ZnCl2 activated electrospun carbon nanofiber for capacitive desalination

Desalination 344 (2014) 446–453

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ZnCl2 activated electrospun carbon nanofiber for capacitive desalination Jianyun Liu a,⁎, Shiping Wang a, Jianmao Yang b, Jinjin Liao a, Miao Lu a, Haojie Pan a, Le An a a b

College of Environmental Science and Engineering, Donghua University, Shanghai 201620, China Research Center for Analysis & Measurement, Donghua University, Shanghai 201620, China

H I G H L I G H T S

G R A P H I C A L

A B S T R A C T

• The carbon nanofiber was fabricated by simple electrospinning technique. • The wettability and surface area of ZnAECNF were improved with ZnCl2 activation. • The capacitance and conductivity of ZnAECNF electrode were enhanced significantly. • The Zn-AECNF based capacitor shows high desalination ability.

a r t i c l e

i n f o

Article history: Received 22 December 2013 Received in revised form 10 April 2014 Accepted 11 April 2014 Available online 4 May 2014 Keywords: Electrospun carbon nanofiber Zinc chloride activation Capacitive desalination

a b s t r a c t The ZnCl2 activated electrospun carbon nanofiber (Zn-AECNF) was fabricated by the electrospinning of polyacrylonitrile (PAN) nanofiber, followed by the preoxidation, carbonization and ZnCl2 post-treatment. The activated carbon nanofiber was characterized by scanning electron microscopy (SEM), contact angle, X-ray photoelectron spectroscopy, infrared spectroscopy and nitrogen adsorption. The carbon nanofiber obtained with ZnCl2 activation exhibits good flexibility, high hydrophilicity, and improved specific surface area. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) demonstrated the enhanced capacitance and improved electric conductivity due to the treatment of ZnCl2. The Zn-AECNF was used for capacitive desalination successfully. With the higher ZnCl2 concentration used for carbon fiber impregnation, the specific surface area of the obtained carbon fiber was increased, and the desalination performance was enhanced accordingly. The salt adsorption of 10.52 mg/g was achieved when 60% ZnCl2 solution was used. The Zn-AECNF electrodes offer high potential for use in capacitive desalination technology. © 2014 Elsevier B.V. All rights reserved.

1. Introduction Freshwater is one of earth's most valuable resources, but many regions suffer from an increasingly limited freshwater supply due to the development of the industrial economy. In the face of the global water scarcity, seawater and brackish water desalination holds great promise to effectively increase water supply by generating more freshwater to solve the water crisis, since the sources of seawater and brackish water account for over 97% of all water on the earth [1]. Nowadays, ⁎ Corresponding author. Tel.: +86 21 67792381; fax: +86 21 67792522. E-mail address: [email protected] (J. Liu).

http://dx.doi.org/10.1016/j.desal.2014.04.015 0011-9164/© 2014 Elsevier B.V. All rights reserved.

the major desalination technologies are based on membrane separation via reverse osmosis, membrane distillation and electrodialysis [2–5]. However, high energy consumption, high cost and membrane fouling issues have limited their large scale application. Compared with the traditional technologies mentioned above, capacitive deionization (CDI) as one of the alternative desalination technologies, has attracted much attention for its considerable potentials to effectively improve desalination efficiency while achieving high water recovery due to its low pressure, low energy cost and environmentally-friendly characteristics [6–8]. Biesheuvel has established the useful mathematical models for the optimization of the desalinization process [9,10], and has recently conducted a comprehensive review on the science and technology of

