JP2020149843A - Fuel cell system - Google Patents

Abstract

To provide a fuel cell system capable of improving moisture retention of the cell during high temperature operation.SOLUTION: A fuel cell system comprises a solid polymer fuel cell where an anode gas passageway and a cathode gas passageway include a counterflow structure, a fuel gas supply device for supplying fuel gas to the anode gas passageway, and an oxidant gas supply device for supplying oxidant gas to the cathode gas passageway. The solid polymer fuel cell includes a first gas transport resistance change mechanism for changing the gas transport resistance (A) in the main flow direction of fuel gas, so that the gas transport resistance (A) at the outlet and the inlet of the anode gas passageway becomes lower than the gas transport resistance (A) at the central part of the anode gas passageway, and a second gas transport resistance change mechanism for changing the gas transport resistance (B) in the main flow direction of oxidant gas, so that the gas transport resistance (B) at the outlet and the inlet of the cathode gas passageway becomes lower than the gas transport resistance (B) at the central part of the cathode gas passageway.SELECTED DRAWING: Figure 1

Description

The present invention relates to a fuel cell system, and more particularly to a fuel cell system capable of improving the moisturizing property of a cell during high temperature operation.

The polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in which electrodes including catalyst layers are bonded to both sides of an electrolyte membrane. The electrode generally has a two-layer structure of a catalyst layer and a gas diffusion layer. Further, current collectors (separators) provided with gas flow paths are arranged on both sides of the MEA. The polymer electrolyte fuel cell usually has a structure (cell stack) in which a plurality of single cells composed of such an MEA and a current collector are stacked.

In the polymer electrolyte fuel cell, a perfluorocarbon sulfonic acid resin is generally used as the electrolyte membrane. This resin has a characteristic that the proton conductivity is high in the wet state, but the proton conductivity is low in the dry state. Since the reaction-generated water tends to evaporate during high-temperature operation, the inside of the fuel cell tends to become dry. Therefore, it is important to maintain the wet state inside the fuel cell during high-temperature operation.

Further, the humidity of the oxidant gas is generally low on the inlet side of the cathode gas flow path. However, the oxidant gas is humidified by the reaction-generated water while flowing toward the downstream side of the cathode gas flow path. As a result, the oxidant gas tends to be over-humidified on the outlet side of the cathode gas flow path.
Similarly, the humidity of the fuel gas is generally low on the inlet side of the anode gas flow path. However, the fuel gas is humidified by the water diffused from the cathode side while flowing toward the downstream side of the anode gas flow path. As a result, the fuel gas tends to be over-humidified on the outlet side of the anode gas flow path.

Therefore, various proposals have been made in order to solve this problem.
For example, in Patent Document 1,
The fuel gas flow path and the oxide gas flow path of the cell are arranged so that (a) the stacking direction of the cell is perpendicular to the direction of gravity and (b) the fuel gas flow and the oxidation gas flow are opposite and parallel to each other. The fuel cells in place are disclosed.

In the same document,
(A) When the stacking direction of the cells is the gravity direction, the water discharge property is poor and flooding is likely to occur. On the other hand, when the cell stacking direction is perpendicular to the gravity, water droplets are generated in the gas flow path. Even if it occurs, the water droplets flow downward through the gas flow path due to gravity, which improves the discharge performance.
(B) When the flow of fuel gas and the flow of oxidant gas are opposed to each other, water diffuses from the vicinity of the cathode outlet to the vicinity of the anode inlet through the electrolyte membrane, and at the same time, water flows from the vicinity of the anode outlet to the vicinity of the cathode inlet. The point that the water distribution becomes uniform due to diffusion,
Is described.

Although Patent Document 2 is not intended to maintain or homogenize the wet state,
A separator having a gas flow path in which a groove-shaped flow path whose inlet is closed by a closing member and a groove-shaped flow path whose outlet is closed by another closing member are alternately arranged in a stripe shape is provided. The fuel cell is disclosed.
In the same document,
(A) When a gas flow path whose inlet is closed and a gas flow path whose outlet is closed are adjacent to each other, a pressure difference is generated between the two gas flow paths, so that the reaction gas forms a gas diffusion layer. A point through which one gas flow path flows into the other gas flow path, and
(B) As a result, the reaction gas is supplied to the entire gas diffusion layer, so that the power generation efficiency of the power generation cell is improved.

Although Patent Document 3 is not intended to maintain or homogenize the wet state,
(A) At least one of the cathode gas flow path of the cathode side separator and the anode gas flow path of the anode side separator is provided with a throttle portion for reducing the cross section of the flow path.
(B) The cathode gas flow path and the anode gas flow path have different two-dimensional shapes, and there is an intersection position where the cathode gas flow path and the anode gas flow path intersect.
(C) A fuel cell single cell in which the throttle portion is provided at a position other than the intersection position is disclosed.

In the same document,
(A) If the throttle portion is provided at the intersection position, the contact portion between the two separators is reduced when the single cells are laminated, and the compressive force is excessively concentrated on the other portion.
(B) It is described that such stress concentration can be suppressed by providing a throttle portion at a position other than the intersection position.

