JP3852177B2 - Method for treating cathode for molten carbonate fuel cell - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a method for treating a cathode for controlling the amount of carbonate impregnated in a cathode used in a molten carbonate fuel cell to improve cell performance.
[0002]
[Prior art]
In molten carbonate fuel cells, an electrolyte plate (tile) in which molten carbonate is impregnated into a porous matrix plate as an electrolyte is sandwiched between both cathode (oxygen electrode) and anode (fuel electrode) electrodes. A cell in which a reaction is performed on the cathode side and the anode side by supplying an oxidizing gas to the anode side and a fuel gas to the anode side is made into one cell, and each cell is stacked through a separator to form a stack. It is like that.
[0003]
Among the electrodes of the molten carbonate fuel cell, a cathode mainly composed of nickel oxide (NiO) is conventionally made of 1 to 5% by weight of Ni powder as a raw material powder by a powder metallurgy technique. A binder, also 1 to 5% by weight of a dispersing agent, 1 to 5% by weight of a pore-forming agent, mixed with water to form a slurry, molded into a plate shape, dried, and then reduced to a temperature of 750 ° C. in a reducing atmosphere. It is made to calcinate by making it a Ni metal porous body.
[0004]
When assembling the molten carbonate fuel cell, a carbonate powder is dispersed and placed on a matrix plate constituting an electrolyte plate, and is sandwiched between both cathode and anode electrodes. In this case, the temperature is raised to a temperature higher than the melting point of the carbonate, generally 500 ° C. or higher by flowing an inert gas before the start of operation, and this initial temperature rise is performed under a load that brings these laminates into contact with each other. Occasionally, an impregnation step in which carbonate is impregnated into the matrix plate is performed, and then the cathode is oxidized by flowing air to the cathode side.
[0005]
[Problems to be solved by the invention]
However, when the cathode is oxidized in the battery, depending on the concentration of the oxidizing gas and the heating temperature, the pore structure as the microstructure changes greatly, the porosity decreases, and the pore diameter decreases from 0.1 to 5 μm. As the amount of carbonate in the cathode, that is, the impregnation rate as a volume percentage of the carbonate present in the gap of the cathode, does not match the initial setting value, the battery performance deteriorates. Cause problems. That is, the relationship between the maximum occupied carbonate diameter in the cathode and the overvoltage including Nernstross (hereinafter referred to as overvoltage) in which the carbonate occupies from the small holes in the cathode is generally as shown in FIG. It can be seen that the overvoltage rises sharply when the carbonate is occupied to a large diameter of 4.5 μm or more in pore diameter. However, the overvoltage here means “battery no-load voltage” − “battery voltage under current load when a nearly infinite amount of gas flows” − “voltage loss due to battery internal resistance”. The smaller the value, the better the battery performance (electrode performance). Nernstross can be considered constant when gas conditions and operating conditions (load current, temperature, etc.) are constant.
[0006]
Further, the cathode mainly composed of NiO as described above has a large specific surface area (surface area per unit weight) and a large surface area for dissolving NiO in the molten carbonate by reaction with carbon dioxide in the oxidizing gas. There is a large amount of elution into the molten carbonate, and there is a big problem related to the battery life. In other words, the cathode mainly composed of NiO has NiO + CO ₂ → Ni ²⁺ + CO ₃ ²⁻ due to reaction with carbon dioxide (CO ₂ ) in the oxidizing gas.
In the case of the above-described conventional cathode, since the surface area to be dissolved is large, the amount of dissolution due to the dissolution reaction is large. .
[0007]
The eluted Ni ²⁺ is reduced in the molten carbonate of the electrolyte plate by hydrogen diffused from the anode side and deposited as metal Ni, and the deposited metal Ni causes a short circuit between the cathode and the anode. The battery current is consumed inside the battery, resulting in a problem that the power generation efficiency of the battery is lowered. Also, since the shape is changed to Ni by the reduction reaction in the electrolyte plate, Ni in the carbonate ²⁺ ions do not saturate in the carbonate, and the elution of Ni from the surface of the cathode into the carbonate continues, so the cathode is thinned during battery operation and the pore structure is roughened. There is a problem that the battery life is shortened because it is weakened and is destroyed by the force of compression during battery operation.
[0008]
Incidentally, in the patent publication of Japanese Patent No. 2514748, as a method for starting a molten carbonate fuel cell, a procedure for introducing a gas containing H ₂ to the anode side at the time of starting the cell is shown. However, there is a description that the cathode is introduced with hydrogen at a low temperature, purged with nitrogen and then the oxidizing gas is introduced, but the oxidation treatment of the cathode is not shown.
