Grafting the Charged Functional Groups on Carbon Nanotubes for

Subscriber access provided by UNIV OF LOUISIANA

Functional Nanostructured Materials (including low-D carbon)

Grafting the Charged Functional Groups on Carbon Nanotubes for Improving the Efficiency and Stability of Capacitive Deionization Process Dongya Ma, Yanmeng Cai, Yue Wang, Shichang Xu, Jixiao Wang, and Maaz Ullah Khan ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20588 • Publication Date (Web): 23 Apr 2019 Downloaded from http://pubs.acs.org on April 24, 2019

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 39 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

ACS Applied Materials & Interfaces

Grafting the Charged Functional Groups on Carbon Nanotubes for Improving the Efficiency and Stability of Capacitive Deionization Process Dongya Maa,b,c,d, Yanmeng Caia,b,c,d, Yue Wanga,b,c,d*, Shichang Xua,c,d, Jixiao Wanga,b,c,d, Maaz Ullah Khana,b,c,d a.

Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China

b.

State Key Laboratory of Chemical Engineering, Tianjin 300072, PR China

c.

Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin University, Tianjin 300072, PR China

d.

Collaborative Innovation Center of Chemical Science and Engineering (Tianjin), Tianjin 300072, China

KEYWORDS: Charged functional layer; Electrode oxidation; Potential of zero charge; Cyclic stability; Capacitive deionization ABSTRACT: In the capacitive deionization (CDI) process, the degradation of desalting performance is predominantly due to the co-ions expulsion effect and electrode oxidation. To overcome these complications, carbon nanotubes grafted with amine and sulfonic functional groups respectively were prepared and used as the CDI electrodes. The structural characterizations and performance tests confirmed that a uniform functional layer was formed on the surface of the modified electrodes and it enhanced the ion selectivity and wettability of the electrode surface. Moreover, the effects of the functional layer on the electrode stability were investigated by circulating CV tests and desalination tests. The positive shift value of the potential of zero charge

ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 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

(PZC) for the as-prepared electrodes was tested as a quantitative indication for their possible surface oxidation during cyclic tests. Analysis of the PZC variation and desalting performance demonstrated that the excellent desalting stability was achieved by the Cell N-S assembled with the ammoniated CNTs electrode as anode and sulfonated CNTs electrode as cathode. Because the functional layer could preserve the pores system on the modified electrodes and diminish the parasitic reactions that exacerbate the electrode oxidation. This work provides an effective strategy for promoting the electrode performance and prolonging the life of the electrode. 1. INTRODUCTION The shortage of freshwater resources and contamination of water quality have boosted the development of desalination technologies towards low cost and low energy consumption. As a novel desalination technology, capacitive deionization (CDI) has pulled in much consideration due to its outstanding features such as low external power supply, high energy efficiency and high ion removal rate.1, 2 Typically, the working principle of CDI is similar to that of a capacitor, in which the feed solution is circulated in a channel being in contact with two parallel placed fixed CDI electrodes. When an external voltage (e.g., 1.2 V) is applied across the CDI cell, charged ions in the feed solution are adsorbed and held on the surface or into the pores of the electrodes via the formation of the electrical double layers (EDL) at the electrode/solution interface and the adsorbed ions can be released back to the bulk stream once the electrodes are shorted or open.3, 4 Owing to this reversible adsorption/desorption process, CDI technology is recognized as a promising desalination technology for fresh water.


Page 2 of 39

Page 3 of 39 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


Based upon the above mentioned principle of the CDI process, the chosen electrode material is a critical factor for achieving excellent CDI performance. Currently, considerable progress has been made in the development of electrode materials that possess high specific area, suitable pore structures and superior conductivity.2,

5, 6

Among these researches, carbon materials, such as the activated carbon (AC),7, 8 carbon nanofiber (ACF),9 mesoporous carbon (MC),10 carbon aerogel (CA),11 graphene,12, 13 carbon nanotubes (CNTs),14 hierarchically porous carbon 15, 16 and their composites,17, 18

have been investigated as a suitable candidate for the CDI electrode materials due to

their excellent physical and chemical properties. In fact, it is found in further research that the surface properties of carbon electrodes also have an effect on the desalting performance.19- 21 On the one hand, the hydrophobicity of the carbon electrode surface retards the interaction of the solution with the electrode, thereby decreasing the rate and efficiency of ion adsorption. On the other hand, the co-ions effect on the carbon electrode surface cannot be neglected in the CDI process. In the charging process, the adsorption of the counter-ions (counter charge with the electrode) and undesirable repulsion of the co-ions (same charge with the electrode) always proceed simultaneously on the same electrode.22 This is so called the co-ions effect, which results in the decline of desalting performance and the increment of the energy consumption in the CDI process. To deal with these issues, a novel and effective strategy is proposed to modify the surface chemical properties of the carbon material by grafting charged functional groups. These charged groups on the material surface can improve the surface



wettability and quicken the ions transfer from solution to the carbon electrode.23, 24 Also, the functional layer formed by uniformly grafting a charged group on the electrode surface has the same principle of ion selectivity as that of the membrane capacitive deionization (MCDI) in which the ion exchange membrane is placed on the surface of the electrode in the CDI process. For the MCDI, the ion-exchange membranes efficiently facilitate the adsorption of the counter-ions and decline the expulsion of the co-ions, so the desalination capacity and salt removal efficiency are improved dramatically.25, 26 However, an additional contact resistance is inevitably introduced between the ion-exchange membrane and the electrode, which increases the dissipation of energy during the desalination process. Moreover, the issue of membrane fouling is also a time and cost-consuming tough topic. So the high cost and short service life of the ion exchange membrane restrict the further application of the MCDI technology.27 Compared with the ion exchange membrane, the ion-selective functional layer bonded covalently on the electrode surface is a more stable and cost-effective strategy, which also makes the modified electrodes with less resistance.28 Therefore, the modified carbon materials with the ion-selective functional layer should be extensively explored for the CDI scale application. Although great improvements have been made for the CDI performance by exploring suitable electrode materials, the stability of the CDI electrode is still a critical problem to be addressed for the CDI practical application. The emerging CDI technology is expected to maintain robust and excellent adsorption performance after multi-cyclic adsorption/desorption process. In fact, the CDI cells always suffer from performance


Page 4 of 39

Page 5 of 39 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


degradation after long-term operation.29, 30 The probable parasitic reactions under the applied voltage, such as water splitting and carbon oxidation, occur on the electrodes and lead to a decline in electrode performance.31 During the cyclic desalination tests, the redox reactions are accompanied by the formation of negative oxygen-containing groups on the electrode, which can shift the potential of zero charge (PZC) of the electrode in the positive direction.32 This positive migration of PZC exacerbates the coions effect of the anode and deteriorates the charge efficiency.33, 34 To maintain the longterm stability of the CDI electrode, several effective measures have been implemented, including the optimization of the applied voltage, the adoption of inverted CDI system and the use of ion-exchange membranes in the CDI process.26, 29, 35 Among them, ionexchange membrane applied into the CDI cell is considered to be a viable method that can alleviate the performance degradation by inhibiting the parasitic reactions and coions effect. This interesting phenomenon may also be observed in the CDI cell assembled by the modified electrodes due to the presence of the functional layer on the electrode surface. A few researchers also proposed that the carbon electrode modified by the charged functional groups can effectively suppress the co-ions effect in the desalting process.23,

