Separator for alkaline water electrolysis

Abstract

A separator (1) for alkaline electrolysis comprising a porous support (10), a first porous layer (20b) provided on one side of the porous support and a second porous layer (30b) provided on the other side of the porous support wherein the first and second porous layers are partially impregnated into the porous support and each have a cover layer thickness d1 and d2, respectively, the cover layer thickness is defined as the portion of each porous layer not impregnated into the porous support, characterized in that a) d1 is less than the cover layer thickness (d2) of the second porous layer, and b) d1 is at least 20 [mu] m.

Description

Technical Field

[0001] The present invention relates to a method for producing a separator for alkaline water electrolysis and to a separator obtained by the method. Background Art

[0002] Hydrogen is used today in several industrial processes, for example in the chemical industry as a raw material and in the metallurgical industry as a reducing agent. Hydrogen is a basic building block for the manufacture of ammonia (and therefore fertilizers) and methanol (for the manufacture of many polymers). Oil refineries, where hydrogen is used to process intermediate oil products, are another area of ​​use.

[0003] Hydrogen is also considered an important future energy carrier, which means that it can store and deliver energy in a usable form. The energy is released through an exothermic combustion reaction with oxygen, thereby forming water. No greenhouse gases containing carbon are emitted during such a combustion reaction.

[0004] To realize a low-carbon society, renewable energy using natural energy such as sunlight and wind power is becoming increasingly important.

[0005] The production of electricity from wind and solar power systems is very dependent on weather conditions. Because weather conditions are constantly changing, this leads to an imbalance in electricity demand and supply. In order to store surplus electricity, so-called power-to-gas technologies, in which electrical energy is used to produce gaseous fuels such as hydrogen, have attracted a lot of interest in recent years. As electricity production from renewable energy sources increases, the need for storage and transportation of the produced energy will also increase.

[0006] Alkaline water electrolysis is an important manufacturing method in which electricity can be converted into hydrogen.

[0007] In alkaline water electrolysis cells, so-called separators or diaphragms are used to separate electrodes of different polarity, to prevent short circuits between these electron-conducting parts (electrodes), and to prevent recombination of hydrogen (formed at the cathode) and oxygen (formed at the anode) by avoiding gas crossover. While providing all these functions, the separator should also be a high ionic conductor for transporting hydroxide ions from the cathode to the anode.

[0008] The separator generally comprises a porous support. As disclosed in EP-A 232923 (Hydrogen Systems), such a porous support strengthens the separator, facilitates handling of the separator and introduction of the separator into the electrolyser.

[0009] EP-A 3312306 (Kawasaki / De Nora / ThyssenKrupp) discloses a membrane having a porous support and a polymer porous membrane impregnated in the porous support. Applying a membrane-forming solution to one surface of the support may result in a double-coated membrane having porous layers of significantly different thicknesses on both surfaces of the support.

[0010] EP-A 1776490 (VITO) discloses a method for preparing a reinforced separator. The method produces a membrane with symmetrical properties. The method comprises the steps of providing a porous support in the form of a web and a suitable dope solution, directing the web in a vertical position, coating both sides of the web equally with the dope solution to produce a web-coated support, and applying a symmetrical surface pore forming step and a symmetrical solidification step to the dope-coated web to produce a reinforced membrane.

[0011] WO 2009 / 147084 and WO 2009 / 147086 (Agfa Gevaert and VITO) disclose manufacturing methods for producing reinforced membranes with symmetrical properties as described in EP-A 1 776 490. The porous supports used in these manufacturing methods have a thickness of more than 190 μm.

[0012] An advantage of membranes with symmetrical properties is increased robustness, since defects on one side of the support are not detrimental to the overall performance of the membrane.

[0013] EP-A 3933069 (Agfa-Gevaert) discloses a reinforced separator with symmetrical properties, comprising a porous support with a thickness of 150 μm or less and a membrane thickness of less than 250 μm. The membrane has improved ionic conductivity and adequate mechanical properties. By adjusting the viscosity of the coating solution, a membrane with adequate flatness and less waviness is obtained.

[0014] However, it has been observed that during the manufacturing process of double-sided reinforced separators, the waviness / flatness of the support body may still be problematic, resulting in separators having waviness. Such waviness may lead to performance issues for the electrolyser.

[0015] Therefore, there remains a need for membranes with minimal corrugation, high ionic conductivity and adequate gas separation properties. SUMMARY OF THE INVENTION

[0017] An object of the present invention is to provide a separator with minimal waviness, high ionic conductivity and good gas separation properties. Surprisingly, it was found that when the separator has different cover layer thicknesses d1 and d2 as defined in claim 1, it has minimal waviness, high ionic conductivity and sufficient gas separation properties.

[0018] Another object of the present invention is to provide a method for preparing such a separator.

[0019] Other objects of the present invention will become apparent from the following description. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] Figure 1 An embodiment of a separator according to the invention is shown schematically.

[0022] Figure 2 An embodiment of a method for producing a separator according to the present invention is schematically shown.

[0023] Figure 3 Another embodiment of a method for producing a separator according to the present invention is schematically shown.

[0024] Figure 4 is a microscopic image of a cross section of a separator according to the invention illustrating the cover layer thicknesses d1 and d2 by showing several distances for d1 and d2 represented by d1(1), d1(2), d1(3), d2(1), d2(2), d2(3). Multiple lines are plotted to enable calculation of an average value.

[0025] Figure 5 is an image of a spacer showing waviness.

[0026] Implementation plan description

[0027] Separators for alkaline electrolysis

[0028] The separator (1) for alkaline water electrolysis according to the present invention comprises a porous support (10) and a first porous layer (20b) and a second porous layer (30b) provided on one side and the other side of the porous support, respectively, characterized in that:

[0029] a) the cover thickness (d1) of the first porous layer is less than the cover thickness (d2) of the second porous layer, and

[0030] b) The covering layer thickness (d1) of the first porous layer is at least 20 μm, preferably at least 30 μm, more preferably at least 35 μm, most preferably at least 40 μm.

