KR20090014135A - Stabilized fuel cell flow field - Google Patents

Abstract

A fuel cell (10) includes a cathode catalyst (26) for receiving a first reactant and an anode catalyst (24) for receiving an expected amount of a second reactant. The cathode catalyst (26) and the anode catalyst (24) respectively catalyze the first reactant and the second reactant to produce an electrochemical reaction that generates a flow of electrons between the cathode catalyst (26) and the anode catalyst (24) The amount of the first reactant consumed in the electrochemical reaction corresponds to a threshold amount of the second reactant needed to generate a forward flow of the electrons from the anode catalyst (24) to the cathode catalyst (26). A portion (42) of a fuel cell flow field includes a feature (54, 60, 80, W1, D1) that restricts consumption of the first reactant.

Description

Stabilized Fuel Cell Flow Field {STABILIZED FUEL CELL FLOW FIELD}

The present invention relates generally to fuel cells, and more particularly to flow fields for fuel cells.

Fuel cells are well known and used to generate electricity in various applications. Conventional fuel cells use reactant gases such as hydrogen and oxygen (eg from air) to generate current. Typically, a fuel cell includes adjacent flow fields with flow channels containing respective reactant gases. Each flow field distributes the reaction gas through a gas distribution layer to each anode catalyst or cathode catalyst adjacent to the ion conductive polymer exchange membrane (PEM) to generate a current. Typically, a network of carbon particles supports each of the cathode and anode catalysts and forms part of an external circuit for current.

Conventional fuel cells typically generate electricity from an electrochemical reaction that combines an oxidant, which is oxygen from air, and water, which is a byproduct of fuel, which is typically hydrogen. Oxygen continues to pass over the cathode catalyst and hydrogen passes over the anode catalyst. The anode catalyst separates hydrogen protons from the electrons. Both pass through the PEM, and electrons flow through external electrical circuits. Both recombine with the electrons and react with oxygen in the cathode catalyst to form water byproducts.

Under some conditions, such as low hydrogen partial pressure in the local region of the anode catalyst, there is a shortage of hydrogen to maintain the electrochemical reaction in that region. This can result in local current reversal from the normal fuel cell operating mode described above, resulting in a reaction state that degrades the cathode catalyst, carbon support, or both. As an example, water at both electrodes is electrolyzed to form oxygen, hydrogen and free electrons. Hydrogen protons pass through the PEM, and electrons emitted from the fuel-rich region of the negative electrode flow into the fuel-depleted region of the negative electrode. The absence of proton and electron sources at low potentials in the fuel depletion region increases the potentials in the cathode catalyst, which results in a degradation reaction between water and the carbon support to form carbon dioxide. Degradation ultimately results in loss of catalyst and electrical performance, which in turn lowers fuel cell efficiency.

The present invention addresses the need to reduce or eliminate fuel cell catalyst degradation in order to maintain proper fuel cell operation.

An apparatus of one embodiment for use in a fuel cell includes a cathode catalyst for accommodating a first reactant and an anode catalyst for accommodating an expected amount of a second reactant. The cathode catalyst and the anode catalyst each generate an electrochemical reaction that catalyzes the first and second reactants to generate a flow of electrons between the anode catalyst and the cathode catalyst. Since the kinetics of hydrogen oxidation are very easy, the rate of fuel cell reaction generally depends on the rate of ohmic conduction through the fuel cell's conductive layer and the ratio of the first reactant to the cathode catalyst. Regulated by The amount of first reactant consumed in the electrochemical reaction corresponds to the critical amount of second reactant required to generate a forward flow of electrons from the anode catalyst to the cathode catalyst. Part of the fuel cell flow field includes features that regulate the consumption of the first reactant to keep this threshold below the expected amount of the second reactant.

The method of one embodiment reduces the consumption of the first reactant in the cathode catalyst of the fuel cell to reduce the critical amount of the second reactant required for the anode catalyst to generate a forward flow of electrons from the anode catalyst to the cathode catalyst. Regulating.

The above examples are not intended to be limiting. Additional examples are described below. Those skilled in the art will apparently appreciate the various features and advantages of the present invention from the following detailed description. The drawings that accompany the detailed description can be briefly described as follows.

