EP4731439A1

EP4731439A1 - Process for producing ceria-based multilayer ceramic scaffold with dense and porous layers

Info

Publication number: EP4731439A1
Application number: EP24734922.8A
Authority: EP
Inventors: Elise BONNET; Thierry Le-Mercier; Marc-David BRAIDA; Camille Jourde; Pierre-Marie Geffroy; Jean-Marc Bassat
Original assignee: Centre National de la Recherche Scientifique CNRS; Rhodia Operations SAS; Universite de Bordeaux; Institut Polytechnique de Bordeaux
Current assignee: Centre National de la Recherche Scientifique CNRS; Rhodia Operations SAS; Universite de Bordeaux; Institut Polytechnique de Bordeaux
Priority date: 2023-06-26
Filing date: 2024-06-26
Publication date: 2026-04-29
Also published as: TW202513507A; CN121464039A; WO2025003225A1

Abstract

The invention relates to a process for producing a doped ceria, preferably a gadolinium-doped ceria (GDC) multilayer ceramic scaffold comprising porous and dense layers, wherein the porous layers are obtained from a suspension comprising at least two different pore-forming agents. The invention also relates to a unit solid cell (SOC) comprising said multilayer ceramic scaffold. The invention also relates to the use of a doped ceria, preferably a GDC ceramic suspension comprising at least two different pore-forming agents for reducing the difference in shrinkage during the co-sintering of said multilayer ceramic scaffold.

Description

PROCESS FOR PRODUCING CERIA-BASED MULTILAYER CERAMIC SCAFFOLD WITH DENSE AND POROUS LAYERS

TECHNICAL FIELD

The invention relates to a process for producing a doped ceria multilayer ceramic scaffold and a unit solid oxide cell (SOC) comprising the same. The invention also relates to the use of a doped ceria ceramic suspension comprising at least two different pore-forming agents for reducing the difference in shrinkage during the co-sintering of a multilayer ceramic scaffold.

TECHNICAL BACKGROUND

Solid oxide cells (SOCs), including solid oxide fuel cells (SOFCs) and solid oxide electrolysis cells (SOECs), have gained significant attention in recent years as a promising technology for efficient energy conversion and storage.

However, their development is hindered by various factors and several challenges still need to be addressed to enable their widespread adoption and commerci alizati on .

One problem is linked to their long-term chemical stability at the operating temperatures of the cells. Indeed, conventional SOCs typically require high operating temperatures, often above 800 °C to achieve optimal performances. Such high temperatures, however, result in increased degradation rates of the materials constituting the cell and/or inter-diffusion between the various components of the cell.

Another disadvantage is connected to the high manufacturing costs associated with the numerous shaping steps that are generally needed to assemble the various layers of the cell (i.e., el ectrode/electrolyte/el ectrode layers) as well as the conventional thermal sintering treatments that are performed at high temperatures. Such treatments are necessary to obtain a densified electrolyte layer with the desired density, but at the same time are often detrimental for the electrode microstructure that, in order yield optimal electrochemical properties, need to be maintained with a large specific surface area and porosity.

In this respect, the sintering temperature is a parameter that must be optimized to fully densify the electrolyte layer while keeping a sufficient porosity in the electrodes. Additionally, further important aspects that must be considered when preparing a SOC and that can influence the electrochemical properties of the cell are the thermo-mechanical compatibility between the electrolyte and the electrodes as well as the architecture of the cell (such as the thickness of the different layers). In particular, it has been found that the materials used for the various components of the cell must have coefficient of expansion close to each other to ensure that the resulting scaffold has sufficient mechanical strength, particularly at the various interfaces. Furthermore, it is also necessary to ensure the chemical stability of each of the materials at the operating temperatures and with respect to the gases normally used to operate the cell (i.e., air and fuel). Finally, chemically compatibility between said materials is essential to minimize chemical reactivities.

A widely used family of materials used for this purpose are ceria substituted with gadolinium which have higher ionic conductivity values than the other commonly employed materials such as YSZ, especially at reduced operating temperatures.

In this context, many efforts have been done to provide an all-ceria SOC that can operate with high performance at a reduced temperature, with an increased durability and with a low production cost in terms of materials and process steps.

In two works of the group of L. Guesnet el al., namely “ Shaping of ceriabased SOC cells: development of a combined tape-casting and infiltration route", ECS Transactions, Electrochemical Society, Inc., 2019, 91 (1), pp. 291- 299, and “ Shaping of ceria-based single solid oxide cells combining tapecasting, screen-printing and infiltration''. Journal of the European Ceramic Society 40 (2020) 5662-5669, it is described a process for producing a SOC with porous/dense/porous layers made of the same ceramic material GDC10 (i.e., gadolinium-doped ceria with 10% dopant) in one single step by tape-casting and co-sintering, the porous layers being thereafter infiltrated by catalysts to obtain a dense GDC 10 electrolyte arranged between two porous GDC 10 composite electrodes. Since the same material is used for making up the cell, thermomechanical problems linked to the use of different materials are avoided and the use of tape-casting and single step sintering allows obtaining a cell that can be operated at reduced temperature (500-600 °C) with increased long-term stability and produced in a single step with a densified electrolyte layer while at the same time maintaining a desired porosity of the electrode layers.

However, during the co-sintering of such porous/dense/porous multilayer scaffolds, the large differences in the shrinkage of the porous and dense layers lead to deformation and internal residual stress of the layers which in turn result in cracking and ruptures of the final multilayer ceramic scaffold.

Such a problem has been addressed in the art by several solutions, such as adjusting the initial green density of the porous layer with organic additives or adjusting the kinetics of sintering with the granulometry of powders by additional grinding step of ceramic powder used for the dense layer. However, these techniques are not suitable to adjust the shrinkage differences between the porous and dense layers in case of large differences and variations.

It appears thus clear that there remains the need to find a novel process for producing a multilayer ceramic scaffold comprising porous and dense layers which not only requires fewer shaping and sintering steps but also which advantageously allows to minimize and adjust the differences in shrinkage between the porous and dense layers.

SUMMARY OF THE INVENTION

The present invention relates to a process for the preparation of a multilayer ceramic scaffold, wherein said scaffold comprises at least a dense layer and at least a porous layer. Said dense and porous layers are made of a doped ceria ceramic material, preferably of a gadolinium-doped ceria (GDC) ceramic material.

