High Performance Dye-Sensitized Solar Cells Based on Electrospun MoS2-Carbon Nanofiber Composite Counter Electrode

MoS2-carbon nanofiber composites were prepared via electrospun technique with ammonium thiomolybdate(VI) and polyvinylpyrrolidone. The effects of thermal pretreatment on the morphology of material were studied. The results demonstrate that the composite nanofiber with pretreatment shows a uniform diameter and lots of small size MoS2 particles inside with porous structure. When the composites was used as counter electrodes in dye-sensitized solar cell, a maximum efficiency of 5.7% has been achieved due to the combination of electrical conductivity and catalysis ability from MoS2-carbon nanofiber composites.


INTRODUCTION
For nearly thirty years, due to the advantages of low price and simple preparation, a lot of attention has been paid to dye sensitized solar cells (DSSCs) (Sun et al., 2015;Sun et al., 2020). In a typical DSSC with sandwich structure, electrons excite from the dye and inject to the conduction band of the photoanode, then the oxidized dye could be reduced by a charge mediator in electrolyte (Miao et al., 2015). Counter electrode plays an important role in this process, which take charge of reducing I 3 − to I − in electrolyte and collecting electrons from external circuit (Yue et al., 2014;Yun et al., 2014). Although, the traditional Pt counter electrode has an outstanding catalytic performance, its high price and poor corrosion resistance seriously hinder its mass production process. Therefore, it is of great significance to develop new materials instead of Pt counter electrode (Hauch and Georg, 2001;Olsen et al., 2000).
Benefit from the uniform radial structure and good transmission characteristics, one-dimensional carbon materials are superior to other carbon materials in electrical conductivity (Ramasamy et al., 2008;Poudel and Qiao, 2012;Kim and Yong, 2013). Recently, carbon nanofibers, carbon nanotubes, carbon nanowires with porous structure have been widely used in the DSSCs, secondary batteries or other energy fields Pan et al., 2019;Jia et al., 2020;Pan et al., 2020;Qin et al., 2020;Yuan et al., 2020;Zhu et al., 2021). Joshi et al. prepared carbon nanofiber counter electrode through electrospinning for DSSC, which obtained an efficiency of 5.5% (Zhu et al., 2010). Zou et al. made an one-dimensional DSSC using carbon fiber as counter electrode, with an efficiency of 2.7% (Hou et al., 2011). Guo et al. synthesized Pt-carbon fiber composite counter electrode to obtain a high efficiency of 8.97% . Xiao et al. used single-walled carbon nanotubes combined with Pt as counter electrode, through optimizing the content of carbon nanotubes, with an efficiency of 5.96% (Ruan et al., 2011).
In our previous work, MoS 2 -nitrogen doped carbon shell-core microspheres have been synthesized for Pt-free DSSC (Zhu et al., 2017). Thus it can be expected that MoS 2 -carbon nanofiber composites should be promising counter electrode materials for DSSCs. In this work, a simple and fast electrospinning method was employed to synthesize MoS 2 -carbon nanofiber composites. After thermal treatment, the composites material were used as counter electrodes of DSSCs, with an efficiency of 5.7% under 1 sun illumination.

EXPERIMENTAL Preparing Composites Material and Counter Electrode
In a typical process, a certain amount of ammonium thiomolybdate(VI) (0.2 M) and polyvinylpyrrolidone (1,300,000 g/mol) (8 wt%) were dissolved in N, N-dimethylformamide. The solution was then placed into a needle. The applied voltage was 20 kV, and the distance between the needle and the stainless steel roller was 15 cm. The flow rate was set as 0.6 ml/h. Subsequently, the samples were separated into two parts. One thermally treated at 800°C for 2 h in an N 2 /H 2 atmosphere, named as mesoporous silica nanofiber (MSF). For the other part, a pretreatment process is added before thermal treatment, which is kept in air atmosphere at 280°C for 2 h with a heating rate 2°C min −1 , and named as air-MSF. The as-synthesized composites material were coated onto the cleaned F-doped SnO 2 (resistivity: 14 Ω/□, Nippon Sheet Glass, Japan) glass using the screen printing method, and were annealed at 400°C for 30 min, then used as counter electrodes. The mass of the activated material is ∼5 mg in one counter electrode. The conventional Pt film and pure MoS 2 film were also used as counter electrodes for comparison.

