DOI: 10.1002/cssc.201200647

Ordered Mesoporous Tungsten Suboxide CounterElectrode for Highly Efficient Iodine-Free Electrolyte-BasedDye-Sensitized Solar CellsInyoung Jeong,[a] Changshin Jo,[a] Arockiam Anthonysamy,[a] Jung-Min Kim,[a] Eunae Kang,[a]

Jongkook Hwang,[a] Easwaramoorthi Ramasamy,[b] Shi-Woo Rhee,[a] Jin Kon Kim,[a] Kyoung-Su Ha,[c] Ki-Won Jun,[c] and Jinwoo Lee*[a]

Introduction

Since the first report by Gr�tzel and O’Regan in 1991, dye-sen-sitized solar cells (DSCs) have attracted significant attention asa promising alternative to conventional silicon solar cells withfascinating advantages such as a relatively high efficiency thatexceeds 12 %, a low fabrication cost, easy manufacture andflexible application to various demands.[1] Upon light illumina-tion, electrons excited from the HOMO level of the dye to theLUMO level are injected into the conduction band of the metaloxide in a working electrode (WE), typically TiO2 sensitizedwith dye. Photoexcited dyes are regenerated through redox re-actions at the dye–electrolyte and electrolyte–counter elec-trode (CE) interfaces. Most research in this area has focused onthe optimization of the photovoltaic properties of a DSC

through the development of each of its three parts: WE, elec-trolyte and CE.[2] In particular, although a conventional I3

�/I�

redox-couple electrolyte shows a superior power conversionefficiency (PCE, h), much recent research has focused oniodine-free electrolytes for outdoor use and the commercializa-tion of DSCs, which require long-term stability and high effi-ciency, owing to drawbacks such as evaporation loss, the cor-rosive properties of metal current collectors (Ag, Cu) and thevisible light absorption (ca. 430 nm) properties of the I3

�/I�

redox couple.[3] Among iodine-free electrolytes, various organicdisulfide/thiolate redox-couple systems have recently gainedinterest, as these systems have negligible absorption in thevisible-light region, noncorrosive properties and multielectron-transfer capability.[4–6] Wang et al. exploited a disulfide/thiolate(T2/T�) redox couple derived from 5-mercapto-1-methyltetra-zole, which shows a remarkable 6.4 % PCE with the Z907Nadye.[4] Sun et al. developed a BMT/McMT� (disulfide dimer/thio-late form) redox couple derived from 2-mercapto-5-methyl-1,3,4-thiadiazole (McMT), which showed a PCE of 4.0 % withthe organic TH305 dye.[5a] In addition, the performance of a tet-ramethylformaminium disulfide/tetramethylthiourea (TMFDS2 +

/TMTU) redox couple, which possesses a more positive redoxpotential than I3

�/I� , was reported by Li et al. with the N3 dyeand by Liu et al. with the D131 dye.[6] Although the disulfide/thiolate redox couples in previous studies have promisingqualities as alternatives to iodine electrolytes, DSCs based onthe disulfide/thiolate electrolyte show poor PCE values owingto a low fill factor (FF) caused by a large charge-transfer resist-

A disulfide/thiolate (T2/T�) redox-couple electrolyte, which isa promising iodine-free electrolyte owing to its transparentand noncorrosive properties, requires alternative counter-elec-trode materials because conventional Pt shows poor catalyticactivity in such an electrolyte. Herein, ordered mesoporoustungsten suboxide (m-WO3�x), synthesized by using KIT-6 silicaas a hard template followed by a partial reduction, is used asa catalyst for a counter electrode in T2/T�-electrolyte-baseddye-sensitized solar cells (DSCs). The mesoporous tungstensuboxide, which possesses interconnected pores of 4 and20 nm, provides a large surface area and efficient electrolyte

penetration into the m-WO3�x pores. In addition to the advan-tages conferred by the mesoporous structure, partial reductionof tungsten oxide creates oxygen vacancies that can functionas active catalytic sites, which causes a high electrical conduc-tivity because of intervalence charge transfer between the W5 +

and W6+ ions. m-WO3�x shows a superior photovoltaic perfor-mance (79 % improvement in the power conversion efficiency)over Pt in the T2/T� electrolyte. The superior catalytic activityof m-WO3�x is investigated by using cyclic voltammetry (CV),electrochemical impedance spectroscopy (EIS), and Tafel polari-zation curve analysis.

[a] I. Jeong, C. Jo, Dr. A. Anthonysamy, J.-M. Kim, Dr. E. Kang, J. Hwang,Prof. S.-W. Rhee, Prof. J. K. Kim, Prof. J. LeeDepartment of Chemical EngineeringPohang University of Science and TechnologyKyungbuk 790-784 (Korea)E-mail : jinwoo03@postech.ac.kr

[b] Dr. E. RamasamyCentre for Solar Energy MaterialsInternational Advanced Research Centre for Powder Metallurgyand New MaterialsBalapur (PO), Hyderabad 500 005, AP (India)

[c] Dr. K.-S. Ha, Dr. K.-W. JunKorea Research Institute of Chemical Technology141 Gajeongno, Yuseong, Daejeon, 305-600 (Korea)

Supporting Information for this article is available on the WWW underhttp://dx.doi.org/10.1002/cssc.201200647.

