Cathode-driven or assisted solar cell

Abstract

In one form, a photoelectrochemical cell comprising a p-type sensitized photocathode including a sensitizer dye and a water-based electrolyte. In another form, the sensitizer dye and an adjacent semiconductor may have a reduction potential that is sufficiently high to either reduce a desired chemical feedstock in the cell or reduce protons in the water to hydrogen gas. The semiconductor to which the sensitizer dye is affixed may be nickel oxide. The photoelectrochemical cell can include a sensitized photocathode and an electrolyte that contains an electron acceptor, where light illumination of the sensitized photocathode results in reduction of the electron acceptor. The electrolyte can include water.

Claims

1. A photoelectrochemical cell, comprising: an electrolyte, wherein the electrolyte is water or is water-based; and a single junction p-type sensitized photocathode including a sensitizer, and an anode that is not photoactive; wherein the photoelectrochemical cell operates when the photoelectrochemical cell comprises the second electrode that is not photoactive, and wherein the single p-type sensitized photocathode is configured to form hydrogen from the water when illuminated with light and thereby causing solar-driven water-splitting.

2. The photoelectrochemical cell of claim 1, wherein the sensitizer is a dye, an organic dye or a donor-acceptor dye.

3. The photoelectrochemical cell of claim 1, wherein the sensitizer is a metal complex.

4. The photoelectrochemical cell of claim 1, wherein the sensitizer is a semiconductor nanoparticle or a quantum dot.

5. The photoelectrochemical cell of claim 1, wherein the photocathode is at least partially formed of NiO.

6. The photoelectrochemical cell of claim 1, wherein the photocathode is at least partially formed of WO.sub.3 and/or Fe.sub.2O.sub.3.

7. The photoelectrochemical cell of claim 1, wherein in use a redox reaction proceeding at the anode is oxidation of water to oxygen.

8. The photoelectrochemical cell of claim 1, wherein the anode is provided with a catalyst to promote oxidation of water.

9. The photoelectrochemical cell of claim 8, wherein the catalyst is a manganese complex.

10. The photoelectrochemical cell of claim 1, wherein the sensitizer provided at the single junction p-type sensitized photocathode is capable of absorbing photons in the near-infrared and/or infrared ranges.

11. The photoelectrochemical cell of claim 1, wherein the sensitizer provided at the single junction p-type sensitized photocathode is capable of absorbing photons of a first frequency range.

12. A photoelectrochemical cell, comprising a single junction sensitized photocathode and an electrolyte that contains an electron acceptor, wherein the electrolyte is water or is water-based, and an anode second electrode that is not photoactive, wherein the photoelectrochemical cell is configured to reduce the electron acceptor and generate a fuel when illuminated with light and thereby causing solar-driven water-splitting.

13. The photoelectrochemical cell of claim 12, wherein the single junction sensitized photocathode includes a p-type sensitizer and the fuel is hydrogen.

14. A method of forming hydrogen comprising providing the photoelectrochemical cell of claim 1; illuminating the p-type sensitized photocathode with light; and forming hydrogen from water at the p-type sensitized photocathode without applying an external voltage.

15. The method of claim 14, wherein the electrolyte includes an electron acceptor, and wherein light illumination of the p-type sensitized photocathode results in reduction of the electron acceptor.

16. The method of claim 15, wherein: (i) reduction of the electron acceptor is accomplished by a photoexcited dye as the sensitizer; or (ii) the photoexcited dye is reduced through electron transfer from a semiconductor of the photocathode to a photoexcited state of the photoexcited dye and where the acceptor is reduced by a photoreduced dye molecule; or (iii) a combination of mechanisms (i) and (ii).

Description

BRIEF DESCRIPTION OF FIGURES

(1) Embodiments of the present invention will now be described solely by way of non-limiting example and with reference to the accompanying drawings in which:

(2) FIG. 1 depicts a schematic showing the operation of a known solar-assisted water-splitting dye-sensitized solar cell;

(3) FIG. 2 shows a schematic of an example water-splitting photoelectrochemical cell including a sensitized photocathode;

(4) FIG. 3 is a schematic diagram illustrating some example principles and energy levels according to a particular embodiment;

(5) FIG. 4 is a cyclic voltammogram of an example p-type sensitized photocathode;

(6) FIG. 5 is an expansion of the cyclic voltammogram shown in FIG. 3, in the region of where hydrogen gas is produced;

(7) FIG. 6 shows the photocurrent of an example p-type sensitized photocathode due to hydrogen generation under different wavelengths of light illumination;

(8) FIG. 7 shows the photocurrent of an example p-type sensitized photocathode due to hydrogen generation under different intensities of light illumination;

(9) FIG. 8(a) shows the photocurrent of an example p-type sensitized photocathode due to hydrogen generation over an extended period of time, as can be seen, the photocurrent is sustained and invariant;

(10) FIG. 8(b) shows the formation of bubbles of oxygen gas on the platinum counter-electrode used in an example photoelectrochemical cell in conjunction with an example p-type sensitized photocathode after illumination over an extended period of time;

(11) FIG. 9 illustrates the quantum efficiency of an example p-type sensitized photocathode in water-splitting as a function of the wavelength of the illuminating light;

(12) FIG. 10 shows a schematic of an example water-splitting tandem photoelectrochemical cell including a proton reducing photocathode and a water-oxidising photoanode.

