Membrane electrode assembly and method for evaluating sub-zero starting performance

Abstract

To provide a membrane electrode assembly improved in starting performance below a freezing point, and, an evaluation method capable of accurately evaluating the starting property below the freezing point.SOLUTION: A membrane electrode assembly comprises a cathode catalyst layer which is bonded to one face of an electrolyte membrane. Regarding the membrane electrode assembly, a water generation quantity in start below a freezing point is equal to or more than 0.4 mg / cm2 and equal to or less than 0.8 mg / cm2 and regarding the cathode catalyst layer, a cavity quantity is equal to or more than 0.27 μL / cm2 and equal to or less than 0.75 μL / cm2. The evaluation method for starting property below the freezing point includes: a first step of supplying a humidified gas to an anode passage and a cathode passage of a solid polymer fuel cell and then sealing the anode passage and the cathode passage; a second step of freezing the solid polymer fuel cell; a third step of performing power generation by supplying a non-humidified fuel gas and an oxidant gas to the solid polymer fuel cell; a fourth step of calculating the water generation quantity from a current quantity during the power generation; and a fifth step of determining whether or not the water generation quantity is within a predetermined range.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a membrane electrode assembly and a method for evaluating sub-zero starting performance, and more specifically, to a membrane electrode assembly that exhibits excellent starting performance in sub-zero environments, and to a method for evaluating such sub-zero starting performance. [Background technology]

[0002] A polymer electrolyte fuel cell (MSF) is based on a membrane electrode assembly (MEA) in which electrodes (catalyst layers) are bonded to both sides of a polymer electrolyte membrane. In a polymer electrolyte fuel cell, a gas diffusion layer is generally placed outside the catalyst layer. The gas diffusion layer supplies reaction gases and electrons to the catalyst layer and is made of materials such as carbon paper or carbon cloth. The catalyst layer is the reaction field for the electrode reaction and generally consists of a composite of carbon supporting an electrode catalyst such as platinum and a polymer electrolyte (catalyst layer ionomer). Furthermore, a separator with a gas channel is placed outside the gas diffusion layer.

[0003] It is known that polymer electrolyte fuel cells (MSFs) tend to stop generating power when started in sub-zero temperatures. This is because, when power generation is started in sub-zero temperatures, the water generated by the power generation freezes in the voids of the catalyst layer, blocking the voids and inhibiting the diffusion of the reaction gas within the catalyst layer. Therefore, various proposals have been made to improve the sub-zero starting performance of MSFs.

[0004] For example, Patent Document 1 discloses a polymer electrolyte fuel cell in which the volume ratio of ionomer to catalyst support contained in the cathode catalyst layer is 0.45 or more. The document states: (a) When a fuel cell is started under low temperature conditions below 0°C, water generated by power generation may freeze in the pores of the catalyst layer, which may interfere with the supply of oxidant gas. (b) Increasing the volume ratio of ionomers within the catalyst layer reduces the number of pores within the catalyst layer, but the generated water remains within the ionomer clusters, thus preventing freezing, and (c) Oxygen is transported to the catalyst via fluorine atoms that make up the ionomer and water within the ionomer clusters, so even if the water in the pores freezes, oxygen continues to be supplied to the catalyst. It is stated.

[0005] Patent Document 2 discloses a fuel cell catalyst obtained by spray-drying a slurry containing catalyst-supported particles, a solid polymer electrolyte, and a solvent. The document states: (a) By this method, hollow particles containing catalyst-supported particles and solid polymer electrolytes can be obtained, and (b) When such hollow particles are used as a catalyst for fuel cells, the generated water is retained inside the hollow particles, which suppresses the decrease in power generation performance due to condensation of the generated water during sub-zero starting. It is stated.

[0006] The method described in Patent Document 1 aims to increase the volume ratio of ionomer to catalyst support contained in the cathode catalyst layer, thereby causing the ionomer in the catalyst layer to absorb generated water. However, in order to ensure a practically sufficient amount of water absorption, a relatively large amount of ionomer is required. If there is an excess of ionomer in the catalyst layer, flooding is likely to occur under normal power generation conditions, and sufficient output may not be obtained.

[0007] On the other hand, the method described in Patent Document 2 aims to add a catalyst consisting of hollow particles to the catalyst layer and retain the generated water within the voids of the hollow particles. Using the method described in Patent Document 2 improves sub-zero starting performance. However, the method described in Patent Document 2 requires a spray drying treatment of the slurry, which may increase the manufacturing cost of the catalyst. In addition, the method described in Patent Document 2 may introduce more voids into the catalyst layer than is necessary to improve sub-zero starting performance. Furthermore, in order to manufacture a fuel cell with excellent sub-freezing start-up performance, it is necessary to accurately evaluate the sub-freezing start-up performance. However, there has been no example of a proposed evaluation method that can accurately evaluate the sub-freezing start-up performance in the past.

Prior Art Documents

Patent Documents

[0008]

Patent Document 1

Patent Document 2

Summary of the Invention

Problems to be Solved by the Invention

[0009] The problem to be solved by the present invention is to provide a membrane electrode assembly capable of improving sub-freezing start-up performance without significantly impairing other characteristics. Another problem to be solved by the present invention is to provide a sub-freezing start-up performance evaluation method capable of accurately evaluating sub-freezing start-up performance.

