Solid oxide fuel cell with ceramic interconnector and heating method

Abstract

The invention relates to a solid oxide fuel cell at least having an anode, an electrolyte, a cathode, a ceramic interconnector for which either Si3N4 or MoSi2 is used as matrix and either MoSi2 or Si3N4 is used as secondary phase, and SiC and / or TiC is optionally used as material for the ceramic interconnector.

Description

[0001] 2024PF00132 1 Solid Oxide Fuel Cell and Process The present invention relates to electrically conductive ceramics for interconnects for high-temperature solid oxide fuel cells (SOFCs) and processes. Within the energy transition, the use of hydrogen, and thus the high-temperature solid oxide fuel cell (SOFC), will play an important role due to its high efficiency. For an SOFC to operate successfully, two gases, oxygen (O2) and fuel (e.g., hydrogen (H2)), must be supplied in parallel. The electrochemical process begins at the anode side, where the supplied hydrogen is oxidized by the existing oxygen ions (O2-). This means that each hydrogen atom releases its electron, and the resulting hydrogen protons (H2-) +The electrons react with oxygen ions (O2-) to form water (H2O). The released electrons flow through the electron conductor from the anode to the cathode, thus generating an electric current. At the cathode, the incoming electrons recombine with an oxygen molecule, forming oxygen ions (O2-). These oxygen ions (O2-) diffuse through the electrolyte (often yttrium-doped zirconium dioxide) to the anode, where they react again with hydrogen (H2). This cycle is a continuous loop, generating electrical energy. The mechanism of electrical conductivity in ceramics is based on the movement of charge carriers, such as electrons or ions, through the crystal structure of the material. In ionic ceramics, conduction occurs through the migration of ions along lattice sites, while in electronic ceramics, electrons migrate through the material.Conductivity can also be affected by defects in the crystal structure or by the presence of foreign atoms. One way to increase the electrical conductivity of ceramics is to optimize the crystal structure by doping with foreign elements to increase the mobility of the charge carriers. However, the operating temperatures of SOFCs of 1173 K (900 °C) present various problems with the materials used. The current standard is the use of metallic interconnects, for example, nickel wire or coatings, whose function is to conduct the current generated during operation. Aging of the metallic interconnects leads to a time-dependent loss of efficiency.Due to the differing thermal expansion rates between the metallic interconnect and the ceramic cell body, the metallic interconnect introduces additional stresses into the ceramic cells, which contribute to cracking and destruction of the ceramic cells during manufacturing. Such a fuel cell is described in DE 10033 944 A1. A solid oxide fuel cell stack comprises a first solid oxide fuel cell, a second solid oxide fuel cell, an interconnect, and a joining element. The first solid oxide fuel cell contains a cathode. The second solid oxide fuel cell contains an anode. The interconnect is positioned between the cathode of the first solid oxide fuel cell and the anode of the second solid oxide fuel cell.The joining element connects the intermediate connector to at least one of the two, the cathode of the first solid oxide fuel cell or the anode of the second solid oxide fuel cell, wherein the joining element has a porous substrate. A method for manufacturing a stack of solid oxide fuel cells is also disclosed. The interconnect comprises two interconnect surfaces, each of which is substantially smooth and flat. DE 4242 570 C2 further discloses a composite material for a solid oxide fuel cell for electrically connecting adjacent cell units, each of which has one electrode, one electrolyte, and the other electrode. The composite material is made from a mixture of an alloy containing mainly nickel and chromium with oxide ceramics in an amount of 50% to 85% of the mixture.Previous SOFC systems use metallic interconnectors, which, however, can lead to stresses and cracks in the cells due to aging and differing thermal expansion rates between metal and ceramic. Therefore, the invention aims to propose a novel cell architecture that overcomes the disadvantages of the prior art. This objective is achieved according to the invention by a solid oxide fuel cell (SOFC) with a ceramic interconnector according to claim 1 and a method according to claim 5. To overcome these disadvantages, the present invention proposes the use of electrically conductive ceramic as the interconnector. The ceramic interconnectors exhibit temperature resistance and thermal expansion comparable to previously used ceramic cell materials.Matching the thermal expansion to that of the ceramic cell results in lower stresses during operation and reduced reject rates during manufacturing. Figure 3 illustrates the principle of a cell. It shows a cell consisting of a tube in which the oxygen flows in one direction 10, is deflected, and then flows in the same direction 11 as the fuel 12. The cross-section shows the arrangement by means of the circles with the point 2024PF00132 4 or the cross in the middle, which indicate the flow directions into or out of the plane of the drawing. A layered structure is present between the fuel and the oxygen / air. Figure 1 shows a layered structure 1' of a SOFC layer according to the prior art, namely consisting of anode 2, electrolyte 3, cathode 4, and a two-layer interconnect 5' with nickel plating.According to the invention in Figure 2, the interconnect 5' according to Figure 1 is reduced to a single ceramic interconnect 5 made of one material. Possible material systems for the ceramic interconnect 5 include Si3N4-SiC-MoSi2, Si3N4-TiC-MoSi2, and Si3N4-MoSi2. The matrix is ​​either silicon nitride (Si3N4) or molybdenum disilicide (MoSi2), i.e., the proportions of silicon nitride (Si3N4) or molybdenum disilicide (MoSi2) preferably amount to 5 vol% and 95 vol%, respectively (all values ​​in vol%): MoSi2 as matrix: MoSi2 + 5% - 45% Si3N4 as secondary phase; Si3N4 as matrix: Si3N4 + 5% - 45% MoSi2 as secondary phase. Preferably, the proportion of the secondary phase in the matrix is ​​10 vol% to 35 vol%.

