Pt Nanoparticles Supported on Polypyrrole Nanospheres as a

Sep 29, 2011 - Sang Soo Jeon , Chulwoo Kim , Tae Hyun Lee , Young Woo Lee , Kwangseok Do , Jaejung .... Woo-Yeol Lee , Van-Duong Dao , Ho-Suk Choi...
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Pt Nanoparticles Supported on Polypyrrole Nanospheres as a Catalytic Counter Electrode for Dye-Sensitized Solar Cells Sang Soo Jeon,†,§ Chulwoo Kim,‡,§ Jaejung Ko,*,‡ and Seung Soon Im*,† † ‡

Department of Fiber and Polymer Engineering, Hanyang University, 17 Haengdang-dong, Seoul, 133-791 Korea Department of Advanced Material Chemistry, Korea University, Jochiwon, Chungnam, 339-700 Korea

bS Supporting Information ABSTRACT: Polypyrrole (PPy) nanospheres (∼80 nm) were synthesized chemically within micelles composed of tetradecyl trimethyl ammonium bromide (TTAB) and decyl alcohol. Highly dispersed Pt nanoparticles (∼3.2 nm) supported on PPy nanospheres (PPy Pt) were fabricated using a modified microwave-assisted polyol process in the presence of ethylene glycol, a Pt precursor, and PPy nanospheres as supporting materials in an aqueous medium. PPy Pt was used as a highly efficient catalytic counter electrode for dye-sensitized solar cells (DSSCs). The charge-transfer resistance (Rct) (14.9 Ω) of the PPy Pt counter electrode was lower than that (19.1 Ω) of the conventional Pt counter electrode, indicating higher electrocatalytic activity. The DSSC using a PPy Pt counter electrode with higher surface area and mesoporosity showed enhanced overall power conversion efficiency (8.6%) compared with that of the conventional Pt-based DSSC (8.1%), under 100 mW 3 cm 2, AM 1.5 G sunlight illumination.

’ INTRODUCTION Solar energy has been considered a green and renewable alternative energy source to fossil fuels.1 Photovoltaic systems are among the best means of harvesting energy directly from sunlight. In particular, dye-sensitized solar cells (DSSCs), based on sensitizer dye adsorbed nanocrystalline TiO2 anode, electrolyte, and, traditionally, platinized counter electrodes, are interesting photovoltaic devices because of their powerful sunlightharvesting efficiency, low cost, and ease of fabrication.2,3 Until recently, much effort had been focused on the optimization4 of sensitizer dyes,5 7 photoanode materials,8,9 and electrolytes10 12 for more efficient DSSCs; however, counter electrodes13,14 have received less attention. Generally, platinum (Pt) layered transparent conductive oxide (TCO) substrates produced by sputtering, electrochemical deposition, or thermal deposition methods are currently used as counter electrodes because of their high electrocatalytic activity toward I /I3 redox couples commonly used for DSSCs.13,14 “Champion cells” with power conversion efficiencies higher than 11% also use Pt counter electrodes.15 To further improve the power conversion efficiency of DSSCs, new types of efficient counter electrodes with lower charge-transfer resistance (Rct) and higher reduction rates of redox couples should be developed. Efficient counter electrodes can reduce inner energy losses in DSSCs, improving current density and fill factors. Recently, electrically conducting polymers16 22 and various carbonaceous materials23 27 have been widely studied for potential use as DSSC counter electrodes because of their low material cost and high electrocatalytic activity for reducing I3 ions. To achieve electrocatalytic activity comparable to that of a Pt counter electrode, the layer of carbon has to be several tens of r 2011 American Chemical Society

micrometers thick. Recently, rapid progress has advanced the fabrication of efficient counter electrodes using conducting polymers; however, the power conversion efficiencies of most conducting polymer based DSSCs are still lower than those of DSSCs with conventional Pt counter electrodes. To enhance DSSC performance, conducting polymer based counter electrodes should have highly mesoporous structures and high electrical conductivity. However, if nanostructured Pt and conducting polymers as electrocatalysts are hybridized, we can expect that both of these materials with a large number of catalytic active sites and high electrical conductivity will function as efficient cocatalysts, enhancing power conversion efficiency. However, there have been few reports on a hybrid Pt conducting polymer material or Pt carbonaceous material as a counter electrode for DSSCs.28 Here, we report Pt nanoparticles supported on polypyrrole (PPy) nanospheres as an efficient catalytic counter electrode for DSSCs. Highly dispersed Pt nanoparticles on PPy nanospheres (PPy Pt) were readily fabricated using a microwave-assisted polyol process.29 31

