JP4733788B2 - Fuel cell system - Google Patents

Description

  The present invention relates to a fuel cell system, and more particularly to a fuel cell system suitable as an in-vehicle power source, a stationary small power generator, a cogeneration system, and the like.

  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.

The electrolyte membrane or the catalyst layer electrolyte that constitutes such an MEA includes a perfluorinated electrolyte excellent in oxidation resistance (an electrolyte that does not contain a C—H bond in the polymer chain. For example, Nafion (registered trademark, DuPont). ), Aciplex (registered trademark, manufactured by Asahi Kasei Co., Ltd.), Flemion (registered trademark, manufactured by Asahi Glass Co., Ltd.), etc.) are generally used.
In addition, perfluorinated electrolytes are excellent in oxidation resistance but are generally very expensive. Therefore, in order to reduce the cost of the solid polymer fuel cell, a hydrocarbon electrolyte (an electrolyte that includes a C—H bond and does not include a C—F bond in a polymer chain) or a partial fluorine electrolyte The use of (electrolytes containing both C—H bonds and C—F bonds in the polymer chain) has also been studied.

  However, in order to put the polymer electrolyte fuel cell into practical use as a vehicle-mounted power source, there remain problems to be solved. For example, hydrocarbon electrolytes are less expensive than perfluorinated electrolytes, but have a problem that they are easily degraded by peroxide radicals. In addition, solid polymer electrolytes generally require water to develop electrical conductivity. For this reason, when the polymer electrolyte fuel cell is operated under high temperature and low humidification conditions, the electrical conductivity of the electrolyte membrane decreases, and a high output cannot be obtained.

In order to solve this problem, various proposals have heretofore been made.
For example, Patent Document 1 discloses a highly durable solid polymer electrolyte in which transition metal oxide fine particles such as manganese oxide, ruthenium oxide, and iridium oxide are dispersed in a hydrocarbon electrolyte. This document describes that the oxidation resistance of the membrane is improved by dispersing ruthenium oxide or the like in the hydrocarbon electrolyte membrane.

  Patent Document 2 discloses a solid polymer electrolyte membrane in which a transition metal oxide catalyst such as manganese oxide or ruthenium oxide or a macrocyclic metal complex catalyst such as iron phthalocyanine or copper phthalocyanine is added to a hydrocarbon solid polymer electrolyte membrane. A molecular electrolyte membrane is disclosed. In this document, when a transition metal oxide catalyst or a macrocyclic metal complex catalyst is added to a hydrocarbon-based electrolyte, hydrogen peroxide generated by the side reaction of the electrode reaction is decomposed by the transition metal oxide catalyst or the macrocyclic metal complex catalyst. The point that the oxidation resistance of the hydrocarbon electrolyte is improved is described.

  Patent Document 3 discloses a polymer electrolyte fuel cell in which MEA is impregnated with a non-aqueous solution (for example, formic acid + N-methylacetamide). This document describes that proton conductivity is exhibited even under non-humidified conditions by impregnating a catalyst layer and a solid polymer electrolyte membrane with a non-aqueous solution.

  Patent Document 4 discloses a polymer electrolyte fuel cell in which a molecule having a binding energy smaller than that of a carbon-fluorine bond is mixed in at least one of a fuel gas and an oxidant gas. This document describes that the attack of the fluorine-based electrolyte by radicals is suppressed by mixing molecules that are more easily oxidized than the fluorine-based electrolyte into the fuel gas or the oxidant gas.

Further, Patent Document 5, a polymer electrolyte fuel cell system for supplying and _{H 2} in the anode containing 30～70Mol% of CO ₂ have been disclosed. This document describes that the risk of ignition of hydrogen gas due to static electricity or friction can be reduced by mixing a large amount of CO ₂ gas with hydrogen gas.

JP 2001-118591 A JP 2000-106203 A JP 2002-110192 A JP 2003-109623 A JP 2004-095515 A

Noble metals such as Pt, Ru, Ir, Rh or their oxides have an action of decomposing peroxides. Therefore, when these powders are added to a hydrocarbon-based electrolyte or a partial fluorine-based electrolyte, generation of peroxide radicals is suppressed, and oxidation resistance can be improved. However, this method is not practical because noble metals are low in resources and expensive.
On the other hand, macrocyclic metal complexes and transition metal oxides have an action of decomposing hydrogen peroxide and are cheaper than noble metals, but have low electron conductivity. Therefore, when a large amount of these compounds is added to the electrode in order to improve the durability of MEA, there is a problem that the internal resistance or the contact resistance is increased and the battery performance is lowered.

