Ion-conducting membrane, method for manufacturing such a membrane, electrochemical cell comprising such a membrane and plant comprising such a cell

Abstract

The invention relates to an ion-conducting membrane (10) for an electrochemical device, said membrane comprising a layer of a material comprising: - 5% to 30% by weight of a polymer binder and - 70% to 95% by weight of a powdered ceramic, the powdered ceramic comprising ceramic doped with yttrium oxide and / or ceramic doped with cerium oxide. The invention can be used to produce a non-porous membrane for low-temperature electrolysis (0°C to 150°C).

Description

Description Title of the invention: Ionic conductive membrane, method of manufacturing such a membrane, electrochemical cell comprising such a membrane and installation comprising such a cell Technical field of the invention [1] The technical field of the invention relates to ionic conductive membranes such as those used in particular in water electrolysis devices. More particularly, the invention relates to a membrane (also called a separator) for the alkaline electrolysis of water and a method for manufacturing such membranes. State of the art [2] Hydrogen is used in several industrial processes, including as a raw material in the chemical industry and as a reducing agent in the metallurgical industry. Hydrogen is a fundamental element for the manufacture of ammonia, and therefore fertilizers, and the manufacture of methanol, used in the manufacture of many polymers. Refineries, where hydrogen is used for the processing of intermediate petroleum products, are another area of ​​use. [3] Hydrogen is also an important energy carrier: it can store and provide energy in a usable form. The energy is released by an exothermic combustion reaction with oxygen, thus forming water. During such a combustion reaction, no carbon-containing greenhouse gases are emitted. [4] As electricity production from renewable energy increases, so does the need for energy storage and transmission. Many renewable energy sources, particularly solar and wind, are located far from population centers and only produce electricity intermittently. Hydrogen can be the perfect energy carrier for renewable energy. It can store energy and distribute it where it is needed. [5] Alkaline water electrolysis is an important process for producing hydrogen. In an alkaline water electrolysis cell, a membrane is used to separate the produced gases as well as to separate the electrodes of different polarities to prevent a short circuit between these electronically conductive parts (electrodes) and to prevent the mixing of H2 (formed at the cathode) and O2 (formed at the anode) to avoid explosive mixtures of these gases. While performing all these functions, the membrane is an ionic conductor for the transport of OH ions from the cathode to the anode. [6] In the field of electrolysis, the notion of porosity is important in the qualification and definition of a membrane (gas-tight) or diaphragm (non-gas-tight) type separator. gas-tight), especially when producing hydrogen. For some electrolysis processes, a porous diaphragm is used, i.e. a diaphragm whose pores have an average diameter greater than 1 micrometer (Ipm): the pores are sought to make the diaphragm ionically conductive because the electrolyte penetrates these pores and, under the effect of the electrolysis voltage, allows the circulation of ions (or anions) through the electrolyte absorbed in the pores. But gas bubbles can also easily penetrate these pores, pass through the diaphragm and increase the "crossover" (passage of gas through the diaphragm). For more recent electrolysis processes, a gas-tight membrane (meaning gas bubble-tight) is used, i.e. a membrane whose pores have an average diameter of less than 1 micrometer (Ipm) or even less than 1 nanometer (Inm). In this case, the ionic electrolyte is absorbed by the membrane but the gas bubbles cannot pass through the membrane. The membrane is said to be gas-tight or non-porous. [7] It is known to produce microporous membranes (or separators) comprising hydrophilic inorganic particles, in particular zirconium oxide particles (also called Zirconia). Examples include Zirfon® membranes comprising zirconium oxide particles bound by a polymer. These membranes are microporous (pores with diameters of approximately 1 pm); they also have disadvantages, in particular insufficient mechanical and thermal stability, significant permeability to dihydrogen, higher permeability to dioxygen at temperatures above 100 degrees Celsius, limited ionic conductivity, and therefore limited efficiency. Also, these membranes show a limited lifespan in use due to rapid degradation in an alkaline environment requiring frequent membrane changes. All these disadvantages have an impact on the profitability of large-scale hydrogen production. [8] It is also known from document EP2373833 to produce non-porous membranes consisting solely of zirconia-type ceramic doped with Yttrium oxide, Yttrium oxide being chosen here for its ionic conduction effect at high temperature. These membranes are produced from ceramic grains by a high-temperature sintering process (of the order of 1400 to 1600°C) resulting in a reorganization of the ceramic crystals which fuse together to form a rigid membrane. These membranes, however, have disadvantages: these membranes are brittle because they are rigid, and they can only be used for high-temperature electrolysis (above 600°C). In addition, the manufacturing process for these membranes is particularly difficult and expensive to implement, due to the need for a sintering step. Statement of the invention [9] The invention provides a new membrane which does not have all or part of the drawbacks mentioned above.

