Ultrahigh Desalination Capacity Dual-ion Electrochemical

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Ultrahigh Desalination Capacity Dual-ion Electrochemical Deionization Device Based on Na3V2(PO4)3@C-AgCl Electrodes Weiyun Zhao, Lu Guo, Meng Ding, Yinxi Huang, and Hui Ying Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b14014 • Publication Date (Web): 29 Oct 2018 Downloaded from http://pubs.acs.org on October 31, 2018

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ACS Applied Materials & Interfaces

Ultrahigh Desalination Capacity Dual-ion Electrochemical Deionization Device Based on Na3V2(PO4)3@C-AgCl Electrodes Weiyun Zhao, Lu Guo, Meng Ding, Yinxi Huang and Hui Ying Yang* Pillar of Engineering Product Development, Singapore University of Technology and Design, 8 Somapah Road, 487372, Singapore KEYWORDS: desalination, deionization, NASICON, Na3V2(PO4)3, electrochemical

ABSTRACT

Seawater desalination is a promising way to alleviate water scarcity nowadays. Present capacitive desalination methods have limitation on salt removal capacity. Herein, a new dual-ion electrochemical desalination system with ultrahigh desalination capacity is reported. It is based on Na3V2(PO4)3@C wires as sodium ion Faradaic electrode, AgCl as chloride ion Faradaic electrode, and salt feed solution as electrolyte. When a constant current is applied, redox reactions occur on electrodes, releasing or removing sodium ions and chloride ions. Na3V2(PO4)3 has high sodium specific capacity, and as a sodium super ionic conductor, Na3V2(PO4)3@C wires

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form an ion conductor network providing high sodium ion mobility. Additionally, both wire structure and carbon shell enhance electrical conductivity of Na3V2(PO4)3. Benefiting from these, outstanding desalination performance, rate capability and cycle capability have been achieved with Na3V2(PO4)3@C wire-AgCl device. An ultrahigh desalination capacity of 98.0 mg/g is obtained with current density of 100 mA/g for more than 50 cycles. This system provides a viable dual-ion electrochemical desalination strategy which outperforms most of the existing desalination methods.

Introduction With rapid population growth, spreading environmental pollution, and drastic climate change, fresh water shortage has become one of the most significant global challenges nowadays1. Research on seawater desalination has attracted a lot of attention in recent years 2-3. Various desalination methods such as reverse osmosis (RO)4-6, thermal distillation7-8, electro-dialysis9-11, capacitive deionization (CDI)12-17 and so on have been advanced rapidly. Among these techniques, RO and thermal distillation are well established for seawater desalination application. However, these two methods usually cost a lot of manpower and energy18-19. Electro-dialysis and CDI are developing technologies with some premier features like economical energy consumption, easy operation, eco-friendly processetc20. CDI is the technique of deionization through applying electrical potential between two electrodes21. Carbon based materials are chosen as CDI electrodes frequently, such as activated carbon22-23, carbon aerogels24, carbon nanotubes25-26, porous carbon27, carbon nanofibers28, three-dimensional graphene29, composites of metal oxide and carbon30, etc. The salt removal capacity of CDI is limited by low physical charge adsorption capacity, which is the main bottleneck for the further


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development of CDI. A number of works on novel CDI devices have been reported to enhance the deionization capacity, including membrane capacitive deionization (MCDI)31, flow-through electrode capacitive deionization (FTCDI)32, flow electrode capacitive deionization (FCDI)33, hybrid capacitive deionization (HCDI)34-35 and desalination battery36. Inspired by the works on HCDI and desalination battery, a new concept of electrochemical deionization (EDI) was proposed37-39. The electrode materials for EDI have redox reaction with either sodium or chloride ions with current applied and achieve higher desalination capacities of EDI devices comparing with traditional CDI devices37-38, 40. Thus, materials with high specific capacity of sodium or chloride ions are ideal candidates for EDI, including Prussian blue 37, 41, Na0.44MnO238, 42 and NaTi2(PO4)340, 43 for sodium electrode, and BiOCl38 and Ag/AgCl40, 42 for chloride electrode. Motivated by these works, more candidate materials for EDI electrodes could be explored to expand the scope of EDI application and improve performance. Recently, sodium super ionic conductor (NASICON) structure materials have been employed as advanced cathode materials of sodium battery due to their high theoretical specific capacity and high Na+ conductivity44-50. Na3V2(PO4)3 (NVP), a typical NASICON material, has a high theoretical specific capacity of 117.6 mAh/g and is a potential sodium electrode material for EDI. However, low electrical conductivity of NVP is a drawback of its electrochemical performance51-52. In the past few years, many works have been carried out to enhance NVP’s electrical conductivity hence further improve its electrochemical performance and reduce the resistive loss. Most of them focused on carbon coating methods53-55, while some worked on structural construction56-58. The highest specific capacity of NVP has been optimized almost to the theoretical value 59. Being a high-performance material for sodium ion battery cathode, NVP is expected to be a potential candidate for the sodium electrode of EDI.


