Fuel cell cathode, method for producing same, and fuel cell

Abstract

The invention relates to a fuel cell cathode (1) comprising a first phase (2) and a second phase (3), wherein the first phase (2) comprises fibres and the second phase (3) comprises particles (7), wherein the particles (7) of the second phase (3) are arranged on a surface (4) of the fibres of the first phase (2), wherein the fibres of the first phase (2) comprise 23 to 75% by mass, in particular 35 to 67% by mass and in particular 45 to 60% by mass, of proton-conductive ionomer (5) and 25 to 67% by mass, in particular 33 to 56% by mass and in particular 40 to 50% by mass, of a conductive carbon (6), wherein the particles (7) of the second phase (3) comprise 70 to 100% by mass of at least one catalytically active component (10) containing at least one catalytically active substance (8), and wherein a distance (M) of the catalytically active substance (8) in the particles (7) of the second phase (3) from a surface (9) of the first phase (2), which is closest to the catalytically active substance (8), is 0 nm to 500 nm, in particular 0 nm to 200 nm and in particular 0 to 100 nm.

Description

[0001] Fuel cell cathode, process for its production and fuel cell

[0002] Description

[0003] The invention relates to a fuel cell cathode and a fuel cell with improved efficiency. Furthermore, the present invention also relates to a method for producing a fuel cell cathode.

[0004] In fuel cell cathodes, both the reaction gas (oxygen) and protons must reach the surface of the catalytically active substance for the fuel cell reaction to occur. To ensure proton transport, ionomer is typically added to the cathode (also called the cathode catalyst layer) and mixed with the catalytically active substance. In this way, the ionomer comes into contact with the catalytically active substance and transports the protons to the surface of the catalytically active substance.However, this has two adverse effects: First, sulfonic acid groups contained in the ionomer adsorb onto the surface of the catalytically active substance, occupying the catalytic sites and thus causing a strong reduction in catalytic activity (up to a factor of 4); second, the ionomer covers the surface of the catalytically active substance and inhibits local oxygen transfer, leading to mass transport-induced potential losses, especially at low catalytic electrode loadings (e.g., <0.3 mgPt / cm). 2 ) and high current densities (e.g. >1.5 A / cm 2 ).

[0005] In the prior art, platinum is commonly used as the catalytically active substance. Platinum (Pt) is available in the form of nanoparticles with a diameter of 2 to 8 nm, which are supported on high-surface-area carbons to increase the specific surface area of ​​the Pt catalyst available for the reaction. Carbons are often divided into porous carbons and solid, non-porous carbons. Well-known porous carbons include Ketjen black (KB), oxidized acetylene black, and other furnace blacks. Solid carbons include Vulcan, acetylene black, and highly graphitized carbons. Porous carbons have a high proportion of internal pores in their primary particles and therefore typically have very large specific surface areas (> 500 m 2 / g, measured by BET), while solid carbons have moderate specific surface areas (< 500 m 2 / g). Due to the internal porosity of porous carbons, a large number of Pt nanoparticles can be deposited inside the micropores. Since the ionomer is excluded from these micropores, the surface of the catalyst particles is not poisoned by sulfonic acid groups, and the activity remains high. On the other hand, since these micropores can be deep and tortuous, and their opening is very small, this leads to high resistance for the reacting oxygen to reach the Pt catalyst surface, resulting in high mass transport losses.

[0006] A solution to this problem was found by engineering carbons so that the pores housing the majority of the catalyst particles are not as deep and tortuous, while still excluding the ionomer from penetrating (pore diameter approximately 2 to 7 nm). In this way, mass transport resistances are reduced while avoiding direct adsorption of the sulfonic acid groups on the Pt surface. Such carbons are typically referred to as mesoporous carbons and are described, for example, in Yarlagadda et al., Boosting Fuel Cell Performance with Accessible Carbon Mesopores, ACS Energy Lett., 3, 618 (2018). However, such an approach has two major drawbacks. The first is that the controlled structures of mesoporous carbons are very difficult and expensive to fabricate.The second is that the ionomer can close the entrance of the micropores containing the Pt particles and still offer significant resistance to oxygen transport to the catalyst.

[0007] Another approach to improving gas transport resistance in fuel cell cathodes is to introduce ionomers with high oxygen permeability, sometimes referred to as HOPI (highly oxygen permeable ionomers). Examples of these ionomers are cited in Jinnouchi et al., "The role of oxygen-permeable ionomer for polymer electrolyte fuel cells," Nat. Commun., 12, 4956 (2021). However, like mesoporous carbons, these materials are very expensive to produce and offer only a partial solution, as losses related to mass transport and sulfonic acid group poisoning are improved but not eliminated.

[0008] Therefore, other approaches for structured electrodes have been tried.

[0009] Patent application US2014 / 0051013 A1 describes an electrode consisting of a polymer fiber structure comprising fibers with a diameter of <1 micrometer with a plurality of catalytic particles supported thereon. The polymer fibers are preferably ionically conductive, with the polymer being a Nafion ionomer. The structure can be produced by electrospinning the ionomer fibers and electrospraying the catalytic particles. According to the teaching of this patent application, the fiber structure is polymeric in nature. The catalytic particles must be interconnected to ensure proper electronic conductivity of the electrode, which results in the formation of relatively large agglomerates that could impede gas and proton transport. The examples show that the catalytic particles contain a significant amount of Nafion ionomer. This leads to catalyst poisoning and a decrease in its activity.

[0010] Patent application US2020 / 0365909 A1 describes an electrode consisting of multiple structured units with a core and a shell. The structured units are typically spherical, and the shell has a higher ionomer concentration than the core. Both core and shell typically contain catalyst particles and ionomer. Such structured units can be fabricated by electrospraying a catalyst ink from one or more reservoirs. As a consequence of the core-shell structure type, the catalyst in the outer shell suffers from the limitations imposed by the close contact of the catalyst with the ionomer described above. On the other hand, the catalyst in the inner core is shielded by the outer shell and is not as easily accessible to the reaction gas (oxygen).Furthermore, due to the high amount of ionomer in the shell, the carbon support of the catalyst in this section of the structured unit is prone to corrosion over prolonged fuel cell operation, leading to collapse of the outer layer. Mass transport constraints become even more important in this case, with high resistances for oxygen to enter the core of the structured unit from the outside and water to be transported out of the core.

[0011] From the above description of the prior art, it is apparent that there is still a need for an improved fuel cell cathode structure that simultaneously provides excellent mass transport properties and catalyst activity.

[0012] It is therefore an object of the invention to provide a fuel cell cathode with both excellent mass transport properties and high catalyst activity. Furthermore, it is also an object of the present invention to provide a fuel cell with improved efficiency, wherein the improved efficiency is achieved by the fuel cell cathode with very good mass transport properties and high catalyst activity. Furthermore, it is an object to provide a simplified method for producing a fuel cell cathode, wherein the fuel cell cathode is characterized by a fuel cell cathode with excellent mass transport properties and high catalyst activity. These objects are achieved by the features of the independent claims. The dependent claims contain advantageous developments and refinements of the invention.

[0013] Accordingly, the object is achieved by a fuel cell cathode comprising a first phase and a second phase, wherein the first phase comprises fibers and the second phase comprises particles. The particles of the second phase are arranged on a surface of the fibers of the first phase. Preferably, the first phase is formed from fibers and the second phase is formed from particles.

