EP1529321A1

EP1529321A1 - Control of a fluid flow in an electrochemical cell

Info

Publication number: EP1529321A1
Application number: EP03794778A
Authority: EP
Inventors: Felix Blank; Cosmas Heller
Original assignee: DaimlerChrysler AG
Current assignee: Daimler AG
Priority date: 2002-08-13
Filing date: 2003-08-04
Publication date: 2005-05-11
Also published as: WO2004025763A1; JP4780579B2; AU2003297586A1; JP2005536033A; DE10236998A1; CA2494196A1; DE10236998B4; US20110097648A1

Abstract

The invention relates to an electrochemical cell, especially a proton exchange membrane fuel cell (PEM fuel cell) or an electrolysis cell which displays improved efficiency as a result of improved temperature or moisture distribution and/or reactant distribution inside said cell. The invention is characterized in that in an electrochemical cell, comprising a channel structure for feeding, circulating and discharging fluids necessary for the operation of said cell, at least one element (4, 7, 8, 9-14, 22, 23, 29, 40, 48, 49) modifying the flow cross-section is integrated into at least one channel (2, 15, 26, 27, 37) of the channel structure for automatic control of at least one fluid flow (5, 24, 33, 34).

Description

CONTROLLING A FLUID CURRENT IN AN ELECTROCHEMICAL CELL

The invention relates to an electrochemical cell, in particular a proton exchange membrane fuel cell (PEM fuel cell) or an electrolysis cell, according to the preamble of patent claim 1.

In an electrolytic cell with a cathode and an anode, electrical energy is converted into chemical energy. A chemical compound is broken down by electric current through an ion discharge. When an external voltage is applied, the ions are picked up by the ions in the course of a reduction process. In the course of an oxidation process, the ions emit electrons at the anode. The electrolysis cell is constructed in such a way that reduction and oxidation take place separately.

Fuel cells are galvanic elements with positive and negative poles, or with a cathode and an anode, which convert chemical energy into electrical energy. For this purpose, electrodes are used which interact with an electrolyte and preferably a catalyst. A reduction takes place at the positive pole, resulting in a lack of electrons. Oxidation takes place at the negative pole, causing an excess of electrons. The electrochemical processes take place in the fuel cell as soon as an external circuit is closed.

DE 100 47 248 AI shows a typical structure of a fuel cell. The fuel cell consists of a ca thodenelektrode, an anode electrode and a matrix, which together form a membrane electrode assembly (MEA). The cathode electrode and the anode electrode each consist of an electrically conductive body which serves as a carrier for a catalyst substance. The matrix is arranged between the cathode and anode electrodes and serves as a support for an electrolyte. Several fuel cells are stacked on top of one another with the interposition of separator plates. The supply, circulation and discharge of oxidants, reductants, reactants and coolants takes place via channel systems which are generated with the separator plates. For each liquid or gaseous operating medium, supply collecting channels, distribution channels and discharge collecting channels are provided in the fuel cell stacks, which are separated from one another by sealants. The feed collection channels and discharge collection channels are referred to as ports in English-speaking countries. The cells of a stack are supplied in parallel with an oxidant fluid, a reactant fluid and a coolant via at least one feed collecting channel. The reaction products, excess reactant and oxidant fluid and heated coolant are led out of the cells out of the stack via at least one discharge collecting duct. The distribution channels form a connection between the supply and discharge manifold and the individual active channels of a fuel cell. The fuel cells can be connected in series to increase the voltage. The stacks are closed off by end plates and housed in a housing, with positive and negative poles leading to a consumer on the outside.

Japanese patent application JP 60-041769 A describes a fuel cell system in which a fuel cell stack is surrounded by a thermal insulator. For heat dissipation, the fuel cell stack is surrounded by a good heat-conducting metallic body. U-shaped bimetallic bodies are attached to the body. If the temperature in the fuel cell stack exceeds a predetermined temperature, the bimetallic bodies are lost. forms and come into contact with radiator plates so that there is heat transfer from the heat-conducting metallic body of the fuel cell stack via the bimetallic bodies to the radiator plates. The arrangement is voluminous and the heat dissipation via mechanical contact is imperfect.

