Description

A separator for alkaline water electrolysis

Technical field of the Invention

[001 ] The present invention relates to a separator for alkaline water electrolysis.

Background art for the invention

[002] Nowadays, hydrogen is used in several industrial processes, for example its use as raw material in the chemical industry and as a reducing agent in the metallurgy industry. Hydrogen is a fundamental building block for the manufacture of ammonia, and hence fertilizers, and of methanol, used in the manufacture of many polymers. Refineries, where hydrogen is used for the processing of intermediate oil products, are another area of use.

[003] Hydrogen is also being considered an important future energy carrier, which means it can store and deliver energy in a usable form. Energy is released by an exothermic combustion reaction with oxygen thereby forming water.

During such combustion reaction, no greenhouse gases containing carbon are emitted.

[004] For the realization of a low-carbon society, renewable energies using natural energy such as solar light and wind power are becoming more and more important.

[005] The production of electricity from wind power and solar power generation systems is very much dependent on the weather conditions and therefore variable, leading to an imbalance of demand and supply of electricity. To store surplus electricity, the so-called power-to-gas technology wherein electrical power is used to produce gaseous fuel such as hydrogen, attracted much interest in recent years. As production of electricity from renewable energy sources will increase, the demand for storage and transportation of the produced energy will also increase.

[006] Alkaline water electrolysis is an important manufacturing process wherein electricity may be converted into hydrogen.

[007] In an alkaline water electrolysis cell, a so-called separator or diaphragm is used to separate the electrodes of different polarity to prevent a short circuit between these electronic conducting parts (electrodes) and to prevent the recombination of hydrogen (formed at the cathode) and oxygen (formed at the anode) by avoiding gas crossover. While serving in all these functions, the separator should also be a highly ionic conductor for transportation of hydroxyl ions from the cathode to the anode.

[008] A separator typically includes a porous support. Such a porous support reinforces the separator thereby facilitating the manipulation of the separator and the introduction of the separator in an electrolyser as disclosed in EP-A 232923 (Hydrogen Systems).

[009] EP-A 1776490 (VITO) discloses a process of preparing a reinforced separator. The process leads to a membrane with symmetrical characteristics. The process includes the steps of providing a porous support as a web and a suitable dope solution, guiding the web in a vertical position, equally coating both sides of the web with the dope solution to produce a web coated support, and applying a symmetrical surface pore formation step and a symmetrical coagulation step to the dope coated web to produce a reinforced membrane including a porous support and two porous polymer layers on both sides of the support.

[010] W02009/147084 and W02009/147086 (Agfa Gevaert and VITO) disclose manufacturing methods to produce a reinforced membrane with symmetrical characteristics as described in EP-A 1776490.

[011] EP-A 3312306 (Kawasaki, De Nora Permelec, Thyssen Krupp) discloses a separator manufactured by applying a dope solution on one side of a porous support followed by a symmetrical pore formation step and resulting in a separator having substantial identical pores on both surfaces of the separator.

[012] EP-A 3660188 (Nippon Shokubai) also discloses a separator reinforced with a porous support.

[013] However, it has been observed that voids located at the interface between the porous polymer layers and the porous support may result in gas bubble formation inside the separator. The gas bubble formation may result in a decreased ion conductivity through the separator and therefore a decreased efficiency of the electrolysis. These gas bubbles may accumulate in the upper part of the separator placed in the electrolyzer (lateral migration of gas bubbles in the separator) resulting in so-called hot spots or even burning of the separator.

Summary of the invention

[014] It is an object of the invention to provide a reinforced separator wherewith a more efficient water electrolysis may be realized due to less lateral migration of gas bubbles inside the separator.

[015] This object is realized with the separator as defined in claim 1.

[016] Further objects of the invention will become apparent from the description hereinafter.

Brief description of the drawings

[017] Figure 1 shows schematically a lateral and longitudinal direction of a separator as used in the present invention.

[018] Figure 2 shows schematically an embodiment of a separator according to the present invention.

[019] Figure 3 shows schematically another embodiment of a separator according to the present invention.

[020] Figure 4 shows a SEM picture of a comparative (Top) and inventive (Bottom) separator wherein the occurrence (Top) and absence (Bottom) of voids (150) at the interface between the porous layers and the porous support is illustrated.

[021] Figure 5 shows schematically the lateral diffusion of gas bubbles in a separator in a “zero-gap” electrolysis cell.

[022] Figure 6 shows schematically some examples of a pore diameter distribution in the thickness direction of a separator.

[023] Figure 7 schematically depicts an embodiment of a manufacturing method of a separator as shown in Figure 3. [024] Figure 8 schematically depicts another embodiment of a manufacturing method of a separator as shown in Figure 3.

[025] Figure 9 shows schematically the measurement method to determine the lateral Bubble Point.

Detailed description of the invention

Separator for alkaline water electrolysis

[026] The separator for alkaline electrolysis (1 ) according to the present invention comprises a porous support (100) and a porous layer (200) provided on a side of the porous support, characterized in that a lateral Bubble Point of the separator, measured according to the method described below, is at least 0.2 bar, preferably at least 0.35 bar, more preferably at least 0.5 bar.

