A method of producing metal compounds for coating applications

Abstract

The invention relates to a method for producing metal compounds for coating applications (10), said method comprising providing a metal beta-dicarbonylate, wherein the metal is a metal of the fourth group in the periodic table, providing a metal carboxylate, wherein the metal is said metal of the fourth group in the periodic table, forming a reaction mixture comprising said metal beta-dicarbonylate, said metal carboxylate, and at least one solvent, and reacting the metal beta-dicarbonylate with the metal carboxylate to obtain a metal compound for coating applications (10). The invention also relates to a metal compound for coating applications (10), to a coating solution (12) and to a method of coating a work piece (14).

Description

[0001] Title : A method of producing metal compounds for coating applications

[0002] Specification

[0003] The invention relates to methods of producing metal compounds for coating applications , to metal compounds for coating applications produced by way of such methods , to coating solutions comprising such metal compounds for coating applications , and to methods of coating a work piece with such a coating solution .

[0004] Specifically, the invention relates to metal compounds 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 .

[0005] 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 . Aqueous metal acetate solutions are interesting candidates as metal source for the production of such metal oxide layers because they are typically available at low costs . However, they are generally difficult to apply as a coating due to their poor wetting behavior . Metal beta-dicarbonylates are another possible metal source for metal oxide layers . However , metal beta- dicarbonylates are rather expensive and have limited storage stability in organic solutions .

[0006] It is an obj ect of the present invention to provide a costefficient way of producing metal compounds for coating applications . Said metal compounds for coating applications shall have a high solubility and a high storage stability in organic solvents , particularly in alcohols .

[0007] According to the invention there is provided a method of producing metal compounds for coating applications according to claim 1 . The method comprises the step of providing a metal beta-dicarbonylate , wherein the metal of said metal betadicarbonylate is a metal of the fourth group in the periodic table , i . e . the titanium group in the periodic table . The method further comprises providing a metal carboxylate , wherein the metal of said metal carboxylate is said metal of the fourth group in the periodic table . That is to say, the metal of said metal beta-dicarbonylate and the metal of said metal carboxylate are the same metal . The method further comprises forming a reaction mixture comprising said metal beta-dicarbonylate , said metal carboxylate , and at least one solvent . The method further comprises reacting the metal beta-dicarbonylate with the metal carboxylate to obtain a metal compound for coating applications . Thus , said metal beta-dicarbonylate and said metal carboxylate react with one another under the formation of said metal compound for coating applications . In other words , said metal beta-dicarbonylate and said metal carboxylate are the starting materials , whereas said metal compound for coating applications is the product .

[0008] It was found that metal beta-dicarbonylates and metal carboxylates react with one another under the formation of metal compounds that display high solubility in organic solvents , particularly in alcohols . Thus , metal beta-dicarbonylates can be used as a means of converting inexpensive metal carboxylates into metal compounds that are better suitable for organic coating solutions . Accordingly, the drawbacks of metal carboxylates as metal sources for coating applications can be overcome by reacting the metal carboxylates with corresponding metal beta-dicarbonylates .

[0009] In some embodiments , providing the metal beta-dicarbonylate comprises providing a metal beta-dicarbonylate of the chemical formula M (A) 4 , wherein M represents said metal of the fourth group in the periodic table , and wherein A represents said beta- dicarbonylate . Accordingly, the stoichiometry of metal to beta- dicarbonylate is preferably 1 : 4 .

[0010] The metal carboxylate is preferably provided in a stoichiometry of metal to carboxylate of 1 : 2 . However , in some embodiments , the stoichiometry of metal to carboxylate can also differ from 1 : 2 . Additional anions may be coordinated or bound to the metal of said metal carboxylate ( i . e . , in addition to carboxylate anions ) . This is typically oxygen-containing anions , preferably in the form of hydroxyl groups . Thus , the metal carboxylate may be a metal hydroxide carboxylate . In some preferred embodiments , said reaction mixture is a solution of said metal beta-dicarbonylate and said metal carboxylate in said at least one solvent . However, in some other embodiments , the metal beta-dicarbonylate and / or the metal carboxylate may be suspended in the at least one solvent .

