Cathode catalyst layer, membrane electrode assembly, and fuel cell

Abstract

To provide a cathode catalyst layer capable of reducing poisoning due to an ionomer of an electrode catalyst without reducing proton conductivity, and a membrane electrode assembly and a fuel cell in which the cathode catalyst layer is used.SOLUTION: A cathode catalyst layer includes: a porous base material containing a polymer electrolyte (A) as a main component; and a cathode catalyst containing catalyst particles (A) which are carried on the surface of the base material and which have activity against oxygen reduction reaction. The cathode catalyst layer has a surface concentration (Cionsuf) of an anion group derived from a polymer electrolyte (B) other than the polymer electrolyte (A) of 0.00 nmol / cm2 or more and 0.05 nmol / cm2 or less. A membrane electrode assembly and a fuel cell include such a cathode catalyst layer.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a cathode catalyst layer, a membrane electrode assembly, and a fuel cell, and more particularly to a cathode catalyst layer that exhibits high oxygen reduction reaction activity, and a membrane electrode assembly and a fuel cell that use the same. [Background technology]

[0002] A polymer electrolyte fuel cell is equipped with a membrane electrode assembly (MEA) in which electrodes containing a catalyst are bonded to both sides of an electrolyte membrane. The electrodes usually have a two-layer structure consisting of a catalyst layer and a diffusion layer. Current collectors (separators) equipped with gas flow channels are further arranged on both sides of the MEA. A polymer electrolyte fuel cell usually has a structure (fuel cell stack) in which multiple unit cells each consisting of such an MEA and current collector are stacked.

[0003] At the anode (hydrogen electrode) of a polymer electrolyte fuel cell, the hydrogen oxidation reaction (HOR) occurs, as shown in the following formula (1). The HOR is known to be highly active in an acidic environment. 2H2→ 4H + +4e - …(1) Meanwhile, at the cathode (air electrode), the oxygen reduction reaction (ORR) occurs, as shown in the following formula (2): ORR is known to be more active in alkaline environments than in acidic environments. O2+4H + +4e - → 2H2O …(2)

[0004] In a polymer electrolyte fuel cell, the catalyst layer is made of a composite of an electrode catalyst and a catalyst layer ionomer. To promote the electrode reaction described above, protons and electrons must be supplied to the catalyst surface. That is, the catalyst layer is required to have high proton conductivity, high electronic conductivity, and high gas permeability. However, increasing the amount of catalyst layer ionomer to improve proton conductivity reduces the electronic conductivity and gas permeability of the catalyst layer. On the other hand, decreasing the amount of catalyst layer ionomer reduces the proton conductivity of the catalyst layer.

[0005] Therefore, various proposals have been made in the past to solve this problem. For example, Patent Document 1 states: (a) preparing a nonwoven fabric made of a proton-conductive material using a melt-blowing method; (b) Immersing the nonwoven fabric in a catalyst ink containing platinum-supported carbon; (c) After removing the nonwoven fabric from the catalyst ink, it is dried. The electrode obtained by the above process is disclosed. The same document states: (A) By such a method, an electrode comprising platinum-supported carbon and a nonwoven fabric made of a proton-conductive material supporting the platinum-supported carbon can be obtained; and (B) When the proton-conducting material contained in the electrode is fibrous, the proton conductivity in the longitudinal direction of the fiber is improved, thereby improving the power output of the fuel cell. is stated.

[0006] Patent Document 2 states: (a) A paste containing carbon particles carrying Pt—Ru alloy fine particles, a perfluorocarbon sulfonic acid solution, perfluorocarbon sulfonic acid fibers (average fiber length: 50 μm, average fiber diameter: 200 nm), water, and methoxypropanol was prepared. (b) Apply this paste to the gas diffusion layer and let it dry. The resulting fuel electrode is disclosed. The document states that mixing a fibrous proton conductor into a catalyst layer enables long-distance proton conduction, improves catalyst utilization in the electrode, reduces impedance, and improves output performance.

[0007] Patent Document 3 states: (a) Resin fibers are prepared using a thin film cutting method; (b) passing the catalyst fiber through a dip-coating bath containing a catalyst ink containing catalyst-supported conductive particles, an electrolyte resin, and a solvent to form a catalyst-coated resin fiber; (c) The dried catalyst-coated resin fiber is cut into lengths of 10 to 100 μm. (d) The catalyst-coated resin fibers are heat-pressed to form a nonwoven fabric. The catalyst layer obtained by the above process is disclosed. The same document states: (A) The catalyst layer has a three-dimensional structure of catalyst-coated resin fibers, which ensures gas diffusion paths and discharge paths for generated water. (B) The proportion of polymer electrolyte in the catalyst layer is high, and the polymer electrolyte is fibrous, so high proton conductivity can be maintained. is stated.

[0008] Patent Document 4 discloses an electrode for a fuel cell obtained by integrating catalyst-coated resin fibers, in which a catalyst is embedded in the surface layer of the resin fibers, into a sheet. The same document states: (A) When a catalyst is attached to the surface of an electrolyte resin fiber by a dip coating method, and the catalyst-coated fiber is pressed to form a nonwoven fabric, and this is used as an electrode for a fuel cell, the catalyst falls off from the fiber during power generation, resulting in a decrease in the output of the fuel cell; and (B) When a part of the catalyst is embedded in the surface layer of the resin fiber, the resin fiber with the embedded catalyst is formed into a sheet, and this is used as an electrode for a fuel cell, the catalyst does not easily fall off from the resin fiber, and a decrease in the output of the fuel cell can be suppressed. is stated.

