WO2021121777A1

WO2021121777A1 - Gas diffusion layer for an electrochemical cell

Info

Publication number: WO2021121777A1
Application number: PCT/EP2020/081499
Authority: WO
Inventors: Ulrich Berner; Thilo Lehre; Andreas Gehrold
Original assignee: Robert Bosch Gmbh
Priority date: 2019-12-17
Filing date: 2020-11-09
Publication date: 2021-06-24
Also published as: DE102019219776A1

Abstract

A gas diffusion layer for an electrochemical cell is proposed, wherein the electrochemical cell has a semipermeable membrane, and the gas diffusion layer has a fluid-permeable structure and a multiplicity of fibres; and a predominant proportion of the fibres have, at the two ends thereof, at least one spiral-shaped winding, the arrangement and diameter of which is configured, in relation to the fibres, such that piercing of the membrane by individual ones of these fibres during a process of construction of the electrochemical cell with the gas diffusion layer is impeded.

Description

description

title

Gas diffusion layer for an electrochemical cell

The present invention relates to a gas diffusion layer for an electrochemical cell, which comprises a plurality of fibers with spiral turns.

State of the art

Fuel cells or batteries as examples of electrochemical cells are typically used as electrical power sources for the supply of electric motors or machines. Electric drives are increasingly part of a vehicle drive for electric bicycles,

Electric cars, hybrid vehicles and the like.

In contrast to battery-operated vehicles, fuel cell vehicles are characterized by a significantly longer range and faster refueling times.

In the further development of PEM fuel cells in particular, i. H. a polymer electrolyte membrane fuel cell (PEM; for “polymer electrolyte membrane”), for higher power output, the water management, power dissipation and heat conduction in the half-cells of the electrochemical cells are of particular importance to the performance of the fuel cell to optimize.

Disclosure of the invention

The significantly higher sales prices of fuel cell vehicles compared to combustion vehicles are still a major hurdle for them Mass market introduction. One way to achieve the target cost is to improve or simplify components.

A gas diffusion layer of an electrochemical cell typically has a carbon fiber fleece (GDB = gas diffusion backing) on the channel side to a bipolar plate and a microporous particle layer (MPL = microporous layer) on the catalyst side.

The gas diffusion layer has various tasks within, for example, a fuel cell. This includes the substance distribution of oxygen, hydrogen, water, etc., as well as a power line, a heat conduction and a force distribution of contact pressures. The fiber fleece typically used is very permeable to gases in the plane of the gas diffusion layer and helps with the transverse distribution of the media, the heat and the current under the webs of bipolar plates, which divert the current to the outside.

The carbon fiber structures are brittle and can break during the manufacture of the GDL or during the assembly process of the cells or the stack. The resulting fiber ends or the basically existing ends of unbroken fibers from the GDL can protrude from the MPL and consequently penetrate the very thin (<30 μm) and mechanically sensitive catalyst-coated membrane. This leads to irreparable damage to the electrochemical cell and, in a very short time, to failure of the electrochemical cell.

According to the invention, a gas diffusion layer for an electrochemical cell and an electrochemical cell according to the features of the independent claims are specified, which at least in part achieve the stated objects. Advantageous configurations are the subject matter of the dependent claims and the description below.

According to one aspect, a gas diffusion layer for an electrochemical cell is proposed, the electrochemical cell having a semipermeable membrane, and the gas diffusion layer having a fluid-permeable structure and a multiplicity of electrically conductive fibers. This instructs The majority of the fibers have at least one spiral-like turn at their two ends, the arrangement and diameter of which is set up in relation to the fiber in such a way that it is difficult for individual fibers to pierce the membrane when the electrochemical cell is built with the gas diffusion layer.

The semipermeable membrane serves to separate the two electrode spaces of the fuel cell.

Such a gas diffusion layer can electrically and thermally connect an electrocatalytic reaction layer located on the membrane side to the bipolar plate and ensure the media transport to and from the electrocatalytic reaction layer.