[0011] In some embodiments , the reaction mixture comprises only one solvent . In some other embodiments , the reaction mixture comprises more than one solvent , i . e . , a solvent mixture comprising at least two solvents . The reaction mixture may be formed in a suitable reaction vessel such as a reaction flas k or a crystallisation dish .

[0012] Said metal of the fourth group in the periodic table may be hafnium . Preferably, said metal of the fourth group in the periodic table is zirconium.

[0013] In some preferred embodiments , said beta-dicarbonylate is a beta-diketonate . Thus , both carbonyl groups of the beta- dicarbonylate may be a ketone . Alternatively, one or both of the carbonyl groups of said beta-dicarbonylate may be a carbonyl group that is not a ketone . Particularly, one or both of the carbonyl groups of said beta-dicarbonylate may be an aldehyde , the carbonyl group of a carboxylic acid or the carbonyl group of an ester .

[0014] Preferably, said beta-dicarbonylate is acetylacetonate . Metal acetylacetonates are typically available at comparatively low costs . Moreover, metal acetylacetonates proved to reliably react with corresponding metal carboxylates . Most preferably, said metal beta-dicarbonylate is zirconium acetylacetonate . Accordingly, said metal carboxylate is a zirconium carboxylate.

[0015] In some preferred embodiments, said carboxylate is acetate. Metal acetates are typically inexpensive variants of said metal carboxylate. Alternatively, said carboxylate may be another carboxylate, particularly propionate.

[0016] In some preferred embodiments, said at least one solvent is an alcohol. Alcohols are preferred due to their beneficial dissolving properties. Furthermore, alcohols are typically miscible with water. This facilitates using an aqueous solution of the metal carboxylate as a starting material.

[0017] In some preferred embodiments, said at least one solvent is selected from the group consisting of: ethanol, propanol, isopropanol, octanol and cyclohexanol.

[0018] As mentioned above, the reaction mixture may comprise more than one solvent (i.e., at least a first solvent and a second solvent that is different from the first solvent) . Preferably, the first solvent is an alcohol, e.g. ethanol, propanol or iso-propanol. The second solvent is preferably selected from the group consisting of: alcohols, particularly high-boiling alcohols such as octanol or cyclohexanol, and ethers, particularly diethylene glycol dimethyl ether or diethylene glycol monobutyl ether. The mass ratio of said second solvent to said metal of the fourth group in the periodic table, calculated as metal oxide, in the reaction mixture may be between 0.1:1 and 20:1. As used herein, a "high-boiling alcohol" refers to an alcohol having a boiling temperature of 140 °C or more. Preferably, the molar ratio of metal carboxylate to metal betadicarbonylate in said reaction mixture is between 0 . 5 : 1 and 10 : 1 . Such molar ratios resulted in metal compounds having particularly desirable properties regarding their use in coating applications . Furthermore , due to the significant part of metal carboxylate , the costs are comparatively low .

[0019] In some preferred embodiments , said metal beta-dicarbonylate is provided as a liquid composition comprising said at least one solvent and the metal beta-dicarbonylate , wherein said reaction mixture is formed by adding said metal carboxylate to said liquid composition . Alternatively, said metal beta-dicarbonylate and said metal carboxylate may be mixed with one another as solids , followed by the addition of the at least one solvent .

