Ceramic interconnect for fuel cell stacks

This application claims the benefit of U.S. Provisional Application No. 61/063,643, filed on Feb. 5, 2008 and U.S. Provisional Application No. 61/009,003, filed on Dec. 21, 2007. The entire teachings of the above applications are incorporated herein by reference.

A fuel cell is a device that generates electricity by a chemical reaction. Among various fuel cells, solid oxide fuel cells use a hard, ceramic compound of metal (e.g., calcium or zirconium) oxide as an electrolyte. Typically, in the solid oxide fuel cells, an oxygen gas, such as O₂, is reduced to oxygen ions (O²⁻) at the cathode, and a fuel gas, such as hydrogen gas (H₂) gas, is oxidized with the oxygen ions to form water at the anode.

Interconnects are one of the critical issues limiting commercialization of solid oxide fuel cells. Currently, most companies and researchers working with planar cells are using coated metal interconnects. While metal interconnects are relatively easy to fabricate and process, they generally suffer from high power degradation rates (e.g. 10%/1,000 h) partly due to formation of metal oxides, such as Cr₂O₃, at an interconnect-anode/cathode interface during operation. Ceramic interconnects based on lanthanum chromites (LaCrO₃) have lower degradation rates than metal interconnects partly due to relatively high thermodynamic stability and low Cr vapor pressure of LaCrO₃ compared to Cr₂O₃ formed on interfaces of the metal interconnects and electrode. However, lanthanum chromites generally are difficult to fully densify and require high temperatures, such as at or above about 1,600° C., for sintering. Although certain doped lanthanum chromites, such as strontium-doped and calcium-doped lanthanum chromites, can be sintered at lower temperatures, they tend to be either unstable or reactive with an electrolyte (e.g., a zirconia electrolyte) and/or an anode.

Therefore, there is a need for development of new interconnects for solid oxide fuel cells, addressing one or more of the aforementioned problems.

The invention is directed to a fuel cell, such as a solid oxide fuel cell (SOFC), that includes a plurality of sub-cells and to a method of preparing the fuel cell. Each sub-cell includes a first electrode in fluid communication with a source of oxygen gas, a second electrode in fluid communication with a source of a fuel gas, and a solid electrolyte between the first electrode and the second electrode. The fuel cell further includes an interconnect between the sub-cells. The interconnect includes a first layer in contact with the first electrode of each sub-cell, and a second layer in contact with the second electrode of each sub-cell. The first layer includes a (La,Mn)Sr-titanate based pertovskite represented by the empirical formula of La_ySr_(1−y)Ti_(1−x)Mn_xO_b, wherein x is equal to or greater than zero, and equal to or less than 0.6; y is equal to or greater than 0.2, and equal to or less than 0.8; and b is equal to or greater than 2.5, and equal to or less than 3.5. In one embodiment, the second layer includes a (Nb,Y)Sr-titanate based pertovskite represented by the empirical formula of Sr_{(1−1.5z−0.5k±δk)}Y_zNb_kTi_(1−k)O_d, wherein each of k and z independently is equal to or greater than zero, and equal to or less than 0.2; d is equal to or greater than 2.5 and equal to or less than 3.5; and δ is equal to or greater than zero, and equal to or less than 0.05. In another embodiment, the interconnect has a thickness of between about 10 μm and about 100 μm, and the second layer of the interconnect includes a (Sr)La-titanate based perovskite represented by the empirical formula of Sr_(1−z±δ)La_zTiO_d, wherein z is equal to or greater than zero, and equal to or less than 0.4; d is equal to or greater than 2.5, and equal to or less than 3.5; and δ is equal to or greater than zero, and equal to or less than 0.05.

In the invention, the first layer of (La,Mn)Sr-titanate based perovskite, which is in contact with the first electrode exposed to an oxygen source, can provide relatively high sinterability (e.g., sinterability to over 95% of theoretical density at a temperature lower than about 1,500° C.), stability in the oxidizing atmosphere and/or electrical conductivity. The second layer of (Nb,Y)Sr-titanate based perovskite and/or (La)Sr-titanate based perovskite, which is in contact with the second electrode exposed to a fuel source, can provide high electrical conductivity and stability in the reducing atmosphere. The (La,Mn)Sr-titanate based perovskite and the (Nb,Y)Sr-titanate based perovskite materials have similar thermal expansion coefficients with each other. For example, La_0.4Sr_0.6Ti_0.4Mn_0.6O₃ has an average thermal expansion coefficient of 11.9×10⁻⁶ K⁻¹ at 30° C.-1,000° C. in air, and Sr_0.86Y_0.08TiO₃ has an average thermal expansion coefficient of 11-12×10⁻⁶ K⁻¹ at 25° C.-1,000° C. in air. Thus, both of the first layer of (La,Mn)Sr-titanate based perovskite and the second layer of (Nb,Y)Sr-titanate based perovskite can be co-sintered at the same time, minimizing process steps.

