Patent Application
20160190614
The subject matter of this disclosure relates generally to electrochemical devices such as solid oxide fuel cells, and more particularly, to a solid oxide fuel cell (SOFC) metallic manifold and interconnect structure that provides a dense surface adjacent to a permeable surface with a flat transition perpendicular to the interfacial junction of the surfaces so that both surfaces are in the same plane. Subsequent electrode and electrolyte layers are deposited onto the surface, so that they are parallel to the planar interconnect surface.
A Solid oxide fuel cell (SOFC) converts chemical energy to electrical energy with high efficiency and low emissions. A cathode reduces oxygen on one side and supplies oxygen ions to a hermetic electrolyte. The hermetic electrolyte conducts the oxygen ions at high temperature to an anode, where the oxygen ions oxidize hydrogen to form water. A resistive load connecting the anode and cathode conducts electrons to perform work.
Anode-supported SOFCs based on traditional ceramic sintering technology are limited by the maximum manufacturable cell size at high yields; and sinter-based manufacturing facilities require large capital investment. However, metal interconnect-supported SOFCs utilizing thermal spray deposition offer a variety of manufacturing benefits as well as a more rugged design. As a result, cell sizes can be increased with more success than observed when using sintering manufacturing. The success of thermal spray deposited electrolytes is not only dependent on the intrinsic coating hermeticity, but also the design of the interconnect substrate.
The interconnect surface needs to be relatively smooth to prevent gross defects from forming as electrode, typically, although not exclusively, anode, and the subsequent electrolyte are deposited, which can result in low open circuit voltage (OCV) and poor performance at high fuel utilization (Uf). In addition, the interconnect needs to have a fuel flow field designed to allow sufficient fuel gas to reach the anode and electrolyte interface to minimize mass transport polarization. Typically this is achieved by using large perforations or interconnected porosity. Powder feedstock that is fed into the thermal spray process can range from 100 nm to around 50 um for thermal spray deposition, restricting the interconnect fuel flow field to less than about 100 um porous feature sizes. Porous feature sizes less than about twice the powder size is typically adequate to deposit a complete and uniform coating, where anode and electrolyte powder can adequately bridge the features without the formation of permeable defects. The use of a porous metal foam has shown promise for providing adequate fuel to reach the anode and electrolyte interface, while still maintaining a hermetic electrolyte coating without crack causing defects. However, known designs that require the porous metal to be sealed to a dense metal manifold result in sharp transitional corners and seams that prevent full electrolyte coverage and cause localized areas of high stress concentrations after electrolyte deposition. The stress substantially increases the probability of cracking during operation, leading to the loss of OCV and Uf and can ultimately cause SOFC failure.
Since a SOFC flow field carries fuel gas or air to the electrodes for electrochemical reactions, the gas and air flow fields, in conjunction with their respective manifolds must be separated and sealed to prevent fuel and air mixing. Further, the gas and air flow fields must also be electrically insulated while providing electrical interconnections, conducting electrons from their respective anode or cathode electrodes.
The manifold and flow field must provide pathways for the reactant gas to reach the electrodes. This is typically achieved using perforated, channeled, or corrugated designs. These designs do not offer a substantially smooth and flat substrate for planar coating deposition. Thermal spray deposition techniques, for example, require smooth surfaces for uniform coatings, without sharp features that can cause large local stresses leading to cracks. These qualities are desired to ensure electrolyte coating hermeticity. The electrolyte not only needs to be intrinsically hermetic, but must also form a seal to the dense portion of the manifold.
One known method deposits an electrolyte over an anode with direct bonding to the anode interconnect. This results in a seal between the electrolyte and the anode interconnect only when the electrolyte is sufficiently hermetic, eliminating fluid communication between the fuel and the oxidant (typically air). A monolithic manifold has been proposed with large perforated holes for fluid communication with the electrode and electrolyte. Since the part is monolithic, the bonding of a permeable interconnect to the manifold is not needed. However, to address the challenge of depositing thin anode and electrolyte layers over large fuel openings, the concept suggests using an expendable fugitive material in the interconnect fuel openings. This step requires a high temperature burnout and can interfere or have limited success with high temperature electrolyte deposition processes such as thermal spray deposition.
