BR102014003458A2 - metal plate for a proton exchange membrane fuel cell - Google Patents

Abstract

bipolar metal plate for a proton exchange membrane fuel cell. a metal plate for a proton exchange membrane (pemfc) fuel cell having on at least one surface thereof a coating including: - conductive material fillers; - a polymer used as a binder; - a metal cation absorption compound.

Description

Scope of the Invention The present invention relates to a bipolar metal plate for a fuel cell and, more specifically, for a proton exchange fuel cell. (PEMFC). According to the invention, this plate has a coating consisting of a compound capable of absorbing metal cations, preferably in the form of an ionomer, for example perfluorsulfonic acid polymer (PFSA) type.

A cell with such a plate, especially usable for generating electric power, has better corrosion resistance and better performance as well as a longer life span. This type of cell can be targeted especially as a power source for future large-scale motor vehicles.

State of the Art A fuel cell is an electrochemical device that converts chemical energy directly into electrical energy. A fuel cell comprises a stack of several cells connected in series. Each cell generates a voltage on the order of 0.7V and its stacking allows to generate a higher voltage energy supply, for example on the order of hundreds of volts.

More specifically a proton exchange membrane fuel cell or PEMFC, it comprises at least one unit cell containing an electrode-membrane assembly or MEA, formed of a selectively releasing electron membrane proton and, on either side of this membrane, the anode and the cathode.

[0005] The membrane is generally made of a perfluoricosulfonic ionomer (PFSA) such as Nafion®. The electrodes, also called catalytic or active layers, contain a carbon-supported catalyst, preferably platinum (Pt), and possibly an ionomer, usually the same as the one forming the membrane.

At the anode level, dihydrogen (H2), used as fuel, is ionized to produce protons crossing the membrane. The electrons produced by this reaction migrate toward a flow plate and then cross an electrical circuit outside the cell to generate an electric current. At the cathode level, oxygen (02) is reduced and reacts with the protons that cross the membrane to form water.

Gas diffuser or GDL layers conventionally made of graphite fibers are interposed between the electrodes and the flow plates.

As already mentioned, a PEMFC may comprise a unit cell stack, and thereby a plurality of MEAs arranged between two flow plates. In this configuration, two adjacent unit cells are generally separated by the same plate, one of their surfaces in contact with the cathode compartment of a first MEA and their other surface in contact with the anode compartment of a second MEA (Figure 1). For this reason, PEMFCes flow plates are generally called “bipolar plates”.

With this a bipolar plate performs several functions, including in particular: - the distribution of gases (H2, 02, or air) and the discharge of water formed, possibly through the channels and / or doors formed therein - collection of electrons generated at the anodes of the different unit cells, which implies that the plate is an electrical conductor; - cooling unit cells, possibly through the flow of a coolant through that; - a mechanical support for the MEA.

For cost, size and performance reasons a bipolar plate for PEMFC is generally made of metal.

The bipolar plate is however in a corrosive environment which can cause its degradation over time with as a consequence the crystallization of metal ions in the case of a metal plate.

This corrosion can have several consequences: In extreme cases, the corrosion phenomenon can result in plate perforation, leading to loss of fit between the anode and cathode compartments.

Corrosion of the plate can lead to the formation of an insulating oxide layer in the metal, which increases the contact resistance with the gas diffusion layer, which increases the electrical resistance between the plate and the GDL, and decreases the resistance. fuel cell performance.

In addition, crystallized metal ions may alter the operation of the electrolytic membrane. With this, Wang et al. (Journal of Power Sources 183, 2008, 576-580) and Kelly et al. (Journal of Power Sources 145, 2005, 249-252) has highlighted the adverse influence of metal ions (Fe3 + and Cr3 +, Cu2 + and Ni2 + respectively) on the conductivity of a Nafion® proton exchange membrane. Metal ions can also “contaminate” the catalyst present in the electrodes, occupying the active places and thereby decreasing the catalyst activity. In fact, cell performance and durability can be negatively affected by bipolar plate degradation.

In order to reduce the corrosion of metal plates, the solutions employed in the past have especially understood the protection of said plates by means of a coating also having electrical conducting properties.

