Membrane electrode assembly

Abstract

Membrane electrode assembly (100) comprising: a cathode catalyst layer (10); an anode catalyst layer (20); an electrolyte layer (30) arranged between the cathode catalyst layer (10) and the anode catalyst layer (20); and an intermediate layer (40) arranged between the anode catalyst layer (20) and the electrolyte layer (30), wherein the intermediate layer (40) comprises a support (42) having an insulating property and a recombination catalyst (41) which is supported on the support (42) having the insulating property, characterized in that the intermediate layer (40) is arranged in contact with the anode catalyst layer (20) and the electrolyte layer (30).

Description

BACKGROUND OF THE INVENTION 1. Field of the invention The present invention relates to a membrane electrode assembly, in particular a membrane electrode assembly for water electrolysis. 2. Description of the related prior art In recent years, hydrogen has attracted attention as a CO2-free energy source. Examples of hydrogen production methods include alkaline water electrolysis, polymer electrolyte membrane (PEM) water electrolysis, and others. PEM water electrolysis is particularly noteworthy due to its high efficiency. In water electrolysis, a phenomenon known as "hydrogen crossover" occurs, in which the hydrogen produced in the cathode catalyst layer (hydrogen electrode catalyst layer) penetrates an electrolyte membrane and migrates to the side of the anode catalyst layer (oxygen electrode catalyst layer). As a result, the hydrogen mixes with the oxygen produced in the anode catalyst layer, and the hydrogen concentration in the oxygen increases. Therefore, this increase in the hydrogen concentration in the oxygen on the anode catalyst side due to hydrogen crossover must be suppressed. JP 2019-167 619 A and WO 2018 / 115 821 A1, along with its related patent JP 2020-514 528 A, disclose a technology for suppressing the increase in hydrogen concentration in oxygen in a membrane electrode assembly for PEM water electrolysis by reacting the hydrogen that has migrated through crossover with the oxygen using a recombination catalyst. Furthermore, WO 00 / 24 ​​074 A1 discloses a process for producing a solid polymer electrolyte membrane comprising an ion-conducting polymer, a catalyst, and a support material for the catalyst. Specifically, JP 2019-167619A discloses a laminated electrolyte membrane comprising a first electrolyte membrane, a second electrolyte membrane, and a nanosheet-like laminated catalyst layer positioned between the first and second electrolyte membranes. This laminated structure contains a multitude of nanosheet-like catalysts laminated with gaps between them. In JP 2019-167619A, the nanosheet-like catalyst functions as a recombination catalyst. WO 2018 / 115 821 A1 and JP 2020-514 528 A disclose a catalyst-coated membrane for use in a water electrolysis cell with a laminate structure comprising a first layer containing a first membrane component, wherein the first membrane component has a cathode catalyst layer arranged on a first surface thereof; a second layer containing a second membrane component, wherein the second membrane component has an anode catalyst layer arranged on a first surface thereof; and an intermediate layer arranged between the first and second layers, comprising a third membrane component, wherein the third membrane component has a recombination catalyst layer arranged on a first surface thereof. WO 2018 / 115 821 A1 and JP 2020-514 528 A thus disclose a membrane electrode assembly with the features of the preamble of claim 1. BRIEF SUMMARY OF THE INVENTION As described in JP 2019-167 619 A, WO 2018 / 115 821 A1, and JP 2020-514 528 A, the arrangement of a recombination catalyst in a membrane electrode assembly for water electrolysis enables the suppression of the increase in hydrogen concentration in the oxygen. However, the structures of the membrane electrode assemblies disclosed in JP 2019-167 619 A, WO 2018 / 115 821 A1, and JP 2020-514 528 A are complex and require repeated joining processes, resulting in numerous manufacturing steps. Accordingly, the present inventors have produced a prototype membrane electrode assembly with a simpler structure in which the electrolyte layer on the oxygen electrode side has been removed. This weakened the effects of the recombination catalyst in reducing the hydrogen concentration. The present invention provides a membrane electrode assembly capable of suppressing the increase in hydrogen concentration in oxygen due to hydrogen crossover using a catalyst layer with a simple configuration. A membrane electrode assembly according to a first aspect of the invention comprises a cathode catalyst layer, an anode catalyst layer, an electrolyte layer arranged between the cathode catalyst layer and the anode catalyst layer, and an intermediate layer arranged between the anode catalyst layer and the electrolyte layer. The intermediate layer comprises a