Method for evaluating fuel electrode and method for producing electrochemical cell

Abstract

To provide an evaluation method capable of accurately evaluating performance of a fuel electrode before cell construction.SOLUTION: A fuel electrode evaluation method disclosed herein comprises the steps of: obtaining a cross-sectional SEM image of a fuel electrode 10; acquiring an inter-catalyst particle interface length X, which is a total length of inter-catalyst particle interfaces 12a where catalyst particles 12 contact each other; acquiring a three-phase interface number Y, which is a total number of three-phase interfaces 18 where the catalyst particles 12, ion-conductive particles 14, and internal pores 16 contact each other; and performing a non-defective determination of the fuel electrode based on a ratio X / Y of the inter-catalyst particle interface length X to the three-phase interface number Y. According to this evaluation method, performance of the fuel electrode 10 can be evaluated based on the ratio X / Y without constructing an electrochemical cell and measuring a current density.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present disclosure relates to a method for evaluating an anode and a method for manufacturing an electrochemical cell. [Background technology]

[0002] In recent years, as the introduction of renewable energy has been promoted, hydrogen energy has been attracting attention as a secondary energy source from the perspective of energy storage and transportation. Solid oxide electrolysis cells (SOECs) have been attracting attention as one of the technologies that can generate this hydrogen energy without emitting CO2. SOECs have a configuration in which a fuel electrode and an air electrode face each other with a solid electrolyte layer in between. When using this SOEC, electricity is passed through while supplying water vapor to the fuel electrode. At this time, water (H2O) is reduced at the fuel electrode to produce hydrogen gas (H2) and oxygen ions (O 2- ) are generated. In addition, oxygen ions (O 2- ) moves to the air electrode through the solid electrolyte layer. The oxygen ions that reach the air electrode are then oxidized to oxygen gas (O2). As described above, the SOEC generates oxygen gas and hydrogen gas through the electrolysis reaction of water. This allows electrical energy, which is difficult to store, to be converted into hydrogen energy, which is easy to store and transport. An example of this type of SOEC is disclosed in Patent Document 1.

[0003] On the other hand, some SOEC cells can also be used as solid oxide fuel cells (SOFC). When a cell with the above configuration is used as an SOFC, hydrogen gas is supplied to the fuel electrode and oxygen gas is supplied to the air electrode. At the air electrode of the SOFC, oxygen is reduced to oxygen ions (O 2- ) The oxygen ions move to the fuel electrode through the solid electrolyte layer. When the oxygen ions reach the fuel electrode, they react with hydrogen gas to produce water (H2O). When this water is produced, electrons (e -) are released. In SOFCs, power is generated by supplying these electrons to an external load. In this specification, a cell structure that functions as an SOFC or SOEC is referred to as an "electrochemical cell." An example of such an electrochemical cell is disclosed in Patent Document 2. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent No. 7309642 [Patent Document 2] Japanese Patent Publication No. 2023-140335 Summary of the Invention [Problem to be solved by the invention]

[0005] As mentioned above, electrons move at the anode of an electrochemical cell, regardless of whether it is an SOEC or SOFC. Therefore, the current density of the anode significantly affects the performance of the electrochemical cell. However, the current density of the anode is affected by various factors, such as the state and dispersion of the material. This can lead to variations in current density even among anodes from the same production line. Furthermore, in electrochemical cell manufacturing, anodes with low current density are discarded to prevent the distribution of defective products. However, the current density of the anode due to the electrode reaction can only be measured after the electrochemical cell is assembled. Therefore, if an anode is found to be defective after the cell is assembled, not only the defective anode but also the normal solid electrolyte layer and air electrode must be discarded. This can increase the manufacturing cost of the electrochemical cell. Furthermore, measuring the anode current density requires connecting the electrochemical cell to a power supply and gas supply and heating it to above 700°C. Therefore, measuring the anode current density requires high testing costs and a long testing period of several days.

[0006] The technology disclosed herein has been made in view of the above circumstances, and aims to provide an evaluation method that can accurately and simply evaluate the performance of a fuel electrode before a cell is constructed. [Means for solving the problem]

[0007] The disclosed method for evaluating an anode for an electrochemical cell includes the steps of preparing a test specimen in which the anode is embedded in a resin, polishing the test specimen to expose a cross section of the anode, obtaining a cross-sectional SEM image of the anode using a field emission scanning electron microscope (FESEM), analyzing the cross-sectional SEM image to obtain a catalyst particle-to-catalyst particle interface length X, which is the total length of interfaces where catalyst particles contact each other, analyzing the cross-sectional SEM image to obtain a three-phase interface number Y, which is the total number of interfaces where catalyst particles, ion-conductive particles, and internal pores contact each other, and determining whether the anode is acceptable or not based on the ratio X / Y of the catalyst particle interface length X to the three-phase interface number Y.

