Anion-conducting membrane, method for manufacturing such a membrane, electrochemical cell comprising such a membrane and facility comprising such a cell

Abstract

The invention relates to a layer of catalytic material for an anion-conducting membrane (10) for an electrochemical device; the layer of catalytic material comprises nanoparticles of at least two active components, wherein the active components are selected from a set comprising nickel, a metal oxide and molybdenum, and wherein the nanoparticles are bound by a polymer binder. Such a layer is preferably deposited on the anode side of the membrane. The invention also relates to a membrane comprising a layer of catalytic material and to a method for preparing and depositing a catalytic layer.

Description

Description Title of the invention: Anionic 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 anionic conductive membranes such as those used in particular in water electrolysis devices. More particularly, the invention relates to a membrane 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 for the manufacture of methanol, used in the manufacture of many polymers. Another area of use is refineries, where hydrogen is used for the processing of intermediate petroleum products. [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 may be the perfect carrier for renewable energy. It can store energy and then distribute it where and when it is needed. [5] Alkaline water electrolysis is an important process for producing hydrogen. In an alkaline water electrolysis cell, a membrane is used between two electrodes, a cathode and an anode: the membrane separates the electronically conductive electrodes. Non-porous to gases, the membrane also separates the gases produced at the electrodes to prevent an explosive mixture of hydrogen gas H2 (formed at the cathode) and oxygen gas O2 (formed at the anode). The membrane is also ionically conductive for the transport of OH- ions from the cathode to the anode. [6] It is known to produce proton exchange membranes (or PEM for proton-exchange membrane). The most widely used membrane is the so-called Nation® membrane marketed by the company Dupont de Nemours. The price of this membrane is particularly high. In addition, this membrane generates a very acidic environment, particularly aggressive towards the electrodes. [7] New membranes are being developed, called anion exchange membranes (or AEM membrane for anion-exchange membrane or anion-conducting membrane or anionic conducting membrane). Examples include Zirfon® separators comprising zirconium oxide particles bound by a polymer. Also included are membranes comprising ceramic particles, such as the Boron Carbide membranes described in document Dl = FR3122778 or the Yttrium Zirconia membranes described in document D2 = FR3150048. Made with a polymer and metal oxide and / or ceramic particles, these membranes are fairly simple to manufacture and less expensive. Also, these membranes are used in a non-acidic or weakly acidic environment, which allows the use of common metals for the production of the electrodes, for example Nickel for the cathode and stainless steel for the anode.However, all of these membranes are generally less efficient than membranes with catalysts, which impacts the profitability of large-scale hydrogen production. [8] To improve the efficiency of membranes, it is known to deposit on the membrane a layer of catalytic materials based on rare materials, for example metals from the Platinum group, also called PGM metals (in English, for platinum group metals), a group which includes iridium (Ir), Osmium (Om), platinum (Pt), palladium (Pd), rhodium (Rh) and ruthenium (Ru). The efficiency of the AEM membrane is improved but it is observed in practice that the resistance of the catalytic layers on the membrane is limited in time and depends more strongly on the formulation of the catalytic layers and the process of deposition of these layers, anode side and cathode side. In addition, PGM metals are rare metals, which significantly increase the cost of membranes and therefore reduce the profitability of large-scale hydrogen production. Statement of the invention [9] The invention provides new catalytic layers and a new membrane which do not have all or part of the drawbacks mentioned above.

[0010] To this end, the invention proposes a new layer of catalytic material for an anionic conductive membrane for an electrochemical device, layer of catalytic material comprising nanoparticles of at least two active components, the active components being chosen from a set comprising Nickel, a metal oxide and Molybdenum, the nanoparticles being bound by a polymer binder.

[0011] Advantageously, such a catalytic layer is deposited on an anionic conductive membrane on the anode side.

