Intermediate layer for solid acid compounds

Abstract

A method for forming an interlayer of solid oxide cell units on a surface of a substrate includes the steps of: providing a base interlayer solution comprising a solution of soluble salt precursors of a metal oxide (crystalline) ceramic and crystalline nanoparticles; depositing the base interlayer solution on the surface of the substrate; drying the base interlayer solution to define a nanocomposite sublayer of the soluble salt precursors and the nanoparticles; heating the sublayer to decompose it and form a film of metal oxide containing nanoparticles on the surface; and firing the substrate with the film on the metal surface to form a nanocomposite crystalline layer.

Description

[Technical Field]

[0001] The present invention relates to a method for depositing ceramic films on ceramic or metal surfaces, and in particular to the deposition of submicron thick ceramic films such as films of stabilized zirconia and doped ceria such as CGO (cerium gadolinium oxide).

[0002] The present invention is particularly useful for the fabrication of high and intermediate temperature operating cell units, including solid oxide fuel cells (SOFCs) and metal supported intermediate temperature SOFCs operating in the range of 450-650°C. [Background technology]

[0003] A solid oxide electrolyte cell (SOEC) can have the same structure as a SOFC, but is essentially a SOFC operated in a reverse or regenerative mode to achieve the electrolysis of water and / or carbon dioxide by using a solid oxide electrolyte to produce hydrogen gas and / or carbon monoxide and oxygen.

[0004] The present invention relates to an interlayer for a solid oxide fuel cell unit having a structure suitable for use as an SOEC or SOFC. For convenience, both SOEC or SOFC stacked cell units will be referred to hereinafter as "cell units" (i.e., meaning SOEC or SOFC stacked cell units).

[0005] Fuel cells, fuel cell stack assemblies, fuel cell stack system assemblies, and the like are well known in the prior art, and relevant teachings include U.S. Patent Nos. 5,613,299; 5,729,665; 5,729,685; 5,729,69 ... and 5,729,695, all of which are incorporated herein by reference in their entireties.

[0006] There has been a movement for many years to reduce the operating temperature of SOFCs (solid oxide fuel cells) from the conventional 800-1000°C to below 600°C. It has been recognized that achieving this requires the use of a different set of materials than those traditionally used for SOFCs. In particular, this involves the use of cathode materials with higher catalytic activity than conventional yttria-stabilized zirconia (YSZ) when operating at 450-650°C, and electrolyte materials with higher oxygen ion conductivity.

[0007] Higher performance cathode materials are typically cobalt oxide-based perovskite oxides such as LSCF (lanthanum strontium cobalt ferrite), LSC (lanthanum strontium cobaltite), and SSC (samarium strontium cobaltite).Higher conductivity electrolyte materials are typically (i) rare-earth doped ceria such as SDC (samarium-doped ceria) and GDC (gadolinium-doped ceria), or (ii) lanthanum gallate-based materials such as LSGM (lanthanum strontium magnesium gallate).

[0008] The conductivity of zirconia can also be significantly improved by doping it with scandia rather than yttria, but this is a more expensive material.

[0009] Unfortunately, materials that offer higher performance at low temperatures are often less stable than traditional high temperature materials, and certain problems that frequently arise are: High performance cathode materials react with zirconia to form strontium or lanthanum zirconate, which are very poor ionic conductors, leading to reduced performance. ·LSGM reacts with the nickel oxide normally found in the anode. Doped ceria is partially reduced when exposed to the fuel atmosphere, resulting in mixed ionic and electronic conductivity, which can cause internal short circuits in the cell and reduce operating efficiency. Doped ceria and zirconia can react when treated at temperatures above 1200°C to form mixed phases with poor electrical conductivity.

[0010] To mitigate these undesirable material interactions, it is often desirable for the electrolyte to have a composite electrolyte consisting of a main layer and one or more intermediate layers. The main layer performs the primary function of conducting oxygen ions from the cathode to the anode and providing a gas-tight barrier to physically separate the reactants. The intermediate layer is a thin film of another electrolyte material that separates the main electrolyte layer from one or both electrodes, preventing harmful interactions. Common uses for intermediate layers include: An intermediate layer of doped ceria deposited between the zirconia main electrolyte layer and the cobaltite cathode prevents the formation of zirconates and improves the catalytic activity of the cathode. An intermediate layer of doped ceria, deposited between the main electrolyte and the anode in LSGM, avoids reaction with the nickel oxide found in the anode. Fabrication of thin (<1000 nm), continuous, impermeable films is known to be a nontrivial process for cost-effective fuel cell manufacturing. Considering material quality, reproducibility, and process costs, traditional powder, sintering, and plasma or vacuum spray deposition routes are not attractive for mass production.

