Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
CA 02711210 2010-07-26 Bipolar plate and method for its production The present invention relates to a bipolar plate as well as to a method for its production. The bipolar plate according to the invention can be used in an electrochemical system, for example in a fuel cell system or in an electrolyser. Several types of electrochemical systems are known which make use of a stack of electrochemical cells with a layering of a multitude of electrochemical cells, which are each separated by bipolar plates. The bipolar plates have several functions: - Electrical contacting of the electrodes of the individual electrochemical cells and conducting of electric current to the adjacent cells (serial connection of the cells); - Supplying media or reactants such as e.g. water or gases to the cells and removal of the reaction gases produced in the cell via a corresponding distribution structure, the so called flow field; - Transport of the heat produced in the electrochemical cells; and - Sealing of the different kinds of media and cooling channels of the flow field relative to each other and to the outside. For the intended application at large industrial scale, it is of great importance to be able to produce large numbers of bipolar plates with high quality at low cost. In this context, it is of great importance that variations in dimensional tolerances are not exceeded as failing in doing so can lead to functional and even safety-relevant malfunction. This is particularly important with welded multi-layer bipolar plates. Up to now, positioning holes are used in order to secure the precise positioning of the layers relative to each other. Practice shows that in case these positioning holes, which ascertain the precise relative positioning of the at least two bipolar plates relative to each other, are formed simultaneously with the other through-openings as well as with the cutting of the outer edge, the accuracy and the reproducibility of the positioning of the layers with respect to each other is not sufficient. In particular, an offset between the channel geometries of the Flowfield of the individual layers can be the consequence. With an extreme offset, welding of the bipolar plates takes place at such areas in which the bipolar plates do not contact each other during this welding step, which then leads to destruction of the respective areas by burning. It is therefore the object of the present invention to provide a multi-layer bipolar plate as well as a method for the production of a multi-layer bipolar plate, which allows to produce multi-layer bipolar plates at large scale at high quality and low cost. This object is achieved by the bipolar plate and the method according to the independent claims. A first embodiment of the method for the production of a multi-layer bipolar plate provides that at least a first and a second indentation/protrusion are formed into each of at least two layers and that the layers are positioned one above the other. In the state in which the two layers are fully positioned one above the other, the first indentation/protrusion of the first layer and the first indentation/protrusion of the second layer engage with each other and touch in a plane El in a form-locking manner, thus with positive fit, by establishing the first 1 CA 02711210 2010-07-26 contact area. The second indentation/protrusion of the first layer and the second indentation/protrusion of the second layer simultaneously engage with each other but touch each other in the plane E2 only in sections by forming the second contact area. Its contact portions are arranged on both sides of a virtual straight line, which extends in the main direction of the second indentation/protrusion of the second layer along the longest extension of this indentation/protrusion. There is no enclosing positive fit between the second indentation/protrusion of the first layer and the second indentation/protrusion of the second layer in this plane E2. The at least two layers are positioned in a nested manner with the indentations/protrusions of at least one layer being fixed in complementary indentations/protrusions of a fixation device and the a least two layers there being joined to each other by an adhesive bond. As a consequence of this arrangement, in a completely positioned state, thus in the arrangement of the layers immediately before they are joined, no translatory movement between the layers in the direction of a virtual straight line through both touching positions is possible. However, an adjustment between the layers in the direction perpendicular to this virtual line within the limits of the ratio of the sizes of the respective second indentations/protrusions is possible. Bipolar plates generally show a channel structure with depressions and/or elevations. This channel structure defines the flow field. In case of two layers forming a separator and cooling system, both layers will be structured. In case of three layers forming a separator and cooling system, at least the outer two ones will be structured. The inner layer may be structured but does not have to be. These depressions and/or elevations that from the channel structure of the layers are independent from the indentations and/or protrusions which are used to ascertain the positioning of the layers. This means that the indentations/protrusions are preferably located outside the channel structure. For space- saving reasons, it may however be preferable to arrange them in between the channel structure. It is preferred that the channel-forming depressions and/or elevations are formed into the at least two layers in the same step of the procedure as the indentations/protrusions. The orientation of the channel structure is usually such that its main direction runs in parallel to the direction in which the indentations/protrusions allow for a limited adjustment between the layers of the bipolar plate. The position of the respective second indentation/protrusion relative to the channel structure in both layers is predetermined before it is embossed. This prevents at the outset the formation of an offset between the channels of the corresponding layers perpendicular to their main direction in a reproducible manner. The term indentation/protrusion in the context of this invention is used for a section of a layer of a bipolar plate, which projects from