JP7194936B2 - Solid oxide fuel cell with mixed conductor layers - Google Patents

Description

The present invention relates to a solid oxide fuel cell having an electrolyte layer in which two or more different ion-conducting oxides are stacked.

In recent years, fuel cells, which directly convert chemical energy of fuel into electrical energy, have attracted attention as highly efficient and clean power generation devices. Among them, a solid oxide fuel cell (SOFC), which uses a solid oxide permeable to oxide ions or protons as an electrolyte, has a simpler structure than other fuel cells, and suffers from electrolyte loss and There are advantages such as no corrosion problem (for example, Non-Patent Document 1).

In general, SOFCs have a laminated structure in which a solid electrolyte layer made of an ionically conductive solid oxide is sandwiched from both sides by an air electrode layer (cathode) and a fuel electrode layer (anode). In an SOFC using an oxide-ion conductive electrolyte, oxygen ions generated by a reduction reaction of oxygen at the air electrode move to the fuel electrode through the electrolyte, and the fuel gas (H ₂ , CO, CH ₄ , etc.) to generate water and carbon dioxide. At this time, electrons are generated at the fuel electrode and electrons are consumed at the air electrode, so it is possible to generate an electric current by connecting the two electrodes can. In SOFCs, the ionic conduction resistance of the solid electrolyte is a factor that lowers the output characteristics, so reducing the electrolyte resistance by thinning the solid electrolyte is an effective technique for improving efficiency. Thinning the electrolyte is also an effective technique for lowering the operating temperature.

On the other hand, not only mobile ion species but also electrons and holes can move inside the solid electrolyte. Leakage current (leakage current) occurs in which the pores flow to the counter electrode through the electrolyte, and the current that can be extracted to the outside is reduced, resulting in a decrease in efficiency. Since the deterioration in performance due to leakage current becomes more pronounced as the film thickness of the solid electrolyte decreases, there is a demand for material design and cell design that can suppress the increase in leakage current that accompanies thinning.

Y. Matsuzaki et al., Sci. Rep. , 5, 12640, 2015

An object of the present invention is to design and construct a cell structure that can suppress an increase in leakage current due to the thinning of the electrolyte while reducing the ionic conduction resistance due to the thinning of the electrolyte in a solid oxide fuel cell (SOFC). It is.

As a result of intensive studies to solve the above problems, the present inventors have found that by adopting a structure in which two or more different ion-conductive solid oxides are laminated in the electrolyte layer, leakage current can be reduced by thinning. It was found that a cell capable of suppressing growth can be constructed. In particular, by using multiple materials with different band structures (bandgap width or Fermi level) as the solid oxides used in the electrolyte layer, high electromotive force can be obtained even when solid oxides with high hole conductivity are used. The present inventors have found that the leakage current can be suppressed, and have completed the present invention.

That is, in one aspect of the present invention,
<1> A solid oxide fuel cell having an electrolyte layer formed by stacking two or more solid oxide layers, a cathode, and an anode, wherein the solid oxide layers have band structures different from each other. A solid oxide fuel cell characterized in that it is formed of a material.

Also, in a preferred embodiment, the present invention provides
<2> The solid oxide fuel cell according to <1> above, wherein the solid oxide materials have Fermi levels different from each other;
<3> The solid oxide fuel cell according to <1> or <2> above, wherein the solid oxide material is a proton-conducting oxide or an oxide ion-conducting oxide;
<4> Any one of <1> to <3> above, wherein the electrolyte layer has a structure in which two or more solid oxide layers are stacked in a pn junction type, a pp junction type, or an nn junction type. solid oxide fuel cells;
<5> The electrolyte layer has a pn junction type laminated structure, and the energy level difference of the Fermi level of the solid oxide material forming each solid oxide layer is in the range of 0.3 to 5.0 eV. , the solid oxide fuel cell according to <4>above;
<6> The electrolyte layer has a pp junction type laminated structure, and the energy level difference of the valence band of the solid oxide material forming each solid oxide layer is in the range of 0.3 to 5.0 eV. , the solid oxide fuel cell according to <4>above;
<7> The electrolyte layer has an nn junction type laminated structure, and the energy level difference of the conduction band of the solid oxide material forming each solid oxide layer is in the range of 0.3 to 5.0 eV. The solid oxide fuel cell according to the above <4>; and <8> the solid oxide material is a rare earth element-doped perovskite oxide, a transition metal element-doped titanium-based perovskite oxide, or an alkaline earth element. Metal-doped LaYb-based oxides, LaW-based oxides, LaCe-based oxides, Y-doped Zr-based oxides, rare earth element-doped Ce-based oxides, La-based perovskite-type oxides, and Ba-based perovskite-type oxides Provided is a solid oxide fuel cell according to any one of <4> to <7> above, which is selected from the group consisting of oxides.

