WO2014081316A1

WO2014081316A1 - Bipolar plates, method and use of these plates in polymer electrolyte membrane (pem) fuel cells or other electrochemical cells

Info

Publication number: WO2014081316A1
Application number: PCT/NO2013/050206
Authority: WO
Inventors: Anders ØDEGÅRD; Ole Edvard KONGSTEIN; Hans HUSBY
Original assignee: Sinvent As
Priority date: 2012-11-21
Filing date: 2013-11-21
Publication date: 2014-05-30
Also published as: NO20121388A1

Abstract

The present invention relates to a bipolar plate (BPP) with gas diffusion layer (GDL) for use in PEM fuel cells or other electrochemical cells. Parts of the bipolar plate are in direct contact with the gas diffusion layer, and parts of the bipolar plate are coated with a protective layer in areas where the bipolar plate is not in contact with the gas diffusion layer. The method of producing the combined BPP/GDL involves applying of a protective layer on the bipolar plate and subsequently hot-pressing of the gas diffusion layer onto the coated bipolar plate at elevated temperature and compaction pressure before and during curing of the protective layer. The coated bipolar plate and the gas diffusion layer are glued together, and direct contact between the GDL and the BPP is obtained. Another aspect of the invention is a bipolar plate prepared by applying a protective layer comprising an electrical conductive material on a conductive plate, and subjecting the coated plate to an elevated compaction pressure at elevated temperature before and during curing of the protective layer.

Description

Bipolar plates, method and use of these plates in polymer electrolyte membrane (PEM) fuel cells or other electrochemical cells.

Technical field

The present invention concerns new concepts related to bipolar plates (BPP) for use in polymer electrolyte membrane (PEM) fuel cells or other electrochemical cells.

Background

Polymer electrolyte membrane fuel cells (PEMFC) have gained a lot of interests for converting hydrogen and oxygen into electric energy, heat and water. PEM fuel cells have low emission of pollutants and very high electric energy efficiency. The main purpose of the bipolar plates of a fuel cell is to distribute oxidant and fuel gas and electrons to/from the gas diffusion layer (GDL), and water out of the system. For such bipolar plates, carbon or carbon composite materials have traditionally been used because of their good chemical resistance. However, carbon based bipolar plates have a low mechanical strength, a rather high electrical resistance and high machining cost. Metals on the other hand are desired because of very high electric conductivity, very good mechanical properties, but the chemical resistance is rather poor in the humid, acidic and anodic environment.

Carbon composites have been considered being the standard material for PEM bipolar plates because of its low interfacial contact resistance (ICR) and high corrosion resistance. Unfortunately, carbon and carbon composites are brittle and permeable to gases, and have poor cost effectiveness for high volume

manufacturing processes relative to metals. Since durability and cost represent the two main challenges hindering the fuel technology from penetrating the energy market, considerable attention has been given to metallic bipolar plates for their particular suitability to transportation applications. Metals possess higher mechanical strength, better durability to shocks and vibration, no permeability, and superior manufacturability and cost effectiveness when compared to carbon-based materials. However, the main handicap of metals is the lack of ability to withstand corrosion in the harsh acidic and humid environment inside the PEM fuel cell without dissolution or formation of electric insulating passive layers, which cause considerable power degradation. Considerable attempts are being made using noble metals, stainless steel and various coated materials with nitride and carbide based alloys to improve the corrosion resistance of the metals used without sacrificing interfacial contact resistance and maintaining cost effectiveness.

ICR of a BPP/coating is normally measured in a set-up where a GDL is placed on top of the BPP (with coating), and the compaction pressure on these is varied while interfacial contact resistance between them is measured. The nominal ICR value is often given at around 140 N/cm². The US DoE target for PEMFC ICR is 10 m Ω cm².

In Journal of Power Sources 163 (2007), p 755-767, H. Tawfik, Y. Hung and D. Mahajan presented a review of research work conducted on metal bipolar plates to prevent corrosion while maintaining a low contact resistance. All of the protective coatings described are either metal-based or carbon-based and thus, electrically conductive.

