WO2016175714A1 - Carbon nanotube - cement composite composition for catalysis on counter electrodes of dye sensitized solar cells - Google Patents

Carbon nanotube - cement composite composition for catalysis on counter electrodes of dye sensitized solar cells Download PDF

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WO2016175714A1
WO2016175714A1 PCT/TH2016/000042 TH2016000042W WO2016175714A1 WO 2016175714 A1 WO2016175714 A1 WO 2016175714A1 TH 2016000042 W TH2016000042 W TH 2016000042W WO 2016175714 A1 WO2016175714 A1 WO 2016175714A1
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cement
carbon
counter electrode
carbon nanotubes
electrode structure
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PCT/TH2016/000042
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French (fr)
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Prinya CHINDAPRASIRT
Wirat JERERNBOON
Vittaya AMORNKITBAMRUNG
Nattawat RATCHAPOLTHAVISIN
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Khon Kaen University
The Thailand Research Fund
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Priority claimed from TH1501002551A external-priority patent/TH171264A/en
Application filed by Khon Kaen University, The Thailand Research Fund filed Critical Khon Kaen University
Priority to JP2017554603A priority Critical patent/JP6522152B2/en
Publication of WO2016175714A1 publication Critical patent/WO2016175714A1/en

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    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
    • H01G9/20—Light-sensitive devices
    • H01G9/2022—Light-sensitive devices characterized by he counter electrode
    • H—ELECTRICITY
    • H10—SEMICONDUCTOR DEVICES; ELECTRIC SOLID-STATE DEVICES NOT OTHERWISE PROVIDED FOR
    • H10K—ORGANIC ELECTRIC SOLID-STATE DEVICES
    • H10K30/00—Organic devices sensitive to infrared radiation, light, electromagnetic radiation of shorter wavelength or corpuscular radiation
    • H10K30/80—Constructional details
    • H10K30/81—Electrodes
    • H10K30/82—Transparent electrodes, e.g. indium tin oxide [ITO] electrodes
    • H10K30/821—Transparent electrodes, e.g. indium tin oxide [ITO] electrodes comprising carbon nanotubes
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
    • H01G9/20—Light-sensitive devices
    • H01G9/2027—Light-sensitive devices comprising an oxide semiconductor electrode
    • H01G9/2031—Light-sensitive devices comprising an oxide semiconductor electrode comprising titanium oxide, e.g. TiO2
    • H—ELECTRICITY
    • H01—ELECTRIC ELEMENTS
    • H01G—CAPACITORS; CAPACITORS, RECTIFIERS, DETECTORS, SWITCHING DEVICES, LIGHT-SENSITIVE OR TEMPERATURE-SENSITIVE DEVICES OF THE ELECTROLYTIC TYPE
    • H01G9/00—Electrolytic capacitors, rectifiers, detectors, switching devices, light-sensitive or temperature-sensitive devices; Processes of their manufacture
    • H01G9/20—Light-sensitive devices
    • H01G9/2059—Light-sensitive devices comprising an organic dye as the active light absorbing material, e.g. adsorbed on an electrode or dissolved in solution
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/542—Dye sensitized solar cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02E—REDUCTION OF GREENHOUSE GAS [GHG] EMISSIONS, RELATED TO ENERGY GENERATION, TRANSMISSION OR DISTRIBUTION
    • Y02E10/00—Energy generation through renewable energy sources
    • Y02E10/50—Photovoltaic [PV] energy
    • Y02E10/549—Organic PV cells
    • Y—GENERAL TAGGING OF NEW TECHNOLOGICAL DEVELOPMENTS; GENERAL TAGGING OF CROSS-SECTIONAL TECHNOLOGIES SPANNING OVER SEVERAL SECTIONS OF THE IPC; TECHNICAL SUBJECTS COVERED BY FORMER USPC CROSS-REFERENCE ART COLLECTIONS [XRACs] AND DIGESTS
    • Y02—TECHNOLOGIES OR APPLICATIONS FOR MITIGATION OR ADAPTATION AGAINST CLIMATE CHANGE
    • Y02P—CLIMATE CHANGE MITIGATION TECHNOLOGIES IN THE PRODUCTION OR PROCESSING OF GOODS
    • Y02P70/00—Climate change mitigation technologies in the production process for final industrial or consumer products
    • Y02P70/50—Manufacturing or production processes characterised by the final manufactured product

Definitions

  • aspects of the present disclosure are directed to a carbon - cement based catalyst layer that acts as a photoelectric conversion material or element in a dye-sensitized solar cell (DSSC).
  • DSSC dye-sensitized solar cell
  • Dye-sensitized solar cells have existed since 1991 , as described by B. O'Regan and M. Gratzel (B.O 'Regan and M. Gratzel. A Low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO? films. Nature. 1991: 353; 737-740). The efficiency of DSSCs has reached at least 7%.
  • the typical configuration of a DSSC consists of a working electrode, also called a photo-electrode; an electrolyte solution; and a counter electrode.
  • the working electrode acts as an anode, and is typically formed from a transparent conductive oxide (TCO) glass substrate coated with mesoporous titanium dioxide nanoparticles, and anchored with sensitizer molecules capable of harvesting energy in the visible region of the electromagnetic spectrum (400-700 nm).
  • TCO transparent conductive oxide
  • the liquid electrolyte which is in the middle between both electrodes, contains tri- iodide/iodide as a redox couple.
  • the counter electrode acts as a cathode, and consists of a catalyst layer such as platinum, carbon, conducting polymer, metal oxide, metal sulfide and metal carbide coated on transparent conductive oxide glass.
  • the catalyst is used for reducing tri-iodide ions (I ⁇ ) to iodide ions (/ " ) in an electrolyte.
  • the catalyst material(s) transfer electrons from an external load to an electrolyte solution.
  • the dye molecules in the ground state will absorb photons in the visible spectral region, and then jump to an excited state; while titanium dioxide will absorb higher energy photons outside of the visible spectral region, i.e., the ultra-violet region.
  • the excited dye injects an electron into the conduction band (CB) of the titanium dioxide which is coated on the TCO, and consequently becomes oxidized dye.
  • An electron is transferred to an external circuit via the working electrode, and goes back to the cell via the counter electrode.
  • a tri-iodide molecule is reduced by receiving an electron from the counter electrode, and correspondingly becomes iodide.
  • the oxidized dye will receive an electron from the iodide molecule via a redox reaction, returning back to its original state.
  • the mechanism is closed and still working as long as the cell remains in the aforementioned light illumination condition.
  • the counter electrode consisting of catalyst material(s) coated on the substrate is one important component. Improving the performance of the counter electrode is an effective approach to enhance DSSC performance and reduce the DSSC fabrication cost.
  • a DSSC requires a counter electrode having platinum, which is an expensive rare-earth element, as a catalyst because of its high electrochemical activity and low resistivity. Many efforts have been proposed for solving the issue of requiring this expensive element by replacing it with other efficient yet less expensive materials.
  • Carbon is a suitable candidate because of various special properties, such as high surface area, low cost, high electrical conductivity, and easy synthesis.
  • Several forms of carbon materials can be utilized, such as carbon nanotubes, nanoparticles, and graphene sheets.
  • carbon nanotubes can be used as an efficient catalyst for DSSCs, and the performance of DSSCs that utilize carbon nanotubes as the catalyst can be comparable to that for platinum catalysts.
  • a binder is needed for carbon catalyst layer preparation in order to improve adhesion between carbon nanotubes-carbon nanotubes and carbon nanotubes-TCO glass substrate.
  • the most common binder is a conductive polymer, which unfortunately is not stable at high temperatures, and which is undesirably expensive.
  • At least one type of cementitious material, cement based material, cement material, or cement having good mechanical properties and high porosity, and which can typically include calcined limestone and clay material, and/or which can be formed or provided as a powder that includes various compounds such as alumina, lime, iron oxide, and magnesium oxide that have been burned together in a kiln and subsequently pulverized, is utilized as a binder material or binder for carbon based counter electrodes of DSSCs.
  • the cement can include or be conventional cement that is suitable for use as a physical infrastructure construction material (e.g., a building construction material), such as a conventional type of Portland cement.
  • Various embodiments in accordance with the present disclosure utilize cement as the DSSC counter electrode binder instead of using a conventional conductive polymer binder. More particularly, various embodiments in accordance with the present disclosure provide a carbon based material - cement counter electrode structure or counter electrode for DSSC applications, which is referred to hereafter as a carbon - cement counter electrode structure or carbon - cement counter electrode.
