Fuel cell system

A fuel cell assembly has a fuel cell stack (1) provided with a polymer electrolyte membrane (2), an air electrode (4) and a fuel electrode (3); an air supply device (19) for supplying air; a fuel gas supply device (20) for supplying fuel gas, and a humidity regulation module (5) allowing movement of water from a humid air passage (6) to a dry air passage (7). The air electrode (4) is divided into an upstream air electrode (4a) and a downstream air electrode (4b). The air supplied from the air supply device (19) is supplied to the upstream air electrode (4a) after passing through the dry air passage (7). The air discharged from the upstream air electrode (4a) is supplied to the downstream air electrode (4b) after passing through the humid air passage (6).

FIELD OF THE INVENTION

This invention relates to a fuel cell system and in particular, to water management in the oxygen electrode of the fuel cell, in other words, the cathode.

BACKGROUND OF THE INVENTION

Tokkai 2000-164232 published by the Japanese Patent Office in 2000 discloses a humidity regulation module for a fuel cell. The humidity regulation module comprises a dry air passage disposed in an air supply channel to the air electrode (oxygen electrode) and a humid air passage disposed in a discharge channel from the air electrode. Discharge air having a high moisture content flows through the humid air passage and supplied (fresh) air having a low moisture content flows through the dry air passage. As a result, since the moisture contained in the discharge air is transferred to the supplied air, the air to be supplied to the fuel cell is humidified.

SUMMARY OF THE INVENTION

Oxygen is consumed in response to the rate of the electrochemical reaction for the power generation in the vicinity of the air electrode of the fuel cell according to the reaction (1/202+2H++2e−→H20). The consumption of oxygen results in production of an amount of water which is twice the amount of the consumed oxygen.

In a prior-art fuel cell system, moisture in the air in the humid air passage disposed downstream of the air electrode returns to the dry air passage upstream of the air electrode. Further, in an air channel facing the air electrode, air absorbs water from the air electrode in proximity to the air channel inlet and transports the absorbed water to the vicinity of the air channel outlet. Thus the air channel in the vicinity of the air channel outlet contains moisture added in the dry air passage as well as moisture from the produced water in the air electrode. On the other hand, since oxygen is consumed while passing through the air channel facing the air electrode, the molar fraction of oxygen in the air is small in proximity to the air channel outlet. When an excess in the molar fraction of moisture and a shortfall in the molar fraction of oxygen occur in the air channel in this manner, fuel cell operations are impeded as a result of a decrease in the partial pressure of oxygen in the air channel or as a result of excessive condensation of water in the air electrode.

It is therefore an object of this invention to provide a fuel cell assembly suppressing excessive moisture content and a shortfall in the oxygen content in air in proximity to the air channel outlet.

In order to achieve the above object, this invention provides a fuel cell assembly comprising: a fuel cell having first and second oxygen electrodes, a hydrogen electrode and an electrolyte membrane disposed between the hydrogen electrode and the first and second oxygen electrodes; a hydrogen gas channel for supplying fuel gas containing hydrogen to the hydrogen electrode, the hydrogen gas channel facing the hydrogen electrode; a first oxidant gas channel for supplying oxidant gas to the first oxygen electrode, the first oxidant gas channel facing the first oxygen electrode; a second oxidant gas channel for supplying the oxidant gas which has passed through the first oxidant gas channel to the second oxidant electrode, the second oxidant channel facing the second oxygen electrode; and a dehumidifier for dehumidifying the oxidant gas which has passed through the first oxidant gas channel, the dehumidifier being disposed downstream of the first oxidant gas channel and upstream of the second oxidant gas channel.

