METHOD OF PREPARING PLATINUM-BASED CATALYST AND PLATINUM-BASED CATALYST

Abstract

The invention relates to the method of forming a platinum-based catalytic coating on electrodes for using in electrochemical devices such as fuel cells or electrolysis cells. According to the invention, to produce a platinum-based catalyst, the carrier is preliminary cleaned by ion etching and the catalytic coating is applied onto the cleaned surface from at least one target based on platinum in vacuum in the primary gas plasma with addition of reactive gas, sputtering being done at the power density on the magnetron sputtered target within (0.004-0.17)*10.sup.5 W/m.sup.2 and the ratio of concentrations of the primary and reaction gas of 75-99%. Technical result: increased specific catalytic activity of the electrode's catalytic coating for electrochemical devices (fuel cells and electrolysis cells). 2 primary claims, 8 depending claims, 5 figures.

Claims

1. A method of platinum-based catalyst production based on preliminary cleaning of the carrier by ion etching and further magnetron sputtering of at least one target based on platinum in vacuum in the primary gas plasma with addition of reactive gas characterized in that sputtering is done at the power density on the magnetron sputtered target within (0.004-0.17)*10.sup.5 W/m.sup.2 and the ratio of concentrations of the primary and reaction gas of 75-99%.

2. The method according to claim 1 characterized in that sputtering is done at the reaction gas residual pressure of 6.7-20 Pa.

3. The method according to claim 1 characterized in that argon, nitrogen, hydrogen, helium or any combination thereof is used as the primary gas.

4. The method according to claim 3 characterized in that argon is preferably used.

5. The method according to claim 1 characterized in that oxygen, nitrogen, air or any possible combination thereof is used as the reaction gas.

6. The method according to claim 5 characterized in that oxygen is preferably used.

7. The method according to claim 1 characterized in that at least one additional target and/or combined target based on metal selected from a group comprising palladium, iridium, ruthenium, tungsten, zirconium, niobium, tantalum, antimony, tin, molybdenum, nickel, cobalt, silica, graphite and/or oxides thereof is used during sputtering.

8. The method according to claim 1 characterized in that an intermediate catalyst layer is applied before platinum application, which is based on titanium, niobium and tantalum, palladium, ruthenium, tungsten, zirconium, antimony, tin, molybdenum, nickel, cobalt and/or alloys thereof and/or oxides thereof.

9. The method according to claim 8 characterized in that the intermediate layer is preferably applied by magnetron sputtering.

10. The platinum-based catalyst produced as claimed in claim 1.

Description

SHORT DESCRIPTION OF DRAWINGS

[0017] FIGS. 1A-1D show enlarged images of the front surface of nano-dispersive platinum surface samples on a soot micro-porous layer of carbonic gas-diffusion paper Freudenberg H2315-C2 with the specific platinum content of 22 μg/cm.sup.2 (B) and 150 μg/cm.sup.2 (C,D), and a carbon paper with no platinum applied (A). These pictures were taken with an electronic microscope JSM-6390 LA (JEOL USA, Inc.).

[0018] FIG. 2 shows electronic pictures of the titanium-based platinum catalyst with the coat of 594 μg/cm.sup.2 (×40 000).

EMBODIMENT OF INVENTION

[0019] As seen in FIG. 1A, the initial structure of the micro-porous coating based on soot represents large formations 500-800 nm in size consisting of soot spherical particles up to 100 nm in size. Application of dispersive platinum on this surfaces results in gradual enlargement of particles. It means that the formation of the platinum dispersive coating and its growth starts and occurs directly on the surface of soot particles. It can be seen that this growth results in increased diameter of soot particles due to sedimentation of dispersive platinum. Prominent change in particles diameters is observed starting with the charge of 20 μg/cm.sup.2 (FIG. 1B), and for the charge of 300 μg/cm.sup.2, the diameter of produced particles exceeds the diameter of soot particles by 2-3 times due to the formation of the black soot on the surface (not shown in drawings). The dispersive platinum characteristic color is black. When the platinum charge is increased to above 500 μg/cm.sup.3, platinum particles become able to group into fine nano-fibers (FIG. 2).