Patent application EP2234192 A2 discloses a method of manufacturing a metallic bipolar plate for fuel cells, including (a) preparing a metal plate as a matrix of the metallic bipolar plate; (b) pickling a surface of the metal plate; (c) coating a composition comprising a binder resin, carbon particles and a solvent on the pickled surface of the metal plate and (d) drying the surface of the metal plate on which the composition is coated, at a temperature less than a thermal

decomposition temperature of the binder resin and greater than or equal to a boiling point of the solvent to form a coating layer on the surface of the metal plate, the coating layer having the carbon particles dispersed in a matrix of the binder resin, wherein these processes are performed as a continuous process.

The object of present invention is to provide alternative concepts of using conductive bipolar plates that will resist corrosion while maintaining a low

interfacial contact resistance during operation in a corrosive environment.

For many types of electrochemical cells, contact resistance, electrical conductivity and corrosion protection are important component properties, and thus, the invention can be applied to more than PEM fuel cells. Summary of the invention

The present invention provides a bipolar plate with a gas diffusion layer. The combined BPP/GDL comprises a bipolar plate and a gas diffusion layer wherein parts of the bipolar plate are in direct contact with the gas diffusion layer, and parts of the bipolar plate are coated with a protective layer in areas where the bipolar plate is not in contact with the gas diffusion layer. The combined BPP/GDL is prepared by coating the BPP with any protective layer and directly pressing the GDL to the coated BPP before and during the curing of the protective layer, thereby gluing the coated BPP and the GDL together, and thus, obtaining the direct contact.

A second aspect of the invention is a method for producing a bipolar plate with a gas diffusion layer, wherein a bipolar plate is coated with any protective layer and a gas diffusion layer is pressed onto the coated bipolar plate before and during the curing of the protective layer, thereby gluing the coated BPP and the GDL together. This results in direct contact between the bipolar plate and the gas diffusion layer.

Further, the invention relates to the use of the combined BPP/GDL plate for PEM fuel cells and other electrochemical cells.

Another aspect of the invention is a bipolar plate for PEM fuel cells or other electrochemical cells prepared by applying a protective coating comprising an electrical conductive material on a conductive plate. The coated conductive plate is subjected to a hot-pressing procedure before and during the curing of the protective coating layer.

Figures

Figure 1 shows a cross-section of a traditional BPP/GDL solution where the bipolar plate is entirely made of carbon or carbon composite.

Figure 2 shows a metal-based bipolar plate with a conductive protective layer and a gas diffusion layer on top.

Figure 3 shows a bipolar plate with a gas diffusion layer and a protective layer, and is an embodiment according to the invention. Detailed description

By the terms "carbon / carbon composite / carbon material" in the present application it is meant to include all types of carbon, e.g. graphite, diamond, carbon black (amorphous), nano-structure carbons.

By the term "curing" it is meant the process where liquid coating sets to a solid, and includes drying and hardening.

In Figure 1 , a traditional solution where the bipolar plate is entirely made of carbon or carbon composite is shown. The gas diffusion layer is a carbon based paper/cloth arranged on the bipolar plate when a fuel cell is assembled. It is essential that the electrical contact between the BPP and the GDL is good. This solution is working excellent, but is expensive and because of the poor strength and high gas permeability of the plate, thick plates are required.

Figure 2 shows a solution where the carbon based bipolar plate of figure 1 is replaced by a metallic bipolar plate. The metal core may be selected from stainless steel, aluminum, magnesium, titanium or carbon steel plates. To maintain low interfacial contact resistance and high corrosion resistance, the metal core plate has to have a conductive protective layer. Many different materials for use as protective layer have been described. Chromium nitride, titanium nitride, carbon, electrically conductive polymers are some examples of such materials.

In EP2234192, such a metal separator plate for a fuel cell having a coating layer comprising carbon particles dispersed in a binder resin is described. Coating of the metal may be performed by spray coating, dip coating, roll coating or the like. Drying/hardening of the coating was performed with a suitable temperature and duration.

