DE10254114B4 - Gas diffusion electrode, polymer electrolyte membrane fuel cell and polymer electrolyte membrane fuel cell stack - Google Patents

Abstract

Gas diffusion electrode (4) a membrane-electrode assembly, wherein the gas diffusion electrode (4) of at least one gas diffusion layer (1, 2) and a catalyst layer is formed, wherein in the at least one gas diffusion layer (1, 2) a concentration gradient (8) one of the physical properties the gas diffusion electrode (4) affecting chemical component the gas diffusion layer (1, 2) is formed, and wherein the concentration gradient (8) the chemical component of the gas diffusion layer (1, 2) in FIG Direction perpendicular to the top of the gas diffusion electrode (4) is formed, characterized in that the concentration gradient (8) the chemical component of the gas diffusion layer (1, 2) beyond in at least one direction parallel to the top of the gas diffusion electrode (4) is formed.

Description

The The invention relates to the technical field of gas diffusion electrodes for membrane electrode units. In particular, the invention relates to a gas diffusion electrode according to the preamble it patent claim 1, a fuel cell with this gas diffusion electrode and a fuel cell stack with these gas diffusion electrodes.

The basic structure of a polymer electrolyte membrane fuel cell - PEM-BZ short - is as follows. The PEM-BZ contains a membrane-electrode assembly - MEA short, which is composed of an anode, a cathode and an interposed polymer electrolyte membrane - PEM - constructed. In turn, the MEA is interposed between two separator plates, with one separator plate having fuel distribution channels and the other separator plate having channels for the distribution of oxidant and the channels facing the MEA. The electrodes, anode and cathode, are generally designed as gas diffusion electrodes - in short GDE. These have the function to dissipate the current generated during the electrochemical reaction (eg 2H ₂ + O ₂ → 2H ₂ O) and allow the reactants, starting materials and products to diffuse through. A GDE consists of at least one gas diffusion layer - in short GDL - and a catalyst layer, which faces the PEM and at which the electrochemical reaction takes place.

A Such a fuel cell can operate at relatively low operating temperatures produce high-power electrical power. Real fuel cells are usually stacked into so-called fuel cell stacks - in short stacks -, to achieve a high power output, using instead of the monopolar Separator plates bipolar separator plates, so-called bipolar plates, be used and monopolar separator plates only as end plates of the Stacks.

The reactants used are fuels and oxidizing agents. Most gaseous reactants are used, for example H ₂ or an H ₂ -containing gas (eg reformate gas) as fuel and O ₂ or an O ₂ -containing gas (eg air) as the oxidant. Reactants are understood as meaning all substances participating in the electrochemical reaction, including the reaction products such as H ₂ O. A dry PEM has a low ionic conductivity. To moisten the PEM, therefore, the reactants are usually moistened before they are fed to a PEM fuel cell. The disadvantage of humidification is the associated effort and the additional components required (humidifier), which is contrary to the pursuit of the simplest possible operation and a compact design as possible.

Around generate electric power with high power with PEM fuel cells to be able to will one possible even moistening the PEM sought. De facto, however, PEMs are generally not evenly moistened, i.e. The PEM is generally moist in some areas or even too humid and in other areas dry or even too dry. The optimal thermodynamic state for a PEM, i. the thermodynamic State where an MEA can bring its maximum performance lies generally only in a few areas of PEM before, or even in none.

• – der Gesamtdruck p in der Gasphase der Brennstoffzelle, der die Summe der Partialdrücke p_i ist (p = Σ_ip_i);
• – die Partialdrücke p_i der Komponenten i der Gasphase, z.B. der Wasserdampf-Partialdruck p_H2O, der H₂-Partialdruck p_H2 (wenn als Brennstoff H₂ eingesetzt wird), der O₂-Partialdruck p_O2(wenn als Oxidationsmittel O₂ eingesetzt wird);
• – die Temperatur.

