Solar-driven molecular splitting with surface-modified 3c-sic microparticles and associated electrolysis cell

Abstract

The invention relates to the use of 3C-SiC microparticles having a carbon surface, for the solar-driven splitting of molecules.

Description

[0001] Solar molecular cleavage with surface-modified 3C-SiC microparticles

[0002] The invention relates to the use of 3C-SiC microparticles and an electrolysis cell for solar splitting of molecules.

[0003] To provide energy carriers in a resource-efficient manner, it would be desirable to produce hydrogen through the electrolysis of water, with the energy input coming directly from sunlight. This process is also referred to as solar water splitting and is also known as photoelectrochemical (PEC) water splitting. Ideally, a cost-effective and simple reactor should be available for this purpose, for which, for example, existing commercial reactors could be easily modified.

[0004] Prior art proposals exist that utilize nanocrystalline materials, such as gold, platinum, or silicon carbide (SiC) nanoparticles, to produce hydrogen directly from sunlight. Nanoparticles are defined as objects with three external dimensions, where the object's dimension in at least one dimension is smaller than 100 nm. These proposals exploit the nano-specific properties of the materials. However, due to the nanometer particle size, only those portions of sunlight with wavelengths in the ultraviolet region of the electromagnetic spectrum (i.e., wavelengths smaller than 380 nm) can be used, as the radiation absorption of nanoparticles is limited to such wavelengths. Consequently, for example, the portion of sunlight with wavelengths in the visible or infrared range cannot be used.Nanoparticles with wavelengths greater than 380 nm cannot be used. Therefore, the maximum efficiency of nanoparticle-based systems is limited to approximately 1%.

[0005] Against this background, the object of the invention is to specify possibilities by which the efficiency of solar splitting can be increased.

[0006] This problem is solved by the subject matter of the independent claims. The dependent claims concern specific configurations.

[0007] A key concept of the present invention is to use larger particles, particularly particles in the micrometer range, hereinafter referred to as microparticles, for solar fission, instead of nanoparticles as previously used. Microparticles are defined as objects with three external dimensions, wherein the dimension of the object in at least one dimension, preferably in at least two dimensions, and particularly preferably in all three dimensions, is smaller than 1000 pm. Furthermore, the dimension in none of the three dimensions is smaller than 100 nm. The term microparticles also includes aggregates of small particles, provided the above definition of microparticles applies to the particle aggregate. The microparticles may be crystalline.

[0008] Micrometer particles have the advantage that they can also absorb electromagnetic radiation with wavelengths greater than 380 nm and use it for solar splitting, including the visible and infrared components of sunlight. Furthermore, light scattering by the particles can increase efficiency by allowing the absorption of not only direct but also scattered sunlight. For example, this can increase the efficiency of solar water splitting to approximately 10%.

[0009] A first aspect of the invention relates to the use of 3C-SiC microparticles with a carbon surface for the solar splitting of molecules. The term 3C-SiC microparticle refers to silicon carbide microparticles with a cubic crystal structure, also known as β-SiC. 3C-SiC has a band gap of 2.35 eV and therefore exhibits a band gap that fits optimally into the electromagnetic spectrum of sunlight. Solar water splitting requires a minimum energy plus an overvoltage, so semiconductor materials for solar water splitting should have a band gap of at least 2 eV. However, the band gap must not be too large, as this shifts the usable wavelength range of sunlight further and further into the UV range, significantly reducing efficiency. For example, TiCh is a popular material, but with a band gap of over 3 eV, it splits water very inefficiently in the UV range. Other SiC polytypes, e.g.,Hexagonal or rhombohedral SiC, for example, also have a band gap that is far too large. 3C-SiC is the most chemically stable and best-suited band gap material among all semiconductor materials.

[0010] The function of the 3C-SiC microparticles is to absorb as much sunlight as possible and convert the absorbed energy in such a way that it can be used for molecular cleavage. For this purpose, electrons and holes are generated in the semiconducting 3C-SiC microparticles. These electrons and holes cause a chemical reaction at the interface between the 3C-SiC microparticles and the surrounding electrolyte, which contains or consists of the molecules to be cleaved. This reaction leads to molecular cleavage.

