JP2011028973A - Method for evaluating catalyst layer of solid polymer fuel cell - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a method for evaluating the proton conductivity of the catalyst layer of a solid polymer fuel cell. <P>SOLUTION: The proton conductivity of the catalyst layer of the solid polymer fuel cell including carbon powder where a catalyst is supported and hydrophilic treatment is applied, and proton conductive polymer electrolyte is evaluated in accordance with a proton moving distance coefficient defined by an expression :(proton moving distance coefficient)=d×A×S×r<SP>×</SP>, where d is the thickness (μm) of the catalyst layer, A is the acid amount (mmol/g) per unit weight of the carbon powder where the catalyst is supported and the hydrophilic treatment is applied, S is a carrier shape factor determined by the shape of the carbon powder, r is a weight ratio P/C of the proton conductive polymer electrolyte (P) to the carbon powder (C) where thr catalyst is supported and the hydrophilic treatment is applied, in the catalyst layer, and x is -1. It is Preferable to control the proton moving distance coefficient to be 0.05-0.8. <P>COPYRIGHT: (C)2011,JPO&INPIT

Description

  The present invention relates to a method for evaluating proton conductivity in a catalyst layer of a polymer electrolyte fuel cell. Furthermore, the present invention relates to a polymer electrolyte fuel cell having a catalyst layer optimized using the evaluation method.

  A polymer electrolyte fuel cell is provided with a pair of electrodes on both sides of a proton conductive solid polymer electrolyte membrane, supplies hydrogen gas as a fuel gas to one electrode (fuel electrode: anode), and supplies oxygen gas or air. An electromotive force is obtained by supplying the other electrode (air electrode: cathode) as an oxidant. Solid polymer fuel cells are expected to be put to practical use as mobile vehicles such as electric vehicles and power sources for small cogeneration systems because they are easy to reduce in size and weight in addition to obtaining high battery characteristics. Yes.

  Normally, a gas diffusible electrode used for a polymer electrolyte fuel cell includes a catalyst layer containing a catalyst-supporting carbon powder coated with a proton conductive polymer electrolyte, and supplies a reaction gas to the catalyst layer. And a gas diffusion layer for collecting electrons. In the catalyst layer, there are voids composed of minute pores formed between the carbon secondary particles or the tertiary particles, and the voids function as a reaction gas diffusion channel.

  As the cathode and anode catalyst supported on the carbon powder, a noble metal such as platinum or a platinum alloy is used. For example, platinum-supported carbon powder is prepared by adding sodium hydrogen sulfite to a chloroplatinic acid aqueous solution and then reacting with hydrogen peroxide. The resulting platinum colloid is supported on a powder such as carbon black, washed, and if necessary. It is prepared by heat treatment. The platinum-supported carbon powder is dispersed in a proton conductive polymer electrolyte solution to prepare a catalyst ink. The catalyst ink is applied to a gas diffusion substrate such as carbon paper and dried to produce an electrode. An electrolyte membrane-electrode assembly (MEA) is assembled by sandwiching a solid polymer electrolyte membrane between these two electrodes and performing hot pressing or the like.

  In order to improve the power generation performance of the polymer electrolyte fuel cell, it is desired to efficiently supply protons to the catalyst. Therefore, it is necessary to improve proton conductivity in the catalyst layer. Conventionally, in order to operate the fuel cell efficiently, water vapor is supplied to the catalyst layer together with the reaction gas to keep the polymer electrolyte membrane in a wet state. However, in order to obtain a wet state, a humidifier must be separately provided in the air supply path or the hydrogen supply path, which has the disadvantage that the fuel cell becomes larger and the manufacturing cost increases. Therefore, development of technology that can operate even under low humidification conditions is desired, but under low humidification conditions, the mass transfer resistance of protons in the catalyst layer usually increases and the output performance of the fuel cell tends to decrease. There is. Therefore, improvement of proton conductivity in the catalyst layer becomes a more important issue in operation under low humidification conditions.

