Reaction cell for photoelectrochemical molecule splitting and use of same

Abstract

The invention relates to a reaction cell for photoelectrochemical, in particular solar, molecule splitting, and to the use of said reaction cell.

Description

[0001] Reaction cell for photoelectrochemical molecular splitting and its use

[0002] The invention relates to a reaction cell for photoelectrochemical, in particular solar, molecular splitting and the use of such a reaction cell.

[0003] To provide energy carriers in a resource-efficient manner, it would be desirable to produce hydrogen through the electrolysis of water. Various electrolysis cell designs are known for this purpose, including those described in Hodges, A., Hoang, AL, Tsekouras, G. et al. A high-performance capillary-fed electrolysis cell promises more cost-competitive renewable hydrogen. Nat Commun 13, 1304 (2022). https: / / doi.org / 10.1038 / s41467-022-28953-x.

[0004] In addition to using electricity generated from renewable energy sources such as wind or solar power, directly inputting energy through solar radiation would improve efficiency. Such a process, which uses solar radiation directly for electrolysis, is referred to below as solar water splitting and is also known as photoelectrochemical (PEC) water splitting.

[0005] Other molecules can also be split by photoelectrochemical, especially solar, molecular fission, such as CO2 into carbon monoxide and oxygen. If a mixture of water and CO2 is split, further or different carbon and hydrogen-containing fission products, such as methane, can be formed. It is also possible to directly produce synthesis gas, i.e., a mixture of hydrogen and carbon monoxide, which can be used in a variety of applications, for example, in the chemical industry.

[0006] The commercialization of direct sunlight use for hydrogen synthesis requires modules that meet numerous requirements. For a given efficiency, the output (hydrogen generation per square meter) should be optimized, also in relation to module costs. In addition to long mechanical durability, the material used should be electrolyte-resistant, meaning the electrolyte must not be damaging. Furthermore, the material should be gas-tight, especially for gaseous hydrogen, have the lowest possible weight, and be scalable. The required amount of electrolyte, e.g., its mass, should be minimized. Cell manufacturing should be as simple and cost-effective as possible. The design should be variable with regard to the electrodes used, e.g., their number, geometric shape, etc. Tracking the cell relative to the direction of sunlight should preferably not be necessary.A PEC cell that meets several or even all of these requirements does not currently exist.

[0007] Against this background, it is an object of the invention to provide an improved reaction cell for carrying out solar molecular splitting.

[0008] This problem is solved by the subject matter of the independent claims. The dependent claims concern specific configurations.

[0009] It goes without saying that the special embodiments mentioned below, in particular special design variants and forms or the like, which are only described in connection with one aspect of the invention, also apply accordingly to the other aspects of the invention, without the need for express mention.

[0010] Furthermore, it should be noted that all parameter specifications mentioned below, or similar information, can in principle be determined or ascertained using standardized or explicitly specified determination procedures, or determination methods that are generally familiar to the person skilled in the art.

[0011] A first aspect of the invention relates to a reaction cell for molecular splitting, for example for splitting water and / or CO2.

[0012] The reaction cell has a sandwich structure formed from a base element and a translucent top element, an electrode arranged within the sandwich structure, designed for solar molecular splitting, a supply device designed for supplying an electrolyte to the electrode, and a discharge device designed for removing a reaction product.

[0013] The terms "top element" and "bottom element" refer to the orientation of the reaction cell when used for solar molecular splitting. This means that the top element is located on the side of the reaction cell facing the incident electromagnetic radiation, particularly solar radiation, and the bottom element is located on the side facing away from the incident electromagnetic radiation, particularly solar radiation. Both the top element and the bottom element can be single-layered or multi-layered.

[0014] The term "solar radiation" refers to electromagnetic radiation emitted by the sun. Within the scope of the present invention, solar radiation is preferably used for molecular splitting because it is very cost-effective. However, to ensure the most continuous operation possible of the reaction cell according to the invention, it is also possible to use electromagnetic radiation from artificial radiation sources.

