Waterjet entangled nonwoven fabric gas diffusion layer

Abstract

The present invention relates to a method for producing a gas diffusion layer, in which a web made of carbon fibers or a carbon fiber precursor is entangled with a water-containing fluid jet of a specific water quality, a gas diffusion layer obtainable by said method, and a fuel cell equipped with said gas diffusion layer.

Description

[Technical Field]

[0001] The present invention relates to a method for producing a gas diffusion layer, comprising a method for entangling a web made of carbon fibers or carbon fiber precursors with a water-containing fluid jet of a specific water quality, a gas diffusion layer that can be obtained by this method, and a fuel cell equipped with the gas diffusion layer.

[0002] Background technology Fuel cells generate electrical energy by utilizing the chemical reaction between a fuel, particularly hydrogen, and oxygen, to produce water. In a hydrogen-oxygen fuel cell, hydrogen or a hydrogen-containing gas mixture is supplied to the positive electrode, where electrochemical oxidation occurs and electrons are released (H2 → 2H). + +2e - Protons are transported from the positive electrode chamber to the negative electrode chamber via a membrane that electrically insulates and airtightly separates the reaction chambers. Electrons supplied to the positive electrode are supplied to the negative electrode via an external conductive circuit. Oxygen or an oxygen-containing gas mixture is supplied to the negative electrode, and oxygen is reduced by electron acceptance. The oxygen anions produced at this time react with the protons transported through the membrane to produce water (1 / 2O2 + 2H2). + +2e - →H2O).

[0003] Many applications, particularly in automotive powertrains, utilize low-temperature proton exchange membrane fuel cells (PEMFCs, also known as polymer electrolyte membrane fuel cells). At the core of these fuel cells is the polymer electrolyte membrane (PEM), which contains protons (or oxonium ions H3O). +The membrane allows only water and other liquids to pass through, spatially separating the oxidizing agent (generally oxygen from the atmosphere) from the reducing agent. A catalyst layer is provided on the positive and negative sides of the airtight, electrically insulating, and proton-conducting membrane. This catalyst layer forms the electrodes and usually contains platinum as the catalytically active metal. Actual redox reactions and charge separation occur in this catalyst layer. The membrane and catalyst layer form a single unit, also called a CCM (catalyst coated membrane). Gas diffusion layers (GDLs) are present on both sides of the CCM, stabilizing the cell structure and serving the functions of transporting and distributing reaction gases, water, heat, and electricity. The membrane, electrodes, and gas diffusion layers form a membrane electrode assembly (MEA). A bipolar plate is placed between these membrane electrode assemblies, which has channels for supplying process gases to adjacent negative and positive electrodes, and usually also has internal cooling channels.

[0004] The gas diffusion layer (GDL), located between the flow divider and the catalyst layer, is crucial for the function and performance of the fuel cell. It must transport process components consumed and generated in the electrode reaction through the GDL, distributing them homogeneously from the macrostructure of the flow divider / bipolar plate to the microstructure of the catalyst layer. Electrons generated and consumed in the half-cell reaction must be conducted to the flow divider with minimal voltage loss. Furthermore, the GDL material must possess sufficient thermal conductivity to release the heat generated during the reaction to the coolant at the flow divider. In addition, the GDL must function as a mechanical compensator between the macrostructured flow divider and the catalyst layer.

[0005] A gas diffusion layer for fuel cells typically consists of a carbon fiber substrate that has been hydrophobically treated, usually with a fluorine-based polymer (e.g., PTFE), and then coated with a microporous layer (MPL) in a planar manner. The MPL is usually composed of a fluorine-containing polymer (such as PTFE) as a binder and a porous, conductive carbon material (such as carbon black or graphite powder). Currently, the following three materials are used as carbon fiber substrates for GDLs: - Carbon fiber paper (a nonwoven fabric of carbon fibers that has been chemically bonded by wet-laid processing, containing a chemical binder and being carbonized), - Carbon fiber woven fabric (for example, yarn made from polyacrylonitrile fibers that have been oxidized but not yet carbonized, which is then carbonized or graphitized after weaving), - Carbon fiber nonwoven fabric (for example, a nonwoven fabric composed of oxidized polyacrylonitrile, which is dry-laid, carded, entangled with a water jet, and then carbonized after thickness adjustment).

[0006] Fuel cells are known to be contaminated by the introduction of foreign ions not involved in the electrode process. For example, the impact of bipolar plate materials, and the cations and anions introduced from them into the fuel cell's MEA, on battery performance has been investigated. Other sources of metal cations, in particular, include other materials in the cell, system components such as tanks, heat exchangers, and piping, air supply flow to the cathode, and emissions from hydrogen contaminants during manufacturing and transportation. One potential problem with introduced metal ions is that they can be easily absorbed by the electrolyte membrane. This is because metal cations have a strong affinity for the sulfonic acid groups in perfluorocation exchange membranes, which is generally greater than the affinity of protons for sulfonic acid groups.

[0007] It has now been discovered that GDL can also contribute to the loading of foreign ions onto MEA. Therefore, there is a need for a gas diffusion layer containing only very low concentrations of ions, particularly metal cations, and a method for producing such a layer. In particular, it is desirable that the GDL has very low concentrations of cations such as calcium, magnesium, sodium, and potassium ions, which are commonly found in industrial water. In this regard, it is desirable that the other mechanical properties of the GDL are not unfavorably altered.

[0008] For the manufacture of carbon fiber nonwovens, webs made of carbon fibers or carbon fiber precursors can be entangled by the action of a water-containing fluid jet. Such spunlacing methods for web entanglement by fluid jets or fluid flows (including spunlacing by superheated steam jets) are known to those skilled in the art. One specific method for mechanical entanglement of nonwovens is waterjet entanglement, in which high-pressure water exceeding approximately 20-400 bar is directed at the web to be entangled through a number of nozzles. The impact force of the waterjet mechanically fixes the fibers to the product. So-called nozzle strips can be arranged in one or more rows and serve as tools for this method, with a number of nozzles in each row. The maximum number of nozzles can be up to 20,000 per strip, and the typical nozzle diameter is 0.05-0.3 mm.

