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Journal of Power Sources 272 (2014) 1091e1099

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

Hierarchical porous carbons as a metal-free electrocatalyst of triiodidereduction for dye-sensitized solar cells

Leng-Leng Shao 1, Ming Chen 1, Zhong-Yong Yuan*

Key Laboratory of Advanced Energy Materials Chemistry (Ministry of Education), Collaborative Innovation Center of Chemical Science and Engineering(Tianjin), College of Chemistry, Nankai University, Tianjin 300071, China

h i g h l i g h t s

* Corresponding author.E-mail address: zyyuan@nankai.edu.cn (Z.-Y. Yuan

1 These authors have made an equal contribution t

http://dx.doi.org/10.1016/j.jpowsour.2014.09.0280378-7753/© 2014 Elsevier B.V. All rights reserved.

g r a p h i c a l a b s t r a c t

� Hierarchical porous carbons weresynthesized by a one-step hydro-thermal route.

� Hierarchical porous carbons ascounter electrode materials forDSSCs.

� Excellent catalytic property of hier-archical porous carbons for triiodidereduction.

� Hierarchical pore system promotedthe catalytic activity of carboncounter electrode.

� High energy conversion efficiency of7.22%, comparable to 7.25% of the Pt-based cell.

a r t i c l e i n f o

Article history:Received 9 July 2014Received in revised form9 August 2014Accepted 3 September 2014Available online 16 September 2014

Keywords:CarbonHierarchical porous structureCounter electrodeDye-sensitized solar cells

a b s t r a c t

Hierarchical porous carbons (HPCs) are prepared by a one-step hydrothermal route using soluble resolsas a carbon source, polystyrene spheres and Pluronic F127 as macroporous and mesoporous templates,respectively. The resulting HPCs comprise continuous macroemesoemicropore networks on the carbonblocks, simultaneously exhibiting large surface areas (501e709 m2 g�1), which are particularly evaluatedas counter electrodes (CEs) for dye-sensitized solar cells (DSSCs). Electrochemical impedance spectra(EIS), Tafel polarization curves, and cyclic voltammograms (CVs) demonstrate that the hierarchical poresystem plays a crucial role in promoting the catalytic activities of the HPC CEs. In comparison with otherporous carbon CEs, the HPC CEs with various macropores and appropriate micro/mesopores presentbetter catalytic behaviors due to the easier accessibility for electrolyte ions and better utilization of thecatalytically active sites, resulting in the higher photovoltaic conversion efficiencies of DSSCs. The op-timum photovoltaic conversion efficiency of DSSCs based on HPC CEs is 7.22%, which can even match upto that of the cell with Pt CE (7.25%).

© 2014 Elsevier B.V. All rights reserved.

).o this work.

1. Introduction

Dye-sensitized solar cells (DSSCs) have been considered as oneof the most competitive alternatives to silicon-based solar cellsbecause of their low cost, simple fabrication, and high power con-version efficiency [1,2]. A typical DSSC is composed of a dye-adsorbed TiO2 photoanode, a Pt counter electrode (CE), and an

L.-L. Shao et al. / Journal of Power Sources 272 (2014) 1091e10991092

electrolyte containing I�=I�3 redox couple. The CE is one of thecrucial components of the DSSC inwhich the collection of electronsfrom the external circuit back to electrolyte and the catalysis of thetriiodide reduction to iodide take place. As the common CE mate-rial, Pt has shown outstanding performance due to its high catalyticactivity and good conductivity [3,4]. Nevertheless, its high cost andpoor long-term stability prohibit the large-scale production ofDSSCs, which stimulates the development of the economic, stable,and effective CE materials.

Currently, the alternative carbonaceous CE materials such asactivated carbon [5], mesoporous carbon [6,7], macroporous car-bon [8], carbon nanotube [9,10], carbon black [11], graphene [12],and carbon fiber [13] have attracted considerable attention due totheir low cost, good catalytic activity, and strong anti-corrosion.However, the overall reduction catalytic activities of these carbo-naceous materials still cannot match up to that of Pt, which needto be further improved. With regard to the carbon CEs, the porestructure and surface area are two crucial physical characteristicsin affecting the catalytic activity toward the triiodide reduction bymediating the diffusion of electrolyte ions and the quantity ofcatalytically active sites [13,14]. The development of well-definedpore structure and high surface area in the carbon materialscould be an effective strategy for fabrication of high-performancecarbon CEs.

