JPH033338B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Technical Field of the Invention] The present invention relates to a method for manufacturing a molten carbonate fuel cell that can maintain stable cell characteristics over a long period of time.

[Technical background of the invention and its problems]

As is well known, a fuel cell is a system in which a gas that is easily oxidized, such as hydrogen, and a gas that has oxidizing power, such as oxygen, react in the presence of an appropriate electrolyte to obtain a direct current output through an electrochemical process. It is broadly classified into phosphoric acid type, molten carbonate type, and solid electrolyte type depending on the electrolyte used.

Among these types of fuel cells, molten carbonate type fuel cells operate 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 the 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.
A pair of gas diffusion electrodes (fuel electrode and oxidizer electrode) 2 made of nickel alloy are provided on both sides of this electrolyte layer 1.
a and 2b are provided to constitute a unit battery 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 a function of electrically connecting each unit battery 3 and a function of dividing gas to be introduced into each electrode plate 2a, 2b. In addition, side wall members 6a, 6b, 7a, and 7b made of stainless steel are brazed in parallel to both sides of each surface in order to form gas flow paths in directions orthogonal to each other. 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).
Furthermore, corrugated plates 8a and 8b made of stainless steel are fitted into each of these gas passages in order to substantially divide the gas flowing therein. 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,
The ends of 7a and 7b and the end of the electrolyte layer 1 constitute a wet seal part, which is structured to prevent leakage of gas introduced into the gas flow path. This wet seal has, for example, an electrolyte of Li ₂ CO ₃ /K ₂ CO ₃ ,
In the case of a two-element eutectic composition with a 62/38 molar ratio, the electrolyte layer 1 is melted at 488°C.

By the way, in order to always obtain stable output from such a molten carbonate fuel cell, it is necessary that at least the bipolar separator 4 described above satisfies the following conditions. In other words, in order to ensure reliable continuity between each unit battery,
It has high electrical conductivity, and because it comes into contact with evaporated molten carbonate at high temperatures in the presence of oxidizing or fuel gases, it is not susceptible to carbonate attack even under these conditions. This method requires a continuous supply path for the reactant gas, uniform contact with the electrode, and low contact resistance.

In order to meet these requirements, conventional bipolar separators have been made of austenitic stainless steel, such as SUS-316, which has excellent conductivity and corrosion resistance.

However, even though austenitic stainless steel has excellent conductivity and corrosion resistance, during battery operation, high-temperature air oxidation occurs on the oxidant electrode side that comes into contact with high-temperature oxidant gas, forming a non-conductive oxide film. Ru. For this reason, there was a problem in that the contact resistance between the bipolar separator and the oxidizer electrode inevitably increased over time. Also, the increase in contact resistance between each electrode plate and the bipolar separator due to the strong corrosive force of the evaporated molten carbonate electrolyte could not be ignored.

Therefore, in order to solve this problem, conventionally, austenitic stainless steel is immersed in molten alkali metal containing at least lithium salt and heat treated in air to contain lithium on the surface. It has been attempted to use a bipolar separator with an oxide layer formed thereon. in this way,
If an oxide layer containing lithium is formed on the surface of the bipolar separator, it will not deteriorate over time due to the electronic conductivity of lithium and the corrosion resistance of the oxide layer, and will have excellent conductivity. bipolar separators can be formed.

However, when a lithium-containing oxide layer is formed by the above method, there is a problem that lithium in the formed oxide film is distributed only on the surface, making it impossible to obtain the desired conductivity.

In addition, the oxide layer formed by the above method does not have good adhesion with the base material, and the oxide layer may peel or crack due to thermal stress when the temperature of the fuel cell increases. It was hot. For this reason, corrosion due to the carbonate progresses in the areas where the oxide layer has peeled off or cracked, resulting in a problem of deterioration of the characteristics of the molten carbonate fuel cell over time.

[Purpose of the invention]

The present invention was made in view of these problems, and its purpose is to maintain the conductivity and corrosion resistance of a bipolar separator over a long period of time, and to produce a molten carbonate with little deterioration of properties over time. An object of the present invention is to provide a method for manufacturing a salt fuel cell.

[Summary of the invention]

The present invention involves immersing at least one side of a conductive metal plate in an aqueous solution containing lithium, forming a hydroxide layer containing lithium on the surface of the metal plate through an electrochemical process, and then applying the hydroxide layer to the surface of the metal plate. A bipolar separator was formed by heat treatment to convert the hydroxide layer into a conductive oxide layer, and the bipolar separator was used to construct a molten carbonate fuel cell. It is characterized by

〔Effect of the invention〕

According to the present invention, after a conductive metal plate is immersed in a lithium-containing aqueous solution, a lithium-containing hydroxide layer is formed on the surface of the metal through an electrochemical process. Lithium can be uniformly diffused into the oxide layer. Therefore, the obtained oxide layer can obtain high electronic conductivity because lithium is uniformly diffused.

Furthermore, when a hydroxide layer is formed by an electrochemical process and this hydroxide layer is further converted into an oxide layer, the obtained oxide layer can be used as a conductive metal plate. Excellent adhesion and elasticity between the layers. Therefore, the oxide layer does not peel off or crack due to thermal stress during temperature rise, unlike in the conventional case. Therefore, it is possible to manufacture a molten carbonate fuel cell with less increase in contact resistance due to corrosion of the bipolar separator and less deterioration of characteristics over time.

[Embodiments of the invention]

Example 1 Two plate members 11 as shown in FIG. 1 were manufactured using conductive metal plates made of SUS-430 (ferritic stainless steel). Note that this plate-like member 11 has the same configuration as the bipolar separator 4 described above, so the same parts as in FIGS. I will omit the explanation.

