Method for making metal oxide electrode and electrode obtained thereof

Abstract

The present invention relates to a method for making metal oxide electrode, the method comprising the subsequent steps of: a- dissolving one or more metal salts in a solution, b- adding silica powder to said solution, c- applying said solution comprising said silica powder and said dissolved one or more metal salts on a conductive substrate thus forming a coated substrate, d- thermally decomposing the one or more salts by curing said coated substrate at a temperature comprised between 350oC and 950oC thus obtaining a cured substrate, e- optionally, repeating steps c- and d- to arrive to a predefined thickness and / or metal-load, f- treating said cured substrate with a strong basic aqueous solution adapted to remove silica from said substrate.

Description

[0001]INDUSTRIE DE NORA S.p.A.- 1 -METHOD FOR MAKING METAL OXIDE ELECTRODE AND ELECTRODEOBTAINED THEREOF ------ TECHNICAL FIELD The present invention relates to a method for makingmetal oxide electrodes, the electrodes obtained thereofand the use of the electrodes in anion exchange membranewater electrolysis or alkaline water electrolysis.PRIOR ARTMetal oxide electrodes are widely used in variouselectrochemical reactions both as anodes and cathodes due to their efficacy, stability, and long lifetime.Metal oxide electrodes are made of substrates such asplates or meshes coated with one or more metal oxides.Mixtures of several metal oxides can be applied on thesubstrate to form mixed metal oxide (MMO) electrodes. Inthe MMO electrode, precious metal oxides catalyzetargeted electrochemical reactions such aselectrochlorination or water electrolysis. Some metaloxides can be mixed with the precious metal oxides toprovide support, minimize the corrosion duringelectrolysis or to reduce the overall cost of theelectrode. Various techniques and modifications have been adopted by the industry in order to improve the performance of M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 2 -the electrodes including:- combining different metal oxides from the same ordifferent groups of the periodic table,- doping the catalytic coating with small amounts ofspecific metals,- creating non-stoichiometric defects in the lattice ofthe oxides,- increasing the porosity and the surface area of thecatalytic coating.Elevated porosity and surface area of the metal oxidesin the catalytical layer are particularly critical toachieve good performance for some electrochemicalreactions especially at high current such as the anionexchange membrane electrolysis AEM.One of the common methods to obtain metal oxideelectrodes with high porosity and surface area is theuse of pre-formed metal oxide nanoparticles, which arepreviously synthesized using methods known in the art.Pre-formed metal oxide nanoparticles are then applied tothe substrate using a binder. The drawbacks of thismethod are the need of a binder which is expensive anddamaging to the environment, and the fact that metaloxide nanoparticles are not mechanically stable underharsh working conditions, thus not suitable for allelectrochemical applications. Another common method to obtain metal oxide electrodeswith high porosity and surface area is through the directM / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 3 -growth of the metal oxide on the substrate. For example,hydrothermal growth and electrodeposition. The drawbacksof the direct growth are also the mechanicallyinstability of the metal oxides obtained therefrom andthe fact that those methods can be only applied tocertain types of substrates that can tolerate theconditions of the direct growth.A common method for making metal oxide electrodesinvolves applying metal-salt solutions on a substrate,then curing the coated substrate under air to transformmetal salts to metal oxides. The main drawback of thismethod is that the catalytic layer of metal oxides thusobtained has low porosity and low surface area.Another major drawback of this method is itsincompatibility with 3-dimensional substrates such asfelts or foams. Indeed, applying a metal-salt solutionon a 3-dimensional substrate leads to the infiltrationof the solution into the inner surfaces of the substrateand consequently, undesirable inner deposits of metaloxides are formed instead of the proper deposition ofmetal oxides on the outer surface of the substrate, whichcauses the reduction in efficiency of the catalyticlayer. Thus, it would be desirable to provide a method formaking metal oxide electrodes that can solve theaforementioned drawbacks without affecting theperformance or the lifetime of the electrodes.M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 4 -SUMMARY OF THE INVENTIONFrom what is stated above, the present application aimsat providing a method for making metal oxide electrodeswhich can efficiently be applied to any kind of substrateincluding 3-dimensional substrates and can producemechanically stable electrodes with elevated porosityand surface area.This result is achieved using the method described inthe appended claims, wherein silica powder is added tothe metal