Patent Application
20110158880
A fuel cell is a device that generates electricity by a chemical reaction. Typically, in a fuel cell, an oxygen gas, such as O2, is reduced to oxygen ions (O2−) at the cathode, and a fuel gas, such as H2, is oxidized with the oxygen ions to form water at the anode. Among various types of fuel cells, solid oxide fuel cells (SOFCs) use hard ceramic compounds of metal oxides (e.g., calcium or zirconium oxides) to form components of the fuel cell, such as, for example, the anode, cathode, electrolyte, and interconnect. Fuel cells are generally designed as stacks, whereby subassemblies, each including a cathode, an anode and a solid electrolyte between the cathode and the anode, are assembled in series by locating an electrical interconnect between the cathode of one subassembly and the anode of another.
One SOFC cell design consists of five layers. Two of these five layers are relatively thick layers: anode bulk and cathode bulk. A relatively thin layer of electrolyte and relatively thin functional anode and cathode layers are sandwiched between the bulk layers. Typically, the thin layer thickness is only about 1/100th of the bulk layer thickness. The most important mechanism in stress generation in a SOFC co-fired cell is the step of cooling down from the sintering temperature, typically 1300-1400° C., to room temperature, due to the mismatch in the coefficients of thermal expansion (CTE) between the materials of the five layers. Where there is sufficient mismatch of CTE among the layers, cooling, or any temperature change that is too rapid, can cause fracture and consequent failure of the SOFC. Because of the much larger thickness, the majority of the stress is generated by the mismatch between the CTEs of the anode bulk layer and the cathode bulk layer. Since most ceramics show linear elastic stress-strain behavior up to failure in the temperature range that a SOFC operates, for a fixed geometry design, there are only two material properties that affect the thermal mismatch stress: modulus and coefficient of thermal expansion of the materials.
In order to reduce the thermal mismatch stress, it is desirable to have both the cathode and anode materials of a SOFC have a CTE that is as close as possible to the CTE of the electrolyte, which is typically made of yttria-stabilized zirconia (YSZ). A typical CTE of YSZ generally is in a range of between about 10.5·10−6° C.−1 and 11·10−6° C.−1, which is much lower than the CTE of most cathodes. Specifically, while a typical anode material has a CTE of 11.3·10−6° C.−1, the most commonly used cathode material for SOFCs is lanthanum strontium manganite (LSM), La0.2Sr0.8MnO3 (LSM20/80), which has a CTE in a range of between about 12.2·10−6° C.−1 and about 12.4·10−6° C.−1 (average CTE between room temperature and 1200° C.). See L. Kindermann, et al., Synthesis and properties of La—Sr—Mn—Fe—O perovskites, Proceedings of the 3rd European solid oxide fuel cell forum, 1998, pp. 123. The difference between the CTEs of the LSM and YSZ materials would generate a large thermal mismatch stress in the SOFC. With a Sr content increase above 0.2, the CTE of the LSM material would further increase. Id. On the other hand, it is often undesirable to use an LSM with Sr content lower than 0.2, due to diminished electrochemical performance.
Therefore, there is a need to overcome or minimize the above-mentioned problems.
The invention generally is directed to a cathode having an anisotropic CTE and a method for forming the anisotropic CTE cathode of a solid oxide fuel cell.
In one embodiment, a cathode of a solid oxide fuel cell includes a sintered layer having an anisotropic coefficient of thermal expansion (CTE) defined by an out-of-plane CTE and an in-plane CTE with a difference of at least 1×10−6° C.−1 between the out-of-plane CTE and the in-plane CTE. The sintered layer can be a perovskite. In some embodiments, the perovskite can be lanthanum strontium manganite (LSM). In these specific embodiments, the LSM can have an in-plane CTE in a range of between about 10.6×10−6° C.−1 and about 11.8×10−6° C.−1, preferably in a range of between about 11.0×10−6° C.−1 and about 11.4×10−6° C.−1. In some embodiments, the out-of-plane CTE of the LSM can be about 13.0×10−6° C.−1 and the in-plane CTE of the LSM is about 12.0×10−6° C.−1. In other embodiments, the out-of-plane CTE of the LSM can be about 14.0×10−6° C.−1 and the in-plane CTE of the LSM is about 10.6×10−6° C.−1. The anisotropic CTE cathode can exhibit a non-linear stress-strain curve at a temperature in a range of between about 20° C. and about 50° C.
