carbon nanotube composites as cathode material for performance enhancing of capacitive deionization technology

carbon nanotube composites as cathode material for performance enhancing of capacitive deionization technology

Desalination 354 (2014) 62–67 Contents lists available at ScienceDirect Desalination journal homepage: www.elsevier.com/locate/desal Polypyrrole/ca...

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Desalination 354 (2014) 62–67

Contents lists available at ScienceDirect

Desalination journal homepage: www.elsevier.com/locate/desal

Polypyrrole/carbon nanotube composites as cathode material for performance enhancing of capacitive deionization technology Yue Wang a,b,c,⁎, Liwen Zhang a,b,c, Yafei Wu a,b,c, Shichang Xu a,b, Jixiao Wang a,b,c a b c

Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China Tianjin Key Laboratory of Membrane Science and Desalination Technology, Tianjin 300072, PR China Collaborative Innovation Center of Chemical Science and Engineering, Tianjin 300072, PR China

H I G H L I G H T S • Polypyrrole/carbon nanotube composites were prepared and used as cathode in CDI. • Specific capacitance of the composites was about 4.6 times that of carbon nanotubes. • Enhanced desalination performance for CDI cell was achieved.

a r t i c l e

i n f o

Article history: Received 25 July 2014 Received in revised form 15 September 2014 Accepted 17 September 2014 Available online xxxx Keywords: Polypyrrole Carbon nanotubes Cathode material Capacitive deionization

a b s t r a c t Polypyrrole/carbon nanotube (PPy/CNT) composites were prepared via chemical oxidation method. By choosing sodium dodecyl benzene sulfonate (SDBS) as the dopant in the preparation process, the obtained composites have the ability of selective adsorption for cations and are suitable for application as the cathode material of capacitive deionization cell. The scanning electron microscope and transmission electron microscopy analysis showed that the PPy/CNT composites were in nanotube morphology with the CNTs wrapped uniformly by the PPy layer. Electro-chemical characteristics and desalting performance of PPy/CNT composites were tested and analyzed. The results indicated that specific capacitance of PPy/CNT composites increased more than 3 times compared with the CNTs. The saturated adsorbing capacity of PPy/CNT–CNT cell (PPy/CNTs used as cathode) was evaluated as 43.99 mg/g, which is much higher than that of CNT–CNT cell (about 11.00 mg/g). So the PPy/CNT composites doped with DBS− have a great potential as the high-performance cathode material for capacitive deionization technology. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Capacitive deionization (CDI) based on the electrochemical method has emerged as a promising technology for water desalination due to its significant advantages such as no secondary pollution, low operating cost and high recovery rate [1,2]. The CDI process basically consists of two stages. The first being an ion adsorption stage, ions from feed solution are adsorbed on porous electrodes when a cell voltage is applied. In the following ion desorption stage, the adsorbed ions are released back into the bulk solution as the cell voltage is reversed or removed and thus the electrodes is regenerated [3]. Usually, for obtaining good performance of the CDI cell, an excellent electrode material is essential and significant. ⁎ Corresponding author at: Chemical Engineering Research Center, School of Chemical Engineering and Technology, Tianjin University, Tianjin 300072, PR China. Tel.: +86 22 27406889. E-mail address: [email protected] (Y. Wang).

http://dx.doi.org/10.1016/j.desal.2014.09.021 0011-9164/© 2014 Elsevier B.V. All rights reserved.

As the typical electrode materials, porous carbons such as activated carbon (AC) [4,5], carbon aerogels [6,7] and carbon nanotubes (CNTs) [8,9] have been extensively developed and used in CDI process due to their high specific surface area, good electrical conductivity and high mechanical strength. However, the low specific capacitance of carbon material determined by the electric double layer principle limits its adsorption capacity. So, combinations of carbon material and faradic capacitance material such as conducting polymers and metal oxides were proposed to solve the problem. C. Yan et al. [10] used single-walled carbon nanotubes and polyaniline composites as CDI electrode material, and the salt removal efficiency of the composites based electrodes was improved by 12% compared to that of the singlewalled carbon nanotube based electrodes. J. Yang et al. [11] had successfully adopted MnO2/nanoporous carbon composites as CDI electrode materials, finding that the composites' specific capacitance of 204.7 F/g was much higher than that of the AC (98.6 F/g) and the desalting performance of the composites electrodes achieved was about 3 times of AC electrodes.

