WO2005103329A2

WO2005103329A2 - Manufacturing method

Info

Publication number: WO2005103329A2
Application number: PCT/GB2005/001524
Authority: WO
Inventors: Jan Augustynski; Martine Ulmann; Renata Solarska
Original assignee: Hydrogen Solar Ltd
Priority date: 2004-04-21
Filing date: 2005-04-21
Publication date: 2005-11-03
Also published as: WO2005103329A3; AU2005235787A1; GB0408887D0; GB2413337A

Abstract

An embodiment of the invention pertains to a method of making an electrode which comprises a WO3 film, the method comprising the steps of: (a) forming a colloidal solution of a mixture of tungstic acid and at least one organic material, (b) ultrasonically stirring said colloidal solution; (c) applying a layer of said solution to a conductive substrate, and (d) heattreating the said substrate at a temperature of at least 350°C. Another embodiment of the invention pertains to a method of making an electrode which comprises a WO3 film, the method comprising the steps of: (a) forming a colloidal solution of a mixture of tungstic acid and at least organic material, (b) adding to said mixture a solution comprised of a soluble Lithium salt; (c) applying a layer of said solution to a conductive substrate, and (d) heattreating the said substrate at a temperature of at least 350°C. An electrode and photoelectrical systems using that electrode are also disclosed in conjunction with methods for making colloidal solutions for use in the aforementioned electrode manufacturing methods.

Description

MANUFACTURING METHOD Field of the Invention A particularly preferred embodiment of the invention relates to a method of making an electrode comprising a tungsten trioxide (WO₃) film. A further embodiment of the invention relates to an electrode manufactured by the aforementioned process, and a related embodiment pertains to a process for producing a colloidal solution for use in the aforementioned electrode manufacturing process. Electrodes of this ilk can be used in a number of different applications. For example, these electrodes can be used as photoanodes in photoelectrochemical cells for the photoelectrolysis of water, as photoanodes in cells for the photoelectrochemical degradation of organic waste, or indeed in the production of electrochromic display cells. One particularly preferred embodiment of the invention pertains to a photoelectrochemical cell for the photoelectrolysis of water. Background to the Invention For a number of the aforementioned applications, it is necessary for the electrode to be transparent, and as such a glass plate coated with a transparent conductive layer formed from doped tin oxide is typically used as a substrate. International PCT Patent Application No. WO 99/67181 (also pertaining to a manufacturing process) sets out a number of previously proposed methods for fabricating these electrodes. One such previously proposed process included the steps of: preparing a mixture of tungstic acid in solution with an organic material, forming a colloidal solution of this mixture, depositing the colloidal solution in the form of a thin layer onto a conductive glass plate, and heat treating the plate at a temperature of at least

350°C. Another previously proposed process, again as disclosed in the aforementioned

