DE102004024845A1 - Catalyst layer`s manufacture for high-temperature fuel cells, involves applying electrode paste on substrate under blade pressure to form catalyst layers, drying layers above room temperature and soaking dried layers in acid - Google Patents

Abstract

The method involves applying an electrode paste (20) on a substrate (21) under blade pressure for forming catalyst layers, where the pressure amounts to 3 bars. The catalyst layers are dried under temperature effect above the room temperature. The dried catalyst layers are soaked in an acid. The drying process takes place dependent on a catalyst loading in the vacuum or under normal temperature.

Description

The The invention relates to a method for producing a catalyst layer for electrochemical Cells, especially for High-temperature fuel cells with a polymer membrane, according to the preamble of claim 1

During the Operation of a Polymer Electrolyte Membrane (PEM) Fuel Cell is an oxidizing agent of the cathode and a reducing agent of Anode supplied to the fuel cell. As process gases become common Air as well as a hydrogen-rich reformate or pure hydrogen used. Anode and cathode are through an ion-conductive fuel cell membrane separated. At the anode, the electrochemical oxidation of the Hydrogen instead, at the cathode, the reduction of oxygen. By the direct conversion of chemical into electrical energy can be achieved in fuel cells high efficiency.

The PEM fuel cell technology developed at present most widely based on fuel cell membranes of Nafion ^® as the electrolyte. The electrolytic conduction takes place via hydrated protons, whereby the proton conductivity of the fuel cell membrane is coupled to the presence of liquid water. This limits the operating temperature at normal pressure below 100 ° C.

at Temperatures higher are 80-95 ° C, Performance deteriorates significantly due to fluid loss. To maintain conductivity The fuel cell membrane above 100 ° C are due to the temperature dependence the vapor pressure of water very large amounts of water for humidification the fuel cell membrane needed. In systems with a process gas pressure greater than the normal pressure can the operating temperature can be increased in principle, but at a cost the efficiency, size and weight the overall system. For operation well above 100 ° C would be the needed Pressure increase dramatically.

operating temperatures but greater than 100 ° C are for a variety of reasons First of all, the electrokinetics as well as the catalytic activity for both Electrodes increase with increasing temperature. Besides that is the tolerance Impurities in the fuel gas, e.g. Carbon monoxide (CO) higher. CO is common contained in hydrogen-rich reformate and must with great effort are removed before the reformate of the fuel cell are supplied can. Furthermore, for The use in a vehicle is as high a temperature as possible the fuel cell and thus a large temperature difference to Ambient temperature for the exhaustion the waste heat desirable.

In order to achieve higher operating temperatures it has already been proposed to use fuel cells with fuel cell membranes based on basic polymers from the group of polyazoles whose functionality is not linked to the presence of water. This results in significant simplifications in terms of the water balance compared to the Nafion ^® -based system described above. However, in systems with such fuel cell membranes, only low power densities of typically less than 0.4 W / cm ^{2 have been achieved} at a voltage of 0.6V.

Centerpiece of the fuel cell is their membrane electrode unit, which consists of a fuel cell membrane with double-sided arranged gas diffusion electrode, the each comprises a catalyst layer. The catalyst layer is either on a gas-permeable Substrate or applied directly to the fuel cell membrane. At the catalytic surface the catalyst layer receives the anodic oxidation of the reducing agent Protons or the cathodic reduction of the oxidant instead. Usually Adjacent to the catalyst layer, on both sides of the fuel cell membrane is arranged, the gas diffusion layer. The gas diffusion layer serves both the distribution of the reactants and the current dissipation. Each one of these elements and their specific interaction comes a big one Significance in the achievable power density of fuel cells to.

From WO 01/31725 A1 a screen printing method for producing a layered electrocatalyst for Nafion ^® -based fuel cell is known, is applied to the fuel cell membrane in which an electrode paste and is compressed and then dried by supplying heat with the fuel cell membrane. To prevent swelling of the fuel cell membrane, the electrodes paste a non-polar solvent is added.

task The present invention is a process for the preparation a catalyst layer for electrochemical cells, in particular for high-temperature fuel cells with polymer membrane, provide at operating temperatures up to 200 ° C can work while maintaining improved power density during operation of the electrochemical cell allows.