[0011] "With a carbon surface" means that the 3C-SiC microparticles have at least a partial, and preferably a complete, surface formed exclusively by carbon atoms. In other words, no silicon atoms are present in the carbon surface region. Preferably, the carbon surface can have or consist of graphite and / or graphene. The carbon surface can reduce or even prevent unwanted oxidation of the SiC surface. Furthermore, the carbon surface can improve charge separation. This can further increase the efficiency of solar molecular splitting. Additionally, co-catalysts can be omitted.

[0012] For example, the molecules to be split can be water molecules and / or CO2 molecules, which are then split into hydrogen and oxygen or carbon monoxide and oxygen, respectively. If a mixture of water and CO2 is split, further or different carbon and hydrogen-containing 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.

[0013] Solar cleavage can be achieved, for example, in an electrolysis cell where 3C-SiC microparticles act as the electrode. The electrolysis cell is designed such that sunlight can strike the 3C-SiC microparticles directly and / or indirectly. The 3C-SiC microparticles are surrounded by an electrolyte that contains or consists of the molecules to be cleaved, thus enabling the desired cleavage to occur as described.

[0014] According to various embodiments, the particle size of the 3C-SiC microparticles can range from 1 pm to 150 pm, preferably from 40 pm to 130 pm. The term particle size refers here to at least one dimension, preferably at least two dimensions, and particularly preferably all three dimensions of the particle. In other words, in a particularly preferred embodiment, all three dimensions of the particles can have a size between 1 pm and 150 pm. The particle size can be determined, for example, by laser diffraction or static laser scattering (SLS). The specified particle size means that the median value or d50 value of the measured particles lies within the stated range, i.e., for example, between 1 pm and 150 pm. The particle size analysis can be performed, for example, according to ISO 13320:2020-01.

[0015] According to further embodiment variants, the 3C-SiC microparticles can be undoped and, in particular, may have neither donor doping with, for example, nitrogen nor acceptor doping with, for example, aluminum.

[0016] This has the advantage of simple and cost-effective particle production, as a doping step can be omitted.

[0017] According to further design variants, the 3C-SiC microparticles can be doped.

[0018] Doping the 3C-SiC microparticles can further increase their efficiency compared to undoped 3C-SiC microparticles, as a greater proportion of sunlight can be used for solar fission. The 3C-SiC microparticles can be doped with, for example, boron, chromium, vanadium, and / or iron.

[0019] Preferably, the 3C-SiC microparticles can be doped with boron. Boron induces a so-called "low" energy level in the 3C-SiC band gap, thus additionally enabling the use of the infrared spectral range of sunlight. Compared to undoped 3C-SiC microparticles, the efficiency can be increased from a theoretical 10% to up to 63%.

[0020] The proportion of the dopant(s) or the doping density, e.g. the boron content, can be at least 10 17 atoms per cm 3 3C-SiC microparticles, preferably between 10 18 and 10 2 ° atoms per cm 3The concentration of the dopant(s) in the 3C-SiC microparticles is determined by their solubility limit in the 3C-SiC. The aim is to create a dopant band, also known as an intermediate band (IB), within the band gap of 3C-SiC, enabling multiple optical excitations from the valence band (VB) to the dopant band and from the dopant band to the conduction band (LB). The excitation energies are preferably in the infrared and red spectral ranges. According to further embodiments, the concentration of impurities in the 3C-SiC microparticles can be less than 0.1 ppm, preferably less than 0.01 ppm. For example, the nitrogen content can be below 0.1 ppm or 0.01 ppm.

[0021] By using particles of the highest purity, the efficiency of molecular splitting can be further increased. For example, parasitic effects, such as unwanted donor-acceptor recombination luminescence as a competing process to the desired water splitting, can be prevented.

[0022] According to further design variants, the 3C-SiC microparticles can be in powder form or used as a powder.

[0023] This can allow for the direct use of the previously produced particles. Complex coating processes, etc., can be eliminated.

[0024] According to further design variants, a co-catalyst can be used.