[0031] The covering layer of the porous layer is defined as the portion of the porous layer that is not immersed in the porous support (see Figure 1 ). The cover thickness of the porous layer can be determined microscopically as described further.

[0032] Surprisingly, it was found that when the cover layer thicknesses d1 and d2 are different from each other, the waviness can be reduced. The cover layer thickness d1 of the first porous layer is at least 20 μm. When d1 is less than 20 μm, fibers from the porous support may protrude from the cover layer of the porous layer, resulting in a too high gas permeability. Too high a gas permeability can lead to too high a flow rate of electrolyte, which can bring dissolved gases to the wrong side of the electrolyzer, resulting in an increase in HTO (volume % hydrogen present in the oxygen formed at the anode).

[0033] Preferably, the ratio d1 / d2 of the cover layer thickness is less than 0.8, more preferably less than 0.6, most preferably less than 0.4. If d1 / d2 is greater than 0.8, the separator may have waviness.

[0034] The total thickness (D) of the separator is preferably 250 μm or less, more preferably 225 μm or less, most preferably 175 μm or less, and particularly preferably 150 μm or less. If the thickness of the separator is less than 100 μm, its physical strength may become insufficient, and when the thickness is greater than 250 μm, the electrolysis efficiency may decrease.

[0035] The gas permeability of the membrane is preferably between 1L / min.cm 2 and 7L / min.cm 2 between, more preferably between 1.5L / min.cm 2 and 6.5L / min.cm 2 Between, most preferably between 2L / min.cm 2 and 5.5L / min.cm 2 Porolux can be used at 5 bar TM 1000 device measures gas permeability.

[0036] In a 30 wt% KOH aqueous solution at 80°C, the separator preferably has a relative humidity of less than 0.1 ohm.cm 2 , more preferably less than 0.07ohm.cm 2 The ionic resistance (also called area specific resistance) can be measured using a conductivity cell available from VWR (Avantor) equipped with a TetraCon 925 conductivity cell available from Xylem. The ionic resistance was determined using a Multi 9310IDS instrument.

[0037] As described in more detail below, the separator according to the invention is preferably prepared by applying a coating solution on both surfaces of the porous support. The coating solution, also referred to as a coating solution, generally comprises a polymer resin, hydrophilic inorganic particles and a solvent. A porous layer is then obtained after a phase inversion step, wherein the polymer resin forms a three-dimensional porous polymer network.

[0038] A separator according to the invention having different covering layer thicknesses on both sides of the porous support is preferably obtained by adjusting the flow rate at the slot coating die, as will be described in more detail further on.

[0039] When the coating solution is applied on both surfaces of the porous support, the coating solution impregnates the porous support. The porous support is preferably completely impregnated with the coating solution.

[0040] The first porous layer and the second porous layer provided on the porous support may have the same or different compositions, and may have the same or different bubble points.

[0041] After the phase inversion, the impregnation of the porous support ensures that the three-dimensional porous polymer network also extends into the porous support. This results in good adhesion between the porous layer and the porous support.

[0042] A preferred spacer (1) is schematically shown in Figure 1 middle.

[0043] On both sides of the porous support (10) a coating solution has been applied and the porous support is completely impregnated with the applied coating solution. The applied coating layers are referred to as 20a and 30a.

[0044] After the phase inversion step (50), a separator (1) is obtained, comprising a porous support (10) and a porous layer (20b, 30b) on each side of the support.

[0045] The separator comprises pores with a pore size small enough to prevent gas crossover. On the other hand, to ensure efficient transport of hydroxide ions from the cathode to the anode, larger pore sizes are preferred. Efficient transport of hydroxide ions requires efficient penetration of the electrolyte into the separator.

[0046] The pores are preferably characterized using the bubble point test method described in the American Society for Testing and Materials Standard (ASMT) method F316. This technique is based on displacing the wetting liquid embedded in the spacer by applying an inert pressurized gas. Only through-pores are measured in this way.

[0047] The most challenging part for the gas to dislodge the liquid along the entire pore path is the most constricted part of the pore, also known as the pore throat. The diameter of the pore measured by the bubble point test method is the diameter of the pore throat, regardless of the location of the pore throat in the pore path.

[0048] The pore sizes mentioned herein are measured using the bubble point test method described above.

[0049] The maximum pore diameter (PDmax) of the separator is preferably between 0.05 μm and 2 μm, more preferably between 0.10 μm and 1 μm, and most preferably between 0.2 μm and 0.6 μm.

[0050] The average pore size of the separator is preferably between 0.01 μm and 1 μm, more preferably between 0.02 μm and 0.5 μm, most preferably between 0.05 μm and 0.25 μm.

[0051] The bubble point test method can be adapted to measure the maximum pore diameter (PDmax) on both sides of the spacer by using a grid supporting one side of the spacer during the measurement.Another measurement is then made using a grid supporting the other side of the spacer.

[0052] The two sides of the separator can have the same or different maximum pore sizes.

[0053] The preferred insulator has a maximum pore size PDmax(1) on one side and a maximum pore size PDmax(2) on the other side, wherein PDmax(1) and PDmax(2) are both 0.05μm to 2μm, more preferably 0.10μm to 1μm, most preferably 0.15μm to 0.5μm, and wherein the ratio PDmax(1) / PDmax(2) is 0.9 to 1.1, more preferably 0.95 to 1.05.

[0054] However, the spacer may also have maximum pore diameters PDmax(1) and PDmax(2) that are very different from each other, for example to avoid that gas bubbles are trapped in the spacer. For example, the spacer may have a maximum pore diameter PDmax(1) of 0.05 μm to 0.3 μm, more preferably 0.08 μm to 0.25 μm, most preferably 0.1 μm to 0.2 μm on one side, and a maximum pore diameter PDmax(2) of 0.2 μm to 6.5 μm, more preferably 0.2 μm to 1.50 μm, most preferably 0.2 μm to 0.5 μm on the other side.

