WO2022002932A1

WO2022002932A1 - Gas diffusion layer for a fuel cell having properties which vary along an areal extent

Info

Publication number: WO2022002932A1
Application number: PCT/EP2021/067839
Authority: WO
Inventors: Harald Bauer
Original assignee: Robert Bosch Gmbh
Priority date: 2020-07-02
Filing date: 2021-06-29
Publication date: 2022-01-06
Also published as: DE102020208276A1; EP4176477A1

Abstract

The invention relates to a gas diffusion layer for a fuel cell, the gas diffusion having an areal extent and a layer thickness measured perpendicular to the areal extent, and the gas diffusion layer having predefined physical properties which comprise at least a permeability and a hydrophobicity. According to the invention, at least one of the physical properties changes in at least one direction along the areal extent from a starting value in a starting region to an end value in an end region.

Description

description

Title:

Gas diffusion layer for a fuel cell with variable properties along a surface extension

The present invention relates to a gas diffusion layer for a fuel cell, a fuel cell with such a gas diffusion layer and a method for producing a gas diffusion layer.

State of the art

PEM fuel cell stacks usually comprise a plurality of stacked fuel cells, each of which has a hydrophilic membrane with a catalyst layer on both sides and adjoining hydrophobized gas diffusion layers which are in contact with a flow field for distributing educts. Due to the gas diffusion layers, the membrane can, on the one hand, have a certain humidity and, on the other hand, gaseous starting materials can reach the catalyst layer of the membrane in the opposite direction. In the electrochemical process carried out, hydrogen reacts with oxygen and an electrical voltage is provided at the electrodes of the fuel cells. This creates water at the cathode. If not all of the water vapor is removed, water can condense under unfavorable conditions. The fuel cell in question could be flooded locally as a result and the educts can no longer come into contact with the catalyst, at least locally, due to the water barrier created as a result. The performance of the cell is reduced locally and thus the overall performance of the system is reduced. Such flooded areas could be zones or starting points for degradation effects, such as corrosion. To prevent excessive condensation, gas diffusion layers made of a dense hydrophobic particulate layer can be arranged on a carbon fiber fleece and placed between the flow field and the membrane with its catalyst layer, so that gas and water vapor are always allowed through and still sufficient liquid water is held on the membrane, with additional required or excess liquid water is passed through. The gas diffusion layers can also be used as a gas diffusion electrode for diverting the generated current.

Educts that are fed to the membrane via the flow fields and via the gas diffusion layer always require a certain pressure to overcome the flow resistance along the entire flow path. The local pressure over the membrane is not constant over the area even with constant operating parameters. A method is known for producing gas diffusion layers, in particular the particulate hydrophobic layer, in which integrally optimized properties are to be set. Either statistically uniform properties (e.g. cracks, pores, holes) are set in the area, or a gradient profile perpendicular to the area is provided.

A gas diffusion layer or a gas diffusion electrode must, on the one hand, hold a certain amount of liquid water on the membrane and, on the other hand, remove the excess water produced in the fuel cell process and allow the gaseous reactants to reach the membrane. This can be done with a porous structure.

Disclosure of the invention

The object of the invention is therefore to propose a gas diffusion layer with which local condensation can be avoided, but at the same time the desired moisture content of the membrane is maintained and the supply of the starting materials is not hindered.

The object is achieved by a gas diffusion layer for a fuel cell with the features of independent claim 1. Advantageous embodiments and further developments can be found in the subclaims and the following description.

A gas diffusion layer for a fuel cell is proposed, the gas diffusion layer having a flat extent and a layer thickness measured perpendicular to the flat extent, and the gas diffusion layer having predetermined physical properties that include at least one permeability and one hydrophobicity. According to the invention, it is provided that at least one of the physical properties changes from an initial value in an initial area to an end value in an end area spaced therefrom in at least one direction along the two-dimensional extent.

A core of the invention is therefore not to keep the determining physical properties of the gas diffusion layer with regard to the accumulating water constant in the area of the respective fuel cells, i.e. in the cell plane perpendicular to a stacking direction. They therefore change essentially with the pressure profile along the flow field, so that, despite the pressure differences, the functionality of the gas diffusion layer, in particular for the retention capacity of liquid water and the permeability of gaseous educts, is almost the same everywhere. The gas diffusion layer can thus be loaded equally everywhere in order to achieve high performance and low degradation with high power density in a stacked system.

