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Facile synthesis of hierarchical nanostructured polypyrrole and its application in the counter electrode of dye-sensitized solar cells
Materials Letters 214 (2018) 158–161
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Facile synthesis of hierarchical nanostructured polypyrrole and its application in the counter electrode of dye-sensitized solar cells Guiqiang Wang ⇑, Weinan Dong, Chao Yan, Shuo Hou, Wei Zhang School of New Energy, Bohai University, Jinzhou 121013, China
a r t i c l e
i n f o
Article history: Received 22 October 2017 Received in revised form 22 November 2017 Accepted 29 November 2017 Available online 1 December 2017 Keywords: Hierarchical nanostructure Microstructure Polymer Counter electrode Solar energy materials
a b s t r a c t Hierarchical nanostructured polypyrrole (HNPPy) of spherical polypyrrole (PPy) nanoparticles anchored on the surface of PPy nanofiber network is prepared by a facile process and incorporated into dye-sensitized solar cells (DSCs) as Pt-free counter electrode. This unique hierarchical nanostructure can provide the fast electron-transport pathway and abundant electrocatalytic active sites simultaneously. Electrochemical impedance spectroscopy analysis demonstrates that the electrocatalytic activity of HNPPy electrode for the I3 reduction is higher than that of PPy nanoparticles (PPyNPs) electrode and comparable to that of Pt electrode. As a consequence, the DSC fabricated with HNPPy counter electrode achieves a high conversion efficiency of 6.78%, which is close to the efficiency using Pt counter electrode. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, the third-generation photovoltaic devices have been widely studied as the emerging technologies, leading to great advances in the efficiency and stability [1]. Dye-sensitized solar cells (DSCs) are the remarkable third-generation photovoltaic devices due to their attractive features of low cost and high conversion efficiency [2,3]. As a key component of DSCs, the counter electrode gathers the electrons and catalyzes the reduction reaction of I3 in the electrolyte, thus affects the photovoltaic performance of DSCs greatly. Platinum (Pt) is a preferred counter electrode material so far for high-efficiency DSCs [4,5]. However, the high price and unstability in the corrosive I /I3 electrolyte of Pt incentivize the great efforts to develop cost-effective alternatives to Pt for the counter electrode in DSCs [6]. Recently, polypyrrole (PPy) has attracted more and more interests as the potential candidate for Pt counter electrode in DSCs due to its high stability, good electrochemical performance, and facile preparation. PPy nanoparticles (PPyNPs) counter electrodes have displayed good photovoltaic performance [7–10]. Nevertheless, the high electron-transport resistance of PPyNPs deriving from the abundant particle boundaries severely restricts the further improvement of the photovoltaic performance. This limitation impelled to construct the PPyNPs/multi-walled carbon nanotubes composite [11,12], which combined both the excellent electrocat⇑ Corresponding author. E-mail address: [email protected] (G. Wang). https://doi.org/10.1016/j.matlet.2017.11.129 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
alytic performance and high electron-transport efficiency in one material and then greatly enhanced the photovoltaic performance of the corresponding device. In this work, hierarchical nanostructured polypyrrole (HNPPy) of spherical PPyNPs anchored on the surface of PPy nanofibers network was prepared by a facile process and incorporated into DSCs as Pt-free counter electrode. HNPPy can provide both the fast electron-transport pathway and the abundant electrocatalytic active sites. As a result, the as-prepared HNPPy counter electrode exhibited superior performance to PPyNPs as the counter electrode of DSCs.
2. Experimental HNPPy was prepared by a facile process. Briefly, 1 g of pyrrole was dissolved in 60 ml of 1 M HCl solution by stirring. Then 10 mg of V2O5 nanofibers was dispersed in the above solution. The mixture was stirred for 4 h at room temperature. Then, 5 ml of 3 mM FeCl36H2O aqueous solution was added drop by drop. After reaction at room temperature for 1 h, the obtained PPy precipitate was filtered and washed with double distilled water and ethanol. For comparison, PPyNPs were also prepared by using similar procedure without V2O5 nanofibers. 100 mg of the obtained PPy sample was dispersed in the mixed solution consisting of 10 ml of n-butanol and 15 ll of tetrabutyl titanate by grinding and sonication. The PPy dispersion was deposited onto fluorine-doped tin oxide (FTO) glass by a spraying
G. Wang et al. / Materials Letters 214 (2018) 158–161
159
Fig. 1. Schematic illumination of the preparation of HNPPy (a); SEM images of HNPPy (b) and PPyNPs (c).
