An electrode, a method of producing it, and a fuel cell with such electrode

Abstract

An electrode, a method of producing it, and a fuel cell with such elec- trode In a method of producing electrodes (20), for example for fuel cells, an aqueous coating (15, 16) is applied onto a microporous substrate (10) for a gas diffusion layer. The coating comprises a catalyst as well as a first polymeric binder that is only water soluble below a first temperature T1 and a second polymeric binder that is only water soluble above a second temperature T2, wherein T2>T1. When preparing the coating, the first and second polymeric binders are added to an aqueous base in separate stages, where the temperature of the aqueous base is below T1 when adding the first polymeric binder and above T2 when adding the second polymeric binder. By heating the coating, the two polymers are cross linked, which prevents dissolution of the two polymeric binders.

Description

[0001] An electrode, a method of producing it, and a fuel cell with such electrode

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a method of producing electrodes, especially electrodes for fuel cells. In particular, it relates to production methods for electrodes in which aqueous coatings are applied onto porous hydrophobic electroconductive substrates, wherein the coatings comprise an electroconductive filler, a catalyst, water soluble hydrophilic polymeric binders, and advantageously a hydrophobic polymer binder dispersed in water.

[0004] BACKGROUND OF THE INVENTION

[0005] High temperature proton-exchange membrane, HT-PEM, fuel cells operate at temperature range of 150 to 200°C and may utilize reformate gas as a fuel. The key part of HT- PEM fuel cell is the membrane-electrode assembly, MEA, which consists of two electrodes, anode and cathode, sandwiching a phosphoric acid doped polymer membrane, typically made of polybenzimidazole PBI, see [Ref. 1] in the reference list at the end of the description.

[0006] Typically, each electrode has a three-layer structure, which is a highly porous gas-diffusion layer, GDL, produced from carbon cloth or carbon paper and a less porous but also electroconductive microporous layer, MPL, and a catalyst layer, CL. The GDL and MPL must be highly hydrophobic in order to provide effective release of water from the GDL. This water is produced in the CL, which contains a noble metal-based catalyst for acceleration of the electrochemical processes. The GDL must also prevent leaching of phosphoric acid. Normally, fluoropolymers like polytetrafluoroethylene PTFE and fluorinated ethylene propylene FEP are used as a binder to provide enough hydrophobicity for the MPL, see [Ref. 2],

[0007] US2005 / 064275 discloses carbon paper as GDL, which is coated or impregnated with a hydrophobizing dispersion of a fluoropolymer, typically polytetrafluoroethylene, PTFE. However, highly hydrophobic properties of the MPL create challenges when coating with aqueous catalyst inks, where water is used as solvent or dispersing media. Therefore, preparation of such electrodes, where the CL is produced from water-based ink, is non-trivial task including manipulation of the MPL hydrophobicity and reduction of the surface tension for the catalyst ink as compared to pristine water. For instance, temporal reduction of MPL hydrophobicity becomes possible when plasma or corona treatment is applied to its surface, see [Ref. 3, 4] and also US2003 / 044671, US9431665, W02005 / 117176 and US2005 / 064275.

[0008] However, for plasma or corona treatment, expensive equipment is needed, especially if a continuous large-scale production is considered. It would be helpful to find other methods than plasma or corona treatment in connection with water based coatings on hydrophobic substrates.

[0009] US2013 / 0122402A1 discloses an electrode for a fuel cell and a method of preparing the electrode. The electrode includes an electrode support, for example carbon paper or carbon cloth. A catalyst layer on the support includes a catalyst material and a waterbased binder, for example cellulose derivatives or composites of organic polymer materials and inorganic oxides. The catalyst layer formed on the electrode support during a drying step where water evaporates.

[0010] A problem associated with water soluble binders is longevity, as these are exposed to aqueous conditions when in use, for example in fuel cells. Accordingly, it would be desirable to find methods for improving longevity of electrodes, despite use of aqueous coatings.

[0011] DESCRIPTION / SUMMARY OF THE INVENTION

[0012] It is therefore an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide a production method for electrodes, especially electrodes in fuel cells, in which hydrophobic substrates are coated with aqueous coatings. Another objective is increasing lifetime and robustness of electrodes produced by using aqueous coatings. In particular, it is an objective to provide a production method for electrodes which is suitable for large-scale, low cost production. This objective and further advantages are achieved with a method of producing electrodes for fuel cells as described below.

[0013] In the following, the following abbreviations are used:

[0014] CL : catalyst layer

[0015] CB : carbon black

[0016] CMC : carboxymethyl cellulose

[0017] EC: ethyl cellulose

[0018] FEP : fluorinated ethylene propylene

[0019] GDL : gas-diffusion layer

[0020] HEC : hydroxyethyl cellulose

[0021] HPC : hydroxypropyl cellulose

[0022] HPMC : hydroxypropylmethyl cellulose

[0023] HT-PEM : high temperature proton-exchange membrane

[0024] MEA : membrane-electrode assembly

[0025] MC: methyl cellulose

[0026] MPL : microporous layer

[0027] PBC : polybutylene carbonate

[0028] PBI : polybenzimidazole

[0029] PEC : polyethylene carbonate

[0030] PPC : polypropylene carbonate

[0031] PTFE : polytetrafluoroethylene

[0032] PVA: polyvinyl alcohol

[0033] PVP: polyvinyl pyrrolidone

[0034] Herein, a production method is described for providing a porous hydrophobic electro- conductive substrate with a catalyst layer, CL, which is provided as a aqueous coating that contains a catalyst. The catalyst-containing coating is applied to the substrate as an aqueous coating blend comprising a catalyst, and a polymeric binder. The coating is then heated to evaporate the liquid, which is mainly water, but potentially also containing some other liquid, such as a surfactant. The aqueous coating is further heated to temperatures so that the binder melts and binds the catalyst. Advantageously, in order to apply the aqueous coating, the substrate is temporarily hy- drophilized by using sacrificial hydrophilic polymers.