[0015] At least the cover element consists of a translucent material, e.g., glass such as quartz glass. A material is described as "translucent" if it is transparent or allows at least a portion of electromagnetic radiation, in particular solar radiation, e.g., visible light with a wavelength in the range between 380 nm and 780 nm, i.e., it has a transmittance of at least 80%, preferably at least 90%, more preferably at least 95%, and most preferably at least 99%.

[0016] Preferably, the cover element should be transparent to visible light and at least partially to infrared radiation. Infrared radiation, in this context, refers to radiation with a wavelength between 780 nm and 1 mm. Additionally, the cover element can also be at least partially transparent to ultraviolet radiation. Ultraviolet radiation, in this context, refers to radiation with a wavelength between 200 nm and 380 nm.

[0017] The supply device serves to deliver the electrolyte, e.g., water, to the electrode. The discharge device serves to remove one or more reaction products, e.g., hydrogen and oxygen. Preferably, the reaction products can be collected and discharged separately.

[0018] Within the scope of the invention, it may be provided that the reaction cell has several, in particular 2 to 500, preferably 5 to 300, preferably 10 to 100, electrodes.

[0019] Preferably, several electrodes can be arranged at a distance from each other within the sandwich structure. This allows for efficient solar molecular splitting in a small space.

[0020] The multiple electrodes are preferably arranged parallel to each other in the main plane of the reaction cell.

[0021] The proposed reaction cell is characterized by its exceptional durability and low weight. Furthermore, the reaction cell can be manufactured cost-effectively. Depending on the design, the base and top elements can be bonded together, for example, by welding or fusing.

[0022] This prevents unwanted ingress of foreign substances and can contribute to increasing the service life of the reaction cell.

[0023] The base element and the cover element are preferably directly connected to each other, in particular directly by a material bond. In this way, a simple and virtually defect-free connection between the base element and the cover element is achieved, and defined cavities for the electrode(s) are created.

[0024] According to further embodiments, the electrode can be made of or consist of silicon carbide, preferably silicon carbide with a cubic crystal structure (Crista II). Silicon carbide with a cubic crystal structure is also referred to as "3C-SiC" or "β-SiC". 3C-SiC has a band gap of 2.35 eV and therefore exhibits a band gap that is optimally matched to the electromagnetic spectrum of solar radiation and simultaneously provides the necessary energy for molecular splitting, in particular for the splitting of water. 3C-SiC is the most chemically stable and band-gap-matched semiconductor material among all semiconductor materials.

[0025] Furthermore, the silicon carbide may be doped. If doped, the silicon carbide can be either p- or n-doped. If doped, it has proven effective to use elements from the group consisting of nitrogen, phosphorus, boron, aluminum, indium, iron, vanadium, and mixtures thereof.

[0026] Preferably, according to the invention, the electrode has a pn junction or a pin junction, preferably a pin junction. In this context, it is preferably provided that the electrode comprises exclusively doped and / or undoped silicon carbide as the semiconductor material. Preferably, therefore, the p-doped material, the n-doped material, and optionally the undoped material of the electrode comprise or consist of silicon carbide.

[0027] According to further design variations, the electrode can be porous. The pore size can, for example, range from 1 pm to 100 pm. The porosity, i.e., the ratio of void volume to total electrode volume, can, for example, range from 10 to 60%, e.g., 50%. This porosity allows for capillary action, enabling the electrolyte to be supplied to the electrode. Furthermore, a porous electrode structure, particularly in conjunction with doping of the electrode material and the electrical potential, acts as a membrane, allowing the separation of product gases, especially hydrogen and oxygen, on both sides of the electrode.

[0028] According to further design variants, the cover element can have a structure on a side opposite the base element, i.e., on the side facing the incident electromagnetic radiation, in particular solar radiation.

[0029] This structuring can be achieved, for example, by rolling the glass plates of the cover element from the melt. The structuring enables the deflection of incident electromagnetic radiation, particularly solar radiation, preferably towards the electrode(s). For example, the structuring can be designed such that light incident on the cover element is refracted towards the electrode.