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efficiency was only about 60%. Contrasted to the high salt removal efficiency of the MCDI cell, the current efficiency was still limited by its added resistivity arising from the weak contact adhesion between the electrodes and ion-exchange membranes. So, ion-exchange polymer layers [14,15] were proposed to coat directly on the electrode surface as a replacement of ion-exchange membranes. Recently, C. Nie et al. [16] reported a composite of carbon nanotubes and polyacrylic acid as the CDI cathode material, in which polyacrylic acid served as the cation-exchange polymer. By assembling the composite electrode and the CNT electrode, the obtained CDI cell exhibited a high NaCl removal efficiency of 83%, which was 51% higher than that of based on pure CNT electrodes. However, the poor electrical conductivity of the ion-exchange polymer discussed above may still be a problem of the electrodes that needs to be dealt with. As a typical conducting polymer, polypyrrole (PPy) not only possesses a good electrical conductivity, but also exhibits superior ion exchange capacity. Cation exchange is found to take place primarily on PPy modified with large anions due to the immobility of these ions in the PPy chain [17–19]. So, it is expected to introduce the PPy material into the CDI cell system for eliminating the co-ion effect without increasing the electrode resistivity considerably. In this study, the polypyrrole/carbon nanotube (PPy/CNT) composites doped with dodecyl benzene sulfonate (DBS−) were prepared using chemical oxidation method. The textural and electrochemical properties of the composites were investigated. Also, by using the PPy/CNT composite electrode as the cathode, the assembled asymmetric CDI cells were tested and evaluated by their desalination performances and cyclic stabilities.

2. Experimental Fig. 1. Elemental mapping of the PPy/CNT electrode.

2.1. Synthesis of PPy/CNT composite material Though an increasing salt removal efficiency was achieved by improving the capacitance of electrode materials in the above studies, the co-ion effect, meaning that the counter-ion (opposite charge of electrode) adsorption and co-ion (same charge of electrode) expulsion happen simultaneously, still existed and can lead to lower desalination performance [12]. A common strategy for eliminating the co-ion effect is to place ion-exchange membranes as charge barriers on the electrode surface. The membranes allow counter-ions to permeate through and reject the co-ions, thus reducing the unregulated movements of the co-ions. Haibo Li et al. [13] developed a novel MCDI device using walled carbon nanotube electrodes and ion-exchange membranes. The results indicated that the MCDI cell exhibited a high NaCl removal efficiency of 97%, which was much higher than the CDI whose salt removal

PPy/CNT composites were synthesized through chemical oxidation method. Firstly, pyrrole (Aladdin Industrial Corporation) was purified by distillation with N2 protection to obtain the pyrrole monomer sample. CNTs (0.1 g, Chengdu Organic Chemicals Co. Ltd., Chinese Academy of Science, Multi-Wall, diameter of 20–30 nm, length of 10– 30 μm) were dispersed into alcohol under ultraphonic oscillation to make the CNT suspending solution. Then the sodium dodecyl benzene sulfonate (SDBS) solution, pyrrole monomer sample and the oxidant FeCl3 were injected into the solution orderly and reacted for 12 h with an environmental temperature of 0 °C. As the final step, the reaction solution was filtered and rinsed thoroughly with deionized water to collect the precipitate product of PPy/CNT composites. Then the composites were dried in vacuum at 60 °C for 24 h.

Fig. 2. SEM images of PPy/CNT composites (a) and CNTs (b).

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Fig. 3. TEM images of PPy/CNT composites (a) and CNTs (b).

2.2. Fabrication of PPy/CNT composite electrode The PPy/CNT composite electrode was fabricated by compression molding method. According to the mass ratio of 85:10:5, the PPy/CNT composite material (active component), polyvinylidene fluoride (PVDF, binder) and graphite powder (conductivity agent) were mixed and grinded adequately. Then the well mixed powders were put into the self-manufactured mold and pressed at a pressure of 10 MPa for 5–10 min. After mold-releasing, the PPy/CNT plate, which was 15 mm in diameter and 0.1– 0.2 mm in thickness, was adhered on the disposed titanium plate to obtain the PPy/CNT electrode shown in Fig. 1. The CNT electrode was fabricated with the same procedure. Before used, the PPy/CNT electrode should be pretreated to eliminate the interference of Cl− which might be introduced through FeCl3 oxidant and doped with the PPy/CNT composites during the polymerization process. In the paper, potentiostatic method was adopted to remove Cl− in the three-electrode system with the potential of −0.7 V. 3. Results and discussion 3.1. Characterization of PPy/CNT materials 3.1.1. Morphology SEM images of the PPy/CNT composites and the CNTs are given in Fig. 2. From the images, it can be found that PPy/CNTs present nanowire morphology with a uniform diameter. Furthermore, the diameter of

the PPy/CNT composites is about 50 nm, which is larger than that of the CNTs (about 30 nm), proving that the PPy has been deposited on the CNTs successfully. Fig. 3 compares typical TEM images of the PPy/CNTs with the CNTs. It illustrates that the PPy layer can be clearly distinguished wrapping uniformly along the center core CNTs.