precursors and subsequently leached out fromthe matrix after depositing and curing the coatedelectrode, leaving behind a porous catalytic layer.These and other objects and advantages of the invention will become obvious from the following detailed description. BRIEF DESCRIPTION OF THE FIGURESFigure 1 shows J-V curve for comparative Ni55Fe45-oxideanode prepared without adding silica and Ni55Fe45-oxideanode prepared according to the method of the presentinvention.Figure 2 shows IR-free J-V curve for comparative NiFe45-oxide anode prepared without adding silica and NiFe45-oxide anode prepared according to the method of theM / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 5 -present invention.Figure 3 shows J-V curve for comparative Ru80Pr20-oxidecathode prepared without adding silica and Ru80Pr20-oxidecathode prepared according to the method of the presentinvention.Figure 4 shows IR-free J-V curve for comparative Ru80Pr20-oxide cathode prepared without adding silica and Ru80Pr20-oxide cathode prepared according to the method of thepresent invention.Figure 5 shows SEM images of comparative NiFe45-oxideprepared without adding silica (left side) and NiFe45-oxide prepared according to the method of the presentinvention (right side).Figure 6 shows SEM images of comparative Ru80Pr20-oxideprepared without adding silica (left side) and Ru80Pr20-oxide prepared according to the method of the presentinvention (right side).Figure 7 shows cross-sectional SEM image of NiFe45-oxideprepared according to the method of the present invention. Figure 8 shows cross-sectional SEM image of comparative NiFe45-oxide prepared without adding silica. M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 6 -DETAILED DESCRIPTION OF THE INVENTION A first object of the invention therefore relatesto a method for making metal oxide electrodes, the methodcomprising the subsequent steps of:a- dissolving one or more metal salts in a solution,b- adding silica powder to the solution,c- applying the solution comprising said silicapowder and said dissolved one or more metal salts on aconductive substrate thus forming a coated substrate, d- thermally decomposing the one or more salts bycuring the coated substrate at a temperature comprisedbetween 350oC and 950oC, preferably comprised between350oC and 650oC thus forming a cured substrate,e- optionally, repeating steps c- and d- to arriveto a predefined thickness and / or metal-load,f- treating, preferably immersing, the curedsubstrate with a strong basic aqueous solution adaptedto remove silica from the substrate.As explained, silica powder is added to the metal precursors and subsequently leached out from the matrix after depositing and curing the coated electrode leavingbehind a porous catalytic layer.The electrodes obtained according to the method of the present invention have high porosity and surfacearea combined with excellent electrochemical efficacyand mechanical stability under electrolysis. Additionally, the method of the present invention is M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 7 -particularly advantageous for 3-dimensinoal substratesbecause silica ensures the deposition of the porouscatalytic layer of metal oxides on the surface of thesubstrates. Advantageously, the metal of the metal salts areselected from a group consist of bismuth, calcium,cerium, chromium, cobalt, copper, gold, hafnium, iridium, iron, lanthanum, lithium, manganese, molybdenum, nickel, niobium, palladium, platinum, praseodymium, rhenium, rhodium, ruthenium, scandium, silver, strontium, tantalum, tin, titanium, yttrium,zirconium and combinations thereof.The solution in step a- can be an aqueous solutionor an alcohol-based solution. Preferably, a volatilesolution, more preferably an alcohol-based solution suchas iso-propyl alcohol. The use of alcohol-basedsolution, in combination with silica, seems to drive thedeposition of the metal oxides on the 3-dimensinoalsubstrates surface.Any kind of silica can be used, preferably silicahaving a surface area of at least 20 m2 / g measured byBET, more preferably silica having a surface areacomprised between 100 and 400 m2 / g measured by BET, evenmore preferably fumed silica having a surface areacomprised between 100 and 400 m2 / g measured by BET. Itis noted that silica does not dissolve, rather it produces a colloidal suspension. M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 8 -The amount of silica to be added to the solution isbased on the amount of metals. Advantageously, the amountof silica to be added to the solution comprised between5 wt% and 50 wt%, preferably comprised between 15 wt%and 35 wt% with respect to the weight of the metals ofthe metal salts in the solution. In a preferred embodiment, the method furthercomprises after step of adding silica powder to thesolution and before applying the solution on a substratea step of: b2- sonicating the solution comprising said silicapowder and said dissolved one or more metal salts so asto disperse said silica powder, preferably for at least1 minute.The substrate can be of any shape including 3-dimensional substrates such as felts or foams, or 