[0008] A typical anode has a three-phase interface where catalyst particles, ion-conducting particles, and internal pores come into contact. When an electrochemical cell is used as an SOEC, the three-phase interface of the anode receives sufficient water vapor through the internal pores, and oxygen ions (O 2- ) is transferred to other regions. Therefore, at the three-phase interface of the anode, the reduction decomposition of water (H2O + 2e - →H2+O 2-) occurs appropriately. Therefore, the current density of the anode tends to increase as the number of three-phase interfaces present in the anode increases. On the other hand, during the fabrication of the anode, catalyst particles are prone to aggregation. At this time, the interfaces where catalyst particles contact each other are less likely to receive water vapor and less likely to transfer oxygen ions resulting from decomposition, making it difficult for water to be reductively decomposed. That is, the current density of the anode tends to decrease as the length of the contact interfaces between catalyst particles increases. Based on the above findings, the inventors believed that the current density of the anode could be evaluated without constructing an electrochemical cell by using the ratio X / Y of the length of the contact interfaces between catalyst particles (catalyst particle interfacial length X) to the number of three-phase interfaces between the catalyst particles, ion-conducting particles, and internal pores (three-phase interface number Y). Then, after repeated experiments, the inventors discovered a tendency for the current density of the anode to decrease as the ratio X / Y decreases. The evaluation method disclosed herein was developed based on the above findings. This evaluation method allows the current density of the anode to be evaluated without constructing an electrochemical cell. Therefore, the technique disclosed herein makes it possible to accurately and easily evaluate the performance of a fuel electrode before it is used in an electrochemical cell.

[0009] Another aspect of the technology disclosed herein provides a method for manufacturing an electrochemical cell, comprising the steps of preparing a paste containing catalyst particles, ion-conductive particles, and a pore-forming material, baking the paste to form a test anode, subjecting the test anode to the above-described anode evaluation method, and, if the test anode is determined to be a non-defective product by the evaluation method, fabricating an anode using the paste used to fabricate the test anode, and constructing an electrochemical cell using the anode.

[0010] Typical electrochemical cell manufacturing sites employ a batch-type manufacturing method in which several hundred anodes are produced from a single raw material paste. If the raw material paste for the anode has any defects, all of the mass-produced electrochemical cells may be defective. In contrast, with the manufacturing method described above, before mass production of electrochemical cells begins, test anodes are evaluated for pass / fail based on the ratio X / Y of the catalyst particle interface length X to the number of three-phase interfaces Y. Then, with this manufacturing method, electrochemical cells are manufactured using test anodes that are determined to be pass / fail. This enables stable production of high-performance electrochemical cells while reducing the risk of mass disposal of defective products. [Brief explanation of the drawings]

[0011] [Figure 1] FIG. 1 is a cross-sectional view schematically illustrating an example of an electrochemical cell. [Figure 2] FIG. 2 is a flowchart illustrating a method for evaluating an anode according to one embodiment. [Figure 3] FIG. 3 is a diagram showing a schematic structure of the fuel electrode. [Figure 4] FIG. 4 is a flowchart illustrating a method for manufacturing an electrochemical cell according to one embodiment. [Figure 5] FIG. 5 is a cross-sectional SEM image of the anode of Example 1. [Figure 6] FIG. 6 is a cross-sectional SEM image of the anode of Example 3. [Figure 7] FIG. 7 is an enlarged image of the catalyst obtained by binarizing the cross-sectional SEM photograph of FIG. [Figure 8] FIG. 8 is an image obtained by performing noise removal processing on the catalyst image of FIG. [Figure 9] FIG. 9 is an image obtained by performing image processing on the catalyst image of FIG. 8 to separate the interfaces of the catalyst particles. DETAILED DESCRIPTION OF THE INVENTION

[0012] Preferred embodiments of the technology disclosed herein are described below. Matters necessary for implementing the technology disclosed herein, other than those specifically mentioned in this specification, can be understood as design matters for a person skilled in the art based on the prior art in the relevant field. The technology disclosed herein can be implemented based on the content disclosed in this specification and the technical common sense in the relevant field. In the following drawings, components and parts that perform the same function are denoted by the same reference numerals, and redundant explanations may be omitted or simplified. In this specification, when a numerical range is described as "A to B (A and B are arbitrary numerical values)," this means "greater than A and less than B (including a range greater than A but less than B)."

[0013] As used herein, the term "electrochemical cell" refers to a device capable of converting chemical energy into electrical energy, and a device capable of converting electrical energy into chemical energy, and refers to a device having an anode, a cathode, and an electrolyte. As used herein, the term "anode" refers to an electrode from which electrons flow into an external circuit. As used herein, the term "cathode" refers to an electrode into which electrons flow from an external circuit. An example of an electrochemical cell is an electrolytic cell such as an SOEC. An electrolytic cell uses electrical energy to decompose water (e.g., water vapor) to produce hydrogen gas and oxygen gas. Another example of an electrochemical cell is a fuel cell such as an SOFC. A fuel cell generates electricity using hydrogen gas and oxygen gas.

[0014] 1. Electrochemical Cell The following describes an anode that is the subject of evaluation using the evaluation method disclosed herein, and an electrochemical cell including the anode. Figure 1 is a cross-sectional view of an electrochemical cell 1. Note that the following description shows an example of the subject of evaluation using the evaluation method disclosed herein, and is not intended to limit the subject of evaluation to anodes or electrochemical cells with a specific structure.

[0015] In the electrochemical cell 1 shown in Figure 1, an anode 10 and an cathode 30 face each other via a solid electrolyte layer 20. When this electrochemical cell 1 is used as an SOEC, the anode 10 serves as the cathode and the cathode 30 serves as the anode. The structure of each layer will be described below assuming that the electrochemical cell 1 shown in Figure 1 is used as an SOEC.