Patent Document 4 describes a fuel cell separator having a flow path structure in which a gas flow path groove is formed in the separator, and the opening width of the gas flow path groove and the separator base material in the gas flow path groove extension direction. The gas flow path groove depth is constant, the gas flow path cross section is changing, and the change in the gas flow path cross section is due to the gas flow path cross section on the downstream side in the gas flow direction being on the upstream side. A fuel cell separator having a flow path structure that is a change of less than or equal to the cross section is disclosed.

In Patent Document 5, a plurality of power generation units having an electrolyte membrane electrode assembly, a gas diffusion layer arranged so as to sandwich the electrolyte membrane electrode assembly, and a pair of separator units arranged outside the gas diffusion layer are laminated. A fuel cell is disclosed in which the downstream flow path cross-sectional area of the flow path space formed between the separator unit and the gas diffusion layer is smaller than the upstream flow path cross-sectional area.

Patent Document 6 includes a power generation cell having an electrolyte membrane and a separator facing the power generation cell, and has a plurality of grooves in which a fuel gas or an oxidant gas flows on a surface of the separator on the power generation cell side. A gas flow path in which the flow path is formed in a striped shape is formed, and the cross-sectional area of at least a part of the groove-like flow path continuously increases or decreases in the length direction of the groove-like flow path. A fuel cell is disclosed, wherein the change in cross-sectional area of the adjacent groove-shaped flow paths is opposite.

Further, Patent Document 7 describes an electrode having an effective power generation range in which the electrolyte film is sandwiched and a power generation reaction is generated in a fuel cell that generates power by moving hydrogen ions between the electrolyte films, and along the electrode. A plurality of reaction gas flow paths for passing the reaction gas, and a flow path cross-sectional area reducing member for reducing the cross section of the reaction gas flow path, which is arranged near the upstream end of the effective power generation range in the reaction gas flow path. Disclosed is a solid polymer fuel cell characterized in that the height of the flow path cross-sectional area reducing member when the power generation surface of the electrode is the bottom surface is gradually lowered along the flow of the reaction gas. There is.

As described in Patent Document 1, when the flows of the fuel gas and the oxidant gas are opposed to each other, the humidity distributions on the anode side and the cathode side are opposite to each other. Therefore, on the inlet side of the anode gas flow path, water is transported from the cathode side to the anode side via the electrolyte membrane. Similarly, on the inlet side of the cathode gas flow path, water is transported from the anode side to the cathode side via the electrolyte membrane. As a result, the generated water circulates in the cell surface, and the performance during high temperature operation is improved.
However, even when the conventional method is used, the moisturizing property becomes insufficient under the condition where the temperature is higher. As a result, the electrolyte membrane dries and the performance deteriorates.

In the case of the methods described in Patent Documents 4 and 5, the gas flow velocity is improved by reducing the cross section of the gas flow path on the downstream side, the function of discharging liquid water is promoted, and as a result, the gas on the downstream side is promoted. Transport resistance may be reduced. However, the function of reducing the gas transport resistance on the upstream side is not realized, and the moisturizing effect is poor.

In the case of the method described in Patent Document 6, a differential pressure is generated because the cross-sections of the adjacent flow paths are different, and the gas flows under the rib portion due to the differential pressure, so that the gas transport resistance is reduced. .. However, this method does not realize the function of reducing the gas transport resistance at the inlet and outlet of the gas flow path with respect to the central portion, and the moisturizing effect is poor.

Further, the method described in Patent Document 7 is a method of increasing the gas transport resistance in the upstream portion of the gas flow path by a mechanism for temporarily increasing the pressure in the upstream portion of the gas flow path. However, this method does not realize the moisturizing effect by promoting the in-plane water cycle.

Japanese Unexamined Patent Publication No. 2002-184428 JP-A-2010-272541 JP-A-2017-228482 Japanese Patent No. 3956864 Japanese Unexamined Patent Publication No. 2005-327532 Japanese Unexamined Patent Publication No. 2006-114387 Japanese Patent No. 4007093

An object to be solved by the present invention is to provide a fuel cell system capable of improving the moisturizing property of a cell during high temperature operation.

In order to solve the above problems, it is a gist that the fuel cell system according to the present invention has the following configurations.
(1) The fuel cell system is
A polymer electrolyte fuel cell in which the anode gas flow path and the cathode gas flow path have a countercurrent structure,
A fuel gas supply device that supplies fuel gas to the anode gas flow path and
It is provided with an oxidant gas supply device that supplies the oxidant gas to the cathode gas flow path.
(2) The polymer electrolyte fuel cell is
Along the mainstream direction of the fuel gas, the gas transport resistance (A) at both the outlet and the inlet of the anode gas flow path is lower than the gas transport resistance (A) at the center of the anode gas flow path. A first gas transport resistance changing mechanism that changes the gas transport resistance (A),
Along the mainstream direction of the oxidant gas so that the gas transport resistance (B) at both the outlet and the inlet of the cathode gas flow path is lower than the gas transport resistance (B) at the center of the cathode gas flow path. It is provided with a second gas transport resistance changing mechanism that changes the gas transport resistance (B).