In another embodiment, the present invention is directed to a method of forming a fuel cell that includes a plurality of sub-cells. The method includes connecting each of the sub-cells with an interconnect. Each sub-cell includes a first electrode in fluid communication with a source of oxygen gas, a second electrode in fluid communication with a source of a fuel gas, and a solid electrolyte between the first electrode and the second electrode. The interconnect includes a first layer that includes a (La,Mn)Sr-titanate-based perovskite represented by the empirical formula of La_ySr_(1−y)Ti_(1−x)Mn_xO_b, wherein x is equal to or greater than zero and equal to or less than 0.6, y is equal to or greater than 0.2 and equal to or less than 0.8, and b is equal to or greater than 2.5 and equal to or less than 3.5. The first layer is in contact with the first electrode of each sub-cell. The interconnect also includes a second layer that includes a (Nb,Y)Sr-titanate-based perovskite represented by the empirical formula of Sr_{(1−1.5z−0.5k±δ)}Y_zNb_kTi_(1−k)O_d, wherein each of k and z independently is equal to or greater than zero and equal to or less than 0.2, d is equal to or greater than 2.5 and equal to or less than 3.5, and δ is equal to or greater than zero and equal to or less than 0.05. The second layer is in contact with the second electrode of each sub-cell. In one embodiment, the method includes forming at least one component of each sub-cell. In another embodiment, the method includes forming at least one of the electrodes of each sub-cell, and forming the interconnect. In yet another embodiment, at least one of the electrodes of each sub-cell is formed independently from the formation of the interconnect, and at least one of the electrodes of each sub-cell is formed together with the formation of the interconnect. In one embodiment, the first electrode of a first sub-cell of the plurality of sub-cells is formed together with the first and the second layers of the interconnect, and the formation of the first electrode, the first layer and the second layer includes disposing a second-layer material of the interconnect over the second electrode of a first sub-cell, disposing a first-layer material of the interconnect over the second-layer material, disposing a first-electrode material of a second sub-cell over the first-layer, of the interconnect, and heating the materials such that the first-layer and second-layer materials of the interconnect form the first and second layers of the interconnect, respectively, and that the first-electrode material forms the first electrode.

In another embodiment, the present invention is directed to a method of forming a fuel cell that includes a plurality of sub-cells, comprising the step of connecting each of the sub-cells with an interconnect having a thickness of between about 10 μm and about 100 μm. Each sub-cell includes a first electrode in fluid communication with a source of oxygen gas, a second electrode in fluid communication with a source of a fuel gas, and a solid electrolyte between the first electrode and the second electrode. The interconnect includes a first layer that includes a (La,Mn)Sr-titanate-based perovskite represented by the empirical formula of La_ySr_(1−y)Ti_(1−x)Mn_xO_b, wherein x is equal to or greater than zero and equal to or less than 0.6, y is equal to or greater than 0.2 and equal to or less than 0.8, and b is equal to or greater than 2.5 and equal to or less than 3.5. The first layer is in contact with the first electrode of each sub-cell. The interconnect also includes a second layer that includes a (La)Sr-titanate based perovskite represented by the empirical formula of Sr_(1−z±δ)La_zTiO_d, wherein z is equal to or greater than zero and equal to or less than 0.4, d is equal to or greater than 2.5 and equal to or less than 3.5, and δ is equal to or greater than zero and equal to or less than 0.05. The second layer is in contact with the second electrode of each sub-cell. In one embodiment, the method includes forming at least one component of each sub-cell. In another embodiment, the method includes forming at least one of the electrodes of each sub-cell, and forming the interconnect. In yet another embodiment, at least one of the electrodes of each sub-cell is formed independently from the formation of the interconnect, and at least one of the electrodes of each sub-cell is formed together with the formation of the interconnect. In one embodiment, the first electrode of a first sub-cell of the plurality of sub-cells is formed together with the first and the second layers of the interconnect, and the formation of the first electrode, the first layer and the second layer includes disposing a second-layer material of the interconnect over the second electrode of a first sub-cell, disposing a first-layer material of the interconnect over the second-layer material, disposing a first-electrode material of a second sub-cell over the first-layer of the interconnect, and heating the materials such that the first-layer and second-layer materials of the interconnect form the first and second layers of the interconnect, respectively, and that the first-electrode material forms the first electrode.

This invention has many advantages. Bi-layer ceramic interconnects of the invention meet all the major requirements for solid oxide fuel cell (SOFC) stack interconnects. (La,Mn)Sr-titanate based perovskite is stable and its electrical conductivity is high in an oxidizing atmosphere, and therefore this material can be used on the air side in the bi-layer ceramic interconnect. (Nb,Y)Sr-titanate based perovskite and (La)Sr-titanate based perovskite is stable and its electrical conductivity is high in a reducing atmosphere, and therefore this material can be used on the fuel side in the bi-layer ceramic interconnect. These materials also have the advantage that, containing no chromium, they do not have the problems associated with lanthanum chromites (LaCrO₃). The present invention can be used in a solid oxide fuel cell (SOFC) system, particularly in planar SOFC stacks. SOFCs offer the potential of high efficiency electricity generation, with low emissions and low noise operation. They are also seen as offering a favorable combination of electrical efficiency, co-generation efficiency and fuel processing simplicity. One example of a use for SOFCs is in a home or other building. The SOFC can use the same fuel as used to heat the home, such as natural gas. The SOFC system can run for extended periods of time to generate electricity to power the home and if excess amounts are generated, the excess can be sold to the electric grid. Also, the heat generated in the SOFC system can be used to provide hot water for the home. SOFCs can be particularly useful in areas where electric service is unreliable or non-existent.