[0014] To achieve very good proton conductivity, the fibers of the first phase contain 23 to 75 mass%, in particular 35 to 67 mass%, and especially 45 to 60 mass% of proton-conductive ionomer. This can be one proton-conductive ionomer or a mixture of two or more proton-conductive ionomers; in the case of two or more ionomers, the above-mentioned amounts refer to the sum of all ionomers. The ionomer(s) are not specifically limited, as long as they possess suitable proton conductivity. An ionomer in the sense of the present invention is understood to be a plastic component characterized by a content of ionic groups.Suitable ionic groups are acid groups, preferably sulfonic acid groups. Ionomers are typically obtained by polymerizing monomers containing a polar functional group and nonpolar monomers. The functional groups of the polymers can be converted into an ionic, proton-conducting form after polymerization. The ionomers can comprise fluorinated units. Fluorinated polymers are known to have intrinsically higher gas permeability compared to fluorine-free polymers. Fluorinated ionomers are therefore preferred for high oxygen transport in the fuel cell cathode.

[0015] So-called HOPIs can also be used. The abbreviation HOPI (highly oxygen permeable ionomers) refers to ionomers with a sterically demanding polymer backbone, which typically contains cyclic units, designed to improve oxygen permeability through the polymer compared to standard ionomers without such a sterically demanding polymer backbone.

[0016] In addition, to improve electrical conductivity, the fibers of the first phase comprise 25 to 67 mass%, in particular 33 to 56 mass% and in particular 40 to 50 mass% of one or more conductive carbons and optionally one or more catalytically active components containing at least one catalytically active substance. The particles of the second phase comprise 70 to 100 mass% of at least one catalytically active component. According to the present invention, a catalytically active component is understood to mean a component that comprises one or more catalytically active substances, in particular metallic substances or elements and compounds of these elements and optionally one or more carbon components and / or optionally one or more carrier substances such as, for example, inorganic oxides or organic compounds such as, for example, perylenes.Thus, the catalytically active component used in the second phase according to the invention contains at least one catalytically active substance.

[0017] If the catalytically active component comprises one or more carbon components or carrier substances in addition to the catalytically active substance, the 70 to 100 mass% refers to the entire catalytically active component, including the one or more carbon components and the one or more carrier substances.

[0018] In the second phase, one catalytically active component can be used alone or two or more catalytically active components can be used. If two or more catalytically active components are used, they differ in their composition, i.e., in the contents of the catalytically active substance(s) used for the respective catalytically active component and / or any carbon components and / or carrier substances contained therein and / or in the chemical composition of the respective catalytically active components.

[0019] According to one example, a catalytically active component is used which comprises at least one catalytically active substance and optionally at least one carbon component and / or at least one carrier substance.

[0020] If more than one catalytically active component is used in the second phase, the amounts stated above (70 to 100 mass%) apply to the total mass of all catalytically active components used in the second phase.

[0021] The catalytically active component is present in particulate form in the particles of the second phase.

[0022] The phase arrangement in the cathode according to the invention ensures that essentially no catalyst sites are occupied by the ionomer, thus reducing the mass transport resistance of oxygen. Furthermore, in the case of sulfonic acid groups in the ionomer, the occupancy of the catalytically active sites is reduced, thus preventing a reduction in catalytic activity.

[0023] In the fuel cell cathode according to the invention, a distance of the catalytically active substance in the particles of the second phase to a surface of the first phase which is closest to the catalytically active substance is 0 nm to a maximum of 500 nm, in particular 0 nm to 200 nm and in particular 0 to 100 nm.

[0024] In other words, this means that for more than 50 mass%, in particular for more than 75 mass%, and especially for more than 80 mass% of all catalytically active substances in the second phase, the maximum distance to the nearest surface of the first phase is 500 nm. A distance of 0 nm means that the catalytically active substance lies directly on a surface of the first phase.

[0025] The shorter the distance between the catalytically active substance in the particles of the second phase and the nearest surface of the first phase, the higher the efficiency of the cathode, meaning the more efficiently the fuel cell's cathode reaction takes place, since oxygen is transported directly to the catalytically active substance, avoiding oxygen mass transport resistance. Furthermore, resistance losses due to proton transport are low due to the short distances, even if the second phase has only a low proton conductivity, e.g., due to a low ionomer content.

[0026] In order to prevent as far as possible an occupation of catalytically active sites of the catalytically active substance in the catalytically active component of the second phase by ionomer of the first phase, a minimum distance between the catalytically active substance in the particles of the second phase and the nearest surface of the first phase is preferably 3 nm. In other words, this means that for more than 50 mass% and in particular for more than 70 mass% of all catalytically active substances in the second phase, a minimum distance to the nearest surface of the first phase is 3 nm.

[0027] The distance between the catalytically active substance in the particles of the second phase and the first phase is determined from the surface of the catalytically active substance to the nearest surface of the first phase. This applies both to catalytically active components in which the catalytically active substance is present in the form of individual catalytically active particles and in the form of agglomerated particles, and even more so to catalytically active components which, in addition to the catalytically active substance(s), comprise one or more carbon components and / or, if appropriate, one or more carrier substances. The distance is determined between each catalytically active surface, e.g., between each primary particle of each catalytically active substance, whereby for more than 50 mass% of all catalytically active substances in the second phase, the distance to the nearest surface of the first phase is a maximum of 500 nm.This distance can be determined by geometrically measuring the distances using transmission electron microscopy. Typical magnifications range from 100 kJ to 400 kJ. Catalytically active substances, usually metal-containing catalysts, can be easily identified with the electron beam due to their large cross section.

[0028] Thus, the transport of protons to the surface of the easily accessible catalytically active substance is guaranteed by the short distance to the first phase, which is a maximum of 500 nm for each selected particle of the catalytically active substance. The short distance of a large proportion of the catalytically active surfaces or particles of the catalytically active substance of the second phase, namely at least 50 mass% and more preferably at least 75 mass%, to the surface (or interface) of the first phase, is achieved in particular by the design and realization of the second phase, in which the particles of the second phase have very small characteristic dimensions (diameter), which can be represented by an equivalent diameter.

[0029] The very short distance between the first phase and the catalytically active substance in the second phase significantly increases the efficiency of the fuel cell cathode and promotes conversion to water in the second phase due to the high proton conductivity in the first phase. This particularly short distance promotes the transport of protons to the catalytically active substance, minimizes losses due to proton transport, especially within the second phase, and ensures a particularly efficient cathode reaction.

[0030] According to a particularly advantageous development, the proton-conductive ionomer of the first phase and / or the ionomer of the second phase comprises a hydrocarbon-based ionomer. If two or more proton-conductive ionomers are used in the first phase, it is further preferred if all ionomers are hydrocarbon-based ionomers. For the purposes of the present invention, a hydrocarbon-based ionomer is understood to mean an ionomer whose fluorine content is 5% by mass or less, based on the total mass of the ionomer.

[0031] Thus, it is preferred that both the ionomer present in the first phase and the ionomer present in the second phase be a hydrocarbon-based ionomer. Hydrocarbon-based ionomers can be selected from the group mentioned below, and the respective mass fractions correspond to the previously mentioned ranges. A cathode according to this embodiment has an improved environmental profile because it does not contain perfluoroalkyl compounds and does not require perfluoroalkyl chemistry in its manufacture.