28

Unfortunately, the cyclic stability of the modified carbon

electrodes has not been investigated in the CDI process. Especially, the effects of the functional layer on the electrode stability still remain unclear so far. In this work, the ammoniated CNTs and sulfonated CNTs were prepared by the chemical modification method and used as the CDI electrode material. The structure and hydrophilicity of the modified CNTs materials were characterized in terms of the



spectroscopic and dynamic contact angle analyses. And the effects of the grafted functional groups on the modified electrodes were investigated by the electrochemical tests, desalting tests and electrosorption kinetics analysis. In our research, it is found that the grafted uniformly amine and sulfonic groups as the functional layer could enhance the wettability and ion selectivity of the electrode. These features made the modified electrodes with the excellent electrochemical performance and desalting performance. Moreover, the cyclic stability of the modified electrodes was measured by repeating CV tests and desalting tests. And the potential of zero charge (PZC) was invoked to evaluate the oxidation degree of the electrode. Results obtained from these tests indicated that the uniform functional layer made the modified electrodes with oxidation resistance, thereby improving the electrode stability. 2. EXPERIMENTAL SECTION 2.1 Chemical modification of carbon nanotubes. Before modification, the raw CNTs (multi-walled carbon nanotubes, > 98% purity, diameter 20-30 nm, length 10-30 μm) were treated by 10 M nitric acid at 80 ℃ for 4 h, which not only removed the metal catalysts and amorphous carbon, but also achieved partially oxidized CNTs. Figure 1 depicts a schematic illustration for the chemical modification of CNTs. For the ammoniation treatment, the pretreated CNTs was modified by using 3aminopropyltriethoxysilane (APTES), as shown in reaction step (1)-(4). APTES was represented by the following formula: H2N-(CH2)3-Si-(OC2H5)3. First of all, (OC2H5)3 in APTES was easily hydrolyzed to form a trisilanol in that it was the hydrolyzable alkoxy group. Then, these hydroxyl groups of the hydrolyzed APTES could be


Page 6 of 39

Page 7 of 39 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


covalently bonded to the carboxyl and hydroxyl groups produced on the partially oxidized CNTs.23, 36 Typically, 0.2 g of pretreated CNTs was dispersed in the acetone for 30 min. Then 10 mL APTES was dropped into the above solution and reacted at 85 ℃ for 4 h. After that, the obtained mixture was filtered and washed repeatedly with acetone to remove any APTES remnant. The cake was dried at a temperature of 80 ℃ for 3 h to evaporate the excess solvent. The amine group (-NH3+) was introduced on the surface of CNTs to increase the amount of positive functional group. In the sulfonation treatment, the chemical reduction method was adopted.37 And defects on the surface of the pretreated CNTs promoted the sulfonation modification reaction.38 As shown in reaction step (5)-(7), the 4-benzenediazonium sulfonate was first synthesized by dissolving 5.2 g of sulfanilic acid into1 M HCl solution and then slowly adding 10% excess of 1 M NaNO2 to the above solution under an ice bath. After stirring for 1 h, the white precipitate was formed and separated by filtration. Then, the obtained 4-Benzenediazonium sulfonate was added into 0.2 g of pretreated CNTs suspension solution under an ice bath. Subsequently, 90 mL 50 wt% hypophosphorous acid (H3PO2) solution as a reducing agent was added dropwise to this suspension solution. In the end, the obtained sulfonated CNTs was washed with deionized water and dried at 80 ℃ overnight. The sulfonic group (-SO3-) was introduced on the surface of CNTs to increase the amount of negative functional group.



Figure 1. Schematic illustration for the chemical modification of CNTs, and (1)-(4) and (5)-(7) are the reaction steps of ammoniation process and sulfonation process. 2.2 Morphology characterizations. The morphology and structure of ammoniated CNTs and sulfonated CNTs were characterized by field emission scanning electron microscope (SEM, Nanosem 430, Netherlands), field emission transmission electron microscope (TEM, Tecnai G2 F20, Netherlands) with electron energy loss spectroscopy (EELS), Fourier transform infrared (FTIR) spectroscopy (MultiGas 2030, America) and X-ray photoelectron spectroscopy (XPS) (PHI5000 Versa Probe, Japan). The surface wettability of modified CNTs was tested by a dynamic contact angle analysis method by using the DataPhisics contact angle analyzer (DataPhysics Instruments, Germany). 2.3 Preparation of the modified CNTs electrodes. The electrodes used in this paper were prepared by the coating method.39 Taking the ammoniated CNTs electrode, for example, the electrode slurry was prepared by mixing the ammoniated CNTs (active


Page 8 of 39

Page 9 of 39 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


component, 0.8 g), graphite powder (conductivity agent, 0.1 g) and polyvinylidene fluoride (binder, 0.1 g) with 5 g N-methyl-2-pyrrolidone (NMP). After stirring for 12 h, the uniform electrode slurry was coated onto the graphite paper by using the H-type coating applicator with a groove depth of 0.4 mm. The resulting electrode with a dimension of 80 mm×160 mm was dried at 40 ℃ for 12 h to remove the NMP residual. The CNTs and sulfonated CNTs electrodes were also prepared by using the same procedure. For the three as-prepared electrodes, the dried thickness of the electrode material coating on the graphite paper was around 0.1 mm. 2.4 Measurements of electrochemical performance. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were investigated by using the electrochemical workstation (PARSTATTM 4000A) and a three-electrode system. The CNTs electrode, ammoniated CNTs electrode and sulfonated CNTs electrode, resized to 50 mm × 25 mm (length × width), were set as the working electrode in turn, while a platinum electrode and a standard calomel electrode (SCE) were used as the counter electrode and the reference electrode, respectively. The mass of the active component in the three as-prepared electrodes was all about 17 mg. Both CV and EIS tests were all performed in 1 M NaCl solution. The CV test of three electrodes was conducted with the potential range of -0.2 - 0.6 V at the scan rate of 5 mV/s. The specific capacitance (C, F·g-1) was calculated from CV curves based on the following equation (1)

C=



E2

E1

idE

2mv( E 2 − E1)


(1)


Page 10 of 39

where E1, E2 (V) are the initial and final potential respectively, i (A) is the response current, v (V/s) is the scan rate and m (g) is the mass of the active component in the electrode. The EIS test of three electrodes was performed at a potential of 0.0 V with a frequency range of 10 KHz to 0.01 Hz. The specific capacitance (Ci, F·g-1) was derived from the angular frequency (ω) and imaginary part of the impedance spectrum ( Z＂) based on the following equation (2)

Ci=

1  Z ''

(2)

