JP2003514354A - Electrochemical fuel cell stack - Google Patents

Abstract

The present invention relates to an electrochemical fuel cell stack having at least one membrane electrode unit consisting of an anode, a cathode and an electrolyte membrane disposed therebetween, comprising: A gas inlet region, an anode gas outlet region, and at least one anode-side gas delivery structure having a channel for guiding anode gas from the anode gas inlet region to the anode gas outlet region, wherein the anode gas comprises hydrogen; A cathode gas delivery structure having an inlet region, a cathode gas outlet region, and a channel for guiding cathode gas from the cathode gas inlet region to the cathode gas outlet region, wherein the cathode gas comprises oxygen and water vapor, Medium inlet area, cooling medium outlet area And having a cooling medium delivery structure having a channel for guiding the cooling medium from the cooling medium inlet region to the cooling medium outlet region, to an electrochemical fuel cell stack. According to the invention, the coolant inlet region and the cathode gas outlet region at least partially overlap, so that condensation of water vapor in the cathode gas takes place in the region of this overlap.

Description

Detailed Description of the Invention

The present invention relates to an electrochemical fuel cell stack according to the preamble of claim 1.

Prior art fuel cell stacks include at least one, and usually a plurality of individual fuel cells, which are stacked next to each other or one above the other. Each individual battery cell is composed of a so-called membrane electrode unit, which is composed of an anode, a cathode, and a proton-conducting electrolyte membrane arranged between them, or MEA for short. On the anode side there is a gas distribution structure with channels for guiding the hydrogen-containing anode gas. On the cathode side there is yet another gas delivery structure with channels for guiding the cathode gas. The cathode gas contains, inter alia, oxygen as well as water, which water is partly generated by electrochemical reactions at the cathode. These gas distribution structures are usually realized, for example, as channel structures on the surface of metal plates. The gas distribution structure on the anode side of individual battery cells and the gas distribution structure on the cathode side of adjacent battery cells are usually formed on both sides of the same plate. In this case, it is called a pipolar plate.

There are cooling chambers inside the stack for temperature control of the fuel cells, which cooling chambers are flowed by a liquid or gaseous cooling medium. These are arranged at arbitrary positions inside the stack and inside the individual battery cells. For example,
A cooling chamber can be assigned to each individual battery cell. However, it is also possible to assign a plurality of individual battery cells to one cooling chamber.

FIG. 1 shows a fuel cell stack having convection liquid water cooling according to the prior art. For cooling the fuel cell stack, a liquid cooling medium, in particular water, flows through a cooling chamber located inside the stack. After leaving the stack, the warmed cooling water is guided to the cooler to discharge the absorbed heat. In order to keep the cooler for exhausting fuel cell residual heat as compact as possible,
These known fuel cell stacks are preferably operated with a slight temperature difference between the inlet and outlet of the cooling water of approximately 10 ° C.

These fuel cell stacks are more advantageously 60% to obtain effective efficiency.
Since it is operated at temperatures above ° C, only a very small amount of cathode water vapor is condensed inside the fuel cell stack in the known configuration. Therefore, an additional heat exchanger is arranged outside the fuel cell stack, by which the water vapor present in the cathode gas is condensed. This is used, for example, to maintain a water balance across the entire fuel cell system.

It is an object of the present invention that the major part of the water vapor present in the cathode gas is already condensed inside the stack, so that the condensation in the downstream condenser is greatly reduced or even more It is to provide a fuel cell stack that is superfluous.

The above problem is solved by the configuration described in the characterizing part of claim 1. Advantageous embodiments of the invention as well as methods for the operation of the fuel cell stack of the invention are the subject of the dependent claims.

According to the invention, the region in which the cooling medium enters the fuel cell stack and the region in which the cathode gas leaves the fuel cell stack at least partially overlap each other, so that this overlap Condensation of water vapor of the cathode exhaust gas is performed in the region. Thus, condensation of water vapor inside the stack is achieved, so that a separate condenser for the condensation can be omitted, or this separate condenser is much smaller than known systems and designed to To be done.

