WO2003032421A2 - Flow module and fuel cell having said flow module - Google Patents

Abstract

The invention relates to a flow module for a fuel cell comprising an polymer electrolyte membrane (PEM), said module supporting compensation of partial water vapor pressure using simple means thereby countering the drying of areas in the membrane even when the process gases fed to the fuel cell are only slightly humid or not humid at all. The flow module comprises at least one inlet (1) and at least one outlet (2) for a gas that is to be guided by means of an electrode of the fuel cell. The surface of the flow module that faces the electrode is structured in such a way that at least one channel (3) connecting the inlet (1) and the outlet (2) is formed between the flow module and the electrode. According to the invention, the at least one inlet (1) and the at least one outlet (2) are arranged and the at least one channel (3) is guided in such a way that the segments of the channel having a higher relative humidity are arranged in the vicinity of the segments of the channel having a lower relative humidity so that the humidity gradient on the PEM is balanced out.

Description

DaimlerChrysler AG FTP / U-RUECKERTS

Flow module and fuel cell with such a flow module

The invention relates to a flow module for conducting a gas over at least one electrode of a fuel cell. Furthermore, the invention relates to a fuel cell equipped with such a flow module.

The fuel cell comprises a polymer electrolyte membrane (PEM), on the one hand the cathode and on the other hand the anode of the fuel cell is arranged. The flow module is positioned over one of the electrodes. It includes at least one inlet and at least one outlet for the gas. In addition, the surface of the flow module facing the electrode is structured such that there is at least one channel connecting the inlet and the outlet between the flow module and the electrode. The longer the gas flows over the electrode, the more the moisture level of the gas changes.

The membrane of a PEM fuel cell must have a certain moisture content in order to ensure the proton conductivity required to operate the fuel cell. For this reason, the process gases are usually humidified before they are introduced into the fuel cell. Humidification involves considerable technical effort, especially if the water required for this is separated from the process gases in an autonomous system. The option of water storage in frost complicates and increases the cost of the humidifier. Fuel cells are also known which are equipped with flow modules made of a porous material at least on the cathode side. As a result, the water vapor partial pressure is compensated via the flow module, so that unhumidified process air can also be supplied to such fuel cells. At the cathode inlet, the water vapor partial pressure is low due to the unsaturated inflowing air flow. The air flow is gradually enriched with water due to the product water that forms along the cathode channel, which increases the water vapor partial pressure. At the cathode outlet, the water vapor partial pressure has generally risen to such an extent that water condenses out and is absorbed by the porous material of the flow module. The water gets into the dry area of the cathode entrance by diffusion. This prevents the membrane from drying out in this area. On the other hand, the air stream entering at the cathode inlet is humidified.

The known flow modules made of porous material, which are realized in the form of bipolar plates, prove to be problematic in several respects. In addition to the relatively high costs of such a flow module, the porosity of the material also increases the thickness of the bipolar plates compared to the commonly used bipolar plates and thus the overall space requirement of the fuel cell. In addition, porous bipolar plates are relatively sensitive to high cell temperatures, as can be expected when the cooling system of the fuel cell is downsized.

With the present invention, a flow module for a fuel cell is proposed which supports a compensation of the water vapor partial pressure in the flow module with simple means and thus counteracts a region-wise drying out of the membrane, even if the process gases supplied to the fuel cell are only slightly or not at all humidified. This is achieved according to the invention in that the at least one inlet and the at least one outlet are arranged and the at least one duct is guided such that duct sections with a higher relative humidity are arranged in the vicinity of duct sections with a lower relative humidity, so that the moisture gradient over the PEM compensates.

According to the invention, it has been recognized that the water vapor partial pressure cannot be compensated exclusively via the flow module but also via the membrane structure, in particular if the moisture gradient is sufficiently large. In order to favor the equalization of the water vapor partial pressure in this way, it is proposed according to the invention to choose the arrangement of the inlets and outlets and the channel routing so that the highest possible moisture gradient in the areas of the membrane particularly affected by dehydration, for example in the area of the inlets exists between adjacent channel sections.