[0009] International Publication No. 2021 / 170608 describes a method for manufacturing spunbond nonwoven fabrics, in which the spunbond nonwoven fabric is subjected to water jet confluence and then washed in a further sequence. Fresh water can be supplied to the water jet confluence, and wastewater from the water jet confluence can be supplied to the washing process. Generally, completely desalinated water can be used as the fresh water.

[0010] U.S. Patent Application Publication 2003 / 182730 describes a nonwoven fabric with a low ionic impurity content, which is achieved by washing with water with a low ionic concentration. This nonwoven fabric is particularly used for cleaning cloths and protective clothing for cleanrooms. The manufacture of gas diffusion layers for fuel cells is not described.

[0011] International Publication No. 0231841 describes a conductive nonwoven fabric obtained from pre-oxidized carbon fiber fluff by entangling the fiber fluff with a high-pressure fluid jet at a pressure of 100-300 bar, compressing the entangled fiber web, and then carbonizing and / or graphitizing it in a protective gas atmosphere at a temperature of 800°C-2500°C.

[0012] German Patent Application Publication No. 102006060932 describes a temperature-stable structure including fibers and a coating, the coating being covalently bonded to the fiber surface. In particular, this is a conductive nonwoven fabric suitable as a gas diffusion layer for fuel cells, which is coated with a plasma coating of fluorinated hydrocarbons. To manufacture the conductive nonwoven fabric, carbon fibers or carbon fiber precursors are laminated to form fiber fluff, which is then entangled by the action of a high-pressure fluid jet, followed by pre-drying, calendering, and carbonization.

[0013] U.S. Patent Application Publication No. 2019 / 0165379 describes a material for a gas diffusion layer based on a carbon fiber nonwoven fabric, the carbon fiber nonwoven fabric having areas of high and low basis weight in a plane, and at least one surface of the nonwoven fabric having a textured pattern with depressions and depressions independent of the weight distribution of the fibers. The manufacture of the nonwoven fabric includes a waterjet process.

[0014] None of the last four references mentioned contain any information regarding the water quality used in water jet treatment.

[0015] The unpublished international application PCT / EP2021 / 085452 (International Publication No. 2022 / 128895) describes a high-purity gas diffusion layer and a method for producing the same. In this method, a web made of carbon fibers or carbon fiber precursors is entangled with a water-containing fluid jet, and the water has a conductivity of at most 250 microsiemens / cm at 25 °C. There is no description regarding the pH value of the water used for the water jet treatment.

[0016] Now, it has been found that when a dry-laid web made of carbon fibers or carbon fiber precursors is entangled by the action of a water-containing fluid jet, a high-quality and particularly high-purity carbon fiber non-woven fabric can be produced. The quality of the water used for entanglement is extremely important.

[0017] Here, for example, one of the important parameters for removing unnecessary web accompanying substances during wet entanglement without using a detergent is the pH value of the water. At the same time, it is desirable that the additives already added before this treatment step are substantially retained. By optimizing the pH value, it is also possible to reduce or prevent damage to the non-woven fabric caused by water treatment.

[0018] Another important parameter is the ion concentration, that is, the ratio of the dissociated substances dissolved in a certain amount of water. Surprisingly, it is possible to produce a high-purity non-woven fabric with a very low ion concentration by the water jet entanglement method, and this non-woven fabric can be further processed into a GDL with a very low ion concentration as well. Advantageously, the obtained non-woven fabric is characterized by a very small number of so-called nozzle strip defects. Such defects can occur when the individual nozzles of the nozzle strip are blocked.

[0019] Summary of the Invention The first main subject of the present invention is 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 fiber fluff manufacturing process; c) Entangle the fiber fluff by the action of a water-containing fluid jet to form a non-woven fabric. At this time, the water used shall have a pH value in the range of 5.5 to 8.0. d) If necessary, subject the non-woven fabric obtained in step c) to heat treatment and / or mechanical treatment for drying and / or further entanglement. e) A method in which, when the fiber composition used in step a) contains a carbon fiber precursor, the non-woven fabric is subjected to pyrolysis at a temperature of at least 1000 °C.

[0020] In a specific embodiment, the water used in step c) has a conductivity of at most 250 microsiemens / cm at 25 °C.

[0021] In a further specific embodiment, the non-woven fabric obtained in step c), d) or e) is finished with a hydrophobizing agent (i.e., following the last of these steps, depending on which of these steps is carried out) (= step f)).

[0022] In a further specific embodiment, the non-woven fabric obtained in step c), d), e) or f) is coated with a microporous layer (i.e., following the last of these steps, depending on which of these steps is carried out) (= step g)).

[0023] The present invention further relates to fiber fluff with a very low ion concentration (water jet entangled non-woven fabric) entangled by the action of a water-containing fluid jet. Therefore, the subject matter of the present invention also includes the following: a) Provide a fiber composition containing carbon fibers and / or carbon fiber precursors. b) Subject the fiber composition provided in step a) to a fiber fluff manufacturing process. c) Entangle the fiber fluff by the action of a water-containing fluid jet to form a non-woven fabric. At this time, the water used shall have a pH value in the range of 5.5 to 8.0. A non-woven fabric obtainable by the method of.

[0024] For steps a), b), and c), refer entirely to the following descriptions of those steps.

[0025] A further subject of the present invention is a gas diffusion layer defined above and below, or obtainable by the methods defined above and below.

[0026] A further subject of the present invention is a fuel cell comprising at least one gas diffusion layer as defined above and below, or which can be obtained by the methods defined above and below.

