Electrochemical cell, solid oxide electrolysis cell, cell stack, hot module, and hydrogen production device

Abstract

An electrolysis cell 21 comprises: a solid electrolyte layer 211; a fuel electrode layer 213 that is disposed as a stack on one surface side of the solid electrolyte layer 211; and an air electrode layer 212 that is disposed as a stack on the other surface side of the solid electrolyte layer 211. The fuel electrode layer contains Fe. When a surface 213a1 on the solid electrolyte layer side of the fuel electrode layer is defined as a first surface and a surface 213b2 on the side opposite from the solid electrolyte layer side is defined as a second surface, n points pk (where k is an integer of 1 to n) are set in the fuel electrode layer along the thickness direction at intervals such that a point p1 is located at the first surface, a point pn is located at the second surface, and k increases in the direction from the first surface toward the second surface, and when a value obtained by dividing, by n, a cumulative value of the Fe concentration at each of the points from the point p1 to the point pk is defined as a normalized cumulative value Ck of the Fe concentration at the point pk, a normalized cumulative value Cn is 0.118-0.367 wt%.

Description

Electrochemical cells, solid oxide electrolysis cells, cell stacks, hot modules, and hydrogen production devices

[0001] The present invention relates to an electrochemical cell, a solid oxide electrolysis cell, a cell stack, a hot module, and a hydrogen production device.

[0002] Solid oxide electrochemical cells using a solid oxide as an electrolyte have been known for some time (see, for example, Patent Document 1). Solid oxide electrochemical cells are characterized by performing electrochemical reactions with high efficiency in high-temperature environments and can be used as solid oxide electrolysis cells (SOECs) or solid oxide fuel cells (SOFCs). Solid oxide electrolysis cells are electrolysis devices that use electrical energy to decompose water vapor into hydrogen and oxygen. Solid oxide fuel cells are power generation devices that generate electrical energy through a chemical reaction between hydrogen and oxygen.

[0003] Japanese Patent Application Laid-Open No. 2019-8914

[0004] The electrochemical cell can be configured to include a solid electrolyte layer, a fuel electrode layer laminated on one side of the solid electrolyte layer, and a cathode layer laminated on the other side of the solid electrolyte layer. The fuel electrode layer can be configured to include metal particles as an electrode catalyst and an ion-conductive oxide (also referred to as an oxide-ion conductor). For example, Ni (nickel) can be used as the metal particles, and YSZ (yttria-stabilized zirconia) can be used as the ion-conductive oxide.

[0005] When an electrochemical cell containing Ni in the fuel electrode layer is operated for a long time under a harsh operating environment (high temperature, high humidity, high current), migration and aggregation of Ni within the fuel electrode layer occurs. When Ni migrates and aggregates within the fuel electrode layer, the conduction paths within the fuel electrode layer decrease, increasing internal resistance. Furthermore, the number of three-phase interfaces (boundaries between the fuel gas, Ni, and the electrolyte (YSZ)) serving as reaction fields decreases, increasing reaction resistance in the electrochemical reaction (in other words, reducing reaction efficiency). This increase in internal resistance and reaction resistance reduces the performance of the electrochemical cell, making it impossible to achieve the desired performance. This is not limited to configurations in which the metal particles are Ni and the ion-conductive oxide is YSZ, but also applies to cases in which other metal particles and ion-conductive oxides are used. Therefore, there is a need for the development of technology that can maintain the desired performance (i.e., suppress deterioration in durability) even when the electrochemical cell is operated for a long time. On the other hand, the introduction of technology to suppress deterioration in durability tends to reduce the initial performance of the electrochemical cell. Therefore, it is essential to suppress deterioration in durability while maintaining the initial performance. The initial performance refers to the performance of the electrochemical cell after an initial reduction treatment, which is a process for reducing oxides of metal particles contained in the green sheet of the fuel electrode layer to metal particles (e.g., reducing NiO (nickel oxide) to Ni) during the manufacturing process of the electrochemical cell.

[0006] The present invention has been made to address the above-mentioned problems, and an object of the present invention is to provide a technique that can suppress a decrease in durability while maintaining the initial performance of an electrochemical cell.

[0007] The electrochemical cell (21) according to the present invention comprises a solid electrolyte layer (211), a fuel electrode layer (213) laminated on one side of the solid electrolyte layer, and an air electrode layer (212) laminated on the other side of the solid electrolyte layer. the anode layer contains Fe, and a surface (213a1) of the anode layer facing the solid electrolyte layer is defined as a first surface, and a surface of the anode layer opposite the solid electrolyte layer is defined as a second surface (213b2); n points pk (k: an integer from 1 to n) are set in the anode layer along the thickness direction at intervals such that point p1 is located on the first surface and point pn is located on the second surface, and k increases in a direction from the first surface to the second surface; and the value obtained by dividing the integrated value of the mass percent concentration of Fe at each point from point p1 to point pk by n is defined as a normalized integrated value Ck of the mass percent concentration of Fe at point pk; the normalized integrated value Cn is equal to or greater than 0.118 wt% and equal to or less than 0.367 wt%.

[0008] In the electrochemical cell according to the present invention, the fuel electrode layer contains iron (Fe). Fe has the property of suppressing the migration and aggregation of metal particles (electrode catalysts contained in the fuel electrode layer). However, because Fe also acts as a sintering aid, if the Fe concentration is too high, the densification of the fuel electrode layer during molding (sintering) of the fuel electrode layer is promoted, impairing the diffusibility of gas passing through the fuel electrode layer and reducing the initial performance and durability of the electrochemical cell. Furthermore, if the Fe concentration is too low, the effect of suppressing the migration and aggregation of metal particles due to the addition of Fe is reduced, and the deterioration of durability cannot be sufficiently suppressed. In contrast, the electrochemical cell according to the present invention is configured so that the normalized integrated value Cn of the mass percent concentration of Fe on the second surface (the surface of the fuel electrode layer opposite the solid electrolyte layer) is 0.118 wt% or more and 0.367 wt% or less. By including Fe in the fuel electrode layer in this way, the initial performance of the electrochemical cell can be maintained while suppressing deterioration of durability.

