Carbon Nanotube Aerogel as

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3D MoS2 Nanosheet Based Graphene/Carbon Nanotube Aerogel as a Pt-free Counter Electrode for High-efficiency Dye-sensitized Solar Cells Fei Yu, Yan Shi, Xiaojie Shen, Wenhao Yao, Sheng Han, and Jie Ma ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.8b03143 • Publication Date (Web): 06 Nov 2018 Downloaded from http://pubs.acs.org on November 7, 2018

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ACS Sustainable Chemistry & Engineering

3D MoS2 Nanosheet Based Graphene/Carbon Nanotube Aerogel as a Pt-free Counter Electrode for High-efficiency Dye-sensitized Solar Cells Fei Yua, b§, Yan Shib,c§, Xiaojie Shene, Wenhao Yaob, Sheng Hanb, Jie Mac,d* a

College of Marine Ecology and Environment, Shanghai Ocean University, 999 Huchenghuan Road, Shanghai 201306, P. R. China

b School

of Chemical and Environmental Engineering, Shanghai Institute of

Technology, 100 Hai Quan Road, Shanghai 201418, P. R. China c Shanghai

Institute of Pollution Control and Ecological Security, Shanghai, 200092, P. R. China

d State

Key Laboratory of Pollution Control and Resource Reuse, College of

Environmental Science and Engineering, Tongji University, 1239 Siping Road, Shanghai 200092, P. R. China, email:[email protected] e

Department of Environmental Science & Engneering, College of Chemical

Engineering, Beijing University of Chemical Technology, 15 North Third Ring Road, Beijing 100029, P. R. China

*Corresponding author: Tel.: +86 21 65981831; Fax: +86 21 65981831; E-mail address: [email protected] (J. Ma) §These

authors contributed equally to this work. 1

ACS Paragon Plus Environment

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Abstract 3D molybdenum disulfide (MoS2) nanosheets grew on reduced graphene oxide/single-walled carbon nanotubes (GNTs). The resulting reduced graphene oxide/single-walled carbon nanotube/MoS2 (GNT-M) hybrid nanocomposites were used as efficient counter electrodes (CEs) in dye-sensitized solar cells (DSSCs). The microstructural details of the nanohybrids were characterized by transmission electron microscopy (TEM) and field-emission scanning electron microscopy (SEM), which showed an intertwined structure with an abundance of exposed active sites on the edge. X-ray diffraction and Raman spectroscopy were used to further confirm the binding of MoS2/GNT and the nanostructures. The composite electrode-based DSSCs with GNTM10 showed a maximum power conversion efficiency of 8.01%, which is higher than that of a Pt CE (7.21%). Furthermore, GNT-M10 performed stably in the one-week continuous test. Such excellent photoelectric performance can be attributed to the fact that MoS2 nanosheets with a 3D structure have more exposed active sites to promote the reduction and regeneration of I3- ions and the superior electrical conductivity of graphene and single-walled carbon nanotube.

Keywords: Molybdenum disulfide; Graphene; Single-walled carbon nanotubes; Counter electrode

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Introduction Over the past three decades, dye-sensitized solar cells (DSSCs) have been considered third-generation solar cells owing to their easy assembly, low cost, relatively high power conversion efficiency (PCE) and environmentally friendly nature1-3. A typical DSSCs is composed of a ruthenium dye-sensitized semiconductor (TiO2) working electrode (WE), a redox-couple active electrolyte (I3-/I-), and a counter electrode (CE) 4. The CE is an indispensable part of DSSCs, and its main task is to collect electrons from the external circuit and catalyze the reduction of triiodide to iodide at the contact interface between the CE and electrolyte5. Platinum (Pt) is widely used as a common CE material due to its high catalytic reduction for I3-, good chemical stability, and superior conductivity6-9. However, the widespread commercial use of Pt is limited because of its rarity and expense9-10. Therefore, it is important to develop inexpensive Pt-free materials with excellent electrocatalytic activity to substitute Pt CEs. For example, carbonaceous material11-12, alloys13 , conducting polymers14-15 and composite16-17 have been reported. In recent years, transition-metal dichalcogenides (TMDs)18-20 have attracted more attention due to their unique and excellent performances. Molybdenum disulfide (MoS2) is a typical TMD and possesses a two-dimensional (2D) architecture with a Mo layer sandwiched between two S layers due to weak van der Waals forces21. MoS2 has been researched and developed in DSSCs and has demonstrated outstanding electrocatalytic performances. In addition to the catalytic activity of a material, the electric conductivity is also a major consideration for researchers. Many electrode materials with a high catalytic performance do not have a sufficient electron transfer capability for electrochemical applications. Thus, the photoelectric performance of MoS2 will be restricted by the resistance of its conductive inertia. To address this problem, many recent research studies have proposed improving the capacity for conducting electrons by creating hybrids of carbonaceous materials with MoS2. Zhang et al.22 prepared a flowerlike MoS2/multi-wall carbon nanotube (MoS2/MWCNT) hybrid by a lowtemperature hydrothermal method. Owing to the synergistic effects between MoS2 and MWCNT, the MoS2/MWCNT electrode with an optimal MWCNT content exhibits 3



