Electrochimica Acta 190 (2016) 720–728
Contents lists available at ScienceDirect
Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
Electro-polymerization of polypyrrole/multi-wall carbon nanotube counter electrodes for use in platinum-free dye-sensitized solar cells Wenjing Houa,b , Yaoming Xiaoa,b,* , Gaoyi Hana,b,* , Haihan Zhoua,b a b
Institute of Molecular Science, Innovation Center of Chemistry and Molecular Science, Shanxi University, Taiyuan 030006, PR China Key Laboratory of Materials for Energy Conversion and Storage of Shanxi Province, Taiyuan 030006, PR China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 14 July 2015 Received in revised form 1 January 2016 Accepted 2 January 2016 Available online 9 January 2016
In the development of the dye-sensitized solar cell (DSSC), it is critically important, but remains challenging to discover new counter electrode (CE) materials to replace the expensive and precious platinum (Pt). Herein we electro-polymerize the polypyrrole (PPy) on the multi-wall carbon nanotube (MWCNT)-coated F-doped tin oxide substrate by a facile cyclic voltammetry method to obtain the PPy/ MWCNT composite CE. The resultant composite CE shows that the MWCNT is wrapped with 15 nm thickness of PPy nanoparticles, which takes advantage of the high surface area and good conductivity of the MWCNT and good catalytic activity of the PPy for the reduction of triiodide to iodide, providing the fast electron transport and diffusion channels and plenty of interfacial catalytic active sites. The DSSC based on the PPy/MWCNT composite CE shows an excellent photoelectric conversion efficiency of 7.15% under full sunlight illumination (100 mW cm2, AM 1.5 G), which is much higher than that of the MWCNT (1.72%) and PPy (5.72%) based DSSCs, and 92.14% that of the Pt-based DSSC (7.76%). ã 2016 Elsevier Ltd. All rights reserved.
Keywords: dye-sensitized solar cell counter electrode polypyrrole multi-wall carbon nanotube electrocatalytic activity
1. Introduction The dye-sensitized solar cell (DSSC) has been regarded as a promising candidate for the next-generation solar cell due to its low cost, relatively high energy conversion efficiency, and environmentally friendly fabrication process, since its first prototype in 1991 by O’Regan and Grätzel [1–5]. In general, a DSSC consists of a dye-sensitized porous nanocrystalline TiO2 anode, an electrolyte containing the iodide/triiodide (I/I3) redox couple, and a platinized counter electrode (CE). As the most common CE of the DSSC, platinum (Pt) has high conductivity, stability and catalytic activity for the reduction of triiodide (I3) to iodide (I) [6]. However, the noble metal Pt is one of the most expensive components in the DSSC [6,7], which directly limit the widely commercial application of DSSCs especially for the largearea solar cells [4,8,9]. Meanwhile, the PtI4 will be produced, due to Pt could be dissolved in the I/I3 redox couple electrolyte, which will resort to a negative effect on the long term stability of the DSSC [10]. Based on above defects, various CE materials have been explored as promising low-cost alternatives to the Pt catalyst, such as carbon materials [11–13], transition-metal compounds [14–20],
* Corresponding author. Tel.: +86 351 7010699; fax: +86 351 7016358 E-mail addresses:
[email protected] (Y. Xiao),
[email protected] (G. Han). http://dx.doi.org/10.1016/j.electacta.2016.01.012 0013-4686/ ã 2016 Elsevier Ltd. All rights reserved.
conducting polymers [21–25], and metal alloys [7,26,27]. The universalities of these candidates are comparable electrocatalytic performances for the triiodide reduction and high electrontransfer ability. Among them, on the one hand, carbon nanotubes reveal some of the most potential for use as an alternative CE due to their features of very high accessible surface area, superior electronic conductivity and excellent mechanical strength [28]. However, the conversion efficiency of the DSSC based on the carbon nanotube CE is relatively low due to the poor catalytic activity for I3 reduction [11]. On the other hand, the polypyrrole (PPy) nanostructures, on account of easy to fabricate, high catalytic activity for the I3 reduction and considerable environmental stability, have been considered as one of the most promising CE materials [21–23]. To further improve the catalytic activity, the PPy/multi-wall carbon nanotube (MWCNT) composite CE, combing the good conductivity and the high surface area of MWCNTs with the high catalytic activity of PPy for I3 reduction, has improved the photoelectric conversion efficiency of the DSSC [29–32]. Early works in fabricating PPy/MWCNT composites are focusing on chemical [29–31] and electrochemical copolymerization approaches [32]. Peng and coworkers prepared PPy/MWCNT composite CE by the chemical polymerization and drop-casting method and employed in the flexible DSSC [29], giving a conversion efficiency of 4.04%. The lower conversion efficiency might contribute to the poor contact between the PPy and MWCNT when prepared by the
W. Hou et al. / Electrochimica Acta 190 (2016) 720–728
chemical polymerization method. The electrochemical copolymerization approach is also not convenient to prepare the PPy film around the MWCNT [32]. Herein, we fabricated the PPy/MWCNT CE by a facile cyclic voltammetry method to electro-polymerize the PPy nanoparticles on the MWCNT-coated F-doped tin oxide (FTO) substrate. The resultant composite CE showed that the MWCNT was wrapped with 15 nm thickness of PPy nanoparticles. Furthermore, the contact among the PPy, MWCNT, and FTO substrate was enhanced. Due to the high active surface and facile electron transport from the MWCNT and intrinsically excellent electrocatalytic activity from the PPy, a high energy conversion efficiency of 7.15% was achieved for the DSSC based on the PPy/MWCNT CE, which was much higher than that of MWCNT (1.72%) and PPy (5.72%) based DSSCs under full sunlight illumination (100 mW cm2, AM 1.5 G). 2. Experiment
721
mL1 was spin-coated on the FTO glass substrate, and dried at 60 C in a drying oven (Suzhou Jiangdong Precision Instrument Co., Ltd., China) to obtain the MWCNT CE. The PPy was electrodeposited on the MWCNT CE by the cyclic voltammetry (CV) measurement from an aqueous solution containing 10 mM Py, 20 mM SDS, and 20 mM LiClO4, which was sonicated for 15 min prior to the experiment. A three-electrode cell with an Electrochemical Workstation (CHI660D, Shanghai Chenhua Device Company, China) was used, and the MWCNT CE and Pt sheet (2 cm2) were used as the working electrode and counter electrode, respectively. The CV electro-polymerization of the PPy on the MWCNT CE was carried out at a potential window of 0.4 V to 0.8 V for 12 cycles vs. Ag/AgCl at a scan rate of 0.05 V s1, in which the CV parameters were optimized in advance. The resultant PPy/MWCNT CE was rinsed in distilled water and dried at 60 C. For comparison, the PPy CE based on same CV parameters were also manufactured, and a thermal decomposition Pt CE was employed.