  Techniques for increasing the proton conductivity of the catalyst layer have been conventionally proposed. For example, Patent Document 1 discloses a catalyst layer for a fuel cell including micro / nanocapsules in which catalyst-supported carbon particles subjected to a hydrophilic treatment are coated with a polymer electrolyte. By the hydrophilization treatment, the catalyst-supported carbon particles are covered with a relatively uniform polymer electrolyte layer, and an area having excellent proton conductivity is formed in the vicinity of the particle surface. However, this technique is not sufficient in that the proton conductivity in the individual catalyst-supported carbon particles is examined, and the proton conductivity in the entire catalyst layer is evaluated and not optimized.

Further, (Patent Document 2) includes a membrane-electrode assembly including an electrolyte membrane having proton conductivity and a first electrode joined on the electrolyte membrane, wherein the first electrode includes a catalyst, And a first ionomer (proton conductive polymer electrolyte) that covers the catalyst and functions as a proton exchange group. In the first electrode, the amount of water produced (mol / min) at the rated output point of the membrane-electrode assembly ) / First ionomer volume (cm ³ ) is 1350 or more, a membrane-electrode assembly is disclosed. With this configuration, the water concentration in the first ionomer becomes sufficiently high and the proton transfer resistance can be lowered. However, as a technique for evaluating the proton conductivity in the entire catalyst layer and optimizing the configuration of the catalyst layer, There was room for improvement.

JP 2006-286329 A JP 2008-192490 A

  Accordingly, an object of the present invention is to provide a method for evaluating proton conductivity in a catalyst layer of a polymer electrolyte fuel cell in view of the above conventional situation. Another object of the present invention is to provide a catalyst layer having excellent proton conductivity and high power generation performance by utilizing the evaluation method.

  In response to the above problems, the present inventor has defined a “proton transfer distance coefficient” based on the thickness of the catalyst layer, the acid amount of the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment, and the like. As a result of investigating the relationship between the battery performance and the battery performance, a correlation was observed, and the present invention was completed based on this finding. That is, the gist of the present invention is as follows.

(1) The proton conductivity in the catalyst layer of a solid polymer fuel cell containing a catalyst-supported carbon powder subjected to hydrophilic treatment and a proton-conductive polymer electrolyte is expressed by the following formula (proton transfer distance coefficient) = d × A × S × r ^x
[Wherein, d is the thickness of the catalyst layer (μm), A is the amount of acid per unit weight (mmol / g) of the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment, and S is the shape of the carbon powder. The carrier form factor determined, r is the weight ratio P / C of the proton conductive polymer electrolyte (P) and the carbon powder (C) on which the catalyst is supported and subjected to the hydrophilic treatment in the catalyst layer, x is −1]
The evaluation method is based on the proton transfer distance coefficient defined in 1.

(2) The proton conductivity in the catalyst layer of a solid polymer fuel cell containing a catalyst-supported carbon powder that has been subjected to hydrophilic treatment and a proton-conducting polymer electrolyte is expressed by the following formula (proton transfer distance coefficient) = W x A x S x r ^x
[Where, W is the amount (mg / cm ² ) per unit area of the catalyst layer of the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment, and A is the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment. Acid amount per unit weight of the powder (mmol / g), S is a carrier form factor determined by the shape of the carbon powder, r is a catalyst layer in which the proton conductive polymer electrolyte (P) and the catalyst are supported and hydrophilic treatment Weight ratio P / C of carbon powder (C) to which is applied, x is −1]
The evaluation method is based on the proton transfer distance coefficient defined in 1.

(3) A polymer electrolyte fuel cell having a catalyst layer having a proton transfer distance coefficient of 0.05 to 0.8 as described in (1) above.

  According to the present invention, it is possible to evaluate proton conductivity in the catalyst layer of a polymer electrolyte fuel cell by adopting a new concept of proton transfer distance coefficient. Moreover, by controlling the proton transfer distance coefficient to be 0.05 to 0.8, the efficiency of proton conduction to the catalyst particles can be improved, and the power generation performance can be improved. In particular, since proton conductivity generally tends to decrease in an environment where there is little moisture in the MEA, it can be said that the catalyst layer optimized by the present invention greatly contributes to the improvement of power generation performance.

