JPH033339B2 - - Google Patents

Description

[Detailed description of the invention]

[Technical Field of the Invention] The present invention relates to a molten carbonate fuel cell that provides stable cell characteristics over a long period of time. [Technical background of the invention and its problems] As is well known, a fuel cell generates electricity by reacting a gas that is easily oxidized, such as hydrogen, with a gas that has oxidizing power, such as oxygen, in the presence of an appropriate electrolyte. Direct current output is obtained through a chemical process, and there are three main types depending on the electrolyte used: phosphoric acid type, molten carbonate type, and solid electrolyte type. Among these fuel cells, molten carbonate type fuel cells are operated at high temperatures of 600 to 700°C, so electrode reactions occur more easily and they do not require expensive precious metal catalysts. It is highly anticipated as a next-generation energy source. The main parts of such a molten carbonate fuel cell are constructed as shown in FIGS. 5 and 6. That is, 1 in the figure is an electrolyte layer formed in a flat plate shape, and is constructed by holding a carbonate electrolyte such as lithium carbonate or potassium carbonate with a ceramic holding material such as lithium aluminate. On both sides of this electrolyte layer 1, a pair of gas diffusion electrodes (fuel electrode and oxidizer electrode) 2a and 2b made of a nickel alloy are provided to constitute a unit cell 3. A plurality of such unit cells 3 are stacked with bipolar separators 4 interposed therebetween, which will be described below, to constitute a fuel cell. The bipolar separator 4 has side wall members 6a, 6b, 7a, and 7b made of stainless steel on each side of the separator body 5, which is made of stainless steel, for example, in order to form gas flow paths perpendicular to each other on both sides of the plate main body 5. It is brazed parallel to both sides. The grooves formed by these side wall members 6a, 6b, 7a, 7b and the surface of the separator body 5 serve as the gas flow paths (fuel gas flow path and oxidant gas flow path). In addition, each of these gas channels is provided with a stainless steel corrugated plate 8 in order to substantially separate the gas flowing therein.
a and 8b are fitted. Further, each end face of the side wall members 6a, 6b, 7a, 7b is provided with a stepped portion for fitting the gas diffusion electrodes 2a, 2b, respectively. Then, the gas diffusion electrodes 2a, 2b are fitted into the stepped portions, and the side wall members 6a, 6b, 7
The ends of a and 7b and the end of the electrolyte layer 1 constitute a wet seal part, which is structured to prevent leakage of the gas introduced into the gas flow path. This wet seal has electrolytes such as Li ₂ CO ₃ /K ₂ CO ₃ , 62 /
In the case of a two-element eutectic composition with a molar ratio of 38,
This is done by melting the electrolyte layer 1 at 488°C. However, when the end surfaces of the side wall members 6a, 6b, 7a, and 7b of the bipolar separator 4 are brought into direct contact with the molten carbonate to form a wet seal portion as described above, the strong corrosive force of the molten carbonate causes the above-mentioned There is a problem that the side wall members 6a, 6b, 7a, 7b are corroded. Such corrosion not only causes leakage of reaction gas but also structural deterioration. Therefore, a gasket made of asbestos, mica, silver, aluminum, etc. foil is interposed in the wet seal part.
Efforts are made to prevent corrosion of the bipolar separator 4. However, gaskets made of asbestos and mica have a problem in that they cannot withstand long-term use because they have poor mechanical strength. Furthermore, a silver foil gasket is expensive and cannot be put to practical use. Gaskets made of aluminum foil tend to melt and agglomerate at temperatures above 650°C, which is the operating temperature of the battery, and molten aluminum metal can be removed from the outer edge of the electrolyte layer 1 or from the portions of the electrolyte layer 1 where the electrolyte has escaped. There was a problem that this caused a short circuit accident between the gas diffusion electrodes 2a and 2b tubes. Furthermore, in separating the molten carbonate and the side wall members 6a, 6b, 7a, 7b using such a gasket,
