JPS6077365A - Operation of fuel cell - Google Patents

Abstract

PURPOSE:To increase operation life of a fuel cell by intermittently exchanging reaction gases supplied to each of electrodes by exhausting reaction gases in gas flow lines with inactive gas. CONSTITUTION:After a fuel cell is operated for a specified time, a fuel gas supply valve 10a and an oxidizing gas supply valve 11a are closed to stop gas supply. Inactive gas supply valve 12a and 12b are opened and inactive gas, for example, nitrogen is supplied to pipe lines and a fuel cell 5 to exhaust reaction gases in arrow directions B and D. Then valves 12a and 12b are closed, the valve 10a is opened, the valve 10b is closed, the valve 10c is opened. The valve 11a is opened, the valve 11b is closed, the valve 11c is opened. When reaction gases are supplied, former anode operates as a cathode, and former cathode operates as an anode and electricity having opposite polarity is generated in electrodes M and N and supplied to a load after changing polarity with a polarity exchanger 12.

Description

DETAILED DESCRIPTION OF THE INVENTION A unit battery is prepared by forming a unit battery comprising a set of electrodes supporting an active catalyst and having water resistance against reaction product water, and an electrolyte layer interposed between these electrodes. The present invention relates to a method for operating a fuel cell in which a plurality of fuel cells are stacked and provided with a gas flow system for supplying different reaction gases to each electrode.

[Prior art and its problems]

A fuel cell directly converts the chemical energy of fuel into electrical energy.The basic structure of the unit cell is a pair of porous electrodes sandwiching an electrolyte layer, and a reactant gas is injected into these electrode layers. This is done by supplying fuel gas and oxidizing gas respectively. FIG. 1 shows an example of the structure of the unit battery described above.

In Figure 1, one fuel pole (
(hereinafter referred to as anode) 1 air electrode (hereinafter referred to as cathode)
2 and an electrolyte layer 3 is sandwiched between them to constitute a unit battery,
Electrically conductive and gas-impermeable grates 4 are arranged in contact with the anode 1 and the cathode 2, respectively, and the fuel gas flows through the plurality of rows of grooves 4a of the plate 4, and the oxidizing gas flows through the plurality of rows of grooves 4b in directions perpendicular to each other. The reactant gas and electrolyte come into contact with each other at the electrodes of each porous element, causing an electrochemical reaction to generate electricity and extract energy from the conductive grating.

FIG. 2 is an exploded perspective view of a fuel cell in which unit cells as described above are stacked. In FIG. 2, a fuel gas supply manifold 6 and an exhaust manifold 7 are arranged on opposite sides of the battery stack 5, and an oxidizing gas supply manifold 7 is mounted on the opposite side of the battery stack 5. An exhaust manifold 8 and an exhaust manifold 9 are arranged. The fuel gas enters the manifold 6 through the inlet pipe 6a of the manifold 6, flows through the plurality of rows of grooves 4a in the battery stack 5 in the directions of arrows A and B, is collected in the manifold 7, and then flows out from an outlet pipe (not shown). Exhausted. - Power oxidation residue enters the manifold 8 through an inlet chamber (not shown) of the manifold 8, takes the flow of fuel gas through the plurality of rows of grooves 4b of the battery stack 5, flows in the direction of right-angled arrows C and D, and flows into the manifold 8. 9 and exhausted through the outlet pipe 9a, these reaction gases undergo an electrochemical reaction within the unit cell to generate electricity. And the entrance...
The outlet pipe and the manifold together with the grooves in the plate described above constitute a waste flow system.

Now, in conventional technology, the performance of a fuel cell usually drops rapidly after a certain period of operation, and the reason for this drop in performance is the deterioration of the catalyst.

