Pt Nanourchins as Efficient and Robust Counter Electrode Materials

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Pt Nanourchins as Efficient and Robust Counter Electrode Materials for Dye-Sensitized Solar Cells Van-Duong Dao, and Ho-Suk Choi ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b11097 • Publication Date (Web): 18 Dec 2015 Downloaded from http://pubs.acs.org on December 19, 2015

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ACS Applied Materials & Interfaces

Pt Nanourchins as Efficient and Robust Counter Electrode Materials for Dye-Sensitized Solar Cells Van-Duong Dao†, Ho-Suk Choi†* †

Department of Chemical Engineering, Chungnam National University, 220 Gung-Dong,

Yuseong-Gu, Daejeon 305-764, Republic of Korea ABSTRACT This study reports on the synthesis of Pt nanourchins (PtNUs) on FTO glass surfaces and its application as an efficient and robust counter electrode (CE) in dye-sensitized solar cells (DSCs). PtNUs with sizes in the range of 100–300 nm are successfully synthesized on FTO surfaces via a simple room temperature chemical reduction of H2PtCl6 using formic acid. Note that the PtNUs have numerous Pt nanowires with 2 nm diameters and 12 nm lengths. The PtNU CE exhibits very low charge-transfer resistance for DSCs. The efficiency of DSCs fabricated with PtNU CEs is 9.39%, which is higher than that of devices assembled with Pt-sputtered CEs (8.51%). Keywords: nanourchin, platinum, formic acid, counter electrode, dye-sensitized solar cells INTRODUCTION Dye-sensitized solar cells (DSCs) have garnered significant attention as next-generation solar cells due to their having a lot of advantages such as low-cost fabrication, environmentally friendly manufacturing process, glossy transparency, and relatively high power conversion efficiency (PCE)1. To date, the best efficiency of 13% has been achieved in a laboratory2. In order to be competitive in the large power market with conventional solid-state photovoltaic (PV) 1 ACS Paragon Plus Environment


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cells, DSC’s efficiency must be increased. Among key issues in DSCs, the improvement in the reduction of triiodide to iodide at counter electrode (CE) can be an important issue. It is well known that Pt is the best material for CEs of DSCs3. In general, both high electrical conductivity and excellent catalytic activity are required for ideal CE materials. However, it is not easy to simultaneously satisfy these two aspects. For example, a Pt layer directly sputtered on the FTO glass substrate usually achieves excellent electrical conductivity, but it has low catalytic activity because the active surface area is not large enough. Furthermore, the use of vacuum processing and an expensive target can also require a high production cost. Although Pt nanoparticles (NPs) can supply high catalytic activity due to their large active area, they contain many abundant grain boundaries and defects4. Thus, Pt NPs usually exhibit low electric conductivity. In order to resolve this issue, three-dimensional (3D) nanostructures have been recently developed for CEs5-10. For example, Pt hollow spheres have been fabricated on FTO using polystyrene (PS) templates5. Furthermore, Pt nanocup arrays were prepared through UV-based nanoimprint lithography6. Wang et al. deposited Pt NPs on 3D FTO structure on FTO glass as an efficient CE for DSCs7. Large-area Pt CE platforms using commercial TiO2 paste and poly(dimethylsiloxane) (PDMS) nanostamps were presented by Kim et al.8. The imprinted Ti plates was fabricated through the anodization technique with the aid of TiO2 nanotubes (TNT) 9. Pt nanoneedles grown on carbon material were prepared via a redox reaction and then coated onto an FTO glass as CEs for DSCs10. These previously proposed technologies require templates, such as PS, 3D FTO conductive glass, holey polymer templates, PDMS nanostamps, TNT, and carbon materials, in order to form 3D CEs. Further drawbacks of developing an economic and continuous process for device fabrication include the expensive vacuum equipment, nanoimprint