This can be explained as follows. The thermodynamic state of the thermodynamic fuel cell system can be described by thermodynamic parameters. These thermodynamic parameters are in the case of a fuel cell, which is operated with gaseous reactants - in short reaction gases - for example:

• The total pressure p in the gas phase of the fuel cell, which is the sum of the partial pressures p _i (p = Σ _i p _i );
• - The partial pressures p _{i of} the components i of the gas phase, for example, the water vapor partial pressure p _H2O , the H ₂ partial pressure p _H2 (if as fuel H _{2 is} used), the O ₂ partial pressure p _O2 (if used as the oxidant O ₂ becomes);
• - the temperature.

These change thermodynamic parameters due to the electrochemical reaction that takes place in an MEA flow direction the reactants along the MEA surface. That is, at every point along a MEA another thermodynamic state exist can. Overall, therefore, the thermodynamic conditions are within a fuel cell, in particular along an MEA, inhomogeneous.

conventional MEAs, on the other hand, are usually homogeneously formed, which is due to has the appropriate manufacturing process, and therefore can not to a homogenization of the thermodynamic conditions contribute, so that the inhomogeneous thermodynamic conditions through the GDE of an MEA can also affect its PEM.

However, inhomogeneous thermodynamic conditions on a PEM are disadvantageous for several reasons. For example, the partial pressure of the reactants decreases as a result of the electrochemical reaction in the flow direction, which can set different reaction rates at different locations, which in turn lead to different current densities and thermi can lead to gradients. The consequence of this can be equalizing currents and hot spots as well as locally different humidifying states of the PEM, which can not only diminish the performance of an MEA or fuel cell, but also physically damage the fuel cell.

All in all considered MEAs of prior art as a result of inhomogeneous thermodynamic conditions generally only locally below optimal conditions, just where the thermodynamic conditions fortuitously are optimal. This means that an MEA, or a fuel cell, which has this MEA, is generally operated suboptimal and only part of the theoretically possible maximum power supplies.

Regarding moistening a PEM results in the situation that in the Near the Input port for an oxidizing agent (preferably air) for water tends to to evaporate because the oxidant there relatively dry is so that the PEM in this area tends to dehydration. At the Stream through the channels of Separator plate absorbs the oxidant water from the MEA, as a result of which its moisture increases and, on the way, further from the input port to the output port, increasingly less water from the Oxidizing agent is recorded or can be included, so that even the situation can arise at the starting port that the product water produced during the electrochemical reaction not sufficiently dissipated can.

Becomes now moisten the oxidant before entering a fuel cell, the situation may arise that the MEA is in the area of Entry ports while wet enough to dry out the PEM to prevent, however, then the humidity of the oxidizing agent be too high at the exit port, so that no more water can be removed there can and is liquid Water forms. This can clog the GDE of an MEA and the diffusion impede the reactants within the GDE, leaving the MEA has a poor performance.

Furthermore the problem arises in every fuel cell of a fuel cell stack other thermodynamic state can prevail, the in one Fuel cell respectively present thermodynamic state is dependent on the location of the fuel cell in the fuel cell stack. The reason for that is, that the fuel cells in the stack exchange heat with each other and fuel cells, which are further arranged in the middle of the stack are, a higher one Have temperature as fuel cells, rather at the outer ends of the Stack are arranged. With higher Temperatures aggravated also the humidification problem, so the PEM warmer, in the stack rather centrally arranged fuel cells stronger Dehydration, as PEM cooler, in the stack rather outside arranged Fuel cells. These stack-based differences in Temperature and humidification come the o.g. Differences along a single MEA, making it desirable would not be only one MEA available to have that to the changing ones thermodynamic conditions within a single fuel cell adapted, but many MEAs that are designed so that with them also the location of the fuel cell within a fuel cell stack considered is.

