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polyaniline nanocomposites as counter electrodes for dye-sensitized solar cells
Accepted Manuscript Ultrasonic irradiation preparation of graphitic-C3N4/polyaniline nanocomposites as counter electrodes for dye-sensitized solar cells Mohaddeseh Afshari, Mohammad Dinari, Mohamad Mohsen Momeni PII: DOI: Reference:
S1350-4177(17)30593-X https://doi.org/10.1016/j.ultsonch.2017.12.023 ULTSON 4007
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
5 October 2017 14 December 2017 14 December 2017
Please cite this article as: M. Afshari, M. Dinari, M. Mohsen Momeni, Ultrasonic irradiation preparation of graphiticC3N4/polyaniline nanocomposites as counter electrodes for dye-sensitized solar cells, Ultrasonics Sonochemistry (2017), doi: https://doi.org/10.1016/j.ultsonch.2017.12.023
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Revised Ultrasonic
irradiation
preparation
of
graphitic-
C3N4/polyaniline nanocomposites as counter electrodes for dyesensitized solar cells
Mohaddeseh Afshari, Mohammad Dinari*, Mohamad Mohsen Momeni*
Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Islamic Republic of Iran
*
Corresponding author. Tel.; +98-31-3391-3270; FAX: +98-31-3391-2350.
E-mail address: [email protected], [email protected]. (M. Dinari). *
Corresponding author. Tel.; +98-31-3391-3283; FAX: +98-31-3391-2350.
E-mail address: [email protected] (MM. Momeni).
1
Abstract In this research, polyaniline/ graphitic carbon nitride (PANI/g-C3N4) nanocomposites were synthesized via in-situ electrochemical polymerization of aniline monomer whit different number of cyclic voltammetry scans (10, 20 and 30 cycles) after electrode surface prepreparation
using
a
potential
shock
under
ultrasonic
irradiation.
PANI/g-C3N4
nanocomposites with two values of g-C3N4 (0.010%w and 0.015%w) were deposited on the surface of the transparent conducting film (FTO glass) by immersing FTO into the aniline solution and g-C3N4 during the electro-polymerization. The resulting PANI/g-C3N4 films were characterized by Fourier transformed infra-red (FTIR), power X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) techniques. The prepared electrodes were applied as counter electrode in dye-sensitized solar cells. Among them, the prepared electrode with 10 cycles and 0.01%w gC3N4 showed the best efficiency. These hybrids show good catalytic activity in elevating triiodide reduction and due to the synergistic effect of PANI and g-C3N4, PANI/g-C3N4 nanocomposite electrode shows power conversion efficiency about 1.8%.
Keywords: Graphitic carbon nitride (g-C3N4); Ultrasonic Irradiation; Polyaniline; Dyesensitized solar cell
2
1. Introduction Dye-sensitized solar cells (DSSC) have concerned attention as cheering, low cost systems, which convert the solar energy into electricity with no emission [1-8]. Conventional DSSCs made from three main components: platinum (Pt) counter electrode (CE) layer, a liquid electrolyte containing dissolved tri iodides (I3-)/ iodide (I-) couple and nanocrystalline titanium dioxide (TiO2) coated with dye molecules as a photo-electrode layer (PE) [9,10]. The high cost of Pt CE in DSSCs devices has limited the commercialization and expansion of the use of DSSCs. In order to overcome this limitation, Pt electrode should be replaced by cost-effective CE candidates with suitable electrocatalytic activity against I3- reduction and fast charge transfer [11, 12]. To reduce the cost of fabrication of DSSCs, substitute materials such as composites, conducting polymers, or carbonaceous materials, have been investigated through replacement of Pt used in DSSCs [13, 14]. Among the mentioned compounds, carbonaceous materials due to their high electrical conductivity, high stability, and low cost are one of the best materials used to replace Pt in DSSC devices. Various types of carbon materials, such as graphene [15], graphene quantum dots [16], carbon nanotubes [17, 18], carbon black [19], and porous carbon [20], have been studied as the CE in DSCs. A literature survey in this field shows that carbonaceous materials have interesting performance in DSSC for example, Hwang et al. have reported DSSC free Pt counter electrodes with high conductivity, good electrochemical activity, and low cost based on carbon nanotubes [21] and Bora et al. have synthesized polythiophene/graphene composite as a highly efficient platinum-free counter electrode whit power 4.8% conversion efficiency [22]. Also Zhang et al. have fabricated the bifacial DSSCs from covalent-bonded polyaniline-multiwalled carbon nanotube complex as a CE with maximum energy conversion efficiency of 9.24%, which is higher than 8.08% for pure PANI as a CE [23].
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Despite all these benefits, the electrocatalytic activity of these carbon materials is still not able to compete with Pt [24]. In recent years, researchers improved the electrocatalytic activity and improve the surface hydrophilicity of these compounds through the doping of nitrogen inside the carbon frameworks [25]. Nitrogen-doped carbon materials exhibit greater electrocatalytic activity and better chemical stability than Pt [26]. Easwaramoorthi et al. have synthesized
soft-template
of
simple
ordered
mesoporous
titanium
nitride-carbon
nanocomposite for high performance DSSC Counter Electrodes [27]. Also, Xu et al. have prepared the liquid-based growth of polymeric carbon nitride layers and their use in a mesostructured polymer solar cell with Voc exceeding 1 V [28]. Sonication due to various advantages including reducing of reaction time and decrease reaction temperature considered as multi propose method for nanomaterial preparation. The ultrasound waves produce high local pressure (500atom) and temperatures (5000K) owing to the formation of acoustic cavities. During acoustic cavitation, the cavities growth and created bubbles in a liquid which lead to local temperature and pressure. The ultrasound waves could reduce particles size and change the morphology of nanoparticles (NPs). Finally, ultrasonication provides a better dispersion of nanoparticles which provide a homogenous NP distribution in the polymer matrix [29-33]. Graphitic carbon nitride (g-C3N4), a two-dimensional (2D) structure, as a low-cost nitrogen rich precursor exhibited strong attraction due to its superior chemical and thermal stability, interesting electronic structure and good biocompatibility and considered as a potential candidate for nanocomposites preparation [34-37]. Wang et al. have reported the gC3N4/MWCNTs composite as Pt-free counter electrode for high-efficiency dye-sensitized solar cells [38]. Wu et al. have reported the synthesis of two dimensional graphitic-phase C3N4 as a multifunctional protecting layer to enhance short-circuit photocurrent in DSSC [39]. Due to the high percentage of nitrogen content, g-C3N4 shows high active
4
electrocatalytic sites which leads to an increase in its capacity for I3- reduction reaction [40, 41]. However, the electrocatalytic activity of g-C3N4 limited due to its low electrical conductivity [24]. Doping metals in the structure or using carbon-based conductors such as conductive carbon black composite is an effective way to improve the transfer of electrons within the network and increase the electrocatalytic activity of g-C3N4. Conducting polymers have been extensively studied, and have proved to be promising candidates for Pt replacement. Among conducting polymers, polyaniline (PANI) is the most noteworthy material to replace Pt CE due to simple synthesis, substantial catalytic activity, and well environmental stability [42-46]. However, PANI is an organic semiconductor and inherently has the relatively low ability to charge transfer [16]. Nevertheless, its electrocatalytic activity and long-term stability of pure PANI still cannot compete with Pt, as a result, the energy conversion efficiency of these systems is low. The coupling of PANI with carbon nanomaterials is the most appropriate choice to increase its conductivity [13-16, 47, 48]. Herein, PANI/g-C3N4 hybrids were synthesized under ultrasonic irradiation via in situ polymerization of aniline monomers in the presence of various percentages of g-C3N4 on transparent surfaces of FTO to improve the electrocatalytic activity and increase the speed and capacity of charge transfer. The catalytic activity and morphology of PANI/g-C3N4 hybrids as counter electrodes was investigated.
