Gas diffusion layer made of water jet entangled nonwovens

Abstract

The present invention relates to a method for producing a gas diffusion layer, wherein nonwovens made of carbon fibers or carbon fiber precursors are subjected to entanglement with water-containing fluid jets of a certain water quality. The invention also relates to the gas diffusion layer obtainable according to the method and to a fuel cell that contains such a gas diffusion layer.

Description

[0001] Gas diffusion layer made of hydroentangled nonwovens

[0002] Description

[0003] The present invention relates to a method for producing a gas diffusion layer, in which nonwovens made of carbon fibers or carbon fiber precursors are subjected to consolidation with water-containing fluid jets of a certain water quality, the gas diffusion layer obtainable by this method and a fuel cell containing such a gas diffusion layer.

[0004] BACKGROUND OF THE INVENTION

[0005] Fuel cells use the chemical reaction of a fuel, particularly hydrogen, with oxygen to produce water to generate electrical energy. In hydrogen-oxygen fuel cells, hydrogen or a hydrogen-containing gas mixture is fed to the anode, where electrochemical oxidation takes place with the release of electrons (H2 2 H ++ 2 e-). The protons are transported from the anode compartment to the cathode compartment via a membrane that separates the reaction chambers from each other in a gas-tight manner and electrically insulates them. The electrons provided at the anode are conducted to the cathode via an external conductor circuit. Oxygen or an oxygen-containing gas mixture is supplied to the cathode, whereby the oxygen is reduced and the electrons are absorbed. The oxygen anions formed react with the protons transported across the membrane to form water (1 / 2 O2 + 2 H + +

[0006] For many applications, especially in automotive powertrains, low-temperature proton exchange membrane fuel cells (PEMFCs, also known as polymer electrolyte membrane fuel cells) are used, whose core is a polymer electrolyte membrane (PEM) that is only suitable for protons (or oxonium ions HsO +) and water and spatially separates the oxidizing agent, generally atmospheric oxygen, from the reducing agent. A catalyst layer is applied to the anode and cathode sides of the gas-tight, electrically insulating, proton-conducting membrane; this layer forms the electrodes and usually contains platinum as the catalytically active metal. The actual redox reactions and charge separation take place in the catalyst layers. The membrane and catalyst layers form a unit, also known as a CCM (catalyst coated membrane). On both sides of the CCM there is a gas diffusion layer (GDL), which stabilizes the cell structure and takes on the transport and distribution functions for reaction gases, water, heat and electricity. The membrane, electrodes and gas diffusion layer form the membrane electrode assembly (MEA).Flow distribution plates (so-called bipolar plates) are arranged between the membrane electrode units, which have channels for supplying the adjacent cathode and anode with process gases and usually also internal cooling channels.

[0007] The gas diffusion layers located between the flow distributor plates and the catalyst layers are crucial for the function and performance of the fuel cell. The process components consumed and generated in the electrode reactions must be transported through the gas diffusion layer and distributed homogeneously from the macroscopic structure of the flow distributor plates / bipolar plates to the microscopic structure of the catalyst layers. The electrons generated and consumed in the half-cell reactions must be conducted to the flow distributor plates with the lowest possible voltage loss. The heat generated during the reaction must be dissipated to the coolant in the flow distributor plates, so the materials of the gas diffusion layers must also have sufficient thermal conductivity.In addition, the GDL must also act as a mechanical balance between the macrostructured flow distribution plate and the catalyst layers.

[0008] Gas diffusion layers for fuel cells typically consist of a carbon fiber substrate, which is usually hydrophobically treated with fluoropolymers (e.g., PTFE) and then coated with a microporous layer (MPL). The MPL typically consists of a fluorine-containing polymer as a binder (e.g., PTFE) and a porous and electrically conductive carbon material (e.g., carbon black or graphite powder). The following three materials are currently used as carbon fiber substrates for GDL:

[0009] Carbon fiber papers (wet-laid and chemically bonded carbon fiber nonwovens with chemical binders that are carbonized), carbon fiber fabrics (e.g. made from yarns of oxidized but not yet carbonized polyacrylonitrile fibers that are carbonized or graphitized after weaving),

[0010] Carbon fiber nonwovens (e.g. dry-laid, carded and hydroentangled nonwovens made of oxidized polyacrylonitrile, which are subsequently thickness calibrated and carbonized).

[0011] It is known that fuel cells can be contaminated by the introduction of foreign ions that are not involved in the electrode processes. For example, the influence of the materials of the bipolar plates and the cations and anions introduced from these into the fuel cell's MEA on cell performance was investigated. Other sources, especially for metallic cations, include emissions from the other cell materials, system components such as the tank, heat exchanger, piping, etc., the air supply stream to the cathode, and contamination of the hydrogen during production or transport. One potential problem with introduced metallic ions is that they can be easily absorbed by the electrolyte membrane. The reason for this is the strong affinity of the metal cations for the sulfonic acid groups of the perfluorinated cation exchange membrane, which is generally greater than the affinity of the protons for the sulfonic acid groups.

[0012] It has now been discovered that GDL can also contribute to the loading of the MEA with foreign ions. Therefore, there is a need for gas diffusion layers that have only very low concentrations of ions, especially metal cations, as well as for processes for their production. The GDL should specifically have low concentrations of cations commonly found in water for technical applications, such as calcium, magnesium, sodium, and potassium ions. The other mechanical properties of the GDL should not be adversely affected.

