Fuel cell stack

Abstract

Fuel cell stack (100) comprising a plurality of stacked cells, each having separators (17, 18), wherein: a base material of each separator used for an end cell (20) located at one end of the positive side of the fuel cell stack (100), and a base material of each separator used for cells other than the end cell (30), are different metallic materials, and the base material of each separator used for the end cell (20) is a material with higher corrosion resistance than the base material of each separator used for the cells other than the end cell (30).

Description

Technical field

[0001] The present invention relates to a fuel cell stack. State of the art

[0002] Fuel cells are often used as one of the environmentally friendly energy sources. For example, a polymer electrolyte fuel cell mainly comprises a membrane electrode assembly (MEA), which contains an electrolyte membrane with catalyst layers surrounding opposite surfaces, gas diffusion layers located on opposite outer surfaces of the catalyst layers, and a pair of separators surrounding the entire body on the right and left sides. Multiple such cells are stacked and electrically connected in series to form the fuel cell stack.

[0003] When a fuel cell generates energy or electricity, the cell is exposed to a corrosive environment. Therefore, a SUS with excellent corrosion resistance is typically used as the base material for each separator in the cell. However, if the cell voltage in the corrosive environment becomes high (greater than or equal to approximately 1 V), the metal would inevitably dissolve. This is because the cooling water for the fuel cells flows through the fuel cell stack in such a way that the water comes into contact with each cell. Therefore, an equivalent circuit, as described below, is formed. Furthermore, since the cells in the fuel cell stack are connected in series,The cells are arranged in series, the voltage is high, and a high corrosion current flows through a plurality of cells located at the end of the positive side of the fuel cell stack, thus increasing the voltage within them. Consequently, corrosion (dissolution) occurs in the base material of the separators (for example, SUS). It should be noted that in the present invention, "corrosion" or "dissolution" describes a condition in which a metallic material is ionized and thus dissolves in such a way that the thickness of the plate decreases.

[0004] Methods have been proposed to prevent such corrosion (dissolution) of the separator base material. For example, patent literature 1 describes a fuel cell stack formed by stacking a plurality of unit cells, each having metal separators of the same type. These separators are obtained by applying a surface treatment to the metal separators of one or more cells located at the end of the positive side of the fuel cell stack, thereby increasing the corrosion resistance of the metal separators relative to those of the metal separators used for the other cells.Examples of surface treatments to provide high corrosion resistance include a plating process using precious metals such as gold or silver, or a plating process that forms a thick plating, and examples of surface treatments to provide low corrosion resistance include a plating process that forms a thinner plating.

[0005] Patent literature 2 describes a fuel cell stack with an arrangement or configuration in which a pair of terminal plates are arranged at opposite ends of a cell stack containing a plurality of stacked unit cells. These terminal plates incorporate flow channels for supplying or discharging reaction gas and cooling water to / from the cell stack, with only one inlet and one outlet opening penetrating the terminal plate on the negative side. This means that, since oxidation current flows through the terminal plate on the positive side, the cell stack is configured such that the cooling water or moisture from the reaction gas flowing through the flow channels does not touch or contact the end plate on the positive side. This improves the corrosion resistance of the terminal plate on the positive side and reduces costs.Patent literature 2 also discloses the arrangement of a separating plate for separating the moisture permeation or moisture passage between the connecting plate of the positive side and the cell stack.

