WO2016180494A1

WO2016180494A1 - Method for producing a metallic coating with macro-pores, coated substrate with such a coating and use of such a substrate

Info

Publication number: WO2016180494A1
Application number: PCT/EP2015/060630
Authority: WO
Inventors: Michael CASPERSEN; Sune Daaskov EGELUND; Per Möller
Original assignee: Siemens Aktiengesellschaft
Priority date: 2015-05-13
Filing date: 2015-05-13
Publication date: 2016-11-17

Abstract

The invention is related to a method for producing a coating (13) comprising a metallic matrix (17) on a substrate (11) by electrochemical deposition. There are graphite particles (14) embedded into the metallic matrix (17) of the coating (13) which leads to the creation of macro-pores (15) in the coating. The coating (13) can be completed by a surface layer (18) which also extends into the macro-pores (15) and therefore provide a catalytic surface with a great surface area. Additionally, the surface layer can be activated by a leaching process for further raising its surface area.

Description

Specification

Method for producing a metallic coating with macro-pores, coated substrate with such a coating and use of such a sub- strate

The invention relates to a method for producing a coating comprising a metallic matrix on a substrate by electrochemical deposition wherein carbon comprising particles are dis- persed in the deposition bath whereby the carbon comprising particles are embedded into the metallic matrix and pores are located in the coating enlarging the surface of the coating. The enlargement of the coating is the consequence of the fact that the pores are open to the surroundings of the coating and become therefore part of the surface area of the coating by being reachable by a medium (gas or liquid) which is in contact with the coating.

The invention also relates to a coated substrate whereby the coating comprises a metallic matrix in which the carbon comprising particles are embedded and in which a metallic matrix comprises pores enlarging the surface of the coating. Last the invention also relates a use of such coated substrates. Coated substrates as mentioned before are used for a lot of purposes. Especially because of the huge surface area provid^¬ ed by the pores substrates like these can be used as catalyt^¬ ic reaction accelerators. In alkaline electrolysis and catalysis in general, electrodes with a large surface area per geometric area is agreed to be desirable due to many available reaction sites for a given chemical reaction thus resulting in a more effective cata^¬ lyst. This is known to be important both for anodic and ca- thodic reactions.

There are methods of optimizing the surface area. For exam^¬ ple, anodes performing the OER (oxygen evolution reaction) and cathodes performing the HER (hydrogen evolution reaction) of alkaline water electrolysis are typically made from non- activated pure nickel or nickel plated steel as these elec^¬ trodes tend to be long-term stable. They can be activated by a thermal procedure where a precursor is decomposed directly on top of the electrode leading to a desired oxide phase as a thermally prepared film. The disadvantage when using this procedure is that the necessary calcination temperature is high and that the structure is rather brittle and fragile. It is also only small fraction of the active oxides which can be prepared in this way.

The state of the art according to alkaline electrolysis are so-called Raney electrodes. To obtain Raney electrodes a se- lective leaching of a material, typically aluminum or zinc, from a metal matrix of nickel, produces a highly porous structure with microscopic sized cavities and hence a large surface area. This structure, regardless of the applied manu^¬ facturing technique, has however been reported to lack me- chanical stability. This raises the danger of a delamination of the coating from the substrate. So, another hurdle in ob^¬ taining suitable electrodes lies in the creation of suffi^¬ cient adherence between the activated coating and the sub^¬ strate. Because of such difficulties plain nickel or stain- less steel electrodes are utilized in large scale plants in order to obtain durability with lowered efficiency as a com^¬ promise .

In general it is known to produce a porous coating by incor- porating graphite particles into an electroplated coating. In DE 2900453 a coating like that can be used in a heat exchang^¬ er to enhance the boiling capabilities of the surface provid^¬ ed for a heat exchange. The challenge is especially to produce electrodes that can endure the harsh environment and operating conditions of the alkaline electrolysis. The electrodes shall remain active and stable during electrolysis, heat-up and cool-down cycles and at open circuit without any severe delamination . Therefore, the problem to be solved by the invention is to provide and produce a catalytic surface on substrates which has a large surface area combined with low-cost production capabilities, durability and stability, i.e. chemical resistance as well as mechanical strength.