In addition, the potential of zero charge (PZC) information was determined by the capacitance data under different potential that was collected by EIS measurements. The potential range of EIS measurements was about -0.6-0.6 (V vs SCE). The PZC was obtained from the minimum value of the capacitance plot (as a function of potential) at the frequency of 0.01 Hz. 2.5 Measurements of CDI performance. To explore the effects of surface modification on the electrode performance, especially the electrode stability, four CDI cells were configured by using the as-prepared electrodes. The corresponding information about the four CDI cells was displayed in Table 1. Table 1. Electrode assembling of the four CDI cells Cell name

Anode

Cathode

Type

Cell C-C Cell N-C Cell C-S Cell N-S

CNTs Ammoniated CNTs CNTs Ammoniated CNTs

CNTs CNTs Sulfonated CNTs Sulfonated CNTs

Symmetric Asymmetric Asymmetric Asymmetric

The CDI tests were conducted using the experimental setup as shown in Figure 2. The working voltage of 1.2 V and 0 V controlled by the power controller was applied


Page 11 of 39 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


on the CDI cell in the adsorption and desorption processes respectively. 80 mL of NaCl solution (500ppm, ~1050 μS/cm) contained in the tank was recirculated through the CDI cell by the peristaltic pump at a flow rate of 25 mL/min. The on-line conductivity meter was connected at the outlet of the cell to track the changes in conductivity. Moreover, the size of the anode and cathode was both 100 mm × 80 mm (length × width) and the quality of the electrode materials on each graphite paper was about 0.1 g for these four CDI cells. The salt-removal capacity (mg/g) can be derived from the equation (3)

mt =

 ( k 0 − kf ) V m

(3)

Where α is the coefficient between the conductivity and concentration of NaCl solution (5.2×10−4 mg/(μS·cm2)); V (mL) is the volume of NaCl solution; m (g) is the total quality of the electrode materials on graphite papers. The corresponding charge efficiency (Λ, %) can be calculated by the equation (4)

=

F 100 M

(4)

Where F is the Faraday’s constant (96,485 C/mol); M is the molecular weight of NaCl (58.5 g/mol); Σ (charge, C/g) is achieved by integrating the corresponding current curve.



Figure 2. Flow diagram of the CDI evaluation system. 3. RESULTS AND DISCUSSION 3.1 Morphological and structural characterizations. The morphology of CNTs, ammoniated CNTs and sulfonated CNTs was investigated by SEM, as illustrated in Figure 3a-c. It can be clearly observed that the ammoniated CNTs and sulfonated CNTs still retain the main features of CNTs after the modification treatment. Whereas, the surface roughness of two modified CNTs is larger than that of CNTs. The corresponding elemental mapping of two modified CNTs in Figure 3d-e shows that the N element and S element are uniformly distributed on the surface of the ammoniated CNTs and sulfonated CNTs, respectively. These characteristics confirm that the amine and sulfonic groups are successfully attached on the CNT surface by covalent bond.


Page 12 of 39

Page 13 of 39 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


Figure 3. SEM images of CNTs (a), ammoniated CNTs (b) and sulfonated CNTs (c), and the corresponding elemental mappings of ammoniated CNTs (d) and sulfonated CNTs (e). The functional groups on the surface of the modified CNTs were confirmed by FTIR spectra as shown in Figure 4a. Compared with CNTs, in addition to the presence of three major signals about O-H, C-H, and C=C stretching vibrations observed at ~3421, ~2914/2839 and ~1663 cm-1 respectively, the newly emerging peaks in the spectra of the modified CNTs indicate the changes on the surface groups of CNTs after the chemical treatments.40 In the ammoniated CNTs spectrum, the peaks detected at 3480 cm-1 (overlapped signals for N-H and O-H stretching vibrations), 1593 cm-1 (N-H bending vibration) and 1134 cm-1 (C-N stretching vibration) reveal the presence of NH2 group in the ammoniated CNTs. For sulfonated CNTs, the new peaks at 571, 1034 and 1172 cm-1 are ascribed to the stretching vibration of S-O, the stretching vibration and symmetric stretching of S=O, indicating the formation of sulfonic group (-SO3-) in



the sulfonated CNTs.23, 41 XPS measurements were carried out to analyze the surface species and atomic ratio of the three materials. As can be seen from Figure 4b, two typical peaks of C1s (285 eV) and O1s (531 eV) are all presented in the spectra of the three materials. The newly formed peaks at 400 eV and 102 eV corresponding to N1s and Si2p respectively are only detected in the XPS spectrum of the ammoniated CNTs. Also, the additional S2p peak at 168 eV can be found in the XPS spectrum of the sulfonated CNTs.20 The surface atomic content of these materials is shown in Table 2, which clearly shows that the nitrogen content of the ammoniated CNTs and sulfur content of the sulfonated CNTs are 2.98% and 1.19%, respectively. Compared with CNTs, the atomic ratio of C decreases whereas the content of O increases for the modified CNTs, which specified an increased amount of O-containing functional groups after the chemical modification. The existence of N element in ammoniated CNTs and S element in sulfonated CNTs confirms that the modified CNTs materials with the amine group or sulfonic group have been successfully prepared. It is corresponding to the result of FTIR. Due to the presence of the functional groups, the surface of the modified electrodes possesses a charging property, which contributes to improve the electrode performance.


Page 14 of 39

Page 15 of 39 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


Figure 4. FTIR spectra (a) and XPS spectra (b) of CNTs, ammoniated CNTs and sulfonated CNTs, and the inset is S2p peak. Table 2. Surface atomic content of the three materials sample CNTs Ammoniated CNTs Sulfonated CNTs

Content percentage (%) C1s

O1s

N1s

S2p

Si2p

96.49 82.89 94.41

3.51 10.13 4.40

0 2.98 0

0 0 1.19

0 4.00 0

In order to evaluate the effect of the functional groups on the wettability of the electrode surface, the water contact angle measurements were conducted and shown in Figure 5. As is well known, when the static contact angle is greater than 90°, the electrode surface is considered to be hydrophobic, and when it is less than 90°, the electrode surface is hydrophilic. Also, the smaller the contact angle is, the better the hydrophilicity of the materials is. Obviously, the initial contact angle of water droplet on the CNTs electrode (97°) is higher than that of the ammoniated CNTs electrode (50°) and sulfonated CNTs electrode (81°), indicating the enhanced wettability of modified electrodes. The difference in wettability between the ammoniated CNTs and sulfonated CNTs electrodes mainly result from the content of the functional groups. Based upon the results of XPS tests, the nitrogen content of the ammoniated CNTs is higher than the sulfur content of the sulfonated CNTs. Higher content of the amine group is more conducive to promoting the wettability of the ammoniated CNTs electrode. Moreover, the remaining hydroxyl groups on the hydrolyzed APTES also promotes the wettability of the ammoniated CNTs electrode. As can be seen from Figure 5, the adsorption time at which the water droplets reached a zero contact angle on the surface of the ammoniated CNTs electrode (4 minutes) and the sulfonated CNTs electrode (5 minutes)



Page 16 of 39

is shorter than that of the CNTs electrode (8 minutes). The improved wetting rate of the modified electrodes can be largely ascribed to the interaction of the functional groups with polar hydroxyl groups in the water molecules.23,

42

The superior solution

compatibility for the modified electrodes is beneficial to the ion transportation, which will enhance the capacitance and desalination performance.

