Method of producing an electrochemical cell

Abstract

The invention relates to a method to produce an electrochemical cell (1), in particular an electrolysis cell, comprising a catalyst-coated membrane (2) and porous transport layers (3) arranged on both sides of the membrane (2). At least one side of at least one porous transport layer (3), preferably the side facing the catalyst-coated membrane (2), is subjected to laser treatment, in which surface contamination (4) is removed with the aid of a laser (5).

Description

[0001] Description

[0002] Method of producing an electrochemical cell

[0003] The present invention relates to a method of producing an electrochemical cell, e.g. an electrolysis cell or a fuel cell. Preferred fields of application of the invention are electrolysis systems or fuel cell systems.

[0004] Background

[0005] An electrochemical cell, in particular an electrolysis cell, has a layered construction, typically comprising:

[0006] An electrically insulating and chemically separating, but ion conducting membrane with a respective catalyst layer provided on either side thereof, forming an anode and a cathode, often referred to as catalyst-coated membrane (CCM).

[0007] A porous transport layer (PTL) on either side of the CCM for carrying electric current to the electrodes, while allowing electrolyte to be supplied to and products carried away from a respective electrode of the CCM. The PTL can be provided as a metal foam, (sintered) metal powder, metal fibers / whiskers felt or mesh, (woven or non-woven) carbon fibers and the like. Effective porosity, average pore size, tortuosity, as well as electric conductance are all relevant characteristics of the PTL.

[0008] A frame-like holding plate on either side of the CCM for sealing and supporting the CCM. The holding plates have a central opening for receiving the respective PTL. Besides, they are provided with media ports and in-plane media channels which extend from the media ports to a central opening. Seals on both sides of the holding plate separate the different media from each other.

[0009] A metal bipolar plate (BPP) shared between and mutually separating adjacent cells for coupling electric current into these cells.

[0010] The porous transport layer (PTL) is a key component in an electrolysis cell, in particular in a PEM-electrolysis cell. Due to the combination of low pH and high potential, it is commonly made of titanium (Ti). A critical issue is the contamination of the PTL, in particular the contamination of the surface of the PTL, which has direct contact with the catalyst-coated membrane (CCM). Most critical are metal contaminations, e.g. containing iron (Fe), copper (cu), yttrium (Y), magnesium (Mg), calcium (Ca), nickel (Ni), chrome (Cr), led (Pb), sodium (Na), Potassium (K) and / or silicium (Si), because they can degrade the CCM and thus reduce the lifetime of the electrolysis cell.

[0011] A common solution is to apply an etching process for dissolving the contamination. In this etching process aggressive chemicals like hydrogen fluoride (HF) are used. Unfortunately, these chemicals not only dissolve the contamination but also the titanium fibers and / or particles of which the PTL is made. Thereby, the structure of the PTL is damaged which results in a reduced mechanical strength of the PTL. Another problem is that titanium contains a low percentage of iron (Fe) which is also dissolved. Due to the porous structure of the PTL, it is nearly impossible to remove completely the dissolved iron, e.g. by rinsing. Thus, the contamination created by the etching process will initially remain in the porous structure of the PTL and later on, during operation, migrate to the CCM where it will promote its degradation.

[0012] Contamination is also an issue after resistance welding. Resistance welding is often used for bonding the metal components of an electrolysis cell, e.g. bipolar plates, separator plates and / or porous transport layers. Bonding by resistance welding ensures low interfacial contact resistance between the bonded metal components. Another solution having the same effect is coating the metal components with a noble metal on both sides. However, this is very expensive solution. During resistance welding, the metal components to be bonded are positioned between two electrodes. The two electrodes are pressed against the metal components and a high current is applied to the electrodes. Thereby the joint is heated up to a temperature, at which metal melts and the components are forged together. Because the electrodes used for resistance welding are commonly made of copper or another metal with high electrical conductivity, electrode material tends to remain on the surfaces of the components thereby contaminating their surfaces.

[0013] The object of the present invention is to prevent degradation of the catalyst- coated membrane of an electrochemical cell, in particular of an electrolysis cell, due to surface contamination of a porous transport layer. The method can also be used for other components of the stack, like the bipolar plate, the mesh or the interlayer for the same purpose.

[0014] The object is solved by the method according to claim 1 . The dependent claims relate to preferable embodiments of the invention.

[0015] Disclosure of the invention

[0016] Proposed is a method to produce an electrochemical cell, in particular an electrolysis cell, comprising a catalyst-coated membrane and porous transport layers arranged on both sides of the membrane. According to the invention, at least one side of at least one porous transport layer, preferably the side facing the catalyst-coated membrane, is subjected to laser treatment, in which surface contamination is removed with the aid of a laser.

[0017] The laser treatment proposed for removing surface contamination replaces the etching process mentioned at the beginning. In contrast to the etching process, the laser treatment is restricted to the surface of the porous transport layer. Thus, the structure of the porous transport layer is not affected, so that its mechanical strength is retained. Furthermore, the laser treatment can be limited to one side of the porous transport layer, which should then be the side in direct contact with the catalyst- coated membrane. This is because surface contamination on this side of the porous transport layer can accelerate the degradation of the CCM. Removing the surface contamination extends the lifetime of the CCM.

[0018] By dispensing with the etching process, the production of an electrochemical cell can be simplified and made more cost-effective. In addition, no harmful aggressive chemicals are required, which then have to be disposed of. Thus, the proposed method also helps protecting the environment.

