This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and

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1 This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier s archiving and manuscript policies are encouraged to visit:

2 Nano Energy (2013) 2, Available online at journal homepage: RAPID COMMUNICATION A microporous platinum counter electrode used in dye-sensitized solar cells Ziying Tang, Jihuai Wu n, Min Zheng, Jinhao Huo, Zhang Lan Engineering Research Center of Environment-Friendly Functional Materials, Ministry of Education, Institute of Materials Physical Chemistry, Huaqiao University, Quanzhou , China Received 9 June 2013; received in revised form 4 July 2013; accepted 27 July 2013 Available online 9 August 2013 KEYWORDS Dye-sensitized solar cells; Microporous platinum; Counter electrode; Rapid thermal decomposition Abstract A microporous platinum film is prepared by a facile rapid thermal decomposition method. Scanning electronic microscope observes a large amount of interconnected and uniform micropores with a pore diameter of nm on the microporous platinum electrode. The cyclic voltammetry measurements indicate that the microporous platinum electrode has higher catalytic activity and smaller resistance than the conventional platinum electrode does. By using the microporous platinum film as counter electrode, a dye-sensitized solar cell (DSSC) is fabricated, the DSSC achieves a high light-to-electric energy conversion efficiency of 8.15% under a simulated solar light irradiation with a intensity of 100 mw cm 2. The efficiency is increased by 21.28% compared to that of the DSSC with conventional platinum counter electrode. & 2013 Elsevier Ltd. All rights reserved. Introduction Since the first prototype of a dye-sensitized solar cell (DSSC) was reported in 1991 by O'Regan and Gratzel [1], it has aroused intensive interests over the past decade due to its low cost, simple preparation procedure [1,2] and high lightto-electric conversion efficiency over 12% [3,4]. Generally, a DSSC consists of three main components: a photoanode made of a mesoporous dye-sensitized TiO 2 film on transparent conductive glass substrate, an iodide/triiodide redox n Corresponding author. Tel.: , fax: address: jhwu@hqu.edu.cn (J. Wu). couple in an organic solvent as an electrolyte, and a platinized conductive glass substrate as a counter electrode to collect electrons and catalyze the iodide/triiodide (I /I 3 ) redox reaction. The operation principle of the DSSC involves that the photoanode receives electrons from the photoexcited dye which is thereby oxidized, and which in turn oxidizes the iodide in the electrolyte to triiodide. The triiodide is reduced at the counter electrode with the circuit being completed via electron migration through the external circuit [3,4]. Counter electrode, as one important component in DSSCs, should have low resistance and high electrocatalytic activity for I /I 3 redox reaction to decrease overvoltage and charge recombination [4]. Platinum has low resistance and high electrocatalytic activity for I /I 3 redox couple, which is /$ - see front matter & 2013 Elsevier Ltd. All rights reserved.

3 Microporous platinum counter electrode 623 widely used in DSSC as a counter electrode. Usually, platinum counter electrode is prepared by electroplating, vacuum coating, sputter coating, hydrogen reduction, and thermal decomposition of complex compounds [5]. However, the platinum counter electrodes prepared by these methods are mostly compact, which limits the penetration of the electrolyte into the electrode and has a faulty contact between platinum particle and electrolyte. Here, a microporous platinum electrode was prepared by using a simple thermal decomposition method and was used to fabricate a DSSC. Scanning electron microscope observation, cyclic voltammetry measurements, electrochemical impedance spectroscopy and photovoltaic tests were carried out. It is expected that the performance of DSSC be improved. Experimental Material Titanium (IV) isopropoxide and 4-tert-butylpyridine (TBP) were purchased from Fluka and used without further treatment. The organometallic compound sensitized dye N-719 [RuL 2 (NCS) 2, L=4,4 -dicarboxylate-2,2 -bipyridine] was obtained from Solaronix SA (Switzerland). Chloroplatinic acid (H 2 PtCl 6 ), OP emulsification agent (Triton X-100) and other reagents were obtained from Shanghai Chemical Agent, PR China and used without further treatment. Conductive glass substrates (fluorine doped tin oxide overlayer (FTO glass)) were purchased from Hartford Glass, U.S., and were used as substrates for precipitating porous TiO 2 films and platinum counter electrodes. The substrate was cut into cm 2 sheets and were cleaned sequentially in acetone, ethanol and deionized water for 20 min to remove organic pollutants prior to using [6]. Fabrication of microporous platinum counter electrode A microporous platinum electrode was prepared by a rapid thermal decomposition method. A cleaned FTO glass was heated at 450 1C for 8 min to remove residual organic impurities. Then the heated FTO glass was naturally cooled to 270 1C, 0.4 ml (8 drops) of H 2 PtCl 6 (1.0 wt%) isopropanol solution containing OP emulsification agent (1.0 wt%) was rapidly dropped onto the surface of the heated FTO glass. At this time, the H 2 PtCl 6 immediately decomposed and a mirro-like platinum thin layer was solidified on the FTO glass. Simultaneously, the isopropanol (boiling point temperature at C) boiled and rapidly volatilized. Because of the rapidly boiling and volatilizing of isopropanol, a large amount of pores were in situ generated in the platinum layer. Finally, the FTO glass was sintered at 450 1C for 30 min to remove some residual organic compounds in the platinum layer, a microporous platinum electrode thus was prepared. For comparison, a conventional platinum counter electrode was prepared by thermal decomposition method. In a typical experiment, conventional platinum counter electrode was prepared by dropping H 2 PtCl 6 isopropanol solution onto the FTO glass and then thermal decomposing H 2 PtCl 6 at 450 1C for 30 min under air ambient [6,7]. The assembling of DSSCs A thin TiO 2 blocking layer was deposited on the FTO glass substrate by immersing the glass in 0.15 M TiCl 4 isopropanol solution for 12 h, followed by sintering at 450 1C for 30 min in the air [5,8]. Subsequently, a TiO 2 layer with a particle size of nm and thickness of 10 μm was covered on the blocking layer by using a doctor blade method, then sintering at 450 1C for 30 min in the air. The thickness of the film was controlled by pasting a plastic tape around the edge of the FTO substrate. A dye was loaded by immersing the TiO 2 film in a 0.3 mm dye N719 ethanol solution at room temperature for 24 h. Thus a dye-sensitized TiO 2 film anode was obtained. A dye-sensitized solar cell was fabricated by injecting a liquid electrolyte (5 M I 2, 0.1 M LiI, 0.6 M tetrabutylammonium iodide and 0.5 M TBP in acetonitrile) in the aperture between the dye-sensitized TiO 2 electrode and the platinum counter electrode. The two electrodes were clipped together and a cyanoacrylate adhesive was used as sealant. Epoxy resin was used for further sealing the cell. The detailed procedure for preparing TiO 2 Figure 1 SEM images of conventional (A) and microporous (B) platinum electrodes.

4 624 Z. Tang et al. Current density (ma cm -2 ) Conventional Pt electrode Microporous Pt electrode Current density (ma cm -2 ) Conventional Pt electrode Microporous Pt electrode Voltage (V) Cycle times Figure 2 (A) Cyclic voltammogram for the conventional and microporous Pt counter electrodes using acetonitrile solution containing 0.1 M LiClO 4, 1 M LiI, 01 M I 2 as supporting electrolyte, scan rate=50 mv s 1, Cycle time=11. (B) The relationship between the cycle times and the maximum redox peak currents for conventional and microporous Pt counter electrodes, scan rate=50 mv s Current density (ma cm -2 ) mv s mv s mv s mv s mv s -1 Current density (ma cm -2 ) Reduction peak current Oxidation peak current Voltage (V) (Scan rate) 1/2 / (V s -1 ) 1/2 Figure 3 (A) Cyclic voltammogram for the microporous Pt counter electrode in supporting electrolyte solution with different scan rates (from inner to outer: 50,100, 250 mv s 1 ), inserting: the magnification of redox current peeks. (B) The redox peaks current versus square root of