Preparing Dye Sensitized Solar Cells
The photoanodes were prepared of P25 paste on F-doped SnO 2 glass by screen printing method. After sintered at 450°C for 30 min and cooled to 80°C, photoanodes were immersed into 0.5 mM N719 dye solution for 24 h. The redox electrolyte contained of 0.6 M 1-butyl-3-methylimidazolium iodide, 30 mM I 2 , 0.5 M tert-butylpyridine, and 0.1 M guanidinium thiocyanate in a solvent mixture of 85% acetonitrile with 15% FIGURE 1 | Field emission scanning electron microscope images of (A) air-mesoporous silica nanofiber and (B) mesoporous silica nanofiber.

RESULTS AND DISCUSSION
The FESEM of air-MSF with pretreatment are shown in Figure 1A. As illustrated in Figure 1A, a large number of one-dimensional carbon nanofiber are interactively connected together, forming a three-dimensional network structure, which is conducive to the transmission of electrons. Figure 1B shows FESEM image of MSF. It can be observed that MSF has serious adhesion, bulky fiber and is not separable, which is not conducive to electron transmission.
The detailed morphologies of air-MSF and MSF are shown by TEM in Figure 2. It can be clearly seen from Figure 2A Figure 2B are short and thick and uneven in diameter due to adhesion (Zhu et al., 2010). Figures 3A,B are high-magnification TEM images of air-MSF. It can be found from Figure 3A that a large number of MoS 2 nanoparticles are uniformly distributed in the carbon nanofiber. In the further enlarged Figure 3B, the lattice fringes with interplanar spacing of 0.27 nm correspond to the (100) crystal plane of hexagonal MoS 2 . Figures 3C-F are the element distribution diagrams of air-MSF, where the white dots in Figure 3D are carbon elements, the green dots in Figure 3E are sulfur, and the red dots in Figure 3F are molybdenum. The distribution diagram can illustrate that a large number of MoS 2 nanoparticles are uniformly distributed in the carbon nanofiber.    The XRD patterns of air-MSF, MSF and pure MoS 2 prepared by microwave methods are shown in Figure 4. Both air-MSF and MSF have diffraction peaks near ∼33.5°and ∼68°, corresponding to the (100) hexagonal crystal form of MoS 2 , which is consistent with the (110) crystal plane and the PDF card (JCPDS card no.73-1508). It shows that the pretreatment does not affect the formation of MoS 2 crystals. However, the XRD patterns of the MoS 2 crystal synthesized by electrospun and the MoS 2 crystal prepared by the microwave method are not exactly the same. Smaller MoS 2 crystals through electrospun grow preferentially in a specific direction, with low crystallinity and disordered structure. Due to the low carbon content in the two composite samples, the C (002) peak around ∼26°is relatively weak (Qin et al., 2015).
The electrochemical performance of air-MSF, MSF, MoS 2 and Pt electrodes are shown by CV curves in Figure 5. Although both composite electrodes show catalytic ability to iodine-based electrolyte, the peak current density of MSF electrode is very close to or even lower than MoS 2 , which indicates that the electrochemical catalytic capacity of MSF without pretreatment has not been enhanced. It should be resulted by excessive adhesion of carbon nanofiber with weak conductivity. The current density of the pretreated air-MSF has been improved, which shows that the carbon nanofiber composite MoS 2 electrode has a higher catalytic ability to iodine-based electrolyte than the MoS 2 electrode. This is because the carbon nanofiber network is conducive to the transmission of electrons. Combined with the catalytic ability of MoS 2 , air-MSF electrode with stronger performance is obtained (Li et al., 2010;Wang et al., 2011). Figure 6 shows the Nyquist spectrum of the four electrodes, and the inset shows its equivalent circuit. A typical electrochemical impedance spectra curve of a DSSC has two semicircles. The large semicircle