DE2638987A1 - SCREEN PRINTED ELECTROLYTE MATRIXES - Google Patents

Description

Electrolyte matrices made by screen printing.

The present invention relates to fuel cells and in particular on novel processes for the production of electrolyte matrices.

Fuel cells for producing electrical energy from one Fuel and an oxidizer are well known. These known cells generally comprise a housing, a Oxidizing agent electrode, a fuel electrode and an electrolyte lying between these electrodes. As an electrolyte can be a solid, a melted paste, a free flowing Liquid or a liquid in a die can be used. The present invention relates to such fuels cell electrolyte matrices.

To ensure optimal performance in liquid fuel cells, the die must have various properties. ^{So Ί} ϊε matrix should be hydrophilic. The die should also not have any flaws in order to avoid gas transfer and mixing of the reaction gases in the fuel cell. The template should be as thin as possible to keep the power losses through the electrolyte minimal Zn. Intimate contact between the die and the electrode surfaces is necessary to obtain maximum catalyst utilization. Also should the

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Die have a uniform thickness because they are uneven ^; Electrodes cause poor current distribution and thus a reduction in performance. The pore size distribution of the die should also be adjusted to avoid gas transfer and to ensure good distribution of the electrolyte in the cell.

It's not easy to get these properties, and that Problem is compounded by the fact that choosing the most appropriate Materials is limited. The materials must be chemically and thermally stable at the cell operating temperatures, they must not poison the catalyst and must have a high electronic resistance. Also should be the die be able to be produced in an economical process if possible.

In an economically known method of making matrices, papermaking processes have been used where the The die is in the form of a sheet and is more like a fuel cell or a fuel cell system between the electrodes was attached. US Pat. No. 3,407,249 describes the manufacture of sheets or foils from fibrillated polytetrafluoroethylene described. US Pat. No. 3,627,859 to Mesite et al describes the manufacture of a matrix from cellulose fibers in admixture described with a fluorocarbon plastic. The US patent 3 694 310 by Emanuelson et al describes the production of mats from phenolic resin fibers which are made with a phenolic additive resin are coated.

The mechanical attachment of the die between the electrodes has the disadvantage, regardless of the die material selected, that there is not necessarily intimate contact between the die and the electrode over the entire surface of the electrode

the

and the die is obtained by the papermaking process The matrices produced also have the disadvantage that the properties are lost in the case of thin matrices avoid gas transfer. Even if the thinnest possible die material could be made, it would be extremely difficult or even impossible to use this sheet or slide. handle.

The disadvantages of papermaking processes can be due to a

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Processes are avoided according to which the die is placed directly on the surface of an electrode by dipping the electrode in an aqueous solution of the matrix material as in Blanc et al U.S. Patent 3,022,244. Also can Matrix material, applied to the surface of the electrode by spraying or applying with a brush or: a brush will. Although this procedure makes the handling separate However, if it contains matrix foils, it is difficult to obtain a uniform matrix thickness. Since a uniform thickness is not it is necessary to allow undesirably thick places to ensure that there is enough in the thinnest places Die material is present.

Although the properties that electrolyte matrices have, should have been known to those skilled in the art for a long time, has not been a satisfactory process for economic up to now Production of such a die is described.

A well known method of applying thin layers of different materials to different substrates is this Screen Printing Process · US Pat. No. 2,779,975 to Lee et al discloses the use of screen printing to manufacture electrical components such as capacitors, coils, resistors, printed circuits, and the like, where a each component is made up of different layers and the layers are electrically connected. As shown in US Pat. No. 3,577,915 to Thompson, screen printing can of course also be used to apply decorative coatings or dye the silk.

Regardless of the various uses of the screen printing process were known.wurde the use of this method for the application of die material on the surface of a Electrode neither described nor suggested. In the past, those skilled in the art believed that no suitable die could be produced by this Method could be applied to an electrode.

The present invention describes an economical process for the production of a fuel cell electrolyte matrix. The electrolyte matrices of the present invention are better than

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the known matrices. The electrolyte matrices applied to the electrode surface by the method of the invention are thinner than known matrices but have all the necessary matrix properties.

According to the present invention, the electrolyte matrix

material by screen printing onto the surface of an electrode
brought with aqueous application solution is used.

It was found / that thinner, more uniform, improved matrices with respect to the known ones through the screen printing process Matrices can be made.