2. Experimental 2.1. Materials Aniline (purity>99%), N,N-dimethylformamide (DMF, purity>99.5%), cyanuric chloride (purity>99%), N,N-diisopropylethylamine (98%), melamine (purity>99%), and sulfuric acid (H2SO4, 98%) were purchased from Sigma Aldrich and Merck chemical company. The monomer of aniline was distilled under reduced pressure and then stored at low temperature
5
before use. Distilled water was used for the preparation of all the solutions. The other of material was used without further purification.
2.2. Apparatus Electrochemical
experiments
were
fulfilled
using
a
computerized
potentiostat/galvanostat (SAMA 500, Iran). The electrocyclic voltammetry was conducted in a solution of acetonitrile containing 10 mM KI, 1 mM I2 and 0.1 M LiClO4. The electrochemical experiments were performed in a three-electrode cell assembly. The platinum plate as a counter electrode, saturated calomel (SCE) as a reference electrode and all potentials are measured relative to the reference electrode. A Qsonica XL 2000 with the power of 100 W at 50/60 kHz was utilized in the synthesis procedure. Fourier transform infrared (FT-IR) spectra of samples were carried out with a Jasco-680 spectrometer (TokyoJapan) in the wavelength range of 400–4000 cm–1, using KBr pallet technique by performing 60 scans at 4 cm−1. X-ray diffraction (PXRD) profiles were obtained with a Bruker advanced powder X-ray diffraction system (D8 Advance, Germany) at 45 kV and 100 mA using Cu Kα radiation. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) was performed on a Philips CM120 microscope (Eindhoven, Netherland) and a HITACHI S-4160 (Tokyo-Japan), respectively, to observe the surface morphology and structure of obtained nanoparticles and nanocomposites. Thermogravimetric analysis (TGA) of the sample was performed on a STA503 TA instrument (Hullhorst, Germany) under nitrogen atmosphere with a heating rate of 10 °C.min−1 from 298 K to 1073 K.
2.3. Synthesis of g-C3N4
6
Graphitic-C3N4 was prepared via the multi-step reaction between cyanuric chloride and melamine in the presence of N,N-diisopropyleethylamine according to a recently published article [48]. In order to separate the g-C3N4 layers from each other and maintain the network structure, the use of ultrasonic waves replaced the utilization of chemical solvents or harsh acidic condition. Accordingly, the obtained product was sonicated for 10 hours with a power of 100 W at 50/60 kHz frequency distribution and then dried at ambient temperature for 24 hours (Fig. 1). Fig. 1 2.4. Preparation of PANI/g-C3N4 films Prior to assembly, fluorine-doped tin oxide (FTO) glass substrates (8 ohm sq-1, Solaronix) were cleaned by immersing in the cleaning solution containing 30% isopropyl alcohol, 30% acetonitrile and the rest double-distilled water under sonication with power of 100 W and Frequency of about 60 kHz. Ultrasonic waves, in addition to improving the washing process and eliminating surface contaminations, by creating a smooth surface will produce a uniform and non-stressed film during polymerization process. Aniline monomer was conducted on glass electrode in acidic solution by electro-polymerization technique. For the preparation of CE, 0.1 M of aniline monomer was added to 0.5 M H2SO4 solution and then 0.01 or 0.015 wt. % of g-C3N4 was added to the mixture solution. The mixture was exposed to ultrasound irradiations with the power of 100 W and 60 kHz frequency due to severe dispersing the gC3N4 in acidic solution and forming a sustained suspension for the next steps. Electropolymerization occurred through 10, 20 or 30 consecutive cyclic voltammetry scans in the 0.0 to 0.8 V potential ranges. The temperature adjusted at 25 oC. The experimental conditions for different samples are summarizing in Table 1. The experimental setup for preparation FTO and synthesize the PANI/g-C3N4 films have been shown in Fig. 2a and 2b respectively. Table 1
7
Fig. 2 2.5. Fabrication and cell assembly To fabricate solar cells, TiO2 film was deposited on FTO by doctor blade technique, followed by sintering at 500 oC for 30 min. For DSSC construction, the samples were immersed in 0.3 mM ethanol solution of dye N719 for 24 h to be dye-sensitized with Ru based N719 dye (cisbis
(isothiocyanato)
bis
(2,
2-bipyridyl
4,4-dicarboxylato)
ruthenium
(II)
bis-
tetrabutylammonium) (Solaronix SA, Switzerland) and used as the photo-anode of the DSSC. The prepared PANI-g-C3N4/FTO glass electrodes were used as the photocathode. Sandwich cells were assembled by sealing the anodic electrode of TiO2 together with the photocathode using a thermo plastic spacer with a thickness of 60 microns at 120-150 oC. An electrolyte solution of 0.5 M LiI, 0.05 mM I2, and 0.5 M tert-butylpyridine in 3-methoxypropionitrile was introduced into the cell through a hole in the FTO electrode. The current-voltage characteristics of the cells were measured under AM 1.5, 100 mW/cm2 simulated light radiations.
3. Results and discussion 3.1. FT-IR study The FT-IR spectra of the g-C3N4, pure PANI and hybrids of PANI/g-C3N4 are shown in Fig. 3. In the FT-IR spectrum of g-C3N4, the N-H groups show the broad peak at 30003400 cm-1. The characteristic peaks for stretching modes of C=N and C-N heterocycles appear at 1241 and 1570 cm-1. Moreover, the characteristic breathing modes of triazine units were observed at 808 and 879 cm-1 [49]. As shown in Fig. 3, the main characteristic peaks of pure PANI can be assigned as follows: the band at 3360 cm-1 was attributed to hydrogen bonding between the N–H of amine and imine sites. The bands at 1562 and 1481 cm-1 can be assigned to C=N and C=C stretching of the quinonoid and benzenoid rings, respectively. The
8
bands at 1302 and 1240 cm-1 were attributed to the C–N stretching mode for the benzenoid unit, while the band at 1111 cm-1 to the quinonoid unit of PANI. The peak at 767 cm-1 is associated with C-C and C-H in the benzenoid unit [16]. In the FT-IR spectrum of the PANI/g-C3N4 sample, the characteristic peaks from PANI around 1302, 1481 and 1562 cm-1 shift to higher wave numbers of 1305, 1483 and 1567 cm-1 after the g-C3N4 is introduced. By the way, the hydrogen bond absorption at 3368 cm-1 is strengthened. All of the above peaks can be seen from spectrum of PANI/g-C3N4 composite film, showing PANI is existent in the hybrid NCs. Fig. 3 3.2. Thermogravimetric analysis (TGA) Thermal behavior of prepared g-C3N4 was investigated by TGA technique. The analysis was performed at 25 to 800 °C and heating rate of 10 °C min-1 under nitrogen atmosphere. As seen in Fig. 4, g-C3N4 shows a one weight loss about 700 °C which proved high thermal stability of prepared nanoparticles. Fig. 4.