As mentioned above, the physical properties include at least the permeability and the hydrophobicity. The hydrophobicity can have a comparatively low value where there is still no water. In an area opposite this, in which the greatest amount of water can be expected, it makes sense to select a higher hydrophobicity. The hydrophobicity can be achieved, for example, by a content of water-repellent material incorporated therein that changes along the extent of the surface. This material can include, for example, PTFE. The use of the hydrophobic material can prevent or reduce the excess water. The permeability, which can be controlled, for example, by the porosity of the gas diffusion layer, can also allow adaptation to the pressure curve. In an area in which there is a high pressure in the flow field, a lower permeability can be provided than in an area with low pressure. With an ideal adaptation of the permeability to the pressure curve, a uniform volume flow can be achieved over the entire surface of the gas diffusion layer.

In the case of an applied microporous layer, its thickness can be influenced, which among other things also leads to a changing local depth of penetration into the carrier layer. The microporous layer could be applied locally differently by means of multi-layer printing. The electrical contact resistance, the electrical volume resistance, the wetting angle and the hydrophobicity can be adjusted, for example by using different local densities or contents of a hydrophobizing material. The microporous layer could be printed onto the carrier layer, with different pore shapes and thus variable properties being able to be produced by printing different areas with different contents of hydrophobing material, uneven drying, different masses or the like. When using a laser, the porosity can also be adjusted by means of a variable drilling pattern.

In an advantageous embodiment, the gas diffusion layer has a carrier layer made of a fiber fleece, in which at least one of the hydrophobicity, layer thickness and fiber density changes along the two-dimensional extent. The fiber fleece can have carbon fibers. The hydrophobicity can be achieved through the use of a hydrophobic material that can be integrated into the carrier layer. Influencing the layer thickness and the fiber density allow the permeability to be adjusted via the two-dimensional expansion of the gas diffusion layer. For example, the carrier layer could have a base layer on which several sections of nonwovens with different fiber densities are applied next to one another, so that as a result, a fiber density profile can be realized overall. The local thickness and the local fiber density can control the permeability.

It is advantageous if a local content of a hydrophobizing material changes in or on the carrier layer along the two-dimensional extent. The hydrophobic material, such as PTFE, leads to a contact angle of more than 90 ° with respect to water, so that a water-repellent effect is achieved. The local PTFE content or the lack of water-repellent material can be adjusted via the local variation of the properties of the carrier layer.

It is also advantageous if the gas diffusion layer has a microporous layer in which at least one of the hydrophobicity, layer thickness and porosity changes along the two-dimensional extent. Such a microporous layer can be applied to an aforementioned carrier layer or provided as a separate layer. The microporous layer can comprise one or more layers of carbon allotropes. The hydrophobic properties of the microporous layer can also be influenced by the selection of the carbon allotropes.

The microporous layer could also be arranged on the carrier layer and at least partially penetrate into the carrier layer. Consequently, the gas diffusion layer has several layers which form a coherent mechanical unit. The adaptation of the physical properties can be achieved by combining different measures. The carrier layer could have a changing permeability, while for example the microporous layer can have different particle sizes or particle densities over the two-dimensional extent, so that the porosity changes. The carrier layer and / or the microporous layer can each be interspersed with a hydrophobizing material, its density or the local content also being able to change over the two-dimensional extent.

The properties of the carrier layer are preferably largely constant and the determining properties such as hydrophobicity and permeability are essentially set by the microporous layer. When building the The properties of both layers are superimposed on the gas diffusion layer from the carrier layer and the microporous layer. The microporous layer can accordingly generate the essential, pressure-dependent adaptations of the gas diffusion layer even with a constant structure of the carrier layer.

It is also advantageous if the microporous layer has a penetration depth that changes along the two-dimensional extent. This also allows at least the permeability and hydrophobicity to be set.

It is advantageous if the change in the at least one physical property takes place at least in several stages. The gas diffusion layer can consequently have a plurality of sections in which the corresponding physical properties are in each case constant. This can simplify production. However, it is also conceivable that the properties change continuously over the area. An even better adaptation to the pressure curve to be expected could thus be achieved. For example, a simplified approach to generating a gas diffusion layer can be used in which the fuel cell in question is thought to be subdivided into several small fuel cells, each of which has a gas diffusion layer with constant properties. The division could be made into 2, 3, 4, 5, 6, 7, 8, 9, 10 or more individual imaginary fuel cells.

However, the change in the at least one physical property preferably takes place continuously.

The invention further relates to a fuel cell with a membrane which is surrounded by two gas diffusion layers according to one of the preceding claims, which are in fluid connection with the flow fields surrounding the gas diffusion layers.

It is particularly preferred if the at least one of the physical properties changes as a function of a change in pressure in each case along a flow channel of the flow fields. Depending on the selected shape of the flow channel, the pressure change, ie in particular the pressure drop, meander-shaped, spiral-shaped, in strips or in some other way over the flow field. The properties of the gas diffusion layer are adapted to this. The starting area is a region of highest pressure from which the flow channel extends to an end region as a region of lowest pressure. The pressure changes along the distance covered and the intended properties of the gas diffusion layer change accordingly.