2.1. Materials 2.3. Fabrication of mesoporous dye-sensitized TiO2 anodes Pyrrole (Py, Sigma–Aldrich), MWCNTs (Purity: >99.5%; Average diameter: 40–60 nm; Length: >10 um; Golden Innovation Business Co.), tetrabutyl titanate, acetic acid, nitric acid, sodium dodecylsulpfate (SDS), polyvinylpyrrolidone K-30 (PVP), triton X-100, iodine, lithium iodide, lithium pechlorate (LiClO4), tetrabutyl ammonium iodide, 4-tert-butylpyridine, ganidine thiocyanate, acetonitrile, acetone, and ethanol were purchased from Shanghai Chemical Agent Ltd., China (Analysis purity grade). Dye N719 [cisdi(thiocyanato)-N,N-bis(2,2-bipyridyl-4-carboxylic acid-4-tetrabutyl ammoniumcarboxylate) ruthenium (II)] was purchased from Dyesol, Australia. The above agents were used without further purification. F-doped tin oxide (FTO) glass substrates were purchased from NSG, Japan (12 V sq1).
FTO glass substrates with the settled size of 2 cm 1.5 cm were thoroughly rinsed in the order by deionized water and anhydrous ethanol. The TiO2 colloid and mesoporous TiO2 anodes were prepared according to our previous reports [4,7,24]. The thickness of the TiO2 film was controlled by the thickness of the adhesive type around the edge of the FTO substrate. After drying at room temperature, the TiO2 anodes were sintered in a muffle furnace at 450 C for 30 min. The resultant mesoporous TiO2 anodes were immersed in a 2.5 104 M solution of dye N719 in absolute ethanol solution for 24 h to adsorb the dye adequately, followed by cool air drying. 2.4. Assembly of DSSCs
2.2. Electro-polymerization of PPy/MWCNT CEs Raw MWCNTs were refluxed in a 3:1 mixture of H2SO4 (98%) and HNO3 (78%) at 120 C for 15 min to oxidize the graphitic sp2 carbon into a COOH functional group on the side walls of the MWCNTs. The oxidized MWCNTs were further filtered by suction filtration, washed thoroughly with deionized water, and suspended in a 1:1 mixture of acetone and ethanol by ultrasonication for 2 h. The MWCNT suspension with a concentration of 0.5 mg
The DSSC was fabricated by sandwiching redox electrolyte between a dye-sensitized TiO2 anode and a CE under an open system without any sealing. The redox electrolyte composed of 0.60 M tetrabutyl ammonium iodide, 0.10 M lithium iodide, 0.10 M iodine, 0.10 M ganidine thiocyanate, and 0.50 M 4-tert-butylpyridine in acetonitrile. The redox electrolyte was also employed in the electrochemical impedance spectroscopy (EIS) and Tafel polarization measurements.
Fig. 1. (a) FTIR and (b) UV–vis absorption spectra of the PPy, MWCNT and PPy/MWCNT.
