Method for making metal oxide electrode and electrode obtained thereof

Abstract

The present invention relates to a method for making an electrode provided with a catalytic coating for electrocatalytic processes, preferably gas evolution electrode, comprising: a- providing a precursor comprising salts of metals, wherein said metals are selected from a group consisting of combinations of one or more noble metal with one or more rare earth metal, b- heating said precursor to a temperature comprised between 300 and 700oC under oxygen or air, thus obtaining a dried powder comprising oxides of said metals, c- preferably, subjecting the dried powder to a process of particle size reduction, d- optionally storing or transferring said dried powder, e- dispersing said dried powder in a liquid thus obtaining a catalytic dispersion, f- applying said catalytic dispersion by a coating technique on a thermally unstable substrate thus forming a coated substrate, g- drying said coated substrate thus forming said electrode provided with a catalytic coating

Description

[0001] INDUSTRIE DE NORA S . p . A .

[0002] - 1 -

[0003] METHOD FOR MAKING METAL OXIDE ELECTRODE AND ELECTRODE

[0004] OBTAINED THEREOF

[0005] TECHNICAL FIELD

[0006] The present invention relates to a method for making electrodes for electrocatalytic processes , preferably gas evolution electrodes , the electrodes obtained thereof and the use of the electrodes in chlor-alkali reaction, anion exchange membrane water electrolysis , proton exchange membrane electrolysis , or alkaline water electrolysis .

[0007] PRIOR ART

[0008] Catalytically active metal-based electrodes such as metal oxide electrodes are widely used in various electrochemical reactions both as anodes and cathodes due to their ef ficacy, stabil ity, and long li fetime . Those kinds of electrodes are made of substrates such as plates or meshes coated with one or more catalytically active metal .

[0009] Mixtures of several metals or their oxides can be applied on the substrate to form the catalytic layer . In the catalytic layer, catalytically active metals catalyze targeted electrochemical reactions such as electrochlorination or water electrolysis . Some metals can be mixed with the active metals to provide support

[0010] M / 66045 -PCT ( 412 PCT ) INDUSTRIE DE NORA S.p.A. or to minimize the corrosion during electrolysis. Mixtures of noble metals with rare earth metals are particularly effective as electrocatalysts. Patent application WO2011042484A1 in the name of the applicant discloses the use of mixtures of noble metals with rare earth metals as electrocatalysts.

[0011] Various techniques and modifications have been adopted by the industry in order to improve the performance of the electrodes including:

[0012] - combining different metals from the same or different groups of the periodic table,

[0013] - doping the catalytic coating with small amounts of specific metals,

[0014] - creating non-stoichiometric defects in the lattice of the catalysts,

[0015] - increasing the porosity and the surface area of the catalytic coating, providing a porous interlayer to improve the conductivity and the robustness of the electrode.

[0016] A common method used in the industry for making catalytically active metal-based electrodes involves applying a precursor of the catalytically active metal on a substrate, then heating the substrate to a temperature comprised between 300 and 700°C under air to thermally decomposes the precursor into oxides of catalytically active metal.

[0017] The advantage of this method is the proper adhesion of

[0018] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S.p.A. the catalytic layer on the substrate. The drawbacks of this method involve:

[0019] - Difficulties in storing or transporting the catalytic layer separately,

[0020] Inability to include polymeric binder material to tune the properties of the catalytic layer,

[0021] - The need of heating the substrate to high temperatures, thus the method cannot be used with thermally unstable substrates such as carbon paper or carbon cloth,

[0022] - the oxidation of the backside of the substrate and resultant increase in contact resistance when welding the electrode is not desired

[0023] - The need to repeat the thermal decomposition several times to arrive at a specific metal load.

[0024] Thus, it would be desirable to provide a method for making electrodes that can solve the aforementioned drawbacks without affecting the performance or the lifetime of the electrodes.

[0025] Khan et al., Coatings, 2021, vol. 11, 257. describes catalysts based on silver oxide and praseodymium oxide supported on y-alumina for the electro-oxidation of ammonia in alkaline medium. The catalysts are prepared from nitrate precursors, calcined at temperatures around 600 °C, and then dispersed in an alcohol solution containing Nafion before being applied to a glassy carbon electrode .

[0026] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S.p.A.