In the international application WO 00/14816 A2, a balanced water balance of an MEA, ie a homogenization of the thermodynamic state at the PEM, is achieved in that a gas diffusion structure perpendicular to the membrane has a gradient in the gas permeability. To achieve the gradient, the gas diffusion structure is adjusted in its pore volume. In this regard, a method of manufacturing a gas diffusion structure is provided in which substantially two or more layers, each of different porosity, are disposed adjacent to each other. This is intended to ensure that the gradients in the O ₂ and H ₂ O partial pressures do not develop in parallel, but perpendicular to the PEM surface, as a result of which the thermodynamic state over the entire membrane surface should be constant. The disadvantage of this production method or the resulting gas diffusion structure is that the complex structure with multiple layers, each with different porosities is difficult and can be realized with great effort.

In the international application WO 01/17047 A1 is a PEM-BZ with discloses an electrode in which the hydrophobicity in one direction varies along the thickness of the electrode or in one direction along the surface.

US Pat. No. 6,365,293 B1 discloses a fuel cell having a cathode having a water permeation adjusting layer. The water permeability of this layer changes along the cathode from the oxidant input port into the fuel cell toward the output port. The different water permeability is achieved by locally different concentrations of a water repellent agent in the layer. With regard to the manufacturing method, it is merely stated that in a region of an electrode in which a low water permeability is desired, more water-repellent agent is introduced or that in a range of Electrode in which a greater water permeability is desired, less water-repellent agent is introduced.

The DE 199 62 686 A1 discloses a membrane electrode assembly for a PEM fuel cell comprising a noble metal-containing electrocatalyst layer. This document describes that the pressure of the process gases and / or the temperature and thus the conversion rate of the process gases vary over an active cell area of the PEM fuel cell. In regions of the active cell surface in which a high process gas pressure and a high temperature prevail, a high concentration of catalyst or noble metal is thus required to achieve a good efficiency, while in poorly flown areas of the active cell surface a lower catalyst or noble metal occupancy is sufficient , The electrocatalyst layer and / or the noble metal concentration in this layer is / are therefore asymmetric, ie the thickness and / or the height of the layer and / or the concentration of the noble metal in the layer varies depending on the requirements in the respective membrane region.

The DE 197 37 389 A1 describes a gas diffusion electrode for fuel cells comprising an anisotropic gas diffusion layer and a catalytic layer. The anisotropic gas diffusion layer consists of a porous carbon matrix in which carbon particles and polyethersulfone are distributed so that the matrix is homogeneously porous in the lateral direction to the gas flow, whereas the porosity of the gas diffusion layer decreases in the direction of gas flow.

From the DE 197 09 199 A1 For example, there is known a reduced diffusivity gas diffusion electrode for water which contains in one portion a filler which reduces the effective diffusion constant of the electrode for water as compared to the unfilled electrode. The subregion extends over the entire surface of the electrode, but not over its entire thickness and thus does not extend to the surfaces of the electrode.

The DE 100 56 537 A1 describes a fuel cell having an anode and a cathode formed in the form of reactive layers on which gas reactions take place. In the direction of a gas flow flowing over the anode or the cathode, the anode and / or the cathode has / have a stepwise or continuously varying reactivity.

The Properties of an MEA are determined by the characteristics of their GDE and determined by their PEM. The present invention deals with the GDE.

task Therefore, it is the object of the present invention to provide a GDE represent the inhomogeneous thermodynamic conditions within a PEM-BZ at least partially compensated, so along the PEM at least approximately optimal thermodynamic conditions prevail and an MEA into which the GDE is installed, thereby bringing a good performance.

One The first object of the present invention is accordingly a GDE for an MEA, wherein the GDE consists of at least one GDL and a catalyst layer is formed, wherein in the at least one GDL a concentration gradient a chemical that influences the physical properties of the GDE Component of the GDL is formed. This concentration gradient The chemical component of the GDL is in at least one direction parallel to the top of the GDE and a direction perpendicular to the Top of the GDE trained.

Of the Concentration gradient is at least partially or in sections different from zero along the said directions. It can both the directions as well as the course of the concentration gradient be arbitrarily predetermined.