Means for Solving the Problems

[0010] In order to solve the above problems, the membrane electrode assembly according to the present invention has the following configuration. (1) The membrane electrode assembly includes an electrolyte membrane containing a resin (A) having proton conductivity, a cathode catalyst layer joined to one surface of the electrolyte membrane, and an anode catalyst layer joined to the other surface of the electrolyte membrane and is provided with. (2) The cathode catalyst layer includes an electrode catalyst (B) in which catalyst particles (B) are supported on a conductive carrier (B), a resin (B) having proton conductivity and includes, the resin (B) includes a high oxygen permeable ionomer. (3) The membrane electrode assembly generates 0.4 mg / cm³ of water when started at sub-zero temperatures. 2 The above 0.8 mg / cm³ 2 The following applies: (4) The cathode catalyst layer has a void volume of 0.27 μL / cm² per unit area. 2 More than 0.75μL / cm 2 The following applies:

[0011] The sub-zero starting performance evaluation method according to the present invention is A first step involves supplying humidified gas to the anode channel and cathode channel of a polymer electrolyte fuel cell, and then sealing the anode channel and cathode channel. A second step of freezing the aforementioned polymer electrolyte fuel cell, A third step involves supplying unhumidified fuel gas and oxidizer gas to the aforementioned polymer electrolyte fuel cell and generating electricity. A fourth step involves calculating the amount of water produced from the amount of current during power generation, A fifth step is to determine whether the amount of water produced is within a predetermined range. It is equipped with. [Effects of the Invention]

[0012] When a frozen polymer electrolyte fuel cell is supplied with unhumidified fuel gas and oxidizer gas and used to generate electricity, the amount of water produced can be calculated from the amount of current generated during power generation. This amount of water produced serves as an indicator of sub-zero starting ability. Generally, a low amount of water produced indicates that the voids in the catalyst layer are more likely to become blocked by the freezing of the water produced during sub-zero starting. On the other hand, a higher amount of water produced indicates better sub-zero starting ability, but excessive water production may lead to a decrease in fuel cell characteristics, especially in the high-temperature range. Therefore, by optimizing the amount of voids in the cathode catalyst layer so that the amount of generated water falls within a predetermined range, it is possible to improve sub-zero starting performance without significantly impairing other characteristics. [Brief explanation of the drawing]

[0013] [Figure 1] This figure shows an example of the results of a sub-zero starting acceleration test. [Figure 2] Figure 2(A) is an image of the catalyst layer cross-section. Figure 2(B) is the same image as Figure 2(A), but with the threshold level increased until areas that can be considered voids appear black. Figure 2(C) is the same image as Figure 2(B), but with the threshold level decreased to the value just before areas that can be considered carbon cross-sections appear black. Figure 2(D) is a binarized image of Figure 2(C), with the remaining white areas in the voids filled in black. [Figure 3] This figure shows the relationship between the amount of void space and the amount of water produced. [Figure 4] This figure shows the relationship between tap density and void volume. [Modes for carrying out the invention]

[0014] One embodiment of the present invention will be described in detail below. [1. Membrane electrode assembly] The membrane electrode assembly according to the present invention has the following configuration. (1) The film electrode assembly is An electrolyte membrane containing a resin (A) having proton conductivity, A cathode catalyst layer bonded to one side of the electrolyte membrane, an anode catalyst layer bonded to the other side of the electrolyte membrane and It is equipped with. (2) The cathode catalyst layer is An electrode catalyst (B) in which catalyst particles (B) are supported on a conductive carrier (B), A resin (B) having proton conductivity and Includes, The aforementioned resin (B) contains a highly oxygen-permeable ionomer. (3) The membrane electrode assembly generates 0.4 mg / cm³ of water when started at sub-zero temperatures. 2 The above 0.8 mg / cm³ 2 The following applies: (4) The cathode catalyst layer has a void volume of 0.27 μL / cm² per unit area. 2 More than 0.75μL / cm 2 The following applies:

[0015] [1.1. Electrolyte membrane] [1.1.1. Materials] The electrolyte membrane contains a proton-conducting resin (A). The material of resin (A) is not particularly limited, and the most suitable material can be selected depending on the purpose. For example, resin (A) is: (a) Perfluorocarbon sulfonate polymers such as Nafion®, Flemion®, Aciprex®, and Acquivion®, (b) Hydrocarbon polymers such as sulfonated polyether ketones, sulfonated polyether sulfones, sulfonated polyether ether sulfones, sulfonated polysulfides, and sulfonated polyphenylenes. (c) High oxygen permeable ionomers, as described later, These are some examples.

[0016] [1.1.2.EW] The EW of resin (A) affects sub-zero starting performance. During sub-zero starting, some of the water generated in the catalyst layer diffuses and is absorbed into the electrolyte membrane. The lower the EW of resin (A) (i.e., the higher the amount of acid groups in resin (A)), the more water is absorbed into the electrolyte membrane, and the less water is absorbed into the catalyst layer, making it less likely for the voids in the catalyst layer to become blocked. In an electrolyte membrane made of resin (A) with a high EW, this water absorption effect is small, and it is not possible to continue generating enough power for sub-zero starting. Therefore, the EW of resin (A) is preferably 1100 g / eq or less. Preferably, the EW is 1000 g / eq or less, and more preferably 900 g / eq or less. On the other hand, if the EW becomes too small, the swelling of the electrolyte membrane may increase significantly during operation at high humidity, potentially reducing the durability of the electrolyte membrane. Therefore, an EW of 600 g / eq or more is preferable. Preferably, the EW is 650 g / eq or more, and more preferably, 700 g / eq or more.