Similarly, some techniques have co-sintered metal interconnect structures with electrolyte, where the electrolyte is in contact with and forms a seal over the junction of porous and dense regions. The sealed interface requires electrolyte only to be in contact with the porous and dense interface, leaving a discontinuous and incomplete anode electrode coverage. The incomplete anode coverage and co-sintering of metal, cermet and ceramic can disadvantageously create substantial residual stresses in coating and is not desirable.
Another technique that has been used to provide manifold fluid contact to the electrodes uses a microporous porous metal. Porous metal with pore sizes less than about 100 microns have been adequate for providing complete anode coverage with low roughness values. Subsequent electrolyte deposition has been shown to be without discontinuities and stress related cracking. However, the perimeter of the porous metal, where in contact with the dense portion of the manifold, is a critical location that can cause defective coatings. Laser welding has been used to attach planar metallic porous metal to the dense manifold for thermal spray applications. Anode deposition was followed by the deposition of electrolyte, which formed a hermetic seal to the dense metal. The laser welding technique resulted in sharp corners that were undesirable, leading to high local stresses and often resulted in catastrophic electrolyte cracks adjacent to the permeable interconnect.
In view of the foregoing, a need exists for a metal interconnect-supported electrochemical device that further minimizes any chance for the formation of defects that may adversely affect the open circuit voltage and/or fuel utilization.
According to one embodiment, an electrochemical device, such as a solid oxide fuel cell (SOFC) manifold and interconnect structure, comprises a manifold that comprises a dense and hermetic planar surface; and a porous material that comprises a permeable planar surface that is in lateral contact with the dense and hermetic planar surface to form an electrode interconnect, wherein the interfacial junction between the dense and hermetic planar surface and the permeable planar surface comprises an exposed planar surface that is substantially flat and devoid of discontinuities, corners and seams, and further wherein the dense and hermetic planar surface, the permeable planar surface and the exposed planar surface of the junction lay in a single plane common to the dense and hermetic planar surface, the permeable planar surface and the exposed planar surface of the junction. The electrochemical device manifold and interconnect structure may further comprise an anode material that is thermally sprayed onto and fully covers both the exposed planar surface of the interconnect junction and the permeable planar surface; and an electrolyte thermally deposited onto and fully covering the anode material, wherein the electrolyte is thermally bonded to the anode to hermetically seal the anode material and provide a fluid bather between an oxidant and predetermined fuel gases associated with the electrochemical device. The electrochemical device may further comprise a cathode electrode material thermally deposited onto the electrolyte, and a second electrode interconnect in fluid contact with the cathode electrode material.
Large metal supported and thermally sprayed electrode and electrolyte require coatings without regions with residual stress that can cause crack propagation. Therefore, it is beneficial to entirely coat a metal interconnected substrate face with a uniformly thick electrode. Full electrode coverage provides a low modulus interface between the metal and electrolyte, allowing for some compliance and a lower stress state in the electrolyte coating.
The success of the seal created by the interconnect junction between dense/hermetic manifold and the permeable material relies on the dense portion that is impermeable to the fuel gas that is adjacent to a porous region created by the permeable material. The porous region allows for the reducing gas to reach the anode and electrolyte interface. In addition, the surface of the interconnect junction is smooth and seamless, providing an ideal substrate for deposition of thin electrode and electrolyte layers and minimizing the chance for defects to form.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings, wherein:
While the above-identified drawing figures set forth particular embodiments, other embodiments of the present invention are also contemplated, as noted in the discussion. In all cases, this disclosure presents illustrated embodiments of the present invention by way of representation and not limitation. Numerous other modifications and embodiments can be devised by those skilled in the art which fall within the scope and spirit of the principles of this invention.
