A liquid composition for coating applications

Abstract

The invention relates to a liquid composition (10) for coating applications, said liquid composition (10) at least comprising: based on the total mass of the liquid composition (10), at least 10 mass-% water; one or more metals, said one or more metals at least comprising a first metal that is capable of adopting oxidation state +4; and at least one nitrogen-containing cation, wherein the ratio of the molar concentration of said nitrogen-containing cation in the liquid composition (10) to the molar concentration of the one or more metals in the liquid composition (10) is at least 0.05 : 1.00. The invention also relates to a method of producing liquid compositions for coating applications, to a method of coating a workpiece (12) and to an electrochemical cell.

Description

[0001]

[0002] Title: A liquid composition for coating applications

[0003] Specification

[0004] The invention relates to the field of coating technologies. Particularly, the invention relates to liquid compositions for coating applications, to methods of producing liquid compositions for coating applications, to methods of coating a workpiece with such liquid compositions, and to electrochemical cells that are coated by way of such methods.

[0005] Specifically, the invention relates to liquid compositions for use in electrochemical cells, in particular fuel cells and electrolyser cells. Fuel cells are energy conversion devices that allow for conversion of electrochemical fuel to electricity. Electrolyser cells may be considered as fuel cells running in reverse mode, i.e., using electricity to decompose a compound into its constituent parts, for example water into hydrogen and oxygen. Reversible cells are capable of operating in both modes.

[0006] Such electrochemical cells typically comprise cell chemistry layers that may be applied as thin coatings. The cell chemistry layers commonly include an electrolyte layer that is sandwiched between two electrode layers. In addition, the cell chemistry layers may comprise an electron barrier layer, e.g. between a cathode layer and the electrolyte layer. For such electron barrier layers, metal oxide coatings have proven advantageous.

[0007] The object of the invention is to provide a liquid composition that allows for achieving a greater layer thickness per coating process.

[0008] According to the invention there is provided a liquid composition for coating applications according to claim 1.

[0009] The liquid composition comprises, based on the total mass of the liquid composition, at least 10 mass-% of water.The liquid composition further comprises one or more metals. Said one or more metals at least comprise a first metal that is capable of adopting oxidation state +4. The first metal may already exhibit oxidation state +4 in the liquid composition. Alternatively, the first metal may exhibit another oxidation state in the liquid composition, other than oxidation state +4, e.g. oxidation state +3. Preferably, the first metal is capable of stably adopting oxidation state +4 by heating the first metal in an oxygencontaining atmosphere. Such a heat treatment is typically performed during the coating process to dry the formed layer. The one or more metals may comprise the first metal as the sole metal. Alternatively, the one or more metals may comprise at least one additional metal, in addition to the first metal. Said at least one additional metal may be capable of adopting oxidation state +4. Alternatively, said at least one additional metal may not be capable of adopting oxidation state +4.

[0010] The liquid composition further comprises at least one nitrogen-containing cation. As used herein, the term "nitrogen-containing cation" refers to a positively charged substance where nitrogen is the central atom holding the positive charge. The term includes nitrogen-containing cations having a permanent positive charge, e.g. tetraalkylammonium ions, as well as nitrogen-containing bases. As used herein, the term "nitrogen-containing base" refers to a substance comprising a nitrogen-containing functional group having a pKa value between 6 and 15, as measured for its conjugate acid. The ratio of the molar concentration, i.e., the molarity, of said nitrogen-containing cation in the liquid composition to the molar concentration, i.e., the molarity, of the one or more metals in the liquid composition is at least 0.05 : 1.00. Thus, the concentration of the one or more metals is at most 20 times larger than the concentration of the nitrogen-containing cation. Where the one or more metals comprises only the first metal, the ratio of the concentration of said nitrogen-containing cation in the liquid composition to the concentration of the first metal is at least 0.05 : 1.00. Where the one or more metals comprises at least one additional metal, in addition to the first metal, the ratio of the concentration of said nitrogencontaining cation in the liquid composition to the combined concentration of metals in the liquid composition is at least 0.05 : 1.00. Preferably, the ratio of the concentration of the nitrogen-containing cation in the liquid composition to the concentration of the one or more metals in the liquid composition is at least at least 0.05 : 1.00 and at most 4.00 : 1.00.