J. Liu et al. / Desalination 344 (2014) 446–453

water desalination by capacitor from electrode materials to various theoretical–conceptual approaches to understand the phenomenon of capacitive deionization [11]. Aurbach et al. [12–14] developed a control system by integrating various pieces of equipment in order to effectively study the multitude of parameters. All of these researches gave helpful guidelines for the practical development of capacitive water deionization processes. Electrode materials with high electrosorption capacity are the key to ensure the high deionization efficiency of CDI. An ideal electrode material should possess high specific surface area and good electrical conductivity and ease of manufacturing. The suitable pore size and good electrolyte wettability for easy ionic migration are also important to improve the sorption capacity. The carbon based materials, such as activated carbon [15,16], carbon aerogel [17,18], carbon nanofiber [19], carbon nanotube [20,21] and graphene [22,23] have been applied extensively as the CDI electrode. Activated carbon fibers (ACFs) are typical microporous materials with large surface areas and good conductivity, and are easy to manufacture. Thus, they can serve as a candidate for the active component of CDI electrodes. Besides, the ACFs with non-woven mat form are more popular due to their low cost, easy handling without the extra processing steps and being free of binders [24]. Recently, electrospinning, a simple and inexpensive technique, has attracted significant attention in the preparation of the carbon nanofiber mat with large surface-to-volume ratio and good electronic properties. The electrospun carbon fibers have been applied in many fields, such as electrodes in fuel cells and electrochemical double layer capacitors (EDLC) [25–27]. Usually, polyacrylonitrile (PAN) polymer is used as the electrospinning precursor to get the carbon nanofiber, due to its good spinnability in solution [28] and its relatively high carbon yield [29]. However, the channel between carbon basal planes in PAN will be filled during the carbonization process of the electrospun PAN fibers, resulting in a low surface area. Many efforts have been made to activate the carbon surface in order to enhance the specific surface area, during the carbonization of PAN. Physical activation, such as CO2 [30,31] and steam [24,32,33] activation were used to create new pores in order to improve the porosity. Chemical activation by chemical agents such as ZnCl2 [34–36], KOH [37], and H3PO4 [38,39] is the preferred way because it can lead to higher yield, and larger surface area with low operating temperature. Among the chemical agents, ZnCl2 is one of the mostly used agents for enhancing the specific surface area of many kinds of carbon materials [34,35] with mild treatment condition. ZnCl2 can not only catalyze the oxidation stabilization and dehydration, thus developing the micropores during the carbonization [24], but also promote the formation of oxygen-containing groups on the carbon surface facilitating the adsorption of ions [34,40]. Therefore, the purpose of this study was to improve the electrosorption performance of the electrospun carbon fiber electrode by ZnCl2 activation. The ZnCl2 activated carbon nanofiber (Zn-AECNF) was fabricated by electrospinning polyacrylonitrile (PAN) nanofiber,

447

followed by pre-oxidation, carbonization and ZnCl2 post-treatment. The ZnCl2 activated carbon nanofiber is used for the first time as the electrode material in CDI cell to investigate electrochemical behavior and desalination performance. The excellent desalination performance demonstrates that it is promising to develop a good CDI electrode material for capacitor desalination application.

2. Experimentals 2.1. Fabrication of ZnCl 2 activated electrospun carbon nanofiber (Zn-AECNF) Polyacrylonitrile (PAN) (Mw = 150,000, Aldrich) was dissolved in N,N-dimethyl formamide (DMF) at a concentration of 9.0 wt.% and stirred continuously at 50 °C for 12 h to obtain a homogeneous PAN solution. For electrospinning, the PAN solution was placed in a 10 ml syringe with 25 gauge stainless steel needle through which the solution was pushed by a syringe pump (Baoding Longer Precision Pump Co., Ltd, China). The anode of the high-voltage power supply (DW-P303-1ACF0, Dongwen High Voltage, Tianjin, China) was connected to the syringe needle tip, and the cathode was connected to an Al foil, which is electrically connected to the ground. The electrospun fibers were collected on the Al foil. The electrospinning parameters, including voltage, flow rate of solution and the distance between the nozzle to collector were carefully adjusted in order to get the uniform and continuous fibers (see SI figures). The optimal electrospinning conditions are as follows: The applied voltage of 15 kV, flow rate of 0.5 ml/h and a receiving distance of 20 cm. After being dried under vacuum for 12 h, the spun fibers were preoxidized in air by heating at 250 °C for 90 min at a rate of 2 °C/min, followed by carbonization at 800 °C (heating rate 5 °C/min) under nitrogen atmosphere with a flow rate of 80 ml/min. After being held at this temperature for 1 h, the sample was cooled under the nitrogen. Then the electrospun carbon nanofiber (ECNF) was obtained. In order to adjust the loading of Zn in carbon fiber, three ZnCl2 aqueous solutions (20%, 40% and 60% (w/v)) were used to treat the ECNF. After immersion in ZnCl2 solution for 24 h, the carbon fiber samples were dried at 90 °C for 24 h. the high ZnCl2 concentration will result in the increased ZnCl2 loading on the carbon fiber surface. For activation, the ZnCl2-impregnated carbon fiber was heat-treated under N2 atmosphere with a flow rate of 80 ml/min. The temperature was raised from room temperature up to 500 °C at a rate of 5 °C/min. The sample was held at 500 °C for 90 min before cooling down to room temperature. Then it was immersed in 1 mol/L hydrochloric acid overnight, followed by thorough washing with 1 mol/L hydrochloric acid and hot distilled water to remove the residual zinc compounds until the pH value of the washing solution was neutral. After drying in the oven at 90 °C overnight, the ZnCl2 activated electrospun carbon nanofiber

