Membrane separator for electrolysis of alkaline water

Abstract

The present invention relates to a symmetric separator membrane for electrolyzing alkaline water and having a uniform pore distribution.

Description

[0001] To prevent global warming, many projects for reducing carbon dioxide emissions have emerged worldwide.

[0002] For a long time, there has been controversy surrounding sustainable alternatives to conventional hydrogen production using renewable energy and being a "zero-carbon" energy source. From the movement of renewable energy to energy storage, this research aims to ensure that green hydrogen becomes a viable option for emissions reduction.

[0003] Green hydrogen has been the focus of international attention, aiming to achieve carbon emission reduction and energy transition goals. In addition to meeting the requirements of low-carbon emission thresholds, green hydrogen is also produced using renewable energy sources (such as photovoltaic, wind, or hydropower energy).

[0004] In the latter activity, fuel cells are used in various applications, such as in vehicles and power systems. Today, the most common industrial hydrogen production process is the steam reforming process. However, this method has problems of emitting carbon dioxide and other pollutants due to its use of fossil gases as resources. Therefore, alkaline water electrolysis using renewable energy as the driving force is attracting worldwide attention as a means of hydrogen supply because it does not emit carbon dioxide simultaneously.

[0005] In the alkaline water electrolysis process,

[0006] The alkaline electrolyte enters the anode and cathode regions on both sides of the membrane, and water molecules can pass through the membrane to the other side. Due to the electric current, the water molecules in the electrolyte in the cathode region combine with electrons to form molecular hydrogen and hydroxide ions. In addition, in the anode region, the hydroxide ions lose electrons to produce oxygen and water. Due to the obstruction of the permeable membrane, the gases generated by electrolysis cannot pass through the separator to the other side, and the generated gases are discharged from the processing chamber together with the electrolyte.

[0007] In an alkaline water electrolysis system, one of the key materials affecting hydrogen production efficiency is the diaphragm or separator.

[0008] The separator used for alkaline water electrolysis needs to:

[0009] - Have high resistance to the cross-permeation of hydrogen and oxygen; and

[0010] - Have low ohmic resistance and high ionic conductivity generated.

[0011] In addition, considering that alkaline water electrolysis operates under ultra-extreme conditions (such as high temperature and high alkaline concentration), few separators can be used for alkaline water electrolysis.

[0012] Previously used separators were usually made of asbestos mesh. This material is now prohibited in most countries worldwide, making it mandatory to produce separators without this carcinogenic compound.

[0013] Select a synthetic separator consisting of a supported polymer membrane.

[0014] Various types of these separator membranes are already available on the market.

[0015] Although having good properties in terms of ionic resistance and resistance to oxidative environments, these separator membranes show the drawback that they must be stored in a humid environment (see, for example, patent EP3933069A1). The drawbacks of storing the membrane in a humid environment include additional transportation costs and storage costs as well as greater operational difficulties for the separator user.

[0016] Advantageously and unexpectedly, the separator membrane obtained by the method of the present invention overcomes the drawbacks of the prior art products and can be stored dry without water. Advantageously, due to the method of obtaining the separator membrane of the present invention, the separator membrane of the present invention can be stored under these conditions. Such a method does not provide the use of water as a whole.

[0017] The fact that it can be stored without water also shows the advantage of minimizing the risk of bacterial / fungal contamination of the separator membrane.

[0018] In addition, the separator membrane obtained by the method of the present invention has additional advantages over the prior art products and the products available on the market. In fact, the separator membrane obtained by the method of the present invention provides a material with a double-layer lining, a symmetric surface structure, and a uniform porosity. This results in the improvement of the following properties compared to the prior art:

[0019] - Electrolyte absorption,

[0020] - Ohmic resistance;

[0021] - Ionic conductivity; and

[0022] - Effective gas separation.

[0023] The following is a detailed description of the present invention.

[0024] Definition - Method

[0025] Hydrophilic

[0026] The expression "hydrophilic" membrane is used to denote a membrane in which the contact angle between a liquid (e.g., a water droplet) and the surface of the polymer forming the membrane is less than 90°.