’ EXPERIMENTAL SECTION Synthesis of PPy Nanoparticles. PPy nanoparticles were synthesized using chemical oxidative polymerization within micelles.32,33 Tetradecyl trimethyl ammonium bromide (TTAB; 3.4 g, 10 mmol) and decyl alcohol (1.6 g, 10 mmol) were dissolved in distilled water Received: July 11, 2011 Revised: September 28, 2011 Published: September 29, 2011 22035

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The Journal of Physical Chemistry C (100 mL) with stirring for 1 h. The resulting milky white mixture was ultrasonicated with an Ultrasonic Processor GEX-500 probe sonicator for 20 min at room temperature until it became transparent. Then, distilled and purified pyrrole monomer (2.0 g, 30 mmol) was added to the solution and stirred for 1 h. After further ultrasonication for 10 min, FeCl3 solution (70 mmol dissolved in 5 mL of distilled water) was added to the above solution. After polymerization at 25 °C for 3 h, the resulting PPy nanoparticles were washed with methanol and distilled water and then dried in a freezedryer for 12 h. Synthesis of PPy Pt. PPy Pt was synthesized using a modified microwave-assisted polyol process.29 31 PPy nanoparticles (0.13 g) were added to a deionized water/2-propanol solution (v/v = 9/1, 400 mL) and ultrasonicated for 30 min. Aqueous H2PtCl6 3 H2O solution (0.05 M, 5.7 mL) was added and stirred for 1 h. Ethylene glycol (400 mL) was added to the above mixture, and the pH was adjusted between 7 and 8 with 0.4 M KOH. The mixture was heated in a microwave (Tong Yang Magic MW-2381A, 2450 MHz, 800 W) for 7 min and then stirred for 12 h. The resulting PPy Pt were thoroughly washed with distilled water several times and then dried in a freeze dryer for 12 h. Preparation of PPy Pt Nanoparticle Counter Electrodes. PPy Pt nanoparticles (3 wt %) were dispersed in acetone and sonicated for 2 h. The resulting PPy Pt dispersions (10 μL) were then drop-cast onto an 8  8 mm2 area of a fluorine-doped tin oxide (FTO) glass substrate (Pilkington, TEC-8, 8 Ω/sq, 2.3 mm thick) framed in tape. Finally, the counter electrodes were dried in a vacuum oven at 60 °C for 2 h. To improve the electrical conductivity of the PPy nanoparticles, the PPy Pt/ FTO counter electrodes were postdoped with HCl vapor. The electrodes were placed in a 500-mL round flask with an air inlet/ outlet valve and exposed to nitrogen gas flow with HCl vapor supplied via a bubbler containing HCl aqueous solution (35 wt %) at room temperature for 1 min. For comparison, a conventional Pt counter electrode was prepared by coating a drop of H2PtCl6 solution (2 mg of Pt in 1 mL of ethanol) onto an FTO glass substrate and heating at 400 °C for 15 min. Fabrication of DSSCs. Photoanodes were prepared according to a previously reported procedure.5,7 Precleaned FTO glass plates were immersed in 40 mM TiCl4 (aqueous) at 70 °C for 30 min and then washed with water and ethanol. A transparent nanocrystalline layer was prepared on the FTO glass plates using doctor-blade printing using a paste composed of 20-nm anatase TiO2 particles (Solaronix, Ti-Nanoxide T/SP) and then dried at 25 °C for 2 h. The TiO2 electrodes were heated gradually under an air flow at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 15 min. Scattering layers were deposited by doctor-blade printing a paste containing 400-nm anatase TiO2 particles (CCIC, PST-400C) and then dried at 25 °C for 2 h. The TiO2 electrodes were heated gradually under flowing air at 325 °C for 5 min, at 375 °C for 5 min, at 450 °C for 15 min, and at 500 °C for 15 min. The resulting film was composed of a 10-μm-thick transparent layer and a 4-μm-thick scattering layer. The TiO2 electrodes were treated again with TiCl4 at 70 °C for 30 min and sintered at 500 °C for 30 min. Then, the TiO2 electrodes were immersed in N719 dye (Solaronix) solution (0.3 mM in CH3CN) and kept at room temperature for 12 h. The dye-adsorbed TiO2 electrodes and various counter electrodes were assembled in a sealed sandwichtype cell with a 25-μm gap of a hot-melt ionomer film (Surlyn, DuPont). Then, the electrolytes were introduced into the cell,

