JP6862792B2 - Method of manufacturing electrode catalyst - Google Patents

Description

The present invention relates to a method for producing an electrode catalyst.

In recent years, in response to social demands and trends against the background of energy and environmental problems, fuel cells that operate even at room temperature and can obtain high output density have been attracting attention as power sources for electric vehicles and stationary power sources. A fuel cell is a clean power generation system in which the product of the electrode reaction is water in principle and has almost no adverse effect on the global environment. In particular, polymer electrolyte fuel cells (PEFCs) are expected as power sources for electric vehicles because they operate at relatively low temperatures. The structure of a polymer electrolyte fuel cell generally has a structure in which an electrolyte membrane-electrode assembly (MEA) is sandwiched between separators. The electrolyte membrane-electrode assembly is formed by sandwiching a polymer electrolyte membrane between a pair of electrode catalyst layers and a gas diffusible electrode (gas diffusion layer; GDL).

In a polymer electrolyte fuel cell having an electrolyte membrane-electrode assembly (MEA) as described above, the reaction formulas described below are described below according to the polarity of both electrodes (cathode and anode) sandwiching the polymer electrolyte membrane. The electrode reaction indicated by is advanced to obtain electrical energy. First, hydrogen contained in the fuel gas supplied to the anode (negative electrode) side is oxidized by the catalyst component to become protons and electrons (2H ₂ → 4H ⁺ + 4e ⁻ : reaction 1). Next, the generated protons pass through the solid polymer electrolyte contained in the electrode catalyst layer and the solid polymer electrolyte membrane in contact with the electrode catalyst layer, and reach the cathode (positive electrode) side electrode catalyst layer. Further, the electrons generated in the electrode catalyst layer on the anode side are the carbon carrier constituting the electrode catalyst layer, the gas diffusion layer in contact with the side different from the solid polymer electrolyte film of the electrode catalyst layer, the separator and the external circuit. It reaches the cathode side electrode catalyst layer through. Then, the protons and electrons that reach the cathode side electrode catalyst layer react with oxygen contained in the oxidant gas supplied to the cathode side to generate water (O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O: Reaction 2). .. In a fuel cell, electricity can be taken out to the outside through the above-mentioned electrochemical reaction.

Here, in order to improve the power generation performance, improving the activity of the electrode catalyst in the electrode catalyst layer is an important key. For example, Patent Document 1 discloses a carbon carrier obtained by heat-treating carbon particles on which catalyst particles A (for example, cobalt oxide particles) that promote carbon oxidation are supported. Patent Document 1 also supports catalyst particles B (for example, platinum particles) that catalyze the above-mentioned electrochemical reaction using carbon particles having concave dents formed on the surface of the catalyst particles A as a carrier. Electrode catalysts for fuel cells are disclosed.

Japanese Unexamined Patent Publication No. 2006-187744

Patent Document 1 describes that the carbon carrier is useful as an electrode catalyst for a fuel cell, which is desired to stably exhibit high catalytic activity for a long period of time. Further, in the electrode catalyst for a fuel cell described in Patent Document 1, the catalyst particles that catalyze the above-mentioned electrochemical reaction in the concave recess formed on the surface of the carbon particles by the catalyst particles A (for example, cobalt oxide particles). B (for example, platinum particles) is carried. It is described that the catalyst particles B can be supported in a highly dispersed manner.

As described above, by using the carbon carrier described in Patent Document 1, it is possible to obtain an electrode catalyst for a fuel cell that exhibits high catalytic activity for a long period of time. However, in the embodiment of Patent Document 1, in order to obtain a carbon carrier having a concave recess formed on the surface, carbon particles on which cobalt oxide (catalyst particles A) are supported are prepared as a first step. .. Then, cobalt oxide (catalyst particles A) is removed from the carbon carrier (carbon particles), and platinum (catalyst particles B) is supported on the carbon carrier again. In such a method, since the step of supporting the metal particles on the carbon carrier (carbon particles) is included a plurality of times, there is room for improvement from the viewpoint of process cost.

Therefore, the present invention has been made in view of the above circumstances, and an object of the present invention is to provide a method for producing an electrode catalyst, which can obtain a highly active electrode catalyst at a low cost.

The present inventors have conducted diligent research in order to solve the above problems. As a result, it has been found that the above-mentioned problems can be solved by a production method including heating an electrode catalyst precursor in which a platinum-containing catalyst metal is supported on a carbon carrier under predetermined conditions, and the present invention has been solved.

According to the present invention, a highly active electrode catalyst can be obtained at low cost. In the present invention, since the electrode catalyst precursor on which the catalyst metal that promotes the oxidation reaction of hydrogen or the reduction reaction of oxygen is supported in advance is subjected to the heat treatment, it is not necessary to support the metal particles a plurality of times, and the process cost is suppressed. Be done. Further, it is considered that by heating such an electrode catalyst precursor under predetermined conditions, a highly active electrode catalyst in which the catalyst metal is appropriately embedded in the carbon carrier can be obtained.

It is a schematic sectional drawing which shows the basic structure of the polymer electrolyte fuel cell which concerns on one Embodiment of this invention. It is an electron microscope image of the electrode catalyst obtained in Example 1. It is an electron microscope image of the electrode catalyst obtained in Comparative Example 1.

[Manufacturing method of electrode catalyst]
In one embodiment of the present invention, an electrode catalyst precursor in which a platinum-containing catalyst metal is supported on a carbon carrier is prepared, and the electrode catalyst precursor is heated in an oxygen-containing atmosphere to obtain the electrode catalyst precursor. The present invention relates to a method for producing an electrode catalyst, which comprises reducing the weight by a rate of more than 0% and less than 10%.

Another embodiment of the present invention is to prepare an electrode catalyst precursor in which a platinum-containing catalyst metal is supported on a carbon carrier, and to prepare the electrode catalyst precursor at a temperature at which the following relational expression 3 is satisfied in an oxygen-containing atmosphere. The present invention relates to a method for producing an electrode catalyst, which comprises heating with.

However, in the above equation _{3, T heat} (℃) represents the temperature in the _{heating; T carbon} (℃) represents the combustion temperature of the carbon support.

According to the above method, since the electrode catalyst precursor on which the catalyst metal that promotes the oxidation reaction of hydrogen or the reduction reaction of oxygen is supported in advance is subjected to the heat treatment, it is not necessary to support the metal particles a plurality of times. Therefore, the electrode catalyst can be manufactured at low cost.

Further, according to the above method, a highly active electrode catalyst can be obtained. The technical scope of the present invention is not limited, but it is presumed that this is due to the following mechanism.

By heating the electrode catalyst precursor in which the platinum-containing catalyst metal is supported on the carbon carrier under the above conditions, oxidative decomposition of the carbon carrier proceeds in the portion in contact with the platinum-containing catalyst metal, and the catalyst metal becomes a carbon carrier. An electrode catalyst having an appropriately buried structure can be obtained. As a result, sintering of the catalyst metal is prevented, that is, the electrode catalyst in which the catalyst metal is highly dispersed and supported is obtained. In particular, an electrode catalyst having such a structure has advantages from the following viewpoints when the catalyst metal is used in a polymer electrolyte fuel cell.