  • DSSCs using a carbon - cement counter electrode which includes a carbon based catalyst combined or mixed with cement as the counter electrode binder, and which can omit or exclude a conductive polymer binder.
  • a carbon - cement counter electrode in accordance with embodiments of the present disclosure can be inexpensively fabricated and can achieve high power conversion efficiency, as further detailed below.
  • a carbon - cement counter electrode in accordance with various embodiments of the present disclosure includes three layers, namely: (i) a carbon based or carbon material mixed with cement as a catalyst layer; (ii) a TCO layer; and (iii) a transparent substrate (e.g., a glass substrate).
  • the carbon material which can include or be in the form of carbon nanomaterials, nanostructures, or nanoparticles such as carbon nanotubes, exhibits high catalytic activity as well as low resistivity, similar to a conventional platinum catalyst; while the cement is used to improve adhesion between the carbon nanotubes and the TCO layer carried by the substrate, and can further aid adhesion between carbon nanotubes and the substrate itself.
  • An objective of particular embodiments in accordance with the present disclosure is the use of carbon nanotubes as a DSSC counter electrode catalyst material, and cement as the counter electrode binder.
  • a DSSC having a carbon material mixed with cement as its catalyst layer can achieve high power conversion efficiency generally comparable or comparable to that for conventional DSSCs that utilize a platinum catalyst.
  • cement as binder provides a suitable, simple, reliable, easily manufacturable, and inexpensive replacement for conventional conductive polymer binders.
  • a counter electrode structure for a dye-sensitized solar cell includes: (i) a carbon - cement catalyst layer that includes cement comprising calcined limestone and a clay material, and carbon nanotubes mixed with the cement; (ii) a transparent conductive oxide (TCO) layer in contact with the catalyst layer; and (iii) a substrate that carries the TCO layer and the catalyst layer, wherein the cement serves as a binder material that facilitates adhesion of the carbon nanotubes to the TCO layer.
  • TCO transparent conductive oxide
  • Such a DSSC counter electrode structure need not include, and can exclude, electrically conductive polymer.
  • the carbon - cement catalyst layer acts as a catalyst for electrolyte based tri-iodide and iodide redox couples, and the TCO layer acts as an electron carrier from a load to the carbon - cement catalyst layer.
  • the carbon - cement catalyst layer typically has a thickness of 10.0 - 100.0 microns.
  • the cement typically includes silicon dioxide, aluminum oxide, calcium oxide, sulfur trioxide, magnesium oxide, and iron oxide.
  • the cement can include carbon, silicon, oxygen, aluminum, iron, sulfur, calcium, and magnesium at respective volumetric concentrations of 10.0-20.0%, 2.0-10.0%, 30.0-60.0%, 1.0-5.0%, 0.2-5.0%, 0.5-5.0%, 10.0-40.0%, and 0.1-1.0% in relative amounts that sum to 100%.
  • the carbon - cement catalyst layer includes carbon, oxygen, silicon, aluminum, iron, sulfur, magnesium, and calcium.
  • the carbon nanotubes typically include at last one of single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs).
  • the relative weight percentage ratio of carbon nanotubes to cement is selected to fall within a range of 27:73 to 37:63.
  • the weight percentage ratio of carbon nanotubes to cement can be selected to provide a particular, target, or minimum power conversion efficiency, such as a power conversion efficiency greater than or equal to approximately 9.3%.
  • the weight percentage ratio of carbon nanotubes to cement is 29.0:71.0.
  • the TCO layer can include one of fluorine doped tin oxide, indium doped tin dioxide, aluminum doped zinc oxide, gallium doped zinc oxide, germanium doped indium oxide, titanium metal, and stainless steel metal.
  • a process for fabricating a carbon - cement counter electrode structure for a dye-sensitized solar cell includes: (i) grinding carbon nanotubes together with a cement powder to obtain a homogeneous carbon nanotube - cement powder mixture; (ii) adding deionized (DI) water to the homogenous carbon nanotube - cement powder mixture to form a paste; (iii) grinding the paste to form a homogeneous paste; (iv) providing a substrate that carries a transparent conductive oxide (TCO) layer thereon; (v) coating the homogeneous paste onto the substrate; and (vi) drying the paste coated on the substrate.
  • DI deionized
  • TCO transparent conductive oxide
  • Adding DI water can include adding 8 parts by volume of DI water to one part by volume of the homogeneous carbon nanotube - cement powder mixture. Drying the paste typically involves or includes air drying the paste. For instance, drying the paste can occur by way of air drying the paste at an ambient temperature between 17 - 35 degrees Celsius.
  • FIG. 1 is a cross-sectional view illustrating a representative carbon - cement counter electrode structure in accordance with an embodiment of the present disclosure.
  • FIGs. 2(a) - 2(g) show Scanning Electron Microscope (SEM) images corresponding to various representative counter electrodes, as follows: (a) Comparative Example 2, (b) Example 1, (c) Example 2, (d) Example 3, (e) Example 4, (f) Example 5, and (g) cross- sectional SEM image of Example 2.
  • SEM Scanning Electron Microscope
  • FIG. 3 shows Nyquist plots corresponding to Dye-Sensitized Solar Cells (DSSCs) fabricated using the counter electrodes of Examples 1 to 5 and Comparative Examples 1 to 3.
  • DSSCs Dye-Sensitized Solar Cells
  • FIG. 4 shows variations in current density depending on voltage for DSSCs using the counter electrodes of Examples 1 to 5 and Comparative Examples 1 to 3.
  • FIG. 5 is a cyclic voltammogram showing variations in current density versus voltage for DSSCs using the counter electrodes of Example 2, Comparative Example 1 , and Comparative Example 2.
  • FIG. 6 is a graph showing estimated linearized power conversion efficiency versus relative weight percentage of carbon nanotubes to cement corresponding to DSSCs fabricated using the counter electrodes of Examples 1 to 5, and a representative range of such relative weight percentages that can be identified or selected to provide a particular, target, or minimum power conversion efficiency in accordance with an embodiment of the present disclosure.
  • FIG. 1 illustrates a representative counter electrode structure or counter electrode 5 in accordance with the present disclosure, which is a stacked or sandwiched structure that includes a catalyst layer 10, a transparent conductive oxide (TCO) layer 20, and a transparent substrate 30, as further detailed hereafter.
  • TCO transparent conductive oxide
  • the catalyst layer 10 which can be defined as the top layer of the counter electrode 5, includes or acts as the catalyst material for tri-iodide-iodide ) redox couples.
  • the catalyst layer 10 is coated on the TCO layer 20, and includes at least one type of carbon based material, carbon material, or carbon (hereafter carbon for purpose of brevity and simplicity) that includes or which is provided in the form of carbon nanomaterials or carbon nanostructures (e.g., intentionally or specifically engineered carbon nanomaterials or nanostructure) such as carbon nanotubes, and which is combined or mixed with cement. Consequently, the catalyst layer 10 can be referred to as a carbon - cement catalyst layer or film 10.
  • the carbon can include or be formed of at least one of single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs).
  • the carbon can additionally or alternatively include one or more of buckyballs, carbon nanobuds, and graphene.
  • the catalyst layer 10 contains several elements, including carbon, silicon, oxygen, aluminum, iron, sulfur, calcium, and magnesium.
  • cement acting as the binder can include or typically includes silicon dioxide (Si0 2 ), aluminum oxide (AI 2 O 3 ), calcium oxide (CaO), sulfur trioxide (SO3), magnesium oxide (MgO) and iron oxide (Fe 2 0 3 ).
  • the volumetric content of silicon, oxygen, aluminum, iron, sulfur, calcium, and magnesium in the cement are in the range of 2.0-10%, 30.0-60.0%, 1.0-5.0%, 0.2-5.0%, 0.5-5.0%, 10.0-40.0% and 0.1 -1.0%), respectively, in relative amounts that sum to 100%), i.e., in a manner that forms a 100% cement composition, material, or product, in a manner readily understood by individuals having ordinary skill in the relevant art.
  • the fractional or percentage ratio of carbon to cement can be in the range of 5- 50:50-95 by weight, depending upon embodiment details; and in particular representative embodiments (e.g., in which the carbon includes or is in the form of carbon nanotubes), an appropriate weight fraction or weight percentage ratio of carbon to cement giving good, high, or highest DSSC efficiency is within a range of approximately 27:73 to 31 :69, e.g., approximately 29:71 in particular embodiments.
