WO2018224448A1

WO2018224448A1 - Gas diffusion layer

Info

Publication number: WO2018224448A1
Application number: PCT/EP2018/064649
Authority: WO
Inventors: Ruben DE BRUYCKER; Frank De Ridder; Davy Goossens; Kris SYNHAEVE; Jérémie DE BAERDEMAEKER
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 2017-06-07
Filing date: 2018-06-04
Publication date: 2018-12-13
Anticipated expiration: 2019-12-07

Abstract

A gas diffusion layer for the cathode side of a proton exchange membrane electrolyser consists out of a nonwoven layer of stainless steel fibers. The average equivalent diameter of the stainless steel fibers is less than 50 µm. The cohesion between the stainless steel fibers in the nonwoven layer is provided by fiber entanglement. The relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is less than 16.7 when the nonwoven layer of stainless steel fibers is under no compressive forces.

Description

Gas diffusion layer

Description

Technical Field

[1] The invention relates to the field of gas diffusion layers in electrochemical cells. More particularly, the invention relates to gas diffusion layers used at the cathode side of a proton exchange membrane (PEM) electrolyser.

Background Art

[2] It is known to use carbon paper or needle felted carbon fiber nonwovens in the gas

diffusion layer at the cathode side of proton exchange membrane electrolysers.

US2015/318558 describes the use of needle felted carbon fiber nonwovens in gas diffusion layers and a method to make such nonwovens.

[3] US5,565,072 relates to the provision in an electrochemical cell of deformable layers with sufficient residual resiliency under compression, as required to compensate unavoidable planarity defects of the various components in the cell. The patent document indicates that a certain degree of residual resiliency is also necessary to maintain under constant pressure the electrode/membrane structures in order to compensate the thermal expansion of the various components during start-up, shut down and electric load variations. Therefore, structures similar to a tridimensional network of metal wires are used, preferably fixed to each other in interconnecting points.

[4] US2006/0159982A1 suggests the use of a needle punched metal fiber sheet in a fuel cell.

[5] WO2015/19321 1A1 discloses a gas diffusion layer to be placed between a bipolar plate and an electrode of an electrochemical cell. The gas diffusion layer comprises at least two layers layered over each other. One of the layers is designed as a spring component that has a progressive spring characteristic curve.

[6] US4,331 ,523 discloses a method of electrolyzing water. A thin layer of an electrically conductive fibrous assembly, e.g. a felt, is used at the cathode side of an ion-exchange membrane. The felt can be obtained by needle-punching to strengthen the fiber entanglement.

Disclosure of Invention

[7] The first aspect of the invention is a gas diffusion layer for the cathode side of a proton exchange membrane electrolyser. The gas diffusion layer comprises or consists out of a nonwoven layer of stainless steel fibers. The average equivalent diameter of the stainless steel fibers is less than 50 μιτι, preferably less than 30 μιτι; and more preferably less than 20 μιτι. As an example, the stainless steel fibers can have an equivalent diameter of 12 μιη. With equivalent diameter of a fiber is meant the diameter of a circle having the same surface area as the cross section of the fiber. The cohesion between the stainless steel fibers in the nonwoven layer is provided by fiber entanglement. It is meant that no metallurgical bonds such as sinter bonds or welded bonds are present in the nonwoven layer of stainless steel fibers, with the possible exception of rare cold welds between fibers in the nonwoven layer. The relative thickness of the nonwoven layer compared to sheet material with the same surface mass (in g/m²) and out of the same stainless steel is less than 16.7, preferably less than 14, when the nonwoven layer of stainless steel fibers is under no compressive forces. It is meant that the relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is less than 16.7, preferably less than 14.

The relative thickness of the nonwoven layer compared to sheet material with the same surface mass (in g/m²) and out of the same stainless steel is related to the porosity of the nonwoven layer. This relative thickness is calculated as the thickness of the nonwoven layer divided by the thickness of the of sheet material with the same surface mass (in g/m²) and out of the same stainless steel alloy as the nonwoven layer. The relative thickness r is related to the porosity p of the nonwoven layer as follows: r=1/(1-p).

