Patent Application
20230061956
The present disclosure is concerned with methods for the deposition of ceramic films on ceramic or metallic surfaces, particularly the deposition of sub-micron thickness ceramic films such as films of stabilized zirconia and doped ceria such as CGO (cerium gadolinium oxide).
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 the likes of WO 02/35628, WO 03/075382, WO 2004/089848, WO 2005/078843, WO 2006/079800, WO 2006/106334, WO 2007/085863, WO 2007/110587, WO 2008/001119, WO 2008/003976, WO 2008/015461, WO 2008/053213, WO 2008/104760, and WO 2008/132493, all of which are incorporated herein by reference in their entirety.
There has been a drive over a number of years to lower the operating temperature of SOFCs (Solid Oxide Fuel Cells) from the traditional 800-1000° C., down to 600° C. or below. It has been recognized that achieving this requires the use of a different set of materials from those traditionally used for SOFCs. In particular, this entails the use of cathode materials with increased catalytic activity and electrolyte materials with higher oxygen ion conductivity than the traditional yttria-stabilized zirconia (YSZ) when operating between 450-650° C.
The higher-performance cathode materials are typically perovskite oxides based on cobalt oxide, such as LSCF (lanthanum strontium cobalt ferrite), LSC (lanthanum strontium cobaltite) and SSC (samarium strontium cobaltite). The more conductive electrolyte materials are typically either (i) rare-earth-doped ceria such as SDC (samarium-doped ceria) and GDC (gadolinium-doped ceria), or (ii) materials based on lanthanum gallate, such as LSGM (lanthanum-strontium-magnesium gallate).
The conductivity of zirconia can also be significantly improved by doping with scandia rather than yttria, although this is a more costly material.
Unfortunately, materials with higher performance at lower temperatures are frequently less stable than the traditional high-temperature materials. Particular problems frequently encountered are:
To mitigate these undesirable material interactions, it is frequently desirable to have a composite electrolyte in which the electrolyte consists of a main layer and one or more interlayers. The main layer performs the primary functions of conducting oxygen ions from the cathode to the anode, and providing a gas-tight barrier to physically separate the reactants. The interlayer(s) are thin film(s) of another electrolyte material which separate the main electrolyte layer from one or both electrodes, preventing detrimental interactions. Typical uses of interlayers include:
However, it is widely reported that the deposition of interlayers within an electrolyte can be difficult, particularly by means of conventional sintering processes. This is particularly the case if there is a requirement for the interlayer to be dense, or if there is a limit on the maximum permissible sintering temperature. Such limits apply if the cell is supported on a metal substrate (preferably sintering <1100° C.), or when trying to sinter doped ceria and zirconia together without forming a non-conductive phase (preferably sintering <1200° C.).
Thus, the deposition of interlayers within an electrolyte presents fundamental problems when it is desired for the interlayer to be dense, when the interlayer is to form part of a metal-supported solid oxide fuel cell, and where doped ceria and zirconia are to be sintered together. These problems are even more substantial when the interlayer is to be formed within an electrolyte of a metal-supported intermediate temperature solid oxide fuel cell, the maximum manufacturing process temperature being <1100° C.
US 2017/146481A discloses a method for manufacturing an electrolyte for a SOFC, the method including applying a liquid containing nanoparticles and metal compounds to an electrode, decomposing the nanoparticles and metal compound to form a metal oxide film, and repeating steps of applying an decomposing to build up a layer thickness. However, application of this method to a metal-supported SOFC may result in metal ion species migration, at each of the decomposition steps, from the substrate into the fuel cell electrolyte and/or electrodes, which may be detrimental to their performance. Each of decomposition step also results in unnecessary oxide layer growth on the substrate.
US 2005/153171 discloses a method for production of a metal oxide layer in which a nanoparticle suspension is spun onto a substrate and dried. Subsequent layers may be added by repeating the spinning and drying steps. The accumulated layers are fired at high temperatures 1200-1400° C. to form a metal oxide layer. However, the layers produced do not involve any ceramic component and so do not function as a cell unit. Furthermore, the required firing temperature is unsuitable for a metal-supported cell unit.