support with insulating properties and a recombination catalyst carried on the support with insulating properties. The intermediate layer is in contact with the anode catalyst layer and the electrolyte layer. In the membrane electrode assembly according to the first aspect of the invention, the recombination catalyst can be platinum. In the membrane electrode assembly according to the first aspect of the invention, the recombination catalyst can be a platinum alloy containing platinum and an alloying element. The alloying element can be at least one metallic element selected from the group consisting of cobalt, nickel, iron, manganese, tantalum, titanium, hafnium, tungsten, zirconium, niobium, aluminum, tin, molybdenum, and silicon. In the membrane electrode unit according to the first aspect of the invention, the support with the insulating property can be at least one metal oxide selected from a group consisting of tin oxide, titanium oxide, niobium oxide, molybdenum oxide and tungsten oxide. In the membrane electrode assembly according to the first aspect of the invention, the electrical conductivity of the support with insulating properties, on which the recombination catalyst is carried, can be 2.7 × 10-3 Scm-1 or lower. In the membrane electrode assembly according to the first aspect of the invention, the electrical conductivity of the support with insulating properties, on which the recombination catalyst is carried, can be 2.7 × 10-5 Scm-1 or lower. In the membrane electrode assembly according to the first aspect of the invention, the electrical conductivity of the support with insulating properties, on which the recombination catalyst is carried, can be 1.2 × 10-8 Scm-1 or lower. In the membrane electrode assembly according to the first aspect of the invention, the intermediate layer can contain an ionomer with proton conductivity. In the membrane electrode assembly according to the first aspect of the invention, the support with the insulating property can be particulate, and the particle size of the support with the insulating property can be 0.01 µm to 1 µm. According to the membrane electrode unit of the invention, the increase in hydrogen concentration in oxygen due to hydrogen crossover can be suppressed. BRIEF DESCRIPTION OF THE DRAWINGS Features, advantages and technical and industrial significance of embodiments of the invention are described below with reference to the accompanying drawings, in which the same symbols denote the same elements and in which: Fig. 1 is a schematic cross-sectional view of a membrane electrode assembly 100; and Fig. 2 is a schematic cross-sectional view of a membrane electrode assembly 200. DETAILED DESCRIPTION OF THE EXECUTION EXAMPLES Membrane electrode assembly A membrane electrode assembly (MEA) according to the present invention is described with reference to a membrane electrode assembly 100 according to an exemplary embodiment. Fig. 1 shows a schematic cross-sectional view of the membrane electrode assembly 100. As shown in Fig. 1, the membrane electrode assembly 100 has a cathode catalyst layer 10, an anode catalyst layer 20, an electrolyte layer 30 and an intermediate layer 40, wherein the electrolyte layer 30 is arranged between the cathode catalyst layer 10 and the anode catalyst layer 20 and the intermediate layer 40 is arranged between the anode catalyst layer 20 and the electrolyte layer 30. Cathode catalyst layer 10 The cathode catalyst layer (hydrogen electrode catalyst layer) 10 contains a cathode catalyst capable of generating hydrogen through water electrolysis. Examples of cathode catalysts include, but are not limited to, metal catalysts. Examples of metal catalysts include those containing at least one type of metal selected from platinum (Pt), ruthenium (Ru), rhodium (Rh), osmium (Os), iridium (Ir), palladium (Pd), and gold (Au). The metal catalyst may consist of oxides of these metals. The cathode catalyst may consist of a single metal catalyst or a mixture of several types of metal catalysts. The cathode catalyst can be an electrically conductive support carrying a metal catalyst (metal-supported catalyst). Examples of the support include carbon supports, but this is not the only option. The amount of metal catalyst on the support ranges, for example, from 5 to 90 wt%, but is not limited to this. In some embodiments, the cathode catalyst can be a platinum-containing carbon catalyst. The form of the cathode catalyst is usually powder, but not exclusively limited to this. The particle size of the powder can be adjusted according to the intended use. The cathode catalyst layer 10 can contain a proton-conducting ionomer. The specific ionomer is not defined. Examples include proton-conducting polymers. Examples of proton-conducting polymers are fluoroalkyl polymers such as polytetrafluoroethylene, and perfluoroalkyl sulfonate polymers, and so on. The weight ratio of the metal catalyst to the ionomer in the cathode catalyst layer 10 is, for example, in the range of 