ES2961789B2 - SYSTEM FOR DISSOCIATING MOLECULES FROM A LIQUID MEDIUM BY ELECTROLYSIS - Google Patents

Description

DESCRIPTION

SYSTEM FOR DISSOCIATING MOLECULES FROM A LIQUID MEDIUM BY ELECTROLYSIS

FIELD OF INVENTION

The present invention belongs to the field of systems for dissociating molecules from a liquid medium. In particular, the invention is applicable to the production of hydrogen by electrolysis, by dissociating water or another liquid with hydrogen atoms, at ambient temperature and pressure or under other temperature and pressure conditions. It also relates to electrolysers.

BACKGROUND OF THE INVENTION

Through dissociation, molecules in a medium are separated into simpler chemical components, such as smaller molecules or atoms. When dissociation is carried out by means of electricity, it is called electrolysis.

In particular, if the molecules to be dissociated are water, we speak of electrohydrolysis. Specifically, electrohydrolysis to produce green hydrogen is of great interest since it constitutes a clean source of energy. The hydrogen obtained can be used as fuel without emitting CO<2>.

In electrohydrolysis, the hydrogen atoms present in the molecules of a liquid medium (electrolyte), usually alkaline water (for example, distilled water to which potassium hydroxide has been added), are separated by the application of an external electric field.

Electrohydrolysis is carried out using electrodes made of conductive wires or meshes of conductive wires, both for the positive electrode, usually the anode, and for the negative electrode, usually the cathode. The electrolyte is located between the cathode and the anode. By establishing a potential difference between the two electrodes, an electric field is produced between them. Near the electrodes, the electric field increases exponentially with a maximum at the surface of said electrodes. The electric field near the electrodes can be sufficiently high (several eV in the case of water) which favors the separation of the liquid molecules into dissociated components (hydrogen and oxygen in the case of water). When alkaline water is used, this method is called Alkaline Water Electrolysis (AWE).

It is desirable to make the electrolysis process more sustainable. One way involves using energy sources with low CO<2> emissions, for example renewable sources. Another way is to reduce the energy required, which involves reducing the potential difference between the electrodes.

The smaller the diameter of a conducting wire, the greater the electric field in its vicinity. Therefore, one way to require less energy is to use a smaller electrode, since this increases the ability to dissociate molecules. However, there is a drawback to this approach. Very small electrode sizes generally mean less mechanical robustness. In practice, this means that miniaturized electrodes cannot withstand a high electric field.

BRIEF DESCRIPTION OF THE INVENTION

In light of the problems of the state of the art set out in the previous section, one object of the present invention relates mainly to a system for dissociating molecules of a liquid medium into components using electrolysis. The applications are diverse, although the production of hydrogen for energy generation can be highlighted. However, there may be other applications that benefit from the advantages of the present invention. Among them, mention should be made of the deposition of atoms in layers or the chemical synthesis of molecules, both for the chemical industry. Also in the electrochemical reduction of CO2 into methanol.

The proposed system manages to minimize the energy required to obtain dissociated molecules (e.g. hydrogen) by concentrating the electric field in the vicinity of the electrodes, decreasing the voltage that needs to be applied between them. To do this, the system for dissociating molecules by electrolysis includes two conductive structures that act as electrodes, each with a region exposed to the liquid medium (electrolyte). The conductive structures define two chambers (for the anode and cathode). If a voltage difference is applied between the conductive structures, an electric field is generated and the system minimizes the value necessary to cause the dissociation of the liquid medium. This is achieved thanks to a concentrator structure associated with the conductive structures that allows the formation of regions that concentrate the electric field extraordinarily to produce the dissociation.