[0035] The method described herein has the advantage of using water based ink for the coating and, one the one hand, overcomes the problem of applying and binding the water based ink to the hydrophobic surface of the substrate, and, on the other hand, overcomes the problem of using water soluble binders without the risk of the binders being dissolved later when used in fuel cells. As it will become more apparent in the following, the method as described herein solves multiple problems.

[0036] Details are explained in the following.

[0037] Providing a catalyst layer on a GDL-forming substrate

[0038] As an offset, a substrate is provided which for the final electrode includes the function of a hydrophobic gas-diffusion layer, GDL, for the electrode. A typical material for the GDL is porous carbon cloth or porous carbon paper.

[0039] A catalyst layer CL is provided as a catalyst coating on the substrate. The catalyst coating is applied as a catalyst-containing aqueous coating blend comprising, in addition to a catalyst, multiple polymeric binders that are subsequently cross-linked by heating in order to bind the catalyst to the substrate.

[0040] As mentioned in the introduction, the substrate of the electrode needs to be hydrophobic for proper functioning in a fuel cell, especially a HT-PEM fuel cell. However, this may impose challenges when applying an aqueous catalyst coating on the substrate surface. As discussed above, temporal reduction of hydrophobicity can be achieved by plasma treatment of the substrate surface. However, as plasma treatment of the substrate surface requires costly equipment, an alternative technical solution is presented below after describing the formation of the catalytic coating. This alternative solution comprises application of a first aqueous coating blend to form a first coating on the substrate and which hydrophilizes the substrate and which acts as a hydrophilic foundation for the subsequent catalyst-containing second aqueous coating blend which forms a second coating. However, in principle plasma treatment could be used as alternative for hy- drophilizing the substrate for the catalyst-containing aqueous coating. In order to solve the problem of using water soluble binders for the catalytic layer, CL, without the risk of the binders being dissolved later when used in hot aqueous environment in fuel cells, the following technical solution is provided. In this case, the catalystcontaining aqueous coating blend contains, dissolved therein, a first water soluble polymeric binder and a second water soluble polymeric binder, which after drying are cross linked for binding the catalyst onto the substrate.

[0041] The first polymeric binder, for example HPC, is selected among binders that are only water soluble below a first temperature Tl, and the second polymeric binder, for example PVA, is selected among binders that are only water soluble above a second temperature T2, where T2>T1. Accordingly, there is a gap in between Tl and T2 in which both are insoluble in water. However, more importantly, the two polymers are not soluble at the same temperature. This implies that at least one of the binder polymers is insoluble at all temperatures to which the cross-linked co-polymeric binder is exposed. In particular, as the first and second polymeric binders are cross-linked, it is not possible to dissolve one of the polymer binders without the other. And due to the different temperatures necessary for dissolution of the binders in water, none of them can be dissolved in water.

[0042] In more detail, the first polymeric binder is not water soluble at temperatures above the lower temperature Tl and, thus, prevents the second polymeric binder, to which it is cross-linked, from being removed from the substrate by dissolution at temperatures above Tl and in particular above the higher temperature T2. On the other hand, the second polymeric binder is not soluble at temperatures below T2, which is the higher of the two temperatures Tl and, thus prevents the first polymeric binder from being removed from the substrate by dissolution in water at temperatures below T2 and in particular at temperatures below Tl.

[0043] In popular terms, it may be expressed such that one polymeric binder protects the other from dissolution in aqueous media, such as in fuel cells.

[0044] In an exemplified embodiment, the first polymeric binder is HPC, which is only soluble in water below Tl=40°C, and the second polymeric binder is fully hydrolysed or at least 97% hydrolysed PVA, which is only soluble at temperatures above T2=80°C. By providing an aqueous solution at a temperature in the range of 80-99°C, the second polymeric binder can be dissolved in the aqueous base of the coating, and at temperatures below 40°C, the first polymeric binder can be dissolved in the aqueous base.

[0045] By changing the temperature of the base between regions where one or the other of the two polymeric binders is soluble, both polymeric binders can be dissolved in the base, one after the other. The later cross linking by heat prevents the binders from becoming dissolved again.

[0046] In more general terms, the method comprises preparing an aqueous base for the catalystcontaining aqueous coating and adding the first and second polymeric binders to the base in two steps with controlled temperature. When adding the first polymeric binder to the base, the temperature of the aqueous base is not higher than Tl, and when adding the second polymeric binder, the temperature is not lower than T2. This way, both polymeric water soluble binders are dissolved in the catalyst-containing aqueous coating, which can then be applied to the substrate prior to heating the substrate to a temperature for the cross-linking and final fixing of the catalyst to the substrate.

[0047] In the above example with HPC and PVA, Tl and T2 are spaced 40 degrees, which is not strictly necessary, but a spacing of at least 10 degrees between Tl and T2 is useful when selecting appropriate polymeric binders. Accordingly, in useful embodiments, Tl is at least 5 or rather at least 10 degrees lower than T2.