[0009] Furthermore, Patent Document 5 states: (a) A catalyst ink was prepared containing platinum-loaded carbon with a hydrophobic coating, a polymer electrolyte solution, proton-conducting fibers (average fiber length: 1.5 μm), and a solvent. (b) The catalyst ink is applied to the substrate and dried. The electrode catalyst layer obtained by the above process is disclosed. The same document states: (A) The electrode catalyst layer thus obtained has the following advantages: the proton-conductive fibers are entangled, so that the occurrence of cracks is suppressed; (B) The catalyst-supporting particles having a hydrophobic coating have no affinity for the hydrophilic fibrous material (proton-conducting fiber), and therefore are separated from the hydrophilic fibrous material; and (C) This structure allows the electrode catalyst layer, which has enhanced water retention, to expel water generated by the electrode reaction in the high current region. is stated.

[0010] When a nonwoven fabric made of a proton-conductive material is used as the base material of the catalyst layer, or when short fibers made of a proton-conductive material are added to the catalyst layer, the proton conductivity in the catalyst layer or the gas diffusibility of the catalyst layer is improved. However, in the catalyst layers obtained by the conventional methods, most of the surface of the electrode catalyst is in contact with the catalyst layer ionomer, and therefore, when a fuel cell is fabricated using such a catalyst layer, the electrode catalyst is poisoned by the ionomer, resulting in a problem of reduced power generation performance. [Prior art documents] [Patent documents]

[0011] [Patent Document 1] Japanese Patent Application Laid-Open No. 2007-220416 [Patent Document 2] Japanese Patent Application Laid-Open No. 2008-276990 [Patent Document 3] Japanese Patent Application Laid-Open No. 2009-026698 [Patent Document 4] Japanese Patent Application Laid-Open No. 2012-124046 [Patent Document 5] Japanese Patent Publication No. 2020-061248 Summary of the Invention [Problem to be solved by the invention]

[0012] The problem to be solved by the present invention is to provide a cathode catalyst layer that can reduce the poisoning of the electrode catalyst by an ionomer without reducing the proton conductivity. Another object of the present invention is to provide a novel membrane electrode assembly and a fuel cell using such a cathode catalyst layer. [Means for solving the problem]

[0013] In order to solve the above problems, the cathode catalyst layer according to the present invention has the following configuration. (1) The cathode catalyst layer is a porous substrate containing a polymer electrolyte (A) as a main component; a cathode catalyst containing catalyst particles (A) active in an oxygen reduction reaction supported on the surface of the substrate; It is equipped with: (2) The cathode catalyst layer has a surface concentration of anion groups (C ion suf ) is 0.00nmol / cm 2 More than 0.05nmol / cm 2 The following is the result. However, the surface concentration of anionic groups (C ion suf )" refers to the number of moles of anionic groups derived from the polymer electrolyte (B) other than the polymer electrolyte (A) per surface area of ​​the cathode catalyst.

[0014] The membrane electrode assembly according to the present invention has the following configuration. (1) The membrane electrode assembly is an electrolyte membrane made of a polymer electrolyte (C); an anode bonded to one surface of the electrolyte membrane; a cathode bonded to the other surface of the electrolyte membrane; It is equipped with: (2) The anode comprises an anode catalyst layer containing a polymer electrolyte (D) and catalyst particles (B) active in the hydrogen oxidation reaction. (3) The cathode comprises the cathode catalyst layer according to the present invention.

[0015] Furthermore, the fuel cell according to the present invention comprises the membrane electrode assembly according to the present invention. [Effects of the Invention]

[0016] When a catalyst ink containing a cathode catalyst is sprayed onto the surface of a porous substrate made of a polymer electrolyte (A) using an electrospray method to support the cathode catalyst on the surface of the porous substrate, high activity can be obtained under both high and low humidity conditions by setting the content of polymer electrolyte (B) in the catalyst ink below a certain critical value. This is thought to be because, by attaching a cathode catalyst that is not substantially covered with polymer electrolyte (B) to the surface of the substrate responsible for ion transport, catalyst poisoning can be avoided without affecting ion transport. [Brief explanation of the drawings]

[0017] [Figure 1] 1 shows the current values ​​per platinum mass at 0.84 V for the fuel cells obtained in Example 1 and Comparative Examples 1 and 2. [Figure 2] FIG. 1 is a graph showing the relationship between the number of potential cycles and the ECSA retention rate at a cell temperature of 60° C. and a relative humidity of 80% RH for the fuel cells obtained in Example 1 and Comparative Example 2. [Figure 3] 1 is an SEM image of the catalyst layer obtained in Example 1. [Figure 4] 1 shows a comparison of oxygen diffusion resistance (Rother×RF) per surface area of ​​catalyst particles in the catalyst layers obtained in Example 1 and Comparative Examples 1 and 2, which is independent of total pressure. [Figure 5]Figure 5(A) is an SEM image of the nonwoven fabric obtained in Comparative Example 3 immediately after electrospinning. Figure 5(B) is an SEM image of the nonwoven fabric after immersion in the solvent of the catalyst ink. Figure 5(C) is an SEM image of the nonwoven fabric after immersion in the catalyst ink. DETAILED DESCRIPTION OF THE INVENTION

[0018] An embodiment of the present invention will be described in detail below. [1. Cathode catalyst layer] The cathode catalyst layer according to the present invention comprises: a porous substrate containing a polymer electrolyte (A) as a main component; a cathode catalyst containing catalyst particles (A) active in an oxygen reduction reaction supported on the surface of the substrate; It is equipped with:

[0019] [1.1. Base material] Structure The substrate is made of a porous structure containing the polymer electrolyte (A) as a main component. In the present invention, a porous structure is used for the substrate in order to improve gas permeability. The substrate is not particularly limited as long as it exhibits the above-mentioned functions. Examples of the substrate include nonwoven fabric, porous body, three-dimensional lattice structure, and columnar structure.