Fig. 1. SEM images of (A) ECNF and (B) Zn-AECNF. The high-magnification images are at the upper left of (A) and (B). Left-bottom of (B) shows the photo image of the flexible Zn-AECNF sheet.

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J. Liu et al. / Desalination 344 (2014) 446–453

Fig. 2. Contact angle of water droplet on (A) ECNF and (B) Zn-AECNF mat.

(Zn-AECNF) was obtained. The Zn-AECNF treated with 20%, 40% and 60% ZnCl2 were named as Zn-20, Zn-40 and Zn-60, respectively.

2.2. Characterization of ZnCl2 activated carbon nanofiber The Zn-AECNF sample treated with 60% ZnCl2 (unless otherwise noted) was used for characterization unless otherwise described. The surface morphology of the obtained ECNF and Zn-AECNF was examined using scanning electron microscopy (SEM, Hitachi, S-4700, Japan).

A

The wettability of the samples was determined by water contact angle measurement using JGW-360B (Xihuayi Technology Co. Ltd., Beijing, China). Samples were placed on a sample stage and a single drop (20 μL) of distilled water was dropped at three different places and the average contact angle was then measured. The specific surface area was evaluated by the nitrogen adsorption with an accelerated surface area and porosimeter (ASAP2020, Micromeritics, USA). X-ray photoelectron spectroscopy (XPS) measurement was performed on an AXIS Ultra DLD spectrometer (Kratos Co., JP.) with Al K-

B C 1s

Zn 2p

ECNF

Zn-AECNF O 1s N 1s

Zn2p 3/2

C 1s

Zn2p 1/2

Zn-AECNF O 1s

N 1s

0

200

400

Zn2p 3/2,1/2

600

800

1000 1200

1010

1030

Binding Energy/eV

C

D C 1s

ECNF

280

285

290

Binding Energy /eV

1070

E ECNF

N 1s

ECNF

O 1s

Zn-AECNF

275

1050

Binding Energy/eV

Zn-AECNF

295

525

530

535

Binding Energy/eV

540

Zn-AECNF

390

395

400

405

Binding Energy /eV

Fig. 3. XPS spectra of ECNF and Zn-AECNF in survey scan (A), and the core level scans of Zn 2p (B), C 1s (C), O 1s (D) and N 1s (E).

410

J. Liu et al. / Desalination 344 (2014) 446–453

449

Table 1 C, O and N distributions obtained by the peak resolution of XPS spectra. Atom form

C-I 284.6

C-II 286.1

C-III 287.6

O-I 531.6

O-II 532.9

O-III 534.1

N-6 398.4

N-5 400

N-Q 400.7

N-O 404

Atom% (ECNF) Atom% (Zn-AECNF)