[0030] However, it is also possible that, in a multi-layered, particularly two-layered, structure of the cover element, the structuring constitutes a layer of the cover element, which is preferably bonded to the other layer(s) by a material bond. The structuring can, in particular, be designed as a film, which is bonded to at least one, preferably exactly one, further layer of the cover element.

[0031] Structuring allows for a concentration of electromagnetic radiation, particularly solar radiation, and thus a higher efficiency. Therefore, tracking the orientation of the reaction cell relative to the direction of the solar radiation is unnecessary, keeping costs low while maintaining high robustness.

[0032] Within the scope of the present invention, it is particularly preferred if the structuring comprises one or more, preferably several, optical lenses, in particular one or more, preferably several, Fresnel lenses. Within the scope of the present invention, it is therefore preferred if the structuring comprises optical lenses, in particular Fresnel lenses. Structuring with optical lenses, in particular Fresnel lenses, ensures that the highest possible proportion of the solar radiation striking the cover element actually reaches the electrode and can be used there for the photochemical reaction.

[0033] A structuring element comprising one or more, preferably several, optical lenses, in particular one or more, preferably several, Fresnel lenses, can also be formed as a layer of the top element, especially a film. According to further embodiments, the bottom element can be transparent. This allows excess electromagnetic radiation, especially solar radiation, to pass through the reaction cell and, after passing through the reaction cell, be further utilized, influenced, absorbed, etc. This can contribute to improved utilization of the incident electromagnetic radiation, especially solar radiation. The bottom element can be flat on a side opposite the top element or have a structuring.

[0034] According to a particularly preferred embodiment, the reaction cell has a reflective structure, in particular a reflective layer, preferably a mirror layer. The reflective structure, in particular the mirror layer, is preferably arranged on or in the base element. For example, the underside, i.e., the side of the base element furthest from the electrode, can be mirrored, e.g., by a thin metal layer. However, it is also possible for the base element to have several layers, and for the reflective structure, in particular the mirror layer, to be arranged in or form one of the middle layers of the base element.

[0035] The reflective structure, in particular a reflective layer, preferably a mirror layer, is preferably designed to direct the incident electromagnetic radiation, in particular solar radiation, specifically onto the electrode. For this purpose, the reflective structure, in particular a mirror layer, has a three-dimensional structure. The structure can be generated, for example, by a special design of the underside of the base element, which is subsequently provided with a mirror structure, in particular a reflective layer, preferably a mirror layer.

[0036] According to further embodiments, a cooling device can be arranged on the side of the base element opposite the top element. The cooling device allows the reaction cell to be temperature-controlled, enabling photoelectrochemical, and in particular solar, molecular splitting to be carried out within a preferred temperature range at high efficiency. Optionally, the cooling device can be part of a solar thermal system, allowing excess heat to be dissipated from the reaction cell and reused for other applications. By coupling the reaction cell with a solar thermal system, the infrared component of electromagnetic radiation, especially solar radiation, can be utilized in addition to the radiation used for photoelectrochemical, and in particular solar, molecular splitting. For example, the reaction cell can be operated at a temperature in the range between 80 °C and 95 °C, e.g.,90 °C, i.e. in a temperature range that is also preferred for solar thermal applications.

[0037] According to further design variants, the sandwich structure can have one or more cavities, in particular electrode spaces, for arranging one or more electrodes.

[0038] For example, a single cavity, particularly an electrode cavity, can be divided into a reactant cavity and a product cavity by means of an electrode. The cavity(ies) allow for a protected arrangement of the electrode(s), thus improving the robustness of the reaction cell.

[0039] Due to the preferably high porosity and capillary action of the electrode, it is possible to operate the reaction cell preferably with almost no free electrolyte in an electrode compartment, in particular in a product and / or reactant compartment of the reaction cell. Preferably, the product compartment, i.e., the compartment on the electrode side in which the desired product, in particular hydrogen, is collected, is free of electrolyte. Likewise, it is preferred that the reactant compartment, i.e., the compartment on the electrode side in which the electrolyte is usually supplied and, if applicable, the byproduct of the reaction, in particular hydrogen, is collected, is free or almost free of electrolyte. Preferably, the reactant compartment contains no more than 20 vol%, in particular no more than 10 vol%, and preferably no more than 5 vol%, of electrolyte by volume of the reactant compartment.