[0020] The nonwoven fabric can be obtained, for example, by electrospinning a solution containing the polymer electrolyte (A) and, if necessary, a carrier polymer. In this case, the density, thickness, fiber diameter, etc. of the nonwoven fabric are not particularly limited, and optimal values ​​can be selected depending on the purpose. A porous body is one in which approximately half of the volume is void, and nonwoven fabrics are also a type of porous body. Porous bodies can be obtained, for example, by electrospinning or by using a pore-forming agent. In this case, the density, thickness, pore size, etc. of the porous body are not particularly limited, and optimal values ​​can be selected depending on the purpose.

[0021] The term "three-dimensional lattice structure" refers to a three-dimensional structure resembling a jungle gym, which is created using a metal organic framework (MOF) or the like as a template. The "columnar structure" refers to a structure in which multiple micro- to nano-sized columns are erected from the electrolyte membrane toward the cathode catalyst layer, and can increase the interfacial area between the electrolyte membrane and the cathode catalyst layer. The columnar structure can be fabricated using, for example, anodized alumina or an etched material (silica, various metals, resins, etc.) as a template.

[0022] [1.1.2. Materials] The substrate contains a polymer electrolyte (A) as a main component. "Main component" means that the content of the polymer electrolyte (A) is 90 mass % or more. In the present invention, the material of the polymer electrolyte (A), which is the main component of the substrate, is not particularly limited.

[0023] The polymer electrolyte (A) may be any material that exhibits proton conductivity. Among these, the polymer electrolyte (A) is preferably a cation exchange resin because it has better moldability than other materials (e.g., oxides and phosphoric acid-based liquid materials). Among cation exchange resins, the polymer electrolyte (A) is preferably a perfluorosulfonic acid polymer (A), since the main chain is fluorinated and the polymer is stable in the fuel cell environment.

[0024] In order to obtain high proton conductivity, the substrate preferably consists of only the polymer electrolyte (A), but may contain other components as required. Other ingredients include, for example, (a) a carrier polymer for imparting spinnability to the polymer electrolyte (A); (b) unavoidable impurities; (c) a radical quencher (e.g., Ce, Ag, etc.) that eliminates radicals generated during fuel cell operation; (d) Conductive additives (e.g., carbon fiber, CNT, etc.) for adding conductivity; etc.

[0025] [1.2. Cathode catalyst] The cathode catalyst contains catalyst particles (A) that are active in the oxygen reduction reaction. The cathode catalyst is supported on the surface or in the gaps of a substrate. The cathode catalyst may consist of only the catalyst particles (A), or may further include a carrier for supporting the catalyst particles (A).

[0026] [1.2.1. Catalyst particles (A)] In the present invention, the catalyst particles (A) are composed of catalyst particles having activity in the oxygen reduction reaction (ORR) shown in the following formula (3): The catalyst particles (A) are not particularly limited as long as they have the ORR activity shown in formula (3). O2+H2O+4e - → 4OH - …(3)

[0027] Examples of the catalyst particles (A) include: (a) Pt or Pt alloy, (b) a metal or alloy containing one or more base metal elements selected from the group consisting of Fe, Ni, Co, and Mn; (c) a porphyrin encapsulating one or more base metal elements selected from the group consisting of Fe, Ni, Co, and Mn; (d) nitrogen-doped carbon; etc. The catalyst particles (A) may consist of any one of these, or may consist of two or more of them.

[0028] 1.2.2. Carriers The catalyst particles (A) may be supported on the surface of a carrier made of a conductive material. By supporting the catalyst particles (A) on the surface of a carrier, the fine catalyst particles (A) can be stably dispersed, thereby reducing the amount of catalyst used. In the present invention, the material of the carrier is not particularly limited, and examples of the carrier include carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[0029] The carrier preferably has a hydrophobic surface. When a carrier having a hydrophilic surface is used, the carrier has good dispersibility in a hydrophilic solvent. However, when catalyst particles (A) are supported on the surface of a hydrophilic carrier, the drainage property of the cathode catalyst layer is reduced. As a result, the performance is reduced, particularly under highly humidified conditions. In contrast, if a support with a hydrophobic surface is used, the drainage of the cathode catalyst layer is improved, and as a result, flooding under highly humidified conditions can be suppressed.

[0030] [1.2.3. Average particle size] The "average particle size of the cathode catalyst" refers to the average particle size of the entire cathode catalyst (i.e., the entire carrier supporting the catalyst particles (A)) when the cathode catalyst is composed of catalyst particles (A) supported on the surface of a carrier (particularly, a carbon carrier). The term "average particle size" refers to the average value of the maximum dimension of 100 or more cathode catalyst particles measured from an image obtained by an electron microscope.

