DE202005010590U1 - Formation of catalytic anode and cathode layers on a fuel cell membrane uses a roller printing process - Google Patents

Abstract

Fuel cell membrane (2) is to have anode and cathode layers on opposite sides. The layers are produced using a printing process in which rollers (7) transfer coating material onto the surfaces. The material is a catalytic powder that is electrically charged and transferred from magazines (8) onto the rollers and then onto the membrane.

Description

The The invention relates to a device for applying a catalytic Layer on a membrane, for the production of a membrane electrode assembly for one Fuel cell.

fuel cells are electrochemical systems with which the chemical energy of Fuels in an electrochemical reaction directly into electrical Energy is implemented. To operate the fuel cell is the Anode-acting electrode is hydrogen gas or hydrogen-containing Gas supplied. At the cathode acting as the electrode is oxygen or oxygen-containing Gas supplied. Hydrogen is catalytically oxidized at the anode. The thereby released Electrons are over the electrode is discharged to the consumer and the resulting protons migrate through the electrolyte to the cathode side, where they are with Oxygen can also be catalytically converted to water. The necessary for this Electrons are over the electrode is supplied. In a polymer electrolyte fuel cell, the electrolyte exists from a proton-conducting membrane. From the electron surplus on the anode side and the electron deficiency on the cathode side of the electrolyte results in a voltage gradient, which is for the production of electrical Energy (DC) can be used.

One Fuel cell stack consists of several in a row switched cells, each through a so-called bipolar plate are separated from each other. By engraved in this plate channels the fuel or the oxidate are distributed. It also serves the bipolar plate for cooling the cell. In addition to the pure hydrogen-oxygen fuel cell There are other cell types that are primarily related to the electrolyte differ.

Because of their high efficiency, their compact design and their short startup times polymer electrolyte membrane fuel cells (PEMFC: Polymer-Electrolyte Membrane Fuel Cell) increasingly gaining importance as an alternative energy source for mobile or decentralized power generation. In particular, in motor vehicles for driving or for the electrical supply of future vehicle electrical systems or as smaller stationary systems for power generation, they have a high application potential and are constantly being developed further. PEMFCs are low-temperature cells for typical working temperatures of 60-120 ° C. Recent developments, for example described in the DE 20 2004 000 365 U1 , also work with temperatures up to 250 ° C.

Centerpiece of PEM fuel cells is a stack of so-called membrane-electrode assemblies (MEA: Membrane Electrode Assembly). An MEA consists of a solid ion-conducting foil-shaped electrolyte membrane, each with a catalytically active electrode layer on both sides, which is the anode, or the cathode form, is provided. These highly porous catalyst layers consist essentially of an electrically conductive substrate, for example, soot, and from an electric catalyst, preferably platinum (possibly in an alloy with other metals), which is deposited in highly dispersed form on the carrier material. That concludes outward usually a porous one Gas diffusion layer (GDL), for example made of carbon fiber paper, to, the good access of the reaction gases to the electrodes and a good derivation of the cell current allows. The GDL forms with the catalyst layer, a gas diffusion electrode (GDE). The catalyst layers can first be applied to the GDL and then be joined to the membrane or you can be applied directly to the membrane, in which case the GDL then or is applied in a joint operation.

to Preparation of the catalytic layer is, for example, first Mixture of a binder and a pore former containing the catalyst (e.g. 30 weight percent platinum on carbon black) with a solvent processed into a paste or ink and then onto the membrane, or the GDL. The binder can later, for example by heating, be removed again. about the pores in the GDL and in the catalytic layer is the dispersed electro-catalyst (Platinum) with the supplied Working gases (hydrogen at the anode, or oxygen or air at the cathode) directly in contact.

From the quality the catalyst layer (s) applied to the PEM critically depends on the Performance of the PEMFC. Therefore comes in the production of the fuel cell the coating process of particular importance.

From the DE 196 11 510 A1 For example, a spraying technique for applying a catalytic layer to a membrane is known. In this case, the prepared as a dispersion layer material is sprayed onto a non-acidic membrane at a temperature between 130 ° C to 170 ° C and then dried.

From the US 5 211 984 a brush technique is known in which first sprayed a suspension on a support, then hot pressed with a membrane and then the carrier is withdrawn.

Furthermore, from the US 5 211 984 a white The spray process is carried out via a template in order to obtain a specific electrode size.

From the DE 100 37 074 A1 It is known that a catalytic layer can be applied by rolling, doctoring or printing (screen printing, stencil printing or offset printing) on a membrane, which does not address the individual methods.

adversely in the known methods is that they are relatively expensive in cost, Time and complexity during execution and number of operations are. Furthermore, it is particularly difficult with the known techniques, very thin (10 μm and below) uniform layer thicknesses to create. Another disadvantage is that with the mentioned Method no positionally accurate coating of the membrane or the Gas diffusion layer possible is.

task The present invention is to provide a device which in effectiveness, Accuracy and cost is improved and the generation of catalytic Coatings with layer thicknesses in the range of 5 to 50 microns, preferably 10 to 20 μm allowed.