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lithography, and target. Despite numerous studies on the subject, the 3D CE remains an issue for DSC technology. Here, a facile one-pot method for synthesizing Pt nanourchins (NUs) on FTO glass substrates and its application to CEs of DSCs are reported. In this approach, PtNUs were grown on FTO glass substrates via a simple room temperature chemical reduction of H2PtCl6 using formic acid. The developed 3D CE exhibits enhanced catalytic activity toward triiodide ion reduction as a CE in DSCs, which increases the PCE to 9.39% under 1 Sun (100 mWcm-2) irradiation, in comparison with the conventional Pt-sputtered CE (8.5%). Furthermore, as a result of the proposed preparation process being easy without requiring high temperatures or expensive equipment, the method has significant potential for large-scale production. EXPERIMENTAL Materials H2PtCl6.xH2O (≥37.5% Pt basic) and formic acid were purchased from Sigma-Aldrich. FTO glass (~8 Ω/□) for conductive transparency electrodes was bought from Pilkington (USA). These substrates were cleaned in acetone (Fluka) with ultrasonic irradiation and then dried in an argon flow. A transparent TiO2 paste and a ruthenium-based cis-[Ru(dcbpyH)2(NCS)2](NBu4)2 (N719) were received from Solaronix, Switzerland. A scattering TiO2 paste (DSL 18 NR-AO) was purchased from Dyesol-Timo, Australia. All chemicals were received from Sigma-Aldrich, unless stated otherwise. The dye solution was prepared from a 0.3 mM solution of mixed acetonitrile and tert-butyl alcohol (a volume ratio of 1:1). The solution of 0.60 M 1-methyl-3butylimidazolium iodide, 0.03 M I2, 0.10 M guanidinium thiocyanate, and 0.50 M 4-tert-



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butylpyridine in a mixed solvent of acetonitrile, and valeronitrile (a volume ratio of 85:15) was used as electrolyte in DSCs. Synthesis and characterization of PtNUs The hierarchical PtNUs were grown directly on the FTO glass substrates using a simple room temperature chemical reduction of H2PtCl6 using formic acid. The substrate was cleaned via sonication in acetone, methanol, and deionized water; then, it was dried under a N2 flow. The aqueous solution was prepared after adding 0.032 g H2PtCl6.xH2O into 20 ml of H2O, followed by mixing with 1 ml formic acid at room temperature. Then, the FTO glass substrates were placed parallel to the bottom of the reactor. Note that the conductive surface faced upward. The reaction was maintained at room temperature for 16 h and at 80 °C for 15 min11, 12. After that, the samples were removed and washed with ethanol, and then dried in air. The morphology of the PtNUs was characterized using field-emission scanning electron microscopy (FESEM) (JSM 7000F, JEOL, Japan). For the transmission electron microscopy (TEM) analysis of the PtNUs, part of the PtNU film was scratched, dispersed in an ethanol suspension, and transferred to a holey carbon grid13. CE preparation Two different CEs were prepared for comparison. The first one contained a sputtered Pt CE with a thickness of 100 nm as described previously5. The second CE was PtNUs on FTO glass substrates that were prepared as described above. Preparation of the working electrodes and DSC assembly


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We coated two type of TiO2 layer on FTO glass substrate through screen-printing (200 T mesh) as working electrodes. The first one was the transparent film of 20 nm TiO2 particles with the thickness of 12 µm. The second layer was the scattering layer of TiO2 paste with the thickness of 4 µm. Next, they were sintered at 325 oC for 5 min, at 375 oC for 5 min, at 450 oC for 15 min, and finally at 500 oC for 15 min under ambient condition. Note that the blocking layers were fabricated before screen-printing the TiO2 layer on FTO glass and after sintering the TiO2 layer through immersing in 40 mM of TiCl4 solution at 70 oC for 30 min. After reheating the layers at 500 oC for 30 min and cooling to 80 oC, the sample was immersed into the dye solution for 24 h at room temperature. The details of the assembly of the DSCs have been described previously13. Measurements The photocurrent-voltage (J-V) characteristics were recorded on an IviumStat device. All DSCs were illuminated by a Sun 3000 solar simulator consisting of a 1000 W mercury-based Xe arc lamp and AM 1.5-G filters. The incident monochromatic photon-to-electric current conversion efficiency (IPCE) was measured in the range of 300 to 800 nm using a specifically designed IPCE system for DSCs (HS Technologies, Korea). A 150 W xenon lamp was used as the light source for the generation of the monochromatic beam. Calibration of system was conducted using a PECSI02-calibrated silicon photodiode as the standard. The cyclic voltammograms (CVs) of three electrode electrochemical cells were used to evaluate redox behaviors of the electrodes under study. A Pt mesh and Hg/Hg2+ electrodes functioned as the CE and reference electrodes, respectively. The electrolyte consisted of 10 mmol L-1 LiI, 1 mmol L-1 I2, and 1 mmol L-1 LiClO4. The CVs were recorded in the range of 600 to –300 mV with various scan rates from 10 to 50 mV s-1. The impedance spectroscopy of the DSCs was measured under constant light illumination (100 mWcm-2) biased in an open-circuit condition with a frequency range of 100 5 ACS Paragon Plus Environment