It is a schematic diagram which shows the state in a catalyst layer for demonstrating the evaluation method of this invention. It is a schematic diagram which shows the state in a catalyst layer for demonstrating the evaluation method of this invention. It is a schematic diagram which shows the state in a catalyst layer for demonstrating the evaluation method of this invention. It is a graph which shows the relationship between acid treatment time and the acid amount of catalyst carrying | support carbon powder. It is a graph which shows the relationship between an acid treatment temperature and the acid amount of catalyst carrying | support carbon powder. It is a graph which shows the relationship between the acid amount of a catalyst carrying | support carbon powder, and a proton conduction overvoltage. It is a graph which shows the change of the power generation performance with respect to a proton movement distance coefficient.

Hereinafter, the present invention will be described in detail.
In the method of the present invention, in evaluating the proton conductivity in the entire catalyst layer of a solid polymer fuel cell including a carbon powder carrying a catalyst and subjected to a hydrophilic treatment, and a proton conductive polymer electrolyte, Attention is paid to the thickness of the catalyst layer and the acid amount of the catalyst-supported carbon powder subjected to the hydrophilic treatment.

The relationship between the thickness, acid amount and proton conductivity will be described with reference to FIGS. 1 and 2 schematically showing the state of the catalyst layer. As shown in FIG. 1, the catalyst layer includes a carbon powder 1 on which a catalyst 2 is supported and subjected to a hydrophilic treatment, and a proton conductive polymer electrolyte 3 surrounding the carbon powder 1. At this time, the movement distance of proton H ⁺ increases or decreases depending on the thickness d of the catalyst layer. That is, it can be seen that as the catalyst layer thickness d increases, the proton transfer distance increases. In addition, as shown in FIG. 2, the carbon powder 1 carrying the catalyst 2 is given a hydrophilic group such as a carboxyl group on the surface by the action of an acid, etc. The amount of this hydrophilic group (acid amount) When there is a large amount (FIG. 2 (a)), the surface of the catalyst-supporting carbon powder is similarly thinly and uniformly covered with the proton conductive polymer electrolyte 3 having a hydrophilic group, and as a result, H ₂ contacts the catalyst 1. Proton H ⁺ generated in this way travels nonlinearly (arrow) along the thin proton-conductive polymer electrolyte 3 layer, and the proton movement distance increases. On the other hand, when the amount of acid is small (FIG. 2 (b)), the surface of the catalyst-carrying carbon powder is non-uniformly coated with the proton conductive polymer electrolyte 3, and as a result, a conduction path in which the proton moving distance is reduced. Is formed (arrow). Therefore, it is preferable to control the acid amount to be small from the viewpoint of reducing the proton movement distance.

Based on the above findings, the present inventor has found that the ease of proton movement in the catalyst layer can be expressed using the “proton transfer distance coefficient” defined below. That is, the proton transfer distance coefficient is
(Proton transfer distance coefficient) = d × A × S × r ^x (1)
[Wherein, d is the thickness of the catalyst layer (μm), A is the amount of acid per unit weight (mmol / g) of the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment, and S is the shape of the carbon powder. The carrier form factor determined, r is the weight ratio P / C of the proton conductive polymer electrolyte (P) and the carbon powder (C) on which the catalyst is supported and subjected to the hydrophilic treatment in the catalyst layer, x is −1]
Sought by.

Here, the acid amount (mmol / g) can be determined, for example, from the amount of alkali required for neutralizing the catalyst-supported carbon powder that has been subjected to the hydrophilic treatment and neutralizing the same. Further, the carrier form factor indicates the degree of bending of the proton transfer path relative to Ketjen EC (trade name; manufactured by Ketjen Black International), and the carrier having a larger specific surface area tends to have a larger value. The value fluctuates in the range of 2. As specific examples, the carrier form factor of Ketjen EC (specific surface area: about 800 m ² / g) is 1, Vulcan (manufactured by Cabot, specific surface area: about 250 m ² / g), 0.3, acetylene black (Electrochemical Industry Co., Ltd.) Manufactured, specific surface area: about 60 m ² / g) is 0.1, and Ketjen EC 600JD (manufactured by Ketjen Black International, specific surface area: about 1200 m ² / g) is 1.5. These carrier form factors can be determined based on an approximate ratio of specific surface areas of the respective carriers.