3.3. X-ray diffraction The PXRD patterns of the g-C3N4, PANI, and PANI/g-C3N4 nanocomposites are shown in Fig. 5. The PXRD pattern of pure g-C3N4 contains clear peaks in 12.6 and 27.4, which respectively relate to the (100) plane diffraction arising from the in-plane repeating motifs of the sequential triazine network and the (002) plane arising from the stacking of the conjugated aromatic system [50]. In the pure PANI, the crystalline plates of (011), (020) and (200) are marked with special peaks at 15.8o, 20.5o and 26.3o [50]. When g-C3N4 was incorporated into the PANI matrix, the diffraction peak of g-C3N4 at 27.4 o (002) overlapped with the peak of PANI which results in the broad and intense peak in the composite. The data
9
indicates that no additional crystalline order has been introduced into the composite. Compared with functionalized g-C3N4, the obvious characteristic peaks in PANI/ g-C3N4 can be ascribed to the formation of crystal appearing on the outer layers of g-C3N4. Fig. 5 3.4.
Morphology study: TEM and FE-SEM and BET analysis The morphology and microstructure of the synthesized g-C3N4 were investigated by
FE-SEM and TEM techniques and their results are presented in Fig. 6. The FE-SEM images of g-C3N4 show random stacking clubbed morphology with anomalous smooth nanorods structure (Figs. 6a and 6b). Also, according to the TEM analysis, the g-C3N4 shows platelet layered morphology analogous to graphene like materials (Figs. 6c and 6d). Also, the BET analysis was employed to investigate the active surface area of prepared g-C3N4. Brunauer– Emmett–Teller (BET) surface area of 75.2 m2.g-1 and average pore diameter equal 6.1nm was obtained for synthesized g-C3N4. The observed data indicate microporous pores in g-C3N4. Fig. 6. The nanostructure and morphology of the pure PANI, as well as PANI/g-C3N4 nanocomposites with 0.01 and 0.015 g of g-C3N4, was investigated by FE-SEM techniques. Pristine PANI showed flake-like agglomerations which were stacked by spherical particles (Fig. 7). After incorporating g-C3N4 to the PANI, the g-C3N4 nanosheets have a tendency to be present in the form of small nanosheets. For PANI/g-C3N4 nanocomposites with 0.01 of gC3N4, the composite show spherical structure where g-C3N4 particles get deposited on the surface of PANI (Figs. 8a and 8b). But, for PANI/g-C3N4 nanocomposites with 0.015 of gC3N4, road-like morphology was observed (Figs. 8c and 8d). Fig. 7 Fig. 8
10
3.5. Electrochemical and photovoltaic characterization Electro-polymerization of PANI/g-C3N4 as counter electrode using cyclic voltammetry in a 0.1 M of aniline solution and 0.5 M of H2SO4 in the presence of 0.01 or 0.015 wt. % of gC3N4 at scan rate of 10 mV s-1 occurred in two steps. At the first step, in order to activate the FTO surface and requires fewer cycles to achieve a relatively certain thickness of polyaniline film, the polymerization reaction was performed in the potential range of 0.0-1.3 V and the second-step it occurred in the potential range of 0.0-0.8 V. Diagrams of cyclic voltammetry is shown in Fig. 9. The results of using two-step applying potential on the CV diagram indicate a significant increase in the current density in the second potential interval for the number of cycles of 10, 20, or 30, which prove the activation of FTO surface in preelectropolymerization. The formation and growth of PANI chains can be investigated using the intensity of the redox peaks in the CV curves. It is clearly seen that after each cycle the peak current density increased (Fig. 9a). As mentioned earlier, g-C3N4 cause faster and easier transfer of electrons from the electrolyte solution to polymer and performed the redox reactions. As a result, the PANI/g-C3N4 electrode has a faster increase in the peak intensities relative to PANI electrode (Figures 9b and 9c). Fig. 9 To elucidate the distinction in electrocatalytic properties and reaction kinetics between the Pt CEs and various PANI/g-C3N4 composite, CV curves were recorded, of which the detailed results are shown in Figure. 10. In all of the CV patterns, two pairs of the redox peaks are well observed. The left pairs are related to the oxidation of I-/I3- (eq. 1) and the right pairs have explained the reduction of I2/I3- (eq. 2). 3I- ↔ I3- + 2e-
(1)
2I3- ↔ 3I2 + 2e-
(2)
11
The pairs peak observed on the left side of the CV curve shows the I3- to I- reaction. Therefore, the current density of these peaks is investigated to determine the efficiency of the auxiliary electrodes. The catalytic activity of DSSCs based on PANI/g-C3N4 CEs were found to be better than that of Pt due to its higher peak current density. According to this result, the PANI/g-C3N4 nanocomposite could be used as an effective CE in DSSC devices. Fig. 10 Photocurrent density-photovoltage (J–V) curves of PANI/g-C3N4 composite as CEs in DSSCs devices are shown in Fig. 9. The corresponding photovoltaic parameters for these CEs are summarized in Table 2. The photovoltaic performance of the DSSCs based on different PANI/g-C3N4 counter electrodes showed in Fig. 11. When PANI/g-C3N4 composites are used as CE, the highest short-circuit current density (JSC) 7.6 mA cm-2 and a modest fill factor (FF) 0.39, and open-circuit voltage (VOC) 0.44 V, resulted in a conversion efficiency of 1.79%, (in sample A) this is attributed to the higher electrochemical catalytic activity of this electrode in comparison with other samples. The results of this work were compared with the previous work in Table 3. Fig. 11 Table 2 Table 3
4. Conclusions PANI/g-C3N4 nanocomposites with different concentration of g-C3N4 in PANI and various cycle numbers were successfully synthesized by in-situ polymerization. The various hybrids were coated on the FTO substrates to construct CEs for DSSCs. According to FESEM results, the hybrids show road-like morphology. Three cycle numbers (10, 20 and 30) were employed for the preparation of electrodes, which the minimum cycle number shows
12
the best efficiency. The better efficiency was observed for PANI/g-C3N4 counter electrode comparing with PANI counter electrode. The obvious feature of easy synthesis, the multilateral electrical and photovoltaic performance of multilayer films based on conductive polymers, puts into be good CE materials in DSSCs. The profound advantages in easy synthesis, versatile electrical and photovoltaic performances promise the multilayer films to be good CE materials in DSSCs.
Acknowledgments This work was partially funded by the Research Affairs Division of Isfahan University of Technology (IUT), and Iran Nanotechnology Initiative Council (INIC).
