| PRECURSOR INFILTRATION AND COATING METHOD STATEMENTREGARDINGFEDERALLY SPONSORED RESEARCH ORDEVELOPMENT
This invention was made with government support under Grant (Contract) No. DE- AC02-05CH11231 awarded by The United States Department of Energy. The government has certain rights to this invention. BACKGROUND OF THE INVENTION
Field of the Invention
The present - invention pertains generally to the field of solid state electrochemical devices. This invention relates to coatings on the surfaces of porous structures suitable for use in such devices to form composites. Such composites have applications for electrochemical systems such as fuel cells and oxygen generators, catalysts for hydrocarbon reforming and many other reactions, protective coatings for metals, ceramics, or polymers, and applications where an electronically conductive and/or an ionically conductive or an insulating layer is needed. Description of Related Art
Solid state electrochemical devices are often implemented as cells including two porous electrodes, the anode and the cathode, and a dense solid electrolyte and/or membrane which separates the electrodes. For the purposes of this application, unless otherwise explicit or clear from the context in which it is used, the term "electrolyte" should be understood to include solid oxide membranes used in electrochemical devices, whether or not potential is applied or developed across them during operation of the device, hi many implementations, such as in fuel cells and oxygen and syn gas generators, the solid membrane is an electrolyte composed of a material capable of conducting ionic species, such as oxygen ions, or hydrogen ions, yet has a low electronic conductivity. In other implementations, such as gas separation devices, the solid membrane is composed of a mixed ionic electronic conducting material ("MIEC"). In each case, the electrolyte/membrane must be dense and pinhole free ("gas-tight") to prevent mixing of the electrochemical reactants. hi all of these devices a lower total internal resistance of the cell improves performance. The ceramic materials used in conventional solid state electrochemical device implementations can be expensive to manufacture, difficult to maintain (due to their brittleness) and have inherently high electrical resistance. The resistance may be reduced by operating the devices at high temperatures, typically in excess of 900°C. However, such high temperature operation has significant drawbacks with regard to the device maintenance and the materials available for incorporation into a device, particularly in the oxidizing environment of an oxygen electrode, for example.
The preparation and operation of solid state electrochemical cells is well known. For example, a typical solid oxide fuel cell (SOFC) is composed of a dense electrolyte membrane of a ceramic oxygen ion conductor, a porous anode layer of a ceramic, a metal or, most commonly, a ceramic-metal composite ("cermet"), in contact with the electrolyte membrane on the fuel side of the cell, and a porous cathode layer of a mixed ionically/electronically-conductive (MIEC) metal oxide on the oxidant side of the cell. Electricity is generated through the electrochemical reaction between a fuel (typically hydrogen produced from reformed methane) and an oxidant (typically air). This net electrochemical reaction involves charge transfer steps that occur at the interface between the ionically-conductive electrolyte membrane, the electronically-conductive electrode and the vapor phase (fuel or oxygen). The contributions of charge transfer step, mass transfer (gas diffusion in porous electrode), and ohmic losses due to electronic and ionic current flow to the total internal resistance of a solid oxide fuel cell device can be significant.
Previous work in the field has seen the development of a technique for fabrication of such solid state electrochemical device fabrication that involves the formation of a composite, or mixed, electrode, typically the cathode in a SOFC for example. A mixed cathode comprises ionically and electronically conductive components. It has been found to be advantageous to infiltrate a porous structure formed from the ionically conductive component with a suspension of solution of a precursor for the electronically conductive component in the formation of the mixed electrode.
However, conventional infiltration does not result in a connected network of the electronically conductive component after a single infiltration, and so typically several infiltration and heat cycles are required to form a connected network. Prior infiltration techniques may also yield a low-purity electronically conductive component. Also, some conventional sintered electrodes require high temperatures, well matched thermal expansion coefficients, and chemical compatibility. The high firing temperature of conventional electrodes (greater than 10000C) results in relatively large particle size, lower surface area and therefore lower area for electrochemical reactions to take place. The high firing temperatures also limit the choice of materials.
At present, most solid oxide fuel cells (SOFCs) use 8 mol% yttria stabilized zirconia (YSZ) as the electrolyte, Ni-YSZ as the supporting anode, and La1-xSrxMnθ3. s (LSM)-YSZ as the cathode. The cells are typically operated at or above 800 C to achieve high specific power densities. Lowering cell-operation temperatures expands the materials choices, potentially suppressing degradation of SOFC components, and extending cell lifetimes. The lower temperatures do, however, require measures to minimize ohmic losses and to enhance oxygen reduction reaction catalysis. Thin-film electrolytes as well as alternative electrolytes with higher oxide-ion conductivity than that of YSZ have been extensively explored and have effectively reduced electrolyte ohmic losses.
At low temperatures the typical composite LSM-YSZ cathode becomes a major factor limiting cell performance because the catalytic activity of the cathode for oxygen reduction decreases dramatically. Various models have been proposed to develop a relationship between cathode performance, e.g., characterized by an effective charge transfer resistance, and its structure and catalytic properties. After some structural assumptions and simplifications, an effective charge transfer resistance, RJef , was derived by Virkar et al. (C. Tanner, K. Fung, and A. Virkar, J. 
which Rct is the intrinsic averaged charge transfer resistance; L is the periodicity of the structural model, and could be taken to be the electrode pore spacing, and P is the electrode porosity; and σQΪ^ is the ionic conductivity of electrolyte phase. In this model, the catalyst is assumed to form a thin, uniform layer on the pore walls of the electrode's YSZ network, which does not quite correspond to the usual structure of an YSZ-LSM composite cathode. Further, the oxygen ion conductivity, σ 2. , of the
YSZ in composite electrodes is affected by other structural factors, such as the network connectivity that is in turn affected in the co-firing process by the presence of the LSM. An advantageous approach would therefore be first to form a well-connected oxygen ion-conducting network that can later be infiltrated with electrocatalysts well below the usual co-firing temperatures.
Catalyst infiltration is common practice for polymer membrane fuel cell electrodes, and has recently been introduced for SOFC electrodes. This method expands the set of viable electrode materials combinations, because of the elimination of thermal expansion mismatch and the suppression of possible deleterious reactions among the electrode materials if sintered at the high temperatures required for co-firing. Materials such as LSM provide not only catalytic sites for the oxygen reduction reaction, but also have high electronic conductivity. The latter requires, of course, a continuous LSM structure, and previously multiple infiltrations were necessary to infuse enough electrocatalysts in the electrodes for sufficient electron conduction (see, e.g., Y. Huang, J.M. Vohs, RJ. Gorte, J. Elechtrochem. Soc, 151 (4), A646 (2004), US 5,543,239 and US 2005/0238796). Such multiple processing steps have hindered the practical application of infiltration approaches.
