Solid oxide fuel cell

Abstract

To provide a solid oxide fuel cell that can control the reactivity of the power generation reaction and achieve a uniform power generation distribution within the cell surface. [Solution] A solid oxide fuel cell having an electrolyte layer 20 having ionic conductivity that allows oxide ions to pass through, an anode layer 22A provided on one side of the electrolyte layer 20, and a cathode layer provided on the other side of the electrolyte layer 20. The anode layer 22A includes an electron conduction phase 32 that conducts electrons, an ion conduction phase 34 that conducts ions, and a void phase 36 that diffuses fuel gas. The electron conduction phase 32, the ion conduction phase 34, and the void phase 36 constitute an anode three-phase interface 38 at the point where these three phases are in contact. The density of the anode three-phase interface 38 at the downstream portion of the anode layer 22A located downstream in the direction of fuel gas flow is greater than the density of the anode three-phase interface 38 at the upstream portion of the anode layer 22A located upstream in the direction of fuel gas flow.

Description

[Technical Field]

[0001] The present invention relates to a solid oxide fuel cell comprising an electrolyte layer, an anode layer, and a cathode layer. [Background technology]

[0002] A known solid oxide fuel cell (hereinafter sometimes referred to as "fuel cell") has an electrolyte layer that is ionically conductive and allows oxide ions to pass through, an anode layer (fuel electrode) provided on one side of the electrolyte layer, and a cathode layer (air electrode) provided on the other side of the electrolyte layer. In such a fuel cell, a fuel gas (e.g., hydrogen) is supplied to the anode layer, and an oxidizing gas (e.g., air) is supplied to the cathode layer.

[0003] In this solid oxide fuel cell, in the cathode layer, Reaction on the cathode layer (air electrode) side: 1 / 20² + 2e - → O 2- ...(1) The following reaction occurs, and the oxide ions generated on the cathode layer side flow through the electrolyte layer to the anode layer side, and in the anode layer, Reaction on the anode (fuel electrode) side: H2 + O 2- → H2O + 2e - ...(2) The following reaction occurs, and electricity is generated through this power generation reaction.

[0004] In such fuel cell cells, the power generation reaction is concentrated on the inflow side where the concentration of fuel gas (e.g., hydrogen) is high, and resistive heating (ohmic heating) occurs at the power generation point where the generated current flows. As a result, the temperature on the inflow side of the fuel cell tends to become locally high. When the temperature gradient becomes large locally on the inflow side of the fuel cell, thermal stress is generated in that area, which may cause damage to the fuel cell (electrolyte layer, anode layer, or cathode layer).

[0005] Therefore, in order to suppress the rise in the local temperature gradient on the inlet side of the fuel cell, a fuel cell has been proposed in which the porosity of the outlet side portion of the anode layer is greater than that of the inlet side portion (see Patent Document 1). In this fuel cell, the porosity of the anode layer is small on the inlet side portion and large on the outlet side portion. By configuring it in this way, the resistance to the movement of hydrogen to the reaction interface in the anode layer and the release of water vapor (H2O gas) generated by the power generation reaction is reduced in the outlet side portion. As a result, the diffusivity of the fuel gas in the downstream portion of the anode layer is increased, the reactivity of the outlet side portion of the anode layer where the fuel gas concentration is low is increased, and the power generation distribution in the anode layer is made more uniform.

[0006] Furthermore, instead of changing the porosity of the anode layer in the direction of fuel gas flow, a method has been proposed in which the thickness of the anode layer of the fuel cell is changed (see, for example, Patent Document 2). In this fuel cell, the thickness of the anode layer is thicker at the inflow side and thinner at the outflow side, thereby increasing the diffusivity of the fuel gas at the outflow side and aiming to equalize the power generation distribution in the anode layer. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2012-94427 [Patent Document 2] Patent No. 6118694 [Overview of the Initiative] [Problems that the invention aims to solve]

[0008] In conventional solid oxide fuel cell cells, the porosity or thickness of the anode layer is varied in the direction of fuel gas flow to equalize the power generation distribution in the anode layer. However, there has been a desire to realize a fuel cell that can control the reactivity of the power generation reaction using factors other than porosity and thickness.

[0009] The object of the present invention is to provide a solid oxide fuel cell that can control the reactivity of the power generation reaction and achieve uniformity of the power generation distribution within the cell surface (anode layer and / or cathode layer). [Means for solving the problem]

[0010] The solid oxide fuel cell according to claim 1 of the present invention is a solid oxide fuel cell having an electrolyte layer having ionic conductivity that allows oxide ions to pass through, an anode layer provided on one side of the electrolyte layer, and a cathode layer provided on the other side of the electrolyte layer, The anode layer comprises an electron conduction phase that conducts electrons, an ion conduction phase that conducts ions, and a void phase that diffuses fuel gas, and the electron conduction phase, the ion conduction phase and the void phase constitute an anode three-phase interface at the portion where these three phases are in contact. The density of the anode three-phase interface in the downstream portion of the anode layer, located downstream in the flow direction of the fuel gas, is greater than the density of the anode three-phase interface in the upstream portion of the anode layer, located upstream in the flow direction of the fuel gas.

