JP2023139576A - Fuel battery cell - Google Patents

Abstract

To provide a fuel battery cell capable of achieving satisfactory power generation performance in wide temperature range operation by using a gas diffusion layer that suppresses electrolyte membrane drying and drains generated water.SOLUTION: A fuel battery cell 1 is provided with a laminate in which catalyst electrode layers 3a, 3c are laminated on both sides of an electrolyte membrane 2 and gas diffusion layers 4a, 4c are further laminated on its both sides, and has a configuration that the laminate is sandwiched by an anode side separator 5a defining a flow path 6a for supplying gas containing hydrogen to the gas diffusion layer on the anode side of the gas diffusion layers and a cathode side separator 5c defining a flow path 6c for supplying gas containing oxygen to the gas diffusion layer on the cathode side of the gas diffusion layers and discharging generated water. A degree of curvature of the anode side gas diffusion layer is 3.7 or less.SELECTED DRAWING: Figure 3

Description

The present invention relates to a structure of a fuel cell, and more particularly to a structure related to drainage performance of water generated by a power generation reaction in a fuel cell.

A fuel cell basically consists of a membrane electrode assembly consisting of an electrolyte membrane sandwiched between a pair of catalyst electrode layers on both sides, a membrane electrode assembly sandwiched between a pair of gas diffusion layers, and hydrogen gas on both sides. It has a structure in which a pair of separators are sandwiched between each other to form a flow path for oxygen gas (air) or water. Various configurations have been proposed for such fuel cells to further improve power generation performance. For example, in Patent Document 1, water generated on the cathode side during high humidification and high current density operation accumulates in the cathode catalyst layer, is inhibited by the generated water, and reactants are not sufficiently supplied to the cathode catalyst layer. In order to avoid the phenomenon (flooding) in which the battery output is reduced due to the water not being supplied to the cathode, the gas permeability and water permeability in the gas diffusion layer on the cathode side are the conditions for properly discharging generated water from the cathode side. It is proposed to make the water level almost the same. Furthermore, in Patent Document 2, in a non-humidified state and in a high temperature environment, water is excessively discharged from the cathode side, drying the electrolyte membrane, and reducing the proton conductivity of the electrolyte membrane. As a condition for avoiding a drop in the voltage of the fuel cell, the gas diffusion layer includes a base material and a water-repellent layer provided between the base material and the catalyst layer, and the cathode is water-repellent. It has been proposed that the water vapor diffusion coefficient of the layer be 0.01 or more, and that the water vapor diffusion coefficient of the water repellent layer of the anode electrode be equal to or less than the water vapor diffusion coefficient of the water repellent layer of the cathode electrode.

JP2008-98066 JP2010-146756

“Method for measuring the oxygen diffusion coefficient of microporous materials” Yoshiro Utaka et al., Transactions of the Japan Society of Mechanical Engineers (B edition), Vol. 74, No. 739, pp. 655-661 (March 2008)* T. MASHIO, et al., ECS Transactions, 11(1) 529-540 (2007) “Gas diffusion phenomenon in porous solid oxide fuel cell electrodes” Kota Miyoshi, Energy and Resources Vol.35 No.4, p. 250-253 (2014)

In the fuel cell described above, the electrolyte membrane that conducts protons from the anode side to the cathode side must be constantly moistened with liquid water, and when the operating temperature of the fuel cell is high, the electrolyte membrane When the membrane dries, proton resistance within the membrane increases, resulting in a decrease in power generation performance. However, on the cathode side, water is constantly generated as a result of the power generation reaction, so especially when the operating temperature of the fuel cell is low, water tends to accumulate inside the cell, and the accumulated water can be used to generate gas. It becomes a resistance to diffusion, which also causes a decrease in power generation performance. Therefore, in fuel cells, during high-temperature operation, the electrolyte membrane should be kept moist by avoiding drying, and during low-temperature operation, where water tends to accumulate, it is possible to improve the drainage of generated water. It is preferable to be able to control the degree of drainage from the cell (drainability) so as to achieve both performance and performance during low-temperature operation.

Thus, one object of the present invention is to suppress drying and maintain wettability during high-temperature operation in a fuel cell, while increasing drainage of generated water during low-temperature operation. be.

