Ceramic inter-electrode separator

Abstract

Disclosed are robust, thin, flexible and / or porous ceramic inter-electrode separators comprising porous metal substrates, that are suitable for use in electrochemical cells, and methods of fabrication thereof. In one example there is disclosed a ceramic inter-electrode separator for an electrochemical cell, comprising a porous metal substrate and a porous sintered bed comprising metal oxide particles of at least one metal oxide, wherein the metal oxide particles are attached to each other and to the porous metal substrate. The porous sintered bed, taken in cross-section, has an overall thickness of less than 0.20 mm. The separator is flexible having a bend diameter of less than 6 cm. The porous sintered bed has a porosity in the range of 55% to 95%. The metal oxide particles can comprise a single metal oxide, with one or more distinct particle size, or can comprise two or more different metal oxides having distinct particle sizes.

Description

CERAMIC INTER-ELECTRODE SEPARATOR TECHNICAL FIELD

[0001] The invention broadly relates to novel inter-electrode separators for use in electrochemical cells. Example embodiments of the invention more particularly relate to robust, thin, flexible, and porous ceramic inter-electrode separators incorporating porous metal support structures that are suitable for use in electrochemical cells with zero-gap architectures. BACKGROUND

[0002] Many commercial electrochemical cells employ a ‘zero-gap’ cell architecture, in which two electrodes are tightly compressed and sandwiched up against opposite sides of a thin inter-electrode separator. A separator may also be referred to as a ‘spacer’, an ‘electrode spacer’, ‘a porous spacer’, a ‘porous electrode spacer’, a ‘membrane spacer’, a ‘membrane electrode spacer’, ‘a porous membrane spacer’, a ‘porous membrane electrode spacer’, a ‘membrane separator’, a ‘membrane electrode separator’, ‘a porous membrane separator’, a ‘porous membrane electrode separator’, and in other terms not listed here. The combination of the two electrodes and the separator may typically be referred to as a ‘membrane electrode assembly’, or an MEA. In this work, the combination of the two electrodes and the separator may also be referred to as the ‘electrode-separator-electrode’ assembly.

[0003] An inter-electrode separator is typically a thin sheet membrane, diaphragm or ionomer that has the unusual property that it may have inherently high ionic conductance, or may be porous and filled or imbued with a liquid solution (known as an ‘electrolyte’) having a high ionic conductance, while simultaneously displaying very high resistance to electron transport; i.e. it simultaneously exhibits very high electrical resistance. The high electrical resistance is needed to prevent an electrical current flowing between the electrodes, which would comprise a short- circuit of the cell. The high ionic conductance is needed to ensure that ions (i.e. charged atoms or molecules) can readily flow between the electrodes during the electrochemical reaction that is carried out by the cell. Such ions may flow via specially created ion-transport pathways in the separator, or via a liquid electrolyte that fills or is imbued inside the structure of the separator, where that liquid allows ions to move through it.

[0004] Examples of commercially-available inter-electrode separators that are widely used in, for example, hydrogen-generating water electrolysis cells or hydrogen-oxygen fuel cells, include but are not limited to: (1) Agfa’s Zirfon PERL®UTP 500, which is used as an inter- electrode separator in, for example, alkaline electrolysis cells; (2) Chemours’ Nafion®membrane separators such as Nafion 115 or Nafion 117 membrane separators, which are used, for example, in commercial Polymer Electrolyte Membrane (PEM) electrolysis cells and fuel cells; (3) Dioxide Materials’ X37-50, which is an anion exchange membrane (AEM) separator used in anion exchange membrane (AEM) electrolysis cells or fuel cells, and (4) asbestos diaphragms and cloths of the type manufactured by companies like Raybestos, whose use was discontinued many years ago due to safety concerns.

[0005] Separators such as those described in (1)-(3) above typically comprise of polymeric materials and may incorporate inorganic particles. Thus, for example, Zirfon PERL®UTP 500 comprises a porous polysulfone-zirconia structure that becomes filled with a highly ionically conductive alkaline liquid electrolyte during operation and thereby becomes itself highly ionically conductive. By contrast, the polymeric Nafion and AEM separators contain ionizable chemical groups that facilitate ion migration from one side of the separator to its other side. That is, the Nafion and AEM separators comprise a polymeric chemical structure that makes them inherently ionically conductive. Nafion and AEM separators are typically dense and non-porous.

[0006] Due to the polymeric nature of modern separators, they can typically only be employed in electrochemical cells operating below certain maximum temperatures, in the range 60-110oC. However, to achieve the highest possible electrochemical performance, many electrochemical cells would ideally operate at higher temperatures, for example at 110-250oC. In this temperature range the polymeric separators would degrade and fail. For this reason, there has been interest in developing fully inorganic sheet membranes that may serve as inter-electrode separators for electrochemical cells capable of operating at more elevated temperatures. Of particular interest are ‘ceramic’ inter-electrode membrane separators that are capable of operating at 110-250oC. The term ‘ceramic’ is used herein to describe materials that are capable of withstanding very high temperatures, for example above 600oC, without discernible degradation. As such ‘ceramic’ inter-electrode separators cannot, by definition, comprise ionizable groups that facilitate ion migration, they must instead rely on a porous structure that can contain and imbue a liquid electrolyte having a high ionic conductivity.

[0007] US 4,559,124 describes the historical development of porous inter-electrode separators incorporating metal support structures, that may be ceramic in character. It also describes and claims nickel oxide-based ceramic separators, with or without incorporation of titanium dioxide, that can be used to separate the electrodes in alkaline electrolysis cells. By virtue of being able to imbue an ionically conductive liquid electrolyte in their porous structure, such inter-electrode separators displayed the high ionic conductance needed in an inter-electrode separator. However, the outer surfaces of these separators, that contact the electrodes were ceramic in nature, and electrically insulating, meaning that these separators also displayed the high electrical resistance that is needed. Their fabrication process involved packing nickel powder, with or without titanium powder, about a nickel net and then heating and sintering the resulting mass in an oxygen-containing atmosphere at 700-1200oC. In the process, the nickel / titanium particles were sintered together and the outer surfaces of the nickel or titanium became coated with a ceramic layer comprising their corresponding Ni oxide / Ti oxide, both of which are electrically insulating. This resulted in the separators displaying a very high overall electrical resistance, in the MΩ range.

[0008] As noted above, it is critically important that an inter-electrode separator in an electrochemical cell is electrically insulating, i.e. electrons must never be able to move through the separator. This is needed to prevent the electrodes that are sandwiched tightly against the opposite sides of the inter-electrode separator, from short-circuiting with each other. If, at any point in time, or at any location on the separator, an electrical circuit flows through the separator, and electrons move from one electrode to the other through the separator, then a short-circuit current will pass between the electrodes. Such a short-circuit current may severely damage the cell or create a serious safety hazard, for example in the form of an electrical arc. Such an electrical arc may ignite any combustible materials present, such as, for example, the hydrogen gas that is produced in an electrolyser or used in a fuel cell. A hydrogen detonation, in the presence of pure oxygen, which is also produced in an electrolyser and may be used in a fuel cell, may constitute a catastrophic event.

[0009] A critical issue with the nickel oxide-based ceramic inter-electrode separators disclosed in US 4,559,124 (and, in fact, with all other such ceramic inter-electrode separators developed to date) is that, while all the outer surfaces of the separators are electrically non- conductive, those surfaces are underlaid by metals (nickel / titanium) that are electrically highly conductive. The underlying metals comprise a body of the original Ni or Ti powder which is notconverted into its corresponding electrically insulating metal oxide during heating to 700-1200oC under an oxygen-containing atmosphere. Any cracking or fracturing or crumbling or collapsing or flaking off or scraping off, or loss, diminution, degradation, or corrosion, of the non-conducting, ceramic outer surface layers, at any location where the separator contacted an electrode during operation of the cell, therefore risks direct metal-to-metal contact between the electrodes via the separator through which electrons could move, creating a short circuit. Such a separator-facilitated short circuit could have exceedingly serious safety and other implications if it were used in an electrolysis cell that produced hydrogen and oxygen, or a fuel cell that used hydrogen and oxygen. For this reason, the commercial electrolyser industry does not currently use such ceramic inter- electrode separators.

[0010] The possibility that portions of the non-conducting surface layer on the ceramic inter- electrode separator would crack or fracture or crumble or collapse or flake off or be scraped off or would be lost, diminuted, degraded, or corroded, during operation of the cell, also limits the thickness and flexibility of the separators. Thus, it was not possible to fabricate workable inter- electrode separators that were very thin overall. It also limited the flexibility of such ceramic inter- electrode separators to relatively large bend diameters. That is, separators could not be bent around a cylindrical structure having a specified diameter or less without their non-conducting surface layer losing its structural integrity in at least one location on the separator.

[0011] To maximize electrochemical performance, the porosity of such a porous ceramic inter-electrode separator needs to be high. Ceramic materials are, however, generally too brittle to be readily fabricated in so highly porous a form, especially ones that could, additionally, avoid cracking or fracturing or crumbling or collapsing or flaking off or scraping off, or loss, diminution, degradation, or corrosion of the outer ceramic layers, when tightly sandwiched by two electrodes in an operating electrochemical cell. This was most particularly the case when the ceramic separator was very thin and / or very flexible. That is, it has not proved possible to date to fabricate very thin and / or very flexible ceramic separators that avoid cracking or fracturing or crumbling or collapsing or flaking off or scraping off, or loss, diminution, degradation, or corrosion of the outer ceramic layers, when tightly sandwiched by two electrodes in an operating electrochemical cell. This is more demanding when one considers that, in commercial use, inter-electrode separators are generally expected to operate for lifetimes exceeding 90,000 hours of continuous operation.

[0012] Accordingly, a clear need exists for robust ceramic inter-electrode separators that address or ameliorate one or more of the problems discussed above. For example, there is a need for ceramic inter-electrode separators that are robust, thin, and / or flexible that are capable of high electrochemical performance and safe operation at elevated temperatures in commercial electrochemical cells.

[0013] The reference in this specification to any prior publication (or information derived from it), or to any matter which is known, is not, and should not be taken as an acknowledgment or admission or any form of suggestion that the prior publication (or information derived from it) or known matter forms part of the common general knowledge in the field of endeavour to which this specification relates. SUMMARY

[0014] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify all of the key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

[0015] The inventors have realised that for improved electrochemical performance, inter- electrode separators need to be thin, typically less than 2 mm thick overall. A very thin separator maximises the rate at which ions can flow between the electrodes, through the separator (i.e. it provides the highest possible ionic conductance). A high ionic conductance provides for low energy losses in an electrochemical cell during operation. However, even when very thin, the separator must, during operation of the cell, and at all locations on the separator, be electrically insulating (i.e. it must exhibit a high electrical resistance) to thereby prevent electrical short- circuits between the electrodes. This requires that the separator be robust. Even when robust, the minimum overall thickness of a particular separator may depend on how its electrical resistance is affected by its thickness. The high electrical resistance of many known separators may stay relatively constant until the separator becomes too thin overall, at which point it suddenly declines and becomes unviable. This may occur because tight compression of the electrodes against opposite sides of the separator (in a ’zero-gap’ configuration) may cause the structure of known separators, if made too thin, to readily become deformed or damaged.

[0016] The inventors have also realised that inter-electrode separators need to be flexible and not rigid, to accommodate non-uniformities when the electrodes are compressed against its opposite sides. Operation of an electrochemical cell may involve fluctuating mechanical and thermally induced stresses that the separator must accommodate. During such flexing and any associated movement in the electrode-separator-electrode assembly, it is critically important that the separator remains electrically highly resistant at all locations on the separator, and at all times during operation of the cell. This also requires the separator to be robust. Even when robust, the maximum flexibility that may be created in a separator may also depend on its electrical resistance. The high electrical resistance of many known separators may stay relatively constant until the separator becomes too flexible, at which point it suddenly declines and becomes unviable. This may occur because tight compression of the electrodes against opposite sides of the separator (in a ’zero-gap’ configuration) may cause the structure of known separators, if not flexible enough, to readily become deformed or damaged.

