Novel material, films and membranes produced from said material, and use of such membranes as a diaphragm in an alkaline electrolyser

Abstract

The present invention relates to a novel material comprising an organic binder consisting of a thermoplastic polymer, selected from the group consisting of polyethylene, polypropylene, polystyrene, acrylonitrile-butadiene-styrene, poly vinyl halide or poly vinylidene halide or mixtures thereof, a hydrophilic inorganic filler and a porosity agent. This material can be used for the manufacture of a film which, after treatment, will provide a membrane suitable for use as a diaphragm in an alkaline electrolyser, allowing the production of hydrogen.

Description

New material, films and membranes made from it and use of such membranes as a diaphragm of an alkaline electrolyzer.

[0001] Description

[0002] The present invention relates to the technical field of alkaline electrolysers and more particularly to membranes used as diaphragms in such an electrolyser. According to a first of its aspects, the invention relates to a new material, film and membrane made of this material as well as its use as a diaphragm for an alkaline electrolyser. Another aspect of the present invention relates to a method for manufacturing a membrane.

[0003] Indication of prior art

[0004] Dihydrogen (H2) is a particularly clean energy vector. However, it is rarely found in nature in its molecular state and must therefore be produced before use or storage. This is achieved through various processes: reforming or gasification of hydrocarbons, electrolysis of water, or thermochemical dissociation of water or biomass. Almost all of the hydrogen currently available comes from the reforming of natural gas. In the current context of reducing the use of fossil fuels, the production of hydrogen, and more specifically dihydrogen, by electrolysis seems a particularly suitable solution.

[0005] Among the different types of hydrolysis, alkaline electrolysis is the most widely used process in industry. These electrolyzers use an aqueous solution of potassium hydroxide (KOH), or even sodium hydroxide (NaOH), whose concentration varies depending on the temperature (typically from 25% by mass at 80°C to 40% at 160°C) and can produce more than 1000 Nm^ of hydrogen per hour. Potassium hydroxide is preferred to sodium hydroxide, mainly for reasons of higher ionic conductivity at equivalent temperature, and better control of impurities such as chlorides or sulfates. Electrolyzers generally include: a power supply, electrolysis cells, a water purification unit, a gas dehumidification unit, a hydrogen purification unit, a compressor and a control system. Some electrolyzer technologies operate directly under pressure.Small-capacity modules typically operate at 3 to 30 bar. The energy efficiencies of such electrolysers are in the order of 75 to 90%, with a lifetime of more than 80,000 hours of operation, up to 160,000 hours (approximately 18 years). Alkaline electrolysers operate by transporting hydroxide ions (OH') through the electrolyte from the cathode to the anode, with hydrogen being generated on the cathode side. Operating at up to 160°C, alkaline electrolysers use a liquid alkaline solution of potassium hydroxide (KOH) as the electrolyte. In this process, the anode and cathode may be separated by a diaphragm.

[0006] The diaphragm, also called the porous separator, must fulfill three functions. Firstly, it must ensure electrical insulation between the cathode and the anode, but also sealing against gases in order to avoid a mixture of the oxygen and hydrogen produced which could cause a risk of explosion, and finally it must ensure the conduction of hydroxide ions from the cathode to the anode. The use of these diaphragms in extreme conditions (concentrated alkaline solution at high temperature) significantly restricts the choice of materials which can be used in their construction.

[0007] The prior art has seen many developments in such diaphragms. Historically, the diaphragms of the first alkaline electrolyzers were asbestos sheets. These suffered from a lack of resistance caused by the highly alkaline environment at high temperatures. In addition, asbestos is classified as a carcinogenic material and its commercial use is prohibited in many countries. More recently, asbestos has been replaced by organic matrices made, for example, of polysulfone (PSU), polyphenylene sulfide (PPS) or polytetrafluoroethylene (PTFE) formed into sheets with a tight braiding. Thanks to the tight braiding, the diaphragm limits, but does not completely eliminate, the transfer of H2 and O2 gases generated during hydrolysis.The braiding and the space between the fibers is such that the purity of the gases is not optimized to the point of allowing direct use and a post-purification step which is costly in terms of time and energy is necessary.