[0031] If the average particle size of the cathode catalyst is too large, it becomes difficult to supply protons to the surface of the catalyst particles (A). Therefore, the average particle size of the cathode catalyst is preferably 2000 nm or less. The average particle size is more preferably 1500 nm or less, and even more preferably 1000 nm or less. In terms of facilitating proton supply, the smaller the average particle size of the cathode catalyst is, the better. For example, when the cathode catalyst includes a support, the average particle size of the cathode catalyst that can be produced at present is about 50 nm.

[0032] [1.3. Polyelectrolyte (B)] The term "polymer electrolyte (B)" refers to a polymer electrolyte other than the polymer electrolyte (A) that constitutes the substrate. More specifically, "polymer electrolyte (B)" refers to a polymer electrolyte that may be added to a catalyst ink containing a cathode catalyst when the catalyst ink is electrosprayed. When a catalyst ink containing a relatively large amount of polymer electrolyte (B) is electrosprayed, the majority of the catalyst particles (A) are usually coated with polymer electrolyte (B).

[0033] As will be described later, in the present invention, a catalyst ink containing a cathode catalyst is electrosprayed onto the surface of a porous substrate. When the catalyst ink is a mixture of a hydrophilic solvent and a hydrophobic cathode catalyst, it is preferable to add a component (so-called surfactant) to improve the dispersibility of the cathode catalyst. In this case, organic compounds other than polymer electrolytes can also be used as surfactants. However, organic compounds other than polymer electrolytes are decomposed by radicals (such as OH) generated during fuel cell operation, and the decomposition products may poison the catalyst. Therefore, it is desirable to use a material with high chemical stability as the surfactant.

[0034] In contrast, adding polymer electrolyte (B) to the catalyst ink and drying it results in a composite in which the surface of the cathode catalyst is coated with polymer electrolyte (B). Polymer electrolyte (B) not only functions as a proton conductor, but also as a surfactant to disperse the hydrophobic cathode catalyst in a hydrophilic solvent. In addition, like other surfactants, polymer electrolyte (B) is also a substance that can poison the catalyst. However, a small amount of polymer electrolyte (B) does not poison the catalyst particles (A) and actually improves the dispersibility of the cathode catalyst in the catalyst ink. Therefore, it is preferable to add polymer electrolyte (B) to the catalyst ink as needed.

[0035] When the polymer electrolyte (B) is added to the catalyst ink, the polymer electrolyte (B) may be the same material as the polymer electrolyte (A), or may be a different material. The polymer electrolyte (B) is particularly preferably a perfluorosulfonic acid polymer (B). The perfluorocarbon sulfonic acid polymer (B) has high resistance to radicals and is therefore particularly suitable as a surfactant for dispersing the cathode catalyst.

[0036] 1.4. Surface concentration of anionic groups Surface concentration of anionic groups (C ion suf )" refers to the number of moles of anionic groups derived from the polymer electrolyte (B) other than the polymer electrolyte (A) per surface area of ​​the cathode catalyst. In other words, the surface concentration of anionic groups (C ion suf )" refers to the number of moles of the anionic groups per surface area of ​​the cathode catalyst, assuming that the surface of the cathode catalyst is uniformly coated with the polymer electrolyte (B) having anionic groups. The term "surface area of ​​the cathode catalyst" used herein refers to the BET surface area of ​​the cathode catalyst, regardless of the agglomeration form of the cathode catalyst. Also, the term "BET surface area" refers to the surface area determined by a gas adsorption method using nitrogen gas. When the cathode catalyst includes a carrier, the "surface area of ​​the cathode catalyst" refers to the sum of the surface area of ​​the catalyst particles (A) and the surface area of ​​the carrier.

[0037] C ion suf affects the activity and durability of the cathode catalyst layer and the output of the fuel cell using the cathode catalyst layer. When the catalyst particles (A) are poisoned by the anion groups of the polymer electrolyte (B), the activity, durability, and / or output decrease. Therefore, C ion suf is 0.05 nmol / cm 2 To prevent catalyst poisoning, C ion suf The smaller the better, zero nmol / cm 2 It may be. Such a low C ion sufThis can be achieved by adding no polymer electrolyte (B) to the catalyst ink or by adding only the minimum amount of polymer electrolyte (B) to the catalyst ink.

[0038] [1.5. Characteristics] [1.5.1. ECSA retention rate] "ECSA retention rate (%) after 10,000 cycles (hereinafter also referred to as "ECSA retention rate @ 10,000 cycles")" refers to the ratio (= x1 × 100 / x0) of ECSA (x1) after a durability test in which 10,000 cycles of potential fluctuation are applied to ECSA (x0) before the durability test. The "durability test of applying 10,000 cycles of potential fluctuation" refers to a test in which a square wave of 0.6 V (3 s) ⇔ 1.0 V (3 s) is applied 10,000 cycles to a polymer electrolyte fuel cell including the cathode catalyst layer.

[0039] Generally, when a potential cycle is applied to a cathode catalyst layer, dissolution and redeposition of the catalyst particles (A) occur. Therefore, the ECSA generally decreases as the number of cycles increases. However, the cathode catalyst layer according to the present invention has a higher ECSA retention rate after potential cycling than conventional cathode catalyst layers. This is because, in the cathode catalyst layer according to the present invention, C ion suf This is thought to be because the C ion suf The fact that the value is low indicates that the anion groups (e.g., -SO3 - As a result, it is thought that the pH on the surface of the catalyst particles (A) becomes high, and the dissolution of the catalyst particles (A) is suppressed.