In addition to being difficult to perform complete separation, there is also the problem that the manufacturing and assembly process of fuel cells becomes complicated and complicated. On the other hand, apart from the problem of corrosion of the bipolar separator 4, there was a problem of electrolyte movement and dissipation at the wet seal portion. That is, when fuel gas is present inside the wet seal portion and oxidizing gas is present outside the wet seal portion, carbonate ions are accumulated on the outside as a result of an electrochemical reaction. However, since the mobility of carbonate ions is lower than that of lithium cations and potassium cations, lithium and potassium cations are essentially pulled out. This causes carbonate migration to occur as described above. Carbonates that migrate outward along the wet seal surface are absorbed by the alumina or zirconia mats used in the manifold seal and dissipate from the electrolyte region, increasing cell resistance and causing fuel gas leakage. There was a problem in that the amount of cross-mixing with the oxidant gas increased, resulting in a significant decrease in battery performance. [Object of the Invention] The present invention was made in view of these problems, and its purpose is to prevent corrosion of parts that come in direct contact with molten carbonate, such as bipolar separators and gas manifolds, and to prevent corrosion in these parts. An object of the present invention is to provide a molten carbonate fuel cell that can effectively prevent the movement and escape of carbonate and provide stable cell performance over a long period of time. [Summary of the Invention] The present invention provides the use of boron carbide, boron nitride, or hafnium nitride in a portion of a bipolar separator or gas manifold that directly contacts the molten carbonate electrolyte layer to form a wet seal. It is characterized by the provision of an anti-corrosion layer made of Ride, molybdenum borate, aluminum nitride or silicon nitride. In particular, it is desirable that the anticorrosion layer be formed on a base layer whose composition is a plurality of substances selected from nickel, chromium, aluminum, cobalt, yttrium, and molybdenum. [Effect of the invention] The above-mentioned boron carbide, boron nitride, hafnium nitride, molybdenum borate, aluminum nitride, or silicon nitride can be used at 650°C, which is the operating temperature of a fuel cell.
Even above, it has excellent corrosion resistance against molten carbonate and also has high mechanical strength. Therefore, the conventional problems of supply gas leakage and structural weakening due to corrosion of bipolar separators and the like can be solved. These materials are also substantially non-electronically conductive and have low wettability with carbonates. For this reason,
Electrochemical reactions in the wet seal portion are less likely to occur, and as a result, movement and escape of carbonates can be prevented. Therefore, it is possible to suppress an increase in battery resistance and an increase in the amount of cross-mixing of reactant gases due to a decrease in carbonate over time. As a result of obtaining such various effects, the molten carbonate fuel cell according to the present invention can obtain stable cell performance over a long period of time. [Embodiments of the Invention] Hereinafter, embodiments of the present invention will be described with reference to the drawings. Example 1 FIG. 1 is a diagram showing the main parts of a fuel cell according to this example, and the same parts as in FIGS. 5 and 6 are given the same reference numerals. Therefore, the explanation of duplicate parts will be omitted. This fuel cell differs from conventional ones in that side wall members 6a, 6b, 7a, 7
An anticorrosive layer 12, which will be described later, is formed on the surface facing the electrolyte layer b with a base layer 11 interposed therebetween. In this example, 150 mm x 150 mm of SUS316L stainless steel with a thickness of 0.3 mm and nickel clad on one side was used.
It was cut out to a size of , and this was used as the separator main body 5.
Also, SUS316L with thickness 2.5mm, width 15mm, and length 150mm
Side wall members 6a, 6b, 7a, 7b made of stainless steel
This was used as a separator and was joined to the edge of the separator body 5 by brazing. Next, a base layer 11 made of a Co-29Cr-6Al-1Y alloy (weight ratio) is applied to the surface of the side wall members 6a, 6b, 7a, and 7b facing the electrolyte layer 1 to a thickness of 150 μm by plasma spraying. It was formed. Furthermore, on top of this base layer 11, boron carbide with a purity of 99.9% is applied by CVD.
(chemical vapor deposition)
The anticorrosion layer 12 was formed to have a thickness of 300 μm. Three layers of unit cells were stacked using the bipolar separator 4 thus obtained, and a reactant gas manifold, end plates, tightening bars, etc. were provided by conventional means to assemble a fuel cell. Example 2 Instead of the anticorrosion layer 12 made of boron carbide in Example 1 described above, boron nitride was formed by CVD to an average thickness of 320 μm,