[0027] Detailed description of the invention Unless otherwise specified below, the pH values ​​shown are for a temperature of 25°C.

[0028] The pH value can be determined by conventional methods known to those skilled in the art. Preferably, this determination is carried out by an electrometric method based on the measurement of the voltage of a unit of an electrochemical cell, where one of two half-cells is the measuring electrode and the other is the reference electrode. The potential of the measuring electrode depends on the pH value of the solution being measured. A commercially available pH measuring unit based on a pH electrode and a reference electrode can be used to determine the pH value, for example, in the form of a combined measuring unit. A suitable method for determining the pH value is described in DIN EN ISO 10523-C5:2012-04 (Water quality - Measurement of pH value).

[0029] Measuring devices for measuring pH values ​​using proton activity, particularly measuring devices using electrical measurement methods such as commercially available pH measurement units, generally have an automatic or manual temperature compensation unit to compensate for the temperature dependence of water ion products.

[0030] The gas diffusion layer obtained by the method according to the present invention has the following advantages: - By optimizing the pH value of the water used for waterjet contangling, unwanted web-associated material can be removed during wet contangling without the use of detergents. At the same time, additives already added before this process, such as hardeners, softeners, antistatic agents, hydrophobic agents, antibacterial, antifungal or antifungal finishing agents, flame retardants, and other additives, are substantially retained. - By optimizing the pH value, it is also possible to further reduce or prevent damage to the nonwoven fabric caused by water treatment. - Carbon fiber nonwovens obtained from dry-laid carbon fibers by waterjet contanglement, and GDLs based on them, are characterized by their extremely low ion concentration. - Nonwoven fabrics and GDLs based on them, obtained from dry-laid carbon fiber precursors by water-jet confluence and subsequent carbonization or graphitization, are also characterized by very low ion concentrations. - The nonwoven fabric obtained by water jet entanglement using the method of the present invention has a very small number of so-called nozzle strip defects. - Compared to GDLs conventionally used in prior art, the GDL according to the present invention has equivalent and excellent mechanical properties. - The GDL-based fuel cell according to the present invention has a longer service life than conventional GDL-based fuel cells.

[0031] The gas diffusion layer according to the present invention and the method thereof comprises a carbon fiber nonwoven fabric as a planar conductive material. The carbon fiber nonwoven fabric and the gas diffusion layer are planar structures having a substantially two-dimensional and planar extent but with a relatively small thickness. The gas diffusion layer has a base surface, which typically substantially corresponds to the base surface of an adjacent membrane having a catalyst layer and the base surface of an adjacent flow divider plate in a fuel cell. The shape of the base surface of the gas diffusion layer may be, for example, polygonal (n-sided polygon with n≧3, e.g., triangle, quadrilateral, pentagon, hexagon, etc.), circular, circular segmented (e.g., semicircular), elliptical, or elliptical segmented. Preferably, the base surface is rectangular or circular.

[0032] Manufacturing of a gas diffusion layer Process a) In step a) of the method according to the present invention, a fiber composition comprising carbon fibers and / or carbon fiber precursors is provided.

[0033] Preferred carbon fibers consist of at least 90% by weight, preferably at least 92% by weight, of carbon. In certain embodiments, graphitized carbon fibers can be used. These carbon fibers have a higher carbon content, and in particular consist of at least 95% by weight of carbon.

[0034] Suitable precursors for carbon fibers are synthetic or naturally derived fibers that can be converted into carbon fibers by one or more processing steps (carbonization). These include, for example, fibers made from polyacrylonitrile homo- and copolymers (PAN fibers), phenolic resins, polyesters, polyolefins, cellulose, aramids, polyether ketones, polyether ester ketones, polyether sulfones, polyvinyl alcohols, lignin, pitch, and mixtures thereof. Preferably, the fiber composition provided in step a) includes 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) includes PAN fibers and other fibers, which are advantageously selected from fibers made from phenolic resins, polyesters, polyolefins, cellulose, aramids, polyether ketones, polyether ester ketones, polyether sulfones, polyvinyl alcohols, lignin, pitch, and mixtures thereof. Such additional polymers are advantageously included in the carbon fiber precursor in an amount of up to 50% by weight, particularly preferably up to 25% by weight, relative to the carbon fiber precursor. In a second preferred embodiment, the fiber composition provided in step a) consists solely of PAN fibers.

[0035] Suitable PAN fibers are selected from PAN homopolymers, PAN copolymers, and mixtures thereof. A PAN copolymer comprises at least one comonomer introduced by polymerization, which is advantageously selected from (meth)acrylamide, alkyl acrylate, hydroxyalkyl acrylate, alkyl ether acrylate, polyether acrylate, alkyl vinyl ether, vinyl halide, vinyl aromatic compounds, vinyl esters, ethylenically unsaturated dicarboxylic acids, their mono- and diesters, and mixtures thereof. For example, comonomers are 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, monomethyl itaconic acid, monolauryl itaconic acid, dimethyl fumarate, styrene, vinyl acetate, vinyl bromide, vinyl chloride, etc. When polyacrylonitrile copolymer fibers are used as carbon fiber precursors in step a), the proportion of comonomers is up to 20% by weight, preferably up to 10% by weight, relative to the total weight of monomers used in polymerization. Preferably, polyacrylonitrile homopolymer fibers are used as carbon fiber precursors in step a).

[0036] PAN polymers can be spun into filaments, for example, by wet spinning and coagulation as a solution, and then bundled together into string-like materials (fiber bundles). PAN copolymers often have lower melting points than PAN homopolymers, making them suitable for use in both wet spinning and melt spinning. The resulting PAN fibers are typically subjected to oxidative cyclization (also called oxidation or stabilization) in an oxygen-containing atmosphere at a high temperature of approximately 180-300°C. The resulting chemical crosslinking improves the dimensional stability of the fibers.

