JPH06267545A - Phosphoric acid type fuel cell - Google Patents

Abstract

PURPOSE:To obtain an optimum distribution of an electrolyte inside a battery, and to increase the area of a three-phase interface to be the area of reaction, by specifying the volume of the catalyst layer of a positive electrode corresponding to the diameter of a pore with the diameter not larger than 100mum. CONSTITUTION:As a gas diffusion layer 6, a porous plate which consists of a carbon structure to which a polytetrafluoroethylene (PTFE) is coated is used, and a catalyst layer 5 consisting of a catalyst and PTFE is provided on one side surface of the diffusion layer 6, it is baked in the air, and a positive electrode is manufactured. Of the pores with the diameter less than 100mum of the catalyst layer 5, the volume of the pores with the diameter less than 0.1mum, and the volume of the pores with the diameter 0.1 to 1.0mum are made 42%, and the volume of the pores with the diameter 10 to 100mum is made 11% or more. As a result, an optimum distribution of the electrolyte is obtained by the positive electrode catalyst layer 5 and the gas diffusion layer 6 to compose the fuel cell, and the control of the porosity. Consequently, a high performance of fuel cell in which the area of a three-phase interface to be the area of reaction is increased can be obtained.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION The present invention relates to a phosphoric acid fuel cell using phosphoric acid as an electrolyte.

[0002]

2. Description of the Related Art In order to improve the efficiency of a fuel cell system, it is necessary to improve the performance of the fuel cell body. The fuel cell reaction is carried out in three phases (reaction gas-
The distribution of the electrolyte in the pores of the catalyst layer is very important because it proceeds at the interface between the electrolyte and the catalytically active component. Especially, since the oxygen reduction reaction rate of the positive electrode is small, the performance is strongly influenced by the distribution of the electrolyte. Therefore, polytetrafluoroethylene (hereinafter referred to as PTFE) is added as a water-repellent material to the electrode of a fuel cell using phosphoric acid as an electrolyte. The amount of PTFE added and the firing conditions are optimized to control the electrolyte distribution in the catalyst layer. In this regard, for example, Battery Handbook, Maruzen (1990), p. 373.

[0003]

However, when the performance of various catalysts was evaluated, it was found that the cell performance varies depending on the catalyst even if the amount of PTFE added is the same and the firing conditions are the same. As the cause, the catalyst layer of the electrode,
It was considered that the distribution of pores in the matrix was different.
Therefore, when the pore distributions of various electrode catalyst layers were measured, a correlation was observed with the cell performance.

The object of the present invention is to obtain the optimum distribution of the electrolyte inside the electrode by controlling the pore distribution and porosity of the positive electrode catalyst layer, the gas diffusion layer, etc. constituting the fuel cell, and to provide a reaction site. It is to provide a high performance fuel cell having an increased area of the phase interface.

[0005]

A battery is composed of a positive electrode, a matrix and a negative electrode, and the positive electrode and the negative electrode are composed of a gas diffusion layer and a catalyst layer. The electrolyte is retained in the positive and negative electrode catalyst layers and the matrix, but the pores of the matrix are 100
% Filled with electrolyte. On the other hand, in the electrode catalyst layer, the reaction does not proceed without the diffusion of the reaction gas, so that the electrolyte must be filled with the electrolyte without blocking the diffusion path of the reaction gas. This makes it difficult to improve the battery performance. In particular, since the reaction rate of the negative electrode is high, even if the reaction area changes a little, the influence on the performance is small. However, since the reaction rate of the positive electrode is small, the reaction area size directly causes the performance to be high or low. Therefore, the distribution of the electrolyte inside the positive electrode catalyst layer is important.

Therefore, in order to solve the above problems, the pore distribution of the catalyst layer of the positive electrode was measured. After removing the phosphoric acid as the electrolyte by ultrasonic cleaning, the pore distribution was measured by a mercury porosimeter. In addition, since the pore size distribution changes depending on the particle size distribution (secondary particle size distribution) of the catalyst used and the amount of platinum supported, the pore size distribution can be measured by variously changing the catalyst and the positive electrode single electrode at that time. Performance and battery performance were evaluated. As a result, of the catalyst layer pore volume of 100 μm or more, the diameter of the pores was 0.1 μm or less, and the volume of the pores in the range of 0.1 to 1.0 μm was small, and the volume of the pores in the range of 10 to 100 μm was small. It was found that the larger the size, the higher the performance of the positive electrode single electrode. In order to obtain a high-performance electrode, the diameter of the pores is 0.1 μm or less, and the volume of pores in the range of 0.1 to 1.0 μm is 42% or less, and the volume of pores in the range of 10 to 100 μm is 11% or more, It became clear that it was necessary.
In addition, a battery was assembled in the same manner and the battery performance was evaluated. Also in this case, the same result as the positive electrode single electrode performance was obtained.

