WO2002030810A1

WO2002030810A1 - Hydrogen source for operating a fuel cell and fuel cell provided within said source

Info

Publication number: WO2002030810A1
Application number: PCT/EP2001/011770
Authority: WO
Inventors: Arthur Koschany
Original assignee: Manhattan Scientifics, Inc.
Priority date: 2000-10-12
Filing date: 2001-10-11
Publication date: 2002-04-18
Also published as: AU2002220616A1; DE10050554A1

Abstract

The invention relates to a hydrogen source for operating a fuel cell (1), comprising a hybrid (10) designated as an ionic or chemical hybrid such as, lithium hybrid, for example, which reacts with water and forms hydrogen. The aim of the invention is to prevent uncontrollable, perhaps even explosive operations, whereby the chemical hybrid (10, 21) is bonded to an organic substance (22), particularly a polymer or a fat. Said organic substance (22) should be a hydrophobic substance which does not react with the chemical hybrid (10) and should contain pores as gas passages (23). The above mentioned fuel cell (1) contains a hydrogen source of said type, which is arranged directly in the anode chamber (3) of the fuel cell (1).

Description

Hydrogen source for operating a fuel cell, and fuel cell equipped with it

The invention relates to an apparatus for producing gaseous hydrogen for use as fuel in fuel cells, and more particularly relates to a hydrogen source for a fuel cell, with a chemical hydride which reacts with water to form gaseous hydrogen, and one with such a hydrogen source Equipped fuel cell with an electrolyte that delimits an anode compartment on the one hand and a cathode compartment on the other.

A fuel cell is an electrochemical device for generating electricity. It has an electrolyte, a cathode and an anode. The cathode becomes an oxidizing agent, e.g. B. oxygen, and the anode is a fuel, e.g. B. hydrogen supplied. Fuel cells can be manufactured with a polymer electrolyte membrane (PEM). This is provided on both sides with a catalytically active layer and is located between two gas diffusion layers. It is also possible for the two gas diffusion layers to be provided with a catalyst layer instead of the membrane. At the anode form from the

Hydrogen in the presence of the catalyst Protons that cross the electrolyte and combine with oxygen to water in the cathode-side catalyst layer. This process creates a potential difference between the two catalyst layers, which is used in an external circuit.

It is known that ionic or salt-like hydrides react by hydrolysis with water to form gaseous hydrogen and a hydroxide. Most suitable as hydrogen sources are the binary compounds of hydrogen and alkali or alkaline earth elements, especially othium hydride, sodium hydride or calcium hydride, which are commercially available. Likewise, the ternary compounds of hydrogen with alkali and alkaline earth elements which contain aluminum or boron, especially othium aluminum hydride, lithium borohydride, sodium aluminum hydride or sodium borohydride. These hydrides are referred to below as chemical hydrides, mainly for

Differentiation from metallic hydrides, which are mainly used for the storage of gaseous hydrogen by absorption or which release hydrogen by thermal decomposition. In principle, hydrolysis is also possible with these hydrides, but it takes place very slowly and because of the high atomic masses of the metals, these compounds contain only comparatively little hydrogen per mass. The hydrogen produced during the hydrolysis is very pure and therefore very well suited for use as a fuel in a fuel cell. It contains no organic impurities and no carbon monoxide that would block the action of the catalyst. Since the hydrolysis reaction described is highly exothermic runs, so releases a lot of heat, there is a risk that the hydrogen produced ignites when there is contact with atmospheric oxygen. Therefore, and to avoid excessive gas evolution, appropriate measures must be taken to control the reaction, that is, the amount of water that can react with the hydride must be limited.

A possible control of the reaction is described in U.S. Patents 4,155,712 and 4,261,955. For this purpose, liquid water is stored separately from the chemical hydride used in a reservoir. The Tnnschicht is a porous membrane made of a hydrophobic material that only for

Water vapor is permeable, but not for liquid water. The supply of water to the hydride is therefore only in vapor form, not liquid. The regulation of the hydrogen production takes place via the height of the water level in the water reservoir. A disadvantage of these arrangements is the risk of crack formation in the membrane, which cannot be ruled out, due to strong mechanical loads, as a result of which liquid water can then come into contact with the chemical hydride and trigger uncontrolled reactions. Another disadvantage is that this control method is sensitive to external influences such as tilting of the device or mechanical shocks, since this causes the liquid water in the reservoir to move and the surface of the porous, hydrophobic membrane that is in contact with the water changes constantly. This type of hydrogen generation is therefore unsuitable for mobile applications.

