DE102004063457A1 - Membrane electrode unit for fuel cell, has catalyst containing layer arranged with polymer-intermixture between fuel cell membrane and gas diffusion layers in such a manner that water is stored and kept in electrode unit and/or membrane - Google Patents

Abstract

The electrode unit has a fuel cell membrane (12), which is arranged between two gas diffusion layers (15, 16). The cell membrane is formed on the basis of acidic beverage polymers. A catalyst containing layer (14) is arranged with a polymer-intermixture between the fuel cell membrane and the gas diffusion layers in such a manner that water is stored and kept in the electrode unit and/or the membrane.

Description

The The invention relates to a membrane electrode assembly of a fuel cell according to the preamble of claim 1.

During the Operation of a Polymer Electrolyte Membrane Fuel Cell (PEM Fuel Cell) becomes an oxygen-containing gas of the cathode and a hydrogen containing gas supplied to the anode. At the anode, the electrochemical oxidation of hydrogen takes place instead, at the cathode the reduction of oxygen. By the direct Implementation of chemical into electrical energy can be done in fuel cells independently from a Carnot limitation a high efficiency can be achieved.

The PEM fuel cell technology developed at the time the most based on Nafion ^® membranes as the electrolyte. The electrolytic conduction takes place via hydrated protons, whereby the proton conductivity of the membrane is coupled to the presence of liquid water. This limits the operating temperature at normal pressure below 100 ° C. At temperatures higher than 80-95 ° C, the performance of the fuel cell deteriorates significantly due to the loss of fluid. To maintain the conductivity of the membrane above 100 ° C due to the temperature dependence of the vapor pressure of water very large amounts of water to moisten the membrane necessary. In systems with a pressure greater than normal pressure, the temperature can be increased at the expense of the overall system's efficiency, size, and weight. For operation well above 100 ° C, the required pressure would increase dramatically.

operating temperatures greater than 100 ° C are off for a variety of reasons desirable. The electrokinetics as well as the catalytic activity for both Electrodes increase with increasing temperature. Besides that is the tolerance to contamination the operating gases used, for example with respect to carbon monoxide, higher. For use in a vehicle is due to the low heat emission into the exhaust air a big temperature difference advantageous to the ambient temperature.

One promising approach, like one with no or very little Humidification at operating temperatures from 120 ° C to 180 ° C working fuel cell can be realized relates to a fuel cell type, in the conductivity the membrane on the content of liquid, electrostatically bound to the polymer backbone of the membrane-bound acid, which is also almost complete Dryness of the membrane above the boiling point of water without additional Humidification of the operating gases takes over the proton conductivity. The one described here Fuel cell type, as is known in the art is generally as a high temperature polymer electrolyte membrane fuel cell (HTM fuel cell) designated. Polybenzimidazole (PBI) is known as a material for such Membranes which are impregnated, for example, with phosphoric acid as a liquid electrolyte.

Of the However, use of such membranes requires an adaptation commercial electrodes to the polymer membrane. This is in the Usually achieved by placing the electrodes on the polymer membrane side facing with electrolyte soaked, then the connection allowed to the polymer membrane. Another possibility exists in that one by hot pressing at appropriate pressures and temperatures obtained MEA (Membrane Electrode Assembly) for impregnation as a whole in acid is inserted.

Critical This type of membrane has the effect of lowering the operating temperature below the boiling point of water, such as during a cold start the fuel cell or when shutting down the fuel cell system is necessary. The water-soluble Electrolyte can by the liquid Product water released from the cell and be discharged from the MEA. This leads to irreversible damage the polymer membrane, there afterwards not enough anymore charge carrier for proton transport to disposal stand.

The Therefore, based on this type of membrane fuel cells must therefore held until the boiling temperature of water de-energized to prevent electrolyte leakage. A performance requirement This type of fuel cell must not be used until temperatures which ensures that as a result of the fuel cell reaction resulting product water is obtained in vapor form.

Around To keep the mechanical stress of the components low, this must be a time about 30 minutes before the system is ready is what the suitability of this type of membrane for mobile applications so far has severely limited. On the other hand, this membrane type is due to the increased operating temperature and the low moisture requirement a particularly advantageous proton conductor for mobile Applications dar.