20:1 to 1:2, but is not limited to this range. The weight ratio of the metal-supported catalyst to the ionomer in the cathode catalyst layer 10 is, for example, in the range of 1:1 to 1:20, but is not limited to this range. In cathode catalyst layer 10, the areal weight of the metal catalyst, for example, ranges from 0.01 mg to 2.0 mg, but is not specifically limited to this range. In cathode catalyst layer 10, the weight per unit area of ​​the metal support catalyst, for example, ranges from 0.015 mg to 40 mg, without being specifically limited to this range. The thickness of the cathode catalyst layer 10, for example, is in a range of 0.1 µm to 20 µm, but is not limited to this. Anode catalyst layer 20 The anode catalyst layer (oxygen electrode catalyst layer) 20 contains an anode catalyst capable of generating oxygen by water electrolysis. Examples of anode catalysts include, but are not limited to, metal catalysts. Examples of metal catalysts are those containing at least one type of metal selected from Pt, Ru, Rh, Os, Ir, Pd, and Au. The metal catalyst may consist of oxides of these metals. The anode catalyst may consist of a single metal catalyst or a mixture of several types of metal catalysts. In some embodiments, the anode catalyst may consist of iridium oxide. The anode catalyst can be an electrically conductive support carrying a metal catalyst (metal-supported catalyst). Examples of support types include, but are not limited to, titanium oxide supports. The proportion of the metal catalyst on the support ranges from 5 to 90 wt%, but is not limited to this. The anode catalyst is usually in powder form, but this is not limited to it. The particle size of the powder can be adjusted according to the intended use. The anode catalyst layer 20 can contain an ionomer with proton conductivity. The ionomer is not specifically restricted. The ionomer can, for example, be selected from the ionomers used for the cathode catalyst layer 10. The weight ratio of the metal catalyst to the ionomer in the anode catalyst layer 20 is, for example, in the range of 1:5 to 100:1, but is not limited to this range. The weight ratio of the metal-supported catalyst to the ionomer in the anode catalyst layer 20 is, for example, in the range of 1:5 to 100:1, but is not limited to this range. In the anode catalyst layer 20, the areal weight of the metal catalyst is in a range of, for example, 0.1 mg to 5 mg, but is not specifically limited to this range. Similarly, in the anode catalyst layer 20, the areal weight of the metal-supported catalyst is in a range of, for example, 0.1 mg to 5 mg, without being specifically limited to this range. The thickness of the anode catalyst layer 20, for example, is in a range of 0.1 µm to 20 µm, but is not limited to this. Electrolyte layer 30 The electrolyte layer 30 is not particularly limited in its composition, as long as it consists of a polymer electrolyte with a sulfonic acid group. For example, the electrolyte layer 30 can consist of a dry resin with an ion exchange capacity of 0.5 meq / g to 3.0 meq / g, a dry resin with an ion exchange capacity of 0.7 meq / g to 2.5 meq / g, or a dry resin with an ion exchange capacity of 2.5 meq / g. This is because a dry resin with an ion exchange capacity of less than 0.5 meq / g does not have sufficient conductivity, and a dry resin with an ion exchange capacity of more than 3.0 meq / g forms a gel, preventing the formation of a membrane. Furthermore, from the standpoint of durability, the polymer electrolyte can be a fluorinated polymer or a perfluorocarbon polymer (which may contain an ether oxygen atom). Alternatively, the polymer electrolyte can also be a perfluorocarbon polymer containing a sulfonic acid group. Examples of the perfluorocarbon polymer include a perfluorocarbon polymer with a side chain containing a sulfonic acid group, expressed as -(OCF₂CFX)m-Op-(CF₂)n-SO₃H (where m is an integer from 0 to 3, n is an integer from 1 to 12, p is 0 or 1, and X is a fluorine atom or a trifluoromethyl group), although this is not specifically limited to this. The thickness of the electrolyte layer 30, for example, ranges from 1 µm to 400 µm, but is not limited to this range. If the thickness of the electrolyte layer 30 is less than 1 µm, the influence of hydrogen crossover increases, and the hydrogen concentration in the oxygen on the side of the anode catalyst layer tends to increase. If the thickness of the electrolyte layer 30 exceeds 400 µm, the conductivity of the protons tends to decrease. Intermediate shift 40 The intermediate layer 40 is arranged between the anode catalyst layer 20 and the electrolyte layer 30. More precisely, the intermediate layer 40 is in contact with the anode catalyst layer 20 and the electrolyte layer 30. The intermediate layer 40 contains a recombination catalyst 41, which is applied to a support 42 that has insulating properties. The recombination catalyst 41 is not specifically limited as long as it can catalyze the reaction of