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ance if a conventional Pt counter electrode is used. This indi-cates that alternative catalytic materials are necessary becausePt is not an appropriate catalytic material for the reduction ofdisulfide in a CE. In the BMT/McMT� and TMFDS2+/TMTU redoxcouples, poly(3,4-ethylenedioxythiophene) (PEDOT),[5b] CoS,[5c]

and carbon black[6] were introduced as alternative CE materialsto Pt in each electrolyte, and showed improved PCEs. In theT2/T� redox couple, which showed a superior photovoltaic per-formance over the other various disulfide/thiolate electrolyte-based DSCs, PEDOT,[7] vanadium carbide embedded in meso-porous carbon (VC-MC) composite,[8] and graphite[9] were usedas CE materials, and a PCE higher than that of a Pt CE was ach-ieved, which opens the opportunity to use more suitable cata-lyst materials in a T2/T� redox couple.

Ideal CE materials should possess not only high catalytic ac-tivity but also high electrical conductivity. For the materials,previously, carbon materials[10] and conducting polymers[11]

have been studied as low-cost and efficient Pt-free CE materi-als.[12] Recently, various inorganic compounds, which includetransition metal nitrides, carbides, oxides and sulfides, were in-vestigated owing to their attractive merits such as Pt-like cata-lytic activity, a wide range of materials and better thermal sta-bility and showed comparable performances to Pt CE.[12, 13]

In addition to searching for other catalytic materials, it hasbeen confirmed that a mesoporous structure in the materialcan enhance the catalytic activity of a CE, which is attributedto a high surface area, an interconnected wall and accessiblepores for the electrolytes.[14] However, most of the previous re-search on mesoporous CEs has focused on carbon materials.[14]

To the best of our knowledge, very few mesoporous inorganicmaterials have been investigated as CEs for DSCs even thoughthere is a chance to improve their intrinsic catalytic activity.In previous work by our group and other co-workers, an or-dered mesoporous tungsten suboxide was successfully synthe-sized by using KIT-6 silica as a hard template followed by a re-duction process and employed as electrodes for batteries andsupercapacitors, which showed excellent results attributed tothe advantages of an ordered mesoporous structure and a par-tial reduction effect.[15] The effect of the partially reduced metaloxide induces high electrical conductivity owing to its mixed-valence characteristics, in which electrons can be effectivelydistributed through an electron hopping mechanism, and cre-ates defect sites through the removal of oxygen that can pro-mote chemical reactions.[16] The mesoporous tungsten subox-ide developed by our group possesses all the qualifications(catalytic active sites, high electrical conductivity, and a meso-porous structure) needed for an ideal CE for a DSC.

In this context, we adopted a highly ordered mesoporoustungsten suboxide (m-WO3�x) as a CE for a T2/T�-electrolyte-based DSC and investigated its catalytic activity for the reduc-tion of disulfide (T2) compared to that of Pt. m-WO3�x-CE-basedDSCs with an N719 Ru-based dye showed a 79 % higherenergy conversion efficiency (5.86 %) than that with a Pt CE(3.27 %), which is attributed to a nearly twofold higher FF(0.630 vs. 0.327). Interestingly, both bulk tungsten suboxide (b-WO3�x) and mesoporous tungsten oxide (m-WO3), which weretested for comparison with m-WO3�x CEs, also exhibited

a better performance (5.40 % and 3.97 %, respectively) than thePt CE. The better catalytic activities of m-WO3�x, m-WO3, and b-WO3�x in the T2/T� redox couple were evaluated by usingcyclic voltammetry (CV), electrochemical impedance spectros-copy (EIS) and Tafel polarization curve analysis. Through thesemeasurements, we confirmed that the partial reduction ofmetal oxide (WO3�x) strongly influences the high catalytic activ-ity as a result of a higher electrical conductivity, which iscaused by an intervalence charge transfer between W5 + andW6 + ions and the catalytically active sites for T2 reduction.In addition, the interconnected mesoporous structure playsa role in a good CE material as it provides a high active surfacearea, efficient electron pathway and improved electrolyte pen-etration. In this study, we developed highly catalytically activeinorganic CEs by controlling both the intrinsic (partial reduc-tion) and extrinsic (mesoporous structure) properties of thematerial at the same time.

