Field of the invention

The present invention relates to a method for preparing a gas diffusion layer for fuel cell comprising a heating treatment with a laser light source, and in particular with a vertical-cavity surface-emitting laser (VCSEL). The invention also concerns a gas diffusion layer for fuel cell electrode prepared according to the method of the invention, as well as to an electrode for fuel cell comprising a gas diffusion layer according to the invention. Finally, the invention is directed to a fuel cell comprising an electrode according to the invention, and to a vehicle, an electronic device, or a stationary power generating device, comprising a battery according to the invention.

Technological background

Fuel cells are electrochemical cells that convert the chemical energy of a fuel (generally hydrogen) and an oxidizing agent (generally oxygen) into electricity through a pair of redox reactions. In a fuel cell electrode, hydrogen is decomposed into hydrogen ions and electrons. The hydrogen ions migrate toward the air electrode through the electrolyte membrane, and the electrons migrate toward the air electrode through conductive lines and loads constituting an external circuit. An oxidizer, generally air, is supplied to the air electrode. In the air electrode, the hydrogen ions and the electrons react with oxygen in the air to generate water. Fuel cells have high energy efficiency and only emit water, and thus are expected to serve as clean energy. Therefore, when the electrical load increases, that is, when the current to be extracted outside the cell is increased, a large amount of water (water vapor) is produced. At a low temperature, such water vapor is condensed into water drops, blocking pores of the gas diffusion electrode. As a result, the amount of gas (oxygen or hydrogen) supplied to the microporous layer decreases, and when all the pores are blocked eventually, power generation ceases (this phenomenon is called "flooding"). In order to minimize the occurrence of flooding, in other words, in order to maximize the current value that causes flooding, a gas diffusion electrode is required to have water drainage. As a means for enhancing the water drainage, usually, a gas diffusion electrode including an electrically conductive porous substrate that has been subjected to a water-repellent treatment is used.

Technical problem of the invention

Inside the fue! cel! electrode, the microporous layer (MPL) has the function of ensuring a uniform gas distribution and to prevent water accumulation (flooding) near the membrane. Typically, the MPL is coated as an ink on the gas diffusion layer (GDL) substrate, and then dried to remove the ink solvents (for example, water). Since the ink contains surfactant agents to keep the ink stable during the coating process, an additional heat treatment step after solvent drying is necessary to eliminate the surfactants and give the MPL its necessary hydrophobic properties. It is therefore essential that the surfactant be completely removed from the MPL. The heat treatment is generally achieved in conventional ovens, but in this case, the heat treatment process is relatively long (100's of seconds) and requires high equipment costs. In addition, the long heat treatment times can result in an uncontrolled re-distribution of some of the components (such as fluorinated resin) of the MPL. In particular, polytetrafluoroethylene (PTFE) can migrate from the MPL layer into the GDL, thereby reducing the water removal ability of the MPL. Therefore, it would be advantageous to have a heat treatment process which can completely remove the surfactant of the MPL in a short period of time and avoids migration of essential components to maintain the MPL water management function as its optimal design point.

Summary of the invention

It has now been found that the use of a specific type of laser source to substitute the high temperature oven during the heat treatment can lead in a reduction in the process time to less than 1 second. This heating process relies on energy transfer by radiation instead of convection and/or conduction, as it is the case with conventional ovens. An additional benefit of using the specific type of laser source of the invention for the heat treatment of MPL is that given its high energy transfer rate, it prevents the re-distribution of components such as PTFE into the GDL thereby improving the water removal ability of the MPL in fuel cell electrodes. Another advantage of the invention is that the method flow of device preparation of GDL is simplified, and the equipment cost significantly reduced.

Summary of the invention

In a first aspect, the present invention aims at a method for preparing gas diffusion layer (GDL) for fuel cell comprising a heating treatment of a microporous layer (MPL) on a GDL substrate with a laser light source.

In another aspect, the invention relates to a GDL for fuel cell prepared according to the method of the invention, as well as an electrode for fuel cell comprising a GDL according to the invention and a fuel cell comprising an electrode according to the invention.

Finally, the invention also relates to a vehicle, an electronic device, and a stationary power generating device, comprising a fuel cell according to the invention.

Brief description of the Figures

Figure 1 represents images made by SEM-EDS showing the difference between a GDL structure according to the invention (VCSEL) and a GDL structure representative of the prior art (OVEN).