[0016] <Fuel electrode 10> In the case of an electrochemical cell 1 for an SOEC, water (H2O) such as water vapor is supplied to the fuel electrode 10. At this fuel electrode 10, hydrogen gas (H2) and oxygen ions (O 2- ) occurs. As shown in FIG. 3, the anode 10 has catalyst particles 12, ion-conducting particles 14, and internal pores 16. The catalyst particles 12 function as a catalyst to promote the water decomposition reaction. Examples of such catalyst particles include metals such as nickel (Ni), copper (Cu), iron (Fe), cobalt (Co), and ruthenium (Ru), as well as metal oxides containing these metals. Among the above-mentioned catalyst materials, nickel oxide (NiO) is preferred because it exhibits high catalytic function and is inexpensive. The ion-conducting particles 14 form ion-conducting paths for transmitting oxygen ions generated by the reductive decomposition of water to other parts. Examples of such ion-conducting particles include gadolinium-doped ceria (GDC), samarium-doped ceria (SDC), scandia-stabilized zirconia (ScSZ), and yttria-stabilized zirconia (YSZ). The internal pores 16 serve as channels through which water vapor supplied to the anode 10 passes.

[0017] <Solid electrolyte layer 20> The solid electrolyte layer 20 is interposed between the fuel electrode 10 and the air electrode 30. In the electrochemical cell 1 shown in FIG. 1, oxygen ions (O 2-) is transferred to the air electrode 30 through the solid electrolyte layer 20. The solid electrolyte layer 20 is a dense layer without pores. This prevents gas exchange between the anode 10 and the air electrode 30. The material of the solid electrolyte layer 20 is not particularly limited, and any conventionally known solid electrolyte that can be used in electrochemical cells can be used without particular limitation. Examples of such solid electrolytes include oxides containing elements such as zirconium (Zr), cerium (Ce), magnesium (Mg), scandium (Sc), titanium (Ti), aluminum (Al), yttrium (Y), calcium (Ca), gadolinium (Gd), samarium (Sm), barium (Ba), lanthanum (La), strontium (Sr), gallium (Ga), bismuth (Bi), niobium (Nb), and tungsten (W).

[0018] <Air electrode 30> The air electrode 30 absorbs oxygen ions (O 2- ) to generate oxygen gas (O2). Although not shown, the air electrode 30 has metal oxide particles and internal pores. The metal oxide particles function as a catalyst to promote the oxidation reaction of oxygen ions. Examples of these metal oxide particles include perovskite oxides containing La, Sr, and Mn (LSM), perovskite oxides containing La, Sr, and Co (LSC), and perovskite oxides containing La, Sr, Co, and Fe (LSCF). The internal pores of the air electrode 30 serve as channels for releasing the generated oxygen gas to the outside.

[0019] <External power supply 40> The electrochemical cell 1 shown in FIG. 1 also includes an external power source 40. The external power source 40 is connected to the anode 10 and the cathode 30 via current collecting members 50 and 60. When the external power source 40 passes current between the anode 10 and the cathode 30, the generation of hydrogen gas and oxygen gas begins. Specifically, the current from the external power source 40 causes electrons (e - ) is supplied to the fuel electrode 10. This allows the reduction and decomposition of water (H2O+2e - →H2+O 2-As a result, hydrogen gas (H2) is generated at the fuel electrode 10. Then, oxygen ions (O 2- ) are transferred to the air electrode 30 through the solid electrolyte layer 20. Then, oxygen ions (O 2- ) is oxidized to produce oxygen gas (O2).

[0020] 2. Evaluation method of fuel electrode An embodiment of the anode evaluation method disclosed herein will now be described. FIG. 2 is a flowchart illustrating the anode evaluation method according to this embodiment. FIG. 3 is a diagram schematically illustrating the structure of an anode. As shown in FIG. 2, the evaluation method according to this embodiment includes a preparation step S10, a polishing step S20, a photographing step S30, a first measurement step S40, a second measurement step S50, and an evaluation step S60.

[0021] (1) Preparation process S10 In the preparation step S10, a test specimen is prepared in which the anode 10 is embedded in resin. The resin used in this step can be any conventionally known resin material used to prepare samples for cross-sectional SEM observation, without any particular restrictions. Examples of such resins include thermosetting resins such as epoxy resin, urethane resin, and methacrylate resin, and UV-curable resins such as styrene resin. Note that the test specimen prepared in this step is not limited to a specific form as long as it includes the anode 10. For example, the test specimen may be one in which only the anode 10 is embedded in resin, or one in which a laminate including the anode 10, solid electrolyte layer 20, and air electrode 30 is embedded in resin.

[0022] (2) Polishing process S20 In the polishing step S20, the test specimen is polished so that the cross section of the anode 10 is exposed. The method for polishing the test specimen is not particularly limited, and any conventionally known polishing method can be used without any particular restrictions. Examples of such polishing methods include ion milling, ion beam processing, and mechanical polishing. Note that in a typical anode 10, the catalyst particles 12 and the ion-conductive particles 14 are not imparted with a specific orientation. Therefore, the cross section exposed in this step is not limited to a cross section along a specific direction. However, polishing the test specimen perpendicular to the thickness direction of the anode 10 (the stacking direction in the electrochemical cell 1) is more preferable because it allows evaluation of bias in the distribution of the particulate material (catalyst particles 12, ion-conductive particles 14) due to sedimentation and separation within the raw material paste.

