fuel cell system

Abstract

To provide a fuel cell system that can suppress a deterioration of a solid polymer electrolyte, without lowering a power generation performance.SOLUTION: A fuel cell system includes a solid polymer type fuel cell, a fuel gas supply device, an oxidant gas supply device, and a control device. The solid polymer type fuel cell includes a membrane electrode assembly in which fine particles of a tungsten compound are added to a cathode catalyst layer and / or an anode catalyst layer. The control device includes: determination means for determining whether to carry out a detoxification treatment of a metal ion mixed into the membrane electrode assembly; and adsorption means for controlling a flux Nc of the metal ion at any timing in a period in which the fuel cell system operates after it is determined that the detoxification treatment should be executed, thereby moving the metal ion to a cathode catalyst layer side which contains a tungsten compound or a cathode catalyst layer side which contains a tungsten compound, and causing the metal ion to be adsorbed to the tungsten compound.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a fuel cell system, and more particularly to a fuel cell system equipped with a means for detoxifying harmful metal ions contained in a polymer electrolyte fuel cell. [Background technology]

[0002] A polymer electrolyte fuel cell comprises a membrane electrode assembly (MEA) in which catalyst layers are bonded to both sides of an electrolyte membrane. A gas diffusion layer is usually disposed on the outside of the catalyst layer. Furthermore, a current collector (separator) with gas flow channels is disposed on the outside of the gas diffusion layer. A polymer electrolyte fuel cell typically comprises a structure (fuel cell stack) in which a plurality of unit cells each consisting of such an MEA, gas diffusion layer, and current collector are stacked.

[0003] The electrolyte contained in the MEA is said to be attacked and deteriorated by radicals generated directly by the direct reaction of oxygen and hydrogen or by electrochemical reactions, or by radicals generated via hydrogen peroxide. In solid polymer fuel cells, radical attack is known to increase the resistance of the electrolyte membrane, increase cross-leakage, and shorten the lifespan due to thinning. Furthermore, degradation products generated by radical attack can poison the catalyst, potentially reducing electrolytic and cell performance.

[0004] Therefore, various proposals have been made in the past to solve this problem. For example, Patent Document 1 discloses an electrolyte membrane for a polymer electrolyte fuel cell, which is made of a cation exchange membrane in which part of the sulfonic acid groups have been ion-exchanged with cerium ions. The document describes that resistance to hydrogen peroxide or peroxide radicals is improved when part of the sulfonic acid groups of the electrolyte membrane is ion-exchanged with cerium ions.

[0005] In addition, Patent Document 2 states: (a) A catalyst ink containing 0.25 times the molar amount of H2WO4 and 0.05 times the molar amount of Ce(NO3)3 relative to the molar number of sulfonic acid groups in Nafion (registered trademark), or (b) Catalyst ink containing 0.25 times the molar amount of H2WO4 and 0.05 times the molar amount of Mn(NO3)3 relative to the molar number of sulfonic acid groups in Nafion (registered trademark). MEAs fabricated using the method are disclosed.

[0006] The same document states: (A) When a W compound and a transition metal compound are dispersed or dissolved in a suitable solvent, a mixture of the W compound and the transition metal compound, or a composite compound which is a reaction product of the W compound and the transition metal compound, is obtained. These compounds function as a degradation inhibitor for the solid polymer electrolyte, and (B) MEAs fabricated using catalyst inks containing degradation inhibitors (H2WO4, Ce(NO3)3, Mn(NO3)3) have a higher molecular weight retention rate of the solid polymer electrolyte than MEAs fabricated using catalyst inks that do not contain degradation inhibitors. is stated.

[0007] Patent Document 1 describes that the durability of an electrolyte membrane is improved by ion-exchanging some of the protons in the sulfonic acid groups of the electrolyte membrane with cerium ions. However, ion-exchanging some of the protons in the sulfonic acid groups with cerium ions reduces the proton conductivity of the electrolyte membrane, resulting in a decrease in fuel cell performance. Furthermore, because cerium ions are cations, they can be locally unevenly distributed due to potential gradients, potential sweeps, humidity gradients, and the like, potentially causing further performance degradation.

[0008] On the other hand, Patent Document 2 describes that a mixture or composite compound of a W compound and a transition metal compound functions as a degradation inhibitor for a solid polymer electrolyte. However, there are cases where simply adding a degradation inhibitor to a solid polymer electrolyte does not sufficiently inhibit the degradation of the solid polymer electrolyte. This is because the causative substance (i.e., Fe) that causes degradation of the solid polymer electrolyte is not necessarily present in the vicinity of the degradation inhibitor.3+ , Cu 2+ This is thought to be because metal ions active in the Fenton reaction (such as ammonium nitrate) are not necessarily present. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] International Publication No. 2005 / 124911 [Patent Document 2] Patent No. 5194448 Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to provide a fuel cell system that can suppress degradation of the solid polymer electrolyte without reducing power generation performance. [Means for solving the problem]

[0011] In order to solve the above problems, the fuel cell system according to the present invention has the following configuration. (1) The fuel cell system is a polymer electrolyte fuel cell; a fuel gas supply device for supplying a fuel gas to the anode of the polymer electrolyte fuel cell; an oxidant gas supply device that supplies an oxidant gas to the cathode of the polymer electrolyte fuel cell; a control device that controls the fuel cell system; It is equipped with: (2) The polymer electrolyte fuel cell includes a membrane electrode assembly in which fine particles of a tungsten compound are added to the cathode catalyst layer and / or the anode catalyst layer. (3) The control device a determination means for determining whether or not to perform a detoxification treatment of metal ions active in the Fenton reaction that have been mixed into the membrane electrode assembly; an adsorption means for moving the metal ions to the cathode catalyst layer side containing the tungsten compound or the cathode catalyst layer side containing the tungsten compound by controlling a flux Nc of the metal ions in the membrane electrode assembly at any timing during operation of the fuel cell system after it has been determined that the detoxification treatment should be performed, and for adsorbing the metal ions to the tungsten compound; It is equipped with: [Effects of the Invention]

