JP5002928B2 - Fuel cell system - Google Patents

Description

  The present invention relates to a fuel cell system, a fuel cell vehicle, a portable device, and a transportation device, and particularly to a fuel cell system including a polymer electrolyte fuel cell and a fuel cell vehicle.

Fuel cell technology is attracting attention as a means of solving the recent energy resource problems and global warming problems associated with CO ₂ emissions. In a fuel cell, fuel such as hydrogen, methanol, or other hydrocarbons is electrochemically oxidized in the cell, thereby converting the chemical energy of the fuel directly into electrical energy and taking it out. For this reason, the fuel cell has been attracting attention as a clean electric energy source without generating NO _X or SO _X due to fuel combustion in an internal combustion engine such as thermal power generation and automobile.

  There are several types of fuel cells. Among them, the polymer electrolyte fuel cell (hereinafter referred to as PEFC) has attracted the most attention and is being developed. PEFC is (1) easy to start and stop due to low temperature operation, (2) high theoretical voltage and high conversion efficiency, and (3) flexible cell structure such as vertical because there is no liquid phase in the electrolyte. (4) At the ion exchange membrane / electrode interface, there are various advantages such as that a large current can be taken out by securing a three-phase interface as a reaction field and a high output density can be obtained. .

  However, even though PEFC has received the most attention, there are still many problems. Among them, polymer electrolyte membrane technology is one of the most important issues. At present, perfluorosulfonic acid polymers represented by Nafion (registered trademark) membranes, which are the most widely used electrolyte membranes and are commercially available from DuPont, USA, are generated at the air electrode (positive electrode) of fuel cells. It has been developed as a membrane resistant to active oxygen. However, according to the results of a long-term durability test, it cannot be said that there is sufficient resistance yet.

The operating principle of a fuel cell is based on two electrochemical processes, namely, H ₂ oxidation at the fuel electrode (negative electrode) and water generation by 4-electron reduction of molecular oxygen (O ₂ ) at the air electrode shown in Formula (1). It is made up.

O ₂ + 4H ⁺ + 4e ⁻ → 2H ₂ O (1)
In practice, however, side reactions occur simultaneously. A typical example is the production of hydrogen peroxide (H ₂ O ₂ ) by two-electron reduction of O _{2 at} the air electrode shown in Formula (2).

O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O ₂ Formula (2)
Hydrogen peroxide has a weak oxidizing power but is stable and has a long life. Hydrogen peroxide is decomposed according to the following reaction formulas (3) and (4), and radicals such as hydroxy radicals (.OH) and hydroperoxy radicals (.OOH) are generated upon decomposition. These radicals, particularly hydroxy radicals, have a strong oxidizing power, and even perfluorosulfonic acid polymers used as electrolyte membranes may be decomposed by long-term use.

H ₂ O ₂ → 2.OH Formula (3)
H ₂ O ₂ → · H + · OOH (4)
In addition, when there are low-valence ions of transition metals such as Fe ²⁺ , Ti ³⁺ , and Cu ⁺ , Harbor Weiss (Haber −) in which hydrogen peroxide is reduced by one electron with these metal ions to generate a hydroxy radical. Weiss) reaction occurs. Hydroxyl radicals are known to be most reactive among active oxygens and have very strong oxidizing power. When the metal ion is an iron ion, the Harbor Weiss reaction is known as the Fenton reaction shown in Formula (5).

Fe ²⁺ + H ₂ O ₂ → Fe ³⁺ + OH ⁻ + · OH Formula (5)
As described above, when metal ions are mixed in the electrolyte membrane, hydrogen peroxide is changed into hydroxy radicals in the electrolyte membrane by the Harbor Weiss reaction, and the electrolyte membrane may be deteriorated by the hydroxy radicals (Non-Patent Document). 1).

Thus, as a method for preventing oxidation of the electrolyte membrane by hydroxy radicals, for example, a method has been proposed in which a compound having a phenolic hydroxyl group is blended in the electrolyte membrane, and peroxide radicals are trapped and inactivated (patent) References 1 and 2). In addition, there has been proposed a method of eliminating generated radicals by blending an electrolyte membrane with a phenol compound, an amine compound, a sulfur compound, a phosphorus compound, or the like as an antioxidant (see Patent Document 3). Furthermore, as a molecule having a binding energy smaller than that of the carbon-fluorine bond, for example, lauryl alcohol is contained in a catalyst layer disposed adjacent to the electrolyte membrane, and this molecule reacts preferentially with a hydroxy radical. A method for protecting the electrolyte membrane has been proposed (see Patent Document 4). In addition, a method of supplying ascorbic acid as a fuel from a negative electrode has been proposed (see Non-Patent Document 2).
JP 2000-223135 A JP 2001-118591 A JP 2004-134269 A JP 2003-109623 A New Energy and Industrial Technology Development Organization Subcontractor, Graduate School of Engineering, Kyoto University, “2001 Results Report, Research and Development of Solid Polymer Fuel Cells, Research on Degradation Factors of Polymer Electrolyte Fuel Cells, Basics on Degradation Factors Study (1) Electrode catalyst / electrolyte interface degradation factors ", March 2002, p.13, 24, 27 Naoko Fujiwara and five others, “Electorochemical and Solid-State Letters”, Vol. 6, No. 12, p. A257-A259, 2003, American Electrochemical Society

