Patent Publication Number: US-2023163338-A1

Title: Cell, module and module housing device

Description:
TECHNICAL FIELD 
     The present disclosure relates to a cell, a module and a module housing device. 
     BACKGROUND ART 
     Recently, various fuel cells, and cell stack devices including a plurality of fuel cells have been proposed as next-generation energy sources. A fuel cell is a type of cell capable of obtaining electrical power by using a fuel gas such as a hydrogen-containing gas and an oxygen-containing gas such as air. 
     In such a fuel cell, for example, an intermediate layer may be provided between a solid electrolyte layer and an air electrode in an element portion in the fuel cell (see Patent Document 1). 
     CITATION LIST 
     Patent Literature 
     Patent Document 1: JP 2015-35416 A 
     SUMMARY OF INVENTION 
     According to one aspect of the embodiments, a cell includes an element portion including a fuel electrode, a solid electrolyte layer, an air electrode, and an intermediate layer located between the solid electrolyte layer and the air electrode. The solid electrolyte layer or the intermediate layer includes a first site, and a second site that is located closer to the air electrode or closer to a center part of the element portion than the first site and that has a smaller porosity or a lower density than the first site. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1 A  is a horizontal cross-sectional view illustrating an example of a cell according to an embodiment. 
         FIG.  1 B  is a side view of the example of the cell according to the embodiment, as viewed from an interconnector side. 
         FIG.  2 A  is a perspective view illustrating an example of a cell stack device according to the embodiment. 
         FIG.  2 B  is a cross-sectional view taken along line X-X illustrated in  FIG.  2 A . 
         FIG.  2 C  is a top view illustrating the example of the cell stack device according to the embodiment. 
         FIG.  3    is a side view of the example of the cell according to the embodiment, as viewed from an air electrode side. 
         FIG.  4    is a diagram for illustrating an outer peripheral part and center part of an element portion according to the embodiment. 
         FIG.  5    is an enlarged cross-sectional view illustrating an example of the element portion according to the embodiment. 
         FIG.  6    is a diagram showing the porosity of each region in the outer peripheral part and center part of the element portion. 
         FIG.  7    is an enlarged horizontal cross-sectional view illustrating the example of the cell according to the embodiment. 
         FIG.  8    is a side view illustrating another example of the element portion according to the embodiment. 
         FIG.  9    is a side view illustrating another example of the element portion according to the embodiment. 
         FIG.  10    is an exterior perspective view illustrating an example of a module according to an embodiment. 
         FIG.  11    is an exploded perspective view schematically illustrating an example of a module housing device according to an embodiment. 
         FIG.  12    is a cross-sectional view illustrating a cell according to a first variation of the embodiment. 
         FIG.  13    is a perspective view illustrating a flat plate cell according to a second variation of the embodiment. 
         FIG.  14    is a diagram for describing an outer peripheral part and a center part of an element portion according to a second variation of the embodiment. 
         FIG.  15    is a bottom view illustrating the example of the element portion according to the second variation of the embodiment. 
         FIG.  16    is a bottom view illustrating another example of the element portion according to the second variation of the embodiment. 
         FIG.  17    is a bottom view illustrating another example of the element portion according to the second variation of the embodiment. 
         FIG.  18    is a bottom view illustrating another example of the element portion according to the second variation of the embodiment. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a cell, a module and a module housing device disclosed in the present application will now be described in detail with reference to the accompanying drawings. The present invention is not limited by the following embodiments. 
     Note that the drawings are schematic, and the relationship between the dimensions of each element, the ratio of each element, or the like may differ from reality. There may be cases in which portions of the drawings differ from each other in relation to each other in terms of dimension, ratio, etc. 
     Recently, various fuel cells, and cell stack devices including a plurality of fuel cells have been proposed as next-generation energy sources. A fuel cell is a type of cell capable of obtaining electrical power by using a fuel gas such as a hydrogen-containing gas and an oxygen-containing gas such as air. 
     In such a fuel cell, for example, an intermediate layer may be provided between a solid electrolyte layer and an air electrode in an element portion in the fuel cell. 
     However, in the fuel cell described above, the durability of the fuel cell can be improved. 
     Thus, the achievement of a technology capable of enhancing the durability of the fuel cell is expected. 
     Configuration of Cell 
     Referring to  FIGS.  1 A and  1 B , an example of a solid oxide fuel cell will be described as a cell according to an embodiment. 
       FIG.  1 A  is a horizontal cross-sectional view illustrating an example of a cell  1  according to an embodiment.  FIG.  1 B  is a side view of the example of the cell  1  according to the embodiment, as viewed from the interconnector  4  side.  FIG.  1 A  and  FIG.  1 B  are enlarged views illustrating portions of the respective configurations of the cell  1 . The example of the cell  1  viewed from an air electrode  8  side will be described later. 
     In the example illustrated in  FIGS.  1 A and  1 B , the cell  1  is a hollow and flat plate, elongated in shape. As illustrated in  FIG.  1 B , the overall shape of the cell  1  viewed from the side surface is, for example, a rectangle in which the side length in a length direction L is from 5 cm to 50 cm and the length in a width direction W perpendicular to the length direction L is from 1 cm to 10 cm. The overall thickness in a thickness direction T of the cell  1  is between 1 mm to 5 mm. 
     As illustrated in  FIG.  1 A , the cell  1  includes a conductive support substrate  2 , an element portion  3 , and an interconnector  4 . The support substrate  2  has a pillar shape with a pair of opposing flat surfaces n 1 , n 2  and a pair of side surfaces m in circular arc shape connecting the flat surfaces n 1 , n 2 . 
     The element portion  3  is provided on the flat surface n 1  of the support substrate  2 . The element portion  3  such as that described above includes a fuel electrode  5 , a solid electrolyte layer  6 , an intermediate layer  7 , and an air electrode  8 . In the example illustrated in  FIG.  1 A , the interconnector  4  is provided on the flat surface n 2  of the cell  1 . 
