METHOD AND DEVICE FOR FORMING A CATALYTICALLY-ACTIVE MEMBRANE OR A MEMBRANE-ELECTRODE-ASSEMBLY

Abstract

Described herein is a method for manufacturing a catalytically-active membrane-electrode-assembly (20) with one or more, particularly two electrodes, the method comprising at least the steps of: i) depositing a heterogenous layer (3) on a substrate (5), the heterogeneous layer (3) comprising a base metal (1) and a noble metal (2) heterogeneously distributed in the heterogenous layer (3), ii) leaching of the base metal (1) out of the heterogeneous layer (3), such that a first self-supporting nanoporous catalyst layer (4) comprising the noble metal (2) is formed on the substrate (5), iii) adding of at least one kind of proton-conductive ionomers (40) and/or at least one kind of hydrophobic particles (41) and/or an ionic liquid (42) to the first self-supporting nanoporous catalyst layer (4), and iv) forming a catalytically-active membrane-electrode-assembly (20) by attaching the self-supporting nanoporous catalyst layer (4) to a first side of a membrane (10), such that a catalytically-active membrane-electrode-assembly (20) with one electrode is formed.

Claims

1. A method for manufacturing a catalytically-active membrane-electrode-assembly (20) with one or more, particularly two electrodes, the method comprising at least the steps of: i) depositing a heterogenous layer (3) on a substrate (5), the heterogeneous layer (3) comprising a base metal (1) and a noble metal (2) heterogeneously distributed in the heterogenous layer (3), ii) leaching of the base metal (1) out of the heterogeneous layer (3), such that a first self-supporting nanoporous catalyst layer (4) comprising the noble metal (2) is formed on the substrate (5), iii) adding of at least one kind of proton-conductive ionomers (40) and/or at least one kind of hydrophobic particles (41) and/or an ionic liquid (42) to the first self-supporting nanoporous catalyst layer (4), and iv) forming a catalytically-active membrane-electrode-assembly (20) by attaching the self-supporting nanoporous catalyst layer (4) to a first side of a membrane (10), such that a catalytically-active membrane-electrode-assembly (20) with one electrode is formed.

2. The method according to claim 1, wherein a second self-supporting nanoporous catalyst layer (4) on the substrate (5) is formed according to step ii) and iii), e.g. either simultaneously with or sequentially to the first self-supporting nanoporous catalyst layer (4), or by separating a portion of the first self-supporting nanoporous catalyst layer (4) that forms the second self-supporting nanoporous catalyst layer (4), and wherein in step iv) or in a subsequent step, the second self-supporting nanoporous catalyst layer (4) is attached to a second side of the membrane (10) such that a catalytically-active membrane-electrode-assembly (20) with two electrodes is formed.

3. The method according to claim 1, wherein attaching the first and/or the second self-supporting nanoporous catalyst layer (4) is carried out by pressing the substrate (5) with the first and/or second self-supporting nanoporous catalyst layer (4) on to the membrane (10) and/or by decal transferring the first and/or second self-supporting nanoporous catalyst layer (4) from the substrate (5) to the membrane (10).

4. The method according to claim 1, characterized in that the at least one kind of proton-conductive ionomer (40) and/or at least one kind of hydrophobic particles (41) and/or an ionic liquid (42) is added to the first and/or second self-supporting nanoporous catalyst layer (4) by at least one of the following methods: spraying, ultrasonic spraying, decal-transferring, immersing, drop casting, filtering.

5. The method according to claim 1, wherein the substrate (5) comprises or is a gas-permeable electrode (50) and/or an electrically non-conductive or conductive transfer substrate (51) configured for decal transferring the first and/or second self-supporting nanoporous catalyst layer (4) onto the membrane (10).

6. The method according to claim 1, wherein in step i), the base metal (1) and/or the noble metal (2) is deposited by physical vapor deposition method (PVD).

7. The method according to claim 6, characterized in that the PVD includes or is a sputtering method, particularly an alternating magnetron sputtering method (7).

8. The method according to claim 6, characterized in that the substrate (5) is moved back and forth along at least one direction during the PVD.

9. The method according to claim 1, characterized in that step ii) is carried out in an electrolyte (35) arranged between a first leaching electrode (31) and a second leaching electrode (32), wherein the heterogeneous layer (3) and the first leaching electrode (31) are electrically connected and a leaching current is applied between the first and the second leaching electrode (31,32), such that the base metal (1) is leached out of the heterogenous layer (3), particularly wherein the heterogeneous layer (3) on the substrate (5), particularly on the transfer substrate (51) is arranged within the electrolyte (35).