In the liquid fuel cell system shown in Japanese patent application JP 61-058173 A, cooling air from a fan flows around a fuel cell stack. The cooling air flow can be controlled by means of fins which can be pivoted in the cooling air path with a coupling rod. The coupling rod is actuated with a bimetal which is in thermal contact with anode liquid. When the temperature of the anode liquid changes, the bimetal deforms so that the fins open more or less the cooling air path. The cooling system is arranged on the outside of a fuel cell stack and thus increases the size of a fuel cell system. The cooling system is unable to compensate for temperature inhomogeneities within a fuel cell stack. Only the total cell temperature is controlled at a time.

Solutions are also known which use a fluid dynamic flow against a fuel cell stack through a cooling air flow. In the solution according to Japanese patent application JP 58-100372 A, the flow resistance of the cooling air is reduced by a special design of an inflow area. Japanese patent application JP 58-017964 A describes a uniform distribution of cooling air on fuel cells by air baffles. Japanese patent application JP 1185871 A shows a special flow of cooling air.

In all of these solutions, attempts are made to design the cooling air flow in such a way that the individual cells are optimally temperature-controlled without, however, adapting the cooling flow to local needs. The object of the invention is to develop an electrochemical cell which has an improved efficiency due to an improved temperature or humidity distribution and / or reactant distribution within the cell.

The object is achieved with an electrochemical cell which has the features of claim 1. Advantageous refinements result from the subclaims.

The invention permits control or regulation of fluid flows in the area of a single cell. By using at least one element that changes the flow cross section within at least one channel, the temperature distribution or moisture distribution, which depends on the cooling medium and operating state of the cell, can be set as desired.

A major advantage of the arrangement according to the invention is that each channel can be regulated individually, ie, by varying the pressure loss in the individual channels, the volume flows of the individual channels are varied and are supplied or disposed of with gas via collecting and distribution channels become. There is a homogenization of the temperature or humidity between the channels if a homogeneous temperature or humidity distribution is desired. If a specific temperature or humidity profile is required in more complex fuel cell systems, this can be achieved with an appropriate arrangement of the elements that change the flow cross-sections.

An uneven temperature distribution in a fuel cell results, among other things, from an inhomogeneous heat discharge. For example, the heat emitted to the surroundings is greater in the edge cells of a fuel cell stack than in the case of cells located on the inside. In the case of air cooling in particular, there is an uneven heat discharge due to the heating of the cooling fluid. Furthermore, the reactions take place within a cell not everywhere to the same extent, so that the heat sources are unevenly distributed. The reactions depend, among other things, on the local temperature, the local partial pressures and the local humidity.

With the elements that change the flow cross-sections, e.g. Bimetal strip, the coolant flow can be regulated in each cooling channel. This results in an optimized temperature distribution.

Furthermore, the elements changing the flow cross section can be used to control or regulate the local gas composition by influencing the gas flows. For example, For example, bimetal strips can be provided in the fluid channels of one or both reaction gases. If the fluid channels are connected to one another, a gas exchange can take place between the channels. This leads to locally increased cell reactions and locally higher temperatures. Higher temperatures result in a narrowing of the cross-section of the gas channels through the bimetallic strips, which means that there are fewer reaction gases locally in this cell area and the gas flow increases in other areas. The cell reaction is reduced by the lowering of the gas flow, with the reactions intensifying in the more supplied areas. This results in an even reaction distribution.