excellent electrochemical properties. Yue et al.23 reported a porous molybdenum sulfide-carbon (MoS2-C) hybrid film synthesized via an in situ hydrothermal route showed a decent PCE of 7.69% as an efficient CE in DSSCs, and further research demonstrated that the MoS2-C CE possesses the capability to quickly transfer charge on the electrolyte-electrode interface. In these above works, the conductivity of the MoS2 composite materials was increased to some extent because of the rapid electron transmission provided by the carbon-based material. However, in the above works, the researchers did not deal with the exposure of more MoS2 active sites. From theoretical and experimental conclusions, it has been confirmed that the catalytic activity of MoS2 is associated with its sulfur edge24-25. With abundant exposed edges, MoS2 has higher electrochemical and electrocatalysis activities26. Inspired by the above analysis, in this work, we synthesized a 3D nanocomposite of reduced graphene oxide/single-walled carbon nanotube/MoS2 (GNT-M) as an efficient Pt-free CE. On account of the high specific surface area (SSA) of graphene/single-walled carbon nanotubes (GNTs)27, the GNTs were used as the 3D anchored templates, and MoS2 was grown on the surface. Thus, more MoS2 active sites were created. Finally, the effect of the quality ratio of the reaction precursors on the MoS2 structure and resulting photoelectric properties has been studied and discussed based on the results.

Experimental Materials Chemical reagents including ammonium tetrathiomolybdate powder ((NH4)2MoS4), polyvinylpyrrolidone with an average molecular weight of 4000 (PVPk30), ethanol, ethanediol and hydrochloric acid (HCl) were acquired from Sinopharm Chemical Reagent Co., Ltd. in Shanghai, China. Graphite oxide was prepared via an improved Hummers method, while the present single-walled carbon nanotubes (SWCNTs) were prepared using a catalytic chemical vapor deposition method28 and purified using a nondestructive approach29. The SWCNT sample contained >95% SWCNTs with an outer diameter of approximately 10 nm, and the number of walls was approximately 5. 4


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Sensitized dye N719, the fluorine-doped tin oxide (FTO) transparent glass substrates and Pt-CE were obtained from Dalian HeptaChroma SolarTech Co., Ltd. Synthesis of the 3D GNT aerogel A simple hydrothermal reduction and freeze-drying method were used to prepare the 3D GNT aerogel30-31. In a typical synthesis, 100 mg of graphite oxide was first added to 100 mL of deionized (DI) water in an ultrasonic bath for 4 h. Then, 200 mg of the SWCNTs was dispersed in the GO solution. After that, 200 mg of reduced glutathione was dispersed in the above solution and placed into a 95 °C water bath for 11 h to form the reduced graphene oxide (RGO)/SWCNT hydrogels. Then, the 3D GNT aerogel was obtained by freeze-drying. Finally, the above aerogel was annealed at 800 °C for 2 h with argon as the protective gas. Synthesis of the GNT-M nanocomposite The steps to synthesize GNT-M are as follows. GNT and (NH4)2MoS4 were dispersed in an ethanol solution (20 %) at a mass ratio of 1:1 with constant stirring and sonication for 2 h. Subsequently, 5 mL of concentrated hydrochloric acid (37.5 %) was added to the aforementioned solution, which was stirred for 2 h. The solution was allowed to stand until a black intermediate product mixture was obtained. The above black solution was collected by centrifugation and dried under vacuum, and the solid black precipitates were washed several times with DI water and ethanol, dispersed in DI water and sonicated for 4 h to obtain a uniform and stable solution. Finally, the GNT-M nanocomposite was obtained after freeze-drying and was annealed at 650 °C under hydrogen/argon in a slow speed atmosphere, which is referred to as GNT-M1. Using the same method, the rest of the sample was prepared by changing the content of (NH4)2MoS4 to prepare GNT/(NH4)2MoS4 ratios of 5:1 and 10:1, which are referred to as GNT-M5 and GNT-M10, respectively. Preparation of the GNT-M CE Then, 30 mg of the above samples of GNT and GNT-M were added to 5 ml of ethanediol solvent followed by 10 mg of PVPk30 and sonication for 1 day until the mixture was smooth. The dispersion slurry was coated on FTO transparent glass substrates via dropping technology. Subsequently, the above samples were vacuum 5