Further, this invention provides a fuel cell system comprising: a fuel cell having an electrolyte membrane, an air electrode and a fuel electrode; the electrolyte membrane being disposed between the air electrode and the fuel electrode; an air supply device for supplying air to the air electrode; a fuel gas supply device for supplying fuel gas to the fuel electrode; and a humidity regulation module having a dry air passage and a humid air passage and allowing movement of water from the humid air passage to the dry air passage. The air electrode is divided into an upstream section and a downstream section with respect to a flow of supplied air. Air supplied from the air supply device passes through the upstream section of the air electrode after passing through the dry air passage of the humidity regulation module, and air discharged from the upstream section of the air electrode is supplied to the downstream section of the air electrode after passing through the humid air passage of the humidity regulation module.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring toFIG. 1andFIG. 2, a first embodiment of this invention will be described. The fuel cell stack1is a stack of solid polymer fuel cells. The fuel cell stack1comprises a plurality of unit cells30separated by a bipolar plate16(separator). The unit cell30is provided with a membrane electrode assembly (MEA)35which has a polymer electrolyte membrane2, a fuel electrode3(in other words, hydrogen electrode) and an air electrode4(in other words, oxygen electrode). The fuel electrode3and the air electrode4sandwich the polymer electrolyte membrane2so that the polymer electrolyte membrane2is disposed between the fuel electrode3and the air electrode3. Each electrode is a gas diffusion electrode provided with a thin platinum catalytic layer making contact with the polymer electrolyte membrane2and a porous gas diffusion layer on the outer side of the platinum catalytic layer.

Referring toFIG. 1, the fuel cell stack1is formed by alternate lamination of a MEA35and a bipolar plate16. The section of the bipolar plate16in proximity to the air electrode4forms the upstream oxidant gas channel50aand downstream oxidant gas channel50bwhich are separated from each other. The upstream oxidant gas channel50aand the downstream oxidant gas channel50bare disposed in an adjacent parallel orientation. Precisely, the upstream oxidant gas channel50aand the downstream oxidant gas channel50bare separated by a plate16adisposed in the central section of the bipolar plate16in proximity to the air electrode4. The flow of air in the upstream oxidant gas channel50ais in the opposite direction to the flow of air in the downstream oxidant gas channel50b. Furthermore hydrogen supplied from the fuel gas supply inlet11aflows through the fuel electrode3in an orthogonal orientation to the flow of air flowing through the upstream oxidant gas channel50aand the downstream oxidant gas channel50b, and is discharged from the fuel gas discharge outlet11b.

FIG. 2shows a fuel cell system having a fuel cell stack1and a humidifying-dehumidifying system. For the sake of simplicity, only a single unit cell30is shown in the fuel cell stack1. Fuel gas containing hydrogen is supplied to the fuel electrode3through a fuel gas channel40(hydrogen gas channel) from a fuel gas supply device20. Air is supplied as a gaseous oxidant to the air electrode4through the upstream oxidant gas channel50aand the downstream oxidant gas channel50bfrom an air supply device19. The upstream oxidant gas channel50ais positioned upstream of the downstream oxidant gas channel50brelative to the flow of oxidant gas. The fuel cell stack1performs power generation operations as a result of the migration of protons in the polymer electrolyte membrane2from the fuel electrode3to the air electrode4.

The unit cell30of the fuel cell stack1comprises an upstream air electrode4a(first oxygen electrode) opposed to the upstream oxidant gas channel50a(first oxidant gas channel) and a downstream air electrode4b(second oxygen electrode) opposed to the downstream oxidant gas channel50b(second oxidant gas channel). The air electrode4is divided into the upstream air electrode4aand the downstream air electrode4bwith respect to a flow of supplied air. In this embodiment, the upstream air electrode4aand the downstream air electrode4bare physically connected. However the upstream air electrode4aand the downstream air electrode4bare functionally separated by the section4cof the air electrode which is not supplied with oxygen resulting in the absence of electrochemical reactions. Therefore, the upstream air electrode4aand the downstream air electrode4bserve as respectively independent electrodes.

The fuel cell stack1is provided with a humidity regulation module which has a humid air passage6and a dry air passage7. The air supplied from the air supply device19is introduced into the dry air passage7. Air which is discharged from the dry air passage7is supplied to the upstream oxidant gas channel50athrough an air supply passage77. Furthermore the air discharged from the upstream oxidant gas channel50ais introduced into the humid air passage6through an air passage81and the air discharged from the humid air passage6is supplied to the downstream oxidant gas channel50bthrough an air passage83.

The fuel cell system is provided with a controller15for controlling auxiliary devices such as pumps or gas flow rate control valves in the fuel system. The controller21comprises a microprocessor provided with a central processing unit (CPU), a read-only memory (ROM), a random access memory (RAM) and an input/output interface (I/O interface).

