Electrospinning of Photocatalytic Electrodes for Dye-sensitized Solar Cells

Summary

The overall goal of this project was to use electrospinning to fabricate a photoanode with improved performance for dye-sensitized solar cells.

Abstract

This work demonstrates a protocol to fabricate a fiber-based photoanode for dye-sensitized solar cells, consisting of a light-scattering layer made of electrospun titanium dioxide nanofibers (TiO₂-NFs) on top of a blocking layer made of commercially available titanium dioxide nanoparticles (TiO₂-NPs). This is achieved by first electrospinning a solution of titanium (IV) butoxide, polyvinylpyrrolidone (PVP), and glacial acetic acid in ethanol to obtain composite PVP/TiO₂ nanofibers. These are then calcined at 500 °C to remove the PVP and to obtain pure anatase-phase titania nanofibers. This material is characterized using scanning electron microscopy (SEM) and powder X-ray diffraction (XRD). The photoanode is prepared by first creating a blocking layer through the deposition of a TiO₂-NPs/terpineol slurry on a fluorine-doped tin oxide (FTO) glass slide using doctor blading techniques. A subsequent thermal treatment is performed at 500 °C. Then, the light-scattering layer is formed by depositing a TiO₂-NFs/terpineol slurry on the same slide, using the same technique, and calcinating again at 500 °C. The performance of the photoanode is tested by fabricating a dye-sensitized solar cell and measuring its efficiency through J-V curves under a range of incident light densities, from 0.25-1 Sun.

Introduction

Dye-sensitized solar cells (DSSCs) are an interesting alternative to silicon-based solar cells¹ thanks to their low cost, relatively simple manufacturing process, and ease of large-scale production. Another benefit is their potential to be incorporated into flexible substrates, a distinct advantage over silicon-based solar cells². A typical DSSC utilizes: (1) a nanoparticulate TiO₂ photoanode, sensitized with a dye, as a light-harvesting layer; (2) a Pt-coated FTO, used as a counter electrode; and (3) an electrolyte containing a redox couple, such as I^-/I₃^-, placed between the two electrodes, working as a "hole-conducting medium."

Although DSSCs have surpassed efficiencies of 15%³, the performance of nanoparticle-based photoanodes is still still hindered by a number of limitations, including slow electron mobility⁴, poor absorption of low-energy photons⁵, and charge recombination⁶. The electron collection efficiency strongly depends upon the rate of electron transport through the TiO₂ nanoparticle layer. If the charge diffusion is slow, the probability of recombination with I₃^- in the electrolyte solution increases, resulting in a loss of efficiency.

It has been shown that replacing nanoparticulate TiO₂ with one-dimensional (1D) TiO₂ nanoarchitectures can improve charge transport by reducing the scattering of free electrons from the grain boundaries of the interconnected TiO₂ nanoparticles⁷. As 1D nanostructures provide a more direct pathway for charge collection, we can expect that electron transport in nanofibers (NFs) would be significantly faster than in nanoparticles^8,9.

Electrospinning is one of the most commonly used methods for the fabrication of fibrous materials with sub-micron diameters¹⁰. This technique involves the use of high voltage to induce the ejection of a polymer solution jet through a spinneret. Due to bending instability, this jet is subsequently stretched many times to form continuous nanofibers. In recent years, this technique has been extensively used to fabricate polymeric and inorganic materials, which have been used for numerous and diverse applications, such as tissue engineering¹¹, catalysis¹², and as electrode materials for lithium ion batteries¹³ and supercapacitors¹⁴.

The use of electrospun TiO₂-NFs as the scattering layer in the photoanode can increase the performance of DSSCs. However, photoanodes with nanofibrous architectures tend to have poor dye absorption due to surface-area limitations. One of the possible solutions to overcome this is to mix NFs and nanoparticles. This has been shown to result in additional scattering layers, improving light absorption and overall efficiency¹⁵.

The protocol presented in this video provides a facile method to synthesize ultralong TiO₂ nanofibers through a combination of electrospinning and sol-gel techniques, followed by a calcination process. The protocol then illustrates the use of the TiO₂-NFs in combination with nanoparticulate TiO₂ for the fabrication of a dual-layer photoanode with enhanced light-scattering capability using doctor blading techniques, as well as the subsequent assembly of a DSSC using such a photoanode.