method. Then, the obtained PPy electrodes were sintered at 150 °C under Ar atmosphere for 20 min. The dye-sensitized TiO2 photoanodes were prepared as previous report [13]. A DSC was fabricated by clipping a dye-sensitized photoanode, a drop of electrolyte, and a counter electrode into a sandwich structure. The 3-methoxypropionitrile solution consisting of 0.2 M LiI, 0.5 M 1-hexyl-3-methylimidazolium iodide, 0.05 M I2, and 0.4 M 4-tert-butylpyridine was used as the electrolyte. The morphology of PPy samples were investigated by a scanning electron microscopy (SEM, Hitachi S-4800). Fourier transform infrared (FTIR) spectra were recorded on a Scimitar 2000 FT-IR spectrometer. X-ray diffraction was performed on a D8 Advance diffractometer (Bruker). The electrocatalytic activity of PPy electrode was investigated by electrochemical impedance spectroscopy (EIS), which was carried out on a Solartron 1287/1255 electrochemical system using symmetric thin-layer cells. The photocurrent density–voltage curves were recorded by a Keithley 2400 source meter under light irradiation of 100 mW cm 2 (AM 1.5).
3. Results and discussion The synthesis procedure of HNPPy is schematically illuminated in Fig. 1a. When V2O5 nanofiber mixed with pyrrole, pyrrole monomers were adsorbed and oxidatively polymerized on the surface of V2O5 nanofibers. V2O5 nanofibers acted as both the template and the initiator, resulting in the production of PPy nanofibers with smooth surface (SEM image in Fig. 1a); meanwhile, V2O5 was converted into soluble vanadium salt and then washed away during product washing. When FeCl3 was added, pyrrole monomers were initiated and polymerized on the surface of PPy nanofibers, forming HNPPy. The SEM image of HNPPy (Fig. 1b) displays that the spherical PPy nanoparticles with the size of about 100 nm anchor on the surface of interconnected PPy nanofibers to form a hierarchical architecture. Without the presence of V2O5 nanofibers, the polymerization of pyrrole monomers was initiated by FeCl3 and formed PPyNPs (as shown in Fig. 1c). The PPy nanofibers can provide the fast electron-transport pathway, and then greatly improve
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G. Wang et al. / Materials Letters 214 (2018) 158–161
Fig. 3. Nyquist plots of HNPPy, PPyNPs, and Pt electrode (a); the photocurrent density–voltage curves of DSCs with HNPPy, PPyNPs, and Pt counter electrode.
Fig. 2. FTIR (a) and XRD (b) curves of HNPPy and PPyNPs.