[0009] In one aspect of the present invention, the normalized integrated value Cm at a point pm (mεk) located midway in the thickness direction of the fuel electrode layer (213) is 84% ​​or more of the normalized integrated value Cn.

[0010] The normalized integrated value Cm being 84% or more of the normalized integrated value Cn means that the integrated value of the Fe concentration at each point from point p1 to point pm is 84% ​​or more of the integrated value of the Fe concentration at each point from point p1 to point pn (because it is a ratio, the total number n of points pk is excluded). With this configuration, the Fe concentration in the thickness direction of the fuel electrode layer is relatively low in the range from the middle to the second surface. Because gas enters the fuel electrode layer from the second surface and diffuses inside, by configuring the fuel electrode layer so that the integrated value gradient of Fe in the fuel electrode layer satisfies Cm / Cn ≥ 84%, it is possible to more reliably avoid the promotion of densification in the range from the middle to the second surface during formation of the fuel electrode layer, and more appropriately suppress the deterioration of diffusivity of gas passing through this range. As a result, the initial performance of the electrochemical cell can be better maintained while more appropriately suppressing deterioration in durability.

[0011] In one aspect of the present invention, the number n is 200 or more, and the n points pk are set at equal intervals from one another.

[0012] This configuration can improve the reliability of the normalized integrated value Cn.

[0013] The solid oxide electrolysis cell (21) according to the present invention comprises an electrochemical cell according to the present invention.

[0014] According to the above configuration, it is possible to provide a solid oxide electrolysis cell in which the initial performance is maintained while deterioration in durability is suppressed.

[0015] The cell stack (20) according to the present invention is formed by stacking the solid oxide electrolysis cells (21) according to the present invention.

[0016] According to the above configuration, it is possible to provide a cell stack in which the initial performance of the solid oxide electrolysis cell is maintained while deterioration in durability is suppressed.

[0017] The hot module (10) of the present invention comprises a cell stack (20) of the present invention, a vaporizer (30) that generates steam to be supplied to the cell stack, a heat exchanger (40) that exchanges heat with gas supplied to the cell stack, a heater (50) that heats the cell stack, and a thermal insulator (60) in which the cell stack, the vaporizer, the heat exchanger, and the heater are arranged.

[0018] According to the above configuration, it is possible to provide a hot module in which the initial performance of the solid oxide electrolysis cell is maintained while deterioration in durability is suppressed.

[0019] The hydrogen production device (1) according to the present invention comprises the hot module (10) according to the present invention.

[0020] According to the above configuration, it is possible to provide a hydrogen production device in which the initial performance of the solid oxide electrolysis cell is maintained while deterioration in durability is suppressed.

[0021] In the above description, in order to facilitate understanding of the invention, the symbols used in the embodiments are added in parentheses to the constituent elements of the invention corresponding to the embodiments, but each constituent element of the invention is not limited to the embodiments defined by the symbols.

[0022] 1 is a block diagram of a hydrogen production device. 2 is a perspective view of a cell stack. 3 is a cross-sectional view taken along line III-III in FIG. 2. 4 is a cross-sectional view in the thickness direction of an electrolysis cell. 5 is a graph defining the relationship between the distance d from the interface S1 in the fuel electrode layer and the normalized integrated value Ck. 6 is a graph defining the relationship between the distance percentage d / T from the interface S1 in the fuel electrode layer and the normalized integrated value percentage Ck / Cn.

[0023] Hereinafter, an embodiment of the present invention will be described with reference to the drawings. Fig. 1 is a block diagram of a hydrogen production device 1 according to this embodiment. The hydrogen production device 1 according to this embodiment is a device that produces hydrogen by electrolyzing water vapor. As shown in Fig. 1, the hydrogen production device 1 includes a hot module 10 and a condenser 90.

[0024] The hot module 10 is constructed by covering with insulating material the main components that become hot among the elements that make up the hydrogen production device 1, and is a device in which the main components are concentrated within the insulating material so that the high temperature state of the main components is maintained. This hot module 10 includes a cell stack 20, a vaporizer 30, a heat exchanger 40, a heater 50, and insulating material 60.

[0025] The vaporizer 30 contains water (H ２ 0) is supplied to the vaporizer 30. The vaporizer 30 is configured to heat the supplied water to a temperature of 100°C or higher by a heat source. Therefore, the water supplied to the vaporizer 30 evaporates within the vaporizer 30, generating water vapor. The water vapor generated in the vaporizer 30 is introduced into the heat exchanger 40.

[0026] In addition to the water vapor, air is introduced into the heat exchanger 40. The heat exchanger 40 also receives high-temperature hydrogen (H ２ ) and high-temperature oxygen (O ２ ) is introduced into the heat exchanger 40. The high-temperature gas exchanges heat with the steam and air in the heat exchanger 40, thereby heating the steam and air introduced from the vaporizer 30.

[0027] The water vapor and air heated by the heat exchanger 40 are further heated by the heater 50 to the operating temperature of the cell stack 20 (i.e., the temperature required to operate the cell stack 20).The water vapor and air are then introduced into the cell stack 20.

[0028] The cell stack 20 is formed by stacking solid oxide electrolysis cells. The cell stack 20 is heated to an operating temperature by a heat source (such as a burner) not shown. A predetermined voltage is applied to the cell stack 20. As a result, water vapor introduced into the cell stack 20 is electrolyzed to produce hydrogen and oxygen. The hydrogen produced in the cell stack 20 is introduced into the heat exchanger 40 together with unreacted water vapor, where it is used to heat the water vapor and air introduced into the heat exchanger 40 from the vaporizer 30, and then introduced into the condenser 90. The unreacted water vapor is condensed in the condenser 90. The condensed water produced in the condenser 90 is introduced into the vaporizer 30. Meanwhile, hydrogen separated by condensation of the water vapor in the condenser 90 is recovered. The oxygen produced in the cell stack 20 is introduced into the heat exchanger 40, where it is used to heat the water vapor and air, and then introduced into the vaporizer 30 to heat the water supplied to the vaporizer 30. The oxygen discharged from the vaporizer 30 is then recovered (or released to the atmosphere).