[0055] The ratio between PDmax(2) and PDmax(1) is preferably 1.1 to 20, more preferably 1.25 to 10, and most preferably 2 to 7.5. Smaller PDmax(1) ensures efficient separation of hydrogen and oxygen, while PDmax(2) ensures good penetration of electrolyte in the separator, resulting in sufficient ionic conductivity.

[0056] The porosity of the separator is preferably between 30% and 80%, more preferably between 50% and 70%. A separator having a porosity within the above range generally has excellent ion permeability and excellent gas barrier properties because the pores of the separator are continuously filled with the electrolyte solution.

[0057] Porous support

[0058] The porous support is used to reinforce the separator to ensure its mechanical strength.

[0059] It has been observed that as the thickness of the porous support decreases, the ionic conductivity through the reinforced separator increases. Therefore, the thickness of the porous support is preferably 150 μm or less, more preferably 125 μm or less, most preferably 100 μm or less, particularly preferably 75 μm or less.

[0060] However, in order to ensure sufficient mechanical properties of the reinforced separator, the thickness of the porous support is preferably 20 μm or more, more preferably 40 μm or more, most preferably 50 μm or more.

[0061] The porous support may be selected from porous fabrics and porous ceramic plates.

[0062] The porous support is preferably a nonwoven fabric, a woven fabric, a net or a felt, more preferably a nonwoven fabric or a woven fabric.

[0063] The porous support is preferably a porous fabric, more preferably a porous polymer fabric.

[0064] Porous polymer fabrics can be woven or nonwoven. Woven fabrics generally have better dimensional stability and uniformity of open area and thickness. However, the manufacture of woven fabrics with a thickness of 100 μm or less is more complicated, resulting in more expensive fabrics. The manufacture of nonwoven fabrics is less complicated, even for fabrics with a thickness of 100 μm or less. In addition, nonwoven fabrics can have a larger open area.

[0065] The open area of ​​the porous support is preferably between 30% and 80%, more preferably between 40% and 70%, to ensure good penetration of the electrolyte into the support.

[0066] The fiber diameter of the fabric is preferably 20 μm to 200 μm, more preferably 40 μm to 150 μm, and most preferably 60 μm to 100 μm. Thin fabrics preferably have a smaller fiber diameter. For example, the fiber diameter of a fabric having a thickness of 150 μm or less is preferably 75 μm or less, more preferably 50 μm or less, and most preferably 35 μm or less.

[0067] To further reduce the thickness of the fabric, the ratio of web thickness to fiber diameter is preferably less than 2.0, more preferably 1.7 or less, most preferably 1.4 or less. Thinner fabrics make it possible to prepare thinner separators.

[0068] Suitable porous polymer fabrics are prepared from polypropylene (PP), polyethylene (PE), polysulfone (PSU), polyphenylene sulfide (PPS), polyamide / nylon (PA), polyethersulfone (PES), polyphenylsulfone (PPSU), polyethylene terephthalate (PET), polyetheretherketone (PEEK), sulfonated polyetheretherketone (s-PEEK), monochlorotrifluoroethylene (CTFE), copolymers of ethylene and tetrafluoroethylene (ETFE) or chlorotrifluoroethylene (ECTFE), polyimide, polyetherimide and meta-aramid.

[0069] Preferred polymer fabrics are made from polypropylene (PP), polyetheretherketone (PEEK) or polyphenylene sulfide (PPS), most preferably made from polyetheretherketone (PEEK) or polyphenylene sulfide (PPS).

[0070] The porous support based on PPS or PEEK has high tolerance to high temperature, high concentration alkaline solution and high chemical stability against active oxygen escaping from the anode during the water electrolysis process. In addition, PPS or PEEK can be easily processed into various forms, such as woven fabrics or nonwoven fabrics.

[0071] The density of the porous support is preferably between 0.1 g / cm 3 and 0.7g / cm 3 between.

[0072] The porous support is preferably a continuous web to enable the manufacturing methods as disclosed in EP-A 1776490 and WO 2009 / 147084.

[0073] The width of the web is preferably between 30 cm and 300 cm, more preferably between 40 cm and 200 cm.

[0074] Polymer resin

[0075] The porous layer preferably comprises a polymer resin.

[0076] As described below, the polymer resin forms a three-dimensional porous network as a result of a phase inversion step in the preparation of the separator.

[0077] The polymer resin may be selected from fluorine resins such as polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE); olefin resins such as polypropylene (PP); and aromatic hydrocarbon resins such as polyethylene terephthalate (PET) and polystyrene (PS). The polymer resin may be used alone or in combination of two or more polymer resins.

[0078] PVDF and vinylidene fluoride (VDF) copolymers are preferred for their anti-oxidation / reduction properties and film-forming properties. Among these, terpolymers of VDF, hexafluoropropylene (HFP) and chlorotrifluoroethylene (CTFE) are preferred for their excellent swelling properties, heat resistance and adhesion to electrodes.

[0079] Another preferred polymer resin is an aromatic hydrocarbon resin because they have excellent heat resistance and alkali resistance. Examples of aromatic hydrocarbon resins include polyethylene terephthalate, polybutylene terephthalate, polybutylene naphthalate, polystyrene, polysulfone, polyethersulfone, polyphenylene sulfide, polyphenylene sulfone, polyacrylate, polyetherimide, polyimide and polyamide-imide.

[0080] Particularly preferred polymer resins are selected from the group consisting of polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone and polyphenylene sulfone, with polysulfone being most preferred.

