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Low-cost counter electrodes based on nitrogen-doped porous carbon nanorods for dye-sensitized solar cells
Materials Science in Semiconductor Processing 63 (2017) 190–195
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Low-cost counter electrodes based on nitrogen-doped porous carbon nanorods for dye-sensitized solar cells
MARK
⁎
Guiqiang Wang , 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
A BS T RAC T
Keywords: Nitrogen doping Porous carbon nanorods Counter electrode Dye-sensitized solar cells
Nitrogen-doped porous carbon nanorods (N-PCNRs) with high accessible surface area are prepared by carbonization of polyaniline (PANI) nanorods and subsequent chemical activation, and explored as the counter electrode in dye-sensitized solar cells (DSCs). SEM and TEM images demonstrate that the nanorod morphology of PANI is preserved after pre-carbonization and chemical activation treatment. The unique combination of the porosity with high accessible surface area, nitrogen doping, and nanorod structure endows the N-PCNR electrode with an excellent electrocatalytic activity for the I3- reduction, which is illuminated by the electrochemical measurements. Under simulated AM 1.5 illumination (100 mW cm−2), the DSC based on NPCNR counter electrode achieves a conversion efficiency of 7.01%, which is nearly close to that of the cell based on Pt counter electrode (7.25%).
1. Introduction Since reported by Grätzel's group in 1991, dye-sensitized solar cells (DSCs) have attracted wide-spread attention and been considered as one of the most promising alternative to Si solar cells due to their low cost, simple fabrication process, relatively high conversion efficiency, and good stability [1–5]. In general, a DSC is consisted of a dyesensitized TiO2 photoanode, an electrolyte containing I-/I3- redox couple, and a catalytic counter electrode. The counter electrode plays an important role in determining the photovoltaic performance of DSCs in term of collecting the electrons from external circuit and catalyzing the reduction of I3- at the electrolyte/counter electrode interface. Therefore, the ideal counter electrode should possess high electrical conductivity and good electrocatalytic activity for the reduction of I3- [6]. So far, the most comment materials used for the counter electrode of DSCs is Pt due to its excellent conductivity and superior electrocatalytic performance [7–10]. However, the scare reserve in nature and the associated high price hinder its large-scale application in DSCs. Therefore, some cost-effective alternatives to Pt in the counter electrode of DSCs have been extensively studied, such as polyaniline [11–15], carbon-based materials [16], and metal sulfide [17]. Recently, many efforts have been made to replace Pt using carbonaceous materials such as carbon nanotubes [18,19], porous carbon [20–22], graphene [23–25], carbon fibers [26], and carbon black [27] in the counter electrode of DSCs owing to their low cost, good electrocatalytic performance, and high conductivity. However, the
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current performances of these carbonaceous materials are still lower than that of Pt and should be further improved for practical application in DSCs. The electrocatalytic performance of carbonaceous materials relies heavily on their morphology and microstructure, which greatly influences the effective electrocatalytic surface area and the electrolyte diffusion within the catalyst layer. It has been reported recently that the nitrogen-doping and porous structure of carbonaceous materials can enhance their conductivity, surface hydrophilicity, and electrocatalytic activity, respectively, and then are of great benefit to the counter electrode application in DSCs [28–30]. Xue et al. [31] prepared a nitrogen-doped 3D graphene foam and demonstrated its application as an efficient electrocatalyst to replace Pt in the counter electrode of DCSs, leading to a conversion efficiency up to 7.07%. Li et al. [32] reported the synthesis of nitrogen-doped macro/mesoporous carbon with 3D interconnected open-pore structure as the efficient counter electrode material. The nitrogen-doped macro/mesoporous carbon showed a high conversion efficiency of 7.27%, which was even slightly better than that of Pt electrode. These results indicate that nitrogendoped carbon nanomaterial with suitable porous structure is an ideal low-cost counter electrode material in DSCs. In this work, nitrogen-doped porous carbon nanorods (N-PCNRs) with high accessible surface area were prepared by carbonization of polyaniline (PANI) nanorods and subsequent chemical activation, and employed as the Pt-free counter electrode in DSCs. The one-dimensional nanorod structure provides direct pathway for the electron transport and then enhances the electron transport within the N-
Corresponding author. E-mail address: [email protected] (G. Wang).
http://dx.doi.org/10.1016/j.mssp.2017.02.018 Received 29 November 2016; Received in revised form 8 February 2017; Accepted 13 February 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
Materials Science in Semiconductor Processing 63 (2017) 190–195
G. Wang et al.
Fig. 1. Process used for the preparation of nitrogen-doped porous carbon nanorods (N-PCNRs).
using N2 sorption at 77 K. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCAlab 220I-XL spectrometer with Al Kα radiation. Electrochemical measurements were carried out on Solartron1255/1287 system. Electrochemical impedance spectroscopy (EIS) was conducted with symmetric thin-layer cells in the frequency range of 0.1–105 Hz at 0 V bias and 10 mV amplitude. Cyclic voltammetry (CV) measurements were performed in a three-electrode system using a Pt wire as counter electrode and Ag/AgCl electrode as reference electrode in acetonitrile solution containing 0.01 M LiI, 0.001 M I2, and 0.1 M LiClO4. The scan rate is 50 mV s−1. The photocurrentvoltage curves of DSCs were recorded by a Keithley 2400 source meter under a simulated AM 1.5 illumination at 100 mW cm−2.
PCNR layer. The nitrogen-doping as well as the porosity with high accessible surface area provides numerous electrocatalytic active sites for I3- reduction. This combination of characteristics means that NPCNR electrode possesses an excellent electrocatalytic activity for the reduction of I3-. The assembled DSC with N-PCNR counter electrode exhibits a high conversion efficiency of 7.01%, which is very close to that of the cell with Pt counter electrode. 2. Experimental 2.1. Preparation of N-PCNR electrodes PANI nanorods were firstly prepared by polymerization of aniline in aqueous solution using ammonium persulfate (APS) as the initiator. Briefly, 10 g of aniline and 7 g of citric acid were mixed in 500 mL of distilled water. 20 g of ammonium persulfate was dissolved in 150 mL of distilled water. Then, ammonium persulfate solution was added into the mixture solution of aniline and citric acid with vigorous stirring. The resulting solution was left standing at about 3 °C for 20 h. The obtained dark green sample was filtered and washed with distilled water, and then dried under vacuum at 50 °C. The as-prepared PANI nanorods were pyrolyzed at 600 °C under nitrogen atmosphere for 3 h to obtained nitrogen-doped carbon nanorods (N-CNRs). N-PCNRs were prepared by activating N-CNRs. In brief, 3 g of N-CNRs were mixed with 9 g of KOH in aqueous solution by stirring and subsequent sonication, followed by evaporation at 90 °C. Then, above mixture was transferred into a tube furnace for activation at 800 °C under nitrogen atmosphere with duration of 2 h. The obtained product was rinsed with 1 M HCl solution and distilled water until the pH value of effluent reached about 7, and then dried at 80 °C in air. 150 mg of as-prepared N-PCNRs were ground with 10 mL isopropanol and 10 μL tetrabutyl titanate in a mortar to obtain N-PCNRs paste. The N-PCNR electrodes were fabricated by spraying the N-PCNR paste onto pre-cleaned fluorine-doped tin oxide (FTO) glass, followed by sintering at 400 °C for 15 min. N-CNR electrodes were also prepared through the same procedure. The thickness of the carbon catalyst layer was about 9 µm, which was controlled by the spray time.
