High efficient Pt counter electrode prepared by homogeneous deposition method for dye-sensitized solar cell

Applied Energy 100 (2012) 132–137

Contents lists available at SciVerse ScienceDirect

Applied Energy journal homepage: www.elsevier.com/locate/apenergy

High efficient Pt counter electrode prepared by homogeneous deposition method for dye-sensitized solar cell Min Young Song, Kiran N. Chaudhari, Jinsol Park, Dae-Soo Yang, Jung Ho Kim, Min-Sik Kim, Kimin Lim, Jeajung Ko, Jong-Sung Yu ⇑ Dept. of Advanced Materials Chemistry, WCU Research Team, Korea University, 208 Seochang, Jochiwon, ChungNam 339-700, Republic of Korea

h i g h l i g h t s

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

" Homogeneous deposition (HD) and

A combined urea-assisted HD–EG method is used for fabricating a Pt counter electrode for the DSSC. It gives better control over size and dispersion of Pt NPs generated compared to conventional methods with lower resistance and higher efficiency.

"

"

"

"

reduction by ethylene glycol (EG) were tested. A Pt counter electrode for DSSC was prepared by the combined HD–EG method. The HD–EG method demonstrates excellent control over size and dispersion of Pt NPs. The Pt reveals high electrochemical surface area and electrocatalytic reduction of I 3. The HD–EG will bring great impacts on the development of efficient electrocatalysts.

a r t i c l e

i n f o

Article history: Received 26 December 2011 Received in revised form 25 April 2012 Accepted 4 June 2012 Available online 12 July 2012 Keywords: Homogeneous deposition Dye-sensitized solar cell Urea Counter electrode Platinum

a b s t r a c t Platinum counter electrodes in DSSC are generally prepared by thermal reduction method. Poor control over Pt particle size, agglomeration and bad adhesion to the transparent conducting substrate, are some of the shortcomings affecting overall efficiency of the cell. In order to achieve better dispersion and control over the particle size, we explored direct deposition of metal nanoparticles (NPs) from their precursor solution onto the FTO glass substrate by using a simple and effective urea-assisted homogenous deposition (HD) method. This method works at substantially lower temperature for hydrolysis of urea and uses mild reducing agent, ethylene glycol (EG), giving better control over particle size along with better adhesion and negligible agglomeration compared to the thermal reduction method. DSSC prepared from such Pt counter electrode exhibited much enhanced photovoltaic performance compared to ones prepared by thermal and EG reduction alone, with better light-to-electricity conversion efficiency of 9.34% under one sun illumination. This excellent conversion efficiency is attributed to the uniform dispersion and smaller size of the Pt NPs prepared by the novel, combined urea-assisted HD–EG synthesis method. Ó 2012 Elsevier Ltd. All rights reserved.

⇑ Corresponding author. Tel./fax: +82 44 860 1331. E-mail address: [email protected] (J.-S. Yu). 0306-2619/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.apenergy.2012.06.017