in the low frequency region corresponds to the reaction at the photoanode/electrolyte interface reaction, and the small semicircle in the high frequency region corresponds to the reaction on counter electrode/electrolyte. So the high-frequency semicircle is the focus of characterizing the performance of the counter electrode. According to the equivalent circuit, R s is the series resistance of the cell. R ct is the charge transfer resistance at the counter electrode/electrolyte interface. And R w is the charge transfer resistance at the photoanode/electrolyte interface. It can be seen from the fitting that the R ct value (25.9 Ω) of the MSF counter electrode is higher than the MoS 2 counter electrode (22.3 Ω), which indicates that the MSF counter electrode does not perform well, which is consistent with the CV results. The R ct value of the air-MSF counter electrode is about 16.6 Ω, significantly lower than MoS 2 , which indicates that the electrochemical performance of the pretreated carbon nanofiber composite MoS 2 counter electrode is better than that of the pure MoS 2 counter electrode (Rick et al., 2014). Figure 7 shows J-V curves of DSSCs based on composite counter electrodes, pure carbon nanofiber, pure MoS 2 , and traditional Pt counter electrode. Comparing the current density (J sc ), open circuit voltage (V oc ), fill factor (FF) and conversion efficiency (η) of pure carbon nanofiber and pure MoS 2 counter electrode (as shown in Table 1), it can be found that MSF with severe adhesion of carbon nanofiber has a lower FF (43.3%) and η (4.4%) than the pure MoS 2 counter electrode (50.8%, 5.0%), and only slightly higher than the pure carbon nanofiber counter electrode (41.5%, 4.2%). The pre-processed air-MSF counter electrode exhibits more impressive photovoltaic performance, with FF and η reaching 55.0 and 5.7%, respectively, which are higher than the one-dimensional DSSC made by Zou (Hou et al., 2011). This shows that the pre-heat treatment of carbon nanofiber is beneficial to the morphology of the sample, thereby improving the conductivity of the sample. Moreover, the comparison with the pure MoS 2 counter electrode shows that the combination of carbon nanofiber and MoS 2 benefits from the close adhesion, which can combine the excellent electrical conductivity of carbon nanofiber with the good catalytic ability of MoS 2 to obtain a composite electrode with better performance. Although the air-MSF electrode does not exceed the Pt counter electrode in photovoltaic performance, the simpler and faster method of electrospinning will benefit the industrial production of DSSCs .

CONCLUSION
The electrospinning method is used to prepare carbon nanofiber and MoS 2 composite material with ammonium tetrathiomolybdate and polyvinylpyrrolidone as raw materials. Through characterization, it is found that the MoS 2 particles synthesized by this method are small in size and densely distributed among the carbon nanofiber. The influence of the pretreatment on the formation of carbon nanofiber was discussed. Through the comparison of several kinds of electrodes, it can be found that the efficiency of the pretreated air-MSF composite counter electrode can reach 5.7%, which is higher than that of pure carbon nanofiber and pure MoS 2 counter electrode.

DATA AVAILABILITY STATEMENT
All datasets presented in this study are included in the article.

AUTHOR CONTRIBUTIONS
HS and XZ proposed ideas and wrote papers. Experiment was done by HS. WY, YG, RZ, and JL modified articles. HW, YG, and ZW provided financial and technical support.

FUNDING
The authors declare that this study received funding from 2020 State Grid Corporation Headquarters Technical Service (546816200012). The funder was not involved in the study design, collection, analysis, interpretation of data, the writing of this article or the decision to submit it for publication.