According to the present invention, by adopting a structure in which two or more different ion-conducting solid oxides are laminated in the electrolyte layer, a depletion layer of electrons and holes is formed at the interface of each solid oxide layer in the electrolyte layer. can suppress an increase in leakage current due to thinning. In addition, by using an electrolyte layer having such a laminated structure, a high electromotive force can be obtained even when a solid oxide having high hole conductivity is used. Even an oxide material can be used as an excellent electrolyte layer, which greatly contributes to the practical use of solid oxide fuel cells.

FIG. 1 is a schematic diagram of a band structure showing the formation of a charge depletion layer when solid oxide material layers are bonded according to the present invention. FIG. 1(a) shows the band structure for the pn junction type, and FIG. 1(b) shows the band structure for the pp junction type. FIG. 2 is a conceptual diagram of the solid oxide fuel cell of the present invention produced in Examples. FIG. 3 is current-voltage and current-power curves for a solid oxide fuel cell of the invention. FIG. 4 is a schematic diagram showing the band structures of LWO and LYO assumed from the XPS measurement results.

Preferred embodiments of the present invention are described below. The scope of the present invention is not restricted by these descriptions, and other than the following examples can be appropriately changed and implemented without impairing the gist of the present invention.

The solid oxide fuel cell of the present invention has an electrolyte layer formed by stacking two or more solid oxide layers, a cathode, and an anode, and the solid oxide layers have different band structures (bandgap widths). or Fermi level) is formed of a solid oxide material.

In the solid oxide fuel cell of the present invention, in a single cell, a cathode (air electrode) and an anode (fuel electrode) are bonded to an electrolyte layer so as to face each other. A single cell may have various shapes such as a flat plate type and a cylindrical type. Moreover, as the single cell having each shape, in addition to the one using the electrolyte layer as the support, the one using the thickly formed fuel electrode as the support can be used.

1. Electrolyte Layer As described above, the solid oxide fuel cell of the present invention is characterized in that the electrolyte layer has a structure in which two or more different ion-conducting solid oxides are laminated. More specifically, the electrolyte layer has a structure in which two or more solid oxide layers are laminated, and the solid oxide materials forming these solid oxide layers have different band structures ( It is characterized by having a bandgap width or Fermi level).

In this specification, "different band structures" or "different band structures" for solid oxide materials means that each solid oxide material has a different bandgap width or a different Fermi level. means that it is in at least one of the states of Here, "different bandgap widths are different" means that the energy level difference between the valence band and the conduction band is different. Generally, solid oxide materials having different bandgap widths thus have different Fermi levels. However, even if the energy levels of the valence band and the conduction band are the same (that is, even if the bandgap width is the same), there may be solid oxide materials that have different Fermi levels. It shall be included in the case of "different band structure" or "different band structure" in .

FIG. 1 is a schematic diagram of a band structure showing the formation of a charge depletion layer when solid oxide material layers having different band structures used in the present invention are joined. As shown in FIG. 1, at the interface where materials with different Fermi levels are joined, transfer of electrons and holes as carriers occurs, and a charge depletion layer with a significantly reduced carrier concentration is formed near the interface. Then, the formation of the depletion layer reduces the electron/hole conductivity due to the low carrier concentration, thereby suppressing the leakage current. FIG. 1(a) shows the band structure for the pn junction type, and FIG. 1(b) shows the band structure for the pp junction type.

In general, solid electrolyte materials are classified into p-type and n-type according to their electron/hole conductivity types. It can be applied to any combination of stacked structures of pp junction type (FIG. 1(b)) and nn junction type. The laminated structure of the electrolyte layer may be a two-layer structure or a multilayer structure such as a superlattice structure.

