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Electrochimica Acta 123 (2014) 248–253

Contents lists available at ScienceDirect

Electrochimica Acta

j our na l ho me pa g e: www.elsev ier .com/ locate /e lec tac ta

lectronically and Catalytically Functional Carbon Cloth as aermeable and Flexible Counter Electrode for Dye Sensitizedolar Cell

ukta Tathavadekara, Mandakini Biswala, Shruti Agarkara,b,∗,ingamallu Giribabuc, Satishchandra Ogalea,b,∗

Center of Excellence in Solar Energy, Physical and Materials Chemistry Division, CSIR-National Chemical Laboratory, Pune 411008, and Network Institutef Solar Energy (CSIR-NISE), New Delhi, IndiaAcademy of Scientific and Innovative Research, Anusandhan Bhawan, 2 Rafi Marg, New Delhi-110 001, IndiaInorganic & Physical Chemistry Division, CSIR-Indian Institute of Chemical Technology, Hyderabad 500607, India

r t i c l e i n f o

rticle history:eceived 17 October 2013eceived in revised form5 December 2013ccepted 28 December 2013vailable online 9 January 2014

a b s t r a c t

In this work we demonstrate that a conducting carbon cloth obtained by one-step pyrolysis of easily avail-able cellulose fabric competes favorably with the commonly employed expensive platinum/FTO systemas a counter electrode for dye sensitized solar cell (DSSC). In view of the low carbonization temperature(1000 ◦C) which forbids full graphitization, the nature of carbon in this case is represented by topolo-gically randomly assembled nanoscale graphene units (turbostratic carbon). This morphology has highdensity of edge states and oxygen containing surface groups rendering multitude of catalytic sites for the

−

eywords:SSCounter electrodeonducting carbon clothithout drilling

reduction of I3 . Moreover the cloth is permeable to the dye and/or liquid electrolyte and its absorptionproperties also help retain the electrolyte. A fairly high efficiency of 5.8% is achieved using such cloth as acounter electrode as against 7% with the conventional Pt/FTO system. Moreover, the permeable propertyof the cloth eliminates a complete step of drilling hard TCO substrates for final dispensing of electrolyteinto the device and the sealing process is also facile. In fact even the dye can be dispensed through thecloth. Finally, the flexibility of the carbon cloth can adapt easily to flexible/wearable DSSC designs.

. Introduction

Counter electrodes in dye sensitized solar cells (DSSCs) are keylayers in determining the efficiency of energy conversion and thus

lot of efforts are put into the designing of such electrodes. Pt (sup-orted on ITO or FTO) has thus far proved to be the best in thisegard, but clearly there is a need to replace this expensive andare material. To be a viable counter electrode the material musthow good catalytic activity towards reduction of I3− present inhe electrolyte, be stable, and also preferably earth abundant. Car-on has been found to be a suitable candidate in this respect and hashe advantages of low cost in addition to its good catalytic activity,hermal and chemical stability [1].

Carbon in its various forms like activated carbon [2], graphene

3], fullerene derived [4], CNTs [5] etc. has been used as a counterlectrode material in DSSC. Activated carbon was used by Imotot al. as DSSC counter electrode, wherein it was shown that by

∗ Corresponding authors. Tel.: +91 20 25902260; fax: +91 20 25902636.E-mail addresses: sa.agarkar@ncl.res.in (S. Agarkar), sb.ogale@ncl.res.in

S. Ogale).

013-4686/$ – see front matter © 2014 Elsevier Ltd. All rights reserved.ttp://dx.doi.org/10.1016/j.electacta.2013.12.175

© 2014 Elsevier Ltd. All rights reserved.