[0027] Macroporous

[0028] According to the IUPAC (International Union of Pure and Applied Chemistry) classification, if a membrane has pores with a diameter greater than 50 nm, the membrane is called macroporous (Mulder et al., "Basic Principles of Membrane Technology", Kluwer Academic Publishers, Dordrecht, 2nd edition, p. 159, 1996). In measurements using advanced porosity determination methods, the diameter of the flow pores of the target-sized membrane is in the range of 0.1 µm to 5.23 µm.

[0029] Isotropic / Anisotropic

[0030] Isotropic / symmetric membranes have a "completely uniform composition and structure"; such membranes can be porous or dense.

[0031] In contrast, anisotropic (or asymmetric) membranes consist of several layers, each with different structures and permeabilities. A typical anisotropic membrane has a relatively dense and thin surface layer supported on an open and much thicker microporous substrate (Baker et al., "Membrane Technology and Application", Wiley and Sons, 3rd edition, 2012).

[0032] BP

[0033] The points where air flow can be seen indicate that the pressure is high enough to eject the liquid from the largest pores. The pressure at which a constant bubble flow appears in this test is the bubble point pressure (ASTM F316–03; Standard Test Method for Pore Size Characteristics of Membrane Filters by Bubble Point and Mean Flow Pore Testing).

[0034] Pore Size Distribution / Average Pore Size

[0035] By performing advanced dehumidification of the pores via the permanent porosity determination method using gas / liquid, information on various parameters for measuring the pore size can be obtained.

[0036] The pore size can actually be defined as "at the bubble point" and "at the average value" (Dapeng et al., "Characterization of nanofibrous membranes with capillar flow porometry", Journal of Membrane Science, Elsevier, Vol. 286, Nos. 1-2, pp. 104-114, 2006; M. Khayet and T. Matsuura "Membrane Distillation: Principles and Applications" Elsevier 2011, Chapter 8 - Membrane characterisation MD).

[0037] To measure the parameters related to the pore size, in the membranes according to the invention, the WET-UP / DRY-DOWN porosimetry method was used, using a certified advanced capillary flow porometer (obtained from PMI Corporation). In this method, first, a membrane of a defined area is immersed in a suitable wetting liquid and placed under an increasing gas pressure. For each applied gas pressure, the flow rate of the gas volume is measured. The point at which the gas flow can be measured indicates that the pressure is high enough to eject the liquid from the largest pores (bubble point pressure); this pressure is used to define the largest pore size, called "at the bubble point". As the pressure is further increased, the liquid is even ejected from the smallest pores and the flow rate increases until all the pores are emptied.

[0038] In the second part of the characterization, the gas flow rate is measured as a function of the pressure in the dry membrane. The point at which the flow rate through the wet sample is the same as that through the dry sample is the smallest pore size. The average pore size "at the average flow rate" at 50% of the "wet flow rate" (during the wet measurement) is determined relative to the gas flow rate through the dry membrane (dry) at the same pressure.

[0039] The pore size of the membranes prepared using the method according to the invention at the average flow rate is from 0.1 μm to 5.2 μm. By comparing the dry flow rate with the wet flow rate at each pressure (corresponding to a specific porosity ratio according to Laplace's equation), the pore size distribution can be obtained.

[0040] Ohmic Resistance / Ionic Conductivity

[0041] Regarding the measurement of membrane resistance, a linear sweep voltage (potentiodynamic sweep) is usually used. If the voltage (V) is traced relative to the current (A), the result of the voltage sweep is a straight line, the slope of which is the resistance (Ω) (the voltage sweep is reported in Figure 3 ).

[0042] To determine the calculated membrane resistance and conduction resistance, the resistance R must be measured precisely. 电池 ) This resistance of the target test environment can be determined by assembling a cell without the sample to be tested. R 电池 is a function of the conductivity of the solution under discussion

[0043] R 测量的 = R 样品 + R 电池

[0044] The conductivity of the sample is:

[0045]

[0046] (Nourani et al., “Elucidating Effects of Faradaic Imbalance on Vanadium Redox Flow Battery Performance: Experimental Characterization”; Journal of The Electrochemical Society Vol. 166, No. 15, 2019)

[0047] Antioxidant Property

[0048] Chemical stability under alkaline and oxidative means is an important characteristic in the practical application of the membranes of the present invention. The oxidative stability of the samples of the present invention and parallel samples was estimated by using Fenton's reagent by observing the weight loss of the membranes.