[027] Figure 1 schematically depicts the lateral (L1) and longitudinal (L2) direction of a separator (1 ) as used herein.

[028] Figure 2 schematically depicts an embodiment of a separator according to the present invention wherein a porous layer (200) is provided on a side of a porous support (100). The porous layer (200) is preferably provided on a side of a porous support as described below.

[029] Figure 3 schematically depicts another embodiment of a separator according to the present invention wherein a first (250) porous layer is provided on one side of a porous support (100) and a second (250’) porous layer is provided on the other side of the porous support (100). The first (250) and second (250’) porous layers may be identical or different from each other. The porous layers are preferably provided on both sides of the porous support as described below.

[030] The thickness of the separator (t2) is preferably from 50 to 750 pm, more preferably from 75 to 500 pm, most preferably from 100 to 250 pm, particularly preferred from 125 to 200 pm. Increasing the thickness of the separator typically results in a higher physical strength of the separator. Flowever, increasing the thickness of the separator typically also results in a decrease of the electrolysis efficiency due to an increase of the ionic resistance.

[031] As the lateral bubble point is influenced by shrinkage phenomena (see below), the thickness of the separator may have an influence on the lateral bubble point. A lower thickness typically results in a higher lateral bubble point.

[032] The separator preferably has an ionic resistance at 80°C in a 30 wt% aqueous KOFI solution of 0.1 ohm. cm ² or less, more preferably of 0.07 ohm. cm ² or less. The ionic resistance may be determined with an Inolab® Multi 9310 IDS apparatus available from VWR, part of Avantor, equipped with a TetraCon 925 conductivity cell available from Xylem.

[033] As described below in more detail a separator according to the present invention is preferably prepared by the application of a coating solution, also referred to herein as a dope solution, on one or both sides of a porous support.

[034] The dope solution preferably comprises a polymer resin, hydrophilic inorganic particles and a solvent. [035] A porous layer is then obtained after a phase inversion step wherein the polymer resin forms a three-dimensional porous polymer network.

[036] Upon application of the dope solution(s) on one or both sides of the porous support, the dope solution(s) preferably impregnate the porous support. The porous support is more preferably completely impregnated with the dope solution(s). Such impregnation of the dope solution(s) into the porous support ensures that after phase inversion the three-dimensional porous polymer network also extends into the porous support, resulting in an improved adhesion between the porous layer and porous support.

[037] However, it has been observed that during preparation of the separator voids may be formed at the interface between the porous layer and the porous support, probably due to shrinking phenomena. The occurrence of such voids (150) at the interface between a porous support (100) and the porous layers (250, 250’) provided on both sides of the support is clear from the SEM picture in Figure 4 (Top).

[038] Gas bubbles may be formed in such voids when the latter are large enough. These gas bubbles may rise to the top of the electrolysis cell when the voids are interconnected in a lateral direction of the separator. Such a diffusion of gas bubbles “inside” the separator to the top of an electrolysis cell, in this case having a zero-gap configuration, is shown in Figure 5 (LB).

Accumulation of gas bubbles at the top of the electrolysis cell may result in a higher ionic resistance in that part of the cell. A temperature rise as a result of a less efficient cooling by the electrolyte in that area of the electrolysis cell may even result in burning of the separator.

[039] The lateral Bubble measured as described below is determined by the diffusion of gas bubbles in the lateral direction (L1) inside the separator. This in contrast to the commonly known bubble point also described below that is a measure of the diffusion of gas bubbles in the longitudinal direction (L2) of the separator.

[040] It has now been observed that with a separator having a lateral Bubble Point, measured according to the method described below, of at least 0.2 bar, preferably at least 0.35 bar, more preferably at least 0.5 bar, less gas bubbles are formed inside the separator and less gas bubbles are therefore accumulated at the top of the electrolysis cell that may negatively influence the electrolysis efficiency.

[041] It has been found that the lateral Bubble Point referred to above may be influenced by:

- the composition of the dope solution;

- the composition and structure of the porous support;

- residual solvent in the porous support.

These parameters will be described more in detail below.

[042] The separator includes pores having a pore diameter that is sufficiently small to prevent recombination of hydrogen and oxygen by avoiding gas crossover in the longitudinal direction of the separator. On the other hand, to ensure efficient transportation of hydroxyl ions from the cathode to the anode the pore diameter may not be too small to ensure an efficient penetration of electrolyte into the separator.

[043] The pores are preferably characterized using the Bubble Point Test method described in American Society for Testing and Materials Standard (ASMT) Method F316. This technique is based on the displacement of a wetting liquid embedded in the separator by applying an inert pressurised gas. Only through-pores are measured in this way.

[044] The most challenging part for the gas to displace the liquid along the entire pore path is the most constricted section of the pore, also known as pore throat. The diameter of a pore measured with the Bubble Point Test method is the diameter of that pore throat, regardless of where the pore throat is positioned in the pore path.