[0011] The inventors found that the above-described liquid composition allows for achieving a greater layer thickness per coating process. This is attributed to the presence of the nitrogen-containing cation in theliquid composition that stabilizes the formation of thick layers comprising the first metal. The presence of water in the liquid composition contributes to good solubility of the nitrogen-containing cation.

[0012] Preferably, where the one or more metals comprises at least one additional metal, in addition to the first metal, the first metal constitutes a major portion of the one or more metals. Thus, the concentration of the first metal constitutes to at least 50% of the combined concentration of metals in the liquid composition.

[0013] Preferably, the liquid composition for coating applications contains no substance that is classified as CMR. More preferred is that the liquid composition for coating applications contains only substances that are not classified as CMR, not classified as hazardous to health (GHS08), and not classified as toxic (GHS06).

[0014] The counterions of the nitrogen-containing cations are preferably selected from the group consisting of: nitrate, carboxylates such as formate, acetate, propionate, butyrate or iso-butyrate, hydroxycarboxylates such as glycolate or lactate, and halides.

[0015] The liquid composition may comprise at least one acid, e.g. at least one carboxylic acid. The preferred pH of the liquid composition (as corresponding to an aqueous system) is between 0.8 and 7.

[0016] In some preferred embodiments, the liquid composition is a solution. In these embodiments, the liquid composition may be used for coating as such. Such liquid composition will also be referred to as coating solutions herein. In other embodiments, at least one substance of the liquid composition may be suspended in the liquid composition. In these embodiments, the liquid composition may be an intermediate composition that is not yet suitable to be used for coating as such. Thus, additional measures have to be taken to obtain a coating solution from an intermediate composition.

[0017] In some preferred embodiments, the liquid composition comprises, based on the total mass of the liquid composition, at most 75 mass-% of water. Such a restriction of the mass concentration of water may improve the wetting behaviour of the liquid composition. Preferably, the liquid composition comprises, based on the total mass of the liquid composition, at most 50 mass-% of water, more preferably at most 25 mass-% of water.In some preferred embodiments, said first metal is zirconium. Zirconium is well suitable for the formation of layers in electrochemical cells, e.g. for the formation of electron blocking layers.

[0018] In some preferred embodiments, the ratio of the molar concentration of said nitrogen-containing cation in the liquid composition to the molar concentration of the one or more metals in the liquid composition is at least 0.1 : 1.0 and at most 0.6 : 1.0. This ratio proved especially effective where the first metal is zirconium.

[0019] Preferably, the pH value of the liquid composition is between 0.8 and 1.5. Such strongly acidic pH values proved especially suitable where the first metal is zirconium.

[0020] In some preferred embodiments, said first metal is cerium. Cerium is also well suitable for the formation of layers in electrochemical cells, e.g. for the formation of electron blocking layers.

[0021] In some preferred embodiments, the ratio of the molar concentration of said nitrogen-containing cation in the liquid composition to the molar concentration of the one or more metals in the liquid composition is at least 0.4 : 1.0 and at most 2.0 : 1.0. This ratio proved especially effective where the first metal is cerium.

[0022] Preferably, the pH value of the liquid composition is between 4.0 and 6.0. Such mildly acidic pH values proved especially suitable where the first metal is cerium.

[0023] In some preferred embodiments, said first metal is hafnium.