[0040] In the cathode catalyst layer according to the present invention, the structure of the substrate, the type of the cathode catalyst, C ion suf By optimizing the above, the ECSA maintenance rate @ 10,000 cycles will be 55% or more.

[0041] [1.5.2. Total pressure-independent oxygen diffusion resistance per surface area of ​​catalyst particle (R other ×RF)] In a polymer electrolyte fuel cell equipped with a cathode catalyst layer, the oxygen transfer resistance R in the cathode catalyst layer total is expressed by the following equation (1). R total =R mole +R other =R mole +(R knud +R iono ) …(1) however, R mole is the molecular diffusion resistance, R other is the other resistance component.

[0042] R other is the oxygen diffusion resistance independent of total pressure. R other is the Knudsen diffusion resistance R, which is the resistance to transport in the water-repellent layer of the gas diffusion layer and in the small pores in the cathode catalyst layer. knud and the oxygen transfer resistance R within the ionomer and at the interface (the interface between the ionomer and the gas phase or platinum). iono It is expressed as the sum of and. Furthermore, the "oxygen diffusion resistance per surface area of ​​the catalyst particle independent of total pressure" is R other This refers to the value obtained by multiplying this by RF (roughness factor = surface area of ​​catalyst particles per geometric area of ​​catalyst layer).

[0043] The cathode catalyst layer according to the present invention is ion suf is low (i.e., there is less polymer electrolyte (B) covering the catalyst particles (A)), so R iono As a result, R other In the cathode catalyst layer according to the present invention, the structure of the substrate, the type of cathode catalyst, C ion suf By optimizing the above, R other The value of ×RF is 1100 s / m or less.

[0044] [2. Manufacturing method of cathode catalyst layer] The cathode catalyst layer according to the present invention can be produced by various methods. For example, in the case where the substrate of the cathode catalyst layer is made of a nonwoven fabric, the manufacturing method thereof is as follows: (a) A first method in which a step of preparing a nonwoven fabric by electrospinning a solution containing a polymer electrolyte (A) and a step of electrospraying a catalyst ink containing a cathode catalyst onto the surface of the nonwoven fabric are repeated; (b) A second method in which the preparation of a nonwoven fabric by electrospinning of the polymer electrolyte (A) and the electrospraying of a catalyst ink containing a cathode catalyst are carried out simultaneously; etc. Among these, the second method is suitable as a method for producing a cathode catalyst layer because it can reduce the proportion of electrically isolated cathode catalysts.

[0045] Whichever method is used, the catalyst ink may or may not contain a polymer electrolyte (B). However, when a cathode catalyst having a hydrophobic surface is dispersed in a hydrophilic solvent, it is preferable to add a polymer electrolyte (B) that functions as a surfactant to the catalyst ink. In this case, the amount of polymer electrolyte (B) added is C ion suf By minimizing the amount of polymer electrolyte (B) added, a catalyst ink with good dispersibility can be obtained, and at the same time, catalyst poisoning by anionic groups can be reduced.

[0046] [3. Membrane electrode assembly] The membrane electrode assembly according to the present invention comprises: an electrolyte membrane made of a polymer electrolyte (C); an anode bonded to one side of the electrolyte membrane; a cathode bonded to the other side of the electrolyte membrane; It is equipped with:

[0047] [3.1. Electrolyte membrane] The electrolyte membrane is made of a polymer electrolyte (C). The polymer electrolyte (C) may be made of the same material as the polymer electrolytes (A) and (B) contained in the cathode or the polymer electrolyte (D) contained in the anode, or may be made of a different material. Other aspects of the polymer electrolyte (C) are the same as those of the polymer electrolytes (A) and (B), and therefore further explanation will be omitted.

[0048] 3.2. Anode An anode is bonded to one side of the electrolyte membrane. The anode includes an anode catalyst layer. The anode may consist of only the anode catalyst layer, or may further include an anode gas diffusion layer disposed on the outside of the anode catalyst layer.

[0049] The anode catalyst layer comprises a polymer electrolyte (D) and an anode catalyst. The anode catalyst may consist solely of catalyst particles (B) active in the hydrogen oxidation reaction (HOR), or may comprise catalyst particles (B) supported on the surface of a carrier. The polymer electrolyte (D) may be the same material as the polymer electrolytes (A) and (B) contained in the cathode or the polymer electrolyte (C) constituting the electrolyte membrane, or may be a different material. Other aspects of the polymer electrolyte (D) are the same as those of the polymer electrolytes (A) to (C), and therefore further explanation will be omitted.

[0050] The catalyst particles (B) may be any particles that are active in the hydrogen oxidation reaction (HOR). When the catalyst particles (B) are supported on the surface of a carrier, the carrier may be any particles that are electronically conductive. Examples of the catalyst particles (B) include Pt nanoparticles. Examples of the carrier include carbon carriers.

[0051] 3.3. Cathode A cathode is bonded to the other surface of the electrolyte membrane. The cathode includes a cathode catalyst layer. The cathode may consist of only the cathode catalyst layer, or may further include a cathode gas diffusion layer disposed on the outside of the cathode catalyst layer. In the membrane electrode assembly according to the present invention, the cathode comprises the cathode catalyst layer according to the present invention. Details of the cathode catalyst layer are as described above, and therefore will not be described here.

