WO2015139880A1

WO2015139880A1 - Bipolar plate

Info

Publication number: WO2015139880A1
Application number: PCT/EP2015/052571
Authority: WO
Inventors: Michael Götz; Jürgen KRAFT
Original assignee: Elringklinger Ag
Priority date: 2014-03-17
Filing date: 2015-02-09
Publication date: 2015-09-24
Also published as: DE102014103611A1

Abstract

The invention relates to a bipolar plate for an electrochemical assembly, in particular for a low-temperature fuel cell, said plate comprising a metal carrier with a single-layer or multi-layer coating. The outermost layer of the coating contains an electrically conductive, monomeric organic compound which is oxidation-resistant, temperature-stable and substantially water-insoluble.

Description

bipolar

The present invention relates to a bipolar plate for an electrochemical arrangement, in particular for a low-temperature fuel cell, comprising a metallic carrier with a single-layer or multi-layer coating.

Bipolar plates, which are also referred to as interconnectors, allow in electrochemical arrangements of various types, the series connection of several individual elements in a stack. In fuel cells, the individual cells are combined in this way to form a fuel cell stack in order to provide a corresponding multiple of the cell voltage.

In a fuel cell stack, the bipolar plate performs several essential functions, namely the production of the electrical connection between the adjacent electrodes and the supply of the fuel to the anode on one side and the oxidizer to the cathode on the other side. Furthermore, the bipolar plate is often designed in two parts and includes a space through which a coolant flows and ensures a temperature of the stack. Basic prerequisites for the effective operation of the fuel cell are, inter alia, that the bipolar plate has the lowest possible electrical contact resistance and is substantially gas-tight.

In the polymer electrolyte fuel cell (PEFC), which is a widely used type of low temperature fuel cell, the bipolar plates account for a substantial portion of the total weight of the stack as well as manufacturing costs (see, for example, S. Karimi et al .: A Review of Metallic Bipolar Plates for Proton Exchange Membrane Fuel Cells: Materials and Fabrication Methods, Advances in Materials Science and Engineering, 2012, 1-22). The bipolar plates usually comprise a metal support plate, for example stainless steel, which is provided with a coating to reduce the contact resistance. In addition, this coating must be durably resistant under the operating conditions of the PEFC to ensure efficient and efficient operation In particular, it must be sufficiently resistant to temperature and oxidation.

Coatings for metallic bipolar plates known in the art include, in particular, noble metal coatings, such as gold, as well as carbon-based coatings in various modifications. However, such coatings are too expensive for many applications, since their production is possible only with expensive processes such as physical or chemical vapor deposition (PVD or CVD). In the case of precious metals, the material costs are added, which are considerable despite the relatively small layer thickness. The same applies to coatings based on conductive ceramics such as titanium or chromium nitrides.

The invention is therefore based on the object to propose a bipolar plate for an electrochemical arrangement, which can be produced more cheaply.

This object is achieved in the bipolar plate of the aforementioned type according to the invention that an outermost layer of the coating contains an electrically conductive monomeric organic compound which is resistant to oxidation, temperature and substantially water insoluble.

With the help of organic monomers having the stated properties, bipolar plates having a high electrochemical resistance and a low contact resistance can be produced relatively simply and inexpensively. This is especially true in comparison to electrically conductive polymers, such as polyaniline and polypyrrole, in which there is usually no sufficient oxidation resistance.

The electrically conductive compound is favorably temperature resistant up to at least 100 ° C, preferably up to at least 130 ° C. This is an assignment the bipolar plate in low-temperature fuel cells and in many other electrochemical arrangements possible.

As organic compounds having the required properties, in particular polycyclic aromatic hydrocarbons or derivatives thereof can be used. In these compounds, the electrical conductivity is based on the lone pairs of electrons of the conjugated aromatic system, which are delocalized over a relatively wide range within the molecule.

It is particularly favorable if the electrically conductive compound is a perylene derivative, in particular the dianhydride or a diimide of 3,4,9,10-perylenetetracarboxylic acid.