PREFERRED EMBODIMENTS

(13) The following modes, given by way of example only, are described in order to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments.

Example 1

A Water-Splitting P-Type Sensitized Photocathode with a Conventional or Catalytic Anode in a Solar-Driven or Solar-Assisted Process

(14) The working principle of a known dye-sensitized solar cell 10 that splits water is shown in FIG. 1 (prior art). As can be seen, the anode 12 of the solar cell 10 includes an n-type Ruthenium sensitizer dye 14 which is attached to TiO.sub.2 film 16, and also interfaces with an IrO.sub.2 catalyst. Sunlight 17 is incident on anode 12. The cathode 18 is a simple platinum electrode.

(15) In a general form, and referring to FIG. 2, the principle of the current invention is the reverse of this arrangement. FIG. 2 shows a photoelectrochemical cell 20 including a sensitized photocathode 24, 22. A sensitizer dye 22, such as a perylene or a related dye, is attached to, affixed or provided as part of cathode material 24 to form the photocathode 24, 22. The cathode material 24 is preferably, though not necessarily, a semiconductor. Sunlight 26 is absorbed by the sensitizer dye 22, which is typically, but not necessarily, chemically attached to a nano-structured electrode 24, for example NiO. Upon illumination with light 26, electrons are transferred from the NiO valance band to the Highest-Occupied Molecular Orbital (HOMO) of the photo-excited dye (or the low-energy SOMO in the excited state), creating a negatively charged, reduced dye. A hole is then left in the valance band of the NiO. The reduction potential of the electron on the reduced dye is around −1.3 V, which is much higher than the standard redox potential of proton reduction to hydrogen. The conversion of protons to hydrogen therefore proceeds efficiently in clean water 28 without the use of any additional catalyst.

(16) The holes in the NiO are collected on a transparent electrode (for example fluorine-doped TiO.sub.2 at no additional applied potential). Electrons are induced to move from the anode 32 to the photocathode 24, 22 through the external circuit 30 to quench the holes. As a result, electrons are abstracted from molecules of water 28 at the counter electrode (anode) 32, for example made of platinum, generating oxygen gas. Due to the overpotential for water oxidation at platinum, an additional, moderate external voltage (ca. 0.2 V) needs to be supplied in this specific arrangement of materials. That is, when NiO is used as a semiconductor electrode 24 with a perylene-based p-type sensitizing dye 22, using pH 7 water 28 as electrolyte, and with Pt as the counter electrode (anode) 32, then the cell 20 is solar-assisted, not solar-driven. The cell 20 can be housed, positioned or encapsulated in a range of different housings, frameworks or bodies to contain the electrolyte and/or protect the anode and photocathode. It may also be desirable that the cell is structured to allow the electrolyte to flow past the anode and/or photocathode so that the electrolyte can be refreshed.

(17) In one example, the sensitizer is a p-type dye (e.g. donor-acceptor dye) and may be a triaryl-oligothiophene-perylene dye. In another example, the sensitizer is a semiconductor nanoparticle (quantum dot), such as but not limited to WO.sub.3, or Fe.sub.2O.sub.3. An additional small voltage is needed to drive the cycle for these materials. The system can be made solar-driven by, amongst others: (i) using a sensitizing dye with a larger reduction potential, or (ii) modifying the conduction band of the NiO to incorporate within its lattice another semiconductor phase, such as Fe.sub.2O.sub.3 or WO.sub.3.

(18) Thus, in a broad form there is provided a photoelectrochemical cell that comprises a p-type sensitized photocathode which includes a sensitizer, and where the cell includes an electrolyte. The photocathode can be formed of, or at least partially formed of, a range of different materials or a composite of materials. The sensitizer can be a range of materials or chemical compounds that facilitate hole donation to the cathode material or part thereof.

(19) The electrolyte can include an electron acceptor, which again can be a range of different chemicals or molecules that provide the function of electron acceptance. Light illumination of the p-type sensitized photocathode results in reduction of the electron acceptor.