[0012] The catalytic layer according to the invention deposited on the surface of the membrane significantly improves the efficiency of the electrochemical reactions which take place at the interfaces of the membrane and the electrodes, and the yields obtained are generally more interesting than a catalytic deposition on one or other of the electrodes of a cell using an AEM membrane without catalytic deposition. Also, the yields obtained are not degrade much less over time. Finally, the Nickel and / or Molybdenum used in the catalytic layer according to the invention are not rare materials and make it possible to produce a membrane with a catalytic layer at a reasonable cost, which makes it possible to produce hydrogen on a large scale at a reasonable cost.

[0013] The invention also relates to an anionic conductive membrane comprising at least one catalytic layer as described above, a cell comprising an anionic conductive membrane and a water electrolysis installation comprising an anionic conductive membrane. The invention finally relates to a method for depositing a catalytic layer as described above on an anionic conductive membrane. Presentation of figures

[0014] The invention will be better understood, and other characteristics and advantages of the invention will appear in light of the following description of examples of implementation of the invention. These examples are given without limitation. The description should be read in conjunction with the appended drawings in which: • [Fig. 1] shows a cell suitable for water electrolysis application • [Fig. 2] shows a simplified diagram of a water electrolyser • [Fig. 3], [Fig. 4], [Fig. 5] and [Fig. 6] show results of tests carried out on catalytic layers and membranes according to the invention Detailed description

[0015] As previously stated, the invention relates to a layer of catalytic material and an anionic conductive membrane 10 for an electrochemical device. Said membrane can be used to produce a cell for an electrochemical device, for example a water electrolysis installation; the cell comprises: - a 30 anode - a cathode 20 and - between the anode and the cathode, a membrane 10.

[0016] The membrane comprises a main layer known elsewhere. The main layer is for example a main layer comprising metal oxide particles such as zirconium oxide such as that described in D2, or a main layer comprising ceramic particles such as that described in D1.

[0017] Unless otherwise specified locally, the term "nanoparticles" is used here and throughout the description to refer to particles with an average size value between 1 and 100 nm, where size is defined as the largest dimension of a particle. Nanoparticle size measurements can be performed by scanning electron microscopy (SEM) or transmission electron microscopy (TEM). Each nanoparticle is an agglomeration of several atoms or molecules of the same or different components.

[0018] In order to compare the performance of the catalytic layers and the membranes on which they are deposited, tests were carried out with a test cell comprising a 30 cm membrane 2 , a nickel cathode and a stainless steel anode; the electrolyte used is potassium hydroxide KOH, also known as caustic potash or simply potash. Several types of membranes were tested, including membranes whose main layer comprises ceramic particles of the Boron Carbide, Boron Nitride, Zirconia or Yttria Zirconia type. The tests described below were carried out at a temperature T = 90°C and a KOH concentration of 4 mol / l.

[0019] To compare membrane performance, R, the HHV (High Heating Value) efficiency of a water electrolysis cell including the membrane, is calculated. To do this, with the membrane placed in the test cell, a current density I is imposed between the cathode and the anode per unit area of membrane, and the voltage V (T, I) is measured between the cathode and the anode of the cell. For a given current density, we seek to obtain the lowest possible voltage to obtain the dissociation of a given quantity of water molecules.

[0020] The minimum theoretical voltage for water dissociation reactions to begin is V0 = 1.48V and, by convention, corresponds to an efficiency R of 100% HHV. The efficiency R is calculated by the relation R = VO / V(T, I), where V(T, I) is the voltage across the cell, for a given temperature T and a given current density I.

[0021] Since the heat produced by electrolysis itself allows the temperature to be increased and then the system to be maintained at a constant temperature, the efficiency calculation above accurately reflects the efficiency of a cell. To calculate the overall efficiency of an electrolyser including such a cell, the amount of energy consumed for the overall operation of the system should be subtracted from the energy consumed directly by electrolysis. In this document, only the efficiency of the cells is analysed.

