JP2007258024A - Fuel cell system - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a fuel cell system capable of self-starting even when a fuel cell is exposed to freezing point atmosphere during the stop of operation, and having no risk of breaking of an MEA caused by freezing. <P>SOLUTION: The fuel cell system includes a polymer electrolyte fuel cell having a solid polymer electrolyte membrane; a fuel gas supply exhaust means for supplying/exhausting fuel gas to/from a fuel electrode of the polymer electrolyte fuel cell; an oxidant gas supply/exhaust means for supplying/exhausting oxidant gas to/from an air electrode of the polymer electrolyte fuel cell; and a drying means for drying the polymer electrolyte membrane, installed in the oxidant gas supply exhaust means and/or the fuel gas supply exhaust means, and the drying means removes a part of moisture contained in the solid polymer electrolyte membrane after the stop of power generation so that the water content of the solid polymer electrolyte membrane becomes W<SB>60</SB>or more and W<SB>RH'(Tmin)</SB>×1.1 or less when the minimum temperature (Tmin) reaches freezing point after the stop of power generation of the polymer electrolyte fuel cell. <P>COPYRIGHT: (C)2008,JPO&INPIT

Description

  The present invention relates to a fuel cell system, and more particularly to a fuel cell system including means for preventing freezing of water contained in an electrolyte membrane after the fuel cell is stopped.

  A solid polymer fuel cell has a membrane electrode assembly (MEA) in which electrodes are bonded to both surfaces of a solid polymer electrolyte membrane as a basic unit. In the polymer electrolyte fuel cell, the electrode generally has a two-layer structure of a diffusion layer and a catalyst layer. The diffusion layer is for supplying reaction gas and electrons to the catalyst layer, and carbon paper, carbon cloth, or the like is used. The catalyst layer is a part that becomes a reaction field for electrode reaction, and generally comprises a composite of carbon carrying an electrode catalyst such as platinum and a solid polymer electrolyte (electrolyte in the catalyst layer).

  For the electrolyte membrane or the catalyst layer electrolyte that constitutes such an MEA, a fluorine-containing electrolyte having excellent oxidation resistance (for example, Nafion (registered trademark, manufactured by DuPont)), Aciplex (registered trademark, Asahi Kasei Co., Ltd.) And Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.)) are generally used. In addition, the fluorine-containing electrolyte is excellent in oxidation resistance, but is generally very expensive. Therefore, in order to reduce the cost of the polymer electrolyte fuel cell, the use of a hydrocarbon-based electrolyte has been studied.

  However, there are still problems to be solved in order to put the polymer electrolyte fuel cell into practical use as an in-vehicle power source. For example, a polymer electrolyte fuel cell is desired to be able to start up independently even if it is left in an environment below freezing after the operation is stopped. However, if it is left below the freezing point after the operation is stopped, the water in the electrode freezes. As a result, gas diffusion is hindered, making it difficult to start the fuel cell. In addition, when water freezes at the interface between the electrolyte membrane and the electrode, the electrode may be separated from the electrolyte membrane.

In order to solve this problem, various proposals have heretofore been made.
For example, in Patent Document 1, before stopping the operation, the humidification of air is first stopped, and then the non-humidified air is supplied to the cathode electrode for a predetermined time, and then the operation of the fuel cell power generator is stopped. A method of operating a molecular fuel cell power generator is disclosed. This document describes that the electrolyte membrane can be dried by performing a non-humidifying operation before stopping the fuel cell, so that structural breakdown due to freezing inside the electrode during cold can be prevented.

  Patent Document 2 discloses a fuel cell device that drives a fuel cell when the temperature outside the device becomes below the freezing point and heats circulating water circulating in the fuel cell. In this document, when the fuel cell is driven when the temperature outside the apparatus is below the freezing point, the circulating water is heated by the heat generated from the fuel cell. It describes that freezing can be prevented.

