This invention relates generally to electrochemical cell structures and more specifically to substrates for depositing electrochemical cell components and methods of making the same.
Electrochemical cells are energy conversion devices that are usually classified as either electrolysis cells or fuel cells. Electrolysis cells can function as hydrogen generators by electrolytically decomposing water to produce hydrogen and oxygen gases. Fuel cells electrochemically react a hydrogen gas with an oxidant across an exchange membrane or electrolyte to generate electricity and produce water. Fuel cells, such as solid oxide fuel cells, have demonstrated a potential for high efficiency and low pollution and have many potential applications including large-scale power generation, distributed power and use within automobiles.
One of the key challenges related to electrochemical cell advancement is to develop cost effective processes to manufacture electrode and electrolyte materials, especially with large surface areas.
Accordingly there is a need in the art for improved electrochemical cell design and associated fabrication techniques.
An electrochemical cell support structure comprises a conductive base defining a plurality of holes or openings passing through the conductive base and a microporous cellular substrate disposed on the conductive base. In another embodiment, an electrochemical cell support structure comprises a conductive base defining a plurality of holes passing through the conductive base, a coarse microporous cellular substrate disposed on the conductive base and a fine microporous cellular substrate disposed on the coarse microporous cellular substrate.
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 in which like characters represent like parts throughout the drawings, wherein:
An electrochemical cell support structure 10 comprising a conductive base 12 defining a plurality of holes 14 passing through the conductive base 12 and a microporous cellular substrate 16 disposed on the conductive base 12, is shown in
Conventionally electrochemical cell structures, especially SOFC structures, are processed using sintering methods. There are, however, significant limitations of these sintering methods, especially when used in large cell fabrication. Direct deposition techniques, as discussed below, overcome many of the limitations of these sintering methods, especially in regards to large cell fabrication.
Electrochemical cell support structure 10 is ideally suited for use with deposition processes, as microporous cellular substrate 16 provides a relatively smooth surface to support deposition techniques such as chemical vapor deposition (CVD), plasma enhanced CVD (PE-CVD), physical vapor deposition (PVD), electron beam physical vapor deposition (EB-PVD), sputter deposition, ion beam deposition, molecular beam epitaxy (MBE), spray pyrolysis or particulate deposition techniques such as plasma spray, flame spray, or high-velocity oxygen fuel thermal spray process (HVOF) or other deposition techniques.
Conductive base 12 typically comprises a plate-type structure that includes a series of holes 14 that pass through, allowing fluid flow, for example fuel or oxidant flow, to flow from one side of conductive base 12 to an opposite side of conductive base 12. Alternatively, conductive base 12 includes a series of flow-optimized pillars that allow reactants to flow thereabout. Accordingly, conductive base 12 can comprise a perforated solid piece or a series of pillar supports that permit flow through the thickness or parallel to the base respectively. In one embodiment, the holes have a diameter between about 0.25 inches to about 1.0 inch. As discussed herein, the term “holes” refers to any type of void, slot, groove or other cavity arrangement that allows fluid flow to pass from one side of conductive base 12 to the opposite side. Conductive base 12 is typically made of metals that oxidize slowly under the operating conditions associated with solid oxide fuel cell (SOFC) and solid oxide electrolyzer cell (SOEC) applications, namely, high temperatures and pressures. Conductive base 12 is typically a metallic interconnect made of iron, chromium, nickel, tin, combinations thereof or other materials conventionally used as interconnect materials, especially in SOFC and SOEC applications. Such materials conventionally include for example, chromium-containing alloys, iron chromium (FeCr), nickel chromium (NiCr), or nickel iron chromium (NiFeCr) based alloys or ferritic stainless steels such as E-Brite or other stainless steels. The dimensions of conductive base 12 and the accompanying holes 14 can vary greatly depending on the application and size requirements.