(20) Thus, in another general form, there is provided a photoelectrochemical cell comprising a sensitized photocathode and an electrolyte that contains an electron acceptor. Light illumination of the sensitized photocathode results in reduction of the electron acceptor and consequently generation of a fuel. If the sensitized photocathode includes a p-type sensitizer and the electrolyte is water-based, then the generated fuel would be hydrogen. However, a range of other fuels could be generated if different electrolytes or electron acceptors were utilised.

(21) Most preferably, the electrolyte is water or is water-based. Other chemicals, compounds, materials, catalysts, etc. can be provided in the electrolyte if desired. Illumination of the p-type sensitized photocathode with light, which need not be visible light but can be from a non-visible part of the electromagnetic spectrum, results in reduction of water and formation of hydrogen.

(22) Preferably, the sensitizer is a dye, an organic dye and/or a donor-acceptor dye. A mixture of different sensitizers or dyes can be utilised. For example this could allow the photocathode to be photoactive over a broader range of frequency spectrum by utilising a mixture of different dyes or other types of sensitizers. For example, the sensitizer could be a metal complex, a semiconductor nanoparticle or a quantum dot.

(23) The sensitizer dye(s) can be one or more of the following types or bases of dye: perylene, naphthalene, anthracene, porphyryns, indolines, coumarins, donor-acceptor type organic dyes, and combinations thereof. The photocathode can be at least partially formed of a semiconductor to which the sensitizer is attached to, affixed or provided as part of, and can be one or more of the following types of semiconductor: NiO, p-CdSe, p-CdTe, p-InP, GaAs, CuInSe.sub.2, Fe.sub.2O.sub.3, SiC, ZnSe, and combinations thereof.

(24) In one form the second electrode (i.e. anode) is not photoactive. In use, a redox reaction proceeding at the second electrode is oxidation of water to oxygen. The second electrode could be provided with a type of catalyst to promote oxidation of water. Although there are a variety of potential catalysts, one example is that the catalyst is a manganese complex.

(25) In different mechanisms, reduction of the electron acceptor is accomplished by a photoexcited dye as the sensitizer, or the photoexcited dye is reduced through electron transfer from a semiconductor of the photocathode to a photoexcited state of the photoexcited dye and where the acceptor is reduced by a photoreduced dye molecule, or alternatively by a combination of these two mechanisms.

(26) In a particular example, the photocathode is at least partially formed of NiO. In another example, the photocathode is at least partially formed of WO.sub.3 and/or Fe.sub.2O.sub.3. Furthermore, the photocathode can be provided with a type of catalyst that promotes the reduction of the electron acceptor. A number of different catalysts are possible. For example, the photocathode catalyst is one that promotes the reduction of water to hydrogen. In an alternative embodiment, CO.sub.2 could be used as the electron acceptor.

(27) FIG. 3 is a schematic diagram illustrating some example principles and energy levels according to a particular embodiment using a NiO cathode including a p-type sensitizer dye, a Pt counter electrode and water electrolyte at pH 7.

(28) FIG. 4 depicts a cyclic voltammogram of the above described example photocathode in the presence (light) and absence (dark) of illumination with light.

(29) FIG. 5 enlarges the portion of FIG. 4 in the region where hydrogen gas is produced. As can be seen, a clear and definite photocurrent is observed.

(30) FIG. 6 shows the photocurrent using illuminating light of different wavelengths. As can be seen, the photoelectrochemical cell (or system) 20 displays a significant photocurrent deep into the visible spectrum, even upon illumination at a wavelength of 550 nm.

(31) FIG. 7 depicts the photocurrent under different illumination intensities. The data for 1 sun is marked as such on the figure. The numbers shown relate to the extent of transmission through filters applied between the illuminating light and the cell 20. For example, the number 2 signifies the presence of roughly 2 filters in the illumination path, whereas the number 0.3 signifies the presence of a single, highly transmissive filter.

(32) FIG. 8(a) shows the photocurrent due to hydrogen generation over an extended period of time. As can be seen, while there is a large initial current upon commencement of the illumination, the photocurrent flattens out and becomes invariant over time. The photocurrent remains stable for a long period of time. No n-type sensitized photoanode is known to display so stable and sustained a photocurrent.

(33) FIG. 8(b) is a photograph showing the formation of bubbles of oxygen at the Pt anode of the cell 20.

(34) FIG. 9 graphs the quantum efficiency of the cell 20 as a function of the wavelength of the illuminating light for three different semiconductor-dye combinations, NiO alone, NiO with dye “28”, and NiO with dye “32”. The dyes “28” and “32” are proprietary dyes.