[0023] (3) Shooting process S30 In the photographing step S30, a field emission scanning electron microscope (FESEM) is used to obtain a cross-sectional SEM image of the anode 10 (see FIGS. 5 and 6). As shown in FIGS. 3, 5, and 6, three phases, namely, catalyst particles 12, ion-conducting particles 14, and internal pores 16, are observed in the cross section of the anode 10. In this step, it is preferable to adjust the photographing conditions so that each of these three phases can be clearly observed.

[0024] For example, the magnification of the cross-sectional SEM image is preferably 3000x or more, more preferably 3500x or more, even more preferably 4000x or more, and particularly preferably 4500x or more. On the other hand, the magnification of the cross-sectional SEM image is preferably 7000x or less, more preferably 6500x or less, even more preferably 6000x or less, and particularly preferably 5500x or less. Setting the magnification of the cross-sectional SEM image within the above range makes it easy to observe the three phases consisting of the catalyst particles 12, the ion-conducting particles 14, and the internal pores 16.

[0025] The acceleration voltage of the FESEM is preferably 1 kV or higher, more preferably 2 kV or higher, even more preferably 3 kV or higher, and particularly preferably 4 kV or higher. Meanwhile, the magnification of the cross-sectional SEM image is preferably 15 kV or lower, more preferably 10 kV or lower, even more preferably 8 kV or lower, and particularly preferably 6 kV or lower. This allows for clearer observation of the catalyst particles 12, ion-conducting particles 14, and internal pores 16.

[0026] The resolution of the cross-sectional SEM image is preferably 96 dpi or more, more preferably 110 dpi or more, even more preferably 220 dpi or more, and particularly preferably 256 dpi or more. As the resolution of the cross-sectional SEM image increases, the accuracy of image analysis in the first measurement step S40 and the second measurement step S50 described below tends to improve. On the other hand, the upper limit of the resolution of the cross-sectional SEM image is not particularly limited, and may be 1200 dpi or less, 1000 dpi or less, 800 dpi or less, or 600 dpi or less.

[0027] Furthermore, in this step, it is preferable to acquire a backscattered electron image as a cross-sectional SEM image so that the three phases (catalyst particles 12, ion-conducting particles 14, and internal pores 16) in the anode 10 have different brightness values. This further improves the accuracy of image analysis in the first measurement step S40 and the second measurement step S50. For example, the brightness value of the catalyst particles 12 in the cross-sectional SEM image is preferably set in the range of 40 to 150 (preferably 80 to 140). The brightness value of the ion-conducting particles 14 is preferably set in the range of 150 to 255 (preferably 160 to 200). The brightness value of the background (internal pores 16) is preferably set in the range of 0 to 40 (preferably 0 to 30). This allows accurate separation of the three phases in the anode 10 by image analysis. Note that the "brightness value" in this specification refers to the brightness value when an 8-bit (256-level) SEM image is used.

[0028] Furthermore, in the photographing step S30, it is preferable to perform a binarization process on the cross-sectional SEM image of the anode 10. This binarization process can obtain a catalyst image showing only the catalyst particles 12, an ion-conductive material image showing only the ion-conductive particles 14, and a pore image showing only the internal pores 16. As will be described in detail later, preparing these three types of binarized images enables more accurate performance evaluation of the anode 10. The specific steps of this binarization process are as follows: First, a brightness histogram of the cross-sectional SEM photograph is created using image analysis software (Image-J). This histogram includes a brightness peak attributed to the catalyst particles 12, a brightness peak attributed to the ion-conductive particles 14, and a brightness peak attributed to the internal pores 16. Based on this, the brightness of the valley portion between the peak attributed to the ion-conductive particles 14 and the peak attributed to the catalyst particles 12 is set as a first threshold value. Next, the brightness of the valley portion between the peak attributed to the catalyst particles 12 and the peak attributed to the internal pores 16 is set as a second threshold value. Then, by creating a binarized image by extracting pixels having a brightness equal to or greater than the first threshold, an ion conductive material image showing only the ion conductive particles 14 can be obtained. Furthermore, by extracting pixels having a brightness less than the first threshold and equal to or greater than the second threshold, a catalyst image showing only the catalyst particles 12 can be obtained. Then, by extracting pixels having a brightness less than the second threshold, a pore image showing only the internal pores 16 can be obtained. An example of a catalyst image after binarization (an image showing only the catalyst particles 12) is shown in Figure 7.

[0029] (4) First measurement step S40 In the first measurement step S40, the inter-catalyst particle interface length X, which is the total length of the interfaces where catalyst particles 12 come into contact with each other, is obtained by analyzing the cross-sectional SEM image. As shown in FIGS. 3 to 5, there are areas inside the anode 10 where catalyst particles 12 come into contact with each other. In this specification, these contact areas between catalyst particles 12 are referred to as "inter-catalyst particle interfaces 12a." These inter-catalyst particle interfaces 12a are not in contact with either the internal pores 16 through which water vapor is supplied or the ion-conducting particles 14 that serve as ion conduction paths. For this reason, an anode 10 with many inter-catalyst particle interfaces 12a tends to have a low current density. In this step, the inter-catalyst particle interface length X, which is the total length of the inter-catalyst particle interfaces 12a that could be a factor in this decrease in current density, is obtained.