2-dimensional substrates like meshes. The substrates canbe made of nickel or any other conductive material. Adventagiously, the conductive substrate of the method of the present invention is a 3-dimensionalsubstrate having an inner surface and an outer surface,preferably the conductive substrate is a felt substrateor a foam substrate. The method of present invention isparticularly advantageous in 3-dimensional substrates asit favors the deposition of the metal oxides on the outersurface of the substrate and prevents or at least greatlyM / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 9 -reduces the deposition of metal oxides within the innersurfaces of the substrate.Thus, a second object of the present inventionrelates to a metal oxide electrode which comprises aconductive substrate and one or more metal oxides deposited via a thermal decomposition of their metalsalts on the substrate, wherein the conductive substrateis a felt substrate or a foam substrate having an innersurface and an outer surface, wherein at least 90%,preferably at least 95% of the one or more metal oxidesare deposited on the outer surface of the conductivesubstrate measured by cross-sectional SEM imageanalysis. Step c- of applying the solution on a substrate canachieved by any method known to the person skilled inthe art. Spray-coating is particularly advantageous.Curing the coated substrate must be made at atemperature comprised between 350oC and 950oC, preferablycomprised between 350oC and 650oC. the optimumtemperature depends on the metal or metals in the metalprecursors. It is noted that curing the coated substrateat lower temperatures leads to the formation of mechanically unstable metal oxides, which cannot be used as electrodes. Step f- of leaching silica from the matrix isachieved using a strong basic aqueous solution,M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 10 -preferably, an aqueous solution of sodium hydroxide,more preferably, the sodium hydroxide concentration insaid aqueous solution of sodium hydroxide is at least 2%by weight with respect to the total weight of saidaqueous solution of sodium hydroxide.The leaching step can take minutes or hours untilthe partial or complete removal of silica is achieved.Preferably, the leaching step lasts for a periodcomprised between 2 minutes and 4 hours, preferablybetween 2 minutes and 1 hour, under a temperaturecomprised between 30oC and 100oC preferably comprisedbetween 40oC and 90oC.A third object of the present invention relates to a metal oxide electrode obtained according to the method of the present invention. A fourth object of the present invention relates toan electrolyzer which comprises one or more metal oxideelectrode obtained according to the method of the present invention. A fifth object of the present invention relates tothe use of the metal oxide electrode obtained accordingto the method of the present invention in anion exchangemembrane water electrolysis, proton exchange membraneelectrolysis, or alkaline water electrolysis.The following examples are provided forM / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 11 -illustrative purposes only of the present invention and must not be understood as limiting the scope of protection defined by the appended claims. EXAMPLESExample 1 – Synthesis of NiFe45-oxide anode according tothe present inventionSolution preparation: Nickel (II) nitrate hexahydrate and Iron (III) nitrate nonahydrate are added in a ratio of 55:45 (with respectto the metals, not the metal-salts) to a plastic jar. Tothe metal-salts, isopropyl alcohol is added so that the final concentration of metal in solution is 60 mg / ml. Additionally, fumed silica (commercially available asCAB-O-SIL® EH-5) is added to the solution so that it is10 wt% with respect to the metal-salts. Using a probe sonicator, the solution is sonicated for 1 minute in an ice bath. Coating application and heat-treatment:The sonicated solution from step 1 is applied via an airspray-gun to a Ni felt substrate in amounts so that the gain rate is near 10 gsm per coat. The electrode is thenplaced in a curing oven at 450oC for 25 minutes to formthe catalytic layer, then removed and cooled down underambient conditions. The process described above is M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 12 -repeated until the desired loading is achieved. SiO2leaching:The electrode is added to 30 wt% KOH at 95oC for 6 hoursto remove the SiO2 from the catalytic layer.Example 2 – Synthesis of Ru80Pr20-oxide cathode accordingto the present invention Solution preparation: Ruthenium (III) nitrosyl nitrate and Praseodymium (III)nitrate hexahydrate are added in a ratio of 80:20 (withrespect to the metals, not the metal-salts) to a plasticjar. To the metal-salts, isopropyl alcohol is added so that the final concentration of metal in solution is 60 mg / ml. Additionally, fumed silica (commercially available as CAB-O-SIL® EH-5) is added to the solution so that it is 10 wt% with respect to the metal-salts.Using a probe sonicator, the solution is sonicated for1 minute in an ice bath. Coating application and heat-treatment:The