[0037] The fibers obtained during oxidative cyclization can be used as carbon fiber precursors in step a) without further work-up. The fibers obtained during oxidative cyclization can also be advantageously subjected to at least one work-up step selected from a combination of at least two of these processes: cleaning, coating with at least one sizing agent, drying, and other related processes. To clean the fibers after electrochemical oxidation, they can be subjected to a washing process. This washing is performed specifically to remove fiber fragments. Typically, a drying step follows washing. To modify 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. The fibers can be passed through a sizing bath, for example, for coating. The sizing agent can be at least partially removed from the fibers during waterjet entanglement in step c). If the water used for entanglement of fiber fluff in step c) is at least partially recycled, it may be advantageous to subject the wastewater generated by waterjet entanglement to a work-up to partially or completely remove the sizing agent contained in the wastewater.

[0038] After the fibers have been coated with at least one sizing agent, they are usually subjected to (further) drying. Drying can be carried out using, for example, hot air, a hot plate, a heating roller, or a radiant heater.

[0039] The carbon fiber precursor thus obtained can be used as a fiber composition in step a) of the method according to the present invention and further processed. Alternatively, a fiber composition containing or consisting of PAN fibers can be subjected to thermal decomposition at a temperature of at least 1000°C, in which case the PAN precursor is converted into carbon fibers. For thermal decomposition conditions, refer to the following description regarding step e). The carbon fibers thus obtained can similarly be used as a fiber composition in step a) of the method according to the present invention and further processed.

[0040] Step b) In step b) of the method according to the present invention, the fiber composition provided in step a) is subjected to a fiber fluff (carbon fiber web or carbon fiber precursor web) manufacturing process. Suitable methods for manufacturing nonwoven fabrics are known to those skilled in the art, and are described, for example, in H. Fuchs, W. Albrecht, Vliesstoffe, 2. Aufl. 2012, p. 121 ff., Wiley-VCH. These include, for example, dry, wet, extrusion, and solvent methods. In a preferred embodiment, fiber fluff is produced by subjecting the fiber composition provided in step a) to a dry-laid process in step b). The production of dry-laid nonwoven fabrics can, in principle, be carried out using a carding process or an aerodynamic process. In the carding process, fiber fluff is formed by carding or combing, whereas in the aerodynamic process, web formation is carried out using air from the fibers. If desired, the fiber fluff can be stacked in several layers to form a web. The dry-laid process in step b) may include, for example, modification of properties by stretching the web. This can, for example, calibrate the web thickness and / or pre-entangle the fiber fluff.

[0041] Process c) In step c) of the method according to the present invention, the fiber fluff obtained in step b) is entangled by the action of a water-containing fluid jet to form a nonwoven fabric. Here, the water-containing fluid jet also includes a fluid stream and a steam jet.

[0042] In principle, the mechanical entanglement method, also known as the spunlace method, is suitable for waterjet entanglement for this purpose. In principle, the so-called steamjet technique, which uses a superheated steam jet for web entanglement, is also suitable. Such methods are known to those skilled in the art. In certain mechanical entanglement methods for nonwoven fabrics, high-pressure water of about 20 to 500 bar is directed onto the web to be entangled through a number of nozzles. The nozzles are arranged in one or more rows on so-called nozzle strips. These nozzle strips have a number of nozzles in each row. The maximum number of nozzles can be up to 20,000 per strip, and the typical nozzle diameter is 0.05 to 0.5 mm. The nozzle pore diameter usually has a very small tolerance, for example, less than 2 mm. To obtain a nonwoven fabric without defects, it is necessary that the nozzle pore diameter does not change during operation, and in particular, that the nozzles do not become clogged.

[0043] It has been found that the pH value of the water used to entangle the fiber fluff (web) is important for the quality of the resulting gas diffusion layer for fuel cell applications. Therefore, a key feature of the method according to the present invention is that the water used to entangle the nonwoven fabric in step c) has a pH value in the range of 5.5 to 8.0, preferably 5.5 to 7.0, and particularly preferably 6.0 to 6.9 (at 25°C).

[0044] Furthermore, it has been found that the conductivity of the water used to entangle the fiber fluff (web) is also important for the quality of the resulting gas diffusion layer for fuel cell applications. Therefore, advantageously, the water used to entangle the nonwoven fabric in step c) has a conductivity of up to 250 microsiemens / cm (μS / cm) at 25°C. Particularly preferable, the water used in step c) has a conductivity of up to 200 microsiemens / cm at 25°C, especially up to 150 microsiemens / cm at 25°C, and especially up to 100 microsiemens / cm at 25°C.

[0045] Conductivity is an indicator that shows the ion concentration, that is, the ratio of the dissociative substances dissolved in a certain amount of water. Here, conductivity depends particularly on the concentration of the dissolved substances, their degree of dissociation, the valences and mobilities of the formed cations and anions, as well as the temperature. The measurement of conductivity is based on the determination of the ohmic resistance of the water sample to be analyzed, or the conductivity (unit Siemens S = Ω -1 ) which is the reciprocal of the resistance. For the measurement of conductivity, a commercially available conductivity meter (conductometer) can be used. In this case, the measured values are usually shown in units of S / cm (Siemens per centimeter), and for water samples with low ion load, they are shown in units of microsiemens per centimeter.

