WO2005060038A1

WO2005060038A1 - Fuel cell

Info

Publication number: WO2005060038A1
Application number: PCT/GB2004/005176
Authority: WO
Inventors: Jeremy Daniel Mckendrick Watson
Original assignee: The Boc Group Plc
Priority date: 2003-12-18
Filing date: 2004-12-10
Publication date: 2005-06-30
Also published as: GB0329240D0

Abstract

A proton exchange membrane (PEM) fuel cell comprises a proton exchange membrane, an anode formed on a first surface of the membrane for exposure to a fuel and a cathode formed on a second surface of the membrane for exposure to an oxidant. In order to utilise a hydrocarbon such as methanol as a fuel without pre-conditioning, and thus simplify fuel preparation, the electrodes are coated with to a titanium dioxide photocatalyst irradiated by a source of ultra violet radiation.

Description

FUEL CELL

The invention relates to a fuel cell, and in particular to a proton exchange membrane (PEM) fuel cell.

Fuel cells are energy conversion devices, which combine a fuel, such as hydrogen, and an oxidizing agent, such as air, to produce electrical power. There are various types of fuel cell, and these are defined by the nature of the electrolyte used in the cell. Examples include aqueous alkaline fuel cells, molten carbonate fuel cells and proton exchange membrane (PEM) fuel cells.

PEM fuel cells produce electricity by combining fuel (e.g. hydrogen) and oxygen (from air). With reference to Figure 1 , a known PEM fuel cell 1 comprises a porous PEM electrolyte 2, such as Nafion® (manufactured by DuPont), sandwiched between a porous anode 3 and a porous cathode 4. Catalysts 5,6, such as palladium or platinum particles, are typically provided on the inner surfaces of the anode 3 and cathode 4. Hydrogen fuel is supplied to the anode 3 through a first channel extending between the anode 3, and housing plates 7 and 9, and oxygen is supplied to the cathode 4 through a second channel extending between the cathode 4, and housing plates 8 and 10. In the presence of the anode catalyst 5, hydrogen atoms split into protons (hydrogen ions) and electrons at the anode 3:

2H₂ - AH + 4e (Equation 1)

The PEM electrolyte 2 allows protons to flow therethrough, but not electrons. As a result the protons and electrons proceed along separate paths to the cathode 4. The protons reach the cathode via the electrolyte 2, whilst the electrons create a separate current through an electrical circuit 11 thus providing electricity.

In the presence of the cathode catalyst 6, the oxygen gas (0₂) forms oxygen atoms at the cathode 4. The negative charge of these atoms attracts the protons passing through the PEM electrolyte 2, where they combine with the oxygen atoms and the electrons to form water molecules:

0₂ + AH + Ae → 2H₂0 _(Equation 2)

In order to maximize the contact area between the hydrogen, oxygen, electrodes and electrolyte, and in order to minimize the distance that the protons need to travel between the electrodes, the electrodes and electrolyte are usually made to be flat and thin with large relative surface area

There are a number of problems associated with the use of gaseous hydrogen as the fuel for a PEM fuel cell. Terrestrial hydrogen is not abundant in its elemental state, and must be generated either at the fuel cell location or remotely. Various hydrocarbon fuels may be catalytically reformed into a hydrogen rich mixture. However, this can require complex systems, and the thus-formed mixture requires pre-conditioning to remove by-products, such as carbon monoxide, which could otherwise significantly reduce the efficiency of the fuel cell catalysts. When generated remotely, hydrogen fuel must be transported, stored and delivered to the fuel cell.

Furthermore, there are also a number of problems associated with the use of palladium or platinum for the fuel cell catalysts. Such catalytic materials are expensive, can be subject to "poisoning", and fuel cells using such catalysts have a relatively slow rate of reaction. Moreover, when hydrocarbon fuels such as methanol are used, 'crossover' effects can impair efficiency of PEM fuel cells.

It is an aim of at least the preferred embodiment of the present invention to solve these and other problems.

The present invention provides a fuel cell comprising a proton exchange membrane, an anode provided on a first surface of the membrane for exposure to a fuel, an anode catalyst for catalysing the fuel, a cathode provided on a second surface of the membrane for exposure to an oxidant, and a cathode catalyst for catalysing the oxidant, wherein at least one of the catalysts comprises photocatalytic material.

The invention involves the illumination of photocatalytic particles with electromagnetic radiation to promote photoexcitation of valence band electrons and holes. These electrons and holes migrate to the surface of the catalytic particles and participate in reduction/oxidation (redox) reactions with adsorbed species.

In a preferred embodiment, the photocatalytic material comprises a semiconductor photocatalyst, for example, a metal oxide such as SnO₂, TiO₂, Nb₂O₅, Ta₂O₅, WO₃, ZrO₂, BaTiO₃ or SrTiO₃. For example, titanium dioxide absorbs ultra violet radiation having a wavelength of about 400 nm or less, to thereby excite the electrons and holes thereof. When the resultant electrons and holes reach the surfaces of titanium oxide particles, the electrons and holes are combined with adsorbed molecules of the fuel gas and/or oxidant gas, thereby generating various radicals.