Figure 2 shows the ThermoGravimetric Analysis (TGA) results for two different samples: GDL of Example 1 (invention) (dotted curve) and GDL of Comparative Example 1 (prior art) (solid curve).

Figures 3, 4 and 5 shows current-voltage curves obtained for two different samples and at three different % of relative humidity (% RH): - Sample 1: GDL having undergone a heat treatment in a traditional oven

(GDL of Comparative Example 1),

- Sample 2: GDL having undergone a heat treatment with VCSEL (GDL of

Example 1).

Detailed description of the invention

The present invention relates to a method for preparing a gas diffusion layer (GDL) for fuel cell comprising the following steps:

(i) forming a microporous layer (MPL) on a GDL substrate made of carbon fibers, by applying a carbon aqueous dispersion, and

(ii) heating the MPL formed in step (i) with a laser light source, preferably with a semiconductor laser diode, and more preferably with a vertical-cavity surface- emitting laser (VCSEL).

The GDL substrate on which the MPL is formed is a conductive porous substrate advantageously made of a porous material having a pore diameter ranging from 0.5 to 20 pm. The GDL substrate contains carbon fibers such as a carbon fiber woven fabric, a carbon fiber paper sheet, a carbon fiber nonwoven fabric, carbon felt, carbon paper, and carbon cloth. Conductive porous substrates containing carbon fibers, such as carbon felt, carbon paper, and carbon cloth are preferred in view of their excellent corrosion resistance.

During step (i), the GDL substrate is subjected to a coating treatment by applying a carbon aqueous dispersion. To this end, the carbon aqueous dispersion is impregnated onto the substrate to form a GDL having a multi ^¬ layered microporous structure with a uniform thickness.

In an embodiment, the carbon aqueous dispersion applied in step (i) comprises carbon powder, at least one polymer resin, at least one fluorinated resin, and at least one surfactant, dispersed in at least one aqueous solvent. The mixture is stirred to disperse the carbon powder. The mixing speed may be from 500 to 10 000 rpm. If the mixing speed is less than 500 rpm, the carbon powder may not be effectively dispersed. On the other hand, if the mixing speed exceeds 10 000 rpm, excess heat may be generated in the carbon aqueous dispersion, thereby changing the composition of the aqueous dispersion.

The carbon aqueous dispersion includes carbon powder with porosity and electrical conductivity. The carbon powder may be active carbon, possibly in the form of active carbon fiber, carbon black, carbon aerosol, carbon nanotube, carbon nanofiber, natural or synthetic graphite, or a mixture thereof, preferably with an average particle size ranging from 20 to 5 000 nm. In a preferred embodiment, the carbon powder is advantageously carbon black since it has a very high absorption rate, which makes heating by radiation very effective.

The polymer resin can be selected from polyethers such as polyethylene oxide, polyethylene glycol, or polyacetaldehyde; polysulfides; polyesters; polycarbonates; ethylene-propylene-elastomers (EPDMs); polyethylenes; polypropylenes; polyvinyls such as PVCs and polyvinylfluorides; and polysaccharides such as celluloses, cellulose derivatives, and starches; and mixtures thereof. The polymer resin is preferably a polyether resin selected from polyethylene oxide, polyethylene glycol, polyacetaldehyde, and mixture thereof, and more preferably polyethylene oxide.

The polymer resin is used to adjust the rheological properties of the carbon aqueous dispersion and to facilitate the dispersion of the carbon powder. The polymer resin of the invention is different from the fluorinated resin defined below. The polymer resin as defined in the invention is therefore a non-fluorinated resin.

The fluorinated resin is advantageously selected from polyethylene, polypropylene, polytetrafluoroethylene (PTFE), styrene-butadiene rubber, tetrafluoroethylene-hexafluoropropylene copolymers, tetrafluoroethylene- hexafluoropropylene copolymers (FEP), tetrafluoroethylene-perfluoroalkyl vinyl ether copolymers (PFA), vinylidene fluoride-hexafluoropropylene copolymers, vinylidene fluoride-chlorotrifluoroethylene copolymers, ethylene- tetrafluoroethylene copolymers (ETFE resins), polychlorotrifluoroethylene (PCTFE), vinylidene fluoride-pentafluoropropylene copolymers, propylene- tetrafluoroethylene copolymers, ethylene-chlorotrifluoroethylene copolymers (ECTFE), vinylidene fluoride-hexafluoropropylene-tetrafluoroethylene copolymers, vinylidene fluoride-perfluoromethyl vinyl ether- tetrafluoroethylene copolymers, ethylene-acrylic acid copolymers, and mixtures thereof. Polytetrafluoroethylene (PTFE) is the most preferred fluorinated resin.

