JP5560743B2 - Electrode catalyst, membrane electrode assembly and fuel cell - Google Patents

Description

  The present invention relates to an electrode catalyst for a fuel cell, a membrane electrode assembly including the electrode catalyst, and a fuel cell.

  A fuel cell of a polymer electrolyte fuel cell includes a membrane electrode assembly (MEA: an ion permeable electrolyte membrane) and electrode catalyst layers (electrode catalysts) on the anode side and the cathode side that sandwich the electrolyte membrane. Membrane Electrode Assembly), gas diffusion layers (GDL) for promoting gas flow and increasing current collection efficiency are provided outside each electrode catalyst to form electrode bodies (MEGA: MEA and GDL assembly) A separator is arranged outside the gas diffusion layer to form a fuel cell. Actually, these fuel cells are stacked in the number corresponding to the power generation performance to form a fuel cell stack.

  In the fuel cell described above, hydrogen gas or the like is provided as fuel gas to the anode electrode, oxygen or air is provided as oxidant gas to the cathode electrode, and each electrode has its own gas channel layer or gas channel groove of the separator. The gas flows in the in-plane direction, and then the gas diffused in the gas diffusion layer is guided to the electrode catalyst to cause an electrochemical reaction. In this electrochemical reaction, hydrogen ions and water generated at the anode electrode pass through the electrolyte membrane in a hydrated state to reach the cathode electrode, and generated water is generated at the cathode electrode. Therefore, depending on the movement mode of water in the membrane electrode assembly and the generation mode of water generated by electrochemical reaction, the anode electrode is easily dried with the progress of power generation, and in some cases, it is dry up, while the cathode electrode is likely to be excessively watery. In some cases, there is a problem that it is easy to lead to flatting. In the case of dry-up, since the hydrogen gas is dry, the proton conductivity of the electrolyte membrane, which is an ion exchange membrane, decreases, and the power generation performance of the fuel cell decreases. Since water stays in the cathode side gas diffusion layer and gas flow path layer (or the gas flow path groove of the separator), the flow of the oxidant gas is hindered, and sufficient oxidant gas is not provided to the membrane electrode assembly. In addition, the power generation performance of the fuel cell decreases.

  In the fuel cell described above, both the oxidant gas provided on the cathode side and the fuel gas provided on the anode side are provided in the fuel cell while being humidified by the humidification module. However, the presence of the humidification module increases the overall size of the fuel cell system including the fuel cell and the humidification module, and further increases the weight of the system. Development of fuel cells that can be self-humidified and can be reduced in weight has been underway. Here, the “self-humidification” means that the generated water generated on the cathode side is back-diffused to the anode side, and the accompanying water is moved to the cathode side along with the proton movement from the cathode side. It is intended for water circulation.

  Thus, in order to improve the power generation performance of the fuel cell and further realize its self-humidification, it is extremely important to further improve the performance of the electrode catalyst (catalyst layer) constituting the membrane electrode assembly, Among them, one of the important catalyst performances for self-humidification is the water retention of the electrode catalyst, and improving the water retention leads to an improvement in proton conductivity. For example, imparting a hydrophilic functional group to the catalyst-supporting carrier is one effective measure for improving water retention.

  As described above, while water retention of the electrode catalyst is important, if its water retention performance is too high, this may cause the flatting described above, and conversely lead to a decrease in the performance of the fuel cell.

  Therefore, in particular, when self-humidifying the fuel battery cell, it is necessary to adjust the water retention performance of the electrode catalyst within an appropriate range, and identifying this appropriate range is one of the important issues in the technical field. It has become.

  Here, to turn to the conventional published technology, an electrode catalyst in which the water vapor adsorption amount of the electrode powder forming the electrode catalyst (catalyst layer) is defined within a predetermined range, and a fuel cell including the same are disclosed in Patent Document 1. Is disclosed. In particular, this technique quantifies the hydrophilicity of the electrode catalyst directly and defines a quantitative evaluation index. However, the present inventors have determined the acidic functional group of the catalyst support (catalyst) and the electrode catalyst. Both the sulfonic acid groups of the polymer electrolyte (ionomer) to be formed are hydrophilic, and therefore the water retention of the electrode catalyst takes into account both the acidic functional groups of these catalyst-supported carriers and the sulfonic acid groups of the polymer electrolyte. More specifically, more specifically, as an index for determining the water retention performance of the electrode catalyst, the amount of acidic functional groups (or acid group density) of the catalyst-supported carrier and the amount of sulfonic acid groups of the polymer electrolyte It has been found that the interphase between (or sulfonic acid group density) is defined, and that the optimum water retention performance of the electrode catalyst can be ensured thereby, leading to the present invention.