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kHz to 100 mHz and a perturbation amplitude of 10 mV. We used the Z-view software (2.8d, Scribner Associates, Inc., USA) to fit the obtained spectra. The Tafel measurements were performed at a scanning rate of 5 mVs-1 in the potential range of 0.6 to – 0.6 V. The data were analyzed using the IviumStat device in order to obtain the exchange current density. RESULTS & DISCUSSION Figure 1 compares the surface morphologies of the Pt-sputtered and PtNU CEs using scanning electron microscopy (SEM) plane images. As seen in Fig. 1(a), the Pt-sputtered film, which was well adhered to the FTO surface, uniformly covered the surface. The uniform covering along the grain boundaries of the SnO2:F substrate with grain size variation of 100–300 nm is clearly visible in the plane image (Fig. 1(a), and Fig. S1). Figures 1(b) and 1(c) present the morphology of PtNUs on the FTO surface. It is clearly observed that the PtNUs were successfully synthesized on the FTO surface under room temperature without using surfactants and templates. The PtNUs, which were formed densely on the FTO surface, clearly exhibited a 3D sea urchin-like shape. The size of the individual NUs was in range of 100 to 300 nm. Because the individual urchin structure consisted of a large number of Pt nanowires, the surface of the NUs was very rough. The thickness of the PtNU layer was approximately 265 nm, as depicted in Fig. 1(d). This 3D network structure of the NU layers is good for the electrochemical catalytic activity of CEs for DSCs because it provides numerous channels for the triiodide ions across the NU film. Figure 1(e) presents the morphology of a Pt nanowire on a Cu grid, which scratched part of the PtNU films on the FTO glass substrate. It was found that the nanowire diameter was 2 nm and the length of the nanowire was ~12 nm. Figure 1(f) presents the high magnification of a section of Fig. 1(e). The lattice spacings of a heavy element were 1.96 and 2.22 Å, which coincide well


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with the {200} and {111} planes, respectively, of the face center cubic (fcc) Pt structure (JCPDS 04-0802). Figure 1(g) depicts the Fast Fourier Transform (FFT) patterns. As can be seen, the [111] and [200] were growth directions of Pt. FFT pattern also shows that the structure of Pt crystal is distinctly observed. The formation of Pt nanowires was further confirmed by energy dispersive X-ray spectroscopy (EDS), which was measured during the TEM analyses (Fig. 1(e)), as depicted in Fig. S2 (Supporting Information). The insets of Figs. 1(a) and 1(b) are macroscopic images of the Pt-sputtered and PtNU CEs, respectively; these were used to compare the uniformity of the two CE surfaces. As can be seen in the insets, there was a difference in the colors of the two samples. This results from the difference in the thickness and roughness of the two Pt surfaces. Although the uniformity of the Pt-sputtered surface was slightly better than that of the PtNU surface, both surfaces were almost equivalently uniform and reproducible. Note that the proposed method is cost effective for largescale applications. The growth mechanism of the PtNUs can be explained using three steps. First, the FTO is used as a support in the synthesis of the PtNUs. It is known that the F atoms on FTO can provide free electron pairs because of their hybrid orbital configuration of 2s22p5 14, and H2PtCl6 is a type of acid

15

. Therefore, there is a Lewis acid-base interaction between the H2PtCl6 and SnO2:F. At

room temperature, many Pt nuclei were first formed through the chemical reduction of H2PtCl6 using formic acid

16, 17

. The second step is the formation of the Pt nanowires as a result of the

decrease in the reduction rate, which favors the growth of {111} planes 11, 18. The third step is the formation of the sea urchin-like nanostructures through continuous growth and densification 11, 18.