The Task is according to the invention with the features of claim 1.

The inventive method for producing a catalyst layer for an electrochemical cell By screen printing provides an electrode paste under pressure to apply a substrate, under the influence of temperature above room temperature to dry and to soak the dried catalyst layer with an acid. Advantageous is that a separate hot pressing after avoid the coating and / or drying of the electrode pastes can. A preferred substrate is a gas diffusion layer conductive carbon fabric. Another preferred substrate is a plastic based Fabric or felt material. A particularly preferred substrate, in particular for one Series production, is a fuel cell membrane, in particular a based on a basic polymer from the group of polyazoles Fuel cell membrane, preferably predominantly polybenzimidazole, Poly (pyridines), polybenzoxazoles or mixtures thereof and / or others consisting of suitable polymers. Essential process parameters, such as about squeegee pressure, squeegee speed, number of print passes, number flood and pressure repetitions per print run, as well as a Mesh size of the screen for coating the substrate may be in dependence on the composition of the electrode paste to be processed as well dependent on chosen from the substrate become. When floods leads the squeegee the electrode paste the sieve from the end of printing to the beginning of printing without pressure too. This allows the doctor blade flooding, i. the filling the mesh openings in the sieve before the next Print. The drying is advantageously carried out under reduced pressure. Optionally, a drying under inert gas is conceivable.

The Electrode paste preferably comprises a catalyst powder of a preferably carbon-supported Catalyst, a solvent, is preferred an organic, especially dipolar, aprotic solvent, at least one pore-forming material and a polymer solution. The solvent is expediently for producing the polymer solution as well as used to suspend the paste components. The catalyst powder preferably has a carbon-supported noble metal catalyst, in particular platinum or another noble metal such as iridium or Ruthenium or other suitable substances. The pore-forming material preferably consists of one member of the group of inorganic Salts, in particular carbonates and / or inorganic azides, in particular from the group of ammonium carbonate, sodium azide and / or calcium carbonate. Alternatively or in addition For example, the pore-forming material may be an inert component with a large internal Be surface in particular at least one member of the group carbon, in particular a correspondingly modified graphite, silicon dioxide, and / or titanium dioxide. In addition to its kind are in the pore-forming Material its distribution and its content, its particle size in the Electrode paste before drying the electrode paste each parameter, with which the pore formation can be controlled well. Furthermore, the Conditions for increasing the temperature during drying of the electrode paste, the viscosity the electrode paste, its temperature gradient and its moisture content other well manageable factors influencing the training of the desired pore structure.

When favorable Solvent, in particular a dipolar aprotic solvent, one of the group N, N-dimethylformamide, N, N-dimethylacetamide and / or a Acid, especially a strong acid like trifluoroacetic acid, phosphoric acid be used. With that you can specifically gas channels be introduced into the catalyst layer, which transport to support reactants and allow an improved power density of the electrochemical cell.

Preferably is the pressure, in particular the squeegee pressure, when doctoring the electrode paste to the substrate at least 3 bar, more preferably between 4 and 5 bar.

advantageous Further developments of the invention are specified in subclaims.

A favorable number of printing passes when applying the electrode paste 20 at least 2, preferably 2 to 4. It can be done at each print run a certain number of flood and pressure repeats. It is advantageous if the number of flood and pressure repeats lies between 2 and 5, in particular between 2 and 4. The parameters for the screen printing and the catalyst-containing layer with gas channels obtained can be different for an anode side and cathode side of the electrochemical cell and according to need to be selected.

In better Continuing the coated substrate under vacuum at a dried first drying temperature of more than 100 ° C, preferably at more than 140 ° C, more preferably in the range of 150 ° C ± 5 ° C. A favorable negative pressure is included below 50 mbar, in particular in the range of at 20 ± 5 mbar. conveniently, the first drying temperature will work for at least 12 hours, preferably 20 ± 5 H. Optionally, it is also possible to dry in an inert gas atmosphere.