[0012] By adding fine particles of a tungsten compound to the cathode catalyst layer and controlling the operating conditions (i.e., the metal ion flux Nc), the metal ions contained in the MEA can be migrated to the cathode catalyst layer. As a result, the metal ions are trapped by the tungsten compound added to the cathode catalyst layer, increasing the probability that the metal ions will be detoxified. Alternatively, adding fine particles of a tungsten compound to the anode catalyst layer and controlling the operating conditions can move metal ions contained in the MEA to the anode catalyst layer, which increases the probability that the metal ions will be trapped by the tungsten compound added to the anode catalyst layer and rendered harmless. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a diagram showing the amount of Fe3+ adsorbed in various samples (Examples 1 to 4). [Figure 2] FIG. 2 is a cross-sectional view of a cell used in the experiment. [Figure 3] FIG. 10 is a diagram showing the cation substitution rate of each membrane after a cell provided with a laminated membrane of a membrane containing Co2+ and a membrane not containing Co2+ was maintained under various conditions (Examples 5 to 6, Comparative Example 1). [Figure 4] Figure 1 shows the relationship between the WO3·H2O content and the normalized total F-elution amount. DETAILED DESCRIPTION OF THE INVENTION

[0014] An embodiment of the present invention will be described in detail below. [1. Fuel Cell System] The fuel cell system according to the present invention has the following configuration.

[0015] [Configuration 1] A fuel cell system comprising: (1) The fuel cell system is a polymer electrolyte fuel cell; a fuel gas supply device for supplying a fuel gas to the anode of the polymer electrolyte fuel cell; an oxidant gas supply device that supplies an oxidant gas to the cathode of the polymer electrolyte fuel cell; a control device that controls the fuel cell system; It is equipped with: (2) The polymer electrolyte fuel cell includes a membrane electrode assembly in which fine particles of a tungsten compound are added to the cathode catalyst layer and / or the anode catalyst layer. (3) The control device a determination means for determining whether or not to perform a detoxification treatment of metal ions active in the Fenton reaction that have been mixed into the membrane electrode assembly; an adsorption means for moving the metal ions to the cathode catalyst layer side containing the tungsten compound or the cathode catalyst layer side containing the tungsten compound by controlling a flux Nc of the metal ions in the membrane electrode assembly at any timing during operation of the fuel cell system after it has been determined that the detoxification treatment should be performed, and for adsorbing the metal ions to the tungsten compound; It is equipped with:

[0016] [Configuration 2] the cathode catalyst layer contains the tungsten compound, The adsorption means has a current density of 1.0 A / cm 2 The fuel cell system according to configuration 1, further comprising a current density control means for generating electricity under the conditions maintained as above.

[0017] [Configuration 3] the fuel cell system further includes a humidifier for humidifying the oxidant gas; the anode catalyst layer contains the tungsten compound, The adsorption means is (a) Current density is 0.02 A / cm 2 Maintained below, and (b) The difference between the cathode side relative humidity (RHc) and the anode side relative humidity (RHa) (ΔRH = RHc - RHa) is maintained at 40% RH or more. 3. The fuel cell system according to claim 1 or 2, further comprising humidity control means for generating electricity under the conditions.

[0018] [Configuration 4] 4. The fuel cell system according to any one of configurations 1 to 3, wherein the control device includes a means for executing the adsorption means when the fuel cell system is idling, starting up, and / or stopping.

[0019] [Configuration 5] the fuel cell system further includes a power storage device for storing surplus power; 5. The fuel cell system according to any one of configurations 1 to 4, wherein the control device includes means for storing, in the power storage device, the electric power generated when the adsorption means is executed.

[0020] [Configuration 6] 6. The fuel cell system of any one of Configurations 1 to 5, wherein the tungsten compound includes at least one selected from the group consisting of tungsten oxide (+IV, +V, +VI), tungsten oxide (+VI)n hydrate (n=1 to 3, n includes non-integer), tungsten carbide, and tungstate.

[0021] [Configuration 7] 7. The fuel cell system according to any one of configurations 1 to 6, wherein the metal ions include ions of one or more metal elements selected from the group consisting of Fe, Cu, Ni, Al, Co, and Ti.

[0022] [1.1. Polymer electrolyte fuel cell] A polymer electrolyte fuel cell generally has the following characteristics: (a) A membrane electrode assembly (MEA) in which an anode catalyst layer is bonded to one side of an electrolyte membrane and a cathode catalyst layer is bonded to the other side; (b) an anode gas diffusion layer disposed on the outside of the anode catalyst layer, and a cathode gas diffusion layer disposed on the outside of the cathode catalyst layer; and (c) an anode separator disposed on the outside of the anode gas diffusion layer, and a cathode separator disposed on the outside of the cathode gas diffusion layer; It is equipped with: A polymer electrolyte fuel cell has a structure (stack structure) in which a plurality of unit cells each consisting of such an MEA, a gas diffusion layer, and a separator are stacked.