dried at 60 °C for 12 h. Finally, after annealing at 400 °C for 1 h under an argon atmosphere, the GNT-M CE was successfully obtained. Fabrication of the DSSCs In a typical TiO2 photoanode preparation, the TiO2 photoanode was dye-sensitized by immersion in a 3 mM N719 solution at 60 °C for 20 h. Subsequently, the dyesensitized TiO2 photoanode was washed several times and dried at 60 °C with ethanol to remove the residual dye on the surface. After that, the sandwich-like DSSCs were assembled with a dye-sensitized TiO2 photoanode, various CEs, including a Pt electrode as a comparison, and an iodide-based electrolyte with a 25-μm-thick hot-melt surlyn film between the two layers. Characterizations The microstructure and morphology of the samples were characterized by transmission electron microscopy (TEM, JEOL-2010F) and field-emission scanning electron microscopy (SEM, Hitachi S-4800). Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) analyses were performed with an electrochemical workstation (CHI Instruments 660D). The CV curves were measured in a three-electrode system in which a Pt foil and Ag/AgCl served as the counter and reference electrodes, respectively, in an acetonitrile solution containing 10 mM LiI, 1 mM I2, and 0.1 M LiClO4. The EIS spectra were measured by simulating an opencircuit at ambient atmosphere with an applied bias voltage and AC amplitude of 0 V and 10 mV, respectively, with the frequency varying from 100 kHz to 0.1 Hz. The EIS measurements were conducted using symmetric dummy cells.

Results and discussion The detailed fabrication approach for GNT-M is illustrated in Scheme 1. A large amount of oxygen-containing functional groups on the graphene oxide (GO) and singlewalled carbon nanotube (SWCNT) surface facilitates aggregation of the MoS2 nanoplatelets.32-33 On the other hand, the single-walled carbon nanotubes increase the 3D spatial extension of the material and act as fast electron-transport channels. 6


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Scheme 1. Schematic illustration of the preparation process for GNT-M hybrid nanocomposites Fig. 1 shows the TEM micrographs of the GNT and GNT-M5 nanostructures after annealing, respectively. As shown in Fig. 1a and 1b, the graphene sheets are very thin with single-walled carbon tubes interspersed among them and play a supporting role as a scaffold for the 3D nanocomposite, which acts as a “highway” to effectively provide an electronic transmission channel. Fig. 1c-e show the TEM image of GNT-M5 at different magnifications. It is easy to observe the porous microstructure of the GNTM5 skeleton, which illustrates its strong 3D stereoscopic sense. The SWCNTs that are loaded with MoS2 are purer in contrast with those in the images of GNT, which may be the result of purification by HCl treatment. Interestingly, a number of small black translucent pieces grew at the intersections of the SWCNTs. At a high magnification, we can see a small piece of a thin layer hanging on the SWCNT surface, which is embedded on the surface. The high-resolution TEM image (Fig. 1f) reveals a well layered crystal structure with an interlayer distance of 0.27 nm was consistent with the lattice parameter in the (100) plane and of 0.62 nm was consistent with the lattice parameter in the (002) plane, which is equal to the hexagonal lattice of the 2H–MoS2 phase34. This 3D morphology of the GNT can serve as an attractive substrate for the growth of MoS2 to provide sufficient catalytic sites for the reduction of I3- to I-.

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Fig. 1. TEM images of the GNT 3D hybrid nanostructures (a, b); GNT-M5 (c, d, e). high-resolution TEM image of GNT-M5 nanostructures (f).