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[33] G. H. Yang, A. Abulizi, and J. J. Zhu, Sonochemical fabrication of gold nanoparticles– boron nitride sheets nanocomposites for enzymeless hydrogen peroxide detection, Ultrason. Sonochem. 21 (2014) 1958-1963. [34] V. Lau, M. Mesch, V. Duppel, V. Blum, J. Senker, B. Losch, Low-Molecular-Weight Carbon Nitrides for Solar Hydrogen Evolution, J. Am. Chem. Soc. 137 (2015) 1064-1072. [35] H. Zhang, L. Zhao, F. Geng, L.H. Guo, B. Wan, Y. Yang, Carbon dots decorated graphitic carbon nitride as an efficient metal-free photocatalyst for phenol degradation, Appl. Catal., B 180 (2016) 656-662. [36] S. Liu, D. Li, H. Sun, H.M. Ang, M.O. Tade, S. Wang, Oxygen functional groups in graphitic carbon nitride for enhanced photocatalysis, J. Colloid Interface Sci. 468 (2016) 176-82. [37] G. Wang, S. Kuang, J. Zhang, S. Hou, S. Nian, Graphitic carbon nitride/multiwalled carbon nanotubes composite as Pt-free counter electrode for high-efficiency dye-sensitized solar cells, Electrochim. Acta 187 (2016) 243-248. [38] D. Wu, K. Cao, F. Wang, H. Wang, Z. Gao, F. Xu, Y. Guo, K. Jiang, Two dimensional graphitic-phase C3N4 as multifunctional protecting layer for enhanced short-circuit photocurrent in ZnO based dye-sensitized solar cells, Chem. Eng. J. 280 (2015) 441–447. [39] Y. Zheng, J. Liu, J. Liang, Graphitic carbon nitride materials: controllable synthesis and applications in fuel cells and photocatalysis, Energy Environ. Sci. 5 (2012) 6717-6731. [40] J. Xu, Y. Wang, Y. Zhu, Nanoporous Graphitic Carbon Nitride with Enhanced Photocatalytic Performance, Langmuir 29 (2013) 10566-10572. [41] Q. Tang, H. Cai, S. Yuan, X. Wang, Counter electrodes from double-layered polyaniline nanostructures for dye-sensitized solar cell applications J. Mater. Chem. A 1 (2013) 317323.
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[42] K.H. Park, S.J. Kim, R. Gomes, High performance dye-sensitized solar cell by using porous polyaniline nanotubes as counter electrode, A. Bhaumik, Chem. Eng. J. 260 (2015) 393-398. [43] R. K. Sonker, B. C. Yadav, G. I. Dzhardimalieva, Preparation and Properties of Nanostructured PANI Thin Film and Its Application as Low Temperature NO2 Sensor, J. Inorg. Organomet. Polym. 26 (2016) 1428–1433. [44] S. Ghani, R. Sharif, S. Shahzadi, N. Zafar, A.W. Anwar, A. Ashraf, A.A. Zaidi, A.H. Kamboh, S. Bashir, Simple and inexpensive electrodeposited silver/polyaniline composite counter electrodes for dye-sensitized solar cells, J. Mater. Sci. 50 (2015) 1469–1477. [45] R. Tas, M. Gulen, M. Can, S. Sönmezoglu, Effects of solvent and copper-doping on polyaniline conducting polymer and its application as a counter electrode for efficient and cost-effective dye-sensitized solar cells, Synth. Met. 212 (2016) 75-83. [46] R. K. Sonker, B. C. Yadav, A. Sharma, M. Tomar, V. Gupta, Experimental investigations on NO2 sensing of pure ZnO and PANI–ZnO composite thin films, RSC Adv. 6 (2016) 56149–56158. [47] A. Bora, K. Mohan, P. Phukan, and S. K. Dolui, A low cost carbon black/ polyaniline nanotube composite as efficient electro-catalyst for triiodide reduction in dye sensitized solar cells, Electrochim. Acta 259 (2018) 233-244. [48] M. Dinari, M.M. Momeni, M. Afshari, Fabrication and characterization of hybrid films based on polyaniline and graphitic carbon nitride nanosheet, J. Appl. Polym. Sci. 133 (2016) 44059. [49] J.H. Sun, J.S. Zhang, M.W. Zhang, M. Antonietti, X.Z. Fu, X.C. Wang, Bioinspired hollow semiconductor nanospheres as photosynthetic nanoparticles, Nat. Commun. 2 (2012) 1139.
18
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19
Table 1: The experimental parameters for the synthesized samples. Samples A
B
C
D
E
F
Electro-polymerization solution
Electro-polymerization condition
0.5 M H2SO4 solution containing of
10 successive cyclic voltammetry scans, scan rate of 10 mV s-1
0.1 M aniline and 0.01 wt.% g-C3N4 0.5 M H2SO4 solution containing of
20 successive cyclic voltammetry scans, scan rate of 10 mV s-1
0.1 M aniline and 0.01 wt.% g-C3N4 0.5 M H2SO4 solution containing of
30 successive cyclic voltammetry scans, scan rate of 10 mV s-1
0.1 M aniline and 0.01 wt.% g-C3N4 0.5 M H2SO4 solution containing of
10 successive cyclic voltammetry scans,
0.1 M aniline and 0.015 wt.% g-C3N4
scan rate of 10 mV s-1
0.5 M H2SO4 solution containing of
20 successive cyclic voltammetry scans,
0.1 M aniline and 0.015 wt.% g-C3N4
scan rate of 10 mV s-1
0.5 M H2SO4 solution containing of
30 successive cyclic voltammetry scans,
0.1 M aniline and 0.015 wt.% g-C3N4
scan rate of 10 mV s-1
Table 2: DSSCs performance. J sc (mA/cm2)
Voc (v)
η [%]
Sample A
7.56
0.59
1.786
Sample B
1.97
0.44
0.349
Sample C
0.93
0.45
0.153
Sample D
1.01
0.39
0.143
Sample E
0.96
0.37
0.110
Sample F
0.58
0.38
0.083
JSC: short-circuit current, VOC: open-circuit voltage, η: efficiency
20
Table 3: Comparison efficiency of PANI/g-C3N4 electrode with previous literature reports. Composite J sc (mA/cm2) Voc (v) η [%] Ref 4-ATP/PANI/(CD)
8.340
0.682
4.500
51
ZnO/PANI/(100 mg/L)
2.324
0.566
0.640
52
ZnO/PANI/(200 mg/L)
1.692
0.610
0.430
52
PAA/g-CTAB/PANI
14.20
0.657
6.680
53
PVA/(0.25%)MWCNT/PAni
6.700
0.370
1.140
54
EB-SWCNTs/ZnO
0.155
0.555
0.170
55
Sample A
7.560
0.590
1.786
current research
21
Figures Captions: Fig. 1. Preparation of g-C3N4 nanosheets by ultrasonic sonication. Fig. 2. Schematic procedure of preparation PANI/g-C3N4 films a: cleaning process b: electropolymerization. Fig. 3. FT-IR spectra of g-C3N4, PANI and PANI/g-C3N4 nanocomposite. Fig. 4. Thermogram of g-C3N4 under N2 atmosphere. Fig. 5 PXRD pattern of g-C3N4, PANI and PANI/g-C3N4 nanocomposite. Fig. 6 FE-SEM (a, b) and TEM (c, d) images of g-C3N4. Fig. 7 FE-SEM photographs with different Magnify of synthesized PANI. Fig. 8 FE-SEM images FE-SEM photographs of PANI/g-C3N4 CEs in the presence of (a, b) 0.010 g-C3N4 and in the presence of (c, d) 0.0l5 g-C3N4 with different magnification. Fig. 9 CV curves of solution of 0.5 M H2SO4 containing 0.1 M aniline monomers during electrochemical deposition at a scan rate of 10 mV s-1 (a) without g-C3N4, (b) in the presence of 0.01 wt% of g-C3N4 and (c) in the presence of 0.015 wt% of g-C3N4. Fig. 10 CV curves of I-/I3- redox reaction on sample 1-3 (a) and sample 4-6 (b) in an acetonitrile solution containing 10 mM KI, 1.0 mM I2, and 0.1 M LiClO4. Scanning rate, 50 mV s-1. Fig. 11 Photovoltaic characteristics of DSSCs using various PANI and PANI/g-C3N4 counter electrodes.
