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graphene
Electrochimica Acta 215 (2016) 543–549
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
An efficient dye-sensitized solar cell with a promising material of Bi4Ti3O12 nanofibers/graphene Yuhong Yu, Haiwu Zheng* , Xinan Zhang, Xiao Liang, Gentian Yue* , Fengzhu Li, Mingsai Zhu, Tianfeng Li, Jianjun Tian, Guosheng Yin Key Laboratory of Photovoltaic Materials of Henan Province, Institute of Microsystem, School of Physics and Electronics, Henan University, Kaifeng 475004, China
A R T I C L E I N F O
Article history: Received 29 May 2016 Received in revised form 29 July 2016 Accepted 18 August 2016 Available online 31 August 2016 Keywords: Bi4Ti3O12 Graphene Counter Electrode Dye-sensitized Solar Cells
A B S T R A C T
In this work, the composites of Bi4Ti3O12/graphere (BTO/Gr) are used as counter electrode (CE) through facile method based on sol-gel and electrospinning techniques to improve the efficiency of dyesensitized solar cells (DSSCs). The structure and morphology of the composites with different ratios are characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The optical bandgap of the composites decreases as Gr content increases. The electrochemical performance of BTO/Gr CE is characterized by cyclic voltammetry curves, electrochemical impedance spectroscopy, Tafel polarization curves and photocurrent density-voltage character curves. Compared with pure BTO CE, BTO/Gr CE has a higher electrocatalytic activity and a lower charge transfer resistance in the electrolyte/electrode interface. The power conversion efficiency of DSSCs with BTO/Gr CE (contained 2.5 wt.% Gr), has reached 9.70% under a simulated solar illumilnation of 100 mW/cm2, which is much larger than that of BTO CE. The power conversion efficiency amounts to 79.6% of the Pt CE and 86.2% of the graphene CE. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) have attracted much interest owing to their significant advantages such as low cost, high efficiency, simple manufacture and environmental friendliness [1–3]. The typical structure of DSSCs contains conducting optical glass, photoanode, dyes, electrolyte and counter electrode (CE) [4,5]. Platinum (Pt), as an extremely expensive and rare metal material, is usually used as CE in DSSCs because of its excellent catalytic activity and high electrical conductivity. However, several disadvantages of Pt severely restrict the rapid development of DSSCs, such as the cost, reliability of sealing and easily poisoned by the redox species [6]. Hence, looking for materials that can be used to replace Pt in DSSCs has become an urgent issue. It is reported that some materials can substitute Pt as CE of DSSCs, for example, carbon-based materials, metal sulfides, nitrides, polymers and oxides [7,8]. Especially, a limited number of oxides have been endowed with expectation in that they contain earth-abundant elements and have stable chemical properties.
* Corresponding authors. E-mail addresses: [email protected], [email protected] (H. Zheng), [email protected] (G. Yue). http://dx.doi.org/10.1016/j.electacta.2016.08.086 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
Bismuth titanate (Bi4Ti3O12, BTO) can be considered as a wideband gap (3.21 eV) semiconductor material, which belongs to a family of layered perovskite compounds with good chemical stability. Although ferroelectric, piezoelectric, and photocatalytic properties of BTO have been extensively investigated [9,10], its electrochemical and photovoltaic properties have been less reported. In our previous work, we have investigated the DSSCs based on BTO acting as photoanode, however, the power conversion efficiency (PCE) is still very low [11]. Therefore, it is interesting to investigate the potential applications of BTO as CE in DSSCs for higher photoelectric conversion efficiency. In addition, graphene (Gr) is one of the most promising potential CE materials in DSSC due to its inherent features such as large surface area, high optical transmittance, excellent electrical conductivity and thermal conductivity [12,13]. Although composite CEs based on Gr have been reported by many groups, they mainly focused on sulfide, noble metal and polymers [14–17]. Consequently, it is worthy of us to explore the composite CE combined ternary oxides with Gr in order to improve the photo-electrochemical properties of functional oxides. In this paper, on account of the low charge mobility and inferior catalytic activity of BTO CE, we designed BTO/Gr composites as CE for DSSCs, hoping that Gr could promote the catalytic activity of
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BTO/Gr composite CE and improve the photovoltaic property of the DSSC. This work may broaden the potential applications of multifunctional oxides in the area of photo-electrochemistry. 2. Experimental 2.1. Material synthesis Bismuth Nitrate (Bi(NO3)3.5H2O 99.0%), Acetylacetone (C5H8O2 99.5%), Acetic acid glacial (C2H4O2 99.5%) and N,N-Dimethylformamide (DMF 99.5%) were put into a beaker stirring until all solids dissloved. Afterwards, Tetrabutyl titanate (C16H36O4Ti 98.0%) was dropwise added into the solution. Then polyvinylpyrrolidone (PVP, molecular weight of about 1300000) was added and continuously stirred for 12 h to form a yellow and transparent precursor. During this process, all the solvents and solutes were analytically pure without further purification. The prepared precursor was electrospun into nanofibers by home-made electrospinning equipment at a 10 kV accelerated voltage and a collecting distance of about 15 cm. The as-collected nanofibers were calcined in muffle furnace at 690 C in air for 1 h with a heating rate of 2 C/min. Finally, BTO nanofibers were produced. 2.2. Preparation of BTO/Gr CEs BTO/Gr CEs were prepared with commercial available Gr in weight ratio ranging from 0 wt.%, 0.5 wt.%, 1.5 wt.%, 2.5 wt.% and 100 wt.