These The object is in connection with the preamble of claim 1 solved by that two xerographic printer units are arranged opposite one another, by means of which the membrane can be coated on both sides in one working pass is.

The xerographic printer units enable the production of very thin electrode layers high quality, i.e. with a defect-free coating and a very uniform layer thickness. By having two xerographic units (drums, corona units) are provided, a simultaneous double-sided coating the membrane allows the effectiveness of the manufacturing process of a membrane-electrode assembly increases. The individual components can inexpensive the construction of the corresponding known from laser printers components be ajar. in principle it goes without saying also possible with this device or with a corresponding device with only a xerographic unit, (laser printer) a one-sided Apply coating and the two membrane sides in succession to coat.

According to one preferred embodiment of The invention is a first cartridge of a catalytic coating material for the production an anode layer of a membrane side surface and a second cartridge a catalytic coating material for producing a cathode layer the other membrane side surface fed.

Thereby, that two cartridges are provided, it is possible to use the two drums for the anode side and the cathode side in the double-sided coating on simple Way with toner powder to feed. Furthermore, it is possible for performance optimization or for future Developments of membrane-electrode assemblies, on the anode side and to apply different catalytic layers to the cathode side.

The Application of a catalytic layer to a membrane can after a method in which initially a substantially from an ionomer (proton conductive polymer material) and a Support material e.g. Carbon black, to which an electrocatalyst, e.g. Platinum is deposited, a fine catalytic powder produced and provided becomes. Possibly. can still further additives such as waxes, pore formers, metal oxides and a polymeric binder contained therein.

Under a xerographic process becomes an electrostatic printing process understood, is used in the dry toner powder.

at the xerographic coating process will be in a first step the catalytic powder negatively charged in a developer unit and applied to a rotating drum on which the powder sticks. In a second step, the membrane passed between the drum and a positively charged corona unit, wherein pulled by the positive tension of the catalytic toner on the membrane becomes. In a shooting third step is in a fixing unit of the still loose toner by means of pressure and / or heat firmly connected to the membrane. The xerographic coating allows over conventional Rolling, printing (planographic printing, screen printing) and spraying techniques, in particular uniform catalytic Layers with small layer thicknesses of up to less than 10μm and one consistently high quality standard the coatings at a reduced cost.

According to one preferred embodiment of Invention, the coating process by means of a xerographic Print procedure performed. there For example, the coating process may be through a laser printer or Copier performed become. this makes possible a positionally accurate application of the electrode layer on the membrane.

In this case, the catalytically prepared toner powder is preferably pre-loaded in a toner cartridge assigned to the laser printer or the copier will hold. This type of provision at the same time allows easy storage and shipment of the toner material.

Of the Coating process can be particularly cost effective and easy in one Device that is based on a laser printer or copier to be performed. A laser printer or copier has a photosensitive one on the drum Layer arranged initially over a Charging corona unit charged negatively and then by targeted Exposure over an exposure unit (e.g., laser) at the exposed locations is neutralized again. This will leave the toner in the exposed areas only stick (and is repelled by the unexposed areas) and a latent, differentiated image emerges. In the inventive method can this can be exploited to make the underlay (membrane) with a catalytic Layer with exactly specified surface dimensions and exact positioning to coat. For example, such a predetermined frame area the membrane remain uncoated. Elaborate templates or positioning devices can thereby omitted, whereby further costs are saved. Also are basically certain Coating pattern possible.

In However, the simplest form of the method can also be applied to the photosensitive Layer of the drum and dispense with the laser unit (exposure unit), making a very cost-effective electrostatic Pressure device is suitable. By a simultaneous bilateral Coating the membrane is an additional time savings possible.

The prerequisite for the coating according to the invention of a membrane having an electrocatalytic layer is a corresponding mechanical and thermal membrane stability. Therefore, the method according to the invention is particularly suitable for the production of membrane-electrode assemblies, which, as in DE 20 2004 000 365 U1 described to be used in the temperature range up to 250 ° C.

Further Details of the invention will become apparent from the following detailed Description and attached Drawings in which preferred embodiments of the invention are exemplified.

In show the drawings:

1 : A construction diagram of a membrane-electrode assembly in a perspective view and

2 : A schematic representation of a device according to the invention with two xerographic printer units in a perspective view.

A device for applying a catalytic layer ( 3 . 4 ) on a membrane ( 2 ), to produce a membrane-electrode assembly 1 for a fuel cell consists essentially of two xerographic printer units ( 6 . 6 ' ).

