JP4771327B2 - Solid oxide fuel cell - Google Patents

Description

  The present invention relates to a solid oxide fuel cell, and more particularly to a solid oxide fuel cell capable of efficient operation at a low temperature.

  In recent years, fuel cells have attracted attention as clean energy sources that can perform high energy conversion and are friendly to the global environment. Among various fuel cells, the solid oxide fuel cell has advantages such as high operating efficiency due to its high operating temperature and low cost without using expensive materials such as noble metals.

  A general solid oxide fuel cell uses oxide ceramics having oxide ion conductivity as an electrolyte, and oxide ions that have moved from the air electrode side to the fuel electrode side through the electrolyte are fuel. React with. In other types of fuel cells, such as solid polymer type and phosphoric acid type, hydrogen in the fuel becomes ions and moves in the electrolyte and reacts with oxygen on the oxygen electrode side. In solid oxide fuel cells, Since the oxide ions move through the electrolyte and react with the fuel on the fuel electrode side, the fuel used for the reaction may be any material that can be oxidized by the oxide ions. Therefore, in the solid oxide fuel cell, not only hydrogen but also hydrocarbon fuel such as methane and carbon monoxide can be used as fuel.

  The solid oxide fuel cell having the advantages as described above is usually operated at a high temperature of about 800 to 1000 ° C. in order to ensure the ionic conductivity of the electrolyte. Therefore, high heat resistance is required not only for the fuel cells but also for the entire fuel cell system such as piping, which is one of the factors that hinder cost reduction. Moreover, since warm-up operation for a long time is required at the time of start-up and the start-stop property is poor, there are many adverse effects due to high operation temperature, such as being limited to applications that do not involve frequent start-stop.

  Therefore, in order to operate the solid oxide fuel cell at a low temperature, research for ensuring oxide ion conductivity in the electrolyte at a temperature lower than that in the past has been conducted. This is possible by reducing the thickness of the electrolyte layer. However, in the conventional solid oxide fuel cell, the electrolyte layer is generally formed on the porous electrode. It was difficult to form a thin electrolyte layer.

  When a film having a thickness equal to or less than the diameter of the opening is formed on the porous body having the opening, a part of the film enters the opening, and a recess is formed in the film. May be damaged. Such damage to the membrane causes problems such as gas cross-leak in the fuel cell, leading to a decrease in power generation performance of the fuel cell. In addition, if the porosity of the porous body of the electrode serving as the substrate is reduced in order to form a thin electrolyte layer, the gas permeability of the electrode is reduced and the power generation efficiency of the fuel cell is also reduced. A thick electrolyte layer was formed on the porous body having a high thickness.

  Patent Document 1 discloses a solid oxide fuel cell in which an electrolyte layer is formed on a gas permeable dense layer having an opening and / or void of 10 μm or less on the surface. However, the dense layer has openings and / or voids for allowing the oxidant gas or fuel gas to pass therethrough, and a thin film (for example, about 1 μm) is formed on such a layer having an opening of about 10 μm. Is extremely difficult to form.

JP 2004-119108 A JP 2003-331866 A

  The present invention has been made in view of the above problems, and a main object of the present invention is to provide a solid oxide fuel cell that can be operated at a low temperature and has excellent start / stop characteristics. .

  In order to achieve the above object, the present invention provides an electrolyte layer having oxide ion conductivity, a first electrode layer disposed on one side of the electrolyte layer, and a first electrode disposed on the other side of the electrolyte layer. A solid oxide fuel cell comprising at least two electrode layers, comprising a dense layer provided on the electrolyte layer side of the second electrode layer and having oxide ion conductivity and airtightness An oxide fuel cell is provided.

  In the present invention, since the dense layer having airtightness is a dense layer having no opening, by using this dense layer as a substrate for forming the electrolyte layer, the electrolyte layer is thin and has no defects. Can be formed. Further, since the dense layer has oxide ion conductivity, it can be used without impairing the oxide ion conductivity between the electrolyte layer and the electrode layer.

  Moreover, in the said invention, it is preferable that the said 2nd electrode layer is a porous body, and the film thickness of the said dense layer is larger than the aperture diameter of the said porous body. By forming the dense layer with such a film thickness, defects or the like of the dense layer due to the opening of the lower porous body can be prevented.