[0002] 2024PF00132 5 Tested and good results were achieved with (all values ​​in vol%): 1. MoSi2+ 5% Si3N4 2. MoSi2+ 10% Si3N4 3. MoSi2+ 12.5% ​​Si3N4 4. MoSi2+ 15% Si3N4 5. MoSi2+ 20% Si3N4 6. MoSi2+ 22.5% Si3N4 7. MoSi2+ 25% Si3N4 8. MoSi2+ 30% Si3N4 9. MoSi2+ 32.5% Si3N4 10. MoSi2+ 35% Si3N4 11. MoSi2+ 40% Si3N4 12. MoSi2+ 45% Si3N4 or 1. Si3N4+ 7% MoSi2 2. Si3N4+ 12% MoSi2 3. Si3N4+ 17% MoSi24. Si3N4+ 22% MoSi25. Si3N4+ 27% MoSi26. Si3N4+ 32% MoSi27. Si3N4+ 34% MoSi28. Si3N4+ 37% MoSi29. Si3N4+ 39% MoSi210. Si3N4+ 42% MoSi211. Si3N4+ 44% MoSi2. Additives such as silicon carbide (SiC) or titanium carbide (TiC) can preferably also be added to adjust the electrical conductivity. The effect of TiC and SiC is virtually identical. SiC or TiC can be pre-mixed as a powder with the ceramic of the matrix or the ceramic of the secondary phase. The proportions of silicon carbide (SiC) or titanium carbide (TiC) are then, individually or combined, between 0.5 vol% and 2024PF00132 6 5 vol%, partially replacing the corresponding secondary phase: MoSi2 as matrix: MoSi2 + (5 vol% - 45 vol%) Si3N4 and (0% to 5 vol%) (TiC or SiC) as secondary phase, where: 5 vol% <= Si3N4 + TiC + SiC <= 45 vol%. Si3N4 as matrix: Si3N4 + (5 vol% - 45 vol%) MoSi2 and (0% to 5 vol%) (TiC or SiC) as secondary phase, where: 5 vol% <= MoSi2 + TiC + SiC <= 45 vol%. Preferably, the proportion of the secondary phase in the matrix is ​​10 vol% to 35 vol%. The following combinations were tested and showed good results (all values ​​in vol%): 1. MoSi2+ 5% Si3N4+ 0.5% TiC / SiC 2. MoSi2+ 10% Si3N4+ 1% TiC / SiC 3. MoSi2+ 15% Si3N4+ 1.5% TiC / SiC 4. MoSi2+ 20% Si3N4+ 2% TiC / SiC 5. MoSi2+ 25% Si3N4+ 2.5% TiC / SiC 6. MoSi2+ 30% Si3N4+ 3% TiC / SiC 7. MoSi2+ 32.5% Si3N4+ 3.25% TiC / SiC 8. MoSi2+ 35% Si3N4+ 3.5% TiC / SiC 9. MoSi2+ 37.5% Si3N4+ 3.75% TiC / SiC 10. MoSi2+ 40% Si3N4+ 4% TiC / SiC 11. MoSi2+ 22.5% Si3N4+ 2.25% TiC / SiC 12. MoSi2+ 27.5% Si3N4+ 2.75% TiC / SiC 13. MoSi2+ 17.5% Si3N4+ 2.125% TiC / SiC 14.MoSi2+ 7.5% Si3N4+ 0.75% TiC / SiC 15. MoSi2+ 12.5% ​​Si3N4+ 1.25% TiC / SiC 16. MoSi2+ 32% Si3N4+ 3.2% TiC / SiC 17. MoSi2+ 34% Si3N4+ 3.4% TiC / SiC 18. MoSi2+ 36% Si3N4+ 3.6% TiC / SiC 19. MoSi2+ 38% Si3N4+ 3.8% TiC / SiC or 1. Si3N4+ 5% MoSi2+ 0.5% TiC / SiC 2. Si3N4+ 10% MoSi2+ 1% TiC / SiC 3. Si3N4+ 15% MoSi2+ 1.5% TiC / SiC 4. Si3N4+ 20% MoSi2+ 2% TiC / SiC 5. Si3N4+ 25% MoSi2+ 2.5% TiC / SiC 6. Si3N4+ 30% MoSi2+ 3% TiC / SiC 2024PF00132 7 7. Si3N4+ 32.5% MoSi2+ 3.25% TiC / SiC 8. Si3N4+ 35% MoSi2+ 3.5% TiC / SiC 9. Si3N4+ 37.5% MoSi2+ 3.75% TiC / SiC 10. Si3N4+ 40% MoSi2+ 4% TiC / SiC 11. Si3N4+ 22.5% MoSi2+ 2.25% TiC / SiC 12. Si3N4+ 27.5% MoSi2+ 2.75% TiC / SiC 13. Si3N4+ 17.5% MoSi2+ 2.125% TiC / SiC For TiC / SiC, relative proportions from 0% to 100 vol% were varied in 10% increments. This means that for the example above, 1 vol% TiC / SiC could be: 1.0 vol% SiC, 0.1 vol% TiC + 0.9 vol% SiC, ... 