It may in particular be a coating containing noble metals or carbon in a polymer matrix.

In this regard, EP 2469634 describes a bipolar metal plate having a discontinuous film containing a noble metal (Au, Pt, Ir, Ru) and an oxide film formed on the part of the plate that does not comprise the discontinuous film.

EP 2112250 describes a bipolar plate having, on its surface, a passive chromium-containing layer and then a film made of metal nitride (MNx) metal / metal nitride (M / MNx), metal carbide (MCy) , or metal boride (MBz).

In addition, US 7699916 describes a bipolar plate having a polymer film coating containing conductive particles smaller than 10 micrometers.

FR 2971628 describes the insertion of layers made of a polyurethane matrix having fillers of conductive materials included therein between the metal bipolar plate and the gas diffusion layer.

In practice, coatings developed in the past technique physically protect the metal bipolar plate from the corrosive environment of the fuel cell.

There is, however, a continuing need to develop new technical solutions to limit the degradation of metal plates to PEMFCs and above all to improve the durability of the performance of such cells. Brief Description of the Invention The present invention fits in this context. In an original manner the Depositor developed a coating for a PEMFC sheet metal, forming both a physical and chemical barrier. The plate is thus protected from the corrosive environment which prevents it from being degraded. In addition, the cell nucleus, that is, the membrane as well as the electrodes, are protected from the possible release of metal ions, which allows them to maintain cell performance and durability.

Accordingly, and according to a first aspect, the invention relates to a metal plate having on at least one of its surfaces a coating comprising: - conductive material fillers; - a polymer used as a binder; - a metal cation absorber compound.

In the structure of the invention, "metal plate" means a plate containing metal, or even made of metal, preferably stainless steel. The materials used to form metal-containing bipolar plates are presented in Hermann et al. (International Journal of Hydrogen Energy 30 (2005) 1297-1302).

[0027] The metal plate is thereby made of an electrically conductive material particularly allowing to collect the electrons generated at the anode of the cell. Therefore in the framework of the invention it may be called a "flow plate" as well as the "current collection plate" or "bipolar plate".

In the context of its use in a fuel cell, this plate preferably has a rectangular or even square shape, with a surface area in the range of 100 to 1000 cm2, for example 220 cm2. Its thickness can vary between 50 and 500 pm and can be, for example, 200 pm. It should be noted that a plate may actually be two or more stacked plates, preferably welded, which allow cooling channels to form between the plates.

Again, in relation to its use in a cell, such a plate is preferably provided with teeth and / or channels. Better yet, the teeth are located on the surface of the plate in contact with the electrode to collect the current produced, while the channels allow gases to enter and discharge the produced water.

Typically according to the invention, such a plate is provided with a coating on at least one of its surfaces, preferably that facing to be in contact with the gas diffusion layer. In practice and in the case of stacking an MEA and a bipolar plate, the metal plate is coated on both surfaces. According to a specific context the coating forms a continuous layer over the entire surface of the plate.

Preferably the coating appears in the form of a continuous layer having a preferably uniform thickness. Also preferably also the coating thickness is in the range of from 1 to 50 micrometers or even from 10 to 20 micrometers.

Such a coating comprises at least one polymer forming a polymer matrix and / or behaving as a binder capable of giving the coating an acceptable mechanical strength. In addition, such a polymer must withstand the operating conditions of a cell. A polymer blend may also be included.

According to a specific context, this polymer is of the polyurethane type, ie a urethane polymer. In practice, the polymer may be obtained by condensation between a polyol (e.g. Bayers Bayhydrol®) and a polyisocyanate, preferably a diisocyanate (e.g. Bayer Bayhydur®).

To give the desired electrical conductive character to the coating, this coating also comprises fillers of conductive materials, preferably included in the polymer matrix. These fillers may be made of carbon black, graphite, carbon fibers, or a mixture thereof.

Preferably, the fillers appear in the form of particles having a size smaller than one micrometer. As an example, carbon black having a nanometer grain size preferably less than 100 nanometers may be used. The fillers may particularly appear in the form of clusters of carbon black nanoparticles.

The filler mass ratio is preferably in the range of 35 to 55% (dry weight of the fillers relative to the dry weight of the total coating) or even 40 to 50%.