PCT application, included the steps of: passing through a cation-exchange resin an aqueous solution of sodium tungstate, which becomes converted into tungstic acid, mixing the collected acid with an organic material, forming a colloidal solution of the mixture, depositing the colloidal solution in successive layers onto a conductive glass plate, and heat-treating after each deposition (500° to 600°C in an oxygen-rich atmosphere). This second method enabled a mesoporous structure to be obtained, where crystalline WO₃ nanoparticles with a diameter of from 20 to 50 nm were aggregated together, with each layer having a thickness of about 500 nm. Such a structure, enabled electrodes with good electrochromic and photoelectrochemical characteristics to be produced. The aforementioned PCT patent application improved upon these previously proposed processes by enhancing the industrial applicability of the processes. In particular, the aforementioned PCT application sought to improve the adhesion of the WO₃ film to its substrate, thereby avoiding problems associated with the structure rapidly disintegrating to a point where the electrode is unusable. A further subsidiary aim was to improve the shelf life of the colloidal solution to a point where it was stable enough to be usable for several hours, or even several days. In an effort to meet these aims, the aforementioned PCT patent application disclosed a process for manufacturing an electrode comprising a WO film, the process including the steps of: forming a colloidal solution comprising a mixture made using tungstic acid and an organic material, depositing a thin layer of the solution onto a conductive glass plate, and heat-treating the said plate at a temperature of at least 350°C. The method disclosed did indeed provide the improvements sought whilst maintaining, or even enhancing, the electrochemical properties of the film. The present invention has been conceived with the aim of yet further enhancing the method disclosed in the aforementioned PCT patent application, the contents of which are incorporated herein by reference, to thereby enable the provision of films that provide even better photocurrents. Summary of the Invention In pursuit of this aim a presently preferred embodiment of the present invention provides a method of making an electrode which comprises a WO₃ film, the method comprising the steps of: (a) forming a colloidal solution of a mixture of tungstic acid and at least one organic material, (b) ultrasonically stirring said colloidal solution; (c) applying a layer of said solution onto a conductive glass plate, and (d) heat-treating the said plate at a temperature of at least 350°C. In a preferred modification of this embodiment, following step (a) and prior to step (b), the method may additionally comprise the step of adding to said mixture a solution comprised of a soluble Lithium salt. In accordance with another aspect of the invention, there is provided a method of making an electrode which comprises a WO₃ film, the method comprising the steps of: (a) forming a colloidal solution of a mixture of tungstic acid and at least one organic material, (b) adding to said mixture a solution comprised of a soluble Lithium salt; (c) applying a layer of said solution onto a conductive glass plate, and (d) heat-treating the said plate at a temperature of at least 350°C. In a preferred modification of this embodiment, following step (b) and prior to step (c), the method may additionally comprise the step of ultrasonically stirring the colloidal solution and added soluble lithium salt solution. As will hereafter be described in detail, the embodiments described herein provide significant improvements to the photocurrents achievable with these films. Other aspects of the invention are defined in the other independent claims, and preferred features of these and the aforementioned aspects of the invention are defined in the dependent claims. Further advantages of embodiments of the invention will become apparent following a consideration of the following detailed description. Brief Description of the Drawings Preferred embodiments of the present invention will now be described, by way of illustrative example only, with reference to the accompanying figures, in which: Fig. 1 is a schematic representation of a conductive glass plate bearing a film obtained in accordance with the teachings of the invention; Fig. 2 illustrates the various steps of a process; Fig. 3 represents the variation in the photocurrent density of various films as a function of the potential; Fig. 4 is electron micrographs of an electrode film manufactured in accordance with the teachings of the aforementioned prior art PCT patent application; Fig. 5, for comparison, is an electron micrograph of an electrode film manufactured in accordance with the teachings of the present invention, Fig. 6 is another view of an electrode film, at a greater magnification, manufactured in accordance with the teachings of the aforementioned prior art PCT patent application; Fig. 7 is another view, again for comparison, of an electrode film, at a greater magnification, manufactured in accordance with the teachings of the present invention; Fig. 8 is a schematic representation, for illustration, of a system employing an electrode made in accordance with the teaching provided herein; and Fig. 9 is a schematic representation, again for illustration, of another system employing an electrode made in accordance with the teaching provided herein. Detailed Description of Preferred Embodiments Before embarking on a detailed description of the preferred embodiment, it is useful at this juncture to provide a general description of the teachings of the present invention. In general terms, as explained above, the present invention is founded in modifications of the process disclosed in the aforementioned PCT patent application - which modifications - in isolation or indeed in combination - provide remarkable and wholly unexpected improvements in the photocurrent achievable for the electrode. In general terms, the modifications comprise the addition of lithium to the colloidal solution, and the substitution of ultrasonic stirring whilst the colloid is aged for a magnetic stirring step in the method previously disclosed. Both of these modifications provide a surprising and hitherto unexpected improvement in the achievable photocurrent, and may be employed in isolation or, to provide an even greater improvement, in combination. The scope of the present invention, as claimed, extends to both of these modifications in isolation; but for efficacy the following description will present these modifications in combination. It should be remembered, however, that the scope of the present invention extends to each modification of the aforementioned process in isolation. Referring now to Fig. 1, there is depicted a schematic representation of a conductive glass plate 10 which includes a transparent conductive layer 12 that has been coated with a WO₃ film 14. Such plates (including the conductive layer 12) are sold, for example, by the LOF Company (the former Libbey-Owens-Ford Company - now owned by Pilkington) under the trade name Conductive Glass. The film 14 is formed from an aggregate of WO₃ particles, which are rigidly attached together to define a mesoporous structure - this structure being obtained by alternately depositing layers and heat-treating those layers. In a preferred arrangement 6 to 9 layers are provided. Depending on the application, however, the number of layers may be reduced to only a single layer, or increased up to 12 or more layers. The mesoporous nature of the film structure is advantageous as it allows the area of contact between the electrode and the electrolyte to be considerably increased. As shown in Figure 2, the process according to the invention includes several steps, identified by an upper case letter, which are the following: A. Production of tungstic acid B. Addition of a first organic material C. Concentration to form a colloidal solution D. Introduction of lithium and a second organic material E. Ultrasonic stirring of the resultant solution F. Deposition of the colloidal solution onto a substrate G. Formation of a thin layer of the solution H. Heat treatment. Each of these steps will now be described in greater detail with reference to the quantities used for laboratory preparation of a film. In an industrial scenario, the volumes used will of course be greater.