scan rates. nanocrystalline photoanodes and fabricating DSSCs were described by us elsewhere [9,10]. Characterizations and measurements Morphologies of the microporous platinum counter electrode and conventional platinum counter electrode were observed by using a scanning electron microscope (SEM, Hitachi S-5200, Japan). The cyclic voltammetry measurements (CVs) of microporous platinum counter electrode and conventional platinum counter electrode were carried out in a three-electrode electrochemical cell by using platinum electrode as working electrode, a Pt-foiled as counter electrode and an Ag/AgCl as reference electrode dipped in an acetonitrile solution containing 1 M LiI, 01 M I 2 and 0.1 M LiClO 4. The measurements of CVs were performed by using CHI660B electrochemical measurement system (scan condition: mv s 1 ) [11,12]. Electrochemical impedance spectroscopy (EIS) was measured by using a sandwich cell configuration with symmetric films in an acetonitrile electrolyte containing 5 M I 2, 0.1 M LiI, 0.6 M tetrabutylammonium iodide and 0.5 M TBP. A 50 μm Surlyn film was used to separate the films and to seal the cells. The magnitude of the alternating signal was 5 mv, and the frequency range was 100 khz to 0.1 Hz, the active area of the electrodes about 0.6 cm 2. Unless otherwise specified, all measurements were carried out at a temperature of 20 1C. Photovoltaic test The photovoltaic test of DSSC was carried out by measuring the J V characteristic curves under irradiation of a simulated solar light with an intensity of 100 mw cm 2 from a 100 W xenon arc lamp (XQ-500 W, Shanghai Photoelectricity Device Company, China) in ambient atmosphere. The fill factor (FF) and light-to-electric energy conversion efficiency (η) of the cell were calculated according to the following Eq. [13]:

5 Microporous platinum counter electrode Conventional Pt electrode Microporous Pt electrode Z''/ Ω Z'/ Ω Figure 4 EIS spectra of the cells with two electrodes. The conventional Pt electrode and microporous Pt electrode were used as the working electrode, respectively. (Rs is the ohmic serial resistance, Cdl is the constant phase element, Rct is the charge-transfer resistance, Zw is the diffusion bimpedance.) Figure 5 Photocurrent voltage curves of DSSCs with conventional and microporous Pt counter electrodes. Table 1 Electrochemical parameters for two Pt electrodes. FF ¼ V max J max V oc J sc ð1þ Electrode Reduction peak current (ma cm 2 ) a Oxidation peak current (ma cm 2 ) a Rs (Ω) b Rct (Ω) b ηð%þ¼ V max J max 100% ¼ V oc J sc FF 100% ð2þ P in P in where J SC is the short-circuit current density (ma cm 2 ), V OC is the open-circuit voltage (V), P in is the incident light power, and J max (ma cm 2 )andv max (V) are the current density and voltage in the J V curves at the point of maximum power output, respectively. Results and discussions Morphologies of platinum counter electrodes Figure 1 is the SEM images of conventional (A) and microporous (B) platinum electrodes. From Figure 1A, it can be seen that the surface of conventional platinum electrodes is compact, smooth, and of few pores, which limits the penetration of the electrolytes into the electrode and has a faulty contact between platinum particles and electrolytes. From Figure 1B, a large amount of interconnected and uniform micropores (pore diameter nm) can be seen. The interconnected and uniform micropores largely increase the specific surface area of platinum catalyst and are availed for the adsorption of liquid electrolyte [11], consequently resulting in a great improvement of electrocatalytic activity for I /I 3 redox reaction. The generation of interconnected and uniform microporous platinum possibly comes from three factors: (a) when the H 2 PtCl 6 solution contacts the heated glass, the H 2 PtCl 6 is rapidly decomposed into Pt (H 2 PtCl 6 -Pt+2HCl+2Cl 2 ). Owing to the rapid decomposition of H 2 PtCl 6 solution, the platinum exists in smaller particles and avoids the aggregation and growth of platinum particles; (b) when the H 2 PtCl 6 isopropanol solution contacts the heated glass, the solvent isopropanol immediately boils and vaporizes, which leaves behind a large amount of pores and holes in the platinum; (c) The thermal decomposition of remained organic compounds, and the volatilization HCl and Cl 2 produce many voids inside of platinum. Cyclic voltammetry measurements (CVs) Figure 2 shows the electrochemical properties of conventional and microporous platinum counter electrodes. In DSSCs, electrons are injected into photo-oxidized dye from I ions in the electrolyte [Eq. (3)], and the produced I 3 ions are reduced on the counter electrode [Eq. (4)] [3]. 