Enhanced Capacitive Deionization by Dispersion of CNTs in Activated

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Enhanced Capacitive Deionization by Dispersion of CNTs in Activated Carbon Electrode Bin Lee, Namsoo Park, Kyung Suk Kang, Ho Jin Ryu, and Soon Hyung Hong ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b01750 • Publication Date (Web): 01 Dec 2017 Downloaded from http://pubs.acs.org on December 6, 2017

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ACS Sustainable Chemistry & Engineering

Enhanced Capacitive Deionization by Dispersion of CNTs in Activated Carbon Electrode

Bin Lee1, Namsoo Park2, Kyung Suk Kang2, Ho Jin Ryu3,4, and Soon Hyung Hong3

1

Manufacturing Technology Office, Korea Institute for Rare Metal, Korea Institute of

Industrial Technology, Get-Pearl Tower, Gaetbeol-ro 12, Yeonsu-gu, Incheon 21999, (Republic of Korea) 2

Siontech Co., Ltd, 530 Yongsan-dong, Yuseong -gu, Daejeon 34025, (Republic of Korea)

3Department

of Materials Science and Engineering, Korea Advanced Institute of Science and

Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, (Republic of Korea) 4Department

of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and

Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 34141, (Republic of Korea)

Corresponding author : Prof. Soon Hyung Hong Tel.: +82-42-350-3327, E-mail: [email protected] Co-Corresponding author : Prof. Ho Jin Ryu Tel.: +82-42-350-3812, E-mail: [email protected]

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Abstract Energy-effective, eco-friendly desalination is a technology in universal demand due to the global water scarcity. Capacitive deionization (CDI) is a promising method that has those advantages, but it is still necessary to enhance desalination performance in order to desalinate high-concentration raw salt water. In this work, carbon nanotubes (CNTs) are used as a conductive agent of the CDI electrode. In order to use CNTs as a conductive agent, we examine the effect of the dispersion status of the CNTs within activated carbon active material on the CDI performance. Acid treatment-functionalization of CNTs created a better dispersion status than CNTs without any treatment. Homogeneously dispersed CNTs showed enhanced electrochemical and desalination performance. Interestingly, desalination tests with highly concentrated raw salt water achieved more notable improvement with 13.9% at only 1 wt.% of CNT dispersion. The improvement mechanism with dispersing CNTs such as increment of surface area and decrement of electrode resistivity are analyzed. Keywords: Capacitive

deionization,

CDI, Carbon nanotube, Activated

carbon,

Desalination,

Functionalization

Introduction Because of increasing water evaporation due to global warming, the drastic increase in population, the increase in water needed for industrial use, desalination of saline water has become one of the most demanded technologies. Existing technologies such as thermal evaporation or reverse osmosis (RO) have advantages for large-scale desalination of seawater. However, they are associated with high energy consumption. An ionexchange technology is a widely used technique for desalination. It separates ionic matter in saline water, but after the process, it is necessary to regenerate the resins, which creates more energy and waste fluid.1 Capacitive deionization (CDI) has been proposed as a promising alternative method for desalination process method, due to its good performance.2 Furthermore it has been proposed to be low energy consumption and an environment-friendly process due to its regenerative electrodes. This process uses electrical double layers (EDLs) to adsorb ions of the saline water. Carbon materials, such as activated carbon, carbon aerogels, carbon nanotubes, carbon nanofibers, and graphenes are promising materials due to their high specific surface area, high electrical conductivity, and hydrophilicity.3–5


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Therefore, recent papers on CDI have focused on the carbon nanomaterials as electrode materials. They have a large number of mesopores and a large specific surface area. Furthermore, they possess extraordinary electrical conductivity due to their sp2 carbon structure. Li et al. fabricated a CNT and reduced graphene oxide (RGO) composite, and used it as an electrode for the CDI system.3 They made an interesting theoretical analysis of CNT and RGO composites and their use in the CDI system. Wilmalasiri et al. also studied the CNT/graphene composite for the CDI electrode, and it demonstrated that it had an extraordinary electrochemical and desalination performance.4 Dengsong Zhang group have been actively analyzing the effects of nanostructures and surface properties on the desalination properties through the use of graphene, CNT and meso/micro porous carbon. These efforts have been expanding the possibilities of future CDI electrodes using carbon nanomaterials. 6–10 There are many other research studies of emerging carbon nanomaterials such as CNTs, graphenes, graphene aerogels, carbon nanofibers, nitrogen doped graphene sponges and porous carbon spheres, for use as CDI electrode active materials (electrosorption materials), and those ideal nanomaterials are showing promising desalination performances.11–22 However, regardless of great performance of carbon nanomaterials, only activated carbon made through commercial CDI systems, since it is much cheaper than other ideal carbon nanomaterials. They are abundant and can be obtained from nature. Although the theoretical efficiency of the material has more disadvantages than other carbon materials, the commercial advantage of activated carbon is overwhelming. The activated carbon CDI system is already being briskly commercialized for the water and sewage industry, ultrapure water and household water softeners. However, it still needs more improvement of efficiency to be used in high concentration desalination areas such as seawater or factory wastes. In this research, we focused on combining those two advantages of the ideal properties of carbon nanomaterials and the extremely low cost of activated carbon. The goal of this research was to enhance the commercially existing activated carbon CDI electrode by adding a limited amount of carbon nanomaterial, CNTs. We changed the conventional point of: We used CNTs as the conductive agent for the CDI electrode. The typical CDI electrode contains active materials, binders, and conductive agents. Even some of the studies using carbon nanomaterials as electrode active materials included a small amount of graphite powder or carbon black as the conductive agent.3,4,18 However, we believe that CNTs have more potential for use as a conductive agent due to their excellent electrical conductivity and unique 1D structure. In this research, we used CNTs as a conductive agent in order to reduce several kinds of resistance of the electrode, namely resistance between each of activated