producing water from hydrogen and oxygen (recombination reaction). For example, the recombination catalyst 41 is platinum or a platinum alloy. In the platinum alloy, the alloying element can be at least one metallic element selected from the group consisting of cobalt (Co), nickel (Ni), iron (Fe), manganese (Mn), tantalum (Ta), titanium (Ti), hafnium (Hf), tungsten (W), zirconium (Zr), niobium (Nb), aluminum (Al), tin (Sn), molybdenum (Mo), and silicon (Si). The recombination reaction activity is high when the alloying element of the platinum alloy is selected from the aforementioned metallic elements. The support 42 is not limited in particular as long as it has an insulating property; examples of this are metal oxide supports with an insulating property. The degree of insulation of the support 42 is such that the electrical conductivity is 10⁻² Scm⁻¹ or lower, preferably 10⁻³ Scm⁻¹ or lower, more preferably 10⁻⁵ Scm⁻¹ or lower, and even more preferably 10⁻⁷ Scm⁻¹ or lower. The lower the electrical conductivity of the support 42, the greater the effect of the reduction in hydrogen concentration. A method for measuring the electrical conductivity of the support 42 (the support 42 on which the recombination catalyst 41 is located) is as follows. First, the support 42 is pressurized to 2 MPa to produce a green pellet. Then, a direct current of 10 mA is applied to the green pellet, and the electrical resistance R is determined from the voltage at that time using Ohm's law. The electrical conductivity is then calculated using the following expression. The metal oxide support can be a metal oxide that is stable under a high electrical potential (1.2 V or higher). In particular, the support can be at least one metal oxide selected from a group consisting of tin oxide, titanium oxide, niobium oxide, molybdenum oxide, and tungsten oxide. Iron oxide, for example, dissolves under high electrical potential. The amount of recombination catalyst 41 applied to the support 42 is, for example, in the range of 5 to 90 wt.%, but is not limited to this range. The form of the carrier 42 is usually powdery, but not limited to this. That is to say, the carrier 42 is particulate. The particle size of the powder (the particles) can be adjusted according to the intended use. The particle size is, for example, 0.01 µm to 1 µm. The intermediate layer 40 can contain an ionomer 43 that possesses proton conductivity. The ionomer 43 is not specifically restricted. The ionomer can, for example, be selected from the ionomers used for the cathode catalyst layer 10. The weight ratio of the recombination catalyst 41 to the ionomer in the interlayer 40, for example, is in the range of 1:5 to 100:1, but is not limited to this range. Similarly, the weight ratio of the support 42 to the ionomer in the interlayer 40 is in the range of 1:5 to 100:1, but is not limited to this range. In the intermediate layer 40, the areal weight of the recombination catalyst 41, for example, lies in the range of 0.01 mg to 1 mg, but is not limited to this range. In the anode catalyst layer 20, the areal weight of the support 42, for example, lies in the range of 0.01 mg to 1 mg, but is not limited to this range. Method for manufacturing a membrane electrode assembly 100 The membrane electrode assembly 100 is manufactured by laminating each layer onto the electrolyte layer 30 in a suitable manner. The method for laminating the individual layers onto the electrolyte layer 30 is not specifically restricted, and a known method can be used. Examples include spray coating, inkjet coating, printing coating, centrifugal coating, and so on. The electrolyte layer can be manufactured by a known method. Alternatively, the electrolyte layer can also be a commercially available product. The following describes an example of the manufacturing process for the Membrane Electrode Unit 100. First, catalyst layer inks are produced by dispersing the components of each layer in suitable dispersion media. There are known methods for producing catalyst layer inks. Next, each layer is deposited onto the electrolyte layer. The coating process described above can be used as the lamination method. The electrolyte layer onto which the catalyst layers are deposited is then heated, thereby bonding the electrolyte layer and the catalyst layers together. In this way, the Membrane Electrode Unit 100 can be manufactured. Effects One of the features of the membrane electrode assembly 100 is that the recombination catalyst 41 is supported on a substrate that has insulating properties. The effects of this property are described below. Fig. 2 shows a schematic cross-sectional view of a membrane electrode assembly 200 using a recombination catalyst 141 that is not mounted on a support. As shown in Fig. 2, the membrane electrode assembly 200 has a cathode catalyst layer 110, an anode catalyst layer 120, an electrolyte layer 130, and an intermediate layer 140, with the electrolyte layer 130 being arranged between the cathode