There are 81 possible causes, such as deterioration of the electrolyte composition and corrosion of the side slopes of the battery structure, but it is thought that gas diffusion is largely □ in electrodes that have porous gas diffusivity. That is, this is caused by the electrolytic solution gradually moving into the electrode pores, causing wetting of the electrode catalyst layer and hindering gas diffusion. For example, using an acidic electrolyte,
In the case of a fuel cell that uses hydrogen as the fuel gas and air as the oxidizing gas, water vapor, which is reaction product water, is generated on the cathode side due to the chemical reaction of the fuel cell, and this generated water vapor is transferred to the electrolyte impregnated in the matrix. Pull the liquid towards the cathode. Furthermore, in normal fuel cells, in order to ensure sufficient cathode reaction, a large amount of air is supplied to the cathode, which is several times to several times as large as the fuel gas flow rate. A portion of the electrolyte present in the layer is carried out to the air passage by scattering or evaporation along with water vapor. Along with this, the electrolyte moves from the anode side to the cathode side through the matrix, and at this time, the electrolyte is trapped in some of the pores in the cathode, causing so-called wetting of the cathode catalyst layer, and the diffusion of air as reaction scum. Inhibition occurs and performance decreases. Particularly in fuel cells using an acidic electrolyte, the polarization of the cathode is greater than the polarization of the anode, so deterioration in characteristics due to poor gas diffusion at the cathode greatly affects the characteristics of the entire cell.

Therefore, in order to maintain the life of the fuel cell for a long time, K
It is desirable to prevent the pores in the electrode from being blocked by the electrolyte and to maintain good diffusion of fuel gas and oxidant gas.

For this reason, some devices have a structure in which poor gas diffusion within the electrode does not occur even during long-term operation by increasing the water resistance of the electrode. However, water repellent agents 2, such as polytetrafluoroethylene, which are added to increase this water resistance, adhere to the active sites of the catalyst and reduce the active surface area of the catalyst, so excessive addition will reduce the characteristics of the fuel cell. cause In addition, during long-term operation, once the water resistance of the senior official decreases due to operating conditions or changes in the electrode material, electrolyte and electrolyte permeate into the pores in the electrode, resulting in poor diffusion of gas. was no longer likely to regain its properties.

[Purpose of the invention]

In view of the above-mentioned drawbacks of the prior art, an object of the present invention is to provide an operating method that can maintain good diffusion of reaction gas, maintain its performance even during long-time fuel cell operation, and extend the operating life of the fuel cell. With the goal.

[Summary of the invention]

According to the present invention, this purpose is achieved by forming a set of electrodes that support a catalyst that is active on both fuel gas and oxidant gas and that has water resistance against reaction product water, and interposing an electrolyte layer between these electrodes. In a fuel cell formed by stacking a plurality of unit cells each having a gas distribution system for supplying a different reaction gas to each of the sets of electrodes, the reaction gas in the gas distribution system is extracted by an inert gas. This is accomplished by intermittent exchange of the reactant gases supplied to each of the sets of electrodes throughout the process.

As mentioned above, for example, in a fuel cell that uses an acidic electrolyte, water vapor as reaction product water is generated at the cathode, and this water vapor pulls the electrolyte impregnated into the matrix toward the cathode, causing the cathode catalyst to Wetting of the layer occurs, which inhibits the diffusion of air as a reactive gas. This wetting of the electrode catalyst layer by the electrolyte can be reduced to some extent by providing the electrode with water resistance; on the other hand, if the electrode does not have water resistance, wetting of the electrode catalyst layer will progress rapidly. In the present invention, in which the electrodes of the set are used alternately as anodes and cathodes, it is important that the electrodes of the set have water resistance against the water produced by the reaction. The catalyst used must be active at both the fuel electrode and the air electrode.

[Embodiments of the invention]

The present invention will be described in detail below based on the drawings.

FIG. 3 is a characteristic diagram showing changes over time in the output characteristics of a fuel cell obtained by carrying out the operating method of the present invention, and FIG. 4 is a circuit diagram showing the system of the fuel cell operating method according to the present invention. It is. In the figure, the same parts as in FIGS. 1 and 2 are given the same reference numerals.

FIG. 3 shows an example of a unit cell of a phosphoric acid fuel cell. Platinum is supported as a catalyst that is active against both gases and oxidizing gases, and electrodes that have water resistance of 50 wt % with respect to this catalyst are used as anodes and cathodes. The reactant gases supplied using the gas flow system containing the gas flow system are intermittently exchanged with an inert gas through a step of removing the reactant gas from the gas flow system, and the anode is used as the cathode, and the cathode is used as the anode. A characteristic curve P shows the relationship between the output voltage and the elapsed time when the device is operated as follows.