  • the thickness of the catalyst layer is in the range of 10-100 microns.
  • the TCO layer 20 located between the catalyst layer 10 and transparent substrate 30 serves to transfer electrons from an external load to the catalyst layer 10.
  • the TCO layer 20 can include or be formed of fluorine-doped tin dioxide (FTO), indium doped-tin dioxide (ITO), aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO), germanium-doped indium oxide (GIO), titanium metal, stainless steel, or another suitable material.
  • FTO fluorine-doped tin dioxide
  • ITO indium doped-tin dioxide
  • AZO aluminum-doped zinc oxide
  • GZO gallium-doped zinc oxide
  • GIO germanium-doped indium oxide
  • titanium metal stainless steel, or another suitable material.
  • the transparent substrate 30 can be defined as the bottom layer of the counter electrode 5.
  • Transparent substrates 30 each carrying a TCO layer 20 were cleaned using a series of acetone, methanol, and deionized (Dl) water in an ultrasonic cleaner for 30 minutes for each of the acetone, methanol, and Dl water.
  • cement conventional Portland cement having a conventional grade
  • Deionized (Dl) water was added to each mixed powder at the ratio of Dl water to mixed powder of 8: 1 , and then further mixed to from a corresponding paste until homogenous.
  • Each paste was coated on a support structure or member that included a TCO layer 20 coated on transparent glass 30, using a conventional painting method, and air dried for 24 hours, to form carbon - cement counter electrodes 5 having the different weight percentages of carbon nanotubes to cement as set forth above. More particularly, the carbon - cement counter electrodes 5 having percentage by weight of carbon to cement of 17:83, 29:71, 38:62, 44:56, and 50:50 were defined as Example 1 , Example 2, Example 3, Example 4, and Example 5, respectively. Each sample, i.e., each carbon - cement counter electrode 5 corresponding to Examples 1 to 5, was then ready for a DSSC assembly process.
  • Comparative Example 1 was a conventional counter electrode structure having a conventional platinum catalyst layer.
  • a TOC glass substrate was provided.
  • a platinum film was coated on the substrate by spin coating 20 mM of chloroplatinic acid, i.e., H 2 PtCl 6 H 2 0 (Sigma Aldrich) and 0.01 g of ethylcellulose (Sigma Aldrich) in ethanol, and annealing at 500 °C for 1 hour in an ambient environment.
  • Comparative Example 2 was a counter electrode having carbon nanotubes coated on a TCO glass substrate, without cement.
  • carbon nanotubes were ground until homogenous, and DI water was added to form a paste. The paste was then coated on the substrate.
  • Comparative Example 3 was cement coated on a TCO substrate, without the intentional addition of carbon nanotubes or other conductive carbon materials.
  • cement was ground until homogenous, and DI water was added to form a paste. The paste was then coated on the substrate. 3.
  • Representative Manufacturing Process for Forming a T1O2 Working Electrode A Ti0 2 working electrode was prepared using a conventional screen printing method. Briefly, transparent and scattering Ti0 2 films were fabricated using Ti0 2 pastes. The transparent layer was firstly coated on a TCO substrate, at 10-20 microns of thickness and 10-30 nm particle size.
  • a scattering layer with thickness of 2-5 micron was then coated to improve adhesion of titanium dioxide particles.
  • the Ti0 2 films coated on TCO substrate were sintered at 400 - 600 °C for 1 hour, and cooled down to 25 - 80 °C.
  • the Ti0 2 films were subsequently treated with Ultra- Violet radiation for 10-20 minutes.
  • Dye residuals were removed by rinsing in acetonitrile.
  • the as-prepared carbon - cement counter electrodes 5 corresponding to Examples 1 to 5 set forth above were assembled with the Ti0 2 working electrode and an insulating film, and an electrolyte was placed in the middle between both electrodes to create space.
  • An electrolyte solution consisting of 0.05 M iodine (I ), 0.10 M lithium iodide (Lil), 0.0025 M lithium carbonate (Li 2 C0 3 ), 0.50 M 4-tert-butylpyridine (TBP) and 0.60 M l-methyl-3-propylimidazolium iodide (MPI) in acetonitrile was injected into the space between both electrodes, such that each DSSC was completely fabricated.
  • Electrodes The morphology of the catalyst layers corresponding to Examples 1 to 5, which included carbon - cement catalyst layers 10 in which carbon was mixed with cement at different weight ratios of 17:83, 29:71, 38:62, 44:56, and 50:50, respectively, was observed using a scanning electron microscope (SEM), and compared with pure carbon catalyst and pure cement layer samples respectively corresponding to Comparative Examples 2 (shown in the SEM image of FIG. 2(a)) and 3 (not shown in the SEM images of FIGs. 2(b) - 2(f)). As shown in the SEM images of FIGs.
  • SEM scanning electron microscope
  • each of the carbon - cement catalyst layers 10 of Examples 1 to 5 have a porous structure, but some uncertainty exists with respect to the detailed structure or micro-architecture of such layers.
  • rod-shape crystal structures are observed.
  • the size of the rod-shape structure(s) tends to increase with increasing carbon - cement mixing weight percentage ratios, i.e., 44:56 and 50:50 percent by weight corresponding to FIGs. 2(e) and 2(f).
  • FIG 2(g) shows that the thickness of a representative carbon - cement catalyst layer was 45 microns.
  • the impedances of the DSSCs using the counter electrodes of Examples 1 to 5 and Comparative Example 1 to 3 were measured using electrochemical impedance spectroscopy (EIS, Gamry REF 3000, USA), varying frequency from 0.1 Hz to 100,000
  • FIG. 3 shows Nyquist plots of these DSSCs, which have identical types of working electrodes, but different counter electrode structures.
  • Y and X axes represent reactance (Z im ) and resistance (Z re ) values, respectively. Both of such values are proportional to charge transfer resistance (R r) an ⁇ ⁇ capacitance (C) of electrons moving through the interface between two materials, in a manner readily understood by individuals having ordinary skill in the relevant art. It was found that the semicircle radii of Nyquist plots of the DSSCs obtained from the electrodes of Examples 1 and 2 exhibit a decreasing trend. The semicircle radii of Nyquist plots of the DSSC obtained from the electrode of Example 2 has the smallest semicircle that is close(est) to that of the electrode of Comparative Example 1, i.e., the electrode having the conventional platinum catalyst.
  • carbon - cement catalyst layer or film 10 of Example 2 exhibits low electrical resistance and high catalytic activity for an electrolyte solution containing tri-iodide and iodide redox couples.
  • the size of semicircle slightly increased for higher carbon nanotube content, i.e., the counter electrodes corresponding to Examples 3 to 5. This result is expected to correspond to the increased size of rod-shape structures present in the counter electrodes of Examples 3 to 5 as shown in FIGs. 2(d)-(f), resulting in an increase in total impedance.
  • a J-V curve measurement was performed by using standard conditions including a light source from a solar simulator with an intensity of 100 mW/cm 2 and ambient environmental temperature of 25 °C.
  • the overall efficiency ( ⁇ ) was calculated based on the following equation:
  • Pmax is the maximum output power density (W/m 2 )
  • Jsc is the short circuit current density (A/m 2 )
  • Voc is the open circuit voltage (V)
  • Jmax is the maximum current at the maximum output power (A/m 2 )
  • V nix is the maximum voltage at the maximum output power (V)
  • ⁇ is the power conversion efficiency (%) As per equation [1] above, it can be seen that the power conversion efficiency of a solar cell (e.g., a DSSC) is proportional to Jsc, Voc and FF. This means that high power conversion efficiency can be obtained from high Jsc, Voc and FF. Power conversion efficiency testing of DSSCs using carbon - cement counter electrodes (i.e., carbon based material that can act as a catalyst, mixed with cement as a binder) corresponding to Examples 1 to 5 was conducted for comparison with the performance of the platinum counter electrode of Comparative Example 1, as well as the pure carbon nanotube counter electrode of Comparative Example 2, and the pure cement counter electrode of Comparative Example 3.
  • carbon - cement counter electrodes i.e., carbon based material that can act as a catalyst, mixed with cement as a binder
  • FIG. 4 illustrates J- V characteristics for the DSSCs corresponding to Examples 1 to 5 and the DSSC corresponding to Comparative Examples 1 to 3.
  • the solar cell parameters i.e., Jsc, ⁇ oc, FF and overall efficiency of DSSCs using carbon - cement counter electrodes corresponding to Examples 1 to 5 are provided in Table 1 , and compared to results from other publications.