3. Results and discussion N-PCNRs were prepared by carbonization of PANI nanorods at 600 °C and subsequent chemical activation at 800 °C (see Fig. 1). PANI is an ideal precursor for preparing nitrogen-doped carbonaceous materials due to its high content of nitrogen and uniformly distributed nitrogen atoms in its framework. In addition, PANI with different morphologies, such as nanofiber, nanotube, and nanosphere, can be easily prepared by controlling the synthesis conditions [33–35], and these original morphologies can be preserved even after high-temperature treatment. Accordingly, nitrogen-doped carbon nanorods (NCNRs) were prepared from PANI nanorods through high-temperature carbonization. By activating N-CNRs, N-PCNRs were prepared. Fig. 2 displays SEM and TEM images of original PANI nanorods, N-CNRs, and N-PCNRs. As shown in Fig. 2a and b, the PANI exhibits a nanorodlike morphology with rough surface. The diameters of PANI nanorods are in the range of 150–200 nm and the lengths range from 0.5 to 3 µm. After carbonization and chemical activation treatment, the nanorod morphology is preserved except that the aspect ratio of nanorod decrease slightly as shown in Fig. 2(c–f). The high-resolution TEM image shown in Fig. 2g demonstrates that N-CNRs possess a smooth surface. During the chemical activation, KOH corroded carbon nanorods at high temperature to develop new pores on the surface of carbon nanorods. Therefore, the as-prepared N-PCNRs exhibit the well-developed pore structure as shown in Fig. 2h. The pore structure and surface area of N-CNRs and N-PCNRs were determined by N2 sorption measurements. Fig. 3 shows the N2 adsorption-desorption isotherms and pore-size distribution curves of N-CNRs and N-PCNRs. As displayed in Fig. 3a, the isotherms of NCNRs are of type I, and the adsorption branch nearly overlap with desorption branch over a wide range of relative pressure. In addition, the nitrogen adsorption capacity of N-CNRs is small. These results demonstrate that N-CNRs are poorly porous. The Brunauer-EmmettTeller (BET) surface area and pore volume of N-CNRs are 116.3 m2 g−1 and 0.10 cm3 g−1, respectively. N-PCNRs presents a combined type I/ IV isotherms with an obvious hysteresis loop at the relative pressure 0.4–0.8, demonstrating a characteristic of both the mesoporosity and the microporosity. The pore-size distribution curve shown in Fig. 3b further demonstrates that N-PCNRs have both micropores and welldeveloped mesopore. The significant increase in the nitrogen uptake in the entire pressure range for N-PCNRs is a result of the increase in porosity created by chemical activation. The BET surface area and pore volume of N-PCNRs are 2580.5 m2 g−1 and 1.36 cm3 g−1, respectively,
2.2. Fabrication of DSCs TiO2 electrodes were prepared by coating 9 µm-thick TiO2 film on FTO glass using doctor-blade method and then sintering at 450 °C for 30 min. The TiO2 electrodes sensitized with N719 dye were used as the photoanodes. A solution containing 0.5 M 1-hexyl-3-methylimidazolium iodide, 0.2 M LiI, 0.05 M I2, and 0.4 M 4-tert-butylpyridine in 3methoxypropionitrile was used as the electrolyte. The DSC was fabricated by clapping a photoanode, an electrolyte, and a counter electrode into a sandwich structure. 2.3. Measurements and characterization The morphology of samples was observed by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM2010). The surface area and pore structure of the samples were measured by a Micromeritics ASAP 2020 instrument 191
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Fig. 2. SEM and TEM images for PANI nanorods (a and b), N-CNRs (c and d), and N-PCNRs (e and f); the high-resolution TEM images of N-CNRs (g) and N-PCNRs (h).
The cross-section SEM image of N-PCNR electrode in Fig. 5 shows that the carbon catalyst layer has a thickness of about 9 µm. In order to eliminate the effect of TiO2 photoanode, the symmetric thin-layer cells consisting of two identical electrodes were used for EIS measurements. Fig. 6a displays Nyquist plots of the symmetric cells based on N-CNR and N-PCNR. For comparison, the Nyquist plot for Pt electrode was also exhibited in Fig. 6a. Pt electrodes were prepared by thermal decomposition of H2PtCl6 on FTO glass at 390 °C for 15 min. From Fig. 6a, it can be seen that all Nyquist plots show two semicircles in the high and low frequency regions, corresponding to the charge-transfer process at the electrode/electrolyte interface and Nernst diffusion process in the electrolyte, respectively. The corresponding equivalent circuit is shown in the inset in Fig. 6a, in which Rs is the ohmic series resistance, Rct is the charge-transfer resistance, CPE is the constant phase element, ZN is the Nernst diffusion impedance. The EIS parameters are obtained by fitting Nyquist plots and listed in Table 2. Rct is a significant parameter to evaluate the electrocatalytic activity of the electrode for the reduction of I3-. The smaller Rct value is, the higher electrocatalytic activity could be. As shown in Table 2, Rct for Pt, N-CNR, and N-PCNR electrodes are 1.6, 8.9 and 2.2 Ω cm2, respectively. Clearly, the electrocatalytic activity of N-PCNR electrode is much higher than that of N-CNR electrode, and nearly close to that of Pt electrode. This greatly enhanced electrocatalytic activity of NPCNR electrode can be attributed to the unique porous nanorod structure. As shown in Fig. 6b, the nanorod structure can provide a direct pathway for fast electron transport, and the porosity with high
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which are much higher than those of N-CNRs. XPS measurements were conducted to determine the element compositions and states of N-CNRs and N-PCNRs. Fig. 4 exhibits XPS survey spectra and N 1 s peaks for N-CNRs and N-PCNRs. As expected, XPS survey spectra of N-CNRs and N-PCNRs (Fig. 4a) show three peaks at binding energy of 288.1, 400.1, and 533.2 eV, corresponding to C 1 s, N 1 s, and O 1 s, respectively. The elemental content calculated from XPS survey spectra are summarized in Table 1. The nitrogen and oxygen contents of N-CNRs are 8.67 at% and 9.77 at%, respectively, while the nitrogen and oxygen contents of N-PCNRs are 2.03 at% and 14.62 at%, respectively. Clearly, the chemical activation leads to a decrease in nitrogen content and an increase in oxygen content. The N 1 s peak for N-CNRs can be deconvoluted into three peaks centered at binding energy of 398.6, 400.1, and 401.2 eV (Fig. 4b), assigning to pyridinic nitrogen (N-6), pyrrolic nitrogen (N5), and quaternary nitrogen (N-Q), respectively [36]. The deconvolution of N 1 s peak for N-PCNRs (Fig. 4c) displays three peaks at the same binding energies, however, with different relative contributions. From Table 1, it can be seen that after chemical activation, the relative content of N-6 increase greatly, while the relative content of N-Q decrease considerably. This indicates that N-6 and N-5 are predominant in N-PCNRs. N-6 and N-5 locate at the edge of the graphene layer, and then can provide the electrochemically active sites to improve the electrocatalytic performance of carbon materials [37]. EIS measurements were employed to characterize the electrocatalytic activity of N-CNR and N-PCNR electrodes for the I3- reduction.
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Fig. 3. N2 adsorption-desorption isotherms (a) and pore-size distribution curves (b) of N-CNRs and N-PCNRs.