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In this work, we successfully developed a new dual-ion EDI (DEDI) system and proposed to realize ion conductor network in electrodes which lead to superior desalination performance of the DEDI system. The DEDI system consisted of carbon coated NASICON NVP (NVP@C) as a sodium ion Faradaic electrode, AgCl as a chloride ion Faradaic electrode, and salt feed solution as electrolyte. Three types of NVP samples were synthesized through a solvothermal method with different reaction time hence three types of morphology, including sphere (NVPS), flower (NVPF) and wire (NVPW). The as-prepared NVP samples were coated with polydopamine (PDA) uniformly to form a core-shell structure to protect the sample from agglomeration during annealing. NASICON structure NVP core and amorphous carbon shell can be obtained after annealing. Among three types of NVP@C samples, the one with quasi-one-dimensional wire structure form an ion conductor network and achieved best desalination performance. The DEDI with NVP@C wires obtained an ultrahigh desalination capacity of 98.0 mg/g with current density of 100 mA/g after 50 cycles. The outstanding desalination performance demonstrates the promising application of NVP@C-AgCl DEDI device for seawater desalination in the future.

Experimental Vanadium oxide (V2O5), N,N-Dimethylformamide (DMF), sodium phosphate monobasic (NaH2PO4), oxalic acid (H2C2O4), dopamine hydrochloride, tris base, silver chloride (AgCl), carbon black, polyvinylidene fluoride (PVdF), 1-Methyl-2-pyrrolidinone (NMP) and sodium chloride (NaCl) used in this work were all purchased from Sigma-Aldrich. In a typical synthesis of NVPW, 1 mmol V2O5 was first added into 30 ml DMF. The solution was then heated to 80℃ and stirred for 30 minutes. 3 mmol NaH2PO4and 3 mmol H2C2O4with 5 ml deionized water were added into the above solution and stirred for another 30 minutes at


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80℃. After that the whole solution was transferred into a 50 ml Teflon-lined stainless-steel autoclave and kept at 180 ℃ for 24 hours. The product was washed with acetone for 3 times and dried in an oven at 60 ℃. NVPS and NVPF were synthesized with the same method but kept at 180 ℃ for 0.5 hour and 10 hours respectively. NVP@C was fabricated using poly-dopamine (PDA). First, 2 mmol tris base was dissolved in 200 ml deionized water to adjust pH value of the solution to 8.5. 200 mg dopamine hydrochloride and 100 mg as-prepared NVP samples were added into above tris buffer solution and stirred 2 hours to let PDA attach well onto the surface of NVP. The mixture solution was filtered to collect NVP@PDA and dried at 60 ℃. After that, the powder was annealed in Ar atmosphere first at 400 ℃ for 4 hours as pre-heating and followed with 750 ℃ for 8 hours with a heating rate of 2 ℃/min. The NVP@C and AgCl electrodes were prepared with 80 wt% of NVP@C or AgCl powder,10 wt% of carbon black and 10 wt% of PVdF. The well-mixed slurry was pasted on graphite paper and dried in a vacuum oven under 60 ℃ overnight. In all the desalination tests of this work, the mass ratio between two electrodes was controlled around 1:1, each with around 10 mg. The prepared electrodes were assembled into an EDI cell with anion exchange membrane, cation exchange membrane and a spacing piece. The morphology and nanostructures of the samples were characterized using field emission scanning electron microscopy (FESEM) (JEOL JSM7600), transmission electron microscopy (TEM) (JEOL JEM-2100F) and X-ray diffraction (XRD) (Bruker D8 Powder). The carbon content ratios were detected using thermogravimetric analysis (TGA). The surface areas of the samples were tested by Brunauer-Emmett-Teller (BET) method. The cyclic voltammetry (CV)