[0030] The catalyst particle-to-catalyst particle interface length X is preferably measured based on the catalyst image described above. Specifically, the first measurement step S40 preferably includes a first step of acquiring a first interface length, a second step of acquiring a second interface length, and a third step of acquiring the catalyst particle-to-catalyst particle interface length X by calculating the difference between the second interface length and the first interface length. Specifically, the "first interface length" here refers to the total length of the interface A between the catalyst particle 12 and the ion-conductive particle 14 and the interface B between the catalyst particle 12 and the internal pore 16 (see FIG. 3). This first interface length is obtained by measuring the length of the interface between the black portion (catalyst particle) and the white portion (other portion) of the catalyst image after binarization processing. Next, the "second interface length" refers to the total length of the interface A between the catalyst particle 12 and the ion-conductive particle 14, the interface B between the catalyst particle 12 and the internal pore 16, and the interface C between the catalyst particles 12 themselves. This second interface length is obtained by using the Watershed function of image analysis software (ImageJ) on the binarized catalyst image to display interface C (see Figure 3) between adjacent catalyst particles 12, and then measuring the total length of each of interfaces A to C. An example of an image displaying interface C between adjacent catalyst particles 12 is shown in Figure 9. Then, by calculating the difference between the second interface length, which is the total length of interfaces A to C, and the first interface length, which is the total length of interfaces A and B, the length of interface C between catalyst particles 12 (catalyst particle interface length X) can be calculated.

[0031] Furthermore, in this process, before measuring the catalyst particle interfacial length X, it is preferable to perform a noise removal process on the binarized catalyst image to remove minute white pixels and black pixels. This allows the interface D between the ion-conducting particles 14 and the internal pores 16 to be removed from the catalyst image (see FIG. 8). By using the image after this noise removal process, the above-mentioned calculation process of the catalyst particle interfacial length X can be performed more accurately. Note that this noise removal process preferably removes white pixels of 1 pixel or less and black pixels of 4 pixels or less. This allows the interface D between the ion-conducting particles 14 and the internal pores 16 to be appropriately removed.

[0032] (5) Second measurement step S50 In the second measurement step S50, the cross-sectional SEM image is analyzed to obtain the number Y of three-phase interfaces, which is the total number of interfaces at which the catalyst particles 12, the ion-conducting particles 14, and the internal pores 16 come into contact. As shown in FIGS. 3, 5, and 6, three-phase interfaces 18 exist within the anode 10, at which the catalyst particles 12, the ion-conducting particles 14, and the internal pores 16 come into contact. At these three-phase interfaces 18, water is supplied through the internal pores 16, and oxygen ions (O 2- ) is transferred to other parts through the ion-conducting particles 14. That is, at the three-phase interface 18, the water decomposition reaction (H2O + 2e - →H2+O 2- ) occurs appropriately. Therefore, an anode 10 having many three-phase interfaces 18 tends to have a high current density. In this step, the number Y of three-phase interfaces, which is the total number of three-phase interfaces 18 that is a factor in improving the current density, is obtained.

[0033] The number Y of three-phase boundaries is preferably measured based on the catalyst image, ion-conductive material image, and pore image obtained by the binarization process described above. Specifically, in the second measurement step S50, the following image processing is performed on each of the catalyst image, ion-conductive material image, and pore image using image analysis software. First, for the catalyst image, a first contour line is drawn around the outside of the catalyst particle 12 so as to follow the outer edge of the catalyst particle 12. For the ion-conductive material image, a second contour line is drawn around the outside of the ion-conductive particle 14 so as to follow the outer edge of the ion-conductive particle 14. For the pore image, a third contour line is drawn around the outside of the internal pore 16 so as to follow the outer edge of the internal pore 16. The thickness of these contour lines is preferably about 1 to 5 pixels. Next, a layered image is created by superimposing the catalyst image, ion-conductive material image, and pore image after image processing. In this layered image, there are overlapping portions of the first to third contour lines. The overlapping portions of the contours of the phases are regarded as three-phase interfaces 18. Then, by counting the number of three-phase interfaces 18, the number of three-phase interfaces Y can be obtained. By employing this method, the number of three-phase interfaces Y can be automatically calculated on the image analysis software.

[0034] (6) Judgment step S60 In the evaluation step S60, first, the ratio X / Y of the catalyst particle-to-particle interface length X to the three-phase interface number Y is calculated. Then, the quality of the anode 10 is evaluated based on this ratio X / Y. As described above, the catalyst particle-to-particle interface 12a is a region that does not contact either the internal pores 16 through which water vapor is supplied or the ion-conducting particles 14 that serve as ion conduction paths, which reduces the current density of the anode 10. On the other hand, the three-phase interface 18, which contains the catalyst particles 12, the ion-conducting particles 14, and the internal pores 16, increases the current density. Therefore, an increase in the three-phase interface number Y and a decrease in the catalyst particle-to-particle interface length X (i.e., a decrease in the ratio X / Y) improves the current density of the anode 10. By evaluating the quality of the anode 10 based on this ratio X / Y, the current density of the anode 10 can be evaluated without constructing the electrochemical cell 1. Therefore, the evaluation method according to this embodiment can prevent an increase in manufacturing costs due to the disposal of normal solid electrolyte layers 20 and cathodes 30. Furthermore, because there is no need for preprocessing (such as connecting to various devices or heating the cell) required for measuring current density, it is possible to easily evaluate the performance of the anode 10. Furthermore, because the evaluation method disclosed herein can be performed using only image analysis of SEM photographs, it is also possible to evaluate the performance of the anode 10 whose materials and manufacturing conditions are unknown.