     As illustrated in  FIG.  1 B , the interconnector  4  may extend to the upper and lower ends of the cell  1 . At a lower end portion of the cell  1 , the interconnector  4  and the solid electrolyte layer  6  are exposed on the surface. As illustrated in  FIG.  1 A , the solid electrolyte layer  6  is exposed at the surface at the pair of side surfaces m in a circular arc shape of the cell  1 . The interconnector  4  need not extend to the lower end of the cell  1 . The cell  1  may include a reinforcing layer  9  described below in the region between the lower end and the interconnector  4 . 
     Hereinafter, each of the components constituting the cell  1  will be described. 
     The support substrate  2  includes gas-flow passages  2   a  in which gas flows. The example of the support substrate  2  illustrated in  FIG.  1 A  includes six gas-flow passages  2   a . The support substrate  2  has gas permeability and allows the fuel gas flowing through the gas-flow passages  2   a  to pass through to the fuel electrode  5 . The support substrate  2  may be electrically conductive. The support substrate  2  having conductivity collects electricity generated in the element portion  3  to the interconnector  4 . 
     The material of the support substrate  2  includes, for example, an iron group metal component and an inorganic oxide. For example, the iron group metal component may be Ni (nickel) and/or NiO. The inorganic oxide may be a specific rare earth element oxide. 
     Generally known materials can be used for the fuel electrode  5 . The fuel electrode  5  may be a porous electrically conductive ceramic, for example, a ceramic that contains ZrO 2  in which calcium oxide, magnesium oxide or rare earth element oxide are in solid solution, and Ni and/or NiO. As the rare earth element oxide, for example, Y 2 O 3  or the like is used. ZrO 2  in which calcium oxide, magnesium oxide or rare earth element oxide are in solid solution is sometimes called stabilized zirconia. The stabilized zirconia also includes partially stabilized zirconia. 
     The solid electrolyte layer  6  is an electrolyte that bridges ions between the fuel electrode  5  and the air electrode  8 . At the same time, the solid electrolyte layer  6  has a gas blocking property and hardly causes leakage of fuel gas and oxygen-containing gas. 
     The material of the solid electrolyte layer  6  may be, for example, ZrO 2  in which 3 to 15 mole % of a rare earth element oxide is in solid solution. As the rare earth element oxide, for example, Y 2 O 3  or the like is used. As long as the solid electrolyte layer  6  has the above characteristics, other materials may be used for the solid electrolyte layer  6 . 
     The intermediate layer  7  functions as a diffusion prevention layer. When strontium (Sr) contained in the air electrode  8 , which will be described later, diffuses into the solid electrolyte layer  6 , a resistive layer of SrZrO 3  is formed on the solid electrolyte layer  6 . The intermediate layer  7  suppresses the diffusion of Sr and makes it difficult to form SrZrO 3 . 
     The material of the intermediate layer  7  is not particularly limited as long as generally used for the diffusion prevention layer of Sr. The material of the intermediate layer  7  includes, for example, cerium oxide (CeO 2 ) in which a rare earth element except Ce (cerium) is in solid solution. As the rare earth element, gadolinium (Gd), samarium (Sm), or the like are used. 
     The material of the air electrode  8  is not particularly limited as long as generally used for the air electrode. The material of the air electrode  8  may be, for example, an electrically conductive ceramic such as a so-called ABO 3 -type perovskite oxide. 
     The material of the air electrode  8  may be, for example, a composite oxide in which Sr and La coexist at the A site. Examples of such a composite oxide include La x Sr 1-x Co y Fe 1-y O 3 , La x Sr 1-x MnO 3 , La x Sr 1-x FeO 3 , and La x Sr 1-x CoO 3 . Note that x satisfies 0&lt;x&lt;1 and y satisfies 0&lt;y&lt;1. 
     The air electrode  8  has gas permeability. The open porosity of the air electrode  8  may be, for example, 20% or more, and is particularly in the range of from 30% to 50%. 
     The material of the interconnector  4  may be a lanthanum chromite-based perovskite oxide (LaCrO 3 -type oxide) or a lanthanum strontium titanium-based perovskite oxide (LaSrTiO 3 -based oxide). These materials are electrically conductive and are not reduced or oxidized upon contact with a fuel gas such as a hydrogen-containing gas, and an oxygen-containing gas such as air. 
     The interconnector  4  is dense and hardly causes leakage of fuel gas flowing through the gas-flow passage  2   a  inside the support substrate  2  and oxygen-containing gas flowing outside the support substrate  2 . The interconnector  4  may have a relative density of 93% or more, particularly 95% or more. 
     Configuration of Cell Stack Device 
     The cell stack device  10  according to the present embodiment using the cell  1  described above will be described with reference to  FIGS.  2 A to  2 C .  FIG.  2 A  is a perspective view illustrating an example of a cell stack device  10  according to the embodiment.  FIG.  2 B  is a cross-sectional view taken along line A-A illustrated in  FIG.  2 A .  FIG.  2 C  is a top view illustrating an example of the cell stack device  10  according to the embodiment. 
     As illustrated in  FIG.  2 A , the cell stack device  10  includes a cell stack  11  including a plurality of cells  1  arranged (stacked) in the thickness direction T (see  FIG.  1 A ) of the cell  1 , and a fixing member  12 . 
     The fixing member  12  includes a fixing material  13  and a support member  14 . The support member  14  supports the cells  1 . The fixing material  13  fixes the cells  1  to the support member  14 . The support member  14  includes a support body  15  and a gas tank  16 . The support body  15  and the gas tank  16 , which constitute the support member  14 , are made of a metal and have electrical conductivity. 
     As illustrated in  FIG.  2 B , the support body  15  includes an insertion hole  15   a  into which the lower end portions of the plurality of cells  1  are inserted. The lower end portions of the plurality of cells  1  and the inner wall of the insertion hole  15   a  are joined by the fixing material  13 . 
     The gas tank  16  includes an opening portion for supplying a reactive gas to the plurality of cells  1  through the insertion hole  15   a , and includes a recessed groove  16   a  provided around the opening portion. One end portion of the support body  15  is joined to the gas tank  16  by a bonding material  21  filled in the recessed groove  16   a  of the gas tank  16 . 