  Furthermore, in the said invention, it is preferable that the said 1st electrode layer is a fuel electrode containing a metal, and is provided with a reaction suppression layer between the said electrolyte layer and the said 1st electrode layer. By providing the reaction suppression layer, it is possible to suppress the reaction between the metal that constitutes the fuel electrode and the electrolyte that constitutes the electrolyte layer, and the deterioration of the electrolyte layer can be suppressed.

  ADVANTAGE OF THE INVENTION According to this invention, it is possible to drive the solid oxide fuel cell which can use various fuels at low temperature, and the use of a solid oxide fuel cell is expanded, and cost is suppressed. There is an effect that can be.

  In the solid oxide fuel cell (hereinafter sometimes simply referred to as a fuel cell), the present invention forms an electrolyte layer on a dense layer having oxide ion conductivity and airtightness. The thin film is intended to operate the fuel cell at a low temperature.

Hereinafter, the fuel cell of the present invention will be described with reference to the drawings.
FIG. 1 is a schematic sectional view showing an example of the configuration of the fuel cell of the present invention. As shown in FIG. 1, in the fuel cell 1 of the present invention, an electrolyte layer 4 is formed on a dense layer 3 provided on the second electrode layer 2. A reaction suppression layer 5 is disposed on the electrolyte layer 4, and a first electrode layer 6 (a fuel electrode in this example) is disposed on the reaction suppression layer 5.

  In the present invention, the dense layer is a dense layer having no opening. Therefore, by forming an electrolyte layer on the dense layer, the electrolyte is significantly thinner than conventional ones and has no defects. Layers can be easily formed. In the power generation reaction, when the electrolyte layer is thick, the oxide ions move a long distance in the electrolyte layer unless the operating temperature of the fuel cell is raised to maintain high oxide ion conductivity in the electrolyte layer. I can't. However, in an electrolyte layer with a small film thickness, such as the electrolyte layer in the present invention, the moving distance of oxide ions in the electrolyte layer is short, and it is possible to sufficiently ensure oxide ion conductivity at a temperature lower than the conventional one. Therefore, the operating temperature of the fuel cell can be lowered.

By operating the fuel cell at a low temperature, it is possible to suppress heat loss when starting and stopping. In addition, since the time required for starting and stopping can be greatly shortened and the starting and stopping characteristics of the fuel cell can be improved, the use of the fuel cell can be expanded. Furthermore, if the operating temperature of the fuel cell is low, the heat resistance required for the members constituting the fuel cell system is alleviated, so that the material selectivity is widened and the fuel cell system can be manufactured at a lower cost. In addition, the fuel cell can be miniaturized by reducing the thickness of the electrolyte layer.
Hereafter, each structural member which comprises such a fuel cell of this invention is each demonstrated separately.

1. Dense layer In the present invention, the dense layer is not particularly limited as long as it has oxide ion conductivity and airtightness. In the present invention, the dense layer only needs to move the oxide ions generated at the air electrode of the fuel cell to the electrolyte layer, or move the oxide ions that have moved in the electrolyte layer to the fuel electrode. Gas permeability is not required. Therefore, there is no need to have an opening that allows gas to pass through like an electrode, so that a dense film having airtightness can be used as the dense layer.

  The dense layer used in the present invention only needs to have oxide ion conductivity equal to or higher than that of the electrolyte layer used together, and the numerical value of the oxide ion conductivity is not particularly limited. The dense layer has airtightness. Here, having airtightness means that gas does not pass at all under a differential pressure of about 0.1 atm. Furthermore, the dense layer is a dense layer with very few openings, and even when there are openings, the opening diameter is preferably 10 μm or less, and more preferably 1 μm or less.

The material for forming such a dense layer is not particularly limited as long as it has oxide ion conductivity, and a material having electron conductivity can also be used. For example, La (Sr) Ga (Me) O ₃ (where Me is Co, Fe, Cr or Ni), La (Sr) Co (Fe) O ₃ , La (Sr) Fe (Co) O ₃ , Ba ( La) CoO ₃ , La ₂ NiO ₄ , SrFeO _3, or the like generally called a mixed conductor can be used. In particular, those having high oxide ion conductivity at low temperatures are often mixed conductors. Among these, La (Sr) Ga (Fe) O ₃ , La (Sr) Ga (Co) O ₃ and La (Sr) Co (Fe) O ₃ are preferably used, and particularly La (Sr) Ga (Fe). ) O ₃ is preferred.