[045] The pores preferably have a maximum pore diameter (PDmax) measured with the Bubble Point Test method of from 0.05 to 2 pm, more preferably from 0.10 to 1 pm, most preferably from 0.15 to 0.5 pm.

[046] Figure 6 schematically depicts so called through-pores a to e having various shapes. Through-pores referred to herein are pores that enables transport from one side to the separator to the other side of the separator. The pore throat (p) is shown for the different pore shapes.

For clarity reasons the porous support and porous layer(s) of the separator are not separately shown in Figure 6. The separator in Figure 6 may be the separator shown in Figure 2 or Figure 3.

[047] The pore throat may be situated:

- at the outer surface(s) of the separator of the separator (a);

- “inside” the separator (b, c, e); or

- both at one outer surface of the separator and “inside” the separator (d).

[048] According to a preferred embodiment, the pore throat is situated at a distance d3 and/or d4 from one or both outer surfaces of the separator. The distances d3 and d4 may be identical or different from each other. The distances d3 and d4 are preferably from 0 to 15 pm, more preferably from 0 to 10 pm from respectively the outer surfaces A” and B” of the separator.

[049] The pore diameter at both outer surfaces may be substantially identical or different from each other. Substantially identical referred to herein means that a ratio of the pore diameter of both surfaces is from 0.9 to 1.1. Pore diameters at the outer surface of a separator may also be measured with Scanning Electrode Microscopy (SEM) as disclosed in EP-A 3652362.

[050] For a pore shape (a) in Figure 6 the pore diameter measured with SEM at the outer surfaces of the separator will correspond with the maximum pore diameter PDmax measured with the Bubble Point Test method.

[051 ] When a pore throat is however situated inside the separator (see pore shapes (b), (c), (d) and (e) in Figure 6) the maximum pore diameter (PDmax) measured with the Bubble Point Test method will be smaller compared to the pore diameter measured at the outer surfaces with SEM.

[052] The Bubble Point Test method may be adapted to measure a maximum pore diameter (PDmax) on both sides of a separator by using a grid supporting one side of the separator during the measurement. Another measurement is then carried out using the grid supporting to other side of the separator.

[053] Also, the PDmax measured for both sides of the separator may be substantially identical or different from each other.

[054] A preferred separator of which both sides have substantial identical pore diameters measured with the Bubble Point Test method is disclosed in EP-A 1776480, W02009/147084 and EP-A 3312306. [055] A preferred separator of which both sides have different pore diameters measured with the Bubble Point Test method is disclosed in EP-A 3652362. The maximum pore diameter at the outer surface of a first porous layer PDmax(1) is preferably between 0.05 and 0.3 pm, more preferably between 0.08 and 0.25 pm, most preferably between 0.1 and 0.2 pm and the maximum pore diameter at the outer surface of a second porous layer PDmax(2) is preferably between 0.2 and 6.5 pm, more preferably between 0.2 and 1.50 pm, most preferably between 0.2 and 0.5 pm. The ratio between PDmax(2) and PDmax(1) is preferably between 1.1 to 20, more preferably between 1.25 and 10, most preferably between 2 and 7.5. The smaller PDmax(1) ensure an efficient separation of hydrogen and oxygen while PDmax(2) ensures a good penetration of the electrolyte in the separator resulting in a sufficient ionic conductivity.

[056] The porosity of the separator is preferably between 30 and 70 %, more preferably between 40 and 60 %.

[057] A separator having a porosity within the above ranges typically has excellent ion permeability and excellent gas barrier properties because the pores of the diaphragm are continuously filled with an electrolyte solution. A porosity of 80 % or higher would result in a too low mechanical strength of the separator and a too high permeation of electrolyte, the latter resulting in an increase of the HTO (wt% hydrogen present in the oxygen formed at the anode).

[058] The separator preferably has a water permeability from 200 to 800 l/bar.h.m ² , more preferably from 300 to 600 l/bar.h.m ² .

Porous support

[059] The porous support (100) is used to reinforce the separator to ensure its mechanical strength.

[060] A thickness of the porous support (t1 ) is preferably 350 pm or less, more preferably 200 pm or less, most preferably 100 pm or less, particularly preferred 75 pm or less.

[061] It has been observed that the ion conductivity through a reinforced separator increases when the thickness of the porous support decreases.

[062] However, to ensure sufficient mechanical properties of the reinforced separator, the thickness of the porous support is preferably 20 pm or more, more preferably 40 pm or more.

[063] The porous support may be selected from the group consisting of a porous fabric and a porous ceramic plate.

[064] The porous support is preferably a porous fabric, more preferably a porous polymer fabric. Such a porous polymer fabric is often referred to as a polymer mesh.

[065] The porous polymer fabric may be woven or non-woven. Woven fabrics typically have a better dimensional stability and homogeneity of open area and thickness. However, the manufacture of woven fabrics with a thickness of 100 pm or less is more complex resulting in more expensive fabrics. The manufacture of non-woven fabrics is less complex, even for fabrics having a thickness of 100 pm or less. Also, non-woven fabrics may have a larger open area.

