Fuel cell system and method for suppressing fuel cell deterioration

Abstract

To provide a technology that suppresses oxygen crossover when using an oxidant gas with a higher oxygen partial pressure than air in a fuel cell system.SOLUTION: A fuel cell system 2 includes a cell 6 having an anode 12 and a cathode 18 with an electrolyte layer 10 in between, and further includes a fuel cell 4 that supplies an oxidant gas with a higher oxygen partial pressure than air to the cathode 18, and the electrolyte layer 10 has a thickness such that the crossover amount of oxygen in the oxidant gas supplied to the cathode 18 is equal to or less than a predetermined amount.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The technology disclosed in this specification relates to a fuel cell system and a method for suppressing deterioration of a fuel cell. [Background technology]

[0002] Fuel cells are constructed as a stack of many stacked cells. For example, a polymer electrolyte fuel cell (PEFC) cell has a membrane electrode assembly (MEA) as a power generation section, which integrates an anode (fuel electrode) with a catalyst layer to which hydrogen as fuel gas is supplied, sandwiching a polymer electrolyte membrane that is permeable to hydrogen ions, and a cathode (air electrode) with a catalyst layer to which air or other oxidant gas is supplied.

[0003] In this type of fuel cell, air is generally used as the oxidant gas, but in some cases, high-concentration oxygen, such as pure oxygen, which has a higher oxygen partial pressure than air, is used instead (Patent Document 1). The oxygen partial pressure in air is about 21%. Therefore, when pure oxygen is used, for example, its oxygen partial pressure is at least about five times that of air. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Japanese Patent Application Publication No. 2018-147622 Summary of the Invention [Problem to be solved by the invention]

[0005] The inventors have found that supplying an oxidant gas with a high oxygen partial pressure can cause oxygen crossover, in which oxygen passes through the polymer electrolyte membrane and moves to the anode. They also found that using an oxidant gas with a high oxygen partial pressure promotes an oxidation reaction of the carbon material at the cathode that is different from the original fuel cell reaction, resulting in fuel cell degradation.

[0006] That is, the present inventors have found that oxygen crossover causes a reaction at the anode that consumes hydrogen ions and electrons (see, for example, formula (A)). 1 / 2O2+2H + +2e - →H2O (A)

[0007] It was also found that oxygen crossover promotes the oxidation reaction of the carbon material (see, for example, formula (B)) that supplies hydrogen ions to the anode for the reaction of formula (A), thereby further promoting the reaction represented by formula (A). 1 / 2C+H2O → 1 / 2CO2+2H + +2e - (B)

[0008] The present specification provides a technology that can suppress oxygen crossover in a fuel cell system even when an oxidant gas with a higher oxygen partial pressure than air is used. [Means for solving the problem]

[0009] The technology disclosed in this specification relates to a fuel cell system. The fuel cell system includes a cell having an anode and a cathode sandwiching an electrolyte layer, and is equipped with a fuel cell that supplies an oxidant gas having an oxygen partial pressure higher than that of air to the cathode. The electrolyte layer has a thickness that ensures that the amount of oxygen crossover in the oxidant gas supplied to the cathode is equal to or less than a predetermined amount.

[0010] The inventors have found that oxygen crossover can be suppressed by increasing the thickness of the electrolyte layer as the oxygen partial pressure of the oxidizer gas increases. By setting the thickness of the electrolyte layer so that the amount of oxygen crossover in the oxidizer gas is equal to or less than a predetermined amount, oxygen crossover can be easily and effectively suppressed even when an oxidizer gas with a higher oxygen partial pressure than air is used. As a result, the consumption of hydrogen ions and electrons at the anode caused by oxygen crossover can be suppressed, and the oxidation of carbon materials at the cathode can also be suppressed. As a result, deterioration of the fuel cell system can be effectively suppressed when an oxidizer gas with a higher oxygen partial pressure than air is used.