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measurement was carried out using electrochemical workstation (VMP3, Biologic, France) in a two-electrode mode with NVP@C and AgCl electrodes. The desalination performance tests were carried out using a continuously recycling system and a battery tester. The recycling system consisted of an EDI device, a conductivity monitor and a peristaltic pump. Feed solution was prepared by dissolving NaCl in deionized water. The initial salt concentration of feed solution was around 1000 mg/L and the flow rate was 100 ml/min. 50 ml feed solution was in a beaker and covered with parafilm to prevent evaporation. The feed solution was being stirred throughout the whole test. During the electrical deionization process, a constant current was applied on EDI device via the battery analyser (Neware, Shenzhen, China). The voltage window was set as -1.4 ~ 1.4 V. Current density, which is current per gram of NVP@C, was set as 100, 150, 200, 300 and 500 mA/g in rate capability test and 100 mA/g in cycle capability test. The conductivity of feed solution and voltage were recorded simultaneously and independently. . Results and discussion The growth mechanism of NVP during solvothermal process was studied by FESEM. Figure 1A-F shows the morphology of as-prepared NVP samples with different solvothermal reaction duration of 0.5, 10 and 24hours. With a short reaction time of 0.5 hour, NVP first assembled into microspheres (NVPS) rapidly as shown in Figure 1A and D. It can be observed from SEM images that the surface of NVPS is rough, and the diameter is around 5-6 µm. As reaction time increased to 10 hours, complex surface structures appeared via Ostwald ripening57 and NVP became flower-like (NVPF) microstructures as shown in Figure1B and E. With reaction time further prolonged, the metastable nanopedals on the surface began to recrystallize into nanowires


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(Figure S1). Upon further prolongation of the reaction time to 24 hours, all the flowers were turned into nanowire networks (NVPW) as shown in Figure 1C and F. It should be noticed that the NASICON structure NVP can only appear after high temperature (750℃) annealing. To confirm the phase of NVP samples, XRD measurements were conducted on the three types of annealed NVP samples (NVPW, NVPF and NVPS). As shown in Figure 2, all the three samples are NASICON structure NVP (JCPDS No. 53-0018). NVPW and NVPF produced much sharper peaks in XRD spectrum as compared with NVPS, indicating better crystallinity. With prolonged reaction time, the phase of NVP is more uniform which is beneficial for high crystallinity. Since higher crystallinity reduces the possible scattering arising from charged impurities and grain boundaries inside the nanostructures, electron mobility of crystalline sample will be higher leading to better electrical conductivity. To preserve the delicate structure of NVP samples during annealing process as well as enhancing electrical conductivity, carbon coating is an efficient method. In this work, asprepared NVP samples were coated with PDA via in situ polymerization in buffer solution. During the annealing process, a slow heating rate of 2 ℃/min was also helpful to protect the structure. After annealing, PDA was carbonized and formed an amorphous carbon layer on the surface of NVP. The morphology of NVPS@C, NVPF@C and NVPW@C was investigated using SEM as shown in Figure S2A, S2B and S2C respectively. Even after high temperature (750℃) annealing, the structures of all samples were still maintained well. The element mapping images (Figure S3) confirm the uniform distribution of elements Na, V, P and C on NVPW@C. The carbon content of three NVP samples were detected by TGA as shown in Figure S4. Generally, with the same coating method, the carbon content of all three samples were similar. NVPW@C contained 34 wt% of carbon, while carbon contents were found to be 36 wt% for


