METHOD OF MANUFACTURING AN ISOLATED POROUS MATERIAL AND AN ISOLATED POROUS MATERIAL

TECHNICAL FIELD

[0001]The present invention relates to a method of manufacturing an isolated porous material and an isolated porous material obtained via said method.

BACKGROUND

[0002]Porous materials, including foams, are core components in next-generation electrochemical systems including electrolyzers, batteries, fuel cells, ion-selective separations and others. They may also be applied in general chemical systems as well as ultra-light-weight manufacturing and high surface area applications such as sensors, filters or heat exchangers. They must fulfil a set of seemingly contradictory requirements including the facilitation of transport of reactants and products, provide active sites for reactions and conduct electrons and heat. Optimizing these coupled phenomena necessitates the development of very controlled electrode geometries and chemical compositions.

[0003]In general, a foam is a dispersion in which a large proportion of gas by volume in the form of gas bubbles, is dispersed in a liquid, solid or gel. Examples of non-electrochemical and electrochemical methods for the preparation of metal foams include selective dissolution, templating, combustion, and the sol-gel method. Dynamic hydrogen bubble templated (DHBT) electrodeposition is a relatively newly developed, yet very promising method of the preparation of metal foams.

[0004]Electrodeposition of metals using DHBT creates characteristic porous materials (foams). They feature a high porosity and a duality in their pore structure where networks of secondary pores are arranged in such a way that they form larger, primary pore structures. The overall morphology is sometimes referred to as honeycomb-like. Furthermore the primary pore structures feature a pore size gradient from the top of the foam to the bottom with large pores at the top and small pores at the bottom of the material. Both of these features have been shown to improve mass transport phenomena in electrochemical devices such as fuel cells.

[0005]The fundamental idea of DHBT is that the generated H₂ bubbles disrupt the growth of the metal layer, acting as a dynamic template for the electrodeposition process. Secondary pores, which are sometimes also called microscopic pores or micropores, in the submicron range (and primary pores, also called macroscopic pores or macropores, in the 5-1000 μm size range are formed as a result of the growth of metal around small or coalesced bubbles generated on the surface, blowing up the specific surface area.

[0006]When applying the DHBT method, high cathodic overpotentials are used, so that certain reaction rates become comparable and decisive for the obtained foam structure. Apart from the reaction rates, however, other factors such as the nucleation, growth and detachment of the surface-generated bubbles, the intensive stirring and the related convective effects caused by bubble formation, the local alkalination of the near-electrode solution layers and its consequences on the chemistry of metal deposition, complex formation, the addition of additives, etc. may also determine the surface morphology of the deposited foam.

[0007]The term DHBT has been used only in recent years. Other terms for this method of manufacturing a porous layer on a substrate include hydrogen evolution assisted (HEA) electroplating, electrochemical deposition mediated by hydrogen bubbles, nanodendritic micro-porous structure, mesoporous foams created using template-assisted electrodeposition, electrodeposition method with in-situ grown dynamic gas bubble templates, and gas bubble dynamic template.

[0008]US20220085390A1 discloses a porous transport layer having a plurality of sintered porous layers with a permeability for gaseous and liquid substances. The multilayer porous transport layer is assembled between a bipolar plate and a catalyst layer of an electrochemical cell.

[0009]EP1991824A1 discloses a method for forming a surface layer on a substrate wherein the surface layer is deposited by a controlled electrodeposition process or a controlled gas phase deposition process.

[0010]State of the art production of this type of material results in the porous material (the foam) being bonded to the substrate (also known as base plate) it its deposited onto. This limitation of the current material prevents a large variety of further treatments and analysis methods to be applied to the porous material and most importantly its use in a vast range of applications is restricted by the presence of the substrate. Examples of such applications include the application as diffusion media, filter material, and catalyst applications. Detachment of the porous material from the substrate is challenging as the structure is quite fragile compared to the strength of attachment to the substate. In prior art, the term “self-standing foam” is often used to refer to a foam that is connected to a substrate. The “self-standing” then refers to the fact that no additional mechanical support is required for the foam to maintain its structure, but the foam is not isolated (separated) from the substrate.