Figure 5. The contact angles (θ) of water droplets on the surface of CNTs, sulfonated CNTs and ammoniated CNTs electrodes as a function of contact time. 3.2 Electrochemical characterizations. The electrochemical measurements were conducted to evaluate the electrosorption performance of the CNTs, ammoniated CNTs and sulfonated CNTs electrodes. Figure 6a shows the CV curves of the three asprepared electrodes in the potential range from -0.2 V to 0.6 V. These CV curves maintain quasi-rectangular shapes and no evident oxidation/reduction peaks are observed in the scope of the given potential window, implying that the adsorption of ions on the electrode follows the ideal capacitive electrical double layer (EDLC) mechanism.43 After the forward and reverse potential sweeps, the closed CV curves are still symmetric about the X-axis, indicating a quick and efficient charge propagation capability of the electrodes. Also, the closed area of the CV curve can be used to calculate the specific capacitance. Obviously, the encircled area of CNTs electrode is


Page 17 of 39 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


smaller than that of the modified electrodes. According to the equation (1), the specific capacitances of the CNTs, ammoniated CNTs and sulfonated CNTs electrodes are 30.04 F/g, 56.54 F/g and 49.36 F/g, respectively. The specific capacitances of the ammoniated CNTs and sulfonated CNTs electrodes are about 1.9 times and 1.6 times that of CNTs electrode. For the modified electrodes, the significant improvement in capacitance can be ascribed to the following facts: firstly, good wettability of the modified electrodes promotes the interaction between electrode and salt solution, thereby enhancing the transportation of electrons and ions; 44 secondly, the modified electrodes with different charged groups greatly reduce the diffusion resistance of ions by selectively transferring ions, which is proved by the EIS experiment as shown below.

Figure 6. Electrochemical properties for the CNTs, ammoniated CNTs and sulfonated CNTs electrodes tested in 1M NaCl solution: CV curves at the scanning rate of 5 mV/s (a); Charging resistance as a function of the frequency (b), and the inset is the capacitance in the frequency range from 0.01 Hz to 10 kHz. EIS measurements were further performed to assess the charging resistance and dynamic capacitance for the three as-prepared electrodes. The charging resistance of the CNTs, ammoniated CNTs and sulfonated CNTs electrodes as a function of the



frequency are shown in Figure 6b. This charging resistance characterizes the diffusion and transfer of the ions from the electrolyte into the electrode pores, which is calculated by subtracting the impedance at a high frequency of 10 kHz from the real impedance at a certain frequency.45, 46 In Figure 6b, the charging resistance of the two modified electrodes is lower than that of the CNTs electrodes especially in the low frequency range between 0.01 Hz and 1 Hz. The significant decrease in charging resistance for the modified electrodes is mainly due to the good wettability and ion selectivity on the electrode surface, which is consistent with the conclusions of CV tests. While in the high frequency range from 1 Hz to 10 kHz, the advantages of the two modified electrodes in terms of charging resistance are not more prominent than the low frequencies and this can be explained by the fact that the charging and discharging processes of the ions exist only on the electrode surface rather than inside the electrode. Based on the equation (2), the dynamic capacitance for the three electrodes is exhibited in the inset of Figure 6b. The capacitances of the three electrodes decline sharply in the low frequency range and tend to be flat in the high frequency range. The high capacitance is also due to the fact that the ions can reach the internal adsorption site of the electrode at low frequencies. Furthermore, the capacitances of the modified electrodes are higher than that of the CNTs electrode, which is in accordance with the result of CV tests. All of the above electrochemical tests illustrate that the functional layer on the electrode surface can enhance the electrochemical properties of the modified electrodes. At last, the difference in electrochemical properties for the ammoniated CNTs and sulfonated CNTs electrodes depends on the content of the


Page 18 of 39

Page 19 of 39 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


functional groups. 3.3 The desalting performance of four CDI cells. To investigate the desalting behavior of the modified electrodes, four CDI cells were assembled and measured in 500 ppm NaCl solution at a constant voltage of 1.2 V, and the electrode combinations of these CDI cells are shown in Section 2.5. For the CDI cells, the variation curves of conductivity over time are depicted in Figure 7a. The obvious decrease of the conductivity for all CDI cells can be observed in the initial 1500 s and then the conductivity changes gently, which suggest that the salt ions are effectively adsorbed onto the electrodes. The corresponding current curves of four CDI cells are presented in Figure 7b. These current curves decline as the adsorption process proceeds. According to the equation (3) and (4), the electrosorption capacity and charge efficiency for these CDI cells are calculated and listed in Table 3. It clearly displays that the enhanced desalting performance can be achieved when the CDI cells contain the ammoniated CNTs electrode as the anode or the sulfonated CNTs electrode as the cathode. For instance, the electrosorption capacity and charge efficiency of the Cell NC and Cell C-S are higher than those of Cell C-C. Especially, the Cell N-S possesses the largest electrosorption capacity (6.18 mg/g) and charge efficiency (62.4%), which is 1.4 and 2.2 times those of Cell C-C, respectively. These results further confirm that the functional layer on the modified electrode surface plays a positive effect on the desalting performance. The ammoniated CNTs electrode with positive surface charge preferentially adsorbs anions and repels cations near the electrode to eliminate the coion effect. When the ammoniated CNTs electrode is used as the anode in the CDI



Page 20 of 39

process, the anions in the solution will be adsorbed under the driving force from the surface functional group and the applied potential. Simultaneously, the additional cations in the solution will also be adsorbed on the opposite electrode to keep the solution electrically neutral, thereby improving the performance of the CDI cell.47 Similarly, the sulfonated CNTs electrode with the negative surface charge tends to adsorb cations when used as the cathode. And the performance of the anode will also be promoted at the same time. Moreover, the excellent wettability of the modified electrodes makes it easier for the salt solution to interact with the electrode, which can also facilitate the desalting performance.3

Figure 7. CDI performance for four CDI cells: conductivity curves (a), current curves (b), and pseudo-first-order kinetics curves (c) in 500 ppm NaCl solution at 1.2 V. Table 3. The desalting performance and parameters of electrosorption kinetics for four CDI cells CDI assembly

Electrosorption capacity (mg/g)

Charge efficiency (%)

Cell C-C

4.28

27.9%

Cell N-C Cell C-S Cell N-S

4.85 4.73 6.18

43.2% 38.4% 62.4%

k (s-1)

R2

4.646×10-4 5.601×10-4 5.167×10-4 8.207×10-4

0.98299 0.99254 0.99568 0.99613

To further investigate the effects of the functionalized groups on desalination performance, the electrosorption kinetics is often adopted to analyze the electrosorption


Page 21 of 39 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


rate during the CDI process. The experimental data can be fitted by the pseudo-firstorder kinetics equation (5) and pseudo-second-order kinetics equation (6):42, 48 log(qe − qt ) = log qe −

k 1t 2.303

t 1 t = + 2 qt k2 qe qe

(5) (6)