722
W. Hou et al. / Electrochimica Acta 190 (2016) 720–728
2.5. Characterizations The surface feature of the CE was observed using a scanning electron microscopy (SEM, JEOL-JSM-6701F) operating at 10 kV. Fourier transform infrared spectra (FTIR) of samples were recorded on a Perkin Elmer Spectrum Gx FTIR Spectrometer using KBr as pellets. UV–vis absorbtion measurement was carried out with an Agilent 8453 UV–vis diode array spectrophotometer. Cyclic voltammograms (CVs) for I/I3 system were measured in an acetonitrile solution consisting of 0.05 M lithium iodide, 0.01 M iodine, and 0.05 M lithium pechlorate, the potential window was 0.6 V to 0.8 V (vs. Pt) with different scan rates (50, 75, 100, and 125 mV s1) using a computer-controlled CHI660D in a threeelectrode electrochemical cell at a constant temperature of 20 C, the resultant CE acted as the working electrode, a Pt-foil and Ptwire as the counter electrode and reference electrode. The EIS measurement was conducted with two identical CEs by using a CHI660D electrochemical measurement system under simulating open-circuit conditions at an ambient atmosphere, and the impedance data covered a frequency range of 0.1–105 Hz with 5 mV of amplitude and 0 V bias potential. The resultant impedance spectra were analyzed by means of the Z-view software. Tafel polarization curves for the CEs were measured using the CHI660D system from 1.0 V to 1.0 V at a scan rate of 10 mV s1. 2.6. Photoelectric measurements The photovoltaic tests of DSSCs were carried out by measuring the current–voltage characteristic curves using the CHI660D Electrochemical Workstation, under irradiation of the CELS500 simulated solar light at an ambient atmosphere. The incident light intensity was set under 100 mW cm2 (AM 1.5 G), and a black mask (0.30 cm2) was used on top of the device to control the active cell area for the light irradiation. Current density as a function of time for the DSSC held at 0 V forward bias
was measured using the CHI660D system under AM1.5 simulated sun light. 3. Results and discussion Fig. 1a shows the FTIR spectra of the PPy, MWCNT, and PPy/ MWCNT, respectively. In the PPy curve, the peaks at 1541 and 1456 cm1 are from the fundamental stretching vibrations of pyrrole rings, indicating the ring structure was not affected by polymerization. The bands at 1037 and 773 cm1 can be attribute to the N-H deformation vibration and C H wagging vibration, respectively. The presence of strong bands at about 1153 and 966 cm1 indicates the formation of the doped PPy [32]. The low absorption band at 670 cm1 is due to the C H out of plane bending of the pyrrole moiety in the PPy [29]. In the MWCNT curve, the absorption bands at 1581 and 1205 cm1 are associated with stretching of the carbon nanotube backbone. Two bands appearing at 1720 and 1401 cm1 can be attributed to the C¼O stretching vibration and hydroxyl group ( OH) bending deformation of the carboxylic group ( COOH) from the acidoxidized MWCNT, respectively [33,34]. All the characteristic bands of the PPy and MWCNT are reserved in the spectrum of the PPy/MWCNT composite, demonstrating that the backbone structure of the PPy and MWCNT was not damaged and the PPy/MWCNT composite was successfully prepared by the electrochemical polymerization. The band shift can be interpreted by covalent interactions (probably the p–p coupling interaction) between the PPy and MWCNT [32,35], which is expected to give a facile electron transport between the PPy and MWCNT. The UV–vis absorption spectra of PPy, MWCNT, and PPy/ MWCNT CEs are shown in Fig. 1b. The PPy CE exhibits a characteristic absorbance peak at 482 nm, which is associated with p–p* electron transition of conjugated PPy chains [36]. And a broad absorption onset ranging between 850 and 1000 nm, that could be related to the conductive form of PPy (dication) [37]. In
Fig. 2. SEM images of the (a) bare FTO, (b) PPy, (c) MWCNT, and (d) PPy/MWCNT CEs in high magnification of 50000 times with a bar of 100 nm, respectively.
W. Hou et al. / Electrochimica Acta 190 (2016) 720–728
723
Scheme 1. Schematic diagram of the (a) MWCNTs preparing on the FTO substrate, (b) pyrrole monomer absorbing on the surface of the MWCNTs, (c) CV electropolymerization of PPy onto the MWCNTs and FTO substrate. (c) Catalytic mechanism of PPy/MWCNT CE.
case of the PPy/MWCNT composite CE, the spectrum exhibits distinct absorption peak around 504 nm. Compared with the single PPy CE sample at 482 nm, the red shift is a typical characteristic of a delocalization of electrons [23], which might resulting from the p–p coupling due to the doping of MWCNTs [38]. At the same time, an increase in the electronic density of the p band after polymerizing the PPy film has been reported [37]. The relative intensity of the absorption peak around 482 nm is much stronger than the PPy. This enhancement arises from the delocalized electron induced by the interaction between the PPy and carboxyl group in MWCNT [23]. The delocalized electron facilitates the
transfer of charges that the catalytic reaction rate at the interface of electrolytes and electrodes could be enhanced. Fig. 2b shows the SEM image of the PPy nanoparticles and the overall appearance is supported by the FTO surface feature (Fig. 2 a). In Fig. 2d, the out wall of the MWCNT is wrapped with the PPy nanoparticles and the total diameter is about 80 nm, which is larger than that of the original MWCNT (50 nm, Fig. 2c). Thus the PPy thin film around the MWCNT is about 15 nm in thickness. Scheme 1 shows the possible formation mechanisms of MWCNTs preparing on the FTO substrate (Scheme 1a), and CV electropolymerization of PPy onto the MWCNTs and FTO substrate
Fig. 3. CVs of the Pt, PPy, MWCNT, and PPy/MWCNT CEs at a scan rate 75 mV s1, respectively.