  • Example 1 mixed cement in 17.85 0.790 0.560 7.88 ratio of 17:83
  • Example 2 mixed cement in 18.66 0.800 0.640 9.60 ratio of 29:71
  • Example 3 mixed cement in 17.72 0.800 0.610 9.21 ratio of 38:62
  • Example 4 mixed cement in 17.86 0.810 0.590 8.62 ratio of 44:56
  • Example 5 mixed cement in 15.58 0.820 0.580 7.47 ratio of 50:50
  • Voc is the open circuit voltage (Volt)
  • ⁇ is the power conversion efficiency (%)
  • the results obtained by Dingwen Zhang et al. indicate that the power conversion efficiencies of DSSCs can vary based upon the type of carbon nanotubes incorporated therein. More particularly, Dingwen Zhang et al. obtained power conversion efficiencies of 7.61 %, 8.30%, and 7.06% for DSSCs using SWCNTs, DWCNTs, and MWCNTs, respectively. It should be noted that SWCNTs and DWCNTs products are more expensive than MWCNTs products.
  • the MWCNTs utilized in representative Example embodiments in accordance with the present disclosure are well suited for mixture with cement, which is a multicomponent metal oxide material, as will be readily understood by individuals having ordinary skill in the relevant art. Individuals having ordinary skill in the art will also recognize that SWCNTs, DWCNTs, and mixtures of SWCNTs, DWCNTs, and/or MWCNTs are also suitable for use in embodiments in accordance with the present disclosure.
  • FIG. 5 shows the cyclic voltammogram of the carbon - cement counter electrode of Example 2 compared to that of the platinum counter electrode of Comparative Example 1 and the pure carbon counter electrode of Comparative Example 2, as tested using conventional cyclic voltammetry. It can be seen that the spectra for all samples show two pairs of redox peaks; i.e., redox reaction 1 corresponding to oxidation reaction (Ox) 1 and reduction reaction (Red) 1 ; and redox reaction 2 corresponding oxidation reaction (Ox) 2 and reduction reaction (Red) 2. Theoretically, redox reaction 1 and 2 represent + 2e ⁇ ⁇ -> 3I ⁇ and 3/ 2 + 2e ⁇ ⁇ 21 , respectively.
  • the counter electrode of Example 2 has higher current compared to that of the counter electrode of Examples 1 , 3, 4 and 5, and especially compared to the platinum counter electrode of Comparative Example 1.
  • carbon - cement counter electrode of Example 2 is suitable or well-suited for use in DSSCs.
  • the platinum counter electrode of Comparative Example 1 has AE P ⁇ and ⁇ £ ⁇ 2 of 200 mV and 130 mV, respectively.
  • Redl is the reduction reaction of the first redox reaction
  • Red2 is the reduction reaction of the second redox reaction
  • O l is the oxidation reaction of the first redox reaction
  • 0x2 is the oxidation reaction of the second redox reaction
  • a carbon - cement catalyst layer 10 in accordance with embodiments of the present disclosure is suitable or well-suited for use in DSSCs.
  • DSSC having a carbon - cement counter electrode 5 in accordance with particular embodiments of the present disclosure can provide solar performance that is nearly equal to that for DSSCs with conventional counter electrodes that utilize a platinum catalyst.
  • carbon - cement counter electrodes 5 in accordance with embodiments of the present disclosure can be easily and cost effectively manufactured without the need for a conductive polymer binder, as cement is a readily available and inexpensive material compared to conductive polymer binders. 8. Representative Carbon Nanotube to Cement Weight Percentage Range Selection
  • FIG. 6 is a graph showing estimated linearized power conversion efficiency versus relative weight percentage of carbon nanotubes to cement corresponding to DSSCs fabricated using the counter electrodes of Examples 1 to 5, and a representative range of such relative weight percentages that can be identified or selected to provide a particular, target, or minimum power conversion efficiency in accordance with an embodiment of the present disclosure. As indicated in Table 1 and shown in FIG. 6, among the DSSCs fabricated using the counter electrodes of Examples 1 to 5, the DSSC using the counter electrode of Example 2 obtained a highest or peak power conversion efficiency of approximately 9.6%.

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Abstract

In accordance with embodiments of the present disclosure, a catalyst layer for electrolyte based tri-iodide/iodine redox couples of a dye- sensitized solar cell (DSSC) includes carbon nanotubes and cement, such as a conventional type of cement suitable for use as an infrastructure construction or building material. Representative example catalyst layers were prepared by mixing carbon nanotube powders with cement powder as a binder, and such catalyst layers were coated on a transparent conducting oxide (TCO) glass substrate using a painting technique without heating. By mixing carbon nanotubes and cement at different carbon nanotube to cement weight percentage ratios, i.e., 17:83, 29:71, 38:62, 44:56 and 50:50, to establish an appropriate cement fraction, catalyst layers with low electrical resistivity, high electro-catalytic activity, and excellent photoelectric conversion efficiency have been fabricated, where the photoelectric conversion efficiency has been measured as 85.56% of that of a DSSC having a conventional platinum catalyst based counter electrode.

Description

CARBON NANOTUBE - CEMENT COMPOSITE COMPOSITION FOR
CATALYSIS ON COUNTER ELECTRODES OF DYE SENSITIZED SOLAR CELLS
TECHNICAL FIELD
Aspects of the present disclosure are directed to a carbon - cement based catalyst layer that acts as a photoelectric conversion material or element in a dye-sensitized solar cell (DSSC). BACKGROUND
Dye-sensitized solar cells (DSSCs) have existed since 1991 , as described by B. O'Regan and M. Gratzel (B.O 'Regan and M. Gratzel. A Low-cost, high-efficiency solar cell based on dye-sensitized colloidal TiO? films. Nature. 1991: 353; 737-740). The efficiency of DSSCs has reached at least 7%. The typical configuration of a DSSC consists of a working electrode, also called a photo-electrode; an electrolyte solution; and a counter electrode. The working electrode acts as an anode, and is typically formed from a transparent conductive oxide (TCO) glass substrate coated with mesoporous titanium dioxide nanoparticles, and anchored with sensitizer molecules capable of harvesting energy in the visible region of the electromagnetic spectrum (400-700 nm).
The liquid electrolyte, which is in the middle between both electrodes, contains tri- iodide/iodide as a redox couple.
The counter electrode acts as a cathode, and consists of a catalyst layer such as platinum, carbon, conducting polymer, metal oxide, metal sulfide and metal carbide coated on transparent conductive oxide glass. The catalyst is used for reducing tri-iodide ions (I^) to iodide ions (/") in an electrolyte. In other words, the catalyst material(s) transfer electrons from an external load to an electrolyte solution. When the cell is under sunlight illumination, the dye molecules in the ground state will absorb photons in the visible spectral region, and then jump to an excited state; while titanium dioxide will absorb higher energy photons outside of the visible spectral region, i.e., the ultra-violet region. The excited dye injects an electron into the conduction band (CB) of the titanium dioxide which is coated on the TCO, and consequently becomes oxidized dye. An electron is transferred to an external circuit via the working electrode, and goes back to the cell via the counter electrode. A tri-iodide molecule is reduced by receiving an electron from the counter electrode, and correspondingly becomes iodide. Meanwhile, the oxidized dye will receive an electron from the iodide molecule via a redox reaction, returning back to its original state. At this stage, the mechanism is closed and still working as long as the cell remains in the aforementioned light illumination condition.
At present, the three main DSSC components, i.e., working electrode, electrolyte solution, and counter electrode, have been intensively developed to improve DSSC performance. The counter electrode consisting of catalyst material(s) coated on the substrate is one important component. Improving the performance of the counter electrode is an effective approach to enhance DSSC performance and reduce the DSSC fabrication cost. A DSSC requires a counter electrode having platinum, which is an expensive rare-earth element, as a catalyst because of its high electrochemical activity and low resistivity. Many efforts have been proposed for solving the issue of requiring this expensive element by replacing it with other efficient yet less expensive materials. Carbon is a suitable candidate because of various special properties, such as high surface area, low cost, high electrical conductivity, and easy synthesis. Several forms of carbon materials can be utilized, such as carbon nanotubes, nanoparticles, and graphene sheets.
