JPH02824B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of the Invention] The present invention relates to a fuel cell using an alkaline electrolyte represented by a caustic potash (KOH) aqueous solution, and in particular to a fuel cell configured in a so-called free electrolyte type.

[Prior art and its problems]

This type of fuel cell is well-known as a highly efficient and highly reliable fuel cell installed in Apollo spacecraft, etc. However, carbon dioxide gas is contained in the fuel gas and oxidizing gas supplied to the battery. This has the disadvantage that the alkaline electrolyte reacts with the alkaline electrolyte to form a precipitate, which deteriorates the battery's function. There is no problem with using pure hydrogen as the fuel gas, but using pure oxygen as the oxidizing gas would be expensive, so there is a strong desire to use air as the oxidizing gas. However, as is well known, air always contains carbon dioxide gas, albeit in a relatively small amount.
When this type of fuel cell is operated for a long period of time using air as an oxidizing gas, it is impossible to avoid the accumulation of solid phase carbonate in the electrolyte, which is the reaction between the alkali metal in the electrolyte and carbon dioxide gas. It is known that when approximately 40% of the alkaline electrolyte becomes carbonated, the performance of the battery decreases rapidly. For this reason, a strategy is adopted in which the fuel cell is made into a so-called free electrolyte type, and the electrolyte is temporarily taken out of the cell and returned to the cell after removing the solid-phase reaction products as described above.

On the other hand, the fuel gas and the oxidizing gas electrochemically react within the battery to produce electricity, but the reaction product usually produces water as a byproduct. Since this water produced by the reaction naturally dilutes the electrolyte, it is necessary to remove the produced water by some method. However, since it is not easy to concentrate the electrolyte that has been diluted, measures are taken to extract the water produced by the reaction from the gas supply system. For this reason, in the gas supply system, an amount of gas in excess of the fuel and oxidizing gas consumed by the battery for power generation, for example several times the consumption amount, is sent into the battery to evaporate the reaction product water into the gas. The reaction product water is removed from the battery by condensing this evaporated water in a gas system outside the battery body. Sending several times the amount of gas needed by the battery to the battery is not a problem if the oxygen gas is air, since the supply source is infinite, but in a fuel gas system, the gas will of course be wasted. So,
The fuel gas supply system is usually configured as a circulation system so that the fuel gas is passed through the battery many times.

A conventional fuel cell constructed in this manner is shown in FIG. In the figure, the fuel cell main body is indicated by 1, which includes a fuel gas supply system 10, an oxidizing gas supply system 20, an electrolyte system 30, and a condensed water system 4.
0 is attached and provided. A fuel cell 1 is shown conceptually, with positive and negative electrodes 2, 2 serving as locations for generating electricity through an electrochemical reaction.
Then, the positive and negative terminals that send power to the outside are 3,
3. The inside of the battery main body 1 is divided into three compartments, a fuel gas chamber 11, an oxidizing gas chamber 21, and an electrolyte chamber 31, with the electrodes 2, 2 as boundaries. Of course, the battery body 1 in this figure is shown as a unit battery, and in reality, as is well known, many unit batteries are electrically connected in series.
It should be understood that the gas and liquid systems are stacked in parallel.

Fuel gas F is supplied to the fuel gas system 10 from the direction of the arrow on the left side of the figure, and the ejector 1 for fuel gas uses the pressure that this fuel gas normally has.
3 is driven, and the fuel gas that comes out from the ejector 13 to the right in the figure passes through the fuel gas chamber 11 and heat exchanger 12 in the battery main body 1 and returns to the ejector 1.
3 and heads toward the fuel cell main body 1 again. As a result, the fuel gas supply system constitutes one circulation system, and only the amount of fuel gas consumed by the fuel cell main body 1 is supplied from the left side by the action of the ejector 13. Several times as much fuel gas is passed through the gas chamber 11. On the other hand, the oxidizing gas system 20 is configured as a simple one-way path, and oxidizing gas, for example, air A is supplied from the direction of the arrow on the left side of the figure, passes through the carbon dioxide remover 23, and the oxidizing gas inside the fuel cell main body 1 is It enters the chamber 21, from where it passes through the heat exchanger 22 and is discharged to the atmosphere in the direction of the left arrow.

