JP3285708B2 - Oxygen generating electrode - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an improvement in an oxygen generating electrode in an electrochemical cell such as a high-temperature steam electrolyzer or a solid electrolyte oxygen pump.

[0002]

2. Description of the Related Art The operation principle of a high-temperature steam electrolysis cell is shown in FIG. The high-temperature steam electrolysis cell has a structure in which porous electrodes are located on both sides of an oxygen ion conductive solid electrolyte. Generally, several mol of Y ₂ O ₃ is used as a solid electrolyte in a constituent member.
% Solid solution ZrO ₂ is used, and Pt or conductive ceramics is used for the electrode. 900 ° C to 10 ° C
When heated to a high temperature of 00 ° C., steam is sent to one electrode and air is sent to the other electrode, and a voltage is applied between the two electrodes, the steam is electrolyzed on the cathode surface to generate hydrogen gas. Oxygen atoms move as ions in the electrolyte and become oxygen gas on the anode surface. The oxygen pump has the same structure as the high-temperature steam electrolysis cell, and can continuously maintain a certain oxygen partial pressure by moving oxygen from the cathode side to the anode side.

[0003] The efficiency of high temperature steam electrolysis and oxygen pumps is determined by the magnitude of the internal resistance of the cell. That is, when the same voltage is applied to a cell, a cell having a lower internal resistance has a lower overvoltage and can flow a larger current. The overvoltage is divided into two types, a resistance overvoltage due to oxygen ion resistance in the electrolyte and an electrical resistance in the electrode, and a reaction overvoltage due to the electrode reaction, and both greatly depend on the performance of the electrode material and the manufacturing method thereof.

The material of the anode of these devices, that is, the material of the oxygen generating electrode is 900 to 1000 in an oxidizing atmosphere such as air.
Because of exposure to ° C, platinum and conductive ceramics are generally used. There are a sputtering method, a CVD method, a solid phase sintering method and the like as a method of forming an electrode, and the solid phase sintering method is advantageous from the viewpoint of production cost. The solid phase sintering method is a method in which a slurry (or paste) of a material powder is directly baked at a high temperature on an electrolyte.

[0005]

The following two points are required as the performance required for the oxygen generating electrode. Low resistance and electrical resistance (including contact resistance) due to reaction overvoltage. (0.5Ω · cm or less) The resistance does not change even if used for a long time. Although the reaction overvoltage of the oxygen generating electrode using platinum is sufficiently low, there is a problem that the raw material cost is high. On the other hand, a perovskite-type oxide such as La _1-x Sr _x MnO ₃ is often used as the conductive ceramic, but the electrode is easily peeled off by energization, and as a result, the resistance tends to increase. Such deterioration phenomenon is caused by La _1-x Sr
_It occurs only when xMnO ₃ is used as an anode (oxygen generating electrode), and does not occur when it is used as a cathode (air electrode of a fuel cell). The cause of the deterioration of the electrode is considered to be a stress due to oxygen gas generated at the solid electrolyte / electrode interface or a chemical change due to an increase in the oxygen partial pressure near the interface, but it has not been sufficiently clarified.

[0006]

Means for Solving the Problems The present inventors have studied the relationship between the electrode material and the performance of an oxygen generating electrode for high-temperature steam electrolysis or an oxygen pump. As a result, Y ₂ O ₃ stable type Z was added to the powder of La _1-x Sr _x MnO _3.
It has been found that an electrode formed by mixing rO ₂ fine powder to form a film has a low overvoltage and can improve durability. The present invention has been completed on the basis of this finding, and is a water vapor in which an oxygen generating electrode is provided as an anode on one side and a cathode is provided on the other side of Y ₂ O ₃ stable ZrO ₂ which is an oxygen ion conductive solid electrolyte. Oxygen generation in electrolysis cells or oxygen pump cells
A raw electrode comprising La _1-x Sr _x MnO ₃ (x = 0.1
0.4) in an oxygen generating electrode characterized by comprising consist Y ₂ O ₃ stabilized ZrO ₂ from 40 to 60 wt% blended material.