In one embodiment, microporous substrate 16 comprises a porous nickel (Ni) layer disposed on conductive base 12. The nickel substrate 16 has a good bond to the conductive base 12, typically a metallic interconnect, which bond is attained in a variety of ways including by using the methods discussed below. Additionally, the nickel substrate 16 comprises a sufficient porosity to allow gas flow needed to ensure proper cell functionality and is robust enough to enable deposition of the cell layers, including the electrode and electrolyte layers. In one embodiment, the microporous substrate 16 comprises iron, chromium, nickel, tin, or combinations thereof or other materials conventionally used a interconnect materials.
In one embodiment, nickel (Ni) layer 16 is created as depicted in the method steps of
Next, in method step S2, an agent that acts as an initial adhesive or glue and subsequently serves as a bonding agent is applied to the surface of conductive base 12. The agent can comprise a variety of metal organic compounds including Ni acetyl acetonate, nickel octoate, nickel decanoate, and the like. The metal organics can be used to prepare solutions that can be applied to the metal surface using methods such as painting, dipping, or spinning or the like, that typically result in a thin layer being applied. The applied solution imparts tackiness to the surface so that subsequent layers, such as tape cast material, will adhere to the surface. Furthermore, the second and more critical role of the metal-organic layer is that it will leave behind a very thin layer of metal upon subsequent heat treatment, which layer facilitates the bonding between the base metal and the porous metal structure.
Optionally, method 50 may include a surface-plating step S3. In S3, the surface of the conductive base is coated with Ni-plating to enhance the bonding with the microporous cellular substrate 16. Additionally, Ni-plating also provides a protective layer against oxidation of the conductive base 12 during the subsequent heat treatments or during reduction of NiO.
Next, in method step S4, the NiO tape formed in S1 is applied to the surface of the conductive base 12. The NiO tape can be applied or laminated to the conductive base 12 using a variety of methods including, for example, manual rolling, warm pressing, and vacuum bagging. Vacuum bagging offers some advantage over other methods because it allows the lamination of the NiO tape to flat as well as irregular surfaces using a uniform pressure. Additionally, vacuum bagging ensures that gases evolving during the process are continuously removed and consequently reducing the probability of blister formation at the interface between the NiO tape and the conductive base 12.
Finally, in method step S5, the laminated substrate is put through a heat treatment cycle that uses an initial burnout phase followed by reduction and sintering phases. Typically the binder burnout phase is conducted in either air or wet hydrogen environments. The reduction and the sintering phases are typically conducted in a wet hydrogen environment. In this step, the NiO is reduced to Ni, the Ni is sintered and the bond between the Ni layer and the conductive base 12 forms, resulting in an electrochemical cell structure 10 having a substantially uniform microporous substrate 16 that is ideally suited as a substrate for deposition techniques in the manufacture of electrochemical cells, especially SOFC or SOEC cells.
A few examples of electrochemical cell support structure 10 formation using method 50 are shown in
In another embodiment of the instant invention, the microporous cellular substrate 16 comprises a porous medium made of nickel (if used with a reducing gas) or any corrosion resistant metal or electronically conductive ceramic. For example, when used to enclose a reducing gas, nickel or any corrosion resistant metal or electronically conductive ceramic can be used as the microporous cellular substrate 16, followed by an anode, an electrolyte and a cathode. These layers can be applied using deposition techniques or by using any other thin film coating process. If the cathode is the first layer of film coating, typically only corrosion resistant metal or electronically conductive metals can be used as the microporous cellular substrate 16 as this layer will be subjected to an oxidizing environment. Microporous cellular substrate 16 can either be manufactured integrally with the conductive base 12 or it can be pre-fabricated and adhered to the conductive base 12 using a variety of techniques including, for example, brazing to avoid coarsening of the microstructure, chemical bonding or solid-state diffusion bonding. Once electrochemical cell structure 10 is formed, additional layers of cell can be deposited on the microporous cellular substrate 16 to create a cell 100, as shown in
In
In an alternative embodiment, cell 100 may further comprise a perforated screen 108 interposed between the conductive base 12 and the microporous substrate 16, which screen 108 is typically comprised of the same material as that of the conductive base 12. Screen 108 is attached to conductive base 12 atop openings 14, or flow channels, using metallic joining methods, for example brazing. Screen 108 serves as a substrate for the deposition of the microporous substrate 16 across the openings 14 in conductive base 12 as a gas diffusion layer or as an electrode. Screen 108 is optionally used in this embodiment for increased structural durability.