724
W. Hou et al. / Electrochimica Acta 190 (2016) 720–728
Table 1 Epp,Dn, and best-fit values for RS, RCT, YCPE, Wcat, and W of the equivalent circuits to the impedance spectra in Fig. 5a. CE Pt PPy MWCNT PPy/MWCNT
EPP (V) 0.71 0.93 – 0.86
Dn (cm2 s1) 6
7.62 10 3.89 106 4.32 107 4.99 106
RS (V cm2)
Wcat (V cm2)
RCT (V cm2)
YCPE (mF cm2)
W (V cm2)
12.06 13.92 15.94 13.60
– 2.07 3.09 4.65
3.22 60.09 245.12 38.15
0.75 0.80 0.89 0.87
0.65 2.03 6.61 1.42
(Scheme 1b and c). Scheme 1b presents that the pyrrole monomer could be absorbed on the surface of the functionalized MWCNTs due to possible interactions (such as p–p coupling [23] and electrostatic interaction [33]) between the pyrrole monomer and MWCNT, and then electro-polymerized to form the PPy/MWCNT composite (Scheme 1c). Moreover, the PPy nanoparticles were also electro-deposited on the bare FTO substrate, which could enhance the cohesiveness among the MWCNT, PPy, and FTO substrate. This could prevent the direct contact between the electrolyte and the bare FTO substrate to reduce the electron recombination. The composite structure would result in good electron dislocation, facile electron transfer, and plenty of interfacial catalytic active sites. Encouraged by the special morphology, CV measurements were carried out to elucidate the electrocatalytic activities of the Pt, PPy, MWCNT, and PPy/MWCNT CEs in the I/I3 system and the distance between working electrode and counter electrode was fixed at 0.70 cm. As shown in Fig. 3, all the CEs show a typical oxidation and reduction peaks owing to the redox reaction of I3 + 2e $ 3I, which directly affects the DSSC performance [7]. In theory, the electrocatalytic activity of a CE dependents on two factors: the reduction peak current density and the peak-to-peak separation (Epp). The high reduction peak current density suggests an enhanced electroreductive behavior to I/I3 redox, and the Epp value depends not only on the standard electrochemical rate constant of a redox reaction, but also the electrode porosity [7,33,39]. However, no reduction peak was found for the MWCNT CE in this potential interval, indicating the catalystic activity of the
MWCNT CE in reduction reaction of I3 to I obviously slow. The catalytic performance of CEs is in a similar order of the reduction peak current density, which is MWCNT < PPy < PPy/MWCNT < Pt. The Epp with an order of Pt (0.71 V) < PPy/MWCNT (0.86 V) < PPy (0.93 V) (shown in Table 1), demonstrates that the CEs have an inverse order of the catalytic activity. The PPy/MWCNT CE shows an enhance electrocatalytic activity compared to that of the PPy and MWCNT CEs. This is because of that the large active surface of the MWCNT provides a lot of catalytic activity sites and improves the conductivity of the PPy CE. Fig. 4a–d show CVs of the I3/I system on the Pt, PPy, MWCNT, PPy/MWCNT CEs at different scan rates, respectively, and one can find an outward extension of all peaks. In Fig. 4e, the anodic (or cathodic) peak current density scales linearly with the square root of the scan rate, which illustrates that the redox reaction is controlled by the diffusion of iodide species either on the surface of the CEs, suggesting no specific interaction between I/I3 redox couple and CEs [7,33,34]. Moreover, the diffusion coefficient (Dn) in the Randles–Sevcik equation (Eq. (1)) can be calculated from the correlation between the peak current density (Jred) and scan rate (v). Where K represents the constant of 2.69 105, n stands for the number of electrons devoting to the charge transfer, A is the electrode area, and C is the bulk concentration of I3 species. As listed in Table 1, the Dn increases in the order of MWCNT (4.32 107 cm2 s1) < PPy (3.89 106 cm2 s1) < PPy/MWCNT (4.99 106 cm2 s1) < Pt (7.62 106 cm2 s1), confirming the catalytic activity of the CEs with an order of MWCNT < PPy < PPy/ MWCNT < Pt. The PPy/MWCNT CE has a high catalytic activity,
Fig. 4. CVs of the (a) Pt, (b) PPy, (c) MWCNT, and (d) PPy/MWCNT CEs at different scan rates (from inner to outer: 50, 75, 100, and 125 mV s1, respectively), respectively; (e) relationship between all the redox peak currents and scan rates.
W. Hou et al. / Electrochimica Acta 190 (2016) 720–728
725
Fig. 5. (a) Nyquist plots and (b) Tafel curves of the symmetrical Pt, PPy, MWCNT, and PPy/MWCNT CEs, respectively.
which is presumably originating from the MWCNT provides a channel for I3 diffusion and the catalytic activity sites of PPy nanoparticles. pffiffiffiffiffiffiffiffiffiffiffiffiffiffi Jred ¼ KAC n3 vDn ð1Þ To further understand the electrocatalytic activities of different CEs on the I3 reduction, EIS measurements were carried out using symmetric cells (Fig. 6a). The Nyquist plots (Fig. 5a) for symmetric cells with different CEs illustrate impedance characteristics. The commonly used equivalent circuit model (Fig. 6b) and electronic elements (ohmic series resistance (RS), charge-transfer resistance (RCT), Warburg impedance (W), and constant phase element (CPE)) have been already reported and defined elsewhere [7,24,33,34,40,41]. However, in this paper, an element (Wcat) representing the Warburg impedance from the charge transport resistance in the thick catalyst film should be incorporated into the equivalent circuit [25,34,42]. Fig. 6c illustrates the equivalent circuit models employed to simulate the resultant spectra of PPy, MWCNT, and PPy/MWCNT CEs. The impedance parameters fitted
Fig. 6. (a) Symmetric cell, (b) Equivalent circuit model employed to simulate the resultant spectra of Pt CE, (c) Equivalent circuit models employed to simulate the resultant spectra of PPy, MWCNT, and PPy/MWCNT CEs.