SUMMARY

[0011]It is an object of the present invention to provide an improved porous material that is isolated (“self-standing”).

[0012]It is an object of the present invention to provide a porous material with improved permeability and/or flexibility.

[0013]It is a further object of the present invention to provide a method of manufacturing an isolated porous material. With isolated is meant that it is free-standing, i.e. that is not connected to a substrate.

[0014]It is a further object of the present invention to provide an improved method of manufacturing a porous material.

[0015]It is a further object of the present invention to provide a method of manufacturing a porous material that leads to a porous material with improved permeability and/or flexibility.

[0016]One or more of these objects are achieved by porous material according to a first aspect of the invention. In a first aspect, the invention relates to a porous material comprising a porous wall structure defining and separating primary pores that are interconnected across its thickness dimension. The primary pores have a diameter greater than 5 μm and less than 1000 μm. The diameter of the primary pores gradually increases across its thickness dimension while their number decreases in its thickness dimension. The porous wall structure comprises or consists of secondary pores that are interconnected throughout the material. The secondary pores have a diameter smaller than 5 μm.

[0022]In a third aspect, the invention relates to a porous material obtainable with a method of manufacturing according to the second aspect.

[0023]In a fourth aspect, the invention relates to the use of a porous material according to the first or third aspect in a chemical or electrochemical system. Embodiments of one aspect are applicable correspondingly to each of the other aspects of the present invention.

[0024]Specifically, the inventors have surprisingly found that applying an electrically conductive intermediate layer on at least part of a surface of the substrate allows for the porous material that is deposited to be disconnected from the substrate by removal of the intermediate layer.

DESCRIPTION OF EMBODIMENTS

[0025]As stated above, the present invention relates in a first aspect to a porous material comprising a porous wall structure defining and separating primary pores that are interconnected across its thickness dimension, wherein the primary pores have a diameter greater than 5 μm and less than 1000 μm and wherein the diameter of the primary pores gradually increases across its thickness dimension while their number decreases in its thickness dimension, wherein the porous wall structure comprises or consists of secondary pores that are interconnected throughout the material, wherein the secondary pores have a diameter smaller than 5 μm.

[0026]In an embodiment, the secondary pores have diameter smaller than 1 μm.

[0027]In an embodiment, the primary pores are regularly spaced, sized and shaped.

[0028]In general, the diameter of the secondary pores may be for example one order of magnitude smaller than the diameter of the primary pores.

[0029]The size and shape of the pores may e.g. be influenced by the addition of additives. For example, addition of a small amount of HCl may drastically change the morphology of the resulting porous material, while still maintaining the duality in pore size (primary vs. secondary pores) and a size gradient in the primary pores.

[0030]In an embodiment of the first aspect, the porous material is foldable on itself without breaking, up to a bend radius equal to or below 2 mm.

[0031]In an embodiment of the first aspect, the material is permeable by gases and liquids continuously in all directions.

[0032]In an embodiment of the first aspect, the porous material has a porosity of at least 85%. Porosity for the material could also be at least 90%, or at least 95% and can reach also up to 98%. The material further has a pore size gradient of the primary pores across its thickness dimension. The porosity can be determined via weighting of the resulting material and, based on the material composition, calculating the porosity.

[0033]In an embodiment of the first aspect, the porous material has a surface area of at least 0.1 m²/g. In a specific embodiment, the surface area is between 0.5 m²/g-3.5 m²/g. Surface area can be determined via the BET theory.