Conveniently, the previously acid-impregnated catalyst layer is at a second drying time drying, wherein the second drying temperature can be adjusted depending on a catalyst loading of the catalyst powder or the catalyst paste mixed with the catalyst paste. The acid used is preferably phosphoric acid (H ₃ PO ₄ ) or its derivatives, in particular with a concentration between 60% and 99%, preferably with a concentration of 70%. An acid content per catalyst layer of from 0.01 to 0.3 ml / cm ² , preferably from 0.05 to 0.2 ml / cm ^2, is advantageous. It proves to be advantageous if the second drying temperature is higher with a low catalyst loading of the catalyst powder than with a high catalyst loading of the catalyst powder. It is particularly favorable to dry under vacuum and at higher catalyst loading under vacuum at low catalyst loading. Preferably, the second drying temperature for a low catalyst loading greater than 100 ° C and for a high catalyst loading less than 100 ° C, in particular, the drying temperature for the low catalyst loading about 110 ° C and for the high catalyst loading about 50 ° C. Under low catalyst loading is a ratio of catalyst to support of up to 25%, preferably up to 20%, and under high catalyst loading, a ratio of up to 70%, preferably understood to 60% (percentages in weight percent). It is also conceivable as an option to dry at high catalyst loading under inert gas atmosphere, in each case the most appropriate atmosphere can be selected.

It is advantageous, at a high catalyst loading, to dry the substrate coated with the catalyst layer under reduced pressure, in particular below 50 mbar, preferably in the range of 20 ± 5 mbar. A favorable drying time in both cases is more than 12 h, preferably 15-20 h. A favorable content of acid in the catalyst layer or the electrode after drying is more than 0.005 g / cm ² , preferably 0.01-0.015 g / cm ² .

The Invention leaves especially for high-temperature fuel cells, but also for electrolysis cells use, preferably a membrane of a basic polymer from the group of 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 FIG. 2 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 fuel cell, FIG.

2 a schematic representation of a screen printing process according to the invention.

1 shows for illustrative purposes a fuel cell unit 10 with a plurality of fuel cells 11 which are arranged in a stacking direction. Each fuel cell has a fuel cell membrane formed as a polymer fuel cell membrane 12 on, on their anode side between an anode compartment 15 and the fuel cell membrane 12 a gas diffusion layer 14 and one to the fuel cell membrane 12 subsequent, layered catalyst layer 13 has and symmetrical to the fuel cell membrane 12 on its cathode side between a cathode compartment 18 and the fuel cell membrane 12 a gas diffusion layer 17 with a to the fuel cell membrane 12 subsequent layered catalyst layer 16 having. fuel cell membrane 12 , Gas diffusion layers 14 . 17 as well as catalyst layers 13 . 16 form a so-called membrane electrode assembly (MEA).

2 outlines a sequence of the method according to the invention for producing a catalyst layer 13 . 16 for fuel cells 11 by screen printing, with which an electrode paste 20 on a substrate 21 is applied. The substrate 21 a gas diffusion layer is particularly preferred. Optionally, the substrate 21 also be a fuel cell membrane. The electrode paste 20 has as catalyst material preferably a catalyst powder with carbon-supported platinum.

The electrode paste 20 is depressurized on a sieve during the flooding process 22 painted, using the mesh with the electrode paste 20 be flooded. The sieve 22 has a covering with a suitable fabric having a mesh size which is preferably 21-140 or 27-120, the first number indicating the number of threads per cm ² and the second number indicating the diameter of the thread in μm.

The electrode paste 20 is applied in a number of pressure passes, preferably a maximum of three pressure passes, conveniently one to two pressure passes are provided. The electrode paste 20 is doing by the squeegee 23 under pressure with a squeegee pressure of about 4 to 5 bar in the direction of application 24 on the substrate 21 applied. The squeegee 23 moves at a speed between preferably 44 to 56 mm / s. The The number of flood and pressure repetitions per print run is expediently between 2 and 4.