% which were labeled as BG0, BG1, BG2, BG3 and Gr for convenience, respectively. Firstly, the series samples (BG0, BG1, BG2, BG3, Gr) and polyethylene glycol 2000 were placed and ground in an agate mortar with the proportion of 4:1. Secondly, the composites were mixed with 1 ml anhydrous ethanol, keeping grind until the mixture became colloidal suspension. The suspension was coated on fluorine-doped tin oxide conductive glass substrate to form film [18], followed by heating at 400 C for 1 h under the protection of argon. The BTO/Gr CEs were obtained when cooling to room temperature. 2.3. Assembly of DSSCs DSSCs were sandwich structure consisting of anode, CE and electrolyte. A TiO2 anode was prepared as previously described [19]. 10 ml Tetrabutyl titanate and 100 ml deionized water were stirred for 30 min to form A solution. Yellow powders were obtained from A solution by suction filtration. Afterwards, the powders were put in 150 ml deionized water, 10 ml acetic acid glacial and 1 ml nitric acid stirring at 80 C for about 12 h to compound B solution. After B solution became light blue, P25 (TiO2
0.2 g) was added. Then the above solution was added into autoclave heating at 200 C for 12 h to form C solution. When it cooling to room temperature, 0.8 g polyethylene glycol and 1 ml emulgator were joined, followed by ultrasonic cleaning for 30 min to compound D solution. Finally, D solution was stirred at 80 C until thickened in order to prepare TiO2 paste. The above TiO2 paste was evenly spread on FTO substrate, dried naturally and annealed at 450 C for 30 min in air. In order to gain dye-sensitized TiO2 electrode, the photoanode needed to be soaked in a 0.3 mM ethanol solution of N719 dye for 24 h. The dye-sensitized TiO2 electrode and the as-prepared CE were clipped together, the electrolyte (0.05 M I2, 0.1 M LiI, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), and 0.5 M 4-tert-butyl pyridine in acetonitrile) was injected into the aperture between the two electrodes with effective area of 0.25 cm2. 2.4. Characterizations The structures were characterized by a DX-2700 X-ray diffractometer (XRD) equipped with monochromatized Cu Ka radiation (l = 0.1542 nm) and confocal microscopy laser Raman spectrometer (Renishaw, RM-1000). The morphologies were measured by field emission scanning electron microscope (SEM, JSM-7001F) and transmission electron microscope (TEM, JEM2010). The absorption spectrum was observed by a UV-vis-NIR photospectrometer (Varian Cary 5000). Monochromatic incident photo-to-current conversion efficiency (IPCE) curves were gauged by an IPCE measurement system (Qtest Station 500ADX). The electrochemical characteristics of the DSSCs were measured by electrochemical workstation at room temperature (Wuhan CorrTest Instrument Co., Ltd.). Additionally, lithium perchlorate (0.8022 g), iodine (0.01269 g) and lithium iodine (0.06692 g) were added into 50 ml acetonitrile to form the electrolyte for the measurements of the cyclic voltammetry. The potentiostat that we used was CEL-S500/350/150. The potential was from 0.3 V to 0.8 V at a scanning rate of 30 mV/s and the reference electrode was AgCl. Moreover, the frequency of electrochemical impendence spectroscopy (EIS) is ranging from 10 kHz to 10 MHz and IPCE measurements were performed from 300 nm to 800 nm with a 150 W bromine-tungsten lamp and a 10 nm scanning step. 3. Results and disscussion Fig. 1a shows the SEM image of BTO nanofibers. It can be seen that their average diameter is in the range of 100–200 nm and the surface of the nanofibers is smooth. The inset in Fig. 1a presents the SEM image of the nanofibers under higher magnification. The HRTEM image of BTO nanofibers is indicated in Fig. 1b, the lattice spacing of which can be estimated as 0.272 nm. Compared with the
Fig. 1. (a) SEM of BTO nanofibers, the illustration in (a) exhibits the SEM image under greater magnification. (b) HRTEM of BTO nanofibers, the inset of (b) indicates the TEM image of BTO.
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Fig. 2. (a) Energy dispersive x- ray spectrum (EDX) of BTO. (b) TEM of Gr.
standard card of orthorhombic BTO, it coincides with the (200) plane. The illustration in Fig. 1b measured by TEM further describes the fibers structure. The energy dispersive x- ray spectrum (EDX) pattern of BTO is shown in Fig. 2a, in which the Bi, Ti and O elements can be detected and their atomic ratio is 26.1:13.9:60.0. This is close to the ideal stoichiometric ratio of BTO. From the TEM of Gr in Fig. 2b, it is noted that the Gr has a single flake structure. The TEM image of BG3 is exhibited in Fig. 3a, the black particles characterize the BTO nanoparticles and the reticulation denotes Gr, it can be inferred that BTO and Gr are adequately mixed without any other change in crystalline structure. This can be further confirmed by the selected area electron diffraction (SAED) image of BG3 in Fig. 3b, which appears the diffraction spots of BTO and the diffraction rings of Gr. Moreover, the crystal plane spacing is calculated to be 0.336 nm and 0.272 nm by the formula of Rd = L,
matching with the (002) plane of Gr and the (200) plane of BTO, respectively. Fig. 4a shows the XRD pattern of BTO nanofibers. From which we can see, all diffraction peaks correspond well with the JCPDS database card number 35-0795. No impurity diffraction peaks are observed and the XRD pattern matches well with the orthorhombic single-phase perovskite structure. It indicates that BTO has a good crystallinity, which is consistent with the previous HRTEM results. Moreover, no diffraction peaks for Gr are found within the limitation of the XRD equipment due to its tiny amount of concentration. The XRD results of BG1 and BG2 are almost identical with that of BG3 and thus are ignored. Raman scattering spectra is given in Fig. 4b for the sake of further ascertaining the phase composition of the samples. From this figure, there are six apparent peaks of BTO which are located at
Fig. 3. (a) TEM of BTO and Gr composites (BG3). (b) Selected area electron diffraction (SAED) of BTO and Gr composites (BG3).