[0052] [4. Fuel cell] The fuel cell according to the present invention includes the membrane electrode assembly according to the present invention. Separators each having a gas flow path are disposed on both sides of the membrane electrode assembly. The fuel cell typically has a structure (stack structure) in which a plurality of unit cells each consisting of a membrane electrode assembly and a separator are stacked. In the present invention, the separators and stack structure are not particularly limited, and the optimum ones can be selected depending on the purpose. The details of the membrane electrode assembly are as described above, and therefore will not be described here.

[0053] [5. Effect] A catalyst layer is typically manufactured by preparing a catalyst ink containing catalyst particles and an ionomer (polymer electrolyte), applying the catalyst ink to the surface of a substrate, and drying the ink. In the catalyst layer thus obtained, the catalyst particles are coated with the ionomer. This can result in the catalyst particles being poisoned by the ionomer, which can reduce the activity and durability of the catalyst particles and / or the output of the fuel cell. On the other hand, reducing the amount of ionomer added to the catalyst ink to prevent poisoning can impede ion transport and reduce power generation capacity.

[0054] This also applies to the case where a catalyst layer is produced using a method that combines electrospinning of a nonwoven fabric made of a polymer electrolyte with electrospraying of a catalyst ink containing an ionomer. That is, even when such a method is used, if the production conditions are inappropriate, deterioration in the activity and durability of the catalyst particles due to poisoning, and deterioration in the output of the fuel cell due to poisoning, etc., may occur.

[0055] In contrast, when a catalyst ink containing a cathode catalyst is sprayed onto the surface of a porous substrate made of polymer electrolyte (A) using electrospraying to support the cathode catalyst on the surface of the porous substrate, if the content of polymer electrolyte (B) in the catalyst ink is kept below a certain critical value, high activity can be obtained under both high and low humidity conditions. This is thought to be because by attaching cathode catalyst particles that are not substantially coated with polymer electrolyte (B) to the surface of the substrate responsible for ion transport, catalyst poisoning can be avoided without affecting ion transport. [Example]

[0056] (Example 1, Comparative Examples 1 and 2) [1. Preparation of cathode catalyst layer] 1.1. Preparation of fiber solution (electrospinning solution) The polymer electrolyte (A) was a Nafion (registered trademark) solution (D2020, manufactured by Chemours Corporation). The carrier polymer was polyethylene oxide (PEO, M W ~1,000,000) was used. Furthermore, methanol was used as the solvent. A Nafion (registered trademark) solution and PEO were added to methanol to prepare a fiber solution. The mass ratio of polymer electrolyte (A) / PEO was 99 / 1. The solid content (polymer electrolyte (A) + PEO) of the fiber solution was 6 mass%.

[0057] 1.2. Preparation of catalyst ink 1.2.1. Example 1 The electrode catalyst used was platinum-supported carbon Pt / C (Tanaka Kikinzoku Kogyo K.K., TEC10V30E, platinum weight ratio: 30 mass%). The polymer electrolyte (B) used was Nafion (registered trademark) solution (Chemours Inc., D2020). Furthermore, the solvent used was a mixed solvent of water / ethanol = 10 / 90 (mass ratio). A catalyst ink was prepared by adding Pt / C and Nafion (registered trademark) solutions to the mixed solvent. The I / C ratio was set to 0.1. The concentration of the solid content (polymer electrolyte (B) + Pt / C) of the catalyst ink was set to 1.0 mass%. In this case, the surface concentration of anionic groups (C ion suf ) is 0.05nmol / cm 2 It was.

[0058] [1.2.2. Comparative Examples 1 and 2] The surface concentration of anionic groups (C ion suf ) is 0.20nmol / cm 2 (Comparative Example 1), or 0.40 nmol / cm 2 A catalyst ink was prepared in the same manner as in Example 1, except that the I / C of the catalyst ink was changed to provide (Comparative Example 2).

[0059] C. Electrospinning and Electrospraying Using an electrospinning device, the electrospinning of fibers and electrospraying of catalyst ink were simultaneously performed on the surface of a substrate with a water-repellent paper diffusion layer (GDL). Electrospinning and electrospraying were performed by applying a voltage of 15 kV and feeding the catalyst ink at 2.0 mL / h and the fiber solution at 0.5 mL / h.

[0060] 2. Test Method 2.1. Cell Preparation The anode catalyst layer was transferred onto one side of the electrolyte membrane using a hot press. Next, the cathode catalyst layer with GDL, the electrolyte membrane with anode catalyst layer, and the anode-side GDL were placed in a cell and clamped with a current collector plate with gas channels and an end plate. The GDL thickness in the cell was controlled by the thickness of a polytetrafluoroethylene (PTFE) gasket. The GDL compression rate was approximately 15%. The gas channels had parallel grooves with a groove width of 0.4 mm, a rib width of 0.2 mm, and a groove depth of 0.5 mm. The gas flow between the anode and cathode was cross-flow.

[0061] The cell was evaluated using an electrochemical measurement device manufactured by Toho Giken Co., Ltd. All potentials were set relative to the anode, and converted to RHE at 1 atmosphere of hydrogen using the Nernst equation using the hydrogen partial pressure under each condition.

[0062] 2.2. Evaluation of IV performance (a) Cell temperature 80°C, relative humidity 100% RH, 80% RH, or 30% RH, or (b) Cell temperature: 60°C, relative humidity: 165%RH Under this condition, air (21% O2 / 79% N2, 2000 ccm) was passed through the cathode and H2 (500 ccm) through the anode. The cathode potential was swept at 10 mV / s in the range of the natural potential to 0 V relative to the anode, and a cyclic voltammogram was obtained. The gas pressure was adjusted so that the oxygen partial pressure was 20 kPa. Of the three cycles measured, the anodic sweep in the third cycle was taken as the IV performance of the cell.