In one family of embodiments, the concentrator structure itself is used to perform several additional functions. One, to act as a separator structure and to separate both electrically and physically the conductive structures (electrodes) from each other. Two, to prevent bubbles of dissociated gaseous molecules, produced in the liquid medium by one of the conductive structures, from passing to the other conductive structure. Three, to allow the passage of the liquid medium from one side of the concentrator structure to the other. On each side the concentrator structure is joined to a conductive structure. The assembly has holes small enough to prevent (or at least minimize) the exchange of bubbles from one side of the concentrator structure to the other (between the anode and cathode chambers) and, at the same time, to allow some liquid exchange. The holes themselves also help to concentrate the electric field and produce an increase in a marginal region of the conductive structure exposed to the liquid medium. This increase is greater when a portion of the concentrator structure narrows part of the hole. This marginal region includes the edge of the hole in the conductive structure where a high value for the electric field is reached. Various geometries (single wedge, double wedge, etc.) can be defined for the portion for the hole profile in the section corresponding to the concentrator structure. Also for the contour of the hole.

In this family of embodiments, the electrodes constituting the conductive structures can be made as a mesh of wires. At the nodes of the mesh there can be a larger connecting element. They can also be extremely thin sheets, for example, between 10 nanometers and 100 microns, preferably, between 10 nanometers and 10 microns, more preferably, between 10 nanometers and 1 micron. The advantage of sheets over meshes is their greater robustness. In both cases, the electrodes are extremely close to each other.

In another family of embodiments of the proposed system, the separator structure and the concentrator structure are differentiated. The electrodes are further apart in this case. The concentrator structure is composed of two electrically insulating substructures, immersed in the liquid medium. Each substructure integrates its corresponding conductive structure (electrode). The substructures include portions of material that form a series of cavities or valleys inside which a conductive element is housed. The valleys are separated from each other by ridges. This conductive element is part of the conductive structure that functions as an electrode (for example, a grid of conductive wires). The conductive element can be partially exposed to the liquid medium and partially embedded in the substructure for greater robustness. With this design, the field lines are also concentrated in the region of the cavity where said conductive element is located.

Aspects of both families can be combined. For example, valleys and ridges can be designed with geometric shapes that help to better concentrate the electric field in the vicinity of the conductive element. For example, by modifying the profile for the channel for the field. Each channel is delimited by a valley separated by adjacent ridges and can have different shapes. For example, single wedge, double wedge or simply rectangular.

DESCRIPTION OF THE DRAWINGS

The characteristics and advantages will be fully understood from the detailed description of the invention, as well as from the exemplary embodiments referred to in the attached figures, which are described in the following paragraphs.

FIG. 1A schematically shows an electrode formed by a conducting wire with its associated electric field lines. FIG. 1B shows a graph of the electric field intensity as a function of the distance from the center of the electrode.

FIG. 2 is a comparative graph of the current density per unit area for an embodiment according to the state of the art and according to the present invention.

FIG. 3A schematically shows a front view of a first embodiment of the present invention. FIG. 3B schematically shows a side section of a first embodiment of the present invention.

FIG. 4 schematically shows a side section of a second embodiment of the present invention.

FIG. 5A schematically shows a side section of a third embodiment of the present invention. FIG. 5B schematically shows a variation of the third embodiment with an insulating structure with an inner conductive sheet at an intermediate voltage.

FIG. 6A schematically shows a front view of a fourth embodiment of the present invention. FIG. 6B shows a rear perspective view.

FIG. 7A schematically shows a section of a fifth embodiment of the present invention. FIG. 7B shows an enlargement of a part of the fifth embodiment.

Numerical references used in the drawings:

I Electric field line.

5 Conductive element.

5a Edge of the conductive element.

8 Hole.

10 Electrode.

10a Exposed surface of the electrode (to the liquid medium).

I I First conductive sheet.

11a Edge of the hole of the first conductive sheet.

12 Second conductive sheet.

12a Edge of the hole of the second conductive sheet.

13 Insulating sheet.

13th Portion.

131 Inner conductive sheet (of the concentrator structure).

132 Insulating material layer (of the concentrator structure).

14 Fourth separating structure.

21 First conductive mesh.

21a Edge of the first conductive mesh.

22 Second conductive mesh.

2a Edge of the second conductive mesh.