[0017] The inventors have further realised that where an inter-electrode separator is porous, such that it can be filled or imbued with a liquid electrolyte having a high ionic conductance, the porosity of such a porous inter-electrode separator should ideally be high to thereby maximise its electrochemical performance. A very porous separator increases the rate at which ions can flow between the electrodes, through the separator (i.e. it provides a higher possible overall ionic conductance), if it can be filled or imbued with a larger volume of liquid electrolyte having a high ionic conductance. A high overall ionic conductance provides for low energy losses in the cell during operation. However, even when very porous, such a separator must, at all times during operation of the cell, and at all locations on the separator, be electrically insulating (i.e. it must exhibit a high electrical resistance) to thereby prevent electrical short-circuits between the electrodes. This requires that the separator be robust. Even when robust, the maximum porosity of a particular separator may depend on how its electrical resistance is affected by its porosity. The high electrical resistance of many known separators may stay relatively constant until the separator becomes too porous, at which time it suddenly declines and becomes unviable. This may occur because tight compression of the electrodes against opposite sides of separator (in a ’zero-gap’ configuration) may cause the structure of a very porous separator to readily become deformed or damaged.

[0018] The inventors have developed ceramic inter-electrode separators that are very thin (preferably less than 0.2 mm overall), and / or highly flexible (preferably having a bend diameter of less than 6 cm), and / or highly porous (preferably having a porosity in the range of 55-95%), and / or are robust and that are suitable for use in electrochemical cells. The inventors have, surprisingly, found that such ceramic inter-electrode separators may be obtained by, for example: - using a porous metal substrate, that is preferably thin, moderately open and finely structured, and that may be sheet-like, wherein “moderately open” is defined by a moderately open screening area as discussed herein and finely structured includes, but is not limited to, components such as thin wires or strands; - attaching, coating or embedding the porous metal substrate with or within a porous sintered bed, that is preferably thin, comprising metal oxide particles of at least one metal oxide, attached to each other and attached to the porous metal substrate.

[0019] The porous sintered bed, in a preferred aspect, is purely a metal oxide ceramic that contains no underlying or extraneous metal powder, as was used in, for example, US 4,559,124 and earlier, to prepare ceramic inter-electrode separators. That is, the porous sintered bed in preferred embodiments is created by directly sintering metal oxide particles instead of powdered metals during its formation. The at least one metal oxide particle size is, preferably but not exclusively, sub-micron (i.e. smaller than 1 ^m). In other examples, more than one metal oxide particle size is present and at least one of the particle sizes is, preferably but not exclusively, sub- micron. In still other examples, more than one metal oxide particle size is present, for example two distinct particle sizes are present, and both the particle sizes are, preferably but not exclusively, sub-micron. In yet other examples, more than one metal oxide particle size is present, and a multiplicity of the particle sizes are, preferably but not exclusively, sub-micron. The porous metal substrates may include but are not limited to, substantially 2D structures like metal meshes or substantially 3D structures like metal foams. The surface of the porous metal substrate may be partially or wholly covered by a thin layer of its corresponding oxide.

[0020] In one example, the ceramic inter-electrode separators may be fabricated by selecting a porous metal substrate, that is preferably thin, moderately open, and finely structured, and then coating, embedding or encasing the porous metal substrate in a slurry, that is preferably thinly- applied, comprising a carbonaceous polymer binder (preferably but not exclusively, together with a carbonaceous dispersant and pore former), and particles of the metal oxide / s, followed bysintering under inert or reducing or oxidising atmosphere or air atmosphere, at temperatures preferably above 600oC. The carbonaceous binder, dispersant and pore former are burned off at such high temperatures. In an example, metal oxide particles of a single metal oxide are present in the porous sintered bed, wherein the metal oxide particles of the single metal oxide comprise particles of at least one distinct particle size. In another example, metal oxide particles of two or more metal oxides are present in the porous sintered bed, wherein the metal oxide particles of at least one of the metal oxides comprises metal oxide particles of at least one distinct particle size. In another example, metal oxide particles of two or more metal oxides are present in the porous sintered bed, wherein the metal oxide particles of at least one of the metal oxides in the porous sintered bed have a distinct particle size that is different to the particle size of metal oxide particles of another one of the metal oxides in the porous sintered bed.

[0021] The resulting ceramic inter-electrode separators may be surprisingly highly robust, as demonstrated in the ceramic inter-electrode separators being surprisingly thin overall, and / or surprisingly highly flexible, and / or surprisingly highly porous. They may be highly electrically insulating in all the above forms. The term ‘robust’ is used herein to denote a capacity by the ceramic inter-electrode separator to avoid any discernible cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion of the ceramic metal oxide layer, even when subjected to relatively severe mechanical and / or thermal stresses, for example as may occur in an electrochemical cell when electrodes are tightly compressed against their opposite sides.

[0022] The inventors have found that the use of a porous metal substrate, that is thin and having a moderately open and fine structure, with application of a porous sintered bed, that is also thin, dramatically and surprisingly amplifies the adhesion between the porous metal substrate and the porous sintered bed. Preferably, the presence of at least one distinct particle size of the metal oxide particles of preferably but not exclusively sub-micron size, in the porous sintered bed further provides for high adhesion between the porous sintered bed and the porous metal substrate. Preferably but not exclusively, the porous metal substrate is covered by a thin layer of its corresponding metal oxide, and this provides for still further increased adhesion between the porous sintered bed and the porous metal substrate. Preferably, the presence of the at least one distinct particle size of the metal oxide particles of preferably but not exclusively sub-micron size, formed of at least one metal oxide, in the porous sintered bed provides for additional high adhesion between the metal oxide particles in the porous sintered bed itself. Preferably, the structure of theporous sintered bed within the ceramic inter-electrode separator is in ‘compression’, and this further amplifies its adhesion and robustness. The term ‘compression’ refers to the phenomenon wherein the ceramic component (i.e. the porous sintered bed) is under internal compressive strain, which may be created, for example, when the metal component (i.e. the porous metal substrate) within a metal-ceramic assembly that has been fabricated at very high temperature, undergoes greater thermal contraction during cooling to a lower temperature, than the ceramic component. The porous sintered bed within the ceramic inter-electrode separator may additionally be placed in compression by chemical reduction of the metal substrate’s thin surface covering of its corresponding metal oxide, during high temperature fabrication of the ceramic inter-electrode separator under chemically reductive conditions. Compression of these types may, individually or cumulatively, and / or in combination with the other adhesive effects described above, counteract any cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion of the ceramic inter-electrode separator, making it still more robust. Preferably, the cumulative adhesive effects described above in combination with the compressive effects described above, are such that the porous metal substrate coated with the porous sintered bed of metal oxide particles having at least one distinct particle size, is highly robust and flexible. Preferably, but not exclusively, it is sufficiently robust and flexible to remain safe during operation at elevated temperatures in commercial electrochemical cells, even over long periods of operation (e.g.90,000 hours).

[0023] In one example aspect, embodiments relate to a ceramic inter-electrode separator for an electrochemical cell comprising: a porous metal substrate; and a porous sintered bed comprising metal oxide particles of at least one metal oxide, the metal oxide particles attached to each other and to the porous metal substrate; and wherein the porous sintered bed, taken in cross-section, has an overall thickness of less than 0.2 mm. Preferably, the metal oxide particles in the porous sintered bed comprise at least one distinct particle size.

[0024] In another example aspect, embodiments relate to a ceramic inter-electrode separator for an electrochemical cell comprising: a porous metal substrate; and a porous sintered bed comprising metal oxide particles of at least one metal oxide, the metal oxide particles attached to each other and to the porous metal substrate; wherein the ceramic inter-electrode separator, including the porous sintered bed, has a bend diameter of less than 6 cm. Preferably, the ceramic inter-electrode separator, including the porous sintered bed, is flexible.

[0025] In another example aspect, embodiments relate to a ceramic inter-electrode separator for an electrochemical cell comprising: a porous metal substrate; and a porous sintered bed comprising metal oxide particles of at least one metal oxide, the metal oxide particles attached to each other and to the porous metal substrate; wherein the porous sintered bed, taken in cross-section, has an overall thickness of less than 0.2 mm, and the ceramic inter-electrode separator, including the porous sintered bed, is flexible and has a bend diameter of less than 6 cm.

[0026] In another example aspect, embodiments relate to a ceramic inter-electrode separator for an electrochemical cell comprising: a porous metal substrate; and a porous sintered bed comprising metal oxide particles of at least one metal oxide, the metal oxide particles attached to each other and to the porous metal substrate; wherein the porous sintered bed in the ceramic inter-electrode separator, has a porosity in the range of 55% to 95%.

[0027] In another example aspect, embodiments relate to a ceramic inter-electrode separator for an electrochemical cell comprising: a porous metal substrate; and a porous sintered bed comprising metal oxide particles of at least one metal oxide, the metal oxide particles attached to each other and to the porous metal substrate; wherein the porous sintered bed, taken in cross section, has an overall thickness of less than 0.2 mm, and the porous sintered bed in the ceramic inter-electrode separator has a porosity in the range of 55% to 95%.

[0028] In another example aspect, embodiments relate to a ceramic inter-electrode separator for an electrochemical cell comprising: a porous metal substrate; and a porous sintered bed comprising metal oxide particles of at least one metal oxide, the metal oxide particles attached to each other and to the porous metal substrate; wherein the ceramic inter-electrode separator, including the porous sintered bed, has a bend diameter of less than 6 cm, and the porous sintered bed in the ceramic inter-electrode separator has a porosity in the range of 55% to 95%.

[0029] In another example aspect, embodiments relate to a ceramic inter-electrode separator for an electrochemical cell comprising: a porous metal substrate; and a porous sintered bed comprising metal oxide particles of at least one metal oxide, the metal oxide particles attached to each other and to the porous metal substrate; wherein the porous sintered bed, taken in cross section, has an overall thickness of less than 0.2 mm, the ceramic inter-electrode separator, including the poroussintered bed, has a bend diameter of less than 6 cm, and the porous sintered bed in the ceramic inter-electrode separator has a porosity in the range of 55% to 95%.

[0030] In another example aspect, the metal oxide particles comprise a single metal oxide, and the metal oxide particles of the single metal oxide comprise more than one distinct particle size.

[0031] In another example aspect, the metal oxide particles comprise two or more different metal oxides.

[0032] In another example aspect, the metal oxide particles comprise two or more different metal oxides, and the metal oxide particles of at least one of the metal oxides have a distinct particle size when compared to the metal oxide particles of another one of the metal oxides.

[0033] Preferably but not exclusively, the metal oxide particles in the porous sintered bed have at least one of the distinct particle sizes smaller than 1 ^m. In another example, preferably but not exclusively, the metal oxide particles in the porous sintered bed have more than one distinct particle size and the more than one distinct particles size is smaller than 1 ^m.

[0034] Preferably, but not exclusively, the porous sintered bed within the ceramic inter-electrode separator, taken in cross section, has an overall thickness of less than 0.15 mm, less than 0.1 mm, less than 0.08 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.02 mm.

[0035] Preferably, but not exclusively, the ceramic inter-electrode separator, including the porous sintered bed, has a bend diameter of less than 5 cm, less than 4 cm, less than 3 cm, less than 2 cm, less than 1 cm, less than 0.5 cm, less than 0.4 cm, less than 0.3 cm, less than 0.2 cm, less than 0.1 cm or less than 0.05 cm.

[0036] Preferably, but not exclusively, the porous sintered bed in the ceramic inter-electrode separator has a porosity of more than 60%, more than 65%, more than 70%, more than 75%, more than 80%, more than 85%, or more than 90%. Preferably, but not exclusively, the porous sintered bed in the ceramic inter-electrode separator has a porosity in the range of 60% to 95%, in the range of 65% to 95%, in the range of 70% to 95%, in the range of 75% to 95%, in the range of 80% to 95%, in the range of 85% to 95%, or in the range of 90% to 95%.

[0037] Preferably, but not exclusively, the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has a thickness of less than 0.20 mm.

[0038] Preferably, but not exclusively, ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has a thickness of less than 0.15 mm, less than 0.1 mm, less than 0.08 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.02 mm.

[0039] Preferably, but not exclusively, the porous sintered bed within the ceramic inter-electrode separator is under compressive strain.

[0040] Preferably, but not exclusively, the compressive strain is due to a mismatch or discrepancy in the thermal contraction of the metal and the ceramic components following fabrication of the ceramic inter-electrode separator.

[0041] Preferably, but not exclusively, the compressive strain is due to chemical reduction of a metal oxide layer on a surface of the porous metal substrate during high temperature fabrication of the ceramic inter-electrode separator.