[0035] In the determination step S60, it is preferable to determine a threshold value for the ratio X / Y in advance, and to determine whether an anode 10 having a ratio below the threshold value is a non-defective product. The threshold value is not limited to a specific value, but is set appropriately depending on various factors such as the type of electrochemical cell, the material of the anode, and the structure of the anode. For example, it is preferable to conduct a preliminary test to examine the correlation between the ratio X / Y and the current density of the anode 10, and set the threshold value in the determination step S60 based on the results of the preliminary test.

[0036] In addition, in the evaluation method according to this embodiment, a binarization process is performed on a cross-sectional SEM image to obtain a catalyst image, an ion-conductive material image, and a pore image. Then, the catalyst particle-to-catalyst particle interface length X and the number of three-phase interfaces Y are obtained based on these catalyst image, ion-conductive material image, and pore image. This allows for more accurate measurement of the catalyst particle-to-catalyst particle interface length X and the number of three-phase interfaces Y. However, this method does not limit the technology disclosed herein. For example, as shown in FIGS. 5 and 6, even in a cross-sectional SEM image that has not been subjected to binarization process, the interfaces between the catalyst particles 12, the ion-conductive particles 14, and the internal pores 16 can be confirmed. Therefore, the catalyst particle-to-catalyst particle interface length X and the number of three-phase interfaces Y can also be measured directly from the cross-sectional SEM image.

[0037] 3. Manufacturing method of electrochemical cells The method for evaluating an anode having the above configuration can be used in the manufacture of electrochemical cells. This method for manufacturing electrochemical cells makes it possible to stably manufacture electrochemical cells with excellent performance while reducing the risk of mass disposal of defective products. This method will be described in detail below.

[0038] 4 is a flowchart illustrating a method for manufacturing an electrochemical cell according to this embodiment. As shown in FIG. 4, the manufacturing method according to this embodiment includes a preparation step S110, a test piece fabrication step S120, an inspection step S130, and a cell construction step S140.

[0039] (1) Preparation step S110 In the preparation step S110, a paste containing catalyst particles, ion-conductive particles, and a pore-forming material is prepared. The paste for the anode can be prepared by mixing and stirring catalyst particles, ion-conductive particles, the pore-forming material, and a dispersion medium (such as an organic solvent). The catalyst particles and ion-conductive particles have already been described, so a repeated description will be omitted. The pore-forming material is flammable beads. This pore-forming material forms internal pores 16 in the anode 10 by burning off during the firing process. Examples of such pore-forming material include resin beads containing acrylic resin, methacrylic resin, polystyrene resin, cellulose resin, polyvinyl acetal resin, polyester resin, polyurethane resin, epoxy resin, phenolic resin, polyvinyl acetate resin, polyvinyl alcohol resin, etc. Other examples of pore-forming materials include carbon particles and starch particles.

[0040] (2) Test piece preparation process S120 In the test specimen preparation step S120, the paste described above is fired to prepare a test fuel electrode. For example, in this step, a portion of the paste to be used in manufacturing the electrochemical cell 1 may be sampled. The test fuel electrode can then be prepared by applying this paste to a substrate and firing it. Note that in this step, it is preferable to apply and fire the paste under the same conditions as the actual manufacturing conditions. This allows for accurate evaluation of the performance of the resulting electrochemical cell.

[0041] (3) Inspection process S130 In the inspection step S130, the above-described anode evaluation method is performed on the test anode. The outline of this step is as follows. First, a cross-sectional SEM image of the test anode is acquired, and the catalyst particle interfacial interface length X and the number of three-phase interfaces Y are acquired based on the cross-sectional SEM image. Then, the ratio X / Y of the catalyst particle interfacial interface length X to the number of three-phase interfaces Y is calculated, and the calculation result is compared with a predetermined threshold value to determine whether the test anode is good or bad. In order to avoid duplication, detailed explanations of each process in this step will be omitted.

[0042] (4) Cell construction process S140 If the test anode is determined to be a non-defective product in the inspection step S130, an anode 10 is fabricated using the paste used to fabricate the test anode in the cell construction step S140, and an electrochemical cell 1 is constructed using the anode 10. This allows for the stable production of electrochemical cells with excellent performance while reducing the risk of mass disposal of defective products. Specifically, the production of electrochemical cells 1 typically employs a batch production method in which several hundred anodes are fabricated from a single type of raw material paste. In this case, if the raw material paste is defective, all of the produced electrochemical cells may be defective. In contrast, in the production method according to this embodiment, the test anode is evaluated for its non-defectiveness before mass production of electrochemical cells begins. Then, electrochemical cells are manufactured using the paste from the test anode determined to be a non-defective product. This allows for the stable production of electrochemical cells with excellent performance while reducing the risk of mass disposal of defective products.

[0043] 4. Other Embodiments One embodiment of the technology disclosed herein has been described above. However, the technology disclosed herein is not limited to the above-described embodiment. For example, the above-described embodiment is directed to an electrochemical cell used as an SOEC. However, the technology disclosed herein can be broadly applied to all electrochemical cells having a fuel electrode, and is not limited to SOECs. Specifically, in the fuel electrode of an SOFC, oxygen ions (O 2- Electricity is generated by the reaction between the catalyst particles and hydrogen gas (H2). On the other hand, the above-mentioned power generation reaction hardly occurs at the interfaces between catalyst particles. Therefore, just like the anode for SOEC, the anode for SOFC can be judged as non-defective based on the ratio X / Y of the interface length X between catalyst particles to the number Y of three-phase interfaces.