Typically, according to the invention, the coating also has the property of absorbing metal ions, in particular metal cations capable of being crystallized by the metal plate.

In the case of steel plate, these are particularly iron ions (Fe3 +), chromium ions (Cr3 +) and possibly nickel (Ni2 +) or molybdenum (Mo2 +) ions.

According to a first context, said compound may be activated carbon which is believed to absorb metal ions due to its high porosity.

Preferably the metal cation absorption compound has tonic and / or ionizable functions or groups. In the specific case where the compound is a polymer, it may comprise ionic or ionizable units, thereby forming an ionomer.

Given the cationic nature of the metal ions present, the metal ion absorber compound preferably resists anionic functions and preferably sulfonic functions (S03H / S03-).

In practice an acid base reaction occurs between the M + cation and the S03- function of the compound, in equilibrium strongly shifting towards the formation of an intimate ionic pair. Metal cations have very low mobility since these ions have a strong base affinity due to the fact that they form a 'soft' acid unlike 'hard' protons (whereby a low affinity and greater mobility).

According to the specific context, the metal cation absorption function can be directly ensured by the polymer behaving as a binder, which preferably amounts to 45 to 65% by weight of the final coating (polymer dry weight relative to total dry weight). preferably from 50 to 60%. In this particular case, the coating comprises, or is made of, an ionomer capable of absorbing metal cations and behaving as a binder, as well as fillers of conductive material.

According to another context the polymer behaving as a binder and the metal cation absorption compound are two different molecules. Generally the metal cation absorbing compound amounts to 1 to 10% by coating weight (dry weight of the metal cation absorbing compound relative to the total coating dry weight) or even 5 to 10%.

According to a preferred context, the metal cation absorber compound is also polymeric, thereby forming an ionomer, preferably a cation absorber ionomer. In the context of the invention a "cation absorbing ionomer" means an ionomer having its ionized and / or ionizable groups particularly allowing to absorb positively charged metal ions.

In this case, the polymer used as a binder and the metal cation absorber polymer preferably amount from 45 to 65% by weight of the final coating (polymer dry weight relative to the total coating dry weight) more preferably from 50 to 65%. 60%. Within the polymer component of the coating, the metal cation absorber ionomer accounts for from 1 to 10% by weight of the final coating (dry weight of the metal cation absorber ionomer relative to the total coating dry weight) even better from 5 to 10%.

This ionomer may particularly be a polymer having its backbone comprising CFn groups (n = 1, 2), in addition to accepting sulfonic acid groups in the form of acid or salt formed. It may be particularly a perfluorsulfonic polymer or perfluorsulfonic acid (PFSA). PFSAs are ionomers derived from perfluorsulfonic acid, that is comprising SO3 sulfonate groups. They are also fluorinated polymers.

With this, unexpectedly, the Depositor has shown that ionomers currently used in the context of membrane-contained PEMFCs to ensure proton transport and possibly catalytic layers could be incorporated into the coatings according to the invention. These ionomers used for their ability to absorb protons (H +) are thus used on the plate to absorb metal cations.

Generally, PFSA polymers have a formula - (CF2-CF2) x - (CF (O-RI-SO3 H) -FC2) where Rf is a fluorinated carbon group, x and y positive integers.

Such ionomers are commercially available, for example under the registered name Nafion® (Dupont), Aquivion® (Solvay), Flemion® (Asahi Glass), Fumion® (Fumatech) or Aciplex® (Asahi Chemical Company).

A particularly adapted ionomer is a sulfonated tetrafluoroethylene derivative, for example sold under the tradename Nafion® (Dupont).

As an example Nafion® D2020 product number CAS31175-20-9 may be used in the context of the present invention.

Such an ionomer preferably has the following chain footprint structure.

[0054] k being a positive integer.

Another suitable ionomer is tetrafluoroethylene and copolymer Vinyl Fluoride Sulfonyl, for example, sold under the name Aquivion (Solvay), preferably under CAS number 111173-25-2.

Such an ionomer preferably has the following chain footprint structure.