A. The process begins with a solution 20 of sodium tungstate (Na₂WO₄) in a first container 21, distilled water 22 in a second container 23, and a cation- exchange resin 24 (one example of which is the W 50 HCR-2 100-200 mesh sold by the Dow Chemical Company, Michigan, USA under the trade name of Dowex®) housed in a column 25 and imbibed with distilled water. The solution 20 is metered so as to provide 20 ml of 0.5 M sodium tungstate. The solution 20 is poured into the column 25 and flows through the cation- exchange resin 24, pushing before it the water initially contained in the column, which is eliminated. It is itself entrained towards the column outlet with distilled water 22, which acts as eluent. The sodium tungstate 20 is acidified by cation exchange with the resin 24, which retains the sodium cations and releases protons, thus forming tungstic acid 26.

B. The tungstic acid 26 thus produced is collected at the outlet of the column 25 into a container 27 initially containing 20 ml of a first organic material 28 such as ethanol.

A magnetic stirrer 29 is spun inside the container 27 in order to mix the tungstic acid 26 and the organic material 28, which together form a solution. Acid 26 is added to the container 27 until it contains approximately 50 to 60 ml of solution (the volume of solution being greater than the sum of the volumes of sodium tungstate and organic material due to the fact that water becomes mixed with the sodium tungstate as it flows through the column 25).

C. The mixture thus obtained is placed in a rotary evaporator 30 to concentrate it by evaporation under reduced pressure. This operation is performed at a temperature of from 40° to 70°C, typically 60°C, until a colloidal solution of roughly 20 ml of 0.5M tungstic acid is obtained.

D. Once the colloidal solution 31 has been obtained, the next step is to add Lithium to the solution. Experiments have been conducted where 0.1 to 0.5 atomic percent of Lithium (i.e. 1 to 5 lithium ions per 1000 tungsten atoms) were added to the solution, and the best results (in terms of photocurrent) were obtained with an addition of roughly 0.1 to 0.2 at.%, preferably roughly 0.2 at.% (for which a 15 to 20% improvement in photocurrent was obtained). In the preferred arrangement, Lithium is introduced to the colloidal solution as a 0.02 mol/1 solution 32 of lithium perchlorate in ethanol. In an alternative arrangement, lithium can be introduced by adding a 0.5 mol/1 aqueous solution of lithium nitrate. In general terms, lithium can be added as a solution of one of its soluble salts (e.g. perchlorate, nitrate, acetate) either in water or ethanol.

The next step is to add, to the colloidal solution 31 and lithium 32, a second organic material 33. As an illustrative example, the second organic material may comprise a modulator, such as polyethylene glycol 300 (known as PEG 300). In the preferred arrangement 5 ml of PEG 300, corresponding to a tungstic acid/ PEG weight weight ratio of roughly 0.5, is added to the mixture. More or less PEG may be added if desired, and workable results have been obtained with tungstic acid/ PEG weight/weight ratios of between 0.3 and 0.8.

Once the second organic material 33 has been added, the next step in the process is to place the container 27 in an ultrasonic bath 50 for ageing of the mixture 34 under continuous stirring - ageing being required before the mixture can be applied to a substrate.