  It is known that the mixed conductor exhibits oxide ion conductivity at a temperature lower than that of an electrolyte generally used in a solid oxide fuel cell. However, since the mixed conductor has conductivity as well as oxide ion conductivity, it cannot be used as a material for forming an electrolyte membrane of a fuel cell. Therefore, in the present invention, the dense layer is formed using the mixed conductor having the above characteristics, and the electrolyte layer is formed on the dense layer, thereby enabling the electrolyte layer to be thinned.

  As described above, since the mixed conductor exhibits oxide ion conductivity at a low temperature, even if a dense layer made of the mixed conductor is formed between the electrode layer and the electrolyte layer of the fuel cell, There is no loss of oxide ion conductivity. In addition, since an insulating electrolyte layer is disposed between the two electrode layers of the fuel cell, a conductive layer can be used as a dense layer without causing a short circuit between the electrode layers. Therefore, a conductive material can also be used as a material for forming the dense layer.

  The film thickness of the dense layer is not particularly limited as long as the surface smoothness suitable for the formation of the electrolyte layer can be ensured, but it is the lower layer of the dense layer and the pore diameter of the electrode layer that is a porous body Is preferably larger. This is because by forming a dense layer with a film thickness larger than the pore diameter of the lower porous body, a uniform dense layer can be formed without defects and the like. Specifically, the film thickness of the dense layer is preferably in the range of 10 to 500 μm, more preferably in the range of 10 to 100 μm, and particularly preferably in the range of 50 to 100 μm. The dense layer can also be used as a substrate on which an electrolyte layer is formed. Alternatively, the first electrode layer or the second electrode layer may be used as the substrate, and the dense layer and the electrolyte layer may be formed thereon. .

  The method for forming the dense layer is not particularly limited as long as the dense layer can be formed with a desired film thickness, and the general method for forming a film is similar to the method used for forming the electrolyte layer. It can be formed by a general method. For example, sputtering, EB vapor deposition, ion plating, plasma spraying, AD (aerosol deposition), gas deposition, laser ablation and other PVD methods (physical vapor deposition), CVD ( Chemical vapor deposition method), a wet method such as a screen printing method, a slurry coating method, a spray method, a sol-gel method, a green compact method, a solid phase reaction method, or the like.

2. Electrolyte layer In the present invention, the electrolyte layer can be formed thin by using the dense layer as described above and forming the electrolyte layer on the dense layer. The thickness of the electrolyte layer at this time is not particularly limited, but is preferably in the range of 0.5 to 10 μm, more preferably in the range of 1 to 10 μm, and particularly preferably in the range of 1 to 5 μm. By setting the thickness of the electrolyte layer within the above range, it becomes possible to operate the fuel cell at a low temperature, diversifying the use of the solid oxide fuel cell that can use various fuels, and reducing the cost. Reduction can be realized. It is also possible to reduce the size of the fuel cell.

The material for forming such an electrolyte layer is not particularly limited as long as it has oxide ion conductivity, insulating properties, and gas impermeability, and is generally used for solid oxide fuel cells. The materials that are used can be used. Examples of materials that can be used include LaGaO ₃ such as La (Sr) Ga (Mg) O ₃ , La (Sr) Ga (Mg, Me) O ₃ (where Me is Co, Fe, Cr, or Ni). Solid solution of perovskite, neodymium oxide (Nd ₂ O ₃ ), samarium oxide (Sm ₂ O ₃ ), yttria (Y ₂ O ₃ ), scandium oxide (Sc ₂ O ₃ ), gadolinium oxide (Gd ₂ O ₃ ), etc. And stabilized zirconia, ceria (CeO ₂ ) -based solid solution, bismuth oxide solid solution, and the like. Of these, La (Sr) Ga (Mg) O ₃ and La (Sr) Ga (Mg, Me) O ₃ , particularly La (Sr) Ga (Mg) O _3, are preferably used.

In addition, the dense layer and the electrolyte layer used together are preferably close in crystal structure to the material forming them. For example, when an electrolyte layer formed of La (Sr) Ga (Mg) O ₃ is used, La (Sr) Ga (Me) O ₃ (having a crystal structure close to La (Sr) Ga (Mg) O ₃ ) ( Here, it is preferable to use a dense layer formed of Me, Co, Fe, Cr, or Ni. It can also be used for LaFeO ₃ system.