22
Fig. 1
23
Fig. 2
24
Fig. 3.
Fig. 4.
25
Fig. 5
Fig. 6
26
Fig. 7
Fig. 8
27
a
2000
2
i (µA/cm )
1000
0
-1000 0
0.2
0.4
0.6
0.8
1
1.2
1.4
E (V vs. SCE)
b
10000
2
i (µA/cm )
6000
2000
-2000
-6000 0
0.2
0.4
0.6
0.8
E (V vs. SCE)
28
1
1.2
1.4
c
8000
2
i (µA/cm )
4000
0
-4000 0
0.2
0.4
0.6
0.8
E (V vs. SCE)
Fig. 9
29
1
1.2
1.4
a
2200
I 1400
III
2
i (µA/cm )
II 600
-200
-1000 -1
-0.5
0
0.5
1
1.5
2
E (V vs. SCE)
b I II
600
2
i (µA/cm )
III
-200
-1000 -1
-0.5
0
0.5 E (V vs. SCE)
30
1
1.5
2
2200
600
i (µA/cm2)
c
200
I
-200
1400 -600 -0.7
0.1
0.9
1.7
2
i (µA/cm )
E (V vs. SCE)
600
II
-200
-1000 -1
-0.5
0
0.5
1
1.5
2
E (V vs. SCE)
Fig. 10
8
6
Current/ mA cm-2
4
2
0 0
0.1
0.2
0.3
0.4
-2
0.5
f
-4
-6
Voltage /V
Fig. 11
31
0.6
0.7
a
A new kind of low cost Pt-free counter electrode for DSSCs was prepared PANI/g-C3N4 NC was prepared by in-situ electrochemical polymerization under sonication PANI/g-C3N4 NCs how good catalytic activity in promoting tri-iodide reduction Physicochemical characterization was carried out by different techniques The DSSC NC electrode exhibits an energy conversion efficiency of 1.8%
32
33
S1350-4177(17)30593-X https://doi.org/10.1016/j.ultsonch.2017.12.023 ULTSON 4007
To appear in:
Ultrasonics Sonochemistry
Received Date: Revised Date: Accepted Date:
5 October 2017 14 December 2017 14 December 2017
Please cite this article as: M. Afshari, M. Dinari, M. Mohsen Momeni, Ultrasonic irradiation preparation of graphiticC3N4/polyaniline nanocomposites as counter electrodes for dye-sensitized solar cells, Ultrasonics Sonochemistry (2017), doi: https://doi.org/10.1016/j.ultsonch.2017.12.023
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Revised Ultrasonic
irradiation
preparation
of
graphitic-
C3N4/polyaniline nanocomposites as counter electrodes for dyesensitized solar cells
Mohaddeseh Afshari, Mohammad Dinari*, Mohamad Mohsen Momeni*
Department of Chemistry, Isfahan University of Technology, Isfahan, 84156-83111, Islamic Republic of Iran
*
Corresponding author. Tel.; +98-31-3391-3270; FAX: +98-31-3391-2350.
E-mail address: [email protected], [email protected]. (M. Dinari). *
Corresponding author. Tel.; +98-31-3391-3283; FAX: +98-31-3391-2350.
E-mail address: [email protected] (MM. Momeni).
1
Abstract In this research, polyaniline/ graphitic carbon nitride (PANI/g-C3N4) nanocomposites were synthesized via in-situ electrochemical polymerization of aniline monomer whit different number of cyclic voltammetry scans (10, 20 and 30 cycles) after electrode surface prepreparation
using
a
potential
shock
under
ultrasonic
irradiation.
PANI/g-C3N4
nanocomposites with two values of g-C3N4 (0.010%w and 0.015%w) were deposited on the surface of the transparent conducting film (FTO glass) by immersing FTO into the aniline solution and g-C3N4 during the electro-polymerization. The resulting PANI/g-C3N4 films were characterized by Fourier transformed infra-red (FTIR), power X-ray diffraction (PXRD), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM) techniques. The prepared electrodes were applied as counter electrode in dye-sensitized solar cells. Among them, the prepared electrode with 10 cycles and 0.01%w gC3N4 showed the best efficiency. These hybrids show good catalytic activity in elevating triiodide reduction and due to the synergistic effect of PANI and g-C3N4, PANI/g-C3N4 nanocomposite electrode shows power conversion efficiency about 1.8%.
Keywords: Graphitic carbon nitride (g-C3N4); Ultrasonic Irradiation; Polyaniline; Dyesensitized solar cell
2
1. Introduction Dye-sensitized solar cells (DSSC) have concerned attention as cheering, low cost systems, which convert the solar energy into electricity with no emission [1-8]. Conventional DSSCs made from three main components: platinum (Pt) counter electrode (CE) layer, a liquid electrolyte containing dissolved tri iodides (I3-)/ iodide (I-) couple and nanocrystalline titanium dioxide (TiO2) coated with dye molecules as a photo-electrode layer (PE) [9,10]. The high cost of Pt CE in DSSCs devices has limited the commercialization and expansion of the use of DSSCs. In order to overcome this limitation, Pt electrode should be replaced by cost-effective CE candidates with suitable electrocatalytic activity against I3- reduction and fast charge transfer [11, 12]. To reduce the cost of fabrication of DSSCs, substitute materials such as composites, conducting polymers, or carbonaceous materials, have been investigated through replacement of Pt used in DSSCs [13, 14]. Among the mentioned compounds, carbonaceous materials due to their high electrical conductivity, high stability, and low cost are one of the best materials used to replace Pt in DSSC devices. Various types of carbon materials, such as graphene [15], graphene quantum dots [16], carbon nanotubes [17, 18], carbon black [19], and porous carbon [20], have been studied as the CE in DSCs. A literature survey in this field shows that carbonaceous materials have interesting performance in DSSC for example, Hwang et al. have reported DSSC free Pt counter electrodes with high conductivity, good electrochemical activity, and low cost based on carbon nanotubes [21] and Bora et al. have synthesized polythiophene/graphene composite as a highly efficient platinum-free counter electrode whit power 4.8% conversion efficiency [22]. Also Zhang et al. have fabricated the bifacial DSSCs from covalent-bonded polyaniline-multiwalled carbon nanotube complex as a CE with maximum energy conversion efficiency of 9.24%, which is higher than 8.08% for pure PANI as a CE [23].
