High Salt Removal Capacity of Metal–Organic Gel Derived Porous

Research Article pubs.acs.org/journal/ascecg

High Salt Removal Capacity of Metal−Organic Gel Derived Porous Carbon for Capacitive Deionization Zhuo Wang, Tingting Yan, Guorong Chen, Liyi Shi, and Dengsong Zhang* Research Center of Nano Science and Technology, Shanghai University, No. 99 Shangda Road, BaoShan District, Shanghai 200444, P. R. China S Supporting Information *

ABSTRACT: Fresh water shortage poses serious threats to humanity. Capacitive deionization (CDI) holds promise for water desalination. Here, porous carbon derived from Al-based metal−organic gels (MOGs) upon calcination has been originally developed as electrodes for capacitive deionization. The obtained material with a large specific surface area, a large percentage of micropore, and a suitable pore size distribution favors slat ion accessibility. Its desalination performance is investigated under various operation conditions. Excitingly, this material displays a remarkable salt removal capacity of 25.16 mg g−1 in a 500 mg L−1 aqueous sodium chloride solution at 1.4 V, superior to those of the recently reported carbon materials. Moreover, the obtained electrode material also exhibits a high salt removal rate and an excellent recycling stability. The results demonstrate that MOG-derived carbon is an appealing candidate as an efficient electrode material in the CDI process for brackish and seawater desalination. KEYWORDS: Metal−organic gels, Porous carbon, Capacitor, Water deionization

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to be a class of ideal precursors to prepare porous carbons.18−25 So far, a number of MOF-derived carbons, including MOF-5,18 ZIF-8,19 Al-PCP,20 and isoreticular MOFs,21 have been reported and have shown promising applications in supercapacitors,18,22 gas adsorption,19,21 and fuel cells,23−25 etc. Considering their regular pores and open channels suitable for the entrance of ions, several MOF-derived carbons have been designed and produced as an electrode material applied in the CDI process.26−31 For example, MOF-5-templated carbon displayed a salt removal capacity as high as 9.39 mg g−1,26 and a salt removal capacity of 13.86 mg g−1 was achieved based on ZIF-8-templated carbon.27 We reported that bimetallic Zn-CoZIF-templated carbon showed an excellent performance as a CDI electrode for removing salt (16.63 mg g−1).28 Most recently, our studies have also demonstrated that porous carbon derived from the modified ZIF-8 can remarkably improve the salt removal capacity (20.05 mg g−1) compared to that of the normal ZIF-8-templated sample.31 These suggest the potential of such MOF-derived carbons as electrode materials for CDI. However, it should be noted that the synthesis of MOFs or PCPs always involves costly, complex procedures and toxic reagents such as diethylformamide, methanol, and dimethylformamide. In addition, the surface areas of carbon products often lead to a drastic decrease when the processes are scaled

INTRODUCTION The development of highly efficient desalination technology is crucial to meet the ever growing demand for clean water. Capacitive deionization (CDI) represents a very promising technology that can be used to remove salt ions from aqueous solutions and is a more cost-effective and energy-efficient process in comparison with conventional desalination methods like reverse osmosis and thermal distillation.1,2 CDI technology utilizes an electrostatic field induced adsorption process, in which the cations and anions of salt water electromigrate and are electrostatically held in the electrical double layer on the electrode/water interface. The release of adsorbed ions by canceling the voltage results in the regeneration of the electrodes.3−5 As is known, the electrode material is the most important component of CDI devices and largely determines its salt removal capacity. Carbon materials, with characteristics of good electrical conductivity, low fabrication cost, large surface area, and environmental benignity, are usually the favored materials for CDI applications. Various carbon materials, such as activated carbon (AC),6,7 carbon nanotubes (CNTs),8,9 ordered mesoporous carbon (OMC),10,11 and graphene (GR),12−14 have been developed as electrode materials for CDI. Metal−organic frameworks (MOFs) or porous coordination polymers (PCPs), which are crystalline compounds consisting of metal ions connected by organic ligands, have been of considerable interest owing to their diverse structures, tunable pore channels, high surface area, and their enormous applications.15−17 Recently, MOFs or PCPs have been proven © 2017 American Chemical Society