The diimides mentioned correspond in particular to the following general formula (I),

wherein R 1 and R _{2 are the} same or different and are each selected from a hydrogen atom, an alkyl radical, an aryl radical or an aryl-alkyl radical, each of which may be substituted by one or more alkyl, alkyloxy, halogen, OH or CN groups.

Some of the compounds according to the formula I are used as color pigments on a larger scale and are therefore readily available. These are, in particular, the perylene pigments with the CI designations PR29, PR123, PR149, PR178, PR179, PR190, PR224, PBk31 and PBk32 Advantage can be used as electrically conductive compounds for the bipolar plate according to the invention.

According to the invention, the electrically conductive compound in the outermost layer of the coating (which may also be their only location), since the electrical properties of this outermost layer for the contact resistance to the adjacent element, in particular an electrode, are crucial. In order to reduce the contact resistance, it is favorable if the layer thickness is as small as possible, wherein the thickness of the outermost layer of the coating can be in particular in the range of 1 to 20 μm.

The outermost layer of the coating can be formed according to an embodiment of the invention exclusively from the electrically conductive compound. In this case, a very small layer thickness will be sufficient to achieve the stated advantages. The electrically conductive compound can be applied to the metallic carrier by means of PVD, for example.

Often, however, it is preferred that the outermost layer of the coating comprises a binder in which the electrically conductive compound is dispersed. By means of such a binder, an optimal adhesion of the coating to the metallic carrier can be achieved. It is also particularly advantageous in this case that the coating in liquid form can be applied to the carrier by means of simple methods such as spraying or dipping. In this case, either a solvent for the binder can be used or a liquid binder which hardens after application chemically, thermally or by UV radiation.

Binders which can preferably be used in the context of the invention are selected from epoxy resins, acrylic resins, phenolic resins, urethane resins, melamine resins, fluorine resins and silicone resins. In order to achieve the desired low contact resistance of the bipolar plate, a sufficient amount of the electrically conductive compound in the binder must be dispersed, with smaller layer thicknesses tend to be lower amounts sufficient. Preferably, the compound is contained in a proportion of 30 to 90 wt.% In the outermost layer of the coating, more preferably in a proportion of 50 to 80 wt.%. Excessive levels may result in insufficient adhesion of the coating.

If the coating comprises more than one layer, it is particularly preferred if under the outermost layer with the electrically conductive compound a further layer is provided which contains carbon particles. The carbon particles are preferably dispersed in a binder. In this case, the outermost layer desirably has a smaller layer thickness than the layer with the carbon particles. Since the latter usually have better conductivity than the abovementioned organic compounds, the total resistance of such a coating with the same total thickness is substantially lower than a single-layer coating with the organic compound. Compared to a pure coating based on carbon particles, however, has the significant advantage that the organic compounds are much more resistant to oxidation and thus protect the underlying carbon particles from oxidation.

The carbon particles are preferably formed from graphite, carbon black or carbon nanotubes.

The metallic carrier is advantageously formed in the bipolar plate according to the invention from stainless steel or from an alloy based on aluminum, copper, titanium or tantalum, with stainless steel being preferred.

The best possible corrosion resistance of the metallic carrier is important in that it can not be ruled out in practice that the coating does not or not completely cover individual regions of the carrier. However, it is of course desirable that the coating be so to form uniformly as possible, since in this way the contact resistance is reduced.

The bipolar plate according to the invention may be constructed in one or two parts, depending on which variant is more advantageous for the relevant electrochemical arrangement. In the first case, the metallic carrier comprises a single metal plate having on one or both sides the single or multi-layer coating. Preferably, the metal plate has an embossing structure, are formed by the flow channels for the fuel on one side and for the oxidizer on the other side of the plate. The respective raised areas of the embossing are then in contact with the adjacent anode or cathode.

In the two-part embodiment, the metallic support comprises two metal plates, which are at least partially bonded together, in particular welded or glued, and at least on their outer sides have the one or more layers coating. Again, both plates preferably have an embossed structure, wherein between the two plates additional flow channels can be provided for a coolant.