In other words, the output voltage of the fuel cell was initially A volt due to the flow of the reactant gas, but as time passed, the output voltage began to drop before point B, and the reactant gas continued to flow. If this continues, the output voltage will rapidly drop as shown by the broken line BD line. However, at point B, after removing the reactive gas from the gas distribution system with an inert gas, the reactive gas is replaced, and the fuel gas that was flowing to the anode is transferred to the cathode, and the oxidizing gas that was flowing to the cathode is transferred to the anode. If we reverse the direction of the electrochemical reaction in the unit cell by using the conventional anode as the cathode and the cathode as the anode to generate electricity, the output voltage will recover at point B without dropping and the characteristic curve will change. Output-1 pressure is maintained as between POBC. However, after driving for a certain period of time, the output voltage starts to drop rapidly again in front of C, so if you replace the reactant gas at point 6 as before and return to the initial state, the output voltage will not drop suddenly. The output voltage is maintained. If we exchange the flow of reactant gases, the anode acts as a cathode and the cathode acts as an anode, and the electrochemical reaction is reversed, so the polarity is reversed in the characteristic diagram, and the voltage is displayed as 0. Note that the output voltage At the point when the value decreased, the presence or absence of permeation of reactive gas from the anode to the cathode and from the cathode to the anode in the unit cell in the conventional operating method and the operating method of the present invention was confirmed, but gas permeation was not detected in either case. First, the deterioration in characteristics in the conventional operating method is thought to be due to wetting of the electrodes, as described above. Therefore, according to the operating method of the present invention, in which the direction of the electrochemical reaction in the unit cell is intermittently switched by exchanging the supplied reaction gas,
It is thought that by changing the direction of movement of the reaction products, the electrolytic solution that was biased toward one electrode is pulled back, suppressing wetting of the electrode and maintaining good gas diffusion.

The effect of the present invention in reducing diffusion inhibition of reactive gases is
In order to confirm that the reaction gas is obtained by exchanging the supplied reaction gas, at point B, where the output voltage begins to decrease, only the operation of removing the reaction gas from the gas distribution system with inert gas is performed. , we measured the change in output voltage when the same reaction gas continued to flow, but the results were as follows.
As shown by curve B-8 in the figure, the output voltage tended to drop, and no recovery was observed as when the reactant gas was replaced.

Next, referring to FIG. 4, a method of operating a fuel cell according to the present invention will be explained. In FIG. 4, reference numeral 5 denotes a fuel cell in which unit cells are stacked. First, the fuel gas is its source 1
The fuel passes through the multi-fuel cell 5 as indicated by arrow A, via valve 10a and valve 10b, and is exhausted as indicated by arrow B. In this case valve 10c
Close the valve 12a to prevent it from flowing in the direction of arrow E.
The inert gas supply source 12 is also closed, and the flow of inert gas from the inert gas supply source 12 is stopped. The oxidizing gas is supplied from its source 11 to the valve 11.
a.

The fuel flows to the multi-fuel cell 5 in the direction of arrow C via valve 11b and is exhausted in the direction of arrow C. At this time, as in the case of fuel gas, valve 11c is closed to prevent it from flowing in the direction of arrow F. The valve 12b for supplying the moss inert gas is closed.

In this state, the fuel cell undergoes an electrochemical reaction between the reactant gas and the unit cell, producing positive and negative electricity at the electrodes M and N of the current collector plate of the fuel cell.

N is connected to the load 13 via the polarity switching device 12. However, after a certain period of operation time has elapsed, the time schedule device (not shown) closes the fuel gas supply valve 10a and the oxidation gas supply valve 11a to stop the flow. Then, open the inert gas supply valves 12a and 12b, and flow an inert gas, such as nitrogen, into the fuel gas and oxidizing gas pipes and the fuel cell, as indicated by the arrows B and 12b, respectively. D
The reaction gas is discharged in the direction of. After removing the reaction gas from the gas distribution system with inert gas, inert gas supply valve 1
2a and 12b are closed, and the fuel gas supply valve 10a is closed.
open, close valve 10b, open valve 10c, open valve 11a for supplying the oxidizing gas, close valve 11b, and close valve l.
When the reactant gas is supplied with the ie opened, the fuel gas flows through the pipe in the direction of arrow E, and inside the fuel cell in the direction of arrows C and D.
, while the oxidizing gas flows through the pipe in the direction of arrow F.
The fuel flows in the directions of arrows A and B within the fuel cell, and the anode at this point is the cathode.