[0011] Furthermore, in the solid oxide fuel cell according to claim 2 of the present invention, the cathode layer comprises an electron conduction phase that conducts electrons, an ion conduction phase that conducts ions, and a void phase that diffuses an oxidizing gas, wherein the electron conduction phase, the ion conduction phase, and the void phase constitute a cathode three-phase interface at the portion where these three phases are in contact, and the density of the cathode three-phase interface in the downstream portion of the cathode layer located downstream in the direction of oxidizing gas flow is greater than the density of the cathode three-phase interface in the upstream portion of the cathode layer located upstream in the direction of oxidizing gas flow.

[0012] Also, in the solid oxide fuel cell according to claim 3 of the present invention, the cathode layer includes an electron-ion conduction phase that conducts electrons and ions, and a void phase that diffuses an oxidant gas. The electron-ion conduction phase and the void phase form a cathode two-phase interface at a site where these two phases contact, and the density of the cathode two-phase interface at the downstream portion of the cathode layer located on the downstream side in the flow direction of the oxidant gas is greater than the density of the cathode two-phase interface at the upstream portion of the cathode layer located on the upstream side in the flow direction of the oxidant gas.

[0013] Also, the solid oxide fuel cell according to claim 4 of the present invention is a solid oxide fuel cell having an electrolyte layer having oxide ion conductivity, an anode layer provided on one side of the electrolyte layer, and a cathode layer provided on the other side of the electrolyte layer, The anode layer includes an electron conduction phase that conducts electrons, an ion conduction phase that conducts ions, and a void phase that diffuses a fuel gas. In the downstream portion of the anode layer located on the downstream side in the flow direction of the fuel gas, the ion conduction phase and the electron conduction phase have a high reactivity ratio, and in the upstream portion of the anode layer, the ion conduction phase and the electron conduction phase have a low reactivity ratio.

[0014] Furthermore, in the solid oxide fuel cell according to claim 5 of the present invention, the cathode layer includes an electron conduction phase that conducts electrons, an ion conduction phase that conducts ions, and a void phase that diffuses an oxidant gas. In the downstream portion of the cathode layer located on the downstream side in the flow direction of the oxidant gas, the ion conduction phase and the electron conduction phase have a high reactivity ratio, and in the upstream portion of the cathode layer, the ion conduction phase and the electron conduction phase have a low reactivity ratio.

Advantages of the Invention

[0015] According to the solid oxide fuel cell described in claim 1 of the present invention, the electron conduction phase, ion conduction phase, and void phase of the anode layer constitute an anode three-phase interface (where the power generation reaction takes place) at the point where these three phases are in contact. Since the density of the anode three-phase interface in the downstream part of the anode layer is greater than the density of the anode three-phase interface in the upstream part of the anode layer, the power generation performance in this downstream part is enhanced, and as a result, the power generation distribution of the anode layer in the direction of fuel gas flow can be made more uniform.

[0016] Furthermore, according to the solid oxide fuel cell described in claim 2 of the present invention, the electron conduction phase, ion conduction phase, and void phase of the cathode layer constitute a cathode three-phase interface at the point where these three phases are in contact. Since the density of the cathode three-phase interface downstream of the cathode layer is greater than the density of the cathode three-phase interface upstream of the cathode layer, the reactivity in the downstream portion of the cathode layer can be increased, and as a result, the reaction distribution of the anode layer in the flow direction of the oxidant gas can be made more uniform.

[0017] Furthermore, according to the solid oxide fuel cell described in claim 3 of the present invention, the electron / ion conduction phase and the void phase of the cathode layer constitute a cathode two-phase interface at the point where these two phases are in contact. Since the density of the cathode two-phase interface in the downstream portion of the cathode layer is greater than the density of the cathode two-phase interface in the upstream portion, the reactivity in the downstream portion of the cathode layer can be increased, and the reaction distribution of the anode layer in the flow direction of the oxidant gas can be made more uniform.

[0018] Furthermore, according to the solid oxide fuel cell described in claim 4 of the present invention, the ion conduction phase and the electron conduction phase are highly reactive in the downstream portion of the anode layer, while the ion conduction phase and the electron conduction phase are less reactive in the upstream portion of the anode layer. As a result, the reactivity in the downstream portion of the anode layer is increased, and consequently, the power generation distribution of the anode layer in the direction of fuel gas flow can be made more uniform.