[0029] The cell stack 20, vaporizer 30, heat exchanger 40, and heater 50 are disposed inside a thermal insulation material 60. This suppresses heat radiation from each of the components 20, 30, 40, and 50. Heat-resistant fibers such as ceramic wool, refractory ceramic fiber (RCF), and biosoluble fiber (AES), and / or a heat-resistant container formed from these heat-resistant fibers, may be used for the thermal insulation material 60. The heat-resistant fibers are disposed so as to fill gaps between the cell stack 20, vaporizer 30, heat exchanger 40, and heater 50.

[0030] FIG. 2 is a perspective view of the cell stack 20, and FIG. 3 is a cross-sectional view taken along line III-III in FIG. 2 . As shown in FIGS. 2 and 3 , the cell stack 20 comprises an electrolysis unit group including a plurality of rectangular flat-plate-shaped electrolysis units Ue stacked in the thickness direction (up-down direction), and a pair of end plates 27, 28 disposed on the upper and lower surfaces of the electrolysis unit group, respectively. The end plates 27, 28 are rectangular flat-plate-shaped members having the same outer shape as the electrolysis units Ue, and each has a rectangular opening formed in its center. The electrolysis unit group and the end plates 27, 28 are fastened to each other at their four corners by bolts B inserted through the electrolysis units 27, 28 in the thickness direction and nuts (not shown). The end plates 27, 28 are made of metal (for example, stainless steel) and function as an anode and a cathode, respectively, when a voltage is applied. Note that for ease of explanation, the proportions of the components in the drawings may differ from the actual proportions.

[0031] The electrolysis unit Ue will be described with reference to Fig. 3. As shown in Fig. 3, the electrolysis unit Ue comprises a solid oxide electrolysis cell 21 (hereinafter simply referred to as "electrolysis cell 21"), an interconnector 22, a separator 23, a cathode frame 24, an anode frame 25, and a current collector 26.

[0032] The electrolysis cell 21 is the smallest unit of the SOEC and includes a solid electrolyte layer 211, an air electrode layer 212, and an anode layer 213. The air electrode layer 212 is laminated on the upper surface of the solid electrolyte layer 211 so as to contact the upper surface of the solid electrolyte layer 211. The air electrode layer 212 has a smaller outer shape than the solid electrolyte layer 211 and the anode layer 213, and is disposed in the center of the upper surface of the solid electrolyte layer 211 in a plan view of the electrolysis cell 21. Therefore, the upper surface of the outer periphery of the solid electrolyte layer 211 is exposed. The anode layer 213 is laminated on the lower surface of the solid electrolyte layer 211 so as to contact the lower surface of the solid electrolyte layer 211.

[0033] The interconnector 22 is a rectangular metal (for example, stainless steel) member that has a rectangular current collecting part 22a that protrudes downward from the center of its lower surface. A pair of interconnectors 22 is arranged on both sides of the electrolysis cell 21 in the thickness direction. Two adjacent electrolysis units Ue, Ue share one interconnector 22. The interconnector 22 also functions as a separator that separates the two adjacent electrolysis units Ue, Ue. The lower surface of the current collecting part 22a is in contact with the upper surface of the air cathode layer 212 of the electrolysis cell 21. The lower electrolysis unit Ue includes a pair of interconnectors 22, 29 instead of the pair of interconnectors 22, 22. The interconnector 29 is arranged at the bottom end of the cell stack 20 and differs from the interconnector 22 in that it does not have a current collecting part 22a.

[0034] The separator 23 is a rectangular plate-shaped metal (e.g., stainless steel) member with a rectangular opening formed in its center. The periphery of the opening of the separator 23 is brazed to the upper surface of the outer periphery of the solid electrolyte layer 211 of the electrolysis cell 21 with a brazing material (e.g., Ag brazing) (not shown). The separator 23 prevents mixing of oxygen generated in the air electrode layer 212 by electrolysis of water vapor and hydrogen generated in the fuel electrode layer 213.

[0035] The cathode frame 24 is a rectangular plate-shaped insulating member and may be formed of, for example, a mica sheet. A rectangular opening is formed in the center of the cathode frame 24. The cathode frame 24 is disposed between the separator 23 and the interconnector 22 above it.

[0036] The fuel electrode frame 25 is a rectangular plate-shaped metal (e.g., stainless steel) member having a rectangular opening at its center, and is disposed between the separator 23 and the interconnector 22 below it.

[0037] The internal space of the electrolysis unit Ue is partitioned into an air chamber Sa and a fuel chamber Sf by the separator 23. The air chamber Sa is a space that allows the flow of oxygen generated in the air electrode layer 212, and is defined by a space surrounded by the separator 23, the interconnector 22 above the separator 23, the air electrode frame 24, and the electrolysis cell 21. The fuel chamber Sf is a space that allows the flow of hydrogen generated in the fuel electrode layer 213, and is defined by a space surrounded by the separator 23, the interconnector 22 (or interconnector 29) below the separator 23, the fuel electrode frame 25, and the electrolysis cell 21.

[0038] The current collector 26 is a rectangular porous member made of metal (for example, nickel) that is smaller than the anode layer 213 in a plan view and allows hydrogen to pass through. The current collector 26 is arranged in the fuel chamber Sf so as to be in contact with the lower surface of the anode layer 213 and the upper surface of the lower interconnector 22. Two adjacent electrolysis cells 21 are stacked in the thickness direction so as to share the interconnector 22 via the current collector 26, thereby electrically connecting the multiple electrolysis cells 21 in series.

