Chemical Engineering Science 61 (2006) 428–433www.elsevier.com/locate/ces

NaCl adsorption in multi-walled carbon nanotube/active carboncombination electrode

Kai Daia,∗, Liyi Shia,b,c,1, Dengsong Zhanga,b, Jianhui Fangb

aSchool of Material Science and Engineering, Shanghai University, 99 Shangda Road, Shanghai 200444, PR ChinabCollege of Science, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China

cNano-Science & Technology Center, Shanghai University, 99 Shangda Road, Shanghai 200444, PR China

Received 3 April 2005; received in revised form 18 July 2005; accepted 18 July 2005Available online 30 August 2005

Abstract

In this paper, we proposed a new process for fabricating electrochemical double layer capacitors employing active carbon and multi-walled carbon nanotubes to adsorb Na+ and Cl− from NaCl solution. Due to their unique mesoporosity, active carbons have high abilityto desalt NaCl solution. But they have many defects such as high electrical resistance, high-energy consumption and low intensity. Sincecarbon nanotube is a new material which has high intensity and low resistance, we can composite the merits of active carbon and carbonnanotube and develop carbon nanotube/active carbon materials combination electrode. It was tested that when carbon nanotube contentin carbon materials is 10%, the characteristics of combination electrode is the best for us to desalt brackish water because of their highdesalination characterization and low energy consumption. Though there are a few technical problems to be solved, our results show apromising technique for desalting salt water.� 2005 Elsevier Ltd. All rights reserved.

Keywords:Carbon nanotube; Active carbon; Salt water; Desalination

1. Introduction

In recent years, a great interest has been focused on elec-trochemical double layer (ECDL) capacitors because of theirhigh energy density and long life cycle (Jayalakshmi et al.,2004; Conway et al., 1997; Michael and Prabaharan, 2004;Zheng, 2004).

Active carbon (AC) has received increasing attentionfor application as electrode materials in the ECDL capac-itors because of their large specific surface area and lowmass density, morphology and high BET-specific surfacearea (Ghenciu, 2002; Wei et al., 2000; Tarasevich et al.,2003; Robert et al., 2003; Motoo et al., 1997; Haichao

∗ Corresponding author. Tel.: +86 21 66134852; fax: +86 21 66135066.E-mail addresses:sly@snpc.org.cn, daikai940@163.com(K. Dai).

1Also for correspondence.

0009-2509/$ - see front matter� 2005 Elsevier Ltd. All rights reserved.doi:10.1016/j.ces.2005.07.030

et al., 2004). But it has many defects such as high resis-tance, high energy consumption and unfavored mechanicalproperties. Carbon nanotube (CNT) has been studied exten-sively and found to be promising as a new nanoscale materialfor a variety of potential applications because of their ex-cellent electrical properties, good mechanical strength, andlow resistance (Zhanhong et al., 2003; Lamari et al., 2002;Sarangi et al., 2002; Penza et al., 2004), we can compositethe merits of AC and CNT and develop CNT/AC materialscombination electrode.

The purpose of this study was to desalt NaCl solution us-ing CNT/AC materials combination electrode. AC and CNTwere treated at first, we compared the performance of ECDLcapacitors with electrodes made from different CNT con-tent, it was found that 10% content of CNT is the best elec-trode both for desalting NaCl solution and low energy con-sumption, and the regeneration of the electrode was alsostudied.

K. Dai et al. / Chemical Engineering Science 61 (2006) 428–433 429

2. Experimental

2.1. Preparation and treatment of AC

ACs were the carbonization products from coconut(Zhonghua and Srinivasan, 1999). The carbonization pro-cess involved heating the source material up to 900◦C,usually in the absence of air, to drive out the volatile sub-stances and leaving a charred residue. In order to increasethe surface area, the carbon was activated by carbon dioxideat 850◦C and immersed in HCl for 3 h, and then the mixturewas washed several times with double distilled water on asintered glass filter until the washings showed no acidity.At last further activated carbon were obtained after dryingin an oven at 100◦C for 2 h.