[0063] 2.3. Durability evaluation At a cell temperature of 60°C and a relative humidity of 80%, N2 (101 kPa) was supplied to the cathode. abs , 1000 ccm, 10% H2 (N2 balance) on the anode, N2 (101 kPa abs With a current of 1000 ccm (1000 ccm) flowing through the cathode, a square wave potential cycle of 0.6 V for 3 seconds and then 1.0 V for 3 seconds was applied 10,000 times. Every 500 cycles, the ECSA was calculated using the method described below to determine how much the ECSA decreased as the number of cycles increased.

[0064] The ECSA was calculated as follows. A cyclic voltammogram was obtained by sweeping the potential range of 0.05 to 1.00 V at 50 mV / s. The peak of the hydrogen desorption wave observed around 0.2 V during the anodic sweep was integrated to determine the charge amount, and a conversion coefficient of 210 μC / cm was used. 2 is used to calculate the geometric area of ​​the catalyst layer (=1 cm 2 ) was calculated. By normalizing this surface area by the platinum weight, the electrochemically effective platinum surface area per platinum mass, ECSA (m2 Further, the ECSA retention rate was calculated by dividing the ECSA after the potential cycle by the initial ECSA value.

[0065] [2.4. R other × RF measurement] Oxygen transfer resistance in the catalyst layer R total and the total pressure-independent oxygen transfer resistance R other was calculated from the results of limiting current measurements. total As described above, is expressed by the following equation (1). R total =R mole +R other =R mole +(R knud +R iono ) …(1) however, R mole is the molecular diffusion resistance, R other is the other resistance component (oxygen diffusion resistance independent of total pressure), R knud is the Knudsen diffusion resistance, R iono is the oxygen transfer resistance within the ionomer and at the interface.

[0066] R total is the diffusion limiting current density I lim (A / m 2 ) was calculated using the following formula (2). R total =4FP O2 / I lim RT...(2) however, F is the Faraday constant (C / mol), P O2 is the partial pressure of oxygen (kPa), R is the gas constant (J / (mol K)), T is the absolute temperature (K).

[0067] I lim (A / m 2The measurement of anodic potential was carried out as follows. The total pressure was set to 110 to 150 kPa (5 points at 10 kPa intervals), and the N2 partial pressure was adjusted so that the oxygen partial pressure in the atmosphere was constant (1.5 kPa) at each total pressure. In this gas atmosphere, a potential sweep was performed three times in the range of 0.1 V to 0.9 V at a rate of 10 mV / s, and the maximum current of the third anodic sweep was measured as I lim (A / m 2 The above measurements were carried out at 80% RH. I wanted lim From R total was calculated using equation (2). This was plotted against the total pressure, and R was calculated from the Y-intercept when linearly approximated. other asked for.

[0068] This R other It is known that the RF is almost inversely proportional to the surface area of ​​the catalyst per geometric area of ​​the catalyst layer (Reference 1). In other words, even if the resistance of the platinum-ionomer interface is small, RF will be larger if the platinum particle size is smaller or the platinum coverage is high. Therefore, the apparent R other will look small. So, R other By multiplying by RF, R other is the value per unit surface area of ​​platinum (R other This allows the differences in the weight per unit area and particle size to be ignored, making it possible to compare differences in the resistance value of the platinum-ionomer interface purely. [Reference 1] J. Phys. Chem. Lett. 2016, 7, 1127-1137

[0069] Strictly speaking, R other R knud Therefore, R other The R knud is R iono Since it is about 1 / 10 of the knud This can be neglected unless a carrier or GDL with a large Based on the above, in the present invention, the oxygen diffusion resistance at the platinum-ionomer interface is determined as R other ×RF was used.

[0070] [3. Results] [3.1. IV performance] FIG. 1 shows the current values ​​per platinum mass at 0.84 V for the fuel cells obtained in Example 1 and Comparative Examples 1 and 2. From FIG. 1, it can be seen that Example 1 has a larger current value than Comparative Examples 1 and 2, regardless of the relative humidity. This is because C ion suf This is thought to be because catalyst poisoning was reduced due to the small

[0071] [3.2. Durability] Figure 2 shows the relationship between the number of potential cycles and the ECSA retention rate at a cell temperature of 60°C and a relative humidity of 80%RH for the fuel cells obtained in Example 1 and Comparative Example 2. Figure 2 shows that Example 1 exhibited a slower decrease in ECSA (smaller slope) than Comparative Example 2, and exhibited a higher ECSA retention rate after 10,000 cycles. This suggests that the dissolution of platinum in Example 1 was less than that in Comparative Example 1.

[0072] The amount of sulfonic acid group coverage in Example 1 (C ion suf ) is smaller than that of Comparative Example 2, and as a result, the pH of the platinum surface is high, which is thought to be the reason for the high durability. The area retention rate of the catalyst particles is also affected by the particle size of the catalyst particles, but the particle size of the active species used in this application is quite small (2 to 3 nm), and particles smaller than this are not practical due to durability issues.

[0073] FIG. 3 shows an SEM image of the catalyst layer obtained in Example 1. It can be seen from FIG. 3 that the catalyst layer is composed of fibers and a cathode catalyst. In Example 1, the diameter of the cathode catalyst was approximately 2 μm. If the diameter of the cathode catalyst were larger than this, protons may not be transported to the active species at positions far from the fibers. Therefore, the diameter of the cathode catalyst is preferably 2 μm or less.