carbon powder and the contact resistance between activated carbon slurry and a graphite foil. Low-dimensional nanomaterials like CNTs have unique properties due to their extremely high aspect ratio. They also suffer from the agglomeration due to their high aspect ratio. The Van der Waals force plays a more crucial role in low-dimensional materials on a nano scale.23,24 In order to prevent agglomeration problems, the CNTs require a functionalization process. In this research, CNTs were functionalized by an acid treatment process.25,26 A small amount of CNTs with and without the functionalization process were dispersed in the activated carbon active material as a conductive agent. To determine the improvement mechanism of the CNTs in the CDI electrode, we determined the electrochemical properties with a cyclic voltammetry (CV) test and electrochemical impedance spectroscopy (EIS) test. Desalination properties at low and high concentrations of saline water were characterized to interpret the effect of the dispersion stability of the CNTs on the CDI system.

Experimental section Fabrication and characterization of a CNT/activated carbon composite slurry Multi-walled CNTs fabricated by chemical vapor deposition were obtained from Hanwha Nanotech (CM-95). The CNTs were acid treated with H2SO4 and HNO3. For the 0.2g of CNT, 75ml of H2SO4 and 25ml of HNO3 were used to generate functional groups on the surfaces of the CNTs. After a 12 hr sonication process in the acid, the functionalized CNTs were centrifuged and washed for neutralization. The functionalized CNTs were sonicated for 12 hr in 20g of N,N-dimethylacetamide (DMAc, anhydrous, 99.8%, Sigma-Aldrich). The amount of the CNTs was varied between 0.01g (0.1 wt.%) and 0.1g (1 wt.%). The same amount of CNTs without acid treatment also dispersed in DMAc for a control experiment. After the dispersion of the acid-treated CNTs, 10g of activated carbon and 0.437g of PVdF were dissolved in the solution as an active material and a binder. The solution was mixed with a planetary centrifugal mixer (THINKY, ARE-310) at a 2000 rpm rotation speed and 30 min holding time. The microstructure and surface area measurement of the slurry were characterized by scanning electron microscope (Hitachi S-4800, Nova 230) and BET analyzer (Micromeritics, Tristar 3000). FT-IR spectra were analyzed using the attenuated total reflectance (ATR) method (Jasco FT/IR-4100 type-A spectrometer); Raman spectra were determined using a high resolution dispersive Raman microscope (LabRAM HR UV/Vis/NIR, excitation at 514 nm). And Zeta potential values were obtained with the ELS-Z2 (Otsukael, Japan) instrument.


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Those measurements were conducted on CNTs before and after acid treatment.

Electrochemical characterization and CDI experiments of the CNT/activated carbon electrode The slurry was coated on graphite film (0.2 mm thickness, KUKIL INNTOT Co., LTD.) using an auto mini-coater (Hohsen, MC-20) with 101 μm. The CNT/activated carbon coated graphite foil electrode was dried at 80℃ for 1 hr in air. Electrochemical characterization of the electrodes was performed by Zive SP1 (Wonatech) and a plate test cell (PTC1) with a 1 cm2 working area. All electrochemical measurements were done with a threeelectrode method. A CNT/activated carbon coated graphite electrode served as the working electrode, while an Ag/AgCl electrode and a graphite rod were used as a reference and a counter electrode, respectively. The characterizations were carried out in a 0.5M KCl aqueous electrode. EIS measurements were performed in a frequency range from 10 kHz to 0.01 Hz. A cyclic voltammetry test was done between -0.5 and 0.5 V (Vs Ag/AgCl reference electrode) at different scan rates of 5 mV and 50 mV. The specific capacitance (in F/g) were calculated from the CV curves according to C=

∫ 𝑖𝑑𝑡 𝑀𝑣

(1)

where 𝑖 is the average current (A), 𝑀 is the measured mass of the working electrode material loaded on the graphite foil (g) and v is the scan rate (V/s) of the test.3 Contact angle of activated carbon electrode without ion-selective binder coating was measured by SEO Phoenix300, in order to confirm the hydrophilicity change of CDI electrodes,. Contact angles of 0, 3, 5, 10 seconds were measured.

