDI operation are 1.6 V applied potential and 0.2 M initial salt concentration, under which faradaic reactions are not being triggered, and concentration polarization does not significantly affect ion transfer. Furthermore, a comparative study between FCDI and ED indicated that ED has a different dependence on the electrolyte concentration and applied potential, in which the desalination can be linearly enhanced with increasing potential but greatly limited at high concentrations. Due to the presence of carbon particles in FCDI, the enhanced charge/ion transfer is probably the main reason for the different desalination performance of FCDI and ED. The optimal operating parameters obtained in this work could be used as basic test conditions for further development of new carbon-based materials for FCDI.

Desalination behavior and performance of flow-electrode capacitive deionization under various operational modes

Flow-electrode capacitive deionization (FCDI) has attracted growing attention due to its superior desalination capacity and continuous operation. This study, for the first time, systematically investigated the desalination behaviors of FCDI under a variety of operational modes and made a detailed comparison of relevant operation to provide useful information for selecting appropriate FCDI operational mode. Five operational modes with respect to the flow of electrodes and feed water were studied with both constant voltage and constant current applied. Results revealed that the effluent conductivity during charging decreased continuously in batch mode while decreased quickly and then leveled off in single-pass mode. pH of flow-electrodes fluctuated differently for the operational modes of isolated closed-cycle (ICC), short-circuited closed-cycle (SCC) and open cycle (OC), while pH of the effluent stream maintained stable for all operational modes. The variation of current or voltage could be explained by the change in the resistance of FCDI mainly induced by the middle-chamber salt concentration. Based on the three performance indicators of average salt removal rate, charge efficiency and removed salt normalized energy consumption, ICC/single-pass and SCC/single-pass are the two most superior operational modes, followed by OC/single-pass, ICC/batch-mode and SCC/batch-mode. Further considering the advantage of SCC in the continuous charge neutralization and electrode regeneration, SCC/single-pass operational mode can be deemed optimal.

Carbon Black Flow Electrode Enhanced Electrochemical Desalination Using Single-Cycle Operation.

Flow-electrode electrochemical desalination (FEED) processes (e.g., flow-electrode capacitive deionization), which use flowable carbon particles as the electrodes, have attracted increasing attention, holding the promise for continuous desalination and high desalting efficiency. While it is generally believed that carbon particles with abundant microporous and large specific capacitances (e.g., activated carbon, AC) should be ideal candidates for FEED electrodes, we provide evidence to the contrary, showing that highly conductive electrodes with low specific surface area can outperform microporous AC-based electrodes. This study revealed that FEED using solely high surface area AC particles (∼2000 m2 g-1, specific capacitance of ∼44 F g-1, average salt adsorption rate of ∼0.15 μmol cm-2 min-1) was vastly outperformed by electrodes based solely on low-surface area carbon black (CB, ∼70 m2 g-1, ∼0.5 F g-1, ∼0.75 μmol cm-2 min-1). Electrochemical impedance spectroscopy results suggest that the electrode formed by CB particles led to more effective electronic charge percolation, likely contributing to the improved desalination performance. In addition, we propose and demonstrate a novel operation mode, termed single cycle (SC), which greatly simplified the FEED cell configuration and enabled simultaneous charging and discharging. Using SC mode with CB flow electrodes delivered an increased average salt removal rate relative to the more traditional short-circuited closed cycle (SCC) mode, achieving up to 1.13 μmol cm-2 min-1. Further investigations demonstrate that up to 50% of energy input would be avoided when using CB flow electrodes operated under SC mode as compared to that of AC flow electrodes operated under SCC mode. In summary, the FEED process presented in this study provided an innovative and promising approach toward high-efficient and low-cost brackish water desalination.

Flow-electrode capacitive deionization (FCDI) scale-up using a membrane stack configuration

Flow-electrode capacitive deionization (FCDI) is an attractive variant of CDI with distinct advantages over fixed electrode CDI including the capability for seawater desalination, high flow efficiency and easy management of the electrodes. Challenges exist however in increasing treatment capacity with this attempted here through use of a membrane stack configuration. By comparison of standardised metrics (in particular, average salt removal rate (ASRR), energy normalized removed salt (ENRS) and productivity), results show that that an FCDI system with two pairs of ion exchange membranes had the highest efficiency in desalting a brackish influent (1000 mg L−1) to potable levels (∼150 mg L−1) at higher ASRR and ENRS. Further increase in the number of membrane pairs resulted in a decrease in current efficiency, likely as a result of the dominance of electrodialysis. Results of this study provide proof of concept that (semi-)continuous desalination can be achieved in FCDI at high energy efficiency (13.8%–20.2%) and productivity (> 100 L m−2 h−1) and, importantly, provide insight into possible approaches to scaling up FCDI such that energy-efficient water desalination can be achieved.

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.

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.

Heteroatom doping modified hierarchical mesoporous carbon derived from ZIF-8 for capacitive deionization with enhanced salt removal rate

Heteroatom doped carbonaceous material has attracted enormous attentions in electrochemical research community due to the replacement of some carbon atoms in the carbon skeleton result in charge redistribution and thus the modification of physiochemical properties. In this work, we proposed a facial approach to synthesize nitrogen and titanium co-doped hierarchical mesoporous carbon ((N, Ti)-HMCs) by employing ZIF-8 as procures for capacitive deionization (CDI) with enhanced performance. The as-prepared (N, Ti)-HMCs exhibits novel cubic structure, high doping content and high specific surface area of 1025.93 m2/g. The Raman spectra revealed the presence of abundant structural defects in (N, Ti)-HMCs rather than HMCs which is beneficial to promote the electrosorption. Besides, the electrochemical analysis demonstrated that (N, Ti)-HMCs owns improved specific capacitance and lower resistivity as compared to that of HMCs. Moreover, by examining the CDI performance of (N, Ti)-HMCs in a batch mode experiment, it illustrated high salt removal capacity of 13.89 mg/g at 1.2 V in a 500 mg/L NaCl solutions and outstanding adsorption rate of 3.53 mg/(g·min). Thus, (N, Ti)-HMCs should be proposed as superior electrode materials for enhanced CDI. Our work also strengthens the emphasis on heteroatom doped carbonaceous materials in CDI.

Continuous Lithium Extraction from Aqueous Solution Using Flow-Electrode Capacitive Deionization

Flow-electrode-based capacitive deionization (FCDI) is a desalination process that uses electrostatic adsorption and desorption of ions onto electrode materials. It provides a continuous desalination flow with high salt removal performance and low energy consumption. Since lithium has been regarded as an essential element for the last few decades, the efficient production of lithium from the natural environment has been intensively investigated. In this study, we have extracted lithium ions from aqueous solution by using FCDI desalination. We confirmed that lithium and chloride ions could be continuously collected and that the salt removal rate depends on various parameters, including feed-flow rate and a feed saline concentration. We found that the salt removal rate increases as the feed-flow rate decreases and the feed salt concentration increases. Furthermore, the salt removal rate depends on the circulation mode of the feed solution (continuous feed stream vs. batch feed stream), which allows control of the desalination performance (higher capacity vs. higher efficiency) depending on the purpose of the application. The salt removal rate was highest, at 215.06 μmol/m−2s−1, at the feed rate of 3 mL/min and the feed concentration of 100 mg/L. We believe that such efficient and continuous extraction of lithium chloride using FCDI desalination can open a new door for the current lithium-production industry, which typically uses natural water evaporation.