[0034]In an embodiment of the first aspect, the porous material has an in-plane resistivity of at most 10×10⁻⁵ Ωm. For example, the in-plane resistivity can be between 0.1×10⁻⁵ and 10×10⁻⁵ Ωm. In a specific embodiment, the in-plane resistivity is at most 6×10⁻⁵ Ωm, such as between 0.5×10⁻⁵ and 6×10⁻⁵ Ωm. The resistivity is determined by measuring the resistance across different lengths of material and using the resulting slope and knowledge of the cross sectional area to exclude effects of contact and lead resistances.

[0035]In an embodiment of the first aspect, the porous material is sintered. Sintering is explained further below.

[0036]In an embodiment of the second aspect of the present invention, dynamic bubble templating is dynamic hydrogen bubble templating.

[0037]In a further embodiment of the second aspect, the electro-deposited surface layer comprises a porous wall structure defining and separating primary pores that are interconnected in the general direction normal to the surface of the intermediate layer, wherein the primary pores have a diameter greater than 5 μm and less than 1000 μm and wherein the diameter of the pores gradually increases with distance from the intermediate layer. In a specific embodiment of this, the primary pores have a diameter greater than 5 μm and less than 100 μm. More specifically, the primary pores may have a diameter greater than 5 μm and less than 50 μm. Of course, a diameter of e.g. 50 μm corresponds to a radius of 100 μm.

[0038]The direction normal to the surface of the intermediate layer corresponds to the thickness dimension of the resulting porous material. It is this same direction wherein the pore diameter of the primary pores gradually increase. At the same time, the number of primary pores decreases in this direction. The porosity of the material may stay the same throughout the thickness dimension, as the increasing pore diameter and the decreasing pore numbers balance each other out. In an embodiment of the second aspect, the intermediate layer is a metal with a lower standard electrode potential than the material of the surface layer. For instance, when the surface material is made is of copper, the metal for the intermediate layer can be Zn, Fe or Ni for example. Alternatives such as conductive polymers or carbon based variations would also be possible. For example, it is possible for the intermediate layer to be carbon particles held together by a polymer sprayed onto the substrate and later removed by an organic solvent.

[0039]In an embodiment of the second aspect, the intermediate layer reduces or eliminates the direct attachment of the porous surface layer to the substrate.

[0040]When the step of removing the intermediate layer takes place during deposition of the porous surface layer (e.g. intermediate layer made of zinc), this removal may be done for instance by the chemical acting as hydrogen source (e.g. sulfuric acid) or the metal salt used to form the foam (e.g. copper sulphate) present in the plating solution or by an additive (e.g. solvent in case the conductive intermediate layer is comprised of carbon particles held in place by a polymer) that is not partaking in the foam formation process itself.

[0041]When the step of removing the intermediate layer takes place after deposition of the porous surface layer, this removal may be done for instance by chemicals not used during the foam formation step such as different acids or bases or by other means such as thermal degradation of the intermediate layer by UV-light irradiation or by selective chemical degradation of the intermediate layer or through the use of solvents selective to the interlayer material. Suitable acids for etching include sulfuric acid and hydrochloric acid. Suitable solvents for dissolving include acetone, benzene, and alcohols.

[0042]After removal of the intermediate layer, any remaining attachment between the foam and the substrate can be removed for instance by slicing with a straight edge blade which can be inserted in the gap between the foam and the substrate created by the intermediate layer, optionally while the material is submerged in a liquid, by etching with an acid, by dissolving in a solvent, by increasing the temperature, by UV-light irradiation or by selective chemical degradation of the intermediate layer. Suitable acids for etching include sulfuric acid and hydrochloric acid. Suitable solvents for dissolving include acetone, benzene, and alcohols. It is noted that the porous material will likely separates already by itself from the substrate and means such as the razor just acts as a transport vehicle.

[0043]In an embodiment of the second aspect, the step of removing the intermediate layer takes place after deposition of the porous surface layer, by slicing with a straight edge blade, optionally while the material is submerged in a liquid, by etching with an acid or by dissolving in a solvent.