US Pat. No. 5,833,934 describes a process for the production of hydrogen

Hydrolysis of chemical hydrides is disclosed, water being supplied from a water reservoir by means of hydrophilic structures. To control the hydrogen production, the water is displaced by the increasing hydrogen gas pressure during the reaction. A membrane is also used, which is located between the water reservoir and the hydride. She has that here The task is to let water pass through, but to be impermeable to hydrogen. As a result, a pressure can build up in the hydride container, which prevents further water from entering the hydride space. This system is also endangered by the formation of cracks in the membrane, since then there can no longer be a pressure difference between the water reservoir and the hydride space and the water reaches the hydride unhindered.

The invention is intended to provide a largely fail-safe device for generating hydrogen gas. This device should be used directly as a fuel source for a fuel cell.

This is achieved according to the invention in that the chemical hydride is bound in an organic substance; and in that the hydrogen source, ie the bound chemical hydride, is arranged in the anode space in the fuel cell.

Preferred developments of the invention can be found in the respective subclaims.

Further details, advantages and developments of the invention result from the following description of preferred exemplary embodiments with reference to the drawings. Show it:

1: the cross section through a fuel cell according to a first embodiment;

2 shows a cross section through a fuel cell according to a second

Embodiment; 3 shows a cross section through a hydrogen source that is not directly integrated into a fuel cell; 4 shows a cross section through an amount of a bound hydride. 1 consists of a polymer electrolyte membrane 2, a gas-tight anode space 3 with an anodic gas diffusion electrode 4, and a cathode space with a cathodic gas diffusion electrode 5. In the anode space 3 there is a bound chemical hydride 10 and a water reservoir 11 , which is enclosed in a porous, hydrophobic membrane 12. The fuel cell has current leads 15 from the anode and cathode and the anode compartment 3 has a pressure relief valve 16. The anode compartment 3 is encapsulated in a housing 19.

As the chemical hydride 10, which is placed directly in the anode compartment 3 of the cell for use as a hydrogen source for the fuel cell, lithium hydride, utium aluminum hydride or lithium borohydride is preferably used.

The hydride 10 reacts in the hydrolysis with the liberation of hydrogen

Water that is supplied in the fuel cell 1 according to FIG. 1 in the form of water vapor. The water vapor is generated by evaporation from the reservoir 11 with liquid water at a temperature below the boiling point of the water.

The water is supplied in the form of water vapor from the water reservoir 11 through the membrane 12, which is a porous, hydrophobic membrane, preferably made of PTFE. The reservoir 11 is thus realized in that a certain amount of water is enclosed in the porous, hydrophobic PTFE membrane 12 through which the water vapor can diffuse to the hydride 10. The possibly increased water temperature in the reservoir 11 results from the operating temperature of the fuel cell and the heat development of the hydride-water reaction.

The advantage of placing the chemical hydride 10 in the anode compartment is in the possibility of a compact, multi-room construction of a unit consisting of a hydrogen source and a fuel cell.

As stated above, the supply of water to the chemical hydride must be restricted to control the hydrolysis reaction. For this, as in Fig.

4 schematically shows the chemical hydride 10 in the form of powder particles 21 which are bound in an organic substance 22. The term binding of the chemical hydride 10 here means that the particles 21 of the powdered chemical hydride, which preferably have a size of between 1 and 100 μm, particularly preferably between 10 and 50 μm, are enveloped by the organic substance 22 and are held together , The organic substances used are hydrophobic and do not react with the chemical hydride.

The organic substance 22 with which the hydride 10 is bound can be a

Polymer. For example, silicone rubber, polyethylene or epoxy resin are suitable as possible polymers. The hydride bound with the polymer is a porous material such that the hydride particles 21 are surrounded by the polymer 22 and passages 23 remain free between the resulting grains, through which gas, in particular water vapor and

Hydrogen can diffuse. These passages arise from the initial evolution of gas caused by the reaction of the hydride with contaminants of the uncured polymer or with the moisture in the ambient air that takes place before the polymer envelops the hydride particles. This gas evolution can also be achieved by adding a blowing agent. The water vapor must therefore diffuse through the gas passages in the polymer and through a relatively thin covering layer consisting of the polymer material in order to be able to react with the hydride. The cladding layer represents the main diffusion barrier and the gas passages ensure that the hydrogen is uniform to the maximum possible surface of the coated hydride particles arrives.