The catalyst is usually powder, and polytetrafluoroethylene (OTEF) is added thereto to form a catalyst layer. The catalyst powder forms secondary particles, and the distribution of pores in the catalyst layer varies depending on the size of the secondary particles or the distribution state of the particle diameters. Therefore, the distribution of the secondary particle diameter of the catalyst powder used is also a very important factor in optimizing the pores of the catalyst layer. Some catalysts may contain large secondary particles formed by aggregation of particles during preparation. In this case, the state of pore distribution becomes non-uniform inside the catalyst layer, resulting in a state of insufficient performance. Therefore, in order to bring out the performance sufficiently, it is necessary not to include large secondary particles. In order to confirm this, a catalyst powder containing large secondary particles was irradiated with ultrasonic waves to disintegrate the secondary particles, and the irradiation time was changed to prepare a catalyst having a different particle size distribution. The positive electrode single electrode performance was measured using these. As a result, it was found that the performance was improved by using a catalyst powder having a particle size distribution close to the normal distribution. As expected, the performance of the positive electrode using the catalyst powder containing large secondary particles was poor.

Further, since the pore distribution and water repellency of the gas diffusion layer and the phosphoric acid reservoir adjacent to the catalyst layer affect the phosphoric acid distribution of the catalyst layer, the gas diffusion layer and the phosphoric acid reservoir are also optimized. did. The gas diffusion layer suppresses the migration of phosphoric acid from the catalyst layer to the gas diffusion layer, retains sufficient phosphoric acid in the catalyst layer, and maintains sufficient gas diffusivity in the gas diffusion layer. Was increased by PTFE coating, and the amount of PTFE was also investigated variously to clarify the optimum amount. In the phosphoric acid reservoir, since phosphoric acid easily moves to the positive electrode side, the phosphoric acid reservoir of the positive electrode was treated with PTFE in order to enhance water repellency.

[0009]

The electrolyte distribution in the electrode, especially in the positive electrode catalyst layer, has a great influence on the performance. This is because the battery reaction is the reaction gas
This is because the reaction proceeds only at the three-phase interface of the electrolyte-catalyst active component (the region where the above three coexist). In order to improve the battery performance, it is necessary to increase this reaction area. When the electrolyte is absorbed by the catalyst layer, it is affected by the degree of water repellency of the surface of the material, but is first absorbed by the fine pores of the catalyst layer having a small diameter by capillary force. When the fine pores are filled with the electrolyte, the large pores are sequentially filled with the electrolyte. However, when the pores have a certain size, the electrolyte is not filled by the capillary force, and the absorption of the electrolyte stops at a certain pore diameter. As a result, a gas diffusion passage is secured in the catalyst layer and a three-phase interface is formed. However, the distribution of the catalyst layer pores causes a difference in the three-phase interface region, resulting in different performance. If the volume of large pores that will not be filled with electrolyte is small, the performance will be poor due to poor gas diffusion, while if the volume of large pores that will not be filled with electrolyte will be too large, the electrolyte will be insufficient. Causes performance degradation. When the catalyst layer is formed by using a general catalyst, the small-diameter pores are mainly the pores of the catalyst and are sufficiently present in the catalyst layer. However, pores having a large diameter are formed by PTFE and PTFE and a catalyst, and generally account for a small proportion of the total pore volume. Therefore, it was considered that a catalyst layer having a large proportion of pores with a large diameter was preferable.

Therefore, a method for supporting platinum as an active ingredient,
When electrodes were prepared for catalysts with different preparation methods and the correlation between the catalyst layer pore distribution and performance was examined, as expected, a positive electrode with a high electrode having a catalyst layer with a large proportion of pores with a large diameter It was clarified that it exhibits monopolar performance and battery performance. It is presumed that the gas diffusivity was improved and the three-phase interface region was expanded.

Further, the large-diameter pores (pores not filled with the electrolyte) are made of PTFE and PTFE as described above.
And formed by the catalyst. When the catalyst particles aggregate to form large secondary particles, the pore distribution changes and the mixing with PTFE becomes insufficient. As a result, the distribution of phosphoric acid is unbalanced, the distance between the catalyst containing the electrolyte and the large pores increases, and the gas cannot reach the catalyst sufficiently. As a result, the three-phase interface region is reduced, causing performance degradation. Therefore, the catalyst powder ideally preferably exhibits a normal distribution, and the peak position of the powder particle size is also an important factor.