In the preferred arrangement the ultrasonic bath operates at roughly 35 KHz, and experiments conducted have shown that an ageing of roughly two hours provides good results (a stirring of only one hour giving scattered results, and a stirring of four hours giving no real improvement over a mixture stirred for two hours). It will be appreciated, however, that with different bath frequencies the ageing process may take longer than two hours, or be concluded in less time than two hours.

The use of an ultrasonic stirrer differs markedly from the process described in the aforementioned PCT patent application. In this prior process a conventional magnetic stirrer stirred the mixture, and the mixture had to be left to age (under continuous stirring) for roughly twelve hours before it could be applied to a substrate. Thus, the process proposed herein provides a marked reduction of the ageing time required before the mixture can be applied to a substrate. Even more surprisingly, we have found that the use of an ultrasonic stirrer provides a massive improvement (circa 15%) in the photocurrent achievable with the electrode. We surmise that this improvement, as will be explained later in conjunction with the SEMs shown in Figs. 4 and 5, occurs primarily because ultrasonic stirring provides a film with a nanostructure that is more conducive to charge carrying than the film nanostructure previously achievable with the method described in the aforementioned PCT patent application.

F. Once the mixture 34 of the colloidal solution 31, lithium 32 and organic material 33 has been stirred and aged, it is maintained under continuous magnetic stirring and is then ready for deposition on a substrate. In an illustrative method shown in Fig. 2e, the mixture 34 is deposited in the form of a drop 35 using a pipette 36 onto the conductive layer 12 of a conductive glass plate 10.

G. The drop 35 is spread on the plate 10 to form a band, and then drawn over its entire surface by means of a glass plate 37.

H. Once a layer has been formed on the plate, it is fired in an oven 38 under an oxidizing atmosphere (typically a flow of pure oxygen) at a temperature of between 350° and 650°C, preferably between 400° and 600°C and more preferably between 500 and 550°C, for 15 to 60 minutes, preferably between 30 and 60 minutes. During this operation, the organic materials burn and become volatilized, and the WO₃ crystallizes on the substrate into a porous structure.

As an alternative where it is not necessary to provide a (highly) transparent layer it is possible to anneal the layers in air instead of oxygen. Annealing in air will cause the layers to be slightly opaque, but has no real effect on the photocurrent achievable in the finished electrode. Steps F to H are then repeated to build up layers on the substrate to form the electrode. An effective electrode requires, in general terms, 6 or so layers to be deposited on the electrode. Alternative applications may require more or less layers, in some instances one layer and in others 12 or more layers.