Fig. 4. (a) XRD patterns of various samples. (b) Raman spectra of samples, the inset of (b) illustrates the enlarged views in the wavelength range of 1100–1800 nm.
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Fig. 5. (a) UV–vis diffuse absorption spectra of various samples. Diagram (b) indicates the numerical value of band gap for various samples.
225, 261, 328, 539, 615, 847 cm 1, respectively. The modes observed at 225, 328 and 539 cm 1 are attributed to the internal vibrational mode of TiO6 and belong to B2g and B3g symmetry originating from the lifting of Eg degeneracy [20]. The modes at 261, 615 and 847 cm 1 are ascribed to the stretching of the TiO6 octahedron and can be assigned as A1g character [21]. As seen from the inset in Fig. 4b, there are two prominent peaks of Gr at 1352 cm 1 and 1575 cm 1, which are assigned to the disordered (D) and graphitic (G) bands, respectively. The D peak is derived from the structure defect of lattice, while G peak belongs to the E2g phonon of C sp2 atoms. As a consequence, the BTO and Gr are successfully composited from the results of Raman spectrum and SAED images. It can be inferred that the specific area of the composites enhance with the increase of Gr content, which will probably facilitate the generation and transport of the photogenerated carriers, leading to a higher electrochemical properties [22]. The UV–vis absorbance spectra of the samples are illustrated in Fig. 5a. Fig. 5b presents the fitting curves based on direct band gap. As is shown in the diagram, BTO nanofibers exhibit a good lightharvesting capability in the range of 300 to 400 nm. By means of linear extrapolation method, the optical band gap of BTO nanofibers can be approximately estimated as 3.21 eV, which comes up to the published in the previous literature [23]. Whereas, the effective band gaps of BG1, BG2 and BG3 are severally estimated at about 3.06 eV, 2.99 eV, 2.86 eV, indicating larger light-harvesting capacity than that of pure BTO, which probably arises from the smaller optical band gap of Gr (1.57 eV). Therefore, it can be
concluded that the photo-absorption ability and photo-generated carriers’ numbers can be enhanced through the combination of Gr. The cyclic voltammograms of various CEs are shown in Fig. 6a, which is measured by using a three-electrode system with the Pt sheet as CE, saturated sliver chloride electrode as reference electrode and various CEs as working electrode. In general, there are two pairs of redox peaks, while the pair of peaks on the low potential area has a significant impact on the photovoltaic properties of DSSCs. The over-potential (DEp) is inversely correlated with the standard electrochemical rate constant of a redox reaction, illuminating that a smaller DEp represents a better catalytic activity [24]. For comparison, the DEp for Pt and Gr are measured and displayed in the inset of Fig. 6a. From Fig. 6a, the values of DEp for Pt, Gr, BG3, BG2, BG1 and BG0 are 288 mV, 332 mV, 346 mV, 402 mV, 476 mV and infinity, respectively. It reflects the catalytic activity of various CEs following the orders of Pt > Gr > BG3 > BG2 > BG1 > BG0, suggesting that BG3 CE has a decent catalytic activity, which is derived from Gr because it possesses a larger specific surface area which can vastly enhance the assessibility of the electrolyte to the electrode, thus improving interfacial charge transfer and increasing the number of active catalytic sites. Additionally, twenty cycles of CV curves in Fig. 6b are used to demonstrate the stability of BG3 CE. It turns out that BG3 CE is stable for catalyzing triiodide. The above experimental results imply that the incorporation of Gr can indeed increase the catalytic activity of BTO. Fig. 7a, b, and c exhibit the electrochemical impedance spectroscopy (EIS), which is invoked to investigate the properties
Fig. 6. (a) Cyclic voltammetry (CV) curves of triiodide/iodide redox species of various CEs, inset of (a) denotes the CV curves of Pt and Gr CEs. (b) Twenty cycles of CV curves from BG3 CE at a scan rate of 30 mV/s.
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Fig. 7. (a), (b) and (c) represent the Nyquist plots of various CEs, and inset of (b) denotes the partial enlarged drawing of diagram (b). (d) shows the Tafel curves for symmetric cells fabricated with assorted CEs.
and quality of devices. The corresponding parameters are listed in Table 1. The intercept on the horizontal axis represents the series resistance (Rs), a reflection of conductive substrate resistance and lead resistance. The real radius of the first semicircle in the high frequency domain denotes the charge-transfer resistance (Rct) at CE/electrolyte interface which characterizes the reduction of triiodide, while the second arc refers to the Nernst diffusion impedance corresponding to the diffusion resistance of the redox couples in the electrolyte. It is noteworthy that Rs and Rct are the significant parameters for evaluating the performance of CE. As we know, a smaller Rs stands for a higher conductivity and the smaller Rct, the lower DEp, resulting in a faster electron transferring from CE to electrolyte, thus the electrocatalytic activity can be accordingly increased. From Table 1, it can be deduced that the Rct and Rs of the BTO/Gr CE are becoming smaller with the increasing of Gr, which presumably originates from the lamellar structure of graphene effectively facilitating the electron transport and the diffusion of the redox electrolyte within the CE, further leading to a better catalytic activity. The implication of a better catalytic activity infers to a higher PCE. Thus, when the mass fraction of Gr reaches to 2.5%, the PCE (9.70%) is much higher than that of pure BTO. This indicates that BG3 CE possesses a larger active surface area, better catalytic activity and superior electrolyte ab sorption property, demonstrating composites based on traditional ferroelectric oxides combined with Gr are one of the promising CEs in DSSC.