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which consisted of 0.03 M guanidinium thiocyanate (GuSCN), 0.6 M 3-hexyl-1,2-dimethyl imidazolium iodide (HDMII), 0.05 M I2, 0.1 M LiI, and 0.5 M tert-butylpyridine (TBP) in acetonitrile. Characterization. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM), high-resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray (EDX) analyses were performed with a field-emission transmission electron microscope (FE-TEM; FEI, Tecnai F30ST) operated at 300 kV, installed at the Korea Basic Science Institute (KBSI). The surface and cross-sectional morphologies of the counter electrodes fabricated with PPy Pt nanoparticles were observed using scanning electron microscopy (SEM; JEOL, JSM6340). Wide-angle X-ray diffraction (WAXD) measurements were carried out using a Rigaku X-ray generator (Rigaku, D/MAX-2500) with Cu Kα (λ = 1.54 Å) radiation operated at 40 kV and 100 mA. The surface resistivity of the counter electrodes was measured using a four-point probe technique (Advanced Instrument Technology, CMT-100M). Cyclic voltammetry (CV; Autolab potentiostat/galvanostat) was measured in a three-electrode system with Pt/FTO and PPy Pt/FTO working electrodes, a Pt wire counter electrode, and an Ag/AgCl reference electrode dipped in an acetonitrile solution containing 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 in the range (1.0 V at a scan rate of 50 mV 3 s 1.12,19,33 The photocurrent density voltage (J V) curves of solar cells were measured under illumination of 100 mW/cm2 using a 450 W xenon arc lamp (Ushio, UXL-75XE), whose power of an AM 1.5 Oriel solar simulator was calibrated by using a KG5 filtered Si reference solar cell (National Renewable Energy Laboratory (NREL) certified Si diode, area = 3.937 cm2) to reduce the mismatch in the region of 350 750 nm between the simulated light and AM 1.5 to less than 2%. Before measurement to reduce scattered light from the edge of the glass electrodes of the TiO2 layer, light shading masks were used on the DSSCs, so the average active area of a DSSC was 0.18 cm2. The incident-photon-to-current conversion efficiency (IPCE) was measured as a function of wavelength from 300 to 1000 nm using a Model QEX7 system (PV Measurements, Inc.) equipped with a 75 W xenon lamp, monochromator, and optical chopper. Light intensity was calibrated using an NIST-calibrated photodiode G425 as a standard. Electrochemical impedance spectroscopy (EIS) measurements were carried out using an impedance analyzer (Parstat 2273, Princeton) at zero bias potential and 10 mV of amplitude over the frequency range 0.1 100 kHz at 20 °C.

’ RESULTS AND DISCUSSION Parts a and b of Figure 1 show the SEM and TEM images of assynthesized PPy nanospheres, respectively. The PPys were spherical with diameters between 75 and 90 nm. The polymerization yield of PPy nanospheres was as high as 95%. The electrical conductivity of PPy nanospheres using a four-point probe method was 9.5 S 3 cm 1. These morphological and electrical characteristics indicate that electrically conducting PPys were successfully synthesized using a microemulsion polymerization process. Figure 1c shows a TEM image of PPy Pt. Pt nanoparticles were homogeneously and densely dispersed over the surface of PPy nanospheres. The amount of Pt supported on the PPy nanospheres was roughly calculated by weighing both samples before and after the microwaveassisted polyol process. The actual weight of Pt loaded on the PPy nanospheres was to be ca. 28.4 wt % (theoretically, 22036

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Figure 3. SEM images of the (a) surface and (b) cross section of a counter electrode coated with a 3 wt % PPy Pt colloidal solution onto FTO glass.