[0044]In an embodiment of the second aspect, the deposited material is a metallic material, preferably wherein the material is a metal chosen from the group consisting of Fe, Ni, Co, Cu, Cr, Au, Mg, Mn, Al, Ag, Ti, Pt, Sn, Zn and any alloys thereof, more preferably wherein the material is Cu or Ni.

[0045]In an embodiment of the second aspect, the substrate and the surface layer are comprised of the same or different material. The nature of the substrate material is not critical as long is at is compatible with the application of the intermediate layer, which in turn needs to be compatible with the surface layer. In general the substrate needs to be electrically conductive. Ideally, it is resistant or passive towards the chemicals used in the deposition solution. Examples could include Fe, Ni, Co, Cu, Cr, Au, Mg, Mn, Al, Ag, Ti, Pt, Sn, Zn and any alloys thereof, as well as carbon based materials such as glassy carbon (vitreous carbon).

[0046]In an embodiment of the second aspect, the deposition takes place using a solution comprising copper sulphate, and this solution is mixed with sulfuric acid. In a specific embodiment, the copper sulphate solution has a concentration below 0.2 M and the sulfuric acid concentration is above 0.1 M. In a more specific embodiment, the copper sulphate solution has a concentration below 0.1 M and the sulfuric acid concentration is above 1.5 M.

[0047]Other suitable solutions for deposition include solutions containing metal chlorides or acids that act as hydrogen source.

[0048]In an embodiment, the potential during deposition is at least 4 V. This potential ensures a stable foam. In a specific embodiment, this potential is at least 6 V.

[0049]A person skilled in the art will know the settings and conditions to generate the desired foam morphology. These can include a wide range of electrical currents, potentials, operation modes (constant current, constant potential, pulsed, switching polarity etc.), deposition solution compositions (in terms of metal salts, hydrogen source, supporting salts, further additives that alter the structure in the desired way), deposition solution flow rates, temperatures, pressures, setup orientation and electrode distance and deposition time.

[0050]In an embodiment of the second aspect, the porous surface layer is Cu, and the deposition takes place using a solution of copper sulphate mixed with sulfuric acid, and wherein the intermediate layer is made of zinc and has a thickness between 200 nm and 500 nm.

[0051]In an embodiment of the second aspect, the porosity, binary pore size distribution and the pore-size gradient across the thickness dimension of the porous material are substantially unchanged by removal of the intermediate layer.

[0052]In an embodiment of the second aspect, the method of manufacturing according to the present invention further comprises the step of washing the isolated porous material with a low surface tension liquid. This step may be preceded by a first washing with a non-low surface tension liquid. Washing prevents any residual salt from the deposition solution to crystalize.

[0053]In an embodiment of the second aspect, the method of manufacturing according to the present invention further comprises the step of drying the isolated porous material. This drying takes place after washing.

[0054]The method according to the second aspect of the invention may also include the step of “thickening” or enhancement of the structure by electrochemical or chemical post treatments such as application of an additional metal layer on the entire surface by electrodeposition or coating of the structure in a protective layer or application of a polymer thin film to alter the wettability.

[0055]In an embodiment of the second aspect, the method of manufacturing according to the present invention further comprises the step of sintering the isolated porous material by heat treatment. This may increase a grain size of the nanoparticles and reduce a boundary effect of the nanoparticles.

[0056]Heat treatment in the context of the present invention relates to the sintering of the material by maintaining it at an elevated temperature for a duration of time under the exclusion of oxygen. Heat treatment may result in an increase of mechanical stability, while still maintaining the duality in pore size of the overall material. Without wishing to be bound by theory, the inventors believe that this may due to the merging of fine metal grains in the domains of the secondary pores. The optimal sintering temperatures and duration will depend on the desired characteristics of the resulting material, as well as on the type of metal used. Selection of suitable temperature and duration is within the ability of a person skilled in the art.

[0057]As stated above, the present invention relates in a third aspect to a porous material obtainable with a method of manufacturing according to the second aspect.