A nonwoven layer of stainless steel fibers can be made by means of needle punching. However, after needle punching, the relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is much higher, typically about 25. Therefore, - for use in the invention - a specific additional process step is required to compress the nonwoven further, e.g. by means of calendaring or in a press. This compression to relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is less than 16.7, preferably less than 14, can e.g. be done by cold calendaring.

It has been noticed that the performance of a proton exchange membrane electrolyser having a gas diffusion layer according to the first aspect of the invention is improved compared to when using a similar needle punched stainless steel fiber nonwoven layer that has not been compressed to the relative thickness values as in the invention. The reasons for the beneficial results are not fully understood, but it is believed that the compression to the lower relative thickness values has a positive effect on the properties of the nonwoven layer relevant for the operation of a PEM electrolyser. An electrolyser operates at lower compression forces of the gas diffusion layer at the cathode compared to the compressive forces that have been used in manufacturing the nonwoven layer of stainless steel fibers of the gas diffusion layer of the invention. The compressive forces used in manufacturing the nonwoven layer of stainless steel fibers have resulted in permanent reduced thickness. This permanent reduced thickness results in a lower electrical resistance through the thickness of the nonwoven layer of stainless steel fibers of the gas diffusion layer of the invention, contributing to the improved performance of the electrolyser.

Preferably, the relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is more than 6.67.

Preferably, the nonwoven layer of stainless steel fibers is a needle felt. Needle punching - which is the process of making needle felt - creates observable "holes" in the nonwoven by the passage of the needles. These holes create preferential channels for gas flow. It is believed that the compressive forces exerted on a needle punched nonwoven layer to provide it with the relative thickness as in the invention has the benefit of creating more regular properties of the nonwoven layer, e.g. by homogenizing the structure of the nonwoven layer by removing to a large extent the otherwise observable "holes". Such homogenizing is beneficial for the performance of the electrolyser in which the gas diffusion layer is used.

Preferably, the nonwoven layer has a specific weight of at least 500 g/m²; and more preferably of at least 750 g/m².

Preferably, the nonwoven layer has a specific weight of less than 1700 g/m²; and more preferably of less than 1 100 g/m².

Preferably, the electrical conductivity in the nonwoven layer of stainless steel fibers is obtained by contacts of the stainless steel surface of contacting stainless steel fibers. Preferably, the stainless steel fibers are bundle drawn fibers. Bundle drawing is disclosed e.g. in US-A-2050298, US-A-3277564 and in US-A-3394213. Metal wires are forming the starting material and are covered with a coating such as iron or copper. A bundle of the covered wires is subsequently enveloped in a metal pipe. Thereafter the thus enveloped pipe is reduced in diameter via subsequent wire drawing steps to come to a composite bundle with a smaller diameter. The subsequent wire drawing steps may or may not be alternated with an appropriate heat treatment to allow further drawing. Inside the composite bundle the initial wires have been transformed into thin fibers which are embedded separately in the matrix of the covering material. Such a bundle preferably comprises not more than 2000 fibers, e.g. between 500 and 1500 fibers. Once the desired final diameter has been obtained the covering material can be removed e.g. by solution in an adequate leaching agent. The result is a bundle of metal fibers, which can be further processed into a nonwoven layer of stainless steel fibers.

The second aspect of the invention is a proton exchange membrane electrolyser cell. The cell comprises a proton exchange membrane; and a gas diffusion layer as in any embodiment of the first aspect of the invention. The gas diffusion layer is in contact at the cathode side with the proton exchange membrane. An electrolyser operates under compressive forces, as the layers of the cell are pressed together and as the cells operate under gas pressure. However, when removing the cathode side gas diffusion layer, the nonwoven layer of stainless steel fiber will elastically recover a major part of its thickness. As proton exchange membrane, a Nafion membrane of Dupont can e.g. be used.

