WO2022167880A1

WO2022167880A1 - Particularly compact and efficient assembly with separator and electrodes to be used in the electrolysis of water for the production of hydrogen at high pressure

Info

Publication number: WO2022167880A1
Application number: PCT/IB2022/050398
Authority: WO
Inventors: Massimiliano BOCCIA; Roberto Cremonese; Daniele VERARDO
Original assignee: CTS H2 S.r.l.
Priority date: 2021-02-04
Filing date: 2022-01-18
Publication date: 2022-08-11
Also published as: EP4288584A1

Abstract

The present invention is relative to a separator/electrodes assembly to be positioned within an appropriate electrolytic cell for the production of hydrogen at high pressure, alternative to the classic MEA, provided with a particularly compact and sturdy structure that makes it possible to achieve an excellent efficiency of the system. In practice, the structure is designed to reduce, or even eliminate the so-called "crossover" of oxygen into the hydrogen and vice-versa, so as to allow the increase in pressure directly in the cell, without compressors, and to facilitate its production and the maintenance operations, thanks to the fact that the apparatus consists of a single metallic piece (One Piece Core, "OPC"). This apparatus is used in the scope of the "AES" (Solid State Alkaline) technology, which shares the principles of the alkaline polymeric membrane (AEM) technology, without resorting to the use of polymeric membrane.

Description

“PARTICULARLY COMPACT AND EFFICIENT ASSEMBLY WITH SEPARATOR AND ELECTRODES TO BE USED IN THE ELECTROLYSIS OF WATER FOR THE PRODUCTION OF HYDROGEN AT HIGH PRESSURE’’

DESCRIPTION

TECHNICAL FIELD OF INVENTION

[001] The present invention refers to a negative electrode/separator/positive electrode assembly, in other words a cathode/separator/anode structure to be inserted in an electrolytic cell based on the same principle of operation as the AEM (Alkaline Electrolyte Membrane) technology in place of the classic MEA (Membrane Electrode Assembly). In practice, the structure has been conceived to sustain high operating pressures with the purpose of improving the yield and at the same time considerably reduce the so-called “crossover” of hydrogen in the oxygen and eliminate the crossover of oxygen in the hydrogen. Obviously, it is clear that the invention may also be used in sports shoes similar to a ski boot, such as for example snowboard or ski mountaineering boots, cross-country ski shoes, mountain or rock-climbing shoes, shoes for ice or roller skates, cycling shoes and other types of sports shoes.

PRIOR ART

[002] The prior-art electrolyzers consist of an internal structure of the electrolytic cell made up of two catalytic layers (electrodes) that respectively are the anode and the cathode. The anode is the electrode where the reaction of production of oxygen takes place and the cathode is the electrode where the reaction of production of hydrogen takes place. The electrodes are arranged to form a sandwich structure enclosing an ion exchange polymeric membrane (known with the acronym MEA, or Membrane and Electrodes Assembly), acting as a solid electrolyte in addition to being a separator of the two compartments, and thus of the two gases, in the PEM (Proton Electrolyte Membrane) technology and in the AEM (Alkaline Electrolyte Membrane) technology, whereas in the former AEL (Alkaline Electrolyte Liquid) technology a plastic diaphragm is used as a separator and the electrolytic action is performed by a highly conductive liquid, like soda (NaOH) or potash (KOH) at high concentrations (25-30% of weight by weight w/w).

[003] As is widely known, electrolysis is based on the decomposition of water by means of an electrical potential. Hydrogen is generated on the cathode (-) and oxygen on the anode (+). Between the electrodes, the electrolyte acts as ionic conductor. The ions that transmigrate between the electrodes are both H⁺ and OH'. The electrolytic membrane separates the H2 and O2 gases that are generated between the two electrodes. Moreover, it must satisfy important requirements, such as stability in operating conditions, effective separation of the gases, mechanical separation of the electrodes, ionic conduction and mechanical support for the pressure differences between the two sides of the cell, generally lower than 30 bar.