[0042] Preferably, but not exclusively, the porous metal substrate comprises a metal mesh, a metal net, a perforated metal plate, or a substantially 2D metal substrate.

[0043] Preferably, but not exclusively, the metal mesh, the metal net, the perforated metal plate, or the substantially 2D metal substrate, has an open screening area of 25% to 65%.

[0044] Preferably, but not exclusively, the metal mesh, the metal net, the perforated metal plate, or the substantially 2D metal substrate, has an open screening area of more than 65%, more than 70%, or more than 75%.

[0045] Preferably, but not exclusively, the metal mesh, the metal net, the perforated metal plate, or the substantially 2D metal substrate, comprises wires or strands of diameter 0.015 to 0.19 mm.

[0046] Preferably, but not exclusively, the porous metal substrate is fully or partially embedded within the porous sintered bed.

[0047] Preferably, but not exclusively, the porous metal substrate is embedded within the porous sintered bed in the ceramic inter-electrode separator, and the porous metal substrate has a thickness equal to or less than 0.19 mm.

[0048] Preferably, but not exclusively, the porous metal substrate is embedded within the porous sintered bed in the ceramic inter-electrode separator, and the porous metal substrate has a thickness less than 0.15 mm, less than 0.1 mm, less than 0.08 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.02 mm.

[0049] In another aspect, preferably, but not exclusively, the porous sintered bed is substantially located on only one side of the porous metal substrate.

[0050] Preferably, but not exclusively, the porous sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, and the porous metal substrate is a metal foam or similar 3D structure.

[0051] Preferably but not exclusively, the ceramic inter-electrode separator is electrically insulating.

[0052] Preferably but not exclusively, the separator as a whole, including the porous sintered bed and porous metal substrate, has an electrical resistance of more than 1 kΩ, more than 10 kΩ, more than 50 kΩ, more than 100 kΩ, more than 200 kΩ, more than 300 kΩ, more than 500 kΩ, more than 750 kΩ, more than 1 MΩ, more than 10 MΩ, or more than 100 MΩ.

[0053] Preferably but not exclusively, the electrical resistance of the porous sintered bed in the ceramic inter-electrode separator is more than 1 kΩ, more than 10 kΩ, more than 50 kΩ, more than 100 kΩ, more than 200 kΩ, more than 300 kΩ, more than 500 kΩ, more than 750 kΩ, more than 1 MΩ, more than 10 MΩ, or more than 100 MΩ.

[0054] Preferably but not exclusively, the porous sintered bed in the ceramic inter-electrode separator, when fully imbued with 30 wt% potassium hydroxide aqueous solution, has an area specific ionic resistance (ASR) at 21oC of less than 0.4 Ω cm2, less than 0.35 Ω cm2, less than 0.3 Ω cm2, less than 0.25 Ω cm2, less than 0.2 Ω cm2, less than 0.15 Ω cm2, less than 0.1 Ω cm2, less than 0.09 Ω cm2, less than 0.08 Ω cm2, less than 0.07 Ω cm2, less than 0.06 Ω cm2, less than 0.05 Ω cm2, less than 0.04 Ω cm2, less than 0.03 Ω cm2, less than 0.02 Ω cm2, or less than 0.01 Ω cm2.

[0055] Preferably but not necessarily, when the sintered bed is substantially coated on one side of the porous metal substrate in the ceramic inter-electrode separator, the porous metal substrate is a metal foam or similar 3D structure, and the porous sintered bed in the ceramic inter-electrode separator has a thickness less than 0.20 mm. Preferably but not necessarily, when the sintered bed is substantially coated on one side of the porous metal substrate in the ceramic inter-electrode separator, the porous sintered bed in the ceramic inter-electrode separator, has a porosity of 55% to 95%, an electrical resistance of more than 1 kΩ, and / or, when fully imbued with 30 wt% potassium hydroxide aqueous solution, the porous sintered bed in the ceramic inter-electrode separator has an area specific ionic resistance (ASR) at 21oC of less than 0.2 Ω cm2.

[0056] Preferably but not exclusively, the at least one metal oxide is a zirconium oxide or a titanium oxide, these being chemically unaffected by high molarity alkaline electrolytes of the types used in alkaline electrolyzers and fuel cells.

[0057] Preferably but not exclusively, the porous metal substrate is formed of a material selected from the group of: nickel, titanium, zirconium, stainless-steel, a metal alloy containing nickel, a metal alloy containing titanium, a metal alloy containing zirconium, and a metal alloy containing stainless-steel.

[0058] In another example aspect, there is provided an electrochemical cell including the ceramic inter-electrode separator described above.

[0059] In another example aspect, there is provided an electro-synthetic or electro-energy cell including the ceramic inter-electrode separator described above.

[0060] In another example aspect, there is provided an alkaline water electrolysis cell or alkaline fuel cell including the ceramic inter-electrode separator described above. In a further aspect, there is also provided other electrochemical cells, including but not limited to: a battery, a redox flow battery, a rechargeable battery, a galvanic or a voltaic battery, or an electro plating cell, including the ceramic inter-electrode separator described above.

[0061] In another example aspect, there is provided a method of fabricating a ceramic inter- electrode separator comprising the steps of: coating a porous metal substrate with a slurry orembedding the porous metal substrate within the slurry, to form a coated or embedded substrate, wherein the slurry contains a polymer binder and metal oxide particles comprising at least one metal oxide; and firing the coated or embedded substrate in a furnace, under inert or reducing or oxidising atmosphere or an air atmosphere, at a temperature greater than 600oC.

[0062] Preferably but not necessarily, in the fabricated ceramic inter-electrode separator the coated or embedded substrate, taken in cross-section, has an overall thickness of less than 0.20 mm. Preferably but not necessarily, in the fabricated ceramic inter-electrode separator the coated or embedded substrate is flexible and has a bend diameter of less than 6 cm. Preferably but not necessarily, in the fabricated ceramic inter-electrode separator the coated or embedded substrate has a porosity in the range of 55% to 95%. Preferably but not necessarily, in the fabricated ceramic inter-electrode separator the metal oxide particles comprise a single metal oxide, and the metal oxide particles of the single metal oxide comprise more than one distinct particle size. Preferably but not necessarily, in the fabricated ceramic inter-electrode separator the metal oxide particles comprise two or more different metal oxides. Preferably but not necessarily, in the fabricated ceramic inter-electrode separator the metal oxide particles of at least one of the metal oxides have a distinct particle size when compared to the metal oxide particles of another one of the metal oxides.

[0063] Optionally, the method further comprises the initial step of cleaning and / or pre-treating the porous metal substrate. Optionally, the step of cleaning and / or pre-treating the porous metal substrate includes one or more of the following steps: cleaning the porous metal substrate by washing the porous metal substrate with, for example, an alcoholic solution, and thereafter drying the porous metal substrate; cleaning the porous metal substrate by pickling the porous metal substrate in an acidic solution; and / or heating the porous metal substrate in an oxygen atmosphere.

[0064] In another example aspect, when the porous metal substrate has a 3D structure and the porous sintered bed is coated on substantially only one side of the porous metal substrate, cleaning and / or pre-treating the porous metal substrate comprises: filling the internal cavities of the porous metal substrate with a wax, paraffin, or ‘blocking material’ that prevents the subsequently applied slurry from penetrating into the 3D structure of the porous metal substrate, wherein the wax, paraffin or ‘blocking material’ is driven off and lost during the subsequent firing step, thereby leaving the porous metal substrate coated with the porous sintered bed on substantially only one side.

[0065] In another example aspect, there is provided a method of fabricating a ceramic inter- electrode separator, wherein the method comprises the following sequence of steps: cleaning and / or pre-treating a porous metal substrate; coating the porous metal substrate with a slurry or embedding the porous metal substrate within a slurry, using a technique such as, but not limited to, dip-coating, knife coating, printing, or painting, wherein the slurry contains a polymer binder and at least one metal oxide in at least one distinct particle size; and firing the resulting coated or embedded substrate assembled in a furnace under inert or reducing or oxidising atmosphere or air atmosphere at a temperature above 600oC. BRIEF DESCRIPTION OF THE FIGURES

[0066] Illustrative embodiments will now be described solely by way of non-limiting examples and with reference to the accompanying figures. Various example embodiments will be apparent from the following description, given by way of example only, of at least one preferred but non- limiting embodiment, described in connection with the accompanying figures.

[0067] Figure 1 depicts a scanning electron micrograph of a porous sintered bed of an example ceramic inter-electrode separator of the type disclosed herein, wherein two different particle sizes of metal oxide particles (in this example both being BaTiO3) were used in its fabrication.

[0068] Figure 2 depicts scanning electron micrographs of: (left image) a slurry that has been thinly applied to a metal mesh prior to sintering; (right image) the porous sintered bed that resulted when the slurry was subjected to a sintering process, as described in Example 3. DETAILED DESCRIPTION

[0069] The following modes, features or aspects, given by way of example only, are described to provide a more precise understanding of the subject matter of a preferred embodiment or embodiments. Definitions

[0070] An ‘electro-energy cell’ is a commercial electrochemical cell that generates electrical power continually or continuously, over indefinite periods of time, for use outside of the cell. Electro-energy cells may require a constant external supply of reactants during operation. The products of the electrochemical reaction may also be constantly removed from such cells during operation. An example of an electro-energy cell is a hydrogen-oxygen fuel cell. Other examples of electro-energy cells include but are not limited to: (i) an ammonia fuel cell for producing electrical energy from ammonia and oxygen, (ii) a direct methanol fuel cell for producing electrical energy from methanol, and (iii) a direct ethanol fuel cell for producing electrical energy from ethanol.

[0071] An ‘electro-synthetic cell’ is a commercial electrochemical cell that manufactures one or more chemical materials continually or continuously, over indefinite periods of time, for use outside the cell. The chemical materials may be in the form of a gas, liquid, or solid. Like an electro-energy cell, an electro-synthetic cell may also require a constant supply of reactants and a constant removal of products during operation. Electro-synthetic cells may generally further require a constant input of electrical energy during operation. An example of an electro-synthetic cell is a water electrolysis cell. Other examples of electro-synthetic cells include but are not limited to: (i) a nitrogen reduction cell for producing ammonia from nitrogen and hydrogen or oxygen; (ii) a chlor-alkali cell for producing chlorine, hydrogen and caustic from brine; (iii) an oxygen- depolarized chlor-alkali cell for producing chlorine and caustic from brine; (iv) a cell for recycling hydrochloric acid to produce chlorine and hydrogen; and (v) a cell for extracting pure hydrogen from gas mixtures containing hydrogen.

[0072] Electro-energy and electro-synthetic cells differ from other types of electrochemical cells, such as ‘galvanic cells’, which are defined herein to be cells that do incorporate within the cell body all or some of the reactants they require to operate, and all or some of the products they generate during operation. Illustrative examples of galvanic cells include but are not limited to batteries, air-batteries, sensors and the like. Unlike a battery, an electro-energy cell does not store chemical or electrical energy within it. Similarly, while some electrochemical sensors may consume reactants and generate products in limited quantities during the sensing operation, all / some of these are stored within the cell body itself.

[0073] A ‘zero-gap’ electrochemical cell is a cell in which there is no gap between the electrodes and the inter-electrode spacer. That is, in a ‘zero-gap’ cell, the electrodes are tightly sandwiched and compressed against, or abut, opposite sides of the inter-electrode spacer.

[0074] The terms ‘particle size’ and ‘distinct particle size’ refer to a set of particles whose size is distributed about a distinct average size. ‘Two different particle sizes’ refers to two distinct sets of particles, each having a particle size distribution about a different average size. The term ‘particle size’ generally refers to the average diameter or average width of a set of particles. Other equivalent terms that may be used and that fall within the definitions above are: median, percentiles, mean, mode and Span or polydispersity index, as well as Equivalent Diameter, relative proportions, overall distribution percentiles.

[0075] The term ‘ceramic’ is defined herein to refer to a material that is capable of withstanding high temperatures, for example above 600oC, without discernible degradation. The term ‘ceramic’ may include terms like inorganic, non-metallic, traditional ceramics, engineering ceramics, and glass ceramics.

[0076] The term ‘thickness’ is defined herein as the structural thickness taken on average, over a cross section of the structure.

