Porous hydrophilic separator, its method of production and an alkaline electrolyzer with such separator

Abstract

Porous hydrophilic separator, its method of production, and an alkaline electrolyzer with such separator In an alkaline electrolyzer (12), especially for production of hydrogen gas, the separator (11) has larger pores in layers (8, 9) on its outer sides (7A, 7C), facing the electrodes (13, 14), than in the bulk layer (10). In a practical embodiment, the separator (11) is composed of two diaphragms (7, 7'), each with asymmetric pore structure, where the diaphragms (7, 7') are oriented such that largest pores are on the outer sides of the sep- arator (11).

Description

[0001] Porous hydrophilic separator, its method of production and an alkaline electrolyzer with such separator

[0002] FIELD OF THE INVENTION

[0003] The present invention relates to a porous separator for electrolysis and a method of producing the separator, as well an electrolyzer for alkaline water electrolysis, for example for hydrogen production.

[0004] BACKGROUND OF THE INVENTION

[0005] Alkaline electrolysis for production of hydrogen is constantly subject for improvement, especially with respect to the microporous separators between the electrodes. Different types of separator concepts exist, including diaphragms, ion-solvating membranes and anion exchange membranes. To this date, the most matured separators are porous hydrophilic diaphragms.

[0006] Separators for electrolysis should have high ionic conductivity and good gas separation capabilities. Porosity of the separators causes electrolyte passing through the separator, and with it also gas that is dissolved in the electrolyte. As a consequence, gas crossover from the cathode to the anode and from the anode to the cathode occurs. Gas crossover is a security concern, as the crossover of gases may result in accumulation of gas mixtures of hydrogen and oxygen that are critical with respect to explosion limits. Alkaline electrolyzers have to be shut down before such limits are reached. Furthermore, hydrogen crossing over to the oxygen-producing anode side is a loss of the desired product, and oxygen crossing over to the hydrogen-producing cathode side results in impurity of the hydrogen stream. The latter is unwanted and is usually cleared by catalytic conversion of hydrogen and oxygen to water, but the use of two moles of hydrogen per mole of oxygen that is crossing over results in further losses of the desired product.

[0007] Overall, gas crossing over, either hydrogen to the anode side of the separator or oxygen to the cathode side of the separator, causes a reduction in efficiency by the loss of electrochemically produced hydrogen. Accordingly, substantial efforts are put into production of useful separators for alkaline electrolysis.

[0008] Often used and cited as state of the art is a type of separator membrane that is commercially available by the international company AGFA® under the initial trademark Zir- fon®. Such membranes are produced from a polymer dope that is exposed to phase inversion, also called phase separation. Original production techniques involved coating of the polymer dope onto a solid support, for example glass plate, and then exposing the uncovered side of the dope to the phase inversion, prior to lifting the final separator material off the glass plate. This production method results in membranes having an asymmetrical pore size distribution with larger pores on one side than on the other. The side exposed to the phase inversion appears covered by a skin layer with much smaller pores than the pores in the bulk and the pores on the side that was covered by the glass plate during phase inversion.

[0009] In order to provide membranes with similar small pore size on opposite sides of the membrane, European patent EP2296825B1, and the corresponding US8496989, assigned to AGFA Graphics NV, disclose production methods in which both sides of a porous supporting web are impregnated with a polymer dope from opposite sides prior to phase inversion in a bath. The exposure of the dope to phase inversion simultaneously on both sides of the impregnated web results in a web-supported separator having outer sides with a skin-layer having the smaller pores as compared to the pores in the bulk. A discussion of phase inversion and corresponding asymmetry or symmetry is also found in this disclosure.

[0010] In contrast to the technologies that are used in alkaline electrolysis, polymer electrolyte membranes have been disclosed in the prior art, such as W02022 / 26400A1, with a three layer structure where the outer layers have larger pores in the range of 5-5000 micrometer than in the middle layer, which has pores in the range of 0.01-100 micrometers. However, PEM electrolysis is in nature very different from alkaline electrolysis, and other considerations apply, why Zirfon® membranes are still regarded to state of the art for alkaline electrolysis. However, despite the achievements in the field of separators in alkaline electrolysis for hydrogen production, there is still room for improvement, especially with respect to improvements of the separator and the reduction of gas crossover.

[0011] DESCRIPTION / SUMMARY OF THE INVENTION

[0012] It is therefore an objective of the invention to provide an improvement in the art. In particular, it is an objective to provide improved separators with reduced gas crossover in electrolysis for alkaline hydrogen production. This objective and further advantages are achieved with a separator that has larger pores in the range of 1 to 10 micrometer on its outer sides than in the bulk, where the average pore size is not larger than 0.3 micrometer, for example in the range of 0.03 to 0.3 micrometer. In a practical embodiment, the separator is composed of two diaphragms, each with asymmetric pore structure, where the diaphragms are oriented such that largest pores are on the outer sides of the separator.

[0013] This invention relates to alkaline electrolysis and is based on a surprising finding, that asymmetric pore structure in a diaphragm significantly reduces the gas crossover in one direction, namely from the side of the larger pores towards the smaller pores. It appears that the larger pores block for gas transport whereas the smaller pores promote transport. Accordingly, when two such diaphragms are combined such that the skin layers with the smaller pores are positioned back-to-back with the smaller pores in the center and the larger pores on the surface, the gas crossover is substantially reduced in both directions. This is surprising in that it circumvents the general belief in the prior art that the skin layer with the smaller pores should be directed outwards in alkaline electrolysis.

[0014] Details are described below.

[0015] In the following, the following abbreviations have been used for simplicity:

[0016] NIPS - nonsolvent-induced phase separation

[0017] LIPS - nonsolvent liquid induced phase separation

[0018] VIPS - nonsolvent vapor induced phase separation EIPS - evaporation induced phase separation DEC - Diethyl Carbonate DMAc - Dimethylacetamide

[0019] DMC - Dimethyl Carbonate

[0020] DMF - Dimethylformamide

[0021] DMI - Dimethylisosorbide

[0022] DMSO - Dimethylsulfoxide

[0023] GVL - Gamma-valerolactone

[0024] NBP - N-Butyl-2-Pyrrolidone

[0025] NEP - N-Ethyl-2-Pyrrolidone

[0026] NMP - N-Methyl-2-Pyrrolidone

[0027] PA - Polyamide

[0028] PAI - Polyamide-Imide

[0029] PAN - Polyacrylonitrile

[0030] PE - Polyethylene

[0031] PEEK - Poly ether ether ketone

[0032] PEKK - Polyetherketoneketone

[0033] PES - Polyethersulfone

[0034] PI - Polyimide

[0035] PMP - Polymethylpentene

[0036] Polarclean™ - Pentanoic acid, 5-(dimethylamino)-2-methyl-5-oxo-, methyl ester

[0037] PP - Polypropylene

[0038] PPS - Polyphenylene sulfide

[0039] PPSU - Polyphenylsulfone

[0040] PS - Polystyrene

[0041] PSU - Polysulfone

[0042] PVP - Polyvinylpyrrolidone

[0043] SEB - Styrene-Ethylene-Butylene

[0044] SEBS - Styrene-Ethylene-Butylene- Styrene

[0045] SEM - Scanning Electron Microscope

[0046] All percentages for substances given herein are by weight (“w%”) unless indicated otherwise.