[0023] [1.1.1. Membrane electrode assembly] [A. Electrolyte membrane] The electrolyte membrane contains a solid polymer electrolyte. In the present invention, the type of solid polymer electrolyte contained in the electrolyte membrane is not particularly limited. Examples of solid polymer electrolytes include: (a) fluorine-based electrolytes such as Nafion®, Flemion®, Aquivion®, and Aciplex®; (b) Polyetheretherketone, polysulfone, polyethersulfone, polyimide, polyphenylene, polyamide, polyamideimide, or a wholly aromatic hydrocarbon-based electrolyte comprising a derivative thereof, into which an acid group such as a sulfonic acid group has been introduced; (c) Partially aromatic hydrocarbon electrolytes having an aromatic ring in a part of the polymer chain of an aliphatic hydrocarbon electrolyte; etc.

[0024] The electrolyte membrane may contain only a solid polymer electrolyte, or may be a composite of a solid polymer electrolyte and a reinforcing material. Examples of reinforcing materials include: (a) Porous membranes and nonwoven fabrics of fluorine-based resins such as polytetrafluoroethylene (PTFE), perfluoroalkoxyalkane (PFA), perfluoroethylenepropene copolymer (FEP), and ethylenetetrafluoroethylene copolymer (ETFE), (b) Porous membranes and nonwoven fabrics of hydrocarbon resins such as polyethylene (PE) and polypropylene (PP), etc.

[0025] [B. Anode catalyst layer] An anode catalyst layer is bonded to one side of the electrolyte membrane. The anode catalyst layer includes an electrode catalyst (anode catalyst) active in the hydrogen oxidation reaction and a catalyst layer ionomer. The anode catalyst may consist solely of catalyst particles (A) active in the hydrogen oxidation reaction, or the catalyst particles (A) may be supported on the surface of a carrier. In the present invention, the materials for the catalyst layer ionomer, catalyst particles (A), and carrier are not particularly limited, and the most suitable materials can be selected depending on the purpose.

[0026] The catalyst layer ionomer on the anode side may be made of the same material as the solid polymer electrolyte constituting the electrolyte membrane, or may be made of a different material. Examples of catalyst particles (A) include noble metals, alloys containing two or more noble metals, and alloys of one or more noble metals and one or more base metals. Examples of the carrier include carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[0027] [C. Cathode Catalyst Layer] A cathode catalyst layer is bonded to the other surface of the electrolyte membrane. The cathode catalyst layer includes an electrode catalyst (cathode catalyst) active in the oxygen reduction reaction and a catalyst layer ionomer. The cathode catalyst may consist only of catalyst particles (B) active in the oxygen reduction reaction, or the catalyst particles (B) may be supported on the surface of a carrier. In the present invention, the materials for the catalyst layer ionomer, catalyst particles (B), and carrier are not particularly limited, and the most suitable materials can be selected depending on the purpose.

[0028] The cathode-side catalyst layer ionomer may be made of the same material as the solid polymer electrolyte constituting the electrolyte membrane, or may be made of a different material. Examples of catalyst particles (B) include precious metals, alloys containing two or more precious metals, and alloys containing one or more precious metals and one or more base metals. Examples of the carrier include carbon black, carbon nanotubes, carbon nanohorns, activated carbon, natural graphite, mesocarbon microbeads, and glassy carbon powder.

[0029] [1.1.2. Anode gas diffusion layer, cathode gas diffusion layer] An anode gas diffusion layer and a cathode gas diffusion layer are usually disposed on both sides of the MEA. In the present invention, the structure of the gas diffusion layer is not particularly limited, and an optimum structure can be selected depending on the purpose. A gas diffusion layer typically comprises a substrate made of carbon paper or the like and a microporous layer formed on the surface of the substrate. The microporous layer typically comprises a composite of conductive particles such as carbon and a water-repellent resin such as polytetrafluoroethylene.

[0030] [1.1.3. Anode separator, cathode separator] In a polymer electrolyte fuel cell, an anode separator is placed on the outside of the anode gas diffusion layer, and a cathode separator is placed on the outside of the cathode gas diffusion layer. Gas flow channels for the flow of reactant gases are formed on the surface of each separator facing the MEA. In the present invention, the material and structure of the separator are not particularly limited, and the optimum material and structure can be selected depending on the purpose.

[0031] 1.1.4. Tungsten compounds In the present invention, fine particles of a tungsten compound are added to the cathode catalyst layer and / or the anode catalyst layer. The tungsten compound may be added to either the anode catalyst layer or the cathode catalyst layer, or may be added to both. When the tungsten compound is added to both the anode catalyst layer and the cathode catalyst layer, high durability can be obtained with a relatively small amount of the tungsten compound.

[0032] [A. Materials] In the present invention, the term "tungsten compound" refers to a compound containing W that has the effect of detoxifying metal ions that are active in the Fenton reaction (a reaction that generates hydroxyl radicals (·HO) from hydrogen peroxide). Examples of tungsten compounds having such an effect include: (a) Tungsten oxide (+IV, +V, +VI), (b) tungsten oxide (+VI)n hydrate (n = 1 to 3, including non-integer n); (c) tungsten carbide, (d) Tungstate etc.

[0033] Tungsten oxide may be anhydrous or hydrated. The tungsten compound is preferably WO2, WO3, WO3·(1 / 3)H2O, WO3·H2O, or WO3·2H2O. Compared to other tungsten compounds, WO2 or WO3, or their hydrates, have significantly higher hydrogen peroxide decomposition ability and metal ion trapping ability, making them particularly suitable as materials for microparticles. Examples of tungstates include Al2(WO4)3, BaWO4, FeWO4, Ni2WO4, Cu2WO4, ZnWO4, and Ce2(WO4)3.

