Gas channel forming plate for fuel cell and fuel cell stack

A gas channel forming plate includes protrusions, which extend parallel with each other, gas channels that are respectively located between each adjacent pair of the protrusions, and water channels, which are respectively formed on the back surface of each protrusion. Each protrusion includes first communication portions and second communication portions. Each first communication portion includes a first opening. Each second communication portion includes a second opening. The second communication portions of each protrusion constitute an expanding region, in which the opening area of the second opening in each second communication portion is greater than the opening area of the first opening of each first communication portion, to limit introduction of water to the water channel on the back side of the protrusion using capillary action by the second communication portions.

RELATED APPLICATION

The present application claims priority to Japanese Application No. 2015-163694, filed Aug. 21, 2015, which is hereby incorporated herein in its entirety by reference.

BACKGROUND OF THE INVENTION

The present invention relates to a gas channel forming plate that is arranged between a membrane electrode assembly and a plate-shaped flat separator and constitutes a separator of a single cell in a fuel cell, and to a fuel cell stack formed by stacking single cells.

For example, a solid polymer fuel cell includes a fuel cell stack, which is formed by stacking single cells. Each single cell includes a membrane electrode assembly and a pair of separators. The membrane electrode assembly is placed between the separators.

As such a separator, there is a separator including a plate-shaped flat separator and a gas channel forming plate that is arranged between a membrane electrode assembly and the flat separator (for example, refer to Japanese Laid-Open Patent Publication No. 2015-15218).

Grooves that extend parallel with each other are formed on a surface of the gas channel forming plate that faces the membrane electrode assembly. The grooves constitute gas channels, through which fuel gas or oxidant gas flows. The gas channel forming plate includes protrusions that are each formed between the corresponding adjacent pair of gas channels. In addition, grooves are formed on the back surfaces of the protrusions. The grooves constitute water channels for discharging water produced during power generation. The projections include communication passages through which the gas channels communicate with water channels and water in the gas channels is introduced to the water channels using capillary action.

In the fuel cell stack, water produced during power generation in the membrane electrode assembly flows out to the gas channels of the gas channel forming plate and is introduced to the water channels through the communication passages. The water is then discharged to the exterior of the water channels by the flow pressure of fuel gas or oxidant gas (hereinafter, referred to as gas) that flows through the water channels.

In the fuel cell stack, dry gas is introduced into the gas channels. Thus, a portion of the membrane electrode assembly that is close to the entrances of the gas channels is easily dried. Especially, at a low load time at which a small amount of power is generated, the amount of water produced during power generation in the membrane electrode assembly decreases. Thus, the membrane electrode assembly is more easily dried. As a result, movement of protons via water is hampered in the membrane electrode assembly, and it becomes a cause of decrease in the power generation performance.

SUMMARY OF THE INVENTION

It is an objective of the present invention to provide a gas channel forming plate for a fuel cell and a fuel cell stack that reduce water staying in the gas channels, while limiting decrease in the power generation performance caused by the dry membrane electrode assembly.

To achieve the objective, a gas channel forming plate for a fuel cell is arranged between a membrane electrode assembly and a plate-shaped flat separator to constitute a separator of a single cell of a fuel cell. Fuel gas and oxidant gas are supplied to the membrane electrode assembly to generate power and produce water. The gas channel forming plate includes a surface that faces the membrane electrode assembly, a plurality of protrusions that is formed on the surface and extends parallel with each other in an extending direction, a plurality of groove-shaped gas channels, each of which is located between an adjacent pair of the protrusions, and a plurality of groove-shaped water channels, each of which is formed on a back surface of one of the protrusions. One of the fuel gas and the oxidant gas flows into the membrane electrode assembly through the gas channels, and the water produced during power generation in the membrane electrode assembly flows out to the gas channels. Each protrusion includes a plurality of communication portions, which is formed to be separated from each other in the extending direction. The communication portions of each protrusion are configured to allow at least one gas channel adjacent to the protrusion to communicate with a water channel on a back side of the protrusion and to introduce water in the at least one gas channel adjacent to the protrusion to the water channel on the back side of the protrusion using capillary action. The communication portions include a plurality of first communication portions and a plurality of second communication portions, which is located upstream of the first communication portions in a flowing direction of gas. Each first communication portion includes a first opening, which opens in the surface that faces the membrane electrode assembly. Each second communication portion includes a second opening, which opens in the surface that faces the membrane electrode assembly. The second communication portions of each protrusion constitute an expanding region, in which an opening area of the second opening in each second communication portion is greater than an opening area of the first opening of each first communication portion, to limit introduction of water to the water channel on the back side of the protrusion using capillary action by the second communication portions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference toFIGS. 1 to 9D, one embodiment will now be described.

