DE102013020876A1 - Membrane electrode arrangement for use between bipolar plates of PEM-fuel cell used in motor car, has anode including selective membrane between catalyst and gas diffusion layers, where membrane is impermeable for nitrogen, oxygen and water - Google Patents

Abstract

The arrangement has a proton-exchange-membrane (PEM) (1) arranged between an anode and a cathode. The anode and cathode include catalyst layers (2) and gas diffusion layers (3), where the catalyst layers are arranged at the PEM. The anode includes a hydrogen-permeable selective membrane (4) between the catalyst layers and the gas diffusion layers, where the selective membrane is impermeable for nitrogen, oxygen and water. The selective membrane is provided with a carrier layer for a metal layer and/or a nanoporous carbon layer. The carrier layer is made of aluminum oxide or titanium oxide.

Description

The invention relates to a membrane electrode assembly and a fuel cell. Furthermore, the invention relates to a fuel cell stack and a motor vehicle.

In fuel cells, electrical energy is generated directly from chemical energy sources without the detour via the conversion into heat and power as in an internal combustion engine. In vehicles, especially proton exchange membrane (PEM) fuel cells are used in a membrane electrode assembly (MEA) in which the electrodes are separated by a semipermeable proton-conducting membrane. The electrolyte (polymer) membrane is usually coated on both sides with a catalyst layer, followed by gas diffusion layers coated as electrodes. These MEAs are usually taken between two bipolar plates, which serve to regulate the temperature and channels for the distribution of gases to the gas diffusion layers, the z. B. can consist of porous carbon paper or fiber fabric.

The catalyst layers comprise catalyst metals such as platinum or platinum alloys with ruthenium, nickel or cobalt, or have a mixture of carbon (graphite or carbon black) and catalyst metals.

Out DE 11 2004 002 549 T5 For example, a fuel cell is known in which an electrode (anode) is formed by a hydrogen-permeable metal layer without a gas diffusion layer. The hydrogen-permeable metal layer is formed on one side of a proton-conducting electrolyte layer, on the other side of which a catalyst layer and a gas diffusion layer (cathode) adjoin. The hydrogen-permeable metal layer comprises at least one layer of palladium (Pd) or Pd alloy and may optionally comprise a multilayer membrane having a base layer of a group of V metal such as vanadium, niobium or tantalum or their alloys, and Pd -containing layer which is formed at least on the side facing away from the electrolyte layer side of the base layer. In this case, this fuel cell has a temperature distribution compensation section, with which an uneven temperature distribution in the fuel cell during operation can be compensated.

The energy sources that provide energy in a fuel cell by means of electrochemical reaction are (air) oxygen and a fuel, often using hydrogen; But there are also fuel cells that use organic compounds such as methane or methanol as fuel.

On the anode side, the oxidation reaction / electron donation occurs, ie in the case of hydrogen as fuel, the dissociation of H ₂ molecules to two electrons and two protons, which diffuse through the membrane, while the electrons perform electrical work in an external circuit and on the Reduce the cathode side oxygen, so that together with the transported through the membrane protons water is formed. Due to unwanted mass transport of oxygen, nitrogen and process water from the cathode side to the anode side, the hydrogen supplied there is diluted and must therefore be supplied to ensure a surface reaction at the anode in excess. In order to limit the loss of hydrogen, the exiting gas stream of hydrogen and impurities is usually recirculated. Furthermore, the channel geometries in the bipolar plates are variable only within certain limits, since a certain pressure loss is required to discharge liquid water can.

Furthermore, downtimes of the fuel cell are problematic because then oxygen can diffuse to the anode side, which causes increased potentials when starting the cell, which damage the catalyst.

Based on this prior art, it is an object of the present invention to provide a membrane electrode assembly for a fuel cell with improved water and start management.

This object is achieved by a membrane electrode assembly having the features of claim 1.

Further developments are set forth in the subclaims.

Another object is to provide a fuel cell that can use bipolar plates with alternative channel geometries. This object is achieved by a fuel cell with the features of claim 6.

Further objects of the invention are a fuel cell stack with the features of claim 7 and a motor vehicle having a fuel cell stack as an electrical energy source, with the features of claim 8.

