JP2011258452A - Electrode catalyst, membrane electrode assembly and polymer electrolyte fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrode catalyst capable of decreasing both an activation overvoltage loss and a gas diffusion overvoltage loss, which are the main factors of a voltage loss, by regulating a polymer electrolyte relative to the electrode catalyst composed of conductive carriers having various specific surface areas, within an appropriate range which is uniquely defined, to improve power generation performance of a fuel cell, and to provide a membrane electrode assembly having the electrode catalyst as well as a polymer electrolyte fuel cell having the membrane electrode assembly.SOLUTION: The electrode catalyst 20 of a polymer electrolyte fuel cell is composed of a catalyst carrying carrier 3 and a polymer electrolyte 4, the amount of the polymer electrolyte 4 in the electrode catalyst 20 being within a scope of 22 to 27 vol.%.

Description

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

  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 layer, and an electrode body (MEGA: MEA and GDL joined body) is formed. And a fuel cell is formed by arranging a separator outside the gas diffusion layer. Actually, these fuel cells are stacked in the number corresponding to the power generation performance to form a fuel cell stack.

  The conventional method for forming a catalyst layer described above includes, for example, a catalyst-supported carrier supporting a catalyst, a polymer electrolyte (ionomer), and a dispersion solvent on the surface of a substrate such as a Teflon sheet (Teflon: registered trademark, DuPont). A catalyst layer is formed by applying a catalyst solution (catalyst ink) and then drying the surface of the catalyst solution with a hot plate or the like (wet coating method). In addition, in this coating operation, there are a method of applying by spray, a method of using a doctor blade, and the like.

  By the way, not all of the energy generated by the combustion of fuel gas such as hydrogen contributes to the power generation of the fuel cell, and some of the energy is lost as heat loss due to multiple types of voltage loss elements.

  This voltage loss includes activation overvoltage loss, gas diffusion overvoltage loss, resistance overvoltage loss, etc., but according to the present inventors, by adjusting the amount of the polymer electrolyte forming the electrode catalyst, It has been found that both the activation overvoltage loss and the gas diffusion overvoltage loss can be controlled to small values, and the voltage loss can be reduced as much as possible.

Here, "activation overvoltage", for example, the cathode side of the electrode ^{_{catalyst, 1 / 2O 2 + 2H +}} + 2e by oxygen gas provided a conduction protons from the anode side ^- → H ₂ 0 electrochemical The reaction takes place, which is the energy required to cause this reaction during this electrochemical reaction.

  When there is a region that is not sufficiently covered with the polymer electrolyte on the surface of the catalytic metal supported on the conductive support, the above-described electrochemical reaction does not occur in this region, and the region covered with the polymer electrolyte is not covered. The electrochemical reaction will be concentrated. And in this area | region where an electrochemical reaction concentrates, activation energy will increase too much, in other words, activation overvoltage will increase.

  Accordingly, this activation overvoltage tends to increase when the amount of the polymer electrolyte is too small and an insufficiently coated region with the polymer electrolyte is formed on the surface of the catalyst metal.

  On the other hand, “gas diffusion overvoltage” refers to a region where the thickness of the polymer electrolyte covering the surface of the catalytic metal is too large, that is, a region where the oxygen diffusion distance is too long. This is the load energy that is increased in this region because the catalytic reaction is difficult to occur without catching up with the region, and the catalytic reaction is performed only in other regions.

  Therefore, this gas diffusion overvoltage tends to increase when the amount of the polymer electrolyte is too large and a region where the oxygen diffusion distance is too long is formed on the surface of the catalyst metal.

  From the above, in order to reduce both the activation overvoltage and the gas diffusion overvoltage, the amount of the polymer electrolyte may be controlled within a certain appropriate range.

  In addition, there are various specific surface areas of conductive carriers used for the formation of the electrode catalyst, but the amount of the polymer electrolyte described above is the mass of the electrode catalyst (the total amount of the polymer electrolyte and the catalyst-supported carrier). If the mass fraction of the polymer electrolyte with respect to (mass) is to be defined, the appropriate range is completely different depending on the specific surface area. More specifically, in the case of a conductive carrier having a small specific surface area, the appropriate range of ionomer shifts to a relatively small mass fraction range, and in the case of a conductive carrier having a large specific surface area, the appropriate range of ionomer. Shift to a relatively large mass fraction range. Therefore, it becomes necessary to define an appropriate range of ionomer for which both the activation overvoltage and the gas diffusion overvoltage are small values for each specific surface area of the conductive carrier.