[0024] In some preferred embodiments, the at least one nitrogen-containing cation comprises one or more nitrogen-containing cations selected from the group consisting of: ammonium ions, particularly substituted ammonium ions such as primary ammonium ions, preferably (2-hydroxyethyl)ammonium ions, secondary ammonium ions, tertiary ammonium ions or quaternary ammonium ions, and guanidinium ions, particularly substituted guanidinium ions such as 1,1,3,3-tetramethylguanidine or 2-tert-butyl-l,l,3,3-tetramethylguanidine. Substituted ammonium ions may comprise different substituents such as alkyl groups or hydroxyalkyl groups. Substituted guanidinium ions may alsocomprise different substituents such as alkyl groups or hydroxyalkyl groups. Most preferably, where the first metal is zirconium, the liquid composition comprises dimethylammonium ions. Most preferably, where the first metal is cerium, the liquid composition comprises tetramethyl guanidinium ions.

[0025] In some preferred embodiments, said one or more metals comprise, in addition to said first metal, at least one additional metal. The additional metal can be used to modify the crystal structure of the first metal in a layer formed from the liquid composition. Thus, the additional metal may act as a dopant metal.

[0026] In some preferred embodiments, the one or more metals comprise scandium and / or yttrium as an additional metal. Those additional metals proved especially effective where the first metal is zirconium.

[0027] In some preferred embodiments, the one or more metals comprise gadolinium and / or praseodymium as an additional metal. Those additional metals proved especially effective where the first metal is cerium.

[0028] In some preferred embodiments, the molar concentration of the one or more metals in the liquid composition is at least 0.05 mmol / g and at most 1.00 mmol / g. Where the one or more metals only comprise the first metal, the concentration of the first metal in the liquid composition is at least 0.05 mmol / g and at most 1.00 mmol / g. Where the one or more metals comprise at least one additional metal, in addition to the first metal, the combined concentration of metals in the liquid composition is at least 0.05 mmol / g and at most 1.00 mmol / g.

[0029] In some preferred embodiments, the liquid composition comprises at least one organic solvent. The addition of an organic solvent may improve the wetting behaviour of the liquid composition. Preferably, the liquid composition comprises at least one polar organic solvent. These organic solvents are typically miscible with water. In some embodiments, the at least one organic solvent comprises only one organic solvent, preferably an alcohol. In some other embodiments, the at least one organic solvent comprises several different organic solvents, i.e., at least a first organic solvent and a second organic solvent. Where the at least one organic solvent comprises several different organic solvents, one of the organic solvents may be a primary solvent, i.e., this organic solvent constitutes a major amount among the organic solvents, and another one of the organic solvents may be a secondary solvent, i.e., this organicsolvent constitutes a minor amount among the organic solvents. Preferably, the primary solvent is an alcohol.

[0030] Preferably, the liquid composition comprises, based on the total mass of the liquid composition, at least 40 mass-% of organic solvent or organic solvents.

[0031] Preferably, the organic solvent or organic solvents have a standard boiling point of at most 250 °C, more preferably at most 200 °C. This has the advantage that the organic solvents are easily evaporated in a heat treatment that may be performed during coating or subsequently to coating. Most preferably, at least 90 mass-% of the liquid substances have a standard boiling point of at most 125 °C.

[0032] In some preferred embodiments, the at least one organic solvent comprises at least one organic solvent that is selected from the group consisting of: alcohols, particularly methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, iso-butanol, tert-butanol, pentanol, hexanol or isomers thereof, glycols, particularly ethylene glycol or 1,2-propylene glycol, glycol ethers, particularly methylglycol, ethylglycol, propylglycol, isopropylglycol, butylglycol or isomers thereof or isomers of propylene glycol monomethylether, glycol esters, particularly 2-methoxyethyl acetate or propylene glycol methyl ether acetate, carboxylic acids, particularly formic acid, acetic acid or propionic acid, carboxylic acid amides, particularly formamide or acetamide including mono- and dialkyl derivatives, nitriles, particularly acetonitrile or propionitrile, and ketones, particularly acetone or MEK.

[0033] In some preferred embodiments, the liquid composition comprises nitrate ions. Nitrate ions are preferred as counterions, because they decompose into non-hazardous gases under heating, namely into nitrogen oxides and oxygen. Additionally, nitrates of the first metals typically have high solubility in organic solvents such as alcohols.