When the gas flow path has a countercurrent structure, the humidity distributions on the anode side and the cathode side are opposite to each other. Therefore, on the inlet side of the anode gas flow path, water is transported from the cathode side to the anode side via the electrolyte membrane. Similarly, on the inlet side of the cathode gas flow path, water is transported from the anode side to the cathode side via the electrolyte membrane. However, with only the countercurrent structure, the electrolyte membrane dries under conditions of higher temperature, and the performance during high temperature operation deteriorates.

On the other hand, if a mechanism is provided at the outlet and inlet of the gas flow path so that the gas transport resistance is smaller than that at the center of the gas flow path, in addition to the countercurrent structure, the gas is provided at the outlet and inlet of the gas flow path. The main direction in which the gas flows changes from the mainstream direction to the thickness direction of the electrode. Therefore, the reaction gas is easily transported to a region close to the surface of the electrolyte membrane. This also facilitates water transport between the MEA and the gas flow path at the outlet and inlet of the gas flow path. On the other hand, in the central portion of the gas flow path, the generated water is less likely to evaporate into the gas flow path and is easily retained inside the MEA. As a result, the moisturizing property of MEA is improved, and the performance during high temperature operation is improved.

FIG. 1A is a plan view of the anode-side separator provided with the first throttle portion. FIG. 1B is a plan view of the cathode side separator provided with the second throttle portion. It is a top view of the cathode side separator provided with the tapering mechanism (B) in which the gas transport resistance changes stepwise or continuously. The gas transport resistance of the fuel cell of Comparative Example 1 (FIG. 3 (A)), the relative humidity of the electrolyte membrane (FIG. 3 (B)), and the amount of water transferred from the anode to the cathode (FIG. 3 (C)).

The gas transport resistance of the fuel cell of Comparative Example 2 (FIG. 4 (A)), the relative humidity of the electrolyte membrane (FIG. 4 (B)), and the amount of water transferred from the anode to the cathode (FIG. 4 (C)). The gas transport resistance of the fuel cell of Example 1 (FIG. 5 (A)), the relative humidity of the electrolyte membrane (FIG. 5 (B)), and the amount of water transferred from the anode to the cathode (FIG. 5 (C)). The gas transport resistance of the fuel cell of Example 2 (FIG. 6 (A)), the relative humidity of the electrolyte membrane (FIG. 6 (B)), and the amount of water transferred from the anode to the cathode (FIG. 6 (C)). These are cell voltages during high-temperature operation of the fuel cells of Examples 1 and 2 and Comparative Examples 1 and 2.

Hereinafter, an embodiment of the present invention will be described in detail.
[1. Fuel cell system (1)]
The fuel cell system according to the present invention
A polymer electrolyte fuel cell in which the anode gas flow path and the cathode gas flow path have a countercurrent structure,
A fuel gas supply device that supplies fuel gas to the anode gas flow path and
It is provided with an oxidant gas supply device that supplies the oxidant gas to the cathode gas flow path.

[1.1. Polymer electrolyte fuel cell]
The polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in which electrodes including catalyst layers are bonded to both sides of an electrolyte membrane. The electrode generally has a two-layer structure of a catalyst layer and a gas diffusion layer. Further, current collectors (separators) provided with gas flow paths are arranged on both sides of the MEA. The polymer electrolyte fuel cell usually has a structure (cell stack) in which a plurality of single cells composed of such a MEA and a current collector are stacked.

In the present invention, the polymer electrolyte fuel cell has a countercurrent structure.
Here, the "countercurrent structure" is
(A) The direction of the fuel gas flow (mainstream direction) in the anode gas flow path arranged on one surface of the MEA and the flow of the oxidant gas in the cathode gas flow path arranged on the other surface of the MEA. Is almost parallel to the direction of (mainstream direction), and
(B) A structure in which the direction of fuel gas flow and the direction of oxidant gas flow are opposite to each other.

In other words, what is a "countercurrent structure"?
(A) On the inlet side (or outlet side of the cathode gas flow path) of the anode gas flow path, excess water contained in the oxidant gas discharged from the MEA is discharged from the cathode side to the anode side via the electrolyte membrane. It is possible to transport to and
(B) On the outlet side of the anode gas flow path (or the inlet side of the cathode gas flow path), excess water contained in the fuel gas discharged from the MEA is transferred from the anode side to the cathode side via the electrolyte membrane. A structure that can be transported toward.
The mainstream direction (the direction of gas flow along the gas flow path) is as long as excess water can be transported towards the opposite surface of the MEA on both the outlet and inlet sides of the gas flow path. It does not have to be perfectly parallel, and the gas flow paths do not necessarily have to be straight.

The structure of other parts of the polymer electrolyte fuel cell and the materials constituting the same are not particularly limited, and the optimum structure and material can be selected according to the purpose.

[1.2. Fuel gas supply device]
The fuel gas supply device is for supplying fuel gas to the anode gas flow path. The structure of the fuel gas supply device is not particularly limited as long as a predetermined amount of fuel gas can be supplied to the anode gas flow path.