[0019] According to a preferred embodiment of the invention, the surface contamination is removed with the aid of an ultra-short pulse laser. An ultra-short pulse laser can be used to ablate very thin surface layers in order to remove the contamination. This minimizes the loss of material.

[0020] Preferably, the laser treatment is carried out using a shielding gas and with the following process parameters: pulse time from 1 ps to 100 ns power density of 1 to 200 W / cm2.

[0021] The pulse time and power density must be kept low to prevent heating up the material, which could result in distortion or even burning, due to the trapped oxygen in the porous structure in case of a PTL.

[0022] Within the proposed ranges metal is evaporated and not molten during the laser treatment. Thus, the risk of damaging the titanium fibers or particles under the surface contamination of the porous transport layer during laser treatment is minimized.

[0023] According to a preferred embodiment of the invention, a cleaning process with an acid compound follows the laser treatment. The additional cleaning process after laser treatment serves to remove contaminants within the porous structure of the porous transport layer. In contrast to the etching process mentioned in the beginning, the pH value of the applied acid is higher, in particular no fluorides or other harmful acids are used. Thus, acid cleaning neither dissolves the titanium nor the iron contained in the titanium the porous transport layer is made of.

[0024] According to another preferred embodiment of the invention, at least two metallic components of the electrochemical cell are bonded by means of resistance welding. During resistance welding, an oxide layer is removed from the surfaces of the components to be bonded, which increases the electrical conductivity of the components. An expensive coating of the surfaces with a noble metal to increase the electrical conductivity can therefore be omitted.

[0025] Preferably, the proposed laser treatment for removing surface contamination of the PTL follows the resistance welding for removing surface contamination caused by the resistance welding process. This applies regardless of whether the porous transport layer is one of the metallic components that has been bonded to another metallic component by means of resistance welding or not, because more than only one surface can be subjected to laser treatment. As an example, the surface of the porous transport layer contacting the catalyst-coated membrane as well as the surface of another metallic component, e.g. a bipolar plate forming the outer layer of the electrochemical cell, can be subjected to laser treatment for removing surface contamination regardless of the origin of the contamination.

[0026] Resistance welding leads to surface contamination in the form of copper deposits, whereby the copper comes from the electrodes used in resistance welding. In case the electrodes are made of another material, this material can be removed as well.

[0027] Besides, resistance welding can create spots of molten fibers and / or particles on the surface of the porous transport layer. Even these spots can be removed with the aid of the laser during the proposed laser treatment. In this way, the permeability of the porous structure of the porous transport layer in the area of the CCM is improved. Preferred embodiments of the invention are explained in conjunction with the enclosed figures. The figures show:

[0028] Figure 1 a cross section through an electrochemical cell in form of an electrolysis cell,

[0029] Figure 2 a cross section through a porous transport layer during laser treatment, and

[0030] Figure 3 a cross section through metallic components of an electrolysis cell bonded by resistance welding and subjected to laser treatment.

[0031] Detailed description of the figures

[0032] Figure 1 shows the typical structure of an electrochemical cell 1 , in this case an electrolysis cell. In the center is a membrane coated on both sides with a catalyst material, the so-called catalyst-coated membrane (CCM) 2. On both sides of the CCM 2, there are porous transport layers (PTL) 3, which usually consist of titanium fibers and / or titanium particles. Further metallic components 6 in the form of bipolar plates form the two outer layers.

[0033] As shown as an example in Figure 1 , the left PTL 3 has surface contamination 4 on its side contacting the CCM 2. Because this kind of surface contamination 4 promotes degradation of the CCM 2, it should be removed.

[0034] According to the present invention, the surface of the contaminated PTL 3 is cleaned with the aid of a laser 5. As shown in figure 2, the laser 5 strips a thin layer while it is moved parallel to the contaminated surface (see the arrow in figure 2), thereby removing the surface contamination 4.

[0035] The surface contamination 4 of the PTL 3 can have various causes. For example, if the PTL 3 has been joined to another metallic component 6 by means of resistance welding, the outer surfaces of the joint may be contaminated with copper from the electrodes used in this process. The outer surfaces then can be subjected to laser treatment for removing the surface contamination 4 as shown in figure 3. In figure 3, the other metallic component 6 is a bipolar plate forming an outer layer of the electrochemical cell 1 . Between the PTL 3 and the bipolar plate, another layer 7 with a porous structure is arranged. Thus, the proposed laser treatment for cleaning the surface of a PTL 3 may also be used for cleaning further surfaces if they are contaminated.

Claims

Claims1 . Method to produce an electrochemical cell (1), in particular an electrolysis cell, comprising a catalyst-coated membrane (2) and porous transport layers (3) arranged on both sides of the membrane (2), characterized in that at least one side of at least one porous transport layer (3), preferably the side facing the catalyst-coated membrane (2), is subjected to laser treatment, in which surface contamination (4) is removed with the aid of a laser (5).

2. Method according to claim 1 , characterized in that the surface contamination (4) is removed with the aid of an ultra-short pulse laser (5).

3. Method according to claim 1 or 2, characterized in that the laser treatment is carried out using a shielding gas and with the following process parameters: pulse time from 1 ps to 100 ns and power density of 1 to 200 W / cm2.

4. Method according to any one of the preceding claims, characterized in a cleaning process with an acid compound follows the laser treatment.

5. Method according to any one of the preceding claims, characterized in that at least two metallic components (6) of the electrochemical cell (1) are bonded by means of resistance welding.

6. Method according to claim 5, characterized in that the laser treatment follows the resistance welding for removing surface contamination (4) caused by the resistance welding process.

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