In an advantageous variant of the flow module according to the invention, at least one channel section adjoining an outlet is guided into the area of at least one inlet and in the vicinity of the channel section adjoining this inlet. As already explained in connection with the prior art, the water vapor partial pressure at the channel inlet is low due to the unsaturated inflowing gas stream. The gas flow is gradually enriched with water by the electrolytic reaction on the membrane along the channel, so that the water vapor partial pressure may even rise to such an extent that water condenses in the area of the channel outlet. In the variant of a flow module according to the invention in question here, the water then reaches the dry area of the adjacent channel inlet by diffusion in the membrane structure. The flow module according to the invention can be designed so that a compensation of the water vapor partial pressure takes place between channel sections of a channel (type 1), and also that a compensation of the water vapor partial pressure takes place between channel sections of several channels (type 2).

In an advantageous variant of the flow module of the first type according to the invention, the channel is led from the inlet area into the outlet area, at least once again back into the inlet area and then back into the outlet area, so that channel sections with a higher relative humidity in the vicinity of channel sections with a lower one relative humidity are arranged.

In a flow module of the second type, at least two channels are designed such that sections with a higher relative humidity of one channel are arranged in the vicinity of sections with a lower relative humidity of the other channel. In this context, it is advantageous if the inlet and the outlet of at least two channels are arranged close to each other.

It is sensible to limit the number of inlets and outlets, both for manufacturing reasons and to simplify the connection situation. In this case, an arrangement according to the invention of duct sections with a higher relative humidity in the vicinity of duct sections with a lower relative humidity can be realized simply by splitting the individual ducts into several duct branches behind the inlet region and merging the duct branches of a duct in the outlet region become. It is possible to arrange the channel sections so that they intersect, ie that they are guided one above the other, which is a preferred embodiment of the present invention. The flow module according to the invention can easily be implemented in the form of a bipolar plate, like the flowfields known from practice. The compensation of the water vapor partial pressure can additionally be supported in that at least parts of the flow module are formed from a water vapor and / or liquid water-conducting material, such as a porous material, so that in addition, a compensation of the water vapor partial pressure, for example by capillary forces, also takes place via the flow module , he follows.

The compensation of the water vapor partial pressure, which is supposed to take place via the membrane according to the invention, can be favored by a suitable structure of the membrane. It proves to be advantageous if the structure of the PEM comprises at least one layer that conducts water vapor and / or at least one layer that conducts liquid water. Finally, it should also be mentioned that the compensation of the water vapor partial pressure can also take place via a gas diffusion layer arranged between the PEM and the electrode.

As already discussed in detail above, there are different possibilities for advantageously designing and developing the teaching of the present invention. For this purpose, reference is made on the one hand to the claims subordinate to claims 1 and 10 and on the other hand to the following description of several exemplary embodiments of the invention with reference to the drawings.

Figures 1 and 2 each show an inventive flow module of the first type in plan view.

Fig. 3 shows a first variant of a flow module of the second type according to the invention in plan view.

Fig. 4 shows a second variant of a flow module of the second type according to the invention in plan view. FIG. 5 shows a further flow module of the second type according to the invention in plan view with channels that split and intersect.

6 shows a section through a flow module in the form of a bipolar plate with intersecting channels.

All of the flow modules shown in FIGS. 1 to 6 serve to conduct a cathode gas, for example air or oxygen, via the cathode of a fuel cell with a polymer electrolyte membrane (PEM). The flow modules are realized in the form of bipolar plates that are positioned over the cathode. Each of the flow modules shown comprises at least one inlet 1 and at least one outlet 2 for the cathode gas. In addition, the respectively shown surface of the flow module facing the cathode is structured such that there is at least one channel 3 connecting the inlet 1 and the outlet 2 between the flow module and the cathode. In all of the flow modules shown, the at least one inlet 1 and the at least one outlet 2 are arranged and the at least one channel 3 is guided in such a way that channel sections with a higher relative humidity due to the product water formed on the cathode side during the electrolyte reaction in the vicinity of Channel sections are arranged with a lower relative humidity. As a result, the water vapor partial pressure is compensated for via the PEM.