Accordingly, improved techniques for forming mixed electrodes for solid state electrochemical devices, and the resulting structures and devices are needed. In particular, an effective single-step infiltration technique to prepare high quality LSM-YSZ composite cathodes and other composite structures would be desirable. These techniques could also be applicable in other contexts to improve other devices and procedures. SUMMARY OF THE INVENTION
The present invention provides a method of forming a composite (e.g., a mixed electrode) by infiltration of a porous structure (e.g., one formed from an ionically conductive material) with a solution of a precursor (e.g., for an electronically conductive material) that results in a particulate layer on and within the porous structure with a single infiltration. The method involves forming a solution comprising at least one metal salt and a surfactant; heating the solution to substantially evaporate solvent and form a concentrated salt and surfactant solution (e.g., to between about 70 and 130°C); infiltrating the concentrated solution into a porous structure to create a composite; and heating the composite to substantially decompose the salt and surfactant to oxide and/or metal particles (e.g., to greater than 500°C, but below 10000C5 for example 800°C). The result is a particulate layer on the pore walls of the porous structure. In a preferred embodiment, the particulate layer is a continuous network.
This invention eliminates many of the deleterious elements of a mixed electrode consisting of a mixture of predominately electronically conductive catalytic particles and ionically conducting particles. It allows for lower electrode material sintering temperatures and therefore a larger possible material set. In addition the fine scale of the coating allows for the use of materials with thermal expansion coefficients that are not well matched. Separating the firing step of the porous ionic conducting framework (the porous electrolyte structure into which the electronically conductive catalyst precursor is infiltrated) also allows for optimizing the properties of the porous ionic network (for example, firing YSZ at higher temperatures results in improved ionic conductivity through the porous network). An additional advantage is that a very low volume percent (or weight percent) of an electronically conductive material is required to obtain an electronically connected network within a porous structure. This allows for the infiltration of complex compositions into porous structures that results in a continuous network after conversion of the precursor to an oxide, metal, mixture of oxides, or mixtures of metals and oxides.
While a single infiltration step resulting in a continuous network within a porous structure is beneficial to reducing the processing cost, the invention is not limited to only a single infiltration and include the possibility of multiple infiltrations wherein each infiltration is of a continuous network.
The invention also enables novel structures to be fabricated. For example, FeCrAlY alloys are well known in the art for their resistance to oxidation at high temperatures, however the high electronic resistance of the Al2O3 scaled formed during oxidation prevents their application as electronically conductive portions of electrochemical devices such as solid oxide fuel cells. The infiltration of a continuous electronically conductive networks allows a porous support structure to be fabricated from the FeCrAlY or FeAl or Fe3Al or Ni3Al or similar Al2O3 forming alloy. A porous ionic conducting layer in contact with a dense ionically conducting layer can be applied to this porous Al2O3 forming alloy and the continuous electronically conducting layer, such as Cu or Co or Ni with or without doped ceria, or LSM can then be infiltrated. These and other aspects ands advantages of the present invention are more fully described and exemplified in the detailed description below with reference to the figures. BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows a schematic of a process in accordance with the present invention resulting in a continuous network of LSM inside a YSZ pore.
Fig. 2 shows a SEM micrograph of a continuous LSM network within a porous YSZ framework in contact with a dense YSZ electrolyte (SOFC cathode structure) formed in accordance with the infiltration technique of the present invention.
Fig. 3 shows XRD patterns of the decomposition products from LSM precursors without (a) and with the surfactant (Triton X-IOO) (b) processed in accordance with the infiltration technique of the present invention.
Fig. 4 is a plot of voltage and power vs. current density at 923K for a cell with an infiltrated LSM-YSZ cathode in accordance with the present invention.
Fig. 5 shows plots of impedance spectra at 923K for a cell with a non-infiltrated cathode (a) and with the infiltrated LSM-YSZ cathode in accordance with the present invention (b).
Fig. 6 shows a schematic cross-sectional view through support and electrode in contact with dense electrolyte layer for an alternative embodiment using the infiltration technique of the invention.
Fig. 7 is a plot of voltage and power vs. current density at 973K for a cell with an infiltrated LSF cathode in accordance with the present invention.
Fig. 8 shows plots of impedance spectra at 923K for a cell with a LSF infiltrated cathode (a) and with the infiltrated LSF infiltrated with additional Co in accordance with the present invention (b).
Fig. 9 is a plot of voltage and power vs. current density at 973K for a cell with an infiltrated Ag cathode in accordance with the present invention.
Fig. 10 is a plot of voltage and power vs. current density at 923K for a cell with infiltrated LSM, Ag, and LSM-Ag cathodes in accordance with the present invention. DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS Introduction
As noted above, infiltration of precursors into porous structures is well known in the art. However, repeated infiltration and firing steps have been needed to create an interconnected network of the infiltrated material. What is needed is a method of forming a high quality continuous network of fine particles on the pore walls of a porous structure in a single step.
The present invention provides a method of forming a composite, such as a mixed electrode for an electrochemical device, by infiltration of a porous structure with a solution of a precursor that results in a particulate layer on the walls of the porous structure with a single infiltration. The method involves forming a solution comprising at least one metal salt and a surfactant; heating the solution to substantially evaporate solvent (e.g., the temperature of the solution is raised near or above the solvent (e.g., water) boiling point to remove as much solvent as possible) and form a concentrated salt and surfactant solution; infiltrating the concentrated solution into a porous structure to create a composite; and heating the composite to substantially decompose the salt and surfactant to oxide and/or metal particles. The result is a particulate layer on the pore walls of the porous structure. In a preferred embodiment, the particulate layer is a continuous network.
This combination of heat, surfactant, and concentrated salt solution provides improved results in terms of single step coating coverage that was not previously attainable. This technique also produces a pure (single phase) coating material that provides superior performance, hi a preferred implementation, the porous structure is an ionically conductive material (e.g., YSZ) that is infiltrated with a solution of a precursor for an electronically conductive material with a single infiltration, hi other embodiments, the porous substrate can be a mixed ionic-electronic conductor MIEC (e.g., a composite LSM/YSZ substrate) or an electronic conductor (e.g., a porous metal), such as detailed in the Examples below.
Infiltration Method and Structures
An important aspect of the present invention is the particular way in which a surfactant is combined with one or more metal salts prior to infiltration of the porous structure. Surfactants are known to improve the wetability of solutions infiltrated into porous structures. It has now been found that by heating an infiltrate solution containing a metal salts(s) and surfactant near to or above the boiling point of the solution's solvent to remove most or all of the solvent prior to infiltration has beneficial results. Typically a solution of infiltrate is formed from metal salt(s), a solvent (typically water or an alcohol) and a surfactant. Substantial removal of the solvent prior to infiltration has been found to improve the infiltration such that coverage resulting in the formation of a continuous network of the infiltrated material after firing of the composite can be achieved with a single infiltration step. In addition, the quality of the resulting continuous network has been found to be high; in particular, single phase (phase pure) perovskite has been found to result from this process when LSM forming metal salts are infiltrated in this way. These results have been obtained for a variety of substrate and infiltrate materials including ionically, MIEC and electronically conducting porous substrates; and infiltrate solutions formed from a single or multiple metal salts and a variety of surfactants. The scope of the invention encompasses all these instances, as well as others.