Those skilled in the art feared that the screen image could remain on the surface of the stencil after screen printing, which would result in a non-uniform thickness of the stencil. In addition, the experts assumed that the large particles (around 5 JU m in diameter and larger) of the matrix material could clog the sieve openings. It was also believed that the screen printed matrices would not have all of the desirable properties.

However, although these fears were proven wrong the first attempts to apply matrices to electrodes by screen printing were unsuccessful. So the die material settled in the Use of water as the application liquid from the mixture quickly causing the sieve openings to be clogged and a continuous application became impossible. Inconsistent die thicknesses were also obtained here. The well-known order solutions which can be used in most screen printing processes, cannot be used to apply matrix material used on electrodes as they attack or poison the catalyst. Some of the well-known application solutions even burn at room temperature in the presence of platinum or another noble metal catalyst. Some order solutions require one Decreasing the contact angle between the electrolyte and the hydrophobic material in the electrode causing a flooded Catalyst layer is obtained. In the literature it was still no application solution which could be used for the production of electrolyte matrices in the screen printing process is described.

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However, despite these problems, an economical process has been found. Mission resolution proved key to this problem. It was found that using a mixture of ethylene glycol and water as the application solution
a very thin die with a uniform thickness and all the desired properties could be produced. Although
Glycol, even in small quantities, must not be present in the die, as it poisons the catalyst _/ this was not a problem, since the glycol and water could be completely removed from the die in a heat treatment. j

Although the glycol-water mixture is the first application solution
presented with the help of which a matrix could be applied to the electrode surface by the screen printing process
However, this application solution also has some undesirable properties. On the one hand, it is desirable to produce the dies continuously, one die after the other
made as quickly as possible on the same machine
will. However, the die material sets quite quickly
from the glycol-water application solution so that the matrices produced after the first matrix are not the same uniform
Composition like the first die. The settling of the matrix material from the application solution also blocked the sieve openings so that the sieve had to be removed and washed after 1 to 2 matrices had been produced. A
Another disadvantage of the glycol-water application solution is that it has to be completely removed from the die, so that a number of heat treatment and drying stages are required. Thus, the production of such a matrix mixed with the glycol water is rather tedious.

Thus, efforts have been made to obtain a better job solution. Unexpectedly, it was found that an aqueous solution of polyethylene oxide ensures far better results. One of the main advantages of the polyethylene oxide solution
is the fact that the die material lasts for a long time
remains in dispersion. The sieve openings are not clogged with the matrix material, even after a long period of time, so that an almost unlimited number of electrolyte matrices can be used without washing the

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Sieves can be manufactured. It was also found that polyethylene oxide does not poison the catalyst. This eliminates the need for various drying and heat treatment stages. Most of the polyethylene oxide evaporates when the die is sintered and any residues cannot affect the operation of the fuel cell. By the method of the present invention, continuous matrix layers from 50 to 178 ^ in thickness were applied with the thickness varying only about 25 µm. In comparison, the minimum average thickness of a _{die produced by the papermaking process (} die is approximately 203 ^ m with fluctuations of approximately 50 μm. Matrices with a thickness of 121H m can be sprayed on or applied with a brush, but the fluctuations in thickness are approximately It has been proven that fuel cells with matrices according to the invention have a better performance than fuel cells which are operated with known matrices.

For a better understanding of the invention, reference is made to the following description, the examples and the accompanying one Figure which shows a cross-section, not to scale, of a fuel cell.

According to Figure 1, the fuel cell 10 consists of a pair Electrode / die elements 12 which abut one another along surface 14 and between a pair of gas separation plates 16, 18 being held. Each electrode / die element 12 consists of an electrode 19 and an electrolyte die 20. Die Electrode 19 consists of a substrate 22 with a catalyst layer 24. According to the present invention, the die 20 applied to the catalyst layer 24 on the electrode 19 by screen printing. According to this embodiment, the plate forms 16 a fuel chamber 26 on the side of the electrode 19 facing away from the electrolyte, and the plate 18 forms an oxidizing agent chamber 28 on the side of the electrode 19 facing away from the electrolyte. Fuel, such as hydrogen, from a Fuel supply 30 is fed through line 34 to inlet 32 of chamber 26. An oxidizing agent, e.g. Air, from a supply 36, is supplied through line 40 to the inlet

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_ "7 -

38 of the chamber 28 out. The electrode! 19 are over a charge 42 connected.