increasing the surface roughness of carbon the efficiency of DSSCcan be enhanced [2]. There are several reports on the effects ofdifferent particle sizes of carbon on the efficiency of DSSC. It hasbeen shown that nanocarbon gives better efficiency as comparedto micro carbon [2]. Laser synthesized carbon has also been usedas counter electrode with good success [6]. Another form of car-bon that has been widely used for DSSC counter electrode is CNTs.New techniques have also been explored to grow the films of CNTsfor this application which include spray coating, screen printingand modified CVD techniques [7,8]. Comparison between SWNT,DWNT (double walled) and MWNT as counter electrode materialis also made wherein SWNT leads the way due to effectiveness incharge transfer and high catalytic activity [9]. Graphene, the 2Dcrystalline form of carbon, and its functionalized forms have alsobeen used as counter electrodes in DSSCs [10,11]. Functionalizedgraphene has shown efficiency very close to Pt [10]. This reportshows that by systematically adjusting C/O ratio, the catalytic activ-ity of the functionalized graphene material can be increased [10].

Several interesting reports have also used carbon composites aselectrode materials [1,12,13]. A hybrid of highly porous MoS2 andcarbon was used by Yue et al. to achieve an efficiency of 7.69% ascompared to 6.7% for Pt [14]. In a recent interesting report, Wang

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t al. have used bio-inspired integrated carbon counter electrodeor DSSC to obtain an impressive efficiency close to that with Pt15]. Ordered meso-porous carbon (OMC) synthesized by using aimple direct tri-constituent co-assembly method has also yieldedfficiency comparable to Pt [16]. Carbon is thus one of the mostesirable and appropriate materials for DSSC counter electrode. It

s inexpensive, light and corrosion-resistant. However, almost all ofhese carbon forms have to be deposited on some substrate so as toe used as a counter electrode. This has to be followed by drilling of aole in counter electrode for electrolyte injection and finally sealingf the counter and working electrodes with a low melting polymer.hese steps make the cell fabrication process time consuming andedious.

In this work we demonstrate that a conducting carbon clothbtained by one-step pyrolysis of easily available cellulose fabricompetes favourably with the commonly employed expensive plat-num/FTO system as a counter electrode for dye sensitized solar cellDSSC). This carbon cloth has turbostratic nature, which representsopologically randomly displaced assembly of nanoscale graphenenits. A high efficiency of 5.8% is achieved using only such carbonloth counter electrode without any further processing or surfaceoating, as against 7% with conventional Pt/FTO system. Impor-antly, the permeable property of the cloth allows an alternate andimpler device fabrication strategy that excludes a complete step ofrilling hard TCO substrates for final dispensing of electrolyte intohe device and easy sealability. Moreover, the flexibility of carbonloth can adapt easily to flexible/wearable DSSC designs.

. Experimental

.1. Synthesis of conducting carbon cloth

Cellulose fabric was used for preparation of the conductingarbon cloth. It was placed on an alumina plate and was subjectedo pyrolysis at 1000o C for 4 Hr in a split tube furnace underontinuous flow of argon gas. The heating rate was 10 ◦C per min.fter the pyrolysis a black coloured conducting carbon cloth wasbtained.

.2. Fabrication of the DSSC

Fabrication of the DSSC solar cells was done by doctor bladingethod using the commercially available TiO2 nanopowder (Sigmaldrich, 20 nm) with ethyl cellulose and �-terpineol as binder andurfactant, respectively. After coating, the films were annealed at50 ◦C for 60 min. The thickness was kept at around ∼12 �m. Thelms were subjected to TiCl4 treatment at 70 ◦C for 30 min followedy annealing at 450 ◦C for 30 mins. Then they were dipped in ruthe-ium N719 dye solution (1:1 v/v acetonitrile + t-Butyl alcohol) anduthenium Z907 dye solution (in ethanol) for 24 h at room tem-erature. The samples were then rinsed with ethanol to removehe excess dye on the surface. This was followed by redox elec-rolyte addition and top contact of carbon cloth or Pt coated FTO (foromparison) as counter electrodes. Pt counter electrode was pre-ared by drop casting 0.6 mM H2PtCl6 ethanolic solution on cleanTO. It was allowed to dry at room temperature then heated at50 ◦C for 15mins. The iodine electrolyte used with N719 dye was.6 M 1-propyl-2, 3-dimethyl-imidazolium iodide, 0.05 (M) LiI, 0.05M) I2, and 0.5 (M) 4-tertbutylpyridine in acetonitrile/valeronitrileolution (v/v 1:1). The cobalt electrolyte used with Z907 dye forSSC was 0.1 M [Co(II)], 0.01 M, [Co(III)], 0.05 M LiClO4, and 0.25 M

-tert-butylpyridine in acetonitrile.