     In the example illustrated in  FIG.  2 A , fuel gas is stored in an internal space  22  formed by the support body  15  and the gas tank  16 , which constitute the support member  14 . A gas flow pipe  20  is connected to the gas tank  16 . The fuel gas is supplied through the gas flow pipe  20  to the gas tank  16 , and is supplied from the gas tank  16  to the gas-flow passage  2   a  (see  FIG.  1 A ) inside the cell  1 . The fuel gas supplied to the gas tank  16  is generated by a reformer  102  (see  FIG.  10   ) described later. 
     The hydrogen-rich fuel gas can be produced by steam reforming the raw fuel. When the fuel gas is generated by steam reforming, the fuel gas contains water vapor. 
     In the example illustrated in  FIG.  2 A , two rows of cell stacks  11  having a plurality of cells  1  are provided, and each row of cell stacks  11  is fixed to the support body  15 . Two through holes are provided on the upper surface of the gas tank  16 . Each support body  15  is disposed in the two through holes so as to be aligned with the insertion hole  15   a . The internal space  22  is formed of one gas tank  16  and two support bodies  15 . 
     The insertion hole  15   a  has, for example, an oval shape in top surface view. The insertion hole  15   a  has, for example, a length in the arrangement direction or thickness direction T of the cell  1 , which is greater than the distance between the two end current collectors  17  located at opposite ends of the cell stack  11 . The width of the insertion hole  15   a  is greater than, for example, the length in the width direction W of the cell  1  (see  FIG.  1 A ). 
     As illustrated in  FIG.  2 B , the fixing material  13 , which has solidified, fills the joint between the inner wall of the insertion hole  15   a  and the lower end portion of each cell  1 . Thus, the inner wall of the insertion hole  15   a  and each of the lower end portions of the plurality of cells  1  are joined and fixed, and the lower end portions of the cells  1  are joined and fixed to each other. The gas-flow passage  2   a  of each cell  1  communicates at the lower end portion with the internal space  22  of the support member  14 . 
     The fixing material  13  and the bonding material  21  may be of low conductivity, such as a glass. The specific material may be an amorphous glass or the like, or a crystallized glass. 
     As the crystallized glass, for example, a SiO 2 —CaO system, a MgO—B 2 O 3  system, a La 2 O 3 —B 2 O 3 —MgO system, a La 2 O 3 —B 2 O 3 —ZnO system or a SiO 2 —CaO—ZnO system can be adopted, but any material of a SiO 2 —MgO system may be used. 
     As illustrated in  FIG.  2 B , a conductive member  18  is interposed between adjacent cells  1  among the plurality of cells  1 . The conductive member  18  electrically connects the fuel electrode  5  of one adjacent cell  1  and the air electrode  8  of the other adjacent cell  1  in series. The details of the conductive member  18  connected between the adjacent cells  1  will be described later. 
     As illustrated in  FIG.  2 B , the end current collector  17  is connected to each of the outermost cells  1  in the arrangement direction of the plurality of cells  1 . The end current collector  17  is connected to a conductive portion  19  protruding outside the cell stack  11 . The conductive portion  19  collects electricity generated by the power generation of the cells  1  and draws it to the outside. In  FIG.  2 A , the end current collectors  17  are not illustrated. 
     As illustrated in  FIG.  2 C , the cell stack device  10 , in which two cell stacks  11 A and  11 B, both formed by the cells  1  in a line, are connected in series, functions as a single battery. Thus, the conductive portions  19  of the cell stack device  10  are distinguished into a positive terminal  19 A, a negative terminal  19 B, and a connection terminal  19 C. 
     The positive terminal  19 A is a positive electrode for outputting the electrical power generated by the cell stacks  11  to the outside, and is electrically connected to the end current collector  17  on the positive electrode side of the cell stack  11 A. The negative terminal  19 B is a negative electrode for outputting the electrical power generated by the cell stacks  11  to the outside, and is electrically connected to the end current collector  17  on the negative electrode side of the cell stack  11 B. 
     The connection terminal  19 C electrically connects the end current collector  17  on the negative side in the cell stack  11 A and the end current collector  17  on the positive side in the cell stack  11 B. 
     Details of Element Portion 
     Details of the element portion  3  according to the embodiment will be described with reference to  FIGS.  3  to  9   .  FIG.  3    is a side view of an example of the cell  1  according to the embodiment, as viewed from the air electrode  8  side. 
     As illustrated in  FIG.  3   , the intermediate layer  7  is formed over the entire surface of the solid electrolyte layer  6  as viewed from the air electrode  8  side, except for the upper and lower end portions of the cell  1 . In other words, third sites  30  in which the intermediate layer  7  is not located on the surface of the solid electrolyte layer  6 , are provided at the upper and lower end portions of the cell  1  respectively, as viewed from the air electrode  8  side. 
     Such third sites  30  are provided along at least two sides of the cell  1  respectively, and each have a predetermined width, for example, about 5 mm, from each side. In the embodiment, the third sites  30  are formed with approximately equal widths along the top and bottom sides as the two sides of the cell  1 , respectively. 
     A reinforcing layer  9  is provided between the solid electrolyte layer  6  and the intermediate layer  7  above the third site  30  at the lower end portion of the cell  1 . When the lower end portion of the cell  1  is fixed by a fixing member  12  (see  FIG.  2 A ), the cell  1  may be damaged by stress from the fixing member  12 . The reinforcing layer  9  reduces the likelihood of the cell  1  being damaged when the cell  1  is fixed. 
     When the interconnector  4  does not extend to the lower end portion of the cell  1 , the reinforcing layer  9  may be provided on the flat surface  2   n  of the support substrate  2  in the region between the lower end portion of the cell  1  and the interconnector  4 . 
     The reinforcing layer  9  is made of, for example, ZrO 2  in which 3 to 15 mole % of a rare earth element oxide is in solid solution. As the rare earth element oxide, for example, Y 2 O 3  or the like is used. As long as the reinforcing layer  9  has the above characteristics, the reinforcing layer  9  may be formed using other materials or the like. 