Hierarchical porous carbon combines the efficient mass trans-port frommacropores and high surface area frommicro/mesopores[15,16], offering a great adaptability toward the CE application[17,18]. For example, Ko's group [17] fabricated the multimodalporous carbon by nanocasting hierarchical nanostructured silica,demonstrating the fantastic catalytic activity as CE in DSSC. Fanget al. [18] reported the synthesis of hierarchical porous carbonthrough nanocasting solid core/mesoporous shell silica, exhibitinga considerably improved catalytic activity and good chemical sta-bility. Although the incorporation of hierarchical pores into thecarbon could be realized by the hard-template strategy (nano-casting), the synthesis process involving in the preparation andremoval of silica template is complex and time-consuming, whichis inappropriate for the large-scale production of hierarchicalporous carbon [17,19]. Therefore, exploring the effectivemethods todirectly prepare the hierarchical porous carbons is important forthe practical application. The soft template strategy with thesimplified synthesis process is highly proposed and scarcelyreported.

On the other hand, based on the fact that the redox reactions areimplemented on electrode/electrolyte interfaces in pore channelsof carbon CEs, the accessibility of the electrolyte ions into the poresof the electrode greatly affects their catalytic activity [20,21]. Forthe sake of facilitating the diffusion of electrolyte ions and gettinghigher utilization of the interior surface areas of CEs, the size andshape of pores in the electrode material should be adapted to theelectrolyte ions. However, less effort has been devoted to system-atically investigate the influence of pore structure on the electro-catalytic performance of carbons in DSSCs.

Herein, we directly synthesized the hierarchical porous carbonmonoliths by a one-step low-temperature hydrothermal route, andapplied them as CE materials for DSSCs. The resultant carbons withhierarchical porous system and large surface area demonstrate highcatalytic activities toward the triiodide reduction. Moreover, tospecifically study the correlation between the pore architecture andthe catalytic performance, the porous carbons with correspondingmacroporous, mesoporous and microporous frameworks were alsoprepared, respectively. The evaluation of catalytic behaviors amongthese porous carbon CEs may provide the feasible direction toconstruct CE materials with unique porous architectures for highlyefficient DSSCs.

2. Experimental

2.1. Preparation of polystyrene spheres

Mono-disperse polystyrene (PS) spheres were prepared ac-cording to the procedure reported previously [22]: A certainamount of styrene (A.R.) was extracted at 70 �C by vacuum distil-lation before use. A three-necked, 500 ml round-bottomed flaskcontaining 40 g of styrene, 0.33 g of sodium dodecyl sulfate (A.R.),and 364 ml of water was heated to 70 �C under magnetic stirring(200 rpm). A thermometer, condenser, and pipet were attached tothe flask. Nitrogen was firstly bubbled for 30 min to deaerate themixture. In a separate beaker, 0.41 g of potassium persulfate (A.R.)initiator was added to 28 ml of water and heated to 70 �C, and thenadded to the above styrene mixture. The final solution was main-tained at 70 �C and stirred for 24 h. The resulting suspension wasdemulsified by 2.72 g of CaCl2, and then filtered. Ethanol was usedto wash away any unreacted styrene. Finally, the obtained PSspheres were dried at 60 �C.

2.2. Preparation of hierarchical porous carbons

Hierarchical porous carbons were synthesized by employing PSspheres and Pluronic F127 as dual templates, and soluble resols as acarbon source. 3.3 g of resorcinol and 5.0 g of F127 were dissolvedin a mixture of 30 ml water and 30 ml ethanol, stirred for 30 minuntil the solution turned into colorless. Then 0.4 g of HCl (37 wt%)was added to act as a catalyst. After 1 h of stirring, 5.0 g of 37 wt%formaldehyde solution was added dropwise under tempestuouslystirring. The reaction mixture was further vigorously stirred foranother 1 h, followed by the addition of 0.83, 1.65 or 3.30 g of PSspheres dispersed in 20 ml mixture of water and ethanol (v/v ¼ 1:1). The final solution was stirred for 1 h, transferred to aTeflon-lined autoclave and heated at 80 �C for 2 days. The resultingpolymeric monolith was washed with ethanol and water, and driedin an oven at 60 �C for 12 h. Carbonization was carried out in atubular furnace under an inert atmosphere (N2 flow) with a heatingrate of 1 �C min�1, and then keeping the temperature at 800 �C for3 h. For convenience, the obtained hierarchical porous carbon wasdenoted as HPC-x (x¼ 10, 20 or 40, representing the weight ratio ofPS to resols used). For comparison, other porous carbons were alsoprepared in the same procedure except the templates. The mac-roporous carbon (MAC) was fabricated with the use of 1.65 g of PSspheres. The ordered mesoporous carbon (MEC) was prepared inthe presence of 5.0 g of F127. The microporous carbon (MIC) wassynthesized without using any templates.