Received: August 30, 2017 Revised: October 25, 2017 Published: October 31, 2017 11637

DOI: 10.1021/acssuschemeng.7b03015 ACS Sustainable Chem. Eng. 2017, 5, 11637−11644

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The resulting wet gel was allowed to age in an oven at 80 °C for 24 h. The gel was further washed with ethanol overnight in a Soxhlet extractor to remove the excess unreacted starting materials and subsequently dried overnight at 80 °C. The resulting purified gel was subsequently thermally treated in nitrogen for 4 h at a heating rate of 3 °C·min−1. The obtained sample was washed extensively by a 2 mol L−1 HCl solution to remove the deposited Al species. Finally, the product was collected by centrifugation, washed with deionized water, and dried at 120 °C overnight prior to use. Characterization. The materials were investigated by TEM, SEM, HRTEM, XRD, Raman spectroscopy, nitrogen sorption isotherms, cyclic voltammetry (CV), electrochemical impedance spectroscope (EIS), and galvanostatic charge−discharge (GC) tests. Details of the structural characterization can be found in the Supporting Information. CDI Experiments. The electrodes were prepared by the same procedure as the electrochemical tests. All of the active electrodes were 0.16 g in total mass, 66 mm × 70 mm in size, and 0.12 mm in thickness. The total volume (35 mL) and temperature (298 K) of the NaCl aqueous solution were kept in each experiment. The different applied voltages (1.0−1.4 V), initial concentrations (100−500 mg L−1) of NaCl aqueous solution, and flow rates (10−30 mL min−1) were explored in the CDI process. The salt removal capacity and the charge efficiency (Λ) of the active electrode material were calculated by the following equations, respectively:

up,20 which is not beneficial for CDI. Therefore, an alternative simple and cost-effective approach of new electrode material synthesis at large quantities is imperative for further development of CDI technology. Metal−organic gels (MOGs), as a novel class of extended MOF material, can be obtained under mild conditions (for example, routine solvent, low temperature, short reaction time, and neutral condition).32,33 It is interesting that MOGs also possess desirable characteristics of high thermal stability, large interior surface area, and suitable porous structure, although its crystallinity is poor.32,33 These features make MOGs attractive templates and sacrificing agents for porous carbon preparation. Despite the fact that several MOGs for gas absorption, catalysis, solar cell, and absorbents have been reported,34−37 studies on MOG-templated porous carbon are still scarce.38,39 No relevant work concerning the CDI performance of such materials has been reported. Herein, for the first time, the facile synthesis of porous carbon from MOGs has been developed, and the obtained porous carbon has been demonstrated as an efficient electrode material for CDI applications. An aluminum-BTC MOG (BTC = 1,3,5-benzene tricarboxylate), which is assembled from the coordination 1,3,5-benzenetricarboxylate and Al3+ ions, is selected because aluminum is a low cost and naturally abundant material.33 In this work, the Al-BTC gels were synthesized and then directly calcined to construct the porous carbons (Scheme 1). The precursor was calcined at 600, 800, and 1000 °C, and

salt removal capacity (mg g −1) = (C0 − Ce)V /m

(1)

Λ = (Γ × F )/Σ

(2)

where C0 and Ce are the initial and equilibrated NaCl concentrations in solution (mg L−1), respectively, V is the total volume of solution (L), and m is the total mass of the active electrodes (g), Γ is the salt removal capacity (mol g−1), F is the Faraday constant, and ∑ (charge, C g−1) is calculated by integrating the corresponding current. The salt removal rate was calculated by dividing the salt adsorption capacity over the charging time.

Scheme 1. Schematic Illustration for the Preparation of Porous Carbon from MOGs

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RESULTS AND DISCUSSION Characteristics. Figure 1 depicts the transmission electron microscopy (TEM) and high-resolution TEM (HRTEM)

corresponding obtained products were designated as PC600, PC800, and PC1000, respectively. The PC800 exhibits a remarkable salt removal capacity (25.16 mg g−1 in a 500 mg L−1 aqueous sodium chloride solution) that is better than those of the recently reported MOF-templated carbon and other carbon materials. This work shows that the MOG-derived carbon is a promising candidate as an electrode material for high performance water desalination.