In another embodiment, microporous cellular substrate 16 is grown on conductive base 12 using chemical or environmental heat treatments or a combination thereof. In yet another embodiment, microporous cellular substrate 16 is pre-manufactured and attached to the conductive base 12. Microporous cellular substrate 16 can be manufactured using a variety of methods. In one embodiment, microporous cellular substrate 16 is made by sintering a metallic powder using either gravity fed sintering or compacted sintering. This sintering is typically conducted in a vacuum atmosphere at temperatures high enough. for metal sintering. In another embodiment, microporous cellular substrate 16 is made by filling in macroporous foam with a metallic paste followed by thermal and/or environmental treatment so as to generate an appropriate microporous structure. For example, macroporous nickel foam has pore sizes that are typically too large for fuel cell deposition processes. In this embodiment, nickel foam is coated with a NiO paste that is prepared by mixing NiO powder with an organic binder such as polyvinyl butryl and solvents. Alternatively, a NiO slurry can be cast on top of the macroporous foam. The foam is then heat treated to burn out the organics contained within the paste, followed by a treatment in a reducing environment at temperatures that are high enough to create a partially sintered porous Ni filler structure within the macroporous foam. In yet another embodiment, microporous cellular substrate 16 is made using spray deposition or tape casting of metallic or metal-precursor particles such as metal oxides.
The thickness, pore volume, and pore size distribution within microporous cellular substrate 16 is typically determined by the deposition particle size and the allowed interfacial and/or interlaminar stress developed during the construct of the cell 100, given the strength of the electrolyte layer 102. Typically, the preferred pore size within the microporous cellular substrate 16 is between about 5 to about 30 microns, and more preferably between about 10 to about 20 microns. Additionally, the preferred porosity is typically within about 40 to about 85 percent, more preferably between about 50 to about 75 percent.
Given a set of material constants,
In order to provide foam properties for specific material sets, transfer functions between buckling loads and substrate properties have been developed mathematically through the use of composite laminate theory applied to the conditions that cell 100 may encounter. Three of the most significant parameters related to the buckling failures of the substrate are: thickness; porosity; and heat flux of the deposited structure. The laminate theory is used for each material for certain heating conditions to obtain transfer functions that aid in determining substrate mechanical properties for the given conditions.
In deposited fuel cell environments, the surface of the substrate can get extremely hot due to molten particulates, especially in process such as air plasma, vacuum plasma, chemical vapor deposition or other deposition techniques. Due to reduced thermal conductivity in the substrate and the thickness of the substrate over certain unsupported regions, the thermal gradient can drive buckling failure.
An electrochemical cell structure 200 comprising a conductive base 202 defining a plurality of holes 204 passing through the conductive base 202, a coarse microporous cellular substrate 206 disposed on the conductive base and a fine microporous cellular substrate 208 disposed on the coarse microporous cellular substrate 206, is shown in
In an alternative embodiment, the coarse microporous cellular substrate 206 at least partially interpenetrates into conductive base 202, as shown in
Next, in step S8, a chemical glue, typically in the form of a Ni-metalorganic compound is applied over the conductive base 202 surface. The metalorganic compound is selected from Ni-acetyl acetonate, Ni-octoate, Ni-decanoate, or from a number of Ni-alkoxides. Alternatively, Ni or NiO paste, which is prepared by mixing Ni or NiO powder with organic solvents and binders, can be used as a gluing layer.
Next, in step S9, a porous open cell structure or foam with a typical relative density of about 0.6 to about 0.9 and a pore size of between about 150 to about 300 microns is placed on top of the conductive base 202.