[004] These are the reactions that take place by alkaline electrolysis on the cathode and on the anode:

[005] The obtainment of large quantities of hydrogen and its storage as a source of energy has always been a subject of great interest, thanks to its high ecocompatibility, especially since the attention presently given to environmental problems, and therefore to highly polluting fossil fuels, has gained a fundamental importance at the global level.

[006] Among the above-mentioned technologies, the electrolyzers based on liquid alkaline electrolyte (AEL) are the most common because they have the best “performance/price” ratio, thanks to the low cost of components and to the scalability in the large dimensions (large diameter of the electrolytic cell). Thus, this technology is the current standard for large-scale electrolysis, while the polymeric-membrane based protonic technology (PEM) has the main advantage of simplicity of layout of the system thanks to the high purity achieved, with a mild system of purification.

[007] However, in recent years there has been increasing interest in the membrane-based alkaline technology (AEM) which is interposed between the liquid alkaline technology (AEL) and the protonic polymeric technology (PEM), as it combines the advantages of both. The extreme simplicity of the system of this technology makes it possible to increment the pressure of the hydrogen directly within the electrolytic cell so as to obtain hydrogen with a high degree of purity already under pressure, ready to be stored without the help of the compressor, with clear advantage in terms of increased total efficiency of the system and of the lower costs connected with the purchase and maintenance of the compressor.

[008] One of the main problems connected with the increased pressure within these cells, if said purpose is kept in mind, is the phenomenon of “crossover” of each gas from its formation compartment to the other. There is, in fact, a threshold quantity of hydrogen that can mix with the oxygen so as to avoid having an explosive mixture, both in one direction and in the opposite direction with respect to the formation compartment of both gases. One of the functions of the polymeric membrane consists in fact of separating the gases. Although a total separation is ideal, in reality a small quantity of gas passes past the membrane through its pores; this is necessary to absorb water and ensure ionic transport. The quantity is proportional to the pressure of the gas, but naturally, if the pressure in increased this phenomenon of migration or “crossover” would also be increased, and this would affect the functionality of the system and make it hazardous.

SUMMARY OF THE INVENTION

[009] The technical problem at the basis of the present invention is therefore to devise a system that makes it possible to operate under high pressures while avoiding the problem of the “crossover” phenomenon and the breakage of the polymeric membrane that must sustain high pressure levels.

[0010] This problem is solved by an electrolytic cell provided with a compact basic “negative electrode/separator/positive electrode” structure that is resistant to high pressures.

[0011] Thus, a first objective of the present invention is an assembly provided with electrodes that are catalyzers consisting of common (i.e., not noble), and therefore low- cost metals, capable of being used in an alkaline environment for the generation of oxygen at the anode and of hydrogen at the cathode.

[0012] A second objective is an apparatus that uses a very resistant separator, selective to the ionic exchange capable of being used in place of membranes or diaphragms used in the prior art alkaline technology with the polymeric-based membrane.

[0013] A third objective is an electrolytic cell comprising said apparatus.

[0014] A further objective is a particularly efficient hydrogen-producing electrolysis process that uses said electrolytic cell.

BRIEF DESCRIPTION OF THE FIGURES

[0015] Further characteristics and the advantages of the apparatus according to the invention will become more evident from the following description of some embodiments given purely by way of non-limiting examples with reference to the enclosed figure 1 , which is a schematic view of a cell of the electrolyzer comprising the assembly according to the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0016] With reference to figure 1 , the electrolytic cell according to the invention is schematically indicated in its more internal components, with particular reference to the assembly 10, indicated with reference numeral 1. The electrolytic cell 1 has a sandwich structure in which a sealed container 2 comprises an anodic compartment 3 where the breakdown of water takes place to form oxygen, a cathodic compartment 4 where hydrogen is formed, a separator 5 interposed between said cathodic and anodic compartments, a source of electric current 6 connected to the cathode 40 and to the anode 30 of the respective compartments through appropriate and conventional “current collectors” (not shown in figure 1 ), that have the function of conducting the current from the source to the electrodes.