[0048] In order for the first and second polymeric binders being soluble in the aqueous coating, the coating should not be frozen and also not boiling. Accordingly, Tl and T2 are the range of 1-99 °C at atmospheric pressure.

[0049] Hydrophilizing the substrate

[0050] As mentioned in the introduction, the substrate of the electrode needs to be hydrophobic for proper functioning in a fuel cell, especially a HT-PEM fuel cell. However, this may impose challenges when using aqueous coatings on the substrate surface, in particular an aqueous catalyst coating. As discussed above, temporal reduction of hydrophobicity can be achieved by plasma treatment of the substrate surface. However, plasma treatment of the substrate surface requires costly equipment, especially if a continuous process is considered. Herein, a different technical solution is presented for a hydrophobic substrate, which is useful for a fast, low cost, continuous, large-scale production.

[0051] For coating the hydrophobic surface of the substrate, especially a GDL, it is firstly provided with a first aqueous coating that comprises at least one hydrophilizing sacrificial polymer for temporarily reducing the hydrophobicity of the hydrophobic substrate surface, typically, in addition to a non-sacrificial hydrophobic polymer.

[0052] The one or more, for example two or three, hydrophilizing sacrificial polymer(s) is / are selected among polymers that is / are different from the first and second polymeric binders. The hydrophilizing sacrificial polymer(s) has / have a decomposition temperature lower than decomposition temperatures of the first and second binders, as the first and second binders should remain for binding the catalyst for the electrode, whereas the sacrificial binder(s) is / are sacrificed by decomposition and evaporation for forming a microporous layer, MPL, between the GDL and the CL in the electrode.

[0053] Optionally, the at least one hydrophilizing sacrificial polymer comprises at least one of polypropylene carbonate PPC, polyethylene carbonate PEC, polybutylene carbonate PBC, hydroxyethyl cellulose HEC, hydroxypropyl cellulose HPC, carboxymethyl cellulose CMC, ethyl cellulose EC, methyl cellulose MC.

[0054] As it appears from the above, the cellulose type HPC can be used either as first polymeric binder of the CL or as sacrificial polymer in the hydrophilizing coating for forming the MPL, however, not in the same embodiments of an electrode, due to the fact that the sacrificial polymers in the MPL must be different from the polymeric binders in CL and must have lower decomposition temperatures. For example, if HPC is used as sacrificial binder in the MPL with a corresponding decomposition temperature, for example around 249°C, the first and second polymer binders in the CL have to be selected differently, for example as hydroxypropyl methyl cellulose, HPMC and PVA, which both have higher decomposition temperatures at around 309°C and 275°C, respectively, and for which the decomposition temperature is even higher after crossbinding. It should be noted that decomposition temperature here we consider as onset of thermal degradation of polymeric bonds, but not mass losses due to the removal of bounded and non-bounded water molecules.

[0055] A useful embodiment has been found in using HEC and PPC in combination for the MPL. HEC is useful for increasing viscosity and protect against sedimentation of the ingredients of the aqueous coating. The blend of PPC and HEC has been found useful, as the polymers form OH-groups which are believed to function as anchors for adsorption of water molecules and making the substrate surface more hydrophilic. Additionally, the decomposition temperatures are relatively low.

[0056] In some practical embodiments, the at least one hydrophilizing sacrificial polymer, for example the combination of two or more hydrophilizing sacrificial polymers, has a concentration in the dried and cross-linked coating which is 10-60 wt.% relatively to the total weight of the electroconductive filler in the aqueous coating.

[0057] The coatings which contain the hydrophilizing sacrificial polymer, on the one hand, and the first and second polymeric binders, on the other hand, are applied to the substrate prior to heating the coatings to various temperatures at which liquid evaporates and then the at least one hydrophilizing sacrificial polymer decomposes and evaporates from the aqueous coating and the first and second polymeric binders cross-link.

[0058] Firstly, the liquid is evaporated, which is mainly water, at 100°C. At further increased temperature, for example up to 120°C, also potentially used surfactants will evaporate. At even higher temperature, for example in the range of 180-240°C, the sacrificial polymers decompose and evaporate, and at possibly even higher temperature, for example in the range of 240-274°C, the polymeric binders melt and cross-link.

[0059] As already indicated above, advantageously, the aqueous coating is applied to the substrate in multiple stages, including providing a first aqueous coating blend for the MPL and a second coating blends for the CL, the blends being mutually different. The first aqueous coating blend contains the at least one hydrophilizing sacrificial polymer and the second aqueous coating blend contains the first and second polymeric binders. In such embodiment, the first aqueous coating with the at least one hydrophilizing sacrificial polymer is applied to the substrate surface first for temporarily reducing the hydrophobicity of the surface. This prepares the otherwise hydrophobic surface of the substrate for good adhesion of the subsequent second aqueous coating with the catalyst. While the substate is more hydrophilic, the second aqueous coating blend that contains the first and second polymeric binders and the catalyst is applied onto the hydrophilized substrate surface, typically after drying the first aqueous coating blend. Only after both coating blends have been applied and also the second coating blend has been dried, the resulting combined aqueous coating is heated to the temperatures needed for decomposition and evaporation of the hydrophilizing sacrificial polymer, which reverts the surface to hydrophobicity, and for cross-linking the polymeric binders.

[0060] Advantageously, the first coating that is used for producing the MPL between the GDL substrate and the CL also contains a hydrophobic polymer, for example fluorinated ethylene propylene, FEP, or polytetrafluoroethylene, PTFE, for increasing hydrophobicity again after evaporation of the at least one hydrophilizing sacrificial polymer and after applying the cross-linked binders. The hydrophobic polymer also assists binding the filler to the substrate.