CDI characterization and CDI experiments of CNT/activated carbon electrode In order to characterize the CDI performance of the electrodes, the prepared electrodes were dip-coated in an ion-selective binder synthesized by Innochemtech Co., Ltd., Republic of Korea (Negative ion-selective binder : INC-ITA (aminized), Positive ion-selective binder : INC-ICS (sulfonated), both based on poly phenylene oxide). Positive and negative ion selective activated carbon electrodes were fabricated dip-coating process. Both coating solutions were maintained 25oC temperature and 1000cp viscosity. Activated carbon coated graphite



electrodes were dipped 10 seconds and dried in hot air of 80oC for 30 minutes. As shown in figure 1, two electrodes were connected to a potentiostat and placed against each other. Furthermore, for effective recovery of the electrodes, each electrode was dip-coated with aforementioned ionselective binders. The saline water was supplied to the cell using a pump with a flow rate of 30 ml/min. The NaCl concentrations of saline were 250 ppm and 1000 ppm. A voltage of 1.2V was applied to the electrode system during the saline water flow. Conductivity measurement system was connected to the outlet of the cell, to characterize the NaCl concentration of the desalinated solution.

Figure 1. Equipment of the CDI experiment. (a) whole CDI system, (b) CDI unit cell, and (c) the schematic diagram of the CDI unit cell Results and discussion In this study, two kinds of CNTs were inserted as conductive agents for the activated carbon electrode. One was pristine CNTs without any functional treatment (NCNT), the other was nitric and sulfuric acid-treated CNTs (functionalized CNT, FCNT). Although the FCNTs were covalent functionalized and had many defects


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which can lower the electrical conductivity of the CNT, 27 this research suggests the dispersion status of the conductive agents is more important than their intrinsic property (conductivity). Figure 2 shows the scheme of the structure of the electrode that was used in this research. The CDI electrode contained three layers, the current collector, the active layer, and the ion selective polymer layer. Graphite foil was used as a current collector. For the active layer, activated carbon was used as the active material. The two kinds of CNTs were dispersed in the active layer to enhance the conductivity of the active layer. Finally, positive (sulfonated) or negative (aminized) ion selective polymer layers were coated on the activated carbon active layer. Without the CNTs, the electrode consisted of a commercialized CDI system from Siontech Co., Ltd.

Figure 2. Schematic design for the structure of the CDI electrode in this research

Firstly, we conducted acid treatment of the CNTs. Acid treatment was processed using sulfuric and nitric acid, and following ultrasonication process. In order to precise control of the test parameters, the CNT length before and after the acid treatment should not be significantly different. From the SEM image, it might be confirmed that our acid treatment conditions did not change CNT length significantly (Figure S1).



In order to figure out the result of the acid treatment of CNTs, FT-IR, Raman and Zeta potential analysis were conducted. The CNTs that we used(CM-95, Hanwha Chemical) already contain small amount of functional groups such as C=O(1715 cm-1) peak during the fabrication process (Figure S2). After further acid treatment functionalization conducted, some characteristic peaks were appeared. The newly appeared C-H(around 620cm1

) and C-O(around 1075cm-1) peaks show CNTs were further functionalized. Another evidence for

functionalization is obtained from Raman analysis(Figure S3). Raman spectroscopy from CNT has two characteristic peaks; which are G band and D band. G band generally indicates the graphitization and crystallinity of the CNT and D band shows the defect of the CNT. Defect of CNT is usually related with the functionalization of the CNT. The ID/IG ratio of NCNT and FCNT was calculated. After acid treatment, the I D/IG of the CNT is increased from 0.92 to 0.97. Those results clearly show the acid treatment of CNT using sulfuric and nitric acid has been successful. Dispersion stability and status of the CNTs in the DMAc solution was conducted by Zeta potential analysis. The absolute value of Zeta potential before and after acid treatment increased from -24.90mV to -43.76mV. Through the acid treatment, entangled CNTs became untangled, and dispersion stability was increased due to newly formed functional groups. It is expected that the acid treated CNTs which have high dispersion stability can cause a higher electric conductivity improvement of the electrode than the pristine CNTs. Figure 3a shows the microstructure of activated carbon powders with a size of around 10 μm and porous structures. After the mixing process with the activated carbon, DMAc, PVdF and CNTs using the planetary centrifugal mixer, the slurry was coated on a graphite foil. The graphite foil had a 300 μm thickness and the coated slurry formed a 150 μm uniform thickness. Figures 3c and 3d show SEM microstructure images of 1 wt.% NCNT and 1 wt.% FCNT dispersed activated carbon slurry. Figure 3c shows the activated carbon slurry, which contains CNTs without any treatment and has agglomerated CNTs on the surface of the activated carbon. However, FCNT dispersed slurry does not have entangled CNTs due to their covalent functional groups. Homogeneously dispersed FCNTs might have connected to activated carbon particles in the slurry, and this phenomenon would increase the electrical conductivity of the electrode. The change of hydrophilicity of the activated carbon coated electrode by CNT addition was evaluated with contact angle measurement, Figure S4. However, there was no significant change in contact angle with or without CNTs and with or without acid treatment process. It is considered that the small amount of CNT(0.1~1wt.%) does not contribute significantly to change of the contact angle.