[0017] The cathodic compartment 4 includes in turn the cathode 40 formed by the catalyzer properly supported, having a first surface 41 in contact with said separator 5. Advantageously, a second surface 42 of the cathode, opposite to said first surface, is in contact with a first surface 70 of a layer 7 permeable only to hydrogen (H2). This layer 7 is structured in such a manner as to have a porosity whose pores have a diameter smaller than 0.26 nanometers, preferably included between 0.1 and 0.19 nanometers. Preferably, said layer is made of sintered metallic material capable of sustaining high pressures in the order of 300 bar and above.

[0018] The above selective porosity has been studied in order to allow the passage into the cathodic compartment of hydrogen alone but not water.

[0019] A second surface 71 of the layer 7 is then turned toward a chamber 43 of the cathodic compartment 4. In said chamber is released hydrogen at high pressure, which will be sent, for example to storage through an outlet 44 of the same cathodic compartment.

[0020] Advantageously, according to the present invention, the separator 5 is interposed as in a sandwich between the cathode 40 and the anode 30 of the respective compartments and is permeable to water. In particular, the pores of the separator have a diameter between 0.2 and 0.28 nanometers so that they can trap a very diluted aqueous solution of KOH (potassium hydroxide) as electrolyte for the electrolysis reaction.

[0021] Preferably, the separator is metallic, that is consisting of a sintered metal layer. More preferably, the separator metal is nickel-based, possibly coated with carbon-based materials, such as for example graphene and carbon nanotubes.

[0022] Alternatively, the separator can be made up of the above-mentioned carbon-based materials or oxides such as for example transition metals of the “d” or “f” block.

[0023] In addition, this separator is capable of sustaining pressures higher than 350 bar, as it must maintain a maximum positive pressure delta in the order of 30-50 bar between the anodic compartment with respect to the cathodic compartment.

[0024] In any case, the separator 5 is preferably made by sintering, Atomic Layer Deposition (ADL) or additive laser printing. Each of these methodologies is known in the field and the adjustment of the respective operating parameters such as for example, temperature and pressures, are within the common knowledge of the technician to achieve the above-mentioned dimensions of the pores of the final separator.

[0025] In the case of sintering, the preferred method is thermomechanical sintering, which is carried out inside a mold that works with pressures between 200 and 500 bar and temperatures between 600 and 2,000°C.

[0026] The anodic compartment 3 comprises an anode 30 having a first surface 31 in contact with said separator 5 and a second surface 32 opposite to the first surface and facing a chamber 33 of the anodic compartment itself.

[0027] In particular, the chamber 33 includes an inlet 34 to feed at high pressure, over 300 bar, an aqueous mixture di potassium hydroxide (KOH) or sodium hydroxide (NaOH). It should be noted that thanks to the structure of the electrolytic cell of the invention, the aqueous solution of KOH or NaOH can advantageously be included between 1% and 5% in weight (w/w).

[0028] Further, the chamber 33, but also the entire cell 1 , of the invention is built so as to sustain high pressures in the order of 350 bar and above, with precision machining processes of the contact surfaces with a very low tolerance and with high performance gaskets which in contact with a perfectly smooth surface and properly and evenly tightened guarantee their air tightness.

[0029] In fact, as described below, the above aqueous solution is fed at high pressure into the inlet 34, and at the outlet 35 a liquid-gas separator 36 makes it possible to recover the oxygen produced in the electrolysis reaction by the aqueous solution of potassium or sodium hydroxide; the latter will be reinserted into the inlet through a conventional circuit (non shown).

[0030] The cathode 40 and the anode 30 of the cell according to the present invention are entirely conventional. Thus, they are electronic conductors provided with a catalytic surface adapted to favor the release of hydrogen and oxygen, respectively, and adapted to offer a large surface between a catalyzer and an electrolyte. Moreover, they must have sites that are suitable for the formation of gas bubbles and properties that favor the detachment of gas bubbles so that they separate from the electrolyte when the operating voltage of the cell is achieved. [0031] The support materials of the anode and the cathode are typically steel and steel cladded with nickel. The large relative surfaces are obtained with the use of sintered structures, screens, perforated plates and plates with electrochemically corrugated surfaces machined by laser.