[0061] The result is a catalytic layer, CL, in which the first and second polymeric binders are cross linked and binding the catalyst to the surface of the MPL, containing the hydro- phobic polymer for its binding with the surface of the GDL substrate.

[0062] In the embodiments with multiple coating stages, as an option, the hydrophobic polymer is provided as a powder, or water-powder dispersion, and then dispersed in the first aqueous coating blend.

[0063] In some embodiments, the method comprises selecting a third polymeric binder among polymers that reduces viscosity and surface tension, for example PVP, and crosslinking the first, second and third polymeric binders in the coating. Use of PVP is in particularly useful when PVA contains a high mount of hydrolysed groups which cannot be directly crosslinked with HPC but, then, can be cross linked via PVP. In the embodiments with multiple coating stages, as an option, the third polymeric binder is dissolved in the second aqueous coating blend. As an example, the catalytic coating forming the CL on the MPL / GDL foundation, after evaporating the volatile components from the aqueous coating, comprises, relatively to the total weight of the solids in the CL, polymeric binders in the range of 5-30 wt.% and catalyst in the range of 70 to 95 wt.%. Typically, the total content of polymeric binder and catalyst therein is at least 95% of the total weight of the CL.

[0064] As an example, the coating for the MPL on the GDL substrate, after evaporating and decomposing the volatile components from the aqueous coating, comprises relatively to the total weight of MPL, hydrophobic polymer in the range of 5-50 wt.% and elec- troconductive filler in the range of 50-95 wt.%.

[0065] For example, the content of sacrificial polymer in the coating for the MPL is within the range of 10-60 wt.%, relatively to the weight of electroconductive filler.

[0066] For example,

[0067] - the first polymeric binder is HPC,

[0068] - the second polymeric binder is PVA,

[0069] - the third polymeric binder is PVP,

[0070] - the at least one sacrificial polymer comprises PPC and HEC;

[0071] - the hydrophobic particulate polymer is FEP;

[0072] - the electroconductive filler comprises carbon black and graphite;

[0073] - the catalyst is platinum based.

[0074] Use of the electrodes

[0075] An advantageous use of the method is for providing an electrode for a fuel cell, the electrode comprising a porous electroconductive hydrophobic substrate as GDL carrying an MPL and a CL as described above. Also, a fuel cell with such electrode fulfils the objective of the invention as initially stated.

[0076] The above described electrode is useful for high temperature polymer electrolyte membrane fuel cell, (HT-PEM), which operates above 120 degrees centigrade, differentiating HT-PEM fuel cell from low temperature PEM fuel cells, the latter operating at temperatures below 100 degrees, for example at 70 degrees. The normal operating temperature of HT-PEM fuel cells is the range of 120 to 200 degrees centigrade, for example in the range of 160 to 180 degrees centigrade. Such HT-PEM fuel cells are advantageous for compact fuel cell systems, for example for automobile industry.

[0077] SHORT DESCRIPTION OF THE DRAWINGS

[0078] The invention will be explained in more detail with reference to the drawing, where FIG. 1 illustrates the two temperature regimes in which PVA and HPC are insoluble; FIG. 2 is an exemplary scheme for the formation of hydrogen bonds between polymers in the MPL and the CL;

[0079] FIG. 3 is a scheme for an exemplified continuous preparation process of an MPL and CL on a GDL substrate;

[0080] FIG. 4 illustrates Voltage increase for a current density of at 0.4 A / cm2with time of testing (activation procedure) for a freshly made MEA based on electrodes produced by a method describe herein;

[0081] FIG. 5 illustrates a polarization curve (dependence of cell voltage on current density) for the same MEA as in FIG. 4 recorded after 6 days of galvanostatic testing at 0.4 A / cm2;

[0082] FIG. 6 illustrates changes in hydrophobicity of MPL before (A) and after (B) thermal decomposition and evaporation of hydrophilizing sacrificial binders namely PPC and HEC, thereout.

[0083] DETAILED DESCRIPTION / PREFERRED EMBODIMENT

[0084] In the following, various stages for the preparation will be explained in detail. The procedure starts out with a porous hydrophobic substrate, which will become the hydro- phobic gas diffusion layer, GDL, for the final electrode, and includes the stages of hydrophilizing the surface of a hydrophobic substrate by providing a first coating on the substrate and drying the first coating, and then by a second coating onto the hydrophilic dried first coating, providing a catalyst layer, CL, after heat treatment. For the CL, there are used different polymeric binders that have different and mutually separate temperature ranges in which they are water soluble. Preparation of an ink for creating the MPL by coating the GDL

[0085] As a substrate, a GDL material is provided, which, typically, is porous carbon cloth or porous carbon paper. The GDL material should be hydrophobic when used as part of an electrode but needs hydrophilic properties during the coating with aqueous solutions that contain water soluble polymers. For this reason, the substrate is temporarily hy- drophilized as explained in the following.

[0086] The first coating from which the microporous layer, MPL, is created is in the general terminology in the technical field is called ink and is produced as an aqueous solu- tion / dispersion with an electroconductive filler and three different polymer binders. It is also possible to use more polymeric binders, or even less, although, at least two are useful, namely one for temporarily hydrophilizing the surface of the GDL and one that enhances the hydrophobicity again in the final stage.