The surfactant is advantageously selected from anionic surfactants, cationic surfactants, amphoteric surfactants, nonionic surfactants, and mixtures thereof, which can disperse the carbon powder and which have good compatibility with the fluorinated resin. The surfactant may be selected from cationic surfactants such as alkyltrimethylammonium salts, alkyldimethylbenzylammonium salts, or amine phosphates; anionic surfactants such as polyoxyalkylenealkylethers, polyoxyethylene derivatives, alkylamineoxides, or polyoxyalkyleneglycols; amphoteric surfactants such as alanines, imidazoliumbetains, amidepropylbetains, or aminodiproionates; or nonionic surfactants such as alkylarylpolyetheralcohols. In a preferred embodiment, the surfactant is a nonionic surfactant. Polyoxyethylene (Cs- C22)alkyl ethers, such as polyoxyethylene (10) tridecylether, are the most preferred surfactants.

The solvent is advantageously water and/or an alcohol such as n- propanol, isopropanol, or a mixture thereof. Water is the most preferred solvent.

In a preferred embodiment, the carbon aqueous dispersion applied in step (i) comprises:

(a) from 5 to 30 wt%, and preferably from 10 to 20 wt%, of carbon powder,

(b) from 0.01 to 10 wt%, and preferably from 0.05 to 8 wt%, of at least one polymer resin,

(c) from 0.05 to 8 wt%, and preferably from 0.1 to 5 wt%, of at least one fluorinated resin,

(d) from 0.1 to 5 wt%, and preferably from 0.5 to 3 wt%, of at least one surfactant, and

(e) from 50 to 95 wt%, and preferably from 60 to 90 wt%, of at least one aqueous solvent. In another preferred embodiment, the carbon aqueous dispersion applied in step (i) comprises:

(a) from 5 to 30 wt%, and preferably from 10 to 20 wt%, of carbon black,

(b) from 0.01 to 10 wt%, and preferably from 0.05 to 8 wt%, of polyethylene oxide,

(c) from 0.05 to 8 wt%, and preferably from 0.1 to 5 wt%, of polytetrafluoroethylene (PTFE),

(d) from 0.1 to 5 wt%, and preferably from 0.5 to 3 wt%, of polyoxyethylene (10) tridecylether, and

(e) from 50 to 95 wt%, and preferably from 60 to 90 wt%, of water.

The carbon aqueous dispersion can be easily coated due to its soft characteristics, thereby making it possible to use a common coating method such as die coating, roll coating, gravure coating, or knife coating. The carbon aqueous dispersion is preferably applied by slot die coating.

The microporous layer (MPL) formed in step (i) can be dried, for example at a temperature ranging from 100 to 250°C, and preferably from 150 to 220°C, before the heating step (ii). The MPL can be dried in a conventional oven. After drying, no macro-cracks and micro-cracks can be observed on the MPL.

The MPL on a GDL substrate obtained at the end of step (i) is then submitted to a heating step (ii). During this thermal treatment, the MPL formed in step (i) is heated with a laser light source.

In a preferred embodiment, the heating treatment of step (ii) is performed with a semiconductor laser diode.

In a more preferred embodiment, the heating treatment of step (ii) is carried out with a vertical-cavity surface-emitting laser (VCSEL). VCSEL, is a type of semiconductor laser diode with laser beam emission perpendicular from the top surface, contrary to conventional edge-emitting semiconductor lasers (also in-plane lasers) which emit from surfaces formed by cleaving the individual chip out of a wafer. VCSEL has the advantage that it can be tested at several stages throughout the process to check for material quality and processing issues.

The heating step (ii) is advantageously carried out at an energy density of at least 10 W.cm ² , more advantageously at an energy density from 10 to 140 W.cm ² , and even more advantageously at an energy density from 50 to

140 W.cm ² .

In an embodiment, the heating step (ii) is carried out at a wavelength ranging from 900 to 1 100 nm, and more advantageously at a wavelength ranging from 950 to 1 000 nm. In an embodiment, the heating step (ii) is carried out in less than one second, preferably in less than 0.8 second, and more preferably in less than or equal to 0.4 second. VCSEL units have higher energy densities (in W/cm ² ) compared to traditional Infra-Red (IR) lamps, so the heat treatment time can be as low as or equal to 0.4 second (compare to 160 seconds for conventional oven).

