WO2004035464A2

WO2004035464A2 - Hydrogen fuel cell systems

Info

Publication number: WO2004035464A2
Application number: PCT/GB2003/004508
Authority: WO
Inventors: Rui Chen
Original assignee: Loughborough University Enterprises Limited
Priority date: 2002-10-17
Filing date: 2003-10-17
Publication date: 2004-04-29
Also published as: WO2004035464A3; AU2003271976A1; AU2003271976A8; GB0224204D0

Abstract

A hydrogen generation apparatus, in which a hydride is decomposed by a catalyst to produce hydrogen and waste products, comprises: a) a store of solid hydride material (1); b) dissolution means to dissolve at least part of the solid hydride to produce a hydride solution; c) delivery means to deliver the hydride solution to a catalyst (4) for evolution of hydrogen from the solution and production of a waste solution; and d) waste recovery means to remove waste products from contact with a catalyst (6).

Description

HNdrogen Fuel Cell Systems

This invention relates to fuel cells and is particularly, although not exclusively, related to polymer electrolyte fuel cells.

A fuel cell is an electrochemical cell that can continuously convert the chemical energy of a fuel and an oxidant to electrical energy. Unlike heat engines, fuel cells are not limited by the Carnot cycle and most of the chemical energy in the fuel may be converted into electricity, typically at an efficiency up to 40-60%.

A polymer electrolyte fuel cell, PEFC, is basically a low-temperature electrochemical reactor. It generates electric power by consuming hydrogen as a fuel, and oxygen from air as an oxidant. It has been the most favoured technology by the majority of automotive manufactures due to its high power densities, zero emission potential, mechanical robustness, fast start-up and dynamic response characteristics. The present invention is particularly applicable to PEFC systems but can be used in conjunction with other systems (for example solid oxide fuel cells).

Using fuel cells for vehicular propulsion, one of the major issues that has to be solved is hydrogen storage on-board vehicles. A number of potential H₂ storage technologies have been proposed or used. These include:-

• use of compressed hydrogen at high pressures;

• use of liquefied hydrogen;

• use of hydrogen adsorption onto metal substrates;

• storage and reforming of natural gas, alcohols and hydrocarbons; • use of catalytic reduction of water with metal; and,

• use of slush hydrogen.

The US Department of Energy (DOE) has published a Hydrogen Plan calling for a ratio of stored hydrogen weight to system weight of at least 6.5wt% and a volumetric density of at least 62kg H₂/m . Research results published so far indicated that none of these technologies can reach this standard and all have other potential difficulties involving, for example, safety and/or costs. A much more efficient and low environmental impact H₂ storage technology is needed.

International Patent Application no. WO01/51410 (Millennium Cell, LLC) discloses a hydrogen storage technology using stabilized metal hydride solution as a store for hydrogen in conjunction with a catalyst to release hydrogen from the metal hydride solution. This patent application describes many of the disadvantages to previous storage systems mentioned above. It is also highly relevant towards an understanding of the present invention and is incorporated herein by reference.

The metal hydrides referred to in WOO 1151410 are complex metal hydrides and include, for example, NaBH₄, LiBH₄, KBH₄, NH₄BH₄, (CH₃) NBH₄, NaAlH₄, LiAlH₄, KA1H₄, and mixtures thereof. The present invention is not restricted to these hydrogen-generating materials but is applicable to any hydrogen generating material that can be provided as a solution and that decomposes in contact with a catalyst to generate hydrogen. In the following the invention is described with reference to sodium borohydride, which is a preferred hydride, but the invention is not limited thereto.

Sodium borohydride (NaBH₄) is a white solid stable in dry air up to temperature of 300°C. It decomposes slowly in moist air or in vacuum at 400°C [Encyclopedia of Chemical Engeering, Fourth Edition, V13, 616-624.]. Sodium borohydride is soluble in many solvents including water. It reacts with water to produce hydrogen (H₂) and a by-product sodium borate (NaBO₂):

NaBH₄+2H₂O-»4H₂+NaBO₂

The uncatalysed reaction is slow, but under catalytic conditions the reaction is relatively fast but smoothly controllable [Schlesinger, H. I., et al., "Recent developments in the chemistry of the boron hydrides", Chemical Reviews, vol 31, 1-41, 1942]. About 4.7kg of NaBH₄ can produce 1kg of H₂.

