Flexible, Low Cost, and Platinum-Free Counter Electrode for Efficient

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Flexible, Low Cost, and Platinum-Free Counter Electrode for Efficient Dye-Sensitized Solar Cells Abid Ali, Khurram Shehzad, Faiz Ur-Rahman, Syed Mujtaba Shah, Muhammad Khurram, Muhammad Mumtaz, and Rizwan Ur Rehman Sagar ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08826 • Publication Date (Web): 05 Sep 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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ACS Applied Materials & Interfaces

Flexible, Low Cost, and Platinum-Free Counter Electrode for Efficient Dye-Sensitized Solar Cells

Abid Ali†‡, Khurram Shehzad§*, Faiz-Ur-Rahman‡, Syed Mujtaba Shah†*, Muhammad Khurram†, Muhammad Mumtaz∆, Rizwan Ur Rehman Sagar┬

†

Department of Chemistry, Quaid-i-Azam University Islamabad 45320, Pakistan

‡

Department of Chemistry, Fudan University, 220 Handan Road, Shanghai 200433, China

§

College of Information Science and Electronic Engineering and State Key Laboratory of

Silicon Materials, Zhejiang University, Hangzhou, Zhejiang, 310027, China ∆

Materials Research Laboratory, Department of Physics FBAS, International Islamic

University, Islamabad 44000, Pakistan ┬

College of Materials Science & Engineering, and College of Optoelectronic Engineering,

Shenzhen University, Shenzhen 518060, PR China ABSTRACT A platinum-free counter electrode composed of surface modified aligned multi-walled carbon nanotubes (MWCNTS) fibers was fabricated for efficient flexible dye-sensitized solar cells (DSSCs). Surface modification of MWCNTS fibres with simple one step hydrothermal deposition of cobalt selenide (CoSe) nanoparticles, confirmed by scanning electron microscopy (SEM) and X-ray diffraction (XRD), provided a significant improvement (~2 times) in their electrocatalytic activity. Cyclic voltammetry and electrochemical impedance spectroscopy suggest a photoelectric conversion efficiency of 6.42% for our modified fibers, higher than 3.4% and 5.6% efficeincy of prisitne MWCNTS fiber and commonly used Pt wire, respectively. Good mechanical and performance stability after repeated bending, and

*

Corresponding Authors’ Emails: [email protected] and [email protected]

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high output voltage for in-series connection suggest that our surface modified MWCNTS fiber based DSSCs may find applications as flexible power source in next-generation flexible/wearable electronics.

Keywords: Multi-walled carbon nanotube fiber; CoSe nanoparticles; Counter electrode; Dye sensitized solar cell; Platinum-free; Flexible INTRODUCTION Dye-sensitized solar cells (DSSCs), a charming alternate for conventional planner silicon solar cells, have been widely studied due to their low cost, easy fabrication and versatile designing.1-3 Plane solar cells can not fulfil the demands in portable and flexible devices for next-generation electronics.4-5 To this end, fiber-shaped solar cells are promising candidates for future applications with various directiones.6-9 Generally, for fiber-shaped DSSCs, modified titanium wire and platinum (Pt) based materials are used as photoanode and counter electrodes, respectively.9-10 Though, Pt is one of the most selective materials for catalysing the reduction of I3− to I− due to its superior electrocatalytic activity, stable to react and excellent conductivity,11 however, as a noble metal its high cost disfavour the scale up production for fiber-shaped device. Introduction of carbon based materilas12-13 including carbon nanotubes,14-17 graphene12, 18-19 and carbon fiber20 provides some new opportunities to replace the platinm metal as elctrode materials. Due to the excellent electrical and mechanical properties and good stability, carbon nanotubes (CNTs) in the form of fiber could be the best replacement for expensive platinum based materials.21-23 However, performance for CNT based materials is still lower as compare to Pt wire. While aiming at high performance, a combination of good electrocatalytic activity and reasonable conductivity is required for CNT based fibrous materials. Therefore, attempts were made to enhance the electrocatalytic activity of electrodes based on carbon nanomaterials by using electrochemically platinized 2 ACS Paragon Plus Environment