3
Despite all these benefits, the electrocatalytic activity of these carbon materials is still not able to compete with Pt [24]. In recent years, researchers improved the electrocatalytic activity and improve the surface hydrophilicity of these compounds through the doping of nitrogen inside the carbon frameworks [25]. Nitrogen-doped carbon materials exhibit greater electrocatalytic activity and better chemical stability than Pt [26]. Easwaramoorthi et al. have synthesized
soft-template
of
simple
ordered
mesoporous
titanium
nitride-carbon
nanocomposite for high performance DSSC Counter Electrodes [27]. Also, Xu et al. have prepared the liquid-based growth of polymeric carbon nitride layers and their use in a mesostructured polymer solar cell with Voc exceeding 1 V [28]. Sonication due to various advantages including reducing of reaction time and decrease reaction temperature considered as multi propose method for nanomaterial preparation. The ultrasound waves produce high local pressure (500atom) and temperatures (5000K) owing to the formation of acoustic cavities. During acoustic cavitation, the cavities growth and created bubbles in a liquid which lead to local temperature and pressure. The ultrasound waves could reduce particles size and change the morphology of nanoparticles (NPs). Finally, ultrasonication provides a better dispersion of nanoparticles which provide a homogenous NP distribution in the polymer matrix [29-33]. Graphitic carbon nitride (g-C3N4), a two-dimensional (2D) structure, as a low-cost nitrogen rich precursor exhibited strong attraction due to its superior chemical and thermal stability, interesting electronic structure and good biocompatibility and considered as a potential candidate for nanocomposites preparation [34-37]. Wang et al. have reported the gC3N4/MWCNTs composite as Pt-free counter electrode for high-efficiency dye-sensitized solar cells [38]. Wu et al. have reported the synthesis of two dimensional graphitic-phase C3N4 as a multifunctional protecting layer to enhance short-circuit photocurrent in DSSC [39]. Due to the high percentage of nitrogen content, g-C3N4 shows high active
4
electrocatalytic sites which leads to an increase in its capacity for I3- reduction reaction [40, 41]. However, the electrocatalytic activity of g-C3N4 limited due to its low electrical conductivity [24]. Doping metals in the structure or using carbon-based conductors such as conductive carbon black composite is an effective way to improve the transfer of electrons within the network and increase the electrocatalytic activity of g-C3N4. Conducting polymers have been extensively studied, and have proved to be promising candidates for Pt replacement. Among conducting polymers, polyaniline (PANI) is the most noteworthy material to replace Pt CE due to simple synthesis, substantial catalytic activity, and well environmental stability [42-46]. However, PANI is an organic semiconductor and inherently has the relatively low ability to charge transfer [16]. Nevertheless, its electrocatalytic activity and long-term stability of pure PANI still cannot compete with Pt, as a result, the energy conversion efficiency of these systems is low. The coupling of PANI with carbon nanomaterials is the most appropriate choice to increase its conductivity [13-16, 47, 48]. Herein, PANI/g-C3N4 hybrids were synthesized under ultrasonic irradiation via in situ polymerization of aniline monomers in the presence of various percentages of g-C3N4 on transparent surfaces of FTO to improve the electrocatalytic activity and increase the speed and capacity of charge transfer. The catalytic activity and morphology of PANI/g-C3N4 hybrids as counter electrodes was investigated.
2. Experimental 2.1. Materials Aniline (purity>99%), N,N-dimethylformamide (DMF, purity>99.5%), cyanuric chloride (purity>99%), N,N-diisopropylethylamine (98%), melamine (purity>99%), and sulfuric acid (H2SO4, 98%) were purchased from Sigma Aldrich and Merck chemical company. The monomer of aniline was distilled under reduced pressure and then stored at low temperature
5
before use. Distilled water was used for the preparation of all the solutions. The other of material was used without further purification.
2.2. Apparatus Electrochemical
experiments
were
fulfilled
using
a
computerized
potentiostat/galvanostat (SAMA 500, Iran). The electrocyclic voltammetry was conducted in a solution of acetonitrile containing 10 mM KI, 1 mM I2 and 0.1 M LiClO4. The electrochemical experiments were performed in a three-electrode cell assembly. The platinum plate as a counter electrode, saturated calomel (SCE) as a reference electrode and all potentials are measured relative to the reference electrode. A Qsonica XL 2000 with the power of 100 W at 50/60 kHz was utilized in the synthesis procedure. Fourier transform infrared (FT-IR) spectra of samples were carried out with a Jasco-680 spectrometer (TokyoJapan) in the wavelength range of 400–4000 cm–1, using KBr pallet technique by performing 60 scans at 4 cm−1. X-ray diffraction (PXRD) profiles were obtained with a Bruker advanced powder X-ray diffraction system (D8 Advance, Germany) at 45 kV and 100 mA using Cu Kα radiation. Transmission electron microscopy (TEM) and field emission scanning electron microscopy (FE-SEM) was performed on a Philips CM120 microscope (Eindhoven, Netherland) and a HITACHI S-4160 (Tokyo-Japan), respectively, to observe the surface morphology and structure of obtained nanoparticles and nanocomposites. Thermogravimetric analysis (TGA) of the sample was performed on a STA503 TA instrument (Hullhorst, Germany) under nitrogen atmosphere with a heating rate of 10 °C.min−1 from 298 K to 1073 K.
2.3. Synthesis of g-C3N4
6
Graphitic-C3N4 was prepared via the multi-step reaction between cyanuric chloride and melamine in the presence of N,N-diisopropyleethylamine according to a recently published article [48]. In order to separate the g-C3N4 layers from each other and maintain the network structure, the use of ultrasonic waves replaced the utilization of chemical solvents or harsh acidic condition. Accordingly, the obtained product was sonicated for 10 hours with a power of 100 W at 50/60 kHz frequency distribution and then dried at ambient temperature for 24 hours (Fig. 1). Fig. 1 2.4. Preparation of PANI/g-C3N4 films Prior to assembly, fluorine-doped tin oxide (FTO) glass substrates (8 ohm sq-1, Solaronix) were cleaned by immersing in the cleaning solution containing 30% isopropyl alcohol, 30% acetonitrile and the rest double-distilled water under sonication with power of 100 W and Frequency of about 60 kHz. Ultrasonic waves, in addition to improving the washing process and eliminating surface contaminations, by creating a smooth surface will produce a uniform and non-stressed film during polymerization process. Aniline monomer was conducted on glass electrode in acidic solution by electro-polymerization technique. For the preparation of CE, 0.1 M of aniline monomer was added to 0.5 M H2SO4 solution and then 0.01 or 0.015 wt. % of g-C3N4 was added to the mixture solution. The mixture was exposed to ultrasound irradiations with the power of 100 W and 60 kHz frequency due to severe dispersing the gC3N4 in acidic solution and forming a sustained suspension for the next steps. Electropolymerization occurred through 10, 20 or 30 consecutive cyclic voltammetry scans in the 0.0 to 0.8 V potential ranges. The temperature adjusted at 25 oC. The experimental conditions for different samples are summarizing in Table 1. The experimental setup for preparation FTO and synthesize the PANI/g-C3N4 films have been shown in Fig. 2a and 2b respectively. Table 1
7
Fig. 2 2.5. Fabrication and cell assembly To fabricate solar cells, TiO2 film was deposited on FTO by doctor blade technique, followed by sintering at 500 oC for 30 min. For DSSC construction, the samples were immersed in 0.3 mM ethanol solution of dye N719 for 24 h to be dye-sensitized with Ru based N719 dye (cisbis
(isothiocyanato)
bis
(2,
2-bipyridyl
4,4-dicarboxylato)
ruthenium
(II)
bis-
tetrabutylammonium) (Solaronix SA, Switzerland) and used as the photo-anode of the DSSC. The prepared PANI-g-C3N4/FTO glass electrodes were used as the photocathode. Sandwich cells were assembled by sealing the anodic electrode of TiO2 together with the photocathode using a thermo plastic spacer with a thickness of 60 microns at 120-150 oC. An electrolyte solution of 0.5 M LiI, 0.05 mM I2, and 0.5 M tert-butylpyridine in 3-methoxypropionitrile was introduced into the cell through a hole in the FTO electrode. The current-voltage characteristics of the cells were measured under AM 1.5, 100 mW/cm2 simulated light radiations.