[0027] Wang et al., Sci. China Mater., 2021, vol. 64, 2193- 2201, describe highly stable Ir-based oxygen evolution reaction (OER) catalysts. A praseodymium iridium oxide catalyst is prepared by mixing praseodymium acetate with iridium chloride followed by calcination at 1200 °C. The resulting material is tested as an OER catalyst in acidic medium using glassy carbon as the working electrode.

[0028] SUMMARY OF THE INVENTION

[0029] From what is stated above, the present application aims at providing a method for making metal oxide electrode for electrocatalytic processes, preferably a gas evolution electrode, which provides an efficient electrode and allows to effortlessly store and transport the catalytic layer as well as easily applying the catalytic layer on substrates, especially thermally unstable substrates.

[0030] This result is achieved using the method described in the appended claims, wherein salts of one or more noble metal with one or more rare earth metal are thermally decomposed under air or oxygen to obtain a catalytic powder comprising the metal oxides, which is then stored, transported or directly used to prepare the electrode.

[0031] These and other objects and advantages of the present invention will become obvious from the following detailed description.

[0032] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S.p.A.

[0033] BRIEF DESCRIPTION OF THE FIGURES

[0034] Figure 1 shows SEM images of comparative example 1 (a. and b.) , comparative example 2 (c. and d.) and example 1 according to the invention (e. and f.) .

[0035] Figure 2 shows the J-V performance of the cathodes of the examples according to the invention and cathodes of the comparative examples.

[0036] DETAILED DESCRIPTION OF THE INVENTION

[0037] A first object of the invention therefore relates to a method for making an electrode provided with a catalytic coating for electrocatalytic processes, preferably gas evolution electrode, comprising: a- providing a precursor comprising salts of metals, wherein said metals are selected from a group consisting of combinations of one or more noble metal with one or more rare earth metal, b- heating said precursor to a temperature comprised between 300 and 700°C under oxygen or air, thus obtaining a dried powder comprising oxides of said metals , c- preferably, subjecting the dried powder to a process of particle size reduction, d- optionally storing or transferring said dried powder, e- dispersing said dried powder in a liquid thus obtaining a catalytic dispersion,

[0038] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S . p . A . f- applying said catalytic dispersion by a coating technique on a thermally unstable substrate thus forming a coated substrate g- drying said coated substrate thus forming said electrode .

[0039] Steps a, b, e , f and g are in chronological order . Preferably, steps a to g are in chronological order .

[0040] It was surprising found that the method of the present invention solves all the above-mentioned drawbacks because it allows the easy storing or transporting of the powder which can be easily turned to a catalytic coating to be applied on the substrates at relatively low temperatures . Another advantage of the method of the present invention is the ability to include polymeric binder material with the catalytic coating to tune the properties of the catalytic layer . Another advantage of the method is its compatibility with thermally unstable substrates such as carbon paper or carbon cloth since thermal decomposition of the salts of metals takes place in the former steps . Yet another advantage of the method of the present invention is the ability to arrive at a speci fic metal load by a single application without the need to repeat the thermal decomposition several times to arrive at the metal load .

[0041] Preferably, the precursor is a chloride- free aqueous solution comprising the salts of the metals , more preferably an acidic aqueous solution .

[0042] M / 66045 -PCT ( 412 PCT ) INDUSTRIE DE NORA S . p . A .

[0043] Advantageously, the salts of metals are acetates of metals . Acetate ligands are preferred to alternatives as they act as caping agents to inhibit the growth of the metal-oxide particles , keeping the particle si ze low and enabling a higher surface area catalyst .

[0044] The acetates of metals can be obtained by mixing salts of the metal s other than acetates with an aqueous solution comprising acetic acid at a concentration higher than 30% by weight .

[0045] Advantageously, the one or more noble metal is selected from a group consisting of platinum, palladium, ruthenium and combinations thereof , said rare earth metal is praseodymium, more preferably said noble metal is a mixture of platinum and palladium, said rare earth metal is praseodymium or said noble metal i s ruthenium, said rare earth metal is praseodymium . The noble metal allows for ef ficient HER catalysts while the rare earth metal can improve the catalysts robustness and decrease the amount of noble metal required .

[0046] Advantageously, the precursor further comprises carbon powder . Carbon is used as a support for the electrocatalyst .

[0047] Preferably, the heating temperature of step b- i s comprised between 350 and 550°C .

[0048] M / 66045 -PCT ( 412 PCT ) INDUSTRIE DE NORA S.p.A.