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graphene fiber,4 deposition of Pt nanoparticles through magnetron sputtering on the surface of carbon fiber,24-25 electrochemically deposition of Pt nanoparticles onto CNT fiber26 and a composite of graphene/CNT-Pt.27 Although a significant improvement has been achieved in the electrocatalytic activity but with complex processing, and still the Pt compulsion remained the major drawback. Recently some inorganic compounds like selenides,28 nitrides,29 Oxides30-31, carbides32, and sulphides33-34 were used for the modification of conducting glass to boost the electrocatalytic activity. Among all of these, selenium based nanoparticles were the best candidates due to their unique electronic properties, low cost materials, amiable chemical behaviour and a large variety of applications.28, 35-36 With aim to combine the excellent electrocatalytic properties of selenium based nanoparticles and the excellent electrical and mechanical properties of the CNT fibers for more efficient and costeffective flexible fiber shaped DSSC, herein, we report a CNT fiber decorated with CoSe nanoparticles via a simple hydrothermal process. Hydrothermally treated CoSe nanoparticles decorated fiber showed much better electrocatalytic acitivity in fiber-shaped DSSCs. This surface treated CNT fiber was used as counter electrode twisted around TiO2/N719 modfied Ti wire (photoanode) to fabricate a flexible fiber-shaped DSSC, as illustrated in Figure 1. CoSe/MWCNTS composite fiber electrode showed an effective electrocatalytic activity with enhanced efficiency upto 6.42% compared with 3.4% and 5.6% derived from prisitne MWCNTS fiber and Pt wire, respectively. The low cost and abundant sources for both cobalt and selenium precursores made this idea more feasible for the scale up preparation. RESULTS AND DISCUSSION The surface treated CNT fiber used as counter electrode twisted around TiO2/N719 modfied Ti wire (photoanode) to fabricate a flexible fiber-shaped DSSC, as illustrated in the Figur 1. Figure 2 provides the SEM images of the fabricated DSSC. A simple one step hydrothermal method was followed for the in-situ growth of metal selenide nanoparticles on the surface of 3 ACS Paragon Plus Environment


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MWCNTS fiber. The crucial concept in this work is to eliminate the Pt based expensive materials with low cost and efficient carbon based composite nanomaterials. In-situ growth of metal selenide nanoparticles was performed on the surface of aligned MWCNTS fiber from precursor mixture solution (details of the process are provided in the experimental section). Modest temperature (only 120 ºC) and no post treatment requirement made this nanocomposite fiber more ideal for the scale up production of metal free electrode materials with excellent performance. XRD pattern for CoSe nanoparticles are given in Figure 3d. Peak heights predicted the good crystallinity and peak position values were well indexed with the standard JCPDS No. 52-1008 for CoSe nanoparticles.28 After surface modification, the flexibility remains almost the same as for the original fiber which is important for fibershaped devices. SEM and optical images of pristine and CoSe nanoparticles modified MWCNTS fibers, shown in Figures 3a-3c, confirmed the successful decoration of the nanoparticle on the MWCNTS fiber surface. These unique patterned MWCNTS are ultra-light with 1 µgcm-3 and linear density 10 µg m-1. Specific strength and stiffness of CNT fibers reported in the literature is much higher than the strongest and stiffest engineering fiber of T1000 and M70J.37-39 Alignment in MWCNTS fibers boosted their electrocatalytic properties, making them excellent candidate to replace the current Pt based materials with comparable performance. Surface adsorbed semiconducting nanomaterials on MWCNTS fibers plays a critical role in enhancing the surface area and provide new sites in solid state which proved an ideal phase to catalyse the electrons transfer even more effectively. Triiodide (I3−) get electrons more effectively from hybrid (CoSe/MWCNTS) electrode which improved the cell performance. We also employed MWCNTS fibers from 12 µm to 68 µm (Figure 3c) and found that with increasing diameter of fibers, PCE improves from 1.21% to 3.97% and 2.1% to 6.7% for both pristine and CoSe modified MWCNTS fibers respectively. However, for a given diameter, performance of modified fibers was always better compared to the pristine