[0081] The molecular weight (Mw) of the polymer resin is preferably between 10000 and 500000, more preferably between 25000 and 250000. When the Mw is too low, the physical strength of the porous layer may become insufficient. When the Mw is too high, the viscosity of the coating solution may become too high.

[0082] Examples of polysulfones, polyethersulfones and combinations thereof are disclosed in EP-A 3085815, paragraphs

[0021] to

[0032] .

[0083] Inorganic hydrophilic particles

[0084] The hydrophilic layer preferably contains hydrophilic inorganic particles.

[0085] Preferred hydrophilic inorganic particles are selected from metal oxides and metal hydroxides.

[0086] Preferred metal oxides are selected from zirconium oxide, titanium oxide, bismuth oxide, cerium oxide and magnesium oxide.

[0087] Preferred metal hydroxides are selected from zirconium hydroxide, titanium hydroxide, bismuth hydroxide, cerium hydroxide and magnesium hydroxide. Particularly preferred magnesium hydroxide is disclosed in EP-A 3660188, paragraphs

[0040] to

[0063] .

[0088] Other preferred hydrophilic inorganic particles are barium sulfate particles as disclosed in EP-A 3994295.

[0089] Other hydrophilic particles that may be used are nitrides and carbides of elements from Group IV of the Periodic Table.

[0090] Hydrophilic inorganic particles D 50The particle size is preferably 0.05 μm to 2.0 μm, more preferably 0.1 μm to 1.5 μm, most preferably 0.15 μm to 1.00 μm, particularly preferably 0.2 μm to 0.75 μm. 50 The particle size is preferably less than or equal to 0.7 μm, preferably less than or equal to 0.55 μm, more preferably less than or equal to 0.40 μm.

[0091] D 50 Particle size is also called median diameter or median of particle size distribution. It is the particle size value at 50% of the cumulative distribution. For example, if D 50 =0.1 μm, then 50% of the particles are larger than 1.0 μm and 50% are smaller than 1.0 μm.

[0092] D is preferably measured using laser diffraction (eg using a Mastersizer from Malvern Panalytical). 50 granularity.

[0093] The amount of hydrophilic particles is preferably at least 50% by weight, more preferably at least 75% by weight, relative to the total dry weight of the porous layer.

[0094] The weight ratio of hydrophilic particles to polymer resin is preferably greater than 60 / 40, more preferably greater than 70 / 30, and most preferably greater than 75 / 25.

[0095] Preparation of spacers

[0096] The preferred method for preparing the separator described above comprises the following steps:

[0097] - applying a coating solution as described below on both sides of the porous substrate; and

[0098] - subjecting the applied coating solution to a phase inversion, thereby forming a porous layer and a first porous layer on one side and the other side of the porous support, respectively, characterized in that

[0099] a) The covering layer thickness (d1) of the first porous layer is smaller than the covering layer thickness (d2) of the second porous layer.

[0100] The separator according to the present invention is preferably obtained by the above method.

[0101] In a preferred embodiment, the cover layer thickness (d1) of the first porous layer is at least 20 μm, preferably at least 30 μm, more preferably at least 35 μm, most preferably at least 40 μm.

[0102] Preferred methods of making reinforced separators are disclosed in EP-A 1776490, WO 2009 / 147084 and EP-A 3652362. These methods produce web-reinforced separators in which the web (i.e., the porous support) is well embedded in the separator without the web appearing at the surface of the separator.

[0103] Other methods of manufacture which may be used are disclosed in EP-A 3272908.

[0104] Coating solution

[0105] The coating solution preferably comprises a polymer resin as described above, hydrophilic inorganic particles as described above, and a solvent.

[0106] The solvent of the coating solution is preferably an organic solvent in which the polymer resin can be dissolved. In addition, the organic solvent is preferably miscible in water.

[0107] The solvent is preferably selected from N-methylpyrrolidone (NMP), N-ethylpyrrolidone (NEP), N-butylpyrrolidone (NBP), N,N-dimethylformamide (DMF), formamide, dimethyl sulfoxide (DMSO), N,N-dimethylacetamide (DMAC), acetonitrile and mixtures thereof.

[0108] For health and safety reasons, a highly preferred solvent is N-butylpyrrolidone (NBP).

[0109] The coating solution may further comprise other ingredients in order to optimize the properties of the obtained polymer layers, such as their porosity and the maximum pore size at their outer surface.

[0110] The coating solution preferably contains additives to optimize the pore size on the surface and inside the porous layer.Such additives may be organic compounds or inorganic compounds, or a combination thereof.

[0111] Organic compounds that can affect the formation of pores in the porous layer include polyethylene glycol, polyethylene oxide, polypropylene glycol, ethylene glycol, tripropylene glycol, glycerol, polyols, dibutyl phthalate (DBP), diethyl phthalate (DEP), diundecyl phthalate (DUP), isononanoic acid or neodecanoic acid, polyvinyl pyrrolidone, polyvinyl alcohol, polyvinyl acetate, polyethylene imine, polyacrylic acid, methyl cellulose and dextran.

[0112] Preferred organic compounds which can influence the formation of pores in the porous layer are selected from polyethylene glycol, polyethylene oxide and polyvinyl pyrrolidone.

[0113] Preferred polyethylene glycols have a molecular weight of 10,000 to 50,000, preferred polyethylene oxides have a molecular weight of 50,000 to 300,000, and preferred polyvinyl pyrrolidone has a molecular weight of 30,000 to 1,000,000.

[0114] A particularly preferred organic compound which can influence the formation of pores in the porous layer is glycerol.

[0115] The amount of the compound capable of influencing pore formation is preferably between 0.1% and 15% by weight, more preferably between 0.5% and 5% by weight, relative to the total weight of the coating solution.

[0116] Inorganic compounds that can affect pore formation include calcium chloride, magnesium chloride, lithium chloride, and barium sulfate.

[0117] Combinations of two or more additives that affect pore formation may be used.