the plane of the respective layer. It depends on the point of view whether this projection is considered as an indentation or as a protrusion. If the complementary term protrusion/indentation is used here, this is done in order to stress the inverse direction. A second embodiment of the method for the production of a multi-layer bipolar plate provides that in at least two layers at least a first, a second and a third indentation/protrusion is formed and the layers are positioned one above the other. In a 2 CA 02711210 2010-07-26 completely positioned state, the respective first indentations/protrusions of the first and second layer engage into each other by forming a first contact area, the respective second indentations/protrusions of the first and second layer engage into each other by forming a second contact area and the respective third indentations/protrusions of the first and second layer engage into each other by forming a third contact area. Doing so, the first and second layers touch each other in the planes E2, E3 and E4 only in sections, namely in at least two contact portions without resulting in a positive fit between the first and second layer. These contact portions are arranged in such a manner that in the plane E2, E3 and E4, respectively, they extend on both sides of a virtual straight line, which extends in the main direction of the respective second indentation/protrusion of the second layer. These virtual straight lines through the first and second contact areas run in general essentially parallel to each other. "Essentially parallel" in the context of this invention means an angle between -10 and +10 . The virtual straight line through the third contact area runs essentially perpendicular to the former two virtual straight lines. In the context of this invention, "essentially perpendicular" means an angle between 80 and 100 . The at least two layers are connected to each other by joining, also referred to here as adhesive bonding, where the indentations/protrusions of at least one layer are fixed in complimentary protrusions/indentations of a fixation device in order to guarantee an exact positioning of the layers with respect to each other. Joining in the context of this invention comprises gluing, brazing, soldering and welding, especially laser welding. As a consequence, the limited adjustability provided by the two contact areas with generally parallel orientation of the virtual straight lines mentioned beforehand is distributed in a more regular manner over the complete area of the plate. Especially with a central arrangement of the other contact portion, the one with essentially perpendicular arrangement of the virtual straight line, the adjustment of the forming tolerances of both layers with respect to each other is effected on both sides of this contact area. Even with a lack of forming accuracy, a sufficient freedom for adjustments of the layers relative to each other is given, which provides for the position of the layers relative to each other being still exactly determined. This solution shows its advantages especially with an almost square flow field area. In this embodiment, too, it is preferred that the channel-forming depressions and/or elevations are formed into the at least two layers in the same step of the procedure as the indentations/protrusions, in order to provide predefined distances between the channel structures and the contact portions in the respective layer. In both embodiments mentioned above it is advantageous for an optimized positioning before the connection of the layers if at least one indentation/protrusion shows a through- opening through which a centring bolt of the fixation device can reach. Preferred methods for the connection of the layers with each other comprise gluing and welding, most preferably laser welding. The bipolar plates and their layers, respectively, preferably comprise metal, most preferably steel or consist of it. This allows them to be shaped by embossing, deep drawing, hydroforming, adiabatic forming, such as forging and high energy rate forming, or roll forming. 3 CA 02711210 2010-07-26 The planes El and E2 or E2, E3 and E4, respectively, are planes which are oriented essentially perpendicular to the stacking direction of the fuel cell stack in the respective plane of contact between the two layers. These planes El and E2 or E2, E3 and E4, respectively, extend parallel to the plane E of the bipolar plate, which bipolar plate apart from the structures necessary for its functioning as a rule is flat; the plane E is defined as extending essentially centred between the two - outer - layers of the bipolar plate. The planes El and E2 on the one hand and planes E2, E3 and E4 on the other hand are parallel to each other, namely in case the contact is achieved at different contact portions in an offset manner. They can however also be identical, which results from the absence of such an offset. A first embodiment of a bipolar plate according to the invention provides that this bipolar plate consists of at least two layers with channel structures, with at least two layers each showing at least a first and a second indentation/protrusion where the first indentation/protrusion of the first layer and the first indentation/protrusion of the second layer in a completely positioned state of the layers engage with each other and contact each other in plane El in a form-locking manner, thus with positive fit, by forming a first contact area. In contrast, the second indentation/protrusion of the first layer and the second indentation/protrusion contact each other only in sections in plane E2, namely along at least two short, in some cases even only point-shaped, sections while forming the second contact area. These contact portions are located on both sides of a virtual straight line extending in the main extension direction of the second indentation/protrusion of the second layer. The section-wise contact between the engaged second indentations/protrusions of the first and second layer, respectively, however, does not cause a positive fit of the layers in the plane E2. In contrast, in the direction of the virtual straight line mentioned, a limited translational adjustment is possible parallel to plane E2. The interaction between the two contact areas prevents however from a rotational adjustment. This provides for both layers being exactly positioned