The present inventors have now developed a method for coating a bipolar plate with a carbon composite coating by a spraying technique, wherein the density and quality of the coating is improved by a subsequent hot-pressing step before and during the curing of the coating. Considerable improvement of the contact resistance was obtained. Experiments showed that a contact resistance of 9.8 mQ cm² was measured at a compaction pressure of 125 N/cm² for a coated plate, whereas the same coating deployed without the subsequent step of hot- pressing had values of about 400 mQ cm². The solution described above requires use of an electrically conductive coating composition. The composition contains an electrically conductive material, a polymer resin acting as a binder and a solvent.

The polymer resin may be selected from acrylic resins, phenolic resins, urethane resins, melanin resins, fluorine resins, silicone resins, epoxy resins, or a combination thereof.

When the electrically conductive material is a carbon material a typical composition in which carbon particles are used as conductive material comprises 30-95 wt% of carbon particles, 5-70 wt% of polymer binder resin. Other electrically conductive materials are also possible to use in the coating compositions, such as transition metal carbides, nitrides and borides.

The solvent to be used as a coating solution of the composition may include any solvent compatible with the polymer resin.

The base material of the bipolar plates may be selected from stainless steel, low alloyed steel, carbon steel, aluminum, magnesium, titanium or other conductive materials.

The bipolar plates may be pretreated and cleaned by etching, pickling or grinding before coating deposition.

Both preformed bipolar plates with flow structures and plain metals sheets can be coated. In the former case, the hot-pressing step may include a negative die of the flow structure to ensure compression of the coating in the indentation. In the latter case, the hot-pressing can be applied to whole surface by a flat "die" and the flow structure(s) is "applied" to the plate in a subsequent step, by e.g.

stamping or hydroforming.

The coating may be performed by spray coating, dip coating, roll coating, or the like.

According to the invention a post treatment including hot-pressing is performed. During parts or the whole curing process of the coating material, the coated plate is pressed between two plates (typical pressure range 100 - 100 000 N/cm²). The curing process may also be accelerated by elevating the temperature (typical temperature range 50-200 °C), depending on the polymer resin.

Because it is essential to maintain good contact resistance, a coating comprising a conductive material has been regarded necessary. However, the present inventors have found that when a gas diffusion layer (GDL) is added on top of a coated bipolar plate before the protective material has cured, also non- conductive materials may be used as coating. A bipolar plate is coated with any protective layer by a spraying technique, and a GDL is placed on the BPP. A post treatment of the coated bipolar plate and gas diffusion layer by a compaction pressure and temperature sufficient to obtain direct electric contact between the components is performed. The pressure and temperature to be used depend on the type of gas diffusion layer and the anti-corrosive coating material. If carbon- based GDLs are used, the pressure during curing should not exceed 200 N/cm² _. This method will result in a combined BPP and GDL, where the coating material will prevent corrosion in the exposed areas, i.e. the areas where GDL and BPP are not in contact, while the GDL is glued to the conductive plate resulting in a direct electrical contact to the bipolar plate. This is illustrated in Figure 3. Because direct contact is obtained between BPP and GDL, the protective layer may comprise any anti-corrosive coating material such as the electrically conductive composite compositions described above, but also non-conductive paints and glues.

EXPERIMENTS

Example 1 : Bipolar plate coated with a carbon-polymer composite coating

A carbon-polymer composite coating for SS 316L bipolar plate substrates was investigated. The coating consisted of 45 vol % graphite, 5 vol % carbon black and 50 vol % epoxy binder. The coating was deployed by a spraying technique followed by hot-pressing at 1210 N/cm² and 1 10°C for three hours while the binder cured. A contact resistance of 9.8 mQ cm² was measured at a compaction pressure of 125 N/cm² for a coated plate, whereas the same coating deployed without the subsequent step of hot-pressing had values of about 400 mQ cm². Coated plates were electrochemically tested in a 1 mM H₂S0₄ solution at 75 °C, with measurements of contact resistance before and after polarization experiments. The coating seemed to protect the substrate from degradation at potentials of 0.0191 and 0.6191 V vs. SHE, but not at a potential of 1 .0 V vs. SHE. At 1 .0 V vs. SHE the corrosion current density from the coated plates were higher than for the bare SS 316L plates (probably due to corrosion of carbon fillers), and the increase in contact resistance after 16 hours of polarization was equally large as for the bare SS plates.