M.Y. Song et al. / Applied Energy 100 (2012) 132–137

1. Introduction

2. Materials and methods

Dye-sensitized solar cells (DSSCs) have attracted extensive attention as highly efficient power generators from renewable source due to their simple fabrication process and low-cost solar electricity [1]. A typical DSSC unit consists of a photoanode, sensitizer dye, electrolyte and a counter electrode. The photoanode is usually fluorine doped tin oxide (FTO) glass plate coated with a film of TiO2 nanoparticles (NPs), while the counter electrode is also FTO coated with a thin layer of platinum [2]. When dye molecules adsorbed on the TiO2 surface are exposed to sunlight, photo-excited dye readily injects electrons into the conduction band of TiO2, and holes are shuttled to the counter electrode through I =I 3 redox couple electrolyte. When the electron reaches the counter electrode and reduces I 3 at the electrode–electrolyte interface, the electrical circuit is completed [3,4]. Reduction of I 3 at the counter electrode is a very important reaction step for efficient light-to-electricity conversion [1]. Platinum (Pt) as a catalyst is widely used as counter electrodes in DSSCs due to its high electrocatalytic affinity towards reduction of I 3 and excellent electrical conductivity. Conversely, its limited natural availability and excessive cost possess hindrance in its generous utility [1]. Therefore, developing a practical and cost-effective replacement has become a priority in terms of minimizing the Pt usage and is being extensively investigated for future development of DSSCs [5,6]. Currently most of literature uses thermal reduction method for preparation of counter electrode by depositing Pt solution on a FTO glass substrate, followed by heat treatment at high temperature of around 400 °C [7]. The Pt NPs formed during the thermal treatment aggregate, and the morphology of NPs is difficult to control and is sensitive to the experimental conditions. NP aggregates of large size reduces the available active surface area and increases resistance, naturally affecting effective electron transfer, thus limiting the utility of as-prepared counter electrode. Recently, polyol process, usually employing ethylene glycol (EG) demonstrated enhanced ability over partial control of the particle size and dispersion of the supported metal NPs on the carbon support. Rapid and homogeneous in situ generation of reducing species in this process results in much smaller and uniform metal nanoparticle deposition on the support. It was also reported that the Pt NPs prepared by EG reduction improved efficiency of the counter electrode in DSSC [3]. However, large aggregates of Pt species were still found, indicating difficulty in controlling Pt particle size by EG alone. Urea has been recently used in the preparation of oxide-supported Au catalysts for organic reactions [8], unsupported metal oxide NPs [9], SBA-15-supported Pt for toluene hydrogenation [10], and CNF-supported Pt–Ru catalysts with low metal loadings [11]. The urea-assisted homogeneous deposition method permits in situ gradual and homogeneous generation of OH species throughout the solution by urea hydrolysis taking place around 90 °C and above to interact with cationic metal precursor, allowing homogenous deposition (HD) of metal hydroxides [12]. The precipitating hydroxide ions generated by urea hydrolysis help in deposition of metal precursor on the support. Recently, a combination of urea-assisted HD method and subsequent reduction by polyol process resulted in better control over size and dispersion of Pt NPs on commercial Vulcan XC-72 carbon black and CNT for fuel cell applications. [13,14]. Herein, we extended this concept to achieve better control over size and dispersion of Pt NPs on FTO glass. DSSC prepared with Pt NPs synthesized using this approach exhibited lower resistance and higher solar cell efficiency compared to those prepared by the conventional methods. Our current approach greatly contributes in increasing the overall efficiency of the DSSCs.

2.1. Preparation of Pt counter electrode by urea-assisted HD–EG method

133

Pt counter electrodes were prepared by urea-assisted HD–EG process. A typical synthesis route for a Pt counter electrode is as follows: 0.01 M solution of H2PtCl6 was prepared by dissolving proper amount of H2PtCl6 in ethanol. Urea solution (0.1 M) was prepared by dissolving urea in 100 mL of DI-water. The urea solution was added to the Pt solution under stirring. The mixture was heated at 90 °C for 30 min to induce urea hydrolysis. A thin Pt layer was formed by drop casting of the Pt solution on a FTO glass substrate (8 X cm2) of 1.9  1.9 cm2 size at room temperature. The yellow colored Pt layer was formed on the FTO glass after drying at 80 °C for 10 min. The electrode was cooled to room temperature, and EG was drop casted on the Pt layer as a second step. The electrode was heat-treated at 180 °C for 15 min to induce reduction of Pt hydroxide to metallic Pt. For the purpose of comparison, Pt counter electrodes were also prepared according to thermal [7] and EG reduction methods [3]. For the thermal reduction method, 0.01 M Pt in ethanol was drop casted on the FTO glass substrate at room temperature. Subsequently, electrodes were heat-treated at 400 °C for 15 min to induce thermal reduction of Pt layer. For EG reduction, a Pt layer was formed by drop casting 0.01 M ethanol solution of H2PtCl6 and drying at 80 °C for 10 min after solvent vaporization. When the electrode got cooled to room temperature, EG was added on the Pt layer followed by heat treatment at 180 °C for 15 min to reduce the ionic Pt species to metallic Pt. For all the synthesis methods, the amount of deposited Pt only was 4.5  105 lg/cm2. 2.2. Characterization of Pt nanoparticles The scanning electron microscopy (SEM) images were obtained using a Hitachi S-4700 microscope operated at an acceleration voltage of 10 kV. X-ray photoelectron spectroscopy (XPS) analyses were carried out with an AXIS-NOVA (Kratos) X-ray photoelectron spectrometer using a monochromatic Al KR (150 W) source under a base pressure of 2.6  109 Torr. 2.3. Solar cell fabrication TiO2 anode was prepared as reported elsewhere [15]. Typically, FTO glass plates were immersed in 40 mM aqueous TiCl4 solution at 70 °C for 30 min and then washed with water and ethanol. A 10 lm TiO2 film in thickness was fabricated on the treated FTO glass plates using a doctor blade and TiO2 paste (DYESOL, 18NRT) and then dried at 25 °C for 2 h. The TiO2 electrodes were heated gradually under air flow at 325 and 374 °C for 5 min, respectively, at 450 °C for 15 min, and at 500 °C for 15 min. Scattering layer was deposited by doctor blade method using a paste containing 350– 450 nm size anatase particles (DYESOL, 18NR-AO) and dried. After being sintered under flowing air at 325 and 374 °C and at 450 and 500 °C, respectively for 15 min, the film was once again treated with a 40 mM of aqueous TiCl4 solution followed by heat treatment at 500 °C for 30 min. The dye was loaded by immersing the TiO2 films into N719 dye prepared in 1:1 acetonitrile:ethanol (0.4 mM) solution. The dye-adsorbed TiO2 electrode and each of the as-prepared counter electrodes were immediately assembled into a sealed sandwich-type cell, separated by 50 lm-thick hotmelt ionomer film (Surlyn) as a spacer. Electrolyte containing 0.03 M guanidinium thiocyanate (GuSCN), 0.6 M 1-butyl-3-methylimidazoliumiodine (BMII), 0.05 M I2, 0.1 M LiI, and 0.5 M 4-tertbutylpyridine was prepared in acetonitrile and introduced in the