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EXPERIMENTAL SECTION

Chemicals. Aluminum nitrate nonahydrate (Al(NO3)3·9H2O), 1,3,5-benzenetricarboxylic acid (H3BTC), ethanol (C2H5OH), and hydrochloric acid (HCl) were purchased from Sinopharm Chemical Reagent Co. Ltd. (China). All chemicals and solvents were used without any further purification. Deionized water was used in all of the experimental processes. Synthesis of MOG-Templated Porous Carbon. The aluminum based metal−organic gel was first prepared according to a recent report with minor modifications.33 In a typical synthesis, 0.015 mol of Al(NO3)3·9H2O and 0.01 mol of H3BTC were dissolved in 70 mL of ethanol with stirring for 15 min at room temperature, and the mixture was then transferred to a sealed container and heated to 393 K for 1 h.

Figure 1. TEM images (a,b,d,e,g,h) and HRTEM images (c,f,i) of PC600 (a−c), PC800 (d−f), and PC1000 (g−i). The micropores on the HRTEM images are labeled by circles. 11638

DOI: 10.1021/acssuschemeng.7b03015 ACS Sustainable Chem. Eng. 2017, 5, 11637−11644

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Figure 2. (a) XRD patterns, (b) Raman spectra, (c) adsorption−desorption isotherms (solid and open symbols represent adsorption and desorption isotherms, respectively), and (d) corresponding pore size distributions of PC600, PC800, and PC1000. The dV/dD values of PC800 and PC600 are offset by 0.1 and 0.2 cm3·g−1 nm−1, respectively.

Table 1. Surface Areas and Total Pore Volumes of PC600, PC800, and PC1000 samples

SBET (m2·g−1)

Smicro (m2·g−1)

Smicro/SBET (%)

Vpore (cm3·g−1)

Vmicro (cm3·g−1)

Vmicro/Vpore (%)

PC600 PC800 PC1000

594 1587 141

537 1440 104

90.4 90.7 73.7

0.237 0.621 0.066

0.215 0.580 0.045

90.7 93.4 68.2

surface area of all materials are evaluated by the N2 adsorption isotherm. As shown in Figure 2c, all the samples present the type I shape of isotherms with sharp N2 uptakes in the low pressure range and the saturation region at the high pressure, manifesting that the materials are microporous. In contrast, the sample PC800 shows a significant N2 uptake increase at a very low pressure, implying the presence of a large density of micropores in PC800. The BET specific surface areas and pore volumes are 594 m2 g−1, 0.237 cm3 g−1, and 1587 m2 g−1, 0.621 cm3 g−1 for the PC600 and PC800 samples (Table 1), respectively. Obviously, the BET specific surface area and pore volume increase when the temperature rises from 600 to 800 °C. However, further increase in calcination temperature has deleterious effects on the carbon structures. The BET specific surface area and pore volume of PC1000 fall to 141 m2 g−1 and 0.066 cm3 g−1, respectively, owing to a highly agglomerated structure at a calcination temperature of 1000 °C. Pore size distributions show that sharp peaks of PC800 are mainly centered at 0.55, 1.18, and 1.42 nm (Figure 2d). Pore apertures in PC800 are much larger than the hydrated ion sizes of Na+ (0.358 nm) and Cl− (0.332 nm),40 which are suited for hydrated ions to accommodate inside the pores. It has been demonstrated that micropores with a pore size ranging from 1 to 2 nm are favorable for most electrochemical processes.41,42 It is worth noting that the surface areas of the obtained PC800 are higher than that of the reported Al-PCP-templated carbons and comparable to that of MIL-100(Al)-templated carbons.43,44 Because of a large specific surface area, a large amount of micropores (micropore volume accounts for 93.4% of the total