Applicant's earlier patent application WO2009/090419 discloses a method for depositing at least one layer of metal oxide crystalline ceramic upon a surface of a substrate the method comprising the steps of:
(i) depositing a solution of a soluble salt precursor of a metal oxide crystalline ceramic onto said surface of said substrate to define a layer of said solution of said soluble salt precursor on said surface, said surface being selected from the group consisting of: a metallic surface and a ceramic surface;
(ii) drying said solution of said soluble salt precursor to define a layer of said soluble salt precursor on said surface;
(iii) heating said soluble salt precursor on said surface to a temperature of between 150 and 600° C. to decompose it and form a layer of metal oxide film on said surface;
(iv) repeating steps (i)-(iii) at least one additional time, said solution of said soluble salt precursor being deposited onto said metal oxide film, such that said metal oxide film on said surface comprises a plurality of layers of metal oxide; and
(v) firing said substrate with said metal oxide film on said surface at a temperature of 500-1100° C. to crystallize said metal oxide film into a layer of metal oxide crystalline ceramic bonded to said surface of said substrate, wherein each of steps (ii), (iii) and (v) is performed in an air atmosphere.
Subsequent to the heating step (iii) and prior to the repeat of deposition step (i), the substrate and metal oxide film is cooled to below the decomposition temperature used in heating step (iii).
As discussed in WO2009/090419, each layer produced in steps (i)-(iii) is around 100-150 nm thick. Steps (i)-(iii) are repeated to define a plurality of layers of metal oxide film on the surface. After the completion of steps (i)-(iv) the metal oxide film has a thickness of around 400-600 nm.
As further discussed in WO2009/090419, it is usually desired to provide a thicker layer of metal oxide crystalline ceramic, in which case it is possible to repeat steps (i)-(v), this time the surface being the layer of metal oxide crystalline ceramic previously produced. However, it is typically desirable to avoid additional sintering steps in order to avoid unnecessary metal ion species migration from the substrate into the fuel cell electrolyte and/or electrodes and also to avoid unnecessary oxide layer growth on the substrate.
WO2009/090419, discloses that by “solution” it is meant a true solution comprised of at least one substance (the solute) in at least one other substance (the solvent), i.e. excludes the presence of solid particles and thus excludes liquid colloidal dispersion, colloidal solutions, and mechanical suspensions. That document further discloses that experiments have shown that the presence of any solids in the layer of step (i) generate stress points which result in cracking and therefore loss of layer integrity, and that a layer made from a sol-gel mix or suspension containing solid particles will tend to dry an uneven way and also sinter in a non-homogeneous way with the suspension areas drying faster than those around the particle or gel, creating mechanical drying and annealing stresses which can lead to cracking.
Both drying and decomposition (heating step (iii)) lead to significant shrinkage of the layer of soluble salt precursor. If the layer is sufficiently thin, the shrinkage stresses that build up as a result of drying and/or decomposition do not result in cracking or mechanical failure and a dense, defect-free metal oxide film is formed. However, if the layer is too thick then the shrinkage stresses can lead to cracking or even delamination and thus failure of the resulting layer of metal oxide crystalline ceramic.
Further shrinkage occurs on crystallization, and the maximum metal oxide film thickness which can be deposited and decomposed before a crystallization is that required to avoid cracking on crystallization. The metal oxide film thickness is determined by the number of successive depositions and decompositions performed before crystallization, and the thickness of each of these layers is limited as described above.
The actual maximum allowable metal oxide film thickness before crystallization may be determined by factors such as the material being deposited and its degree of shrinkage on crystallization, the level of residual material such as carbon left behind from the decomposition process, and the evenness of the deposited layer.
Thus, it is desirable to have an interlayer deposition process that dries and anneals with low risk of cracking. The interlayer is made up of a plurality of sublayers. Each sublayer is associated with a manufacturing cost in terms of time and space. Thus, it is desirable to be able to deposit the interlayer thickness using the fewest number of sub-layers. As a result, there is a need for interlayer deposition processes that allow deposition of thicker sub-layers.
The present disclosure is particularly useful in the manufacture of high and intermediate temperature operating cell units including solid oxide fuel cells (SOFC) and also metal supported intermediate temperature SOFC operating in the 450-650° C. range.
A solid oxide electrolyzer cell (SOEC) may have the same structure as an SOFC but is essentially that SOFC operating in reverse, or in a regenerative mode, to achieve the electrolysis of water and/or carbon dioxide by using the solid oxide electrolyte to produce hydrogen gas and/or carbon monoxide and oxygen.
The present disclosure is directed at an interlayer for a solid oxide fuel cell unit having a structure suitable for use as an SOEC or an SOFC. For convenience, SOEC or SOFC stack cell units will both be referred to herein as “cell units” (i.e. meaning SOEC or SOFC stack cell units).