[0118] The coating solution provided on each side of the porous support may be the same or different.

[0119] Applying coating solution

[0120] The coating solution may be applied to the surface of the substrate, preferably a porous support, by any coating or casting technique.

[0121] The preferred coating technique is extrusion coating.

[0122] In a highly preferred embodiment, the coating solution is applied by a slot die coating technique, wherein two slot coating dies ( Figure 2 and 3 , 200 and 300) are located on each side of the porous support.

[0123] The slot coating die is capable of maintaining the coating solution at a predetermined temperature, uniformly distributing the coating solution on the support, and adjusting the coating thickness of the applied coating solution.

[0124] In 100s -1 The viscosity of the coating solution measured at a shear rate of 1.0 Pa.s and a temperature of 20° C. is preferably between 1.0 Pa.s and 30.0 Pa.s, more preferably between 5.0 Pa.s and 20.0 Pa.s, most preferably between 7.5 Pa.s and 15.0 Pa.s.

[0125] The coating solution is preferably shear thinning. -1 The viscosity at the shear rate of 100s -1 The ratio of the viscosities at the shear rates of 2 and 4 is preferably at least 2, more preferably at least 2.5, most preferably at least 5.

[0126] The flow rates Q of the coating solutions at the slot coating die may be the same or different.

[0127] In order to prepare the separator according to the present invention, the coating solution is preferably applied in the following manner: the flow rate Q of the slot coating die is adjusted so that the flow rate Q1 at the first slot coating die (200) is different from the flow rate Q2 at the second slot coating die (300). The difference in flow rate Q1-Q2 is called ΔQ. The sum of the flow rates Q1+Q2 is called Q TOT .

[0128] If there is no difference in flow (ΔQ is zero) or only a small difference, there is a high probability that there is waviness in the spacer.

[0129] If the difference in flow rate is too large, the cover thickness (d1) of the first porous layer may be less than 20 μm, resulting in too high a gas permeability of the separator. TOT , the substrate can be coated on one side only.

[0130] The flow difference ΔQ is preferably greater than zero and less than Q TOT The difference in flow rate at the slot coating die is preferably such that Q1 / Q2 is greater than 0 and less than 1. Q1 / Q2 is preferably from 0.1 to 0.9, more preferably from 0.2 to 0.8, most preferably from 0.3 to 0.7.

[0131] The open area of ​​the porous support can determine how large the difference in flow can be. When there is a larger open area, it may be desirable for ΔQ to be larger to reduce waviness. When there is a smaller open area in the porous support, ΔQ can be smaller in order to obtain minimal waviness.

[0132] The insulator according to the present invention is preferably obtained by the following method: wherein the flow rate Q of the coating solution at the slit coating die is adjusted so that the flow rate Q1 at the first slit coating die (200) is different from the flow rate Q2 at the second slit coating die (300), more preferably wherein Q1 / Q2 is 0.1 to 0.9, more preferably 0.2 to 0.8, and most preferably 0.3 to 0.7.

[0133] The porous support is preferably a continuous web such as Figure 2 and 3 As shown in , it is conveyed downwardly between slot coating dies (200, 300).

[0134] Immediately after application, the porous support becomes impregnated with the coating solution.

[0135] Preferably, the porous support becomes completely impregnated with the applied coating solution.

[0136] Phase inversion step

[0137] After applying the coating solution to the porous support, the applied coating solution is subjected to phase inversion. In the phase inversion step, the applied coating solution is converted into a porous hydrophilic layer.

[0138] In a preferred embodiment, the two coating solutions applied to the porous support are subjected to a phase inversion.

[0139] Any phase inversion mechanism may be used to produce the porous hydrophilic layer from the applied coating solution.

[0140] The phase inversion step preferably comprises a so-called liquid induced phase separation (LIPS) step, a vapour induced phase separation (VIPS) step or a combination of a VIPS step and a LIPS step. The phase inversion step preferably comprises both a VIPS step and a LIPS step.

[0141] Both LIPS and VIPS are non-solvent-induced phase inversion processes.

[0142] In the LIPS step, the porous support coated on both sides with the coating solution is contacted with a non-solvent miscible with the solvent of the coating solution.

[0143] Typically, this is done by immersing the porous support, coated on both sides with the coating solution, in a non-solvent bath, also called a coagulation bath.

[0144] The non-solvent is preferably water; a mixture of water and an aprotic solvent selected from N-methylpyrrolidone (NMP), N-ethylpyrrolidone (NEP), N-butylpyrrolidone (NBP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO) and dimethylacetamide (DMAC); an aqueous solution of a water-soluble polymer such as PVP or PVA; or a mixture of water and an alcohol (e.g., ethanol, propanol or isopropanol).

[0145] Most preferably, the non-solvent is water.

[0146] The temperature of the coagulation bath is preferably between 20°C and 90°C, more preferably between 40°C and 70°C.

[0147] The transfer of solvent from the coated polymer layer to the non-solvent bath and into the polymer layer results in phase inversion and the formation of a three-dimensional porous polymer network. Impregnation of the applied coating solution into the porous support results in the resulting hydrophilic layer being fully adhered to the porous support.

[0148] In a preferred embodiment, Figure 2 and 3 As shown in , a continuous web (100) coated on each side with a coating solution is conveyed downwardly in a vertical position toward a coagulation bath (800).

[0149] In the VIPS step, the porous support coated with the coating solution is exposed to a non-solvent vapor, preferably humid air.

[0150] Preferably, the solidification step comprises both a VIPS step and a LIPS step. Preferably, the VIPS step is performed before the LIPS step. In a particularly preferred embodiment, the porous support coated with the coating solution is first exposed to humid air (VIPS step) and then immersed in a water bath (LIPS step).

[0151] exist Figure 2 In the manufacturing method shown in , VIPS is performed in a region 400 between the slot coating die (200, 300) and the non-solvent surface in the coagulation bath (800), and the region 400 is isolated from the environment by, for example, an insulating metal plate (500).