[0009]
Therefore, the present invention provides a method for treating a molten carbonate fuel cell cathode, which is capable of reducing a change in microstructure and reducing a decrease in porosity even when an oxidation treatment is performed. Is to be able to improve.
[0010]
[Means for Solving the Problems]
In order to solve the above-mentioned problems, the present invention provides a Li / K · CO ₃ binary carbonate having an Li component of 62 mol % or more on the matrix plate constituting the electrolyte plate , or an Li component of 52 mol %. A carbonate powder having a composition with more Li component than the above Li / Na · CO ₃ binary carbonate and a composition in which solid phase Li ₂ CO ₃ does not coexist at the operating temperature of the fuel cell is dispersed and placed. Assemble the fuel cell in a state where Ni is held between the cathode and the anode made of a porous Ni metal, and heat the carbonate to 570 to 650 ° C. in an inert gas atmosphere and melt it before starting operation. For the molten carbonate fuel cell, the cathode is impregnated, and then the cathode is maintained at 570 to 650 ° C. in an oxidizing gas atmosphere having an O ₂ concentration of 20% or less to oxidize before starting operation. Cathode The processing method.
[0011]
Since the cathode is maintained at 570 to 650 ° C. in an oxidizing gas atmosphere with an O ₂ concentration of 20% or less using a carbonate having a composition with a large amount of Li component, the structure is oxidized before the start of operation. A cathode with little reduction in rate is obtained.
[0012]
As the carbonate, the Li component exceeds 62 mol% , and the upper limit is Li / K · CO ₃ binary carbonate having a composition in which the solid phase Li ₂ CO ₃ does not coexist at the operating temperature of the fuel cell , or the Li component is It is preferable to use Li / Na · CO ₃ binary carbonate having a composition exceeding 52 mol % and having an upper limit of the composition at which the solid phase Li ₂ CO ₃ does not coexist at the operating temperature of the fuel cell .
[0013]
DETAILED DESCRIPTION OF THE INVENTION
Hereinafter, embodiments of the present invention will be described with reference to the drawings.
[0014]
FIG. 1 shows a procedure of a method for treating a cathode for a molten carbonate fuel cell according to the present invention.
[0015]
When the cathode is oxidized in the battery, first, a carbonate powder is dispersed and placed on the matrix plate constituting the electrolyte plate, and this is sandwiched between the cathode and anode electrodes. (Battery assembly process I).
[0016]
The cathode is an Ni metal porous body, an MgO-added Ni metal porous body, or an Fe ₂ O ₃ -added Ni metal porous body. As the carbonate, as many compositions of Li component, in the case of L i / K · CO ₃ binary carbonate, Li component and 62 mol% or more, Li / Na · CO ₃ binary carbonate In this case, a carbonate having a Li component of 52 mol % or more is used, and the upper limit is a composition in which solid phase Li ₂ CO ₃ does not coexist at the operating temperature of the fuel cell. In the case of Li / K · CO ₃ binary carbonate, the upper limit is 90 mol% at 700 ° C., 83 mol% at 650 ° C., 77 mol% at 600 ° C., 69 mol% at 550 ° C., and Li / Na · CO _In the case of ternary carbonates, the upper limit is 90 mol% at 700 ° C., 80 mol% at 650 ° C., 70 mol% at 600 ° C., and 60 mol% at 550 ° C.
[0017]
In the above state, before operating the fuel cell, air or air + N ₂ is flowed to the cathode side in advance as an oxidizing gas having an O ₂ concentration of 20% or less, and carbonate in an oxidizing gas atmosphere. The temperature is raised to 400 to 450 ° C., which is a temperature at which the electrolyte does not melt, so that the binder contained in the matrix plate constituting the electrolyte plate is removed (binder removal step II).
[0018]
Next, the oxidizing gas is purged with N ₂ as an inert gas, and heated to 570 to 650 ° C. which is a temperature equal to or higher than the melting point of the carbonate in an inert gas atmosphere to melt the carbonate, The molten carbonate is impregnated into the matrix plate constituting the electrolyte plate and the pores of the cathode (carbonate impregnation step III).
[0019]
Thereafter, as an oxidizing gas having an O ₂ concentration of 20% or less, air, air + N ₂ , or air + N ₂ + CO ₂ is flowed to the cathode side to purge the inert gas, and the above heating is performed in an oxidizing gas atmosphere. The cathode is totally oxidized while maintaining a temperature of 570 to 650 ° C. (oxidation step IV).