[0020] Preferably, said metal carboxylate is provided as an aqueous solution . Aqueous solutions of metal carboxylates are typically commercially available and inexpensive . As mentioned above , the stoichiometry of metal to carboxylate can differ from 1 : 4 . In case the metal carboxylate is provided as an aqueous solution, the chemical formula of the metal carboxylate may be

[0021] M (CH3COO )x(OH ) y, wherein M represents said metal of the fourth group in the periodic table , and wherein x + y is 4 or approximately 4 . Thus , the metal carboxylate is present in said aqueous solution as a metal acetate hydroxide . The concentration of the metal of said metal carboxylate in said aqueous solution is preferably between 15 wt . -% and 20 wt . -% . As mentioned above , it is preferred that said metal of the fourth group in the periodic table ( i . e . , the metal of said metal beta- dicarbonylate and said metal carboxylate ) is zirconium. Thus , preferably an aqueous zirconium carboxylate solution is provided as the metal carboxylate , most preferably an aqueous zirconium acetate solution .

[0022] In some preferred embodiments , the metal beta-dicarbonylate and the metal carboxylate are reacted at room temperature . Thus , reacting said metal beta-dicarbonylate with said metal carboxylate comprises keeping the reaction mixture at room temperature , particularly stirring the reaction mixture at room temperature . It was found that said metal beta-dicarbonylate and said metal carboxylate may readily react to form said metal compound for coating applications at room temperature .

[0023] In some preferred embodiments , the metal beta-dicarbonylate and the metal carboxylate are reacted at a heating temperature of at least 40 ° C . Thus , the reaction mixture may be heated to at least 40 ° C in order to react said metal beta-dicarbonylate with said metal carboxylate . Even though the reaction may be carried out at room temperature , heating the reaction mixture is preferred to increase the reaction kinetics . Preferably, the metal beta-dicarbonylate and the metal carboxylate are reacted at a heating temperature of at least 40 ° C and at most 150 ° C, most preferably at a heating temperature of at least 40 ° C and at most 100 ° C .

[0024] In some preferred embodiments , the liquids are partially or completely removed from said reaction mixture . Complete removal of the liquids yields the metal compound for coating applications as a solid . In the solid form, the metal compound for coating applications is particularly storage-stable . Removal of the liquids may comprise evaporating the liquids under increased temperature and / or under reduced pressure . Removal of the liquids may also comprise precipitating the metal compound for coating applications and removing the liquids by filtration . Partial removal of the liquids yields the metal compound for coating applications as a concentrated liquid composition, particularly a concentrated solution .

[0025] In some preferred embodiments , the liquids are removed at a temperature of at least 50 ° C and at most 180 ° C , preferably at a temperature of at least 70 ° C and at most 130 ° C . Removal of the liquids may comprise heating the reaction mixture in an oven, particularly in a convection oven .

[0026] The invention also relates to a metal compound for coating applications according to claim 16 . The metal compound for coating applications is produced by way of a method as described above . The metal compound may be a solid or a part of a solid composition . Alternatively, the metal compound may be a part of a liquid composition . Specifically, the metal compound may be dissolved or suspended in said liquid composition .

[0027] The invention also relates to a coating solution as disclosed in the claims . The coating solution comprises a metal compound for coating applications as described above . The coating solution further comprises at least one solvent . The metal compound for coating applications is dissolved in said at least one solvent .

[0028] Where the above-described method for producing metal compounds for coating applications yields the metal compound for coating applications as a part of a solution, said solution may readily constitute the coating solution . Preferably, the coating solution is an alcoholic coating solution, i . e . the coating solution comprises at least one alcohol as a solvent .

[0029] In some preferred embodiments , the concentration of said metal compound for coating applications , calculated as metal oxide , in the coating solution is between 5 wt . -% and 40 wt . -% . A coating solution comprising the metal compound in said concentration range has the advantage that a layer with a comparatively high layer thickness can be obtained with each coating step . In consequence , the number of coating steps may be reduced compared with the use of a less concentrated coating solution . Preferably, the concentration of said metal compound, calculated as metal oxide , in the coating solution is between 10 wt . -% and 20 wt . -% , most preferably between 12 wt . -% and 17 wt . -% .