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Figure 3. Microstructures of the electrode measured by SEM. (a) activated carbon active materials before being mixed, (b) Activated carbon coated on a graphite foil, (c) 1 wt.% NCNT dispersed electrode, and (d) 1 wt.% FCNT dispersed electrode

Electrochemical behavior of the CNT dispersed activated carbon electrodes Based on the above suggestion, it is expected that the FCNT-dispersed activated carbon electrode would show enhanced conductivity. The superior electrical conductivity of the electrode indicates that it possesses easy electrolyte (saline water) penetration and low diffusion resistance of cations and anions to the pores.28,29 From



these characteristics we might expect an enhanced CDI performance in the activated carbon electrode with homogeneously dispersed CNTs. In order to confirm the electrical conductivity enhancement of the CNT dispersed electrode, EIS tests were performed (Figure 4). In this research, electrochemical properties were characterized with a fragment of the commercial electrode without an ion-selective binder-coated layer. EIS results of the CNT dispersed electrodes show significantly reduced resistivity.

Figure 4. (a) EIS analysis of pure, NCNT and FCNT dispersed electrode (b) magnified 0.1 and 1 wt.% FCNT dispersed activated carbon electrode Due to the unstable interface property of the electrode material (activated carbon or CNT dispersed activated carbon) and the current collector (graphite film), the EIS results did not show the typical shape of other EDLC capacitors. However, the EIS results of the electrodes with or without CNTs are still powerful enough to confirm the effect of dispersion status of the CNTs on the resistivity of the electrodes. In this research, the initial point of the EIS graph (at 104 Hz) was assumed as the resistivity of each electrode.


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Without any functionalization process, the enhancement of the conductivity of NCNTs were limited due to their agglomeration and entanglement. Nevertheless, because of the high intrinsic electrical conductivity of CNTs,30 electrodes with NCNTs have still shown increase in conductivity, but the effect was only marginal (Figure 4(a)). For the 0.1 wt.% functionalized CNT dispersed electrodes, one-third of the resistivity was decreased compared to the intrinsic activated carbon electrode. However, when the amount of functionalized CNTs increased up to 1 wt.%, the resistivity changed slightly from 9.7Ω to 9.4Ω (Figure 4(b)). It seems that 0.1 wt.% is a sufficient amount of CNTs in the activated carbon electrode to achieve efficient reduction in resistance.

Figure 5. Cyclic voltammetry analysis of the electrodes (a) and (b) at a 5 mV/s scan rate, and (c) and (d) indicate 50 mV/s which indicates a high scan rate

Specific capacitance measurement was conducted by using CV charge-discharge experiment with a 3 electrodes method (figure 5). At a 50 mV/s scan rate, it seems that the effect of reduced resistivity on specific



capacitance of the electrode takes dominance over that of the surface area. When 1 wt.% of FCNTs was added, the specific capacitance was increased by 13 %, and when only 0.1 wt.% of FCNTs dispersed, the specific capacitance increased by 10.2% over the pure activated carbon electrode. However, for a low scan rate (5 mV/s), the specific capacitance was not drastically enhanced by adding CNTs. Furthermore, for the 1 wt.% NCNT added electrode, the specific capacitance was 3.4 % decreased even though it had enhanced electrical conductivity. Figures 2 and 3c, which indicate an agglomerate microstructure of NCNT added activated carbon electrode can interpret this specific capacitance drop. Homogeneously dispersed FCNTs can enhance not only the electrical conductivity, but also the surface area. But in the NCNT added electrode, CNTs were entangled, and they could be blocking the pores which can act as electrochemically active sites.

1600

Surface area (m2/g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Total BET surface area Mesopore surface area

1400 1200 1000 800 600 400 Pure

0.1NCNT

0.1FCNT

1NCNT

1FCNT

Types of electrodes

Figure 6. BET specific surface area analysis of the electrodes

Those suppositions which explain the relativity of the dispersion status of CNTs and capacitance can be interpreted by the BET specific surface area measurement. The BET surface area results of the activated carbon based electrodes show the expected data. The total specific surface area of the electrode was not drastically increased or decreased (Figure 6). The CNTs were agglomerated and they clogged the micropores, which reduced the specific surface. When the CNTs were homogeneously dispersed, the electrodes showed an increased specific surface area due to the added ‘surface’ by the CNTs.31


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Those results of electrochemical properties can be related to the performance of CDI. From the results, we suggest a few mechanisms of CNTs in the CDI electrode as conductive agents. By homogeneously dispersed CNTs, electrochemical properties and pore structure improve. Firstly, the EIS test revealed that the resistivity of the electrode had been drastically decreased. Although the shape of the curves was not clearly performed due to the irregular surface of the graphite current conductor, it is confirmed the resistivity of the electrode would decrease, and the effective voltage which can be applied to each part of the activated carbon active material would be higher. Second, which can be related to the first mechanism, owing to the reduced resistivity of the activated carbon powders, the ions can easily penetrate the inner mesopores of the activated carbon powders.32 For a low scan ratio (5 mV/s), the ions have sufficient time to be adsorbed into the inner mesopores of the activated carbon powders; however, for a high scan ratio (50 mV/s), the existence of homogeneously dispersed highly conductive CNTs play a greater role than that of a low scan ratio. Last, the increased specific surface area due to dispersed CNTs could be positively affected by the adsorption property of the electrode.31 However, for the NCNT dispersed electrode, which has saturated NCNTs, then the NCNTs could be aggregated, which would lower the specific surface area and the electrochemical properties. Those enhancing or degrading mechanisms of FCNTs and NCNTs are summarized in Figure S5.