In a preferred proton exchange membrane electrolyser cell, the nonwoven layer of stainless steel fibers has - when removed from the cell - a relative thickness compared to sheet material with the same surface mass and out of the same material less than 16.7; more preferably less than 14. Preferably, the nonwoven layer of stainless steel fibers has when removed from the cell a relative thickness compared to sheet material with the same surface mass and out of the same material of more than 6.67. An electrolyser operates under compressive forces, as the layers of the cell are pressed together and as the cells operate under gas pressure. However, when removing the cathode side gas diffusion layer, the nonwoven layer of stainless steel fiber will elastically recover a very substantial part its thickness.

A preferred proton exchange membrane electrolyser cell comprises a proton exchange membrane; and a gas diffusion layer as in any embodiment of the first aspect of the invention; in contact at the cathode side with the proton exchange membrane. The electrolyser cell further comprises a layer comprising or consisting out of sintered titanium fibers or comprising or consisting out of sintered titanium powder. The layer comprising or consisting out of sintered titanium fibers or sintered titanium powder is at the anode side in contact with the proton exchange membrane; and acts as gas diffusion layer. In an electrolyser, the hydrogen (produced at the cathode) is often at higher pressure than the gas at the anode side. The overpressure at the cathode side ensures contact between the gas diffusion layer at the anode side and the proton exchange membrane. It is a benefit of the invention that the nonwoven layer of stainless steel fibers forming the gas diffusion layer or which is part of the gas diffusion layer at the cathode side can elastically cope with pressure variations; while the gas diffusion layer at the anode side is stiff.

The third aspect of the invention is a method of operating a proton exchange membrane electrolyser cell as in the second aspect of the invention. The method comprises the steps of

- producing a needle felt nonwoven out of stainless steel fibers;

- compressing the needle felt nonwoven out of stainless steel fibers to a first relative thickness level r1 compared to sheet material with the same surface mass and out of the same stainless steel and letting it elastically recover to a second relative thickness level r2 compared to sheet material with the same surface mass and out of the same stainless steel; and

- operating the proton exchanger membrane electrolyser cell under conditions such that the nonwoven out of stainless steel fibers is compressed to a third relative thickness r3 compared to sheet material with the same surface mass and out of the same stainless steel; wherein r2 is larger than r3 and wherein r3 is larger than r1.

The fourth aspect of the invention is a method of operating a proton exchange membrane electrolyser cell as in any embodiment of the second aspect of the invention or as in any embodiment of the third aspect of the invention. The proton exchange membrane electrolyser cell is operated such that the relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is more than 6.67, preferably more than 7.69. Preferably, the proton exchange membrane electrolyser cell is operated such that the relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is less than 12.5. As an example, the proton exchange membrane electrolyser cell is operated such that the relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel equals 10 (which is equal to 90% porosity).

Brief Description of Figures in the Drawings

[22] Figure 1 shows a proton exchange membrane electrolyser cell according to the invention. Mode(s) for Carrying Out the Invention

[23] Figure 1 shows a proton exchange membrane electrolyser cell 100 according to the

invention. The proton exchange membrane electrolyser cell comprises a proton exchange membrane 1 10; a gas diffusion layer 120 at the cathode side 130 in contact with the membrane; and a sintered nonwoven layer 140 out of titanium fibers in contact at the anode side 150 with the proton exchange membrane. The gas diffusion layer at the anode side is a nonwoven layer of bundle drawn stainless steel fibers contacting the proton exchange membrane. The average equivalent diameter of the stainless steel fibers is 12 μητι. The nonwoven layer of stainless steel fibers is a needle felt of 1000 g/m². The cohesion between the stainless steel fibers in the nonwoven layer is provided by fiber entanglement obtained via needle punching. After needling, the thickness of the nonwoven was 3 mm. The nonwoven layer has then been calendared to 1.6 mm thickness. The relative thickness of this nonwoven layer of 1 .6 mm thickness compared to sheet material with the same surface mass and out of the same stainless steel is 13.53, when the nonwoven layer of stainless steel fibers is under no compressive forces. The relative thickness 13.53 means porosity 92%.