In a variant of the invention, the desired reaction distribution can be set by arranging bimetals and connections between the gas channels. For this purpose, a flow field for a fluid can be divided into different areas, whereby communication of fluids over different areas is possible. The fluid channels in the areas can be parallel to one another, the elements for changing the cross sections of the channels advantageously being integrated in downstream areas. Another possibility to regulate both a cooling air flow and reaction gas flows locally results from the use of materials or components that change their volume or shape depending on the humidity. Depending on the reactants, a phase change occurs in the fuel cell on the cathode side in the course of the gas flow between the entrance and the exit of a channel, ie liquid water can occur. The amount of water generated depends on the reaction, since the water is a reaction product. If the said materials or components are used in such a way that they narrow duct cross sections depending on the humidity, the same effect can be achieved as with the use of bimetal strips.

When regulating the local heat, bimetal strips can be used in the ducts on the bottom and cathode sides and in the coolant ducts. When controlling depending on the humidity, the cross-sectional changing materials or components are introduced directly into the cathode channels. If the anode fluid flow and / or the cooling fluid flow are also to be controlled as a function of moisture, then the moisture in the cathode fluid flow must be recorded in order to achieve a change in the cross-section of the duct on the anode side or on the cooling fluid side.

The invention will be explained in more detail below on the basis of exemplary embodiments, in which:

Fig. 1: a cooling channel of a fuel cell with a bimetal plate arranged on the channel bottom at low

Cooling fluid temperature,

2: the cooling duct according to FIG. 1 at high cooling fluid temperature,

3: a cooling channel of a fuel cell with a bimetal plate integrated on the channel bottom at a low cooling fluid temperature, 4: the cooling duct according to FIG. 3 at high cooling fluid temperature,

5: a cooling channel of a fuel cell with a large number of bimetal plates arranged on the channel bottom at a high cooling fluid temperature,

6: the cooling duct according to FIG. 5 at a low cooling fluid temperature,

7: a cathode channel of a fuel cell with moisture-dependent swelling bodies in plan view with a dry cathode fluid flow,

8: the cathode channel according to FIG. 7 with a moist cathode fluid flow,

9 and 10: a cathode channel of a fuel cell with moisture-dependent swelling bodies in plan view between two fluid channels at two different temperatures of a cooling fluid, and

Fig. 11-13: different arrangements of bimetallic elements in the flow field of a cooling fluid in a separator plate.

1 and 2 show a section of a separator plate 1 of a fuel cell with a rectangular cooling channel 2. A rectangular bimetal plate 4 is also attached to the channel bottom 3 at one end. The bimetallic plate 4 essentially has the width of the cooling channel 2, the width extending perpendicular to the plane of the drawing. A cooling fluid 5 circulates in the cooling channel 2. If the cooling fluid 5 has a temperature which is too low for the operation of the fuel cell, then the bimetal plate 4 bends up, so that the flow cross section of the cooling channel 2 is narrowed. In an extreme case, the bimetallic plate 4 bends so far that, as shown in FIG. 1, it completely closes the cooling channel 2. If the cooling fluid 5 does not flow or only flows slightly, then the cooling fluid 5 heats up the processes taking place in the fuel cell. The bimetal plate 4 bends with its free end in the direction of the channel bottom 3 and increases the flow cross section. The cooling fluid 5 can flow in the indicated direction 6 without great resistance.

In the description below, the same reference numerals from elements already described are used for elements with an equivalent function.

Figures 3 and 4 show a section of a separator plate 1 of a fuel cell with a rectangular cooling channel 2. On the channel bottom 3 there is a tongue-shaped notch 7, which is freely movable at one end. Over the length, the notch 7 is connected on the channel side to a metallic, rectangular plate 8. The plate 8 has a coefficient of thermal expansion different from the material of the notch 7, so that the notch 7 and the plate 8 form a bimetallic element. When the cooling fluid 5 is cool, the notch 7, as shown in FIG. 3, bends together with the plate 8 away from the channel bottom 3 and narrows the flow cross section. 4 shows the state when the cooling fluid 5 is heated. The notch 7, together with the plate 8, lies back in the channel bottom 3, so that almost the entire flow cross section is released.