JP 2001-332281 A JP-A-11-214025

  If the humidification operation is performed before the fuel cell is stopped, the electrolyte membrane can be dried. However, if moisture is removed too much, the proton conductivity of the electrolyte membrane is significantly reduced, making it difficult to restart. Also, the method of driving the fuel cell when the temperature outside the apparatus becomes below freezing point is wasteful of fuel gas and is not efficient. Furthermore, even when the fuel cell is exposed to a temperature below freezing point, an example in which the self-sustained activation is possible and the necessary and sufficient drying conditions for suppressing electrode peeling due to freezing, etc. have been shown. Not.

The problem to be solved by the present invention is to provide a fuel cell system that can start up independently even when the fuel cell is exposed to below freezing point while the fuel cell is stopped.
In addition, another problem to be solved by the present invention is that even when the fuel cell is exposed to below freezing point while the fuel cell is stopped, the MEA due to water freezing in the fuel cell is damaged. It is to provide a fuel cell system without the above.

In order to solve the above problems, a fuel cell system according to the present invention supplies a solid polymer fuel cell having a solid polymer electrolyte membrane, and supplies and discharges fuel gas to the fuel electrode of the solid polymer fuel cell. Fuel gas supply / discharge means, oxidant gas supply / discharge means for supplying / discharging oxidant gas to / from the air electrode of the polymer electrolyte fuel cell, oxidant gas supply / discharge means and / or fuel A drying means for drying the solid polymer electrolyte membrane provided in the gas supply / discharge means, wherein the drying means has a minimum temperature (Tmin) reached after freezing of the solid polymer fuel cell is below freezing point. A portion of the water contained in the solid polymer electrolyte membrane after power generation is stopped so that the water content of the solid polymer electrolyte membrane is W ₆₀ or more and W _{RH ′ (Tmin)} × 1.1 or less. It consists of something that removes.
However,
W ₆₀ = maximum water content that the solid polymer electrolyte membrane can have when the relative humidity RH ′ (= saturated vapor pressure of ice × 100 / saturated vapor pressure of supercooled water) is 60% in the presence of ice below freezing point .
W _{RH ′ (Tmin)} = maximum water content that the solid polymer electrolyte membrane can have in the relative humidity in the presence of ice corresponding to the minimum temperature (Tmin).

When the solid polymer electrolyte membrane is dried after the operation of the fuel cell is stopped, if the water content of the solid polymer electrolyte membrane is made to be W ₆₀ or more, the fuel cell is exposed to a freezing point after the operation is stopped. Even if it exists, it can be started independently. In addition, when the solid polymer electrolyte membrane is dried so that the water content is W _{RH ′ (Tmin)} × 1.1 or less, the MEA due to freezing is obtained even when the fuel cell is exposed to below freezing after the operation is stopped. Can be prevented from being damaged.

Hereinafter, an embodiment of the present invention will be described in detail.
The fuel cell system according to the present invention includes a polymer electrolyte fuel cell, fuel gas supply / discharge means, oxidant gas supply / discharge means, and drying means.

  Although not shown, the polymer electrolyte fuel cell includes a membrane electrode assembly (MEA) in which electrodes (fuel electrode, air electrode) are bonded to both surfaces of a solid polymer electrolyte membrane, and a separator that sandwiches both surfaces of the MEA. It has. A gas flow path for supplying a fuel gas or an oxidant gas to the electrode is provided on the surface of the separator on the MEA side. A polymer electrolyte fuel cell is generally formed by stacking a plurality of unit cells composed of such MEAs and separators.

  For the polymer electrolyte membrane constituting the MEA, perfluorocarbon sulfonic acid polymer represented by Nafion (registered trademark, manufactured by DuPont) and various hydrocarbon electrolytes are used. The electrode generally has a two-layer structure of a catalyst layer bonded to the surface of the solid polymer electrolyte membrane and a diffusion layer bonded to the outside thereof. The diffusion layer is generally composed of carbon paper, carbon cloth or the like, and the catalyst layer is generally composed of a composite of carbon carrying an electrode catalyst such as platinum and a solid polymer electrolyte (electrolyte in the catalyst layer). The catalyst layer electrolyte is generally made of the same material as the solid polymer electrolyte membrane, but a different material may be used. Furthermore, carbon is generally used for the separator, but the use of various metal materials is also being studied.