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It is known that the rate-limiting step of the I3-/I- redox process is the rate of desorption of the reduced I- ions

19

. There are numerous factors that affect the rate of desorption, such as the

desorption energy, CE surface structure, surface morphology, CE roughness factor, and double layer thickness

20-23

. As seen in Fig. 1, the PtNU CEs have a high roughness due to the 3D

structure with numerous nanowires on the surface. Note that the surface of the Pt-sputtered electrode is very smooth, as described in a previous study

24

. This indicates a thinner double

layer in the PtNUs as opposed to the thick double layer in the smooth surface of the Pt-sputtered film 23. The diffusion of iodide ions near the electrode surface can be blocked by a thick double layer22. Therefore, an increase in the diffusion of I3- and I- ions can be facilitated through an increasing in the CE roughness, which subsequently increases in the rate of the I3-/I- redox process. The fast desorption of I- ions from the PtNU surfaces, which exhibits minimal surface energy, also results in high catalytic activity of the PtNUs. Based on this, it is expected that there is high electrochemical catalytic activity in the PtNU CEs for the regeneration of iodide from the triiodide ions compared with that of the Pt-sputtered CE. In order to support these expectations, a comparative study of the electrochemical catalytic activity of PtNU and Pt-sputtered CEs was conducted. In order to evaluate the catalytic activity of the CEs, CVs were conducted. The results are presented in Fig. 2(a). The catalytic activity of the electrodes was estimated through the reduction current (Jred) and peak-to-peak separation (∆E). The data are listed in Table 1. As seen in table, the Jred value of the PtNU electrode was higher than that of the Pt-sputtered electrode, which indicates that a large amount of redox reaction occurs on the PtNU CEs compared with the Pt-sputtered CEs. Moreover, the total electric charge that flows between the electrolyte and CE during the reduction of triiodide ions was also calculated. The detailed calculations are provided 8 ACS Paragon Plus Environment

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in Fig. S3 of the Supporting Information. The total electric charge values are also listed in Table 1. As seen in the table, the total electric charge value of the PtNU electrode was higher than that of the Pt-sputtered electrode. This result is in good agreement with the trend in the Jred values. It is known that a higher total electric charge indicates a larger surface active area 6. The catalytic activities of the electrodes were further confirmed through analyzing the values of the surface active areas calculated from the CV data. The obtained data of the surface active areas are provided in Table 1. The results are in a good agreement with data on the total electric charge and the Jred values. The ∆E values of the two electrodes also are presented in Table 1. As seen in the table, the ∆E value of the PtNU electrode was 290 mV, while 360 mV was obtained for the Ptsputtered electrode. It should be noted that a lower ∆E indicates a faster rate of I3-/I- redox reaction with an increasing catalytic active area

25

. The two sets of data extracted from the CV

curves correlate well with the charge-transfer resistance (Rct) and the open-circuit voltage (Voc) in the DSCs 26. In order to understand the charge-transfer mechanism, we tested the CV performances at various scan rates. The results are presented in Figs. S4 and S5 (Supporting Information) for the PtNU and Pt-sputtered electrodes, respectively. Figure 2(b) illustrates the relationship between the peak current and square root of the scan rates. As seen in Fig. 2(b), both electrodes exhibit linearity in the peak currents with respect to the square root of the scan rates. The detailed parameters are presented in Table S1 (Supporting Information). The results indicate that charge transfer in the redox step is controlled through the diffusion of charge in the film 27. One important demand for the application of PtNUs to CEs in DSCs is chemical stability in the electrolyte solution. For this purpose, the stability of the current-voltage curves were measured over time for the redox system with PtNU working electrodes and Pt mesh CEs, as depicted in the inset of Fig. 2(c). The 9 ACS Paragon Plus Environment