  However, in the method in which the electrolyte membrane only contains the compound that prevents oxidation as described above, this compound loses the effect of preventing the oxidation of the electrolyte membrane by oxidizing the compound itself by reducing the hydroxy radical. For this reason, it is impossible to maintain the antioxidation effect of the electrolyte membrane semipermanently. Moreover, since these compounds are hardly soluble and have a large molecular weight, it is impossible to send a compound that prevents oxidation through the electrode into the electrolyte. In addition, the compound may become unstable radicals or peroxides by reaction with hydroxy radicals and become a new oxidation initiator, which may cause deterioration of the electrolyte membrane. In addition, the fuel cell may generate a potential of 1.23 [V] (NHE) at the maximum, but most of the compounds that have been studied so far prevent oxidation are 1.23 [V] (NHE). Since this compound has the following redox potential, this compound is electrolytically oxidized at the air electrode. For this reason, even if these compounds may function as fuel for the fuel cell, they do not exhibit the antioxidant effect of the electrolyte membrane.

The present invention has been made to solve the above-described problems. A fuel cell system according to a first aspect of the present invention includes a fuel cell reaction system in which a fuel cell reaction occurs, and a hydroxyl radical in the fuel cell reaction system. Which has an oxidation potential higher than 1.23 [V] (NHE), and benzene, benzene derivatives, biphenyl, acenaphthene, naphthalene, naphthalene derivatives, pyridine, The gist is that the compound is selected from the group consisting of pyridine derivatives, styrene, alkylamines, acetamides, amide derivatives, anilines, aniline derivatives and benzyl alcohol.

  A fuel cell vehicle according to a second aspect of the invention is characterized in that the fuel cell system according to the first aspect of the invention is mounted.

  The gist of a portable device according to a third invention is that the fuel cell system according to the first invention is used.

  The gist of a transportation device according to a fourth invention is that the fuel cell system according to the first invention is used.

  According to the first invention, since the compound existing in the fuel cell reaction system has an oxidation potential higher than 1.23 [V] (NHE), the compound is not subjected to electrolytic oxidation accompanying the reaction at the air electrode, and the hydroxyl radical Is inactivated. For this reason, the fuel cell system excellent in durability can be realized.

  According to the second to fourth inventions, since the fuel cell system according to the first invention is used, it is possible to realize a fuel cell vehicle, portable equipment, and transportation equipment that can withstand continuous operation for a long time.

  Hereinafter, details of a fuel cell system and a fuel cell vehicle according to the present invention will be described based on embodiments.

  FIG. 1 is a schematic explanatory view illustrating an embodiment of a fuel cell system according to the present invention. As shown in FIG. 1, the fuel cell system according to the present embodiment is disposed outside a fuel cell stack (fuel cell) 1 and the fuel cell stack 1, and hydroxy radicals are reduced in the fuel cell stack 1. And a compound supply means 11 for continuously supplying the inactivated compound.

  As shown in FIG. 1, a fuel cell stack 1 constituting the fuel cell system according to the present embodiment is configured by laminating a plurality of single cells 2 of a fuel cell, which is a basic unit for generating power by an electrochemical reaction. And a fuel cell stack (not shown) configured by disposing end flanges (not shown) at both ends after lamination and fastening the outer periphery with fastening bolts (not shown). The unit cell 2 includes an air electrode 5 and a fuel electrode 6 that sandwich a solid polymer electrolyte membrane 4, and one of the air electrode 5 and the fuel electrode 6 constitutes an electrode, and the membrane electrode joint. An air electrode side separator 7 that is disposed on the air electrode 5 side of the body 3 and forms an air flow path 8 between the membrane electrode assembly 3 and a surface on the fuel electrode 6 side of the membrane electrode assembly 3. And a fuel electrode side separator 9 that forms a fuel gas flow channel 10 between the electrode assembly 3 and the electrode assembly 3. The fuel cell stack 1 includes a hydrogen supply line (not shown) for supplying a fuel gas containing hydrogen such as hydrogen gas to each single cell 2 and an air supply line (not shown) for supplying air as an oxidant gas. (Not shown) and a cooling water supply line (not shown) for supplying cooling water are provided.

  As the solid polymer electrolyte membrane 4 used for the single cell 2, a perfluorocarbon polymer membrane having a sulfonic acid group (trade name; Nafion (registered trademark), DuPont, USA) and the like can be used. Then, a catalyst layer of platinum catalyst-carrying carbon is bonded to one surface of the solid polymer electrolyte membrane 4 as an air electrode 5 and the other surface as a fuel electrode 6 to form a membrane electrode assembly 3.

  As shown in FIG. 2, the air electrode side separator 7 and the fuel electrode side separator 9 are formed by forming carbon or metal into a plate shape, and a gas channel and a cooling water channel are formed on the surface thereof. The air flow path 8 is formed between the air electrode 5 and the air electrode side separator 7, and supplies air as a reaction gas to the air electrode 5. The fuel gas channel 10 is formed between the fuel electrode 6 and the fuel electrode side separator 9, and supplies hydrogen as a reaction gas to the fuel electrode 6. The fuel gas channel 10 functions as a moisture replenishment passage by humidifying the fuel gas, and the air channel 8 also functions as a generated water removal passage. A gas diffusion layer formed of carbon paper or carbon nonwoven fabric is appropriately disposed between the separators 7 and 9 and the electrodes 5 and 6.