relative to each other with a limited degree of freedom of the layers for adjustment purposes, namely in a direction essentially perpendicular to a virtual straight line through both contact portions of the second contact area. One should be aware that bipolar plates in general comprise channel structures. These channel structures, especially at the outer surfaces of the bipolar plate, provide the guidance of reaction media, such as molecular hydrogen on the one hand and air/oxygen on the other hand, and especially in between the layers, provide the guidance of cooling media. In order to do so, the channel structures of two adjacent bipolar plate layers on the surfaces facing each other form a flowfield, usually for cooling media. In the same way, a flowfield is formed on the respective opposite side of the respective bipolar plate layer, this flowfield provides the distribution of the reactants and removal of reaction products. With metallic bipolar plates, the structures on both sides of a layer are usually complementary which means that a protrusion on the upper side results in a depression on the lower side. Apart from parallel and/or serpentine-shaped arranged continuous channel structures, other distribution structures, which allow a transition between virtual parallel streaming lines, are feasible as well. The latter are also referred to as channel structures in the context of this invention. Positioning of the layers using positioning embossments is particularly advantageous for metallic bipolar plates, as the layers of which tend to show form tolerances resulting from 4 CA 02711210 2010-07-26 the forming process, especially due to spring back. Simultaneous formation of the channel structures and the indentations/protrusions provides a predictable, reproducible distance between these structures. Thus, the invention allows to minimize the offset of the individual layers of a bipolar plate relative to each other and therefore to achieve an exact positioning of the - preferably embossed - channel structures of the individual layers, especially also while joining the layers in a welding device such as a laser welding device. This results in the following advantages: Better and faster welding, reduction of the number of defective parts at welding and during other processes due to better positioning. This allows to increase the tolerance of the total cut of the plate layer. In return, a cost reduction is achieved without any impact on the quality, especially since simpler cutting methods, such as punching, can be applied. In order to prevent the multi-layer bipolar plate from bloating, it is recommended that at least the two outermost layers of the bipolar plate are welded to each other, especially by laser welding. One should however pay attention to welding the correct sections of the fine and/or small channel structure. This is extremely important as it ascertains the tightness as well as the controlled flow of the cooling media between the layers of the bipolar plate. It is once more stressed that the welding of the layers of the bipolar plate is not exclusively performed along the outer periphery of the plates, but in its inner area as well, namely in between the different channel structures, and especially for the latter, an extremely precise positioning of the bipolar plate layers relative to each other is required. A further alternative of the invention provides that it comprises at least two layers with at least two layers each showing at least a first, a second and a third indentation/protrusion, respectively. The first indentations/protrusions of the first and second layer, respectively, engage with each other while forming the first contact area. The second indentations/protrusions of the first and second layer, respectively, engage with each other while forming the second contact area. The third indentations/protrusions of the first and second layer, respectively, engage with each other while forming the third contact area. The engagement is always given in the completely arranged state. The corresponding indentations/protrusions touch each other only in sections in the planes E2, E3 and E4. The pairs of contact portions of a contact area are located in the plane E2, E3 and E4, respectively on both sides of a virtual straight line, which extends in the main direction of the respective indentation/protrusion in the second layer. These virtual straight lines run in parallel to each other as to the first and second contact portion while the one extending along the main direction of the third contact portion runs essentially perpendicular to the former two virtual straight lines. In the following, preferred embodiments of the bipolar plate according to the invention are described in an exemplified manner. One embodiment provides the indentations/protrusions of the first layer to be self-centring to the respective indentations/protrusions of the second layer. This makes it unnecessary to align the plates, especially in their height direction, thus the stacking direction of a fuel cell stack, as the layers centre themselves with respect to each other. A further advantageous embodiment of the invention requires the first and/or second indentation/protrusion of the first and/or second layer to comprise a through- hole. These CA 02711210 2010-07-26 through-holes are preferably arranged centred. It is most advantageous that holes are provided in both layers. Such through-holes allow the engagement of fixation or centring bolts, e.g. of a fixation device. On the other hand, holes with different sizes or holes in only one layer allow the visual control of a correct combination of layers. A further advantageous embodiment provides that the first indentations/protrusions of the first and second layer, which engage with each other, have circular shape. In the same way, it can be provided that the second indentation/protrusion of the first layer is circular as well, while the second indentation/protrusion of the second layer is oblong or has the shape of a rounded polygon but not circular. In general, the indentations/protrusions can have all shapes which can be produced by embossment with rounded shapes being preferred for production and tooling reasons. It is essential that the geometry of the fixation device and of the indentation/protrusion are adapted to each other. Accordingly, in the context of this invention oblong