[0034] In some preferred embodiments, the ratio of the molar concentration of nitrate ions in the liquid composition to the molar concentration of the first metal in the liquid composition is at least 1.0 : 1.0 and at most 2.0 : 1.0. This molar concentration of nitrate may be sufficient to achieve a high solubility of the first metal in organic solvents. Additionally, such lower values of at most 2.0 : 1.0 enable more moderate pH values, particularly where the first metal is zirconium.In some preferred embodiments, the liquid composition further comprises at least one carboxylic acid having 3 carbon atoms or more. The at least one carboxylic acid having 3 carbon atoms or more preferably comprises at least one carboxylic acid selected form the group consisting of: aliphatic carboxylic acids including unbranched carboxylic acids, particularly tridecanoic acid, and branched carboxylic acids, particularly pivalinic acid, and oxygen-substituted carboxylic acids, particularly having the formula CH3(OCH2)nCOOH with 10 > n > 1 or CH3(OCH2)oCH2COOH with 10 > o > 1. Particularly preferred are oxygen-substituted carboxylic acids of either one of the two formulas above with 4 > n or o > 2. One particularly preferred example is [2-(2-methoxyethoxy)ethoxy]acetic acid.

[0035] In some preferred embodiments, the liquid composition is void of a beta-dicarbonylate. Thus, no betadicarbonylate such as acetylacetonate is present in the liquid composition.

[0036] In some other preferred embodiments, the liquid composition comprises a beta-dicarbonylate. In such embodiments, the ratio of the molar concentration of the first metal, multiplied by the value of the positive charge of said first metal in the liquid composition, to the molar concentration of beta-dicarbonylate in the liquid composition is at least 10 : 1. For example, the positive charge of zirconium ions may be +4. Thus, where the liquid composition comprises zirconium ions exhibiting oxidation state +4 in the liquid composition, the ratio of the molar concentration of zirconium cations to the molar concentration of beta-dicarbonylate is at least 10 / 4 : 1, i.e., 2.5 : 1. Preferably, the ratio of the molar concentrations is at least 20 : 1.

[0037] In some preferred embodiments, the liquid composition further comprises at least one organic polymer. This may further improve the characteristics of the liquid composition. Specifically, the presence of at least one organic polymer may further increase the layer thickness that can be realised. Furthermore, the formation of cracks in formed layers may be prevented.

[0038] Preferably, the at least one organic polymer comprises at least one organic polymer having a molecular weight between 500 g / mol and 5,000,000 g / mol, more preferably between 5,000 g / mol and 500,000 g / mol.Preferably, the liquid composition for coating applications comprises, based on the total mass of the liquid composition, between 0.02 mass-% and 20 mass-% of organic polymer, more preferably between 0.05 mass-% and 5 mass-%.

[0039] In some preferred embodiments, the at least one organic polymer comprises one or more organic polymers selected from the group consisting of: polyvinylpyrrolidone, poly(vinylpyrrolidone-co-vinyl acetate), poly acrylic acid, polyacrylamides, poly(2-ethyl-2-oxazoline), cellulose and derivatives of cellulose, and polyethylene glycol.

[0040] The invention also relates to a method of producing liquid compositions for coating applications according to claim 20.

[0041] The method comprises providing starting materials, said starting materials at least comprising water, at least one nitrogen-containing cation, and one or more metals, said one or more metals at least comprising a first metal that is capable of adopting oxidation state +4.

[0042] The method further comprises combining said starting materials such that a liquid composition is obtained. Said liquid composition comprises, based on the total mass of the liquid composition, at least 10 mass-% of water. Further, the ratio of the molar concentration of said nitrogen-containing cation in the liquid composition to the molar concentration of the one or more metals in the liquid composition is at least 0.05 : 1.00, more preferably at least 0.05 : 1.00 and at most 4.0 to 1.0. Thus, the starting materials are used in suitable quantities to realize a liquid composition with the above features.

[0043] Additional constituents may be added as described above in the context of the liquid composition.