A process flow noting relevant aspects of an infiltration method in accordance with the present invention is:
Step 1 : Provide a porous structure.
Step 2: Create a concentrated precursor solution by heating a mixture of metal salt(s) with a surfactant, such as Triton X-IOO (Union Carbide Chemicals and Plastics Co., Inc.), or other appropriate surfactant, to remove solvent (e.g., water) from the solution.
Step 3: Infiltrate the concentrated precursor solution into the porous structure, preferably by vacuum infiltration.
Step 4: Convert the precursor to a coating by decomposing the precursors by heating above 5000C (e.g., about 500-8000C, such as about 8000C) in air or by reducing the precursor to a metal by heating above 200°C in a reducing atmosphere (e.g., H2).
The result is a particulate layer, that is preferably a continuous network in many embodiments, on the pore walls of the porous structure.
Step 2 above should occur at a temperature above the melting point of the surfactant and at least some of the metal salt(s) and near (e.g., slightly above) the boiling point of the solvent, but preferably below the boiling point of the liquid metal salts so that the metal salts are not decomposed prior to infiltration. The melting points (MP) and boiling points (BP) of several typical materials used in accordance with the present invention are shown below:
.230C MP Triton X-IOO
•37°C MP Mn(NO3)2
.400C MP La(NOs)2
•100°C BP H2O
•126°C BP Nitrates (stop before boiling point of nitrates, to infiltrate)
•270°C BP Triton X-IOO
«570°C MP Sr(NO3)2 (Use H2O to dissolve)
Suitable heating temperatures for step 2 are typically in the 70 to 130°C range, depending upon the solvent and salts used.
Triton X-IOO (octylphenol ethoxylate) is a nonionic surfactant noted above as suitable for use in accordance with the present invention. Any suitable surfactant may be used in accordance with the present invention including nonionic, anionic, cationic, and polymeric surfactants. Other examples include polymethylmetacrylic ammonium salt (PMMA) (e.g., Darvan C, R.T. Vanderbilt Co.) and polyethylene glycol.
While the invention is not limited by any particular theory of operation, it is believed that lowering the surface tension of the solution and/or foaming of the surfactant in the infiltrated metal salt solution during decomposition of the heated metal salts plays a role in the superior performance of the method of the present invention. The foaming is believed to arise from outgassing from the metal salts during their decomposition. The precursor preferentially wets and adheres to the surfaces of the porous material during the outgassing resulting in a coating.
The invention will now be described in further detail with reference to specific embodiments in which mixed cathodes are fabricated for a solid oxide fuel cell. It should be understood, however, that the invention is applicable more generally to the infiltration of porous substrates in conjunction with the fabrication of other electrochemical devices and devices and structures of other types.
Referring to Fig. 1, a schematic of a process in accordance with the present invention resulting in a continuous network of LSM (electronically conductive material) inside a YSZ (ionically conductive material) pore. With reference to the process flow above, steps 3 (infiltration) and 4 (reaction) and the final product are shown. The porous structure of step 1 is composed of YSZ; typically a porous coating of YSZ on a dense layer of YSZ electrolyte. The concentrated precursor solution of step 2 is a LSM (La85Sr 15MnO3) (electronically conductive material) precursor solution that can be prepared by adding lanthanum nitrate, strontium nitrate, manganese nitrate hydrate, Triton X-IOO and enough water to dissolve the nitrates. The solution is then heated (e.g., to about 11O0C or 1200C) to evaporate most or all of the water in the solution (both the water added to the solution and that held by the nitrates).
Referring to the figure, in the first image the hot solution (e.g., about 1000C) is then infiltrated in the YSZ pores. This can be accomplished by drop wise addition to the porous YSZ layer followed by vacuum impregnation. In the second image, after infiltration the porous structure is fired at a relatively low temperature (e.g., 8000C) to react the precursors in the solution to form the continuous network of LSM in the YSZ pores shown in the final image.
Fig. 2 shows a SEM micrograph of a continuous LSM network within a porous YSZ framework in contact with a dense YSZ electrolyte (SOFC cathode structure) formed in accordance with the infiltration technique of the present invention described above. The cathode is composed of YSZ grains, pores, and infiltrated LSM particles with a size of about 30 -100 run. The LSM particles appear preferentially to coat the pore walls of the YSZ network, forming in may instances a fairly densely packed, single layer of nanosized LSM particles, as shown in the inset. The LSM particles are generally in intimate connect with each other, allowing for sufficient electronic connectivity. The layer of the nanoparticles is interesting, since with sufficient ionic conductivity the entire surface of the particles can participate in catalysis. These morphologies can be far more effective than those in some conventional cathodes where at about 50-50 wt% of the LSM and YSZ form large-scale interpenetrating structures. In contrast, the infiltrated LSM produced here is only about 6 wt% of the YSZ network.
Fig. 3 shows XRD patterns of the decomposition products from LSM precursors without (a) and with the surfactant (Triton X-100) (b) processed in accordance with the present invention described above. Post infiltration heating was in air at 1073K for 1 hour. (P) Peaks corresponding to perovskite phase. As indicated in (a), directly decomposing nitrate precursors at 1073K does not yield a phase-pure LSM perovskite. In contrast, with the use of the concentrated precursor solution containing surfactant, the majority of characteristic peaks in (b) correspond to the perovskite phase.
The performance of LSM-YSZ mixed cathodes fabricated in accordance with the present invention as described herein was measured. Results are shown in Fig. 4 (I-V curves) and Fig. 5 (impedance plots including a spectrum corresponding to a non-infiltrated cell). Fig. 4 is a plot of voltage and power vs. current density at 923K for a cell with an infiltrated LSM-YSZ cathode in accordance with the present invention. The LSM-YSZ cathode displays a promising performance at 923K; cell open circuit voltage is about 1.1 V, and maximum power density is about 0.27W/cm2. Fig. 5 shows plots of impedance spectra at 923K for a cell with a non-infiltrated cathode (a) and with the infiltrated LSM-YSZ cathode (b). The impedance for the non-infiltrated cell at near-OCV. The cell ohmic resistance (Rr), determined from the high-frequency intercept on the real axis, combines the ohmic loss from the cell anode, electrolyte, and cathode. The infiltrated cell has an Rr of ~0.3Ω *cm2, while the Rr for the non-infiltrated cell is ~3.4Ω *cm2. Since both cells have similar anodes, electrolytes and porous YSZ networks, this significant difference in the R/s implies that the infiltrated LSM particles in the porous YSZ network impart sufficient electronic conductivity to the resulting LSM-YSZ cathode. In addition, the polarization resistance for the infiltrated cell is ~2.9Ω *cm2, strikingly smaller than the ~1100 *cm2 for the non-infiltrated cell. Therefore, it is the infiltrated LSM, not the Pt electrode paste that provides sufficient active reaction sites for electrochemical reduction of oxygen.