Figure 1. Typical (a) SEM and (b) TEM images of as-synthesized PPy nanospheres. (c) Representative TEM image of PPy Pt. (d) HR-TEM image of PPy Pt. (e) HAADF-STEM image of PPy Pt. (f) EDX spectrum of PPy Pt.

Figure 4. Cyclic voltammograms for Pt and PPy Pt electrodes in 10 mM LiI, 1 mM I2, and 0.1 M LiClO4 acetonitrile solution at a scan rate of 50 mV 3 s 1.

Figure 2. XRD patterns of PPy and PPy Pt.

30 wt %). The measured electrical conductivity of PPy Pt was as high as 50.8 S 3 cm 1, which might be due to the densely dispersed Pt nanoparticles with excellent conductance on the semiconducting PPy nanospheres. The HR-TEM image (Figure 1d) together with Figure 1c shows that Pt nanoparticles (∼2 4 nm) were deposited on the surface of PPy nanospheres. The lattice distance (white arrows) in Figure 1d is about 2.2 Å, corresponding well with the d111 interplanar distance (2.3 Å) of face-centered-cubic (fcc) crystalline Pt. Figure 1e shows the HAADF-STEM image of PPy Pt. It can be clearly observed that Pt nanoparticles were uniformly dispersed on the PPy nanosphere surface. Figure 1f shows the EDX spectrum of PPy Pt, clearly indicating the presence of carbon, nitrogen, and chlorine in chloride anion doped PPy and Pt components.

XRD patterns of the PPy nanospheres and PPy Pt are shown in Figure 2.The diffraction peaks at 2θ ≈ 25.2° for both samples correspond to the (001) crystal plane, which is associated with the average face-to-face distance of the inter-pyrrole rings in the π-stacks.34,35 For the XRD pattern of PPy Pt, it is seen that new peaks at 2θ ≈ 39.8, 46.1, and 67.9° appeared in addition to the background diffraction of PPy, which is ascribed to the facets (111), (200), and (220) characteristic of fcc crystalline Pt (JCPDS, Card No. 04-0862), respectively. This XRD result and X-ray photoelectron spectroscopy (XPS) data for PPy Pt (see Supporting Information, Figure S1) indicate that Pt species were reduced successfully to the metallic state by ethylene glycol. The average grain size of Pt, calculated by the Debye Scherrer equation using the XRD peak of the (111) crystalline plane, was ca. 3.2 nm, in agreement with the above TEM results. The surface and cross section of the PPy Pt counter electrode drop-cast onto FTO glass are presented in Figure 3. The surface exhibited a highly mesoporous structure with a pore diameter less than 60 nm, and the thickness of the PPy Pt layer was ∼3 μm over the entire range. Considering the dimensions (width = ∼3 Å and length = ∼6 Å) of I3 ions,36 they can readily diffuse into the mesopores and also be reduced in the PPy Pt counter electrode. Moreover, the electrocatalytic activity toward 22037

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Figure 5. J V curves of the DSSCs using Pt/FTO and PPy Pt/FTO counter electrodes.

Table 1. Charge Transfer Resistivity and Photovoltaic Performances of DSSCs Based on Pt and PPy Pt Counter Electrodes photovoltaic performance counter electrode Rct (Ω) Voc (mV) Jsc (mA 3 cm 2) ff (%) η (%) Pt

