METHOD TO PRODUCE A GAS DIFFUSION LAYER AND FUEL CELL COMPRISING A GAS DIFFUSION LAYER

Abstract

A method of manufacturing gas diffusion layers (GDL) with a defined pattern of hydrophobic and hydrophilic regions is used to produce electrically conductive porous materials with distributed wettability. The method includes a) Coating the external and internal surfaces of a porous base material made of carbon fiber or Titanium with Fluoroethylene-Propylene (FEP) and/or perfluoroalkoxy (PFA) and/or Ethylene-Tetrafluoroethylene (ETFE) or any other hydrophobic polymer; b) Exposing the coated material to irradiation through a blocking mask such that only parts of the coated porous material are exposed; and c) Immersing the previously exposed material in a monomer solution and heating to a temperature higher than 45° C., resulting in the graft co-polymerization of monomers on the FEP layer.

Claims

1-18. (canceled)

19. A method for producing electrically conductive porous materials with distributed wettability, the method comprising the following steps: a) coating external and internal surfaces of a porous base material made of carbon fiber or titanium with at least one of fluoroethylene-propylene (FEP) or perfluoroalkoxy (PFA) or ethylene-tetrafluoroethylene (ETFE) or any other hydrophobic polymer; b) exposing the coated material to irradiation through a blocking mask causing only parts of the coated porous material to be exposed; and c) immersing the previously exposed material in a monomer solution and heating to a temperature higher than 45° C., resulting in a graft co-polymerization of monomers on the FEP layer.

20. The method according to claim 19, wherein the base material is a porous material or an electrically conductive or non-conductive material.

21. The method according to claim 19, which further comprises distributing the applied coating over a thickness in any configuration or homogeneously distributing the applied coating over a thickness.

22. The method according to claim 19, which further comprises carrying out the irradiation step by using photon-type radiation.

23. The method according to claim 22, wherein the irradiation is provided in an energy range from UV to gamma.

24. The method according to claim 19, which further comprises using a particle beam to generate initiators including plasma, accelerated and heavy ions.

25. The method according to claim 19, which further comprises regulating a penetration depth of the irradiation to obtain a targeted modification depth.

26. The method according to claim 25, which further comprises modifying previously produced stacks of various materials including gas diffusion layers and micro porous layers by the targeted modification depth.

27. The method according to claim 19, which further comprises defining a patterned design including parallel slits or circles.

28. The method according to claim 19, which further comprises using lithography methods to define exposed regions.

29. The method according to claim 19, wherein the monomer solution of step c) includes a radically activated monomer resulting in graft copolymerization of a hydrophilic polymer, including N-nivylformamide, acrylic acid, methacrylic acid, styrene, sufonated styrene, vinyl pyridine or glycerol methacrylate.

30. The method according to claim 19, which further comprises using the monomer solution as a pure liquid or diluting the monomer solution with at least one of a solvent or other compounds and one or more additional monomers.

31. The method according to claim 19, which further comprises performing an additional step d) of further exposing the material to solutions and different pressure and temperature conditions to pursue additional chemical reaction to further reduce a contact angle or implement another desired property, including hydrolysis or sulfonation.

32. The method according to claim 19, which further comprises additionally applying negative pressures in step c) to allow penetration of the solution into material pores.

33. The method according to claim 19, which further comprises adding a surfactant or agents lowering surface tension to the monomer solution to allow penetration into material pores.

34. The method according to claim 19, which further comprises exposing the porous material to a vapor phase of the monomer or another mixture of gases.

35. A fuel cell, comprising: an anode side gas diffusion layer including hydrophilic parts for achieving a distributed wettability; and an anode flow plate having water channels partly aligned with said gas diffusion layer to be connected to a water inlet for flooding with water; said water channels and said hydrophilic parts of said gas diffusion layer forming a water supply network for humidifying the fuel cell.

36. The fuel cell according to claim 35, wherein heat removal by water evaporation is sufficient to remove all heat produced in the fuel cell, suppressing a need for an additional cooling system.

37. The fuel cell according to claim 35, wherein said water supply network is placed on a cathode side.

38. The fuel cell according to claim 36, wherein said water supply network is configured to supply water in an electrolyser stack or in a stack of fuel cells.

Description

[0029] Preferred embodiment of the present invention are hereinafter described in more detail with respect to the attached drawings which depict in:

[0030] FIG. 1 schematically the relation between the pathways for liquid water and for reactant gases in porous materials;

[0031] FIG. 2 schematically a synthesis method according to the present invention for the porous material with patterned wettability;

[0032] FIG. 3 schematically the distribution of different components in the gas diffusion layer visualized by SEM-EDX;

[0033] FIG. 4 schematically the fine distribution of water supply using engineered hydrophilic channels in the gas diffusion layer; and

[0034] FIG. 5 schematically a fuel cell stack without coolant channels and using monolithic plates made of carbon (left) or stamped stainles steel (right).

[0035] The present invention discloses a method of manufacturing gas diffusion layers (GDL) with a defined pattern of hydrophobic and hydrophilic regions. The interest of such materials for fuel cell applications is their potential use on the cathode side to improve the transport of oxygen in presence of liquid water, as discussed in the literature and illustrated in

[0036] FIG. 1. FIG. 1 illustrates the relation between the pathways for liquid water and for reactant gases in porous materials. Left: in standard hydrophobic materials. Right: in the new proposed material according to the present invention.

[0037] A further possibility of application is to use this invention as a help for evaporation cooling in fuel cells and electrolyzers, using the hydrophilic pathways as a help to guide water injection. Other applications relying on porous supports that require through plane conduction of water or any other liquid can benefit from this material, such as microfluidics (dialysis).