[0017] [1.2. Cathode Catalyst Layer] A cathode catalyst layer is bonded to one side of the electrolyte membrane. In the present invention, the cathode catalyst layer is An electrode catalyst (B) in which catalyst particles (B) are supported on a conductive carrier (B), A resin (B) having proton conductivity and Includes.

[0018] [1.2.1. Electrocatalyst (B)] The electrode catalyst (B) consists of catalyst particles (B) supported on a conductive carrier (B).

[0019] [A. Conductive carrier (B)] [A.1. Materials] In the present invention, the material of the conductive carrier (B) is not particularly limited. Examples of conductive carriers (B) include: (a) Carbon carriers such as carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder. (b) Metal carrier made of pure metal or alloy, (c) an oxide carrier made of a conductive metal oxide or composite metal oxide, These are some examples. The shape of the conductive carrier (B) is not particularly limited, and the optimal shape can be selected according to the purpose.

[0020] [A.2. Tap Density] "Tap density" refers to a value measured in accordance with JIS Z 2512. The conductive support (B) typically has a primary particle size of 10 nm to 300 nm. This is to ensure that the catalyst particles (B) are highly dispersed on the surface of the conductive support (B). In the cathode catalyst layer, these fine primary particles do not usually exist individually, but rather as aggregated secondary particles. The degree of primary particle aggregation affects the amount of voids in the cathode catalyst layer.

[0021] The degree of aggregation of primary particles can be represented by the tap density. Generally, the smaller the tap density of the conductive carrier (B), the more voids can be introduced into the cathode catalyst layer. Therefore, generally, the lower the tap density, the better the sub-zero startability. In order to ensure the void volume described later, the conductive carrier (B) preferably has a tap density of 0.17 g / cm 3 or less. The tap density is preferably 0.15 g / cm 3 or less.

[0022] However, if the tap density becomes too small, the thickness of the cathode catalyst layer required to make the coating weight of the catalyst particles (B) a certain value or more increases, and the proton resistance of the cathode catalyst layer increases. Therefore, the tap density is preferably 0.05 g / cm 3 or more. The tap density is more preferably 0.06 g / cm 3 or more.

[0023] [B. Catalyst Particles (B)] In the present invention, the material of the catalyst particles (B) is not particularly limited. Examples of the material of the catalyst particles (B) include (a) noble metals (Pt, Au, Ag, Pd, Rh, Ir, Ru, Os), (b) alloys containing two or more noble metal elements, (c) alloys containing one or two or more noble metal elements and one or two or more base metal elements (e.g., Fe, Co, Ni, Cr, V, Ti, etc.). and the like.

[0024] Among these, the catalyst particles (B) are preferably Pt or a Pt alloy because they have high activity for the electrode reaction of the fuel cell. Examples of the Pt alloy include Pt-Fe alloy, Pt-Co alloy, Pt-Ni alloy, Pt-Pd alloy, Pt-Cr alloy, Pt-V alloy, Pt-Ti alloy, Pt-Ru alloy, Pt-Ir alloy, etc.

[0025] The particle size of catalyst particles (B) is not particularly limited, and the optimal particle size can be selected according to the purpose. Generally, if the particle size of catalyst particles (B) is too small, the catalyst particles (B) will dissolve easily. Therefore, the particle size of catalyst particles (B) is preferably 1 nm or larger. On the other hand, if the particle size of catalyst particles (B) becomes too large, the mass activity decreases. Therefore, the particle size of catalyst particles (B) is preferably 20 nm or less. Preferably, the particle size of catalyst particles (B) is 10 nm or less, and more preferably 5 nm or less.

[0026] [1.2.2. Catalyst layer ionomer (B)] The catalyst layer ionomer (B) contained in the cathode catalyst layer consists of a proton-conducting resin (B).

[0027] [A. Materials] In the present invention, the material of resin (B) is not particularly limited. Resin (B) may be a perfluorocarbon sulfonic acid polymer, a hydrocarbon polymer, or a highly oxygen-permeable ionomer. In order to reduce oxygen transport resistance within the cathode catalyst layer, it is preferable that resin (B) contains a highly oxygen-permeable ionomer.

[0028] The resin (B) may consist solely of a highly oxygen-permeable ionomer. In this case, oxygen transfer to the catalyst particles (B) is facilitated, making the catalytic reaction more likely to occur and thus offering the advantage of improved performance. Alternatively, resin (B) may contain other ionsomers (e.g., perfluorocarbon sulfonic acid polymer) in addition to the highly oxygen-permeable ionomer. In this case, cracking of the catalyst layer becomes less likely, which has the advantage of increasing the productivity of the catalyst layer. However, if the content of the high-oxygen-permeable ionomer is too low, the performance may deteriorate. Therefore, the mass percentage of the high-oxygen-permeable ionomer in resin (B) is preferably 70 mass% or more. Preferably, the mass percentage of the high-oxygen-permeable ionomer is 80 mass% or more, and more preferably 90 mass% or more.