[0012]

【Example】

Example 1 As a gas diffusion layer, a porous plate made of carbon fiber coated with 2.5 wt% of PTFE was used, and the catalyst and polytetrafluoroethylene (PT
A catalyst layer made of FE) was provided and baked in air at 360 ° C. for 30 minutes to prepare a positive electrode. As the catalyst, several powders in which platinum was supported on carbon powder in an amount of 20 to 30% by weight were used.
The PTFE content in the catalyst layer was about 45%. After absorbing the same amount of phosphoric acid into the catalyst layer of this positive electrode as during battery operation, the positive electrode single electrode performance was measured. The temperature is 200 ℃,
Air was excessively supplied to the gas diffusion layer side of the positive electrode. Also,
After the positive electrode single electrode performance measurement, the electrode was washed with water to remove the contained phosphoric acid, the catalyst layer and the gas diffusion layer were separated, and the pore distribution of the catalyst layer was measured by a mercury porosimeter. FIG. 1 is a diagram showing the relationship between the pore size distribution of the catalyst layer in the positive electrode and the unipolar performance. The pore distribution was divided into four according to the size of the pores.
That is, the diameter is 0.1 μm or less, 0.1 to 1.0 μm, 1.0
It was divided into 10 μm and 10 to 100 μm. The value obtained by dividing the pore volume in each range by the pore volume of 100 μm or less was used as an index. As is clear from Fig. 1, there is a correlation between the pore size distribution and the performance, and the diameter is 0.1
Higher performance is shown when the pore volume is less than or equal to μm, and the pore volume of 0.1 to 1.0 μm is small, and the pore volume of 10 to 100 μm is large. Diameter less than 0.1 μm, and 0.1
Pore volume of up to 1.0 μm is 42% or less, 10 to 100 μm
It is suitable that the pore volume of m is 11% or more. It should be noted that the samples having substantially the same surface area of platinum in the electrodes were compared. Also PT
Since the FE content and the firing conditions are the same, it is considered that the performance is governed by the pore distribution.

Similar results were obtained even when the amount of PTFE coating on the gas diffusion layer was varied between 2.0 and 3.0% by weight. The gas diffusion layer has a porosity of 75% or more, and the volume of pores having a diameter of 1.0 μm or more is 9% of the total pore volume.
It accounted for 0% or more.

Example 2 Considering that the size and distribution of the secondary particles of the catalyst affect the performance, water was added to the catalyst and ultrasonic treatment was carried out. The particle size distribution of the catalyst particles before and after ultrasonic treatment,
It was measured by a particle size analyzer by forward scattering and side scattering using a laser light source and a halogen light source. The result is shown in Figure 2.
Shown in. FIG. 2 is a diagram showing the particle size distribution of the catalyst powder. When ultrasonic treatment is not carried out, shoulders are recognized in a portion having a diameter of 10 μm or more, and it is considered that secondary particles are agglomerated with each other. However, when the catalyst is subjected to ultrasonic treatment, this shoulder becomes smaller as the treatment time increases. Further, the peak of the distribution becomes sharper by the ultrasonic treatment for 1 hour or more.

Then, in order to investigate the change in performance due to ultrasonic treatment, a catalyst treated with ultrasonic treatment was used in Example 1.
The positive electrode single electrode performance was measured in the same manner as. FIG. 3 is a diagram showing the relationship between ultrasonic treatment time and positive electrode single electrode performance. As can be seen from the figure, the positive electrode single electrode performance is improved by increasing the ultrasonic treatment time. In this case, the only difference between the catalysts is the ultrasonic treatment, and it can be said that the catalyst particle size distribution changes and the performance is improved. Therefore, in order to improve the performance, a catalyst having a particle size distribution after 1 hour and 3 hours of ultrasonic treatment in FIG. 2 may be used. In the particle size distributions after 1 hour and 3 hours of ultrasonic treatment, the proportion of particles having a size of 10 μm or more was 1.4% and 0.3%, respectively.

Example 3 Using the positive electrode having the highest performance in Example 1, a large cell having a side of 600 mm was produced. FIG. 4 is a diagram showing the structure of a phosphoric acid fuel cell. The positive electrode catalyst layer 5 and the negative electrode catalyst layer 3 are arranged on both sides of the matrix 4 holding the phosphoric acid electrolyte. The positive electrode gas diffusion layer 6 and the negative electrode gas diffusion layer 2, and the positive electrode phosphoric acid reservoir 7 and the negative electrode phosphoric acid reservoir 1 are sequentially arranged on the outer side thereof. In practice, this battery is sandwiched between separators and used in a number of layers, but in this example, the performance was evaluated using a single battery. In addition, the matrix holding the phosphoric acid electrolyte,
A material composed of silicon carbide and PTFE was used. The negative electrode gas diffusion layer is the same as that of the positive electrode. Regarding the phosphoric acid reservoir, the positive electrode is treated with PTFE, but the negative electrode is not treated. Since phosphoric acid easily moves to the positive electrode side, the water repellency of the phosphoric acid reservoir on the positive electrode side is enhanced. The battery was heated to 205 ° C. A gas of hydrogen 60%, carbon dioxide 15%, and water vapor 25% was supplied as a fuel gas at a utilization rate of 80%. Also, air is used as the oxidant gas at a utilization rate of about 50.
Supplied in%. In addition, the current density is 300 mA /
The value is based on cm ² . As a result, it was confirmed that the battery performance predicted from the positive electrode single-pole performance was obtained.