the conductivity of HNPPy. The electrical conductivity of HNPPy measured by a four-probe method is 12.36 S cm 1, which is higher than that of PPyNPs (1.46 S cm 1). This feature is favorable for improving the electrocatalytic performance of HNPPy. Fig. 2a shows FTIR spectra of HNPPy and PPyNPs. Both HNPPy and PPyNPs display the typical bands of PPy at 1568, 1303, 1206, 1046, and 937 cm 1, assigning to stretching vibration of pyrrole ring, the C–H in-plane deformation vibration, the C–N stretching vibration, the C–H in-plane bending vibration, and the C–H outof-plane vibration [14], respectively. Fig. 2b exhibits XRD patterns of the as-prepared HNPPy and PPyNPs. XRD patterns display a broad peak centered at 2h = 24.7°, which is the typical characteristic of amorphous PPy material [15]. No diffraction peaks corresponding to V2O5 are observed from the XRD curve of HNPPy, which demonstrates that V2O5 nanofibers are removed completely. The electrocatalytic performance of PPy electrodes were evaluated by EIS measurement. Fig. 3a displays Nyquist plots of HNPPy, PPyNPs and Pt electrodes. All Nyquist plots display a semicircle in the high-frequency region and an oblique line in the low-frequency region. The high-frequency semicircle corresponds with the charge-transfer process at the electrode/electrolyte interface. Based on the equivalent circuit (the inset in Fig. 3a), the intercept of the high-frequency semicircle on the real axis represents the series resistance (Rs); the diameter of the high-frequency semicir-
Table 1 Photovoltaic parameters of DSCs based on HNPPy, PPyNPs, and Pt counter electrodes. Counter electrode
Voc (V)
Jsc (mA cm
HNPPy PPyNPs Pt
0.69 0.68 0.70
15.37 14.74 15.88
2
)
FF
g (%)
0.64 0.54 0.66
6.78 5.41 7.33
cle corresponds to the charge-transfer resistance (Rct). Fig. 3a clearly indicates that the Rs value of HNPPy electrode is smaller than that of PPyNPs electrode, mainly due to higher electrical conductivity of HNPPy compared to PPyNPs. The Rct values of HNPPy, PPyNPs, and Pt electrodes determined from Fig. 3a are 1.6, 7.8, and 1.1 X cm2, respectively. Obviously, HNPPy electrode performs better than PPyNPs electrode and exhibits comparable electrocatalytic activity to Pt electrode. The enhanced electrocatalytic activity of HNPPy electrode can be attributed to its unique hierarchical nanostructure. PPy nanofibers avoid the grain-boundary barrier and then provide the fast electron-transport pathway. PPyNPs anchored on the surface of PPy nanofibers provide large effective surface area on the electron pathway. Consequently, HNPPy can provide fast electron-transport pathway and abundant active sites simultaneously, which contributes to the improvement of the electrocatalytic activity of HNPPy electrode. Fig. 3b shows the photocurrent density–voltage curves of DSCs fabricated with HNPPy, PPyNPs, and Pt counter electrodes. The
G. Wang et al. / Materials Letters 214 (2018) 158–161
photovoltaic parameters determined from Fig. 3b are listed in Table 1. The open-circuit voltage (Voc), the short-circuit current density (Jsc), the fill factor (FF), and the conversion efficiency (g) of the HNPPy electrode-based DSC are 0.69 V, 15.37 mA cm 2, 0.64, and 6.78%, respectively, which are higher than those of the PPyNP electrode-based DSC. The photovoltaic performance here is mainly related to the electrocatalytic performance of the counter electrodes. EIS analysis indicates that the Rct and Rs of HNPPy electrode are lower than those of PPyNPs electrode. The integration of lower Rct and Rs can effectively facilitate the electron-transfer process at the electrode/electrolyte interface and then decrease the energy loss on the counter electrode, which contributes to the improvement of the photovoltaic parameters for HNPPy electrode. The DSC with Pt counter electrode achieves a g of 7.33%. The photovoltaic performance of HNPPy electrode is close to that of Pt electrode. Furthermore, HNPPy counter electrode was compared with