Further, the proton transfer distance coefficient can be obtained even when the weight ratio r of the proton-conductive polymer electrolyte / hydrocarbon-treated catalyst-supported carbon powder is not constant and varies in the thickness direction of the catalyst layer. it can. Specifically, the proton transfer distance coefficient of the entire catalyst layer can be obtained by calculating the proton transfer distance coefficient individually for the thickness portions where the weight ratio r can be considered constant, and adding them together. For example, as shown in FIG. 3, the catalyst layer has a portion of thickness d ₁ where the weight ratio of proton conductive polymer electrolyte / hydrophilic treated catalyst-supported carbon powder is r ₁ , and the weight ratio is r If composed of the thickness d ₂ of the parts is _two, proton transfer distance coefficient of the entire catalyst layer is given by the following equation.
(Proton transfer distance coefficient) = d ₁ × A × S × r ₁ ^x + d ₂ × A × S × r ₂ ^x (2)

  In order to exhibit sufficient proton conductivity in the catalyst layer and to obtain high power generation performance, the proton transfer distance coefficient obtained by the above formula is 0.05 to 0.8, particularly 0.1 to 0.5. It is preferable to design a catalyst layer. These preferable ranges apply similarly to any of the catalyst layers on the anode side and the cathode side. The proton transfer distance coefficient of the catalyst layer is based on the above formula, the thickness of the catalyst layer, the conditions for hydrotreating the catalyst-supported carbon powder (acid treatment time, acid treatment temperature, acid concentration, etc.), the type of carbon powder, The weight ratio of the proton conductive polymer electrolyte / catalyst-supported carbon powder can be arbitrarily controlled. For example, the conditions for obtaining a proton transfer distance coefficient of 0.05 to 0.8 include a catalyst layer thickness of 0.3 to 2.5 μm and an acid amount of 0.20 to 0.35 mmol / g (acid The treatment temperature is preferably 80 to 90 ° C. and the acid treatment time is 0 to 1 hour, but is not limited thereto. The conventionally known catalyst layer has, for example, a thickness of 3 μm or more, an acid amount of about 0.4 mmol / g, and a proton transfer distance coefficient of about 1.0 or more.

In addition, since the thickness d of the catalyst layer in the above formula is proportional to the amount W of the catalyst-supported carbon powder subjected to the hydrophilic treatment per unit area of the catalyst layer, d may be replaced with W. That is, the proton transfer coefficient can be obtained according to the following equation.
(Proton transfer distance coefficient) = W × A × S × r ^x (3)
[Where, W is the amount (mg / cm ² ) per unit area of the catalyst layer of the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment, and A is the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment. Acid amount per unit weight of the powder (mmol / g), S is a carrier form factor determined by the shape of the carbon powder, r is a catalyst layer in which the proton conductive polymer electrolyte (P) and the catalyst are supported and hydrophilic treatment Weight ratio P / C of carbon powder (C) to which is applied, x is −1]

  When the proton transfer distance coefficient is obtained using the above formula (3), the preferred range is 0.0025 to 0.04, preferably 0.005 to 0.025, from the viewpoint of obtaining high power generation performance.

The carbon powder for supporting the catalyst can be appropriately selected in consideration of the value of the proton transfer distance coefficient obtained by the above formula, and is not particularly limited. The specific surface area is preferably 200 m ² / g or more. Carbon black is generally used. In addition, acetylene black, graphite, carbon fiber, activated carbon, carbon nanotube, and the like are applicable. Preferable examples include Ketjen EC (Ketjen Black International) and Vulcan (Cabot).

  Further, as a catalyst to be supported on carbon powder, platinum, cobalt, palladium, ruthenium, gold, rhodium, osmium, iridium, or other metals, or alloys composed of two or more of the above metals, metals and organic compounds or inorganic compounds A complex, a metal oxide, etc. can be mentioned.

  As the proton conductive polymer electrolyte used in the catalyst layer, a fluorine-containing ion exchange resin or the like is applicable, and in particular, a sulfonic acid type perfluorocarbon polymer is preferably used. A suitable example is Nafion (manufactured by DuPont).