[0046] Process water and service water for industrial processes are usually supplied from the public drinking water supply network or pumped from wells, rivers and lakes. Drinking water and process water used in processes where water quality is important usually have their components managed and are subjected to water treatment processes as required. Here, the requirements for the purity of water vary extremely depending on each application field. For example, drinking water is supplied as a colorless and transparent liquid, contains no odor, harmful microorganisms or substances, but is rich in essential minerals and salts. This water has food quality but is not necessarily suitable for many industrial application fields. According to the German Drinking Water Ordinance (TrinkwV 2001, revised version on March 10, 2016), German drinking water must have a pH value in the range of 6.5 to 9.0, usually 7.0 to 8.5. According to the German Drinking Water Ordinance, the limit value of conductivity is 2790 microsiemens / cm at 25°C. The conductivity of tap water supplied by the German water supply system is 250 to 1000 microsiemens / cm at 25°C, depending on the hardness level. The main part of the inorganic cations is occupied by Na + , K + , Ca 2+ and Mg 2+ .

[0047] Preferably, the water used in step c) has a Na + ion content of at most 200 weight ppm, particularly preferably at most 25 weight ppm.

[0048] Preferably, the water used in step c) contains a maximum of 200 ppm by weight of K, and particularly preferably a maximum of 10 ppm by weight of K. + It contains ions.

[0049] Preferably, the water used in step c) contains a maximum of 10 ppm by weight of Mg 2+ It contains ions.

[0050] Preferably, the water used in step c) contains up to 200 ppm by weight of Ca, and particularly preferably up to 40 ppm by weight of Ca. 2+ It contains ions.

[0051] In order to provide the water used in step c) according to the present invention, available drinking water or process water can be subjected to workup for pH adjustment and / or ion concentration reduction. This includes membrane treatments such as ion exchange, electrodeionization, nanofiltration, reverse osmosis, and electrodialysis, and heat treatments such as distillation and flash evaporation.

[0052] In a first preferred embodiment, nanofiltration, reverse osmosis, or a combination of these processes are used to reduce the ion concentration. Both nanofiltration and reverse osmosis are based on the principle that water to be treated passes through a semipermeable membrane under a pressure higher than the osmotic pressure to obtain a permeate with reduced ion concentration. Here, nanofiltration is performed at a lower pressure than reverse osmosis, and therefore its purification capacity is lower than that of a reverse osmosis membrane, but is often sufficient. It is also possible to perform preliminary purification by nanofiltration and then further reduce the ion concentration by reverse osmosis.

[0053] In a second preferred embodiment, an ion exchange process is used to reduce the ion concentration. For this purpose, available drinking water or process water (raw water) is brought into contact with at least one cation exchange resin and at least one anion exchange resin, typically. In a suitable embodiment, the raw water is first treated with at least one strongly acidic cation exchanger to convert the cations present in the water into hydrogen ions (H+ The water obtained in this way is then treated with at least one strong basic anion exchanger to remove negatively charged ions from hydroxide ions (OH). - ) is exchanged with the cation exchanger. If necessary, the water obtained after contact with the cation exchanger may be further contacted with at least one weak basic anion exchanger before the strong basic exchanger. If necessary, water may be subjected to carbon dioxide degassing treatment after the cation exchanger, or between the weak basic anion exchanger and the strong basic anion exchanger, if present.

[0054] To adjust the properties of the water used in step c), it is also possible to mix two or more starting waters with different compositions. In this case, they differ in at least one property, such as pH value or the content of certain ions. In certain embodiments, to adjust the pH value of the water used in step c), a mixture of at least one water with a pH lower than the target value and at least one water with a pH higher is used. In very specific embodiments, to adjust the pH value of the water used in step c), a mixture of at least one low pH water obtained by nanofiltration or reverse osmosis and at least one high pH water obtained by ion exchange is used.

[0055] Regarding the pH value of the water used in step c), the important factors are the CO2 content and the dissociation of carbon dioxide produced from it. Here, the first dissociation step from CO2 and water in the pH range of 4.3 to 8.2 results in the formation of bicarbonate anions (HCO₃⁻). 3- ) and oxonium ions (H3O +The formation of ) is particularly important for the method according to the present invention. In drinking water or process water, the dissolved carbon dioxide concentration ranges from a few milligrams to over 20 mg / l per liter, depending on the source. The concentration of bicarbonate anions in drinking water or process water can be several hundred g / l, depending on the source. Because carbon dioxide is absorbed from the surrounding air, pure water usually has a slightly acidic pH, and distilled water can reach an acidic pH of about 5.8. Only freshly produced distilled water has a pH of about 7, but it becomes slightly acidic within a few hours of contact with carbon dioxide.

[0056] The effect of dissolved carbon dioxide on the pH value depends on the method chosen for the workup of the raw water. Bicarbonate anions can be effectively separated from the raw water by processes such as ion exchange, nanofiltration, and reverse osmosis. This is not the case with dissolved CO2, which is not substantially retained by membranes used in nanofiltration or reverse osmosis, for example. As a result, the permeates from nanofiltration and reverse osmosis membranes generally have an acidic pH value due to the dissociation of dissolved carbon dioxide, which can be well below pH 6. Furthermore, since bicarbonate is subsequently generated from the dissolved CO2, the conductivity increases accordingly. Therefore, in certain embodiments, the raw water can be subjected to carbon dioxide degassing treatment before being treated with nanofiltration or reverse osmosis membranes.

[0057] For carbon dioxide degassing, a CO2 degasser can be used, for example. In this case, water is flowed through a column and strip air is supplied in a countercurrent, thereby removing CO2 from the water. Alternatively or additionally, a membrane degassing process can be used for carbon dioxide degassing.

[0058] In certain embodiments of the method according to the present invention, the water used to entangle the fiber fluff in step c) is partially or completely recycled. Thus, the method according to the present invention makes it possible to reduce the amount of fresh water required for waterjet entanglement and the amount of wastewater to be discarded. In this case, it is ensured that the water used to treat the fiber fluff always has conductivity within the range of the present invention, and that contamination of the fiber fluff by components contained in the waterjet entanglement wastewater is avoided. For this purpose, the waterjet entanglement wastewater can be partially or completely used for workup and / or replaced.

[0059] Workup and / or replacement of wastewater from water jet entanglement can be performed continuously or at intervals.