Where qe (mg/g) and qt (mg/g) are the adsorption amounts of NaCl at equilibrium and time t (s), respectively. k1 (s-1) and k2 (g·mg-1·s-1) are the electrosorption rate constants of the pseudo-first-order and pseudo-second-order kinetics equations, respectively. Figure S1 shows the linear fitting between the two equations and experimental data. And the fitting parameters are presented in Table S1 (Supporting Information). For the four CDI cells, the correlation coefficient R2 of the pseudo-first-order kinetics model is higher than that of the pseudo-second-order kinetics model, which indicates that the pseudo-first-order kinetics model is better to simulate the kinetics behavior of the four CDI cells rather than the pseudo-second-order kinetics model. Therefore, the pseudofirst-order kinetics model is selected to describe the electrosorption rate during the CDI process. Based on the pseudo-first-order kinetics equation, the fitting curves and parameters are displayed in Figure 7c and Table 3. Obviously, all the fitting curves between equation (5) and experimental data possess excellent regression fits due to the high correlation coefficients (R2 > 0.98), which confirms the correctness of the kinetic assumption and the adsorption of ions is controlled by external diffusion mass transfer in the CDI process. Since the electrosorption rate constant is determined by the slope of the fitting curves, the electrosorption rate constants of the Cell C-C, Cell N-C, Cell C-S and Cell N-S are 4.646×10-4 s-1, 5.601×10-4 s-1, 5.167×10-4 s-1 and 8.207×10-4 s-1,



respectively. As expected, the maximum rate value is achieved by the Cell N-S assembled with the ammoniated CNTs electrode as the anode and the sulfonated CNTs electrode as the cathode. This can be ascribed to the wettability and ion selectivity of both modified electrodes with the functional groups, which makes ions quick diffuse and transfer onto the surface of the electrode. 3.4 Electrochemical and desalting stability evaluation. The cyclic stability of the electrode is of great importance for the practical application of CDI technology. So the electrochemical and desalting stability of the modified electrodes are mainly studied by repeating CV tests and cyclic desalination tests. Figure 8 displays the capacitance of the CNTs, ammoniated CNTs and sulfonated CNTs electrodes as a function of the cycle number, and the inset gives the capacitance retention of the three electrodes after 100 cycles. It can be observed that the ammoniated CNTs and sulfonated CNTs electrodes possess excellent cyclic stability with 90.2% and 84.6% of the capacitance retention rate after 100 cycles, respectively. Whereas the CNTs electrodes exhibit an obvious decrease in the capacitance after 100 cycles (only 61.4% of the capacitance retention rate). These results confirm the facts as follows. Primarily, after cyclic tests, the electrochemical performance of the three electrodes all declines at different degrees, mainly because the oxygen-containing groups generated from the parasitic oxidation reactions on the electrode surface can reduce the surface area and adsorption sites of the electrode. Furthermore, the functional groups (NH3+/SO3-) are highly stable in the repeating tests in that they are covalently bonded on the surface of the modified electrodes. This contribute to keep the high capacitance of the modified electrodes.49, 50


Page 22 of 39

Page 23 of 39 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


As importantly, the uniformly dispersed functional group is served as a protective layer for the modified electrodes, so that the modified electrodes can retain higher electrochemical stability than the CNTs electrode due to their oxidation resistance.

Figure 8. The capacitance of the CNTs, ammoniated CNTs and sulfonated CNTs electrodes as a function of the cycle number at the scanning rate of 5 mV/s and the inset is the capacitance retention of the three electrodes after 100 cycles. The oxidation resistance of the modified electrodes is further evaluated by the potential of zero charge (PZC) that can characterize the charge from the functional groups on the surface of the electrode. The PZC is defined to be a characteristic potential where there is no net charge on the electrode surface.34, 45 When the electrode surface is grafted by the positively charged groups, additional negative potential is required to keep no net charge on the surface of the electrode. As a result, the PZC will shift to the negative potential direction. Likewise, when the electrode surface is grafted by the negatively charged groups, additional positive potential is required to keep no net charge on the surface of the electrode. As a result, the PZC will shift to the positive potential direction.51, 52 So the PZC variation of the electrode needs to be monitored



during the repetitive CV experiments. For the pre-prepared electrodes, the PZC values before and after 100 cycles of CV tests are obtained by the normalized differential capacitance curves in Figure 9. Based on the initial PZC (0.1V) of the CNTs electrode, the initial PZC for the ammoniated CNTs electrode (-0.1 V) and sulfonated CNTs electrode (0.2 V) indicates that the amine group and sulfonic group are respectively grafted on the CNTs electrode after chemical modification. Compared with the initial electrodes, the PZC values of the three electrodes are all shifted in the positive direction after 100 cycles of CV tests, suggesting the increase of the negatively charged groups on the electrode surface. So the migration value of PZC can reflect the oxidation extent of the corresponding electrode. From Figure 9, it is also noted that the PZC of the CNTs electrode is positively shifted by 0.15 V after 100 cycles, which is evidently higher than that of the ammoniated CNTs electrode (0.06 V) and sulfonated CNTs electrode (0.10 V). This result characterizes that the amount of negatively charged groups on the surface of the CNTs electrode is larger than that of the modified electrodes after cyclic CV tests. It is further confirmed that the functional layer uniformly formed by the modified groups can effectively alleviate the oxidation reactions on the surface of the modified electrodes.53 For the ammoniated CNTs and sulfonated CNTs electrodes, the difference in the shifted value of PZC also indicates the oxidation resistance of the electrode is proportional to the content of the surface functional groups.


Page 24 of 39

Page 25 of 39 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


Figure 9. Normalized differential capacitance curves for the CNTs electrode (a), ammoniated CNTs electrode (b) and sulfonated CNTs electrode (c) before and after 100 cycles of CV tests; the shift of zero charge potential for three electrodes after 100 cycles (d). To investigate the oxidation resistance of the modified electrodes during the CDI process, the desalting stability of the four CDI cells was measured by the cyclic adsorption/desorption experiments. And the adsorption/desorption time of the four CDI cells was set as 25 min and 15 min, respectively. Figure 10 shows the cyclic adsorption/desorption curves of the four CDI cells in ten cycles. It is clearly found that, during every adsorption process, the decline extent of the solution conductivity for the four CDI cells follows the order: Cell N-S > Cell N-C > Cell C-S > Cell C-C, which is in agreement with the ion removal performance in Section 3.3. After ten cycles of the desalting tests, there is a significant difference in the recyclability of these CDI cells.



Compared with the adsorbing capacity in the first cycle, the decay rate for the Cell CC can almost reach up to 15.8%, while the slight decay rate is achieved by the Cell NC (10.7%), Cell C-S (12.0%) and Cell N-S (9.1%). It is clearly showed that the CDI cells containing the modified electrode display better desalting stability than the Cell C-C. For these CDI cells, the destruction of the electrode pores and the decrease of the specific surface are the main reasons for the degradation of desalination stability, which results from the oxidation of the electrode.30, 54 Among these results, it can be also concluded that the CDI cells containing the ammoniated CNTs electrode as anode or sulfonated CNTs electrode as cathode present a good desalting stability. Especially, the Cell N-S remains the best desalting stability due to the synergistic effect between the ammoniated CNTs electrode and sulfonated CNTs electrode. Therefore, the functional groups on the surface of the modified electrodes not only contribute to the selective adsorption of the counter-ions but also slow down the electrode attenuation.