  In each single cell 2 of the fuel cell stack 1 configured as described above, when air and hydrogen gas are respectively supplied to the air flow path 8 and the fuel gas flow path 10, the air and hydrogen gas are respectively on the air electrode 5 and fuel electrode 6 sides. The reaction of the following formulas (6) and (7) occurs.

Fuel electrode side: H ₂ → 2H ⁺ + 2e ⁻ Expression (6)
Air electrode side: (1/2) O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O (7)
As shown in FIG. 3, when hydrogen gas is supplied to the fuel electrode 6 side, the reaction of the formula (6) proceeds and H ⁺ (proton) And e ⁻ (electrons) are generated. H ⁺ moves in the solid polymer electrolyte membrane 4 in a hydrated state and flows to the air electrode 5 side. On the air electrode 5 side, H ⁺ , e ⁻ and oxygen gas in the supplied air represent the formula ( The reaction of 7) proceeds to produce water. At this time, an electromotive force is obtained by electrons generated at the fuel electrode 6 moving to the air electrode 5 via the external circuit 17 shown in FIG. As described above, the fuel cell system according to the embodiment of the present invention has the fuel cell reaction system in which the reactions of formulas (6) and (7), that is, the fuel cell reaction occurs, The membrane electrode assembly 3 is included.

As shown in Equation (7), the reaction at the air electrode 5 is the generation of water by four-electron reduction of molecular oxygen (O ₂ ). In addition to this 4-electron reduction of oxygen, side reactions occur simultaneously, superoxide (O ₂ ⁻ ), which is a one-electron reductant of oxygen, hydroperoxy radical (· OOH), a conjugate acid of superoxide, and a two-electron reductant. Active oxygen such as hydrogen peroxide (H ₂ O ₂ ), a three-electron reduced form, or a hydroxy radical (.OH) is generated. The generation mechanism of each active oxygen is presumed to be a complex reaction through a plurality of elementary reaction processes represented by the following formulas (8) to (12).

O ₂ + e ⁻ → O ₂ ⁻ (8)
O ₂ ⁻ + H ⁺ → · OOH Formula (9)
O ₂ + 2H ⁺ + 2e ⁻ → H ₂ O ₂ Formula (10)
H ₂ O ₂ + H ⁺ + e ⁻ → H ₂ O + · OH Formula (11)
H ₂ O ₂ → 2.OH Formula (12)
The generated active oxygen is presumed to be finally reduced to water through the elementary reaction processes of the following formulas (13) to (15). Note that E ₀ is represented by a normal oxidation-reduction potential (Normal Hydrogen Electrode; NHE).

OOH + H ⁺ + e ⁻ → H ₂ O ₂ E ₀ = 1.50 [V] (13)
H ₂ O ₂ + 2H ⁺ + 2e ⁻ → 2H ₂ O E ₀ = 1.77 [V] (14)
OH + H ⁺ + e ⁻ → H ₂ O E ₀ = 2.85 [V] (15)
The problem here is a hydroxy radical having a high oxidation-reduction potential of 2.85 [V] and a strong oxidizing power. Hydroxyl radicals are the most reactive of active oxygen, have a very short lifetime of 1 millionth of a second, and have strong oxidizing power. For this reason, hydroxy radicals react with other molecules unless they are rapidly reduced and inactivated. It is speculated that most of the oxidative degradation that is a problem in fuel cells is caused by this hydroxy radical. This hydroxy radical continues to be generated via the above formulas (8) to (12) while the fuel cell is generating power. On the other hand, hydroperoxy radicals and hydrogen peroxide are less oxidative than hydroxy radicals, but may pass through hydroxyl radicals in the process of being reduced to water. Thus, the generation of hydroxy radicals continues semipermanently as long as power is generated by the fuel cell. For this reason, unless a compound that inactivates hydroxy radicals is supplied to the fuel cell, the solid polymer electrolyte membrane may be oxidized and deteriorated by the hydroxy radicals that continue to be generated.

Since the fuel cell system according to the present embodiment has a compound that reduces and deactivates hydroxy radicals in the fuel cell reaction system, the hydroxy radicals generated by the fuel cell reaction oxidize the solid polymer electrolyte membrane. Reacts with the aforementioned compounds. The reaction for reducing the hydroxy radical by this compound is as shown in the formulas (16) and (17).

Formula (16) shows a reaction for inactivating a hydroxy radical by a reduction reaction with compound AH. Formula (17) represents a reaction that is inactivated by adding a hydroxy radical to Compound B. AH, A and B are optional substituents, and k is a reaction rate constant. Hydroxyl radicals are reduced and inactivated by the reactions of formulas (16) and (17), so that deterioration of the solid polymer electrolyte membrane due to oxidation can be suppressed. Further, since this compound has an oxidation potential higher than 1.23 [V] (NHE), the compound is not electrolytically oxidized in the fuel cell reaction, and the effect of preventing the solid polymer electrolyte membrane from being oxidized can be obtained.