Table 1 The photo-electrochemical parameters derived from EIS plots, Tafel curves and J-V curves. CE
VOC (V)
JSC (mA/cm2)
FF
h%
Rs (V)
Rct (V)
BG0 BG1 BG2 BG3 Gr Pt
0.520 0.623 0.711 0.724 0.744 0.744
11.96 18.52 19.01 20.89 22.78 23.97
0.130 0.185 0.431 0.641 0.664 0.683
0.81 2.14 5.83 9.70 11.25 12.18
33.17 9.10 8.99 8.40 8.38 5.69
... ... 12400 1.35 0.90 0.73 0.69
Tafel polarization curves are used to further verify the catalytic activity of various CEs. As demonstrated by Fig. 7d, the exchange current density (J0) is approximately calculated by Tafel linear extrapolation method. That is, the intersection of the cathodic branch and the equilibrium potential line. The limiting current density (Jlim) depends on the intersection of the cathodic branch and the vertical axis. The J0 and Jlim are closely related to the catalytic activity of catalysts, which can severally assess the reduction capability and the diffusion capability of the iodide/ triiodide redox couple on CE materials. As we know, a larger slope implies a higher J0. Here we can see the catalytic ability of various CEs obey the rules of Pt > Gr > BG3 > BG2 > BG1 > BG0, indicating that Gr is favorable for enhancing the interfacial contact and reducing the charge recombination rate by providing rapid electron transport at CE/electrolyte interface, thus increasing the catalytic capability of pure BTO CE for the reduction of triiodide, which is in agreement with the previous EIS and CV results. It is definite to expound the photovoltaic characteristics of DSSCs based on various CEs from Fig. 8a. The detailed photovoltaic parameters are described in Table 1. The photovoltaic performance parameter of DSSCs is gradually meliorated with the increased Gr contents. According to the vast photoelectric performance tests, when the mass fraction of Gr exceeds 2.5%, the PCE of DSSC keeps almost unchanged. It can be observed that the DSSC with BG3 CE acquires a PCE of 9.70%, Voc of 726 mV, Jsc of 20.87 mA/cm2 and FF of 0.640, which is ten times larger than that of the DSSC based on the pristine BTO CE (PCE = 0.81%, Voc = 594 mV, Jsc = 12.09 mA/cm2 and FF = 0.113). As a result, Gr can promote the electrocatalytic activity of the BTO CE which is in accordance with the previous measurements (EIS and Tafel curves). The reason for which is the positive effect of Gr’s better electrocatalytic performance. The contact frequency between the redox couple in electrolyte and the electrode can be quicken because of the great specific surface area of Gr, so as to improve the reaction speed. Our work clearly indicates that small amount of Gr can strikingly raise the electrochemical and photovoltaic properties of BTO. In addition, it is reported that other binary oxides or polynary oxides composite Gr as CEs for DSSC in previous literatures, for instance, ZnO/Gr CE
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Fig. 8. Photocurrent density-voltage (J-V) curves (a) and IPCE spectrum (b) of the DSSCs based on various CEs.
has a PCE of 8.12% which is nine times larger than pure ZnO CE (0.80%), the PCE of La0.65Sr0.35MnO3 CE is raised from 5.35% to 6.57% by the addition of 8% Gr and in the case of 33.33% Grincorporated SiO2 CE, its PCE increases from 4.04% to 6.82% [25–27]. Compared the present experimental results with these literatures, it is apparent that the incorporation of BTO with Gr can likewise provide a simple and effective approach to evidently improve the PCE of BTO. Although Gr can be widely employed to increase the photoelectrochemical properties of some metal oxides such as SiO2, TiO2 and NiCo2O4 [27–29], to the best of our knowledge, it is the first time for us to report Gr enhancing electrocatalytic activity for ternary ferroelectric oxides. Fig. 8b presents the incident photon-to-current efficiency (IPCE) of the DSSCs based on the assorted CEs. IPCE measurements are carried out to investigate the contribution of each monochromatic light to the photocurrent [30]. The values of IPCE are mainly determined by dye loading capacity and electron collection efficiency in general. As is shown in Fig. 8b, the strong photoelectric responses of DSSC with various CEs are all located at about 530 nm, and the photoelectric response enhances with the increasing of Gr content. The DSSC assembled with BG3 CE indicates a photoelectric response of 36% which is higher than that of the DSSC on the basis of BG0 CE (28%), the enhanced IPCE values at about 380 nm and 530 nm are probably due to the increased generated carrier amount derived from the increasing light absorption. And the trend of IPCE is consistent with the J-V curves [31].Therefore, it can be distinctly discerned that after Gr incorporation, more active sites are provided for the absorption of dye molecules and the photocurrent in the external circuit is enhanced, which increases the charge collection efficiency. In consequence, the BTO/Gr CEs have a faster electron transmission and a higher dye absorption capacity. Generally speaking, the variation trend of the IPCE measurement is in line with the foregoing EIS and J-V results [32,33]. 4. Conclusions Bi4Ti3O12/graphene (BTO/Gr) CEs were prepared by a facile method based on sol-gel and electrospinning techniques. Extensive experiments indicate that BTO/Gr CE has a significant enhancement of photovolatic performance in comparison with pristine BTO CE, which embodies in higher catalytic activity for the reduction of triiodide, larger specific surface area and lower charge transfer resistance on the electrolyte-electrode interface, in which Gr plays an important role owing to its inherent features. Under the optimum conditions, the DSSC based on BTO/Gr CE exhibits much larger PCE (9.70%) than that of BTO CE. The work offers a new and
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Electrochimica Acta journal homepage: www.elsevier.com/locate/electacta