[0047] In the following, an electrolyzer is described for producing hydrogen gas by splitting water into oxygen and hydrogen by electrolysis. Generally, the electrolyzer contains an alkaline electrolyte and an anode and a cathode separated by an ion-transporting hydrophilic porous separator.

[0048] On a first side, which is facing the anode, the separator comprises a first porous layer with a first average pore size. On a second, opposite, side, which is facing the cathode, the separator comprises a second porous layer with a second average pore size. In between the first and second layers, the separator comprises a third porous layer with a third average pore size that is smaller than the first and the second average pore sizes. Accordingly, the separator has largest pores on its outer layers as compared to an inner layer.

[0049] When a web is coated from both opposite sides, as disclosed in US8496989, mentioned in the introduction, and the two opposite sides are exposed to the phase inversion, both achieve an outer skin layer, and the smallest pores of the final product are at its two surfaces and with larger pores inside the bulk towards the supporting web. In the invention, the principle of pore size asymmetry is circumvented, which brings about some advantages as discussed in more detail in the following.

[0050] In particular, it has been found in experiments that the hydrogen and oxygen crossover through an asymmetric diaphragm film is smaller in a direction from the side with the larger pores towards the side with the smaller pores as compared to the opposite direction. In other words, orienting such asymmetric diaphragm film with the larger pores towards the anode, reduces the oxygen transport through the asymmetric diaphragm film, and orienting such asymmetric diaphragm film with the larger pores towards the cathode reduced the hydrogen transport through the diaphragm film.

[0051] In particular, it was found that a combination of two such asymmetric diaphragm films back-to-back with the sides of the smaller pores facing each other has a much lower crossover of both hydrogen and oxygen than a combination of diaphragms with the sides with the larger pores facing each other. More precisely in this useful example, each of the first and second diaphragm films has an average pore size larger on its one side than on its opposite side, and the sides with the smaller average pore sizes face each other and the sides with the larger pore size face the anode and cathode, respectively. Advantageously, the first and the second diaphragms are attached to each other to form a single-piece multilayer separator.

[0052] For example, the first and second asymmetric diaphragm films are identical and have an average pore size smaller on one side than on the opposite side. The sides with the smaller average pore size are attached to each other, forming the third layer.

[0053] In more general terms, the separator comprises a combination of a first porous diaphragm and a second porous diaphragm, wherein the first porous diaphragm on its one side comprises the first layer and on its other side comprises the third layer. Further, the second diaphragm on its one side comprises the second layer, and typically also a porous layer with smaller pores on its opposite side.

[0054] In the example above, the first and second diaphragms are asymmetric in that each of them has an average pore size larger on its one side than on its opposite side and the sides with the smaller average pore sizes face each other and the sides with the larger pore size face outwards to the anode and cathode, respectively.

[0055] Following this principle, it is potentially also possible to achieve good effects if the separator comprises a combination of multiple porous diaphragms, for example three, of which a first diaphragm comprises the first layer with larger pores and a second diaphragm comprises the second layer with larger pores, and wherein the third layer is provided by a third diaphragm with smaller pores, sandwiched between the first and the second diaphragms, for example centered between the first and second diaphragm. Advantageously, the first, second, and third diaphragms are attached to each other to form a single-piece multilayer separator.

[0056] In order to achieve a useful crossover reduction, it has been found useful if the average pore size of the third layer is at least a factor of 3, rather at least a factor of 5 smaller than the average pore size of the first and second layers. A possible production method for such diaphragm films is based on a mix of a polymer solution, a porogen, wherein the porogen is a metal salt or a polymer, and at least one hydrophilic component selected among the group of a hydrophilic polymer, an alkoxide of an inorganic metal as a precursor, and sub-micron particles of a metal oxide, metal hydroxide or metal sulfate, and then extruding the mix as a dense film, and dissolving or chemically etching the porogen in a liquid bath to create the porous hydrophilic diaphragm film. Examples for the production of dense films may include extrusion of molten polymer mixes or, alternatively, solvent-based coatings on a substrate followed by evaporation of the solvent to perform evaporation induced phase separation, EIPS.

[0057] An alternative production method is based on a mix of a polymer solution and at least one hydrophilic component selected among the group of a hydrophilic polymer, an alkoxide of an inorganic metal as a precursor, and sub-micron particles of a metal oxide, metal hydroxide or metal sulfate, and subsequently coating a supporting substrate with a film of the mix and converting the film into porous hydrophilic diaphragms by nonsolvent induced phase-separation, NIPS. For the phase inversion, a useful method is using liquid, which is known as liquid induced phase separation, LIPS. Alternatively, vapor is used, known as vapor induced phase separation, VIPS.

[0058] The term nonsolvent is used for the substance in which the polymer for the membrane, for example PSU, is not soluble, irrespective of the nonsolvent, for example water, being a solvent for other chemical compounds.

[0059] In a concrete embodiment, a polymer solution is provided by dissolving a polymer resin, for example PSU, for the porous membrane in a solvent, for example NMP. Furthermore, particles of a hydrophilic metal oxide or metal hydroxide are added for optimization of the desired hydrophilic properties, for example TiCh. Alternatively, an alkoxide of an inorganic metal is selected, for example titanium-butoxide Ti(OBu)4, as a precursor for conversion into hydrophilic metal oxide or metal hydroxide particles, for example TiCh. Alternatively or additionally, a hydrophilic polymer is added to the mix to further optimize the hydrophilicity of the diaphragm, for example Polyvinylpyrrolidone. A layer of the mix is then provided on a rigid or flexible supporting substrate, typically by casting the mix onto the substrate. Potentially, a slot-die dispenser is used for a continuous process. Then, the layer is exposed to nonsolvent-induced phase separation, NIPS, for providing pores in the polymer matrix. If a precursor is used, this is converted into metal oxide or metal hydroxide, typically sub-micrometer particles, by hydrolyzing the precursor.