[0008] To improve this situation, it has been proposed to use a diaphragm consisting of a membrane with finer porosity. However, the organic matrix materials used until now have a fairly pronounced hydrophobic character and the implementation of pores of a reduced dimension would be counterproductive because the hydrophilic character of the electrolyte would prevent it from wetting the pores and, ultimately, the electrolyte would not be able to pass through these pores. To overcome this drawback and increase the ionic conductivity of the diaphragm, it has been proposed to add highly hydrophilic species to the composition of the membrane material. These hydrophilic species, often based on ZrC>2, MgO, TiC^, CeC>2 or BaSO / , modify the polarity of the pores and then allow the transfer of the electrolyte. Such a membrane is known, for example, from document WO-A2-2006 / 015462.This membrane, which can be used as a diaphragm, is marketed under the name ZIRFON. However, the production method of this diaphragm involves the use of highly toxic solvents. Furthermore, the membrane is marketed at a high price, which significantly impacts the overall cost of the electrolyzer. The preparation of this membrane requires dissolving the membrane material, generally in a highly toxic solvent such as dimethylformamide, and then forming a film by evaporating this solvent. Documents CN 116 024825 A and CN 114432 905 A disclose such membranes and their preparation.

[0009] It would therefore be desirable to propose a new material capable of forming a membrane, said membrane being suitable for use as a diaphragm in an alkaline electrolyzer. Ideally, this diaphragm would have properties similar to those of the diaphragms of the prior art without, however, exhibiting the aforementioned defects.

[0010] Statement of the invention

[0011] According to the invention, this new material comprises an organic binder consisting of a thermoplastic polymer, a hydrophilic inorganic filler and a porosity agent. The thermoplastic polymer is chosen from the group consisting of polyethylene, polypropylene, polystyrene, acrylonitrile-butadiene-styrene, polyvinyl or vinylidene halide (optionally overhalogenated) or mixtures thereof.

[0012] The thermoplastic polymer has a melting point below 200°C, preferably below 190°C and even more preferably below 180°C, so it is possible to use solvents that are significantly less toxic than those used in the prior art. It is also possible to simply disperse the thermoplastic polymer without necessarily dissolving it. In addition, being able to work at low temperatures makes it possible to preserve the various additives and the support used.

[0013] This material can be formed into a film. It then constitutes a nanoporous membrane capable of being used as a diaphragm for an alkaline electrolyzer which does not have the disadvantages of known diaphragms.

[0014] The thermoplastic polymer is provided in powder form. Suitable thermoplastic polymers may be selected from the group consisting of polyethylene, polypropylene, polystyrene, acrylonitrile-butadiene-styrene, polyvinyl or vinylidene halide (optionally overhalogenated, such as C-PVC); or mixtures thereof. According to an advantageous embodiment, the thermoplastic polymer is a polyvinyl or vinylidene halide, preferably fluorinated, or polypropylene. Advantageously, the thermoplastic polymer consists of polyvinylidene fluoride, preferably a high molecular weight polyvinylidene fluoride. It is indeed preferable to use such a high molecular weight polymer, thus having a high viscosity, to prevent, during the formation of the membrane, the composition from flowing, thus generating inhomogeneities in the membrane.It is considered according to the invention that a molecular weight greater than 250k Daltons is suitable, that a molecular weight greater than 350 Daltons is preferable and that a molecular weight close to 500k Daltons is particularly suitable. A preferred example of a fluoropolymer according to the present invention is a polyvinylidene fluoride marketed by the company Solvay under the name SOLEF 1015. This polymer has a molecular weight close to 500k Daltons. According to another very advantageous embodiment, polypropylene is used.

[0015] The hydrophilic inorganic filler consists of particles of an inorganic compound resistant to very high pH and with very polarization properties favorable. It comprises a metal oxide or sulfate, preferably chosen from the following compounds: ZrO2, MgO, TiC^, CeC>2 or BaSO / preferably ZrC>2 or BaSO / According to another embodiment, talc, montmorillonite, a zeolite or halloysite-type nano-clays can be used as hydrophilic inorganic filler. The filler particles can have, for example, an average size of 1 to 5 μm. Said hydrophilic inorganic filler is distributed homogeneously in the organic binder.

