Journal ofMaterials Chemistry A

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aKey Laboratory of Ningxia for Photovoltaic

750021, China. E-mail: Haib.lee@gmail.c

0951 2062414bInstitut National de la Recherche Scientiq

9A9, CanadacPillar of Engineering Product Development

Design, 20 Dover Drive, Singapore 138682,

† Electronic supplementary informa10.1039/c3ta14322h

Cite this: J. Mater. Chem. A, 2014, 2,3484

Received 24th October 2013Accepted 17th December 2013

DOI: 10.1039/c3ta14322h

www.rsc.org/MaterialsA

3484 | J. Mater. Chem. A, 2014, 2, 348

A high charge efficiency electrode by self-assembling sulphonated reduced graphene oxideonto carbon fibre: towards enhanced capacitivedeionization†

Haibo Li,*a Francois Zaviska,b Sen Liang,a Jin Li,a Lijun Hea and Hui Ying Yangc

Sulphonated reduced graphene oxide (SRGO) is self-assembled onto carbon fibre cloth (CFC) by

electrophoresis deposition as a composite (CFC–SRGO) electrode for high-performance capacitive

deionization (CDI). The resulting CFC–SRGO composite shows a cross-linked nanotubular structure and

the individual carbon fibre is fully encapsulated by SRGO nano-sheets, forming a cylindrical-shell

microstructure. The electrosorption performance of the CFC–SRGO composite electrode is determined

by carrying out a batch-mode electrosorption experiment, showing a great improvement in charge

efficiency of more than twice that of a bare CFC electrode. Remarkably, this result is comparable with

that of membrane enhanced capacitive deionization. Further, the low exponential decay constant of the

CFC–SRGO composite electrode derived from the electrosorption kinetics demonstrates that the

deposited SRGO favors a decrease in the internal electrode resistivity and therefore improves the ion

electrosorption rate which contributed to the enhanced charge efficiency significantly. Another possible

reason accounting for the high charge efficiency of the CFC–SRGO electrode is that the functionalized

SRGO is analogous to a cation-exchange membrane which can block the co-ions during

electrosorption. In addition, the electrosorption isotherm of the CFC–SRGO film follows Langmuir

adsorption, indicating the monolayer adsorption mechanism. Meanwhile, the thermodynamic analyses

imply that the electrosorption of salt ions onto the CFC–SRGO electrode is driven by a physisorption

process.

Introduction

The potable drinking water crisis is expected to be one of themain challenges globally in the next few decades due to theincreased population and environmental contamination.1–3

Solving this problem demands a tremendous amount ofresearch effort to develop new methods of purifying water atlower cost and with lower driving energy, while concurrentlyminimizing the use of chemicals and their impact on theenvironment. Capacitive deionization (CDI), as a typical appli-cation of electrosorption, is a powerful deionization techniquethat removes deleterious charged species without high powerconsumption and secondary pollution as compared to other

Materials, Ningxia University, Ningxia,

om; Fax: +86 0951 2062414; Tel: +86

ue, 490, rue de la Couronne, Quebec G1K

, Singapore University of Technology and

Singapore

tion (ESI) available. See DOI:

4–3491

conventional approaches.4–10 It has been developed as a poten-tial technique for removing inorganic ions from aqueous solu-tions. The principle of CDI is based on imposing an externalelectrostatic eld between the electrodes in order to forcecharged ions to move toward oppositely charged electrodes(see Fig. 1). The charged ions can be held within the electricaldouble layer formed between the solvent and the electrodeinterface.11–16

Fig. 1 The schematic principle of CDI.

This journal is © The Royal Society of Chemistry 2014

Fig. 2 Schematic diagram of the preparation of CFC–SRGO byelectrophoresis deposition and the SEM image of graphite, SRGO, CFCand CFC–SRGO.