Page 26 of 39

Page 27 of 39 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


Figure 10. The cyclic adsorption/desorption curves of the four cells in ten cycles. To better understand the impact of the modified electrodes on the desalting stability, the difference in the desalting stability for the four CDI cells is further explored based on the PZC of anode and cathode. After cyclic desalination tests, the changes of anodic PZC and cathodic PZC for the assembled CDI cells are determined by the normalized differential capacitance curves of anode and cathode, as displayed in Figure 11. The corresponding initial PZC (derived from Figure 9) and PZC shift value of the anode and cathode are presented in Table 4. From Figure 11 and Table 4, it can be found that, for all CDI cells, the PZC of the anode and cathode shifts to the positive potential direction after periodic tests. Since the PZC largely depends on the chemical charge on the electrode surface, the negatively charged groups are generated on the surface of both the anode and cathode after prolonged experiments. These negatively charged groups mainly consisting of carboxyl are derived from the parasitic reactions of dissolved oxygen reduction, carbon oxidation and water splitting shown in the following equations,30, 31, 55 which complicates the desalination process. O2 + H2O + 4e- → 4OH-

(7)

Csurf + H2O + e- → C-H + OH-

(8)

Csurf + 6OH- → CO32- + H2O + 4e-

(9)

Csurf + H2O → C=Osurf + 2H+ + 2e-

(10)

Csurf + H2O →C-Osurf + 2H+ + 2e-

(11)

It is worth noting that the positive shift of the anodic PZC is larger than that of the cathodic PZC, which indicates that the carbon oxidation reactions (9) - (11) have less



Page 28 of 39

effect on the cathode compared to the anode. The parasitic reactions (7) and (8) usually occur at the cathode and result in the local basic environment near the cathode surface. Considering the local alkaline solution can transfer from the cathode to the anode, the oxidation of the anode is accelerated in return due to the presence of the parasitic reaction (9). This fact can be confirmed by the PZC shift values of the anode and cathode for the Cell C-C and Cell C-S. For the cell C-C, the anodic PZC and cathodic PZC positively shift about 0.30 V and 0.05 V, respectively. Whereas, for the Cell C-S, the anodic PZC and cathodic PZC positively shift about 0.15 V and 0.02 V, respectively. The decline of the PZC shift value, especially anodic PZC shift value, suggests that the grafted group (SO3-) as a functional layer contributes to protect the electrode adsorption site and enhance the adsorption rate of cations, thereby inhibiting the parasitic reactions (7) and (8) at the cathode. And the degree of anodization is also alleviated due to a decrease in the local alkaline solution. Compared with cell C-C, the anodic PZC in the Cell N-C has just 0.05 V positive shift. It is easy to find that the ammoniated CNTs electrode used as anode effectively overcomes the oxidation of anode. As expected, the shift of anodic PZC and cathodic PZC for the Cell N-S all reach the minimum owing to the synergistic protection of the functional layer on the two modified electrodes. For the four CDI cells, the positive shift of anodic PZC follows the order of Cell N-S < Cell N-C < Cell C-S < Cell C-C. On the contrary, the cyclic desalting stability shown in Figure 10 follows a decreasing order compared to the positive shift of anodic PZC. Based on the above results, the PZC shift of the anode and cathode can be acted as a quantitative indication for their possible surface oxidation during


cyclic

Page 29 of 39 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


adsorption/desorption tests.

Figure 11. Normalized differential capacitance curves of anode and cathode for the four cells after ten cycles. Table 4. The PZC for cathode and anode of the four cells before and after cyclic adsorption/desorption Initial anode

Initial cathode

Cycled anode

Cycled cathode

△PZC+

△PZC-

PZC (V)

PZC (V)

PZC (V)

PZC (V)

(V)

(V)

Cell C-C

0.10

0.10

0.40

0.15

0.30

0.05

Cell N-C

-0.10

0.10

-0.05

0.13

0.05

0.03

Cell C-S

0.10

0.20

0.25

0.22

0.15

0.02

Cell N-S

-0.10

0.20

-0.06

0.21

0.04

0.01

CDI cell

4. CONCLUSIONS In this paper, the ammoniated CNTs and sulfonated CNTs were successfully prepared by the chemical modification method and used as the CDI electrode material. The enhanced wettability and ion selectivity of the electrode surface after grafting functional



groups significantly promoted the electrochemical performance of the modified electrodes. The specific capacitance of the ammoniated CNTs and sulfonated CNTs electrodes were 1.9 and 1.6 times that of the CNTs electrode. Lower charging resistance was found in the modified electrodes as compared to the CNTs electrode. The effects of the modified groups on the desalting performance were investigated using four CDI cells combined with the as-prepared electrodes. The remarkably improved adsorption capacity and charge efficiency were achieved in the Cell N-S with the values of 6.18 mg/g and 62.4%. And the fastest electrosorption process and highest correlation for the Cell N-S were observed in the electrosorption kinetics analysis. In addition, the long-term stability of the modified electrodes was measured by repeating CV tests and desalting tests. The excellent electrochemical and desalting stability were obtained by the Cell N-S, which was explained by the fact that the uniformly grafted amine and sulfonic groups as the protective layer on the electrode surface effectively retarded the parasitic reactions of the anode and cathode, thereby weakening the oxidation of anode and boosting the electrode stability. Therefore, the study of the modified electrodes can enhance the ion-selectivity and stability of the CDI cell and contribute to advance their practical application. ASSOCIATED CONTENT Supporting Information Supporting Information Available: [The electrosorption kinetics analysis for the four CDI cells based on the pseudo-first-order kinetic equation and pseudo-second-order kinetic equation.]


Page 30 of 39

Page 31 of 39 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


AUTHOR INFORMATION Corresponding author * E-mail address: [email protected] (Y. Wang). ORCID Yue Wang: 0000-0003-3005-3480 Notes The authors declare no competing financial interest. ACKNOWLEDGEMENT This research is supported by the National Natural Science Foundation of China (No.21576190). 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, 38453856. (2) Ahmed, M. A.; Tewari, S. Capacitive Deionization: Processes, Materials and State of the Technology. J. Electroanal. Chem. 2018, 813, 178-192. (3) 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, 1388-1442. (4) Sun, Z.; Chai, L.; Liu, M.; Shu, Y.; Li, Q.; Wang, Y.; Qiu, D. Effect of the Electronegativity on the Electrosorption Selectivity of Anions during Capacitive