- The cathode still reacts as an anode, the electrode M.

Electricity of opposite polarity is generated at N, and the polarity is switched by the polarity switching device 12 and connected to the load.

In this way, the reaction gas is exchanged repeatedly at predetermined time intervals, and the fuel cell is operated. Note that while the reactant gas flow system is being replaced with an inert gas, the electric circuit is opened in the polarity switching device 12 and the connection with the load is disconnected. However, if necessary, the load can be supplied with electricity from another power source, such as a plurality of fuel cells, by operating them in parallel at the time of switching. In addition, the predetermined time for exchanging the reaction gas can be set to a time period in which the output characteristics can be activated by exchanging the reaction gas again if the output characteristics are deteriorated due to the exchange of the reaction gas.

In the description of the above embodiment, an example in which the reactant gas supplied at a predetermined period is exchanged is explained, but the present invention is not limited to this, and for example, the output voltage value of the fuel cell is constantly monitored. The reaction gas supplied may be replaced when the output voltage value falls below a predetermined value. In the first embodiment, the case where the operating method of the present invention was applied to a phosphoric acid fuel cell was explained, but the present invention can also be applied or implemented to an alkaline fuel cell, and in this case, the reaction product is the anode. Occurs on the side.

〔Effect of the invention〕

As explained above, according to the present invention, the reaction gas supplied to each set of electrodes is intermittently exchanged through a process of removing the reaction gas from the gas flow system using an inert gas. The method has the significant effect of extending the operating life of the fuel cell. As mentioned above, this effect is based on the fact that by changing the direction of movement of the water generated by the reaction within the battery, it has a kind of activating effect or function recovery effect, in which wetting of the electrode is suppressed, and is caused by intermittent replacement. By repeating this process repeatedly, the lifespan of the fuel cell can be more than doubled during its long operating period.

With the demand for increasing the capacity of fuel cells, that is, increasing the size of the electrode surface, the operating life of the battery tends to be shortened. In particular, deterioration in characteristics due to poor gas diffusion in the cathode greatly affects the battery life, and this deterioration tends to accelerate over time.

The operating method according to the present invention has the effect of preventing the problem of poor gas diffusion, or at least preventing the acceleration of the deterioration of characteristics due to poor gas diffusion, and as fuel cells become larger in capacity, they will be put into practical use. It has important implications.

In addition, according to the present invention, in which the reaction gas supplied to each set of electrodes is intermittently exchanged, when the catalyst supported on the fuel electrode is poisoned by scum containing poisonous substances, the catalyst is removed. The accompanying effect of cleaning by oxidation removal using oxidant gas can also be expected.

[Brief explanation of the drawing]

FIG. 1 is an exploded perspective view showing the basic structure of a unit cell of a fuel cell, FIG. 2 is an exploded perspective view of a fuel cell in which unit cells are stacked, and FIG. 3 is a unit box of a fuel cell according to the operating method of the present invention.
FIG. 4 is a characteristic diagram showing changes in the output characteristics of the pond over time, and FIG. 4 is a circuit diagram showing the system of the operating method of the present invention. 1 near node, 2: cathode, 3: electrolyte layer, 5: fuel cell, 12: polarity switching device. 1 figure, 2 figures, 3 figures

Claims (1)

[Claims] 1) A set of electrodes that support a catalyst that is active on both fuel gas and oxidizing gas and have water resistance against reaction product water, and three layers of electrolyte between these electrodes. In a fuel cell in which a plurality of unit cells are stacked with gas distribution systems for supplying different reaction gases to each of the sets of electrodes, the reaction gas in the gas distribution system is replaced by an inert gas. A method of operating a fuel cell, characterized in that the reactant gas supplied to each of the two sets of electrodes is intermittently replaced through a withdrawal process. 2. In the operating method according to claim 1, -
A method of operating a fuel cell characterized by exchanging the reactant gas supplied to a set of electrodes at a predetermined period 3) The method of operating a fuel cell according to claim 1, comprising: -
A method of operating a fuel cell, characterized in that a reaction gas supplied to a set of electrodes is converted into a fuel cell. 4) The method of operating a fuel cell according to claim 3, wherein the performance value representing the performance of the fuel cell is an output voltage value of the fuel cell.