Therefore it has the potential to be used as an efficient hydrogen storage medium provided that the following two questions are addressed:

• Does the ratio of stored hydrogen weight to system weight meet technical requirements (e.g. the DOE Hydrogen Plan figure of greater than 6.5wt%)? • Can the by-product NaBO₂ be disposed of efficiently and in an environmentally safe manner?

Turning first to generation density, NaBH₄ has been proposed as H₂ storage medium for fuel cell applications by a number of researchers. Kaufman et al [Kaufman, C. M., "Catalytic generation of hydrogen from the hydrolysis of sodium-borohydride application in a hydrogen/oxygen fuel cell", Louisiana State University, USA, PhD Thesis, 1981] used acidic catalyst and diluted NaBH₄ solution. The non-recoverable catalyst-was potentially an environmental problem.

Amendola et al (inventors of WOOl/51410) used Ru catalyst, but a diluted solution (e.g. <20%) was confirmed to be essential since much higher concentrations (35% or more) leads to clogging of the catalyst by the NaBO₂ [Amendola, S. C, et al., "An ultrasafe hydrogen generator: aqueous, alkaline borohydride solutions and Ru catalyst", Journal of Power Sources, vol 85, ppl86-189, 2000. and Amendola, S. C, et al., "SUV powered by on-board generated H2", SAE paper 2000-01-1541, 2000.]. The by-product, NaBO₂, thus causes problems. Sufficient water is needed to dissolve and remove it from the catalytic reaction bed. Although potentially not harmful to environment, simple release to the environment by the vehicle is certainly not an acceptable solution. It needs to be accumulated and stored on-board vehicle, and by doing so, more water is needed.

Due to the extra weight of the water used for diluting the NaBH₄ solution, the ratios of stored hydrogen weight to system weight of these existing technologies are much lower than the US DOE requirement. [WOOl/51410 calculates efficiencies ignoring the amount of water that has to be carried in solution].-

So a first problem posed by WOOl/51410 and other documents relating to the use of NaBH₄ as a storage medium is, how does one reduce the amount of water present so as to increase hydrogen generation efficiency without clogging of the catalyst?

WOOl/51410 describes the use of slurries of NaBH₄ as a means of preventing the NaBH₄ from drying out, but the applicants have realised that if the NaBH₄ can be stored as a solid and prepared as a solution as needed, then the amount of water required can be drastically reduced. Further, the need for stabilizing agents will be reduced as the solid is likely to be more stable than a solution.

Accordingly, in a first aspect, this invention provides a hydrogen generation apparatus in which a hydride is decomposed by a catalyst to produce hydrogen and waste products, the apparatus comprising:- a) a store of solid hydride material; b) dissolution means to dissolve at least part of the solid hydride to produce a hydride solution; c) delivery means to deliver the solution of hydride material to a catalyst for evolution of hydrogen from the solution and production of a waste solution; and d) waste recovery means to remove waste products from contact with the catalyst.

This apparatus also provides the means to solve a second problem in that the waste products may then be delivered to a store for waste products that can then be emptied or removed as appropriate.

Having produced waste products there is the need to dispose of them. The second part of this invention relates to recycling the by-product, for example NaBO₂.

Since the discovery of sodium borohydride, two main technologies have been developed and widely used for its industrial production. One is to produce NaBH₄ from sodium hydride and trimethyl borate in a mineral oil medium at a temperature of about 275°C. The other process uses finely ground borosilicate glass, sodium, and hydrogen in atmosphere of H₂ at 300kPa and 400-500°C. Although the production rates for both methods are high, these technologies cannot be directly adopted for NaBO₂ re-circulation.

NaBO₂ is just a simple empirical representation generally accepted for sodium metaborate. It is actually composed of sodium cations and trimeric borate-ring ions in which the boron atom is three-coordinated with oxygen. When a trimeric borate-ring ion is attacked by a highly active negative hydrogen, its electron cloud is shifted so that a new B-H bond is formed and the B-O bond is ultimately cleaved. Therefore, active negative hydrogen provides a means for converting NaBO₂ back into NaBH₄. The negative hydrogen can be formed from the reaction between hydrogen and metal sodium since sodium is an extremely active electron donor. Alternatively, sodium hydride can be directly used to supply the active negative hydrogen. Unfortunately, these agents are generally unstable and expensive. They are not suitable for wider industrial scale to recirculate NaBO₂ back into NaBH₄.

Aluminium is relatively active, but inexpensive, stable and safe to handle. It has the potential to be used as the electron donor for the reaction. In this invention, a new method is provided,, which can be used to convert waste products into hydrides (e.g. NaBO₂ into NaBH₄) at industrial scale with relatively low cost.