5 and 6 show a section of a separator plate 1 of a fuel cell with a rectangular cooling channel 2. A plurality of rectangular bimetallic plates 9-14 are fastened at one end to the channel bottom 3. The fastening ends of the bimetallic plates 9-14 point in the same direction. The bimetallic plates 9-14 can essentially have the width of the cooling channel 2 or a plurality of such bimetallic plates 9-14 can have the width of the Cooling channel 2 lie side by side. The lengths L of the bimetallic platelets 9-14 are significantly smaller compared to the height H of the cooling channel 2. 5 shows the state of the bimetal plates 9-14 when the cooling fluid 5 is too warm. Because of the high temperature of the cooling fluid 5, the bimetal plates 9-14 are set up. In this state, the bimetallic plates 9-14 which have been set up increase the effective heat-dissipating surface of the channel bottom 3. The bimetallic plates 9-14 which are set up increase the roughness of the wall and thus improve the heat transfer into the material of the separator plate 1. Because of the short length of the bimetallic plates 9-14 the flow cross section of the cooling channel 2 is only insignificantly reduced. The bimetallic plates 9-14 can of course also be arranged on the other channel walls of the cooling channel 2 in addition to the channel bottom 3. At a low temperature of the cooling fluid 5, the bimetallic plates 9-14, as shown in FIG. 6, lie against the channel bottom 3, as a result of which the contact area with the cooling fluid 5 is reduced. In this case, the cooling fluid 5 is cooled only slightly via the channel bottom 3.

7 shows a top view of a cathode channel 15 of a cathode channel system of a fuel cell, which is formed by a separator plate 16. The cathode channel 15 is delimited by webs 17, 18 which abut a membrane-electrode unit. The cathode gas 19 flowing through the cathode channel 15 contacts the membrane-electrode unit and there undergoes a chemical reaction with the formation of product water. The cathode channel 15 has a width B and a depth which runs in the direction perpendicular to the plane of the drawing. Source bodies 22, 23 are arranged opposite one another on the side walls 20, 21 of the cathode channel 15. The swelling bodies 22, 23 consist of an elastic material which swells when moisture is present. If, as shown in FIG. 7, the cathode gas 19 has a low water content, then the swelling bodies 22, 23 are drawn together, so that the flow cross section for the cathode gas 19 is hardly restricted. There is a large cathode gas flow 24, which favors the reaction at the membrane electrode assembly. The strong reaction produces more product water. This causes the swelling bodies 22, 23 to swell, as shown in FIG. 8. In this situation, the source bodies 22, 23 reduce the flow cross section, so that the cathode gas flow 24 is reduced. During normal operation of the fuel cell, there is a balance between the flow rate and water content of the cathode gas 19 in or between the cathode channels 15 of the cathode channel system, so that homogenization or adaptation to a selected profile of the temperature or humidity between the channels 15 occurs. The swelling bodies 22, 23 can be present multiple times in a cathode channel 15.

9 and 10 show part of a separator plate 25 in which a cathode channel 26 and a cooling channel 27 are formed, which are separated from one another by a web 28 made of the material of the separator plate 25. This arrangement of cathode channel 26, web 28 and cooling channel 27 is present several times on a separator plate 25. In the web 28, a swelling body 29 is installed, which has a wall 30 made of elastic, water-impermeable material on the side of the cooling channel 27 and a wall 31 made of rigid water-permeable material on the side of the cathode channel 26. The wall 30 can be made of rubber and the wall 31 can be made of metal mesh. Depending on the water content of the cathode gas 32 in the cathode channel 26, the swelling body 29 swells more or less. As shown in FIG. 9, there is little water in the cathode gas flow 33, so that the swelling body 29 is contracted and the wall 30 is contracted. The cooling fluid flow 34 can flow almost unhindered in the cooling channel 27, so that the cooling effect is increased in this area of a membrane-electrode unit. When the active area of the membrane electrode assembly is cooled, the saturated state of the cathode gas 32 is reached until there is water discharge in the cathode channel 26. The water passes through the wall 31 to the swelling body 29, which thereby swells as shown in FIG. 10. Due to the volume increase of the swelling body 29, the wall 30 expands in the direction of the cooling channel 27 and narrows its cross section. The narrowing of the cross section causes the cooling fluid flow 34 to decrease. In normal operation of the fuel cell, a balance is established between the water content of the cathode gas 32 in the cathode channels 26 and the flow rate in the cooling channels 27, so that homogenization or adaptation to a selected profile of the Temperature or humidity between the channels 26, 27 occurs.