[0032] In an alternative preferred embodiment, the ionomer present in the first phase is a hydrocarbon-based ionomer, while the optional ionomer present in the second phase is a fluorinated ionomer, most preferably a fluorinated HOPI ionomer or a low equivalent weight ionomer (approximately EW < 900 g / mol). Typically, hydrocarbon ionomers are not used in fuel cell cathodes due to their low oxygen permeability. However, in the present electrode configuration, this is not a disadvantage, as the hydrocarbon ionomer does not hinder oxygen access to the catalyst surface in the second phase, as it is present in the separate first phase. Electrodes consisting exclusively or largely of hydrocarbon ionomer are preferred from an environmental point of view, as this avoids or at least reduces the use of fluorinated substances.Examples of sulfonated, non-fluorinated, i.e. hydrocarbon-based ionomers include sulfonated polyether ketones (sPEK), sulfonated polyether ether ketones (sPEEK), sulfonated polyketone ketones (sPKK), sulfonated polyether sulfones (sPES), sulfophenylated polyphenylenes (sPPX), sulfonated polyarylether ketones (sPAEK), sulfonated polyphenylene oxides (sPPO) and sulfonated polysulfones (sPSU).

[0033] According to an advantageous further development, the ionomer present in the first phase is a hydrocarbon-based ionomer and the second phase is free of ionomer.

[0034] According to an advantageous development, the first phase also comprises a catalytically active component which contains at least one catalytically active substance, wherein a mass fraction of the catalytically active component, based on the total mass of the first phase, is 0 to 40 mass%, in particular 2 to 30 mass%, in particular 5 to 20 mass% and in particular less than 10 mass%.

[0035] The catalytically active component of the first phase is defined in the same way as the catalytically active component of the second phase. This means that the catalytically active component of the first phase can consist of one or more catalytically active substances. Optionally, it can contain one or two or more carbon components or other carrier substances.

[0036] If two or more catalytically active components are used in the first phase, they differ in the contents of the catalytically active substance(s) and / or the optional carbon components and / or the optional carrier substances and / or the chemical composition of the respective catalytically active components.

[0037] The mass fraction stated above refers to the total mass of all catalytically active substances, carbon components and carrier substances in all catalytically active components in the first phase.

[0038] Preferably, the precious metal content of the catalytically active component of the first phase is 5 to 60 mass%. The precious metal content corresponds to the mass of the catalytically active substance relative to the total mass of the catalytically active component, in other words, the sum of the catalytically active substance and the optional carbon component and / or the optional carrier substance.

[0039] Further advantageously, the mass fraction of catalytically active substance in the first phase, based on the total mass of the first phase, is less than 20 mass%, preferably less than 10 mass%, more preferably less than 5 mass%, and in particular 0 mass%. This achieves a functional separation between the first phase and the second phase, thus promoting the efficiency of the fuel cell cathode. While the first phase improves proton conductivity and thus also mass transport, the high proportion of catalytically active substance in the second phase ensures very high catalytic activity there. Catalytically active sites of the catalytically active substance in the second phase are blocked neither by carbon nor by ionomer.

[0040] For the above reason, that the catalytic activity of the catalytically active substance in the second phase is not hindered by occupation of the catalytically active sites by foreign substances, it is further advantageous that the particles of the second phase further comprise, if at all, only a very small proportion of proton-conductive ionomer. Accordingly, the mass fraction of ionomer in the particles of the second phase, based on the total mass of the second phase, is preferably 0 to 30 mass% and in particular 0 to 20 mass%. However, the ionomer can act as a binder in the second phase in small amounts. Alternatively, the mass fraction of ionomer in the particles of the second phase can also be 0 mass%, thereby maximizing the catalytic activity of the second phase.

[0041] Preferably, the catalytically active substance of the catalytically active component of the first phase and the second phase (optionally in the first phase and essential in the second phase) is a Pt or Pt alloy catalyst supported on carbon. Pt or Pt alloy metal is preferably in the form of nanoparticles with a size of 2 to 10 nm, preferably 3 to 7 nm (measured by transmission electron microscopy), which are dispersed on the surface of the carbon. If platinum or a platinum alloy is present as a continuous metal coating on a support, a layer thickness of this metal coating is in particular from 0.2 to 10 nm, preferably from 0.5 to 5 nm and particularly preferably less than 3 nm for high noble metal utilization. The measurement of the particle size or the thickness of the metal coating is carried out by transmission electron microscopy, wherein the particle size orThe layer thickness of the metal coating is determined geometrically as the average value of 100 measuring points. In particular, the respective layer thickness of at least 50% (Dv50) of the measuring points is from 0.1 to 15 nm, preferably from 0.25 to 7.5 nm and particularly preferably below 4.5 nm. The content of platinum (Pt) or platinum alloy is preferably 10 to 65 mass%, in particular 40 to 65 mass% and particularly preferably 45 to 60 mass%, based on the total mass of the catalytically active component of the second phase. This achieves a high catalyst density in the second phase, which makes the second phase more compact and smaller without reducing efficiency. A compact phase also reduces effects that are detrimental to efficiency, such as mass transport losses and proton resistance.

[0042] Preferred metal alloys are PtCo and PtNi, but other noble and / or transition metals can also be part of the alloy. Core-shell metal particles can also be used, for example, Pt shells on Pd cores. The described catalysts for use in fuel cells are well known to those skilled in the art.

[0043] Further advantageously, the catalytically active component in the second phase comprises a conductive carbon with an average specific surface area of ​​less than 500 m 2 / g, measured by BET nitrogen adsorption. Because the catalytically active substance and the ionomer are in different phases, it is not necessary to introduce the catalytically active substance into the pores of a conductive carbon. This is advantageous for the gases to be converted, as they can now reach the catalytically active substance much more easily and unhindered, which is beneficial to the efficiency of the cathode. Rather, the gases do not have to penetrate the pores of the conductive carbon to be converted. This reduces the mass transport resistance.

[0044] Further advantageously, the porosity of the fuel cell cathode, measured by BET nitrogen adsorption, is at least 20 vol%, in particular at least 30 vol%, and in particular at least 50 vol%. This allows reaction gases to be transported more easily through the fuel cell cathode to the surface of the catalytically active substance. The use of conductive carbons in the second phase, as described in detail above, typically ensures a correspondingly high porosity, particularly when the binder content is low, i.e., with a sum of ionomer and polymer content of, in particular, a maximum of 30 mass%, preferably a maximum of 20 mass%, more preferably a maximum of 10 mass%. The design of the present invention, according to which proton transport through the cathode is primarily assigned to the first phase, allows the total binder content in the second phase to be kept low.This, in turn, leads to high porosities and improved mass transport properties of the second phase. In other words, not only can local mass transport near the catalytically active substance be improved, since no ionomer is adsorbed on the surface of the catalytically active substance, but also the overall diffusion of reactants (oxygen) and products (water) through the cathode due to high porosity values ​​in the second phase, which contains the catalytically active substance. Regarding the removal of product water, this can either pass through the second phase in liquid or vapor form, or by rapid diffusion to the surface of the first phase and permeation through this ionomer-rich first phase to the surface of the cathode, thus coming into contact with the gas diffusion layer and / or the membrane of a fuel cell.