10. The method according to claim 9, wherein an electrical potential of the first leaching electrode (31) is controlled by a third leaching electrode (33) that is immersed in the electrolyte next to the first leaching electrode (31), wherein a voltage between the first and the third leaching electrode (31,33) is determined.

11. The method according to claim 10, characterized in that during step ii), the leaching current between the first and the second leaching electrode (31,32) is controlled such that the voltage between the first leaching electrode (31) and the third leaching electrode (33) reaches a selectable value.

12. The method according to claim 9, characterized in that step ii) is carried out in a leaching chamber (30), the leaching chamber (30) comprising: the first leaching electrode (31), wherein the first leaching electrode (31) is electrically connected to the heterogeneous layer (3), particularly the heterogeneous layer (3) deposited on the substrate (5), the second leaching electrode (32), as well as the electrolyte (35) arranged between the first and the second leaching electrode (31,32), wherein the heterogeneous layer (3) is arranged between the first and the second leaching electrode (31,32).

13. The method according to claim 12, characterized in that at least during step ii), a flow of at least one non-reactive gas, particularly N.sub.2 and/or Ar is conducted through the leaching chamber (30).

14. The method according to claim 12, characterized in that at least during step ii), a flow of at least one reactive gas, particularly H.sub.2 or O.sub.2 is conducted through the leaching chamber (30), particularly above the first leaching electrode (31).

15. A membrane-electrode-assembly (20) for a proton exchange membrane (PEM) fuel cell or an electrolyzer comprising the following components: a first gas-permeable electrode layer (5, 5-1), a first self-supporting nanoporous catalyst layer (4, 4-1) comprising a noble metal (2), a membrane (10), wherein the first self-supporting nanoporous catalyst layer (4, 4-1) extends between the first gas-permeable electrode layer (5, 5-1) and the membrane (10), characterized in that the first self-supporting nanoporous catalyst layer (4, 4-1) is formed by a plurality of grains (100) of a first catalyst compound, wherein gaps (101) are formed in between the grains (100) such as to form an increased surface area of the first self-supporting nanoporous catalyst layer (4, 4-1) for enhancing catalytic reactions.

16. The membrane-electrode-assembly (20) according to claim 15, wherein the assembly further comprises a second self-supporting nanoporous catalyst layer (4, 4-2), arranged on a second gas-permeable electrode layer (5, 5-2) on a side of the membrane (10) facing away from the first self-supporting nanoporous catalyst layer (4, 4-1), such that the second self-supporting nanoporous catalyst layer (4, 4-2) extends between the second gas-permeable electrode layer (5, 5-1) and the membrane (10), wherein the second self-supporting nanoporous catalyst layer (4, 4-2) is formed by a plurality of grains comprising a second catalyst compound, wherein gaps are formed in between the grains such as to form an increased surface area of the second self-supporting nanoporous catalyst layer (4, 4-1) for enhancing catalytic reactions.

17. The membrane-electrode-assembly (20) according to claim 15, wherein a median equivalent spherical diameter of the plurality of grains (100) of the first and/or the second self-supporting nanoporous catalyst layer (4-1, 4-2) is in the range of 0.1 m to 1.0 m.

18. The membrane-electrode-assembly (20) according to claim 15, wherein an equivalent spherical diameter of the plurality of grains of the first and/or the second self-supporting nanoporous catalyst layer is in the range of 50 nm to 1.500 nm.

19. The membrane-electrode-assembly (20) according to claim 15, wherein a ratio of a volume comprising the gaps (101) to a volume comprising the grains (100) of the first and/or the second self-supporting nanoporous catalyst layer (4, 4-1, 4-2) is in the range of 0.3 to 5.

20. The membrane-electrode-assembly (20) according to claim 15, wherein the first catalyst compound comprises or is iridium.

21. The membrane-electrode-assembly (20) according to claim 15, wherein the first gas-permeable electrode layer (5-1) comprises or is titanium (5-1).

22. The membrane-electrode-assembly (20) according to claim 16, wherein the second catalyst compound comprises or is platinum

23. The membrane-electrode-assembly (20) according to claim 16, wherein the second gas-permeable electrode layer (5-2) comprises or is carbon (5-1).

24. The membrane-electrode-assembly (20) according to claim 15, wherein the membrane (10) is a proton-permeable membrane configured to retain at least hydrogen, oxygen.