Desalination properties and the mechanisms of the FCNT dispersed electrodes. For measuring desalination properties of CNT dispersed electrodes, pure activated carbon electrode and 1 wt.% CNT dispersed electrodes were used. Two types of characterizations were performed: one was the 250 ppm of NaCl dissolved D.I. water which was a relatively low concentration, and the other one was 1000 ppm of NaCl solution which was chosen for high-concentrated saline water to be desalinated. The experiments were conducted at the Siontech Co., Ltd. Photographs and a schematic description of the desalination system are shown in Figure 1. Interestingly, unlike the other CDI desalination electrode system, we used not the ion-selective membrane but positive and negative ion-selective polymers which were dip-coated on the surface of the electrode. (The schematic image of the ion-selective binder-coated electrode is depicted in Figure 2) As the saline water penetrates between the negative and positively charged electrodes, the sodium and chloride ions attach to the electrode, and the desalination property is measured via checking the conductivity of the outlet.



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Figure 7. Desalination properties of pure and 1 wt.% CNT dispersed electrode (a) at 250 ppm concentration NaCl, (b) at 1000 ppm concentration Figure 7 indicates the desalination property of the electrodes in a 250 ppm NaCl solution. The CNT dispersed electrode showed enhanced desalination. In this research, 5-180 seconds were set as one cycle. In this research, removal capacity of the electrodes was calculated by following equation 𝑃𝑒𝑟𝑐𝑒𝑛𝑡𝑎𝑔𝑒 𝑜𝑓 𝑠𝑎𝑙𝑡 𝑟𝑒𝑚𝑜𝑣𝑎𝑙 =

𝐶𝑎 𝐶0

× 100

(2)

Where 𝐶0 is an initial saline water concentration, and 𝐶𝑎 is an average value of desalinated salt during the given time(5-180 seconds). Average values of 5-180 seconds after the voltage was applied to pure activated carbon and 1 wt.% FCNT dispersed electrodes were 46.5 and 28.8 ppm, respectively. They show 81.4% and 88.5% desalination. Only 1 wt.% of CNTs can enhance about 7% desalination for a 250 ppm NaCl solution. However, 1 wt.% of a NCNT mixed electrode, which has a poor dispersion status, had a 42.1 ppm average concentration, which limited enhancement on CDI. The result can be compared to other results, and it is summarized in table S1. Novel carbon-based


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nanomaterials such as CNT and graphene have extraordinary specific surface area, electric conductivity, and easily controllable hydrophilicity. Therefore, some previous results which used carbon-based nanomaterials as the active material, show higher removal capacity than current result. However, the important point of this study is that the addition of a very small amount of CNT(0.1wt.%) to the activated carbon electrode, which is being commercialized on the due to its low cost, greatly improves desalination performance. The result with a 1000 ppm NaCl solution, which is high concentration saline water, shows a more significant finding (Figure 7(b)). If the desalination is calculated the same way as 250 ppm saline water (average of 5-180 seconds), desalination performance of the 1 wt.% FCNT dispersed electrode was not significantly enhanced and was from 34.1% to 34.4%. However, the desalting performance around the first 30 seconds was significantly enhanced. The average desalination between 5 and 30 seconds was increased from 66.8% to 80.7% by adding FCNTs. This graph gives a clear demonstration of the effect of homogeneous dispersed CNTs on desalination. The improvement mechanisms of the CNTs for desalination are similar to that of the electrochemical property. Due to reduced resistivity of the electrode, sodium and chloride ions can easily penetrate into the small mesopores of the activated carbon. Furthermore, there are more enhancing mechanisms by the distribution of CNTs. S. Porada et al. minutely explained that there are several important electrochemical reactions and processes in CDI electrodes.2,33 They are capacitive ion storage, ion kinetics, and chemical surface charge which are classified as non-Faradaic processes, and carbon redox reactions, water chemistry and carbon oxidation, which are classified as Faradaic reactions. Of course, the main reaction of the mechanism of CDI is capacitive ion storage, which is determined by the surface area and pore distribution. As mentioned earlier, CNTs can increase the surface area of the electrode. Acid-treated functionalized CNTs had carboxyl functional groups on their surfaces. Because of them, a “chemical surface charge” mechanism might occur and they accelerate H/OH transport rates around the micropores, and it can also influence the flux of sodium and chloride ions. This process can explain the drastically enhanced desalination performance in high concentration raw water. A durability test of two electrodes was also determined with 250 ppm saline water. The FCNT dispersed electrode showed enhanced desalination performance during the repetition (Figure 8). After 10 cycles of tests were repeated, desalination performances did not decrease after the first cycle. Our experiment confirmed that the addition of a small amount of homogeneously dispersed CNTs into commercially available electrodes can improve desalination performance without any side effects such as reduction in durability of the electrodes.



