Patent Application
20080124610
The invention relates to a bipolar plate, in particular for a fuel cell, in particular a PEM fuel cell (PEM=polymerelectrolyte), a direct-methanol fuel cell or some other suitable fuel cell.
The use of fuel cells to convert chemical energy to electrical energy represents an efficient and environmentally friendly method for obtaining electrical power from the elements hydrogen and oxygen. In this case, two physically separate electrode reactions normally take place, in which electrons are released and/or bonded. The reactants oxygen and hydrogen may be provided in the form of various fluids, and they need not necessarily be in a pure form. The use of pure, molecular oxygen and hydrogen is, for example, just as possible as the use of oxygen from the air and methane. A first example of two corresponding electrode reactions in a polymer electrolyte fuel cell (PEMFC or PEM fuel cell for short) comprises the following reactions:
H2=>2H++2e− (anodic reaction)
2H++2e−+½O2=>H2O (cathodic reaction)
The type of reaction depends on the type of fuel cell and on the fluids used. In the case of a solid oxide fuel cell (SOFC for short), by way of example, the following reactions can be observed:
H2+O2−=>H2O+2e− (anodic reaction I)
CO+O2−=>CO2+2e− (anodic reaction II)
O2+4e−=>2O2− (cathodic reaction)
Some other fuel cell types have different reactions. One common feature of all fuel cells is on the one hand the transport of an ion type through an electrolyte and on the other hand the transport, in parallel, of electrons through an outer conductor, in order to return the ions to an electrically neutral state after the transport process.
As a result of an electrical connection of the physically separate reaction zones, some of the reaction enthalpy that is released in this case can be obtained directly as electrical power. Normally, a plurality of fuel cells which are connected electrically in series are stacked one on top of the other, and a stack that has been formed in this way is used as a power source.
An single fuel cell in this case comprises an electrolyte unit such as a membrane and two electrodes covered with catalyst material. The membrane is located between the reactants, isolating them, with the reactants being, in particular, hydrogen and oxygen in the case of the PEM fuel cell or hydrogen/carbon-monoxide and oxygen in the case of a solid-oxide fuel cell, or a methanol/water mixture and oxygen in the case of a direct-methanol fuel cell, and has an ion conductivity, for example an H+-proton conductivity in the case of a DMFC or PEM fuel cell, or O2 conductivity in the case of a solid-oxide fuel cell.
The electrodes are required, inter alia, in order to tap off the electrical power produced by the fuel cell.
The fluids (also referred to as reactants, reaction media or working fluids), for example hydrogen and oxygen, and the reaction product of water flow through fluid channels into the areas of the reaction zones, and out of them. A channel system of fluid channels for a specific fluid is in general also referred to as a flow field.
In order to achieve optimum efficiency, the geometry of the flow fields for the respective reaction media is matched to the respective type of flow medium (for example transport of hydrogen/reformate (gaseous) and transport of methanol/water (liquid/gaseous) and of oxygen (air) and the resultant product water).
Various concepts of fuel cell stacks (individual fuel cells stacked one on top of the other) are known, in which the waste heat is transported away via one of the two reaction media, so that there is no need for any specific cooling flow field for each individual cell. Particularly in the case of direct-methanol fuel cells, the temperature of the individual fuel cells can easily be managed by the liquid fuel fluid (methanol/water mixture).
In addition to achieving particularly high efficiency, the provision of a bipolar plate which can be produced at the minimum possible cost is nowadways a priority development aim. The invention is therefore based on the object of specifying a bipolar plate which is better than the prior art, requires as little production effort as possible to manufacture, and is particularly highly efficient. A further aim is to specify a better fuel cell stack.
With regard to the bipolar plate, the object is achieved according to the invention by the features of claim 1. With regard to the fuel cell stack, the object is achieved according to the invention by the features of independent claims 21 and 22.
Advantageous developments of the invention are the subject matter of the dependent claims.
According to one fundamental idea of the invention, a bipolar plate for a fuel cell stack comprises two disc parts which are electrically conductively connected to one another and of which a first disc part only partially covers a second disc part in the stacking direction of the fuel cell stack. There is therefore no longer any need for a seal between the first and the second disc part in some circumstances, so that the bipolar plate does not itself any longer need to be leak-tested, as is the case with known bipolar plates which have two completely covering disc parts.