Results and Discussion

Material characterization

Figure 1 shows XRD patterns of m-WO3�x, m-WO3 and b-WO3�x.The crystal structure of m-WO3�x has a cubic phase (JCDPS 46-1096) identical to that of nonporous b-WO3�x, whereas m-WO3

shows a tetragonal phase XRD pattern (JCDPS 89-1287). Thedifferent phases are ascribed to the formation of oxygen va-cancies and the co-presence of W5 + and W6+ ions in WO3�x

through a partial reduction process under H2/Ar mixed-gasconditions. The difference in XRD patterns between m-WO3�x

and b-WO3�x is a peak broadening owing to a nanometre-scalewall thickness of m-WO3�x and a bigger crystal size of b-WO3�x.The TEM images in Figure 2 a and b show the uniform, highlyordered mesoporous structures of m-WO3 and m-WO3�x. TheSEM image in Figure 2 c shows the ordered mesopores thatcan be observed all over the m-WO3�x particles. The SEMimage of b-WO3�x (Figure S1, Supporting Information) confirms

Figure 1. XRD patterns of m-WO3�x (c), b-WO3�x (c) and m-WO3 (c).

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the bulk structure of b-WO3�x, which was synthesized withouta template.

The BET surface area and pore size characterizations of m-WO3�x, m-WO3 and b-WO3�x are summarized in Table 1. The re-sults were measured from N2 adsorption–desorption isotherms(Figure 3 a). Figure 3 b shows the pore size distributions calcu-lated from adsorption isotherms by using the Barrett–Joyner–Halenda (BJH) method. Both m-WO3�x and m-WO3 synthesizedby using a KIT-6 template have two types of mesopores of ap-proximately 4 and 20 nm. The 4 nm pores are produced fromthe exact negative replicas of KIT-6, as the wall thickness ofKIT-6 is 3.6 nm.[17] The large 20 nm pores can be explained bythe ordered asymmetric networks of WO3�x in the KIT-6 pores(7.8 nm, Figure S2 b) after the removal of KIT-6, caused by theselective filling of phosphotungstic acid into one of two chiralchannels of KIT-6.[15b] As shown in Table 1, m-WO3�x hasa much higher surface area (51 m2 g�1) than b-WO3�x

(5.1 m2 g�1) owing to its ordered mesopores with a large porevolume. The electrical conductivities of m-WO3�x and b-WO3�x

were also measured to analyse their catalytic activities as CEsmore exactly as materials with a high electrical conductivitycan transport electrons faster, which results in an efficient re-duction of T2 at the CE. The high conductivity of m-WO3�x

(1.76 S cm�1) is comparable to that of ordered mesoporouscarbon (3.0 S cm�1),[15b] which is attributed to a single-crystal-line wall structure and intervalence charge transfer between

W5 + and W6+ ions in m-WO3�x. However, the electrical conduc-tivity of b-WO3�x is 50.8 S cm�1, which is much higher than thatof m-WO3�x. This indicates that the high conductivity mighthave a strong effect on the catalytic activity of a b-WO3�x CE.With the exception of the Pt CE, the electrodes were preparedby spray coating onto fluorine-doped tin oxide (FTO) glassafter ball-milling the same amount (30 mg) of each material.The coated glass was subsequently heated at 500 8C for 30 minfor good adhesion to the FTO glass substrate. The thickness ofthe m-WO3�x and m-WO3 CEs (ca. 2 mm) is thicker than that ofthe b-WO3�x CE (ca. 1.3 mm) (Figure S3) as a result of the lowdensity caused by a mesoporous structure and large porevolume. The magnified SEM image (Figure S3 d) of the m-WO3�x CE confirms the existence of a mesoporous honey-comb-like structure, which was preserved after spray coatingand heat treatment.

CV of the WOx and Pt CEs forthe T2/T� electrolyte

CV has been widely used to in-vestigate the catalytic activity ofvarious CEs in DSCs. Figure 4shows CVs of m-WO3�x, m-WO3,b-WO3�x and Pt electrodes in T�

(10 mm), T2 (1 mm) and LiClO4

(0.05 m) in acetonitrile/propylene

Figure 2. TEM images of a) m-WO3 and b) m-WO3�x, c) SEM image of m-WO3�x, and d) magnified SEM image of m-WO3�x.

Figure 3. a) N2 adsorption–desorption isotherms and b) pore size distributioncalculated from N2 adsorption branches of the isotherms by using the BJHmethod for m-WO3 (*), m-WO3�x (~) and b-WO3�x (&).

Table 1. Summary of N2 adsorption–desorption isotherms and electrical conductivity of m-WO3�x, m-WO3, andb-WO3�x.

Material Surface area[a] [m2 g�1] Pore diameter [nm] Pore volume [cm3 g�1] Conductivity[b] [S cm�1]

m-WO3�x 51 3.6 and 20 0.21 1.76m-WO3 46 3.9 and 20 0.21 –b-WO3�x 5.1 – 0.02 50.8

[a] The surface area was calculated by using the BET method. [b] The electrical conductivity was measured byusing Van der Pauw four-probe methods.