33 Insulating substructure.

33a Valleys.

33b Ridges.

DETAILED DESCRIPTION OF THE INVENTION

A detailed description of the invention is set forth below, referring to different preferred or advantageous embodiments based on Figures 1 to 7. Said detailed description is provided for illustrative purposes and is not limiting with respect to the scope of the claimed invention.

Several designs of structures are presented that concentrate the electric field lines in the vicinity of the electrodes in a region exposed to the liquid medium to be dissociated.

In FIG. 1A a section of an electrode10formed by a conducting wire and the associated electric field lines1is shown, to which a potential difference has been applied with another electrode (not shown). It can be seen that the density of electric field lines1is greater as one approaches the electrode10. The maximum occurs on the exposed surface10a.

FIG. 1B is a graph of the electric field intensity as a function of the distance from the center of the electrode for the above case. The x-axis represents the distance to the electrode surface. The y-axis represents the electric field intensity. It is clearly seen how the value for the electric field intensity increases exponentially in the vicinity of the electrode surface x=a.

As will be seen in the following embodiments, an insulating structure, suitably arranged in the vicinity of regions of a conductive structure, makes it possible to create a region for each electrode that promotes dissociation. A further increase in the concentration of the electric field occurs. This region functions as a channel to direct and concentrate the electric field.

FIG. 2 is a comparative graph of current density per unit area for an electro-hydrolysis process according to the state of the art in dotted lines and according to an embodiment of the present invention in solid lines. The x-axis represents the voltage measured in Volts (V). The y-axis represents the electric current density per unit area or surface current density, measured in Amps per square centimeter (A/cm2). The units shown are not precise and are only used to illustrate favorable performance.

It can be seen from the comparison between the curves that the proposed system manages to produce hydrogen by dissociating the water molecule by applying voltages of lower value. This result can be extended to other chemical reactions of electrolysis, replacing the water with another liquid composed of other molecules, and choosing the appropriate electrolyte for said reaction. From these liquid molecules, the atomic elements that compose them or simpler molecules would be obtained.

FIG. 3A and FIG. 3B are both a schematic representation of a front view and a side section of an embodiment of the system. For clarity, the liquid medium with the molecules to be dissociated is not shown. The system includes an insulating sheet 13 that is externally joined on one side to a first conductive sheet 11 and on the other opposite side to a second conductive sheet 12. This three-layer structure has multiple holes 8. One of such holes is shown in the side section of FIG. 3B (hidden in the front view of FIG. 3A). The conductive sheets 11, 12 form the electrodes (anodes on one side and cathodes on the opposite side). The holes 8 are through and communicate one side (anode chamber) with the other (cathode chamber) allowing the passage of molecules to be dissociated in a liquid state, but preventing the passage of gaseous bubbles with molecules produced in the dissociation.

By applying a potential difference between the two electrodes (first conductive sheet 11, and second conductive sheet 12), the electric field lines 1 are concentrated around the holes 8 as shown in FIG. 3B. Due to the narrowing of the passage channel, an increase in the concentration of the electric field occurs in the center of the hole 8 and, therefore, near the edge 11a of the conductive sheet 11, 12 which is exposed to the liquid medium and in its vicinity.

In FIG. 3B it can be seen that a channel is formed which passes through said hole 8 and which concentrates, as it passes through the interior, the electric field. When this channel is modified and made additionally narrower with a portion 13a (in the form of a projection) of the insulating sheet 13 which extends in a direction transverse to the direction of the electric field, the electric field which must pass through the hole 8 is further concentrated and thus, on the edges 11a of each hole 8 of the conductive sheet 11. In this way, the electrolysis reaction is favoured.

The profile shown by FIG. 3B is called a double-wedge profile. That is, the diameter of the hole8 progressively decreases from the faces of the insulating sheet13 (each face next to one of the conductive sheets11,12) to an inner plane. For example, to a plane dividing the insulating sheet13 into two halves of equal thickness. That is, it would be spindle-shaped. However, other shapes would be possible for the inner channel portion of the hole8 (surrounded by insulation) as will be shown below. Some shapes have advantages in particular configurations.