1 schematically illustrates selected portions of a fuel cell stack.

FIG. 2 is a cross-sectional view taken along the line shown in FIG. 1 illustrating an embodiment having features that regulate the amount of reactant gas consumed. FIG.

3 is a cross-sectional view similar to FIG. 1 showing another exemplary embodiment.

4 is a cross-sectional view illustrating an embodiment of a gas distribution layer having a filler material.

5 is a cross-sectional view illustrating an embodiment of a catalyst having a reduced amount of catalyst material.

6 is a cross-sectional view illustrating an embodiment having a channel of variable width.

7A is a cross-sectional view illustrating one embodiment having channels and parallel channels engaged with each other.

FIG. 7B is a cross-sectional view along the cut line shown in FIG. 6A illustrating the channel depth of the parallel channel.

FIG. 7C is a cross-sectional view along the cut line shown in FIG. 6A illustrating the channel depth of the channels engaged with each other.

8 is a sectional view illustrating an embodiment having a cooling passage.

1 schematically illustrates selected portions of an exemplary fuel cell 10 for generating electricity. In this embodiment, the cathode side 12 receives the reaction gas R ₁ and the anode side 14 receives the reaction gas R ₂ to generate a current using a known reaction. Each of the cathode side 12 and the anode side 14 comprises a flow field plate 16, such as a forming plate, a perforated plate or another type of plate, which flow channel plate 16 and the channel wall 17 It is provided with a channel 18 extending between the channel walls 17 for distributing the reaction gases R ₁ and R ₂ over the cathode side 12 and the anode side 14.

In the illustrated embodiment, a gas exchange layer 20 is disposed adjacent each of the flow field plates 16. A polymer exchange membrane (PEM) 22 separates the cathode catalyst 24 from the anode catalyst 26 between the gas exchange layers 20. In some embodiments, each of the cathode catalyst 24 and the anode catalyst 26 includes a catalyst material deposited on a support such as carbon cloth.

FIG. 2 shows the anode side 14 flow field plate 16 along the cut line shown in FIG. 1 to illustrate one feature of the operation of the fuel cell 10. Reactant gas R ₂ is supplied into channel 18 through inlet 38 and flows through channel 18 toward outlet 40. Reactant gas R ₂ in channel 18 diffuses at least partially toward anode catalyst 26 through gas exchange layer 20 (FIG. 1). Similarly, reactant gas R ₁ flows through channel 18 on cathode side 12 and diffuses toward cathode catalyst 24. Cathode catalyst 24 and anode catalyst 26 catalyze the respective reaction gases R ₁ and R ₂ to provide a current (i.e., flow of electrons) between cathode catalyst 24 and anode catalyst 26. It generates an electrochemical reaction that results in the generation of.

In general, when the cathode catalyst 24 and the anode catalyst 26 consume reaction gases R ₁ and R ₂ in the electrochemical reaction, respectively, partial pressures of the reaction gases R ₁ and R ₂ (or, Alternatively, by way of example, the concentration decreases along channel 18 from inlet 38 to outlet 40. Other indicators of the partial pressure, concentration or gas amount of reactant gases R ₁ and R ₂ along the channel 18 can be estimated in a known manner.

In some parts of the fuel cell 10, such as near the inlet 38, the partial pressure is relatively high, resulting in an electrochemical reaction that results in an intended forward flow of electrons from the anode catalyst 26 to the cathode catalyst 24. . The amount of reaction gas R ₁ consumed by the cathode catalyst 24 corresponds to the critical amount of reaction gas R ₂ required for the anode catalyst 26 to generate a forward flow of electrons. In other parts of the fuel cell 10, such as near the outlet 40 or near the local uneven distribution of the reactant gas, at least the partial pressure of the reactant gas R ₂ is relatively lower. When the partial pressure of the reaction gas R ₂ is below the threshold (eg, lack of fuel), the electrochemical reaction generates an unintentional reverse flow of electrons from the cathode catalyst 24 to the anode catalyst 26.