[0060] A preferred method involves entangling the fiber fluff with a water-containing fluid jet to form a nonwoven fabric, discharging a wastewater stream from the fiber fluff treatment, defining a target value for the conductivity of the wastewater stream, examining the actual conductivity of the wastewater stream, and, after reaching a limit value for the deviation of the actual value from the target value, subjecting at least a portion of the wastewater stream to work-up and / or exchange with water with a lower ion concentration, and returning at least a portion of the wastewater stream to the fiber fluff treatment.

[0061] For work-up purposes, the wastewater stream can be subjected to the ion concentration reduction described above. Furthermore, the wastewater stream can be subjected to further purification to remove, for example, fibers and fiber fragments.

[0062] Step d) If necessary, the nonwoven fabric obtained in step c) may be subjected to thermal and / or mechanical treatment for drying and / or further entanglement. Suitable drying methods include convection drying, contact drying, radiant drying, and combinations thereof.

[0063] Preferably, the nonwoven fabric obtained in step c) is subjected to calendering. Calendering allows for further thermal entanglement of the nonwoven fabric and simultaneously allows for thickness calibration. Multiple web layers can also be bonded together. In certain embodiments, the nonwoven fabric obtained in step c) contains thermoplastic fibers, which function as bonding fibers and are generally carbonizable. In this case, thermal bonding of the nonwoven fabric by calendering can be performed in step d) to form bonding sites, where the fibers are plasticized and fused (thermal bonding).

[0064] Process e) If the fiber composition used in step a) contains carbon fiber precursors, the nonwoven fabric is subjected to thermal decomposition at a temperature of at least 1000°C in step e). Depending on the temperature during thermal decomposition, it is classified into carbonization and graphitization. Carbonization is treatment at approximately 1000-1500°C under an inert gas atmosphere, where volatile products are decomposed. Graphitization, i.e., heating to approximately 2000-3000°C under an inert gas, yields so-called high-elasticity fibers or graphite fibers. The carbon content increases during thermal decomposition, for example, from approximately 67% by weight when treated at temperatures below 1000°C to approximately 99% by weight when treated at temperatures above 2000°C. In particular, the fibers obtained by graphitization are highly pure, lightweight, high-strength, and have excellent electrical and thermal conductivity.

[0065] Process f) Optionally, following step c), d), or e), the nonwoven fabric may be finished with at least one additive. The additive is advantageously selected from hydrophobic agents f1), conductivity-enhancing additives f2), further additives f3) different from f1) and f2), and mixtures thereof.

[0066] Preferably, the nonwoven fabric is coated and / or impregnated (finished) with a hydrophobic agent f1) containing at least one fluorine-containing polymer. Preferably, the fluorine-containing polymer is selected from polytetrafluoroethylene (PTFE), tetrafluoroethylene-hexafluoropropylene copolymer (FEP), perfluoroalkoxy polymer (PFA), and mixtures thereof. Perfluoroalkoxy polymers are, for example, copolymers of tetrafluoroethylene (TFE) and perfluoroalkoxy vinyl ethers such as perfluorovinylpropyl ether. Preferably, polytetrafluoroethylene is used as the fluorine-containing polymer.

[0067] Preferably, the mass percentage of the fluorine-containing polymer f1) is 0.5 to 40%, particularly preferably 1 to 20%, and especially 1 to 10%, relative to the mass of the nonwoven fabric. In certain embodiments, the fluorine-containing polymer is PTFE, and its mass percentage is 0.5 to 40%, preferably 1 to 20%, and especially 1 to 10%, relative to the mass of the nonwoven fabric.

[0068] In many cases, nonwoven fabrics already possess good electrical and thermal conductivity due to the carbon fibers used, even without conductivity-enhancing additives. However, to improve electrical and thermal conductivity, the nonwoven fabric can be further finished with at least one conductivity-enhancing additive f2). Preferably, the nonwoven fabric is finished with a conductivity-enhancing additive f2) selected from metal particles, carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof. Preferably, the conductivity-enhancing additive f2) contains or consists of carbon black. Finishing of the nonwoven fabric with at least one conductivity-enhancing additive f2) can be carried out, for example, with a polymer f1) and / or further additives f3). Preferably, an aqueous dispersion is used for finishing the nonwoven fabric.

[0069] Preferably, the mass percentage of the conductivity-improving additive f2) is 0.5 to 45%, preferably 1 to 25%, relative to the mass of the nonwoven fabric. In certain embodiments, the conductivity-improving additive f2) contains or consists of carbon black, and its mass percentage is 0.5 to 45%, preferably 1 to 25%, relative to the mass of the nonwoven fabric.

[0070] The nonwoven fabric may be further finished with at least one additional additive f3). These include, for example, polymer binders, surfactants, etc., that are different from components f1) and f2). A suitable binder f3) is, for example, a furan resin. In particular, the nonwoven fabric may be further finished with at least one polymer different from f1), preferably a high-performance polymer. The additional polymer f3) is preferably selected from polyaryletherketones, polyphenylene sulfide, polysulfone, polyethersulfone, semi-aromatic (co)polyamide, polyimide, polyamideimide, polyetherimide, and mixtures thereof. The finishing of the nonwoven fabric with at least one additive f3) can be carried out, for example, together with polymer f1) and / or conductivity-enhancing additive f2). The binder f3) can be cured thereafter as needed. This can be carried out, for example, together with drying and / or sintering after the finishing with polymer f1), or separately therefrom.

[0071] Preferably, the total mass percentage of the additional additive f3) is 0 to 80%, preferably 0 to 50%, relative to the mass of the nonwoven fabric. If the nonwoven fabric contains at least one further additional additive f3), the total mass percentage of the additional additive f3) is 0.1 to 80%, preferably 0.5 to 50%, relative to the mass of the nonwoven fabric.