[0016] The porosity agent is intended to allow, when the material is formed as a film and in a subsequent washing step with water or alkali, the formation of pores of an appropriate size in the matrix constituted by the binder. This agent disappears more or less completely during the manufacture of the membrane. Generally, it is soluble in a neutral or alkaline medium. It is, for example, an acid. For example, phosphoric acid or phosphorous acid are used. Acidic metal oxides capable of dissolving in an alkaline medium can also be used.

[0017] Another aspect of the invention relates to a method of preparing the material.

[0018] According to the method of the invention, the organic binder is dispersed in a liquid vehicle. In this case, the liquid vehicle acts as a dispersing agent for the organic binder. The hydrophilic inorganic filler and the porosity agent are also added to the liquid vehicle. In this case, the liquid vehicle acts as a solvent, suspending agent or dispersing agent for the hydrophilic inorganic filler and the porosity agent. The incorporation of these three components can be carried out in any order. Advantageously, the liquid vehicle will be chosen such that its boiling point is lower than the melting temperature of the thermoplastic polymer to allow partial melting of the organic binder during drying. Furthermore, it is also preferable that these temperatures are chosen in such a way as not to degrade the constituents of the material present, such as for example the porosity precursors.

[0019] Other ingredients can also be added to the mixture as porosity agents. These ingredients are preferably surfactants. They can be in the form of micelles in the formulation and leave a void once washed, they can trap air during mixing of the formulation and therefore naturally create empty cavities and they promote the dispersion of the different constituents in the mixture. These can be ionic surfactants (for example sodium lauryl sulfate) or non-ionic surfactants (for example a 4-octyl-phenol polyethoxylate such as TRITON X-45 or X100) or surfactants with a steric effect (for example high molecular weight alkylammonium copolymers, such as BYK9076 or BYK9077). Advantageously, ionic and non-ionic surfactants are used in combination. Water can also be added.These other ingredients can also be added in any order (before, after, or at the same time as the previous ingredients).

[0020] The liquid vehicle serves to allow the dispersion and at least partial fusion, but not the dissolution, of the organic binder and the dissolution, suspension or dispersion of all the other ingredients of the material. The inventors have observed that for a high molecular weight polyvinylidene fluoride, a very suitable liquid vehicle is l-methoxy-2-propyl acetate. It can be used alone or in a mixture with other organic compounds such as propyl, butyl or pentyl acetates. This vehicle is also suitable for most hydrophilic inorganic fillers and porosity agents as well as for any other ingredients (surfactants for example). Its relatively high boiling point ensures sufficiently slow evaporation so as not to generate excessive porosity or defects such as bubbling during drying.In addition, these vehicles ensure efficient heat transfer from the furnace air to the particles to be melted and fused. Ideally, the boiling point of the liquid vehicle is close to the melting point of the organic binder consisting of a fluoropolymer. However, it should be kept in mind that this characteristic is not absolutely essential. Indeed, it is not mandatory to achieve complete melting of the polymer.

[0021] The organic binder dispersed in the liquid vehicle containing the porosity agent and the inorganic filler as well as any other ingredients is heated until said binder melts or partially melts.

[0022] This mixture can be formed into a film by depositing it on a smooth, non-reactive support (e.g. PTFE) which is removed (and possibly recycled) after the vehicle has dried and cooled. This produces a self-contained flexible film (without support). Any suitable coating technique can be used to deposit the film. For example, a roller coating system or a slot-die controlled liquid supply can be used.

[0023] It is also possible to impregnate said mixture with an organic matrix made for example of polysulfone (PSU), polyphenylene sulfide (PPS) or polytetrafluoroethylene (PTFE) formed into a ribbon with a tight braiding which remains present in the final film. When the invention is implemented in this configuration (new material absorbed onto an organic matrix), it is preferable to use a thermoplastic polymer having a lower melting temperature than that of the material constituting the matrix. A supported flexible film with excellent mechanical strength is then obtained. For example, the device described in document WO-A2-2006 / 015462 can be used. In both cases, upon evaporation of the liquid vehicle, a solid film is formed.

[0024] In a further step, the solid film is washed with water or a concentrated alkaline solution. This can be, for example, potassium or sodium hydroxide. This washing step can be carried out, for example, for two hours in a solution 30% (by weight) potassium hydroxide at 70°C. This washing step can be carried out by rinsing, soaking, immersion or any other known washing technique. During this step, acids, surfactants and, in general, all agents soluble in neutral or alkaline media are eliminated and generate pores in the solid film. In the case of rinsing with water, the last compounds soluble only in alkaline media will be dissolved in the electrolyser. The pores thus generated have a diameter of 20 to 50 nm.