  The catalyst can be supported on the carbon powder by a conventional method. Specifically, for example, carbon powder is suspended in water or the like, a catalyst metal compound such as chloroplatinic acid is added dropwise thereto, and a reducing agent is added dropwise to deposit the catalyst on the carbon powder. Although the ratio of the catalyst to the carbon powder in the obtained catalyst-supporting carbon powder varies depending on the type of the catalyst and the like, it is generally preferable that catalyst: carbon powder = 1: 9 to 5: 5 by weight ratio. In the case of supporting an alloy, another metal is precipitated on the carbon powder and heat-treated at a high temperature to form an alloy.

  The carbon powder carrying the catalyst is subjected to a hydrophilic treatment to impart hydrophilic groups such as carboxyl groups to the surface in order to improve the covering property with the proton conductive polymer electrolyte. Examples of the hydrophilic treatment include treatment with an acid such as nitric acid, ozone oxidation, plasma treatment, and heat treatment in an oxidizing atmosphere, but are not limited thereto. The various conditions (acid treatment conditions, heat treatment conditions, etc.) in each of the hydrophilic treatments mentioned above are determined by the relationship with other factors affecting the value of the proton transfer distance coefficient, and as described above, the value of the proton transfer distance coefficient Can be appropriately set so as to be within a preferable range. As an example, preferable conditions for treating the catalyst-supported carbon powder with a nitric acid aqueous solution are a nitric acid aqueous solution concentration of 0.01 to 5 N, an acid treatment temperature of room temperature to 100 ° C., and an acid treatment time of 0.5 to 50 hours. However, the present invention is not limited to this.

  Subsequently, the catalyst-supporting carbon powder subjected to the hydrophilic treatment and the proton conductive polymer electrolyte are added to the solvent, and dispersion treatment is performed by ultrasonic irradiation, bead mill, or the like, thereby producing a catalyst ink. Examples of the solvent used here include water, alcohols such as ethanol, ethylene glycol, and propylene glycol, fluorine-containing alcohols, and fluorine-containing ethers. And a catalyst layer can be formed by apply | coating catalyst ink to the carbon electrolyte etc. which become the polymer electrolyte membrane of a fuel cell, or a gas diffusion layer, and making it dry. Alternatively, the catalyst layer may be formed on the polymer electrolyte by transferring a catalyst prepared by applying the catalyst ink onto a separately prepared substrate and drying it.

  In the catalyst ink, the weight ratio of the proton conductive polymer electrolyte to the catalyst-supported carbon powder subjected to the hydrophilic treatment is 0.5 to 1. 5 is preferable. Further, the weight ratio of water to the entire catalyst ink is preferably 20 to 50% by weight.

  The thickness of the cathode and anode catalyst layers in the fuel cell can be set as appropriate. Generally, it is 1 to 30 μm, preferably 2 to 15 μm.

  As a material for the polymer electrolyte membrane sandwiched between the cathode and anode catalyst layers, any material that exhibits good proton conductivity under wet conditions can be used. Examples thereof include a perfluorocarbon polymer having a sulfonic acid group, a polysulfone resin, a perfluorocarbon polymer having a phosphonic acid group or a carboxylic acid group. Among these, sulfonic acid type perfluorocarbon polymers are preferably used. The polymer electrolyte membrane may be the same resin as the proton conductive polymer electrolyte contained in the catalyst layer, or may be composed of a different resin.

  When the catalyst layer is formed on the gas diffusion layer, the electrolyte membrane-electrode assembly (MEA) is assembled by bonding the catalyst layer and the polymer electrolyte membrane by adhesion or hot pressing. Further, when the catalyst layer is formed on the polymer electrolyte membrane, the anode and the cathode may be constituted only by the catalyst layer, and further, the gas diffusion layer is disposed adjacent to the catalyst layer so as to serve as the anode and the cathode. Also good.

  On the outside of the anode and the cathode, a separator having a gas flow path is usually disposed, and the polymer electrolyte fuel cell of the present invention is produced. Electric power is generated by supplying a gas containing hydrogen to the anode and a gas containing oxygen or air to the cathode.