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NVPF@C and 32% wt% for NVPS@C. Higher BET surface areas (Figure S5) of 215.3 m2/g and 183.0 m2/g were achieved on NVPW@C and NVPF@C comparing with 79.0 m2/g of NVPS@C owing to the nanowire and flower structures. TEM and high-resolution TEM (HRTEM) were used to examine the structure of NVPW@C. Figure 3A is TEM image of NVPW@C networks. Figure 3B shows a single nanowire of the NVPW@C sample. Clear core-shell structure and a uniformly thick carbon coating layer can be observed from the image. The NVPW core is around 50 nm in diameter and the thickness of amorphous carbon shell is around 15-20 nm. Figure 3C and D are HRTEM images of NVPW@C. The inset of Figure 3C is the magnified HRTEM of NVP core. The interfringe spacings shown in Figure 3C and D are around 0.28 and 0.37 nm, corresponding to (116) and (113) interplanes of NASICON-type NVP respectively. XRD pattern of NVPW@C is shown in Figure 3E. NASICON structure still can be detected and a broad peak around 24° is corresponding to amorphous carbon. Figure 4 shows the schematics of NVPW@C network and NVP atomic structure60. Since sodium ions can move through the NASICON structure freely and NVPW network provides an ion conductor network, sodium ions are highly mobile in the NVPW sample. Additionally, the amorphous carbon shell provides electron transport channels which help enhance the electrical conductivity and further boost the desalination performance of NVPW. The experimental setup and desalination mechanism are shown in Figure S6A, Figure S6B and Figure 5A. A typical EDI cell consisted of NVP@C electrode on graphite paper, cation exchange membrane (CEM), spacer, anion exchange membrane (AEM) and AgCl electrode on graphite paper (FigureS6A). 50 ml feed solution was pumped into EDI device and circled back continuously with a flow rate of 100 ml/min (Figure S6B). Different weight ratios between NVP@C and AgCl were used first to identify the optimal ratio. Although we have known that


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the theoretical specific capacity of AgCl (187.0 mAh/g) is higher than NVP (117.6 mAh/g), it should be noted that AgCl is not as good an ionic conductor as NVP is. Therefore, the ions tend to react with the surface part of AgCl and the capacity of AgCl in experiment may be not as high as its theoretical value. As shown in Figure S6C, the highest salt removal capacity of 29.4 mg/g was achieved with the weight ratio of 1:1 with a current density of 500 mA/g. Using weight ratio of 1:2 and 2:1 instead, lower removal capacities of 28.8mg/g and 22.3mg/g respectively were found in experiment. Thus, the weight ratio of two electrodes was controlled to be 1:1. Here, the loading mass of NVP@C and AgCl of all DEDI devices were around 10 mg and the total mass of one electrode was controlled to be 12-13 mg, including 80 wt% of NVP@C or AgCl, 10 wt% of carbon black, 10 wt% of PVdF. Additionally, various initial salt concentrations were also tested to find an appropriate choice for this DEDI system (Figure S6D). With the increase of salt concentration, the removal capacity was increased at first and converged after 1000 mg/L. Hence, initial concentration of 1000 mg/L was used for all tests in this work. Cyclic voltammetry (CV) curve of NVPW-AgCl two electrodes system with 1 M NaCl aqueous solution is shown in Figure 5B. During anodic part, oxidization peaks were detected around 0.04, 0.24 and 0.50 V. In cathodic part, reduction peaks were located around -0.09, 0.14 and 0.36 V. The two main peaks at 0.50 V and 0.36 V are owing to sodium ions extraction and re-intercalation into NVP electrodes. The small peaks may be assigned to some side-reactions, such as the structural reorganization of NVP during ion extraction and insertion61, surfacelimited process62 and H+ intercalation due to the aqueous solution63. Reactions occurred on each electrode are shown below: Sodium electrode: Na3V2(PO4)3⇌ Na3-xV2(PO4)3 + xNa+ + xeChloride electrode: AgCl + e-⇌ Ag + Cl-