images of the obtained carbon materials. As observed by TEM images (Figure 1), all the obtained samples have a similar 3D sponge-like morphology with a small pore size, which can be demonstrated in the following N2 adsorption analysis. This result is supported by the scanning electron microscopy (SEM) images (Figure S1, Supporting Information). The HRTEM images (Figure 1c,f) further reveal graphitic microstructures with oriented multilayer graphene layer domains for the PC600 and PC800 samples. However, the samples are prone to agglomerate at elevated temperatures. PC1000 contains only a few graphene edges (Figure 1i). Figure 2a shows the X-ray diffraction (XRD) profiles of PC samples. Only two broad peaks along (002) and (101) lattice planes at around 23° and 43° can be indexed to the typical peaks of carbon.22 The low intensity and broad peaks for PC1000 indicate a low graphitization degree, which is consistent with the TEM data. No other diffraction peaks are observed, indicating the successful synthesis of the pure carbons. Figure 2b shows the Raman spectra of PC600, PC800, and PC1000. Two distinctive bands at around 1590 cm−1 (G band) and 1323 cm−1 (D band) in all the spectra are observed. The G band arises due to the sp2 hybridized carbon vibrations, while the D band indicates the presence of the disordered and defected carbons.22,23 The relative intensity (IG/ ID) ratios provide a direct measurement of the graphitization degree of carbon. The relative ratio in PC1000 decreases to 0.83 in contrast to that of PC600 (1.34) and PC800 (1.05), which shows a decrease in the graphitization of the carbon materials with increasing calcination temperature. The result coincides with the TEM and XRD analyses. The porosity and 11639

DOI: 10.1021/acssuschemeng.7b03015 ACS Sustainable Chem. Eng. 2017, 5, 11637−11644

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Figure 3. Electrochemical properties of the PC600, PC800, and PC1000 electrodes. (a) Cyclic voltammograms at a scan rate of 1 mV s−1, (b) specific capacitance at different scan rates, (c) Nyquist electrochemical impedance spectra (the inset is the enlarged view of the high frequency region), and (d) galvanostatic charge−discharge at a current density of 200 mA g−1. All the curves were obtained in a 0.5 M NaCl solution.

Figure 4. (a) Plots of solution conductivity vs time and (b) CDI Ragone plots of PC600, PC800, and PC1000, compared with the recently reported ZIF-67 derived carbon,28 ZIF-8 derived carbon,27 bimetallic Zn−Co-ZIF derived carbon,28 surfactant-template ZIF-8 derived carbon,31 3D GR,13 3D hierarchically porous GR,14 and N-doped carbon.46 The error bars represent the standard deviations from three parallel sets of experiments.

a small semicircle at high frequencies (Figure 3c). At low frequencies, the slopes of PC600 and PC800 are larger than that of PC1000, indicating faster salty ion diffusion. At high frequencies, the real-axis intercept represents the equivalent series resistance (ESR),45 which also represents the interfacial charge transfer resistance.28,30,46 PC800 (0.53 Ω) has lower ESR than that of PC600 (0.74 Ω) and PC1000 (0.81 Ω). The fast ion diffusion and small ESR of PC800 result in an enhanced capacitive performance. All galvanostatic charge−discharge profiles at 200 mA g−1 exhibit a nearly triangular shape (Figure 3d), indicating an ideal capacitive nature as well. PC800 has a longer discharge time than PC600 and PC1000, indicating that PC800 offers larger specific capacitance than PC600 and PC1000, in agreement with CV analyses. The potential drop (iR drop) is lower in PC800 (0.055 V) as compared to that in PC600 (0.078 V) and PC1000 (0.084 V), suggesting that the resistance of PC800 is smaller than that of the other two electrodes. Low resistance is beneficial for its CDI application. CDI Performance. The salt removal performances of PC600, PC800, and PC1000 electrodes were investigated systematically. When the external voltage is imposed between the two electrodes, ions are forced toward the electrodes, and the conductivity drops correspondingly. Figure 4a shows the conductivity variation of aqueous sodium chloride solution with

pore volume), and a reasonable pore size distribution, PC800 should be promising for CDI applications. Electrochemical Performance. Figure 3a presents cyclic voltammograms (CVs) of PC600, PC800, and PC1000 electrodes in a potential range of −0.5−0.5 V. At 1 mV s−1, all CV profiles have a rectangular shape, which is a typical electric double layer character. In contrast, the curve of PC800 is more rectangular than that of others, indicating that the micropores in PC800 provide better diffusion and accession of ions throughout the porous electrode. The specific capacitances of PC600, PC800, and PC1000 calculated from CV curves are 213.1, 129.1, and 114.3 F g−1, respectively. The specific capacitance of PC800 is higher than that of the recently developed carbon materials (Table S1, Supporting Information). Figure 3b shows the effect of scan rate on specific capacitance. The error bars represent the standard deviations of three parallel sets of experiments. The specific capacitance of the three electrodes has similar variations with increasing scan rates. The specific capacitance of PC800 is higher than that of others at all tested scan rates. However, the small difference in their specific capacitances is observed at scan rates above 30 mV s−1 due to the limited ion penetration into the micropores. The Nyquist profiles of PC600, PC800, and PC1000 electrodes display similar shapes with a straight line at low frequencies and 11640