[0030] Preferably, the coating solution comprises at least one solvent selected from the group consisting of : alcohols , particularly ethanol , propanol , iso-propanol , octanol or cyclohexanol , ethers , particularly diethylene glycol diethyl ether , diethylene glycol monobutyl ether or l-methoxy-2-propanol , carboxylic acids , organic amines , amides , particularly dimethylformamide or N-methylpyrrolidone , and esters , particularly ethyl acetate . The coating solution may also comprise a mixture of at least two solvents of the solvents listed above .

[0031] Preferably, the coating solution comprises , in addition to the metal compound for coating applications , at least one metal salt . The metal of said metal salt is preferably scandium, yttrium, lanthanum or a rare earth metal . The anion of said metal salt is preferably a halide , a nitrate , a sulphate , a carboxylate or an alcoxide. The molar ratio of the metal of said metal compound for coating applications and the metal of said metal salt is preferably between 100:1 and 1:1, most preferably between 50:1 and 2:1.

[0032] Preferably, the coating solution comprises water. Where water is present in the coating solution, the concentration of water is preferably between 0.1 wt . -% and 35 wt . -% .

[0033] Preferably, the coating solution comprises at least one dispersant such as an organic polymer, particularly polyvinyl alcohol, polyvinyl acetate, polyvinyl pyrrolidone or corresponding copolymers, or a carboxylic acid, particularly trioxadecanoic acid. The mass ratio of said dispersant to the metal compound for coating applications, calculated as metal oxide, in the coating solution is preferably between 2% and 150%, most preferably between 5% and 50%.

[0034] The invention also relates to a method of coating a work piece with the features of claim 19. The method comprises providing a work piece to be coated and coating the work piece with a coating solution as described above. Coating the work piece may comprise a number of subsequent coating steps. Thus, the coating may be obtained by subsequently forming a number of stacked layers on the work piece.

[0035] In some preferred embodiments, an electrode for an electrochemical cell, preferably a solid oxide fuel cell or a solid oxide electrolyser cell, is provided as the work piece.

[0036] In some embodiments, an electrolyte layer for an electrochemical cell, preferably a solid oxide fuel cell or a solid oxide electrolyser cell , is provided as the work piece and coated with the coating solution . The electrolyte layer may be one of a plurality of sub layers .

[0037] Further embodiments are derivable from the following description and the drawings .

[0038] In the drawings :

[0039] Figure 1 shows a flow chart of a method for producing metal compounds for coating applications ; and

[0040] Figure 2 shows a flow chart of a method for coating a work piece .

[0041] Figure 1 shows a flow chart of a method of producing metal compounds 10 for coating applications .

[0042] In a first step 101 , a metal beta-dicarbonylate is provided, wherein the metal of said metal beta-dicarbonylate is a metal of the fourth group in the periodic table . Preferably, the metal is zirconium or hafnium. The beta-dicarbonylate may be a beta- diketonate . In this example , the beta-dicarbonylate is acetylacetonate .

[0043] In this example , said metal beta-dicarbonylate is provided as a liquid composition comprising said metal beta-dicarbonylate and at least one solvent . Preferably, the at least one solvent is iso-propanol . The metal beta-dicarbonylate is dissolved in the at least one solvent . Alternatively, the metal beta- dicarbonylate may be suspended in the at least one solvent . The concentration of said metal beta-dicarbonylate in said liquid composition may be 25 wt . -%, for example.

[0044] In a second step 103, a metal carboxylate is provided, wherein the metal of said metal carboxylate is said metal of the fourth group in the periodic table. In this example, the carboxylate of said metal carboxylate is acetate. In this regard, zirconium acetate is provided in the second step 103 as the metal carboxylate .

[0045] In this example, said metal carboxylate is provided as an aqueous solution. Thus, the metal carboxylate, i.e. zirconium acetate, is dissolved in water. The concentration of metal carboxylate in said aqueous solution may be between 15 wt . -% and 20 wt . -%, for example.

[0046] In a third step 105, said liquid composition and said aqueous solution are mixed to form a reaction mixture. Mixing may be performed in a suitable reaction vessel such as a reaction flask or a crystallisation dish. The molar ratio of metal carboxylate to metal beta-dicarbonylate in said reaction mixture may be between 0.5:1 to 10:1.