  Since the generation of active oxygen continues semipermanently as long as power is generated in the fuel cell stack, the compound is continuously supplied as a compound solution to the air electrode or the fuel electrode by the compound supply means 11 provided outside the fuel cell stack 1. However, it is preferable to supply from the fuel electrode 9 side of the fuel cell stack 1. In this case, even if the fuel cell stack 1 continues to generate power and active oxygen continues to be generated, the hydroxyl radicals are surely deactivated and erased to prevent deterioration of the solid polymer electrolyte membrane. Can do. Even in an environment where the compound is easily oxidized, the efficiency of inactivating the hydroxy radical does not change because the compound is continuously supplied from the outside.

  When the compound is supplied from the fuel electrode 9 side, for example, as shown in FIG. 1, the compound supply means 11 includes a compound solution tank 12 in which the compound solution is sealed, and the compound solution as the fuel of the fuel cell stack 1. A liquid feed pump 13 that supplies the electrode 6, a compound solution pipe 14 that connects the compound solution tank 12 and the liquid feed pump 13, and a compound supply pipe that connects the liquid feed pump 13 and the fuel gas flow path 10. The road 15 is constituted. As described above, on the surfaces of the air electrode side separator 7 and the fuel electrode side separator 9 of the fuel cell stack 1, there are the air flow path 8 and the fuel gas flow path 10 for supplying air or hydrogen as a reaction gas. Each is formed. As shown in the exploded perspective view of the single cell 2 of the fuel cell stack 1 that constitutes the fuel cell system shown in FIG. 2, each reaction gas is humidified by a bubbler (not shown) and the air flow path 8 and the fuel gas flow. Pass through road 10. At this time, the compound solution is supplied from the compound solution tank 12 to the fuel gas channel 10 via the compound solution line 14 and the solution supply line 15 by the driving force of the liquid feed pump 13. The compound solution supplied to the fuel gas channel 10 is diffused from the fuel electrode 6 side to the solid polymer electrolyte membrane 4 as shown in the explanatory view for explaining the movement of the substance in the membrane electrode assembly 3 shown in FIG. It is uniformly distributed in the air electrode 5 according to the concentration gradient.

  As described above, the reason why it is preferable to supply the compound as a compound solution from the fuel electrode 6 side is that the possibility of generating active oxygen such as hydroxy radicals is high near the three-phase interface of the air electrode. . As shown in the schematic diagram showing the three-phase interface in the air electrode shown in FIG. 4, the vicinity of the three-phase interface of the air electrode is an environment where oxygen in the air and platinum as an electrode catalyst are present and are easily oxidized. For this reason, when the compound is introduced from the air electrode, the compound itself may be oxidized and lost at the three-phase interface of the air electrode regardless of the presence or absence of hydroxy radicals, and the deactivation efficiency of active oxygen is reduced. there's a possibility that.

Further, as described above, the air flow path 8 of the air electrode side separator 7 also functions as a generated water removal passage. For this reason, an excessively supplied compound that has become unnecessary, an oxidized form of a compound that has been inactivated by deactivating active oxygen, is oxidized by a catalyst on the three-phase interface, and CO ₂ , H ₂ O, N _2, etc. Then, it can be discharged from the discharge pipe 16 shown in FIG. For this reason, it can be prevented that the compound becomes unstable radicals or peroxides and becomes a new oxidation reaction initiator to cause deterioration of the electrolyte membrane.

  When preparing the compound solution, it may be difficult to dissolve in the air electrode, but it is important to dissolve in the solvent. In the case of insolubility, the entry and exit of hydrogen radicals becomes impossible, and the effect of inactivating the hydroxy radicals is not sufficiently exhibited. For this reason, it is necessary to select a solvent in which the compound is dissolved, and an organic solvent alone or a mixed solvent of an organic solvent and water is used as necessary. And when using an organic solvent, it is necessary to confirm beforehand that it does not affect a power generation performance. For this reason, if the compound can be dissolved, an aqueous solution is preferable from the viewpoint of power generation performance.

The compound is preferably a hydrocarbon compound composed of four elements of carbon, oxygen, nitrogen and hydrogen. Other elements other than carbon, oxygen, nitrogen, and hydrogen may poison the platinum in the electrode and hinder the power generation performance of the fuel cell. In the case of a base metal element, there is a possibility of promoting the generation of hydroxy radicals. Considering the case where the compound is oxidized and discharged at the air electrode, it is composed of only four elements of carbon, oxygen, nitrogen and hydrogen and is a hydrocarbon compound that is decomposed into CO ₂ , H ₂ O and N ₂ Is preferred.

  The compound preferably has a molecular weight of 400 or less, more preferably 200 or less, and even more preferably 100 or less. In consideration of supplying the compound from the fuel electrode into the electrolyte, the compound may be hardly soluble, but is water-soluble, and the smaller the molecular weight, the easier the penetration into the electrolyte through the electrode.

  It is preferable that an oxidant obtained by oxidation of a compound with a hydroxy radical is chemically stable or is hydrolyzed and chemically stable. If the oxidant is unstable, the oxidant may become a new side reaction initiator and oxidize the electrolyte membrane or the like.