2.2. Preparation and treatment of CNT

CNTs were produced by chemical vapor depositionmethod using CH4 and La2NiO4 as the carbon source andcatalyst (Hou et al., 2002), the aperture of the CNTs is40–60 nm and the length is about several micrometers. Theraw materials were dispersed by using ultrasound for 4 h in40 wt% Nitric acids and immersed in 20 wt% Nitric acidsfor 48 h, and then the mixture was washed several timeswith double distilled water on a sintered glass filter until thewashings showed no acidity. Finally, purified CNTs wereobtained after drying at 100◦C for 2 h.

2.2.1. Preparation of electrodeCarbon materials used in this study were presented as

xCyA, where C is CNT, A is AC andx andy is the weightratios of each component, for example, 90C10A is the mix-ture carbon materials of 90 wt% CNT and 10 wt% AC. Andthen the carbon materials mixed with binder powders in aweight ratio of 80:20 was mounded under 20 MPa pressureat 150◦C for 15 min, where the binders were made up ofphenolic resin (95%) and a minute of urotropine (5%). Fi-nally, the samples were obtained by carbonization at 850◦Cfor 2 h under nitrogen environment. All of the electrodeswere cut into 115.0 × 75.0 × 1.5 mm specimens.

2.2.2. PrincipleThe key part of desalinator is flow-through capacitor, ide-

ally, the electrochemical double layer formed at the elec-trode and brackish water interfaces in the drive of directcurrent because chemical potential of positive and negativeions is different in polarized electrodes and electrolyte, theions moved to electrode which has reverse polarity (Pauloand Manuel, 2004). The pores of electrodes are used to storeions. Therefore, fresh water is obtained, asFig. 1(a) shows.When ions are saturated in the electrodes, we change thepolarity of electrodes, asFig. 1 (b) shows, the ions can fleefrom the electrodes by repulsive force, and the electrodesregenerated.

Cl- Na+

NaCl solution NaCl solution

Fresh water High salt water

Cl- Na+

(a) (b)

Fig. 1. Assembly drawing of desalination theory. (a) The process of NaCladsorption; (b) the process of regeneration.

The capacitance of electrochemical double layer is calcu-lated according toNishino (1988):

C =∫

�0� dS

4��, (1)

whereC is the capacitance in Faraday (F),�0 is the dielec-tric constant of vacuum,� is the dielectric constant of solu-tion, � is the distance between the surface of electrode andthe center of ion,S is the surface area of electrode. The ca-pacitance of electrode is the primary factor to desalt the saltwater. According to Eq. (1),� is the constant in a settledcapacitor, so a high value ofS is required. ACs and CNTshave high surface area and high chemical inertness, whichare highly desirable properties as an electrode material forflow-through capacitor.

2.2.3. Analytical and testing instrumentThe salt concentration was measured by a conductivity

meter of Cyerscan CON200 (EUTECH) at the outlet ofthe apparatus, Scanning electron microscopy (SEM) of theelectrodes was performed using JSM-6700F (JEOL), theBrunauer–Emmett–Teller (BET) specific surface area valueswere performed by ASAP2100 (Micromeritics), voltage wasperformed by DF1730SB5A (ZHONGCE).

3. Results and discussion

3.1. Further activation

Fig. 2 shows the N2 adsorption isotherms of untreatedand treated AC at 77 K. The BET surface area and porespecific volume data of two samples were obtained fromN2 adsorption data using the BET equation, and they werelisted inTable 1.

CO2 has been employed here to improve the surface areaand pore volume, the molecules activation mechanism has

430 K. Dai et al. / Chemical Engineering Science 61 (2006) 428–433

8007006005004003002001000250

300

350

400

450

500

550

Vo

lum

e ad

sorb

ed (

ST

P)(

cm3 /

g)

Relative pressure(P/P0)

untreated AC treated AC

Fig. 2. Isotherms for N2 adsorption of untreated and treated AC.