[0049] Fenton's reagent can generate free radicals and cause significant degradation of the membranes. The oxidative stability of the AWE membranes was carried out in 50 ppm of Fe 2 + , 5% H2O2 at pH = 3 at room temperature. After 72 hours, a gradual weight reduction of the membranes was observed over time. The presence of cracks or dissolution of the membranes could also be analyzed. (Based on the method of the article by Zhanga et al., “Development of a high-performance anion exchange membrane using poly(isatin biphenylene) with flexible heterocyclic quaternary ammonium cations for alkaline fuel cells”, Journal of Materials Chemistry A, No. 12, 2019 and internal verification)

[0050] Scanning Electron Microscope (SEM)

[0051] An intuitive view of the morphological structure of the membrane can be obtained by means of a scanning electron microscope (SEM). The upper and lower surfaces of the membrane according to the present invention are examined.

[0052] Double - sided Tape Casting

[0053] This is a method known and commonly used by those skilled in the art, which can be carried out vertically or horizontally using a doctor blade, die coating or dip coating with a double-edged knife. The coating solution is then immersed in a coagulation bath for the conversion step.

[0054] Open Area

[0055] The open area is the total area of the openings divided by the total area of the material, expressed as a percentage. BRIEF DESCRIPTION OF THE DRAWINGS

[0056] Figure 1 ： Figure 1 (1.1, 1.2, 1.3 and 1.4) report images of both sides (designated as "top" and "bottom") obtained by SEM of two samples (symmetric GVS1 and GVS 4) of a separator membrane obtained using the method of the present invention.

[0057] Figure 2 ： Figure 2 (2.1 and 2.2) report images of both sides (designated as "top" and "bottom") obtained by SEM of samples of a commercially available separator membrane.

[0058] Figure 3 ： Figure 3 A voltage scan graph is reported.

[0059] Figure 4 ： Figure 4 Results of a simulation test of the aging and deterioration of the membrane of the subject matter of the present invention are reported.

[0060] For this test, samples of the present invention (GVS) were considered and compared with samples currently available on the market (competitive product 3).

[0061] The following is a detailed description of the present invention. DETAILED DESCRIPTION

[0062] In one embodiment, the present invention relates to a symmetric separator membrane for alkaline water electrolysis having a uniform pore distribution, said symmetric separator membrane being obtained by the following method:

[0063] a) dissolving a thermoplastic polymer in a dispersion comprising an inorganic filler and an organic solvent,

[0064] Wherein:

[0065] - The thermoplastic polymer is selected from: polysulfone (PSU), polyethersulfone, polyphenylene sulfide, polyetheretherketone (PEEK), polyurethane (PU), polyvinylidene fluoride (PVDF), polyacrylonitrile (PAN), polyvinyl alcohol (PVA), polyvinyl acetate (PVAc);

[0066] - The inorganic filler is selected from: zirconia, zirconium hydroxide, yttrium-doped zirconia, magnesium oxide, magnesium hydroxide, titanium oxide, titanium hydroxide, and barium sulfate;

[0067] - The organic solvent is selected from: dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), N-ethylpyrrolidone (NEP), N-butylpyrrolidone (NBP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO);

[0068] And wherein the components are present in the following ranges:

[0069] - Thermoplastic polymer: 7% to 18% (weight / weight);

[0070] - Inorganic filler: 20% to 35% (weight / weight);

[0071] - Organic solvent: 48% to 72% (weight / weight)

[0072] Wherein the sum of the components is equal to 100% (weight / weight)

[0073] b) Degasify the solution obtained in point a);

[0074] c) Produce a film by applying the solution obtained in step b) to a permeable medium positioned in the center using the "double-sided casting" technique in a coagulation bath,

[0075] Wherein:

[0076] - The permeable medium is selected from: paraphenylene sulphide (PPS), polypropylene, polyethylene, polyetheretherketone (PEEK),

[0077] - The coagulation bath consists of a solvent and an alcohol, wherein:

[0078] o The solvent is selected from: dimethylacetamide (DMAc), N-methylpyrrolidone (NMP), N-ethylpyrrolidone (NEP), N-butylpyrrolidone (NBP), dimethylformamide (DMF), dimethyl sulfoxide (DMSO);

[0079] o The alcohol is selected from: ethanol, isopropanol, methanol;

[0080] d) Wash the membrane obtained in point c) with alcohol.

[0081] e) Dry the membrane obtained in point d).