[0013] To produce carbon fiber nonwovens, nonwovens made of carbon fibers or carbon fiber precursors can be bonded by exposure to water-containing fluid jets. Such spunlace processes for bonding nonwovens using fluid jets and streams, including bonding with superheated steam jets, are known to those skilled in the art. A special method for mechanically bonding nonwovens is hydroentanglement, in which water at an elevated pressure of approximately 20 to over 400 bar is directed through a multitude of nozzles onto the nonwoven to be bonded. The impulse force of the water jets leads to a mechanical anchoring of the fibers in the product. The tool used for this method is so-called jet strips, which can be arranged in one or more rows. Each row has a multitude of nozzles. The maximum number of nozzles can be up to 20.000 nozzles per strip, with typical nozzle diameters ranging from 0.05 to 0.3 mm. WO 2021 / 170608 A1 describes a process for producing spunbonded nonwovens, in which they are subjected to hydroentanglement and subsequent washing. Fresh water can be fed to the hydroentanglement process, and the wastewater from the hydroentanglement process can be fed to the washing process. It is generally mentioned that demineralized water can be used as fresh water.

[0014] US 2003 / 182730 A1 describes a nonwoven fabric with a low content of ionic impurities, achieved by washing with water with a low ion concentration. These nonwoven fabrics are used, among other things, in cleaning wipes and protective clothing for cleanrooms. A use for the production of a gas diffusion layer for a fuel cell is not described.

[0015] WO 0231841 A2 describes a conductive nonwoven fabric obtained from a fiber web of pre-oxidized fibers for carbon fibers by consolidating the fiber web with high-pressure fluid jets at pressures of 100 to 300 bar, compacting the consolidated fiber web and subsequent carbonization and / or graphitization under a protective gas atmosphere at temperatures of 800 °C to 2500 °C.

[0016] DE 10 2006 060 932 A1 describes temperature-stable structures comprising fibers and a coating, with this coating being covalently bonded to the surface of the fibers. Specifically, these are conductive nonwovens that have been subjected to a plasma coating with fluorinated hydrocarbons and are suitable as gas diffusion layers for fuel cells. To produce the conductive nonwoven, carbon fibers or carbon fiber precursors are laid into a fiber web and consolidated by exposure to high-pressure fluid jets. The fibers are then pre-dried, calendered, and carbonized.US 2019 / 0165379 A1 describes a material for a gas diffusion layer based on a carbon fiber nonwoven fabric that has regions with high and low basis weights in the plane, and wherein at least one of the surfaces of the nonwoven fabric has an uneven pattern with indentations and elevations that is independent of the weight distribution of the fibers. The production of the nonwoven fabric involves a waterjet process.

[0017] None of the last four documents contains information on the quality of the water used in waterjet treatment.

[0018] The unpublished international application PCT / EP2021 / 085452 (WO 2022 / 128895 A1 ) describes a high-purity gas diffusion layer and a process for its production, in which nonwovens made of carbon fibers or carbon fiber precursors are subjected to consolidation with water-containing fluid jets, the water having a conductivity of no more than 250 microsiemens / cm at 25 °C. No information is available on the pH value of the water used in the water jet treatment.

[0019] It has now been discovered that high-quality and, in particular, high-purity carbon fiber nonwovens can be produced by subjecting dry-laid nonwovens made of carbon fibers or carbon fiber precursors to consolidation by exposure to water-containing fluid jets. The quality of the water used for consolidation is critical.

[0020] The pH of the water is a critical parameter, for example, to remove unwanted nonwoven contaminants during wet bonding without the aid of a detergent. At the same time, additives added prior to this treatment step should be largely retained. pH optimization also makes it possible to reduce or prevent damage to the nonwoven fabric during water treatment. Another critical parameter is the ion concentration, i.e., the proportion of dissociated substances dissolved in a specific amount of water.

[0021] Surprisingly, a hydroentanglement process is able to produce highly pure nonwovens with a very low ion concentration, which can be further processed into GDL with a similarly very low ion concentration. The resulting nonwovens are advantageously characterized by a very low number of so-called jet strip defects. These can occur when individual nozzles in the jet strip become clogged.

[0022] SUMMARY OF THE INVENTION

[0023] A first subject matter of the invention is a process for producing a gas diffusion layer for a fuel cell, in which a) a fiber composition is provided which comprises carbon fibers and / or precursors of carbon fibers, b) the fiber composition provided in step a) is subjected to a process for producing a fiber web, c) the fiber web is consolidated to form a nonwoven fabric by the action of water-containing fluid jets, wherein the water used has a pH in the range of 5.5 to 8.0, d) optionally the nonwoven fabric obtained in step c) is subjected to a thermal and / or mechanical treatment for drying and / or further consolidation, e) if the fiber composition used in step a) comprises precursors of carbon fibers, the nonwoven fabric is subjected to pyrolysis at a temperature of at least 1000 °C.In a specific embodiment, the water used in step c) has a conductivity of not more than 250 microsiemens / cm at 25 °C.

[0024] In a further specific embodiment, the nonwoven fabric from step c), d) or e) (i.e., depending on which of these steps is carried out, following the last of these steps) is treated with a hydrophobicizing agent (= step f)).

[0025] In a further specific embodiment, the nonwoven fabric from step c), d), e) or f) (i.e., depending on which of these steps are carried out, following the last of these steps) is coated with a microporous layer (= step g)).

[0026] The invention further relates to a fiber web consolidated by exposure to water-containing fluid jets (water-jet consolidated nonwoven fabric) with a very low ion concentration. The invention therefore also relates to a nonwoven fabric obtainable by a process comprising a) providing a fiber composition comprising carbon fibers and / or carbon fiber precursors, b) subjecting the fiber composition provided in step a) to a process for producing a fiber web, c) consolidating the fiber web into a nonwoven fabric by exposure to water-containing fluid jets, wherein the water used has a pH in the range of 5.5 to 8.0.

[0027] With regard to steps a), b), and c), reference is made in full to the following explanations of these steps. The invention further relates to a gas diffusion layer as defined above and below, or obtainable by a process as defined above and below.

[0028] A further subject of the invention is a fuel cell comprising at least one gas diffusion layer as defined above and below, or obtainable by a process as defined above and below.

[0029] DESCRIPTION OF THE INVENTION

[0030] Unless otherwise stated below, the pH values ​​given refer to a temperature of 25°C.