[0060] Examples of the polymer resin are found among PSU, PES, PPS, PPSU, PEEK, PEKK, PP, PE, PMP, PI, PAI, PA, PAN, PS, SEB, SEBS, and mixtures thereof. Optionally, the mix contains 6w% to 26w% polymer resin, for example 8w% to 21w%, optionally 10w% to 17w%.

[0061] Optionally, the metal of the liquid precursor or the particulate metal oxide or hydroxide is selected from the group of Zr, Ti, Ce, Hf, Ba, Al, Cu, Li, Na, K, Si, V, Sc, Y, Ca, Mg, Sr, optionally from the group of Zr, Ti, Ce, Hf, Ba, Al.

[0062] In case of the use of a liquid precursor that is converted into particles during the process, candidates for the alkoxide / alcoholate for the precursor include methoxides, ethoxides, propoxides, iso-propoxides, butoxides, tert-butoxides, pentoxides, hexoxides and mixtures thereof. Typically, the concentration of the precursor in the mix is in the range of 8w% to 25w% of the mix.

[0063] An alternative production method comprises of a multi-stage NIPS treatment of a polymer mix into a diaphragm. Each NIPS treatment step produces a diaphragm with porous layers having various pore sizes. As an offset, a layer of the mix is provided on a rigid or flexible supporting substrate. Then, the layer is exposed to multiple NIPS treatment steps.

[0064] In some embodiments, the method comprises providing the separator as a single diaphragm with multiple diaphragm layers having various average pore sizes, the method comprising providing a polymer solution on a substrate, the polymer solution having a first side facing away from the substrate and a second side facing and abutting the substrate and a bulk of polymer solution in between the first and second side, and wherein the method comprises exposing only the first side to phase inversion, the phase inversion involving a first phase inversion treatment, creating first pores in the diaphragm, and at least one subsequent, different, further phase inversion treatment, wherein the further phase inversion treatment creates further pores in the diaphragm, wherein a portion of the further pores are created at the second side. This portion of further pores was not created during the first phase inversion treatment.

[0065] Advantageously, the first and further phase inversion treatments in combination provide a first porous layer having a first average pore size not smaller than 1 micrometer on the first side of the separator, for example a first average pore size having a value in the range of 1 to 10 micrometer, and / or a second porous layer having a second average pore size not smaller than 1 micrometer on as second, opposite side of the separator, for example a second average pore size having a value in the range of 1 to 10 micrometer. Furthermore, as an advantageous option, a third porous layer is provided with a third average pore size in the range of 0.03 to 0.3 micrometer in between the first and second layers.

[0066] As an alternative to multiple NIPS treatments from only one side of the layer, namely from the exposed side, it is also possible to use various NIPS on opposite sides of the layer, for example by a first NIPS treatment on a first side of the layer, then a lift-off of the layer from the substrate, followed by a second NIPS treatment on the opposite side.

[0067] For example, when using multiple phase inversion treatments, VIPS is used in a first and LIPS in a second NIPS treatment. The first NIPS treatment step provides porous layers, including the first porous layer, corresponding to the first diaphragm in the above examples, and the second NIPS treatment step provides other porous layers, including the second porous layer, corresponding to the second diaphragm in the above examples. Alternatively, three or more NIPS treatments provide multiple porous layers, similar to the three or more diaphragms in the examples above. As a result of the film formation from the same layer of polymer mix, the porous layers are produced in analogy to multiple separately produced diaphragms that are abutting each other.

[0068] As a further alternative production method, two or more polymer mixes by means of multiple coating steps on a rigid or flexible substrate provide the two or more layers of the final diaphragm by exposing the abutting coating layers to NIPS. The NIPS is done with a NIPS-agent, be it in liquid form for LIPS or in vapor form for VIPS. VIPS may further be achieved by exposing the layer to a mist of a NIPS-agent, for example by spraying techniques. The NIPS-agent comprises a NIPS-nonsolvent selected among water, methanol, ethanol, propanol, isopropanol, butanol, iso-butanol, hexanol, acetone, methyl-ethyl ketone, formic acid, acetic acid, propionic acid, butyric acid, acetylacetone, ethyl acetate, ammonium acetate, ethylenediaminetetraacetic acid, potassium hydroxide and mixtures thereof. In some instances, it has been found advantageous, if the NIPS-agent, in addition to the nonsolvent, comprises a minor portion of solvent. For example, such NIPS-solvent is selected among NMP, DMSO, NEP, NBP, DMC, DEC, DMAc, DMF, DMI, GVL, Cyrene, Polarclean™ or mixtures thereof. This portion of NIPS-solvent is less than NIPS-nonsolvent, and typically in the order of 1- 50%. In experiments, NMP was mixed with water.

[0069] The polymer mix is optionally applied onto a rigid substrate. However, for large-scale production, the casting onto a flexible substrate is advantageous, for example a belt that is guided through various stations, for example comprising a slot-die station and one or more baths. Optionally, the flexible substrate further comprises a reinforcing substrate, which becomes part of the final diaphragm.

[0070] Typically, the thickness of the final diaphragm film on the supporting substrate is in the range 100-1200 micrometer, for example 150-1000 micrometer, optionally 200-700 micrometer.