In addition, in the present invention, it is known that by forming an electrolyte layer having a laminated structure of solid oxide materials having different band structures, the hole conductivity or electron conductivity is high, and conventionally, sufficient electromotive force cannot be obtained. It is also characterized by obtaining superior electromotive force derived from the material with the lowest hole conductivity in the two or more solid oxide layers, even when using the materials with the lowest hole conductivity. Although not necessarily bound by theory, it is believed that by setting the difference in bandgap width of each solid oxide material within an appropriate range, the valence band and conduction band between the materials can be controlled to a predetermined energy level. As a result, it is thought that only hole conduction (or electron conduction) is suppressed at the interfaces where these materials are bonded. This idea is based on combining materials with low ionic conductivity but different band structures, even when using materials with high ionic conductivity but large leakage current, that is, materials with poor electron transport properties (low electromotive force). By stacking layers (high electromotive force), it is possible to raise the performance to the same level as a single layer with good performance. This is fundamentally different from the traditional concept of using multiple solid oxides.

From the viewpoint of obtaining a high electromotive force while suppressing the leak current, in a preferred embodiment of the present invention, the difference in the band structure of each solid oxide material is preferably within an appropriate range. More specifically, when the electrolyte layer has a pn junction type laminated structure, the energy level difference of the Fermi level of the solid oxide material forming each solid oxide layer is 0.3 to 5.0 eV. is preferably in the range of Further, when the electrolyte layer has a pp junction type laminated structure, the energy level difference of the valence band of the solid oxide material forming each solid oxide layer is in the range of 0.3 to 5.0 eV. is preferred. Furthermore, it is preferable that the electrolyte layer has an nn junction type laminated structure, and that the energy level difference of the conduction band of the solid oxide material forming each solid oxide layer is in the range of 0.3 to 5.0 eV. .

The solid oxide material used as the electrolyte layer in the present invention can be a proton-conducting oxide or an oxide ion-conducting oxide. Preferably, proton-conducting oxides are used. A fuel cell using a proton-conducting oxide as an electrolyte (p-SOFC) provides higher power generation efficiency and a lower operating temperature than a fuel cell using an oxide ion-conducting oxide.

The combination of the solid oxide materials used as the electrolyte layer in the present invention is not particularly limited as long as they have band structures different from each other as described above. transition metal element-doped titanium-based perovskite-type oxides, alkaline-earth metal-doped LaYb-based oxides, LaW-based oxides, LaCe-based oxides, Y-doped Zr-based oxides, rare-earth element-doped A material selected from the group consisting of Ce-based oxides, La-based perovskite-type oxides, and Ba-based perovskite-type oxides can be used. Each solid oxide layer can therefore be a combination of these materials. In some cases, solid solutions of these solid oxide materials can also be used.

Specific examples of the p-type and n-type proton-conducting oxides include the materials shown in the table below. In the table, x is 0≤x≤1, preferably 0.05≤x≤0.5. y is 0≤y≤1, preferably 0.1≤y≤0.8. also,
δ is the amount of oxygen deficiency, typically 0.025≦δ≦0.5.

Specific examples of the p-type and n-type oxide ion-conducting oxides include the materials shown in the following table. In the table, x is 0≤x≤1, preferably 0.05≤x≤0.5. y is 0≤y≤1, preferably 0.1≤y≤0.8. δ is the amount of oxygen deficiency, typically 0.025≦δ≦0.5.

When the electrolyte layer has a pn junction type laminated structure, a preferred combination of solid oxide materials is [La28 _- xW4 ₊ xO54 _+3x/ 2v2-3x _/2 and BaCe1 _- xNd _{xO3 -} δ], [La28 _- xW4 ₊ xO54 _+3x/ 2v2-3x _/2 and LaYb1 _{- xTixO3 -δ} ], [ _{SrZr1 - xYxO3} -[ _delta ] and _{SrCe1 - xYbxO3-} [ _delta ]], [ _{BaTi1 - xYxO3-} [delta] and SrTi1O3-[ _delta ]].

When the electrolyte layer has a pp junction type laminated structure, preferred combinations of solid oxide materials include [La28 _- xW4 ₊ xO54 _+3x/ 2v2-3x _/2 and _LaYbO3 ], [ _La28 -xW4 ₊ xO54 _{+3x/ 2v2-3x/2} and La2 ₊ xCe2 _- xO7 _-δ ], [BaZr1 _{- xYxO3 -δ} and _{SrZr1- x} Y _x O _3-δ ], [BaZr _1-x Y _x O _3-δ and LaYbO ₃ ], and [BaZr _1-x Yb _x O _3-δ and LaYbO ₃ ].

When the electrolyte layer has an nn junction type laminated structure, preferred combinations of solid oxide materials include [La _1-x Sr _x TiO ₃ and SrTi _1-x Nb _x O _3-δ ], [Zr _1-xy Y _x NbyO _2-δ and SrTiO ₃ ].