[0034] The cathode catalyst layer or the anode catalyst layer may contain any one of these, or two or more of them. Furthermore, when a tungsten compound is added to both the cathode catalyst layer and the anode catalyst layer, the same type of tungsten compound may be added to the cathode catalyst layer and the anode catalyst layer, or different types of tungsten compounds may be added to the cathode catalyst layer and the anode catalyst layer.

[0035] [B. Content] When a tungsten compound is added to the anode catalyst layer or the cathode catalyst layer and the catalyst particles are supported on a support, the "tungsten compound content" refers to the ratio of the mass of the tungsten compound (Ww) to the mass of the support (C) (=Ww / C). In the present invention, the content of the tungsten compound is not particularly limited, and an optimum content can be selected depending on the purpose.

[0036] When a tungsten compound is added to a catalyst layer and catalyst particles are supported on a carrier, the durability of the MEA generally improves as the content of the tungsten compound increases. To achieve this effect, the content of the tungsten compound (=Ww / C) is preferably 0.01 or more. The content is more preferably 0.1 or more. On the other hand, if the content of the tungsten compound is excessive, it may inhibit the electron conduction in the catalyst layer or become the starting point for mechanical cracking or tearing of the electrolyte membrane. Therefore, the content of the tungsten compound (= Ww / C) is preferably 0.24 or less. The content is more preferably 0.16 or less.

[0037] [C. Average particle size] The "average particle size of fine particles of a tungsten compound" refers to the average value of the maximum dimension of 200 or more fine particles randomly selected under microscope observation.

[0038] The average particle size of the microparticles is not particularly limited, and an optimal average particle size can be selected depending on the purpose. Generally, if the average particle size of the microparticles is too small, the microparticles will aggregate, reducing the surface area and making it difficult to efficiently decompose hydrogen peroxide. Therefore, the average particle size of the microparticles is preferably 1 nm or more. The average particle size is more preferably 5 nm or more, and even more preferably 8 nm or more. On the other hand, when fine particles are added to an electrolyte membrane, if the average particle size of the fine particles is too large, the mechanical strength of the electrolyte membrane may decrease, causing the membrane to crack. Furthermore, if the fine particles are arranged so as to penetrate the electrolyte membrane, gas leakage may occur. Therefore, the average particle size of the fine particles is preferably 50 nm or less. The average particle size is more preferably 40 nm or less.

[0039] 1.1.5. Metal ions Metal ions may be present as impurities in some part of the membrane electrode assembly due to unavoidable circumstances. In the present invention, the term "metal ions" refers to impurity ions that are mixed into the membrane electrode assembly due to unavoidable circumstances and that have activity in the Fenton reaction.

[0040] Immediately after production, polymer electrolyte fuel cells may contain impurity ions resulting from the manufacturing process. Furthermore, polymer electrolyte fuel cells that have deteriorated over time may contain impurity ions contained in the reaction gases or humidifying water, or impurity ions eluted from piping, etc. These impurity ions typically undergo ion exchange with protons in the acid groups of the electrolyte membrane or catalyst layer ionomer. These impurity ions not only reduce the proton conductivity of the electrolyte membrane or catalyst layer ionomer, but also cause the generation of hydroxyl radicals (·OH) from hydrogen peroxide, which deteriorates the polymer electrolyte.

[0041] The fuel cell system according to the present invention is equipped with a means for detoxifying such impurity ions with a tungsten compound, which is different from conventional systems. Metal ions that can be rendered harmless using the means according to the present invention include, for example, ions of metal elements such as Fe, Cu, Ni, Al, Co, or Ti. The MEA may contain any one of these ions, or may contain two or more of these ions.

[0042] [1.2. Fuel gas supply device, oxidant gas supply device] The term "fuel gas supply device" refers to a device for supplying fuel gas to the anode of a polymer electrolyte fuel cell. The term "oxidant gas supply device" refers to a device for supplying an oxidant gas to the cathode of a polymer electrolyte fuel cell. In the present invention, the structures of the fuel gas supply device and the oxidant gas supply device are not particularly limited, and an optimum structure can be selected depending on the purpose.

[0043] [1.3. Humidifier] The fuel cell system according to the present invention may further include a humidifier for humidifying the oxidant gas. When fine particles of a tungsten compound are added to the anode catalyst layer, optimizing the humidity of the oxidant gas using the humidifier allows metal ions in the MEA to migrate to the anode catalyst layer and be adsorbed onto the tungsten compound.

[0044] As will be described later, the metal ion flux Nc depends not only on the potential gradient and the metal ion concentration gradient, but also on the humidity gradient between the anode and cathode. Therefore, by optimizing the potential gradient and concentration gradient and simultaneously increasing the relative humidity on the cathode catalyst layer side compared to the anode catalyst layer side, the humidity gradient facilitates the migration of metal ions from the cathode side to the anode side. As a result, the metal ions can be adsorbed onto the tungsten compound added to the anode catalyst layer.

[0045] [1.4. Energy storage devices] The fuel cell system according to the present invention may further include a power storage device for storing surplus power. In the present invention, the power storage device is used not only to store surplus power generated during normal operation of the fuel cell system, but also to store surplus power generated during the detoxification of metal ions. This is different from conventional systems. In the present invention, the type of power storage device is not particularly limited, and an optimum one can be selected depending on the purpose. Examples of power storage devices include secondary batteries and capacitors. The power storage device may be any one of these, or a combination of two or more of them.

[0046] 1.5. Control Devices "Control device" refers to a device for controlling a fuel cell system. The control device is equipped with means for controlling the operation of various devices required to operate the fuel cell system during normal operation, such as a fuel gas supply device and an oxidant gas supply device.