JP 2009-26602 A

  The present invention has been made in view of the above problems, and is based on the phase between the acid group density of the catalyst-supporting carrier (or the catalyst that is a component of the catalyst support) and the sulfonic acid group density of the polymer electrolyte that forms the electrode catalyst. Thus, it is an object of the present invention to provide an electrode catalyst that guarantees proper water retention performance, a membrane electrode assembly and a fuel battery cell including the electrode catalyst, in order to realize a fuel cell capable of self-humidification.

  In order to achieve the above object, the electrode catalyst according to the present invention relates to a catalyst-supported carrier and a polymer electrolyte that form an electrode catalyst, wherein the catalyst-supported carrier has an acid group density of X and the polymer electrolyte has a sulfonic acid group density of Y. In this case, (1) Y = −0.285X + 0.99, (2) Y = −0.285X + 1.09, an electrode having an acid group density and a sulfonic acid group density within the range defined by the following two formulas It is a catalyst.

  As described above, the acidic functional group of the catalyst-supporting carrier (catalyst) and the sulfonic acid group of the polymer electrolyte (ionomer) forming the electrode catalyst are both hydrophilic, so that the water retention of the electrode catalyst is appropriate. In order to control the range, it is necessary to consider both the acidic functional group of the catalyst-supporting support and the sulfonic acid group of the polymer electrolyte. For example, when the acid group density of the catalyst-supporting carrier is increased, it is necessary to suppress the sulfonic acid group density of the polymer electrolyte to a certain amount or less.

  On the other hand, it is easy to understand that a certain amount of polymer electrolyte is required from the viewpoint of coating the catalyst-supporting carrier with the polymer electrolyte and ensuring good proton conductivity. Therefore, if a phase between the acid support density of the catalyst-supporting support and the sulfonic acid group density of the polymer electrolyte can be found within the optimum range of water retention of the electrode catalyst, both acid group densities and The density of the sulfonic acid group can be adjusted, and it is possible to ensure good water retention performance of the electrode catalyst.

  As a result of the verification by the present inventors based on this knowledge, when the acid group density of the catalyst-supporting carrier is X and the sulfonic acid group density of the polymer electrolyte is Y, it is within the range defined by the above two formulas. It has been specified that the good water retention performance of the electrode catalyst can be guaranteed by using the acid group density and the sulfonic acid group density.

  Here, a method for producing an electrode catalyst will be outlined. First, a catalyst supporting carrier and a polymer electrolyte adjusted to have acid group density and sulfonic acid group density within the range defined by the above two formulas are put into a dispersion solvent, and stirred to produce a catalyst solution (catalyst ink). To do. The produced catalyst solution is stretched into a layer on the electrolyte membrane as a base material by a coating blade, for example, to form a coating film, and dried in a hot press, a hot air drying furnace, etc. The electrode catalyst (catalyst layer) is formed on the anode side and the cathode side, and the membrane electrode assembly is obtained by forming the electrode catalyst of both electrodes.

  A fuel cell is formed by disposing a gas permeable layer (gas diffusion layer or gas flow path layer) on the anode side and cathode side of the membrane electrode assembly and further disposing a separator on both sides thereof. In the fuel cell comprising the above-described electrode catalyst of the present invention, the water retention performance of the electrode catalyst is controlled in the optimum range, and therefore, self-humidification is possible and there is no risk of fluttering. It is a fuel cell with extremely low risk. Therefore, in addition to improving the performance of the fuel cell itself, by enabling self-humidification, the humidification module can be eliminated from the fuel cell system and its overall weight can be significantly reduced. This is suitable for fuel cells for electric vehicles and hybrid vehicles that require higher performance and light weight for in-vehicle devices.

  As can be understood from the above description, according to the electrode catalyst of the present invention, good water retention performance is ensured by the defined phase difference between the acid group density of the catalyst-supporting carrier and the sulfonic acid group density of the polymer electrolyte, Therefore, the fuel cell is provided for a fuel cell that has excellent power generation performance and enables self-humidification.