In another embodiment, a method for forming a low coefficient of thermal expansion (CTE) cathode of a solid oxide fuel cell includes the steps of placing a layer of perovskite powder between two platens, and sintering the layer while applying pressure to the platens, thereby forming the anisotropic CTE cathode. The layer can have a thickness in a range of between about 0.5 mm and about 4 mm. In some embodiments, the layer can be sintered at a temperature in a range of between about 1200° C. and about 1400° C. for a time period in a range of between about 15 minutes and about 3 hours. In certain embodiments, the layer can be sintered at a temperature in a range of between about 1270° C. and about 1290° C. for a time period in a range of between 15 minutes and about one hour. The pressure applied to the platens can be in a range of between about 0.02 MPa and about 20 MPa. In some embodiments, the pressure applied onto the platens is in a range of between about 0.02 MPa and about 12 MPa.
This invention has many advantages, including maintaining low thermal mismatch stresses in the cell and stack of a SOFC. As will be further described below, it has been found that compression of LSM during heat treatment lowers the CTE of the LSM, thereby minimizing or significantly reducing failure of SOFCs that employ LSM during use.
The foregoing will be apparent from the following more particular description of example embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention. The teachings of all patents, published applications and references cited herein are incorporated by reference in their entirety.
The invention generally is directed to a cathode having an anisotropic CTE and a method for forming the anisotropic CTE cathode of a solid oxide fuel cell. A SOFC stack can include the anisotropic CTE cathode, wherein the anisotropic CTE cathode can be made separately before stack fabrication or processed simultaneously with other layers of the SOFC stack. As used herein, the term solid oxide fuel cell includes a solid oxide electrolyser, that is, a solid oxide fuel cell operated in a regenerative mode for the electrolysis of water.
In one embodiment, illustrated in
Layer 10 can have a thickness in a range of between about 0.5 mm and about 4 mm. Layer 10 can be green-formed from the perovskite powder by, for example, die pressing, tape casting/lamination, extrusion, and gel casting.
In some embodiments, layer 10 can be sintered at a temperature in a range of between about 1200° C. and about 1400° C., preferably between about 1270° C. and about 1290° C., for a time period in a range of between about 15 minutes and about 3 hours, preferably in a range of between about 15 minutes and about one hour. The pressure applied to platens 20 can be in a range of between about 0.02 MPa and about 20 MPa, preferably in a range of between about 0.02 MPa and about 12 MPa, with a constant pressure being applied to platens 20 during the sintering time period.
The resulting low CTE cathode can have a CTE in a range of between about 10.6×10−6° C.−1 and about 11.8×10−6° C.−1, preferably in a range of between about 11.0×10−6° C.−1 and about 11.4×10−6° C.−1. The CTEs listed herein are linear secant CTEs between room temperature and about 900° C., to avoid complications from continued sintering above about 1000° C.
The CTE can be anisotropic. As illustrated in
The cathode can exhibit a non-linear stress-strain curve at a temperature in a range of between about 20° C. and about 50° C.
All samples were sintered at about 1280° C. for about 30 minutes in air unless otherwise noted. For the hot-pressed (HPed) samples whose results are shown in
To demonstrate the effect of hot-pressing the LSM samples, three sets of samples were made from the same batch of CREE LSM20. Set A was free-sintered in a box oven with a spacer covering the top and bottom surfaces. Set B was free-sintered in a hot-press oven with a sapphire spacer covering the top and bottom surfaces, and Set C was hot-pressed. Each set of samples consisted of at least 4 samples and two rounds of repeated tests. The CTE results were:
Set A (free-sintered in box oven, covered by spacer)—12-12.2 ppm
Set B (free-sintered in hot-press oven, covered by spacer)—11.9-12.2 ppm
Set C (hot-pressed)—11-11.2 ppm
A comparison of differential CTEs between hot-pressed and free sintered CREE LSM20 powder is shown in
Additional results for low CTE LSM were obtained from a series of pre-sintered (bisque-fired) cathode bulk samples. Three sets of identical 1 inch (25 mm) disks were first free-sintered. Two of them were then hot-pressed under different temperatures and pressures, listed in Table 3 below. Their CTEs, also listed in Table 3, were measured afterwards. As shown in Table 3, hot-pressing lowered the CTE of the LSM material.
An additional sample made from Praxair LSM20 and calcined before hot-pressing also showed a low CTE after hot-pressing.
Hot-pressed CREE LSM20 was subjected to thermal cycle tests in which the samples were exposed to 900° C. for an accumulated time period of about 16 hours. The CTE before the cyclic test (during the initial heat-up) and after the test (during final cycle cool-down) were calculated. The two CTE values were the same for all three types of samples tested (11-11.2 ppm for hot-pressed CREE LSM20, 11.9-12.2 ppm for free-sintered Praxair LSM20 and free-sintered CREE LSM20). Therefore, the relatively short-term stability of the CTE is demonstrated at 900° C.