by using the Z-view software were listed in Table 1. Generally, the RCT is an index to represent the electrocatalytic activity of the CE, the smaller RCT corresponds to the less overpotential required for the electron transferring from a CE to the electrolyte. Thus the RCT increases in the order of Pt (3.22 V cm2) < PPy/MWCNT (38.15 V cm2) < PPy (60.09 V cm2) < MWCNT (245.12 V cm2), indicating an inverse order of catalytic performance [43,44]. Nevertheless, the W is of highly dependence on the electrical conduction of a CE [42]. The electrical conduction of a CE material can be evaluated by the RS value, therefore the W value follows an order of Pt < PPy/ MWCNT < PPy < MWCNT. These reveal that the I3 can be rapidly reduced to the I under the catalyst of the PPy/MWCNT to speed up the diffusion of the I3 as well as the Pt CE. In addition, the YCPE increases in the order of Pt (0.75 mF cm2) < PPy (0.80 mF cm2) < PPy/MWCNT (0.87 mF cm2) < MWCNT (0.89 mF cm2). Basically, the larger YCPE value corresponds to larger active surface area, therefore introducing the MWCNT in the PPy/MWCNT CE could increase its interfacial catalytic active sites to provide enhanced current response for the I3 reduction reaction. Because of the Pt film is very thin, thus the Wcat is not incorporated in the equivalent circuit for the Pt CE. However, the diffusion length and resisting force for the I or I3 diffusing to or away from the CE surface during the reaction are consequently increased for the thick films of the PPy, MWCNT, and PPy/MWCNT CEs, thus the Wcat in these catalysts could not be ignored. The Wcat value is of highly dependence on the thickness of the CE, therefore the largest Wcat value of the PPy/MWCNT CE can be attribute to its thickest film among the PPy, MWCNT, and PPy/MWCNT CEs. Tafel polarization curves were further performed with the dummy cells similar to those in EIS measurements, and the corresponding pictures can be shown in Fig. 5 b. Theoretically, the Tafel curve can be divided into three zones [23]. The curve at very low potential is polarization zone, arising from the electrochemical reaction. The limiting diffusion zone is the curve at very high potential, which depends on the transport of triiodide and iodide in the electrolyte. The limiting diffusion current density (Jlim) is in an order of Pt > PPy/MWCNT > PPy > MWCNT CEs, which is corresponding to the catalyst activities. The curves at relatively low potential but higher than 0.10 V always be defined as the Tafel zone, the log J is a linear function of voltage (E) according to Eq. (2). The larger slope for the anodic or cathodic branch indicates a higher exchange current density (J0) on the electrode and better catalytic activity toward I3 reduction [7]. The Pt CE displays the highest J0, followed by the PPy/MWCNT, PPy, and MWCNT CEs. This reveals the PPy/MWCNT CE has the highest electrocatalytic activity
726
W. Hou et al. / Electrochimica Acta 190 (2016) 720–728
Fig. 7. Photocurrent density-voltage characteristics of DSSCs based on the Pt, PPy, MWCNT, and PPy/MWCNT CEs, respectively.
compared to the PPy and MWCNT CEs. The similar RCT trends can be achieved from EIS according Eq. (3). E¼
2:3RTðlgJ lgJ0 Þ anF
RCT ¼
RT nFJ0
ð2Þ
ð3Þ
Where R is the gas constant, T is the temperature, a is the distribution coefficient, n is the number of electrons involved in the reaction at the electrode, and F is Faraday’ constant. Scheme 1c also shows the possible catalytic mechanism of the PPy/MWCNT CE for I3 reduction. The working mechanism of I3 reduction on PPy electrode is recently explained in detail by Lu and co-workers [36]. Overall, the electrocatalysis mechanism includes three steps: 1) formation of the weakly bonded iodine species at the PPy surface (such as PPy-I3 and PPy-I); 2) formation of the electric neutral PPy-I2 intermediates, 3) reduction of PPy-I2 into the PPy and I (or I3). As a good adhesion substrate for I3/I, PPy exhibits good electrocatalytic activity. However, the electrocatalytic activity of PPy particles was influenced by its lower conductivity and lower surface area. In the hybrid structure of PPy/ MWCNT CE, the PPy could enhance the cohesiveness among the MWCNT, PPy, and FTO substrate. The composite structure provides the fast electron transport and diffusion channels and plenty of interfacial catalytic active sites due to the the high surface area and good conductivity of the MWCNT and good catalytic activity of the PPy for the reduction of triiodide to iodide. Therefore, the PPy/ MWCNT CE displays the highest catalytic activity toward the reduction of I3 of the PPy and MWCNT CEs.Fig. 7 compares the photovoltaic performances of DSSCs based on various CEs under full sunlight illumination (100 mW cm2, AM 1.5). At least 3 pieces of DSSCs for each counter electrode were tested and the average values on photovoltaic parameters were summarized in Table 2. The DSSC with the MWCNT CE shows a lowest open-circuit voltage