According to a specific context, the ionomer present in the coating according to the invention is the same as that forming the cell membrane, and possibly as present in the catalytic layers of said cell.

As mentioned above the metal ion absorbing compound preferably an ionomer plus a PFSA such as Nafion® may add from 1 to 10% by weight of the coating (dry weight of the compound relative to the dry weight of the total coating) or even 5 to 10%.

Where so. the same ionomer as the one that formed the cell membrane, the amount of ionomer present in the coating can be expressed as a% of the weight of the membrane weight. Preferably the mass ratio of the ionomer in the coating is from 0.5 to 5% of the cell membrane weight.

Therefore preferably the metal plate coating according to the invention comprises, in dry weight relative to the dry weight of the coating: - from 35 to 55% of the conductive material filler - from 45 to 65% of the polymer, including from 1 to 10% of the metal cation absorbing compound preferably when said compound is an ionomer.

According to a preferred context of the invention the coating comprises or is made of: - from 40 to 50% of the fill of conductive material, preferably carbon black; - from 50 to 60% of polymers, including from 5 to 10% of the metal cation absorbing compound, preferably polyurethane and PFSA as Nafion®.

The coating may also comprise at least one additive, which may be selected from a group containing: a surface active agent such as SDS (sodium dodecyl sulfate) or Triton X100, to promote dispersion of the filler in the solution giving the coating and / or - a surface agent, such as Byk33, for leveling the surface of the coating, thereby giving better electrical contact.

These additives may however have the disadvantage of contaminating the membrane. Notably, it has been observed in the context of the invention that the presence of the metal cation absorbing compound could make it possible to do without a surface active agent and thereby avoid possible membrane contamination. This is particularly true when the metal cation absorbing compound is an ionomer having a hydrophobic chain and a hydrophilic group, particularly a PFSA such as Nafion®. Accordingly and in a specific context, the coating according to the invention does not contain any additional active surface agents.

In the adapted manner, such plate is for fuel cell, proton exchange membrane fuel cells preferably.

Accordingly, in another aspect, the present invention is directed to a proton exchange membrane fuel cell (PEMFC) comprising at least one metal plate as defined above.

Preferably, in this context, the metal plate has a coating at least on its surface in contact with the gas diffusion layer (GDL). In the case of a bipolar plate and a cell stack, the plate comprises a coating as defined above on each surface.

As already mentioned, the metal cation absorbing compound is preferably the same ionomer as that forming the membrane and / or that present in the catalytic layers preferably a PFSA such as Nafion®.

According to another aspect, the present invention relates to a composition and method giving a coated metal plate as defined above.

Typically, this composition comprises: fillers of conductive materials - a polymer designed to behave as a binder or precursors thereof; a metal cation absorbing compound preferably an ionomer; - a solvent.

The catalyst solvent should allow polymer solubilization to behave as a binder and / or ionomer and may be water and / or alcohol (ethanol, propanol ...). It may, of course, be a solvent mixture. In the composition according to the invention, the dry matter mass ratio is typically in the range of 1 to 20% particularly depending on the deposition method, the solvent then constituting from 80 to 99% by mass of the composition.

The mass ratios, in composition, fillers of conductive material, binder polymer, and metallic cation absorber compound are such that, after evaporation of the excipient solvent, their mass ratios in the coating are as indicated above. .

It should be noted that the polymer behaving as a binder can be obtained directly by polymerization in the composition even in situ on the plate. In this case the monomers or precursors adapted or may be used instead of the final polymer. In the case of polyurethane, for example, the composition may contain a polyol (such as Bayerol® marketed by Bayer) and a polyisocyanate (such as Bayhydur® marketed by Bayer). In practice the polymerization occurs by heat treatment typically under the following conditions: - at a temperature in the range of 130 ° C to 170 ° C, preferably equal to 135 ° C. - For a duration of a few minutes to a few hours, preferably 30 to 60 minutes.

As mentioned with respect to the coating, as a composition may also comprise additives.

It may particularly comprise a surface active agent, such as SDS (Sodium Dodecyl Sulphate) or Triton X100. It is actually known that a surface active agent helps to disperse carbon black and the binder in the solvent phase. The mass ratio of the surface active agent in the composition should however be as low as possible since, as already mentioned, it can contaminate the membrane by crystallization. It is preferably less than 1%, typically between 0.1 and 1%.