[0047] In the present invention, the control device includes means for controlling the operation of various devices, as well as: (a) a determination means for determining whether or not to perform a detoxification treatment of metal ions active in the Fenton reaction that have been mixed into the membrane electrode assembly; (b) an adsorption means for controlling the flux Nc of metal ions in the membrane electrode assembly at any timing during the operation of the fuel cell system after it has been determined that detoxification treatment should be performed, thereby moving the metal ions to the cathode catalyst layer side containing a tungsten compound or to the cathode catalyst layer side containing a tungsten compound, and causing the metal ions to be adsorbed onto the tungsten compound; It also has:

[0048] [1.5.1. Judgment method] The "determination means" refers to a means for determining whether or not to perform a treatment to render harmless metal ions active in the Fenton reaction that have been mixed into the membrane electrode assembly. The means for determining whether or not to execute the detoxification process is not particularly limited, and an optimum method can be selected depending on the purpose.

[0049] For example, the determining means may determine the cumulative operating time T total is equal to or greater than the first critical value ε1 (or exceeds ε1). Generally, the longer the operation time of a polymer electrolyte fuel cell, the greater the amount of impurity ions that enter from the outside. Therefore, regardless of whether degradation is actually occurring, T total When the detoxification treatment is carried out when the value of the metal ions is equal to or greater than ε1, the metal ions can be detoxified before the solid polymer electrolyte is deteriorated by radical attack.

[0050] Alternatively, the determination means may be a means for sequentially acquiring the current and voltage of the polymer electrolyte fuel cell, which change from moment to moment, and determining whether the voltage (reference voltage) when the current value is a certain value (reference current value) is less than or equal to the second critical value ε2 (or less than ε2). When protons in the acid groups of the electrolyte membrane or catalyst layer ionomer are replaced by metal ions, the proton conductivity of the electrolyte membrane or catalyst layer ionomer decreases. As a result, the reference voltage decreases as the amount of replacement by metal ions increases. Therefore, by performing a detoxification treatment when the reference voltage is equal to or less than ε2, the metal ions can be detoxified before the solid polymer electrolyte deteriorates due to radical attack.

[0051] [1.4.2. Adsorption means] The "adsorption means" refers to means for controlling the flux Nc of metal ions in the membrane electrode assembly at any timing during the operation of the fuel cell system after it has been determined that detoxification treatment should be performed, thereby moving the metal ions to the cathode catalyst layer side containing a tungsten compound or to the cathode catalyst layer side containing a tungsten compound, and causing the metal ions to be adsorbed onto the tungsten compound.

[0052] [A. Timing of sanitization process execution] If the determining means determines that a detoxification process (i.e., a metal ion adsorption process) should be performed, the detoxification process is performed at some timing during the operation of the fuel cell system. As will be described later, the detoxification process is performed by maintaining the current density at a predetermined condition. The timing for performing the detoxification process is not particularly limited as long as it is possible to maintain the current density at the predetermined condition.

[0053] However, during normal operation, the power required by the fuel cell often changes from moment to moment. Therefore, it is generally difficult to maintain the current density at a value suitable for the detoxification process during normal operation. Therefore, it is preferable that the control means includes a means for executing the adsorption means when the fuel cell system is idling, starting up, and / or stopping.

[0054] [B. Flux Nc] Metal ions contained in the membrane electrode assembly migrate to the anode or cathode depending on the potential gradient within the membrane electrode assembly, the metal ion concentration gradient, and the humidity gradient between the anode and cathode. In this case, the "flux Nc" of metal ions within the membrane electrode assembly is specifically expressed by the following formula (1):

[0055]

number

[0056] however, u c is the mobility of the metal ion, C c is the concentration of the metal ion, φ is the ionic potential, D c is the diffusion coefficient of the metal ion, ξ c is the electroosmotic coefficient of the metal ion, z c is the valence of the metal ion, F is the Faraday constant, μ w is the chemical potential of water.

[0057] The first term on the right side of equation (1) represents the contribution of the potential gradient (or current density gradient) to the flux Nc. The second term on the right side of equation (1) represents the contribution of the metal ion concentration gradient to the flux Nc. Furthermore, the third term on the right side of equation (1) represents the contribution of the humidity gradient between the anode and cathode to the flux Nc.

[0058] Generally, when a fuel cell generates electricity, the potential gradient causes metal ions to move more easily to the cathode side. On the other hand, if metal ions segregate to the cathode side, the concentration gradient causes them to move more easily to the anode side. Furthermore, if there is a humidity gradient between the anode and cathode, water molecules tend to move more easily from the high humidity side to the low humidity side, and in doing so, metal ions also tend to move more easily from the high humidity side to the low humidity side, accompanying the water molecules. Therefore, if the operating conditions (i.e., potential gradient, concentration gradient, and / or humidity gradient) are controlled so that the direction of the flux Nc is toward the cathode, metal ions can be migrated toward the cathode. Conversely, if the operating conditions are controlled so that the direction of the flux Nc is toward the anode, metal ions can be migrated toward the anode.

[0059] [C. Migration of metal ions to the cathode] When the cathode catalyst layer contains a tungsten compound, the metal ions need to be transferred to the cathode catalyst layer side in order to be rendered harmless. The method for transferring the metal ions to the cathode catalyst layer side is not particularly limited, and an optimum method can be selected depending on the purpose.