     The air electrode  8  is provided on the surface of the intermediate layer  7  in a region between the third site  30  on the upper side of the cell  1  and the reinforcing layer  9 . 
     The element portion  3  of the cell  1  according to the embodiment includes an outer peripheral part  1   a  and a center part  1   b . The outer peripheral part  1   a  is a region located near each side when the cell  1  is viewed from the air electrode  8  side, and the center part  1   b  is a central region surrounded by the outer peripheral part  1   a  when the cell  1  is viewed from the air electrode  8  side. 
     The outer peripheral part  1   a  may be a region located near the contour of the element portion  3  or near the contour of the air electrode  8  when the cell  1  is viewed from the air electrode  8  side. In that case, the outer peripheral part  1   a  includes the contour of the element portion  3  or the air electrode  8 . The outer peripheral part  1   a  may include the inside of the contour of the element portion  3  in the cell  1  or may include the outside of the contour of the element portion  3 . 
     As illustrated in  FIG.  4   , in the element portion  3 , the outer peripheral part  1   a  is a region in which the distance from the long side along the length direction L is no more than a predetermined distance X 1  and the distance from the short side along the width direction W is no more than a predetermined distance X 2 .  FIG.  4    is a diagram for describing the outer peripheral part  1   a  and the center part  1   b  of the element portion  3  according to the embodiment. 
     In the embodiment, the distance X 1  is, for example, 25% of the length W 1  of the short side of the element portion  3 . The distance X 2  is, for example, 15% of the length L 1  of the long side of the element portion  3 . 
       FIG.  5    is an enlarged cross-sectional view illustrating an example of the element portion  3  according to the embodiment, which is an enlarged cross-sectional view of the intermediate layer  7  and its vicinity. As illustrated in  FIG.  5   , in the element portion  3 , the solid electrolyte layer  6  is layered on the fuel electrode  5  (see  FIG.  1 A ), the intermediate layer  7  is layered on the solid electrolyte layer  6 , and the air electrode  8  is layered on the intermediate layer  7 . 
     The intermediate layer  7  includes a first region  7   a  near the interface with the solid electrolyte layer  6  and a second region  7   b  near the interface with the air electrode  8 . The first region  7   a  is a region in which the distance from the interface between the solid electrolyte layer  6  and the intermediate layer  7  is no more than a predetermined distance X 3 . 
     The second region  7   b  is a region where the distance from the interface between the intermediate layer  7  and the air electrode  8  is equal to or less than a predetermined distance X 4 . In the embodiment, such distances X 3  and X 4  may be each, for example, ⅓ of the thickness T 1  of the intermediate layer  7 . 
     The solid electrolyte layer  6  includes a third region  6   a  near the interface with the intermediate layer  7  in a cross-sectional view. The third region  6   a  is a region where the distance from the interface between the solid electrolyte layer  6  and the intermediate layer  7  is no more than a predetermined distance X 5 . In the embodiment, such a distance X 5  may be, for example, ⅓ of the thickness T 1  of the intermediate layer  7 . 
     In the embodiment, the porosity of the first region  7   a , the second region  7   b , and the third region  6   a  described above is controlled to enhance the durability of the cell  1 .  FIG.  6    is a diagram showing the porosity of each region in the outer peripheral part  1   a  and the center part  1   b  of the element portion  3 . 
     As shown in  FIG.  6   , in the embodiment, the porosity of the intermediate layer  7  in the first region  7   a  near the interface with the solid electrolyte layer  6  is larger than the porosity of the intermediate layer  7  in the second region  7   b  near the interface with the air electrode  8 . 
     The intermediate layer  7  in the first region  7   a  near the interface with the solid electrolyte layer  6  is an example of the first site, and the intermediate layer  7  in the second region  7   b  near the interface with the air electrode  8  is an example of the second site. 
     In the embodiment, the porosity of the first region  7   a  is larger than that of the second region  7   b  in both the outer peripheral part  1   a  and the center part  1   b  of the element portion  3 . 
     Thus, by increasing the porosity of the first region  7   a  in the intermediate layer  7 , the first region  7   a  of the intermediate layer  7  can function as a stress relieving layer, so that the intermediate layer  7  can be made difficult to peel off from the solid electrolyte layer  6 . 
     By reducing the porosity of the second region  7   b  in the intermediate layer  7 , the function as the diffusion prevention layer of Sr can be maintained. 
     That is, in the embodiment, the intermediate layer  7  can maintain the function as the diffusion prevention layer of Sr, and can hardly be separated from the solid electrolyte layer  6 . Thus, according to the embodiment, the durability of the cell  1  can be enhanced. 
     In the embodiment, the porosity of the intermediate layer  7  in the outer peripheral part  1   a  of the element portion  3  is larger than the porosity of the intermediate layer  7  in the center part  1   b  of the element portion  3 . The intermediate layer  7  in the outer peripheral part  1   a  of the element portion  3  is another example of the first site, and the intermediate layer  7  in the center part  1   b  of the element portion  3  is another example of the second site. 
     In the embodiment, the porosity of the outer peripheral part  1   a  is larger than that of the center part  1   b  in both the first region  7   a  and the second region  7   b  of the intermediate layer  7 . 
     Thus, by increasing the porosity of the intermediate layer  7  in the outer peripheral part  1   a  of the element portion  3 , the intermediate layer  7  of the outer peripheral part  1   a , which contributes less to power generation than the center part  1   b , can be made to function with emphasis on the stress relieving effect rather than the diffusion prevention effect. 
     That is, in the embodiment, by increasing the porosity of the outer peripheral part  1   a , the intermediate layer  7  can be made difficult to peel off from the outer peripheral part  1   a  of the element portion  3 . Thus, according to the embodiment, the durability of the cell  1  can be enhanced. 
     In the embodiment, the porosity of the intermediate layer  7  may range from 5% to 30%. By setting the porosity of the intermediate layer  7  to 5% or more, the stress relieving effect due to the voids can be sufficiently obtained, and even when the cell  1  is subjected to a temperature cycle and a thermal stress is applied to the intermediate layer  7 , the intermediate layer  7  hardly peels off. 