In the illustrated embodiment, portion 42 of fuel cell 10 generally receives an expected partial pressure or concentration of reactant gas R ₂ below the partial pressure near inlet 38. In this example, the portion 42 is a reaction gas reaction gas consumed at the cathode catalyst 24 during the electrochemical reaction in order to remain below the estimated partial pressure in the (R ₂₎ reaction gas (R ₂₎ a threshold amount of the (R ₁₎ It includes features that regulate the amount of. This provides the benefit of maintaining the intended forward flow of electrons from the anode catalyst 26 to the cathode catalyst 24.

3 illustrates one exemplary feature for controlling the consumption of reactant gas R ₁ . In this example, the cathode side 12 includes a barrier layer 54 between the cathode catalyst 24 and the PEM 22. In one example, barrier layer 54 comprises a carbon ionomer material.

Barrier layer 54 inhibits mass transport of catalyzed reactant gas R ₁ from cathode catalyst 24 to PEM 22 (ie, oxygen transmembrane through PEM 22). cross-over is regulated]. This reduces the amount of catalyzed reaction gas (R ₁ ) available for the electrochemical reaction and, in turn, reduces the critical amount of reaction gas (R ₂ ) needed to generate a forward flow of electrons. This provides the benefit of maintaining a threshold below the expected partial pressure of reactant gas R _{2 in} the portion 42 to avoid unintentional reverse electron flow, which reduces the likelihood of catalyst and carbon support degradation.

4 illustrates another feature for controlling the consumption of reaction gas R ₁ . In this example, the gas exchange layer 20 on the cathode side 12 is a porous cloth layer 56 such as a carbon nonwoven with fibers 58. The voids between the fibers 58 provide passages 59 between the fibers 58 that allow the reactant gas R ₁ to diffuse from the channel 18 to the cathode catalyst 24. Filler 60 at least partially blocks diffusion of reactant gas R ₁ through the voids. In the illustrated embodiment, filler 60 suppresses diffusion by forcing reactant gas R ₁ to diffuse along a longer path around filler 60.

In one embodiment, filler 60 comprises a relatively inert material suitable for regulating reactant gas R ₁ flow. In another embodiment, filler 60 includes an oxide material, such as tin oxide or other metal oxide. In another embodiment, filler 60 includes a polymeric material, such as polytetrafluoroethylene. In another embodiment, filler 60 includes a carbon graphite material.

In one embodiment, the filler material 60 is deposited on the fabric layer 56 in a known manner such as a cloud tower or screen printing method.

In another embodiment, the cloth layer 56 covers the portion 42 and at least one other portion of the fuel cell 10. The other portion is shielded prior to the deposition of the filler 60 to selectively deposit the filler 60 only on the portion 42 of the cloth layer 56.

In another embodiment, shown in FIG. 5, the cathode catalyst 24 utilizes a reduced amount of catalyst material, such as platinum, in the catalyst portion 61 compared to the other portion 62 of the cathode catalyst 24 in the fuel cell 10. Include. This reduces the amount of catalyzed reaction gas (R ₁ ) available for the electrochemical reaction, which in turn reduces the critical amount of reaction gas (R ₂ ) required to generate the forward flow of electrons as described above.

In one embodiment, the catalytic material is deposited on the carbon support in a known manner. As an example, the deposition rate is controlled to deposit the required amount of catalyst material on the carbon support. In some embodiments, selected portions of the carbon support are shielded to achieve different amounts of catalytic material in different regions of the carbon support.

6 illustrates an example of a modified portion 42 of the flow field. In this embodiment, channel 18 ′ of portion 42 includes a first width W ₁ . In section 42 the other parts of the flow field 66 of the upstream from the channel 18 has a first width smaller than the second width (W ₁₎ (W _2). In one embodiment, portions 42 and 66 have the same pitch (ie, the combined width of channel 18 and channel wall 17 shown in P), but with different channel widths.

In the illustrated embodiment, the relatively wider channel 18 'and narrower channel wall 17' of the portion 42 reduces the amount of catalyzed reaction gas R ₁ available for the electrochemical reaction. In turn, the threshold amount of reactant gas (R ₂ ) required to generate a forward flow of electrons is reduced.