other PPy counter electrode reported in the literature. The efficiency of HNPPy electrode is also superior to the previous reported efficiency of free-standing PPy nanotube [16] and vapour-phase-polymerization PPy particle [10] electrodes. These results demonstrate that HNPPy is a promising low-cost alternative to Pt for the counter electrode of DSCs. 4. Conclusions HNPPy was prepared by a facile process and investigated as the high-efficiency counter electrode for DSCs. The unique hierarchical nanostructure of HNPPy provides the fast electron-transport pathway and abundant electrocatalytic active sites simultaneously. As a result, HNPPy electrode showed an excellent electrocatalytic
161
activity for the I3 reduction reaction. Under illumination of 100 mW cm 2, the DSC with HNPPy counter electrode achieved a conversion efficiency of 6.78%, which is close to that of the cell with Pt counter electrode. These results suggested that the as-prepared HNPPy could be a promising substitute for Pt in the counter electrode of DSCs. Acknowledgement This work is supported by Natural Science Foundation of Liaoning (201601011). References [1] Z. Zang, A. Nakamura, J. Temmyo, Opt. Express 21 (2013) 11448–11456. [2] S. Mathew, A. Yella, P. Gao, R. Humphery-Baker, B. Curchod, M.K. Nazeeruddin, M. Grätzel, Nat. Chem. 6 (2014) 242–247. [3] A. Yella, H. Lee, H. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, M.K. Zakeeruddin, M. Gratzel, Science 334 (2011) 629–634. [4] N. Fu, Y. Fang, Y. Duan, X. Zhou, Y. Lin, ACS Nano 6 (2012) 9596–9605. [5] Y. Zhu, C. Gao, Q. Han, H. Zheng, M. Wu, J. Catal. 346 (2017) 62–69. [6] M. Wu, T. Ma, ChemSusChem 5 (2012) 1343–1357. [7] S. Yun, A. Hagfeldt, T. Ma, Adv. Mater. 26 (2014) 6210–6237. [8] C. Bu, Q. Tai, Y. Liu, S. Guo, X. Zhao, J. Power Sources 221 (2013) 78–83. [9] S. Jeon, C. Kim, J. Ko, S. Im, J. Mater. Chem. 21 (2011) 8146–8151. [10] J. Xia, L. Chen, S. Yanagida, J. Mater. Chem. 21 (2011) 4644–4649. [11] S. Peng, Y. Wu, P. Zhu, V. Thavasi, S. Mhaisalkar, S. Ramakrishna, J. Photochem. Photobiol., A 223 (2011) 97–102. [12] G. Yue, L. Wang, X. Zhang, J. Wu, Q. Jang, M. Huang, J. Lin, Energy 67 (2014) 460–467. [13] G. Wang, S. Kuang, W. Zhang, Mater. Lett. 174 (2016) 14–16. [14] J. Wang, B. Wei, F. Kang, RSC Adv. 4 (2014) 199–202. [15] T. Yao, T. Cui, J. Wu, Q. Chen, S. Lu, K. Sun, Polym. Chem. 2 (2011) 2893–2899. [16] T. Peng, W. Sun, C. Huang, Z. Dai, S. Guo, X. Zhao, ACS Appl. Mater. Interfaces 6 (2014) 14–17.
Contents lists available at ScienceDirect
Materials Letters journal homepage: www.elsevier.com/locate/mlblue
Facile synthesis of hierarchical nanostructured polypyrrole and its application in the counter electrode of dye-sensitized solar cells Guiqiang Wang ⇑, Weinan Dong, Chao Yan, Shuo Hou, Wei Zhang School of New Energy, Bohai University, Jinzhou 121013, China
a r t i c l e
i n f o
Article history: Received 22 October 2017 Received in revised form 22 November 2017 Accepted 29 November 2017 Available online 1 December 2017 Keywords: Hierarchical nanostructure Microstructure Polymer Counter electrode Solar energy materials
a b s t r a c t Hierarchical nanostructured polypyrrole (HNPPy) of spherical polypyrrole (PPy) nanoparticles anchored on the surface of PPy nanofiber network is prepared by a facile process and incorporated into dye-sensitized solar cells (DSCs) as Pt-free counter electrode. This unique hierarchical nanostructure can provide the fast electron-transport pathway and abundant electrocatalytic active sites simultaneously. Electrochemical impedance spectroscopy analysis demonstrates that the electrocatalytic activity of HNPPy electrode for the I3 reduction is higher than that of PPy nanoparticles (PPyNPs) electrode and comparable to that of Pt electrode. As a consequence, the DSC fabricated with HNPPy counter electrode achieves a high conversion efficiency of 6.78%, which is close to the efficiency using Pt counter electrode. Ó 2017 Elsevier B.V. All rights reserved.