[0020] The structure of such coats represents a combination of fibers whose length and thickness are increased together with the platinum specific content. When the charge exceed 2500 μg/cm.sup.2, their thickness is 10-14 nm, and the length is 200-300 nm. Irrespective of the platinum content, all coats had a characteristic black color, which indicates their large specific surface area.

[0021] As one or several platinum components within the mixed catalyst, palladium, iridium, ruthenium, tungsten, zirconium, niobium, tantalum, antimony, tin, molybdenum, nickel, cobalt, silica, graphite and their oxides are used. To produce mixed catalysts, the sputtering is done from one or more targets, one of which is based on platinum. For this purpose, combined targets of the above materials are used. The flows of the sputtered coat can be directed to the substrate, or the substrate can move relative to the targets by using a carousel mechanism. The composition and structure of the catalyst is adjusted by changing the process parameters: magnetron current, substrate moving speed relative to the magnetron, substrate inclination angle relative to the magnetron target, ratios of areas of the combined target of the sputtered material.

[0022] In order to enhance the platinum bonding with the electrode surface, such as a soot-based surface, an intermediate layer of the catalyst is applied, preferably by magnetron sputtering in vacuum, followed by catalyst and carrier layers applied in series. As an intermediate layer material, titanium, niobium and tantalum, their alloys and oxides are used.

[0023] As a primary gas, nitrogen, hydrogen, helium, argon or any possible combination thereof is used. Preferably, argon is used.

[0024] As a reaction gas, oxygen, nitrogen, air or any possible combination thereof is used. If electronegative gases are used in relative sputtering (such as nitrogen or oxygen), negatively charged ions are formed. Oxygen is the preferable reaction gas. By means of oxide formation on the target surface, it affects the speed of target sputtering, and, consequently, design features of catalyst's dispersive structures with low energies of platinum particles.

[0025] During magnetron platinum DC sputtering, the range of positively charge ions in the mixture of argon and oxygen is vast and includes Ar.sup.+, Ar.sup.+2 ions (possible ionized platinum oxides). All positive ions are formed in the plasma volume, since otherwise they would accelerate back towards the target in the dark cathode space (p. 107, E. V. Berlin, L. A. Seydman. Plasma ion processes in thin-film technology. Moscow: Teckhnosfera, 2010. Pp. 528).

[0026] Unlike positive ions, the range of negatively charged ions is much simpler. It contains only oxygen ions O.sup.2−, O.sup.− (possible ionized platinum oxides). Unlike positive ions, negative ones are accelerated by cathode potential and acquire several hundreds of electron volts. When comparing the fluxes of positive and negative ions, it can be noted that the flux of positively charged ions exceeds the flux of negatively charged ones by 1-2 orders. However, thanks to their high energy, negative ions can produce defects in the film and cause re-sputtering of atoms from the surface of growing film, which promotes the formation of the dispersive structure. The number of negatively charged ions increases with the oxygen partial pressure in the chamber-not directly proportional, but much faster. This is caused by oxide formation on the substrate surface.

[0027] In this manner, the oxide on the platinum target affects the intensity and composition of the platinum atomic steam obtained by means of bombarding the target surface with Ar.sup.+ ions. The formation of electronegative gas negative ions can produce defects in the film and cause re-sputtering of atoms from the surface of growing film, which promotes the formation of the dispersive structure.

[0028] In order to illustrate various aspects of the invention embodiment, below are examples of the suggested method embodiment that are explanatory only and not restrictive of the disclosure. Selective, examples from Table 1 are described below. In examples 1-6, the specific activity in Table 1 is given for a catalytic coating on a titanium substrate.

