Electrochemical system

The present invention relates to an electrochemical system, as well as to a bipolar plate for use in such a system.

The electrochemical system may for example be a fuel cell system or an electrochemical compressor system, in particular an electrolyser with which, by way of applying a potential, apart from the production of hydrogen and oxygen from water, these gases are simultaneously compressed under pressure. Apart from this, electrochemical compressor systems such as electrochemical hydrogen compressors are also known, to which gaseous, molecular hydrogen is supplied and in which this is electrochemically compressed by applying a potential. This electrochemical compressing lends itself in particular for small quantities of hydrogen to be compressed, since a mechanical compression of the hydrogen here would require significantly more effort.

Electrochemical systems are known, with which an electrochemical cell stack is constructed with a layering of several electrochemical cells, which in each case are separated from one another by bipolar plates. With this, the bipolar plates have several tasks:

the electrical contacting of the electrodes of the individual electrochemical cells (e.g. fuel cells) and conveying the current further to the adjacent cell (series connection of the cells),

the supply of the cells with media, i.e. reaction gases, and the removal of reaction products via a channel structure, which is arranged in an electrochemically active region (gas distribution structure/flowfield),

the further conveying the waste heat arising during the reaction in the electrochemical cell, as well as

the sealing of the different media channels or cooling channels against one another and to the outside.

For the supply and removal of media from the bipolar plates to the actual electrochemical cells, these e.g. are MEAs (membrane electrode assembly) with a gas diffusion layer in each case orientated towards the bipolar plates (e.g. of a metal non-woven or carbon non-woven), and the bipolar plates may have openings for cooling, or for the supply and removal of media.

With known bipolar plates, the gas distribution along the MEA or the gas diffusion layer is effected via channel structures or meander structures on both sides of the bipolar plate.

It is known, in particular for metallic bipolar plates, to stamp channel structures into these bipolar plates and thereby also to stamp a boundary wall at the same time unitary with the bipolar plate, which surrounds the electrochemically active region. The boundary wall thereby often has a bead-like shaping. Now it has been found in series of trials that such bipolar plates may display large fluctuations with regard to their performance, which appear to originate from an insufficient distribution of media.

It is therefore the object of the present invention to provide an electrochemical system or a bipolar plate which do not have fluctuations of performance due to insufficient distribution of media in the electrochemically active region.

This object is achieved by the subject-matters of the independent claims.

Firstly, this is an electrochemical system consisting of a layering of several cells which are in each case separated from one another by bipolar plates, wherein the bipolar plates comprise openings for cooling or for the removal and supply of media to the cells, and the layering may be set under mechanical compressive stress, wherein at least one cell comprises an electrochemically active region which is surrounded by a boundary wall of the bipolar plate, and a channel structure of the bipolar plate for the uniform distribution of media is provided within the electrochemically active region, wherein at least one gas diffusion layer is provided for micro-distribution of the medium. Limitation elements are provided in the border region between the channel structure as well as the boundary wall, for avoiding media flow between the channel structure and the boundary wall. Thereby, the gas diffusion layer covers the channel structure and/or at least parts of the limitation elements.

The bipolar plate for use in an electrochemical system according to the invention is characterised in that this bipolar plate has a base plane, and a channel structure (flowfield) projecting from this base plane, as well as openings for the supply and removal of media are provided, and the channel structure as well as the openings are surrounded by a boundary wall (under certain circumstances provided with openings for leading through fluid) and that at least one limitation element for preventing media bypassing in the border region between the boundary wall and the channel structure is provided in the region between the boundary wall and the outer edge of the channel structure.

Thus a bypass of media at the channel structure is largely prevented with the limitation elements according to the invention. A uniform distribution of media over the channel structure

is achieved by way of this, and the undesired performance fluctuations are eliminated in this manner.

Thereby, it is particularly advantageous if the gas diffusion layer not only covers the channel structure, but also at least parts of the limitation elements. On account of this, an additional compression of the gas diffusion layer occurs in this region, which is greater than the compression in the region of the "normal" channel structure. Thus a bypass of medium in the questionable region between the channel structure and the boundary wall is therefore prevented in an even greater manner.

Advantageous further embodiments of the present invention are described in the dependent claims.