also comprises oval and rounded polygonal, but explicitly excludes circular shapes. A further advantageous embodiment provides that at least one of the layer comprises a channel structure and the longitudinal direction of the second indentation/protrusion is arranged in parallel to the channel structure. This ascertains a parallel shift along the channel structures which are connected to each other in case of an extension of the plate which may be due to e.g. heat or production, especially different spring back of the individual layers after embossment. This allows that the "deepest points" of the channel in both layers are always arranged one on top of the other and the desired areas are in contact with each other. This is especially advantageous during welding, especially laser welding, as burning damages due to overheating are prevented. Another advantageous embodiment provides the indentations/protrusions of the first layer to have a steeper or smaller conical angle than the conical angle of the complementary indentation/protrusion of the second layer. This allows a linear, circumferential contact between the indentations/protrusions which in turn results in a sufficient surface pressure. This effect is further increased by different heights of the indentations/protrusions in the adjacent layers. In the following, a particularly preferred embodiment of the invention is described with slightly different words, which shall however not be understood as limiting the invention: In order to achieve an exact positioning of the bipolar plates relative to each other, conical embossments are formed into the layers. These embossments can be considered to be indentations or protrusions, depending on the point of view. It is advantageous if e.g. the bipolar plate layer facing the anode side - the anode-sided layer - shows two circular embossments and the bipolar plate layer facing the cathode side - the cathode- sided layer - shows a circular and an oblong embossment. These positioning embossments should have a shape and size which allows e.g. the anode-sided layer to be nested to the cathode-sided layer and that the cone of the embossments allows a form-locking centring of the layers to each other. The advantage of the oblong embossment results from the layer system not being over-determined which allows an adjustment between the anode- and cathode-sided layers. Therefore, it is advantageous if the main direction, thus the longest extension of the 6 CA 02711210 2010-07-26 oblong embossment, runs in parallel to the main direction of the channels of the flow field. This prevents from an offset in y-direction and thus from an offset perpendicular to the channel structure. It is a further property of the positioning embossment that it can also be used for form- locking positioning in a device, such as a welding device. For the same reasons as described beforehand, the retainer in the device preferably shows a complementary but slightly increased shape compared to the corresponding indentation/protrusion of the bipolar plate layer. Thus, if an oblong positioning embossment is used, the retainer shows an oblong shape as well, but its size is larger than the one of the embossment in the respective layer, see for instance section C-C in figure 3e. A round retainer with an increased size is used for a round positioning embossment. In order to allow a rough pre-centring of the individual layers in the device, it can be advantageous to provide both a bore and an oblong hole in the middle of the positioning embossment as well as a corresponding retainer pins or centring bolts in the device. Such a pre-centring via aretainer pin provides for a smooth insertion of the layers into the retainer of the device. There are various options for the design of corresponding positioning embossments in corresponding layers. On the one hand it is possible that protrusions projecting relative to the remaining plane of the layer are formed into both layers and the protrusion in the layer arranged below the other layer engages in the indentation which results on the lower side of the protrusion of the upper layer. When considering the other structural parts of the respective bipolar plate or its layers, it is however often not possible to form the protrusion with its entire height in only one direction as the protrusions then would project above e.g. the bead necessary for sealing the layers relative to each other and thus would prevent the sealing of the plates. In such a situation, it is recommended that indentations with a larger diameter are formed into both layers and that the protrusions are embossed into the surface of these indentations, respectively. In order to result in a protrusion, the height of the indentation is smaller than the total height of the actual protrusion. This allows a distribution of the height of the indentation/protrusion to both sides of the plane of the respective bipolar plate. In total, symmetric height distributions to both sides are possible as well as asymmetrical ones. The design of the positioning embossment can be chosen in such a way that one layer shows a steeper cone than the other one. Moreover, it is advantageous to have different heights of the embossments in both layers so that the layers contact each other in the contact portion only in the respective flank of the cone either in sections or completely circumferentially. It is further useful that the circular or oblong retainer of the device at its open side is provided with a radius, which receives the slope of the outer cone of a bipolar plate layer. Nevertheless, sufficient force needs to be applied during positioning in order to achieve a centring with positive fit. The bipolar plate according to the invention comprises, as already laid down, at least two layers, but may comprise further layers. This can especially be advantageous if cooling media are guided at the inside of the bipolar plate. Especially with an unstructured intermediate layer constituting a third layer, it is advantageous to integrate the latter in such a manner into the bipolar plate that it is cut out in the area of the indentations/protrusions of the 7 CA 02711210 2010-07-26 outer layers so that the indentations/protrusions of one outer layer may engage with the protrusions/indentations of the other outer layer while reaching through the cut-out. It is further possible for bipolar