Conventionally, in order for the electrochemical reaction in a polymer electrolyte fuel cell to proceed, it is considered necessary that the polymer electrolyte (ionomer) as a proton path and the catalyst metal as a reaction catalyst are in contact with each other. Has been done. However, in the electrode catalyst layer for a fuel cell containing the electrode catalyst and the polymer electrolyte (ionomer), the present inventors form a three-phase interface with water even when the electrode catalyst does not come into contact with the polymer electrolyte. By doing so, it was found that the electrode catalyst can be effectively used. Here, since the polymer electrolyte has a property of being easily adsorbed on the surface of the catalyst metal, the access of the reaction gas to the catalyst metal can be hindered by the polymer electrolyte. Therefore, by using an electrode catalyst having a structure in which the catalyst metal is appropriately embedded in the carbon carrier for the fuel cell electrode catalyst layer of the polymer electrolyte fuel cell, it is possible to prevent the polymer electrolyte from coming into contact with the catalyst metal. .. It is considered that this prevents the reactive area of the catalyst metal surface from decreasing, and further, by forming a three-phase interface with water, the electrode catalyst can be effectively used.

On the other hand, if the heat treatment is excessively performed beyond the scope of the present invention, the catalytic activity is rather lowered. It is presumed that this is because the platinum-containing catalyst metal has a higher decomposition activity of the carbon carrier than the cobalt oxide and the like described in Patent Document 1. That is, if the heat treatment is excessively performed beyond the scope of the present invention, the oxidative decomposition in the heat treatment of the carbon carrier at the portion in contact with the platinum-containing catalyst metal proceeds excessively, and the contact between the catalyst metal and the carbon carrier It is considered that the electron transfer path is narrowed due to the decrease in area. Further, in such an electrode catalyst in which oxidative decomposition of the carbon carrier has progressed excessively, the catalyst metal is completely buried in the carbon carrier, whereby the accessibility of the reaction gas to the catalyst metal and the three-phase interface with water Is considered to be insufficiently formed.

Hereinafter, embodiments of the present invention will be described. The present invention is not limited to the following embodiments. In addition, the dimensional ratios in the drawings are exaggerated for convenience of explanation and may differ from the actual ratios.

In the present specification, "X to Y" indicating a range means "X or more and Y or less". Unless otherwise specified, operations and physical properties are measured under the conditions of room temperature (20 to 25 ° C.) / relative humidity of 40 to 50% RH.

An electrode catalyst precursor in which a platinum-containing catalyst metal used in the present invention is supported on a carbon carrier (hereinafter, "an electrode catalyst precursor in which a platinum-containing catalyst metal is supported on a carbon carrier" is also simply referred to as an "electrode catalyst precursor". (Referred to as) is not particularly limited. The electrode catalyst precursor may be prepared by supporting a platinum-containing catalyst metal on a carbon carrier by, for example, a method described later, or a commercially available product may be used. Examples of such commercially available products include, but are not limited to, TEC10V30E, TEC10V40E, TEC10V50E (all manufactured by TANAKA Holdings, Inc.) and the like.

The platinum-containing catalytic metal that can be used in the present invention has a function of catalyzing an electrochemical reaction. For example, as long as it is a catalyst metal used for the anode catalyst layer of a fuel cell, there is no particular limitation as long as it has a catalytic action on the oxidation reaction of hydrogen, and a known platinum-containing catalyst metal can be used in the same manner. Further, as long as the catalyst metal is used for the cathode catalyst layer of the fuel cell, there is no particular limitation as long as it has a catalytic action on the reduction reaction of oxygen, and a known platinum-containing catalyst metal can be used in the same manner.

In the present invention, a catalyst metal containing platinum is used. The platinum-containing catalytic metal has advantages such as excellent catalytic activity, toxicity resistance to carbon monoxide and the like, and heat resistance. A platinum-containing catalytic metal (hereinafter, "platinum-containing catalytic metal" is also simply referred to as "catalytic metal") is a platinum-containing alloy composed of platinum atoms and non-platinum metal atoms, even if it is substantially composed of platinum atoms. There may be. The above-mentioned "substantially composed of platinum atoms" refers to atoms having a platinum atom content of more than 90 atomic% among the atoms constituting the catalyst metal, preferably having a platinum atom content of 99 atoms. % Or more (upper limit: 100 atomic%). The composition of the alloy depends on the type of metal to be alloyed, but the content of platinum is preferably 30 to 90 atomic%, and the content of metal to be alloyed with platinum is preferably 10 to 70 atomic%. In addition, an alloy is generally a general term for a metal element to which one or more kinds of metal elements or non-metal elements are added and which has metallic properties. The structure of the alloy includes a eutectic alloy, which is a mixture in which the component elements are separate crystals, a solid solution in which the component elements are completely fused, and a compound in which the component elements are intermetallic compounds or compounds of metals and non-metals. Some of them are formed, and any of them may be used in the present application. More specifically, non-platinum metal atoms include ruthenium, iridium, rhodium, palladium, osmium, tungsten, lead, iron, copper, silver, chromium, cobalt, nickel, manganese, vanadium, molybdenum, gallium, and aluminum. One or more selected from the above can be exemplified.

The carbon carrier functions as a carrier for supporting the catalyst metal and as an electron conduction path involved in the transfer of electrons between the catalyst metal and other members. In the present specification, the term "carbon carrier" refers to a carrier containing a carbon atom as a main component, and is a concept including both a carbon atom and a substantially carbon atom. In some cases, elements other than carbon atoms may be contained in order to improve the characteristics of the fuel cell. In addition, the fact that it is substantially composed of carbon atoms means that impurities of about 2 to 3% by weight or less can be mixed.

Specific examples of the carbon carrier include carbon blacks such as acetylene black, channel black, oil furnace black, gas furnace black (for example, Vulcan (registered trademark)), lamp black, thermal black, and Ketjen black (registered trademark). Black Pearl®; Graphitized acetylene black; Graphitized channel black; Graphitized oil furnace black; Graphitized gas furnace black; Graphitized lamp black; Graphitized thermal black; Graphitized Ketjen black®; Graphitized black pearl (registered trademark); carbon nanotubes; carbon nanofibers; carbon nanohorns; carbon fibrils; activated carbon; coke; natural graphite; artificial graphite and the like. Further, as the carbon carrier, zeolite template carbon (ZTC) having a structure in which nano-sized band-shaped graphene is regularly linked in a three-dimensional manner can also be mentioned.

The BET specific surface area of the carbon carrier is preferably 10 m ² / g carrier or more ^{and less than 800 m 2} / g carrier, more preferably 20 to 700 m ² / g carrier, and further preferably 50 to 600 m ² / g carrier. .. With such a BET specific surface area, it is possible to prevent the catalyst metal from being excessively embedded in the carbon carrier by heat treatment while supporting (highly dispersing) a sufficient catalyst metal on the carbon carrier. The "BET specific surface area (m ² / g carrier)" of the carrier is measured by the nitrogen adsorption method.