3.1.2. BET specific surface area The BET specific surface areas of the PPy/CNT and CNT electrodes were measured as 185.21 m2/g and 200.74 m2/g, respectively, through a specialized instrument (Tirstar 3000, MICROMERITICS INSTRUMENT CORP, USA), which were characterized by the N2 adsorption/desorption isotherm. The results show a slight decrease of the BET specific surface area for PPy/CNT electrode, which is probably because the wrapping of the PPy layer on the CNTs increases the diameter of the nanotube composites.

3.1.3. FTIR Fig. 4 shows the FTIR spectra of the CNTs and PPy/CNTs. The FTIR spectra peak of the CNTs appears at about 1580 cm−1 due to the C_C backbone stretching of CNTs [20]. The peaks of PPy/CNTs at 1640 cm−1, 1035 cm−1 and 895 cm−1 were consistent with the C_C stretching vibration in benzene ring, the S_O vibration and benzene ring plane vibration, respectively, which demonstrate that DBS− has doped in PPy successfully [21]. Peaks at 1296 cm−1 and 1167 cm−1 come from the conjugation effect and vibration of C and N respectively, and the peak at 1544 cm− 1 represents the vibration of N\O bond, which suggested a strong interaction between PPy and CNTs [22].

100 PPy/CNTs CNTs

1580

90

70 4000

3000

2000

Wavenumbers (cm-1)

Fig. 4. FTIR spectra of PPy/CNT composites and CNTs.

895

1167

75

1035

80

1400 1296

1544

1640

85

3431

Transmittance (a.u.)

95

1000

Fig. 5. EDS of PPy/CNTs before pretreatment.

Y. Wang et al. / Desalination 354 (2014) 62–67 2x105

-O1s

-F1s

1x105

-F KLL

-O KLL

8x104

3.2.2. Conductivity The conductivity of the electrodes was studied by a four-probe electro-conductivity instrument (RTS-9 type, Guang Zhou four-probe technology company). The average conductivity of PPy/CNT electrode is 6.37 S/cm, while that of the CNT electrode is 11.36 S/cm. The conductivity decrease of PPy/CNT electrode can be explained as the resistance of electron transportation in PPy is larger than that in CNTs.

-N1s

c/s

1x105

-C KLL

2x10

1x105

in PPy/CNT electrode before the pretreatment as shown in Fig. 5. The disappearance of the Cl peak in Fig. 6 illustrates that the Cl− has been removed thoroughly after pretreatment. Therefore, the effects of Cl− can be ignored in the succeeding experiments.

-C1s

5

65

6x104 4x104 2x104 0

1000

800

600

400

200

0

Binding Energy(eV) Fig. 6. XPS survey scans for PPy/CNTs after pretreatment.

0.020 PPy/CNTs CNTs

0.015

Current (A)

0.010 0.005 0.000

3.2.3. Cyclic voltammetry Fig. 7 gives the cyclic voltammetry (CV) behaviors of the PPy/CNT and CNT electrodes tested in 1.0 M NaCl solution within a potential range of − 0.2 V to 0.6 V vs. SCE (saturated calomel electrode) and at the potential sweep rate of 5 mV/s. The CV curve of PPy/CNT electrode presents nearly rectangular in shape under the test conditions. The response current of PPy/CNT electrode is evidently much larger than that of the CNTs, promising a good charge transportation between the solution and the PPy/CNT electrode interface and fine electron transportation in the electrode. This phenomenon accords with the results of electrode specific capacitance (C, F/g) evaluated by formula (1), as 106 F/g for PPy/CNT electrode and 23 F/g for CNT electrode.

-0.005

Z

-0.010

E

2

E1

-0.015



-0.020 -0.2

0.0

0.2

0.4

Fig. 7. CV behavior of PPy/CNT electrode and CNT electrode.

3.2. Characterization of PPy/CNT electrode 3.2.1. Test of Cl− content before and after pretreatment To make sure that Cl− has been extracted thoroughly in the pretreatment process (in 2.2), EDS (energy dispersive spectrometer) test before pretreatment and XPS (X-ray photoelectron spectroscopy analysis) after pretreatment were implemented and depicted in Figs. 5 and 6 respectively. The strong peak of Cl in EDS proves that there exists Cl−

PPy/CNT Electrode

ð1Þ

where E1, E2 (V) are the initial and final potential respectively, i (A) is the response current, v (V/s) is the potential scan rate, and m (g) is the mass of active component in the electrode. The high specific capacitance of the PPy/CNT electrode can be ascribed to the synergistic effect of PPy and CNTs. Firstly, the polymerization of PPy on CNTs can effectively increase the pseudo-capacitance function of the electrode. Secondly, the large specific surface area of CNTs can provide plenty of adsorbing sites, which is beneficial for the transportation of electrolyte ions and charge transfer. Finally, CNTs in the composites not only can buffer the volume change of PPy during the charging and discharging processes, but also preserve the relative high conductivity of the whole electrode [23].

