EP1774060A2

EP1774060A2 - Photoelectrochemical system

Info

Publication number: EP1774060A2
Application number: EP05744959A
Authority: EP
Inventors: Julian Martin c/o Hydrogen Solar Limited KEABLE; Davi Hugh c/o Hydrogen Solar Limited AUTY; Andrew c/o Hydrogen Solar Limited STEVENSON; John c/o E-Synergy Limited WHITE; Scott c/o Hydrogen Solar Limited VOORHEES
Original assignee: Hydrogen Solar Production Co Ltd
Current assignee: Hydrogen Solar Production Co Ltd
Priority date: 2004-05-18
Filing date: 2005-05-18
Publication date: 2007-04-18
Also published as: WO2005113859A3; AU2005245663A1; WO2005113859A2; JP2008507464A

Abstract

A photoelectrochemical system for the cleavage of an electrolyte into hydrogen by light, the system comprising: a photoelectrochemical device that comprises first and second electrically connected cells, said first cell including a photoactive electrode that is operable when in contact with said electrolyte to absorb light and generate protons, said second cell being operable to absorb light to generate a voltage bias for driving the reduction of protons generated by said first cell to hydrogen, said hydrogen being evolved at a second electrode immersed in said electrolyte; and means for circulating electrolyte through the system.

Description

PHOTOELECTROCHEMICAL SYSTEM

Field ofthe Invention This invention relates to a photoelectrochemical system that is operable, on illumination with light, to cleave an electrolyte to yield gaseous products, one or more of which may be used for fuel. Other embodiments of the invention pertain to a photoelectrochemical cell for use in such a system, as well as to an array of such cells. In most cases, for practical efficacy, the electrolyte is an aqueous electrolyte (such as sea water, or water with a suitable electrolyte added) and the system is operable on illumination to cleave water in the electrolyte into oxygen and hydrogen (with usually only the hydrogen being collected for fuel). It will be appreciated, however, that other electrolytes may be used and hence that gasses other than oxygen may be evolved, and as such references or examples herein to the electrolysis of an aqueous electrolyte should not be construed as limiting the scope of the invention only to an aqueous electrolyte. For the purposes ofthe present invention it is immaterial which gas or gases in addition to hydrogen are evolved on illumination of the cell. Accordingly, in the context of the present application the term "electrolyte" should be construed as any electrolytic solution which includes (as a minimum) hydrogen to thereby enable hydrogen to be evolved on operation of the cell. Background to the Invention The mechanisms by which hydrogen (and in addition other gaseous products) can be produced by means of the photoelectrolysis of an electrolyte are well known. The core component of any photoelectrochemical system for cleaving water of an aqueous electrolyte is a photoelectrochemical cell, and International PCT Patent Application No. WO 01/02624 - the contents of which are incorporated herein by reference - discloses one such cell (known colloquially as the "Tandem Cell") which proves, at least experimentally, that the production of hydrogen in this way is a viable proposition. As is described in detail in the aforementioned PCT application, the Tandem Cell uses visible light to cleave water of an aqueous electrolyte into hydrogen and oxygen, and in general terms consists of two photosystems that are electrically connected in parallel. Fig. 1 is an illustrative representation of a Tandem Cell. As shown, light travels through a glass plate 10 and enters the first photosystem of the cell via a compartment which contains an aqueous electrolyte 12 that is subjected to water photolysis. The light crosses the electrolyte and impinges upon the front face of a semiconducting WO₃ electrode (the electrode comprising a glass plate 14, a conducting layer 16 provided on the plate, and a