  Here, as a conventional published technique, Patent Document 1 discloses an electrode in which the content of the polymer electrolyte in the catalyst layer is adjusted to 0.1 or more by mass ratio with respect to the electron conductive substance (conductive support). There is a catalyst disclosure.

In this method, the amount of ionomer is defined by the mass ratio with respect to the conductive carrier. According to the present inventors, the mass ratio of the ionomer to the conductive carrier having a small specific surface area of about 100 m ² / g or less is 0.00. It has been found that when the number is 1 or more, the coating thickness of the ionomer increases and the gas diffusibility decreases.

  On the other hand, as another open technology, a membrane electrode assembly of a direct methanol fuel cell has a multi-layer structure in which the catalyst layer constituting the layer is composed of a polymer electrolyte-rich layer and a few polymer electrolyte layers. Discloses a membrane / electrode assembly in which the polymer electrolysis mass of a small number of layers is less than 30% by volume.

  This is a fuel cell that does not use hydrogen as a fuel gas but methanol as a fuel liquid, and is therefore a subject that is significantly different from the polymer electrolyte fuel cells described so far. However, by defining the amount of the polymer electrolyte in terms of volume fraction, the appropriate range of the polymer electrolyte may change depending on the specific surface area of the conductive support, as in the case of defining in the mass fraction described above. Thus, the appropriate range of the polymer electrolyte can be uniquely determined.

  Patent Document 3 discloses a catalyst ink for fuel cells in which the volume ratio of catalyst particles to ionomer is 55:45 to 90:10.

  This technique also defines the control range of the ionomer in terms of volume fraction, but according to the present inventors, this numerical range does not take into account any voids in the catalyst layer, and therefore, the ionomer in the catalyst layer is not considered. It is unclear to which range the volume fraction of is actually controlled. In addition, this technology does not raise the problem of reducing both the activation overvoltage loss and the gas diffusion overvoltage loss, which are the main causes of voltage loss, in order to improve the power generation performance. It does not provide an appropriate range of molecular electrolytes.

JP 2008-4298 A International Publication No. 2006/123529 Pamphlet JP 2009-152112 A

  The present invention has been made in view of the problems described above, and the voltage is controlled by controlling the polymer electrolyte to an appropriate range that is uniquely determined for an electrode catalyst composed of a conductive support having various specific surface areas. Electrode catalyst capable of reducing both the activation overvoltage loss and gas diffusion overvoltage loss, which are the main causes of loss, and thereby improving the power generation performance of the fuel cell, and a membrane electrode assembly including this electrode catalyst, this membrane An object of the present invention is to provide a polymer electrolyte fuel cell provided with an electrode assembly.

  In order to achieve the above object, the electrode catalyst according to the present invention is an electrode catalyst of a solid polymer fuel cell comprising a catalyst-supporting carrier and a polymer electrolyte, and the amount of the polymer electrolyte in the electrode catalyst is 22 to 27 volumes. % Adjusted.

  The electrode catalyst of the present invention is used in a polymer electrolyte fuel cell using hydrogen as a fuel gas, and is suitable for a polymer electrolyte (ionomer) that can reduce both an activation overvoltage loss and a gas diffusion overvoltage loss. The range is defined as 22 to 27% by volume.

  Here, the numerical range concerning the volume fraction of the polymer electrolyte described above is the ratio of the polymer electrolyte to the total volume of the electrode catalyst composed of the catalyst-supporting carrier, the polymer electrolyte, and the voids.

  As described above, when the amount of the polymer electrolyte is too small, an insufficiently coated region with the polymer electrolyte is formed on the surface of the catalyst metal, and the activation overvoltage is likely to increase. When the amount is too large, a region where the oxygen diffusion distance is too long is formed on the surface of the catalyst metal, and the gas diffusion overvoltage tends to increase.