Table 1Specific surface area and pore specific volume data of untreated andtreated AC

Sample SBET (m2/g) V (cm3/g)

Untreated AC 1235.0 0.537Treated AC 1434.4 0.751

SBET: surface area calculated using the BET equation;V: total porevolume estimated by converting the amount of N2 gas adsorbed at arelative pressure of N2.

been suggested as follows:

C + CO2 → 2CO,

C + CO2 → C(O) + CO,

C(O) → CO,

CO+ C → C(CO).

The porosity of AC, after activated treatment, is de-veloped by burning off part of the carbonaceous mate-rial and removing the mineral matter and opening upclosed pores, the carbon pore walls begin to react awaywhich lead to enlargement of the pore size at first andthen partial collapse of the pore structure and reductionin the number of pores. Therefore, the surface area andpore volume of further activated carbon are obviouslyincreased.

3.2. Desalination characterization

Fig. 3shows the desalination curve of different electrodes,where the starting salt concentration is 5000 mg/L, voltageis 1.2V, water current is 10 ml/min and the number of elec-trode sheet is 10. FromFig. 3, we can see that different elec-trodes have different NaCl adsorption capacity. According toEq. (1), a linear relation is found between the capacitance of

0 10 15 20 25 30 35 40 45 50

3000

3500

4000

4500

5000

Sal

t co

nce

ntr

atio

n (

mg

/L)

Time (min)

100C0A 80C20A 50C50A 20C80A 10C90A 0C 100A

5

Fig. 3. Desalination curve of different electrodes.

0

50

100

150

200

250

300

350V

olu

me

adso

rbed

(S

TP

) (c

m3 /

g)

Relative pressure(P/P0)

0C100A 10C90A 20C80A 50C50A 90C10A 100C0A

-100 0 100 200 300 400 500 600 700 800

Fig. 4. Isotherms for N2 adsorption of different electrodes.

electrochemical double layer and surface area of electrode,and NaCl adsorption in electrodes is closely connected tothe capacitance of electrochemical double layer. Therefore,NaCl adsorption performance is closely linked with surfacearea of electrode.Fig. 4 shows the N2 adsorption isothermsof different electrodes at 77 K. The BET surface area andpore-specific volume data of electrodes were listed inTable 2. As indicated inTable 2, 10C90A has the maxi-mum value of specific surface area and the highest NaCladsorption capacity.

Fig. 5 shows SEM images of 100C0A, 0C100A and10C90A. FromFigs. 5(b) and6 we can find that there areenormous gaps between the AC particles. When a smallamount of CNTs were put in AC, asFig. 7(c) shows, CNTsoccupied the space of the gaps, which would not influencedesalination of AC. Besides, the desalination capacity ofcombination materials increased slightly after some CNTs

K. Dai et al. / Chemical Engineering Science 61 (2006) 428–433 431

Table 2Specific surface area and pore specific volume data of different electrodes

Sample SBET (m2/g) V (cm3/g)

0C100A 1219.5 0.89610C90A 1309.1 0.94020C80A 590.5 0.29150C50A 328.0 0.18790C10A 138.4 0.128100C0A 129.2 0.132

were added to adsorb NaCl. When the CNT content is 10%,electrode amounts to its best adsorption capacity. As theCNT content increases continuously, quantity of AC de-creases, and specific surface area decreases rapidly, so doesadsorption capacity.

In order to describe NaCl adsorption capacity in differ-ent CNT/AC materials combination electrode more clearly,NaCl adsorption capacity in 1 g of electrodes, in other words,specific NaCl adsorption capacity in electrodes are broughtforward, and specific NaCl adsorption capacity in electrodesis calculated according to

M = (P0 − P)V/m, (2)

whereP0 is the starting salt concentration,P is the finalsalt concentration,V is the volume of desalted water,m isthe mass of the electrode. According to Eq. (2) andFig. 4,we can get the relation between specific NaCl adsorptioncapacity and different CNT content of electrode, asFig. 5shows.