[0006] Furthermore, patent literature 3 describes an arrangement or configuration of a fuel cell stack including power-generating cells and a non-power-generating cell dummy(ies) or dummy cell(s), in which the cell dummy is arranged at one or each end of the plurality of stacked, power-generating cells. The use of a cell dummy can suppress flattening at one or each end of the cells in the stacked direction and a voltage drop due to impurities or contamination. Reference list of patent literature Patent Literature 1: JP 2005-293876 A Patent Literature 2: JP 2005-293874 A Patent Literature 3: JP 2015-69737 A Summary of the invention: Technical problem

[0007] In the fuel cell stack described in patent literature 1, a surface treatment to provide higher corrosion resistance (for example, a plating process using precious metals such as gold or silver, or a plating process capable of forming a thicker film) is applied to the metal separators of the end cell(s), which are located at the end of the positive side of the fuel cell stack. This treatment aims to prevent corrosion (dissolution) of the metal separators, thus extending the service life of the fuel cell stack. However, to effectively prevent corrosion of the metal separators, a plating process capable of forming a film with a thickness on the order of micrometers would be required.Additionally, performing a plating process to form a thick film using a metal, such as gold or silver, as a post-process or finishing step can often increase costs and reduce productivity.

[0008] The fuel cell stack described in patent literature 2 is effective in increasing the corrosion resistance of the positive-side terminal plate. However, patent literature 2 does not describe how to prevent corrosion of the metal separators of the end cell(s), which are located at the end of the positive side of the fuel cell stack.

[0009] The present invention has been made in light of the foregoing circumstances. An objective of the present invention is to provide a fuel cell stack comprising a plurality of stacked cells, each having separators, in which the corrosion (dissolution) of the base material of each separator, which is used for an end cell comprising one or more end cells arranged at the end of the positive side of the fuel cell stack, can be prevented at lower cost without reducing productivity. Solution to the problem

[0010] A fuel cell stack according to the present invention is a fuel cell stack with a plurality of stacked cells, each having separators in which the base material of each separator used for an end cell arranged at the end of the positive side of the fuel cell stack and the base material of each separator used for cells other than the end cell are different metallic materials, and the base material of each separator used for the end cell is a material with a higher corrosion resistance than the base material of each separator used for the cells other than the end cell.

[0011] As described above, “corrosion” as defined in the present invention means a condition in which a metallic material, which is the base material of a separator, is ionized and therefore dissolves, with the result that the thickness of the plate decreases.

[0012] In the fuel cell stack, which consists of a plurality of cells, only the base material of each separator of the end cell, in which corrosion of the separator is likely to occur, is a material with a higher corrosion resistance than that of the base material of each of the separators used for the other cells, whereby the manufacturing costs can be reduced in comparison if the base materials of the separators of all cells are expensive materials with high corrosion resistance.

[0013] In the fuel cell stack according to the present invention, the end cell can be a single cell located at the outermost end on the positive side of the fuel cell stack, or one or more cells including such a cell and its adjacent cell(s). It would be practical to determine, by means of experiments or simulation, up to which cell, counting from the end of the positive side of the fuel cell stack, significant corrosion, generated therein by current generation, is likely to occur, and to designate such cell(s) as the "end cell".

[0014] In the fuel cell stack according to the present invention, all cells are preferably cells that contribute to power generation. It is also possible to use a "dummy cell" or "cell replica," such as the one described in patent literature 3, as one of the end cells, but the use of cells that contribute to power generation for all end cells can obtain sufficient power even if the size of the fuel cell stack is reduced.

[0015] In the fuel cell stack according to the present invention, the base material of each of the separators used for the end cell can have a passivation film formed on its surface. The passivation film can form naturally if the separator base material is left in its natural environment, or it can be artificially formed. Examples of previously used base materials include Ti, Sn, Ta, and similar materials. Such a base material can certainly provide higher corrosion resistance due to the passivation film formed on its surface.

[0016] In the fuel cell stack according to the present invention, a material that does not corrode, even if it lacks a passivation film, can be used as the material with higher corrosion resistance. Examples of such materials include precious metals such as gold, platinum, or silver. However, such precious metals are expensive and cost-disadvantageous. Accordingly, the use of titanium, tin, tantalum, or similar materials, which have a passivation film, as the base material, particularly titanium, is very cost-effective.