[0031] The pH can be determined using conventional methods known to those skilled in the art. Preferably, the determination is carried out using an electrometric method based on measuring the chain potential of an electrochemical cell, with one of the two half-cells serving as a measuring electrode and the other as a reference electrode. The potential of the measuring electrode is a function of the pH of the measurement solution. Commercially available pH measuring chains based on a pH electrode and a reference electrode, for example, in the form of a combination electrode, can be used to determine the pH. Suitable methods for determining the pH are described in DIN EN ISO 10523-05:2012-04 (Water quality - Determination of pH).

[0032] Measuring devices for pH measurement via proton activity, especially those based on an electrometric method, such as commercially available pH measuring chains, usually have automatic or manual temperature compensation to compensate for the temperature dependence of the ion product of water. The gas diffusion layers obtained using the method according to the invention have the following advantages:

[0033] By optimizing the pH value of the water used for hydroentanglement, unwanted nonwoven contaminants can be removed during wet bonding without the use of a detergent. At the same time, additives added prior to this treatment step, such as stiffening finishes, plasticizers, antistatic agents, hydrophobic agents, antibacterial, antifungal, or fungicidal treatments, flame retardants, and other additives, are largely retained.

[0034] By optimizing the pH value, it is also possible to reduce or prevent damage to the nonwoven fabric caused by water treatment.

[0035] The carbon fiber nonwovens obtained from dry-laid carbon fibers by hydroentanglement and the GDL based on them are characterized by a very low ion concentration.

[0036] The nonwovens obtained from dry-laid carbon fiber precursors by hydroentanglement and subsequent carbonization or graphitization and the GDL based on them are also characterized by a very low ion concentration.

[0037] The nonwovens obtained by hydroentanglement according to the process according to the invention have a very small number of so-called jet stripe defects.

[0038] Compared to the GDLs used in the prior art, the GDLs according to the invention have comparably good mechanical properties.

[0039] Fuel cells based on the GDL according to the invention have a longer service life than fuel cells based on conventional GDL.

[0040] The gas diffusion layer according to the invention and obtainable by the process according to the invention comprises a carbon fiber nonwoven fabric as a flat, electrically conductive material. The carbon fiber nonwoven fabric and the gas diffusion layer are sheet-like structures that have a substantially two-dimensional, planar extension and a comparatively smaller thickness. The gas diffusion layer has a base area that generally essentially corresponds to the base area of ​​the adjacent membrane with the catalyst layers and the base area of ​​the adjacent flow distribution plate of the fuel cell. The shape of the base area of ​​the gas diffusion layer can be, for example, polygonal (n-sided with n > 3, e.g., triangular, square, pentagonal, hexagonal, etc.), circular, circular-segment-shaped (e.g., semicircular), elliptical, or elliptical-segment-shaped. The base area is preferably rectangular or circular.

[0041] Production of the gas diffusion layer

[0042] Step a)

[0043] In step a) of the process according to the invention, a fiber composition is provided which comprises carbon fibers and / or precursors of carbon fibers.

[0044] Preferred carbon fibers consist of at least 90 wt.%, preferably at least 92 wt.%, carbon, based on their total weight. In a specific embodiment, carbon fibers that have been subjected to graphitization can be used. These carbon fibers have a higher carbon content and then consist, in particular, of at least 95 wt.% carbon.

[0045] Suitable precursors for carbon fibers are fibers from synthetic or natural sources that can be converted into carbon fibers through one or more treatment steps (carbonization). These include, for example, fibers made of polyacrylonitrile homopolymers and copolymers (PAN fibers), phenolic resins, polyesters, polyolefins, cellulose, aramids, polyether ketones, polyether ester ketones, polyether sulfones, polyvinyl alcohol, lignin, pitch, and mixtures thereof. The fiber composition provided in step a) preferably comprises PAN fibers as precursor fibers or consists of PAN fibers as precursor fibers. In a first preferred embodiment, the fiber composition provided in step a) comprises PAN fibers and fibers other than PAN, which are preferably selected from fibers made of phenolic resins, polyesters, polyolefins, cellulose, aramids, polyether ketones, polyether ester ketones, polyether sulfones, polyvinyl alcohol, lignin, pitch and mixtures thereof.Such additional polymers are preferably present in the carbon fiber precursor in an amount of up to 50 wt.%, particularly preferably up to 25 wt.%, based on the carbon fiber precursor. In a second preferred embodiment, the fiber composition provided in step a) consists exclusively of PAN fibers.

[0046] Suitable PAN fibers are selected from PAN homopolymers, PAN copolymers, and mixtures thereof. PAN copolymers contain at least one comonomer, which is preferably selected from (meth)acrylamide, alkyl acrylates, hydroxyalkyl acrylates, alkyl ether acrylates, polyether acrylates, alkyl vinyl ethers, vinyl halides, vinyl aromatics, vinyl esters, ethylenically unsaturated dicarboxylic acids, their mono- and diesters, and mixtures thereof.For example, the comonomer is selected from acrylamide, methyl acrylate, methyl methacrylate, ethyl acrylate, n-propyl acrylate, n-butyl acrylate, n-octyl acrylate, lauryl acrylate, stearyl acrylate, 2-ethylhexyl acrylate, benzyl acrylate, 2-hydroxyethyl acrylate, 2-hydroxypropyl acrylate, 4-hydroxybutyl acrylate, 2-methoxyethyl acrylate, 4-methoxybutyl acrylate, diethylene glycol ethyl ether acrylate, 2-butoxyethyl acrylate, ethyl vinyl ether, acrylic acid, methacrylic acid, itaconic acid, itaconic acid monomethyl ester, itaconic acid monolauryl ester, fumaric acid dimethyl ester, styrene, vinyl acetate, vinyl bromide, vinyl chloride, etc. If a polyacrylonitrile copolymer fiber is used as the carbon fiber precursor in step a), the proportion of comonomers is at most 20 wt. %, preferably at most 10 % by weight, based on the total weight of the monomers used for polymerization. Polyacrylonitrile homopolymer fibers are preferably used as the carbon fiber precursor in step a).