plates with more than two layers that the layers of the plate are arranged as pairs in the same way as described beforehand for two-layered plates. With positioning embossments which result in a positive fit in the x-y plane, to be more precise the plane El, it is possible just as well to provide the corresponding indentation/protrusion at the same position in all layers, while the positioning embossments which in their respective contact plane E2, E3 or E4 do not lead to a positive fit have to be arranged offset for adjacent pairs of layers. This arrangement is suitable for structured plates, thus embossed intermediate layers, as the positioning embossment can be formed with the remaining structure, but it can also be applied for non-structured intermediate layers. In the following, the invention is explained based on different figures. Identical reference numbers refer to identical elements. In the figures show Figs. la to 1d an exemplary construction of an electrochemical cell stack which comprises at least one bipolar plate; Fig. 2 one layer of a bipolar plate according to the invention; Figs. 3a to 3g views, sections and further details of a first embodiment of a bipolar plate according to the invention; Figs. 4a to 4c top views as well as a section of a second embodiment of a bipolar plate according to the invention; Figs. 5a to 5h different top views of contact portions; Fig. 6 a sectional view of a further embodiment of a bipolar plate according to the invention; Fig. 7 a sectional view of a contact portion of a further embodiment of a three-layered bipolar plate according to the invention; and Fig. 8 details as to the laser welding of a bipolar plate according to the invention. Figures la and 1d show the construction of a fuel cell unit 7. A plurality of such fuel cell units 7 forms the fuel cell arrangement 8 which is stacked with stacking direction z between the endplates 70 and 71, see Fig. 1c. Figure lb shows several fuel cell units 7 stacked one upon the other but without endplates. In figures la and 1d, a fuel cell unit 7 with its usual parts can be seen in an exploded representation and in a section view in plane x-z, respectively. This fuel cell unit 7 for example comprises a polymer membrane 9, which in its central area 9a is provided on both surfaces with a catalyst layer. The fuel cell unit 7 comprises two bipolar plates 1 and 1*, between which the coated polymer membrane is arranged. Moreover, in the area between each bipolar plate 1, 1* and the coated polymer membrane 8 CA 02711210 2010-07-26 9, a gas diffusion layer 10 is arranged. The bipolar plates 1, 1* comprise a bead 4a, which encircles the flow field, thus the cannel structure 4, in a sealing manner. In addition, a further bead 4a is indicated, which seals a media conduct from repeating unit 7' to repeating unit 7'. In the following figures, a representation of the sealing elements has been dispensed with for clarity reasons. In the context of this application the following terms are distinguished: - The actual fuel cell is defined as the ensemble of first gas diffusion layer 10, coated polymer membrane 9 and second gas diffusion layer 10. - The fuel cell unit 7 comprises the anode-sided layer 2 of a bipolar plate 1, the fuel cell as defined beforehand and the cathode-sided layer 3 of a second bipolar plate 1*, as well as further layers of a bipolar plate 1 where required. This means that a bipolar plate 1 is always divided between two fuel cell units 7. This term needs to be distinguished from the so-called repetitive unit 7', which comprises a complete bipolar plate 1 as well as a fuel cell according to the definition above. Figure 1d further indicates that the layers at least in the area of the beads 4a are circumferentially welded to each other, see welding seams 11. In this context, the larger embossment depth of the sealing beads 4a compared to the channel structures 4 becomes obvious. Figure 1d further shows the different compartments of the channel structures 4: Reactant channels 41, 42 on the surfaces of the layers 2, 3 facing away from each other and channels 43 for coolant between the layers 2, 3 which layers contact each other in sections. The representation in figure 1d relates to a non-compressed state of the fuel cell. The following aims on explaining the construction of the bipolar plate 1 as well as a method for its production by way of example. Figure 2 shows one layer 3 of a bipolar plate according to the invention. In the example shown, this is a metallic layer 3 with a continuous, straight channel structure 4 with several parallel channels with the flow field showing an extension in x-direction which is about five times the extension in y-direction. The metallic layer 3 further shows a circular indentation 3a as well as an oblong indentation 3b. The circular indentation 3a in its centre is provided with a circular bore, while the oblong indentation 3b shows a centrally- arranged oblong hole. The extension direction 5, thus a virtual straight line in the direction of the longest extension of the oblong indentation is arranged in parallel to the extension of the channel structure 4, thus to the direction of the fluid conduction. This means that the channel structures guide the fluid - either a coolant medium on the inner side of the future bipolar plate or a medium on the outer surface of the future bipolar plate - exactly in this direction. The layer 3 shown in figure 2 is connected to at least one further layer in order to form a bipolar plate. These layers are joined to each other through laser welding, which does not only take place at the outer edge of the plates, but also at least in portions of the central area, namely at some contact areas of the flow field of the bipolar plate in order to prevent the bipolar plate from bloating at an increased pressure of the coolant on the inside of the bipolar plate. In order to provide a secure and precise welding, it is advantageous that the extension direction 5 of the oblong indentation runs in parallel to the channel structure. Figure 3a shows a top view of a joined bipolar plate 1 according to the invention. The bipolar plate consists of at least two layers 2 and 3, which are joined by laser welding, as is indicated 9 CA 02711210 2010-07-26 by the welding seams 11, 11' and 11". The laser welding line 11 constitutes a continuous line. This means that the area encircling the