EXAMPLES Hereinafter, although an Example and a comparative example demonstrate this invention further in detail, it is not limited to this.
<Preparation of catalyst-supported carbon powder>
Carbon black Ketjen EC (trade name; manufactured by Ketjen Black International) was used as the carbon powder to carry Pt. The carrier form factor is 1. 2.0 to 4.5 g of this carbon black was suspended and stirred in distilled water, and 3 to 20 g of 15% chloroplatinic acid was added dropwise. Next, 5 to 30 g of ethanol was dropped as a reducing agent to deposit Pt on the carbon. The mixture was filtered and the solid was dried to obtain Pt-supported carbon powder. These production conditions vary depending on the weight ratio of Pt to carbon (Pt / C ratio) in the catalyst-supported carbon powder to be prepared and the amount of catalyst-supported carbon powder per batch. Each of the above conditions is a condition when Pt / C = 1/9 to 6/4, 5 g / batch is used as a guide.

<Hydrophilic treatment>
Next, the obtained Pt-supported carbon powder was immersed in an aqueous nitric acid solution and stirred while being controlled at a low temperature to perform a hydrophilic treatment.

<Measurement of acid amount>
0.5 g of the Pt-supported carbon powder subjected to the above hydrophilic treatment was added to 20 ml of 0.1N sodium hydroxide, and the mixture was subjected to ultrasonic stirring for 20 minutes, followed by filtration to separate the Pt-supported carbon powder. To 5 ml of the filtrate, 0.05 ml of the indicator methyl orange was added with stirring, titrated with 0.05N hydrochloric acid, and the acid amount per unit weight of the Pt-supported carbon powder subjected to hydrophilic treatment (mmol / g) ) FIG. 4 shows changes in the amount of acid (Pt / C ratio is the same) when the stirring time (acid treatment time) in the aqueous nitric acid solution is 0, 21, and 45 hours (all temperatures are 80 ° C.). FIG. 5 shows changes in the amount of acid (Pt / C ratio is the same) when the treatment temperature with the aqueous nitric acid solution is 80, 90, and 95 ° C. (all acid treatment times are 21 hours). From the results of FIGS. 4 and 5, it was found that the acid amount of the Pt-supported carbon powder can be controlled by the acid treatment time and the acid treatment temperature.

<Preparation of catalyst ink>
Distilled water was added to the Pt-supported carbon powder that had been subjected to hydrophilic treatment, and then ethanol was added. A commercially available Nafion solution (manufactured by DuPont) was further added as a proton conductive polymer electrolyte. The weight ratio of water to the total weight at this time was 40% by weight. The weight ratio of the proton conductive polymer electrolyte to the Pt-supported carbon powder subjected to the hydrophilic treatment was set to 1. These mixtures were sufficiently stirred and subjected to a dispersion treatment with an ultrasonic homogenizer for the purpose of atomization and uniform dispersion of particles, thereby preparing a catalyst ink.

<Preparation of catalyst layer>
Each catalyst ink produced above was applied on a Teflon substrate in various thicknesses and dried to form a catalyst layer.

<Production of MEA>
The catalyst layer prepared above is used on the anode side, and one cathode side contains Pt-supported carbon powder (Pt: C = 1: 1 (weight ratio)) and 40% by weight of water, and further has proton conductivity. A catalyst layer formed of a catalyst ink prepared so that the ratio of the polymer electrolyte to the Pt-supported carbon powder was 0.75 was used. For the polymer electrolyte membrane, 20-JT-VIILA (trade name; manufactured by Gore), which is a fluorine-based polymer, is used, and the cathode and anode catalyst layers are bonded to both sides of the polymer electrolyte membrane by 130 ° C. hot pressing, The Teflon substrate was removed. The obtained electrolyte membrane-electrode assembly (MEA) was used for the following power generation performance evaluation. In addition, the thickness of the catalyst layer necessary for calculating the proton transfer distance coefficient is obtained by cutting out a part of MEA and embedding it with resin, producing an ultrathin section using a microtome, and preparing a sample of the ultrathin section from FE-SEM. It was measured by observing the cross section at. And the proton movement distance coefficient in each preparation conditions was computed according to the said (1) formula.