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A prerequisite for the high deionization efficiency of CDI isthe demand for a high electrosorption capacity endowed by theelectrode materials. To attain this objective, it is important toemploy typical materials with a high specic surface area.Carbon aerogels,17,18 activated carbons,19,20 multi-walled carbonnanotubes and nanobers,21,22 ordered mesoporouscarbons,23,24 graphenes25,26 and oxide-incorporated carbons27,28

have been investigated as CDI electrodes. Activated carbon breis supposed to be one of themost attractive potential candidatesfor the high efficiency of CDI electrodes due to its high specicsurface area, high bulk conductivity, mass production and lowfabrication cost.29,30 Graphene is a two-dimensional (2D) struc-ture of carbon atoms with unique electronic, chemical, andmechanical properties. Extensive research has shown thepotential of graphene or graphene related materials to impact awide range of technologies including energy storage, catalysis,sensing, and water purication.31–33 Importantly, the facilechemical method paves the way for mass production of gra-phene in the case of practical applications.

Charge efficiency, dened as the ratio of specic removalcapacity to specic charge, is viewed as one of the most signif-icant parameters to evaluate the CDI process. Until now, CDIhas been proven to be a very energy efficient water desalinationtechnology in solutions with a relatively low ionic strengthbecause of high charge efficiency, such as for brackish water(lower than 10 mM).34,35 However, when the solution concen-tration is further increased, the charge efficiency woulddecrease due to the presence of serious co-ions. To address thisissue, ion-exchange membranes have been introduced into theCDI process resulting in membrane enhanced capacitivedeionization (MCDI) which is a modication of CDI in whichion exchange membranes are placed in front of each elec-trode.36–38 Specically, an anion exchange membrane is placedin front of the anode, and a cation exchange membrane isplaced in front of the cathode. One major advantage of usingion-exchange membranes is to minimize the effect of co-ions bycharging the active functional groups present on themembrane. Another advantage is that a high charge efficiencycould be achieved at any solution concentration due to therestricted co-ions. However, a big problem with the present lab-scale and commercialized MCDI is that it uses an expensive ion-exchange membrane, resulting in the increase of the totalcapital cost of the MCDI desalination plant. Thus, achievinghigh charge efficiency without utilizing this expensive partwould be a big advancement in spreading the usage of CDI atlow cost and with high efficiency in any solution concentration.The possible solutions are summarized as below: onesolution isto prepare a carbon and ion-exchange polymer compositeelectrode to replace the physically combined carbon and ion-exchange membrane electrode. Choi et al. proposed a mixtureof poly(vinyl alcohol) and sulfosuccinic acid as a coating solu-tion to introduce negatively charged ion-exchange groups ontoactivated carbon.39 The resulting composite materials exhibit animproved desalination efficiency when compared with a carbonelectrode without coating. Similar results may be found inothers' work.35,37 Although these results showed enhancedelectrosorption performance, it is still lower than that of the

This journal is © The Royal Society of Chemistry 2014

corresponding MCDI. A big problem with such a compositeelectrode is that the introduced polymer would decrease thespecic surface area and conductivity of the carbon electrode,which is harmful to the lifetime of the CDI device. Anothereffective solution is to design a new carbon electrode with amicrostructure to improve the charge efficiency of CDI withoutsacricing the essential and useful properties of the carbonelectrode.

In this work, a carbon bre cloth (CFC) and sulphonatedreduced graphene oxide (SRGO) composite is prepared byelectrophoretic deposition and then proposed as the electrodefor high performance CDI. Within this novel electrode, the CFCis fully encapsulated by SRGO nano-sheets, forming a cylin-drical-shell structure. The results show that the electrosorptionperformance of such a composite electrode, i.e. removalcapacity, specic charge and charge efficiency, is muchimproved when compared with that of a bare CFC electrode.Meanwhile, these results are comparable with those of MCDI.Thereby, our experiment provides a facile and easy large-scaleproduction method to prepare a novel carbon electrode for highperformance CDI which is very signicant for commercializa-tion of the CDI technique in the near future.