  The present inventors measured the activation overvoltage and the gas diffusion overvoltage using conductive carriers having various specific surface areas, and based on the measurement results, the optimum polymer electrolyte having both the activation overvoltage and the gas diffusion overvoltage decreased. As a range, the above-mentioned 22-27% by volume is specified. That is, when the amount of the polymer electrolyte is less than 22% by volume, the activation overvoltage tends to increase in the conductive carrier having all the specific surface areas of the experiment target, and when the amount of the polymer electrolyte exceeds 27% by volume, all of the specific surface area of the test object is measured. Since the gas diffusion overvoltage tends to increase in these conductive carriers, these numerical values have critical significance of the upper and lower limits that define the appropriate range of the polymer electrolyte.

  A polymer electrolyte and a catalyst-supporting carrier controlled in an appropriate range as described above are put into a dispersion solvent and stirred to produce a catalyst solution (catalyst ink).

  The produced catalyst solution is stretched in a layer form on a base material such as an electrolyte membrane by, for example, a coating blade to form a coating film, and dried in a warm air drying oven or the like to dry at least a cathode-side electrode catalyst ( Catalyst layer) is formed, and a membrane electrode assembly is obtained.

  The polymer electrolyte fuel cell equipped with this membrane electrode assembly has excellent power generation performance because the amount of the polymer electrolyte in the electrode catalyst is controlled within an appropriate range to reduce power generation loss. . Therefore, production of this polymer electrolyte fuel cell has recently been expanded, and it is suitable for electric vehicles and hybrid vehicles that require higher performance for in-vehicle devices.

  As can be understood from the above description, according to the electrode catalyst of the present invention, the membrane electrode assembly including the electrode catalyst on at least the cathode side, and the polymer electrolyte fuel cell including the membrane electrode assembly, The amount of the polymer electrolyte in the electrode is regulated within the range of 22 to 27% by volume, which is an optimal range in which both the activation overvoltage and the gas diffusion overvoltage are reduced, thereby reducing the power generation loss of the electrode catalyst as much as possible. Therefore, the power generation performance of the polymer electrolyte fuel cell can be improved.

(A) is the schematic diagram which showed the membrane electrode assembly, (b) is the schematic diagram which expanded the b section of FIG. 1a. Experimental results of measuring the activation overvoltage and gas diffusion overvoltage of a fuel cell having an electrode catalyst composed of a plurality of conductive supports having different specific surface areas, and the volume of the polymer electrolyte in the electrode catalyst defined based on this result It is a figure which shows the appropriate range of a fraction.

  The electrode catalyst of the present invention will be outlined below with reference to the drawings.

  FIG. 1a is a schematic diagram showing a membrane electrode assembly, and FIG. 1b is a schematic diagram enlarging a part b of FIG. 1a, simulating the microstructure of the electrode catalyst.

  The membrane electrode assembly 40 shown in FIG. 1 a is configured by sandwiching an electrolyte membrane 10 between a cathode-side electrode catalyst 20 and an anode-side electrode catalyst 30.

  For example, as shown in FIG. 1 b, the cathode-side electrode catalyst 20 includes a catalyst-supporting carrier 3 formed by supporting a catalytic metal 2 on a conductive carrier 1, and a moving path of oxygen and protons covering the periphery thereof. And a gap G formed between them. The anode-side electrode catalyst 30 also has the same structure.

  In particular, in the case of the electrode catalyst 20 on the cathode side, the entire area A (the entire area) composed of the catalyst carrier 3, the polymer electrolyte 4, and the gap G within the unit area A (unit volume in consideration of the unit depth) shown in FIG. The volume fraction of the polymer electrolyte 4 in the product) is controlled in the range of 22 to 27% by volume. Note that the polymer electrolyte of the anode-side electrode catalyst 30 is not necessarily controlled within this range.

  By controlling the amount of the polymer electrolyte 4 in the cathode side electrode catalyst 20 in the range of 22 to 27% by volume, it is possible to reduce both the activation overvoltage and the gas diffusion overvoltage, which are the main causes of power generation loss, The power generation performance of the polymer electrolyte fuel cell can be improved.

  Further, by defining the appropriate range of the polymer electrolyte by the volume fraction, the above-described volume range of the polymer electrolyte can be uniformly applied to various conductive carriers having different specific surface areas.