[0011] The present disclosure also relates to a method for suppressing degradation of a fuel cell. The fuel cell includes a cell having an anode and a cathode sandwiching an electrolyte layer, and supplies an oxidizer gas having a higher oxygen partial pressure than air to the cathode. In this suppression method, the thickness of the electrolyte layer is adjusted so that the amount of oxygen crossover in the oxidizer gas is equal to or less than a predetermined amount. The thickness of the electrolyte layer is increased according to the oxygen partial pressure of the oxidizer gas to suppress oxygen crossover. In this way, even when an oxidizer gas having a higher oxygen partial pressure than air is used, oxygen crossover from the cathode to the anode via the electrolyte layer can be easily and effectively suppressed, thereby suppressing fuel cell degradation. [Brief explanation of the drawings]

[0012] [Figure 1] FIG. 1A is a diagram showing an example of a fuel cell (cell) in a fuel cell system according to an embodiment, and FIG. 1B is a diagram showing the operation of this fuel cell. [Figure 2] 1 is a graph showing the relationship between the thickness of the solid electrolyte membrane (X-axis) and the oxygen partial pressure (Y-axis) of the oxidant gas that reaches the target oxygen crossover amount (the allowable upper limit of the oxygen crossover amount). [Figure 3] 3 is a graph based on the diagram shown in FIG. 2, in which the oxygen partial pressure of the oxidant gas is plotted on the X axis and the thickness of the solid electrolyte membrane required to keep the oxygen crossover amount below a target amount is plotted on the Y axis. DETAILED DESCRIPTION OF THE INVENTION

[0013] The fuel cell system disclosed in this specification includes a cell having an anode and a cathode with an electrolyte layer sandwiched therebetween, and is equipped with a fuel cell that supplies an oxidant gas having an oxygen partial pressure higher than that of air to the cathode, and the electrolyte layer has a thickness that prevents oxygen crossover in the oxidant gas supplied to the cathode from exceeding a predetermined amount. This fuel cell system can further adopt the following embodiments.

[0014] In one embodiment of any of the above fuel cell systems, the fuel cell is a polymer electrolyte fuel cell, and the thickness of the electrolyte layer can satisfy the following formula (1): Polymer electrolyte fuel cells are advantageous in that oxygen crossover is easily promoted and they can generate electricity at operating temperatures close to room temperature. y≧0.32x+1.51 (1) (where x represents the oxygen partial pressure (kPa) of the oxidant gas, and y represents the thickness (μm) of the electrolyte layer.)

[0015] In one embodiment of any of the above fuel cell systems, the oxidant gas may be high-concentration oxygen with an oxygen concentration of 99.9% or more. This embodiment can easily ensure the power generation performance of the fuel cell by suppressing the adverse effects of oxygen crossover that occur when using such high-concentration oxygen.

[0016] One embodiment of any of the above fuel cell systems includes supplying hydrogen at a mass ratio of 1.1 to 1.3 relative to the amount of hydrogen to be supplied to the anode. According to this embodiment, by supplying hydrogen at this range, crossover oxygen can be consumed at the anode. This also makes it possible to suppress oxidation reactions of carbon materials at the cathode that involve the generation of hydrogen ions.

[0017] In this specification, the fuel cell is not particularly limited and can be any of various known fuel cells. For example, a polymer electrolyte fuel cell (PEFC) may be preferable. In this specification, a fuel cell generally has at least the form of a stack in which a plurality of cells are stacked. The stacking form is not particularly limited. In addition, in this specification, the thickness of the electrolyte layer refers to the average thickness of the portion of the electrolyte layer sandwiched between the anode and cathode.

[0018] The fuel cell system disclosed in this specification will be described below with reference to the accompanying drawings. Figure 1(a) shows an example of a polymer electrolyte fuel cell cell included in the fuel cell system, and Figure 1(b) shows the operation of the cell shown in Figure 1(a). Figures 2 and 3 each show the relationship between the thickness of the solid electrolyte membrane of a solid electrolyte fuel cell and the oxygen partial pressure of the oxidant gas.