In FIG. 4 another geometry is illustrated, called a simple wedge, for a portion 13a of the channel formed internally by the hole 8 which would be conical in shape. As in the previous embodiment, the system has a conductive sheet 11, 12 that is joined to a different face of the insulating sheet 13. With the simple portion 13a the diameter of the hole 8 progressively decreases from one of the faces of the insulating sheet 13 to the other opposite face.

In FIG. 5A another geometry is illustrated, called cylindrical where the inner channel defined by the hole 8 is cylindrical. A rectangular portion of the insulating sheet 13 can be seen extending into the inner channel in the section through which the insulating sheet 13 passes. It can be seen that it narrows with respect to the conductive sheets 11, 12 located on the outside. The portion of the insulating sheet 13 reduces the free space for the passage of the electric field through the hole 8 and thus increases the electric field at the edge 11 of the hole 8 in the conductive sheet 11, and similarly at the edge 12 of the hole 8 in the conductive sheet 12.

In FIG. 5B the same geometry as in FIG. 5A is illustrated but using a different insulating structure. Instead of a uniform insulating sheet a sandwich structure is used where an inner conducting sheet 131 is covered with a layer 132 of insulating material on each side, which prevents the inner conducting sheet 131 from coming into contact with the conducting sheets 11, 12. Thus each element 11, 12, 131 can be at a different voltage. In particular it is shown that voltage V1 is applied to the first conducting sheet 11, voltage V2 is applied to the second conducting sheet 12 and voltage V3 is applied to the inner conducting sheet 131. It follows that V1>V2>V3.

A schematic representation of another embodiment quite similar to that shown in FIG. 3A is illustrated in FIG. 6A and FIG. 6B. In this embodiment, instead of a conductive foil, a mesh 21 with multiple annular conductors 7 and wires 9 between them is used. As can be seen, the meshes 21, 22 cover much less surface of the insulating foil 13 than one foil. Analogously to the previous embodiment, there is an inner portion (not shown) of the insulating foil 13 which narrows the passage through the hole 8 and concentrates the electric field at the edge 21a of the hole 8 of the annular conductor 7 of the conductive mesh 21.

In FIG. 6B the other side can be seen, corresponding to the hidden face of FIG. 6A, with the other conductive mesh22. Each hole8 is surrounded on both sides of the insulating sheet13 by an annular conductor7, one corresponding to the conductive mesh21, another to the conductive mesh22 on the other side. There is also an edge22a defined by the hole8 that coincides with the inner diameter of the annular conductors7 present in the conductive mesh22.

Regarding the aforementioned geometries in FIGS. 3A, 4 and 5 for the insulating portion responsible for reducing the channel and concentrating the electric field, it should be noted that they are equally applicable to this embodiment with a mesh instead of a sheet. Some characteristic properties of each geometry are mentioned below.

A double wedge has the advantage of further concentrating the field lines and is easier to fabricate by etching techniques. A single wedge close to the electrode that produces the dissociated molecule of interest (e.g., hydrogen) has the advantage of making recombination of the dissociated molecule into the molecule to be dissociated (e.g., water) less likely.

Any additional portion of insulation to allow the electric field to be concentrated in the vicinity of the electrodes with the holes would be applicable. Other geometries could be used to reduce the passage channel to different degrees (e.g. 25%, 35%, 50%, 75%, 90%, etc. with respect to the contour of the hole in the conductor).

One of the difficulties of systems that carry out electrolysis is recombination. In particular, when it comes to hydrolysis, recombination involves a mixture of oxygen and hydrogen gas bubbles. In the above embodiments, the oxygen and hydrogen gas bubbles are separated by the very design of the field line concentrator structure. As an additional function, the concentrator structure is also a separator and allows the passage of bubbles to be reduced; the holes are small in size and limit or prevent the passage of bubbles. Thus, it is important to establish an adequate size of the holes so that the exchange of molecules of the liquid medium (water, in the case of electro-hydrolysis) is allowed but the exchange of dissociated gaseous molecules (molecular hydrogen, H<2>) is prevented.