The second disc part is advantageously provided with one or more inlets and/or outlets, which are in the form of apertures, for a first reaction medium, and with an area of the second disc part which is not covered by the first disc part comprising a part of the circumferential rim, in particular the entire circumferential rim of at least one or all of the apertures. By way of example, sealing by means of a sealing element between the second disc part and an adjacent component of the fuel cell stack, for example an electrolyte unit then contributes in a preferred manner to there no longer necessarily being any need for sealing between the first and the second disc part for functional reliability of the bipolar plate.
A bipolar plate for a fuel cell stack is preferably composed of two disc parts which are joined to one another on the inside such that, if required, fluid and/or gas can pass through, at least in places, with internal cavities being formed, and which are functionally tolerant if the joint is incomplete in places. In this case, functional tolerance refers to a state in which the associated fuel cell can be operated functionally and reliably irrespective of the characteristic of the joint in places. In particular, a fluid is a liquid or some other flowing medium, allowing a fluid to pass through means that a liquid can pass through. For this purpose, the two disc parts are of different size. In this case, the disc parts each have at least one channel structure which, at least in places, forms at least one associated flow field. The flow field of the respective disc part is in the form of an outer flow field on the outside, facing away from the other disc part, with a medium, in particular reaction medium, for example air, water, hydrogen, methanol, air, mixtures of them and possibly with reaction products, flowing through the relevant channel structure. The first reaction medium mentioned above is then preferably associated with the channel structure or the flow field of the second disc part.
During the production of the bipolar plate by joining the two disc parts together, for example by welding, soldering, pressing, this avoids the need for complex leak testing of the joint bead, which would otherwise be necessary. If a reaction medium enters the internal cavities which may result from the joining process between the two disc parts, this is irrelevant to the operation and operational reliability of the fuel cell. This is achieved in that the area of the bipolar plate to be sealed is fitted on the outside and with respect to the reaction media on a first, larger-area disc part which for this purpose is provided, for example, with an elastomer seal which surrounds the second, smaller-area disc part. In particular, a disc part means a flat component with or without recesses, a component with surface structures and/or recesses, or any formed component with predetermined dimensions.
In one advantageous embodiment, the two disc parts have different external dimensions. In other words: the bipolar plate essentially comprises two disc parts, for example half shells, of different size which are joined together and connected to one another by a suitable joining process, for example welding, soldering or adhesive bonding. The external dimensions of one of the disc parts expediently correspond at least to the external dimensions of the associated flow field, and thus to the area of the active cell surface. The external dimensions of the other, and in particular larger, disc part correspond at least to the external dimensions of the associated flow field and of a frame surrounding this flow field, together with inlet and outlet apertures (“ports”) for the reaction media and the required sealing areas. Reducing the size of one of the disc parts of the bipolar plates saves both material and manufacturing costs. Furthermore, particularly for small-format bipolar plates and fuel cell stacks, this considerably reduces the effort required for quality assurance and leak testing, and therefore the test time.
The larger disc part is expediently provided in the area of the frame with inlets for supplying reaction media, in particular hydrogen and air, and corresponding outlets for carrying away reaction products, and residues of reaction products, in particular water and carbon dioxide. In order to transfer the reaction media into and out of the associated flow field, transfer elements are preferably provided in the area of the inlets and outlets. The transfer elements (also referred to as bridging elements) are in this case used for the reaction media to pass through and to support the inlets and outlets, which open into the channel structure of the respective flow field. In other words: the transfer elements mean that a sealing element which surrounds the inlet and outlet, for example an elastomer seal, does not interrupt the inlet and outlet processes.
The transfer elements may be designed in various ways. In one possible embodiment, the transfer element is in the form of a separate insert element (also referred to as an inlay) which extends in the longitudinal direction between an inlet or an outlet and one or more of the flow channels and is arranged transversely with respect to them. In the simplest case, a transfer element such as this is composed of a bending-resistant piece of sheet metal which has corresponding structures, in particular recesses and/or beads. Alternatively, the transfer elements may be arranged on one of the two or on both of the disc parts, in particular by being sprayed onto them. In a further alternative embodiment, the transfer elements may be integrated in a sealing element.