Deionization. Chemosphere 2018, 195, 282-290. (5) Bao, S.; Duan, J.; Zhang, Y. Recovery of V(V) from Complex Vanadium Solution Using Capacitive Deionization (CDI) with Resin/Carbon Composite Electrode. Chemosphere 2018, 208, 14-20. (6) Wang, Z.; Yan, T.; Chen, G.; Shi, L.; Zhang, D. High Salt Removal Capacity of Metal–Organic Gel Derived Porous Carbon for Capacitive Deionization. ACS Sustainable Chem. Eng. 2017, 5, 11637-11644. (7) Sun, Z.; Chai, L.; Liu, M.; Shu, Y.; Li, Q.; Wang, Y.; Wang, Q.; Qiu, D. Capacitive Deionization of Chloride Ions by Activated Carbon Using a Three-Dimensional Electrode Reactor. Sep. Purif. Technol. 2018, 191, 424-432. (8) Zornitta, R. L.; Srimuk, P.; Lee, J.; Kruner, B.; Aslan, M.; Ruotolo, L. A. M.; Presser, V. Charge and Potential Balancing for Optimized Capacitive Deionization Using Lignin-Derived, Low-Cost Activated Carbon Electrodes. ChemSusChem 2018, 11, 2101-2113. (9) Huang, Z. H.; Wang, M.; Wang, L.; Kang, F. Y. Relation between the Charge Efficiency of Activated Carbon Fiber and Its Desalination Performance. Langmuir 2012, 28, 5079-5084. (10) Xu, F.; Chen, Y.; Tang, M.; Wang, H.; Deng, J.; Wang, Y. Acid Induced SelfAssembly Strategy to Synthesize Ordered Mesoporous Carbons from Biomass. ACS Sustainable Chem. Eng. 2016, 4, 4473-4479. (11) Biener, J.; Stadermann, M.; Suss, M.; Worsley, M. A.; Biener, M. M.; Rose, K. A.; Baumann, T. F. Advanced Carbon Aerogels for Energy Applications. Energy Environ.


Page 32 of 39

Page 33 of 39 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


Sci. 2011, 4, 656-667. (12) Wang, H.; Zhang, D.; Yan, T.; Wen, X.; Shi, L.; Zhang, J. Graphene Prepared via a Novel Pyridine–Thermal Strategy for Capacitive Deionization. J. Mater. Chem. 2012, 22, 23745–23748 . (13) Yan, T.; Liu, J.; Lei, H.; Shi, L.; An, Z.; Park, H. S.; Zhang, D. Capacitive Deionization of Saline Water Using Sandwich-Like Nitrogen-Doped Graphene Composites via a Self-Assembling Strategy. Environ. Sci.: Nano 2018, 5, 2722-2730. (14) Bao, S.; Duan, J.; Zhang, Y. Characteristics of Nitric Acid-Modified Carbon Nanotubes and Desalination Performance in Capacitive Deionization. Chem. Eng. Technol. 2018, 41, 1793-1799. (15) Xing, R.; Zhou, T.; Zhou, Y.; Ma, R.; Liu, Q.; Luo, J.; Wang, J. Creation of Triple Hierarchical Micro-Meso-Macroporous N-doped Carbon Shells with Hollow Cores Toward the Electrocatalytic Oxygen Reduction Reaction. Nano-Micro Lett. 2018, 10:3 (16) Han, J.; Shi, L.; Yan, T.; Zhang, J.; Zhang, D. Removal of ions from saline water using N, P co-doped 3D hierarchical carbon architectures via capacitive deionization. Environ. Sci.: Nano 2018, 5, 2337-2345. (17) 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, 58907-58915. (18) Cao, J.; Wang, Y.; Chen, C.; Yu, F.; Ma, J. A Comparison of Graphene Hydrogels Modified with Single-Walled/Multi-Walled Carbon Nanotubes As Electrode Materials for Capacitive Deionization. J. Colloid Interface Sci. 2018, 518, 69-75.



Page 34 of 39

(19) Zhang, J.; Fang, J. H.; Han, J. L.; Yan, T. T.; Shi, L. Y.; Zhang, D. S. N, P, S CoDoped

Hollow

Carbon

Polyhedra

Derived

from

MOF-Based

Core-Shell

Nanocomposites for Capacitive Deionization. J. Mater. Chem. A 2018, 6, 15245-15252. (20) Jo, H.; Kim, K. H.; Jung, M.-J.; Park, J. H.; Lee, Y.-S. Fluorination Effect of Activated Carbons on Performance of Asymmetric Capacitive Deionization. Appl. Surf. Sci. 2017, 409, 117-123. (21) Wu, T.; Wang, G.; Dong, Q.; Qian, B.; Meng, Y.; Qiu, J. Asymmetric Capacitive Deionization Utilizing Nitric Acid Treated Activated Carbon Fiber As the Cathode. Electrochim. Acta 2015, 176, 426-433. (22) Wang, Y.; Zhang, L.; Wu, Y.; Xu, S.; Wang, J. Polypyrrole/Carbon Nanotube Composites As Cathode Material for Performance Enhancing of Capacitive Deionization Technology. Desalination 2014, 354, 62-67. (23) Liu, P. Y.; Wang, H.; Yan, T. T.; Zhang, J. P.; Shi, L. Y.; Zhang, D. S. Grafting Sulfonic and Amine Functional Groups on 3D Graphene for Improved Capacitive Deionization. J. Mater. Chem. A 2016, 4, 5303-5313. (24) Pan, L. K.; Wang, X. Z.; Gao, Y.; Zhang, Y. P.; Chen, Y. W.; Sun, Z. Electrosorption of Anions with Carbon Nanotube and Nanofibre Composite Film Electrodes. Desalination 2009, 244, 139-143. (25) Fritz, P. A.; Zisopoulos, F. K.; Verheggen, S.; Schroën, K.; Boom, R. M. Exergy Analysis of Membrane Capacitive Deionization (MCDI). Desalination 2018, 444, 162168. (26) Wang, M.; Xu, X.; Li, Y.; Lu, T.; Pan, L. Enhanced Desalination Performance of


Page 35 of 39 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


Anion-Exchange Membrane Capacitive Deionization via Effectively Utilizing Cathode Oxidation. Desalination 2018, 443, 221-227. (27) Gu, X.; Deng, Y.; Wang, C. Fabrication of Anion-Exchange Polymer Layered Graphene–Melamine Electrodes for Membrane Capacitive Deionization. ACS Sustainable Chem.Eng. 2016, 5, 325-333. (28) Yang, J.; Zou, L.; Choudhury, N. R. Ion-selective Carbon Nanotube Electrodes in Capacitive Deionisation. Electrochim. Acta 2013, 91, 11-19. (29) Lu, D.; Cai, W.; Wang, Y. Optimization of the Voltage Window for Long-Term Capacitive Deionization Stability. Desalination 2017, 424, 53-61. (30) Cohen, I.; Avraham, E.; Bouhadana, Y.; Soffer, A.; Aurbach, D. The Effect of the Flow-Regime, Reversal of Polarization, and Oxygen on the Long Term Stability in Capacitive Deionization Processes. Electrochim. Acta 2015, 153, 106-114. (31) Gao, X.; Omosebi, A.; Holubowitch, N.; Liu, A.; Ruh, K.; Landon, J.; Liu, K. Polymer-coated Composite Anodes for Efficient and Stable Capacitive Deionization. Desalination 2016, 399, 16-20. (32) Zhang, H.; Liang, P.; Bian, Y.; Sun, X.; Ma, J.; Jiang, Y.; Huang, X. Enhancement of Salt Removal in Capacitive Deionization Cell through Periodically Alternated Oxidation of Electrodes. Sep. Purif. Technol. 2018, 194, 451-456. (33) Cohen, I.; Avraham, E.; Bouhadana, Y.; Soffer, A.; Aurbach, D. Long Term Stability of Capacitive De-Ionization Processes for Water Desalination: The Challenge of Positive Electrodes Corrosion. Electrochim. Acta 2013, 106, 91-100. (34) Omosebi, A.; Gao, X.; Landon, J.; Liu, K. Asymmetric Electrode Configuration