The process according to the present invention comprises the steps of: a) preparing a first suspension by mixing a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder, with a mixture of at least two different pore-forming agents, an organic solvent, a dispersant, and a binder, wherein said pore-forming agents are independently selected from the group consisting of: carbon, an organic polymer, and a combination thereof; b) preparing a second suspension by mixing a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder, with an organic solvent, a dispersant, and a binder, wherein said second suspension is free of any pore-forming agents; c) assembling a green layer stack comprising at least a porous green layer obtained from the suspension of step a) and at least a dense green layer obtained from the suspension of step b); d) co-sintering the green layer stack obtained in step c).

According to an embodiment of the invention, the multilayer ceramic scaffold comprises a dense layer arranged between two porous layers so that step c) of the process according to the present invention comprises assembling a green layer stack comprising a dense green layer arranged between two porous green layers, wherein said dense green layer is obtained from the suspension of step b) and said porous green layers are obtained from the suspension of step a).

According to an embodiment of the invention, said dense layer can contain a thinner layer made of a different material than said doped ceria ceramic material, as a barrier layer. According to said embodiment, step c) of the process of the present invention comprises assembling said barrier layer within the dense layer.

According to a particularly preferred embodiment, said at least two different pore-forming agents are characterized by at least two different mean particle sizes. Preferably said at least two different pore-forming agents comprise a first pore-forming agent characterized by a first mean particle size comprised between 10 and 500 nm, preferably between 10 and 100 nm, and at least a second pore-forming agent, different from the first pore-forming agent, characterized by a second mean particle size comprised between 1 and 100 pm, preferably between 1 and 50 pm.

The present invention also relates to the use of a suspension comprising a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder and a mixture of at least two different pore-forming agents in the presence of a dispersant and a binder to reduce and adjust the shrinkage during co-sintering of green layer stacks comprising at least a porous layer obtained from said suspension and at least a dense layer obtained from a second suspension comprising a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder in the presence of a dispersant and a binder, wherein said second suspension is free of any pore-forming agents.

The present invention further relates to a process for the preparation of a unit solid oxide cell (SOC) comprising a dense layer, which is the electrolyte layer, arranged between two porous layers which have a larger thickness than the dense layer. For the purposes of the present invention, said dense layer is also called “thin dense layer”. Both the dense and the porous layer are made of doped ceria ceramic material, preferably of gadolinium-doped ceria (GDC) ceramic material.

Said process comprises the step of: e) impregnating the porous layers of the scaffold obtained after step d) of the process according to the present invention as described above with a catalyst so to obtain composite electrode layers.

The present invention also concerns a multilayer ceramic scaffold obtainable or obtained by the process according to the present invention as described above.

Said multilayer ceramic scaffold comprises at least a porous layer and at least a dense layer. According to an embodiment, said multilayer ceramic scaffold comprises a dense layer arranged between two porous layers.

Said at least a porous layer is also preferably characterized by a total porous volume of at least 35 vol%.

Preferably, said at least a porous layer is also characterized by a porosity which is an interconnected porosity and, preferably, by a pore size between 300 nm and 600 nm, preferably between 350 nm and 600 nm, more preferably of about 500 nm. The size of the pores may notably be determined by SEM, in particular by applying a color contrast to discriminate the pores from the material. The average size of the pores may then calculated from the measurement of each pore size; the largest dimension of the pore may be used for each. For the SEM observations and/or measurements referred to in the present application, a JEOL IT300 scanning electron microscope may in particular be used.

According to the preset invention, the multilayer ceramic scaffold is characterized by a variation of the surface level which is < 200 nm as measured by a mechanical profilometer on a 3 cm scanning length. For the purposes of the present invention, this indicates the surface flatness of the multilayer ceramic scaffold.

The present invention solves the criticalities and disadvantages mentioned above by providing a process which allows producing a multilayer ceramic scaffold comprising porous and dense layers in a low-cost and simple one-step process by co-sintering the layers and obtaining a dense layer having the desired density and porous layers which maintain the desired porosity.

Moreover, the present invention advantageously allows to minimize the differences in shrinkage between the porous and dense layers during the cosintering so that cracks, deformation, and defects in the final multilayer ceramic scaffold are avoided and the desired surface flatness (measured in terms of variation of the surface level) is obtained. Indeed, the solution of using a mixture of at least two different pore-forming agents according to the present invention allows to easily adjust the shrinkage of porous layers with the shrinkage of the dense layer so that the co-sintering of multilayer ceramic scaffolds with porous and dense layer is improved and the final material is obtained with the desired surface flatness and without cracks, deformations, defects and/or residual internal stresses.

BRIEF DESCRIPTION OF THE FIGURES

Figures la and lb are photos taken with a digital camera showing the surface flatness and the absence of cracks and surface defects of the porous/dense/porous scaffold of GDC10 as obtained as described in Example 1.

Figure 2 is a graphic of the evolution of the shrinkage of porous and dense layers during co-sintering (dL/Lo% measured with a co-sintering cycle at 1400 °C/2hours (5°C/min)) as described in Example 2.

Figures 3a and 3b are photos taken with a digital camera showing a deformed and uneven surface of the scaffold obtained according to Example 4.

Figures 4a and 4b show examples of multi-layer battery architectures produced by tape casting and co-sintering with adjustment of kinetics and shrinkage during sintering using a formulation based on the use of at least two different pore forming agents.

DETAILED DESCRIPTION OF THE INVENTION

Before the issues of the invention are described in detail, the following should be considered:

It is to be understood that this invention is not limited to particular embodiments described, since such embodiments may, of course, vary. It is also to be understood that the terminology used herein is not intended to be limiting, since the scope of the present invention will be limited only by the appended claims.

As used herein, the singular forms “a”, “an”, and “the” include both singular and plural referents unless the context clearly dictates otherwise. By way of example, “a compound” means one compound or more than one compound.

The terms “comprising”, “comprises”, and “comprised of’ as used herein are synonymous with “including”, “includes” or “containing”, “contains”, and are inclusive or open-ended and do not exclude additional, non-recited members, elements or method steps. It will be appreciated that the terms “comprising”, “comprises” and “comprised of’ as used herein comprise the terms “consisting of’, “consists”, and “consists of’.

Throughout this application, the term “about” is used to indicate that a value includes the standard deviation of error for the device or method being employed to determine the value.

As used herein, the term “average” refers to number average unless indicated otherwise.

As used herein, the terms “% by weight”, “wt.-%”, “weight percentage”, or “percentage by weight”, are used interchangeably. The same applies to the terms “% by volume”, “vol.-%”, “volume percentage”, or “percentage by volume”, or “% by mol”, “mol-%”, “mol percentage”, or “percentage by mol”.

As used herein, the terms “%o by weight” or “wt.-%o” are used interchangeably to indicate the “per mille” (i.e. “per thousand”) amount. The same applies to the terms “%o by volume”, “vol.- %o”, or “%o by mol”, “mol- %o”.