[0071] SHORT DESCRIPTION OF THE DRAWINGS

[0072] The invention will be explained in more detail with reference to the drawing, where FIG. 1 is a sketch illustrating the production process;

[0073] FIG. 2A illustrates a diaphragm film with an asymmetric pore distribution;

[0074] FIG 2B illustrates a combination of two diaphragm films of the type of FIG. 2A;

[0075] FIG. 3 is a sketch of an electrolyzer system;

[0076] FIG. 4A is a principle sketch of gas crossover for an asymmetric diaphragm film having the side with the larger pores towards the anode; FIG. 4B is a principle sketch of gas crossover for an asymmetric diaphragm film having the side with larger pores towards the cathode;

[0077] FIG. 4C is a principle sketch of gas crossover for a combination of two asymmetric diaphragm films having the side with smaller pores towards the electrodes;

[0078] FIG. 4D is a principle sketch of gas crossover for a combination of two asymmetric diaphragm films having the side with larger pores towards the electrodes;

[0079] FIG. 5 A shows measurement results for oxygen crossover through an asymmetric diaphragm film;

[0080] FIG. 5B shows measurement results for hydrogen crossover through an asymmetric diaphragm film;

[0081] FIG. 6 shows measurement results for oxygen crossover through a combination of two asymmetric diaphragm films having the side with smaller pores towards the electrodes; FIG. 7 shows measurement results for oxygen crossover through a combination of two asymmetric diaphragm films having the side with larger pores towards the electrodes;

[0082] FIG. 8A shows measurement results for oxygen crossover through an asymmetric diaphragm film of thickness 0.17 mm;

[0083] FIG. 8B shows measurement results for hydrogen crossover through an asymmetric diaphragm film of thickness 0.17 mm;

[0084] FIG. 9 shows measurement results for oxygen crossover through a combination of two asymmetric diaphragm films of thickness 0.17 mm having the side with larger pores towards the electrodes;

[0085] FIG. 10A is an SEM image of pores in a diaphragm as on the covered side;

[0086] FIG. 10B is an SEM image of pores in a diaphragm as on the side exposed to NIPS;

[0087] FIG. 11A shows measurement results for oxygen crossover through an asymmetric TiCh-containing diaphragm of thickness 0.44 mm;

[0088] FIG. 1 IB shows measurement results for oxygen crossover through a combination of two asymmetric TiCh-containing diaphragm films of 0.44 mm thickness having the side with larger pores towards the electrodes;

[0089] FIG. 12 illustrates a three-layer embodiment with three diaphragm films;

[0090] FIG. 13 A is an SEM image of pores in a diaphragm produced by multi-stage NIPS treatment on the side of the first NIPS treatment;

[0091] FIG. 13B is an SEM image of pores in a diaphragm produced by multi-stage NIPS treatment on the covered side; FIG. 14 shows measurement results for oxygen crossover through two separator samples that are produced by multi-stage NIPS treatment;

[0092] FIG. 15 shows a schematic illustration of the pore structure of a separator prepared by multi-stage NIPS treatment.

[0093] DETAILED DESCRIPTION / PREFERRED EMBODIMENT

[0094] Experimentally, porous membranes were produced according to the following method 1, which is illustrated in FIG. 1. Initially, PSU was dissolved in pure NMP in a weight ratio between 15% to 20% PSU and 85% to 80% NMP. Pure PVP was added as a hydrophilic component, with a ratio of PVP to PSU between 0.16 to 0.99 for different samples. A preferentially investigated sample contained of PVP at a ratio of 0.50. The PVP was of the type K90 with a molecular weight of 360.000 g / mol.

[0095] This polymer dope mix 2 was cast onto a clean glass plate as a supporting substrate 3, and distributed well by a doctor blade 4 to obtain a layer of well-defined height. Glass plate 3 and mix 2 were subsequently placed in a bath 5 containing a nonsolvent, in particular water, 6 to obtain a porous hydrophilic diaphragm film 7. Films of thicknesses between 160 and 500 micrometer were achieved by this method. These films were cut in order to be used as diaphragms 7 for a separator 11.

[0096] FIG. 2A illustrates such diaphragm 7 in a stylistic manner. When films for diaphragms 7 for separators 11 are produced this way by providing a respective polymer dope 2 with its first side 7 A on a support 3, such as a glass plate, and with its opposite side 7B only exposed to the phase inversion in the nonsolvent 6, the exposed side 7B has a layer 10 with smaller pores as compared to the first side 7A of the material, which is protected by the support, such as the glass plate. The exposed side 7B has a similarity to a mi- croporous skin-layer, why also this terminology of a skin-layer is often used. Accordingly, diaphragms 7 produced from such films comprises an asymmetric average pore size distribution in that it has a first layer 8 on its first side 7A having larger pores and a further layer 10 on the opposite side 7B with smaller pores.

[0097] It is emphasized herein that other asymmetric diaphragms 7 could be used as well, for example produced by the various methods described in the prior art. In particular, it should be noted that the productions presented herein were experimental, only, and for optimization, typically, also metal oxides or metal hydroxides would be added to the polymer as inorganic hydrophilic additives, which is also explained in detail in the prior art.

[0098] As will become apparent below, advantages were found when two of such diaphragms for a separator were attached to each other back-to-back with the sides 7B of the smaller pores facing each other and the sides 7A with the larger pores facing outwards, which would be facing the electrodes when incorporated in an electrolyzer. Such combination of two diaphragms 7, 7’ is illustrated in FIG. 2B. The used diaphragm films for the two diaphragms 7, 7’ are advantageous identical, but this is not strictly necessary. The resulting hydrophilic separator 11 has a first side 7A, which that in operation is facing the anode of the electrolyzer and comprises a first porous layer 8 with a first average pore size, and a second, opposite, side 7C that in operation is facing the cathode and comprises a second porous layer 9 with a second average pore size. These electrode-facing sides 7A, 7C and layers 8, 9 have the larger of the pores of the separator 11. The sides 7B of the diaphragms 7, 7’ which have the smaller pores and which face and abut each other in the combined films and which are in between the first layer 8 and the second layer 9, form a third porous layer 10, with a third average pore size. As already described, this third average pore size of the third porous layer 10 is smaller than the first and the second average pore sizes of the two outer layers 8, 9.

[0099] The experimentally produced porous hydrophilic films, as described above, were investigated by Scanning Electron Microscope (SEM), and pore sizes were found differently on opposite sides, as expected. Such pore size distribution being smaller on one side than the other is typical. Examples of photos taken in a Scanning Electron Microscope (SEM) are shown in FIG 10A on one side of the diaphragm, which was attached to the glass substrate, and FIG. 10B on the opposite side of the film, which was exposed to the NIPS. As expected, these diaphragm films show an asymmetrical pore size distribution in which one side had larger pores than the other. In particular, one side had pores in the range between 1.0 and 5.0 micrometer, while the other had pores in the range of less than 0.2 micrometer. A potential electrolyzer 12 arrangement is illustrated in FIG. 3. By applying current from a power supply 18 to the anode 13 and the cathode 14 on opposite sides of the iontransporting hydrophilic porous separator 11, the hydrogen gas is produced from the alkaline electrolyte 16 by splitting water into hydrogen and oxygen by the electrolysis process, the gases being collected in corresponding containers 19, 20.