[0058]The present invention relates in a fourth aspect to the use of a porous material according to the first or third aspect in a chemical or electrochemical system. In an embodiment of the fourth aspect, the chemical or electrochemical system is selected from electrolyzer, sealed battery, flow battery, fuel cell, and ion-selective separation, a sensor, a filter or a heat exchanger.

[0059]Other variations to the disclosed embodiments can be understood and effected by those skilled in the art in practicing the claimed invention, from a study of the drawings, the disclosure, and the appended claims. In the claims, the word “comprising” does not exclude other elements or steps, and the indefinite article “a” or “an” does not exclude a plurality. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measured cannot be used to advantage.

[0060]The scope of the present invention is defined by the appended claims. One or more of the objects of the invention are achieved by the appended claims.

EXAMPLES

[0061]The present invention is further elucidated based on the Examples below which are illustrative only and not considered limiting to the present invention.

DHBT Deposition and Isolation of Copper Foam

Deposition of the Porous Copper Layer

[0062]In this example, zinc was used as the material of the intermediate layer. The material of the porous surface layer (the foam) was copper.

[0063]The DHBT reactor was prepared and it was ensured that all parts are clean and especially the copper parts are free of oxide buildup. The PTFE guides where placed on the side of the counter electrode. The reactor was assembled by placing the mask on the gasket, followed by a zinc coated substrate with the zinc coated side facing into the reactor. Next, the cathode current collector was placed on top of the substrate, and a PTFE spacer was placed on top of the current collector, after which the PTFE backplate was placed on the spacer. The assembly was then connected to a power source. The reactor was set up in a tilted fashion so the exit tube is higher than the inlet tube, at an angle between 20 to 40 degree, and the zinc coated substrate (cathode) was on the bottom side of the reactor assembly with the counter electrode above it.

[0064]The specific settings depend on the desired material properties and the deposition solution. In the present example, the deposition solution used was a 0.1 M solution of CuSO₄ mixed with a 1.5 M solution of H₂SO₄. The further settings applied were a voltage 6 V, a charge of at least 100 C/cm² and a flow rate of 30 ml/min and a deposition area of 4 cm².

[0065]When the deposition was finished, the plating solution was removed from the system and air was let in.

[0066]The substrate with the porous surface layer (foam) was placed in a solution of 0.1 M sulfuric acid to remove any remaining zinc from the substrate. After this, the substrate with the foam were placed in a water bath to remove any remaining acid.

Isolation of the Porous Material

[0067]The substrate with the porous surface layer were placed in water. A straight edge razor, was used to pass between the foam and the substrate and aid in separating the foam from the substrate. Specifically, removal was started in a corner of the foam, and sawing motions were avoided. The angel of the razor to the surface was around 20 degrees.

Post-Treatment

[0068]After isolation of the foam, the foam was washed using isopropyl alcohol, ensuring displacement of all the water from the foam. After this, drying took place in a vacuum oven at 50° C. for at least 30 minutes.

Heat Treatment

[0069]Sintering was carried out for 10 h at 500 degree Celsius under nitrogen atmosphere. This temperature was reached by incrementally increasing it by 50 K per hour. Afterwards, the samples were removed from the oven and stored in petri dishes. To prevent oxidation of the copper samples during sintering, nitrogen was continuously flowed through the oven tube, and the samples were placed between two glass plates to limit their exposure to oxygen.

[0070]This resulted in an increase of mechanical stability by merging of the fine copper grains in the microporous (secondary porosity) domains while still maintaining the duality in pore size of the overall material.

Method Overview

[0071]Several of the figures provide a graphical overview of the method.

[0072]FIG. 1B is a depiction of the formation processes of the secondary pores through dendrites, the primary pores through gas formation and the overall DHBT foam over time.