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In deintercalation process, sodium ions and chloride ions were extracted from NVP and AgCl, which leads to a higher salt concentration in feed. In intercalation process, sodium ions and chloride ions in feed are inserted into electrodes with the recovery of NVP and AgCl, which results in the decrease of salt concentration in feed. The variation of salt concentration can be detected and converted from electrical conductivity measurements. A conductivity meter was placed in the feed to measure the electrical conductivity during the whole salination/desalination process. Figure 5C is the curve of voltage vs. time and corresponding electrical conductivity during salination and desalination processes of NVPW@C-AgCl EDI device with current density of 100 mA/g. When a positive current was applied, the electrical conductivity was increasing due to sodium ions and chloride ions extract ion from electrodes. When negative current was applied, the electrical conductivity kept decreasing due to sodium ions and chloride ions insert ion into electrodes. The conductivity curve shows excellent stability and demonstrates good regeneration of our DEDI system in aqua condition. Rate capability of the NVP@C-AgCl systems with NVPW@C, NVPF@C and NVPS@C as cathode materials were tested and compared as shown in Figure 6A. The salt removal performance of three EDI devices were tested with different current densities (100, 150, 200, 300 and 500 mA/g). Constant current was applied to charge (discharge) electrodes till a bias of 1.4 V (-1.4 V) between electrodes was reached. Then removal capacity (Γ) was calculated via the equation: Γ = (C0-Cf) × V/Mt where C0 is initial NaCl concentration, Cf is final NaCl concentration, V is volume of feed solution and Mt is total mass of electrodes materials (including NVP@C, AgCl, carbon black and PVdF). The capacity stabilized for 10 cycles. At all measured current densities, better ion


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mobility and electrical conductivity as well as larger surface area allow superior performance of NVPW@C as compared to other samples. With high current density however, reactions may not be complete due to faster charging/discharging and the capacity is possibly limited by factors other than the morphologies, such as surface effect, carbon shell, or performance of AgCl electrode. These factors may narrow the difference among samples with different morphologies at high current density. It is worth to point out that when the current density returned to 100 mA/g after high current density cycling, the removal capacity of all the measured samples almost recovered to the initial values. With current density of 100 mA/g, the removal capacity of the DEDI device with NVPW@C is around 102.1 mg/g, while 83.1 mg/g of NVPF@C and 65.6 mg/g of NVPS@C. Even the device with NVPS@C, which has the lowest removal capacity among the three, also achieved decent performance, showing outstanding desalination potential of NASICON-type NVP. The good performance may be attributed to high sodium ionic conductivity of NASICON structure NVP as well as electrical conductivity enhancement from amorphous carbon shell. The DEDI with NVPW@C shows highest removal capacity at all measured current densities. As we have discussed, NVP wires can form ion conductor network which leads high ionic mobility. In addition to this unique structure, NVPW has best crystallinity according to XRD result and largest surface area according to BET result. Therefore, NVPW@C manifests best salt removal performance. It is worth to mention that, among the above three factors, i.e. structure, crystallinity and surface area, the key factor of excellent desalination performance should be the unique structure. As shown in XRD pattern, both NVPW and NVPF have much better crystallinity as compared to NVPS. Similar trend is also found for BET surface areas, where the surface area of NVPF@C is only a little lower than that of NVPW@C, but much higher than that


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of NVPS@C. However, when it comes to desalination performance, we find that the difference between NVPW@C and NVPF@C (i.e. 102.1 mg/g vs. 83.1 mg/g with current density 100 mA/g) is close to the difference between NVPF@C and NVPS@C (i.e. 83.1 mg/g vs. 65.6 mg/g with current density 100 mA). Unlike the cases of surface area and crystallinity, in desalination tests NVPW@C outperforms both NVPF@C and NVPS@C significantly. This implies that crystallinity and surface area may affect the desalination performance, but they are not the most important factors in this work. If crystallinity or surface area is the dominating mechanism behind performance, NVPF@C should have much higher desalination capacity than that of NVPS@C but only slightly lower desalination capacity than that of NVPW@C. This does not agree with our experimental observations. Hence, we expect the key factor to be the unique structure of ion conductor network since among the three samples, only NVPW@C has such feature. To shed light on the contribution of carbon shell, we tested the rate capability of a bare NVP sample. The bare sample was obtained after annealing NVPW precursor directly without PDA coating. As shown in Figure S7, desalination capacities of bare NVP were lower than NVPW@C at all measured current densities. Although the initial salt removal capacity of bare NVP was as high as 94.0 mg/g, it decreased to about 69.0 mg/g when it was stable. The carbon coating enhances sample performance in two possible ways. First, carbon shell can protect wire structure well to form the ion conductor network. Second, carbon shell can help overcome the poor electrical conductivity of NVP. The outstanding desalination performance of NVPW@C was also shown in cycling test. Figure 6B is the salination/desalination capacity of NVPW@C-AgCl EDI device for 50 cycles with the current density 100 mA/g. Desalination capacity of the first cycle was up to 124.0 mg/g


