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SynthesisandCharacterizationofSemiconductingPorousCarbonforEnergyApplicationsandCO2Adsorption

ARTICLE·SEPTEMBER2015

DOI:10.1016/j.jiec.2015.09.002

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7AUTHORS,INCLUDING:

VinodhRajangam

HanseoUniversity

26PUBLICATIONS173CITATIONS

SEEPROFILE

AzizAbidov

HanseoUniversity

11PUBLICATIONS10CITATIONS

SEEPROFILE

Availablefrom:VinodhRajangam

Retrievedon:20December2015

Journal of Industrial and Engineering Chemistry xxx (2015) xxx–xxx

G Model

JIEC-2635; No. of Pages 9

Synthesis and characterization of semiconducting porous carbon forenergy applications and CO2 adsorption

Rajangam Vinodh a,b, Aziz Abidov a,b, Mei Mei Peng a,b, Cadiam Mohan Babu a,b,Muthiahpillai Palanichamy a,b, Wang Seog Cha b,c, Hyun-Tae Jang a,b,*a Department of Chemical Engineering, Hanseo University, Seosan 360-706, South Koreab Korea Carbon Capture and Sequestration R and D Centre, 360 Daegok-ri, Haemi-myun, Seosan-si 356 706, Chungcheongnam-do, South Koreac School of Civil and Environmental Engineering, Kunsan National University, Kunsan 573-701, South Korea

A R T I C L E I N F O

Article history:

Received 27 February 2015

Received in revised form 11 August 2015

Accepted 6 September 2015

Available online xxx

Keywords:

Hyper branched polymers

Graphene

Supercapacitor

Transmission electron microscopy

Phloroglucinol

A B S T R A C T

In this paper we report a semiconducting porous carbon material for supercapacitor application.

Semiconducting carbon material was prepared by two step method. First, hypercross-linked polymers

(HCPs) of phloroglucinol and a,a0-dichloro-p-xylene was prepared by a simple Friedel Crafts alkylation,

and second, the synthesized microporous HCPs was subjected into carbonization at 700 8C in a helium

atmosphere. The carbonized sample was characterized into cyclic voltammetry, galvanostatic charge–

discharge and electrochemical impedance spectroscopy in 1 M sodium sulphate aqueous solution for

supercapacitor applications. The specific capacitance of carbonized materials is found to be 86 F g�1 at a

current density of 0.1 A g�1.

� 2015 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights

reserved.

Contents lists available at ScienceDirect

Journal of Industrial and Engineering Chemistry

jou r n al h o mep ag e: w ww .e lsev ier . co m / loc ate / j iec

Introduction

Supercapacitors or electrochemical capacitors have been thesubject of great interest because of their very important role inenergy storage, which is very desirable for applications in electricvehicles and hybrid electric vehicles [1–4]. At present, the vastmajority of electrode materials for supercapacitors are made ofcarbonaceous materials. These materials are regarded as beingunavoidable and irreplaceable in the modern era owing to theirexcellent properties, including, a high surface area, good electricalconductivity, well defined morphology, controllable pore size, andtunable surface functionality [5–8]. This said large numbers ofmicropores have been regarded as being inaccessible electrolytesolutions, mainly because of their random pore connectivity andclosure characteristics [9,10]. This dramatically slows down thetransfer of ions from the electrolyte solution to the depth of thepores, thus leading to deterioration in the high rate charge–discharge properties [11]. Metal oxides, conducting polymers and

* Corresponding author at: Department of Chemical Engineering, Hanseo

University, Seosan 360-706, South Korea. Tel.: +82 041 660 1423;

fax: +82 041 688 4143.

E-mail address: htjang@hanseo.ac.kr (H.-T. Jang).

Please cite this article in press as: R. Vinodh, et al., J. Ind. Eng. Chem

http://dx.doi.org/10.1016/j.jiec.2015.09.002

1226-086X/� 2015 The Korean Society of Industrial and Engineering Chemistry. Publis

their composites, have also been used for supercapacitor applica-tions, but they have quite often encountered low electricalconductance and poor cycling stability, limiting their practicalapplications [12].