Regarding this point, in the research conducted by the inventor of the present invention, it was found that the drainage and water retention performance of the electrolyte membrane in a fuel cell depends on the "degree of curvature" of the gas diffusion layer on the anode side. . Here, the "degree of curvature" is the ratio (L/d) of the distance L that gas moves in the anode side gas diffusion layer to the thickness d of the anode side gas diffusion layer in the gas transmission direction (see Figure 2). ). The inventor of the present invention conducted various studies and found that, as will be explained in detail later, if the "curvature" of the anode side gas diffusion layer is set to 3.7 or less, preferably 3 or less, drainage and water retention will be improved. Through experiments, it was discovered that the structure is compatible with both functions, and that the cell's power generation performance can be maintained without any trade-offs during high-temperature or low-temperature operation. This knowledge is utilized in the present invention.

According to the present invention, the above problem can be solved by providing a laminate in which a catalyst electrode layer is laminated on both sides of an electrolyte membrane, and a gas diffusion layer is further laminated on both sides, An anode-side separator that defines a flow path for supplying a hydrogen-containing gas to a gas diffusion layer on an anode side, and water generated when supplying a gas containing oxygen to a gas diffusion layer on a cathode side of the gas diffusion layer. This is achieved by a fuel cell having a configuration in which the gas diffusion layer is sandwiched between cathode-side separators that define a flow path for discharging gas, and the degree of curvature of the anode-side gas diffusion layer is 3.7 or less. .

In the above configuration, the "fuel cell" may be a single cell of a so-called "polymer electrolyte fuel cell", and includes a laminate consisting of an electrolyte membrane, a catalyst electrode layer, and a gas diffusion layer, a separator, and a separator. The frame member, etc. that surrounds and supports the stacked body therebetween may basically be of a form normally used for fuel cells. The "hydrogen-containing gas" may be a hydrogen gas commonly used in this field, and the "oxygen-containing gas" may be air or an oxygen-containing gas commonly used in this field. As mentioned above, the "curvature degree" is the ratio (L/d) of the distance L of the path through which gas moves in the anode side gas diffusion layer to the thickness d of the anode side gas diffusion layer in the gas transmission direction. As the anode side gas diffusion layer, a gas diffusion layer whose material is adjusted so that its degree of curvature is 3.7 or less is used.

As described above, in the present invention, in a so-called "polymer electrolyte fuel cell", a pair of gas diffusion layers are stacked on both sides of a stacked body with an electrolyte membrane and a catalyst electrode layer sandwiched therebetween. Among these, members are selected so that the characteristics of the anode side gas diffusion layer have a bending degree of 3.7 or less. It has been found that with such a configuration, high power generation performance can be obtained or maintained in both high temperature and low temperature ranges, as will be explained in detail in the embodiment section below. has been done. This is because the degree of curvature is related to the ease with which fluid passes through the gas diffusion layer (generally speaking, the higher the degree of curvature, the longer the fluid movement path becomes, making it more difficult for fluid to pass through). By adjusting the degree of curvature of the anode side gas diffusion layer as described above, water can be properly discharged during low-temperature operation, the flow resistance to gas can be kept low, and the high gas concentration in the gas diffusion layer can be suppressed. This is thought to be due to the fact that transparency was ensured. Thus, according to the above configuration of the present invention, by appropriately controlling the degree of curvature in the characteristics of the anode side gas diffusion layer, it is possible to obtain a fuel cell with high power generation performance over a wide operating temperature range. becomes possible.

In the above configuration of the present invention, the gas diffusion layer has a base material layer made of carbon paper or the like, and the MPL layer is provided on the side of the base material layer in contact with the catalyst electrode layer. (Micro porous layer) may have a laminated structure. The MPL may be formed by applying a paste containing carbon black, a water repellent, or even a radical quenching agent to a base layer and firing.

Furthermore, in preparing or selecting the material of the anode side gas diffusion layer, the degree of curvature of the material may be determined using the following formula.
τ=Rgas・D・ε/d…(1)
Here, τ is the degree of tortuosity, Rgas is the gas diffusion resistance, D is the diffusion coefficient of oxygen in the air, ε is the porosity of the gas diffusion layer, and d is ( This is the thickness of the gas diffusion layer (when no load is applied). The gas diffusion resistance Rgas is a quantity defined by d/Deff from the oxygen diffusion coefficient Deff in the gas diffusion layer and the thickness d of the gas diffusion layer. (see Reference 1). The porosity ε is the ratio of the volume of voids in the gas diffusion layer to the volume of the gas diffusion layer, and is measured by mercury intrusion method using a mercury porosimeter. The thickness d of the gas diffusion layer may be measured using a micrometer or the like. That is, in the configuration of the present invention, an anode-side gas diffusion layer whose degree of curvature is determined using equation (1) may be used. Equation (1) is calculated as follows between the oxygen diffusion coefficient Deff in the gas diffusion layer and the oxygen diffusion coefficient D in the air: Deff=(ε/τ)D...(2)
It was derived from the relationship (Non-Patent Document 2).