[0044] In some preferred embodiments, the method comprises providing an aqueous first metal nitrate solution, e.g. an aqueous zirconium nitrate solution, an aqueous cerium nitrate solution or an aqueous hafnium nitrate solution, or an aqueous first metal oxonitrate solution, e.g. an aqueous zirconium oxonitrate solution or an aqueous hafnium oxonitrate solution, as a starting material. In these embodiments, the first metal typically already has a high solubility in organic solvents such as alcohols.In some preferred embodiments, the method comprises providing an aqueous first metal carboxylate solution, e.g. an aqueous zirconium carboxylate solution, an aqueous cerium carboxylate solution or an aqueous hafnium carboxylate solution, as a starting material. Aqueous carboxylate solutions of the first metal are typically available at low costs. Further, providing the first metal as a carboxylate has the advantage that the amount of nitrate present in the liquid composition can be reduced to a desired value. Preferably, the aqueous first metal carboxylate solution is an aqueous first metal acetate solution.

[0045] In some preferred embodiments, the method further comprises adding nitric acid. The addition of nitric acid is especially beneficial where an aqueous first metal carboxylate solution is used as a starting material. Due to its low pKa value of around -1.3, nitric acid protonates carboxylate ions present in the aqueous first metal carboxylate solution upon addition. Thus, nitrate ions are associated as counterions to the first metal, thereby increasing the solubility of the first metal in organic solvents such as alcohols. Nitric acid may be added as concentrated nitric acid, i.e., an aqueous solution containing 68% to 70% nitric acid. Alternatively, a more diluted aqueous solution of nitric acid may be added, e.g. containing 50% nitric acid, containing 20% nitric acid or containing 10% nitric acid. Preferably, nitric acid is added in a quantity such that the ratio of the molar concentration of nitrate ions in the liquid composition to the molar concentration of the first metal in the liquid composition is at least 1.0 : 1.0 and at most 2.0 : 1.0.

[0046] In some embodiments, the method may comprise heating the liquid composition at a heating temperature of at least 40 °C. A heat treatment of the liquid composition may be advantageous where an aqueous first metal carboxylate solution is used as a starting material. Specifically, an excess of carboxylic acid present in the starting material and / or formed due to the addition of nitric acid may be removed or partially removed by heating the liquid composition.

[0047] The invention also relates to a method of coating a workpiece according to claim 24. The method comprises providing a workpiece to be coated and coating the workpiece with a liquid composition as described above. Coating the workpiece may comprise a number of subsequent coating steps. Thus, the coating may be obtained by subsequently forming a number of stacked layers on the workpiece.

[0048] In some preferred embodiments, the workpiece is made of ceramic or metal. Preferably, the workpiece is made of ceramic.In some preferred embodiments, the workpiece is a support plate for an electrochemical cell, preferably a solid oxide fuel cell or solid oxide electrolyser cell. In these embodiments, coating the workpiece preferably comprises forming at least one layer, particularly at least one electrode layer, at least one electron barrier layer and / or at least one electrolyte layer, on the support plate.

[0049] The invention also relates to an electrochemical cell with the features of claim 26. The electrochemical cell comprises a support plate that is coated with at least one layer. Said layer is formed on the support plate by a method as described above.

[0050] Further embodiments are derivable from the following description and the drawings.

[0051] In the drawings:

[0052] Figure 1 shows a flow chart of a method of producing liquid compositions for coating applications; and

[0053] Figure 2 shows a flow chart of a method of coating a workpiece.

[0054] Figure 1 shows a flow chart of a method of producing liquid compositions 10 for coating applications.

[0055] In a first step 101, starting materials for the liquid composition 10 for coating applications are provided. The starting materials at least comprise water, at least one nitrogen-containing cation, and one or more metals, said one or more metals at least comprising a first metal that is capable of adopting oxidation state +4.

[0056] The first metal may be zirconium, cerium or hafnium, for example. In this example, the first metal is zirconium.

