1782 Phys. Chem. Chem. Phys., 2013, 15, 1782--1787 This journal is c the Owner Societies 2013 Cite this: Phys. Chem. Chem. Phys., 2013,
15, 1782
Electrophoresis of randomly and vertically embedded
graphene nanosheets in activated carbon film as a
counter electrode for dye-sensitized solar cells†
Mao-Sung Wu* and Yu-Jun Zheng
A new approach has been developed to randomly and vertically embed the graphene nanosheets (GNs) in the activated carbon (AC) film in an applied electric field. The activated carbon (AC) nano-particles in suspension during electrophoresis play an important role in supporting the GNs perpendicular to the FTO (fluorine-doped tin oxide) glass. Insufficient amount of AC nanoparticles might result in a deposition of GNs parallel to the FTO glass, leading to incomplete utilization of the surface area accessible to electrolyte ions. An AC cathode with randomly and vertically embedded GNs facilitated electrolyte penetration and electron conduction. The photoelectron conversion efficiency of the cell was increased to 7.50% by employing the AC cathode with randomly and vertically embedded GNs. The dye-sensitized solar cell (DSC), which is one of the most promising devices converting light to electricity, has been receiving much attention due to its simplicity and low fabrication cost.1Platinum is a widely used cathode material due to its high electrocatalytic activity for a redox couple (I/I3). Although the electrocatalytic activity of Pt is excellent, it is too expensive for commercialization. Therefore, investigation focuses on alternative materials that are inexpensive and still exhibit electrocatalytic activity similar to the Pt. Carbon-based materials such as activated carbon (AC), carbon nanotubes (CNTs), and graphene nanosheets (GNs) are reported to be highly prospective materials to replace the Pt counter electrode in DSCs.2–10
The preparation of carbon and its deposition onto the FTO (fluorine-doped tin oxide) surface determine the performance of DSCs. Several processes including dip coating, screen printing, spin coating, and electrophoretic deposition (EPD) have been used to fabricate the carbon layer or composite carbon layer on the FTO surface for DSCs. Among these methods, the EPD method is an effective way to obtain a reliable carbon layer for
improving the cell performance.11–15 Moreover, with the EPD method it is also possible to align particles and clusters on the conductive substrate in a desired direction.15 Recently, graphene-based materials are considered as the potential materials for applications in energy storage devices and DSCs because of their unique physicochemical and electronic properties.16,17 The graphene–CNT composite electrodes have been shown to improve the electrocatalytic behavior of a redox couple (I/I3).12,18,19 Nitrogen-doped graphene has been reported to have positive effects on the optoelectronic conversion and oxygen reduction.20,21
Generally, stacked GNs are parallel to the current collector during deposition. In this case, the electroactive surface area cannot be completely utilized because some of the regions are inaccessible to the electrolyte ions.22Therefore, the formation of vertically and/or randomly aligned GNs on the current collector has become a key issue for improving the electrode performance. Electric field alignment is a powerful technique that has been shown to orient CNTs along a particular direc-tion.23–26 Vertically aligned CNTs have been demonstrated to have a larger accessible surface area and better catalytic properties.27–29 The application of electric field may enable orientation of GNs on the electrode surface in a desired direc-tion. In this work, the GNs were randomly and vertically embedded in a homogeneous AC film using the EPD method. The photovoltaic properties of DSCs employing GN-embedded carbon cathodes were characterized.