As shown inFIG. 1, a solid polymer electrolyte fuel cell includes a fuel cell stack formed by stacking single cells10. The single cell10on the upper side inFIG. 1has a cross-section taken along such a line that first and second water channels26and36, which will be described later, are visible while the single cell10on the lower side has a cross-section taken along such a line that first and second gas channels25and35, which will be described later, are visible.

Each single cell10includes a rectangular first frame11and second frame12. The first and second frames11and12hold the outer edge of a rectangular sheet-shaped membrane electrode assembly13of a known type in between.

The membrane electrode assembly13includes a solid polymer electrolyte membrane14. First and second electrode catalyst layers15and16of a known type hold the solid polymer electrolyte membrane14in between. First and second gas diffusion layers17and18of a known type are arranged on the surfaces of the first and second electrode catalyst layers15and16, respectively.

A first separator20and a second separator30sandwich the membrane electrode assembly13from the cathode-side (the lower side ofFIG. 1) and the anode-side (the upper side ofFIG. 1), respectively.

The first separator20includes a plate-shaped first flat separator21and a first gas channel forming plate22, which is arranged between the first flat separator21and the membrane electrode assembly13.

The second separator30includes a plate-shaped second flat separator31and a second gas channel forming plate32, which is arranged between the second flat separator31and the membrane electrode assembly13.

The flat separators21and31and the gas channel forming plates22and32are each formed of a metal plate.

The first frame11and the first flat separator21define a first supply passage41for supplying oxidant gas from an oxidant gas supply source (not shown) to a first gas channel25, which will be described later, and a first discharge passage42for discharging oxidant gas that has not been used for power generation to the exterior of the first gas channel25.

The second frame12and the second flat separator31define a second supply passage51for supplying fuel gas from a fuel gas supply source (not shown) to a second gas channel35, which will be described later, and a second discharge passage52for discharging fuel gas that has not been used for power generation to the exterior of the second gas channel35.

In the portion shown inFIG. 1, the second gas channel forming plate32of the second separator30has a shape obtained by inverting the first gas channel forming plate22of the first separator20vertically and horizontally. Thus, hereinafter, the first gas channel forming plate22of the first separator20will be described, and the repetitive description of the second gas channel forming plate32of the second separator30will be omitted by giving reference numerals “3*”, which are obtained by adding 10 to reference numerals “2*” of components of the first gas channel forming plate22of the first separator20, to corresponding components of the second gas channel forming plate32of the second separator30. In addition, the repetitive description will be omitted by giving reference numerals “37*” and “38*,” which are obtained by adding 100 to reference numerals “27*” and “28*.” The sign * indicates one of numbers from zero to nine.

The structure of the first gas channel forming plate22will now be described.

As shown inFIG. 2, the first gas channel forming plate22has a corrugated cross section and is formed by, e.g., rolling one metal plate such as a stainless steel plate. Inner protrusions23, which extend parallel with each other, are formed on the upper surface of the first gas channel forming plate22, i.e., the surface of the first gas channel forming plate that faces the membrane electrode assembly13. The top surfaces of the inner protrusions23are in contact with the membrane electrode assembly13. A groove-shaped first gas channel25, through which oxidant gas flows, is formed between each adjacent pair of inner protrusions23.

Outer protrusions24, which extend parallel with each other, are formed on the lower surface of the first gas channel forming plate22. The top surfaces of the outer protrusions24are in contact with the flat separator21. A groove-shaped first water channel26for discharging water produced during power generation in the membrane electrode assembly13is formed on the back surface of each inner protrusion23. Thus, each outer protrusion24is located between the corresponding adjacent pair of first water channels26and defines the water channels26.

Each inner protrusion23includes first and second communication portions27and28, which allow the corresponding first gas channel25and first water channel26to communicate with each other, in the extending direction L of the inner protrusions23.

As shown inFIGS. 2, 3, and 6, the first communication portions27are located downstream of the second communication portions28in the flowing direction of gas. Each first communication portion27includes two first slits271, which are formed at a predetermined interval L1in the extending direction L of the inner protrusions23. Each first slit271extends perpendicular to the extending direction L of the inner protrusions23and has a preset width A1. The width A1of the first slit271is set to be the same measurement as the interval L1between each adjacent pair of first slits271(L1=A1). The width A1of each first slit271is set to have a measurement that allows water in the corresponding first gas channel25to be introduced to the corresponding first water channel26using capillary action.