The flow modules 10 and 20 shown in FIGS. 1 and 2 each comprise only an inlet 1 arranged on one side of the bipolar plate and an outlet 2 arranged on the opposite side of the bipolar plate. In the case of FIG. 1, four channels 3 are formed, which the Connect inlet 1 to outlet 2. The variant shown in FIG. 2 comprises five such channels 3. In both cases, channel sections with a higher relative humidity and channel sections are cuts with a lower relative humidity of the same channel arranged side by side. For this purpose, the channel sections in the outlet area are led back locally into the inlet area, so that these channel sections lie in the immediate vicinity of channel sections in the inlet area. In the variant shown in FIG. 2, the channels 3 are led from the inlet area into the outlet area, back again into the inlet area and then again into the outlet area, while the channels 3 in the variant shown in FIG. 1 run twice from the inlet area into the outlet area and are brought back again. In addition, in both cases, the duct sections of the individual ducts 3 adjoining the outlet 2 are each arranged next to the duct sections of the adjacent duct 3 adjoining the inlet 1, so that duct sections with a higher relative humidity of a duct 3 in the vicinity of Channel sections with a lower relative humidity of the adjacent channel 3 are arranged.

The variant of a flow module shown in FIG. 3 also

30 comprises only one inlet 1 arranged on one side of the bipolar plate and one outlet 2 arranged on the opposite side of the bipolar plate. Between the inlet 1 and the outlet 2 there are five channels 3 of equal length, which ensure uniform distribution of the cathode gas and essentially one Make a straight line connection between inlet 1 and outlet 2. However, each channel 3 comprises a deflection area 31 with six deflections. The deflection areas 31 of adjacent channels 3 are arranged offset from one another, so that each channel 3 in its deflection area

31 compared to the or the adjacent parallel channels 3 is extended. Due to the greater length of the corresponding channel section, a larger amount of process water is formed in the deflection area 31 of a channel than in the corresponding channel section of an adjacent channel 3 which is guided in parallel. Accordingly, the deflection areas 31 in the exemplary embodiment shown here provide channel sections with a ner higher relative humidity, while the parallel duct sections adjacent to the deflection regions 31 have a lower relative humidity, so that here a compensation of the moisture content takes place between these adjacent duct sections of the flow module 30.

The flow module 40 shown in FIG. 4 comprises six inlets 1 and six outlets 2, which are arranged on two opposite sides of the bipolar plate. Inlets 1 are connected via straight-line channels 4 and 5 to the corresponding outlets arranged on the opposite side of the bipolar plate. In the variant shown here, an inlet 1 and an outlet 2 are arranged alternately on each side of the bipolar plate, so that the flow direction alternates between adjacent channels 4 and 5. Accordingly, the dry inlet areas of channels 4 and 5 are located directly next to the moist outlet areas of adjacent channels 5 and 4.

5 shows a flow module 50 in which adjacent channels or channel sections are likewise flowed through in the opposite direction. However, the number of inlets and outlets is reduced here compared to the flow module 40 shown in FIG. 4. Thus, only one inlet 1 and one outlet 2 are arranged on each side of the bipolar plate, the two corresponding ports, ie inlet and corresponding outlet, being arranged opposite one another. From each of the two inlets 1 there is only one channel 4, 5, which, however, then splits into three parallel channel branches. These channel branches are then brought together again in the outlet area, so that only one channel 4, 5 opens into the outlets 2. The branches of channels 4 and 5 are indicated by dots in FIG. 5. All six channel branches are arranged parallel to one another, the three channel branches of channel 4 flowing through in the opposite direction to the three channel branches of channel 5. According to the invention, the channel branches of channel 4 are always alternating arranged with the channel branches of channel 5. To do this, the individual channel branches must cross. The crossing points are marked by an arch in the channel.

FIG. 6 illustrates the implementation of such an intersection area or a corresponding intersection structure 61 in a bipolar plate 60. The top of this bipolar plate 60 is intended to form the cathode side of a fuel cell, while the underside of the bipolar plate 60 can serve as the anode side of another adjacent fuel cell. Grooves 6 are formed in the upper side which form channels for the cathode gas in connection with a PEM. Grooves 9 for an anode gas are formed in the underside symmetrically to the grooves 6. Two intersecting channels 7 and 8 lie one above the other in the intersection area. Since the crossing structure 61 extends over the entire thickness of the bipolar plate 60, there are no grooves 9 in this area in the underside of the bipolar plate 60.