Experimental

The details in terms of preparation and characterization ofSRGO are listed in the ESI†. CFC (Jiangsu Kejing Carbon FibreCo., Ltd, China) with detailed specications, i.e. density: 0.04 gcm�3, specic surface area: 1800 m2 g�1, total mass in CDI unit:2.4 g, geometry size: 14 cm � 7 cm, is used as the substrate forthe electrophoresis deposition of SRGO. Fig. 2 shows the sche-matic diagram of the electrophoresis deposition of SRGO ontoCFC and the respective SEM image of graphite, SRGO, CFC andthe CFC–SRGO composite. In detail, 50 mg of SRGO and 5 mg ofAl(NO3)3 were dispersed in a 50 mL ethanol/acetone mixturewith a volume ratio of 1 : 1, then they were subjected to highpower ultrasonic bath for several hours until they were homo-geneously dispersed. Aer that, two pre-cleaned CFC electrodeswere vertically placed in solution at a distance of approximately1 cm as shown in Fig. 2. Electrophoresis deposition was carriedout for 30 min with an electric voltage set at 20 V. To get a

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uniform SRGO lm, the as-prepared solution was gently stirredduring electrophoresis deposition. The digital image of theCFC–SRGO lm is given in Fig. S1(a).† It can be seen that thesurface of the CFC–SRGO lm is quite uniform without anyvisible stack and the CFC–SRGO lm is very exible which iseasy to bend without any damage. Finally, the as-prepared CFC–SRGO electrode was directly assembled in the CDI unit cell(80 mm wide � 100 mm long � 0.2 mm thick). It should bementioned that both the anode and cathode electrodes weremade of the as-prepared CFC–SRGO lm.

Batch-mode experiments were conducted in a continuouslyrecycling system including a CDI/MCDI unit cell, conductivitymonitor and current recorder, as shown in Fig. S1(b).† As for theMCDI unit, it comprises one pair of pure CFC electrodes, wherethe ion exchange membrane was placed in front of each CFClm. Please note that the ion exchange membrane was fabri-cated by us. For/during each experiment, the solution wascontinuously pumped by a peristaltic pump into the unit celland the effluent returned to the feed tank. The aqueous solutionwas prepared using analytically pure sodium chloride (NaCl)with an initial concentration of 250 mg L�1 (conductivity of500 mS cm�1). The volume and temperature of the solution weremaintained at 80 mL and 298 K, respectively. The relationshipbetween conductivity and concentration was obtained accord-ing to a calibration table made prior to the experiment. A directvoltage of 0.6–1.4 V was applied between the electrodes. Itshould be noticed that hydrolysis of water was not observedwhen the voltage between the two electrodes was more than1.2 V because of the existence of resistance in the whole circuit.The variations in conductivity and current were recordedsimultaneously and independently.

The concept of charge efficiency is derived from the classicGouy–Chapman–Stern theory which is based on an assump-tion34,35 that if each electron charge is fully charge-balanced bycounter ion adsorption, the transfer of an electron from oneelectrode to another is accompanied by the removal of preciselyone salt molecule from the bulk solution. However, in the actualcase, the co-ions (ions with the same polarity as the corre-sponding electrode) are simultaneously expelled from thedouble layer accommodating the adsorbed ions, which has anegative impact on the charge efficiency.40 Please note that theco-ion is dened as the ion of the same charge as the electrodeand is the ion that is repelled out of the electrode. The coun-terions are those of opposite charge to that of the electrodesurface and are attracted into the electrode. Thus a cation is thecounterion in the cathode and is the co-ion in the anode.Ideally, if the counterion adsorption can fully compensate theelectron charge, the charge efficiency would be close to 1.Therefore, the charge efficiency L is a very useful tool to eval-uate how much the electrical voltage contributes to adsorptionand it is experimentally dened as

L ¼ G� F

S(1)

where F is the Faraday constant (96 485 C mol�1). G (removalcapacity, mol g�1) and S (charge, C g�1) are obtained from eqn(2) and (3), respectively:

3486 | J. Mater. Chem. A, 2014, 2, 3484–3491

G ¼ ðCt � C0Þ � V � 103

58:5�M(2)

S ¼

ðidt

M(3)

where C0 and Ct are the initial concentration (mg L�1) and theconcentration at time t, respectively, V is the solution volume(mL), M is the total mass of both electrodes (g) and i is thecurrent response during electrosorption (A).