[0019] The fuel cell system 2 includes a polymer electrolyte fuel cell 4 and a control unit (not shown) that is, for example, a computer or the like. The control unit can appropriately control the supply amounts of fuel cell gas containing hydrogen and oxidant gas containing oxygen to the fuel cell 4 according to the required output. The fuel cell system 2 includes known elements (not shown), such as a fuel gas supply system containing hydrogen, an oxidant gas supply system containing oxygen, and a cooling system. The fuel cell system 2 is installed, for example, in a part of a mobile object that includes a motor driven by power supplied by the fuel cell system 2 under various environments.

[0020] The fuel cell 4 includes a stack (not shown) of multiple cells 6 shown in FIG. 1(a). As shown in FIG. 1(a), each cell 6 includes an MEA 8, with separators (not shown) interposed between adjacent MEAs 8. The MEA 8 includes a solid electrolyte membrane 10, and an anode 12 and a cathode 18 arranged opposite each other with the solid electrolyte membrane 10 sandwiched therebetween. The solid electrolyte membrane is an example of the electrolyte layer disclosed in this specification.

[0021] As shown in FIG. 1(a), the anode 12 includes a catalyst layer 14, and in addition to the catalyst layer 14, a gas diffusion layer 16. The cathode 18 includes a catalyst layer 20 and a gas diffusion layer 22. Note that the gas diffusion layers 16 and 22 may both be part of the catalyst layers 14 and 20.

[0022] The solid electrolyte membrane 10 has a thickness that prevents the crossover of oxygen in the oxidant gas supplied to the cathode 18 from occurring at a predetermined level or less.

[0023] The relationship between the thickness of the electrolyte membrane of a solid oxide fuel cell and the oxygen partial pressure of the oxidant gas, as determined by the present inventors, will now be described with reference to Figures 2 and 3. The graph shown in Figure 2 is based on the results of evaluations conducted by preparing fuel cells 4 equipped with cells 6 in which the thickness of the solid electrolyte membrane 10 was varied within a range of 20 μm to 70 μm, and supplying oxidant gases with various oxygen partial pressures to each fuel cell 4.

[0024] More specifically, a cell 6 was constructed including a solid electrolyte membrane 10 of a predetermined thickness containing a fluorine-based polymer having sulfonic acid groups, an anode 12 containing a ruthenium-platinum alloy catalyst, and a cathode 18 having platinum on a carbon-based support such as carbon black, and the cell 6 was then supplied with an oxidizer gas having a gradually increasing oxygen partial pressure, and the oxygen partial pressure of the oxidizer gas was recorded when the allowable upper limit of the oxygen crossover amount (target oxygen crossover amount) was detected. Here, the target oxygen crossover amount is an example of a "predetermined amount of oxygen crossover amount" in this specification.

[0025] 2, the X-axis represents the thickness of the solid electrolyte membrane 10, and the Y-axis represents the oxygen partial pressure at which the target oxygen crossover amount is reached. The graph in FIG. 2 shows that as the thickness of the solid electrolyte membrane 10 increases, the oxygen partial pressure of the oxidizer gas at which the target oxygen crossover amount is reached also increases.

[0026] Here, the detection of oxygen crossover is not particularly limited, and may be performed, for example, by measuring the amount of oxygen or water in the anode 12. Alternatively, oxygen crossover may be detected by measuring the amount of carbon dioxide in the cathode 18. Alternatively, oxygen crossover may be detected by measuring changes in the oxygen partial pressure in the anode 12 and / or the cathode 18. Stable isotopes may also be used to detect the amount of oxygen. By detecting these amounts, a person skilled in the art can set a target oxygen crossover amount to ensure the intended battery performance.

[0027] The graph shown in FIG. 3 is a graph in which the X-axis (thickness of the solid electrolyte membrane 10) of FIG. 2 is swapped with the Y-axis (oxygen partial pressure at which the target oxygen crossover amount is reached). The graph in FIG. 3 with swapped coordinate axes shows that increasing the oxygen partial pressure of the oxidizer gas used also increases the thickness of the solid electrolyte membrane 10 at which the target oxygen crossover amount is reached. The graph in FIG. 3 also shows that even if the oxygen partial pressure of the oxidizer gas is increased, oxygen crossover can be suppressed by increasing the thickness of the solid electrolyte membrane 10. Furthermore, the graph line in FIG. 3 indicates the thinnest thickness of the solid electrolyte membrane 10 that does not exceed the target oxygen crossover amount.