2.3. Fabrication of DSSCs

TheporouscarbonCEswerepreparedwithasimpledoctor-bladingmethodas follows:Amixtureof 130mgof carbon sampleand15mgofPEG-2000 was ultrasonically dispersed in 3 ml of deionized wateralong with the subsequent magnetic stirring for 5 h. In a separatecontainer, 20mgof TiO2 (P25, Degussa, Germany) in 3ml ofwaterwasultrasonically dispersed for 30 min and stirred to form a white TiO2colloid. Then the TiO2 colloid was added to the above sample solutionand stirred continually for 3 h to form the final paste. The paste wasscrapedontoanFTO(fluorine-dopedtinoxide)conductiveglass (sheetresistance 14 U sq�1, Nippon Sheet Glass) by the doctor-bladingmethod and then dried at 60 �C overnight, resulting in the porouscarbon CEs. For comparison, Pt CEs were fabricated by conventionalthermal decomposition of 30mMH2PtCl6$6H2O solution spin-coatedon the surface of FTO at 400 �C for 30 min.

The dye-sensitized TiO2 photoanodes were prepared with twokinds of TiO2 pastes. Typically, a ca. 5 mm thick layer of 20 nm-sized

L.-L. Shao et al. / Journal of Power Sources 272 (2014) 1091e1099 1093

TiO2 particles (Degussa, Germany) was firstly printed on FTO by thedoctor-blading technique and dried at 125 �C for 15 min. The ob-tained layer was further coated with a ca. 3 mm thick scatteringlayer of 200 nm-sized TiO2 particles and calcined at 500 �C for30 min. The calcined electrode was treated with 40 mM TiCl4aqueous solution at 70 �C for 30 min, washed with distilled water,recalcined at 500 �C for 30 min. The as-made TiO2 films wereimmersed in a 3 � 10�4 M N719 dye (Solaronix SA, Switzerland)ethanol solution for 24 h to obtain dye-sensitized TiO2 films. TheDSSC was assembled with a dye-sensitized TiO2 photoanode, acounter electrode, and an acetonitrile electrolyte containing 0.05 MI2, 0.5 M LiI, 0.3 M 1,2-dimethyl-3-propylimidazoliumiodide(DMPII), and 0.5 M 4-tert-butyl pyridine. The apertures of thescotch tapes coated on both FTO were used as the electrolytestorage spacer. The two electrodes were clipped together and theactive area was 0.2 cm2.

Fig. 2. The small-angle XRD patterns of HPCs and MEC materials.

2.4. Characterization

X-Ray diffraction (XRD) patterns were recorded on a Bruker D8Focus diffractometer with Cu Ka radiation (l ¼ 1.5406 Å) operatedat 40 kV and 40 mA. Scanning electron microscopy (SEM) wascarried out on a JOEL JSF-7500L at an acceleration voltage of 5.0 kV.Nitrogen adsorptionedesorption isotherms were measured on aQuantachrome NOVA 2000e sorption analyzer at 77 K. All thesamples were degassed in a vacuum at 200 �C overnight prior to themeasurements. The BrunauereEmmetteTeller (BET) method wasutilized to calculate the specific surface areas (SBET). The pore sizedistributions were derived from the adsorption branches of iso-therms using BarretteJoynereHalenda (BJH) method, and the totalpore volumes (Vtotal) were estimated from the adsorbed amount ata relative pressure P/P0 of 0.980.