70.1 48.8

11.7 30.3

18.1 20.9

43.2 44.9

38 46.2

18.8 8.9

41.9 39.9

31.8 16.5

31.2 37.1

2.6 6.5

2.3. Capacitor assembly and desalination testing The above prepared Zn-AECNF mat is very soft and flexible. It was cut into a 60 mm × 40 mm size, and assembled into the CDI cell. Capacitor desalination was performed in a 500 mg/L NaCl aqueous solution (50 ml) in batch mode. A peristaltic pump was used to control the flow rate of the solution. All the desalination experiments were done at room temperature with a flow rate of 10 ml/min. The desalination performance of the capacitor was measured by constant current charge–discharge test using the computer-controlled battery test system (LANHE, CT2001A, Wuhan, China). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) measurements were carried out using a Princeton 2273 electrochemical system with two-electrode configuration (same as the capacitor cell) in 1 mol/L NaCl solution. For EIS, the frequency range was from 10 MHz to 1 mHz with a 5 mV amplitude voltage. 3. Results & discussion 3.1. Characterization of the electrospun carbon fiber Fig 1(A) and (B) show the SEM images of ECNF and Zn-AECNF, respectively. It can be seen that the non-activated ECNF possesses a smooth surface with a uniform size (diameter ca. 250 nm). After activation with ZnCl2, the resultant Zn-AECNF still keeps its original fibrillous structure, with only a little decrease of the fiber diameters due to the shrinkage of fibers during the dehydration activation of ZnCl2. By comparing the high-magnification SEM images (upper left of Fig. 1), we can see that the Zn-AECNF surface became a little rough with some pores randomly distributed on the fiber surface. It is worthy to be noted that the obtained Zn-AECNF mat is still bendable, as shown from the photographic image of the Zn-AECNF sheet in the left bottom of Fig. 1(B). The flexibility of the fibers mat is very important as a freestanding electrode for capacitor assembly. The wettability of the formed Zn-AECNF was studied by watercontact angle test. Fig. 2(A) and (B) show the photographic images of water droplets on the surface of the ECNF and Zn-AECNF sheets, respectively. The contact angle of ECNF without ZnCl2 activation was ca. 133.7° ± 0.6° (Fig. 2B), consistent with the high hydrophobicity of the general carbon [41]. After ZnCl2 activation, the contact angle of Zn-AECNF was down to ca. 21.1° ± 3°. Obviously, the hydrophilicity of Zn-AECNF is significantly increased. This good water wettability is very beneficial to the diffusion and adsorption of ions in the solution to the fiber surface, resulting in the improved surface utilization [42].

The XPS experiments were performed to investigate the atom components on the carbon surface. The XPS survey scan curves of ECNF and Zn-AECNF shown in Fig. 3(A) reveal the presence of characteristic C, N and O peaks in both samples, while a small Zn 2p peak was observed in the Zn-AECNF compared to ECNF. The core level spectrum shown in Fig. 3(B) exhibited typical Zn 2p peaks with the binding energy at 1021.65 and 1044.65 eV. It indicates that there is still a trace amount of Zn compound left in the Zn-AECNF even though a thorough wash was given. It is possible that the melting Zn salt is intercalated into the carbon layer by forming the ZnO or Zn complexes during the high temperature treatment [34], resulting in difficult removal. The core level XPS spectra of C, O and N elements in ECNF and Zn-AECNF were shown in Fig. 3(C), (D) and (E), respectively. The C 1s XPS spectra of the samples have been resolved into the individual component peaks with the strong peak at 284.6 eV (C-I) representing the graphitic carbon. The peak at 286.1 eV (C-II) represents the C\O band in the phenolic, alcohol or C–N group, and the peak at 287.6 eV (C-III) comes from C_O in the carbonyl or quinone group [43]. The O 1s spectra showed three resolved peaks: at 531.6 eV (O-I, from the carbonyl or quinine group), at 532.9 eV (O-II, from the hydroxyl and ether groups), and at 534.1 eV (O-III, from the carboxyl group) [44]. In the N 1s XPS spectra, the peak with binding energy at 398.4 eV (N-6) originates from pyridine N, and a resolved peak at 400 eV (N-5) attributes to pyrrolic N, pyridoic or amine moieties. The peak at 400.7 eV (N-Q) is ascribed to the nitrogen substituents in the aromatic graphene structures (quaternary nitrogen), and the weak peak at 404 eV (N-O) may be the pyridine-N-oxides or ammonia groups. The distribution of these elements was collected in Table 1. Clearly, through the ZnCl2 activation, the concentration of N-Q increased with the decrease of N-6 and N-5, indicating the transformation of the N-6 and N-5 form into N-Q during the activation process [45]. And the carbon surface possesses more O-containing groups such as OH or C–O group. Rufford et al. [46] and Xiang et al. [40] have also proved that ZnCl2 activation had a large effect on the oxygen and nitrogen functional groups on the carbon surface. The presence of a small amount of Zn elements and the variation of oxygen and nitrogen functional groups on carbon fiber surface result in the increase of surface hydrophilicity, which may contribute to the wettability of the Zn-AECNF. FTIR analysis was done to examine the functional groups on the carbon surface. The FTIR spectra of ECNF and Zn-AECNF were shown in Fig. 4. The band of O\H stretching vibration at 3420 cm− 1 is due to the existence of surface hydroxyl groups or chemisorbed water. The