[0087] I the exemplified embodiment, fluorinated ethylene propylene, FEP, is used for forming hydrophobic properties in a final stage of the process. Accordingly, it has a role of a hydrophobic main binder. Alternatively, other fluorinated polymers are useful, for example PTFE.

[0088] Additionally, hydrophilizing sacrificial polymers are used. Various such polymers are available, but in the present example, polypropylene carbonate, PPC, and hydroxy ethyl cellulose, HEC are used. This blend of PPC and HEC has OH-groups in its structure, playing role as anchors for adsorption water molecules and making the surface of the substate more hydrophilic, see [Ref. 2, 5, 6], This hydrophilization of the substrate surface is only temporary, as will be explained in the following.

[0089] HEC and PPC have relatively low decomposition temperature, namely between 180 and 240°C, see [Ref. 7, 8], i.e. they may be considered as sacrificial functional binder to not only temporally making the substate hydrophilic by the coating but also for creating additional porosity in the coating itself afterwards in order to provide the MPL. HEC also helps to stabilize the viscosity of the ink.

[0090] Instead of PPC, other binders with similar properties may be applied, for example, polyethylene carbonate PEC and / or polybutylene carbonate PBC, while different cellulose ethers may be used instead of HEC, for example hydroxypropyl cellulose HPC, carboxymethyl cellulose CMC, ethyl cellulose EC and / or methyl cellulose MC.

[0091] PPC as sacrificial binder is added in a quantity of 10-60 wt.%, optionally 20-40 wt.% relatively to the weight of the electroconductive filler. HEC as hydrophilic agent to reduce the hydrophobicity may be thermally decomposed together with PPC and is added in quantity of 10-60 wt.%, optionally 20-40 wt.%, relatively to the amount of the hydrophobic main binder FEP.

[0092] High molecular weights of HEC are, for example, above 500,000 or even above 1,000,000, in order to obtain strong hydrophilic properties at low concentration.

[0093] The content of FEP after drying and heat treating the first coating to create the MPL is advantageously in range from 5 to 50 wt.%, optionally 10 to 30 wt.%, relatively to the weight of electroconductive filler.

[0094] Both PPC and FEP are available on the market in form of aqueous emulsion and dispersion, respectively, see [Ref. 9, 10], which makes their addition into the water-based ink easy.

[0095] Sub-micron powder of graphite is used as electroconductive main filler in this recipe of the first coating material because of its high electroconductivity and good chemical stability, while carbon black CB is advantageously used as electroconductive additive filler to reduce the resistivity of the MPL, which is possible due to the small size of its particles, typically tens to hundreds of nanometres, filling interparticle gaps of the graphite. The relative amount of CB in the mixture with graphite may vary from 0.1 to 50 wt.%, but is advantageously 1 to 30 wt.%, optionally 5 to 20 wt.%.

[0096] Other allotropic forms of carbon, for example, amorphous carbons, carbon fibres CF, carbon nanotubes CNT, or graphene may be used, optionally in various combinations.

[0097] Normally, some amount of surfactants is presence in the first coating material, which contains an aqueous PPC emulsion and a FEP dispersion. The amount has to be regulated to the needs, and if the electroconductive filler has hydrophobic nature, an additional amount of surfactant may be needed in the first coating, which is an MPL ink, in order to improve the dispersity of the carbon particles.

[0098] Examples of surfactants include Capstone™ FS-50, Surfadone™ LP-100, Triton™ X- 100, butoxynol-5 carboxylic acid, laureth-11 carboxylic acid, and oleth-6 carboxylic acid. The content of surfactants in the MPL ink may vary in the range of 0.01 to 1 wt.%, but is advantageously in the range of 0.02 to 0.2 wt.%.

[0099] An example of an advantageous first aqueous coating blend (ink composition) for the first coating for the MPL with changeable hydrophobicity / hydrophilicity is presented in Table 1 below.

[0100] Table 1. Components used in a first aqueous blend for hydrophilizing the MPL

[0101] As a conclusion, the first coating changes the hydrophobic nature of the substrate to increased hydrophilicity in order for the first coating to readily attach to the otherwise hydrophobic substrate and also provides a hydrophilic foundation for the subsequent second coating, which finally results in the CL. As will be explained below, in relation to FIG. 3, the hydrophilic first coating is converted into a hydrophobic MPL during final heat treatment where the sacrificial polymers decompose and evaporate. As will be apparent, FEP may play an important role for the final hydrophobicity.

[0102] Preparation of ink for the CL

[0103] Reduction of surface tension in water media for the second coating, which is a CL ink, is achieved by use of hydrophilic polymeric binders and surfactants. In principle, it is possible to use a blend of only two polymeric binders. However, in the example given below a binder composition includes the combination of three water-soluble polymers, namely polyvinyl alcohol PVA, polyvinyl pyrrolidone PVP, and hydroxypropyl cellulose HPC.

[0104] It is important to note that mixing such polymers together increases thermal stability of binder composition compared to single polymers due to their cross-linking, see [Ref. 11, 12].

[0105] In this ternary polymer blend, PVA is a main binder because of its good adhesive properties, see [Ref. 13], Addition of PVP leads to reduction of viscosity and surface tension of PVA solutions, see [Ref. 14, 15], which helps to disperse the catalyst, in particular Pt-based catalyst, more effectively. The use of HPC includes a role as thickening agent, see also [Ref. 16 - US3485915] in the finalized ink composition.

[0106] It is possible to use low molecular weight PVP, for example below 30,000 or even below 15,000. However, for HPC, high molecular is preferable, for example, above 500,000 or even above 1,000,000.