The total thickness of the electrolyte layer in the solid oxide fuel cell of the present invention can be similar to that in typical solid oxide fuel cells, and can be, for example, 3-30 μm. The individual thickness of each solid oxide layer making up the electrolyte layer can typically range from 1 to 10 μm.

2. Cathode and Anode As the cathode (air electrode) in the solid oxide fuel cell of the present invention, electrode materials known in the art can be used, but typically include metal oxides. Specifically, the cathode uses metal oxide particles having a perovskite crystal structure, such as (Sm,Sr)CoO ₃ , (La,Sr)MnO ₃ , (La,Sr)CoO ₃ , (La, Sr) (Fe, Co) O ₃ , (La, Sr) (Fe, Co, Ni) O ₃ , (Ba, Sr) (Fe, Co) O ₃ and other metal oxide particles, You may use the said metal oxide individually or in mixture of 2 or more types. Materials forming the cathode may also include noble metals such as platinum, ruthenium, and palladium. Furthermore, lanthanum manganite doped with strontium, cobalt, iron, or the like is used as a material for forming the cathode. For example, the cathode includes SrFe _0.95 Nb _0.05 O _3-δ (SFN), La _0.8 Sr _0.2 MnO ₃ (LSM), La _0.6 Sr _0.4 Co _0.8 Fe _0.2 O ₃ (LSCF), and the like.

As the anode (fuel electrode) in the solid oxide fuel cell of the present invention, electrode materials known in the art can be used, but typically metal catalysts, or those formed from metal catalysts and solid electrolytes. can be exemplified. Specific examples of metal catalysts include Ni, Ni alloys, Co, Ru, Pt, Pd, Nb alloys, and V alloys. Two or more of these may be mixed. On the other hand, a stabilized zirconia, a ceria-based solid solution, or the like can be used as the solid electrolyte constituting the mixture. The ratio (mass ratio) between the metal catalyst and the solid electrolyte in the mixture is preferably within the range of catalyst:solid electrolyte=30:70 to 70:30.

3. Method for Producing Solid Oxide Fuel Cell, etc. A single cell of the solid oxide fuel cell of the present invention can be produced, for example, as follows. The solid oxide material is molded into a desired shape such as a flat plate by a press molding method, a tape molding method, or the like, and sintered at an optimum temperature depending on the composition to form an electrolyte layer. Next, a slurry containing an anode material is applied to one surface of the electrolyte layer by a screen printing method, a doctor blade method, a brush coating method, a spray method, a dipping method, or the like, and baked at an optimum temperature according to its composition. to form a fuel electrode. Similarly, the slurry containing the cathode material is applied to the other surface of the electrolyte layer and sintered to form an air electrode.

A plurality of single cells can be used by electrically connecting them in series via separators (interconnectors). The separator is made of a conductive material in which gas channels are formed. Examples of the material constituting the separator include stainless steel such as Cr steel, and heat-resistant metal materials such as Cr-based alloys and Ni-based alloys. A plate-like material made of these materials can be used as a separator by forming gas flow paths by pressing, etching, or the like. A cell stack can be constructed by alternately stacking unit cells and separators with appropriate current collectors interposed therebetween.

The solid oxide fuel cell of the present invention can operate at temperatures between 600 and 1000°C. After the temperature is raised to the operating temperature, fuel gas is supplied to the fuel electrode and air is supplied to the air electrode through the gas flow path of the separator. Examples of the fuel gas include hydrogen, methane, or those diluted with an inert gas such as nitrogen, or town gas.

EXAMPLES The present invention will be described in more detail with reference to examples below, but the scope of the present invention is not limited to the following examples.

1. Materials Used Anode: NiO + Gd 10% doped CeO2 (1: ₁ wt.%, NiO + GDC10)
Cathode: SrFe _0.95 Nb _0.05 O _3-δ (SFN)
Electrolyte: La28 _- xW4 ₊ xO54 _+3x/ 2v2-3x _/2 (La/W=6.7, LWO67), _LaYbO3 (LYO).

2. Cell production method <Production of substrate (anode support)>
NiO and Gd 10% doped CeO ₂ (manufactured by Daiichi Kigenso Kagaku Kogyo Co., Ltd.) were mixed at a weight ratio of 1:1 and mixed with ethanol in a ZrO ₂ ball mill for 2 hours. After mixing, ethanol was evaporated and the mixture was classified with a sieve having a mesh size of 35 μm. The classified powder was molded into pellets of 10 mmΦ using a tablet molding machine, and then fired at 1200°C for 1 hour to obtain an anode support.
The fired anode support was mirror-polished (#2000), and the surface was cleaned by ultrasonic cleaning. Acetone and ethanol were used as solvents, and the tests were performed alternately twice for 5 minutes each. The cleaned surface was scrubbed with a cotton swab impregnated with acetone.