[0029] Here, "high oxygen permeable ionomer" refers to a polymer compound that contains acidic groups and cyclic structures within its molecular structure. Because high oxygen permeable ionomers contain cyclic structures within their molecular structure, they have a high oxygen permeability coefficient. Therefore, when used as an ionomer, the oxygen transport resistance at the interface with the catalyst becomes relatively small. In other words, a "high oxygen permeability ionomer" refers to an ionomer whose oxygen permeability coefficient is higher than that of perfluorocarbon sulfonate polymers, such as Nafion (registered trademark).

[0030] Generally, the performance of a fuel cell is limited by the diffusion of oxygen to the catalyst surface. In contrast, coating the surface of the electrode catalyst with a highly oxygen-permeable ionomer improves the oxygen permeability of the catalyst layer, thereby improving the performance of the fuel cell. The molecular structure of a highly oxygen-permeable ionomer is not particularly limited, as long as it exhibits relatively low oxygen transport resistance. In particular, ionsomers containing a cyclic structure (aliphatic ring structure) within their molecular structure are suitable as coatings for electrode catalysts because they have lower oxygen transport resistance than ionsomers without a cyclic structure.

[0031] Examples of high-oxygen-permeability ions include, (a) an electrolyte polymer comprising a perfluorocarbon unit having an aliphatic ring structure and an acid group unit having perfluorosulfonic acid as a side chain, (b) an electrolyte polymer comprising a perfluorocarbon unit having an aliphatic ring structure and an acid group unit having a perfluoroimide as a side chain, (c) An electrolyte polymer containing a unit in which perfluorocarbon having an aliphatic ring structure is directly bonded to perfluorosulfonic acid, These include (see references 1-4). [Reference 1] Japanese Patent Publication No. 2003-036856 [Reference 2] International Publication No. 2012 / 088166 [Reference 3] Japanese Patent Publication No. 2013-216811 [Reference 4] Japanese Patent Publication No. 2006-152249

[0032] [B. EW] The EW of resin (B) affects sub-zero starting performance, proton conductivity and / or gas diffusion of the cathode catalyst layer, etc. Generally, if the EW of resin (B) is too low, the water content increases, making resin (B) more easily soluble in water. Therefore, an EW of 588 g / eq or higher is preferable. Preferably, the EW is 600 g / eq or higher, and more preferably, 620 g / eq or higher. On the other hand, if the EW of resin (B) becomes too high, the proton conductivity of the cathode catalyst layer decreases. Also, resin (B) has the effect of absorbing generated water. Therefore, if the EW becomes too high, water will remain in the voids of the catalyst layer during sub-zero starting, and the water in the voids will be more likely to freeze. Accordingly, an EW of 900 g / eq or less is preferable. Preferably, the EW is 850 g / eq or less, and more preferably, 830 g / eq or less.

[0033] [Gelation of C. ionoma] Ionomer molecules generally consist of a main chain and side chains bonded to the main chain, with acidic groups attached to the ends of the side chains. When such ionomer molecules are dispersed in water, they form a structure (micelle structure) in which the outer layer consists mainly of hydrophilic side chains and the inner layer consists mainly of a hydrophobic main chain.

[0034] When a dispersion is prepared using an ionomer with such a micelle structure, the micelle structure remains in the dispersion. Dispersions containing micelle structures generally have low viscosity. When a cathode catalyst layer is prepared using such a catalyst ionomer, the adhesion strength (interfacial adhesion strength) between the cathode catalyst layer and the electrolyte membrane may decrease. This is thought to be because there is less molecular entanglement at the interface between the electrolyte membrane and the cathode catalyst layer.

[0035] On the other hand, when preparing a dispersion using an ionomer with a micelle structure, if the ionomer is treated appropriately, it will not take on a micelle structure in the dispersion but will be uniformly dispersed (this state is also called "gelation").

[0036] Dispersions containing gelled ionomers generally have high viscosity. When a cathode catalyst layer is fabricated using such a catalyst layer ionomer, relatively high interfacial adhesion strength can be obtained. This is thought to be because molecular entanglement increases at the interface between the electrolyte membrane and the cathode catalyst layer. To obtain high interfacial adhesion strength, it is preferable that the cathode catalyst layer be manufactured using the gelled resin (B) mentioned above. The degree of gelation can be expressed by the storage modulus. To obtain a catalyst layer with high interfacial adhesion strength, it is preferable that the resin (B) has a storage modulus of 40 Pa or more and 140 Pa or less when vibration at a frequency of 1 Hz is applied and the strain is 1%.

[0037] [1.2.3. Average I / C values] The "average I / C ratio" of the cathode catalyst layer refers to the ratio of the total mass of resin (B) to the total mass of conductive carrier (B) contained in the cathode catalyst layer. The average I / C value of the cathode catalyst layer affects sub-zero starting performance, proton conductivity, and / or gas diffusion of the cathode catalyst layer. Generally, if the average I / C value of the cathode catalyst layer is too low, the proton conductivity of the cathode catalyst layer decreases. Therefore, an average I / C value of 0.80 or higher is preferable. More preferably, an average I / C value of 0.95 or higher is preferable. On the other hand, if the average I / C of the cathode catalyst layer becomes too high, the amount of voids within the cathode catalyst layer decreases, and the gas diffusivity of the cathode catalyst layer decreases. As a result, sub-zero starting performance may decrease. Therefore, the average I / C is preferably 1.8 or less. Preferably, the I / C is 1.5 or less.