An efficient dye-sensitized solar cell with a promising material of Bi4Ti3O12 nanofibers/graphene Yuhong Yu, Haiwu Zheng* , Xinan Zhang, Xiao Liang, Gentian Yue* , Fengzhu Li, Mingsai Zhu, Tianfeng Li, Jianjun Tian, Guosheng Yin Key Laboratory of Photovoltaic Materials of Henan Province, Institute of Microsystem, School of Physics and Electronics, Henan University, Kaifeng 475004, China
A R T I C L E I N F O
Article history: Received 29 May 2016 Received in revised form 29 July 2016 Accepted 18 August 2016 Available online 31 August 2016 Keywords: Bi4Ti3O12 Graphene Counter Electrode Dye-sensitized Solar Cells
A B S T R A C T
In this work, the composites of Bi4Ti3O12/graphere (BTO/Gr) are used as counter electrode (CE) through facile method based on sol-gel and electrospinning techniques to improve the efficiency of dyesensitized solar cells (DSSCs). The structure and morphology of the composites with different ratios are characterized by X-ray diffraction, scanning electron microscopy and transmission electron microscopy. The optical bandgap of the composites decreases as Gr content increases. The electrochemical performance of BTO/Gr CE is characterized by cyclic voltammetry curves, electrochemical impedance spectroscopy, Tafel polarization curves and photocurrent density-voltage character curves. Compared with pure BTO CE, BTO/Gr CE has a higher electrocatalytic activity and a lower charge transfer resistance in the electrolyte/electrode interface. The power conversion efficiency of DSSCs with BTO/Gr CE (contained 2.5 wt.% Gr), has reached 9.70% under a simulated solar illumilnation of 100 mW/cm2, which is much larger than that of BTO CE. The power conversion efficiency amounts to 79.6% of the Pt CE and 86.2% of the graphene CE. ã 2016 Elsevier Ltd. All rights reserved.
1. Introduction Dye-sensitized solar cells (DSSCs) have attracted much interest owing to their significant advantages such as low cost, high efficiency, simple manufacture and environmental friendliness [1–3]. The typical structure of DSSCs contains conducting optical glass, photoanode, dyes, electrolyte and counter electrode (CE) [4,5]. Platinum (Pt), as an extremely expensive and rare metal material, is usually used as CE in DSSCs because of its excellent catalytic activity and high electrical conductivity. However, several disadvantages of Pt severely restrict the rapid development of DSSCs, such as the cost, reliability of sealing and easily poisoned by the redox species [6]. Hence, looking for materials that can be used to replace Pt in DSSCs has become an urgent issue. It is reported that some materials can substitute Pt as CE of DSSCs, for example, carbon-based materials, metal sulfides, nitrides, polymers and oxides [7,8]. Especially, a limited number of oxides have been endowed with expectation in that they contain earth-abundant elements and have stable chemical properties.
* Corresponding authors. E-mail addresses: [email protected], [email protected] (H. Zheng), [email protected] (G. Yue). http://dx.doi.org/10.1016/j.electacta.2016.08.086 0013-4686/ã 2016 Elsevier Ltd. All rights reserved.
Bismuth titanate (Bi4Ti3O12, BTO) can be considered as a wideband gap (3.21 eV) semiconductor material, which belongs to a family of layered perovskite compounds with good chemical stability. Although ferroelectric, piezoelectric, and photocatalytic properties of BTO have been extensively investigated [9,10], its electrochemical and photovoltaic properties have been less reported. In our previous work, we have investigated the DSSCs based on BTO acting as photoanode, however, the power conversion efficiency (PCE) is still very low [11]. Therefore, it is interesting to investigate the potential applications of BTO as CE in DSSCs for higher photoelectric conversion efficiency. In addition, graphene (Gr) is one of the most promising potential CE materials in DSSC due to its inherent features such as large surface area, high optical transmittance, excellent electrical conductivity and thermal conductivity [12,13]. Although composite CEs based on Gr have been reported by many groups, they mainly focused on sulfide, noble metal and polymers [14–17]. Consequently, it is worthy of us to explore the composite CE combined ternary oxides with Gr in order to improve the photo-electrochemical properties of functional oxides. In this paper, on account of the low charge mobility and inferior catalytic activity of BTO CE, we designed BTO/Gr composites as CE for DSSCs, hoping that Gr could promote the catalytic activity of
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BTO/Gr composite CE and improve the photovoltaic property of the DSSC. This work may broaden the potential applications of multifunctional oxides in the area of photo-electrochemistry. 2. Experimental 2.1. Material synthesis Bismuth Nitrate (Bi(NO3)3.5H2O 99.0%), Acetylacetone (C5H8O2 99.5%), Acetic acid glacial (C2H4O2 99.5%) and N,N-Dimethylformamide (DMF 99.5%) were put into a beaker stirring until all solids dissloved. Afterwards, Tetrabutyl titanate (C16H36O4Ti 98.0%) was dropwise added into the solution. Then polyvinylpyrrolidone (PVP, molecular weight of about 1300000) was added and continuously stirred for 12 h to form a yellow and transparent precursor. During this process, all the solvents and solutes were analytically pure without further purification. The prepared precursor was electrospun into nanofibers by home-made electrospinning equipment at a 10 kV accelerated voltage and a collecting distance of about 15 cm. The as-collected nanofibers were calcined in muffle furnace at 690 C in air for 1 h with a heating rate of 2 C/min. Finally, BTO nanofibers were produced. 2.2. Preparation of BTO/Gr CEs BTO/Gr CEs were prepared with commercial available Gr in weight ratio ranging from 0 wt.%, 0.5 wt.%, 1.5 wt.%, 2.5 wt.% and 100 wt.