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carbonate (1:1 v/v) electrolyte. Although a typical CV of a Ptelectrode displays two pairs of redox peaks in the I3

�/I� elec-trolyte, all four electrodes have only one pair of redox peaks inthe T2/T� electrolyte, which is consistent with previous re-sults.[4, 8, 9] In the case of the tungsten oxide electrodes, there isan additional anodic peak at around �0.2 V. We also measuredCVs under the same conditions without T2 and T� species toidentify this peak (Figure S4). We conclude that this peak iscaused by the reaction between LiClO4 used as a supportingelectrolyte and the WO3 electrodes, which is well known inelectrochromic systems.[18] The details are presented and dis-cussed in the Supporting Information (Figure S4). The CE ina T2/T�-electrolyte-based DSC catalyses the reduction of T2 toT� ions according to Equations (1) and (2).[9]

T2 þ 2e� ¼ 2T� ð1Þ

T2 þ e� ¼ T� þ T� ð2Þ

As shown in Figure 4, the Pt electrode shows a cathodicpeak at �0.23 V, which can be ascribed to the reduction of T2,and an anodic peak at 0.39 V, which can be assigned to the re-verse reaction (oxidation to T2). The other three electrodes (m-WO3�x, m-WO3 and b-WO3�x) have a similar peak position tothat of Pt but a much higher current density, indicating thatm-WO3�x, m-WO3 and b-WO3�x have better catalytic activitiesand reversibility than Pt and that these electrodes are moresuitable as CEs in T2/T�-electrolyte-based DSCs.[11c, 19] Generally,not only the current density of each peak but also the peak-to-peak separation (DEP) between the potentials of the anodicand cathodic peaks is an important factor that can determinethe degree of catalytic activity of the materials. In particular,DEP is negatively correlated with the standard electrochemicalrate constant of the redox reaction.[10d, 20] A small DEP reflectsa small overpotential and a high electron transfer rate. Theorder of DEP is m-WO3�x (640 mV)<b-WO3�x (710 mV) <m-WO3 (750 mV), indicating that tungsten suboxide (WO3�x),

which is partially reduced from tungsten oxide (WO3), can pro-vide advantages in the redox reaction of T2. The enhanced cat-alytic activity is caused by the co-presence of W5 + and W6 +

ions, and the defect sites created by the removal of oxygenatoms in the tungsten oxide lattice as the intervalence chargetransfer between W5 + and W6 + provide a high electrical con-ductivity and can act as active catalytic sites. The higher cur-rent density of m-WO3�x and m-WO3 than that of b-WO3�x indi-cates a higher accessible surface area, which is attributed tothe mesoporous structure.[21] The CV results reveal that m-WO3�x has a strong catalytic activity for the reduction reactionof T2 as an alternative CE material to Pt in a T2/T� electrolytesystem.

Photovoltaic performance of DSCs with WOx and Pt CEs

Figure 5 shows the current density–voltage (J–V) curves of fourDSCs made by using spray-coated m-WO3, m-WO3�x and b-WO3�x, and thermally platinized FTO glass CEs under standardillumination (AM 1.5, 100 mW cm�2). The detailed photovoltaicparameters from the J–V curves are summarized in Table 2. The

cell employing a conventional Pt CE shows a short-circuit cur-rent density (Jsc) of 15.12 mA cm�2, an open circuit voltage (Voc)of 662 mV and a poor h of 3.27 %, which is attributed to a lowFF of 0.327. The cell with the m-WO3�x-based CE exhibits a su-perior photovoltaic performance with a Jsc of 13.96 mA cm�2,Voc of 666 mV, and FF of 0.630, which result in a higher efficien-cy of 5.86 % than the Pt-CE-based cell (3.27 %). The 79 % im-provement in the power conversion efficiency of the m-WO3�x

CE compared to that of the Pt CE is a result of a nearly twofoldhigher FF, which implies that m-WO3�x has a higher catalyticactivity for the reduction of T2. To test the additional effects ofthe mesoporous structure and the partial reduction of tung-sten oxide, b-WO3�x and m-WO3 were also used in DSCs, andthe cell performances were measured. Interestingly, the m-WO3

Figure 4. CV of m-WO3 (c), m-WO3�x (c), b-WO3�x (c) and Pt (c)electrodes in T� (10 mm), T2 (1 mm), and LiClO4 (0.05 m) in acetonitrile/pro-pylene carbonate (1:1 v/v) at a scan rate of 10 mV s�1. Ag/AgCl and Pt wireelectrodes are used as reference and counter electrodes, respectively.

Figure 5. J–V curves of DSCs based on different CEs: m-WO3 (c), m-WO3�x

(c), b-WO3�x (c) and Pt (c). The active area of all electrodes is0.25 cm2. (solid line: measured under standard illumination (AM 1.5,100 mW cm�2 ; dashed line: dark measurement).