WO₃ film 18 formed on the conducting layer). The WO₃ film absorbs the blue and green part of the solar spectrum, and transmits the red and yellow part to the second photosystem of the cell - which in this instance is provided behind the back face ofthe WO₃ electrode. The second photosystem contains a dye-sensitised mesoporous TiO film 20 which functions as a light driven electric bias that is operable to increase the electrochemical potential of the electrons that emerge from the WO₃ film l8. The TiO₂ film 20 is formed on a transparent conductor 22 which has been formed on the back face of the glass plate 14 of the WO electrode, and the film is in contact with an organic redox electrolyte 24 that is provided between the film 20 and a transparent counter electrode 26 which is rendered conductive on the side facing the redox electrolyte by means of an applied conductive layer. Behind the counter electrode there is provided a chamber 30 bounded by a glass plate 28 in which an electrolyte (of the same composition as that provided in the first cell) is provided, the electrolytes being in fluid communication with one another by means of a glass frit 32 or ion conducting membrane. On illumination of the Tandem Cell with visible light, the tungsten trioxide film ofthe first photosystem ofthe cell is operable to cleave water of the electrolyte to evolve oxygen, and the second photosystem of the cell is operable to generate a voltage bias which drives the reduction of protons from the tungsten trioxide film 18 into hydrogen that is evolved at a cathode 34 immersed in the chamber 30 provided behind the counter electrode 26. Evolved hydrogen, and optionally oxygen, can be collected at the cathode and anode respectively, and then used - for example to generate electricity. Whilst it is the case that the Tandem Cell has been shown to function adequately in experimental conditions, a working system (as might be suitable for home installation for example) has not yet been proposed. Accordingly, a first aim of the present invention is to provide a practical working system for the production of hydrogen. Another aspect of the present invention is concerned with devising an arrangement which makes use of a greater proportion of the electromagnetic spectrum of incident light. In the Tandem Cell, for example, whilst the WO₃ film is responsive to blue and green light and the TiO₂ film is responsive to red and yellow light, the remainder of the incident electromagnetic spectrum is largely unused. Clearly it would be highly advantageous if some of this hitherto unused solar energy could be utilised so that a greater proportion of the incident solar spectrum is put to use in the system. Accordingly, it is a further object of the present invention to devise a means for making use of a greater proportion of the incident solar spectrum. Another aspect of the present invention pertains to the fact that whilst the Tandem Cell itself is well suited for laboratory experimentation (for example to prove the principle of cleaving water by light), it is not particularly well suited as the core component of a practical working system. For example, whist the biasing voltage provided by the sensitised mesoporous TiO₂ film is sufficient to drive the photoelectrolysis of water, more efficient hydrogen production may be provided by increasing the biasing voltage. Other aspects of the Tandem Cell which would merit some improvement (for a practical working system) are the durability of the cell and the cost of its manufacture. A further aim ofthe present invention is, therefore, to provide a cell which mitigates some of these disadvantages. Other aspects ofthe present invention are directed towards making use of generated system heat that would otherwise be wasted, and providing a means for supplying hydrogen at a suitable pressure for a variety of applications. Statement ofthe Invention In accordance with a first aspect of the invention, there is provided a photoelectrochemical system for the cleavage of an electrolyte into hydrogen by light, the system comprising: a photoelectrochemical device that comprises first and second electrically connected cells, said first cell including a photoactive electrode that is operable when in contact with said electrolyte to absorb light and generate protons, said second cell being operable to absorb light to generate a voltage bias for driving the reduction of protons generated by said first cell to hydrogen, said hydrogen being evolved at a second electrode immersed in said electrolyte; and means for circulating electrolyte through the system. In accordance with another aspect of the invention, there is provided a photoelectrochemical device for the system, the device comprising first and second electrically connected cells, said first cell including a photoactive tungsten trioxide electrode (or equivalent) that is operable when in contact with said electrolyte to absorb light and generate protons, said second cell comprising a photovoltaic device that is operable to absorb light and generate a voltage bias for driving the reduction of protons generated by said first cell to hydrogen, said hydrogen being evolved at a second electrode electrically coupled to said second cell and immersed in said electrolyte. A further aspect of the invention provides another photoelectrochemical device for the system, the device comprising first and second electrically connected cells, said second cell comprising a photovoltaic device that is encapsulated between first and second conducting layers, the first conducting layer having a photoactive tungsten trioxide electrode (or equivalent) deposited on the other surface thereof, and the second conducting layer having a conducting film deposited thereon as an electrode, wherein the tungsten trioxide layer is operable when in contact with said electrolyte to absorb light and generate protons, and the photovoltaic device is operable to absorb light and generate a voltage bias for driving the reduction of protons generated by said tungsten trioxide electrode to hydrogen, said hydrogen being evolved at the electrode deposited on said second conducting layer when said electrode is immersed in said electrolyte. Yet another aspect of the invention relates to a reel-to-reel manufacturing method for a photoelectrochemical device, the method comprising encapsulating a flexible photovoltaic device in transparent flexible conducting material, coating one side of the material in front of said device with a photoactive semiconducting film to form a first electrode, and coating the other side of the material behind said device with a conductor to form a second electrode. Preferred features of each of these aspects of the invention are set out in the accompanying claims, and further features and advantages of the invention will be apparent following a reading ofthe application. Brief Description ofthe Drawings Various preferred embodiments of the present invention will now be described, by way of illustrative example only, with reference to the accompanying drawings, in which: Fig. 1 is a schematic representation of a Tandem Cell as disclosed in the aforementioned PCT patent application; Fig. 2 is a schematic representation of an alternative photoelectrochemical cell; Fig. 3 is a schematic cross-sectional view (along the line B — B of Fig. 4) of an array of photoelectrochemical cells; Fig. 4 is a schematic lateral cross-sectional view (along the line A — A of Fig. 3) ofthe array; Fig. 5 is a diagrammatic representation of a photoelectrochemical system; and Fig. 6 is a representation of another photoelectrochemical system; Fig. 7 is a schematic idealised plot of hydrogen/metal hydride ratio against equilibrium pressure; and Fig. 8 is a schematic representation of a system that employs a hydride pressure cascade to increase the pressure ofthe hydrogen supplied. Detailed Description of Preferred Embodiments As mentioned above, Fig. 1 is an illustrative representation of the aforementioned previously proposed Tandem Cell. This arrangement, whilst being eminently suitable for proving the feasibility of producing hydrogen by photoelectrolysis of an electrolyte, is less suitable for commercial production.