  On the other hand, when a certain organic substance is added to the fuel gas or the oxidant gas, the deterioration of the electrolyte due to the peroxide radical can be suppressed to some extent. However, the performance deterioration of the fuel cell can be caused not only by the deterioration of the electrolyte but also by the deterioration of the carbon material contained in the catalyst layer and the diffusion layer. Further, there has never been an example in which means for positively suppressing such deterioration of the carbon material has been proposed.

  The problem to be solved by the present invention is to provide a fuel cell system capable of suppressing the deterioration of the carbon material and the deterioration of the fuel cell performance resulting therefrom. Another object of the present invention is to provide a fuel cell system that is practical, has high battery performance, and is excellent in durability.

In order to solve the above problems, a fuel cell system according to the present invention includes a polymer electrolyte fuel cell having a fuel electrode and an air electrode, and first CO ₂ concentration control means for controlling the CO ₂ concentration on the air electrode side. And the gist of this.
In this case, the first CO ₂ concentration control means supplies CO ₂ to the air side when the polymer electrolyte fuel cell is in an open circuit state and the operating temperature is 60 ° C. or higher. preferable.

Since the air electrode side of the polymer electrolyte fuel cell is usually in a high potential state, the carbon material is easily oxidized by water in the fuel cell. Therefore, if at least the CO ₂ concentration on the air electrode side is set to a certain value or more, the oxidation reaction of the carbon material can be suppressed.
Further, this oxidation reaction becomes significant at the air electrode where the polymer electrolyte fuel cell is in an open circuit state and the operating temperature is 60 ° C. or higher. Therefore, in such a case, if CO ₂ is supplied only to at least the air electrode side, the consumption of the carbon material and the resulting deterioration in the performance of the fuel cell can be efficiently suppressed without using a large amount of CO _2. Can do.

  Hereinafter, an embodiment of the present invention will be described in detail. FIG. 1 shows an example of a fuel cell system. In FIG. 1, the fuel cell system 10 includes a polymer electrolyte fuel cell 20, a fuel gas supply device 30, an oxidant gas supply device 50, and a humidification path 70.

Although not shown, the polymer electrolyte fuel cell 20 includes a membrane electrode assembly (MEA) in which electrodes (fuel electrode and air electrode) are bonded to both surfaces of a solid polymer electrolyte membrane, and a separator that holds both surfaces of the MEA. And. 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. The polymer electrolyte fuel cell 20 is generally formed by stacking a plurality of unit cells composed of such MEAs and separators.
The fuel gas (for example, hydrogen gas) supplied from the fuel gas supply device 30 is distributed to the fuel electrode side of each MEA via an intake manifold (not shown), and the gas discharged from each fuel electrode is exhausted. It is discharged through a manifold (not shown). Similarly, an oxidant gas (for example, air) supplied from the oxidant gas supply device 50 is distributed to the air electrode side of each MEA via an intake manifold (not shown), and is discharged from each air electrode. The gas is discharged through an exhaust manifold (not shown).
Furthermore, the polymer electrolyte fuel cell 20 is provided with various sensors (for example, a voltage measuring device 22, a temperature measuring device 24, etc.) for monitoring and controlling the operation state.

  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, although carbon is generally used for the separator, the use of metal materials such as Ti, stainless steel, and Sn alloys is also being studied.

  The fuel gas supply device 30 includes an intake system and an exhaust system. The intake system includes a hydrogen cylinder 32 that stores hydrogen as a fuel gas, and a hydrogen intake pipe 34 a that connects the hydrogen cylinder 32 and an intake manifold (not shown) on the fuel electrode side of the polymer electrolyte fuel cell 20. ing. The hydrogen intake pipe 34a includes a pressure adjustment valve 34b for adjusting the pressure of hydrogen gas flowing into the hydrogen intake pipe 34a from the hydrogen cylinder 32, and a hydrogen intake valve for adjusting the amount of hydrogen supplied to the fuel electrode side. 34c is provided.