During the heating step (ii), the distance between the laser source and the MPL may range from 10 mm to 30 cm, preferably from 15 mm to 25 cm, and more preferably from 20 mm to 20 cm.

In an embodiment, the MPL thickness ranges from 0.1 to 50 pm, preferably from 0.5 to 40 pm, and more preferably from 2 to 30 pm, after the heating step (ii). The MPL thickness may be measured, for example, by Scanning Electron Microscopy (SEM). Advantageously, the MPL has an area density (or a weight per unit area) ranging from 0.5 to 15 mg/cm ² , preferably from 0.5 to 10 mg/cm ² , and more preferably from 0.5 to 5 mg/cm ² . In an embodiment, the GDL thickness ranges from 50 pm to 500 pm, and preferably from 80 to 200 pm. The GDL thickness may be measured, for example, by Scanning Electron Microscopy (SEM). Advantageously, the GDL has an area density (or a weight per unit area) ranging from 1 to 40 mg/cm ² , preferably from 1 to 30 mg/cm ² , and more preferably from 1 to 20 mg/cm ² . Only the carbon powder, the polymer resin and the fluorinated resin are left in the MPL formed on the GDL substrate, after the thermal treatment of step (ii).

The present invention also provides a GDL for fuel cell prepared according to the method of the invention, comprising:

(a) a gas diffusion layer substrate made of carbon fibers, covered by

(b) a MPL made of carbon powder, at least one polymer resin, and at least one fluorinated resin, said GDL being free from surfactant.

In a preferred embodiment, the GDL for fuel cell according to the invention comprises a MPL (b) made of carbon powder, at least one polymer resin, and at least one fluorinated resin, wherein the fluorinated resin of the MPL (b) is in the form of agglomerates of at least 0.05 pm, preferably from 0.05 to 1.2 pm, more preferably from 0.2 to 1 pm, and even more preferably from 0.3 to 0.8 pm. The size of the fluorinated resin agglomerates can be measured, for example, by Scanning Electron Microscopy with Energy Dispersive Spectroscopy (SEM-EDS).

On the contrary, in the GDL for fuel cell of the prior art, the fluorinated resin is melted into the GDL. This difference of GDL structure is shown in Figure 1 where the presence of the fluorinated resin in the form of agglomerates is observed in the GDL of the invention (VCSEL) whereas the fluorinated resin is completely melted in the GDL of the prior art (OVEN) (images made by SEM-EDS).

An electrode for fuel cell comprising a GDL according to the invention, as described above, is also part of the invention.

In the sense of the invention, the expression "electrode for fuel cell" refers to an anode or a cathode used in a fuel cell. Oxidation for fuel occurs at the anode, and reduction of oxygen occurs at the cathode. In both the anode and the cathode, the electrode is made of a catalyst layer, a MPL and a GDL.

The invention also concerns a fuel cell comprising an electrode for fuel cell according to the invention. In the sense of the invention, a fuel cell comprises:

- an anode,

- a cathode, and

- a hydrogen ion conductive electrolyte membrane, and

- two bipolar plates (on one side of the anode and on one side of the cathode), wherein at least one of the anode and the cathode includes a GDL prepared according to the method of the invention.

The two bipolar plates have flow fields. They supply fuel to the anode and oxidant to the cathode, and provide electrical conductivity.

Lastly, the invention relates to a vehicle, an electronic device, or a stationary power generating device, comprising a fuel cell according to the invention.

Any combination of the above described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context. Thus, all features and embodiments described herein in particular as applicable, advantageous or preferred in the context of the invention are to be construed as being applicable in combination with one another, in preferred embodiments of the invention.

Examples

Example 1: Preparation of a GDL for fuel cell electrode according to the method of the invention (with a heat treatment with a VCSEL)

A fuel cell electrode was prepared according to the method of the invention as follows:

Preparation of a carbon aqueous dispersion for the MPL:

774 g of ultrapure water were added to 160 g of acetylene black and stirred in a high speed mixer (2 000 rpm, 30 minutes). 16 g of a surfactant (polyoxyethylene (10) tridecylether) was added to the solution, and the mixture was stirred for another 30 minutes. 10 g of a 1.2% aqueous solution of polyethylene oxide were added, and the mixture was stirred for another 30 minutes. Finally, 47 g of a 60% aqueous PTFE dispersion sere added, and the mixture was stirred in a low speed mixer (500 rpm) for 2 hours to obtain a carbon aqueous dispersion.