All of the exemplary embodiments of the invention described above are based on the idea of compensating for the moisture gradient in the flow module via the membrane of the fuel cell. The channeling according to the invention guides the water that accumulates along the channel - especially at the end of the channel - into the vicinity of dry areas - in particular to the channel entrance. This results in a moisture balance between the non-humidified and heavily humidified areas of the flow module and the membrane, which has a positive effect on the performance of the fuel cell.

If the flow module according to the invention is used on the cathode side of a fuel cell, a non-humidified cathode gas takes up water along its path in the fuel cell due to the electrolyte reaction, diffusion and osmosis processes of the membrane structure. If duct sections with a high and a low relative humidity are close to each other, this moisture gradient can vary over the membrane structure and any gas diffusion level position. This compensation results in a uniform moistening of the membrane in the entire fuel cell, in particular also at the cathode entrance.

DaimlerChrysler AG FTP / U-RUECKERTS

reference numeral

1 entry

2 outlet

3 channel

4 channel

5 channel

6 groove (cathode gas)

7 channel

8 channel

9 slot (anode gas)

10 flow module (Fig.l)

20 flow module (Fig. 2)

30 flow module (Fig. 3) 31 deflection area (Fig. 3)

40 flow module (Fig. 4) 50 flow module (Fig. 5)

60 bipolar plate (Fig. 6)

61 crossing structure (Fig. 6)

Claims

claims 1. Flow module for conducting a gas over at least one electrode of a fuel cell, the fuel cell comprising a polymer electrolyte membrane (PEM) on which the cathode and on the other hand the anode of the fuel cell are arranged, and the flow module is positioned above the electrode, the flow module comprises at least one inlet (1) and at least one outlet (2) for the gas and the surface of the flow module facing the electrode is structured such that at least one connecting the inlet (1) and the outlet (2) between the flow module and the electrode Channel (3), and the moisture level of the gas changes the longer the gas flows over the electrode, characterized in that the at least one inlet (1) and the at least one outlet (2) are arranged in this way and the at least one channel (3) is designed so that duct sections with a higher relative humidity are arranged in the vicinity of duct sections with a lower relative humidity, so that the moisture gradient over the PEM compensates. 2. Flow module according to claim 1, characterized in that at least one duct section adjoining an outlet (2) is guided into the region of at least one inlet (1) and in the vicinity of the duct section adjoining this inlet (1). 3. Flow module (10; 20) according to one of claims 1 or 2, characterized in that the channel (3) connecting an inlet (1) and an outlet (2) from the inlet area into the outlet area, at least once again back into the inlet area and is then led back into the outlet area so that duct sections with a higher relative humidity are arranged in the vicinity of duct sections with a lower relative humidity. 4. Flow module according to one of claims 1 to 3, characterized in that at least two channels (3) connecting an inlet (1) and an outlet (2) are guided such that sections with a higher relative humidity of the one channel (3) are arranged in the vicinity of sections with a lower relative humidity of the other channel (3). 5. flow module (40; 50) according to any one of claims 1 to 4, characterized in that in each case the inlet (1) and the outlet (2) of at least two channels (4, 5) are arranged close to each other and these two channels ( 4, 5) are guided such that the sections with a higher relative humidity of the one channel (4, 5) are arranged in the vicinity of the sections with a lower relative humidity of the other channel (5, 4). 6. Flow module (50) according to one of claims 1 to 5, characterized in that the channel (4, 5) splits into a plurality of channel branches behind the inlet area and that the channel branches are brought together again in the outlet area. 7. flow module (50) according to any one of claims 1 to 6, characterized in that intersecting channel sections are provided. 8. Flow module according to one of claims 1 to 7, characterized in that it is realized in the form of a bipolar plate. 9. Flow module according to one of claims 1 to 8, characterized in that at least parts of the flow module are formed from a water vapor and / or liquid water conductive material. 10. Fuel cell with a flow module according to one of claims 1 to 9. 11. Fuel cell according to claim 10, characterized in that the structure of the PEM comprises at least one water vapor conductive layer. 12. Fuel cell according to one of claims 10 or 11, characterized in that the structure of the PEM comprises at least one liquid water-conducting layer. 13. Fuel cell according to one of claims 10 to 12, characterized in that at least one gas diffusion layer is arranged between the PEM and the electrode and that the moisture gradient between the channel sections with a higher relative humidity and the channel sections with a lower relative humidity also over the Compensates for gas diffusion.