1. Introduction Recently, the third-generation photovoltaic devices have been widely studied as the emerging technologies, leading to great advances in the efficiency and stability [1]. Dye-sensitized solar cells (DSCs) are the remarkable third-generation photovoltaic devices due to their attractive features of low cost and high conversion efficiency [2,3]. As a key component of DSCs, the counter electrode gathers the electrons and catalyzes the reduction reaction of I3 in the electrolyte, thus affects the photovoltaic performance of DSCs greatly. Platinum (Pt) is a preferred counter electrode material so far for high-efficiency DSCs [4,5]. However, the high price and unstability in the corrosive I /I3 electrolyte of Pt incentivize the great efforts to develop cost-effective alternatives to Pt for the counter electrode in DSCs [6]. Recently, polypyrrole (PPy) has attracted more and more interests as the potential candidate for Pt counter electrode in DSCs due to its high stability, good electrochemical performance, and facile preparation. PPy nanoparticles (PPyNPs) counter electrodes have displayed good photovoltaic performance [7–10]. Nevertheless, the high electron-transport resistance of PPyNPs deriving from the abundant particle boundaries severely restricts the further improvement of the photovoltaic performance. This limitation impelled to construct the PPyNPs/multi-walled carbon nanotubes composite [11,12], which combined both the excellent electrocat⇑ Corresponding author. E-mail address: [email protected] (G. Wang). https://doi.org/10.1016/j.matlet.2017.11.129 0167-577X/Ó 2017 Elsevier B.V. All rights reserved.
alytic performance and high electron-transport efficiency in one material and then greatly enhanced the photovoltaic performance of the corresponding device. In this work, hierarchical nanostructured polypyrrole (HNPPy) of spherical PPyNPs anchored on the surface of PPy nanofibers network was prepared by a facile process and incorporated into DSCs as Pt-free counter electrode. HNPPy can provide both the fast electron-transport pathway and the abundant electrocatalytic active sites. As a result, the as-prepared HNPPy counter electrode exhibited superior performance to PPyNPs as the counter electrode of DSCs.
2. Experimental HNPPy was prepared by a facile process. Briefly, 1 g of pyrrole was dissolved in 60 ml of 1 M HCl solution by stirring. Then 10 mg of V2O5 nanofibers was dispersed in the above solution. The mixture was stirred for 4 h at room temperature. Then, 5 ml of 3 mM FeCl36H2O aqueous solution was added drop by drop. After reaction at room temperature for 1 h, the obtained PPy precipitate was filtered and washed with double distilled water and ethanol. For comparison, PPyNPs were also prepared by using similar procedure without V2O5 nanofibers. 100 mg of the obtained PPy sample was dispersed in the mixed solution consisting of 10 ml of n-butanol and 15 ll of tetrabutyl titanate by grinding and sonication. The PPy dispersion was deposited onto fluorine-doped tin oxide (FTO) glass by a spraying
G. Wang et al. / Materials Letters 214 (2018) 158–161
159
Fig. 1. Schematic illumination of the preparation of HNPPy (a); SEM images of HNPPy (b) and PPyNPs (c).
method. Then, the obtained PPy electrodes were sintered at 150 °C under Ar atmosphere for 20 min. The dye-sensitized TiO2 photoanodes were prepared as previous report [13]. A DSC was fabricated by clipping a dye-sensitized photoanode, a drop of electrolyte, and a counter electrode into a sandwich structure. The 3-methoxypropionitrile solution consisting of 0.2 M LiI, 0.5 M 1-hexyl-3-methylimidazolium iodide, 0.05 M I2, and 0.4 M 4-tert-butylpyridine was used as the electrolyte. The morphology of PPy samples were investigated by a scanning electron microscopy (SEM, Hitachi S-4800). Fourier transform infrared (FTIR) spectra were recorded on a Scimitar 2000 FT-IR spectrometer. X-ray diffraction was performed on a D8 Advance diffractometer (Bruker). The electrocatalytic activity of PPy electrode was investigated by electrochemical impedance spectroscopy (EIS), which was carried out on a Solartron 1287/1255 electrochemical system using symmetric thin-layer cells. The photocurrent density–voltage curves were recorded by a Keithley 2400 source meter under light irradiation of 100 mW cm 2 (AM 1.5).