channel structure - where necessary interrupted by supply and removal lines - is accordingly welded in a tight manner. Further, the areas of protrusions and indentations- which one is considered as which depends from the point of view, see reference numbers 2a and 2b - are joined to each other, see the dashed lines 11'. Further, the two layers of the bipolar plate 1 are also connected to each other in the area of the flow field by means of segmented straight welding seams, the position of which is indicated as an example by arrow 11". The figure shows an upper first layer 2, below which the layer 3 can only be identified in sections, namely in the area of the bores. The diameter of the bores in the area of the indentations/protrusions is smaller in the lower layer 3, which provides a small section to be visible, as can be seen in the upper right part of figure 3a. This allows - either by visual or automated inspection - to verify whether the correct layers have been assembled in a correct manner. The following focuses on the indentations/protrusions of the contact areas 23a and 23b, especially also by means of the sectional views B-B, see figure 3b, C-C and D- D, see figures 3a, 3e and 3f. The corresponding cut-out schematic top-views to the areas D1 and D2 are depicted in figures 3c and 3g. In addition, figure 3d explains the relationship of the angles of the indentations/protrusions of figure 3b. In this context, elements of the lower layer being in fact covered by the upper layer and therefore not visible, such as the oblong indentation/protrusion, are indicated by continuous lines. The same applies also to figure 4a. Figure 3a demonstrates that at the contact area 23a in both layers 2 and 3, circular indentations/protrusions 2a and 3a are provided. In contrast, contact area 23b shows a circular indentation/protrusion 2b in layer 2 and an oblong indentation/protrusion 3b in layer 3. These interlocking indentations/protrusions 2b and 3b show two contact portions 30b and 31b. Compared to the circular protrusion 2b in layer 2, the corresponding contact portions 30b and 31b only extend in an extremely short section of the circumference, they are almost point-shaped, only. The contact portions 30b and 31b are situated on opposing sides of the circle and oppose each other. A virtual straight line through the centre points of the respective contact portions, 30b and 31b indicates a direction along which no movement is possible. This direction is essentially perpendicular both to the main direction of the channel structure 4 and the direction of the welding seams 11'. However, in the direction perpendicular to this virtual straight line, along line 5b, both layers can move in a limited range relative to each other which allows limited adjustment of tolerances or relative movements along the channel structures of both layers which are due to heat induced extension or due to the forming process. Figure 3b shows a bipolar plate according to figure 3a, which is introduced into the retainer 6c of a fixation device 6, e.g. the fixation device of a welding apparatus. The fixation device further shows a centring bolt 6a as well as radius 6b. The figure shows a detailed view according to section B-B, where below a first layer 2 - in the installed state the anode-sided layer - a second layer 3 is inserted, which second layer in the installed state is the cathode- sided layer. Both layers are made from thin metallic sheet, especially steel sheet metal. A first protrusion 2a of the first layer 2 is inserted into a first indentation 3a of the second layer 3. The mentioned indentation 3a and protrusion 2a are arranged in a way that they come CA 02711210 2010-07-26 into positive-fit in a plane El parallel to plane E, which means that no translational movement is possible in whatever direction of the plane. The plane E is defined as the plane of the plate which corresponds to the plane x-y according to figure 2. These facts are further underlined by the orthogonal double arrows in the simplified representation of the area D1 without bores in figure 3c. There it is obvious that in the plane E and El, respectively, no movement is possible. The first protrusion and the first indentation 3a are thus arranged in a self-centring manner relative to each other. As is obvious from figure 3b, both the first protrusion 2a and the first indentation 3a are provided with a circular bore tax and 3ax, respectively. The diameter of this bore 3ax of the first indentation 3a is smaller than the diameter of the bore tax of the first protrusion 2a. The same ratio applies for the corresponding areas of the bores. The fixation device itself in the area of the centring bolt shows the actual receiving section 6c, which passes upwards with a rounded radius 6b towards plane E. The receiving section here shows the shape of a circular cylinder. As can clearly be seen in figure 3d in a detailed view according to section F1 of figure 3b, the angle of the cone a2 between the vertical and the outer shell surface of the protrusion 2a is slightly smaller than the angle a3 between this vertical and the inner shell surface of the indentation 3a. The vertical here corresponds to the direction of the main axis of the centring bolt 6a. This allows only for a linear, circumferential contact of the outer shell surface of the first protrusion 2a with the inner shell surface of the first indentation 3a, which leads to the positive fit in the plane El. It is further obvious that the height h2 of the first protrusion 2a is smaller than the height/depth h3 of the indentation 3a. This causes that the first layer 2 does not rest on the second layer 3 in the area surrounding the centring bolt 6a and that the two layers 2 and 3 only have contact to each other in the contact area corresponding to the circumferential contact line. Figure 3e further shows a sectional view according to C-C in the area around a second centring bolt 6d of the fixation device 6. It is complemented by the section D- D in figure 3f which is orthogonal to the former. One can see a second protrusion 2b of layer 2, which is circular and shows a circular, centred opening 2bx, which is arranged concentric to the outer shell of the second centring bolt 6d in the receiving section 6f. This second protrusion 2b engages with a second indentation 3b in the layer 3 arranged below the first layer 2. As in figure 3b, the cross