The present disclosure provides a method for forming an interlayer of a solid oxide cell unit upon a surface of a substrate, the method including the steps of: Providing a base interlayer solution comprising a solution of a soluble salt precursor 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 sub-layer of the soluble salt precursor and nanoparticles; Heating the sub-layer to decompose it and form a film of metal oxide comprising nanoparticles on the surface; Firing the substrate with the film on the metal surface, to form a nanocomposite crystalline layer.
The solid oxide cell unit may be a cell unit of a SOFC or a SOEC. In a solid oxide cell (SOFC or SOEC), the use of a nanocomposite approach to fabricate a thin film of dense doped-zirconia on top of a CGO electrolyte by deposition of a solution of metal salts combined with a dispersion of electrochemically active or passive nanoparticles. Once dry, this forms a nanocomposite layer consisting of crystalline nanoparticles surrounded by amorphous organometallic matrix.
The film may be converted to a film of metal oxide by heat treatment to decompose the salts. The presence of nanoparticles provides reinforcement to the organometallic matrix, especially during the heat treatment to distribute the shrinkage-induced stresses that lead to cracking. Presence of the nanoparticles also reduces the amount of organic matter that needs to be removed in the heat treatment, thus reducing shrinkage-induced stresses that lead to cracking. The matrix provides a percolating network to enable ionic conductivity through the layer. Preferably, the nanoparticles will also exhibit ionic conductivity.
The surface of the substrate may be selected from the group consisting of: a metallic surface and a ceramic surface. Thus, the base interlayer solution may be deposited on the metal surface (which may be the metal support in a metal supported SOFC or SOEC) or on the ceramic surface (which may be the electrolyte of a SOFC or SOEC) by, for example, spraying, spin-coating, dip-coating, or ink-jet printing, and allowed to dry to form a thin film. The resulting nanocomposite layer may be thermally decomposed into an amorphous mixed oxide by thermal decomposition, this may be by use of an infrared heater. The process may be repeated sufficient times to build up a layer of, for example, approximately 600 nm, and then the layer may be heat treated in a furnace at 500-1100° C. to convert it to a crystalline layer of 10Sc1YSZ or 8YSZ. The whole process may be repeated to obtain a final film thickness of, for example, 1200 nm.
The base interlayer solution may comprise a solution of a soluble salt precursor of a metal oxide (crystalline) ceramic and crystalline nanoparticles at 5-30 mol %.
In an aspect, there is provided a method for forming an interlayer of a solid oxide cell unit upon a surface of a substrate. The method may include steps of: i. Providing a base interlayer solution comprising a solution of a soluble salt precursor of a metal oxide (crystalline) ceramic and crystalline nanoparticles at 5-30 mol %; ii. Depositing the base interlayer solution on the surface of the substrate, the surface optionally being selected from the group consisting of: a metallic surface and a ceramic surface; iii. Drying the base interlayer solution to define a nanocomposite sub-layer of the soluble salt precursor and nanoparticles; iv. Heating the sub-layer to a temperature of between 150 and 600° C., to decompose it and form a film of metal oxide comprising nanoparticles on the surface; v. Firing the substrate with the film on the metal surface at a temperature of 500 to 1100° C., to form a nanocomposite crystalline layer. In this way, the final nanocomposite layer may be composed of 5-30% nanoparticles by volume of the fired film.
The film so formed at step iv from the sub-layer may have a minimum thickness of 130 nm. Due to the inherent nature of the nanocomposite, it is capable of yielding a thicker sub-layer in a single pass, so that the resulting final nanocomposite crystalline layer can be produced with a smaller number of passes.
Steps of depositing, drying, and heating. may be repeated at least one additional time before the step of firing, the base interlayer solution being deposited onto the sub-layer, such that the film of metal oxide comprising nanoparticles is formed from a plurality of sub-layers
At the step of heating, the film so formed from each sub-layer may have a thickness of at least 130 nm, preferably within the range of 150 to 500 nm, more preferably 150 to 200 nm, even more preferably 175 to 200 nm. In an example, the thickness of each sub-layer in the film has a thickness of 200-300 nm.
The nanoparticles may comprise doped zirconia nanoparticles. In an example, the nanoparticles doped zirconium (IV) dioxide nanoparticles. In an example, the nanoparticles are yttria stabilized. In an example, the nanoparticles are 8YSZ or 10Sc1YSZ nanoparticles.