The inventor of the present invention has discovered that in a fuel cell, the drainage performance from the fuel cell is determined not by the gas permeability and water permeability in the gas diffusion layer, but by the degree of curvature of the anode side gas diffusion layer. Focusing on the point that can be controlled by adjusting the It has been found that good power generation performance can be obtained both at low temperatures and during high temperature operation. As will be understood from the experimental results described below, by controlling the degree of curvature of the anode side gas diffusion layer as described above, the moist state of the membrane can be maintained during high temperature operation, while during low temperature operation. This is thought to be because fluid circulation on the anode side is appropriately improved, thereby suppressing stagnation in gas supply due to water stagnation. According to the configuration of the present invention, it is expected that suitable power generation performance can be obtained during operation in a wide temperature range. Further, such a configuration is advantageous in that preferred effects can be easily achieved simply by appropriately selecting the material of the anode side gas diffusion layer.

Other objects and advantages of the invention will become apparent from the following description of preferred embodiments of the invention.

FIG. 1 is a schematic cross-sectional view of one aspect of the fuel cell according to this embodiment. FIG. 2 is a schematic diagram of a gas diffusion layer for explaining the definition of the degree of curvature. FIGS. 3A and 3B are plots of changes in the generated cell voltage when the anode side gas diffusion layer is set to various bending degrees in the fuel cell. (A) shows the case where the cell temperature setting is 40° C. (during low temperature operation), and (B) shows the case where the cell temperature setting is 105° C. (during high temperature operation). The cell voltage is a value when a certain reference current is applied. FIG. 3(C) is a value obtained by multiplying the generated voltages of FIG. 3(A) and FIG. 3(B) for each example.

1... Fuel cell 2... Electrolyte membrane 3a, 3c... Anode side catalyst electrode layer, cathode side catalyst electrode layer 4a, 4c... Anode side gas diffusion layer, cathode side gas diffusion layer 5a, 5c... Anode side separator, cathode side separator 6a, 6c...Anode side flow path, cathode side flow path 7a, 7c...Anode side base material layer, cathode side base material layer 8a, 8c...Anode side MPL, cathode side MPL

BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now be described in detail with reference to preferred embodiments and with reference to the accompanying drawings. In the figures, the same reference numerals indicate the same parts.

Basic Structure of Fuel Cell Referring to FIG. 1, in the basic structure of fuel cell 1, electrolyte membrane 2 is sandwiched between anode side and cathode side catalyst electrode layers 3a and 3c, and A laminate sandwiched between the side gas diffusion layers 4a, 4c is formed, and this laminate and a frame member (not shown) arranged to surround the laminate are connected to the anode side and cathode side separators 5a, 5c. sandwiched between. The electrolyte membrane 2 is typically an ion exchange membrane with a thickness of several micrometers and has proton conductivity formed of a polymer electrolyte material, such as a perfluorosulfonic acid polymer having a sulfo group at the end of a side chain. It may be formed from The catalytic electrode layers 3a and 3c are made of carbon particles supporting a catalytic metal (for example, platinum or a platinum alloy consisting of platinum and another metal such as ruthenium) that promotes the water production reaction, and a polymer having proton conductivity. A material containing an electrolyte is applied in a layered form with a thickness of approximately several μm. The gas diffusion layers 4a and 4c have base layers 7a and 7c made of carbon material such as carbon cloth and carbon paper, and carbon black, a water repellent, or a radical quencher on the side in contact with the catalyst electrode layers 3a and 3c. It is a layer of a porous material having MPL (Micro porous layer: water repellent layer) 8a and 8c formed by applying and sintering a paste containing an agent (for example, cerium oxide). On the sides of the separators 5a and 5c facing the gas diffusion layers 4a and 4c, passages (channels) 6a and 6c through which fluid flows are provided, respectively, and although not shown, the periphery of the stacked body is surrounded by a frame. The member seals between the separators 5a and 5c. The separators 5a and 5c may be made of carbon paper or metal plates.

In the power generation operation of the fuel cell described above, hydrogen gas and air are supplied through the anode side flow path 6a and the cathode side flow path 6c, respectively, and pass through the gas diffusion layers 4a and 4c to the catalyst electrode layer 3a. , reaches the catalyst electrode layer 3c, hydrogen gas emits electrons in the catalyst electrode layer 3a, becomes protons, passes through the electrolyte membrane 2, reaches the catalyst electrode layer 3c, and there, air It reacts with the oxygen inside to generate water, and the generated water is collected and discharged to the cathode side flow path 6c.