[0047] In a fourth step 107, the metal beta-dicarbonylate and the metal carboxylate are reacted, which results in the formation of a metal compound 10 for coating applications. In this example, reacting the metal beta-dicarbonylate and the metal carboxylate comprises heating the reaction mixture at a heating temperature of at least 40 °C and at most 150 °C. Alternatively, reacting the metal beta-dicarbonylate and the metal carboxylate may comprise keeping the reaction mixture at room temperature. In an optional fifth step 109 , the liquids ( i . e . , iso-propanol and water ) are , preferably completely, removed from the reaction mixture , thus obtaining the metal compound 10 for coating applications as a solid . The liquids may be removed by heating the reaction mixture in a convection oven, for example . As an alternative , the liquids may be removed only partially . Furthermore , the fifth step 109 may be omitted .

[0048] Figure 2 shows a flow chart of a method of coating a work piece 14 .

[0049] In a first step 201 , a coating solution 12 comprising the metal compound 10 for coating applications is provided . In this example , the metal compound 10 for coating applications is provided as a solid . Providing the coating solution 12 comprises dissolving the metal compound 10 for coating applications in at least one solvent . Preferably, ethanol is used as the at least one solvent . The concentration of said metal compound 10 for coating applications , calculated as metal oxide , in the coating solution 12 may be between 10 wt . -% and 20 wt . -% .

[0050] In case that the step 109 of the method shown in Figure 1 is omitted or the liquids are only partially removed in the step 109 , the metal compound 10 for coating applications is obtained as a part of a liquid composition . In that case , this liquid composition may directly be used as the coating solution 12 without addition of further substances . However, it is preferred to add additional substances such as at least one solvent , a metal salt or a dispersant .

[0051] In a second step 203 of the method shown in Figure 2 , a work piece 14 to be coated is provided . In this example , the work piece 14 is an electrode 16 or an electrolyte layer for an electrochemical cell , particularly a solid oxide fuel cell or a solid oxide electrolyser cell .

[0052] In a third step 205 , the work piece 14 is coated with the coating solution 12 . Thus , the work piece 14 is provided with a coating 18 . In the example shown in Figure 2 , only one side of the work piece 14 is coated with the coating solution 12 . Alternatively, several sides of the work piece 14 may be coated .

[0053] Conventional coating techniques may be used for providing work piece 14 with coating 18 . Preferably, coating 18 is provided on work piece 14 by spray coating, drop coating or doctor blade coating .

[0054] Coating the work piece 14 with the coating solution 12 may comprise several coating steps . Thus , the coating 18 may be build up by several layers that are stacked upon one another . As a result of the comparatively high concentration of the metal compound 10 for coating applications in the coating solution 12 , layers with a comparatively high layer 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 coating solutions .

[0055] Examples

[0056] Example 1 : Production of a metal compound for coating applications

[0057] 12 g of zirconium acetylacetonate were added to 50 mL of isopropanol and stirred under heating . 56 g of aqueous zirconium acetate solution were added gradually under heating, yielding a clear solution as a reaction mixture. Said reaction mixture was stirred for 20 min in an open crystallising dish at a heating temperature of 80 °C. Successively, the open crystallising dish was transferred to a convection oven and stirred for 90 min at a heating temperature of 110 °C, leading to removal of volatile components from the reaction mixture. The above procedure resulted in a reaction of zirconium acetylacetonate with zirconium acetate. 27.2 g of a solid metal compound for coating applications were obtained. Said metal compound displayed high solubility in alcoholic solvents and proved stable in alcoholic solvents. The calculated molar mass of said metal compound was 205 g / mole.

[0058] Example 2 : Production of an alcoholic coating solution

[0059] 10 g of the above-mentioned solid metal compound, 2.9 g of Sc(NO3)3and 3 g of trioxadecanoic acid where dissolved in 30 g of ethanol (96%) under heat. The resulting mixture was stirred for 2 h at a heating temperature of 75 °C, resulting in a clear and storage-stable alcoholic coating solution. The solid contend in said coating solution was 14.5 wt . -%, calculated as oxide.