[0017] For a separator with a passivation film formed on it, it is preferable to use a configuration or arrangement in which a surface of the passivation film forms a protective layer with a thickness in the range of 5 nm to 500 nm and the ability to further improve corrosion resistance. A plated layer of NiSn or Au can be used as an example of such a protective layer. In a separator with such a configuration, since the passivation film forms on the surface of the separator's base material, a certain level of corrosion resistance is already provided, and therefore the expected performance can be fully achieved even if the thickness of the protective layer that forms on the surface of the passivation film is as thin as 5 nm to 500 nm.Furthermore, even when using an expensive precious metal like Au, the thickness of the resulting plate is on the order of nanometers. Therefore, cost reductions and productivity improvements are achieved compared to conventionally clad layers with a thickness on the order of micrometers. Advantageous effects of the invention

[0018] According to the fuel cell stack in accordance with the present invention, which comprises a plurality of stacked cells, each having separators, the base material of each separator used for the end cell, which comprises one or more cells arranged at the end of the positive side of the fuel cell stack, can be preserved from corrosion (dissolution) at low cost and without reduced productivity. Brief description of the drawing The Fig. 1A and Fig. Figure 1B shows a schematic perspective view and a side view illustrating a fuel cell; The Fig. 2A, Fig. 2B, and Fig. 2C are views which represent a reason why corrosion occurs in separators of an end cell of a fuel cell stack; Fig. Figure 3 is a schematic view which represents an example of a fuel cell stack in accordance with the present invention; Fig. Figure 4 is a schematic perspective view showing the configuration or arrangement of an end cell; Fig. Figure 5 is a schematic perspective view showing the configuration of each cell except the end cell; Fig. 6 is a diagram in which the voltage is plotted against the pH of Ti; The Fig. 7A and Fig. 7B are views of the simulation results, each representing the degree of corrosion of a separator; Fig. Figure 8 is a cross-sectional view showing the configuration of a simulated cell; and Fig. Figure 9 is a circuit diagram of a simulated cell. Description of the embodiments

[0019] Embodiments of the present invention are described below with reference to the drawing. First, with reference to the Fig. 1 and Fig. 2 described one reason why corrosion occurs in some separators of the cells on the positive side of a fuel cell stack.

[0020] The Fig. 1 are a schematic perspective view ( Fig. 1A) and a side view ( Fig. 1B), each of which represents an example of a power-generating cell of a fuel cell stack. In this example, a power-generating cell 10 is a power-generating solid-polymer cell, and the power-generating cell 10 comprises a membrane electrode assembly 14, which includes an electrolyte membrane 11, an electrode arranged on one of the surfaces of the electrolyte membrane 11 (an anode-side catalyst layer 12), and an electrode arranged on the other surface of the electrolyte membrane 11 (a cathode-side catalyst layer 13); diffusion layers 15 and 16, which are arranged on opposite surfaces of the membrane electrode assembly 14; and a pair of separators 17 and 18, which surround the two diffusion layers 15 and 16 from their outer sides.In the example shown in the drawing, flow channels 19a and 19b are formed on the sides of the separators 17 and 18 opposite the diffusion layers 15 and 16, and hydrogen gas as fuel gas is supplied to one of the flow channels 19a, while air as oxidizing gas is supplied to the other flow channel 19b. Subsequently, the power-generating cell 10 produces heat and also water through the process of generating electricity.

[0021] The fuel cell stack comprises a plurality of such power-generating cells 10, which are stacked while being electrically connected in series. The fuel cell stack is provided with a flow channel for circulating the cooling water through a distributor or manifold formed in each power-generating cell to manage the heat generated during power generation. The cooling water (FCC) is in contact with all power-generating cells in the fuel cell stack, and the cooling water (FCC) acts as a solvent, such that an equivalent circuit, like the one in Fig. 2A is formed. It should be noted that in the example shown in Fig. As shown in Figure 2A, 370-ch power-generating cells form a fuel cell stack.