The heat exchangers 12 and 22 described above are for condensing the reaction product water that has evaporated into the fuel gas and oxidizing gas in the battery body 1 onto the water-cooled pipes to which the cooling water W is supplied, and these heat exchangers The condensed water from the vessel enters the condensate tank 41 of the condensate system 40 below. This condensed water tank 41 is provided with an overflow pipe 41a, and is configured such that a certain amount of condensed water always remains in the condensed water tank 41.

The electrolyte system 30 is also a kind of circulation system, and the electrolyte in the electrolyte chamber 31 of the battery body 1 is constantly or intermittently transferred to the electrolyte tank 3 through the pipe 31.
2 and from there returns to the electrolyte chamber 31 again by the action of the electrolyte pump 33. As mentioned above, water in excess of the generated reaction product water, that is, an amount of water including water in the electrolyte, is removed from the battery body 1 by the fuel and oxidizing gas, and is stored in the condensed water tank 41. Water can be stored in the electrolyte system 3 on a regular basis.
I need to replenish it to 0. This make-up water control is performed in the electrolyte tank 32, and the liquid level 32a is maintained at a predetermined level 32b at all times.
The drop in a is detected by the liquid level control switch 32c, in this example a float switch, and the condensed water system 40
The electrolyte tank 32 is supplied with condensed water in the condensed water tank 41 by opening the solenoid valve 42 inside. In this electrolytic solution system 30, a means for removing the solid phase carbonate described above, such as a filter, is provided in the pipe 31 or on the outlet side pipe of the pump 33.

In the fuel cell described above, even if air containing carbon dioxide is used as the oxidizing gas, if an appropriate carbon dioxide remover 23 is used, almost all the carbon dioxide in the oxidizing gas supplied to the cell body 1 can be removed. It is completely removed, and even if a small amount of carbon dioxide gas enters the battery body 1 and forms carbonate, it is removed by means such as a filter, so it is considered that the countermeasure against carbon dioxide gas is almost complete. It was getting worse. However, when such a fuel cell system was actually operated for a long period of time, it was found that the electrolyte deteriorated more quickly than expected, necessitating complete replacement of the electrolyte.

In order to investigate the cause of this, we conducted a thorough analysis of the gas supply system, including the battery itself, the gas in the electrolyte, and the impurities contained in the solution.As a result, we surprisingly found that the cause was in the electrolyte tank. was ascertained. The electrolyte tank 32 has a small opening 32d that communicates with the atmosphere, albeit slightly, as shown in the figure, and is the only exception in the almost completely sealed battery body and related system, and through this small opening 32d. In the electrolyte tank 32, carbon dioxide gas in the air diffuses into the electrolyte from the electrolyte surface 32a at a considerable rate, accelerating the deterioration of the solution. As a result of investigating the cause, various countermeasures were taken, but the most difficult point in taking countermeasures was that the electrolyte tank could not be completely sealed. That is, when the electrolyte tank is closed and operated, the gas in the space above the electrolyte surface 32a is naturally not released to the outside, so the amount of gas gradually increases and pushes the electrolyte surface downward. It becomes practically impossible to control the amount of electrolyte. The electrolyte is constantly in contact with fuel gas and oxidizing gas in the electrodes 2, 2, which are configured as porous membranes in the battery body 1, and absorbs a considerable amount of gas, and this dissolved gas is transferred to the electrolyte system 30. If it is not released somewhere, the electrolyte volume control will no longer work as described above.

[Purpose of the invention]

The purpose of the present invention is to prevent carbon dioxide gas from entering the electrolyte system, thereby creating a fuel cell in which the deterioration rate of the electrolyte is low even when cheap air is used as the oxidizing gas, and there is almost no need to replace the electrolyte. It's about getting.

[Key points of the invention]

According to the present invention, the above purpose is to extract a very small amount of fuel gas from the fuel gas circulation system, introduce it into an electrolyte tank for controlling the amount of liquid in the electrolyte system, and cover it on the surface of the electrolyte. This is achieved by flowing it as a gas and then releasing it from the electrolyte tank as a purge gas to the outside of the system.