sonicated solution from step 1 is applied via an airspray-gun to a felt substrate in amounts so that thegain rate is near 10 gsm per coat. The electrode is thenplaced in a curing oven at 400oC for 25 minutes to formthe catalytic layer, then removed and cooled down underM / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 13 -ambient conditions. The process described above is repeated until the desired loading is achieved. SiO2leaching:The electrode is immersed in 30 wt% KOH at 95oC for 5hours to remove the SiO2 from the catalytic layer.Example 3 (comparative) – Synthesis of NiFe45-oxide anodewithout the addition of silicaThe anode of example 3 is prepared using the same methoddescribed in example 1, while skipping the steps of adding and leaching of silica.Example 4 (comparative) – Synthesis of Ru80Pr20-oxidecathode without the addition of silicaThe cathode of example 4 is prepared using the samemethod described in example 2, while skipping the steps of adding and leaching of silica. Characterization studies Figures 1 and 2 show an example of the observed performance difference in current-voltage (J-V) curvesbetween anodes made with and without the presence of SiO2incorporated metal-salt solutions according to themethod of example 1 and example 3. The cathode is common between the tests and is 30 wt% Pt / C (5 gsmPt) coated on M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 14 -carbon paper. The anode / cathode pairs are assembled into an electrolyzer with a commercial anion exchangemembrane (Ionomr Aemion+ ®, Reinforced, 75um – AF3-HWK9-75-X), and the anode is supplied with 1 molar potassium hydroxide at 60oC. During the JV curve, the current density is stepped in 10 mA / cm2increments from 10 mA / cm2to 100 mA / cm2, and then in 100 mA / cm2increments from 100 mA / cm2to 1 A / cm2. Each current density is held for 60 seconds, and the data points shown represent the average of 60 individual data points at each current density. Electrochemical Impedance Spectroscopy (EIS) is used to make the IR-correction (Figure 2). Figures 3 and 4 show an example of the observed performance difference in current-voltage (JV) curves between cathodes made with and without the SiO2incorporated metal-salt solutions according to example2 and example 4. The anode is common between the tests and is uncoated nickel felt. The anode / cathode pairs are assembled into an electrolyzer with a commercial anionexchange membrane (Ionomr Aemion+ ®, Reinforced, 75um –AF3-HWK9-75-X), and the anode is supplied with 1 molar potassium hydroxide at 60oC. During the JV curve, the current density is stepped in 10 mA / cm2increments from 10 mA / cm2to 100 mA / cm2, and then in 100 mA / cm2increments from 100 mA / cm2to 1 A / cm2. M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 15 -Each current density is held for 60 seconds, and the data points shown represent the average of 60 individual data points at each current density. Electrochemical Impedance Spectroscopy (EIS) is used to make the IR- correction (Figure 4).In summary, figures 1-4 show increase in performancedifference between electrodes made according to the method of the present invention compared with those madewithout adding silica in the initial metal-salt coatingsolution, and how the differences correlate with the SEMimages of figures 5 and 6.The IR-free plots show that the performance difference is not due to ohmic resistance differences, rather something intrinsic to the electrodes themselves.Figure 7 shows cross-sectional SEM image of NiFe45-oxideprepared according to the method of the presentinvention. The coated Ni felt is mounted in epoxy andits cross-section is viewed using a scanning electronmicroscope (SEM). The image demonstrates that the entiremetal oxides coating resides at the outer surface of thesubstrate. Figure 8 shows cross-sectional SEM image of comparativeNiFe45-oxide prepared without adding silica. The coatedNi felt is mounted in epoxy and its cross-section is viewed using a scanning electron microscope (SEM). Theimage demonstrates that very little metal oxides coatingM / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 16 -resides at the surface of the substrate, rather it isdeposited randomly throughout the inner surfaces of thesubstrate.The improved electrochemical performance of theelectrodes of the present invention is not entirelyunderstood. While not wishing to be bound by any theory, the Applicant believes that both the increase of porosity and the surface area, and the controlled deposition ofthe metal oxides on the outer of the substrates causedby the adding and leaching of silica contribute to suchimprovement. In the present patent application, all the operating conditions reported in the text must be understood as preferred conditions even if not expressly declared. For the purposes of the present discussion the term "to comprise" or "to include" also comprises the term "to consist in" or "essentially consisting of". For the purposes of the present discussion the definitions of the ranges always comprise the extreme values unless otherwise specified. M / 66015-PCT (407PCT)