[0022] The Fig. 2B and Fig. Figures 2C each represent the current and voltage distributions in each power-generating cell of such a fuel cell stack. In the fuel cell stack, the power-generating cells 10 are arranged in series as described above, and a large current flows as a corrosion current on the positive side, as shown in Figure 2C. Fig. 2B is shown, and therefore the voltage is high here, as in Fig. 2C is shown. Therefore, in the fuel cell stack, a corrosion phenomenon in which a metallic material is ionized and thus dissolves or decomposes will probably occur in the separators of the majority of power-generating cells on the positive side (hereinafter referred to as "end cells") at a voltage above 1 V.

[0023] To prevent the type of corrosion in the fuel cell stack described in patent literature 1, where all power-generating cells have separators made of the same metal, a plating process using a precious metal is applied to the separators of the end cell(s) on the positive side. Therefore, it is suspected that production costs could increase and productivity could deteriorate, as described above.

[0024] The fuel cell stack according to the present invention prevents such corrosion of the separators of the end cell(s) on the positive side, based on a method differing from JP 2005-293876 A. This is described in particular below. Fig. Figure 3 is a schematic view illustrating an example of a fuel cell stack according to the present invention. A fuel cell stack 100 basically comprises a plurality of power-generating cells 10. Each power-generating cell 10 can be a power-generating cell with the [missing information]. Fig. The configuration shown in Figure 1 is different from the ordinary fuel cell stack, as described in patent literature 1, in which the metallic materials of the separators 17a and 18a of one or more cells, which are located at the outermost end on the positive side of the fuel cell stack 100 (in the example in Figure 1), are Fig. 3, four cells counted from the positive side; hereinafter referred to as “end cells 20”) are arranged and the metallic materials of separators 17b and 18b of cells 30, except for the end cells, are different metallic materials.

[0025] It should be noted that a pair of terminal plates or end plates 41, 41 for power consumption are usually arranged on the positive and negative sides of the fuel cell stack 100, and a pair of end plates 43, 43 are arranged on the outer sides of the end plates 41, 41 with insulating plates 42, 42 inserted between them, and furthermore the entire fuel cell stack is compressed or pressed together.

[0026] In particular, the base material of each of the separators used for the end cells 20 is a material with higher corrosion resistance than the base material of each separator used for the cells 30 other than the end cells. For example, separators 17a and 18a of the end cells 20 are Ti separators, as shown in Fig. 4 shown, and the separators 17b and 18b of cells 30 except the end cells are separators made of SUS, as in Fig. 5 shown.

[0027] In such a case, a passivation film of TiO2 is formed on the surface of each of the Ti separators 17a and 18a of the end cells 20 such that the separators exhibit superior corrosion resistance compared to the SUS separators 17b and 18b of the other cells 30. The passivation film on the Ti surface forms naturally when Ti remains exposed to the atmosphere, and thus no special means are required to form a passivation film. Therefore, the use of Ti as the base material of each separator used for the end cell 20 is highly advantageous. Sn or Ta can also be used as a material of the same type.

[0028] Fig. Figure 6 is a graph of the voltage versus pH of Ti. As in Fig. As shown in Figure 6, a passivation film (TiO2) is formed in a normal operating environment of the fuel cells, specified by "FC environment", whereby Ti can be kept in a stable state even when Eh is 1.0 V, and therefore it is found that the use of Ti for the separators of the end cells 20 is effective.

[0029] Even if a passivation film is not provided, a precious metal such as Au, Pt, or Ag can be used as the base material for each of the end-cell separators, exhibiting superior corrosion resistance compared to SUS. However, the use of such materials is disadvantageous in that it incurs higher costs than the use of Ti.

[0030] As described above, if the separators of the end cells 20 are formed using a material that is less prone to corrosion (dissolution), the end cells 20 consume a current, and thus a corrosion current flowing through the cells 30, which are formed using a cost-effective material (for example, SUS), is suppressed. Accordingly, the probability of corrosion occurring in the separators of the cells 30 other than the end cells can be reliably suppressed.