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Fig. 4. (a) XPS scanning spectra of N-CNRs and N-PCNRs; deconvoluted N 1 s peaks of N-CNRs (b) and N-PCNRs (c). Table 1 Oxygen and nitrogen content in N-CNRs and N-PCNRs determined by XPS. Sample
N-CNRs N-PCNRs
C content (at%)
O content (at%)
81.56 83.35
9.77 14.62
Table 2 EIS parameters of the symmetric thin-layer cells with Pt, N-CNR, and N-PCNR electrodes and photovoltaic parameters of DSCs with Pt, N-CNR, and N-PCNR counter electrodes.
N content (at%) Total
N-6
N-5
N-Q
8.67 2.03
2.89 0.96
3.73 0.81
2.05 0.26
Electrode
Rct (Ω cm2)
Rs (Ω cm2)
CPE (μF cm−2)
Voc (V)
Jsc (mA cm−2)
FF
η (%)
N-CNR N-PCNR Pt
8.9 2.2 1.6
16.8 17.1 16.5
23.6 41.2 5.8
0.678 0.702 0.698
15.01 15.85 15.98
0.58 0.63 0.65
5.91 7.01 7.25
doping, and the nanorod structure endows the N-PCNR electrode with an excellent electrocatalytic activity. The Rs values of Pt, N-CNR, and N-PCNR electrode do not vary significantly, demonstrating a good binding of carbon nanorods with FTO substrate and a fast electron transport through carbon nanorods network. As expect, N-PCNR electrode exhibits the highest CPE value among the three electrodes due to the largest surface area. Cyclic voltammetry (CV) measurements were also carried out using a three-electrode system to examine the electrocatalytic activity of the electrodes. Fig. 7 shows CV curves for Pt, N-CNR, and N-PCNR electrodes. Two pair redox peaks can be observed in CV curves. The pair at more positive potentials is assigned to the redox reaction of I3-/ I2 couple (3I2 +2e ↔ 2I3-), and the pair at more negative potentials is attributed to the redox reaction of I3-/I- (I3- +2e ↔ 2I-). The function of the counter electrode in a DSC is to catalyze the reduction of I3- to I-. Therefore, the characteristics of the negative pair are usually employed to evaluate the electrocatalytic activity of the counter electrode. From Fig. 7, it can be observed that the peak current densities of N-PCNR electrode are higher than those of the N-CNR electrode and comparable
Fig. 5. The cross-section SEM image of N-PCNR electrode.
accessible surfaces area as well as the nitrogen doping of N-PCNRs provides more electrocatalytic active sites. Therefore, it is the combination of the porosity with high accessible surface area, the nitrogen 10 2Rct
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Z' (ohm cm ) Fig. 6. (a) Nyquist plots of the cell based on Pt, N-CNR, and N-PCNR electrodes; (b) diagram of the electrochemical reaction process in N-CNRs and N-PCNRs. The inset in (a) is the equivalent circuit.
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alternative to Pt in the counter electrode of DSCs. 2
4. Conclusions
Pt N-PCNR N-CNR
N-PCNRs with high accessible surface area were prepared by carbonization of PANI nanorods and subsequent chemical activation and successfully investigated as the efficient counter electrode for DSCs. The as-prepared N-PCNR electrode exhibited an excellent electrocatalytic activity for I3- reduction and showed a very close photovoltaic performance to Pt electrode in terms of Voc, Jsc, FF, and η, which was attributed to the ideal combination of the porosity with high accessible surface area, nitrogen doping, and nanorod structure. This work could provide a promising strategy for designing a low-cost and high-performance counter electrode material in DSCs.
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Acknowledgement
Fig. 7. CV curves of Pt, N-CNR, and N-PCNR electrodes obtained by a three-electrode system at scan rate of 50 mV s−1.
This work was supported by National Natural Science Foundation of China (grant number 21273137) and Liaoning Science & Technology Project (grant number 201601011)
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References
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[1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 film, Nature 353 (1991) 737–739. [2] S. Mathew, A. Yella, P. Gao, R. Humph ry-Baker, B. Curchod, N. Ashari-Astani, I. Tavernrlli, U. Rothlisberger, M.K. Nazeeruddin, M. Grätzel, Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers, Nat. Chem. 6 (2014) 242–247. [3] A. Yella, H. Lee, H. Tsao, C. Yi, A. Chandiran, M.K. Nazeeruddin, E.E. Diau, C. Yeh, S.M. Zakeeruddin, M. Grätzel, Porphyrin-sensitized soalr cells with cobalt (III/II)based redox electrolyte exceeding 12 percent efficiency, Science 334 (2011) 629–634. [4] J. Wu, Z. Lan, J. Lin, M. Huang, Y. Huang, L. Fan, G. Luo, Electrolyte in dyesensitized solar cells, Chem. Rev. 115 (2015) 2136–2173. [5] J. Halme, P. Vahermaa, K. Miettunen, P. Lund, Device physics of dye solar cells, Adv. Mater. 22 (2010) E210–E234. [6] N. Papageorgiou, Counter-electrode function in nanocrystalline photoelectrochemical cell configurations, Coord. Chem. Rev. 248 (2004) 1421–1446. [7] H. Elbohy, A. Aboagye, S. Sigdel, Q. Wang, M.H. Sayyad, L. Zhang, Q. Qiao, Graphene-embedded carbon nanofibers decorated with Pt nanoneedles for highefficiency dye-sensitized solar cells, J. Mater. Chem. A 3 (2015) 17721–17727. [8] V. Dao, H. Choi, Pt nanourchins as efficient and robust counter electrode materials for dye-sensitized solar cells, ACS Appl. Mater. Interfaces 8 (2015) 1004–1010. [9] M. Yeh, S. Chang, L. Lin, H. Chou, R. Vittal, B. Hwang, K. Ho, Size effect of Pt nanoparticles on the electrocatalytic ability of the counter electrode in dyesensitized solar cells, Nano Energy 17 (2015) 241–253. [10] K. Lee, L. Lin, C. Chen, V. Suryanarayanan, C. Wu, Preparation of high transmittance platinum counter electrode at an ambient temperature for flexible dye-sensitized solar cells, Electrochim. Acta 135 (2014) 578–584. [11] W. Hou, Y. Xiao, G. Han, D. Fu, R. Wu, Serrated, flexible and ultrathin polyaniline nanoribbons: an efficient counter electrode for dye-sensitized solar cell, J. Power Sources 322 (2016) 155–162. [12] Y. Xiao, G. Han, Y. Li, M. Li, Y. Chang, High performance of Pt-free dye-sensitized solar cells based on two-step electropolymerized polyaniline counter electrodes, J. Mater. Chem. A 2 (2014) 3452–3460. [13] Y. Xiao, W. Wang, S. Chou, T. Lin, J. Lin, In situ electropolymerization of polyaniline/cobalt sulfide decorated carbon nanotube composite catalyst towards triiodide reduction in dye-sensitized solar cells, J. Power Sources 266 (2014) 448–455. [14] Y. Xiao, J. Lin, J. Wu, S. Tai, G. Yue, T. Lin, Dye-sensitized solar cells with highperformance polyaniline/multi-wall carbon nanotube counter electrode electropolymerized by a pulse potentiostatic technique, J. Power Sources 233 (2013) 320–325. [15] Y. Xiao, J. Lin, W. Wang, S. Tai, G. Yue, J. Wu, Enhanced performance of low-cost dye-sensitized solar cells with pulse-electroplymerized polyaniline counter electrode, Electrochim. Acta 90 (2013) 468–474. [16] S. Yun, A. Hagfeldt, T. Ma, Pt-free counter electrode for dye-sensitized solar cells with high efficiency, Adv. Mater. 26 (2014) 6210–6237. [17] M. Wu, T. Ma, Platinum-free catalysts as counter electrodes in dye-sensitized solar cells, ChemSusChem 5 (2012) 1343–1357. [18] G. Wang, J. Zhang, S. Kuang, S. Zhuo, Nitrogen-doped porous carbon prepared by a facile soft-templating process as low-cost counter electrode for high-performance dye-sensitized solar cells, Mater. Sci. Semicond. Process. 38 (2015) 234–239. [19] Y. Jo, J. Cheon, J. Yu, H. Jeong, C. Han, Y. Jun, S. Joo, Highly interconnected ordered mesoporous carbon-carbon nanotube nanocomposite: pt-free high efficient durable counter electrode for dye-sensitized solar cells, Chem. Commun. 48 (2012) 8057–8059. [20] B. Zhao, H. Huang, P. Jiang, H. Zhao, X. Huang, P. Shen, D. Wu, R. Fu, S. Tan,
12 8
Pt N-PCNR N-CNR PANI
4 0
0.0
0.2
0.4
0.6
0.8
Voltage (V) Fig. 8. Photocurrent density-voltage curves of the DSCs with Pt, N-CNR, N-PCNR, and PANI electrodes measured under simulated AM 1.5 illumination.