[0045] The conductive carbon of the second phase (and also of the first phase described above) can be a type commonly used in fuel cell cathodes and described in the literature for use in fuel cell electrodes. Non-limiting examples include Ketjenblack (KB), furnace blacks, Vulcan, acetylene black, oxidized acetylene black, highly graphitized carbons, mesoporous carbons, graphene, carbon nanotubes, etc. The carbon improves the electronic conductivity and / or porosity of the second (and first) phase, contributing to overall improved electrode performance, both in terms of electrical conductivity and mass transport properties.A suitable carbon can be selected by a person skilled in the art by measuring the cathode performance using common carbon types and evaluating whether an increase in electrical conductivity or mass transport occurs. For example, if an increase in electrical conductivity is required, graphene or carbon nanotubes in small amounts might be sufficient, whereas for an increase in mass transport, a highly structured carbon such as Ketjenblack or Vulcan, for example, more than 20 vol% or more than 40 vol%, is preferred, which imparts a certain porosity to the first phase.

[0046] Further advantageously, the first phase and / or the second phase further comprises a polymer, wherein a mass fraction of the polymer, based on the total mass of the first phase or the second phase (i.e. the respective phase in which the respective polymer is contained), is 0 to 30 mass%, in particular 1 to 20 mass% and in particular 2 to 5 mass%.The polymer is a non-ionomeric polymer and is in particular selected from the group consisting of polyalkylene oxides, polyethylene oxides, polyvinylpyrrolidone, polyacrylic acid, polyvinyl alcohol, polyvinylidene fluoride, polyamide 6,6, polyurethanes, polybenzimidazoles, polycarbonate, polyacrylonitrile, polylactic acid, polyaniline, polystyrene, polyvinylcarbazole, polyethylene terephthalate, polymethacrylate, polymethyl methacrylate, polyamides, polyvinyl chloride, cellulose acetate, polycaprolactone, polyetherimide, polysulfone and polyethersulfone, wherein the polymer is in particular selected from polyethylene oxides, polyvinylpyrrolidone, polyacrylic acid, polyvinyl alcohol and polyvinylidene fluoride.

[0047] The polymer in the first phase can be the same as in the second phase. However, different polymers can also be used. One or more non-ionomeric polymers can also be used in the first phase and / or the second phase.

[0048] As already explained, the polymer of the first and / or second phase is a non-ionomeric polymer, which is advantageously used for the preparation of the phases and can either remain in the electrode or be removed from it before the electrode is put into use. According to a third possibility, the polymer can also be eliminated during operation of the electrode in the fuel cell.

[0049] For example, the polymer may be water-soluble, and the cathode may be treated in water or an acidic aqueous solution before further processing or before incorporation into a fuel cell to remove the polymer from the cathode. This prevents the polymer from being leached out with the wastewater during cell operation. If the amount of polymer is very small, e.g., less than 1% by mass of the total weight of the cathode, it may remain in the cell and be leached out during operation.

[0050] Alternatively, the polymer is water-insoluble, a preferred option, and remains permanently in the electrode. In this case, it is preferred that the polymer be stable under the operating conditions of a fuel cell and can impart additional beneficial properties to the cathode, such as increased cathode durability.

[0051] As already explained, the polymer in the first phase and in the second phase, if present in both phases, can be the same or different. For example, a water-soluble polymer can be used for the first phase, while a water-insoluble polymer can be used for the second phase, particularly if no ionomer is used in the second phase, so that the polymer in the second phase can act as a binder for the catalytically active component, leading to improved mechanical properties and avoiding the loss of catalyst particles during processing and operation of the cathode. To this end, it is advantageous that the sum of ionomer and polymer in the second phase is at least 2% by mass, preferably at least 3% by mass, and particularly preferably at least 5% by mass (in each case based on the total mass of the second phase).Most preferably, the second phase contains no ionomer (0 mass%) and the polymer is present in the second phase at least in the stated amounts.

[0052] PVDF is a particularly preferred polymer for the second phase because it exhibits good and stable bonding properties in the absence or with only very small amounts of ionomer. Furthermore, PVDF exhibits very good electrochemical stability and hydrophobic properties, which allow for good water removal from the second phase. This good water removal maintains good mass transport and ensures low corrosion of the second phase.

[0053] The polymer in the first phase is preferably present in an amount of less than 30% by mass, based on the sum of ionomer and polymer in the first phase. A higher amount than this reduces the proton conductivity of the first phase and the entire cathode due to dilution by the polymer.

[0054] More preferably, the amount of polymer, based on the sum of ionomer and polymer in the first phase, is a maximum of 25 mass% and even more preferably a maximum of 20 mass%.

[0055] In particular, a total proportion of ionomer and polymer in the particles of the second phase, based on the total mass of the second phase, is 2 to 30 mass%, in particular 3 to 20 mass% and in particular 5 to 10 mass%, whereby a maximum catalytic activity is achieved by keeping the catalytically active sites of the catalytically active substance free of ionomer.

[0056] To further improve the catalytic activity of the fuel cell cathode, it is advantageous that the equivalent diameter of the particles of the second phase is at least 10 nm, in particular at least 20 nm, and in particular at least 30 nm, and in particular a maximum of 1000 nm, in particular a maximum of 500 nm, in particular a maximum of 200 nm, and in particular a maximum of 100 nm. An equivalent diameter of the particles of the second phase can be considered if the diameter of the particles is not exactly round. The equivalent diameter can be calculated as the ratio between the volume and area of ​​the particles multiplied by six: (6 x V / A).

[0057] The mass transport properties as well as the catalytic activity of the fuel cell cathode can be improved by the advantageous further development in which a contact area between the first phase and the second phase is at least 1 m 2 per cm3 of the cathode volume, in particular at least 3 m 2 per cm 3 of the cathode volume and in particular at least 10 m 2 per cm 3 of the cathode volume and a maximum of 300 m 2 per cm 3 of the cathode volume, in particular a maximum of 150 m 2 per cm 3 of the cathode volume and in particular a maximum of 100 m 2 per cm 3 of the cathode volume. Thus, the transport distances of the reactants are particularly short, which improves the efficiency of the cathode reaction.

[0058] Particularly advantageously, the catalytically active substance in the second phase comprises nanoparticulate whiskers, in other words a nano-whisker catalyst. A nano-whisker catalyst is a catalyst in which the individual catalyst particle consists of a support in the form of a nano-whisker which is continuously coated with the catalytic metal (in particular platinum or a platinum alloy). The nano-whisker support has a high aspect ratio, e.g. >10 or >15 or even 20 or more, and can be cylindrical (rod-shaped) or flattened (lath-shaped). A high aspect ratio is understood to be a ratio of length to width in which the length has much higher values ​​than the width or diameter. A rod-shaped support particle can, for example, have a diameter between 5 and 30 nm and a length at least 10 times that of the support.For example, a lath-shaped support particle can be 5 to 30 nm high, 10 to 50 nm wide, and >100 nm, preferably >500 nm, long. The support material can be conductive or non-conductive, as long as it is chemically and electrochemically stable in the fuel cell environment. The continuous metal coating can have a thickness between 0.2 and 10 nm, preferably between 0.5 and 5 nm, more preferably below 3 nm for high noble metal utilization. The continuous metal coating can have some roughness, e.g., if it consists of a large number of adjacent crystal particles, but it should have continuous coverage to enable good electrical and proton conductivity along the surface of the catalytic metal, even if the support material is non-conductive.