[0057] The nitrogen-containing cation may be an ammonium cation or a guanidinium cation, for example. In this example, the nitrogen-containing cation is provided by a dimethylammonium salt, e.g. dimethylammonium chloride.In a second step 103, the starting materials are combined, thus forming the liquid composition 10 for coating applications. Preferably, combining the starting materials yields the liquid composition 10 as a clear solution that can be used for coating as such, i.e., as a coating solution. The quantities of the starting materials are chosen such that the liquid composition 10 comprises, based on the total mass of the liquid composition 10, at least 10 mass-% of water, preferably at least 10 mass-% of water and at most 25 mass-% of water. Further, the ratio of the molar concentration of said nitrogen-containing cation in the liquid composition 10 to the molar concentration of the one or more metals in the liquid composition 10 is at least 0.05 : 1.00.

[0058] The starting materials may be provided as separate entities. Alternatively, a composition comprising at least two of the starting materials may be provided.

[0059] In one example, an aqueous zirconium nitrate solution is provided in step 101, said aqueous zirconium nitrate solution comprising water and the first metal, i.e., zirconium.

[0060] In another example, an aqueous zirconium carboxylate solution, e.g. an aqueous zirconium acetate solution, is provided in step 101, said aqueous zirconium carboxylate solution comprising water and the first metal, i.e., zirconium.

[0061] Where an aqueous first metal carboxylate solution is provided, the method preferably further comprises adding nitric acid. Nitric acid may be added to the aqueous first metal carboxylate solution or to the liquid composition 10. The addition of nitric acid has the effect that carboxylate ions are protonated, thereby forming carboxylic acids and nitrate ions. Hence, at least some of the carboxylate ions associated with the first metal ions are exchanged by nitrate ions. This typically increases the solubility of the first metal in organic solvents such as alcohols.

[0062] Preferably, additional substances are provided as starting materials and added to the liquid composition 10.

[0063] In some examples, the method further comprises adding at least one organic solvent, preferably an alcohol. Preferably, the organic solvent is added in a quantity such that the liquid composition 10comprises, based on the total mass of the liquid composition 10, at least 40 mass-% of organic solvent or organic solvents.

[0064] In some examples, the method further comprises adding at least one additional metal. Thus, the one or more metals comprise the first metal, e.g. zirconium, and at least one additional metal. The additional metal may be scandium, yttrium, gadolinium and / or praseodymium. Where the first metal is zirconium, it is preferred that scandium and / or yttrium are added as additional metals.

[0065] In some examples, the method further comprises adding at least one organic polymer. Preferably, the organic polymer is added in a quantity such that the liquid composition 10 comprises, based on the total mass of the liquid composition 10, between 0.02 mass-% and 20 mass-% of organic polymer.

[0066] Figure 2 shows a flow chart of a method of coating a workpiece 12.

[0067] In a first step 201, a liquid composition 10 is provided as described above.

[0068] In a second step 203, a workpiece 12 to be coated is provided. In this example, the workpiece 12 is a support plate 14 for an electrochemical cell, particularly a solid oxide fuel cell or a solid oxide electrolyser cell.

[0069] In a third step 205, the workpiece 12 is coated with the liquid composition 10. Thus, the workpiece 12 is provided with a coating 16. In the example shown in Figure 2, only one side of the workpiece 12 is coated with the liquid composition 10. Alternatively, several sides of the workpiece 12 may be coated. In the example shown in Figure 2, the coating 16 may be an electrode layer, an electrolyte layer or an electron barrier layer.

[0070] Conventional coating techniques may be used for providing workpiece 12 with coating 16. Preferably, coating 16 is provided on workpiece 12 by spray coating, drop coating or doctor blade coating.

[0071] Coating the workpiece 12 with the liquid composition 10 may comprise several coating steps. Thus, the coating 16 may be build up by several layers that are stacked upon one another. As a result of the presence of the nitrogen-containing cation in the liquid composition 10, layers with a comparatively highlayer thickness can be obtained in each coating step. In consequence, the number of coating steps may be reduced compared with the use of less concentrated liquid compositions.