FIG. 2 is a diagram simulating a catalyst-supporting carrier and a polymer electrolyte that form an electrode catalyst, in which (a) shows a case where the acid group density of the catalyst-supporting carrier is low, and (b) shows a catalyst-supporting carrier. It is the figure which showed the case where the acid group density of is high. FIG. 6 is a graph showing experimental results of measuring the power generation voltage value of a fuel cell equipped with an electrode catalyst obtained by changing the acid group density of the catalyst-supporting carrier and changing the sulfonic acid group density of the polymer electrolyte for each acid group density. is there. The acid group density and the sulfone, which are based on the experimental results shown in FIG. 2 and guarantee the optimum water retention performance between the acid support density of the catalyst-supporting carrier and the sulfonic acid group density of the polymer electrolyte and the electrode catalyst. It is the graph which prescribed | regulated the control range of the acid group density.

  Hereinafter, an embodiment of an electrode catalyst of the present invention will be described with reference to the drawings.

  FIG. 1 is a diagram simulating a catalyst supporting carrier and a polymer electrolyte forming an electrode catalyst, FIG. 1a is a diagram showing a case where the acid group density of the catalyst supporting carrier is low, and FIG. 1b is a diagram of the catalyst supporting carrier. It is the figure which showed the case where an acid group density is high.

  In the electrode catalyst constituting the membrane electrode assembly of the fuel cell, a polymer electrolyte 20 (ionomer) serving as a conduction path for protons and oxygen covers and connects a large number of catalyst-supporting carriers 10 to form a layer having a predetermined thickness. It is a thing. The catalyst-supporting carrier 10 is obtained by attaching a catalytic metal 2 (catalyst) to the surface of the conductive carrier 1, and further has an acidic functional group 11 formed on the surface of the conductive carrier 1.

  On the other hand, the polymer electrolyte 20 has a sulfonic acid group 21, and the acidic functional group 11 of the catalyst-supporting carrier 10 and the sulfonic acid group 21 of the polymer electrolyte 20 are both hydrophilic.

  Therefore, in order to control the water retention of the electrode catalyst within an appropriate range, it is necessary to consider both the acidic functional group 11 of the catalyst-supporting carrier 10 and the sulfonic acid group 21 of the polymer electrolyte 20. For example, as shown in FIG. 1a, when the amount of the acidic functional group 11 of the catalyst-supporting carrier 10, ie, the acid group density is low, the amount of the sulfonic acid 21 of the polymer electrolyte 20, ie, the sulfonic acid group density is increased. If the acid density of the catalyst-supporting carrier 10 is high as shown in FIG. 1b, the density of the sulfonic acid group of the polymer electrolyte 20 needs to be low. As described above, it is important to increase the water retention capacity for improving the performance of the electrocatalyst. On the other hand, the fact that the water retention performance is too high is based on the fact that it can be a factor of flatting. is there.

  As described above, the acid group density of the catalyst-supporting carrier 10 and the sulfonic acid group density of the polymer electrolyte 20 are controlled as desired, and a method for producing an electrode catalyst having these as its main components will be outlined below.

  First, the catalyst-supported carrier and the polymer electrolyte, in which the acid group density of the catalyst-supporting carrier and the sulfonic acid group density of the polymer electrolyte are previously adjusted as desired, are charged into the dispersion solvent accommodated in the prepared container. A catalyst solution (catalyst ink) is obtained by stirring using an ultrasonic homogenizer, a bead mill, a ball mill, or the like.

  Here, the polymer electrolyte forming the solution is an ion exchange resin having a skeleton of an organic fluorine-containing polymer, which is a proton conductive polymer, such as perfluorocarbon sulfonic acid resin, sulfonated polyether ketone, sulfonated polymer. Ether sulfone, sulfonated polyether ether sulfone, sulfonated polysulfone, sulfonated polysulfide, sulfonated plastic electrolytes such as sulfonated polyphenylene, sulfoalkylated polyetheretherketone, sulfoalkylated polyethersulfone, sulfoalkylated polyetherether Examples thereof include sulfoalkylated plastic electrolytes such as sulfone, sulfoalkylated polysulfone, sulfoalkylated polysulfide, and sulfoalkylated polyphenylene. Examples of commercially available materials include Nafion (registered trademark, manufactured by DuPont) and Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.).

  In addition, a PVD method such as vapor deposition, sputtering, or ion braiding, or a CVD method such as plasma CVD or thermal CVD is applied to attach a catalytic metal to the surface of the conductive carrier to obtain a catalyst-carrying carrier. Here, examples of the conductive support include carbon materials such as carbon black, carbon nanotubes, and carbon nanofibers, as well as carbon compounds typified by silicon carbide. Examples of the catalytic metal include platinum and Any one of platinum alloy, palladium, rhodium, gold, silver, osmium, iridium and the like can be used, and preferably platinum or platinum alloy is used. Further, examples of the platinum alloy include platinum, aluminum, chromium, manganese, iron, cobalt, nickel, gallium, zirconium, molybdenum, ruthenium, rhodium, palladium, vanadium, tungsten, rhenium, osmium, iridium, titanium, and lead. An alloy with at least one of them can be mentioned.