[0039] 2 and 3 , four paths Pfi, Pfo, Pai, and Pao are formed as gas flow paths around the outer periphery of the cell stack 20. These paths Pfi, Pfo, Pai, and Pao are each formed to penetrate members of the cell stack 20 in the thickness direction, excluding the "end plate 27" and the "upper interconnector 22 of the upper electrolysis unit Ue."

[0040] The path Pfi is formed near one corner of the side E1, which is one of the four sides that make up the outer periphery of the cell stack 20. The path Pfo is formed near the other corner of the side E2 that faces the side E1 (the corner located diagonally from the one corner of the side E1). As shown in FIG. 3 , the path Pfi communicates with the fuel chamber Sf via a horizontal hole 25a formed in the anode frame 25 of each electrolysis unit Ue. The path Pfo communicates with the fuel chamber Sf via a horizontal hole 25b formed in the anode frame 25 of each electrolysis unit Ue.

[0041] The path Pai is formed near one corner of the side E2. The path Pao is formed near the other corner of the side E1. The path Pai and the path Pao each communicate with the air chamber Sa via a horizontal hole (not shown) formed in the air electrode frame 24 of each electrolysis unit Ue.

[0042] Next, the configuration of the electrolysis cell 21 will be described in more detail with reference to FIG. 4 . The sizes and thicknesses of each layer of the electrolysis cell 21 shown below are merely examples and are not limited to these values. FIG. 4 is a cross-sectional view of the electrolysis cell 21 in the thickness direction. As described above, the electrolysis cell 21 includes a solid electrolyte layer 211, an air electrode layer 212, and an anode layer 213. The solid electrolyte layer 211 is a rectangular, flat layer measuring 150 mm square and 6 μm thick. The solid electrolyte layer 211 is configured to contain ceramic particles and is formed by sintering. The ceramic particles are oxide particles with ion conductivity (hereinafter also simply referred to as "ion-conductive oxide particles"), and YSZ is used in this embodiment. The solid electrolyte layer 211 has high oxide ion conductivity. The solid electrolyte layer 211 is a dense layer and is designed to prevent leakage between the atmosphere on the air electrode layer 212 side (air atmosphere) and the atmosphere on the anode layer 213 side (reducing atmosphere). The ceramic particles (ion-conductive oxide particles) in the solid electrolyte layer 211 are not limited to YSZ, but may be, for example, GDC (gadolinia-doped ceria).

[0043] The air electrode layer 212 is laminated on the upper surface of the solid electrolyte layer 211 so as to contact the upper surface of the solid electrolyte layer 211. The air electrode layer 212 is a rectangular flat layer with a thickness of 108 μm, and is configured to contain a perovskite oxide such as LSCF (lanthanum strontium cobalt iron oxide), and is formed by sintering. The air electrode layer 212 has a functional layer and a current collecting layer. The current collecting layer is thicker than the functional layer and is disposed on the upper surface of the functional layer. The air electrode layer 212 has high electronic conductivity and effectively collects electrons from the current collecting layer. The air electrode layer 212 is a porous layer and has pores inside.

[0044] The anode layer 213 is a rectangular flat plate-like layer measuring 150 mm on each side, and is formed so that its thickness T is greater than the thicknesses of the solid electrolyte layer 211 and the air cathode layer 212, e.g., T = 420 μm. The solid electrolyte layer 211 and the air cathode layer 212 are supported by the anode layer 213. In other words, the electrolysis cell 21 is an anode-supported cell.

[0045] The fuel electrode layer 213 includes a functional layer 213a and a support layer 213b. The functional layer 213a is laminated on the lower surface of the solid electrolyte layer 211 so as to contact the lower surface of the solid electrolyte layer 211. As a result, an interface S1 is formed between the upper surface 213a1 of the functional layer 213a and the lower surface of the solid electrolyte layer 211. The support layer 213b is laminated on the lower surface of the functional layer 213a so as to contact the lower surface 213a2 of the functional layer 213a. As a result, an interface S2 is formed between the upper surface 213b1 of the support layer 213b and the lower surface 213a2 of the functional layer 213a. That is, the functional layer 213a and the support layer 213b are laminated in this order on the lower surface of the solid electrolyte layer 211. The interface S1 is both the upper surface 213a1 of the functional layer 213a and the lower surface of the solid electrolyte layer 211. The interface S2 is also the upper surface 213b1 of the support layer 213b and the lower surface 213a2 of the functional layer 213a.

[0046] The thickness of the support layer 213b is significantly thicker than the thickness of the functional layer 213a. In this embodiment, the functional layer 213a has a thickness of 20 μm, and the support layer 213b has a thickness of 400 μm. However, the thicknesses of the functional layer 213a and the support layer 213b are not limited to these values, and the ratio of the thickness of the support layer 213b to the thickness of the functional layer 213a can be set to, for example, approximately 16 to 40 times. The upper surface 213a1 of the functional layer 213a and the lower surface 213b2 of the support layer 213b correspond to the upper and lower surfaces of the fuel electrode layer 213, respectively. The upper surface 213a1 and the lower surface 213b2 correspond to examples of the "first surface" and the "second surface," respectively.

[0047] The functional layer 213a is mainly composed of a cermet of Ni as a catalytic metal and YSZ as ceramic particles (ion-conductive oxide particles), and also contains a small amount of Fe as other components.

[0048] Like the functional layer 213a, the support layer 213b also contains a cermet of Ni as a catalytic metal and YSZ as ceramic particles as its main component, with a small amount of Fe as other components, although the proportion of Fe in the support layer 213b is significantly lower than that in the functional layer 213a.

[0049] The functional layer 213a and the support layer 213b are both porous layers configured to have a porous shape including a plurality of micropores. The diameters of the micropores in the functional layer 213a and the support layer 213b are on the order of several μm, thereby ensuring water vapor permeability (gas diffusibility). However, the functional layer 213a is formed more densely than the support layer 213b. In other words, the porosity of the functional layer 213a is smaller than the porosity of the support layer 213b. Like the solid electrolyte layer 211 and the air electrode layer 212, the fuel electrode layer 213 is also formed by sintering.