The size of the carbon carrier is not particularly limited, but the primary average particle size is 5 to 200 nm from the viewpoint of ease of carrying, catalyst utilization, and control of the thickness of the electrode catalyst layer within an appropriate range. It is preferably about 10 to 100 nm. The "average particle size of the carrier" is the average crystallite diameter obtained from the half-value width of the diffraction peak of the carrier particles in X-ray diffraction (XRD) and the average particle size of the carrier examined by a transmission electron microscope (TEM). Can be measured as a value. In the present specification, the "average particle size of the carrier" is the average particle size of the carrier particles examined from a transmission electron microscope image for a statistically significant number of samples (for example, at least 200, preferably at least 300). The value. Here, the "particle size" means the maximum distance among the distances between any two points on the contour line of the particles.

The catalyst metal can be supported on the carbon carrier by a known method, and is not particularly limited. For example, known methods such as an impregnation method, a liquid phase reduction-supporting method, an evaporation to dryness method, a colloidal adsorption method, a spray pyrolysis method, and a reverse micelle (microemulsion method) can be used. When a platinum-containing alloy is supported, platinum ions and non-platinum metal ions may be sequentially supported on a carbon carrier. Alternatively, a mixed solution containing a platinum precursor and a non-platinum metal precursor is prepared, a reducing agent is added to the mixed solution, and platinum ions derived from the platinum precursor and non-platinum metal ions derived from the non-platinum metal precursor are simultaneously added to the mixed solution. It may be carried on a carbon carrier. As an example of the method for supporting the catalyst metal on the carbon carrier, the support by the liquid phase reduction support method will be described in detail below, but the support method is not limited to the following.

First, a platinum precursor is added to a solvent to prepare a platinum precursor-containing liquid. Here, the platinum precursor is not particularly limited, but a platinum salt and a platinum complex can be used. More specifically, chloroplatinic acid (typically its hexahydrate; H ₂ [PtCl ₆ ], 6H ₂ O), nitrates such as dinitrodiammine platinum, sulfates, ammonium salts, amines, tetraammine platinum and Use ammine salts such as hexaammine platinum, carbonates, bicarbonates, halides such as platinum chloride, inorganic salts such as nitrites and oxalic acid, carboxylates such as formates, hydroxides, and alcoholides. Can be done. The platinum precursor may be used alone or as a mixture of two or more.

The solvent used for preparing the platinum precursor-containing liquid is not particularly limited, and is appropriately selected depending on the type of platinum precursor used. The form of the platinum precursor-containing liquid is not particularly limited, and includes a solution, a dispersion liquid, and a suspension. From the viewpoint of uniform mixing, the platinum precursor-containing liquid is preferably in the form of a solution. Specific examples thereof include organic solvents such as water, methanol, ethanol, 1-propanol and 2-propanol, acids and alkalis. Of these, water is preferable, and pure water or ultrapure water is particularly preferable, from the viewpoint of sufficiently dissolving the platinum precursor (ionizing platinum efficiently). The solvent may be used alone or in the form of a mixture of two or more.

The concentration of the platinum precursor in the platinum precursor-containing liquid is not particularly limited, but is preferably 0.1 to 50% by mass, more preferably 0.5 to 45% by mass in terms of metal.

Next, a carbon carrier and a reducing agent are added to the platinum precursor-containing liquid, and platinum ions derived from the platinum precursor are reduced and supported on the carbon carrier to produce platinum-supported precursor particles. Here, the order in which the carbon carrier and the reducing agent are added to the platinum precursor-containing liquid is not particularly limited, but the carbon carrier first contains the platinum precursor in consideration of the ease of dispersion of the platinum particles on the carbon carrier. It is preferable to add the reducing agent after adding to the liquid.

The amount of the carbon carrier added to the platinum precursor-containing liquid is not particularly limited, but is preferably adjusted to be 0.1 to 70% by weight based on the total amount of the carrier. It is preferable to set the loading concentration in such a range because the aggregation of the catalyst metals can be suppressed and the increase in the thickness of the electrode catalyst layer can be suppressed. It is more preferably 0.5 to 60% by weight, and even more preferably 2 to 50% by weight.

After adding the carbon carrier to the platinum precursor-containing liquid, the mixture may be stirred. As a result, the platinum precursor and the carbon carrier are uniformly mixed, so that the catalyst metal particles can be uniformly dispersed on the carbon carrier. Here, the stirring conditions are not particularly limited as long as they can be mixed uniformly. For example, uniform dispersion and mixing can be performed by using an appropriate stirrer such as a stirrer or a homogenizer, or by applying ultrasonic waves such as an ultrasonic disperser. The stirring temperature is preferably 0 to 50 ° C. The stirring time may be appropriately set so that dispersion is sufficiently performed, and is usually 1 to 60 minutes.

The reducing agent is not particularly limited, and the same reducing agent as before can be used. For example, formic acid such as ethanol, methanol, propanol, formic acid, sodium formate and potassium formate, citrate such as formaldehyde, sodium thiosulfate, citric acid, sodium citrate, trisodium citrate, sodium borohydride (NaBH _4). ) And hydrazine (N ₂ H ₄ ) and the like can be used. Of the above reducing agents, trisodium citrate dihydrate can also act as an anti-aggregation agent. These may be in the form of hydrates. Further, two or more types may be mixed and used. The form of addition of the reducing agent is not particularly limited, and the reducing agent may be added to the platinum precursor-containing solution as it is, or may be added to the platinum precursor-containing solution in the form of a reducing agent solution previously dissolved in a solvent. The solution form is preferable because it can be easily and uniformly mixed.

The amount of the reducing agent added is not particularly limited as long as it is sufficient to reduce the platinum ions. Specifically, the amount of the reducing agent added is preferably 5 to 90 mol with respect to 1 mol (metal equivalent) of platinum ions. With such an amount, platinum ions can be sufficiently reduced. When two or more kinds of reducing agents are used, the total addition amount of these agents is preferably in the above range.

The reduction reaction conditions of the platinum precursor are not particularly limited as long as the catalytic metal ions such as platinum ions can be sufficiently reduced and supported on the carbon carrier. For example, the reduction reaction temperature is preferably near the boiling point of the solvent (solvent boiling point ± 10 ° C., more preferably solvent boiling point ± 5 ° C.). Alternatively, the reduction reaction temperature is preferably 40 to 150 ° C, more preferably 60 to 100 ° C. The reduction reaction time is not particularly limited as long as the platinum precursor and the reducing agent can be uniformly mixed, but is preferably 0.5 to 30 hours, more preferably 1 to 8 hours. Under such conditions, the catalyst metal can be highly dispersed and supported by a carbon carrier.