[0100] The produced films were evaluated with respect to gas crossover. For this purpose, firstly, a single diaphragm film was used as separator, oriented with the small pore size side towards the cathode, which is stylistically illustrated in FIG. 4A. In another approach, a single diaphragm film was used as separator, oriented with the small pore size side towards the anode, see FIG. 4B. As will be confirmed below by evidence from measurements, the orientation of the diaphragm had influence of the oxygen transport and the hydrogen transport through the diaphragm in that it reduced gas transport from the side with the larger pores towards the side with the smaller pores. This is indicated by arrows with different lengths in FIG. 4, a shorter arrow indicating reduced gas flow as compared to the longer arrow.

[0101] In a further experiment, two diaphragm films were positioned against each other and used as separator. In this configuration, as illustrated in FIG. 4C, the two diaphragm films were combined so that those sides that had the larger pores were facing each other, and the small pore size sides were facing the electrodes on opposite sides of the separator.

[0102] When a web is coated from both opposite sides, as disclosed in US8496989, as mentioned in the introduction, and the two opposite sides are exposed to the phase inversion, both achieve an outer skin layer, and the smallest pores of the final product are at its two surfaces and with larger pores inside the bulk towards the supporting web. In line with this, the configuration of FIG. 4C resembles the concept as described in the aforementioned US8496989 and to the membrane marketed as Zirfon®.

[0103] As an experimental alternative, two diaphragm films were positioned against each other and the combination used as separator for electrolysis, as illustrated in FIG. 4D, where the two sides facing each other have the smaller pore. In this configuration, the large pore size sides are facing the electrodes. As indicated by the arrows in FIG. 4C and FIG. 4D, the configuration in FIG. 4D was found to have a reduced gas flow in both directions through the separator as compared to the configuration in FIG. 4C. These findings have been confirmed by experimental measurements presented below.

[0104] With the aim of improving hydrogen production by electrolysis these findings are useful and have consequences for the production, especially, if this should be a large scale continuous production. From the findings presented herein, a double sided coating of a web and phase inversion from opposite sides of the so coated web, which would result in a configuration as in FIG. 4C with the skin layer towards the electrodes, is not optimum, seeing that the experiments indicate the most desired configuration being the one shown in FIG. 4D. Instead, for a configuration as in FIG. 4D, the skin layers of the diaphragms should face each other.

[0105] For the produced diaphragm films, the gas crossover of hydrogen from the cathode side to the anode side into the oxygen (HTO), and crossover of oxygen from the anode side to the cathode side and into the hydrogen (OTH), were evaluated in electrolyzer test cells. An alkaline electrolyzer test station was used to measure the gas crossover. The product gas streams of Hz and O2 from the cathode side to the anode side and from the anode side to the cathode side, respectively, were measured by gas sensors. Each gas sensor was flushed with pure nitrogen for at least 30 minutes before each test to evaluate the zero-level of the sensors. During cell testing in 30 w% KOH at 80 G, the nitrogen stream was kept, and the hydrogen-to-oxygen, HTO, flow and the oxygen-to-hydrogen, OTH, flow were measured and recalculated accounting for the nitrogen stream. HTO and OTH were evaluated under steady-state polarization, in steps at current densities of 50, 100, 200, 300, and 400 mA / cm2, and in some instances up to 600 mA / cm2. At each step, 2 hours were passed for the system to generate a stable steady-state.

[0106] The orientation of the small pores towards the cathode, as schematically illustrated in FIG. 4A, resulted in a relatively large hydrogen flow into the oxygen on the anode side but a small flow of oxygen in the opposite direction. If the diaphragm was oriented with the small pores towards the anode, as illustrated in FIG. 4B, the opposite was observed with a larger oxygen flow towards the cathode and a smaller hydrogen flow towards the anode. Large flows in both directions were observed for the configuration as illustrated in FIG. 4C where two diaphragms were oriented with the larger pores back-to-back and the smaller pores towards the electrodes. Most surprisingly, though, was the observation with a configuration of FIG. 4D, where two diaphragms were oriented with the smaller pores back-to-back with the larger pores towards the electrodes. In this configuration, gas crossover was drastically reduced.

[0107] For a 0.4 mm thick experimentally produced diaphragm, FIG. 5A shows the flux-density measurements for oxygen crossover from the anode side into the H2 gas on the cathode side. The series 1 and series 3 correspond to the situation as illustrated in FIG. 4A with the larger pores towards the anode, and series 2 and 4 corresponds to the situation illustrated in FIG. 4B with the smaller pores towards the anode. The diaphragm was swapped from a first orientation in series 1 to a second orientation in series 2, then back to the first orientation in series 3 and once more to the second orientation in series 4 in order to check for change and reproducibility. It is clearly observed that the orientation with the larger pores towards the anode, in series 1 and 3, reduce the flow of oxygen.

[0108] For the same diaphragm, FIG. 5B shows the flux-density measurements of hydrogen crossover from the cathode side into the O2 gas on the anode side. It clearly shows that there is less H2 flux density measured for series 2 and series 4 when the larger pores are oriented towards the cathode.

[0109] The surprising finding from the measurements of FIG. 5 A and 5B were tested further by combining two 0.4 mm thick diaphragms of the same type as in FIG. 5A back-to- back with the larger pores in the center, which corresponds to the configuration in FIG. 4C. Such measurements are illustrated in FIG. 6. Series 1 and 2 refer to shift of the orientation of the double-layer combination, which, however, had no influence on the measurements, as expected, as the combination is symmetric.

[0110] Comparative measurements were made by combining two such type of diaphragms back-to-back with the smaller pores in the center and the larger pores towards the electrodes, measurements of which are illustrated in FIG. 7, corresponding to the configuration of FIG. 4D. Series 1 and 2 refer to the orientation of the double-layer combination, which had no influence on the measurements, as expected, as the combination is symmetric.

[0111] When comparing FIG. 6 to FIG. 7, it is observed that the oxygen crossover is a factor of 2 to 4 less in the configuration of FIG. 4D as compared to the configuration of FIG. 4C. For hydrogen crossover, a factor in the range of 5-10 less was observed, although with a slightly higher uncertainty in the measurements.

[0112] For a thinner diaphragm produced similarly and having a thickness of 0.175 mm, oxygen flux and hydrogen flux measurements are shown in FIG. 8A and 8B for two orientations series 1, corresponding to the configurations illustrated in FIG. 4B, and series 2, corresponding to the configurations illustrated in FIG. 4A. Comparing the dependency of the flux on the orientation with FIG. 5A and 5B, a similar directional behavior for the gas flow is observed for this thinner diaphragm. Accordingly, the one-directional effect is achieved with various thickness of the asymmetric diaphragm.