According to this embodiment, the substrates 22 passed Carbon paper electrodes impregnated with a hydrophobic Agents such as polytetrafluoroethylene. Each catalyst layer 24 consisted of a mixture of catalyst and fluoroplastic. The catalyst layer 24 can be made by any known one Methods such as spraying, filtration transfer, etc. can be applied to the carbon paper substrate 22. the Catalyst layer 24 does not extend all the way to the edge of the substrate, so that a catalyst-free all around each substrate Surface 44 remains. The matrix layer 22 covers the catalyst layer 24 completely and also extends to the edge of the substrate 22 so that the catalyst-free surface 44 is covered by the die. The uncatalyzed edges of the substrate 22 are treated so that they are exposed to the in the Matrix 20 containing electrolytes are wettable. In this way, a moist seal is obtained all around the edge of the cell 10 whereby an escape of the reaction gases is avoided. This seal is described in detail in U.S. Patent 3,867,206 to Trocciola et al.

It should be noted that the present invention does not rely on a special fuel cell structure, e.g. described above, limited. The present invention relates generally only to a novel manufacturing process for electrolyte matrices.

According to the present invention, the matrix is applied to the surface of an electrode by screen printing. The screen printing process can be on a screen printing machine available on the market such as "Compact 4" from Argon Service Ltd., Milano, Italy. The electrode on which the The die to be applied is placed on the table of the screen printing machine attached. The sieve is lowered onto the surface of the electrode. A mixture of application solution and die material . is made by mixing the appropriate matrix material with made of a liquid application solution. Then the sieve covered with the mixture of application solution and matrix material.

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A slight pressure is exerted on the screen so that the matrix material and the application solution through the openings of the Sieves to get onto the electrode. This also ensures that the sieve is completely covered with the application mixture. A stronger pressure is now exerted on the screen, e.g. with the help of a flexible spatula (i.e. rubber dryer). This process is of course well known from the screen printing process. The application mixture thus passes from the sieve to the Surface of the electrode. Steps 4 to 6 can be repeated a few times in order to obtain the desired die thickness. As is known, the spatula is moved back and forth in the sieve, the number of passes depending on the viscosity of the application mixture, the thickness of the screen, the size of the screen openings, the pressure exerted on the rubber dryer, and the evenness the electrode surface on which the die is applied should be depends. The sieve is removed from the electrode. The electrode is removed from the machine, dried and Heat treated to remove most of the application solution and to maintain the desired matrix properties.

Unexpectedly, it was found that an aqueous solution of ethylene glycol or polyethylene oxide, with polyethylene oxide is preferred, is excellently suited as an application solution for the screen printing process described above.

Example I.

An aqueous solution of ethylene glycol was used as an application solution in the manufacture of an electrolyte matrix. A novel matrix material ₍ which is described in a further application by the applicant ₍ was used ie 96% by weight silicon carbide powder mixed with a polytetrafluoroethylene binder). The matrix material was mixed with the application solution in a weight ratio of 2 parts of matrix material to one part of application solution here from 32 parts by weight of water and 20% by weight of ethylene glycol. The silicon carbide used was Green 1000 Grit ^ from the Carborundum Company and the polytetrafluoroethylene used was TFE 3170 from Dupont, which is a mixture of poly-

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- 9 is tetrafluoroethylene with a surfactant.

The above-described relationships make the viscosity | determined by the application mix. These ratios were selected for this example because good results are achieved here _; could. The ratios for other embodiments depend] on the substrate, the matrix material, the Great of Oeffnunger} of the wire and other variables. These ratios can

can easily be determined experimentally by those skilled in the art. ;

The application mixture described above was in a screen printing!

method used with which a matrix layer on the

Surface of a gas diffusion electrode _; with a catalyst

coating of 0.5 mg / cm platinum on carbon substrate, ' was applied. The sieve used in the procedures consisted of
Nylon and had openings of about 0.250 mm. After that
Die material had been applied to the electrode
the electrode dried in the air at 66 ° C to remove the water
remove. The electrode / die unit was then
heated to 204 ° C. for 2 hours to remove the glycol from the die
to remove. The die was then held in for 15 minutes
Washed isopropanol to remove the surfactant and dried again in the air at 66 C to remove the isopropanol. The electrode / matrix unit
was then washed in water for 15 minutes
remove remaining isopropanol and leave in the air at 66 C
dried to remove the water. The die thus obtained
was sintered at a temperature of 260 ° C. for 13 minutes
whereby good adhesion of the silicone carbide particles to the
Binder was obtained. According to the present invention
; the term "sintering" means an elevated temperature so that the matrix particles combine with the binder.