Lee and his research group (W.J. Lee et al. Efficient Dye-sensitized Solar Cells with Catalytic Multiwall Carbon Nanotube Counter Electrodes. Applied Materials & Interfaces 2009: 1(6): 1145-1149) fabricated DSSCs with multi-walled carbon nanotubes (MWCNTs) as the catalyst. The MWCNTs were mixed with a binder, and then coated on a transparent conductive oxide glass substrate using a screen printing technique. The substrates coated with MWCNT were sintered at 50 °C for 10 hours. The samples were then applied as counter electrodes of DSSCs. It was found from performance testing that a DSSC with the MWCNT catalyst gave an efficiency of 7.67% which is comparable to that of the Pt catalyst (7.85%). Zhang and his research group (Zhang D el al. Performance of Dye-Sensitized Solar Cells with Various Carbon Nanotube Counter Electrodes. Microchimica Acta 2011: 174; 73- 79) studied the efficiency of DSSCs having various forms of carbon, i.e., multi-walled carbon nanotubes (MWCNTs), double-walled carbon nanotubes (DWCNTs), and single- walled carbon nanotubes (SWCNTs) as catalysts. The efficiency of DSSCs with the DWCNT as catalyst was 8.03% while cells with SWCNT and MWCNT showed efficiency of 7.61% and 7.06%, respectively, which can comparable to what this group measured for the Pt catalyst (8.49%). As mentioned above, carbon nanotubes can be used as an efficient catalyst for DSSCs, and the performance of DSSCs that utilize carbon nanotubes as the catalyst can be comparable to that for platinum catalysts. However, a binder is needed for carbon catalyst layer preparation in order to improve adhesion between carbon nanotubes-carbon nanotubes and carbon nanotubes-TCO glass substrate. The most common binder is a conductive polymer, which unfortunately is not stable at high temperatures, and which is undesirably expensive.
SUMMARY
In accordance with aspects of the present disclosure, at least one type of cementitious material, cement based material, cement material, or cement (hereafter cement for purpose of brevity and simplicity) having good mechanical properties and high porosity, and which can typically include calcined limestone and clay material, and/or which can be formed or provided as a powder that includes various compounds such as alumina, lime, iron oxide, and magnesium oxide that have been burned together in a kiln and subsequently pulverized, is utilized as a binder material or binder for carbon based counter electrodes of DSSCs. The cement can include or be conventional cement that is suitable for use as a physical infrastructure construction material (e.g., a building construction material), such as a conventional type of Portland cement. Various embodiments in accordance with the present disclosure utilize cement as the DSSC counter electrode binder instead of using a conventional conductive polymer binder. More particularly, various embodiments in accordance with the present disclosure provide a carbon based material - cement counter electrode structure or counter electrode for DSSC applications, which is referred to hereafter as a carbon - cement counter electrode structure or carbon - cement counter electrode. DSSCs using a carbon - cement counter electrode, which includes a carbon based catalyst combined or mixed with cement as the counter electrode binder, and which can omit or exclude a conductive polymer binder. A carbon - cement counter electrode in accordance with embodiments of the present disclosure can be inexpensively fabricated and can achieve high power conversion efficiency, as further detailed below.
In general, a carbon - cement counter electrode in accordance with various embodiments of the present disclosure includes three layers, namely: (i) a carbon based or carbon material mixed with cement as a catalyst layer; (ii) a TCO layer; and (iii) a transparent substrate (e.g., a glass substrate). The carbon material, which can include or be in the form of carbon nanomaterials, nanostructures, or nanoparticles such as carbon nanotubes, exhibits high catalytic activity as well as low resistivity, similar to a conventional platinum catalyst; while the cement is used to improve adhesion between the carbon nanotubes and the TCO layer carried by the substrate, and can further aid adhesion between carbon nanotubes and the substrate itself.
An objective of particular embodiments in accordance with the present disclosure is the use of carbon nanotubes as a DSSC counter electrode catalyst material, and cement as the counter electrode binder. A DSSC having a carbon material mixed with cement as its catalyst layer can achieve high power conversion efficiency generally comparable or comparable to that for conventional DSSCs that utilize a platinum catalyst. Furthermore, the use of cement as binder provides a suitable, simple, reliable, easily manufacturable, and inexpensive replacement for conventional conductive polymer binders. In accordance with an aspect of the present disclosure, a counter electrode structure for a dye-sensitized solar cell (DSSC) includes: (i) a carbon - cement catalyst layer that includes cement comprising calcined limestone and a clay material, and carbon nanotubes mixed with the cement; (ii) a transparent conductive oxide (TCO) layer in contact with the catalyst layer; and (iii) a substrate that carries the TCO layer and the catalyst layer, wherein the cement serves as a binder material that facilitates adhesion of the carbon nanotubes to the TCO layer. Such a DSSC counter electrode structure need not include, and can exclude, electrically conductive polymer. The carbon - cement catalyst layer acts as a catalyst for electrolyte based tri-iodide and iodide redox couples, and the TCO layer acts as an electron carrier from a load to the carbon - cement catalyst layer. The carbon - cement catalyst layer typically has a thickness of 10.0 - 100.0 microns.
The cement typically includes silicon dioxide, aluminum oxide, calcium oxide, sulfur trioxide, magnesium oxide, and iron oxide. For instance, the cement can include carbon, silicon, oxygen, aluminum, iron, sulfur, calcium, and magnesium at respective volumetric concentrations of 10.0-20.0%, 2.0-10.0%, 30.0-60.0%, 1.0-5.0%, 0.2-5.0%, 0.5-5.0%, 10.0-40.0%, and 0.1-1.0% in relative amounts that sum to 100%. The carbon - cement catalyst layer includes carbon, oxygen, silicon, aluminum, iron, sulfur, magnesium, and calcium. The carbon nanotubes typically include at last one of single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs).
In various embodiments, the relative weight percentage ratio of carbon nanotubes to cement is selected to fall within a range of 27:73 to 37:63. The weight percentage ratio of carbon nanotubes to cement can be selected to provide a particular, target, or minimum power conversion efficiency, such as a power conversion efficiency greater than or equal to approximately 9.3%. In some embodiments, the weight percentage ratio of carbon nanotubes to cement is 29.0:71.0.
The TCO layer can include one of fluorine doped tin oxide, indium doped tin dioxide, aluminum doped zinc oxide, gallium doped zinc oxide, germanium doped indium oxide, titanium metal, and stainless steel metal.
In accordance with an aspect of the present disclosure, a process for fabricating a carbon - cement counter electrode structure for a dye-sensitized solar cell (DSSC) includes: (i) grinding carbon nanotubes together with a cement powder to obtain a homogeneous carbon nanotube - cement powder mixture; (ii) adding deionized (DI) water to the homogenous carbon nanotube - cement powder mixture to form a paste; (iii) grinding the paste to form a homogeneous paste; (iv) providing a substrate that carries a transparent conductive oxide (TCO) layer thereon; (v) coating the homogeneous paste onto the substrate; and (vi) drying the paste coated on the substrate.
Adding DI water can include adding 8 parts by volume of DI water to one part by volume of the homogeneous carbon nanotube - cement powder mixture. Drying the paste typically involves or includes air drying the paste. For instance, drying the paste can occur by way of air drying the paste at an ambient temperature between 17 - 35 degrees Celsius.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional view illustrating a representative carbon - cement counter electrode structure in accordance with an embodiment of the present disclosure.
FIGs. 2(a) - 2(g) show Scanning Electron Microscope (SEM) images corresponding to various representative counter electrodes, as follows: (a) Comparative Example 2, (b) Example 1, (c) Example 2, (d) Example 3, (e) Example 4, (f) Example 5, and (g) cross- sectional SEM image of Example 2.
FIG. 3 shows Nyquist plots corresponding to Dye-Sensitized Solar Cells (DSSCs) fabricated using the counter electrodes of Examples 1 to 5 and Comparative Examples 1 to 3.
FIG. 4 shows variations in current density depending on voltage for DSSCs using the counter electrodes of Examples 1 to 5 and Comparative Examples 1 to 3. FIG. 5 is a cyclic voltammogram showing variations in current density versus voltage for DSSCs using the counter electrodes of Example 2, Comparative Example 1 , and Comparative Example 2.
FIG. 6 is a graph showing estimated linearized power conversion efficiency versus relative weight percentage of carbon nanotubes to cement corresponding to DSSCs fabricated using the counter electrodes of Examples 1 to 5, and a representative range of such relative weight percentages that can be identified or selected to provide a particular, target, or minimum power conversion efficiency in accordance with an embodiment of the present disclosure.