[0019] Furthermore, according to the solid oxide fuel cell described in claim 5 of the present invention, the cathode layer includes an electron conduction phase that conducts electrons, an ion conduction phase that conducts ions, and a void phase that diffuses the oxidizing gas. In the downstream portion of the cathode layer, the ion conduction phase and the electron conduction phase are highly reactive, while in the upstream portion of the cathode layer, the ion conduction phase and the electron conduction phase are less reactive. This increases the reactivity in the downstream portion of the cathode layer and makes the reaction distribution of the anode layer more uniform in the direction of oxidizing gas flow. [Brief explanation of the drawing]

[0020] [Figure 1] A perspective view showing an example of a cell stack using a reference embodiment of a solid oxide fuel cell. [Figure 2] A simplified cross-sectional view of the cell stack in Figure 1. [Figure 3] Figure 2 is a magnified cross-sectional view of a solid oxide fuel cell. [Figure 4] An enlarged cross-sectional view schematically showing a magnified portion of the anode layer. [Figure 5] An explanatory diagram illustrating the curvature of the electron conduction phase, ionic conduction phase, and void phase of the anode layer. [Figure 6] An enlarged cross-sectional view schematically showing a portion of the anode layer in a first embodiment of a solid oxide fuel cell according to the present invention. [Figure 7] This figure shows the relationship between the length ratio of the anode three-phase interface in the anode layer and the power generation performance of the fuel cell. [Figure 8] This figure shows the relationship between the volume fraction of the ion-conducting phase in the anode layer and the overpotential of the anode layer. [Figure 9] This diagram shows the changes in temperature and power generation current from the inlet to the outlet in a reference example of a solid oxide fuel cell. [Figure 10] This figure shows the changes in temperature and power generation current from the inlet to the outlet in a comparative example solid oxide fuel cell. [Modes for carrying out the invention]

[0021] Hereinafter, a reference embodiment of a solid oxide fuel cell and embodiments of the present invention will be described with reference to the attached drawings.

[0022] First, a reference embodiment of a solid oxide fuel cell will be described with reference to Figures 1 to 5. Figures 1 and 2 show a cell stack using the solid oxide fuel cell of the reference embodiment. In Figures 1 and 2, the illustrated cell stack 2 comprises a rectangular stack body 4, and a plurality of solid oxide fuel cell cells 6 are assembled in a stacked state on this stack body 4. The stack body 4 is provided with a first inlet manifold 8 and a first outlet manifold 10. Fuel gas (e.g., hydrogen) from the fuel supply passage is distributed in the first inlet manifold 8 and then supplied to the anode chambers 12 of the plurality of fuel cell cells 6, and fuel off-gas from the anode chambers 12 is discharged through the first outlet manifold 10.

[0023] The stack body 4 is further provided with a second inlet manifold 14 and a second outlet manifold 16. Oxidizer gas (e.g., air) from the oxidizer gas supply channel is distributed in the second inlet manifold 14 and then supplied to the cathode chambers 18 of the multiple fuel cell cells 6, and the oxidizer off-gas from the cathode chambers 18 is discharged through the second outlet manifold 16.

[0024] Next, the solid oxide fuel cell cell 6 shown in the figure will be described. Multiple solid oxide fuel cell cells 6 have substantially the same configuration, and one of them will be described below. Referring mainly to Figure 3, the solid oxide fuel cell cell 6 shown in the figure comprises an electrolyte layer 20, an anode layer 22 (fuel electrode) disposed on one side of the electrolyte layer 20 (the bottom side in Figures 2 and 3), and a cathode layer 24 (air electrode) disposed on the other side of the electrolyte layer 20 (the top side in Figures 2 and 3). In this configuration, the electrolyte layer 20 is provided on the surface side of the anode layer 22, which serves as a support base, and the cathode layer 24 is provided on the surface side of the electrolyte layer 20.

[0025] Interconnectors 26 are provided between adjacent fuel cell cells 6 (see also Figure 2), and these interconnectors 26 are electrically connected to the anode layer 22 and cathode layer 24 of the adjacent fuel cell cells 6. Each anode chamber 12 is formed between the anode layer 22 of the fuel cell cell 6 and the interconnector 26, and the cathode chamber 18 is formed between the cathode layer 24 of the fuel cell cell 6 and the interconnector 26.

[0026] The electrolyte layer 20 is formed from, for example, yttria-stabilized zirconia (also called "YSZ"), scandia-stabilized zirconia (ScSZ), etc. This electrolyte layer 20 is ionically conductive and has the effect of passing oxide ions generated on the cathode layer 24 (air electrode) side to the anode layer 22 side. The anode layer 22 (fuel electrode) is formed from, for example, nickel-yttria-stabilized zirconia (also called "Ni-YSZ"), and the reaction of equation (2) above takes place in this anode layer 22. The cathode layer 24 (air electrode) is formed from, for example, lanthanum strontium manganite-yttria-stabilized zirconia (La 0.6 Sr 0.4 MnO) (also known as "LSM-YSZ"), lanthanum strontium cobaltite ferrite [(La,Sr)(Co,Fe)O3] (LSCF), lanthanum strontium cobaltite (La 0.6 Sr 0.4 Formed from CoO3)(LSC), the cathode layer 24 undergoes the reaction described in equation (1) above. As will be described in detail later, LSM-YSZ is a material having a three-phase cathode interface, while LSCF and LSC are materials having a two-phase cathode interface.

[0027] In this solid oxide fuel cell cell 6, the power generation distribution of the anode layer 22 in the direction of fuel gas flow is further homogenized as follows. Referring mainly to Figures 4 and 5, when the anode layer 22 (fuel electrode) is formed from, for example, Ni-YSZ, the Ni phase 32 functions as an electron conduction phase that conducts electrons, the YSZ phase 34 functions as an ion conduction phase that conducts ions, and the void phase 36 functions as a gas diffusion phase that diffuses fuel gas (e.g., hydrogen).