catalyst layer 110 and the anode catalyst layer 120. The intermediate layer 140 also contains the recombination catalyst 141 and an ionomer 143. The provision of the intermediate layer 140 in the membrane electrode assembly 200 enables a recombination reaction between hydrogen, which penetrates and migrates from the side of the cathode catalyst layer 110 through the electrolyte layer, and oxygen, which is generated at the anode catalyst layer 120. This suppresses an increase in the hydrogen concentration in the oxygen on the side of the anode catalyst layer 120. On the other hand, the intermediate layer 140 borders the anode catalyst layer 120, which has a high electrical potential. Accordingly, the intermediate layer 140 can exhibit electrical conductivity with the anode catalyst layer, which has a high electrical potential, and the surface of the recombination catalyst 141 can become oxidized. Fig. 2 shows a state in which the surface of the recombination catalyst 141 is oxidized (a state in which an oxide film 142 is formed on the surface of the recombination catalyst 141). When the surface of the recombination catalyst 141 is oxidized, the recombination reaction is inhibited by the oxide film 142, and the recombination reaction efficiency decreases. Consequently, the increase in the hydrogen concentration in oxygen on the side of the anode catalyst layer 120 due to hydrogen crossover from the side of the cathode catalyst layer 110 cannot be sufficiently suppressed. On the other hand, the membrane electrode assembly 100 employs an arrangement in which the recombination catalyst 41 is supported on the substrate 42, which has insulating properties. Accordingly, surface oxidation due to electrical conduction with the anode catalyst layer, which has a high electrical potential, can be suppressed. Thus, according to the membrane electrode assembly 100, the recombination catalyst 41 can maintain a state in which the recombination reaction efficiency is high. Consequently, an increase in the hydrogen concentration in the oxygen on the side of the anode catalyst layer 20 due to hydrogen from the side of the cathode catalyst layer 10 can be sufficiently suppressed by the membrane electrode assembly 100. Addendum Although an intermediate layer is provided in the membrane electrode assembly 100, the number of intermediate layers in the invention is not limited to this. Two or more intermediate layers can be provided. For example, intermediate layers can be arranged between the anode catalyst layer and the electrolyte layer, as well as between the cathode catalyst layer and the electrolyte layer. Intermediate layers can also be arranged between the anode catalyst layer and the electrolyte layer, as well as within the electrolyte layer. Thus, according to the membrane electrode unit of the invention, the increase in the hydrogen concentration in the oxygen on the side of the anode catalyst layer due to hydrogen crossover from the side of the cathode catalyst layer can be suppressed. The present revelation will be described in more detail below using examples. Manufacturing a membrane electrode assembly Example 1 In a membrane electrode assembly according to Example 1, an intermediate layer is provided between the anode catalyst layer and the electrolyte layer. The manufacturing processes are described below. 1. Production of recombination catalyst ink for intermediate layers Initially, 10 g of a platinum-bearing tin oxide catalyst (platinum content of 20%), 1.5 g of a proton-conducting ionomer (20% Nafion (registered trademark) dispersion solution DE2020 (manufactured by Fujifilm Wako Chemical Corporation)), 37 g of deionized water, and 56 g of ethanol were weighed, mixed in a beaker, and then dispersed using an ultrasonic homogenizer to produce a recombination catalyst ink. At this stage, the recombination catalyst ink was prepared such that the weight ratio of platinum to ionomer was 1:0.15. 2. Coating of the electrolyte layer with recombination catalyst ink for the intermediate layer The recombination catalyst ink produced in step 1 was applied to one side of the electrolyte layer (NR212, manufactured by WL Gore & Associates GK) using a spray applicator and dried for five minutes at 80°C. The coating was applied at a density of 0.1 mg of platinum per unit area. This layer was then applied to the electrolyte layer. 3. Production of the catalyst ink for the anode (oxygen electrode) Initially, 5.2 g of an iridium oxide catalyst (ElystIr750520, manufactured by Umicore), 6.8 g of a proton-conducting ionomer (20% Nafion (registered trademark) dispersion solution DE2020 (manufactured by Fujifilm Wako Chemical Corporation)), 3.6 g of deionized water, and 6.4 g of 1-propanol were weighed, mixed in a beaker, and then dispersed using an ultrasonic homogenizer to produce an anode (oxygen electrode) catalyst ink. At this stage, the anode catalyst ink was prepared such that the weight ratio between the iridium oxide catalyst and the ionomer was 1:0.3. 