[0072] The nonwoven fabric advantageously has a thickness in the range of 50 to 500 μm, particularly preferably 100 to 400 μm. This thickness refers to the uncompressed state of the nonwoven fabric in its unfinished state, i.e., the state before the GDL is incorporated into the fuel cell.

[0073] The finishing treatment of the nonwoven fabric with components f1), f2), and / or f3) can be carried out using methods known to those skilled in the art, particularly coating and / or impregnation. Advantageously, coating and / or impregnation of the nonwoven fabric can be performed using methods selected from padding, doctor blade processing, spraying, slope padding, and combinations thereof.

[0074] In the padding process, the nonwoven fabric is passed through a padder (immersion tank) with a solution or dispersion containing the additive, and then squeezed with a pair of rollers that allow for pressure and, if necessary, gap adjustment to obtain the desired amount of additive.

[0075] In the doctor blade process, gravure printing and screen printing are distinguished. In gravure printing, the doctor blade is used, for example, a polished, blade-shaped steel plate with or without a backup blade. This is used to scrape off excess solutions or dispersions containing additives that adhere to the web of the impression cylinder (removal by the doctor blade). In screen printing, on the other hand, the doctor blade is usually made of rubber or plastic, and its edges are polished to be sharp or rounded.

[0076] In spray coating, a solution or dispersion containing additives is applied to the nonwoven fabric to be finished using at least one nozzle, particularly at least one slot nozzle.

[0077] The slope padding process (kiss roll) is advantageously used to coat the underside of web material that extends horizontally. The coating medium can be applied in the same direction as or opposite to the web. A transfer roller allows for indirect coating with small amounts of material.

[0078] In certain embodiments, the nonwoven fabric that has been finished with components f1), f2), and / or f3) in step f) of the method according to the present invention is subjected to drying and / or heat treatment. Processes suitable for drying and / or heat treating nonwoven fabrics coated and / or impregnated with a solution or dispersion containing additives are known in principle. Preferably, the drying and / or heat treatment is carried out at a temperature in the range of 20 to 250°C, particularly preferably 40 to 200°C. Furthermore, drying can be carried out under reduced pressure.

[0079] Process g) In a preferred embodiment, the gas diffusion layer according to the present invention consists of a two-layer composite based on a nonwoven fabric and a microporous layer (MPL) provided on one surface of the nonwoven fabric. Accordingly, in the production of the gas diffusion layer, the nonwoven fabric obtained in step c), d), e), or f) can be coated with the microporous layer.

[0080] In contrast to macroporous nonwovens, MPLs are microporous, typically having pore sizes significantly smaller than 1 micrometer, preferably up to 900 nm, particularly preferably up to 500 nm, and especially up to 300 nm. The average pore size of MPLs is advantageously in the range of 5–200 nm, particularly preferably 10–100 nm. The average pore size can be measured by mercury porosimetry. MPLs contain conductive carbon particles, advantageously carbon black or graphite, in a matrix of polymer binders. Preferred binders are the fluorine-containing polymers mentioned earlier, particularly polytetrafluoroethylene (PTFE).

[0081] The microporous layer has a thickness of preferably 10 to 100 μm (micrometers), and particularly preferably 20 to 50 μm. This thickness refers to the incompressible state of the microporous layer B), i.e., the state before the GDL is incorporated into the fuel cell.

[0082] The gas diffusion layer according to the present invention has a thickness (total thickness of the nonwoven fabric and MPL) that is advantageously in the range of 80 to 1000 μm, particularly preferably in the range of 100 to 500 μm. This thickness relates to the uncompressible state of the GDL, i.e., the state of the GDL before it is incorporated into the fuel cell.

[0083] A further subject of the present invention is a fuel cell comprising at least one gas diffusion layer as defined above or obtainable by the method defined above. In principle, the gas diffusion layer according to the present invention is suitable for all types of conventional fuel cells, and is particularly suitable for low-temperature proton exchange membrane fuel cells (PEMFCs). The above embodiments concerning the structure of the fuel cell are referred to in full. [Brief explanation of the drawing]

[0084] [Figure 1] This figure shows the metal content (Ca2+, Na+, Mg2+, and K+) of untreated nonwoven fabrics and nonwoven fabrics contangled with water of various conductivity levels. [Figure 2] This figure shows the Ca2+ and Na+ ion content of a base nonwoven fabric entangled with water of various conductivity levels, a carbon fiber nonwoven fabric obtained by carbonizing the base nonwoven fabric, and a gas diffusion layer obtained by applying MPL. [Figure 3] This figure, similar to Figure 2, shows the total content of Ca2+ and Na+ ions.

[0085] The present invention will be described by the following examples, but these are not intended to be understood as limiting examples.

[0086] Examples Base nonwoven fabric made of oxidized polyacrylonitrile fibers, carbonized nonwoven fabric obtained therefrom, and metal content (Ca 2+ kaNa + Mg 2+ and K +The measurement was performed using the ICP-AES method (Inductively Coupled Argon Plasma - Atomic Emission Spectrometry). Sample pretreatment (for decomposition) can be carried out according to EPA Method 3050A for acid decomposition of sediments, sludge, and soil. This process includes the following steps: 1.) Decomposition with nitric acid is carried out at 95°C for 15 minutes. 2.) Add nitric acid and continue the decomposition for another hour. 3.) Remove from the hot plate and add deionized water and 20% hydrogen peroxide solution. 4.) Heat again on the hot plate for about 15 minutes, and once the bubbling has completely stopped, remove from the hot plate again. 5.) Add concentrated hydrochloric acid and allow to decompose again for 1 hour.

[0087] For water jet confounding, water with the conductivity shown in Table 1 was used. Comparative water 1 corresponds to the process water commonly used in conventional water jet web confounding processes. In water batches 2 and 3, the ion concentration was reduced by nanofiltration.