By the above reduction reaction, the catalyst metal ion is reduced and supported on the carbon carrier, and an electrode catalyst precursor particle-containing liquid is obtained. Here, if necessary, the electrode catalyst precursor particles may be isolated from the reaction solution. Here, the isolation method is not particularly limited, and the electrode catalyst precursor particles may be filtered and dried. If necessary, the electrode catalyst precursor particles may be filtered and then washed (for example, washed with water). Moreover, the above-mentioned filtration and washing step may be repeated if necessary. In addition, the electrode catalyst precursor particles may be dried after filtration or washing. Here, the electrode catalyst precursor particles may be dried in air or under reduced pressure. The drying temperature is not particularly limited, but can be, for example, in the range of 10 to 100 ° C., preferably room temperature (25 ° C.) to 80 ° C. The drying time is also not particularly limited, but can be, for example, in the range of 1 to 60 hours, preferably about 5 to 50 hours.

In order to keep the average particle size of the catalyst metal in a desired range, the catalyst metal may be supported on a carrier and then the electrode catalyst precursor may be annealed in a reducing atmosphere. At this time, the annealing treatment temperature is preferably in the range of 300 to 1200 ° C., more preferably in the range of 500 to 1150 ° C., and particularly preferably in the range of 700 to 1000 ° C. The reducing atmosphere is not particularly limited as long as it contributes to the grain growth of the catalyst metal, but it is preferably performed in a mixed atmosphere of the reducing gas and the inert gas. The reducing gas is not particularly limited, but hydrogen (H ₂ ) gas is preferable. The inert gas is not particularly limited, but helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), nitrogen (N ₂ ) and the like can be used. The inert gas may be used alone or in the form of a mixed gas of two or more kinds. The heat treatment time is preferably 0.1 to 2 hours, more preferably 0.5 to 1.5 hours.

The average particle size (diameter) of the platinum-containing catalyst metal in the electrode catalyst precursor is not particularly limited, but is, for example, 1 to 30 nm, preferably 2 nm or more and less than 10 nm, and more preferably more than 3 nm and less than 10 nm. .. By setting the average particle size (diameter) of the platinum-containing catalyst metal in the above range, the platinum-containing catalyst metal can be appropriately embedded in the carbon carrier without requiring a complicated process while ensuring the specific surface area of the catalyst metal. .. The average particle size (diameter) of the platinum-containing catalyst metal is a value measured as follows. First, n particles are observed with a transmission electron microscope (TEM), and the particle diameter (equivalent circle diameter) when the area is a perfect circle is calculated back from the projected area of each particle, and the particles of each particle are calculated. The diameter (d (nm)) is measured. Using the particle size (d (nm)) of the particles thus obtained, the average particle size (nm) of the particles is calculated by the following formula (A). The number of particles (n) measured is not particularly limited, but is preferably a number that does not have a statistically significant difference, and is, for example, at least 300.

From the viewpoint of further improving the catalytic activity, the weight ratio of the catalyst metal to the total weight of the catalyst metal and the carbon carrier (supporting ratio of the catalyst metal, metal conversion) is preferably 5 to 45 weight in the electrode catalyst precursor. %. More preferably, in the electrode catalyst precursor, the loading ratio (metal equivalent) of the catalyst metal is 10 to 35% by weight. When the amount of the catalyst metal supported is a value within such a range, the balance between the dispersity of the catalyst component on the carbon carrier and the catalyst performance can be appropriately controlled. The amount of the catalyst metal carried can be examined by a conventionally known method such as inductively coupled plasma emission spectrometry (ICP atomic emission spectrometry), inductively coupled plasma mass spectrometry (ICP mass spectrum spectrum), and fluorescent X-ray analysis (XRF). it can.

In the method according to the present invention, the electrode catalyst precursor prepared as described above is heated (heat-treated) in an oxygen-containing atmosphere. This reduces the weight of the electrode catalyst precursor. In one embodiment of the present invention, the heating is performed so that the weight of the electrode catalyst precursor is reduced by more than 0% and less than 10%. In another embodiment of the present invention, heating is performed at a temperature satisfying the above relational expression 3. By heating the electrode catalyst precursor under the above conditions, oxidative decomposition of the carbon carrier proceeds in the portion in contact with the platinum-containing catalyst metal, and the catalyst metal is appropriately embedded in the carbon carrier. Can be obtained. Thereby, a highly active electrode catalyst can be obtained.

In the method according to the present invention, the heat treatment of the electrode catalyst precursor is performed in an oxygen (molecular oxygen) -containing atmosphere. By performing the heat treatment in an oxygen (molecular oxygen) -containing atmosphere, oxidative decomposition proceeds on the surface of the carbon carrier in contact with the platinum-containing catalyst metal. The heat treatment may be performed in an atmosphere containing a gas other than oxygen such as helium, argon, and nitrogen in addition to oxygen. From the viewpoint of promoting the oxidative decomposition reaction, the oxygen content of the atmosphere in the heat treatment is, for example, 1 to 80% by volume, preferably more than 5% by volume and 50% by volume or less, and more preferably more than 10% by volume. 40% by volume or less. In one preferred embodiment, the heating is performed in an air atmosphere.

The heat treatment may be carried out by using a conventionally known device and is not particularly limited, but may be carried out by using, for example, an oven, a furnace, a low temperature dryer or the like.

In one embodiment, the rate of weight loss of the electrode catalyst precursor by heat treatment is more than 0% and less than 10%. Preferably, the weight of the electrode catalyst precursor is reduced by 6% or more and less than 10% in heating. More preferably, the weight of the electrode catalyst precursor is reduced by more than 6% and less than 10% by heating. A particularly highly active electrode catalyst can be obtained by performing the heat treatment at the rate of weight reduction as described above. In the present specification, the rate of weight loss (weight loss rate) of the electrode catalyst precursor is a value calculated by the method described in the examples.

In one embodiment, the heat treatment of the electrode catalyst precursor is carried out below _{the combustion temperature (T carbon) of the carbon carrier.} This makes it possible to prevent the carbon carrier from disappearing due to combustion. That is, in one embodiment of the present invention, the temperature during heating is lower than the combustion temperature of the carbon carrier. The combustion temperature of the carbon carrier can be determined as the exothermic peak temperature of the differential thermal analysis (DTA) by performing thermogravimetric analysis of the carbon carrier in an atmosphere where heat treatment is performed, as described in Examples. .. From the viewpoint of the activity of the electrode catalyst, in one preferred embodiment, the heating is performed at a temperature satisfying the following relational expression 2.

However, in the above equation _{2, T heat} (℃) represents the temperature in the _{heating; T carbon} (℃) represents the combustion temperature of the carbon support.

In one embodiment, from the viewpoint of the activity of the obtained electrode catalyst, the electrode catalyst precursor is heat-treated at a temperature equal to or higher than the lower limit temperature, with a temperature obtained by subtracting 150 ° C. from _{the combustion temperature (T carbon) of the carbon carrier as the lower limit.} Do. That is, in the embodiment, the heating is performed at a temperature satisfying the following relational expression 1.

However, in the above equation _{1, T heat} (℃) represents the temperature in the _{heating; T carbon} (℃) represents the combustion temperature of the carbon support.