[0107] Another important role for HPC is protection of PVA from dissolution in water steam when used in fuel cells, because HPC is insoluble in water at temperatures above 40°C, see [Ref. 17], If PVA is used which has a high degree of hydrolysis acetate groups to alcohol, for example 97% or higher, its dissolution in water becomes possible only at temperatures above 80°C, see [Ref. 18], Consequently, an enhanced chemical stability of PVA-PVP-HPC mixture that is cross-linked is reached in wide temperature range as compared to single polymers.

[0108] The total binder content in the final CL, according to the recipe that is described herein, may vary from 5 to 30 wt.%, optionally from 7 to 15 wt.%, in order to reach a right balance between adhesion and electroconductivity. The amount of PVA in the blend, as a main binder, is higher than 50 wt.% or even higher than 60 wt.%, while amounts of PVP and HPC are typically in the range of 20 to 25 wt.%, but may also differ from this percentage ratio depending on the requirements for the final viscosity of the catalyst ink. It should also be mentioned here that improved adhesion between the MPL and the CL is also achieved by cross-binding by means of hydrogen bonding of polymers used herein, namely via forming additional HPC-PPC, PVP-HEC and PVA-PPC bonds, see [Ref. 19-21], Simplistically, such polymer blends in MPL and CL can be presented by the scheme in FIG. 2.

[0109] Surfactant is another useful component for the water-based ink, as it decreases surface tension below 40 dyne / cm or even below 30 dyne / cm. Amphoteric, non-ionic, anionic surfactants as well as their combinations may be used for this purpose, for example Capstone™ FS-50, Surfadone™ LP-100, Triton™ X-100, butoxynol-5 carboxylic acid, laureth-11 carboxylic acid, and / or oleth-6 carboxylic acid. Surfactant contents in the ink may vary from 0.01 to 1 wt.%, but are typically in the range of 0.02 to 0.2 wt.%.

[0110] Optionally, certain amounts of phosphoric acid may be added to the catalyst ink, for example, in the range of 0.1 wt.% to 5 wt.%, optionally from 0.5 to 3 wt.%, in order to improve the ionic electrical conductivity of the CL.

[0111] Pt-based materials for the CL may include nanosized Pt or Pt-containing alloys with noble or non-noble metals (Au, Ir, Pd, Ru, Cr, Co, Cu, Ni, Fe, Sn) on different carbon supports, for example amorphous carbons, carbon fibers CF, carbon black CB, carbon nanotubes CNT, graphene, and / or graphite. The Pt content in such catalyst may vary in the range of 5 to 80 wt.%, optionally 10 to 70 wt.% or 20 to 60 wt.%.

[0112] Another component in the second aqueous coating blend, also called ink, is a solvent, which was demineralized water in our case. The solid content in the ink, including Ptbased catalyst, PVA, PVP and HPC, may vary by adding less or more water therein. Typically, it is from 1 to 30 wt.%, optionally 5 to 15 wt.% , in order to make it suitable for the coating process with respect to the ink viscosity.

[0113] An example of such recipe for CL is given in Table 2 below. Table 2. Components used in water-based ink for the CL

[0114] Preparation of electrodes

[0115] In the following, the preparation stages for the aqueous coatings are described, including the steps to hydrophilize the substrate by depositing a first coating for the MPL and then applying a second coating to obtain the CL on top of the first coating.

[0116] Reference is made to FIG. 3, which shows a continuous production method with an endless substrate 10, for example on a conveyor (not shown) with a direction of movement as indicated by arrow 21. After production, the resulting electrode (20) is cut into desired dimensions. Alternatively, the substrate is provided in pieces and transported through the various stations 3, 4, 8, 9.

[0117] The substrate 10 comprises the material for the final GDL 13 of the produced electrode 20. For example, a carbon paper is used as substrate 10. As an example of a substrate, H23 type of carbon paper from Freudenberg Performance Materials is used, see [Ref. 22], However, the substrate 10 could also be a paper cloth or a combination of hydro- phobic porous layers. For the production, the substrate 10 should be self-supporting and preferably flexible.

[0118] A two-stage coating procedure 17, 18 with two different coating blends 11, 12 supplied via two subsequent dispensers 3, 8 is used, each coating procedure 17, 18 followed by heating in heating stages 4, 9, which results in the combination of the MPL 14 on the GDL 13 and with the CL 19 on top, which in combination are making up the material of the electrode 20.

[0119] It is emphasized that the first coating 15 also enters the pores of the substrate 10 to a certain extent, while components of the second coating 16 may enter the pores of the MPL 14 during its creation by heating, but not the pores of the GDL substrate 10, as the first layer for the MPL 14 prevents this.

[0120] The components for the first and second aqueous coating blends in this two-stage coating process 17, 18 are mixed according to Table 1 and Table 2, respectively. Notice that the ingredients of the first and second coating blends 11, 12 for the two containers 1 and 5, respectively, are examples only and can be subject to change.

[0121] As an example, for preparing the first aqueous coating blend 11 from a first set of ingredients, the following blending procedure has been used experimentally with success. Demineralized water in container 1 is stirred and, during the stirring, an aqueous FEP dispersion is added. When the FEP microparticles and / or sub-micrometre particles are uniformly distributed in the aqueous base, an aqueous emulsion of PPC is added to obtain a uniform distribution, which normally takes 1-10 minutes. Then, a certain amount of surfactant (e.g. FS-50) is added under continuous stirring. CB and graphite are added next and, after this, the ultrasonic bath 2 is switched-on together with a mechanical stirrer for 5-60 minutes in container 1 in order to disperse the powder and break larger agglomerates. Finally, HEC is added to increase viscosity and protect the MPL ink from sedimentation. The stirring time required for complete dissolution of HEC is, typically, in the range of 5 to 60 minutes, but typically half an hour.