Figure 8. Durability test of the electrodes. (b) is an expanded graph of (a)

A Charge efficiency (η) is a useful tool to show the correlation between the amounts of charge applied and the amount of ion actually adsorbed. However, there was no significant difference in charge efficiency with and without CNT due to (from 97.4 to 97.6). In this study, ion-selective positive and negative binders were coated on electrodes to control external factors including dissolved oxygen. Therefore, it is considered that most of the applied current is used for the adsorption of ions because there are very few external elements that can reduce charge efficiency. The detailed calculation process about the charge efficiency is demonstrated the supporting information (Figure S6).

Conclusion In this research, we determined the effect of the dispersion status of CNTs as a conductive agent in the CDI electrode on the desalination property of the CDI process. Electrochemical properties of agglomerated (NCNTs) and homogeneously dispersed functionalized CNTs (FCNTs) containing an activated carbon electrode were compared in order to interpret the effect of the dispersion effect. Homogeneously dispersed CNTs reduced


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the resistivity of the electrode and increased the specific surface area of the electrode. However, without the functionalization process of the CNTs, they easily agglomerated and blocked the internal pores which could adsorb the ions. Specific capacitances of FCNTs dispersed were increased 6.6% (at 5 mV/s) and 13.3% (at 50 mV/s) by adding 1 wt.% FCNTs. We suppose that homogeneously dispersed CNTs reduce the resistivity and increase the specific surface area of the electrode. Furthermore, the FCNT dispersed electrode showed enhanced desalination performance. Two kinds of measurements were conducted: one was low-concentrated (250 ppm) the other was high-concentrated (1000 ppm) saline water. Above all, the high concentrate desalination more drastically improved the performance 13.9% at 1 wt.% of FCNTs. Increased surface areas due to homogeneously dispersed FCNTs, their surface functional groups and reduced electrode resistivity can upgrade the existing activated carbon electrode by adding a small amount of CNTs. The bottom line of this research is based on the commercial activated carbon electrode. Grafting with a small amount of nanomaterial (CNTs), desalination can be conspicuously improved. This concept will affect to the future application of CDI technology.

Acknowledgements This research was supported by Nano·Material Technology Development Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Science, ICT and Future Planning (2016M3A7B4905609).

Supporting Information SEM images, FT-IR, and Raman spectroscopy of CNTs; wettability of activated carbon electrodes; schematic illustration of the ion adsorbing mechanisms of CNT dispersed activated carbon electrode; current change during the desalination tests; and comparison of percentage of salt removal of the CDI electrodes from the literature



References (1)

Anderson, M. A.; Cudero, A. L.; Palma, J. Capacitive deionization as an electrochemical means of saving energy and delivering clean water. Comparison to present desalination practices: Will it compete? Electrochim. Acta 2010, 55 (12), 3845–3856. DOI : 10.1016/j.electacta.2010.02.012

(2)

Porada, S.; Zhao, R.; Van Der Wal, A.; Presser, V.; Biesheuvel, P. M. Review on the science and technology of water desalination by capacitive deionization. Prog. Mater. Sci. 2013, 58 (8), 1388–1442. DOI : 10.1016/j.pmatsci.2013.03.005

(3)

Li, H.; Liang, S.; Li, J.; He, L. The capacitive deionization behaviour of carbon nanotube and reduced graphene oxide composite. J. Mater. Chem. A 2013, 1, 6335–6341. DOI : 10.1039/c3ta10681k

(4)

Wimalasiri, Y.; Zou, L. Carbon nanotube/graphene composite for enhanced capacitive deionization performance. Carbon N. Y. 2013, 59, 464–471. DOI : 10.1016/j.carbon.2013.03.040

(5)

Jia, B.; Zhang, W. Preparation and Application of Electrodes in Capacitive Deionization (CDI): a Stateof-Art Review. Nanoscale Res. Lett. 2016, 11 (1), 64. DOI : 10.1186/s11671-016-1284-1

(6)

Peng, Z.; Zhang, D.; Yan, T.; Zhang, J.; Shi, L. Three-dimensional micro/mesoporous carbon composites with carbon nanotube networks for capacitive deionization. Appl. Surf. Sci. 2013, 282, 965– 973. DOI : 10.1016/j.apsusc.2013.06.107

(7)

Peng, Z.; Zhang, D.; Shi, L.; Yan, T. High performance ordered mesoporous carbon/carbon nanotube composite electrodes for capacitive deionization. J. Mater. Chem. 2012, 22 (14), 6603. DOI 10.1039/c2jm16735b

(8)

Wang, Z.; Yan, T.; Shi, L.; Zhang, D. In Situ Expanding Pores of Dodecahedron-like Carbon Frameworks Derived from MOFs for Enhanced Capacitive Deionization. ACS Appl. Mater. Interfaces 2017, 9 (17), 15068–15078. DOI : 10.1021/acsami.7b02712