With Top of a GDE is a surface of the GDE meant to one Reactant faces, while the bottom of a GDE of a PEM or a catalyst layer is facing. If the GDE is installed in an MEA and this in turn into a PEM-BZ, the top of the GDE is a separator plate or bipolar plate, or the channels on a separator plate or bipolar plate, facing.

The GDE according to the invention is to the inhomogeneous thermodynamic conditions within one PEM-BZ adapted. It compensates for these inhomogeneous thermodynamic conditions at least partially, thereby causing at least PEM along the PEM nearly optimal thermodynamic conditions prevail so that an MEA, those with a GDE invention equipped, performs well.

The Performance of an MEA, e.g. with a GDE according to the invention is in the context of the present invention with reference to their i-U characteristic. An MEA delivers good performance, if its i-U characteristic is high, i. when the MEA high current densities i at high voltages U supplies, and the decrease of the voltage U with increasing current density is low.

A preferred variant of the GDE according to the invention has a concentration gradient of a chemical component of the GDL, the we at least one direction parallel to the top of the GDE, which is formed by a connecting line connecting the input port for a reactant in the PEM-BZ with the exit port for the reactant from the PEM-BZ.

The To make concentration gradients so that he at least one Direction from the inlet port to the outlet port of a reactant has, is advantageous, since this is the general flow direction of the reactant is (serpentine etc. neglected channels), along this direction the strongest change in the thermodynamic Conditions are expected.

A Further preferred variant of the GDE according to the invention has a concentration gradient on, which has at least two sections in which the concentration different courses has, wherein the course of the concentration is selected from the group comprising linear increasing, nonlinear increasing, linear decreasing, nonlinear decreasing and staying constant.

By These fine gradations of concentration of chemical substance dependent on from the place in a GDE it is possible To adjust the physical properties of the GDE very finely depending on the location.

at a still further preferred variant of the GDE invention the concentration gradient has one or more jumps or one or more kinks, or both one or more jumps as also one or more kinks. That the concentration gradient can discontinuities such. Jumps or kinks have, where jumps and kinks can also coexist. Particularly preferred while kinks.

By these jumps or kinks, this inventively preferred GDE spatially accurate placeable jump in their physical properties.

such GDE can e.g. have predetermined areas within the diffusion layer, which are deliberately kept anhydrous, and other areas that are considered Water reservoir or serve for water distribution.

At this point should be mentioned that the physical properties of a GDE are also different activities as by a concentration gradient of a chemical component the GDL can be influenced. Further suitable measures are e.g. the variation of the layer thickness of the GDE; the production a layer thickness profile and / or porosity profile by stacking multiple GDL with different layer thicknesses and / or porosities.

As mentioned, affects the chemical component of the GDL physical Properties of a GDE. The component of the GDL is preferred a chemical substance that has the following physical properties the GDE influences: permeability the GDE for Gases, permeability the GDE for Liquids, Wetting behavior of the GDE, heat conduction the GDE, mechanical stability the GDE, rigidity of the GDE and connectivity of the GDE with others Materials.

The preferred according to the invention GDE, where the o.g. physical properties at least partly spatially are targeted, are of particular advantage because they to the inhomogeneous thermodynamic conditions within a fuel cell and the special burdens for the Materials and materials are particularly well adapted. fuel cells, which are equipped with such GDE, provide a special good performance and show improved behavior Defects due to material fatigue.

In In this context, it is preferable if it is the physical Properties of the GDE influencing component of the GDL by one chemical substance that determines the permeability of the GDE to gases and liquids, preferably the permeability the GDE for Water and in particular the permeability of the GDE for water vapor influenced.

Under the permeability from e.g. Steam is understood to mean the amount of water vapor, by a unit area the diffusion layer migrates when the water vapor partial pressure on one side of the diffusion layer from the water vapor partial pressure on the other side differs the diffusion layer. The more Water vapor can migrate through the diffusion layer, the larger it is the permeability for water vapor.