11 shows a separator plate 1 on which a flow field for a cooling fluid is formed. Collection channels 35.1, 35.2, 36.1, 36.2 are provided for supplying and removing anode and cathode fluids. Cooling channels 37 are embossed in the separator plate for the passage of a cooling fluid. There are webs 38 between the cooling channels 37. Viewed in the direction of flow 39 of the cooling fluid, there are bimetal strips 40 at the outlet of the cooling channels 37, which are designed as described for FIG. 1. Since the heat dissipation in a fuel cell varies greatly depending on the operating conditions and ambient conditions from cooling channel 37 to cooling channel 37, it is advantageous if the cooling fluid flow can be regulated to the optimum temperature in each individual cooling channel 37. If air is used as the cooling fluid, then the air is forced through the cooling channels 37 with a compressor. Depending on the heating of the bimetal strips 40, the bimetal strips 40 are bent up to different heights and narrow the respective cooling channel 37 in such a way that the desired volume flows are established. This means that the temperatures in the individual channels 37 or cell areas are homogenized or adapt to a selected profile. In contrast to FIG. 11, the flow field for a cooling fluid in FIG. 12 has openings 41 between the cooling channels 37. This embodiment can be used advantageously if the heat on a separator plate 1 is not homogeneously distributed due to a non-homogeneous reaction or an inhomogeneous heat dissipation is or does not correspond to a desired profile.

In the separator plate 1 shown in FIG. 12, heat is generated to a greater extent in the last third of the cooling channels 37, as seen in the direction of flow 39. It is therefore only necessary here to regulate the volume flows with bimetal strips 40 which are arranged in this third. Because the cooling channels 37 are connected to one another via the openings 41, cross-flows 42 of the cooling fluid between the openings 41 occur at different positions of the bimetal strips 40.

In the separator plate 1 shown in FIG. 13, channels 37 are each interrupted by two openings 43, 44. Viewed in flow direction 39, three sections 45-47 are created for each cooling channel 37. In the two downstream sections 46, 47, a bimetal strip 48, 49 is arranged in each cooling channel 37. In this way, the temperature on the surface of a membrane electrode unit can be regulated individually in each section 46, 47.

The distribution of the bimetallic elements 4, 7, 8, 9-14, 40, 48, 49 or cross-sectional constricting elements 22, 23, 29 for controlling or regulating the moisture content or the temperature of fluids is only given as an example in the figures and the description , The distribution of the elements can be adapted to the respective conditions in an electrochemical cell, in particular the temperature and humidity distribution. List of the reference symbols used