Device fabrication with carbon cloth as counter electrode: Inhis modified and simpler device fabrication protocol, carbon cloths fixed on photo anode (active area 0.25 cm2) with the help of epoxy

ica Acta 123 (2014) 248–253 249

resin sealant. Then electrolyte is poured on the carbon cloth. Thecloth has high capacity for holding the liquid electrolyte because ofits porous nature. Finally sealant is applied to FTO and it is placedover the carbon cloth containing electrolyte. This cell assembly waskept to settle for an hour and used for further studies.

3. Characterizations

Various Characterization techniques such as X-ray diffraction(XRD, Philips X’Pert PRO), Scanning electron microscopy (FE-SEM)were used for the determination of various properties of thematerials used. Raman Spectroscopy was carried out on HoribaJoevin instrument using 632 nm laser source. I-V characteristicswere measured under irradiation at 100 mW/cm2 (150 W xenonlamp, Oriel Instruments), 1 sun AM 1.5, simulated sunlight (solarsimulator). Impedance measurement was recorded using autolabpotentiostat.

4. Results and discussion

Fig. 1 (a) shows the images of cellulose fabric which on pyrol-ysis forms carbon cloth. Fig. 1 (b-d) shows the FE-SEM images ofthe carbon cloth. Fig. 1 (b) clearly shows that the woven natureof the cellulose fabric is retained even after pyrolysis. Fig. 1 (c)and (d) show the morphology of carbon cloth on different scalesof magnifications. The images indicate that the cellulose fabric onpyrolysis gives woven and crumpled form of conducting carbon.Fig. 1(d) shows high magnification image of carbon cloth show-ing rough surface with pores. Fibers of a few hundred micronsin length exhibiting interconnecting network with a fairly rough(porous) surface are noted. The average density of pyrolised car-bon cloth was found to be 0.5 g/cm3. Just for comparison, this ismuch lower than a typical graphitized mesoporous carbon (AldrichGMC, 1.828 g/cm3). Fig. 1 (e) shows the schematic of turbostraticdisorder present in the carbon cloth due to partial graphitization(as confirmed by X-ray diffraction).

Fig. 2 (a) shows the X-ray diffraction (XRD) data for the as-prepared carbon cloth. The peaks at 2� values near 24◦ and 44◦

correspond to (002) and (101) planes of turbostatic carbon, respec-tively, having turbostratic nature [17]. Note that these are shiftedto lower 2� as compared to those for pure graphite located at 26.5o

and 44.3o. Such carbon is known to have lots of defects (especiallyedge states) due to imperfect structure and assembly [18]. To studywhether such defects are actually present in our carbon cloth itwas characterized by Raman spectroscopy, the data being shown inFig. 2 (b). Both the D and G bands of carbon are seen. The presence ofG band shows graphitic nature of the cloth while the high intensityof D band proves the presence of defects in this graphitic network,as expected. As stated earlier turbostratic disorder can contributea host of edge states which can be very helpful for surface catalyticactivity, e.g. for iodide-tri-iodide reactions.