[0022] One embodiment of the invention relates to a layer of catalytic material adapted to be positioned on the main layer of the membrane, on the anode side. In this embodiment, the particles of the catalytic layer comprise a mixture of nickel nanoparticles and metal oxide nanoparticles, a mixture of nickel nanoparticles and molybdenum nanoparticles, a mixture of metal oxide nanoparticles and molybdenum nanoparticles or a mixture of nickel nanoparticles, metal oxide nanoparticles and molybdenum nanoparticles. When the nanoparticles with a catalytic effect comprise metal oxide particles, these are preferably iron oxide particles, and even more preferably iron 11 / 111 oxide particles. The nanoparticles of active components used for the implementation of the invention can be purchased from a catalog; for example, for the tests carried out, the Nickel, Molybdenum, Cobalt nanoparticles were purchased from the Chinese company HEBEI Fiance Nanotechnoloy Co. Ltd and the metal oxide nanoparticles are those of the Sigma Aldriche brand.

[0023] The metal oxide nanoparticles are preferably particles with a so-called perovskite crystal structure. Perovskites have an ABX3 crystal structure, where A, B, and X represent different ions. The structure is cubic, with a B-type ion at the center of a cube formed by A-type ions, and X-type ions occupying the corners of the cube. Perovskites have interesting electrical and magnetic properties, making them useful in particular here as catalyst materials. Their particular crystal structure also makes them useful as catalyst supports.

[0024] In terms of efficiency for water electrolysis, good HHV efficiencies have been obtained with a membrane comprising a main layer comprising Yttria-containing Zirconia and a catalytic layer in which the nanoparticles of active components comprise, per 100% by weight of particles, 30 to 70% by weight of Nickel, 15 to 35% by weight of Iron oxide and 15 to 35% by weight of Molybdenum, and better, 40 to 60% by weight of Nickel, 20 to 30% by weight of Iron oxide and 20 to 30% by weight of Molybdenum. As an example, Figure 3 shows the results of tests carried out with a membrane having a layer of catalytic material at the anode comprising a mixture of nanoparticles comprising 50% by weight of Nickel, 25% by weight of Iron oxide and 25% by weight of Molybdenum. During the tests, the current density is varied and the voltage across the electrodes is measured. With a current density of 1A / cm 2, the voltage obtained is 1.95V, i.e. an efficiency R = 1.48 / 1.95 = 76%. With a current density of 0.7A / cm 2 , the voltage obtained is 1.80V, i.e. an efficiency R = 1.48 / 1.80 = 82%. Other tests were carried out with a membrane having a layer of catalytic material at the anode comprising a mixture of nanoparticles comprising 50% by weight of iron oxide, 25% by weight of nickel and 25% by weight of molybdenum; an efficiency of 75% HHV was obtained for a current density of 1A / cm 2 and an efficiency of 79% HHV was obtained for a current density of 0.7A / cm 2 applied between the electrodes of the test cell.

[0025] Also, in terms of efficiency for water electrolysis, good HHV yields have been obtained with a membrane comprising a main layer comprising Yttria-containing Zirconia and a catalytic layer in which the nanoparticles comprise, per 100% by weight of particles, 35 to 65% by weight of Nickel and 35 to 65% by weight of Molybdenum, preferably 40 to 60% by weight of Nickel and 40 to 60% by weight of Molybdenum, and even more preferably 50% by weight of Nickel and 50% by weight of Molybdenum. As an example, Figure 4 shows results of tests carried out with a membrane having a layer of catalytic material at the anode comprising a mixture of nanoparticles comprising 50% by weight of Nickel and 50% by weight of Molybdenum. During the tests, the current density is varied and the voltage across the electrodes is measured. With a current density of 1A / cm 2, the voltage obtained is 1.86V, i.e. an efficiency R = 1.48 / 1.86 = 80% HHV. With a current density of 0.7A / cm 2 , the voltage obtained is 1.77V, i.e. an efficiency R = 1.48 / 1.77 = 84% HHV.

[0026] The efficiency of the membranes depends in particular on the quantity of nanoparticles present in the catalytic layer on the anode side. The best efficiencies have been obtained for membranes in which the layer of catalytic material comprises, on the surface, 0.1 to 7 mg / cm 2 of nanoparticles or, more precisely, 0.1 to 7 mg of nanoparticles in a volume corresponding to 1 cm 2 membrane surface area multiplied by membrane thickness. The catalytic layer preferably has a thickness of 10 pm to 100 pm.