In a further preferred embodiment, the flow fields are each surrounded by a sealing element. In this case, the sealing elements seal the flow fields from one another and from the outside. For this purpose, the sealing element preferably completely surrounds the respective flow field of the disc part and the inlets and outlets. Joints are therefore safely avoided in the inlet and outlet area of the disc parts. In consequence, there is no need to carry out leak testing in the inlet and outlet area of the bipolar plate. All that is needed is leak testing of the individual disc parts, in particular of the metal sheets, at open points, for example holes, by visual examination.
A sealing element is preferably arranged on each of the two sides, in the area of the frame of the larger disc part. In this case, the sealing element which is arranged in the direction of the smaller disc part completely surrounds the small disc part. In other words: the smaller disc part is located in the sealing element. The outer flow fields are therefore sufficiently well sealed from one another. Furthermore, this saves material and costs.
The two disc parts can advantageously be positioned with respect to one another in advance of the joining process by means of structures which engage in one another, for example studs and beads.
The sealing element is expediently in the form of a further disc part composed of a deformable material. For example, the sealing element is a molding, in particular an elastomer seal. When the bipolar plate is being joined to adjacent components, for example the membrane electrode assembly, the elastomer seal rests in an interlocking manner on the webs, which project out of the disc parts, of the respective flow field and on the outer contours of the inlets and outlets, thus sealing the outer flow fields from one another and from the outside, such that media cannot pass through.
Depending on the requirement, the sealing element may be arranged on, in particular sprayed on, one of the disc parts, in particular on the larger disc part. Alternatively, the sealing element may be arranged on, in particular sprayed on, a membrane electrode assembly which is adjacent to the disc parts on the outside. One of the sealing elements may also be arranged on the disc part, and the other on the membrane electrode assembly.
Each sealing element is expediently formed with a locally different height. In consequence, the sealing element is matched as well as possible to the surface contour of the respective disc part. Furthermore, the frame can be provided with a recess for holding the sealing element. This allows the sealing element to be fixed and held easily before and during the joining of the fuel cell stack.
In a further embodiment, one of the disc parts may be provided with openings, in particular tapping holes, slots, rectangles, squares, and with the internal cavity between the two disc parts being filled with a relevant reaction medium. In this case, in particular, the smaller disc part is provided with openings. In the event of a disturbance in the supply of reaction medium, the reaction medium that is buffered there can maintain the fuel cell reaction, at least to an extent restricted to the size of the buffer and the diffusion capability of the reactant.
The disc parts are formed metal parts, in order to form different flow fields and different numbers of them. The forming of the disc parts results in an associated flow field for reaction media on the respective outside, and the respective negatives of the outer flow fields form the internal cavities. In one particular embodiment, the disc parts are, in particular, formed identically, that is to say they are provided with mutually corresponding channel structures such that, when the stack is pressed together, no damage occurs to the membrane electrode assembly, for example by shearing off as a result of channel-web geometries of adjacent or mutually engaging bipolar plates being pressed onto one another. In this case, the respective channel structure is formed for example by introducing beads with a width of 0.5 mm to 3 mm and with a depth of 0.1 mm to 2 mm into the disc parts. The beads may have a meandering profile. The flow field is formed by these beads and by webs which are located between two beads and connect the respective inlet and outlet on different paths. Any other structural forming can also be provided, in order to form a flow channel. The bead shape is particularly simple and cost-effective to manufacture in the case of a disc part in the form of a metal sheet.
For reaction effectiveness that is as sufficiently good as possible, the inlets and outlets provided for the flow fields are arranged at the edges and opposite one another, in particular diagonally opposite one another, and in particular are incorporated in the large disc part.
The advantages achieved by the invention are, in particular, that the disc parts of a bipolar plate which are joined in a functionally tolerant manner on the inside with regard to the capability for fluid and/or gas to pass through mean that there is no need for extensive leak testing of the joint bead. For sealing which is sufficiently good towards the outside and separates the two reaction media from one another, the external dimensions of one of the two disc parts are smaller than the other disc part, and it is embedded in a completely surrounding sealing element. Furthermore, material is saved by the disc parts being of different size. In addition, this makes it possible to provide a bipolar plate which is particularly compact and can be produced variably for a bipolar fuel cell stack and which, in turn, can be produced at particularly low cost because of the considerably reduced number of leak tests.