[0011] However, it has been widely reported that deposition of interlayers in electrolytes can be difficult, especially by conventional sintering processes. This is especially true when the interlayer needs to be dense or when there are limitations on the maximum allowable sintering temperature. Such limitations also apply when the cell is supported on a metal substrate (preferably sintered below 1100°C) or when doped ceria and zirconia are to be sintered together without forming non-conductive phases (preferably sintered below 1200°C).

[0012] Thus, deposition of the interlayer within the electrolyte presents fundamental problems when the interlayer is desired to be dense, when the interlayer forms part of a metal-supported solid oxide fuel cell, and when doped ceria and zirconia are sintered together. These problems are even more severe when the interlayer is formed within the electrolyte of a metal-supported intermediate-temperature solid oxide fuel cell, where the maximum manufacturing process temperature is less than 1100°C.

[0013] Patent Document 15 discloses a method for producing an electrolyte for a SOFC, which involves applying a liquid containing nanoparticles and a metal compound to an electrode, decomposing the nanoparticles and metal compound to form a metal oxide film, and repeating the application and decomposition steps to increase the layer thickness. However, when this method is applied to a metal-supported SOFC, each decomposition step can cause migration of metal ion species from the substrate to the fuel cell electrolyte and / or electrodes, potentially adversely affecting their performance. Each decomposition step also results in the growth of an unwanted oxide layer on the substrate.

[0014] Patent Document 16 discloses a method for producing a metal oxide layer by spin-coating a nanoparticle suspension onto a substrate and drying it. The spin-coating and drying steps can be repeated to add the next layer. The deposited layer is then fired at a high temperature of 1200-1400°C to form a metal oxide layer. However, the resulting layer does not function as a cell unit because it does not contain any ceramic components.

[0015] Furthermore, the firing temperatures required are inappropriate for metal supported cell units.

[0016] Applicant's earlier application, US Pat. No. 5,699,999, discloses a method for depositing at least one layer of a metal oxide crystalline ceramic on the surface of a substrate, the method comprising: (i) depositing a solution of a soluble salt precursor of a metal oxide crystalline ceramic onto the surface of the substrate to define a layer of the solution of the soluble salt precursor on the surface, the surface being selected from the group consisting of a metal surface and a ceramic surface; (ii) drying the solution of the soluble salt precursor to define a layer of the soluble salt precursor on the surface; (iii) heating the soluble salt precursor on the surface to a temperature of 150-600°C to decompose it and form a layer of metal oxide film on the surface; (iv) repeating steps (i)-(iii) at least one additional time to deposit the solution of the soluble salt precursor onto the metal oxide film such that the metal oxide film on the surface comprises multiple layers of metal oxide; (v) firing the substrate having the metal oxide film thereon at a temperature of 500-1100°C to crystallize the metal oxide film into a layer of metal oxide crystalline ceramic bonded to the surface of the substrate, wherein steps (ii), (iii) and (v) are performed in an air atmosphere.

[0017] Following the heating step (iii), the substrate and metal oxide film are cooled to below the decomposition temperature used in the heating step (iii) before repeating the deposition step (i).

[0018] As discussed in U.S. Patent No. 6,277,999, each layer produced in steps (i)-(iii) is approximately 100-150 nm thick. Steps (i)-(iii) are repeated to define multiple metal oxide films on the surface. After steps (i)-(iv) are completed, the metal oxide films have thicknesses of approximately 400-600 nm.

[0019] As further discussed in U.S. Patent No. 6,277,999, it is usually desirable to provide a thicker layer of metal oxide crystalline ceramic, in which case steps (i)-(v) can be repeated, this time with the surface being a previously produced layer of metal oxide crystalline ceramic. However, it is typically desirable to avoid an additional sintering step to avoid migration of unwanted metal ion species from the substrate to the fuel cell electrolyte and / or growth of unwanted oxide layers on the electrodes and substrate.