[0027] The efficiency of the membranes also depends on the catalytic activity of the nanoparticles, which increases when the number of electrochemical reaction sites for dissociating water molecules increases. The sites of the chemical reactions are present on the surfaces of the catalytic materials. To increase the number of reaction sites on particles such as nickel, iron oxide or molybdenum particles, tests have shown that an effective solution consists of increasing the specific surface area (ratio between the surface area and the mass) of the particles. For this purpose, in the context of the invention, the nanoparticles have a size of less than 50 nm, preferably less than 20 nm and even more preferably less than 5 nm. The particles have a substantially convex polyhedral general shape and their size corresponds to their largest dimension.

[0028] The tests also showed that, for the active components used in the invention and for a given particle size, the catalytic activity of the particles is improved by improving the particle size homogeneity of the particle mixture. Tests have further shown that the homogeneity of a particle mixture (in terms of the distribution of particles of different metals in the particle mixture) further improves the catalytic activity. The method according to the invention described below makes it possible to obtain this homogeneity of size and distribution of the nanoparticles.

[0029] The performance of the membranes still depends on the good cohesion of the nanoparticles between them, the good cohesion of the catalytic layer to the main layer of the membrane and the durability of this cohesion over time. For this purpose, the polymer binder used is an ionomer chosen for its resistance over time in aggressive chemical environments and at high temperatures up to 120°C, but also for its ionic conduction properties. By polymerizing, the binder acts as a glue in the catalytic layer and holds the particles and nanoparticles of this layer together. It also allows good adhesion of the catalytic layer with the membrane. Finally, an essential point in the context of the invention, due to its ionic conduction, the ionomer allows the catalytic layer to remain electrically and ionically conductive.The preferred ionomer is known as Nation®, derived from the polymerization of a monomer called tetrafluoroethylene-perfluorosulfonic acid. The proportion of polymer binder in the layer of catalytic material is a compromise between a maximum quantity of particles having catalytic activity and a minimum quantity of polymer binder adapted to ensure over time the cohesion of the particles between them and the cohesion of the catalytic layer on the membrane. Tests have shown that a proportion of polymer binder of 10% to 40% of the weight of the binder / nanoparticle mixture is a good compromise.

[0030] The performance of membranes with a catalytic layer also depends on the method of producing the catalytic layer on the main layer of the membrane.

[0031] To this end, the invention proposes a method for depositing a catalytic layer on an anionic conductive membrane, catalytic layer comprising nanoparticles of at least two active components and a polymer binder, method comprising a step of preparing a liquid ink comprising the following steps consisting of: - mixing the nanoparticles in a liquid, the liquid being water, a solvent or a mixture of water and solvent, the solvent being for example an alcohol, - grind the mixture of particles in a liquid medium until a homogeneous mixture is obtained, - add the liquid polymer binder.

[0032] In the example of the catalyst according to the invention, the particles are chosen from a group comprising Nickel, a metal oxide and Molybdenum; the metal oxide used is for example an Iron II / III oxide.

[0033] The liquid is water, a solvent, or a mixture of water and solvent. The solvent is suitable for diluting the binder. The solvent is, for example, an alcohol. Successful tests have been carried out, for example, with water, with ethanol, with a mixture of water and isopropanol.

[0034] The quantity of liquid chosen is adapted to wet the solid nanoparticles, so as to facilitate the following grinding step and to limit an operator's exposure to the nanoparticle dust which will be generated during the grinding step. The amount of liquid is also chosen so that the resulting ink can be projected onto the membrane. In the successful tests carried out, a quantity of liquid was used whose weight corresponded to 10 to 25 times the weight of the solid particles.

[0035] The grinding of nanoparticles is carried out in an aqueous medium, for example in a planetary mill / mixer (or ball miller in English). Grinding a mixture of particles of at least two different components (one can also speak of co-grinding) makes it possible to obtain smaller nanoparticles, of the chosen size, less than 50 nm, preferably less than 20 nm and even more preferably less than 5 nm, and particles of the most homogeneous average size possible.