[066] Suitable porous polymer fabrics are prepared from polypropylene (PP), polyethylene (PE), polysulfone (PS), polyphenylene sulfide (PPS), polyamide/nylon (PA), polyether sulfone (PES), polyphenyl sulfone (PPSU), polyethylene terephthalate (PET), polyether ether ketone (PEEK), sulfonated polyether ether keton (s-PEEK), monochlorotrifluoroethylene (CTFE), copolymers of ethylene with tetrafluorethylene (ETFE) or chlorotrifluorethylene (ECTFE), polyimide, polyether imide, polyarylene and m-aramide.

[067] The porous support preferably has a high resistance to high temperatures and to highly concentrated alkaline solutions and a high chemical stability against active oxygen evolved from an anode during the water electrolysis process.

[068] A preferred porous support is prepared from polypropylene (PP), poly phenylene sulphide (PPS) and polyether-ether ketone (PEEK).

[069] It has been observed that the lateral Bubble Point of the separator increases when it includes a PEEK or a polyarylene porous support.

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

[071] The porous support is preferably a continuous web to enable a manufacturing process as disclosed in EP-A 1776490 and W02009/147084.

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

[073] It has been observed that using porous supports comprising residual solvent(s) for the preparation of separators may result in an increased lateral Bubble Point.

[074] The residual solvent(s) may be the result of the manufacturing process of the porous support or may be the result of a treatment of the porous support with one or more solvents.

[075] The residual solvent is preferably selected from the group consisting of toluene, phenol, butyrolacton, dichlorobenzene, N-methyl pyrrolidone, N-ethyl pyrrolidone, N-butyl pyrrolidone, chloro-N-methylaniline and oleicamide.

[076] The residual solvent is more preferably selected from the group consisting of butyrolacton, N-methyl pyrrolidone, N-ethyl pyrrolidone, N-butyl pyrrolidone, chloro-N-methylaniline and oleicamide.

[077] The residual solvent is most preferably selected from N-methyl pyrrolidone, N-ethyl pyrrolidone, N-butyl pyrrolidone, chloro-N-methylaniline and oleicamide.

[078] The amount of residual solvent is preferably at least 25 ppm, more preferably at least 50 ppm, most preferably at least 75 ppm, particularly preferred at least 100 ppm.

[079] A method of treating a porous support preferably includes the steps of:

- immersing a porous support into at least one solvent as described above,

- partly removing the solvent from the porous support.

[080] The temperature of the solvent(s) in the immersion step is dependent on the type of solvent(s), in particular its boiling point. The immersion is preferably carried out at a temperature of at least 50 °C, more preferably at a temperature of at least 75°C, most preferably at a temperature of at least 100 °C.

[081] The immersing time is preferably 12 hours or more, more preferably 1 day or more, most preferably 1 week or more. [082] The solvent is then partly removed from the porous support to obtain a desired residual solvent amount. The removal may be carried out by wiping the solvent from the porous support and/or drying the porous support.

Polymer resin

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

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

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

[086] PVDF and vinylidenefluoride (VDF)-copolymers are preferred for their oxidation/reduction resistance and film-forming properties. Among these, terpolymers of VDF, hexanefluoropropylene (FIFP) and chlorotrifluoroethylene (CTFE) are preferred for their excellent swelling properties, heat resistance and adhesion to electrodes.

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

[088] A particular preferred polymer resin is selected from the group consisting of polysulfone, polyethersulfone and polyphenylsulfone, polysulfone being the most preferred.

[089] The molecular weight (Mw) of polysulfones, polyether sulfones and polyphenyl sulfones 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 dope solution may become too high.

[090] Examples of polysulfones, polyether sulfones and combinations thereof are disclosed in EP-A 3085815, paragraphs [0021] to [0032]

[091 ] It has been observed that the amount of polymeric resin in the dope solution may have an influence on the lateral bubble point. A too low amount of polymer resin may result in void formation and/or a too high porosity both resulting in a too low lateral bubble point.

Inorganic hydrophilic particles

[092] The porous layer preferably comprises hydrophilic particles. Upon phase inversion, a hydrophobic porous polymer layer is obtained. To ensure sufficient wetting of the separator by the electrolyte, a sufficient amount hydrophilic particles are added to the porous layer.

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

[094] Preferred metal oxides are selected from the group consisting of zirconium oxide, titanium oxide, bismuth oxide, cerium oxide and magnesium oxide. [095] Preferred metal hydroxides are selected from the group consisting of zirconium hydroxide, titanium hydroxide, bismuth hydroxide, cerium hydroxide and magnesium hydroxide. A particularly preferred magnesium hydroxide is disclosed in EP-A 3660188, paragraphs [0040] to [0063]

[096] Other preferred hydrophilic particles are barium sulfate particles.

[097] Other hydrophilic particles that may be used are nitrides and carbides of Group IV elements of the periodic tables.