Transmittance %

alpha X-ray radiation as the X-ray source for excitation. The operating pressure in the analysis chamber was below 10−9 Torr with an analyzer pass energy of 20 eV. High resolution scans were recorded with a step of 0.1 eV. Fourier transform infrared spectroscopy (FTIR) studies were carried out in the infrared range of 400–4000 cm−1 using a Nicolet iN10 MX spectrometer (ThermoFisher, USA). The carbon fiber sample was milled in an agate mortar and then mixed with KBr in a mass ratio of 1:500. After careful milling again, the mixture was pressed with 10 tons of force load to make about 0.5 mm thick clear pellets, the transmission of which was measured in the FTIR spectrometer after drying at 120 °C. Each spectrum was collected by accumulating 100 scans at a resolution of 4 cm−1.

1616

1152

ECNF Zn-AECNF 3420

400

1200

2000

2800

3600

Wavenumber (cm-1) Fig. 4. FTIR spectra of ECNF (dotted line) and Zn-AECNF (solid line).

J. Liu et al. / Desalination 344 (2014) 446–453

B 180

pore volume (cm3/g)

Quantity Adsorbed(cm3/g STP)

A d 120

c b 60

0.4

0.3

510 micropore volume mesopore volume surface area 340

0.2 170 0.1

Surface area (m2/g)

450

a 0

0

0.4

0.8

1.2

0.0

ECNF

Zn-20

Zn-40

Zn-60

0

Relative Pressure(p/p0) Fig. 5. (A) N2 adsorption–desorption isotherms of the ECNF (a) and Zn-AECNF (b–d). Curves b–d correspond to Zn-20, Zn-40 and Zn-60, respectively. (B) Summary of BET specific surface area and pore volume of the Zn-AECNF treated with different concentrations of ZnCl2. For comparison, the results of ECNF are also shown.

band located at about 1616 cm−1 is attributed to the carbonyl group. The bands between 1275 cm−1 and 1100 cm−1 could be assigned to C\O and C\N vibrations. The obvious increase of the band at 1152 cm−1 could be attributed to the increase of ester, ethers, or phenol, alcohol group [47]. This result is consistent with that of XPS. The N2 adsorption–desorption isotherms were measured in order to determine the specific surface area and pore-size distribution of the ECNF and Zn-AECNF. As shown in Fig. 5(A), the adsorbed volume of the ECNF is pretty low, indicating its nonporous characteristics. With ZnCl2 activation, the obtained Zn-AECNF samples show a characteristic type I isotherm with a drastic nitrogen uptake at low relative pressure area, indicating that the activated carbon is predominantly microporous [48]. And the observed H4 hysteresis loop is often associated with slitlike pores [48]. A little uptilting at higher relative pressure (0.5 P/P0) of the isotherm is due to the presence of mesopores, as confirmed from SEM image (vide supra). Specific surface areas and pore volumes were calculated by BET and t-plot methods, respectively [48,49], as shown in Fig. 5(B). The non-treated ECNF possesses the negligible value of the BET specific surface area and pore volume. Comparatively, the pore volume and specific surface area of the Zn-AECNF have an obvious increase with ZnCl2 concentration, which is consistent with that of the ZnCl2 activated granular carbon [34]. Too high ZnCl2 concentration for carbon fiber impregnation resulted in poor mechanical properties of the electrospun carbon fiber and difficulty in further handling in the CDI cell. 3.2. Electrochemical characterization As mentioned, the electrospun carbon fiber still keeps good flexibility after ZnCl2 activation, so it can be used as a free-standing electrode for