Important to the understanding of the invention is the fact that the basic mass conversion of the electrochemical reaction occurs in principle in the cathode gas inlet region. Due to the inventive overlap of the cooling medium inlet region and the cathode gas outlet region, a low dew point is achieved in the region of this overlap (typically below 40 ° C.).
These low dew points result in the desired strong condensation of water vapor in the cathode gas. On the other hand, in the cathode gas inlet region, i.e. where the basic conversion of the fuel cell reaction takes place, only a slight heat absorption of the cooling medium takes place.

Thus, essentially the desired condensation of water vapor is achieved without any loss of efficiency of the fuel cells. Approximately the same battery cell voltage is obtained as compared with the above-mentioned fuel cell stack of the prior art which is operated with a slight temperature difference.

Advantageously, the fuel cells of the invention are operated with a gaseous cooling medium. This gaseous cooling medium has a much lower heat capacity than commonly used cooling media such as water or Gykol. In order to obtain a larger temperature difference, it is not possible to adjust the temperature difference uniformly over each battery cell of the fuel cell stack with a cooling medium having a large heat capacity without an excessively high control cost. This is because only a very small and therefore difficult to regulate cooling medium flow is required for this. On the other hand, choosing a cooling medium with a relatively small heat capacity requires a relatively large cooling medium flow that can be adjusted much more easily to maintain the same temperature difference.

Furthermore, it is possible to adapt the heat exchange between the cathode gas and the cooling medium to various operating conditions of the fuel cell in order to optimize the condensation of the produced water inside the cell. This adaptation is carried out by locally defined means in the region of overlap of the cooling medium inlet region and the cathode gas outlet region or in a complementary manner to the other region of the battery cell. These measures are, for example, a geometrical adaptation of the channels. In this case, inter alia, the channel cross section, the number of channels per area or the arrangement of channels is spatially varied. Yet another possible geometrical adaptation concerns the control of the contact surface by means of ribs, bars, grooves or needles in the flow-through channel.

Yet another possible measure is the intended spatial use of materials with special heat-conducting properties. For example, a material having good thermal conductivity may be provided in the condensation area in the inlet area of the cooling medium and / or a material having poor thermal conductivity may be provided in other areas of the battery cell. These materials may be applied in layer form on the surface of the channels or incorporated into the carrier material itself.

By means of one of the above measures or a combination of the above measures, an increased heat exchange between the cathode gas and the cooling medium takes place, for example locally in the condensation zone of the cooling medium inlet zone, This increases the amount of water condensed over time.

[0015]   The present invention will be described in detail based on embodiments with reference to the drawings.

FIG. 1 is a known fuel cell stack as described above, FIGS. 2 to 4 are examples of the fuel cell stack of the present invention, and FIG. 5 is compared with a known fuel cell stack. FIG. 6 is a voltage-current density characteristic curve of the fuel cell stack of the present invention, FIG. 6 is an embodiment of the fuel cell stack of the present invention having a locally adapted channel geometry in a condensation region, FIG. 7 is an embodiment of a fuel cell stack of the present invention having a locally adapted use of thermally conductive / thermally insulating material, and FIG. 8 is a book having separate guides for cathode gas and cooling medium. FIG. 9 is an embodiment of a fuel cell stack of the invention, and FIG. 9 shows a fuel cell stack of the present invention in which a part of the cooling medium passes through the cooling medium distribution structure and then flows through the cathode side gas distribution structure as a cathode gas. It is an example of a cell stack.

FIG. 2 is a schematic view showing a first embodiment of the fuel cell stack of the present invention. Shown is a plate made of metal, for example, on the surface of which the cathode-side gas distribution structure for the cathode gas has been incorporated. The gas distribution structure is only shown here schematically. This gas distribution structure consists of one or more serpentine or meandering channels as is well known to those skilled in the art. Cathode gas enters the battery cell through a hole, passes through one or more flow-through channels, and exits the battery cell through a diagonally opposite hole.