[0038] [1.2.4. Void volume] In this invention, "void volume per unit area (P)" refers to the value expressed by the following formula (1). P(μL / cm 2 ) = S × t / S0 ... (1) however, S0 is the area of ​​the field of view when observing the cross-section of the cathode catalyst layer with a microscope. S is the area of ​​the void included in the field of view. t is the thickness of the cathode catalyst layer.

[0039] The amount of voids per unit area of ​​the cathode catalyst layer (hereinafter simply referred to as "void amount") affects sub-zero starting performance. If the void amount is too low, water generated during sub-zero starting may freeze within the voids of the cathode catalyst layer, potentially hindering the diffusion of the oxidizer gas. Therefore, the void amount should be 0.27 μL / cm². 2 The above is necessary. The void volume is preferably 0.3 μL / cm 2 That's all. On the other hand, if the void volume is excessive, the cathode catalyst layer needs to be thickened to obtain a certain basis weight of electrode catalyst (B). If the cathode catalyst layer becomes excessively thick, the proton conductivity of the cathode catalyst layer may decrease. Therefore, the void volume should be 0.75 μL / cm². 2 The following is required: The void volume is preferably 0.6 μL / cm². 2 More preferably, 0.5 μL / cm 2 The following applies:

[0040] Furthermore, the amount of voids depends not only on the microstructure of the conductive support (B) but also on the manufacturing conditions of the cathode catalyst layer. For example, when manufacturing a catalyst ink by dispersing an electrode catalyst (B) with sufficient voids in a solvent, excessive dispersion treatment may destroy the microstructure of the electrode catalyst (B) during the dispersion treatment. If such a catalyst ink is used to form the cathode catalyst layer, the required amount of voids may not be obtained. Therefore, when manufacturing the cathode catalyst layer, it is preferable to select the material of the conductive support (B) and the manufacturing conditions of the cathode catalyst layer so as to ensure the necessary amount of void space.

[0041] [1.3. Anode Catalyst Layer] An anode catalyst layer is bonded to the other side of the electrolyte membrane. In the present invention, the anode catalyst layer is An electrode catalyst (C) in which catalyst particles (C) are supported on a conductive carrier (C), A resin (C) having proton conductivity and Includes.

[0042] [1.3.1. Electrocatalyst (C)] The electrode catalyst (C) consists of catalyst particles (C) supported on a conductive carrier (C). In the present invention, the electrode catalyst (C) is not particularly limited, and the most suitable one can be selected depending on the purpose. The conductive carrier (C) contained in the anode catalyst layer may be the same as or different from the conductive carrier (B) contained in the cathode catalyst layer. Similarly, the catalyst particles (C) contained in the anode catalyst layer may be the same as or different from the catalyst particles (B) contained in the cathode catalyst layer. Other aspects of the conductive carrier (C) and catalyst particles (C) are the same as those of the conductive carrier (B) and catalyst particles (B), respectively, so their explanation will be omitted.

[0043] [1.3.2. Catalyst layer ionomer (C)] The catalyst layer ionomer (C) contained in the anode catalyst layer is made of a proton-conducting resin (C). In the present invention, the catalyst layer ionomer (C) is not particularly limited, and the most suitable one can be selected depending on the purpose. The resin (C) contained in the anode catalyst layer may be the same as the resin (A) that constitutes the electrolyte membrane or the resin (B) contained in the cathode catalyst layer, or it may be different. Other aspects of resin (C) are the same as those of resin (A) or resin (B), so the explanation is omitted.

[0044] [1.3.3. Average I / C values] The "average I / C ratio" of the anode catalyst layer refers to the ratio of the total mass of resin (C) to the total mass of conductive carrier (C) contained in the anode catalyst layer. The average I / C value of the anode catalyst layer affects the performance of the film electrode assembly. Generally, if the average I / C value becomes too small, the proton conductivity of the anode catalyst layer decreases. Therefore, an average I / C value of 0.95 or higher is preferable. On the other hand, if the average I / C value becomes too large, the void amount in the anode catalyst layer decreases, and the gas diffusivity of the anode catalyst layer decreases. Therefore, the average I / C value is preferably 1.8 or less. Preferably, the average I / C value is 1.5 or less.

[0045] [1.4. Water production volume when starting at sub-zero temperatures] "The amount of water generated when starting at sub-zero temperatures" refers to (a) After humidifying both the anode and cathode electrodes of the polymer electrolyte fuel cell with a gas at a predetermined temperature and dew point, the gas flow path of the polymer electrolyte fuel cell is sealed. (b) Freeze the polymer electrolyte fuel cell with the gas flow path sealed, (c) Power is generated at a predetermined voltage for a predetermined time while supplying unhumidified hydrogen gas / air to the anode / cathode of a frozen polymer electrolyte fuel cell. (d) Convert the current density during power generation into the total amount of water produced per unit area. This refers to the value obtained by [the method described]. As a specific example, "the amount of water produced when starting at sub-zero temperatures" means: (a) After humidifying both the anode and cathode electrodes of the polymer electrolyte fuel cell with a gas at a temperature of 82°C and a dew point of 40°C, the gas flow path of the polymer electrolyte fuel cell is sealed. (b) Freeze the polymer electrolyte fuel cell at -20°C with the gas flow path sealed. (c) Power is generated at 0.6V for 120 seconds while supplying unhumidified hydrogen gas / air to the anode / cathode of a frozen polymer electrolyte fuel cell. (d) Convert the current density during power generation into the total amount of water produced per unit area. This refers to the value obtained by [the method described].