Similarly for the preparation of a composition intended for the manufacture of a conventional coating, the composition is prepared as follows: dissolving the surface active agent in the solvent, preferably while stirring; addition of conductive material fillers preferably carbon black; - addition of polymer binder; adding the metal cation absorbing compound, preferably an ionomer, most preferably a PFSA such as Nafion®;

Possible dispersion of fillers of conductive materials, preferably carbon black, for example with a magnetic bar or ultrasound.

Notably, the metal cation absorbing compound, preferably an ionomer, most preferably a PFSA such as Nafion® may help to disperse conductive material fillers in the solvent system.

According to a specific context, the composition of the invention as well as the coating obtained therefrom does not contain any other surface active agent.

In this case the composition preparation protocol is simplified: - dissolving the metal cation absorption compound, preferably an ionomer, more preferably a PFSA such as Nafion®, in the solvent, preferably while stirring; addition of conductive material fillers, preferably carbon black; - addition of polymer binder; possible dispersion of fillers of conductive materials, preferably carbon black, for example with magnetic bar or ultrasound.

As already mentioned, other additives may be present in this composition, such as surface agents allowing to smooth the coating surface.

The present invention also relates to a method of preparing the metal plate comprising a coating as defined above comprising the steps of: depositing the composition described on a metal plate; - drying of the metal plate.

Drying is performed at a temperature allowing evaporation of the present solvent system, for example, between 60 and 80 ° C to a water / ethanol mixture.

In the event that the polymer behaves as a binder not in its polymerized form, a heat treatment may be carried out under the conditions described above. These heat treatments can be performed after the plate has dried, or it can replace drying.

Deposition of the composition to form the coating may be accomplished by any conventional method known to those skilled in the art, particularly by spreading, spraying, spreading, roller or silk screen.

The invention and the resulting advantages will appear better from the following drawings and examples, given as a non-limiting illustration of the invention. Description of the Drawings Figure 1 illustrates the corrosion phenomenon within a proton exchange membrane fuel cell.

Context Examples 1 / Deterioration of a fuel cell;

Figure 1 illustrates a fuel cell comprising a stack of MEAs (membrane-electrode assemblies) separated by a bipolar plate (7).

Each MEA is formed of: An anode (3) where hydrogen oxidizes to give two protons and two electrons: H2 -> 2H- + 2e-; - a proton exchange membrane (2); - a cathode (1) where oxygen is reduced in water: 2H + + 2e- + 1/2 02 -> H2).

Each electrode is supported by a current collector, in this case a bipolar plate (7). This bipolar plate (preferably two plates welded together to have cooling channels between them) (7) has a structure that allows the passage of reactive gases (H2 and 02 or air) but also a coolant (8).

In this specific context the metal bipolar plate is arranged to allow cooling of a fuel cell comprising a unit cell stack.

During fuel cell operation, metal bipolar plates (7) may degrade due to the corrosion phenomenon, particularly caused by the presence of an oxidizer (oxygen) or an acid medium (protons originating from the anode reaction).

As already indicated, corrosion of the metal bipolar plate (7) generates metal ions that can: - (5) poison the catalysts present in the electrodes (1, 3); - (4) decrease the membrane proton conductivity (2); - (6) cause formation of the passivating film on the bipolar plate (7).

The metal bipolar plate constituting the object of the invention makes it possible to overcome these drawbacks due to the nature of its coating, in particular the presence of a compound, preferably an ionomer, capable of capturing metal ions.

In the absence of such a coating, corrosion may cause perforation (9) of the proton exchange membrane (2) and even bipolar plate (7) as shown in Figure 1.

The bipolar plate carries the electrons generated at the anode (3) towards the cathode (1). It also allows to strongly separate the anode from the cathode to avoid any contact between gases (H2 and 02 or air) in the electrodes.

In addition, the metallic bipolar plate gives the cell a mechanical support. It may also allow the distribution of gases (H2 and 02 or air) in the electrodes.

The water formed in the cathode may also be realized due to the structure of the bipolar metal plate.