EXAMPLE 1

Manufacturing an Electrode-Catalyst for a Fuel Cell

[0029] By using a platinum target (P199.93) 1 mm thick and Kraudion 1M magnetron sputtering unit, the platinum nano-structured coating was applied on the surface of the soot low-porous layer of Freudenberg H2315-C2 carbon paper and of the titanium foil in order to determine the catalyst's activity and specific surface area. The air was removed from the chamber until reaching the partial pressure below 2×10.sup.−3 Pa. The vacuum chamber was then charged with a plasma-forming gas—argon—until reaching the pressure of 1.7 Pa, which was maintained at the given level during the entire sputtering process. The substrates were then subjected to ion etching by a flux of argon ions with the average energy up to 1.5 keV. Then platinum target was then exposed to voltage and a plasma charge with the power density of (0.004)*10.sup.5 W/m.sup.2 was excited. This was followed by the oxygen reaction gas supplied to the vacuum chamber until reaching 7 Pa. The oxygen percentage was 77%. With the fixed oxygen pressure and stabilized sputtering process, the protective screen was opened and the platinum was sputtered on the surface of the low-porous layer of the carbon paper and of the titanium foil. The process took 400 seconds. The platinum specific content was 21.6 μg/cm.sup.2. The oxygen activity was then determined. The catalyst specific activity (according to the specific current of adsorbed hydrogen oxidation) was 26.1 A/g of Pt. The titanium-based catalyst activity was determined by a potentiodynamic method in 0.5 M H.sub.2SO.sub.4 within a three-electrode electrochemical cell. The activity criterion was the maximum oxidation peak of adsorbed hydrogen reduced to the amount of platinum in the catalyst. This specific current reliably characterizes the catalyst's electrochemical surface, which is proportional to the electrode activity when working in the fuel cell.

EXAMPLE 2

Manufacturing an Electrode-Catalyst for a Fuel Cell

[0030] By using a platinum target (P199.93) 1 mm thick and Kraudion 1M magnetron sputtering unit, the platinum nano-structured coating was applied on the surface of the soot low-porous layer of Freudenberg H2315-C2 carbon paper and of the titanium foil in order to determine the catalyst's activity and specific surface area. The air was removed from the chamber until reaching the partial pressure below 2×10.sup.−3 Pa. The vacuum chamber was then charged with a plasma-forming gas—argon—until reaching the pressure of 1.7 Pa, which was maintained at the given level during the entire sputtering process. The substrates were then subjected to ion etching by a flux of argon ions with the average energy up to 1.5 keV. The platinum target was then exposed to voltage and a plasma charge with the power density of (0.021)*10.sup.5 W/m.sup.2 was excited. This was followed by the oxygen reaction gas supplied to the vacuum chamber until reaching 7 Pa. The oxygen percentage was 77%. With the fixed oxygen pressure and stabilized sputtering process, the protective screen was opened and the platinum was sputtered on the surface of the low-porous layer of the carbon paper and of the titanium foil. The process took 80 seconds. The platinum specific content was 22 μg/cm.sup.2. The oxygen activity was then determined. The catalyst specific activity (according to the specific current of adsorbed hydrogen oxidation) was 29.8 A/g of Pt. The titanium-based catalyst activity was determined by a potentiodynamic method in 0.5 M H.sub.2SO.sub.4 within a three-electrode electrochemical cell. The activity criterion was the maximum oxidation peak of adsorbed hydrogen reduced to the amount of platinum in the catalyst. This specific current reliably characterizes the catalyst's electrochemical surface, which is proportional to the electrode activity when working in the fuel cell. The photographs of the platinum catalyst sputtered on the soot substrate are given in FIG. 1(A,B).