[0036] In the method according to the invention, the grinding also makes it possible to obtain a ground material comprising not only particles comprising atoms or molecules of the first active component, and particles comprising atoms or molecules of a second active component but also, and this surprisingly, particles comprising atoms or molecules of two (or more) different active components. For example, grinding a mixture of nickel particles and molybdenum particles produces a ground material comprising nickel particles, molybdenum particles and particles of a Ni-Mo alloy. Grinding thus physically agglomerates particles together by physically compressing the particles against each other. The resulting ground material thus has greater catalytic activity than the mixture of particles before grinding.

[0037] The mixing of nanoparticles in the liquid also allows for a homogeneous mixture (in terms of distribution of particles of different natures) of the materials, particularly when two or more types of nanoparticles are used (for example Nickel, metal oxide and / or Molybdenum here, or Nickel and Cobalt).

[0038] The preparation step described above makes it possible to obtain an ink comprising suspended catalytic nanoparticles, a composition which will also be called “catalytic ink”.

[0039] The catalytic ink thus prepared is sprayed by ultrasound onto the main layer of an anionic conductive membrane. The deposition of the catalytic ink by an ultrasonic spraying process, combined with the fluidity of the ink, allows the nanoparticles to be well separated from each other to avoid their agglomeration during spraying. This process also allows the ink to be kept homogeneous throughout the spraying. This spraying process also allows, by a pressure effect, to obtain good adhesion of the ink to the main layer of the membrane. The result is a catalytic layer where the distribution of the nanoparticles is particularly homogeneous and regular; also, the layer obtained has a very low thickness, of the order of 10 pm to 100 pm at the anode for a layer comprising Nickel, a metal oxide and / or Molybdenum, and particularly constant over the entire surface of the membrane.Finally, this process makes it possible to obtain a porous catalytic layer which thus presents a large specific surface area (ratio between the active surface area and the weight of the catalytic layer), from 5m2 / g to 200m2 / g depending on the formulation of the catalytic ink.

[0040] After complete drying, the membrane can be stored. Drying allows the liquid, water or a mixture of water and solvent, to evaporate. The catalytic layer deposited on the membrane then comprises only the particles bound by the polymer binder.

[0041] Figures 5 and 6 show test results carried out with the test cell described above comprising a 30 cm membrane 2, a Nickel cathode and a Stainless steel anode; the electrolyte used is potassium hydroxide KOH at a concentration of 4 mol / l; the main layer of the membrane is covered on the anode side with a catalytic layer according to the invention comprising Nickel particles and Molybdenum particles in a proportion of 50% / 50%. The tests were carried out at a temperature T = 90°C, an applied current density of 0.7A / cm2 and over a period of several tens of hours to assess the evolution over time of the cell's efficiency.

[0042] In the case of Figure 5, the catalytic layer was deposited on the membrane using a conventional process: a catalytic layer was produced by mixing particles of the two active components, Nickel and Molybdenum, by adding the binder and mixing again, then the catalytic ink was deposited on one side of the membrane. It is noted that, over time, the voltage across the cell terminals gradually increases so that the efficiency decreases over time.

[0043] In the case of Figure 6, the catalytic layer was made according to the method of the invention, by adding a step of grinding the particles of mixed active components. It is noted that, over time, the voltage across the cell terminals gradually decreases so that the efficiency increases over time.

[0044] The method according to the invention can be implemented to deposit a first catalytic layer on a first face of the main layer of the membrane, then, optionally, a second catalytic layer on a second face of the main layer of the membrane.

[0045] Thus, the method according to the invention can be implemented to produce an anionic conductive membrane 10 comprising a main layer covered by: - a layer of a first catalytic material comprising nanoparticles of at least two active components, the active components being chosen from a set comprising Nickel, a metal oxide and Molybdenum, the nanoparticles being bound by a polymer binder, - or, on one side of the main layer, a layer of the first catalytic material and, on the other side of the main layer, a layer of a second catalytic material, for example a second catalytic material comprising Nickel particles and / or Cobalt particles. The catalytic layers on either side of the main layer can be made using the same process.