[0028] In this embodiment, attention is paid to the curvature of the void phase 36 to equalize the power generation distribution of the anode layer 22 in the direction of fuel gas flow. In such a fuel cell cell 6, the concentration of fuel gas (e.g., hydrogen) on the inflow side is high, and the power generation distribution tends to be large on the inflow side where the concentration is high and small on the outflow side where the concentration is low. For this reason, the curvature of the void phase 36 in the upstream portion 22a of the anode layer 22, which is located on the upstream side when viewed in the direction of fuel gas flow, is configured to be greater than the curvature of the void phase 36 in the downstream portion 22b of the anode layer 22, which is located on the downstream side when viewed in the direction of fuel gas flow.

[0029] To explain the degree of flexibility (τ), it is an index that indicates the degree to which conduction and diffusion of a substance are suppressed. The relationship between this degree of flexibility (τ) and the phase structure is as shown in Figure 5, for example. In a linear structure as shown in Figure 5(a), the degree of flexibility (τ) is "1" (τ=1). In a structure that extends at a 45-degree angle as shown in Figure 5(b), the degree of flexibility (τ) is "1.4" (τ=1.4). In a linear structure as shown in Figure 5(c), the degree of flexibility (τ) is greater than "1", but the value is relatively small (τ=small). In a bent and connected structure as shown in Figure 5(d), the degree of flexibility (τ) is relatively large (τ=large). In a non-continuously connected structure (in other words, a broken structure) as shown in Figure 5(e), the degree of flexibility (τ) is infinite (τ=∞).

[0030] The relationship between the curvature (τ1) of this void phase 36 and the effective diffusion coefficient (De1) of the fuel gas considering its structure is as follows: The effective diffusion coefficient De1 = (ε1 / τ1) × D1 ··· (3) ε1: Volume fraction of the void phase τ1: Tortuosity of the void phase D1: Theoretical diffusion coefficient of the fuel gas As represented by this equation (3) and understood from this equation (3), the smaller the tortuosity (τ1) of the void phase 36, the larger the effective diffusion coefficient (De1) of the fuel gas.

[0031] In this embodiment, the tortuosity (τ 1a ) of the void phase 36 in the upstream portion 22a of the anode layer 22 is configured to be larger than the tortuosity (τ 1b ) of the void phase 36 in the downstream portion 22b thereof (τ 1a > τ 1b ), and the tortuosity (τ 1a ) of the upstream portion 22a is, for example, about 5 to 20, and the tortuosity (τ 1b ) of the downstream portion 22b is configured to be, for example, about 1 to 15.

[0032] With such a configuration, the effective diffusion coefficient of the downstream portion 22b of the anode layer 22 becomes larger than the effective diffusion coefficient of the upstream portion 22a thereof, the gas diffusibility of the downstream portion 22b becomes higher, and the fuel gas easily flows to the reaction field (near the interface between the anode layer 22 and the electrolyte layer 20). Therefore, the power generation reactivity (that is, the power generation performance) of the downstream portion 22b of the anode layer 22 (fuel electrode) becomes high. As a result, the power generation distribution of the anode phase 22 in the flow direction of the fuel gas can be made uniform, and a local temperature rise of the anode layer 22 can be suppressed.

[0033] In the above-described embodiment, the tortuosity (τ1) of the void phase 36 of the anode layer 22 is changed. However, instead of this void phase 36, the tortuosity (τ2) of the electron conduction phase (Ni phase 32) or the tortuosity (τ3) of the ion conduction phase (YSZ phase 34) of the anode layer 22 may be changed.

[0034] To explain the case where the degree of bending (τ2) of the electron conduction phase (Ni phase 32) of the anode layer 22 is changed, in this case the relationship between the degree of bending (τ2) of this electron conduction phase and the effective electronic conductivity (σe2) considering its structure is as follows: Effective electronic conductivity σe² = (ε² / τ²) × σ²···(4) ε2: Volume fraction of the electron conduction phase τ2: Curvature of the electron conduction phase σ²: Electronic conductivity of the electronically conducted phase As can be seen from equation (4), the smaller the degree of bending (τ²) of this electron conduction phase, the larger the effective electron conductivity (σe²).

[0035] In this case, as described above, the degree of bending (τ) of the electron conduction phase 32 in the upstream portion 22a of the anode layer 22. 2a ) is the degree of bending of the electron conduction phase 36 in the downstream portion 22b (τ 2b (τ) is configured to be larger than 2a >τ 2b ). By configuring it in this way, the effective electronic conductivity of the downstream portion 22b of the anode layer 22 becomes greater than the effective electronic conductivity of the upstream portion 22a, making it easier for electrons to flow in this downstream portion 22b. Therefore, even with this configuration, the power generation reactivity (power generation performance) of the downstream portion 22b of the anode layer 22 (fuel electrode) is increased, and the power generation distribution of the anode layer 22 in the direction of fuel gas flow can be made more uniform.