This problem is solved by the invention in such a kind that the coating is coated with a surface layer of a catalytic ma- terial whereby the surface layer extend into or interlocks in the pores which have the dimensions of macro-pores whereby at least 80 % of the macro-pores have a pore diameter in a range from 3 to 30 ym. This distribution of the pore diameter shall apply for the term macro-pores in the context of this inven- tion. The problem also is solved by a coated substrate which especially can be produced by the mentioned method whereby the pores in the metallic matrix are macro-pores and the coating comprises a surface layer which extends into the macro-pores. This coated substrate especially can be used as electrodes in electrochemical processes like the electrolysis of water. The fact that the pores are accessible to the me^¬ dium (gas, liquid) in which a reaction shall be catalysed is advantageous because the surface area of the coated substrate with the surface layer is enlarged and facilitates the cata- lytic abilities of the coated substrate.

The advantage of the proposed inventive method is that a pro^¬ cedure is provided for the production of a low cost metallic substrate suitable for post-coating via electrochemical or thermal procedures hereby preparing metallic surfaces which is well adhered and mechanically stable an serves as cata^¬ lyst. The process involves the addition of a carbon component in form of carbon comprising particles which functions as a macro-pore creator. Controlled stirring and dosing makes it possible to obtain different loadings of carbon comprising particles in the metallic matrix and great thicknesses of the coating. The macro-pores provide a comparatively large sur^¬ face area for the catalyst. This can be explained by the ob- servation that the electrically conductive carbon comprising particles which are co-deposited with the metal matrix act as antennas for further reduction of the metal ions of metal ions presented in the electrolyte and the layer is slowly build-up until a sufficient thickness is reached. This is the reason why the carbon comprising particles are fully embedded into the metallic matrix of the coating. The network struc^¬ ture of the carbon comprising particles constitutes a macro porous base structure for the further coating build-up of any kind, thus making it possible to tailor a desired material deposition with a macro porous structure by varying the com^¬ position of the electrolyte and the plating parameters as well. Therefore a procedure is provided for the production of a low cost metallic substrate suitable for post-coating via electrochemical or thermal procedures hereby preparing metal^¬ lic surfaces which is well adhered on the metallic matrix of the coating and mechanically stable an serves as catalyst.

According to an advantageous refinement of the invention the carbon comprising particles (14) consist from boron carbide, silicon carbide, graphite, carbon, activated graphite or ac^¬ tivated carbon. Advantageous is the use of activated parti^¬ cles. Due to its intrinsic high roughness factor for example activated carbon or graphite powders are employed to be in- corporated while electroplating into the metallic matrix of the coating and contribute to the build-up of a 3D porous Ni- C lattice structure with an increased macro-pore densitiy. A model for the deposition mechanism is that graphite particles are attracted to the negative electrode in the deposition bath, upon which the metal ions, i. e. nickel, can nucleate. This structure can be used as a precursor layer for a subse^¬ quent deposition of catalytic coatings.

The obtained coating can be used for both heterogeneous ca- talysis and electrocatalysis in both gas and liquid. The method can especially used for manufacturing of anodes for alkaline water electrolysis but the invention is not solely limited to this purpose. It could also be applied for the cathodes of the alkaline water electrolysis or other types of electrocatalysis .

The metallic coating can consist of several metals. However, a deposition without additives has to be possible due to the addition of carbon comprising particles. So it needs to be a metal which can be deposited with ease even without additives like nickel, silver or copper. These metals can be coated with a surface layer which shows a comparably good adhesion on these metals.

According to an advantageous refinement of the invention at least 90 % of the carbon comprising particles have a size in a range from 3 to 40 ym. Carbon comprising particles of this size range show a distinct behavior as a macro-pore producing agent. That means that the macro-porosity can be produced on a high level and a maximum surface area can be provided for catalytic purposes. According to another advantageous refinement of the invention the coating is produced with a thickness in a range of 5 to 200 ym. In this range of thickness the build-up of pores is possible in a manner that at least the mayor portion of the pores has direct or indirect access to the outer surface of the coating and therefore can be used as catalytic surface.