[0020] The '17 patent states that "solution" means a true solution of at least one substance (solute) in at least one other substance (solvent), i.e., excluding the presence of solid particles and therefore excluding liquid colloidal dispersions, colloidal solutions, and mechanical suspensions. The '17 patent further states that the presence of solids in the layer of step (i) creates stress points, cracks, and loss of layer integrity, and that layers made from sol-gel mixtures or suspensions containing solid particles tend to dry unevenly, and that regions of the suspension tend to dry faster than surrounding particles or gels, resulting in uneven sintering and mechanical drying and annealing stresses that can lead to cracking. [Prior art documents] [Patent documents]

[0021] [Patent Document 1] WO 02 / 35628 [Patent Document 2] International Publication No. 03 / 075382 [Patent Document 3] International Publication No. 2004 / 089848 [Patent Document 4] International Publication No. 2005 / 078843 [Patent Document 5] International Publication No. 2006 / 079800 [Patent Document 6] International Publication No. 2006 / 106334 [Patent Document 7] International Publication No. 2007 / 085863 [Patent Document 8] International Publication No. 2007 / 110587 [Patent Document 9] International Publication No. 2008 / 001119 [Patent Document 10] International Publication No. 2008 / 003976 [Patent Document 11] International Publication No. 2008 / 015461 [Patent Document 12] International Publication No. 2008 / 053213 [Patent Document 13] International Publication No. 2008 / 104760 [Patent Document 14] International Publication No. 2008 / 132493 [Patent Document 15] U.S. Patent Application Publication No. 2017 / 146481 [Patent Document 16] U.S. Patent Application Publication No. 2005 / 153171 [Patent Document 17] International Publication No. 2009 / 090419 Summary of the Invention [Problem to be solved by the invention]

[0022] Both the drying and decomposition steps (heating step (iii)) cause significant shrinkage of the soluble salt precursor layer. If the layer is sufficiently thin, the shrinkage stresses accumulated as a result of the drying and / or decomposition steps will not cause cracking or mechanical failure, resulting in the formation of a dense, defect-free metal oxide film. However, if the layer is too hot, the shrinkage stresses can cause cracking and delamination, leading to failure of the resulting metal oxide crystalline ceramic layer.

[0023] Further shrinkage occurs during crystallization, and the maximum thickness of the metal oxide film that can be deposited and decomposed before crystallization is that required to avoid cracking during crystallization. The thickness of the metal oxide film is determined by the number of successive depositions and decompositions performed before crystallization, with the thickness of each of these layers limited as described above.

[0024] The practical maximum allowable metal oxide film thickness before crystallization may be determined by factors such as the material being deposited and the degree of shrinkage during crystallization, the level of residual material such as carbon left behind from the decomposition process, and the uniformity of the deposited layer.

[0025] Thus, it is desirable to have an interlayer deposition process that dries and anneals with a low risk of cracking. The interlayer is composed of multiple sublayers. Each sublayer has associated manufacturing costs in terms of time and space. Therefore, it is desirable to be able to deposit the interlayer thickness using a minimum number of sublayers. As a result, there is a need for an interlayer deposition process that allows for the deposition of thicker sublayers. [Means for solving the problem]

[0026] The present invention provides a method for forming an interlayer of solid oxide cell units on a surface of a substrate, the method comprising the steps of: providing a base interlayer solution comprising a solution of a metal oxide (crystalline) ceramic and crystalline nanoparticles; dispersing the base interlayer solution on the surface of the substrate; drying the base interlayer solution to define a nanocomposite sublayer of the soluble salt precursor and nanoparticles; heating the sublayer to decompose it and form a metal oxide film containing nanoparticles on the surface; and firing the substrate with the film on the metal surface to form a nanocomposite crystalline layer.

[0027] The solid oxide cell unit can be a SOFC or SOEC cell unit. A solid oxide cell (SOFC or SOEC) uses a nanocomposite approach to fabricate a thin film of densely doped zirconia on a CGO electrolyte by depositing a solution of metal salts combined with a dispersion of electrochemically active or passive nanoparticles. Once dried, a nanocomposite layer is formed consisting of crystalline nanoparticles surrounded by an amorphous metal-organic matrix.