Since the oxidation-reduction potential of the hydroxyl radicals is very high, most of the four elements that consists hydrocarbon compounds are believed to act as a reducing agent to thermodynamically hydroxyl radicals. Each compound is considered to have a difference in reducing ability in terms of kinetics. In view of the high reactivity of the hydroxy radical, the reaction between the compound and the hydroxy radical is preferably faster in terms of kinetics. Therefore, the reaction rate of the hydroxy radical and the compound is preferably such that the reaction rate constant k is larger than 5 × 10 ⁻⁸ [M ⁻¹ sec ⁻¹ ] at normal temperature, and k is 10 × 10 ⁻⁸ [M ⁻¹ sec ^{−]. 1} ] is more preferable. When k is 5 × 10 ⁻⁸ [M ⁻¹ sec ⁻¹ ] or less, the reaction rate with the hydroxy radical is insufficient, and the oxidation of the electrolyte membrane is prioritized. When k is larger than 10 × 10 ⁻⁸ [M ⁻¹ sec ⁻¹ ], a greater antioxidant effect can be expected.

  As a form of reaction with the hydroxy radical, there is no problem if the hydroxy radical is inactivated, and a reduction reaction of the hydroxy radical shown in the formula (16) or an addition reaction of the hydroxy radical shown in the formula (17) may be used. Absent. In this case, an oxidized form obtained by oxidizing the compound with a hydroxy radical or an adduct obtained by adding a hydroxy radical to the compound is chemically stabilized or hydrolyzed to prevent side reactions from occurring. It is preferable that it is stable.

  Examples of compounds that reduce and inactivate hydroxy radicals include, for example, benzene, benzene derivatives, biphenyl, acenaphthene, naphthalene, naphthalene derivatives, pyridine, pyridine derivatives, cyclohexane, cyclohexane derivatives, C3-C10 linear olefins, cyclohexene, cyclohexene Selected from the group consisting of derivatives, styrene, alkylamines, acetamides, amide derivatives, anilines, aniline derivatives, C2 to C8 primary to tertiary alcohols, alcohol derivatives, cyclohexanol, benzyl alcohol, C2 to C8 ethers and cyclic ethers Compounds.

  As described above, the fuel cell system according to the embodiment of the present invention has a fuel cell reaction system in which a fuel cell reaction occurs and a compound that inactivates hydroxy radicals in the fuel cell reaction system. And since this compound has an oxidation potential higher than 1.23 [V] (NHE), the fuel cell system excellent in durability can be implement | achieved.

  In the fuel cell system according to the present embodiment, the fuel cell system is not limited to the type of fuel as long as it is a fuel cell that uses a proton conductive polymer electrolyte membrane. It can be used for any fuel cell such as a solid polymer fuel cell and a direct hydrocarbon solid polymer fuel cell.

  Further, the fuel cell system according to the present invention is not limited to a fuel cell vehicle as its application. For example, the fuel cell system is used in a fuel cell cogeneration power generation system, a fuel cell home appliance, a fuel cell portable device, a fuel cell transport device, and the like. It is possible to apply.

  Hereinafter, the fuel cell system according to the present invention will be described more specifically with reference to Examples 1 to 4 and Comparative Examples 1 to 4. However, the scope of the present invention is not limited thereto. These examples are for examining the effectiveness of the fuel cell system according to the present invention, and show examples of fuel cell systems using different compounds.

  In order to investigate the effect of supplying a compound that reduces and deactivates hydroxy radicals from the fuel electrode, an open circuit standing test was conducted, and the power generation durability time of a single cell was measured.

Example 1
As a solid polymer electrolyte membrane, a DuPont Nafion (registered trademark) 211 membrane (thickness 25 [μm]) was cut into 1 [cm] squares and used. The Nafion (registered trademark) membrane is pretreated according to NEDO PEFC R & D project standard treatment, boiled in 3% hydrogen peroxide water for 1 hour, then boiled in distilled water for 1 hour. M] The solution was boiled in an aqueous solution of sulfuric acid for 1 hour, and finally boiled in distilled water for 1 hour.

After the pretreatment, a single cell was prepared for an open circuit standing test. First, TEC10E50E manufactured by Tanaka Kikinzoku was used as platinum-supporting carbon, and a 5 wt% Nafion (registered trademark) solution manufactured by DuPont was used as an electrolyte binder. After the platinum-supporting carbon was sufficiently moistened with water, an electrolyte binder was added, and isopropyl alcohol was added to disperse the platinum-supporting carbon and the electrolyte binder in isopropyl alcohol. Thereafter, the mixture was mixed for 3 hours with a homogenizer and homogenized, and then defoamed to obtain a catalyst ink. Thereafter, the catalyst ink was applied to a Teflon (registered trademark) sheet so that the amount of platinum was 0.4 [mg / cm ³ ] to prepare two catalyst sheets. This catalyst sheet is arranged on the fuel electrode side and air electrode side of the Nafion (registered trademark) 211 membrane, respectively, and hot pressed at 2 [MPa], 132 [° C.] and 10 [minutes] to form the catalyst sheet as Nafion (registered trademark). The film was transferred to a 211 membrane to form an electrode catalyst layer, and CCM (catalyst coated membrane) was obtained. CCM was sandwiched between gas diffusion layers, and then sandwiched between a separator and a current collector plate to produce a single cell with a catalyst area of 5 × 5 [cm ² ].