1 separator plate

2 cooling channels

3 channel floor

4 bimetal plates

5 cooling fluid

6 direction

7 notch

8 tiles

9-14 bimetal plates

15 cathode channel

16 separator plate

17, 18 bridge

19 cathode gas

20, 21 side wall

22, 23 swelling bodies

24 cathode gas flow

25 separator plate

26 cathode channel

27 cooling channel

28 footbridge

29 swelling bodies

30, 31 wall

32 cathode gas

33 cathode gas flow

34 cooling fluid flow

35.1, 35.2, 36.1, 36.2 collecting channel

37 channel

38 footbridge

39 Flow direction

40 bimetal strips

41 breakthrough

42 cross flow 43, 44 breakthrough 45-47 section 48, 49 bimetal strips

Claims

claims

1. Electrochemical cell with a channel structure for the supply, circulation and removal of fluids necessary for the operation of the cell, characterized in that for the automatic control of at least one fluid stream (5, 24, 33, 34) at least one that changes the flow cross-section Element (4, 7, 8, 9-14, 22, 23, 29, 40, 48, 49) is integrated in at least one channel (2, 15, 26, 27, 37) of the channel structure.

2. Electrochemical cell according to claim 1, characterized in that in the case of a cell with a channel (2, 37) formed in a separator plate (1) in the channel (2, 37) at least one bimetal element (4, 7, 8, 9-14 , 40, 48, 49) is provided.

3. Electrochemical cell according to claim 2, characterized in that at least one of the cross-section of the channel (2, 37) adapted bimetal element (4, 7, 8, 9-14, 40, 48, 49) is provided, with the fluid being reduced Temperature the bimetallic element (4, 7, 8, 9-14, 40, 48, 49) reduces the flow cross-section of the channel (2, 37) due to a thermally induced change in shape.

4. Electrochemical cell according to claim 3, so that a separate, plate-shaped bimetal element (4) is attached at one end to a channel wall (3).

5. Electrochemical cell according to claim 3, so that the bimetallic element consists of a tongue-shaped notch (7) on a channel wall (3) and a flat-shaped element (8) connected to the notch (7).

6. Electrochemical cell according to claim 2, characterized in that in the channel (2) a plurality of bimetallic elements (9-14) are each fixed at one end to a channel wall (3), the bimetallic elements (9-14) when the temperature rises of the fluid (5).

7. Electrochemical cell according to claim 1, characterized in that in a cell with a channel (15) formed in a separator plate (16) in the channel (15) at least one element (22, 23, 29) is provided, which at Increased moisture experiences an increase in volume.

8. Electrochemical cell according to claim 7, so that the element (22, 23) is fastened to a channel wall (20, 21).

9. Electrochemical cell according to claim 7 and 8, characterized in that two elements (22, 23) are arranged in pairs opposite one another in the channel (15).

10. Electrochemical cell according to claim 7, so that the element (29) is integrated in a channel wall (28).

11. Electrochemical cell according to claim 10, characterized in that the channel wall (28) of a fuel cell separates a cathode fluid channel (26) from a cooling fluid channel (27), the element (29) on the side of the cathode fluid channel (26) a water-permeable material, preferably a metal grid (31), and on the side of the cooling fluid channel (27) consists of an elastic, water-impermeable material (30).

12. The electrochemical cell according to claim 1, which also means that in a cell with parallel channels (37) for each cooling fluid, at least one element (40, 48, 49) is assigned to each channel (37).

13. Electrochemical cell according to claim 1, so that the elements (40, 48, 49) are integrated in channels (37) of a channel structure consisting of several areas (45-47).

14. The electrochemical cell according to claim 13, so that at least one of the fluids has a connection (41, 43, 44) between the channels (37) for communication (42) via different areas (45-47).

15. Electrochemical cell according to claim 14, characterized in that the communication (42) between the different Areas (45-47) can be controlled by means of the elements (48, 49).

16. Electrochemical cell according to claim 13, characterized in that the channels (37) in the direction of flow (39) of the fluid run parallel in several areas (45-47) and after each area (45, 46) cross-connections (43, 44) of the channels (37) exist, the elements (48, 49) being arranged in downstream regions (46, 47) for the area-wise control of the fluid flows.

17. Electrochemical cell according to claim 16, characterized in that the channels (37) run parallel in a first area, are connected to one another via an opening (41) in a second area and run again in parallel in a third area, the elements ( 40) are arranged in the channels (37) in the third area.