electrochemical application. Without the complicated manufacturing processes, eliminating the addition of non-active material such as polytetrafluoroethylene binder, the resultant carbon fiber electrode should have improved electrochemical properties. In the present study, the CV and EIS behaviors of the carbon fiber electrode were investigated in order to get the information of these electrode materials about their conductivity and capacity. Fig. 6 shows the cyclic voltammograms of the ECNF and Zn-AECNF in a 1 mol/L NaCl aqueous solution at a scan rate of 10 mV/s, which represent the charging–discharging process of a typical electric double layer capacitor. The pristine ECNF exhibits a very low current response. Based on the formula C = I / v (C: the specific capacitance (F/g), I: current density (A/g), v: scan rate (V/s)), the capacitance of the pristine ECNF was calculated to be 2.1 F/g. The low capacitance is due to the limited ion-adsorption sites and poor ionic diffusion, related to its nonporous structure and its hydrophobic property demonstrated previously. With the ZnCl2 solution treatment, the capacitance current of the resultant Zn-AECNF electrodes enhanced obviously, and an ideal rectangular shape was seen on the Zn-60 electrode, representing an excellent double layer capacitor behavior. The specific capacitance values calculated from the CV curves were 10.1, 34.5 and 39 F/g for the Zn-20, Zn-40 and Zn-60 samples, respectively. It suggests that with ZnCl2 activation, the porous structure of the Zn-AECNF contributes greatly to the formation of double-layer capacitance, which is much beneficial to the ion adsorption during the charging process [46]. The electrochemical behavior of the electrospun carbon nanofiber electrodes with and without ZnCl2 activation could be more clearly understood by EIS measurements. The complex impedance plots of different carbon fiber electrodes are shown in Fig. 7. In the high frequency region, the diameter of the semicircle corresponds to the charge transfer

50

d c

0.1

b

Zim (ohm)

Current density (A/g)

0.3

a

-0.1

-0.3 -0.3

30

0.1

0.3

0.5

Potential (V) Fig. 6. Cyclic voltammograms of the ECNF (a) and Zn-AECNF (b–d) electrodes in 1 mol/L NaCl aqueous solution. Curves b–d correspond to Zn-20, Zn-40 and Zn-60, respectively. Scan rate: 10 mV/s.

c

b

a 10

-10 -0.1

d

0

5

10

15

20

Zre (ohm) Fig. 7. Complex impedance curves of the ECNF (a) and Zn-AECNF (b–d) electrodes in 1 mol/L NaCl aqueous solution. Curves b–d correspond to Zn-20, Zn-40 and Zn-60, respectively.

J. Liu et al. / Desalination 344 (2014) 446–453

A

B Salt Adsorption (mg/g)

Conductivity (µS/cm)

1020

a

980

b c

940

900

d 860

0

4

8

12

12 10 8 6 4 2 0

16

451

ECNF

Zn-20

Zn-40

Zn-60

t (min) Fig. 8. (A) Ionic adsorption behavior of ECNF (a) and Zn-AECNF (b–d) electrodes in CDI cell in 500 mg/L NaCl solution. Curves b–d correspond to Zn-20, Zn-40 and Zn-60, respectively. Data were obtained after 5 charging–discharging cycles. (B) The salt adsorption per cycle on different electrodes derived from the average of 5 charging–discharging cycles.