Protocol

1. Precursor Solution Preparation

NOTE: Please consult all relevant material safety data sheets (MSDS) before use. Several of the chemicals used in this procedure are harmful and/or toxic to humans. Nanomaterials may have additional hazards compared to their bulk counterpart. Please use the appropriate safety measures and personal protective equipment.

1. Place 5 g of titanium(IV) n-butoxide, 1 g of polyvinylpyrrolidone (PVP), 1 mL of glacial acetic acid, and 10 mL of absolute ethanol into a sample vial.
2. Use a magnetic stirring plate to mix the solution until it has become homogeneous and no bubbles can be observed.

2. Electrospinning and Calcination of the Nanofibers

1. Prepare the needle used for the electrospinning process by cutting off the tip of a 21 G needle and sanding it down using moderate-grade sandpaper until the tip is completely flat.
2. Mount the needle on a disposable 10 mL syringe.
3. Load some of the precursor solution into the syringe and place it on the syringe pump.
4. Wrap the collector plate in aluminum foil and place it directly in front of the needle tip.
   NOTE: The distance from the needle to the plate should be 20 cm.
5. Connect the collector plate to the ground and the needle to the high-voltage power source.
6. Place the protective shield around the setup.
7. Set the flow rate on the syringe pump to 1 mL/h and begin pumping.
8. As soon as some solution appears at the tip of the needle, turn on the high-voltage source and set it to 15 kV.
   NOTE: At this point, fibers are going to collect on the plate. The setup be left running for as long as necessary to achieve the desired thickness of the fiber mat.
9. After the spinning is completed, turn off the high-voltage source and syringe pump. Remove the foil from the collector plate.
10. Let the fibers rest overnight and then peel them off the aluminum foil.
11. Place the peeled-off fibers in a crucible and place that into a muffle furnace.
12. Calcinate the fibers by setting up a temperature ramp of 5°/min up to 500 °C and holding for 2 h to remove the PVP and to produce pure TiO₂ nanofibers.
13. Once the calcination process is completed, leave the furnace closed until the temperature reaches below 80 °C to avoid any thermal shock, which may damage the fibers.

3. Electrode Fabrication

1. Preparation of the slurries
   1. Add 500 mg of titanium dioxide paste to 20 mL of ethanol in a round-bottomed flask.
   2. In a separate flask, mix 500 mg of the electrospun TiO₂-NFs with another 20 mL of ethanol.
   3. Sonicate the solutions for 2 h using a bath sonicator.
   4. Once uniform mixtures are obtained, add 2 mL of terpineol to each flask and sonicate for another 15 min.
   5. Evaporate the solvent from both flasks using a rotary evaporator to obtain the slurries.
2. Doctor blading and sintering
   1. Using a diamond glass cutter, cut an FTO-conductive glass slide into a 2 cm x 2 cm square.
   2. Secure the FTO slide to the work area by placing adhesive tape on the glass slide, leaving a 0.4-cm² area exposed in the center. To avoid an irregular coating, place the tape on two parallel sides first and then on the other two.
   3. Deposit a few drops of the TiO₂-NP slurry on the exposed center of the slide.
   4. Use a razor blade to spread the slurry evenly over the exposed area.
   5. Once a uniform coating is achieved, carefully remove the adhesive tape.
   6. Place the coated slide in a furnace and sinter at 500 °C for 2 h.
   7. Repeat steps 3.2.2-3.2.6 on the same FTO slide, this time using the TiO₂-NF slurry instead of the nanoparticles, to obtain the photoanode.

4. NF Characterization

1. SEM characterization
   1. Prepare the sample for SEM by attaching a strip of adhesive carbon tape to a microscope stub. Carefully place a small quantity of nanofibers on the tape.
   2. Mount the stub onto a sample holder and load it into the exchange chamber of the instrument.
   3. Set up the instrument conditions and parameters: set the accelerating voltage to 20 kV and the working distance to 10 mm.
   4. Collect images of the sample, making sure that they display the overall morphology of the material.
2. XRD characterization
   1. Gently grind some nanofibers into a fine powder and spread them evenly on an XRD stage.
   2. Load the sample into the diffractometer.
   3. Set up the acquisition parameters: use a start angle of 10°, an end angle of 80°, and a step size of 0.015°.
   4. Start the acquisition of the XRD data.