The structure of the fuel cell stack1will be described referring toFIG. 3A-3E. The outer appearance of the fuel cell stack11is shown inFIG. 3A-3E. The arrow inFIG. 3A-3Eshows the direction of lamination80in the fuel cell stack. InFIG. 3A-3E, air is supplied from the air supply inlet10ato the upstream oxidant gas channel50aand is discharged from the medial air outlet10b. An air manifold91ais connected between the air supply inlet10aand the plurality of upstream oxidant gas channels50aof the fuel cell stack1so as to distribute air to the plurality of upstream oxidant gas channels50a. Similarly, an air manifold91bis connected between the medial air outlet10band the plurality of upstream oxidant gas channels50aof the fuel cell stack1so as to collect air from the plurality of upstream oxidant gas channels50a. Thereafter the air is introduced into the downstream oxidant gas channel50bfrom the medial air inlet10cafter passing through humidity regulation module5. The oxygen in the air diffuses into the upstream air electrode4afrom the upstream oxidant gas channel50aand into the downstream air electrode4bfrom the downstream oxygen gas channel50b. After passing through the downstream air electrode4b, the air is discharged from the air discharge outlet10d. An air manifold91cis connected between the medial air inlet10cand the plurality of downstream oxidant gas channels50bof the fuel cell stack1so as to distribute air to the plurality of downstream oxidant gas channels50b. Similarly, an air manifold91dis connected between the air discharge outlet10dand the plurality of downstream oxidant gas channels50bof the fuel cell stack1so as to collect air from the plurality of downstream oxidant gas channels50b.

On the other hand, fuel gas from the fuel gas supply inlet11ais introduced into the fuel electrode3. The fuel gas is discharged from the fuel gas discharge outlet11bafter hydrogen contained therein is used in power generation operations. A fuel gas manifold95ais connected between the fuel gas supply inlet11aand the plurality of fuel gas channels40of the fuel cell stack1so as to distribute fuel gas to the plurality of fuel gas channels40. Similarly, a fuel gas manifold95bis connected between the fuel gas discharge outlet11band the plurality of fuel gas channels40of the fuel cell stack1so as to collect fuel gas from the plurality of fuel gas channels40. The direction of flow of the fuel gas in the fuel electrode3is orthogonal to the direction of airflow in the air electrode4.

During power generation operations, protons can migrate in the electrolyte membrane2as a result of conversion into hydrated protons. For this reason, it is necessary to maintain the humidity characteristics of the polymer electrolyte membrane2. There is a known method of humidifying at least one of the air and the fuel gas. At the air electrode4of the fuel cell stack1, two moles of water are produced for each mole of oxygen consumed as shown by the reaction (1/2O2+2H++2e−→H2O). The humidity regulation module5can use water produced by the air electrode4in order to humidify air to be supplied to the air electrode4before it is introduced into the fuel cell stack1.

FIG. 4is a schematic diagram showing the humidity regulation module5. In the humidity regulation module5, a plurality of humid air passages6are alternated in parallel to a plurality of dry air passages7. A water permeable membrane8allowing selective permeation of water is disposed between the humid air passage6and the dry air passage7so as to separate the humid air passage6and the dry air passage7. Air used in power generation operations, in other words, air to be supplied to the air electrode4flows through the dry air passage7. Air discharged from the upstream oxidant gas channel50athrough the medial air outlet10bflows to the humid air passage6. The water permeable membrane8and the humid air passage6constitute a dehumidifier for dehumidifying the air which has passed through the upstream oxidant gas channel50a. In contrast, the water permeable membrane8and the dry air passage7constitute a humidifier for humidifying air to be supplied to the upstream oxidant gas channel50a. The respective air inlets are disposed on the fuel cell stack1so that the direction of flow of air in the dry air passage7is opposite to the direction of flow of air in the humid air passage6.

Referring toFIG. 5A-5E, the assembly60of the fuel cell stack1and the humidity regulation module5will be described. The arrow inFIG. 5A-5Eshows the direction of lamination80in the fuel cell stack. The air supply device19is connected by a pipe to the inlet7aof the dry air passage7. Similarly, the air supply inlet10afor the upstream air electrode4ais connected by a pipe used as the air supply passage77to the outlet7bof the dry air passage7. Furthermore the inlet6aof the humid air passage6is connected to the medial air outlet10bfor the upstream air electrode4athrough a pipe used as the air passage81. The medial air inlet10cand the outlet6bof the humid air passage6are connected by a pipe used as the air passage83.