  The conductive carrier has a structure of an aggregate (primary aggregate) in which fine spherical primary particles are adhered, and an agglomerate (secondary aggregate) formed by physical adsorption of the aggregate. The development process of gates and agglomerates differs depending on the type of conductive carrier and post-treatment. When ionomer is included in this, the ionomer mainly contains secondary pores of the conductive carrier (pores surrounded by the conductive carrier constituting the aggregate, pores surrounded by the adjacent aggregate, etc.). In order to fill, the size of the agglomerate of the conductive carrier is the main factor determining the volume of the electrode catalyst.

  Here, the manufacturing method of the membrane electrode assembly 40 shown in the figure will be outlined. First, the conductive carrier 1 and the precursor of the catalyst metal 2 are put into a dispersion solvent accommodated in a container (not shown), heat treatment is performed with stirring, and a reducing agent is added thereto as necessary. Thus, the catalyst metal 2 is reduced and deposited from the precursor and supported on the surface of the conductive support 1 to obtain a powder of the catalyst support 3.

  Here, examples of the conductive carrier 1 include carbon materials such as carbon black, carbon nanotubes, and carbon nanofibers, as well as carbon compounds typified by silicon carbide, etc. Examples of the catalyst metal include: Any one of platinum, platinum alloy, palladium, rhodium, gold, silver, osmium, iridium and the like can be used, and platinum or a platinum alloy is preferably 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-supporting carrier 3 is put into a dispersion solvent, and further, a polymer electrolyte 4 is put and stirred using an ultrasonic homogenizer, a bead mill, a ball mill, etc. ) Is generated.

  Examples of the polymer electrolyte 4 include proton exchange polymers, ion-exchange resins having an organic fluorine-containing polymer as a skeleton, such as perfluorocarbon sulfonic acid resin, sulfonated polyether ketone, sulfonated polyethersulfone, sulfone. Sulfonated plastic electrolytes such as sulfonated polyetherethersulfone, sulfonated polysulfone, sulfonated polysulfide, sulfonated polyphenylene, sulfoalkylated polyetheretherketone, sulfoalkylated polyethersulfone, sulfoalkylated polyetherethersulfone, sulfoalkyl And sulfoalkylated plastic electrolytes such as sulfonated polysulfone, sulfoalkylated polysulfide, and sulfoalkylated polyphenylene. As commercially available materials, Nafion (registered trademark, manufactured by DuPont), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), and the like can be used.

  The produced catalyst solution is applied to any one of the electrolyte membrane 10, the gas diffusion layer, and the support film, which is a base material, and is heated on a base layer by hot air drying or hot pressing. Catalyst) is formed. Here, the electrolyte membrane 10 is, for example, a fluorine-based 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 non-fluorine 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. A commercially available material such as a Teflon sheet (Teflon: registered trademark, DuPont) may also be used.

  A gas diffusion layer is arranged on both sides of the membrane electrode assembly 40 shown in FIG. 1a, and a separator (not shown) is arranged outside thereof to constitute a fuel cell, and a fuel cell of a radix corresponding to the power generation performance is stacked. As a result, a polymer electrolyte fuel cell is formed.

[Experiment and results of measuring activation overvoltage and gas diffusion overvoltage of fuel cells]
The inventors of the present invention use five types of conductive carriers having different specific surface areas, change the volume fraction of the ionomer to each of the conductive carriers, and form membrane electrode assemblies. A fuel cell was fabricated by attaching a gas diffusion layer to the body. And fuel gas and oxidant gas were provided with respect to this fuel battery cell, the electrochemical reaction was started, and the activation overvoltage and gas diffusion overvoltage in that case were measured.