The exhaust system of the fuel gas supply device 30 includes a hydrogen exhaust pipe 36a connected to an exhaust manifold (not shown) on the fuel electrode side of the polymer electrolyte fuel cell 20, and an exhaust gas exhausted through the hydrogen exhaust pipe 36a. Are provided with a hydrogen gas-liquid separator 38 for separating water contained in the water and a hydrogen pump 40 for increasing the pressure of the off-gas in order to recycle and use the hydrogen off-gas.
A valve 36 b for adjusting the back pressure is provided on the downstream side of the hydrogen pump 40. The exhaust gas discharged through the hydrogen pump 40 and the valve 36b is diluted with the exhaust gas on the air electrode side and discharged into the atmosphere.

Further, a water pipe 40a is connected to the hydrogen gas-liquid separator 38, and the water pipe 40a is provided with an opening / closing valve 40b for opening and closing the pipe. The hydrogen gas-liquid separator 30 is configured to discharge water separated from the exhaust gas from the water distribution pipe 40a to the outside of the system.
Further, the pressure adjusting valve 34b-hydrogen intake valve 34c of the hydrogen intake pipe 34a and the hydrogen pump 40-valve 36b of the hydrogen exhaust pipe 36a are connected by a connecting pipe 42a provided with a check valve 42b. The SUS check valve 42b is for preventing air from entering from the exhaust side.

The oxidant gas supply device 50 includes an intake system and an exhaust system. The intake system includes an air intake pipe 52 connected to an intake manifold (not shown) on the air electrode side of the polymer electrolyte fuel cell 20, and a humidifier 54 for humidifying the air flowing into the air intake pipe 52. And an air compressor 56 for pressurizing the humidified air to a predetermined pressure.
The exhaust system of the oxidant gas supply device 50 is exhausted through an air exhaust pipe 58 connected to an exhaust manifold (not shown) on the air electrode side of the polymer electrolyte fuel cell 20 and the air exhaust pipe 58. And an air-gas-liquid separator 60 for separating moisture contained in the exhaust gas. The exhaust gas on the air electrode side from which moisture has been separated is led to an exhaust system for diluted hydrogen.
The air / gas separator 60 is connected to the humidifier 54 via a recovery pipe 62a. The air-liquid separator 60 has a structure for discharging water separated from the exhaust gas to the recovery pipe 62a. The recovery pipe 62a is provided with a three-way valve 62b for switching the water of the air-gas / liquid separator 60 to the humidifier 54 side or the drain side, and an open / close valve 62c for opening and closing the recovery pipe 62a.

  The humidification path 70 is for supplying water and / or water vapor to the polymer electrolyte fuel cell 20 and / or for collecting water and / or water vapor discharged from the solid polymer fuel cell 20. . In the fuel cell system 10 illustrated in FIG. 1, the humidification path 70 includes an air intake pipe 52, a humidifier 54, an air compressor 56, a polymer electrolyte fuel cell 20, an air exhaust pipe 58, and an air / liquid separator 60. , A recovery pipe 62a, a three-way valve 62b, and an on-off valve 62c.

In the fuel cell system shown in FIG. 1, the recovery pipe 62a and the opening / closing valve 62c may be omitted. In this case, the humidification path 70 is configured only by the intake system of the oxidant gas supply device 50 and the polymer electrolyte fuel cell 20.
In the fuel cell system 10 shown in FIG. 1, a humidifier for humidifying the fuel gas may be provided in any of the hydrogen intake pipes 34a. In this case, the hydrogen intake pipe 34a, the fuel electrode side humidifier, and various valves provided in the hydrogen intake pipe 34a are components of the humidification path 70.
Further, in the fuel cell system 10 shown in FIG. 1, a circulation path for collecting water discharged from the fuel electrode side and using it for humidification of the polymer electrolyte fuel cell 20 may be provided. In this case, in addition to the intake system of the fuel gas supply device 30, a part of the exhaust system (for example, the hydrogen exhaust pipe 36 a, the hydrogen gas-liquid separator 38, the open / close valve 40 b, the hydrogen pump 40) is also connected to the humidification path 70. It becomes a component.
Furthermore, in the fuel cell system 10 shown in FIG. 1, the hydrogen cylinder 32 is used as a fuel source, but a reformer system may be used instead. Further, air is used as the oxidant gas, but oxygen or a mixed gas of oxygen and nitrogen may be used instead.

  For each component of the humidification path 70, stainless steel, Ti alloy, Pb alloy, ceramic material, glass, heat-resistant engineering plastics (for example, polyether ether ketone (PEEK) resin, etc.), fluorine-based resin are used depending on the purpose. Various materials such as polytetrafluoroethylene (PTFE), PFA, ethylenetetrafluoroethylene copolymer (ETFE) and the like are used.