[0047] PAN polymers can, for example, be spun from solution into filaments by wet spinning and coagulation, and then combined into tows (fiber bundles). PAN copolymers often have a lower melting point than PAN homopolymers and are therefore suitable for use not only in wet spinning processes, but also in melt spinning processes. The resulting PAN fibers are typically subjected to oxidative cyclization (also referred to as oxidation or stabilization) in an oxygen-containing atmosphere at elevated temperatures of approximately 180 to 300 °C. The resulting chemical crosslinking improves the dimensional stability of the fibers.

[0048] The fibers obtained in the oxidative cyclization can be used as carbon fiber precursors in step a) without further processing. It is also possible to subject the fibers obtained in the oxidative cyclization to at least one processing step, preferably selected from cleaning, coating with at least one sizing agent, drying, and combinations of at least two of these treatment steps. To clean the fibers after the electrochemical oxidation, they can be subjected to a washing process. Washing specifically serves to remove fiber fragments. Washing is generally followed by a drying step. To modify the surface properties, the fibers can be at least partially coated with at least one sizing agent. The sizing agent can be used, for example, in the form of a solution in a suitable solvent or in the form of a dispersion.For the application of the coating, the fibers can be passed through a sizing bath, for example. The sizing can be at least partially removed from the fibers during hydroentanglement in step c). If the water used to consolidate the fiber web in step c) is at least partially recycled, it may be advantageous to subject the hydroentanglement wastewater to a treatment process in which the sizing contained in the wastewater is partially or completely removed.

[0049] After coating the fibers with at least one sizing agent, they are typically subjected to (further) drying. Drying can be carried out using hot air, hot plates, heated rollers, or heat lamps, for example.

[0050] The carbon fiber precursors thus obtained can be used as a fiber composition in step a) of the process according to the invention and further processed. Alternatively, a fiber composition containing PAN fibers or consisting of PAN fibers can be subjected to pyrolysis at a temperature of at least 1000°C, whereby the PAN precursors are converted into carbon fibers. Regarding the pyrolysis conditions, reference is made to the following explanations regarding step e). The carbon fibers thus obtained can also be used as a fiber composition in step a) of the process according to the invention and further processed.

[0051] Step b)

[0052] In step b) of the process according to the invention, the fiber composition provided in step a) is subjected to a process for producing a fiber web (carbon fiber nonwoven or carbon fiber precursor nonwoven). Suitable processes for nonwoven production are known to the person skilled in the art and are described, for example, in H. Fuchs, W. Albrecht, Vliesstoffe, 2nd ed. 2012, p. 121 ff., Wiley-VCH. These include, for example, dry processes, wet processes, extrusion processes, and solvent processes. In a preferred embodiment, the fiber composition provided in step a) is subjected to a dry-laying process in step b) to produce a fiber web. The production of dry-laid nonwovens can in principle be carried out using a carding process or an aerodynamic process.After the carding process, a fiber web is formed using a carding machine or carding machine, whereas in aerodynamic processes, nonwovens are formed from fibers with the aid of air. If desired, the fiber webs can be stacked in several layers to form a nonwoven. The dry-laying process in step b) can include a modification of the properties, e.g., by stretching the nonwoven. This can, for example, lead to web thickness calibration and / or pre-consolidation of the fiber web.

[0053] Step c)

[0054] In step c) of the process according to the invention, the fiber web obtained in step b) is consolidated into a nonwoven fabric by exposure to water-containing fluid jets. Water-containing fluid jets also include fluid streams and steam jets.

[0055] In principle, the known mechanical bonding processes, also known as spunlace processes, are suitable for hydroentanglement. So-called steamjet technology, in which superheated steam jets are used to bond the nonwoven fabric, is also suitable in principle. Such processes are known to those skilled in the art. In a special method for the mechanical bonding of nonwovens, water at an increased pressure of around 20 to 500 bar is passed through a large number of nozzles onto the nonwoven fabric to be bonded. The nozzles are arranged in one or more rows in so-called nozzle strips. These nozzle strips have a large number of nozzles in each row. The maximum number of nozzles can be up to 20,000 nozzles per strip, with typical nozzle diameters being in a range of 0.05 to 0.5 mm. The hole diameters of the nozzles generally have very small tolerances, e.g. less than 2 mm.To achieve defect-free nonwovens, it is necessary that the hole diameters of the nozzles do not change during operation and, in particular, that the nozzles do not close.

[0056] It has been found that the pH of the water used to consolidate the fiber web (nonwoven) is essential for the quality of the gas diffusion layers produced therefrom for use in fuel cells. Therefore, a critical feature of the process according to the invention is that the water used to consolidate the nonwoven in step c) has a pH (relative to 25°C) in the range of 5.5 to 8.0, preferably in the range of 5.5 to 7.0, particularly preferably in the range of 6.0 to 6.9.

[0057] It was further found that the conductivity of the water used to consolidate the nonwoven fabric is also essential for the quality of the gas diffusion layers produced therefrom for use in fuel cells. Therefore, the water used to consolidate the nonwoven fabric in step c) preferably has a conductivity of at most 250 microsiemens / cm (pS / cm) at 25°C. Particularly preferably, the water used in step c) has a conductivity of at most 200 microsiemens / cm at 25°C, in particular of at most 150 microsiemens / cm at 25°C, especially of at most 100 microsiemens / cm at 25°C.

[0058] Electrical conductivity is a sum indicator of the ion concentration, i.e., the proportion of dissociated substances dissolved in a specific amount of water. Conductivity depends, among other things, on the concentration of the dissolved substances, their degree of dissociation, the valence and mobility of the formed cations and anions, as well as the temperature. Conductivity measurement is based on the determination of the ohmic resistance of the water sample being analyzed or the reciprocal of the resistance, the electrical conductance (unit Siemens S = Q). 1 Commercially available conductivity meters (conductometers) can be used to measure conductivity. The measured values ​​are usually given in S / cm (Siemens per centimeter) or, for water samples with a low ion load, in microSiemens per centimeter.