% which were labeled as BG0, BG1, BG2, BG3 and Gr for convenience, respectively. Firstly, the series samples (BG0, BG1, BG2, BG3, Gr) and polyethylene glycol 2000 were placed and ground in an agate mortar with the proportion of 4:1. Secondly, the composites were mixed with 1 ml anhydrous ethanol, keeping grind until the mixture became colloidal suspension. The suspension was coated on fluorine-doped tin oxide conductive glass substrate to form film [18], followed by heating at 400 C for 1 h under the protection of argon. The BTO/Gr CEs were obtained when cooling to room temperature. 2.3. Assembly of DSSCs DSSCs were sandwich structure consisting of anode, CE and electrolyte. A TiO2 anode was prepared as previously described [19]. 10 ml Tetrabutyl titanate and 100 ml deionized water were stirred for 30 min to form A solution. Yellow powders were obtained from A solution by suction filtration. Afterwards, the powders were put in 150 ml deionized water, 10 ml acetic acid glacial and 1 ml nitric acid stirring at 80 C for about 12 h to compound B solution. After B solution became light blue, P25 (TiO2
0.2 g) was added. Then the above solution was added into autoclave heating at 200 C for 12 h to form C solution. When it cooling to room temperature, 0.8 g polyethylene glycol and 1 ml emulgator were joined, followed by ultrasonic cleaning for 30 min to compound D solution. Finally, D solution was stirred at 80 C until thickened in order to prepare TiO2 paste. The above TiO2 paste was evenly spread on FTO substrate, dried naturally and annealed at 450 C for 30 min in air. In order to gain dye-sensitized TiO2 electrode, the photoanode needed to be soaked in a 0.3 mM ethanol solution of N719 dye for 24 h. The dye-sensitized TiO2 electrode and the as-prepared CE were clipped together, the electrolyte (0.05 M I2, 0.1 M LiI, 0.6 M 1,2-dimethyl-3-propylimidazolium iodide (DMPII), and 0.5 M 4-tert-butyl pyridine in acetonitrile) was injected into the aperture between the two electrodes with effective area of 0.25 cm2. 2.4. Characterizations The structures were characterized by a DX-2700 X-ray diffractometer (XRD) equipped with monochromatized Cu Ka radiation (l = 0.1542 nm) and confocal microscopy laser Raman spectrometer (Renishaw, RM-1000). The morphologies were measured by field emission scanning electron microscope (SEM, JSM-7001F) and transmission electron microscope (TEM, JEM2010). The absorption spectrum was observed by a UV-vis-NIR photospectrometer (Varian Cary 5000). Monochromatic incident photo-to-current conversion efficiency (IPCE) curves were gauged by an IPCE measurement system (Qtest Station 500ADX). The electrochemical characteristics of the DSSCs were measured by electrochemical workstation at room temperature (Wuhan CorrTest Instrument Co., Ltd.). Additionally, lithium perchlorate (0.8022 g), iodine (0.01269 g) and lithium iodine (0.06692 g) were added into 50 ml acetonitrile to form the electrolyte for the measurements of the cyclic voltammetry. The potentiostat that we used was CEL-S500/350/150. The potential was from 0.3 V to 0.8 V at a scanning rate of 30 mV/s and the reference electrode was AgCl. Moreover, the frequency of electrochemical impendence spectroscopy (EIS) is ranging from 10 kHz to 10 MHz and IPCE measurements were performed from 300 nm to 800 nm with a 150 W bromine-tungsten lamp and a 10 nm scanning step. 3. Results and disscussion Fig. 1a shows the SEM image of BTO nanofibers. It can be seen that their average diameter is in the range of 100–200 nm and the surface of the nanofibers is smooth. The inset in Fig. 1a presents the SEM image of the nanofibers under higher magnification. The HRTEM image of BTO nanofibers is indicated in Fig. 1b, the lattice spacing of which can be estimated as 0.272 nm. Compared with the
Fig. 1. (a) SEM of BTO nanofibers, the illustration in (a) exhibits the SEM image under greater magnification. (b) HRTEM of BTO nanofibers, the inset of (b) indicates the TEM image of BTO.
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Fig. 2. (a) Energy dispersive x- ray spectrum (EDX) of BTO. (b) TEM of Gr.
standard card of orthorhombic BTO, it coincides with the (200) plane. The illustration in Fig. 1b measured by TEM further describes the fibers structure. The energy dispersive x- ray spectrum (EDX) pattern of BTO is shown in Fig. 2a, in which the Bi, Ti and O elements can be detected and their atomic ratio is 26.1:13.9:60.0. This is close to the ideal stoichiometric ratio of BTO. From the TEM of Gr in Fig. 2b, it is noted that the Gr has a single flake structure. The TEM image of BG3 is exhibited in Fig. 3a, the black particles characterize the BTO nanoparticles and the reticulation denotes Gr, it can be inferred that BTO and Gr are adequately mixed without any other change in crystalline structure. This can be further confirmed by the selected area electron diffraction (SAED) image of BG3 in Fig. 3b, which appears the diffraction spots of BTO and the diffraction rings of Gr. Moreover, the crystal plane spacing is calculated to be 0.336 nm and 0.272 nm by the formula of Rd = L,
matching with the (002) plane of Gr and the (200) plane of BTO, respectively. Fig. 4a shows the XRD pattern of BTO nanofibers. From which we can see, all diffraction peaks correspond well with the JCPDS database card number 35-0795. No impurity diffraction peaks are observed and the XRD pattern matches well with the orthorhombic single-phase perovskite structure. It indicates that BTO has a good crystallinity, which is consistent with the previous HRTEM results. Moreover, no diffraction peaks for Gr are found within the limitation of the XRD equipment due to its tiny amount of concentration. The XRD results of BG1 and BG2 are almost identical with that of BG3 and thus are ignored. Raman scattering spectra is given in Fig. 4b for the sake of further ascertaining the phase composition of the samples. From this figure, there are six apparent peaks of BTO which are located at
Fig. 3. (a) TEM of BTO and Gr composites (BG3). (b) Selected area electron diffraction (SAED) of BTO and Gr composites (BG3).