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and b-WO3�x CE cells also exhibited a better performance thanthe Pt CE cell, which is also attributed to a higher FF. In a com-parison between m-WO3�x, m-WO3 and b-WO3�x, the order ofPCE is m-WO3�x>b-WO3�x>m-WO3, which corresponds withthe order of the FF and indicates that m-WO3�x is a more pow-erful catalytic material than b-WO3�x or m-WO3. These resultsare consistent with the DEP values in CV. The Pt and m-WO3

CEs showed S-shaped J–V curves that resulted in low FFs,which is attributed to large internal series resistance (Rs).

[7, 14a] Inaddition to the results achieved under standard illumination,dark currents were observed for each cell (Figure 5). Compar-ing the dark currents of DSCs with the same WE compositionbut different CE materials, a different dark current is attributedto the degree of catalytic activity of each material. In the darkunder a high forward bias (>600 mV), the electrons diffusethrough the TiO2 WE and reduce the T2.[22] This dark currentcan be decreased by the depletion of T2 concentration in thevicinity of the TiO2 film, which can be caused by the poor cata-lytic activity of the CE materials.[23] Therefore, the large darkcurrent observed for m-WO3�x also confirms its superior cata-lytic activity. The tungsten suboxides (m-WO3�x and b-WO3�x)are prepared by a reduction process under H2/Ar gas at 600 8C.One of the main factors to control the degree of reduction isthe temperature. We investigated the W5 +/W6 + ratio of m-WO3�x, b-WO3�x, m-WO3 and other m-WO3�x samples reducedat different temperatures by fitting the XPS results (Figure S5).As we expected, the W5 +/W6+ ratio is increased on increasingthe reduction temperature from 0.14 at 550 8C to 0.19 at650 8C, and the J–V curves (Figure S6) prove that the highly re-duced material has a higher catalytic activity as a result of thepresence of more catalytic active sites. By checking whetherthe ratio of m-WO3�x is the same as that of b-WO3�x, we canconfirm that the mesoporous structure contributes to the en-hanced catalytic activity of m-WO3�x compared with that of b-WO3�x. Interestingly, the W5 +/W6 + ratio of b-WO3�x (0.24) ishigher than that of m-WO3�x (0.17). This may be attributed tothe smaller accessible surface of m-WO3�x to the reducing gas(H2/Ar) because m-WO3�x is reduced in the WO3/KIT-6 silicacomposite, whereas b-WO3�x is formed without the KIT-6 tem-plate. Despite the smaller W5+/W6 + ratio, the catalytic activityof the m-WO3�x CE is higher than that of the b-WO3�x CE. Thisconfirms that the advantages of the mesoporous structure sur-pass the deficient reduced state of m-WO3�x compared withthat of b-WO3�x. Compared with b-WO3�x, the well-developed,ordered, porous structure of m-WO3�x facilitates the permea-

tion of electrolyte into the electrode to make large electrolyte–CE interfaces. Also, interconnected networks in sub-microme-tre-sized m-WO3�x provide an efficient electron transport path-way.[24] The 5.86 % efficiency of m-WO3�x is one of the highestefficiencies among previous research on the T2/T� electrolyteand Pt-free CEs.[7–9, 24, 25] Notably, most previous research usinginorganic-compound-based CEs have used MC as an electricalconductor to improve their PCE,[8, 24, 25] whereas our m-WO3�x

CE has accomplished a large improvement without MC be-cause a electrical conductivity and high surface area are al-lowed by partial reduction and the mesoporous structure.

EIS, LSV and Tafel polarization analysis

To investigate the relationship between the electrochemicalcharacteristics of the four electrodes and the photovoltaic per-formance of each cell, EIS was used for a symmetric cell com-posed of two identical electrodes. Figure 6 c shows Nyquistplots obtained from the EIS of the symmetric cell composed ofm-WO3�x, m-WO3, b-WO3�x and Pt electrodes under 0 V bias.In general, the symmetric cells show two well-defined semicir-cles, which can be fitted well by a Randles circuit (Fig-ure S7 b).[26] At a high frequency of approximately 100 kHz, theonset of the first semicircle determines the ohmic serial resist-ance (Rh), which mainly comes from the sheet resistance of theFTO glass substrate. The first semicircle in the high-frequencyregion represents the charge transfer resistance (Rct) and thecorresponding constant phase angle element (CPEdl) at the CE–electrolyte interface, whereas the second semicircle in the low-frequency region reflects the Nernst diffusion impedance (ZN)of the T2 in the electrolyte. If bias is applied in a symmetriccell, Rct exponentially decreases until the diffusion-limitedregion is reached as expressed by the Butler–Volmer equation,whereas ZN increases owing to the depletion of oxidized ions(here, T2) at the cathode.[26] As shown in Figure 6 a and b, byapplying bias, the Nyquist plot of the Pt electrode shows gen-eral trends that imply that the Randles circuit is applicable.However, the plot for the m-WO3�x electrode shows that thefirst semicircle in the high-frequency region is unchanged andthe second semicircle in the low-frequency region dwindleddrastically, whereas a third semicircle appears on the applica-tion of bias. This demonstrates that the second semicircle inthe low-frequency region for m-WO3�x represents Rct and thefirst small semicircle in the high-frequency region representsanother resistance. We applied an appropriate equivalent cir-

Table 2. Photovoltaic performance of DSCs, EIS parameters and overall cell resistances (Rcell) obtained from LSV based on m-WO3�x, b-WO3�x, m-WO3 andPt CEs.