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Fig. 2. SEM images of the GNT 3D hybrid nanostructures (a, b); GNT-M1 (c, d). SEM was used to further investigate the morphology and nanostructural details of GNT and GNT-M1, as shown in Fig. 2. In Fig. 2a and 2b, the GNT shows a 3D intertwined structure among agglomerated and overlapping stacks of graphene sheets (GNSs) with one-dimensional SWCNTs. It can be seen that the surfaces of the GNSs and SWCNTs are quite pure. Fig. 2c shows a low-magnification image of GNT-M1, and the high-magnification SEM image is shown in Fig. 2d. We can see clearly that many small fuzzy MoS2 nanosheets have randomly grown on the SWCNT and GNS surfaces, and the 3D GNT played an important role as a framework. In contrast to the GNT, when MoS2 was added, the boundary between the carbon material becomes less clear. It is conceivable that the 3D GNT can provide a considerable surface area for MoS2 assembly, and GNT-M has more exposed edges. (a)

(100) (103) 

(110) 

20

30

40 50 2 degree

(c)

60

70

80

50

0 0.0

dV /cm3 g-1

150

100

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Pore Diameter /nm

0.2

0.4

1000

1500

2000

2500

3000

Raman shift (cm-1)

GNT-M1

200

500

Volume adsorped (cm3 g-1 STP)

10

2D D

0.6

0.8

1.0

Pressure (P/P0)

(d)

GNT-M5

600

400

dV /cm3 g-1



GNT GNT-M10

Intensity (a.u.)

Intensity (a.u.)



G

(b)

GNT GNT-M1 GNT-M5 GNT-M10

(002)


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


200

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Pore Diameter /nm

0 0.0

0.2

9


0.4

0.6

Pressure (P/P0)

0.8

1.0

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800

(e)

GNT-M10

600

400

200

0 0.0

dV /cm3 g-1

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60



0.5

0.2

1.0

1.5

2.0

2.5

3.0

3.5

Pore Diameter /nm

0.4

4.0

0.6

4.5

Pressure (P/P0)

0.8

1.0

Fig. 3. XRD patterns (a) and Raman spectra (b) of the GNT and various GNT-M nanocomposites; N2 adsorption-desorption isotherm and pore size distribution (insert) of GNT-M1 (c), GNT-M5 (d) and GNT-M10 (e). The X-ray diffraction (XRD) patterns of GNT and the GNT-M10 nanocomposites prepared with different proportions are shown in Fig. 3a. The diffraction peak of GO shows a peak of 2θ = 10.9°, which can be attributed to the reaction of GNS build-up, which means that the reduction of GO was not complete and GO retains a few oxygencontaining functional groups35. Simultaneously, a broad diffraction peak can be observed at 23.6° and corresponds to the RGO d-spacing of 0.37 nm calculated by the Bragg equation, which was larger than that of graphite (0.33 nm). This result also suggests that the stacking between adjacent GNSs was inhibited, and that was most likely because more oxygen-containing functional groups were removed during the hydrothermal reduction and self-assembly of the GNSs. It was also noted that the GNTM material systems show a weak diffraction peak at 2θ = 26.4°, which corresponds exactly to the (002) peak of the carbon nanotubes. In the XRD pattern of GNT-M, the peaks at 14.48°, 32.72°, 39.55°, and 58.35° can be well assigned to the (002), (100), (103), and (110) planes, which correspond with the hexagonal phase structure of MoS2 (JCPDS card no.37-1492), and this is further evidence of the successful combination of MoS2 and GNT. Furthermore, the characteristic peaks of MoS2 gradually weakened with the decrease in the MoS2 loading. The crystal peak located at (002) shows a widening phenomenon, which revealed that the stacking of MoS2 on the GNT along the z-axis is limited, and thus, the MoS2 lamellae can be randomly oriented along the GNS and SWCNT surfaces in other directions. This can effectively increase the interface 10