[098] The hydrophilic particles preferably have a D50 particle size of 0.05 to 2.0 pm, more preferably of 0.1 to 1.5 pm, most preferably of 0.15 to 1.00 pm, particularly preferred of 0.2 to 0.75 pm. The D50 particle size is preferably less than or equal to 0.7 pm, preferably less than or equal to 0.55 pm, more preferably less than or equal to 0.40 pm.

[099] The D50 particle size is also known as the median diameter or the medium value of the particle size distribution. It is the value of the particle diameter at 50% in the cumulative distribution. For example, if D50 = 0.1 urn, then 50% of the particles are larger than 1.0 urn, and 50% are smaller than 1.0 urn.

[0100] The D50 particle size is preferably measured using laser diffraction, for example using a Mastersizer from Malvern Panalytical.

[0101 ] The amount of the hydrophilic particles relative to the total dry weight of the porous layer is preferably at least 50 wt%, more preferably at least 75 wt%.

[0102] The weight ratio of hydrophilic particles to polymer resin is preferably more then 60/40, more preferably more than 70/30, most preferably more than 75/25.

Preparation of the separator

[0103] A preferred preparation method of a separator according to a first embodiment comprises the steps of:

- applying a dope solution as described below on a side of a porous support(100); and

- performing phase inversion on the applied dope solution thereby forming a a porous layer (200).

[0104] The applied dope solution preferably completely impregnates the porous support before performing the phase inversion.

[0105] A preferred method of manufacturing a reinforced separator is disclosed in EP-A 1776490 and W02009/147084 for symmetric separators and EP-A 3652362 for asymmetric separators. These methods result in web-reinforced separators wherein the web, i.e. the porous support, is nicely embedded in the separator, without appearance of the web at a surface of the separator.

[0106] Other manufacturing methods that may be used are disclosed in EP-A 3272908, EP-A 3660188 and EP-A 3312306.

Dope solution

[0107] The dope solution preferably comprises a polymer resin as described above, hydrophilic particles as described above and a solvent.

[0108] The solvent of the dope solution is preferably an organic solvent wherein the polymer resin can be dissolved. Moreover, the organic solvent is preferably miscible in water. [0109] The solvent is preferably selected from N-methyl-pyrrolidone (NMP), N-ethyl-pyrrolidone (NEP), N-butyl-pyrrolidone (NBP), N,N-dimethyl- formamide (DMF), formamide, dimethylsulfoxide (DMSO), N,N-dimethyl- acetamide (DMAC), acetonitrile, and mixtures thereof.

[0110] Highly preferred solvents, for health and safety reasons, are N-butyl- pyrrolidone (NBP) and methyl 5-(dimethylamino)-2-methyl-5-oxopentanoate.

[0111] The type of solvent used may have an influence on the lateral bubble point.

[0112] The dope solution may further comprise other ingredients to optimize the properties of the obtained polymer layers, for example their porosity and the maximum pore diameter at their outer surface.

[0113] The dope solution preferably comprises an additive to optimize the pore size at the surface and inside of the porous layer. Such additives may be organic or inorganic compounds, or a combination thereof.

[0114] Organic compounds which may influence the pore formation in the porous layers include polyethylene glycol, polyethylene oxide, polypropylene glycol, ethylene glycol, tripropylene glycol, glycerol, polyhydric alcohols, dibutyl phthalate (DBP), diethyl phthalate (DEP), diundecyl phthalate (DUP), isononanoic acid or neo decanoic acid, polyvinylpyrrolidone, polyvinyl- alcohol, polyvinylacetate, polyethyleneimine, polyacrylic acid, methylcellulose and dextran.

[0115] Preferred organic compounds which may influence the pore formation in the porous layers are selected from polyethylene glycol, polyethylene oxide and polyvinylpyrrolidone.

[0116] A preferred polyethylene glycol has a molecular weight of from 10000 to 50 000, a preferred polyethylene oxide has a molecular weight of from 50000 to 300000, and a preferred polyvinylpyrrolidone has a molecular weight of from 30000 to 1 000000.

[0117] A particularly preferred organic compound which may influence the pore formation in the porous layers is glycerol.

[0118] The amount of compounds which may influence the pore formation is preferably between 0.1 and 15 wt%, more preferably between 0.5 and 5 wt% relative to the total weight of the dope solution.

[0119] Inorganic compounds which may influence the pore formation include calcium chloride, magnesium chloride, lithium chloride and barium sulfate.

[0120] A combination of two or more additives that influence the pore formation may be used.

[0121] The dope solutions provided on either side of the porous support may be the same or different.

Applying the dope solution

[0122] The dope solution may be applied on a side of a porous support by any application technique.

[0123] A well know technique includes the impregnation of a porous support in a dope solution that has been applied on a temporary support. After a phase separation step, the temporary support is removed resulting in a separator (1) comprising a porous suppor

[0124] t (100) and a porous layer (200) provided on a side of the porous support.

[0125] The dope solution is preferably applied on a side of a porous support by any coating or casting technique.