The present invention is characterized in that the fuel cell system 10 described above includes first CO ₂ concentration control means for controlling the CO ₂ concentration on the air electrode side.
The CO ₂ concentration on the air electrode side is preferably 10,000 ppm or less. When the CO ₂ concentration becomes excessive, excessive CO ₂ may inhibit the electrode reaction. Moreover, addition of CO ₂ more than necessary requires a large amount of CO ₂ , and a relatively large apparatus is required for generation and / or storage of CO ₂ , which causes a decrease in efficiency. The optimum CO ₂ concentration varies depending on the operating conditions of the fuel cell, but is preferably at least 500 ppm or more in order to suppress the deterioration of the carbon material.

There are various methods for controlling the CO ₂ concentration on the air electrode side. Of these, the 1 CO ₂ concentration control means, CO ₂ and CO ₂ supply means for generating or storing gas, CO ₂ containing gas and / or humidification water an air electrode including CO ₂ gas supplied from the supply means It is preferable to include a CO ₂ addition means to be supplied to. Specifically, the first CO ₂ concentration control means having such a configuration includes the following.

In the first specific example of the first CO ₂ concentration control means, a reformed gas is generated by thermally decomposing naphtha, natural gas, petroleum coke gas or the like as a fuel gas supply source instead of the hydrogen cylinder 32 described above. In the case of using the “reformed gas generation system”, a separation means (CO ₂ supply means) for separating a part of the CO ₂ gas by-produced in large quantities from the reformed gas, and the separated CO ₂ as an oxidant gas And / or CO ₂ addition means for adding to the humidified water.

Specifically, the CO ₂ gas contained in the reformed gas is
(1) Glass such as lithium silicate, ceramic membrane such as zeolite or graft polymer membrane (membrane separation method),
(2) PSA (pressure swing adsorption method) using an adsorbent,
(3) Cryogenic separation system that cools to low temperature,
(4) An absorption / vaporization method in which CO ₂ is absorbed by an oxide such as an alkaline solution or CaO, and vaporized by heating or decompression,
Etc. can be separated. The separated CO ₂ gas may be directly introduced into the air intake pipe 52 or a first water tank provided in the humidifier 54 for holding humidified water converted into steam or mist. (Not shown) or provided outside the humidifier 54 and may be introduced into a second water tank (not shown) for retaining humidified water to be supplied to the humidifier 54. Further, the CO ₂ gas may be introduced into the first water tank and / or the second water tank at the same time when the CO ₂ gas is introduced into the air intake pipe 52.
When CO ₂ gas is introduced into the water tank, a part thereof is dissolved in the water in the water tank. Other CO ₂ gas is introduced into the air supply pipe 52 together with water vapor or mist generated by the humidifier 54. The amount of CO ₂ added to the oxidant gas and / or humidified water (ie, the CO ₂ concentration on the air electrode side) can be adjusted by controlling the flow rate of the CO ₂ gas from the separation means to the humidification system. it can.

A second specific example of the 1 CO ₂ concentration control means, CO ₂ and CO ₂ supply means comprising a cylinder, CO ₂ added the addition of CO ₂ gas supplied from the CO ₂ cylinder to the oxidizing gas and / or humidification water Means. In this case, (1) CO ₂ gas supplied from the CO ₂ cylinder directly, may be introduced into the air intake pipe 52, or, alternatively or additionally to this, the first water tank and / or The point that may be introduced into the second water tank, and (2) the amount of CO ₂ added to the oxidant gas and / or humidified water is controlled by controlling the flow rate of CO ₂ gas from the gas cylinder to the humidifying system. The points that can be adjusted are the same as in the first specific example.

A third specific example of the 1 CO ₂ concentration control means includes a storage means for storing a pre-dissolved CO ₂ gas water (CO ₂ supply means), with water to dissolve the CO ₂ gas in the humidifying water And CO ₂ addition means for adding the vapor or mist to the oxidant gas.
In this case, the amount of CO ₂ gas added to the oxidant gas can be adjusted by controlling the amount of dissolved CO ₂ gas and / or the amount of humidification contained in the humidified water. Alternatively, a plurality of types of humidified water having different amounts of dissolved CO ₂ gas may be prepared and used depending on the output of the fuel cell.

A fourth specific example of the first CO ₂ concentration control means is an electrolytic infusion means (CO ₂ supply means) that generates CO ₂ gas by electrolyzing water using an anode made of a carbon material and a counter electrode. And CO ₂ addition means for supplying an oxidant gas containing the generated CO ₂ gas and / or humidified water to the air electrode.