Claims

INDUSTRIE DE NORA S.p.A.- 17 -CLAIMS 1. Method for making metal oxide electrode, themethod comprising the subsequent steps of:a- dissolving one or more metal salts in a solution,b- adding silica powder to said solution,c- applying said solution comprising said silicapowder and said dissolved one or more metal salts on aconductive substrate thus forming a coated substrate,d- thermally decomposing the one or more salts bycuring said coated substrate at a temperature comprisedbetween 350oC and 950oC thus obtaining a cured substrate,e- optionally, repeating steps c- and d- to arriveto a predefined thickness and / or metal-load,f- treating said cured substrate with a strong basicaqueous solution adapted to remove silica from saidsubstrate.

2. Method according to claim 1, wherein saidsolution in step a- is an alcohol-based solution.

3. Method according to claim 1 or 2, wherein saidmetal of said metal salts are selected from a groupconsist of bismuth, calcium, cerium, chromium, cobalt,copper, gold, hafnium, iridium, iron, lanthanum, lithium, manganese, molybdenum, nickel, niobium, palladium, platinum, praseodymium, rhenium, rhodium, ruthenium, scandium, silver, strontium, tantalum, tin,titanium, yttrium, zirconium and combinations thereof.M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 18 -4. Method according to any one of the precedingclaims, wherein said silica has a surface area of atleast 20 m2 / g, preferably a surface area comprisedbetween 100 and 400 m2 / g measured by BET.

5. Method according to any one of the precedingclaims, wherein said conductive substrate is a 3-dimensional substrate having an inner surface and anouter surface, preferably the conductive substrate is afelt substrate or a foam substrate.

6. Method according to any one of the precedingclaims, wherein the method further comprises after stepb- and before step c- a step of:b2- sonicating said solution comprising said silicapowder and said dissolved one or more metal salts so asto disperse said silica powder, preferably for at least1 minute.

7. Method according to any one of the precedingclaims, wherein said strong basic aqueous solution is anaqueous solution of sodium hydroxide.

8. Metal oxide electrode comprising a conductivesubstrate and one or more metal oxides deposited via athermal decomposition of salts of said metal or metalson said substrate, wherein said conductive substrate isa felt substrate or a foam substrate having an innersurface and an outer surface, wherein at least 90% of M / 66015-PCT (407PCT)INDUSTRIE DE NORA S.p.A.- 19 -said one or more metal oxides are deposited on the outersurface of said conductive substrate measured by cross-sectional SEM image analysis.

9. Electrolyzer comprising one or more metal oxideelectrode according to claim 8.

10. Use of the metal oxide electrode of claim 8 inanion exchange membrane water electrolysis, protonexchange membrane electrolysis, or alkaline waterelectrolysis. M / 66015-PCT (407PCT)

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