section of the opening 3bx of the second indentation 3b - at least in this sectional view - is slightly smaller than the one of the corresponding opening of the second protrusion 2b. However, the second indentation 3b has an oblong shape, which causes the second protrusion in figure 3e to be movable in a limited range from left to right which means that there is no positive fit. In contrast to this, in section D-D of figure 3f, a contact is given between the protrusion 2b and the indentation 3b at the contact portions 30b, 31b in the plane E2. This is emphasised in the simplified sketch of area D2 in figure 3g, in which a representation of the bores has been desisted from. There, the vertical double arrow indicates that a movement of the second protrusion 2b within the second indentation 3b in 11 CA 02711210 2010-07-26 y-direction is not possible. In contrast, the horizontal double arrow shows the option for a movement in x-direction for compensation purposes. The following intends to indicate the typical scale of the invention. The extension of the indentation/protrusion in the x- or y-direction usually is between 2 and 25 mm, preferably from 4 to 15 mm. The depth of the receiving section 6f around the centring bolt corresponds to about 0.5 to 1 mm, the diameter of the receiving section 6f in the fixation device 6 preferably ranges between 2 to 30 mm. The clearance of the openings, thus for example of the opening 2bx or 3bx relative to the centring bolt is usually between 0.1 and 3 mm, preferably between 0.1 and 1 mm. Figures 3a to 3g thus show a bipolar plate 1, which comprises at least two layers 2, 3 with at least two layers 2, 3, each showing a first and a second indentation/protrusion, where the first indentation/protrusion 2a of the first layer 2 and the first indentation/protrusion 3a of the second layer 3 in a completely positioned state of the layers 2, 3 interlock with each other and contact in the plane El with positive fit, whereas the second indentation/protrusion 2b of the first layer 2 and the second indentation/protrusion of the second layer 3 in the completely positioned state of the layers 2, 3 interlock with each other but contact each other only in at least two sections 30b, 31b, with the contact portions 30b, 31b being arranged in such a manner that they are situated on both sides of a virtual straight line 5b, which extends in the main direction of the indentation/protrusion 3b in the second layer 3, and where in the plane E2 no positive fit between the indentations/protrusions 2b and 3b is established. An alternative embodiment of a bipolar plate according to the invention is shown in figures 4a to 4c. The explanations made above apply here, too, except for the differences mentioned in the following. Figure 4a shows a top view of a bipolar plate 1' with the layer 3' being arranged as upper layer. The course of section E-E is explained in figure 4c in a top view on a plate which shows only the positioning embossments. The sectional view E-E itself is given in figure 4b. Figure 4b shows how the protrusions interlock with the indentations, namely protrusion 2a' with indentation 3a', protrusion 2b' with indentation 3b' and protrusion 2c' with indentation 3c'. There are thus three contact areas 23a', 23b' and 23c', each with a pair of indentations/protrusions comparable to figures 3e to 3g. This means that the indentations/protrusions of a contact area, e.g. of the contact area 23b' in the plane E3 contact each other in two sections, respectively, with these contact portions 30b' and 31b' being situated opposite to each other on both sides of the virtual straight line 5b. The same applies for the contact area 23a' in the plane E2 with the contact portions 30a' and 31a' and the virtual straight line 5a as well as for the contact area 23c' in the plane E4 with contact portions 30c' and 31c' and the virtual straight line 5c. Planes E2, E3 and E4 are planes extending parallel to plane E, they are however not indicated in the figures. The virtual straight line 5a at the contact area 23a' and the virtual straight line 5b at the contact area 23b' in the example shown extend essentially in parallel and therefore allow a limited adjustment in this direction, namely in parallel to the main direction of the channel structure 4 and therefore also in the direction of the welding seams 11". The virtual straight line 5c runs essentially orthogonal to the former two virtual straight lines 5a and 5b and is arranged 12 CA 02711210 2010-07-26 essentially in the middle of the contact areas 23a' and 23b'. This virtual straight line 5c indicates the adjustment direction at the contact portion 23c'. Figures 4a to 4c thus show a bipolar plate 1' which comprises at least two layers 2', 3', with the at least two layers 2', 3' each comprising a first, a second and a third indentation/protrusion. The first indentation/protrusion of the first layer interlocks with the first indentation/protrusion of the second layer, the second indentation/protrusion of the first layer interlocks with the second indentation/protrusion of the second layer and the third indentation/protrusion of the first layer interlocks with the third indentation/protrusion of the second layer. This leads to the layers contacting each other in the planes E2, E3 and E4 only in sections, namely in at least two sections 30a', 31a', 30b', 31b', 30c' and 31c' with these contact sections 30a', 31a, 30b', 31b', 30c' and 31c' being arranged in such a way that they are located on opposite sides of a virtual straight line 5a, 5b and 5c, respectively, which straight lines extend in the main direction of the indentations/protrusions 3a', 3b' and 3c', respectively. No positive fit is established between the corresponding indentations/protrusions, 2a' and 3a', 2b' and 3b' as well as 2c' and 3c' in the planes E2, E3 and E4, respectively. The virtual straight lines 5a and 5b extend under an angle of -10 to 10 to each other, while the virtual straight line 5c runs at an angle between 80 and 100 to the virtual straight lines 5a and 5b. Possibilities for the design of the interlocking indentations/protrusions 2a, 3a, 2b, 3b, 2a', 3a', 2b', 3b', 2c' and 3c' of the first and second layer 2, 3, respectively, are depicted in figure based on eight different examples. The examples in figures 5a to 5d show examples, which lead to a positive fit between the indentations/protrusions in their