According to another advantageous refinement of the invention the catalytic material of the surface layer is chosen from the group of zinc, tin, copper, aluminum, molybdenum, silver, iron and cobalt, manganese, chromium, tungsten, zirconium, titanium, sulfur or alloys of these elements. These metals can be deposited on the metallic matrix of the coating. This can be done by electrochemical deposition or, alternatively, by physical vapor deposition or chemical vapor deposition or thermal spraying which is commonly known in the art. These metals or metal alloys can diffuse into the metal matrix whereby nickel, silver and copper are favorite metals to be used as metal matrix. The diffusion can be facilitated by a heat treatment of the compound comprising the metallic matrix of the coating and the surface layer. The respective metal diffusion couples form some sort of leachable phases which can be partially removed by a leaching procedure (this acti- vation procedure for example uses KOH) .

The result is a build-up of meso-pores and/or micro-pores which significantly raise the catalytic surface by forming a skeletal catalyst structure in the surface layer and the re- gion of the metal matrix which was involved in the diffusion process of the material of the surface layer. The meso-pores are characterized in that at least 80 % of the meso-pores have a pore diameter in a range from 1 to 3 ym. The micro^¬ pores are characterized in that at least 80 % of the micro- pores have a pore diameter in a range less than 200 nm. This distribution of the pore diameter of meso-pores and micro^¬ pores shall apply for the term micro-pores and meso-pores re^¬ spectively in the context of this invention. The rest of the metal matrix provides a stabile lattice with said macro- pores. So, the complete structure is advantageously long term stabile according to its mechanical properties also in harsh environments like water electrolysis.

There are also alternative possibilities to produce said sys- tern of meso-pores and/or micro-pores. One could co-deposit for example a nickel zinc alloy which is suitable for leach^¬ ing or other leachable alloys as surface layer. This would make needless a heat treatment of the surface layer before leaching. Also the leaching step can become needless depend- ing on the case of use. If the coated substrate is used in a reactive environment like KOH (which is the case performing an alkaline water electrolysis) a leaching occurs during the intended use of the coated substrate as electrode.

According to another advantageous refinement of the invention a sheet, a perforated plate, a lattice or a mesh is used as a substrate. By this measure the surface area can be raised in macroscopic dimensions. A lattice or a mesh can be provided as 3-dimensional structure. The relation between the mesh size or the lattice interspaces and the substrate material can be chosen with view to a optimum according to the acces- sibility by the medium which includes the reaction partners of the reaction to be catalyzed.

Another advantageous refinement of the invention includes the deposition of a base layer, especially of nickel, copper or silver, which is plated on the substrate before starting the mentioned simultaneous deposition of the particles and the metallic matrix. This base layer can be used as a adhesion promoter between the substrate and the metallic matrix of the coating .

Further details of the invention can be found in the drawings whereby similar drawing elements are denoted by the same ref^¬ erence numerals respectively and are only explained once in below unless there are to be specified differences between the figures.

Figures 1 - 4 show several process steps according to an ex^¬ ample of the inventive method depicting an example of the in^¬ ventive coated substrate as cross-sectional view and

Figure 5 shows an example of the inventive use of the coated substrates as electrodes of a water electrolysis application as cross-sectional view. In Figure 1 there is depicted a substrate 11 which was coated with a base layer 12 consisting from nickel. The process pa^¬ rameters are specified below.

In Figure 2 there is depicted the next process step whereby one can find three examples for possible process parameters below for plating a coating 13 comprising carbon comprising particles which are activated graphite particles 14 in this example. This coating 13 is plated by an electrochemical coating process whereby the graphite particles 14 are laid down to the surface of the base layer 12 in the beginning of the deposition and later are contacted to each other building up a network or lattice which is electrically conductive. This network serves to influence the electric field in the surrounding of the base layer 12 in that kind that the nickel ions from the plating bath preferably are attached to the graphite particles 14. The graphite particles are fully em^¬ bedded this way into a metallic matrix 17 whereby the network of graphite particles 14 are not fully filled with nickel and therefore a build-up of macro-pores 15 occures. The pore di^¬ ameter of the macro-pores 15 lies in a range from 50 to 500 ym. The pore diameters are directly influenced by the choice of the graphite particles in a certain size range. A size range in which 90 % of the graphite particles have a size from 3 - 40 ym results in a favourable pore size distribu^¬ tion .

In Figure 3 there is depicted the next process step which consists of a coating of a surface layer 18 which constitutes at least a part of a outer surface 19 provided by the coating 13. The surface layer 18 also reaches into the macro pores 15 whereby the surface area provided by the surface layer 18 is raised compared to the case that the surface layer would be coated on a flat substrate.