  The method for forming the electrolyte layer is not particularly limited, and can be formed by a general method used for forming the electrolyte layer. For example, PVD methods (physical vapor deposition methods) such as sputtering, EB vapor deposition, ion plating, plasma spraying, AD (aerosol deposition), gas deposition, laser ablation, CVD ( Chemical vapor deposition method) and wet methods such as a screen printing method, a slurry coating method, a spray method, and a sol-gel method.

3. First electrode layer and second electrode layer In the present invention, the first electrode layer may be a fuel electrode, the second electrode layer may be an air electrode, the first electrode layer may be an air electrode, The electrode layer may be a fuel electrode. When a dense layer, an electrolyte layer, and a fuel electrode are formed on the air electrode, oxide ions generated in the air electrode move in the dense layer and then in the electrolyte layer, reach the fuel electrode, and become fuel. react. On the other hand, when a dense layer, an electrolyte layer, and an air electrode are formed on the fuel electrode, oxide ions generated in the air electrode move in the electrolyte layer and then in the dense layer and reach the fuel electrode. Reacts with fuel.
Hereinafter, the fuel electrode and the air electrode used as the first electrode layer or the second electrode layer in the present invention will be described.

a. Fuel Electrode The fuel electrode used in the present invention is not particularly limited, and those used in ordinary solid oxide fuel cells can be used. For example, from cermets such as Ni, Ni—Fe + Ce (Sm) O ₂ , Ni—Ce (Gd) O ₂ , Ni—Ce (Sm) O ₂ , Ni—La (Sr) Ga (Fe) O ₃ , platinum, etc. Those formed can be used, and among these, those formed from cermets of Ni or Ni—Fe + Ce (Sm) O ₂ are preferably used. The film thickness of the fuel electrode is not particularly limited, but is usually in the range of 10 to 50 μm, preferably in the range of 10 to 20 μm.

b. Air electrode The air electrode used in the present invention is not particularly limited, and those used in ordinary solid oxide fuel cells can be used. For example, those formed from La (Sr) CoO ₃ , Sm (Sr) CoO ₃ , La (Sr) MnO ₃ , Ba (La) CoO _3, etc. can be used, and above all, formed from Sm (Sr) CoO _3. What was made is used suitably. Although the film thickness of an air electrode is not specifically limited, Usually, the thing within the range of 1-500 micrometers, Preferably it is within the range of 10-100 micrometers is used.

4). Reaction Suppression Layer In the present invention, a reaction suppression layer may be provided between the fuel electrode and another component adjacent to the fuel electrode. That is, when the dense layer is provided on the air electrode side of the electrolyte layer, between the fuel electrode and the electrolyte layer, when the dense layer is provided on the fuel electrode side of the electrolyte layer, the fuel electrode and the dense layer are provided. The reaction suppression layer can be provided between the layers. Many of the fuel electrodes normally used in solid oxide fuel cells contain a metal. When such a metal-containing layer is used as the fuel electrode, the metal in the fuel electrode may react with the electrolyte in the electrolyte layer adjacent to the fuel electrode, leading to deterioration of the electrolyte layer. By providing the reaction suppression layer between the fuel electrode and the electrolyte layer, the above-described problem can be prevented.

  The reaction suppression layer is also effective for preventing adhesion peeling due to the difference in coefficient of thermal expansion between the fuel electrode and other adjacent constituent members. Since the fuel electrode often contains a component different from that of other constituent members used in the fuel cell, physical properties such as a coefficient of thermal expansion are often different from those of other constituent members. If the coefficient of thermal expansion of adjacent components in the fuel cell stack is different, adhesive peeling may occur with expansion and contraction due to heat, and the oxide ion conductivity etc. of the peeled part will be significantly reduced, and power generation performance will be reduced. Incurs a decline. Such a problem is also caused by providing a reaction suppression layer having a thermal expansion coefficient intermediate between those of the fuel electrode and other adjacent constituent members (electrolyte layer or dense layer) and adjacent to each other. It can suppress by making the difference of the thermal expansion coefficient between members small.

The material for forming the reaction suppression layer is not particularly limited as long as it has oxide ion conductivity and does not react with the fuel electrode used together. For example, the reaction suppression layer can be formed using Ce (La) O ₂ , Ce (Sm) O ₂ , Y ₂ O ₃ —ZrO ₂ , and Ce (La) O ₂ is particularly preferably used. The Ce (La) O ₂ also has a role of relaxing the lanthanum concentration gradient in the two layers when, for example, an Ni catalyst fuel electrode and an La (Sr) Ga (Mg) O ₃ electrolyte layer are used. End.