The FTIR-ATR data was also recorded for this carbon cloth. Fig. 2(c) shows characteristic C = C peaks at 1562 cm−1 and 1618 cm−1,respectively. The =C-H stretching modes are observed at 838 cm−1

and 928 cm−1. The C = O stretching modes are also observedat around 1800 cm−1. Frequencies for asymmetric alkynes areobserved at 2100 cm−1. These data suggest that the formed carbonhas high concentration of defects and surface functional groups.XPS data (SI, Figure S1) was also recorded for this pyrolysed carboncloth. It clearly shows that carbon 1s spectrum can be fitted withthree major peaks which correspond to C = C (binding energy284.6 eV), C-C (binding energy 285.67 eV) and C = O (binding

energy 287.8 eV). Oxygen 1s spectrum also consists of 3 peakswhich belong to C-O (binding energy 533.1 eV), C = O (bindingenergy 531.26 eV) and chemisorbed oxygen (binding energy534.6 eV) [19,20]. This shows that the carbon cloth has oxygen

250 M. Tathavadekar et al. / Electrochimica Acta 123 (2014) 248–253

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ig. 1. (a) Images of cellulose fabric which on pyrolysis forms carbon cloth, (b-d) imhe carbon cloth.

ontaining groups on the surface which can be useful for exhibitingood catalytic activity. We also measured the mean resistivity ofhe cloth by sandwiching it between two copper electrodes. Theominal mean resistivity was estimated to be about 1 �-cm. This

onducting carbon cloth was used as counter electrode for DSSCs.

Fig. 3(a) shows the cyclic voltametry (CV) data for the carbonloth using three electrode system at a scan rate of 100 mV/s ran-ing from -1 V to 1.2 V. Thermally deposited Pt or carbon cloth was

Fig. 2. (a) XRD data, (b) Raman data, and (c

of carbon cloth on different scales, e) schematic of turbostratic disorder present in

used as working electrode with area ∼1 cm2, Ag/AgCl as a referenceelectrode and Pt foil as a counter electrode. The electrolyte used forthis study was 10 mM LiI, 1 mM I2 and 0.1 M LiClO4 in acetonitrile.The CV data display two sets of peaks which relate to the redox

peaks of I-/I3− and I2/I3−. The redox reactions taking place at thecounter electrode surface are as follows:

I3− + 2e− ↔ 3I− (1)

) FTIR-ATR data for the carbon cloth.

M. Tathavadekar et al. / Electrochimica Acta 123 (2014) 248–253 251

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and Table 2. In this case the open circuit voltage and current densityare found to be almost similar in both the cases but the fill factoris found to improve considerably with the use of the carbon clothcounter electrode. Since a number of factors (catalytic efficiency,

Table 1I-V characteristics of DSSC devices using Pt and carbon cloth counter electrodes iniodide tri iodide electrolyte.

Counter Electrode Voc (V) Jsc (mA/cm2) FF (%) � (%)

Pt 0.77 14.4 61 7.0Carbon Cloth 0.73 11.6 65 5.8

Table 2I-V characteristics of DSSC devices using Pt and carbon cloth counter electrodes inCobalt electrolyte.

ig. 3. (a) Cyclic voltametry data for Pt and carbon cloth as counter electrodes in iolectrodes in cobalt redox shuttle electrolyte, (c) DSSC device I-V curves for Pt and curves for Pt and carbon cloth as counter electrodes in cobalt redox shuttle electrol

3− ↔ I2 + I− (2)

For an efficient counter electrode material, the process of holextraction from electrolyte should be faster. The kinetics of conver-ion of I3− to I− decides the performance of the counter electrode21]. The peak at around -0.14 V is assigned the reduction reac-ion of I3− to I−. From the CV data it is clear that the carbonloth shows good catalytic activity towards the reduction reac-ion of the I3−. The current density observed for the carbon cloths higher than that for Pt which is due to its higher thickness,urface roughness, porosity and absorption properties. There isn intimate contact of this carbon cloth counter electrode withhe electrolyte which gives rise to a capacitive behaviour lead-ng to higher current. Also the catalytic activity of the carbonloth and Pt counter electrode were compared by determininghe peak to peak separation (�Epp) values. �Epp for Pt and car-on cloth counter electrode were found to be 560 mV and 480 mV,espectively, with iodide/tri iodide electrolyte. As �Epp is inverselyroportional to the catalytic activity of counter electrode mate-ial, carbon cloth proves to be superior counter electrode due toresence of oxygen containing groups as well as defects in its net-ork.