  An open circuit standing test was conducted at a cell temperature of 90 [° C.] and a humidification temperature of 60 [° C.], and OCV (open circuit voltage) was monitored to examine the durability of the single cell. Hydrogen gas humidified at a humidity of 30 [%] was supplied to the fuel electrode, and oxygen gas humidified at a humidity of 30 [%] was supplied to the air electrode at a flow rate of 500 [ml / min]. Isopropyl alcohol was used as a compound for inactivating the hydroxy radical. Isopropyl alcohol was supplied from the fuel electrode via a liquid feed pump at a flow rate of 0.1 [ml / min]. The OCV was measured while flowing hydrogen gas, oxygen gas, and isopropanol at the above flow rates.

Example 2
In Example 2, isopropyl alcohol was supplied to the fuel electrode side of the single cell at a flow rate of 1.0 [ml / min], and the other conditions were the same as in Example 1.

Example 3
In Example 3, ethyl alcohol was allowed to flow instead of isopropyl alcohol in Example 1, and the other conditions were the same as in Example 1.

Example 4
In Example 4, 1,4-dioxane was allowed to flow instead of the isopropyl alcohol of Example 1, and the other conditions were the same as in Example 1.

Comparative Example 1
In Example 1, the case where the compound solution was not passed was set as Comparative Example 1, and the other conditions were the same as those in Example 1.

Comparative Example 2
In Comparative Example 2, methyl alcohol was allowed to flow instead of isopropyl alcohol in Example 1, and the other conditions were the same as in Example 1.

Comparative Example 3
In Comparative Example 3, acetone was allowed to flow instead of isopropyl alcohol in Example 1, and the other conditions were the same as in Example 1.

Comparative Example 4
In Comparative Example 4, acetic acid was allowed to flow instead of isopropyl alcohol in Example 1, and the other conditions were the same as in Example 1.

  It is not easy to specify the deactivation reaction of the hydroxy radical in the fuel cell because the disappearance rate of the hydroxy radical is extremely high and the concentration of the compound that inactivates the hydroxy radical is extremely low. Therefore, for Examples 1, 3, 4, and Comparative Examples 1 to 4, a model experiment was conducted in a beaker in order to predict the reaction for inactivating the hydroxy radicals in the fuel cell.

In the experiment, an aqueous solution of H ₂ O ₂ : 0.5 [%], FeCl ₂ : 0.2 [ppm], and a compound that inactivates a hydroxyl radical: 1 [%] was prepared in a beaker, and an electrolyte membrane was used. A Nafion® 211 membrane was immersed in a beaker. After standing for 9 hours, the change of the compound that inactivates the hydroxy radical and the amount of fluorine ion eluted from the electrolyte membrane were confirmed by ion chromatography using a gas chromatograph.

Table 1 shows the types of compounds supplied to the fuel electrode in Examples 1 to 4 and Comparative Examples 1 to 4, the molecular weight of the compounds, the supply amount of the compounds, and the reaction rate between the hydroxy radical and the compounds in Table 2. The result of the open circuit standing test, that is, the time during which the OCV maintains 0.8 [V] and the time during which power generation is disabled are shown. Table 3 shows the compounds used in the model experiment using a beaker, main compounds newly detected after the test, and detected fluorine ion concentrations.

  FIG. 5 shows the results of the single cell accelerated durability test. In both Example 1 and Comparative Example 1, the OCV was about 1.0 [V] after the start of the test, and the OCV decreased to about 0.95 [V] after 5 hours. In Comparative Example 1 in which a compound that reduces hydroxy radicals was not added, the OCV decreased rapidly after 15 hours from the start of the test, was below 0.8 [V] in 20 hours, and was unable to generate power in 40 hours. In contrast, in Example 1 in which isopropyl alcohol was supplied as a compound for reducing hydroxy radicals from the fuel electrode side, OCV was 0.8 [V] or higher until 50 hours passed, and power generation continued until 80 hours later. .

From Table 1, when Fc / Fc ⁺ is used as a reference electrode, isopropyl alcohol has an oxidation-reduction potential of 2.50 [V] (Electrical Society edited by Electrochemical Handbook, 5th edition, Maruzen Co., Ltd., 2002) April 30, p298) and higher than 1.23 [V] (NHE). In addition, isopropyl alcohol has a reaction rate constant k at room temperature with a hydroxy radical of 13 × 10 ⁻⁸ , and is known to have a high inactivation reaction rate of the hydroxy radical (Kay Yu Ingold ( KUIngold), "Free Radicals", Wiley Interscience, p82). Thus, the durability of the single cell was improved by reducing and inactivating the hydroxy radical and supplying a compound having a redox potential higher than 1.23 [V] (NHE) from the fuel electrode.