[0046] A low amount of generated water (hereinafter also simply referred to as "amount of generated water") during sub-zero starting indicates low power generation during sub-zero starting, that is, the voids in the cathode catalyst layer are blocked by the freezing of the generated water, making it difficult for the oxidizing gas to diffuse into the electrode catalyst (B). If the voids are completely blocked by freezing during sub-zero starting, power generation stops, and no more generated water is produced. In order to continue power generation during sub-zero starting, the amount of generated water should be 0.4 g / cm³. 2 The above is necessary. The amount of water produced is preferably 0.45 g / cm³. 2 That's all.

[0047] On the other hand, a large amount of generated water means that the amount of power generated when starting at sub-zero temperatures is high, that is, even if some of the voids in the cathode catalyst layer are blocked by the freezing of the generated water, the oxidizing gas is supplied to the electrode catalyst (B) through the other voids. When more power is generated when starting at sub-zero temperatures, the ice in the voids melts quickly due to the heat generated during power generation. However, an excessively large amount of generated water means that there are more voids than necessary in the cathode catalyst layer. If the amount of voids in the cathode catalyst layer becomes excessive, the proton conductivity of the catalyst layer decreases, which may reduce the power generation performance when operating at high temperatures. Therefore, the amount of generated water should be 0.8 g / cm³. 2 The following is preferable: The amount of water produced is preferably 0.75 g / cm³. 2 More preferably, 0.70 g / cm³ 2 The following applies:

[0048] [2. Method for evaluating sub-zero starting performance] The sub-zero starting performance evaluation method according to the present invention is A first step involves supplying humidified gas to the anode channel and cathode channel of a polymer electrolyte fuel cell, and then sealing the anode channel and cathode channel. A second step of freezing the aforementioned polymer electrolyte fuel cell, A third step involves supplying unhumidified fuel gas and oxidizer gas to the aforementioned polymer electrolyte fuel cell and generating electricity. A fourth step involves calculating the amount of water produced from the amount of current during power generation, A fifth step is to determine whether the amount of water produced is within a predetermined range. It is equipped with.

[0049] [2.1. 1st step] First, humidified gas is supplied to the anode and cathode channels of the polymer electrolyte fuel cell, and then the anode and cathode channels are sealed (first step). The type of gas used for humidification is not particularly limited. Examples of humidifying gases include air, nitrogen gas, and hydrogen gas.

[0050] It is preferable to select optimal values ​​for the humidifying gas temperature and dew point (or relative humidity), as well as the humidification time, depending on the evaluation purpose. For example, in order to quickly simulate the humidified state of a real polymer electrolyte fuel cell, the temperature of the humidifying gas is preferably between 30°C and 100°C. Furthermore, in order to reproduce the frozen state that simulates the actual machine in a short time, the relative humidity of the humidifying gas should preferably be between 10% and 90%. Furthermore, the humidification time is not particularly limited, as long as it is the time it takes for the system to reach equilibrium. After humidification under predetermined conditions, the anode and cathode channels are sealed.

[0051] [2.2. 2nd process] Next, the polymer electrolyte fuel cell is frozen (second step). The freezing temperature is not particularly limited, as long as it is possible to freeze the polymer electrolyte fuel cell. Specifically, the freezing temperature is preferably between -50°C and 0°C. More preferably, the freezing temperature is between -40°C and -10°C.

[0052] [2.3. Third step] Next, unhumidified fuel gas and oxidizer gas are supplied to the polymer electrolyte fuel cell to generate electricity (third step). When generating electricity, unhumidified fuel gas and oxidizer gas are supplied. This is to prevent water from freezing in the piping and to ensure stable testing.

[0053] It is preferable to select the optimal power generation conditions according to the evaluation objective. Generally, if the voltage during power generation is too low, the amount of water generated per unit time becomes excessive, causing water to accumulate in the voids of the catalyst layer and making it prone to freezing. On the other hand, if the voltage during power generation is too high, the amount of heat generated decreases, and a sufficient temperature rise in the catalyst layer may not be achieved. Therefore, a voltage of 0.3V to 0.75V during power generation is preferable. Generally, if the power generation time is too short, the test may be terminated even though freezing is progressing, making it difficult to accurately evaluate sub-zero starting performance. On the other hand, extending the power generation time unnecessarily is not practical. Therefore, a power generation time of 30 seconds to 500 seconds is preferable.

[0054] [2.4. 4th step] Next, the amount of water produced is calculated from the amount of current during power generation (step 4). The amount of water generated can be calculated by measuring the current every second and using the following formula. Amount of water produced (mg / cm 2 ) = current density (A / cm 2 ) ÷ 2 × time (sec) ÷ 96500 (C / mol: Faraday constant) × 18 (g / mol: molecular weight of water) × 1000

[0055] [2.5. 5th step] Next, it is determined whether the amount of generated water is within a predetermined range (5th step). The threshold value for the amount of water produced can be selected to the optimal value depending on the evaluation purpose. For example, the first step includes a step of humidifying a polymer electrolyte fuel cell using the gas having a temperature of 82°C and a dew point of 40°C. The second step includes a step of cooling the polymer electrolyte fuel cell to -20°C. If the third step includes a step of generating electricity under the conditions of cell voltage: 0.6V and power generation time: 120 seconds, Step 5 involves generating water at a rate of 0.4 mg / cm³. 2 The above 0.8 mg / cm³ 2Preferably, the system includes a step to determine whether or not the following conditions are met.