In the proposed method, instead of reacting Al with H₂ directly, which is relatively difficult to achieve, a chemically active medium such as NaOH is introduced.

Further aluminium can react with the H₂ to form^' sodium hydride.

2Al+3H₂+3Na₂O->6NaH+Al₂O₃

The produced sodium hydride together with the Al and supplied H₂ reduces NaBO₂ into NaBH₄.

6Al+9H₂+6NaH+6NaBO₂- 6NaBH₄+6NaAlθ₂

The overall reaction is:

10Al+9H₂+6NaOH+6NaBO₂^ 6NaBH₄+6NaAlO₂+2Al₂O3

Theoretical investigation shows that the overall reaction is exothermic. No extra external energy sources are necessary. The reaction conditions are generally milder than other methods. Therefore, the overall production can be a relatively economic industrial process. The final products will include NaBH , NaAlO₂ and Al₂O₃. NaBH₄ is the required H₂ supplier. The other by-products, Al₂O₃ and NaAlO₂ are primary aluminium production materials. They can be recycled by aluminium industry.

As can be seen from the above, this process is particularly applicable to the reduction of sodium borates to produce sodium borohydride but the invention is not limited thereto. Accordingly, the present invention provides in a second aspect a method for the production of complex metal hydrides, the process comprising the step of contacting aluminium, a chemically active medium, and a waste product from oxidation of a complex metal hydride under a hydrogen atmosphere.

The chemically active medium may be a hydroxide.

The closed loop of hydride/waste product generation thus results in the overall conversion of aluminium, hydrogen and fuel cell waste products into a hydride for use in a fuel cell; and the conversion of said hydride in a fuel cell into said fuel cell waste products in the fuel cell.

The invention is illustrated in the following description with^' reference to the attached drawing. Throughout the following reference is made to NaBH₄ but it is to be understood that other hydrides may be used.

The drawing shows a fuel cell and associated hydrogen generating apparatus. This comprises a hydride storage cartridge 1 ; a diluter 2; a heat exchanger 3; a catalytic reactor 4; and electric heater 5; a waste storage cartridge 6; a fuel cell 7; a water storage tank 8; a condenser 9; and a steam separator 11.

In operation, water from water storage tank 8 is metered to the hydride storage cartridge 1 via water delivery valve 15.

The hydride storage cartridge 1 is internally separated by a solution permeable membrane 12. One side 13 of the membrane is used to store the hydride (e.g. NaBH₄) in its solid state, and the other side space is used to contain NaBH₄ solution. [The NaBH₄ solution will normally be saturated but as solubility varies with temperature may on occasion become over saturated or under saturated - in the following it will be referred to as saturated NaBH₄]. The saturated NaBH₄ solution is passed via one-way valve 16 to diluter 2. Flow through the one-way valve is controlled according to the desired H₂ generation rate.

The hydride storage cartridge 1 comprises a water delivery pipe and a solution removal pipe. These pipes may be separate pipes as shown in the drawing or by use of suitable valves can be disposed one within the other so as to provide only a single connection. When disposed one within the other, the water delivery pipe may be inside the solution removal pipe or vice versa as-is convenient.

Extra water from water storage tank 8 is fed into the diluter 2 via dilution water valve 17 and mixed with the saturated NaBH₄ solution to form a much-diluted NaBH solution ready for the decomposition reaction in catalytic reactor 4.

The much-diluted NaBH₄ solution is pumped by pump 18 to pass through the heat exchanger 3 to raise its temperature. Heat for the heat exchanger 3 is provided both by waste water

(typically steam) from the fuel cell 7 and by water (typically steam) extracted from the waste products of the hydrogen generator in a manner described below.

The heat exchanger 3 both raises the temperature of the diluted NaBH₄ solution (which improves the rate of the decomposition reaction in the catalytic reactor 4), and assists in recovering the water produced from the fuel cell 7 by condensing any steam in its exhaust.

High temperature is favourable to the hydrolysis reaction between NaBH₄ and water. Even a slight increase can accelerate the reaction and benefit the H₂ production rate (this dependence upon temperature is discussed in WO 01/51410).

The heat exchanger also serves to reduce the temperature of the exhaust fluid from the fuel cell 7. For reasons given below it is important to condense steam and recover water produced by the fuel cell 7.