This was used as the anticorrosion layer 12. A fuel cell was assembled using the bipolar separator 4 on which the anticorrosion layer 12 was formed in the same manner as described above. Example 3 Instead of the anticorrosion layer 12 made of boron carbide in Example 1 described above, hafnium nitride was formed by CVD to an average thickness of 320 μm,
This was used as the anticorrosive layer 12. A fuel cell was assembled using the bipolar separator 4 on which the anticorrosion layer 12 was formed in the same manner as described above. Example 4 Instead of the anticorrosion layer 12 made of boron carbide in Example 1 described above, a mixed solution of 1 part molybdenum borate and 1 part chromium oxide aqueous solution was used.
It was coated to a thickness of 280 μm and heat-treated at 550° C., and this was used as the anticorrosion layer 12. A fuel cell was assembled using the bipolar separator 4 on which the anticorrosion layer 12 was formed in the same manner as described above. Example 5 Instead of the anticorrosion layer 12 made of boron carbide in Example 1, aluminum nitride was formed by CVD to an average thickness of 250 μm, and this was used as the anticorrosion layer 12. A fuel cell was assembled using the bipolar separator 4 on which the anticorrosion layer 12 was formed in the same manner as described above. Example 6 Instead of the anticorrosion layer 12 made of boron carbide in Example 1 described above, silicon nitride was formed by CVD to an average thickness of 300 μm,
This was used as the anticorrosive layer 12. A fuel cell was assembled using the bipolar separator 4 on which the anticorrosion layer 12 was formed in the same manner as described above. Comparative Example 1 Side wall members 6a, 6 in Example 1 described above
A fuel cell was assembled without forming the base layer 11 and anticorrosion layer 12 on the layers b, 7a, and 7b. Comparative Example 2 (Conventional Example) Side wall members 6a, 6 in Example 1 described above
b, 7a, 7b, base layer 11 and anti-corrosion layer 12
Instead, an aluminum gasket with a width of 1.5 mm and a thickness of 0.5 mm is formed on the side wall members 6a, 6.
A fuel cell was assembled by interposing the electrolyte layer 1 between the electrolyte layer 1 and the electrolyte layer 1.b, 7a, 7b. The following measurements were performed on each of the fuel cells of Examples 1 to 6 and Comparative Examples 1 and 2 configured as described above. That is, as shown in FIG. 2, gas manifolds 16a, 16b, 16c, and 16d were applied to the four sides of a fuel cell stack 15, which is a sample, and this was housed in an electric Matsufuru furnace 17.
After heating to 650℃, air is 70: carbon dioxide gas is 30 (volume ratio) from gas manifold 16a to 16b.
Oxidizing gas P consisting of a mixed gas of 1500 (N・
ml/min), and a fuel gas Q consisting of a mixed gas of 80% hydrogen gas and 20% carbon dioxide gas (volume ratio) is passed through the gas manifolds 16c to 16d.
was passed through the tube at a flow rate of 1000 (N·ml/min). Then, the nitrogen concentration in the exhaust fuel gas was measured by the gas chromatograph 18 at the gas outlet of the gas manifold 16d, and the amount of gas leakage was determined from this measured value. As a result, the fuel cell stacks 15 of Comparative Examples 1 and 2
As shown in FIGS. 3A and 3B, there was a significant increase in the amount of gas leakage over time, but in Examples 1-
The fuel cell stack 15 of No. 6 is C, D, E, F,
As shown in G and H, the increase in the amount of gas leakage over time was extremely small. In addition, the lowermost single cell of the fuel cell stack 15
The 1kHz AC resistance was measured using a milliohmmeter 19. As a result, the fuel cell stacks 15 of Comparative Examples 1 and 2
As shown in FIG. 4A and B, the AC resistance value increased significantly over time, but in the fuel cell stacks 15 of Examples 1 to 6, C, D, E, F, As shown in G and H, the increase in AC resistance value over time was extremely small. In addition, in Comparative Example 2 using an aluminum gasket, a short circuit of the gas diffusion electrode occurred due to condensation on the gasket after 850 hours.Furthermore, after each of the above samples was left for 1000 hours, it was disassembled and the side wall members 6a, 6b, The degree of corrosion on the end faces of 7a and 7b was observed. As a result, the degree of corrosion of each sample was as shown in the following table.