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fibers. Actually, a modified fiber with lower diameter showed much better performance compared to the pristine fiber with higher diameter. For example, modified fiber with six layers MWCNTS sheets (38 µm diameter) showed better electrocatalytic activity compared to the pristine fiber with twelve layers (68 µm in diameter) MWCNTs. These results indicate that to improve the electrocatalytic performance of fibers, modification with CoSe nanoparticle is much more effective strategy than increasing the thickness of diameter. However, it is important to mention that a thicker modified fiber did show a better performance compared to a thinner modified fiber. Short circuit current density, fill factor and charge transfer resistance values (elaborated in Table. 1 and 3) were improved up to a significant extent with surface modification and ultimately the overall performance of the cell. We tested and compared the photovoltaic parameters for three different counter electrodes (pristine MWCNTS fibers, surface modified MWCNTS fibers, and Pt-wire based). Photovoltaic parameters for the solar cells device with different counter electrodes were characterized under AM 1.5 illumination. J-V curves in Figure 4a elaborated a comparative photovoltaic performance for three different counter electrodes with the same photoanode. The corresponding performance for each electrode with different parameters summarized in the following Table 1. While comparing the performance of the pristine and modified MWCNTS fiber based counter electrodes, two key parameters determined their efficiency; one was the surface modification and other the diameter of the fibers. Increase in diameter and the modification both improved the efficiency of the MWCNTS fiber based electrodes. For example, Fill factor (FF), a major parameter in J-V curves, improved from 0.45 to 0.65 from pristine to modified fibers. Similarly, for the single layer MWCNTS fibers, upon modification, short circuit current



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density (Jsc) improved from 0.33 to 7.34 mAcm-2 with the same open circuit voltage (Voc) value. For single layer MWCNTS fiber performance was low while for 12 layers (68 µm diameter), however, for modified fiber, both current density and FF improved significantly. Higher surface area and more interactional sites improved the flow of charges from composite fiber to electrolyte. By increasing the diameter of pristine MWCNTS fiber from 12 µm to 68 µm, both the FF and Jsc improved from 0.29 to 0.55 and 6.5 to 11.96 mAcm-2, respectively (Figure 4b). Similarly, for modified MWCNTS fibers, fill factor and Jsc improved from 0.33 to 0.65 and 7.34 to 14.94 mAcm-2 with the same variation in diameter as shown in Figure S3. Here we consider the six layers MWCNTS fiber with almost 37 µm diameter instead of 12 layers (68 µm) because Jsc value for the 12 layers is not in the trend as compared with other MWCNTS layers (1, 2, 4 and 6 layers), it may be due to the more shaded area and consequently reduce the illuminated part of photoanode for 12 layers MWCNTS fiber [15]. Cell in which counter electrode was CoSe/MWCNTS composite fiber showed the highest power conversion efficiency (PCE) up to 6.42% with short circuit current density ( Jsc) value of 13.78 mAcm-2, which is higher compared to Pt-wire counter electrode with 12.15 mAcm-2 short circuit current density and almost the same open circuit voltage (0.72 V). Fill factor with modified MWCNTS fiber was almost the same as Pt wire electrode (0.65) which reflects that CoSe/MWCNTS composite fiber electrode would be a better replacement for Pt based expensive materials in future. These results suggest a faster electron transfer towards the reduction of I3¯ for modified fiber compared to the pristine fibers. Here we also compared this efficiency with pervious reported work in Table 2. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) were used to investigate the mechanism for charge transfer behaviour from counter electrode to electrolyte.