Such fibers can be produced according to the basic materials used for the production of the fibers with the spiral-like turns. Carbonized fibers with spiral-like windings can be produced, for example, in that the starting materials used for the carbonization are brought into the desired shape and a canonization process then follows. This is shown, for example, in DOI: 10.5772 / 51125, chapter 3.3 including the references therein and in the Journal of Materials Science 34 (1999) 5519 - 5524.

Such spiral-like structures of the fibers are much less likely to break due to their elasticity. In the event that free fiber ends are present, these fiber ends can yield elastically due to the spiral structure and mechanical damage to adjacent layers, such as the semipermeable membrane, for example, can thereby be almost completely ruled out.

These spiral-like fibers can be mechanically significantly more resistant to bending or compression fractures. Advantageously, the use of fibers with spiral-like windings results in a thermal conductivity and electrical conductivity comparable to, for example, carbon particles within the plane of the gas diffusion layer, but due to the three-dimensionality of the structure in this regard improved properties through the layers, as fewer contact points between structures over a certain amount Distance are necessary. According to one aspect, it is proposed that a predominant proportion of the fibers additionally have spiral-like turns over the entire length of the fiber.

This advantageously creates a three-dimensional structure that can further improve the electrical and thermal conductivity through the gas diffusion layer and, due to the increased porosity, transport processes from and to the membrane-side reaction layer, also in areas of the gas diffusion layer that are covered by the webs of a bipolar plate, in the sense of homogenization , towards an evenly supplied electrode.

According to one aspect, it is proposed that the spiral-like turns are shaped and arranged such that a cavity for the transport of fluids is created within the spiral-like turns.

If during the production of the gas diffusion layer, in particular by selecting the size of the additional carbon particles, it can be ensured that cavities result within the spiral-like fibers, the media can be distributed in an improved manner through these cavities. This applies in particular to the areas of the gas diffusion layer that lie under the webs of a bipolar plate.

According to one aspect, it is proposed that the proportion of fibers with spiral-like windings in the gas diffusion layer is at least 90%. The percentage here relates to a ratio of closed structures with at least one passage to a smooth or non-spiral fiber portion of the gas diffusion layer.

Such a high proportion of fibers with spiral-like windings results in a reduction of damage to the semipermeable membrane of the fuel cell by smooth or non-spiral-like fibers by a factor of 10. By using a correspondingly higher percentage of these fibers with spiral-like windings in the gas diffusion layer damage to the semipermeable membrane can be prevented to an even greater extent. The percentage given therefore relates to a ratio of smooth, non-spiral-like fibers to fibers with spiral-like turns. According to one aspect, it is proposed that the fibers have a diameter between 1 pm and 20 pm, and preferably have a diameter between 2 pm and 10 pm.

According to one aspect, it is proposed that the spiral-like turns have a diameter between 1 μm and 100 μm, and the turns preferably have a diameter between 3 μm and 20 μm. These diameters allow ideal embedding in the gas diffusion layer with a target height of 20 μm to 200 μm, so that a sufficiently high number of fiber layers is stacked to ensure mechanical stability.

According to one aspect, it is proposed that the spiral-like windings have a distance from one another between 0 pm and 100 pm, and the windings preferably have a distance between 0.1 pm and 5 pm. The preferred distance allows the use of typical particle-like carbon materials with particle sizes greater than 5 μm as a further filler for the gas diffusion layer, without these being able to penetrate into the spirals and thus block the fluid transport paths.

According to one aspect, it is proposed that the spiral-like turns have a free inner diameter between 0.2 μm and 100 μm, and the spiral-like turns preferably have a free inner diameter between 1 μm and 10 μm. These diameters allow an ideal fluid transport, since here the passages are sufficiently large that limitation by reaching a pore size in the area of Knudsen diffusion is avoided.

According to one aspect, it is proposed that at least some of the plurality of fibers with the spiral-like windings give the structure of the gas diffusion layer.