The surface layer can be made from zinc for example which can be plated in a plating bath mentioned below. However, also other coating methods can be performed, for example chemical vapor deposition, physical vapor deposition or thermal spraying, especially dynamic cold gas spraying.

As depicted in figure 4 an activation of the surface layer 18 can be executed to further raise the surface area provided by the coating 13. For that purpose a heat treatment results in a diffusion of zinc into the metallic matrix of nickel where^¬ by inter-metallic phases are generated. The activation of the surface is performed by leaching the coating 13 with KOH. Leaching results in dissolving the inter-metallic phases in the surface layer which, after heat treatment, can be de^¬ scribed as a diffusion zone of the metallic matrix 17 con^¬ taining the inter-metallic phases. After dissolving the sur^¬ face layer 18 comprises a system of meso-pores 20 and micro^¬ pores 21 which significantly raise the surface area of the coating 13. This surface area can be provided as a catalytic converter .

As depicted in figure 5 the coated substrate 11 of figure 4 can be used as electrodes (anodes 22 and cathodes 23) for the electrolysis of water. The anodes 22 and cathodes 23 are placed in a container 24 comprising KOH and are connected with a voltage supply source 25. The anodes 22 and cathodes 23 could be divided by diaphragms 26 from each other. Apply^¬ ing a voltage by the voltage supply source 25 leads to the production of oxygen O2 on the anodes 22 and hydrogen H₂ on the cathodes 24.

Experimental

All electrolytic deposited electrodes were degreased and ac^¬ tivated using the following procedure prior to deposition of subsequent coatings. The pretreatment procedure was as fol^¬ lows :

Step treatment Bath Parameters

1. Anodic degreasing 55,5 g/1 NaOH 5 min, 5 V

14 g/1 Na₂Si0₃

2. Washing H₂0

3. Activation 100 g/1 5% NaHF₂ 5 min, no

Pickling and 95% NaHS0₄ polarization 4. Washing H₂0

5. Woods 100 g/1 NiCl₂-6H₂0 5 min,

nickel-strike 100 ml/1 HC1 (37%) 0,050 Acm^"2

The following parameters where chosen for the plating an treating steps for

1. the base layer on the substrate,

2. particle containing coating and

3. the surface layer

4. the pretreatment of the surface layer for activation (raising the surface area by building meso-pores and7or micro-pores ) Plating bath for base layer: Watts Ni

300 g/1 NiS04 · 6H20

45 g/1 NiC12 · 6H20

38 g/1 H3BO3

pH: -3-4

Temp: 50 °C

20 min, 0, 05 Acm^"2

Plating bath for particle containing coating example I

100 - 450 g/1 NiS0₄ * 6H₂0

20 - 70 g/1 NiCl₂

20 - 50 g/1 H₃BO₃

5 - 120 g/1 activated carbon powder

pH: ~3 - 4

Temp: 50 °C

20 min, 0, 05 Acm^"2 example II

100 - 450 g/1 NiS0₄ * 6H₂0

100 - 450 g/1 C0SO₄ * 7H₂0

20 - 70 g/1 NiCl₂

20 - 50 g/1 H₃BO₃ 5 - 120 g/1 activated carbon powder

pH: ~3 - 4

Temp: 50 °C

20 min, 0, 05 Acm^"2 example III

180 g/1 CuS0₄ * 5H₂0

60 g/1 H₂S0₄

1 g/1 NaCl

5 - 120 g/1 boron carbide powder (B₄C)

pH: ~0 - 1

Temp: 25 °C

5-10 min, 0,01 - 0,1 Acm^"2 Plating bath for surface layer

Bright Zinc of (supplier: Schlotter Galvanotechnik)

SLOTANIT OT-1 of (supplier: Schlotter Galvanotechnik) pH: ~5

Temp: 50 °C

90 min, 0, 02 Acm^"2

Activation of surface layer

Heat treatment 4h at 350 °C

Activation 2 h in 30 wt% KOH at 25 - 80 °C

A three-electrode cell was used for studies of the half-cell reactions and the overpotentials of these. The activity was characterized at room temperature in order to avoid disturb^¬ ance of the reference electrodes. The three-electrode setup was used for evaluation of OER and HER overpotential record^¬ ing. For long-term evaluation of lab results a test electro- lyser stack manufactured by Greenhydrogen.dk was used. The electrodes were in several occasions up-scaled and implement^¬ ed into this electrolyser .