[0126] A preferred coating technique is extrusion coating. [0127] In a highly preferred embodiment, the dope solutions are applied by a slot die coating technique wherein two slot coating dies (Figures 7 and 8, 600 and 600’) are located on either side of a porous support.

[0128] The slot coating dies are capable of holding the dope solution at a predetermined temperature, distributing the dope solutions uniformly over the support, and adjusting the coating thickness of the applied dope solutions.

[0129] The viscosity of the dope solutions measured at a shear rate of 100 s ¹ and a temperature of 20°C is at least 20 Pa.s, more preferably at least 30 Pa.s, most preferably at least 40 Pa.s.

[0130] The dope solutions are preferably shear-thinning. The ratio of the viscosity at a shear rate of 1 s ¹ to the viscosity at a shear rate of 100 s ¹ is preferably at least 2, more preferably at least 2.5, most preferably at least 5.

[0131] The porous support is preferably a continuous web, which is preferably transported downwards between the slot coating dies (600, 600’) as shown in Figures 7 and 8.

[0132] Immediately after the application, the porous support becomes impregnated with the dope solutions.

[0133] Preferably, the porous support becomes fully impregnated with the applied dope solutions.

[0134] When a dope solution is applied only on one side of the porous support, a single slot coating die located on one side of the porous support may be used.

Phase inversion step

[0135] After applying a dope solution onto a porous support, the applied dope solution is subjected to phase inversion. In the phase inversion step, the applied dope solution is transformed into a porous layer.

[0136] In a preferred embodiment, both dope solutions applied on a porous support are subjected to phase inversion.

[0137] Any phase inversion mechanism may be used to prepare the porous layers from the applied dope solutions.

[0138] The phase inversion step preferably includes a so-called Liquid Induced

Phase Separation (LIPS) step, a Vapour Induced Phase Separation (VIPS) step or a combination of a VIPS and a LIPS step. The phase inversion step preferably includes both a VIPS and a LIPS step.

[0139] Both LIPS and VIPS are non-solvent induced phase-inversion processes.

[0140] In a LIPS step the porous support coated with the dope solution(s) is contacted with a non-solvent that is miscible with the solvent of the dope solution.

[0141] Typically, this is carried out by immersing the porous support coated with the dope solutions into a non-solvent bath, also referred to as coagulation bath.

[0142] The non-solvent is preferably water, mixtures of water and an aprotic solvent selected from the group consisting of N-methylpyrrolidone (NMP), dimethylformamide (DMF), dimethylsulfoxide (DMSO) and dimethyl- acetamide (DMAC), water solutions of water-soluble polymers such as PVP or PVA, or mixtures of water and alcohols, such as ethanol, propanol or isopropanol.

[0143] The non-solvent is most preferably water.

[0144] The temperature of the coagulation bath is preferably between 20 and 90°C, more preferably between 40 and 70°C. [0145] The transfer of solvent from the coated polymer layer towards the non solvent bath and of non-solvent into the polymer layer leads to coagulation of the polymer resin and the formation of a three-dimensional porous polymer network.

[0146] In a preferred embodiment, the continuous web (100) coated with a dope solution is transported downwards, in a vertical position, towards the coagulation bath (800) as shown in Figures 7 and 8.

[0147] In a VIPS step, the porous support coated with the dope solution(s) is exposed to non-solvent vapour, preferably humid air.

[0148] Preferably, the coagulation step included both a VIPS and a LIPS step.

Preferably the VIPS step is carried out before the LIPS step. In a particular preferred embodiment, the porous support coated with the dope solutions is first exposed to humid air (VIPS step) prior to immersion in a water bath (LIPS step).

[0149] In the manufacturing method shown in Figure 7, VIPS is carried out in the area 400, between the slot coating dies (600, 600’) and the surface of the non-solvent in the coagulation bath (800), which is shielded from the environment with for example thermal isolated metal plates (500).

[0150] The extent and rate of water transfer in the VIPS step can be controlled by adjusting the velocity of the air, the relative humidity and temperature of the air, as well as the exposure time.

[0151] The exposure time may be adjusted by changing the distance d between the slot coating dies (600, 600’) and the surface of the non-solvent in the coagulation bath (800) and/or the speed with which the elongated web 100 is transported from the slot coating dies towards the coagulation bath.

[0152] The relative humidity in the VIPS area (400) may be adjusted by the temperature of the coagulation bath and the shielding of the VIPS area (400) from the environment and from the coagulation bath.

[0153] The speed of the air may be adjusted by the rotating speed of the ventilators (420) in the VIPS area (400).

[0154] The VIPS step carried out on one side of the separator and on the other side of the separator, resulting in the second porous polymer layer, may be identical (Figure 7) or different (Figure 8) from each other.

[0155] After the phase inversion step, preferably the LIPS step in the coagulation bath, a washing step may be carried out.

[0156] After the phase inversion step, or the optional washing step, a drying step is preferably carried out.