As an alternative application method to that disclosed in F above, the mixture 34 (which is relatively viscous) may be screen printed onto the substrate. However, in order to preserve adherence the amount of mixture applied must be relatively small, for example an amount providing for the formation of a layer which is roughly 0.5 microns thick. Following application of each layer, as mentioned above, the substrate and layer should be annealed as described in H above. As a yet further alternative, layers may be applied by spraying the mixture 34 onto the substrate at room temperature (i.e. onto a cold plate), followed by annealing as described in H above. For spraying it is necessary to dilute the mixture 34 twice by adding roughly 20 ml of ethanol. This method allows smooth layers to be produced, and is particularly well suited for producing relatively thin (roughly 1 - 1.5 micron) WO₃ films (to be used, for example, in electrochromic devices). As mentioned below, a number of organic materials provide films with the desired results. The reasons as to why the process according to the invention, or indeed according to the prior art PCT application, makes it possible to obtain an adhesive mesoporous structure are, however, not entirely explained. What is immediately clear, however, is that the process described in the present application provides a significant improvement to the photocurrent achievable with an electrode made by the process described. Fig. 3 represents the variation in the photocurrent density of various films as a function of the potential. Specifically, Curve 1 shows variations in the photocurrent density vs. potential of a film produced by the method disclosed in the aforementioned prior art PCT patent application. Curve 2, which as shown clearly provides a significant improvement in photocurrent, shows variations in the photocurrent density vs. potential of a film which has been produced by a process which includes the addition of 0.2 atomic percent of lithium. Curve 3, which offers a yet greater improvement, shows variations in the photocurrent density vs. potential of a film which has been produced by a process which includes the addition of 0.2 atomic percent of lithium to the colloidal solution, which has also been ultrasonically stirred for two hours. Not shown in Fig. 3, but tested by us, is a film which has been produced by a process that includes the step of ultrasonically stirring the mixture for two hours but does not include the step of adding lithium. That film, as mentioned above, also provides an improvement in photocurrent which is in the order of 15% or so. As mentioned above, whilst we are not certain as to why these improvements occur, we have our suspicions and have obtained scanning electron micrographs of both a film produced in accordance with the prior art method, and a film produced in accordance with the method described above. Fig. 4 is an electron micrograph of an electrode manufactured in accordance with the teachings of the aforementioned prior art PCT patent application, and Fig. 5, for comparison, is an electron micrograph of an electrode manufactured in accordance with the teachings of the present invention. In each instance the settings for the electron microscope were the same. Figs. 6 and 7 are further electron micrographs at a higher magnification, of the old and new films respectively. It can be clearly seen by comparing Figs. 6 and 7 that in Fig. 6 the largest grains are in the order of 100 nanometres in diameter, whereas in Fig. 7 the grains are in the order of half this size, namely 50 nanometres of so. As is clearly visible by comparing Figs. 4 and 5, and Figs. 6 and 7; the film nanostructure of Figs. 4 and 6 includes relatively large grains with relatively large gaps between grains of the film. In stark contrast, the nanostructure of the film depicted in Figs. 5 and 7 is that much more compact with much smaller grains, and much smaller gaps between grains of the film. Our conclusion is that it is this more compact nature of the nanostructure which provides the improvement in photocurrent. As has been mentioned above, the electrode of the invention finds utility in a number of different applications. Fig. 8 is an illustrative representation of one such application. Fig. 8 is a schematic representation of a so-called Tandem Cell (similar in structure to that described in PCT application no. WO 01/02624 - the contents of which are incorporated herein by reference) for the cleavage of water into hydrogen and oxygen by visible light. The device consists of two photo systems electrically connected in parallel. The cell on the left (as depicted) comprises a compartment 60 which contains an aqueous electrolyte that is subjected to water photolysis. In the preferred arrangement the electrolyte comprises water to which an electrolyte has been added for ionic conduction, or seawater. Light enters from the left side of the cell through a glass window 62. The light then crosses the electrolyte and impinges upon the front face of a WO₃ electrode (64, 66, 68) which has been produced in accordance with the process described above (the electrode comprising a glass plate 64, a conducting layer 66 and a WO₃ film 68). The WO₃ film 68 absorbs the blue and green part of the solar spectrum, and transmits the red and yellow part to a second cell which in this instance is provided behind the back face of the WO₃ electrode. The second cell contains a dye sensitised mesoporous TiO₂ film 70 