Example 2: Combined BPP/GDL with protective layer.

A BPP made of SS 316L (stainless steel) was spray coated with carboxane epoxy and pressed to a GDL at a pressure of 125 N/cm² at room temperature to obtain direct electric contact between the BPP and the GDL. The interfacial contact resistances of the two plates made by gluing the GDL with carboxane epoxy before running a fuel cell accelerated stress test were 36.5 (anode) and 26.8 (cathode) mQ cm² at a compaction pressure of 125 N/cm². After performing a fuel cell accelerated stress test, the interfacial contact resistances of the plates were 24.4 (anode) and 26.8 (cathode) mQ cm² at the same compaction pressure. The values are listed in Table 1 together with values for plain uncoated stainless steel (316L) plates put through the same test routine. Table 1 shows the interfacial contact resistances (at compaction pressure of 125 N/cm²) before and after performing a fuel cell accelerated stress test for the concept plates with gluing of BPP/GDL, and stainless steel plates for comparison.

Table 1

Interfacial contact resistance Interfacial contact resistance

(mQ cm 2) (mQ cm²)

Glued BPP/GDL Stainless steel 316 L

The concept plates (glued BPP/GDL) showed no degradation in interfacial contact resistance. The interfacial contact resistance of the anode plate decreased for some reason while the cathode plate had an unchanged value. It can be concluded that there was little degradation of the BPP/GDL contact. The interfacial contact resistances were quite high initially for these plates, but it should be possible to obtain a lower value when manufacturing the BPP/GDL plates. The stainless steel plates had a larger increase in interfacial contact resistance after the fuel cell accelerated stress test.

Claims

1 . Bipolar plate (BPP) with gas diffusion layer (GDL) for use in PEM fuel cells or other electrochemical cells, wherein parts of the bipolar plate are in direct contact with the gas diffusion layer, and parts of the bipolar plate are coated with a protective layer in areas where the bipolar plate is not in contact with the gas diffusion layer.

2. Bipolar plate (BPP) with gas diffusion layer (GDL) according to claim 1 , wherein the protective layer is selected from any anti-corrosive material.

3. Bipolar plate (BPP) with gas diffusion layer (GDL) according to claims 1 or 2, wherein the protective layer is an electrically non-conductive paint or glue.

4. Bipolar plate (BPP) with gas diffusion layer (GDL) according to claims 1 or 2, wherein the protective layer is an electrically conductive composite material.

5. Bipolar plate (BPP) with a gas diffusion layer (GDL), according to claims 1 to 4, wherein the bipolar plate comprises a metal plate cleaned before it is coated with the protective layer.

6. Bipolar plate (BPP) and gas diffusion layer (GDL), according to claims 1 to 5, wherein the gas diffusion layer is carbon-based.

7. Method of producing a bipolar plate with a gas diffusion layer wherein a bipolar plate is coated with a protective layer and a gas diffusion layer is pressed onto the coated bipolar plate at elevated compaction pressure before and during curing of the protective layer, thereby gluing the coated bipolar plate and the gas diffusion layer together, and resulting in direct contact between parts of the bipolar plate and the gas diffusion layer.

8. Method according to claim 7, wherein the bipolar plate is coated with protective layer by spray coating, dip coating, roll coating, or the like.

9. Method according to claims 7 or 8, wherein the protective layer is selected from electrically conductive and non-conductive anti-corrosive materials.

10. Use of the bipolar plate with gas diffusion layer according to claims 1 to 6 in PEM fuel cells and other electrochemical cells.

1 1 . Polymer electrolyte membrane fuel cell, PEM, comprising the bipolar plates (BPP) with gas diffusion layer (GDL) according to the claims 1 to 6.

12. Bipolar plate for PEM fuel cells or other electrochemical cells, prepared by applying a protective coating comprising an electrical conductive material on a conductive plate, and subjecting the coated plate to an elevated compaction pressure before and during curing of the protective layer.

13. Use of the bipolar plate according to claim 12 in PEM fuel cells and other electrochemical cells.