[0031] If a material with a passivation film such as Ti on its surface is used as the base material of each separator of the end cells 20, it would be preferable to form a surface protection layer capable of further improving the corrosion resistance on the surface of the passivation film. In particular, a plated layer of NiSn or Au can be used as the surface protection layer. In such a case, the surface protection layer supports power consumption, so that the number of cells whose separators are formed using a base material with higher corrosion resistance than that of the base material of each of the separators used for the other cells 30, i.e., the number of end cells, can be reduced.

[0032] Furthermore, if a material with a passivation film formed on its surface, such as Ti, is used for each of the separators 17a and 18a of the end cells, the expected performance can be achieved even if the aforementioned surface protection layer has a thickness in the range of 5 nm to 500 nm. Even if a noble metal such as Au is used as the material for the surface protection layer, a thickness in the range of 5 nm to 500 nm would be sufficient, and thus lower costs can be achieved compared to using the ordinary end cell, which requires a surface protection layer with a thickness on the order of µm, as described, for example, in patent literature 1.Furthermore, since the surface protective layer is thin, there is no particularly adverse effect on the body of the separator, even if the surface protective layer, which is a plated layer, has defects in the plating, which is advantageous.

[0033] In the fuel cell stack 100 according to the present invention, a certain degree of advantageous effects can be achieved even if the end cell 20 is a single cell at the end of the positive side of the fuel cell stack. Preferably, separators of more than one cell, counting from the end of the positive side, are desirablely formed using a material with high corrosion resistance. The specific number of cells required in the end cell can be effectively determined by generating a corrosion current and a voltage such as those which are respectively in the Fig. 2B and Fig. The number of cells represented by 2C can be determined through simulations or experiments and by selecting the number of cells in which a current or voltage greater than a predetermined value (for example, greater than or equal to 1 V) is expected to be generated. According to the inventors' experiments, it was found that forming separators of approximately 2 to 5 cells, counting from the outermost end of the positive side, using a material with higher corrosion resistance than that of the separators of the other cells, is practical and effective in terms of both cost and productivity.

[0034] In the fuel cell stack according to the present invention, the end cell 20 (or a cell located at the outermost end on the positive side if the end cell comprises a plurality of cells) can be manufactured as a dummy cell, without a power-generating feature or function such as that described in patent literature 3. However, the use of a dummy cell without a power-generating function would reduce the ratio of the amount of power generated to the total volume of the fuel cell stack. Furthermore, in the fuel cell stack according to the present invention, a voltage drop during power generation can be suppressed to a certain degree by using separators for the end cells 20 made of a material with higher corrosion resistance than those of the separators of the cells 30 other than the end cells.Therefore, in the fuel cell stack 100 according to the present invention, there is no inconvenience even if all cells 10 are designed as cells which contribute to power generation (that is, cells, each comprising a membrane electrode unit), and consequently sufficient power can be obtained even if the size of the fuel cell stack 100 is reduced.

[0035] Next, the results of the simulation of a voltage in each cell of the fuel cell stack will be presented in relation to the Fig. 7 described if SUS separators are assumed or adapted for all 370 cells and if separators of 4 cells at the end of the positive side of 370 cells were replaced with separators, each formed by plating a Ti base material with NiSn with a thickness of 500 nm.

[0036] Fig. 7A presents a case in which SUS separators are assumed or adapted for all cells. It was found that when the separators were held at a voltage applied to the respective cells for a given time period, the voltages in the cells up to the fifth cell from the end became high voltages of over 1 V, and thus the SUS separators of the cells up to the fifth cell dissolved. Meanwhile, it is stated that Fig. 7B presents the results when the separators of 4 cells at the end of the positive side were replaced by separators formed by plating a Ti base material with NiSn, and shows that the separators of the cells of up to 4 cells from the end did not corrode even under high voltages, and furthermore, the SUS separators of the fifth cell also did not corrode.