[0025] Such a membrane can be used as a diaphragm in an alkaline electrolyzer.

[0026] Examples of achievements

[0027] Al to A39. In the following examples, the various constituents of the new material were mixed in a reactor. This mixture is left stirring until a homogeneous dispersion is obtained. This liquid mixture is adapted in terms of viscosity by the addition of more or less vehicle to be applied to the chosen support, namely a PTFE film (examples Al to A30) or polymeric fabrics of PSU, PPS (examples A31 to A34) or polypropylene (examples A35 to A39). This viscosity adaptation allows the dispersion to fill the mesh of the fabric well while avoiding gravity flow. The product is applied in the laboratory using techniques such as bar-coating or dip-coating which simulate industrial application with a roll-coater or slot-die. The coated support is then dried in a convection oven to gradually evaporate the solvent and coalesce the polymer particles.In the context of examples A1 to A19, the polymer is polyvinylidene fluoride (SOLEF 1015) whose melting temperature is approximately 175°C. The inventors observed that under the conditions used, the coalescence of the composition begins around 130°C. From this temperature, the fusion of the particles is sufficient to create a mechanically solid film. The temperature and residence time in the oven are chosen to allow time for the particles to coalesce without excessively melting and flowing. It should be noted that the PVDF used is chosen with a high molecular weight which ensures a high viscosity above the melting point. Upon leaving the oven, the coated fabric cools and the structure sets. This cooling can be done naturally in air or forced with air blowing or water jets. If the support is a PTFE film, the coated material detaches easily and the result is a "free" film.If the coated support is a fabric, the result is the impregnated fabric. At this stage, the material is not very porous. The porosity still needs to be developed and the porosity agents removed. To do this, it is necessary to wash the film. This can be done with water or an alkaline solution. This step can be carried out on the coating line, during cooling for example, or off-line in a specific installation or even in the electrolyzer. In the latter case, the acids are dissolved and neutralized and the surfactants dissolved or even decomposed. In the following examples, the washing was carried out in a 30% by mass KOH solution in water at 70°C for 2 hours.

[0028] Table I (Application on PTFE film; Free film thickness 150 pm dry, Cooking 5 min at 200°C)

[0029] In Table I above, the first column indicates the reference of the example film. Examples marked with an asterisk (*) are provided for comparison. Columns two to five show the amounts of the liquid vehicle (LV), organic binder (OB), hydrophilic inorganic filler (HIF), and porosity agent, respectively. The amounts are expressed in parts by weight. The dry thicknesses are approximately 150 μm dry. Regarding the liquid vehicle: MPAC means l-methoxy-2-propyl acetate, MPOH means l-methoxy-2-propanol (Bp=118-119°C). For the porosity agent, the amount added relative to the other ingredients has sometimes been expressed in grams. AS means 96% sulfuric acid. AP means phosphoric acid. APous means phosphorous acid. AO means octanoic acid. BYK76 means Byk9076; It is a steric effect surfactant of an alkylammonium salt of a high molecular weight copolymer from the company BYK.X100 stands for Triton X100 from Dow Chemicals; it is a non-ionic surfactant. FC4430 is a non-ionic fluorosurfactant. LauS stands for sodium lauryl sulfate. MSS stands for sodium metasilicate, OAI stands for aluminum oxide 90 or Aeroxide Alu C from EVONIK, Ae380 stands for AEROSIL 380 from EVONIK which is a hydrophilic fumed silica with a specific surface area of ​​380 m^ / g, Hil-Sil is a micron silica from PPG. The sixth column shows the ionic resistivity (IR) of the film expressed in O.cm^ and measured at 70°C (when the measurement was carried out at a temperature other than 70°C, it is indicated in parentheses) as follows. Note that the parameter we are trying to improve is ionic conductivity. We can easily measure ionic resistivity which is inversely proportional to ionic conductivity.

[0030] A double-compartment measuring cell is used. The film separates the two compartments. Resistivity measurement is always done in two stages: - Measurement of ohmic drop without film - Ohmic drop measurement with film.