1, the water vapor originates from the reservoir 11, which is separated from the hydride 10 by the porous, hydrophobic membrane 12, the bound hydride offers the advantage that if there is a crack in the membrane, no liquid water is in direct contact with the hydride comes what could trigger an uncontrolled, explosive development of hydrogen and thus a strong pressure increase in the anode compartment 3, which can destroy the fuel cell. Furthermore, the binding with the polymer counteracts an increase in volume during the conversion of the hydride into hydroxide. Mechanical stability and better machinability or formability are also advantageous.

The solid, powdered chemical hydride can also be bound by being distributed in a viscous or liquid organic substance. Such organic substances include, for example, silicone grease or

Silicone oil, paraffin or paraffin oil in question. Here too, the hydride particles 21 are enveloped by the organic substance 22. This organic too

Substance has the property of not reacting with the hydride and being highly hydrophobic. Therefore, no liquid water in direct

Contact with the chemical hydride when using this mixture

Water comes into contact. In order that water vapor can penetrate through diffusion, the mixture is used as a thin layer, or a porous substance is introduced into the mixture which is not wetted by the organic substance, so that gas passages arise.

As an alternative or in addition to the supply from the reservoir 11 shown in FIG. 1, the product water which arises during operation of the fuel cell can also be used to react with the chemical hydride 10 and to form hydrogen. The product water diffuses out the cathode compartment, where it is created, through the water-permeable polymer electrolyte membrane 2 of the fuel cell 1 into the anode compartment 3. Even if the water vapor is provided by the product water of the fuel cell, the bound hydride offers a high level of safety since it does not cause spontaneous ignition comes even if the membrane electrode

Unit of the fuel cell is injured and liquid water and oxygen can penetrate from the outside.

Likewise, the humidity of the ambient air can be used to supply water, which, like the product water from the cathode compartment, which is in contact with the ambient air, diffuses through the membrane 2 into the anode compartment 3. This possibility is preferably suitable for starting the hydrogen development if an arrangement is used in which only the product water of the fuel cell is used for the water supply. Because here no hydrogen and therefore no product water generated by the fuel cell, which is required for the production of the hydrogen, is produced until water is supplied to the hydride 10 from the outside.

Another option for water supply is so-called "dry water". It consists of a hydrophobic powder made of synthetic silica, such as hydrophobic AEROSIL R972 from Degussa-Hüls AG, which can absorb a large proportion of water, but which retains its powdery consistency when the water is broken down into fine droplets in the presence of AEROSIL. The hydrophobic silica envelops the drops and prevents them from coming together, which leads to a powdery substance. This powder is placed in the anode compartment 3 and the water vapor evaporating therefrom can react with the chemical hydride 10 to form hydrogen.

In an example according to FIG. 2, the water reserve voir 11 separated from the bound chemical hydride 10 by a porous, hydrophobic membrane 24 forming part of the reservoir housing and is connected to the water in a reservoir 25. In the reservoir 25 there is a stamp 26 with a spring 27 which exerts pressure on the water surface.

The bound chemical hydride can be used in conjunction with any of the water supply methods mentioned above as a hydrogen source that is not directly integrated into a fuel cell. Figure 3 shows such an arrangement. Here is the bound chemical hydride

10 in a gas-tight container 31, from which the hydrogen can be removed through a valve 32. As in FIG. 2, it is separated from the water reservoir 11 by the porous, hydrophobic membrane 24, which is connected to the reservoir 25. Due to increasing gas pressure in the gas-tight container 31, the water under the membrane 24 is pressed back into the reservoir 25. Thus, water vapor can no longer diffuse through the membrane 24 and the hydrogen evolution is interrupted. The hydrogen can be removed through the valve 32.

In the case of such a hydrogen source, the moisture in the ambient air can also be used for the possible additional supply of water by placing the bound chemical hydride in a container in which at least one wall is formed by a membrane which is permeable to water vapor.

Further details and embodiments of the hydrogen source according to the invention are described below by way of example.

Example 1:

Lithium hydride (LiH) is used as chemical hydride, which works well with the two components of the S-Uikon rubber SYLGARD 170 from Dow Corning GmbH are mixed. The proportion of lithium hydride in the mixture is between 20 and 80 percent by weight, preferably between 30 and 50 percent by weight. This mixture is filled into a mold and cured at an elevated temperature. The temperature is preferably between 100 and 180

Centigrade. Water is enclosed in a porous PTFE membrane to supply water. For this purpose, a bag made of PTFE film is filled with a stoichiometric amount of water and sealed. The PTFE film has a polypropylene carrier structure that can be melted and thus seals the bag watertight. The bonded lithium hydride 10 and the PTFE

Water reservoirs 11 are placed in the anode space 3 of the fuel cell 1, which is sealed gas-tight with the polymer electrolyte membrane 2 of a membrane electrode unit. Power is taken from the fuel cell via suitable current leads 15 on the cathode and anode. In order that no overpressure builds up when no power is drawn from the fuel cell, the anode compartment has the overpressure valve 16. The arrangement is shown in FIG. 1.