Manufacturing of the separator

[0157] Figures 7 and 8 schematically illustrates a preferred embodiment to manufacture a separator according to the present invention.

[0158] The porous support is preferably a continuous web (100).

[0159] The web is unwinded from a feed roller (700) and guided downwards in a vertical position between two coating units (600) and (600’).

[0160] With these coating units, a dope solution is coated on either side of the web. The coating thickness on either side of the web may be adjusted by optimizing the viscosity of the dope solutions and the distance between the coating units and the surface of the web. Preferred coating units are described in EP-A 2296825, paragraphs [0043], [0047], [0048], [0060],

[0063], and Figure 1. [0161] The web coated on both sides with a dope solution is then transported over a distance d downwards towards a coagulation bath (800).

[0162] In the coagulation bath, the LIPS step is carried out.

[0163] The VIPS step is carried out before entering the coagulation bath in the VIPS areas. In Figure 7, the VIPS area (400) is identical on both sides of the coated web, while in Figure 8, the VIPS areas (400(1)) and (400(2)) on either side of the coated web are different.

[0164] The relative humidity (RH) and the air temperature in de VIPS area may be optimized using thermally isolated metal plates. In Figure 7, the VIPS area (400) is completely shielded from the environment with such metal plates (500). The RH and temperature of the air is then mainly determined by the temperature of the coagulation bath. The air speed in the VIPS area may be adjusted by a ventilator (420).

[0165] In Figure 8 the VIPS areas (400(1 )) and (400(2)) are different from each other. The VIPS area (400(1)) on one side of the coated web including a metal plate (500(1)) is identical to the VIPS area (400) in Figure 7. The VIPS area (400(2)) on the other side of the coated web is different from the area (400(1)). There is no metal plate shielding the VIPS area (400(2)) from the environment. However, the VIPS area (400(2)) is now shielded from the coagulation bath by a thermally isolated metal plate (500(2)). In addition, there is no ventilator present in the VIPS area 400(2). This results in a VIPS area (400(1)) having a higher RH and air temperature compared to the RH and air temperature of the other VIPS area (400(2)).

[0166] A high RH and/or a high air speed in a VIPS area typically result in a larger maximum pore diameter.

[0167] The RH in one VIPS area is preferably above 85%, more preferably above 90%, most preferably above 95% while the RH in another VIPS area is preferably below 80%, more preferably below 75%, most preferably below 70%.

[0168] After the phase separation step, the reinforced separator is then transported to a rolled up system (750).

[0169] A liner may be provided on one side of the separator before rolling up the separator and the applied liner.

Electrolvser

[0170] The separator for alkaline water electrolysis according to the present invention may be a used in an alkaline water electrolyser.

[0171] An electrolysis cell typically consists of two electrodes, an anode and a cathode, separated by a separator. An electrolyte is present between both electrodes.

[0172] When electrical energy (voltage) is supplied to the electrolysis cell, hydroxyl ions of the electrolyte are oxidized into oxygen at the anode and water is reduced to hydrogen at the cathode. The hydroxyl ions formed at the cathode migrate through the separator to the anode. The separator prevents mixing of the hydrogen and oxygen gases formed during electrolysis.

[0173] An electrolyte solution is typically an alkaline solution. Preferred electrolyte solutions are aqueous solutions of electrolytes selected from sodium hydroxide or potassium hydroxide. Potassium hydroxide electrolytes are often preferred due to their higher specific conductivity. The concentration of the electrolyte in the electrolyte solution is preferably from 20 to 40 wt%, relative to the total weight of the electrolyte solution. The temperature of the electrolyte solution is preferably from 50°C to 120°C, more preferably from 75°C to 100°C.

[0174] An electrode typically include a substrate provided with a so-called catalyst layer. The catalyst layer may be different for the anode, where oxygen is formed, and the cathode, where hydrogen is formed.

[0175] Typical substrates are made from electrically conductive materials selected from the group consisting of nickel, iron, soft steels, stainless steels, vanadium, molybdenum, copper, silver, manganese, platinum group elements, graphite, and chromium. The substrates may be made from an electrically conductive alloy of two or more metals or a mixture of two or more electrically conductive materials. A preferred material is nickel or nickel- based alloys. Nickel has a good stability in strong alkaline solutions, has a good conductivity and is relatively cheap.

[0176] The catalyst layer provided on the anode preferably has a high oxygen generating ability. The catalyst layer preferably includes nickel, cobalt, iron, and platinum group elements. The catalyst layer may include these elements as elemental metals, compounds (e.g., oxides), composite oxides or alloys made of multiple metal elements, or mixtures thereof. Preferred catalyst layers include plated nickel, plated alloys of nickel and cobalt or nickel and iron, complex oxides including nickel and cobalt such as LaNiCb, LaCoCb, and N1C02O4, compounds of platinum group elements such as iridium oxide, or carbon materials such as graphene.

[0177] The Raney nickel structure is formed by selectively leaching aluminium or zinc from a Ni-AI or Ni-Zn alloy. Lattice vacancies formed during leaching result in a large surface area and a high density of lattice defects, which are active sites for the electrocatalytic reaction to take place.