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and decreased to 98.0 mg/g after 50 cycles. Both desalination capacity and salination capacity were quite stable over 50 cycles. Furthermore, salination capacity was almost identical with desalination capacity. The corresponding electric charge capacity of NVPW@C-AgCl device is shown in Figure S8. Electric charge capacity was calculated using the specific capacity of NVPW@C, which is directly obtained from a battery analyser. With current density of 100 mA/g, initial discharge capacity was 104.9 mAh/g and it decreased to 89.2 mAh/g at 50th cycle. High electrical charge capacity indicates that a large amount of sodium and chloride ions were able to intercalate into electrodes. Besides, electrical conductivity is also an important factor for electrochemical stability and reversibility of samples. Here, the amorphous carbon shell played an important role to improve electrical conductivity of NVPW and yielded superior stability and reversibility of NVPW@C-AgCl in aqueous conditions. For comparison, cycling tests with AgCl were also conducted using NVPF@C and NVPS@C (Figure S9). The first cycle desalination capacity of NVPF@C and NVPS@C devices was 94.4 mg/g and 77.5 mg/g, and decreased to 70.4 mg/g and 58.8 mg/g after 50 cycles, respectively. A longer cycling test on NVPW@C-AgCl DEDI device was performed with a current density of 500 mA/g for 500 cycles, as shown in Figure 6C. The initial discharge capacity was 34 mg/g and decreased to 26 mg/g after 500 cycles. The removal capacity of our DEDI device is much higher than that of previous works, such as activated carbon CDI22,

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, CNT CDI26, graphene CDI29,

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, graphene aerogel/TiO2 CDI30,

carbon aerogel FTCDI32, Na4Mn9O18 HCDI34, Na2FeP2O7 HCDI35 etc., which are less than 30 mg/g. This work also shows a higher salt removal capacity as compared with other DEDI works, including 68.5 mg/g with NaMn0.44O2-BiOCl38 and 57.4 mg/g with NaMn0.44O2-AgCl42. The curve of salt concentration change of NVPW@C at different current densities is shown in Figure 6D. The salt concentration curve was changed almost linearly with all the current


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densities due to the constant current applied. It suggests that most electric charges combined with ions during deintercalation and intercalation processes. Kinetics deionization rates can be calculated as Γ / Δt, where Γ is salt removal capacity and Δt is time duration of salt removal process. The calculated rates based on Figure 5C are 0.04, 0.05, 0.06, 0.09, 0.14 mg g-1 s-1 with current density 100, 150, 200, 300, 500 mA/g respectively, demonstrating rapid desalination rates of NVPW@C. The reactions mechanism on both electrodes was further studied by XRD. The two electrodes materials on graphite paper were first charged to 1.4 V with current density of 100 mA/g, followed by XRD measurement. Then they were applied negative current to -1.4 V with 100 mA/g and underwent XRD measurement again. Figure 7A shows XRD patterns of initial NVPW, state after sodium extraction and state after sodium recovery. After deintercalation, peaks of NVP shifted to sodium-poor phase. The peak (116) and (300) shifted to higher angles, indicating smaller d-spacings after sodium ions extraction. After intercalation, sodium ions were recovered and peaks of NVP almost returned to the initial positions. Figure 7D shows XRD patterns of initial AgCl, chloride extraction state, and chloride recovery state. In deintercalation process, chloride ions were extracted from AgCl. The peaks corresponding to Ag appeared after deintercalation located around 2 theta 38.1° and 44.3°. When the voltage went to -1.4 V, the relative peak strength of Ag was reduced obviously compared to that of AgCl. Besides, the stability of morphology of NVPW@C and AgCl electrodes (with PVdF and carbon black) were studied as well since new phases were detected during desalination/salination processes. Figure 7B and C show the morphology of NVPW@C electrode before and after 50 cycles with current density 100 mA/g. It can be found that the wire networks of NVPW@C still can be detected after cycling. Also, a similar morphology of AgCl electrode can be found before (Figure 7E) and


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after (Figure 7F) 50 cycles with current density 100 mA/g. The SEM images before and after cycling show the excellent morphological stability of two electrode materials of this DEDI system.