The fuel gas supply / discharge means is means for supplying / discharging the fuel gas to / from the fuel electrode of the polymer electrolyte fuel cell. Fuel gas supplied from a fuel gas supply device (for example, a hydrogen cylinder, a reformed gas generator, etc.) is distributed to the fuel electrode of each MEA via an intake manifold, and the gas discharged from each fuel electrode is exhausted. It is discharged through the manifold.
The oxidant gas supply means is means for supplying and discharging the oxidant gas to the air electrode of the polymer electrolyte fuel cell. Oxidant gas supplied from an oxidant gas supply device (for example, an oxygen cylinder, a compressor for supplying air, etc.) is distributed to the air electrode of each MEA via the intake manifold, and is discharged from each air electrode. The gas is discharged through an exhaust manifold.

The drying means is means for drying the solid polymer electrolyte membrane, and includes the solid polymer electrolyte membrane when the minimum temperature (Tmin) reached after the solid polymer fuel cell power generation is stopped is below freezing point. This means that part of the water contained in the solid polymer electrolyte membrane is removed after power generation is stopped so that the amount of water is not less than W _{60 and not} more than W _{RH ′ (Tmin)} × 1.1. “After power generation is stopped” refers to after the supply of fuel gas for power generation is stopped.
The drying means may be provided in one of the oxidant gas supply / discharge means and the fuel gas supply / discharge means, or may be provided in both. On the air electrode side of the polymer electrolyte fuel cell, water is generated by the electrode reaction and is in a wet state than the fuel electrode side. Therefore, the drying means is at least the air electrode side, that is, the oxidizing gas supply / discharge means. It is preferable to provide in any part.

  Whether or not the minimum temperature (Tmin) reached after power generation stops below the freezing point may be determined by the operator of the fuel cell system based on experience, or measured current weather data and accumulated past weather. Whether or not the temperature is below freezing point may be determined or predicted based on the data. In any case, when it is clear that the minimum temperature (Tmin) is below freezing, or when there is a high probability that the minimum temperature (Tmin) will be below freezing, the drying means is operated after power generation is stopped, and the electrolyte membrane Remove some of the water in the water.

Further, when the drying means is operated, the electrolyte membrane is not completely dried, but is dried so that the water content is W ₆₀ or more and W _{RH ′ (Tmin)} × 1.1 or less. The water content of the electrolyte membrane at the end of drying is more preferably _{WRH ′ (Tmin)} × 1.0 or less.
Here, “W ₆₀ ” refers to the maximum water content that the solid polymer electrolyte membrane can have when the relative humidity RH ′ under ice coexistence under freezing is 60%. The “relative humidity in the presence of ice” below freezing is the ratio of the saturated vapor pressure of ice to the saturated vapor pressure of supercooled water at a certain temperature (= saturated vapor pressure of ice × 100 / saturated water vapor pressure of supercooled water). The relative humidity when the ice is in an equilibrium state in an environment where ice coexists. The relative humidity in the presence of ice is uniquely determined by the temperature.
“W _{RH ′ (Tmin)} ” refers to the maximum water content that the solid polymer electrolyte membrane can have in the relative humidity in the presence of ice corresponding to the minimum temperature (Tmin).
It should be noted that the water content of the electrolyte membrane when the relative humidity in the presence of ice below freezing point is x% is almost the same as the water content when the relative humidity is 25% and the relative humidity is x%. This point will be described later.