DETAILED DESCRIPTION
1. Representative Carbon - Cement Counter Electrode Structure and Composition FIG. 1 illustrates a representative counter electrode structure or counter electrode 5 in accordance with the present disclosure, which is a stacked or sandwiched structure that includes a catalyst layer 10, a transparent conductive oxide (TCO) layer 20, and a transparent substrate 30, as further detailed hereafter.
Catalyst layer: The catalyst layer 10, which can be defined as the top layer of the counter electrode 5, includes or acts as the catalyst material for tri-iodide-iodide ) redox couples. The catalyst layer 10 is coated on the TCO layer 20, and includes at least one type of carbon based material, carbon material, or carbon (hereafter carbon for purpose of brevity and simplicity) that includes or which is provided in the form of carbon nanomaterials or carbon nanostructures (e.g., intentionally or specifically engineered carbon nanomaterials or nanostructure) such as carbon nanotubes, and which is combined or mixed with cement. Consequently, the catalyst layer 10 can be referred to as a carbon - cement catalyst layer or film 10. Depending upon embodiment details, the carbon can include or be formed of at least one of single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs). In certain embodiments, the carbon can additionally or alternatively include one or more of buckyballs, carbon nanobuds, and graphene. In general, because of the presence of cement acting as a binder, the catalyst layer 10 contains several elements, including carbon, silicon, oxygen, aluminum, iron, sulfur, calcium, and magnesium. For instance, cement acting as the binder can include or typically includes silicon dioxide (Si02), aluminum oxide (AI2O3), calcium oxide (CaO), sulfur trioxide (SO3), magnesium oxide (MgO) and iron oxide (Fe203). In representative embodiments, the volumetric content of silicon, oxygen, aluminum, iron, sulfur, calcium, and magnesium in the cement are in the range of 2.0-10%, 30.0-60.0%, 1.0-5.0%, 0.2-5.0%, 0.5-5.0%, 10.0-40.0% and 0.1 -1.0%), respectively, in relative amounts that sum to 100%), i.e., in a manner that forms a 100% cement composition, material, or product, in a manner readily understood by individuals having ordinary skill in the relevant art. In general, the fractional or percentage ratio of carbon to cement can be in the range of 5- 50:50-95 by weight, depending upon embodiment details; and in particular representative embodiments (e.g., in which the carbon includes or is in the form of carbon nanotubes), an appropriate weight fraction or weight percentage ratio of carbon to cement giving good, high, or highest DSSC efficiency is within a range of approximately 27:73 to 31 :69, e.g., approximately 29:71 in particular embodiments. In various embodiments, the thickness of the catalyst layer is in the range of 10-100 microns.
TCO layer. The TCO layer 20 located between the catalyst layer 10 and transparent substrate 30 serves to transfer electrons from an external load to the catalyst layer 10. The TCO layer 20 can include or be formed of fluorine-doped tin dioxide (FTO), indium doped-tin dioxide (ITO), aluminum-doped zinc oxide (AZO), gallium-doped zinc oxide (GZO), germanium-doped indium oxide (GIO), titanium metal, stainless steel, or another suitable material.
Transparent substrate: The transparent substrate 30 can be defined as the bottom layer of the counter electrode 5.
2. Manufacturing Process for Representative Carbon - Cement Counter Electrodes
Examples 1 - 5
A process for preparing, providing, or manufacturing carbon - cement counter electrodes 5 in accordance with particular embodiments of the present disclosure is described as follows:
Transparent substrates 30 each carrying a TCO layer 20 were cleaned using a series of acetone, methanol, and deionized (Dl) water in an ultrasonic cleaner for 30 minutes for each of the acetone, methanol, and Dl water.
Carbon in carbon nanotube form MWCNTs derived from the Nanomaterials Research Unit CMU, Chiang Mai university was ground with cement (conventional Portland cement having a conventional grade) in different percentages by weight, namely, 17:83, 29:71, 38:62, 44:56, and 50:50 weight percentages, until a homogenous powder was obtained or formed. Deionized (Dl) water was added to each mixed powder at the ratio of Dl water to mixed powder of 8: 1 , and then further mixed to from a corresponding paste until homogenous. Each paste was coated on a support structure or member that included a TCO layer 20 coated on transparent glass 30, using a conventional painting method, and air dried for 24 hours, to form carbon - cement counter electrodes 5 having the different weight percentages of carbon nanotubes to cement as set forth above. More particularly, the carbon - cement counter electrodes 5 having percentage by weight of carbon to cement of 17:83, 29:71, 38:62, 44:56, and 50:50 were defined as Example 1 , Example 2, Example 3, Example 4, and Example 5, respectively. Each sample, i.e., each carbon - cement counter electrode 5 corresponding to Examples 1 to 5, was then ready for a DSSC assembly process.
Comparative Example 1
Comparative Example 1 was a conventional counter electrode structure having a conventional platinum catalyst layer. In order to prepare Comparative Example 1 , a TOC glass substrate was provided. A platinum film was coated on the substrate by spin coating 20 mM of chloroplatinic acid, i.e., H2PtCl6H20 (Sigma Aldrich) and 0.01 g of ethylcellulose (Sigma Aldrich) in ethanol, and annealing at 500 °C for 1 hour in an ambient environment.
Comparative Example 2
Comparative Example 2 was a counter electrode having carbon nanotubes coated on a TCO glass substrate, without cement. In the preparation of Comparative Example 2, carbon nanotubes were ground until homogenous, and DI water was added to form a paste. The paste was then coated on the substrate.
Comparative Example 3
Comparative Example 3 was cement coated on a TCO substrate, without the intentional addition of carbon nanotubes or other conductive carbon materials. In the preparation of Comparative Example 3, cement was ground until homogenous, and DI water was added to form a paste. The paste was then coated on the substrate. 3. Representative Manufacturing Process for Forming a T1O2 Working Electrode A Ti02 working electrode was prepared using a conventional screen printing method. Briefly, transparent and scattering Ti02 films were fabricated using Ti02 pastes. The transparent layer was firstly coated on a TCO substrate, at 10-20 microns of thickness and 10-30 nm particle size.
A scattering layer with thickness of 2-5 micron was then coated to improve adhesion of titanium dioxide particles.
The Ti02 films coated on TCO substrate were sintered at 400 - 600 °C for 1 hour, and cooled down to 25 - 80 °C.
The Ti02 films were subsequently treated with Ultra- Violet radiation for 10-20 minutes.
The Ti02 films were immersed in 0.1-0.5 mM of ([RuL2(NCS)2]: 2 TBA (L=2,2'- bipyridyl-4,4'-dicarboxylic acid; TBA=tetra-n-butylammonium)) dye (N719, Solaronix, Switzerland) in 50:50 percent by volume of acetonitrile per tert-butanol solution for 24 hrs.
Dye residuals were removed by rinsing in acetonitrile.
4. Representative Dye-Sensitized Solar Cell Fabrication or Assembly
To assemble the DSSCs, the as-prepared carbon - cement counter electrodes 5 corresponding to Examples 1 to 5 set forth above were assembled with the Ti02 working electrode and an insulating film, and an electrolyte was placed in the middle between both electrodes to create space. An electrolyte solution consisting of 0.05 M iodine (I ), 0.10 M lithium iodide (Lil), 0.0025 M lithium carbonate (Li2C03), 0.50 M 4-tert-butylpyridine (TBP) and 0.60 M l-methyl-3-propylimidazolium iodide (MPI) in acetonitrile was injected into the space between both electrodes, such that each DSSC was completely fabricated.
5. Representative Morphology Characterization of Carbon - cement Counter
Electrodes The morphology of the catalyst layers corresponding to Examples 1 to 5, which included carbon - cement catalyst layers 10 in which carbon was mixed with cement at different weight ratios of 17:83, 29:71, 38:62, 44:56, and 50:50, respectively, was observed using a scanning electron microscope (SEM), and compared with pure carbon catalyst and pure cement layer samples respectively corresponding to Comparative Examples 2 (shown in the SEM image of FIG. 2(a)) and 3 (not shown in the SEM images of FIGs. 2(b) - 2(f)). As shown in the SEM images of FIGs. 2(b)-(f), it was found that each of the carbon - cement catalyst layers 10 of Examples 1 to 5 have a porous structure, but some uncertainty exists with respect to the detailed structure or micro-architecture of such layers. In the case of carbon nanotubes mixed with cement at a carbon to cement weight percentage ratio of 38:62 corresponding to FIG. 2(d), rod-shape crystal structures are observed. Furthermore, the size of the rod-shape structure(s) tends to increase with increasing carbon - cement mixing weight percentage ratios, i.e., 44:56 and 50:50 percent by weight corresponding to FIGs. 2(e) and 2(f). The formation of such rod-shape structures can be a weakness or drawback because large rod-shape structures will generally exhibit reduced surface area as well as reduced active sites for tri-iodide reduction compared to smaller or small rod- shaped structures. This can correspondingly limit or adversely affect DSSC performance. FIG 2(g) shows that the thickness of a representative carbon - cement catalyst layer was 45 microns.