Conductivity Electrode

+ start

E2 −E1 mv

0.6

Potential (V)

CNT Electrode

iðEÞdE

Temperature Sensor

12μ S/ cm

stop CDI Cell

Conductivity Meter

Power Controller Fig. 8. Flow diagram of the CDI desalting system.

Computer

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1000

43.99 mg/g which is about fourfold of the CNT–CNT cell (11.00 mg/g). The enhanced desalination performance of the PPy/CNT–CNT cell, on the one hand, can be attributed to the larger specific capacitance of PPy/CNT electrode. On the other hand, the special cation exchange ability of PPy/CNT cathode also plays an important role in improvement of desalination performance by eliminating the co-ion effect of the CDI system. For further studying the cyclic property of the PPy/CNT–CNT cell, a multiple adsorption/desorption cycle experiment was carried out and the adsorption time and desorption time were both set as 900 s. From Fig. 10, it can be seen that the reduction of solution conductivity for PPy/CNT–CNT cell was always larger than that for CNT–CNT cell, which was consistent with Fig. 9 on principle and promised that the good performance of PPy/CNT–CNT cell could be remained for multiple cycling operations. The similar cyclic‘V’ shape conductivity curve for PPy/CNT–CNT cell also proved an excellent recyclability of the cell with a regeneration rate of about 96% after 5 cycles. So the PPy/CNT composites doped with DBS− have a great potential as the highperformance cathode material for capacitive deionization technology.

CNT-CNT Cell PPy/CNT-CNT Cell

Conductivity(μS/cm)

980 960 940 920 900 880 0

1000

2000

3000

4000

5000

Time(s) Fig. 9. Saturated adsorbing curve of PPy/CNT–CNT and CNT–CNT cell.

4. Desalination performance test

5. Conclusion

By using the PPy/CNT electrode as cathode and CNT electrode as anode, an asymmetric PPy/CNT–CNT cell was assembled and tested in a NaCl aqueous solution that has an initial conductivity of 1000 μS/cm. Fig. 8 gives the flow diagram of the experiments. Here, the working voltage of the cell was set as 1.4 V which was provided by the power controller. Regeneration of the electrodes was achieved by reversing their polarities. Conductivity of the NaCl solution was monitored and collected in real-time during the experiments. In addition, the specific adsorption capacity of the cell was calculated through the following formula (2): mt ¼

ðc0 −c1 ÞV m

ð2Þ

where mt (mg/g) is the specific adsorption capacity of cell to NaCl; c0 and c1 (mg/L) are the initial and final NaCl concentrations respectively; V (L) is the volume of the feed solution; and m (g) is the mass of active component in the electrode. Fig. 9 gives the saturated adsorbing curves of the PPy/CNT–CNT cell and the CNT–CNT cell. It can be found that a rapid reduction of the solution conductivity appears for both cells at the beginning stage, and then the solution conductivity tends to get constant at 1500 s for CNT–CNT cell and about 2500 s for PPy/CNT–CNT cell. As a result, the saturated adsorption capacity of the PPy/CNT–CNT cell is calculated as 1020

CNT-CNT Cell PPy/CNT-CNT Cell

Conductivity(μS/cm)

1000 980 960 940 920 0

2000

4000

6000

8000

10000

Time(s) Fig. 10. Adsorption/desorption curves of PPy/CNT–CNT and CNT–CNT cell at 5 cycles.

In this paper, PPy/CNT composites doped with DBS− were synthesized and used as cathode for CDI. As shown in the SEM and TEM images, the as-prepared composites were in nanotube morphology with the CNTs wrapped uniformly by the PPy layer. FTIR test suggested a strong interaction between PPy and CNTs. In addition, the specific capacitance of PPy/CNT composites increased to 106 F/g from 23 F/g for CNTs in a 1 M NaCl solution. The saturated adsorbing capacity of the PPy/CNT–CNT cell was further improved as compared with that of the CNT–CNT cell and reached an outstanding value of 43.99 mg/g. So the resultant PPy/CNT composites were demonstrated to be a promising cathode material for CDI technology.

Acknowledgment This research is supported by the National Natural Science Foundation of China (No. 21276178) and the State Key Laboratory of Chemical Engineering (No. SKL-ChE-14B02).

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