From the US 6,124,060 A1 It is known to use an electrolyte with a higher binding tendency to the polymer of the polymer membrane and phosphoric acid by alkyl phosphoric acid or Phe to replace nylphosphoric acid. However, the reported performances and conductivities are below the phosphoric acid impregnated membranes.

It The object of the invention is a membrane electrode unit for a fuel cell in which a discharge of a liquid electrolyte decreases becomes.

The inventive solution this The object is the characterizing features of the main claim, advantageous embodiments of the invention describe the dependent claims.

A Membrane electrode unit according to the invention with a arranged between two gas diffusion layers fuel cell membrane, wherein the fuel cell membrane based on an acid-doped Polymer is formed points to the respective gas diffusion layer in each case at least one catalyst-containing layer with a polymer additive so that water in the membrane electrode assembly and / or the Fuel cell membrane is held and / or acid is stored.

Preferably can the polymer additive of the catalyst-containing layer of the Selected group of polyazoles in particular at least one component from the group of Polybenzimidazole, poly (pyridines), polybenzoxazoles or mixtures selected from it be. It is cheap, though between 0.001 to 0.06 wt.%, In particular between 0.001 to 0.04 Wt.% Of the polymer additive based on 1 g of catalyst powder in the catalyst-containing layer are present. Preferably the polymer additive Hydrophobic acid. Especially the polymer additive acid acts fixing.

In a particularly advantageous embodiment is between the gas diffusion layer and the catalyst-containing layer having a microporous layer arranged a hydrophobicizing additive. Conveniently, up to 45 % By weight, preferably between 20 and 40% by weight of the additive in the microporous layer available. The hydrophobizing additive preferably comprises PTFE.

Advantageous effect the additives twice so that acid not from the fuel cell membrane and / or the membrane electrode assembly is discharged. The PTFE additive, or a similar acting additive, in the microporous layer keeps the water in the membrane electrode unit or from the fuel cell membrane back. The PBI addition, or a similar acting additive, acts as acid storage, so that in each case no acid discharge takes place.

The Invention leaves not only for Fuel cells, especially for High-temperature fuel cells, for .... As well Use electrolysis cells, preferably a membrane of a basic polymer from the group of the polyazole-based fuel cell membrane, preferably predominantly from polybenzimidazole, poly (pyridines), polybenzoxazoles or mixtures thereof and / or with other suitable polymers.

Further Embodiments and aspects of the invention will be independent of a summary in the claims without limitation of the Generality explained in more detail below with reference to a drawing. there demonstrate

1 a schematic representation of a fuel cell unit formed from a plurality of fuel cells arranged in a stacking direction with a detailed view of a membrane electrode assembly;

2 a detail of a preferred gas diffusion layer.

1 shows by way of illustration a preferred fuel cell unit 10 with a plurality of individual fuel cells sequentially arranged in a stacking direction. Release agents between the individual fuel cells, such as bipolar plates or the like, are not shown. Each fuel cell comprises a membrane-electrode unit 11 with a fuel cell membrane 12 polymer-based, each impregnated with an acid. The fuel cell membrane 12 is between two gas diffusion layers 15 . 16 arranged. The fuel cell membrane is preferred 12 formed from a polyazole (PBI). Between the two gas diffusion layers 15 . 16 and the fuel cell membrane 12 is each a layer 14 respectively. 13 arranged to be formed so as to leak the liquid electrolyte from the fuel cell membrane 12 difficult. The gas diffusion layers 15 . 16 with the barrier layers 14 . 13 are part of the electrodes of the membrane-electrode unit 11 ,

2 shows a detail of a gas diffusion layer 15 with a first layer arranged on its outer surface 14 , which is formed as a catalytic layer and is formed as a barrier layer against a discharge of the liquid electrolyte of the fuel cell membrane, and one between the first layer 14 and the gas diffusion layer 15 arranged second layer 17 which is microporous and also comprises a barrier layer. So the electrode as a whole advantageously acts as a barrier layer.

In the production of electrodes with a catalyst layer for a preferred fuel cell electrode with a fuel cell membrane based on basic polymers from the group of polyazoles, in particular for the membrane electrode unit 11 a fuel cell, at least one preferably powdered catalyst material is mixed with a solvent, a pore-forming material and a polymer solution and processed into an electrode paste in a substantially homogeneously mixed state.