3. Results and discussion The synthesis procedure of HNPPy is schematically illuminated in Fig. 1a. When V2O5 nanofiber mixed with pyrrole, pyrrole monomers were adsorbed and oxidatively polymerized on the surface of V2O5 nanofibers. V2O5 nanofibers acted as both the template and the initiator, resulting in the production of PPy nanofibers with smooth surface (SEM image in Fig. 1a); meanwhile, V2O5 was converted into soluble vanadium salt and then washed away during product washing. When FeCl3 was added, pyrrole monomers were initiated and polymerized on the surface of PPy nanofibers, forming HNPPy. The SEM image of HNPPy (Fig. 1b) displays that the spherical PPy nanoparticles with the size of about 100 nm anchor on the surface of interconnected PPy nanofibers to form a hierarchical architecture. Without the presence of V2O5 nanofibers, the polymerization of pyrrole monomers was initiated by FeCl3 and formed PPyNPs (as shown in Fig. 1c). The PPy nanofibers can provide the fast electron-transport pathway, and then greatly improve
160
G. Wang et al. / Materials Letters 214 (2018) 158–161
Fig. 3. Nyquist plots of HNPPy, PPyNPs, and Pt electrode (a); the photocurrent density–voltage curves of DSCs with HNPPy, PPyNPs, and Pt counter electrode.
Fig. 2. FTIR (a) and XRD (b) curves of HNPPy and PPyNPs.
the conductivity of HNPPy. The electrical conductivity of HNPPy measured by a four-probe method is 12.36 S cm 1, which is higher than that of PPyNPs (1.46 S cm 1). This feature is favorable for improving the electrocatalytic performance of HNPPy. Fig. 2a shows FTIR spectra of HNPPy and PPyNPs. Both HNPPy and PPyNPs display the typical bands of PPy at 1568, 1303, 1206, 1046, and 937 cm 1, assigning to stretching vibration of pyrrole ring, the C–H in-plane deformation vibration, the C–N stretching vibration, the C–H in-plane bending vibration, and the C–H outof-plane vibration [14], respectively. Fig. 2b exhibits XRD patterns of the as-prepared HNPPy and PPyNPs. XRD patterns display a broad peak centered at 2h = 24.7°, which is the typical characteristic of amorphous PPy material [15]. No diffraction peaks corresponding to V2O5 are observed from the XRD curve of HNPPy, which demonstrates that V2O5 nanofibers are removed completely. The electrocatalytic performance of PPy electrodes were evaluated by EIS measurement. Fig. 3a displays Nyquist plots of HNPPy, PPyNPs and Pt electrodes. All Nyquist plots display a semicircle in the high-frequency region and an oblique line in the low-frequency region. The high-frequency semicircle corresponds with the charge-transfer process at the electrode/electrolyte interface. Based on the equivalent circuit (the inset in Fig. 3a), the intercept of the high-frequency semicircle on the real axis represents the series resistance (Rs); the diameter of the high-frequency semicir-
Table 1 Photovoltaic parameters of DSCs based on HNPPy, PPyNPs, and Pt counter electrodes. Counter electrode
Voc (V)
Jsc (mA cm
HNPPy PPyNPs Pt
0.69 0.68 0.70
15.37 14.74 15.88
2
)
FF
g (%)
0.64 0.54 0.66
6.78 5.41 7.33
cle corresponds to the charge-transfer resistance (Rct). Fig. 3a clearly indicates that the Rs value of HNPPy electrode is smaller than that of PPyNPs electrode, mainly due to higher electrical conductivity of HNPPy compared to PPyNPs. The Rct values of HNPPy, PPyNPs, and Pt electrodes determined from Fig. 3a are 1.6, 7.8, and 1.1 X cm2, respectively. Obviously, HNPPy electrode performs better than PPyNPs electrode and exhibits comparable electrocatalytic activity to Pt electrode. The enhanced electrocatalytic activity of HNPPy electrode can be attributed to its unique hierarchical nanostructure. PPy nanofibers avoid the grain-boundary barrier and then provide the fast electron-transport pathway. PPyNPs anchored on the surface of PPy nanofibers provide large effective surface area on the electron pathway. Consequently, HNPPy can provide fast electron-transport pathway and abundant active sites simultaneously, which contributes to the improvement of the electrocatalytic activity of HNPPy electrode. Fig. 3b shows the photocurrent density–voltage curves of DSCs fabricated with HNPPy, PPyNPs, and Pt counter electrodes. The