25. A membrane-electrode-assembly (20), wherein the assembly (20) is formed using the method according to any of the claim 1.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0079] Particularly, exemplary embodiments are described below in conjunction with the Figures. The Figures are appended to the claims and are accompanied by text explaining individual features of the shown embodiments and aspects of the present invention. Each individual feature shown in the Figures and/or mentioned in the text of the Figures may be incorporated (also in an isolated fashion) into a claim relating to the method according to the present invention.

[0080] FIG. 1 shows a membrane-electrode-assembly formed by the method according to the invention.

[0081] FIG. 2 shows an embodiment of the method according to the invention, wherein a base metal and a noble metal are deposited on a substrate, forming a heterogeneous layer that may be processed to a catalytically-active electrode.

[0082] FIG. 3 shows an embodiment of the method according to the invention, wherein the base metal is leached out of the heterogeneous layer, forming a catalytically-active electrode.

[0083] FIG. 4 shows an embodiment of the method according to the invention, wherein the base metal is leached out of the heterogeneous layer in a leaching chamber comprising an electrolyte and three leaching electrodes.

[0084] FIG. 5 shows a schematic cross-section through a membrane-electrode-assembly according to the invention.

[0085] FIG. 6 shows SEM-recordings of a membrane-electrode-assembly according to the invention.

[0086] FIG. 7 shows a histogram of a size distribution of the grains on the catalyst layer.

DETAILED DESCRIPTION

[0087] FIG. 1 shows an exploded view of a membrane 10 sandwiched between two self-supporting nanoporous catalyst layers 4 comprising ionomers 40 and two gas-permeable electrodes 50, representing a membrane-electrode-assembly 20 that may be formed using the method according to the invention. The membrane-electrode-assembly 20 may particularly find application in PEM fuel cells and electrolyzers.

[0088] According to the method of the present invention, the self-supporting nanoporous catalyst layer 4 may be formed by in a first step depositing a heterogeneous layer 3 (not shown in FIG. 1) on a substrate 5, wherein the heterogeneous layer 3 preferably comprises a base metal 1 and a noble metal 2.

[0089] The base metal 1 may for example comprise or be at least one of the following: Co, Cu, Fe, Ni, Zn, Al, Mg, Cr, Mo, Gd, Ta, Ti, W, Nb or Mn.

[0090] The noble metal 2 may for example comprise or be at least one of the following: Pt, Ir, IrO2, Ru, RuO2, Au.

[0091] The substrate 5 may comprise or be a gas-permeable electrode 50, for example comprising titanium, tantalum or carbon. The substrate 5 may also comprise or be a transfer substrate 51, for example comprising Kapton, Teflon or carbon foil.

[0092] In a second step, the base metal 1 is preferably leached out of the heterogenous layer, such that a self-supporting nanoporous catalyst layer 4 comprising the noble metal 2 is formed on the substrate 5.

[0093] The leaching may for example be performed chemically or electrochemically, for example within a leaching chamber 30 comprising an electrolyte 35 as further explained in FIG. 4.

[0094] In a third step, at least one kind of proton-conductive ionomers 40 and/or at least one kind of hydrophobic particle 41 and/or an ionic liquid 42 is preferably added to the self-supporting nanoporous catalyst layer 4.

[0095] For example, the at least one kind of hydrophobic particles 41 may comprise or be one of the following: carbon, sulfonated tetrafluoroethylene-based fluoropolymer-copolymer (CAS number: 31175-20-9), titanium oxide or poly(1,1,2,2-tetrafluoroethylene) (CAS number: 9002-84-0).

[0096] According to an embodiment of the present invention, the at least one kind of proton-conductive ionomers 40 and/or at least one kind of hydrophobic particles 41 and/or an ionic liquid 42 may be added for example by spraying, ultrasonic spraying, decal-transferring, immersing, drop casting and/or filtering.

[0097] Finally, in a fourth step of the method according to the invention, a catalytically-active membrane or a membrane-electrode-assembly 20 may be formed by pressing and/or decal transferring the self-supporting nanoporous catalyst layer 4 on the substrate 5 with a membrane 10 or by pressing and/or decal transferring two self-supporting nanoporous catalyst layers 4 on their substrates 5 with the membrane 10, wherein the membrane 10 is preferably arranged between the two self-supporting nanoporous catalyst layers 4.

[0098] According to an embodiment of the present invention, the pressing and/or decal transferring may be carried out at a pressure between 1 bar and 75 bar and/or at a substrate temperature within 115 C. and 145 C., particularly for a time between 15 s and 600 s.

[0099] Optionally, in case a transfer substrate 51 is used, the transfer substrate 51 may be detached from the catalytically-active membrane 10 or the membrane-electrode-assembly 20 after pressing and/or decal transferring.