The principal reason for this is that the dye-sensitised mesoporous cell is a relatively complex arrangement that is necessarily quite expensive to produce. In addition, whilst the voltage bias generated by that film is sufficient to evolve hydrogen, it would be better for a commercial product for the voltage to be increased so that hydrogen can be evolved more efficiently. Fig. 2 is a schematic representation of a cell that goes some way to avoiding these problems. The principal difference between the cell of Fig. 1 and that of Fig. 2 is that in the cell of Fig. 2, the aforementioned dye sensitised mesoporous TiO₂ film 20 has been 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 semiconducting film). Such an arrangement provides an increase in biasing voltage (and hence an increase in the volume of evolved hydrogen), improvements in durability, and is significantly less expensive to manufacture. As is well known in the art, a simple photovoltaic cell 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. 2 shows a photoelectrochemical cell 40 which employs a photovoltaic cell 42 (such as one of the aforementioned silicon photovoltaic cells) in place ofthe dye sensitised mesoporous titanium dioxide film 20 used in the cell of Fig. 1. As shown, the cell on the left (as depicted) comprises a compartment 44 which contains an electrolyte that is subjected to 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 46. The light then crosses the electrolyte 44 and impinges upon the front face of a WO₃ electrode comprising a glass plate 48, a conducting layer 50 and a WO film 52. The WO film 52 absorbs the blue and green part of the solar spectrum, and transmits the red and yellow part to a photovoltaic cell 42 which in this instance is provided behind the back face of the WO₃ electrode. As with the arrangement of Fig. 1, the second (photovoltaic) cell 42 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 54 bounded by a glass plate 56 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 58 or ion conducting membrane. As is depicted in Fig. 2, incident light is used to cleave the electrolyte (which in the preferred arrangement is an aqueous electrolyte) so that Oxygen is evolved from the compartment in the first cell, and hydrogen is evolved at a cathode 60 immersed in the chamber 54 provided behind the second cell. This arrangement is advantageous as compared to that of Fig. 1 as simple photovoltaic cells are much less expensive and more readily available than relatively complex dye sensitised mesoporous cells, and because the increase in bias voltage provided by the photovoltaic cells provides for an increased evolution of hydrogen. One potential drawback with the arrangement of Fig. 2 is that as the rear face of the photovoltaic cell 42 is in contact with the electrolyte, the electrolyte cannot be of a composition which might degrade or otherwise damage or react with the photovoltaic cell, or indeed the electrical contacts that are typically provided thereon. One simple solution to this problem would be to coat the rear part ofthe photovoltaic cell with a suitable material that is impervious to the electrolyte. Another slightly more complex arrangement (which is advantageous for other reasons over the arrangement depicted in Fig. 2) is depicted in Fig. 3. Fig. 3 is a schematic longitudinal cross-sectional view (along the line B — B of Fig. 4) of an array of cells for the photoelectrolysis of an electrolyte to yield hydrogen. The one cell 70 of the array that is visible is depicted as being inclined, as it would normally be (for example when mounted on a rooftop) to increase its exposure to incident solar radiation. In general, the array will need to be tilted by at least 10 degrees to the horizontal to allow gas to gather at the top of the Cell for collection. The array may also be tilted a few degrees from side-to-side to aid gas collection and electrolyte circulation. The cell 70 of this arrangement comprises a flexible photovoltaic panel 72, for example of amorphous silicon, that is sandwiched between first and second conducting layers 74, 76 and sealed top and bottom from the electrolyte by a suitable sealant material 77. Each of the conducting layers 74, 76 are transversely conductive (as opposed to longitudinally conductive as in the cells abovementioned) and at least the first conducting layer is transparent - at least to light of a wavelength to which the photovoltaic panel is responsive. Such layers could comprise, for example, conducting glass or more preferably conducting epoxy glue of the type commonly used in the electronics industry (for example for attaching semiconductor devices to a substrate). A photoactive semiconducting film 78 (which could be for example a tungsten trioxide film as aforementioned, an iron (iii) oxide film or any one of a number of other semiconducting films that are photoactive) is deposited on the front of the first layer 74, and a suitable conducting layer 80 (for example of platinum, a platinised conducting material or similar) is deposited on the underside ofthe second conducting layer 76 (namely the side opposite to that which is attached to the panel 72) to form a cathode. In a similar manner to the cells aforementioned, light incident on the cell strikes the semiconducting film 78 and a gas - for example oxygen - is evolved. Light passing through the film, passes transversely through the conducting layer and strikes the flexible photovoltaic panel. The illuminated panel acts as a light driven electric bias which is operable to increase the electrochemical potential of the electrons that emerge from the photoactive film 78 deposited on the first layer, and hydrogen is evolved at the cathode formed by the conducting layer formed on the underside of the second conducting layer 76. The chief advantage of this arrangement, as compared to that of Fig. 2, is that the flexible nature of the various layers enables a roll-to-roll manufacturing process. This means that the cells can be manufactured very economically. To increase the structural rigidity of the cells, one or more stiffer conducting layers can be included - for example between the deposited photoactive film 78 and the first conducting layer 74, or between the first conducting layer 74 and the flexible photovoltaic panel 72. In a highly preferred arrangement, a stiffer conducting layer 82 (such as a metal or carbon sheet or film) is provided between the flexible photovoltaic panel 72 and the second conducting layer 76, and this stiffer layer can be extended (as shown) at the base ofthe cell to provide a baffle that stops hydrogen passing round the bottom ofthe cell and recombining with the gas produced in the front half of the cell. The cell is located in an enclosure 84 (such as a pressed steel enclosure) in use. As shown in Fig. 3, the enclosure 84 is closed by a glass front sheet which is sealed at either end with the enclosure in a suitably resilient gasket 86 before the cell is filled with electrolyte. In this highly preferred arrangement, the top end ofthe enclosure 84 is shaped to have an inward step 88 against the underside of which the second conducting layer 76 is sealed. By virtue of this arrangement, the cell effectively functions as a manifold to direct hydrogen and, for example, oxygen to respective outlets 90. In addition to the gas outlets 90, the enclosure is also provided with an electrolyte inlet 92 and an electrolyte outlet 94 to enable electrolyte to be circulated through the cell. One particular disadvantage of the cells depicted in Figs. 1 and 2 is that the return ionic path from the cathode to the anode (i.e. the tungsten trioxide film) is relatively long. To mitigate this problem it is proposed to provide the cell of Fig. 3 with a plurality of through-holes 96 to thereby reduce the length of the ionic conduction pathways in the cell between the cathode and the anode. In the preferred arrangement the through holes are of a relatively small diameter so as not to overly decrease the active area of the cell, and are hooded by means of a shield 98 which extends below the level of the cathode so that as hydrogen bubbles up the cathode to the outlet it cannot flow from one side ofthe cell to the other. As mentioned briefly above, a large number of photoactive materials are known to those persons of skill in the art, and any of these materials may be used as the photoactive film in the cells described above. It is also the case that the techniques described in either of our co- pending UK patent application nos. 0405751.9 and 0408887.8 (the contents of each of which are incorporated herein by reference) may be employed to provide yet further improvements in the photocurrent achievable from the photoactive film. Fig. 4 is a schematic lateral cross-sectional view (along the line A — A of Fig. 3) of the array. As depicted, in this instance the array comprises three identical cells placed side-by-side in the enclosure 84. Depending on the required output, however, any number of cells may be provided. As mentioned above, the stiffer conducting layer 82 is preferably downwardly extended at the base of the cell to provide a baffle. It is also preferred for the stiffer conducting layer 82 of each cell to be laterally downwardly extended (as depicted in Fig. 4) to form a lateral skirt 85 that extends below the cathode 80 and into abutment with the base of the enclosure 84 at either end of a transverse depression 83 formed in the base thereof (see Fig. 3). The skirt 85 functions to space the cell from the enclosure 84, to provide a channel 87 through which electrolyte can flow, and to prevent hydrogen formed at the cathode from passing round the sides ofthe cell and recombining with oxygen, for example, formed at the anode. An advantage of the skirt arrangement depicted is that ionic pathways can still extend around either side of the cell (as well as at the bottom of the cell and through the through-holes 96) between the cathode and anode by virtue of the aforementioned depression 83. Fig. 5 is a schematic representation of a system 100 in which any of the cells described above may be employed. The system is shown in situ on the roof 101 of a building, and has been designed with the core aim of making use of a greater proportion of solar energy incident on the cell/array. To this end the system depicted in Fig. 5, as will hereafter be described, proposes to make use of the significant amounts of heat generated (principally by the infrared component of the spectrum) on illumination of a photoelectrochemical cell. The core component of the system depicted in Fig. 5 is a photoelectrochemical device 102. In the preferred arrangement, the photoelectrochemical device 102 comprises an array of cells, as depicted in