Recently, three dimensional polymeric and carbonaceousnetworks have attracted great interest because of their outstand-ing pore accessibility and very fast mass transport. However, as yet,only a few papers have been produced on this subject. For example,Liang et al. have studied porous network structured carbon (PNSC)materials for organic electrolyte supercapacitors, including largecapacitances up to 210 F g�1 and excellent cycling stability[11]. Further, Li et al. have presented a novel class of polymericand carbonaceous networks based on the union of self-assemblyand hypercross-linking. This involved a poly(methyl methacry-late)-b-polystyrene (PMMA-b-PS) diblock copolymer being firstself-assembled into uniform PMMA@PS core–shell nanospheres,and then the resulting nanospheres being used as building blocksto construct polymeric nanoscale networks by hypercross-linking[13].

We have been working on hypercross-linked polymers (HCPs)for the past few years [14–17]. These polymers being one of anumber of new materials that have been developed for use insupercapacitors, solar cell devices, sensors and sorption ofgases. During synthesis, they develop as spheres built with hyper

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hed by Elsevier B.V. All rights reserved.

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cross-linked conjugated quinonoids with abundant micropores.When they are synthesized at 80 8C, they are semiconductingand luminescent, but on heat treatment from 700 to 900 8C inhelium, they lose their luminescence and become conductingcarbon based materials. To do this, we have been using aromaticmonomers like benzene, naphthalene, anthracene etc., andcross-linking agents such as dimethoxy methane, paraformalde-hyde and 1,3,5-trioxane. The synthesis of semi-conductingcarbon material has been attempted by several researchers, asit is essential for electronic applications [18–21]. Compared to allthe reported literature in respect of the synthesis of semicon-ducting porous carbon, for example, partial hydrogenation ofgraphene [22] and chemical vapor deposition (CVD) [23], ourmethod of Friedel Crafts alkylation appears to be very simple andcheap. To convert graphene into a semiconducting one, a few C–C bonds are hydrogenated to suppress electron cloud delocali-zation. In our Friedel Crafts alkylation, none the carbons ofaromatic monomers are subjected to electrophilic reaction norare all the –CH2– bridges dehydrogenated. As a result, both thesechoices yield only semiconducting carbon based materials.Though all our earlier methods yielded semiconducting carbonspheres, our present method also develops porous semiconduct-ing nanotubes and nanorods as a result of applying the FriedelCrafts alkylation of phloroglucinol and a,a0-dichloro-p-xylene at80 8C.

Experimental

Materials

Phloroglucinol and a,a0-dichloro-p-xylene were purchasedfrom Sigma Aldrich, USA. Other chemicals like concentrated HCl,1,2-dichloroethane and methanol were purchased from DaejungChemicals, South Korea.

Synthesis of hypercross-linked polymers and carbonized HCPs

1.0 g of phloroglucinol was placed in a two necked roundbottom (RB) flask. 20 mL of 1,2-dichloroethane was added to it andthe mixture stirred at 200 rpm. 3 g of ferric chloride was added toit under stirring. 0.5 g of a,a0-dichloro-p-xylene in 10 mL of 1,2-dichloroethane was added in drops to the mixture for 30 min, andthe mouth of the flask was closed with the calcium chloride guardtube. After 20 min of stirring the temperature of the mixture wasraised to 40 8C. After 4 h the temperature was raised to 80 8C, andthe reaction continued for 20 h. The flask was cooled to roomtemperature and the contents were filtered under suction. Theresidue was washed with methanol, 5% HCl in acetone watermixture (1:1), water and finally with acetone, and dried in theoven at 100 8C for 12 h. The same experimental procedure wasrepeated varying the quantity of cross-linker, a,a0-dichloro-p-xylene. The detailed reaction protocol is depicted in Table 1. Theschematic representation of the synthesis procedure is illustratedin Scheme 1. Hypercross-linked a,a0-dichloro-p-xylene was alonesynthesized (without phloroglucinol) and tested for the compari-son purpose (Fig. S1). Also, the synthesized HCPs were subjected

Table 1Reaction protocol of phloroglucinol and a,a0-dichloro-p-xylene.