[0032] In addition, a conventional command and control system (not shown in figure 1 ) comprises pressure sensors mounted on the respective anode and cathodic compartments so as to constantly record the pressure and send corresponding signals to a control unit that will regulate the pressure increase of said aqueous solution in the anodic compartment following the pressure increase in the cathodic compartment due to the buildup of hydrogen during the operation of the cell.

[0033] From what has been described above, the electrolytic cell is characterized mainly by said assembly 10 consisting of a sandwich-like structure having a separator interposed between the anode and the cathode, in which the separator is a metallic layer 5 with a porosity between 0.2 and 0.28 nanometers so as to only allow the passage of an aqueous solution of KOH or NaOH, but not the oxygen (O2), and the cathode 40 is cladded on its opposite surface 42 with respect to the one in contact with the layer by a layer 7 with a porosity lower than 0.26 nanometers, preferably included between 0.1 and 0.19 nanometers so as to allow the passage to the outside of the apparatus of the hydrogen alone, but not of said aqueous solution.

[0034] A further objective of the invention is an electrolysis process comprising the steps of:

- providing an electrolytic cell as previously described having said sandwich-like structure of the assembled separator;

- feeding into the anodic compartment 3 an aqueous solution of potassium hydroxide or sodium hydroxide at a concentration of 1 -5% w/w at an electronically controlled pressure having a delta positive (overpressure) with respect to the cathodic compartment of 30-50 bar;

- allowing the selective passage of said solution through the anodic compartment as far as a metallic separator permeable to said solution;

- switching on a current generator to allow current to flow between a cathode and an anode so as to obtain the electrolysis of the solution contained in said separator;

- allowing the passage of hydrogen generated by the electrolysis through the cathode to a cathodic compartment;

- recovering the hydrogen in the cathodic compartment at a pressure of 300 bar or more.

[0035] In particular, it should be noted that the aqueous solution permeates with a pressure difference between the anodic compartment and the cathodic compartment of more than 30 bar, preferably between 30 bar and 50 bar, passing from the anodic compartment through the anode and the separator permeable to water until it reaches the cathode. Once the reaction to the cathode has occurred, the hydrogen released is pushed outside the cathode due to the overpressure of the aqueous solution through the layer permeable to the hydrogen. The ions OH’ generated are returned toward the anode by the effect of electroosmotic entrainment, where oxygen is formed and mixes with the starting aqueous solution.

[0036] A turbulent motion is promoted by an appropriate speed of the pump that feeds the aqueous solution into the anodic compartment, so as to advantageously obtain the rapid removal of the oxygen formed, which cannot permeate through the separator thanks to the above-mentioned characteristics of the separator itself. Moreover, the above- mentioned high pressure in the anodic compartment is controlled so that there is always the pressure delta between 30 and 50 bar mentioned above during the entire operation of the cell. In other words, since the cathodic compartment is closed by a conventional valve (not shown in figure 1 ), as the formation of hydrogen increases the pressure within it also increases and, as a consequence, the pressure within the anodic compartment must also increase to maintain the delta 30-50 bar necessary to ensure the correct functioning of the electrolysis and of the system as designed.

[0037] The ionic conduction is ensured by the presence of the liquid electrolyte, present in small quantities thanks to the compact structure of the electrodes.

[0038] From what has just been described, it is clear that the problems of the prior-art electrolytic cells have been overcome in relation to the pressure increase directly within the cell and important advantages have been achieved.

[0039] First of all, the structure of the cell allows a considerable compacting to the advantage of systems in which more cells are used together to produce considerable quantities of hydrogen.

[0040] Moreover, the cell is sturdier, since the elimination of polymeric membranes substituted with a sintered separator makes it possible to sustain decidedly high pressures.

[0041] At the same time, the selective permeability of the water of said separator combined with the selective permeability of the hydrogen of the sintered layer on the cathode makes it possible to reduce, if not completely eliminate, the problem of the “crossover”.