[0074] [3.3. R other ×RF] FIG. 4 shows the oxygen diffusion resistance (R other 4 shows a comparison of R × RF of Example 1. other It can be seen that ×RF is smaller than that of Comparative Examples 1 and 2. It is known that ionomers on the catalyst surface act as resistance to oxygen transport, but it is thought that the oxygen diffusion resistance is smaller in Example 1 than in Comparative Example 1 because there is less ionomer on the catalyst surface.

[0075] (Comparative Example 3) 1. Sample Preparation A catalyst layer was produced by simulating the method described in Patent Document 1. That is, first, a nonwoven fabric was produced by electrospinning in the same manner as in Example 1, except that no catalyst ink was used. Next, the obtained nonwoven fabric was immersed in the catalyst ink or the solvent of the catalyst ink (a water / ethanol mixed solvent). After a predetermined time had passed, the nonwoven fabric was pulled out of the catalyst ink or the solvent and dried.

[0076] [2. Test Methods and Results] The nonwoven fabric was observed by SEM before and after immersion in the catalyst ink or solvent. Figure 5(A) shows an SEM image of the nonwoven fabric obtained in Comparative Example 3 immediately after electrospinning. Figure 5(B) shows an SEM image of the nonwoven fabric after immersion in the catalyst ink solvent. Furthermore, Figure 5(C) shows an SEM image of the nonwoven fabric after immersion in the catalyst ink.

[0077] From Figure 5, (a) When a nonwoven fabric with many voids is immersed in the solvent of the catalyst ink, the fibers of the nonwoven fabric swell with the solvent, reducing the voids; and (b) Even if the nonwoven fabric is immersed in the catalyst ink for a predetermined time, the catalyst ink does not penetrate into the interior of the nonwoven fabric, and catalyst particles accumulate on the upper surface of the swollen nonwoven fabric; You can see that. That is, it was found that the method of Patent Document 1 cannot obtain a catalyst layer having the structure shown in FIG.

[0078] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above-described embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0079] The cathode catalyst layer according to the present invention can be used as a catalyst layer on the cathode side of various electrochemical devices such as fuel cells.

Claims

1. A cathode catalyst layer comprising: (1) The cathode catalyst layer comprises: a porous substrate containing a polymer electrolyte (A) as a main component; a cathode catalyst containing catalyst particles (A) active in an oxygen reduction reaction and supported on the surface and in the gaps of the substrate; Equipped with The cathode catalyst comprises the catalyst particles (A) supported on the surface of a carbon support. (2) The cathode catalyst layer has a surface concentration of anion groups (C ion suf ) is 0.00 nmol / cm 2 0.05nmol / cm or more 2 The following is the result. However, the "surface concentration of anionic groups (C ion suf ")" refers to the number of moles of anionic groups derived from the polymer electrolyte (B) other than the polymer electrolyte (A) per surface area of ​​the cathode catalyst. (3) The substrate is made of a nonwoven fabric.

2. 2. The cathode catalyst layer according to claim 1, wherein the polymer electrolyte (B) is a polymer electrolyte that coats the surface of the cathode catalyst.

3. The average particle size of the cathode catalyst is 2000 nm or less. The cathode catalyst layer according to claim 1 or 2.

4. 4. The cathode catalyst layer according to claim 1, wherein the ECSA retention rate after 10,000 cycles is 55% or more. However, "ECSA retention rate (%) after 10,000 cycles" refers to the ECSA (x 0 ) after a durability test in which potential fluctuations are applied for 10,000 cycles. 1 ) ratio (= x 1 x100 / x 0 ) The "durability test of applying 10,000 cycles of potential fluctuation" refers to a test in which a square wave of 0.6 V (3 s) ⇔ 1.0 V (3 s) is applied 10,000 cycles to a polymer electrolyte fuel cell including the cathode catalyst layer.

5. The oxygen diffusion resistance (R other 5. The cathode catalyst layer according to claim 1, wherein the surface roughness (RF) is 1100 s / m or less.

6. The catalyst particles (A) are (a) Pt or a Pt alloy; (b) a metal or alloy containing one or more base metal elements selected from the group consisting of Fe, Ni, Co, and Mn; (c) a porphyrin encapsulating one or more base metal elements selected from the group consisting of Fe, Ni, Co, and Mn, or (d) nitrogen-doped carbon; The cathode catalyst layer according to any one of claims 1 to 5, comprising:

7. 7. The cathode catalyst layer according to claim 1, wherein the polymer electrolyte (A) comprises a perfluorosulfonic acid polymer (A).

8. 8. The cathode catalyst layer according to claim 1, wherein the polymer electrolyte (B) comprises a perfluorosulfonic acid polymer (B).

9. A membrane electrode assembly having the following configuration: (1) The membrane electrode assembly is an electrolyte membrane made of a polymer electrolyte (C); an anode bonded to one surface of the electrolyte membrane; a cathode bonded to the other surface of the electrolyte membrane; It is equipped with: (2) The anode comprises a polymer electrolyte (D) and an anode catalyst layer containing catalyst particles (B) active in the hydrogen oxidation reaction. (3) The cathode comprises the cathode catalyst layer according to any one of claims 1 to 8.

10. A fuel cell comprising the membrane electrode assembly according to claim 9.

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