[0050] The ceramic particles (ion-conductive oxide particles) in the functional layer 213 a and the support layer 213 b are not limited to YSZ and may be, for example, GDC. Furthermore, the types of ceramic particles in the solid electrolyte layer 211 and the functional layer 213 a may be different from each other, and the types of ceramic particles in the functional layer 213 a and the support layer 213 b may be different from each other.

[0051] The fuel electrode layer 213 is formed so that the mass percent concentration of Fe (hereinafter simply referred to as "concentration") is approximately the same at the same position in the thickness direction, and has a concentration gradient such that the concentration is highest at its upper surface 213a1 and lowest at its lower surface 213b2 (hereinafter a position in the thickness direction will be simply referred to as a "thickness position"). In this embodiment, attention is focused on the integrated value obtained by integrating the Fe concentration from the upper surface 213a1 to the lower surface 213b2 of the fuel electrode layer 213 according to a predetermined rule. This integrated value is well represented by a normalized integrated value C (%). The normalized integrated value C is calculated as follows. That is, first, n points pk (k: an integer from 1 to n) are set in the fuel electrode layer 213 along the thickness direction. In this case, point p1 is located on the upper surface 213a1 of the fuel electrode layer 213, point pn is located on the lower surface 213b2, and points pk are set at equal intervals such that k increases in the direction from the upper surface 213a1 to the lower surface 213b2. In this embodiment, n = 200, but this value is not limited to this. Next, the integrated value of the Fe concentration at each point from point p1 to point pk is divided by n (the total number of points pk). This value is the normalized integrated value Ck at point pk. By dividing the integrated value of the Fe concentration by n, the influence of the number n of points pk on the normalized integrated value Ck is eliminated. The progression of the normalized integrated value Ck from k = 1 to n indicates the gradient of the integrated value of the Fe concentration. The shape of this gradient of the integrated value can be controlled in the manufacturing process of the electrolysis cell 21, which will be described later.

[0052] In this embodiment, the normalized integrated value Cn (i.e., the normalized integrated value at the lower surface 213b2 of the anode layer 213) is configured to be equal to or greater than 0.118 wt % and equal to or less than 0.367 wt %. Furthermore, the normalized integrated value Cm at the intermediate thickness position point pm (m∈k) of the anode layer 213 is configured to be equal to or greater than 84% of the normalized integrated value Cn. Hereinafter, Cm / Cn is defined as the ratio Rm. The ratio Rm is equal to the proportion of the mass of Fe from the upper surface 213a1 to the intermediate thickness position to the total mass of Fe in the anode layer 213. Furthermore, in this embodiment, the total number n of points pk can be configured to be equal to or greater than 200, and the intervals between adjacent points pk can be configured to be equal.

[0053] The intermediate thickness position point pm of the fuel electrode layer 213 is a point set at a position moved in the thickness direction from a position on the upper surface 213a1 of the fuel electrode layer 213 toward the lower surface 213b2 by a distance half the thickness of the fuel electrode layer 213, and is a point located in the middle in the thickness direction of the fuel electrode layer 213. Note that the intermediate thickness position point pm is defined as a point where m is (n+1) / 2 when n is an odd number, and as a point where m is n / 2 when n is an even number.

[0054] As described above, the ratio Rm is preferably 84% or greater, but as long as the normalized integrated value Cn satisfies 0.118 wt %≦Cn≦0.367 wt %, the ratio Rm does not necessarily have to be 84% or greater and may be configured to be, for example, 52% or greater. In this case, the initial performance will be slightly lower than when Rm≧84%, but this will not particularly affect the operation of the electrolytic cell 21.

[0055] The Fe concentration at point pk can be calculated by measuring the Fe concentration at multiple locations on a cross section of the electrolytic cell 21 cut along the thickness direction at a thickness position corresponding to point pk and averaging these measured values. The Fe concentration at any given location can be measured from an element mapping image obtained by area analysis using an EPMA device (in this embodiment, a device manufactured by JEOL Ltd. is used).

[0056] FIG. 5 is a graph in which n normalized integrated values ​​Ck are approximated by a curve, and shows the integrated value gradient of the anode layer 213. The horizontal axis of the graph represents the distance d (mm) from the interface S1 (i.e., the upper surface 213a1 of the anode layer 213), and the vertical axis represents the normalized integrated value Ck (wt%). d=0 corresponds to point p1, and d=T=0.42 corresponds to point pn. Point pk can be expressed as pk=T(k-1) / (n-1) using T, k, and n. As shown in FIG. 5, the normalized integrated value Cn of the anode layer 213 is 0.118 wt%, which is within the above-mentioned range. If the thickness position at the interface S2 (i.e., the surface located 20 μm away from the interface S1) is defined as point ps2 (s2∈k), the normalized integrated value Cs2 is 2.08×10 －２ It is wt%.

[0057] Figure 6 is a graph in which the vertical and horizontal axes of the graph in Figure 5 are converted into percentages. d / T = 50% corresponds to point pm. As shown in Figure 6, the ratio Rm (= Cm / Cn) of the fuel electrode layer 213 is 86.1%, which is within the above-mentioned range. Furthermore, if the ratio of the normalized integrated value Cs2 to the normalized integrated value Cn is defined as ratio Rs2, then the ratio Rs2 (= Cs2 / Cn) of the fuel electrode layer 213 is 17.6%.

[0058] The intervals between the points pk do not necessarily have to be equal. For example, the intervals between the points pk may be set to increase as the distance d increases.

[0059] The operation of the cell stack 20 will be described. First, a voltage is applied between the end plates 27, 28 of the cell stack 20. Next, high-temperature steam is supplied from the path Pfi. The steam supplied to the path Pfi flows into the fuel chamber Sf of each electrolysis unit Ue via the horizontal holes 25a. In addition, high-temperature air is supplied from the path Pai. The air supplied to the path Pai flows into the air chamber Sa of each electrolysis unit Ue via a horizontal hole (not shown). The reason for supplying high-temperature air to the air chamber Sa is to control the temperature of the cell stack 20.