DOI: 10.1021/acssuschemeng.7b03015 ACS Sustainable Chem. Eng. 2017, 5, 11637−11644

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Figure 5. CDI performance of the PC800 electrode. (a) The variation of solution conductivity along with time and (b) CDI Ragone plots in a 500 mg L−1 NaCl solution at different applied voltages with a flow rate of 20 mL min−1; (c) the salt removal capacity curves and (d) CDI Ragone plots in different concentrations of NaCl solution at 1.4 V with a flow rate of 20 mL min−1; (e) conductivity variation with time and (f) CDI Ragone plots in a 500 mg L−1 NaCl solution at 1.4 V with the different flow rates. The error bars represent the standard deviations of three repeat experiments of each testing condition.

behavior of electrode materials. A comparison of the salt removal capacity and rate for the present three electrodes, MOF-derived porous carbons, graphene, and the recently developed hierarchically porous carbon is provided in Figure 4b. The plot of PC800 is located more in the right upper side of the Ragone plot, which indicates that the PC800 electrode has much higher salt removal capacity and higher salt removal rate than the PC600 and PC1000 ones. Compared to the recently developed carbon materials, the PC800 electrode is also in the right upper side of the plot, indicating better CDI performance. The superior salt removal performance of PC800 is credited to it unique structural properties such as large specific surface area and large pore volume, which provides numerous porous channels for the accumulation of more salt ions, fast ion diffusion, and penetration. Further, an appropriate pore size distribution allows ions to pass through the pores more easily and thus results in more accessible surface area for the adsorption of ions. To further evaluate the performance of the PC800 electrode, the effects of various operation parameters, including applied voltages, initial concentrations of aqueous NaCl solutions, and flow rates were investigated. Plots of the solution conductivity versus time at 1.0−1.4 V are displayed in Figure 5a. As shown in Figure 5a, the solution conductivity curves keep nearly the same downward trend. At 1.4 V, the decrease in solution

time during the charge process. As shown in Figure 4a, the solution conductivity of the PC800 electrode decreases significantly in the first 20 min, indicating quick and easy adsorption of salt ions from the solution to the electrode. With time, the solution conductivity declines slowly and reaches a stable value within 60 min, indicating that the electrosorption reaches equilibrium. In contrast, a slight decrease in the solution conductivity is observed for PC600 and PC1000 samples. Obviously, the desalination amount on the PC800 electrode is much larger than that of the PC600 and PC1000 electrodes. The salt removal capacity of 25.16 mg g−1 is obtained for the PC800 electrode, which is significantly higher than that of PC600 (10.39 mg g−1) and is more than three times greater than that of PC1000 (8.02 mg g−1). The salt removal capacity of PC800 is also higher than that of the recently developed ZIF-67 derived carbon (11.38 mg g−1),28 ZIF-8 derived carbon (13.86 mg g−1),27 bimetallic Zn−Co-ZIF derived carbon (16.63 mg g−1),28 surfactant-template ZIF-8 derived carbon (20.05 mg g−1),31 3D GR (13.72 mg g−1),13 3D hierarchically porous GR (14.7 mg g−1),14 N-doped carbon (12.95 mg g−1),46 and other carbon materials (Table S2, Supporting Information). It is noted that a high salt removal rate is also crucial to the electrodes. The CDI Ragone diagram, in which the salt removal rate is plotted against the salt removal capacity, provides a visual representation of the salt removal 11641

DOI: 10.1021/acssuschemeng.7b03015 ACS Sustainable Chem. Eng. 2017, 5, 11637−11644