The electrical contact between the two metal plates of the carrier takes place, if they are welded, over the welded areas, which may be individual spot welds. If the plates are joined by another method, or if the welded portions are insufficient, the total resistance of the bipolar plate can be favorably reduced by having the single-layered or multi-layered coating also on its inner sides facing each other. As a result, the lowest possible contact resistance between the plates can be achieved even in the non-welded areas.

The present invention further relates to the use of a bipolar plate according to the invention in a low-temperature fuel cell, in particular in a polymer electrolyte fuel cell (PEFC). In the formation of a sponding fuel cell stack alternate each a bipolar plate and an electrode-membrane unit (membrane electrode assembly, MEA), said MEA comprising respectively an anode, a cathode and an interposed ^¬ associated proton exchange membrane.

The invention may also find application in electrochemical arrangements of another kind, in which bipolar plates are provided. In particular, the invention also relates to the use of the bipolar plate in an electrolysis device.

The embodiment described below is used for closer Erläu ^¬ esterification of the invention.

example

This example describes the single-layer coating of a 0.1 mm thick sheet of stainless steel 1.4404 as a metallic support with the commercially available pigment perylene red (IC PR149) as an electrically conductive monomeric organic compound. In perylene red is the compound N, N'-bis (3,5-dimethylphenyl) -3,4,9,10-perylene bisimide, according to the general ^¬ my formula I wherein Ri and _R2 are each a 3.5 -Dimethylphenylrest are.

Before the application of the coating, the surface of the stainless steel support is first activated in order to free them from metal oxides. According to a method described in US 4,422,906, the support is first immersed for 5 minutes in a first activating solution. Immediately thereafter, the carrier is immersed in a second activating solution and subjected to a cathodic current density of 0.14 A / cm ² for 7 minutes.

The two activation solutions have the following compositions: Activation solution 1:

Hydrochloric acid 35% 8% by volume

Citric acid 5% by weight

Nitric acid 68% 2.5% by volume

Sulfuric acid 75% 6 vol.%

Polyoxyethylene (40) stearate 2% by weight

N-ethyl-2-pyrrolidone 3% by weight

Acetic acid 90% 2 vol.%

2-Pentin-1,4-diol 2% by weight

Activation solution 2:

Phosphoric acid 85% 20% by volume

Citric acid 5% by weight

Oxalic acid 3% by weight

Nitric acid 68% 5% by volume

Sulfuric acid 75% 5% by volume

Polyoxyethylene (40) stearate 2% by weight

Gluconic acid 50% 10 vol.%

N-ethyl-2-pyrrolidone 5% by weight

2-Pentin-1,4-diol 3% by weight

Immediately after the galvanic activation of the carrier is rinsed with degassed deionized water, dried by means of a cellulose cloth and immediately coated.

The binder used for the coating formulation is an epoxy-based two-component system available from Mankiewicz Gebr. & Co. under the names Seevenax Coating 110-37 and Seevenax-Hardener 115-37. Further, the thinner 903-76 of the same manufacturer as well as Perylenrot of the Fa. Clariant International AG under the name Pigment PV Fast Red B was used. The coating formulation has the following quantitative composition:

17 g Seevenax coating 110-37

8 g of Seevenax hardener 115-37

2000 g thinner 903-76

100 g of pigment PV Fast Red B

All ingredients are homogenized with a disperser (Ika Ultra-Turrax T 18 with dispersing tool S18N-19G) at a speed of 15000 / min for 60 seconds and then sprayed on both sides of the activated stainless steel carrier.

The coating cures at room temperature within 24 hours.

A stainless steel plate which has been coated on both sides according to this example has an average coating thickness of approx. 15 pm a resistance of 20 ΓΠΩΟΓΠ ² , which is well below the value of the uncoated steel plate. By varying the production parameters, in particular the application rate of the coating and the concentration of the electrically conductive compound, the electrical properties of the coated carrier can be further optimized as required.