[0036] Furthermore, regarding the case where the degree of curvature (τ3) of the ionic conducting phase (YSZ phase 34) of the anode layer 22 is changed, in this case, the relationship between the degree of curvature (τ3) of this ionic conducting phase and the effective ionic conductivity (σe3) considering its structure is as follows: Effective ionic conductivity σe3 = (ε3 / τ3) × σ3···(5) ε3: Volume fraction of the ionic conduction phase τ3: Degree of curvature of the ionic conduction phase σ3: Ionic conductivity of the ionic conduction phase As can be seen from equation (5), the smaller the degree of bending (τ2) of this ionic conducting phase, the greater the effective electronic conductivity (σe3).

[0037] In this case, as described above, the degree of curvature (τ) of the ion conducting phase 34 in the upstream portion 22a of the anode layer 22. 3a ) is the degree of curvature (τ) of the ion conducting phase 32 in the downstream portion 22b. 3b (τ) is configured to be larger than 3a >τ 3b ). By configuring it in this way, the effective ionic conductivity of the downstream portion 22b of the anode layer 22 becomes greater than that of the upstream portion 22a, making it easier for ions to flow in this downstream portion 22b. Therefore, even with this configuration, the power generation reactivity (power generation performance) of the downstream portion 22b of the anode layer 22 (fuel electrode) is increased, and the power generation distribution of the anode layer 22 in the direction of fuel gas flow can be made more uniform.

[0038] In the above-described reference embodiment, the degree of curvature of one of the electron conduction phase 32 (Ni phase), ionic conduction phase 34 (YSZ phase), and void phase 36 in the anode layer 22 is varied between its upstream portion 22a and its downstream portion 22b. However, the degree of curvature of any two of the electron conduction phase 32, ionic conduction phase 34, and void phase 36 may be varied, or the degree of curvature of all three phases may be varied.

[0039] In the above-described reference embodiment, the degree of curvature of the electron conduction phase 32 (Ni phase), ion conduction phase 34 (YSZ phase), and void phase 36 in the anode layer 22 is varied between its upstream portion 22a and its downstream portion 22b. However, the degree of curvature of the cathode layer 24 may also be varied in addition to the anode layer 22.

[0040] For example, when the cathode layer 24 is formed from LSM-YSZ, this LSM-YSZ has a cathode three-phase interface, where the LSM phase functions as the electron conduction phase and the YSZ phase functions as the ion conduction phase, and in addition to these electron conduction and ion conduction phases, it also contains a void phase. In order to homogenize the reaction of the cathode layer 24 in the direction of flow of the oxidizing gas (e.g., air), similar to the anode layer 22, the curvature of the void phase (or electron conduction phase, ion conduction phase) in the upstream portion 24a of the cathode layer 24, which is located upstream in the direction of flow of the oxidizing gas, is configured to be greater than the curvature of the void phase (or electron conduction phase, ion conduction phase) in the downstream portion 24b of the cathode layer 24, which is located downstream in the direction of flow of the oxidizing gas.

[0041] With this configuration, the effective diffusion coefficient (or effective electronic conductivity, effective ionic conductivity) of the downstream portion 24b of the cathode layer 24 becomes larger than that of the upstream portion 24b, and the gas diffusivity (or electronic conductivity, ionic conductivity) of this downstream portion 24b becomes higher than that of the upstream portion 24a. Consequently, the reactivity of the downstream portion 24b of the cathode layer 24 (air electrode) becomes higher, and as a result, the reaction distribution of the cathode phase 24 in the flow direction of the oxidizing gas can be made more uniform.

[0042] In the above-described reference embodiment, the degree of curvature of one of the void phase, electron conduction phase, and ion conduction phase in the upstream portion 24a of the cathode layer 24 is made greater than the degree of curvature of the void phase, electron conduction phase, and ion conduction phase in the downstream portion 24b. However, the degree of curvature of any two of the void phase, electron conduction phase, and ion conduction phase in the upstream portion 24a may be made greater, or the degree of curvature of all three phases may be made greater.

[0043] Furthermore, for example, if the cathode layer 24 is formed from LSCF (LSC), this LSCF (LSC) has an anode two-phase interface, and the LSCF phase (LSC phase) functions as an electron-ionic conduction phase (having the functions of both an electron conduction phase and an ionic conduction phase) in which both electrons and ions are conducted, and in addition to this electron-ionic conduction phase, it also contains a void phase. In this case, in order to homogenize the reaction of the cathode layer 24, the curvature of the void phase (or electron / ion conduction phase) in the upstream portion 24a of the cathode layer 24 is configured to be greater than the curvature of the void phase (or electron / ion conduction phase) in the downstream portion 24b of the cathode layer 24. With this configuration, similar to that having a three-phase interface, the effective diffusion coefficient (or effective electronic conductivity / effective ionic conductivity) of the downstream portion 24b of the cathode layer 24 becomes greater than the effective diffusion coefficient (or effective electronic conductivity / effective ionic conductivity) of the upstream portion 24b, and the gas diffusivity (or electronic conductivity / ionic conductivity) of this downstream portion 24b becomes higher. Consequently, the reactivity of the downstream portion 24b of the cathode layer 24 (air electrode) becomes higher, and as a result, the reaction distribution of the cathode layer 24 in the direction of oxidizing gas flow can be homogenized.