6. Representative Electrochemical Impedance Spectroscopy and Performance Analyses
6. 1 Electrochemical Impedance Spectroscopy of DSSC
The impedances of the DSSCs using the counter electrodes of Examples 1 to 5 and Comparative Example 1 to 3 were measured using electrochemical impedance spectroscopy (EIS, Gamry REF 3000, USA), varying frequency from 0.1 Hz to 100,000
Hz and using an AC amplitude of 10 mV. FIG. 3 shows Nyquist plots of these DSSCs, which have identical types of working electrodes, but different counter electrode structures.
Y and X axes represent reactance (Zim) and resistance (Zre) values, respectively. Both of such values are proportional to charge transfer resistance (R r) an<^ capacitance (C) of electrons moving through the interface between two materials, in a manner readily understood by individuals having ordinary skill in the relevant art. It was found that the semicircle radii of Nyquist plots of the DSSCs obtained from the electrodes of Examples 1 and 2 exhibit a decreasing trend. The semicircle radii of Nyquist plots of the DSSC obtained from the electrode of Example 2 has the smallest semicircle that is close(est) to that of the electrode of Comparative Example 1, i.e., the electrode having the conventional platinum catalyst. This means that carbon - cement catalyst layer or film 10 of Example 2 exhibits low electrical resistance and high catalytic activity for an electrolyte solution containing tri-iodide and iodide redox couples. However, the size of semicircle slightly increased for higher carbon nanotube content, i.e., the counter electrodes corresponding to Examples 3 to 5. This result is expected to correspond to the increased size of rod-shape structures present in the counter electrodes of Examples 3 to 5 as shown in FIGs. 2(d)-(f), resulting in an increase in total impedance.
6.2 Performance Testing
The following test was used to evaluate the performance of DSSCs with various counter electrodes obtained from Examples 1 to 5, and Comparative Example 1 to 3 as references:
A J-V curve measurement was performed by using standard conditions including a light source from a solar simulator with an intensity of 100 mW/cm2 and ambient environmental temperature of 25 °C. The overall efficiency (η) was calculated based on the following equation:
P J V J V FF χ QQ 1
where Pm is the input power of the solar simulator (W/m )
Pmax is the maximum output power density (W/m2)
Jsc is the short circuit current density (A/m2)
Voc is the open circuit voltage (V)
Jmax is the maximum current at the maximum output power (A/m2)
V nix is the maximum voltage at the maximum output power (V)
FF is fill factor with maximum value of 1
η is the power conversion efficiency (%) As per equation [1] above, it can be seen that the power conversion efficiency of a solar cell (e.g., a DSSC) is proportional to Jsc, Voc and FF. This means that high power conversion efficiency can be obtained from high Jsc, Voc and FF. Power conversion efficiency testing of DSSCs using carbon - cement counter electrodes (i.e., carbon based material that can act as a catalyst, mixed with cement as a binder) corresponding to Examples 1 to 5 was conducted for comparison with the performance of the platinum counter electrode of Comparative Example 1, as well as the pure carbon nanotube counter electrode of Comparative Example 2, and the pure cement counter electrode of Comparative Example 3.
FIG. 4 illustrates J- V characteristics for the DSSCs corresponding to Examples 1 to 5 and the DSSC corresponding to Comparative Examples 1 to 3. The solar cell parameters, i.e., Jsc, ^oc, FF and overall efficiency of DSSCs using carbon - cement counter electrodes corresponding to Examples 1 to 5 are provided in Table 1 , and compared to results from other publications.
It can be seen in FIG. 4 and Table 1 that Jsc and η of DSSCs using the carbon - cement counter electrodes of Examples 1 to 5 are significantly higher than Jsc and η of the pure carbon counter electrode of Comparative Example 2. In case of the DSSC of Example 2 compared to Example 1 , the Jsc and η values increased. This result can be due to the decrease in internal resistance as shown in FIG. 3. However, it was found that Jsc, FF, and η have a decreasing trend when the carbon to cement weight ratio fraction or percentage reached 38:62, 44:56, and 50:50. This may be due to the presence of rod-shape crystal structures and increasing rod size, which cause or are expected to cause increased internal resistance of the carbon - cement catalyst film 10 in the manner shown in FIG. 3.
JSC Voc n
Example Counter electrode FF
(niA/cm2) (Volt) (%)
Comparative Example 1 Platinum 20.83 0.790 0.675 1 1.22 Comparative Example 2 Carbon 16.98 0.790 0.370 4.99
Comparative Example 3 Cement 6.09 1.000 0.04 0.22
Carbon nanotubes
Example 1 mixed cement in 17.85 0.790 0.560 7.88 ratio of 17:83
Carbon nanotubes
Example 2 mixed cement in 18.66 0.800 0.640 9.60 ratio of 29:71
Carbon nanotubes
Example 3 mixed cement in 17.72 0.800 0.610 9.21 ratio of 38:62
Carbon nanotubes
Example 4 mixed cement in 17.86 0.810 0.590 8.62 ratio of 44:56
Carbon nanotubes
Example 5 mixed cement in 15.58 0.820 0.580 7.47 ratio of 50:50
Prakash Joshi et al. ACS
Applied
Carbon nanofibers 12.60 0.76 0.57 5.50
Material and Interfaces. 2010;
2(12): 3572-3577
Takurou N. et al. Journal of
The Electrochemical Society. Carbon black 16.80 0.79 0.69 9.10
2006; 153(12); A2255-A2261
D. W. Zhang et al. Carbon.
Graphene 16.99 0.75 0.54 6.81 2011; 49: 5382-5388 Gratzel M. and Kay A., Solar
Carbon black
Energy Materials and Solar 1 1.34 0.83 0.71 6.70 mixed graphite
Cells. 1996; 44: 99-117
Dingwen Zhang et al.
Microchim Acta. 2011; 174; SWCNT 14.94 0.80 0.64 7.61 73-79
Dingwen Zhang et al.
Microchim Acta. 2011; 174: DWCNT 15.43 0.80 0.65 8.30 73-79
Dingwen Zhang et al.
Microchim Acta. 2011; 174: MWCNT 15.25 0.80 0.56 7.06 73-79
Won Jae Lee et al. Letter.
MWCNT 16.20 0.74 0.64 7.67 2009: 1(6); 1145-1149
Table 1 : Measured DSSC Performance and Compared to Other Publications Where Jsc is the short circuit current density (mA/cm )
Voc is the open circuit voltage (Volt)
FF is fill factor with maximum value of 1
η is the power conversion efficiency (%)
As shown in Table 1, the results obtained by Dingwen Zhang et al. indicate that the power conversion efficiencies of DSSCs can vary based upon the type of carbon nanotubes incorporated therein. More particularly, Dingwen Zhang et al. obtained power conversion efficiencies of 7.61 %, 8.30%, and 7.06% for DSSCs using SWCNTs, DWCNTs, and MWCNTs, respectively. It should be noted that SWCNTs and DWCNTs products are more expensive than MWCNTs products. The MWCNTs utilized in representative Example embodiments in accordance with the present disclosure are well suited for mixture with cement, which is a multicomponent metal oxide material, as will be readily understood by individuals having ordinary skill in the relevant art. Individuals having ordinary skill in the art will also recognize that SWCNTs, DWCNTs, and mixtures of SWCNTs, DWCNTs, and/or MWCNTs are also suitable for use in embodiments in accordance with the present disclosure.