(9)

Zhang, D.; Yan, T.; Shi, L.; Peng, Z.; Wen, X.; Zhang, J. Enhanced capacitive deionization performance of graphene/carbon nanotube composites. J. Mater. Chem. 2012, 22 (29), 14696. DOI : 10.1039/c2jm31393f

(10)

Xu, D.; Tong, Y.; Yan, T.; Shi, L.; Zhang, D. N,P-Codoped Meso-/Microporous Carbon Derived from


Page 18 of 30

Page 19 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Biomass Materials via a Dual-Activation Strategy as High-Performance Electrodes for Deionization Capacitors. ACS Sustain. Chem. Eng. 2017, 5 (7), 5810–5819. DOI : 10.1021/acssuschemeng.7b00551 (11)

Zornitta, R. L.; Lado, J. J.; Anderson, M. A.; Ruotolo, L. A. M. Effect of electrode properties and operational parameters on capacitive deionization using low-cost commercial carbons. Sep. Purif. Technol. 2016, 158 (54), 39–52. DOI : 10.1016/j.seppur.2015.11.043

(12)

Emmenegger, C.; Mauron, P.; Sudan, P.; Wenger, P.; Hermann, V.; Gallay, R.; Züttel, A. Investigation of electrochemical double-layer (ECDL) capacitors electrodes based on carbon nanotubes and activated carbon materials. J. Power Sources 2003, 124 (1), 321–329. DOI : 10.1016/S0378-7753(03)00590-1

(13)

Li, H.; Pan, L.; Nie, C.; Liu, Y.; Sun, Z. Reduced graphene oxide and activated carbon composites for capacitive deionization. J. Mater. Chem. 2012, 22 (31), 15556. DOI : 10.1039/c2jm32207b

(14)

Zhang, D.; Wen, X.; Shi, L.; Yan, T.; Zhang, J. Enhanced capacitive deionization of graphene/mesoporous carbon composites. Nanoscale 2012, 4 (17), 5440. DOI : 10.1039/c2nr31154b

(15)

Borenstien, A.; Noked, M.; Okashy, S.; Aurbach, D. Composite Carbon Nano-Tubes (CNT)/Activated Carbon Electrodes for Non-Aqueous Super Capacitors Using Organic Electrolyte Solutions. J. Electrochem. Soc. 2013, 160 (8), A1282–A1285. DOI : 10.1149/2.103308jes

(16)

Liu, Y.; Chen, T.; Lu, T.; Sun, Z.; Chua, D. H. C.; Pan, L. Nitrogen-doped porous carbon spheres for highly efficient capacitive deionization. Electrochim. Acta 2015, 158, 403–409. DOI : 10.1016/j.electacta.2015.01.179

(17)

Dong, Q.; Wang, G.; Wu, T.; Peng, S.; Qiu, J. Enhancing capacitive deionization performance of electrospun activated carbon nanofibers by coupling with carbon nanotubes. J. Colloid Interface Sci. 2015, 446, 373–378. DOI : 10.1016/j.jcis.2014.12.065

(18)

Xu, X.; Sun, Z.; Chua, D. H. C.; Pan, L. Novel nitrogen doped graphene sponge with ultrahigh capacitive deionization performance. Sci. Rep. 2015, 5, 11225. DOI : 10.1038/srep11225

(19)

Xu, X.; Wang, M.; Liu, Y.; Lu, T.; Pan, L. Metal–organic framework-engaged formation of a hierarchical hybrid with carbon nanotube inserted porous carbon polyhedra for highly efficient capacitive deionization. J. Mater. Chem. A 2016, 4 (15), 5467–5473. DOI : 10.1039/C6TA00618C



(20)

Macías, C.; Rasines, G.; Lavela, P.; Zafra, M. C.; Tirado, J. L.; O. Ania, C. Mn-Containing N-Doped Monolithic Carbon Aerogels with Enhanced Macroporosity as Electrodes for Capacitive Deionization. ACS Sustain. Chem. Eng. 2016, 4 (5), 2487–2494. DOI : 10.1021/acssuschemeng.5b01444

(21)

Zhang, H.; Liang, P.; Bian, Y.; Jiang, Y.; Sun, X.; Zhang, C.; Huang, X.; Wei, F. Moderately oxidized graphene–carbon nanotubes hybrid for high performance capacitive deionization. RSC Adv. 2016, 6 (64), 58907–58915. DOI : 10.1039/C6RA10088K

(22)

Li, Y.; Qi, J.; Li, J.; Shen, J.; Liu, Y.; Sun, X.; Shen, J.; Han, W.; Wang, L. Nitrogen-Doped Hollow Mesoporous Carbon Spheres for Efficient Water Desalination by Capacitive Deionization. ACS Sustain. Chem. Eng. 2017, 5 (8), 6635–6644. DOI : 10.1021/acssuschemeng.7b00884

(23)

Rance, G. A.; Marsh, D. H.; Bourne, S. J.; Reade, T. J.; Khlobystov, A. N. Van der waals interactions between nanotubes and nanoparticles for controlled assembly of composite nanostructures. ACS Nano 2010, 4 (8), 4920–4928. DOI : 10.1021/nn101287u