The membrane electrode assembly according to the invention is based on a proton exchange membrane, which is arranged between an anode and a cathode. Both the anode and the cathode have a catalyst layer arranged on the proton exchange membrane and a gas diffusion layer. According to the invention, the Anode further between the catalyst layer and the gas diffusion layer on a hydrogen-permeable selective membrane, which is impermeable at least for nitrogen, oxygen and water. Thus, the mass transport of the cathode side, comprising the atmospheric nitrogen and oxygen and product water, effectively prevented in the anode-side gas diffusion layer; the selective membrane allows only hydrogen to reach the anode-side catalyst layer. The product water formed on the cathode side is removed only by the cathode-side gas diffusion layer. Advantageously, since there is only hydrogen on the anode side, the anode recirculation can be dispensed with and an anode water separator can be dispensed with. Further, the number of purge processes by which impurities in the MEA are removed, especially at the catalyst layer, can be reduced, and also the catalyst degradation can be significantly reduced. In addition, since the presence of liquid water in the hydrogen injection is prevented, the risk of icing drops significantly at corresponding ambient temperatures and can optionally be completely avoided.

The hydrogen-permeable selective membrane may consist of a metal layer or a nanoporous carbon layer. However, it is also conceivable that the hydrogen-permeable selective membrane consists of more than one layer, i. H. has more than one metal layer or more than one carbon layer or metal and carbon layers in combination.

Palladium, nickel, titanium or zirconium may be used to form the metal layer of the hydrogen-permeable selective layer. However, it is also possible to use palladium alloys, especially with nickel, titanium and / or zirconium, but also also with silver, copper, cerium, yttrium and / or gold. Alternatively, the metal layer may be a zirconium, vanadium and / or niobium-based alloy, preferably a group comprising nickel-niobium-zirconium alloys, vanadium-aluminum alloys and vanadium-nickel alloys.

Furthermore, the hydrogen-permeable selective membrane may have a carrier layer for the at least one metal layer and / or carbon layer.

Such a carrier layer consists of an electrically conductive, hydrogen-permeable material, preferably selected from the group comprising electrically conductive ceramics, for example Al ₂ O ₃ , TiO ₂ , sintered metal and stainless steel.

The fuel cell according to the invention has a membrane electrode arrangement according to the invention between two bipolar plates. Since the membrane electrode arrangement according to the invention with the hydrogen-permeable selective layer prevents the material flows of water, oxygen and nitrogen from the cathode side into the gas diffusion layer of the anode side, the channel geometries of the bipolar plates, in particular of the anode-side bipolar plate, can be based primarily on the request for the distribution of the hydrogen, and, where appropriate, the cooling requirements are adjusted. The channels no longer have to be designed to produce a certain pressure loss in order to discharge liquid water. Also, the components for the anode recitation can be dispensed with.

A fuel cell stack according to the invention is composed of a plurality of fuel cells according to the invention.

In a motor vehicle according to the invention, at least one fuel cell stack according to the invention is present as an electrical energy source, but preferably the vehicle will have a plurality of fuel cell stacks which supply an electric motor with power.

These and other advantages are set forth by the following description with reference to the accompanying figures. The reference to the figures in the description is to facilitate understanding of the subject matter. Articles or parts of objects which are substantially the same or similar may be given the same reference numerals. The figures are merely a schematic representation of an embodiment of the invention.

Showing:

1 a simplified side sectional view through a membrane electrode assembly of the prior art,

2 a simplified side sectional view through a membrane electrode assembly according to the invention.

The invention relates to a fuel cell membrane electrode assembly whose structure is a hydrogen-selective membrane, d. H. permeable only to hydrogen to optimize the mass flows within the membrane electrode assembly.

1 Fig. 2 shows a simplified construction of an MEA for a PEM fuel cell of the prior art: the proton exchange membrane 1 close the catalyst layers 2 to which the reaction gases - hydrogen H ₂ on the anode side and Oxygen O ₂ on the cathode side - through the respective gas diffusion layers 3 be transported. The resulting product water H ₂ O is on the cathode side through the gas diffusion layer 3 transported in channels of the bipolar plate, not shown. These substance transports are, as necessary for the fuel cell reaction, wanted and in 1 indicated by white block arrows.