One of the advantages of using conductive sheets is that the energy required to produce electrolysis is lower, since on the one hand, there is a concentration of field lines near the hole, and on the other hand, the distance between the electrodes can be much smaller because it is more robust. In this way, the field can be higher than for conductive meshes.

It should be noted that different techniques can be used to manufacture the different embodiments. In particular, the manufacturing process for the insulating sheet can be very varied. For example, starting from an insulating sheet, such as mylar or kapton, a thin layer of copper or another low-cost conductor is deposited, in which a perforation is subsequently made by chemical etching to make the holes. To obtain a regular mesh of holes, for example, a template with holes would be used through which the substance that carries out the chemical etching for the perforation would pass, but which is not affected by said substance.

FIG. 7A schematically shows a horizontal section of a different embodiment where the concentrator structure is divided into two insulating substructures 33. Each insulating substructure 33 incorporates its corresponding electrode. For clarity, the liquid medium whose molecules are to be dissociated is not shown. The vertical direction (perpendicular to the plane of the figure) coincides with the direction of Earth's gravity. This embodiment allows the use of a separator structure 14 that can be conventional. The separator structure 14 prevents bubbles of gaseous dissociated molecules from passing through it and allows the exchange of liquid.

Each insulating substructure 33 forms valleys 33a and ridges 33b to concentrate the electric field lines near the electrodes. The electrodes are formed with a conductive structure 31, 32 (dotted boxes) composed of a wire 9 (although it can also be a ribbon, the wire can form a three-dimensional grid) that connects a series of larger conductive elements 5 integrated in the insulating substructure 33.

Each valley 33a forms a cavity where the conductive element 5 is housed, partially bordered by insulating material. In the valley 33a, the lines 1 of the electric field towards the conductive element 5 are concentrated. At the edge 5a of said conductive element 5 exposed to the liquid medium, a greater increase in the electric field is achieved, which favors molecular dissociation.

In an analogous embodiment, instead of using electrically insulating material for the entire concentrator structure, a conductive material subjected to a lower potential than the electrodes could be used. In this case, the electrode would have to be separated from the conductive material concentrating the field lines by an insulator (e.g. completely covered by an insulating layer, e.g. similar to FIG. 5B) or by a distance.

Electrodes can be deposited in the valley portion by various methods, e.g. chemical vapor deposition, sputtering, etc.

In all embodiments, the insulating material can be any insulating material, which can be arranged in thin but robust sheets, such as kapton, mylar or even a very thin sheet of aluminium, operating at very low voltages, for example between 0.5 V and 1 V, or between 1 V and 2 V. The material of the electrodes can also be very diverse such as copper or any low cost but robust conductive compound. The shape of the holes and valleys is preferably circular but could also be hexagonal or any other polygonal shape.

For illustrative purposes, some measurements and ranges are provided for various elements described here.

The thickness of the insulating film is generally between 1 micron and 100 microns. It can also be several tens of nanometers, for example 90, 50 or 30 nanometers.

The inlet or outlet diameter of the holes can be between 1 micron and 50 microns.

The voltage ranges for different electrolysis reactions are given below:

Water (hydrolysis): Between 1.23 V and 1.8 V

Electrochemical reduction of CO2 to methanol: Between 1 V and 1.5 V

Claims (11)