[0072] Examples

[0073] Example 1: A first liquid composition for coating applications

[0074] 119 g of an aqueous zirconium oxonitrate solution containing 35% zirconium oxonitrate, 17.3 g YfNOaJa, 42 g water, 20 g acetic acid, 10 g of an aqueous dimethylamine solution containing 40% dimethylamine, 6 g polyvinylpyrrolidone 55,000, 280 g ethanol and 70 g l-methoxy-2-propanol were used as starting materials.

[0075] A clear solution, i.e., the first liquid composition 10 for coating applications, was obtained by combining the starting materials.

[0076] Example 2: A second liquid composition for coating applications

[0077] 80 g cerium nitrate, 16.5 g gadolinium nitrate, 120 g water, 22 g acetic acid, 8 g tetramethyl guanidine, 6 g polyvinylpyrrolidone 55,000, 250 g ethanol and 50 g l-methoxy-2-propanol were used as starting materials.

[0078] A clear solution, i.e., the second liquid composition 10 for coating applications, was obtained by combining the starting materials.

[0079] Example 3: A third liquid composition for coating applications

[0080] 100 g zirconium acetate in diluted acetic acid, 22 g ammonium nitrate, 17.3 g YfNOaJa, 80 g of an aqueous solution of acetic acid containing 20% acetic acid, 6 g polyvinylpyrrolidone 55,000, 286 g ethanol and 54 g l-methoxy-2-propanol were used as starting materials.

[0081] A clear solution, i.e., the third liquid composition 10 for coating applications, was obtained by combining the starting materials.

Claims

Claims1. A liquid composition (10) for coating applications, said liquid composition (10) at least comprising:based on the total mass of the liquid composition (10), at least 10 mass-% water; one or more metals, said one or more metals at least comprising a first metal that is capable of adopting oxidation state +4; andat least one nitrogen-containing cation, wherein the ratio of the molar concentration of said nitrogen-containing cation in the liquid composition (10) to the molar concentration of the one or more metals in the liquid composition (10) is at least 0.05 : 1.00, preferably at least 0.05 : 1.00 and at most 4.00 : 1.00.

2. The liquid composition (10) according to claim 1, wherein the liquid composition (10) is a solution.

3. The liquid composition (10) according to any of the preceding claims, wherein the liquid composition (10) comprises, based on the total mass of the liquid composition (10), at most 75 mass-% of water, more preferably at most 50 mass-% of water, more preferably at most 25 mass-% of water.

4. The liquid composition (10) according to any of the preceding claims, wherein said first metal is zirconium.

5. The liquid composition (10) according to the preceding claim, wherein the ratio of the molar concentration of said nitrogen-containing cation in the liquid composition (10) to the molar concentration of the one or more metals in the liquid composition (10) is at least 0.1 : 1.0 and at most 0.6 : 1.0.

6. The liquid composition (10) according to any of the preceding claims, wherein said first metal is cerium.

7. The liquid composition (10) according to the preceding claim, wherein the ratio of the molar concentration of said nitrogen-containing cation in the liquid composition (10) to the molarconcentration of the one or more metals in the liquid composition (10) is at least 0.4 : 1.0 and at most 2.0 : 1.0.

8. The liquid composition (10) according to any of the preceding claims, wherein the at least one nitrogen-containing cation comprises at least one nitrogen-containing cation that is selected from the group consisting of: ammonium ions, particularly substituted ammonium ions, and guanidinium ions, particularly substituted guanidinium ions.

9. The liquid composition (10) according to any of the preceding claims, wherein said one or more metals comprise, in addition to said first metal, at least one additional metal.

10. The liquid composition (10) according to the preceding claim, wherein the one or more metals comprise scandium and / or yttrium as an additional metal.

11. The liquid composition (10) according to claim 9, wherein the one or more metals comprise gadolinium and / or praseodymium as an additional metal.

12. The liquid composition (10) according to any of the preceding claims, wherein the molar concentration of the one or more metals in the liquid composition (10) is at least 0.05 mmol / g and at most 1.00 mmol / g.