The conception of specific energy consumption isbrought forward to explain energy consumption of NaCladsorption in electrode. Here, specific energy consump-tion is energy consumption in treating 1 g salt water.Fig. 7shows the relation between specific energy consump-tion in 5000 mg/L NaCl solution and different CNT contentof electrode. FromFig. 7, we can find that energy con-sumption decreased sharply as CNT content increased whenCNT content is less than 10%, and energy consumptiondecreased smoothly when CNT content is over 10%. En-ergy is almost consumed in electric resistance of electrode.There are two factors which can influence electric resis-tance of electrode: one is carbon material self-resistanceand the other is contact resistance. Contact resistancecontains AC–AC contact resistance, CNT–CNT contact re-sistance and AC–CNT contact resistance. AC–AC contactresistance is larger than AC–CNT contact resistance, andAC–CNT contact resistance is much larger than CNT–CNTcontact resistance. Therefore, when CNTs were added inelectrode, energy consumption decreased sharply at firstbecause CNTs have good conductivity, and low AC–CNTcontact resistance and CNT–CNT contact resistance willtake place of some AC–AC contact resistance. When theCNT content is 10%, almost all the CNTs contact one byone, which forms a continuous linked current-conducting

Fig. 5. SEM images of (a) 100C0A, (b) 0C100A and (c) 10C90A.

path, and electrons can get through by this way. Thereis also micro-space among some uncontacted CNTs, tun-nel current will come forth to transit the space. Althoughthe CNT content increases continuously, contact resis-tance changes indistinctively and energy consumption fallsgradually.

432 K. Dai et al. / Chemical Engineering Science 61 (2006) 428–433

-10 10 20 30 40 50 60 70 80 90 100 1100

1

2

3

4

5

6

Sp

ecif

ic d

esal

inat

ion

(m

g/g

)

CNT content (wt%) 0

Fig. 6. Relation between specific NaCl adsorption capacity and CNTcontent.

100 1104

6

8

10

12

14

16

Sp

ecif

ic e

ner

gy

cost

(J/

g)

CNT content (wt%)

-10 0 10 20 30 40 50 60 70 80 90

Fig. 7. Relation between specific energy consumption and CNT content.

3.3. Removal-regeneration of desalination

In this study, regeneration of the electrodes was very im-portant, we used the method by reversing voltage of eachelectrode, and the adsorbed ions could come out from elec-trodes by electrostatic forces.Fig. 8 shows the characteris-tics of NaCl adsorption and regeneration of 10C90A, wherethe starting concentration is 5000 mg/L, voltage is 1.2V, wa-ter current is 10 mL/min and the number of electrode pieceis 40. FromFig. 8, we can find that the process of regener-ation could be carried out easily in a short time.

4. Conclusion

It is possible to efficiently remove Na+ and Cl− fromdilute NaCl solution using CNT/AC combination electrode

Fig. 8. Adsorption and desorption curve of 10C90A.

by electric adsorption. It was confirmed that the amountof removal is generally dependent on the surface area andpore volume of the electrode, and energy consumption isalso an important factor to fabricate electrode. Furthermore,10C90A was tested as the best electrode to adsorb NaClfrom salt water because of its high NaCl adsorption capacityand low energy consumption. And the electrode was regen-erated easily with high efficiency. Therefore, it is availableto adsorb NaCl by multi-walled carbon nanotube/active car-bon combination electrode.

Acknowledgements

The authors acknowledge the supports of the NationalHigh Technology Research and Development Program (863Program) of China (2002AA302302) and special nanome-ter fund of Shanghai science and technology committee(0215nm001).

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