[0028] The film can be converted to a metal oxide film by heat treatment to decompose the salt. The presence of nanoparticles strengthens the organometallic matrix, particularly in dissipating shrinkage-induced stresses that lead to cracking during heat treatment. The presence of nanoparticles also reduces the amount of organic material that needs to be removed during heat treatment, reducing shrinkage-induced stresses that lead to cracking. The matrix provides a permeable network, allowing ionic conduction through the layer. Preferably, the nanoparticles also exhibit ionic conductivity.

[0029] The substrate surface can be selected from the group consisting of metal and ceramic surfaces. Therefore, the base interlayer solution can be deposited onto a metal surface (which can be the metal support of a metal-supported SOFC or SOEC) or a ceramic surface (which can be the electrolyte of a SOFC or SOEC) by, for example, spraying, spin coating, dip coating, or inkjet printing, and then dried to form a thin film. The resulting nanocomposite layer can be pyrolyzed into an amorphous mixed oxide by thermal decomposition, which can be achieved using an infrared heater. This process can be repeated a sufficient number of times to form a layer of, for example, approximately 600 nm. The layer is then heat-treated in a furnace at 500–1100 °C to convert it into a crystalline layer of 10Sc1YSZ or 8YSZ. The entire process can be repeated to obtain a final film thickness of, for example, 1200 nm.

[0030] The base interlayer solution can include a 5-30 mole % solution of a soluble salt precursor of a metal oxide (crystalline) ceramic and crystalline nanoparticles.

[0031] In one embodiment, a method for forming an interlayer of solid oxide cell units on a surface of a substrate is provided. The method includes: i. preparing a base interlayer solution containing 5-30 mol% of a solution of soluble salt precursors of a metal oxide (crystalline) ceramic and crystalline nanoparticles; ii. depositing the base interlayer solution on the surface of the substrate, the surface being selected from the group consisting of a metal surface and a ceramic surface; iii. drying the base interlayer solution to define a nanocomposite sublayer of the soluble salt precursors and nanoparticles; iv. heating the sublayer to a temperature of 150-600°C to decompose it and form a metal oxide film containing nanoparticles on the surface; and v. firing the substrate with the film on the metal surface at a temperature of 500-1100°C to form a nanocomposite crystalline layer. In this manner, the final nanocomposite layer may be composed of 5-30% nanoparticles by volume of the fired film.

[0032] The film so formed from the sublayers in step iv can have a minimum thickness of 130 nm. The inherent properties of nanocomposites allow for thicker sublayers to be produced in a single pass, thereby allowing the resulting final nanocomposite crystalline layer to be produced in fewer passes.

[0033] The depositing, drying and heating steps may be repeated at least one more time before the firing step, in which the base interlayer solution is deposited onto the sublayers such that a film of metal oxide containing nanoparticles is formed from multiple sublayers.

[0034] In the heating step, the film so formed from each sublayer can have a thickness of at least 130 nm, preferably 150 to 500 nm, more preferably 150 to 200 nm, and even more preferably in the range of 175 to 200 nm. In one example, each sublayer of the film has a thickness of 200 to 300 nm.

[0035] The nanoparticles may include doped zirconia nanoparticles. In one example, the nanoparticles are doped zirconium (IV) dioxide nanoparticles. In one example, the nanoparticles are yttria-stabilized. In one example, the nanoparticles are 8YSZ nanoparticles or 10Sc1YSZ nanoparticles.

[0036] Nanoparticles are YSZ / yttria stabilized zirconia ((ZrO2) 1-x (Y2O3) x ), stabilization of zirconium dioxide nanoparticles with yttria increases cation mobility, facilitates sintering, and improves the microstructure of the sintered layer. YSZ can offer the advantage of allowing for the deposition of thicker layers while providing ionic conductivity.

[0037] The nanoparticles may be spherical with an average diameter of 1 to 100 nm. In one example, they may have an average diameter of 1 to 10 nm, preferably 3 to 6 nm, and more preferably 3 to 5 nm. In another example, the nanoparticles may have an average diameter of 1 to 50 nm, 50 to 150 nm, or 100 to 150 nm.

[0038] In one example, the nanoparticles exhibit ionic conductivity. In one example, the crystalline nanoparticles are a dispersion in an aqueous solvent, and the providing step further comprises the substep of solvent exchanging the nanoparticles into a non-aqueous medium that contains the nanoparticles in suspension.