1. System for dissociating molecules from a liquid medium by electrolysis comprising: - a first conductive structure (11,21,31) with a plurality of edges (11a,12a,5a) exposed to the liquid medium on a first side; - a second conductive structure (12,22,32) with a plurality of edges (11a,12a,5a) exposed to the liquid medium on a first side; where, in operation, both conductive structures (11,12) act as electrodes and there is a voltage difference between the first conductive structure (11,21) and the second conductive structure (12,22) that generates an electric field; characterized in that it comprises: - a third concentrator structure (13,33) configured to concentrate the electric field, where the third concentrator structure (13,33) is coupled with both conductive structures (11,12) by a second face opposite the first face, where the third concentrator structure (13,33) comprises a plurality of portions (13a,33a) exposed to the liquid medium and associated with the plurality of edges (11a,12a,5a) of the conductive structures (11,12) where the portions (13a, 33a, 33b) cause an increase in concentration of the electric field at the edges (11a,12a,5a), such that the voltage difference threshold necessary to dissociate the molecules of the liquid medium into dissociated gaseous molecules of lower complexity is reduced; - a fourth separator structure (14) configured to separate the first conductive structure (11,21) from the second conductive structure (12,22), to allow the exchange of molecules of the liquid medium, and to limit a mixture of gaseous dissociated molecules produced in each conductive structure (11,12,21,22). 2. System for dissociating molecules from a liquid medium by electrolysis according to claim 1, wherein the third concentrator structure (13,33) is made of insulating material. 3. System for dissociating molecules from a liquid medium by electrolysis according to claim 1, wherein the third concentrator structure (13,33) is made of conductive material (131) partially coated with a layer of insulating material (132), where, in operation, an intermediate voltage between the voltage value of the first conductive structure (11) and the second conductive structure (12) is applied to the conductive material of the third concentrator structure (13,33). 4. System for dissociating molecules from a liquid medium by electrolysis according to any one of claims 1 to 3, - where the first structure is a first conductive sheet (11), the second structure is a second conductive sheet (12), where the third concentrator structure (13) is also the fourth separator structure; where the three structures (11,12,13) are joined and each comprise a plurality of through holes (8), where each hole (8) has an associated edge (11a,12a) in each conductive sheet (11,12). 5. System for dissociating molecules from a liquid medium by electrolysis according to any one of claims 1 to 3, - where the first conductive structure is a first conductive mesh (21), where the second conductive structure is a second conductive mesh (22), where each conductive mesh (21,22) comprises a plurality of annular conductors (7) and conductive wires (9); - where the third concentrator structure (13) is also the fourth separator structure (14) and where the third concentrator structure is insulating and is connected to both conductive meshes (21,22); where the first conductive mesh (21), the second conductive mesh (22) and the third insulating sheet (13) comprise a plurality of holes (8) passing through the annular conductors (7), where each hole (8) has an associated edge (11a, 12a) in each annular conductor (7) of each conductive sheet (11, 12). 6. System for dissociating molecules from a liquid medium by electrolysis according to claim 4 or 5, wherein a channel defined by the hole (8) in the insulating sheet (13) has a profile that is narrowed by a protruding portion (13a). 7. System for dissociating molecules from a liquid medium by electrolysis according to claim 6, wherein the profile of the channel defined by the hole (8) is in the shape of a single wedge, a double wedge or a rectangular wedge. 8. System for dissociating molecules from a liquid medium by electrolysis according to any one of claims 1 to 3, where each conductive structure (31,32) comprises a plurality of conductive elements (5) and conductive wires (9); where the third concentrator structure (13) comprises two insulating substructures (33), each associated with a different electrode, where the portions (33a, 33b) in each insulating substructure (33) form a valley (33a) and a ridge (33b), where the valley (33a) houses a conductive element (5) at least partially exposed to the liquid medium by an edge (5a). 9. System for dissociating molecules from a liquid medium by electrolysis according to claim 8, wherein the profile for a channel defined by the valley (33a) separated by two crests (33b) is in the shape of a single wedge, a double wedge or a rectangular wedge. 10. System for dissociating molecules from a liquid medium by electrolysis according to any one of claims 1 to 9, wherein the liquid medium to be dissociated is water. 11. System for dissociating molecules from a liquid medium by electrolysis according to any one of claims 1 to 9, wherein the liquid medium to be dissociated is methanol.