[0059] The catalytically active substance thus consists of a multitude of such individual catalytic particles that are in close contact with one another. The best-known examples of this type of nano-whisker catalyst are those developed by 3M, which are described, for example, in the review article by M. Debe, "Tutorial on the Fundamental Characteristics and Practical Properties of Nanostructured Thin Film (NSTF) Catalysts," Journal of The Electrochemical Society, 160 (6) F522-F534 (2013). This review, which is incorporated herein by reference, also describes methods for the economical mass production of nano-whisker catalysts.

[0060] In the case of nano-whisker catalysts in the second phase, the first phase preferably contains a small amount of catalytically active component, which comprises at least one catalytically active substance. A small amount here means a weight fraction of the catalytically active component in the first phase of 0 to 40 mass%, in particular 2 to 30 mass%, in particular 5 to 20 mass% and in particular less than 10 mass%. The presence of this catalytically active component in the first phase leads to improved performance of the electrode, in particular under conditions where a lot of water is present in the CCM, e.g. under rather cold operating conditions and medium to high current densities or under a cold start below the freezing point of water. The catalytically active component can be a nano-whisker catalyst or a conventional supported catalyst, e.g.Pt nanoparticles dispersed on a support such as carbon or another material.

[0061] A third phase may also be present in the fuel cell cathode according to the invention if additional functional components are required in the cathode. These additional functional components are preferably introduced within the first phase and / or the second phase. If, for example, catalyst corrosion protection additives such as oxygen evolution catalysts or hydrophobic additives (e.g., hydrophobic polymers) are desired, these are advantageously added within the second phase in the vicinity of the catalytically active substance.

[0062] The total platinum surface weight of the cathode is 0.01 to 0.5 mg(Pt) / cm 2 , in particular 0.02 to 0.2 mg(Pt) / cm 2 and in particular 0.03 to 0.15 mg(Pt) / cm 2 .

[0063] Furthermore, the invention also describes a fuel cell comprising a fuel cell cathode as disclosed above. Due to the use of the highly efficient and high-performance fuel cell cathode with very good mass transport properties and high catalytic activity, the fuel cell according to the invention is also characterized by high performance and efficient mass conversion.

[0064] The advantages, advantageous effects and configurations described for the fuel cell cathode according to the invention also apply to the fuel cell according to the invention.

[0065] Furthermore, the invention also describes a method for producing the fuel cell cathode according to the invention, which comprises a step of electrospinning the first phase and a step of electrospraying the second phase. This forms a fibrous first phase and a particulate second phase, wherein the maximum distance between the first phase and the catalytically active substance of the second phase is 500 nm, in particular 200 nm, and in particular 100 nm.

[0066] Further details, advantages, and features of the present invention will become apparent from the following description of exemplary embodiments with reference to the drawings. It shows:

[0067] Fig. 1 shows a schematic process sequence according to a first embodiment of the process according to the invention and

[0068] Fig. 2 shows a section through a fuel cell cathode according to a second embodiment. Only the essential elements of the present invention are shown in the figures. All other elements have been omitted for clarity. Furthermore, like reference numerals refer to like elements.

[0069] Examples of the production of the fuel cell cathode according to the invention are given below with reference to Fig. 1:

[0070] FIRST PREFERRED MANUFACTURING METHOD (first-phase electrospinning and second-phase electrospraying)

[0071] Preferred methods for producing the fuel cell cathode structure of the present invention are spinning techniques and spraying techniques by which the first phase material is spun into fiber-like structures and the second phase material is deposited as particles on the first phase fibers.

[0072] Electrospinning is particularly preferred due to its ability to produce extremely low diameter fibers within the meaning of the present invention with good productivity.

[0073] General descriptions of the electrospinning process can be found, for example, in "Polymer Nanofibers Assembled by Electrospinning," Frenot et al., Current Opinion in Colloid and Interface Science, vol. 8, pp. 64-75, (2003), and in "A review on polymer nanofibers by electrospinning and their applications in nanocomposites," Huang et al., Composites Science and Technology, vol. 63, pp. 2223-2253, (2003). The details will not be discussed here.

[0074] Step 1.1 of the electrospinning process, as shown in Fig. 1, comprises dissolving or dispersing the solid materials of the first phase, namely the ionomer, an electrically conductive carbon, optionally (but preferably) a polymer, and optionally a catalytically active substance, in a suitable solvent or liquid medium to provide a solution or dispersion for electrospinning (hereinafter referred to as the first ink or first electrospinning ink). Based on the sum of all solids present in the first ink, the proportion of the ionomer is 23-75 mass%, the conductive carbon 25-67 mass%, the polymer 0-30 mass%, and the optional catalytically active substance 0-70 mass%. Some of these solid components are dissolved in the solvent, others merely dispersed. Typically, the polymer is molecularly dissolved, while the carbon and catalytically active component are dispersed.The state of the ionomer can vary from a true solution to a colloidal dispersion, depending on the ionomer's affinity for the solvent, the dissolution conditions, the interaction with the other components of the mixture, etc. Suitable solvents for electrospinning are known to those skilled in the art, and their suitability depends at least in part on the properties of the polymers (ionomer and second polymer). Such solvents can be, for example, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-methylpyrrolidone (NMP), ethanol, methanol, isopropyl alcohol, acetone, methyl ethyl ketone, water, tetrahydrofuran (THF), methylene chloride (MC or dichloromethane), formic acid or acetic acid, and combinations thereof. The skilled person can also choose other solvents suitable for a particular process.

[0075] The solids content in the first ink used for electrospinning can be determined by a person skilled in the art, for example, based on the desired viscosity. Typical solids contents can range from about 1% to about 30% by mass of the ink, preferably from about 5% to about 20% by mass, and more preferably from about 8% to about 15% by mass of the ink. Such solids content values ​​generally result in the ink having a suitable viscosity for electrospinning. Concentrations outside the above ranges can also be used if the resulting ink is suitable for electrospinning. Ideal viscosities for electrospinning Ink 1 are between about 0.1 and 2 Pa s. The viscosity can be measured using a rheometer with a plate-on-plate geometry, a plate diameter of 50 mm and a separation of 0.1 mm at 23°C.Here, a flow curve is measured in rotational mode (as opposed to oscillatory mode) at a shear rate of 0 to 1000 s'. 1 and 1000 to 0 s -1 The viscosity value is the measured value of the return curve at 50 s' 1 A suitable device is, for example, the MCR302 model from Anton Paar.