[0060] When metal ions are moved to the cathode side, the adsorption means is used at a current density of 1.0 A / cm 2 It is preferable that the power generation device includes a current density control means for generating electricity under the conditions maintained as above. When power generation is performed under high current density conditions, the contribution of the first term on the right side of equation (1) becomes larger than the contributions of the second and third terms. As a result, the direction of the flux Nc becomes toward the cathode. This also causes metal ions to migrate toward the cathode catalyst layer, making them more likely to be adsorbed by the tungsten compound on the cathode catalyst layer.

[0061] [D. Migration of metal ions to the anode] When the anode catalyst layer contains a tungsten compound, the metal ions need to be transferred to the anode catalyst layer in order to be rendered harmless. The method for transferring the metal ions to the anode catalyst layer is not particularly limited, and an optimum method can be selected depending on the purpose.

[0062] When metal ions are moved to the anode side, the adsorption means is (a) Current density is 0.02 A / cm 2 Maintained below, and (b) The difference between the cathode side relative humidity (RHc) and the anode side relative humidity (RHa) (ΔRH = RHc - RHa) is maintained at 40% RH or more. Preferably, the device includes a humidity control means under which electricity is generated. The current density is more preferably 0.01 A / cm 2 The following is the result.

[0063] When power generation is performed under low current density conditions, the contributions of the first and second terms on the right side of equation (1) become relatively small. Increasing the relative humidity on the cathode side increases the contribution of the third term on the right side compared to the first and second terms. As a result, the direction of the flux Nc becomes toward the anode. This also causes metal ions to migrate toward the anode catalyst layer, making them more likely to be adsorbed by the tungsten compound on the anode catalyst layer. When such humidity control is performed, the fuel cell system preferably further includes a humidifier for humidifying the oxidant gas. If the fuel cell system further includes a humidifier, the degree of freedom in controlling ΔRH increases, and the process of detoxifying metal ions by humidity control becomes even easier.

[0064] [E. Processing Time] It is preferable to select the optimum processing time for the detoxification process depending on the purpose. Generally, if the processing time is too short, the detoxification process will be insufficient. On the other hand, if the processing time is longer than necessary, there will be no difference in the effect and there will be no practical benefit. Therefore, it is preferable to select the optimum processing time taking these points into consideration.

[0065] [F. Surplus Power Disposal] When performing detoxification processing while the fuel cell system is idling, starting up, and / or shutting down, if surplus power is generated during the detoxification processing, the surplus power generated during the detoxification processing must be consumed in some way.

[0066] For example, if the fuel cell system further includes a power storage device for storing surplus power, the control device preferably includes a means for storing the power generated when the potential control means is executed in the power storage device. In addition, if the surplus electricity can be processed in other ways (for example, if the surplus electricity can be processed by operating supply equipment such as air compressors and humidifiers included in the fuel cell power generation system), the power storage device can be omitted.

[0067] [2. Effect] Tungsten compounds have the effect of decomposing hydrogen peroxide into harmless water and oxygen, as well as trapping metal ions that are active in the Fenton reaction and rendering them harmless. Trapping of metal ions by tungsten compounds is as follows: (a) A tungsten compound reacts with hydrogen peroxide to produce tungsten peroxide, (b) α-Keggin type polytungstic acid is produced from tungsten peroxide, (c) Metal ions are trapped in the Keggin shell of α-Keggin-type polytungstic acid, forming heterotungstic acid. It is thought that this progresses as a result of Furthermore, in addition to the above-mentioned function of trapping metal ions via tungsten peroxide, tungsten compounds also have the function of directly adsorbing or trapping metal ions without passing through tungsten peroxide, thereby rendering the metal ions harmless.

[0068] Therefore, adding a tungsten compound to any part of the MEA traps and neutralizes metal ions, thereby suppressing the generation of peroxide radicals due to the reaction between the metal ions and hydrogen peroxide, and thus suppressing electrolyte degradation. In addition to the above, the consumption of hydrogen peroxide in the generation of tungsten peroxide is thought to have the effect of suppressing the generation of peroxide radicals. However, when a tungsten compound is added to an MEA, if the distance between the tungsten compound and the metal ions is large, the tungsten compound cannot efficiently trap the metal ions.

[0069] In contrast, adding fine particles of a tungsten compound to the cathode catalyst layer and controlling the operating conditions (i.e., the metal ion flux Nc) can migrate the metal ions contained in the MEA to the cathode catalyst layer. As a result, the metal ions are trapped by the tungsten compound added to the cathode catalyst layer, increasing the probability that the metal ions will be neutralized. Alternatively, adding fine particles of a tungsten compound to the anode catalyst layer and controlling the operating conditions can move metal ions contained in the MEA to the anode catalyst layer, which increases the probability that the metal ions will be trapped by the tungsten compound added to the anode catalyst layer and rendered harmless.

[0070] Furthermore, although tungsten compounds can trap metal ions, there is a limit to the amount that can be trapped. To solve this problem, it is possible to increase the amount of tungsten compound added to the catalyst layer. However, simply increasing the amount of tungsten compound may result in a decrease in performance.

[0071] In contrast, if a tungsten compound is added to both the cathode catalyst layer and the anode catalyst layer and the operating conditions are controlled, the metal ions can be migrated to either the cathode catalyst layer side or the anode catalyst layer side. As a result, even if the amount of tungsten compound added is relatively small, the amount of trapped metal ions can be increased without causing a decrease in performance. [Example]

[0072] Examples 1 to 4 [1. Test Method] 1.1. Examples 1 to 3 60 mg of a tungsten compound and 10 mg of iron (III) chloride (FeCl3) were added to 1 mL of 0.1 M aqueous perchloric acid solution, and the solution was stirred for 18 hours at room temperature in the dark. The solution was then centrifuged to separate the supernatant and precipitate. The Fe 3+ The concentration of Fe adsorbed on the tungsten compound was determined using an absorption spectrophotometer. 3+ The tungsten compound used was WO3 (Example 1), WO3·H2O (Example 2), or WO3·2H2O (Example 3).