[0059] Process and service water for industrial processes usually comes from the public drinking water network or is pumped from wells, rivers and lakes. Drinking water and process water for processes where water quality is critical is generally checked for its constituents and, if necessary, subjected to water treatment processes. The requirements for water purity vary extremely depending on the respective area of ​​application. Drinking water is supplied as a clear, colourless liquid, free of odours and harmful microorganisms and substances, but enriched with vital minerals and salts. This water is of food quality but is not necessarily suitable for many technical applications. In Germany, drinking water must therefore be of the highest purity according to the Drinking Water Ordinance (TrinkwV 2001, revised version from 10 January 2001).March 2016) have a pH value of 6.5 to 9.0, but it usually lies in a range of 7.0 to 8.5. According to the Drinking Water Ordinance, the limit for conductivity is 2790 microsiemens / cm at 25°C. Tap water supplied by German waterworks has a conductivity of 250 to 1000 microsiemens / cm at 25°C, depending on the hardness. The majority of inorganic cations are Na. + , K + , Ca 2+ and Mg 2+ out of.

[0060] Preferably, the water used in step c) has a nitrate content of at most 200 ppm by weight, particularly preferably at most 25 ppm by weight.

[0061] Preferably, the water used in step c) has a content of K + -ions of at most 200 ppm by weight, particularly preferably at most 10 ppm by weight. The water used in step c) preferably has a content of Mg 2+ - ions of not more than 10 ppm by weight.

[0062] Preferably, the water used in step c) has a content per gallon of at most 200 ppm by weight, particularly preferably at most 40 ppm by weight.

[0063] To provide the water used in step c) according to the invention, available drinking or process water can be subjected to treatment to adjust the pH and / or reduce the ion concentration. These include ion exchange, electrodeionization, membrane processes such as nanofiltration, reverse osmosis, and electrodialysis, and thermal processes such as distillation, flash evaporation, etc.

[0064] In a first preferred embodiment, nanofiltration, reverse osmosis, or a combination of these processes is used to reduce the ion concentration. Both nanofiltration and reverse osmosis rely on the water to be treated being passed through a semipermeable membrane under pressure that exceeds the osmotic pressure, resulting in a permeate with a reduced ion concentration. Nanofiltration occurs at lower pressures than reverse osmosis and thus has a lower purification efficiency than reverse osmosis, but is sufficient in many cases. Pre-purification by nanofiltration followed by a further reduction of the ion concentration by reverse osmosis is also possible.

[0065] In a second preferred embodiment, an ion exchange process is used to reduce the ion concentration. For this purpose, an available drinking or process water (raw water) is generally brought into contact with at least one cation exchange resin and at least one anion exchange resin. In a suitable embodiment, the raw water is first treated with at least one strongly acidic cation exchanger, so that an exchange of cations present in the water for hydrogen ions (H + ). The water thus obtained is then treated with at least one strongly basic anion exchanger to exchange negatively charged ions for hydroxide ions (OH _). If necessary, the water obtained after contact with the cation exchanger can also be contacted with at least one weakly basic anion exchanger before the strongly basic exchanger. If necessary, the water can be subjected to carbon dioxide degassing after the cation exchanger or, if present, between the weakly basic and the strongly basic anion exchanger.

[0066] To adjust the properties of the water used in step c), it is also possible to mix two or more starting waters of different compositions. These differ in at least one property, such as the pH value or the content of a specific ion type. In a special embodiment, a mixture of at least one water with a lower pH value and at least one water with a higher pH value than the target value is used to adjust the pH of the water used in step c). In a very special embodiment, a mixture of at least one water obtained by nanofiltration or reverse osmosis with a lower pH value and at least one water obtained by ion exchange with a higher pH value is used to adjust the pH of the water used in step c).

[0067] The pH value of the water used in step c) depends on the CO2 content and the dissociation of the carbonic acid formed from it. For the process according to the invention, the first dissociation stage of CO2 and water to hydrogen carbonate anions (HCO3) and oxonium ions (H2O) is particularly important. +) is relevant, which takes place in the range between pH 4.3 and pH 8.2. In drinking or process water, the concentration of dissolved carbon dioxide can range from a few milligrams per liter to over 20 mg / l, depending on the source. The concentration of bicarbonate anions in drinking or process water can be several hundred g / l, depending on the source. Due to the absorption of carbon dioxide from the ambient air, pure water usually has a slightly acidic pH, and distilled water can reach acidic pH values ​​of around 5.8. Only freshly produced distilled water has a pH of around 7, but as soon as it comes into contact with carbon dioxide, a slightly acidic pH is established within a few hours.

[0068] The influence of dissolved carbon dioxide on the pH value depends on the process chosen to treat the raw water. For example, bicarbonate anions can be effectively separated from the raw water using processes such as ion exchange, nanofiltration, and reverse osmosis. This is not the case for dissolved CO2, which is essentially not retained by the membranes used for nanofiltration and reverse osmosis, for example. Thus, the permeate from nanofiltration or reverse osmosis generally has an acidic pH due to the dissociation of the dissolved carbon dioxide, which can easily reach values ​​below pH 6. In addition, bicarbonate is formed from the dissolved CO2, thus increasing the conductivity accordingly. In a special embodiment, the raw water can therefore be subjected to carbon dioxide degassing before treatment in nanofiltration or reverse osmosis.

[0069] For carbon dioxide degassing, for example, a CO2 trickle system can be used. The water is trickled in a column, and stripping air is passed in countercurrent, which leads to the removal of CO2 from the water. Alternatively or additionally, a membrane degassing process can be used for carbon dioxide degassing. In a specific embodiment of the process according to the invention, the water used in step c) to consolidate the fiber web is partially or completely recycled. The process according to the invention thus makes it possible to reduce the fresh water requirement and the amount of wastewater to be disposed of for hydroentanglement. This ensures that the water used to treat the fiber web always has a conductivity within the inventive range, and that contamination of the fiber web with components contained in the wastewater from hydroentanglement is otherwise avoided.For this purpose, the wastewater from hydroentanglement can be partially or completely reprocessed and / or replaced.