[0044] In the above-described reference embodiment, the degree of curvature of either the void phase or the electron / ion conduction phase in the upstream portion 24a of the cathode layer 24 is made greater than the degree of curvature of the void phase and the electron / ion conduction phase in the downstream portion 24b. However, the degree of curvature of both the void phase and the electron / ion conduction phase in the upstream portion 24a may be made greater.

[0045] Next, a first embodiment of a solid oxide fuel cell according to the present invention will be described. In the above-described reference embodiment, attention is focused on the degree of curvature of the void phase, electron conduction phase, and ion conduction phase of the anode layer, but in this first embodiment, attention is focused on the density of the anode three-phase interface of the anode phase to achieve uniformity of the power generation distribution. In the following embodiments of the present invention, components that are substantially the same as those in the above-described reference embodiment will be given the same numbers, and their descriptions will be omitted.

[0046] In this first embodiment, the basic configuration of the solid oxide fuel cell is the same as in the reference embodiment described above, but the structure of the anode layer and cathode layer differs from that of the reference embodiment. In this first embodiment, the anode layer 22A of the solid oxide fuel cell 2A is formed from, for example, Ni-YSZ, similar to the reference embodiment, with the Ni phase 32 functioning as the electron conduction phase, the YSZ phase 34 functioning as the ion conduction phase, and the void phase 36 functioning as the gas diffusion phase that diffuses the fuel gas (for example, hydrogen). The point where the three phases, the electron conduction layer 32 (Ni phase), the ion conduction phase 34 (YSZ phase), and the void phase 36, come into contact becomes the anode three-phase interface 38, where the power generation reaction takes place.

[0047] The ratio of the lengths of this anode three-phase interface 38 and the maximum power density (W / cm²) 2 The relationship shown in Figure 7, for example (quoted from Japanese Patent Publication No. 2015-176774 as a reference), is that as the ratio of the lengths of the anode three-phase interface 38 of the anode layer 22A increases, the maximum power density also increases. In other words, as the density of this anode three-phase interface 38 increases, the generated current of the anode layer 22A also increases. Note that 40 μm, 20 μm, 10 μm, and 5 μm in Figure 7 are the sizes of the ion conduction phase 34 (YSZ layer), and the maximum power density can be further improved by further miniaturizing the size of the ion conduction phase 34.

[0048] The density of the anode three-phase interface in this anode layer 22A can be adjusted by changing, for example, the particle size of the electron conduction phase 32 (Ni), the particle size of the ion conduction phase 34 (YSZ), the pore-forming material (void diameter), or the firing time and / or firing temperature when fabricating the electrode (anode layer). Generally, the smaller the particle size of the material used, the higher the density of the anode three-phase interface tends to be.

[0049] For this reason, in the first embodiment of the present invention, the density of the anode three-phase interface 38 is varied so that the density of the anode three-phase interface 38 in the downstream portion of the anode layer 22A is greater than the density of the anode three-phase interface 38 in the upstream portion, and the density of the anode three-phase interface 38 in the upstream portion is, for example, 2 to 4 μm / μm 3 To that extent, and the density of the anode three-phase interface 38 in the downstream portion is, for example, 3-5 μm / μm 3 It was structured to achieve a certain degree.

[0050] By configuring the anode layer 22A in this way, the density of the anode three-phase interface 38 in the downstream portion is greater than in the upstream portion, increasing the power generation responsiveness (power generation performance) of this downstream portion. As a result, the power generation distribution of the anode layer 22A in the direction of fuel gas flow can be made more uniform.

[0051] In the first embodiment described above, the density of the anode three-phase interface 38 in the anode layer 22A is varied between its upstream and downstream portions. However, it is also possible to vary the density of the cathode three-phase interface or two-phase interface of the cathode layer in addition to the anode layer 22A.

[0052] For example, when the cathode layer is formed from LSM-YSZ, as described above, this LSM-YSZ includes an LSM phase as an electron conduction layer, a YSZ phase as an ion conduction layer, and a void phase as a gas diffusion phase. The points where these three phases—electron conduction phase, ion conduction phase, and void phase—come into contact constitute the cathode three-phase interface, and the reaction on the cathode layer side takes place at this cathode three-phase interface. In such a cathode layer, in order to homogenize the reaction in the flow direction of the oxidizing gas (e.g., air), the cathode three-phase interface density in the downstream portion of the cathode layer is configured to be greater than the density of the cathode three-phase interface in the upstream portion, similar to the anode layer 22A.

[0053] By configuring it in this way, the reactivity in the downstream portion of the cathode layer becomes higher than in the upstream portion, and the reaction distribution of the cathode layer in the direction of oxidant gas flow can be made more uniform.

[0054] Furthermore, for example, when the cathode layer is formed from LSCF (LSC), this LSCF (LSC) contains an LSCF phase (LSC phase) as an electron / ion conduction phase and a void phase as a gas diffusion phase. The area where the electron / ion conduction phase and the void phase come into contact constitutes the cathode two-phase interface, and the reaction on the cathode layer side takes place at this cathode two-phase interface. In this case, the density of the cathode two-phase interface in the downstream portion of the cathode layer is configured to be greater than the density of the cathode two-phase interface in the upstream portion.