[0100] FIG. 2 refers to step i) of the method according to the invention, wherein a heterogenous layer is formed on a substrate 5, the heterogeneous layer 3 comprising a base metal 1 and a noble metal 2.

[0101] To this end, the substrate 5 may be placed on a substrate holder 6.

[0102] According to the embodiment of the present invention, the deposition of the base metal 1 and/or the noble metal 2 may be performed by PVD. The PVD may particularly include sputtering, particularly (alternating) magnetron sputtering 7.

[0103] As shown in FIG. 2, the PVD may be directed from a sputtering target 8 comprising a rectangular target area to the substrate 5, particularly with a pressure between 2 Pa and 10 Pa, particularly by using a RF or a DC plasma source.

[0104] To cover large areas of the substrate 5, the substrate 5 may be moved back and forth relative to a resting point, particularly periodically in time, as indicated by an arrow. For example, a substrate 5 with a lateral dimension L, particularly a width or a length, may be moved back and forth by an offset of at least L/4 relative to the resting point.

[0105] Moving the substrate 5 in this manner, particularly in combination with the rectangular target area advantageously minimizes the required target material and is scalable to different sputtering sizes.

[0106] According to an embodiment of the present invention, the deposition of the base metal 1 and the noble metal 2 may be alternated at least three times. That is to say, that the deposition of the base metal 1 and the noble metal 2 is performed one after another in a series of cycles to achieve the desired loading in a specific layer thickness. Preferably, the weight ratio between the deposited base metal 1 and the deposited noble metal 2 may be between 0.5:1 and 20:1.

[0107] These parameters advantageously result in a highly heterogenous layer that may further be processed to a self-supporting nanoporous catalyst layer 4.

[0108] FIG. 3 depicts an embodiment of the method according to the invention, particularly referring to steps i) and ii) of the method according to the invention.

[0109] In this embodiment, the base metal 1 and the noble metal 2 may be deposited on the substrate 5 placed on the substrate holder 6 by magnetron sputtering 7, wherein the substrate 5 placed on the substrate holder 6 is moved relative to the sputtering target 8. The deposition of the base metal 1 and the noble metal 2 may be alternated by moving the substrate 5 placed on the substrate holder 6 relative to the respective sputtering target 8 comprising the base metal 1 and the noble metal 2 and then starting the sputtering process.

[0110] The resulting heterogeneous layer 3 may further be processed to a self-supporting nanoporous catalyst layer 4 by leaching the base metal 1 out of the heterogeneous layer 3, such that a self-supporting nanoporous catalyst layer 4 comprising the noble metal 2 is formed.

[0111] FIG. 4 shows an embodiment of the method according to the invention, referring to the leaching of the base metal 1 out of the heterogeneous layer 3 as proposed by step ii) of the method according to the invention.

[0112] Preferably, the leaching is carried out in an electrolyte 35 arranged between a first leaching electrode 31 and a second leaching electrode 32, wherein the heterogeneous layer 3 and the first leaching electrode 31 are electrically connected and a leaching current is applied between the first and the second leaching electrode 31,32, such that the base metal 1 is leached out of the heterogenous layer 3. The heterogeneous layer 3 on the substrate 5, particularly on the transfer substrate 51 is preferably arranged within the electrolyte 35. An electrical potential within the electrolyte 35 may further be controlled by an optional third leaching electrode 33.

[0113] The electrolyte 35 may comprise an acid, for example at least one of the following: HClO.sub.4, H.sub.2SO.sub.4, HNO.sub.3. The electrolyte 35 may also comprise a base, for example at least one of the following: KOH or NaOH. The concentration of the acid or the base in the electrolyte 35 may for example be within 0.1 mol/l and 8 mol/l.

[0114] For the leaching process, the leaching current is applied preferably between the first leaching electrode 31 and the second leaching electrode. A third leaching electrode 33 may serve as a reference electrode arranged closer to the first leaching electrode 31 than to the second leaching electrode 32. As such, the third leaching electrode 33 may be configured to control the potential of the first electrode 31 and to allow a voltage measurement between the first and the third electrode 31,33. The third leaching electrode 33 may comprise a platinum wire in hydrogen atmosphere. Preferably, for the leaching process, the leaching current between the first and the second leaching electrode 31,32 may be controlled such that a potential difference between the first leaching electrode 31 and the third leaching electrode 33 corresponds to a predetermined voltage, preferably a voltage within 0.2 V and 1.5 V.