  FIG. 5 shows a gas chromatogram obtained in the model experiment of Example 1. Here, the detection limit is 0.05 [ppm]. In FIG. 5, the peak indicated by A is a slightly confirmed side reaction peak, the peak indicated by B is an acetone peak, and the peak indicated by C is an isopropyl alcohol peak. From this result, acetone was confirmed as a main product in addition to isopropyl alcohol in the solution 9 hours after the electrolyte membrane was immersed in the beaker in Example 1. From this, it is presumed that the reaction mechanism occurring in the beaker is as shown in the reaction equation shown in equation (18).

CH ₃ CHOHCH ₃ + 2 · OH → CH ₃ COCH ₃ + 2H ₂ O (18)
The amount of fluorine ions contained in the solution after 9 hours from the immersion of the electrolyte membrane in the beaker was below the detection limit. From these results, it was confirmed that isopropyl alcohol was oxidized by reaction with a hydroxy radical to become acetone, and the hydroxy radical was reduced and inactivated to suppress the oxidation of the electrolyte membrane. On the other hand, in Comparative Example 1, 2.0 [ppm] fluorine ion was detected, and it was confirmed that the oxidation of the electrolyte membrane was proceeding with hydroxy radicals.

  In Example 2, the time to decrease below the redox potential of 0.8 [V] was 40 hours, which was shorter than that in Example 1, but the time when power generation was disabled was improved to 120 hours. This is considered to be a result of increasing the supply amount of isopropyl alcohol.

In Example 3, the time to decrease below the redox potential of 0.8 [V] was 40 hours, which was shorter than that in Example 1, but the time during which power generation was disabled was improved to 80 hours. When Fc / Fc ⁺ is used as the reference electrode, ethyl alcohol has an oxidation-reduction potential of 2.61 [V] (Electrical Society of Japan, Electrochemical Handbook, 5th edition, Maruzen Co., Ltd., April 30, 2002) Day, p298) and higher than 1.23 [V] (NHE). Further, ethyl alcohol has a reaction rate constant k at room temperature with a hydroxy radical of 11 × 10 ⁻⁸ , and is known to have a high inactivation reaction rate of the hydroxy radical (Kay Yu Ingold ( KUIngold), "Free Radicals", Wiley Interscience, p82). Thus, the durability of the single cell was improved by reducing and inactivating the hydroxy radical and supplying a compound having a redox potential higher than 1.23 [V] (NHE) from the fuel electrode. As a result of model experiments in a beaker, the reaction mechanism occurring in the beaker was estimated as shown in the following reaction formula, and acetaldehyde and acetic acid were confirmed as main products.

CH ₃ CH ₂ OH + 2.OH → CH ₃ CHO + 2H ₂ O Formula (19)
CH ₃ CHO + 2.OH → CH ₃ COOH + H ₂ O Formula (20)
In the model experiment of Example 3, the fluorine ions were below the detection limit as in Example 1.

In Example 4, the time to decrease below the redox potential of 0.8 [V] was 60 hours, and the time when power generation was disabled was improved to 100 hours. Although the literature potential could not be found for the oxidation potential of dioxane, its chemical structure is expected to have an oxidation potential higher than the intended 1.23 [V] (NHE). In addition, the reaction rate constant k at room temperature with a hydroxy radical is 2.0 × 10 ⁻⁷ , and it is known that the inactivation reaction rate of the hydroxy radical is high (KU Ingold). , "Free Radicals", Wiley Interscience, p82). Thus, the durability of the single cell was improved by reducing and inactivating the hydroxy radical and supplying a compound having a redox potential higher than 1.23 [V] (NHE) from the fuel electrode. As a result of conducting a model experiment in a beaker, the mechanism of the reaction occurring in the beaker was estimated as shown in the following reaction formula, and dioxanol and dioxanone were confirmed as main products.

Further, in the model experiment of Example 4, as in Example 1, fluorine ions were below the detection limit.

In Comparative Example 2, the time to decrease below the redox potential of 0.8 [V] was 30 hours, and the time during which power generation was disabled was 50 hours, both of which were shorter than Example 1. When Fc / Fc ⁺ is used as a reference electrode, methyl alcohol has an oxidation-reduction potential of 2.73 [V] (Electrical Society edited by Electrochemical Handbook, 5th edition, Maruzen Co., Ltd., April 30, 2002) Day, p298) and higher than the intended 1.23 [V] (NHE). On the other hand, methyl alcohol has a reaction rate constant k with a hydroxy radical at room temperature of 5.0 × 10 ⁻⁸ (KU Ingold, “Free Radicals”, Willy ・Interscience (see Wiley Interscience, p82), and the inactivation reaction rate of hydroxy radicals is slower than in Examples. In this way, even when the hydroxy radical is reduced and inactivated and a compound having a redox potential higher than 1.23 [V] (NHE) is supplied from the fuel electrode, the reaction rate constant with the hydroxy radical When it was low, the durability was slightly improved as compared with Comparative Example 1. Moreover, as a result of conducting a model experiment in a beaker, formic acid and formaldehyde were confirmed as main products. Further, fluorine ions were also confirmed to be 0.2 [ppm], and it was found that the effect of suppressing the oxidation of the electrolyte membrane was weak.