[0076] Generally, for proper nanofiber formation by electrospinning, the electrospinning ink must contain a certain amount of a fiber-forming polymer, often referred to as an electrospinning polymer or carrier polymer, that can actually dissolve and, upon evaporation of the solvent, create sufficient entanglements to allow the spun fiber to stretch without breaking. "Actually dissolved" means that the polymer is dissolved at the molecular level. For this purpose, the first ink preferably contains a polymer soluble in the ink solvent, which acts as a fiber-forming polymer. This is because ionomers typically do not possess good properties as electrospinning or carrier polymers.The following polymers can advantageously be used as polymers in the first ink: polyalkylene oxides (PAO), polyethylene oxide (PEO), polyvinylpyrrolidone (PVP), polyacrylic acid (PAA), polyvinyl alcohol (PVA), polyvinylidene fluoride (PVDF), polyamide 6,6 (PA 6,6), polyurethanes (PU), polybenzimidazole (PBI), polycarbonate (PC), polyacrylonitrile (PAN), polylactate (PLA), polyaniline (PANI), polystyrene (PS), polyvinylcarbazole, polyethylene terephthalate (PET), polymethacrylate (PMA), polymethyl methacrylate (PMMA), polyamide (PA), polyvinyl chloride (PVC), cellulose acetate (CA), polycaprolactone (PCL), polyetherimide (PEI), polysulfone (PSU), polyethersulfone (PES).

[0077] Additional components may be added to the first ink as defined by processing or functional considerations, e.g., processing aids, surface tension and viscosity modifiers, polymers that impart hydrophobic properties, pore formers, electrochemically active additives, etc., without deviating from the general scope of the present invention. If necessary, small amounts of organic or inorganic salts and / or electrolytes may be added to adjust the conductivity of the ink. This can be useful for improving the stability of fiber formation during the electrospinning process.

[0078] In step 1.2 of Fig. 1, the first ink is delivered to at least one electrospinning needle or nozzle (or electrode in the case of nozzleless electrospinning). Hereinafter, the needle / nozzle / electrode is referred to as the "needle." In a preferred embodiment, the ink is pumped to the electrospinning needle. Any other suitable means for delivering the ink to the electrospinning needle may be used.

[0079] In step 1.3 of Fig. 1, the first electrospinning needle releases the first ink onto a grounded substrate in the form of a fine stream with a diameter in the range of tens or hundreds of nanometers. In a typical electrospinning device, the electrospinning needle is connected to a high-voltage power supply, while the substrate is grounded. This creates an electromagnetic field between the electrospinning needle and the grounded substrate. The first ink moves toward the substrate through electrostatic attraction, assisted by the force of the electromagnetic field.

[0080] The voltage applied to the electrospinning needle may depend on the components and viscosity of the first ink. In an exemplary embodiment, the voltage applied to the electrospinning needle is between about 3 kV and about 50 kV or between about 10 kV and about 40 kV.

[0081] The distance between the tip of the electrospinning needle and the substrate may depend on the diameter of the ink jet and its viscosity. In general, a finer stream and lower viscosity may require a shorter distance between the electrospinning needle and the substrate. In some exemplary embodiments, the distance between the tip of the electrospinning needle and the substrate may be between about 1 cm and about 100 cm, preferably between about 3 cm and about 50 cm, more preferably between about 5 cm and 35 cm. The speed of electrospinning may depend on the ink. In general, the faster the ink sets or dries, the higher the electrospinning speed that can be used. For example, the ink may be dispensed through the electrospinning needle at a flow rate of about 0.01 to about 50 ml / hour, preferably about 0.1 to about 30 ml / hour, more preferably about 0.5 to about 10 ml / hour.Advantageous needle inner diameters are between approximately 0.05 mm and 2 mm, preferably between approximately 0.1 mm and 1 mm. For hybrid electrospinning, larger needle diameters up to approximately 3 mm are preferred.

[0082] Voltage, needle-to-substrate distances, flow rates and needle diameters within the ranges specified here generally ensure uniform fiber formation, no needle clogging and no electrical discharge.

[0083] An electrospinning needle can be used to spin the first ink onto the substrate. However, it may also be desirable to use multiple electrospinning needles (hereinafter referred to as first-type needles) to increase the production rate of the electrode. The electrospinning needles can be connected in parallel or in series. A parallel arrangement means that, if the substrate is, for example, a continuous roll running in one direction under the spinning head, the fibers ejected by two or more needles reach the same point along the length of the substrate at approximately the same time, i.e., they are deposited simultaneously. In a series configuration, the fibers ejected by two or more needles reach successive points on the substrate at the same time, i.e.,The fibers are spun on top of each other, allowing for multiple layers and a greater final electrode thickness. A combination of parallel and serial needles can be used, allowing for a wide width and reasonable electrode thickness with high productivity. Reasonable thickness refers to a final electrode thickness in the range of 1 micrometer to 20 micrometers, preferably 5 micrometers to 15 micrometers. It is easy to understand that by supplying inks with different compositions to needles arranged in an in-line configuration, a graded or layered first phase inside the electrode in the thickness direction can be achieved.

[0084] The first ink containing the first-phase material may form a continuous nanofiber, or the nanofiber may break, and multiple or individual nanofibers may form on the substrate. In step 1.4 of Fig. 1, the solid materials of the second phase, namely a catalytically active component, optionally an ionomer (ionomeric polymer), preferably not an ionomer, and optionally a polymer, are dissolved or dispersed in a suitable solvent or liquid medium to provide a solution or dispersion for electrospraying (hereinafter referred to as the second ink or second electrospray ink).

[0085] Based on the sum of all solid materials present in the second ink, the catalytically active component is present in an amount of 70-100 mass%, the ionomer in an amount of 0-30 mass%, and the polymer in an amount of 0-30 mass%, with the sum of ionomer and polymer not exceeding 30 mass%. Analogous to the first ink, some of the solid components of the second ink may actually be dissolved in the solvent, while others are merely dispersed.

[0086] Typically, the catalytically active component is dispersed, the polymer is dissolved to obtain a molecular solution, while the state of the ionomer can vary from a true solution to a colloidal dispersion, depending on the ionomer's affinity for the solvent, the dissolution conditions, the interaction with the other components of the mixture, etc. Suitable solvents for electrospraying are known to those skilled in the art, and their suitability depends at least in part on the properties of the polymeric substances (polymer, ionomer). Suitable solvents include, for example, N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), N-methylpyrrolidone (NMP), ethanol, methanol, isopropyl alcohol, acetone, methyl ethyl ketone, water, tetrahydrofuran (THF), methylene chloride (MC or dichloromethane), formic acid, acetic acid, and combinations thereof. Those skilled in the art may also choose other solvents suitable for a particular process.

[0087] Similar to the first ink, additional components can be added to the second ink based on processing or functional considerations. Metal oxide nanoparticles are advantageously added to the second ink to improve electrode behavior under dry conditions, for example, dispersed SiO2 nanoparticles with an average diameter in the range of 5-30 nm. The particle size is measured using transmission electron microscopy, with the particle size being determined geometrically as the average value of 100 measurements.

[0088] The solvent of the second ink can be the same or different from the solvent of the first ink. If the solvents are different, it is preferable that they have similar evaporation properties, so that evaporation during the spinning process and the spraying process occurs at a similar rate, preventing either premature drying of one of the two phases or, conversely, one of the two phases landing on the substrate still wet. However, solvents with different evaporation properties can also be used, for example, by adjusting the distances of the spinning needles / spraying needles from the substrate accordingly to give one of the solvents more or less time to evaporate before reaching the substrate.