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between the material and the electrolyte and release more catalytic sites. The material characterization of the GNT and GNT-M10 were further confirmed by the Raman spectroscopy results, as shown in Fig. 3b. The D peak at 1240 cm-1 is the characteristic peak of structural defects in carbon materials, and the G peak at 1603 cm-1 correlates with the graphitization of E2g, which is related to the graphitization of the carbon material. Normally, the ratio of the intensity of the D peak to the G peak can be used to measure the number of graphene defects and the quality of the lamellar structure. A higher ID/IG value indicates that the carbon material has more structural defects and poorer graphitization structure. From the graph, the ID/IG ratios of GNT and GNT-M10 samples were 0.12 and 0.04, respectively, which indicated that the GNT samples have more defects than GNT-M10. This is further evidence of a decrease in the number of sp2-hybridized carbon atoms in GNT-M10, which is accompanied by an increase in the G-peak with the formation of more graphitic carbon. Additionally, in the Raman spectra of GNT-M10, the peak appearing at 250 cm-1 indicates the presence of SWCNTs in GNTs. The N2 adsorption-desorption isotherms and pore size distribution of GNT-M1, GNT-M5 and GNT-M10 are shown in Fig. 3c, 3d and 3e, respectively. The pore structure analysis of GNT-Ms showed a microporous structure. The mean pore diameters of GNT-M1, GNT-M5 and GNT-M10 were 23.22 nm, 18.67 nm and 17.04 nm, respectively, but the total pore volumes of GNT-M5 (0.99 cm3 g-1) and GNT-M10 (1.02 cm3 g-1) showed greater than that of GNT-M1 (0.34 cm3 g-1), which was consistent with the analysis of the (002) peak in XRD. The SSA of GNT-M10 based on the Brunauer-Emmett-Teller (BET) model were 238.25 m² g-1, which is much larger than that of GNT-M1 (57.89 m² g-1) and GNT-M5 (211.85 m² g-1). This may be due to excessive MoS2 loaded which causes some small-sized MoS2 to be inserted into the GNT gap and then results in re-construction of the pore volume structure. The large SSA of GNT-M10 will certainly provide more active sites for the catalysis of I3-, thereby increasing the catalytic properties.

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3.0

0.002

(a)

GNT-M1 GNT-M5 GNT-M10 Pt

(b)

△Ep Ox(i)

2.5

Ox(ii) 0.001

0.000 Red(i) -0.001

2Rs

Red(ii)

-0.4

-0.2

0.0

0.2

0.4

0.6

0.8

Nd

1/2CPE

1.5 1.0

Pt GNT-M1 GNT-M5 GNT-M10

-0.002 -0.6

2Rct

2.0

-Z''/

Current Density (mA cm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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0.5 0.0 20

1.0

21

22

23

Potential (V.vsAgCl)

24

25

26

27

28

Z'/Ω

Fig. 4. CV curves for the different CEs in I-/I3- electrolyte at a scan rate of 20 mV s-1 (a); Nyquist plots of symmetrical dummy cells with different CEs (b). Table 1 Rs, Rct, and Nd values of different CEs. CE

Rs (Ω cm2)

Rct (Ω cm2)

Nd

Pt

10.56

0.7

0.36

GNT-M1

10.22

2.03

0.27

GNT-M5

12.31

0.49

1.25

GNT-M10

11.98

0.35

0.85

Cyclic voltammetry (CV) is often used as a very convincing and powerful method to characterize the catalytic performance of electrodes. Therefore, to investigate the reduction performance of GNT, GNT-M, and Pt CEs for I3-, CV was carried out at a scan rate of 20 mV s-1 from -0.5 to 1 V in an electrolyte containing the I3-/I- redox couple, and the test results are shown in Fig. 4a. Within DSSCs, the main redox reaction can be described by Eqs. (1, 2). The pair of peaks at a higher potential indicate the redox reaction of I2/I3- (Eq. (1)), and the pair of peaks at a lower potential is associated with the redox reactions of I3-/I- (Eq. (2))36. I- is oxidized to I3- at the photoanode by the oxidation-state sensitizing dye, and then, I3- diffuses to the counter electrode to receive the external circuit electrons catalytically reduced by the counter electrode to I-. 3 I2 + 2 e- = 2 I3- (Ox(i) / (Red(i))

(1)

I3- + 2 e- = 3 I- (Ox(ii) / Red(ii))

(2)

Normally, the left pair of peaks (associated with Eq. (2)) is the key factor used to analyze the CE catalytic ability, and the value of the peak spacing (△Ep) between Ox(ii) and Red(ii) is inversely proportional to the charge transfer rate constant (ks). 12