Results and discussion

Fig. 2 shows the SEM image of the CFC–SRGO sample (moreSEM images can be found in Fig. S2†). Obviously, SRGO nano-sheets with a geometrical surface area far less than 1 mm2 areclosely distributed on the surface of individual carbon bres,forming a core-cylinder structure. On the other hand, theCFC–SRGO bre is randomly entangled with a cross-linkednanotubular structure. It is widely accepted that such a networkstructure permits easy access for ions to the electrode/electro-lyte interface, which is crucial for a non-faradic capacitiveelectrode material.36 Fig. S2(f)† presents the EDX spectra ofCFC–SRGO, evidencing the presence of elements, i.e. C, O, Al,P and S, where S is derived from SRGO. Due to the strong p–p

interaction, easy restacking between individual RGO layersduring the preparation leads to a relatively low specic surfacearea and low conductivity of the as-prepared graphene.41,42

Ideally, GO must be rigorously reduced aer exfoliation torecover the desirable properties of graphene. To that end,polymer-coupled “graphene nanosheets” could be obtained byreducing exfoliated GO in the presence of a polymericdispersing agent giving intriguing composite materials.However, the presence of a polymeric dispersing agent in thegraphene composite oen scarices the essential and usefulproperties of graphene which may be undesirable for applica-tion in CDI.26 To avoid this, functional SRGO was synthesized bycovalently functionalizing the partially reduced graphene oxidesurface with a controllable amount of –SO3H groups. In thiswork, SRGO has been prepared in an improved manner asdescribed in ref. 43 and 44. The –SO3H groups prevent thegraphitic sheets from aggregating in solution aer the nalreduction stage of the GO thereby yielding isolated sheets oflightly SRGO with improved water solubility and conductivity,which are not only benecial to electrophoresis deposition, butalso greatly enhance the conductivity of the CFC–SRGO elec-trode. Both Fig. 2 and S2(g), (h)† show the SEM image of SRGO,in which a number of isolated pieces of ultra-thin SRGO atsheets can be observed very clearly, indicating the minimizedaggregations. Furthermore, the sheet conductivity of the SRGOsample is observed to be 2.5 � 10�4 S m�1 using a four pointprobe meter which is higher than that of GO directly reduced byhydrazine (2 � 10�4 S m�1). In addition to conductivity, sul-phonation of RGO has two more advantages. One is that SRGOhas an improved specic surface area when compared with RGOwithout sulphonation which is favorable for providing as manyvacancies as possible to accommodate ions during the CDI

This journal is © The Royal Society of Chemistry 2014

Fig. 3 (a) The electrosorption–desorption performance of the CFC–SRGO electrode in NaCl solution with an initial conductivity of 500 mScm�1 by varying the cell voltage from 0.6 to 1.4 V, (b) current response,(c) removal capacity and specific charge with respect to the cellvoltage, (d) charge efficiency at each cell voltage.

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process. Fig. S3† depicts the N2 adsorption–desorptionisotherms and BJH pore size distribution plots of RGO with andwithout sulphonation. Both samples exhibit representativetype-IV curves with obvious capillary condensation steps, sug-gesting a mesoporous structure. Table S1† gives the texturalproperties of SRGO and RGO. The BET surface areas and totalpore volumes of RGO with and without sulphonation are 439and 254 m2 g�1, 0.28 and 0.17 cm3 g�1, respectively, demon-strating the above analysis. Since sulphonation is one of thecommon steps to prepare the cation-exchange membrane,another possible advantage of sulphonation is that it xes thestrong negatively charged groups (–SO3