Next, in step S10, suitable pressure is then applied to the structure and the foam is partially compressed into the holes 204 of the conductive base 202, thereby increasing the relative density of the supported regions. The pressure can be applied using a press or any load giving apparatus, typically having the structures positioned between two smooth high stiffness plates. A smooth outer surface is obtained after this process.
Next, in step S11, the assembly is heated in a reducing atmosphere to a temperature in the range between about 900 C to about 1050 C, to create a diffusion bond between the foam and the conductive base 202.
Next, in step S12, green NiO tape is placed upon the assembly as the next layer and a low stiffness roller is used across the entire surface to create an initial bond. Typically, prior to attaching the NiO tape, the surface of the macroporous foam is painted with Ni-metalorganic solution that initially acts as a glue that holds the tape and substrate together. The metalorganic layer also enhances bonding between the macroporous foam and porous Ni layer that results from the reduction of the NiO tape by leaving behind a thin Ni layer that results from the reduction of the NiO tape by leaving behind a thin layer of metal upon subsequent heat treatment. The metalorganic compound is selected from Ni-acetyl acetonate, Ni-octoate, Ni-decanoate, or from a number of Ni-alkoxides. Alternatively, NiO can be applied to the assembly by screen-printing a NiO paste. This may require the application of several layers until the desired thickness is built up.
Next, in step S13, the assembly is placed into a vacuum bag and heated to between about 150 C to about 200 C to laminate the NiO tape to the assembly. This step is not necessary if the NiO paste is used.
Finally, in step S14, the complete adhesion of the tape or paste layer to the assembly is accomplished by the final heat treatment, which heat treatment typically consists of binder burnout and reduction/sintering segments. Binder burnout from the green NiO layer is achieved by heating in air to between about 350 C to about 400 C. This is followed by heating in a reducing atmosphere in the range between about 800 C to about 1000 C to reduce NiO to Ni and sinter the Ni into a porous layer.
In another embodiment, the fine microporous cellular substrate 208 at least partially interpenetrates into the coarse microporous cellular substrate 206, as shown in
In yet another embodiment, the coarse microporous cellular substrate 206 at least partially interpenetrates into conductive base 202 and the fine microporous cellular substrate 208 at least partially interpenetrates into the coarse microporous cellular substrate 206, as shown in
As discussed herein, the electrolyte materials may comprise any conventional electrolyte material including, for example: yttria-stabilized zirconia; lanthanum gallate; doped cerium oxide; ceria-stabilized zirconia; a stabilized zirconia like CaO-stabilized zirconia, MgO-stabilized zirconia, M2O3 stabilized zirconia, where M is taken from the group of Y, Sc, Yb, Nd, Sm, or Gd; lanthanum gallate having a general composition of La1−x−wSrx−wGa1−yMgy+zO3−0.5(x+y+5w−2z) where, 0.3≧x≧0.1; 0.3≧y≧0.1; 0.04≧w≧0.01; 0.15≧z≧0.03; doped cerium oxide where CeO2 is doped with one or a mixture from La2O3, Y2O3, Sm2O3, Gd2O3, other rare earth oxides, Gd2O3+Pr2O3, CaO, SrO; certain stabilized Bismuthsesquioxides, Bi2O3-MO, where M is calcium, strontium or barium; Pyrochlore oxides of the general formula A2B2O7, particularly Ln2Zr2O7, where Ln is a lanthanoid such as Gd2(ZrxTi1−x)2O7 (GZT) and Y2(ZrxTi1−x)2O7 (YZT); or Perovskite structures such as BaCe0.9Gd0.1O3, CaAl0.7Ti0.3O3, SrZr0.9Sc0.1O3; or combinations thereof.