(VOC) value of 0.71 V. One explanation might be due to the poor catalytic activity of MWCNT CE for I3 reduction, and other possible explanation is the adsorption of electrolyte, like 4-tertbutylpyridine onto the carbon surface [45]. The short-circuit current density (JSC) of the DSSCs based different CEs increases in an order of MWCNT < PPy < Pt < PPy/MWCNT, which can be explained that the MWCNT can provide high surface area, fast electron transport and diffusion channels and the PPy nanoparticles wrapped on MWCNT surface can provide plenty of interfacial active sites. The fill factor (FF) is generally determined by the RS of the DSSC [4,7,17,24,33]. Therefore, the comparable RS matches to its corresponding FF. The difference in FF can be attributed to the good electrical conductivity of the MWCNT reduce the charge-transfer resistance at the CE/electrolyte interface. It can be seen that when the PPy/MWCNT composite film on the FTO glass is employed as the DSSC CE, the device exhibits a VOC of 0.74 V, JSC of 17.56 mA cm2, FF of 0.55, and an energy conversion efficiency of 7.15%, which is much higher than that of the MWCNT (1.72%) and PPy (5.72%) based DSSCs, and 92.14% that of the Ptbased DSSC (7.76%). It is believed that the good photovoltaic performance of the PPy/MWCNT is attributed to the p–p coupling interaction between the PPy and MWCNT, arising from the delocalized electrons of the MWCNT and the aromatic rings of the PPy. The resultant CE can take advantage of the synergic effect of higher electronic conductivity of the MWCNT and superior catalytic activity of the PPy. This can reduce the internal series resistance and enhance FF and JSC values, which in turn lead to
Table 2 Comparison of JSC, VOC, FF, and h in the resultant DSSCs. CE
JSC (mA cm2)
VOC (V)
FF
h (%)
Pt PPy MWCNT PPy/MWCNT
17.25 16.09 14.24 17.56
0.75 0.74 0.71 0.74
0.60 0.48 0.17 0.55
7.76 5.72 1.72 7.15
W. Hou et al. / Electrochimica Acta 190 (2016) 720–728
727
Fig. 8. Current density as a function of time for the DSSC held at 0 V forward bias under AM1.5 simulated sun light.
higher conversion efficiency. Therefore, this PPy/MWCNT hybrid structure demonstrates an ideal CE material. Additionally, the current density (J)–time (s) curves of the DSSCs based different CEs were held at 0 V forward bias and measured under continuous illumination over 1000 s to study the stability. As shown in Fig. 8, generally, all of the current densities decrease as time goes on, this could be due to that all of the DSSCs were assembled in a sandwich-like architecture under an open system without any sealing, resulting in an evaporation of the electrolyte. Moreover, the PPy, MWCNT, and PPy/MWCNT CEs show an interesting phenomenon that the current densities decrease firstly and then increase finally become stable, this could be owing to that the films of these CEs are more thick than that of the Pt CE, leading to a longer diffusion length and needing more time to become stable. Furthermore, the PPy/MWCNT based DSSC exhibits a good stability with the current density retention of about 83.4% after full sunlight illuminating (100 mW cm2, AM 1.5 G) for 1000 s, which is competitive to the stability of Pt (86.4%) CE and much higher than that of PPy (67.3%) and MWCNT (62.0%) CEs. The long-term stability would be enhanced by strictly packaging for the device application. 4. Conclusion In summary, the PPy/MWCNT CE was fabricated by a facile cyclic voltammetry electro-deposition the PPy on the MWCNT spin-coated FTO substrate, in which the MWCNT were uniformly wrapped with a 15 nm thickness of PPy nanoparticles, making full use of the good conductivity and the high surface area from the MWCNTand the high catalytic activity from the PPy for the I3 reduction. Because of the formation of p–p coupling was conductive to electron delocalization and transport, this PPy/MWCNT CE showed an excellent photovoltaic performance, giving the photoelectric conversion efficiency of 7.15%, which was much higher than that of MWCNT (1.72%) and PPy (5.72%) CEs, and 92.14 percent of what the Pt CE (7.76%) under full sunlight illumination (100 mW cm2, AM 1.5 G). As a result, the PPy/MWCNT CE is a promising candidate as highly efficient and Pt-free CE material.