In the case that the nanoparticles are YSZ/yttria stabilized zirconia ((ZrO2)1-x(Y2O3)x), Stabilization of zirconium dioxide nanoparticles with yttria improves the microstructure of the sintered layer by encouraging cation mobility and enhancing sintering. YSZ, can offer the benefits of enabling thicker layers to be deposited while simultaneously offering ionic conductivity.
The nanoparticles may be spherical with an average diameter of between 1 and 100 nm. In an example, they may have an average diameter of between 1 and 10 nm, preferably between 3 and 6 nm, more preferably between 3 and 5 nm. In another example the nanoparticles may have average diameter of between 1 and 50 nm, between 50 and 150 nm or between 100 and 150 nm.
In an example, the nanoparticles exhibit ionic conductivity. In an example the crystalline nanoparticles are dispersions in an aqueous solvent, and step of providing further comprises the sub-step of: solvent exchange of the nanoparticles into a non-aqueous media comprising the nanoparticles in suspension.
Solvent exchange describes process of changing the environment of the nanoparticle. This may involve the steps of, starting with nanoparticle in isopropanol or water, step 1: addition of ethylene glycol to the nanoparticle solution, a condensation reaction occurs to form water and a gel, an alternative to ethylene glycol is dipropylene glycol. During the condensation reaction, heat may be applied. The gel traps the nanoparticle in a structure which limits agglomeration and maintains particle size. Step 2: addition of acetic acid to reduce viscosity of the gel. Step 3: form a dispersion using EtOH (80%) and 1-methoxy-2-propanol, MEP (20%) and a binder, such that the nanoparticles are 5% by weight of resulting solution. This dispersion is stirred on hotplate. The binder may be polyvinyl butyral, for example 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). In general any polymer system which is soluble in the solvents used and burns out cleanly without residue during heat treatment may be used.
In an example, the crystalline nanoparticles are dispersions in a non-aqueous solvent. In an example, the heating step involves heating the sub-layer to a temperature of between 150 and 600° C.
In an example, the firing at step v. is at a temperature of between 500 and 1100° C. In an example, the firing at step v. may be between 750 and 850° C., in another example the firing at step v. may be at around 800° C.
In an example, the nanocomposite crystalline layer may be at least 90% dense. The nanocomposite crystalline layer may be at least 95% dense. The nanocomposite crystalline layer may be at least 97% dense.
In an example, the surface of the substrate is an electrolyte layer. In an example, the surface is a mixed ionic electronic conducting electrolyte material. In an example, the surface is a CGO electrolyte layer.
In an example, the metal oxide crystalline ceramic is selected from the group consisting of: doped stabilized zirconia, and rare earth oxide doped ceria.
In an 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).
In an 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.
In an example, the solvent for said 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.
In an example, the prior to step iii the step of allowing said solution deposited onto said surface to stand for a period of at least 5, 10, 15, 20, 25, 30, 35, 40, 45, 50, 55 or 60 seconds.
In an example, the method of forming an at least one layer of an air separation device electrolyte.
In an example, having deposited upon it at least one layer of metal oxide crystalline ceramic comprising nanoparticles according to the process of any of claims 1-21.
These and other features of the present disclosure will now be described in further detail, by way of various embodiments, and just by way of example, with reference to the accompanying drawings (which drawings are not to scale, and in which the height dimensions are generally exaggerated for clarity), in which:
A ferritic stainless steel foil substrate 202 (as shown in e.g.
The results of the method detailed above are shown in the following figures.
A nanocomposite crystalline layer comprising nanoparticles, is then formed on top of the CGO layer by performing steps (a)-(f) below. The interlayer 250 may also comprise crystalline ceramic scandia yttria co-stabilized zirconia (10Sc1YSZ; (Sc2O3)0.1(Y2O3)0.01(ZrO2)0.89) The addition of 1% Yttria stabilizes the material in the desired cubic fluorite crystal structure and helps avoid the crystallite phase instability which can occur in the ScSZ system, particularly a tendency to form rhombohedral crystals at around 500° C. which have much lower oxygen ion conductivity that cubic ones.
The steps may be:
(a) air atomized spraying, jetting or ink-jet printing of a layer of base interlayer solution. The base interlayer solution being of 0.1 M cation concentration solution of Sc(NO3)3 and Y(NO3)3 and Zr(C5H7O2) in 90% volume ethanol and 10% volume methoxypropanol (soluble salt precursors which will form the scandia yttria co-stabilized zirconia), and comprising 8YSZ or 10Sc1YSZ nanoparticles such that the final crystallized layer comprises 5-30% 8YSZ or 10Sc1YSZ nanoparticles by volume, at RTP onto the CGO layer. The base interlayer solution may comprise a solution of a soluble salt precursor of a metal oxide (crystalline) ceramic and crystalline nanoparticles at 5-30 mol %.