3I 2e ¼ I 3 ð3þ I 3 Conventional Pt electrode Microporous Pt electrode a The data come from Figure 2. b The data come from Figure 4. þ 2e ¼ 3I ð4þ Using I /I 3 redox as supporting electrolyte, the cyclic voltammogram curves (scan rate of 50 mv s 1 ) for the two counter electrodes are shown in Figure 2A. The cathodic peak (negative current) is assigned to the reduction of I 3, and the anodic peak (positive current) is assigned to the oxidation of I [11,14]. Figure 2A shows a much larger current density of the I 3 reduction peak current and I oxidation peak current for microporous platinum electrode than that of conventional platinum electrode. The reduction peak current density of the microporous platinum electrode is 2.02 ma cm 2, which is obviously larger than

6 626 Z. Tang et al. that of the conventional platinum electrode ( 1.22 ma cm 2 ), which means that the microporous platinum electrode is easier to catalyze the reduction of I 3 to I. On the other hand, the oxidation peak current density on the microporous platinum electrode is 0.75 ma cm 2, which is obviously larger than that of the conventional platinum electrode (0.30 ma cm 2 ), which means that the microporous platinum electrode is easier to catalyze the oxidation of I to I 3. This suggests that the microporous platinum has a higher catalytic activity and the I 3 /I has a faster reaction rate on it. In other words, the charge-transfer resistance (Rct) for the I 3 /I redox reaction is lower on the microporous platinum electrode, which results in a lower energy loss for the I 3 /I redox reaction [11,15]. Logically, the microporous platinum is an efficient electrocatalyst and has a good electrocatalytic activity for the I 3 /I redox reaction. Figure 2B shows 11 consecutive CV cycles of the platinum counter electrodes. During 11 successive CV cycles at a scan rate of 50 mv s 1, no observable peak current change is found, suggesting that the microporous platinum is stably immobilized on the electrode. On 11 successive scans, though the peak current density changes with the scan voltage, the oxidation and reduction peak current scarcely show any change, indicating that the microporous platinum electrode is uniform, homogeneous, stably immobilized on the FTO glass and has an excellent electrochemical stability [11]. Figure 3 shows the cyclic voltammogram cycles of microporous platinum electrode in different scan rates. It can be seen from Figure 3A that the absolute values of cathodic and anodic peaks current increase with the increase of scan rates from 100 to 250 mv s 1. Meanwhile, the cathodic peak gradually and regularly shifts to the negative direction and the corresponding anodic peak shifts to the positive direction with an increasing scan rate. Figure 3B illustrates a relationship between the peak current density and the square root of the scan rate, the current density versus v 1/2 plots is almost linear. The good linear relationship indicates the diffusion limitation of the I 3 /I redox reaction on platinum electrode, which may be due to the transportation of iodide ions in microporous platinum electrode [11,12,16]. This phenomenon shows that the adsorption of iodide ions is hardly affected by the redox reaction on the microporous electrode surface, and suggests that there is no specific interaction between I 3 /I redox couple and microporous platinum electrode [17]. Electrochemical impedance spectroscopy (EIS) measurements Electrochemical impedance spectroscopy (EIS) measurements were carried out to compare the characteristics of charge transfer and ion transport in the conventional and microporous platinum electrodes. The EIS measured results (Figure 4) show well-defined single semicircles over the high