While a single infiltration step resulting in a continuous network within a porous structure is beneficial to reducing the processing cost, the invention is not limited to only a single infiltration and include the possibility of multiple infiltrations wherein each infiltration is of a continuous network.
The invention also enables novel structures to be fabricated. For example, FeCrAlY alloys are well known in the art for their resistance to oxidation at high temperatures, however the high electronic resistance of the Al2O3 scaled formed during oxidation prevents their application as electronically conductive portions of electrochemical devices such as solid oxide fuel cells. The infiltration of a continuous electronically conductive networks allows a porous support structure to be fabricated from the FeCrAlY or FeAl or Fe3Al or Ni3Al or similar Al2O3 forming alloy. A porous ionic conducting layer in contact with a dense ionically conducting layer can be applied to this porous Al2O3 forming alloy and the continuous electronically conducting layer, such as Cu or Co or Ni with or without doped ceria, or LSM can then be infiltrated.
Fig. 6 illustrates such an alternative embodiment using the infiltration technique of the invention. A schematic cross-sectional view through support and electrode in contact with dense electrolyte layer is shown. Infiltration in accordance with the invention forms a continuous electronically conductive network. In this drawing the support is an electronically insulating material such as oxidized FeCrAlY, though an electronically conductive material could also be used.
Alternatively, superior electrocatalysts such as lanthanum strontium cobalt oxide (LSC) could be infiltrated into a porous YSZ or CGO network to form high-performance cathodes for intermediate temperature SOFCs.
Advantages
This invention eliminates many of the deleterious elements of a mixed electrode consisting of a mixture of predominately electronically conductive catalytic particles and ionically conducting particles. It allows for lower electrode material sintering temperatures and therefore a larger possible material set. In addition the fine scale of the coating allows for the use of materials with thermal expansion coefficients that are not well matched. Separating the firing step of the porous ionic conducting framework (the porous electrolyte structure into which the electronically conductive catalyst precursor is infiltrated) also allows for optimizing the properties of the porous ionic network (for example, firing YSZ at higher temperatures results in improved ionic conductivity through the porous network). An additional advantage is that only a very low volume percent (or weight percent) of an electronically conductive material is required to obtain an electronically connected network within a porous structure. This allows for the infiltration of complex compositions into porous structures in a single step that results in a continuous network after conversion of the precursor to an oxide, metal, mixture of oxides, or mixtures of metals and oxides. Finally, the technique of the invention has been found to produce a high quality continuous network of single phase perovskite on a porous substrate.
Examples
The following examples provide details relating to the practice and advantages of an infiltration method in accordance with the present invention. It should be understood the following is representative only, and that the invention is not limited by the detail set forth in these examples.
EXAMPLE 1 - Fabrication of anode supported SOFC with LSM infiltration
The anode portion of an anode/electrolyte/cathode structure was formed by tape casting a mixture of NiO(50%)/YSZ(50 wt%). The mixture of NiO/YSZ was prepared by ball milling 12.5 g of NiO (Nickelous Oxide, Green (available from Mallinckrodt Baker, Phillipsburg, NJ), 12.5 g of YSZ (Tosoh TZ-8Y (available from Tosoh Ceramics, Boundbrook, NJ) and 1 mL of Duramax D-3005 (available from Rohm and Haas, Philadelphia, PA) in 16 mL of water for 1 day. Afterwards 6 mL of Duramax B- 1000 and 4 mL of Duramax HA- 12 (both available from Rohm and Haas, Philadelphia, PA) are added and all excess water was evaporated while the solution was stirred in an air environment. The solution was then tape casted and allowed to dry overnight. The resulting green tape was cut into 1.5 inch diameter Disks. The disk was fired to burn out the binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 6000C at 1°C per min., hold for 1 hour, 6000C to 11000C at 3°C per min., hold for 4 hours, cool HOO0C to RT at 5°C per min.
After cooling, a thin coating of YSZ (the ionically-conductive electrolyte material) was applied to the NiO/YSZ disk by uniformly spraying an YSZ suspension by an aerosol spray method. The suspension was prepared by attritor milling 2 g of YSZ, 0.1 g of fish oil (fish oil from Menhaden (available from Sigma- Aldrich, St. Louis, MO) and O.Olg dibutyl phthalate (available from Mallinckrodt Baker) in 50 mL of Isopropyl Alcohol (IPA), for 1 hour. The suspension was sprayed while the NiO/YSZ disk was held at 1500C (0.037 g of final dried YSZ was deposited, typically yielding a sintered YSZ electrolyte membrane about LO μm thick). The disk was fired to burn our binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 6000C at 3°C per min., 6000C to 14000C at 50C per min., hold for 4 hours, cool 14000C to RT at 50C per min.
After cooling, a suspension of YSZ (35 vol%, ion-conductive material), and graphite (65 vol%, fugitive pore-forming material) was uniformly sprayed, by aerosol spray method, to a 1 cm2 area on the electrolyte surface. The suspension was prepared by attritor milling 1.28 g YSZ (Tosoh TZ-8Y), 0.1 g fish oil (fish oil from Menhaden (Sigma- Aldrich) and 0.01 g dibutyl phthalate in 50 mL of IPA, for 1 hour. Afterwards 0.72 g of graphite (KS4 (available from Timcal Group, Quebec, Canada) was added and sonicated for 5 min. The electrolyte surface has been covered to only reveal a 1 cm2 area which was then uniformly sprayed with the suspension, while being held at 150°C (0.007 g of final dried YSZ/graphite was deposited, typically yielding a sintered porous YSZ membrane about 10 μm thick). The disk was fired to burn our fugitive pore formers and binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 600°C at 3° C. per min., 600°C to 13000C at 50C per min., hold for 4 hours, cool 1300°C to RT at 5°C per min.
After cooling, the porous YSZ layer was infiltrated with an LSM (La185Sr 15MnO3) (electronically conductive material) precursor solution. The solution was prepared by adding 3.144 g La(NO3)3«6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 0.271 g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 2.452g Mn(NO3)2»6H2O (Manganese (II) nitrate hydrate, 98% (available from Sigma-Aldrich) and 0.3 g Triton X-100 (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates). The solution was then heated to 1200C to evaporate the water in the solution (both the water added to the solution and that held by the nitrates). When the solutions internal temperature begins to rise above 100°C all of the water has been evaporated. The hot solution (about 100°C) was then added drop wise to the porous YSZ layer (the remaining electrolyte surface has again been covered to limit the infiltration area to 1 cm2) and vacuum impregnated. After drying at 12O0C for 30 min. the disk was fired according to the following schedule: heat room temperature (RT) to 800°C at 3°C per min., hold for 1 hour, cool 800°C to RT at 5°C per min.