resistance (Rct). It was observed that with the increase of the ZnCl2 concentration, the Rct of the resultant Zn-AECNF much decreased compared with the high Rct value of pristine ECNF. It indicates that the electronic conductivity and charge transfer in the ZnCl2 activated carbon fiber electrode was facilitated, which is consistent with the improvement of wettability of fiber. Furthermore, the sloping line in the low frequency region, which is associated with the ion diffusion inside the electrode, inclined steeply to the imaginary axis, indicating that the kinetics of the diffusion and adsorption of ions between solution and the electrode surface were rapid on the Zn-AECNF, resulting in an excellent capacitive behavior. This is also supported by the changes in CV curves as described above. 3.3. CDI application of the Zn-AECNF electrodes The desalination performance of the activated Zn-AECNF electrodes was evaluated by assembling the CDI cell with the Zn-AECNF electrode. The cell was first cycled with the 500 mg/L NaCl aqueous solution to arrive at the physical adsorption balance. During the charging– discharging process, the conductivity of the solution was recorded. Fig. 8(A) shows the conductivity variation of the solution in the CDI flow cell corresponding to different electrodes during the constantcurrent charging step. The conductivity decreased with charging time, indicating the adsorption of ions from the solution to the electrode. Compared with the pristine ECNF, more and more ions are removed with the increase of the ZnCl2 concentration. The average desalination amount per cycle is calculated and summarized in Fig. 8(B). The desalination amount of 10.52 mg/g was obtained at the Zn-60 electrodes, which is much higher than that of pristine ECNF (1.8 mg/g), and is twice as high as the CO2-activated ECNF (4.64 mg/g) [30] and steamactivated ECNF (3.5 mg/g) [32]. Since these two research groups used the low initial salt concentration which will lead to low ion removal, different carbon-based CDI cell performance was collected in Table 2 for comparison. Our result is comparable to the commercial activated carbon electrode [54] and the titanium carbide-derived carbon [56].

As mentioned previously, with the activation of ZnCl2 to the electrospun carbon nanofiber, the increase in specific surface area and wettability improvement contributed a lot to the enhancement of the salt adsorption performance. It concludes that our ZnCl2 activated electrospun carbon nanofiber electrode is a good candidate for desalination application. The regeneration and stability of the electrode were investigated by recycling the CDI cell with a charging and discharging alternate mode in the 500 mg/L NaCl solution. After three days of charging–discharging recycling, the capacitor can still keep its good desalination properties. The continuous charging–discharging process was monitored by simultaneously recording conductivity and voltage (shown in Fig. 9). During the charging process, the voltage increases while the conductivity decreases, for the reason that the ions were adsorbed inside the carbon fiber electrode. In the zero-current discharging process, the cell voltage decreases with the ions desorbing from the electrode, and electrode regeneration is achieved with the final discharge voltage close to zero. Repeated charging–discharging test indicates that the ZnCl2AECNF electrodes are very stable and can be regenerated completely, which is very important for practical desalination application. 4. Conclusions The ZnCl2 activated carbon nanofiber was successfully fabricated from the electrospun PAN by carbonization and ZnCl2 activation. The resultant ZnCl2 activated ECNF possesses high specific surface area and improved wettability compared with the non-treated ECNF. XPS and FTIR demonstrated the variation of functional groups on the carbon fiber surface. The excellent electrochemical double layer capacitor performance was obtained. The salt-removal tests in the capacitor with Zn-AECNF as the electrodes showed a high electrosorption capacity. The higher ZnCl2 concentration for activation leads to higher capacitance and thus higher salt adsorption of the resultant Zn-AECNF. Multiple cycling tests demonstrate its good stability and regeneration performance during the charge and discharge process. The ZnCl2 activated electrospun carbon nanofibers are of potential in electrochemical

Table 2 Comparison of the desalination performance on different electrode materials. Carbon material

Initial NaCl conc. (mg/L)

Operation voltage (V)

Duration per cycle (min)

Salt adsorption (mg/g)

Ref.

Carbon aerogel TiO2-carbon cloth Carbon nanotubes/carbon fiber Sulfonated graphite Activated carbon powder Graphene/carbon nanotube Activated carbon Carbide-derived carbon Electrospun carbon fiber

100 5844 119 250 292 780 936 290 500

1.2 1.0 1.2 2.0 1.2 1.2 1.2 1.2 1.2

60 600 60 75/only adsorption step 60 62 50/only adsorption step 33 33

2.9 4.3 3.3 8.6 10.9 18 2.34 10.4 10.5

[50] [51] [52] [53] [54] [55] [19] [56] This work

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J. Liu et al. / Desalination 344 (2014) 446–453

3.2

1080 Potential

1000

1.6

920

0.8

840

0

0

75

150

225

300

375

450

Conductivity (µS/cm)

Voltage (V)

Conductivity

2.4

760

t (min) Fig. 9. Variation of the voltage and conductivity with time under the continuous constant current charge–discharge mode of the Zn-AECNF electrode.

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