No. Phloroglucinol (g) a,a0-Dichloro p-xylene (g) Fe

HCP-1 1.0 0.5 3

HCP-2 1.0 1.62* 3

HCP-3 1.0 2.0 3

HCP-4 1.0 2.5 3

* Stoichiometric quantity.

Please cite this article in press as: R. Vinodh, et al., J. Ind. Eng. Chem

into thermal dehydrogenation under helium atmosphere at700 8C.

Characterizations

FTIR

The chemical structure of the synthesized hypercross-linkedpolymers and thermally treated materials were analyzed using FT-IR spectroscopy. The FT-IR spectra of all the samples were recordedin transmission mode using Nicolet IR 200 spectrometer by placingin KBr windows ranging the wave number from 4000 to 400 cm�1.

DRS-UV visible spectroscopy

Ultraviolet–visible diffuse reflectance (DRS UV–vis) spectrawere recorded on SCINCO Neosys 2000 spectrometer between thewavelength 200 and 800 nm.

Thermogravimetric analysis

Thermal stability and the decomposition temperature of theprepared hypercross-linked polymers and carbonized materialscan be assessed easily by using this technique. The thermalanalysis was carried out by using the SCINCO TGA N-100 model.The measurements were conducted by heating the samples fromroom temperature to 800 8C at a heating rate of 10 8C min�1 undernitrogen and air atmosphere.

Scanning electron microscopy

A scanning electron microscopy was used to investigate thesurface morphology of the prepared HCPs and semiconductingporous carbon materials. SEM micrographs were obtained with thehelp of a JOEL instrument operating at 30 keV with an associatedenergy dispersive X-ray analyzer.

Transmission electron microscopy

TEM images are formed using transmitted electrons (instead ofthe visible light), which can produce magnification details up to1000,000� with resolution better than 10 A. The sample prepara-tion involved the dispersion of hypercross-linked polymers intoethanol, and then the slicing of the samples with a diamond knifeto obtain thin sections which were then placed on copper grids forTEM analysis.

N2 sorption isotherm

The nitrogen adsorption–desorption isotherms were measuredat 196 8C with a Belsorp mini II sorption analyzer. Prior to eachadsorption measurement the samples were degassed at 150 8Cunder vacuum (p < 105 mbar) in the de-gassing port. The surfacearea was determined from the linear part of the Brunauer–Emmett–Teller (BET) equation.

Raman spectroscopy

The Raman spectra were obtained based on Nano RamanSpectroscope (NT-MDT NTEGRA) System and by using a 520 mmworking-length objective. The excitation source was 532 nm withradiation from a He–Ar laser with a power of 0–90 mW at thesurface of the sample and a spot of ca. 3 mm on the sample surface.

rric chloride (g) 1,2-Dichloroethane (mL) Temperature (8C)

.5 30 80

.5 30 80

.5 30 80

.5 30 80

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OH

OHHO

CH2Cl

CH2Cl

+1.2-DCEFeCl3, 80 OC

Scheme 1. Schematic representation of hypercross-linked polymers of phloroglucinol and a,a0-dichloro-p-xylene synthesis.

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The treated and untreated hypercross-linked polymers wereplaced in a specially designed quartz in situ Raman sample cell.

X-ray diffraction studies

Powder X-ray diffraction (XRD) patterns were recorded using aRigaku D/Max 2200 + Ultima diffractometer with Cu-Ka radiation(l = 0.154 nm). The diffraction data were recorded in the 2u range5–508 with a step of 0.028 s�1.