Acknowledgements The authors appreciate funding from National Natural Science Foundation of China (61504076, 21574076 and U1510121), National Natural Science Foundation of Shanxi Province (2015021129 and 2014011016-1), Scientific and Technologial Innovation Programs of Higher Education Institutions in Shanxi (2014103), and Scientific Research Start-up Funds of Shanxi University (203533801003). References [1] B. O’Regan, M. Grätzel, A low-cost, high-efficiency solar cell based on dyesensitized colloidal TiO2 films, Nature 353 (1991) 737. [2] A. Yella, H. Lee, H. Tsao, C. Yi, A. Chandiran, M. Nazeeruddin, E. Diau, C. Yeh, S. Zakeeruddin, M. Grätzel, Porphyrin-sensitized solar cells with cobalt (II/III)based redox electrolyte exceed 12 percent efficiency, Science 334 (2011) 629. [3] L. Han, A. Islam, H. Chen, C. Malapaka, B. Chiranjeevi, S. Zhang, X. Yang, M. Yanagida, High-efficiency dye-sensitized solar cell with a novel co-adsorbent, Energy Environ. Sci. 5 (2012) 6057. [4] J. Wu, Y. Xiao, G. Yue, Q. Tang, J. Lin, M. Huang, Y. Huang, L. Fan, Z. Lan, S. Yin, T. Sato, A large-area light-weight dye-sensitized solar cell based on all titanium substrates with an efficiency of 6.69% outdoors, Adv. Mater. 24 (2012) 1884. [5] S. Mathew, A. Yella, P. Gao, R. Humphry-Baker, B. Curchod, N. Ashari-Astani, I. Tavernelli, U. Rothlisberger, M. Nazeeruddin, M. Grätzel, Dye-sensitized solar cells with 13% efficiency achieved through the molecular engineering of porphyrin sensitizers, Nature Chem. 6 (2014) 242. [6] A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Dye-sensitized solar cells, Chem. Rev. 110 (2010) 6595. [7] Y. Xiao, G. Han, Y. Li, M. Li, J. Lin, Three-dimensional hollow platinum-nickel bimetallic nanoframes for use in dye-sensitized solar cells, J. Power Sources 278 (2015) 149. [8] Y. Xiao, J. Wu, G. Yue, J. Lin, M. Huang, L. Fan, Z. Lan, The surface treatment of Ti meshes for use in large-area flexible dye-sensitized solar cells, J. Power Sources 208 (2012) 197. [9] Y. Xiao, J. Wu, J. Lin, M. Huang, L. Fan, Z. Lan, G. Han, S. Li, Low temperature fabrication of high performance p-n junction on the Ti foil for use in large-area flexible dye-sensitized solar cells, Electrochim. Acta 117 (2014) 1. [10] E. Olsen, G. Hagen, S. Lindquist, Dissolution of platinum in methoxy propionitrile containing LiI/I2, Sol. Energy Mater. Sol. Cells 63 (2000) 267. [11] K. Suzuki, M. Yamaguchi, M. Kumagai, S. Yanagida, Application of carbon nanotubes to counter electrodes of dye-sensitized solar cells, Chem. Lett. 32 (2003) 28. [12] H. Wang, Y. Hang, Graphene as a counter electrode material for dye-sensitized solar cells, Energy Environ. Sci. 5 (2012) 8182.
728
W. Hou et al. / Electrochimica Acta 190 (2016) 720–728
[13] P. Poudel, Q. Qiao, Carbon nanostructure counter electrodes for low cost and stable dye-sensitized solar cells, Nano Energy 4 (2014) 157. [14] G. Li, J. Song, G. Pan, X. Gao, Highly Pt-like electrocatalytic activity of transition metal nitrides for dye-sensitized solar cells, Energy Environ. Sci. 4 (2011) 1680. [15] M. Wu, Q. Zhang, J. Xiao, C. Ma, X. Lin, C. Miao, Y. He, Y. Gao, A. Hagfeldt, T. Ma, Two flexible counter electrodes based on molybdenum and tungsten nitrides for dye-sensitized solar cells, J. Mater. Chem. 21 (2011) 10761. [16] H. Sun, D. Qin, S. Huang, X. Guo, D. Li, Y. Luo, Q. Meng, Dye-sensitized solar cells with NiS counter electrodes electrodeposited by a potential reversal technique, Energy Environ. Sci. 4 (2011) 2630. [17] Y. Xiao, J. Wu, J. Lin, S. Tai, G. Yue, Pulse electrodeposition of CoS on the MWCNT/Ti as a high performance counter electrode for the Pt-free dyesensitized solar cell, J. Mater. Chem. A 1 (2013) 1289. [18] Y. Xiao, J. Wu, J. Lin, G. Yue, J. Lin, M. Huang, Y. Huang, Z. Lan, L. Fan, A high performance Pt-free counter electrode of nickel sulfide/multiwall carbon nanotubes/titanium used in dye-sensitized solar cells, J. Mater. Chem. A 1 (2013) 13885. [19] H. Chuang, C. Li, M. Yeh, C. Lee, R. Vittal, K. Ho, A coral-like film of Ni@NiS with core-shell particles for the counter electrode of an efficient dye-sensitized solar cell, J. Mater. Chem. A 2 (2014) 5816. [20] Y. Xiao, G. Han, H. Zhou, Y. Li, J. Lin, Nickel sulfide counter electrodes enhanced by hydrosulphuric acid hydrothermal treatments for use in Pt-free dyesensitized solar cells, Electrochim. Acta 155 (2015) 103. [21] J. Wu, Q. Li, L. Fan, Z. Lan, Pi. Li, J. Lin, S. Hao, High-performance polypyrrole nanoparticles counter electrode for dye-sensitized solar cells, J. Power