Figs. 3 and 4. It will be appreciated, however, that a single cell may instead be provided - or indeed that the array may include different types of cell (such as those depicted in Figs. 1 or 2) or indeed a combination of different types of cells (for example one each of those depicted in Figs. 1, 2 and 3. Each device 102 is provided with an inlet 104 that is coupled to an electrolyte supply manifold 106. The electrolyte supply manifold 106 is coupled to an electrolyte reservoir 108, and in this particular example the electrolyte in the system is pressurised by means of an electrolyte header tank 110 located above the reservoir 108. If the electrolyte is an aqueous electrolyte, then the header tank may be coupled to the mains water supply and provided with a float valve (for example of the type provided in a toilet cistern) so that any water fluid loss in the system is automatically replenished. The provision of a header tank arrangement is advantageous, not only because the tank pressurises the electrolyte, but also because the tank provides overpressure protection for the electrolyte part ofthe system as a whole. To permit circulation of electrolyte, each device is provided with an outlet 112 that is coupled to an electrolyte return manifold 114, and the return manifold is in turn coupled to the reservoir 108 to return electrolyte thereto once it has circulated through the device 102. This arrangement is particularly advantageous as it allows the electrolyte to circulate past the anode and cathode, thereby facilitating the evolution of gas. Hydrogen evolved on illumination of the device 102 is collected at a hydrogen outlet 116 for subsequent use. For an aqueous electrolyte, the gas