The concept of fluid inlet and outlet regions in the sense of the present invention does not only mean here the direct regions of these pores, but is also actually measured along the fluid flow. It is taken to mean the perimeter in the vicinity of these holes. In this region, very little or practically no material conversion due to electrochemical fuel cell reactions is present. In the example shown, for example, the section from the last turning point of the flow-through channel to the hole belongs to the cathode gas outlet region.

As can be seen from the drawing, the cooling medium enters the battery cells in this embodiment essentially through the entire length of the sides of the plate and flows as a transverse flow to the cathode gas (the cooling medium is the back side of the plate shown). Flow in the delivery structure). The cooling medium inlet region and the cathode gas outlet region overlap each other in the main part. This overlap region where condensation occurs is surrounded by a dashed line. Ambient air is used as the cooling medium in this embodiment.

Similarly, typical temperatures for the cathode gas and the cooling medium at the inlet and outlet are shown. It can be seen that the temperature difference between the inlet and outlet of both fluids is relatively large compared to known fuel cells. The temperature difference is in the range of 30 to 45 ° C. A dew point below 40 ° C is achieved at the cathode gas outlet. This saves a separate condenser for the condensation of water in the cathode gas.

FIG. 3 shows another embodiment of the present invention. This embodiment differs from the embodiment shown in FIG. 2 only in the basically different gas distribution structure. It is configured here as a parallel gas distribution structure. Gaseous cooling medium (eg ambient air) enters the battery cells essentially through the entire length of the sides of the plate and flows in the opposite direction to the cathode gas (cooling medium in the delivery structure behind the plate shown). Flow). The cooling medium inlet region and the cathode gas outlet region overlap in the main part. This overlap region where condensation occurs is surrounded by a dashed line.

Air cooling, as illustrated in the embodiment of FIG. 2 or 3, is preferably performed via a radiator. FIG. 4 shows a corresponding embodiment for this. The radiator is arranged immediately before the fuel cell stack, and blows air to the cooling channel or the cooling chamber of the fuel cell stack. In yet another embodiment not shown here, the cooling air supplied to the stack is sent from the radiator to the stack via lines.

FIG. 5 shows a comparison of voltage / current density characteristic curves of a known water-cooled fuel cell stack and an air-cooled fuel cell stack of the present invention. The values shown were obtained by measurements on a battery cell stack consisting of 10 fuel cells ("10 cellular"). Graphite was used as the basic material of the bipolar plate. It can be seen that the two characteristic curves are almost the same. Therefore, the condensation of the steam obtained by the present invention in the stack can be realized without loss of efficiency.

FIG. 6 shows still another embodiment of the present invention. A plate is shown, on the back side of which the observer cannot see, there is a gas delivery structure for guiding the cathode gas shown in FIG. 2 or 3. On the side visible to the observer, the delivery structure for the cooling medium is shown. Individual parallel channels separated from each other by a bar can be identified. Cathode gas enters the battery cell through the holes in the plate, in this case passing through one or more flow-through channels, which are indistinguishable, and exits the battery cell through the diagonally opposite holes. . The cooling medium enters the cooling channels from the bottom side of the plate and exits from these cooling channels on the opposite side. The area of overlap between the cooling medium inlet region and the cathode gas outlet region, in which condensation basically takes place, is surrounded by a line. In this overlap region, additional ribs are arranged inside the channel to increase the contact surface. This increases the heat exchange between the cooling medium and the cathode gas in this region and thus advantageously controls the condensation.

FIG. 7 shows an embodiment of the invention in which the local heat exchange across the battery cell surface is modified by additional means. The plate structure corresponds to that shown in FIG. 6 except that there are no ribs in the area of condensation, so see FIG. 6 to avoid repetition. As an additional measure, a thermal insulation layer is present on the channel surface within the overlap region of the cooling medium inlet region and the cathode gas outlet region. Heat exchange is reduced in this region by appropriate selection of the layer thickness and the layer material, so that possibly undesirably large temperature gradients in the condensation region are avoided. This layer may be, for example, a free-standing layer or foil adhered to the surface. However, it is also possible to apply a thin lacquer layer or to incorporate additional materials directly into the carrier layer.