frequency range, followed by short straight lines in the lowfrequency region for the platinum electrodes. The ohmic serial resistance (Rs) can be determined to the impedance where the phase is zero and the frequency is about 100 khz. In the frequency range of khz, the impedance was dominated by the RC network of the electrolyte electrode interface, consisting of the charge-transfer resistance (Rct) and capacitance of electrical double layer (Cdl) [12,17]. The both Rs and Rct of microporous platinum electrode (13.01, 1.05 Ω, respectively) are lower than those of conventional platinum electrode (14.41, 1.79 Ω, respectively), which is consistent with the CV results in Figure 2. The lower Rs and Rct are attributed to the increase of surface area for microporous platinum electrode. Therefore, in view of good electrocatalytic activity, lower resistance and more facile diffusion, it can be expected that the DSSC based on this microporous platinum counter electrode could achieve a good photovoltaic performance. Photovoltaic performance of DSSC Figure 5 shows the photocurrent voltage curves of DSSCs based on conventional and microporous platinum counter electrode under irradiation of a simulated solar light with a intensity of 100 mw cm 1. The DSSC with microporous platinum counter electrode achieves a short-circuit current density (J SC )of15.1macm 2, open circuit voltage (V OC )of 760 mv, fill factor (FF) of and light-to-electrical energy conversion efficiency (η) of 8.15%, while the J SC, V OC, FF and η for the DSSC with conventional platinum counter electrode is 13.7 ma cm 2, 761 mv, 0.646, and 6.72%, respectively. The J SC and FF increase for the DSSC with microporous platinum electrode is due to that the electrode possesses microporous structure and good electrochemical properties (Table 1), which results in the enhancement of light-to-electrical energy conversion efficiency [18,19]. Figure 5 also shows that the smaller dark current for the DSSC with the microporous platinum electrode than that for the DSSC with conventional platinum electrode. The dark current in DSSC is attributed to the I 3 combination with conduction band electrons of TiO 2 at the TiO 2 electrolyte interface. [Eq. (5)] [20]. The smaller dark current indicates that the reduction of I 3 on the TiO 2 electrolyte interface is retarded [8,21,22] and the reduction of I 3 on the electrolyte counter-electrode interface is more efficient [15]. I 3 þ 2e cb ðtio 2Þ¼3I ð5þ Two factors result in the suppressed dark current in this paper. Firstly, as the CVs in Figure 2 and EIS in Figure 4 illustrated, the microporous platinum counter electrode has a higher electrocatalytic activity for the I 3 /I redox reaction, larger CVs reduction peaks and lower charge-transfer resistance (Rct), which results in an acceleration of the I 3 /I redox reaction on the microporous platinum electrode, decrease of the I 3 reduction on TiO 2. Secondly, due to the increased surface roughness for the microporous platinum counter electrode as shown in SEM image in Figure 1, it is conceivable that more I 3 /I electrolytes are filled in the aperture of the counter electrode of the DSSC. As the result, there are more redox shuttles working in the microporous platinum counter electrode [18 22]. These two factors above lead to a decreased dark-current and an increased light-current. Conclusions A microporous platinum conductive film was prepared by a facile rapid thermal decomposition method. By using

7 Microporous platinum counter electrode 627 the microporous platinum conductive film as counter electrode, a DSSC is fabricated. The DSSC achieves a high lightto-electric energy conversion efficiency of 8.15%, which is increased by 21.28% compared to the DSSC with a conventional platinum counter electrode. The high efficiency for the microporous platinum counter electrode is due to its unique microporous structure, larger specific surface area, lower charge resistance and excellent catalytic activity for I 3 /I redox reaction. The preparation method of microporous platinum film is simple and effective, which provides a novel idea for fabrication of microporous film and can be widely applied in other fields. Acknowledgments