As shown inFIGS. 2 to 4B, first ribs272(first intermediate structure portions), which face the first slits271, are formed inside each first water channel26. The first ribs272extend in the direction perpendicular to the extending direction L of the inner protrusions23(hereinafter, referred to as a width direction W). Each first rib272is formed by shear-bending a part of the corresponding inner protrusion23when a metal plate is rolled in the width direction W to form the first gas channel forming plate22. In other words, each first slit271is formed by forming the corresponding first rib272. As shown inFIG. 4A, each first rib272is located inside the corresponding first water channel26. As shown inFIG. 4B, each outer protrusion24includes communication grooves273, which allows the corresponding pair of first water channels26that is adjacent to each other via the outer protrusion24to communicate with each other.

As shown inFIG. 2, in each inner protrusion23, the first communication portions27are formed at intervals L3in the extending direction L of the inner protrusions23. The interval L3between each adjacent pair of first communication portions27is set to be greater than the interval L1between each pair of first slits271of the first communication portions27(L3>L1). In addition, each first communication portion27of each inner protrusion23is located in the middle point between the corresponding adjacent pair of first communication portions27of an adjacent inner protrusion23.

The second communication portions28are located upstream of the first communication portions27in the flowing direction of gas. Each second communication portion28includes a pair of second slits281, which is formed at a predetermined interval L2, in the extending direction L of the inner protrusions23.

Each second slit281extends perpendicular to the extending direction L of the inner protrusions23and has a preset width A2. The width A2of each second slit281is set to be greater than width A1of each first slit271of the first communication portions27(A2>A1). In other words, each first slit271constitutes a first opening that opens in the surface that faces the membrane electrode assembly13(the upper surface) in the corresponding first communication portion27. Each second slit281constitutes a second opening that opens in the surface that faces the membrane electrode assembly13(the upper surface) in the corresponding second communication portion28. The opening area of each second opening of the second communication portions28is expanded to be greater than the opening area of each first opening of the first communication portions27. In other words, the second communication portions28, which are located upstream in the flowing direction of gas, constitute an expanding region, in which the opening area of the second openings that open in the surface that faces the membrane electrode assembly13of each second communication portion28is greater than the opening area of the first openings of each first communication portion27, which are located downstream in the flowing direction of gas. The width A2of each second slit281is set to be the same measurement as the interval L2between each pair of second slits281(L2=A2).

Second ribs282(second intermediate structure portions), which face the second slits281, are formed inside each first water channel26. The second ribs282are formed in a method similar to the first ribs272of the first communication portions27.

A partition284between each pair of the second slits281that constitutes the corresponding second communication portion28in the corresponding inner protrusion23is located inside the corresponding first water channel26.

In each inner protrusion23, the second communication portions28are formed at a predetermined interval L4in the extending direction L of the inner protrusions23. The interval L4of each adjacent pair of second communication portions28is set to be greater than the interval L2between each pair of second slits281of the second communication portions28(L4>L2). In addition, each second communication portion28of each inner protrusion23is located in the middle point between the corresponding adjacent pair of second communication portions28of an adjacent inner protrusion23.

The first and second ribs272and282of the first and second communication portions27and28are arranged closer to the top surfaces of the outer protrusions24than the top surfaces of the inner protrusions23in the thickness direction of the first gas channel forming plate22(the vertical direction ofFIGS. 4A, 4B, 6, and 7). Even when the cross-sectional area of each first gas channel25and the cross-sectional area of each first water channel26at a location at which the first and second ribs272and282do not exist in the extending direction L are set to be the same as each other, the pressure loss in the entire first water channel26is greater than the pressure loss in the entire first gas channel25due to the first and second ribs272and282. The shapes and sizes of the first and second slits271and281are set such that the pressure loss at the first and second slits271and281is greater than the pressure loss in the first gas channel25. Therefore, oxidant gas mainly flows through the first gas channel25with smaller pressure loss.

Operation of the present embodiment will now be described.

As indicated by the single cell10on the lower side inFIG. 1, when fuel gas is supplied into each second gas channel35through the second supply passage51, the fuel gas flows into the second gas diffusion layer18through the second gas channel35. The fuel gas is then supplied to the second electrode catalyst layer16by passing through the second gas diffusion layer18and being diffused.