More preferably, the electrode catalyst precursor is heat-treated at a temperature equal to or higher than the lower limit temperature, with the temperature obtained by subtracting 130 ° C. from the combustion temperature (T _{carbon) of the carbon carrier as the lower limit.} That is, in the embodiment, the heating is performed at a temperature satisfying the following relational expression 1a:

However, in the above relational expression 1a, The _heat (° C.) and T _carbon (° C.) are the same as those in the above relational expression 1.

In one embodiment, heating is performed at a temperature that satisfies the above relational expression 3. In a more preferable embodiment, the electrode catalyst precursor is heated at a temperature satisfying the following relational expression 3a from the viewpoint that a particularly highly active electrode catalyst can be obtained.

However, in the above relational expression 3a, _Heat (° C.) and T _carbon (° C.) are the same as those in the above relational expression 3.

In a more preferred embodiment, the electrode catalyst precursor is heated at a temperature satisfying the following relational expression 3b.

However, in the above relational expression 3b, _Heat (° C.) and T _carbon (° C.) are the same as those in the above relational expression 3.

For example, when Vulcan® is used as the carbon carrier, the electrode catalyst precursor may be heated at a temperature in the range of 200 to 320 ° C., and in one preferred embodiment, in the range of 280 to 320 ° C. Perform at the temperature inside.

The pressure may be reduced or pressurized during the heat treatment, but the atmospheric pressure during the heat treatment may be, for example, in the range of 0.05 to 0.5 MPa, preferably atmospheric pressure.

The required heating time at the above heating temperature is not particularly limited, but may be determined, for example, by determining the time required for the electrode catalyst precursor weight to become a steady state by thermogravimetric analysis. More specifically, the heat treatment time is, for example, more than 0.5 hours and less than 24 hours, preferably 1 to 7 hours, and more preferably more than 1 hour and less than 5 hours. By setting the heat treatment time within the above range, it is possible to reduce the weight of the electrode catalyst precursor at a desired reduction rate and suppress an increase in production cost due to a long heating time.

The rate of temperature rise during the heat treatment is also not particularly limited, but is preferably 5 to 30 ° C./min. By setting the temperature rise rate to 5 ° C./min or more, temperature control becomes easy, and side reactions due to overshoot (a phenomenon in which the temperature temporarily becomes higher than the target value) can be prevented. By setting the heating rate to 30 ° C./min or less, there is an advantage that side reactions can be prevented and the heat treatment step can be prevented from being excessively lengthened. From the above viewpoint, the rate of temperature rise during the heat treatment is more preferably 10 to 20 ° C./min.

In the electrode catalyst produced by the method of the present invention, the catalyst metal is highly dispersed on the carbon carrier. Further, when it is used for a polymer electrolyte fuel cell (PEFC), it is possible to prevent the polymer electrolyte from coming into contact with the catalyst metal, and it is possible to secure an electrochemically effective surface area of the catalyst metal. Therefore, the electrode catalyst obtained by the production method according to the present invention can be suitably applied to fuel cell applications that require higher performance, such as power sources for home use and mobile body driving. That is, the membrane electrode assembly and the fuel cell having the electrode catalyst produced by the method of the present invention in the catalyst layer are excellent in power generation performance.

That is, one embodiment of the present invention provides a membrane electrode assembly (MEA) and a fuel cell comprising a catalyst layer containing an electrode catalyst produced by the method of the present invention. Hereinafter, a membrane electrode assembly (MEA) and a fuel cell including a catalyst layer containing an electrode catalyst according to an embodiment of the present invention, and a method for producing these will be described.

[Electrolyte Membrane-Electrode Assembly (MEA)]
The electrode catalyst produced by the method of the present invention can be suitably used for an electrolyte membrane-electrode assembly (MEA). That is, the present invention also provides an electrolyte membrane-electrode assembly (MEA) containing the electrode catalyst according to the present invention, particularly an electrolyte membrane-electrode assembly (MEA) for a fuel cell. The electrolyte membrane-electrode assembly (MEA) according to the present invention can exhibit high power generation performance (particularly area specific activity).

The electrolyte membrane-electrode assembly (MEA) of the present invention can be applied with the same configuration except that the electrode catalyst (catalyst) obtained by the production method according to the present invention is used instead of the conventional electrode catalyst. Hereinafter, preferred forms of MEA of the present invention will be described, but the present invention is not limited to the following forms.

The MEA is composed of an electrolyte membrane, an anode catalyst layer and an anode gas diffusion layer, and a cathode catalyst layer and a cathode gas diffusion layer, which are sequentially formed on both surfaces of the electrolyte membrane. Then, in this electrolyte membrane-electrode assembly, the electrode catalyst of the present invention is used for at least one of the cathode catalyst layer and the anode catalyst layer.

(Electrolyte membrane)
The electrolyte membrane is composed of, for example, a solid polymer electrolyte membrane. This solid polymer electrolyte membrane has, for example, a function of selectively permeating protons generated in the anode catalyst layer during operation of a fuel cell (PEFC or the like) into the cathode catalyst layer along the film thickness direction. Further, the solid polymer electrolyte membrane also has a function as a partition wall for preventing the fuel gas supplied to the anode side and the oxidant gas supplied to the cathode side from being mixed.

The electrolyte material constituting the solid polymer electrolyte membrane is not particularly limited, and conventionally known findings can be appropriately referred to. For example, the fluorine-based polymer electrolyte and the hydrocarbon-based polymer electrolyte described as the polymer electrolyte in the following catalyst layer can be used in the same manner. At this time, it is not always necessary to use the same polymer electrolyte as that used for the catalyst layer.

The thickness of the electrolyte membrane may be appropriately determined in consideration of the characteristics of the obtained fuel cell, and is not particularly limited. The thickness of the electrolyte membrane is usually about 5 to 300 μm. When the thickness of the electrolyte membrane is a value within such a range, the balance between the strength during film formation, the durability during use, and the output characteristics during use can be appropriately controlled.

(Catalyst layer)
The catalyst layer is a layer in which the battery reaction actually proceeds. Specifically, the oxidation reaction of hydrogen proceeds in the anode catalyst layer, and the reduction reaction of oxygen proceeds in the cathode catalyst layer. Here, the electrode catalyst of the present invention may be present in either the cathode catalyst layer or the anode catalyst layer. Considering the necessity of improving the oxygen reduction activity, it is preferable to use the electrode catalyst of the present invention at least in the cathode catalyst layer. However, the catalyst layer according to the above embodiment is not particularly limited, for example, it may be used as an anode catalyst layer, or may be used as both a cathode catalyst layer and an anode catalyst layer.

The catalyst layer contains the electrode catalyst and electrolyte according to the present invention. The electrolyte is not particularly limited, but is preferably an ionic conductive polymer electrolyte. The polymer electrolyte is also called a proton conductive polymer because it plays a role of transmitting protons generated around the catalytically active material on the fuel electrode side.

The polymer electrolyte is not particularly limited, and conventionally known findings can be appropriately referred to. Polymer electrolytes are roughly classified into fluorine-based polymer electrolytes and hydrocarbon-based polymer electrolytes according to the type of ion exchange resin as a constituent material.