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According to one embodiment, the dense metallic structure 32 is continuous and completely encircles the permeable metallic structure 22 such that only a single interconnect junction 30 exists between the dense metallic structure 32 and the permeable metallic structure 22. Other embodiments may comprise a plurality of porous areas/permeable metallic structures tiled within a dense metallic structure to form a plurality of interconnect junctions, based on a particular application.
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The success of the seal 34 relies on a dense portion 32 that is impermeable to a fuel gas 36 and that is adjacent to the porous region 22. The porous region 22 allows for the fuel 36 to reach the anode 26 and electrolyte 28 interface. In addition, the surface 38 of the interconnect junction 30 is smooth and seamless, providing an ideal substrate for deposition of the thin electrode 26 and electrolyte 28 layers and minimizing the chance for defects to form.
Further, the anode deposited over the porous interconnect support surface is exposed to fuel during SOFC operation. According to one embodiment, the SOFC anode comprises nickel and a yttria-stabilized zirconia cermet. During operation, the anode nickel exposed to the fuel remains reduced, while the anode nickel close to the perimeter of the cell is oxidized to nickel oxide. The oxidized portion is substantially dense and impermeable to further seal fuel from air.
The interconnect surface 38 is relatively smooth to prevent gross defects from forming as the anode 26 and electrolyte 28 are deposited, which can result in low open circuit voltage (OCV) and poor performance and fuel utilization (Uf), as stated herein. Further, the interconnect has a fuel flow field designed to allow sufficient fuel gas to reach the anode 26 and electrolyte 28 interface. Typically this is achieved by using large perforations or interconnected porosity. Powder feedstock can range from 100 nm to around 100 um for thermal spray deposition. Preferred embodiments use a feedstock with maximum average particle size less than 50 um, restricting the interconnect fuel flow field to less than about 100 um porous feature sizes. Feature sizes smaller than the powder particle size are desired so that anode and electrolyte powder can adequately bridge the features without the formation of permeable defects, as stated herein. The use of a porous metal has shown promise for providing adequate fuel to reach the anode and electrolyte interface, while still maintaining a hermetic electrolyte coating without crack causing defects. Less than 25 micron porous features are employed according to a preferred embodiment.
Previous designs that require the porous metal support to be sealed to a dense metal manifold result in sharp transitional corners and seams that prevent full electrolyte coverage and cause localized areas of high stress concentrations after electrolyte deposition. The stress substantially increases the probability of cracking after deposition, leading to the loss of OCV and Uf and can ultimately cause SOFC failure.
In summary explanation, an electrochemical device such as a solid oxide fuel cell (SOFC) manifold and interconnect structure 20 provides a substantially planar dense surface adjacent to a substantially planar permeable surface with a smooth/gradual transition at the junction 30 between the surfaces to avoid the discontinuities and sharp corners generally associated with known anode supported SOFC manifold and interconnect structures. More specifically, the electrochemical device/SOFC manifold and interconnect structure 20 comprises a dense and hermetic structure 32 comprising a first planar surface 21, a permeable material 22 comprising a second planar surface 23 in lateral contact with the first planar surface 21 to form a first electrode interconnect. An interconnect junction 30 between the first and second planar surfaces 21, 23 comprises an exposed planar surface 38 that is substantially flat and devoid of discontinuities, corners and seams. A first electrode material 26 is deposited on and fully covers the exposed planar surface 38 of the interconnect junction 30, the permeable material 22, and the manifold support surface 24. An electrolyte 28 is deposited on and fully covers the first electrode material 26. Further, the electrolyte 28 substantially seals the first electrode material 26 and provides a fluid barrier between oxidant and fuel gases 36 associated with the electrochemical device/SOFC. The electrochemical device/SOFC may further comprise a second electrode material deposited onto the electrolyte 28 to form a second electrode, and a second electrode interconnect in fluid contact with the second electrode.
According to a preferred embodiment, the first electrode comprises, but is not limited to, an anode nickel and yttria-stabilized zirconia cermet. The anode material can also be chosen from doped cerium oxides, including gadolinium and samarium doped ceria; lanthanum-based perovskite oxides; stabilized zirconia; nickel or cobalt; strontium titanate and doped-strontium titanate; and mixtures thereof. It can be appreciated that alternatively, a cathode can be deposited first onto the interconnect material to coat over the interfacial junction of the dense and porous interconnect surface. The electrolyte would then subsequently be deposited, followed by the anode.