[0031] The membrane resistance is calculated by simple difference. We then move on to the ionic resistivity, taking the surface area into account.

[0032] An alternating current (GEIS mode) at variable frequency is passed between the working electrode WE PI and the counter electrode CE P2 (Ti / Pt electrodes). The working electrode (WE) and the counter electrode (CE) are placed in the compartments on either side of the membrane. The potential is measured between the sense electrode S2 and sense electrode S3, S2 and S3 being located on either side of the membrane. These electrodes are Pt wires or a Ni grid. The Bio-Logic potentiostat is used to record Ece (S2-S3). Knowing Ece and the current flowing between WE and CE, it is possible to calculate, as a function of the frequency of the imposed current (GEIS mode), the impedance of the membrane, therefore its resistance if we know the frequency at which the inductive and capacitive components of the impedance are zero (in general we choose to measure at 1 kHz).

[0033] The seventh and eighth columns show the results of a wettability test, respectively with water (M(water)) or with a 30% by weight solution of potassium hydroxide (M(KOH)). When the wettability test results are different for the two sides of the film, it is specified in column 1 whether it is the smooth side (FL) or the rough side (FR). The test determines whether a drop of the test liquid is absorbed directly (AbDir) or gradually (AbProg). Wettability was measured with a DROP SHAPE ANALYZER - DSA100S device from Krüss.

[0034] The film of Example A30 was observed to be brittle, which is not compatible with use as a membrane.

[0035] The following conclusions can be drawn from examining Table I: The greater the quantity of acid, the more the ionic conductivity of the layer decreases. The most effective acids are strong acids. The addition of surfactant further reduces the conductivity. The combination of an ionic and non-ionic acid is even more effective. The use of hydrophilic particles provides performance in terms of hydrophilicity. The choice of their size can slightly influence the result on the porosity but to a lesser extent than acids or surfactants. The choice of vehicle will rather affect the mechanical strength of the final membrane. It also has an influence on the solubility of the compounds that can be included in the formulation.

[0036] Ibis Table (Application on 250pm EPS fabric; Thickness of the membrane on fabric: 500pm; Cooking 2 min at 180°C)

[0037] The abbreviations used in Table Ibis are the same as those in Table I. In addition, PP31 means Polypropylene PROPYLMATTE 31 from MPI, PP31HD means Polypropylene PROPYLMATTE 31 high density from MPI. Different hydrophilic inorganic fillers have been used: Mica M2 / 1 or Talc Jetfine 1 from IMERYS.

[0038] Iter Table (Application on 280pm Polypropylene fabric; Total membrane thickness with 500pm dry impregnation, Cooking 2 min at 160°C)

[0039] The abbreviations used in Table Ibis are the same as those in Tables I and Ibis. In addition, PP450 and PP500 mean respectively Polypropylene PROPYLMATTE 450 and 500 from MPI, PP3815 means Polypropylene 3815 from DEUREX Different hydrophilic inorganic fillers were used: Talc Jetfine 1 from IMERYS, montmorillonite K10 from SIGMA-ALDRICH, Zeolite ZEO-CO2 from Micropowders technologies and nano clay Halloysite from SIGMA-ALDRICH.

[0040] B1 to B10. In the following examples, the various constituents of the new material were mixed in a reactor. A 250 μm thick polyphenylene sulfide (PPS) fabric is impregnated with the material to a given thickness. The impregnated fabric is placed in an oven at 180°C (120°C for example B10) for 5 min. It is observed that the organic binder melts and then coalesces upon evaporation of the solvent. The supported film is then washed for two hours in a 30% (by weight) potassium hydroxide solution at 70°C (for twenty-four hours in a 30% (by weight) potassium hydroxide solution at 80°C for example B10).

[0041] Table II

[0042] In Table II above, the first column indicates the membrane reference. The second column gives the thickness of the fabric impregnation. Where applicable, the third column indicates the reference of the film used in example series A. Film BO is a commercial film of ZIRFON PERL 500 (film impregnated on a PPS fabric). Columns four to seven show the quantities of the liquid vehicle (LV), the organic binder (OL), respectively. of the hydrophilic inorganic filler (Cl H) and the porosity agent. The quantities are expressed in parts by weight. The abbreviations used have the same meaning as in Table I; X45 means Triton X45 from Dow Chemicals; it is a non-ionic surfactant. The eighth column indicates the ionic resistivity (RI L) of the free film expressed in O.cm^ and measured at 30°C as described above. The ninth column indicates the ionic resistivity (RI lmp) of the membrane consisting of the film impregnated on canvas expressed in O.cm^ and measured at 30°C as described above.