respective contact plane, while the examples of figures 5e to 5h do not result in a positive fit between interlocking indentations/protrusions. A representation of possible bores for centring bolts or for the control of a correct assembly of the layers has been dispensed with for clarity of the drawings. Figure 5a shows how two circular indentations/protrusions interlock. The circular indentations/protrusions 2a, 3b contact each other circumferentially, as already shown in figures 3b and 3c. Figure 5b shows a triangular indentation/protrusion 2a, which interlocks with a circular indentation/protrusion 3a and contacts the latter at the corners of the triangle. These three contact points are sufficient for providing a positive fit between the indentations/protrusions 2a and 3a. A larger number of contacting corners of a polygon also results in a positive fit, as is shown on the example of figure 5c. There, a rectangular indentation/protrusion interlocks into a circular indentation/protrusion and contacts the latter in the contact plane at is four corners. In order to prevent unnecessary abrasion of the tools, polygons with rounded corners are preferred over such ones with sharp corners. Figure 5d indicates that interlocking of an oblong indentation/protrusion 2a into a circular indentation/protrusion may lead to a positive fit, too, provided that the dimensions fit. Inversely, a circular indentation/protrusion interlocking with an oblong indentation/protrusion can only lead to a local contact of the interlocking indentations/protrusions but not to a positive fit, as follows from figure 5e and had already been demonstrated in figures 3e to 3g. 13 CA 02711210 2010-07-26 Interlocking of an indentation/protrusion with the shape of an equilateral polygon, e.g. a square 2b, into an oblong indentation/protrusion 3b, will lead to contact portions at the corners as shown in figure 5f, or to contact portions at the lateral edges, not shown here. Thus, it depends on the relative orientation of the indentations/protrusions which situation applies. It is also possible that two oblong indentations/protrusions 2b, 3b engage with each other and - provided their respective width fits - contact each other along their lateral edges. In this situation it depends on the choice of the respective length of the indentations/protrusions whether a positive fit is established or not. As long as the outer extension of the engaging indentation/protrusion is smaller than the inner extension of the receiving indentation/protrusion, no positive fit results. In addition, figure 5h shows that the contact of the interlocking indentations/protrusions does not have to be limited to two portions, being situated on opposite sides of the virtual straight line 5b, but that a different number of contact portions or contact points, especially even a different number on both sides of the virtual straight line 5b, is possible when realizing the engagement without positive fit. Figure 6 shows an embodiment of the bipolar plate 1 according to the invention, in which the positioning embossment of both layers is formed in such a way that it protrudes beyond both sides of the plate plane E. In the upper layer for instance a protrusion 3a* with a height h3x is formed, which - the figure shows the circular positioning embossment - has a diameter d3. Inside this circular protrusion 3a*, a circular indentation 3a+ is arranged, which has a height h3i with the height h3i being larger than h3x, which means that the indentation 3a+ is lowered relative to the plate plane E. The positioning embossment of layer 2 is designed in a comparable manner except for the diameter d2 of the protrusion 2a*, d2, being smaller than the one of the protrusion 3a*, d3, so that the upper protrusions 2a*, 3a* only cause an optimization of the height ratios within the layer but do not contribute to the actual positioning or positive fit, respectively. Figure 6 further demonstrates that the total height of the positioning embossment in both layers, he, essentially corresponds to the total height of the channel structure 4, hf, so that the positioning embossment does not impair the sealing in the area of the outer edge of the bipolar plate, which edge is however not shown here. In the embodiment shown in figure 7 the bipolar plate 1 shows in total three layers 2, 100 and 3. The central layer 100 in the area of the contact areas 23a and 23b, respectively, shows recesses, the extension of which is sufficiently large in order to allow the indentations 3a and 3b, respectively, to pass through into the indentations 2a and 2b, respectively. The positioning of both outer layers 2, 3 is effected in the manner described beforehand for two- layered embodiments but with the third layer being kept in between. Such a solution is possible no matter whether the indentations 2a, 2b, 3a, 3b are provided with through holes as depicted or not. Figure 8 shows an example of welding a first layer 2 to a second layer 3 in the area of the channel structure 4 using a laser beam 12. Both layer 2 and layer 3 comprise channel structures with the sections to be welded to each other, e.g. the narrow portion in the middle of figure 8, lying flat one on the other. As an example of the scale, the width of the flat section of the first layer 2 is specified here to be 200 pm, while the contact face of layer 3 has a width of 170 pm. The width of the laser beam 12 in this area is about 50 pm. The 14 CA 02711210 2010-07-26 invention provides for the contact areas to have an overlapping area with a width of at least 100 m, which causes an joining connection - a welding connection - being securely located within the contact area between the first layer 2 and the second layer 3, even with a slight imprecision of the course of the laser beam. To do so, the layers 2 and 3 are arranged one on top of the other as shown in figures 3a to 3g using corresponding centring bolts 6a and 6d, respectively, using the herein described positioning embossments and the foregoing described receiving section of the fixation device 6 and the layers are welded to each other. The description of the foregoing examples is only to be understood as exemplary. It is stressed that the combinations of all embodiments shown here is in the frame of the invention and that the subjects of the dependent claims, as far as not explicitly excluded, can be combined in any order.