[0152] The extent and rate of water transfer during the VIPS step can be controlled by adjusting the air velocity, the relative humidity and temperature of the air, and the exposure time.

[0153] The exposure time can be adjusted by varying the distance d between the slot coating die (200, 300) and the surface of the non-solvent in the coagulation bath (800) and / or the speed at which the elongated web 100 is transported from the slot coating die toward the coagulation bath.

[0154] The relative humidity in the VIPS area (400) can be adjusted by the temperature of the coagulation bath and the isolation of the VIPS area (400) from the environment and from the coagulation bath.

[0155] The speed of the air can be adjusted by the rotation speed of the ventilator (420) in the VIPS area (400).

[0156] The VIPS step performed on one side of the separator and the VIPS step performed on the other side of the separator to produce the second porous polymer layer may be identical to each other ( Figure 2 ) or different ( Figure 3 ).

[0157] After the phase inversion step, preferably after the LIPS step in a coagulation bath, a washing step may be performed.

[0158] After the phase inversion step or the optional washing step, an optional drying step is carried out.

[0159] Manufacturing of spacers

[0160] Figure 2 and Figure 3 A preferred embodiment for producing a separator according to the present invention is schematically illustrated.

[0161] The porous support is preferably a continuous web (100).

[0162] The web is unwound from a feed roll (600) and directed downwardly in a vertical position between two coating units (200) and (300).

[0163] Using these coating units, the coating solution is applied to each side of the web. The coating thickness on each side of the web can be adjusted by optimizing the flow rate, the viscosity of the coating solution and the distance between the coating unit and the web surface as described above. Preferred coating units are described in EP-A 2296825 (paragraphs

[0043] ,

[0047] ,

[0048] ,

[0060] ,

[0063] and Figure 1 )middle.

[0164] The web coated on both sides with the coating solution is then conveyed downwardly a distance d toward a coagulation bath (800).

[0165] The LIPS step was performed in a coagulation bath.

[0166] The VIPS step is performed in the VIPS area before entering the coagulation bath. Figure 2 In , the VIPS areas (400) on both sides of the coated web are the same, while in Figure 3 , the VIPS areas (400(1)) and (400(2)) on each side of the coated web are different.

[0167] Insulated metal panels can be used to optimize relative humidity (RH) and air temperature in the VIPS area. Figure 2 In the embodiment, the VIPS area (400) is completely isolated from the environment by such a metal plate (500). Then the RH and temperature of the air are mainly determined by the temperature of the coagulation bath. The air speed in the VIPS area can be adjusted by the ventilator (420).

[0168] exist Figure 3 In the embodiment, the VIPS areas (400(1)) and (400(2)) are different from each other. The VIPS areas (400(1)) on one side of the coated web (including the metal plate (500(1))) are different from each other. Figure 2. The VIPS area (400(2)) on the other side of the coated web is different from area (400(1)). There is no metal plate to isolate the VIPS area (400(2)) from the environment. However, the VIPS area (400(2)) is now isolated from the coagulation bath by the insulating metal plate (500(2)). In addition, there is no ventilator in the VIPS area 400(2). This results in the VIPS area (400(1)) having a higher RH and air temperature than the RH and air temperature of the other VIPS areas (400(2)).

[0169] High RH and / or high air velocity in the VIPS region generally results in a larger maximum pore size.

[0170] The RH in one VIPS zone is preferably above 85%, more preferably above 90%, most preferably above 95%, while the RH in another VIPS zone is preferably below 80%, more preferably below 75%, most preferably below 70%.

[0171] After the phase separation step, the reinforced separator is then transported to a roll-up system (700).

[0172] The spacer may be provided with a liner on one side and then rolled up with the spacer and applied liner.

[0173] Electrolyzer

[0174] The separator for alkaline water electrolysis according to the present invention can be used in an alkaline water electrolyzer.

[0175] An electrolytic cell generally consists of two electrodes (anode and cathode) separated by a separator. An electrolyte is present between the two electrodes.

[0176] When an electric current is supplied to the electrolytic cell, the hydroxide ions of the electrolyte are oxidized to oxygen at the anode, and water is reduced to hydrogen at the cathode. The hydroxide ions formed at the cathode migrate to the anode through the separator. The separator prevents the hydrogen and oxygen formed during electrolysis from mixing.

[0177] The electrolyte solution is typically an alkaline solution. A preferred electrolyte solution is an aqueous solution of an electrolyte selected from sodium hydroxide or potassium hydroxide. Potassium hydroxide electrolytes are typically preferred due to their higher specific conductivity. The concentration of the electrolyte in the electrolyte solution is preferably 20 to 40 weight percent relative to the total weight of the electrolyte solution.

[0178] The temperature of the electrolyte is preferably 50 to 120° C., more preferably 75 to 100° C., most preferably 80 to 90° C. However, higher temperatures (eg, at least 100° C., more preferably 125 to 165° C.) may result in more efficient electrolysis.

[0179] The electrodes usually comprise a substrate provided with a so-called catalyst layer. The catalyst layer can be different for the anode, where oxygen is formed, and for the cathode, where hydrogen is formed.

[0180] Typical substrates are prepared from conductive materials selected from nickel, iron, mild steel, stainless steel, vanadium, molybdenum, copper, silver, manganese, platinum group elements, graphite and chromium. The substrate can be prepared from a conductive alloy of two or more metals or a mixture of two or more conductive materials. The preferred material is nickel or a nickel-based alloy. Nickel has good stability in strongly alkaline solutions, has good electrical conductivity and is relatively cheap.