[0049] Advantageously, the process of particle size reduction is milling or grinding. Physically reducing the particle size increases the catalysts surface area thereby improving the electrolysis efficiency.

[0050] Preferably, the applying of said catalytic coating by a coating technique is:

[0051] - spray-coating, slot-die coating or blade-coating the catalytic coating on the substrate. Other techniques such as brush-coating or roller-coating are also feasible .

[0052] Drying of the coated substrate can be performed at a suitable second temperature below 300 °C at which the substrate is thermally stable. Preferably, the drying of said coated substrate is: heating the coated substrate to a second temperature comprised between 30 and 90 °C, preferably comprised between 40 and 60 °C.

[0053] For the purposes of the present invention, the term "thermally unstable substrate" refers to a material used as an electrode substrate or support in electrocatalytic processes that undergoes significant physical, chemical, or structural degradation at a temperature of decomposing the salts of the metals. Typically, said substrate is thermally unstable at a temperature of the heating step of the method of the present invention, i.e. at a temperature comprised between 300 and 700°C under an oxygen-rich or air atmosphere. As used herein,

[0054] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S.p.A. the thermally unstable substrate is, however, thermally stable at lower temperatures, specifically at temperatures employed during the drying step of the method of the present invention. Thus, the substrate is typically stable at a temperature of 90° and below, preferably below 300 °C. Such substrates include conductive polymer-based substrates, conductive metalorganic frameworks (MOFs) or covalent organic frameworks (COFs) , or conductive organic fiber-based substrates, e.g. conductive cellulose papers or fabrics doped with conductive materials. More preferably said substrate is carbon paper or carbon cloth.

[0055] In contrast to electrode substrates such a glassy carbon, substrates like carbon paper or carbon cloth can be coating without requiring a polymeric binder in the coating in order to obtain a stable coating.

[0056] However, in certain embodiments of the invention, the liquid used for dispersing the dried powder further comprises a polymeric binder. The inclusion of a polymeric binder provides several technical advantages: it improves the adhesion of the catalytic particles to the substrate, enables control over the porosity and hydrophobicity of the catalytic coating, and increases the mechanical stability of the electrode during operation. Suitable polymeric binders include, but are not limited to, polytetrafluoroethylene (PTFE) , polyvinylidene fluoride (PVDF) , Nafion®, and polybenzimidazole (PBI) . In further embodiments, the

[0057] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S . p . A . polymeric binder is an anion-exchange ionomer, which not only acts as a binder but also provides ionic conductivity for hydroxide ions . Such binders are particularly advantageous in anion exchange membrane (AEM) electrolyzers operated with very dilute or even no supporting electrolyte , as they facilitate ionic transport across the catalytic layer . The choice of binder depends on the target electrochemical process and the desired wetting properties of the electrode surface . The polymeric binder may be added in the form of a dispersion or solution in the liquid phase together with the dried oxide powder .

[0058] The electrode can be a cathode or an anode , preferably a cathode , more preferably a cathode for chlor-alkali reaction, anion exchange membrane water electrolysis , proton exchange membrane electrolysis , or alkaline water electrolysis .

[0059] A second aspect of the present invention related to the electrode obtained according to the method of the present invention .

[0060] A third aspect o f the present invention related to an electrode comprising a substrate and a catalytic coating, wherein said substrate is a thermally unstable substrate , wherein said catalytic coating comprises oxides of metals , wherein said metal s are selected from a group consisting of combinations of one or more noble metal with one or more rare earth metal . Preferably said

[0061] M / 66045 -PCT ( 412 PCT ) INDUSTRIE DE NORA S . p . A . thermally unstable substrate is a carbon paper or a carbon cloth .

[0062] Preferably said catalytic coating further comprises carbon powder .

[0063] Preferably said catalytic coating further comprises a polymeric binder .

[0064] Preferably said oxides of metals are obtained by thermally decomposing acetates of the metals .

[0065] Advantageously, the one or more noble metal is selected from a group consisting of platinum, palladium, ruthenium and combinations thereof , said rare earth metal is praseodymium, more preferably said noble metal is a mixture of platinum and palladium, said rare earth metal is praseodymium or said noble metal i s ruthenium, said rare earth metal is praseodymium .

[0066] A fourth aspect of the present invention related to an electrolyzer which comprises one or more electrode according to the present invention .