to those of the Pt electrode. This indicates that the electrocatalytic activity of N-PCNR electrode is higher than that of N-CNR electrode and comparable to that of Pt electrode, displaying that N-PCNRs can be used as an efficient counter electrode material for application in DSCs. The current density-voltage curves of DSCs fabricated with Pt, NCNR, and N-PCNR counter electrodes are displayed in Fig. 8. The corresponding photovoltaic parameters, such as the open-circuit voltage (Voc), the short-circuit current density (Jsc), the fill fact (FF), and the conversion efficiency (η), are summarized in Table 2. The DSC fabricated with N-CNR counter electrode exhibits a Voc of 0.678 V, a Jsc of 15.01 mA cm−2, a FF of 0.58, and a conversion efficiency of 5.91%. When N-PCNR is used as the counter electrode, the photovoltaic parameters of the corresponding DSC are Voc=0.702 V, Jsc=15.85 mA cm−2, FF=0.63, and η=7.01%. Obviously, the photovoltaic performance of the DSC with N-PCNR counter electrode is much better than that of the cell with N-CNR counter electrode, and very close to that of the device with Pt counter electrode. This enhanced photovoltaic performance can be assigned to the excellent electrocatalytic activity of N-PCNR electrode for I3- reduction derived from combination of the porosity with high accessible surface area, nitrogen doping, and nanorod structure. Previous works indicated that PANI can be employed as the Pt-free counter electrode in DSCs [11,17]. Therefore, we also fabricated the PANI nanorod electrode-based DSC and measured its photovoltaic performance. As shown in Fig. 8, the conversion efficiency of DSC with N-PCNR counter electrode is much higher than that of the cell with PANI counter electrode (5.11%). These results demonstrate that N-PCNRs could be a promising cost-efficient
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[21]
[22]
[23] [24]
[25] [26]
[27]
[28]
[29]
Flexible counter electrodes based on mesoporous carbon aerogel for high-performance dye-sensitized solar cells, J. Phys. Chem. C 115 (2011) 22615–22621. G. Wang, C. Huang, W. Xing, S. Zhuo, Micro-meso hierarchical porous carbon as low-cost counter electrode for dye-sensitize solar cells, Electrochim. Acta 56 (2011) 5459–5463. P. Srinivasu, A. Islam, S. Singh, L. Han, M.L. Kantam, S.K. Bhargava, Highly efficient nanoporous graphitic carbon with tunable textural properties for dyesensitized solar cells, J. Mater. Chem. 22 (2012) 20866–20869. J.D. Roy-Mayhew, I.A. Aksay, Graphene materials and their use in dye-sensitized solar cells, Chem. Rev. 114 (2014) 6323–6348. W. Yang, X. Xu, Y. Gao, Z. Li, C. Li, W. Wang, Y. Chen, G. Ning, L. Zhang, J. Kong, Y. Li, High-surface-area nanomesh graphene with enriched edge sites as efficient metal-free cathodes for dye-sensitized solar cells, Nanoscale 8 (2016) 13059–13068. H. Wang, Y. Hu, Graphene as counter electrode materials for dye-sensitized solar cells, Energy Environ. Sci. 5 (2012) 8182–8188. P. Joshi, L. Zhang, Q. Chen, D. Galipeau, H. Fong, Q. Qiao, Electrospun carbon nanofibers as low-cost counter eelctrode for dye-sensitized solar cells, ACS Appl. Mater. Interfaces 12 (2010) 3572–3577. T.N. Murakami, S. Ito, Q. Wang, M.K. Nazeeruddin, P. Liska, R. Humphery-Baker, P. Comte, M. Grätzel, Highly efficient dye-sensitize solar cells based on carbon black counter electrodes, J. Electrochem. Soc. 153 (2006) A2255–A2261. S. Hou, X. Cai, H. Wu, X. Yu, M. Peng, K. Yan, D. Zou, Nitrogen-doped graphene for dye-sensitized solar cells and the role nitrogen states in triiodide reduction, Energy Environ. Sci. 6 (2013) 3356–3362. L. Song, Q. Luo, F. Zhao, Y. Li, H. Lin, L. Qu, Z. Zhang, Durally functional, N-doped
[30]
[31]
[32]
[33] [34]
[35]
[36] [37]
195
porous graphene foams as counter electrodes for dye-sensitized solar cells, Phys. Chem. Chem. Phys. 16 (2014) 21820–21826. M. Chen, L. Shao, Y. Liu, T. Ren, Z. Yuan, Nitrogen-doped ordered cubic mesoprous carbons as counter electrodes for dye-sensitized solar cells, J. Power Sources 283 (2015) 305–313. Y. Xue, J. Liu, H. Chen, R. Wang, D. Li, J. Qu, L. Dai, Nitrogen-doped graphene foams as metal-free counter electrodes in high-performance dye-sensitized solar cells, Angew. Chem. 124 (2012) 12290–12293. L. Li, C. Wang, J. Liao, A. Manthiram, Dual-template synthesis of N-doped macro/ mesoporous carbon with an open-pore structure as metal-free catalysts for dyesensitized solar cells, J. Power Sources 300 (2015) 254–260. H. Guan, L. Fan, H. Zhang, X. Qu, Polyaniline nanofibers obtained by interfacial polymerization for high-rate supercapacitor, Electrochim. Acta 56 (2010) 964–968. Z. Huang, E. Liu, H. Shen, X. Xiang, Y. Tian, C. Xiao, Z. Mao, Preparation of polyaniline nanotubes by a template-free self-assembly method, Mater. Lett. 65 (2011) 2015–2018. Z. Lei, M. Zhao, L. Dang, A. Lo, N. Yu, S. Liu, Structure evolution and electrocatalytic application of nitrogen-doped carbon shell synthesized by pyrolysis of near-monodisperse polyaniline nanospheres, J. Mater. Chem. 19 (2009) 5985–5995. D. Hulicova, J. Yamashita, Y. Soneda, H. Hatori, M. Kodama, Supercapacitors prepared from melamine-based carbon, Chem. Mater. 17 (2005) 1241–1247. L. Chen, X. Zhang, H. Liang, M. Kong, Q. Guan, P. Chen, Z. Wu, S. Yu, Synthesis of nitrogen-doped porous carbon nanofibers as an efficient electrode material for supercapacitors, ACS Nano 6 (2012) 7092–7102.