Throughout this application, the expression “pore-forming agent(s)” is used as a synonym of “porogen(s)”.

The recitation of numerical ranges by endpoints includes all integer numbers and, where appropriate, fractions subsumed within that range (e.g. 1 to 5 can include 1, 2, 3, 4 when referring to, for example, a number of elements, and can also include 1.5, 2, 2.75, and 3.80, when referring to, for example, measurements). The recitation of end points also includes the end point values themselves (e.g. from 1.0 to 5.0 includes both 1.0 and 5.0). Any numerical range recited herein is intended to include all sub-ranges subsumed therein.

For the purposes of the present invention, the expression “a suspension free of any pore-forming agents’" means that said suspension does not comprise any pore-forming agents. Analogously, the expression “a multilayer ceramic scaffold free of cracks, deformations and/or defects" means that said scaffold does not comprise any cracks, deformations and/or defects.

All references cited in the present specification are hereby incorporated by reference in their entirety. In particular, the teachings of all references herein specifically referred to are incorporated by reference.

Unless otherwise defined, all terms used in disclosing the invention, including technical and scientific terms, have the meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. By means of further guidance, term definitions are included to better appreciate the teaching of the present invention. In the following passages, different alternatives, embodiments, and variants of the invention are defined in more detail. Each alternative and embodiment so defined may be combined with any other alternative and embodiment, and this for each variant unless clearly indicated to the contrary or clearly incompatible when the value range of a same parameter is disjoined. In particular, any feature indicated as being preferred or advantageous may be combined with any other feature or features indicated as being preferred or advantageous.

Furthermore, the particular features, structures, or characteristics described in the present description may be combined in any suitable manner, as would be apparent to a person skilled in the art from this disclosure, in one or more embodiments. Furthermore, while some embodiments described herein include some but not other features included in other embodiments, combinations of features of different embodiments are mean to be within the scope of the invention, and form different embodiments, as would be understood by those in the art.

The present invention refers to a process for the preparation of a multilayer ceramic scaffold, wherein said scaffold comprises at least a dense layer and at least a porous layer made of a doped ceria ceramic material, preferably a gadolinium-doped ceria (GDC) ceramic material.

Throughout this application, said multilayer ceramic scaffold comprising at least a dense layer and at least a porous layer is also called porous/dense multilayer ceramic scaffold.

The process according to the present invention comprises the steps of: a) preparing a first suspension by mixing a doped ceria powder with a mixture of at least two different pore-forming agents, an organic solvent, a dispersant, and a binder, wherein said pore-forming agents are independently selected from the group consisting of: carbon, an organic polymer, and a combination thereof; b) preparing a second suspension by mixing a doped ceria powder with an organic solvent, a dispersant, and a binder, wherein said second suspension is free of any pore-forming agents; c) assembling a green layer stack comprising at least a porous green layer and at least a dense green layer, wherein the porous green layer is obtained from the first suspension of step a) and the dense green layer is obtained from the second suspension of step b); d) co-sintering the green layer stack obtained in step c).

Throughout this application, said green layer stack comprising at least a porous green layer and at least a dense green layer of step c) is also called a dense/porous green layer stack.

According to a preferred embodiment of the present invention, said multilayer ceramic scaffold comprises a dense layer arranged between two porous layers (also called “porous/dense/porous multilayer ceramic scaffold”) and the process described above is characterized by the fact that step c) comprises assembling a green layer stack comprising a dense green layer arranged between two porous green layers (also called “porous/dense/porous green layer stack”), wherein said porous green layers are obtained from the first suspension of step a) and the dense green layer is obtained from the second suspension of step b).

According to an embodiment of the invention, said dense layer can contain a thinner layer made of a different material than said doped ceria ceramic material, as a barrier layer. According to said embodiment, step c) of the process of the present invention comprises assembling said barrier layer within the dense layer. More specifically, according to said embodiment, the process of the present invention comprises a step b’) of preparing a third suspension by mixing a barrier layer powder material with an organic solvent, a dispersant, and a binder, wherein said third suspension is free of any pore-forming agents. Said barrier layer powder material is preferably doped zirconia, more preferably yttrium-stabilized zirconia.

According to this embodiment, step c) comprises assembling a green layer stack comprising at least a porous green layer, at least a dense green layer and at least a barrier green layer, wherein the porous green layer is obtained from the first suspension of step a), the dense green layer is obtained from the second suspension of step b), and the barrier green layer is obtained from the third suspension of step b’). More preferably, said barrier green layer is assembled with the other layers so to be arranged within the dense green layer.

Preferably, the at least two different pore-forming agents of step a) of the process according to the present invention are independently selected from the group consisting of: carbon black, carbon fibers, graphite, polymethacrylate, rice starch, potato starch, and a combination thereof.

According to an embodiment of the present invention, said mixture of at least two different pore-forming agents is in an amount comprised between 35 vol% and 50 vol%, preferably between 40 vol% and 45 vol%, with respect to the total volume of the first suspension of step a).

Said mixture of at least two different pore-forming agents can preferably comprise a first pore-forming agent in an amount comprised between 3 vol% and 25 vol%, preferably between 5 vol% and 15 vol%, with respect to the total volume of said mixture of pore-forming agents.

Without wishing to be bound to a specific theory, it has been found that using a mixture of at least two different pore-forming agents as disclosed in step a) of the process according to the present invention, and, preferably, in the preferred amounts described above, allows to advantageously adjust the shrinkage of the porous layer(s) to the shrinkage of the dense layer during the cosintering step so that a multilayer ceramic scaffold can be obtained without deformation, cracks, defects and/or internal residual stresses.

According to a particularly preferred embodiment of the present invention, said at least two different pore-forming agents are characterized by at least two different mean particle sizes.

Preferably, said at least two different pore-forming agents comprise a first pore-forming agent characterized by a first mean particle size comprised between 10 and 500 nm, preferably between 10 and 100 nm, and at least a second poreforming agent, different from the first pore-forming agent, characterized by a second mean particle size comprised between 1 and 100 pm, preferably between 1 and 50 pm.

The mean particle size is measured, for the purposes of the present invention, by means of Scanning Electron Microscopy (SEM).

Preferably, said mixture of at least two different pore-forming agents is a mixture of two different pore-forming agents. For the purposes of the present invention, said mixture of two different pore-forming agents is also called a binary mixture.

Preferably, within said binary mixture, the first pore-forming agent is characterized by a mean particle comprised between 10 and 500 nm, more preferably between 10 and 100 nm, and/or the second pore-forming agent, different from the first one, is characterized by a mean particle size between 1 and 100 pm, more preferably between 1 and 50 pm.