In addition, however, other material transports also take place - represented by the patterned block arrows - which limit or even prevent the desired substance transports. Thus, part of the air introduced nitrogen migrates N ₂ through the membrane 1 On the anode side, as well as a part of the product water H ₂ O. This takes place on the anode side, a dilution of the hydrogen H ₂ , so that it must be supplied in excess (stoichiometric) to over the entire catalyst surface 2 to be able to ensure a reaction. In order not to lose too much hydrogen, the exiting gas stream is usually recirculated.

Furthermore, the process water occurring unintentionally on the anode side requires a specific channel geometry in the bipolar plate, not shown, in order to generate a certain pressure loss, so that liquid water can be discharged. The tasks of the bipolar plate are in addition to the electrical contacting of the electrodes and the connection of adjacent cells therein to direct the reaction gases in the most favorable flow field in the gas diffusion layers and dissipate process water. In some cases, conflicting requirements are imposed on the channel geometries in the bipolar plates. While in order to prevent blockages of the channels by water droplets, the resulting liquid water must be removed, for which a high pressure loss is helpful, low pressure loss is advantageous for low peripheral power consumption. Furthermore, the reaction gases should be transported as evenly as possible in the gas diffusion layer, and for the electrical contacting the largest possible contact surface available. The design options of the channel geometries to accommodate these requirements are therefore limited.

Further, in the prior art, when the cell is stopped, oxygen O ₂ diffuses on the anode side. This creates when starting the fuel cell, the situation that at one part of the anode (in particular at the cell entrance) hydrogen H ₂ comes to the reaction, while at another part of the anode (at the cell exit) still oxygen O _{2 is} present. This creates increased potentials that damage the catalyst.

In the 2 The membrane electrode assembly according to the invention also has a proton exchange membrane 1 to which the anode and cathode side catalyst layers 2 connect. While the cathode side with the to the catalyst layer 2 subsequent gas diffusion layer 3 As may be provided in the prior art, the anode side according to the invention between the catalyst layer 2 and the gas diffusion layer 3 a selective membrane 4 which is permeable to hydrogen H ₂ but not to nitrogen N ₂ , oxygen O ₂ and water H ₂ O.

Again, the reaction gases H ₂ and O ₂ through the gas diffusion layers 3 to the respective catalyst layer 2 transported and the resulting product water H ₂ O is on the cathode side through the gas diffusion layer 3 transported in channels of the bipolar plate, not shown. As in 1 Here, too, these wanted material transports are represented by white block arrows. However, in the membrane electrode assembly according to the invention in 2 unwanted mass transport (patterned block arrows) into the anode-side gas diffusion layer 3 through the selective membrane 4 prevented or at least significantly restricted. In this way, nitrogen N ₂ , process water H ₂ O and oxygen O _{2 are} not or only to a very limited extent in the anode-side gas diffusion layer 3 and the channels of the bipolar plate, not shown.

Advantageously, the supplied hydrogen H _{2 is} not or only slightly contaminated so that its recirculation is not required and corresponding device components can be omitted.

In addition, the purge can be reduced to a minimum, or Purgeintervalle be increased. Purgen refers to the cyclic discharge of hydrogen, wherein the pressure on the anode side is rapidly degraded rapidly to discharge accumulated contaminants and water droplets. Since this is largely avoided in a membrane electrode assembly according to the invention, the purge and the associated loss of hydrogen (purge loss) can be significantly reduced. Purification will not be completely dispensed with in practice, since, in spite of the selective membrane according to the invention, it is possible on the anode side to accumulate impurities over time which, for B. come from the hydrogen storage tank or get to a small extent from the cathode side through the selective membrane.

Also for the flow field design through the channels of the bipolar plates, in particular the anode-side bipolar plate, there are advantages: Due to the prevented dilution of the hydrogen, the supplied hydrogen stream can be reduced; At the same time, the uniform distribution of homogeneously composed gases (without condensable water) is much easier to implement. The requirement for the channels of the anode-side bipolar plate with respect to the removal of the process water in order to prevent a blockage of the channels, is eliminated, so that the channel geometries can be optimized with respect to the other requirements.

On the other hand, by virtue of the fact that almost all the process water produced is discharged on the cathode side on the cathode side, the cathode humidifier required for the reaction works better, ie. H. be designed smaller for the same performance.