[0028] From the above, in the fuel cell system 2, the solid electrolyte membrane 10 can have a thickness that keeps the amount of oxygen crossover in the oxidant gas supplied to the cathode 18 at or below a predetermined amount, depending on the oxygen partial pressure of the oxidant gas supplied to the cathode 18. In this way, oxygen crossover can be suppressed.

[0029] Furthermore, for example, oxygen crossover can be suppressed by providing a solid electrolyte membrane 10 having a thickness that satisfies formula (1) based on the graph of FIG. 3.2YX≧4.7 (1) (where X represents the oxygen partial pressure (kPa) of the oxidant gas, and Y represents the thickness (μm) of the electrolyte layer.)

[0030] According to the above formula (1), for example, when the oxygen partial pressure (X) of the oxidant gas is 60 kPa, oxygen crossover can be suppressed by setting the thickness (Y) of the solid electrolyte membrane 10 to 21 μm or more. Also, when the oxygen partial pressure (X) of the oxidant gas is 100 kPa, oxygen crossover can be suppressed by setting the thickness of the solid electrolyte membrane 10 to 33 μm or more.

[0031] The oxygen partial pressure of the oxidant gas supplied to the cathode 18 of the fuel cell 4 is not particularly limited. For example, it is 20 kPa or more, 30 kPa or more, 40 kPa or more, 50 kPa or more, 60 kPa or more, 80 kPa or more, 100 kPa or more, 120 kPa or more, 140 kPa or more, or 170 kPa or more. The oxygen partial pressure is, for example, 200 kPa or less, or 180 kPa or less.

[0032] This can be changed as appropriate depending on the oxygen concentration of the oxidizer gas and the total pressure of the oxidizer gas supplied. The oxygen concentration of the oxidizer gas is not particularly limited, but various oxygen gases can be used, such as air (oxygen partial pressure of approximately 21.3 kPa at 1 atmosphere) and high-concentration oxygen with a purity of 99.9% or more (oxygen partial pressure of 101 kPa or more at 1 atmosphere). Oxygen crossover may contain nitrogen, water vapor, etc. in addition to oxygen. The pressure at which the oxidizer gas is supplied is also not particularly limited and is set as appropriate.

[0033] Furthermore, the fuel cell system 2 may be configured to supply hydrogen at a mass ratio of 1.1 to 1.3 relative to the amount of hydrogen to be supplied to the anode 12. For example, it may be 1.2 to 1.3. In some cases, the excess hydrogen in the anode 12 directly consumes crossover oxygen, thereby suppressing the reaction that promotes oxidation of the carbon material in the cathode 18.

[0034] Next, the effect of using the cell 6 having the solid electrolyte membrane 10 with the above-described thickness in the fuel cell 4 will be described. As shown in FIG. 1(b), in the fuel cell system 2, the thickness of the solid electrolyte membrane 10 is set according to the oxygen partial pressure of the oxidant gas supplied to the cathode 18 (it is made larger than when air is used as the oxidant gas). The thickness of the solid electrolyte membrane 10 can be set, for example, by evaluating the required thickness of the solid electrolyte membrane 10 using the target oxygen crossover amount as an index, as exemplified in the above-mentioned FIGS. 2 and 3. Furthermore, for example, the solid electrolyte membrane 10 is configured to satisfy the above-mentioned formula (1).