[0010] To this end, the invention proposes a novel ionic conductive membrane for an electrochemical device, membrane comprising a layer of a material comprising: - 5 to 30% by weight of a polymer binder and - 70 to 95% by weight of a powdered ceramic, the powdered ceramic comprising ceramic doped with Yttrium oxide and / or ceramic doped with Cerium oxide.

[0011] A doping process consists of adding impurities (in the context of the invention, Yttrium oxide or Cerium oxide) to a pure substance (in the context of the invention, a ceramic) in order to modify the crystallographic structure and ultimately the physicochemical properties of the pure substance. In the context of the invention, the doped substances used, in this case the Yttrium oxide-doped ceramic and / or the Cerium oxide-doped ceramic, are thus substances which are different from the initial pure substance (the ceramic) from the point of view of their crystallographic structure and which therefore have at least one distinct physicochemical property compared to the initial ceramic.

[0012] Surprisingly, tests have shown that, in use, in a membrane according to the invention, consisting of a polymer binder and ceramic doped with Yttrium oxide and / or ceramic doped with Cerium oxide, the polymer binder is not degraded during operation of the electrolyser by the solvated or free electrons present in the electrolyte of an electrolyser or by the particularly aggressive nascent oxygen atoms "O'". The tests thus show that the ceramic doped with Yttrium oxide and the ceramic doped with Cerium oxide have a protective effect against oxidation for the polymer binder. The lifetime of the membrane according to the invention is thus greater than that of membranes known elsewhere, without it being necessary to add a layer to protect the membrane against oxidation.

[0013] In embodiments of the membrane according to the invention, the powdered ceramic may comprise: - zirconia doped with Yttrium oxide, - zirconia doped with cerium oxide, or - a mixture of zirconia doped with Yttrium oxide and zirconia doped with Cerium oxide. Zirconia (or zirconium dioxide ZrO2) doped with Yttria oxide, also called Yttria Stabilized Zirconia or YSZ (for Yttria Stabilized Zirconia), is a ceramic made of zirconium dioxide (ZrO2) whose crystalline structure is stabilized in its cubic or tetragonal form at room temperature by yttrium oxide. Zirconia has very good ionic conductivity, so that the electrolysis efficiency is improved. Doping zirconia with Yttrium oxide gives doped zirconia a particular antioxidant effect that is interesting in the context of the invention. Doping with Cerium oxide also provides an interesting antioxidant effect, and Cerium oxide is less expensive than Yttrium oxide.

[0014] In other embodiments, the powdered ceramic may comprise: - a mixture of zirconia doped with Yttrium oxide and Boron nitride, - a mixture of zirconia doped with cerium oxide and boron nitride, or - a mixture of zirconia doped with Yttrium oxide, zirconia doped with Cerium oxide and Boron nitride. Boron nitride, due to its good ionic conductivity, is a good complement to zirconia.

[0015] The polymer binder can be a hydrophobic thermoplastic polymer. The polymer's tendency to repel water thus strengthens its resistance to free electrons and further improves the membrane's lifespan.

[0016] The polymer binder is preferably polysulfone (PSU), polyethersulfone (PES), polyphenylene sulfone (PPSU), polytetrafluoroethylene (PTFE), or a mixture of two or more of these polymers. These polymers are chosen for their excellent resistance to heat, oxidation, and hydrolysis in aqueous and alkaline media. This improves the membrane's lifespan. These polymer binders also have good electrical properties, which improves the efficiency of the hydrogen production reaction.