�) onto the RGO back-bone which may allow the passage of cations but reject theanions. Therefore, the resulting composite electrode preparedby coupling cation-exchange SRGOwith carbon bre is probablyanalogous to a micro-MCDI which is capable of achieving highcharge efficiency when compared with a pure CFC electrode. Incontrast to traditional MCDI, this composite electrode hasmany advantages, i.e. low cost, simplicity, avoidance of the useof membranes and easy large-scale production. It should bementioned that we have made a calculation that the cost offabricating one pair of CFC–SRGO electrodes is around 20 centswhich is signicantly lower than that of MCDI electrodes. Theevidence of various functional groups presented on SRGO isconrmed by FTIR spectra (Fig. S4†). A broad absorption peakranging from 2850 to 3600 cm�1, a phenyl absorption peak at1621 cm�1, and an epoxy group absorption peak at 1250 cm�1

were observed. In addition, a relatively broad peak ranging from2850 to 3600 cm�1 was observed due to the presence of hydroxylgroups in water molecules.

In a complete CDI process, ions are forced toward theCFC/CFC–SRGO electrodes from the aqueous solutions while adirect voltage is applied to the electrodes and the CDI perfor-mance can be examined by the conductivity variation of solu-tions during the charging process. In the case of the dischargingprocess, the electrode is short-circuited accompanied by anincrease of the solution conductivity until it reaches the initiallevel. The typical CDI experiment was conducted in NaCl solu-tion with an initial conductivity of around 500 mS cm�1 (corre-sponding to 250 mg L�1). Fig. 3(a) and (b) show theelectrosorption–desorption cycles and the correspondingcurrent response for the CFC and CFC–SRGO composite elec-trode on applying a cell voltage ranging from 0.6 to 1.4 V,respectively. In all cases, the solution conductivity began todecrease drastically for any cell voltage applied. Aer 60 min,the electrode was fully saturated, and the conductivity remainedat a constant value. The cell was then short-circuited, and theconductivity rapidly returned to the initial value because ofdesorption of the adsorbed ions. The corresponding transientcurrent showed changes similar to those of conductivity. Inaddition, the conductivity variations showed reproducibleresults for several cycles of electrosorption and desorption,indicating goodregeneration. The salt removal efficiency, as animportant indication to evaluate the sorption performance of aCDI electrode, is oen dened as (C0 � C) � 100%/C0, where C0

and C are the initial and the nal concentrations. Obviously, thesalt removal efficiency increased with the electrical voltage.

This journal is © The Royal Society of Chemistry 2014

Specically, the salt removal efficiencies were 20% and 60%when the electrical voltage was set as 0.6 V and 1.4 V, respec-tively. On the other hand, the respective removal capacity (mmolg�1) and the specic charge (C g�1) of both electrodes are pre-sented in Fig. 3(c). Obviously, at both electrodes, the higher thecell voltage, the higher the removal capacity is, which isconsistent with other studies.19–21 This is due to the high elec-trostatic force resulting from a high cell voltage (see eqn (4)).More importantly, at each cell voltage, the removal capacity andthe specic charge as well as the charge efficiency of the CFC–SRGO electrode are much higher than those of the CFC elec-trode (Fig. 3(d)), indicating that the co-ions have been restrictedby the CFC–SRGO electrode.45

As is well-known, CDI is recognized as a very efficient desa-lination technique in salt solutions with relatively low ionicstrengths because of high charge efficiency. So, obtaining highcharge efficiency in high salt solution is one of the most urgentchallenges for CDI study. As shown in Fig. S5,† with increase inthe ionic strength, the charge efficiency of both electrodes isfound to be decreased and this can be explained by a simpliedGouy–Chapman–Stern theory.46,47 In principle, the ion distri-bution close to the planar electrodes can be classied as thesum of an adsorbed electrical double layer (EDL, Stern EDL) anda diffuse EDL. A brief qualitative explanation is therefore given.Ions in the diffuse electrical double layer reach the equilibriumbetween the diffusion and electrostatic force which could bereasonably described by the Poisson–Boltzmann equation andthe Gouy–Chapman solution for ion concentrations andpotential. At a long distance from the electrode the charge iscompletely held, so that the electric eld is present only insidethe diffuse EDL. The relationship between the surface chargedensity s and the potential difference 4 between the electrodeand the bulk solution is given by

4 ¼ 2KBT

esinh�1

�sffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi

8CNA303rKBTp

�(4)

J. Mater. Chem. A, 2014, 2, 3484–3491 | 3487

Fig. 4 (a) The electrosorption kinetics of CFC and CFC–SRGO elec-trodes in NaCl solution with an initial conductivity of 500 mS cm�1 at acell voltage of 1.2 V, (b) the current response and equivalent circuit(inset).