Specifically, there are the following drying means.
The first specific example of the drying means includes a gas having a relative humidity lower than the relative humidity RH ′ corresponding to the minimum temperature (Tmin) (hereinafter referred to as “dry gas”) as fuel gas supply / discharge means and This is a scavenging means for supplying the oxidant gas supply / discharge means and scavenging the polymer electrolyte fuel cell.
FIG. 1A is a conceptual diagram of a fuel cell system provided with scavenging means. In FIG. 1 (a), a system including scavenging means on both the fuel electrode side and the air electrode side is shown, but this is merely an example, and either the fuel electrode side or the air electrode side is shown. As described above, the scavenging means may be provided only on one side and the scavenging means is preferably provided at least on the air electrode side.

The relative humidity RH of the dry gas only needs to be lower than the relative humidity RH ′ (x%) in the presence of ice at the lowest temperature (Tmin). In general, the lower the relative humidity RH of the dry gas, the shorter the water content of the electrolyte membrane can be made to a predetermined value or less.
As the dry gas, it is preferable to use air in which humidification is stopped. For example, when scavenging the air electrode side, it is preferable to continue introducing the humidified air to the oxidant gas supply system as it is. On the other hand, when scavenging the fuel electrode side, means for introducing air into the fuel gas supply system (air introduction means) is provided, and after the fuel gas supply is stopped, humidification is stopped in the fuel gas supply system Air is preferably introduced.

The dry gas is not limited to air, and other gases (for example, N ₂ gas, CO ₂ gas, etc.) may be used. In this case, a cylinder filled with these gases is installed in the fuel cell system, and the gas is supplied from the cylinder after the operation is stopped. Further, for example, when scavenging the fuel electrode side, a fuel gas in which humidification is stopped may be used as the dry gas. In this case, since the fuel gas is discharged outside the fuel cell without being consumed in the reaction, it is preferable to provide a means (for example, a combustion means) for treating the exhausted fuel gas in the exhaust system.

As the time (scavenging time) for operating the scavenging means, an optimum time is selected according to the relative humidity of the drying gas, the drying gas flow rate, the minimum temperature (Tmin), and the like.
For example, when the fuel cell system is used only in a limited area, the minimum temperature (Tmin) is almost uniquely determined by the area of use. In such a case, the scavenging time may be controlled by “time control” that is performed only for a predetermined time.
On the other hand, when the minimum temperature (Tmin) is difficult to predict, the scavenging time is controlled by monitoring the degree of drying of the electrolyte membrane and performing scavenging until the moisture content of the electrolyte membrane is within a certain range. Is preferably performed. In this case, the degree of drying of the electrolyte membrane can be known by measuring the membrane resistance of the electrolyte membrane, the relative humidity of the gas after scavenging, and the like.

A second specific example of the drying means is a moisture absorption means disposed in the fuel gas supply / discharge means and / or the oxidant gas supply / discharge means.
FIG. 1 (b) shows a conceptual diagram of a fuel cell system provided with moisture absorption means. In FIG. 1 (b), a system provided with moisture absorbing means only on the air electrode side is shown, but this is merely an example, and only the fuel electrode side or both the fuel electrode side and the air electrode side are shown. As described above, the water absorbing means may be provided on the air electrode and the water absorbing means is preferably provided at least on the air electrode side.

In FIG.1 (b), the water | moisture-content absorption means is provided in the exhaust system of oxidant gas. The moisture absorbing means may be provided in the intake system. Further, it is preferable to provide the moisture absorbing means at a position as close to the electrolyte membrane as possible.
Specifically, the moisture absorbing means is composed of a container filled with a moisture absorbent, and this container is connected to the exhaust system via an on-off valve. The opening / closing valve shuts off (closes) the moisture absorbing means and the exhaust system during normal operation, and allows the moisture absorbing means and the exhaust system to communicate (open) when the electrolyte membrane is dried after the operation is stopped. .

  When the on-off valve is opened, the oxidant gas supply / discharge system may be an open system in which both the inlet and the outlet are open, or a closed system in which both the inlet and the outlet are closed. In the case of an open system, when the moisture absorption means is provided in the exhaust system, the oxidant gas supply means may be used to scavenge the air. When the air is scavenged to the air electrode side, the water in the electrolyte membrane can be efficiently conveyed to the water absorption means. In the case of a closed system, circulation means for circulating air may be provided on the air electrode side, and the air may be circulated in the sealed oxidant gas supply / discharge system. When the air on the air electrode side is circulated, the moisture in the electrolyte membrane can be efficiently carried to the moisture absorbing means.