5. Solar-cell Fabrication

1. Treat the photoanode with an aqueous solution of TiCl₄ at 75 °C for 45 min. After treatment, wash it with deionized water and dry it.
2. Sensitize the photoanode by submerging it in a 0.5-mM solution of ruthenium dye N719 in absolute ethanol for 24 h under dark conditions.
3. Place a sheet of sealing film on top of the sensitized photoanode to serve as a thermoplastic gasket between the photoanode and the counter-electrode.
4. Place a Pt-coated FTO counter-electrode with a pre-drilled hole in the center, on top of the sealing film, such that both sides face each other.
5. Heat the assembled cell to 100 °C for 15 min to seal the gasket.
6. Deposit a few drops of a redox mediator, consisting of a solution of 1-propyl-3-methylimidazolium iodide (0.8 M), iodine (0.1 M), and benzimidazole (0.3 M) in 3-methoxypropionitrile, on top of the pre-drilled hole of the counter-electrode.
7. Place the cell in a vacuum desiccator to let the redox mediator fill the internal space of the assembled cell.

6. J-V Curve Characterization

1. Acquire the J-V curves using a digital source meter under 100 mW/cm² illumination from a xenon-arc source passed through an AM1.5G filter.

Results

The TiO₂ nanofibers were characterized using SEM, X-ray photoelectron spectroscopy (XPS), and XRD. The nanostructure of the photoanode was characterized using SEM. The performance of the assembled DSSC was tested using a solar simulator and a source measure unit.

The SEM image in Figure 1A shows that the nanofibers synthesized using this protocol have a porous structure and a high aspect ...

Discussion

The methods presented in this work describe the fabrication of efficient nanofibrous photoanodes for photocatalytic devices such as DSSCs. Electrospinning is a very versatile technique for the fabrication of nanofibers, but a certain level of skill and knowledge is required to obtain materials with optimal morphologies. One of the most critical aspects to obtaining good nanofibers is the preparation of the precursor solution: there are some key factors, such as the concentration of the carrier polymer and the choice of t...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
titanium(IV) n-butoxide	Sigma-Aldrich	244112
Polyvinylpyrrolidone	Sigma-Aldrich	437190
glacial acetic acid	Sigma-Aldrich	A6283
Ethanol, absolute	Fisher Scientific	E/0650DF/17
20 mL Sample vials	(any)		(or larger volume)
disposable 21G needle	(any)
P150 grit sandpaper	(any)
disposable 10mL syringe	(any)		(or larger volume)
magnetic stirrer + stirring bar	(any)
PHD 2000 syringe pump	Harvard Apparatus	71-2002	(or any other syringe pump capable of outputting a 1mL/hr flow
Aluminium foil	(any)
Stainless steel collector plate	(custom built)
High Voltage Power Source	Gamma High Voltage Research, Inc	ES30P-10W	(or any other power supply capable of outputting +15 kV
Polycarbonate protective shield	(custom built)
Ceramic crucible	(any)
Muffle furnace	(any)
Titanium dioxide, nanopowder	Sigma-Aldrich	718467
50 mL 1-neck round bottom flasks	(any)
bath sonicator	(any)
Terpineol	Sigma-Aldrich
Rotary evaporator	(any)
FTO glass	Solaronix	TCO30-10/LI
Adhesive tape	(any)
razor blade	(any)
SEM	JEOL	6500F
XRD	PANalytical	X'pert Pro
Titanium Tetrachloride	Sigma-Aldrich	89545
Ruthenizer  535-bisTBA	Solaronix	N719
sealing film	Dyesol	Meltonix 1170-25
Pt-coated FTO	Solaronix	TCO30-10/LI
1-propyl-3-methylimidazolium iodide	Sigma-Aldrich	49637
Iodine	Sigma-Aldrich	207772
benzimidazole	Sigma-Aldrich	194123
3-Methoxypropionitrile	Sigma-Aldrich	65290
Digital source meter	Keithley	2400
Solar Simulator	Abet technologies	10500