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where KB is the Boltzmann constant, T is the temperature, e isthe electron charge, NA is the Avogadro constant, 30 is thedielectric constant, 3r is the relative dielectric constant, and C isthe concentration; the equation is valid for a symmetric,monovalent electrolyte, such as NaCl. To well describe theexperimental results, Brogioli rewrote eqn (4) in terms of elec-

trical double layer effective thickness L, shown as 4 ¼ Ls303r

and L

is dened as L ¼ ls sinh�1ffiffiffiffiffiffiffiffiffic=C

p� �,46 where ls ¼ 2KBT303r/(es)

and c ¼ s2

8N303rkB.46 Therefore, the electrical double layer

thickness L is directly related to the electrostatic forces anddiffusion. Obviously, the electrical double layer thickness isinversely proportional to the solution concentration. Speci-cally, with a lower concentration, the electric eld can extend toa longer distance since the solution is less effective in holdingthe charge. Once the concentration changes, the ions movetowards the new equilibrium, resulting in a dynamical changein the length L. Fig. S5† shows the electrosorption performanceof the CFC and CFC–SRGO electrode in NaCl solutions with theinitial conductivity ranging from 100 mS cm�1 to 2000 mS cm�1.Clearly, the CFC–SRGO electrode exhibits higher salt removalefficiency than that of the pure CFC electrode at a certain initialconcentration. Further, the charge efficiency of the CFC–SRGOelectrode is higher than that of the CFC electrode at each givensolution conductivity. At a relatively lower conductivity, i.e. at100 mS cm�1, the charge efficiencies of both CFC and CFC–SRGO electrodes are close to 1, demonstrating that the CDI isenvisioned to be a highly efficient desalination technique whenthe ionic strength is extremely low. With increase in the ionicstrength, the charge efficiencies of both electrodes start todecline, in which the decay of the CFC electrode is more fasterthan that of the CFC–SRGO electrode. At a relatively highconductivity, i.e. at 2000 mS cm�1 (which nearly corresponds to1000 mg L�1), the charge efficiency of the CFC electrode is onlyaround 0.1 while the corresponding value for theCFC–SRGOelectrode still remains at 0.4, indicating that the co-ions havebeen blocked by the CFC–SRGO electrode. To make a compar-ison, we have explored the charge efficiency of the CFC-basedMCDI by taking into account 1000 mS cm�1 as the initial solu-tion conductivity. Fig. S6† shows the electrosorption perfor-mance of CFC-based MCDI at the cell voltages of 1.0, 1.2 and1.4 V. According to eqn (1), (2) and (3), the charge efficiencies ofCFC-basedMCDI at 1.0, 1.2 and 1.4 V are 0.462, 0.475 and 0.483,respectively, which are comparable with those of the CFC–SRGOelectrode, revealing that CFC–SRGO could be utilized to replacethe physically mixed carbon and ion-exchange membraneelectrode, thereby having great potential in spreading theapplication of CDI without using a high cost membrane.

The electrosorption kinetics of CFC–SRGO electrodes hasbeen understood by considering the adsorption kinetics model,i.e. Lagergren's pseudo-rst-order kinetic model. The linearizedform of the pseudo-rst-order kinetic model can be expressedas26

logðqe � qtÞ ¼ logqe � kt

2:303(5)

