JP2513638B2 - Method for producing molten carbonate corrosion resistant material - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Field of Application) The present invention relates to a method for producing a molten carbonate corrosion resistant material used in a molten carbonate fuel cell.

[Prior Art] A molten carbonate fuel cell is considered as a second-generation fuel cell next to a phosphoric acid fuel cell because of its high power generation efficiency. This molten carbonate fuel cell has a structure in which an electrolyte plate mainly composed of carbonate is sandwiched between a cathode and an anode, and further various metal members are provided for uniformly distributing gas to the cathode and the anode, respectively. are doing.

By the way, conventionally, SUS316, for example, has been adopted as the metal material used for the portion that directly contacts with the molten carbonate, that is, the electrode, punching metal, current collector, and separator. However, since the operating temperature of the molten carbonate fuel cell is as high as 650 ° C., these metal members are significantly corroded by elution into the molten carbonate and reaction with the molten carbonate. Further, these metal members are exposed to hydrogen, carbon monoxide and water vapor produced as an electrode reaction product on the anode side, and carbon dioxide gas and oxygen gas on the cathode side. receive.

Such high temperature oxidation and corrosion due to molten carbonate significantly deteriorate the characteristics of the molten carbonate fuel cell. That is,
The oxide film formed by high-temperature oxidation has a remarkably large electric resistivity, so that the contact resistance between the electrode and punching metal, between punching metal and current collector, and between current collector and separator is significantly increased, which in turn increases the internal resistance of the battery. It becomes difficult to increase the large amount of electric power obtained by the battery reaction to the outside. Further, the elution of the metal material into the molten carbonate is
Not only does the effective thickness of the member decrease, the strength of the component decreases, but the reaction with the molten carbonate causes depletion of the molten carbonate as the electrolyte. Furthermore,
With the progress of these corrosions, cracks may occur in the electrodes and the electrolyte plate, and hydrogen gas on the anode side and oxygen gas on the cathode side may mix with each other to cause explosion noise, which may cause a serious accident.

As described above, the metallic materials used in molten carbonate fuel cells are exposed to extremely severe environments, but SUS316, which is currently commonly used, has neither oxidation resistance nor molten carbonate corrosion resistance. It is enough.

[Problems to be Solved by the Invention] The present invention has been made to solve the above-mentioned problems, and produces a molten carbonate corrosion resistant material having high corrosion resistance to molten carbonate under operating conditions. The purpose is to provide a method of obtaining.

(Structure of Invention) [Means and Actions for Solving Problems] The method for producing a molten carbonate corrosion-resistant material of the present invention contains at least one of pure metal made of iron, nickel or copper, or iron or nickel. Alloy concentration 30-80% by weight
The surface treatment is carried out at a treatment temperature of 100 to 200 ° C.

The object of the method of the present invention is a pure metal or alloy which is a structural member of a molten carbonate fuel cell. Specifically, examples of the pure metal include Fe, Ni, Cu and the like. Further, as the alloy, for example, stainless steel, Fe-based alloy such as Incoloy,
Examples include Ni-based alloys such as Inconel. The method of the present invention is effective for an alloy containing at least one of iron and nickel among these metallic materials.

In the method of the present invention, for example, an alkaline aqueous solution containing at least one of LiOH, NaOH and KOH and having a concentration of 30% by weight or more is heated to 100 ° C. or more. Next, the above-mentioned pure metal or alloy required to have molten carbonate corrosion resistance is immersed in this alkaline aqueous solution.

By this surface treatment, an oxide film mainly containing the constituent elements of pure metal or alloy is formed. For example, the reaction mechanism for Fe is shown below.

Oxidation of iron surface: 2Fe + (3/2) O ₂ → Fe ₂ O ₃ Generation of sodium ferrate: Fe ₂ O ₃ ＋ 4NaOH + (1/2) O ₂ → 2Na ₂ FeO ₄ ＋ 2H ₂ Fe by reduction of sodium ferrate Generation of ₃ O ₄ : 3Na ₂ FeO ₄ + 5H ₂ → Fe ₃ O ₄ + 6NaOH + 2H ₂ O The film thus formed is thinner and denser than the oxide film obtained by high temperature oxidation in the atmosphere.

The coating film as described above serves as a barrier layer that prevents the elution of the metal element from the base material made of a pure metal or an alloy and the reaction between the metal element and the molten carbonate. It should be noted that when the coating film is rough, the action as a barrier layer cannot be sufficiently exhibited, but since the coating film formed by the method of the present invention is dense, it effectively acts as a barrier layer. Therefore, the strength of the structural member is not reduced and the molten carbonate as an electrolyte is not depleted.