On the other hand, as shown in FIG.
An aqueous solution mixed at a molar ratio of (1molKOH+
1molLiOH/l) 13, and this aqueous solution 13
The two plate-like members 11 were immersed in the solution while facing each other. Then, the plate member 11 and each electrode of the constant potential pulse generator 14 are connected, and the constant potential pulse generator 14 is driven to generate +0.6V (vs-Hg/HgO) as shown in FIG. ) 6 sec, -1.1 V (vs-Hg/HgO) 1 sec, constant voltage pulse electrolysis was performed for 50 hours to form a hydroxide layer containing lithium on each opposing surface of the plate member 11. After that, the plate member 11 is washed and dried at 200°C for 1 hour, and then at 500°C for 2 hours.
A heat treatment for a period of time was applied to convert the hydroxide layer into a lithium-containing oxide layer, forming two bipolar separators. The obtained bipolar separator and a small unit cell of 40 mm square were stacked alternately so that the above oxide layer was placed on the oxidizer electrode side, and the reaction gas manifold and end were stacked by normal means. plate,
A fuel cell was formed by assembling tightening bars and the like.

Example 2 The plate member 11 in Example 1 described above,
A bipolar separator was formed using a conductive member made of SUS-316 (ferritic stainless steel) in the same manner as in Example 1, and a fuel cell was assembled using this bipolar separator.

Example 2 The plate member 11 in Example 1 described above was formed of a conductive member made of pure nickel, and hydroxide was applied to the surface of the plate member by the same constant voltage pulse electrolysis method as in Example 1. formed a layer. This was washed with water, dried at 100°C for 1 hour, and heat treated at 480°C for 2 hours to obtain a bipolar separator. A fuel cell was assembled using this bipolar separator in the same manner as in Example 1 above.

Comparative Example A plate member 11 shown in FIG. 1 was formed from a conductive member made of SUS-430, and was used as a bipolar separator to assemble a fuel cell.

Conventional Example 1 A plate member 11 shown in FIG. 1 was formed from a conductive member made of SUS-316, and was used as a bipolar separator to assemble a fuel cell.

Conventional Example 2 A plate-like member 11 shown in FIG. 1 is formed using a conductive metal made of SUS-316, and this plate-like member 11 is
is immersed in a melt of carbonate, which is a mixture of lithium carbonate and potassium carbonate in a weight ratio of 1:1,
Heat treatment was performed in air at 700°C for 3 hours to form a bipolar separator with an oxide layer formed on the surface. A fuel cell was assembled using the obtained bipolar separator.

Each fuel cell obtained as above was heated to 650°C.
The fuel cell was operated by supplying an oxidizing gas of 70% Air/Co ₂ and a fuel gas of 80% H ₂ /Co ₂ to the gas manifold. And 150mA/
When we investigated the cell voltage at cm ² and the change in alternating current resistance at AC 1 KHz over time, we obtained the results shown in Figure 4.

As is clear from this figure, the cell voltages A, B, and C of the fuel cells manufactured by the methods described in Examples 1 to 3 are the cell voltages D of the comparative example, the cell voltages of the conventional example 1,
Compared to cell voltages E and F of No. 2, the decrease over time was less. Furthermore, the AC resistances A', B', and
C' increased less over time than the AC resistance D' of the comparative example and the AC resistances E' and F' of Conventional Examples 1 and 2.

In this way, it was confirmed that the fuel cells according to Examples 1 to 3 described above could maintain stable cell characteristics over a long period of time.

Note that the present invention is not limited to the embodiments described above.

For example, in Examples 1 to 3 above, the lithium-containing hydroxide layer was formed by constant-potential pulse electrolysis, but the hydroxide layer was formed by constant-current pulse electrolysis. You can do it like this. Furthermore, the pulse voltage, pulse width, etc. can be changed in various ways. Further, the method for manufacturing a conductive oxide layer according to the present invention can also be applied to a cathode porous body.

[Brief explanation of the drawing]

1 to 4 are diagrams for explaining the embodiment method of the present invention, in which FIG. 1 is a perspective view of a plate-like member constituting a bipolar separator, and FIG. 2 is a surface view of the plate-like member. Figure 3 is a waveform diagram showing the pulses applied in the same process, and Figure 4 is a diagram for explaining the constant potential pulse electrolysis process for forming a lithium-containing hydroxide layer on the lithium-containing hydroxide layer. Figure 5 is an exploded perspective view showing the main parts of a conventional molten carbonate fuel cell; The figure 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, 5...
... Separation plate main body, 6a, 6b, 7a, 7b ... Side wall member, 8a, 8b ... Corrugated plate, 11 ... Plate member, 12 ... Electrolytic cell, 13 ... Aqueous solution, 14 ...
Constant potential pulse generator.

Claims (1)

[Claims] 1. At least one side of a conductive metal plate is immersed in an aqueous solution containing lithium to form a hydroxide layer containing lithium on the surface of the metal plate by an electrochemical process, and then This is then heat-treated to convert the hydroxide layer into a conductive oxide layer to form a bipolar separator, and a plurality of unit cells are stacked via the bipolar separator. A method for manufacturing a molten carbonate fuel cell. 2. The molten carbonate according to claim 1, wherein the conductive metal plate is made of austenitic stainless steel, ferritic stainless steel, pure nickel, or a layered plate of two selected from these. A method for manufacturing a type fuel cell. 3. The method for manufacturing a molten carbonate fuel cell according to claim 1, wherein the bipolar separator has the oxide layer formed only on the side in contact with the oxidant gas. 4 The electrochemical process involves periodically changing the potential to
2. The method for manufacturing a molten carbonate fuel cell according to claim 1, wherein the step is a constant potential pulse electrolysis method in which electrolysis is performed by anodic polarization.