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Cyclic voltammogram, shown in Figure 5, provides iodide/triiodide redox couple for CoSe modified MWCNTS fiber pristine MWCNTS fiber, and Pt wire as counter electrode. The increasing trend for Jsc could be predicted by the higher anodic and cathodic peak current (Ip) values and lower peak to peak separation (∆Ep) for the first redox couple. For each electrode two pair of redox couple (Ox-1/Red-1, Ox-2/Red-2) were observed. First couple represents the conversion of I¯ to the I3¯, while the second couple is related with the reduction of I2 to I3¯, as given in the following equations; I3¯ I2

+ 2e¯ + 2e¯

↔

3I¯

(1)

↔

2I3¯

(2)

In DSSCs, counter electrode is responsible for catalysing the triiodide to iodide ion, so first (OX-1/Red-1) couple represented by equation 1 would be the concerned redox couple.40 Anodic and cathodic peak current values, and their respective peak to peak separation for the first redox couple are useful parameters to obtain the mechanism details of electron transfer from pristine and modified MWCNTS fibers (Table 3). Higher peak current and lower peak to peak separation favoured the better catalytic activity. CoSe/MWCNTS composite fiber with 0.284 V ∆Ep value was much better as compared to the Pt wire and pristine MWCNTS fiber with 0.599 V and 0.531 V, respectively. ∆Ep value for pristine MWCNTs (0.531) and Pt (0.599) have a marginal difference. So catalytic activity is nearly same for pristine MWCNT and Pt. Surface area for MWCNTs is much higher as compare to Pt at which more triiodide (I3¯) can convert into (I¯), so their peak current is also higher. In the case of current voltage (J-V) characteristics, conductivity of the electrode is another important factor. Conductivity of Pt is much higher as compared to MWCNTs fiber. It is also revealed from impedance spectra where first semicircle for Pt start from lower resistance as compared to MWCNTs which shows that Pt has very low resistance even with 7 ACS Paragon Plus Environment


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low catalytic activity. So higher conductivity and near ∆Ep value of platinum metal led to the performance as good as MWCNTs, even though it had higher ∆Ep value. Lower ∆Ep for this reported modified MWCNTS fibers showed excellent catalytic activity, compared to all previous reported modified counter electrodes for fiber-shaped devices.4-5, 24-25, 41-44 Adsorbed CoSe nanoparticles with solid state phase proved to be more efficient for catalytic reduction of I3−. Therefore, higher current and low ∆Ep values indicated the improvement for the electron transfer by composite materials. MWCNTS fiber with different diameters were prepared, and effect of change in the diameter on the catalytic behaviour was also investigated. With increase in the diameter of modified MWCNTS fibers from 12 µm to 68 µm, Ip values increased from 41 µA to 127 µA (reduction peak current for 1st couple) with almost the same ∆Ep, as shown in Figure 5c, which indicated that more triiodide converted to iodide at the counter electrode. Qualitatively similar increase in Ip was observed for pristine MWCNTS fiber (from 16.39 µA to 71.97 µA in Figure S4) when diameter was increased from 12 µm to 68 µm, however, quantitatively the increment was lower compared to modified fiber, as discussed above. Six layer modified fiber (37 µm diameter) with ∆Ep 0.284 V, compared with the twelve layers pristine MWCNTS fiber having 0.493 V ∆Ep with comparable current values. Even 12 layers pristine MWCNTS have double surface area but still showed the poor catalytic activity as compared to modified fiber with six layers (Figure 5d). Hence surface modification is much worthy art as compared to just increase the diameter of MWCNTS fiber. The charge transfer behaviour from composite fiber to counter electrode was also tested by the electrochemical impedance spectroscopy (EIS). Two semicircle in Nyquist plot illustrate impedance behaviour of the symmetric cell with different counter electrodes which were used in devices shown in Figure 5b. First semicircle at higher frequency region shows the interfacial charge transfer resistance (Rct) between counter electrode (MWCNTS fiber) and 8 ACS Paragon Plus Environment