G. Wang et al. / Materials Letters 214 (2018) 158–161
photovoltaic parameters determined from Fig. 3b are listed in Table 1. The open-circuit voltage (Voc), the short-circuit current density (Jsc), the fill factor (FF), and the conversion efficiency (g) of the HNPPy electrode-based DSC are 0.69 V, 15.37 mA cm 2, 0.64, and 6.78%, respectively, which are higher than those of the PPyNP electrode-based DSC. The photovoltaic performance here is mainly related to the electrocatalytic performance of the counter electrodes. EIS analysis indicates that the Rct and Rs of HNPPy electrode are lower than those of PPyNPs electrode. The integration of lower Rct and Rs can effectively facilitate the electron-transfer process at the electrode/electrolyte interface and then decrease the energy loss on the counter electrode, which contributes to the improvement of the photovoltaic parameters for HNPPy electrode. The DSC with Pt counter electrode achieves a g of 7.33%. The photovoltaic performance of HNPPy electrode is close to that of Pt electrode. Furthermore, HNPPy counter electrode was compared with other PPy counter electrode reported in the literature. The efficiency of HNPPy electrode is also superior to the previous reported efficiency of free-standing PPy nanotube [16] and vapour-phase-polymerization PPy particle [10] electrodes. These results demonstrate that HNPPy is a promising low-cost alternative to Pt for the counter electrode of DSCs. 4. Conclusions HNPPy was prepared by a facile process and investigated as the high-efficiency counter electrode for DSCs. The unique hierarchical nanostructure of HNPPy provides the fast electron-transport pathway and abundant electrocatalytic active sites simultaneously. As a result, HNPPy electrode showed an excellent electrocatalytic
161
activity for the I3 reduction reaction. Under illumination of 100 mW cm 2, the DSC with HNPPy counter electrode achieved a conversion efficiency of 6.78%, which is close to that of the cell with Pt counter electrode. These results suggested that the as-prepared HNPPy could be a promising substitute for Pt in the counter electrode of DSCs. Acknowledgement This work is supported by Natural Science Foundation of Liaoning (201601011). References [1] Z. Zang, A. Nakamura, J. Temmyo, Opt. Express 21 (2013) 11448–11456. [2] S. Mathew, A. Yella, P. Gao, R. Humphery-Baker, B. Curchod, M.K. Nazeeruddin, M. Grätzel, Nat. Chem. 6 (2014) 242–247. [3] A. Yella, H. Lee, H. Tsao, C. Yi, A.K. Chandiran, M.K. Nazeeruddin, M.K. Zakeeruddin, M. Gratzel, Science 334 (2011) 629–634. [4] N. Fu, Y. Fang, Y. Duan, X. Zhou, Y. Lin, ACS Nano 6 (2012) 9596–9605. [5] Y. Zhu, C. Gao, Q. Han, H. Zheng, M. Wu, J. Catal. 346 (2017) 62–69. [6] M. Wu, T. Ma, ChemSusChem 5 (2012) 1343–1357. [7] S. Yun, A. Hagfeldt, T. Ma, Adv. Mater. 26 (2014) 6210–6237. [8] C. Bu, Q. Tai, Y. Liu, S. Guo, X. Zhao, J. Power Sources 221 (2013) 78–83. [9] S. Jeon, C. Kim, J. Ko, S. Im, J. Mater. Chem. 21 (2011) 8146–8151. [10] J. Xia, L. Chen, S. Yanagida, J. Mater. Chem. 21 (2011) 4644–4649. [11] S. Peng, Y. Wu, P. Zhu, V. Thavasi, S. Mhaisalkar, S. Ramakrishna, J. Photochem. Photobiol., A 223 (2011) 97–102. [12] G. Yue, L. Wang, X. Zhang, J. Wu, Q. Jang, M. Huang, J. Lin, Energy 67 (2014) 460–467. [13] G. Wang, S. Kuang, W. Zhang, Mater. Lett. 174 (2016) 14–16. [14] J. Wang, B. Wei, F. Kang, RSC Adv. 4 (2014) 199–202. [15] T. Yao, T. Cui, J. Wu, Q. Chen, S. Lu, K. Sun, Polym. Chem. 2 (2011) 2893–2899. [16] T. Peng, W. Sun, C. Huang, Z. Dai, S. Guo, X. Zhao, ACS Appl. Mater. Interfaces 6 (2014) 14–17.