3. Results and discussion 3.1. FT-IR study The FT-IR spectra of the g-C3N4, pure PANI and hybrids of PANI/g-C3N4 are shown in Fig. 3. In the FT-IR spectrum of g-C3N4, the N-H groups show the broad peak at 30003400 cm-1. The characteristic peaks for stretching modes of C=N and C-N heterocycles appear at 1241 and 1570 cm-1. Moreover, the characteristic breathing modes of triazine units were observed at 808 and 879 cm-1 [49]. As shown in Fig. 3, the main characteristic peaks of pure PANI can be assigned as follows: the band at 3360 cm-1 was attributed to hydrogen bonding between the N–H of amine and imine sites. The bands at 1562 and 1481 cm-1 can be assigned to C=N and C=C stretching of the quinonoid and benzenoid rings, respectively. The
8
bands at 1302 and 1240 cm-1 were attributed to the C–N stretching mode for the benzenoid unit, while the band at 1111 cm-1 to the quinonoid unit of PANI. The peak at 767 cm-1 is associated with C-C and C-H in the benzenoid unit [16]. In the FT-IR spectrum of the PANI/g-C3N4 sample, the characteristic peaks from PANI around 1302, 1481 and 1562 cm-1 shift to higher wave numbers of 1305, 1483 and 1567 cm-1 after the g-C3N4 is introduced. By the way, the hydrogen bond absorption at 3368 cm-1 is strengthened. All of the above peaks can be seen from spectrum of PANI/g-C3N4 composite film, showing PANI is existent in the hybrid NCs. Fig. 3 3.2. Thermogravimetric analysis (TGA) Thermal behavior of prepared g-C3N4 was investigated by TGA technique. The analysis was performed at 25 to 800 °C and heating rate of 10 °C min-1 under nitrogen atmosphere. As seen in Fig. 4, g-C3N4 shows a one weight loss about 700 °C which proved high thermal stability of prepared nanoparticles. Fig. 4.
3.3. X-ray diffraction The PXRD patterns of the g-C3N4, PANI, and PANI/g-C3N4 nanocomposites are shown in Fig. 5. The PXRD pattern of pure g-C3N4 contains clear peaks in 12.6 and 27.4, which respectively relate to the (100) plane diffraction arising from the in-plane repeating motifs of the sequential triazine network and the (002) plane arising from the stacking of the conjugated aromatic system [50]. In the pure PANI, the crystalline plates of (011), (020) and (200) are marked with special peaks at 15.8o, 20.5o and 26.3o [50]. When g-C3N4 was incorporated into the PANI matrix, the diffraction peak of g-C3N4 at 27.4 o (002) overlapped with the peak of PANI which results in the broad and intense peak in the composite. The data
9
indicates that no additional crystalline order has been introduced into the composite. Compared with functionalized g-C3N4, the obvious characteristic peaks in PANI/ g-C3N4 can be ascribed to the formation of crystal appearing on the outer layers of g-C3N4. Fig. 5 3.4.
Morphology study: TEM and FE-SEM and BET analysis The morphology and microstructure of the synthesized g-C3N4 were investigated by
FE-SEM and TEM techniques and their results are presented in Fig. 6. The FE-SEM images of g-C3N4 show random stacking clubbed morphology with anomalous smooth nanorods structure (Figs. 6a and 6b). Also, according to the TEM analysis, the g-C3N4 shows platelet layered morphology analogous to graphene like materials (Figs. 6c and 6d). Also, the BET analysis was employed to investigate the active surface area of prepared g-C3N4. Brunauer– Emmett–Teller (BET) surface area of 75.2 m2.g-1 and average pore diameter equal 6.1nm was obtained for synthesized g-C3N4. The observed data indicate microporous pores in g-C3N4. Fig. 6. The nanostructure and morphology of the pure PANI, as well as PANI/g-C3N4 nanocomposites with 0.01 and 0.015 g of g-C3N4, was investigated by FE-SEM techniques. Pristine PANI showed flake-like agglomerations which were stacked by spherical particles (Fig. 7). After incorporating g-C3N4 to the PANI, the g-C3N4 nanosheets have a tendency to be present in the form of small nanosheets. For PANI/g-C3N4 nanocomposites with 0.01 of gC3N4, the composite show spherical structure where g-C3N4 particles get deposited on the surface of PANI (Figs. 8a and 8b). But, for PANI/g-C3N4 nanocomposites with 0.015 of gC3N4, road-like morphology was observed (Figs. 8c and 8d). Fig. 7 Fig. 8
10
3.5. Electrochemical and photovoltaic characterization Electro-polymerization of PANI/g-C3N4 as counter electrode using cyclic voltammetry in a 0.1 M of aniline solution and 0.5 M of H2SO4 in the presence of 0.01 or 0.015 wt. % of gC3N4 at scan rate of 10 mV s-1 occurred in two steps. At the first step, in order to activate the FTO surface and requires fewer cycles to achieve a relatively certain thickness of polyaniline film, the polymerization reaction was performed in the potential range of 0.0-1.3 V and the second-step it occurred in the potential range of 0.0-0.8 V. Diagrams of cyclic voltammetry is shown in Fig. 9. The results of using two-step applying potential on the CV diagram indicate a significant increase in the current density in the second potential interval for the number of cycles of 10, 20, or 30, which prove the activation of FTO surface in preelectropolymerization. The formation and growth of PANI chains can be investigated using the intensity of the redox peaks in the CV curves. It is clearly seen that after each cycle the peak current density increased (Fig. 9a). As mentioned earlier, g-C3N4 cause faster and easier transfer of electrons from the electrolyte solution to polymer and performed the redox reactions. As a result, the PANI/g-C3N4 electrode has a faster increase in the peak intensities relative to PANI electrode (Figures 9b and 9c). Fig. 9 To elucidate the distinction in electrocatalytic properties and reaction kinetics between the Pt CEs and various PANI/g-C3N4 composite, CV curves were recorded, of which the detailed results are shown in Figure. 10. In all of the CV patterns, two pairs of the redox peaks are well observed. The left pairs are related to the oxidation of I-/I3- (eq. 1) and the right pairs have explained the reduction of I2/I3- (eq. 2). 3I- ↔ I3- + 2e-
(1)
2I3- ↔ 3I2 + 2e-
(2)
11
The pairs peak observed on the left side of the CV curve shows the I3- to I- reaction. Therefore, the current density of these peaks is investigated to determine the efficiency of the auxiliary electrodes. The catalytic activity of DSSCs based on PANI/g-C3N4 CEs were found to be better than that of Pt due to its higher peak current density. According to this result, the PANI/g-C3N4 nanocomposite could be used as an effective CE in DSSC devices. Fig. 10 Photocurrent density-photovoltage (J–V) curves of PANI/g-C3N4 composite as CEs in DSSCs devices are shown in Fig. 9. The corresponding photovoltaic parameters for these CEs are summarized in Table 2. The photovoltaic performance of the DSSCs based on different PANI/g-C3N4 counter electrodes showed in Fig. 11. When PANI/g-C3N4 composites are used as CE, the highest short-circuit current density (JSC) 7.6 mA cm-2 and a modest fill factor (FF) 0.39, and open-circuit voltage (VOC) 0.44 V, resulted in a conversion efficiency of 1.79%, (in sample A) this is attributed to the higher electrochemical catalytic activity of this electrode in comparison with other samples. The results of this work were compared with the previous work in Table 3. Fig. 11 Table 2 Table 3
4. Conclusions PANI/g-C3N4 nanocomposites with different concentration of g-C3N4 in PANI and various cycle numbers were successfully synthesized by in-situ polymerization. The various hybrids were coated on the FTO substrates to construct CEs for DSSCs. According to FESEM results, the hybrids show road-like morphology. Three cycle numbers (10, 20 and 30) were employed for the preparation of electrodes, which the minimum cycle number shows
12
the best efficiency. The better efficiency was observed for PANI/g-C3N4 counter electrode comparing with PANI counter electrode. The obvious feature of easy synthesis, the multilateral electrical and photovoltaic performance of multilayer films based on conductive polymers, puts into be good CE materials in DSSCs. The profound advantages in easy synthesis, versatile electrical and photovoltaic performances promise the multilayer films to be good CE materials in DSSCs.