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Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp
Low-cost counter electrodes based on nitrogen-doped porous carbon nanorods for dye-sensitized solar cells
MARK
⁎
Guiqiang Wang , 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
A BS T RAC T
Keywords: Nitrogen doping Porous carbon nanorods Counter electrode Dye-sensitized solar cells
Nitrogen-doped porous carbon nanorods (N-PCNRs) with high accessible surface area are prepared by carbonization of polyaniline (PANI) nanorods and subsequent chemical activation, and explored as the counter electrode in dye-sensitized solar cells (DSCs). SEM and TEM images demonstrate that the nanorod morphology of PANI is preserved after pre-carbonization and chemical activation treatment. The unique combination of the porosity with high accessible surface area, nitrogen doping, and nanorod structure endows the N-PCNR electrode with an excellent electrocatalytic activity for the I3- reduction, which is illuminated by the electrochemical measurements. Under simulated AM 1.5 illumination (100 mW cm−2), the DSC based on NPCNR counter electrode achieves a conversion efficiency of 7.01%, which is nearly close to that of the cell based on Pt counter electrode (7.25%).
1. Introduction Since reported by Grätzel's group in 1991, dye-sensitized solar cells (DSCs) have attracted wide-spread attention and been considered as one of the most promising alternative to Si solar cells due to their low cost, simple fabrication process, relatively high conversion efficiency, and good stability [1–5]. In general, a DSC is consisted of a dyesensitized TiO2 photoanode, an electrolyte containing I-/I3- redox couple, and a catalytic counter electrode. The counter electrode plays an important role in determining the photovoltaic performance of DSCs in term of collecting the electrons from external circuit and catalyzing the reduction of I3- at the electrolyte/counter electrode interface. Therefore, the ideal counter electrode should possess high electrical conductivity and good electrocatalytic activity for the reduction of I3- [6]. So far, the most comment materials used for the counter electrode of DSCs is Pt due to its excellent conductivity and superior electrocatalytic performance [7–10]. However, the scare reserve in nature and the associated high price hinder its large-scale application in DSCs. Therefore, some cost-effective alternatives to Pt in the counter electrode of DSCs have been extensively studied, such as polyaniline [11–15], carbon-based materials [16], and metal sulfide [17]. Recently, many efforts have been made to replace Pt using carbonaceous materials such as carbon nanotubes [18,19], porous carbon [20–22], graphene [23–25], carbon fibers [26], and carbon black [27] in the counter electrode of DSCs owing to their low cost, good electrocatalytic performance, and high conductivity. However, the
⁎
current performances of these carbonaceous materials are still lower than that of Pt and should be further improved for practical application in DSCs. The electrocatalytic performance of carbonaceous materials relies heavily on their morphology and microstructure, which greatly influences the effective electrocatalytic surface area and the electrolyte diffusion within the catalyst layer. It has been reported recently that the nitrogen-doping and porous structure of carbonaceous materials can enhance their conductivity, surface hydrophilicity, and electrocatalytic activity, respectively, and then are of great benefit to the counter electrode application in DSCs [28–30]. Xue et al. [31] prepared a nitrogen-doped 3D graphene foam and demonstrated its application as an efficient electrocatalyst to replace Pt in the counter electrode of DCSs, leading to a conversion efficiency up to 7.07%. Li et al. [32] reported the synthesis of nitrogen-doped macro/mesoporous carbon with 3D interconnected open-pore structure as the efficient counter electrode material. The nitrogen-doped macro/mesoporous carbon showed a high conversion efficiency of 7.27%, which was even slightly better than that of Pt electrode. These results indicate that nitrogendoped carbon nanomaterial with suitable porous structure is an ideal low-cost counter electrode material in DSCs. In this work, nitrogen-doped porous carbon nanorods (N-PCNRs) with high accessible surface area were prepared by carbonization of polyaniline (PANI) nanorods and subsequent chemical activation, and employed as the Pt-free counter electrode in DSCs. The one-dimensional nanorod structure provides direct pathway for the electron transport and then enhances the electron transport within the N-
Corresponding author. E-mail address: [email protected] (G. Wang).
http://dx.doi.org/10.1016/j.mssp.2017.02.018 Received 29 November 2016; Received in revised form 8 February 2017; Accepted 13 February 2017 1369-8001/ © 2017 Elsevier Ltd. All rights reserved.
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Fig. 1. Process used for the preparation of nitrogen-doped porous carbon nanorods (N-PCNRs).
using N2 sorption at 77 K. X-ray photoelectron spectroscopy (XPS) analysis was carried out on an ESCAlab 220I-XL spectrometer with Al Kα radiation. Electrochemical measurements were carried out on Solartron1255/1287 system. Electrochemical impedance spectroscopy (EIS) was conducted with symmetric thin-layer cells in the frequency range of 0.1–105 Hz at 0 V bias and 10 mV amplitude. Cyclic voltammetry (CV) measurements were performed in a three-electrode system using a Pt wire as counter electrode and Ag/AgCl electrode as reference electrode in acetonitrile solution containing 0.01 M LiI, 0.001 M I2, and 0.1 M LiClO4. The scan rate is 50 mV s−1. The photocurrentvoltage curves of DSCs were recorded by a Keithley 2400 source meter under a simulated AM 1.5 illumination at 100 mW cm−2.
PCNR layer. The nitrogen-doping as well as the porosity with high accessible surface area provides numerous electrocatalytic active sites for I3- reduction. This combination of characteristics means that NPCNR electrode possesses an excellent electrocatalytic activity for the reduction of I3-. The assembled DSC with N-PCNR counter electrode exhibits a high conversion efficiency of 7.01%, which is very close to that of the cell with Pt counter electrode. 2. Experimental 2.1. Preparation of N-PCNR electrodes PANI nanorods were firstly prepared by polymerization of aniline in aqueous solution using ammonium persulfate (APS) as the initiator. Briefly, 10 g of aniline and 7 g of citric acid were mixed in 500 mL of distilled water. 20 g of ammonium persulfate was dissolved in 150 mL of distilled water. Then, ammonium persulfate solution was added into the mixture solution of aniline and citric acid with vigorous stirring. The resulting solution was left standing at about 3 °C for 20 h. The obtained dark green sample was filtered and washed with distilled water, and then dried under vacuum at 50 °C. The as-prepared PANI nanorods were pyrolyzed at 600 °C under nitrogen atmosphere for 3 h to obtained nitrogen-doped carbon nanorods (N-CNRs). N-PCNRs were prepared by activating N-CNRs. In brief, 3 g of N-CNRs were mixed with 9 g of KOH in aqueous solution by stirring and subsequent sonication, followed by evaporation at 90 °C. Then, above mixture was transferred into a tube furnace for activation at 800 °C under nitrogen atmosphere with duration of 2 h. The obtained product was rinsed with 1 M HCl solution and distilled water until the pH value of effluent reached about 7, and then dried at 80 °C in air. 150 mg of as-prepared N-PCNRs were ground with 10 mL isopropanol and 10 μL tetrabutyl titanate in a mortar to obtain N-PCNRs paste. The N-PCNR electrodes were fabricated by spraying the N-PCNR paste onto pre-cleaned fluorine-doped tin oxide (FTO) glass, followed by sintering at 400 °C for 15 min. N-CNR electrodes were also prepared through the same procedure. The thickness of the carbon catalyst layer was about 9 µm, which was controlled by the spray time.