After cooling, all excess LSM was removed from the cathode surface and a thin layer of platinum paste (available from Heraeus, Inc.) was applied to the anode face and to the 1 cm2 cathode face. The platinum paste was dried under heat lamp for 30 min. Afterwards, platinum mesh was attached to the anode and cathode faces with platinum paste, to serve as a current collector. The cell assembly was then fired according to the following schedule: heat room temperature (RT) to 800°C at 30C per min., hold for 1 hour, cool 800°C to RT at 50C per min.
The single cells were sealed onto an alumina tube using Aremco-552 cement, and current-voltage characteristics were obtained, using 97%H2+3% H2O as the fuel and air as the oxidant. The cell performance was determined from 600-800°C with a Solartron 1255 frequency response analyzer combined with a Solartron 1286 electrochemical interface. The impedance spectra were measured under near-open circuit conditions (OCV), using a 1OmV amplitude AC signal over a frequency range of 0.1Hz to 1 MHz. The DC current- voltage (I- V) performance was recorded with a potentiostat-galvanostat (Princeton Applied Research Model 371). After the electrochemical characterization the cells were fractured and the microstructures were examined with a JEOL 6400 scanning electron microscope (SEM). In addition, the phase formation was examined using a diffractometer (Siemens D-500) with CuK0, radiation in the 2Θ range from 20° to 80°. Results are illustrated in Figs. 3, 4 and 5, discussed above.
EXAMPLE 2 - Fabrication of thick electrolyte infiltration of anode and cathode
An anode/electrolyte/cathode structure was prepared on an electrolyte-supported cell, which was formed by pressing a 1 ' inch diameter disk from 0.9g of YSZ. The YSZ was prepared by attritor milling 25 g of YSZ (Tosoh TZ8Y) and 0.625g each of fish oil (Sigma-Aldrich), dibutyl phthalate (Mallinckrodt Baker) and poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (available from Sigma-Aldrich) with 100 mL of (IPA), for 1 hour. The mixture was dried and then ground and sieved through a 100 mesh. The disk was fired to burn out the binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 600°C at 30C per min., 600°C to 1400°C at 50C per min., hold for 4 hours, cool 1400° C. to RT at 5°C per min.
After cooling, a suspension of YSZ (35 vol%, ion-conductive material), and graphite (65 vol%, fugitive pore-forming material) was uniformly sprayed, by aerosol spray method, to a 1 cm2 area on both sides of the electrolyte surface. The suspension was prepared by attritor milling 1.28 g YSZ (Tosoh TZ-8Y), 0.1 g fish oil (fish oil from Menhaden (Sigma-Aldrich) and 0.01 g dibutyl phthalate (Mallinckrodt Baker) in 50 mL of IPA, for 1 hour. Afterwards 0.72 g of graphite (KS4 (available from Timcal Group, Quebec, Canada) was added and sonicated for 5 min. The electrolytes surfaces have been covered to only reveal 1 cm2 areas which are then uniformly sprayed with the suspension, while being held at 150°C (0.007 g of final dried YSZ/graphite was deposited, typically yielding a sintered porous YSZ membrane about 10 μm thick). The disk was fired to burn our fugitive pore formers and binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 6000C at 3°C per min., 6000C to 13000C at 5°C per min., hold for 4 hours, cool 13000C to RT at 5°C per min.
After cooling, one porous YSZ layer was infiltrated with an LSM (La85Sr !5MnO3) (electronically conductive material) precursor solution. The solution was prepared by adding 3.144 g La(NO3)3»6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 0.271 g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 2.452g Mn(NO3)2*6H2O (Manganese (II) nitrate hydrate, 98% (available from Sigma-Aldrich) and 0.3 g Triton X-100 (available from VWR, West Chester, PA) in 10 niL of water (enough to dissolve the nitrates). The solution was then heated to 1200C to evaporate the water in the solution (both the water added to the solution and that held by the nitrates). When the solutions internal temperature begins to rise above 1000C all of the water has been evaporated. The hot solution (about 1000C) was then added drop wise to the porous YSZ layer (the remaining electrolyte surface has again been covered to limit the infiltration area to 1 cm2) and vacuum impregnated. The disk was then dried at 1200C for 30 min. The other porous YSZ layer was then infiltrated with NiO/CeO2 (50-50 wt%)(anode material) precursor material. The solution was prepared by adding 2.520 g Ni(NO3)2*6H2O (Nickel (II) nitrate; Reagent (available from Johnson Matthey Catalog Company, London, England), 1.214 g Ce(NO3)3*6H2O (Cerium (III) nitrate, hexahydrate 99% (available from Sigma-Aldrich) and 0.3 g Triton X-100 (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates). The solution was then infiltrated in the same method as LSM was on the opposite electrode. After drying the disk was fired according to the following schedule: heat room temperature (RT) to 8000C at 30C per min., hold for 1 hour, cool 8000C to RT at 5°C per min.
After cooling, all excess LSM and NiOZCeO2 was removed from the electrode surfaces and a thin layer of platinum paste (available from Heraeus, Inc.) was applied to both of the 1 cm2 electrode faces. The platinum paste was dried under heat lamp for 30 min. Afterwards, platinum mesh was attached to the anode and cathode faces with platinum paste, to serve as a current collector. The cell assembly was then fired according to the following schedule: heat room temperature (RT) to 8000C at 30C per min., hold for 1 hour, cool 8000C to RT at 50C per min. EXAMPLE 3 - Porous disk
A porous structure was formed by pressing a 0.5 inch diameter disk from 0.3 g of a mixture of YSZ (35 vol%, ion-conductive material), and graphite (65 vol%, fugitive pore-forming material). The mixture of YSZ/graphite was prepared by attritor milling 10 g YSZ (Tosoh TZ-8Y), with 0.36 g each of fish oil (fish oil from Menhaden (Sigma-Aldrich), dibutyl phthalate (Mallinckrodt Baker) and polyvinyl butyral-co-vinyl alcohol-co-vinyl acetate) (Sigma-Aldrich) in 100 mL of IPA, for 1 hour. Afterwards 5.67 g of graphite (KS4 (Timcal Group) was added and sonicated for 5 min. The mixture was dried and then ground and sieved through a 100 mesh. The disk was fired to burn out the binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 600° C. at 3° C. per min., 6000C to 1250°C at 5°C per min., hold for 4 hours, cool 1250°C to RT at 50C per min.