[0060] The water vapor that flows into the fuel chamber Sf passes through the support layer 213b of the fuel electrode layer 213 and travels to the functional layer 213a. In the functional layer 213a, the water vapor reacts with electrons supplied from the end plate 28 via the current collector 26 and is decomposed into hydrogen and oxide ions (water vapor electrolysis reaction). The hydrogen generated by the water vapor electrolysis reaction diffuses within the fuel chamber Sf, is discharged through the horizontal hole 25b via path Pfo, and is recovered by a well-known method. At this time, unreacted water vapor can be discharged along with the hydrogen via path Pfo. Meanwhile, the oxide ions travel through the solid electrolyte layer 211 to the air electrode layer 212 in the air chamber Sa, release electrons in the functional layer of the air electrode layer 212, and become oxygen. The oxygen diffuses within the air chamber Sa and, together with the air that flowed into the air chamber Sa, is discharged through path Pao via a horizontal hole (not shown) and is recovered (or released to the atmosphere) by a well-known method. Electrons emitted from the functional layer of the air electrode layer 212 are collected by the current collecting portion 22a of the interconnector 22 via the current collecting layer, and circulate from the end plate 27 to the end plate 28 via the external power source.

[0061] The cell stack 20 operates as described above, and hydrogen is produced in the hydrogen production device 1 .

[0062] As described above, when an electrolytic cell containing Ni in the anode layer is operated for a long time under a severe operating environment (high temperature, high humidity, high current), migration and aggregation of Ni in the anode layer occurs, resulting in a problem of reduced durability of the electrolytic cell. On the other hand, the introduction of a technology to suppress the reduction in durability tends to reduce the initial performance of the electrolytic cell.

[0063] In the electrolysis cell 21 according to this embodiment, the anode layer 213 contains Fe. Fe has the property of suppressing the migration and aggregation of Ni within the anode layer 213. Additionally, the anode layer 213 is configured such that the normalized integrated value Cn of the mass percent concentration of Fe on the lower surface 213b2 of the anode layer 213 is 0.118 wt% or more and 0.367 wt% or less. Having the normalized integrated value Cn of 0.118 wt% or more appropriately suppresses the migration and aggregation of Ni. Furthermore, having the normalized integrated value Cn of 0.367 wt% or less suppresses the promotion of densification within the anode layer 213 during molding (sintering) of the anode layer 213, thereby suppressing deterioration of the diffusivity of gas passing through the anode layer 213. Therefore, the electrolysis cell 21 according to this embodiment can suppress a decrease in durability while maintaining initial performance.

[0064] Furthermore, when the ratio Rm is 84% ​​or higher, the Fe concentration in the range from the intermediate thickness position to the lower surface 213b2 of the anode layer 213 is relatively low. This more reliably prevents the anode layer 213 from densifying in the range from the intermediate thickness position to the lower surface 213b2 during formation of the anode layer 213, thereby more appropriately suppressing deterioration in the diffusivity of gas passing through this range. As a result, the initial performance of the electrolysis cell 21 can be better maintained while more appropriately suppressing deterioration in durability.

[0065] Furthermore, when the number n is 200 or more, it is possible to cover almost the entire distribution of the mass percent concentration of Fe along the thickness direction of the fuel electrode layer 213. Furthermore, when the n points pk are set at equal intervals, it is possible to reduce the variation in the normalized integrated value Cn caused by the points pk being set unevenly in the thickness direction of the fuel electrode layer 213. Therefore, when the number n is 200 or more and the n points pk are set at equal intervals, it is possible to improve the reliability of the normalized integrated value Cn.

[0066] (Examples) 1. Sample Preparation NiO powder and YSZ powder were mixed in a predetermined ratio and stirred for a predetermined time using a ball mill or the like. Next, butyral resin, polyvinyl acetal resin (G-260, manufactured by Sekisui Chemical Co., Ltd.) as a plasticizer, a known dispersant, a mixed solvent of toluene and MEK (methyl ethyl ketone), and optionally a pore-forming agent (typically organic beads) were added to the mixed powder in predetermined ratios and mixed in a ball mill to prepare a slurry. Then, a green sheet of a functional layer having a predetermined thickness was formed from the slurry using a doctor blade method.

[0067] NiO powder and YSZ powder were mixed in a predetermined ratio and stirred for a predetermined time using a ball mill or the like. Next, butyral resin, polyvinyl acetal resin as a plasticizer, a known dispersant, a mixed solvent of toluene and MEK, and optionally a pore-forming agent (typically organic beads) were added to the mixed powder in predetermined ratios and mixed in a ball mill to prepare a slurry. Then, a green sheet of a support layer having a predetermined thickness was formed from the slurry using a doctor blade method.

[0068] A butyral resin, a polyvinyl acetal resin as a plasticizer, a known dispersant, and a mixed solvent of toluene and MEK were added to the YSZ powder in predetermined proportions and mixed in a ball mill to prepare a slurry, which was then molded into a green sheet of a solid electrolyte layer having a predetermined thickness using a doctor blade method.

[0069] Next, a green sheet for the functional layer and a green sheet for the support layer were stacked in this order on one side of the green sheet for the solid electrolyte layer. These stacked green sheets were then pressed together under high pressure using a press while being heated and evacuated. This resulted in the formation of a laminate including the green sheet for the solid electrolyte layer, the green sheet for the functional layer, and the green sheet for the support layer.

[0070] The laminate formed as described above was then degreased at a predetermined temperature (e.g., 200 to 300°C). The laminate was then fired at a predetermined first temperature for a predetermined time (primary firing). This resulted in a primary sintered body having a solid electrolyte layer and a fuel electrode layer stacked on one side of the solid electrolyte layer.

[0071] Next, the primary sintered body is placed so that the solid electrolyte layer faces downward (in other words, so that the support layer faces upward). Then, Fe(NO ３ ) ２ ・9H ２ The primary sintered body was then dried and heat-treated at a predetermined temperature (for example, 1200° C.).