which functions as a light driven electric bias which is operable to increase the electrochemical potential of the electrons that emerge from the WO₃ film. The TiO₂ film is formed on a transparent conductor 72 which has been formed on the back face of the glass plate 64 of the WO₃ electrode. The film is in contact with an organic redox electrolyte 74 that is provided between the film 70 and a transparent counter electrode 76 which is rendered conductive on the side facing the electrolyte by means of an applied conductive layer. Behind the counter electrode 76 there is provided a chamber bounded by a glass plate 78 in which an electrolyte 80 (of the same composition as that provided in the first cell) is provided, the two electrolytes 60, 80 being in fluid communication with one another by means of a glass frit 82 or ion conducting membrane. As depicted in Fig. 8, incident light is used to cleave water so that Oxygen is evolved from the compartment 60 in the first cell, and hydrogen is evolved at a cathode 84 immersed in the chamber provided in the second cell. The principal advantage associated with using'the principles taught herein to manufacture an electrode for use in the aforementioned cell is that the increase in achievable photocurrent increases the yield of hydrogen and oxygen when the cell is illuminated. In a further embodiment of the invention, the aforementioned dye sensitised mesoporous TiO₂ film may be replaced with a photovoltaic cell, such as a conventional silicon photovoltaic cell (or some other photovoltaic cell which is chosen for its response to the particular wavelengths of light that are transmitted by the WO₃ film). Such an arrangement provides an increase in biasing voltage, improvements in durability, and is less expensive to manufacture. A simple example of one photovoltaic cell, as is well known in the art, comprises an n-type silicon layer and a p-type silicon layer which have been abutted to form a P-N junction therebetween. Current is extracted from the silicon cell on illumination by means of a contact grid abutted against the n-type layer, and a conductive back plate abutted against the p-type layer. Fig. 9 illustrates a photoelectrochemical cell for water cleavage which employs a photovoltaic cell 86 (such as one of the aforementioned silicon photovoltaic cells) in place of the dye sensitised mesoporous titanium dioxide film used in the cell of Fig. 8. As shown, the cell on the left (as depicted) comprises a compartment which contains an aqueous electrolyte that is subjected to water photolysis. In the preferred arrangement the electrolyte comprises water to which an electrolyte has been added for ionic conduction, or seawater. Light enters from the left side of the cell through a glass window 62. The light then crosses the electrolyte and impinges upon the front face of a WO₃ electrode (64, 66, 68) which has been produced in accordance with the process described above (the electrode comprising a glass plate 64, a conducting layer 66 and a WO₃ film 68). The WO₃ film 68 absorbs the blue and green part of the solar spectrum, and transmits the red and yellow part to a photovoltaic cell 86 which in this instance is provided behind the back face of the WO₃ electrode. As before, the second (photovoltaic) cell 86 functions as a light driven electric bias which is operable to increase the electrochemical potential of the electrons that emerge from the WO₃ film. Behind the second cell there is provided a chamber bounded by a glass plate 78 in which an electrolyte (of the same composition as that provided in the first cell) is provided, the two electrolytes being in fluid communication with one another by means of a glass frit 82 or ion conducting membrane. As depicted in Fig. 8, incident light is used to cleave water so that Oxygen is evolved from the compartment in the first cell, and hydrogen is evolved at a cathode 84 immersed in the chamber provided in the second cell. It can be seen, therefore, that the teachings of the present invention provide significant improvements over previously proposed processes. It will also be understood that whilst preferred embodiments of the invention have been described above in detail, modifications and alterations may be made to these embodiments without departing from the scope of the invention as set out in the claims. For example, whilst -the foregoing description mentions the use of ethanol and PEG 300 as preferred organic materials, any of the following may alternatively be used: (a) ethanol, methanol and other volatile alcohols, (b) dimethyl sulphoxide, (c) ethylene glycol, (d) polyethylene glycol 200, 300, 600 and 1000, (e) maltose and glucose, (f) glycerol, (g) mannitol, and (h) myo-inositol (or simply inositol). In the embodiment just described, the first organic material 28 was chosen from products a) and b), and the second organic material 32 was chosen from products c) to h). If, in a variant of the embodiment described, the first organic material 28 is chosen from products c) to h) it is no longer necessary to subsequently introduce the second organic material 32. A final point to note is that whilst particular features and combinations of features have been listed in the accompanying claims, the scope of the present invention is not so limited and instead extends to any combination or permutation of features described herein irrespective of whether that particular combination or permutation has been explicitly enumerated in the accompanying claims.