[0113] Furthermore, 2 pieces of the 0.175 mm diaphragm were sandwiched together such that the small pore sides face each other, corresponding to the configuration of FIG. 4D and repeating the measurement from FIG 7, which is illustrated in FIG 9 for oxygen. It is observed that the oxygen crossover level for the sandwich is in the order of 0.5 x 10'9mol s'1cm'1bar'1, which is similar to the reduced oxygen crossover of a single diaphragm with the large pores towards the anode, see FIG. 8A. Moreover, this level of oxygen gas crossover is similar to the level observed in FIG. 7 for the combination of two thicker diaphragms. From these similar results of reduced oxygen gas crossover when the sides with the small pores are attached to each other, it is understood that the effect is not due to the separator thickness. A similar trend was observed for the hydrogen crossover, albeit with a slightly higher uncertainty in the measurements. In particular, the same trend for drastically reduced HTO and OTH as in FIG 7 is observed, despite having an overall thickness of only 0.35 mm, as compared to the thickness of 0.8 mm for the measurements in FIG. 7. It is therefore expected that the surprising finding about drastically reduced HTO and OTH is due to the orientation of the pore sizes, and not due to the increased separator thickness by sandwiching two diaphragms. In order to further evaluate the effect, a new diaphragm containing a second hydrophilic component was produced. In particular, PSU was dissolved in pure NMP in a weight ratio between 15% to 20% PSU and 85% to 80% NMP. Pure PVP was added as a hydrophilic component with a weight ratio of PVP to PSU in the range from 0.16 to 0.99 for different samples. Furthermore, an alkoxide was added with a weight ratio of alkoxide to PSU in the range from 0.4 and 2.5. A preferentially investigated sample contained of PVP at a ratio of 0.45 relatively to PSU and of alkoxide at a weight ratio of 1.33 relatively to PSU. The PVP was of the type K90 with a molecular weight of 360.000 g / mol. The alkoxide was of the type titanium butoxide.

[0114] The diaphragm was subsequently produced in a similar fashion as described above by NIPS. The preferentially investigated sample was subject to a NIPS treatment in a nonsolvent bath containing 95% water and 5% acetic acid, measured by mass. The water in the non-solvent bath converted the alkoxide into TiCh in a hydrolysis reaction.

[0115] The oxygen flux measurements for the TiCb-containing diaphragm, experimentally produced at a thickness of 0.44 mm, are shown in FIG 11 A. Series 1 indicates the measurement which corresponds to FIG 4B, and series 2 corresponds to the configuration of FIG 4A. In a similar fashion to the diaphragms free of TiCh above, the asymmetric TiCh-containing diaphragm shows a similar one-directional crossover, in which the oxygen crossover is lowest when the side with the larger pores is facing the anode. A similar trend was observed for the hydrogen crossover, in which the hydrogen crossover is lowest when the side with the larger pores is facing the cathode.

[0116] An additional measurement with two of such diaphragms oriented with the smaller pore sides facing each other and thereby following the configuration of FIG 4D is shown for oxygen in FIG 1 IB. Series 1 and 2 refer to the orientation of the double-layer combination, which had no influence on the measurements, as expected, as the combination is symmetric. It is observed that the oxygen crossover level for the sandwich of the TiCh- containing diaphragm is in the order of 0.5 x 10'9mol s'1cm'1bar'1, which is similar to the reduced oxygen crossover of the diaphragms above not containing TiCh. A similar trend with reduced crossover was observed for the hydrogen crossover of the sandwich, thereby repeating the findings independently on the composition of the diaphragm. This once more shows the dependence of reduced crossover on the pore orientation of the diaphragm, in which the crossover is lowest when the small pores of a diaphragm are at its center.

[0117] An alternative configuration for a separator is shown in FIG. 12. In this case, the separator 11 comprises a combination of multiple porous diaphragms 7, 7’, 7”, of which a first diaphragm 7 comprises the first layer 8 and a second diaphragm 7’ comprises the second layer 9, and wherein the third layer 10 is provided by a third diaphragm 7” sandwiched between the first and the second diaphragms 7, 7’.

[0118] As a further alternative, a multilayer separator can comprise more than three diaphragm films.

[0119] In the following, an alternative production method is explained relatively to producing three separate diaphragms and combining these into a separator with three porous layers, having the smaller pore size in the bulk as compared to the surface layers. In this alternative production method, production has been demonstrated of an analogue combination of porous layers in a single piece separator, where the three diaphragms described above have been substituted by three diaphragm layers of a single diaphragm. Single-piece separators were produced, each made from a single diaphragm that had pore structures with larger outer pores and smaller inner pores, similar to the combined triple-diaphragm structure. Verifying experimental results are explained in the following.

[0120] In such experiment, multi-stage NIPS treated separators were produced. PSU was dissolved in a mixture of solvents comprising of DMSO, GVL and DEC, with a weight ratio between 15% to 20% PSU and 85% to 80% solvent mixture. Pure PVP was added as a hydrophilic component with a weight ratio of PVP to PSU in the range from 0.16 to 0.99 for different samples. Furthermore, an alkoxide was added with a weight ratio of alkoxide to PSU in the range from 0.4 and 2.5. A preferentially investigated sample contained of PVP at a ratio of 0.45 relatively to PSU and of alkoxide at a weight ratio of 1.25 relatively to PSU. The PVP was of the type K90 with a molecular weight of 360.000 g / mol. The alkoxide was of the type titanium butoxide. The first preferentially investigated sample with multi-stage NIPS treatment was cast onto a clean glass plate. In a first NIPS treatment step, the uncovered surface of the sample was exposed to a mist of a NIPS-agent comprising of 80% water, 15% isopropanol and 5% acetic acid, measured by mass, by means of an airbrush spray effecting a VIPS treatment to produce a porous layer in the sample, resembling a first diaphragm layer with varying pore size having largest pores at the surface and smaller pores in the bulk. The amount of NIPS-agent applied was between 25 g / m2 and 200 g / m2, in particular 100 g / m2 for the preferentially investigated sample. In a second NIPS treatment step, while the sample was still on the glass plate, the uncovered side of the sample was exposed to LIPS in a bath containing 80% water, 15% isopropanol and 5% acetic acid, measured by mass. This second NIPS treatment further changed the porous structure inside the sample so that various porous layers were obtained with larger pore sizes at the exposed surface and at the opposite side, which was abutting the glass plate, resulting, all in all, to similar structures as in the above examples of multiple diaphragms that abut each other. Accordingly, the second NIPS step finalized the production of the separator by producing porous layers, analogous to a second diaphragm that abuts the first diaphragm. Due to the one-sided contact of the layer to the NIPS-treatments, it is expected that the LIPS treatment formed an asymmetric second diaphragm portion underneath the VIPS formed first diaphragm portion, in which the LIPS formed second diaphragm portion produces a skinlayer that is abutting the VIPS formed first diaphragm portion. The single piece separator comprising the two diaphragm portions had a thickness of 0.31 mm.