[0044] [Test example] Test examples relating to the technology disclosed herein will be described below. Note that the test examples are not intended to limit the technology disclosed herein to the following content.

[0045] 1. Sample Preparation In this test, three types of anode pastes (Examples 1 to 3) were prepared, the detailed preparation conditions of which were unknown. The anode pastes contained catalyst particles, ion-conductive particles, a pore-forming agent, a dispersant, a binder resin, and a solvent. The anode pastes were then applied to a support (a fired mixture of NiO and YSZ) using screen printing. Next, a solid electrolyte paste containing YSZ as the main component and a cathode paste containing LSCF as the main component were prepared. The solid electrolyte paste and the cathode paste were then applied in sequence onto the dried anode paste, followed by firing. This resulted in the production of a test cell for Example 1.

[0046] 2.Evaluation Test (1) Acquiring SEM images In this test, cross-sectional SEM images of the anode of each example were obtained according to the procedure described above. A cross-sectional SEM image of Example 1 is shown in Figure 5, and a cross-sectional SEM image of Example 5 is shown in Figure 6. The conditions for obtaining the SEM images were as follows:

[0047] Accelerating voltage: 5 kV Magnification: 5000x Resolution: 512dpi Catalyst particle brightness value: 80~140 Ion conductive particle brightness value: 160~200 Background (internal pore) brightness value: 0 to 30

[0048] (2) Measurement of catalyst particle interfacial length / number of three-phase interfaces Next, binarization processing was performed on the cross-sectional SEM photographs of each example, and catalyst images, ion-conductive material images, and pore images were obtained. Then, following the procedure described above, the catalyst particle-to-particle interface length X and the number of three-phase interfaces Y for each example were measured from the catalyst image, ion-conductive material image, and pore image. The catalyst particle-to-particle interface length / number of three-phase interfaces (X / Y) was calculated based on the measurement results. The calculation results are shown in Table 1. For reference, Figure 7 shows an enlarged portion of the catalyst image created from the cross-sectional SEM photograph in Figure 5. Figure 8 shows the catalyst image after noise removal processing, which removes the interfaces between the ion-conductive particles and the internal pores from the catalyst image. Figure 9 shows the catalyst image when the interfaces between adjacent catalyst particles were separated using the Watershed function of image analysis software (ImageJ).

[0049] (3) Current density measurement Next, the initial current density of the test cell for each example was measured. Specifically, the test cell was first reduced at 700°C. Next, H2 gas containing 50% water vapor was supplied to the fuel electrode (cathode) at a flow rate of 150 ml / min, and air was supplied to the oxygen electrode (anode) at a flow rate of 150 ml / min. The current density at which the voltage reached 1.3 V was defined as the initial current density (A / cm2). 2 The results are shown in Table 1. The initial current densities in Table 1 are relative values ​​with the initial current density in Example 1 being used as the reference value.

[0050] [Table 1]

[0051] As shown in Table 1, a correlation was confirmed in which the initial source density improved as the catalyst particle interfacial length / three-phase interface number (X / Y) decreased. This shows that by measuring the catalyst particle interfacial length / three-phase interface number (X / Y) for the anode, it is possible to appropriately evaluate the performance of the anode even when the composition of the anode raw material paste is unknown or before the electrochemical cell has been constructed.

[0052] The technology disclosed herein has been described in detail above, but these are merely examples and do not limit the scope of the claims. The technology described in the claims includes various modifications and alterations of the specific examples exemplified above. In other words, the technology disclosed herein encompasses the aspects described in items 1 to 7 below.

[0053] <Item 1> 1. A method for evaluating an anode for an electrochemical cell having catalytic particles, ion-conducting particles, and internal pores, comprising: preparing a test specimen in which the anode is embedded in a resin; polishing the test specimen so that a cross section of the anode is exposed; obtaining a cross-sectional SEM image of the anode using a field emission scanning electron microscope (FESEM); A step of acquiring an inter-catalyst particle interface length X, which is the total length of the interfaces where the catalyst particles contact each other, by analyzing the cross-sectional SEM image; acquiring a three-phase boundary number Y, which is the total number of boundaries at which the catalyst particles, the ion-conducting particles, and the internal pores contact, by analyzing the cross-sectional SEM image; a step of determining the quality of the fuel electrode based on a ratio X / Y of the length of the interface between the catalyst particles X to the number Y of the three-phase interfaces; The method for evaluating an anode comprises:

[0054] <Item 2> 2. The method for evaluating an anode according to item 1, wherein the catalyst particles are metal particles or metal oxide particles containing at least one selected from the group consisting of nickel, copper, iron, cobalt, and ruthenium.

[0055] <Item 3> 3. The method for evaluating an anode according to item 1 or 2, wherein the ion-conductive particles include at least one selected from the group consisting of gadolinium-doped ceria, samarium-doped ceria, scandia-stabilized zirconia, and yttria-stabilized zirconia.