According to a particularly preferred embodiment of the present invention, said two different pore-forming agents (constituting the binary mixture) are carbon black and graphite. Preferably, within said binary mixture of carbon black and graphite, the carbon black is in amount comprised between 3 vol% and 25 %, more preferably between 5 vol% and 15 vol% with respect to the total volume of the binary mixture of carbon black and graphite.

Preferably, within said binary mixture of carbon black and graphite, the carbon black is characterized by a mean particle comprised between 10 and 500 nm, more preferably between 10 and 100 nm, even more preferably of about 40 nm and/or the graphite is characterized by a mean particle size between 1 and 100 pm, more preferably between 1 and 50 pm, even more preferably of about 1.5 pm.

According to the present invention, the doped ceria powder is not particularly limited. For the purposes of the present invention, said doped ceria powder is also called “doped ceria ceramic powder”.

According to an embodiment, the doped ceria powder used in the suspensions of steps a) and b) of the process according to the present invention can be the same doped ceria powder or a different one.

Preferably, said doped ceria powder is a gadolinium-doped ceria (GDC) powder. For the purposes of the present invention, said GDC powder is also called “GDC ceramic powder”.

More preferably, said GDC powder is independently selected from the group consisting of a gadolinium-doped ceria powder with 10% gadolinium (GDC 10), a gadolinium-doped ceria powder with 20% gadolinium (GDC20), and a combination thereof.

Preferably, the dispersant used to prepare the suspensions of steps a) and b) of the process according to the present invention is independently selected from the group consisting of: phosphate ester, phosphate poly ether ester, polymethacrylate, polysulfonate, and a combination thereof.

According to a particularly preferred embodiment of the invention, the dispersant used in the first suspension of step a) is a first dispersant (DI) which differs from the second dispersant (D2) used in the second suspension of step b).

Without wishing to be bound to a specific theory, it has been found that it is advantageous that the first dispersant used in the first suspension of step a) is a dispersant (DI) which enables a good dispersion of the ceramic powder in the suspension and has a higher purity than the second dispersant (D2) used in the second suspension of step b) (e.g., DI being a dispersant with a lower content of sodium and potassium impurities than D2). Moreover, it has been found that it is advantageous that the second dispersant (D2) is a dispersant which only partially disperses the ceramic powder in the second suspension of step b) thus leading to the formation of aggregates, and/or that the second dispersant (D2) is a dispersant which contains a higher level of impurities such as sodium and potassium (which can settle on the surface of the ceramic particles in the suspension) than dispersant DI.

Without wishing to be bound to a specific theory, it has been found that both said aggregates and impurities slow the sintering of the layer obtained from said suspension b) and this further contributes to minimize the differences in shrinkage between the porous and dense layers during the co-sintering so that cracks, deformation and/or defects in the final multilayer ceramic scaffold are avoided.

Preferably said dispersant DI is a non-ionic dispersant of the phosphate ester type.

More preferably said dispersant DI is a dispersant of formula RO-PO2R’- OH, wherein R’ is H or R, R is A-(O-CH2-CH2)x-, wherein A is -CnEhn+i with n = 1 to 11 or wherein A is -(CH2-C_n’H2n’+i-O)_m with n’ = 1 to 4.

According to an embodiment of the invention, said dispersant DI is preferably a dispersant having the commercial name Beycostat-C213.

Preferably said dispersant D2 is a dispersant of the phosphate polyether ester potassium salt type.

More preferably, said dispersant D2 is a dispersant of formula C6H6O4P.CH3.K.

According to an embodiment of the invention, dispersant D2 is preferably a dispersant having the commercial name Triton™ H-66 from Dow.

According to the present invention the binder is not particularly limited and is preferably independently selected from polyvinyl butyral, polymethacrylate, or a combination thereof. As before, the binder used in the first suspension of step a) and in the second suspension of step b) can be the same binder of a different one.

Preferably, the first and second suspensions of steps a) and b) of the process according to the present invention further comprise a plasticizer. Also in case of said plasticizer, this is not particularly limited and it is preferably independently selected from the group consisting of: dibutyl phthalate, polyethylene glycol (PEG), glycerol, or a combination thereof. As before, the plasticizer that can be used in the first and second suspensions of steps a) and b) can be the same plasticizer of a different one.

According to the present invention, the organic solvent used for the first and second suspensions of steps a) and b) is not particularly limited and can be the same for both suspensions or a different one. Preferably, said organic solvent is a methyl-ethyl-ketone (MEK)/ab solute ethanol azeotropic mixture, preferably an azeotropic mixture of 60 vol% of methyl-ethyl-ketone and 40 vol% of absolute ethanol.

The suspensions according to any of the embodiments described before can be obtained by: i) milling the (ceramic) powder in the organic solvent with the dispersant and the binder; ii) optionally adding a plasticizer; and iii) in case of the suspension of step a), addition to the resulting slurry of steps i) and, optionally, ii), the mixture of at least two different poreforming agents followed by additional organic solvent to adjust the viscosity.

According to an embodiment of the invention, the dense/porous green layer stack of step c) of the process as described above is preferably characterized by a thickness of the at least a porous green layer comprised between 20 and 150 pm and/or by a thickness of the at least a dense green layer comprised between 20 and 150 pm.

According to another embodiment of the invention, the porous/dense/porous green layer stack of step c) of the process as described above is preferably characterized by a thickness of the first porous green layer comprised between 150 and 900 pm, a thickness of the dense green layer comprised between 20 and 30 pm, and a thickness of the second porous green layer comprised between 20 and 30 pm.

Preferably, the porous layer of step c) according to the process of the invention is obtained by casting, preferably tape-casting the first suspension of step a).

Preferably, the dense layer of step c) according to the process of the invention is obtained by casting, preferably tape-casting the second suspension of step b).

In other words, according to a preferred embodiment of the present invention, step c) is a step of assembling a green layer stack comprising at least a porous green layer obtained by casting, preferably tape-casting the first suspension of step a) and at least a dense green layer obtained by casting, preferably tape-casting the second suspension of step b).

Analogously, according to the embodiment described above wherein step c) is a step of assembling a green layer stack comprising a dense green layer arranged between two porous green layers, said porous green layers are preferably obtained by casting, more preferably tape-casting the first suspension of step a), and said dense green layer is preferably obtained by casting, more preferably tape-casting the second suspension of step b).