[0056] [3. Effect] Fuel cells (FCs) have a problem where the generated water freezes during power generation (startup) in sub-zero environments, causing power generation to stop. To solve this problem, a starting method has been proposed that uses self-heating during power generation to raise the temperature of the fuel cell cells and break through sub-zero temperatures. However, conventional methods have not always provided sufficient sub-zero starting performance. In contrast, in order to continue power generation in sub-zero environments, it is necessary to ensure that voids for the diffusion of reaction gases are secured within the catalyst layer, even when the generated water freezes. The membrane electrode assembly according to the present invention optimizes the amount of voids within the catalyst layer, making it possible to generate a sufficient amount of water to ensure the self-heating necessary to break through the sub-zero temperature. Therefore, the membrane electrode assembly according to the present invention has superior sub-zero starting performance compared to conventional membrane electrode assemblies equipped with a catalyst layer.

[0057] Specifically, when a frozen polymer electrolyte fuel cell is supplied with unhumidified fuel gas and oxidizer gas and power is generated, the amount of water produced can be calculated from the amount of current generated during power generation. The amount of water produced at this time serves as an indicator of sub-zero starting performance. Generally, a small amount of water produced indicates that the voids in the catalyst layer are more likely to become blocked by the freezing of the water produced during sub-zero starting. On the other hand, a larger amount of water produced indicates better sub-zero starting performance, but if the amount of water produced is excessive, it may lead to a decrease in fuel cell characteristics, especially in the high-temperature range. Therefore, by optimizing the amount of voids in the cathode catalyst layer so that the amount of generated water falls within a predetermined range, it is possible to improve sub-zero starting performance without significantly impairing other characteristics. [Examples]

[0058] (Examples 1-14, Comparative Examples 1-3) [1. Sample Preparation] [1.1. Fabrication of the cathode catalyst layer] Ten different types of Pt / C electrodes with varying carbon supports were used as the electrode catalysts in the cathode catalyst layer. Furthermore, a highly oxygen-permeable ionomer with an EW of 623 g / eq to 810 g / eq was used as the ionomer for the cathode catalyst layer. Table 1 shows the specifications of the carbon supports and ionomers used. Table 1 also shows the I / C of the cathode catalyst layer, the void volume and water volume of the cathode catalyst layer as measured by the method described later, and the EW of the electrolyte membrane used.

[0059] [Table 1]

[0060] Water was added to 1 g of Pt / C and stirred with a spatula. Ionomer solution was added to this and the mixture was dispersed using an ultrasonic homogenizer for 5 minutes. Ethanol was then added to this dispersion and the mixture was dispersed using an ultrasonic homogenizer for 10 minutes. Subsequently, a 3-minute centrifugal degassing treatment was performed to obtain a catalyst ink. The obtained catalyst ink had a Pt basis weight of 0.2 mg / cm³. 2 To achieve this, the catalyst layer was obtained by coating a polytetrafluoroethylene (PTFE) sheet with an applicator and drying it.

[0061] [1.2. Fuel Cell Fabrication] The cathode catalyst layer described above was transferred to one side of the electrolyte membrane, and the anode catalyst layer was transferred to the other side. The electrode area was 1 cm². 2 The electrolyte membrane used was a fluorine-based electrolyte membrane with an EW of 850 g / eq or 1030 g / eq. The anode catalyst layer used a composite of Pt / C (Pt load: 30 mass%) and Nafion®. The I / C ratio of the anode catalyst layer was set to 1.0 in all cases. The basis weight of Pt was 0.1 mg / cm³ in all cases. 2 That's what I decided.

[0062] [2. Test Method] [2.1. Sub-zero starting acceleration test] The anode and cathode electrodes of the fuel cell were humidified under conditions of a temperature of 82°C and a dew point of 40°C, then sealed and cooled to -20°C. Subsequently, while supplying unhumidified hydrogen gas and air to the anode and cathode, respectively, power was generated at 0.6V for 120 seconds, and the amount of water produced (cumulative amount of water produced) at sub-zero startup was measured.

[0063] Figure 1 shows an example of the results of a sub-zero starting acceleration test. In the example shown in Figure 1, the current density decreased sharply despite the voltage being maintained at 0.6V. This indicates that the voids in the cathode catalyst layer were blocked by the freezing of water, inhibiting the diffusion of air. However, since the cumulative amount of water generated increased monotonically even after the sharp decrease in current density, it is considered that power generation did not completely stop and continued.

[0064] [2.2. Porosity] Each catalyst layer was cross-sectionally prepared using an Ar ion beam, and then observed using a scanning electron microscope (SEM). The observation was performed under conditions of an acceleration voltage of 2kV and a working distance of approximately 3mm, and images were acquired at magnifications of 30,000 to 50,000 times for 3 to 5 fields of view. The acquired images were binarized using ImageJ. First, the acquired images were converted to 256-level monochrome images, and these were automatically adjusted using ImageJ's automatic brightness / contrast adjustment (Auto) function. Next, noise reduction was performed on the images using the Non-local Means Denoising function, which was added as a plugin function. After these image processing steps, the images were binarized.