[0007] Regarding the cathode material of a solid oxide fuel cell
LaSrMnO_ThreeTo Y_TwoO_ThreeStable ZrO_TwoTo 0
To 30% by weight (preferably 10 to 20% by weight)
There is a thing which lowers resistance of an electrode by doing. (Japanese Patent Laid-Open No. 3-2
No. 01369) However, these materials are converted to high-temperature steam electricity.
Even if used for oxygen generating electrode of solution or oxygen pump,
Poor durability, peeling off after several hours of electricity, making it impractical
It turns out. Good durability and low enough reaction overvoltage
Y to get oxygen generating electrode_TwoO_ThreeStable ZrO _TwoTo 40
６０60% by weight must be added.

[0008] The oxygen generating electrode of the present invention is used as a conductive material.
Lavskite oxide La_1-xSr _xMnO_ThreeUse
ing. The conductivity of this material increases as the amount of solid solution of Sr increases.
It is known that when x becomes 0.4 or more,
Complex oxides other than lobskite-type oxides may appear
Harumi Yokokawa et al., “Electrochemistry” 5
8, No. 2 (1990), p. 162-171}, x =
0.1 to 0.4 is appropriate. Addition of oxygen generating electrode
The material is Y, which is a solid electrolyte having oxygen ion conductivity._TwoO _ThreeCheap
Standard ZrO_TwoPowder is used. Oxygen ion conductivity
Usually 8mol% Y to raise_TwoO_ThreeY with added_TwoO
_ThreeStable ZrO_TwoIs used.

[0009]

The mixed material of the perovskite-type oxide powder and the solid electrolyte powder according to the present invention adheres firmly to the solid electrolyte substrate during firing, so that separation due to energization can be suppressed. By suppressing peeling, the performance change of the oxygen generating electrode is reduced, and the durability is improved. As shown in Table 1, there is a slight difference in thermal expansion coefficient between the solid electrolyte substrate and the material of the oxygen generating electrode.
The electrode is easily peeled off by thermal stress, but by dispersing the solid electrolyte in the oxygen-generating electrode, the thermal stress can be alleviated and peeling can be suppressed. Further, by increasing the contact area between the electrolyte and the electrode, the overvoltage generated near the solid electrolyte / electrode interface during the operation of steam electrolysis or the oxygen pump is reduced, and the performance is improved.

[Table 1]

[0010]

Examples of the present invention will be described below.

(Examples 1 and 2 and Comparative Examples 1, 2, and 3)
The synthesis of La _0.9 Sr _0.1 MnO ₃ as one material of the present invention was performed by a thermal decomposition method of an aqueous nitrate solution. First, La, Sr,
0.9 mol, 0.1 mol of each nitrate of Mn
, 1 mol was weighed and stirred until it was completely dissolved in 2 liters of distilled water to prepare a nitrate aqueous solution. This aqueous solution was transferred to an evaporating dish, heated in air at 50 ° C. for 5 hours, evaporated and dried, and then calcined at 500 ° C. for 5 hours. After pulverizing the calcined particles with a ball mill for 12 hours, 800 to 120
The main baking was performed at 0 ° C. for 10 hours. Both calcination and main calcination were performed in air. The sample after the main calcination was pulverized again with a ball mill for 12 hours, and the average particle size was 0.5 to 2 μm (SEM
(Observation) perovskite oxide powder was obtained.

The thus obtained perovskite oxide powder has an average particle size of 0.2 μm as an additive (according to SEM observation).
YSZ powder was added in an amount of 40 and 60% by weight, and sufficiently dispersed in an organic solvent (a mixed material of 80% by weight of terpineol and 20% by weight of ethyl cellulose) by a roll mill to form a slurry. (Examples 1 and 2) As a comparative example, YS
A slurry was prepared by mixing 10% and 20% by weight with a slurry to which no Z powder was added. (Comparative Examples 1, 2, 3) The ratio between the material powder and the organic solvent was 1: 1 by weight. The slurry is applied on an electrolyte substrate and dried at room temperature for 12 hours.
Further, terpineol was evaporated at a temperature of 50 to 100 ° C. for about 1 hour in a dryer. The electrolyte substrate was transferred to an electric furnace and fired in air at 1200 to 1300 ° C. for 2 hours.