Without wishing to be bound to a specific theory, it has been found that tape-casting is particularly preferred as it allows assembling and shaping the green layer stack (and, thus, the multilayer ceramic scaffold obtained after cosintering said green layer stack) in one single step. Furthermore, tape-casting also allows controlling the thickness of the porous and dense layers and the interface quality between them (i.e., between the porous and the dense layers).

This in turn allows preparing multilayer ceramic scaffolds and unit solid oxide cells comprising a dense electrolyte layer a with the desired low thickness (i.e., thin dense electrolyte layer) and thicker porous electrode layers, likely corresponding to the best compromise between good mechanical properties and high electrochemical performances of the cells.

Step c) of the process according to the present invention preferably comprises thermocompressing the green layer stack according to any of the embodiments described above. Said thermocompressing is preferably carried out at a temperature comprised between 80°C and 120 °C and, preferably, under a pression comprised between 20 MPa and 150 MPa.

According to an embodiment of the invention, said dense layer can contain a thinner layer made of a different material than said doped ceria ceramic material, as a barrier layer. According to said embodiment, step c) of the process of the present invention comprises assembling said barrier layer within the dense layer. More specifically, according to said embodiment, the process of the present invention comprises a step b’) of preparing a third suspension by mixing a barrier layer powder material with an organic solvent, a dispersant, and a binder, wherein said third suspension is free of any pore-forming agents. Said barrier layer powder material is preferably doped zirconia, more preferably yttrium-stabilized zirconia. According to this embodiment, step c) comprises assembling a green layer stack comprising at least a porous green layer, at least a dense green layer and at least a barrier green layer, wherein the porous green layer is obtained from the first suspension of step a), the dense green layer is obtained from the second suspension of step b), and the barrier green layer is obtained from the third suspension of step b’). More preferably, said barrier green layer is assembled with the other layers so to be arranged within the dense green layer.

According to an embodiment, the process according to the present invention comprises, after step c) and before step d), a step c’) of debinding the green layer stack.

Preferably, said debinding step c’) is carried out at a temperature comprised between 400 and 900 °C.

According to an embodiment, step d) of the process according to the present invention is preferably carried out at a temperature lower than 1500 °C, preferably between 1350 °C and 1500 °C.

Another object of the present invention is the use of a suspension comprising a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder, and a mixture of at least two different pore-forming agents, in the presence of a dispersant and a binder, to reduce the difference in shrinkage during co-sintering of a green layer stack comprising at least a porous green layer and at least a dense green layer, wherein the porous green layer is obtained from said suspension (also called, for the purposes of the present invention a “suspension forming the porous green layer(s)” and/or “porous suspension”) and the dense layer is obtained from a second suspension (also called, for the purposes of the present invention, a “suspension forming the dense green layer” and/or “dense suspension”) comprising a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder in the presence of a dispersant and a binder, wherein said second suspension is free of any pore-forming agents.

According to an embodiment, the present invention relates to the use of a suspension comprising a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder and a mixture of at least two different pore-forming agents in the presence of a dispersant and a binder, to reduce the difference in shrinkage during co-sintering of a green layer stack comprising a dense green layer arranged between two porous green layers, wherein the porous green layers are obtained from said suspension (also called, for the purposes of the present invention a “suspension forming the porous green layer(s)” and/or “porous suspension”) and the dense layer is obtained from a second suspension (also called, for the purposes of the present invention, a “suspension forming the dense green layer” and/or “dense suspension”) comprising a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder in the presence of a dispersant and a binder, wherein said second suspension is free of any poreforming agents.

Preferably, according to both embodiments, said suspensions, the doped ceria powder, dispersant, binder, organic solvent, pore-forming agents, and green layer stack along with the porous green layers and dense green layer and the cosintering conditions, are as described above according to any one of the previously described embodiments of the present invention.

According to a particularly preferred embodiment, the dispersant of the porous suspension differs from the dispersant of the dense suspension.

Preferably, the dispersant of the porous suspension is a first dispersant (DI) which differs from the dispersant of the dense suspension which is a second dispersant (D2) according to any of the above-described embodiments.

Without wishing to be bound to a specific theory, it has been advantageously found that the use of said suspension comprising at least two- different pore-forming agents according to the present invention, allows to reduce the difference(s) in shrinkage during co-sintering of a green layer stack comprising dense and porous layers or, in other words, to adjust the shrinkage of the porous layers to that of the dense layer so that the difference(s) in shrinkage between the dense and porous layers are minimized (or even removed). Without wishing to be bound to a specific theory, it has been found that by employing the above-described dispersants and, preferably, a first dispersant for the “porous suspension” forming the porous green layer(s) and a second dispersant different from the first dispersant for the “dense suspension” forming the dense green layer, the shrinkage differences between the dense and porous green layers can be further advantageously minimized.

The present invention also relates to the process for the preparation of a unit solid oxide cell (SOC) comprising a dense layer which is the electrolyte layer, arranged between two porous layers, which are the electrode layers, wherein said dense and porous layers are made of a doped ceria ceramic material, preferably of a gadolinium-doped ceria (GDC) ceramic material and wherein said porous layer have a larger thickness than the dense layer. Said process comprises the step of: e) impregnating the porous layers of the scaffold obtained in step d) of the process according to the present invention as described above according to any one of the embodiments, with a catalyst so to obtain composite electrode layers.

Analogously, said process can be used for the preparation of a half-cell comprising at least a dense layer which is the electrolyte layer and at least a porous layer which is the electrode layer.

The composite electrode layers are composites of doped ceria, preferably of GDC, and the catalyst.

Preferably, the amount of the catalyst within the composite electrode layers is of at least 30% by mass (for each layer). Without wishing to be bound to a specific theory, it has been found that the porosity of the porous layers of the scaffold obtained with the process of the present invention allows the effective infiltration of the catalyst within the scaffold in terms of amount and distribution which in turn results to achieve the desired electrochemical properties.

Said catalyst used for impregnating the porous layers of the scaffold is in form of an aqueous solution comprising a precursor of the catalyst, preferably in the form of a salt, more preferably in the form of a nitrate.

According to an embodiment, said catalyst is the same catalyst for both porous layers or a different catalyst for each of the two porous layers.

According to an embodiment, said catalyst is praseodymium(III,IV) oxide (PrO_x), wherein x = 1.5 - 2, preferably PrCh, or perovskite type materials, preferably selected from the group consisting of: simple type, double type, Ruddlesden-Popper type materials, or a combination thereof; and the precursor thereof used for infiltrating the porous layer(s) of the scaffold are metallic salts thereof, preferably nitrates.

Preferably, said simple type perovskite materials are selected from the group consisting of: LSM ((Lao.8oSro.2o)o.95Mn03-s), LSC (Lai-xSrxCoCh-s), LSFC (Lao,6Sro,4Coi-_xFe_x03-5), or a combination thereof.