[0038] The proposed method to synthesize such a material is based on radiation induced graft polymerization. The originally hydrophobic material is exposed to a beam which is masked in certain regions. Alternatives to define the exposed regions are to use lithography or a scanning pencil beam. The regions which have been exposed are “activated”. Consequently the graft copolymerization process will occur only in these regions. The synthesis process is summarized in FIG. 2. FIG. 2 shows the synthesis method according to the present invention for the porous material with patterned wettability. (a): Base porous material (hydrophobized). (b): Irradiation using a mask. (c): Locally irradiated base material. (d): Impregnation with grafting solution. (e) Resulting material with patterned wettability.

[0039] Currently, the realization of a first GDL hydrophobized using

[0040] Fluoro-Ethylene-Propylene and with localized grafting of Acrylic Acid as a hydrophilic compound has been demonstrated. For this example a gas diffusion layer commercially available Toray TPGH-090 was coated with 30% weight of FEP and sintered. The material was then irradiated with an electron beam of 1.5 MeV energy and 50.0 kGy dose. A mask of 2 mm thickness stainless stell was used with drilled slits of 500 μm. The irradiated material was placed into a reactor of 50 mL and 50 mbar pressure. The grafting solution (15% vol Acrylic acid in water) was pumped in after 60 min of nitrogen bubling. The reaction was brought to a temperature of 60° C.

[0041] The distribution of the different components visualized by SEM-EDX is shown in FIG. 3. The current method uses vacuum for a correct wetting of the material with the grafting solution, but the process can be realized in an also cost effective way, for example using a surfactant or vapor phase grafting. FIG. 3 shows SEM-EDX mappings of a gas diffusion layer hydrophobized using FEP and locally grafted with Acrylic Acid according to the present invention. Left: C (carbon) signal showing the carbon fibers. Middle: F (fluorine) signal showing the distribution of coating. Right: Na (sodium) signal showing the grafted Acrylic Acid, after exchange in NaOH for visualization purposes.

[0042] It is believed that this method is of significant interest, since the performance of the fuel cell is improved by the use of this material. Moderate additional costs are expected due the simplicity of the process and the low cost of the chemicals involved.

[0043] General thoughts on patentability [0044] In particular, the use of FEP as hydrophobizing agent prior to grafting is not published yet. The use of FEP is a critical part of the process due to better radiation resistance and grafting kinetics of this material compared to the PTFE normally used for hydrophobization. The patent application therefore covers all GDL using FEP as hydrophobizing agent which is grafted afterwards.

[0045] In addition, the implementation of the evaporation cooling concept using the material above can be realized. The direct injection of liquid water in a fuel cell for the purpose of humidification and cooling has important advantages: [0046] Operation of the cell with dry gases, suppressing the bulky and costly external humidifiers [0047] Efficient cooling, potentially suppressing the need for an additional coolant circulation

[0048] The practical implementation of evaporation cooling is difficult. Our studies on differential cells have shown that, on the local scale, humidification from the anode side is much more effective than from the cathode side. However, due to the permeation through the membrane and the electro osmotic drag, water injected in the anode inlet will be transported to the cathode side. FIG. 4 shows fine distribution of water supply using engineered hydrophilic channels in the GDL. Below the water soaked hydrophilic regions, hydrogen is transported to the electrode through the hydrophobic micro-porous layer (MPL) usually present in fuel cells.

[0049] The present invention now uses a design that provides some of the channels on the anode side (for example, each 3.sup.rd channel) as water supply channels being completely flooded with water (these supply channels are connected to another manifold than the hydrogen supply channels). To transport the water efficiently to the regions between the water supply channels, elongated hydrophilic water pathways are created in the GDL (using the method described above). To avoid gas bubbling into the water supply channels, those can optionally be covered by a hydrophilic region as illustrated in FIG. 4. As the transport of gas is not very critical on the anode side, a significant portion of the GDL volume (e.g. 50% or even 75%) can be used for the transport of water. Besides their role in distribution the water away from the water supply channels, the finely spaced hydrophilic channels are appropriate to increase the water surface for a more efficient evaporation.

[0050] The key interest of the present invention resides in the fact that the use of the common components can be maintained and nothing is particularly added compared to the classical fuel cell technology. The extra cost of the present GDL modification method is offset by the system cost reduction due to the suppression of external humidifiers. Even further, if the evaporation cooling is, as targeted, sufficient to remove all the heat produced, a further cost and size reduction by suppressing the coolant circulation and by using monolithic separator plates can be realized. Such a design is illustrated in FIG. 5. Due to these advantages, the present idea has a large potential compared to the existing designs for evaporation cooling. FIG. 5 illustrates a stack based on the present idea, without coolant channels and using monolithic plates made of carbon (left) or stamped stainles steel (right).

[0051] The present invention provides a significant advantage compared to other proposed ideas due to the following features: [0052] The use of an industrially available irradiation method (e-beam) and low cost chemicals, providing a cost advantage to other proposed methods (e.g. deposition inkjet or screen printing) [0053] The use of a chemical modification method with a better durability as compared to other physical deposition methods [0054] Compared to some of the methods (e.g. perforation), the present design offers a significantly higher flexibility for the design of the hydrophilic area/volumina of a gas diffusion layer.

[0055] On the evaporation cooling (application) disclosure, the significant advantage of the present inventions resides in the fact that no additional layer is included, and that no layer is significantly bulkier or expensive than in classical fuel cell designs (as opposed, for example, to the water transport plates of the UTC design). Thus, the following advantages are realizable: [0056] Reduced stack and system cost, which is critical for automotive applications [0057] Reduced system size (=increase of power density), which is essential as well for automotive applications