(24)

Gobre, V. V; Tkatchenko, A. Scaling laws for van der Waals interactions in nanostructured materials. Nat. Commun. 2013, 4, 2341. DOI : 10.1038/ncomms3341

(25)

Bikiaris, D.; Vassiliou, A.; Chrissafis, K.; Paraskevopoulos, K. M.; Jannakoudakis, A.; Docoslis, A. Effect of acid treated multi-walled carbon nanotubes on the mechanical, permeability, thermal properties and thermo-oxidative stability of isotactic polypropylene. Polym. Degrad. Stab. 2008, 93 (5), 952–967. DOI : 10.1016/j.polymdegradstab.2008.01.033

(26)

Cha, S. I.; Kim, K. T.; Arshad, S. N.; Mo, C. B.; Hong, S. H. Extraordinary Strengthening Effect of Carbon Nanotubes in Metal-Matrix Nanocomposites Processed by Molecular-Level Mixing. Adv. Mater. 2005, 17 (11), 1377. DOI : 10.1002/adma.200401933

(27)

Li, Y.; Li, H.; Petz, A.; Kunsági-Máté, S. Reducing structural defects and improving homogeneity of nitric acid treated multi-walled carbon nanotubes. Carbon N. Y. 2015, 93, 515–522. DOI : 10.1016/j.carbon.2015.05.068

(28)

Lou, X. Growth of ultrathin mesoporous Co 3 O 4 nanosheet arrays on Ni foam for high-performance electrochemical capacitors. Energy Environ. Sci. 2012, 1–9. DOI : 10.1039/c2ee21745g

(29)

Deng, M.-J.; Wang, C.-C.; Ho, P.-J.; Lin, C.-M.; Chen, J.-M.; Lu, K.-T. Facile electrochemical


Page 20 of 30

Page 21 of 30 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


synthesis of 3D nano-architectured CuO electrodes for high-performance supercapacitors. J. Mater. Chem. A 2014, 2 (32), 12857. DOI : 10.1039/C4TA02444C (30)

Datsyuk, V.; Kalyva, M.; Papagelis, K.; Parthenios, J.; Tasis, D.; Siokou, A.; Kallitsis, I.; Galiotis, C. Chemical oxidation of multiwalled carbon nanotubes. Carbon N. Y. 2008, 46 (6), 833–840. DOI : 10.1016/j.carbon.2008.02.012

(31)

Kim, Y. K.; Cha, S. I.; Hong, S. H.; Jeong, Y. J. A new hybrid architecture consisting of highly mesoporous CNT/carbon nanofibers from starch. J. Mater. Chem. 2012, 22 (38), 20554–20560. DOI : 10.1039/c2jm32631k

(32)

Xue, Y.; Ding, Y.; Niu, J.; Xia, Z.; Roy, A.; Chen, H.; Qu, J.; Wang, Z. L.; Dai, L. Rationally designed graphene-nanotube 3D architectures with a seamless nodal junction for efficient energy conversion and storage. Sci. Adv. 2015, 1 (8), e1400198. DOI : 10.1126/sciadv.1400198

(33)

Suss, M. E.; Porada, S.; Sun, X.; Biesheuvel, P. M.; Yoon, J.; Presser, V. Water desalination via capacitive deionization: what is it and what can we expect from it? Energy Environ. Sci. 2015, 8 (8), 2296–2319. DOI : 10.1039/C5EE00519A



For Table of Contents Use Only

Desalination performance of commercial activated carbon capacitive deionization (CDI) electrode was enhanced by adding a small amounts of homogeneously dispersed CNTs.


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Figure 2. Schematic design for the structure of the CDI electrode in this research 105x72mm (300 x 300 DPI)


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Figure 3. Microstructures of the electrode measured by SEM. (a) activated carbon active materials before being mixed, (b) Activated carbon coated on a graphite foil, (c) 1 wt.% NCNT dispersed electrode, and (d) 1 wt.% FCNT dispersed electrode 105x95mm (300 x 300 DPI)



Figure 4. (a) EIS analysis of pure, NCNT and FCNT dispersed electrode (b) magnified 0.1 and 1 wt.% FCNT dispersed activated carbon electrode 84x125mm (300 x 300 DPI)


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Figure 5. Cyclic voltammetry analysis of the electrodes (a) and (b) at a 5 mV/s scan rate, and (c) and (d) indicate 50 mV/s which indicates a high scan rate 105x80mm (300 x 300 DPI)



Figure 6. BET specific surface area analysis of the electrodes 84x62mm (300 x 300 DPI)


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Figure 7. Desalination properties of pure and 1 wt.% CNT dispersed electrode (a) at 250 ppm concentration NaCl, (b) at 1000 ppm concentration 84x123mm (300 x 300 DPI)



Figure 8. Durability test of the electrodes. (b) is an expanded graph of (a) 84x128mm (300 x 300 DPI)


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Enhanced Capacitive Deionization by Dispersion of CNTs in Activated

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