Example 2:

Here, polyethylene powder is used as an organic substance, in which it is bound to hydride. The PE powder and the lithium hydride are mixed well. The proportion of the I-itMu m hydride in the mixture is preferably between 20 and 80 percent by weight, particularly preferably between 30 and 50 percent by weight. This is filled into a mold and at elevated

Pressed temperature so that the polyethylene melts and binds the lithium hydride on subsequent cooling. The temperature is preferably between 100 and 220 degrees Celsius. The pressure exerted is preferably 100 to 200 bar. Liquid water in the anode compartment 3 of the fuel cell 1, which is connected to the storage container 25 outside the anode compartment, is used for the water supply, as shown in FIG. The PE-bonded lithium hydride is separated from the water in the anode compartment 3 by the porous PTFE membrane 24 stretched in the anode compartment, through which the water vapor diffuses to the hydride. Since the anode space 3 is gas-tight, the water is pressed back into the storage container 25 if the hydrogen production is too high due to the increased hydrogen gas pressure, as a result of which water vapor can no longer diffuse through the PTFE membrane 24. In order to ensure that the control mechanism functions independently of its position in the room, that is to say is not influenced by tilting, pressure is exerted on the water in the reservoir 25. The pressure is such that no liquid water is forced through the porous PTFE membrane 24. The stamp 26 with the spring 27, which presses on the water surface in the reservoir 25, can serve as a device for generating this pressure, for example.

Example 3:

The silicone grease BAYSILONE from Bayer AG is used here as a viscous, organic substance for binding -thium hydride. Silicone grease and lithium hydride are mixed well. The proportion of lithium hydride in the mixture is between 20 and 50 percent by weight. This results in a viscous mass which can be filled as a thin layer in the fuel cell arrangements of Examples 1 or 2.

Claims

claims:

1. Hydrogen source for the operation of a fuel cell (1), with a chemical hydride (10) with water to form gaseous

Hydrogen reacts, characterized in that the chemical hydride (10, 21) is bound in an organic substance (22).

2. Hydrogen source according to claim 1, characterized in that the organic substance (22) is a hydrophobic, not with the chemical

Hydride (10) reacting substance.

3. Hydrogen source according to claim 1 or 2, characterized in that the bound hydride (10) contains pores as gas passages (23).

4. Hydrogen source according to one of claims 1 to 3, characterized in that the organic substance (22) is a polymer.

5. Hydrogen source according to claim 3, characterized in that the organic substance is liquid or viscous.

6. Hydrogen source according to one of claims 1 to 5, characterized in that the chemical hydride in powder form (21) with a particle size of 1 to 100 microns, preferably from 10 to 50 microns, is present.

7. Hydrogen source according to one of claims 1 to 6, characterized in that the weight ratio of the chemical hydride (10) to the organic substance (22) is in the range from 20:80 to 80:20, preferably from 30:70 to 50:50 ,

8. Hydrogen source according to one of claims 1 to 7, characterized in that the chemical hydride (10) is lithium hydride, sodium hydride, calcium hydride, UtMumaluminiumhydrid, -tithiumborhydrid, sodium aluminum hydride or sodium borohydride; and / or that the organic substance (22) is silicone rubber, polyethylene, epoxy resin, silicone grease, silicone oil, paraffin or paraffin oil.

9. Fuel cell (1) with an electrolyte (2) which on the one hand delimits an anode space (3) and on the other hand a cathode space, characterized in that a hydrogen source according to one of the claims

1 to 8 is arranged in the anode compartment (3) of the fuel cell (1).

10. Fuel cell according to claim 9, characterized in that for the use of the product water of the fuel cell in operation (1) and / or the ambient humidity at least as part of the water supply for the chemical hydride (10) of the electrolyte (2) a water leakage enabling Solid electrolyte is.

11. Fuel cell according to claim 9 or 10, characterized in that for the use of the moisture in the ambient air at least as part of the water supply for the chemical hydride (10) it is encapsulated by a moisture-permeable housing (19).

12. Fuel cell according to one of claims 9 to 11, characterized in that the hydrogen source in the fuel cell (1) is interchangeable.