[0060] Example 3: Production of an alcoholic coating solution

[0061] 34 g of zirconium acetylacetonate were added to 80 mL of ethanol and stirred at room temperature. 34 g of aqueous zirconium acetate solution were added gradually under stirring, yielding a clear solution as a reaction mixture. 8 g Sc(NO3)3hexahydrate, 1g Y(NO3)3hexahydrate, 9 mL l-methoxy-2-propanol were added. The solution was heated to 45°C and stirred for 1 h.

Claims

Claims1. A method of producing metal compounds for coating applications (10) , said method comprising: a. providing a metal beta-dicarbonylate, wherein the metal is a metal of the fourth group in the periodic table, b. providing a metal carboxylate wherein the metal is said metal of the fourth group in the periodic table, c. forming a reaction mixture comprising said metal beta- dicarbonylate, said metal carboxylate, and at least one solvent, and d. reacting the metal beta-dicarbonylate with the metal carboxylate to obtain a metal compound for coating applications (10) .

2. The method according to claim 1, wherein said metal of the fourth group in the periodic table is zirconium.

3. The method according to any of the preceding claims, wherein said beta-dicarbonylate is a beta-diketonate .The method according to any of the preceding claims, wherein said beta- dicarbonylate is acetylacetonate .

5. The method according to any of the preceding claims, wherein said metal beta-dicarbonylate is zirconium acetylacetonate .

6. The method according to any of the preceding claims, wherein said carboxylate is acetate.

7. The method according to any of the preceding claims, wherein said at least one solvent is an alcohol.

8. The method according to any of the preceding claims, wherein said at least one solvent is selected from the group consisting of: ethanol, propanol, iso-propanol, octanol and cyclohexanol.

9. The method according to any of the preceding claims, wherein the molar ratio of metal carboxylate to metal beta-dicarbonylate in said reaction mixture is between 0.5:1 and 10:1.

10. The method according to any of the preceding claims, wherein said metal beta-dicarbonylate is provided as a liquid composition comprising said at least one solvent and the metal beta-dicarbonylate, wherein said reaction mixture is formed by adding said metal carboxylate to said liquid composition.

11. The method according to any of the preceding claims, wherein said metal carboxylate is provided as an aqueous solution .

12. The method according to any of the preceding claims, wherein the metal beta-dicarbonylate and the metal carboxylate are reacted at room temperature.

13. The method according to any of claims 1 to 11, wherein the metal beta-dicarbonylate and the metal carboxylate are reacted at a heating temperature of at least 40 °C, preferably at a heating temperature of at least 40 °C and at most 150 °C, most preferably at a heating temperature of at least 40 °C and at most 100 °C.

14. The method according to any of the preceding claims, wherein liquids are partially or completely removed from said reaction mixture.

15. The method according to claim 14, wherein the liquids are removed at a temperature of at least 50 °C and at most 180 °C, preferably at a temperature of at least 70 °C and at most 130 °C.

16. A metal compound for coating applications (10) , produced by way of a method according to any of the preceding claims .

17. A coating solution (12) comprising a metal compound for coating applications (10) according to claim 16.

18. The coating solution (12) according to claim 17, wherein the concentration of said metal compound for coating applications (10) , calculated as metal oxide, in the coating solution (12) is between 5 wt . -% and 40 wt . -%, preferably between 10 wt . -% and 20 wt . -%, most preferably between 12 wt . -% and 17 wt . -%.

19. A method of coating a work piece (14) , said method comprising : a. providing a work piece (14) to be coated, b. coating the work piece (14) with a coating solution (12) according to any of claims 17 and 18.

20. The method according to claim 19, wherein an electrode (16) for an electrochemical cell, preferably a solid oxide fuel cell or solid oxide electrolyser cell, is provided as the work piece (14) .

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