[0039] Solvent exchange refers to the process of changing the nanoparticles' environment. This can involve the following steps: Starting with nanoparticles in isopropanol or water, Step 1: Add ethylene glycol, or alternatively, dipropylene glycol, to the nanoparticle solution to cause a condensation reaction to occur, forming water and a gel. Heat may be applied during the condensation reaction. The gel traps the nanoparticles in a structure, limiting aggregation and maintaining particle size. Step 2: Add acetic acid to reduce the viscosity of the gel. Step 3: Form a dispersion using 80% EtOH, 20% 1-methoxy-2-propanol, MEP, and a binder, such that the nanoparticles constitute 5% by weight of the resulting solution. This dispersion is stirred on a hot plate. The binder can be polyvinyl butyral, such as Butvar® B-76. Polyvinyl butyral is a thermoplastic resin that offers a combination of properties for coating or adhesive applications. The use or addition of polyvinyl butyral to a system imparts adhesion, toughness, and flexibility. Other suitable potential binders include polyvinylpyrrolidone (PVP) and polyethylene glycol (PEG). Generally, any polymer system can be used that is soluble in the solvent used and burns off cleanly without leaving any residue during thermal processing.

[0040] In one example, the crystalline nanoparticles are a dispersion in a non-aqueous solvent. In one example, the heating step includes heating the sublayer to a temperature of 150-600°C.

[0041] In one example, the calcination step in step v is performed at a temperature between 500 and 1100° C. In one example, the calcination step in step v is performed at a temperature between 750 and 850° C., and in another example, the calcination step in step v is performed at about 800° C.

[0042] In one example, the nanocomposite crystalline layer can be at least 90% dense. The nanocomposite crystalline layer can be at least 95% dense. The nanocomposite crystalline layer can be at least 97% dense.

[0043] In one example, the surface of the substrate is an electrolyte layer. In one example, the surface is a mixed ionic-electronic conducting electrolyte material. In one example, the surface is a CGO electrolyte layer.

[0044] In one example, the metal oxide crystalline ceramic is selected from the group consisting of doped stabilized zirconia and rare earth oxide doped ceria.

[0045] In one example, the metal oxide crystalline ceramic is selected from the group consisting of scandia-stabilized zirconia (ScSZ), yttria-stabilized zirconia (YSZ), scandia-ceria co-stabilized zirconia (ScCeSZ), scandia-yttria co-stabilized zirconia (ScYSZ), ytterbia-stabilized zirconia (YbSZ), samarium-doped ceria (SDC), gadolinium-doped ceria (GDC), praseodymium-doped ceria (PDC), and samaria-gadolinia-doped ceria (SGDC).

[0046] In one example, the soluble salt precursor is selected from at least one of the group consisting of zirconium acetylacetonate, scandium nitrate, and yttrium nitrate, cerium nitrate, ytterbium nitrate, cerium acetylacetonate, and gadolinium nitrate.

[0047] In one example, the solvent for the soluble salt precursor is selected from at least one of the group consisting of methanol, ethanol, propanol, methoxypropanol, ethyl acetate, acetic acid, acetone, and butyl carbitol.

[0048] In one example, prior to step iii, the method includes the step of depositing the solution on the surface and leaving it for at least 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 seconds.

[0049] One example is a method for forming at least one layer of an air separation unit electrolyte.

[0050] In one example, the process according to any one of claims 1 to 21 comprises depositing thereon at least one layer of metal oxide crystalline ceramic comprising nanoparticles. [Brief explanation of the drawings]

[0051] [Figure 1] FIG. 1 illustrates an exemplary solid oxide cell layer structure that can be achieved with the method of the present invention. [Figure 2] FIG. 2a is a high magnification top view SEM image of the interlayer, and FIG. 2b is a high magnification cross-sectional SEM image of the interlayer achieved by the method of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0052] These and other features of the present invention will be described in further detail, by way of various embodiments and by way of example only, with reference to the accompanying drawings (which are not to scale and height dimensions are generally exaggerated for clarity) in which:

[0053] A ferritic stainless steel foil substrate 202 (e.g., as shown in FIG. 1) is provided, defining a perforated region 201 surrounded by a non-perforated region, on which an anode layer 210 is deposited, and on top of the anode layer, a gas-impermeable, dense CGO electrolyte layer 220, which is 10-15 microns thick, as taught in GB 2434691 (foil substrate 4, anode layer 1a, and electrolyte layer 1e) and WO 02 / 35628. In other embodiments (not shown), a perforated foil substrate is used on which an anode layer and a gas-impermeable, dense electrolyte layer are deposited (GB 2440038, GB 2386126, GB 2368450, U.S. Pat. No. 7,261,969, EP 1,353,394, and U.S. Pat. No. 7,045,243). In a further embodiment (not shown), the graded metal substrate of US Patent Application Publication No. 2007 / 0269701 is used.

[0054] The results of the above method are shown in the following figures: Figure 1 shows the location of the interlayer in a metal-supported electrochemical cell suitable for use as a SOFC or SOEC.

[0055] A nanocomposite crystalline layer containing nanoparticles is then formed on top of the CGO layer by performing the following steps (a)-(f): The intermediate layer 250 is also made of the crystalline ceramic scandia yttria co-stabilized zirconia (10Sc1YSZ; (Sc2O3) 0.1 (Y2O3) 0.01 (ZrO2) 0.89 The addition of 1% yttria helps stabilize the material in the desired cubic fluorite crystal structure and avoids the microcrystalline phase instabilities that occur in ScSZ systems, particularly the tendency to form rhombohedral crystals at about 500°C, where the oxygen ion conductivity is much lower than in cubic crystals.

[0056] The steps are as follows: (a) Air-atomized spraying, jetting, or inkjet printing a layer of base interlayer solution. The base interlayer solution is a 0.1M cation concentration solution of Sc(NO3)3, Y(NO3)3, and Zr(C5H7O2) in 90% by volume ethanol and 10% by volume methoxypropanol (soluble salt precursors for forming scandia yttria co-stabilized zirconia). The solution contains 8YSZ or 10Sc1YSZ nanoparticles at room temperature on the CGO layer, such that the final crystallized layer contains 5-30% by volume of 8YSZ or 10Sc1YSZ nanoparticles. The base interlayer solution may contain a 5-30 mol% solution of soluble salt precursors and crystalline nanoparticles of a metal oxide (crystalline) ceramic. (b) Drying the base interlayer solution in air at room temperature for 60 seconds, during which the soluble salt precursor and nanoparticles become uniform across the surface, followed by further drying at 100°C for 30 seconds. Alternatively, the drying step can be performed at a slightly higher temperature (e.g., 30-50°C) for longer than 30 seconds. (c) Heating the base interlayer solution to above 500 °C using an infrared (IR) heat lamp for a total of 60 seconds to decompose and semi-crystallize the base soluble salt precursors to form a semi-crystalline scandia yttria co-stabilized zirconia film containing 8YSZ or 10Sc1YSZ nanoparticles approximately 200–400 nm thick. (d) optionally repeating steps (a)-(c), cooling the substrate and metal oxide film to a temperature of 35-80° C. before each repetition of step (a) to obtain a metal oxide and semi-crystalline film having a combined thickness of about 500-600 nm, which is crack-free and suitable for further processing. (e) firing in air at 800°C for 60 minutes, forming a fully crystalline ceramic layer 250 of scandia yttria co-stabilized zirconia metal oxide film having a thickness of approximately 400-600 nm and containing nanoparticles of scandia yttria co-stabilized zirconia. (f) Optionally, repeating steps (a)-(e) one more time to achieve a final layer thickness of about 800-1200 nm.

[0057] The next step (g) is to repeat steps (a) through (e) once more, but this time deposit a layer of CGO 260 on top of the previously deposited crystalline ceramic layer of scandia yttria co-stabilized zirconia containing nanoparticles. Specific conditions are as follows: 0.1M cation concentration of C3(C5H7O2) and gadolinium nitrate in 70% ethanol by volume and 30% methoxypropanol by volume, using the same spraying, deposition, and treatment steps as before, but with a final crystallization calcination temperature of 980 °C, resulting in a CGO layer with a final thickness of approximately 250 nm. This layer serves as a barrier layer between the scandia yttria co-stabilized zirconia layer and the subsequently deposited cathode layer 270.