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variation of conductivity in solution with time. As can be seen, the solution conductivity can return to its initial value in the desorption process. No obvious decline in the solution conductivity is observed, which indicates that the PC800 electrode has good stability in consecutive electrosorption− desorption cycling.

conductivity is obviously higher as compared to 1.0 and 1.2 V, indicating the favorable salt removal at high applied voltage owing to the electrostatic force. However, the applied voltage higher than 1.4 V is not recommended because the excessively high voltage will lead to water electrolysis.47 The salt removal capacities of PC800 are 16.26, 21.52, and 25.16 mg g−1 at 1.0, 1.2, and 1.4 V, respectively. Moreover, the slope of the conductivity curves increases as the applied voltage increases, illustrating that the salt removal rate increases. A higher applied voltage shifts the Ragone plot toward the right upper region, suggesting that both the salt removal amount and salt removal rate are increased (Figure 5b). Figure 5c shows the plots of the salt removal capacity versus time at different initial sodium chloride concentrations of 100− 500 mg L−1. In all the curves, similar upward trend profiles can be observed. At a higher initial sodium chloride concentration, the amount of salt removed is higher than that at a lower concentration. This is due to the easy electrical double layer formation. The salt removal capacities are 18.62, 22.11, and 25.16 mg g−1 for 100, 300, and 500 mg L−1. On the other hand, the slope of the curves increases with concentration, indicating high salt removal rate at high concentration. This is due to the increase in conductivity, which accelerates salt ion transportation through their nanochannels. The Ragone plot confirms the higher salt removal amount and higher salt removal rate when the concentration increased (Figure 5d). In addition, the current transient of the electrosorption process was recorded simultaneously (Figure S2, Supporting Information). The charge efficiency is calculated to be 0.66 in 100 mg L−1, 0.58 in 300 mg L−1, and 0.45 in 500 mg L−1. The charge efficiency decreases slightly with an increase in the salt concentration, which is in good agreement with previous reports.48,49 It should be noted that the charge efficiency of the PC800 electrode is almost comparable with that of the other newly reported materials.14,27,28 The effect of the flow rate (10−30 mL min−1) on salt removal behavior was also examined (Figure. 5e,f). It is found that the amount of salt removed first increases with flow rate and then decreases after 20 mL min−1. The salt adsorption capacity reaches a maximum value of 25.16 mg g−1 at 20 mL min−1 and subsequently decreases with further increasing flow rate. So the flow rate of 20 mL min−1 is selected as the optimum parameter. Above 20 mL min−1, both the salt removal amount and rate decrease (Figure 5f). The recycling stability of the electrode is important for practical use. To evaluate the recycle performance of the PC800 electrode, 15 regeneration cycles between electrosorption at 1.4 V and desorption at 0 V were conducted. Figure 6 shows the

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CONCLUSION In conclusion, this work demonstrates a facile approach to produce porous carbons for salt removal. The as-prepared material possesses a high surface area and a suitable pore size distribution. Such unique structural features endow it with excellent CDI performance, that is, with a superior salt removal capacity up to 25.16 mg g−1 in a 500 mg L−1 initial sodium chloride concentration at 1.4 V with a flow rate of 20 mL min−1 and a high salt removal rate, as well as good regeneration behavior. More importantly, the salt removal capacity of the obtained material is much greater than those of the recently reported carbon materials. Because of the convenient, fast, and clean preparation route, PC800 should be a promising candidate for highly efficient CDI electrode materials.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03015. Detailed information on characterization; SEM images of the samples; current transient of PC800 in different concentrations of NaCl solution at 1.4 V; and a comparison of the specific capacitance and salt removal capacity of various carbon materials from the literature (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Dengsong Zhang: 0000-0003-4280-0068 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We acknowledge the financial support from the National Natural Science Foundation of China (21722704), the Science and Technology Commission of Shanghai Municipality (16DZ1204300, 15DZ2281400 and 16JC1401700), and the State Key Research and Development Plan (2017YFB0102200).

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REFERENCES

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Figure 6. Desalination-regeneration profiles of the PC800 electrode in a 100 mg L−1 NaCl solution at 1.4 V with a flow rate of 20 mL min−1. 11642

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DOI: 10.1021/acssuschemeng.7b03015 ACS Sustainable Chem. Eng. 2017, 5, 11637−11644