[0115] Preferably, the first leaching electrode 31 connected to the heterogeneous layer 3 may be kept at positive potential so as to avoid redepositing of base metal 1 on the resulting self-supporting nanoporous catalyst layer 4. The voltage between the first and the third reference electrode 31,33 and the leaching current between the first and the second electrode 31,32 may be constant or alternating in time, particularly alternating periodically in time.

[0116] To avoid cross-contamination of leached cations, the electrolyte 35 is preferably at least partially exchanged or comprises a flow during step ii).

[0117] According to an embodiment of the present invention, the leaching is performed at a temperature of the electrolyte 35 between 10 C. and 95 C.

[0118] Preferably, as shown in FIG. 4, the first, the second and the third leaching electrode 31,32,33 as well as the electrolyte 35 may be arranged at least partially within a leaching chamber 30.

[0119] The leaching chamber 30 may further comprise an inlet 34 and optionally an outlet (not shown) for conducting and controlling a flow of a non-reactive (inert) gas, particularly N.sub.2 and/or Ar or a reactive gas, particularly H.sub.2 and/or O.sub.2 through the leaching chamber 30, so as to create desired electrochemical conditions in the leaching chamber 30.

[0120] The leaching chamber 30 may be configured such that the third leaching electrode 33, that may serve as a reference electrode, may be at least partially arranged within the electrolyte 35, so as to control the electrical potential within the electrolyte 35. The first and the second leaching electrode 31,32 may be arranged such that the electrolyte 35 is located between the first and the second leaching electrode 31,32. Particularly, the first leaching electrode 31, that is preferably connected to the heterogeneous layer 3, may be arranged on top, particularly floating on top of the electrolyte 35, as shown in FIG. 4. The second leaching electrode 32 may for example be arranged on a bottom of the leaching chamber 30. The inlet 34 and the outlet for the optional flow of the non-reactive or reactive gas may be arranged on top of the electrolyte 35, particularly on top of the first leaching electrode 31, such that the flow is directed over the electrolyte 35 and an upside of the first electrode 31 facing away from the electrolyte 35, so as to establish the desired electrochemical conditions in the leaching chamber 30.

[0121] In FIG. 5 a schematic cross-section through an embodiment of the membrane-electrode-assembly 20 according to the invention is shown. In this example, the membrane-electrode-assembly 20 comprises two nanoporous catalyst layers 4-1, 4-2 that are arranged between the membrane 10 and a respective first or second gas-permeable electrode layer 5-1, 5-2.

[0122] The first nanoporous catalyst layer 4-1 consists of a plurality of grains 100 (indicated by black spaces of the fill pattern of the corresponding box) that form the nanoporous catalyst layer 4-1. In between the grains 100, there are gaps 101 (white spaces of the fill pattern), that allows a comparably large surface area of each grain 100 accessible so that the catalyst compound comprised by the grains 100 is exposed to a great degree to the surroundings of the grains, as for example compared to a continuous or quasi continuous catalyst layer.

[0123] The assembly 20 in essence forms a stack that may be used in electrolyzers or fuel cells.

[0124] In FIG. 6 a scanning electron microscope (SEM) image of one embodiment of the assembly 20 is shown. The image is taken at a cross-section such that the layers of the assembly. Recording parameters are 15 kV acceleration energy for the electrons, at a 15.000 magnification. The recording was obtained at a working distance of 7.8 mm. A pixel size of the pixels in the image corresponds to 7 nm; a scale bar indicative of a length 2 m is overlaid on the image.

[0125] In the upper panel A) the SEM recording is shown, wherein in the lower panel B) the same region is recorded showing a false-coloring indicative of the detected compounds.

[0126] As can be seen the first nanoporous catalyst layer 4-1 is formed by a plurality of grains 100 that are made of iridium in this example. Each grain 100 can be recognized as a separate entity or particle of the catalyst layer 4-1, even though the grains may be partially interconnected. An equivalent sphere diameter can be associated to the grains 100, that allow for a comparison and measure of the differing grain shapes. The gaps 101 extend and at least partially separate the grains 100 from each other allowing a comparably large surface area to be exposed to the surrounding of the grains 100. The gaps may be filled with membrane 10 compound.

[0127] An associated thickness of the catalyst layer is in the range of 2 m, which is comparably smaller than layers known in the prior art [1], [2].

[0128] In FIG. 7 a histogram of the relative occurrences of grain sizes from the determined equivalent spherical diameters of the grains is shown, demonstrating the heterogeneity of the grain size distribution as well as the granular structure of the catalyst layer. A median value of the grain size according to this histogram may be in the range of 450 nm to 750 nm, while a range of the grain diameters may extend approximately in the range of 50 nm to 1.300 nm.

REFERENCES

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