(b) drying the base interlayer solution at RTP in air for 60 seconds during which period the soluble salt precursor and nanoparticles even out across the surface, followed by further drying at 100° C. for 30 seconds. In an alternative, the step of drying may be undertaken at a slightly elevated temperature (e.g., 30-50° C.) but for longer than 30 seconds.
(c) heating the base interlayer solution to >500° C. over a total period of 60 seconds using an infra-red (IR) heating lamp which decomposes and semi-crystallizes the base soluble salt precursor it to form a layer about 200-400 nm thick of a semi-crystalline scandia yttria co-stabilized zirconia film comprising 8YSZ or 10Sc1YSZ nanoparticles.
(d) optionally repeating steps (a)-(c), the substrate and metal oxide film being cooled to a temperature of 35-80° C. prior to each repeat of step (a), to give a metal oxide and semi-crystalline film having a total thickness of about 500-600 nm. This film does not have any cracks in it and is suitable for further processing.
(e) firing at 800° C. for 60 minutes in air, the metal oxide film of scandia yttria co-stabilized zirconia forms a fully crystalline ceramic layer 250 of scandia yttria co-stabilized zirconia comprising nanoparticles, having a thickness of about 400-600 nm.
(f) optionally repeating steps (a)-(e) once more to achieve a final layer thickness of about 800-1200 nm
The next steps may be: (g) the repeating of steps (a)-(e) once more but this time depositing a layer 260 of CGO on top of the previously deposited crystalline ceramic layer of scandia yttria co-stabilized zirconia comprising nanoparticles. Example specific conditions are: 0.1 M cation concentration Ce(C5H7O2) and gadolinium nitrate in 70% volume ethanol and 30% volume methoxypropanol and spraying, depositing and processing as before but using a final crystallization firing temperature of 980° C. to achieve a CGO layer with a final thickness of around 250 nm. This layer acts as a barrier layer between the scandia yttria co-stabilized zirconia layer and a subsequently deposited cathode layer 270.
(h) finally, a cathode layer 270 is then deposited on top of the previously deposited interlayer 250 or CGO layer 260. This may be done by screen-printing an LSCF cathode and processing it in accordance with WO2006/079800. This layer may have a thickness of about 50 μm.
The method may be used to manufacture the cell unit of
The nanoparticles may be 8YSZ or 10Sc1YSZ nanoparticles having average particle size, as measured by TEM, of 1-10 nm. The nanoparticles are generally spherical, but need not be, the size quoted is the characteristic diameter of the particles. The nanoparticles may be 3-5 nm in size. Equally, they may be 3-6 nm, 1-10 nm, 1-50 nm, or 50-150 nm in size. Nanoparticles may, for example, be 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 a solution for deposition.
In other possible examples the nanoparticles could be 8YSZ or 10Sc1YSZ particles made by hydrothermal synthesis supplied in an acidified aqueous suspension. In this case it is necessary to perform a solvent exchange to transfer the particles to an organic solvent system before mixing them with the interlayer salt solution for deposition.
There are a number of possible methods for achieving a solvent exchange, but one of the simplest ones is to add a low-volatility 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 before deposition.
Thus, the deposition method allows:
It is known that nanoparticles can be hard to densify especially at lower temperatures however the method results in a layer of higher density than prior art technique.
Comparison, by SEM, of an interlayer formed in accordance with the disclosure using just 4 passes (each sublayer being around 200 nm thick after firing given that total interlayer is 800 nm after firing) with interlayers formed using a prior art technique show pores (which are voids in the interlayer) . Many more pores are evident using the prior art technique. Thus, because of the pores, the prior art interlayers are of lower density than the interlayer formed with the method of the present disclosure. A dense interlayer is desired to improve the gas-impermeability of the electrolyte and interlayer, to prevent mixing of gasses on either side of the solid oxide cell. For example, in the case of a SOFC and with reference to
These and other features of the present disclosure have been described above purely by way of example. Modifications in detail may be made to the disclosure within the scope of the claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2001139.1 | Jan 2020 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2021/050845 | 1/15/2021 | WO |