[0089] The solids content of the second ink used for electrospraying can be determined by a person skilled in the art, for example, based on the desired viscosity. Preferred solids content ranges are the same as those specified for the first ink, and the viscosities most suitable for the electrospray process are between 0.1 and 2 Pa s. Viscosity can be measured using a rheometer with a plate-on-plate geometry, a plate diameter of 50 mm and a separation of 0.1 mm at 23 °C. This involves plotting a flow curve in rotational mode (as opposed to oscillatory mode) at a shear rate of 0 to 1000 s -1 and 1000 to 0 s -1 measured. The viscosity value is the measured value of the return curve at 50 s -1 A suitable device is, for example, the MCR302 model from Anton Paar.

[0090] In step 1.5 of Fig. 1, the second ink is supplied to at least one electrospray needle. In a preferred embodiment, the ink is pumped to the electrospray needle. Any other suitable means for supplying the ink to the electrospray needle may be used.

[0091] In step 1.6 of Fig. 1, the electrospray needle 2 releases the second ink as fine nanodroplets with a diameter in the range of tens or hundreds of nanometers onto the same grounded substrate as the first ink. The electrospray needle is connected to a high-voltage power supply, preferably the same as that used for the first needle, but may also be a separate one. The second ink moves toward the substrate through electrostatic attraction, assisted by the force of the electromagnetic field.

[0092] The voltage applied to the electrospray needle may depend on the components and viscosity of the second ink. In an exemplary embodiment, the voltage applied to the electrospray needle is between about 3 kV and about 50 kV or between about 10 kV and about 40 kV. Preferably, it is the same as or within the same range as the voltage applied to the first-described electrospinning needle, but may also be different, e.g., depending on the distance between the electrospray needle and the substrate. The distance between the tip of the electrospray needle and the substrate may depend on the diameter of the ink jet and its viscosity. In general, a finer stream and lower viscosity may require a shorter distance between the electrospray needle and the substrate.In some exemplary embodiments, the distance between the tip of the electrospray needle and the substrate may be between about 1 cm and about 100 cm, preferably between about 3 cm and about 50 cm, more preferably between about 5 cm and 35 cm.

[0093] Preferably, the distance between the tip of the electrospinning needle / electrospray needle and the substrate is the same for both the first and second needles. The needles are preferably placed close together to ensure good deposition of the second-phase particles onto the first-phase fibers.

[0094] The electrospray speed of the second ink must be adapted to the electrospinning speed of the first ink. For example, the ink can be metered through the electrospray needle at a flow rate of about 0.01 to about 50 ml / hour, preferably about 0.1 to about 30 ml / hour, more preferably about 0.5 to about 10 ml / hour. Given the flow rate of the first ink, the flow rate of the second ink must be adjusted so that the final amount of dry second-phase material on the substrate is in the desired ratio to the dry first-phase material, as indicated above (ratio of the mass of the first phase to the mass of the second phase between 0.2 and 6.6, preferably between 0.3 and 4, more preferably between 0.4 and 2).At this ratio, the flow rate of the second ink depends on the flow rate of the first ink and the solid content of both inks and can be determined by a skilled person using a mass balance calculation.

[0095] In step 1.7 of Fig. 1, the solvent evaporates, and the solid nanofibers of the first phase and particles of the second phase form on the substrate. A network is formed in which the nanofibers derived from the first ink are intimately mixed with the particles derived from the second ink.

[0096] Fig. 2 shows a section of a fuel cell cathode 1 according to a first embodiment, which can be produced by the method shown in Fig. 1. The fuel cell cathode 1 comprises a first phase 2 and a second phase 3. The first phase 2 is a fiber phase, i.e., it is formed from fibers. The second phase 3 consists of particles 7. The particles 7 of the second phase 3 are arranged on a surface 4 of the fibers of the first phase 2.

[0097] The first phase 2 and the second phase 3 have specific compositions. The fibers of the first phase 2 comprise 23 to 75 mass%, in particular 35 to 67 mass%, and in particular 45 to 60 mass% of proton-conductive ionomer 5. The proton-conductive ionomer is a plastic component characterized by its content of ionic groups. Sulfonic acid groups are preferably used as ionic groups. Fluorinated ionomers are preferred with regard to high oxygen transport in the cathode of the fuel cell. HOPI (highly oxygen permeable ionomers) can also be used. One ionomer can be used alone or two or more ionomers can be used in combination. It is important that the first phase 2 comprises at least 23 mass% ionomer.

[0098] The first phase 2 further comprises 25 to 67 mass%, in particular 33 to 56 mass%, and in particular 40 to 50 mass% of a conductive carbon 6. The conductive carbon can comprise, for example, Ketjen black (KB), furnace blacks, Vulcan, acetylene black, oxidized acetylene black, highly graphitized carbons, mesoporous carbons, graphene, and carbon nanotubes. The carbon improves the electronic conductivity and / or porosity of the first phase 2, which contributes to an overall improved performance of the electrode, both in terms of electrical conductivity and mass transport properties. Here, the first phase 2 can contain one conductive carbon e or two or more conductive carbons in combination.

[0099] The first phase 2 can optionally comprise a catalytically active component. If this is the case, the mass fraction of catalytically active component in the first phase 2, based on the total mass of the first phase 2, is in particular 0 to 40 mass%, in particular 2 to 30 mass%, in particular 5 to 20 mass%, and in particular less than 10 mass%, since this ensures a good functional separation between the first phase 2 and the second phase 3, and the phases do not impair each other's functions.

[0100] Thus, it is provided that the particles 7 of the second phase 3 comprise 70 to 100 mass% of at least one catalytically active component 10, which contains at least one catalytically active substance 8 present in the form of particles. The catalytically active component 10 can be present in the form of primary particles or in the form of agglomerates on the surface 4 of the first phase 2. Platinum or one or more platinum alloys are used as the catalytically active substance.

[0101] The particles 7 of the second phase 3 may further comprise an ionomer, wherein the mass fraction of ionomer in the particles 7 of the second phase 3, based on the total mass of the second phase 3, is 0 to 30 mass% and in particular 0 to 20 mass%. The ionomer, if present, is used in the second phase 3 at most as a binder so that the catalytically active component 10, which constitutes the main constituent of the second phase 3, adheres better to the surface 4 of the first phase 2.

[0102] A distance M between the first phase 2 and the catalytically active substance 8 of the second phase 3 is 0 to 500 nm. Preferably, the distance M is 0 to 200 nm and in particular 0 to 100 nm. To determine the distance M, for each of 100 particles 7, the nearest surface 9 of the first phase 2 is determined and then the distance between the nearest surface 9 of a respective catalytically active substance 8 and the nearest surface 9 is measured. For more than 50% of the particles considered, i.e. for at least 51 catalytically active substances 8, the maximum distance is 500 nm.

[0103] To determine the distance M between the catalytically active substance 8 of the catalytically active component 10 of the second phase 3 and the nearest surface 9 of the first phase 2, an image taken with a transmission electron microscope at a magnification of 100,000–400,000 is analyzed. Here, the individual particles of the catalytically active substance 8 of the second phase 3 can be seen. For each of 100 particles, the nearest surface 9 of the first phase 2 is determined, and then the shortest distance M between the surface of the particles of the catalytically active substance 8 and the nearest surface 9 is measured. For more than 50% of the particles considered, i.e., for at least 51 particles of the catalytically active substance 8, the distance M is a maximum of 500 nm.