[0077] A ‘porous material’ is defined herein as a solid material containing open space (‘void’ space) not occupied by the framework of atoms or molecules that make up the structure of the solid.

[0078] The ‘porosity’ of a porous material is defined herein as the ratio of the volume of void space divided by the total volume of the porous material, expressed as a percentage.

[0079] The term ‘compression’ or a ‘compressive state’ or ‘compressive strain’ or ‘compressive stress’ are defined herein to refer to the phenomenon wherein the ceramic component in an inter- electrode separator of the type disclosed herein, is under internal compressive strain. Such internal compressive strain may be created in several ways, including but not limited to, for example: (i) when the metal component within a metal-ceramic assembly that has been fabricated at very high temperature, undergoes greater thermal contraction during cooling to a lower temperature, than the ceramic component, or (ii) as a result of chemical reduction of a metal oxide layer on the surface of the porous metal substrate during firing of the ceramic inter-electrode separator in a reducing environment. The extent of the compressive strain within a ceramic inter-electrode separator of the type described herein, may be measured by measuring the shift of the X-Ray Diffraction peaks of the porous sintered bed relative to the corresponding un-sintered metal oxide particles, or by usingRaman Spectroscopy, which also shows shifts on the peaks to higher wavenumbers when a sintered bed is in a compressive state. Nano indentation could also be used.

[0080] A ‘slurry’ is defined widely herein, as any composition that may be considered a flowable particulate mixture. It may be, for example, a mixture of solid particles suspended in a liquid, or suspended in a gas. The term slurry may be used interchangeably with terms like: a paint, an ink, a ceramic suspension, a pourable ceramic composition, a non-aqueous ceramic dispersion, and other common terms that refer, in essence to a flowable particulate mixture. The technique of coating a substrate using a slurry is also defined widely herein. It may, for example, refer to many different techniques for applying solid particles onto substrate surfaces, including: electrostatic coating, painting, printing, air-brushing, dipping, spin-coating, slot die coating, thermal spraying (e.g., Plasma Spray, High-Velocity Oxygen Fuel (HVOF), Flame Spray), Electrostatic Spray Deposition, Aerosol Deposition Method, Chemical Vapor Deposition (CVD), Physical Vapor Deposition (PVD), Electrophoretic Deposition (EPD), Sol-Gel, Cold Spray (plastic deformation deposition at high velocity), Inkjet Printing / 3D Printing, Atomic Layer Deposition, or other means of coating that relies on the use of a flowable particulate mixture.

[0081] An ‘open screening area’ of a substantially 2D metal substrate such as, but not limited to a metal mesh, is defined herein as the ratio of the geometric area of the vacancies in the mesh / net relative to the total geometric area of the mesh / net, expressed as a percentage.

[0082] A ‘blocking material’ is defined as any sacrificial material that may be used to block access to a location, typically preventing a subsequently applied slurry from penetrating into the 3D structure of the porous metal substrate, wherein the wax, paraffin or ‘blocking material’ is driven off and lost during the subsequent firing step, thereby leaving the porous metal substrate coated with a porous sintered bed on substantially only one side. A ‘blocking material’ may be, for example a wax, a paraffin, tape (e.g. masking tape), adhesive, polymer filler, a coating, a repellent layer or coating, a stencil, Peelable Liquid Maskants, Water-Soluble Maskants, Solvent-Soluble Maskants, UV-Curable Maskants, Wax Maskants, Photolithography, Inkjet Printing (implicit masking), Plugs, Caps, and Fixtures.

[0083] The term ‘sintering driving force’ refers to the thermodynamic favourability of the sintering process, for example, how strongly discrete metal oxide particles are driven to merge during sintering, by forming solid state ‘necks’ with each other. Theoretically, the sintering driving force is quantified by the change in Gibbs free energy as a result of the sinter process. The mostsignificant and universal driving force for sintering is the reduction of the total surface (or interfacial) free energy of the powder. In practice, quantification of the sintering driving force involves measuring the Specific Surface Area that results from the sintering. Another means of quantifying the sintering driving force is to use the concept of sintering stress, which equates the sintering driving force to a mechanical stress that would provide the same densification. Example Embodiments

[0084] Although preferred embodiments have been described in detail, it is to be understood that many modifications, changes, substitutions or alterations will be apparent to those skilled in the art without departing from the scope of the present invention.

[0085] The inventors have found that the use of a porous metal substrate, preferably a thin, porous metal substrate having a moderately open and fine structure, combined with the application of a porous sintered bed, preferably a thin, porous sintered bed, to the porous metal substrate, dramatically and surprisingly amplifies the adhesion between the porous metal substrate and the porous sintered bed, thereby producing a highly robust ceramic inter-electrode separator. The porous sintered bed is a ceramic material. The porous metal substrate is a metallic material, or at least a partially metallic material. The complete inter-electrode separator, including the porous metal substrate and the porous sintered bed, may also be considered a ceramic material. Preferably, the presence of at least one distinct particle size of at least one metal oxide in the porous sintered bed further increases the high adhesion between the porous sintered bed and the porous metal substrate. Preferably but not exclusively, the porous metal substrate is covered with a thin coating of its corresponding metal oxide prior to attachment of the porous sintered bed, to further increase the adhesion between the porous sintered bed and the porous metal substrate. The term ‘thin coating’ preferably but not exclusively, refers to a coating that is less than 0.2 mm thick. In other examples, ‘thin coating’ may refer to a coating that is less than 0.18 mm thick, less than 0.16 mm thick, less than 0.14 mm thick, less than 0.12 mm thick, less than 0.10 mm thick, less than 0.08 mm thick, less than 0.06 mm thick, less than 0.04 mm thick, or less than 0.02 mm thick. Preferably but not exclusively, the presence of the at least one distinct particle size of the at least one metal oxide in the porous sintered bed provides for additional high adhesion between the particles in the porous sintered bed itself. Preferably, the structure of the porous sintered bed within the ceramic inter-electrode separator is in compression, and this further amplifies its adhesion and robustness.The compression involves the ceramic component (i.e. the porous sintered bed) being placed under internal compressive strain, which may be created, for example, when the metal component (i.e. the porous metal substrate) within a metal-ceramic assembly that has been fabricated at very high temperature, undergoes greater thermal contraction during cooling to a lower temperature, than the ceramic component. The porous sintered bed within the ceramic inter-electrode separator may additionally be placed in compression by chemical reduction of the porous metal substrate’s thin surface covering of its corresponding metal oxide during high temperature fabrication of the ceramic inter-electrode separator under reductive conditions. Compression of these types may, individually or cumulatively, or in combination with the other adhesive effects described above, counteract any loss, diminution, degradation, corrosion, flaking, fracturing, crumbling, or cracking of the structure of the ceramic inter-electrode separator, making it remarkably robust. Preferably, the cumulative adhesive effects described above in combination with the compressive effects described above, are such that the porous metal substrate coated with the porous sintered bed of inorganic particles having at least one distinct particle size, is very highly robust and flexible. Preferably but not exclusively, it is sufficiently robust and flexible to be unreservedly safe during operation at elevated temperatures in commercial electrochemical cells, even over long periods of operation (e.g. exceeding 90,000 hours of continuous operation).

[0086] The ceramic inter-electrode separators may be fabricated by coating or encasing the porous metal substrate in a slurry comprising a polymer binder and the metal oxide / s in the at least one distinct particle size, followed by sintering under inert or reducing or oxidising atmosphere or air atmosphere, at a temperature greater than 600oC.

[0087] That is, the inventors have, surprisingly, found that highly robust ceramic inter-electrode separators suitable for use in electrochemical cells, may be obtained by coating, embedding or encasing, a porous metal substrate, preferably a thin, moderately open and finely structured porous sheet-like metal substrate, with or within a porous sintered bed, preferably a thin, porous sintered bed, comprising metal oxide particles of at least one metal oxide attached to each other and attached to the porous metal substrate, wherein the metal oxide particles of the at least one metal oxide include metal oxide particles of at least one distinct particle size. At least one of the metal oxide particle sizes is, preferably but not exclusively, sub-micron, that is less than 1 ^m in average diameter or average width. In other examples, more than one of the metal oxide particle sizes is, preferably but not exclusively, sub-micron. The porous metal substrates may include but are not limited to, substantially 2D structures like metal meshes or substantially 3D structures like metalfoams. The surface of the porous metal substrate may be partially or wholly covered by a thin layer of its corresponding oxide.

[0088] The ceramic inter-electrode separators may be fabricated by selecting a porous metal substrate, for example a thin, moderately open, finely structured porous metal substrate, and then coating or encasing the porous metal substrate in a thinly-applied slurry comprising a carbonaceous polymer binder (along with dispersant and pore former), and particles of the metal oxide / s, wherein the particles of the metal oxide / s comprise at least one distinct particle size, followed by sintering under inert or reducing or oxidising atmosphere or an air atmosphere, at a temperature or temperatures above 600oC. The carbonaceous binder, dispersant and pore former are removed at such high temperatures. The term ‘thinly-applied slurry’ preferably but not exclusively, refers to a slurry that is applied in a layer less than 0.2 mm thick. In other examples, ‘thinly-applied slurry’ may refer to a slurry that is applied in a layer less than 0.18 mm thick, less than 0.16 mm thick, less than 0.14 mm thick, less than 0.12 mm thick, less than 0.10 mm thick, less than 0.08 mm thick, less than 0.06 mm thick, less than 0.04 mm thick, or less than 0.02 mm thick. In one example, metal oxide particles of a single metal oxide are present in the porous sintered bed, wherein the metal oxide particles of the single metal oxide comprise metal oxide particles of at least one distinct particle sizes. In another example, metal oxide particles of two or more metal oxides are present in the porous sintered bed, wherein the metal oxide particles of at least one of the metal oxides comprises metal oxide particles of two or more distinct particle sizes. In another example, metal oxide particles of two or more metal oxides are present in the porous sintered bed, wherein the metal oxide particles of at least one of the metal oxides in the porous sintered bed have a distinct particle size relative to the metal oxide particles of another one of the metal oxides in the porous sintered bed.

[0089] The resulting ceramic inter-electrode separators may be, surprisingly highly robust, as demonstrated by being surprisingly thin, and / or surprisingly highly flexible, and / or surprisingly highly porous. They may be highly electrically insulating in all the above forms. The term ‘robust’ is used herein to denote a capacity by the ceramic inter-electrode separator to avoid any discernible cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion, in the structure of the ceramic inter-electrode separator, even when subjected to relatively severe mechanical and / or thermal stresses, for example by being bent or flexed into a tight arc (known as a ‘bend diameter’) and subjected to repeated, large temperaturechanges, for example during operation within an electrochemical cell over an extended time period with electrodes tightly sandwiched and compressed against opposite sides of the separator.

[0090] In one example, there is provided a ceramic inter-electrode separator for an electrochemical cell. The ceramic inter-electrode separator comprises a porous metal substrate and a porous sintered bed. The porous sintered bed comprises metal oxide particles of at least one metal oxide, where the metal oxide particles are attached to each other and are attached to the porous metal substrate. The metal oxide particles in the porous sintered bed comprise at least one distinct particle size. The porous sintered bed, taken in cross-section, has an overall thickness of less than 0.20 mm.

[0091] In another example, there is provided a ceramic inter-electrode separator for an electrochemical cell. The ceramic inter-electrode separator comprises a porous metal substrate and a porous sintered bed. The porous sintered bed comprises metal oxide particles of a single metal oxide, where the metal oxide particles are attached to each other and are attached to the porous metal substrate. The metal oxide particles of the single metal oxide in the porous sintered bed comprise at least one distinct particle size. The ceramic inter-electrode separator, including the porous sintered bed, is flexible and has a bend diameter of less than 6 cm.

[0092] In another example, there is provided a ceramic inter-electrode separator for an electrochemical cell. The ceramic inter-electrode separator comprises a porous metal substrate and a porous sintered bed. The porous sintered bed comprises metal oxide particles of a single metal oxide, where the metal oxide particles are attached to each other and are attached to the porous metal substrate. The metal oxide particles of the single metal oxide in the porous sintered bed comprise at least one distinct particle size. The porous sintered bed in the ceramic inter-electrode separator has a porosity in the range of 55% to 95%.