[1.3. Oxidizing agent gas supply device]
The oxidant gas supply device is for supplying the oxidant gas to the cathode gas flow path. The structure of the oxidant gas supply device is not particularly limited as long as a predetermined amount of oxidant gas can be supplied to the cathode gas flow path.

[1.4. Gas transport resistance change mechanism]
The polymer electrolyte fuel cell according to the present invention has, in addition to the above-mentioned configuration,
Along the mainstream direction of the fuel gas, the gas transport resistance (A) at both the outlet and the inlet of the anode gas flow path is lower than the gas transport resistance (A) at the center of the anode gas flow path. A first gas transport resistance changing mechanism that changes the gas transport resistance (A),
Along the mainstream direction of the oxidant gas so that the gas transport resistance (B) at both the outlet and the inlet of the cathode gas flow path is lower than the gas transport resistance (B) at the center of the cathode gas flow path. It is provided with a second gas transport resistance changing mechanism that changes the gas transport resistance (B).

Here, the "gas transport resistance" means a resistance to gas transport in the thickness direction of the electrode.
In other words, "gas transport resistance" is
(A) Until the reaction gas reaches the catalyst layer from the gas flow path provided in the current collector through the gas diffusion layer, and
(B) Until the reaction gas that reaches the catalyst layer and is not consumed in the electrode reaction returns to the gas flow path through the gas diffusion layer.
The resistance to gas transportation.
When the reaction gas is flowed in the mainstream direction of the gas flow path, a part of the reaction gas reaches the catalyst layer through the gas diffusion layer, but the rest passes through the gas flow path as it is. In order to promote the supply of the reaction gas to the catalyst layer, the replenishment of water to the MEA via the reaction gas, and the discharge of water from the MEA, it is preferable that the gas transport resistance is low. On the other hand, in order to retain the reaction-generated water in the MEA, it is preferable to suppress the discharge of water from the MEA to the gas flow path via the reaction gas. For that purpose, it is preferable that the gas transport resistance is high.

In the present invention, the polymer electrolyte fuel cell has a gas transport resistance changing mechanism (that is, gas transport resistance is low at the outlet and inlet of the gas flow path, and gas transport resistance is high at the central portion of the gas flow path. Mechanism) is provided.
Therefore, if gas transport resistance changing mechanisms are provided at the outlets and inlets of the gas flow path, water transport is promoted from one surface of the MEA in which water is excessive to the other surface of the MEA in which water is insufficient. Will be done. On the other hand, in the central portion of the gas flow path, water transportation from the MEA to the gas flow path side is suppressed. As a result, the moisturizing property of MEA is improved, and the performance during high temperature operation is improved.

The structure of the gas transport resistance changing mechanism is not particularly limited as long as it functions as described above. Specific examples of the gas transport resistance changing mechanism are as follows. Specific examples of the gas transport resistance changing mechanism shown below may be used alone, or may be used in combination of two or more as long as physically possible.

[1.4.1. 1st aperture]
The first gas transport resistance changing mechanism may be a first throttle portion provided at both the outlet and the inlet of the anode gas flow path for reducing the cross-sectional area of the anode gas flow path.
In this case, the second gas transport resistance changing mechanism is not particularly limited. For example, the second gas transport resistance changing mechanism may be a second throttle portion described later, or may be another mechanism.

FIG. 1A shows a plan view of the anode-side separator provided with the first throttle portion. In FIG. 1 (A), the central portion of the anode-side separator 10 is formed into a corrugated shape, and a groove portion for flowing fuel gas (a portion of the corrugated separator that is not in contact with the gas diffusion layer). 12 and ribs (a portion of the corrugated separator that is in contact with the gas diffusion layer) 14 are arranged alternately. The groove 12 between the adjacent ribs 14 and 14 is the anode gas flow path. First throttle portions 16, 16 ... For reducing the cross-sectional area of the anode gas flow path are provided at the inlet and outlet of the anode gas flow path, respectively.

When the first throttle portions 16, 16 ... Are formed in the anode gas flow path, the positions of the first throttle portions 16, 16 ... In the mainstream direction may be the same as or different from each other.
However, as shown in FIG. 1A, it is preferable that the first throttle portions 16, 16 ... Facing each other across the ribs 14, 14 ... Are arranged so that the positions in the mainstream direction are staggered. When the first throttle portions 16, 16 ... Are formed alternately, a pressure difference is generated between adjacent anode gas flow paths. The differential pressure facilitates the diffusion of fuel gas through the gas diffusion layer into the adjacent anode gas flow path. As a result, the gas diffusion resistance in the vicinity of the first throttle portions 16, 16 ... Can be reduced.

In addition, in FIG. 1A, four first throttle portions 16, 16 ... Are formed on the inlet side and the outlet side of each anode gas flow path, but this is merely an example. The optimum number of the first diaphragm portions 16, 16 ... Can be selected according to the purpose.
Further, the cross-sectional area of the first throttle portions 16, 16 ... Is also not particularly limited as long as the gas transport resistance can be changed. For example, the height of the first drawing portions 16, 16 ... May be the same as the height of the ribs 14, 14 ..., Or may be lower than that.
Further, in FIG. 1A, the width (thickness in the direction perpendicular to the mainstream direction) of the first throttle portions 16, 16 ... Is about 1/3 of the width of the anode gas flow path, but is desired. It may be wider or narrower as long as the effect of is obtained.