<Performance evaluation of fuel cell>
A gas diffusion layer (GDL) composed of a carbon base material and a water repellent layer (PTFE) is disposed outside the MEA to produce a polymer electrolyte fuel cell, and hydrogen is supplied to the anode side and air is supplied to the cathode side. It was made to generate electricity. The performance of the fuel cell was evaluated based on the voltage value with respect to the load current. The humidification conditions were 40% RH for both electrodes with respect to a cell temperature of 80 ° C., and the load current was 1.0 A / cm ² .

FIG. 6 shows the results of evaluating the proton conduction overvoltage (voltage loss caused by proton resistance) while fixing the thickness of the anode catalyst layer and changing the acid amount. In general, the main reasons for the voltage drop are (a) reaction overvoltage (activation overvoltage) on the catalyst surface, (b) oxygen diffusion overvoltage, (c) proton conduction overvoltage, and (d) resistance overvoltage. Can be considered. The battery performance decreases as the total value of these overvoltages increases. The proton conduction overvoltage in FIG. 6 is obtained by assuming that the gas supplied to the cathode is O ₂ , (b) assuming that the oxygen diffusion overvoltage is 0, and from the total overvoltage measured, (a) the activation overvoltage, and (d ) Calculated by removing resistance overvoltage. At this time, the calculation was made assuming that (a) the activation overvoltage was 0.43 V, and (d) the resistance overvoltage was 0.14 V.

  From the result of FIG. 6, as the acid amount of the Pt-supported carbon powder increases, the surface of the Pt-supported carbon powder is thinly and uniformly covered with the proton conductive polymer electrolyte, so that the proton transfer distance increases and the proton conductivity. It was suggested that the overvoltage increased.

  FIG. 7 shows the voltage values of a fuel cell designed to have various proton transfer distance coefficients by changing the thickness of the anode catalyst layer and the amount of catalyst acid. From the results of FIG. 7, it was found that the performance improved as the proton transfer distance coefficient decreased, and that a high voltage value was maintained within the range of 0.05 to 0.8. From this, it became clear that a fuel cell with high power generation performance can be obtained by using a catalyst layer having a proton transfer distance coefficient of 0.8 or less for the anode electrode.

1 Carbon powder 2 Catalyst 3 Proton conductive polymer electrolyte

Claims (3)

The proton conductivity in the catalyst layer of a polymer electrolyte fuel cell including a catalyst-supported carbon powder subjected to hydrophilic treatment and a proton-conducting polymer electrolyte is expressed by the following formula (proton transfer distance coefficient) = d × A × S × r ^x
[Wherein, d is the thickness of the catalyst layer (μm), A is the amount of acid per unit weight (mmol / g) of the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment, and S is the shape of the carbon powder. The carrier form factor determined, r is the weight ratio P / C of the proton conductive polymer electrolyte (P) and the carbon powder (C) on which the catalyst is supported and subjected to the hydrophilic treatment in the catalyst layer, x is −1]
The evaluation method is based on the proton transfer distance coefficient defined in 1. The proton conductivity in the catalyst layer of a solid polymer fuel cell containing a catalyst-supported carbon powder that has been subjected to hydrophilic treatment and a proton-conductive polymer electrolyte is expressed by the following formula (proton transfer distance coefficient) = W × A × S × r ^x
[Where, W is the amount (mg / cm ² ) per unit area of the catalyst layer of the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment, and A is the carbon powder on which the catalyst is supported and subjected to the hydrophilic treatment. Acid amount per unit weight of the powder (mmol / g), S is a carrier form factor determined by the shape of the carbon powder, r is a catalyst layer in which the proton conductive polymer electrolyte (P) and the catalyst are supported and hydrophilic treatment Weight ratio P / C of carbon powder (C) to which is applied, x is −1]
The evaluation method is based on the proton transfer distance coefficient defined in 1.   The polymer electrolyte fuel cell which has a catalyst layer whose proton transfer distance coefficient of Claim 1 is 0.05-0.8.

Images