[0122] The produced first aqueous coating blend 11 is then applied as a coating 15 onto the substate 10 via slot dispenser 3, for example a slot dye, to obtain a hydrophilized surface on the substate 10. After that, the substrate 10 and the wet first coating 15 goes to oven 4 to remove water by applying heat gradually increasing the temperature from 40 to 120°C. The drying of the first coating 15 on the substrate 10 results in a hydrophilic foundation for the subsequent second coating 16. The ink for the CL 19 is prepared in a similar way in the second aqueous coating blend 12. The various ingredients are added to the container 5 under continuous stirring. Advantageously, before adding PVA, the water is heated up to 79-99°C on the heating plate 6 and kept at that temperature for 10-30 minutes for complete dissolution of PVA. When PVA is fully dissolved in water, the solution is cooled-down to room temperature, namely 15-30°C, and other ink components, including PVP, surfactant, and OP A, are added under continuous stirring. After their complete dissolution, a Pt / CB catalyst is added as well to the container 5 and mixture is stirred at least 10 minutes. Once all Pt / CB powder is wet with liquid phase, an ultrasonic bath 7 is switched-on together with mechanical stirrer for 5-60 minutes in container 5 to get good dispersion of the catalyst. Finally, HPC is added to container 5 to increase the viscosity of the ink and protect it from sedimentation. Stirring time required for complete dissolution of HPC is normally 5 to 60 minutes, but typically in the order of 30 minutes. The temperature should not be higher than 39°C due to the solubility of HPC in water only under 40°C.

[0123] The resulting second aqueous coating blend, also called ink, is then coated as a second coating 16 onto the dried first coating 15 via slot dispenser 8, for example a slot dye. After that, the substrate 10 with the dried first coating 15 and the wet second coating 16 goes to oven 9 to remove water and other potential liquids, such as surfactants, by applying heat therein gradually increasing from 40 to 120°C. After this heat treatment, not only the first coating 15 but also the second coating 16 are dry.

[0124] Then, further heat treatment is carried out, for example in the same oven 9, and the temperature increased up to 240-274°C to decompose PPC and HEC (necessary) as well as to melt the FEP (desirable) for creating the MPL. Melted FEP may partially cover pores in the MPL which are formed after decomposition of the sacrificial binders PPC and HEC and increase of the hydrophobicity of the MPL again, in order for the MPL being able to correctly perform its functions, for example in the final MEA.

[0125] In the final electrode 20, the substrate 10 forms the GDL 13, the heat-converted first coating 15 with the FEP results in a hydrophobic MPL 14, and the second coating 16 results in the CL 19 after heat treatment, including the cross-linking of the binders and the pore formation due to the decomposed and evaporated sacrificial binders. Experiments were performed for testing the change of the induced hydrophilicity to the hydrophobicity by the heat treatment. FIG. 6A is a photo of a dried first coating 15 in which the sacrificial binders PPC and HEC have not yet disintegrated. Water was applied to the surface, which was fully adsorbed by the hydrophilic surface. In contrast thereto, FIG. 6B illustrates the surface of the MPL after heat treatment that decomposed and evaporated the sacrificial binders PPC and HEC and left the FEP as part of the MPL. When water was added to it after heat treatment, it is clearly observed by the droplets that the surface exhibits hydrophobicity.

[0126] Moreover, decomposition of PPC and HEC in the first coating 15 formed not only pores for the resulting MPL but also formed pores in the CL due to the release of gaseous products, which, during formation of the MPL, migrate through the CL and then out of the CL. As a result, the branched porous structure makes diffusion of gases easier inside the electrode.

[0127] Additionally, the heating to the high temperatures leads to cross-linking of the polymeric binders, PVA, PVP and HPC, resulting in a more thermostable electrode.

[0128] Performance of the finally resulting MEA, which contains electrodes produced in the described way, sandwiching a phosphoric acid doped polybenzimidazole membrane, is shown in Fig. 4 and Fig. 5. Its electrochemical behaviour is typical for MEAs used in HT-PEM fuel cells, see [Ref. 23], As seen from FIG. 4, the tested MEA demonstrates relatively fast activation by starting to give stable voltage already after first 40 hours of galvanostatic testing at 0.4 A / cm2. Control voltage screenings at different current densities (polarization curves) performed every 24 hours show that maximal performance this MEA has reached in the following 4 days. A polarization curve recorded after 144 h (6 days) of continuous galvanostatic testing at 0.4 A / cm2, as illustrated in FIG. 5, shows linear dependence of voltage on current density in range of last one from 0.2 to 0.8 A / cm2. Power density calculated at 0.55 V is 0.4 W / cm2.

[0129] Conclusions

[0130] In summary, the following features are highlighted: ) inks for the MPL and the CL are based on water for all components, i.e. the process for ink preparation implies low costs and high safety due to the absence of organic solvents; ) a temporary change of the hydrophobicity of the substrate is achieved by coating it with a hydrophilizing polymer, and a change back to hydrophobicity is obtained by thermal treatment, when the hydrophilic additives are decomposed, which is additionally advantageous in that the decomposition forms pore structures for better gas diffusion; ) the CL contains a ternary binder composition based on water soluble polymers, which may form PVA-PVP and PVP-HPC cross-linked blends with enhanced chemical and thermal stability compared to single polymers; ) improved adhesion between the MPL and the CL is achieved by means of multiple hydrogen bonding of polymers used therein, namely such polymer blends may be formed as PVP-HEC, PPC-PVA and PPC-HPC.