A single web can be produced, for example, by means of film extrusion or compression of the mass with a rolling mill or film drawing, as is the case with lithium-ion battery anodes (carbon particles + binder polymer), which have this profile of properties. A fluidizing agent can be used for the production, or a solvent for a binder polymer or the like dissolved therein.

Further measures improving the invention are shown in more detail below together with the description of the preferred exemplary embodiments of the invention with reference to figures.

Working examples

It shows:

1 shows a schematic representation of a fuel cell stack, FIG. 2 shows a plan view of a flow field, FIGS. 3 to 5 different gas diffusion layers.

1 shows a fuel cell stack 2 in a simplified, schematic sectional illustration. The fuel cell stack 2 is delimited at opposite ends by an end plate 4. An electrical voltage is provided at the end plates 4. A plurality of fuel cells 6 are arranged between the end plates 4, each of which is designed as a membrane electrode assembly (MEA). These have a membrane 8 to which electrodes on both sides 10 connect, each of which has a gas diffusion layer 12 and a catalyst layer 14. As indicated in the detailed view, the gas diffusion layer 12 could comprise a carrier layer 16 in the form of a carbon fiber fleece with a microporous layer 18 arranged thereon. The carrier layer 16 could have a uniform thickness in a plane parallel to the membrane 8. The microporous layer 18 can partially penetrate into the carrier layer 16. It has, for example, particles from a carbon allotrope, such as graphite. The gas diffusion layer 12 has a layer thickness d which is preferably constant.

Bipolar plates 20 are arranged between the individual fuel cells 6, each of which has a flow field 22 on both sides with a series of flow channels 24 which are provided for supplying starting materials and removing reaction products. The flow channels 22 are in fluid connection with the respective membrane 8 via the respective gas diffusion layer 12.

In FIG. 2, the flow field 22 is shown with an exemplary meandering flow channel 24. Of course, the explanations relating to the gas diffusion layer 12 are also possible for other shapes of the flow channel 24. These could be designed as a comb distributor, with a band structure, a spiral structure or as a counter-rotating gas distributor. An educt is introduced into the flow field 22 in a first area 26 arranged at the top of the plane of the drawing the second region 28 arranged at the bottom of the plane of the drawing leave the flow field 22. To ensure the flow through the entire flow channel 24, a sufficient initial pressure in the first region 26 is necessary. Depending on the geometric properties of the flow channel 24 and the extraction through the respective fuel cell 6, a certain flow resistance is to be expected. As a result of this, the local pressure of the educt drops continuously from the first area 26 to the second area 28. The final pressure at the end of the flow channel 24 in the second region 28 is then more or less clearly below the initial pressure. Due to the concise pressure profile, excessive condensation could occur locally on the membrane 8, where the pressure and temperature are below a saturated steam curve. In order to avoid this, so that a free flow of educts to the membrane is possible, the same pressure would have to be set everywhere at a constant temperature within a fuel cell via the two-dimensional extension of the flow field 22. Due to the flow resistance, this is only conceivable for very small fuel cells with a generous flow field under laboratory conditions.

According to the invention, the gas diffusion layer 12 is designed in such a way that the physical properties of the gas diffusion layer that determine the water balance and permeability are not constant in the area of the respective fuel cells, but are adapted to the pressure profile of the flow field 22. They change with the pressure profile along the flow channel 24. Where a lot of water could condense out, it is advantageous to set the hydrophobicity of the gas diffusion layer 12 to be particularly high. In places where little or no water could condense out, a lower level of water repellency would make sense. In locations with high pressure, the gas diffusion layer 12 could be less gas-permeable, i.e. permeable, than in locations with lower pressure. Overall, the retention capacity of liquid water and the permeability of gaseous starting materials can then be approximately the same everywhere along the gas diffusion layer 12, despite the pressure differences.

A simplified example of a gas diffusion layer 12a is shown in FIG. 3. The areal extent can be seen here, through which flow paths 30 lie in a direction vertical to the areal extent. The flow paths 30 could be visible as perforation openings, for example. The gas diffusion layer 12a can overall have a porous surface, so that the illustration in FIG. 3 is to be understood schematically. As an alternative to this porous design, however, gas diffusion layers made of a solid material, which are provided with individual bores 30, could also be used.