[0121] FIG. 13 A shows SEM pictures of the first side of the separator which was treated with the mist in the first NIPS-treatment, and FIG. 13B shows SEM pictures of the second side of the separator, which was facing the supporting glass plate. The first side of the separator shows a primary set of pores with an average at the size of 1-10 micrometer and a secondary set of pores with an average of 10-60 micrometer, and the second side of the separator shows an average pore size of 1-10 micrometer. A third porous layer in the center portion of the separator could not be displayed by SEM due to the nature of a single piece separator. In order to evaluate that the pore size of the third layer within the separator is not larger than 0.3 micrometer, a set of different analysis techniques such as Computer Tomography, Mercury Intrusion Porosimetry, or Bubble Point Pressure Measurement can be used to characterize the pore size of the third layer. For the particularly investigated sample, Bubble Point Pressure Measurement was used to analyze the pore size of the third layer.

[0122] The Bubble Point Pressure is governed by the Laplace equation, with d being the maximum pore size of a porous medium, y being the surface tension of the liquid that fills the porous medium, a being the contact angle between the liquid and the porous medium, and p being the pressure required to replace the liquid in the porous medium.

[0123] For a water filled porous separator, it can be assumed that the contact angle of water and separator may not be larger than 85 degrees to ensure the function of the separator in alkaline water electrolysis. With a given surface tension of 72 mN / m for water, a measured Bubble Point Pressure of 0.84 bar or more indicates that the porous separator has a maximum pore size of 0.3 micrometer. Consequently, a separator indicating an average primary pore size of 1-10 micrometer on its first side and 1-10 micrometer on its second side and having a measured Bubble Point Pressure of 0.84 bar or more indicates that the porous separator has a maximum pore size of 0.3 micrometer or less within the bulk of the separator.

[0124] The first preferentially investigated sample was measured to have a Bubble Point Pressure of 3.25 bar, measured by means of a differential pressure over the separator with compressed air required to replace the water inside the sample with air. This measurement indicated that the maximum pore size in the bulk was not more than 0.3 micrometer.

[0125] The second preferentially investigated sample with multi-stage NIPS treatment was cast onto a clean glass plate and in a first NIPS treatment step exposed to a mist of a NIPS- agent comprising of 80% water, 15% isopropanol and 5% acetic acid, measured by mass, by means of an airbrush spray effecting a VIPS treatment to produce porous layers for the first diaphragm portion. The amount of NIPS-agent applied was between 25 g / m2 and 200 g / m2, in particular 100 g / m2 for the preferentially investigated sample. In a second NIPS treatment step, the sample was exposed to a constant flow of humid air at 52±10% relative humidity (measured at room temperature) and 80 °C for 10 minutes to produce the second diaphragm portion underneath and abutting the first diaphragm portion by a combination of VIPS and evaporation of solvent resulting in EIPS. In a third NIPS treatment step, LIPS in a bath containing 80% water, 15% isopropanol and 5% acetic acid, measured by mass, finalized the production of the separator by producing the third diaphragm portion, which abuts the second diaphragm portion. The single piece separator comprising of three diaphragms portions, in which the first diaphragm portion comprises the first porous layer facing the first side of the final separator, the third diaphragm portion comprises the second porous layer facing the second side of the final separator, and the second diaphragm portion, which is between the first and third diaphragm portions, comprises the third layer of the separator, The single piece separator comprising the three diaphragms portions, analogous to a combination of three diaphragms as illustrated in FIG 12, had a thickness of 0.22 mm. The Bubble Point Pressure was measured to 4.75 bar.

[0126] FIG. 14 shows the measurements for OTH of the first and second preferentially investigated samples by multi-stage NIPS treatment. In series 1, the separators are placed with the side of the first NIPS treatment comprising the first side towards the anode, in series 2, the substrate-covered side comprising the second side is placed towards the anode. It is indicative that the multi-stage NIPS treatment did not resolve in a fully symmetrical separator. However, as the strong asymmetric crossover as of FIG 4B and FIG 5A is not seen, and similar low levels of (0.5-2.0) x 10'9mol s'1cm'1bar'1as seen for reduced crossover previously are observed, it can be concluded that the multi-stage NIPS treatment results in a separator wherein the third layer with the smallest pores has an offset from the center of the separator positioned closer to the side of the first NIPS treatment.

[0127] FIG. 15 schematically indicates the pore structure of the separator produced by multistage NIPS treatment. Notice the larger pores towards the anode and the cathode and the smaller pores in the bulk.

[0128] These findings are surprising in that these circumvent the general belief in the prior art that the skin-layers with the smaller pores should be directed outwards.

Claims

CLAIMS1. An electrolyzer (12) for producing hydrogen gas by splitting water into oxygen and hydrogen by electrolysis, wherein the electrolyzer (12) contains an anode (13) and a cathode (14) separated by an ion-transporting hydrophilic porous separator (11), wherein the porous separator (11) on a first side (7 A), which is facing the anode (13), comprises a first porous layer (8) with a first average pore size, and wherein the porous separator on a second, opposite, side (7C), facing the cathode (14), comprises a second porous layer (9) with a second average pore size, and wherein the separator (11) comprises a third porous layer (10), in between the first and second layers (8, 9), with a third average pore size that is smaller than the first and the second average pore sizes, characterized in that the electrolyzer (12) contains alkaline electrolyte (16) and the third average pore size is not larger than 0.3 micrometer and the first and second average pore sizes are no smaller than 1 micrometer.

2. The electrolyzer according to claim 1, wherein first or second average pore sizes or both are in the range of 1 to 10 micrometer.