3. Results and discussion N-PCNRs were prepared by carbonization of PANI nanorods at 600 °C and subsequent chemical activation at 800 °C (see Fig. 1). PANI is an ideal precursor for preparing nitrogen-doped carbonaceous materials due to its high content of nitrogen and uniformly distributed nitrogen atoms in its framework. In addition, PANI with different morphologies, such as nanofiber, nanotube, and nanosphere, can be easily prepared by controlling the synthesis conditions [33–35], and these original morphologies can be preserved even after high-temperature treatment. Accordingly, nitrogen-doped carbon nanorods (NCNRs) were prepared from PANI nanorods through high-temperature carbonization. By activating N-CNRs, N-PCNRs were prepared. Fig. 2 displays SEM and TEM images of original PANI nanorods, N-CNRs, and N-PCNRs. As shown in Fig. 2a and b, the PANI exhibits a nanorodlike morphology with rough surface. The diameters of PANI nanorods are in the range of 150–200 nm and the lengths range from 0.5 to 3 µm. After carbonization and chemical activation treatment, the nanorod morphology is preserved except that the aspect ratio of nanorod decrease slightly as shown in Fig. 2(c–f). The high-resolution TEM image shown in Fig. 2g demonstrates that N-CNRs possess a smooth surface. During the chemical activation, KOH corroded carbon nanorods at high temperature to develop new pores on the surface of carbon nanorods. Therefore, the as-prepared N-PCNRs exhibit the well-developed pore structure as shown in Fig. 2h. The pore structure and surface area of N-CNRs and N-PCNRs were determined by N2 sorption measurements. Fig. 3 shows the N2 adsorption-desorption isotherms and pore-size distribution curves of N-CNRs and N-PCNRs. As displayed in Fig. 3a, the isotherms of NCNRs are of type I, and the adsorption branch nearly overlap with desorption branch over a wide range of relative pressure. In addition, the nitrogen adsorption capacity of N-CNRs is small. These results demonstrate that N-CNRs are poorly porous. The Brunauer-EmmettTeller (BET) surface area and pore volume of N-CNRs are 116.3 m2 g−1 and 0.10 cm3 g−1, respectively. N-PCNRs presents a combined type I/ IV isotherms with an obvious hysteresis loop at the relative pressure 0.4–0.8, demonstrating a characteristic of both the mesoporosity and the microporosity. The pore-size distribution curve shown in Fig. 3b further demonstrates that N-PCNRs have both micropores and welldeveloped mesopore. The significant increase in the nitrogen uptake in the entire pressure range for N-PCNRs is a result of the increase in porosity created by chemical activation. The BET surface area and pore volume of N-PCNRs are 2580.5 m2 g−1 and 1.36 cm3 g−1, respectively,
2.2. Fabrication of DSCs TiO2 electrodes were prepared by coating 9 µm-thick TiO2 film on FTO glass using doctor-blade method and then sintering at 450 °C for 30 min. The TiO2 electrodes sensitized with N719 dye were used as the photoanodes. A solution containing 0.5 M 1-hexyl-3-methylimidazolium iodide, 0.2 M LiI, 0.05 M I2, and 0.4 M 4-tert-butylpyridine in 3methoxypropionitrile was used as the electrolyte. The DSC was fabricated by clapping a photoanode, an electrolyte, and a counter electrode into a sandwich structure. 2.3. Measurements and characterization The morphology of samples was observed by scanning electron microscopy (SEM, Hitachi S-4800) and transmission electron microscopy (TEM, JEOL JEM2010). The surface area and pore structure of the samples were measured by a Micromeritics ASAP 2020 instrument 191
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Fig. 2. SEM and TEM images for PANI nanorods (a and b), N-CNRs (c and d), and N-PCNRs (e and f); the high-resolution TEM images of N-CNRs (g) and N-PCNRs (h).
The cross-section SEM image of N-PCNR electrode in Fig. 5 shows that the carbon catalyst layer has a thickness of about 9 µm. In order to eliminate the effect of TiO2 photoanode, the symmetric thin-layer cells consisting of two identical electrodes were used for EIS measurements. Fig. 6a displays Nyquist plots of the symmetric cells based on N-CNR and N-PCNR. For comparison, the Nyquist plot for Pt electrode was also exhibited in Fig. 6a. Pt electrodes were prepared by thermal decomposition of H2PtCl6 on FTO glass at 390 °C for 15 min. From Fig. 6a, it can be seen that all Nyquist plots show two semicircles in the high and low frequency regions, corresponding to the charge-transfer process at the electrode/electrolyte interface and Nernst diffusion process in the electrolyte, respectively. The corresponding equivalent circuit is shown in the inset in Fig. 6a, in which Rs is the ohmic series resistance, Rct is the charge-transfer resistance, CPE is the constant phase element, ZN is the Nernst diffusion impedance. The EIS parameters are obtained by fitting Nyquist plots and listed in Table 2. Rct is a significant parameter to evaluate the electrocatalytic activity of the electrode for the reduction of I3-. The smaller Rct value is, the higher electrocatalytic activity could be. As shown in Table 2, Rct for Pt, N-CNR, and N-PCNR electrodes are 1.6, 8.9 and 2.2 Ω cm2, respectively. Clearly, the electrocatalytic activity of N-PCNR electrode is much higher than that of N-CNR electrode, and nearly close to that of Pt electrode. This greatly enhanced electrocatalytic activity of NPCNR electrode can be attributed to the unique porous nanorod structure. As shown in Fig. 6b, the nanorod structure can provide a direct pathway for fast electron transport, and the porosity with high
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which are much higher than those of N-CNRs. XPS measurements were conducted to determine the element compositions and states of N-CNRs and N-PCNRs. Fig. 4 exhibits XPS survey spectra and N 1 s peaks for N-CNRs and N-PCNRs. As expected, XPS survey spectra of N-CNRs and N-PCNRs (Fig. 4a) show three peaks at binding energy of 288.1, 400.1, and 533.2 eV, corresponding to C 1 s, N 1 s, and O 1 s, respectively. The elemental content calculated from XPS survey spectra are summarized in Table 1. The nitrogen and oxygen contents of N-CNRs are 8.67 at% and 9.77 at%, respectively, while the nitrogen and oxygen contents of N-PCNRs are 2.03 at% and 14.62 at%, respectively. Clearly, the chemical activation leads to a decrease in nitrogen content and an increase in oxygen content. The N 1 s peak for N-CNRs can be deconvoluted into three peaks centered at binding energy of 398.6, 400.1, and 401.2 eV (Fig. 4b), assigning to pyridinic nitrogen (N-6), pyrrolic nitrogen (N5), and quaternary nitrogen (N-Q), respectively [36]. The deconvolution of N 1 s peak for N-PCNRs (Fig. 4c) displays three peaks at the same binding energies, however, with different relative contributions. From Table 1, it can be seen that after chemical activation, the relative content of N-6 increase greatly, while the relative content of N-Q decrease considerably. This indicates that N-6 and N-5 are predominant in N-PCNRs. N-6 and N-5 locate at the edge of the graphene layer, and then can provide the electrochemically active sites to improve the electrocatalytic performance of carbon materials [37]. EIS measurements were employed to characterize the electrocatalytic activity of N-CNR and N-PCNR electrodes for the I3- reduction.