Formation of a M PL on a GPL substrate bv applying a carbon aqueous

A spool of GDL substrate (Freudenberg Performance Materials SE & Co. KG) with an area density of 11 mg/cm ² and an average GDL thickness of 145 pm was coated with the MPL carbon aqueous dispersion prepared above by using a slot die coater (Coatema® Coating Machinery GmbH). The coated GDL was then dried inside a hot-air oven at 180°C for 1 minute.

The GDL area density was measured as follows: cutting a GDL substrate sample with a 25 mm diameter handheld punch, then measuring the weight of the sample with a high precision scale, and dividing the area of the sample by its weight to obtain a weight per unit area (in mg/cm ² ).

The GDL thickness was determined by SEM.

Heat treatment of the MPL with a VCSEL:

A 2.2 kW VCSEL module (Trumpf GmbH) with 12 emitters was used for the heat treatment of the GDL coated with the MPL prepared above, as well as a suction unit to remove any vapour during heat treatment.

Parameters of the heat treatment:

- Wavelength: 980 nm,

- Duration: 0.4 second,

- Working distance between the laser source and the MPL: 50 mm,

- Energy density: 46 W/cm ² , and

- Substrate speed: 8 m/min.

The heat-treated MPL has an area density of 2 mg/cm ² and an average MPL thickness of 5 pm on top of the GDL. The MPL area density was measured as follows: cutting a MPL substrate sample with a 25 mm diameter handheld punch, then measuring the weight of the sample with a high precision scale, and dividing the area of the sample by its weight to obtain a weight per unit area (in mg/cm ² ).

The MPL thickness was determined by SEM.

After heat treatment, the contact angle at the surface of the MPL went from 90° to 150°, proving that the treatment removes the remaining surfactant and renders the MPL layer completely hydrophobic.

Comparative Example 1: Preparation of a GDL for fuel cell electrode according to a method of the prior art (with a conventional drying in an oven)

The same protocol as described in Example 1 was repeated, except that the heat treatment was carried out on a conventional oven (hot air) at a temperature a 120°C for 120 seconds.

The GDLs of Example 1 and Comparative Example 1 were compared by ThermoGravimetric Analysis (TGA). The results are shown on Figure 2 where the dotted curve represents the GDL of Example 1 and the solid curve represents the GDL of Comparative Example 1. The curve for Comparative Example 1 shows two breaks, the first break observed between 300 and 400°C corresponding to the decomposition of surfactant residue present in the GDL. On the contrary, on the dotted curve of the GDL of the invention no weight loss is observed as the surfactant was already removed during the heat treatment by VCSEL.

Assembly of a fuel cell according to the invention:

A fuel cell membrane (Nafion™ NR212, The Chemours Company) was coated on both sides with a platinum catalyst dispersion, and assembled inside a custom-made 1 cm ² cell with the M PL-coated GDL of the invention prepared above on both sides (anode and cathode), with the MPL side facing the membrane.

After assembly, the fuel cell was subjected to a conditioning (activation) protocol consisting of: - a membrane hydration step by flushing H2 and air at 5 normal liters per minute (nlpm) for 30 minutes, followed by

- a N ₂ purging step for another 30 minutes, and

- a proton pumping step at a voltage rate of 10 mV. s ¹ . Current-voltaae test protocol:

A current was drawn from the fuel cell, and the voltage was measured at each corresponding current density and at three different % of relative humidity.

The test was carried out on square-shaped samples of 1 cm ² . The two following samples were assessed:

- Sample 1: GDL having undergone a heat treatment in a traditional oven

(solide curves),

- Sample 2: GDL having undergone a heat treatment with VCSEL (dotted curves). The current-voltage curves, also named "polarization" curves, obtained for Sample 1 and Sample 2, are presented on Figures 3, 4 and 5, wherein % RH refers to the relative humidity at which the curves were obtained. Increased performances (higher current-voltage curves) were observed for the GDL of the invention (Sample 2 treated by VCSEL) at the three different % of relative humidity.