An anode gas inlet side manifold 18 is provided on the upper left of the anode side separator 10. Further, an anode gas outlet side manifold 20 is provided at the lower right of the anode side separator 10.
The anode gas inlet side manifold 18 and the anode gas outlet side manifold 20 communicate with each other in the anode flow path (groove portion 12). Therefore, when the fuel gas is supplied to the anode gas inlet side manifold 18, the fuel gas is distributed to each anode gas flow path and flows from the left to the right of the paper surface. Further, the residual gas discharged from the outlet of the anode gas flow path is discharged to the outside of the MEA through the anode gas outlet side manifold 20.

On the other hand, a cathode gas inlet side manifold 22 is provided on the upper right of the anode side separator 10. Further, a cathode gas outlet side manifold 24 is provided at the lower left of the anode side separator 10. However, neither the cathode gas inlet side manifold 22 nor the cathode gas outlet side manifold 24 communicates with the anode flow path (groove portion 12). Therefore, even if the oxidant gas is supplied from the cathode gas inlet side manifold 22, the oxidant gas does not flow into the cathode flow path (groove 12).

When the fuel gas is flowed through the anode side separator 10 having such a configuration, the inlet side and the inlet side and the outlet side of the anode gas flow path are provided with the first throttle portions 16, 16 ... On the outlet side, the resistance of gas flow in the mainstream direction increases. The direction of flow of some of the fuel gas that has lost its place is changed from the mainstream direction to the thickness direction of the electrodes. As a result, the gas transport resistance (A) becomes smaller on the inlet side and the outlet side of the anode gas flow path.
On the other hand, such a throttle portion is not provided in the intermediate portion of the anode gas flow path. Therefore, the gas transport resistance (A) increases in the intermediate portion of the anode gas flow path.

Further, in the anode gas flow path, the direction of the gas flow is opposite to that of the cathode gas flow path (not shown). Therefore, on the inlet side of the anode gas flow path, excess water contained in the oxidant gas discharged from the MEA is transported from the cathode side to the anode side via the electrolyte membrane.
On the other hand, on the outlet side of the anode gas flow path, excess water contained in the fuel gas discharged from the MEA is transported from the anode side to the cathode side via the electrolyte membrane. As a result, the moisturizing property of MEA is improved, and the performance during high temperature operation is improved.

[1.4.2. 2nd aperture]
The second gas transport resistance changing mechanism may be a second throttle portion provided at both the outlet and the inlet of the cathode gas flow path for reducing the cross-sectional area of the cathode gas flow path.
In this case, the first gas transport resistance changing mechanism is not particularly limited. For example, the first gas transport resistance changing mechanism may be the first throttle portion described above, or may be another mechanism.

FIG. 1B shows a plan view of a cathode side separator provided with a second throttle portion. In FIG. 1B, the cathode side separator 30a has a corrugated central portion, and a groove portion for flowing an oxidant gas (a portion of the corrugated separator that is not in contact with the gas diffusion layer). ) 32 and ribs (a portion of the corrugated separator that is in contact with the gas diffusion layer) 34 are arranged alternately. The groove 32 between the adjacent ribs 34, 34 is the cathode gas flow path. Second throttle portions 36a, 36a ... For reducing the cross-sectional area of the cathode gas flow path are formed at the inlet and the outlet of the cathode gas flow path, respectively.

An anode gas inlet side manifold 38 is provided on the upper left of the cathode side separator 30a. Further, an anode gas outlet side manifold 40 is provided at the lower right of the anode side separator 30a. However, neither the anode gas inlet side manifold 38 nor the anode gas outlet side manifold 40 communicates with the cathode gas flow path (groove portion 32). Therefore, even if the fuel gas is supplied from the anode gas inlet side manifold 38, the fuel gas does not flow into the cathode gas flow path (groove 32).

On the other hand, a cathode gas inlet side manifold 42 is provided on the upper right of the cathode side separator 30a. Further, a cathode gas outlet side manifold 44 is provided at the lower left of the cathode side separator 30.
The cathode gas inlet side manifold 42 and the cathode gas outlet side manifold 44 communicate with each other in the cathode gas flow path (groove portion 32). Therefore, when the oxidant gas is supplied from the cathode gas inlet side manifold 42, the oxidant gas is distributed to each cathode gas flow path, and contrary to FIG. 1 (A), from right to left on the paper surface. It flows toward. Further, the residual gas discharged from the outlet of the cathode gas flow path is discharged to the outside of the MEA through the cathode gas outlet side manifold 44.

Since the other points regarding the cathode side separator 30a are the same as those of the anode side separator 10, the description thereof will be omitted.