Conclusions A new dual-ion electrochemical deionization device has been demonstrated to achieve superior desalination performance with high salt removal capacity, fast desalination rate and excellent stability. It was composed of NVPW@C as sodium ion Faradaic electrode, AgCl as chloride ion Faradaic electrode, and NaCl aqueous solution as electrolyte. The redox reaction occurred on the electrodes when a constant current was applied. On salt release process with positive current applied, sodium ions and chloride ions were released from electrodes to electrolyte. On salt removal process with negative current applied, sodium ions and chloride ions in electrolyte were chemically inserted into electrodes. Owing to its ion conductor network consisting of NVP nanowires, NVPW@C exhibited higher desalination capacity than NVPS@C and NVPF@C. Moreover, the amorphous carbon shell formed by carbonization of PDA coating improved NVP sample’s electrical conductivity and further enhanced its desalination performance. With current density of 100 mA/g, a high salt removal capacity of 102.1 mg/g was obtained on the EDI device with NVPW@C and AgCl. The salt removal capacity was still as high as 98.0 mg/g after 50 cycles with current density of 100 mA/g. A high desalination rate of 0.14 mg g-1 s-1 was obtained with current density of 500 mA/g. The promising desalination properties shown in this work demonstrated the potential of NVPW@C as a candidate material for desalination, and the viability of NVP@C-AgCl DEDI system for seawater desalination in the future.


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Figure 1 SEM images of as-prepared NVP samples. (A) and (D) NVPS, (B) and (E) NVPF, and (C) and (F) NVPW.


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Figure 2 XRD patterns of annealed NVPS, NVPF and NVPW.


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Figure 3 TEM images of (A) NVPW@C networks, and (B) a single NVPW@C. (C) and (D) HRTEM images of NVPW@C. (E) XRD pattern of NVPW@C.


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Figure 4 Schematics of NVPW@C ion conductor network and NVP atomic structure. NVP atomic structure was created using VESTA60.


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Figure 5 (A)Schematics of salination and desalination process during charging and discharging. (B) CV curve of NVPW-AgCl electrode system in 1M NaCl aqueous solution with scan rate of 1 mV/s. (C) Voltage vs. time and corresponding electrical conductivity during salination and desalination process of NVPW@C-AgCl system at current density 100 mA/g.


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Figure 6 Salt removal performance of NVP@C-AgCl DEDI devices. (A) Rate capability comparison of the DEDI system with NVPW@C, NVPF@C, NVPS@C. Cycle capability of NVPW@C-AgCl DEDI with current density (B) 100 mA/g and (C) 500 mA/g. (D) Salt concentration change with different current densities based on NVPW@C-AgCl system.


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Figure 7 XRD patterns of (A) initial NVPW@C sample on graphite paper, after sodium extraction, and after sodium recovery; (D) initial AgCl sample on graphite paper, after chloride extraction, and after chloride recovery. SEM images of NVPW@C electrode (B) before and (C) after 50 cycles, and AgCl electrode (E) before and (F) after 50 cycles.


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ASSOCIATED CONTENT Supporting Information. The following files are available free of charge. Additional SEM images, experimental setup information and property characterizations (pdf). AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] Notes The authors declare no competing financial interest. ACKNOWLEDGMENT This research work is supported by the National Research Foundation of Singapore, Prime Minister’s Office under its Environment & Water Research Programme with Grant No. 1301IRIS-17 and administered by the Environment & Water Industry Programme Office (EWI) of the PUB, Singapore’s national agency.

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TABLE OF CONTENTS GRAPHIC


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Ultrahigh Desalination Capacity Dual-ion Electrochemical

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