According to one aspect, a thermal spray technique is used to apply an electrode, preferably anode, and a subsequent electrolyte. “Thermal spray technique”, as used herein, shall mean a coating process in which melted or heated materials are sprayed onto a surface. The feedstock, i.e., a coating precursor, is heated by an electrical technique or by chemical means. The electrical technique can be, without limitation, plasma or arc-based. The thermal spray technique generally employs some sort of combustion flame or plasma. The coating materials are usually fed into a spraying mechanism in powder form, heated to a molten or semi-molten state, and accelerated toward a substrate in the form of particles, e.g., micrometer-sized particles. In some preferred embodiments, the thermal spray technique is carried out at atmospheric pressure, or very close to atmospheric pressure. Further, in some embodiments, the technique is carried out at a temperature that is sufficient to melt the coating composition or its precursors, during application of the material(s) to the surface of the substrate. Usually, the thermal spray technique is a high-velocity fuel technique or a plasma spray technique. Examples of plasma spray techniques include vacuum plasma spray deposition (VPS), radio frequency plasma, plasma transfer arc, and air or atmospheric plasma spray (APS). APS techniques are preferred in some embodiments. Examples of high-velocity fuel techniques include high velocity oxy-fuel (HVOF), high velocity air fuel (HVAF), and high velocity liquid fuel (HVLF).
In some preferred embodiments, suspension spray techniques are used. The coating feedstock for such techniques is dispersed in a liquid suspension before being injected into the jet stream of a spray gun. Distilled or deionized water, alcohols such as ethanol, or water-alcohol mixtures are usually used as the solvent. A suspension technique may provide considerable advantages, such as, without limitation, easier handling and feeding of very small feedstock particles, e.g. particles having an average size in the range of about 100 nanometers to about 10 microns.
In other embodiments, deposition of anode and electrolyte coatings can be achieved using vacuum plasma spray, chemical vapor deposition processes, and sputtering. In still other embodiments, the anode and electrolyte can be deposited using more traditional ceramic deposition techniques, such as tape casting, screen printing, and spin-coating using slurries, pastes or sol-gel formulations.
A number of methods can be employed to fabricate the metal interconnect and deposition substrate with interconnect junction 30. Powder metallurgical techniques followed by solid state co-sintering can provide a dense metal portion with lateral contact with a porous portion. Welding techniques, such as laser, tungsten inert gas (TIG), and metal inert gas (MIG) welding can be used to join a porous metal foam to dense sheet metal providing metal tolerances are closely controlled for intermit lateral contact. A preferred embodiment uses metallurgical melt infiltration techniques to densify and encircle the porous metal. Melt infiltration utilizes a material with a lower melting temperature than the porous metal to preferentially fill porosity in desired regions using capillary and gravity directed forces. During melt, diffusion of materials rapidly raised the melting temperature to solidify creating the interfacial porous and dense regions described herein. Those skilled in the art can recognize low melting temperature metal alloys relative to the porous metal substrate.
Those skilled in the art will recognize that there are a variety of conductive metals that can be used for high temperature electrochemical devices such as solid oxide fuel cells. Typical designs use metals that oxidize slowly and are corrosion resistant in representative temperature and pressure operating conditions. Such material can include, without limitation, chromium-containing alloys, nickel-containing alloys, iron chromium (FeCr), nickel chromium (NiCr), or combinations thereof, including ferritic stainless steels. In some embodiments, the porous metal can be made from nickel and ferritic stainless steel such as 300 or 400-series stainless steel. The porous metal comprises nickel according to a preferred embodiment.
While the invention has been described in terms of various specific embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the claims.
This invention was made with Government support under sub contract number DE-NT0004109 awarded by the United States Department of Energy. The Government has certain rights in the invention.