[0093] In still another example, there is provided a ceramic inter-electrode separator for an electrochemical cell. The ceramic inter-electrode separator comprises a porous metal substrate and a porous sintered bed. The porous sintered bed comprises metal oxide particles of a single metal oxide, where the metal oxide particles are attached to each other and are attached to the porous metal substrate. The metal oxide particles of the single metal oxide in the porous sintered bed comprise at least one distinct particle size. The porous sintered bed, taken in cross-section, has an overall thickness of less than 0.20 mm, and / or the ceramic inter-electrode separator, including theporous sintered bed, is flexible and has a bend diameter of less than 6 cm, and / or the porous sintered bed in the ceramic inter-electrode separator has a porosity in the range of 55% to 95%.

[0094] In another example, the porous sintered bed comprises particles of two or more metal oxides, where the metal oxide particles are attached to each other and are attached to the porous metal substrate. The metal oxide particles of at least one of the metal oxides in the porous sintered bed comprise a distinct particle size.

[0095] In another example, the porous sintered bed comprises particles of two or more metal oxides, where the metal oxide particles are attached to each other and are attached to the porous metal substrate. The metal oxide particles of at least one of the metal oxides in the porous sintered bed have a distinct particle size when compared to the particles of another one of the metal oxides in the porous sintered bed.

[0096] Preferably but not exclusively, at least one of the distinct particle sizes is smaller than 1 ^m. In another example, preferably but not exclusively, more than one of the distinct particles sizes is smaller than 1 ^m. Preferably but not exclusively, at least two or more distinct particle sizes is smaller than 1 ^m.

[0097] Scanning electron micrographs of the porous sintered bed in the ceramic inter-electrode separator may be used to readily demonstrate the presence of metal oxides of different and distinct particle sizes in the porous sintered bed. Figure 1, for example, depicts a scanning electron micrograph of a sintered bed containing BaTiO3 particles. As can be seen, two different particle sizes are clearly visible. The particle sizes of the particles that were originally used to create the sintered bed can also be inferred from the final particle sizes present using the equations of sintering kinetics for ‘neck’ growth. Example 6 below provides an explanation of how this may be done.

[0098] The robustness and flexibility of a ceramic inter-electrode separator of the type disclosed herein, is most appropriately and most easily demonstrated by its capacity to be very thin and to be flexible, for example by displaying a low bend diameter. A ‘bend diameter’ is the diameter of a cylinder about which the ceramic inter-electrode separator may be bent or flexed without any observable / discernible cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion of the structure. Preferably but not necessarily,the ceramic inter-electrode separator exhibits a bend diameter of less than 6 cm. In other examples, the ceramic inter-electrode separator exhibits a bend diameter of less than 15 cm, less than 10 cm, less than 9 cm, less than 8 cm, or less than 7 cm. In still other examples, the ceramic inter-electrode separator exhibits a bend diameter of less than 5 cm, less than 4 cm, less than 3 cm, less than 2 cm, less than 1 cm, less than 0.5 cm, less than 0.4 cm, less than 0.3 cm, less than 0.2 cm, less than 0.1 cm, or less than 0.05 cm. Preferably but not necessarily, the ceramic inter-electrode separator is highly robust even when it is bent and flexed to an exceedingly small bend diameter. In so doing, the ceramic inter-electrode separator is shown to exhibit excellent mechanical strength. Other techniques may be used to further confirm the robustness and mechanical strength (see Example 3 below).

[0099] One important way to demonstrate high robustness in a ceramic inter-electrode separator is a capacity for its porous sintered bed to be thin without discernible cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion, of the structure. In an example aspect, the porous sintered bed in the ceramic inter-electrode separator has a thickness of less than 0.20 mm. In other examples, the sintered bed has a thickness of less than 0.50 mm, less than 0.40 mm, less than 0.35 mm, less than 0.30 mm, or less than 0.25 mm. In other examples, the porous sintered bed in the ceramic inter-electrode separator has a thickness less than 0.15 mm, less than 0.1 mm, less than 0.08 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.02 mm.

[0100] In another example aspect, the porous metal substrate (for example, a metal mesh, a metal net, a perforated metal plate, or a similar substantially 2D metallic structure) may be embedded or encased within the porous sintered bed, wherein the porous metal substrate is, preferably, fully or partially enclosed by the porous sintered bed. In another example aspect, the porous metal substrate (for example, a metal foam, or a similarly substantially 3D metallic structure) may be coated with the porous sintered bed, wherein the porous sintered bed is, preferably, substantially located on only one side of the porous metal substrate. Optionally, regardless of how the porous sintered bed is configured relative to the porous metal substrate, the surface of the porous metal substrate may be partially or fully covered with a thin layer of its corresponding metal oxide.

[0101] Another important contributing factor to achieving the high robustness described above, is to use a porous metal substrate that is thin, especially when the porous metal substrate is embedded or encased within the porous sintered bed. When the porous metal substrate is embeddedor encased within the porous sintered bed in the ceramic inter-electrode separator, the porous metal substrate is, preferably but not necessarily, a metal mesh, a metal net, a perforated metal plate, or a similar, substantially 2D metal substrate. When the porous metal substrate is embedded or encased within the porous sintered bed in the ceramic inter-electrode separator, the porous metal substrate is, preferably but not necessarily, thin. Preferably but not necessarily, when embedded within the porous sintered bed of the ceramic inter-electrode separator, the porous metal substrate has a thickness less than 0.19 mm. In other examples, when embedded within the porous sintered bed of the ceramic inter-electrode separator, the porous metal substrate has a thickness less than 0.45 mm, less than 0.40 mm, less than 0.30 mm, or less than 0.25 mm. In still other examples, when embedded within the porous sintered bed of the ceramic inter-electrode separator, the porous metal substrate has a thickness less than 0.15 mm, less than 0.1 mm, less than 0.08 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.02 mm.

[0102] A further important contributing factor to achieve the high robustness described above, is to use a porous metal substrate having a moderately open structure (i.e. a moderately open screening area) and finely structured components, such as thin wires or strands. Preferably but not necessarily, the metal mesh / net / perforated plate has an open screening area of 25% to 65%, wherein an ‘open screening area’ is the ratio of the geometric area of the vacancies in the mesh / net relative to the total geometric area of the mesh / net / perforated plate, expressed as a percentage. In other examples, the metal mesh / net / perforated plate has an open screening area of more than 65%, more than 70%, or more than 75%. Preferably but not necessarily, the metal mesh, metal net, or perforated metal plate contains wires or metal strands of diameter 0.015 to 0.190 mm. In other examples, the metal mesh / net / perforated plate contains wires or metal strands of diameter less than 0.100 mm, less than 0.120 mm, less than 0.150 mm, less than 0.170 mm, or less than 0.190 mm.

[0103] An important demonstration of high robustness is a capacity for the overall ceramic inter- electrode separator to be thin. When the porous metal substrate is embedded or encased within the porous sintered bed in the ceramic inter-electrode separator, the full thickness of the ceramic inter- electrode separator, including the porous sintered bed and the embedded or encased porous metal substrate, taken in cross-section, is, preferably but not necessarily, thin, being only a little thicker than the thickness of the porous metal substrate. Preferably but not necessarily, when the porous metal substrate is embedded or encased within the porous sintered bed in the ceramic inter- electrode separator, the full thickness of the ceramic inter-electrode separator including the sinteredbed and the embedded or encased porous metal substrate, taken in cross-section, is, on average, less than 0.20 mm. In other examples, when the porous metal substrate is embedded or encased within the porous sintered bed in the ceramic inter-electrode separator, the full thickness of the ceramic inter-electrode separator including the porous sintered bed and the embedded or encased porous metal substrate, taken in cross-section, is, on average, less than 0.5 mm, less than 0.40 mm, less than 0.3 mm, or less than 0.25 mm. In still other examples, when the porous metal substrate is embedded or encased within the porous sintered bed in the ceramic inter-electrode separator, the full thickness of the ceramic inter-electrode separator including the porous sintered bed and the embedded or encased porous metal substrate, taken in cross-section, is, on average, less than 0.15 mm, less than 0.1 mm, less than 0.09 mm, less than 0.08 mm, less than 0.07 mm, less than 0.06 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.025 mm. Preferably but not necessarily, the ceramic inter-electrode separator is highly robust despite the ceramic inter- electrode separator being thin. That is, the separator displays no discernible cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion, of its outer structure despite being thin.

[0104] Preferably but not necessarily, when the porous metal substrate is embedded or encased within the porous sintered bed of the ceramic inter-electrode separator, the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has a porosity in the range of 55% to 95%. Preferably but not necessarily, the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has a porosity of more than 55%, more than 60%, more than 65%, more than 70%, more than 75%, more than 80%, more than 85%, more than 90%, or more than 95%. Preferably but not necessarily, the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, is highly robust despite having a high overall porosity in the range of 55% to 95%. In other examples, the ceramic inter- electrode separator, including the porous sintered bed and the porous metal substrate, has a porosity in the range of 60% to 95%, in the range of 65% to 95%, in the range of 70% to 95%, in the range of 75% to 95%, in the range of 80% to 95%, in the range of 85% to 95%, or in the range of 90% to 95%.

[0105] Preferably but not necessarily, when the porous metal substrate is embedded or encased within the porous sintered bed of the ceramic inter-electrode separator, the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has an electrical resistance of more than 1 kΩ. In other examples, preferably but not necessarily, the electricalresistance of the ceramic inter-electrode separator as a whole, including the porous sintered bed and the porous metal substrate, is more than 10 kΩ, more than 50 kΩ, more than 100 kΩ, more than 200 kΩ, more than 300 kΩ, more than 500 kΩ, more than 750 kΩ, more than 1 MΩ, more than 10 MΩ, or more than 100 MΩ. Preferably but not necessarily, the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, is highly robust even when it is highly electrically insulating.

[0106] Preferably but not necessarily, when the porous metal substrate is embedded or encased within the porous sintered bed of the ceramic inter-electrode separator, the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has, when fully imbued with 30 wt% potassium hydroxide aqueous solution, an area specific ionic resistance (ASR) at 21oC of less than 0.2 Ω cm2. In other examples, when fully imbued with 30 wt% potassium hydroxide aqueous solution, the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has an area specific ionic resistance (ASR) at 21oC. of less than 0.4 Ω cm2, less than 0.35 Ω cm2, less than 0.3 Ω cm2, or less than 0.25 Ω cm2. In still other examples, when fully imbued with 30 wt% potassium hydroxide aqueous solution, the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has an area specific ionic resistance (ASR) at 21oC of less than 0.19 Ω cm2, less than 0.15 Ω cm2, less than 0.1 Ω cm2, less than 0.09 Ω cm2, less than 0.08 Ω cm2, less than 0.07 Ω cm2, less than 0.06 Ω cm2, less than 0.05 Ω cm2, less than 0.04 Ω cm2, less than 0.03 Ω cm2, less than 0.02 Ω cm2, or less than 0.01 Ω cm2.

[0107] When the porous sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, the porous metal substrate is, preferably but not necessarily, a metal foam or similar substantially 3D structure.

[0108] One important demonstration of high robustness when coated on substantially one side of the porous metal substrate, is a capacity for the porous sintered bed to be thin without discernible cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion, of the outer structure. When the sintered bed is produced on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not necessarily, the porous sintered bed in the ceramic inter-electrode separator has a thickness less than 0.20 mm. In other examples, when the porous sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, the porous sintered bed inthe ceramic inter-electrode separator has a thickness less than 0.45 mm, less than 0.40 mm, less than 0.35 mm, less than 0.30 mm, or less than 0.25 mm. In still other examples, when the porous sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter- electrode separator, the porous sintered bed in the ceramic inter-electrode separator has a thickness less than 0.15 mm, less than 0.1 mm, less than 0.08 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.02 mm. Preferably but not necessarily, the ceramic inter-electrode separator is highly robust despite the porous sintered bed being thin. Preferably, but not exclusively, the porous metal substrate employed, has a moderately open structure and finely structured components, as described above. Preferably, but not exclusively, the moderately open structure may involve a high porosity.