Referring toFIG. 6A, air passes in sequence through the dry air passage7, the upstream oxidant gas channel50aopposed to the upstream air electrode4a, the humid air passage6and the downstream oxidant gas channel50bopposed to the downstream air electrode4b. Air flows in a substantially horizontal direction in the upstream and downstream oxidant gas channels50a,50b. Referring toFIG. 6B, in the fuel cell stack1, fuel gas flows orthogonal with respect to the flow of air and in a substantially vertically downward direction. The fuel gas flow in a substantially vertically downward direction readily transfers water on the fuel electrode3to a water drain passage73extending from the fuel gas discharge passage71, with the assistance of gravity. (Water is usually present on the fuel electrode3because of osmosis of water from the air electrodes4a,4bto fuel electrode3.) If fuel gas flows in a vertically upward direction, the fuel gas blows off the water accumulated in the water drain passage73above a drain valve75. Further, the upstream oxidant gas channel50ais positioned on the downstream side with respect to the flow of the fuel gas and below the downstream oxidant gas channel50b. Oppositely, the downstream oxidant gas channel50bis positioned on the upstream side with respect to the flow of the fuel gas and above the upstream oxidant gas channel50a. This configuration can promote a homogeneous power generation in the MEA35by compensating for the difficulty of generating power on the downstream side of the fuel electrode3and on the downstream air electrode4b. Power generation using fuel gas is relatively impeded on the downstream side with respect to the fuel gas flow because the fuel gas is consumed while flowing through the fuel electrode3. Power generation using oxygen is relatively impeded on the downstream air electrode4bbecause the oxygen amount is lower on the downstream air electrode4bthan on the upstream air electrode4a.

Referring toFIG. 6C, cooling water flows in the opposite direction to the flow of fuel gas. Referring toFIG. 6D, electrical current flows in the direction of lamination in the fuel cell stack1and is collected to the positive terminal17and negative terminal18. An electrical circuit is connected to the positive terminal17and negative terminal18provided on both ends of the fuel cell stack1in order to extract electrical power.

Air supplied to the humid air passage6contains moisture supplied as a result of humidifying air in the humidity regulation module5as well as moisture (water) produced at the upstream air electrode4a. A portion of the moisture in the air flowing through the humid air passage6migrates towards the dry air passage7through the water permeable membrane8and humidifies air flowing through the dry air passage7.

Referring toFIG. 7, the spatial variation in the oxygen amount and the moisture in the air along the air electrode4of the fuel cell stack1according to the first embodiment will be described. For the purposes of comparison,FIG. 8shows the spatial variation in the oxygen amount and the moisture in the air along the air electrode in a prior-art fuel cell system. In the prior-art fuel cell system, the air electrode4is not divided and dehumidifying operations are not performed on air flowing through the fuel cell stack1. In the prior-art fuel cell system, moisture contained in air after it has completely flowed through the fuel cell stack1is removed and used in humidifying operations.

Referring toFIG. 8A-8E, a prior-art technique will be described. As shown inFIG. 8A, the moisture amount (water amount) increases according to the flow of air. In contrast, since the oxygen in the air is consumed by power generation reactions, as shown inFIG. 8B, the oxygen amount is reduced along the direction of airflow. As shown inFIG. 8C, the molar fraction of moisture (or partial vapor pressure) increases downstream. Thus in downstream sections of the air electrode, power generation using oxygen is difficult.

In contrast, in the first embodiment of this invention, the air electrode4is divided into the upstream air electrode4aand the downstream air electrode4b. The humidity regulation module5performs dehumidifying operations on air between the upstream oxidant gas channel50afor supplying air to the upstream air electrode4aand the downstream oxidant gas channel50bfor supplying air to the upstream air electrode4b. Referring toFIG. 7AandFIG. 7C, on the border of the upstream air electrode4aand the downstream air electrode4b, the moisture amount in the air falls by an amount corresponding to the moisture amount reduction which is the moisture amount shifting from the humid air passage6to the dry air passage7. Furthermore as shown inFIG. 7B, the oxygen amount in the air is reduced as the air approaches the air discharge outlet10d(also refer toFIG. 8B).