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developed electrode did not exhibit noticeable changes in the Jred and oxidation current (Joxd) values over the 1000 cycles, as depicted in Fig. 2(c); this indicates that there is no detachment of the PtNU film from the FTO glass. This result indicates the PtNU film is highly stable under electrochemical reaction conditions. In order to investigate the interfacial charge transfer resistance properties of the triiodide/iodide couple on the surface of the CEs, electrochemical impedance spectroscopy (EIS) studies were performed using different symmetrical dummy cells fabricated with two identical electrodes. The Nyquist plots are presented in Fig. 3(a). The equivalent circuit, which is depicted in the inset of Fig. 3(a), was used to fit the spectra with the Z-view software. The parameters of the EIS spectra obtained from the equivalent circuit are summarized in Table 1. As seen in Table 1, the Rh value of the PtNU CE was higher than that of Pt-sputtered CE. The result pointed out that the conductivity of the PtNU was slightly lower than that of Pt-sputtered CE. The Rct value of the PtNU CE was as small as 0.05 Ω, while that of the Pt-sputtered CE was 0.92 Ω. This results from the morphology of the CEs. The obtained results are in accordance with the CV analyses. It is well known that the exchange current density (J0) is calculated using the expression J0 = RT/nFRct, where R, T, n, and F are the gas constant, the temperature, the number of electrons involved in the reduction of iodide electrolyte, and Faraday's constant, respectively15. The J0 values are also listed in Table 1. It is reported that a high J0 value indicates a high fill factor (FF) and short-circuit current density (Jsc) in DSCs

28

. Table 1 also shows that the diffusion

impedance, Ws, of the PtNU CE is 0.51 Ω, which is lower than that of 0.75 Ω for Pt-sputtered CE and means that triiodide ions can be rapidly reduced to iodide ions under 3D nanostructure of PtNU CE. The constant-phase element (CPE = (CPE-T)-1(jw)-(CPE-P), where j2 = –1, w is the frequency, and CPE-T and CPE-P are frequency-independent parameters of CPE) of the CE also 10 ACS Paragon Plus Environment

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supported a larger active surface area of the PtNU CE compared with the Pt-sputtered CE. Indeed, Table 1 illustrates that the value of CPE-T was 965 µF for the PtNU CE, which was significantly larger than the 73.2 µF of the Pt-sputtered CE. A larger CPE-T indicates an increase in the active surface area. The decrease in the Rct resulted in the reduction of the total internal resistance and the increase in the FF of DSCs 29, 30, which confirms the increase in the PCE. In order to further investigate the interfacial charge transfer resistance properties of the triiodide/iodide couples on the surface of CEs, we conducted Tafel measurements. The polarization, i.e. the Tafel and limiting diffusion zones

31

, are indicated in Fig. 3(b). J0 can be

obtained from the Tafel zone, and the values of J0 are listed in Table 1. It was found that the values of J0 exhibited the same trend as those obtained from the CV and EIS analyses. The limiting diffusion current densities (Jlim) of different CEs are also depicted in Fig. 3(b). It is known that the diffusion coefficient (D) can be expressed as follows: D = lJlim/2nFC, where l and C represent the spacer thickness and electrolyte concentration, respectively. A high value of the Jlim indicates a high diffusion coefficient at the same potential, which results in a high diffusion velocity in the redox couple in the electrolyte 31. As seen in Fig. 3(b), the Jlim value of the PtNU CE was higher than that of the Pt-sputtered CE. This result indicates that there is a larger D and higher diffusion velocity for the redox couple in the electrolyte at the PtNU electrode at the same potential compared with the Pt-sputtered electrode. These results support the relatively high photovoltaic performance of the DSC with the PtNU CE compared with the DSC with the Pt-sputtered CE. Figure 4(a) compares the performance of the DSCs using different CEs with PtNU and Ptsputtered CEs. The photovoltaic (PV) parameters are listed in Table 1. It was found that the DSC with a PtNU CE exhibited a PCE of 9.39%, which is higher than that of the DSC with a Pt11 ACS Paragon Plus Environment