[0109] An outcome of the high robustness described above and the manner of their fabrication, is that the ceramic inter-electrode separators disclosed herein, are electrically non-conductive. When the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not necessarily, the porous sintered bed in the ceramic inter-electrode separator is highly electrically insulating, displaying an overall electrical resistance across its full thickness in the kΩ - MΩ range. When the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not necessarily, the electrical resistance of the porous sintered bed is more than 1 kΩ. In other examples, when the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not necessarily, the electrical resistance of the porous sintered bed is more than 10 kΩ, more than 50 kΩ, more than 100 kΩ, more than 200 kΩ, more than 300 kΩ, more than 500 kΩ, more than 750 kΩ, more than 1 MΩ, more than 10 MΩ, or more than 100 MΩ. Preferably but not necessarily, the ceramic inter- electrode separator and the porous sintered bed is highly robust even when it is highly electrically insulating.

[0110] An outcome of the high robustness described above and the manner of their fabrication, is that the ceramic inter-electrode separators disclosed herein, may be highly porous. When the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter- electrode separator, preferably but not necessarily, the porous sintered bed in the ceramic inter- electrode separator, has a high porosity of 55% to 95%. When the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not necessarily, the porous sintered bed in the ceramic inter-electrode separator, hasa porosity of more than 55%, more than 60%, more than 65%, more than 70%, more than 75%, more than 80%, more than 85%, or more than 90%. In other examples, the porous sintered bed in the ceramic inter-electrode separator has a porosity in the range of 60% to 95%, in the range of 65% to 95%, in the range of 70% to 95%, in the range of 75% to 95%, in the range of 80% to 95%, in the range of 85% to 95%, or in the range of 90% to 95%. When the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not necessarily, the ceramic inter-electrode separator as a whole and / or the porous sintered bed in the ceramic inter-electrode separator, is highly robust despite having a high overall porosity of 55% to 95%.

[0111] When the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not necessarily, because of the high porosity and low thickness, the porous sintered bed in the ceramic inter-electrode separator, when fully imbued with 30 wt% potassium hydroxide aqueous solution, has a low area specific ionic resistance (ASR) at 21oC. When the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not exclusively, the porous sintered bed in the ceramic inter-electrode separator, when fully imbued with 30 wt% potassium hydroxide aqueous solution, has an area specific ionic resistance (ASR) at 21oC of less than 0.2 Ω cm2. In other examples, when the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not exclusively, when fully imbued with 30 wt% potassium hydroxide aqueous solution, the porous sintered bed in the ceramic inter-electrode separator, has an area specific ionic resistance (ASR) at 21oC. of less than 0.4 Ω cm2, less than 0.35 Ω cm2, less than 0.3 Ω cm2, or less than 0.25 Ω cm2. In still other examples, when the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, preferably but not exclusively, when fully imbued with 30 wt% potassium hydroxide aqueous solution, the porous sintered bed in the ceramic inter-electrode separator, has an area specific ionic resistance (ASR) at 21oC of less than 0.19 Ω cm2, less than 0.15 Ω cm2, less than 0.1 Ω cm2, less than 0.09 Ω cm2, less than 0.08 Ω cm2, less than 0.07 Ω cm2, less than 0.06 Ω cm2, less than 0.05 Ω cm2, less than 0.04 Ω cm2, less than 0.03 Ω cm2, less than 0.02 Ω cm2, or less than 0.01 Ω cm2.

[0112] Preferably but not necessarily, the porous metal substrate in the ceramic inter-electrode separator is resistant to degradation in the strongly alkaline conditions employed in alkaline electrolysis, wherein the liquid electrolyte may, for example, comprise 30 wt% potassiumhydroxide aqueous solution. Preferably but not necessarily, the porous metal substrate in the ceramic inter-electrode separator comprises substantially of a nickel, a titanium, a zirconium, or a stainless-steel containing metal. For example, the porous metal substrate is formed of nickel, titanium, zirconium, stainless-steel, a metal alloy containing nickel, a metal alloy containing titanium, a metal alloy containing zirconium, or a metal alloy containing stainless-steel.

[0113] Preferably but not necessarily, the metal oxide / s in the porous sintered bed of the ceramic inter-electrode separator is / are resistant to degradation in the strongly alkaline conditions employed in alkaline electrolysis, wherein the liquid electrolyte may, for example, comprise 30 wt% potassium hydroxide aqueous solution. Preferably but not necessarily, the porous sintered bed in the ceramic inter-electrode separator comprises metal oxide particles of at least one metal oxide, being, preferably but not necessarily, a zirconium oxide or a titanium oxide.

[0114] In another example there is provided a method of fabricating a ceramic inter-electrode separator. The method comprising the steps of: (1) coating a porous metal substrate with a slurry or embedding the porous metal substrate within the slurry to form a coated or embedded substrate, wherein the slurry contains a polymer binder and metal oxide particles comprising at least one metal oxide; and (2) firing the coated or embedded substrate in a furnace, preferably under inert or reducing or oxidising atmosphere or an air atmosphere, at a temperature greater than 600oC. Preferably, the metal oxide particles comprise at least one distinct particle size.

[0115] Optionally, the method further comprises the initial step of cleaning and / or pre-treating the porous metal substrate. Optionally, the step of cleaning and / or pre-treating the porous metal substrate includes one or more of the following steps: cleaning the porous metal substrate by washing the porous metal substrate with, for example, an alcoholic solution, and thereafter drying the porous metal substrate; cleaning the porous metal substrate by pickling the porous metal substrate in an acidic solution; and / or heating the porous metal substrate in an oxygen atmosphere.

[0116] In another example, when the porous metal substrate has a 3D structure and the porous sintered bed is coated on substantially only one side of the porous metal substrate, cleaning and / or pre-treating the porous metal substrate comprises: filling the internal cavities of the porous metal substrate with a wax, paraffin, or a ‘blocking material’ that prevents the subsequently applied slurry from penetrating into the 3D structure of the porous metal substrate, wherein the wax, paraffin or‘blocking material’ is driven off and lost during the subsequent firing step, thereby leaving the porous metal substrate coated with the porous sintered bed on substantially only one side.

[0117] In another example there is provided a method of fabricating a ceramic inter-electrode separator. The method comprises the following sequence of steps: (1) Optionally, cleaning and / or pre-treating the porous metal substrate; (2) Coating the porous metal substrate with a slurry or embedding the porous metal substrate within a slurry, using a technique such as, but not limited to, dip-coating, knife coating, printing, or painting, wherein the slurry contains a polymer binder and at least one metal oxide in at least one distinct particle size; and (3) Firing the resulting coated or embedded substrate assembled in a furnace under inert or reducing or oxidising atmosphere or an air atmosphere at a temperature greater than 600oC.

[0118] Preferably but not necessarily, cleaning and / or pre-treating the porous metal substrate may include, in general: Cleaning the porous metal substrate by washing it with, for example, an alcoholic solution, and thereafter drying it; and / or Cleaning the porous metal substrate by pickling it in an acid for a short time to remove any layer of the corresponding metal oxide present on the metal surface; or, alternatively, Heating the porous metal substrate in an oxygen atmosphere to ensure the metal surface of the porous metal substrate is completely and uniformly covered by its corresponding oxide layer.

[0119] In the case where the porous metal substrate has a 3D structure (e.g. a metal foam) and the sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, then the method may include filling the internal cavities of the porous metal substrate with a wax, paraffin, or ‘blocking material’ that prevents the subsequently applied slurry from penetrating into the 3D structure of the porous metal substrate. The wax, paraffin or ‘blocking material’ is driven off and lost during the subsequent firing step, thereby leaving the porous metal substrate coated with the porous sintered bed on substantially only one side.

[0120] Preferably but not necessarily, coating the porous metal substrate with a slurry or embedding or encasing the porous metal substrate within a slurry may include using the polymer binder in the slurry in such proportions as required to ensure that the resulting porous sintered bed has the porosity described above.

[0121] In another example there is provided an electrochemical cell incorporating a ceramic inter-electrode separator as disclosed herein, between the electrodes of the electrochemical cell. In another example, the electrochemical cell is an electro-synthetic or electro-energy cell. In another example, the electrochemical cell is a water electrolysis cell or a hydrogen-oxygen fuel cell. In a further example, the electrochemical cell is an alkaline water electrolysis cell or alkaline fuel cell. Further Examples

[0122] The following examples provide more detailed discussion of particular embodiments. The examples are intended to be merely illustrative and not limiting to the scope of the present invention. Example 1: NiO-coated Ni mesh as the porous metal substrate, embedded within a porous sintered bed containing barium titanate (BaTiO3), with or without cerium oxide (CeO2)

[0123] In a first example, a nickel (Ni) mesh was used as the porous metal substrate. The mesh comprised Nickel 200 (Ni 200). The Ni mesh was 77 ^m thick on average and contained strands (wires) of thickness 50 ^m, with an open screening area of 34%. Meshes described herein were supplied by Haver and Boecker (Germany) or Century Woven (China).

[0124] Barium titanate (BaTiO3) was selected to be the metal oxide used in the porous sintered bed. BaTiO3 has an orthorhombic lattice with a lattice parameter, c = 3.98 Å. Its melting point is 1,625oC. By comparison, Ni has a cubic structure with a lattice parameter, a = 3.48 Å. Its melting point is 1,455oC. The corresponding oxide of Ni is nickel oxide (NiO), and it has a cubic lattice with a lattice parameter, a = 4.19 Å. Its melting point is 1,955oC.

[0125] As NiO has a lattice parameter that is closer in size to BaTiO3 than to Ni, the Ni mesh was initially heated in air to create a uniform but thin NiO coating on its surface. This ensured that during the subsequent sintering process, the BaTiO3coating would bond first (and more strongly) to the NiO on the pre-oxidized mesh. If the sintering was carried out in a reducing environment, the reduction of the NiO surface layer to Ni would further set the BaTiO3 in a compressive state that improves the ceramic-to-metal adherence.

[0126] The Ni mesh was, accordingly, pre-oxidised to create a uniform NiO coating on its surface by maintaining it at 900oC for 1 h in air.

[0127] Thereafter, a sinter slurry was prepared having the following composition:

[0128] Polyvinyl butyral 30 (PVB) from Kremer Pigmente (https: / / shop.kremerpigments.com / us / ) was used both as the dispersant and binder. Cerium oxide (CeO2) was added to illustrate the use of a second metal oxide and test whether it would be uniformly distributed in the final ceramic inter-electrode separator. CeO2has a cubic structure with a lattice parameter, a = 5.47 Å. Its melting point is 2,400oC.

[0129] The BaTiO3components were sourced from TPL, Inc (https: / / tplinc.com / ceramic- nanopowders) and comprised BaTiO3 particles of three distinct particle sizes, namely 400 nm (HPB4000), 200 nm (HPB2000), and 100 nm (HPB1000) in the following ratio:

[0130] The CeO2 had a different particle size to those of the BaTiO3 above. The azeotrope comprised a mixture of two solvents, namely toluene and methanol, in the ratio:

[0131] The Ni mesh, with the coating of NiO on its surface, was then dipped into the sinter slurry using a dip-coating machine set to dip the sample at a specified, preferably slow rate. Multiple dips may be employed.

[0132] As a result of the dipping process, the Ni mesh was coated on both sides with the sinter slurry. To form the ceramic inter-electrode separator, the resulting assembly was laid flat on a flat ceramic crucible within a tube furnace, covered with another flat ceramic plate, where the tube furnace included a reducing gas to create a reducing environment, and fired at 1,100oC for 16-20 h. During the firing, the azeotrope, dispersant and binder were burnt off and lost, leaving only the ceramic and metal components. The tube furnace may be filled, during the firing process, with a reducing gas (e.g. hydrogen) or a gas mixture that includes a reducing gas. In other examples, firing may be done under an inert gas atmosphere (or an oxidising atmosphere or an air atmosphere); that is, the sample may be bathed in an inert gas or a mixture of inert gases (or oxidising gas or air) during the firing.

[0133] The ceramic inter-electrode separator that was produced was exceedingly thin, being only 164 ^m thick overall, and did not exhibit any discernible cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion, even after being operated for some time as an inter-electrode separator in an alkaline electrolysis cell, thereby testifying to its robustness. Its overall thickness also constituted the thickness of the porous sintered bed, as the mesh (77 ^m thick) was fully embedded or encased within the porous sintered bed of metal oxides; that is, it was coated on both sides with the porous sintered bed (double-sided), which also occupied the vacancies in the mesh. The ceramic inter-electrode separator displayed a blue tint, confirming uniform incorporation of the Ce3+donor. Ceria doped oxides demonstrate photoluminescence excitation and violet-blue coloration when irradiated at wavelengths between 350 to 450 nm.