We also tested the catalytic activity of this carbon cloth for cobalt redox shuttle and compared with Pt counter electrodes seen in Fig. 3(b). The cobalt shuttle used for this study washlorophenylterpyridine cobalt complex [22]. The electrolyte wasomposed of 0.001 M Co+2, 0.0001 M Co+3 and 0.01 M LiClO4 incetonitrile. In the measurements Pt wire was used as workinglectrode, Ag/AgCl as a reference electrode and Pt foil as a counterlectrode, at a scan rate of 50 mV/s over the voltage range from 0 to.8 V. The oxidation potential for cobalt electrolyte is around 0.38 Vhile the reduction potential is at 0.3 V. Thus the E1/2 for this cobalt

lectrolyte in acetonitrile vs Ag/AgCl reference electrode is 0.34 V.o check the catalytic activity of carbon cloth counter electrodeame Cyclic Voltametry measurements were done but by replac-

ng Pt wire by carbon cloth or thermally deposited Pt on FTO as

orking electrodes. The carbon cloth clearly shows good catalyticctivity towards redox reaction of the cobalt shuttle due to pres-nce of oxygen containing groups on its surface. The peak to peak

tri-iodide electrolyte, (b) Cyclic voltametry data for Pt and carbon cloth as counter cloth as counter electrodes in iodide tri-iodide electrolyte, and (d) DSSC device I-V

separation i.e. �Epp in cobalt electrolyte for carbon cloth is 200 mVwhile it is 80 mV for thermally deposited Pt counter electrode usedroutinely in DSSCs. Thus this pyrolysed carbon cloth with surfaceoxygen containing groups is an efficient catalyst even for cobaltredox shuttle.

Further, we tested this carbon cloth as counter electrode forDSSC with iodide/triiodide and its performance was compared withthe usual thermally deposited Pt on FTO counter electrode. The cor-responding results are shown in Fig. 3 (c) and Table 1. From thesedata it is clear that the current density and open circuit voltage aresomewhat less for this carbon cloth counter electrode as comparedto Pt/ITO in iodide electrolyte. This could be attributed to higherseries resistance as seen in the impedance measurements discussedin the next section. Interestingly the fill factor is somewhat higherin the case of the carbon cloth counter electrode.

We also examined the carbon cloth electrode in a DSSC devicewith cobalt electrolyte. Admittedly, this is a preliminary evaluationof the comparison between the standard Pt/FTO and carbon cloth,although further optimization for this system may be necessary toget a final picture. The results for this case are shown in Fig. 3(d)

Counter Electrode Voc (V) Jsc (mA/cm2) FF (%) � (%)

Pt 0.76 3.76 66 2.0Carbon Cloth 0.77 3.94 78 2.6

2 ochimica Acta 123 (2014) 248–253

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52 M. Tathavadekar et al. / Electr

ctive site density, charge collection efficiency etc.) contribute tohe fill factor more work is needed to understand the precise causef its significant enhancement as observed.

Impedance measurements were performed using symmetricell assembly on thermally deposited Pt and carbon cloth counterlectrodes in dark under zero bias in iodide/tri-iodide electrolyte.he frequency range used was 105 Hz to 10−1 Hz with AC amplitudef 10 V. Fig. 4 (a) shows the Nyquist plot obtained from impedanceeasurement with the equivalent circuits used to fit the respective

raphs. The parameters to be extracted from the Nyquist plot arehe Rs, Rct, C and the Warburg diffusion. Rs is the series resistance ofhe electrolyte and the electrodes in symmetric cell assembly, Rct