[0055] By configuring it in this way, the reactivity of the downstream portion of the cathode layer becomes higher, similar to that of a cathode three-phase interface, and the reaction distribution of the cathode layer in the flow direction of the oxidizing gas can be made more uniform.

[0056] Next, a second embodiment of the solid oxide fuel cell according to the present invention will be described. In the reference embodiment, attention was paid to the degree of curvature of the void phase, electron conduction phase and ion conduction phase of the anode layer, and in the first embodiment of the present invention, attention was paid to the density of the anode three-phase interface of the anode phase. In this second embodiment, attention is paid to the ratio of reactivity between the ion conduction phase and the electron conduction phase of the anode layer in order to make the power generation distribution of the anode layer more uniform.

[0057] In this second embodiment, the basic configuration of the solid oxide fuel cell is the same as that of the reference embodiment described above and the first embodiment of the present invention, but the ratio of the reactivity between the ion conduction phase (YSZ phase) and the electron conduction phase (Ni phase) in the anode layer differs from that of the reference embodiment and the first embodiment.

[0058] In this second embodiment, although not shown, the anode layer of the solid oxide fuel cell is formed from, for example, Ni-YSZ and contains a Ni phase as the electron conduction phase, a YSZ phase as the ion conduction phase, and a void phase as the gas diffusion phase.

[0059] The volume fraction (Vaε) of the ionic conducting phase (YSZ phase) in this anode layer is: Volume fraction Vaε=V YSZ / (V Ni +V YSZ ) ···(6) V Ni : Volume of the electron conduction phase V YSZ : Volume of the ionic conducting phase It is represented as follows.

[0060] The relationship between the volume ratio of the ion-conducting phase in the anode layer and the overpotential (Arb.unit) of the anode layer is as shown in Figure 8, for example (quoted from Japanese Patent Publication No. 2015-176774 as a reference). When the volume ratio of the ion-conducting phase (Vaε) increases in the range of 0.2 to 0.8, the overpotential (voltage loss) of the anode layer decreases, and the power generation reactivity (power generation performance) of the anode layer increases.

[0061] Given this relationship with respect to the volume ratio (Vaε) of the ion conduction phase, in this third embodiment, the overpotential (voltage loss) due to the volume ratio of the ion conduction phase (YSZ phase) of the anode phase is changed. In other words, the composition ratio relating to the reactivity of the ion conduction phase (YSZ phase) and the electron conduction phase (Ni phase) is changed. The downstream portion of the anode layer is configured to have a high reactivity ratio between the ion conduction phase (YSZ phase) and the electron conduction phase (Ni phase), while the upstream portion is configured to have a low reactivity ratio between the ion conduction phase (YSZ phase) and the electron conduction phase (Ni phase). For example, the ratio of the ion conduction phase to the electron conduction phase in the upstream portion of the anode layer is configured to be approximately 0.2~0.6:0.8~0.4, and the ratio of the ion conduction phase to the electron conduction phase in the downstream portion is configured to be approximately 0.4~0.8:0.6~0.2.

[0062] By configuring the anode layer in this way, the overvoltage (voltage loss) in the downstream portion of the anode layer becomes smaller than in the upstream portion, resulting in higher power generation responsiveness (power generation performance) and enabling a more uniform power generation distribution in the anode layer along the fuel gas flow direction.

[0063] In this second embodiment, in addition to changing the composition ratio of the reactivity between the ionic conduction phase and the electronic conduction phase of the anode layer between its upstream and downstream portions, the composition ratio of the reactivity between the ionic conduction phase and the electronic conduction phase of the cathode layer may also be changed.

[0064] For example, when the cathode layer is formed from LSM-YSZ, this LSM-YSZ contains an LSM phase as an electron conduction layer, a YSZ phase as an ion conduction layer, and a void phase as a gas diffusion phase. In this case, similar to the anode layer, the composition ratio of the reactivity between the ion conduction phase (YSZ phase) and the electron conduction phase (LSM phase) is varied in the cathode layer, so that the ion conduction phase and the electron conduction phase are highly reactive in the downstream portion of the cathode layer, and less reactive in the upstream portion.

[0065] By configuring it in this way, the overvoltage (voltage loss) in the downstream portion of the cathode layer is reduced, increasing reactivity and making the reaction distribution of the cathode layer more uniform in the direction of oxidant gas flow.

[0066] Although reference embodiments and the first and second embodiments of the solid oxide fuel cell have been described above, the present invention is not limited to these embodiments, and various changes and modifications are possible without departing from the scope of the present invention.

[0067] For example, in the above-described reference embodiment, a technique for changing the degree of curvature of the electron conduction phase (ionic conduction phase, void phase) in the upstream and downstream portions of the anode layer was applied and explained; in the above-described first embodiment, a technique for changing the density of the anode three-phase interface in the upstream and downstream portions of the anode layer was applied and explained; and in the above second embodiment, a technique for changing the composition ratio related to the reactivity of the ionic conduction phase and the electron conduction phase in the upstream and downstream portions of the anode layer was applied and explained. However, the present invention does not require the application of these three techniques separately. Any two of the techniques related to the degree of curvature of the anode layer, the anode three-phase interface of the anode layer, and the composition ratio related to the reactivity of the anode layer can be applied in combination, and furthermore, all three of these techniques can be applied in combination.