19.1

791

14.9

69

8.1

PPy Pt

14.9

752

17.4

66

8.6

the I3 /I redox reaction should be enhanced due to the increased surface area and active sites of highly dispersed Pt nanoparticles on PPy nanospheres. Figure 4 compares the cyclic voltammograms of PPy Pt and conventional Pt counter electrodes. The oxidation and reduction peaks marked with dashed circles were assigned to the I3 /I redox reaction.37,38 Although the conventional Pt electrode showed enhanced redox kinetics and sharp oxidation and reduction peaks, the PPy Pt electrode provided much higher current density than that of the Pt electrode toward the I3 /I redox reaction, which may be due to the larger effective surface area. Moreover, the peak-potential separation, ΔE, for the PPy Pt electrode was about 313 mV, while that of the conventional Pt electrode was 532 mV. The significantly lower ΔE indicates that the electrocatalytic activity toward the I3 /I redox reaction was greater for the PPy Pt electrode than that of the conventional Pt electrode, which is primarily attributable to the synergistic cocatalytic effect of PPy and Pt as well as the unique PPy Pt nanostructures that provide a larger surface area for catalysis. Figure 5 shows the current density voltage (J V) curves for DSSCs based on PPy Pt and conventional Pt counter electrodes. The Rct values of the electrochemical cells calculated from Nyquist plots by EIS measurements (see Supporting Information, Figure S2) and photovoltaic parameters derived from the J V curves, such as short-circuit photocurrent density (Jsc), open-circuit voltage (Voc), fill factor (ff), and overall conversion efficiency (η), are summarized in Table 1. Under 1 sun illumination (100 mW 3 cm 2, AM 1.5 G), the DSSC based on the conventional Pt counter electrode gave Jsc = 14.9 mA 3 cm 2, Voc = 791 mV, and ff = 69, corresponding to η = 8.1%. When PPy Pt was used as the counter electrode, the DSSC showed Jsc = 17.4 mA 3 cm 2, Voc = 752 mV, ff = 66, and η = 8.6%. On the other hand, as revealed in our recent study, pure PPy nanoparticle based DSSCs exhibited η = 7.7%.33 Note that the Voc and ff

Figure 6. IPCE spectra of the DSSCs using Pt/FTO and PPy Pt/FTO counter electrodes.

values of both DSSCs were similar, while the Jsc of the PPy Ptbased DSSC was much higher than that of the Pt-based DSSC. Although the Rct (14.9 Ω) of the PPy Pt counter electrode was lower than that (19.1 Ω) of the Pt counter electrode, indicating higher electrocatalytic activity, the much higher surface resistivity of the PPy Pt counter electrode (930 Ω 3 cm 2) compared with the Pt counter electrode (7.3 Ω 3 cm 2) might be responsible for the similar ff values. Consequently, the enhanced conversion efficiency of the PPy Pt-based DSSC was due entirely to the significantly increased Jsc value. This result agreed well with the IPCE data as shown in Figure 6. Moreover, the effective reduction of I3 ions was evaluated using the IPCE at ∼360 nm. Dye molecules cannot effectively absorb light at ∼360 nm because I3 ions absorb light at that wavelength.12,39 Based on IPCE curves (∼360 nm), the higher IPCE of PPy Pt compared with Pt indicates a lower concentration of I3 ions, indicating higher catalytic activity of the PPy Pt counter electrode toward the reduction of I3 ions. Thus, based on the J V and IPCE data, as well as cyclic voltammograms of PPy Pt and conventional Pt electrodes, the composite of nanoscale Pt and PPy with a large surface area as a catalytic counter electrode for DSSCs exhibited higher electrocatalytic activity than the conventional Pt counter electrode, enhancing the current density and overall conversion efficiency.

’ CONCLUSIONS In summary, Pt nanoparticles (∼3.2 nm) supported on polypyrrole nanospheres (∼80 nm) were fabricated using a microwave-assisted polyol process in the presence of ethylene glycol, a Pt precursor, and PPy nanospheres as supporting materials in aqueous medium. PPy Pt was first used as a highly efficient catalytic counter electrode for DSSCs. The PPy Ptbased DSSC showed enhanced conversion efficiency (8.6%) compared with that of the conventional Pt-based DSSC (8.1%). This may have been due to the higher electrocatalytic activity of PPy Pt due to the high surface area and synergistic effect of the cocatalysts of Pt and PPy nanoparticles. Although the DSSC performance based on the PPy Pt counter electrode was slightly higher than that of the DSSC using the conventional Pt counter electrode, further improvements can be expected if the fabrication conditions for the PPy Pt counter electrode are fully optimized, including the electrical conductivity, PPy particle size, particle size and wieght percent of Pt supported on PPy, and layer thickness of PPy Pt on conducting substrates. 22038

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’ ASSOCIATED CONTENT

bS

Supporting Information. X-ray photoelectron spectroscopy survey spectra of pure PPy nanospheres and PPy Pt nanoparticles. Nyquist plots and the equivalent circuit for the electrochemical cells based on Pt and PPy Pt electrodes. This material is available free of charge via the Internet at http://pubs. acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (S.S.I.); [email protected] (J.K). Author Contributions §

These authors contributed equally to this work.