[0035] This suppresses crossover of oxygen in the oxidant gas supplied to the cathode 18. This suppresses the reaction of oxygen that has passed through the solid electrolyte membrane 10 at the anode 12, which is represented by formula (A). This suppresses electron consumption at the anode 12. Furthermore, water production at the anode 12 can also be suppressed. 1 / 2 O2+2H + +2e - →H2O formula (A)

[0036] Furthermore, as a result of suppressing oxygen crossover, the reaction represented by formula (B) is suppressed in the cathode 18, and oxidation of the carbon material accompanied by the generation of hydrogen ions is suppressed. Furthermore, the migration of the generated hydrogen ions to the anode 12 is also suppressed. 1 / 2 C+H2O→1 / 2 CO2+2H + +2e - Formula (B)

[0037] In this way, by suppressing oxygen crossover, the reactions of formulas (A) and (B) are suppressed at the anode 12 and cathode 18, respectively, and thereby the reactions represented by formulas (2) and (3) enable normal operation of the fuel cell 4. H2→2H + +2e - Formula (2) 1 / 2 O2+2H + +2e - →H2O formula (3)

[0038] For example, by supplying excess hydrogen to the anode 12, such as by supplying hydrogen at a mass ratio of 1.1 to 1.3 relative to the hydrogen to be supplied to the anode 12, the reactions expressed by formulas (A) and (B) can be suppressed, making it easier to ensure power generation characteristics. In particular, when high-concentration oxygen is used, it may be effective to increase the stoichiometric ratio of hydrogen.

[0039] In the above embodiment, a solid oxide fuel cell is used as the fuel cell 4, but this is not limited thereto. Oxygen crossover when an oxidant gas with a higher oxygen partial pressure than air is used can also occur in other types of fuel cells, and suppressing oxygen crossover by increasing the thickness of the solid electrolyte layer is similarly effective.

[0040] In the above embodiment, the fuel cell system 2 has been described, but the disclosure of this specification also includes embodiments as a fuel cell 4 and a cell 6 of the fuel cell 4. Furthermore, since methods for manufacturing such fuel cells 4 and cells 6 are well known to those skilled in the art, the disclosure of this specification also includes embodiments as methods for manufacturing the fuel cells 4 and cells 6. Furthermore, the disclosure of this specification also includes embodiments as methods for suppressing deterioration of the fuel cell system 2 and the fuel cell 4.

[0041] Although the embodiments of the present technology have been described in detail above, these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. The technical elements described in this specification or drawings exhibit technical utility alone or in various combinations, and are not limited to the combinations described in the claims at the time of filing. Furthermore, the technology exemplified in this specification or drawings simultaneously achieves multiple objectives, and achieving one of these objectives itself has technical utility. [Explanation of symbols]

[0042] 2: fuel cell system, 4: fuel cell, 6: cell, 8: MEA, 10: solid electrolyte membrane, 12: anode, 18: cathode

Claims

1. 1. A fuel cell system, comprising: The fuel cell system includes a fuel cell including a cell having an anode and a cathode with an electrolyte layer sandwiched therebetween, and supplies an oxidant gas having an oxygen partial pressure higher than that of air to the cathode; The electrolyte layer has a thickness such that an amount of oxygen crossover in the oxidant gas is equal to or less than a predetermined amount.

2. the fuel cell is a polymer electrolyte fuel cell, The system of claim 1 , wherein the thickness of the electrolyte layer satisfies the following equation (1): y≧0.32x+1.51 (1) (where x represents the oxygen partial pressure (kPa) of the oxidant gas, and y represents the thickness (μm) of the electrolyte layer.)

3. The system according to claim 2 , wherein the oxidizer gas is high-concentration oxygen having an oxygen concentration of 99.9% or more.

4. The system according to claim 3, wherein the ratio of the mass of hydrogen supplied to the anode relative to the theoretical value of the mass of hydrogen required for the anode is adjusted to 1.1 or more and 1.3 or less.

5. A method for suppressing deterioration of a fuel cell, comprising: The fuel cell includes a cell having an anode and a cathode with an electrolyte layer sandwiched therebetween, and supplies an oxidant gas having an oxygen partial pressure higher than that of air to the cathode; The method comprises adjusting the thickness of the electrolyte layer so that the amount of oxygen crossover in the oxidant gas is equal to or less than a predetermined amount.

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