[0181] The catalyst layer preferably includes nickel, cobalt, iron and platinum group elements. The catalyst layer may include these elements in the form of elemental metals, compounds (e.g., oxides), composite oxides or alloys prepared from a variety of metal elements, or mixtures thereof. Preferred catalyst layers include plated nickel, plated alloys of nickel and cobalt or nickel and iron, composite oxides including nickel and cobalt (e.g., LaNiO3, LaCoO3 and NiCo2O4), compounds of platinum group elements (e.g., iridium oxide), or carbon materials (e.g., graphene).

[0182] A particularly preferred catalyst layer comprises Raney nickel. The Raney nickel structure is formed by selectively leaching aluminum or zinc from a Ni-Al alloy or a Ni-Zn alloy. The lattice vacancies formed during leaching result in a large surface area and a high density of lattice defects, which are active sites for electrocatalytic reactions.

[0183] Preferred porous electrodes and methods for their production are disclosed, for example, in EP-A 3575442 (paragraphs 23 to 84).

[0184] The pore size of the porous electrode can have an influence on the electrolysis efficiency. For example, in EP-A 3575442 it is disclosed that the preferred pore size of the porous electrode is 10 nm up to 200 nm.

[0185] The catalyst layer may also include organic materials (eg, polymers) to improve durability and adhesion to the substrate.

[0186] In so-called zero-gap electrolyzers, the electrodes are placed in direct contact with the separator, thereby reducing the space between the two electrodes. Mesh-type or porous electrodes are used to enable the separator to be filled with electrolyte and for efficient removal of the oxygen and hydrogen formed. Such zero-gap electrolyzers have been observed to operate at higher current densities.

[0187] However, in such zero-gap electrolytic cells, it has been observed that bubbles formed inside the separator can accumulate on top of the separator. Such bubble accumulation on top of the separator can lead to higher ionic resistance in this part of the cell. The temperature increase due to the inefficient cooling of the electrolyte in this area of ​​the electrolytic cell can even lead to burning of the separator.

[0188] It has been observed that introducing a small distance between one side of the separator and at least one electrode results in less accumulation of gas bubbles inside the separator.The distance between one side of the separator and the anode and the distance between the other side of the separator and the cathode may be the same or different.

[0189] The distance between the separator surface and the at least one electrode is preferably 50 μm to 500 μm, more preferably 100 μm to 250 μm.

[0190] The distance between the separator and the electrode can be achieved using so-called spacers.

[0191] Such spacers are preferably hydrophilic to avoid air bubbles adhering to the spacers (static water contact angle of 90° C. or less, preferably 45° C. or less).

[0192] Such spacers preferably have an open structure to ensure rapid and sufficient discharge of air bubbles.

[0193] A typical alkaline water electrolyser comprises several electrolysis cells, also called an electrolysis cell stack.

[0194] Regarding cell configuration, two types of electrolyzers are commonly used.

[0195] A monopolar (or "tank-type") electrolyzer consists of alternating positive and negative electrodes kept separated by separators. The positive electrodes are all connected in parallel, as are the negative electrodes, and the entire assembly is immersed in a single electrolyte bath (the "tank") to form a unit cell. A factory-scale electrolyzer is then built by electrically connecting these units in series. The total voltage applied to the entire electrolytic cell is the same as that applied to the individual unit cells.

[0196] In a bipolar electrolyzer, on the other hand, metal sheets (or "bipoles") electrically connect adjacent cells in series. The electrocatalyst for the negative electrode is coated on one face of the bipolar body, and the electrocatalyst for the positive electrode of the adjacent cell is coated on the opposite face. In this case, the total cell voltage is the sum of the individual unit cell voltages. Thus, a series-connected stack of such cells forms a module that operates at a higher voltage and lower current than a can-type (monopolar) design. To meet the requirements of large electrolysis plants, these modules are connected in parallel to facilitate increased current.

[0197] A membrane electrode assembly (MEA) can also be used in an electrolyzer. Such an MEA is usually prepared by applying a separator (preferably without a reinforcing support) to at least one porous electrode. Such an MEA is disclosed, for example, in EP-A 2831312 (AgfaGevaert), EP3277862 (De Nora) and WO2020 / 158719 (Nippon Shokubai). Such an MEA can also be used in the electrolysis method according to the invention.

[0198] It is also possible to provide the above-mentioned catalyst layer on the surface of the separator, obtaining a so-called catalyst coated membrane (CCM).

[0199] Such CCMs may have an improved contact surface between the membrane surface and the catalyst layer, resulting in higher electrolysis efficiency.

[0200] The catalyst layer may be applied on the membrane surface by any deposition technique, such as coating, spraying, inkjet printing, gravure printing, screen printing, 3D printing, vapor deposition techniques. Example

[0201] Material

[0202] Unless otherwise stated, all materials used in the following examples are readily available from standard sources such as ALDRICH CHEMICAL Co. (Belgium) and ACROS (Belgium). The water used was deionized water.

[0203] PPS-Fabric: 100 μm thick woven polyphenylene sulfide fabric.

[0204] ZrO2: Zirconium oxide particles with a D50 particle size of less than 1 μm measured with a Mastersizer from Malvern Panalytical.

[0205] Polysulfone: Udel P1700 NT LCD, a polysulfone resin available from SOLVAY.

[0206] Glycerol: a pore-enlarging agent, commercially available from MOSSELMAN.

[0207] NBP: N-butylpyrrolidone, commercially available from Taminco.

[0208] Measurement

[0209] Flatness / Waviness: The flatness / waviness of the separator was assessed by visual inspection. Figure 5 Illustrated is the waviness observed in the spacer.

[0210] Cover thickness: Cover thickness of the porous layer was determined using optical microscopy. Prior to imaging, the membranes were embedded in epoxy resin and mechanically polished. Imaging was performed using a Zeiss Discovery.V12 stereo microscope using a ring light. Calibrated images were analyzed using the measurement tools of ImagePro 10. Figure 4 As shown in , the cover thickness is measured by drawing multiple lines from the edge of the membrane perpendicularly toward the outer edge of the mesh fibers. As an example, Figure 4 Several distances for d1 and d2 are shown, represented by d1(1), d1(2), d1(3), d2(1), d2(2), d2(3). The average values ​​for d1 and d2 were calculated from all measured distances at 10 different points on 5 images of 2 cm long cross sections from 3 different samples.