for Enhanced Membrane Capacitive Deionization. ACS Appl. Mater. Interfaces 2014, 6, 12640-12649. (35) Gao, X.; Omosebi, A.; Landon, J.; Liu, K. Surface Charge Enhanced Carbon Electrodes for Stable and Efficient Capacitive Deionization Using Inverted Adsorption–Desorption Behavior. Energy Enviro. Sci. 2015, 8, 897-909. (36) Kathi, J.; Rhee, K. Y. Surface Modification of Multi-Walled Carbon Nanotubes Using 3-Aminopropyltriethoxysilane. J. Mater. Sci. 2007, 43, 33-37. (37) Viel, P.; Le, X. T.; Huc, V.; Bar, J.; Benedetto, A.; Le Goff, A.; Filoramo, A.; Alamarguy, D.; Noël, S.; Baraton, L.; Palacin, S. Covalent Grafting onto Self-Adhesive Surfaces based on Aryldiazonium Salt Seed Layers. J. Mater. Chem. 2008, 18, 59135920. (38) Yu, D. S.; Kuila, T.; Kim, N. H.; Khanra, P.; Lee, J. H. Effects of Covalent Surface Modifications on the Electrical and Electrochemical Properties of Graphene Using Sodium 4-Aminoazobenzene-4′-Sulfonate. Carbon 2013, 54, 310-322. (39) Wang, Y.; Han, X.; Wang, R.; Xu, S.; Wang, J. Preparation Optimization on the Coating-Type Polypyrrole/Carbon Nanotube Composite Electrode for Capacitive Deionization. Electrochim. Acta 2015, 182, 81-88. (40) Gao, X.; Porada, S.; Omosebi, A.; Liu, K. L.; Biesheuvel, P. M.; Landon, J. Complementary Surface Charge for Enhanced Capacitive Deionization. Water Res. 2016, 92, 275- 282. (41) Cha, J.; Jin, S.; Shim, J. H.; Park, C. S.; Ryu, H. J.; Hong, S. H. Functionalization of Carbon Nanotubes for Fabrication of CNT/Epoxy Nanocomposites. Mater. Design


Page 36 of 39

Page 37 of 39 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


2016, 95, 1-8. (42) Li, Y.; Shen, J.; Li, J.; Sun, X.; Shen, J.; Han, W.; Wang, L. A Protic Salt-Derived Porous Carbon for Efficient Capacitive Deionization: Balance between Porous Structure and Chemical Composition. Carbon 2017, 116, 21-32. (43) Baroud, T. N.; Giannelis, E. P. High Salt Capacity and High Removal Rate Capacitive Deionization Enabled by Hierarchical Porous Carbons. Carbon 2018, 139, 614-625. (44) Zhao, J.; Jiang, Y.; Fan, H.; Liu, M.; Zhuo, O.; Wang, X.; Wu, Q.; Yang, L.; Ma, Y.; Hu, Z. Porous 3D Few-Layer Graphene-like Carbon for Ultrahigh-Power Supercapacitors with Well-Defined Structure-Performance Relationship. Adv. Mater. 2017, 29, 1604569. (45) Ma, D. Y.; Wang, Y.; Han, X. Y.; Xu, S. C.; Wang, J. X. Electrode Configuration Optimization of Capacitive Deionization Cells Based on Zero Charge Potential of the Electrodes. Sep. Purif. Technol. 2017, 189, 467-474. (46) Asquith, B. M.; Meier-Haack, J.; Ladewig, B. P. Cation Exchange Copolymer Enhanced Electrosorption. Desalination 2014, 345, 94-100. (47) Zornitta, R. L.; Ruotolo, L. A. M. Simultaneous Analysis of Electrosorption Capacity and Kinetics for CDI Desalination Using Different Electrode Configurations. Chem. Eng. J. 2018, 332, 33-41. (48) Niu, R.; Li, H.; Ma, Y.; He, L.; Li, J. An Insight into the Improved Capacitive Deionization Performance of Activated Carbon Treated by Sulfuric Acid. Electrochim. Acta 2015, 176, 755-762.



(49) Jeong Tai, K.; Kim, H.-C.; Kim, S.-K.; Kathi, J.; Rhee, K.-Y. 3Aminopropyltriethoxysilane Effect on Thermal and Mechanical Properties of Multiwalled Carbon Nanotubes Reinforced Epoxy Composites. J. Compos. Mater. 2009, 43, 2533-2541. (50) Mahouche-Chergui, S.; Gam-Derouich, S.; Mangeney, C.; Chehimi, M. M. Aryl Diazonium Salts: A New Class of Coupling Agents for Bonding Polymers, Biomacromolecules and Nanoparticles to Surfaces. Chem. Soc. Rev. 2011, 40, 41434166. (51) Avraham, E.; Noked, M.; Bouhadana, Y.; Soffer, A.; Aurbach, D. Limitations of Charge Efficiency in Capacitive Deionization Processes III: The Behavior of Surface Oxidized Activated Carbon Electrodes. Electrochim. Acta 2010, 56, 441-447. (52) Cohen, I.; Avraham, E.; Noked, M.; Soffer, A.; Aurbach, D. Enhanced Charge Efficiency in Capacitive Deionization Achieved by Surface-Treated Electrodes and by Means of a Third Electrode. J. Phys. Chem. C 2011, 115, 19856-19863. (53) Gao, X.; Omosebi, A.; Landon, J.; Liu, K. Dependence of the Capacitive Deionization Performance on Potential of Zero Charge Shifting of Carbon Xerogel Electrodes during Long-Term Operation. J. Electrochem. Soc. 2014, 161, E159-E166. (54) Tang, W.; He, D.; Zhang, C.; Kovalsky, P.; Waite, T. D. Comparison of Faradaic Reactions in Capacitive Deionization (CDI) and Membrane Capacitive Deionization (MCDI) Water Treatment Processes. Water Res. 2017, 120, 229-237. (55) Zhang, C. Y.; He, D.; Ma, J. X.; Tang, W. W.; Waite, T. D. Faradaic Reactions in Capacitive Deionization (CDI) - Problems and Possibilities: A Review. Water Res. 2018,


Page 38 of 39

Page 39 of 39 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


128, 314-330.

Table of Contents


Grafting the Charged Functional Groups on Carbon Nanotubes for

Recommend Documents