[0020]
In the present invention, a carbonate having a composition with a large amount of Li component is used to oxidize the cathode at a high temperature of 570 to 650 ° C. in an oxidizing gas atmosphere having an O ₂ concentration of 20% or less. As can be seen, the change in the cathode microstructure is reduced and the porosity is also reduced. Therefore, a cathode having a carbonate impregnation rate close to a set value can be obtained, the deterioration rate of the cathode can be slowed, and battery performance can be improved.
[0021]
In addition, as a result of the reduction of the carbonate impregnation rate to the cathode, the Ni elution amount is reduced, so that the cathode is not thinned during operation, and the elution amount of the cathode to the outside of the battery is reduced. Therefore, the life of the battery can be extended.
[0022]
In the above case, since the binder removal step II is performed in an oxidizing gas atmosphere before the carbonate impregnation step III, the cathode is oxidized, but the O ₂ concentration is 20% or less and 400 to 450 ° C. Since the temperature is low, the surface is only oxidized by a few percent, and there is no problem as a whole.
[0023]
In the above, since the melting point of the carbonate increases as the Li component increases, the oxidation temperature can be increased. When the oxidation temperature is high, all the carbonate can be dissolved and distributed throughout.
[0024]
【Example】
Next, the results of experiments conducted by the inventor will be described.
An oxidation test was conducted in the presence of carbonate in the atmosphere. The porous Ni—MgO sample was placed in an alumina crucible together with carbonate, placed in a furnace previously set at a predetermined temperature, held for a predetermined time, and then the sample with the crucible was taken out and air-cooled. Oxidation conditions varied holding temperature and holding time. The holding temperature was from 520 ° C. to 620 ° C. every 50 ° C. The carbonate was investigated for a 62/38 mol ratio or a 70/30 mol ratio in the Li ₂ CO ₃ / K ₂ CO ₃ system. As the evaluation of the porous sample, thickness change, porosity change, porous pore size distribution change, and porous specific surface area change were measured, and the phase was identified by X-ray diffraction.
[0025]
FIG. 2 shows the thickness change rate (%) of a sample oxidized 100% in the atmosphere . The vertical axis represents the rate of change in thickness from the initial thickness, and the horizontal axis represents the oxidation holding temperature. In the figure, ○ mark carbonate mol composition 70/30, ● mark is when the carbonate mol composition was allowed to coexist with the sample 62/38.
[0026]
FIG. 3 shows the porosity (%) of a sample that is 100% oxidized in the atmosphere, with the porosity of the sample on the vertical axis and the oxidation holding temperature on the horizontal axis. In the figure, ○ mark carbonate mol composition 70/30, ● mark is when the carbonate mol composition was allowed to coexist with the sample 62/38.
[0027]
Figure 4 shows the median pore diameter (mu m) of the sample oxidized 100% in the air, the median pore diameter of the sample on the vertical axis, taking the oxidation retention time on the horizontal axis. In the figure, ○ mark carbonate mol composition 70/30, ● mark is when the carbonate mol composition was allowed to coexist with the sample 62/38.
[0028]
FIG. 5 shows the surface area per unit weight (specific surface area) (m ² / g) of a sample oxidized 100% in the atmosphere, with the specific surface area of the sample on the vertical axis and the oxidation retention temperature on the horizontal axis. This is a case where the carbonate mol composition is 70/30 coexisting with the sample. In the figure, the x mark is 520 ° C., the □ mark is 570 ° C., and the Δ mark is 620 ° C.
[0029]
Figure 6 shows the pore diameter (mu m) is 2μm or less of the pore existence rate of the samples oxidized 100% in air (pore volume existence ratio) (%) and the ordinate the pore existence rate of the sample In addition, the oxidation retention temperature is plotted on the horizontal axis. In the figure, ○ mark carbonate mol composition 70/30, ● is when carbonates mol composition was allowed to coexist with the sample 62/38. Further, FIG. 7 is a similarly shows the pore diameter (mu m) is 4μm or less of pore existence rate of the samples oxidized 100% in air (pore volume existence ratio) (%), pore existence rate of the sample Is on the vertical axis, and the oxidation retention temperature is on the horizontal axis. In the figure, ○ mark carbonate mol composition 70/30, ● mark is when the carbonate mol composition was allowed to coexist with the sample 62/38.
[0030]
From the results of the experiments shown in FIGS. 2 to 7 , when the cathode is oxidized at a high temperature of 570 ° C. or higher, the thickness change changes thickly, the porosity does not change much, the pore diameter increases, and the specific surface area increases. As a result, the vacancy rate was small and the battery performance was excellent overall.