2.5. Electrochemical measurement

The photovoltaic characteristics of the sealed DSSCs weremeasured under simulated AM 1.5 illumination (I¼ 100 mW cm�2)with a solar simulator (Oriel Sol 2A, Newport). Electrochemicalimpedance spectroscopy (EIS) and Tafel polarization measure-ments were carried out in a symmetrical dummy cell consisted oftwo identical CEs. The geometric area of the symmetrical dummycell was 0.25 cm2. EIS spectrawere obtainedwith an ACmodulation

Fig. 1. Schematic illustration of the synthesis of hierarchic

signal of 10 mV and bias DC voltage of �0.70 V over the frequencyrange of 100 KHz to 100 mHz. Tafel polarization measurementswere performed at a scan rate of 10 mV s�1. Cyclic voltammetry(CV) measurements were conducted in a three-electrode system inacetonitrile solution at a scan rate of 50 mV s�1 using a Zenniumelectrochemical workstation (Zahner, Germany). The electrolytecontained 0.1 M LiClO4, 10 mM LiI, and 1 mM I2 in acetonitrile. Theprepared porous carbon and Pt electrodes were used as workingelectrodes, Pt/FTO as an auxiliary electrode, and Ag/Agþ electrodeas a reference electrode.

3. Results and discussion

3.1. Material synthesis and characteristics

The hierarchical porous carbons were directly synthesized usingPS spheres and F127 as dual templates, and resorcinol-formaldehyderesin as the carbon source (Fig.1). In the process of the preparation ofHPC-x, on one hand, the resorcinoleformaldehyde resin and F127conducted the organiceorganic self-assembly through hydrogenbonding arising from the interaction between hydroxyl groups in

al porous carbon using the dual-templating strategy.

L.-L. Shao et al. / Journal of Power Sources 272 (2014) 1091e10991094

resol and the EO segments of Pluronic F127 template [23]. On theother hand, thepep interaction betweenPS and resorcinolmoleculepossibly plays an important role in the formation of the resorcinol-containing polymer/F127 on the surface of PS spheres [24]. The

Fig. 3. SEM images of PS spheres

highly cross-linked resorcinoleformaldehyde resin/F127/PS com-posite was finally prepared under the hydrothermal treatment. Theresultant carbonprecursor/dual templates hybridswere subjected toa high temperature treatment under N2 atmosphere for the removal

and various porous carbons.

Fig. 4. (a) N2 adsorptionedesorption isotherms and (b) the corresponding pore sizedistribution curves of various porous carbons.

Table 1Structural characteristics of various porous carbons.

Sample SBETa

(m2 g�1)Smicro

b

(m2 g�1)Vtotal

(cm3 g�1)Vmicro

b

(cm3 g�1)DBJH

c

(nm)

HPC-10 592 352 0.37 0.14 3.58HPC-20 709 273 0.56 0.12 4.92HPC-40 501 264 0.34 0.11 4.50MAC-

20514 393 0.35 0.16 e

MEC 625 330 0.54 0.14 6.48MIC 769 684 0.39 0.31 e

a The BET surface area, SBET, was calculated using adsorption data in a relativepressure range P/P0 ¼ 0.05e0.24.

b The micropore surface area Smicro and the micropore volume Vmicro were esti-mated by t-plot method.

c The pore diameter, DBJH, was obtained from the maxima of the pore size dis-tribution curve.

L.-L. Shao et al. / Journal of Power Sources 272 (2014) 1091e1099 1095

of PS and F127 templates and carbonization of resorci-noleformaldehyde resin. In this way, the hierarchical porous carboncould be achieved.

Fig. 2 shows small-angle XRD patterns of HPCs and MEC mate-rials. The XRD pattern of MEC showed a major peak at around 0.74�

(2q), which is indexed to the (1 0 0) reflection of 2-D hexagonalmesostructures (space group p6mm). The resolved diffraction peakswere also observed for the sample HPC-20, suggesting the forma-tion of ordered mesoporous architecture. While the HPC-10 andHPC-40 with the weaker diffraction peaks demonstrate their infe-rior mesopore structures to HPC-20.