Examples of the ion exchange resin constituting the fluoropolymer electrolyte include Nafion (registered trademark, manufactured by DuPont), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.) and the like. Perfluorocarbon sulfonic acid polymer, perfluorocarbon phosphonic acid polymer, trifluorostyrene sulfonic acid polymer, ethylene tetrafluoroethylene-g-styrene sulfonic acid polymer, ethylene-tetrafluoroethylene copolymer, polyvinylidene fluoride-per Fluorocarbon sulfonic acid-based polymers and the like can be mentioned. From the viewpoint of excellent heat resistance, chemical stability, durability, and mechanical strength, these fluorine-based polymer electrolytes are preferably used, and particularly preferably, a fluorine-based polymer electrolyte composed of a perfluorocarbon sulfonic acid-based polymer. Is used.

Specific examples of the hydrocarbon-based electrolyte include sulfonated polyether sulfone (S-PES), sulfonated polyaryletherketone, sulfonated polybenzimidazole alkyl, phosphorylated polybenzimidazole alkyl, sulfonated polystyrene, and sulfonated poly. Ether Etherketone (S-PEEK), sulfonated polyphenylene (S-PPP) and the like can be mentioned. These hydrocarbon-based polymer electrolytes are preferably used from the viewpoint of manufacturing, such as low raw materials, simple manufacturing process, and high material selectivity. As for the above-mentioned ion exchange resin, only one type may be used alone, or two or more types may be used in combination. Further, the material is not limited to the above-mentioned materials, and other materials may be used.

Proton conductivity is important in polymer electrolytes that are responsible for proton transfer. Here, if the EW of the polymer electrolyte is too large, the ionic conductivity of the entire catalyst layer decreases. Therefore, the catalyst layer of this embodiment preferably contains a polymer electrolyte having a small EW. Specifically, the catalyst layer of this embodiment preferably has an EW of 1500 g / eq. It contains the following polymeric electrolytes, more preferably 1200 g / eq. It contains the following polymeric electrolytes, particularly preferably 1000 g / eq. It contains the following polymer electrolytes. On the other hand, if the EW is too small, the hydrophilicity is too high and it becomes difficult for water to move smoothly. From such a viewpoint, the EW of the polymer electrolyte is preferably 600 or more. In addition, EW (Equivalent Weight) represents the equivalent weight of the exchange group having proton conductivity. The equivalent weight is the dry weight of the ion exchange membrane per equivalent of the ion exchange group, and is expressed in units of "g / eq".

Further, the catalyst layer contains two or more kinds of polymer electrolytes having different EWs in the power generation surface, and at this time, the polymer electrolyte having the lowest EW among the polymer electrolytes has a relative humidity of 90% or less of the gas in the flow path. It is preferable to use it in the area of. By adopting such a material arrangement, the resistance value becomes small regardless of the current density region, and the battery performance can be improved. The EW of the polymer electrolyte used in the region where the relative humidity of the gas in the flow path is 90% or less, that is, the polymer electrolyte having the lowest EW, is 900 g / eq. It is desirable that it is as follows. As a result, the above-mentioned effect becomes more certain and remarkable.

Further, it is desirable to use the polymer electrolyte having the lowest EW in a region higher than the average temperature of the inlet and outlet of the cooling water. As a result, the resistance value becomes small regardless of the current density region, and the battery performance can be further improved.

Furthermore, from the viewpoint of reducing the resistance value of the fuel cell system, the polymer electrolyte having the lowest EW is within 3/5 of the gas supply port of at least one of the fuel gas and the oxidant gas with respect to the flow path length. It is desirable to use it in the area of the range.

The catalyst layer may contain, if necessary, a water repellent such as polytetrafluoroethylene, polyhexafluoropropylene, or tetrafluoroethylene-hexafluoropropylene copolymer, a dispersant such as a surfactant, glycerin, or ethylene glycol (EG). ), Polyvinyl alcohol (PVA), propylene glycol (PG) and other thickeners, pore-forming agents and other additives may be included.

The film thickness (dry film thickness) of the catalyst layer is preferably 0.05 to 30 μm, more preferably 1 to 20 μm, and further preferably 2 to 15 μm. The above applies to both the cathode catalyst layer and the anode catalyst layer. However, the cathode catalyst layer and the anode catalyst layer may be the same or different.

(Gas diffusion layer)
The gas diffusion layer (anode gas diffusion layer, cathode gas diffusion layer) has a function of promoting diffusion of gas (fuel gas or oxidant gas) supplied through the gas flow path of the separator into the catalyst layer, and electron conduction. It has a function as a path.

The material constituting the base material of the gas diffusion layer is not particularly limited, and conventionally known findings can be appropriately referred to. Examples thereof include conductive and porous sheet-like materials such as carbon woven fabrics, paper-like paper machines, felts, and non-woven fabrics. The thickness of the base material may be appropriately determined in consideration of the characteristics of the obtained gas diffusion layer, but may be about 30 to 500 μm. If the thickness of the base material is within such a range, the balance between the mechanical strength and the diffusivity of gas, water, etc. can be appropriately controlled.

The gas diffusion layer preferably contains a water repellent for the purpose of further enhancing water repellency and preventing a flooding phenomenon or the like. The water repellent is not particularly limited, but has a high fluorine content such as polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyhexafluoropropylene, and tetrafluoroethylene-hexafluoropropylene copolymer (FEP). Molecular materials, polypropylene, polyethylene and the like can be mentioned.

Further, in order to further improve the water repellency, the gas diffusion layer has a carbon particle layer (microporous layer; MPL, not shown) made of an aggregate of carbon particles containing a water repellent agent on the catalyst layer side of the base material. It may have.

The carbon particles contained in the carbon particle layer are not particularly limited, and conventionally known materials such as carbon black, graphite, and expanded graphite can be appropriately adopted. Among them, carbon blacks such as oil furnace black, channel black, lamp black, thermal black, and acetylene black can be preferably used because of their excellent electron conductivity and large specific surface area. The average particle size of the carbon particles is preferably about 10 to 100 nm. As a result, high drainage can be obtained due to the capillary force, and the contact with the catalyst layer can be improved.

Examples of the water repellent used for the carbon particle layer include the same water repellents as those described above. Among them, a fluorine-based polymer material can be preferably used because it is excellent in water repellency, corrosion resistance during electrode reaction, and the like.

The mixing ratio of the carbon particles and the water repellent in the carbon particle layer is about 90: 10 to 40:60 (carbon particles: water repellent) in consideration of the balance between water repellency and electron conductivity. Is good. The thickness of the carbon particle layer is also not particularly limited, and may be appropriately determined in consideration of the water repellency of the obtained gas diffusion layer.

