Author's Accepted Manuscript

Graphene/SnO2 nanocomposite as an effectiveelectrode material for saline water desalination usingcapacitive deionization

Ahmed G. El-Deen, Nasser A.M. Barakat, KhalilAbdelrazek Khalil, Moaaed Motlak, Hak Yong Kim

PII: S0272-8842(14)00943-2DOI: http://dx.doi.org/10.1016/j.ceramint.2014.06.049Reference: CERI8734

To appear in: Ceramics International

Received date: 28 March 2014Revised date: 24 May 2014Accepted date: 8 June 2014

Cite this article as: Ahmed G. El-Deen, Nasser A.M. Barakat, Khalil Abdelrazek Khalil,Moaaed Motlak, Hak Yong Kim, Graphene/SnO2 nanocomposite as an effectiveelectrode material for saline water desalination using capacitive deionization, CeramicsInternational, http://dx.doi.org/10.1016/j.ceramint.2014.06.049

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Graphene/SnO2 nanocomposite as an effective electrode material for

saline water desalination using capacitive deionization

Ahmed G. El-Deena, Nasser A. M. Barakatb, c,*, Khalil Abdelrazek Khalild,e , Moaaed

Motlakf, and Hak Yong Kimb

aBioNanosystem Department, Chonbuk National University, Jeonju 561-756, South Korea

bOrganic Materials and Fiber Engineering Department, Chonbuk National University, Jeonju

561-756, South Korea cChemical Engineering Department, Faculty of Engineering, Minia University, El-Minia, Egypt

dMechanical Engineering Department, King Saud University, P.O. Box 800, Riyadh 11421, Saudi Arabia

eMaterials Engineering and Design Department, South Valley University, Aswan, Egypt

fDepartment of Physics, College of Science, Anbar University, Anbar 31001, Iraq

Corresponding author:

Nasser A. M. Barakat,

Tel: +82632702363, Fax: +82632702348,

E-mail: nasser@jbnu.ac.kr

2

Abstract

Capacitive deionization (CDI) is a second generation of electrosorption technique for removing

the salt ions from the brackish water. Among the carbonaceous materials, graphene can be

considered a promising CDI electrode due to its exceptional physical properties and chemical

tenability if the specific capacitance could be improved. In this study, graphene/tin dioxide

nanoparticles composites (Gr/SnO2) with different proportions were successfully synthesized via

microwave irradiation; their electrosorption performances in CDI unit were investigated. The

morphology, crystal structure and electrochemical performance were characterized by scanning

electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD),

and cyclic voltammetry. The obtained results indicated that incorporation of SnO2 into graphene

has a great impact for enhancing the electrosorption capacity. Compared to pristine graphene

electrode, higher specific capacitance was observed for all the fabricated Gr/SnO2 composite

electrodes. Specifically, the electrode having 15 wt% SnO2 exhibits distinguished

electrochemical performances; high specific capacitance (323 F/g), excellent cycling stability,

very good salt removal efficiency (83%), and distinct electrosorptive capacity of (1.49 mg/g).

Overall, the present study demonstrates effective and facile route to distinctly improve the

desalination performance of graphene-based electrodes for CDI technology.

Keywords: Capacitive deionization; Graphene/SnO2 nanocomposite; Water Desalination.

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1. Introduction

Beside overcoming the water scarcity dilemma, brackish water desalination is required to

avoid many diseases caused by the salty water including kidney failure, dehydration,

hypertension and gastroenterology diseases[1]. Desalination is a key technology to enhance both

the quantity and quality of water[2]. Although there are several technologies have been

introduced for water desalination such as multistage flash distillation (MSF), electrodialysis (ED)

and reverse osmosis (RO), most of those methodologies need specialized and expensive

infrastructures and require a great deal of energy and maintenance difficulties [3-6]. Moreover,

increasing global awareness of environmental protection and energy conservation has spurred

unprecedented interest in developing low cost, clean and environmentally safe desalination

technologies. Due to the low energy consumption, easily operational and maintenance processes,

and friendly environmental characteristic, capacitive deionization (CDI) can be considered as

one of the promising brackish water desalination technologies and has attracted significant

attention.