[Table] From the above results, the fuel cells according to Examples 1 to 6 of the present invention can effectively prevent the corrosion of the bipolar separator 4 and the movement and dissipation of carbonate at the wet seal portion, and can be used for a long period of time. It was confirmed that stable battery performance could be obtained. Note that the present invention is not limited to the embodiments described above. For example, the bipolar separator may be made of chrome-plated stainless steel 310 or chrome-plated stainless steel 446. Further, the base layer is not limited to those mentioned above, and other alloys may be used as long as the alloy has a composition of multiple substances selected from nickel, chromium, aluminum, cobalt, yttrium, and molybdenum. . Furthermore, it is of course possible to obtain the effects of the present invention even if the anticorrosion layer is directly formed on the surface of the base material of the bipolar separator without using such a base layer. Further, the anticorrosion layer according to the present invention may be formed on the flange portion of the manifold that is in contact with the side surface of the fuel cell stack. By forming the anti-corrosion layer on the manifold in this manner, airtightness between the manifold and the fuel cell stack can be maintained for a long period of time. In short, the present invention can be implemented with various modifications without departing from the gist thereof.

[Brief explanation of the drawing]

FIG. 1 is a longitudinal cross-sectional view of the main parts of a molten carbonate fuel cell according to an embodiment of the present invention, FIG. 2 is a block diagram for explaining a test method for the above fuel cell, and FIG. Figure 4 is a characteristic diagram showing the amount of gas leakage over time in comparison with a comparative example. Figure 4 is a characteristic diagram showing the change in internal resistance over time of the above fuel cell in comparison with a comparative example. FIG. 6 is an exploded perspective view showing the main parts of the carbonate fuel cell, and FIG. 6 is a longitudinal sectional view of the main parts of the fuel cell. DESCRIPTION OF SYMBOLS 1... Electrolyte layer, 2a, 2b... Gas diffusion electrode, 3... Unit battery, 4... Bipolar separator, 6
a, 6b, 7a, 7b... side wall member, 11... base layer, 12... anti-corrosion layer, 15... fuel cell laminate, 16a-16d... gas manifold, P...
...Oxidizing gas, Q...Fuel gas.

Claims (1)

[Scope of Claims] 1. A plurality of unit cells each having a fuel electrode and an oxidizer electrode on both sides of a molten carbonate electrolyte layer formed in a flat plate shape, with a fuel gas flow path and an oxidant gas flow path on both sides. In a molten carbonate fuel cell, a fuel cell stack is constructed by laminating a plurality of layers through metal bipolar separators each having a channel and a gas manifold provided at each end face of the fuel cell stack. , from boron carbide, boron nitride, hafnium nitride, molybdenum borate, aluminum nitride or silicon nitride in the part of the bipolar separator or the gas manifold that is in direct contact with the molten carbonate electrolyte layer. A molten carbonate fuel cell characterized by forming an anti-corrosion layer. 2. A portion of the bipolar separator or the gas manifold that is in direct contact with the molten carbonate electrolyte layer is pre-filled with a plurality of materials selected from nickel, chromium, aluminum, cobalt, yttrium, and molybdenum. 2. The molten carbonate fuel cell according to claim 1, wherein a base layer having the same composition as above is formed.