Electrochemical measurements

The electrochemical measurements were measured with anelectrochemical analyzer CHI 660D under ambient condition bythe three electrode system in 1 M Na2SO4 aqueous solution.Platinum foil and Ag/AgCl electrode were used as the counter andreference electrode, respectively. The three-dimensional porousnickel foam was used as a working electrode with a thickness of1.0 mm and a porosity of 110 pores per inch. Prior to use, the nickelfoam was pressed to 0.6 mm, thoroughly wash with acetone anddeionized water, and dried at ambient temperature. The workingelectrode was prepared by coating the nickel foam with acarbonized HCP-4 mixture without any binding material, dryingat room temperature for 24 h, and annealing at 2508 C for 2 h. Eachworking electrode contained about 3 mg of active materials with ageometric surface area of about circular 1 cm2. Cyclic voltammetry(CV) and galvanostatic charge–discharge measurements wereconducted in a potential window from 0.0 to 0.8 V with respect toAg/AgCl electrode. Electrochemical impedance measurementswere operated in a frequency range from 100 kHz to 10 mHzwith AC amplitude of 5 mV.

CO2 adsorption

CO2 sorption experiment with high purity CO2 (99.999%) and N2

for the samples were performed using a thermogravimetricanalyzer (Scinco TGA N1000). 10 mg of the sample was loaded

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into a platinum sample pan in a thermogravimetric unit and theinitial activation was carried out at 150 8C for 1 h under a N2

atmosphere. Then the temperature of sample was brought down to25 8C for CO2 adsorption. Desorption was conducted by graduallyraising the temperature from 25 to 150 8C by passing N2.

Results and discussion

Fourier transform infra red spectroscopy

The FTIR spectra of the as-synthesized HCPs are shown inFig. 1(a). The broad peak in the high energy region is due to –OHstretching vibration of phloroglucinol. Such groups are notoxidized during polymerization and therefore, the brown colorsof the polymers are exclusively due to their quinonoid content. Asthe O–H stretching vibration is weak for HCP-2 compared toothers, it is suggested that they have lower phloroglucinol contentthan the other polymers. The C55C stretch of the quinonoidstructure is shown by the peak around 1600 cm�1. The broad peakfrom about 1450 and 1200 cm�1 includes bending modes of –CH2–bridges and phenolic C–O stretching vibration. As the –CH2–vibrations just below 3000 cm�1 is very weak in all, most of suchgroups, cross-links, might be dehydrogenated to form linearacenes. For the sake of clarity and better understanding the FTIRspectrum of HCP-2 alone is shown below (Fig. 1(b)).

The intense broad band in the high-energy region is due to the –OH stretching vibration of the phloroglucinol. The peaks just aboveand below 3000 cm�1 illustrate the C–H stretching vibrations ofresidual C–H stretching vibrations of aromatic rings and –CH2–,respectively, and their intensity is very low. The peak at 1615 cm�1

is due to C55C stretch of quinonoid. If it is a p-quinonoid, thisvibration is infrared inactive, but as the intensity is very high, thequinonoid is verified to have additional substitution in the ring. So,electrophilic reaction on a,a0-dichloro-p-xylene might haveoccurred. It is established by the characteristic peaks of 1,2,4-trisubstitution at 826 and about 900 cm�1. So, the intensity of

. (2015), http://dx.doi.org/10.1016/j.jiec.2015.09.002

5001000150020002500300035004000

% T

ran

smit

tan

ce

Wave number (cm-1 )

HCP-1 HCP-2 HCP-3 HCP-4

(a)

(b)

500100015002000250030003500400070

80

90

3000 cm-1 1200 cm-1

1450 cm-1

1600 cm-1

Tra

nsm

itta

nce

(%

)

Wave number (cm-1)

Fig. 1. The FTIR spectra of the (a) as-synthesized HCPs and (b) FTIR spectrum of the

HCP-2 alone for sake of clarity.