Acknowledgments This work was partially funded by the Research Affairs Division of Isfahan University of Technology (IUT), and Iran Nanotechnology Initiative Council (INIC).
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Table 1: The experimental parameters for the synthesized samples. Samples A
B
C
D
E
F
Electro-polymerization solution
Electro-polymerization condition
0.5 M H2SO4 solution containing of
10 successive cyclic voltammetry scans, scan rate of 10 mV s-1
0.1 M aniline and 0.01 wt.% g-C3N4 0.5 M H2SO4 solution containing of
20 successive cyclic voltammetry scans, scan rate of 10 mV s-1
0.1 M aniline and 0.01 wt.% g-C3N4 0.5 M H2SO4 solution containing of
30 successive cyclic voltammetry scans, scan rate of 10 mV s-1
0.1 M aniline and 0.01 wt.% g-C3N4 0.5 M H2SO4 solution containing of
10 successive cyclic voltammetry scans,
0.1 M aniline and 0.015 wt.% g-C3N4
scan rate of 10 mV s-1
0.5 M H2SO4 solution containing of
20 successive cyclic voltammetry scans,
0.1 M aniline and 0.015 wt.% g-C3N4
scan rate of 10 mV s-1
0.5 M H2SO4 solution containing of
30 successive cyclic voltammetry scans,
0.1 M aniline and 0.015 wt.% g-C3N4
scan rate of 10 mV s-1
Table 2: DSSCs performance. J sc (mA/cm2)
Voc (v)
η [%]
Sample A
7.56
0.59
1.786
Sample B
1.97
0.44
0.349
Sample C
0.93
0.45
0.153
Sample D
1.01
0.39
0.143
Sample E
0.96
0.37
0.110
Sample F
0.58
0.38
0.083
JSC: short-circuit current, VOC: open-circuit voltage, η: efficiency
20
Table 3: Comparison efficiency of PANI/g-C3N4 electrode with previous literature reports. Composite J sc (mA/cm2) Voc (v) η [%] Ref 4-ATP/PANI/(CD)
8.340
0.682
4.500
51
ZnO/PANI/(100 mg/L)
2.324
0.566
0.640
52
ZnO/PANI/(200 mg/L)
1.692
0.610
0.430
52
PAA/g-CTAB/PANI
14.20
0.657
6.680
53
PVA/(0.25%)MWCNT/PAni
6.700
0.370
1.140
54
EB-SWCNTs/ZnO
0.155
0.555
0.170
55
Sample A
7.560
0.590
1.786
current research
21
Figures Captions: Fig. 1. Preparation of g-C3N4 nanosheets by ultrasonic sonication. Fig. 2. Schematic procedure of preparation PANI/g-C3N4 films a: cleaning process b: electropolymerization. Fig. 3. FT-IR spectra of g-C3N4, PANI and PANI/g-C3N4 nanocomposite. Fig. 4. Thermogram of g-C3N4 under N2 atmosphere. Fig. 5 PXRD pattern of g-C3N4, PANI and PANI/g-C3N4 nanocomposite. Fig. 6 FE-SEM (a, b) and TEM (c, d) images of g-C3N4. Fig. 7 FE-SEM photographs with different Magnify of synthesized PANI. Fig. 8 FE-SEM images FE-SEM photographs of PANI/g-C3N4 CEs in the presence of (a, b) 0.010 g-C3N4 and in the presence of (c, d) 0.0l5 g-C3N4 with different magnification. Fig. 9 CV curves of solution of 0.5 M H2SO4 containing 0.1 M aniline monomers during electrochemical deposition at a scan rate of 10 mV s-1 (a) without g-C3N4, (b) in the presence of 0.01 wt% of g-C3N4 and (c) in the presence of 0.015 wt% of g-C3N4. Fig. 10 CV curves of I-/I3- redox reaction on sample 1-3 (a) and sample 4-6 (b) in an acetonitrile solution containing 10 mM KI, 1.0 mM I2, and 0.1 M LiClO4. Scanning rate, 50 mV s-1. Fig. 11 Photovoltaic characteristics of DSSCs using various PANI and PANI/g-C3N4 counter electrodes.
22
Fig. 1
23
Fig. 2
24
Fig. 3.
Fig. 4.
25
Fig. 5
Fig. 6
26
Fig. 7
Fig. 8
27
a
2000
2
i (µA/cm )
1000
0
-1000 0
0.2
0.4
0.6
0.8
1
1.2
1.4
E (V vs. SCE)
b
10000
2
i (µA/cm )
6000
2000
-2000
-6000 0
0.2
0.4
0.6
0.8
E (V vs. SCE)
28
1
1.2
1.4
c
8000
2
i (µA/cm )
4000
0
-4000 0
0.2
0.4
0.6
0.8
E (V vs. SCE)
Fig. 9
29
1
1.2
1.4
a
2200
I 1400
III
2
i (µA/cm )
II 600
-200
-1000 -1
-0.5
0
0.5
1
1.5
2
E (V vs. SCE)
b I II
600
2
i (µA/cm )
III
-200
-1000 -1
-0.5
0
0.5 E (V vs. SCE)
30
1
1.5
2
2200
600
i (µA/cm2)
c
200
I
-200
1400 -600 -0.7
0.1
0.9
1.7
2
i (µA/cm )
E (V vs. SCE)
600
II
-200
-1000 -1
-0.5
0
0.5
1
1.5
2
E (V vs. SCE)
Fig. 10
8
6
Current/ mA cm-2
4
2
0 0
0.1
0.2
0.3
0.4
-2
0.5
f
-4
-6
Voltage /V
Fig. 11
31
0.6
0.7
a
A new kind of low cost Pt-free counter electrode for DSSCs was prepared PANI/g-C3N4 NC was prepared by in-situ electrochemical polymerization under sonication PANI/g-C3N4 NCs how good catalytic activity in promoting tri-iodide reduction Physicochemical characterization was carried out by different techniques The DSSC NC electrode exhibits an energy conversion efficiency of 1.8%
32
33