[0072] Thereafter, a material containing LSCF was screen-printed onto the other surface of the solid electrolyte layer of the formed primary sintered body, and the resulting product was fired at a predetermined second temperature (e.g., 900 to 1000°C) for a predetermined time (e.g., 1 to 5 hours) (secondary firing). This produced a sample electrolysis cell including a solid electrolyte layer, an anode layer laminated on one surface of the solid electrolyte layer, and an cathode layer laminated on the other surface of the solid electrolyte layer.

[0073] The primary sintered body is Fe(NO ３ ) ２ ・9H ２ By impregnating the O aqueous solution, Fe penetrates and diffuses into the fuel electrode layer due to gravity from surface A1 of the support layer toward surface A2 of the functional layer (the surface that will become the upper surface 213a1 of the fuel electrode layer 213 when completed). As a result, a concentration gradient is formed in the fuel electrode layer such that the Fe concentration is highest on surface A2 of the functional layer and lowest on surface A1 of the support layer. Note that this impregnation process does not allow Fe to penetrate into the solid electrolyte layer. This is because the solid electrolyte is formed to be much denser than the fuel electrode layer.

[0074] In this example, Fe(NO ３ ) ２ ・9H ２Eight electrolysis cell samples (Samples 1 to 8) with different Fe concentration gradient shapes were prepared by changing the concentration, impregnation amount, and number of impregnations of the O aqueous solution. Samples 1 to 8 were prepared so that the Fe concentration on surface A2 of the functional layer increased as the sample number (1 to 8) increased. Note that, hereinafter, surface A1 of the support layer will be referred to as the "lower surface of the fuel electrode layer," and surface A2 of the functional layer will be referred to as the "upper surface of the fuel electrode layer."

[0075] 2. Calculation of normalized integrated value Cn, ratio Rm, and Rs2 Samples 1 to 8 were cut along a surface along the thickness direction of the electrolytic cell, and the Fe concentrations were measured using the method described above at n points pk on the cut surface of Samples 1 to 8. Then, normalized integrated values ​​Cn, Cm, and Cs2 were calculated using the method described above, and the ratio Rm (Cm / Cn) and the ratio Rs2 (=Cs2 / Cn) were calculated using these values.

[0076] 3. Measurement of Current Density For Samples 1 to 8, the samples were heated to 700°C, and a constant voltage (1.3 V in this example) was applied between the anode layer and the cathode layer while a constant flow rate of steam was supplied to the anode layer side to perform a steam electrolysis reaction. The current per unit area (current density) flowing during the steam electrolysis reaction was measured, and the measured current density was used as an index for evaluating performance. That is, Samples 1 to 8 were operated as SOECs. It can be said that the higher the current density, the better the initial performance.

[0077] 4. Measurement of Durability Degradation Rate For Samples 1 to 8, a durability degradation test was conducted in which the samples were heated to 700°C, a constant flow rate of water vapor was supplied to the anode layer side so that a constant current flowed between the anode layer and the cathode layer, and a voltage was applied between the anode layer and the cathode layer to continuously perform a steam electrolysis reaction. That is, Samples 1 to 8 were operated as an SOEC. When a constant current was continuously flowed during the durability degradation test, the applied voltage increased over time due to an increase in internal resistance within the sample. The difference ΔV (= V1 - V0) between the initially applied voltage (V0) and the voltage (V1) applied after a predetermined time (e.g., 400 hours) was calculated, and the calculated value was converted to the difference between the voltage that would be applied after 1000 hours and the initially applied voltage. The converted value was then divided by the initially applied voltage to calculate the durability degradation rate as a percentage. It can be said that the smaller the durability degradation rate, the higher the durability.

[0078] 5. Evaluation Table 1 shows the normalized integrated value Cn, ratio Rm, ratio Rs2, current density, durability deterioration rate, and overall evaluation obtained for Samples 1 to 8.

[0079] Regarding the current density, the current density was 1.4 A / cm ２ When the current density was 1.3 A / cm or more, the initial performance was evaluated as very good (◎). ２ 1.4A / cm or more ２ When the current density is less than 1.2 A / cm, the initial performance is evaluated as good (◯). ２ 1.3A / cm or more ２ When the current density is less than 1.2 A / cm, the initial performance is evaluated as fair (△). ２When the initial performance was less than 3.0%, the durability was evaluated as poor (×). Regarding the durability degradation rate, when the durability degradation rate was less than 3.0%, the durability was evaluated as good (◯), and when the durability degradation rate was 3.0% or more, the durability was evaluated as poor (×). Regarding the overall evaluation, when the initial performance was very good (◎) and the durability was good (○), the overall evaluation was very good (◎); when the initial performance was good (○) and the durability was good (○), the overall evaluation was good (○); and when at least one of the initial performance or the durability was poor (×), the overall evaluation was poor (×).

[0080] As can be seen from Table 1, for samples 1 to 8, the normalized integrated value Cn increases as the sample number (1 to 8) increases. For samples 3 to 5, the initial performance was very good (◎) and the durability was good (◯), so the overall evaluation was very good (◎). For sample 6, the initial performance and durability were good (◯), so the overall evaluation was good (◯). In contrast, for samples 1 and 2, the initial performance was very good (◎), but the durability was poor (×), so the overall evaluation was poor (×). For sample 7, the initial performance was good (◯), but the durability was poor (×), so the overall evaluation was poor (×). For sample 8, the initial performance was fair (△) and the durability was poor (×), so the overall evaluation was poor (×).