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the electrolyte and the second semicircle with low frequency region reflects the overall impedance for dye/TiO2/Ti.45 Charge transfer resistance (Rct) values for CoSe/MWCNTS counter electrode with 0.22 Ω.cm2 was much lower as compared to pristine MWCNTS fiber (0.48 cm2.Ω) and for Pt wire (1.33 Ω.cm2). Lower Rct value for composite fiber revealed the ease of electron transfer as compared to the pristine MWCNTS fiber and Pt wire with higher Rct. Therefore, a large amount of current and higher fill factor was observed in J-V curves for surface treated fibers. The inserted image with magnified first semicircle clearly elaborated the Rct values. The surface modification in MWCNTS fiber plays an effective role to collect and transport charges to electrolyte with reduced Rct value. The lower Rct value is the in charge for the higher current density and improved fill factor consequently enhance the performance of overall device. Fiber-shaped DSSCs reported in this work demonstrated a good flexibility and voltage enhancement with bending and in-series combination respectively. Both mentioned properties made this device more ideal to put it for practical applications. Different bending angles from 30º to 180º (Figure S5) and bending cycles did not leave a harmful effect with device performance. The efficiency was well maintained above 90% after 100 times bending with stable Voc and little reduction in Jsc as shown in Figures 4(c, d). This kind of fiber-shaped device can also be easily connected in series combination to enhance the output voltage to operate electronic devices with higher voltage as shown in Figure S6. Here we connected three cells in series combination and increased the voltage from 0.72 V to 2.07 V as shown in Figure 6. Therefore, good flexibility and easy to connect in series favoured this kind of device for the next-generation wearable electronics. Furthermore, we also tested our electrodes after a period of 30 days and found no significant change in their performance, which demonstrated their stability and suitability for use over a longer period of time.



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CONCLUSIONS In summary, metal selenides nanoparticles have successfully grown over the surface of highly aligned MWCNTs fiber using a simple one step hydrothermal process. CoSe/MWCNTS composite fiber demonstrated excellent electrocatalytic activity for the reduction of triiodide in DSSC. Device based on composite fiber electrode showed better performance with improve current density and fill factor. In consideration of this mild, low cost and easy to fabricate CoSe/MWCNTS based composite materials could have vast potential to develop a new family for platinum free electrodes in energy conversion and storage devices. EXPERIMENTAL DETAILS Growth of MWCNTS array by chemical vapour deposition method Highly aligned spin-able MWCNTS were synthesized by chemical vapor deposition (CVD) method46. Iron nanoparticles, deposited by physical vapor deposition method were used as catalyst for the growth of MWCNTS arrays. Al2O3 on the silicon wafer act as a buffer layer for the stable growth of iron film, and finally the nanoparticles after annealing.47 A temperature controlled quartz tube furnace was used at 740 ºC for the growth of vertically aligned MWCNTS arrays as shown in supporting Figures S1 (a, b). Precursor gases for the growth of MWCNTS were ethylene, argon and hydrogen with a flow rate of 90, 400, and 30 sccm (standard cubic centimeter per minute), respectively. MWCNTS sheets were pulled out from arrays with a sharp edge and spun with microprobe at the rotation speed of 2000 rpm shown in SEM Figure S1(c) and optical images in Figure S2. Diameter of the fiber increased by adding the multiple MWCNTS sheets. Surface modification with CoSe MWCNTS fiber In-situ growth of CoSe nanoparticles onto the surface of MWCNTS fibers were carried out hydrothermally under high pressure. 0.144 mM of Se powder (99.99% Sigma Aldrich), 0.12 10 ACS Paragon Plus Environment