When the oxidant gas is passed through the cathode side separator 30a having such a configuration, the second throttle portions 36a, 36a ... Are provided at the inlet and the outlet of the cathode gas flow path, respectively, so that the inlet side is provided. And on the outlet side, the resistance of gas flow in the mainstream direction increases. The direction of the flow of some of the oxidant gas that has lost its place is changed from the mainstream direction to the thickness direction of the electrode. As a result, the gas transport resistance (B) becomes smaller on the inlet side and the outlet side of the cathode gas flow path.
On the other hand, such a throttle portion is not provided in the intermediate portion of the cathode gas flow path. Therefore, the gas transport resistance (B) increases in the intermediate portion of the cathode gas flow path.

Further, in the cathode gas flow path, the direction of the gas flow is opposite to that of the anode gas flow path (not shown). Therefore, on the outlet side of the cathode gas flow path, excess water contained in the oxidant gas discharged from the MEA is transported from the cathode side to the anode side via the electrolyte membrane.
On the other hand, on the inlet side of the cathode gas flow path, excess water contained in the fuel gas discharged from the MEA is transported from the anode side to the cathode side via the electrolyte membrane. As a result, the moisturizing property of MEA is improved, and the performance during high temperature operation is improved.

[1.4.3. Anode side gas diffusion layer with varying porosity]
In the first gas transport resistance changing mechanism, the porosity (A) of both the outlet side and the inlet side of the anode gas flow path is larger than the porosity (A) of the central portion of the anode gas flow path. The anode-side gas diffusion layer may have a porosity (A) changed along the mainstream direction.
In this case, the second gas transport resistance changing mechanism is not particularly limited. For example, the second gas transport resistance changing mechanism may be a cathode-side gas diffusion layer in which the porosity (B) is changed, which will be described later, or may be another mechanism.

The method for changing the porosity (A) of the anode-side gas diffusion layer is not particularly limited. For example, when a through hole or a semi-through hole is formed in the thickness direction of the diffusion layer on the anode side by using Kenzan or the like, the porosity (A) of the formed portion can be locally increased. Further, by changing the diameter and pitch of the needle, the porosity (A) of the portion where the hole is formed can be arbitrarily controlled.
Therefore, if a large number of holes are formed in the gas diffusion layer at a short pitch on the outlet side and the inlet side of the anode gas flow path, the porosity (A) on the outlet side and the inlet side can be increased. On the other hand, in the central portion of the anode gas flow path, if no pores are formed in the gas diffusion layer or if a small number of pores are formed at a long pitch, the porosity (A) in the central portion can be reduced.

[14.4. Cathode gas diffusion layer with varying porosity]
The second gas transport resistance changing mechanism is an oxidizing agent gas so that the porosity (B) on both the outlet side and the inlet side of the cathode gas flow path is larger than the porosity (B) at the center of the cathode gas flow path. The cathode side gas diffusion layer may have a porosity (B) changed along the mainstream direction of the above.
In this case, the first gas transport resistance changing mechanism is not particularly limited. For example, the first gas transport resistance changing mechanism may be the anode-side gas diffusion layer in which the porosity (A) is changed as described above, or may be another mechanism.

The method for changing the porosity (B) of the cathode-side diffusion layer is not particularly limited. Further, the porosity (B) of the cathode-side diffusion layer may be the same as or different from the porosity (A) of the anode-side diffusion layer.
The details of the method for changing the porosity (B) are the same as those for the anode-side diffusion layer, and thus the description thereof will be omitted.

[1.4.5. Cathode side separator with gradually decreasing gas transport resistance]
The second gas transport resistance changing mechanism may include a gradual reduction mechanism (B) in which the gas transport resistance (B) is gradually or continuously reduced from the center to the outlet of the cathode gas flow path.
In this case, the first gas transport resistance changing mechanism is not particularly limited. For example, the first gas transport resistance changing mechanism may be the same mechanism as the second gas transport resistance changing mechanism, or may be a mechanism different from the same.

FIG. 2 shows a plan view of a cathode-side separator provided with a tapering mechanism (B) in which the gas transport resistance changes stepwise or continuously. In FIG. 2, the same components as those in FIG. 1 (B) are given the same reference numbers. In FIG. 2, the central portion of the cathode side separator 30b is formed into a corrugated shape, and the groove portions 32 for flowing the oxidant gas and the ribs 34 are alternately arranged. The groove 32 between the adjacent ribs 34 and 34 is the cathode gas flow path.

In FIG. 2, a total of four second throttle portions 36b, 36b ... Are provided on the inlet side of each cathode gas flow path, respectively. The widths of the second throttle portions 36b, 36b ... On the inlet side are the same.
On the other hand, a total of eight second throttle portions 36c, 36c ... Are provided from the center to the outlet of each cathode gas flow path, and these correspond to the gradual reduction mechanism (B). The widths of the second throttle portions 36c, 36c ... On the outlet side become wider toward the outlet side. Therefore, the gas transport resistance (B) of the cathode gas flow path decreases stepwise or continuously from the center of the cathode gas flow path toward the outlet. This point is different from the cathode side separator 30a shown in FIG. 1 (B).
Other points are the same as those of the cathode side separator 30a shown in FIG. 1 (B), and thus the description thereof will be omitted.