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Fig. 4. (a) XPS scanning spectra of N-CNRs and N-PCNRs; deconvoluted N 1 s peaks of N-CNRs (b) and N-PCNRs (c). Table 1 Oxygen and nitrogen content in N-CNRs and N-PCNRs determined by XPS. Sample
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O content (at%)
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9.77 14.62
Table 2 EIS parameters of the symmetric thin-layer cells with Pt, N-CNR, and N-PCNR electrodes and photovoltaic parameters of DSCs with Pt, N-CNR, and N-PCNR counter electrodes.
N content (at%) Total
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N-5
N-Q
8.67 2.03
2.89 0.96
3.73 0.81
2.05 0.26
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Rs (Ω cm2)
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Jsc (mA cm−2)
FF
η (%)
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8.9 2.2 1.6
16.8 17.1 16.5
23.6 41.2 5.8
0.678 0.702 0.698
15.01 15.85 15.98
0.58 0.63 0.65
5.91 7.01 7.25
doping, and the nanorod structure endows the N-PCNR electrode with an excellent electrocatalytic activity. The Rs values of Pt, N-CNR, and N-PCNR electrode do not vary significantly, demonstrating a good binding of carbon nanorods with FTO substrate and a fast electron transport through carbon nanorods network. As expect, N-PCNR electrode exhibits the highest CPE value among the three electrodes due to the largest surface area. Cyclic voltammetry (CV) measurements were also carried out using a three-electrode system to examine the electrocatalytic activity of the electrodes. Fig. 7 shows CV curves for Pt, N-CNR, and N-PCNR electrodes. Two pair redox peaks can be observed in CV curves. The pair at more positive potentials is assigned to the redox reaction of I3-/ I2 couple (3I2 +2e ↔ 2I3-), and the pair at more negative potentials is attributed to the redox reaction of I3-/I- (I3- +2e ↔ 2I-). The function of the counter electrode in a DSC is to catalyze the reduction of I3- to I-. Therefore, the characteristics of the negative pair are usually employed to evaluate the electrocatalytic activity of the counter electrode. From Fig. 7, it can be observed that the peak current densities of N-PCNR electrode are higher than those of the N-CNR electrode and comparable
Fig. 5. The cross-section SEM image of N-PCNR electrode.
accessible surfaces area as well as the nitrogen doping of N-PCNRs provides more electrocatalytic active sites. Therefore, it is the combination of the porosity with high accessible surface area, the nitrogen 10 2Rct
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alternative to Pt in the counter electrode of DSCs. 2
4. Conclusions
Pt N-PCNR N-CNR
N-PCNRs with high accessible surface area were prepared by carbonization of PANI nanorods and subsequent chemical activation and successfully investigated as the efficient counter electrode for DSCs. The as-prepared N-PCNR electrode exhibited an excellent electrocatalytic activity for I3- reduction and showed a very close photovoltaic performance to Pt electrode in terms of Voc, Jsc, FF, and η, which was attributed to the ideal combination of the porosity with high accessible surface area, nitrogen doping, and nanorod structure. This work could provide a promising strategy for designing a low-cost and high-performance counter electrode material in DSCs.
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Acknowledgement
Fig. 7. CV curves of Pt, N-CNR, and N-PCNR electrodes obtained by a three-electrode system at scan rate of 50 mV s−1.
This work was supported by National Natural Science Foundation of China (grant number 21273137) and Liaoning Science & Technology Project (grant number 201601011)
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References
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[1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 film, Nature 353 (1991) 737–739. [2] S. Mathew, A. Yella, P. Gao, R. Humph ry-Baker, B. Curchod, N. Ashari-Astani, I. Tavernrlli, U. Rothlisberger, M.K. Nazeeruddin, M. Grätzel, Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers, Nat. Chem. 6 (2014) 242–247. [3] A. Yella, H. Lee, H. Tsao, C. Yi, A. Chandiran, M.K. Nazeeruddin, E.E. Diau, C. Yeh, S.M. Zakeeruddin, M. Grätzel, Porphyrin-sensitized soalr cells with cobalt (III/II)based redox electrolyte exceeding 12 percent efficiency, Science 334 (2011) 629–634. [4] J. Wu, Z. Lan, J. Lin, M. Huang, Y. Huang, L. Fan, G. Luo, Electrolyte in dyesensitized solar cells, Chem. Rev. 115 (2015) 2136–2173. [5] J. Halme, P. Vahermaa, K. Miettunen, P. Lund, Device physics of dye solar cells, Adv. Mater. 22 (2010) E210–E234. [6] N. Papageorgiou, Counter-electrode function in nanocrystalline photoelectrochemical cell configurations, Coord. Chem. Rev. 248 (2004) 1421–1446. [7] H. Elbohy, A. Aboagye, S. Sigdel, Q. Wang, M.H. Sayyad, L. Zhang, Q. Qiao, Graphene-embedded carbon nanofibers decorated with Pt nanoneedles for highefficiency dye-sensitized solar cells, J. Mater. Chem. A 3 (2015) 17721–17727. [8] V. Dao, H. Choi, Pt nanourchins as efficient and robust counter electrode materials for dye-sensitized solar cells, ACS Appl. Mater. Interfaces 8 (2015) 1004–1010. [9] M. Yeh, S. Chang, L. Lin, H. Chou, R. Vittal, B. Hwang, K. Ho, Size effect of Pt nanoparticles on the electrocatalytic ability of the counter electrode in dyesensitized solar cells, Nano Energy 17 (2015) 241–253. [10] K. Lee, L. Lin, C. Chen, V. Suryanarayanan, C. Wu, Preparation of high transmittance platinum counter electrode at an ambient temperature for flexible dye-sensitized solar cells, Electrochim. Acta 135 (2014) 578–584. [11] W. Hou, Y. Xiao, G. Han, D. Fu, R. Wu, Serrated, flexible and ultrathin polyaniline nanoribbons: an efficient counter electrode for dye-sensitized solar cell, J. Power Sources 322 (2016) 155–162. [12] Y. Xiao, G. Han, Y. Li, M. Li, Y. Chang, High performance of Pt-free dye-sensitized solar cells based on two-step electropolymerized polyaniline counter electrodes, J. Mater. Chem. A 2 (2014) 3452–3460. [13] Y. Xiao, W. Wang, S. Chou, T. Lin, J. Lin, In situ electropolymerization of polyaniline/cobalt sulfide decorated carbon nanotube composite catalyst towards triiodide reduction in dye-sensitized solar cells, J. Power Sources 266 (2014) 448–455. [14] Y. Xiao, J. Lin, J. Wu, S. Tai, G. Yue, T. Lin, Dye-sensitized solar cells with highperformance polyaniline/multi-wall carbon nanotube counter electrode electropolymerized by a pulse potentiostatic technique, J. Power Sources 233 (2013) 320–325. [15] Y. Xiao, J. Lin, W. Wang, S. Tai, G. Yue, J. Wu, Enhanced performance of low-cost dye-sensitized solar cells with pulse-electroplymerized polyaniline counter electrode, Electrochim. Acta 90 (2013) 468–474. [16] S. Yun, A. Hagfeldt, T. Ma, Pt-free counter electrode for dye-sensitized solar cells with high efficiency, Adv. Mater. 26 (2014) 6210–6237. [17] M. Wu, T. Ma, Platinum-free catalysts as counter electrodes in dye-sensitized solar cells, ChemSusChem 5 (2012) 1343–1357. [18] G. Wang, J. Zhang, S. Kuang, S. Zhuo, Nitrogen-doped porous carbon prepared by a facile soft-templating process as low-cost counter electrode for high-performance dye-sensitized solar cells, Mater. Sci. Semicond. Process. 38 (2015) 234–239. [19] Y. Jo, J. Cheon, J. Yu, H. Jeong, C. Han, Y. Jun, S. Joo, Highly interconnected ordered mesoporous carbon-carbon nanotube nanocomposite: pt-free high efficient durable counter electrode for dye-sensitized solar cells, Chem. Commun. 48 (2012) 8057–8059. [20] B. Zhao, H. Huang, P. Jiang, H. Zhao, X. Huang, P. Shen, D. Wu, R. Fu, S. Tan,
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4 0
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Voltage (V) Fig. 8. Photocurrent density-voltage curves of the DSCs with Pt, N-CNR, N-PCNR, and PANI electrodes measured under simulated AM 1.5 illumination.