A series of such porous structures were made and each one was infiltrated with a different catalyst precursor material including the following:
A LSM solution was prepared by adding 3.144 g La(NO3)3«6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 0.271 g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 2.452g Mn(NO3)2«6H2O (Manganese (II) nitrate hydrate, 98% (available from Sigma-Aldrich) and 0.3 g Triton X-100 (available from VVTR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
A SSC solution was prepared by adding 2291% Sm(NO3)3»6H2O (Samarium (III) nitrate hexahydrate, 99.9% (available from Aldrich), 0.729g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 2.507g Co(NO3)2«6H2O (Cobalt (II) nitrate, ACS, 89% (from Alfa Aesar) and 0.3 g Triton X-100 (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
A LSCF (La.60Sr.4QCo.2oFe.8θ03-δ) solution was prepared by adding 2.332 g La(NO3)3»6H2O (Lanthanum (HI) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 0.797 g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 0.522 Co(NO3)2«6H2O (Cobalt (II) nitrate, ACS, 89% (from Alfa Aesar), 2.90Og Fe(NO3)3«9H2O (Iron (III) nitrate nonahydrate 98+% A.C.S reagent (available from Aldrich) and 0.3 g Triton X-100 (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates). A LaCr9Mg11O3 solution was prepared by adding 3.667g La(NO3)3«6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 3.05Og Cr(NO3)3«9H2O (Chromium (III) nitrate nonahydrate, 99% (available from Aldrich), 0.217g Mg(NO3)2«6H2O (Magneseium nitrate hexahydrate 99% A.C.S reagent available from Aldrich) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
MnCo2O4: 2.425 Mn(NO3)2«6H2O (Manganese (II) nitrate hydrate, 98% (available from Sigma- Aldrich), 4.917g Co(NO3)2«6H2O (Cobalt (II) nitrate, ACS, 89% (from Alfa Aesar) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
NiO-CeO2 (50-50 volume%): 2.520 Ni(NO3)2«6H2O (Nickel (II) nitrate, reagent (available from Johnson Matthey Catalog Company) 1.214g Ce(NO3)3«6H2O (Cerium (III) nitrate hexahydrate, REacton 99.5% (REO) (available from Alfa Aesar) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
Ce 8Gd2O3: 3.627g Ce(NO3)3«6H2O (Cerium (III) nitrate hexahydrate, REacton 99.5% (REO) (available from Alfa Aesar), 0.943g Gd(NO3)3»XH2O (X «6) (Gadolinium (III) nitrate hydrate 99.9% (REO) (available from Alfa Aesar) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
Each of the solutions was then heated to 100°C to evaporate most of the water in the solution (both the water added to the solution and that held by the nitrates). When the solution's internal temperature begins to rise above 100°C most of the water has been evaporated. The hot solution (about 100°C) was then added drop wise to the porous YSZ (the remaining electrolyte surface has again been covered to limit the infiltration area to 1 cm2) and vacuum impregnated. After drying at 12O0C for 30 min. the disk was fired according to the following schedule: heat room temperature (RT) to 800°C at 3°C per min., hold for 1 hour, cool 800°C to RT at 5°C per min. EXAMPLE 4 - Anode supported SOFC with LSCF cathode
Preparation for cell support up to infiltration was the same as in Example 1.
After cooling, the porous YSZ layer was infiltrated with an LSCF (La.eoSr.40Co.2oFe,8o03-s) (electronically conductive material) precursor solution. The solution was prepared by adding 2.332 g La(NO3)3*6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 0.797 g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 0.522 Co(NO3)2«6H2O (Cobalt (II) nitrate, ACS, 89% (from Alfa Aesar), 2.90Og Fe(NO3)3»9H2O (Iron (III) nitrate nonahydrate 98+% A.C.S reagent (available from Aldrich) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
Processing after infiltration was the same as in Example 1.
EXAMPLE 5 - Porous metal SOFC with YSZ electrolyte and infiltrated LSM cathode and Ni-CeO2 anode
Stainless steel powder (type Fe30Cr from Ametek) was applied to a porous YSZ layer on both sides of a dense YSZ disk then at 1300°C for 4 hrs in flowing 4%H2/balance Ar. LSM and NiO-CeO2 solutions were prepared as in Example 3 and infiltrated into opposite sides of the coated YSZ disk. The Pt leads were attached to the both sides of the cell which was then sealed at the end of an alumina tube as in Example 1. The electrodes were converted to the oxides during heat up to 600°C. The fuel cell was tested between 600-8000C with air as the oxidant and H2+3%H2O as the fuel. After testing the cell was mounted in epoxy, cut and polished. SEM micrographs showed LSM infiltrated the porous YSZ structure as well as coated the porous metal.
EXAMPLE 6 - LSM precursor solution made using a hydroxide
LSM (La85Sr J5MnO3) precursor solution was produced using a mixture of salts. The solution was prepared by adding 3.144 g La(NO3)3«6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar), 0.340 g Sr(OH)2 -6H2O (Strontium hydroxide Tech. Gr. (available from Johnson Matthey Catalogue Corporation, Ward Hill, MA), 2.452g Mn(NO3)2*6H2O (Manganese (II) nitrate hydrate, 98% (available from Sigma-Aldrich) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the salts). The precursor was fired according to the following schedule: heat room temperature (RT) to 800°C at 30C per min., hold for 1 hour, cool 8000C to RT at 5°C per min. An XRD image of the oxidized powder was similar to that produced with only nitrate salts.
EXAMPLE 7 - Infiltration of dual phase cathode LSM/CeO2
A 2 part LSM (La.85Sr.15MnO3) 1 part lanthanum doped ceria (Ce.8La.2O2) precursor solution was prepared by adding 3.324 g La(NO3)3«6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar), 0.367 g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 2.452g Mn(NO3)2»6H2O (Manganese (II) nitrate hydrate, 98% (available from Sigma-Aldrich), 1.483g Ce(nO3)3«6H2O (Cerium (III) nitrate hexahydrate, 99% (available from Aldrich) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates). The precursor was fired according to the following schedule: heat room temperature (RT) to 800° C. at 3° C. per min., hold for 1 hour, cool 800° C. to RT at 5° C. per min. An XRD image of the oxidized powder showed both LSM perovskite peaks (P) as well as doped ceria peaks (D). EXAMPLE 8 - Fabrication of anode supported SOFC with LSM infiltration using alternative surfactant
An LSM (La. S5Sr, 15MnO3) precursor solution was prepared in the same method as Example 1, except Triton x-100 was replaced by Darvan C (polymethylmetacrylic ammonium salt (PMMA), R.T. Vanderbilt Co.) in the same weight ratio. The precursor was fired according to the following schedule: heat room temperature (RT) to 8000C at 3°C per min., hold for 1 hour, cool 800°C to RT at 5°C per min. An XRD image of the oxidized powder was similar to that produced by Triton X-100.
EXAMPLE 9 - Anode supported SOFC with LSF cathode plus additional Co Catalyst Preperation for the cell support up to infiltration was the same as in Example 1.
After cooling, the porous YSZ layer was infiltrated with an LSF (La.80Sr.2oFeO3-δ) (electronically conductive material) precursor solution. The solution was prepared by adding 2.980 g La(NO3)3«6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 0.20 g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 3.48 Fe(NO3)3*9H2O (Iron (III) nitrate nonahydrate 98+% A.C.S reagent (available from Aldrich) and 0.3 g Triton X-100 (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
Processing after infiltration was the same as in Example 1.
A plot of voltage and power vs. current density exemplifying the performance of the above cell at 700°C is shown in Fig. 7.