134

M.Y. Song et al. / Applied Energy 100 (2012) 132–137

cell via an injection hole on the counter electrode side using vacuum filling. Finally, the electrolyte injection hole was sealed with Surlyn film and a microscope cover glass. The J–V curves of solar cells were measured under illumination of 100 mW cm2 using 1000 W xenon light source, whose power in an AM 1.5 Oriel solar simulator was calibrated by using KG5 filtered Si reference solar cell. 2.4. Electrochemical and photovoltaic characteristics Cyclic voltammetry (CV) measurements were carried out in N2purged acetonitrile solution containing 0.1 M LiClO4, 0.01 M LiI, and 0.01 M I2 at the scan rate of 100 mV s1 in a three-electrode setup. Each of Pt-coated FTO glass electrodes prepared by different methods namely, thermal reduction, EG and HD–EG was used as a working electrode with Pt wire as a counter electrode and Ag/AgCl (3 M) as a reference electrode. Electrochemical impedance spectroscopy (EIS) measurements were carried out with a sandwich cell composed of two identical counter electrodes with an apparent surface area of 0.5 cm2. For EIS measurements, an impedance analyzer (Parstat 2273, Princeton Applied Research) was employed with a zero-bias potential, and 10 mV amplitude was applied over the frequency range of 100 kHz to 0.1 Hz at a potential scan rate of 50 mV s1. 3. Results and discussion 3.1. Preparation and characterization of Pt counter electrode Pt counter electrodes were prepared by urea-assisted HD–EG, EG and thermal reduction methods. Fig. 1 shows SEM images and respective particle size distribution histogram of Pt NPs deposited on FTO glass substrates by thermal reduction (Fig. 1a–c), EG

reduction (Fig. 1d–f), and urea-assisted HD–EG method (Fig. 1g– i). Thermal reduction method resulted in a non-uniform Pt NPs with large variation in sizes varying between 50 and 400 nm. EG reduction generated nanoparticle aggregates of smaller sizes (80– 200 nm) compared to thermal reduction. The urea-assisted HD– EG method resulted into even smaller nanoparticles compared to both of the earlier described methods. The nanoparticles formed are relatively well dispersed with sizes between 50 and 110 nm, while the majority of the NPs are dispersed in a narrow size range between 60 and 85 nm. Fig. 1c, f and i show size distribution for Pt NPs. Histogram of Pt NPs by thermal reduction and EG reduction shows particles of larger size with wider size distribution. It is quite evident that the Pt counter electrode prepared by the ureaassisted HD–EG method has more uniform and homogenous dispersion of Pt NPs with smaller Pt particle size than the other two methods. This is attributed to in situ gradual and homogeneous generation of OH species throughout the solution by the urea hydrolysis, which takes place above 90 °C [11]. The pH value is a key parameter in controlling particle size, shape, and dispersion [16–19]. Usually, the synthesis is performed in a basic solution of pH ranging between 7 and 9 to have small and homogenous deposition of Pt NPs on substrate [18,19]. In urea-assisted HD–EG method, the self-generated OH ions shift the pH of the solution towards higher value approximating to 8, thus controlling the size of Pt ionic species and their dispersion effectively by electrostatic repulsion between as-formed Pt hydroxide species before reduction. Subsequently, ethylene glycol reduces the ionic Pt species into metallic Pt under mild reducing environment with least interference in the earlier distribution of ionic Pt species by homogenous deposition. Thus, urea plays a key role in controlling the nanoparticle size by controlling the formation and distribution of Pthydroxide complex species [13]. Therefore, the homogeneous deposition of Pt NPs with smaller Pt particle size can lead to Pt