Preferably, said Ruddlesden-Popper type materials are nickelates.

The resulting composite electrode layer is a doped ceria/catalyst composite (oxygen electrode layer, cathode). According to said embodiment once impregnated with said precursor of the catalyst (which is a cathode-forming catalyst), the porous layer is calcinated so to obtain an intermediate oxide and subsequently annealed to yield the desired composite electrode layer. According to another embodiment, said catalyst is selected from the group consisting of: Ni, Cu, or a combination thereof; and the precursor thereof used for infiltrating the porous layer is nickel nitrate or copper nitrate, respectively.

The resulting composite electrode layer is a doped ceria/Ni or doped ceria/Cu composite (hydrogen electrode layer, anode). According to said embodiment once impregnated with said precursor of the catalyst (which is an anode-forming catalyst), the porous layer is subjected to reduction under hydrogen to yield the desired composite electrode layer.

According to an embodiment, the process of the present invention is a process for the preparation of a symmetrical solid oxide cell wherein said step e) is a step of impregnating both porous layers with the same type of catalyst, preferably with a cathode-forming or anode-forming catalyst as described above, so to obtain composite electrode layers, preferably composite cathode or anode layers.

According to an embodiment, the process of the present invention is a process for the preparation of an asymmetrical solid oxide cell, wherein said step e) comprises: e') impregnating a first porous layer with a catalyst which is a cathodeforming catalyst so to obtain a composite cathode layer and e”) impregnating the second porous layer with a catalyst which is an anode-forming catalyst so to obtain a composite anode layer.

The present invention further concerns a multilayer ceramic scaffold obtainable or obtained by the process as described above.

According to an embodiment of the present invention, said multilayer ceramic scaffold comprises at least a porous layer and at least a dense layer.

According to another embodiment, said multilayer ceramic scaffold comprises a dense layer arranged between two porous layers.

Preferably, the dense layer has a density higher than 95% of the theoretical density, more preferably, higher than 98% of the theoretical density.

The at least a porous layer of the scaffold according to the present invention is also preferably characterized by a total porous volume of at least 35 vol%, preferably between 40 vol% and 45 vol%, as measured by SEM. Preferably, in the embodiment wherein the multilayer ceramic scaffold comprises a dense layer arranged between two porous layers, said two porous layers are both characterized by a total porous volume of at least 35 vol%, preferably between 40 vol% and 45 vol%, as measured by SEM. The total porous volume is expressed relatively to the total volume of the layer.

The porous volume can notably be calculated by applying a color contrast on the SEM picture to discriminate pores from the material on the picture and determining the percent volume occupied by the pores identified on the picture. Especially, the porous volume may be determined as an average value on the basis of at least 3 SEM pictures.

More preferably, the at least a porous layer of the scaffold according to the present invention is characterized by an interconnected porosity, as measured by SEM. Preferably, in the embodiment wherein the multilayer ceramic scaffold comprises a dense layer arranged between two porous layers, said two porous layers are both characterized by an interconnected porosity, as measured by SEM. Such interconnected porosity may notably be evidenced by a simple observation of the pores network on the SEM picture.

Without wishing to be bound to a specific theory, it has been found that the porosity in the porous layer is produced after removal of the pore-forming agent occurring during the debinding step of the process according to the present invention.

According to a particularly preferred embodiment of the invention, said at least two different pore-forming agents are two different pore-forming agents, preferably carbon black and graphite, wherein the first pore-forming agent, preferably carbon black, has a mean particle size comprised between 10 and 500 nm, preferably between 10 and 100 nm, more preferably of about 40 nm as described above and wherein the second pore-forming agent, preferably the graphite a mean particle size comprised between 1 and 100 pm, preferably between 1 and 50 pm, more preferably of about 1.5 pm. According to such embodiment, the porous layer of the multilayer ceramic scaffold obtained or obtainable with the process of the present invention is characterized by a porosity deriving from the pore-forming agents, preferably carbon black and graphite. More preferably, said porosity is characterized by a pore size between 300 nm and 600 nm, preferably between 350 nm and 600 nm, more preferably of about 500 nm measured by SEM, and, even more preferably, by an interconnected porosity, also measured (evidenced) by SEM. The size of the pores may notably be determined by SEM, in particular by applying a color contrast to discriminate the pores from the material. The average size of the pores may then calculated from the measurement of each pore size on the picture; the largest dimension of the pore may be used for each. To get a statistical analysis, at least 3 SEM pictures may be used.

According to the invention, the multilayer scaffold is characterized by a surface flatness which is measured by a variation of the surface level of < 200 nm as measured by a mechanical profilometer on a 3 cm scanning length.

Without wishing to be bound to a specific theory, it has been found that the process according to the present invention, and in particular the use of at least two different pore-forming agents in the first suspension of step a) (from which the at least a porous layer is obtained) allows minimizing the difference in shrinkage of the porous and dense layers during co-sintering so that the final multilayer ceramic scaffold can be obtained with a desired surface flatness and free (or substantially free) of cracks, deformations and/or defects.

According to a preferred embodiment, the use of a mixture of at least two different pore forming agents in the formulation of layers to produce electrode materials is particularly well suited to the manufacture of all-solid state batteries for energy storage. As shown in the example in Figure 4, the battery can be made up of a three-layer architecture (anode/electrolyte/cathode or anode/solid separator+liquid electrolyte/cathode) which can be produced by layer casting or calendering, followed by a step of consolidating the crude object by co-sintering, which is generally the critical manufacturing step in the process. The advantage of using at least two different pore-forming agents in the layer formulation is that the shrinkage of the electrode materials can be adjusted and controlled to avoid deformation or micro-cracking of the battery during or after the co-sintering stage.

The present invention will now be illustrated by the following examples, which are not intended to be limiting.

EXAMPLES

Materials and methods

All starting materials used in the examples are commercially available. Example 1 - GDC10 scaffold of porous-dense-porous layers

The tape-casting suspensions of GDC 10 powder containing or not poreforming agents (porous suspension and dense suspension, respectively) were prepared by planetary milling in a 45 ml zirconia jar containing six zirconia balls of 1 mm diameter and in the azeotropic mixture of 60 vol% of methyl-ethyl- ketone and 40 vol% of absolute ethanol.

For the suspension without pore-forming agents (dense suspension), the first step was to mix the powder in the solvent with a first dispersant DI (of the non-ionic phosphate ester type) for 2:30 hours at 200 rpm. The second step was to add to the slurry the binder (polyvinyl butyral, PVB B90, from Brenntag) and the plasticizer (dibutyl phthalate, DBP, from Sigma Aldrich) and to mix for 19 hours at 120 rpm.