One advantageous further embodiment envisages the height of the limitation elements being selected in a manner such that the compression of the gas diffusion layer in the contact region to the limitation elements being greater than in the contact region to the channel structure. Thus a particularly good sealing in this border region is achieved. A further advantageous embodiment envisages the limitation elements being designed as extensions of the channel structure, which merge into the boundary walls. This is particularly advantageous with metallic bipolar plates, since thus a combined and unitary embossing of the limitation elements and elements of the channel structure is possible. Basically, it is possible to provide one or more limitation elements. With several limitation elements, these are preferably distanced to one another and particularly preferably here two adjacent limitation elements together with the boundary wall in each case form chambers between the channel structure and the boundary wall, so that bulkheads are formed (as with a freight ship), in order to prevent the bypass as securely as possible.

The repetition distance of individual limitation elements thereby is preferably greater than 2 mm, particularly preferably greater than 5 - 10 mm (with smaller distances, the boundary wall would be mechanically weakened far too much). As an alternative, one could also say that here at least one limitation element is to be provided for 100 mm length of the boundary wall, preferably five to twenty five limitation elements. One further advantageous design envisages the boundary wall running in a serpentine manner, thus making the boundary wall mechanically stronger. With a serpentine course of the boundary wall, in each case the portion of the boundary wall which lies closest to the channel structure may be connected to the channel structure via a limitation element.

It is advantageous, with regard to the limitation elements which are to prevent the bypass between the boundary wall and the channel structure, for these to run essentially transversely to

the boundary wall as well as essentially transversely to the outermost elements of the channel structure.

The constructional shape of the limitation elements is moreover dependent on the respective design of the channel structures. If the channel structures for example are provided as individual elements, then the limitation elements may also be provided in a linear manner, in order to avoid a bypass. In the other case, these limitation elements are also to be provided as individual elements.

A further advantageous embodiment envisages the boundary wall having a greater height with respect to a base plane of the bipolar plate, than the predominant elevation of the channel structure in the vicinity of the boundary wall with respect to this base plane. Vicinity hereby is to be understood as a distance to the boundary wall of maximal 1 cm.

The limitation elements of the bipolar plate should at least have the height of the predominant elevation of the channel structure starting from the base plane. This means that they should therefore preferably have the same height as the channel structure or have a height between the height of the channel structure and the height of the boundary wall.

The limitation elements are preferably provided as embossings in a (preferably metallic) bipolar plate.

A further advantageous design envisages a bipolar plate being constructed of two plates, wherein the at least one limitation element is hollow on the side of the first plate which is distant to the electrochemically active side, and this hollow space acts as a complementary space for inserting the second plate of the bipolar plate.

The boundary walls preferably have the shape of a bead, in particular a full bead or a half bead and thus are an integral component which is unitary with the bipolar plate, but however attachment parts are also possible here.

One particularly advantageous embodiment envisages the openings of the bipolar plate for cooling or for the removal or supply of media, being provided with elastic bead arrangements, wherein these bead arrangements comprise openings for conducting fluid or gaseous media into a hollow space of the bipolar plate, or to the electrochemically active region.

Preferably, the conducting of media through the electrochemically active region is effected in a manner such that the location of the introduction of the medium and the location of the leading-out of the medium is effected at respective maximally distanced points of the

electrochemically active region. By way of this, the basic requirements for obtaining a distribution which is as plane as possible are created, and thereby a meandering leading is useful, even if dead-end layouts are possible. With such ones however, a flow resistance within the electrochemically active region is always to be overcome, so that the medium always seeks "short cuts" or bypasses. For this reason, the present invention with the limitation elements is particularly useful here.

Further advantageous embodiments of the present invention are specified in the remaining dependent claims.

The invention is now explained with the help of several figures. These show:

Fig. Ia to Ic the construction of a fuel cell stack,

Fig. 2a and 2b plan views of differently designed bipolar plates,

Fig. 3a and 3b cross sections through a bipolar plate arrangement according to the invention (Fig. 3a) and a bipolar plate arrangement according to the state ofthe art (Fig. 3b),

Fig. 4 curves of the cathode-side volume flow against back-pressure, with and without limitation elements,

Fig. 5 a plan view of a further embodiment of a bipolar plate with limitation elements,

Fig. 6 a illustration of a centering according to B-B from Fig. 5,

Fig. 7 flow resistance curves of electrochemical systems according to the invention, with different compressions.

Fig. Ia to Ic show the basic construction of an electrochemical system in the form of a fuel cell stack 1. This comprises a layering of several fuel cell arrangements 12 (see Fig. Ib). The layering of these fuel cell arrangements 12 is held together by end plates which e.g. via clamping bolts as shown in Fig, Ic, apply a compressive stress to the layering of the fuel cell arrangements.