(oxygen) evolved at the anode can simply be vented to atmosphere from each cell or array of cells forming a said device. Alternatively, further pipework can be provided to collect the gas or gasses evolved in addition to hydrogen. In this example, hydrogen collected at each device outlet 116 passes to a hydrogen manifold 118 that is coupled to a hydride storage array 120.

Hydride storage arrays, as is known in the art, comprise a stack of interconnected hydride packs which can be used to store hydrogen and to differentially charge (i.e. store hydrogen) and discharge (i.e. output hydrogen). As the electrolyte circulates through the system it is heated by the incident electromagnetic radiation (typically sunlight), and this heat is returned to the electrolyte reservoir as the electrolyte circulates out of the photoelectrochemical device 102. By locating the hydride array 120 in close proximity to the electrolyte reservoir 108, it is possible to regulate the temperature of the array so that it is well suited for the storage of hydrogen.

Commercial data for such arrays mdicate that a heat source at 85 degrees centigrade (which is typically easily achievable with a device in sunlight) enables compression of hydrogen from 7 bar g to 250 bar g for storage. The hydride array can be coupled to a secondary device (not shown) such as for example a proton exchange membrane fuel cell system, and the hydrogen stored in the array can be used to drive the fuel cell system when the hydrogen is released. A particular advantage of the system depicted in Fig. 5 is that as the header tank 110 is located only a relatively short distance above the reservoir