[0017]

EFFECTS OF THE INVENTION In the phosphoric acid fuel cell of the present invention, the gas diffusivity inside the catalyst layer is improved and the area of the three-phase interface is enlarged, so that high cell performance can be obtained.

[Brief description of drawings]

FIG. 1 is a characteristic diagram showing a relationship between a catalyst layer pore size distribution and a single electrode performance in a positive electrode.

FIG. 2 is a characteristic diagram showing a particle size distribution of catalyst powder.

FIG. 3 is a characteristic diagram showing a relationship between ultrasonic treatment time and positive electrode single electrode performance.

FIG. 4 is a perspective view showing the structure of a phosphoric acid fuel cell according to a third embodiment of the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Negative electrode phosphoric acid reservoir, 2 ... Negative electrode gas diffusion layer, 3 ... Negative electrode catalyst layer, 4 ... Matrix holding a phosphoric acid electrolyte,
5 ... Positive electrode catalyst layer, 6 ... Positive electrode gas diffusion layer, 7 ... Positive electrode phosphoric acid reservoir.

 ─────────────────────────────────────────────────── ─── Continued front page (72) Inventor Shohei Uozumi 7-1 Omika-cho, Hitachi-city, Ibaraki Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Akira Kato 7-chome, Omika-cho, Hitachi-shi, Ibaraki No. 1 Hitachi Ltd., Hitachi Research Laboratory (72) Inventor Hiroshi Miyadera 1-1-1, Omika-cho, Hitachi City, Ibaraki Prefecture Hitachi Ltd. Hitachi Research Laboratory (72) Inventor Seiichiro Ono Hitachi City, Ibaraki Prefecture Kokubun 1-1-1 Machi Kokubun Plant, Hitachi, Ltd.

Claims (7)

[Claims] 1. A positive electrode and a negative electrode composed of a catalyst layer and a gas diffusion layer are arranged with the catalyst layer side facing the phosphoric acid electrolyte and sandwiching the phosphoric acid electrolyte, and the gas diffusion layer of the positive electrode contains an oxidant gas. ,
In a phosphoric acid fuel cell in which fuel gas is supplied to the gas diffusion layer of the negative electrode, the volume of pores having a diameter of 0.1 μm or less and the diameter of 0.1 μm or less among the pores having a diameter of 100 μm or less of the catalyst layer of the positive electrode. 42% of the volume of pores from 1 μm to 1.0 μm
And the volume of pores of 10 μm to 100 μm is 1
A phosphoric acid fuel cell, which is 1% or more. 2. The oxidant gas according to claim 1, wherein the oxidant gas is air, and the fuel gas is a gas containing hydrogen as a main component, the catalyst layer is composed of a catalyst and polytetrafluoroethylene, and the main component of the catalyst is platinum. And a phosphoric acid fuel cell that is carbon. 3. The phosphoric acid fuel cell according to claim 2, wherein the secondary particles of the catalyst are in the range of 0.2 to 15 μm in diameter, and the ratio of secondary particles of 10 μm or more is 1.5% or less. . 4. The phosphoric acid fuel cell according to claim 3, wherein the proportion of secondary particles of 10 μm or more in the catalyst is 0.5% or less. 5. The phosphoric acid fuel cell according to claim 2, wherein the catalyst has secondary particles crushed by ultrasonic treatment. 6. The gas diffusion layer according to claim 1, wherein the gas diffusion layer is formed by molding carbon fibers into a sheet shape and coated with 2.2 to 3.3% by weight of polytetrafluoroethylene. Has a porosity of 75% or more, and the volume of pores with a diameter of 1.0 μm or more is 90% of the total pore volume.
The above is a phosphoric acid fuel cell. 7. The porous carbon plate with grooves coated with polytetrafluoroethylene is arranged outside the gas diffusion layer of the positive electrode, and the oxidant gas is outside the gas diffusion layer of the negative electrode according to claim 1. A phosphoric acid fuel cell in which a porous carbon plate with a groove not coated with polytetrafluoroethylene is arranged to form channels for fuel gas, and to collect current and store phosphoric acid.