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Simultaneously, the magnitude of the current density (Jp) of Ox(ii) is also an important parameter. Hence, a lower △Ep and higher Jp mean that the material has a better catalytic capability. As shown in Fig. 4a, the △Ep values of Pt, GNT-M1, GNT-M5, and GNTM10 are 296, 174, 250 and 177 mV, respectively. All GNT-M series CEs exhibit a relatively lower △Ep than the Pt CE, which means the GNT-M CEs have higher catalytic reduction abilities for I3-. Notably, the △Ep value of GNT-M1 is significantly smaller than that of GNT-M5 and GNT-M10. On the other hand, the magnitude of the Jp of GNT-M1 (3.62 mA cm-2) is higher than that of both GNT-M5 (3.08 mA cm-2) and GNT-M10 (3.61 mA cm-2). In other words, GNT-M1 showed a higher catalytic activity because of the higher amount of loaded MoS2 in GNT-M1. However, DSSCs assembled with GNT-M10 CE showed the highest PCE 8.01% in the J-V test. To further research the electrocatalytic properties of the CE samples, an EIS analysis was carried out to characterize the charge-transfer ability and Nernst diffusion resistance (Nd). The Nyquist plots with symmetrical cells and the equivalent circuit model in the inset are presented in Fig. 4b, and the corresponding EIS parameters are shown in Table 1. The ohmic series resistance (Rs) value can be obtained from the intercept on the horizontal axis and reflects the series resistance between the electrodes and electrolyte. The value of the charge-transfer resistance (Rct) can be obtained from the left semicircle located in the high-frequency region and represents the charge transfer resistance at the electrode and electrolyte interface, while another semicircle on the right in the low-frequency region is associated with the Nd for I3-/I- in the electrolyte37. As shown in Fig. 4b and Table 1, the Rs values of the electrodes are very similar because Rs is related to the film thickness on the electrode surface, and the quality of each sheet electrode material on the FTO substrate is the same. Thus, the effect of Rs can be neglected38. The Rct values of GNT-M5 (0.49 Ω cm2) and GNT-M10 (0.35 Ω cm2), which are lower than that of the Pt film (0.7 Ω cm2), show excellent electrocatalytic properties39 due to the 3D porous structure of the GNT-M material, and more space can be provided on the contact surface of the GNT-M electrode and the electrolyte to facilitate I3- diffusion. In particular, the Nd values of GNT-M1 and GNTM10 are only 0.27 Ω cm2 and 0.85 Ω cm2, respectively, which mean that the electronic 13



transmission channel formed by the SWCNTs in GNT-M can rapidly transfer electrons from the external circuit to I3- to promote its reduction. Importantly, GNT-M1 has a higher Rct value than GNT-M5 and GNT-M10. However, its Nd value is still small, indicating that the most important factor affecting the conductivity of a material is the Rct value, which may be due to the decrease in the conductivity caused by the high MoS2 loading. From this conclusion, the loading of MoS2 should be appropriate to maximize the catalytic performance of MoS2 and suppress its electrical defects so that the overall electrochemical performance of the composite is significantly improved. 20

60

(a)

(b)


50

15 40


0 0.0

0.2

20 10

0.4

0.6

0 300

0.8

400

500

(c)

△Ep Rct η

280

700

800

8

2.0

1.5

Ep/mV

600

Wavelength (nm)

Cell Voltage(V)

240

1.0

7

η(%)

5

30

Rct/Ω cm2

10

IPCE (%)

Photocurrent Density (mAcm-2)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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6

200

0.5 5 160

Pt

GNT-M1

GNT-M5

GNT-M10

Fig. 5. The current density-voltage characteristics of DSSCs with different CEs under one sun illumination (AM 1.5G) (a); IPCE curves of DSSCs based on various CEs (b); the peak-to-peak separation (△Ep) from the CV curves and charge-transfer resistance (Rct) from EIS according to the PCE.

Table 2 Photovoltaic parameters of the DSSCs with different CEs. CE

Voc (v)

Pt

0.80

Jsc (mA cm-2)

FF (%)

η (%)