3488 | J. Mater. Chem. A, 2014, 2, 3484–3491

where qe and qt are the number of ions adsorbed (mmol g�1) atequilibrium and at time t (min), respectively and kad (min�1) isthe rate constant. Fig. 4(a) shows the tting between the modelequation and experimental data for the CFC and the CFC–SRGOelectrode. The tting regression coefficients for CFC and CFC–SRGO are 0.9539 and 0.9987, respectively, indicating a goodsimulation since they are very close to 1. The rate constantsobtained from the slope of the tting line are 0.0688 and 0.1158,respectively, corresponding to the CFC and the CFC–SRGOelectrode, revealing that the ion adsorptions are much fasterwithin the CFC–SRGO electrode which is consistent with theobservations from the electrosorption curve. It should bementioned that highly efficient electrosorption and desorptionare very signicant for the practical CDI device since they arefavorable for desalination of as much salty water as possible in acertain period. On the other hand, the current response curvealso provides us with some useful information to explain theenhanced CDI performance of the CFC–SRGO electrode.According to a simplied model, the equivalent circuit of theCDI process is analogous to the combination of an electroderesistor (Rs) with an electrical double capacitor (Cd), where Rs isconnected with Cd in series. Fig. 4(b) shows the currentresponse for CFC and CFC–SRGO and the inset shows theequivalent circuit. Charging with a constant potential, thetransient current follows the equation: �e�s/RsCd where srepresents the exponential decay constant which can beobtained from the current response curve. Thus, the s values are1263 s and 306 s for the CFC and the CFC–SRGO electrode,respectively, indicating that the formation of the EDL is muchfaster within the CFC–SRGO electrode than within the CFCelectrode. This demonstrates that the CFC–SRGO electrodeshows a better capacitive behavior than that of the CFC elec-trode. Further, the Rs and Cd values are calculated as 118.58 U,10.65 F and 32.79 U, 9.33 F, corresponding to the CFC and theCFC–SRGO electrode, in which the electrode resistivity Rs ofCFC is much higher than that of CFC-SRGO while the doublelayer capacities Cd of the two electrodes are almost identical,implying that the deposited SRGO is very benecial to decreasethe internal electrode resistivity without sacricing the doublelayer capacitance. This also conrms that the electrical voltageapplied on CFC–SRGO and the solution interface is signicantlyhigher than that of the pure CFC electrode, resulting in highCDI performance.

This journal is © The Royal Society of Chemistry 2014

Table 1 Parameters determined from various isotherms of the CFC–SRGO electrode

qm (mmol g�1) KL rL2 KF n rF

2

1.0 V 175.14 0.0092 0.97 21.28 3.36 0.831.2 V 185.57 0.0102 0.93 24.62 3.47 0.711.4 V 188.68 0.0104 0.94 24.97 3.48 0.73

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The electrosorption process is oen described using theadsorption isotherm in order to evaluate the adsorptioncapacity of the adsorbent material (electrode) and understandthe inherent adsorption mechanism. The electrosorptionisotherm is related to the amount of adsorbate on the adsorbent(electrode) as a function of its concentration at a constanttemperature. To explore the electrosorption isotherm of CFC–SRGO, batch electrosorption experiments were carried out inNaCl solutions with initial concentrations ranging from 50 to1000 mg L�1 (note: the electrosorption isotherms of the CFC–SRGO electrode at a cell voltage less than 1 V are not presentedbecause of the low charge efficiency). Fig. 5 gives the electro-sorption data and the as-simulated Langmuir isotherms of theCFC–SRGO electrode at the cell voltages of 1.0, 1.2 and 1.4 V,respectively. As shown in the gure, when the solution is verydilute, the amount of adsorbed NaCl declines to zero. Theremoval amount of NaCl increases as the initial concentration israised, which is due to the enhanced mass transfer rate of ionsinside the micropores and the reduced overlapping effect by ahigher concentration of solution. Note, with the increase in theinitial concentration, the charge would be increased accord-ingly. However, the charge efficiency would be decreased due tothe serious effect of co-ions in a highly ionic solution. Langmuirisotherm (6) and Freundlich isotherm (7) are utilized to t theexperimental data,26

q ¼ qmKLC

1þ KLC(6)

q ¼ KFC1/n (7)

where C is the equilibrium concentration (mg L�1), q is theamount of adsorbed NaCl (in micromoles per gram of the CFC–SRGO lm), qm is the maximum adsorption capacity corre-sponding to complete monolayer coverage (mmol g�1). Table 1shows the determined parameters and regression coefficientsrL

2, rF2, KL and KF of the Langmuir and Freundlich isotherms,

respectively. It is observed that the Langmuir regression coef-cients r2 are higher than the Freundlich regression coefficientsr2 at each cell voltage, indicating that the Langmuir isotherm

Fig. 5 The electrosorption isotherm of the CFC–SRGO electrode atthe cell voltages of 1.0, 1.2 and 1.4 V.