Sources 181 (2008) 172. [22] S. Jeon, C. Kim, J. Ko, S. Im, Spherical polypyrrole nanoparticles as a highly efficient counter electrode for dye-sensitized solar cells, J. Mater. Chem. 21 (2011) 8146. [23] C. Bu, Q. Tai, Y. Liu, S. Guo, X. Zhao, A transparent and stable polypyrrole counter electrode for dye-sensitized solar cell, J. Power Sources 221 (2013) 78. [24] 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. [25] Y. Xiao, J. Lin, J. Wu, S. Tai, G. Yue, Pulse potentiostatic electropolymerization of high performance PEDOT counter electrodes for Pt-free dye-sensitized solar cells, Electrochim. Acta 83 (2012) 221. [26] Y. Xiao, G. Han, Efficient hydrothermal-processed platinumenickel bimetallic nano-catalysts for use in dye-sensitized solar cells, J. Power Sources 294 (2015) 8. [27] Y. Xiao, G. Han, R. Wu, Y. Li, M. Li, Efficiently cubic platinum-cobalt bimetallic nano-catalysts for use in low-cost dye-sensitized solar cells, Electrochim. Acta 174 (2015) 770. [28] Y. Feng, S. Huang, Y. Qi, Y. Feng, F. You, Decrease the OFF state current of carbon nanotube field effect transistors via continuous repeated gate sweeping, J. Nanosci. Nanotechno. 11 (2011) 10544. [29] S. Peng, Y. Wu, P. Zhu, V. Thavasi, S. Mhaisalkar, S. Ramakrishna, Facile fabrication of polypyrrole/functionalized multiwalled carbon nanotubes
[30]
[31]
[32]
[33]
[34]
[35]
[36]
[37] [38]
[39]
[40]
[41] [42]
[43]
[44] [45]
composite as counter electrodes in low-cost dye-sensitized solar cells, J. Photochem. Photobiol. A 223 (2011) 97. Z. Yang, T. Chen, R. He, H. Li, H. Lin, L. Li, G. Zou, Q. Jia, H. Peng, A novel carbon nanotube/polymer composite film for counter electrodes of dye-sensitized solar cells, Polym. Chem. 4 (2013) 1680. B. He, Q. Tang, J. Luo, Q. Li, X. Chen, H. Cai, Rapid charge-transfer in polypyrrolesingle wall carbon nanotube complex counter electrodes: Improved photovoltaic performances of dye-sensitized solar cells, J. Power Sources 256 (2014) 170. G. Yue, L. Wang, X. Zhang, J. Wu, Q. Jiang, W. Zhang, M. Huang, J. Lin, Fabrication of high performance multi-walled carbon nanotubes/polypyrrole counter electrode for dye-sensitized solar cells, Energy 67 (2014) 460. Y. Xiao, J. Lin, J. Wu, S. Tai, G. Yue, Dye-sensitized solar cells with highperformance PANI/MWCNT counter electrodes electropolymerized by a pulse potentiostatic technique, J. Power Sources 233 (2013) 320. Y. Xiao, J. Lin, S. Tai, S. Chou, G. Yue, J. Wu, Pulse electropolymerization of high performance PEDOT/MWCNT counter electrodes for Pt-free dye-sensitized solar cells, J. Mater. Chem. 22 (2012) 19919. Y. Saito, W. Kubo, T. Kitamura, Y. Wada, S. Yanagida, I/I3 redox reaction behavior on poly(3,4-ethylenedioxythiophene) counter electrode in dyesensitized solar cells, J. Photochem. Photobiol. A 164 (2004) 153. S. Lu, S. Wang, R. Han, T. Feng, L. Guo, X. Zhang, D. liu, T. He, The working mechanism and performance of polypyrrole as a counter electrode for dyesensitized solar cells, J. Mater. Chem. A 2 (2014) 12805. F. Inoue, R.A. Ando, C.M.S. Izumi, P. Corio, Spectroscopic Characterization of Carbon Nanotube-Polypyrrole Composites, J. Mater. Chem. C 118 (2014) 18240. H. Guo, H. Zhu, H. Lin, J. Zhang, Polypyrrole–multi-walled carbon nanotube nanocomposites synthesized in oil–water microemulsion, Colloid Polym Sci 286 (2008) 587. J. Roy-Mayhew, D. Bozym, C. Punckt, I. Aksay, Functionalized graphene as a catalytic counter electrode in dye-sensitized solar cells, ACS Nano 4 (2010) 6203. X. Wu, H. Ma, S. Chen, Z. Xu, A. Sui, General equivalent circuits for faradaic electrode processes under electrochemical reaction control, J. Electrochem. Soc. 146 (1999) 1847. A.K. Jonscher, Dielectric Relaxation in Solids, Chelsea Dielectric, London, 1983. J. Chen, B. Li, J. Zheng, J. Zhao, H. Jing, Z. Zhu, Polyaniline nanofiber/carbon film as flexible counter electrodes in platinum-free dye-sensitized solar cells, Electrochim. Acta 56 (2011) 4624. A. Fujiwara, Y. Matsuoka, Y. Matsuoka, H. Suematsu, N. Ogawa, K. Miyano, H. Kataura, Y. Maniwa, S. Suzuki, Y. Achiba, Photoconductivity of single-wall carbon nanotube films, Carbon 42 (2004) 919. G. Mor, K. Shankar, M. Paulose, O. Varghese, C. Grimes, Use of highly-ordered TiO2 nanotube arrays in dye-sensitized solar cells, Nano Lett. 6 (2006) 215. T. Murakami, S. Ito, Q. Wang, M. Nazzeruddin, T. Bessho, I. Caser, P. Liska, R. Humphry-Baker, P. Comte, P. Péchy, M. Grätzel, Highly efficient dye-sensitized solar cells based on carbon black counter electrodes, J. Electrochem. Soc. 153 (2006) A2255.