CDI (or electrosorption) is defined as potential induced adsorption of ions on the surface

of the charged electrode. CDI works as electrical capacitors; the adsorbed charged ions are held

in the electrical double layer near the charged surface of a flow-through capacitor and can be

released back into bulk solution by cancelling or changing the polarity of the potentials of

the electrodes. According to the adsorption/desorption process occur on the electrode surface,

most studies focused on developing of electrodes materials. Generally, the optimal CDI electrode

material should possess these characteristics: high specific capacitance with good electrical

conductivity, fast response to ion absorption-desorption, robust chemical and electrochemical

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stability and wettability. Carbonaceous materials are considered the best candidate

especially from the chemical stability point of view, accordingly many materials have been

introduced for enhanced capacitive deionization performance including carbon aerogels[7],

activated carbon and its composites (AC)[8-10], mesoporous carbon (MC)[11], carbon

nanotube[12], modified carbon nanofibers[13, 14] and graphene[15-18]. However, low

specific capacitance is the main problem facing wide exploitation of the carbonaceous

materials. Consequently, the new trend in CDI technology is enhancing the specific

capacitance of the carbonaceous electrodes by incorporation of other materials [18-20].

Graphene has attracted tremendous attention of most researchers due to distinct features;

large specific surface area, excellent electrical conductivity, good chemical inertia and

remarkable mechanical flexibility[21]. Among many other potential applications, graphene has

been widely used in electrochemical energy storage and was introduced as promising electrodes

for CDI technology [22-24]. Recently, aggregation of the graphene sheets which was considered

frustrating problem could be be minimized or totally prevented by incorporation of nanoparticles

into graphene sheet with good distribution [25, 26]. Accordingly, the intercalating of metal

oxides nanoparticles including MnO2, TiO2 and ZnO into graphene sheet has revealed an

alternative efficient approach to improve the specific capacitance [27-29]. Especially, SnO2 as

low cost, eco-friendly and having good chemical stability and high capacitance is considered a

promising candidate for many electrochemical applications[30]. Despite, Gr/SnO2

nanocomposite has been reported as high performance electrode for supercapacitors and Li-ion

batteries applications[31, 32], however, according to our best knowledge, this promising

composite has not been utilized in CDI technology .

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In this study, facile and green synthesis of graphene intercalated with tin dioxide via

microwave irradiations method is introduced. The main focus is to improve the electrosorption

desalination performance of graphene and investigate the effect of SnO2 loading into graphene

sheet on the salt removal efficiency. As CDI electrode, the proposed electrodes revealed distinct

performance in water desalination.

2. Experimental

2.1 Materials

Commercial graphite powder (<20 μm) was utilized as graphene precursor. Hydrogen

peroxide 30 % (w/w), KMnO4, HCl, SnCl2·2H2O, urea and piperidine (Sigma Aldrich) were used

without further purification.

2.2 Preparation of graphitic oxide.

Graphene oxide was synthesized from the natural graphite based on Hummer’s method [33]. In

more details, 6 g of the graphite powder treated twice by 5% HCl was placed in 140 ml of

concentrated H2SO4 then 3g of NaNO3 was added; the produced hot mixture was cooled to 0 °C.

Later on, 18.0 g of KMnO4 was added stepwisely to avoid increasing the reaction temperature

more than 20 °C. After dilution with DI water, the temperature was increased to 98 °C. The

mixture was then re-cooled to room temperature and 60 ml H2O2 (30 wt %) was added. Finally,

the mixture was filtered under vacuum and washed with a 10% aqueous HCl solution several

times and then dried at 50 °C.

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2.3 Synthesis of graphene/SnO2 composite via microwave irradiation.

Gr/SnO2 composite was prepared by one-pot synthesis procedure. The typical route is as

follows: Specific amount of SnCl2·2H2O was dissolved into 100 ml water, then GO powder was

added to the obtained solution to prepare final mixtures having SnO2 weight percentages of 10%,

15% and 20% with respect to dry GO; the solutions were sonicated for 45 min. To every

formulation, 0.5 mg of urea was added and the mixture was stirred for 2h. The obtained

slurries were subjected to microwave radiation at 1000 W for 30 min. The precipitated graphene

sheets were filtered and washed with distilled water then dried at 50 oC for 24 h. The pristine

graphene was synthesized by the same procedure in SnCl2·2H2O-free solution.