The preferred according to the invention GDE with spatial targeted permeabilities, especially for water vapor, are of particular advantage, since thereby the diffusion within the diffusion layer can be controlled specifically and thereby the thermodynamic conditions at the PEM are evened out can, what an increased Performance of an MEA equipped with such a GDE, entails. In a preferred embodiment of the invention is the one influencing the physical properties of the GDE Component of the GDL a chemical substance belonging to the group of Hydrophobing agent selected from pore builder, carbon black and graphite.

The inventively preferred chemi Substances have the advantage that they can be used for example by their own physical properties with targeted dosing at predetermined locations with particular preference for the targeted influencing some very important physical properties in a diffusion layer.

there it is further preferred if it is the physical Properties of the GDE-influencing component of the GDL Hydrophobic agent is that from the group of perfluorinated polymeric hydrocarbons, e.g. Polytetrafluoroethylene - short PTFE, e.g. Teflon, and the fluorinated polymeric sulfonic acids, e.g. Nafion selected is.

These inventively preferred chemical substances can be used excellently for the control of H ₂ O diffusion in diffusion layers, since in them the hydrophobicity is particularly pronounced and therefore they must be used only in relatively small amounts. They also have the advantage of being chemically relatively stable and inert.

The Use of GDE according to the invention in electrochemical cells, preferably in PEM fuel cells, has the merit of producing electrochemical cells that can be to the thermodynamically inhomogeneous conditions prevailing in them are adapted and thus can be operated with improved performance.

One Another merit is that fuel cell stacks are also manufactured can be where each individual fuel cell is stacked in place in its place prevailing special thermodynamic conditions (total pressure, Partial pressures, temperature) are adjusted by each fuel cell contains at least one GDE according to the invention, whereby operated such a fuel cell stack with improved performance can be.

One Another object of the present invention is therefore a PEM fuel cell, comprising at least one GDE as disclosed above. The fuel cell according to the invention have the advantage that the present in them, locally different thermodynamic conditions are at least partially compensated by the GDE become, whereby on the PEM approximately homogenous or even conditions to adjust. Thereby can et al hot spots and unwanted equalizing currents be prevented and evenly moistening the PEM and thus achieves an improved performance of the fuel cell become.

One Another object of the present invention is a PEM fuel cell stack, the at least two PEM fuel cells as disclosed above are, has.

The Fuel cell stack according to the invention have the advantage that the fuel cells that compose them due to the GDE according to the invention, have improved performance, so that the fuel cell stack Overall, an improved performance brings.

One Another advantage of the fuel cell stack according to the invention is that she is more reliable work, i. with fewer failures or defects, as the risk of potential sources of error such as e.g. Melting of PEMs as a result of hot spots, corrosion due to equalizing currents or umpolung, tearing of PEMs due to dehydration, detachment of materials such as e.g. Sealing materials by thermal or chemical stress, is reduced.

One Another advantageous aspect of the fuel cell stack according to the invention is that not only stacks with improved performance are available, but that these stacks with the same volume of construction a higher performance bring, i. a higher one Have power density. This also opens up the possibility Produce stacks with a smaller construction volume and the same power density, which is advantageous in terms of cost and on the other hand in applications where space is limited, especially for mobile applications, e.g. in the area of vehicles.

The The invention will be explained in more detail with reference to the following drawings. there demonstrate:

1 schematic representation of a GDE according to the invention with a variant of a concentration gradient of a hydrophobing agent;

2 iU characteristic curves or TU characteristics of two MEAs which contain GDE according to the invention, wherein the GDEs have different structures for influencing physical properties.