Fig. 1. SEM images at different magnifications with corresponding size distribution histograms of Pt NPs prepared by different methods on FTO glass: (a, b and c) thermal reduction, (d, e and f) EG reduction, and (g, h and i) urea-assisted HD–EG (insert: graphics of Pt NPs size control).

M.Y. Song et al. / Applied Energy 100 (2012) 132–137

135

Fig. 2. XPS survey scans for (a) metallic Pt and (b) H2PtCl6. Narrow range XPS spectra and deconvoluted curves for (c) metallic Pt NPs prepared by urea-assisted HD–EG method and (d) H2PtCl6.

counter electrode with increased electrocatalytically active surface area, which facilitates the efficient electrocatalytic reduction of the I 3 in the electrolyte solution. Formation of metallic Pt was confirmed by XPS. The XPS profiles are illustrated for H2PtCl6 and Pt NPs prepared by urea-assisted HD–EG method in Fig. 2. XPS survey scans were measured along with narrow range XPS spectra showing Pt 4f region. Chemical conversion of H2PtCl6 to metallic Pt in combined HD–EG was confirmed as shown in Fig. 2a, where no signal for Cl was observed, whereas we can clearly see the metallic Pt signal. However, an XPS spectrum for H2PtCl6 shows the presence of Cl 2s and Cl 2p signals as shown in Fig. 2b, revealing the presence of Pt as ionic Pt species. Narrow range XPS spectra of Pt 4f region are provided in Fig. 2c and d along with deconvoluted curves. The two Pt 4f bands at 75.77 and 72.52 eV in H2PtCl6 shift to 74.53 and 71.17 eV, which correspond to the binding energies of metallic Pt [13,20]. The deconvolution analyses of Pt 4f peaks suggest that the metallic Pt consists of 80% of Pt(0), 11% of Pt(II) and 9% Pt(IV), while the H2PtCl6 is composed of 88% of Pt(II) and 12% of Pt(IV). From the above analysis, transformation of H2PtCl6 to metallic Pt by the HD–EG method can be ascertained [13,20]. 3.2. Photovoltaic and catalytic performance of various working electrodes as counter electrodes Cyclic voltammetry (CV) was used to investigate the catalytic activity of Pt counter electrodes in I =I 3 electrochemical system [21]. Fig. 3 shows voltammograms for Pt counter electrodes synthesized in this work. Two pairs of oxidation and reduction peaks are observed. The current density peaks for Lox and Lred are attributed to the redox reaction of I =I 3 , whereas the peaks for Rox and Rred result from the oxidation and reduction of I2 =I 3 [21,22]. As  the reduction of I 3 to I is of our major concern, the characteristics of Lox and Lred peaks are the focus of our analysis. As we can observe in Fig. 3, the Lox and Lred peaks for oxidation and reduction on the Pt electrodes are higher for the urea-assisted HD–EG method than those for EG and thermal reduction. This can be attributed to the large electroactive surface area of homogenously dispersed Pt NPs with smaller particle size synthesized by the urea-assisted

Fig. 3. Cyclic voltammograms of Pt counter electrodes prepared by various methods in the I =I 3 electrochemical system.