As shown inFIG. 7C, the molar fraction of moisture in the air decreases between the upstream air electrode4aand the downstream air electrode4b. After this decrease, the molar fraction of moisture in the air increases as the air re-approaches the air discharge outlet10d. The molar fraction of moisture in the air in proximity to the air discharge outlet10dis small in comparison to that shown inFIG. 8C. Only moisture produced as a result of power generation operations is contained in the air in proximity to the outlet of the downstream oxidant gas channel50b.

Referring toFIG. 7D, the molar fraction of oxygen in the air increases as the amount of moisture decreases. In other words, after the molar fraction of oxygen increases between the upstream air electrode4aand the downstream air electrode4b, it starts to decrease again. However in proximity to the outlet of the air electrode4, the molar fraction of oxygen in the air is a large value in comparison to that shown inFIG. 8D.

As shown inFIG. 7A, the moisture added during humidifying operations is reused since it moves from the humid air passage6to the dry air passage7. In this manner, since the air discharged from the dry air passage7is humidified by the reused water, it is possible perform highly efficient power generation operations in the upstream air electrode4a.

The effect of this embodiment will be described below. The air electrode4is divided into at least two sections and air is supplied to the upstream air electrode4aand to the downstream air electrode4b. After a portion of the water contained in the air discharged from the upstream air electrode4ais removed, the air is supplied to the downstream air electrode4b. In this manner, it is possible to avoid a reduction in the partial pressure of oxygen in proximity to the air discharge outlet of the air electrode4. Consequently it is possible to improve power generation efficiency. In addition it is possible to suppress flooding as a result of the excess condensation of water in proximity to the air discharge outlet10dof the air electrode4.

Power generation operations can be promoted by shifting a portion of the water removed from the air which has passed through the upstream oxidant gas channel50ainto the air to be supplied to the upstream air electrode4a. In this manner, it is possible to improve the efficiency of water use.

The fuel cell assembly is provided with a humidity regulation module5which allows water to move from the humid air passage6to the dry air passage7. The air which is scheduled to be supplied to the air electrode4passes through the dry air passage7. The air discharged from the upstream air electrode4apasses through the humid air passage6and then is supplied to the downstream air electrode4b. In this manner, it is possible to supply water in the air discharged from the upstream air electrode4ato the air to be supplied to the upstream air electrode4a.

When the flow rate of air introduced from the air supply device19is fixed, moisture supplied for humidifying operations upstream of the upstream air electrode4ais removed from the air discharged from the upstream air electrode4a.

Since it is possible to suppress the amount of moisture contained in the air supplied to the downstream air electrode4b, it is possible to suppress flooding which tends to occur in the downstream air electrode4b.

Referring toFIG. 9, a second embodiment of the invention will be described.FIG. 9shows the structure of the humidifying system and the fuel cell stack1according to the second embodiment. For the sake of simplicity, only a single unit cell30is shown in the fuel cell stack1.

In the second embodiment, a pressure regulation valve12which regulates air pressure is provided between an inlet6aof the humid air passage6and an outlet7bof the dry air passage7. Preferably, the pressure regulation valve12is provided in the air supply passage77which connects the air supply inlet10aof the upstream air electrode4awith the outlet7bof the dry air passage7. The pressure regulation valve12may be provided upstream of the medial air outlet10band downstream of the outlet7bof the dry air passage7. The fuel cell stack1and the humidity regulation module5are the same as the components used in the first embodiment. The controller15controls the opening of the pressure regulation valve12in response to the power required for the fuel cell stack1.

Referring to the flowchart inFIG. 10, the control routine for the pressure regulation valve12which is executed by the controller15will be described.

Firstly in a step S1, the power required for the fuel cell stack1is read. When the fuel cell system used in order to drive a vehicle for example, the vehicle accelerator functions as a sensor41for detecting the required power and the required power corresponds to the depression amount of the vehicle accelerator. In a step S2, the opening of the pressure regulation valve12is set in response to the required power read in the step S1. The setting of the opening is performed by looking up a map as shown inFIG. 11which is obtained from experimentation and stored in the ROM of the controller15.