[0134] Scanning electron micrographs of the porous sintered bed on the ceramic inter-electrode separator showed the particle structure of the porous sintered bed. The scanning electron micrographs indicated the presence of three distinct, different particle sizes of BaTiO3in the sintered bed. To determine the sizes of the particles that were originally used to create the sintered bed, one must infer them using the equations of sintering kinetics for ‘neck’ growth. ‘Necking’ is the phenomenon wherein two particles merge together at a particular point, forming a ‘neck’ between them. The ‘neck’ is the structural bridge that forms between particles during sintering. Example 6 describes how one may infer the original particle sizes. In this case, the necking equations confirmed that three distinct, different particle sizes of BaTiO3 had also been used in the creation of the sintered bed.

[0135] The ceramic inter-electrode separator was also highly flexible, displaying a bend diameter of only 1 cm, which could be achieved without any observable damage, like cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion, even after flexing about its bend diameter many times. The ceramic inter-electrode separator was thereby shown to exhibit excellent robustness. Despite being very thin and very flexible, the ceramic inter-electrode separator exhibited an electrical resistance from its one face to the other face of 3 MΩ. When the ceramic inter-electrode separator was fully imbued with and immersed in a 6 M KOH aqueous solution, it displayed an area specific ionic resistance of 0.058 Ω cm2at 21oC. The ceramic inter-electrode separator, including the porous sintered bed and the Ni scaffold, displayed a porosity of about 75%.

[0136] The thermal expansion of BaTiO3 is: - Room temperature: 6×10−6 / °C - 200 °C: 11.3×10−6 / °C - 350°C: 10.8×10−6 / °C - 1,050°C: 17.5×10−6 / °C, whereas the thermal expansion of Ni is: - Room temperature: 13.5×10−6 / °C - 200 °C: 15.4×10−6 / °C - 350°C: 18.7×10−6 / °C - 1,050°C: 20.7×10−6 / °C,

[0137] The compressive strength of the BaTiO3 due to the thermal expansion mismatch or discrepancy was calculated using the Tsui & Stoney equation to be 485 MPa. The theoretical tensile stress due to the Ni reduction was calculated to be 10.5 GPa (which compares with the theoretical tensile stress of Ni, which is about 29.9 GPa).

[0138] The above process could be repeated without the CeO2; i.e. with only the BaTiO3 present in 67%, as the sole and single metal oxide, giving a similar ceramic inter-electrode separator, wherein the porous sintered bed was white in colour. The ceramic inter-electrode separator was 130 ^m thick (which was also the thickness of the porous sintered bed) and was similarly robust. The ceramic inter-electrode separator displayed a porosity of around 70%. The ceramic inter- electrode separator exhibited an electrical resistance from its one side to the other, of 100 kΩ. When the ceramic inter-electrode separator was imbued with and immersed in a 6 M KOH aqueous solution, it displayed an area specific resistance of 0.060 Ω cm2at 21oC.

[0139] The above processes could also be repeated using only a single particle size of the BaTiO3, namely any one of HPB4000 (400 nm particle size), HPB2000 (200 nm particle size), or HPB1000 (100 nm particle size). The ceramic inter-electrode separator produced displayed similar characteristics and was similarly robust to the examples above.

[0140] The above processes could be repeated without the Ni mesh being pre-oxidised to create a uniform NiO coating on its surface. The resulting ceramic inter-electrode separator were moderately less robust.

[0141] The above process could also be repeated with the Ni mesh, in un-oxidised or pre- oxidised form (i.e. with a surface coating of NiO), being sintered under inert atmosphere conditions, producing a similar, albeit moderately less robust, ceramic inter-electrode separator. Example 2: NiO-coated Ni mesh as the porous metal substrate, coated on one side only with a porous sintered bed containing barium titanate (BaTiO3) in three different particle sizes, as one of the metal oxides

[0142] A NiO-coated Ni mesh of thickness 58 ^m had a wax sheet placed on its one side. The wax was melted into the vacancies within the mesh. After cooling to room temperature, the waxsolidified, covering the side on which it was introduced (termed below: the ‘masked’ side), and occupying the vacancies in the mesh.

[0143] The resulting mesh was then dipped into the slurry described in Example 1 using the procedures described there. Following the dipping process, the Ni mesh was coated with the sinter slurry on only its one side, namely the side opposite to the masked side. To form the ceramic inter- electrode separator, this assembly was fired as described in Example 1.

[0144] The ceramic inter-electrode separator that was produced was coated on its one side with the sintered bed, which was 138 ^m thick. As the mesh employed was 58 ^m thick, the total thickness of the ceramic inter-electrode separator was 196 ^m. The porous sintered bed in the ceramic inter-electrode separator, excluding the metal scaffold, displayed a porosity of about 69%. Scanning electron micrographs of the porous sintered bed in the ceramic inter-electrode separator indicated that three different particle sizes of BaTiO3 had been used in its fabrication. It did not display any observable damage, like cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion, even after operating for some time as an inter-electrode separator in an alkaline electrolysis zero-gap cell.

[0145] Despite being so thin, the porous sintered bed within the ceramic inter-electrode separator exhibited an electrical resistance from its one side to the other of 100 kΩ. When imbued with and immersed in a 6 M KOH aqueous solution, the porous sintered bed of the ceramic inter- electrode separator, excluding the resistance of the Ni scaffold, displayed an area specific resistance of 0.038 Ω cm2at 21oC. Example 3: NiO-coated Ni mesh as the porous metal substrate, embedded within a porous sintered bed containing barium titanate (BaTiO3) in three different particle sizes, as one of the metal oxides, with inclusion of a pore former. Other means of pore formation and control.

[0146] The fabrication process described in Example 1 was repeated with inclusion of 10% corn starch as a pore former, and firing at 1,100oC for 3.5 h. A mesh of 93 ^m thickness, was coated on both sides with a coating of 11 ^m, giving the ceramic inter-electrode separator an overall thickness of only 115 ^m (which was also the thickness of the porous sintered bed). During firing of the ceramic inter-electrode separator, the corn starch was burnt off and removed, along with the azeotrope, binder and dispersant. Figure 2 (left image) depicts a scanning electron micrograph ofthe slurry thinly applied on the mesh prior to sintering, while Figure 2 (right image) shows the porous sintered bed that resulted from the sintering process.

[0147] The extraordinary robustness of the ceramic inter-electrode separator was further demonstrated by its remarkable flexibility, displaying a bend diameter without any observable coating damage, of an incredible 0.32 cm. The ceramic inter-electrode separator was thereby shown to exhibit excellent mechanical strength. It did not display any observable damage, like cracking or fracturing or crumbling or collapsing or flaking off or scraping off or loss, diminution, degradation, or corrosion, even after operating for some time as an inter-electrode separator in an alkaline electrolysis zero-gap cell.

[0148] Beyond the inclusion of pore formers, another method of creating and controlling pores and pore sizes in the sintered bed of a ceramic inter-electrode separator, is to carefully select and control the particle sizes (grain size) and particle size distributions, of the metal oxide particles incorporated into the sintered bed, as well as the sintering driving force during formation of the sintered bed. Ceramics are routinely made highly dense by incorporating carefully selected particle sizes into a sintering process and then applying a large sintering driving force. The driving force for the sintering reaction is the reduction in surface energy that results from neck formation between the particles, with resulting densification of the material. Solid state diffusion processes govern the densification rate with fine-grained ceramics tending to have higher densities compared to coarse-grained materials. Conversely therefore, the application of a small sintering driving force, on a mixture of carefully selected particle sizes, may be used to generate porosity and provide a measure of control of pore formation and size. Example 4: Additional demonstrations of ceramic inter-electrode separator robustness

[0149] To further assess the robustness of ceramic inter-electrode separators of the type described herein, an example inter-electrode separator was subjected to aggressive sonication in water for 1 hour at 80 kHz and 100% power (in an Elma Elmasonic P laboratory sonicator). Measurements of the weight of the ceramic inter-electrode separator before and after the sonication, revealed that only 0.18% by weight of the material in the ceramic inter-electrode separator was lost during the sonication. The water in the sonicator remained clear after the sonication was complete.

[0150] By contrast, when subjected to the same sonication treatment, a sample of the commercial separator Zirfon PERL®UTP 500, lost about 18% by weight of the material present in the separator during the first 5 minutes of sonication. The water in the sonicator became distinctly cloudy during the sonication due to the quantity of material released.

[0151] In a second study, an example ceramic inter-electrode separator of the type described herein, was attached to an overhead stirrer and rotated at a rapid 300 rpm for 1 hour while immersed in a beaker of water. Measurements of the weight of the ceramic inter-electrode separator before and after the experiment, revealed no loss of material. Example 5: An alkaline electrolysis cell incorporating a ceramic inter-electrode separator between its electrodes

[0152] The ceramic inter-electrode separators in Example 1 and Example 3 were successfully tested as inter-electrode separators in an alkaline electrolysis cell of so-called ‘capillary-fed’ design. The anode and cathode electrodes and the cell architecture used were those described in the scientific publication in Nature Communications Vol 13 (2022), page 1304. The electrodes were sandwiched and tightly compressed against opposite sides of the ceramic inter-electrode separator in a zero-gap architecture. The separator-electrode assembly was immersed in a 6 M KOH aqueous liquid electrolyte in a beaker. When an electrical current was applied across the anode and cathode electrodes, hydrogen gas was produced at the cathode and oxygen gas at the anode, with the cell requiring 1.54 V to generate a current density of 0.5 A / cm2. This was comparable to the performance obtained using a conventional inter-electrode separator (as described in the above scientific publication). The cell incorporating the ceramic inter-electrode separator exhibited low energy loss and low gas crossover that was also comparable to the above scientific publication (that is, the ceramic inter-electrode separator was as poorly permeable to the gases produced as the conventional separator originally employed). Example 6: Inferring the Size of the Particles used to form a Porous Sintered Bed.

[0153] Figure 2 depicts scanning electron micrographs of a slurry coating before being sintered (left image) and after it had been sintering into a porous sintered bed (right image). Figure 1 similarly depicts a scanning electron micrograph of a sintered bed comprising BaTiO3particles inan example embodiment ceramic inter-electrode membrane. As can be seen, two different particle sizes are clearly visible. However, these particle sizes are the product of, and result of the sintering process. To determine the particle sizes of the particles that were originally used to create the sintered bed, one must infer them using the equations of sintering kinetics for ‘neck’ growth. A ‘neck’ is the structural bridge that forms between particles during sintering. A common empirical and theoretical approach to describing neck growth (x) in the initial stage of sintering is a power- law relationship: (x / a)n=K⋅t⋅exp(−Q / RT) Where: x = Neck radius (necking size), a = Initial particle radius (initial particle size), n = Exponent that depends on the dominant mass transport mechanism, K = Pre- exponential factor, which includes material constants (e.g., surface energy, atomic volume, diffusion coefficients, and geometric factors), t = Sintering time, Q = Activation energy for the dominant mass transport mechanism, R = Universal gas constant, and T = Absolute sintering temperature. Connecting to Different Diffusion Mechanisms and Their 'n' Values: The value of 'n' (and the specific form of K) changes depending on the dominant mass transport mechanism: 1. Surface Diffusion (non-densifying): This mechanism leads to neck growth but generally no shrinkage (densification). (x / a)7=Ks⋅(DsγsVm / (a4kBT))⋅t Where: o Ds = Surface diffusion coefficient o γs = Surface energy o Vm= Molar volume o kB = Boltzmann constant2. Grain Boundary Diffusion (densifying): This is a common mechanism for densification in the initial and intermediate stages. (x / a)6=Kgb⋅(DgbδgbγsVm / (a3kBT))⋅t Where: o Dgb = Grain boundary diffusion coefficient o δgb = Grain boundary width 3. Lattice Diffusion (densifying): This also contributes to densification, especially at higher temperatures. (x / a)5=KL⋅(DLγsVm / (a2kBT))⋅t Where: o DL= Lattice diffusion coefficient Relating to Final Particle Size: While the above equations primarily describe neck growth in the initial stage, relating necking size to final particle size is more complex. The final particle size (often referred to as grain size) is influenced by grain growth, which occurs concurrently with densification, especially in the intermediate and final stages of sintering. Grain growth often follows a separate power-law relationship: Gm−G0m=C⋅t⋅exp(−Qgg / RT) Where:• G = Final grain size • G0 = Initial grain size (often approximated by initial particle size, a) • m = Grain growth exponent (typically 2 for normal grain growth) • C = Temperature-dependent constant related to grain boundary mobility • Qgg = Activation energy for grain growth Limitations and Considerations: • Stages of Sintering: The provided equations are largely for the initial stage of sintering. The sintered beds in the ceramic inter-electrode separators described here are sintered only in this initial stage. As sintering progresses through intermediate and final stages, the geometry changes, and different models are needed to describe pore shrinkage and grain growth. • Densification vs. Coarsening: Surface diffusion primarily leads to coarsening (neck growth without densification), while grain boundary and lattice diffusion contribute to densification (shrinkage). • Idealized Models: The above equations are based on idealized models (e.g., spherical particles, uniform size). Real powder sintering is more complex. • Dominant Mechanism: The dominant mechanism can change with temperature, particle size, and material properties. • Shrinkage: For densifying mechanisms, the neck growth is accompanied by shrinkage. The linear shrinkage (ΔL / L0) or volumetric shrinkage (ΔV / V0) can also be related to time, temperature, and initial particle size by power-law relationships, similar to the neck growth equations but with different exponents and constants.