[0058] (h) Finally, a cathode layer 270 is deposited on top of the previously deposited intermediate layer 250 or CGO layer 260. This can be done by screen printing an LSCF cathode and processing it according to WO 2006 / 079800. This layer can have a thickness of about 50 μm.

[0059] This method can be used to fabricate the cell unit of FIG. 1. The cell unit can be an SOFC or SOEC. In FIG. 1, a ferritic stainless steel metal substrate 200 having an anode layer 210 thereon is provided, and an electrolyte layer 220 is provided. The electrolyte layer 220 surrounds the anode layer 210 to prevent gas flow through the anode 210 between the fuel side 230 and the oxidant side 240. Next, a nanoparticle-containing interlayer (i.e., a nanocomposite interlayer) 250 is deposited on the ceramic CGO layer 220. The nanoparticle-containing interlayer can be a scandia-yttria-co-stabilized zirconia crystalline ceramic layer having yttria-stabilized zirconia (YSZ) or 10Sc1YSZ and containing nanoparticles. A CGO crystalline ceramic layer 260 can then be deposited on the interlayer 250. The spraying steps used to deposit layers 250 and 260 result in a "layer cake" type structure. Following deposition of layers 250 and 260, the cell unit is completed with the addition of cathode assembly 270. When the cell unit operates as a SOFC, 240 represents the oxidant side and 230 represents the fuel side.

[0060] The nanoparticles can be 8YSZ or 10Sc1YSZ nanoparticles having an average particle size of 1-10 nm as measured by TEM. The nanoparticles are generally spherical, but they are not required to be; the sizes mentioned above are characteristic diameters of the particles. The nanoparticles can be 3-5 nm in size. Similarly, the nanoparticles can be 3-6 nm, 1-10 nm, 1-50 nm, or 50-150 nm in size. The nanoparticles can be, for example, 8YSZ or 10Sc1YSZ particles formed by solvothermal processing, supplied as a dispersion in isopropanol, which can be added directly to the interlayer salt solution to form the deposition solution.

[0061] In another possible example, the nanoparticles could be hydrothermally synthesized 8YSZ or 10Sc1YSZ particles delivered in an acidified aqueous suspension, which requires a solvent exchange to transfer the particles to an organic solvent system before mixing them with the interlayer salt solution for deposition.

[0062] There are many ways to accomplish solvent exchange, but one of the simplest is to add a less volatile polar solvent, such as ethylene glycol or propylene glycol, to the aqueous suspension and then heat the suspension to drive off the water, leaving the nanoparticles suspended in a gel with the glycol. The resulting gel can then be dispersed into the interlayer salt solution using high-energy ultrasound prior to deposition.

[0063] Thus, the deposition method according to the invention makes it possible to: Deposition of 200-400 nm thick interlayer in a single pass without cracks or obvious porosity. Starting from already crystallized material (nanoparticles) reduces shrinkage / stress and therefore reduces the chance of crack initiation.

[0064] Nanoparticles are known to be difficult to densify, especially at low temperatures, but this method results in denser layers than prior art techniques.

[0065] SEM comparison of an interlayer formed using a conventional technique with an interlayer formed according to the present invention using only four passes (each sublayer is approximately 200 nm thick after firing, given that the total thickness of the interlayer is 800 nm after firing) shows pores (voids). It is clear that the conventional technique has more pores. Therefore, due to the pores, the interlayer of the conventional technique is less dense than the interlayer formed by the method of the present invention. A dense interlayer is desirable to improve the gas impermeability of the electrolyte and interlayer and prevent gas mixing on both sides of a solid oxide cell. For example, in the case of a SOFC, referring to FIG. 1, a dense interlayer prevents mixing of the oxidant 240 and fuel 230.

[0066] Figure 2 shows an interlayer achieved using the present invention. Figure 2(a) shows a top-view image of the interlayer. Figure 2(a) appears to show many dimples, however, these are only apparent at very high magnification. Figure 2(b) is a cross-sectional view of the same interlayer. Figure 2(b) shows that the dimples are constrained to the surface and do not penetrate the layer. Nanoporosity is observed, but the pores are closed. The film is 750 nm thick and is achieved in a single pass (i.e., only one sublayer forms the interlayer). There are no cracks or defects that penetrate the layer where the electrolyte is exposed.