[0067] A fi fth aspect of the present invention related to the use of the electrode according to the present invention in chlor-alkali reaction, anion exchange membrane water electrolysis , proton exchange membrane electrolysis , or alkaline water electrolysis .

[0068] The following examples are provided for illustrative purposes only of the present invention and

[0069] M / 66045 -PCT ( 412 PCT ) INDUSTRIE DE NORA S.p.A.

[0070] - 12- must not be understood as limiting the scope of protection defined by the appended claims.

[0071] EXAMPLES

[0072] Example 1 according to the present invention

[0073] 21.69 Ruthenium nitrosyl nitrate (18 wt% Ru) is added to a glass beaker and 12 ml of glacial acetic acid and 50.5 ml of 10 wt% acetic acid are added to the beaker and the solution is mixed using a magnetic stir bar. 3.33 g praseodymium nitrate hexahydrate (30 wt% Pr) are ground into a fine powder and moved to another glass beaker. 59.6 ml of 10 wt% acetic acid, 2.4 ml of glacial acetic acid, and 0.6 ml of nitric acid are added to the second beaker and the solution is mixed using a magnetic stir bar. The obtained solutions are combined and mixed in a glass beaker. The mixed solution is transferred to a crucible and placed in an oven at 350 °C for 2 hrs. The obtained product is a powder mixture of ruthenium oxide and praseodymium oxide (with an approximate Ru:Pr ratio of 4:1) .

[0074] The powders are then ball milled for approximately 24 hours .

[0075] After ball milling, they can be stored, transferred or applied to a carbon paper substrate using the process

[0076] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S.p.A. described below:

[0077] 100 mg catalyst (powders) , 25 mg PTFE (Polytetrafluoroethylene) , 0.5 g DI water, and 2.0 g IPA (Isopropyl Alcohol) were mixed thoroughly using a sonication probe.

[0078] Then, they were applied via hand spray to the substrate (resting on a hotplate heated to 60°C) until a loading of 5 gsm of the noble metal is achieved.

[0079] Example 2 according to the present invention

[0080] The synthesis of the electrode is done by using the same steps of example 1.

[0081] 7.89 g of diamine dinitrate platinum (IT) solution (19 wt% Pt) and 22.22 of palladium nitrate solution (4.5 wt% Pd) are combined in a glass beaker. 8.33 g of praseodymium nitrate hexahydrate are ground into a fine powder and added to the above solution. 0.75 ml nitric acid is added to the solution. The solution is diluted with DI water to 50 ml mixed using a magnetic stir bar. Carbon powder is added to the solution to achieve a metal : carbon ratio of 1:1.

[0082] The solution is transferred to a crucible and placed in an oven at 350 °C for 2 hrs. The product is a powder mixture of carbon, platinum oxide, palladium oxide, and

[0083] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S.p.A.

[0084] - 14- praseodymium oxide (with an approximate Pt:Pd:Pr ratio of 1:0.667:1.667) .

[0085] Example 3 according to the present invention

[0086] The synthesis of the electrode of example 3 is done in the same way as of the example 1 according to the present invention with the difference of the catalyst being applied to a nickel felt substrate.

[0087] Comparative example 1

[0088] The synthesis of the electrode of comparative example 1 is done in the same way as of the Example 1 according to the present invention, with the difference of applying the precursor solution to the carbon paper substrate, drying it under air at 80°C, heating the coated substrate at 350°C under air for 15 minutes, then repeating the drying and heating steps until a loading of 5 gsm of the noble metal is achieved.

[0089] Comparative example 2

[0090] The synthesis of the electrode of Comparative example 2 is done in the same way as of the Comparative example 1, with the difference of the substrate being nickel felt substrate .

[0091] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S.p.A.

[0092] - 15 -

[0093] Characterization studies

[0094] SEM Images:

[0095] Figure 1 shows SEM images of Comparative Example 1 (a. and b.) , Comparative Example 2 (c. and d.) and Example 1 ( e . and f . ) .

[0096] The images show the drawbacks of thermally decomposed coatings on carbon paper-based substrates, and how synthesizing the catalyst as a powder alleviates the problem. Specifically, we can see that the thermally decomposed coating on carbon paper (Comparative Example 1, a. and b.) results in catalyst depositing between the fibers of carbon rather than directly on the fibers as is the case the thermally decomposed coating on Ni felt (Comparative Example 2, c. and d.) . As a result of the catalyst residing between fibers, it can lead to transport issues arising from the effective decrease of the substrates porosity. When the catalyst is instead synthesized as a powder and applied (Example 1, e. and f.) , it deposits onto the fibers as desired.