“Electrolytic penetration” refers to the generation of CO ₂ by electrolyzing water with a carbon material as an anode. Part of the generated CO ₂ is dissolved in water, and the other part is released as CO ₂ gas into the gas phase. The electrolysis of water using a carbon material as an anode can be expressed by the following equation (1).
C + 2H ₂ O → CO ₂ + 4H ⁺ + 4e ⁻ (1)

The water for electrolytic welding may be humidified water for humidifying the electrolyte membrane, or water other than the humidified water. When performing electrolytic infusion using water other than humidified water, only the CO ₂ gas released into the gas phase is taken out and added to the oxidant gas and / or humidified water. On the other hand, when performing electrolytic infusion using humidified water, not only CO ₂ gas released into the gas phase but also CO ₂ gas dissolved in the humidified water can be supplied to the air electrode. Therefore, in order to efficiently use the generated CO ₂ gas, it is preferable to perform electrolytic infusion using humidified water.

In normal humidification, generally used water is ion-exchanged water having a low conductivity of about 0.1 to 1 μScm. Accordingly, when electrolytic welding is performed using humidified water, the distance between the electrodes (the shortest electrode between the anode and the counter electrode) is preferably 10 mm or less. In the case of electrolyzing ion-exchanged water having low conductivity, it is not preferable that the distance between the electrodes exceeds 10 mm because the electrolytic voltage becomes high.
In addition, when electrolytic welding is performed using humidified water, water previously saturated with CO ₂ gas may be used as the humidified water. Since water saturated with CO ₂ gas has higher electrical conductivity than ion exchange water, the electrolysis voltage can be lowered. Further, since it does not contain impurity cations, even if it is supplied to the air electrode, the durability of the electrode of the fuel cell and the electrolyte membrane is not lowered.

As the carbon material for the anode, natural graphite, artificial graphite, glassy carbon, resin-molded carbon, or the like can be used. The shape of the anode is not particularly limited, and can take various shapes. For example, the anode may be flat, round bar, or prismatic. The anode may be a spherical or massive carbon material.
However, when a spherical or massive carbon material is used, it is necessary to put the carbon material in a mesh basket made of Ti or Pt in order to ensure conductivity.
Moreover, the carbon electrode fine powder may fall off during electrolysis, and the fine powder may float in water. In such a case, if this water is supplied directly to the humidification system of the fuel cell, the fine powder may block the gas flow path of the fuel cell. Therefore, in such a case, it is desirable to attach a microporous bag filter to the electrode.

On the other hand, the material of the counter electrode (cathode) is not particularly limited, and various materials can be used. For example, the various carbon materials described above may be used for the counter electrode, or other insoluble materials (for example, Pt, Au, Ti, Nb, SUS, etc.) may be used.
The shape of the counter electrode is not particularly limited, and various shapes such as a flat plate shape, a round bar, and a prismatic shape can be taken. Moreover, when the electrolytic cell (water tank) which stores water consists of an electroconductive material, you may use the electrolytic cell itself for a counter electrode.

The current density at the time of electrolytic welding is preferably 1 mA / cm ² or more and 10 mA / cm ² or less. When the current density is less than 1 mA / cm ² , CO ₂ generation efficiency decreases. On the other hand, if the current density exceeds 10 mA / cm ² , the current efficiency may be reduced, or the anode may fall off, resulting in a single life.

Electrolysis methods are (1) DC electrolysis with a constant current or voltage using a carbon material as an anode, (2) PR electrolysis that changes polarity in a timely manner (for example, every 30 seconds to 1 minute), (3) AC electrolysis, Either may be sufficient. In particular, when PR energization is performed, ^ions such as Fe ²⁺ , Ca ²⁺ , and Mg ²⁺ contained in water are deposited and removed as hydroxides on the electrode surface serving as a cathode, so that electrolysis can be performed without switching the polarity. When this is done, it is possible to prevent a decrease in electrolytic efficiency, which is a problem when performed, and electrolyte proton exchange that occurs when humidified water containing impurity cations is supplied to the fuel cell.
Further, when the electrolytic cell itself equipped with a bag filter and a cathode, when energized, Ca ^2+, in addition to ions of Mg ^2+, etc., pipes and separators, the diffusion layer, an impurity cations leached from the catalyst layer or the like in water (e.g., Fe ²⁺ , Ni ^{2+ and the} like) can be collected at the cathode, and the purity of the water can be increased, so that the durability of the electrode of the fuel cell and the electrolyte membrane can be increased.
When a dissolution and infusion system including such a cation removal system is used, long-term durability can be maintained even in a humidification system that recirculates wastewater.