[0046] Figure 1 shows a diagram of a cell according to the invention adapted for a water electrolysis installation for the production of hydrogen H2 and oxygen O2 gas. The cell comprises a cathode 20, an anode 30 and between the two a membrane 30. Figure 2 shows a schematic diagram of a water electrolysis installation comprising a cell according to the invention. The membrane 10 divides a bath in two, bath comprising a solution of water and electrolyte, in one example potash KOH. The membrane is covered on one side by a layer 40 of catalytic material comprising particles of nickel, iron oxide and molybdenum and on the other side by a layer 50 of catalytic material comprising particles of nickel, cobalt and artificial carbon. 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 a good separation of the hydrogen gas produced on the cathode and the oxygen gas produced on the anode. The cathode and the anode are metallic, for example nickel, respectively stainless steel. With such a membrane, with a main layer based on zirconia and with two layers of catalytic material on either side of the main layer, an efficiency of 84% HHV was obtained for a current density of 1A / cm. 2 and an efficiency of 90%HHV was obtained for a current density of 0.7A / cm 2 applied between the cell electrodes.

[0047] A single cell is shown in Figure 1. However, in practice, an industrial installation can include several cells, or even a hundred cells.

Claims

Claims

1. Layer of catalytic material for an anionic conductive membrane (10) for an electrochemical device, layer of catalytic material comprising nanoparticles of at least two active components, the active components being chosen from a set comprising Nickel, a metal oxide and Molybdenum, the nanoparticles being bound by a polymer binder.

2. Layer of catalytic material according to claim 1 in which one of the active components is a metal oxide, preferably a metal oxide having a so-called Perovskite crystalline structure.

3. Layer of catalytic material according to one of claims 1 to 2 in which one of the active components is an iron oxide, preferably an iron 11 / 111 oxide.

4. Layer of catalytic material according to one of claims 1 to 3 in which the active components comprise 30 to 70% by weight of Nickel, 15 to 35% by weight of Iron oxide and 15 to 35% by weight of Molybdenum, preferably 40 to 60% by weight of Nickel, 20 to 30% by weight of Iron oxide and 20 to 30% by weight of Molybdenum, and even more preferably 50% by weight of Iron oxide, 25% by weight of Nickel and 25% by weight of Molybdenum.

5. A layer of catalytic material according to claim 1 wherein the active components comprise 35 to 65% by weight of Nickel and 35 to 65% by weight of Molybdenum, preferably 40 to 60% by weight of Nickel and 40 to 60% by weight of Molybdenum, and even more preferably 50% by weight of Nickel and 50% by weight of Molybdenum.

6. Layer of catalytic material according to one of claims 1 to 5 comprising: - 60% to 90% by weight of active components in the form of nanoparticles, - 10% to 40% by weight of polymer binder.

7. Layer of catalytic material according to one of the preceding claims in which the layer of catalytic material comprises, on the surface, 0.1 to 7 mg / cm 2 of active components.

8. Layer of catalytic material according to one of the preceding claims in which the polymer binder is an ionomer, preferably a fluoropolymer copolymer based on sulfonated tetrafluoroethylene (Nation).

9. Layer of catalytic material according to one of the preceding claims in which the nanoparticles of active components have a size less than 50 nm, and preferably less than 20 nm, and even more preferably less than 5 nm.

10. Anionic conductive membrane (10) for an electrochemical device, membrane comprising a main layer covered by a layer of catalytic material according to one of claims 1 to 9.

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 claim 10.

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

13. A method of depositing a catalytic layer on a main layer of an anionic conductive membrane, the catalytic layer comprising nanoparticles of at least two active components and a polymer binder, the method comprising a step of preparing a liquid ink comprising the following steps: - mixing the nanoparticles in a liquid, the liquid being water, a solvent or a mixture of water and solvent, the solvent being for example an alcohol, - grind the mixture of particles and liquid, - add the polymer binder to the ground material and mix.

14. The method of claim 12 further comprising a step of ultrasonically spraying said liquid ink onto the main layer of the anionic conductive membrane.

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