When oxidant gas is supplied into each first gas channel25through the first supply passage41, the oxidant gas flows into the first gas diffusion layer17through the first gas channel25. The oxidant gas is then supplied to the first electrode catalyst layer15by passing through the first gas diffusion layer17and being diffused.

In this way, when the fuel gas and the oxidant gas are each supplied to the membrane electrode assembly13, power is generated by electrochemical reaction in the membrane electrode assembly13.

At this time, water produced during power generation mainly flows out to the first gas channel25of the first gas channel forming plate22on the cathode-side.

As indicated by white thick arrows inFIGS. 8A and 8B, some of the water produced during power generation flows in each first gas channel25by flow pressure of oxidant gas flowing through the first gas channel25. The water is then discharged to the exterior through the first discharge passage42(refer toFIG. 1). As described above, the pressure loss at the slits271(281) is set to be greater than the pressure loss at the first gas channels25. Thus, as shown inFIG. 8B, the oxidant gas mainly flows through the first gas channels25. This causes most of the water that exists in the first gas channels25to move in the first gas channels25toward the first discharge passage42while being pushed by the oxidant gas. As indicated by thin arrows inFIG. 8B, some of the water is introduced to a first water channel26through the corresponding slits271(281).

At this time, the water that has been introduced to the first water channel26becomes droplets by surface tension acting according to the opening area of the exit opening of the first water channel26. When the first water channel26is in a moistening state and a droplet S stays at a rib272, water in the first water channel26acts as priming water to guide the water in the corresponding first slits271into the first water channel26by capillary action and discharge the water through the exit opening.

When the first water channel26is in a dry state, in which no water that acts as priming water exists in the first water channel26, as shown inFIG. 9A, water is guided into the first slits271by capillary action in the top surface of the corresponding inner protrusion23, which is in contact with the first gas diffusion layer17, and forms droplets S1and S2.

When more water is guided to make the droplets S1and S2larger, as shown inFIG. 9B, the droplets S1and S2are merged to form one droplet S3. Immediately after the droplets S1and S2are merged to form the droplet S3, or the droplet S3becomes larger, the droplet S3contacts the first ribs272. When the droplet S3moves into the gap between the pair of first ribs272as shown inFIG. 9C, the droplet S3is introduced into the first water channel26by being drawn into the gap by capillary action as shown inFIG. 9D.

When the flow velocity of the oxidant gas is low, water is gradually collected in the first water channel26by introducing the droplet S3as described above.

When the flow velocity of the oxidant gas is high, water introduced to the first water channel26moves in the first water channel26toward the first discharge passage42(refer toFIG. 1) while being pushed by flow pressure of oxidant gas flowing in the first water channel26.

As described above, in a fuel cell stack, dry oxidant gas is introduced into each first gas channel25. Thus, a portion of the membrane electrode assembly13that is close to the entrance of the first gas channel25is likely to be dried. Especially, at a low load time, at which a small amount of power is generated, the amount of water produced during power generation in the membrane electrode assembly13is decreased. Thus, the membrane electrode assembly13is more likely to be dried.

In this respect, in the present embodiment, introduction of water to the first water channel26using capillary action by the second communication portions28is restrained by increasing the opening areas of the second communication portions28in the expanding region formed upstream in the flowing direction of gas. This limits drying of the upstream portion of the membrane electrode assembly13, which is likely to be dried. Thus, the membrane electrode assembly13can be kept moist to promote movement of protons.

In a region downstream of the expanding region, water in the first gas channels25is effectively introduced to the respective first water channels26using capillary action by first communication portions27. This restricts the flow of gas from being hampered by water staying in the first gas channels25.

When the amount of water produced in the membrane electrode assembly13is increased, water in the first gas channels25is also introduced to the first water channels26using capillary action by the second communication portions28in the expanding region. This restricts the flow of gas from being hampered by water staying in the first gas channels25.

Some of the water produced during power generation flows out to the second gas channels35of the second gas channel forming plate32through the second electrode catalyst layer16and the second gas diffusion layer18on the anode-side (the upper side ofFIG. 1). In the present embodiment, the second gas channel forming plate32on the anode-side has basically the same configuration as the first gas channel forming plates22on the cathode-side. Thus, the second gas channels35and the second water channels36on the anode-side also provide the same operation as the first gas channels25and the first water channels26on the cathode-side.

The gas channel forming plate for a fuel cell and the fuel cell stack according to the present embodiment, which is described above, achieves the following advantages.