[0068] For example, consider a solid oxide fuel cell in which the curvature of the void phase in the upstream portion of the anode layer, located upstream in the direction of fuel gas flow, is greater than that of the void phase in the downstream portion. Figure 9 shows the temperature and current density conditions from the fuel gas inlet to the outlet in this example.

[0069] On the other hand, using a conventional solid oxide fuel cell in which the curvature of the void phase of the anode layer is constant when viewed in the direction of fuel gas flow as a comparative example, the temperature and current density from the fuel gas inlet side to the outlet side in this comparative example are shown in Figure 10.

[0070] As can be easily understood by comparing Figures 9 and 10, in the reference example where the curvature of the void phase in the downstream portion of the anode layer is made smaller than that of the void phase in the upstream portion, the power generation reaction in the downstream portion of the anode layer can be enhanced. This allows for suppressing the power generation current on the inlet side and increasing the power generation current on the outlet side. By thus homogenizing the power generation current distribution of the anode layer, the temperature rise gradient on the inlet side can be kept small, and the generation of thermal stress can be reduced.

[0071] In contrast, in the comparative example where the curvature of the void phase is constant in the direction of fuel gas flow, the power generation reaction is concentrated on the inlet side where the fuel gas concentration is high. As a result, the generated current is large on the inlet side and low on the outlet side. This causes a localized temperature increase on the inlet side, and the temperature rise gradient becomes large. Such a large temperature rise gradient may lead to damage due to thermal stress. [Explanation of Symbols]

[0072] 2-cell stack 6. Solid oxide fuel cell 20 Electrolytes 22,22A Anode layer (fuel electrode) 24 Cathode layer (air electrode) 32 Ni phase (electron conduction phase) 34 YSZ phase (ionic conduction phase) 36. Void phase (gas diffusion phase) 38 Anode three-phase interface

Claims

1. A solid oxide fuel cell having an electrolyte layer having ionic conductivity that allows oxide ions to pass through, an anode layer provided on one side of the electrolyte layer, and a cathode layer provided on the other side of the electrolyte layer, The anode layer comprises an electron conduction phase that conducts electrons, an ion conduction phase that conducts ions, and a void phase that diffuses fuel gas, and the electron conduction phase, the ion conduction phase and the void phase constitute an anode three-phase interface at the portion where these three phases are in contact. A solid oxide fuel cell characterized in that the density of the anode three-phase interface in the downstream portion of the anode layer located downstream in the direction of fuel gas flow is greater than the density of the anode three-phase interface in the upstream portion of the anode layer located upstream in the direction of fuel gas flow.

2. The solid oxide fuel cell according to claim 1, wherein the cathode layer comprises an electron conduction phase that conducts electrons, an ion conduction phase that conducts ions, and a void phase that diffuses an oxidizing gas, and the electron conduction phase, the ion conduction phase and the void phase constitute a cathode three-phase interface at the portion where these three phases are in contact, and the density of the cathode three-phase interface in the downstream portion of the cathode layer located downstream in the direction of oxidizing gas flow is greater than the density of the cathode three-phase interface in the upstream portion of the cathode layer located upstream in the direction of oxidizing gas flow.

3. The solid oxide fuel cell according to claim 1, wherein the cathode layer comprises an electron-ion conduction phase that conducts electrons and ions, and a void phase that diffuses an oxidizing gas, and the electron-ion conduction phase and the void phase constitute a cathode two-phase interface at the portion where these two phases are in contact, and the density of the cathode two-phase interface in the downstream portion of the cathode layer located downstream in the direction of oxidizing gas flow is greater than the density of the cathode two-phase interface in the upstream portion of the cathode layer located upstream in the direction of oxidizing gas flow.

4. A solid oxide fuel cell having an electrolyte layer having ionic conductivity that allows oxide ions to pass through, an anode layer provided on one side of the electrolyte layer, and a cathode layer provided on the other side of the electrolyte layer, The anode layer comprises an electron conduction phase that conducts electrons, an ion conduction phase that conducts ions, and a void phase that diffuses fuel gas. A solid oxide fuel cell characterized in that, in the downstream portion of the anode layer located downstream in the flow direction of the fuel gas, the ion conduction phase and the electron conduction phase are highly reactive, and in the upstream portion of the anode layer, the ion conduction phase and the electron conduction phase are less reactive.

5. The solid oxide fuel cell according to claim 4, wherein the cathode layer comprises an electron conduction phase that conducts electrons, an ion conduction phase that conducts ions, and a void phase that diffuses an oxidizing gas, and in the downstream portion of the cathode layer located downstream in the flow direction of the oxidizing gas, the ion conduction phase and the electron conduction phase are highly reactive, and in the upstream portion of the anode layer, the ion conduction phase and the electron conduction phase are less reactive.

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