For the suspension containing pore-forming agents (porous suspension), the steps were the same except that the dispersant used was a second dispersant D2 (of the phosphate poly ether ester potassium salt type), different from DI, and a third step was added 3 hours after the second one, which was the addition of the pore-forming agents (graphite 4827 from Asbury Carbons and Carbon Super P from Sigma Aldrich) and additional solvent to adjust the viscosity.

In the following tables (Table 1 and 2) is reported the composition of the dense and porous suspensions used to produce the multilayer ceramic scaffold according to the process of the present invention. The compositions of the suspensions are characterized by the following parameters in Table 1 : the loading rate (%ioadin_g = V_pOwder / Vtotai wherein Vpowder denotes the volume of GDC 10 powder and pore forming agent if any and Vtotai denotes the total volume of the suspension), the organic rate (%_org = V_org / V_org+_Powder wherein V_org denotes the volume of organic compounds and V_Or_g+_Powder denotes the volume of organic compounds, of GDC 10 powder, and pore forming agents, if any, in the suspension), the dispersant content (Cdis_P = mdi_sp / m_powder wherein mdis_P denotes the mass of dispersant and m_powder the mass of GDC 10 powder and pore forming agents if any, in the suspension), the binder/plasticizer ratio (L/P = Vbinder / Vpiast wherein Vbinder denotes the volume of binder and V_piast the volume of plasticizer in the suspension), the porogen’s rate (%_poro = V_poro / V_powder wherein V_poro denotes the volume of pore forming agents, if any, and V_powder denotes the volume of GDC 10 powder and pore forming agents, if any, in the suspension) and the volume ratio of the Carbon in the volume total of porogen (%_carbon in porogen Vcarbon / Vporo wherein Vcarbon denotes the volume of Carbon Super P and Vporo denotes the volume of pore forming agents, if any, in the suspension). In addition, the compositions of the suspensions are characterized by their masses in Table 2.

Table 1

Table 2 *D1= dispersant of the non-ionic phosphate ester type

**D2= dispersant of the phosphate poly ether ester potassium salt type

Subsequently, the suspensions were recovered and rotated for 24 hours.

Their viscosity was 1 Pa. s'¹ under the shear stress of the casting. After casting the suspensions at 1.6 cm. s'¹ with a double blade of 300 pm and 80 pm high for the porous one and 80 pm for the dense one, the tapes (i.e., layers) were dried under confined atmosphere at a temperature inferior to 17 °C.

Thermocompression of the disks of green tape

The dense green tape (dense green layer), with a thickness of approximately 30 pm, and the porous one, with a thickness of approximately 100 pm, were punched into 40-mm-diameter disks. A stack of one disk of the dense green tape placed on the top of nine disks of the porous green tape was thermocompressed (i.e., laminated) at 90 °C under 60 MPa. Without wishing to be bound to a specific theory, it has been found that these conditions allow a good sintering between the disks in the following co-sintering step. Then, the porous green tape, with a thickness of 40 pm, was punched into 24-mm-diameter disks and one of them was laminated in the same manner on the dense layer of the stack.

Debinding and sintering

The debinding of the green stack was performed at 800 °C during 6 hours in air with a heating rate of 0.3 °C/min to slowly remove all organic additives and finally sintered at 1475 °C during 2 hours in air with a heating rate of 5 °C/min and 2.3 g/cm² of the stack.

The obtained porous/dense/porous scaffold of GDC 10 was flat, without cracks (as shown in Figures la and lb) and with a relative density of the dense layer higher than 95% of the theoretical density and a porous volume in the porous layer of approximately 45 vol%, which was measured by SEM.

The (surface) flatness of the porous/dense/porous scaffold was measured by mechanical profilometer method by measuring the variation of the surface level on a 3 cm scanning length. The obtained porous/dense/porous scaffold had a variation of the surface level less than 200 nm.

Example 2 - Impact of the mixture of pore-forming agents

The impact of the use of a mixture of two different pore-forming agents as described in Example 1 on the shrinkage of the porous and dense layers during co-sintering has been measured, in particular the variation in one direction of the measured pellet (dL/Lo%, wherein Lo denotes the initial pellet’s length and dL denotes the difference between the pellet’s length after a co-sintering cycle at 1400 °C/2hours (5°C/min) and the initial pellet’s length Lo). Different multilayer ceramic scaffolds obtained with different suspensions have been compared.

In the following table (Table 3), the compositions of the various suspensions used in the comparative tests are reported. These suspensions have been produced by repeating the steps of Example 1 (but by varying the pore-forming agents, in particular, by varying the amount of carbon black (C) and graphite). These suspensions have been tape-casted to obtain porous green layers and cosintered with a dense layer obtained by tape-casting a dense suspension (without pore-forming agents) as produced in Example 1. The shrinking of the dense and porous layers during co-sintering has been measured, in particular the variation in two directions of the measured pellet: and the results are shown in Table 3.: variation in the thickness “e” of the pellet, noted de/eo% (wherein eo denotes the initial pellet’s thickness and de denotes the difference between the pellet’s thickness after a co-sintering cycle at 1400 °C/2hours (5°C/min) and the initial pellet’s thickness eo), and the variation in the diameter “o” of the pellet, noted do/00% ‘wherein oo denotes the initial pellet’s diameter and do denotes the difference between the pellet’s diameter after a co-sintering cycle at 1400 °C/2hours (5°C/min) and the initial pellet’s diameter Oo). The results are shown in Table 3.

Table 3

*D1= dispersant of the non-ionic phosphate ester type

**D2= dispersant of the phosphate poly ether ester potassium salt type

These results are also shown in Figure 2 and demonstrate that with the suspension comprising two different pore-forming agents, the shrinkage differences between the porous and dense layers are advantageously minimized. Furthermore, without wishing to be bound to a specific theory, it has been also found that with a 7% vol of carbon black with respect to the mixture of carbon and graphite, the best results in term of reduction of shrinkage differences are obtained.

Example 3 - Comparative example: suspension containing only one type of pore-forming agent

This example follows the same preparation as described in Example 1 but with only one type of pore-forming agent (graphite). In the following table (Table 4) is reported the composition of the dense and porous suspensions used to produce the multilayer ceramic scaffold.

Table 4

*D1= dispersant of the non-ionic phosphate ester type

**D2= dispersant of the phosphate poly ether ester potassium salt type

The green stack in GDC 10 was sintered at 1400 °C during 2 hours in air with a heating rate of 5 °C/min and 2.3 g/cm² of the stack. The obtained scaffold was cracked after sintering, which supports the invention regarding the benefice of the mixture of two different pore-forming agents.