CE J–V curve EIS LSVVoc [V] Jsc [mA cm�2] FF h [%] Rh [W cm2] Rtrns [W cm2] CPEtrap [mF] Rct [W cm2] CPEdl [mF] Rs

[a] [W cm2] Rcell [W cm2]

m-WO3�x 0.67 13.96 0.630 5.86 3.31 0.9 60.3 17.5 827 21.7 21.5b-WO3�x 0.68 14.11 0.567 5.40 3.20 0.8 43.0 27.3 84.9 31.3 31.8m-WO3 0.67 14.47 0.412 3.97 3.38 1.5 76.6 92.0 430 96.9 108Pt 0.66 15.12 0.327 3.27 3.66 – – 214 6.54 217 226

[a] Rs = Rh+Rtrns+Rct.

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cuit for our tungsten oxide electrodes as suggested by Kwonet al.[27] The circuit depicted in Figure S7 a contains additionalresistance and capacitance (RC) components (Rtrns and CPEtrap).These components come from the transport resistance andtrap capacitance of the charge in the tungsten oxide electrodelayer itself. Because CPEtrap is caused in a part of the layer andRtrns is correlated with the conductivity of the materials, thesehave smaller values than Rct and CPEdl. Therefore, it is expectedthat a small semicircle that corresponds to Rtrns and CPEtrap willappear in a high-frequency region followed by large semicirclethat corresponds to Rct and CPEdl.

[27] The spectra of the threetungsten oxide CEs were well fitted following the equivalentcircuit with a c2 value for goodness of fit of less than 10�3,whereas the Pt electrode was fitted by using a Randles circuit.The EIS parameters are shown in Table 2. The Nyquist plot ofthe Pt CE displayed in Figure 6 c shows a much larger semicir-cle than that of the other cells, which indicates that the Pt CEhas a large charge transfer resistance (213.75 W cm2) for the re-duction of T2, which results in a low FF and poor h. In the caseof the mesoporous structured CEs, m-WO3�x has a smaller Rct

(17.48 W cm2) than m-WO3 (92.03 W cm2), which reflects thatWO3�x has a better catalytic activity owing to a higher electricalconductivity and a larger number of active catalytic sites fromdefect sites. In addition, although b-WO3�x has a structurally

smaller surface area than m-WO3, the high electrical conductiv-ity and catalytic defect sites of b-WO3�x also lead to a smallerRct than that of m-WO3, which confirms that the partial reduc-tion makes tungsten oxide catalytically active and confersa stronger effect than the structurally high surface area of m-WO3. By comparison of m-WO3�x and b-WO3�x, the Rct of m-WO3�x smaller than that of b-WO3�x (27.3 W cm2) results ina higher FF and PCE despite the order-of-magnitude smallerelectrical conductivity of m-WO3�x compared to that of b-WO3�x. This confirms that the mesoporous structure maximizesthe advantages from WO3�x to provide a higher catalyticallyactive surface area and promote the efficient penetration ofelectrolytes and electron transport as discussed above. Thelarge CPEdl and CPEtrap of m-WO3�x are attributed to its highersurface area and thicker layer.[8, 25a] The trends of Rtrns (Fig-ure 6 d) correspond well with the order of electrical conductivi-ty of b-WO3�x, m-WO3�x, and m-WO3.[27] For Pt, the semicirclecaused by Rtrns and CPEtrap did not appear because the Pt layeris highly conductive and very thin. In Figure 6, the third semi-circle with regard to ZN, which is caused by ion diffusion of T2,is not distinguished clearly at 0 V bias, which indicates that ZN

is very small.[7] The third semicircle is clearly observed whenthe bias voltage is applied, and the ZN value is very low(0.75 W cm2) even at a high bias voltage (0.4 V), which implies

Figure 6. Nyquist plots of symmetric cells composed of two identical a) m-WO3�x and b) Pt electrodes. Applied bias was 0 (&), 0.2 (*) and 0.4 V (~). c) Nyquistplots of four symmetric cells composed of m-WO3 (~), m-WO3�x (^), b-WO3�x (3) and Pt (&) under open circuit conditions (applied bias = 0 V). d) Magnificationof c) that focuses on the high-frequency region. (symbols: experimental data, solid lines: simulated data, c2 values in all cases are below 10�3).