A set of hot-pressed CREE LSM20 were annealed in a dilatometer in air at 900° C. (4 hrs), 1200° C. (4 hrs), 1280° C. (6 hrs), and 1380° C. (2 hrs) respectively. The CTEs before and after annealing were measured. Pre-annealing CTE was measured during the heat-up phase of annealing. Post-annealing CTE was measured both during cool-down phase of annealing and in at least one subsequent separate CTE measurement run. The results are shown in Table 4 below.
The CTE remained unchanged after annealing at and below 1200° C., but increased back to (or almost to) the CTE of free-sintered LSM20 after annealing above 1200° C. The trend can also be seen from the differential CTE plot shown in
The annealing study was also done with the pre-sintered (bisque-fired) samples. The results are listed in Table 5 below.
As shown by the results in Table 5, the low CTE of the LSM obtained after hot-pressing reverses back to the CTE of free-sintered LSM after annealing at or above 1280° C. Without wishing to be bound by any particular theory, it is possible that the CTE reversal might be activated by the mobility of the crystalline structure, similarly to the continued sintering observed in hot-pressed LSM20 above 1100° C.
The CTE of hot-pressed LSM20 was tested in two orthogonal directions, shown in
Four sets of tests were done, with 32 expansion curves recorded, 16 curves for each orientation. The test samples were cut from the usual long dilatometer test bar (25 mm long). The materials included free-sintered, hot-pressed, bisque-fired, and bisque-fired/hot-pressed CREE LSM20. Measurements (expansion curves) are shown in
In another example, a thicker (23 mm) sample was made with a sintering temperature of about 1320° C. and a time of about 30 min under a pressure of about 3 MPa, so that longer (21 mm) CTE bars could be made for measurements along both in-plane and out-of-plane directions. Table 6 shows the CTE values of 3 in-plane and out-of-plane samples, respectively, showing the anisotropy behavior of the samples. The CTE values along the in-plane direction listed in Table 6 are higher than those reported elsewhere. However, the HPed sample used for measurements in Table 6 was much thicker than the regular ones discussed above (23 mm vs. ≦4 mm). Without wishing to be bound by any particular theory, it is reasonable to suggest that within the thick sample the transition of pressure and the correspondent mass diffusion can be different from those within a thin sample. This difference might cause different CTE results, but the anisotropic behavior remained true for both types of samples regardless of thickness.
X-ray diffraction (XRD) characterization was applied on the in-plane and out-of-plane surfaces of HPed samples. As shown in
Hot-Pressing Results with Variable Pressure
The anisotropy results described above strongly indicate that pressure is the cause of low CTE. Hence, a series of samples of CREE LSM20 were hot-pressed, but with different peak pressure: 1 MPa, 0.1 MPa, dead load (˜0.02 MPa), and no pressure (but with sapphire spacer covering). For each pressure level, at least two samples were tested and each test consisted of at least two repeated tests done on different weeks. Also tested was a Praxair fine powder LSM20 that was hot-pressed at 3 MPa. The CTE results are listed in Table 8. Also listed in Table 8 is the diameter expansion of the sample disk after hot-press—to indicate whether deformation has been caused by the pressure. The sample should shrink in diameter during sintering, but if enough pressure was applied to cause deformation (pressure induced mass diffusion), the diameter of the disk could expand.
As demonstrated by the results shown in Table 8 and
To underscore the effect of even relatively low pressure, all 5 batches of tests results, for which samples were cut from the same disk for each pressure level, for CR LSM-(E) (0.02 MPa) and CR LSM-(G) (0.1 MPa) are given in Table 9 below. The samples were deliberately alternated as to which slot in the dilatometer was used for the test. The tests were conducted over a time span of two months.
Non-Linear Stress-Strain Results
LSM20 powder (CREE LSM20 or Praxair LSM20) was hot-pressed or free sintered into 2 inch disks. Bars were then cut from disks with dimensions of 50 mm×4 mm×4 mm. The bars were then subjected to four-point bending tests to measure the load vs. displacement curves. The results are shown in
To verify the effect of hot-press sample preparation, two sets of samples were identically processed with the only difference being that one set was hot-pressed and one set was free-sintered. The powder batch, green forming, and sintering time and temperature were all the same. The result is shown in
The teachings of all references identified above are incorporated herein by reference in their entirety.
While this invention has been particularly shown and described with references to example embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.
This application claims the benefit of U.S. Provisional Application No. 61/335,085, filed on Dec. 31, 2009. The entire teachings of the above application are incorporated herein by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61335085 | Dec 2009 | US |