[0082] Wherein the specific surface area resistance of the separator membrane measured at room temperature and a KOH concentration of 30% is in the range of 0.03 Ω·cm2 to 0.3 Ω·cm2.

[0083] Preferably, the separator membrane is stored in a dry environment without degrading its mechanical properties and electrochemical performance.

[0084] Preferably, the inorganic filler in point a) is selected from: zirconia, yttrium-doped zirconia.

[0085] Preferably, the permeable medium in point c) is polyphenylene sulfide (PPS) having a thickness of 60 μm to 450 μm and an open area of 40% to 60%.

[0086] Preferably, the permeable medium is a "woven" type permeable medium.

[0087] Preferably, in the separator membrane, the pore size is in the range of 0.1 μm to 5.2 μm (according to the WET-UP / DRY-DOWN method).

[0088] Preferably, in the separator membrane, in the oxidation stability test by the Fenton reaction (pH = 3; 5% H2O2 50 ppm Fe 2+ ), the weight loss within the first 96 hours is in the range of 3% to 5%.

[0089] Preferably, before use, the thermoplastic polymer is placed in an oven overnight (8 hours) to remove any water that may have been absorbed during storage (standard laboratory practice).

[0090] Preferably, in the dispersion of zirconia (ZrO2) in a solvent in point a), all of the ZrO2 and a part of the DMAc are used to concentrate the weight percentage of the filler to 42% during the dispersion step. Specifically, a weight ratio of ZrO2 / DMA of 42 / 58 is used. To reach the final % in the Zr solution, the remaining DMA is added after the dispersion step.

[0091] Preferably, the dispersion in point a) is carried out according to techniques known to those skilled in the art, such as using a ball mill, a rotor-stator, a high-shear mixer, etc.

[0092] Preferably, the dissolution of point a) is carried out by stirring the solution for a period of 2 hours to 8 hours, preferably 6 hours. After all the PSU has dissolved and a homogeneous white dispersion has been produced, the solution is considered to be "completed".

[0093] Preferably, the degassing of point b) is carried out for a period of 0.5 hours to 3 hours, preferably 1 hour.

[0094] In such a degassing step, the solution is left standing to allow the bubbles to escape from the solution, thus avoiding defects during deposition on the medium.

[0095] Preferably, the thickness of the permeable medium of point c) is in the range of 60 um to 450 um.

[0096] Preferably, the temperature of the coagulation bath is room temperature (RT, 25 °C).

[0097] The "double-sided casting" technique allows for the obtaining of a symmetric membrane on both sides: a product with polymer on both sides and a medium at the center. This technique provides the use of NIPS (non-solvent induced phase separation). In this method, a liquid solution is transformed into a solid-state membrane by backmixing (where the solvent in the solution moves in the coagulation bath and the non-solvent moves from the coagulation bath to the solution).

[0098] Preferably, the residence time in the coagulation bath of point c) is in the range of 5 minutes to 15 minutes.

[0099] Preferably, the washing of the separator membrane of point d) is carried out for a period of 5 minutes to 15 minutes.

[0100] Preferably, the washing is carried out under static or dynamic conditions.

[0101] Preferably, the drying of point e) is carried out at a temperature of 50 °C to 100 °C, preferably 80 °C for a period of 5 minutes to 10 minutes, preferably 5 minutes to 7 minutes.

[0102] Advantageously, the separator membrane of the present invention can be stored without water, especially due to its implementation process which advantageously does not involve the use of water.

[0103] The characterization of the separator membrane of the present invention is carried out using the following techniques: SEM, feeler gauge, porosity determination, BP, electrolyte absorption, H-cell (where the ohmic resistance is measured, from which the ionic conductivity and the area resistance are obtained), solution density, oxidation stability.

[0104] Advantageously, the separator membranes obtained by the method of the present invention are capable of ensuring the production of pure gas due to the uniform pore distribution.

[0105] Advantageously, the separator membranes obtained by the method of the present invention exhibit high electrolyte retention due to the symmetrical morphology on the product surface and the spongy interior.

[0106] Advantageously, due to the uniform distribution of nanoparticles with <di 40 nm, the separator membranes of the present invention have high ionic conductivity and low ohmic resistance.

[0107] Advantageously, the separator membranes of the present invention allow dry storage, limiting all problems associated with products stored in a humid environment.