[0043] The following conclusions can be drawn from examining Table II: The conclusions are the same as for the free film table (Table I). That is, it has been shown that the addition of a fabric in the matrix does not change the effectiveness of the concept and that the different components always have the same positive influence on the porosity. It is thus possible to obtain membranes with excellent mechanical strength by adding a reinforcing fabric without changing the ionic conductivity performance. Example B10 shows that the thermoplastic polymer is not limited to polyvinylidene fluoride. In this case, this example shows that a membrane suitable for water electrolysis has been obtained with a low molecular weight polypropylene (CERIDUST 6050M from CLARIANT).

Claims

Claims 1. Material comprising - an organic binder consisting of a thermoplastic polymer, chosen from the group consisting of polyethylene, polypropylene, polystyrene, acrylonitrile-butadiene-styrene, polyvinyl or vinylidene halide optionally overhalogenated or their mixtures; - a hydrophilic inorganic filler and - a porosity agent.

2. Material according to claim 1 wherein the thermoplastic polymer consists of polyvinylidene fluoride, preferably a high molecular weight polyvinylidene fluoride or polypropylene.

3. Material according to claim 2 wherein the thermoplastic polymer consists of polyvinylidene fluoride with a molecular weight greater than 250k Daltons, preferably greater than 350k Daltons and particularly preferably close to 500k Daltons.

4. Material according to any one of the preceding claims, in which the hydrophilic inorganic filler comprises a metal oxide, preferably chosen from the following compounds: ZrO2, MgO, TiC^, CeC>2 or BaSO / talc, zeolite, montmorillonite or nano-clays or mixtures thereof.

5. Material according to claim 4 wherein the hydrophilic inorganic filler comprises ZrC>2 or BaSC>4 or mixtures thereof.

6. Material according to any one of the preceding claims, in which the porosity agent is chosen from phosphoric acid, phosphorous acid, surfactants of ionic, non-ionic or steric type and acidic metal oxides capable of dissolving in an alkaline medium or their mixtures.

7. Film made of a material according to any one of the preceding claims.

8. Film according to claim 7 impregnated on a support consisting of fibers of an organic matrix made of polysulfone (PSU), polyphenylene sulfide (PPS), polypropylene or polytetrafluoroethylene (PTFE) formed into a sheet with tight braiding.

9. Film according to claim 8 in which the thermoplastic polymer used in the composition of the material has a melting point lower than that of the material constituting the support.

10. Film according to claim 9 in which the thermoplastic polymer used in the composition of the material has a melting point of less than 200°C, preferably less than 190°C.

11. Film according to claim 8 in which the thermoplastic polymer used in the composition of the material has a melting point of less than 180°C.

12. A method of preparing a material according to any one of claims 1 to 6 comprising - the dispersion of the organic binder in a liquid vehicle, - the dissolution, dispersion or suspension of a hydrophilic inorganic filler and a porosity agent in said liquid vehicle.

13. A method according to claim 12 also comprising dissolving, dispersing or suspending an ionic, non-ionic or steric surfactant or mixtures thereof in the liquid vehicle.

14. Method according to one of claims 12 or 13 further comprising a step of heating the liquid vehicle until the composition coalesces.

15. A method of forming a film comprising impregnating a support consisting of fibers with an organic matrix made of polysulfone (PSU), polyphenylene sulfide (PPS), polypropylene or polytetrafluoroethylene (PTFE) formed into a sheet or strip with tight braiding with the material according to any one of claims 1 to 6, or with the material obtained according to the method according to any one of claims 12 to 14.

16. A method of forming a membrane for an alkaline electrolyzer comprising washing with water or an alkaline solution the film obtained according to the method of claim 15.

17. A method according to claim 16 wherein the alkaline washing step is carried out at a temperature between 20°C and 90°C in water or potassium hydroxide solution.

18. Membrane obtained by the method of claim 16 or 17.

19. Use of the membrane of claim 18 in an alkaline electrolyzer for the production of dihydrogen.

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