SEM imaging and N2 sorption analysis are two effective tools tounderstand the pore structure of various porous carbons, in whichthe SEM images are preferable to investigate the pores with di-ameters exceeding 50 nm and the N2 sorption isotherms are suitedto characterizing mesopores and micropores. Fig. 3 shows the SEMimages of PS spheres and the synthesized porous carbons. The as-prepared PS spheres exhibited uniform size of ca.160 nm, actingas colloidal templates to create the macroporous networks in thesynthesized carbon matrix. SEM images of HPC-x show that theinterconnected macropores with different sizes were well formedin the carbon blocks due to the decomposition of PS spheres duringthe carbonization process. Compared to HPC-10 and HPC-40, HPC-20 exhibited the better-developed interconnected macropores onthe carbon blocks due to the appropriate ratio of PS spheres toresols. While for theMAC obtained with the utilization of the singletemplate of PS spheres, the relatively irregular macropores wereformed due to the hydrophobicity of PS colloidal crystal template,and thus it is still difficult to precisely control the pore size distri-bution of MAC. As for the MEC fabricated without the addition of PSspheres but with the use of F127 as a single template, no macro-pores on the smooth carbon blocks were observed, and its surfaceroughness was much lower, which is different from the porestructures of HPC-x and MAC.

Fig. 4 displays the obtained nitrogen adsorption and desorp-tion isotherms of all the samples, and their corresponding poresize distributions. All the detailed structural parameters ofvarious porous carbons are listed in Table 1. N2 adsorption anddesorption isotherms of all the HPC-x samples present type IVcurves with clear hysteresis loops at a relative pressure P/P0 of0.4e0.7, indicative of well-developed mesoporous networks [25].The large degree of mesoporosity in HPC-x is mostly created bythe mesoporous template of F127. In addition, the large adsorb-ing capacity at the low relative pressure indicates the presence ofpartial micropores due to the physical activation of carbonaceousframework. The pore size distribution curves derived fromadsorption branches using BJH model reveal that the meanmesopore sizes centered in the range of 3.58e4.92 nm. The BETsurface areas and total pore volumes of HPC-x were calculated tobe 501e709 m2 g�1 and 0.34e0.56 cm3 g�1, respectively. Thetextural parameters suggest that the as-prepared HPC-x pos-sesses high specific surface area and large mesopore size, espe-cially for the HPC-20. Compared with the HPC-x, the MEC derivedfrom the single template of F127 exhibited the representativetype IV isotherm with H1 hysteresis loop, characterizing thewell-defined mesopores with the diameter of ca. 6.48 nm. Apartfrom the macropores as shown in Fig. 3, the MAC also presentedlarge amount of micropores and partial mesopores, which mayarise from the pyrolysis of resols during the carbonization step,and the shrinkage of carbon framework due to the removal of PS.For the template-free MIC, the isotherms are of type I accom-panied with a large uptake, demonstrating the characteristics ofmicropores. The burn-out of C, H, and O in phenolic resinframework during pyrolysis at 800 �C is ascribed to the genera-tion of micropores in MIC [26].

3.2. Electrocatalytic activities

With regard to the aimed application of the CEs for DSSCs, theas-prepared HPC-x materials possess hierarchical pore architec-tures and large surface areas, which can contribute to the fastdiffusion of electrolyte ions and abundant catalytically active sites.

Table 2The characteristics of DSSCs based on various CEs and EIS parameters of the sym-metric cells for different CEs.

Counter electrode Voc (V) Jsc (mA cm�2) h (%) FF Rs (U) Rct (U)

Pt 0.690 15.29 7.25 0.69 42.39 15.16HPC-10 0.754 15.16 6.21 0.54 36.23 8.21HPC-20 0.745 14.97 7.22 0.65 31.92 4.68HPC-40 0.744 16.04 7.06 0.59 32.24 5.38MAC-20 0.727 10.59 4.75 0.62 35.29 7.29MEC 0.744 13.54 6.16 0.61 34.52 22.97MIC 0.697 14.77 4.42 0.43 33.08 52.73