The term structuring is to be understood in particular as meaning that the fibers formed in this way build up a macroscopic basic structure with one another, which determines the external shape of the gas diffusion layer. According to one aspect, it is proposed that at least some of the fibers with the spiral-like turns are connected to one another with at least one further fiber by means of a binding agent.

To build up a structure, the respective fibers with the spiral-like windings can be mechanically connected to one another with a binding agent.

Such a binder can be, for example, PTFE. A structure built up in this way can correspondingly replace a carbon fiber fleece made up of elongated carbonized fibers.

According to one aspect, it is proposed that the gas diffusion layer have carbon particles.

By introducing carbon particles into the spiral structures of the fibers, a particle-based microporous layer can be improved with regard to conduction of current and conduction of heat within the gas diffusion layer. The carbon particles are introduced in such a way that they are arranged in the inter-fiber space and leave the intra-fiber space as free as possible.

An overall layer constructed in this way can provide both the function of a microporous layer and the function of an adjacent macroporous layer. A gas diffusion layer constructed in this way thus fulfills both the task of a microporous layer and the tasks of a macroporous layer and can make an additional carbon fiber fleece in the sense of a gas diffusion backing (GDB) obsolete for the construction of a fuel cell.

According to one aspect, it is proposed that the size of the carbon particles and the spatial dimensions of the spiral-like turns of the fibers are determined in such a way that a cavity for the transport of fluids remains free within the spiral-like turns.

In particular, a free inner diameter of the spiral-like fibers can provide a cavity for improved media transport, such as gases and water, within the layer, since an orientation of the spiral-like fibers within the gas diffusion layer can be set during the production of these layers. For this purpose, the characteristic dimensions of the fibers with the spiral-like windings and the dimensions of the carbon particles can be selected in such a way that the inner diameter of the spiral-like fibers remains free.

According to one aspect, it is proposed that the gas diffusion layer is at least partially penetrated by a microporous functional layer in order to provide both a microporous partial area and a macroporous partial area for a structure of the electrochemical cell.

According to one aspect, it is proposed that the plurality of fibers have carbonized starting materials or inorganic ceramics or metals or stainless steel or copper or nickel or titanium or alloys of these metals.

Due to the large selection of materials with which the invention can be implemented, the material suitable for the respective purpose can be selected.

An electrochemical cell is proposed which has a membrane, a catalytic reaction layer and a gas diffusion layer adjoining the reaction layer, as has been described above.

An electrochemical cell constructed in this way can be manufactured more cheaply and have fewer defects, in particular with regard to the semipermeable membrane.

Embodiment

Exemplary embodiments of the invention are illustrated with reference to FIGS. 1 and 2 and are explained in more detail below. It shows:

FIG. 1 shows a fiber which has spiral-like turns over its length; and

FIG. 2 shows a gas diffusion layer made of carbon particles and fibers which have spiral-like windings over their length.

Figures la) to lc) sketch the characteristic dimensions of fibers 100 for a gas diffusion layer 200 for the construction of a electrochemical cell. The figure la) sketches a fiber 100, which has spiral-like turns over its entire length, which are sketched here in an idealized manner symmetrically about a center line. A characteristic length 120 of an individual turn is indicated accordingly.

FIG. 1 b) outlines a single spiral-like turn of a fiber 100, the individual fiber having a thickness 140.

FIG. 1 c) outlines the inner diameter 160 and the outer diameter 180 of a fiber 100 with spiral-like windings from a view along a center line.

FIG. 2 outlines a gas diffusion layer 200 with a characteristic layer thickness 220, which has a multiplicity of fibers 100 with spiral-like windings which are aligned in the gas diffusion layer 200 in the plane of the layer. The sketch of the gas diffusion layer 200 in FIG. 2 does not show a close-up order of an alignment of the fibers; the sketch stands for a random alignment of the fibers with a preferred direction in the plane of the gas diffusion layer.