<Preparation of target (electrolyte target)>
Electrolyte targets were obtained by molding the electrolyte powder into pellets of 20 mmφ using a tablet molding machine and firing them at a temperature appropriate to the sample. The surface of the target was polished with waterproof abrasive paper (#1200).

<Formation of electrolyte membrane>
The electrolyte membrane was deposited using the Pulsed Laser Deposition (PLD) method. A PLD apparatus manufactured by Pascal was used as a film forming apparatus. The film formation conditions were a temperature of 600° C. and an atmosphere of oxygen (oxygen partial pressure: 1 Pa).

3. Experimental Results FIG. 2 shows a conceptual diagram of the fabricated cell. LYO was laminated on the anode side and LWO was laminated on the cathode side, and LYO was laminated as a reaction suppressing layer between the LWO and the cathode.

Table 1 shows the transport properties of LWO and LYO (G. Kojo et al., Solid State Ionics, 306 (2017) 89-96). The transport properties of LYO were derived from the literature on the transport properties of La _0.9 Ca _0.1 YbO _3-δ of similar composition. The proton conductivity is almost the same value, but the hole conductivity of LWO is lower.

FIG. 3 shows a current-voltage curve and a current-output curve obtained as a result of measuring the cell prepared above. The measurement was performed by flowing a 3% humidified 20% H ₂ +Ar gas to the anode side and a 3% humidified 20% O ₂ +Ar gas to the cathode side. The electromotive forces at 800°C and 600°C were 0.82 V and 0.95 V, respectively (Table 2). The electromotive forces when the LWO layer was used as the electrolyte were 0.88 V and 0.94 V at 800°C and 600°C, respectively, and the values were almost the same.

Considering the transport properties of LWO and LYO used as electrolytes in the above cells (Table 1), we originally observed a higher electromotive force in cells made only with LWO because LWO has a lower hole conductivity. Conceivable. However, according to the experimental results obtained, the cell with the combination of the good layer (LWO) and the bad layer (LYO), so to speak, showed the same electromotive force as the cell with only the good layer. This is thought to be due to the functioning of a new transport property control mechanism at the interface between LWO and LYO.

FIG. 4 shows the relationship between the LWO and LYO band structures assumed from the XPS measurement results. LWO and LYO have different band structures. Therefore, the valence bands between the two materials are different, and a so-called p-p junction is formed at the interface where the two materials are joined, and it is thought that only the conduction of holes is suppressed.

Claims (7)

A solid oxide fuel cell having an electrolyte layer formed by stacking two or more solid oxide layers, a cathode, and an anode,
The solid oxide layers are formed of solid oxide materials having Fermi levels different from each other ,
A solid oxide fuel cell, wherein the solid oxide material has ionic conductivity and electronic conductivity . 2. The solid oxide fuel cell of claim 1 , wherein said solid oxide material is a proton-conducting oxide or an oxide ion-conducting oxide. 3. The solid oxide fuel cell according to claim 1, wherein said electrolyte layer has a structure in which two or more solid oxide layers are stacked in a pn junction type, pp junction type, or nn junction type. The electrolyte layer has a pn junction type laminated structure, and the energy level difference of the Fermi level of the solid oxide material forming each solid oxide layer is in the range of 0.3 to 5.0 eV. 4. The solid oxide fuel cell according to 3 . 3. The electrolyte layer has a pp junction type laminated structure, and the energy level difference of the valence band of the solid oxide material forming each solid oxide layer is in the range of 0.3 to 5.0 eV. 4. The solid oxide fuel cell according to 3 . 4. The electrolyte layer has an nn junction type laminated structure, and the energy level difference of the conduction band of the solid oxide material forming each solid oxide layer is in the range of 0.3 to 5.0 eV. The solid oxide fuel cell according to . The solid oxide material is a perovskite-type oxide doped with a rare earth element, a titanium-based perovskite-type oxide doped with a transition metal element, a LaYb-based oxide doped with an alkaline-earth metal, a LaW-based oxide, or a LaCe-based oxide. Zr-based oxide doped with Y, Ce-based oxide doped with a rare earth element, La-based perovskite oxide, and Ba - based perovskite oxide. The solid oxide fuel cell according to any one of 1.

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