HD–EG on the counter electrode. The peak current density could be used to evaluate the catalytic activity of the counter electrodes [23,24]. Thus, the result proves that the HD–EG method is very effective for the preparation of high efficient Pt counter electrode for DSSCs. The catalytic performances of Pt counter electrodes fabricated by different methods were also examined by electrochemical impedance spectroscopy (EIS) [1,25]. To eliminate the TiO2 photo anode effect, EIS analysis is carried out in situ under dark conditions with the devices composed of two symmetrical counter–counter electrochemical half cells, each separated by spacer containing an electrolyte of I =I 3 redox couple. Counter electrode–counter electrode cell design for EIS analysis is shown in Fig. 4a. The equivalent circuit diagram used to fit the impedance spectra and Nyquist plots of the symmetrical Pt–Pt electrochemical cells are shown in Fig. 4b and c, respectively. The series of ohmic resistance (Rs) and the charge transfer resistance of the counter electrode/electrolyte interface (Rct) can be obtained by fitting with the modified equivalent circuit. The Nyquist plots were fitted with the equivalent circuit diagram and the results are summarized in Table 1. The semicircle at the high-frequency range corresponds to the charge transfer resistance (Rct) at the Pt-based electrode/

136

M.Y. Song et al. / Applied Energy 100 (2012) 132–137

Fig. 5. J–V curves of DSSCs with Pt counter electrodes fabricated by various methods.

Table 2 Parameters of DSSCs with Pt counter electrodes prepared by various methods.

Fig. 4. (a) Symmetric cell configuration, (b) equivalent circuit for EIS tests, and (c) Nyquist plots of Pt counter electrodes prepared on FTO glass by various methods.

Table 1 EIS Parameters of DSSCs with Pt counter electrodes prepared by various methods. Electrodes

Rs (O cm2)

Rct (O cm2)

Urea-assisted HD–EG EG reduction Thermal reduction

2.59 2.87 2.60

1.48 1.75 2.21

electrolyte interface related to the electron transfer for the tri-iodide reduction and is used to judge the catalytic efficiency [7,26]. In other words, smaller the semicircle, smaller the charge transfer resistance, resulting in a better catalytic effect. The Rct of Pt electrode by the urea-assisted HD–EG was found to be 1.47 X cm2, which was less than those of the Pt counter electrodes prepared by other methods. When the Rct decreases, the exchange current density increases, resulting in high electrocatalytic performance. Thus, the Pt counter electrode prepared by the urea-assisted HD– EG method is expected to show better catalytic activity towards reduction of tri-iodide due to its homogeneously deposited Pt NPs of smaller size having high electrocatalytic surface area. This is in good agreement with results from the SEM images and CV as shown in Figs. 1 and 3. Current–voltage (J–V) curves of DSSCs equipped with different counter electrodes under AM 1.5 illumination at 100 mW cm2 are shown in Fig. 5. The corresponding photovoltaic parameters are summarized in Table 2. The DSSC with the Pt counter electrode synthesized by the urea-assisted HD–EG exhibited the best photovoltaic performance. The open circuit voltage (Voc) was 740 V, the short circuit current density (Jsc) was 17.48 mA cm2, the fill factor (FF) 0.72, and the energy efficiency (g) was 9.34%, all of which are much better than those by other methods. These results agree very well with our previous discussion of the SEM, CV and Rct measurements. Interestingly, the Voc values for all the three methods are similar in this work. Voc is the parameter mostly influenced by other components of the whole DSSC unit rather than the cathode alone [27,28]. In this paper, fabrication method and materials used for anode and dye are the same, and in case of cathode all the three

Electrodes

Jsc (mA cm2)

Voc (mV)

FF

g (%)