[0097] Electrochemistry :

[0098] Figure 2 shows the J-V performance of the different cathodes while table 1 summarizes the cell-voltages at 1 A / cm2for the different cathodes. For all tests the anode is Ni felt, the membrane is lonomr Aemion 75um, and the electrolyte is IM KOH at 60°C flown only to the anode .

[0099] M / 66045-PCT (412PCT) INDUSTRIE DE NORA S.p.A.

[0100] - 16-

[0101] Table 1

[0102] As can be seen, the electrodes of the present invention have electrochemical performances analogous to those of the comparative examples.

[0103] In the present patent application, all the operating conditions reported in the text must be understood as preferred conditions even if not expressly declared.

[0104] 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".

[0105] For the purposes of the present invention, the definitions of the ranges always comprise the extreme values unless otherwise specified.

[0106] M / 66045-PCT (412PCT)

Claims

INDUSTRIE DE NORA S . p . A .CLAIMS1 . Method for making an electrode provided with a catalytic coating for electrocatalytic processes , preferably gas evolution electrode , comprising : a- providing a precursor comprising salts of metals , wherein said metals are selected from a group consisting of combinations of one or more noble metal with one or more rare earth metal , b- heating said precursor to a temperature comprised between 300 and 700°C under oxygen or air, thus obtaining a dried powder comprising oxides of said metals , c- preferably, subj ecting the dried powder to a process of particle si ze reduction, d- optionally storing or trans ferring said dried powder, e- dispersing said dried powder in a liquid thus obtaining a catalytic dispersion, f- applying said catalytic dispersion by a coating technique on a thermally unstable substrate thus forming a coated substrate , g- drying said coated substrate thus forming said electrode provided with a catalytic coating .2 . Method for making the electrode according to claim 1 , wherein said salts of metals are acetates of metals .M / 66045 -PCT ( 412 PCT )INDUSTRIE DE NORA S . p . A .3 . Method for making the electrode according to claim 1 or 2 , wherein said one or more noble metal is selected from a group consisting of platinum, palladium, ruthenium and combinations thereof , said rare earth metal is praseodymium, preferably said noble metal i s a mixture of platinum and palladium, said rare earth metal is praseodymium or said noble metal is ruthenium, said rare earth metal is praseodymium .4 . Method for making the electrode according to any one of the preceding claims , wherein said precursor further comprises carbon powder .5 . Method for making the electrode according to any one of the preceding claims , wherein said applying of said catalytic dispersion by a coating technique is spray-coating, slot-die coating or blade-coating the catalytic coating on the substrate .6 . Method for making the electrode according to any one of the preceding claims , wherein said drying of said coated substrate is heating the coated substrate to a second temperature comprised between 30 and 90 °C , preferably comprised between 40 and 60 °C .7 . Method for making the electrode according to any one of the preceding claims , wherein said thermally unstable substrate is carbon paper or carbon cloth .M / 66045 -PCT ( 412 PCT )INDUSTRIE DE NORA S.p.A.

8. Method for making the electrode according to any one of the preceding claims, wherein said liquid comprises a polymeric binder.

9. Electrode obtained according to the method of any of the preceding claims.

10. Electrode comprising a substrate and a catalytic coating, wherein said substrate is a thermally unstable substrate, wherein said catalytic coating comprises oxides of metals, wherein said metals are selected from a group consisting of combinations of one or more noble metal with one or more rare earth metal, preferably said oxides of metals are obtained by thermally decomposing acetates of the metals.

11. Electrode according to claim 10, wherein said thermally unstable substrate is a carbon paper or a carbon cloth.

12. Electrode according to any one of claims 10 or11, wherein said catalytic coating comprises carbon powder .

13. Electrode according to any one of claims 10 to12, wherein said catalytic coating a polymeric binder.

14. Electrolyzer comprising one or more electrode according to any one of the claims 9 to 13.M / 66045-PCT (412PCT)INDUSTRIE DE NORA S.p.A.-20-15. Use of the electrode according to any one of the claims 9 to 13 in chlor-alkali reaction, anion exchange membrane water electrolysis, proton exchange membrane electrolysis, or alkaline water electrolysis.M / 66045-PCT (412PCT)

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