JPS6030067B2 - Fuel cell - Google Patents

Description

[Detailed description of the invention] The present invention relates to fuel cells with improved bipolar separators.

Conventionally, in fuel cells that use phosphoric acid as an electrolyte, grooved bipolar separators are used to form gas supply passages for supplying fuel gas and oxidant gas to areas close to the electrodes of the cell. has been used. In addition to the above-mentioned functions, this bipolar separator should have the following functions. It is necessary to use a material with high densities to prevent diffusion and permeation of fuel gas and oxidant gas.

Since it is a structural material that supports the overall structure of a battery body made up of a number of stacked unit cells, it must have sufficient mechanical strength.
In addition, since it is in contact with the fuel electrode and the oxidizer electrode and serves as a medium for passing current associated with electrode reactions, it is required to have high electrical conductivity. In order to efficiently remove the generated heat generated during battery operation, high thermal conductivity is required to conduct heat to a cooling medium (eg, air cooling holes, radiating fins, coolant). As mentioned above, the bipolar separator has a high reaction gas supply property, fineness,
It is necessary to select from a comprehensive viewpoint, taking into consideration factors such as mechanical strength, electrical conductivity, thermal conductivity, other heat resistance, chemical resistance, and manufacturing cost. As a material having such characteristics, sintered graphite or a material prepared by adding a thermosetting phenol resin to graphite as a binder has been used, as disclosed in Japanese Patent Publication No. 50-111355.

These are known to have high density, high strength, and electrical conductivity, and are used as materials for bipolar separators for fuel cells.

A fuel cell using such a bipolar separator is, for example,
Unit batteries as shown in FIG. 1 are stacked in a container (not shown). In Fig. 1, 1 is a fuel electrode catalyst layer;
Reference numeral 2 denotes a support supporting the catalyst layer, and carbon fiber paper made from carbon fibers is used, for example.

3 is an air catalyst layer, and 4 is its support, for example, the same carbon fiber as in 2 is used.

Reference numeral 5 denotes an electrolyte holding layer, which is made of a polymeric nonwoven fabric or an inorganic nonwoven fabric with high phosphoric acid resistance, heat resistance, and hydrogen ion conductivity.

Reference numeral 6 denotes a bipolar separator plate, which has grooves 7 and 8 perpendicular to each other for supplying fuel gas and oxidizer gas such as air supplied from respective supply sources (not shown) on both sides. Unit batteries are stacked via this.

When a fuel cell constructed by stacking unit cells as described above is operated for a long period of time at 15,000 to 190°C using highly concentrated phosphoric acid, the operating characteristics of the fuel cell may gradually deteriorate. Are known. For example, FIG. 2 shows an example of its operating characteristics.

In the figure, the change in voltage of a unit cell when discharging at 150 A' per unit area is shown by a dotted line as a comparative example. As is clear from the figure, the voltage of the unit battery decreases over time. Incidentally, the voltage decrease per 100 field hours corresponds to about 30 V. Conventionally, the main causes of such a decrease in fuel cell output are: [1] Decrease in the effective reaction area of the catalyst due to agglomeration of catalyst particles and recrystallization.
Reduction due to wetting of the three-phase zone active in the gas electrode reaction by the electrode solution or water produced by the reaction '3] Mutual diffusion of fuel gas and oxidant gas through the electrode and the electrolyte holding layer.

Traditionally, attempts to improve the performance and extend the lifespan of fuel cells have mainly focused on
This is a countermeasure to the above-mentioned obstacles, and has achieved some results so far.

On the other hand, the present inventors have discovered that the bipolar separator gets wet with the electrolyte and various adverse effects caused as a result are a major cause of deterioration in battery performance and life span.

Normally, the electrolyte drains from the electrolyte retaining layer through the electrodes and gradually collects in the grooves of the bipolar separator. The electrolyte that accumulates in this way obstructs or blocks the flow of the reactive gas, resulting in insufficient supply of the reactive gas to the electrodes and deteriorating battery performance. In addition, the electrode catalyst may be poisoned if the part of the bipolar separator that is wet with the decomposition solution dissolves or swells while being exposed to high temperature and high concentration phosphoric acid for a long period of time. , the adhesion between the bipolar separator and the electrode or seal may be poor, resulting in leakage of reaction gas or
Phenomena such as mutual diffusion of reactant gases occur, and battery performance gradually deteriorates. In order to prevent such adverse effects, the present inventors have discovered that wetting by the electrolyte can be prevented by subjecting the bipolar separator to water repellent treatment. According to the present invention, by applying a water repellent agent for the electrolytic solution to the surface of the high-density bipolar separator, the electrolyte that has been drained out does not spread or get wet on the surface of the bipolar separator, but instead forms water beads. becomes.