An advantage provided by the use of photocatalytic material is that complex organic molecules, such as hydrocarbons, as well as hydrogen, may be used as fuel without any pre-conditioning, thus simplifying fuel preparation and improving the robustness and efficiency of the catalyst and PEM fuel cell. For example, a fuel comprising a mixture of methanol (CH₃OH) and water would be converted into carbon dioxide at the anode catalyst, with the release of protons and electrons.

The photocatalytic particles may be embedded in or otherwise integral with a matrix of electrode material. For example, the electrode material may be formed from transparent material, the photocatalytic particles being embedded therein. Examples of suitable transparent material include any transparent conductor, for example indium tin oxide or a polymer, such as a semiconducting polymer. Alternatively, photocatalytic particles may be formed on the inner surface of the transparent electrode material, for example, by coating, deposition or other suitable technique. In this case, the electrodes are preferably porous to allow the respective one of the fuel and oxidant to reach the photocatalytic particles.

As an alternative to forming the electrodes from transparent material, the electrodes may be formed from a wire mesh or other material having pores that allow the electromagnetic radiation to pass to the photocatalytic material. This can enable expensive electrodes, incorporating catalytic material such as palladium and platinum, to be replaced by cheaper electrodes, such as metallic wires coated with titanium dioxide.

Whilst sunlight can provide a suitable source of electromagnetic radiation, the cell preferably comprises means for irradiating the photocatalytic material with electromagnetic radiation, preferably ultra violet radiation. For example, the irradiating means may be selected from the group comprising one or more of a fluorescent lamp, a mercury lamp, a xenon lamp, a halogen lamp, a mercury xenon lamp, a metal halide lamp, a light-emitting diode, and a laser.

The invention also provides a fuel cell stack comprising a plurality of fuel cells as aforementioned.

Preferred features of the present invention will now be described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 illustrates a prior PEM fuel cell;

Figure 2 illustrates a perspective view of an embodiment of a PEM fuel cell according to the invention;

Figure 3 illustrates a cross-sectional view of the fuel cell of Figure 2; and Figure 4 illustrates a cross-sectional view of another embodiment of a fuel cell.

With reference to Figures 2 and 3, an embodiment of a PEM fuel cell 100 comprises a porous PEM electrolyte 102, such as Nation® (manufactured by DuPont), sandwiched between a porous anode 103 and a porous cathode 104. The anode 103 and cathode 104 are preferably formed from non-catalytic materials. The inner surfaces of the anode 103 and cathode 104, that is, those surfaces facing the PEM electrolyte 102, are coated with particles of a photocatalyst 106. In this embodiment, the photocatalyst 106 comprises particles of titanium dioxide, although any other suitable photocatalytic material may be used, for example, SnO₂, Nb₂O₅, Ta₂O₅, WO₃, ZrO₂, BaTiO₃ or SrTiO₃.

The illumination of the photocatalytic particles of TiO₂ with electromagnetic radiation at a wavelength of around 400nm or less promotes photoexcitation of valence band electrons and holes, which migrate to the surface of the particles to participate in reactions with adsorbed species. To illuminate the photocatalytic particles provided on the inner surfaces of the anode 103 and cathode 104 with ultra violet radiation, the cell 100 comprises arrays of light emitting diodes 108, 109, each located on or to the side of, the inner surface of a respective housing plate 110, 111. Alternatively, mercury discharge tubes or any other suitable source of ultra violet radiation may be employed. To enable the ultra violet radiation to impact the photocatalytic particles, the anode 103 and cathode 104 are, in this embodiment, formed from material that is transparent to ultra violet radiation. Examples include indium tin oxide and a semiconducting polymer.

In use, a fuel is supplied to the anode 103 through a first channel extending between the photocatalytically-coated anode 103, housing plates 110 and 112, and oxygen is supplied to the photocatalytically-coated cathode 104 through a second channel extending between the cathode 104, housing plates 111 and 114. In the embodiment illustrated in Figure 2, the fuel comprises hydrogen. In the presence of the illuminated photocatalyst on the surface of the anode 103, hydrogen atoms split into protons and electrons: 2H ₂ - AH + Ae (Equation 1)

Any unused fuel may be recycled to the fuel cell. The PEM electrolyte 102 allows protons to flow therethrough, but not electrons. As a result the protons and electrons proceed along separate paths to the cathode 104. The protons reach the cathode 104 via the electrolyte 102, whilst the electrons create a separate current through an electrical circuit 116.