Comparative Example 4 A commercially available platinum powder was mixed with 10% by weight of YSZ powder to form a slurry, and a platinum electrode was formed in the same manner as described above. After firing, the counter electrode on the back side of the electrode film,
A reference electrode was formed on the side surface using a platinum paste, and each electrode was collected with a platinum mesh and a platinum lead wire. FIG. 3 shows the structure of the measuring cell. The measurement of the reaction overvoltage was performed by a current interruption method while keeping the cell at 1000 ° C. The current interruption method is a measurement method of instantaneously interrupting a current flowing through a cell and quantifying an overvoltage due to a reaction and an overvoltage due to an ohmic resistance from a voltage change at that time. In the durability test of the electrode performance, a positive voltage was applied to the measurement electrode and a negative voltage was applied to the counter electrode, and a voltage of 0.2 A / cm ² was applied.
The measurement was performed by keeping the electric current flowing through and recording the potential of the measurement electrode with respect to the reference electrode.

Table 2 shows the measured values of the reaction overvoltage when a current of 0.2 A / cm ² was applied to each electrode shown in Examples and Comparative Examples. FIG. 1 shows the relationship between the mixing amount of the YSZ powder in the La _0.9 Sr _0.1 MnO ₃ electrode and the reaction overvoltage of each electrode. FIG. 2 shows the change over time of the potential of the oxygen generating electrode with respect to the reference electrode when a current of 0.2 A / cm ² is applied to the oxygen generating electrode. The potential in FIG. 2 is the sum of the reaction overvoltage of the oxygen generating electrode and the resistance overvoltage of the solid electrolyte. It can be seen that the electrode with a small amount of YSZ fine powder added deteriorates with the energization time and the reaction overvoltage increases. . From these results, YSZ fine powder to be added to the La _0.9 Sr _0.1 MnO ₃ electrode in order to suppress the reaction overvoltage when applying 0.2 A / cm ² for a long time to 100 mV or less (internal resistance: 0.5 Ω or less). Was found to be about 40 to 60% by weight.

[Table 2] * Reaction overvoltage 12 hours 0.2 A / cm ² when energized after the energization in the reaction overvoltage ** 0.2 A / cm ² at 0.2 A / cm ² current at the start of energization

[0015]

According to the present invention, an electrode having low overvoltage and good durability can be manufactured by using a material which is less expensive than a platinum-based material. Also, since the electrode is formed by slurry coating, CVD
As compared with film forming methods such as PVD and PVD, capital investment and manufacturing costs are low, automation is easy, and it is suitable for mass production.

[Brief description of the drawings]

FIG. 1 is a table showing the relationship between the amount of YSZ powder added to an electrode and current-reaction overvoltage characteristics.

FIG. 2 shows the addition amount of YSZ powder in an electrode and 0.2 A / cm
² is a chart showing a temporal change in overvoltage when current is applied.

FIG. 3 is a structural diagram of a cell used for measuring electrode performance.

FIG. 4 is a principle diagram of a high-temperature steam electrolysis cell.

──────────────────────────────────────────────────続 き Continuation of the front page (56) References JP-A-64-14872 (JP, A) (58) Fields investigated (Int. Cl. ⁷ , DB name) C25B 1/00-15/08

Claims (1)

(57) [Claims] 1. An oxygen ion conductive solid electrolyte, Y ₂
O ₃ oxygen generating electrode as stable anode on one side of the ZrO _2, an oxygen generating electrode in steam electrolytic cell or oxygen pump cell having a cathode on the other side, La _1-X Sr
_X MnO ₃ (x = 0.1-0.4) to Y ₂ O ₃ stable type Z
oxygen generating electrode characterized by comprising consists of and rO ₂ from 40 to 60 wt% blended material.