If a gradual reduction mechanism (B) in which the gas transport resistance (B) gradually or continuously decreases from the center to the outlet of the cathode gas flow path is provided, the performance during high-temperature operation is improved. It is considered that this is because the gradual reduction mechanism (B) can improve the oxygen supply capacity to the catalyst layer while maintaining high moisturizing property.
A mechanism for discontinuously changing the gas transport resistance (B) may be provided on the inlet side of the cathode gas flow path, or the gas transport resistance (B) increases from the inlet to the center of the cathode gas flow path. A gradual increase mechanism may be provided that is increasing stepwise or continuously.
However, reducing the gas transport resistance at the inlet of the cathode gas flow path is effective for the purpose of increasing the amount of water transported from the anode to the cathode, and the purpose is to reduce the gas transport resistance in a narrow range near the inlet. Can be realized with. Therefore, it is not very useful to provide a gradual increase mechanism on the inlet side of the cathode gas flow path.

[14.6. Anode side separator with gradually decreasing gas transport resistance]
The first gas transport resistance changing mechanism may include a gradual reduction mechanism (A) in which the gas transport resistance (A) is gradually or continuously reduced from the center to the outlet of the anode gas flow path.
In this case, the second gas transport resistance changing mechanism is not particularly limited. For example, the second gas transport resistance changing mechanism may be the same mechanism as the first gas transport resistance changing mechanism, or may be a mechanism different from the same.
Since the influence of the hydrogen concentration overvoltage on the fuel cell performance is not so large, the merit of providing the gradual reduction mechanism (A) on the anode gas flow path side is less than that on the cathode side.

[2. Action]
In the polymer electrolyte fuel cell, a perfluorocarbon sulfonic acid resin is generally used as the electrolyte membrane. This resin has a characteristic that the proton conductivity is high in the wet state, but the proton conductivity is low in the dry state. During high-temperature operation, the reaction-generated water tends to evaporate, so that the inside of the fuel cell tends to become dry. Therefore, it is important to maintain the wet state inside the fuel cell during high-temperature operation.

Here, it is known that the humidity at the inlet portion of both the cathode and the anode is low, whereas the humidity at the outlet portion tends to be high because it contains water generated by power generation. Therefore, if the gas flow path has a countercurrent structure, the humidity distributions on the anode side and the cathode side are opposite to each other. Therefore, on the inlet side of the anode gas flow path, water is transported from the cathode side to the anode side via the electrolyte membrane. Similarly, on the inlet side of the cathode gas flow path, water is transported from the anode side to the cathode side via the electrolyte membrane. As a result, the inlet of the gas flow path can be moistened. However, with only the countercurrent structure, the electrolyte membrane dries under conditions of higher temperature, and the performance during high temperature operation deteriorates.

On the other hand, if a mechanism is provided at the outlet and inlet of the gas flow path so that the gas transport resistance is smaller than that at the center of the gas flow path, in addition to the countercurrent structure, the gas is provided at the outlet and inlet of the gas flow path. The main direction in which the gas flows changes from the mainstream direction to the thickness direction of the electrode. Therefore, the reaction gas is easily transported to a region close to the surface of the electrolyte membrane. This also facilitates water transport between the MEA and the gas flow path at the outlet and inlet of the gas flow path. On the other hand, in the central portion of the gas flow path, the generated water is less likely to evaporate into the gas flow path and is easily retained inside the MEA. As a result, the moisturizing property of MEA is improved, and the performance during high temperature operation is improved.

(Examples 1 and 2, Comparative Examples 1 and 2)
[1. Test method]
The gas transport resistance of various fuel cells having a countercurrent structure, the relative humidity of the electrolyte membrane, the amount of water movement from the anode to the cathode, and the cell voltage were obtained by simulation. The evaluation target is
(A) A fuel cell having only a countercurrent structure (Comparative Example 1),
(B) A fuel cell having a countercurrent structure and a mechanism for reducing gas transport resistance over the entire anode gas flow path and cathode gas flow path (Comparative Example 2).
(C) A fuel cell having a countercurrent structure and a mechanism for reducing only the gas transport resistance on the inlet side and the outlet side of the anode gas flow path and the cathode gas flow path (Example 1), and
(D) Countercurrent structure, mechanism for reducing only gas transport resistance on the inlet side and outlet side of the anode gas flow path, mechanism for reducing only gas transport resistance on the inlet side of the cathode flow path, and cathode gas flow path. A fuel cell having a gradual reduction mechanism that gradually or continuously reduces the gas transport resistance from the center to the outlet (Example 2),
And said.