EPD of the AC electrode was carried out in a suspension containing AC (Degussa, Printex L, 20 mg), hydrated nickel nitrate (0.5 mM), poly(N-vinyl-2-pyrrolidone) (PVP, 10 mg), and isopropyl alcohol (IPA, 50 mL) by applying a voltage difference of100 V between a working FTO glass (3.1 mm thick, 13 O/&, Nippon Sheet Glass) and counter platinum (2 cm 2 cm) electrodes. EPD of the GN-embedded carbon composite was similar to that of AC except that the carbon powder was composed of GN (5 mg) and AC (15 mg). The synthesis of GN was described in our previous report.30 After deposition, the electrodes were heat-treated in a nitrogen atmosphere at 500 1C for 1 h. The surface morphology of electrodes was examined
Department of Chemical and Materials Engineering, National Kaohsiung University of Applied Sciences, Kaohsiung 807, Taiwan. E-mail: [email protected]; Fax: +886-9-45614423
† Electronic supplementary information (ESI) available: SEM, TEM images, and AC impedance data. See DOI: 10.1039/c2cp43443a
Received 8th September 2012, Accepted 11th December 2012 DOI: 10.1039/c2cp43443a www.rsc.org/pccp
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1786 Phys. Chem. Chem. Phys., 2013, 15, 1782--1787 This journal is c the Owner Societies 2013 As the overpotential is increased, the Nernst diffusion
impe-dance increases due to a depletion of electrolyte at the elec-trode.34This means that the semicircle in the high-frequency region represents the charge-transfer process at the electrode– electrolyte interface, and another one in the low-frequency region is attributed to the Nernst diffusion (Zd) process of I/I3redox species in bulk and within pores. It was previously reported that the value of Rct plays an important role in determining the photovoltaic performance of the DSCs.35The simulated charge-transfer resistances of cells with randomly embedded GNs and horizontally embedded GNs are 0.50 and 0.79 O, respectively. This result highlights the superior electro-catalytic activity of randomly and vertically embedded GNs electrode over the randomly and horizontally embedded GNs for I/I3. The Nernst diffusion impedances of the AC electrode with randomly embedded GNs and horizontally embedded GNs are 2.51 and 4.12 O, respectively. The diffusion of I/I3is faster within an AC film with randomly and vertically embedded GNs. As expected, AC film with randomly and vertically embedded GNs shows better electrochemical features, including the lower charge-transfer resistance and Nernst diffusion impedance. The new electrode structure provides more pathways for electron conduction and allows the electrolyte to percolate into the inner layers of composite film. Therefore, the photovoltaic properties of a DSC using the AC cathode with randomly and vertically embedded GNs were considerably improved.
In summary, a one-step EPD process has enabled GNs to randomly and vertically embed in AC film in a dc electric field. GNs and AC particles surrounded by PVP-capped nickel ions readily suspend in IPA solution. The alignment of GNs is responsive to the AC content in the suspension. At low AC content, GNs were randomly and horizontally co-deposited on the FTO. At high AC content, the co-deposited GNs stretched out into the solution and oriented themselves perpendicular to the FTO surface. A DSC employing the AC cathode with randomly and vertically embedded GNs showed a high energy-conversion
efficiency of about 7.50%. The enhanced photovoltaic proper-ties could be attributed to the well dispersed and vertically embedded GNs which provide high electrical conductivity for facilitating the electrocatalytic activity toward I/I3 redox reaction. Alignment of GNs opens a new way to achieve the composite film with high electrical conductivity and without significant hindrance to electrolyte transport, resulting in a lower charge-transfer resistance and Nernst diffusion impedance.
Acknowledgements
The authors acknowledge financial support from the National Science Council, Taiwan (Project No: NSC101-2221-E-151-055-MY2).
Notes and references
1 B. O’Regan and M. Gratzel, Nature, 1991, 353, 737–740. 2 T. N. Murakami and M. Gra¨tzel, Inorg. Chim. Acta, 2008, 361,
572–580.
3 L. J. Brennan, M. T. Byrne, M. Bari and Y. K. Gun’ko, Adv. Energy Mater., 2011, 1, 472–485.
4 B. Lee, D. B. Buchholz and R. P. H. Chang, Energy Environ. Sci., 2012, 5, 6941–6952.
5 M. Wu, X. Lin, T. Wang, J. Qiu and T. Ma, Energy Environ. Sci., 2011, 4, 2308–2315.
6 L. Kavan, J. H. Yum and M. Gra¨tzel, ACS Nano, 2010, 5, 165–172.
7 L. Kavan, J.-H. Yum, M. K. Nazeeruddin and M. Gra¨tzel, ACS Nano, 2011, 5, 9171–9178.
8 Y. Y. Dou, G. R. Li, J. Song and X. P. Gao, Phys. Chem. Chem. Phys., 2012, 14, 1339–1342.
9 A. Kaniyoor and S. Ramaprabhu, J. Appl. Phys., 2011, 109, 124308. 10 K. Imoto, K. Takahashi, T. Yamaguchi, T. Komura, J.-i. Nakamura and K. Murata, Sol. Energy Mater. Sol. Cells, 2003, 79, 459–469.