[0073] 1.2. Example 4 6 mg of WO3·H2O and 1 mg of iron(III) chloride (FeCl3) were added to 1 mL of 0.1 M aqueous perchloric acid solution, and the solution was stirred for 18 hours at room temperature in the dark. The solution was then centrifuged to separate the supernatant and precipitate. The Fe contained in the supernatant was 3+ The concentration of Fe adsorbed on WO3·H2O was determined using an absorption spectrophotometer. 3+ I asked for the quantity.

[0074] [2. Results] FIG. 1 shows the Fe content of various samples (Examples 1 to 4). 3+ The adsorption amount is shown in Figure 1. The following can be seen from Figure 1. (1) Example 4 has a higher Fe content than Example 2. 3+ The amount of adsorption decreased. This is thought to be because the amount of each reagent added to the solution was 1 / 10 or less. (2) WO3 and its hydrates both contain Fe 3+ It was found that the amount of crystal water in the Fe-containing solution was increased by 100%. 3+ The adsorption amounts were found to be different. (4) The results of Examples 1 to 4 show that the tungsten compound traps and detoxifies metal ions even in an environment where hydrogen peroxide does not coexist and tungsten peroxide is not produced.

[0075] (Examples 5 to 6, Comparative Example 1) 1. Cell Preparation Figure 2 shows a schematic cross-sectional view of the cell used in the experiment. The electrolyte membrane (NR115) was filled with 1 L of 0.01 M Co 2+ The electrolyte membrane was immersed in the solution at 80°C for 2 hours. The electrolyte membrane was then removed from the solution and washed by immersing it in ultrapure water at room temperature. The ultrapure water was exchanged three times, and the electrolyte membrane was air-dried at room temperature overnight. 2+ A substituted membrane (electrolyte membrane No. 5 in FIG. 2) and three unsubstituted electrolyte membranes (electrolyte membranes No. 2 to 4 in FIG. 2) were laminated to obtain a laminated membrane.

[0076] 4cm on each side 2 The electrolyte membranes with the electrodes bonded thereto (electrolyte membranes No. 1 and No. 6 in Figure 2) were further placed on both sides of the laminated membrane. The resulting laminated membrane was hot-pressed to obtain an MEA. The hot-pressing conditions were: temperature: 140°C, heating time: 5 minutes, pressure: 60 kg / cm. 2 (5.88 MPa), and pressurization time: 5 minutes. 2 was incorporated into the cell.

[0077] 2. Test Method 2.1. Example 5: Current Density Control Cell temperature: 80℃, Both electrodes: Hydrogen, 300mL / min, 80%RH In this environment, the current from the cathode to the anode is 1.0 A / cm 2 The cations were segregated on the cathode side by holding the film at a constant current density of 1000 kJ / cm2 for 1 hour. 2+ ) concentrations were quantified.

[0078] 2.2. Example 6: Humidity Control Cell temperature: 80℃, Anode: Hydrogen, 300 mL / min, 40% RH Cathode: Hydrogen, 300mL / min, 80%RH The membranes were then left in this environment for 1 hour to allow cations to segregate on the anode side. 2+ ) was quantified.

[0079] 2.3. Comparative Example 1 Cell temperature: 80℃, Both electrodes: Hydrogen, 300mL / min, 80%RH After that, the cations (Co 2+ ) was quantified.

[0080] [3. Results] Figure 3 shows the Co 2+ and a film containing Co 2+ 4 shows the cation substitution rate of each membrane after cells equipped with a laminated membrane with a membrane not containing ethylenediaminetetraacetic acid were maintained under various conditions (Examples 5 to 6, Comparative Example 1). The following can be seen from FIG.

[0081] (1) Example 5 corresponds to an example simulating the adsorption treatment of metal ions by controlling the current density. It can be seen from FIG. 3 that when maintained under high current density conditions, cations segregate to the cathode side. This result indicates that when a tungsten compound is added to the cathode catalyst layer in an actual fuel cell and power is generated under high current density conditions, the cations segregated to the cathode side can be neutralized by the tungsten compound added to the cathode side.

[0082] (2) Example 6 corresponds to an example simulating a metal ion adsorption process using humidity control. Figure 3 shows that when the current density is relatively low and the humidity on the cathode side is increased, cations segregate to the anode side. This result indicates that when a tungsten compound is added to the anode catalyst layer in an actual fuel cell and power is generated under conditions of low current density and a high humidity gradient, the cations segregated to the anode side can be neutralized by the tungsten compound added to the anode side.

[0083] (3) Comparative Example 1 corresponds to an example in which the fuel cell was maintained under conditions of low current density and low humidity gradient. Figure 3 shows that when the fuel cell was maintained under conditions of low current density and low humidity gradient, cations diffuse using only the concentration gradient as a driving force, and the cation concentration becomes uniform.

[0084] (Examples 7 to 8, Comparative Example 2) 1. Sample Preparation 1.1. Example 7 A catalyst ink was prepared by mixing an electrolyte solution (D2020, Nafion (registered trademark) dispersion, 1000EW, 20 mass%, manufactured by Chemours), Pt-supported carbon, and WO3·H2O (manufactured by EM Japan). The amount of WO3·H2O blended was determined so that the WO3·H2O content was 7.0 μg / cm3 when the catalyst layer was formed. 2 The catalyst ink was applied to the surface of the substrate to prepare a catalyst layer sheet.