When performing electrolytic welding using humidified water, the electrolytic welding may be performed in the first water tank by providing an electrode in the first water tank inside the humidifier 54, or the humidified water is retained. A second water tank may be provided separately for the second water tank. When electrolysis of humidified water is performed using the carbon material as an anode, CO ₂ gas is generated, and a part of the gas is dissolved in the humidified water. In general, the solubility of CO ₂ gas in water decreases as the temperature increases. Therefore, in order to generate humidified water having a high CO ₂ concentration, it is preferable to perform electrolytic infusion at a relatively low temperature. For example, the solubility of CO ₂ in water near neutrality is 10 ° C .: 2500 ppm, 20 ° C .: 1800 ppm, 40 ° C .: 1000 ppm, 60 ° C .: 700 ppm, 80 ° C .: 500 ppm. Therefore, specifically, the temperature of water at the time of electrolytic infusion is preferably 60 ° C. or less, more preferably 40 ° C. or less, and further preferably 20 ° C. or less.

For example, when the humidifier 54 is a steam generator, the electrolytic welding is performed in the second water tank for retaining the humidified water maintained near the room temperature, rather than performing electrolytic welding inside the steam generator. Is preferred. In this case, the humidifying water, it is possible to dissolve a relatively large amount of CO ₂ gas, without using a CO ₂ gas released into the gas phase, a relatively large amount of CO ₂ gas air Can be supplied to the pole.
On the other hand, when the humidifier 54 is a mist generator, the temperature of the humidified water is generally lower than that of the steam generator, so that only when electrolytically injecting in the second water tank in which the humidified water is retained. However, even when electrolytic welding is performed in the first water tank inside the mist generator, a relatively large amount of CO ₂ gas can be easily supplied to the air electrode.

Part of the CO ₂ gas generated in the first water tank and / or the second water tank is dissolved in the water tank, and the other CO ₂ gas is air supply pipe together with water vapor or mist generated in the humidifier 54. 52. Alternatively, only the CO ₂ gas released into the gas phase may be taken out and directly introduced into the air supply pipe 52. Furthermore, when performing electrolysis in a water tank independent of the humidification system, only the CO ₂ gas released to the gas phase is taken out and introduced directly into the air supply pipe 52. The CO ₂ concentration on the air electrode side can be adjusted by controlling the electrolysis conditions.

The addition of CO ₂ to the air electrode by these first CO ₂ concentration control means may be performed at all times, or may be performed only when the carbon material in the fuel cell is heavily consumed. The oxidation reaction shown in the formula (1) becomes remarkable when the fuel cell is in an open circuit (power generation suspended) state and the operating temperature is 60 ° C. or higher. Therefore, only in such a case, if CO ₂ is supplied to the air electrode side, it is not necessary to constantly generate a large amount of CO _2, and the efficiency of the system can be improved.

On the fuel electrode side, consumption of the carbon material is normally negligible, so it is not always necessary to constantly supply a predetermined amount of CO ₂ to the fuel electrode side, but when an abnormal reaction occurs in the fuel cell, etc. The carbon material can be consumed. Therefore, a means ( _second CO ₂ concentration control means) for controlling the CO ₂ concentration on the fuel electrode side to a predetermined value may be further provided.
In this case, the CO ₂ concentration on the fuel electrode side is preferably 10,000 ppm or less. Unnecessary addition of CO ₂ is not preferable because it may hinder the oxidation reaction of the fuel on the fuel electrode.

As a method for supplying CO ₂ to the fuel electrode side, various methods described above, (1) a method using CO ₂ separated from the reformed gas, (2) a method using a CO ₂ cylinder, (3) For example, a method of dissolving CO ₂ gas in humidified water in advance can be used.
However, the electrolytic welding is not preferable to use as the _second CO ₂ concentration control means because oxygen may be generated as a by-product of electrolysis. Further, in the case of using the method of dissolving CO ₂ gas in humidified water in advance, when changing the CO ₂ concentration between the fuel electrode side and the air electrode side, the fuel electrode side humidification system and the air electrode side humidification system It is preferable to use humidified water having different CO ₂ concentrations.
Since the other points of the 2CO ₂ density control means is the same as in 1 CO ₂ concentration control means, and a description thereof will be omitted.

