JPH0574190B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a fuel cell, and particularly to a fuel cell in which the composition of a catalyst support layer is improved.

[Technical background of the invention and its problems]

A single cell in a fuel cell forms a gas diffusion electrode65
% or more of porous carbon paper or a porous carbon sintered plate, and a catalyst supporting layer between the matrix forming the electrolyte layer.

The catalyst supporting layer is usually formed of conductive fine particles and a hydrophobic polymer.

The configuration of a single cell of a conventional fuel cell will be explained with reference to FIG. 1 is a fuel electrode base material, that is, an anode side electrode, 2 is a catalyst support layer, 3 is a matrix, 4 is an air electrode base material, that is, a cathode side electrode, 5
is a catalyst support layer. FIG. 1b shows a modification of a, in which a fuel supply groove 6 and an air supply groove 7 are provided on the back surfaces of the anode side electrode 1 and the cathode side electrode 4, respectively, so as to be perpendicular to each other.

However, as a result of the inventors' study of the configurations of these conventional single cells, the following problems became clear.

(1) Although it is the same carbon material as the catalyst support layer, it is required to use a material with improved corrosion resistance, such as carbon black or graphite fine particles, but the material with improved corrosion resistance itself has water repellency. Therefore, the water repellency of the catalyst supporting layer increases abnormally and repels the electrolyte, and the contact between the catalyst and the electrolyte is insufficient.Therefore, the reaction area, which is the contact interface between the electrolyte, the catalyst, and the reaction gas, is small. Battery internal resistance increases and electronic performance deteriorates significantly.

(2) Fluctuations in the operating temperature, operating pressure, load, and humidity of the reactant gas of the fuel cell are inevitably accompanied by changes in the volume of the electrolyte.

That is, phosphoric acid, which is an electrolyte, is a reaction product of water and phosphorus pentoxide as shown in the following reaction formula, and is also a highly hygroscopic desiccant. Therefore,
At high temperature 3H ₂ O + 1/2P ₄ O ₁₀ 2H ₃ PO _4Under dry conditions, the above reaction proceeds to the left;
Under conditions of low temperature and high humidity, the above reaction proceeds to the right. Although omitted here for the sake of explanation, many complex condensates of phosphoric acid are produced as the intermediate products. However, the progress of these reactions has the same tendency as explained above.

On the other hand, when a load is applied, water is produced as an electrochemical reaction product. Furthermore, since humidity is determined by the ratio of water vapor partial pressure to total pressure, the equilibrium of the reaction will vary depending on the operating pressure.

Due to such fluctuations, the volume of the electrolyte changes from time to time.

By the way, as the electrolyte changes, if the water repellency of the catalyst support layer is too high, the electrolyte that overflows from the matrix, which is the electrolyte layer, will conflict with the water repellency of the catalyst and will be pushed out through the catalyst support layer and reach the electrodes. However, when the amount of electrolyte in the matrix decreases, the overflowing electrolyte supplies electrolyte to the matrix again, resulting in a so-called "reservoir function" in which a constant amount of electrolyte is always retained in the matrix. inhibited. For this reason, once the electrolytes overflow,
Due to the strong water repellency of the catalyst support layer, it remains isolated and held within the electrode base material.

As a result, the electrolyte in the matrix gradually decreases, the void area increases accordingly, the separator function that prevents the reaction gas of the fuel cell, that is, the fuel gas, and air from mixing is lost, and the mixing of the re-reactant gas occurs. This causes a so-called crossover phenomenon, which significantly reduces battery performance.
It can also cause an explosion.

(3) In order to improve battery performance, the catalyst carrier is required to have fine particles of several tens of microns or less, and the finer the particles, the more the above phenomena (1) and (2) are promoted.

[Purpose of the invention]

The present invention was made in view of the above problems, and an object of the present invention is to provide a fuel cell with improved electronic performance by improving the composition of the catalyst supporting layer and effectively utilizing the reservoir function of the electrolyte of the electrode.

[Summary of the invention]

In order to achieve such an object, the present invention provides a fuel cell comprising a plurality of stacked single cells each having a pair of gas diffusion electrodes arranged opposite to each other with a matrix forming an electrolyte layer interposed therebetween. The catalyst supporting layer disposed between the diffusion electrodes is characterized in that it is formed by binding a mixture of conductive fine particles and phosphoric acidic particles with a hydrophobic polymer.

[Embodiments of the invention]

Hereinafter, embodiments of the present invention will be described with reference to FIG.

The present invention improves the composition of the catalyst support layer 2, and forms it by binding a mixture of conductive fine particles 8 (e.g. carbon black) and phosphoric acid particles 9 (e.g. silicon carbide) with a hydrophobic polymer 10. .

Example 1 The amount of platinum supported was 14% by weight, and the average particle size was
Conductive particles of 0.03μ, acetylene black particles, and silicon carbide particles with an average particle diameter of 5μ, which are phosphoric acid particles, were mixed with the volume ratio of 10% of acetylene black particles to 1% of silicon carbide particles. . Next, a mixture of these volume ratios was kneaded by adding water as a solvent, and further, 40% by weight of a dispersion solution of polytetrafluoroethylene was added as a solid content and kneaded with a mixer.
Porous carbon paper, each forming an electrode with a porosity of 75% and a thickness of 0.4 mm, was applied by suction spraying. After drying at 80°C for 2 hours, it was fired at 320°C for 1 hour. The amount of catalyst is approximately 2.5 mg/cm ² . The thickness of the catalyst supporting layer on the carbon paper was about 80 μm. Using these electrodes, a single cell was formed as shown in FIG. 1a, and the internal resistance of the battery and the load voltage of 220 mA/cm ² were measured, and the results are shown in FIG. As shown in the figure [Silicon carbide particle volume]/
[Carbon particle volume] ratio is less than 2.5%,
It can be seen that if it is larger than 20%, the internal resistance of the battery increases rapidly and the performance of the battery decreases significantly. Therefore, the above volume ratio is within the range of 2.5 to 20%.
It is preferably within the range of 3.3 to 10%. The weight ratio to this volume ratio (2.5-20%) was approximately (30-240%).