1 2 shows a schematic representation of a section through an exemplary GDE (FIG. 4 ). The GDE consists in this example of two GDLs ( 1 ) and ( 2 ), with the upper GDL ( 1 ) faces a reactant, and the lower GDL ( 2 ) of a PEM or a catalyst layer (both not shown). The GDE is provided, for example, as an electrode for an MEA of a PEM-BZ. About the presentation of this GDE ( 4 ) is a diagram in which the concentration of a chemical substance, for example of a hydrophobing agent such as PTFE, in% by weight against the layer thickness of the GDE ( 4 ) is plotted in mm. The representation thus shows, as it were, a depth profile of the concentration gradient ( 8th ) in a direction perpendicular to the top of a GDE according to the invention. In this example, the course of the concentration gradient ( 8th ) at the top of the GDE ( 4 ) at about 0 wt .-%, then increases linearly to the phase boundary ( 5 ) between the GDLs ( 1 ) and ( 2 ), then goes through a point of discontinuity, namely the kink ( 3 ) then increases more non-linearly, then goes through a maximum ( 7 ), then decreases nonlinearly to the bottom of the GDL ( 2 ), which faces a PEM or a catalyst layer (both not shown), and reaches there a second point of discontinuity, namely the jump ( 6 ), at which the concentration decreases abruptly to about 0 wt .-% nonlinear. The course of the concentration gradient ( 8th ) ends at this point. Overall, the course of the concentration gradient ( 8th ) three sections: 1. linearly increasing; 2. increasing non-linear (nearly square); 3. decreasing non-linear (approximately square).

If the chemical is a hydrophobing agent, then the concentration curve shown causes the following. By the concentration maximum ( 7 ), which is also a hydrophobicity maximum, since concentration and hydrophobicity are proportional to each other, is at the bottom of the GDL ( 2 ), ie at the PEM or catalyst layer, resulting water at a diffusion to the upper limit of the GDL ( 1 ) and thereby partially restrained for moistening the PEM. Due to the fact that the hydrophobicity at the boundary to the PEM or catalyst layer (lower side of the GDL ( 2 )) initially starts at a relatively low value, but part of the water formed at the PEM or catalyst layer can be introduced into the GDE (FIG. 4 ), so that the PEM or catalyst layer is not flooded with water, but, as long as only small amounts of H ₂ O are present, the hydrophobicity maximum ( 7 ) initially do not overcome in significant quantities. Only when larger amounts of H ₂ O are present, for example H ₂ O, which is formed during the electrochemical reaction, the driving force for the diffusion of H ₂ O in the upper top direction of the GDL ( 2 ) so large that larger amounts of H ₂ O have the hydrophobicity maximum ( 7 ) can overcome.

The area between concentration leaps ( 6 ) and hydrophobicity maximum ( 7 ) therefore acts as a water reservoir, while the hydrophobicity maximum ( 7 ) acts like a pressure relief valve for H ₂ O or H ₂ O vapor. It is understood that the concentration in the hydrophobicity maximum ( 7 ) must be carefully chosen, on the one hand to achieve a good moistening of the PEM and on the other hand to prevent flooding of the PEM or catalyst layer. Within these limits, however, the pressure at which the "pressure relief valve" opens can vary well.

The above explained one can, taking into account the fact that the H ₂ O molecules at higher temperatures more easily succeed due to the increased Brownian molecular motion, the Hydrophobizitätsmaximum ( 7 ), to use as follows.

In the case of a GDE, which is to be operated at higher temperatures, for example because it is to be used in a warmer fuel cell, which is located, for example, in the middle of a fuel cell stack, the maximum hydrophobicity ( 7 ) by increasing the concentration of hydrophobing agent in this area. This will change the reservoir character of the area between ( 6 ) and ( 7 ) strengthened. The result is a good humidification of the PEM despite increased temperature.

On the other hand, in the case of a GDE, which is to be operated, for example, at the cooler edges of a fuel cell stack, the maximum hydrophobicity ( 7 ), which reduces the removal of H ₂ O from the GDE ( 4 ) favors.