[0154] Throughout this specification and the claims which follow, unless the context requires otherwise, the word "comprise", and variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated integer or step or group of integers or steps but not the exclusion of any other integer or step or group of integers or steps.

[0155] Embodiments and modes of operation may be said to broadly involve the parts, elements and features referred to or indicated herein, individually or collectively, in any or all combinations of two or more of the parts, elements or features, and wherein specific integers are mentioned herein which have known equivalents in the art to which the invention relates, such known equivalents are deemed to be incorporated herein as if individually set forth.

Claims

The Claims.

1. A ceramic inter-electrode separator for an electrochemical cell, comprising: a porous metal substrate; a porous sintered bed comprising metal oxide particles of at least one metal oxide, the metal oxide particles attached to each other and to the porous metal substrate; and wherein the porous sintered bed, taken in cross-section, has an overall thickness of less than 0.20 mm.

2. The ceramic inter-electrode separator of claim 1, wherein the ceramic inter-electrode separator, including the porous sintered bed, is flexible and has a bend diameter of less than 6 cm.

3. The ceramic inter-electrode separator of claim 1 or claim 2, wherein the porous sintered bed in the ceramic inter-electrode separator has a porosity in the range of 55% to 95%.

4. The ceramic inter-electrode separator of any one of claims 1 to 3, wherein the metal oxide particles comprise a single metal oxide, and the metal oxide particles of the single metal oxide comprise more than one distinct particle size.

5. The ceramic inter-electrode separator of any one of claims 1 to 3, wherein the metal oxide particles comprise two or more different metal oxides.

6. The ceramic inter-electrode separator of claim 5, wherein the metal oxide particles of at least one of the metal oxides have a distinct particle size when compared to the metal oxide particles of another one of the metal oxides.

7. The ceramic inter-electrode separator of any one of claims 1 to 6, wherein the metal oxide particles in the porous sintered bed have at least one distinct particle size smaller than 1 ^m.

8. The ceramic inter-electrode separator of any one of claims 1 to 7, wherein the metal oxide particles in the porous sintered bed have more than one distinct particle size and the more than one distinct particle size is smaller than 1 ^m.

9. The ceramic inter-electrode separator of any one of the preceding claims, wherein a surface of the porous metal substrate is partially or fully covered with a layer of the metal oxide particles.

10. The ceramic inter-electrode separator of any one of the preceding claims, wherein the porous sintered bed within the ceramic inter-electrode separator, taken in cross section, has an overall thickness of less than 0.15 mm, less than 0.1 mm, less than 0.08 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.02 mm.

11. The ceramic inter-electrode separator of any one of the preceding claims, wherein the ceramic inter-electrode separator, including the porous sintered bed, has a bend diameter of less than 5 cm, less than 4 cm, less than 3 cm, less than 2 cm, less than 1 cm, less than 0.5 cm, less than 0.4 cm, less than 0.3 cm, less than 0.2 cm, less than 0.1 cm or less than 0.05 cm.

12. The ceramic inter-electrode separator of any one of the preceding claims, wherein the porous sintered bed in the ceramic inter-electrode separator has a porosity in the range of 60% to 95%, in the range of 65% to 95%, in the range of 70% to 95%, in the range of 75% to 95%, in the range of 80% to 95%, in the range of 85% to 95%, or in the range of 90% to 95%.

13. The ceramic inter-electrode separator of any one of the preceding claims, wherein the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has a thickness of less than 0.20 mm.

14. The ceramic inter-electrode separator of any one of the preceding claims, wherein the ceramic inter-electrode separator, including the porous sintered bed and the porous metal substrate, has a thickness of less than 0.15 mm, less than 0.1 mm, less than 0.08 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.02 mm.

15. The ceramic inter-electrode separator of any one of the preceding claims, wherein the porous sintered bed within the ceramic inter-electrode separator is under compressive strain.

16. The ceramic inter-electrode separator of claim 15, wherein the compressive strain is due to a mismatch in thermal contraction of the metal and the ceramic components following fabrication of the ceramic inter-electrode separator.

17. The ceramic inter-electrode separator of claim 15 or claim 16, wherein the compressive strain is due to chemical reduction of a metal oxide layer on a surface of the porous metal substrate during high temperature fabrication of the ceramic inter-electrode separator.

18. The ceramic inter-electrode separator of any one of the preceding claims, wherein the porous metal substrate comprises a metal mesh, a metal net, a perforated metal plate, or a substantially 2D metal substrate.

19. The ceramic inter-electrode separator of claim 18, wherein the metal mesh, the metal net, the perforated metal plate, or the substantially 2D metal substrate, has an open screening area of 25% to 65%.

20. The ceramic inter-electrode separator of claim 18, wherein the metal mesh, the metal net, the perforated metal plate, or the substantially 2D metal substrate, has an open screening area of more than 65%, more than 70%, or more than 75%.

21. The ceramic inter-electrode separator of any one of claims 18 to 20, wherein the metal mesh, the metal net, the perforated metal plate, or the substantially 2D metal substrate, comprises wires or strands of diameter 0.015 to 0.190 mm.

22. The ceramic inter-electrode separator of any one of the preceding claims, wherein the porous metal substrate is fully or partially embedded within the porous sintered bed.

23. The ceramic inter-electrode separator of any one of the preceding claims, wherein the porous metal substrate is embedded within the porous sintered bed in the ceramic inter- electrode separator, and the porous metal substrate has a thickness equal to or less than 0.19 mm.

24. The ceramic inter-electrode separator of any one of the preceding claims, wherein the porous metal substrate is embedded within the porous sintered bed in the ceramic inter- electrode separator, and the porous metal substrate has a thickness less than 0.15 mm, less than 0.1 mm, less than 0.08 mm, less than 0.05 mm, less than 0.04 mm, less than 0.03 mm, or less than 0.02 mm.

25. The ceramic inter-electrode separator of any one of claims 1 to 21, wherein the porous sintered bed is substantially located on only one side of the porous metal substrate.

26. The ceramic inter-electrode separator of any one of claims 1 to 21, wherein, the porous sintered bed is coated on substantially one side of the porous metal substrate in the ceramic inter-electrode separator, and the porous metal substrate is a metal foam or similar 3D structure.

27. The ceramic inter-electrode separator of any one of the preceding claims, wherein the ceramic inter-electrode separator is electrically insulating.

28. The ceramic inter-electrode separator of any one of the preceding claims, wherein the electrical resistance of the ceramic inter-electrode separator, including the porous sintered bed and porous metal substrate, is more than 1 kΩ, more than 10 kΩ, more than 50 kΩ, more than 100 kΩ, more than 200 kΩ, more than 300 kΩ, more than 500 kΩ, more than 750 kΩ, more than 1 MΩ, more than 10 MΩ, or more than 100 MΩ.

29. The ceramic inter-electrode separator of any one of the preceding claims, wherein the electrical resistance of the porous sintered bed in the ceramic inter-electrode separator is more than 1 kΩ, more than 10 kΩ, more than 50 kΩ, more than 100 kΩ, more than 200 kΩ, more than 300 kΩ, more than 500 kΩ, more than 750 kΩ, more than 1 MΩ, more than 10 MΩ, or more than 100 MΩ.

30. The ceramic inter-electrode separator of any one of the preceding claims, wherein the porous sintered bed in the ceramic inter-electrode separator, when fully imbued with 30 wt% potassium hydroxide aqueous solution, has an area specific ionic resistance (ASR) at 21oC of less than 0.4 Ω cm2, less than 0.35 Ω cm2, less than 0.3 Ω cm2, less than 0.25 Ω cm2, less than0.2 Ω cm2, less than 0.15 Ω cm2, less than 0.1 Ω cm2, less than 0.09 Ω cm2, less than 0.08 Ω cm2, less than 0.07 Ω cm2, less than 0.06 Ω cm2, less than 0.05 Ω cm2, less than 0.04 Ω cm2, less than 0.03 Ω cm2, less than 0.02 Ω cm2, or less than 0.01 Ω cm2.

31. The ceramic inter-electrode separator of any one of the preceding claims, wherein the at least one metal oxide is a zirconium oxide or a titanium oxide.

32. The ceramic inter-electrode separator of any one of the preceding claims, wherein the porous metal substrate is formed of a material selected from the group of: nickel, titanium, zirconium, stainless-steel, a metal alloy containing nickel, a metal alloy containing titanium, a metal alloy containing zirconium, and a metal alloy containing stainless-steel.

33. An electrochemical cell including the ceramic inter-electrode separator of any one of the preceding claims.

34. An electro-synthetic or electro-energy cell including the ceramic inter-electrode separator of any one of claims 1 to 32.

35. A water electrolysis cell or a hydrogen-oxygen fuel cell including the ceramic inter- electrode separator of any one of claims 1 to 32.

36. An alkaline water electrolysis cell or alkaline fuel cell including the ceramic inter- electrode separator of any one of claims 1 to 32.

37. A method of fabricating a ceramic inter-electrode separator comprising the steps of: coating a porous metal substrate with a slurry or embedding the porous metal substrate within the slurry, to form a coated or embedded substrate, wherein the slurry contains a polymer binder and metal oxide particles comprising at least one metal oxide; and firing the coated or embedded substrate in a furnace, under inert or reducing atmosphere, at a temperature greater than 600oC.

38. The method of claim 37, wherein the coated or embedded substrate, taken in cross- section, has an overall thickness of less than 0.20 mm.

39. The method of claim 37 or 38, wherein the coated or embedded substrate is flexible and has a bend diameter of less than 6 cm.

40. The method of any one of claims 37 to 39, wherein the coated or embedded substrate has a porosity in the range of 55% to 95%.

41. The method of any one of claims 37 to 40, wherein the metal oxide particles comprise a single metal oxide, and the metal oxide particles of the single metal oxide comprise more than one distinct particle size.

42. The method of any one of claims 37 to 40, wherein the metal oxide particles comprise two or more different metal oxides.

43. The method of claim 42, wherein the metal oxide particles of at least one of the metal oxides have a distinct particle size when compared to the metal oxide particles of another one of the metal oxides.

44. The method of any one of claims 37 to 43, further comprising an initial step of: cleaning and / or pre-treating the porous metal substrate.

45. The method of claim 44, wherein the step of cleaning and / or pre-treating the porous metal substrate includes one or more of the following steps: cleaning the porous metal substrate by washing the porous metal substrate with an alcoholic solution, and thereafter drying the porous metal substrate; cleaning the porous metal substrate by pickling the porous metal substrate in an acid; and / or heating the porous metal substrate in an oxygen atmosphere.

46. The method of claim 44 or 45, wherein, when the porous metal substrate has a 3D structure and the porous sintered bed is coated on substantially only one side of the porous metal substrate, cleaning and / or pre-treating the porous metal substrate comprises:filling the internal cavities of the porous metal substrate with a wax, paraffin, or blocking material that prevents the subsequently applied slurry from penetrating into the 3D structure of the porous metal substrate, wherein the wax, paraffin or blocking material is driven off and lost during the subsequent firing step, thereby leaving the porous metal substrate coated with the porous sintered bed on substantially only one side.

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