As is known, the electrochemical reaction produces water byproducts. For the porous flow field plate 16, water is removed in a known manner over the region of the channel wall 17 to enable diffusion of the reactant gas R ₁ into the cathode catalyst, with the water removal rate being the channel wall 17 Corresponds to an area of). Therefore, the surplus regulates the diffusion of the reaction gas R ₁ into the cathode catalyst 24. In the illustrated embodiment, the relatively narrower channel wall 17 ′ in the portion 42 provides resistance to water removal compared to the wider channel wall 17 of the portion 66, whereby the cathode catalyst The diffusion of the reaction gas R ₁ into the 24 is suppressed to regulate the consumption of the reaction gas R ₁ , thereby keeping the threshold below the expected partial pressure of the reaction gas R ₂ in the portion 42. .

7A illustrates another example portion 42. In this example, the channels 18 'of the part 42 are parallel. As is known, the main means of mass transfer of reactant gas R ₁ to the cathode catalyst 24 in a parallel channel arrangement is diffusion. In the other part 70 of the flow field upstream from the part 42, the channels 18 are engaged with each other. As is known, the main means of mass transfer of reactant gas R ₁ to the cathode catalyst 24 in the interdigitated channel arrangement is forced convection.

FIG. 7B illustrates the channel depth D ₁ of the channel 18 ′ of one exemplary portion 42, and FIG. 7C illustrates the channel depth D ₂ of the portion 70 along the cutting line shown in FIG. 7A. To illustrate. In the illustrated embodiment, the channel depth D ₁ is shallower than the channel depth D ₂ .

In the illustrated embodiment, the parallel arrangement of the selected channels along the portion 42 and the shallower channel depth D ₁ provide a relatively less reactant gas R ₁ flow into the portion 42 than in the portion 70. Thereby lowering the amount of reaction gas (R ₁ ) available for the electrochemical reaction, and subsequently reducing the critical amount of reaction gas (R ₂ ) required to generate the forward flow of electrons.

In some examples, the parallel arrangement of the portions 42 and the shallower channels 18 ′ have a negligible impact on the performance of the fuel cell 10 at low current densities, but at higher current levels the current densities of the portions 42. Will regulate.

8 illustrates an example of a modified fuel cell 10. In this embodiment, the fuel cell 10 includes a cooling passage 80 for delivering a known coolant. The coolant enters the cooling passage 80 through the inlet 82 and exits through the outlet 84. In the illustrated embodiment, the cooling passages 80 are schematically shown in a sand array. In some embodiments, cooling passages 80 are formed in flow field plate 16 as shown in phantom in FIG. 1.

In the illustrated embodiment, the coolant flows through the cooling passage 80 and removes heat generated by the electrochemical reaction. In general, the temperature of fuel cell 10 increases from inlet 38 to outlet 40. The temperature tends to increase the rate of the electrochemical reaction, which increases the critical amount of reactant gas (R ₂ ) needed to generate the forward flow of electrons. In this embodiment, the portion 42 is close to the inlet 82, and the second portion 86 is far from the inlet 82. The temperature of the coolant generally increases from the inlet 82 to the outlet 84 as the coolant absorbs heat from its surroundings. By providing the part 42 near the inlet 82, the part 42 receives a cooler that is relatively cooler than the coolant preheated by another part of the fuel cell 10. In one embodiment, the cooling passage 80 keeps the portion 42 cooler than the portion 86 present upstream from the portion 42 along the flow direction of the reactant R ₂ . This provides a change in temperature as opposed to a conventional arrangement. Again, this embodiment slows the electrochemical reaction in portion 42 and, in turn, reduces the critical amount of reactant gas R ₂ needed to maintain the forward flow of electrons.

The disclosed embodiment is the portion of fuel cell 10 that is expected to have a relatively low partial pressure of reactant gas R ₂ to maintain the forward flow of intended electrons from anode catalyst 26 to cathode catalyst 24. Regulate the consumption of reactive gas (R ₁ ). In some disclosed embodiments, the described features increase the stability of the operation of the fuel cell 10 with relatively high fuel utilization by lowering the threshold for fuel deficiencies without significant negative impact on performance at low to medium current densities. Let's do it.