[0056] <Item 4> performing a binarization process on the cross-sectional SEM image to obtain a catalyst image in which only the catalyst particles are displayed, an ion conductive material image in which only the ion conductive particles are displayed, and a pore image in which only the internal pores are displayed; 4. The method for evaluating a fuel electrode according to any one of items 1 to 3, wherein the catalyst particle interfacial interface length X and the number of three-phase interfaces Y are obtained based on the catalyst image, the ion-conductive material image, and the pore image.

[0057] <Item 5> The step of obtaining the catalyst particle inter-interface length X includes: acquiring a first interface length, which is the total length of an interface between the catalyst particle and the ion-conducting particle and an interface between the catalyst particle and the internal pore, based on the catalyst image; a step of displaying the interfaces between the catalyst particles on the catalyst image by image processing, and acquiring a second interface length which is the total length of the interface between the catalyst particle and the ion conductive particle, the interface between the catalyst particle and the internal pore, and the interface between the catalyst particles; a step of obtaining the catalyst particle inter-interface length X by calculating a difference between the second interface length and the first interface length; 5. The method for evaluating an anode according to Item 4, comprising:

[0058] <Item 6> The step of obtaining the number Y of three-phase interfaces includes: creating a layered image by superimposing the catalyst image, the ion-conductive material image, and the pore image; obtaining the number Y of three-phase interfaces by counting the number of interfaces where the catalyst particles, the ion-conducting particles, and the internal pores are in contact with each other in the layered image; 6. The method for evaluating an anode according to item 4 or 5, comprising:

[0059] <Item 7> preparing a paste containing catalyst particles, ion-conductive particles, and a pore-forming material; producing a test anode by firing the paste; a step of subjecting the test anode to the anode evaluation method described in any one of items 1 to 6; If the product is determined to be a non-defective product by the evaluation method, a step of fabricating a fuel electrode using the paste used to fabricate the test fuel electrode and constructing an electrochemical cell using the fuel electrode. A method for manufacturing an electrochemical cell, comprising: [Explanation of symbols]

[0060] 1. Electrochemical cell 10 Fuel electrode 12 Catalyst particles 12a Catalyst particle interface 14 Ion-conducting particles 16 internal pores 18 Three-phase interface 20 Solid electrolyte layer 30 Air electrode

Claims

1. 1. A method for evaluating an anode for an electrochemical cell having catalytic particles, ion-conducting particles, and internal pores, comprising: preparing a test specimen in which the anode is embedded in a resin; polishing the test specimen so that a cross section of the anode is exposed; obtaining a cross-sectional SEM image of the anode using a field emission scanning electron microscope (FESEM); A step of acquiring an inter-catalyst particle interface length X, which is the total length of the interfaces where the catalyst particles contact each other, by analyzing the cross-sectional SEM image; acquiring a three-phase interface number Y, which is the total number of interfaces at which the catalyst particles, the ion-conducting particles, and the internal pores contact each other, by analyzing the cross-sectional SEM image; a step of determining whether the fuel electrode is good or bad based on a ratio X / Y of the length of the interface between the catalyst particles X to the number Y of the three-phase interfaces; The method for evaluating an anode comprises:

2. 2. The method for evaluating an anode according to claim 1, wherein the catalyst particles are metal particles or metal oxide particles containing at least one selected from the group consisting of nickel, copper, iron, cobalt, and ruthenium.

3. 2. The method for evaluating an anode according to claim 1, wherein the ion-conductive particles include at least one selected from the group consisting of gadolinium-doped ceria, samarium-doped ceria, scandia-stabilized zirconia, and yttria-stabilized zirconia.

4. performing a binarization process on the cross-sectional SEM image to obtain a catalyst image in which only the catalyst particles are displayed, an ion conductive material image in which only the ion conductive particles are displayed, and a pore image in which only the internal pores are displayed; 2. The method for evaluating an anode according to claim 1, wherein the catalyst particle interfacial length X and the number of three-phase interfaces Y are obtained based on the catalyst image, the ion-conductive material image, and the pore image.

5. The step of acquiring the catalyst particle inter-interface length X includes: acquiring a first interface length, which is the total length of an interface between the catalyst particle and the ion-conducting particle and an interface between the catalyst particle and the internal pore, based on the catalyst image; a step of displaying the interfaces between the catalyst particles on the catalyst image by image processing, and acquiring a second interface length which is the total length of the interface between the catalyst particle and the ion conductive particle, the interface between the catalyst particle and the internal pore, and the interface between the catalyst particles; a step of obtaining the catalyst particle inter-particle interface length X by calculating a difference between the second interface length and the first interface length; The method for evaluating an anode according to claim 4 , further comprising:

6. The step of acquiring the number Y of three-phase interfaces includes: creating a layered image by superimposing the catalyst image, the ion-conductive material image, and the pore image; acquiring the number Y of three-phase interfaces by counting the number of interfaces where the catalyst particles, the ion-conducting particles, and the internal pores are in contact in the layered image; The method for evaluating an anode according to claim 4, further comprising:

7. preparing a paste containing catalyst particles, ion-conductive particles, and a pore-forming material; producing a test anode by firing the paste; a step of subjecting the test anode to the anode evaluation method according to any one of claims 1 to 6; If the product is determined to be a non-defective product by the evaluation method, a step of fabricating a fuel electrode using the paste used to fabricate the test fuel electrode and constructing an electrochemical cell using the fuel electrode. A method for manufacturing an electrochemical cell, comprising:

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