The with the electrode paste 20 coated substrate 21 is dried under the action of temperature at a first drying temperature T1 of about 150 ° C with the lowest possible temperature fluctuations for about 15 to 25 h, preferably 20 h, until any decomposing pore-forming additives decompose at the baking temperature, the solvent is expelled and gas channels in the layer have generated and made the on the substrate 21 applied layer of electrode paste 20 the catalyst layer 13 . 16 forms.

The dried and cooled catalyst layer 13 . 16 is drizzled as evenly as possible with high-percentage phosphoric acid. The content of phosphoric acid is per catalyst layer 13 . 16 between 0.05 and 0.2 ml / cm ² . Due to the mostly hydrophobic surface of the substrate 21 does not completely penetrate the phosphoric acid into this. On the catalyst layer surface, a phosphoric acid film is formed, for example, by tilting the substrate 21 can be removed.

The phosphoric acid impregnated catalyst layer 13 . 16 is dried depending on the catalyst loading of the catalyst material at a second drying temperature T2. The second drying temperature T2 is higher at a low catalyst loading of the catalyst material than at a high catalyst loading of the catalyst material. Preferably, the second drying temperature T2 is greater than 100 ° C for a low catalyst loading, in particular at 110 ° C. It is particularly favorable to dry under vacuum and at higher catalyst loading under vacuum at low catalyst loading. At low catalyst loading, in particular a platinum loading to about 20% Pt / C, with the catalyst layer 13 . 16 coated substrate 21 be dried in a standard drying oven. For a high catalyst loading, in particular a platinum loading between about 20% to about 60% Pt / C. this will happen with the catalyst layer 13 . 16 coated substrate 21 dried at about 50 ° C under reduced pressure in the range between 15 and 25 mbar. The drying time is essentially independent of the catalyst loading and is between 15 and 20 h. After drying, the catalyst layer contains 13 . 16 and optionally also the substrate 21 between 0.01 and 0.015 g / cm ^{2 of} phosphoric acid.

10

fuel cell unit

11

fuel cell

12

fuel cell membrane

13

catalyst layer on anode side

14

Gas diffusion layer

15

anode chamber

16

catalyst layer on cathode side

17

Gas diffusion layer

18

cathode space

20

electrode paste

21

substratum

22

scree

23

doctor

24

application direction

Claims (12)

Process for the preparation of a catalyst layer ( 13 . 16 ) for electrochemical cells, in particular for high-temperature fuel cells ( 11 ) with a polymer membrane, by screen printing, wherein an electrode paste ( 20 ) on a substrate ( 21 ) is applied, characterized in that the electrode paste ( 20 ) under a squeegee pressure on the substrate ( 21 ), dried under the action of temperature above room temperature and the dried catalyst layer ( 13 . 16 ) is soaked in an acid. Method according to claim 1, characterized in that that the squeegee pressure is at least 3 bar. A method according to claim 1 or 2, characterized in that a number of printing passes during application of the electrode paste ( 20 ) is at most three. Method according to claim 3, characterized that the number of flood and pressure repeats per print run between 2 and 5 lies. Method according to at least one of the preceding claims, characterized in that the coated substrate ( 21 ) is dried under vacuum at a first drying temperature of more than 100 ° C. Method according to claim 5, characterized in that that the first drying temperature acts for at least 12 hours. Method according to at least one of the preceding claims, characterized in that the acid-impregnated catalyst layer ( 13 . 16 ) is dried at a second drying temperature. Method according to at least one of the preceding claims, characterized in that the second drying temperature depends on a catalyst loading of the catalyst layer ( 13 . 16 ) is admixed with the catalyst material. Method according to at least one of claims 7 or 8, characterized in that the drying depends on a catalyst loading in a vacuum or under normal atmosphere takes place. A method according to claim 9, characterized in that at a high catalyst loading, the catalyst layer ( 13 . 16 ) is dried under reduced pressure. Method according to at least one of claims 8 to 10, characterized in that at a low catalyst loading the catalyst material with the second drying temperature is higher than at a high catalyst loading of the catalyst material. Method according to at least one of claims 8 to 11, characterized in that the second drying temperature for one low catalyst loading greater than 100 ° C and for one high catalyst loading is less than 100 ° C.