Finally the thermal regulation of the cell can be given by the circulation of a coolant inside the metal bipolar plate (8). 2 / Protection of the metal plate by means of a coating 2-1 Composition used for coating deposition An ink capable of coating the invention is prepared according to the following protocol: - Weighing of the metal cation absorber ionomer ( Nafion®) type PFSA: ie 1.6 g dry extract totaling 20% (ie 0.32 g); - Solvent weighing: 7: 1 water / ethanol mixture, in a ratio: 400 g of water + 60 g of ethanol; - Mix the ionomer in the solvent by means of a magnetic bar; - Weigh out carbon black (Vulcan XC72 carbon = conductive material fillers): 4 g; - Mix with a metal bar; - Weighing of binder in solvent phase: product A (polyurethane, Bayer Bayhydrol® UXP 2239, 10 g with 33% dry extract, ie 3.3 g) and product B (Bayer Bayhydur® BL 5335 isocyanate, 6 g with 33% dry extract ie 2g); - Ultrasonic bath dispersion to break carbon pools and magnetic bar mixing.

The final paint comprises a 2% dry extract and the solvent has a water / alcohol ratio of 7.

The coating obtained by this composition has the following mass composition: - conductive particles: Vulcan XC72 carbon: 41.6% - binder: Bayer Bayhydrol® + Bayhydur® polyurethane 55% - metal cation absorbing ionomer: Nafion ® 3.4%. 2-2 Obtaining the coating The coating that is deposited by spraying can also be spread, with rollers, or silkscreen.

The spray nozzles spread throughout the bipolar plate in successive lines. Repeating this cycle allows depositing successive thin layers having a thickness of the order of one pm. The deposition of about twenty layers makes the final deposit.

The coating deposition protocol is as follows: - Cleaning the bipolar plate, for example with acetone and alcohol. - Begin the process and set the tray at 60 ° C. - Fill the tank with paint. - Multi layer deposition in successive cycles. Drying occurs along with paint deposition. - Stop the process and remove the coated plate. - 135 ° C heat treatment to crosslink the polymer for 60 minutes.

Claims (12)

1. Metal plate for a proton exchange membrane fuel cell (PEMFC), characterized in that it has, on at least one of its surfaces, a coating comprising: - conductive material fillers; - a polymer used as a binder; - metal cation absorption compound. Metal plate according to Claim 1, characterized in that the metal cation-absorbing compound is an ionomer, advantageously a perfluorsulfonic polymer (PFSA). Metal plate according to Claim 1 or 2, characterized in that the fillers of conductive material are made of carbon black, graphite, carbon fibers or a mixture thereof. Metal plate according to any one of Claims 1 to 3, characterized in that the polymer used as a binder is polyurethane. Metal plate according to any one of Claims 2 to 4, characterized in that the coating comprises, in dry weight in relation to the dry weight of the coating: - from 35 to 55% fillers of conductive material. - 45 to 65% polymer, including 1 to 10% metal cation absorption compound. Metal plate according to any one of Claims 1 to 5, characterized in that the coating comprises no surface agents. Proton exchange membrane fuel cell (PEMFC), characterized in that it comprises at least one metal plate as defined in any one of claims 1 to 6. Proton exchange membrane fuel cell (PEMFC) according to claim 7, characterized in that the metal plate has a coating at least on its surface in contact with the gas diffusion layer (GDL). Proton exchange membrane fuel cell (PEMFC) according to claim 7 or 8, characterized in that the metal cation absorption compound is preferably the same ionomer as the one forming the membrane and / or the present one. in the catalytic layers. Composition for forming the metal plate coating as defined in claims 1 to 6, characterized in that it comprises: - fillers of conductive material; - polymer intended to act as a binder or precursors thereof; - metal cation absorption compound; - a solvent. Method of preparing the metal plate as defined in any one of claims 1 to 6, characterized in that it comprises the steps of: - depositing the composition as defined in claim 10 on a metal plate, advantageously by spreading, spraying, applying with rolls or by screen printing; - drying of the metal plate. Method of preparing a metal plate according to claim 11, characterized in that, after drying, the plate is heat treated to obtain the polymer from the precursors.