108, the electrolyte in the system is only at a relatively low pressure of something in the region of 1 to 5 poss. (roughly 0.07 to 0.34 bar). This relatively low electrolyte pressure means that the glass plate provided on the front of each device 102 can be kept quite thin (thereby reducing the amount of light absorbed) and that simple plastic pipework can be used for the electrolyte circuit. If hydrogen is evolved at sufficiently low pressures so that diffusion through the pipework is not a significant concern, the pipework for the hydrogen part of the system can be of inexpensive plastic. As an advantageous alternative, the pipework could be metalised on its inner surface so as to avoid any diffusion of hydrogen through the pipework. As the cells themselves are relatively lightweight, the plastic pipework itself may constitute a structural frame for mounting the photoelectrochemical device (for example on the roof of a building), although a separate structural support frame could be provided if desired or expedient. If plastic pipework is used, it would be preferable if the system were to be constructed so that the pipework is located behind the photoelectrochemical device - where it will be protected from degradation caused by ultraviolet light. In a highly preferred arrangement, the photoelectrochemical device attaches and seals to the hydrogen and electrolyte pipe-work by means of quick-release self-sealing connectors. Such connectors are widely used by gardeners, and commonly sold for example under the trade name "Hozelock". This arrangement is especially advantageous as the self-sealing nature of the connectors would permit photoelectrochemical devices to be removed from the system (for example for repair) without first having to shut down and empty the remainder of the system. Each photoelectrochemical device may also include a flow-meter at the hydrogen outlet to provide an indication to the user ofthe system that the device is functioning. The meter need not be expensive or provide any sort of measurement of flow-rate, it is sufficient if the meter merely provides an indication that hydrogen is flowing from the device. In a highly preferred arrangement, means are provided for monitoring the electrolyte so that a user of the system is able to control the chemistry of the electrolyte to thereby maintain the performance ofthe system. In an alternative arrangement depicted schematically in Fig. 6, the system depicted in Fig. 5 may be supplemented by means of a Stirling Engine (or equivalent device) 122. Stirling Engines, as is well known in the art, cause a piston to move by heating and cooling a gas. In a highly preferred arrangement, the "hot" side of the Stirling Engine 122 can be arranged to be in close proximity to the electrolyte reservoir (so that heat can be drawn therefrom), and the "cold" side of the Stirling Engine can be arranged to be in close proximity to the relatively cool feeder pipe from the header tank. Stirling Engines typically include a flywheel, and this could be coupled to a generator to generate electricity. In an alternative arrangement to that depicted in Fig. 5, the electrolyte reservoir could be coupled to a heat exchanger and to a domestic hot water or heating system for example. In such an arrangement, the relatively hot electrolyte would directly heat the water in the domestic system, and that hot water could then be stored in a lagged container for subsequent use - for example for washing or heating. As an alternative to, or indeed an additional feature of, a system wherein electrolyte is pressurised by means of a header tank, the pipe-work could be arranged to provide a natural (thermo-siphon) flow of electrolyte through the device to a heat-exchanger, with cooled electrolyte being returned to the device. In such an arrangement it would be preferred for the electrolyte to circulate at night as well as in daylight to avoid freezing, and a pump could be used for this purpose. Such an arrangement would be useful as it would avoid extreme heat and cold in the system, thereby maintaining a more even temperature and hence reduce stresses and deterioration due to thermal cycling of components. In another arrangement, depicted schematically in Figs. 7 and 8, hydrogen generated by the array may be output to a so-called thermal hydride compressor to enable the relatively low pressure array output to be increased in pressure to a point more suited for commercial applications. Thermal hydride compressors make use ofthe fact that metal hydrides have an equilibrium pressure, also known in the art as a plateau pressure, at a given temperature. If, at a given temperature, the pressure of supplied hydrogen exceeds the equilibrium pressure of the particular metal hydride in which the hydrogen is to be stored, then the hydrogen is absorbed into the hydride. If, on the other hand, the pressure of supplied hydrogen falls below the equilibrium pressure of the particular metal hydride, then the hydride releases hydrogen that it has previously stored. The process of absorbing hydrogen in a metal hydride is exothermic (i.e. it gives out heat), whereas the process of liberating hydrogen from a hydride is endothermic (i.e. it takes in heat). The reversible nature of the reaction coupled with its temperature sensitivity means that a given hydride can be charged and discharged by applying heat or removing heat from the hydride, and if a series of hydrides are coupled together the pressure of the hydrogen can be stepped up quite considerably. Fig. 7 is a schematic plot of the hydrogen metal hydride ratio against equilibrium pressure showing, inter alia, how the equilibrium pressure varies with temperature for a given metal hydride. Fig. 8 is a schematic representation of one potential implementation of an illustrative two-stage thermal hydride compressor. As shown, the compressor 140 comprises, in this instance, two hydride storage canisters 142, 144. The first of these canisters is coupled to a source of hydrogen (e.g. manifold 118) via a first non-return valve "A", and a second non-return valve "B" is coupled between the first and second canisters 142 and 144. The second canister 144 is coupled to a higher-pressure H₂ outlet via a fourth non- return valve "C". The valves may be simple solenoid valves. A large number of different hydrides are known in the art, any of which are suitable for use in this aspect of the invention. It is also the case that the properties of any one halide may be varied by making slight changes to the basic composition. In the preferred arrangement, the hydrides used in each canister are different from one another, and provide different equilibrium pressures at each stage. As will now be described, the system depicted in Fig 8 may be operated to provide higher pressure pulses of Hydrogen at the outlet. In an alternative configuration two parallel banks of canisters may be provided, one bank being heated (to expel hydrogen from respective canisters of that bank) whilst the other is being cooled (to absorb hydrogen into respective canisters of that bank). In such a system by alternately heating and cooling respective banks it is possible to provide a substantially continuous flow of higher pressure hydrogen at the outlet. As depicted, the canisters 142, 144 are each coupled to a coolant circuit 148 and a heater circuit 150. In the preferred embodiment both circuits are for circulating fluid, for example water. Pumps (not shown) are provided to pump the fluid round each circuit and through valves (e.g. solenoid valves) into and out of the canisters. As will be apparent to those persons skilled in the art, the valves prevent coolant fluid from mixing with heater fluid. The coolant circuit may include a heat exchanger (not shown), and that exchanger could be located in the storage tank 110 so that heat drawn from the canisters is transferred to the electrolyte. Similarly, the heating circuit may include a heat exchanger located - in close proximity to the electrolyte outlet manifold 114 so that heated electrolyte heats the fluid running through the heater circuit. Alternatively, the coolant circuit and heater circuit may include electrical coolers/heaters. The compressor works as follows. In a first step, cold fluid is supplied through respective valves to the canisters. Hydrogen is supplied through inlet valve A to the first canister 142, and is absorbed therein generating heat. The generated heat is carried out of the canister by the coolant so that the absorbtion process can continue until the metal halide is saturated. Next the coolant valves and valve A are closed, and the heater valves are opened to circulate heater fluid through the canisters. The heater fluid warms the canister increasing the pressure ofthe hydrogen trapped therein. The heater valves are then closed, and valve B is opened to allow relatively high-pressure hydrogen (as compared to the pressure at the inlet) to flow from the first canister 142 to the second canister 144 for absorption. The canisters are cooled by opening the coolant valves so that the absorption process continues until the metal hydride in the second canister 144 is saturated. At this point the coolant valves are closed, and the heater valves are opened to warm the canisters and increase the pressure of hydrogen stored in the second canister 144. Valve C is then opened, whilst heater fluid continues to be circulated through the heater circuit, to release hydrogen gas at the outlet which is at a higher pressure than the pressure of the gas supplied to the second canister from the first, and a higher pressure than the supply pressure through valve A. Once the stored hydrogen has been released from the second canister, valve C is closed, and the process is repeated. As will be appreciated by those persons skilled in the art, by providing several discrete hydride canisters that are coupled together (and preferably of different hydride materials) it is possible to significantly step-up the pressure ofthe hydrogen output from the system. It can be seen from the above, that the teachings of the present invention provide an effective mechanism for avoiding, or at least mitigating, the problems associated with the prior art. It will also be apparent, and should be noted, that whilst various preferred features have been described above in detail the scope ofthe present invention is not limited to those features and as such that modifications and alterations may be made to the arrangements described herein without departing from the spirit and scope ofthe invention. For example, whilst the cells aforementioned have been described in the context of one cell provided in front of the other, it will be apparent to those persons skilled in the art that the cells could instead be provided side by side (with appropriate electrical connections therebetween). In such an arrangement, the front glass face of the cell could include an optical device that is operable to divert appropriate wavelengths of incident light to whichever of the two cells that is responsive thereto. Furthermore, whilst tungsten trioxide and ferric oxide have been mentioned herein as suitable photoactive materials, a variety of additional photoactive materials are known in the art, any one of which is suitable for use in accordance with the teachings ofthe invention. It will also be apparent to persons skilled in the art that an electrical compressor, for example, could be provided to pressurise hydrogen output by the cell array. A final point to note is that whilst certain features have been identified as being of importance in the accompanying claims, the scope ofthe invention is not limited to those features or indeed to the particular combinations of features that are explicitly enumerated. The scope of the present invention instead extends to any combination of features herein described irrespective of whether that combination has explicitly been claimed in the accompanying claims.