[0031]
【The invention's effect】
As described above, according to the method for treating a cathode for a molten carbonate fuel cell of the present invention, a Li / K · CO ₃ binary system having a Li component of 62 mol % or more on a matrix plate constituting an electrolyte plate. Carbonate or a composition with more Li component than Li / Na · CO ₃ binary carbonate having a Li component of 52 mol % or more and a composition in which solid phase Li ₂ CO ₃ does not coexist at the operating temperature of the fuel cell A fuel cell is assembled in a state where a carbonate powder is dispersed and placed, and this is sandwiched between both cathode and anode electrodes made of a Ni metal porous body, and carbonate is introduced in an inert gas atmosphere before starting operation. Is heated to 570 to 650 ° C. and melted to impregnate the cathode. After that, the cathode is maintained at 570 to 650 ° C. in an oxidizing gas atmosphere with an O ₂ concentration of 20% or less, and oxidized before starting operation. Letting Since it is possible to obtain a cathode having a carbonate impregnation rate close to a set value with little change in the microstructure and little decrease in the porosity, battery performance can be improved, Further, since the rate of impregnation of the carbonate into the cathode can be reduced, an excellent effect that the elution amount of Ni can be reduced and the life of the battery can be extended is exhibited.
[Brief description of the drawings]
FIG. 1 is a block diagram of a work procedure showing a method for treating a cathode for a molten carbonate fuel cell according to the present invention.
FIG. 2 is a diagram of an experimental result showing a relationship between a cathode thickness change rate and an oxidation temperature.
FIG. 3 is a diagram of experimental results showing the relationship between the porosity of the cathode and the oxidation temperature.
FIG. 4 is a diagram of experimental results showing the relationship between the median pore diameter of the cathode and the oxidation temperature.
FIG. 5 is a diagram of experimental results showing the relationship between the specific surface area of the cathode and the oxidation temperature.
FIG. 6 is a diagram of experimental results showing the relationship between the presence rate of pores having a cathode pore size of 2 μm or less and the oxidation temperature.
FIG. 7 is a diagram of experimental results showing the relationship between the presence rate of pores having a cathode pore size of 4 μm or less and the oxidation temperature.
FIG. 8 is a diagram showing the relationship between the maximum occupied carbonate diameter in the cathode and the overvoltage.
[Explanation of symbols]
I Battery assembly process
II Binder removal process
III Carbonate impregnation process
IV Oxidation process

Claims (3)

On the matrix plate constituting the electrolyte plate, Li / K · CO ₃ binary carbonate having an Li component of 62 mol % or more , or Li / Na · CO ₃ binary carbonate having an Li component of 52 mol % or more Carbonate powder having a composition with more Li component than salt and a composition in which solid phase Li ₂ CO ₃ does not coexist at the operating temperature of the fuel cell is dispersed and placed, and this is made into a Ni metal porous body for the cathode and anode. The fuel cell is assembled in a state of being held between both electrodes, and before starting operation, carbonate is heated to 570 to 650 ° C. in an inert gas atmosphere to melt and impregnate the cathode, and then O _2. A method of treating a cathode for a molten carbonate fuel cell, characterized in that the cathode is maintained at 570 to 650 ° C. in an oxidizing gas atmosphere having a concentration of 20% or less and is oxidized before the start of operation. Li component on the matrix plate constituting the electrolyte plate 62 mol% greater, and the upper limit of the composition is solid Li ₂ CO ₃ at the operating temperature of the fuel cell does not coexist Li / K · CO ₃ binary carbonate powder The fuel cell is assembled in a state in which it is placed between the cathode and anode electrodes made of Ni metal porous body, and before the start of operation, the carbonate is 570 to 650 in an inert gas atmosphere. The cathode is heated to melt at 5 ° C. and impregnated in the cathode, and then the cathode is maintained at 570 to 650 ° C. in an oxidizing gas atmosphere having an O ₂ concentration of 20% or less to oxidize before starting operation. A method for treating a cathode for a molten carbonate fuel cell. Li component on the matrix plate constituting the electrolyte plate 52 mol% greater, and the upper limit of the composition is solid _Li 2 CO ₃ at the operating temperature of the fuel cell does not coexist Li / Na · CO ₃ binary carbonate powder The fuel cell is assembled in a state in which it is placed between the cathode and anode electrodes made of Ni metal porous body, and before the start of operation, the carbonate is 570 to 650 in an inert gas atmosphere. The cathode is heated to melt at 5 ° C. and impregnated in the cathode, and then the cathode is maintained at 570 to 650 ° C. in an oxidizing gas atmosphere having an O ₂ concentration of 20% or less to oxidize before starting operation. A method for treating a cathode for a molten carbonate fuel cell.

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