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sputtered CE. Regardless of the illumination conditions, the Voc of the DSC with the PtNU CE was higher than that of the DSC with the Pt-sputtered CE. As discussed above, the difference in the Voc values was supported by ∆E [5]. It was also found that the FF value of the DSC assembled with PtNU CE was slightly larger than that of the DSC with the Pt-sputtered CE. The lower total internal resistance of the DSC with the PtNU CE compared with that of the DSC with the Pt-sputtered CE, as mentioned previously, supports the explanation of this observation. As seen in Table 1, the Jsc of the DSC with the PtNU CE was also higher than that of the DSC with the Pt-sputtered CE. The higher catalytic activity of the PtNU CE compared with that of the Ptsputtered CE explains this result, because the high catalytic activity leads to an increase of the reduction rate of triiodide to iodide ions, which subsequently diffuse into the working electrode for the dye recovery. The result was further confirmed by the IPCE performance as depicted in Fig. 4(b). The IPCE of the DSC with PtNU CEs was higher than that of the DSC based on the Ptsputtered CEs over the visible wavelength range. The current value calculated from the overlap integral of the IPCE spectrum agrees well with the Jsc derived from the J-V characteristics. It is well known that the IPCE is comprised of the light harvesting and charge collection efficiencies, the efficiency of the electron injection from the excited dye into the TiO2, and the efficiency for the dye regeneration. In this study, identical compounds were used to fabricate all devices, except the CEs. Thus, all factors can be excluded except the last one, which is the efficiency for the dye regeneration. Therefore, the increase in the Jsc value can be attributed to the high electrochemical catalytic activity of the PtNU CE. The data are in good agreement with the CV, EIS, Tafel, and J-V analyses. CONCLUSION


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PtNUs were successfully synthesized on FTO glass substrates via a simple room temperature chemical reduction of H2PtCl6 using formic acid. The formation of PtNUs was confirmed via HRSEM, TEM, and EDS. The PtNUs consisted of numerous Pt nanowires with diameters of 2 nm and lengths of 12 nm. The size of the PtNUs was in the range of 100–300 nm. The thickness of the PtNU film was approximately 265 nm. The PtNUs were also applied in the CEs of DSCs. The obtained results exhibited high electrochemical catalytic activity, which was described by a very low Rct (0.05 Ω), toward the reduction of the triiodide ions to iodide ions, and it exhibited potential as a robust CE for DSCs. The cell using the PtNU CE achieved a high PCE of 9.39%, which was higher than that of the device fabricated with a Pt-sputtered CE (8.51%). These features render PtNUs as a highly promising CE material for PV devices as well as catalysts for fuel cells. Because the synthesis process is simple and cost effective for large-scale applications, it is believed that this method can be applied to the synthesis of various 3D nanostructures for a wide range of applications. ASSOCIATED CONTENT Supporting Information. EDS spectra obtained from the sample treated with the Ar plasma, Cyclic voltammetry curve of the developed electrode in an electrochemical reduction solution. The marked area indicates the total charge transfer of the electrode. CV curves of the PtNUs for the I-/I3- redox species at different scan rates. CV curves of the Pt-sputtered for the I-/I3- redox species at different scan rates. CV parameters for different CEs. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author 13 ACS Paragon Plus Environment


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*Email: [email protected] ACKNOWLEDGEMENTS This

research

was

supported

by

a

National

Research

Foundation

(NRF)