The gas diffusion layer 200 also has carbon particles 240 and binder 260. The plurality of fibers 100 with spiral-like windings are thus embedded in the gas diffusion layer 200 in a composite with carbon particles 240 and binder 260 and can improve both heat conduction and electrical conductivity within the layer, since they have to overcome fewer interfaces over their length than Adjacent carbon particles 240. In addition, fluid transport can be improved within the layer plane if, due to the characteristic dimensions of the fibers 100 with the spiral-like windings and the sizes of the carbon particles 240, the inner diameter of the spiral-like fibers 100 remains free. A gas diffusion layer 200 constructed in this way thus fulfills both the function of a microporous layer and the functions of a macroporous layer and can make an additional carbon fiber fleece in the sense of gas diffusion backing (GDB) obsolete for the construction of a fuel cell.

Claims

Expectations

A gas diffusion layer (200) for an electrochemical cell, the electrochemical cell having a semipermeable membrane, and the gas diffusion layer (200) having a fluid-permeable structure; and a plurality of electrically conductive fibers (100); and a predominant proportion of the fibers (100) have at least one spiral-like turn at their two ends, the arrangement and diameter (180) of which in relation to the fiber (100) is set up in such a way that individual fibers (100) can pierce the membrane a construction of the electrochemical cell with the gas diffusion layer (200) is made more difficult.

2. Gas diffusion layer (200) according to claim 1, wherein a predominant proportion of the fibers (100) additionally has spiral-like turns over the entire length of the fiber (100).

3. Gas diffusion layer (200) according to claim 2, wherein the spiral-like turns are shaped and arranged such that a cavity for the transport of fluids is created within the spiral-like turns.

4. Gas diffusion layer (200) according to one of the preceding claims, wherein the proportion of fibers (100) with spiral-like windings in the gas diffusion layer is at least 90%.

5. Gas diffusion layer (200) according to one of the preceding claims, wherein the fibers (100) have a diameter between 1 pm and 20 pm, and preferably have a diameter between 2 pm and 10 pm.

6. Gas diffusion layer (200) according to one of the preceding claims, wherein the spiral-like turns have a diameter between 1 pm and 100 pm, and the turns preferably have a diameter (180) between 3 pm and 20 pm.

7. Gas diffusion layer (200) according to one of the preceding claims, wherein the spiral-like turns have a distance (120) from one another between 0 pm and 100 pm, and the turns preferably have a distance between 0.1 pm and 5 pm.

8. Gas diffusion layer (200) according to one of the preceding claims, wherein the spiral-like turns have a free inner diameter between 0.2 pm and 100 pm, and the spiral-like turns preferably have a free inner diameter (160) between 1 pm and 10 pm.

9. The gas diffusion layer (200) according to any one of the preceding claims, wherein at least a part of the plurality of fibers (100) with the spiral-like turns is structuring for the gas diffusion layer (200).

10. The gas diffusion layer (200) according to claim 9, wherein at least some of the fibers (100) with the spiral-like windings are connected to one another with at least one further fiber (100) by means of a binder (260).

11. Gas diffusion layer (200) according to one of the preceding claims, which comprises carbon particles (240).

12. Gas diffusion layer (200) according to claim 11, wherein the size of the carbon particles (240) and the spatial dimensions of the spiral-like turns of the fibers (100) are determined so that a cavity for the transport of fluids remains free within the spiral-like turns.

13. Gas diffusion layer (200) according to one of the preceding claims, which is at least partially penetrated by a microporous functional layer in order to provide both a microporous partial area and a macroporous partial area for a structure of the electrochemical cell.

14. Gas diffusion layer (200) according to one of the preceding claims, wherein the plurality of fibers (100): carbonized starting materials, inorganic ceramics, metals, stainless steel, copper, nickel or titanium or alloys of stainless steel, copper, nickel or titanium.

15. An electrochemical cell which has a membrane, a catalytic reaction layer and a gas diffusion layer (200) adjoining the reaction layer according to one of the preceding claims.