[0042] In addition, the structure of the cell makes it possible to not only substitute a more fragile polymeric membrane but also to reduce the quantity of electrolyte and of its aqueous concentration.

[0043] Moreover, the compactness of said assembly and the absence of membranes allow easy industrialization and maintenance since the assembly is conceived as a single manufactured article that takes the place of MEA. This assembly takes the name of “One Piece Core”, indicated with the acronym “OPC”. [0044] It follows that the production and manufacturing costs are drastically reduced in return for a reactivation of minimum quantities of electrolyte every 6/12 months.

[0045] Variants of the cell and the electrolysis process of the present invention carried out by a person skilled in the art are possible, without however departing from the scope of patent protection as defined by the enclosed claims.

Claims

9 CLAIMS

1. Assembly (10) to be positioned into an electrolytic cell consisting of a sandwich structure having a separator (5) interposed between a cathode (40) and an anode (30), characterized in that the separator is a layer (5) comprising a material with a porosity between 0.2 and 0.28 nanometers so as to allow only the passage of a KOH or NaOH aqueous solution, but not of oxygen (O2), and the cathode (40) is coated, onto its surface (42) opposite to the one contacting the layer, with a layer (7) with a porosity lower than 0.26 nanometers, so as to only allow the passage of hydrogen outward from the apparatus, but not of said aqueous solution.

2. Assembly (10) according to claim 1 , wherein the porosity of said layer (7) is between 0.1 and 0.19 nanometers.

3. Assembly (10) according to claim 1 or 2, wherein said layer (7) permeable to hydrogen consists of a sintered metal, oxides of transition metals, or carbon-based materials.

4. Assembly (10) according to claim 3, wherein said separator (5) consists of a layer of nickel-based sintered metal possibly covered by carbon-based materials.

5. Assembly (10) according to claim 3 or 4, wherein said layer (7) is capable of sustaining pressures of 350 bar and over.

6. Electrolytic cell (1 ) comprising a sealed container (2) capable of defining with the apparatus (10) according to any one of claims from 1 to 5, an anodic compartment (3) where water decomposition takes place with the formation of oxygen, a cathodic compartment (4) where hydrogen is formed, a source of electric current (6) connected to the cathode (40) and to the anode (30) of the respective compartments.

7. Electrolytic cell (1 ) according to claim 6, wherein said anodic compartment (3) comprises an aqueous solution of KOH or NaOH at 1 -5% w/w.

8. Electrolytic cell (1 ) according to claim 6 or 7, wherein said anodic compartment (3) comprises a chamber (33) configured to receive an aqueous solution of potassium hydroxide or sodium hydroxide at an overpressure with respect to the cathodic compartment (4) included between 30 and 50 bar.

9. Electrolytic cell (1 ) according to any claims from 5 to 8, also comprising a command and control system wherein the pressure sensors mounted in the respective anodic (3) and cathodic (4) compartments constantly record the pressure and send corresponding signals to a control unit, which will monitor the pressure of an aqueous solution in the anodic compartment after the pressure increase in the cathodic compartment due to the buildup of hydrogen during the operation of the cell. Electrolysis process for the production of hydrogen, comprising the steps of:

- providing an electrolytic cell according to any of claims from 6 to 9;

- feeding into an anodic compartment (3) an aqueous solution of potassium hydroxide or sodium hydroxide at a concentration of 1 -5% w/w at an overpressure of 30-50 bar with respect to the cathodic compartment (4);

- allowing the selective passage of said solution through the anodic compartment as far as a separator permeable to said solution, but impermeable to gaseous oxygen;

- allowing the selective passage of the hydrogen generated by the electrolysis through the cathode into a cathodic compartment;

- recovering the hydrogen in the cathodic compartment at a pressure of 300 bar or over Process according to claim 10, wherein the overpressure of 30-50 bar within the anodic compartment with respect to the cathodic compartment is constantly controlled and commanded, increasing the pressure of the aqueous solution as a consequence of the increase of hydrogen within the cathodic compartment (4).