This journal is © The Royal Society of Chemistry 2014

better describes the electrosorption behaviour of the CFC–SRGO electrode and the electrosorption of ions onto the CFC–SRGO electrode follows monolayer adsorption. The qm value ofthe CFC–SRGO electrode calculated by the Langmuir equationis much higher than that of the CFC electrode (see Fig. S7 andTable S2† for details of the CFC electrode), implying enhancedelectrosorption performance of the CFC–SRGO electrode.

To explore the effect of temperature on the electrosorptionprocess of the CFC–SRGO electrode, thermodynamic parame-ters such as standard free energy, standard enthalpy, andstandard entropy are determined. The calculations are per-formed according to the following equations:

DG0 ¼ �RT ln KF (8)

DS0 ¼ DH0 � DG 0

T(9)

lnðKFÞ ¼ �DS0

R� DH0

RT(10)

where R is the gas constant (8.314 � 10�3 kJ mol�1 K�1), KL isthe Langmuir constant, T is the temperature (K) and aredetermined from the slope and intercept of the van't Hoff plotsof ln(KF) versus 1/T.48 The electrosorption isotherm of the CFC–SRGO electrode at a temperature of 273 K is shown in Fig. S8.†The result of the standard free energy change obtained is9.69 kJ mol�1 and 11.38 kJ mol�1 for the CFC–SRGO electrode attemperatures of 273 K and 298 K, respectively. These resultsindicate the presence of an energy barrier at high temperaturesduring electrosorption and also that the electrosorption wasnon-spontaneous and less favourable at high temperatures. Inother words, it conrms that the electrosorption is dominantlydriven by the electrostatic force. The negative value of thestandard enthalpy change is �8.586 kJ mol�1, indicating thatthe interaction of the NaCl molecule with the CFC–SRGO elec-trode is exothermic. Further, the standard enthalpy change islower than 40 kJ mol�1, implying that the electrosorption isphysisorption. In addition, both the negative values of standardenthalpy change and standard entropy change (�67 J K�1

mol�1) support the fact that the reaction is more favorable atlow temperatures.

Conclusions

In this work, a carbon bre cloth and sulphonated reducedgraphene oxide (CFC–SRGO) composite was prepared by elec-trophoretic deposition and then proposed as the electrode forhigh charge efficiency capacitive deionization. The resulting

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carbon bre is fully encapsulated by SRGO nano-sheets, form-ing a core-cylinder microstructure. Batch-mode experimentsdemonstrate that the CFC–SRGO composite electrode exhibitsan improved charge efficiency of more than twice as comparedto that of the bare CFC electrode, indicating that the co-ionshave been blocked by the CFC–SRGO electrode. This is probablydue to the ion selectivity characteristic of SRGO nano-sheetsand a decreased interfacial resistivity. More importantly, theseresults are comparable with that of membrane enhancedcapacitive deionization. Further, the electrosorption isothermof the CFC–SRGO lm follows the Langmuir adsorption model,indicating monolayer adsorption. The thermodynamic analysesimply that the electrosorption of salt ions onto the CFC–SRGOelectrode is driven by a physisorption process. Thus, it isbelieved that such a CFC–SRGO electrode with advantages oflow cost, mass production and high efficiency will contribute tothe improvement of CDI in large-scale applications verysignicantly.

Acknowledgements

The author would like to thank for the nancial support from2012 Ningxia Key Technology R&D Program, 2013 ResearchFund for Higher Education of Ningxia (NGY2013017), NationalNatural Science Foundation of China (11062010, 51302138,61366005) and the Singapore University of Technology andDesign and the grant from the International Design Centre(IDC).

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