[0040] The absence or minimal amounts of electrolyte in the product and reactant compartments ensure better separation of the gases produced, particularly hydrogen and oxygen, and prevent the formation of large gas bubbles that would reduce reaction efficiency. Electrolytic gas generation in a liquid medium leads to gas bubble formation, which can block some of the electrodes from further molecular splitting. Furthermore, it is often difficult to remove these gas bubbles from the surface, especially with porous materials, without additional equipment. For these reasons, the proportion of free electrolyte in the electrode compartment should be kept as low as possible.

[0041] According to further design variants, the discharge device can remove reaction products from a product chamber arranged between the electrode and the cover element.

[0042] According to further embodiments, the supply device can feed the electrolyte into a reactant chamber located between the electrode and the base element. However, it is equally possible for the supply device to feed the electrolyte directly to the electrode; in this way, the reactant chamber can also be free of electrolyte.

[0043] By arranging the product chamber above the electrode and the reactant chamber below the electrode, the different densities of the liquid electrolyte and the gaseous products can be used to separate them. Preferably, however, the proportion of free liquid electrolyte in the reactant and product chambers is kept as low as possible to ensure good separation of the resulting gases and high reaction efficiency.

[0044] According to further embodiments, the feeding device can have a main channel arranged between two cavities and side channels branching off from the main channel. The main channel and / or the side channels can be designed as capillaries.

[0045] This capillary system can enhance the capillary action of the porous electrodes. The advantage is that the gas channels of the exhaust device remain dry and contain only the generated gas.

[0046] Another aspect of the invention relates to the use of a reaction cell according to the above description for photoelectrochemical, in particular solar, molecular splitting, especially for water splitting and / or CCh splitting.

[0047] The aspects and advantages of the reaction cell explained above also apply analogously to the proposed use, so reference is made to the above statements.

[0048] The invention is explained in more detail below with reference to the accompanying figures. Figure 1 shows a schematic representation of an exemplary reaction cell in a top view;

[0049] Fig. 2 is a schematic representation of a cross-section of the reaction cell from Fig. 1;

[0050] Fig. 3 shows a schematic representation of a cross-section of a preferred reaction cell.

[0051] Fig. 1 shows an exemplary reaction cell 1 in a schematic top view. Fig. 2 shows the same reaction cell 1 in a sectional view. The reaction cell 1 can be considered a module, so that, if required, a correspondingly larger capacity for photoelectrochemical, in particular solar, molecular splitting can be created by using several modules.

[0052] The reaction cell 1 has a sandwich structure 2, which is formed from a translucent base element 3 and a translucent top element 4. In the embodiment shown in Fig. 1, both the base element 3 and the top element 4 are made of glass. However, it is also possible for the base element 3 and / or the top element 4 to have a multilayer structure. Different suitable materials can also be used in this case.

[0053] Within the sandwich structure 2 are several cavities 12, each containing an electrode 5 made of 3C-SiC. Alternatively, the electrode 5 can also be made of another material. Preferably, however, the electrode consists of doped and optionally undoped 3C-SiC and has a pn junction or a pin junction, preferably a pin junction. The electrode 5 divides the cavity 12 into a reactant space 14 and a product space 13.

[0054] The electrodes 5 are thin, e.g., less than 1 mm, and narrow, e.g., about 5 cm wide, strips held in a glass structure as a sandwich structure 2. The top view in Fig. 1 is perpendicular to the direction of incidence of the sun or incident electromagnetic radiation or incident light 11. The glass structure consists of a sandwich of two thin glass plates, e.g., 1 mm thick, and has the structuring 9 shown in Fig. 2.