EXAMPLE 3

Manufacturing an Electrode-Catalyst for a Fuel Cell

[0031] By using a platinum target (P199.93) 1 mm thick and Kraudion 1M magnetron sputtering unit, the platinum nano-structured coating was applied on the surface of the soot low-porous layer of Freudenberg H2315-C2 carbon paper and of the titanium foil in order to determine the catalyst's activity and specific surface area. The air was removed from the chamber until reaching the partial pressure below 2×10.sup.×3 Pa. The vacuum chamber was then charged with a plasma-forming gas—argon—until reaching the pressure of 1.7 Pa, which was maintained at the given level during the entire sputtering process. The substrates were then subjected to ion etching by a flux of argon ions with the average energy up to 1.5 keV. The platinum target was then exposed to voltage and a plasma charge with the power density of (0.017)*10.sup.5 W/m.sup.2 was excited. This was followed by the oxygen reaction gas supplied to the vacuum chamber until reaching 7 Pa. The oxygen percentage was 77%. With the fixed oxygen pressure and stabilized sputtering process, the protective screen was opened and the platinum was sputtered on the surface of the low-porous layer of the carbon paper and of the titanium foil. The process took 10 seconds. The platinum specific content was 21.4 μg/cm.sup.2. The oxygen activity was then determined. The catalyst specific activity (according to the specific current of adsorbed hydrogen oxidation) was 17.0 A/g of Pt. The titanium-based catalyst activity was determined by a potentiodynamic method in 0.5 M H.sub.2SO.sub.4 within a three-electrode electrochemical cell. The activity criterion was the maximum oxidation peak of adsorbed hydrogen reduced to the amount of platinum in the catalyst. This specific current reliably characterizes the catalyst's electrochemical surface, which is proportional to the electrode activity when working in the fuel cell.

EXAMPLE 22

Manufacturing an Electrode-Catalyst for a Fuel Cell

[0032] By using a platinum target (P199.93) 1 mm thick and Kraudion 1M magnetron sputtering unit, the platinum nano-structured coating was applied on the surface of the soot micro-dispersive layer of Freudenberg H2315-C2 carbon paper and of the titanium foil in order to determine the catalyst's activity and specific surface area. The air was removed from the chamber until reaching the partial pressure below 2×10.sup.−3 Pa. The vacuum chamber was then charged with a plasma-forming gas—argon—until reaching the pressure of 1.7 Pa, which was maintained at the given level during the entire sputtering process. The titanium substrate was then subjected to ion etching by a flux of argon ions with the average energy up to 1.5 keV. The platinum target was then exposed to voltage and a plasma charge with the power density of (0.021)*10.sup.5 W/m.sup.2 was excited. This was followed by the oxygen reaction gas supplied to the vacuum chamber until reaching 7 Pa. The oxygen percentage was 77%. With the fixed oxygen pressure and stabilized sputtering process, the protective screen was opened and the platinum was sputtered on the surface of the low-porous layer of the carbon paper and of the titanium foil. The process took 240 seconds. The platinum specific content was 151.5 μg/cm.sup.2. The oxygen activity was then determined. The catalyst specific activity (according to the specific current of adsorbed hydrogen oxidation) was 26.1 A/g of Pt. The photographs of the catalyst on titanium foil are given in FIG. 1(C,D).

EXAMPLE 26

Manufacturing an Electrode-Catalyst for an Electrolysis Cell

[0033] By using a platinum target (P1 99.93) 1 mm thick and Kraudion 1M magnetron sputtering unit, the platinum nano-structured coating was applied on the titanium surface. The air was removed from the chamber until reaching the partial pressure below 2×10.sup.−3 Pa. The vacuum chamber was then charged with a plasma-forming gas—argon—until reaching the pressure of 1.7 Pa, which was maintained at the given level during the entire sputtering process. The titanium substrate was then subjected to ion etching by a flux of argon ions with the average energy up to 1.5 keV. The platinum target was then exposed to voltage and a plasma charge with the power density of (0.021)*10.sup.5 W/m.sup.2 was excited. This was followed by the oxygen reaction gas supplied to the vacuum chamber until reaching 7 Pa. The oxygen percentage was 77%. With the fixed oxygen pressure and stabilized sputtering process, the protective screen was opened and the platinum was sputtered on the titanium surface. The process took 640 seconds. The platinum specific content was 594 μg/cm.sup.2. The catalyst specific activity (according to the specific current of adsorbed hydrogen oxidation) was 18.3 A/g of Pt. The photographs (×60 000) of the titanium-based catalyst are given in FIG. 2. It can be seen that when the platinum content increases, filiform platinum formations start forming on the titanium surface.