Although a combination of features is illustrated in the illustrated embodiments, not all of these need to be combined to realize the benefits of the various embodiments of the invention. In other words, a system designed according to an embodiment of the present invention does not need to include all of the features illustrated in any one of the figures or all of the parts schematically shown in the figures. Also, selected features of one example embodiment may be combined with selected features of another example embodiment.

The foregoing description is by way of illustration, not limitation. Those skilled in the art will clearly recognize variations and modifications to the disclosed embodiments without departing from the spirit of the invention. The scope of legal protection given to this invention should only be determined by a consideration of the following claims.

Claims (20)

Is a device used in fuel cells, A cathode catalyst for containing the first reactant, An anode catalyst for receiving an expected amount of a second reactant, A first fuel cell portion, The cathode catalyst and the anode catalyst respectively catalyze the first and second reactants to generate an electrochemical reaction which generates a flow of electrons between the anode catalyst and the cathode catalyst, and the first consumed in the electrochemical reaction. The amount of reactant corresponds to the critical amount of the second reactant required to generate the intended flow of electrons, The first fuel cell portion includes a feature that regulates consumption of the first reactant to maintain the threshold amount within the intended expected amount of the second reactant. The fuel cell of claim 1, wherein the first fuel cell portion and the other portion of the fuel cell each comprise a porous layer having openings for the flow of the first reactant to the cathode catalyst, the features partially blocking the openings. An apparatus for use in a fuel cell, comprising a filler. The apparatus of claim 2, wherein the filler is selected from at least one of an oxide material, a polymer material, or a carbon material. The apparatus of claim 1, wherein the first fuel cell portion comprises a first channel of equal size and the feature comprises at least one second channel of a different size than the first channel. . The fuel cell of claim 1, wherein the feature includes a first fuel cell portion having a parallel channel for receiving a first reactant, and the other portion of the fuel cell flow field includes an interlocked channel for receiving the first reactant. The apparatus used for a fuel cell. The fuel cell of claim 1, comprising a catalytic material, wherein the feature comprises a greater amount of catalyst material proximate the first fuel cell portion relative to the amount of catalyst material proximate the other portion of the fuel cell. Device. The apparatus of claim 1, wherein the feature comprises a first fuel cell portion maintained at a cooler temperature than other portions of the fuel cell. 8. The apparatus of claim 7, wherein the apparatus comprises a cooling passage having a coolant inlet and a coolant outlet, wherein the first fuel cell portion is closer to the coolant inlet than the coolant outlet. The method of claim 1, wherein the first fuel cell portion comprises a porous layer having an opening for the flow of the first reactant to the cathode catalyst, wherein the feature is adapted to regulate the flow of the first reactant to the cathode catalyst. An apparatus for use in a fuel cell, comprising a first fuel cell portion having a barrier layer adjacent the porous layer. 10. The apparatus of claim 9, comprising an ion membrane between the cathode catalyst and the anode catalyst, the barrier layer being between the cathode catalyst and the ion membrane. Method used in fuel cells, Regulating the consumption of the first reactant in the cathode catalyst of the fuel cell to reduce the threshold amount of the second reactant required for the anode catalyst. The method of claim 11, comprising regulating the consumption of the first reactant in a selected portion of the fuel cell stack. 13. The method of claim 12, comprising using a reduced amount of catalytic material in selected portions relative to other portions of the fuel cell. 13. The method of claim 12, comprising using a reduced channel size in the selected portion relative to the channel size of other portions of the fuel cell. 13. The method of claim 12, comprising using parallel flow field channels in selected portions and using flow field channels engaged with each other in other portions of the fuel cell. 13. The method of claim 12, comprising maintaining the selected portion at a cooler temperature than other portions of the fuel cell. The method of claim 11, comprising regulating the flow of the first reactant to the cathode catalyst. 18. The method of claim 17, comprising partially blocking the flow of the first reactant to the cathode catalyst. The method of claim 18, comprising partially blocking flow through openings in the porous layer adjacent the cathode catalyst with a filler. 19. The method of claim 18, comprising partially blocking flow to the barrier layer and the fabric layer.

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