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quinonoid vibration is very much influenced by its symmetrycharacteristic. It is also clearly evident from the FTIR spectrum ofthe polymer derived from a,a0-dichloro-p-xylene (Fig. S2).

In this polymer the C55C vibration is nearly absent confirmingformation of symmetrical quinonoid with substitutions at 1, 2,4 and 5th positions taking into account the steric hindrance ofsubstitution. The peak at 1714 cm�1 is due to quite unexpectedC55O stretch. It is not due to an aldehyde, as its –C–H stretchingvibration at about 2700 cm�1 is absent. Hence, it is onlyketo carbonyl. So, the only possibility is the oxidation of one of

10008006004002000.0

0.5

1.0

Abs

orba

nce

Wave leng th (nm)

HC P-1 HC P-2 HC P-3 HC P-4

(a)

Fig. 2. DRS UV–vis spectra of the (a) as-synthesized hypercross-l

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the –CH2– bridges. Trace amounts of water in the reagents are thecause for such a reaction proceeding through hydration anddehydrogenation. Hence, from the FTIR analysis it is verified thatthe HCP-2 is mainly tri-substituted.

We have already reported that hyper cross-linked conjugatedquinonoids on thermal treatment between 600 and 900 8C undergodehydrogenative condensation to form semiconducting carbonwith broad light absorption characteristic, and loss of IRcharacteristic [17]. To verify it, here too the HCP-2 and HCP-4were heated to 600 8C under helium atmosphere and analyzed byFTIR. The spectra are shown in Fig. S3. Loss of –CH2– vibrations and–C–H aromatic ring vibrations confirm such transformations.Formation of graphene structure is also supported by their FT-Raman spectra (Fig. 6).

Diffuse reflectance UV–vis spectroscopy

The DRS UV–vis spectra of the as-synthesized polymers showeda broad light absorption from about 1200 to 200 nm (Fig. 2(a)). Itevidences delocalized electronic cloud of the structures with aband gap. Here too the intensity is in accordance with the 1,4-dichloro-p-xylene content of the polymers. A gradual decrease inintensity with the increase in wavelength is due to differentdimensions of rings in each structure. These spectra are almostsimilar to those reported for hyper cross-linked conjugatedquinonoid chromophores [14], as delocalized electronic cloud iscommon in both. So, based on such light absorption characteristicwe once again argue that these polymers are as good as the hypercross-linked conjugated quinonoids for the fabrication of solar celldevices. The carbonized (or thermally treated) HCP-2 and HCP-4were analyzed by DRS UV–vis spectroscopy and the results areillustrated in Fig. 2(b). There is an increase in absorbance andfurther shifting of lmax to longer wavelength for both supportingthe formation of a graphene like structure. This characteristic issimilar to that of our previously reported hyper cross-linkedconjugated-quinonoid [14–17].

N2 sorption isotherm

The nitrogen adsorption isotherms of hypercross-linked phlor-oglucinol and a,a0-dichloro-p-xylene polymers are shown inFig. 3. Condensation of nitrogen in the micropores of the polymerssharply raises the adsorption at low P/P0. The volume of adsorbednitrogen increased with the increase in the content of a,a0-dichloro-p-xylene illustrating the importance of fiber content ofthe matrix. As the fibers could randomly orient, chances of

1000800600400200

0.5

1.0

Abs

orba

nce

Wavelength ( nm)

HC P-2 HC P-4(b)

inked polymers and (b) thermally treated HCP-2 and HCP-4.

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0.0 0.2 0.4 0.6 0.8 1.0

0

50

100

150

200

250

Vac

m-3(S

TP)g

-1

p/p0

HCP-1 HCP-2 HCP-3 HCP-4

Fig. 3. N2 sorption isotherms of HCPs.

Table 2Textural properties of HCPs and thermally treated polymers.