After testing, the above cell was infiltrated with a Co (catalyst) precursor solution. A 1 molar solution of Co(NO3)2«6H2O (Cobalt (II) nitrate, ACS, 89% (from Alfa Aesar) and (NH2)2CO (Urea (available from Mallinckrodt) in a (1:1 ratio by weight). The solution was then added dropwise to the now LSF infiltrated porous YSZ layer and heated to 9O0C for 2 hours. After, the disk was fired according to the following schedule: heat room temperature (RT) to 8000C at 3°C per min., hold for 0.5 hour, cool 8000C to RT at 50C per min.
Processing after the infiltration was the same as in Example 1.
AC impedance data is plotted in Fig. 8 to exemplify the improvement that secondary infiltration has on the LSF cell. Fig. 8 shows plots of impedance spectra at 923K for the cell with a LSF infiltrated cathode (a) and with the infiltrated LSF infiltrated with additional Co (b).
EXAMPLE 10 - Anode supported SOFC with infiltrated Ag cathode
The anode portion of an anode/electrolyte/cathode structure was formed by uniaxially pressing a mixture of NiO(50%)/SSZ(50 wt%). The mixture of NiO/SSZ was prepared by attritor milling 12.5 g of NiO (Nickelous Oxide, Green (available from Mallinckrodt Baker, Phillipsburg, NJ), 12.5 g of SSZ ((Sc2O3)0.1(ZrO2)0.9, (available from Daiichi Kigenso Kagakukokyo) and 0.625g each of fish oil (Sigma-Aldrich), dibutyl phthalate (Mallinckrodt Baker) and polyvinyl butyral-co-vinyl alcohol-co-vinyl acetate) (available from Sigma- Aldrich) with 100 mL of (IPA), for 1 hour. The mixture was dried and then ground and sieved through a 100 mesh. A VA inch disk was then uniaxially pressed with 15 KPSI of pressure. The disk was fired to burn out the binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 6000C at 3°C per min., 6000C to 11000C at 5°C per min., hold for 1 hours, cool 1100° C. to RT at 5°C per min.
After cooling, a thin coating of SSZ (the ionically-conductive electrolyte material) was applied to the NiO/SSZ disk by uniformly spraying an SSZ suspension by an aerosol spray method. The suspension was prepared by attritor milling 2 g of SSZ, 0.1 g of fish oil (fish oil from Menhaden (available from Sigma- Aldrich, St. Louis, MO) and 0.0 Ig dibutyl phthalate (available from Mallinckrodt Baker) in 50 mL of Isopropyl Alcohol (IPA), for 1 hour. The suspension was sprayed while the NiO/SSZ disk was held at 15O0C (0.037 g of final dried SSZ was deposited, typically yielding a sintered SSZ electrolyte membrane about 10 μm thick). The disk was fired to burn our binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 6000C at 3°C per min., 600°C to 13500C at 5°C per min., hold for 4 hours, cool 135O0C to RT at 5°C per min.
After cooling, a suspension of SSZ (35 vol%, ion-conductive material), and graphite (65 vol%, fugitive pore-forming material) was uniformly sprayed, by aerosol spray method, to a 1 cm2 area on the electrolyte surface. The suspension was prepared by attritor milling 1.28 g SSZ, 0.1 g fish oil (fish oil from Menhaden (Sigma- Aldrich) and 0.01 g dibutyl phthalate in 50 mL of EPA, for 1 hour. Afterwards 0.72 g of graphite (KS6 (available from Timcal Group, Quebec, Canada) was added and sonicated for 5 min. The electrolyte surface was been covered to only reveal a 1 cm area which was then uniformly sprayed with the suspension, while being held at 1500C (0.007 g of final dried SSZ/graphite was deposited, typically yielding a sintered porous SSZ membrane about 10 μm thick). The disk was fired to burn our fugitive pore formers and binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 6000C at 3° C. per min., 6000C to 125O0C at 5°C per min., hold for 4 hours, cool 12500C to RT at 5°C per min.
After cooling, the porous SSZ layer was infiltrated with an Ag (Ag) (electronically conductive material) precursor solution. The solution was prepared by adding 3.148g AgNO3 (Silver nitrate, ACS, 99.9+% (available from Alfa Aesar) and 0.3 g Triton X-100 (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates). The solution was then heated to approximately 1000C to evaporate the water in the solution (both the water added to the solution and that held by the nitrates). When the solutions internal temperature rises to about 1000C most of the water has been evaporated. The hot solution (about 1000C) was then added drop wise to the porous SSZ layer (the remaining electrolyte surface has again been covered to limit the infiltration area to 1 cm2) and vacuum impregnated. After drying at 12O0C for 30 min. the disk was fired according to the following schedule: heat room temperature (RT) to 9000C at 3°C per min., hold for 0.5 hour, cool 9000C to RT at 50C per min.
A plot of voltage and power vs. current density exemplifying the performance of the above cell at 750 0C is shown in Fig. 9.
EXAMPLE 11 - Anode supported SOFC with LSM
Preparation for cell support up to infiltration was the same as in Example 10. After cooling, the porous SSZ layer was infiltrated with an LSM (La85Sr. ^MnO3) (electronically conductive material) precursor solution. The solution was prepared by adding 3.144 g La(NO3)3«6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 0.271 g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 2.452g Mn(NO3)2«6H2O (Manganese (II) nitrate hydrate, 98% (available from Sigma-Aldrich) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
Processing after infiltration was the same as in Example 10.
A plot of voltage and power vs. current density exemplifying the performance of the above cell at 600°C is shown in Fig. 10. EXAMPLE 12 - Anode supported SOFC with composite Ag and LSM cathode
Preparation for cell support up to infiltration was the same as in Example 10. After cooling, the porous SSZ layer was infiltrated with an Ag-LSM (La85Sr 15MnO3) (50-50 volume%) (electronically conductive material) precursor solution. The solution was prepared by adding 1.934g AgNO3 (Silver nitrate, ACS, 99.9+% (available from Alfa Aesar), 1.214 g La(NO3)3«6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 0.105 g Sr(NO3)2 (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 0.946 g Mn(NO3)2«6H2O (Manganese (II) nitrate hydrate, 98% (available from Sigma-Aldrich) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
Processing after infiltration was the same as in Example 10.
Voltage and power vs. current density were plotted to exemplify the performance of the LSM cell from Example 11, the Ag cell from Example 10, and the LSM-Ag cell in this example at 600 0C. These are all shown in Fig. 10.