[0017] The membrane according to the invention is non-porous. The surface of the ceramic grains ensures the ionic conduction of the membrane. The polymer binder ensures the mechanical connection between the grains; it also makes it possible to obtain a flexible membrane.

[0018] The invention also relates to a ceramic comprising ceramic doped with Yttrium oxide and / or ceramic doped with Cerium oxide, preferably zirconia doped with Yttrium oxide and / or zirconia doped with Cerium oxide, which is particularly advantageous for the manufacture of a non-porous membrane for low temperature electrolysis (0°C to 150°).

[0019] The invention also relates to a cell for an electrochemical device, said cell comprising a membrane as described above; the invention also relates to a water electrolysis installation comprising such a cell.

[0020] The invention finally relates to a method of manufacturing a membrane as described above, using a phase inversion technique. Presentation of figures

[0021] The invention will be better understood, and other characteristics and advantages of the invention will appear in light of the following description of an example of implementation of the invention. This example is given without limitation. The description should be read in conjunction with the appended drawings in which: • [Fig. 1] shows a cell suitable for a water electrolysis application • [Fig. 2] shows a simplified diagram of a water electrolyser • [Fig. 3] shows results of tests carried out on the membrane according to the invention. Detailed description

[0022] As previously stated, the invention relates to an ionically conductive membrane (10) for an electrochemical device, the membrane comprising a layer of a material comprising - 5 to 30% by weight of a polymer binder and - 70 to 95% by weight of a powdered ceramic comprising ceramic doped with Yttrium oxide or ceramic doped with Cerium oxide. Experience shows that the use of doped ceramics makes it possible to neutralize the oxidizing effects of solvated electrons and isolated atoms during electrochemical reactions, oxidizing effects which degrade the membrane, and more particularly the membrane binder.

[0023] In embodiments, the powdered ceramic may comprise: - zirconia doped with Yttrium oxide, - zirconia doped with cerium oxide or a mixture of zirconia doped with yttrium oxide and zirconia doped with cerium oxide. In other embodiments, the powdered ceramic may comprise: - a mixture of zirconia doped with Yttrium oxide and Boron nitride, - a mixture of zirconia doped with cerium oxide and boron nitride, or - a mixture of zirconia doped with yttrium oxide, zirconia doped with cerium oxide and boron nitride

[0024] The polymer binder is a hydrophobic thermoplastic polymer, preferably polysulfone (PSU), polyethersulfone (PES), polyphenylene sulfone (PPSU), polytetrafluoroethylene (PTFE), or a mixture of at least two of these polymers; these polymers are chosen for their mechanical, chemical, and electrical properties.

[0025] As explained above, the material constituting the member comprises 70 to 95% by weight of powdered ceramic, and - 5 to 30% by weight of polymer. In the membrane according to the invention, the surface of the powdered ceramic grains and more particularly the contact surface between the ceramic grains are the seat of the ionic conduction of the membrane, it is therefore important to maximize these contact surfaces to obtain the most efficient membrane possible, in terms of ionic conduction and therefore electrolysis efficiency. Also, in the membrane according to the invention, the polymer ensures the link between the grains and thus the mechanical strength of the membrane. The membrane obtained is non-porous, the ionic conduction taking place on the contact surfaces of the ceramic grains. The proportion by weight of powdered ceramic and polymer thus results from a compromise. Experiments carried out with different proportions of ceramic and polymer have shown that: - the efficiency of the membrane, particularly in terms of ionic conduction, is satisfactory when the material comprises more than 70% by weight of ceramic, - the mechanical strength of the membrane is sufficient for the intended applications when the material comprises more than 5% by weight of polymer binder.