S. No. Surface area,

as (m2 g�1)

Micro pore volume,

Vp (cm3 g�1)

Pore diameter,

dp (nm)

HCP-1 373.35 0.1177 0.7

HCP-2 564.88 0.1692 0.7

HCP-3 480.53 0.1503 0.7

HCP-4 634.21 0.2110 0.7

HCP-2* 528.43 0.1582 0.7

HCP-4* 596.22 0.1927 0.7

* Thermally treated polymers.

80060040020040

50

60

70

80

90

100

Wei

gh

t (%

)

Temperature (deg. C)

HCP-1 HCP-2 (Stoichiometric) HCP-3 HCP-4 Carbonized HCP-4

Fig. 4. Thermogravimetric analyses of HCPs at nitrogen atmosphere.

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mesopores could not be avoided. There is a slight rise in adsorptionaround 0.5 P/P0 in the isotherms of all the polymers. The polymerHCP-4 depicted higher micropore volume than the other polymers;hence, higher content of a,a0-dichloro-p-xylene is a prerequisitefor polymers with high microporosity.

An unexpected result was that desorption curves lay separatelywithout coinciding with that of the adsorption in all. As a result,there might be significant heat of adsorption for nitrogen. So, theinteraction between the delocalized electronic cloud of thepolymer and the lone pair electrons of nitrogen might besignificant. Textural properties of the HCPs and thermally treatedpolymers were depicted in Table 2.

8070605040302010

HCP-3

HCP-2

HCP-4

HCP-1

Inte

nsity

(AU

)

2 theta (deg.)

(a)

Fig. 5. XRD patterns of (a) HCPs and (b)

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Thermogravimetric analysis

The results of TGA in nitrogen atmosphere of the HCPs areillustrated in Fig. 4. The initial weight loss below 100 8C is due todesorption of adsorbed gases entrapped in the pores formed byacenes. The gradual decrease in weight loss from about 200 8C isdue to the dehydrogenation of cross-links. As the energy ofdehydrogenation of –CH2– bridges depends on a number of cross-linked benzene nuclei, weight loss extends from 200 to about600 8C. As a result, a sharp weight loss at any temperature is notmandatory. Since the weight loss is about 45%, the entire weightloss is not due to dehydrogenation of cross-links alone, butincludes the fusion of chains along their lateral side throughdehydrogenation, as shown below. Such a lateral fusion can lead toform graphene nano structures. The percentage weight loss ofHCP-4 is lower than the other polymers, hence it is proved to have ahigher quinonoid content than the other polymers, as suggested innitrogen adsorption isotherms. The residue that remains above800 8C is also high for HCP-4 polymer compared to others. It is alsosupported by the DRS UV–vis spectra of the polymers, as discussedabove.

X-ray diffraction studies

The X-ray diffraction spectra of the as-prepared polymers areshown in Fig. 5(a). As they are free of any lattices, patterns were not

80604020

HCP-4

HCP-2

Inte

nsity

(AU

)

2 theta (deg.)

(b)

thermally treated HCP-2 and HCP-4.

. (2015), http://dx.doi.org/10.1016/j.jiec.2015.09.002

00020051000110000

20000

30000

40000

1440 cm-1

1580 cm-1

Inte

nsi

ty (

A.U

)

Raman Shift (cm-1)

(a) (b) (c ) (d)

Fig. 6. Raman Spectra of the synthesized HCPs of (a) HCP-1, (b) HCP-2, (c) HCP-3 and

(d) HCP-4.

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seen in any of the spectra. So, all of them are amorphous. The broadenvelope between 10 and 408 is due to aggregation of polymerchains and spheres as supported by SEM images. The X-raydiffraction patterns of polymers of thermally treated polymersHCP-2 and HCP-4 are shown in Fig. 4(b). The as-synthesized

Fig. 7. TEM images of HCP-2 ((a) and (b)) and thermally tre

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polymers were of fibers and amorphous, but the present polymersshowed diffraction patterns. The origin of patterns is due to intercross-linking of fibers to form organized planes as seen in the SEMimages. So the planar free rings or in spheres can cross-linkthemselves via dehydrogenation to form enlarged planar matriceswith enhanced delocalized electronic cloud. It also becomes thedriving force for such a reaction. The patterns were different fromthat of graphite, as the planes here might have many defects.