[0065] Figure 2(A) shows an image of the catalyst layer cross-section. Figure 2(B) shows the same image as in Figure 2(A), but with the threshold grayscale increased until areas that can be considered voids appear black. Figure 2(C) shows the same image as in Figure 2(B), but with the threshold grayscale decreased to the value just before areas that can be considered carbon cross-sections appear black. Figure 2(D) shows a binarized image of Figure 2(C), with the remaining white areas in the voids filled in black.

[0066] The threshold for binarization was determined using ImageJ's threshold function as follows: Specifically, for the image shown in Figure 2(A), the threshold was increased until areas that could be considered voids turned black. As a result, some areas that could be considered carbon cross-sections also turned black, as shown in Figure 2(B). Therefore, the threshold was set to the level just before these areas that could be considered carbon cross-sections turned black (Figure 2(C)). Afterward, the remaining white areas in the voids were filled in with black using ImageJ's Paintbrush tool. The resulting image (Figure 2(D)) was used as the binarized image.

[0067] From the obtained binarized images, the area ratio of the void region (the ratio of the void area (S) to the field of view area (S0)) was calculated. In addition, the thickness (t) of the catalyst layer was measured from the SEM image of the cross-section of the catalyst layer. Furthermore, the amount of voids was calculated using equation (1).

[0068] [3. Results] Table 1 shows the void volume of the cathode catalyst layer and the amount of water produced during sub-zero starting. Figure 3 shows the relationship between void volume and water produced. Figure 4 shows the relationship between tap density and void volume. From Table 1 and Figures 3-4, the following can be seen. (1) The larger the amount of voids in the cathode catalyst layer, the larger the amount of water produced during sub-zero starting. This is thought to be because, as the amount of voids increases, the voids in the cathode catalyst layer are less likely to be blocked by the freezing of the produced water. (2) A correlation was observed between tap density and void volume, and it was found that the lower the tap density of the carbon support used, the greater the void volume.

[0069] (3) Comparative Examples 1-3 have less void space and less water produced compared to Examples 1-14. This is thought to be because the tap density of the carbon support is excessively high. (4) Although the cathode catalyst layer specifications are the same in Example 1 and Example 14, Example 1 produces a larger amount of water than Example 14. This is thought to be because, in Example 1, the EW of the electrolyte membrane is smaller than in Example 14, so more of the water produced is absorbed into the electrolyte membrane before it freezes, and the blockage of voids in the cathode catalyst layer is suppressed.

[0070] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0071] The membrane electrode assembly according to the present invention can be used in fuel cells used in on-board power sources, stationary small-scale power generators, and the like.

Claims

1. A membrane electrode assembly having the following configuration. (1) The film electrode assembly is An electrolyte membrane containing a resin (A) having proton conductivity, A cathode catalyst layer bonded to one side of the electrolyte membrane, an anode catalyst layer bonded to the other side of the electrolyte membrane and It is equipped with. (2) The cathode catalyst layer is An electrode catalyst (B) in which catalyst particles (B) are supported on a conductive carrier (B), A resin (B) having proton conductivity and Includes, The resin (B) contains a highly oxygen-permeable ionomer. (3) The membrane electrode assembly has a water generation amount of 0.506 mg / cm² or more and 0.8 mg / cm² when started at sub-zero temperatures. 2 The following applies: (4) The cathode catalyst layer has a void amount of 0.27 μL / cm² per unit area. 2 0.75 μL / cm or more 2 The following applies:

2. The conductive carrier (B) has a tap density of 0.05 g / cm³. 3 0.17g / cm or more 3 The membrane electrode assembly according to claim 1, wherein the following applies:

3. The film electrode assembly according to claim 1 or 2, wherein the resin (B) has an EW of 588 g / eq or more and 900 g / eq or less.

4. The film electrode assembly according to any one of claims 1 to 3, wherein the cathode catalyst layer has an average I / C value of 0.80 or more and 1.8 or less.

5. The film electrode assembly according to any one of claims 1 to 4, wherein the resin (A) has an EW of 600 g / eq or more and 1100 g / eq or less.

6. A first step involves supplying humidified gas to the anode channel and cathode channel of a polymer electrolyte fuel cell, and then sealing the anode channel and cathode channel. A second step of freezing the polymer electrolyte fuel cell, A third step involves supplying unhumidified fuel gas and oxidizer gas to the aforementioned polymer electrolyte fuel cell and generating electricity. A fourth step involves calculating the amount of water produced from the amount of current during power generation, A fifth step is to determine whether the amount of water produced is within a predetermined range. A method for evaluating sub-zero starting performance, equipped with the following features.

7. The first step includes humidifying the polymer electrolyte fuel cell using the gas having a temperature of 82°C and a dew point of 40°C. The second step includes cooling the polymer electrolyte fuel cell to -20°C. The third step includes a step of generating power under the conditions of a cell voltage of 0.6V and a power generation time of 120 seconds. The fifth step is to produce water at a volume of 0.4 mg / cm³. 2 The above 0.8 mg / cm 2 A method for evaluating sub-zero starting performance according to claim 6, which includes a step of determining whether or not the following applies.

Images