  Next, the operation of the fuel cell system according to the present invention will be described. As a catalyst carrier for an electrode used in a fuel cell, a chemically stable carbon-based material such as acetylene black, oil furnace black, activated carbon, and fullerenes is generally used from the viewpoint of durability. As the diffusion layer, a so-called paper diffusion layer obtained by using a carbon fiber woven fabric or carbon fiber by a papermaking technique is used.

However, even when such a carbon-based material is used, its durability is never sufficient when the required level is high. For example, under long-term operating conditions, the surface of the carbon material is oxidized and becomes hydrophilic. When the surface of the carbon material becomes hydrophilic, it becomes difficult to maintain a three-phase interface in the electrode, leading to a decrease in battery performance. Further, when the carbon material is oxidized, it is gasified into CO or CO ₂ and consumed. When the carbon material is consumed, the state of contact with the catalytic metal deteriorates, leading to a decrease in battery voltage.

On the other hand, when CO ₂ is positively added to the humidification system of the fuel cell, it is possible to suppress the consumption of the carbon material and the deterioration of the fuel cell performance due to this. The reason why such an effect is obtained by adding CO ₂ is considered to be as follows.

The inventors of the present application compared the durability when using air on the oxygen electrode side of the humidifying system and when using a mixed gas of 20 vol% oxygen + 80 vol% nitrogen that does not contain CO _2. It is confirmed that it is slightly better. This deterioration is
(1) Carbon materials are not inherently stable at high temperature and high potential, and are consumed as CO or CO ₂ gas.
(2) By-produced hydrogen peroxide attacks the carbon material to generate hydrophilic functional groups such as C═O, C—OH, and COOH on the surface, thereby reducing water repellency.
This is probably because of this.

The oxidation reaction of the carbon material in the high potential state can be expressed by the above-described formula (1). Here, the thermodynamic equilibrium potential E _{25 ° C.} of the equation (1) at _{25 ° C.} is expressed by the following equation (2) from the Pourbaix diagram.
E _{25 ° C.} = 0.207-0.0591 pH + 0.0148 log P (CO ₂ )
... (2)
That is, when the partial pressure of CO ₂ is increased 10 times, the oxidation potential of C increases by about 15 mV (it is difficult to be oxidized).

Here, since the potential of the fuel electrode is usually around 0 V, the consumption of the carbon material can be ignored unless an abnormal reaction occurs, but the potential on the air electrode side is when generating electricity at a high current density. Is about 0.9 to 0.3 V, and the carbon material may be consumed. Actually, it is known that the expression (1) occurs at 0.8 V or more in the presence of overvoltage. Therefore, if CO ₂ gas is positively introduced at least into the humidification system on the air electrode side, the CO ₂ partial pressure in the vicinity of the carbon material increases, and the elution rate of the carbon material can be reduced.

Further, the oxidation rate of this oxidation reaction is very low at a low temperature of room temperature to about 60 ° C. That is, the carbon material is substantially exhausted in the open circuit (power generation halt) state and at the air electrode of 60 ° C. or higher. Therefore, at least if the CO ₂ is supplied only to the air electrode in such a state, the consumption of the carbon material can be efficiently suppressed without using a large amount of CO ₂ . In addition to the air electrode side, if a predetermined amount of CO ₂ gas is positively introduced into the humidification system on the fuel electrode side, it is possible to prevent the carbon material from being consumed when an abnormal reaction occurs. it can.

Furthermore, among the various CO ₂ addition methods described above, electrolytic welding does not require a reformer system as a CO ₂ supply source or a CO ₂ cylinder, and is a fuel cell system using pure hydrogen as fuel. Since it can be easily applied to the system, the system can be made light and compact.

(Examples 1 to 3, Comparative Example 1)
To 0.5 g of 60 wt% Pt / C catalyst, 2.0 g of distilled water, 2.5 g of ethanol, 1.0 g of propylene glycol, and 0.9 g of 22 wt% Nafion (registered trademark) solution (manufactured by DuPont) were added in this order. A catalyst ink was prepared by dispersing with a sonic homogenizer. This was applied onto a polytetrafluoroethylene sheet and dried to obtain a transfer electrode. The amount of Pt used was constant in the range of 0.5 to 0.6 mg / cm ^{2 for} the air electrode and 0.3 to 0.4 mg / cm ^{2 for} the fuel electrode. These electrodes were cut into 36 mm squares and thermocompression bonded (120 ° C., 50 kgf / cm ² (4.9 MPa)) to one surface of the F-based electrolyte membrane to produce a membrane electrode assembly (MEA).