The coating according to the invention ensures a permanently low contact resistance when using the coated carrier as a bipolar plate in an electrochemical arrangement. In addition, the organic compound used is temperature-resistant and at least 130 ° C substantially water-insoluble, and compared to a precious metal coating (for example gold) much cheaper and easier to apply.

In principle, the coating also protects the metallic carrier against oxidation, wherein it is not absolutely necessary for the functionality of the bipolar plate that the coating completely covers and hermetically seals the carrier. table is tight. The coated areas need only be sufficient to achieve the desired low contact resistance. Uncoated areas of the carrier can passivate and are then also protected against further corrosion.

Claims

Patent claims

1. bipolar plate for an electrochemical arrangement, in particular for a low-temperature fuel cell, comprising a metallic carrier with a single- or multi-layer coating,

characterized in that an outermost layer of the coating contains an electrically conductive monomeric organic compound which is oxidation-resistant, temperature-resistant and substantially water-insoluble.

2. Bipolar plate according to claim 1, wherein the electrically conductive compound is temperature-resistant to at least 100 ° C, preferably to at least 130 ° C.

3. A bipolar plate according to claim 1 or 2, wherein the electrically conductive compound is a polycyclic aromatic hydrocarbon or a derivative thereof.

4. The bipolar plate according to claim 3, wherein the electrically conductive compound is a perylene derivative, in particular the dianhydride or a diimide of 3,4,9,10-perylenetetracarboxylic acid.

5. A bipolar plate according to claim 4, wherein the electrically conductive compound corresponds to the following general formula (I),

wherein Ri and R _{2 are the} same or different and each selected are selected from hydrogen, alkyl, aryl or aryl, each of which may be substituted with one or more alkyl, alkyloxy, halogen, OH or CN groups.

6. A bipolar plate according to claim 5, wherein the electrically conductive compound is selected from the perylene pigments with the C.I. Designations PR29, PR123, PR149, PR178, PR179, PR190, PR224, PBk31 and PBk32.

7. Bipolar plate according to one of the preceding claims, wherein the outermost layer of the coating has a layer thickness in the range of 1 to 20 pm.

8. Bipolar plate according to one of the preceding claims, wherein the outermost layer of the coating is formed exclusively of the electrically conductive compound.

9. A bipolar plate according to any one of claims 1 to 7, wherein the outermost layer of the coating comprises a binder in which the electrically conductive compound is dispersed.

10. Bipolar plate according to claim 9, wherein the binder is selected from epoxy resins, acrylic resins, phenolic resins, urethane resins, melamine resins, fluorine resins and silicone resins.

11. A bipolar plate according to claim 9 or 10, wherein the electrically conductive compound is contained in a proportion of 30 to 90 wt.% In the outermost layer of the coating, preferably in a proportion of 50 to 80 wt.%.

12. Bipolar plate according to one of the preceding claims, wherein the coating under the outermost layer with the electrically conductive Ver¬ Bond comprises a further layer containing carbon particles, which are preferably dispersed in a binder.

13. The bipolar plate according to claim 12, wherein the carbon particles are formed from graphite, carbon black or carbon nanotubes.

14. Bipolar plate according to one of the preceding claims, wherein the metallic carrier is made of stainless steel or of an alloy based on aluminum, copper, titanium or tantalum.

15. Bipolar plate according to one of the preceding claims, wherein the metallic carrier comprises a single metal plate having on both sides of the single or multi-layer coating.

16. Bipolar plate according to one of claims 1 to 14, wherein the metallic carrier comprises two metal plates, which are at least partially bonded together, in particular welded or glued, and which have at least on their outer sides of the single or multi-layer coating.

17. The bipolar plate according to claim 16, wherein the two metal plates also have, on their inner sides, which face each other, the single- or multi-layer coating.

18. Use of a bipolar plate according to one of the preceding

Claims in a low-temperature fuel cell, in particular in a polymer electrolyte fuel cell.

19. Use of a bipolar plate according to one of claims 1 to 17 in an electrolysis device.