According to a further fundamental idea of the invention, a bipolar fuel cell stack comprises at least one bipolar plate, a sealing element and at least one electrolyte unit, in particular a membrane electrode assembly, with the bipolar plate comprising two disc parts which are electrically conductively connected to one another, a first disc part of which at least partially covers a second disc part in the stacking direction of the fuel cell stack, and is arranged in an intermediate space between the electrolyte unit and the second disc part, with the second disc part being provided with one or more inlets and/or outlets, which are in the form of apertures, for a first reaction medium, and with the sealing element sealing at least one or all of the apertures from the intermediate space with the first disc part. Once again, this means that there is no longer any need for sealing between the first and the second disc part in some circumstances, so that the bipolar plates no longer need to be leak-tested in their own right before being stacked to form the fuel cell stack.
It is particularly preferable for the sealing element to touch a part of the circumferential rim, in particular the entire circumferential rim of at least one or all of the apertures. This likewise contributes to it no longer being absolutely essential to carry out leak testing on the bipolar plate itself.
Exemplary embodiments of the invention will be explained in more detail with reference to a drawing, in which:
Mutually corresponding parts are provided with the same reference symbols in all the figures.
In detail, the two disc parts 2.1 and 2.2 each have at least one channel structure 4.1 and 4.2 which forms at least one associated flow field F1 and F2 for reaction media, for example hydrogen and oxygen, or a methanol-water mixture and oxygen. The flow fields F1 and F2 are located on the outside, that is to say on the outside facing away from the respective other disc part 2.1 or 2.2. In this case, the flow field F1 of the first half shell or of the first disc part 2.1 has a first reaction medium flowing around it on its outside facing away from the second disc part 2.2, while the flow field F2 of the second disc part 2.2 has a second reaction medium flowing around it on the surface which faces away from the boundary surface to the first disc part 2.1, and in the area of the first disc part 2.1 (frame R) which surrounds the second disc part 2.2. The channel structures 4.1 and 4.2 are introduced into the disc parts 2.1 and 2.2, respectively, by forming, for example by introduction of beads. The beads run essentially parallel to one another in the longitudinal direction and transverse direction of the disc parts 2.1 and 2.2. The negatives of the outer flow fields F1 and F2 form the internal cavities. In one particularly preferred embodiment, the flow field F1 of the smaller disc part 2.1 has fuel flowing over it, for example a methanol-water mixture, and the flow field F2 of the larger disc part 2.2 has an oxidizer, for example oxygen from the air, flowing over it.
In order to form the bipolar plate 1, the two disc parts 2.1 and 2.2 are joined to one another at least in places in the rim area located on one another, and to the inside at least in places in a manner which allows fluid and/or gas to flow through, forming internal cavities. This means that there is no need for leak testing of the joint bead of the bipolar plate 1, as a result of the process of joining the two half-shells. Since the leak testing costs for a single bipolar plate 1 are not inconsiderable, but the bipolar plate size has only a secondary effect on the test time, this is particularly advantageous for small-format bipolar plates, for example bipolar plates with an active cell area of less than 200 cm2. In a further embodiment, the disc part 2.1 is formed in the area of the transition element 8.1 so as to essentially avoid significant transport of the reaction medium flowing over the disc part 2.1 on the side facing away from the membrane electrode assembly, that is to say in the cavities between the disc parts 2.1 and 2.2. This can be achieved by the disc part 2.1 being shaped in the inlet-flow area by means of geometries which engage in one another in a similar manner to the structure in FIG. 1 et seqq. in the prior German application DE 10 2006 037 353.7 or in the form of a carton-of-eggs configuration such that no significant medium transport takes place between the disc parts 2.1 and 2.2.
Furthermore, the two disc parts 2.1 and 2.2 are of different size. In this case, the external dimensions of the disc part 2.1 are smaller than those of the disc part 2.2. This considerably reduces the amount of the stainless steel required for production. In particular, the external dimensions of the disc part 2.1 correspond to the external dimensions of the associated flow field F1. The external dimensions of the larger disc part 2.1 are formed by the external dimensions of a frame R which surrounds the associated flow field F2.