  The film thickness of the reaction suppression layer is not particularly limited as long as the film can exhibit the above function. Specifically, those within the range of 0.1 to 10 μm, particularly those within the range of 0.1 to 1 μm are preferably used.

  The present invention is not limited to the above embodiment. The above-described embodiment is an exemplification, and the present invention has substantially the same configuration as the technical idea described in the claims of the present invention, and any device that exhibits the same function and effect is the present invention. It is included in the technical scope of the invention.

  For example, by using a material having oxide ion conductivity and conductivity, such as a mixed conductor given as an example of the material forming the dense layer, the function of the first electrode layer or the second electrode layer Alternatively, a layer in which the electrode layer and the dense layer are combined, which has the function of the dense layer, may be formed, and the electrolyte layer may be formed on the layer. In this case, since the layer formed from the oxide ion conductive material and the conductive material is dense, a thin electrolyte layer can be easily formed on such a layer. The present invention includes such an embodiment.

Hereinafter, the present invention will be described more specifically with reference to Examples and Comparative Examples.
[Example]
On the air electrode (Sm _0.5 Sr _0.5 CoO ₃ ) formed by the evaporation to dryness method, the dense layer forming material powder is pressed and solidified and then sintered at 1500 ° C. to thereby obtain a 500 μm dense layer (La _0.7 Sr _0.3 Ga _0.6 Fe _0.4 O ₃ ) and a 5 μm electrolyte layer (La _0.9 Sr _0.1 Ga) on the dense layer by PLD (pulse laser deposition) method. _0.8 Mg _0.2 O ₃ ). Furthermore, the fuel electrode forming material mixture was applied on the electrolyte layer by a screen printing method, and sintered at 1000 ° C. to form the fuel electrode (Ni _0.9 Fe _0.1 ) to produce a fuel cell. IV-IP characteristics were examined. The graph of the obtained IV-IP characteristic is shown in FIG.

[Comparative example]
A fuel cell was prepared in the same manner as in the above example except that the dense layer was not formed and the electrolyte layer was formed with a thickness of 500 μm directly on the air electrode, and the IV-IP characteristics were examined in the same manner as in the above example. It was. The obtained IV-IP characteristic grab is shown in FIG.

[Evaluation]
2 and 3, it can be seen that the fuel cell of the above example exhibits superior IV-IP characteristics at a lower temperature than the fuel cell of the above comparative example. In particular, it can be seen that the difference in performance is large at a low temperature of about 500 to 600 ° C.

It is a schematic sectional drawing which shows an example of a structure of the fuel cell of this invention. It is the graph which showed the IV-IP characteristic of the fuel cell of the Example of this invention. It is the graph which showed the IV-IP characteristic of the fuel cell of the comparative example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 ... Fuel cell 2 ... 2nd electrode layer 3 ... Dense layer 4 ... Electrolyte layer 5 ... Reaction suppression layer 6 ... 1st electrode layer

Claims (4)

Solid oxide type comprising at least an electrolyte layer having oxide ion conductivity, a first electrode layer disposed on one side of the electrolyte layer, and a second electrode layer disposed on the other side of the electrolyte layer In fuel cells,
A dense layer provided on the electrolyte layer side of the second electrode layer, having oxide ion conductivity and hermeticity that does not show any gas passage under a differential pressure of 0.1 atm ,
A solid oxide fuel cell, wherein the electrolyte layer has a thickness in a range of 0.5 μm to 10 μm . The second electrode layer is a porous body,
2. The solid oxide fuel cell according to claim 1, wherein a film thickness of the dense layer is larger than an opening diameter of the porous body. The first electrode layer is a fuel electrode containing a metal;
The solid oxide fuel cell according to claim 1, further comprising a reaction suppression layer between the electrolyte layer and the first electrode layer.   The material for forming the dense layer is La (Sr) Ga (Fe) O. ₃ And
  The material for forming the electrolyte layer is La (Sr) Ga (Mg) O. ₃ The solid oxide fuel cell according to any one of claims 1 to 3, wherein the fuel cell is a solid oxide fuel cell.

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