EXAMPLE 13 - Anode supported SOFC with LSM-YSZ sintered cathode infiltrated with LSM
The anode portion of an anode/electrolyte/cathode structure was formed by uniaxially pressing a mixture of NiO(50%)/YSZ(50 wt%). The mixture of NiO/YSZ was prepared by attritor milling 12.5 g of NiO (Nickelous Oxide, Green (available from Mallinckrodt Baker, Phillipsburg, NJ), 12.5 g of YSZ (Tosoh TZ8Y) and 0.625g each of fish oil (Sigma-Aldrich), dibutyl phthalate (Mallinckrodt Baker) and poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (available from Sigma-Aldrich) with 100 niL of (IPA), for 1 hour. The mixture was dried and then ground and sieved through a 100 mesh. A VA inch disk was then uniaxially pressed with 15 KPSI of pressure. The disk was fired to burn out the binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 600°C at 3°C per min., 600°C to HOO0C at 50C per min., hold for 1 hours, cool 1100° C. to RT at 5°C per min.
After cooling, a thin coating of YSZ (the ionically-conductive electrolyte material) was applied to the NiO/YSZ disk by uniformly spraying an YSZ suspension by an aerosol spray method. The suspension was prepared by attritor milling 2 g of YSZ, 0.1 g of fish oil (fish oil from Menhaden (available from Sigma-Aldrich, St. Louis, MO) and 0.0 Ig dibutyl phthalate (available from Mallinckrodt Baker) in 50 mL of Isopropyl Alcohol (IPA), for 1 hour. The suspension was sprayed while the NiO/YSZ disk was held at 150°C (0.037 g of final dried SSZ was deposited, typically yielding a sintered YSZ electrolyte membrane about 10 μm thick). The disk was fired to burn our binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 600°C at 30C per min., 600°C to 1400°C at 5°C per min., hold for 4 hours, cool 1400°C to RT at 5°C per min.
After cooling, a suspension of SSZ ((Sc2O3)0.1(ZrO2)0.9, (available from Daiichi Kigenso Kagakukokyo) and LSM (55 wt%, ion-conductive material), and graphite (45 wt%, fugitive pore-forming material) was uniformly sprayed, by aerosol spray method, to a 1 cm2 area on the electrolyte surface. The suspension was prepared by attritor milling Ig SSZ, Ig LSM, 0.1 g fish oil (fish oil from Menhaden (Sigma-Aldrich) and 0.01 g dibutyl phthalate in 50 mL of IPA, for 1 hour. Afterwards 0.90 g of graphite (KS 6 (available from Timcal Group, Quebec, Canada) was added and sonicated for 5 min. The electrolyte surface has been covered to only reveal a 1 cm2 area which was then uniformly sprayed with the suspension, while being held at 150°C (0.004 g of final dried LSM-S SZ/graphite was deposited, typically yielding a sintered porous LSM-SSZ membrane about 10 μm thick). The disk was fired to burn our fugitive pore formers and binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 600°C at 3° C. per min., 600°C to 1250°C at 5°C per min., hold for 4 hours, cool 125O0C to RT at 5°C per min. After cooling, the porous LSM-SSZ layer was infiltrated with an LSM (La85Sr15MnO3) (electronically conductive material) precursor solution. The solution was prepared by adding 3.144 g La(NO3)3*6H2O (Lanthanum (III) nitrate, 99.9% (REO) (available from Alfa Aesar, Ward Hill, MA), 0.271 g Sr(NO3);, (Strontium Nitrate, ACS, 99.0% min (Assay) (available from Alfa Aesar), 2.452g Mn(NO3)2*6H2O (Manganese (II) nitrate hydrate, 98% (available from Sigma-Aldrich) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
Processing after infiltration was the same as in Example 10.
EXAMPLE 14 - Anode supported SOFC with LSM-SSZ cathode infiltrated with Ag Processing before infiltration was the same as in Example 13.
The porous LSM-SSZ layer was infiltrated with an Ag (electronically conductive material) precursor solution. The solution was prepared by adding 3.148g AgNO3 (Silver nitrate, ACS, 99.9+% (available from Alfa Aesar) and 0.3 g Triton X-100 (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
Processing after infiltration was the same as in Example 10.
EXAMPLE 15 - Anode supported SOFC with LSM-SSZ cathode infiltrated with CGO
Processing before infiltration was the same as in Example 13.
The porous LSM-SSZ layer was infiltrated with Ce.8Gd.2O3 (CGO) precursor solution. The solution was prepared by adding 3.627g Ce(NO3)3«6H2O (Cerium (III) nitrate hexahydrate, REacton 99.5% (REO) (available from Alfa Aesar), 0.943g Gd(NO3)3*XH2O (X*6) (Gadolinium (III) nitrate hydrate 99.9% (REO) (available from Alfa Aesar) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
Processing after infiltration was the same as in Example 10.
EXAMPLE 16 - Ni-YSZ anode infiltrated with CGO
An anode structure was formed by uniaxially pressing a mixture of NiO(50%)/YSZ(50 wt%). The mixture of NiO/YSZ was prepared by attritor milling 12.5 g of NiO (Nickelous Oxide, Green (available from Mallinckrodt Baker, Phillipsburg, NJ), 12.5 g of YSZ (Tosoh TZ8Y) and 0.625g each of fish oil (Sigma- Aldrich), dibutyl phthalate (Mallinckrodt Baker) and poly(vinyl butyral-co-vinyl alcohol-co-vinyl acetate) (available from Sigma- Aldrich) with 100 mL of (IPA), for 1 hour. The mixture was dried and then ground and sieved through a 100 mesh. A VA inch disk was then uniaxially pressed with 15 KPSI of pressure. The disk was fired to burn out the binders and sinter the structure, according to the following schedule: heat room temperature (RT) to 600°C at 3°C per min., 600°C to 14000C at 5°C per min., hold for 1 hours, cool 1400° C. to RT at 50C per min.
The above cell was then infiltrated with Ce.8Gd.2O3 (CGO) precursor solution. The solution was prepared by adding 3.627g Ce(NO3)3«6H2O (Cerium (III) nitrate hexahydrate, REacton 99.5% (REO) (available from Alfa Aesar), 0.943g Gd(NO3)3»XH2O (X =6) (Gadolinium (III) nitrate hydrate 99.9% (REO) (available from Alfa Aesar) and 0.3 g Triton X-IOO (available from VWR, West Chester, PA) in 10 mL of water (enough to dissolve the nitrates).
The cell was then reduced in a hydrogen furnace at 800°C to convert the NiO to Ni.
Conclusion
Although the foregoing invention has been described in some detail for purposes of clarity of understanding, it will be apparent that certain changes and modifications may be practiced within the scope of the appended claims. In particular, while the invention is primarily described with reference to LSM-YSZ composite electrodes for use in solid oxide fuel cells, other material combinations and associated precursors, including those described in the Examples, and others which would be readily apparent to those of skill in the art given the disclosure herein, may be used to form mixed electrodes for SOFCs or other electrochemical devices in accordance with the present invention, hi addition, the infiltration technique of the present invention may find application beyond electrochemical device fabrication. It should be noted that there are many alternative ways of implementing both the process and compositions of the present invention. Accordingly, the present embodiments are to be considered as illustrative and not restrictive, and the invention is not to be limited to the details given herein.
AU references cited herein are incorporated by reference for all purposes. |