When hydrogen is used as the fuel gas, the composition and amount of impurities in the fuel gas in the fuel circulation system are determined from the above-mentioned analysis to determine the cause. It is known that carbon dioxide gas is not detected. It is also known that the amount of these impurities tends to increase over time and gradually accumulate in the fuel gas system, and from this point of view, a portion of the fuel gas is constantly removed from the fuel gas circulation system, even if only in small amounts. It is originally desirable to purge it outside the system. The problem-solving means of the present invention focuses on the above points, and the amount of the cover gas, that is, the purge gas, may be 1% or less of the amount of fuel gas supplied to the fuel gas system, and at most 1%. A certain amount is enough. As a result, the surface of the electrolyte in the electrolyte tank is covered with the reducing fuel gas, so that the risk of carbon dioxide in the air entering the electrolyte is greatly reduced. Furthermore, since the electrolyte tank is operated under atmospheric pressure, the amount of electrolyte can be controlled smoothly, and gas dissolved in the electrolyte can be diffused.

[Embodiments of the invention]

An embodiment of the present invention will be described below with reference to FIG. In FIG. 2, parts common to those in FIG. 1 showing the prior art are given the same reference numerals.

The configuration of FIG. 2 differs from that of FIG. 1 in that fuel gas is extracted from the fuel gas circulation system 10 by an extraction pipe 14 and introduced into the electrolyte tank 32 as shown by arrow F. Of course, the amount of fuel gas extracted from the fuel gas circulation system 10 can be adjusted by inserting a throttle or a regulating valve in the middle of the extraction pipe 14. Further, the location from which the fuel gas is extracted by the extraction pipe 14 is not limited to this embodiment, and may be placed at an appropriate location in the fuel gas circulation system 10 without making a substantial difference.

The extracted fuel gas introduced into the electrolyte tank 32 covers the liquid surface 32a of the electrolyte as a cover gas, is discharged outside the system as a purge gas through a small opening 32d at the top, and is treated by burning or other means if necessary. . However, the amount of this purge gas is usually a small amount, and there is no problem even if it is directly dispersed into the atmosphere.

As a result, the liquid level of the electrolyte tank is under atmospheric pressure and does not substantially fluctuate, so that the liquid level control of the electrolytic liquid, and thus the liquid volume control of the electrolytic liquid, can be performed accurately and smoothly. However, the gas pressure governing the fuel and oxidizing gas chambers 11 and 21 in the battery main body 1,
That is, in relation to the operating pressure of the fuel cell, if it is necessary to maintain the gas pressure above the liquid level in the electrolyte tank 32 at a pressure other than atmospheric pressure, connect a pressure regulating valve to the small opening 32d, The pressure within the electrolyte tank 32 may be maintained at a desired value, and the necessary purge gas may be discharged.

FIG. 3 is a graph comparing the operation results A of the fuel cell according to the present invention and the operation results B of the conventional fuel cell, where the horizontal axis represents the operation time and the vertical axis represents the voltage generated by the cell per unit cell. It is being Carbon dioxide gas enters the electrolyte, approximately 40% of the alkaline electrolyte
When it becomes carbonate, the generated voltage drops to about 0.7 volts, and the electrolyte needs to be replaced. In the figure, a small triangle X indicates the point in time when electrolyte replacement is required. As can be seen from the figure, in the conventional fuel cell, the electrolyte needs to be replaced every 1000 hours, but in the case of the fuel cell according to the present invention, the electrolyte does not need to be replaced for at least 1500 hours. It should be noted that the tendency of the battery generated voltage to decrease over time as shown in the figure is not necessarily due to the single cause of carbonation of the electrolyte, but is due to a combination of other factors such as a decline in the functionality of the battery's catalyst electrode. Therefore, it should be considered that the effects of the present invention are greater than those shown in FIG.