(Manufacturing method of electrolyte membrane-electrode assembly)
The method for producing the electrolyte membrane-electrode assembly is not particularly limited, and a conventionally known method can be used. For example, a method of transferring or applying a catalyst layer to an electrolyte membrane by hot pressing and then joining a gas diffusion layer to the dried material, or a method of joining a gas diffusion layer to the microporous layer side of the gas diffusion layer (when the microporous layer is not included). , One side of the base material layer) is coated in advance and dried to prepare two gas diffusion electrodes (GDE), and the gas diffusion electrodes are bonded to both sides of the solid polymer electrolyte membrane by hot pressing. Can be used. The coating and bonding conditions of hot press and the like may be appropriately adjusted depending on the type of polymer electrolyte (perfluorosulfonic acid type or hydrocarbon type) in the solid polymer electrolyte membrane or the catalyst layer.

[Fuel cell]
The electrolyte membrane-electrode assembly (MEA) described above can be suitably used for a fuel cell. That is, the present invention also provides a fuel cell using the electrolyte membrane-electrode assembly (MEA) of the present invention. The fuel cell of the present invention can exhibit high power generation performance (particularly mass ratio activity) and durability.

Here, the fuel cell includes a membrane electrode assembly (electrolyte membrane-electrode assembly, MEA), an anode-side separator having a fuel gas flow path through which the fuel gas flows, and a cathode having an oxidant gas flow path through which the oxidant gas flows. It has a pair of separators including a side separator. The fuel cell of the present invention has excellent durability and can exhibit high power generation performance.

Hereinafter, an embodiment of a membrane electrode assembly (MEA) having a catalyst layer using the electrode catalyst according to the present invention and a fuel cell will be described in detail with reference to the drawings as appropriate. However, the present invention is not limited to the following embodiments. It should be noted that each drawing is exaggerated for convenience of explanation, and the dimensional ratio of each component in each drawing may differ from the actual one. Further, when the embodiment of the present invention is described with reference to the drawings, the same elements are designated by the same reference numerals in the description of the drawings, and duplicate description will be omitted.

FIG. 1 is a schematic view showing a basic configuration of a polymer electrolyte fuel cell (PEFC) 1 according to an embodiment of the present invention. The PEFC 1 first has a solid polymer electrolyte membrane 2 and a pair of catalyst layers (anode catalyst layer 3a and cathode catalyst layer 3c) sandwiching the solid polymer electrolyte membrane 2. The laminate of the solid polymer electrolyte membrane 2 and the catalyst layers (3a, 3c) is further sandwiched by a pair of gas diffusion layers (GDL) (anode gas diffusion layer 4a and cathode gas diffusion layer 4c). As described above, the solid polymer electrolyte membrane 2, the pair of catalyst layers (3a, 3c) and the pair of gas diffusion layers (4a, 4c) form the electrolyte membrane-electrode assembly (MEA) 10 in a laminated state. To do.

In PEFC1, the MEA10 is further sandwiched by a pair of separators (anode separator 5a and cathode separator 5c). In FIG. 1, separators (5a, 5c) are shown so as to be located at both ends of the illustrated MEA10. However, in a fuel cell stack in which a plurality of MEAs are stacked, the separator is generally used as a separator for adjacent PEFCs (not shown). In other words, in the fuel cell stack, the MEAs are sequentially stacked via the separator to form the stack. In the actual fuel cell stack, the gas seal portion is arranged between the separator (5a, 5c) and the solid polymer electrolyte membrane 2 or between the PEFC 1 and another PEFC adjacent thereto. In FIG. 1, these descriptions are omitted.

The separators (5a, 5c) can be obtained, for example, by pressing a thin plate having a thickness of 0.5 mm or less to form an uneven shape as shown in FIG. The convex portion of the separator (5a, 5c) seen from the MEA side is in contact with the MEA10. This ensures an electrical connection with the MEA 10. Further, the recesses (the space between the separator and the MEA caused by the uneven shape of the separator) seen from the MEA side of the separators (5a, 5c) are gas for flowing gas during the operation of PEFC1. Functions as a flow path. Specifically, a fuel gas (for example, hydrogen) is circulated in the gas flow path 6a of the anode separator 5a, and an oxidant gas (for example, air) is circulated in the gas flow path 6c of the cathode separator 5c.

On the other hand, the recesses of the separators (5a, 5c) viewed from the side opposite to the MEA side are used as a refrigerant flow path 7 for flowing a refrigerant (for example, water) for cooling the PEFC during the operation of the PEFC 1. .. Further, the separator is usually provided with a manifold (not shown). This manifold functions as a connecting means for connecting the cells when the stack is constructed. With such a configuration, the mechanical strength of the fuel cell stack can be ensured.

In the embodiment shown in FIG. 1, the separators (5a, 5c) are formed into an uneven shape. However, the separator is not limited to such an uneven shape, and may be any shape such as a flat plate shape or a partially uneven shape as long as the functions of the gas flow path and the refrigerant flow path can be exhibited. May be good.

(Separator)
The separator has a function of electrically connecting each cell in series when a plurality of single cells of a fuel cell such as a polymer electrolyte fuel cell are connected in series to form a fuel cell stack. The separator also functions as a partition wall for separating the fuel gas, the oxidant gas, and the coolant from each other. In order to secure these flow paths, it is preferable that each of the separators is provided with a gas flow path and a cooling flow path, as described above. As the material constituting the separator, conventionally known materials such as dense carbon graphite, carbon such as carbon plate, and metal such as stainless steel can be appropriately adopted without limitation. The thickness and size of the separator, the shape and size of each flow path provided are not particularly limited, and can be appropriately determined in consideration of the desired output characteristics of the obtained fuel cell and the like.

The method for manufacturing the fuel cell is not particularly limited, and conventionally known knowledge in the field of the fuel cell can be appropriately referred to.

Further, a fuel cell stack having a structure in which a plurality of electrolyte membrane-electrode assemblies are laminated and connected in series may be formed so that the fuel cell can exert a desired voltage. The shape of the fuel cell and the like are not particularly limited, and may be appropriately determined so as to obtain battery characteristics such as a desired voltage.

The PEFC and the electrolyte membrane-electrode assembly described above use a catalyst layer having excellent power generation performance and durability. Therefore, the PEFC and the electrolyte membrane-electrode assembly are excellent in power generation performance and durability.

The PEFC of the present embodiment and the fuel cell stack using the PEFC can be mounted on a vehicle as a driving power source, for example.

The fuel cell as described above exhibits excellent power generation performance. Here, the type of the fuel cell is not particularly limited, and in the above description, the polymer electrolyte fuel cell has been described as an example, but in addition to this, an alkaline fuel cell and a direct methanol fuel cell have been described. , Micro fuel cells and the like. Among them, a polymer electrolyte fuel cell (PEFC) is preferable because it is compact, has high density, and can have high output. Further, the fuel cell is useful as a power source for a mobile body such as a vehicle whose mounting space is limited, as well as a power source for stationary use. Above all, it is particularly preferable to use it as a power source for a mobile body such as an automobile, which requires a high output voltage after a relatively long operation stop.

The fuel used when operating the fuel cell is not particularly limited. For example, hydrogen, methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, secondary butanol, tertiary butanol, dimethyl ether, diethyl ether, ethylene glycol, diethylene glycol and the like can be used. Of these, hydrogen and methanol are preferably used because they can increase the output.