Specifically, as a moisture absorbent,
(1) Inorganic adsorbents such as zeolite (molecular sieve) and silica,
(2) Salts such as lithium chloride and sodium chloride, or aqueous solutions thereof
and so on.
The filling amount of the moisture absorbent in the container is not particularly limited, and can be arbitrarily selected according to the purpose. The filling amount of the moisture absorbent may be equal to or more than the minimum amount (hereinafter, simply referred to as “minimum amount”) necessary for making the electrolyte membrane have a predetermined moisture content.
The time (absorption time) for operating the moisture absorbing means is the optimum time depending on whether it is a closed system, whether air is scavenged or circulated, the distance between the electrolyte membrane and the moisture absorbing means, the material of the moisture absorbent, etc. select. In general, the longer the absorption time, the lower the water content of the electrolyte membrane.
The control of the absorption time may be performed by “time control” as in the first specific example, or may be performed by “water content control”. In addition, when the filling amount of the moisture absorbent is the minimum amount, the control of the absorption time is unnecessary.

  When the filling amount of the moisture absorbent is the minimum amount, or when the filling amount of the moisture absorbent is larger than the minimum amount, when the absorption of water by the moisture absorbent reaches a limit, the regeneration treatment of the moisture absorbent or Replace the moisture absorbent. Specifically, the regeneration treatment is performed by heat desorption of moisture. The above point is the same when the moisture absorbing means is provided on the fuel electrode side.

A third specific example of the drying means is a water condensing means arranged in a low temperature part of the fuel gas supply / discharge means and / or the oxidant gas supply means.
FIG. 1 (c) shows a conceptual diagram of a fuel cell system provided with water condensing means. In FIG. 1 (c), a system provided with water condensing means only on the air electrode side is shown, but this is merely an example, and only the fuel electrode side or both the fuel electrode side and the air electrode side are shown. As described above, the water condensing means may be provided on the air electrode and the water condensing means is preferably provided at least on the air electrode side.

In FIG.1 (c), the water condensing means is provided in the low temperature part of the exhaust system of oxidizing gas. The water condensing means may be provided in the low temperature part of the intake system. The water condensing means is preferably provided as close to the electrolyte membrane as possible.
The “low temperature part” refers to a part that becomes a temperature (T _trap ) lower than the temperature (T _cell ) of the fuel cell after the operation is stopped. In general, since a fuel cell generates heat during power generation, any fuel cell can be a low temperature part as long as it is upstream of the intake manifold or downstream of the exhaust manifold.
Specifically, the water condensing means is composed of a container for condensing water, and this container is connected to the exhaust system via an on-off valve. A cooling means for promoting condensation of water may be provided around the container. The on-off valve shuts off (closes) the water condensing means and the exhaust system during normal operation, and communicates (opens) the water condensing means with the exhaust system when the electrolyte membrane is dried after the operation is stopped. .

  When the on-off valve is opened, the oxidant gas supply / discharge system may be an open system in which both the inlet and the outlet are opened, or a closed system in which both the inlet and the outlet are closed. In the case of an open system, when the water condensing means is provided in the exhaust system, the oxidant gas supply means may be used to scavenge the air. When air is scavenged to the air electrode side, moisture in the electrolyte membrane can be efficiently conveyed to the water condensing means. In the case of a closed system, circulation means for circulating air may be provided on the air electrode side, and the air may be circulated in the sealed oxidant gas supply / discharge system. When the air on the air electrode side is circulated, moisture in the electrolyte membrane can be efficiently conveyed to the water condensing means.