  Furthermore, as a dispersion solvent, in addition to water, alcohols such as methanol, ethanol, 1-propanol, 2-propanol, ethylene glycol, diethylene glycol, acetone, methyl ethyl ketone, dimethylformamide, dimethylimidazolidinone, dimethyl sulfoxide, dimethylacetamide, Examples include N-methylpyrrolidone, propylene carbonate, esters such as ethyl acetate and butyl acetate, and various aromatic or halogen solvents, and these may be used alone or as a mixture. it can.

  The produced catalyst solution is applied to any one of an electrolyte membrane, a gas diffusion layer, and a support film, which is a base material, and is dried on a hot air, hot pressed, etc., to form a catalyst layer (electrode catalyst) on the surface of the base material. ) Is formed. Here, this electrolyte membrane is, for example, a non-fluorine ion exchange membrane having a sulfonic acid group or a carbonyl group, a substituted phenylene oxide, a sulfonated polyaryletherketone, a sulfonated polyarylethersulfone, a sulfonated phenylenesulfide or the like. It is formed from a fluorine-based polymer or the like. The gas diffusion layer is formed from a fired body made of polyacrylonitrile, a fired body made of pitch, carbon materials such as graphite and expanded graphite, nanocarbon materials thereof, stainless steel, molybdenum, titanium, and the like. Furthermore, the support film is a polyethylene film, a polypropylene film, a polytetrafluoroethylene film, an ethylene / tetrafluoroethylene copolymer film, a tetrafluoroethylene / perfluoro (alkyl vinyl ether) copolymer film, a polyvinylidene fluoride film, a polyimide film. , Polyamide film, polyethylene terephthalate film, and the like. Two or more sheets made of these materials may be laminated to form a base material.

[Experiment and results of measuring the power generation voltage value of a fuel cell equipped with an electrode catalyst by changing the acid group density of the catalyst support and changing the sulfonic acid group density of the polymer electrolyte for each acid group density]
The inventors changed the acid group density of the catalyst-carrying support, changed the sulfonic acid group density of the polymer electrolyte for each acid group density, prepared a corresponding catalyst ink, and used each catalyst ink. Thus, an electrode catalyst and a membrane electrode assembly were prepared, and a gas diffusion layer was disposed on the membrane electrode assembly to prepare a fuel cell.

  More specifically, carbon black such as commercially available Ketjen EC (manufactured by Ketjen Black International) or Vulcan (Cabot) is used as the conductive carrier, and this is suspended and stirred in distilled water, and chloroplatinic acid ( A platinum compound) is added dropwise. To this, platinum is deposited on the carbon by dropping a reducing agent such as ethanol. The mixture was filtered, and the solid after the filtration was dried to obtain a catalyst-carrying carrier carrying a platinum catalyst.

  When the acid group density of this catalyst-supported carrier was measured by a back titration method, a catalyst-supported carrier of 0.3 mmol / g-C was obtained. The obtained catalyst-supported carrier was heated with 0.5N nitric acid under the following three conditions: 50 ° C for 0.5 hours, 80 ° C for 0.5 hours, and 80 ° C for 24 hours. The respective catalyst-supported carriers of 0.5 mmol / g-C, 0.7 mmol / g-C, and 1.0 mmol / g-C were obtained by processing. The “acid” here is mainly carboxylic acid (—COOH group).

  Distilled water is added to the resulting catalyst-supported carrier, and a dispersion solvent such as ethanol or 1-propanol is added. Furthermore, as a polymer electrolyte which is a proton conductor, a commercially available Nafion solution (manufactured by DuPont, EW1000) was further added to obtain a solution. Here, the amount of Nafion charged is 0.6, 0.75, 0.85, 0.9, 1.05 in terms of mass ratio with respect to the carbon black whose sulfonic acid group density is the conductive carrier. It adjusted so that it might become 1.2. Finally, the resulting mixture was sufficiently stirred, and dispersion treatment was performed by ultrasonic irradiation, bead milling, etc. to make the particles fine and uniform, thereby producing a catalyst solution (catalyst ink).