Balancing the drainage and water retention functions of the gas diffusion layer In the fuel cell 1 described above, in order for the protons generated on the anode side to be conducted to the cathode side through the electrolyte membrane 2, the electrolyte membrane 2 must be in a liquid state. It must be moistened with water. In particular, when the operating temperature of the fuel cell is high, if water becomes too easily vaporized and discharged, the electrolyte membrane 2 will dry out, proton resistance within the membrane will increase, and power generation performance will decline. Therefore, the gas diffusion layers 4a and 4c need to have water retention ability to maintain the electrolyte membrane 2 in a moist state even during such high-temperature operation. On the other hand, water is generated on the cathode side during the power generation reaction, so when the operating temperature of the cell is lower than the boiling point, the generated water tends to stay in the cell, and in that case, the catalyst electrode layer It is preferable that the water be drained properly, since this may obstruct the flow of gas supplied to the tank and reduce power generation performance. Therefore, in order to prevent the power generation performance of the fuel cell from decreasing over a wide operating temperature range, the electrolyte membrane 2 must be kept wet by avoiding drying during high-temperature operation. However, during low-temperature operation where water tends to accumulate, it is preferable that both drainage and water retention in the cell 1 be achieved so that the drainage performance of generated water can be improved.

Regarding this point, the inventors of the present invention believe that the drainage and water retention performance in the cell 1 is determined by the ease of fluid flow in the gas diffusion layer, and that the ease of fluid flow in the gas diffusion layer is determined by the ease of fluid flow in the gas diffusion layer. Focusing on the fact that it depends on the ``degree of curvature'' in the layer, we conducted various studies and found that appropriate drainage performance can be achieved by appropriately controlling the ``degree of curvature'' of the anode-side gas diffusion layer. Specifically, if the material of the anode side gas diffusion layer is adjusted or selected so that the degree of curvature of the anode side gas diffusion layer is within the range of 3.7 or less, or more preferably 3 or less, high temperature It has been found that good power generation performance can be obtained both during operation and at low temperature operation. By controlling the degree of curvature of the gas diffusion layer on the anode side as described above, the moist state of the membrane can be maintained during high-temperature operation, while the fluid circulation on the anode side can be moderated during low-temperature operation. This is thought to be due to the fact that water retention is eliminated and gas flow improves.

Therefore, in the present embodiment, the gas diffusion layer 4a on the anode side is adjusted such that the degree of curvature of the gas diffusion layer 4a on the anode side is within the range of 3.7 or less, or more preferably within the range of 3 or less. are adjusted or materials are selected.

Regarding the measurement of the degree of curvature of the gas diffusion layer , referring to FIG. is the ratio of the length L of the fluid path passing through the gas diffusion layer (more precisely, the actual fluid path length to the path length (=d) when the fluid permeates the gas diffusion layer in a straight line) ). However, in measuring the degree of curvature, it is difficult to directly measure the length L of the fluid path, so the degree of curvature of the gas diffusion layer is estimated using several measurable quantities. It may be determined by

Specifically, first, the tortuosity τ is approximately related to the oxygen diffusion coefficient Deff in the gas diffusion layer and the oxygen diffusion coefficient D in the air by the following relational expression (Non-patent Document 2 ).
Deff=(ε/τ)D...(2)
Here, ε is the porosity of the gas diffusion layer (the ratio of the volume of voids in the gas diffusion layer to the volume of the gas diffusion layer). Further, regarding the oxygen diffusion coefficient Deff in the gas diffusion layer, the following relationship is given using the gas diffusion resistance Rgas and the thickness d of the gas diffusion layer.
Rgas=d/Deff...(3)
Therefore, the degree of curvature τ is
τ=Rgas・D・ε/d…(1)
is given by

In the above equation (1), a value obtained by any method may be used as the diffusion coefficient D of oxygen in the air. The thickness d of the gas diffusion layer is measured using a micrometer or the like, and the porosity ε is measured by mercury intrusion method using a mercury porosimeter. The gas diffusion resistance Rgas can be measured by a measurement method using a galvanic cell as described in Non-Patent Document 1. In this measurement method, to put it simply, the amount of decrease ΔC in oxygen concentration when a gas diffusion layer is placed in the oxygen supply channel of the cathode of a galvanic cell (gas diffusion in the oxygen supply channel of the cathode) (difference in oxygen concentration between when the layer is not placed and when the layer is placed), and the oxygen consumption Q calculated from the current flowing through the zinc-air battery when the zinc-air battery is placed on the same side as the galvanic battery. The cross-sectional area A of the gas diffusion layer in the direction perpendicular to the gas flow direction is measured, and the gas diffusion resistance Rgas is
Rgas=ΔC/Q×A…(4)
Calculated by