The time for condensing the water condensing means (condensation time) is selected in accordance with whether or not the system is a closed system, whether air is scavenged or circulated, the distance between the electrolyte membrane and the water condensing means, and the like. In general, the longer the condensation time, the lower the water content of the electrolyte membrane.
The control of the condensation time may be performed by “time control” or may be performed by “water content control” similarly to the first specific example. Note that when the system is a closed system, the relative humidity in the fuel cell is a relative humidity that naturally balances with ice, so it is not necessary to control the condensation time.

When the water condensation by the water condensing means reaches the limit, a drain valve (not shown) is opened to discharge the water accumulated in the container. Or you may return the water collect | recovered by the water condensing means to the humidification system of fuel gas and / or oxidant gas.
The above point is the same when the water condensing means is provided on the fuel electrode side.
The scavenging means, moisture absorption means, and water condensing means described above may be used alone or in combination with a plurality of means.

Next, the operation of the fuel cell system according to the present invention will be described.
After the operation of the fuel cell is stopped, when the temperature of the fuel cell is predicted to be below freezing point, the above-described drying means is operated to dry the electrolyte membrane. For example, when the drying means is a scavenging means, dry air is sent to the air electrode and / or the fuel electrode. For example, when the drying means is a moisture absorption means, the opening / closing valve is opened after the operation is stopped, and moisture is adsorbed to the moisture adsorbent while scavenging or circulating air as necessary. Similarly, when the drying unit is a water condensing unit, the open / close valve is opened after the operation is stopped, and water is condensed in a container (trap) while scavenging or circulating air as necessary. When a certain time has elapsed and the water content of the electrolyte membrane has reached a certain range, the drying means is stopped.

If the fuel cell is left below freezing after it is shut down, the bulk water in it will freeze. As shown in JIS Z8806, below the freezing point, the saturated water vapor pressure of ice is lower than the saturated water vapor pressure of supercooled water. Therefore, as shown in FIG. 2, the relative humidity RH ′ under the coexistence of ice below the freezing point decreases as the temperature decreases. The saturated vapor pressure of supercooled water is a concept introduced to define “relative humidity in the presence of ice (relative humidity when in an equilibrium state in an environment where ice coexists)”. This does not mean that there is actually supercooled water.
On the other hand, the water content of the electrolyte membrane decreases as the relative humidity RH decreases. FIG. 3 shows the relationship between the relative humidity RH at 25 ° C. and the water content of the perfluorinated electrolyte membrane. In addition, the vertical axis | shaft of FIG. 3 is the value normalized by setting the water content at 25 degreeC and relative humidity 100% as 1. In FIG. FIG. 3 shows that the water content decreases as the relative humidity RH decreases.

The water content at 25 ° C. and the relative humidity x% correlates with the maximum water content when the “relative humidity RH ′ in the presence of ice” below the freezing point is x%. That is, the above-mentioned W ₆₀ substantially matches the water content at 25 ° C. and relative humidity 60%. Further, when the relative humidity RH ′ in the presence of ice corresponding to the minimum temperature (Tmin) is y%, W _{RH ′ (Tmin)} substantially matches the water content at 25 ° C. and the relative humidity y%.
For example, as shown in FIG. 2, the “relative humidity RH ′ in the presence of ice” at −50 ° C. is 61%. On the other hand, as shown in FIG. 3, the normalized water content W ′ _{61 at} 25 ° C. and relative humidity 61% (corresponding to the relative humidity in the presence of ice at −50 ° C.) is about 0.43.