[0131] Reference list

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Claims

CLAIMS1. A method of producing electrodes (20), for example for fuel cells, the method comprising- providing a microporous, hydrophobic, electroconductive substrate (10) for a gas diffusion layer, GDL, for an electrode, the substrate (10) on its surface comprising a hydrophilic foundation (15) for a catalyst layer, CL (19),- selecting a first polymeric binder, for example HPC, among binders that are only water soluble below a first temperature Tl,- selecting a second polymeric binder, for example PVA, among binders that are only water soluble above a second temperature T2, wherein T2>T1;- providing a catalyst-containing aqueous coatings blend (12) containing the first and second water soluble polymeric binder in dissolved form as well as a catalyst, for example a platinum based catalyst,- applying the catalyst-containing aqueous coating blend (12) as a catalyst-containing coating (16) onto the foundation (15),- binding the catalyst in the catalyst-containing coating (16) by heating the catalystcontaining coating (16) to temperatures that cause evaporation of liquid from the catalyst-containing coating (16) and cross linking the first and second polymeric binders for providing the CL (19) on the foundation (15), characterized in that the catalyst-containing aqueous coating blend (12) is provided by preparing an aqueous base and adding the first and second polymeric binders to the base in separate stages, wherein the temperature of the aqueous base is below Tl, for example Tl=40°C, when adding the first polymeric binder, for example HPC, to the base, and the temperature is above T2, for example T2=80°C, when adding the second polymeric binder, for example PVA.

2. The method of claim 1, wherein Tl is at least 5 degrees lower than T2.

3. The method according to anyone of the preceding claims, wherein the hydrophilic foundation (15) is provided as a first coating (15) added onto the surface of the microporous, hydrophobic, electroconductive substrate (10); wherein the method comprises- selecting at least one hydrophilizing sacrificial polymer, for example HEC and / or PPC, configured for temporarily reducing the hydrophobicity of the surface of the substrate (10), wherein the at least one hydrophilizing sacrificial polymer is different from the first and second polymeric binders and has a decomposition temperature lower than decomposition temperatures of the first and second polymeric binders,- applying a first aqueous coating blend (11) onto the surface of the substrate (10) as a first coating (15), wherein the first aqueous coating blend (11) comprises an electrocon- ductive filler, for example carbon black and graphite, and the at least one hydrophilizing sacrificial polymer,- drying the first coating (15), which forms the foundation (15) with a dry hydrophilic surface for a second coating (16), and then applying the catalyst-containing aqueous coating blend (12) as the second coating (16), which is the catalyst-containing coating (16), onto the dried first coating (15),- heating of the first and second coatings (15, 16) for obtaining the cross linking of the first and second polymeric binders in the catalyst-containing coating (16), and for causing the at least one hydrophilizing sacrificial polymer in the first coating (15), but not the first and second polymeric binders in the catalyst-containing coating (16), to decompose and evaporate from the first coating (15) for converting the first coating (15) into a micro porous layer, MPL (14).

4. The method of claim 3, wherein first aqueous coating blend (11) also has dispersed therein a hydrophobic polymer, for example fluorinated ethylene propylene FEP or polytetrafluoroethylene PTFE, and wherein the method comprises increasing hydrophobicity of the first coating (15) by causing decomposition and evaporation of the at least one sacrificial polymer but not of the hydrophobic polymer.

5. The method of claim 3 or 4, wherein the at least one hydrophilizing sacrificial polymer comprises at least one of polypropylene carbonate PPC, polyethylene carbonate PEC, polybutylene carbonate PBC, hydroxyethyl cellulose HEC, hydroxypropyl cellulose HPC, carboxymethyl cellulose CMC, ethyl cellulose EC, methyl cellulose MC.

6. The method of anyone of the preceding claims, wherein the method comprises selecting HPC as the first polymeric binder and PVA as the second polymeric binder.

7. The method of claim 6 when dependent on claims 3-5, wherein the method comprises selecting ingredients for the first aqueous coating (15) such that- the at least one sacrificial polymer comprises PPC and HEC;- the hydrophobic particulate polymer is FEP;- the electroconductive filler comprises carbon black and graphite.

8. The method according to anyone of the preceding claims, comprising selecting a third polymeric binder among polymers that reduces viscosity and surface tension when added to the aqueous catalyst-containing coating (16), for example PVP, and the method comprises adding this third polymeric binder to the aqueous catalyst-containing coating (16) and crosslinking the first, second and third polymeric binders in the catalyst-containing coating (16).

9. The method according to anyone of the preceding claims, wherein the substrate (10) is made from carbon cloth or carbon paper as a porous gas diffusion layer, GDL (13), in the electrode.

10. The method according to anyone of the preceding claims wherein the method comprises providing the catalyst layer, CL, which relatively to the total weight of solids in the CL, comprises polymeric binder in the range of 5-30 wt.% and catalyst in the range of 70 to 95 wt.%, wherein the content of polymeric binder and catalyst is at least 95 wt.% of the total weight of the CL.

11. An electrode produced by a method according to any one of the claims 1-10.

12. A fuel cell comprising an electrode produced by a method according to any one of the claims 1-10.

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