L.-L. Shao et al. / Journal of Power Sources 272 (2014) 1091e10991096

Thus the performance of the DSSCs based on the hierarchicalporous carbon CEs was examined. Fig. 5 illustrates the obtainedphotocurrent densityevoltage responses of DSSCs with differentCEs, and Table 2 summarizes the detailed photovoltaic parametersderived from Fig. 5. Among the cells based on HPC-x CEs, the onewith HPC-10 CE displayed the power conversion efficiency (h) ofonly 6.21% due to the relatively low fill factor (FF) of 0.54. With thecombination of larger Jsc and FF than that of HPC-10 CE, the cellbased on the HPC-40 CE achieved the higher h of 7.06%. Moreimportantly, the HPC-20 CE based cell displayed larger FF of 0.65,presenting the optimum h of 7.22%. The high h is even comparableto that of the one using Pt CE (7.25%), suggesting its excellent cat-alytic activity. Noticeably, the cell with MEC CE still achieved the h

of 6.16% due to the large surface area and numerous large-sizedmesopores. While the DSSCs with the MAC and MIC CEs exhibi-ted lower h, implying their poor catalytic performance toward thereduction of triiodide ions. On the other hand, Voc depends directlyon the electrocatalytic properties of the CE under the similarelectrolyte and TiO2 semiconductor according to the eq. (1) [27].

Voc ¼���Ef � 4I�=I�3

��� (1)

The larger Voc corresponds to the more positive reduction po-tential of I�=I�3 , thus offering an easy reduction process for the I�3 toI�. The Voc for the DSSCs with HPC-x andMEC CEs is larger than thatof the cell based on Pt, suggesting their excellent catalytic activities.However, the devices based on MAC and MIC CEs displayed therelatively low Voc in comparison with those of the ones with HPC-xand MEC CEs, especially for the MIC CE, further confirming theirinferior catalytic performance. These results suggest that the h isclosely associated with the pore structure, since the catalytic ac-tivity of porous carbons can be regulated by the accessibility of theelectrolyte ions into the pores of the electrode.

In general, the reaction process of the triiodideeiodide redoxcouple at the counter electrodeeelectrolyte interface involves threecrucial stages [28,29]: firstly, the triiodide ions diffuse into thepores of carbon electrodes; secondly, the appropriate adsorptionenergy prompts the triiodide ions to be adsorbed on the surface ofporous carbons; finally, the adsorbed triiodide ions are reduced tothe iodide ions by the catalytically active sites along with the in-jection of electrons. Obviously, the first stage related to the elec-trolyte ions diffusion is an important factor in modulating thecatalytic activity of porous carbon CEs. To achieve the fast electro-lyte ions diffusion and a large ion-accessible surface area, the size

Fig. 5. Photocurrentevoltage characteristics of DSSCs with different CEs under one sunillumination (AM 1.5G).

and shape of pores in the porous carbon materials should beadapted to the electrolyte ions. In view of the size of triiodide ions(>1 nm) [20], highly-developed porous structure with the pore sizeexceeding 1 nm may be desirable for efficient electrolyte diffusion.Thus, the enriched macropores and appropriate mesopores andmicropores (Fig. 6) of HPC-x contribute to the fast diffusion ofelectrolyte ions in the well-developed porous networks andincreased the utilization of active sites distributed on the interiorpore surface, respectively. The hierarchical porous structure ad-vantageously strengthened the electrocatalytic activity of the HPC-x CEs and led to the enhanced photovoltaic performance of DSSCs.Without macropores, MEC cannot display the efficient diffusion ofelectrolyte ions as high as that of HPC-x, and showed lower catalyticactivity which can be evidenced from its decreased h. The MAC andMIC are mainly composed of macroemicropores and single mi-cropores, respectively. The inappropriate collocation of poremodels resulted in the difficulty of electrolyte ions diffusion, whichlowered their catalytic activity due to the insufficient ion-accessibleactive sites. It is believed that the well-developed hierarchicalporous structures in carbon CEs make them more active in thecatalysis of triiodide to the iodide.