The authors acknowledge the financial joint support by the National Natural Science Foundation of China (Nos. U , ) and the specialized research fund for the doctoral program of Higher University, Ministry of Education, China (No ). References [1] B. O Regan, M. Gratzel, Nature 353 (1991) [2] M. Gratzel, Nature 414 (2001) [3] M. Gratzel, Account of Chemical Research 42 (2009) [4] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chemical Review 110 (2010) [5] Y. Lee, C. Chen, L. Chong, C. Chen, Y. Liu, C. Chi, Electrochemistry Communications 12 (2010) [6] S. Kang, J. Kim, H. Kim, H. Koh, J. Lee, Y. Sung, Journal of Photochemistry and Photobiology A 200 (2008) [7] H. Koo, Y. Kim, Y. Lee, W. Lee, K. Kim, N. Park, Advanced Materials 20 (2008) [8] E. Palomares, J. Clifford, S. Haque, T. Lutz, J. Durrant, Journal of the American Chemical Society 125 (2003) [9] J. Wu, Z. Lan, D. Wang, S. Hao, J. Lin, Y. Huang, S. Yin, T. Sato, Electrochimica Acta 51 (2006) [10] J. Wu, S. Hao, Z. Lan, J. Lin, M. Huang, Y. Huang, P. Li, S. Yin, T. Sato, Journal of the American Chemical Society 130 (2008) [11] Q. Li, J. Wu, Q. Tang, Z. Lan, P. Li, J. Lin, L. Fan, Electrochemistry Communications 10 (2008) [12] Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, Journal of Photochemistry and Photobiology A 164 (2004) [13] J. Wu, Z. Lan, J. Lin, M. Huang, S. Hao, T. Sato, S. Yin, Advanced Materials 19 (2007) [14] P. Hasin, M. Alpuche-Aviles, Y. Li, Y. Wu, Journal of Physical Chemistry C 113 (2009) [15] Z. Tang, J. Wu, Q. Li, Z. Lan, L. Fan, J. Lin, M. Huang, Electrochimica Acta 55 (2010) [16] S. Biallozor, A. Kupniewska, Electrochemistry Communications 2 (2000) [17] A. Hauch, A. Georg, Electrochimica Acta 46 (2001) [18] Z. Lan, J. Wu, J. Lin, M. Huang, Journal of Materials Chemistry 22 (2012) [19] J. Wu, G. Yue, Y. Xiao, M. Huang, J. Lin, L. Fan, Z. Lan, J. Lin, ACS Applied Materials and Interfaces 4 (2012) [20] J. Wu, Z. Lan, S. Hao, P. Li, J. Lin, M. Huang, L. Fan, Y. Huang, Pure and Applied Chemistry 80 (2008) [21] S. Ito, P. Liska, P. Comte, R. Charvet, P. Pechy, U. Bach, L. Schmidt-Mende, S. Zakeeruddin, A. Kay, M. Nazeeruddin, M. Gratzel, Chemical Communications 14 (2005) [22] R. Gao, L. Wang, B. Ma, C. Zhan, Y. Qiu, Langmuir 26 (2010) Ziying Tang received her bachelor degree in Chemistry in 2009 and obtained his Master degree in organic chemistry from Huaqiao University, China in He joined the group of Prof. Wu at Huaqiao University in 2009 as a master. His research interests focus on the synthesis of high orderly and high surface area nanostructure functional materials and the application in energy conversion and storage devices. Jihuai Wu received his master degree in Materials Science from Huaqiao University, China in 1988, and then received his Ph. D. degree in Chemistry from Fuzhou University, China in He joined the Institute of Materials Physical Chemistry, Huaqiao University in 1988, and became a professor in Chemistry and Materials in His main research interests focus on functional nanomaterials, dye-sensitized solar cells, superabsorbent polymers, photocatalytic intercalated nanomaterials, and so on. Min Zheng received her bachelor degree in materials from Huaqiao University, China in She is continuing her study as a doctoral student under the supervision of Prof. Jihuai Wu in materials science in Huaqiao University. Her current research interests focus on the hole-transport materiasls used in dey-sensitized soler cells. Jinghao Huo received her bachelor degree in materials from Huaqiao University, China in Now, she is a reading doctorate in material science in Huaqiao University under the supervision of Prof. Jihuai Wu. Her current research involves the synthesis, modification and application of graphene for dye-sensitised solar cells. Zhang Lan received his master degree and Ph.D. degree in Materials Science from Huaqiao University, China in 2006 and 2009, respectively. He joined the Institute of Materials Physical Chemistry, Huaqiao University in 2009, and became an associate professor in His main interests on the synthesis and application of nano-materials in dye/quantum dot sensitized solar cells and organic-inorganic hybrid solar cells.