     On the other hand, by setting the porosity of the intermediate layer  7  to 30% or less, the intermediate layer  7  can be made to have a strength such that the intermediate layer  7  does not peel off. 
     As described above, since the porosity of the intermediate layer  7  ranges from 5% to 30% in the embodiment, the intermediate layer  7  can be made difficult to peel off. Thus, according to the embodiment, the durability of the cell  1  can be enhanced. 
     The porosity of the intermediate layer  7  may specifically range from 10% to 30%, and further from 13% to 25%. The porosity of the first region  7   a  may range from 15% to 30%, particularly from 16% to 27%. The porosity of the second region  7   b  may range from 10% to 25% particularly from 12% to 22%. 
     In the embodiment, the average diameter of the voids formed in the intermediate layer  7  may be in the range from 0.1 μm to 1.0 μm, particularly from 0.2 μm to 0.8 μm. This makes it difficult for the intermediate layer  7  to peel off from the solid electrolyte layer  6  and suppresses the diffusion of Sr. 
     In the embodiment, the porosity of the third region  6   a  of the solid electrolyte layer  6  in the outer peripheral part  1   a  of the element portion  3  is larger than that of the third region  6   a  of the solid electrolyte layer  6  in the center part  1   b  of the element portion  3 . That is, in the embodiment, the third region  6   a  of the solid electrolyte layer  6  in the outer peripheral part  1   a  of the element portion  3  is denser than the third region  6   a  of the solid electrolyte layer  6  in the center part  1   b  of the element portion  3 . 
     Thus, in the outer peripheral part  1   a  of the element portion  3  in which peeling of the solid electrolyte layer  6  tends to occur, the strength of the solid electrolyte layer  6  can be enhanced by making the third region  6   a  dense. That is, in the embodiment, by densifying the solid electrolyte layer  6  of the outer peripheral part  1   a , the solid electrolyte layer  6  can be made difficult to peel off from the outer peripheral part  1   a  of the element portion  3 . 
     Thus, according to the embodiment, the durability of the cell  1  can be enhanced. 
     In the embodiment, it is described above that the third region  6   a  of the solid electrolyte layer  6  in the outer peripheral part  1   a  of the element portion  3  is denser than the third region  6   a  of the solid electrolyte layer  6  in the center part  1   b  of the element portion  3 . However, the entirety of the solid electrolyte layer  6  in the thickness direction of the solid electrolyte layer  6  in the outer peripheral part  1   a  of the element portion  3  may be denser than the entirety of the sold electrolyte layer  6  in the thickness direction of the solid electrolyte layer  6  in the center part  1   b  of the element portion  3 . The solid electrolyte layer  6  in the outer peripheral part  1   a  of the element portion  3  is another example of the first site, and the solid electrolyte layer  6  in the center part  1   b  of the element portion  3  is another example of the second site. 
     As a result, the strength of the solid electrolyte layer  6  on the whole in the outer peripheral part  1   a  of the element portion  3  can be enhanced, so that the solid electrolyte layer  6  hardly peels off from the outer peripheral part  1   a  of the element portion  3 . 
     In the cell  1  according to the embodiment, the location where the solid electrolyte layer  6  having a high density, i.e., a small porosity, is disposed, is not limited to the outer peripheral part  1   a .  FIG.  7    is an enlarged horizontal cross-sectional view illustrating an example of the cell  1  according to the embodiment. 
     As illustrated in  FIG.  7   , the cell  1  includes a curved portion  1   c  adjacent to the outer peripheral part  1   a . The curved portion  1   c  is the portion located near the air electrode  8 , among the side surfaces m having a circular arc shape. The curved portion  1   c  is, in other words, a portion of the side surfaces m where the distance to the air electrode  8  is smaller than the distance to the interconnector  4 . In the embodiment, a solid electrolyte layer  6  denser than the center part  1   b  (see  FIG.  4   ) may also be disposed in the curved portion  1   c.    
     Thus, since the strength of the solid electrolyte layer  6  in the curved portion  1   c  of the cell  1  can be enhanced, the solid electrolyte layer  6  hardly peels off from the curved portion  1   c  of the cell  1 . 
     The porosity and average diameter of the voids in the first region  7   a , the second region  7   b , and the third region  6   a  can be determined, for example, by the following technique. First, the cell  1  is cut to obtain cross sections of the first region  7   a , the second region  7   b , and the third region  6   a.    
     The cross-sections of the first region  7   a , the second region  7   b , and the third region  6   a  are observed by SEM (scanning electron microscope), and a photograph taken at, for example, 3000 times magnification is obtained. The porosity can be obtained by performing image processing on the photograph to calculate the total area of the voids relative to the entire area of the image. 
     By applying image processing to the photograph, the average diameter of the voids can be determined. The average diameter of the voids obtained by the image processing is the average value of the diameters obtained by converting the areas of the voids in the cross-sectional photograph into circles. For example, analysis by binarization can be used for image processing by using analysis software (Image J from Wayne Rasband). 
     In the embodiment, as illustrated in  FIG.  3   , the cell  1  may include a third site  30 , where the intermediate layer  7  is not located on the surface of the solid electrolyte layer  6 , near at least two sides, when viewed from the air electrode  8  side. 
     The intermediate layer  7  may include a dense film with a thickness less than the first region  7   a  at the interface with the solid electrolyte layer  6 . At this time, in the case where the third site  30  is not formed in the cell  1  and the dense film of the intermediate layer  7  is formed in a manner to reach each side of the cell  1 , when an impact is applied to the cell  1  from the outside, the direct impact is applied to the dense film of the intermediate layer  7  that reaches the side where the impact is applied. As a result, a crack may occur in the dense film of the intermediate layer  7 , which is thin and dense and prone to cracking, and the crack may propagate to the element portion  3 . 
     On the other hand, in the embodiment, since the cell  1  includes the third site  30 , even when an external impact is applied to the side on which the third site  30  is formed, the dense film of the intermediate layer  7  can be suppressed from cracking. 