EIS is an intuitive and effective tool to gain insight into the effectof pore structure on the electrocatalytic activities of carbon CEs[30]. Fig. 7 shows the obtained Nyquist plots from the symmetricaldummy cells. In the Nyquist plot, the two important parameters ofseries resistance (Rs) and charge transfer resistance (Rct) fordifferent CEs were obtained from the intercept on the real axis andsemicircle in the high-frequency region, respectively [31,32], whichare listed in Table 2. Specially, the Rct is directly associated with theability of electron exchange between the electrode and the elec-trolyte, and the lower Rct corresponds to the higher electrocatalyticactivity. The HPC-x (x ¼ 10, 20, 40) has an Rct value of 8.21, 4.68 and5.38 U, respectively, which is smaller to that of Pt (15.16 U), indi-cating the higher capacity of electron transfer on the CE/electrolyteinterface than that of Pt. Furthermore, the smallest Rct wasobserved for the HPC-20 CE, demonstrating the enhanced electro-catalytic activity due to the promotion of ions transport capacity viaa fast-passageway network on the multilevel pore structure ofenriched macropores and abundant ordered mesopores, and theaccompanying large ion-accessible surface area [17,18]. As for MAC,the relative low Rct representing the fast redox reaction is due to thequick electrolyte penetration in the macropores. However, thesmall quantities of mesopores cannot effectively bridge the mac-ropores together with the excessive amount of micropores, whichmay decrease the effective ion-accessible surface area and thenumber of catalytically active sites. The large Rct is observed for theMEC, which may be due to that the mesopores (6.48 nm) are muchsmaller than the macropores, resulting in the slow electrolytepenetration after the electrolyte injection. Nevertheless, orderedmesopore channel and high surface area of MEC guaranteed thelarge ion-accessible surface area and huge catalytically active siteswhich accounted for the good performance of the CE. For the MIC,the microporous structure is unfavorable for the penetration of the

Fig. 6. Schematic of DSSCs based on the hierarchical porous carbon CEs.

L.-L. Shao et al. / Journal of Power Sources 272 (2014) 1091e1099 1097

electrolyte in and out of the bulk material, causing abundant un-employed catalytic sites to MIC, which is consistent with its largeRct and poor electrochemical performance.

In addition, the beeline in the low-frequency region representsthe impedance of redox species diffusion through the electrodepores [4]. To further confirm the effect of pore structure on theelectrolyte ions diffusion, the measurements of electrolyte ionsdiffusion coefficients ðDI�3 Þ based on eq. (2) are very necessary [33].

DI�3¼ R2T2

2A2n4F4C2s2(2)

where DI�3 is the diffusion coefficient of I�3 , R is the gas constant, T isthe absolute temperature, A is the surface area of the cathode, n isthe number of electrons involved in the reduction of I�3 at theelectrode, F is the Faraday constant, C is the concentration of I�3 , s isthe Warburg factor. According to the eq. (2), the s varies negativelywith DI�3 , suggesting that the lower s corresponds to the higher-rateions diffusion in the porous carbons. The s can be directly derivedfrom the slope of straight line in the Nyquist plot based on eq. (3).

Z0 ¼ RD þ RL þ su�1=2 (3)

Fig. 7. Nyquist plots of DSSCs using various CEs.

Fig. 8 presents the relationship between Z0 and reciprocal squareroot of angular frequency (u�1/2) in the low-frequency region, inwhich the slope of straight line represents s. The s firstly decreasesand then increases when the CE varied fromHPC-10 to HPC-40, andthe HPC-20 achieved the smallest s, indicating an enhanced elec-trolyte ions diffusion rate due to the optimum combination oflarger-domain macropores and numerous ordered mesopores inthe carbon blocks, which accelerated the diffusion of ions for thewhole porous systems. Moreover, MEC also displays a pretty highdiffusion rate of electrolyte ions owing to its ordered mesoporouschannels, but slower to those of HPC-x CEs. However, the insuffi-cient amount of mesopores in MAC weakened the penetration ofelectrolyte ions through macropores to micropores, resulting in ahigh transport resistance and large ions-inaccessible microporoussurface area. As for MIC, the small-sized and disordered pores areresponsible for its slow-rate ions transport. It is thus furtherconfirmed that the pore structure played an important role inregulating the catalytic performance of CEs.

Fig. 9 shows the Tafel polarization curves of the logarithmiccurrent density (log J) versus voltage, which are used to study the

Fig. 8. The relationship between Z0 and u�1/2 in the low-frequency region for porouscarbon CEs.

Fig. 9. Tafel polarization curves for the symmetric cells composed of various CEs.

Fig. 10. (a) Cyclic voltammograms of various CEs; (b) cyclic voltammogram of HPC-40CE for 30 cycles.