grant

(2014R1A2A2A01006994), the Korea Research Fellowship Program (2015H1D3A1061830), and a Korea CCS R&D Center (KCRC) grant (2014M1A8A1049345). They are all funded by the Ministry of Science, ICT and Future Planning through the National Research Foundation of Korea. REFERENCES (1) O’Regan, B.; Gratzel, M. A Low-Cost, High-Efficiency Solar Cell Based on Dye-Sensitized Colloidal TiO2 Films. Nature 1991, 353, 737-740 (2) Mathew, S.; Yella, A.; Gao, P.; Humphry-Baker, R.; Curchod, B. F. E.; Ashari-Astani, N.; Tavernelli, I.; Rothlisberger, U.; Nazeeruddin, Md. K. and Gratzel, M. Dye-Sensitized Solar Cells with 13% Efficiency Achieved Through the Molecular Engineering of Porphyrin Sensitizers. Nat. Chem., 2014, 6, 242-247 (3) Peter, L. M. Dye-Sensitized Nanocrystalline Solar Cells. Phys. Chem. Chem. Phys., 2007, 9, 2630-2642 (4) Bell, A. T. The Impact of Nanoscience on Heterogeneous Catalysis. Science 2003, 299, 16881691 (5) Dao, V. D.; Kim, S. H.; Choi, H. S.; Kim, J. H.; Park, H. O.; Lee, J. K. Efficiency Enhancement of Dye-Sensitized Solar Cell Using Pt Hollow Sphere Counter Electrode. J. Phys. Chem. C 2011, 115, 25529-25534 14 ACS Paragon Plus Environment

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(6) Jeong, H.; Pak, Y.; Hwang, Y.; Song, H.; Lee, K. H.; Ko, H. C.; Jung, G. Y. Enhancing the Charge Transfer of the Counter Electrode in Dye-Sensitized Solar Cells Using Periodically Aligned Platinum Nanocups. Small 2012, 8, 3757-3761 (7) Wang, M.; Zhao, Y.; Yuan, S.; Wang, Z.; Ren, X.; Zhang, M.; Shi, L.; Li, D. High ElectroCatalytic Counter Electrode Based on Three-Dimensional Conductive Grid for Dye-Sensitized Solar Cell. Chem. Eng. J., 2014, 255, 424-430 (8) Kim, D. J.; Koh, J. K.; Lee, C. S.; Kim, J. D. Mesh-Shaped Nanopatterning of Pt Counter Electrodes for Dye-Sensitized Solar Cells with Enhanced Light Harvesting. Adv. Energy Mater. 2014, 4, 1400414-1400423 (9) Lin, L. Y.; Yeh, M. H.; Chen, W. C.; Ramamurthy, V.; Ho, K. C. Controlling Available Active Sites of Pt-Loaded TiO2 Nanotube-Imprinted Ti Plates for Efficient Dye-Sensitized Solar Cells. ACS Appl. Mater. Interfaces 2015, 7, 3910-3919 (10) Elbohy, H.; Aboagye, A.; Sigdel, S.; Wang, Q.; Sayyad, M. H.; Zhang, L.; Qiao, Q. Graphene-embedded Carbon Nanofibers Decorated with Pt Nanoneedles for Higher Efficiency Dye-sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 17721-17727 (11) Sun, S.; Yang, D.; Villers, D.; Zhang, G.; Sacher, E.; Dodelet, J. P. Template- and Surfactant-free Room Temperature Synthesis of Self-Assembled 3D Pt Nanoflowers from Single-Crystal Nanowires. Adv. Mater. 2008, 20, 571-574 (12) Dao, V. D.; Kim, P.; Baek, S.; Larina, L. L.; Yong, K.; Ryoo, R.; Ko, S. H.; Choi, H. S. Facile Synthesis of Carbon dot-Au Nanoraspberries and Their Application as High-Performance Counter Electrodes in Quantum dot-Sensitized Solar Cells. Carbon 2016, 96, 139-144