The endurance test was performed using fuel electrode gas: H ₂ (100 ml / min), air electrode gas: air (100 ml / min), cell temperature: 90 ° C., humidifier temperature: 90 ° C. (both on the fuel electrode side and air electrode side). A cycle test with an open circuit of 1 minute and 0.1 A / cm ² of 1 minute was conducted for 150 hours, and the voltage value reduction rate at 0.8 A / cm ² before and after the durability test was compared.

The humidification conditions are as follows.
Example 1: humidification water, both electrodes with the ion-exchange water, by using the CO ₂ gas cylinder was 10000ppm added CO ₂ gas to the air electrode.
Example 2: The humidified water on the air electrode side was previously saturated with CO ₂ (concentration: 1500 ppm) at room temperature, and the humidified water on the fuel electrode was used as ion-exchanged water.
Example 3: _Two carbon plates with an electrode area of 100 cm ² covered with a bag filter made of polypropylene were immersed in CO ₂ saturated water, and a constant current was passed at a room temperature of 10 mm, a current density of 5 A / cm ² , and room temperature. The CO ₂ gas thus obtained was supplied to CO ₂ saturated humidified water at the air electrode. Moreover, the humidification water of the fuel electrode was ion exchange water.
Comparative Example 1: A durability test was performed without supplying CO ₂ gas in the form of ion-exchanged water for both the air electrode and the humidified water of the fuel electrode. Table 1 shows the results.

When CO ₂ gas is added to the air electrode (Example 1), when humidified water saturated with CO ₂ is supplied to the air electrode (Example 2), and electrolytically injected CO ₂ is supplied to the air electrode In any case (Example 3), a decrease in voltage was suppressed compared to the case of normal humidification conditions (CO ₂ not added: Comparative Example 1).

  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 applied to an in-vehicle power source, a stationary small power generator, a cogeneration system, and the like.

It is a schematic block diagram which shows an example of a fuel cell system.

Claims (12)

A polymer electrolyte fuel cell having a fuel electrode and an air electrode;
A fuel cell system including a first 1 CO ₂ concentration control means for controlling the CO ₂ concentration of the air electrode side. _2. The fuel cell system according to claim 1, wherein the first CO ₂ concentration control means controls the CO ₂ concentration on the air electrode side to 10000 ppm or less. The first CO ₂ concentration control means includes:
And CO ₂ supply means for generating or storing a CO ₂ gas,
_3. The fuel cell system according to claim 1, further comprising a CO ₂ addition unit configured to supply an oxidant gas including the CO ₂ gas and / or humidified water supplied from the CO ₂ supply unit to the air electrode. . The CO ₂ supply means is an electrolytic infusion means for generating CO ₂ gas by electrolyzing the humidified water using an anode made of a carbon material and a counter electrode,
The fuel cell system according to claim 3, wherein the CO ₂ addition means supplies the oxidant gas containing the CO ₂ gas and / or the humidified water to the air electrode.   The fuel cell system according to claim 4, wherein the electrolytic welding means has a distance between the anode and the counter electrode of 10 mm or less. 6. The fuel cell system according to claim 4, wherein the electrolytic infusion unit performs electrolysis of the humidified water under a current density of 1 mA / cm ² or more and 10 mA / cm ² .   The fuel cell system according to any one of claims 4 to 6, wherein the electrolytic welding means further includes polarity switching means for switching the polarity between the anode and the counter electrode. The fuel cell system according to any one of claims 4 to 7, wherein the electrolytic welding means performs electrolysis using the water previously saturated with CO ₂ gas.   The fuel cell system according to any one of claims 4 to 8, wherein the electrolytic welding means performs electrolysis of the water at 40 ° C or lower. The first CO ₂ concentration control means supplies CO ₂ to the air side when the polymer electrolyte fuel cell is in an open circuit state and an operating temperature is 60 ° C or higher. The fuel cell system according to any one of 1 to 9. The fuel cell system according to any one of claims 1 to 10, further comprising a first 2CO ₂ density control means for controlling the CO ₂ concentration of the fuel electrode side. The fuel cell system according to claim 11, wherein the _second CO ₂ concentration control means controls the CO ₂ concentration on the fuel electrode side to 10000 ppm or less.

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