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mM of Co(NO3)2.6H2O, and 33 mL of deionized water were taken in a 100 mL Teflon lined autoclave. 9 mL hydrazine was added slowly into above mixture with vigorous stirring. As prepared MWCNTS fiber attached to a glass slide was placed inside the autoclave. The autoclave was sealed and placed at 120 ºC for 12 h.28 Autoclave was then taken out from the oven and allowed to cool at room temperature. Modified MWCNTS fiber was detached from the glass slab, washed with deionized water and dried in air. Fabrication of fiber-shaped DSSC Aligned TiO2 nanotube were grown on the surface of titanium wire with127 µm diameter (Alfa Aesar 99.99%) by an electrochemical anodization method.48 Two electrodes electrochemical system was used for the anodization for 6 h at 60 V and 100 mA. The resultant wires were washed with deionized water and annealed at 500 ºC in air for 1 hour to produce the anatase crystalline form of TiO2 nanotubes. These wires were then treated in a 40 mM TiCl4 aqueous solution at 70 ºC for 30 minutes and heated again at 450 ºC for 30 minutes in air for the growth of TiO2 nanoparticles on the TiO2 nanotubes as shown in Figures 2(a, c, d). The temperature was allowed to drop at 120 ºC, wire was then immersed in 0.3 mM N719 solution in a binary solvent of dehydrated acetonitrile and tert-butanol (volume ratio of 1/1) for 16 h. MWCNTS composite fiber was twisted around the modified Ti wire with approximately 0.5 mm pitch to form a fiber-shaped DSSC as shown in SEM image (Figure 2b). The resulted wire was sealed into a plastic tube (diameter of 0.5 mm) and redox electrolyte (composed of 0.1 M lithium iodide, 0.05 M iodine, 0.6m 1, 2-dimethyl-3propylimidazolium iodide and 0.5 M 4-tert butyl-pyridine in dry acetonitrile) was injected with a syringe and sealed with the UV-cure adhesion (HT8803) to prevent from leakage or evaporation of liquid electrolyte. Two electrodes were connected to the external circuit by



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indium metal (99.90% Alfa Aeser) with soldering machine (USM-V Kuroda Techno Co., Ltd). Characterization Multi-walled carbon nanotubes (MWCNTs) in the form of fiber and arrays were characterized by SEM (Hitachi FE-SEM S-4800 operated at 1 kV). The fiber-shaped solar cell efficiency was measured by recording J-V curves with a Keithley 2400 source meter under illumination (100 mWcm-2) of simulated AM1.5G solar light coming from a solar simulator (Oriel-91193 equipped with a 1000 W Xe lamp and an AM1.5 filter). The light intensity was calibrated using a reference Si solar cell (Oriel-91150). Electrochemical Impedance spectroscopy and cyclic voltammetry were performed by electrochemical workstation (CHI-660e) at room temperature, and the experiments were performed through a three-electrode setup in a glass cell. MWCNTS fiber act as counter electrode where Pt-disc electrode and silver/silver chloride act as working and reference electrodes respectively. An acetonitrile solution containing 0.05 M LiClO4, 5 mM LiI, and 0.5 mM I2 was used as the electrolyte to check out the electrochemical behaviour of carbon based material. Supporting Information. Additional electrochemical data, SEM, and optical images

Acknowledgements The authors thanks to Prof. Huisheng Peng for the approval of my visit and providing research facilities in Fudan University, Shanghai, 200438, China and Higher Education Commission (HEC) Pakistan for financial support under International Research Support Initiative Program (IRSIP) with research grant number I-8/HEC/HRD/20143435. This work is also supported by China Postdoctoral Science Foundation (Grant No. 2015M571868), and National Science Foundation of China (Grant No. 51650110494).


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References

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FIGURES

b)

a) Treatment with N719 dye TiO2 nanotube growth

ire) w i (T

ped a h s ber i F (

) SC S D

Packing Electrolyte filling

d)

2 (TiO

) ire w Ti @ e b otu n a n

CoSe/MSCNT fiber twisting

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

Page 16 of 26

Ti) @ 2 /TiO T CN W M / Se o (C

c)

Fig. 1


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Fig. 2



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Fig. 3


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Page 19 of 26

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Fig. 4



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Fig. 5


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Page 21 of 26

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Figure Captions

Fig. 6



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Table 1 Jsc (mAcm-2)

Voc (V)

FF

η (%)

CoSe/MWCNT fibre

13.78

0.72

0.65

6.42 (±0.3)

Pt wire

12.13

0.72

0.64

5.42 (±0.1)

Pristine MWCNT fibre

11.44

0.71

0.45

3.67 (±0.2)

Counter electrode


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Table 2

Counter electrode

Photoanode

Redox couple

Dye

Pt wire

TiO2 NP@ stainless steel wire

I¯ / I3¯

Cu wire


Pt@Ti

Voc

Jsc

(V)

(mAcm-2)

FF

PCE

Ref.