[0055] This structuring 9 can easily be embossed during the production of the glass plates from the melt by rolling. Alternatively, the structuring can also be in the form of a film, which is bonded to the underlying layer of the cover element, in particular by a material bond. In the direction of the incident electromagnetic radiation, especially in the direction of solar radiation, the surface structure or structuring 9 ensures a concentration of the electromagnetic radiation, especially sunlight, so that tracking the orientation of the modules relative to the direction of incidence, especially of the sun, is unnecessary. The back side of the glass double layer, i.e., the underside of the base element 3, can be flat and could, for example, be used for solar thermal energy to additionally utilize the sun's infrared radiation and to cool the reaction cell 1.Experiments show that in the case of SiC, the efficiency is significantly higher at higher temperatures, e.g. 90 °C, which would be optimal for solar thermal operation.

[0056] The "internal" structure of the two glass plates, i.e., the base element 3 and the top element 4, serves to hold the electrodes 5 and to remove the gaseous fission products or reaction products 10 (see Fig. 2, Gas Discharge), e.g., hydrogen and oxygen. In the upper section (not shown here), the separated gases are collected and discharged through openings. In the lower section (also not shown here), the electrolyte 7 is replenished.

[0057] In the sandwich structure 2, main channels 15 in the form of vertical capillaries are located along the inside of the bottom and top elements 3, 4 to deliver the electrolyte 7 to the electrodes 5. Horizontal side channels 16, also designed as capillaries, are used for further distribution. This capillary system supports the capillary action of the porous electrodes 5. The advantage is that the gas channels remain dry and contain only the generated gas.

[0058] Finally, the base element 3 and the top element 4 are joined together by a material bond, e.g., by fusing or welding. A bracket or frame can be used, at least partially, to secure the entire reaction cell 1.

[0059] The distance between and the width of the electrodes 5, as well as the special shape of the sun-facing glass surface of the cover element 3, depend on the efficiency of the reaction cell 1 and the concentrator effect of the glass and its structuring 9.

[0060] Figure 3 shows a preferred embodiment of the reaction cell 1 according to the invention. The reaction cell 1 has a sandwich structure 2 with a base element 3 and a top element 4. The top element 3 is transparent. The top element 4 preferably comprises glass and preferably consists entirely of glass. According to this embodiment, the base element 3 also preferably comprises glass, but does not consist entirely of glass. Within the sandwich structure 2, cavities 12 are formed by the connection of the top element 4 and the base element 3, with an electrode 5 made of silicon carbide, in particular 3C-SiC, preferably doped and optionally undoped 3C-SiC, being arranged in each cavity 12. The electrode 5 divides the cavity 12 into a reactant chamber 14 and a product chamber 13.

[0061] The cover element 4 has a structure 9, which preferably comprises optical lenses, in particular Fresnel lenses. A structure comprising optical lenses, in particular Fresnel lenses, offers the advantage that almost all of the solar radiation 11 striking the reaction cell 1, in particular the cover element 4, can be directed onto the electrode 5 and is thus available for the photochemical reaction.

[0062] In the figure, the cavities 12 are shown with a rectangular cross-section; however, they can also have other geometries, as shown in Fig. 1, and in particular be oval. An oval shape is especially preferable when forming the top and bottom elements from a single glass melt.

[0063] As previously explained, the structuring 9 is preferably designed in the form of Fresnel lenses. Fresnel lenses can be easily structured during the manufacturing process of the cover element 4 from a gas melt. Alternatively, the structuring 9, which preferably comprises optical lenses, in particular Fresnel lenses, can also be designed as a separate layer, in particular a film, which is preferably metallurgically bonded to an underlying layer of the cover element.

[0064] On one side of the cover element 4 facing away from the electrode, a reflective structure, in particular a reflective layer, preferably a mirror layer 17, is arranged either in the cover element 4 or on the underside of the cover element 4.