Fig. 4. (a) XRD patterns of various samples. (b) Raman spectra of samples, the inset of (b) illustrates the enlarged views in the wavelength range of 1100–1800 nm.
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Fig. 5. (a) UV–vis diffuse absorption spectra of various samples. Diagram (b) indicates the numerical value of band gap for various samples.
225, 261, 328, 539, 615, 847 cm 1, respectively. The modes observed at 225, 328 and 539 cm 1 are attributed to the internal vibrational mode of TiO6 and belong to B2g and B3g symmetry originating from the lifting of Eg degeneracy [20]. The modes at 261, 615 and 847 cm 1 are ascribed to the stretching of the TiO6 octahedron and can be assigned as A1g character [21]. As seen from the inset in Fig. 4b, there are two prominent peaks of Gr at 1352 cm 1 and 1575 cm 1, which are assigned to the disordered (D) and graphitic (G) bands, respectively. The D peak is derived from the structure defect of lattice, while G peak belongs to the E2g phonon of C sp2 atoms. As a consequence, the BTO and Gr are successfully composited from the results of Raman spectrum and SAED images. It can be inferred that the specific area of the composites enhance with the increase of Gr content, which will probably facilitate the generation and transport of the photogenerated carriers, leading to a higher electrochemical properties [22]. The UV–vis absorbance spectra of the samples are illustrated in Fig. 5a. Fig. 5b presents the fitting curves based on direct band gap. As is shown in the diagram, BTO nanofibers exhibit a good lightharvesting capability in the range of 300 to 400 nm. By means of linear extrapolation method, the optical band gap of BTO nanofibers can be approximately estimated as 3.21 eV, which comes up to the published in the previous literature [23]. Whereas, the effective band gaps of BG1, BG2 and BG3 are severally estimated at about 3.06 eV, 2.99 eV, 2.86 eV, indicating larger light-harvesting capacity than that of pure BTO, which probably arises from the smaller optical band gap of Gr (1.57 eV). Therefore, it can be
concluded that the photo-absorption ability and photo-generated carriers’ numbers can be enhanced through the combination of Gr. The cyclic voltammograms of various CEs are shown in Fig. 6a, which is measured by using a three-electrode system with the Pt sheet as CE, saturated sliver chloride electrode as reference electrode and various CEs as working electrode. In general, there are two pairs of redox peaks, while the pair of peaks on the low potential area has a significant impact on the photovoltaic properties of DSSCs. The over-potential (DEp) is inversely correlated with the standard electrochemical rate constant of a redox reaction, illuminating that a smaller DEp represents a better catalytic activity [24]. For comparison, the DEp for Pt and Gr are measured and displayed in the inset of Fig. 6a. From Fig. 6a, the values of DEp for Pt, Gr, BG3, BG2, BG1 and BG0 are 288 mV, 332 mV, 346 mV, 402 mV, 476 mV and infinity, respectively. It reflects the catalytic activity of various CEs following the orders of Pt > Gr > BG3 > BG2 > BG1 > BG0, suggesting that BG3 CE has a decent catalytic activity, which is derived from Gr because it possesses a larger specific surface area which can vastly enhance the assessibility of the electrolyte to the electrode, thus improving interfacial charge transfer and increasing the number of active catalytic sites. Additionally, twenty cycles of CV curves in Fig. 6b are used to demonstrate the stability of BG3 CE. It turns out that BG3 CE is stable for catalyzing triiodide. The above experimental results imply that the incorporation of Gr can indeed increase the catalytic activity of BTO. Fig. 7a, b, and c exhibit the electrochemical impedance spectroscopy (EIS), which is invoked to investigate the properties
Fig. 6. (a) Cyclic voltammetry (CV) curves of triiodide/iodide redox species of various CEs, inset of (a) denotes the CV curves of Pt and Gr CEs. (b) Twenty cycles of CV curves from BG3 CE at a scan rate of 30 mV/s.
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Fig. 7. (a), (b) and (c) represent the Nyquist plots of various CEs, and inset of (b) denotes the partial enlarged drawing of diagram (b). (d) shows the Tafel curves for symmetric cells fabricated with assorted CEs.
and quality of devices. The corresponding parameters are listed in Table 1. The intercept on the horizontal axis represents the series resistance (Rs), a reflection of conductive substrate resistance and lead resistance. The real radius of the first semicircle in the high frequency domain denotes the charge-transfer resistance (Rct) at CE/electrolyte interface which characterizes the reduction of triiodide, while the second arc refers to the Nernst diffusion impedance corresponding to the diffusion resistance of the redox couples in the electrolyte. It is noteworthy that Rs and Rct are the significant parameters for evaluating the performance of CE. As we know, a smaller Rs stands for a higher conductivity and the smaller Rct, the lower DEp, resulting in a faster electron transferring from CE to electrolyte, thus the electrocatalytic activity can be accordingly increased. From Table 1, it can be deduced that the Rct and Rs of the BTO/Gr CE are becoming smaller with the increasing of Gr, which presumably originates from the lamellar structure of graphene effectively facilitating the electron transport and the diffusion of the redox electrolyte within the CE, further leading to a better catalytic activity. The implication of a better catalytic activity infers to a higher PCE. Thus, when the mass fraction of Gr reaches to 2.5%, the PCE (9.70%) is much higher than that of pure BTO. This indicates that BG3 CE possesses a larger active surface area, better catalytic activity and superior electrolyte ab sorption property, demonstrating composites based on traditional ferroelectric oxides combined with Gr are one of the promising CEs in DSSC.