[0037] Next, corrosion tests carried out by the inventors using simulated cells, which employ dry cells, will be presented in relation to the Fig. 8 and Fig. 9 described. Fig. Figure 8 is a cross-sectional view of tested, simulated cells in which 10 dry 1.6 V cells were arranged in series. A corrosion-resistant plate (a non-current-generating Au-plated plate) was placed on the outside of the simulated cell on the positive side to provide a dummy cell. Separator materials made of SUS were placed 3 mm apart and subsequently immersed in cooling water. The central area of ​​each separator material was masked with silicone, and the lower end section below the masked area was used as the surface to be evaluated or tested. Fig.Figure 9 is a circuit diagram or circuit plan of the simulated cells, in which Rs represents the metal resistance of the separator material, Rm represents the metal resistance of the corrosion-resistant plate, and Rw represents the resistance of the cooling water. [Test 1]

[0038] When all 10 separators are made of SUS, corrosion was observed in the separators of the three simulated cells from the end of the positive side of the 10 separators after 26 hours in the experiment. [Test 2]

[0039] The separators of the three simulated cells, which exhibited corrosion, were simply replaced with titanium, and a similar experiment was conducted. Subsequently, no corrosion occurred in the separators of the three simulated cells. However, corrosion was observed in the separators of the fourth simulated cell, starting at the positive end. [Test 3]

[0040] An experiment was performed similarly by replacing the separators of the three simulated cells exhibiting corrosion with separators obtained by cladding Ti with NiSn to a thickness of 500 nm. Subsequently, no corrosion appeared in the separators of the three simulated cells. Furthermore, no corrosion was observed in the separators of the fourth cell, nor in the subsequent simulated cells from the positive side. [Test 4]

[0041] A corrosion test was performed as in Test 3 by replacing a NiSn-clad Ti separator with a thickness of 500 nm with an Au-clad Ti separator with a thickness of 10 nm. In this case, no corrosion occurred in the separators of the three simulated cells. Furthermore, no corrosion was observed in the separators of the fourth and also in the subsequent simulated cells at the end of the positive side. [Consideration]

[0042] From the aforementioned experimental results, it is found that if Ti is simply used as the base material of each separator of the end cells, and if a separator is used which is obtained by forming a surface protective layer capable of further improving the corrosion resistance on the surface of Ti, the degree of corrosion of the separator can be improved compared to when all separators are made of SUS. List of reference symbols 100 fuel cell stacks 10 Power-generating cell 11 Membrane electrode unit 12, 13 catalyst layers 14 Membrane electrode unit 15, 16 diffusion layers 17, 18 separators 17a, 18a Ti separators 17b, 18b SUS separators 20 End cell 30 cells except the end cell 41 Connection plate 42 Insulation board 43 End plate

Claims

[1] Fuel cell stack (100) comprising a plurality of stacked cells, each having separators (17, 18), wherein: a base material of each separator used for an end cell (20) located at one end of the positive side of the fuel cell stack (100), and a base material of each separator used for cells other than the end cell (30), are different metallic materials, and the base material of each separator used for the end cell (20) is a material with higher corrosion resistance than the base material of each separator used for the cells other than the end cell (30). [2] Fuel cell stack (100) according to claim 1, wherein the end cell (20) comprises one or more cells. [3] Fuel cell stack (100) according to claim 1, wherein all cells are cells which contribute to energy generation. [4] Fuel cell stack (100) according to one of claims 1 to 3, wherein a surface of the base material of each separator used for the end cell (20) has a passivation film formed thereon. [5] Fuel cell stack (100) according to claim 4, wherein a surface of the passivation film has a surface protective layer formed thereon with a thickness in the range of 5 nm to 500 nm and is capable of further improving the corrosion resistance. [6] Fuel cell stack (100) according to claim 5, wherein the surface protection layer is a plating layer of NiSn or Au. [7] Fuel cell stack (100) according to claim 4 or 5, wherein the base material of each separator used for the end cell (20) is one selected from the group consisting of Ti, Sn, and Ta.

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