2.4 Electrochemical properties of the synthesized electrodes.

Three-compartment cell including graphene composite electrodes, a platinum wire, Ag/AgCl

electrode as working, counter electrode and reference electrode, respectively was utilized. CV

experimental measurement was carried out using different NaCl solutions (0.1, 0.5 and 1 M), the

sweep potential rang was adjusted from -0.4 to 0.6 V [vs. Ag/AgCl]. The specific capacity was

calculated by integrating the full CV cycle to determine the average value according to the

following equation [19, 34]

(1)

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Where Cs is the specific capacitance (F/g), I is the response current (A), V is the potential (V), υ

is the potential scan rate (V/s), and m is the mass of the electro active materials in the electrodes

(g).

2.5 Electrosorptive capacity measurement

The electrodes were prepared by mixing (90 wt %) of the modified graphene as active

component and polytetrafluoroethylene (10 wt %) with ultrasonication for 30 min. The mixture

was coated onto glassy carbon electrode then dried at 80oC overnight in order to investigate the

total electrosorption capacity of the fabricated carbon material.

The salt removal efficiency (η) and the electrosorptive capacity (Sc) of the electrode were

calculated according to the following equations[35]:

η C CC

100% (2)

Sc Co C V/m (3)

Where and (mg/L) are the initial and final concentrations, respectively and V (L) is

the total volume of the NaCl aqueous solutions. Meanwhile, m (g) represents the mass of

the active components in the working electrodes.

2.6 Characterization

The surface morphology and pours structure were studied by a JEOL JSM-5900 scanning

electron microscope (JEOL Ltd., Japan) and field-emission scanning electron microscope

(FESEM Hitachi S-7400, Japan). The phase and crystallinity were characterized using a Rigaku

8

X-ray diffractometer (Rigaku Co., Japan) with Cu Kα (λ=1.54056 Å) radiation over a range of 2θ

angles from 10 to 80o. The electrochemical performance of the electrodes was determined by

cyclic voltammetry (CV) using a VersaStat4 potentiostat device.

3. Results and discussion

3.1 Morphology and Crystal structure.

Beside enhancing the specific capacitance, a great advantage for intercalating SnO2

nanoparticles into graphene sheet is increasing the interfacial contact area between the surface of

electrode and saline solution which leads to accumulate large amount of salts on the electrode

surface. It is noteworthy mentioning that the utilized tin chloride precursor in presence of urea

has strong reducing influence on GO to be converted to Gr. Therefore, formation of SnO2

accompanies with simultaneous reduction of GO to form the sought for SnO2/Gr composite as

can be described by the following reactions [30, 36].

2NH4+ + CO2 + 2OH- (4) CO(NH2)2 + 3H2O

Sn2+ + 2OH- Sn(OH)2 (5)

Sn(OH)2 + 2OH- SnO2 + 2H2O + 2e (6)

GO + 2e Gr (7)

Overall reaction

SnCl2 + GO + H2O Gr + SnO2 + 2HCl (8)

∆ Urea

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However, the microwaves irradiation was carried out to have complete conversion of GO to Gr

[18]. Fig. 1 A and B display the SEM and TEM images, respectively of the synthesized

graphene nanosheets. As shown, complete transformation of bulk graphite layer into nanosheets

can be observed and almost all graphene sheets are separated from each other. Furthermore, the

corrugated and scrolled graphene sheets resemble crumpled silk veil waves. Interestingly, the

existence of graphene layers by this entity easily leads to adsorption of the salt ions from the

saline water. Fig 2 A shows the SEM image of Gr/SnO2 (15% composite). It is clearly observed

that, good and uniform distribution of SnO2 between the graphene sheets. Furthermore,

Energy-dispersive X-ray spectroscopy (EDX) analysis in Fig. 2B elucidates the presence of

C, Sn and O elements in the investigated area from the graphene sheet. TEM images which

are presented in Fig. 2 (C, D) clearly corroborate the ultrathin corrugated graphene layer

encapsulated with very fine SnO2 NPs. It is also worth mentioning that increase the amount

of tin chloride during fabrication facilities the agglomeration of SnO2 nanoparticles into

graphene sheets as shown in Fig. 2 C and D.