[0070] The treatment and / or replacement of the hydroentanglement wastewater can be carried out continuously or at intervals.

[0071] A preferred method is one in which a fiber web is consolidated into a nonwoven fabric by the action of water-containing fluid jets, a wastewater stream is discharged from the treatment of the fiber web, a target value for the conductivity of the wastewater stream is determined, the actual value of the conductivity of the wastewater stream is determined, after reaching a limit value for the deviation of the actual value from the target value, the wastewater stream is at least partially subjected to processing and / or an exchange with water of lower ion concentration and the wastewater stream is at least partially returned to the treatment of the fiber web.

[0072] For treatment, the wastewater stream can be subjected to a reduction in ion concentration, as described above. In addition, the wastewater stream can be subjected to further purification, e.g., to remove fibers and fiber fragments.

[0073] Step d) Optionally, the nonwoven fabric obtained in step c) can be subjected to a thermal and / or mechanical treatment for drying and / or further consolidation. Suitable drying methods include convection drying, contact drying, radiation drying, and combinations thereof.

[0074] Preferably, the nonwoven fabric obtained in step c) is subjected to a calendering treatment. Calendering allows further thermal bonding of the nonwoven fabric and, at the same time, thickness calibration. Multiple nonwoven layers can also be bonded together. In a specific embodiment, the nonwoven fabric obtained in step c) contains thermoplastic fibers that serve as binding fibers and are generally carbonizable. In this case, thermal calendering of the nonwoven fabric can be carried out in step d), forming bonding points where fibers are plasticized and fused together (thermobonding).

[0075] Step e)

[0076] If the fiber composition used in step a) comprises carbon fiber precursors, the nonwoven fabric is subjected to pyrolysis at a temperature of at least 1000°C in step e). Depending on the pyrolysis temperature, a distinction is made between carbonization and graphitization. Carbonization refers to a treatment at approximately 1000 to 1500°C under an inert gas atmosphere, which leads to the release of volatile products. Graphitization, i.e., heating to approximately 2000 to 3000°C under an inert gas, produces so-called high-modulus or graphite fibers. The carbon content increases during pyrolysis, for example, from approximately 67 wt.% during treatment at temperatures below 1000°C to approximately 99 wt.% during treatment at temperatures above 2000°C. The fibers obtained by graphitization, in particular, are highly pure, lightweight, high-strength, and highly conductive to electricity and heat. Step f)

[0077] Optionally, the nonwoven fabric can be treated with at least one additive following step c), d), or e). The additives are preferably selected from hydrophobic agents f1), conductivity-enhancing additives f2), further additives f3) different from f1) and f2), and mixtures thereof.

[0078] The nonwoven fabric is preferably coated and / or impregnated (treated) with a hydrophobizing agent f1) that contains at least one fluorine-containing polymer. The fluorine-containing polymer is preferably selected from polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymers (FEP), perfluoroalkoxy polymers (PFA), and mixtures thereof. Perfluoroalkoxy polymers include, for example, copolymers of tetrafluoroethylene (TFE) and perfluoroalkoxy vinyl ethers, such as perfluorovinyl propyl ether. Polytetrafluoroethylene is preferably used as the fluorine-containing polymer.

[0079] The mass fraction of the fluorine-containing polymer f1) is preferably 0.5 to 40%, particularly preferably 1 to 20%, in particular 1 to 10%, based on the mass of the nonwoven fabric. In a specific embodiment, the fluorine-containing polymer is PTFE, and the mass fraction is 0.5 to 40%, preferably 1 to 20%, in particular 1 to 10%, based on the mass of the nonwoven fabric.

[0080] In many cases, the nonwoven fabric already has good electrical and thermal conductivity due to the carbon fibers used, even without conductivity-enhancing additives. To improve the electrical and thermal conductivity, however, the nonwoven fabric can additionally be treated with at least one conductivity-enhancing additive f2). The nonwoven fabric is preferably treated with a conductivity-enhancing additive f2) selected from metal particles, carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof. The conductivity-enhancing additive f2) preferably comprises carbon black or consists of carbon black. The nonwoven fabric can be treated with at least one conductivity-enhancing additive f2), for example, together with the polymer f1) and / or further additives f3). An aqueous dispersion is preferably used to treat the nonwoven fabric.

[0081] The mass fraction of the conductivity-improving additive f2) is preferably 0.5 to 45%, preferably 1 to 25%, based on the mass of the nonwoven fabric. In a specific embodiment, the conductivity-improving additive f2) comprises carbon black or consists of carbon black, and the mass fraction is 0.5 to 45%, preferably 1 to 25%, based on the mass of the nonwoven fabric.

[0082] The nonwovens can additionally be provided with at least one further additive f3). This includes, for example, polymeric binders, surface-active substances, etc. that differ from components f1) and f2). Suitable binders f3) are, for example, furan resins, etc. Specifically, the nonwovens can additionally be provided with at least one polymer that differs from f1), with high-performance polymers preferably being used. The further polymers f3) are preferably selected from polyaryletherketones, polyphenylene sulfides, polysulfones, polyethersulfones, partially aromatic (co)polyamides, polyimides, polyamideimides, polyetherimides and mixtures thereof. The nonwovens can be provided with at least one additive f3), for example, together with the polymer f1) and / or conductivity-improving additives f2). The binders f3) can optionally be subsequently cured. This can, for example,together with the drying and / or sintering following the treatment with the polymers f1 ) or separately.

[0083] Preferably, the total mass fraction of further additives f3) is 0 to

[0084] 80%, preferably 0 to 50%, based on the mass of the nonwoven fabric. If the nonwoven fabrics additionally contain at least one further additive f3), the total mass fraction of further additives f3) is 0.1 to 80%, preferably 0.5 to 50%, based on the mass of the nonwoven fabric.