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that the diffusion of T2 in the bulk electrolyte is sufficientlyfast.[5b, 7] Therefore, we can conclude that the ZN component isnot a critical factor to compare the catalytic activity of each CEand the FF of the cell is strongly determined by the Rct becausethe FF of a DSC is influenced by internal series resistance ex-pressed as Rs = Rh+Rct+ZN (Rtrns is added for tungsten oxideCEs).[14d, 28]

Figure 7 a shows linear sweep voltammograms (LSV) of thefour symmetric cells magnified at 0 V. The inverse slope at 0 Vrepresents the overall cell resistance (Rcell) following Ohm’s law

and characterizes the catalytic activity of the electrodes.[29] Thegreater slope of m-WO3�x than that of the other CEs impliesthat it has the smallest overall resistance and the highest FF.The calculated Rcell values are presented in Table 2 and matchthe Rs values obtained from EIS well. Figure 7 b shows the Tafelpolarization curves of the logarithmic current density (log J)versus potential (U), which is widely used to confirm the elec-trochemical catalytic activity and relates to the EIS results.Based on the Tafel equation, the exchange current density (J0)is obtained from the y intercept at U = 0 of the tangent to thecathodic and anodic braches. The gentle slope obtained forthe Pt CE indicates a low J0 on the Pt surface, which manifests

in the poor catalytic activity of Pt in a T2/T� electrolyte. Thesteepest slope of the cathodic branches for m-WO3�x com-pared to the other materials indicates that because of a largeJ0 on the surface m-WO3�x effectively catalyses the T2 reduc-tion. J0 can be expressed by Equation (3) and is related toRct.

[8, 30] The trend of J0 of the four electrodes is in good agree-ment with the Rct values obtained from EIS.

J0 ¼RT

nFRct

ð3Þ

Rct is the charge transfer resistance, n is the number of elec-trons involved in the reduction of T2 at the CE, R is the gasconstant, T is the temperature and F is the Faraday constant.

Conclusions

We successfully synthesized a mesoporous tungsten suboxide(m-WO3�x) by using a KIT-6 silica template followed by a reduc-tion process at 600 8C under H2/Ar. The structural and materialproperties of m-WO3�x were investigated and showed a highsurface area with 4 and 20 nm pores and a high electrical con-ductivity comparable to that of mesoporous carbon. Herein,m-WO3�x was exploited as an alternative CE to conventional Ptin a disulfide/thiolate (T2/T�) redox-couple electrolyte andshowed superior catalytic activity for the reduction of T2,which resulted in a 79 % improvement in the overall conver-sion efficiency compared with a Pt CE. To investigate the effectof a mesoporous structure and the partial reduction of tung-sten oxide in detail, b-WO3�x and m-WO3 were also synthe-sized. In addition, their catalytic activities were compared byanalysis of their J–V curves, CVs, EIS and Tafel polarizationcurves. It was confirmed that the partial reduction of tungstenoxide has a powerful effect on the catalytic activity owing toa higher electrical conductivity and the presence of many cata-lytically active sites (defect sites from the removal of oxygen).In addition, the mesoporous structure also plays an importantrole in the efficient reduction of T2, and m-WO3�x has the bestphotovoltaic performance among the four CEs studied (m-WO3�x, b-WO3, m-WO3, and Pt), which is attributed to its highercatalytically active surface area.

Experimental Section

Syntheses of m-WO3�x, m-WO3 and b-WO3�x

Mesoporous silica KIT-6, which was used as a hard template, wassynthesized according to a reported procedure.[31] A two-step im-pregnation of phosphotungstic acid into the pores of the KIT-6was carried out by using a solvent evaporation infiltrationmethod.[15] In the first step, phosphotungstic acid (1.2 g) was dis-solved in ethanol (20 mL), and KIT-6 (0.45 g) was added to the solu-tion with stirring. The mixture was dried at 60 8C and calcined at350 8C in air. In the second impregnation step, phosphotungsticacid (0.6 g) was incorporated into the pores of the compositemade in the first step and calcined at 550 8C again in air to obtainWO3/KIT-6, which had a yellowish green colour. Finally, m-WO3 wasgenerated by HF etching of WO3/KIT-6 to remove silica. To obtain

Figure 7. a) LSV of the region around 0 V and b) Tafel polarization curves forthe four symmetric cells composed of m-WO3 (c), m-WO3�x (c), b-WO3�x (c) and Pt (c). The electrolyte composition is the same as thatused for EIS.

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m-WO3�x, heat treatment of the WO3/KIT-6 composite at 600 8Cunder H2/Ar (4 %) for 4 h was carried out for the reduction toWO3�x, which was followed by HF etching. b-WO3�x was preparedby following the same procedure for the synthesis of m-WO3�x butwithout the KIT-6 template.