11 H. Kim, H. Choi, S. Hwang, Y. Kim and M. Jeon, Nanoscale Res. Lett., 2012, 7, 1–4.
12 G. Zhu, L. Pan, T. Lu, T. Xu and Z. Sun, J. Mater. Chem., 2011, 21, 14869–14875.
13 T. Umeyama and H. Imahori, Energy Environ. Sci., 2008, 1, 120–133.
14 G. Zhu, L. Pan, T. Lu, X. Liu, T. Lv, T. Xu and Z. Sun, Electrochim. Acta, 2011, 56, 10288–10291.
15 H. Choi, H. Kim, S. Hwang, Y. Han and M. Jeon, J. Mater. Chem., 2011, 21, 7548–7551.
16 M. D. Stoller, S. Park, Y. Zhu, J. An and R. S. Ruoff, Nano Lett., 2008, 8, 3498–3502.
17 J. D. Roy-Mayhew, D. J. Bozym, C. Punckt and I. A. Aksay, ACS Nano, 2010, 4, 6203–6211.
18 V. Josef, J. M. Attila, L. Dan, O. David, W. Gordon, B. Ray and Z. Anvar, Nanotechnology, 2012, 23, 085201.
19 H. Choi, H. Kim, S. Hwang, W. Choi and M. Jeon, Sol. Energy Mater. Sol. Cells, 2011, 95, 323–325.
20 Y. Zhang, K. Fugane, T. Mori, L. Niu and J. Ye, J. Mater. Chem., 2012, 22, 6575–6580.
21 Y. Zhang, T. Mori, L. Niu and J. Ye, Energy Environ. Sci., 2011, 4, 4517–4521.
Fig. 6 Nyquist plots of the symmetric cells with two identical GN-embedded AC cathodes. Equivalent circuit used to fit the experimental data is shown in the inset.
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Published on 13 December 2012. Downloaded by National Kaohsiung University of Applied Sciences on 16/10/2014 08:53:56.
This journal is cthe Owner Societies 2013 Phys. Chem. Chem. Phys., 2013, 15, 1782--1787 1787 22 J. J. Yoo, K. Balakrishnan, J. Huang, V. Meunier,
B. G. Sumpter, A. Srivastava, M. Conway, A. L. Mohana Reddy, J. Yu, R. Vajtai and P. M. Ajayan, Nano Lett., 2011, 11, 1423–1427.
23 X. Q. Chen, T. Saito, H. Yamada and K. Matsushige, Appl. Phys. Lett., 2001, 78, 3714–3716.
24 P. V. Kamat, K. G. Thomas, S. Barazzouk, G. Girishkumar, K. Vinodgopal and D. Meisel, J. Am. Chem. Soc., 2004, 126, 10757–10762.
25 Y. Kunitoshi, A. Seiji and N. Yoshikazu, J. Phys. D: Appl. Phys., 1998, 31, L34.
26 M. Senthil Kumar, S. H. Lee, T. Y. Kim, T. H. Kim, S. M. Song, J. W. Yang, K. S. Nahm and E. K. Suh, Solid-State Electron., 2003, 47, 2075–2080.
27 F. Hao, P. Dong, J. Zhang, Y. Zhang, P. E. Loya, R. H. Hauge, J. Li, J. Lou and H. Lin, Sci. Rep., 2012, 2, 368.
28 K. S. Lee, W. J. Lee, N.-G. Park, S. O. Kim and J. H. Park, Chem. Commun., 2011, 47, 4264–4266.
29 S. Roy, R. Bajpai, A. K. Jena, P. Kumar, N. kulshrestha and D. S. Misra, Energy Environ. Sci., 2012, 5, 7001–7006. 30 M.-S. Wu, Y.-P. Lin, C.-H. Lin and J.-T. Lee, J. Mater. Chem.,
2012, 22, 2442–2448.
31 M.-S. Wu, C.-H. Tsai and T.-C. Wei, Chem. Commun., 2011, 47, 2871–2873.
32 C.-S. Chou, C.-M. Hsiung, C.-P. Wang, R.-Y. Yang and M.-G. Guo, Int. J. Photoenergy, 2010, 2010, 902385.
33 H. Wang and Y. H. Hu, Energy Environ. Sci., 2012, 5, 8182–8188.
34 J. D. Roy-Mayhew, G. Boschloo, A. Hagfeldt and I. A. Aksay, ACS Appl. Mater. Interfaces, 2012, 4, 2794–2800.
35 D. W. Zhang, X. D. Li, H. B. Li, S. Chen, Z. Sun, X. J. Yin and S. M. Huang, Carbon, 2011, 49, 5382–5388.
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