[0085] Next, an electrolyte membrane (Nafion®) containing Ce ions was fabricated. The sulfonic acid group substitution rate with Ce ions was set to 4.4%. A catalyst layer containing WO3·H2O was hot-pressed onto both sides of the electrolyte membrane to obtain a membrane-electrode assembly.

[0086] 1.2. Example 8 The amount of WO3·H2O in the catalyst ink was adjusted so that the WO3·H2O content was 1.2 μg / cm when the catalyst layer was formed. 2 A membrane electrode assembly was produced in the same manner as in Example 7, except that the amounts were changed to:

[0087] 1.3. Comparative Example 2 A membrane electrode assembly containing only Ce ions was prepared in the same manner as in Example 7, except that WO3·H2O was not added to the catalyst ink.

[0088] 2. Test Method The fabricated membrane electrode assembly was subjected to an OCV durability test. The test conditions were: temperature: 95°C, humidity: 40% RH, anode gas / cathode gas: H2 / Air, and test time: 90 hours. After the test, the amount of fluoride ions in the recovered water was measured. - The amount of elution was calculated.

[0089] [3. Results] Figure 4 shows the relationship between the WO3·H2O content and the normalized total F - The relationship between the amount of elution and the total amount of normalized F - The "elution amount" is the total F of each sample. - The amount of elution was the total F of Comparative Example 2. - The value is calculated by dividing the amount by the amount of elution. From Figure 4, the following can be seen.

[0090] (1) In Examples 7 and 8, F was lower than that in Comparative Example 2. - The amount of elution decreased. This is thought to be because the addition of WO3·H2O to the catalyst layer promoted the decomposition of H2O2, reducing the amount of radicals generated. (2) Normalized F - To reduce the elution amount to 0.4 or less, the WO3·H2O content must be 0.8 μg / cm2 More than 7.0μg / cm 2 It turns out that the following is sufficient. Also, the normalized F - To make the elution amount 0.3 or less, the WO3·H2O content must be 1.0 μg / cm 2 More than 5.0μg / cm 2 It turns out that the following is sufficient. Furthermore, the normalized F - To reduce the amount of leaching to 0.22 or less, the WO3·H2O content must be 1.1 μg / cm 2 More than 2.0μg / cm 2 I found that the following should work:

[0091] 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 are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0092] The fuel cell system according to the present invention can be used as an in-vehicle power source, a small stationary power generator, etc. Furthermore, the metal ion removal device according to the present invention can be used to initialize a polymer electrolyte fuel cell immediately after manufacture, or to regenerate a polymer electrolyte fuel cell that has deteriorated over time.

Claims

1. A fuel cell system comprising: (1) The fuel cell system includes: a polymer electrolyte fuel cell; a fuel gas supply device for supplying a fuel gas to the anode of the polymer electrolyte fuel cell; an oxidant gas supply device that supplies an oxidant gas to the cathode of the polymer electrolyte fuel cell; a control device that controls the fuel cell system; It is equipped with: (2) The polymer electrolyte fuel cell includes a membrane electrode assembly in which fine particles of a tungsten compound are added to the cathode catalyst layer and / or the anode catalyst layer. (3) The control device a determination means for determining whether or not to perform a detoxification treatment of metal ions active in the Fenton reaction that have been mixed into the membrane electrode assembly; an adsorption means for controlling a flux Nc of the metal ions in the membrane electrode assembly at any timing during operation of the fuel cell system after it has been determined that the detoxification treatment should be performed, thereby moving the metal ions to the cathode catalyst layer side containing the tungsten compound or to the cathode catalyst layer side containing the tungsten compound, and causing the metal ions to be adsorbed onto the tungsten compound; It is equipped with: Here, the "flux Nc" of the metal ions in the membrane electrode assembly refers to a value expressed by the following formula (1). [Equation 1] however, u c is the mobility of the metal ion, C c is the concentration of the metal ion, φ is the ionic potential, D c is the diffusion coefficient of the metal ion, ξ c is the electroosmotic coefficient of the metal ion, z c is the valence of the metal ion, F is the Faraday constant, μ w is the chemical potential of water.

2. the cathode catalyst layer contains the tungsten compound, The adsorption means has a current density of 1.0 A / cm 2 2. The fuel cell system according to claim 1, further comprising a current density control means for generating electricity under the conditions maintained as above.

3. the fuel cell system further includes a humidifier for humidifying the oxidant gas; the anode catalyst layer contains the tungsten compound, The adsorption means is (a) Current density is 0.02 A / cm 2 Maintained below, and (b) The difference between the cathode side relative humidity (RHc) and the anode side relative humidity (RHa) (ΔRH=RHc-RHa) is maintained at 40% RH or more.

10. The fuel cell system of claim 1, further comprising humidity control means for generating electricity under the conditions.

4. 2. The fuel cell system according to claim 1, wherein the control device includes means for executing the adsorption means when the fuel cell system is idling, starting up, and / or stopping.

5. the fuel cell system further includes a power storage device for storing surplus power; 2. The fuel cell system according to claim 1, wherein the control device includes means for storing, in the power storage device, the electric power generated when the adsorption means is operated.

6. 2. The fuel cell system according to claim 1, wherein the tungsten compound comprises at least one selected from the group consisting of tungsten oxide (+IV, +V, +VI), tungsten oxide (+VI) n-hydrate (n = 1 to 3, n includes non-integer numbers), tungsten carbide, and tungstate.

7. 2. The fuel cell system according to claim 1, wherein the metal ions include ions of at least one metal element selected from the group consisting of Fe, Cu, Ni, Al, Co, and Ti.

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