(1) The inner protrusions23, which extend parallel with each other, are formed on the upper surface of the first gas channel forming plate22, i.e., a surface that faces the membrane electrode assembly13in the first gas channel forming plate. Each inner protrusion23includes the first communication portions27and the second communication portions28, which are located upstream of the first communication portions27in the flowing direction of gas. Each first communication portion27includes the first slits271. Each first slit271constitutes the first opening, which opens in the surface that faces the membrane electrode assembly13. Each second communication portion28includes the second slits281. Each second slit281constitutes the second opening, which opens in the surface that faces the membrane electrode assembly13. In each inner protrusion23, the second communication portions28constitute the expanding region, in which the opening area of the second openings of each second communication portion28is larger than the opening area of the first openings of each first communication portion27. This limits introduction of water to the first water channels26on the back sides of the inner protrusions23using capillary action by the second communication portions28.

Such a configuration limits drying of an upstream portion of the membrane electrode assembly13, which is easily dried. This allows the membrane electrode assembly13to be kept moist, and the movement of protons is promoted.

In a region downstream of the expanding region that consists of the second communication portions28, water in the first gas channel25is effectively introduced to the first water channel26using capillary action by the first communication portions27. When the amount of water produced in the membrane electrode assembly13is increased, water in the first gas channel25is also introduced to the first water channel26in the expanding region using capillary action by the second communication portions28. This restricts the flow of gas from being hampered by water staying in the first gas channel25.

Therefore, a decrease in the power generation performance caused by the dry membrane electrode assembly13is limited while water staying in the first gas channel25is reduced.

(2) The width A2of each second slit281, which constitutes a second opening of the corresponding second communication portion28, is set to be greater than the width A1of each first slit271of the first communication portions27, which are located downstream of the expanding region (A2>A1).

Such a configuration allows the opening area of the second opening of the second communication portion28to be easily expanded by setting the width A2of the second slit281wide.

(3) Each inner protrusion23includes the corresponding partitions284, which are formed between the respective pairs of second slits281of the corresponding second communication portions28. The partitions284are located inside the corresponding water channel26on the back side of the inner protrusion23.

In each second communication portion28of each inner protrusion23, the second openings of the pair of second slits281are connected to each other by locating the partition284between the pair of second slits281inside the corresponding water channel26. This increases the opening area of the second communication portion28, allowing the opening area to be easily expanded to be larger than the area of the first openings of each first communication portion27. Accordingly, the configuration allows the opening area of the second communication portion28to be increased without increasing the widths of the second slits281so much.

MODIFICATIONS

The above-illustrated embodiment may be modified in the following forms.

The first and second gas channel forming plates22and32may be formed of a metal plate other than a stainless plate such as a titanium plate.

For example, as indicated by long dashed two short dashed lines inFIG. 7, the partition284of each second communication portion28may be formed to be the same height as the second ribs282. In other words, the partition284and the second ribs282may be integrated.

In the above-illustrated embodiment, each partition284of the second communication portions28does not necessarily need to be located in the corresponding first water channel26. The partition284may be formed to have the same height as the top surface of the corresponding inner protrusion23. This modification also allows the opening area of the second openings, which open in the surface that faces the membrane electrode assembly13, to be increased by setting the width A2of each second slit281larger than the width A1of each first slit271of the first communication portions27.

In the above-illustrated embodiment, the width A2of each second slit281of the second communication portions28may be set to be the same as the width A1of each first slit271of the first communication portions27. Even in this case, the second openings of each pair of second slits281are connected to each other to increase the opening area of the corresponding second communication portion28by locating the corresponding partition284inside the corresponding first water channel26.

The number of slits that constitute a communication portion is not limited to two. In other words, one slit may constitute a communication portion, or three or more slits may constitute a communication portion.

The first and second ribs272and282(first and second intermediate structure portions) may be omitted. The first and second ribs372and382(first and second intermediate structure portions) may be omitted.

In the above-illustrated embodiment, the first and second gas channel forming plates22and32having expanding regions (second communication portions28and38) are arranged on both the cathode-side and the anode-side of the membrane electrode assembly13. Instead of this, a gas channel forming plate with an expanding region may be arranged only on the cathode-side of the membrane electrode assembly13, and a gas channel forming plate without an expanding region may be arranged on the anode-side.

Alternatively, a gas channel forming plate with an expanding region may be arranged only on the anode-side of the membrane electrode assembly13, and a gas channel forming plate without an expanding region may be arranged on the cathode-side.