Claims

1. A method of making a photoactive electrode which comprises a WO₃ film, the method comprising the steps of: (a) forming a colloidal solution of a mixture of tungstic acid and at least one organic material, (b) adding to said mixture a solution comprised of a soluble Lithium salt; (c) applying a layer of said solution to a conductive substrate, and (d) heat-treating the said substrate at a temperature of at least 350°C.

2. A method of making a photoactive electrode which comprises a WO₃ film, the method comprising the steps of: (a) forming a colloidal solution of a mixture of tungstic acid and at least one organic material, (b) ultrasonically stirring said colloidal solution; (c) applying a layer of said solution to a conductive substrate, and (d) heat-treating the said substrate at a temperature of at least 350°C.

3. A method according to Claim 2 comprising, following step (a) and prior to step (b), the step of adding to said mixture a solution comprised of a soluble Lithium salt.

4. A method according to Claim 1 comprising, following step (b) and prior to step (c), the step of ultrasonically stirring the colloidal solution and added soluble lithium salt solution.

5. A method according to any preceding claim, wherein said substrate comprises a glass plate.

6. A method according to any preceding claim, wherein the heat treatment operation is performed at a temperature of between 400 and 600°(. preferably 550 to 600°).

7. A method according to Claim 6, wherein said heat treatment operation is performed for 15 to 60 minutes, preferably 30 to 60 minutes.

8. A method according to any preceding claim, wherein steps (c) and (d) are repeated lto form a film on said substrate that is comprised of a plurality of layers, each layer being heat-treated after application.

9. A method according to Claim 8, wherein said film is comprised of up to 12, preferably more than six, individually applied layers.

10. A method according to Claim 8 or 9, wherein said film is comprised of six to nine layers.

11. A method according to any preceding claim, wherein step (c) includes the steps of forming a bead of colloidal mixture on said substrate and drawing the bead over the surface to form a layer.

12. A method according to any of claims 1 to 10, wherein step (c) includes the step of screen printing said colloidal solution onto the substrate to form a said layer.

13. A method according to any of claims 1 to 10, wherein step (c) includes the step of spraying said colloidal solution onto the substrate at room temperature to form a said layer.

14. A method according to Claim 13, wherein prior to spraying the colloidal solution is diluted, preferably with ethanol.

15. A method according to any of Claims 4 to 14 when dependent on Claim 1 or Claim 3, wherein said soluble lithium salt solution comprises a solution of a soluble lithium salt; such as lithium perchlorate, lithium nitrate, or lithium acetate; in water or ethanol.

16. A method according to any of Claims 5 to 15 when dependent on Claim 2 or Claim 4, wherein said ultrasonic stirring is accomplished with an ultrasonic stirrer or bath rurining at a frequency of roughly 35 KHz.

17. A method according to Claim 16, wherein said stirring is undertaken for between 1 to 4, preferably at least two, hours.

18. A method according to any preceding claim, wherein step (a) comprises the steps of: obtaining a tungstic acid solution by passing an aqueous sodium tungstate solution through a cation-exchange resin, mixing the tungstic acid solution with a first organic material, concentrating the resulting mixture by evaporation under pressure, and adding a second organic material to form said colloidal solution.

19. A method according to any of claims 1 to 17, wherein step (a) comprises the steps of: obtaining a tungstic acid solution by passing an aqueous sodium tungstate solution through a cation-exchange resin, mixing the tungstic acid solution with said organic material, and concentrating the resulting mixture by evaporation under pressure to form said colloidal solution.

20. A method according to Claim 18, wherein said first organic material is chosen from: ethanol, methanol or other volatile alcohols, and dimethyl sulphoxide

21. A method according to Claim 20, wherein said second organic material is chosen from ethylene glycol, polyethylene glycol 200, 300, 600 and 1000, maltose, glucose, glycerol, mannitol, or myo-inositol.

22. A method according to Claim 20 and 21, wherein said first organic material comprises ethanol, and said second organic material comprises polyethylene glycol, preferably polyethylene glycol 300.

23. A method according to Claim 19, wherein said organic material is chosen from ethylene glycol, polyethylene glycol 200, 300, 600 and 1000, maltose, glucose, glycerol, mannitol, or myo-inositol.

24. A method according to Claim 23, wherein said organic material comprises polyethylene glycol, preferably polyethylene glycol 300.

25. A photoactive electrode produced by a method according to any preceding claim.

26. A photoelectrochemical system for the cleavage of water into hydrogen and oxygen by light, the system including first and second electrically connected cells, the first cell including a photoactive electrode according to Claim 25, said electrode being operable when in contact with an aqueous solution of an electrolyte in use to absorb a first range of wavelengths of light to evolve oxygen and generate protons, the second cell including a dye sensitised mesoporous photovoltaic film which is operable on illumination in use by a second range of wavelengths to drive the reduction of said protons to hydrogen.

27. A system according to Claim 26, wherein said first range of wavelengths includes blue and green light, and said second range of wavelengths includes yellow and red light.