The gas diffusion layer 12a has a higher density of flow paths 30 at the upper end in the plane of the drawing, which consequently results in a higher density Porosity or a higher permeability leads. At the lower end, the density of flow paths 30 is significantly lower. Between the two ends, the permeability changes either steadily, ie continuously, or in stages. As a result, the lower end can be placed on the first region 26 of the flow field 22. This area of the gas diffusion layer 12a is called the starting area 27 here. The upper end, which here represents an end area 29, should, however, be placed on the second area 28. The permeability can largely follow inversely proportional to the pressure in the flow channel 24, so that an at least largely uniform volume flow is established over the entire flow channel 24 through the gas diffusion layer 12 to the membrane 8. The course can run in strips from the starting area 27 to the end area 29. In finer gradations, the course could also be set in a meandering shape or at least continuously in one plane direction.

The variable permeability could also be achieved by a changing depth of penetration of the microporous layer 18 shown in FIG. 1 into the carrier layer 16, as indicated in FIG. 4 with a gas diffusion layer 12b. Here the depth of penetration of the microporous layer 18 is greater at the upper end in the plane of the drawing than at the lower end. It is conceivable that the thickness of the microporous layer increases with the depth of penetration, so that the gas diffusion layer 12b overall has a constant thickness. With a higher penetration depth, the permeability could be reduced. In the case of the greater penetration depth, the starting area 27 is formed. In the case of the lower penetration depth, the end region 29.

5 shows a gas diffusion layer 12c into which a hydrophobing material 32 is integrated, for example PTFE. The density or the content of the hydrophobic material 32 changes here along the two-dimensional extent. With the higher density or the higher content, a higher water entry could be countered, so that the density or the content of the hydrophobic material 32 can be selected largely inversely proportional to the pressure curve. The starting area 27 is consequently with the lower content of the hydrophobizing material, the end area 29 with the higher content of the hydrophobizing material 32. The invention is not restricted to the exemplary embodiment discussed above. Rather, modifications are also conceivable which are included in the scope of protection of the following claims. In connection with the invention, for example, a production is also possible that geometrically combines the fiber layer (carbon backbone) and the PTFE particle layer (MPL) with respect to a web and, in total, has the same property profile of previously known gas diffusion layers. This path can also be provided with the property gradient described here. A gas diffusion layer is produced in the same way as a battery carbon anode by coating it with solvent on a base, drying it and then detaching it from the base, then pressing it together as a free-standing web. These layers can contain fibers. In this case, not only PTFE is processed, but other binder polymers can be used, which affects the variation in the PTFE content and thus the setting of the

Wetting properties significantly simplified. The production of such tracks with a gradient is particularly simple since the mixture can be varied in terms of time or location during production, for example the gradient in the direction of the track.

Claims

Expectations

1. Gas diffusion layer (12) for a fuel cell (6), wherein the gas diffusion layer (12) has a planar extent and a layer thickness (d) measured perpendicular to the planar extent and wherein the gas diffusion layer (12) has predetermined physical properties which at least one Permeability and hydrophobicity, characterized in that at least one of the physical properties changes from an initial value in an initial region (27) to an end value in an end region (29) spaced therefrom in at least one direction along the two-dimensional extent.

2. Gas diffusion layer (12) according to claim 1, characterized in that the gas diffusion layer (12) has a carrier layer (16) made of a fiber fleece, in which at least one of the hydrophobicity, layer thickness and fiber density changes along the two-dimensional extent.

3. Gas diffusion layer (12) according to claim 2, characterized in that a local content of a hydrophobizing material (32) changes in or on the carrier layer (16) along the two-dimensional extent.

4. Gas diffusion layer (12) according to one of the preceding claims, characterized in that the gas diffusion layer (12) has a microporous layer (18) in which at least one of the hydrophobicity, layer thickness and porosity changes along the two-dimensional extent.

5. Gas diffusion layer (12) according to claim 2 or 3 and 4, characterized in that the microporous layer (18) is arranged on the carrier layer (16) and at least partially penetrates into the carrier layer (16).

6. Gas diffusion layer (12) according to claim 5, characterized in that the microporous layer (18) has a penetration depth which changes along the two-dimensional extent. 7. Gas diffusion layer (12) according to one of the preceding claims, characterized in that the change in the at least one physical property takes place at least in several stages.

8. Gas diffusion layer (12) according to one of the preceding claims, characterized in that the change in the at least one physical property takes place continuously.

9. Fuel cell (6) with a membrane which is surrounded by two gas diffusion layers (12) according to one of the preceding claims, the flow fields surrounding the gas diffusion layers (12) in fluid communication

(22) stand.

10. The fuel cell (6) according to claim 9, characterized in that the at least one of the physical properties changes as a function of a pressure change in each case along a flow channel (24) of the respective flow field (22).