[0211] Viscosity: measured at 100 s using a Kinexus LAB+ rheometer available from Malvern Panalytical using "Cup & Bob" geometry -1 and the viscosity of the coating solution at 20°C.

[0212] Gas permeability: using Porolux TM The 1000 device measures gas permeability at 5 bar.

[0213] Example 1

[0214] Preparation of separators S-1 to S-3

[0215] The coating solution was prepared by mixing the ingredients in Table 1.

[0216] Table 1

[0217] Element weight% <![CDATA[ZrO2]]> 40.07 Polysulfone 10.02 glycerin 1.0 NBP 48.91

[0218] The coating solution was measured as described above at 100 s -1 The viscosity is 10.50 Pa.s.

[0219] like Figure 2 Separators S-1 to S-3 were prepared as schematically depicted in FIG. The coating solution was coated on both sides of a 1.3 m wide PPS fabric using a slot die coating technique at a speed of 3 m / min. As shown in Table 2, the slot coating die (200 and 300, Figure 2). The coated support is then conveyed towards a water bath (coagulation bath, 800) maintained at 50°C. A VIPS step is carried out in a closed area (400, d=7 cm, RH=98%, ventilation) before entering the water bath. The coated support is then allowed to enter the water bath for 2 minutes, during which liquid induced phase separation (LIPS) occurs. After an in-line washing step at 65°C during 5 minutes in water, the separator obtained is wound without drying and subsequently cut to the desired format.

[0220] The obtained separators S-1 to S-3 had cover layer thicknesses d1 and d2 and total thickness D as shown in Table 2. The gas permeability and waviness evaluation of the separators evaluated as described above are shown in Table 2. The waviness indicated with OK means that minimal waviness exists in the separator.

[0221] Table 2

[0222] S-1(Invention) S-2(Comparison) S-3(Comparison) <![CDATA[Q1(L.min -1 )]]> 0.45 0.55 0.34 <![CDATA[Q2(L.min -1 )]]> 0.65 0.55 0.76 <![CDATA[Q1 / Q2]]> 0.69 1 0.44 d1(μm) 43 60 19 d2(μm) 77 60 96 d1 / d2 0.56 1 0.2 D(μm) 220 220 184 <![CDATA[G.P(L / min.cm 2 )]]> 5.0 4 5.8 Waviness OK NOK OK

[0223] It is clear from the results in Table 2 that by preparing a separator by coating at different flow rates Q1 and Q2 at a slot coating die, a separator having a cover layer thickness d1 that is smaller than a cover layer thickness d2 is obtained.

[0224] If the cover layer thickness d1 is less than 20 μm, the gas permeability is too high. This is because the fibers of the porous support begin to protrude from the porous layer.

[0225] When d1 / d2 is lower than 0.8, the waviness of the film is good enough.

Claims

1. A separator (1) for alkaline electrolysis, comprising a porous support (10), a first porous layer (20b) provided on one side of the porous support, and a second porous layer (30b) provided on the other side of the porous support, wherein the first porous layer and the second porous layer are partially impregnated into the porous support and each has a covering layer thickness d1 and d2, respectively, the covering layer thickness being defined as the portion of each porous layer not impregnated into the porous support, Features a) d1 is less than the covering layer thickness (d2) of the second porous layer, and b) d1 is at least 20 μm.

2. The spacer according to claim 1, wherein d1 / d2 is 0.8 or less. 3 . The separator according to claim 1 , which has a thickness of 250 μm or less. 4 . The separator according to claim 1 , wherein the porous support has a thickness of 50 to 150 μm.

5. A separator according to any one of the preceding claims, wherein the porous support has an open area of ​​30% to 80%.

6. A separator according to any one of the preceding claims having a flow rate of 2 L / min.cm measured at 5 bar. 2 Up to 5.8L / min.cm 2 gas permeability.

7. The separator according to any one of the preceding claims, having a relative humidity of less than 0.1 ohm.cm in a 30 wt% KOH aqueous solution at 80°C. 2 The ionic resistance.

8. The separator according to any one of the preceding claims, wherein the first and second porous layers comprise a polymer resin and hydrophilic inorganic particles. 9 . The separator according to claim 8 , wherein the polymer resin is at least one selected from the group consisting of polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, and polyphenylene sulfone.

10. The separator according to claim 8 or 9, wherein the hydrophilic inorganic particles are at least one selected from the group consisting of zirconium oxide, zirconium hydroxide, magnesium oxide, magnesium hydroxide, titanium oxide, titanium hydroxide, and barium sulfate.

11. A method for manufacturing a separator for alkaline water electrolysis, comprising the steps of: - applying a coating solution comprising a polymer resin, hydrophilic inorganic particles and a solvent on both sides of the porous support (10); and - subjecting the applied coating solution to a phase inversion, thereby forming a first porous layer (20b) on one side of the porous support and a second porous layer (30b) on the other side of the porous support, It is characterized in that The covering layer thickness (d1) of the first porous layer is smaller than the covering layer thickness (d2) of the second porous layer. 12 . The method according to claim 11 , wherein the first coating solution is applied at one side of the porous support at a flow rate Q1 that is smaller than a flow rate Q2 of the second coating solution at the other side of the porous support.

13. The method according to claim 12, wherein Q1 / Q2 is 0.1 to 0.

9.

14. The method according to any one of claims 11 to 13, wherein the cover layer thickness (d1) of the first porous layer is at least 20 μm.

15. An alkaline water electrolysis device comprising a separator as defined in any one of claims 1 to 10 located between a cathode and an anode.

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