N3

0.61

1.20

0.38

0.28

49

CuI

N3

0.31

0.53

0.26

0.42

50

TiO2 NP@ Ti wire

I¯ / I3¯

N719 0.58

16.1

0.52

4.88

51

Pt wire

TiO2 NP@ Ti wire

I¯ / I3¯

N719 0.71

10.1

0.72

5.05

45

PEDOT:PSS@ Carbon fiber

TiO2 NP@ Ti wire

I¯ / I3¯

N719 0.68

12.1

0.69

5.34

42

Pt wire

Graphene@TiO2 NP@ Ti wire

I¯ / I3¯

N719 0.75

6.06

0.70

3.26

52

Pt @ carbon fiber


I¯ / I3¯

N719 0.73

5.78

0.63

2.63

53

Pt wire

TiO2 NT@ Ti wire

I¯ / I3¯

N719 0.67

15.5

0.64

6.72

54

CNT fiber

TiO2 NT@ CNT fiber

I¯ / I3¯

N719 0.64

9.03

0.45

2.6

55

CNT fiber

TiO2 NP@ CNT fiber

I¯ / I3¯

N719 0.54

19.8

0.32

3.4

56

CoSe@ CNT fiber

TiO2 NT@ Ti wire

I¯ / I3¯

N719 0.72

13.78

0.65

6.42

*

*This work



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Table 3 Epa (V)

Epc (V)

∆Ep (V)

Rct (Ω.cm2)

CoSe/MWCNT fibre

0.370

0.086

0.284

0.22

Pt wire

0.399

-0.20

0.599

1.33

Pristine MWCNT fibre

0.435

-0.078

0.531

0.48

Counter electrode


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FIGURE CAPTIONS Figure 1. (a-d) Schematic illustration of fiber shaped DSSC fabrication. Figure 2. SEM images of electrochemically (a) Modified Ti wire (b) Twisting a MWCNT fiber around modified Ti wire (c) Side view of TiO2 nanotubes (d) Top view of TiO2 nanotubes. Figure 3. SEM images of (a, aʹ) pristine and (b, bʹ) CoSe nanoparticles modified MWCNT fiber with different magnification (c) Optical images of MWCNT fibers with different diameter and (d)

XRD pattern of CoSe nanoparticles. Figure 4. J-V curves of fiber-shaped devices (a) Comparative behaviour with different

counter electrodes (b) with different diameter of counter electrodes (d) optical image of bend device and (c) J-V curve of device before and after bending. Figure 5. Cyclic voltammograms for (a, d) comparative behaviour with different counter

electrodes and (c) with different diameter of electrodes in acetonitrile solution contains 5 mM LiI, 0.5 mM I2 and 0.05 M LiClO4 with a scan rate of 50 mV s-1 (b) Nyquist plot for different counter electrodes in device where the frequencies were ranged from 0.1 to 100 kHz with 0.75 V applied voltage in dark. Figure 6. J-V curve for the three fiber-shaped DSSCs connected in series.

Table 1. Photovoltaic parameters of fiber-shaped DSSCs in Figure 5a. Table 2. Performance comparison of reported DSSCs composed of different photo anodes

and counter electrodes. Table 3 Electrochemical parameters of fiber-shaped DSSCs in Figure 5.



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TOC Graphic 266x89mm (96 x 96 DPI)

ACS Paragon Plus Environment

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Flexible, Low Cost, and Platinum-Free Counter Electrode for Efficient

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