The map shown inFIG. 11shows the relationship between the opening of the pressure regulation valve12and the required power. At low levels of power generation, the amount of water produced in the downstream air electrode4bis low and the required humidifying amount for air to be supplied to the upstream air electrode4ais also low. Consequently at low levels of power generation, the humidifying amount in the humidity regulation module5is decreased by reducing the opening of the pressure regulation valve12. The pressure in the humid air passage6to the air electrode4is further reduced by closing the pressure regulation valve12. As a result, the amount of water moving from the humid air passage6to the dry air passage7is reduced. In contrast, at high levels of power generation, the amount of water produced in the downstream air electrode4bis large and the amount of water in the upstream air electrode4ais also large. As a result, at high levels of power generation, the amount water moving from the humid air passage6to the dry air passage7in the humidity regulation module5is increased by enlarging the opening of the pressure regulation valve12in comparison to the opening at low levels of power generation.

In a step S3, the pressure regulation valve12is controlled in order to realize the opening for the pressure regulation valve12set in the step S2. The control routine above may be repeated at a fixed interval or may be executed when the required power is varied.

The effect of the second embodiment will be described below. A pressure regulation valve12is disposed between the outlet7bof the dry air passage7and the inlet6aof the humid air passage6. The pressure in the humid air passage6and the dry air passage7is regulated by regulating the pressure regulation valve12. In this manner, the amount of water movement (amount of reused water) in the humidity regulation module5can be regulated.

Referring toFIG. 12, a third embodiment of this invention will be described. For the sake of simplicity, only a single unit cell30is shown in the fuel cell stack1ofFIG. 12. In the humidity regulation module5according to the third embodiment, a dry fuel gas passage37is provided instead of the dry air passage7according to the first embodiment. Fuel gas is introduced into the dry fuel gas passage37from the fuel gas supply device20and is humidified by water moving from the humid air passage6to the dry fuel gas passage37. Thereafter humidified fuel gas is supplied to the fuel gas supply inlet1la of the fuel gas channel40. In this manner, a portion of the water removed from the air discharged from the upstream oxidant gas channel50ais supplied to the dry fuel gas passage37through the water permeable membrane8. In contrast, air is supplied directly from the air supply device19to the upstream oxidant gas channel50a. Other features are the same as those described with respect to the first embodiment.

Referring toFIG. 13, a fourth embodiment of this invention will be described. For the sake of simplicity, only a single unit cell30is shown in the fuel cell stack1ofFIG. 13. In the fourth embodiment, the upstream air electrode4a(first oxygen electrode) and the downstream air electrode4b(second oxygen electrode) are physically separated. The upstream air electrode4aand the downstream air electrode4bserve as respectively independent electrodes. Oxygen is supplied from the upstream oxidant gas channel50ato the upstream air electrode4aand from the downstream oxidant gas channel50bto the downstream air electrode4b. Other features of structures are the same as those described referring to the first embodiment.

Referring toFIG. 14, the structure of the humidifying system and the fuel cell stack1according to a fifth embodiment will be described. For the sake of simplicity, only a single unit cell30is shown in the fuel cell stack1ofFIG. 14.

The polymer electrolyte membrane2is physically separated into a first portion2aand a second portion2b. The fuel electrode3is also physically separated into a first portion3aand a second portion3b.

A power regulation element13for regulating the power (upstream power) extracted from the upstream air electrode4aand a power regulation element14for regulating the power (downstream power) extracted from the downstream air electrode4bare provided. The power regulation elements13,14comprise transistors such as an Insulated Gate Bipolar Transistor. The upstream air electrode4ais electrically connected to a first power regulation element13and the downstream air electrode4bis electrically connected to a second power regulation element14. The power regulation elements13,14are electrically connected to the positive terminal17. The first portion3aand the second portion3bare directly connected to the negative terminal18.

In this manner, the controller15performs independent control of the upstream power and the downstream power by regulating a PWM signal transmitted to the gate of the power regulation element13,14. For example, the controller15suppresses the downstream power to a smaller value than the upstream power, thereby reducing the amount of water produced in the downstream air electrode4bto prevent flooding which tends to occur downstream.