[0081] The reason why Samples 1 and 2 were rated as poor (×) in durability is thought to be because the Cn values ​​of Samples 1 and 2 were smaller than those of Samples 3 to 6, resulting in an excessively low Fe concentration in the fuel electrode layer (particularly the reaction field, which is the interface of the functional layer on the solid electrolyte layer side and the region nearby), and therefore the effect of Fe in suppressing Ni migration and aggregation was not achieved. The reason why Samples 7 and 8 were rated as poor (×) in durability is thought to be because the Cn values ​​of Samples 7 and 8 were larger than those of Samples 3 to 6, resulting in an excessively high Fe concentration in the fuel electrode layer, which promoted densification inside the fuel electrode layer during molding, thereby worsening gas diffusivity. Furthermore, the reason why Sample 8 was rated as fair (△) is thought to be because the gas diffusivity of Sample 8 was even worse than that of Sample 7, thereby affecting initial performance.

[0082] Therefore, the migration and aggregation of Ni can be appropriately suppressed and deterioration of gas diffusivity can be suppressed when the normalized integrated value Cn is within the range of Samples 3 to 6. That is, when the normalized integrated value Cn is 0.118 wt % or more and 0.367 wt % or less, the initial performance of the electrolytic cell can be maintained while suppressing deterioration in durability.

[0083] Furthermore, the initial performance of Samples 3 to 5 (very good (◎)) is particularly good compared to the initial performance of Sample 6 (good (◯)). This is thought to be because the ratios Rm of Samples 3 to 5 were all 84% or higher, which was significantly greater than the ratio Rm of Sample 6 (52.5%). That is, in Samples 3 to 5, it is possible to more reliably avoid promotion of densification in the range from the mid-thickness position to the bottom surface of the fuel electrode layer during molding, and to more appropriately suppress deterioration of diffusivity of gas passing through this range.

[0084] Regarding the ratio Rs2, the magnitude relationship of the ratio Rs2 matched the magnitude relationship of the ratio Rm for Samples 1 to 6, except for Samples 7 and 8, which had excessively large normalized integrated values ​​Cn. This indicates that for Samples 1 to 6, the larger the normalized integrated value Cm (normalized integrated value at the intermediate thickness position of the fuel electrode layer), the larger the normalized integrated value Cs2 (normalized integrated value at the interface S2).

[0085] In the above examples, the performance of the electrolytic cell was evaluated by operating it as an SOEC, but similar results were obtained when Samples 1 to 8 were fabricated as fuel cells (the cell configuration was the same) and operated as SOFCs. That is, Samples 3 to 5 had very good initial performance (◎) and good durability (○) (overall evaluation (◎)), and Sample 6 had good initial performance (○) and good durability (○) (overall evaluation (○)).

[0086] Although the embodiment of the present invention has been described above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the object of the present invention.

[0087] Furthermore, the present invention may include the following aspects. [1] An electrochemical cell comprising: a solid electrolyte layer; an anode layer laminated on one side of the solid electrolyte layer; and an air cathode layer laminated on the other side of the solid electrolyte layer, wherein the anode layer contains Fe; and defining a surface of the anode layer facing the solid electrolyte layer as a first surface and a surface of the anode layer opposite to the solid electrolyte layer as a second surface, wherein n points pk (k: an integer from 1 to n) are set at intervals in the thickness direction of the anode layer, with point p1 located on the first surface and point pn located on the second surface, and k increasing in a direction from the first surface to the second surface; and defining a value obtained by dividing an integrated value of the mass percent concentration of Fe at each point from point p1 to point pk by n as a normalized integrated value Ck of the mass percent concentration of Fe at point pk, the normalized integrated value Cn is 0.118 wt% or more and 0.367 wt% or less. Electrochemical cell. [2] The electrochemical cell according to [1], wherein a normalized integrated value Cm at a point pm (m∈k) located midway in the thickness direction of the anode layer is 84% ​​or more of a normalized integrated value Cn. [3] The electrochemical cell according to [1] or [2], wherein the number n is 200 or more, and the n points pk are set at equal intervals. [4] A solid oxide electrolysis cell comprising the electrochemical cell according to any one of [1] to [3]. [5] A cell stack formed by stacking the solid oxide electrolysis cells according to [4]. [6] A hot module comprising: the cell stack according to [5]; a vaporizer that generates water vapor to be supplied to the cell stack; a heat exchanger that exchanges heat with a gas supplied to the cell stack; a heater for heating the cell stack; and a thermal insulation material in which the cell stack, the vaporizer, the heat exchanger, and the heater are disposed. [7] A hydrogen production device comprising the hot module according to [6].

Claims

1. An electrochemical cell comprising: a solid electrolyte layer; an anode layer laminated on one side of the solid electrolyte layer; and an air cathode layer laminated on the other side of the solid electrolyte layer, wherein the anode layer contains Fe; and when a surface of the anode layer facing the solid electrolyte layer is defined as a first surface and a surface of the anode layer opposite to the solid electrolyte layer is defined as a second surface, n points pk (k: an integer from 1 to n) are set along the thickness direction of the anode layer, with point p1 located on the first surface and point pn located on the second surface, and k increasing in a direction from the first surface to the second surface; and when the value obtained by dividing the integrated value of the mass percent concentration of Fe at each point from point p1 to point pk by n is defined as a normalized integrated value Ck of the mass percent concentration of Fe at point pk, the normalized integrated value Cn is 0.118 wt% or more and 0.367 wt% or less. Electrochemical cell.

2. An electrochemical cell according to claim 1, wherein the normalized integrated value Cm at a point pm (m∈k) located midway in the thickness direction of the fuel electrode layer is 84% ​​or more of the normalized integrated value Cn.

3. An electrochemical cell according to claim 1 or 2, wherein the number n is 200 or more, and the n points pk are set at equal intervals.

4. A solid oxide electrolysis cell comprising the electrochemical cell of claim 1.

5. A cell stack comprising a stack of solid oxide electrolysis cells according to claim 4.

6. A hot module comprising: a cell stack according to claim 5; a vaporizer that generates steam to be supplied to the cell stack; a heat exchanger that exchanges heat with gas supplied to the cell stack; a heater that heats the cell stack; and a heat insulating material in which the cell stack, the vaporizer, the heat exchanger, and the heater are disposed.

7. A hydrogen production device comprising the hot module according to claim 6.

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