0.9 vol% TiC + 0.1 vol% SiC = 1.0 vol% TiC.The test ceramics can be easily produced using 3D printing (AM) in a matrix where the appropriate proportions are mixed from two to four powder reservoirs. These ceramic interconnects have a temperature resistance and thermal expansion comparable to the ceramic cell materials already in use and can be adjusted accordingly by varying their proportions. This reduces stresses during operation and manufacturing, leading to lower reject rates. Currently, the cell and module are heated by preheating the air that is passed through the cell. A targeted current flow through the conductive ceramic would allow for uniform heating of the cell during system startup. Additionally, the cell can be kept at temperature more energy-efficiently during partial load operation than is currently possible with air preheating.Another advantage of ceramic interconnectors is that they can be heated evenly by targeted current flow. This enables more efficient temperature maintenance during partial load operation than with previous air preheating methods.

Claims

2024PF00132 9 Claims 1. Solid oxide fuel cell comprising at least: an anode, an electrolyte, a cathode, a ceramic interconnect, characterized in that either Si3N4 or MoSi2 is used as the matrix and either MoSi2 or Si3N4 as the secondary phase, and optionally SiC and / or TiC as the material for the ceramic interconnect.

2. Solid oxide fuel cell according to claim 1, wherein the proportion of the secondary phase is between 5 vol% and 45 vol%.

3. Solid oxide fuel cell according to claim 1, wherein the proportion of the secondary phase is between 15 vol% and 35 vol%.

4. Solid oxide fuel cell according to one or more of claims 1, 2 or 3, wherein the secondary phase comprises MoSi2 or Si3N4 with additions of silicon carbide (SiC) and / or titanium carbide (TiC) as a further secondary phase, wherein the proportion of the additions of TiC and / or SiC is between 0.5 vol% and 5.0 vol% .

5. Method for operating a solid oxide fuel cell according to one or more of the preceding claims, wherein the solid oxide fuel cell is heated by preheating the air that is passed through the solid oxide fuel cell.

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