[0073]FIG. 1C shows a schematic of the side view of the foam as well as preferential liquid and gas pathways for wetting and non-wetting cases. Wetting refers to the case where the liquid readily wets the surface of the material resulting in a preferential filling of the smaller, secondary pores over the larger primary pores with liquid. The non-wetting refers to the case where the liquid does not readily wet the surface of the material an requires the application of external pressure to invade the porous structure. In this case the liquid will preferentially fill the larger primary pores first before filling the smaller secondary pores.

[0074]FIG. 1D is an illustration of the state-of-the-art (SOTA) vs the synthesis route to generate the isolated DHBT foams.

[0075]FIG. 2 shows a cross-section image of a flow reactor designed for the formation of the foam material.

Results

[0076]The results are visualized in the remaining figures. These figures show that an isolated porous material is obtained.

[0077]FIG. 1A shows a SEM image of the top side of a DHBT foam indicating the primary and secondary pores (PP and SP).

[0078]FIG. 1E is an exemplary illustration of the difference in size of the secondary and primary pores as well as the increase in size of the primary pores across the thickness of the porous material.

[0079]FIG. 3 presents an image from Scanning Electron Microscopy (SEM) of the top side and cross-section of a typical DHBT foam structure.

[0080]FIG. 4 presents SEM images at different magnifications of the top side of a typical DHBT foam structure and a schematic depicting the intended use in two phase transport of liquid and gas with the liquid wicking in the porous wall structure and the large pores remaining open for gas transport.

[0081]FIG. 5A shows the area density of the generated foams and faradaic efficiency of copper plating for different solutions used during the DHBT process as function of applied potential for a charge density of 75 C cm⁻².

[0082]FIG. 5B present SEM images of the top side of the foams after isolation produced at 1 and 6V from different solutions.

[0083]FIG. 5C shows photographs of an isolated foam produced at 6V from Solution B.

[0084]FIG. 5D presents SEM images at different magnifications of the top side of a DHBT foam according to the invention (top row) and the bottom side of a DHBT foam according to the invention (bottom row), which is only attainable since the foam has been successfully isolated and maintains its shape.

[0085]FIG. 6 shows different views of a DHBT foam according to the present invention: a bottom and top view of a DHBT foam (left column) side view (middle column) and zoom in of the side view towards the top and bottom sections of the foam (right column) indicating the difference in wall structure as the metal deposition mechanism change between the formation of the bottom and the top of the structure.

[0086]FIG. 7A shows interferometry and SEM images of the top side and SEM images of the cross-sections of DHBT foams produced at different surface charge densities (Note that the scale bar for the interferometry acts as index for the thickness simultaneously).

[0087]FIG. 7B shows the area density of the foams and faradaic efficiency of copper plating during DHBT vs total charge (top); Thickness and porosity of DHBT foams vs total charge (middle); Primary pore radius and pore density vs total charge (bottom). Dashed regions indicate the standard deviation measured within a single sample while error bars show the standard deviation between the means of multiple samples.

[0088]FIG. 7C shows the pore density vs inverse square of the pore radius for primary pores. Gray shaded regions indicate 2D porosity values.

[0089]FIG. 7D shows the porosity of the foams vs total charge and breakdown into the contributions of the primary and the secondary pores.

[0090]FIG. 8 shows, on the left hand side, an illustration of the X-ray tomography processing steps to determine porosity and pore radii of a foam produced at a total charge of 100 C cm⁻² from solution B, and on the right hand side in the top graph the distribution of porosity for primary and secondary pores as well as overall porosity as function of thickness, and in the bottom graph the radius of primary pores determined from X-ray tomography as function of distance from the bottom of a DHBT foam sample.

[0091]FIG. 9 shows SEM images of a non-sintered (plain) porous material (top row) and of a sintered porous material (bottom row), on the level of the primary pores (left) and secondary pores (middle), as well as a photographic image of the material.

[0092]These results show that one or more objects of the invention are achieved by the method of manufacturing according to the first aspect of the present invention and/or the porous material according to the second aspect of the present invention.