In 1 is the structure of the membrane-electrode assembly (MEA) 1 shown schematically. The membrane 2 advantageously consists of a proton-conducting polymer material and is formed as a film. On the membrane surfaces are the anode layer 3 and the cathode layer 4 arranged. The electrode layers consist essentially of an ionomer and an electrocatalyst (Pt on carbon black) and optionally further additives, such as binders, pore formers or waxes. A porous gas diffusion pad closes towards the outside 5 for distributing the fuels, which may be designed differently in a particular embodiment for the anode and cathode side. Joined together form the layers 2 to 5 the MEA 1 , The MEA 1 , as in 1 shown, may be enclosed for placement in a fuel cell stack of a frame, not shown. The fuel cell is composed of a stack of MEAs, each separated by a bipolar plate and terminated by two endplates, each unit providing an open circuit voltage of approximately 1V, so that, depending on the number of MEAs, stack voltages of up to several Add one hundred volts.

In the 2 represented xerographic printer units ( 6 . 6 ' ) are based in their design advantageously on known components of a laser printer.

The second xerographic unit 6 ' is the first unit 6 spaced diametrically opposite so that the membrane 2 between two drums 7 . 7 ' can be passed close by, with the drums 7 . 7 ' rotate in opposite directions. The units 6 . 6 ' consist essentially each of a rotatable, advantageously made of aluminum drum 7 . 7 ' , a developer unit 8th . 8th' and a corona unit 9 . 9 ' , About the width of the drum 7 . 7 ' is the xerographic unit 6 . 6 ' to the size of the membrane to be printed 2 customizable. The photoconductor layer applied to the surface of the drum in laser printers may be omitted here (as well as an exposure unit). At the drums 7 . 7 ' are the developer units 8th . 8th' arranged, which (each) a cartridge, or cassette, in which a catalytic powder, which is to form the subsequent electrode layer, is vorratbar. In the cartridges can be the same or different catalytic powders for an anode layer 3 , or a cathode layer 4 be filled. The catalytic powder may be in the developer unit 8th . 8th' charged negatively. The corona unit 9 . 9 ' , advantageously formed from a tungsten wire, is chargeable to a positive voltage and acts as a transfer charger. Here is the unit 6 the corona unit 9 ' assigned and the unit 6 ' the corona unit 9 The transfer loader 9 . 9 ' are suitably shielded, or aligned, so they only on the catalytic powder on the opposite drum 7 . 7 ' are effective. Furthermore, a fixing unit, not shown, arranged by means of which the transferred to the membrane sides catalytic layers 3 . 4 can be fixed and connected to the membrane surface. Finally, a control electronics, also not shown, is provided, via which the charging voltages, the drum drive and the toner supply are controlled in the electrostatic transfer process.

A method of applying a catalytic layer 3 . 4 on a membrane 2 , for the production of a membrane electrode assembly 1 for a fuel cell, is essentially based on the fact that a catalytic coating material as an electrostatically chargeable toner powder xerographically on the membrane ( 2 ) is transmitted.

In a developer unit 8th . 8th' the catalytic powder is negatively charged. Now turns the drum 7 . 7 ' at the developer unit 8th . 8th' past, the catalytic powder is transferred to the drum. On the drum 7 . 7 ' then there is the latent electrode layer. The catalytic powder is then removed by means of a corona unit 9 . 9 ' (Transfer charger) on the membrane 2 transfer. This is done to the transfer charger 9 . 9 ' applied a positive voltage. The membrane 2 will now be between the drum 7 . 7 ' and the corona 9 . 9 ' passed. In this case, the negatively charged catalytic powder on the membrane surface 2 drawn. Finally, the coating in the fuser is firmly connected to the membrane surface via temperature and / or pressure. The coated membrane 2 can then (possibly) in a conventional manner with gas diffusion underlay to the finished MEA 1 , as in 1 shown, joined together.

If one in the plane differentiated electrode layer is desired or edges on the membrane should remain free, or accurate positioning and demarcation of the area to be coated may be required this with an exposure unit and a photoconductive layer (Photoconductive drum) on the laser printer technology itself known manner be realized.

Claims (2)

Device for applying a catalytic layer to a membrane, for producing a membrane electrode assembly for a fuel cell, characterized in that two xerographic printer units ( 6 . 6 ' ) are arranged opposite one another, by means of which the membrane ( 2 ) is coatable on both sides in a single pass. Apparatus according to claim 1, characterized in that from a first cartridge ( 8th ) a catalytic coating material for producing an anode layer ( 3 ) which can be fed to a membrane side surface, and that from a second cartridge ( 8th' ) a catalytic coating material for producing a cathode layer ( 4 ) of the other membrane side surface can be fed.