Claims

1. Gas diffusion layer for the cathode side of a proton exchange membrane electrolyser,

the gas diffusion layer consists out of a nonwoven layer of stainless steel fibers,

wherein the average equivalent diameter of the stainless steel fibers is less than 50 μιτι;

wherein the cohesion between the stainless steel fibers in the nonwoven layer is provided by fiber entanglement;

wherein the relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is less than 16.7, when the nonwoven layer of stainless steel fibers is under no compressive forces.

2. Gas diffusion layer as in claim 1 , wherein the relative thickness of the nonwoven layer

compared to sheet material with the same surface mass and out of the same stainless steel is more than 6.67.

3. Gas diffusion layer as in any of the preceding claims, wherein the nonwoven layer of stainless steel fibers is a needle felt.

4. Gas diffusion layer as in any of the preceding claims, wherein the stainless steel fibers in the nonwoven layer are not bonded to each other by means of metallurgical bonds.

5. Gas diffusion layer as in any of the preceding claims, wherein the nonwoven layer has a

specific weight of at least 500 g/m².

6. Gas diffusion layer as in any of the preceding claims, wherein the nonwoven layer has a

specific weight of less than 1700 g/m².

7. Gas diffusion layer as in any of the preceding claims, wherein electrical conductivity in the nonwoven layer of stainless steel fibers is obtained by contacts of the stainless steel surface of contacting stainless steel fibers.

8. Gas diffusion layer as in any of the preceding claims, wherein the stainless steel fibers are bundle drawn fibers.

9. Proton exchange membrane electrolyser cell, comprising

- a proton exchange membrane;

- a gas diffusion layer as in any of the claims 1 - 8; in contact at the cathode side with the proton exchange membrane.

10. Proton exchange membrane electrolyser cell as in claim 9;

wherein the nonwoven layer of stainless steel fibers has when removed from the cell a relative thickness compared to sheet material with the same surface mass and out of the same material less than 16.7.

1 1. Proton exchange membrane electrolyser cell as in claim 10; wherein the nonwoven layer of stainless steel fibers has when removed from the cell a relative thickness compared to sheet material with the same surface mass and out of the same material of more than 6.67.

12. Proton exchange membrane electrolyser cell as in claim 9 - 1 1 , comprising

- a proton exchange membrane;

- a gas diffusion layer as in any of the claims 1 - 8; in contact at the cathode side with the proton exchange membrane; and

- a layer comprising or consisting out of sintered titanium fibers or comprising or consisting out of sintered titanium powder;

wherein the layer comprising or consisting out of sintered titanium fibers or sintered titanium powder is at the anode side in contact with the proton exchange membrane.

13. Method of operating a proton exchange membrane electrolyser cell as in any of the claims 9 - 12, comprising the steps of

- producing a needle felt nonwoven out of stainless steel fibers;

- operating the proton exchanger membrane electrolyser cell under conditions such that the nonwoven out of stainless steel fibers is compressed to a third relative thickness r3 compared to sheet material with the same surface mass and out of the same stainless steel; wherein r2 > r3 > r1.

14. Method of operating a proton exchange membrane electrolyser cell as in any of the claims 9 - 12 or as is claim 13; comprising the operation of the proton exchange membrane electrolyser cell such that the relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is more than 6.67, preferably more than 7.69.

15. Method of operating a proton exchange membrane electrolyser as in claim 13; comprising the operation of the proton exchange membrane electrolyser cell such that the relative thickness of the nonwoven layer compared to sheet material with the same surface mass and out of the same stainless steel is less than 12.5.