The Catalyst material preferably comprises a carbon-supported noble metal catalyst, in particular Platinum, up. There are exceptions Platinum, other precious metals conceivable, such as iridium or Ruthenium. The selection of the catalytic material is directed according to the type of fuel cell to be produced. Possibly can anode side and cathode side, a different catalyst material be provided.

On the gas diffusion layer 15 . 16 is the screen by screen printing 14 from the electrode material containing the catalyst material applied. The properties of the catalytic layer 14 are essentially determined by the composition of the electrode paste.

The gas diffusion layer 15 . 16 usually consists of a supporting carbon fabric, on which in a preferred embodiment of the invention for improving the electrical connection, the microporous layer 17 is applied. This layer 17 consists of finely divided carbon, which is bound by a polymer, in particular palytetrafluoroethylene (PTFE).

The amount of PTFE in the microporous layer 17 is preferably 20-40 wt.%. The supporting carbon fabric itself can be hydrophobized by the additional treatment with PTFE. Here, the proportion of PTFE is favorably between 3 and 30 wt.%. Due to the special introduction of a hydrophobic material, the water balance of the membrane-electrode unit 11 be specifically influenced. Thus, by a certain arrangement of the polymer layers in the membrane electrode unit 11 be achieved that the product water formed during the fuel cell reaction either in the region of the membrane electrode unit 11 remains stored, or without in the fuel cell membrane 12 penetrate, the membrane-electrode unit 11 leaves. In this way it can be achieved that in the fuel cell membrane 12 and the three-phase boundary of the electrode contained liquid electrolyte can be discharged only to a small extent by the product water.

If the binder used for constructing the electrode consists of a basic polymer from the group of polyazoles, in particular polybenzimidazole, poly (pyridines), polybenzoxazoles or mixtures thereof, their high binding tendency to the common liquid electrolytes causes a storage of the liquid electrolyte in the region of the electrode and restricts it their discharge into the product water at low operating temperatures of the fuel cell unit 10 continue. Advantageously, the polymer content in the catalyst-containing electrode paste is between 0.001 and 0.06% by weight, based on 1 g of catalyst powder, more preferably between 0.01 and 0.06% by weight.

With the membrane electrode unit according to the invention 11 Based on the PBI / H ₃ PO _{4 system} , cyclic operation between room temperature and 160 ° C. of such a fuel cell can be achieved over several hundred hours without any loss of performance of the system being observed.

10

fuel cell unit

11

MEA

12

fuel cell membrane

13

junction

14

junction

15

Gas diffusion layer

16

Gas diffusion layer

17

microporous layer

Claims (8)

Membrane electrode unit with a fuel cell membrane ( 12 ) sandwiched between two gas diffusion layers ( 15 . 16 ), wherein the fuel cell membrane ( 12 ) is formed on the basis of an acid-impregnated polymer, characterized in that between the fuel cell membrane ( 12 ) and the gas diffusion layers ( 15 . 16 ) at least one catalyst-containing layer ( 14 ) is arranged with a polymer additive so that water in the membrane electrode assembly and / or the fuel cell membrane ( 12 ) and / or acid is stored. Membrane electrode unit according to claim 1, characterized in that the polymer additive of the catalyst-containing Layer is selected from the group of polyazoles. Membrane electrode unit according to claim 2, characterized in that at least one component from the group of polybenzimidazole, Polly (pyridine), polybenzoxazoles or mixtures thereof is selected. Membrane electrode unit according to at least one of claims 1 to 3, characterized in that the polymer additive is hydrophobicizing and / or Fixing acid is. Membrane electrode unit according to at least one of claims 1 to 4, characterized in that between 0.001 and 0.06 wt.%, In particular between 0.001 and 0.04 wt.%, Of the polymer additive based on 1 g of catalyst powder in the catalyst-containing layer ( 14 ) available. Membrane electrode unit according to at least one of claims 1 to 5, characterized in that between the gas diffusion layer ( 15 . 16 ) and the catalyst-containing layer ( 14 ) a microporous layer ( 17 ) is arranged with a hydrophobizing additive. Membrane electrode unit according to claim 6, characterized in that in the microporous layer ( 17 ) up to 45% by weight of the additive are present. Membrane electrode unit according to claim 6 or 7, characterized in that the additive is PTFE.