Table 1 The photo-electrochemical parameters derived from EIS plots, Tafel curves and J-V curves. CE
VOC (V)
JSC (mA/cm2)
FF
h%
Rs (V)
Rct (V)
BG0 BG1 BG2 BG3 Gr Pt
0.520 0.623 0.711 0.724 0.744 0.744
11.96 18.52 19.01 20.89 22.78 23.97
0.130 0.185 0.431 0.641 0.664 0.683
0.81 2.14 5.83 9.70 11.25 12.18
33.17 9.10 8.99 8.40 8.38 5.69
... ... 12400 1.35 0.90 0.73 0.69
Tafel polarization curves are used to further verify the catalytic activity of various CEs. As demonstrated by Fig. 7d, the exchange current density (J0) is approximately calculated by Tafel linear extrapolation method. That is, the intersection of the cathodic branch and the equilibrium potential line. The limiting current density (Jlim) depends on the intersection of the cathodic branch and the vertical axis. The J0 and Jlim are closely related to the catalytic activity of catalysts, which can severally assess the reduction capability and the diffusion capability of the iodide/ triiodide redox couple on CE materials. As we know, a larger slope implies a higher J0. Here we can see the catalytic ability of various CEs obey the rules of Pt > Gr > BG3 > BG2 > BG1 > BG0, indicating that Gr is favorable for enhancing the interfacial contact and reducing the charge recombination rate by providing rapid electron transport at CE/electrolyte interface, thus increasing the catalytic capability of pure BTO CE for the reduction of triiodide, which is in agreement with the previous EIS and CV results. It is definite to expound the photovoltaic characteristics of DSSCs based on various CEs from Fig. 8a. The detailed photovoltaic parameters are described in Table 1. The photovoltaic performance parameter of DSSCs is gradually meliorated with the increased Gr contents. According to the vast photoelectric performance tests, when the mass fraction of Gr exceeds 2.5%, the PCE of DSSC keeps almost unchanged. It can be observed that the DSSC with BG3 CE acquires a PCE of 9.70%, Voc of 726 mV, Jsc of 20.87 mA/cm2 and FF of 0.640, which is ten times larger than that of the DSSC based on the pristine BTO CE (PCE = 0.81%, Voc = 594 mV, Jsc = 12.09 mA/cm2 and FF = 0.113). As a result, Gr can promote the electrocatalytic activity of the BTO CE which is in accordance with the previous measurements (EIS and Tafel curves). The reason for which is the positive effect of Gr’s better electrocatalytic performance. The contact frequency between the redox couple in electrolyte and the electrode can be quicken because of the great specific surface area of Gr, so as to improve the reaction speed. Our work clearly indicates that small amount of Gr can strikingly raise the electrochemical and photovoltaic properties of BTO. In addition, it is reported that other binary oxides or polynary oxides composite Gr as CEs for DSSC in previous literatures, for instance, ZnO/Gr CE
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Fig. 8. Photocurrent density-voltage (J-V) curves (a) and IPCE spectrum (b) of the DSSCs based on various CEs.
has a PCE of 8.12% which is nine times larger than pure ZnO CE (0.80%), the PCE of La0.65Sr0.35MnO3 CE is raised from 5.35% to 6.57% by the addition of 8% Gr and in the case of 33.33% Grincorporated SiO2 CE, its PCE increases from 4.04% to 6.82% [25–27]. Compared the present experimental results with these literatures, it is apparent that the incorporation of BTO with Gr can likewise provide a simple and effective approach to evidently improve the PCE of BTO. Although Gr can be widely employed to increase the photoelectrochemical properties of some metal oxides such as SiO2, TiO2 and NiCo2O4 [27–29], to the best of our knowledge, it is the first time for us to report Gr enhancing electrocatalytic activity for ternary ferroelectric oxides. Fig. 8b presents the incident photon-to-current efficiency (IPCE) of the DSSCs based on the assorted CEs. IPCE measurements are carried out to investigate the contribution of each monochromatic light to the photocurrent [30]. The values of IPCE are mainly determined by dye loading capacity and electron collection efficiency in general. As is shown in Fig. 8b, the strong photoelectric responses of DSSC with various CEs are all located at about 530 nm, and the photoelectric response enhances with the increasing of Gr content. The DSSC assembled with BG3 CE indicates a photoelectric response of 36% which is higher than that of the DSSC on the basis of BG0 CE (28%), the enhanced IPCE values at about 380 nm and 530 nm are probably due to the increased generated carrier amount derived from the increasing light absorption. And the trend of IPCE is consistent with the J-V curves [31].Therefore, it can be distinctly discerned that after Gr incorporation, more active sites are provided for the absorption of dye molecules and the photocurrent in the external circuit is enhanced, which increases the charge collection efficiency. In consequence, the BTO/Gr CEs have a faster electron transmission and a higher dye absorption capacity. Generally speaking, the variation trend of the IPCE measurement is in line with the foregoing EIS and J-V results [32,33]. 4. Conclusions Bi4Ti3O12/graphene (BTO/Gr) CEs were prepared by a facile method based on sol-gel and electrospinning techniques. Extensive experiments indicate that BTO/Gr CE has a significant enhancement of photovolatic performance in comparison with pristine BTO CE, which embodies in higher catalytic activity for the reduction of triiodide, larger specific surface area and lower charge transfer resistance on the electrolyte-electrode interface, in which Gr plays an important role owing to its inherent features. Under the optimum conditions, the DSSC based on BTO/Gr CE exhibits much larger PCE (9.70%) than that of BTO CE. The work offers a new and
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