More specifically, a pure platinum-supported carbon black carrier is used, the platinum weight to carbon weight is 3 to 7, the average particle diameter of platinum is 4 nm, and the carrier types are five types of the following Table 1 and the cathode side electrode catalyst. Was made. Moreover, TEC10E30E (manufactured by Tanaka Kikinzoku) was used as the catalyst on the anode side, NRE211CS (manufactured by DuPont) was used as the electrolyte membrane, and DE2020CS (manufactured by DuPont) was used as the ionomer. In addition, regarding the composition of the electrode catalyst, the amount of platinum on the cathode side was adjusted to 0.1 mg / cm ² , the amount of platinum on the anode side was adjusted to 0.05 mg / cm ² , and the ionomer amount was adjusted to 0.7 g with respect to 1 g of the carbon support. did. Also, catalyst, ionomer, water and ethanol are mixed and dispersed with an ultrasonic homogenizer to prepare a catalyst ink. This is cast onto a polymer film and the solvent is dried. The resulting catalyst sheet is then thermocompression bonded to the electrolyte membrane. Integrated.

  Here, the ionomer volume fraction will be described in detail. In the cathode side electrode catalyst, 0.5 g of ionomer was used with respect to 1 g of the carbon support, and the cathode side electrode catalyst was formed by the above method. Further, the thickness of the cathode electrode catalyst was measured by observing a cross section of the membrane electrode assembly with an SEM (transmission electron microscope). In addition, the volume fraction of the ionomer was calculated from the relationship between the specific gravity of the ionomer and the amount of ionomer used separately obtained. Based on these, the amount of ionomer was adjusted so that the 0-50% level of the ionomer's volume fraction fluctuated to obtain an electrode catalyst on the cathode side.

Carbon paper was contacted as a gas diffusion layer to obtain a fuel cell for evaluation. Hydrogen gas having a humidity of 100% RH and a holding pressure of 150 kPa was provided on the anode side of the fuel cell, and air having a humidity of 100% RH and a holding pressure of 150 kPa was provided on the cathode side. And the electrochemical reaction was produced with the electronic load apparatus, and the correction voltage of electrolyte membrane resistance overvoltage was computed from the electrolyte membrane resistance value observed by the voltage value and electric current value when controlling at 0.1 A / cm < ² >. Here, the electrolyte membrane resistance value is obtained by an AC impedance method. The difference between the electromotive force of the fuel cell 1.23V and the electrolyte membrane resistance overvoltage correction voltage was defined as the activation overvoltage.

In addition, in the electrochemical reaction in the measurement of the activation overvoltage, when the gas introduced to the cathode side is oxygen and in the case of air, the voltage value and the electrolyte membrane resistance overvoltage measured when controlled at 1.5 A / cm ² The difference from the correction voltage was defined as the gas diffusion overvoltage. The experimental results are shown in FIG.

  From the figure, it was found that, in each fuel cell having five types of conductive carriers A to E as electrode catalysts, the activation overvoltage increases when the volume fraction of ionomer is less than 22% by volume. This is because there is platinum that is not covered by the ionomer due to the lack of the ionomer.

  Furthermore, it was found that the activation overvoltage increases when the volume fraction of the ionomer exceeds 27% by volume. This is because since there are too many ionomers, the ionomer penetrates between the aggregates, and the electron conduction resistance between the conductive carriers is increased.

  On the other hand, it was found that the gas diffusion overvoltage increases when the volume fraction of the ionomer exceeds 27% by volume. This is because the thickness of the ionomer coating on platinum is increased and the supply of oxygen to platinum is hindered.

  From the figure, both the activation overvoltage and the gas diffusion overvoltage show a downward behavior with respect to the volume fraction of the ionomer, and each minimum value varies depending on the conductive carrier type, but these minimum values are The range of the volume fraction of the ionomer shown is in the range of 22 to 27% by volume, and this range is an appropriate range of the ionomer amount.

  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 ... Electroconductive support | carrier, 2 ... Catalyst metal, 3 ... Catalyst support | carrier, 4 ... Polymer electrolyte (ionomer), 10 ... Electrolyte membrane, 20 ... Electrode catalyst (catalyst layer) 30 ... Cathode side electrode catalyst, 40 ... Membrane electrode assembly, G ... Gap

Claims (3)

An electrode catalyst for a polymer electrolyte fuel cell comprising a catalyst carrier and a polymer electrolyte,
An electrode catalyst in which the amount of the polymer electrolyte in the electrode catalyst is in the range of 22 to 27% by volume.   A membrane electrode assembly comprising the electrode catalyst according to claim 1 at least on a cathode side electrode.   A polymer electrolyte fuel cell comprising the membrane electrode assembly according to claim 2.

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