Example 4 - Comparative example: only one type of dispersant in both porous and dense layers

This example follows the same preparation as described in Example 1 but with the same type of dispersant (DI) for both dense and porous layers. In the following table (Table 5) is reported the composition of the dense and porous suspensions used to produce the multilayer ceramic scaffold.

Table 5

*D1= dispersant of the non-ionic phosphate ester type The green stack in GDC 10 was sintered at 1400 °C during 2 hours in air with a heating rate of 5 °C/min and 2.3 g/cm² of the stack. The obtained scaffold was deformed after sintering (Figure 3), which supports the invention regarding the benefice of the mixture of pore-forming agents combined with two different types of dispersants.

The surface flatness of the scaffold was measured according to the same method as described in Example 2 and the variation of the surface level was of 1.24 mm +/- 0.02 mm (measured with a caliper).

Claims

C L A I M S

1. A process for the preparation of a multilayer ceramic scaffold, wherein said scaffold comprises at least a dense layer and at least a porous layer made of a doped ceria ceramic material, preferably a gadolinium-doped ceria (GDC) ceramic material, said process comprising the steps of: a) preparing a first suspension by mixing a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder with a mixture of at least two different pore-forming agents, an organic solvent, a dispersant, and a binder, wherein said pore-forming agents are independently selected from the group consisting of: carbon, an organic polymer, and a combination thereof, preferably carbon black, carbon fibers, graphite, polymethacrylate, rice starch, potato starch, and a combination thereof; b) preparing a second suspension by mixing doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder with an organic solvent, a dispersant, and a binder, wherein said second suspension is free of any pore-forming agents and wherein said dispersant is a different dispersant than the dispersant used in the first suspension of step a); c) assembling a green layer stack comprising at least a porous green layer and at least a dense green layer, wherein the porous green layer is obtained from the first suspension of step a) and the dense green layer is obtained from the second suspension of step b); d) co-sintering the green layer stack obtained in step c).

2. The process according to claim 1 wherein the multilayer ceramic scaffold comprises a dense layer arranged between two porous layers and wherein said step c) comprises assembling a green layer stack comprising a dense green layer arranged between two porous green layers, wherein said porous green layers are obtained from the first suspension of step a) and the dense green layer is obtained from the second suspension of step b).

3. The process according to claim 1 or 2, wherein said mixture of at least two different pore-forming agents is in an amount comprised between 35 vol% and 50 vol%, preferably between 40 vol% and 45 vol% with respect to the total volume of the first suspension.

4. The process according to any one of claims 1 to 3, wherein said mixture of at least two different pore-forming agents comprises a first pore-forming agent in an amount comprised between 3 vol% and 25 vol%, preferably between 5 vol% and 15 vol% with respect to the total volume of said mixture of pore-forming agents.

5. The process according to any one of the preceding claims wherein said at least two different pore-forming agents are characterized by at least two different mean particle sizes, preferably wherein said at least two different pore-forming agents comprise at least a first pore-forming agent characterized by a first mean particle size comprised between 10 and 500 nm, preferably between 10 and 100 nm, and at least a second poreforming agent, different from the first pore-forming agent, characterized by a second mean particle size comprised between 1 and 100 pm, preferably between 1 and 50 pm.

6. The process according to any one of the preceding claims, wherein said mixture of at least two different pore-forming agents is a mixture of two pore-forming agents, preferably carbon black and graphite, more preferably wherein the carbon black is in amount comprised between 3 vol% and 25 vol%, even more preferably between 5 vol% and 15 vol% with respect to the total volume of the mixture of carbon black and graphite.

7. The process according to any one of the preceding claims, wherein the dispersant of the first suspension of step a) and the dispersant of the second suspension of step b) are independently selected from the group consisting of: phosphate ester, phosphate poly ether ester, polymethacrylate, polysulfonate, and a combination thereof.

8. The process according to any one of the preceding claims, wherein the first and second suspension of steps a) and b) further comprise a plasticizer.

9. The process according to any one of the preceding claims, wherein the suspension is obtained by: i) Milling the powder in the organic solvent with the dispersant and the binder; ii) Optionally adding a plasticizer; iii) In case of the first suspension of step a), adding to the resulting slurry, the mixture of at least two different pore-forming agents and additional organic solvent.

10. The process according to any one of the preceding claims, therein the porous green layer of step c) is obtained by casting, preferably tapecasting the first suspension of step a), and the dense green layer is obtained by casting, preferably tape-casting the second suspension of step b).

11. The process according to any one of the preceding claims, wherein step c) comprises thermocompressing the green layer stack, preferably at a temperature between 80 °C and 120 °C and under a pression of 20 MPa to 150 MPa.

12. The process according to any one of the preceding claims comprising after step c) and before step d), a step c’) of debinding the green layer stack, preferably at a temperature between 400 °C and 900 °C and/or said step d) is preferably performed at a temperature lower than 1500 °C, more preferably between 1350 °C and 1500 °C.

13. Use of a suspension comprising a doped ceria powder, preferably a gadolinium-doped ceria (GDC) powder, and a mixture of at least two different pore-forming agents, in the presence of a dispersant and a binder, to reduce the difference in shrinkage during co-sintering of a green layer stack comprising at least a porous green layer and at least a dense green layer, wherein the porous green layer is obtained from said suspension and the dense layer is obtained from a second suspension comprising a doped ceria powder, preferably a GDC powder, in the presence of a dispersant and a binder, wherein said second suspension is free of any pore-forming agents and wherein said dispersant of said second suspension forming the dense green layer is preferably a different dispersant than the dispersant of the suspension forming the porous green layer.

14. A process for the preparation of a unit solid oxide cell (SOC) comprising a dense layer, which is the electrolyte layer, arranged between two porous layers, which are the electrode layers, wherein said dense and porous layers are made of a doped ceria ceramic material, preferably a gadolinium-doped ceria (GDC) ceramic material and wherein said porous layer have a larger thickness than the dense layer, said process comprising: e) impregnating the porous layers of the scaffold obtained after step d) of the process according to any one of claims 2 to 12 with a catalyst so to obtain composite electrode layers.

15. A multilayer ceramic scaffold obtainable or obtained by the process according to any one of the claims 1 to 12, comprising at least a porous layer and at least a dense layer, wherein said at least a porous layer is characterized by a total porous volume of at least 35 vol% and wherein said scaffold is characterized by a surface flatness measured by a variation of the surface level of < 200 nm.