[0085] The nonwoven fabric preferably has a thickness in the range of 50 to 500 pm, particularly preferably 100 to 400 pm. This thickness refers to the untreated, uncompressed state of the nonwoven fabric, i.e., before the GDL is incorporated into a fuel cell.

[0086] The nonwovens can be coated with components f1), f2), and / or f3) using application methods known to those skilled in the art, such as, in particular, coating and / or impregnation. Preferably, a method selected from the group consisting of padding, doctor blade coating, spraying, splattering, and combinations thereof is used for coating and / or impregnating the nonwovens.

[0087] In the padding process, the nonwoven fabric is passed through a padding tank containing the additive-containing solution or dispersion and then squeezed out to the desired application quantity of additive using a pair of rollers with adjustable pressure and, if necessary, gap.

[0088] In the doctor blade process, a distinction is made between gravure printing and screen printing.

[0089] In gravure printing, for example, a knife-like steel belt with or without a support blade is used as a doctor blade. It serves to wipe off excess additive-containing solution or dispersion from the webs of the printing cylinder (doctoring). In screen printing, however, the doctor blade is usually made of rubber or plastic with a sharp or rounded edge.

[0090] In spray application, the additive-containing solution or dispersion is applied to the nonwoven fabric to be finished using at least one nozzle, especially at least one slotted nozzle. The kiss-roll coating method is preferably used to coat the underside of horizontally running webs. The coating medium can be applied to the web in either the opposite or the same direction. Transfer rollers can be used to achieve indirect coating with small application quantities.

[0091] In a specific embodiment, the nonwoven fabric finished with components f1), f2), and / or f3) in step f) of the process according to the invention is subjected to drying and / or thermal treatment. Suitable processes for drying and / or thermal treatment of nonwoven fabrics coated and / or impregnated with additive-containing solutions or dispersions are known in principle. The drying and / or thermal treatment preferably takes place at a temperature in the range of 20 to 250°C, particularly preferably 40 to 200°C. In addition, drying can take place at a reduced pressure.

[0092] Step g)

[0093] In a preferred embodiment, the gas diffusion layer according to the invention consists of a two-layer composite based on a nonwoven fabric and a microporous layer (MPL) on one of the surfaces of the nonwoven fabric. To produce the gas diffusion layer, the nonwoven fabric obtained in steps c), d), e), or f) can be coated with a microporous layer.

[0094] In contrast to macroporous nonwovens, MPL is microporous, with pore diameters that are generally well below one micrometer, preferably at most 900 nm, more preferably at most 500 nm, and especially at most 300 nm. The average pore diameter of MPL is preferably in the range of 5 to 200 nm, more preferably 10 to 100 nm. The average pore diameter can be determined by mercury porosimetry. MPL contains conductive carbon particles, preferably carbon black or graphite, in a matrix of a polymeric binder. Preferred binders are the aforementioned fluorine-containing polymers, especially polytetrafluoroethylene (PTFE).

[0095] The microporous layer preferably has a thickness in the range of 10 to 100 pm (micrometers), particularly preferably 20 to 50 pm. This thickness refers to the uncompressed state of the microporous layer (B), i.e., before the GDL is incorporated into a fuel cell.

[0096] The gas diffusion layer according to the invention preferably has a thickness (total thickness of nonwoven fabric and MPL) in the range of 80 to 1000 μm, particularly preferably 100 to 500 μm. This thickness refers to the uncompressed state of the GDL, ie, prior to its installation in a fuel cell.

[0097] The invention further relates to a fuel cell comprising at least one gas diffusion layer as defined above or obtainable by a process as defined above. In principle, the gas diffusion layer according to the invention is suitable for all common fuel cell types, especially low-temperature proton exchange membrane fuel cells (PEMFCs). Reference is made in full to the above statements regarding the construction of fuel cells.

[0098] FIGURE DESCRIPTION

[0099] Figure 1 shows the metal contents (Ca 2+ , N / a + , Mg 2+ and K + ) of an untreated nonwoven fabric and a nonwoven fabric consolidated with water of different conductivity.

[0100] Figure 2 shows the Ca content 2+- and nations of a base nonwoven fabric consolidated with water of different conductivity, the carbon fiber nonwoven fabric obtained from it by carbonization and a gas diffusion layer obtained after application of an MPL.

[0101] Figure 3 shows, analogous to Figure 2, the sum of the Ca content 2+ - and Na + - ions.

[0102] The invention is explained using the following non-limiting examples.

[0103] EXAMPLES

[0104] The determination of metal contents (Ca 2+ , N / a + , Mg 2+ and K +The base nonwovens made of oxidized polyacrylonitrile fibers, the resulting carbonized nonwovens, and GDL were analyzed using an ICP-AES (Inductively Coupled Argon Plasma - Atomic Emission Spectrometry) method. Sample pretreatment (digestion) can be performed according to EPA Method 3050A for acid digestion of sediments, sludges, and soils. This process includes the following steps:

[0105] 1 .) Nitric acid digestion at 95°C for 15 minutes.

[0106] 2.) Further addition of nitric acid while continuing the

[0107] Digestion for 1 h.

[0108] 3.) Remove from the hotplate; add deionized water and

[0109] 20% hydrogen peroxide solution.

[0110] 4.) Heat again for about 15 minutes on the hotplate after the

[0111] When bubbling stops completely, remove from the hotplate.

[0112] 5.) Add concentrated hydrochloric acid and digest again for 1 h.

[0113] Water with a conductivity according to Table 1 below was used for hydroentanglement. Reference water 1 corresponds to process water commonly used in conventional waterjet bonding processes. For water batches 2 and 3, the ion concentration was reduced by nanofiltration.