     That is, in the embodiment, the intermediate layer  7  can be made difficult to crack. Thus, according to the embodiment, the durability of the cell  1  can be enhanced. 
     The arrangement of the third site  30  in the cell  1  according to the embodiment is not limited to the example illustrated in  FIG.  3   .  FIGS.  8  and  9    are side views illustrating another example of the element portion  3  according to the embodiment. In  FIGS.  8  and  9   , the air electrode  8  and the reinforcing layer  9  are omitted for ease of understanding. 
     As illustrated in  FIG.  8   , the third site  30  may be provided such that the four corners of the cell  1  viewed from the air electrode  8  side are cut off. As illustrated in  FIG.  9   , the third site  30  may be provided in a rectangular shape on a part of the upper end portion and on a part of the lower end portion of the cell  1  as viewed from the air electrode  8  side. 
     In the embodiment, as illustrated in  FIG.  3   , the intermediate layer  7  may also be formed on the surface of the reinforcing layer  9  where the diffusion prevention layer is not required because the air electrode  8  is not formed. 
     The intermediate layer  7  according to the embodiment may contain at least one element selected from the group consisting of iron (Fe), silicon (Si), sodium (Na), chlorine (Cl), copper (Cu), titanium (Ti) and aluminum (Al) as an impurity. 
     Thus, when the intermediate layer  7  is formed, the growth of crystal grains in the intermediate layer  7  can be suppressed, thus making it difficult for cracks to form in the intermediate layer  7 . Thus, according to the embodiment, the durability of the cell  1  can be enhanced. 
     The intermediate layer  7  according to the embodiment may contain, for example, 1000 ppm (0.1 mass %) or less in total of the above-described impurity elements. For example, the content of each of Fe, Si, Na and Cl may be 200 ppm (0.02 mass %) or less. For example, the content of each of Cu, Ti and Al may be 50 ppm (0.005 mass %) or less. 
     The intermediate layer  7  may include rare earth elements other than Gd and Sm, for example, such as La (lanthanum), Pr (praseodymium), Nd (neodymium), or Y (yttrium)e. For example, in the case where the material of the intermediate layer  7  is cerium oxide (CeO 2 ) with Gd in solid solution, the intermediate layer  7  may contain at least any one of La, Pr, Nd, Sm and Y in a total amount of about 20 ppm (0.002 mass %). 
     The intermediate layer  7  may include Zr (zirconium), Ca (calcium), Sr (strontium), Mg (magnesium), Co (cobalt), Mn (manganese), or Ni (nickel). These elements may diffuse into the intermediate layer  7  from members disposed near the intermediate layer  7 . 
     In the embodiment, the intermediate layer  7  may include flat crystal grains in the first region  7   a . In the first region  7   a  of the intermediate layer  7 , the long diameter of the flat crystal grains may range from 10 nm to 100 μm, and the average value of the long diameter may range from 100 nm to 10 μm. Such flat crystal grains may form the dense film of the intermediate layer  7  described above. 
     The contact length per unit length between the crystal grains of the intermediate layer  7  and the crystal grains of solid electrolyte layer  6  at a boundary portion between the first region  7   a  of the intermediate layer  7  and the solid electrolyte layer  6  may be larger than the contact length per unit length between the crystal grains of the intermediate layer  7  and the crystal grains of air electrode  8  at a boundary portion between the second region  7   b  of the intermediate layer  7  and the air electrode  8 . 
     Module 
     A module  100  according to an embodiment of the present disclosure using the cell stack device  10  described above will be described with reference to  FIG.  10   .  FIG.  10    is an exterior perspective view illustrating the module  100  according to the embodiment.  FIG.  10    illustrates a state in which the front and rear surfaces, which are part of the housing container  101 , are removed and the cell stack device  10  of the fuel cell contained therein is taken out rearward. 
     As illustrated in  FIG.  10   , the module  100  includes a housing container  101  and a cell stack device  10  contained in the housing container  101 . A reformer  102  is disposed above the cell stack device  10 . 
     The reformer  102  reforms raw fuel such as natural gas or kerosene to generate fuel gas and supplies it to the cell  1 . The raw fuel is supplied to the reformer  102  through a raw fuel supply pipe  103 . The reformer  102  may include a vaporizing unit  102   a  that vaporizes water and a reforming unit  102   b . The reforming unit  102   b  includes a reforming catalyst (not illustrated) for reforming raw fuel into fuel gas. The reformer  102  such as that described above can perform steam reforming, which is an efficient reformation reaction. 
     The fuel gas generated by the reformer  102  is supplied to the gas-flow passage  2   a  (see  FIG.  1 A ) of the cell  1  through the gas flow pipe  20 , the gas tank  16 , and the support member  14 . 
     In the module  100  having the above-described structure, the temperature in the module  100  during normal power generation is about 500 to 1000° C. due to combustion of gas and power generation of the cell  1 . 
     As described above, the module  100  is configured by housing the cell stack device  10  having high durability, so that the module  100  having high durability can be obtained. 
     Module Housing Device 
       FIG.  11    is an exploded perspective view illustrating an example of the module housing device  110  according to an embodiment. The module housing device  110  according to the embodiment includes an external case  111 , a module  100  illustrated in  FIG.  10   , and an auxiliary device (not illustrated). The auxiliary device operates the module  100 . The module  100  and the auxiliary device are housed in the external case  111 . Note that in  FIG.  11   , part of the configuration is omitted. 
     The external case  111  of the module housing device  110  illustrated in  FIG.  11    includes a support  112  and an external plate  113 . The dividing plate  114  divides the external case  111  vertically. The space above the dividing plate  114  in the external case  111  is the module housing chamber  115  that accommodates the module  100 , and the space below the dividing plate  114  in the external case  111  is the auxiliary device housing chamber  116  that accommodates the auxiliary device operating the module  100 . In  FIG.  11   , the auxiliary device accommodated in the auxiliary device housing chamber  116  is omitted. 
