DE10017058A1 - Bipolar plate used for a PEM fuel cell stack is made from a first metal with a metallic coating of a second metal - Google Patents

Abstract

Bipolar plate is made from a first metal with a metallic coating of a second metal. Preferred Features: The bulk material (core) of the plate is St37, aluminum or aluminum alloy. The metallic coating is made from gold, tin, a lead-tin alloy or tantalum optionally with additional metallic intermediate layers (e.g. copper or nickel). The metallic coating is applied by galvanic deposition or by sputtering.

Description

Bipolar plates for PEMFC or DMFC stacks have so far usually been made of either metal, from graphite or from a more or less homogeneous mixture of a carbon-containing Composites made. Important requirements are good electrical conductivity, gas tightness, good mechanical workability (turning, milling) or alternatively the possibility of inserting the bipolar plate in to be able to give the desired shape to a kind of stamping. Are in a fuel cell stack usually some of the bipolar plates are also designed such that by means of a Cooling medium, typically water, dissipates the heat generated during operation or can be decoupled for any further use.

To achieve a good performance of a PEM fuel cell stack, the design of the Bipolar plates of great importance, especially with a view to making them as uniform as possible Supply of the reaction zones with the reaction gases hydrogen and oxygen or air at the same time, the best possible electrical contacting of the adjacent gas diffusion layers.

There are several constructive solutions to this task in electrochemical literature. In The prototypes previously manufactured by various companies are generally based on one continuous milling of parallel channels with rectangular ones on both sides of the bipolar plate Cross-section, either in the same direction (front and back) or at a right angle to each other, used. The reacting gases essentially flow from one edge each the bipolar plate to the opposite. A constructive optimization task consists in the To design channel widths or web widths in such a way that on the one hand a good gas supply and on the other hand however, good electrical contacting of the adjacent gas diffusion electrode is also ensured is. The webs that are necessarily present in this approach result in particular in the Difficulty identifying those areas of the gas diffusion electrode or the adjacent catalytic To supply the reaction layer with gas, which lie directly on the webs, since the cross diffusion the gases within the gas diffusion layer are considerably more difficult. There is a possible solution in inserting one or more wire meshes between the bipolar plate and the gas diffusion layer in order to facilitate gas access to the areas mentioned. Aside from the rather cumbersome Assembly of such a structure, which is disadvantageous with regard to automated production, arise when using base metals i. a. further disadvantages due to the possibility of Formation of surface electrodes on the wire mesh or the wire meshes that cover the internal increase the electrical resistance of the cell or the stack and thus lead to a reduction in performance to lead.

To solve the problem described, a surface structure of a bipolar plate is proposed according to the invention, which offers both a good gas supply to the adjacent gas diffusion structure (typically a prepared carbon fleece) and also an improved electrical contacting of the gas diffusion electrode compared to the prior art. An embodiment is shown in Fig. 1.

The bipolar plate shown in Fig. 1 contains bores ( 1 ) and ( 1 a) for the supply of oxygen or air or removal of excess air or O ₂ ; also ( 2 ) and ( 2 a) for the supply of hydrogen or discharge of excess hydrogen, bores ( 3 ) for the passage of the (threaded) rods or screws for stack mounting, milled or etched recesses ( 4 ) on both sides (approx. 0.5 mm deep from the edge), and from the manufacturing process resulting "left standing" islands (5) connecting the electrical contact to the adjacent layer, generally a gas diffusion layer prepared.

For clarification, the "islands" obtained after milling the channels are in cross-section drawn in excessively thick. The exact thickness D of the central area in relation to the thickness d the edge zone results from the materials used for the carbon backing, the membrane and the sealing material that makes the MEE (membrane electrode assembly) gas-tight to the outside completes. The quotient d / D is to be optimized such that, on the one hand, a good electrical Contacting of the gas diffusion electrodes is achieved, but also the sealing after outside or to the other side of the bipolar plate is guaranteed.

Claims (8)

1. Bipolar plate for a PEM fuel cell stack, the bipolar plate made of a metal with a metallic coating consists of a second metal. 2. A bipolar plate according to claim 1, wherein the bulk material (the metallic "core") Bipolar plate St37, aluminum or an aluminum alloy is and the metallic coating made of gold, tin, a lead-tin alloy or tantalum, possibly with additional metallic ones Interlayers (typically copper, nickel) exist. 3. a bipolar plate according to claims 1 and 2, wherein the plate in the inner region, d. that is, where the gas diffusion electrodes or corresponding current collector materials are present, a larger one Thickness than at the edges. 4. a bipolar plate according to claims 1 to 3, wherein the additional plate thickness inside Area results from the thickness of the material used for sealing outside, so that on the one hand, a secure gas-tight seal of the MEA (membrane electrode unit) to the outside, on the other hand, however, an optimal contact pressure of the bipolar plate against the gas diffusion electrode is achieved. 5. A bipolar plate according to claims 1 to 4, wherein the structure in the middle (thicker) area opposite the plate of small rectangular "islands" with a height between 0.1 mm and 2 mm the surrounding area, lateral extent between 0.2 mm square and 2 mm in Square exists, either by precision machining (milling) or by a photolithographic process can be generated with chemical or electrochemical etching. 6. a bipolar plate according to claims 1 to 5, wherein the metallic coating of the (St37) - Aluminum or aluminum alloy plate over the entire surface using a common method of Surface coating, for example galvanic coating or sputtering, applied becomes. 7. a bipolar plate according to claims 1 to 6, wherein the outer part bores and bilateral contains narrow cutouts for gas supply. 8. a bipolar plate according to claim 7, wherein the gas supply channels milled on both sides through thin metal glued into suitable recesses next to the channels plates are covered gas-tight in the direction perpendicular to the plane of the plate.