Raman spectroscopy

The Raman spectra of the as synthesized polymers are shown inFig. 6. All of them were luminescent, as acenes are rigid and carrysemiconducting characteristics. The intensity decreased with theincrease in the content of a,a0-dichloro-p-xylene, as longer chainsformed with high content of a,a0-dichloro-p-xylene lose theirrigidity and carry low bandgap. Though they were luminescent, thespectra carry peaks at about 1440 and 1580 cm�1, characteristicpeaks of carbon based materials. The former peak is due to disorderor defect, and it represents a ring-breathing mode from SP2 carbonrings. The latter peak is due to –C55C– stretch of planar rings.Acenes are not graphenes, but they also carry a fused one-dimensional quinonoid structure, which is also the skeleton ofgraphenes. So, these two peaks should be common to both. Basedon their luminescence and DRS-UV–vis spectra, it could bedepicted that these polymers could also be a strong candidatefor supercapacitor application and solar cell devices.

ated HCP-2 ((c) and (d)) with different magnification.

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Scanning electron microscopy

The SEM images of the as synthesized HCPs are shown in Fig.S4(a–d). They appear like fibers that are randomly oriented. Due topores of different sizes formed by the fibers they adsorb CO2. AsTGA showed weight loss due to dehydrogenation, the HCP-2 andHCP-4 were separately heated to 700 8C in helium and analyzed bySEM. The images are in Fig. S4(e and f), showed organized plates, inaddition to fibers. The plate like morphology is due to inter cross-linking of fibers via dehydrogenation as discussed above. Itconfirms formation of graphene. Though their XRD patternsproved crystallinity, formation of graphite is not well evident.So, the graphene formed in the reaction is largely defective. If theacenes are well dispersed, on heating they can dehydrogenate,rather than condensing along their lateral positions.

Transmission electron microscopy

The TEM images of as-prepared HCP-2 and HCP-4 and theirthermally treated forms are shown in Figs. 7 and 8, respectively.Their features are almost same; there are tubes with differentdimensions. They are semiconducting as evidenced from theirDRS UV–vis spectra. a,a0-Dichloro-p-xylene can cross-link withphloroglucinol, and react to form structures. The tubes do notcarry the features of usual carbon nanotubes, but appear to beformed by the piling of porous ring structures. The rings areseparately formed and then organized to form tubes. It is believedthat the interaction of their p-electronic cloud becomes thedriving force for such organized structures. The spheres are

Fig. 8. TEM images of HCP-4 ((a) and (b)) and thermally tre

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formed by the planar structures of different fused rings. In order toestablish the role of phloroglucinol, a,a0-dichloro-p-xylene alonewas polymerized by the same procedure, and the resultingpolymer was analyzed by TEM (Fig. S5). It also showed an evidencefor nanotubes. The porous structure of a sphere is evident from theatom mapping.

Electrochemical study

To evaluate the electrochemical performance of the obtainedcarbonized HCP-4 as electrode materials for supercapacitors, CV,galvanostatic charge–discharge and electrochemical impedancemeasurements were employed to find its capacitive property bythe three electrode system in 1 M Na2SO4 aqueous electrolyte.

As we wanted to find an optimum potential window, the CVcurves of carbonized HCP-4 was recorded with three differentpotential ranges at a constant scan rate of 50 mV s�1 (Fig. S6). It iswell known that an ideal capacitance behavior of a carbon basedmaterial electrode is expressed in the form of a rectangular shapeon the voltammetry characteristics [24,25]. In Fig. S6(a) showed acomplete rectangular CV curves were obtained between thepotential 0.0 and 0.8 V.