3. The electrolyzer according to claim 1 or 2, wherein the separator (11) comprises a combination of a first porous diaphragm (7) and a second porous diaphragm (7’), wherein the first porous diaphragm (7) on its one side (7 A) comprises the first layer (8) and on its other side (7B) comprises the third layer (10), and wherein the second diaphragm (7’) on its one side (7C) comprises the second layer (9).

4. The electrolyzer according to claim 3, wherein the first and the second diaphragms (7, 7’) are attached to each other to form a single-piece multilayer separator (11).

5. The electrolyzer according to claim 4, wherein the first and second diaphragms (7, 7’) are identical and have an average pore size smaller on one side (7B) than on the opposite side (7 A, 7C), and wherein the sides (7B) with the smaller average pore size are attached to each other, forming the third layer (10).

6. The electrolyzer according to claim 3, wherein each of the first and second diaphragms (7, 7’) have an average pore size larger on its one side (7 A, 7C) than on itsopposite side (7B), and wherein the sides (7B) with the smaller average pore sizes face each other and the sides (7A, 7C) with the larger pore size face the anode (13) and cathode (14), respectively.

7. A hydrophilic porous separator for an alkaline electrolyzer (12) according to any preceding claims, wherein the porous separator (11) on its first side (7 A) comprises a first porous layer (8) with a first average pore size, and wherein the porous separator (11) on its second, opposite, side (7C) comprises a second porous layer (9) with a second average pore size , and wherein the separator (11) in between the first and second layers (8, 9) comprises a third porous layer (10) that has a third average pore size, characterized in that the first and second average pore sizes are not smaller than 1 micrometer and the third average pore size is less than 0.3 micrometer, optionally in the range of 0.03 to 0.3 micrometer.

8. The separator according to claim 6, wherein first or second average pore sizes or both are in the range of 1 to 10 micrometer.

9. The separator according to claim 7 or 8, wherein the separator (11) comprises a combination of a first and a second porous diaphragm (7, 7’), wherein the first diaphragm (7) on its one side (7 A) comprises the first layer (8) and on its opposite side comprises the third layer (10), and wherein the second diaphragm (7’) on its one side (7C) comprises the second layer (9).

10. The separator according to claim 9, wherein the first and the second diaphragms (7, 7’) are attached to each other to form a single-piece multilayer separator (H).

11. The separator according to claim 10, wherein the first and second diaphragms (7, 7’) are identical and have an average pore size smaller on one side (7B) than on the opposite side (7 A, 7C), and wherein the sides (7B) with the smaller average pore size are attached to each other, forming the third layer (10).

12. The separator according to claim 11, wherein the separator (11) comprises a combination of multiple porous diaphragms (7, 7’, 7”), of which a first diaphragm (7)comprises the first layer (8) and a second diaphragm (7’) comprises the second layer (9), and wherein the third layer (10) is provided by a third diaphragm (7”) sandwiched between the first and the second diaphragms (7, 7).

13. A method of production of a separator according to anyone of the claims 7-12, wherein the method comprises A or B, wherein in A, the method comprises providing the separator as a multilayer of diaphragms and combining the diaphragms into a single-piece separator with a first porous layer having a first average pore size not smaller than 1 micrometer on a first side of the separator and a second porous layer having a second average pore size not smaller than 1 micrometer on as second, opposite side of the separator, and a third porous layer that has a third average pore size in the range of 0.03 to 0.3 micrometer in between the first and second layers; in B, the method comprises providing the separator as a single diaphragm with multiple diaphragm layers having various average pore sizes, the method comprising providing a polymer solution on a substrate, the polymer solution having a first side facing away from the substrate and a second side facing and abutting the substrate and a bulk of polymer solution in between the first and second side, and wherein the method comprises exposing only the first side to phase inversion, the phase inversion involving a first phase inversion treatment, creating first pores in the diaphragm, and at least one subsequent, different, further phase inversion treatment, wherein the further phase inversion treatment creates further pores in the diaphragm, wherein a portion of the further pores are created at the second side , wherein the first and further phase inversion treatments in combination provide a first porous layer having a first average pore size not smaller than 1 micrometer on the first side of the separator and a second porous layer having a second average pore size not smaller than 1 micrometer on a second, opposite side of the separator, and a third porous layer that has a third average pore size in the range of 0.03 to 0.3 micrometer in between the first and second layers.

14. The method according to claim 13, wherein the first or the second average pore size or both are in the range of 1 to 10 micrometer.

15. The method according to claim 13 or 14, wherein, in A, the method comprises producing a first porous diaphragm (7) and a second porous diaphragm (7) by phaseinversion (1) of a polymer solution, wherein each of the first and the second porous diaphragms (7, 7’) comprises larger pores with an average size in the range of 1 to 10 micrometer on its first side (7 A, 7C), and smaller pores with an average size in the range of 0.03 to 0.3 micrometer on its opposite, second side (7B) and combining the two diaphragms (7, 7’) into a separator (11) by attaching the two diaphragms (7, 7’) to each other such that the sides (7B) with the smaller pores face each other and the sides (7 A, 7C) with the larger pores face outwards from the separator (11).

16. The method of claim 15, wherein the method comprises providing a mix of a polymer solution and at least one hydrophilic component selected among the group of a hydrophilic polymer, an alkoxide of an inorganic metal as a precursor, and sub-micron particles of a metal oxide, metal hydroxide or metal sulfate, and subsequently coating a supporting substrate (3) with a film (2) of the mix and converting the film (2) into porous hydrophilic diaphragms (7) by nonsolvent induced phase-inversion.

17. The method of claim 13 or 14, wherein, in A, the method comprises providing at least three diaphragms, wherein the first diaphragm (7) comprises the first layer (8), the second diaphragm (7’) comprises the second layer (9), and the third diaphragm (7”) comprises the third layer (10), and combining the three diaphragms (7, 7’, 7”) into a single-piece separator (11) with the third diaphragm (7”) being sandwiched between the first diaphragm (7) and the second diaphragm (7’).

18. The method of claim 13 or 14 or 17, wherein, in A, the method comprises producing at least one of the diaphragms (7) by providing a mix of a polymer solution, a porogen, wherein the porogen is a metal salt or a polymer, and at least one hydrophilic component selected among the group of a hydrophilic polymer, an alkoxide of an inorganic metal as a precursor, and sub-micron particles of a metal oxide, metal hydroxide or metal sulfate, then extruding the mix as a dense film, and dissolving or chemically etching the porogen in a liquid bath to create the porous hydrophilic diaphragm (7).

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