As discussed herein, the electrode materials may comprise anode materials and cathode materials. The anode materials may comprise any conventional anode material including, for example: a mixture comprising an electronically conducting material such as at least one of a metal and a metal oxide that will be subsequently reduced to form the metal, and an ionically conducting material; at least one of nickel, nickel oxide, a platinum-group metal; a single phase of an electronically conducting materials; certain metals including Ni, Co, Pt, Pd or Ru; mixed oxide conductors including the ZrO2—Y2O3—TiO2 system; or combinations thereof. The cathode materials may comprise any conventional cathode materials including, for example: a mixture comprising an electronically conducting material such as at least one of a metal and a metal oxide that will be subsequently reduced to form the metal and an ionically conducting material; at least one of nickel, nickel oxide or a platinum-group metal; lanthanum strontium manganite; doped lanthanum cobaltite; a mixture comprising at least one of a platinum-group metal, lanthanum strontium manganite, doped lanthanum ferrite, and doped lanthanum cobaltite and an electronically conducting material; doped lanthanum manganite; La MnO3 substituted with various cations such as Ba, Ca, Cr, Co, Cu, Pb, Mg, Ni, K, Rb, Na, Sr, Ti, or Y; lanthanum strontium manganite having a general formula of La1-xSrxMnO3, which can also be further doped with Co or Cr; lanthanum cobaltite; LaCoO3 doped with Sr, Ca, Mn, or Ni to adjust conductivity or thermal expansion; doped lanthanum ferrite, for example La0.8Sr0.2FexCoyO3; or combinations thereof.
In addition, while the current invention is discussed in terms of two electrodes, a cathode and an anode, with an electrolyte disposed therebetween, other embodiment may include additional layers. For example, certain embodiments can include buffer layers deposited between the electrodes and the electrolyte. These buffer layers may be included for a variety of reasons including, without limitation, to prevent deleterious chemical interaction between the other layers. For example, some embodiments may include an interlayer of cerium-gadolinium oxide, or the like, that can be used to reduce the interdiffusion and chemical interaction between a layer of YSZ and a layer of lanthanum cobaltite, lanthanum strontium ferrite or mixtures thereof. Similarly, an interlayer of samarium-doped cerium oxide (Ce1-xSmxO2-0.5x) can be used to reduce the interdiffusion and chemical interaction between a composite anode of NiO: CeO2 and LSGM (La1.8Sr0.2Ga1−yMgyO2.9−0.5y, where 0.05<y<0.3).
Several of the embodiments discussed within this specification provide a high porosity support or substrate for deposition, especially for large surface area deposition, with each support tailored for low gas transport resistance and robust to mechanical and thermal stresses. Additionally, several of the embodiments provide a relatively smooth outer surface that is suitable for a variety of deposition methods. The conductive base discussed in this invention is typically an interconnect, and more specifically, a metal interconnect. The porous or microporous support material or substrate material can comprise either the cathode material or the anode materials, or alternatively, the porous or microporous support material can comprise another layer such as a gas diffusion layer. In many of the embodiments, porous or microporous support material is the electrode and provides a relatively smooth surface for deposition of the remaining cell layers. Additionally, many of the embodiments involve the use of a mesh material, like screen, attached to the conductive base. Typically, this mesh material is made of the same or similar material of the conductive base. In the case where the conductive base and the mesh material are each made of an interconnect material, the resulting structures have improved mechanical durability, the current pathway is less sensitive to cracks that typically develop parallel to the electrode/interconnect surface since the current can travel laterally to the mesh material and then to the conductive base. Additionally, the dimensions of the unsupported electrode and electrolyte layers is greatly reduced to the smaller dimensions of the interstices of the mesh material, adding even more mechanical durability to the resultant cell.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. For example, while several of the embodiments of this invention have been discussed in terms of a fuel cell stack arrangement, this is not a limitation of the invention, in fact this invention is contemplated for use within other fuel cell arrangements including, for example, tubular fuel cell bundles or arrangements. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
This application is related to co-pending US Patent Application, Docket Number 145463-1, Ser. No. ______, entitled “Electrochemical Cell Structures And Methods Of Making The Same,” and co-pending US Patent Application, Docket Number 163855-1, Ser. No. ______, entitled “Solid Oxide Fuel Cell Structures, and Related Compositions and Processes,” each filed contemporaneously herewith, which applications are hereby incorporated by reference.