The upstream air electrode4aand the downstream air electrode4bin the fuel cell stack1are electrically connected after passing through respective power regulation elements13,14. Thus it is possible to perform independent control of the upstream and downstream power in the fuel cell and to regulate the amount of water produced in the downstream air electrode4b.

Referring toFIG. 15-17, the assembly60of the fuel cell stack and the humidity regulation module according to a sixth embodiment will be described. The sixth embodiment differs from the first embodiment in that the humidity regulation module is directly connected to a side surface130of the fuel cell stack1which is substantially parallel to the direction of lamination and is integrated with the air manifolds91b,91cshown in FIGS.3-6. In this embodiment, the humidity regulation module105serves as an air manifold for collecting air from the plurality of upstream oxidant gas channels50aand distributing air to the plurality of downstream oxidant gas channels50b, while allowing movement of water from a humid air passage thereof to a dry air passage thereof. It should be noted that pipes used as the air passages81,83are omitted resulting in both a cost reduction and improved structural strength in the assembly60.

Referring toFIG. 15A-15C, the humidity regulation module105comprises a plurality of hollow fiber membranes108each of which forms a dry air passage in its hollow section and a housing110which forms a humid air passage inside. The hollow fiber membranes108is provided in the housing110, and thus the humid air passage is a space120formed between the housing110and the hollow fiber membranes108as well as between the hollow fiber membranes108. Unlike the first embodiment, the humid air passage of the space120is directly connected to the upstream and downstream oxidant gas channels50a,50b. Air discharged from the upstream oxidant gas channels50ais directly introduced into the space120and is dehumidified by the selective permeation of water through the hollow fiber membranes108from the humid air passage of the space120to the dry air passage inside the hollow fiber membranes108. Then, the dehumidified air is discharged from the humid air passage of the space120into the downstream oxidant gas channels50b. Thus, the humidity regulation module105turns the flow direction of air from the upstream oxidant gas channels50ato the downstream oxidant gas channels50b. For the sake of clarity,FIGS. 16A and 17Ashow an exploded perspective view of the assembly60comprising the fuel cell stack1and the humidity regulation module105.

As shown inFIG. 15A, the hollow fiber membranes108may be evenly distributed in the housing110with a substantially constant density. In this case, since air flows mainly near the fuel cell stack1in the humid air passage of the space120, the humidifying efficiency of the hollow fiber membranes108decreases as the distance from the fuel cell stack1increases. Therefore, as shown inFIGS. 15B and 16B, the hollow fiber membranes108may be unevenly distributed in the housing110with the density of the hollow fiber membranes108decreasing away from the fuel cell stack1if the size of the humidity regulation module105is allowed to be enlarged from that ofFIG. 15A. In this case, air can flow sufficiently on the distal side away from the fuel cell stack1because the space between the hollow fiber membranes108is enlarged on the distal side.

Further, as shown inFIG. 15C, the air supply passage77shown as a pipe in the first embodiment passes thorough the inside of the fuel gas manifold95bdisposed between the fuel gas discharge passage71and the plurality of fuel gas channels40, and thus a part of the air supply passage77is integrated with the fuel gas manifold95band the body of the fuel cell stack1. The detailed structure of the fuel gas manifold95bis shown inFIGS. 17A and 17B. Thus the structural strength of the assembly60can be further improved in comparison to the first embodiment.

Modifications and variations of the embodiments described above will occur to those skilled in the art, in light of the above teachings.

In the above embodiments, although the air electrode4has been divided into two sections, it is not limited in this regard and may be divided in multiple sections. Furthermore although the air electrode has been divided into two equal sections, it is not limited in that respect.

Instead of the humidity regulation module5, a dehumidifier may provided. In this case, the water in the air discharged from the upstream air electrode4ais dehumidified in the dehumidifier and recycled to a water tank. The fuel gas channel40, the upstream oxidant gas channel50aand the downstream oxidant gas channel50bmay be a collection a plurality of channels. In other words, the fuel gas channel40, the upstream oxidant gas channel50aand the downstream oxidant gas channel50bmay be divided into a plurality of more narrow channels disposed in a mutually parallel orientation as shown inFIG. 16AandFIG. 17A.

The entire contents of Japanese Patent Application P2002-252738 (filed Aug. 30, 2002) are incorporated herein by reference.

The scope of the invention is defined with reference to the following claims.