4. Coating the electrolyte layer with anode (oxygen electrode) catalyst ink The anode catalyst ink (oxygen electrode) prepared in step 3 was applied to the intermediate layer formed on the electrolyte layer using a spray applicator and dried for five minutes at 80°C. The coating was applied at this stage such that the weight of iridium per unit area was 2.0 mg. This resulted in an anode catalyst layer (oxygen electrode) being deposited onto the intermediate layer. 5. Production of the catalyst ink for the cathode (hydrogen electrode) First, 5 g of a platinum-containing carbon catalyst (platinum content of 20%, manufactured by Cataler Corporation), 6.0 g of a proton-conducting ionomer (20% Nafion (registered trademark) dispersion solution DE2020 (manufactured by Fujifilm Wako Chemical Corporation)), 67.8 g of deionized water, and 34.3 g of ethanol were weighed, mixed in a beaker, and then dispersed using an ultrasonic homogenizer to produce a cathode catalyst ink (hydrogen electrode). At this stage, the cathode catalyst ink was prepared such that the weight ratio of carbon support to ionomer was 1:1.2. 6. Coating the electrolyte layer with catalyst ink for the cathode (hydrogen electrode) The cathode catalyst layer (hydrogen electrode) prepared in step 5 was applied to the surface of the electrolyte layer on the side facing the surface of the electrolyte layer on which the intermediate layer and the anode catalyst layer (oxygen electrode) were formed, using a spray applicator, and dried for five minutes at 80°C. The coating was applied at a density of 0.2 mg of platinum per unit area. Thus, a cathode catalyst layer (hydrogen electrode) was deposited onto the electrolyte layer. 7. Connection The electrolyte layer coated with each of the catalyst layers was hot-pressed for four minutes at a temperature of 130°C and a pressure of 130 kPa, thereby bonding the electrolyte layer and the catalyst layers. In this way, a membrane electrode assembly according to Example 1 was produced. Examples 2 to 4 Membrane electrode assemblies according to Examples 2 to 4 were prepared in the same manner as in Example 1, except that the platinum-supported tin oxide catalyst used in the interlayer of Example 1 was replaced by a platinum-supported tin oxide catalyst having the electrical conductivity given in Table 1. Note that the electrical conductivity used here is the electrical conductivity of the tin oxide on which the platinum is supported. That is to say, the electrical conductivity given in Table 1 indicates the electrical conductivity of the support on which the recombination catalyst is deposited. Comparative example 1 A membrane electrode assembly according to comparative example 1 is an arrangement in which the intermediate layer in the membrane electrode assembly according to example 1 is omitted. The manufacturing processes are described below. 1. Coating of the electrolyte layer with catalyst ink for the anode (oxygen electrode) The anode catalyst ink (oxygen electrode) prepared in Example 1, step 3, was applied to one side of the electrolyte layer (NR212, manufactured by WL Gore & Associates GK) using a spray applicator and dried for five minutes at 80°C. The coating was applied at this stage such that the weight of iridium per unit area was 2.0 mg. This created an anode catalyst layer (oxygen electrode) on top of the electrolyte layer. 2. Coating the electrolyte layer with catalyst ink for the cathode (hydrogen electrode) The cathode catalyst ink prepared in Example 1, section 5, was applied to the surface of the electrolyte layer on the side facing the surface of the electrolyte layer on which the anode catalyst layer (oxygen electrode) was formed, using a spray applicator, and dried for five minutes at 80°C. The coating was applied at a density of 0.2 mg of platinum per unit area. This resulted in a cathode catalyst layer (hydrogen electrode) being deposited onto the electrolyte layer. 3. Connection The electrolyte layer coated with each of the catalyst layers was hot-pressed for four minutes at a temperature of 130°C and a pressure of 130 kPa, thereby bonding the electrolyte layer and the catalyst layers. In this way, a membrane electrode assembly was produced according to Comparative Example 1. Comparative example 2 A membrane electrode assembly according to Comparative Example 2 is an arrangement in which the platinum-bearing tin oxide catalyst in the interlayer of the membrane electrode assembly according to Example 1 is replaced by a non-bearing platinum catalyst (using platinum itself). The manufacturing processes are described below. 1. Production of recombination catalyst ink for intermediate layers Initially, 10 g of an unsupported platinum catalyst, 7.6 g of a proton-conducting ionomer (20% Nafion (registered trademark) dispersion solution DE2020 (manufactured by Fujifilm Wako Chemical Corporation)), 39 g of deionized water, and 59.5 g of ethanol were weighed, mixed in a beaker, and then dispersed using an ultrasonic homogenizer to produce a recombination catalyst ink. At this stage, the recombination catalyst ink was prepared such that the weight ratio of platinum to ionomer was 1:0.15. 