Claims

1. A photoelectrochemical system for the cleavage of an electrolyte into hydrogen by light, the system comprising: a photoelectrochemical device that comprises first and second electrically connected cells, said first cell including a photoactive electrode that is operable when in contact with said electrolyte to absorb light and generate protons, said second cell being operable to absorb light to generate a voltage bias for driving the reduction of protons generated by said first cell to hydrogen, said hydrogen being evolved at a second electrode immersed in said electrolyte; and means for circulating electrolyte through the system.

2. A system according to Claim 1, wherein said photoactive electrode includes a film of photoactive semiconducting material.

3. A system according to Claim 2, wherein said photoactive semiconducting material comprises tungsten trioxide (WO₃) or Iron (iii) Oxide.

4. A system according to any preceding claim, wherein the second cell comprises a photovoltaic cell, such as a silicon photovoltaic cell.

5. A system according to Claim 4, wherein said photovoltaic cell is flexible.

6. A system according to Claim 4 or 5, wherein the photovoltaic cell is encapsulated between first and second conductive layers to thereby avoid contact with the electrolyte.

7. A system according to Claim 5, wherein the first conducting layer is transparent, at least to wavelengths of light to which the photovoltaic cell is responsive.

8. A system according to Claim 6 or 7, wherein the conductive layers are of conducting epoxy.

9. A system according to any of Claims 5 to 8, wherein said photoactive electrode is formed as a film on said first conducting layer.

10. A system according to any of Claims 5 to 9, wherein said cathode comprises a conductive layer formed on a side of said second conductive layer opposite to that which is in contact with said photovoltaic cell.

11. A system according to any of Claims 1 to 3, wherein the second cell comprises a dye sensitised mesoporous Titanium Dioxide film in contact with an organic redox electrolyte.

12. A system according to any preceding claim, wherein said system includes a housing in which said device is located, an electrolyte inlet and an electrolyte outlet being provided in said housing.