In yet another embodiment (not shown), in the apparatus of FIG. 6, a material having good heat conductivity is provided in the overlapping region of the cooling medium inlet region and the cathode gas outlet region, and the cathode gas in this region is provided. Further enhances the heat exchange between the cooling medium and the cooling medium.

In another embodiment, a thermally insulating material can be used in the device of FIGS. 6 or 7 in the region outside the overlap between the cooling medium inlet region and the cathode gas outlet region.

[0028] Thus, the heat exchange is reduced in the region outside the condensation region by the above means,
It is possible to achieve strong local condensation elsewhere in the flow-through region.

All the means shown in FIGS. 6 and 7 have been described in terms of channels for guiding the cooling medium, but are likewise applicable to channels for guiding the cathode gas.

FIG. 8 shows an embodiment of the fuel cell stack of the present invention in which the cathode gas and the cooling medium are guided separately from each other inside the stack. MEA in this schematic
Indicates a membrane electrode unit, which on one side is adjacent to the anode-side gas distribution structure AS and on the other side to the cathode-side gas distribution structure KS. There is a cooling medium distribution structure KK, also called a cooling chamber, between the two individual battery cells of the fuel cell stack and at the beginning and the end of the stack respectively. A cooling medium, for example air, passes through the cooling chamber KK and is supplied to the lower side of the stack and exhausted from the upper side. The cathode gas is an individual cathode side gas delivery structure KS
Flow through. Cathode gas is supplied and exhausted via so-called ports formed by holes aligned with each other inside the individual components KK, KS, AS, MEA.

In order to keep the performance loss due to the pressure loss during the passage of the cooling air flow as small as possible, the cooling channels in the cooling medium distribution structure KK are advantageously as short as possible and therefore in parallel (without meanders or windings). ) Composed. Hereby, a vertically upwardly directed cooling air flow is present in the stack, so that the cooling air flow is supported by the heat rise stream generated.

FIG. 9 shows yet another embodiment of the fuel cell stack of the present invention in which a portion of the cooling medium is used as the cathode gas after flowing through the cooling medium distribution structure. The order of the components inside the stack, the membrane electrode unit MEA, the cathode side gas distribution structure KS, the anode side gas distribution structure AS, and the cooling medium distribution structure KK corresponds to FIG.

First of all, the precompressed cooling air stream is preferably passed from below through the parallel cooling channels of the cooling medium distribution structure and then out of this cooling medium distribution structure before the upper part of the fuel cell stack. Reach the chamber. A portion of the cooling air from this chamber is directed to the cathode side gas delivery structure, supplying oxygen to the cathode and finally through one or more advantageously laterally arranged openings to the fuel cell. Get out of the stack. Excess cooling air flow is discharged from the upper chamber, for example through a throughflow or pressure control valve, and reaches the ambient directly.

In the two embodiments of FIGS. 8 and 9, besides oxygen, another oxygen-containing gas or mixed gas can be used as the cathode gas. However, the prerequisite is that these other oxygen-containing or mixed gases do not pollute the MEA.

It is likewise possible to use another gas instead of air as the cooling medium, for example a gas which occurs at a low temperature inside the fuel cell system.

A guide for the anode gas inside the stack is not shown in the drawings for clarity.

[Brief description of drawings]

[Figure 1]   FIG. 1 shows a known fuel cell stack.

[Fig. 2]   It is an example of a fuel cell stack of the present invention.

[Figure 3]   It is an example of a fuel cell stack of the present invention.

[Figure 4]   It is an example of a fuel cell stack of the present invention.

FIG. 5 is a voltage / current density characteristic curve of a fuel cell stack of the present invention compared with a known fuel cell stack.