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(13) Dao, V. D.; Tran, Q. C.; Ko, S. H.; Choi, H. S. Dry Plasma Reduction to Synthesize Supported Platinum Nanoparticles for Flexible Dye-Sensitized Solar Cells. J. Mater. Chem. A 2013, 1, 4436-4443 (14) Ramaiah, K. S.; Raja, V. S. Structural and Electrical Properties of Fluorine Doped Tin Oxide Films Prepared by Spray-pyrolysis Technique. Appl. Surf. Sci., 2006, 253, 1451-1458 (15) Lan, Z.; Wu, J.; Lin, J.; Huang, M. Morphology Controllable Fabrication of Pt Counter Electrodes for Highly Efficient Dye-Sensitized Solar Cells. J. Mater. Chem., 2012, 22, 39483954 (16) Sun, S.; Yang, D.; Zhang, G.; Sacher, E.; Dodelet, J. P. Synthesis and Characterization of Platinum Nanowire–Carbon Nanotube Heterostructures. Chem. Mater., 2007, 19, 6376-6378 (17) Meng, H.; Xie, F.; Chen, J.; Sun, S.; Shen, K. Morphology Controllable Growth of Pt Nanoparticles/nanowires on Carbon Powders and Its Application as Novel Electro-catalyst for Methanol Oxidation. Nanoscale 2011, 3, 5041-5048 (18) Sakamoto, Y.; Fukuoka, A.; Higuchi, T.; Schimomura, N.; Inagaki, S.; Ichikawa, M. Synthesis of Platinum Nanowires in Organic-Inorganic Mesoporous Silica Templates by Photoreduction: Formation Mechanism and Isolation. J. Phys. Chem. B 2004, 108, 853-858 (19) Tang, Y.; Pan, X.; Zhang, C.; Dai, S.; Kong, F.; Hu, L.; Sui, Y. Influence of Different Electrolytes on the Reaction Mechanism of a Triiodide/Iodide Redox Couple on the Platinized FTO Glass Electrode in Dye-Sensitized Solar Cells. J. Phys. Chem. C 2010, 114, 4160-4167 (20) Mukherjee, S.; Ramalingam, B.; Griggs, L.; Hamm, S.; Baker, G. A.; Fraundorf, P.; Sengupta, S.; Gangopadhyay, S. Ultrafine Sputter-deposited Pt nanoparticles for Triiodide 16 ACS Paragon Plus Environment

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Table 1. Important parameters that characterize the electrocatalytic activity and cell performance of the different CEs. Index Pt-sputtered Jred (mA) -0.993 ∆E (mV) 360 6 Area (x 10 ) 37.2 3 Charge (x 10 ) (C) 3.72 Rh (Ω) 1.46 0.92 Rct (Ω) CPE-T (µF) 73.2 Ws 0.75 J0 (mA) 0.14 a J0 (mA) 9.7 -2 Jsc (mAcm ) 15.22 ± 0.17 760.00 ± 5.00 Voc (mV) FF (%) 73.52 ± 0.49 PCE (%) 8.51 ± 0.15 Jred: peak current density of reduction ∆E: peak-to-peak separation a The data were calculated from the Tafel curve.

PtNUs -1.399 290 65.7 6.57 4.18 0.05 965 0.51 2.67 15.7 16.45 ± 0.15 771.67 ± 2.35 74.11 ± 0.11 9.39 ± 0.10



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Figure 1. (a) HRSEM image of the Pt-sputtered on the FTO glass substrate; (b) and (c) HRSEM images of the PtNUs grown on the FTO glass substrates; (d) cross-sectional SEM of PtNUs on the FTO glass substrate; the inset of Figures 1(a) and 1(b) are photographs of the Pt-sputtered and PtNU surfaces on the FTO lass substrates; (e) TEM image illustrating the Pt nanowire that was scratched from part of the PtNUs; (f) magnified TEM image from 1(e) illustrating the crystal structure of the Pt nanowire; and (g) FFT pattern depicting the growth of [111] and [200]. 20 ACS Paragon Plus Environment

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Figure 2. (a) CV curves with different working electrodes at scan rate of 10 mVs-1; (b) relationship between the peak current and square root of the scan rates. Note that the solid line and dotted line indicate the PtNU and Pt-sputtered CEs, respectively; and (c) the peak current stability as a function of the cycle. The inset of Fig. 2(c) presents the CV curves of 1000 cycles using the PtNUs as working electrodes at a scan rate of 50 mVs-1.

Figure 3. (a) Nyquist plots of the dummy cells fabricated with identical CEs. The inset is the equivalent circuit diagram used to fit the observed impedance spectra in this figure. (b) Tafel curves of the dummy cells similar to those used for the EIS measurements. 22 ACS Paragon Plus Environment

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Figure 4. (a) Characteristic current density-voltage curves of the DSCs with different CEs measured under standard conditions; and (b) IPCE curves for different DSCs.



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Pt Nanourchins as Efficient and Robust Counter Electrode Materials

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