Urea-assisted HD–EG EG reduction Thermal reduction

17.48 16.25 15.03

740 735 736

0.72 0.72 0.72

9.34 8.66 7.99

methods use platinum. The only change is the synthesis methodology used for cathode preparation. As Pt NPs only with different size distributions from different synthesis methods are used as cathode materials for solar cells under otherwise identical experimental conditions, it is also possible to have similar Voc for all the fabrication methods in our case, where anode and electrolyte materials along with sensitizer dye and other experimental conditions in the cells, which could affect Voc are identical except cathode preparation [27–29]. Low Rct values of counter electrode along with high effective surface area realized by homogeneous dispersion of Pt NPs with small particle size can increase Jsc value and subsequently the catalytic activity of the overall DSSC system. The FF of the DSSCs is directly related to series of internal resistance (Rs) of the DSSCs, which is composed of various components for photoelectrochemical conversion [30]. Since Rs and FF were found to be almost similar for all the three Pt counter electrodes, the higher efficiency for the Pt counter electrode synthesized by urea-assisted HD–EG can be mainly attributed to Jsc value, which is the best for this electrode among the different counter electrodes. The high electrochemical surface area of Pt counter electrode fabricated by urea-assisted HD–EG method greatly enhanced the current density and overall light to current conversion efficiency. The present application of our synthesis method is for enhancing the overall energy output in DSSCs by improving/enhancing the cathode performance. Apart from the DSSCs, the most likely future application of our work could be in the devices which will be used in solar based photo catalysis with redox systems like water splitting, CO2 conversion, etc. 4. Conclusions In this study, we fabricated Pt counter electrodes for DSSCs using urea-assisted HD–EG, EG and thermal reduction methods. The urea-assisted HD–EG method realizes homogeneous deposition of Pt NPs on the FTO glass through combined in situ hydrolysis of urea and homogeneous reduction by reducing species generated in EG, resulting in good control over particle size and distribution of Pt NPs at counter electrode. Compared to Pt counter electrodes by other conventional methods, the Pt counter electrode by

M.Y. Song et al. / Applied Energy 100 (2012) 132–137

present urea-assisted HD–EG approach showed higher catalytic activity towards I =I 3 redox couples and lower charge transfer resistance (Rct) at the counter electrode/electrolyte interface, leading to impressive high power conversion efficiency of 9.34%. This simple and novel methodology is expected to contribute greatly towards development of highly efficient DSSC along with reduction in cost.

[12]

[13]

[14]

Acknowledgements [15]

We thank National Research Foundation grant funded by the Korean Research Foundation (KRF 2010-0029245) and WCU Research Program (R31-2011-000-10035-0) for financial support. Special thanks are given to KBSI at Jeonju for SEM analysis measurements.

[16]

[17]

[18]

References [1] Papageorgiou N. Counter-electrode function in nanocrystalline photoelectrochemical cell configurations. Coord Chem Rev 2004;248:1421–46. [2] Xie Y, Joshi P, Ropp M, Galipeau D, Zhang LF, Fong H, et al. Structural effects of core-modified porphyrins in dye-sensitized solar cells. J Porphyrins Phthalocyanines 2009;13:903–9. [3] Sun K, Fan BH, Ouyang JY. Nanostructured platinum films deposited by polyol reduction of a platinum precursor and their application as counter electrode of dye-sensitized solar cells. J Phys Chem C 2010;114:4237–44. [4] Xie Y, Joshi P, Darling SB, Chen Q, Zhang T, Galipeau D, et al. Electrolyte effects on electron transport and recombination at ZnO nanorods for dye-sensitized solar cells. J Phys Chem C 2010;114:17880–8. [5] Fang B, Fan SQ, Kim JH, Kim MS, Kim M, Chaudhari NK, et al. Incorporating hierarchical nanostructured carbon counter electrode into metal-free organic dye-sensitized solar cell. Langmuir 2010;26:11238–43. [6] Fang B, Kim M, Fan SQ, Kim JH, Wilkinson DP, Ko J, et al. Facile synthesis of mesoporous carbon nanofibers with tailored nanostructure as a highly efficient counter electrode in CdSe quantum-dot-sensitized solar cells. J Mater Chem 2011;21:8742–8. [7] Papageorgiou N, Marier WF, Gratzel M. An Iodine/triiodide reduction electrocatalyst for aqueous and organic Media. J Electrochem Soc 1997;144:876–84. [8] Patil NS, Uphade BS, Jana P, Bharagava SK, Choudhary VR. Epoxidation of styrene by anhydrous t-butyl hydroperoxide over reusable gold supported on MgO and other alkaline earth oxides. J Catal 2004;223:236–9. [9] Wang H, Fan Y, Shi G, Liu Z, Liu H, Bao X. Highly dispersed NiW/!-Al2O3 catalyst prepared by hydrothermal deposition method. Catal Today 2007;125:149–54. [10] Chytil S, Glomm WR, Kvande I, Zhao TJ, Walmsley JC, Blekkan EA. Platinum incorporated into the SBA-15 mesostructure via deposition-precipitation method: Pt nanoparticle size estimation and catalytic testing. Top Catal 2007;45:93–9. [11] Toebes ML, van der Lee MK, Tang LM, Huis in’t Veld MH, Bitter JH, van Dillen AJ. Preparation of carbon nanofiber supported platinum and ruthenium