     The dividing plate  114  includes an air flow communication opening  117  for enabling the air of the auxiliary device housing chamber  116  to flow to the module housing chamber  115 . The external plate  113  constituting the module housing chamber  115  includes an exhaust opening  118  for exhausting air in the module housing chamber  115 . 
     In the module housing device  110 , the module housing device  110  having high durability can be obtained by providing the module  100  having high durability in the module housing chamber  115  as described above. 
     Various Variations 
     Next, the element portions according to various variations of the embodiment will be described with reference to  FIGS.  12  to  18   . 
     In the embodiment described above, a so-called “vertically striped type” cell stack device, in which only one element portion including a fuel electrode, a solid electrolyte layer, and an air electrode is provided on the surface of the support substrate, is exemplified. However, the present disclosure can be applied to a horizontally striped type cell stack device with an array of so-called “horizontally striped type” cells, in which a plurality of element portions are provided on the surface of a support substrate at mutually separated locations, and adjacent element portions are electrically connected to each other. 
     Although the present embodiment exemplifies the case where a hollow flat plate-shaped support substrate is used, the present disclosure can also be applied to a cell stack device using a cylindrical type support substrate. As will be described later, the present disclosure can also be applied to a flat plate cell stack device in which so-called “flat plate” cells are stacked in the thickness direction. 
     In the above embodiment, an example in which a fuel electrode is provided on a support substrate and an air electrode is disposed on a surface of a cell, is illustrated. However, the present disclosure can also be applied to a cell stack device that has an opposite arrangement to the above, that is, an arrangement in which an air electrode is provided on a support substrate and a fuel electrode is disposed on a surface of a cell. 
     In the aforementioned embodiments, the “cell”, the “cell stack device”, the “module”, and the “module housing device” are exemplified by a fuel cell, a fuel cell stack device, a fuel cell module, and a fuel cell device, respectively. However, they may be exemplified by an electrolytic cell, an electrolytic cell stack device, an electrolytic module, and an electrolytic device, respectively. 
       FIG.  12    is a cross-sectional view illustrating a cell according to a first variation of the embodiment. As illustrated in  FIG.  12   , the cell stack device  10 A includes a plurality of cells  1 A extending in the length direction L from a pipe  73  that distributes fuel gas. The cell  1 A includes a plurality of element portions  3 A on the support substrate  71 . A gas-flow passage  74  through which the gas flows from the pipe  73  is provided inside the support substrate  71 . Each element portion  3 A on the support substrate  71  is electrically connected by a connection portion (not illustrated). The plurality of cells  1 A are electrically connected to each other via a conductive member  78 . The conductive members  78  are each located between the element portions  3 A each included in a corresponding one of the cells  1 A and electrically connect adjacent cells  1 A to each other. 
     In the first variation, the porosity of the intermediate layer in the first region near the interface with the solid electrolyte layer is larger than the porosity of the intermediate layer in the second region near the interface with the air electrode. Thus, the durability of the cell  1 A can be enhanced. 
     In the first variation, the porosity of the intermediate layer in the outer peripheral part of the element portion  3 A is larger than the porosity of the intermediate layer in the center part of the element portion  3 A. Thus, the durability of the cell  1 A can be enhanced. 
     In the first variation, the third region of the solid electrolyte layer in the outer peripheral part of the element portion  3 A is denser than the third region of the solid electrolyte layer in the center part of the element portion  3 A. Thus, the durability of the cell  1 A can be enhanced. 
       FIG.  13    is a perspective view illustrating a flat plate cell according to a second variation of the embodiment.  FIG.  14    is a diagram for describing the outer peripheral part  1 Ba and the center part  1 Bb in the element portion  90  according to a second variation of the embodiment. 
     As illustrated in  FIG.  13   , the cell  1 B includes an element portion  90  in which a fuel electrode  5 , a solid electrolyte layer  6 , an intermediate layer  7  and an air electrode  8  are stacked. In a cell stack device in which a plurality of flat plate cells are stacked, for example, a plurality of cells  1 B are electrically connected to each other by conductive members  91  and  92  which are metal layers adjacent to each other. The conductive members  91  and  92  electrically connect adjacent cells  1 B and have a gas-flow passage for supplying gas to the fuel electrode  5  or the air electrode  8 . 
     As illustrated in  FIG.  14   , the element portion  90  of the cell  1 B includes an outer peripheral part  1 Ba and a center part  1 Bb. The outer peripheral part  1 Ba is a region located near each side of the element portion  90  when the cell  1 B is viewed in plan view, and the center part  1 Bb is a central region surrounded by the outer peripheral part  1 Ba when the cell  1 B is viewed in plan view. In  FIGS.  13  and  14   , when the cell is viewed in plan view, the outer shape of the cell  1 B coincides with the outer shape of the element portion  90 . The contour of the cell  1 B may be larger than that of the element portion  90 , and the contour of the cell  1 B may be arranged to surround the contour of the element portion  90 . 
     As illustrated in  FIG.  14   , the outer peripheral part  1 Ba is a region in which the distance from one side S 1  is no more than a predetermined distance X 6  and the distance from the other side S 2  is no more than a predetermined distance X 7  in the element portion  90 . 
     In the second variation, the distance X 6  is, for example, 25% of the length L 3  of the other side S 2  of the element portion  90 . The distance X 7  is, for example, 25% of the length L 4  of the one side S 1  of the element portion  90 . 
     In the second variation, the porosity of the intermediate layer  7  in the first region (that is, near the interface with the solid electrolyte layer  6 ) is larger than that of the intermediate layer  7  in the second region (that is, near the interface with the air electrode  8 ). Thus, the durability of the cell  1 B can be enhanced. 
     In the second variation, the porosity of the intermediate layer  7  in the outer peripheral part  1 Ba of the element portion  90  is larger than that of the intermediate layer  7  in the center part  1 Bb of the element portion  90 . Thus, the durability of the cell  1 B can be enhanced. 
     In the second variation, the third region of the solid electrolyte layer  6  in the outer peripheral part  1 Ba of the element portion  90  is denser than the third region of the solid electrolyte layer  6  in the center part  1 Bb of the element portion  90 . Thus, the durability of the cell  1 B can be enhanced. 