7. Catalytic Redox Couple Activity of Representative Carbon - Cement Counter Electrodes
FIG. 5 shows the cyclic voltammogram of the carbon - cement counter electrode of Example 2 compared to that of the platinum counter electrode of Comparative Example 1 and the pure carbon counter electrode of Comparative Example 2, as tested using conventional cyclic voltammetry. It can be seen that the spectra for all samples show two pairs of redox peaks; i.e., redox reaction 1 corresponding to oxidation reaction (Ox) 1 and reduction reaction (Red) 1 ; and redox reaction 2 corresponding oxidation reaction (Ox) 2 and reduction reaction (Red) 2. Theoretically, redox reaction 1 and 2 represent + 2e ~ <-> 3I ~ and 3/2 + 2e ~ <→ 21 , respectively. With respect to the cyclic voltammogram, two parameters, which are current and charge transfer rate constant, of the redox reaction (ks) are considered. Namely, high current value means more reactions occurred; while low rate constant, which is proportional to 1/Δ£ρ, i.e., &s = 1/Δ£ρ, means fast or rapid reaction occurred (AEp is the separation of peak potential of oxidation and reduction reactions).
It can be seen from FIG. 5 that the counter electrode of Example 2 has higher current compared to that of the counter electrode of Examples 1 , 3, 4 and 5, and especially compared to the platinum counter electrode of Comparative Example 1. This means that carbon - cement counter electrode of Example 2 is suitable or well-suited for use in DSSCs. However, it can be seen from Table 2 below that the platinum counter electrode of Comparative Example 1 has AEP \ and Δ£ρ2 of 200 mV and 130 mV, respectively. These lower AEp values mean that charge transfer rate constant of the redox reaction (£s) for 73 " + 2e ~ <→ 3/ ~ and 312 + 2e ~ <→ 2/3 " of the platinum counter electrode is higher than that of the other counter electrodes considered herein, giving higher solar performance. In the case of the counter electrode of Example 2, Δ£ρι and Δ£ρ2 values, which are higher than that of platinum counter electrode of Comparative Example 1 , are compensated by higher current as shown in FIG. 5. Therefore, the performance of the DSSC using the carbon - cement counter electrode of Example 2 is almost equal to performance of DSSC having the platinum counter electrode of Comparative Example 1 , ca. 85% (e 85.56%) of that of the platinum catalyst.
Figure imgf000019_0001
Table 2: Peak Positions of Redl , Red2, Oxl , Ox2, ΔΕρι , and ΔΕρ2
Where Redl is the reduction reaction of the first redox reaction
Red2 is the reduction reaction of the second redox reaction
O l is the oxidation reaction of the first redox reaction
0x2 is the oxidation reaction of the second redox reaction
ΔΕρΐ is the separation of first redox reaction
Δ£ρ2 is the separation of second redox reaction
In view of the foregoing, a carbon - cement catalyst layer 10 in accordance with embodiments of the present disclosure is suitable or well-suited for use in DSSCs. A
DSSC having a carbon - cement counter electrode 5 in accordance with particular embodiments of the present disclosure can provide solar performance that is nearly equal to that for DSSCs with conventional counter electrodes that utilize a platinum catalyst.
Moreover, carbon - cement counter electrodes 5 in accordance with embodiments of the present disclosure can be easily and cost effectively manufactured without the need for a conductive polymer binder, as cement is a readily available and inexpensive material compared to conductive polymer binders. 8. Representative Carbon Nanotube to Cement Weight Percentage Range Selection
FIG. 6 is a graph showing estimated linearized power conversion efficiency versus relative weight percentage of carbon nanotubes to cement corresponding to DSSCs fabricated using the counter electrodes of Examples 1 to 5, and a representative range of such relative weight percentages that can be identified or selected to provide a particular, target, or minimum power conversion efficiency in accordance with an embodiment of the present disclosure. As indicated in Table 1 and shown in FIG. 6, among the DSSCs fabricated using the counter electrodes of Examples 1 to 5, the DSSC using the counter electrode of Example 2 obtained a highest or peak power conversion efficiency of approximately 9.6%. DSSCs having a predetermined, target, or acceptable level of or variation in power conversion efficiency in relation to such a peak power conversion efficiency can be obtained by varying the relative weight percentage of carbon nanotubes to cement in their carbon - cement catalyst layers 10. For instance, for an approximate target variation of 0.3% in power conversion efficiency with respect to the peak power conversion efficiency of 9.6% corresponding to Example 2, which translates to a power conversion efficiency of at least approximately 9.3% (i.e., 9.6% - 0.3% = 9.3%), the relative weight percentage of carbon nanotubes to cement can range from between approximately 27:73 to 37:63. Aspects of particular embodiments of the present disclosure address at least one problem, limitation, and/or disadvantage associated with conventional DSSC counter electrode structures. While features, aspects, and/or advantages associated with certain embodiments have been described in the present disclosure, other embodiments may also exhibit such features, aspects, and/or advantages, and not all embodiments need necessarily exhibit such features, aspects, and/or advantages to fall within the scope of the disclosure. It will be appreciated by a person of ordinary skill in the art that various modifications, alterations, and/or improvements may be made to embodiments that are disclosed herein, where such modifications remain within the scope of the present disclosure and the following claims.

Claims

Claims
1. A counter electrode structure for a dye-sensitized solar cell (DSSC), comprising: a carbon - cement catalyst layer comprising: cement comprising calcined limestone and a clay material; and carbon nanotubes mixed with the cement; a transparent conductive oxide (TCO) layer in contact with the catalyst layer; and a substrate that carries the TCO layer and the catalyst layer, wherein the cement serves as a binder material that facilitates adhesion of the carbon nanotubes to the TCO layer.
2. The counter electrode structure of claim 1 , wherein the counter electrode structure excludes electrically conductive polymer.
3. The counter electrode structure of claim 1 or 2, wherein the carbon - cement catalyst layer acts as a catalyst for electrolyte based tri-iodide and iodide redox couples, and the TCO layer acts as an electron carrier from a load to the carbon - cement catalyst layer.
4. The counter electrode structure of any one of claims 1 - 3, wherein the cement comprises silicon dioxide, aluminum oxide, calcium oxide, sulfur trioxide, magnesium oxide, and iron oxide.
5. The counter electrode structure of any one of claims 1 - 4, wherein the cement comprises carbon, silicon, oxygen, aluminum, iron, sulfur, calcium, and magnesium at respective volumetric concentrations of 10.0-20.0%, 2.0-10.0%, 30.0-60.0%, 1.0-5.0%, 0.2-5.0%, 0.5-5.0%, 10.0-40.0%, and 0.1 -1.0% in relative amounts that sum to 100%.
6. The counter electrode structure of any one of claims 1 - 5, wherein the carbon - cement catalyst layer comprises carbon, silicon, oxygen, aluminum, iron, sulfur, calcium, and magnesium.
7. The counter electrode structure of any one of claims 1 - 6, wherein the carbon nanotubes comprise at least one of single-walled carbon nanotubes (S WCNTs), double- walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs).
8. The counter electrode structure of any one of claims 1 - 7, wherein the weight percentage ratio of carbon nanotubes to cement is between 27:73 to 37:63.
9. The counter electrode structure of claim 8, wherein the weight percentage ratio of carbon nanotubes to cement is selected to provide a power conversion efficiency greater than or equal to 9.3%.
10. The counter electrode structure of claim 8 or 9, wherein the weight percentage ratio of carbon nanotubes to cement is 29.0:71.0.
1 1. The counter electrode structure of any one of claims 1 - 10, wherein the carbon - cement catalyst layer has a thickness of 10.0 - 100.0 microns.
12. The counter electrode structure of any one of claims 1 - 1 1 , wherein the TCO layer comprises one of fluorine doped tin oxide, indium doped tin dioxide, aluminum doped zinc oxide, gallium doped zinc oxide, germanium doped indium oxide, titanium metal, and stainless steel metal.
13. A method for fabricating a carbon - cement counter electrode structure for a dye- sensitized solar cell (DSSC), comprising: grinding carbon nanotubes together with a cement powder to obtain a homogeneous carbon nanotube - cement powder mixture; adding deionized (DI) water to the homogenous carbon nanotube - cement powder mixture to form a paste; grinding the paste to form a homogeneous paste; providing a substrate that carries a transparent conductive oxide (TCO) layer thereon; coating the homogeneous paste onto the substrate; and drying the paste coated on the substrate.
14. The method of claim 13, wherein adding DI water comprises adding 8 parts by volume of DI water to one part by volume of the homogeneous carbon nanotube - cement powder mixture.
15. The method of claim 13 or 14, wherein drying the paste comprises air drying the paste.
16. The method of claim 15, wherein drying the paste comprises air drying the paste at an ambient temperature between 17 - 35 degrees Celsius.
PCT/TH2016/000042 2015-04-28 2016-04-27 Carbon nanotube - cement composite composition for catalysis on counter electrodes of dye sensitized solar cells WO2016175714A1 (en)

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