Example 2

A Water-Splitting Tandem Cell Incorporating a Proton Reducing Photocathode and a Water-Oxidising Photoanode

(35) In Example 1, a “half-cell” operation is discussed in which hydrogen is generated from water, without necessarily requiring an applied electrical bias on the working electrodes. Complete solar driven water-splitting requires an equally efficient anode where water oxidation takes place driven either by the photovoltage generated by the photocathode or by using a photoanode in a tandem, Z-type arrangement, where both electrodes are photo-driven. For most cathodes of interest, an efficient photoanode can be used to generate the photovoltage needed to oxidise water. As noted above, this can be achieved by using p-type oxides other than NiO with higher valance band potential, or band-gap engineering of NiO to increase its oxidation potential.

(36) Referring to FIG. 10, an alternative is to use another type of example water-splitting photoelectrochemical cell 100. Tandem cell 100 includes a cathode material 104 provided with a p-type sensitizer dye 102 as discussed previously, and a water-based electrolyte 108. Additionally, cell 100 employs an anode material 112 and an attached n-type sensitizer dye 114 to provide a photoactive semiconductor as the anode, for example using TiO.sub.2, WO.sub.3 or Fe.sub.2O.sub.3. The photoanode 112, 114 drives water oxidation at low overpotentials, making the overall cell 100 free-standing and not in need of an externally applied potential. In other words, by combining a p-type photocathode 104, 102 of the present invention with a photoanode 112, 114, such as those mentioned above, the cell 100 becomes a solar-driven water-splitting device.

(37) Photoanodes of the above types may be sensitized by the addition of suitable n-type sensitizing dyes, such as the Ru dye depicted in FIG. 1. The sensitizer present at the photoanode may be an organic dye or a metal complex, such as but not limited to a ruthenium polypyridyl complex.

(38) The light absorption of the photoanode and photocathode in tandem cells of these types can be tailored to be complementary. For example, the photoanode 112, 114 may be designed to absorb ultra-violet and high energy visible light 122, whereas the photocathode 104, 102 may be designed to absorb low energy visible light and infra-red light 120. In this way, a larger proportion of the solar spectrum of sunlight 106 may be harvested than is otherwise possible. In another example, a photoelectrochemical cell can have the two electrodes separated by a membrane in which proton diffusion is significantly faster than the diffusion of hydrogen and or oxygen.

(39) Thus, in an example embodiment the second electrode (i.e. anode) is photoactive. The second electrode can thus be an n-type sensitized photoanode. The photoanode can include an n-type sensitizer which could be, for example, a dye, an organic dye, a metal complex, a semiconductor nanoparticle and/or a quantum dot.

(40) The photoanode in such a tandem cell can be at least partially formed of one or more of: TiO.sub.2, ZnO, Fe.sub.2O.sub.3, WO.sub.3, CdS, CdSe, Nb.sub.2O.sub.3, SnO.sub.2, and combinations thereof. The n-type sensitizer dye(s) which can be attached to, affixed or provided as part of the photoanode semiconductor can be one or more of the following: Ru(bipy) (bipy=2,2′-bipyridine), perylene, naphthalene, anthracene, porphyryns, indolines, coumarins, donor-acceptor type organic dyes, and combinations thereof.

(41) In a specific example, the sensitizer provided at the p-type sensitized photocathode is capable of absorbing photons in the near-infrared and/or infrared ranges. Additionally, it could be provided that the sensitizer provided at the n-type sensitized photoanode is capable of absorbing photons in the near-infrared and/or infrared ranges.

(42) In another form, the sensitizer provided at the p-type sensitized photocathode could be capable of absorbing photons of a first frequency range, and the sensitizer provided at the n-type sensitized photoanode could be capable of absorbing photons of a second frequency range that is different to the first frequency range. This allows the photocathode and the photoanode to compliment each other and for the cell in general to absorb or utilise a wider range of photon frequencies than for a single electrode.

(43) In another example, the photocathode and the second electrode, or photoanode, can be separated by a membrane. For example, the membrane could allow proton diffusion to be faster than diffusion of hydrogen and oxygen to promote efficient operation of the cell.

(44) Replacing non-photoactive cathodes, such as platinum, with p-type dye-sensitised photocathodes, results in lower cost, higher efficiency and increased overall spectral response of the water-splitting cell.

Example 3

Other Applications

(45) While the main application of various embodiments is arguably in photoelectrochemical water-splitting devices, embodiments of the invention also may be applied in chemical transformations of other feedstocks. For example, carbon dioxide is reduced at potentials not dissimilar to that of hydrogen. Thus, an adaption of the discussed embodiments can be used to transform carbon dioxide under light-driven or light-assisted conditions.

(46) Optional embodiments of the present invention may also be said to broadly consist in the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

(47) It will be appreciated that the embodiments described above are intended only to serve as examples, and that many other embodiments are possible within the spirit and the scope of the present invention.