28. A system according to Claim 26 or 27, wherein said second cell is provided behind said first cell.

29. A method of making a colloidal solution for use in a method for the manufacture of a photoactive electrode which comprises a W03 film, the method comprising the steps of: obtaining a tungstic acid solution by passing an aqueous sodium tungstate solution through a cation-exchange resin, mixing the tungstic acid solution with a first organic material, concentrating the resulting mixture by evaporation under pressure, adding a second organic material to form said colloidal solution, and ultrasonically stirring said colloidal solution.

30. A method according to Claim 29, comprising prior to the stirring step, the step of adding a solution comprised of a soluble Lithium salt.

31. A method of making a colloidal solution for use in a method for the manufacture of a photoactive electrode which comprises a W03 film, the method comprising the steps of: obtaining a tungstic acid solution by passing an aqueous sodium tungstate solution through a cation-exchange resin, mixing the tungstic acid solution with a first organic material, concentrating the resulting mixture by evaporation under pressure, adding a second organic material, and adding a solution comprised of a soluble Lithium salt to form said colloidal solution.

32. A method according to Claim 31 comprising the additional step of ultrasonically stirring said colloidal solution.

33. A method of making a colloidal solution for use in a method for the manufacture of a photoactive electrode which comprises a W03 film, the method comprising the steps of: obtaining a tungstic acid solution by passing an aqueous sodium tungstate solution through a cation-exchange resin, mixing the tungstic acid solution with an organic material, and concentrating the resulting mixture by evaporation under pressure to form said colloidal solution, and ultrasonically stirring said colloidal solution.

34. A method according to Claim 33, comprising prior to the stirring step, the step of adding a solution comprised of a soluble Lithium salt.

35. A method of making a colloidal solution for use in a method for the manufacture of a photoactive electrode which comprises a W03 film, the method comprising the steps of: obtaining a tungstic acid solution by passing an aqueous sodium tungstate solution through a cation-exchange resin, mixing the tungstic acid solution with an organic material, concentrating the resulting mixture by evaporation under pressure, and adding a solution comprised of a soluble Lithium salt to form said colloidal solution.

36. A method according to Claim 35 comprising the additional step of ultrasonically stirring said colloidal solution.

37. A method according to any of Claims 29 to 32, wherein said first organic material is chosen from: ethanol, methanol or other volatile alcohols, and dimethyl sulphoxide.

38. A method according to Claim 37, wherein said second organic material is chosen from ethylene glycol, polyethylene glycol 200, 300, 600 and 1000, maltose, glucose, glycerol, mannitol, or myo-inositol.

39. A method according to Claim 37 and Claim 38, wherein said first organic material comprises ethanol, and said second organic material comprises polyethylene glycol, preferably polyethylene glycol 300.

40. A method according to any of Claims 33 to 36, wherein said organic material is chosen from ethylene glycol, polyethylene glycol 200, 300, 600 and 1000, maltose, glucose, glycerol, mannitol, or myo-inositol.

41. A method according to Claim 40, wherein said organic material comprises polyethylene glycol, preferably polyethylene glycol 300.

42. A photo electrochemical system for the cleavage of water into hydrogen and oxygen by light, the system including first and second electrically connected cells, the first cell including a photoactive electrode according to Claim 25, said electrode being operable when in contact with an aqueous solution of an electrolyte in use to absorb a first range of wavelengths of light to evolve oxygen and generate protons, the second cell comprising a photovoltaic cell which is operable on illumination in use by a second range of wavelengths to drive the reduction of said protons to hydrogen.

43. A system according to Claim 42, wherein said second cell comprises a silicon photovoltaic cell.

44. A system according to Claim 42 or 43, wherein said second cell is provided behind said first cell.

45. A photoactive electrode produced by a method according to Claim 1 or 3, the photoactive electrode comprising, as active materials, tungsten trioxide laced with lithium; wherein said active materials are laid down upon a suitable substrate in layers; the electrode comprises a plurality of said layers; and the largest individual particles of each said layer each have a diameter which is significantly less than 100 nm or so.

46. A photoactive electrode according to Claim 45, wherein the largest individual particles of each said layer each have a diameter which is in the region of 50 nm or so.