The matrix thus applied to the electrode was examined; and it was found that this matrix had no defects. The average thickness was 76 ½ m with a variation

of no more ais 25 y m. The Bl · asendruck was 0.07 to 0 ^ 0!

kg / cm. The electrolyte matrix could be wetted by absorption with 85% Η-, ΡΟ a wettability of i4 cm i

j 4

; was obtained in 16 hours at room temperature. ;

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A pair of these electrode / die elements were assembled in a cell and tested with 102% strength by weight HoPO ₄ as the electrolyte. In this series of experiments, both H and O _{2 were used} as reactants, as well as RM-I (transformed natural gas with the following composition: 80% H-, 1.7% CO and 38.3% CO ₂ ) and air as the reactant. The ohmic losses of the

2
Cell were 24 mV / 107 mA / cm. The cell's performance is shown in the table below:

Cell voltage (volts)

2 reactants 0 / C * 107 214 321 mA / cm

H ₂ , O ₂ 1.023 0.750 0.687 0.636

RM-I, air 0.980 0.680 0.600 0.532

* 0 / C = open circuit

Cells of this type were used for 7000 hours

operated without substantial matrix replacement. It is believed, that such a matrix structure has a service life of at least 40,000 operating hours.

Example II

Following the procedure of Example I, a silicon carbide electrolyte matrix was made. In this Example, however, an aqueous solution of polyethylene oxide was used as the application solution. The application solution contained 99 wt. Parts water and one part by weight polyethylene oxide.

The matrix material was in a proportion of 60 parts by weight Matrix material · 40 parts by weight of application solution mixed with the application solution.

After applying the die, the electrode / die element dried in an infrared oven below the sintering temperature and at a temperature of 299 C for 2 minutes sintered with good adhesion of the silicon carbide particles to the binder. The die on this electrode was examined and found to be free from defects. The average thickness was 76 υ m with a fluctuation of not more than 25 ^ m. The bladder pressure was

2 '

0.2 to 0.4 kg / cm and the wettability 17.8 cm in 16 hours at room temperature at 85% H-.PO ..

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This wettability of the matrix is significantly better than that of the matrices which are produced by the screen printing process in the presence a water-ethylene glycol application solution can be prepared. This better wettability could not yet be explained.

A pair of these electrode / die members were assembled in one cell. The cell was with the help of 102 wt.% H3PO. as electrolytes and H ₂ and O ₂ as reactants as well as RM-I (transformed natural gas with the following composition: 80% H ₂ , 1.7% GO and 18.3% CO ₃ ) and air as reactants

operated. The ohmic losses of the cells were 16 mV / 107

2
mA / cm. The cell performance is shown in the following table:

Cell power (mV)

Reagent 107 214 321 mA / cm ²

H ₂ , O ₂ 765 715 678

RM-I, air 700 642 594

Cells of this type were used for 15,000 hours operated without significant die decay. It is believed that such a die structure has a life span of minimal 40,000 hours of operation.

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Claims (10)

Claims (1. A method for producing an electrode / die element for fuel cells, characterized by the following Stages: Application of an electrolyte matrix by screen printing on the surface of a fuel cell electrode, a mixture of a suitable aqueous application solution and a matrix material being produced and applied to the surface of the electrode through a screen; and
Heat treatment of the die produced in this way. 2. The method according to claim 1, characterized in that the electrode is a gas diffusion electrode and one Has catalyst layer on the surface to which the die is applied. 3. The method according to claims 1 to 2, characterized in that an aqueous solution ^of ethylene glycol is used as the application solution. 4. The method according to claims 1 to 2, characterized in that the application solution is an aqueous solution of Polyethylene oxide is used. 5. The method according to claims 1 to 4, characterized characterized in that the matrix material comprises a binder. 6. The method according to claim 5, characterized in that silicon carbide is used as the die material. 7. The method according to any one of claims 1 to 6, Hbe characterized by the fact that the heat treatment is the greatest Part of the application solution is removed from the die. 8. The method according to any one of claims 1 to 7, characterized characterized in that the die is sintered during the heat treatment. _: 9. The method according to any one of claims 1 to 8, characterized in that after the heat treatment an equal; massive, uniform matrix is obtained on the electrode and the thickness does not vary by more than 25 / * m. -. 10. The method according to any one of claims 1 to 9, ] characterized in that the die on the electrode has a thickness of less than 178 μm after the heat treatment. 709810/0878