Thus, when configuring the fuel cell according to the present embodiment, for the anode side gas diffusion layer, refer to the bending degree calculated using the values measured as described above as each numerical value of the above formula (1). A layer having a bending degree of 3.7 or less may be used as the anode side gas diffusion layer. The degree of curvature of the anode side gas diffusion layer 4a is determined by the porosity of the carbon base material contained therein, the amount of water repellent applied or added, the amount of resin binder, and the thickness of the gas diffusion layer. It is adjusted by changing variously. The cathode side gas diffusion layer may be appropriately selected by any method.

Confirming the effects of this embodiment Fuel cells were created using a plurality of anode-side gas diffusion layers with different degrees of curvature, and experiments were conducted to check the power generation performance of these fuel cells during high-temperature operation and low-temperature operation. I checked it.

厚さ、空隙率、ガス拡散抵抗は、それぞれ、上記の方法により計測し、屈曲度は、各計測値を用いて、式（１）により算出した。 In the experiment, as an example of the present embodiment and a comparative example, an anode side gas diffusion layer having the following characteristics was prepared and used in a fuel cell. In addition, in the fuel cell, all the same members were used except for the anode side gas diffusion layer.

The thickness, porosity, and gas diffusion resistance were each measured by the methods described above, and the degree of curvature was calculated by equation (1) using each measurement value.

The cell operating conditions were as follows.
・Cell temperature during high temperature operation: 105℃
Dew point temperature: Anode side 45℃, cathode side 74℃
・Cell temperature during low temperature operation: 40℃
Dew point temperature: 45℃ on the anode side, -40℃ on the cathode side
Conditions such as the stoichiometric ratio and pressure of the supplied gas were the same during high-temperature operation and low-temperature operation.

Figures 3 (A) and (B) show examples 1, 2, 3 (Ex. 1, Ex. 2, Ex. 3) and comparative example (Co) used as the anode side gas diffusion layer, respectively. FIG. 4 is a diagram plotting the generated voltages during low-temperature operation and high-temperature operation with respect to the degree of curvature of each gas diffusion layer when the current is set to a certain predetermined value in the cell. As can be understood with reference to the figure, during high-temperature operation, a relatively high generated voltage was obtained for any degree of tortuosity, but when the degree of tortuosity exceeded 3.7 during low-temperature operation (comparative example Co), a decrease in the generated voltage was observed. This is considered to be because when the degree of curvature exceeds 3.7, drainage performance deteriorates and the amount of gas supplied decreases. Moreover, FIG. 3(C) is a diagram in which the value multiplied by the value of the generated voltage in FIGS. 3(A) and (B) is plotted against the degree of curvature of each gas diffusion layer, and in the diagram, The higher the value, the higher the power generation performance in both low-temperature and high-temperature operation. As understood with reference to the figure, high power generation performance was obtained in Examples 1, 2, and 3, that is, when the degree of curvature of the anode side gas diffusion layer was 3.7 or less. Note that from the tendency of the values in FIG. 3(C), it is expected that higher power generation performance can be obtained more stably when the degree of curvature of the anode side gas diffusion layer is lower than 3.0. (According to experiments, it has been shown that high power generation performance can be reliably obtained in a bending degree range of 2.6 to 3.7.) From these results, the teachings of this embodiment Accordingly, when the anode side gas diffusion layer is set such that its degree of curvature is 3.7 or less, preferably 3.0 or less, good power generation performance can be obtained in operation over a wide temperature range. It was shown that

Although the above description has been made in connection with the embodiments of the present invention, many modifications and changes are easily possible to those skilled in the art, and the present invention is limited to the embodiments illustrated above. It will be obvious that the present invention is not limiting and may be applied to a variety of devices without departing from the inventive concept.

Claims (1)

A laminate in which catalyst electrode layers are laminated on both sides of an electrolyte membrane, and gas diffusion layers are further laminated on both sides, and the laminate is transferred to a gas diffusion layer on the anode side of the gas diffusion layers containing hydrogen. an anode-side separator that defines a flow path for supplying oxygen, and a cathode that defines a flow path for supplying oxygen-containing gas to the cathode-side gas diffusion layer of the gas diffusion layers and discharging generated water. A fuel cell having a configuration in which the anode side gas diffusion layer is sandwiched between side separators, and the degree of curvature of the anode side gas diffusion layer is 3.7 or less.

Images