  The produced catalyst solution was applied onto a Teflon substrate with a doctor blade type applicator and vacuum dried at 100 ° C. to form an electrode catalyst (catalyst layer) on the substrate.

  The electrode catalyst formed on the surface of the Teflon substrate is placed on both sides of the electrolyte membrane made of Nafion 112, that is, on both the anode side and the cathode side, and hot-pressed at 130 ° C. to put the electrode catalyst on both sides of the electrode catalyst. The membrane electrode assembly was manufactured by bonding and removing the Teflon substrate.

  A gas diffusion layer composed of a carbon base material and a water repellent layer (consisting of carbon and PTFE) is disposed on both sides of the obtained membrane electrode assembly to produce a fuel cell, and hydrogen is applied to the anode side, Electricity was generated by providing air to the side, and the voltage value at each load current was measured to evaluate the performance of the fuel cell. The humidification condition is 40% RH for both the anode and cathode with respect to the cell temperature.

  In FIG. 2, the acid group density of the catalyst-supported carrier is 0.3 mmol / g-C, 0.5 mmol / g-C, 0.7 mmol / g-C, and 1.0 mmol / g-C, respectively. The measurement result of the voltage value of the fuel cell provided with the electrode catalyst formed by changing the sulfonic acid group density of the polymer electrolyte in the group density is shown.

  From the figure, it is proved that the graph corresponding to the fuel cell of each acid group density is a curve graph having a voltage peak.

  Using the graph of the figure, the present inventors further plotted the sulfonic acid group density at which a voltage peak is obtained against the acid group density, and the acid group density of the catalyst-supported carrier and the polymer electrolyte shown in FIG. An interphase graph of sulfonic acid group density was obtained.

In this figure, this interphase graph is defined by the following formula 1 when the acid group density of the catalyst support is X and the sulfonic acid group density of the polymer electrolyte is Y.
Y = −0.285X + 1.04 (Formula 1)

Here, it is assumed that the output fluctuation is suppressed to a range of ± 5% in consideration of the effect of the fuel cell stack actually stacked and stacked on the vehicle performance of the hybrid vehicle and the electric vehicle. As described above, the spread of a range of ± 5% is provided with respect to the above-described formula 1, and this range (range A in FIG. 3) can be used to give the maximum voltage peak, and the acid group density of the catalyst-supporting support and the polymer electrolyte It was defined as the control range of sulfonic acid group density. That is, the range A formed by adding a range of ± 5% to the above formula 1 is to set the following two formulas 2 and 3 as the lower limit line and the upper limit line of the range A.
Y = −0.285X + 0.99 (Formula 2)
Y = −0.285X + 1.09 (Formula 3)

  Produced while controlling the density so that both the acid group density of the catalyst-supporting carrier and the sulfonic acid group density of the polymer electrolyte are within the range defined by the above formulas 2 and 3, and using these, the electrode By obtaining a catalyst and producing a fuel cell comprising this electrode catalyst, and thus a fuel cell, the water retention performance of the electrode catalyst is controlled within the optimum range, and a fuel cell with excellent power generation performance can be obtained. And by using this fuel cell, it is possible to build a fuel cell that can also self-humidify while effectively suppressing the occurrence of flatting, and further a fuel cell system, excellent output performance, It can be used for hybrid vehicles and electric vehicles equipped with fuel cells that are lightweight and have excellent fuel efficiency.

  The embodiment of the present invention has been described in detail with reference to the drawings. However, the specific configuration is not limited to this embodiment, and there are design changes and the like without departing from the gist of the present invention. They are also included in the present invention.

DESCRIPTION OF SYMBOLS 1 ... Conductive support | carrier, 2 ... Catalytic metal (catalyst), 10 ... Catalyst support | carrier, 11 ... Catalytic acid group, 20 ... Polymer electrolyte, 21 ... Sulphonic acid group

Claims (3)

Regarding the catalyst-carrying support and polymer electrolyte that form the electrode catalyst, when the acid group density of the catalyst-carrying support is X and the sulfonic acid group density of the polymer electrolyte is Y, it falls within the range defined by the following two formulas: Having acid group density and sulfonic acid group density,
(1) Y = −0.285X + 0.99
(2) Y = −0.285X + 1.09,
(Excluding the range of X <0.3)
Electrocatalyst.   A membrane electrode assembly comprising the electrode catalyst according to claim 1 on each of an anode side and a cathode side of an electrolyte membrane.   A fuel cell comprising the membrane electrode assembly according to claim 2.

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