14.97

60.00

7.21

14


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GNT-M1

0.64

13.00

64.41

5.15

GNT-M5

0.71

14.69

65.38

6.82

GNT-M10

0.74

18.14

60.01

8.01

Fig. 5a shows the photocurrent-voltage (J-V) performance of the DSSCs based on the Pt film and GNT-M series CEs. The corresponding photovoltage parameters such as the open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and PCE (η) are summarized in Table 2. The photovoltaic characteristics of the DSSCs based on the GNT-M1 and GNT-M5 CEs exhibit poor conversion efficiencies of 5.15% and 6.82%, respectively, due to excessive loading of MoS2 resulting in weakened electrical conductivity. The corresponding photovoltaic characteristics of GNT-M10 CE showed a superior PCE (η=8.01%) higher than that of the Pt CE (η=7.21%). The results show that the GNT-M10 CE possessed the best photoelectric conversion performance among the CEs. It should be noted that the Jsc value increased from 13.00 mA cm-2 for GNTM1 to 18.14 mA cm-2 for GNT-M10, while the PCE of GNT-M1 was only 5.15%, which can reflect that the increase in the Jsc value promotes an increase in the η of the DSSCs. On the other hand, a greater number of 3D GNT structures can provide more edge sites to increase the contact area of the electrolyte with the electrode surface and further enhance the photovoltaic performance. Based on the CV and EIS analyses, a better catalytic performance results in a poorer conductivity, and the J-V performance tests show that the conductivity of the CE can have a large impact on the PCE of DSSCs. The CE samples were tested by incident photon-to current conversion efficiency (IPCE) tested, and the data were shown in Fig. 5b. Each Pt and GNT-M series CE has an absorption peak in the visible region near 540 nm and ultraviolet region at 340 nm and a higher PCE at the two wavelengths. Additionally, the IPCE curve of DSSCs based on GNT-M10 is higher than that of the other CEs for almost all line segments, which further confirmed that GNT-M10 is a very promising Pt-free CE material.

15



1.0

(a) 8.16%

8.2

PCE

8.0

7.6

7.91% 7.92%

8.01% 7.99%

7.8

0.9

0.74 0.66

0.72

7.87% 0.73

7.87% 0.8 0.73

0.7

0.66

0.66

7.4 0.6

Current Density (mA cm-2)

8.4

Voc/V

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

7.2 7.0

0.002

Page 16 of 20

(b)

0.001

0.000 2nd 12th 22nd 32nd 42nd 52nd

-0.001

-0.002 1

2

3

4

5

6

7

0.5

-0.6

-0.4

-0.2

Time (day)

0.0

0.2

0.4

0.6

0.8

1.0

Potential (V.vsAgCl)

Fig. 6. The values of the corresponding PCE of GNT-M10 over one week (a); For the stability test, CV was performed for 52 cycles at a scan rate of 20 mV s-1. The CV scans were recorded in steps of 10 cycles (b). In practical applications of solar cells, stability is also a key factor in addition to conductivity and catalytic activity. To this end, we conducted a one-week continuous test of GNT-M10. The overall performance of its PCE was stable enough to reach a maximum PCE of 8.16% on the third day, but then, the PCE dropped slightly to 7.87%, which was probably due to volatilization of the liquid I3-/I- electrolyte. The PCE eventually stabilized at approximately 7.87%. Next, another 52 CV tests with the GNT-M10 CE were performed to test the stability of the electrode. The CV scans were recorded every 10 cycles and plotted as Fig. 6b. The cathode peaks (Red (ii)) shifted gradually toward a negative potential, while the anodic peaks (Ox (ii)) moved to a positive potential, which indicated that an increase in △Ep accompanied with a decrease in the catalytic ability. The CV curve shapes were similar, especially at the Ox (ii) and Red (ii) positions, which further indicated that the ability of GNT-M10 to reduce I3- diminished over time but was relatively stable. Conclusion In summary, several 3D GNT-M composites (GNT-M1, GNT-M5 and GNTM10) were prepared by using GNTs as 3D anchored templates for MoS2 growth on the surface. The topographic characterization studies revealed the 3D structure of the material, while the CV and EIS analyses indicated the excellent electrocatalytic properties and low Rct values of GNT-M5 (0.49 Ω cm2) and GNT-M10 (0.35 Ω cm2). The best performing device was fabricated with the GNT-M10 and showed a PCE of 16


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8.01%, which is slightly higher than that of the Pt CE (7.21%). This result may be due to an increase in the number of 3D GNT structures that can provide more edge sites to increase the contact area of the electrolyte with the electrode surface and further reinforce the photovoltaic performance. The present work demonstrates that the GNTM10 sample shows great potential as a Pt-free material for high-efficiency DCCSs devices.

Acknowledgment This research was supported by the National Natural Science Foundation of China (grant nos. 21577099) and the Foundation of Key Laboratory of Yangtze River Water Environment (YRWEF201606), Ministry of Education, Tongji University, China. We are also thankful to anonymous reviewers for their valuable comments to improve this manuscript.

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Page 20 of 20

For Table of Contents Use Only

3D

nanocomposite

of

reduced

graphene

oxide/single-walled

carbon

nanotube/MoS2 as Pt-free CE showed a maximum power conversion efficiency of 8.01%.

20


Carbon Nanotube Aerogel as

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