[0067] These and other features of the invention have been described above purely by way of example, and the invention may vary in detail within the scope of the claims.

Claims

1. 1. A method for depositing a ceramic film of solid oxide cell units onto a ceramic or metallic surface of a substrate, comprising: i. providing a base solution comprising a solvent, a salt soluble in the solvent that is a precursor for forming a metal oxide (crystalline) ceramic, and crystalline nanoparticles dispersed in the solvent; ii. depositing the base solution onto the surface of the substrate; iii. Drying the base solution to define a nanocomposite sublayer of the soluble salt and nanoparticles; iv. heating the sublayer to decompose it and form a metal oxide film containing nanoparticles on the surface; v. firing the substrate with the film on the surface to form a nanocomposite crystalline layer as a deposited ceramic film; A method comprising:

2. 10. The method of claim 1, wherein in step iv, the film formed from the sublayers has a minimum thickness of 130 nm.

3. 6. The method of claim 1, wherein steps ii.-iv. are repeated at least one additional time to deposit the base solution onto the sublayers prior to the calcination step of step v, so that the film of metal oxide comprising nanoparticles is formed from multiple sublayers.

4. 4. The method of claim 3, wherein in step iv, the film formed from each sublayer has a thickness of at least 130 nm.

5. The method of any one of claims 1 to 4, wherein the nanoparticles comprise doped zirconia nanoparticles.

6. The method of claim 5 , wherein the nanoparticles are doped zirconium (IV) dioxide nanoparticles.

7. The method of claim 5 or 6, wherein the nanoparticles are yttria stabilized.

8. The method of any one of claims 5 to 7, wherein the nanoparticles are 8YSZ nanoparticles or 10Sc1YSZ nanoparticles.

9. The method according to any one of claims 1 to 8, wherein the nanoparticles exhibit ionic conductivity.

10. The crystalline nanoparticles are a dispersion in an aqueous solvent, and step i. comprises the following sub-step a: a. Solvent-exchanging the nanoparticles into a non-aqueous solvent containing the nanoparticles in suspension. The method of any one of claims 1 to 9, further comprising:

11. The method of any one of claims 1 to 10, wherein the crystalline nanoparticles are in dispersion in a non-aqueous solvent.

12. The method of any one of claims 1 to 11, wherein the heating step involves heating the sublayer to a temperature of 150 to 600°C.

13. The method according to any one of claims 1 to 12, wherein the calcination step in step v. is carried out at a temperature of 500 to 1100°C.

14. The method of any one of claims 1 to 13, wherein the surface of the substrate is an electrolyte layer.

15. 15. The method of claim 14, wherein the surface is a mixed ionic and electronic conducting electrolyte material.

16. 16. The method of claim 15, wherein the surface is a CGO electrolyte layer.

17. The method of any one of claims 1 to 16, wherein the metal oxide (crystalline) ceramic is selected from the group consisting of doped stabilized zirconia and rare earth oxide doped ceria.

18. 18. The method of any one of claims 1 to 17, wherein the metal oxide (crystalline) ceramic is selected from the group consisting of scandia-stabilized zirconia (ScSZ), yttria-stabilized zirconia (YSZ), scandia-ceria co-stabilized zirconia (ScCeSZ), scandia-yttria co-stabilized zirconia (ScYSZ), ytterbia-stabilized zirconia (YbSZ), samarium-doped ceria (SDC), gadolinium-doped ceria (GDC), praseodymium-doped ceria (PDC), and samaria-gadolinia-doped ceria (SGDC).

19. 19. The method of any one of claims 1 to 18, wherein the soluble salt is selected from at least one of the group consisting of zirconium acetylacetonate, scandium nitrate, and yttrium nitrate, cerium nitrate, ytterbium nitrate, cerium acetylacetonate, and gadolinium nitrate.

20. 20. The method of any one of claims 1 to 19, wherein the solvent is selected from at least one of the group consisting of methanol, ethanol, propanol, methoxypropanol, ethyl acetate, acetic acid, acetone, and butyl carbitol.

21. 21. The method of any one of claims 1 to 20, further comprising the step of depositing the solution on the surface and leaving it for at least 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 seconds before step iii.

22. 22. The method of any one of claims 1 to 21, which is a method for forming at least one layer of an air separation unit electrolyte.

Images