[0034] The catalyst activity test results for other conditions are given in Table 1.

TABLE-US-00001 TABLE 1 Catalyst activity test results for a catalyst produced using the proposed method. Specific activity (according to the Specific Power current of density Catalyst's adsorbed on the specific hydrogen sputtered Chamber content oxidation), Example Catalyst target, pressure, (Pt), Oxygen A/g of Substrate No. material .Math.10.sup.5 w/m.sup.2 Pa μg/cm.sup.2 percentage, % Pt Soot, 1 Pt 0.004 7 21.6 77 26.1 titanium 2 Pt 0.021 7 22.0 77 29.8 3 Pt 0.17 7 21.4 77 17 4 Pt 0.18 7 17 77 16.1 5 Pt 0.003 7 21.2 77 25.3 6 Pt 0.018 7 21.0 77 16.1 7 Pt 0.021 7 20.8 0 11.1 8 Pt 0.021 7 20.6 50 27.5 9 Pt 0.021 7 20.8 70 28.3 10 Pt 0.021 7 22.0 75 29.5 11 Pt 0.021 7 21.2 90 29.7 12 Pt 0.021 7 20.6 99 29.3 13 Pt 0.021 7 20.8 100 16.3 14 Pt 0.021 6 21.4 77 21.0 15 Pt 0.021 15 21.0 77 19.5 16 Pt 0.021 20 21.2 77 17 17 Pt 0.021 21 21 77 13.1 18 Pt 0.021 7 150.4 0 11.7 19 Pt 0.021 7 150.9 50 10.8 20 Pt 0.021 7 151 70 22.9 21 Pt 0.021 7 152.0 75 25.1 22 Pt 0.021 7 151.5 77 26.1 23 Pt 0.021 7 150.1 90 22.1 24 Pt 0.021 7 149 99 21.3 25 Pt 0.021 7 150.6 100 13.3 26 Pt 0.021 7 594 77 18.3

[0035] As seen from the table, the sputtering within the power density on the mangetron's sputtered target within (0.004-0.17)*10.sup.5 W/m.sup.2 and the ratio of concentrations of primary and reaction gases of 75-99% ensures maximum catalyst activity (according to the specific current of adsorbed hydrogen oxidation). In this case, the activity is determined by forming a nano-porous morphology of the catalyst with developed internal surface of the platinum dispersive surface (FIGS. 1A-1D). Increasing the platinum embedding to the catalyst and adopting thick dispersive platinum surfaces (Example 26) results in decreased specific activity due to the formation of structures with larger platinum particles and fibers (FIG. 2).

[0036] When the power density on the sputtered target goes below 0.004*10.sup.5 W/m.sup.2, the specific activity is not increased, but the time of catalyst application is unreasonably high, which makes the process lack its technological effectiveness. When the power density on the sputtered target goes above 0.17*10.sup.5 W/m.sup.2, the specific activity is decreased, but the time of catalyst application is reduced so much that it makes the application process unsteady, e.g., it lacks technological effectiveness.

[0037] The oxygen concentration in the magnetron unit chamber going below the specified range (below 75%) causes the catalyst's specific activity to go down (Examples 7, 8, 9, 18, 19, 20). Increasing the oxygen concentration in the magnetron unit chamber above the specified range (above 75%) causes the catalyst's specific activity to go down due to the decreased concentration of the plasma-forming gas (Examples 13, 25).

[0038] The pressure in the magnetron unit chamber going below the specified range (below 6.7 Pa) causes the catalyst's specific activity to go down (Example 14). The pressure in the magnetron unit chamber going above the specified range (above 20 Pa) causes the catalyst's specific activity to go down (Example 17).

[0039] The essence of this invention is based on the synthesis effect on the substrate surface of dispersive nano-structured catalytic sediments of platinum and its compositions due to low interaction energies of sputtered platinum atoms and compositions thereof in the gas medium containing the optimal balance of plasma-forming (primary) and reactive (electronegative) gas and creating the conditions for the dispersive nano-structured sediments to grow.