After passing through the heat exchanger a wanned diluted NaBH₄ solution enters the catalytic reactor 4. The reactor for the hydrolysis reaction between NaBH₄ and water may be coated with Ru catalyst in like manner to WOOl/51410. A gas conduit 19 delivers the produced H₂ to the fuel cell 7. A waste outlet passes the exhausted solution to waste (NaBO₂) storage. Although the NaBO₂ could be stored as is, that would be inefficient and have a weight penalty. It is therefore preferred to remove water from the waste solution. Accordingly electric heater 5 is used to heat up the solution and steam separator 10 takes the water vapour produced to leave the NaBO₂ at saturated condition for storage. The water used to dissolve and remove NaBO₂ can then be recovered, and passes as steam through reactant waste water conduit 20 for further treatment. Alternative water extraction means can be used (e.g. an osmotic membrane) but a heating step is preferred as this delivers the waste in over saturated condition to the waste storage tank.

The saturated NaBO₂ is accumulated in waste storage cartridge 6. The temperature of the cartridge is ambient. Therefore, the NaBO₂ is accumulated on-board at over saturated condition and will precipitate out to form a slush, a slurry, or a wet solid. The parts where saturated NaBO₂ passes into waste storage cartridge 6 should be arranged so as to prevent precipitation and blockage of the inlet. Heating of these parts may be required.

In the fuel cell 7, which may be any type of fuel cell, although a polymer electrolyte fuel cell is particularly appropriate, hydrogen from gas conduit 19 reacts with air from inlet 21 to produce electricity and, as a waste product, hot water, water vapour, and/or or steam. The hot water, water vapour, and/or steam is removed via fuel cell waste outlet 22. The exhaust fluid from the cell consists mainly of water and nitrogen. The working temperature of a typical PEFC fuel cell is around 90°C although higher temperatures are possible.

Hot water and/or steam from fuel cell waste outlet 22 and reactant waste water conduit 20 pass through the heat exchanger 3 so heating the diluted NaBH₄ solution as discussed above.

The cooled water then passes to a condenser 9 and is pumped via pump 23 back to the water storage tank 8.

The net reaction of hydrogen generator and fuel cell is:-

NaBH₄+ 20₂ -» 2H₂O + NaBO₂

and so there is ample water to maintain the system, while surplus water can be released from the condenser 9 to the environment. Water recovered from both fuel cell exhaust and NaBO₂ solution is stored in the water storage tank and used to provide a source of water to the hydride storage cartridge 1 and diluter 2 in the manner described above.

The cartridge 1 may include one or more contents sensors to indicate when the level of NaBH₄ is below a predetermined level so as to give warning that the cartridge may need replacement. Such sensors may operate-on optical, electronic, acoustic or other principles.

A saturated NaBH₄ sensor may be fitted between the cartridge 1 and the diluter 2, or preferably in the cartridge 1 itself. Such a sensor can be used to determine the amount of saturated NaBH₄ ready for delivery and can be used to control water delivery valve 15 as required.

In the above description the solid NaBH₄ is provided as a cartridge. Of course, other means of supplying the NaBH₄ could be contemplated. The NaBH₄ could be supplied as a loose powder or compressed into a block (in like manner to washing powders).

The NaBH₄ may include stabilizing agents to prevent or reduce degradation of the NaBH₄ from moisture. Such stabilizers are discussed in WOOl/51410 but are less likely to be necessary in the present invention as the hydride is stored as a solid.

The waste storage cartridge 6 may be provided with one or more sensors to indicate the contents of the waste storage cartridge 6 and to give a warning of when it needs removal and replacement.

In the above, the waste has been shown as passing to a waste storage cartridge 6. Of course, this need not be an actual removable cartridge but could simply be a tank with suitable means being provided to permit washing out of the NaBO₂ by, for example, hot water.

Initial theoretical investigation results into the new system show that the system could produce 3.79kg H₂ with an initial weight of total reactants around 30kg and a final weight around 60kg. In other words, the ratio of stored hydrogen weight to system weight can reach the level of 3.79kg/30kg=12.6wt%, and the volumetric density is about 77kg H₂/m³ or more, which are equivalent to about 94% higher on weight ratio and 24% higher in volumetric density than the DOE Hydrogen Plan figures.

In contrast the process of WOOl/51410, which requires the borohydride to be present as a slurry or saturated solution, would have considerably lower ratio of stored hydrogen to weight and volumetric density for a given borohydride due to the need to store water with the borohydride.