If an H ₂ O molecule has the hydrophobicity maximum ( 7 ), it has become useless for the GDE, especially for moistening the PEM. The object now is to remove such H ₂ O molecules, in particular quickly and to remove them from the fuel cell, in order to prevent them from aggregating with other H ₂ O molecules (water condensation, droplet formation) and the GDL ( 1 ) or and ( 2 ) and thus obstructs the desired diffusion of a reactant to the PEM or catalyst layer. The course of the concentration gradient ( 8th ) such that for H ₂ O molecules which have the hydrophobicity maximum ( 7 ), there is a high tendency to go to the top of the GDL ( 1 ) to diffuse there and to carry out the phase transition into the reactant, to finally be transported away. For this purpose, the concentration of the hydrophilic agent inside the GDE is high, falls towards the top of the GDL ( 1 ) and is at the top of the GDL ( 1 ) about 0% by weight. For the sake of simplicity, the course of the concentration gradient ( 8th ) in this area (ie in the GDL ( 1 )) linearly in the direction of the upper side: such a course can be realized more easily.

Overall, the described concentration gradient ( 8th ) within the GDL ( 1 ) and ( 2 ) a structure for influencing the physical properties of the GDE, in which case the physical property is the hydrophobicity of the GDE is, with which in turn the diffusion behavior of H ₂ O molecules is controlled. The structure for influencing physical properties is in this case also a structure for controlling the diffusion behavior of at least H ₂ O, a diffusion control structure.

A concentration gradient such as the concentration gradient described above ( 8th ) in a direction perpendicular to the top of a GDE can of course also be present in at least one direction parallel to the top of the GDE. However, a concentration gradient in this direction can also have a different, predetermined course.

In 2 the iU characteristic curves and the TU characteristic curves of two MEAs with GDE according to the invention are shown (i = current density in A / cm ² , U = voltage in mV, T = temperature in ° C).

gekennzeichnet und ihre T-U-Kennlinie durch

This is a first MEA with a lower content of hydrophobing agent in its GDE of about 11% by weight, based on the hydrophobing agent and the MEA; their iU characteristic is through

and their TU characteristic through

Further this is a second MEA with a higher content of water repellent in their GDE of about 17% by weight, based on the hydrophobing agent and the MEA; their i-U characteristics are marked by ✱ and their T-U characteristic through ♦.

An MEA with GDE according to the invention can be prepared by a process which is described in the applicant's German patent application "Process for the preparation of a GDE having a structure for influencing its physical properties" (US Pat. DE 10254115.9 ) or "gas diffusion electrode with gas diffusion layer with layer thickness gradient" ( DE 10254116.7 ), filed with the DPMA on the same date as the present application, is disclosed. Another production method which is suitable in this context is in DE 100 52 189 A1 disclosed.

The difference between both MEAs consists essentially in the concentration of water repellents in wt .-%, wherein the MEA with the higher content of hydrophobing agent has a higher hydrophobicity maximum (see. 1 , ( 7 )) having.

As can be seen from the i-U curves, both deliver approximately MEA the same performance.

As from the T-U curves, the first MEA (lower Content of water repellent, lower hydrophobicity maximum) their highest Performance at a lower temperature of about 79 ° C, while the second MEA (higher Content of water repellent, higher hydrophobicity maximum) their highest Power at about 83 ° C supplies.

Therefore it is advantageous to use the first MEA at lower temperatures operate ("low-temperature MEA"), approximately in one cooler Fuel cell at the ends of a fuel cell stack while it It is advantageous to operate the second MEA at higher temperatures ("High-temperature MEA"), roughly in one warmer Fuel cell in the middle of a fuel cell stack.