’ ACKNOWLEDGMENT This work was supported by the Basic Science Research Program through a National Research Foundation of Korea (NRF) grant funded by the Ministry of Education, Science and Technology (MEST) of Korea for the Center for Next Generation Dye-Sensitized Solar Cells (No. 2011-0001055), the WCU (the Ministry of Education and Science) program (No. R312008-000-10035-0), and the New & Renewable Energy of the Korea Institute of Energy Technology Evaluation and Planning(KETEP) grant funded by the Korea Government Ministry of Knowledge Economy (No. 2010T100100674). ’ REFERENCES (1) Dresslhaus, M. S.; Thomas, I. L. Nature 2001, 414, 332. (2) O’Regan, B.; Gr€atzel, M. Nature 1991, 353, 737. (3) Gr€atzel, M. Nature 2001, 414, 338. (4) Luo, Y.; Li, D.; Meng, Q. Adv. Mater. 2009, 21, 4647. (5) Choi, H.; Baik, C.; Kang, S. O.; Ko, J.; Kang, M. S.; Nazeeruddin, M. D.; Gr€atzel, M. Angew. Chem., Int. Ed. 2008, 47, 327. (6) Choi, H.; Kang, S. O.; Ko, J.; Gao, G.; Kang, H. S.; Kang, M. S.; Nazeeruddin, M. D.; Gr€atzel, M. Angew. Chem., Int. Ed. 2009, 48, 5938. (7) Choi, H.; Raabe, I.; Kim, D.; Teocoli, F.; Kim, C.; Song, K.; Yum, J. H.; Ko, J.; Nazeeruddin, M. K.; Gr€atzel, M. Chem.;Eur. J. 2010, 16, 1193. (8) D€urr, M.; Schmid, A.; Obermaier, M.; Rosselli, S.; Yasuda, A.; Nelles, G. Nat. Mater. 2005, 4, 607. (9) Koo, H. J.; Kim, Y. J.; Lee, Y. H.; Lee, W. I.; Kim, K.; Park, N. G. Adv. Mater. 2008, 20, 195. (10) Yanagida, S.; Yu, Y.; Manseki, K. Acc. Chem. Res. 2009, 42, 1827. (11) Hamann, T. W.; Ondersma, J. W. Energy Environ. Sci. 2011, 4, 370. (12) Kim, M. J.; Lee, C. R.; Jeong, W. S.; Im, J. H.; Ryu, T. I.; Park, N. G. J. Phys. Chem. C 2010, 114, 19849. (13) Murakami, T. N.; Gr€atzel, M. Inorg. Chim. Acta 2008, 361, 572. (14) Papageorgiou, N. Coord. Chem. Rev. 2004, 248, 1421. (15) Nazeeruddin, M. K.; Angelis, F. D.; Fantacci, S.; Selloni, A.; Viscardi, G.; Liska, P.; Ito, S.; Takeru, B.; Gr€atzel, M. J. Am. Chem. Soc. 2005, 127, 16835. (16) Ahmad, S.; Yum, J. H.; Xianxi, Z.; Gr€atzel, M.; Butt, H. J.; Nazeeruddin, M. K. J. Mater. Chem. 2010, 20, 1654. (17) Ameen, S.; Akhtar, M. S.; Kim, Y. S.; Yang, O. B.; Shin, H. S. J. Phys. Chem. C 2010, 114, 4760. (18) Pringle, J. M.; Armel, V.; MacFalane, D. R. Chem. Commun. 2010, 46, 5367. (19) Lee, K. S.; Lee, H. K.; Wang, D. H.; Park, N. G.; Lee, J. Y.; Park, O. O.; Park, J. H. Chem. Commun. 2010, 46, 4505.

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