The application of the fuel cell is not particularly limited, but it is preferably applied to the vehicle. The electrolyte membrane-electrode assembly of the present invention is excellent in power generation performance and durability, and can be miniaturized. Therefore, the fuel cell of the present invention is particularly advantageous when the fuel cell is applied to a vehicle from the viewpoint of in-vehicle performance.

The effects of the present invention will be described with reference to the following examples and comparative examples. However, the technical scope of the present invention is not limited to the following examples.

(Preparation of electrode catalyst precursor)
Using Vulcan® XC72 (manufactured by Cabot, BET specific surface area: 220 (m ² / g carrier), primary average particle size: 37 nm) as a carbon carrier, platinum having an average particle size (diameter) of 3.4 nm was used. An electrode catalyst precursor was obtained by supporting the product so as to have a loading ratio of 10% by weight. That is, 46 g of a carbon carrier is immersed in 1000 g (platinum content: 46 g) of a dinitrodiammine platinum nitric acid aqueous solution (platinum precursor-containing liquid) having a platinum concentration of 4.6% by mass, and after stirring, 100 ml of 100% ethanol is added as a reducing agent. did. The solution was stirred and mixed at boiling point for 7 hours to support platinum on a carbon carrier. Then, it was filtered and dried to obtain a powder having a loading ratio of 10% by weight. Then, in a hydrogen atmosphere, the temperature was maintained at 900 ° C. for 1 hour to perform an annealing treatment to obtain an electrode catalyst precursor.

The combustion temperature of the carbon carrier (exothermic peak temperature in DTA, T _carbon ) was determined by thermogravimetric analysis and found to be 387 ° C. The thermogravimetric analysis was performed under the following conditions.
Method TG-DTA
Temperature rise rate 5 ° C / min Gas atmosphere Air measurement The sample was placed in a platinum pan and measured.

(Example 1)
Using the above-mentioned electrode catalyst precursor, heat treatment was performed in an electric furnace at 300 ° C. for 2 hours in an air atmosphere (atmospheric pressure), and air cooling was performed to obtain electrode catalyst 1 (heating rate: 10 ° C./). Minutes). An electron microscope image of the electrode catalyst 1 is shown in FIG. 2A. The weight loss rate before and after heating was 8%. The weight loss rate before and after heating was calculated by the following formula.

(Example 2)
The electrode catalyst precursor was heat-treated in the same manner as in Example 1 except that the heating temperature in Example 1 was changed to 250 ° C. (heating rate: 10 ° C./min) to obtain an electrode catalyst 2. The weight loss rate before and after heating was 6%.

(Comparative Example 1)
The above-mentioned electrode catalyst precursor that had been stored at 25 ° C. without heat treatment was used as the comparative electrode catalyst 1. An electron microscope image of the comparative electrode catalyst 1 is shown in FIG. 2B. Compared with FIG. 2A, in FIG. 2B, it can be seen that the white spot-shaped catalytic metal particles are exposed on the carbon carrier.

(Comparative Example 2)
The electrode catalyst precursor was heat-treated in the same manner as in Example 1 except that the heating temperature in Example 1 was changed to 330 ° C. (heating rate: 10 ° C./min) to obtain a comparative electrode catalyst 2. The weight loss rate before and after heating was 10%.

^{The specific surface area (m 2} / g Pt) and area specific activity (μA / cm ² Pt) of the electrode catalysts 1 and 2 and the comparative electrode catalysts 1 and 2 obtained above were measured according to the following methods. The results are shown in the table below.

[Measurement of electrochemical surface area (cm ² Pt) of platinum]
The electrode catalysts of Examples and Comparative Examples are placed on a rotating disk electrode (RDE, geometric area: 0.19 cm ² ) composed of a glassy carbon disk having a diameter of 5 mm, respectively, and the amount of platinum carried per unit area of the rotating disk electrode. There so that the 34 [mu] g / cm ^2, with Nafion® (Nafion (registered trademark) coating amount = 10 [mu] g / cm ^2), was uniformly applied to prepare a RDE electrode performance evaluation.

The performance evaluation RDE electrodes of Examples and Comparative Examples, using an electrochemical measuring apparatus, in N ₂ gas at 25 ° C. in 0.1M perchloric acid saturated with respect to the reversible hydrogen electrode (RHE) Cyclic voltammetry was performed at a scanning rate of ^{50 mVs -1} over a potential range of 0.05 to 1.2 V. ^{The electrochemical surface area (cm 2} Pt) of platinum in each electrode catalyst was calculated from the area of the hydrogen adsorption peak appearing at 0.05 to 0.4 V of the obtained voltammogram.

[Measurement of area specific activity]
For the performance evaluation RDE electrodes of Examples and Comparative Examples, a speed of 10 mV / s from 0.2 V to 1.2 V in 0.1 M perchloric acid at 25 ° C. saturated with oxygen using an electrochemical measuring device. The potential was scanned at. Further, from the current obtained by the potential scanning, the influence of mass transfer (oxygen diffusion) was corrected using the Koutecky-Levich equation, and then the current value at 0.9 V was extracted. Then, the value obtained by dividing the obtained current value by the electrochemical surface area (cm ² Pt) of the platinum was taken as the area specific activity (μA / cm ² Pt). The method using the Koutecky-Levich equation is described in, for example, Electrochemistry Vol. 79, No. 2, p. 116-121 (2011) (Convection Voltamogram (1) Oxygen Reduction (RRDE)), "4 Analysis of Oxygen Reduction Reaction on Pt / C Catalyst".

As shown in the above table, it can be seen that the electrode catalyst obtained by the production method according to the present invention has high catalytic activity (area specific activity). The measured value of the electrochemical surface area of the comparative electrode catalyst 2 was not stable, and the catalytic activity could not be measured. It is considered that this is because the catalyst metal is excessively embedded in the carbon carrier in the comparative electrode catalyst 2.

1 ... Polymer electrolyte fuel cell (PEFC),
2 ... Solid polymer electrolyte membrane,
3 ... catalyst layer,
3a ... Anode catalyst layer,
3c ... Cathode catalyst layer,
4a ... Anode gas diffusion layer,
4c ... Cathode gas diffusion layer,
5a ... Anode separator,
5c ... Cathode separator,
6a ... Anode gas flow path,
6c ... Cathode gas flow path,
7 ... Refrigerant flow path,
10 ... Electrolyte membrane-electrode assembly (MEA).

Claims (2)

An electrode catalyst precursor in which a platinum-containing catalyst metal is supported on a carbon carrier is prepared, and the electrode catalyst precursor is heated in an oxygen-containing atmosphere to reduce the weight of the electrode catalyst precursor by 6% or more and 10%. A method for producing an electrode catalyst, which comprises reducing at a rate of less than. The method for producing an electrode catalyst according to claim 1, wherein the weight of the electrode catalyst precursor is reduced by more than 6% and less than 10% in the heating.

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