1

upper GDL, facing a reactant

2

lower GDL, facing a PEM or catalyst layer

3

discontinuity: kink

4

GDE (Catalyst layer not shown)

5

Phase boundary between ( 1 ) and ( 2 )

6

discontinuity: Leap

7

Hydrophobizitätsmaximum

8th

concentration gradient

Claims (11)

Gas diffusion electrode ( 4 ) of a membrane-electrode unit, wherein the gas diffusion electrode ( 4 ) from at least one gas diffusion layer ( 1 . 2 ) and a catalyst layer, wherein in the at least one gas diffusion layer ( 1 . 2 ) a concentration gradient ( 8th ) one of the physical properties of the gas diffusion electrode ( 4 ) influencing chemical component of the gas diffusion layer ( 1 . 2 ), and wherein the concentration gradient ( 8th ) of the chemical component of the gas diffusion layer ( 1 . 2 ) in the direction perpendicular to the top of the gas diffusion electrode ( 4 ), characterized in that the concentration gradient ( 8th ) of the chemical component of the gas diffusion layer ( 1 . 2 ) in at least one direction parallel to the top of the gas diffusion electrode ( 4 ) is trained. Gas diffusion electrode according to claim 1, characterized in that it is in the at least one direction parallel to the top of the gas diffusion electrode ( 4 ) is a direction formed by a connection line connecting an input port for a reactant with an exit port for the reactant. Gas diffusion electrode according to one of claims 1 or 2, characterized in that the concentration gradient ( 8th ) has at least two sections in which the concentration has different courses, the course of the Kon is selected from the group comprising linearly increasing, nonlinear increasing, decreasing linearly, decreasing nonlinearly and remaining constant. Gas diffusion electrode according to one of claims 1 to 3, characterized in that the concentration gradient ( 8th ) has one or more jumps or one or more kinks, or both one or more jumps and one or more kinks. Gas diffusion electrode according to one of claims 1 to 4, characterized in that the physical properties of the gas diffusion electrode ( 4 ) influencing component of the gas diffusion layer ( 1 . 2 ) is a chemical that determines the permeability of the gas diffusion electrode ( 4 ) for gases, the permeability of the gas diffusion electrode ( 4 ) for liquids, the wetting behavior of the gas diffusion electrode ( 4 ), the heat conduction of the gas diffusion electrode ( 4 ), the mechanical stability of the gas diffusion electrode ( 4 ), the rigidity of the gas diffusion electrode ( 4 ) and the connectivity of the gas diffusion electrode ( 4 ) influenced with other materials. Gas diffusion electrode according to claim 5, characterized in that the physical properties of the gas diffusion electrode ( 4 ) influencing component of the gas diffusion layer ( 1 . 2 ) is a chemical that determines the permeability of the gas diffusion electrode ( 4 ) for gases and liquids, preferably the permeability of the gas diffusion electrode ( 4 ) for water, more preferably the permeability of the gas diffusion electrode ( 4 ) for water vapor. Gas diffusion electrode according to one of claims 1 to 6, characterized in that the physical properties of the gas diffusion electrode ( 4 ) influencing component of the gas diffusion layer ( 1 . 2 ) is a chemical substance selected from the group of water repellents, pore formers, carbon black and graphite. Gas diffusion electrode according to one of claims 1 to 7, characterized in that the physical properties of the gas diffusion electrode ( 4 ) influencing component of the gas diffusion layer ( 1 . 2 ) is a hydrophobing agent and is selected from the group of perfluorinated polymeric hydrocarbons, preferably polytetrafluoroethylene (PTFE), and the fluorinated polymeric sulfonic acids, preferably Nafion. Polymer electrolyte membrane fuel cell, characterized in that the polymer electrolyte membrane fuel cell at least one gas diffusion electrode ( 4 ) according to one of claims 1 to 8. Polymer electrolyte membrane fuel cell stack, characterized in that the polymer electrolyte membrane fuel cell stack at least two polymer electrolyte membrane fuel cells according to claim 9 has. A polymer electrolyte membrane fuel cell stack according to claim 10, characterized in that the polymer electrolyte membrane fuel cell stack comprises different gas diffusion electrodes ( 4 ), wherein the gas diffusion electrodes ( 4 ) in the course of their concentration gradients ( 8th ).