L.-L. Shao et al. / Journal of Power Sources 272 (2014) 1091e10991098

catalytic activity of CEs. Generally, exchange current density (J0)reflects the catalytic ability of CE, which is derived from the yintercept at voltage ¼ 0 of the tangent to the cathodic or anodicbranch [31,33]. A bigger slope in the Tafel polarization zone offers alarger J0, suggesting a higher catalytic activity [34]. The steepestslope representing the largest J0 was observed for the HPC-20among all the carbon CEs, demonstrating the highest catalytic ac-tivity, which can match up to that of Pt. In comparison with theHPC-20, the HPC-10 and HPC-40 showed relatively lower J0 due tothe larger Rct. In contrast, MIC and MAC with the gentle slopeimplied their poor catalytic properties. Nevertheless, the J0 of MACis larger than that of MIC due to the enhanced catalytic activity inthe presence of macropores. On the whole, the regularity of J0 fordifferent CEs is almost consistent with that derived from EIS ac-cording to eq. (4).

J0 ¼ RTnFRct

(4)

Furthermore, the limiting current density (Jlim) is determined bythe diffusion of the redox couple in the porous structure as pre-sented in eq. (5) [34],

DI�3¼ l

2nFCJlim (5)

where l is the spacer thickness, R, T, Rct, F and n have their usualmeaning. The value of Jlim can further confirm the different diffu-sion behaviors of electrolyte ions on the various pore structures,exhibiting the largely similar tendency to the results derived fromEIS.

CV measurements were performed to further understand thecatalytic behaviors of HPC-x CEs. Fig. 10a shows the CVs of HPC-xCEs at the scan rate of 50 mV s�1. Two pairs of redox peaks corre-sponding to the redox reaction 6 (left pair) and 7 (right pair) wereobserved for Pt [33,35].

I�3 þ 2e43I� (6)

3I2 þ 2e42I�3 (7)

However, only one pair of redox peaks assigned to the redoxreaction 6 was observed for the HPC-x CEs, in which the reductionof I�3 to I� is the rate-determining step [20,36]. Specially, differentfrom the Pt CE, the HPC-x CEs show large background current,

which mainly arises from the double-layer capacitance on theinterface of electrode/electrolyte due to the porous structure. Thephenomenon indicates that the large ion-accessible surface areacontributes to the effective adsorption of I�3 on the surface of HPC-xCEs, and thus promotes their electrocatalytic activity toward the I�3reduction, since the adsorption of I�3 on CEs is crucial for the cat-alytic reaction in the high I� concentration electrolyte [37].Therefore, thanks to the strengthened adsorption capacity and theresultant improved catalytic activity, the HPC-20 CE displays largercurrent response than those of HPC-10 and HPC-40 CEs. Moreover,the peak-to-peak separation (DEp) between the anodic andcathodic peaks varies inversely as the electron transfer rate (ks) andthus, smaller DEp leads to higher ks [34,38]. Compared to those ofHPC-x CEs, a smaller DEp is obviously observed for the Pt due to itsintrinsically excellent catalytic activity. Nevertheless, the HPC-x CEsdisplayed excellent cycling behaviors. Taking HPC-40 CE as anexample (Fig. 10b), there is almost no obvious loss in current den-sities after 30 CV cycles, indicating the good stability in the corro-sive electrolyte.

4. Conclusions

Hierarchical porous carbons have been prepared via a hydro-thermal route with the use of PS spheres as colloid crystal templategenerating the macropores, F127 as a soft template creating the

L.-L. Shao et al. / Journal of Power Sources 272 (2014) 1091e1099 1099

mesopores, and soluble resols as a carbon precursor providingcarbon substrate. By combining the efficient ion transport frommacroemesopores and high surface area from microemesopores,the hierarchical porous carbons exhibited superior electrocatalyticactivities when used as CEs for DSSCs. It was also found that thehierarchical porous carbons with easy access to electrolyte andabundant active sites can avoid the insufficient catalytic sites of themacroporous carbon and microporous carbon. More importantly,their ion transport rates were enhanced in comparison with that ofthe mesoporous carbon, due to the assistance of macropores in thecarbon blocks. As a consequence, the hierarchical porous carbonCEs based cells achieved the maximum photovoltaic conversionefficiency of 7.22%, which is very close to 7.25% of the Pt based oneunder the same experimental conditions. The facile preparation,low cost, good stability and high electrocatalytic activity wouldmake the hierarchical porous carbon as a potential CE alternative toPt for DSSCs.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (21073099), the Specialized Research Fund forthe Doctoral Program of Higher Education (20110031110016), theProgram for Innovative Research Team in University (IRT13022),and the 111 Project (B12015).

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