[0088] [Table 1]

[0089] Manufacturing example To produce the base nonwoven fabric, dry-laid fiber fluff made of 100% oxidized polyacrylonitrile fibers was carded. This fiber fluff was fed into an entanglement unit, where the fibers were spunlaced from both sides using high-energy water jets at pressures of approximately 100 bar in the first stage and approximately 200 bar in the second stage, causing them to entangle with each other. The water quality shown in Table 1 was used. The nonwoven fabric was dried and wound, with a basis weight of 150 g / m² after water jet entanglement and drying. 2Subsequently, the thickness of the water-jet entangled nonwoven fabric was reduced to 0.25 mm by subjecting the nonwoven fabric to thickness calibration. Then, the nonwoven fabric was supplied to a carbonization unit, where it was carbonized at approximately 1000-1400°C under a nitrogen atmosphere.

[0090] To apply a finishing treatment to the nonwoven fabric, an impregnation composition containing 70% carbon black and 30% PTFE on a solid basis was used. The finishing treatment was applied at a concentration of 15% (15 g / m²) relative to the mass of the GDL substrate. 2 The process was carried out by padding impregnation with an aqueous dispersion to a finishing weight equivalent to ( ). The material was then dried at 180°C and sintered at 400°C. To the resulting substrate, an MPL paste containing 2.0 wt% PTFE and 7.8 wt% carbon in distilled water was then applied. The nonwoven fabric was then dried at 160°C and sintered at 400°C. The resulting MPL load was 24 g / m². 2 That was the case.

Claims

1. A method for manufacturing a gas diffusion layer for fuel cells, a) Provide a fiber composition comprising carbon fibers and / or carbon fiber precursors, b) The fiber composition provided in step a) is subjected to a fiber web manufacturing process, wherein the fiber web manufacturing process is a dry-laid process. c) The fiber web is entangled by the action of a water-containing fluid jet to form a nonwoven fabric, and the water used in this process has a pH value in the range of 5.5 to 8.

0. d) Optionally, the nonwoven fabric obtained in step c) may be subjected to a thermal and / or mechanical treatment for drying and / or further entanglement. e) If the fiber composition used in step a) contains a carbon fiber precursor, the nonwoven fabric is subjected to thermal decomposition at a temperature of at least 1000°C. A method for at least partially recycling the water used to entangle the fiber web in step c).

2. The method according to claim 1, wherein the water used to entangle the fiber web in step c) has a pH value in the range of 5.5 to 7.

0.

3. The method according to claim 1 or 2, wherein the water used to entangle the fiber web in step c) has an conductivity of up to 250 microsiemens / cm at 25°C.

4. The method according to claim 1, wherein the nonwoven fabric obtained in step c), d), or e) is further finished with at least one additive selected from a hydrophobic agent f1), a conductivity-enhancing additive f2), further additives f3) and mixtures thereof (step f)).

5. The method according to claim 1, wherein the nonwoven fabric obtained in step c), d), e) or f) is further coated with a microporous layer (step g).

6. The method according to claim 1, wherein the fiber composition provided in step a) comprises a carbon fiber precursor selected from non-oxidized polyacrylonitrile fibers, oxidized polyacrylonitrile fibers, and mixtures thereof.

7. The method according to claim 1, wherein the fiber composition provided in step a) further comprises further fibers selected from fibers made of phenolic resin, polyester, polyolefin, cellulose, aramid, polyether ketone, polyether ester ketone, polyether sulfone, polyvinyl alcohol, lignin, pitch, and mixtures thereof.

8. The method according to claim 1, wherein the fiber composition provided in step a) consists of polyacrylonitrile fibers (for example, polyacrylonitrile homopolymer fibers).

9. The method according to claim 1, comprising: entangling a fiber web with the action of a water-containing fluid jet to form a nonwoven fabric; discharging a wastewater stream from the processing of the fiber web; defining a target value for the conductivity of the wastewater stream; examining the actual value of the conductivity of the wastewater stream; after reaching a limit value for the deviation of the actual value from the target value, subjecting the wastewater stream at least partially to workup and / or exchange with water with a low ion concentration; and returning the wastewater stream at least partially to the processing of the fiber web.

10. The method according to claim 1, wherein the nonwoven fabric obtained in step c) is subjected to further entanglement by calendering in step d).

11. The method according to claim 4, wherein the hydrophobic agent f1) comprises at least one fluorine-containing polymer.

12. The method according to claim 4, wherein the conductivity-improving additive f2) is selected from metal particles, carbon black, graphite, graphene, carbon nanotubes (CNTs), carbon nanofibers, and mixtures thereof.

13. The method according to claim 4, wherein the further additive f3) is selected from polymer binders, surfactants, and mixtures thereof that differ from the components f1) and f2).

14. The method according to claim 4, wherein the nonwoven fabric is subjected to heat treatment during or after coating and / or impregnation with the hydrophobic agent in step f).

15. a) A fiber composition comprising carbon fibers and / or carbon fiber precursors, b) The fiber composition provided in step a) is subjected to a fiber web manufacturing process, wherein the fiber web manufacturing process is a dry-laid process. c) The fiber web is entangled by the action of a water-containing fluid jet to form a nonwoven fabric, and the water used in this process has a pH value in the range of 5.5 to 8.

0. d) Optionally, the nonwoven fabric obtained in step c) may be subjected to a thermal and / or mechanical treatment for drying and / or further entanglement. e) If the fiber composition used in step a) contains a carbon fiber precursor, the nonwoven fabric is subjected to thermal decomposition at a temperature of at least 1000°C. A method for manufacturing a fuel cell, comprising incorporating the gas diffusion layer obtained in step c), d), or e) into the fuel cell, A method for manufacturing a fuel cell, comprising at least partially recycling the water used to entangle the fiber web in step c).

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