FIG. 4 shows the relationship between W ′ _{61 at} 25 ° C. and W ′ _{NMR at} −50 ° C. measured for various electrolyte membranes. W ′ _{61 at} 25 ° C. was determined from an adsorption isotherm. In addition, W ′ _{NMR at} −50 ° C. was measured using a ¹ H-nuclear magnetic resonance (NMR) method. W ′ _NMR represents the normalized water content at −50 ° C., that is, the amount of water (relative value) having high mobility contained in the electrolyte membrane at −50 ° C.
The measurement method of W ′ _NMR is as follows. That is, first, an electrolyte membrane was placed in an NMR sample tube having an outer diameter of 5 mm, and the inside thereof was adjusted to be balanced at 25 ° C. and a relative humidity of 100%. After the upper end of the sample tube was sealed with a gas burner, the NMR spectrum at 25 ° C. was measured. Next, the temperature of the sample tube was lowered to −50 ° C. and left overnight, and then the NMR spectrum was measured again. The integrated intensity of both spectra was calculated, and the spectrum relative intensity (W ′ _NMR ) at −50 ° C. when the value at 25 ° C. was taken as 1 was obtained.
Specifically, since it is known that the sensitivity of NMR measurement increases in inverse proportion to the absolute temperature, the spectral intensities at 25 ° C. and −50 ° C. are I (25 ° C.) and I (-50 C), W ′ _NMR = I (−50 ° C.) × (−50 + 273.15) / {I (25 ° C.) × (25 + 273.15)}.
FIG. 4 shows that W ′ _{NMR at} −50 ° C. and W ′ _{61 at} 25 ° C. are in good agreement.

The state after the operation of the fuel cell is considered to correspond to a relative humidity of 100%. If exposed to below freezing, a part of the water in the electrolyte membrane is no longer staying in the electrolyte membrane, but is expected to go out of the membrane and freeze.
In order to confirm this, the electrolyte membrane was brought into an equilibrium state at 25 ° C. and a relative humidity of 100%, and this was put in a transparent polymer bag and sealed. Subsequently, the whole bag was cooled below freezing point, and the surface of the film was observed with an optical microscope. FIG. 5 shows an optical micrograph of the electrolyte membrane cooled to −30 ° C. FIG. 5 shows that ice is generated on the surface of the electrolyte membrane. This ice decreased with increasing temperature and was absorbed inside the membrane, so it is considered that the water originally contained in the electrolyte membrane came out of the membrane below the freezing point and was frozen.
Next, the electrolyte membrane was conditioned at 25 ° C. and 75% relative humidity (corresponding to the relative humidity in the presence of ice at equilibrium at −30 ° C.) in a polymer bag. Sealed and the entire bag was cooled to below freezing. FIG. 6 shows an optical micrograph of the electrolyte membrane cooled at −30 ° C. FIG. 6 shows that when humidity is adjusted at a relative humidity of 75%, ice is not generated even at −30 ° C.

Therefore, when the minimum temperature (Tmin) reached after the fuel cell is stopped is below freezing point, any one of the drying means described above is operated after the operation is stopped, and the water content of the electrolyte membrane is less than W _{RH ′ (Tmin).} When the electrolyte membrane is dried in such a manner, even when the electrolyte membrane reaches the minimum temperature (Tmin), excess water is not discharged from the electrolyte membrane. In reality, even if the water content is excessive by about 10%, the amount of water discharged from the electrolyte membrane is relatively small, so the adverse effects caused by freezing are small.
Therefore, when the electrolyte membrane is dried so that the water content is at most W _{RH ′ (Tmin)} × 1.1 or less, the water exuded from the electrolyte membrane is frozen at the interface with the electrode. Damage can be suppressed. Moreover, since there is little possibility that the exuded water freezes in the electrode, the diffusion of the reaction gas is not hindered. Therefore, it is not necessary to heat the MEA with a heater or to start it up using an auxiliary power source at the time of restarting, and self-starting is facilitated.