Further, the film formed by the method of the present invention, Fe or Ni
Since it is an oxide semiconductor mainly composed of and is extremely thin, electrical conductivity at high temperature is secured. Furthermore, this film reacts with alkali ions in the molten carbonate, that is, Li, for example LiFeO ₂ (lithium ferrite), LiNiO
(Lithium-added nickel oxide) and the like are stabilized, and the electrical conductivity at high temperature is further enhanced. Note that Li
Even if FeO ₂ , LiNiO or the like is generated, the above-mentioned action as the barrier layer is not impaired. Therefore, the contact resistance between the electrode and punching metal, between the punching metal and the current collector, and between the current collector and the separator, which are the main factors of the internal resistance of the molten carbonate fuel cell, can be minimized and high power consumption can be reduced. Can be taken out.

As described above, the concentration of the alkaline aqueous solution used in the method of the present invention is 30% by weight or more, and the treatment temperature is
100 ° C or higher is desirable. This is because when the concentration of the alkaline aqueous solution is less than 30% by weight or when the treatment temperature is less than 100 ° C, it becomes difficult to form a film on the surface of the metal material and the action as a barrier layer cannot be sufficiently exhibited. Is.
More preferably, the concentration of the alkaline aqueous solution is 40% by weight or more, and the treatment temperature is 120 ° C or more. However, if the concentration of the alkaline aqueous solution is too high or if the treatment temperature is too high, the film grows too much, which deteriorates the electrical conductivity and causes cracks in the film to sufficiently act as a barrier layer. It is desirable that the concentration of the alkaline aqueous solution is 80% by weight or less and the treatment temperature is 200 ° C. or less because it may not be possible to exert the effect.

Further, in the method of the present invention, the time for immersing the metal member in the alkaline aqueous solution cannot be unconditionally specified because it varies depending on the concentration of the alkaline aqueous solution and the treatment temperature, but 30 minutes
About 2 hours is desirable. This is because if the treatment time is short, the coating does not grow completely, while if the treatment time is too long, the coating grows more than necessary and cracks occur, which adversely affects the action as a barrier layer and the electrical conductivity. It is to exert.

[Examples] Examples of the present invention will be described below. In the following examples, the electrical resistance was compared with that of NiO, which is the cathode material currently used.

Example 1 First, SUS316 was alkali degreased with an aqueous solution of NaOH, and HCl and H were added.
Pre-treatment was performed by pickling with a mixed acid aqueous solution with NO ₃ . Next, a 50 wt% NaOH aqueous solution was maintained at 145 ° C., and pretreated SUS316 was immersed in this aqueous solution for 1 hour to perform surface treatment.

This SUS316 sample was placed in an O ₂ / CO ₂ (flow ratio 1/2) atmosphere for 6
A corrosion test was performed by semi-immersion in a molten carbonate (Li ₂ CO ₃ : K ₂ CO ₃ = 68: 32) at 50 ° C. and holding for 100 hours. as a result,
The weight loss due to corrosion was 3.0 × 10 ^-2 mg / mm ² or less. Further, the electric resistance was 0.9 with the electric resistance of the NiO sample having the same surface area and the same thickness as 1.

Example 2 First, SUS316 was alkali-degreased with an aqueous solution of NaOH to obtain HCl and H.
Pre-treatment was performed by pickling with a mixed acid aqueous solution with NO ₃ . SUS
After 316 was immersed in a 50% by weight KOH aqueous solution, the temperature was raised to 130 ° C. and maintained for 2 hours for surface treatment.

A corrosion test was performed on the SUS316 sample under the same conditions as in Example 1. As a result, the weight loss due to corrosion is 5.0 × 10
^{It was -2} mg / mm ² or less. The electric resistance was almost the same as that of the NiO sample having the same surface area and thickness.

Comparative Example The same corrosion test as in Example 1 was performed without any treatment on SUS316. As a result, the weight loss due to corrosion is 3.2 × 10
^{It was -1} mg / mm ² .

From the above Examples 1 and 2 and Comparative Example, it can be seen that the surface treatment with the alkaline aqueous solution of the method of the present invention can improve the molten carbonate corrosion resistance of the metal material and can maintain the electrical conductivity. .

(Effects of the Invention) As described in detail above, according to the method of the present invention, a molten carbonate corrosion resistant material having high corrosion resistance to molten carbonate under operating conditions and sufficient electrical conductivity is produced. It is possible.

Claims (2)

(57) [Claims] 1. A pure metal consisting of iron, nickel or copper or an alloy containing at least one of iron and nickel is dipped in an alkaline aqueous solution having a concentration of 30 to 80% by weight for 100 to 2
A method for producing a molten carbonate corrosion resistant material, characterized in that the surface treatment is performed at a treatment temperature of 00 ° C. 2. The molten carbonate corrosion-resistant according to claim 1, wherein the alkaline aqueous solution is an aqueous solution containing at least one of lithium hydroxide, sodium hydroxide and potassium hydroxide. Material manufacturing method.