Contact plate for an electrochemical cell, process and an injection mold for producing the contact plate and contact plate assembly

#The invention relates to a contact plate for electrochemical cells. More particularly, the invention relates to a contact plate of a graphite-plastic composite material for electrochemical cells, in particular fuel cells. The contact plate with structures required to transport reaction media and ensure electrical contact of electrodes, optionally with a seal or a peripheral area and a seal of non-conductive plastic, is constructed in such a way that it can be produced by an injection molding process. The invention also relates to a process and an injection mold for producing the contact plate as well as to a contact plate assembly.

#Fuel cells are devices for direct conversion of chemical energy into electrical energy. A single fuel cell (see FIG. 1) is formed of two electrodes, an anode 2 and a cathode 3, which are separated from each other physically by an electrolytic layer e.g. a proton-conducting polymer membrane 4. The anode 2, the cathode 3 and the membrane 4 together form a membrane electrode assembly (MEA) 5. A fuel, e.g. hydrogen or methanol, is electrochemically oxidized at the anode. The released electrons flow through an external circuit to the cathode. Then, an oxidation agent e.g. oxygen is reduced upon reception of the electrons. Catalysts 6 are provided on interfaces between the electrode and the electrolyte to accelerate the electrode reactions.

#Frequently, a multiplicity of fuel cells is combined in a cell stack to achieve the required performance for practical applications. The cells stacked one upon or behind another are held together by non-illustrated longitudinal bolts. The “stack” is terminated by end plates before the first and after the last cell.

#FIG. 1 is a diagrammatic, exploded, perspective view of a section of a fuel cell stack;

#FIG. 2A is a fragmentary, perspective, sectional view of a bipolar plate with advantageous features of the invention;

#FIG. 2B is an enlarged, fragmentary, cross-sectional view of an area IIb of FIG. 2A, with an advantageous feature of the invention;

#Referring now to the figures of the drawings in detail and first, particularly, to FIGS. 1 and 2A thereof, it is seen that a structure which defines flow paths or channels 17 required for an even distribution of reaction media is recessed into surfaces of a basic body 7′ of contact plates 7 facing electrodes 2, 3 between which a membrane 4 coated with a catalyst 6 is disposed in a membrane electrode assembly (MEA) 5 of a cell 1. This structure firstly includes recessed parts through which there is a flow of reaction media (referred to below as channels 17, but without restriction to a particular geometry), and secondly includes contact structure elements or projections 16 which protrude from the recesses of the media distribution structure and make contact with the electrodes (e.g. lands delimiting the channels or stand-alone projections with, for example, a rectangular base area, referred to below irrespective of their particular geometry as projections). A section of a bipolar contact plate 7 with a winding flow channel 17 is shown in FIG. 2A.

#The projections 16 on the surface of the basic body 7′ of the bipolar plate 7 correspond in an injection mold to recesses in a cavity which must be completely filled. The filling of these recesses becomes more reliably guaranteed, the better their shape is adapted to the flow behavior of the material (fluid mechanical construction). Therefore, it is advantageous to produce all of the projections as being rounded as shown in FIGS. 2A and 2B. This applies both to first transitions 18 from a base 19 to wall surfaces 20 of the channels 17 and to transitions 21 from the channel wall surfaces 20 to surfaces (contact surfaces) 22 of projections 16. A rounding radius of 0.1 to 0.5 mm has proved advantageous. These roundings also facilitate ejection (removal of the plate from the mold). The rounding at the second transition 21 from the channel wall 20 to the surface 22 of the projection 16, in comparison with a projection 23 without a rounded transition (FIG. 2C), reduces an electrical contact surface to the electrode. However, this loss can, if necessary, be compensated by an initially larger surface structure of the projection 16. Such less finely divided structures in turn facilitate the complete filling of the mold. If, in contrast, no rounding is provided, due to incomplete filling of the cavity at the projections 16, undefined forms can result which in turn reduce the electrical contact to the electrode.

#An alternative to this procedure is to construct the projections initially higher at least by the rounding radius than required in the stack and then to remove the additional material from the surface of the projection again so that a projection without roundings is achieved at the transitions from the channel walls 20 to the surface 22.