In addition, in order to confirm the reservoir function, a cycle test of battery operation and shutdown was repeated for batteries prepared under the conditions of point A in Figure 3.
Although a crossover phenomenon occurred after the 100-cycle test, no crossover phenomenon was observed in the battery according to the present invention even after the 100-cycle test, and the battery performance was stable. That is, it is shown that the porous carbon paper has a sufficient effect of absorbing the change in the capacity of phosphoric acid, which is an electrolyte.

The reason for this is thought to be as follows. The catalyst supporting layer is in good contact with the electrolyte and has enough water repellency to prevent the catalyst from being immersed in the electrolyte, and has a passage through which the electrolyte can freely pass, and the electrode base material has a reservoir for the electrolyte. This is because the functions can be fully demonstrated. In addition, the volume ratio of phosphoric acid particles to conductive fine particles is
At 2.5% or more, the phosphoric acidic portion of the catalyst support layer communicates in the thickness direction of the catalyst support layer, allowing the electrolyte to freely pass through the highly water-repellent catalyst support layer. By the way, when the hydrophobic polymer is less than 20% by weight, the contact force between the conductive particles becomes small, and the water repellency of the catalyst supporting layer decreases, causing the catalyst to become wet with the electrolyte. On the other hand, when the value exceeds 45%, the conductivity of the catalyst supporting layer decreases, the internal resistance of the battery increases, and the performance of the battery decreases.

Phosphorophilic acid particles are required to be materials that are stable at high temperatures, in phosphoric acid, and in air; ZrO ₂ ,
It is desirable to use a material that can be used as a matrix material, such as Ta ₂ O ₅ , SiC, (ZnO) ₂ P ₂ O ₇ . These are electronically non-conducting materials. Furthermore, if the phosphoric acid particles have a capacity ratio of 20% or more to the conductive fine particles, the conductivity of the catalyst layer will decrease, which is not preferable.

Example 2 The average particle size with a platinum loading of 10% by weight was
Silicon carbide particles with an average particle size of 5 μm, which are phosphorophilic, were mixed with carbon black particles of 0.3 μm. The volume ratios were 92.3% and 7.7%, respectively.
Next, water is added as a solvent and kneaded, and a dispersion solution of polytetrafluoroethylene is added as a solid content.
40% by weight was added and kneaded. Porosity 75%
It was applied by suction spraying onto porous carbon paper with a thickness of 0.4 mm. After drying at 80℃ for 2 hours, drying at 320℃
It was baked for 2 hours. The amount of catalyst is approximately 5 mg/cm ² .

The catalyst support layer is approximately 0.05 cm thick on the surface of the carbon paper.
Layers were formed with a thickness of ~0.1 mm. constitutes a single cell,
Furthermore, as in Example 1, a battery was made and the operation/stop cycle test was repeated 100 times.
Similarly, no crossover phenomenon was observed, and stable electronic performance was verified.

Similar results were obtained when zirunium oxide powder was used instead of silicon carbide particles.

〔Effect of the invention〕

As explained in detail above, according to the present invention, while the conventional catalyst support layer did not have good affinity with the electrolyte, the catalyst support layer according to the present invention has good affinity with the electrolyte. , the sea-surface three-phase zone of electrolyte-catalyst-reactant gas increases throughout the catalyst support layer, and the internal resistance of the battery decreases, improving electronic performance. By attracting excess electrolyte to the electrodes, it becomes possible to supply the electrolyte from the electrodes when the amount of electrolyte in the matrix decreases, thereby making it possible to maintain the amount of electrolyte in the matrix constant.

[Brief explanation of the drawing]

Figures 1a and b are cross-sectional configuration diagrams of a single cell, Figure 2 is a cross-sectional view of essential parts of a catalyst support layer according to an embodiment of the present invention, and Figures 3a and b are volumes of the material forming the catalyst support layer. FIG. 3 is a diagram showing the relationship between ratio and battery characteristics. 1... Anode side electrode, 4... Cathode side electrode, 2, 5... Catalyst supporting layer, 6... Fuel supply groove,
3...Matrix, 7...Air supply groove, 8...
Conductive fine particles, 9... Phosphoric acidic particles, 10...
Hydrophobic polymer.

Claims (1)

[Claims] 1. In a fuel cell formed by stacking a plurality of unit cells each having a pair of gas diffusion electrodes arranged opposite to each other with a matrix impregnated with phosphoric acid interposed therebetween, a catalyst support layer provided between the matrix and the gas diffusion electrode. A mixture of 80 to 97.5 Vol% of conductive fine particles and 2.5 to 20 Vol% of phosphoric acid particles to form a 100 Vol% mixture is formed, and 20 to 45 wt% of a hydrophobic polymer is added to 50 to 80 wt% of this mixture. A fuel cell characterized in that the fuel cell is formed by