[19]

[20]

[21]

[22] [23]

[24]

[25] [26]

[27] [28]

[29] [30]

137

catalysts: comparison of ion adsorption and homogeneous deposition precipitation. J Phys Chem B 2004;108:11611–9. Burattin M, Che M, Louis C. Molecular approach to the mechanism of deposition-precipitation of the Ni(II) phase on silica. J Phys Chem B 1998;102:2722–32. Fang B, Kim JH, Yu JS. Homogeneous deposition of platinum nanoparticles on carbon black for proton exchange membrane fuel cell. J Am Chem Soc 2009;131:15330–8. Fang B, Kim MS, Kim JH, Song MY, Wang YJ, Wang HJ, et al. High Pt loading on functionalized multiwall carbon nanotube as a highly efficient cathode electrocatalyst for proton exchange membrane fuel cell. J Mater Chem 2011;21:8066–73. Choi H, Baik C, Kang So, Ko J, Kang HS, Kang MS, et al. Highly efficient and thermally stable organic sensitizers for solvent-free dye-sensitized solar cells. Angew Chem Int Ed 2008;47:327–30. Ullah MH, Chung WS, Kim I, Ha CS. PH-selective synthesis of monodisperse nanoparticles and 3D dendritic nanoclusters of CTAB-stabilized platinum for electroanalytic O2 reduction. Small 2006;2:870–3. Ji XH, Song XN, Li J, Bai YB, Yang WS, Peng XG. Size control of gold nanocrystals in citrate reduction: the third role of citrate. J Am Chem Soc 2007;129:13939–41. Kim MS, Lim S, Chaudhari NK, Fang B, Bae TS, Yu JS. Effect of pH on electrocatalytic property of supported Pt Ru catalysts in proton exchange membrane fuel cell. Catal Today 2010;158:354–60. Fang B, Kim JH, Kim MS, Yu JS. Ordered hierarchical nanostructured carbon as a highly efficient cathode catalyst support in proton exchange membrane fuel cell. Chem Mater 2009;21:789–96. Chen J, Herricks T, Geissler M, Xia Y. Single-crystal nanowires of platinum can be synthesized by controlling the reaction rate of a polyol process. J Am Chem Soc 2004;126:10854–5. Baucke FGK, Bertram R, Cruse K. The Iodide–Iodine system in acetonitrile: evaluation of standard thermodynamic data on the association I þ I2 ! I 3 from potentiometric measurements at 25 and 50 °C. J Electroanal Chem 1971;32:1819–26. Boschloo G, Hagfeldt A. Characteristics of the Iodide/triiodide redox mediator in dye-sensitized solar cells. Acc Chem Res 2009;42:1819–26. Roy-Mayhew JD, Bozym DJ, Punckt C, Aksay IA. Functionalized graphene as a catalytic counter electrode in dye-sensitized solar cells. ACS Nano 2010;4:6203–11. Sun H, Luo Y, Zhang Y, Li D, Yu Z, Li K, et al. In situ preparation of a flexible polyaniline/carbon composite counter electrode and application in dyesensitized solar cells. J Phys Chem C 2010;114:11673–9. Hoshikawa T, Kikuchi R, Eguchi K. Impedance analysis for dye-sensitized solar cells with a reference electrode. J Electroanal Chem 2006;588:59–67. Hauch A, Georg A. Diffusion in the electrolyte and charge-transfer reaction at the platinum electrode in dye-sensitized solar cells. Electrochim Acta 2001;46:3457–66. Ramasamy E, Lee WJ, Lee DY, Song JS. Nano Carbon counter electrode for dye sensitized solar cells. Appl Phys Lett 2007;90:170103-3. Huang SY, Schlichthorl G, Nozik AJ, Gratzel M, Frank AJ. Charge recombination in dye-Sensitized nanocrystalline TiO2 solar cells. J Phys Chem B 1997;101:2576–82. Liu R, Yang WD, Wu JF, Qiang LS. Fabrication of the effective counter electrode for dye-sensitized solar cells. Adv Mater Res 2011;297–198:1143–6. Han L, Koide N, Chiba Y, Komiya R, Fuke N, Fukui A, et al. Improvement of efficiency of dye-sensitized solar cells by reduction of internal resistance. Appl Phys Lett 2005;86:213501-1–1-5.