The XRD technique is widely employed to identify the crystalline structure of the

prepared materials. As it is well known, XRD patterns are very useful for detecting the interlayer

distance and the exfoliation degree of graphene. Fig. 3 displays XRD analysis of GO, Gr and

Gr/SnO2 nanocomposite materials. As shown in the figure, the GO has an intense peak at the

2θ=10.35 due to the existence of abundant oxygen-containing groups and thus GO shows a much

larger interlayer distance than graphite. Obviously, a broad weak reflection peak (002) centered

at 2θ=24.3o confirms formation of the pristine graphene and also refers to removing most of the

intercalated oxygenated groups. Notably, top chart indicate formation of SnO2 NP, diffraction

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peaks of SnO2 nanoparticles are clearly distinguishable which correspond to the rutile structure

of SnO2 crystal (JCPDS, no. 41-1445)[37].

3.2 Electrochemical measurements.

Cyclic voltammetry (CV) measurements are strongly sensitive to determine the

electrosorption performance and evaluate the specific capacitance of the fabricated electrode.

Fig. 4 A: C display CV profiles of graphene and Gr/SnO2 composite electrodes at a scan rate of 5

mVs-1 in 0.1, 0.5 and 1 M NaCl aqueous solutions. The influence of the SnO2 contents on the

electrosorption behavior of the Gr/SnO2 composite electrodes was systematically investigated.

Obviously, all the introduced electrodes have retain a rectangular shape over a wide range of the

applied voltage and no appearance of any oxidation/reduction peaks can be detected in the

chosen potential range which indicates that the ions are mainly adsorbed on the electrode surface

by forming an electric double layer due to Coulombic interactions rather than the

electrochemical oxidation/reduction reaction[38]. It is well known that, increasing the area under

CV curve is an indicator of ion adsorption capacity and refers to higher specific capacitance.

Noteworthy, compared to pristine graphene, the Gr/SnO2 composite electrodes display higher

CV area at different NaCl concentrations. Fig. 5 provides CV cures of Gr/SnO2 (15% wt SnO2)

electrode at different scan rates. These CV curves are symmetric in shape along with the current–

potential axes without any distortion at higher sweep potential and no obvious redox peaks or

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Faradic reactions are seen which concludes ideal electric double-layer capacitive behavior and

highly reversible ion adsorption/desorption characteristics of the utilized electrode.

The specific capacitance (Cs) is a distinct tool to demine the behavior of the CDI

electrode material. As aforementioned in the experimental section, the specific capacitances have

been estimated from I-V cycles (Eq. 1). Generally, with increasing the scan rate, the specific

capacitance dramatically decreases. The corresponding specific capacitances plot of the

electrodes versus scan rates are depicted in Fig. 6 A. Obviously, the Gr/SnO2 (15% wt

formulation) electrode submitted a noticeable increase compared to all the introduced electrodes.

Typically, at 5 mV s-1 scan rate, the estimated specific capacitances were 189, 231, 323 and 268

F/g for the pristine graphene, Gr/SnO2 10%, Gr/SnO2 15% and Gr/SnO2 20%, respectively as

shown Fig 6 B. Therefore, one can claim that the optimum loading of SnO2 on Gr sheets is

15wt%. The effect of SnO2 loading behavior can be attributed to (i) enhancing the

electrochemical behavior compared to the pristine graphene, (ii) doping of SnO2 nanoparticles on

graphene sheets increases the specific surface of graphene sheets, (iii) however, on the other

hand, increasing the amount of SnO2 stimulates agglomeration process and increase the internal

resistance which has a negative impact on ions electrosorption process. Consequently, amount of

SnO2 should be optimized.

One of the main requirements for the optimal CDI electrode is recyclability of the active

materials electrodes. The cycle life time of Gr/SnO2 (15 wt%) electrode was examined. As

shown Fig. 6 C, the cyclic stability test displays insignificant decrease in the specific capacitance

over 50 cycles, approximately 4.9% from of the initial value. This finding indicates excellent

capacity retention and recyclability, subsequently the examined Gr/SnO2 composite can be

strongly nominated as effective CDI electrode materials.

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3.3 Desalination performance.

Desalination performances of the fabricated electrodes were investigated by batch mode

experiments which were conducted in NaCl aqueous solution having an initial conductivity of

~61µS/cm at a working voltage of 1.4 V. The concentration variation of NaCl aqueous solution

was detected from the relationship with the conductivity which can estimate the ion adsorption

behavior of the electrode under specific potential. As shown in Fig. 7A, the plot depicts severe

decrease in the conductivity at the initial stage reflecting quick adsorption of the salt ions that

indicates the capability of the investigated material towards ionic adsorption/desorption process.