The construction of a fuel cell arrangement 12 is explained in more detail hereinafter.

Fig. Ia shows the inner construction of a fuel cell arrangement 12 in the form of an exploded drawing. This is firstly a cell (for example a fuel cell) 2, which comprises a polymer membrane which is capable of conducting ions and which at least in an electrochemically active region 6 has a catalytic layer on both sides. Moreover, two bipolar plates 3 are provided in the fuel cell arrangement 12, between which the fuel cell 2 is arranged. Moreover, a gas diffusion layer 9 is arranged in the region between each bipolar plate and the adjacent fuel cell 2. A bead which is not shown and which is essentially peripheral in the edge region of the bipolar plates, forms a boundary wall and thus ensures the sealing of the electrochemically active region 6, so that no cooling fluid or media may exit to the outside from this region or vice versa.

Moreover, the bipolar plates 3 contain supply openings (interface channels) which are aligned to each other. On the one hand this is an opening 4 for leading through cooling fluid, wherein this opening is surrounded by a further bead arrangement. Moreover, an opening 5 for the supply and removal of media to the electrochemically active region is provided, which is limited by a further bead arrangement. Moreover, passage openings are provided for clamping bolts which are not shown in Fig. Ia.

Fig. 2a shows a plan view of a section of a bipolar late according to the invention. Here, an opening for the supply and removal of media 5, which is surrounded by an annular-shaped full bead is shown. This full bead or bead arrangement comprises openings 5.1 for leading through fluid or gaseous media into a hollow space of the bipolar plate or towards the electrochemically active region. The shown bipolar plate 3 is of metal, wherein the channel structure 8 and the boundary wall 7 are designed as embossings unitary with the bipolar plate 3.

Here, only the upper left corner of the bipolar plate is shown in Fig. 2a for illustration. The leading of media through the electrochemically active region 7 is however effected in a manner such that the location of the introduction of the medium and the location of the leading- out of the medium are positioned at points of the electrochemically active region maximally distanced to each other, preferably at the plane diagonals of the surface plane as it is shown in Fig. 2a.

The course of the boundary wall 7 is shown in a serpentine manner in Fig. 2a, at least in the section which is shown at the top on the right in Fig.2a.

Moreover, it is shown that with a serpentine course of the boundary wall 7, a portion of the boundary wall which lies closest to the channel structure 8, is connected via a limitation element 10 to the channel structure.

Here, one may also deduce that the limitation element 10 runs essentially transversely to the boundary wall 7 or also essentially transversely to the elements of the channel structure 8 which are closest to the boundary wall (outermost)

It may also be deduced from Fig. 2a that the limitation element 10 is designed as an extension of the channel structure 8, which merges into the boundary wall 7.

Fig. 2b shows an alternative embodiment of the bipolar plate shown in Fig. 2a.

In contrast to the bipolar plate shown in Fig. 2a, here however several limitation elements 10 are provided. These are distanced to one another, so that two adjacent limitation elements 10 in each case form chambers between the channel structure 8 (thus the outermost elements of the channel structure) and the boundary wall 7.

The repetition distance of individual limitation elements hereby is preferably greater than 2 mm, particularly preferably greater than 5 - 10 mm.

It is therefore evident that limitation elements 10 are provided in the embodiments shown in Fig. 2a or Fig, 2b, which prevent a bypass (thus a shortcut) of medium between the boundary wall 7 as well as the outermost elements of the channel structure 8. This is designed in Fig. 2a as a single transverse web, in Fig. 2b as a multitude of transverse webs which then form corresponding chambers. It is important that these limitation elements 10 assume this function, thus are not designed as supply beads or inlets to cooling channels or media channels.

In this manner, the flow of medium which for example proceeds from the media supply opening 5 respectively 5.1, is forced through the channel structure 8 which is designed in a meandering manner, and this causes an increased backpressure which is thus also an indicator of greater reaction rates of media in the fuel cells.

Fig. 3a shows a cross-sectional view of a construction, which shows two bipolar plates 3 (for example according to Fig. 2a or Fig. 2b). Here, a fuel cell or a polymer electrolyte membrane (PEM) 2 is arranged between two bipolar plates 3. Moreover, a gas diffusion layer 9 is arranged on each side of the PEM 2, in the electrochemically active region. This gas diffusion layer may be premanufactured and be designed as a direct integral component of a membrane electrode assembly, and the gas diffusion layers may also be provided as separate layers.