[0178] The catalyst layer may also include organic substances such as polymers to improve the durability and the adhesion towards the substrate.

[0179] The catalyst layer provided on the cathode preferably has a high hydrogen generating ability. The catalyst layer preferably includes nickel, cobalt, iron, and platinum group elements. To realize the desired activity and durability, the catalyst layer may include a metal, a compound such as an oxide, a complex oxide or alloy composed of a plurality of metal elements, or a mixture thereof. A preferred catalyst layer is formed from Raney Nickel;

Raney alloys made of combinations of multiple materials (e.g. nickel and aluminium, nickel and tin); porous coatings made by spraying nickel compounds or cobalt compounds by plasma thermal spraying; alloys and composite compounds of nickel and an element selected from cobalt, iron, molybdenum, silver, and copper, for example; elementary metals and oxides of platinum group elements with high hydrogen generation abilities (e.g. platinum and ruthenium); mixtures of elementary metals or oxides of those platinum group element metals and compounds of another platinum group element (e.g. iridium or palladium) or compounds of rare earth metals (e.g. lanthanum and cerium); and carbon materials (e.g. graphene).

[0180] For providing higher catalyst activity and durability, the above described materials may be laminated in a plurality of layers, or may be contained in the catalyst layer. [0181] An organic material, such as a polymer material, may be contained for improved durability or adhesiveness to the substrate.

[0182] In a so-called zero gap electrolytic cell (900) the electrodes (cathode 950 and anode 950’) are placed directly in contact with the separator (1) thereby reducing the space between both 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 gases formed. It has been observed such zero gap electrolytic cells operate at higher current densities. Such a zero gap electrolytic cell is schematically shown in Figure 9.

[0183] A typical alkaline water electrolyser include several electrolytic cells, also referred to stack of electrolytic cells, described above.

EXAMPLES

Materials

[0184] All materials used in the following examples were readily available from standard sources such as ALDRICH CHEMICAL Co. (Belgium) and ACROS (Belgium) unless otherwise specified. The water used was deionized water.

[0185] Fabric 1 is a PPS fabric including 60 ppm residual solvent of which 33 ppm from N-methylpyrrolidone, N-ethylpyrrolidone, N-butylpyrrolidone and chloro- N-methylaniline.

[0186] Fabric 2 is a PPS fabric including 149 ppm residual solvent of which 96 ppm from N-methylpyrrolidone, N-ethylpyrrolidone, N-butylpyrrolidone and chloro- N-methylaniline.

[0187] Fabric 3 is a PEEK fabric.

[0188] Fabric 4 is a polyarylene fabric.

Measurements

Laterial Bubble Point

[0189] The method to measure the lateral Bubble Point is schematically depicted in Figure 9.

[0190] The separator was dried with a Mettler moisture analyzer until its weight remained constant for at least 2 minutes and punched to the correct outer circular size (300). Thereafter a second punch with smaller diameter (10 mm) was used to prepared a concentric hole (350) into this sample.

[0191] The washer-shaped sample (310) was covered from its bottom side by a tape or flat closed piece of circular sheet with the same outer diameter (380).

[0192] Then this bottom covered washer-shaped sample was soaked in Porofil™, a porometer wetting fluid, and placed in the measuring cell of a Porometer 3G with the "grid" (390) underneath and the rubber O-ring (370) on top. Both the porometer wetting fluid and the Porometer are commercially available from Quantachrome.

[0193] The increasing gas pressure of the porometer will now push into the lateral direction (see arrows in Figure 9) and a pore size distribution will be measured over the internal (lateral) structure.

Preparation of the separators S-1 to S-7

[0194] The separators S-1 to S-7 were prepared as schematically depicted in Figure 7 using a dope solution comprising 40 wt% polysulfone, 10 wt% Zirconium oxide and 50 wt % N-butyl pyrrolidone or N-ethyl pyrrolidone on a polymer fabric according to Table 1.

[0195] The dope solutions were coated on both sides of the polymer fabric using slot die coating technology at a speed of 3 m/m in.

[0196] The coated fabric was then transported towards a water bath kept at 65°C.

[0197] A VIPS step was carried out before entering the water bath in an enclosed area.

[0198] The coated support then entered the water bath for 2 minutes during which a liquid induced phase separation (LIPS) occurred.

[0199] The thickness of the obtained separators are shown in Table 1.

[0200] The Lateral Bubble Point (LBP) and the Bubble Point (BP) of S-1 to S-7 were measured as described above and are shown in Table 1.

Table 1

[0201] It is clear from the results of Table 1 that the residual solvent content of the polymeric fabric and the type of polymeric fabric influences the lateral bubble point, while the bubble point remains almost constant.

[0202] Also, a thinner separator also results in a higher lateral bubble point.

[0203] It has been observed that the electrolytic efficiency with separators having a lateral bubble point measured as described above of at least 0.2 bar remains sufficient during operation.