2. Coating of the electrolyte layer with recombination catalyst ink for the intermediate layer The recombination catalyst ink produced in step 1 was applied to one side of the electrolyte layer (NR212, manufactured by WL Gore & Associates GK) using a spray applicator and dried for five minutes at 80°C. The coating was applied at a density of 0.1 mg of platinum per unit area. This layer was then applied to the electrolyte layer. 3. Coating the electrolyte layer with catalyst ink for the anode (oxygen electrode) The anode (oxygen electrode) catalyst ink prepared in Example 1, section 3, was applied to the intermediate layer formed on the electrolyte layer using a spray applicator and dried for five minutes at 80°C. The coating was applied at this stage such that the weight of iridium per unit area was 2.0 mg. This created an anode catalyst layer (oxygen electrode) on the intermediate layer. 4. Coating the electrolyte layer with catalyst ink for the cathode (hydrogen electrode) The cathode catalyst ink prepared in Example 1, section 5, was applied to the surface of the electrolyte layer on the side facing the surface of the electrolyte layer on which the intermediate layer and the anode catalyst layer (oxygen electrode) were formed, using a spray applicator, and dried for five minutes at 80°C. The coating was applied at a density of 0.2 mg of platinum per unit area. This resulted in a cathode catalyst layer (hydrogen electrode) being deposited onto the electrolyte layer. 5. Connection The electrolyte layer coated with each of the catalyst layers was hot-pressed for four minutes at a temperature of 130°C and a pressure of 130 kPa, thereby bonding the electrolyte layer and the catalyst layers. In this way, a membrane electrode assembly was produced according to Comparative Example 2. Comparative example 3 A membrane electrode assembly according to Comparative Example 3 was produced in the same way as in Example 1, except that the platinum-containing tin oxide catalyst used in the interlayer of Example 1 was replaced by a platinum-containing carbon catalyst. Evaluation A diffusion layer of carbon fibers was arranged on the cathode (hydrogen electrode) catalyst layer side of the membrane electrode assembly, a diffusion layer of titanium fibers, on whose surfaces platinum was vapor-deposited, was arranged on the anode (oxygen electrode) catalyst layer side, and the diffusion layer of carbon fibers and the diffusion layer of titanium fibers, on whose surfaces platinum was vapor-deposited, are combined in a single cell (in which both anode and cathode are straight channels) with an electrode area of ​​1 cm2.Subsequently, water electrolysis was carried out under cell temperature conditions of 80°C and atmospheric pressure by applying an electric current with a current density of 1 A / cm² using an electronic load device, while water circulated at a volume several times greater than the amount required for water electrolysis, both at the oxygen electrode (anode) and the hydrogen electrode (cathode). The gas and water on the oxygen electrode (anode) side were then separated by a gas-liquid separator, and after the gas component (oxygen) had been collected for 30 minutes, the hydrogen concentration in the gas component was measured by gas chromatography / mass spectrometry (GC / MS). The results are shown in Table 1. The reduction rate given in Table 1, with the hydrogen concentration of comparison example 1 as the reference, represents the reduction rate of the hydrogen concentration of the other test examples. The effects compared to comparison example 2, also given in Table 1, represent the percentages of the reduction rates of the other test examples, again with the reduction rate of comparison example 2 as the reference. Effects compared to comparison example 2 that were 120% or higher were rated as "A", effects compared to that that were 110% or higher and lower than 120% were rated as "B", and effects compared to that that were 100% or higher and lower than 110% were rated as "C". Table 1 Example 1Pt / Oxide 1.2 × 10 -80.278.7%148%A Example 2Pt / Oxide 3.7 × 10 -60.2276.6%144%A Example 3Pt / Oxide 2.7 × 10 -50.2672.3%136%A Example 4Pt / Oxide 2.7 × 10 -30.356 2.8%118%B Comparison example 1--0.940.0%0%- Comparative example 2Pt5.1 × 10 10.4453.2%100%- Comparative example 3Pt / C 1.4 × 10 10.4354.3%102%C * C represents 100% to 110%, B represents 110% to 120%, and A represents 120% or more results. In examples 1 to 4, the hydrogen concentration in the gas component was significantly lower than the hydrogen concentration in the gas component in comparison examples 1 to 3. Conversely, the hydrogen concentration in the gas component was highest in comparison example 1, which lacked an intermediate layer (recombination catalyst). Similarly, in comparison example 2, with no