[0065] In the embodiment shown in the figures, the base element 3 consists of several layers: A first layer 19, which is connected to the top element 4, in particular by bonding or preferably by fusing. On the underside, i.e., the side of the upper layer 19 facing away from the top element, a reflective structure, in particular a mirror layer 17, is arranged. The reflective structure, in particular the mirror layer 17, preferably has a structure that enables it to reflect light 11 passing through the electrode 5 and to reflect it back onto the electrode 5 as reflected light 18 in a targeted manner. According to the embodiment shown in the figures, the structure is achieved by structuring the underside of the upper layer 19 of the base element 3, which is then provided with a mirror layer, in particular a thin metal layer.To protect the reflective structure, in particular the mirror layer 17, the base element has a further layer, in particular the lower layer 20, which protects the reflective structure, in particular the mirror layer 17, from mechanical stress. The lower layer 19 can also be made of glass, but it can also be made of plastic or metal and should, in particular, allow for good heat dissipation. It is also possible that the structure 19 consists of composite materials, for example, glass containing metal to achieve better thermal conductivity. (Reference symbol list reaction cell.)

[0066] Sandwich structure

[0067] floor element

[0068] Cover element

[0069] electrode

[0070] Feeding device

[0071] electrolyte

[0072] Discharge device

[0073] structuring

[0074] reaction product

[0075] incident light

[0076] cavity

[0077] Product space

[0078] Reduction room

[0079] Main channel

[0080] Side channel

[0081] Mirror coating

[0082] reflected light

[0083] upper layer

[0084] lower layer

Claims

Patent claims 1. Reaction cell (1) for photoelectrochemical, in particular solar, molecular splitting, comprising the reaction cell (1): - a sandwich structure (2) formed from a base element (3) and a translucent top element (4), - an electrode (5) arranged within the sandwich structure (2), designed for solar molecular splitting, - a supply device (6) designed to supply an electrolyte (7) to the electrode (5), and - a discharge device (8) designed to discharge a reaction product (10).

2. Reaction cell (1) according to claim 1, wherein the bottom element (3) and the top element (4) are materially bonded together.

3. Reaction cell (1) according to one of the preceding claims, wherein the electrode (5) comprises silicon carbide, preferably silicon carbide with cubic Krista II structure.

4. Reaction cell (1) according to one of the preceding claims, wherein the electrode (5) is porous.

5. Reaction cell (1) according to one of the preceding claims, wherein the top element (4) has a structuring (9) on a side opposite the bottom element (3).

6. Reaction cell (1) according to claim 5, wherein the structuring (9) is designed such that electromagnetic radiation incident on the cover element (4), in particular incident light (11), is refracted in the direction of the electrode (5).

7. Reaction cell (1) according to claim 5 or 6, wherein the structuring (9) comprises one or more, preferably several, optical lenses, in particular one or more, preferably several, Fresnel lenses.

8. Reaction cell (1) according to one of the preceding claims, wherein the bottom element (3) is translucent.

9. Reaction cell (1) according to one of the preceding claims, wherein a cooling device is arranged on the side of the bottom element (3) opposite the top element (4).

10. Reaction cell (1) according to claim 9, wherein the cooling device is part of a solar thermal device.

11. Reaction cell (1) according to one of the preceding claims, wherein the sandwich structure (2) has one or more cavities (12) for arranging one or more electrodes (5).

12. Reaction cell (1) according to one of the preceding claims, wherein the discharge device (8) discharges reaction products (9) from a product space (13) arranged between the electrode (5) and the cover element (4).

13. Reaction cell (1) according to one of the preceding claims, wherein the supply device (6) supplies the electrolyte (7) to a reactant chamber (14) arranged between the electrode (5) and the base element (3).

14. Reaction cell (1) according to one of claims 11 to 13, wherein the feed device (6) has a main channel (15) arranged between two cavities (12) and side channels (16) branching off from the main channel (15).

15. Reaction cell (1) according to claim 14, wherein the main channel (15) and / or the side channels (16) are designed as capillaries.

16. Reaction cell (1) according to one of the preceding claims, wherein several electrodes (5) are arranged spaced apart from each other within the sandwich structure (2).

17. Use of a reaction cell (1) according to one of the preceding claims for photoelectrochemical, in particular solar, molecular splitting.

18. Use according to claim 17, wherein the molecular cleavage is water cleavage and / or CCh cleavage.

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