[0104] The second phase 3 may optionally comprise a conductive carbon 11, in particular with an average specific surface area of ​​less than 500 m 2 / g, measured by BET.

[0105] In the fuel cell cathode 1, the transport of protons to the surface of the easily accessible catalytically active substance 8 is guaranteed by the short distance M to the first phase 2, which is a maximum of 500 nm for each selected particle of the catalytically active substance 8. The short distance M of a large proportion of the catalytically active surfaces or of particles of the catalytically active substance 8 of the second phase 3, namely at least 50 mass% and more preferably at least 75 mass%, to the nearest surface 9 (or interface) of the first phase 3, is achieved in particular by the design and realization of the second phase 3, in which the particles 7 of the second phase 3 have very small characteristic dimensions (diameter), which can be represented by an equivalent diameter.

[0106] The very short distance M between the first phase 2 and the catalytically active substance 8 of the second phase 3 significantly increases the efficiency of the fuel cell cathode 1 and promotes conversion to water due to the high proton conductivity of the first phases and the short proton transport paths in the second phase. The mass transport properties of the fuel cell cathode 1 are therefore particularly good. The high proportion of catalytically active substance 8 in the second phase 3, which preferably contains only a very small proportion of ionomer, also promotes the catalytic activity of the fuel cell cathode 1, since catalytically active sites of the catalytically active substance 8 are not occupied by foreign matter but are freely accessible to the gas to be converted.

[0107] In addition to the above written description of the invention, reference is hereby explicitly made to the graphic representation of the invention in Figs. 1 and 2 for its supplementary disclosure.

[0108] List of reference symbols

[0109] 1 fuel cell cathode

[0110] 2 first phase

[0111] 3 second phase 4 surface of the first phase

[0112] 5 First phase ionomer

[0113] 6 conductive carbon of the first phase

[0114] 7 particles of the second phase

[0115] 8 catalytically active substance of the second phase 9 nearest surface

[0116] 10 catalytically active component of the second phase

[0117] 11 conductive carbon of the second phase

[0118] M distance

Claims

Claims 1. A fuel cell cathode (1) comprising a first phase (2) and a second phase (3), wherein the first phase (2) comprises fibers and the second phase (3) comprises particles (7), wherein the particles (7) of the second phase (3) are arranged on a surface (4) of the fibers of the first phase (2), wherein: - the fibers of the first phase (2) comprise 23 to 75 mass%, in particular 35 to 67 mass% and in particular 45 to 60 mass% of proton-conductive ionomer (5) and 25 to 67 mass%, in particular 33 to 56 mass% and in particular 40 to 50 mass% of a conductive carbon (6), - the particles (7) of the second phase (3) comprise 70 to 100 mass% of at least one catalytically active component (10) containing at least one catalytically active substance (8), wherein a distance (M) of the catalytically active substance (8) in the particles (7) of the second phase (3) to a surface (9) of the first phase (2) which is closest to the catalytically active substance (8) is 0 nm to 500 nm, in particular 0 nm to 200 nm and in particular 0 to 100 nm.

2. Fuel cell cathode (1) according to claim 1, wherein the first phase (2) further comprises a catalytically active component which contains at least one catalytically active substance, wherein a mass fraction of the catalytically active component, based on the total mass of the first phase (2), is 0 to 40 mass%, in particular 2 to 30 mass%, in particular 5 to 20 mass% and in particular less than 10 mass%.

3. Fuel cell cathode (1) according to claim 2, wherein a mass fraction of catalytically active substance in the first phase (2), based on the total mass of the first phase (2), is less than 20 mass%, preferably less than 10 mass%, more preferably less than 5 mass% and in particular 0 mass% and / or wherein the particles (7) of the second phase (3) further comprise an ionomer, wherein a mass fraction of ionomer in the particles (7) of the second phase (3), based on the total mass of the second phase (3), is 0 to 30 mass% and in particular 0 to 20 mass%, wherein in particular a proportion of ionomer in the particles (7) of the second phase (3) is 0 mass%.

4. Fuel cell cathode (1) according to one of the preceding claims, wherein the catalytically active substance (8) in the second phase (3) comprises platinum or a platinum alloy, wherein an average particle size of the platinum or the platinum alloy is in particular 2 to 10 nm and in particular 3 to 7 nm and / or wherein the catalytically active component (10) in the second phase (3) a conductive carbon (11) with an average specific surface area of ​​less than 500 m 2 / g.

5. Fuel cell cathode (1) according to one of the preceding claims, wherein the first phase (2) and / or the second phase (3) further comprises a polymer, wherein a mass fraction of the polymer, based on the total mass of the first phase (2) and / or the second phase (3), is 0 to 30 mass%, in particular 1 to 20 mass% and in particular 2 to 5 mass%, wherein the polymer is in particular a non-ionomeric polymer and is in particular selected from the group consisting of polyalkylene oxides, polyethylene oxides, polyvinylpyrrolidone, polyacrylic acid, polyvinyl alcohol, polyvinylidene fluoride, polyamide 6,6, polyurethanes, polybenzimidazoles, polycarbonate, polyacrylonitrile, polylactic acid, polyaniline, polystyrene, polyvinylcarbazole, polyethylene terephthalate, polymethacrylate, polymethyl methacrylate, polyamides, polyvinyl chloride, cellulose acetate, polycaprolactone, polyetherimide, polysulfone and polyethersulfone, wherein the polymer (8) is in particular selected from polyethylene oxides,Polyvinylpyrrolidone, polyacrylic acid, polyvinyl alcohol and polyvinylidene fluoride, wherein in particular a total proportion of ionomer and polymer in the particles (7) of the second phase (3), based on the total mass of the second phase (30), is 2 to 30 mass%, in particular 3 to a maximum of 20 mass% and in particular 5 to 10 mass% and / or wherein an equivalent diameter of the particles (7) of the second phase (3) is at least 10 nm, in particular at least 20 nm and in particular at least 30 nm and in particular a maximum of 1000 nm, in particular a maximum of 500 nm, in particular a maximum of 200 nm and in particular a maximum of 100 nm.

6. Fuel cell cathode (1) according to one of the preceding claims, wherein a contact surface between the first phase (2) and the second phase (3) is at least 1 m 2 per cm 3 of the cathode volume, in particular at least 3 m 2 per cm 3 of the cathode volume and in particular at least 10 m 2 per cm3 of the cathode volume and a maximum of 300 m 2 per cm 3 of the cathode volume, in particular a maximum of 150 m 2 per cm 3 of the cathode volume and in particular a maximum of 100 m 2 per cm 3 of the cathode volume.

7. Fuel cell cathode (1) according to one of claims 1 to 3, 5 and 6, wherein the catalytically active substance (8) in the second phase (3) comprises nanoparticulate whiskers.

8. Fuel cell cathode (1) according to one of the preceding claims, wherein the proton-conductive ionomer (5) of the first phase and / or the second phase comprises a hydrocarbon-based ionomer.

9. Fuel cell comprising a fuel cell cathode (1) according to one of the preceding claims.

10. A method for producing a fuel cell cathode (1) according to one of claims 1 to 8, comprising a step of electrospinning the first phase (2) and a step of electrospraying the second phase (3).

Images