What is significant is that the height of the limitation elements 10 is selected in a manner such that the compression of the gas diffusion layer 9 in the contact region to the limitation

elements 10 is greater than in the contact region to the channel structure 8. This is also indicated in an illustrated manner in Fig. 3a by the narrower hatching.

It is also to be seen that the boundary wall 7 has a greater height with respect to a base plane 1 1 of the bipolar plate 3 than the predominant elevation of the channel structure with respect to this base plane (this is indicated by the double arrows in Fig. 3a). It is likewise evident that the limitation elements 10, starting from this base plane 1 1 of the bipolar plate 3, have at least the height of the greatest elevation of the channel structure 8, however at the most the height of the boundary wall 7 with respect to the base plane (this also is evident by the double arrows in Fig. 3a).

In contrast to this, Fig. 3b shows an arrangement which has no limitation elements and with which an additional compression of the gas diffusion layer in the outer edge region is not given.

The figures which were referred to until now, in particular the Fig. Ia to Ic, 2a, 2b as well as 3a, thus show a bipolar plate 3, wherein this comprises a base plane 11, and a channel structure 8 projecting from this base plane, as well as openings 5 for the supply and removal of media are provided, and the channel structure as well as the openings are surrounded by a boundary wall 7, and at least one limitation element 10 is provided in the region between the boundary wall and the outer edge of the channel structure, for preventing medium from bypassing in the border region between the boundary wall 7 and the channel structure 8.

Thus what is shown in the previously mentioned figures is also an electrochemical system 1 consisting of a layering of several cells 2 which in each case are separated from one another by bipolar plates 3, wherein the bipolar plates comprise openings for cooling 4 or the removal and supply 5 of operating media to the cells, and the layering may be set under mechanical compressive stress, wherein at least one cell comprises an electrochemically active region 6 which is surrounded by a boundary wall 7 of the bipolar plate, and a channel structure 8 of the bipolar plate is provided within the electrochemically active region for a uniform distribution of media, wherein at least one gas diffusion layer 9 is provided for the micro- distribution of medium, and limitation elements 10 are provided in the border region between the channel structure as well as the boundary wall, for avoiding the fluid bypassing between the channel structure and the boundary wall in the electrochemically active region (thus not the cooling region), and the gas diffusion layer covers the channel structure and/or at least parts of the limitation elements. Thus a clamping or a strong pressing of the gas diffusion layer in the edge region is achieved on account of this covering, and an even better sealing occurs on account of this, since not only the height of the boundary wall, but also the compression of the gas diffusion layer in this region ensures a prevention of the bypass (flowing-past/shortcut).

Fig. 4 shows the diagram of a volume flow in litres per minute against the back-pressure in millibars, for an electrochemical system.

The left graph shows a conventional design on the cathode side (for example with a cross section according to Fig. 3b), with which a flow of medium bypassing the gas diffusion layer is possible. The right graph shows a compression of the gas diffusion layer with limitation elements, so that a bypass is prevented or limited by way of this. It is shown here that with the same volume flow, a much greater back-pressure is present. This is an indication that the medium does not simply pass without being led through the electrochemically active field. By way of this a constant reaction is forced since the reaction medium no longer bypasses (flows past) in a non-used manner.

Fig. 5 shows a further embodiment of a bipolar plate according to the invention. Here, channel structures 8 are provided in the electrochemically active region 6 which is surrounded by a boundary wall, which are mainly designed as disjunct, thus individual raised elements. Here, it is again the case of a bipolar plate with a flowfield (electrochemically active region), with which the reaction medium is led from the top left in a diagonal manner to the bottom right (exit 5 there). Limitation elements 10 are provided at two locations (bottom left and top right), which prevent an undesired bypass.

Fig. 6 shows a cross section through B-B of the plate arrangement of Fig. 5 in an enlarged scale. Here, it is to be seen that there a bipolar plate is constructed of two plates, wherein the at least one limitation element 10 is hollow on the side which is distant to the electrochemically active side, and this hollow space is provided as a complementary space for inserting the second plate of the bipolar plate. In this manner, an additional centering of both plates is carried out, so that the dimensional accuracy of the complete bipolar plate is increased by way of this.

Fig. 7 shows (similarly as it has been shown already above in Fig. 4) the volume flow of (dry) air in litres per minute, against the back-pressure (in millibar). Here, one may also see that with increasing compression values (compressive stress) in the complete assembly (see Fig. Ic) and with an equal volume flow of air, a significantly increased back-pressure arises and that in this manner uniformly reproducible values can be set.