With time going, the conductivity slowly decrease then remained at a constant value with

reaching the adsorption equilibrium due to the electrosorption saturation. Apparently, the

Gr/SnO2 15% electrodes performed faster and higher adsorption amount compared to all

introduced electrode materials. The salt removal efficiency (η) can be calculated from Eq. 2. As

shown in Fig. 7B demonstrates the Gr/SnO2 15% electrode has the greatest adsorption/desorption

capability and high salt removal efficiency reaching to 83%. By applying Eq. 3, Fig. 7C shows

the electrosorptive capacity of the Gr/SnO2 (15 wt%) electrode has the highest

electrosorptive capacity (1.49 mg/g) compared to the other formulations due to good

incorporating into graphene sheet and low agglomeration of SnO2 NPs.

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4. Conclusion

Due to the super electric conductivity and excellent adsorption capacity, utilizing

graphene as a supporter strongly enhances desalination performance. The Gr/SnO2 NPs

composites can be synthesized via microwave method for CDI electrode, the corresponding

desalination performances are distinctly higher than pristine graphene. Typically, incorporation

of SnO2 NPs into graphene has a significant impact in electrosorptive capacity as well as

avoiding graphene sheets aggregation which strongly increase the total surface area; however

amount of SnO2 NPs content should be optimized to overcome self agglomeration which

increases the charge transfer resistance. Generally, all the prepared Gr/SnO2 electrodes have

adsorption and desorption ability of ions salt more than pure graphene. However, Gr/SnO2

electrode having 15 wt% metal oxide exhibits maximum and excellent salt removal efficiency

due to high specific capacitance, good stability and increased electrode wetting ability.

Acknowledgment

The Authors extend their appreciation to the Deanship of Scientific Research at king Saud

University for funding the work through the research group project No. RGP-227.

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Figures captions

Fig. 1 (A) SEM and (B) TEM images for the synthesized pristine graphene.

Fig. 2 (A) SEM image, (B) and corresponding EDX of the obtained Gr/SnO2 (15 wt%) NPs

and (C, D) TEM images of Gr/SnO2 10% and 15 % formulation, respectively

Fig. 3 X-ray diffraction patterns of graphene oxide, pristine graphene and Gr/SnO2 15%.

Fig. 4 (A;C) Cyclic voltammetry of the prepared electrode at sweep rate 5 mV/s with

different NaCl concentrations of 0.1, 0.5 and 1 M.

Fig 5 CV profiles of Gr/SnO2 15% at different scan rates and 1 M NaCl concentration.

Fig. 6 Specific capacitances of the introduced electrodes in 1 M NaCl aqueous solution at

different scan rates;(A), plot of specific capacitance versus loading of SnO2;(B) and

cycle life plots of Gr/SnO2 (15 wt%) electrode measured at 50 mV/s; (C).

Fig. 7 CDI performance; (A), salt removal efficiency; (B) and electrosorptive capacity; (C) for the fabricated electrodes in NaCl solution at 1.4V

17

Fig. 1 (A) SEM and (B) TEM images for the synthesized pristine graphene.

(A) (B)

18

Fig. 2 (A) SEM image, (B) and corresponding EDX of the obtained Gr/SnO2 (15 wt%) NPs

and (C, D) TEM images of Gr/SnO2 10% and 15 % formulation, respectively

(B) (A)

(C) (D)

19

Fig. 3 X-ray diffraction patterns of graphene oxide, pristine graphene and Gr/SnO2 (15 %)

20

Fig. 4 (A:C) Cyclic voltammetry of the prepared electrode at sweep rate 5 mV/s and

different NaCl concentrations of 0.1, 0.5 and 1 M.

(A) (B)

(C)

21

Fig 5 CV profiles of Gr/SnO2 (15%) at different scan rates and 1 M NaCl concentration.

22

Fig. 6 Specific capacitances of the introduced electrodes in 1 M NaCl aqueous solution at

different scan rates;(A), plot of specific capacitance versus loading of SnO2;(B) and

cycle life plots of Gr/SnO2 (15 wt%) electrode measured at 50 mV/s; (C).

(B)(B)(A)

(C)

23

Fig. 7 CDI performance; (A), salt removal efficiency; (B) and electrosorptive capacity; (C) for the fabricated electrodes in NaCl solution at 1.4V

(B)(A)