[0026] In one example, the doped ceramic may comprise: - 75 to 99%, and preferably 75 to 95%, by weight of ceramic, for example zirconia and - 1 to 25%, and preferably 5 to 25%, by weight of Yttrium oxide or Cerium oxide. The whole forms 100% by weight of the doped ceramic. The yttrium oxide is for example yttrium dioxide Y2O3. The ceramic doped with Yttrium oxide or with Cerium oxide has a protective effect against degradation by oxidation of the binder polymer: tests have shown for example that the polymer of a membrane comprising powdered ceramic comprising doped zirconia comprising at least 1% by weight of yttrium oxide degrades little and that the polymer of a membrane comprising powdered ceramic comprising doped zirconia comprising at least 5% by weight of yttrium oxide does not degrade or degrades very little compared to the polymer of a membrane comprising conventional zirconia.Technically, in the material constituting the membrane according to the invention, the doped ceramic could comprise more than 25% by weight of Yttrium oxide; however, the cost of Yttrium oxide being particularly high, the cost of the membrane would become too high compared to the expected efficiency.

[0027] According to a preferred embodiment, the powdered ceramic grains have an average diameter of between 0.1 and 5 μm, and preferably between 0.2 and 2 μm. The ionic conductivity of the membrane is directly related to the exposed surfaces (not covered with binder) of the grains in the membrane and more particularly to the contact surfaces or surfaces close to adjacent grains promoting ionic exchanges between the grains. The smaller the grains, the greater the contact surfaces or surfaces close to adjacent grains. And at the same time, the smaller the grains, the more they are trapped in the binder which limits the ionic exchange surfaces. Tests show that grains with an average diameter of between 0.1 and 5 μm are a good compromise, regardless of the ceramic used. The best test results were obtained with grains with an average diameter of between 0.2 and 2 μm.

[0028] The membrane described above may be produced according to a method comprising a phase inversion step. Such a step here consists of causing the removal of a solvent from a solvent-polymer solution, by immersing a film of solution in a non-solvent. In the context of the invention, such a step allows the transformation of the solvent-polymer solution film into a polymer membrane.

[0029] In more detail, in an example of the production of a PSU (polysulfone) / YSZ (Yttrium oxide doped zirconia) membrane comprising 10% by weight of PSU and 90% by weight of doped ceramic, the yttrium oxide doped zirconia comprising 85% by weight of zirconia and 15% by weight of yttrium oxide, a method according to the invention comprises the following steps.

[0030] First, (step 1) the polymer is dissolved in a water-immiscible solvent to obtain a polymer solution. In the example, a mass of polymer, polysulfone (PSU), is dissolved in a volume of solvent such as dimethylacetamide (DMAc), which appears as an oily liquid. If necessary, the mixture is stirred at a temperature above 70°C until the polymer is completely dissolved and a homogeneous polymer solution is obtained.

[0031] The ceramic powder is then added (step 2) to the polymer solution. The amount of ceramic powder added is such that, relative to the total weight of the polymer and ceramic, the polymer represents 5 to 30% by weight and the ceramic represents 70 to 95% by weight. The ceramic powder comprises ceramic doped with yttria and / or ceramic doped with ceria, preferably zirconia doped with ceria and / or, as in the present example, zirconia doped with yttria (YSZ). The whole is mixed to obtain a viscous homogeneous membrane solution. In the example, to facilitate mixing, a mass of doped Zirconia powder is dispersed in DMAc solvent and then the solution containing the ceramic and the polymer solution obtained in the previous step are mixed for 45 to 60 minutes until a membrane solution is obtained that is more viscous than the two initial solutions.

[0032] The membrane solution is then cast (step 3) onto a support to form a film. The support is preferably flat. If necessary, particularly depending on its viscosity, the membrane solution is stretched simultaneously with casting. The casting parameters are adjusted according to the desired membrane dimensions and the membrane solution parameters (particularly the viscosity of the membrane solution and the shrinkage power of the polymer binder in the following steps).

[0033] Finally, the film-covered support is then immersed in a non-solvent (step 4) to cause phase inversion. During the phase inversion process, the membrane solution precipitates, the solvent is replaced by non-solvent, especially on the surface and inside the film. In the example of the PSU / Zirconia zirconia membrane stabilized with yttria, the support on which the film rests is immersed in water (the non-solvent here), which replaces the DMAc (the solvent) remaining in the membrane film. Initially, in the film, two phases are in equilibrium: a solid phase, rich in polymer which forms the structure of the film and a liquid phase (poor in polymer) which constitutes the pores of the membrane filled with water here. As precipitation progresses, the viscosity of the polymer-rich phase increases until reaching a state where the precipitated polymer is considered a solid.The membrane solution film thus transforms into a membrane, solid, flexible and which traps ceramic grains.