Such a droplet-shaped electrolyte is easily blown away by the reaction gas flow. Therefore, the flow of reactant gas is not obstructed or blocked. In addition, the bipolar separator maintains high electrical conductivity and does not elute or swell due to electrolyte, making it possible to obtain stable battery performance over a long period of time.

Next, one embodiment of the present invention will be explained with reference to FIG.

The figure is a sectional view showing a part of the bipolar separator 16, which is the main part of the structure of the present invention, and shows grooves 17 forming gas passages on both surfaces.
, 18 are provided, and the entire surface has a water-repellent coating 20 against the electrolyte.

The coating material is selected from materials that are thermally and chemically stable at the operating temperature. The bipolar separator 16 is formed by the following process. Graphite 8 as a material for bipolar separators
0%, phenol resin 20%, and a specific gravity of 1.85. The size of the separator is 30 x 30 arcs, 6 fences thick, 2 convex ribs in width, 2 concave ribs in width, and 2 grooves in depth on both sides by molding or cutting. This bipolar separator
It was immersed in a wt% fluorinated ethylene propylene solution for 2 minutes, pulled up, air-dried, and then baked in a nitrogen atmosphere at 300°C for 10 minutes to form a water-repellent coating on the surface.
The water-repellent coating formed on the bipolar separator was diluted with 6
Since a wt% fluorinated ethylene propylene solution is used, the coating is not a densely arranged coating, but is formed by scattered particles, and has sufficient water repellency to the electrolyte. At the same time, the contact portion with the electrode made of carbon fiber paper has sufficient conductivity. Also, as a fuel electrode and air electrode,
Activated carbon with platinum added is coated on carbon fiber paper and then fired, and as shown in FIG.
N area unit batteries were stacked.

A 95% phosphoric acid electrolyte was co-immersed into an electrolyte holding layer made of a phenol resin non-woven fabric using hydrogen gas as a fuel and air as an oxidant gas.

The fuel cell of the present invention constructed in this manner was heated to a temperature of 170°C.
q○, the change in voltage per unit cell when discharged at 150 mA per unit area over a long period of time is shown by a solid line in FIG.

As a comparative example, the voltage change shown by the dotted line is an example in which the bipolar separator was not subjected to knee water treatment as described above. As is clear from the figure, in a fuel cell without water repellent treatment, 1
The voltage drop per 10 feed hours is approximately 30 mV, whereas the voltage drop in the fuel cell of the present invention is 15 skin V per 10 feed hours, demonstrating that the present invention is effective in suppressing the deterioration of battery performance. It shows.

In addition, the following table shows the observation results when the bipolar separator was separated and investigated after 100 feeding hours, 200 loading hours, and 300 leg hours of the fuel cell, which was tested in parallel under the same conditions. .

Each code in the table is -: No change 10: Slightly recognized Ten: clearly recognized 110: something worse than 10 are shown respectively.

As is clear from this table, in the fuel cell shown in the comparative example, as time passes, problems such as clogging of the gas flow grooves due to the electrolyte spilled out, elution of the grooves, swelling, and poor sealing occur.

In contrast, in the bipolar separator according to the present invention, the above-mentioned failures are almost prevented, indicating that the bipolar separator is stable over a long period of time. In the above examples, fluorinated ethylene propylene resin was used as the water repellent, but other phosphoric acid resins such as tetrafluorinated ethylene resin, trifluorochlorinated ethylene resin, vinylidene fluoride, hexafluorinated propylene resin, etc. It is possible to use a resin that has a high surface tension and a stable battery operating temperature.

As described above, when a bipolar separator is coated with a resin having a high surface tension for an electrolytic solution such as a phosphoric acid solution, the reaction gas flow path is blocked by the electrolytic solution, melting of the groove, swelling, etc. Moreover, it is possible to prevent problems such as poor sealing and to stabilize battery performance over a long period of time.

[Brief explanation of drawings]

FIG. 1 is a partially cutaway perspective view showing the structure of a fuel cell;
FIG. 2 is a characteristic diagram showing the voltage change of the fuel cell, and FIG. 3 is a sectional view showing a part of the bipolar separator that constitutes the main part of the present invention. 16...Bipolar separator, 20...Coating. Figure 1 Figure 2 Figure 3

Claims (1)

[Claims] 1. A unit cell comprising an electrolyte retaining layer between a fuel electrode and an oxidizer electrode, and a high-density unit cell in which a passageway for supplying fuel gas and oxidizer gas to each electrode is formed on the surface of the unit cell;
In a fuel cell configured by stacking a plurality of bipolar separators with high electrical conductivity interposed therebetween, at least the surface of the gas supply passage of the bipolar separators is provided with a coating that is water repellent to the electrolyte. A fuel cell characterized by forming.