13. The liquid composition (10) according to any of the preceding claims, wherein the liquid composition (10) comprises at least one organic solvent, preferably at least one polar organic solvent.

14. The liquid composition (10) according to the preceding claim, wherein the at least one organic solvent comprises at least one organic solvent that is selected from the group consisting of: alcohols, particularly methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-butanol, isobutanol, tert-butanol, pentanol or hexanol, glycols, particularly ethylene glycol or 1,2-propylene glycol, glycol ethers, particularly methylglycol, ethylglycol, propylglycol, isopropylglycol, butylglycol including isomers thereof or isomers of propylene glycol monomethylether, glycol esters, particularly 2-methoxyethyl acetate or propylene glycol methyl ether acetate, carboxylic acids, particularly formic acid, acetic acid or propionic acid, carboxylic acid amides, particularly formamide or acetamide including mono- and dialkyl derivatives, nitriles, particularly acetonitrile or propionitrile, and ketones, particularly acetone or MEK.

15. The liquid composition (10) according to any of the preceding claims, wherein the liquid composition (10) comprises nitrate ions.

16. The liquid composition (10) according to the preceding claim, wherein the ratio of the molar concentration of nitrate ions in the liquid composition (10) to the molar concentration of the first metal in the liquid composition (10) is at least 1.0 : 1.0 and at most 2.0 : 1.0.

17. The liquid composition (10) according to any of the preceding claims, wherein the liquid composition (10) comprises at least one carboxylic acid having 3 carbon atoms or more.

18. The liquid composition (10) according to any of the preceding claims, whereinthe liquid composition (10) is void of a beta-dicarbonylate, or wherein the liquid composition (10) comprises a beta-dicarbonylate, wherein the ratio of the molar concentration of the first metal in the liquid composition (10), multiplied by the value of the positive charge of said first metal in the liquid composition (10), to the molar concentration of beta-dicarbonylate in the liquid composition (10) is at least 10.0 : 1.0, preferably at least 20.0 : 1.0.

19. The liquid composition (10) according to any of the preceding claims, wherein the liquid composition (10) comprises at least one organic polymer.

20. A method of producing a liquid composition (10) for coating applications, said method comprising:providing starting materials, said starting materials at least comprising- water,- one or more metals, said one or more metals at least comprising a first metal that is capable of adopting oxidation state +4, and- at least one nitrogen-containing cation; andcombining said starting materials such that a liquid composition (10) is obtained, wherein:- the liquid composition (10) comprises, based on the total mass of the liquid composition (10), at least 10 mass-% water, and- the ratio of the molar concentration of said nitrogen-containing cation in the liquid composition (10) to the molar concentration of the one or more metals inthe liquid composition (10) is at least 0.05 : 1.00, preferably at least 0.05 : 1.00 and at most 4.0 : 1.0.

21. The method according to the preceding claim, wherein the method comprises providing an aqueous first metal nitrate solution or an aqueous first metal oxonitrate solution as a starting material.

22. The method according to claim 20, wherein the method comprises providing an aqueous first metal carboxylate solution as a starting material.

23. The method according to claim 22, wherein the method further comprises adding nitric acid.

24. A method of coating a workpiece (12), said method comprising:providing a workpiece (12) to be coated; andcoating the workpiece (12) with a liquid composition (10) for coating applications according to any one of claims 1 to 19.

25. The method according to the preceding claim, wherein the workpiece (12) is made of ceramic or metal.

26. The method according to any of claims 24 and 25, wherein the workpiece (12) is a support plate (14) for an electrochemical cell, preferably a solid oxide fuel cell or solid oxide electrolyser cell, and wherein coating the workpiece (12) comprises forming at least one layer (16), particularly at least one electrode layer, at least one electron barrier layer and / or at least one electrolyte layer, on the support plate (14).

27. Electrochemical cell, comprising a support plate (14) that is coated with at least one layer (16), said layer (16) being formed on the support plate (14) by a method according to claim 26.

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