[2. result]
3 to 6, respectively, show the gas transport resistance of the fuel cells of Comparative Examples 1 and 2 and Examples 1 and 2 (upper figure), the relative humidity of the electrolyte membrane (middle figure), and from the anode to the cathode, respectively. The amount of water movement (shown below) is shown. Further, FIG. 7 shows the cell voltage of the fuel cells of Examples 1 and 2 and Comparative Examples 1 and 2 during high-temperature operation.
In addition, in FIGS. 4 to 6, the broken line represents the value of Comparative Example 1 (FIG. 3). In FIGS. 3 to 6, the "average generated water amount ratio" means a value obtained by dividing the "local water movement amount" by the "produced water amount calculated from the total current value of the cell". Further, when the "amount of water movement from the anode to the cathode" is negative, it means that water is moving from the cathode to the anode. The following can be seen from FIGS. 3 to 7.

(1) In the case of a fuel cell having only a countercurrent structure (Comparative Example 1), the humidity of the gas at the inlet of both the anode and the cathode is low, but the humidity of the gas from the center to the outlet contains generated water. It will be high. However, during high-temperature operation, the humidity at the inlet of both the cathode and the anode was significantly reduced (see FIG. 3).
(2) In the case of a fuel cell (Comparative Example 2) equipped with a mechanism for reducing gas transport resistance over the entire area, the relative humidity of the electrolyte membrane is rather reduced on the inlet side of the cathode gas flow path (see FIG. 4). .. It is considered that this is because the water generated by the electrodes easily evaporates into the gas flow path as water vapor. As a result, the cell voltage was lower than that of Comparative Example 1 (see FIG. 7).

(3) In the case of a fuel cell (Example 1) provided with a mechanism for reducing only the gas transport resistance at the inlet and outlet of the gas flow path, water transport from the cathode to the anode on the inlet side of the anode gas flow path. Was promoted, and water transport from the anode to the cathode was promoted on the inlet side of the cathode gas flow path (see FIG. 5C). On the other hand, in the central part of the gas flow path, the amount of water movement was near zero, and the generated water was held inside the electrode. From these, it was found that in Example 1, the humidity circulation in the cell surface was promoted and the moisturizing property was improved. In addition, the cell performance during high-temperature operation was improved as compared with Comparative Example 1 (see FIG. 7).
(4) The anode side is equipped with a mechanism for reducing only the gas transport resistance at the inlet and outlet of the gas flow path, and the cathode side is equipped with a mechanism for gently reducing the gas transport resistance from the center of the gas flow path to the outlet. In the case of the fuel cell provided with (Example 2), the relative humidity of the electrolyte membrane was almost the same as that of Example 1 (see FIG. 6), but the cell voltage of Example 2 was higher than that of Example 1. (See FIG. 7). This is because the gas transport resistance is gradually reduced toward the outlet side of the cathode gas flow path, so that the oxygen supply capacity to the catalyst on the outlet side of the cathode gas flow path is improved while maintaining the moisturizing property. This is probably because the oxygen concentration overvoltage could be reduced.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

The fuel cell system according to the present invention can be used for an in-vehicle power source, a stationary generator, or the like.

Claims (6)

A fuel cell system with the following configurations.
(1) The fuel cell system is
A polymer electrolyte fuel cell in which the anode gas flow path and the cathode gas flow path have a countercurrent structure,
A fuel gas supply device that supplies fuel gas to the anode gas flow path and
It is provided with an oxidant gas supply device that supplies the oxidant gas to the cathode gas flow path.
(2) The polymer electrolyte fuel cell is
Along the mainstream direction of the fuel gas, the gas transport resistance (A) at both the outlet and the inlet of the anode gas flow path is lower than the gas transport resistance (A) at the center of the anode gas flow path. A first gas transport resistance changing mechanism that changes the gas transport resistance (A),
Along the mainstream direction of the oxidant gas so that the gas transport resistance (B) at both the outlet and the inlet of the cathode gas flow path is lower than the gas transport resistance (B) at the center of the cathode gas flow path. It is provided with a second gas transport resistance changing mechanism that changes the gas transport resistance (B). The first gas transport resistance changing mechanism is described in claim 1, which is a first throttle portion for reducing the cross-sectional area of the anode gas flow path, which is provided at both the outlet and the inlet of the anode gas flow path. Fuel cell system. The second gas transport resistance changing mechanism is a second throttle portion provided at both the outlet and the inlet of the cathode gas flow path for reducing the cross-sectional area of the cathode gas flow path, claim 1 or 2. The fuel cell system described in. In the first gas transport resistance changing mechanism, the porosity (A) on both the outlet side and the inlet side of the anode gas flow path is made larger than the porosity (A) in the central portion of the anode gas flow path. The fuel cell system according to any one of claims 1 to 3, which is an anode-side gas diffusion layer in which the porosity (A) is changed along the mainstream direction of the fuel gas. The second gas transport resistance changing mechanism is such that the pore ratio (B) of both the outlet side and the inlet of the cathode gas flow path is larger than the pore ratio (B) of the central portion of the cathode gas flow path. The fuel cell system according to any one of claims 1 to 4, which is a cathode-side gas diffusion layer in which the pore ratio (B) is changed along the mainstream direction of the oxidizing agent gas. The second gas transport resistance changing mechanism includes a gradual reduction mechanism (B) in which the gas transport resistance (B) is gradually or continuously reduced from the center to the outlet of the cathode gas flow path. The fuel cell system according to any one of items 1 to 5.

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