s the charge transfer resistance at the electrolyte electrode inter-ace, C is the capacitance of the double layer and W is the Warburgiffusion. Due to the absorption of electrolyte by the carbon clothounter electrode the Warburg parameter for diffusion of ions wasot employed in the equivalent circuit [23]. Rct which is the chargeransfer resistance has an inverse relation to the catalytic reductionf I3− is also less for the carbon cloth counter electrode (0.71 �)han the Pt electrode (1.3 �). This highlights the better catalyticctivity of carbon counter electrode towards reduction of I3− whichs also seen in CV data of Fig. 3(a). The series resistance for Pt counterlectrode is 15.1 � which is less than that of carbon cloth counterlectrode with Rs of 18.7 �. The cause for higher series resistance inhe carbon cloth counter electrode lies in the current way of deviceabrication. In the carbon cloth case we have just used a mechan-cal press contact between the cloth and ITO conducting substratehrough a layer of epoxy adhesive dispensed over the edges, as

hown later in Fig. 5 (II) in the context of device fabrication, andhis clearly needs to be improved further. This contact in the cur-ent design is thus not ideal and efficient as in the other case wheret is thermally deposited directly on FTO substrate.

Fig. 4. (a) Nyquist plot, and (b) Tafel plot for Pt and carbon cloth counter electrodesin iodide tri-iodide electrolyte.

Fig. 5. (I) Regular protocol for DSSC fabrication, (II) Revised simplified protocol for DSSC fabrication with the use of carbon cloth as counter electrode.

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To further analyze the catalytic properties of the carbon clothounter electrode, Tafel polarization measurements were carriedut on carbon cloth counter electrode and Pt/FTO in a symmet-ic cell assembly with DSSC iodide/tri-iodide electrolyte shown inig. 4(b). Scan rate used for this measurement was 10 mV/sec. Tafellot consists of three parts. Curve at lower potential is the polariza-ion zone, curve in the middle with a steep slope is the Tafel zonewhich determines the catalytic properties of electrode) and thehird is the diffusion region at the higher voltage [24].

In the Tafel zone the intersection of the tangent to the cathodicranch with the equilibrium (zero) potential ordinate is Jo, which ishe exchange current density. The comparison in the inset indicateshat the cloth is more effective in catalysing the reduction of I3−. Jos inversely proportional to Rct from the equation:

Jo = RT/nFRct, Where R is constant, T is temperature, n is the num-er of electrons involved in reaction, and Rct is the charge transferesistance. A higher Jo for carbon cloth counter electrode implies aower value of Rct in the impedance measurement.

Another important parameter that can be extracted from thisafel graph is Jlim which is the limiting diffusion current density.

lim for carbon counter electrode is higher than that of Pt. This cane due to the porous nature and absorption characteristics of thearbon counter electrode. Thus the only reason for somewhat lowerfficiency in the case of carbon cloth as compared to Pt appearso be the contact resistance between the cloth and FTO substratesed for collection of charges at the counter electrode side. Fur-her work on optimization and smart sealing of the final cell is inrocess.

The use of carbon cloth as counter electrode indeed makeshe DSSC fabrication process much simpler. The time consum-ng process of drilling holes in FTO is completely removed andlso the tedious process of sealing of cell is rendered facile androuble-free. Although the efficiency is somewhat less than Ptounter electrode (which could be improved by further refinement)he modified fabrication process certainly aids in easy fabricationSSCs. Fig. 5 (I) shows the regular DSSC fabrication protocol whileig. 5 (II) shows the modified fabrication protocol which is clearlyimpler.

. Conclusion

Conducting carbon cloth obtained from pyrolysis of cellu-ose fabric shows good catalytic activity towards redox reactions

nvolved in the electrolyte of DSSCs. DSSC using this carbon clothounter electrode gives a high efficiency of 5.8%. The strategy usedere can certainly reduce the cost and also make the cell fabricationrocess much simpler. Needless to mention that the cloth is easily

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ica Acta 123 (2014) 248–253 253

adaptable to wearable cell designs. Such a catalytically and electri-cally functional cloth can add value to other flexible optoelectronicand actuator device systems as well.

Acknowledgment

Authors acknowledge CSIR’s TAPSUN program and DST’s UK-India APEX program for funding. SA also acknowledges CSIR forfellowship support.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.electacta.2013.12.175.

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