Preparation of WOx and Pt CEs

With the exception of the Pt CE, the CEs were prepared througha spray-coating method. m-WO3�x, m-WO3 or b-WO3�x (30 mg) wasdispersed in ethanol (5 mL) by using zirconium dioxide balls andthen ball milled to completely break up large aggregates and uni-formly disperse the material in ethanol. The solutions were spraycoated onto the FTO glass substrate at 135 8C with a spray gunconnected to N2 carrier gas. To uniformly deposit the materialsonto the substrate, the spray flow rate and distance between thespray nozzle and FTO glass substrate were optimized. After spraycoating, the CEs were patterned to create a 5 mm � 5 mm activearea and sintered at 500 8C for 30 min under N2. The conventionalPt CE was prepared by tape casting a commercial Pt paste (Solaro-nix, Pt catalyst T/SP) onto an FTO glass substrate with subsequentsintering at 400 8C for 30 min in air.

Fabrication of DSCs with various CEs

The WEs were prepared as bilayer films composed of a 15-mm-thick transparent layer with commercial TiO2 paste (Solaronix, tita-nium nanoxide HT/SP) and a 4-mm-thick scattering layer that con-tained 300-nm-sized TiO2 particles by screen printing onto a con-ducting FTO glass substrate (Hartford Glass Co., Newington, CT;sheet resistance of 15 W cm�2), which was subsequently sintered at500 8C for 30 min in air. After TiCl4 (40 mm aqueous solution) treat-ment at 80 8C for 30 min, the electrodes were sintered at 500 8C for30 min in air. After cooling to 80 8C, the electrodes were immersedin a solution of the N719 dye (0.5 mm, Solaronix) in acetonitrile/tert-butanol (1:1 v/v) for 24 h. The sensitized electrodes were rinsedwith acetonitrile and dried under a N2 flow. The geometric area ofthe photoanode was 0.25 cm2. The photoanode was assembled byusing the four CEs in a sandwich configuration by using a 60-mm-thick hot-melt film (Surlyn, Solaronix). The disulfide/thiolate redoxcouple (T2/T�) was synthesized according to a reported procedure(see Supporting Information for details).[4] The T2/T� electrolyte,which is composed of 5-mercapto-1-methyltetrazole N-tetramethy-lammonium salt (NMe4

+T� , 0.4 m), dis-5-(1-methyltetrazole) disul-fide (T2, 0.4 m), 4-tert-butylpyridine (TBP, 0.5 m) and lithium perchlo-rate (LiClO4, 0.05 m) in acetonitrile/ethylene carbonate (3:2 v/v),was put into the cell through predrilled holes in the CE by usinga vacuum-backfilling method followed by sealing the holes withSurlyn and a microscope cover glass.

Characterization

The N2 adsorption–desorption isotherms were measured by usinga Micromeritics Tristar II 3020 system to obtain the BET surfacearea and pore size distribution. The XRD patterns of the sampleswere measured by using a Rigaku D/Max-3C diffractometerequipped with a rotating anode and CuKa radiation (l=0.15418 nm). The external morphologies of m-WO3�x, m-WO3, b-WO3�x and the electrode films were examined by using SEM (JEOLJSM-840A) and the pore images by TEM (JEOL JEM-2010). The J–Vcharacteristics of the DSCs were measured under an air mass of1.5 G simulated sunlight by using a solar simulator (PEC-L11, Pec-

cell Technologies, Inc. Japan). The light intensity was adjusted to100 mW cm�2 by using an NREL-certified silicon reference cellequipped with a KG-5 filter. An EIS investigation of CE–electrolyte–CE symmetric cells was performed over a frequency range of100 kHz to 0.1 Hz under open circuit conditions. The compositionof the electrolyte used for the symmetric cell was the same at thatused for the DSCs. The magnitude of the AC signal was set to10 mV. The EIS parameters were obtained by fitting by using theZView program. CV was performed by using a three-electrode con-figuration in acetonitrile/propylene carbonate (1:1 v/v) solutionthat contained T� (10 mm), T2 (1 mm) and LiClO4 (0.05 m). Ag/AgCland Pt wires were used as REs and CEs, respectively, at a scan rateof 10 mV s�1. LSV was measured at a scan rate of 50 mV s�1. Allelectrochemical characterizations (EIS, CV and LSV) were performedby using a Gamry Reference 600 potentiostat (Gamry Instruments,USA).

Acknowledgements

This work was supported by the Global Frontier R&D Program ofthe Centre for Multiscale Energy System funded by the NationalResearch Foundation of the Ministry of Education, Science andTechnology, Korea, by a National Research Foundation of Koreagrant funded by the Korean Government(2012R1A2A2A01002879), and by a grant from the IndustrialSource Technology Development Programs (10033093) of theMinistry of Knowledge Economy (MKE) of Korea. This work wasfurther financially supported by the KRICT OASIS Project from theKorea Research Institute of Chemical Technology and by thesecond stage of the BK 21 program of Korea.

Keywords: cyclic voltammetry · electrochemistry ·mesoporous materials · platinum · tungsten

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Received: August 31, 2012

Published online on December 23, 2012

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