Fig. 9(a) represents the galvanostatic charge–discharge curvesof carbonized HCP-4 at the current density varying from 0.1 to3 A g�1. It can be observed that the discharging time is almostequal to their corresponding charge counterpart and all galvano-static charge–discharge curves show closely triangular shape,revealing a good electric double layer capacitive behavior andindicating good electrochemical reversibility.

ated HCP-4 ((c) and (d)) with different magnification.

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Fig. 9. (a) Charge–discharge curves of carbonized HCP-4 at different current densities, (b) Cyclic voltammogram of carbonized HCP-4 with varying the scan rate, and (c)

Nyquist plots of carbonized HCP-4.

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With the increase of the scan rate from 5 to 400 mV s�1, thevoltammogram still remains a rectangular shape (Fig. 9(b)),indicating the material possesses good capacitive characteristicsfor quick charging–discharging. The broad obvious pseudocapacitance peaks on the curve could be attributed to the synergyof the electric double layer capacitance [26,27]. Also, one can seefrom the figure that with an increase in the scan rate there is nodistortion. This suggests that this material is adequate for the fastdiffusion and transfer of electrolyte ions. This clearly shows thatthe porous nature of the material is beneficial for the diffusion andtransport of ions. This in turn contributes to the easy penetration ofelectrolyte ions within the macro/mesopore channels andimproves the efficient utilization of the interior surfaces throughlow-resistance pathways [28,29].

The electrochemical impedance spectra (EIS) of the electrodematerials are depicted in Fig. 9(c). It demonstrates that the similarNyquist plot shapes composed of a nearly vertical line at lowfrequency characterized capacitive behavior, a sloping linear

Table 3CO2 adsorption results.

S. No. CO2 (weight %) at 40 8C (%)

HCP-1 4.6

HCP-2 (stoichiometric quantity) 5.0

HCP-3 5.2

HCP-4 6.8

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region at high-to-medium frequency related diffusion resistanceand a semicircle at high frequency presenting the charge–transferresistance [30,31].

CO2 sorption

The CO2 sorption results are depicted in Table 3, as expected thepercentage of CO2 sorption followed the order of a,a0-dichloro-p-xylene content of the polymers: HCP-4 (�7 wt% at 40 8C) showedhigher percentage of CO2 sorption than the others. If the polymerscarry large amount of –CH2– cross links, they can largely associateleaving reduced density of micropores. Generally creation ofmicropores in hyper cross-linked polymers requires rigid buildingblocks, as they do not pack well. Quinonoid structures are morerigid than the non-quinonoid structures; hence, the HCP-4polymer is evidently to have more quinonoid structures thanthe other polymers. It is also evident from the weight loss in thethermograms of the polymers discussed in TGA.

Conclusions

Though the obtained results lower than the existing materials(86 F g�1 at a current density of 0.1 A g�1), the synthesis protocol,Friedel Crafts cross-linking of aromatic monomers such asphloroglucinol with –CH2– bridges and subsequent dehydrogena-tion of the latter is a simple, cheap method of developing porous,nano tubes and nano rods. In future, we plan to enhance surface

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area of the carbonized materials with the incorporation of noblemetal catalysts and evaluate their performance in supercapacitorapplications. As their electronic cloud can be easily distorted, theycan serve as sensors for any gases or liquids that are p electroncloud acceptors. The materials are thermally stable. With theirmicropores they can be good sorbents for hazardous gases such asNOx, SO2 and volatile organic compounds etc. The present studyreveals not only important insights about how to design andsynthesis semiconducting carbon based materials but also opensup new avenues in the solar cell devices.

Acknowledgements

The authors gratefully acknowledge the financial support fromHanseo University and grants from Korea CCS R&D Centre, fundedby the Ministry of Education, Science and Technology of the KoreanGovernment.

Appendix A. Supplementary data

Supplementary data associated with this article can be found, inthe online version, at doi:10.1016/j.jiec.2015.09.002.

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