       FIG.  15    is a bottom view illustrating an example of the element portion  90  according to the second variation of the embodiment, and is a view for illustrating the arrangement of the intermediate layer  7  and the third site  30  in the cell  1 B. As illustrated in  FIG.  15   , in the second variation, the third site  30  may be provided along each of the two sides facing each other in the cell  1 B. 
     Thus, even when an external impact is applied to the two sides on which the third site  30  is formed, the intermediate layer  7  can be made difficult to crack. Thus, according to the second variation, the durability of the cell  1 B can be enhanced. 
     The arrangement of the third site  30  in the cell  1 B according to the second variation is not limited to the example illustrated in  FIG.  15   .  FIGS.  16    to  18  are bottom views illustrating another example of an element portion  90  according to the second variation of the embodiment. 
     As illustrated in  FIG.  16   , the third site  30  may be provided such that the four corners of the cell  1 B viewed from the air electrode  8  (see  FIG.  13   ) side are cut off. As illustrated in  FIG.  17   , the third site  30  may be provided in a rectangular shape in a part of the two sides facing each other in the cell  1 B as viewed from the air electrode  8  side. As illustrated in  FIG.  18   , the third site  30  may be provided along all four sides of the cell  1 B as viewed from the air electrode  8  side. 
     As described above, the cell  1  ( 1 A,  1 B) according to embodiment includes the element portion  3  ( 3 A,  90 ) that includes the fuel electrode  5 , the solid electrolyte layer  6 , the air electrode  8 , and the intermediate layer  7  located between the solid electrolyte layer  6  and the air electrode  8 . The solid electrolyte layer  6  or the intermediate layer  7  includes a first site, and a second site that is located closer to the air electrode  8  or closer to the center part  1   b  of the element portion  3  ( 3 A,  90 ) than the first site and that has a smaller porosity or a lower density than the first site. Thus, the durability of the cell  1  ( 1 A,  1 B) can be enhanced. 
     In the cell  1  ( 1 A,  1 B) according to the embodiment, the porosity of the intermediate layer  7  near an interface between the intermediate layer  7  and the solid electrolyte layer  6  is greater than the porosity of the intermediate layer  7  near an interface between the intermediate layer  7  and the air electrode  8 . Thus, the durability of the cell  1  ( 1 A,  1 B) can be enhanced. 
     In the cell  1  ( 1 A,  1 B) according to the embodiment, the porosity of the intermediate layer  7  in the outer peripheral part  1   a  of the element portion  3  ( 3 A,  90 ) is greater than the porosity of the intermediate layer  7  in the center part  1   b . Thus, the durability of the cell  1  ( 1 A,  1 B) can be enhanced. 
     In the cell  1  ( 1 A,  1 B) according to the embodiment, the porosity of the intermediate layer  7  ranges from 5% to 30%. Thus, peeling of the intermediate layer  7  can be suppressed. 
     In the cell  1  ( 1 A,  1 B) according to the embodiment, the intermediate layer  7  contains cerium oxide in which a rare earth element except Ce is in solid solution. Thus, the intermediate layer  7  can be provided with a function as a diffusion prevention layer for suppressing the formation of a SrZrO 3  resistive layer on the solid electrolyte layer  6 . 
     In the cell  1  ( 1 A,  1 B) according to the embodiment, the intermediate layer  7  contains at least one element of Fe, Si, Na, Cl, Cu, Ti and Al as an impurity. Thus, the durability of the cell  1  ( 1 A,  1 B) can be enhanced. 
     In the cell  1  ( 1 A,  1 B) according to the embodiment, the solid electrolyte layer  6  includes, near at least two sides on the surface of the solid electrolyte layer  6 , the third site  30  on which the intermediate layer  7  is not located, when viewed from the air electrode  8  side. Thus, the occurrence of cracks in the intermediate layer  7  can be suppressed. 
     In the cell  1  ( 1 A,  1 B) according to the embodiment, the solid electrolyte layer  6  located in the outer peripheral part  1   a  of the element portion  3  ( 3 A,  90 ) is denser than the solid electrolyte layer  6  located in the center part  1   b . This makes it difficult for the solid electrolyte layer  6  to peel off from the outer peripheral part  1   a  of the element portion  3  ( 3 A,  90 ). 
     In the cell  1  ( 1 A,  1 B) according to the embodiment, near the interface between the solid electrolyte layer  6  and the intermediate layer  7 , the solid electrolyte layer  6  is denser in the outer peripheral part  1   a  of the element portion  3  ( 3 A,  90 ) than in the center part  1   b . This makes it difficult for the solid electrolyte layer  6  to peel off from the outer peripheral part  1   a  of the element portion  3  ( 3 A,  90 ). 
     The module  100  according to the embodiment includes a cell stack device  10  including the plurality of cells  1  ( 1 A,  1 B) described above, and the housing container  101  configured to house the cell stack device  10 . Thus, the module  100  having high durability can be obtained. 
     The module housing device  110  according to the embodiment includes the module  100  described above, an auxiliary device configured to operate the module  100 , and the external case  111  configured to accommodate the module  100  and the auxiliary device. Thus, the module housing device  110  having a high durability can be obtained. 
     Embodiments disclosed herein are considered exemplary in all respects and not restrictive. Indeed, embodiments described above may be embodied in a variety of forms. The above embodiments may be omitted, substituted, or modified in various forms without departing from the scope and spirit of the appended claims. 
     REFERENCE SIGNS LIST 
     
         
           1 ,  1 A,  1 B Cell 
           1   a ,  1 Ba Outer peripheral part 
           1   b ,  1 Bb Center part 
           3 ,  3 A,  90  Element portion 
           5  Fuel electrode 
           6  Solid electrolyte layer 
           6   a  Third region 
           7  Intermediate layer 
           7   a  First region 
           7   b  Second region 
           8  Air electrode 
           10  Cell stack device 
           30  Third site 
           100  Module 
           101  Housing container 
           110  Module housing device 
           111  External case