Although, when the fuel cell is started, oxygen can still rest against the anodic catalyst layer, but its proportion is markedly lower since no oxygen can be supplied. Even if oxygen has accumulated in the anode channels (eg via the anode seals), it can not reach the catalyst layer through the selective membrane. The small amount of oxygen that may be present in the catalyst layer will react immediately upon start-up so that the period of time during which the critical overvoltage is applied is minimized and catalyst degradation is drastically reduced.

As a material for the hydrogen-permeable selective membrane z. B. dense metal membranes of Pd, Ni, Ti, Zr or of a Pd alloy, especially with Ni, Ti and Zr and also Ag, Cu, Ce, Y and Au in question. Furthermore, based on Zr, V and Nb alloys such. As Ni-Nb-Zr alloys, V-Al alloys, V-Ni alloys are used to form the selective membrane. Furthermore, nanoporous carbon membranes can be used as a selective membrane, since carbon materials also have the required electrical conductivity.

The selective membrane may be applied to the anode-side catalyst layer or gas diffusion layer. However, since both the catalyst layer and the gas diffusion layer are relatively open structures with pores, which are optionally greater than the layer thickness, in a preferred embodiment, the selective metal membrane or carbon membrane is applied to a separate carrier layer. This may for example consist of an electrically conductive ceramic (eg Al ₂ O ₃ , TiO ₂ ), sintered metal or stainless steel. In general, organic polymers are also conceivable as a carrier layer, which, however, must be equipped with suitable electrical conductivity additives.

The application of the selective membrane on the support layer, or on the catalyst layer or gas diffusion layer can - depending on the material - by wet-chemical coating, electroplating, thermal spraying, physical vapor deposition, or chemical vapor deposition take place.

Although the additional selective membrane means increased costs in the preparation of the membrane electrode assembly, but these are more than offset by the savings made possible by the selective membrane in the catalyst material. The system simplifications, for example in the bipolar plates and by eliminating recirculation and water separator on the anode side, and operating costs by less Purgeverluste and less catalyst degradation costs are saved.

QUOTES INCLUDE IN THE DESCRIPTION

This list of the documents listed by the applicant has been generated automatically and is included solely for the better information of the reader. The list is not part of the German patent or utility model application. The DPMA assumes no liability for any errors or omissions.

Cited patent literature

• DE 112004002549 T5 [0004]

Claims (8)

Membrane electrode assembly comprising a proton exchange membrane ( 1 ), which is arranged between an anode and a cathode, wherein the anode and the cathode in each case one at the proton exchange membrane ( 1 ) arranged catalyst layer ( 2 ) and a gas diffusion layer ( 3 ), and wherein the anode between the catalyst layer ( 2 ) and the gas diffusion layer ( 3 ) a hydrogen-permeable selective membrane ( 4 ) impermeable to at least nitrogen, oxygen and water. A membrane electrode assembly according to claim 1, wherein the hydrogen permeable selective membrane ( 4 ) has at least one metal layer and / or a nanoporous carbon layer. Membrane electrode assembly according to claim 2, in which the metal layer - Palladium, nickel, titanium or zirconium or A palladium alloy with nickel, titanium, zirconium, silver, copper, cerium, yttrium and / or gold, or A zirconium, vanadium and / or niobium-based alloy, preferably from the group comprising nickel-niobium-zirconium alloys, vanadium-aluminum alloys and vanadium-nickel alloys, having. Membrane electrode assembly according to claim 2 or 3, wherein the hydrogen-permeable selective membrane ( 4 ) has a carrier layer for the at least one metal layer and / or carbon layer. The membrane electrode assembly of claim 4, wherein the support layer is of an electrically conductive, hydrogen permeable material selected from the group comprising electrically conductive ceramics comprising Al ₂ O ₃ , TiO ₂ , sintered metal, and stainless steel. Fuel cell having a membrane electrode assembly arranged between two bipolar plates, characterized in that the membrane electrode assembly is a membrane electrode assembly according to at least one of claims 1 to 5. Fuel cell stack of a plurality of fuel cells, characterized in that the fuel cells are fuel cells according to claim 6. Motor vehicle having a fuel cell stack as an electrical energy source, characterized in that the fuel cell stack is a fuel cell stack according to claim 7.

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