FIG. 6 is an example of a fuel cell stack of the present invention having a locally adapted channel geometry in the condensation region.

FIG. 7 is an example of a fuel cell stack of the present invention having a locally adapted use of thermally conductive / thermally insulating material.

FIG. 8 is an embodiment of a fuel cell stack of the present invention having separate guides for cathode gas and cooling medium.

FIG. 9 is an example of a fuel cell stack of the present invention in which a portion of the cooling medium passes through the cooling medium delivery arrangement and then flows through the cathode side gas delivery structure as cathode gas.

[Explanation of symbols]

    MEA Membrane electrode unit     KS cathode side gas distribution structure     AS Anode side gas distribution structure     KK Coolant delivery structure

[Procedure for Amendment] Submission for translation of Article 34 Amendment of Patent Cooperation Treaty

[Submission date] September 25, 2001 (2001.25)

[Procedure Amendment 1]

[Document name to be amended] Statement

[Name of item to be amended] Claims

[Correction method] Change

[Contents of correction]

[Claims]

─────────────────────────────────────────────────── ─── Continued front page    (72) Inventor Udo Martin             Bretzfeld-Rot, Germany             Belt-Meier-Strasse 7 F-term (reference) 5H026 AA06 CC03 CC08 [Continued summary] Condensation of water vapor on the cathode gas takes place in It

Claims (9)

[Claims] 1. An electrochemical fuel cell stack having at least one membrane electrode unit (MEA) comprising an anode, a cathode and an electrolyte membrane disposed therebetween, the anode gas inlet region, At least one anode side gas delivery structure (AS) having an anode gas outlet region and a channel for guiding the anode gas from the anode gas inlet region to the anode gas outlet region, the anode gas comprising hydrogen; At least one cathode side gas distribution structure (KS) having a cathode gas inlet region, a cathode gas outlet region and a channel for guiding the cathode gas from the cathode gas inlet region to the cathode gas outlet region, Contains oxygen and water vapor, In an electrochemical fuel cell stack having a cooling medium distribution structure (KK) having a medium inlet region, a cooling medium outlet region and a channel for guiding the cooling medium from the cooling medium inlet region to the cooling medium outlet region. An electrochemical characterized in that the cooling medium inlet region and the cathode gas outlet region at least partly overlap, so that condensation of water vapor in the cathode gas takes place in the region of this overlap. Fuel cell stack. 2. The geometry of the channels in the region of overlap of the cooling medium inlet region and of the cathode gas outlet region is different compared to the region outside of this overlap. Electrochemical fuel cell stack. 3. The geometry of the channel is such that in the region of the overlap the channel arrangement, the cross-section of the channel, the number of channels per area, additional ribs, bars, grooves or needles and other regions outside this overlap. 3. The electrochemical fuel cell stack according to claim 2, characterized in that 4. The material used outside the overlap in terms of heat transfer properties in the heat transfer path between the cooling medium and the cathode gas in the region of the overlap of the cooling medium inlet region and the cathode gas outlet region. Electrochemical fuel cell stack according to claim 2 or 3, characterized in that different materials are present. 5. The cathode side gas distribution structure and the cooling medium distribution structure are configured such that the cathode gas and the cooling medium are guided in an intersecting flow. An electrochemical fuel cell stack according to claim 1. 6. The cathode-side gas distribution structure and the cooling medium distribution structure are configured such that the cathode gas and the cooling medium are guided in an opposite flow. An electrochemical fuel cell stack according to claim 1. 7. The electrochemical fuel cell stack according to claim 1, wherein the cathode gas and the cooling medium flow through the fuel cell stack separately from each other. 8. A part of the cooling medium passes through the cooling medium distribution structure and then flows through the cathode side gas distribution structure as a cathode gas.
7. The electrochemical fuel cell stack according to claim 1. 9. Method for the operation of an electrochemical fuel cell stack according to claim 1, wherein a gas, for example ambient air, is used as cooling medium.