In order to supply the reaction media and to carry them away,
Transfer elements 8.1 and 8.2 in the form of bridging elements are provided between the respective channel structures 4.1 and 4.2 in order to transfer the reaction media from the respective inlet 6.1 and outlet 6.2 into and out of the relevant channel structure 4.1 and 4.2. The transfer of the reaction media from or to the ports 6.1 and 6.2 respectively into and out of the respective active cell surfaces, that is to say into or out of the flow fields F1 and F2, takes place by means of the transfer elements 8.1 and 8.2 which, for example, may be integrated in an insert seal, may be directly inserted, or may be directly connected to one of the two half-shells. In a further particularly preferred embodiment, the transfer elements 8.1 and 8.2 are fitted directly to the respectively bridged disc part 2.1 or 2.2, to be precise in the area of the larger disc part 2.2 as a flap inlay, and in the area of the smaller disc part 2.1 as an extension in the port direction.
Furthermore, the bipolar plate 1 is completely sealed on the outside by two sealing elements 10.1 and 10.2, for example an elastomer seal, so that there is no need for any sealed joint bead between the two disc parts 2.1 and 2.2. Furthermore, the sealing elements 10.1 and 10.2 separate the reaction media from one another. If one reaction medium enters the cavities created by the joining process between the first and second half-shells, this is irrelevant for operation and operational reliability of the fuel cell. In this case, it is necessary to prevent the two reaction media from entering at the same time. In one particularly preferred embodiment, the sealing elements 10.1 and 10.2 which are required for effective sealing of the fuel cell and of the bipolar plate 1 from the outside are sprayed in a manner which will not be described in any more detail onto adjacent membrane electrode assemblies. The process of spraying the sealing elements 10.1 and 10.2 onto the membrane electrode assembly reduces the number of leak-testing steps required for the fuel cell stack structure from three (1×bipolar plate, 1×membrane electrode assembly, 1×fuel cell stack) to two (1×membrane electrode assembly, 1×fuel cell stack), thus leading to a corresponding cost saving.
In a further embodiment, the disc part 2.1 may be provided in a manner which is not described in any more detail with a number of apertures such that the cavities between the two disc parts 2.1 and 2.2 are filled with reaction medium. In the event of a disturbance in the supply of reaction medium, the medium which is stored there can act as a buffer, and can maintain the fuel cell reaction by diffusion processes of the reactant from the cavity into the reaction area, for a limited time.
The channel structure is connected, in a diagonally opposite form that is not illustrated in any more detail, to an inlet for supplying the reaction medium. The external dimensions of the disc part 2.2 in this case correspond to the external dimensions of the frame R which surrounds the channel structure 4.2 with the flow field F2, and the recesses for the inlet 10.1 and the outlet 10.2.
In further preferred embodiments, the respective bipolar plate 1 is sealed from the outside and provides a seal between the reaction media by means of an elastomer seal which is sprayed onto the bipolar plate 1. In one alternative embodiment, the respective bipolar plate 1 may be sealed from the outside and may form a seal between the reaction media by means of an elastomer seal which is sprayed onto the membrane electrode assembly and may have sealing areas of different height.
Seen from the port area, that is to say from the inlets 6.1 and the outlets 6.2, the channel structure 4.1 for the flow field F1 of the smaller disc part 2.1 can start immediately after the sealing element 10.1, which is not shown in any more detail in this
In order to allow the disc part 2.2 with the larger external dimensions to be sealed sufficiently well both on its upper face (see
In order to position the sealing element 10.1, 10.2 before fitting, it is particularly advantageous to provide one sealing element 10.2 with a T-shaped cross section, and the other sealing element 10.1 with a U-shaped cross section, on the side facing the disc part 2.2. (For the sake of simplicity, the sealing elements 10.1, 10.2 illustrated in
If the inlay metal sheet or smaller disc part 2.2 is “raised”, for example by means of the supporting elements 16, that is to say between the zero level of the base sheet or inlay sheet, this results in a certain distance, and the sealing element 10.1 can be placed directly over the smaller disc part 2.1 to be inserted, because the sealing web 18 and the upper face of the disc part 2.1 lie on one plane. A transfer element, for example a flap inlay, which would otherwise be required for this port can therefore in each case be saved in the inlet ports 6.1 for the reaction medium which is intended to flow over the disc part 2.1. The vertical distance between the large disc part 2.2 (=surface of the base sheet) and the lower face of the small disc part 2.1 (=inlay sheet) is at the same time the height of the flow cross section in the constriction between the inlet 6.1 and the flow field F1.
| Number | Date | Country | Kind |
|---|---|---|---|
| 10 2006 054 849.3 | Nov 2006 | DE | national |