Once the borohydride is produced, it can be sent for recycling by reaction with aluminium and a chemically active medium under a hydrogen atmosphere to reform the NaBH₄ in the manner described above.

The hydride solution has been shown as pumped to the catalyst. The hydride can also be delivered to the catalyst by passing it to a tank and immersing the catalyst in the tank when hydrogen generation is required (as in WOOl/51410). It is preferred however to have a flow of the hydride solution over the catalyst to prevent a build up in waste NaBO₂ concentration.

The above description has referred throughout to NaBH₄ as the hydrogen storage medium. Any other hydride with suitable energetics can be used, but NaBH₄ is preferred both because of its high solubility in water (which reduces water storage needs) and because of the innocuous nature of its waste products (NaBO₂) which reduces the environmental risk of the lifetime use cycle from factory, to fuel cell, and back to factory.

Claims

1. A hydrogen generation apparatus in which a hydride is decomposed by a catalyst to produce hydrogen and waste products, the apparatus comprising:- a) a store of solid hydride material; b) dissolution means to dissolve at least part of the solid hydride to produce a hydride solution; c) delivery means to deliver the hydride, solution to a catalyst for evolution of hydrogen from the solution and production of a waste solution; and d) waste recovery means to remove waste products from contact with the catalyst.

2. A hydrogen generation apparatus as claimed in Claim 1, in which the store of hydride material is adjacent a store for hydride solution.

3. A hydrogen generation apparatus as claimed in Claim 1 or Claim 2, in which the store of solid hydride material comprises a removable cartridge containing the solid hydride material.

4. A hydrogen generation apparatus as claimed in Claim 3, in which the cartridge - comprises i) a first chamber storing the solid hydride material ii) a second chamber for storing hydride solution iii) a permeable barrier between the first chamber and the second chamber.

5. A hydrogen generation apparatus as claimed in Claim 3 or Claim 4, in which the cartridge comprises a water inlet pipe and a solution removal pipe disposed one within the other.

6. A hydrogen generation apparatus as claimed in any preceding Claim, in which the dissolution means comprise:- a) means to deliver water to the store of solid hydride material; b) means to remove hydride solution from the store of solid hydride material; c) a diluter to dilute the concentration of the hydride solution to a desired concentration.

7. A hydrogen generation apparatus as claimed in any preceding Claim, in which the delivery means comprise a heat exchanger to warm the hydride solution prior to delivery to the catalyst.

8. A hydrogen generation apparatus as claimed in any preceding Claim, in which the waste recovery means comprise water extraction means to remove water from the waste solution.

9. A hydrogen generation apparatus as claimed in Claim 8, in which the water extraction means comprise means to heat the waste solution and to remove water vapour or steam.

10. A hydrogen generation apparatus as claimed in Claim 9, in which water from the waste recovery means is passed to a heat exchanger to recover heat for heating the hydride solution.

11. A hydrogen generation apparatus as claimed in any preceding Claim, in which the hydride comprises NaBH₄.

12. A hydrogen generation apparatus as claimed in any preceding Claim, in which stabilizing agents are provided to prevent or reduce degradation of the hydride.

13. A fuel cell system comprising a fuel cell and a hydrogen generation apparatus as claimed in any preceding Claim, in which waste water from the fuel cell is recovered to supply water for the hydrogen generation apparatus and hydrogen from the hydrogen generation apparatus supplies fuel for the fuel cell.

14. A fuel cell as claimed in Claim 13, in which the waste water from the fuel cell is passed through a heat exchanger to recover heat for heating the hydride solution.

15. » A cartridge adapted for use in a hydrogen generation apparatus as claimed in any of Claims 3 to 5.

16. A method of generating a hydride suitable for use in hydrogen generation apparatus as claimed in any of Claims 1 to 12, comprising the step of contacting aluminium, a chemically active medium, and a waste product from oxidation of a complex metal hydride under a hydrogen atmosphere.

17. A method as claimed in Claim 16, in which the chemically active medium is a hydroxide.

18. A method as claimed in Claim 16 or Claim 17, in which the waste product from oxidation of a- complex metal hydride is a borate.

19. A method as claimed in Claim 18, in which the borate is NaBO₂, and the hydride is NaBH₄.

20. A method of supplying hydrogen, comprising the steps of supplying a hydride to hydrogen generation apparatus as claimed in any of Claims 1 to 12, recovering waste product from the hydrogen generation apparatus, and regenerating the hydride by the method of any of Claims 16 to 19.