On the other hand, when the electrolyte membrane is dried more than necessary after the operation of the fuel cell is stopped, the membrane resistance of the electrolyte membrane is excessively increased. Increased membrane resistance makes it difficult to start up independently.
FIG. 7 shows the relationship between the relative humidity at 25 ° C. and the proton conductivity of the perfluorinated electrolyte membrane. As shown in FIG. 7, it can be seen that the proton conductivity of the electrolyte membrane changes with the relative humidity of 60% as a boundary. This is because water is coordinated to the sulfonic acid group and used to dissociate the acid in the low humidity region, whereas the acid dissociates and the acid and water move easily in the high humidity region. This is considered to be a state (see, for example, TAZawodzinski et al., J. Electrochem. Soc., 140, 1041 (1991)).
Therefore, in order to facilitate the self-sustained activation, the drying process is stopped in a state where both the acid and the water move easily, that is, the water content of the electrolyte membrane is equal to or higher than the water content corresponding to 25 ° C. and the relative humidity of 60%. Thus, it is preferable to dry the electrolyte membrane.
As described above, the water content at 25 ° C. and a relative humidity of 60% substantially matches the water content W ₆₀ when the “relative humidity in the presence of ice” is 60% below the freezing point. Therefore, when the electrolyte membrane is dried after the operation of the fuel cell is stopped, if the electrolyte membrane is dried so that the water content is W ₆₀ or more, an increase in membrane resistance is suppressed and self-sustained activation is facilitated.

  Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the scope of the present invention.

  The fuel cell system according to the present invention can be used for in-vehicle power sources, stationary small generators, cogeneration systems, and the like.

FIG. 1A, FIG. 1B, and FIG. 1C are schematic configuration diagrams of a fuel cell system that includes scavenging means, moisture absorption means, and water condensation means, respectively. It is a figure which shows the relationship between temperature, saturated vapor pressure, and relative humidity in the presence of ice. It is a figure which shows the relationship between the relative humidity in 25 degreeC, and the water content of an electrolyte membrane. It is a diagram showing the relationship between the ₆₁ 'W in _NMR and 25 ° C.' W at -50 ° C.. It is a SEM photograph of the electrolyte membrane surface when the electrolyte membrane conditioned at 25 ° C. and relative humidity of 100% is cooled to −30 ° C. It is a SEM photograph of the electrolyte membrane surface when the electrolyte membrane conditioned at 25 ° C. and 75% relative humidity is cooled to −30 ° C. It is a figure which shows the relationship between the relative humidity in 25 degreeC, and proton conductivity.

Claims (4)

A polymer electrolyte fuel cell comprising a polymer electrolyte membrane;
Fuel gas supply / discharge means for supplying and discharging fuel gas to and from the fuel electrode of the polymer electrolyte fuel cell;
Oxidant gas supply and discharge means for supplying and discharging oxidant gas to the air electrode of the polymer electrolyte fuel cell;
A drying means for drying the solid polymer electrolyte membrane provided in the oxidant gas supply / discharge means and / or the fuel gas supply / discharge means,
The drying means has a water content of the solid polymer electrolyte membrane of not less than W _{60 and} W _{RH ′ (Tmin)} × when the minimum temperature (Tmin) reached after stopping the power generation of the polymer electrolyte fuel cell is below freezing point. A fuel cell system for removing a part of water contained in the solid polymer electrolyte membrane after power generation is stopped so as to be 1.1 or less.
However,
W ₆₀ = maximum water content that the solid polymer electrolyte membrane can have when the relative humidity RH ′ (= saturated vapor pressure of ice × 100 / saturated vapor pressure of supercooled water) is 60% in the presence of ice below freezing point .
W _{RH ′ (Tmin)} = maximum water content that the solid polymer electrolyte membrane can have in the relative humidity in the presence of ice corresponding to the minimum temperature (Tmin).   The drying means supplies a gas having a relative humidity lower than the relative humidity in the presence of ice corresponding to the minimum temperature (Tmin) to the fuel gas supply / discharge means and / or the oxidant gas supply / discharge means, and the solids The fuel cell system according to claim 1, further comprising a scavenging means for scavenging the polymer fuel cell.   3. The fuel cell system according to claim 1, wherein the drying unit includes a moisture absorption unit disposed in the fuel gas supply / discharge unit and / or the oxidant gas supply unit. The fuel cell system according to any one of claims 1 to 3, wherein the drying unit includes a water condensing unit disposed in a low temperature portion of the fuel gas supply / discharge unit and / or the oxidant gas supply / discharge unit.

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