In Comparative Example 3, the time to decrease below the redox potential of 0.8 [V] was 15 hours, and the time during which power generation was disabled was 30 hours, both of which were shorter than Comparative Example 1. The reaction rate constant k between acetone and hydroxy radicals at room temperature is 0.6 × 10 ⁻⁸ (KU Ingold, “Free Radicals”, Willy Interscience (Wiley Interscience), p82), and the reaction rate for inactivating hydroxy radicals is slower than in the examples. Moreover, as a result of conducting a model experiment in a beaker, formic acid and formaldehyde were confirmed as main products. Further, 4.3 [ppm] of fluorine ions was also confirmed, and the effect of suppressing the oxidation of the electrolyte membrane was not exhibited.

In Comparative Example 4, the time to decrease below the redox potential of 0.8 [V] was 10 hours, the time during which power generation was disabled was 20 hours, and both were shorter than Comparative Example 1. The reaction rate constant k between acetic acid and hydroxy radical at room temperature is 1.4 × 10 ⁻⁸ (KUIngold, “Free Radicals”, Willy Interscience (Wiley) Interscience), p82), and the reaction rate for inactivating hydroxy radicals is slower than in the examples. Moreover, as a result of conducting a model experiment in a beaker, formic acid was confirmed as the main product. Further, fluorine ions were also confirmed to be 5.2 [ppm] and did not show the effect of suppressing the oxidation of the electrolyte membrane.

  As described above, perfluorosulfonic acid polymers represented by the Nafion (registered trademark) membrane, which is the most widely used electrolyte membrane, are sufficiently resistant to active oxygen generated at the air electrode of the fuel cell. Although it could not be said that there was a situation, by continuously supplying a compound that inactivates hydroxy radicals from the fuel electrode, it is possible to inactivate the hydroxy radicals that are continuously generated, resulting in deterioration of the electrolyte membrane. The durability performance of the fuel cell can be improved.

  As mentioned above, although the form of this Embodiment was demonstrated, it should not be understood that the description and drawing which make a part of indication of said Embodiment limit this invention. From this disclosure, various alternative embodiments, examples, and operational techniques will be apparent to those skilled in the art.

1 is a schematic explanatory diagram illustrating an embodiment of a fuel cell system according to the present invention. It is a disassembled perspective view which shows the single cell of the fuel cell which comprises the fuel cell system which concerns on this invention. It is explanatory drawing explaining the movement of the substance in the membrane electrode assembly which comprises a single cell. It is a schematic diagram which shows the three-phase interface in an air electrode. It is a graph showing the result of the single cell accelerated durability test of Example 1 and Comparative Example 1. 2 is a gas chromatogram obtained in the model experiment of Example 1. FIG.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Fuel cell stack 2 Single cell 3 Membrane electrode assembly 4 Solid polymer electrolyte membrane 5 Air electrode 6 Fuel electrode 7 Air electrode side separator 8 Air flow path 9 Fuel electrode side separator 10 Fuel gas flow path 11 Compound supply means

Claims (8)

A fuel cell reaction system in which a fuel cell reaction occurs;
A compound that inactivates hydroxy radicals in the fuel cell reaction system,
The compound has an oxidation potential higher than 1.23 [V] (NHE), and benzene, benzene derivatives, biphenyl, acenaphthene, naphthalene, naphthalene derivatives, pyridine, pyridine derivatives, styrene, alkylamine, acetamide, amide A fuel cell system comprising a compound selected from the group consisting of a derivative, aniline, an aniline derivative and benzyl alcohol. A fuel cell reaction system in which a fuel cell reaction occurs;
A compound that inactivates hydroxy radicals in the fuel cell reaction system,
The fuel cell system, wherein the compound has an oxidation potential higher than 1.23 [V] (NHE) and is dioxane.   The fuel cell system according to claim 1, wherein the compound has a molecular weight of 400 or less.   The fuel cell system according to claim 3, wherein the molecular weight of the compound is 200 or less.   The fuel cell system according to claim 4, wherein the molecular weight of the compound is 100 or less. A membrane electrode assembly comprising a solid polymer electrolyte membrane, an air electrode and a fuel electrode sandwiching the solid polymer electrolyte membrane, and one of the air electrode and the fuel electrode constitutes an electrode; and the membrane electrode junction An air electrode side separator that is disposed on the air electrode side of the body and defines an air flow path between the membrane electrode assembly, and the fuel electrode side surface of the membrane electrode assembly; A fuel cell in which a plurality of single cells including a fuel electrode side separator that defines a fuel gas flow path between the electrode assembly and the electrode assembly;
6. The fuel cell system according to claim 1, further comprising a compound supply unit configured to continuously supply the compound to the fuel cell and contact the electrode. The fuel cell reaction system includes the membrane electrode assembly, and the compound supply means supplies the compound to the fuel electrode and sends it to the air electrode using a concentration gradient to oxidize the unnecessary compound. The fuel cell system according to claim 6, wherein the body is oxidized by a catalyst contained in the air electrode and discharged as CO ₂ , H ₂ O, or N ₂ .   8. The fuel cell according to claim 6, wherein the fuel cell includes a fuel cell selected from a fuel cell group including a hydrogen fuel cell, a direct methanol fuel cell, and a direct hydrocarbon fuel cell. Fuel cell system.

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