Hydrogen can be supplied as a fuel gas and air can be supplied as an oxidizing gas to the fuel cell configured as described above. It is necessary to supply hydrogen gas that does not contain carbon dioxide gas with high purity, and if hydrogen is produced by decomposing hydrocarbons such as natural gas and may contain carbon dioxide gas, an ejector is used in the same way as in the air supply system. It is necessary to insert a carbon dioxide remover on the upstream side of 13. In addition, the fact that hydrazine gas, which is usually of high purity, can be used as a fuel gas means that
Of course.

In the embodiment shown in FIG. 2, an example of a simple flow of the oxidizing gas supply system was introduced, but of course the system is not limited to this and may be configured as a circulation system. For example, if the part after the exit of the carbon dioxide remover 23 is constructed as a circulation system, the efficiency of using oxygen as an oxidizing gas in the air will be increased, and the capacity of the carbon dioxide remover 23 can be selected to be small. In this case, it is necessary to insert the heat exchanger 22 into the oxidizing gas circulation system, but in the case of only one-way path as shown in FIG. The air containing the vapor of reaction product water generated during the reaction may be directly released into the atmosphere. The present invention is applicable and effective in all of these cases within its scope.

[Effects of the Invention] As described above, the present invention includes a fuel gas supply system, an oxidizing gas supply system, and an electrolyte, of which at least the fuel gas supply system is configured as a circulation system, and In a fuel cell in which reaction product water is extracted from the gas supply system as condensed water in excess of the generated amount, and the required amount is supplied to the electrolyte system from this condensed water, a small amount is extracted from the fuel gas circulation system. By extracting the fuel gas and introducing it into the electrolyte tank, it covers the surface of the electrolyte in the tank as a cover gas, so there is no chance for the electrolyte to come into contact with air containing carbon dioxide, and the carbon dioxide is removed. There is less risk of entering the alkaline electrolyte to form carbonates and deteriorating the electrolyte. Furthermore, the extracted fuel gas used as the cover gas mentioned above is discharged from the electrolyte tank as a purge gas to the outside of the system.
Since the electrolyte is discharged, the liquid level in the electrolyte tank and therefore the amount of electrolyte can be controlled under atmospheric pressure, and gases dissolved in the electrolyte can also be diffused. Furthermore, discharging the extracted fuel gas outside the system as a purge gas also has the effect of preventing impurity gas from accumulating in the fuel gas circulation system.

[Brief explanation of drawings]

FIG. 1 is a system diagram showing the gas supply system and electrolyte system of a fuel cell according to the prior art, FIG. 2 is a system diagram showing the gas supply system and electrolyte system of a fuel cell according to the present invention, and FIG. FIG. 2 is a graph diagram illustrating the operational results of the fuel cell according to the invention in comparison with the operational results of the conventional fuel cell. In the figure, 1: fuel cell main body, 10: fuel gas supply system configured as a circulation system, 12, 22: heat exchanger as means for condensing reaction product water in the gas supply system, 14: extraction of fuel gas extraction tube as a means,
20: Oxidizing gas supply system, 30: Electrolyte system, 32:
Electrolyte tank, 32a: Electrolyte level, 32c:
32d: a small opening as a purge gas discharge means; 40: a condensed water system; 42: a solenoid valve as a means for replenishing condensed water and a means for controlling the amount of electrolytic solution. .

Claims (1)

[Scope of Claims] 1. A fuel and oxidizing gas supply system that supplies a reducing fuel gas and an oxidizing gas that may contain carbon dioxide gas to the battery, respectively, and an electrolyte system that controls the amount of alkaline electrolyte. At least the fuel gas supply system of both gas supply systems is configured as a circulation system, and water in excess of the reaction product water generated in the battery is condensed in the system and taken out from the gas supply system, and the condensed water is In an apparatus in which an appropriate amount of water is replenished into an electrolyte system, a small amount of fuel gas is extracted from the fuel gas circulation system and the electrolyte in the tank is transferred to an electrolyte tank for controlling the liquid volume of the electrolyte system. A fuel cell characterized in that the fuel cell is introduced as a cover gas to cover the liquid surface of the electrolyte solution tank, and is discharged from the electrolyte tank as a purge gas to the outside of the system. 2. The fuel cell according to claim 1, wherein the oxidizing gas is air.