to those of the Pt electrode. This indicates that the electrocatalytic activity of N-PCNR electrode is higher than that of N-CNR electrode and comparable to that of Pt electrode, displaying that N-PCNRs can be used as an efficient counter electrode material for application in DSCs. The current density-voltage curves of DSCs fabricated with Pt, NCNR, and N-PCNR counter electrodes are displayed in Fig. 8. The corresponding photovoltaic parameters, such as the open-circuit voltage (Voc), the short-circuit current density (Jsc), the fill fact (FF), and the conversion efficiency (η), are summarized in Table 2. The DSC fabricated with N-CNR counter electrode exhibits a Voc of 0.678 V, a Jsc of 15.01 mA cm−2, a FF of 0.58, and a conversion efficiency of 5.91%. When N-PCNR is used as the counter electrode, the photovoltaic parameters of the corresponding DSC are Voc=0.702 V, Jsc=15.85 mA cm−2, FF=0.63, and η=7.01%. Obviously, the photovoltaic performance of the DSC with N-PCNR counter electrode is much better than that of the cell with N-CNR counter electrode, and very close to that of the device with Pt counter electrode. This enhanced photovoltaic performance can be assigned to the excellent electrocatalytic activity of N-PCNR electrode for I3- reduction derived from combination of the porosity with high accessible surface area, nitrogen doping, and nanorod structure. Previous works indicated that PANI can be employed as the Pt-free counter electrode in DSCs [11,17]. Therefore, we also fabricated the PANI nanorod electrode-based DSC and measured its photovoltaic performance. As shown in Fig. 8, the conversion efficiency of DSC with N-PCNR counter electrode is much higher than that of the cell with PANI counter electrode (5.11%). These results demonstrate that N-PCNRs could be a promising cost-efficient
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[21]
[22]
[23] [24]
[25] [26]
[27]
[28]
[29]
Flexible counter electrodes based on mesoporous carbon aerogel for high-performance dye-sensitized solar cells, J. Phys. Chem. C 115 (2011) 22615–22621. G. Wang, C. Huang, W. Xing, S. Zhuo, Micro-meso hierarchical porous carbon as low-cost counter electrode for dye-sensitize solar cells, Electrochim. Acta 56 (2011) 5459–5463. P. Srinivasu, A. Islam, S. Singh, L. Han, M.L. Kantam, S.K. Bhargava, Highly efficient nanoporous graphitic carbon with tunable textural properties for dyesensitized solar cells, J. Mater. Chem. 22 (2012) 20866–20869. J.D. Roy-Mayhew, I.A. Aksay, Graphene materials and their use in dye-sensitized solar cells, Chem. Rev. 114 (2014) 6323–6348. W. Yang, X. Xu, Y. Gao, Z. Li, C. Li, W. Wang, Y. Chen, G. Ning, L. Zhang, J. Kong, Y. Li, High-surface-area nanomesh graphene with enriched edge sites as efficient metal-free cathodes for dye-sensitized solar cells, Nanoscale 8 (2016) 13059–13068. H. Wang, Y. Hu, Graphene as counter electrode materials for dye-sensitized solar cells, Energy Environ. Sci. 5 (2012) 8182–8188. P. Joshi, L. Zhang, Q. Chen, D. Galipeau, H. Fong, Q. Qiao, Electrospun carbon nanofibers as low-cost counter eelctrode for dye-sensitized solar cells, ACS Appl. Mater. Interfaces 12 (2010) 3572–3577. T.N. Murakami, S. Ito, Q. Wang, M.K. Nazeeruddin, P. Liska, R. Humphery-Baker, P. Comte, M. Grätzel, Highly efficient dye-sensitize solar cells based on carbon black counter electrodes, J. Electrochem. Soc. 153 (2006) A2255–A2261. S. Hou, X. Cai, H. Wu, X. Yu, M. Peng, K. Yan, D. Zou, Nitrogen-doped graphene for dye-sensitized solar cells and the role nitrogen states in triiodide reduction, Energy Environ. Sci. 6 (2013) 3356–3362. L. Song, Q. Luo, F. Zhao, Y. Li, H. Lin, L. Qu, Z. Zhang, Durally functional, N-doped
[30]
[31]
[32]
[33] [34]
[35]
[36] [37]
195
porous graphene foams as counter electrodes for dye-sensitized solar cells, Phys. Chem. Chem. Phys. 16 (2014) 21820–21826. M. Chen, L. Shao, Y. Liu, T. Ren, Z. Yuan, Nitrogen-doped ordered cubic mesoprous carbons as counter electrodes for dye-sensitized solar cells, J. Power Sources 283 (2015) 305–313. Y. Xue, J. Liu, H. Chen, R. Wang, D. Li, J. Qu, L. Dai, Nitrogen-doped graphene foams as metal-free counter electrodes in high-performance dye-sensitized solar cells, Angew. Chem. 124 (2012) 12290–12293. L. Li, C. Wang, J. Liao, A. Manthiram, Dual-template synthesis of N-doped macro/ mesoporous carbon with an open-pore structure as metal-free catalysts for dyesensitized solar cells, J. Power Sources 300 (2015) 254–260. H. Guan, L. Fan, H. Zhang, X. Qu, Polyaniline nanofibers obtained by interfacial polymerization for high-rate supercapacitor, Electrochim. Acta 56 (2010) 964–968. Z. Huang, E. Liu, H. Shen, X. Xiang, Y. Tian, C. Xiao, Z. Mao, Preparation of polyaniline nanotubes by a template-free self-assembly method, Mater. Lett. 65 (2011) 2015–2018. Z. Lei, M. Zhao, L. Dang, A. Lo, N. Yu, S. Liu, Structure evolution and electrocatalytic application of nitrogen-doped carbon shell synthesized by pyrolysis of near-monodisperse polyaniline nanospheres, J. Mater. Chem. 19 (2009) 5985–5995. D. Hulicova, J. Yamashita, Y. Soneda, H. Hatori, M. Kodama, Supercapacitors prepared from melamine-based carbon, Chem. Mater. 17 (2005) 1241–1247. L. Chen, X. Zhang, H. Liang, M. Kong, Q. Guan, P. Chen, Z. Wu, S. Yu, Synthesis of nitrogen-doped porous carbon nanofibers as an efficient electrode material for supercapacitors, ACS Nano 6 (2012) 7092–7102.