[0108] Advantageously, the separator membranes of the present invention exhibit high antioxidant degradation resistance, with a weight loss of <5% in a highly oxidative environment.

[0109] Advantageously, the method shown in the present invention allows the avoidance of using water in all steps for preparing the separator membranes, and the amount of inorganic filler used is reduced due to its nanoscale size compared to the prior art.

[0110] The following are some non-limiting exemplary experiments, which are intended to better describe the technical aspects and advantages of the present invention.

[0111] Example

[0112] The ZrO2 used in the experiment was produced by Inframat Advanced Materials, with a diameter of 20 nm to 30 nm and a purity equal to 99.9 +%.

[0113] The polysulfone used in the experiment of the present invention is Solvay.

[0114] Example 1

[0115] The symmetric membranes according to the present invention are prepared starting from a solid: liquid weight ratio of 1:1.5 (with an inorganic: polymer ratio of 3:1).

[0116] The solid part consists of a chemically resistant polymer (part of the polysulfone family (PSU or PES)) and an inorganic filler composed of zirconia.

[0117] The thickness of the obtained membranes can be in the range of 190 um to 450 um, and they are reinforced with a porous medium made of PPS.

[0118] The obtained membrane allows to have at least the same properties in terms of ionic resistance as the membranes currently available on the market, with the advantage of the fact that it can be stored and used in dry rather than wet form.

[0119] The difference between Samples 1 and 3 of the present invention lies in the type of polymer (PSU and PES), while the difference between 1 and 2 lies in the casting thickness.

[0120] Three product samples currently available on the market were also analyzed, denoted hereinafter as Competitive Product 1, Competitive Product 2, and Competitive Product 3, and the main properties are reported hereinafter:

[0121]

[0122]

[0123] Example 2

[0124] The symmetric membrane according to the present invention is prepared starting from a solid:liquid weight ratio of 1:1.5 (with an inorganic:polymer ratio of 3:1).

[0125] The solid part consists of a chemically resistant polymer (part of the polysulfone family) and an inorganic filler composed of zirconia.

[0126] The thickness of the obtained membrane can be in the range of 190 μm to 450 μm and it is reinforced with a porous medium made of PPS.

[0127] The membrane of the present invention also shows greater resistance in an oxidizing environment.

[0128] Tests were used to simulate the aging and deterioration of the membrane.

[0129] This test shows a decrease in weight over time.

[0130] For this test, the samples of the present invention (GVS) were considered and compared with the samples currently available on the market (Competitive Product 3, as defined above). The results are reported in Figure 4 in.

[0131] Example 3

[0132] The membrane according to the present invention is prepared starting from a solid:liquid weight ratio of 1:1.5 (with an inorganic:polymer ratio of 3:1).

[0133] The solid part consists of a chemically resistant polymer (part of the polysulfone family) and an inorganic filler composed of zirconia.

[0134] The thickness of the obtained membrane can be in the range of 190 um to 450 um, and it is reinforced with a porous medium made of PPS.

[0135] In the following table, the property differences of the formulations of the present invention (as reported in Example 1) when the phase separation technique varies are shown.

[0136] Specifically, new membranes are obtained using NIPS or VIPS+NIPS.

[0137]

[0138] Example 4

[0139] The symmetric membrane according to the present invention is prepared starting from a solid:liquid weight ratio of 1:1.5 (where the ratio of inorganic matter:polymer is 3:1). The solid part consists of a chemical-resistant polymer (part of the polysulfone family) and an inorganic filler composed of zirconia. The thickness of the obtained membrane can be in the range of 190 um to 450 um, and it is reinforced with a porous medium made of PPS.

[0140] The inorganic filler used in the new membrane of the present invention can be doped with % yttrium oxide.

[0141] In the following table, the resistance values obtained with the new filler are reported. The difference between sample GVS1 of the present invention (as reported in Example 1) and GVS 4 lies only in the type of inorganic filler, and specifically, whether the filler is pure zirconia or yttrium-doped zirconia.

[0142]

[0143] Example 5

[0144] The symmetric membrane according to the present invention is prepared starting from a solid:liquid weight ratio of 1:1.5 (where the ratio of inorganic matter:polymer is 3:1).

[0145] The solid part consists of a chemical-resistant polymer (part of the polysulfone family) and an inorganic filler composed of zirconia.