[0034] The membrane-covered support is then extracted from the water (step 5) and the membrane is separated from the support (step 6).

[0035] If necessary, a rolling step (step 7) of the membrane can be carried out to homogenize the surfaces, remove open porosities to further improve the non-porous character of the membrane and / or improve the mechanical strength of the membrane. The rolling step can be carried out at any time after the end of the transformation of the membrane solution film by phase inversion. The rolling step can thus be carried out before extracting the support and the membrane from the water (step 5), or after extracting the membrane from the water. The rolling step also makes it possible to obtain a membrane of regular thickness. The rolling step can be repeated several times if necessary to obtain a membrane having the required properties, in particular a non-porous membrane. Tests have thus shown that the residual pores of a PSU / YSZ membrane according to the invention have a diameter of 0.2 μm, and that the residual pores of a PTFE / YSZ membrane according to the invention have a diameter of 0.1 μm.

[0036] Further post-inversion treatments may be considered to finalize the membrane. For example, a heat treatment step may be performed to strengthen the bond between the polymer and the ceramic grains and / or to further reduce the pore size in the membrane.

[0037] Finally, the membrane can be rolled up and stored.

[0038] The membrane described above may be produced using another method, in particular when it is produced with PTFE, another method comprising the following steps, consisting of: 1. suspend polytetrafluoroethylene (PTFE) polymer in water, 2. adding an amount of powdered ceramic such that, relative to the total weight of the polymer and the ceramic, the polymer represents 5 to 30% by weight and the ceramic represents 70 to 95% by weight, the powdered ceramic comprising ceramic doped with Yttrium oxide and / or ceramic doped with Cerium oxide, preferably zirconia doped with Yttrium oxide and / or zirconia doped with Cerium oxide, and mixing to obtain a homogeneous membrane paste, 3. Roll out the dough on a support to form the membrane of the desired thickness.

[0039] The amount of water used here is limited so that the rolling operation is sufficient to remove the water from the dough.

[0040] The membrane according to the invention as described above can be used to produce an electrochemical cell comprising in particular - a 30 anode - a cathode 20, and - between the anode and the cathode, a membrane 10 as described above.

[0041] Figure 1 shows a diagram of a known cell for a water electrolysis plant for the production of hydrogen H2 and oxygen O2 gas. Figure 2 shows a schematic diagram of a membrane water electrolysis plant. The membrane 10 divides a bath in two, a bath comprising a mixture of water and electrolyte. The cathode 20 and the anode 30 are positioned on either side of the membrane and are connected respectively to the negative and positive terminals of an electrical energy source. The membrane 10 allows good separation of the hydrogen gas produced on the cathode and the oxygen gas produced on the anode. The cathode and / or the anode may be porous. The The cathode and anode can be metallic, for example nickel, stainless steel or metal oxides, especially on the anodic side. Nickel and stainless steel form surface oxides which are catalysts for the release of oxygen. 316L stainless steel is particularly effective thanks to its molybdenum content.

[0042] A single cell is shown in Figure 1. However, in practice, an industrial installation can include several cells, or even a hundred cells.

[0043] Figure 3 shows the results of tests carried out with a PSU / YSZ membrane according to the invention, in an electrolyser cell comprising a bath of a mixture of water and potassium hydroxide (KOH) at 4 mol / l, at a temperature of approximately 90°C. Figure 3 shows more precisely the evolution of the voltage at the terminals of the cell as a function of the current density flowing between the electrodes. For a current density value, the HHV (high heating value) efficiency of the cell can be calculated by dividing 1.48 V by the voltage at the terminals of the cell; 1.48 V is a reference value corresponding to the thermodynamic dissociation voltage of water, a reference value to which a theoretical efficiency of 100% is associated. For a current density of 0.39 A / cm 2 , the voltage across the electrodes is 1.6V, corresponding to an efficiency of 1.48 / 1.6 = 93% HHV. For a current density of 0.70 A / cm 2corresponding to a nominal operating point of the cell, the voltage across the electrodes is 1.72V, corresponding to an efficiency of 86% HHV. For comparison, the voltage across the terminals of a cell comprising a known Zirfon membrane is 2.1V, i.e. an efficiency of 70% HHV. The membrane lifespan has been further tested under normal operating conditions for 2500 hours without any membrane degradation.