[0114] Table 1

[0115] #) determined according to DIN EN ISO 10523-05:2012-04

[0116] Manufacturing example

[0117] To produce a base nonwoven, a dry-laid fiber web made of 100% oxidized polyacrylonitrile fibers was laid down on a carding machine. The fiber web was fed to a bonding unit, where the fibers were swirled and entangled on both sides using high-energy water jets at pressures of approximately 100 bar in the first stage and approximately 200 bar in a second stage. The water qualities used were as per Table 1. The nonwoven was dried and rolled up, with a basis weight of 150 g / m after water jet entanglement and drying. 2 The nonwoven fabric was then subjected to a thickness calibration, reducing the thickness of the hydroentangled nonwoven fabric to 0.25 mm. The nonwoven fabric was then fed to a carbonization unit, where carbonization took place under a nitrogen atmosphere at approximately 1000 to 1400 °C.

[0118] An impregnation composition containing 70% carbon black and 30% PTFE based on the solids was used to finish the nonwoven fabric. The finishing was carried out by padding with an aqueous dispersion containing 15% finishing weight based on the mass of the GDL substrate (corresponding to 15 g / m 2 ). This was followed by drying at 180 °C and sintering at 400 °C. An MPL paste containing 2.0 wt.% PTFE and 7.8 wt.% carbon in distilled water was then applied to the resulting substrate. The nonwoven fabric was then dried at 160 °C and sintered at 400 °C. The resulting MPL loading was 24 g / m 2 .

Claims

Claims: A method for producing a gas diffusion layer for a fuel cell, comprising: a) providing a fiber composition comprising carbon fibers and / or carbon fiber precursors; b) subjecting the fiber composition provided in step a) to a process for producing a fiber nap; c) consolidating the fiber nap into a nonwoven fabric by the action of water-containing fluid jets, wherein the water used has a pH value in the range of 5.5 to 8.0; d) optionally subjecting the nonwoven fabric obtained in step c) to a thermal and / or mechanical treatment for drying and / or further consolidation; e) if the fiber composition used in step a) comprises carbon fiber precursors, subjecting the nonwoven fabric to pyrolysis at a temperature of at least 1000 °C.The method of claim 1, wherein the water used in step c) to solidify the fiber nap has a pH value in the range of 5.5 to 7.0, particularly preferably in the range of 6.0 to 6.

9. The method of claim 1 or 2, wherein the water used in step c) to solidify the fiber nap has a conductivity of at most 250 microsiemens / cm at 25 °C, preferably a conductivity of . of no more than 200 microsiemens / cm at 25 °C, in particular a conductivity of no more than 150 microsiemens / cm at 25 °C.

4. Method according to one of the preceding claims, wherein the nonwoven fabric obtained in step c), d) or e) is additionally equipped with at least one additive (step f)), preferably selected from hydrophobizing agents f1), conductivity-enhancing additives f2), further additives f3) and mixtures thereof.

5. Method according to any of the preceding claims, wherein the nonwoven fabric obtained in step c), d), e) or f) is additionally coated with a microporous layer (step g)).

6. Method according to any of the preceding claims, wherein the fiber composition provided in step a) comprises precursors of carbon fibers selected from non-oxidized polyacrylonitrile fibers, oxidized polyacrylonitrile fibers and mixtures thereof.

7. A method according to any one of claims 1 to 6, wherein the fiber composition provided in step a) additionally comprises further fibers selected from fibers of phenolic resins, polyesters, polyolefins, cellulose, aramids, polyetherketones, polyetheresterketones, polyethersulfones, polyvinyl alcohol, lignin, pitch and mixtures thereof.

8. Method according to any one of claims 1 to 6, wherein the fiber composition provided in step a) consists of polyacrylonitrile fibers, in particular polyacrylonitrile homopolymer fibers.

9. Method according to any of the preceding claims, wherein the fiber composition provided in step a) is subjected in step b) to a drying process to produce a fiber pile. A method according to any one of the preceding claims, wherein the water used in step c) to consolidate the fiber nap is at least partially recycled. A method according to claim 10, wherein a fiber nap is consolidated into a nonwoven fabric by the action of water-containing fluid jets, a wastewater stream is discharged from the treatment of the fiber nap, a target value for the conductivity of the wastewater stream is defined, the actual value of the conductivity of the wastewater stream is determined, after reaching a limit value for the deviation of the actual value from the target value, the wastewater stream is at least partially reprocessed and / or exchanged with water of lower ion concentration, and the wastewater stream is at least partially returned to the treatment of the fiber nap. A method according to any one of the preceding claims, wherein the nonwoven fabric obtained in step c) is further consolidated by calendering in step d).A method according to any one of claims 4 to 12, wherein the hydrophobizing agent f1) contains at least one fluorine-containing polymer. A method according to any one of claims 4 to 13, wherein the conductivity-enhancing additive f2) is selected from metal particles, carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof. A method according to any one of claims 4 to 14, wherein the further additive f3) is selected from polymeric binders, surfactants, and mixtures thereof, different from components f1) and f2).

16. Method according to any one of claims 4 to 15, wherein the nonwoven fabric is subjected to thermal treatment during or after coating and / or impregnation with the hydrophobizing agent in step f1).

17. Nonwoven fabric obtainable by a process in which a) a fiber composition comprising carbon fibers and / or carbon fiber precursors is provided, b) the fiber composition provided in step a) is subjected to a process for producing a fiber nap, c) the fiber nap is solidified into a nonwoven fabric by the action of water-containing fluid jets, wherein the water used has a pH value in the range of 5.5 to 8.0, preferably a pH value in the range of 5.5 to 7.0, and particularly preferably a pH value in the range of 6.0 to 6.

9.

18. Nonwoven fabric according to claim 17, wherein the water used in step c) has a conductivity of at most 250 microsiemens / cm at 25 °C, preferably a conductivity of at most 200 microsiemens / cm at 25 °C, in particular a conductivity of at most 150 microsiemens / cm at 25 °C.

19. Gas diffusion layer obtainable by a method as defined in any one of claims 1 to 16.

20. Fuel cell comprising at least one gas diffusion layer, obtainable by a method as defined in any one of claims 1 to 16.

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