A long-term testing was carried out in full-cells at what was believed to be realizable and realistic conditions using of 50 wt% KOH at 120 °C for alkaline electrolysis. The harsh en- vironment was selected as an electrocatalyst stress enhancer. In the full-cell, where the global potential situation was considered, the anodes were initially ranked against non- activated and polished nickel as cathode material. As for the major achievement in the investigation it was demonstrated that in a cell, equipped with an inventive cathode and a non- activated nickel anode, a global cell potential of approxi^¬ mately 1,67 V after about 3000 hours at 0,4 Acm^"2, 120 °C, 50 wt% KOH could be achieved. The optimized coatings were subse^¬ quently up-scaled and the results were transferred to elec^¬ trode stacks. Here it was verified that a average cell poten^¬ tial of approximately 1,66 V +/- 0,02 V was obtained at a temperature of only 95 °C. Approximately the same activity was determined for different optimized cathodes at 0,4 Acm^-2 in 30 wt% KOH. The inventive coatings were also used as a combined OER-HER coating for both electrodes, the anode and the cathode .

Claims

1. Method for producing a coating (13) comprising a metallic matrix (17) on a substrate (11) by electrochemical deposition wherein carbon comprising particles (14) are dispersed in the deposition bath whereby the graphite particles (14) are em^¬ bedded into the metallic matrix (17) and pores are located in the coating (13) enlarging the surface of the coating (13) characterized in

that the coating (13) is coated with a surface layer (18) of a catalytic material whereby the surface layer (18) extends into or interlocks in the pores which have the dimensions of macro-pores (15) whereby at least 80 % of the macro-pores (15) have a pore diameter in a range from 3 to 30 ym.

2. Method as claimed in claim 1

characterized in

that the metallic matrix (17) comprises nickel, silver or copper as main component.

3. Method as claimed in one of the preceding claims charac^¬ terized in

that at least 90 % of graphite particles (14) which have a size in a range from 3 to 40 ym.

4. Method as claimed in one of the preceding claims

characterized in

that the coating (13) is produced with a thickness in a range of 5 to 200 ym.

5. Method as claimed in one of the preceding claims

characterized in

that the catalytic material is chosen from the group of zinc, tin, copper, aluminum, molybdenum, silver, iron and cobalt, manganese, chromium, tungsten, zirconium, titanium, sulfur or alloys of these elements.

6. Method as claimed in one of the preceding claims characterized in

that the carbon comprising particles (14) consist from boron carbide, silicon carbide, graphite, carbon, activated graph^¬ ite or activated carbon.

7. Method as claimed in one of the preceding claims

characterized in

that a sheet, a perforated plate, a lattice or a mesh is used as the substrate (11) is plated.

8. Method as claimed in one of the preceding claims

characterized in

that a base layer (12), especially of nickel, copper or sil^¬ ver, is plated on the substrate (11) before starting said simultaneous deposition of graphite particles (14) and the metallic matrix (17) .

9. Method as claimed in one of the claims 8 to 10

characterized in

that the surface area of the surface layer (18) is enlarged by leaching whereby meso-pores (20) whereby at least 80 % of the meso-pores (20) have a pore diameter in a range from 1 to 3 ym and/or micro-pores (21) whereby at least 80 % of the mi^¬ cro-pores (21) have a pore diameter in a range less than 200 nm are build.

10. Coated substrate (11) whereby

• the coating (13) comprises a metallic matrix (17) in

which graphite particles (14) are fully embedded

· the metallic matrix (17) comprises pores enlarging the surface of the coating (13)

characterized in

that

• pores in the metallic matrix (17) are macro-pores (15) · the coating (13) comprises a surface layer (18) which extends into the macro-pores (15) .

11. Coated Substrate (11) as claimed in claim 10, characterized in

that the surface layer (18) comprises meso-pores (20) and/or micro-pores (21) enlarging the surface of the surface layer (18) .

12. Coated Substrate (11) as claimed in claim 10,

characterized in

that the substrate (11) is coated as claimed in claims 1 to 9.

13. Use of coated substrates (11) as claimed in one of the claims 10 to 12 as electrodes in electrocatalytic or catalyt ic processes.

14. Use as claimed in Claim 13,

characterized in

that the electrochemical process is an electrolysis of water and the electrodes are used as anode (22) and/or as cathode (23)