platinum in the intermediate layer, the hydrogen concentration in the gas component was lower than in comparison example 1, but higher than in example 1. Furthermore, comparison example 3, which used a platinum-containing carbon catalyst in the intermediate layer, yielded almost identical results to comparison example 2. The results of comparative examples 1 and 2 show that the recombination reaction through the intermediate layer (platinum) has effects that suppress the increase in the hydrogen concentration in the gas component. The results of Examples 1 to 4 and comparative Examples 2 and 3 further show that the application of platinum (recombination catalyst) to the tin oxide support improves the suppression of the hydrogen concentration increase. This is assumed to be due to the suppression of electrical conductivity to the supported platinum, even though the anode catalyst layer, which is at a high electrical potential, and the intermediate layer are in contact with each other, since tin oxide is an insulator, and consequently the formation of an oxide film on the platinum surface is suppressed. Another possible reason is that the effects of suppressing oxide film formation, as described above, are retained because tin oxide is stable at high electrical potential.Accordingly, in Examples 1 to 4, it is assumed that the hydrogen concentration in the gas component was significantly low because the efficiency of the recombination reaction was maintained at a high level due to the suppression of oxide film formation on the platinum. On the other hand, in Comparative Example 2, unsupported platinum was used, and in Comparative Example 3, a platinum-containing carbon catalyst, which is electrically conductive, is used. Therefore, it is assumed that the hydrogen concentration in the gas component was higher compared to Examples 1 to 4 because an oxide film formed on the platinum, reducing the efficiency of the recombination reaction. Furthermore, a comparison of examples 1 to 4 confirmed that the hydrogen concentration was lower the lower the electrical conductivity of the catalyst (support), i.e., the lower the electrical conductivity of the support on which the recombination catalyst was applied. This is presumably due to the fact that the oxidation of platinum is suppressed more strongly the lower the electrical conductivity of the catalyst.

Claims

Membrane electrode assembly (100) comprising: a cathode catalyst layer (10); an anode catalyst layer (20); an electrolyte layer (30) arranged between the cathode catalyst layer (10) and the anode catalyst layer (20); and an intermediate layer (40) arranged between the anode catalyst layer (20) and the electrolyte layer (30), wherein the intermediate layer (40) comprises a support (42) having an insulating property and a recombination catalyst (41) which is supported on the support (42) having the insulating property, characterized in that the intermediate layer (40) is arranged in contact with the anode catalyst layer (20) and the electrolyte layer (30). Membrane electrode assembly (100) according to claim 1, wherein the recombination catalyst (41) is platinum. Membrane electrode assembly (100) according to claim 1, wherein: the recombination catalyst (41) is a platinum alloy containing platinum (Pt) and an alloying element; and the alloying element is at least one metallic element selected from the group consisting of cobalt (Co), nickel (Ni), iron (Fe), manganese (Mn), tantalum (Ta), titanium (Ti), hafnium (Hf), tungsten (W), zirconium (Zr), niobium (Nb), aluminum (Al), tin (Sn), molybdenum (Mo) and silicon (Si). Membrane electrode assembly (100) according to one of claims 1 to 3, wherein the support (42) having the insulating property is at least one metal oxide selected from a group consisting of tin oxide, titanium oxide, niobium oxide, molybdenum oxide and tungsten oxide. Membrane electrode assembly (100) according to one of claims 1 to 4, wherein the electrical conductivity of the support (42) with the insulating property on which the recombination catalyst (41) is supported is 2.7 × 10-3 Scm-1 or less. Membrane electrode assembly (100) according to claim 5, wherein the electrical conductivity of the support (42) with the insulating property on which the recombination catalyst (41) is supported is 2.7 × 10-5 Scm-1 or less. Membrane electrode assembly (100) according to claim 5 or 6, wherein the electrical conductivity of the support (42) with the insulating property on which the recombination catalyst (41) is supported is 1.2 × 10-8 Scm-1 or less. Membrane electrode assembly (100) according to one of claims 1 to 7, wherein the intermediate layer (40) contains an ionomer with proton conductivity. Membrane electrode assembly (100) according to one of claims 1 to 8, wherein: the support (42) with the insulating property is particle-shaped; and a particle size of the support (42) with the insulating property is 0.01 µm to 1 µm.

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