Claims

Claims

1. Ion-conducting membrane (10) for an electrochemical device, the membrane comprising a layer of a material comprising: - 5 to 30% by weight of a polymer binder and - 70 to 95% by weight of a powdered ceramic, the powdered ceramic comprising ceramic doped with Yttrium oxide or ceramic doped with Cerium oxide.

2. A membrane according to claim 1 wherein the powdered ceramic comprises: - zirconia doped with Yttrium oxide, - zirconia doped with cerium oxide, - a mixture of zirconia doped with Yttrium oxide and zirconia doped with Cerium oxide, - a mixture of zirconia doped with Yttrium oxide and Boron nitride, - a mixture of zirconia doped with cerium oxide and boron nitride, or - a mixture of zirconia doped with Yttrium oxide, zirconia doped with Cerium oxide and Boron nitride.

3. Membrane according to one of claims 1 to 2 in which the polymer binder is a hydrophobic thermoplastic polymer, preferably polysulfone (PSU), polyethersulfone (PES), polyphenylene sulfone (PPSU) or polytetrafluoroethylene (PTFE) or a mixture of at least two of said polymers.

4. Membrane according to one of claims 1 to 3 in which the doped ceramic comprises: - 75 to 99%, and preferably 75 to 95%, by weight of ceramic and - 1 to 25%, and preferably 5 to 25%, by weight of Yttrium oxide or Cerium oxide.

5. Membrane according to one of the preceding claims in which the grains of the powdered ceramic have an average diameter of between 0.1 and 5 pm, and preferably of between 0.2 and 2 pm.

6. Membrane according to one of the preceding claims, non-porous to gases.

7. A method of manufacturing a membrane comprising the following steps:

1. dissolve a polymer in a water-immiscible solvent to obtain a polymer solution, 2. adding an amount of powdered ceramic such that, relative to the total weight of the polymer and the ceramic, the polymer represents 5 to 30% by weight and the ceramic represents 70 to 95% by weight, the powdered ceramic comprising ceramic doped with Yttrium oxide and / or ceramic doped with Cerium oxide, preferably zirconia doped with Yttrium oxide and / or zirconia doped with Cerium oxide, and mixing to obtain a homogeneous membrane solution, 3. pour the membrane solution onto a support to form a film covering the support, 4. immerse the film-covered support in water to cause phase inversion.

8. Method according to the preceding claim, also comprising a step of rolling the membrane.

9. A method of manufacturing a membrane comprising the following steps:

1. suspend polytetrafluoroethylene (PTFE) polymer in water, 2. adding an amount of powdered ceramic such that, relative to the total weight of the polymer and the ceramic, the polymer represents 5 to 30% by weight and the ceramic represents 70 to 95% by weight, the powdered ceramic comprising ceramic doped with Yttrium oxide and / or ceramic doped with Cerium oxide, preferably zirconia doped with Yttrium oxide and / or zirconia doped with Cerium oxide, and mixing to obtain a homogeneous membrane paste, 3. Roll out the dough on a support to form the membrane of the desired thickness.

10. Ceramic comprising ceramic doped with Yttrium oxide and / or ceramic doped with Cerium oxide, preferably zirconia doped with Yttrium oxide and / or zirconia doped with Cerium oxide, for the manufacture of a non-porous membrane for low temperature electrolysis (0°C to 150°).

11. Cell for an electrochemical device, cell comprising: - an anode (30) - a cathode (20) and - between the anode and the cathode, a membrane (10) according to one of claims 1 to 6.

12. Water electrolysis installation comprising at least one cell according to claim 11.

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