13. A system according to Claim 12, wherein said housing includes an outlet for hydrogen evolved at said second electrode.

14. A system according to Claim 13, comprising means operable to provide an indication when hydrogen is flowing through said outlet out of the device.

15. A system according to Claim 13 or 14, wherein the device located in the housing acts as a manifold for hydrogen evolved at the second electrode to direct the hydrogen to said outlet.

16. A system according to any of Claims 12 to 15, wherein said means for circulating electrolyte comprises an electrolyte reservoir coupled to said inlet by means of an inlet manifold, and to said outlet by means of an outlet manifold.

17. A system according to Claim 16, wherein said inlet and outlet manifolds are coupled to said housing by means of self-sealing quick release connectors.

18. A system according to Claim 16 or 17, comprising an electrolyte header tank located above the reservoir and in fluid communication therewith, electrolyte in the header tank serving to pressurise electrolyte in the reservoir, and drive the electrolyte through the device.

19. A system according to any of Claims 16 to 18, comprising a hydride array coupled to said hydrogen output means and located in close proximity to said reservoir, heat exfracted from said electrolyte serving to facilitate charging ofthe hydride stack with hydrogen outputted from the device.

20. A system according to Claim 19, wherein the hydride array is coupled to a hydrogen fuel cell system that is operable to generate electricity from hydrogen input thereto.

21. A system according to any of Claims 16 to 18, comprising a Stirling engine having a hot side and a cold side, the engine being arranged so that the hot side is in close proximity to the reservoir.

22. A system according to Claims 18 and 21, wherein the cold side ofthe Stirling engine is in close proximity to the fluid feed from the header tank to the reservoir.

23. A system according to any of Claims 16 to 18, comprising a heat exchanger that is configured to draw heat from the electrolyte in the reservoir, and to supply that heat to a fluid supply system such as a domestic hot water or heating system.

24. A system according to any preceding claim, wherein the electrolyte is an aqueous electrolyte, and the photoactive electrode is operable to cleave water in the electrolyte to evolve oxygen.

25. A system according to any preceding claim further comprising means for pressurising hydrogen output from said photoelecfrochemcial device.

26. A system according to Claim 25, wherein said pressurising means comprises a thermal hydride compressor, having at least two stages, preferably more than two stages.

27. A system according to Claim 25, comprising a compressor, preferably an electrical compressor, for compressing said output hydrogen.

28. A system according to Claim 25 or 26, wherein the pressurising means comprises a heater circuit and a cooler circuit, each said circuit being operable to heat or cool stages of said pressuring means.

29. A photoelectrochemical device for the system of Claim 1, the device comprising first and second electrically connected cells, said first cell including a photoactive electrode that is operable when in contact with said elecfrolyte to absorb light and generate protons, said second cell comprising a photovoltaic device that is operable to absorb light and generate a voltage bias for driving the reduction of protons generated by said first cell to hydrogen, said hydrogen being evolved at a second electrode electrically coupled to said second cell and immersed in said elecfrolyte.

30 A device according to Claim 29, wherein the photoactive elecfrode is of tungsten trioxide or ferric oxide.

31. A photoelectrochemical device for the system of Claim 1, the device comprising first and second electrically connected cells, said second cell comprising a photovoltaic device that is encapsulated between first and second conducting layers, the first conducting layer having a photoactive electrode deposited on the other surface thereof, and the second conducting layer having a conducting film deposited thereon as an elecfrode, wherein the photoactive electrode is operable when in contact with said elecfrolyte to absorb light and generate protons, and the photovoltaic device is operable to absorb light and generate a voltage bias for driving the reduction of protons generated by said photoactive electrode to hydrogen, said hydrogen being evolved at the electrode deposited on said second conducting layer when said elecfrode is immersed in said elecfrolyte.

32. A device according to Claim 31, wherein said photoactive electrode is of tungsten trioxide or ferric oxide

33. A device according to Claim 31 or 32, wherein said first conducting layer is transparent at least to wavelengths of light to which the photovoltaic device is responsive.

34. A device according to any of Claims 31 to 33, wherein the photovoltaic device comprises a flexible photovoltaic panel.

35. A device according to any of Claims 31 to 34, wherein the first and second conducting layers are of conducting epoxy.

36. A device according to any of Claims 31 to 35, comprising one or more electrically conducting layers provided to impart structural rigidity to the device as a whole.

37. A reel-to-reel manufacturing method for a device as claimed in Claim 31, the method comprising encapsulating a flexible photovoltaic device in transparent flexible conducting material, coating one side of the material in front of said device with a photoactive semiconducting film to form a first elecfrode, and coating the other side of the material behind said device with a conductor to form a second electrode.