In the presence of the illuminated photocatalyst 106 on the surface of the cathode 104, the oxygen gas (0₂) splits into oxygen atoms. The negative charge of these atoms attracts the protons passing through the PEM electrolyte 102, where they combine with the oxygen atoms and the electrons from the external circuit to form water molecules:

0₂ + AH + Ae → 2H₂0 _{(Equation 2)}

Thus, the reactions occurring in this embodiment of a fuel cell according to the present invention are the same as those occurring in the prior fuel cell illustrated in Figure 1. One advantage of the fuel cell according to the present invention over the prior fuel cell is that the expensive electrodes of the prior fuel cell, incorporating catalytic material such as palladium and platinum, can be replaced by much cheaper electrodes.

Another advantage provided by the use of photocatalytic material is that complex organic molecules, such as hydrocarbons, may be used as fuel without any preconditioning, thus simplifying fuel preparation and improving the robustness and efficiency of the catalyst and PEM fuel cell. For example, a fuel comprising a mixture of methanol (CH₃OH) and water, in either liquid or vapour state, would be converted into carbon dioxide at the anode catalyst, with the release of protons and electrons as before:

CH₃OH + H₂0 → C0₂ + 6H + 6e _{(Equation 3)}

Again, any unreacted methanol may be recycled to the fuel cell.

In the embodiment shown in Figure 4, the porous transparent material of the anode and cathode 103, 104 is each replaced by a respective mesh of metallic wires having pores or apertures through which the ultra violet radiation passes to the catalysts.

In summary, a proton exchange membrane (PEM) fuel cell comprises a proton exchange membrane, an anode formed on a first surface of the membrane for exposure to a fuel and a cathode formed on a second surface of the membrane for exposure to an oxidant. In order to utilise a hydrocarbon such as methanol as a fuel without pre-conditioning, and thus simplify fuel preparation, the electrodes are coated with a titanium dioxide photocatalyst irradiated by a source of ultra violet radiation.

Claims

C LAI MS

1 . A fuel cell comprising a proton exchange membrane, an anode provided on a first surface of the membrane for exposure to a fuel, an anode catalyst for catalysing the fuel, a cathode provided on a second surface of the membrane for exposure to an oxidant, and a cathode catalyst for catalysing the oxidant, wherein at least one of the catalysts comprises photocatalytic material.

2. A fuel cell according to Claim 1 , wherein the photocatalytic material comprises a semiconductor photocatalyst.

3. A fuel cell according to Claim 1 or Claim 2, wherein the photocatalytic material comprises a metal oxide.

4. A fuel cell according to Claim 3, wherein the metal oxide comprises one of SnO₂, TiO₂, Nb O₅, Ta₂O₅, WO₃, ZrO₂, BaTiO₃ and SrTiO₃.

5. A fuel cell according to any preceding claim, wherein the photocatalytic material is located on the surface of at least one of the cathode and the anode.

6. A fuel cell according to Claim 5, wherein photocatalytic material is located between the cathode and the membrane.

7. A fuel cell according to Claim 6, wherein the cathode comprises material transparent to electromagnetic radiation.

8. A fuel cell according to Claim 7, wherein the cathode comprises material transparent to ultra violet radiation.

9. A fuel cell according to any of Claims 6 to 8, wherein the cathode comprises one of indium tin oxide and a polymeric material.

10. A fuel cell according to Claim 6, wherein the cathode comprises a plurality of pores through which electromagnetic radiation passes towards the photocatalytic material.

11. A fuel cell according to any of Claims 5 to 10, wherein photocatalytic material is located between the anode and the membrane.

12. A fuel cell according to Claim 11 , wherein the anode comprises material transparent to electromagnetic radiation.

13. A fuel cell according to Claim 12, wherein the anode comprises material transparent to ultra violet radiation.

14. A fuel cell according to any of Claims 11 to 13, wherein the anode comprises one of indium tin oxide and a polymeric material.

15. A fuel cell according to Claim 11 , wherein the anode comprises a plurality of pores through which electromagnetic radiation passes towards the photocatalytic material.

16. A fuel cell according to any preceding claim, wherein at least one of the cathode and the anode comprises non-catalytic material.

17. A fuel cell according to Claim 16, wherein at least one of the cathode and the anode comprises carbon or a metal.

18. A fuel cell according to any preceding claim, comprising means for irradiating the photocatalytic material with electromagnetic radiation.

19. A fuel cell according to any preceding claim, comprising means for irradiating the photocatalytic material with ultra violet radiation.

20. A fuel cell according to Claim 18 or Claim 19, wherein the irradiating means comprises one or more from the group comprising a fluorescent lamp, a mercury lamp, a xenon lamp, a halogen lamp, a mercury xenon lamp, a metal halide lamp, a light-emitting diode, a laser.

21. A fuel cell according to any preceding claim, comprising means for exposing the anode to a fuel comprising a hydrocarbon.

22. A fuel cell according to any preceding claim, wherein the fuel comprises methanol.

23. A fuel cell stack comprising a plurality of fuel cells according to any preceding claim.