[0146] The thickness of the obtained membrane can be in the range of 190 um to 450 um, and it is reinforced with a porous medium made of PPS.

[0147] The membrane of the present invention is obtained by coating the same solution on both sides, thus obtaining a symmetric membrane.

[0148] Obtaining symmetric and porous membranes on both sides improves many properties, such as electrolyte absorption, ohmic resistance, and ionic conductivity.

[0149] Figure 1 and 2 reported images obtained by scanning electron microscopy (SEM) of two symmetric membranes (GVS1 and GVS4) of the present invention.

[0150] Example 6

[0151] The membranes according to the invention are prepared starting from a solid:liquid weight ratio of 1:1.5 (with an inorganic:polymer ratio of 3:1).

[0152] The solid part consists of a chemically resistant polymer (part of the polysulfone family) and an inorganic filler composed of stable or unstable zirconia.

[0153] The thickness of the membranes obtained can range from 190 μm to 450 μm and they are reinforced with a porous medium made of PPS.

[0154] The following table reports the electrolyte absorption values, considering prototypes with a thickness in the range of 190 μm to 250 μm and comparing them with competing products in the same thickness range.

[0155] Sample Electrolyte Absorption [%] Competitive Product 3 63.2 GVS1 73.9 GVS 4 74.4

Claims

1. A symmetric separator membrane for electrolyzing alkaline water with a uniform pore distribution, the symmetric separator membrane being obtained by the following method: a) Dissolving a thermoplastic polymer in a dispersion containing an inorganic filler and an organic solvent, wherein: - The thermoplastic polymer is selected from: polysulfone, polyethersulfone, polyphenylene sulfide, polyetheretherketone, polyurethane, polyvinylidene fluoride, polyacrylonitrile, polyvinyl alcohol, polyvinyl acetate; - The inorganic filler is selected from: zirconia, zirconium hydroxide, yttria-doped zirconia, magnesia, magnesium hydroxide, titanium oxide, titanium hydroxide, and barium sulfate; - The organic solvent is selected from: dimethylacetamide, N-methylpyrrolidone, N-ethylpyrrolidone, N-butylpyrrolidone, dimethylformamide, dimethyl sulfoxide; and wherein the components are present in the following ranges: - Thermoplastic polymer: 7% to 18% (weight / weight); - Inorganic filler: 20% to 35% (weight / weight); - Organic solvent: 48% to 72% (weight / weight) wherein the sum of the components is equal to 100% (weight / weight); b) Degassing the solution obtained in point a); c) Producing a membrane by applying the solution obtained in step b) to a permeable medium positioned in the center by the "double-sided casting" technique in a coagulation bath, wherein: - The permeable medium is selected from: polyphenylene sulfide, polypropylene, polyethylene, polyetheretherketone, - The coagulation bath consists of a solvent and an alcohol, wherein: o The solvent is selected from: dimethylacetamide, N-methylpyrrolidone, N-ethylpyrrolidone, N-butylpyrrolidone, dimethylformamide, dimethyl sulfoxide; o The alcohol is selected from: ethanol, isopropanol, methanol; d) Washing the membrane obtained in point c) with alcohol, e) Drying the membrane obtained in point d); wherein in the separator membrane, the specific area resistance measured at room temperature and 30% KOH concentration is in the range of 0.03 Ω·cm2 to 0.3 Ω·cm2.

2. The separator membrane according to claim 1, wherein the separator membrane is stored in a dry environment without reducing mechanical properties and electrochemical performance.

3. The separator membrane according to one or more of the preceding claims, wherein the inorganic filler in point a) is selected from: zirconia, yttria-doped zirconia.

4. The separator membrane according to one or more of the preceding claims, wherein the permeable medium in point c) is polyphenylene sulfide having a thickness of 60 μm to 450 μm and an open area of 40% to 60%.

5. The separator membrane according to one or more of the preceding claims, wherein in the separator membrane, the pore size is in the range of 0.1 μm to 5.2 μm (according to the WET-UP / DRY-DOWN method).

6. The separator membrane according to one or more of the preceding claims, wherein in the separator membrane, in the oxidation stability test by the Fenton reaction (pH = 3; 5% H2O2 50 ppm Fe2+), the weight loss within the first 96 hours is in the range of 3% to 5%.

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