[0025] In this context, a co-catalyst can mean that it provides a combined catalytic effect with the 3C-SiC microparticles, which can be considered the "main catalyst," and / or an additional catalytic effect. The co-catalyst can, for example, be a noble metal catalyst, where the noble metal could be, for instance, gold and / or platinum. Alternatively or additionally, other co-catalysts, such as iron-based or organic compound-based catalysts like carbonitrides, can be used.

[0026] The co-catalyst can advantageously improve charge separation. However, the co-catalyst plays no role in the absorption of sunlight.

[0027] For example, the 3C-SiC microparticles can be fully or partially coated with the co-catalyst. Alternatively or additionally, the co-catalyst can be in the form of nanoparticles.

[0028] According to further embodiment variants, the 3C-SiC microparticles can be produced by heating a precursor composition.

[0029] The precursor composition can be produced by mixing carbohydrates, e.g., sugars, and nitrogen-free silicon dioxide, where the nitrogen-free silicon dioxide can be, for example, pyrogenic silicon dioxide and / or precipitated silicon dioxide. The nitrogen-free silicon dioxide can be doped, e.g., with boron, chromium, vanadium, and / or iron, or undoped. Doping can be achieved, for example, by introducing the corresponding oxides and / or elements into the silicon dioxide. For instance, boron doping can be achieved by introducing boron oxide and / or elemental boron into the silicon dioxide. The use of oxides has the advantage that the oxides can participate in the carbothermal reaction, and consequently, the dopant can be better incorporated into the 3C-SiC structure.However, since the thermal conditions are not suitable in every case, elemental dopants can be used as an alternative or additional method.

[0030] The precursor composition is characterized by an intensive mixture of carbohydrates and silicon dioxide. In other words, silicon dioxide and the carbohydrate molecules are distributed as evenly as possible and are preferably located close to each other on a molecular scale.

[0031] Fumed silica, also known as pyrogenic silica, is a synthetically produced colloidal material consisting of amorphous silicon dioxide particles aggregated into larger units. It is manufactured by the pyrolysis of silicon tetrachloride with oxygen. Fumed silica is available under trade names such as AEROSIL®, HDK®, and CAB-O-SIL®.

[0032] Precipitated silicon dioxide, also known as precipitated silica, is a synthetically produced colloidal material consisting of amorphous silicon dioxide particles aggregated into larger units. It is produced by precipitation from a silica-containing solution. The term "precipitated silicon dioxide" also includes silicon dioxide produced by the Stöber synthesis. The Stöber synthesis allows for the production of monodisperse particles with definable diameters in the nm to pm range, thereby enabling the subsequent production of monodisperse silicon carbide particles in the aforementioned size range.

[0033] Nitrogen-free means that the nitrogen content is less than 1 ppm, for example, less than 0.1 ppm or less than 0.01 ppm. Nitrogen-free fumed silicon dioxide can be produced in a nitrogen-free protective gas atmosphere, e.g., an argon atmosphere. Accordingly, the process can also involve the production of nitrogen-free fumed silicon dioxide in a nitrogen-free protective gas atmosphere, e.g., an argon atmosphere. In this context, a carbohydrate is defined as a chemical compound with the general formula C n H2nO nThis is understood to mean n > 3. Both a specific carbohydrate and a mixture of different carbohydrates can be used. For example, the carbohydrate can be sugar, e.g., a mixture of glucose and fructose, such as invert sugar, which is a mixture of glucose and fructose produced by the breakdown of sucrose. Glucose and fructose can be present, for example, in a molar ratio of 1:1. The carbohydrates can be used dissolved in a solvent, e.g., water. Thus, for example, an aqueous invert sugar solution can be mixed with nitrogen-free pyrogenic silicon dioxide.

[0034] The preparation of the precursor composition can involve drying the mixture of carbohydrates and pyrogenic silicon dioxide. Drying removes solvents, such as the water from the aqueous sugar solution, resulting in a solid, e.g., powdered, precursor composition.

[0035] Furthermore, the mixture can be heated to a temperature between 200 °C and 500 °C, preferably between 300 °C and 400 °C. This heating can be carried out in stages. Heating can contribute to faster drying of the mixture. Depending on the chosen temperature, partial pyrolysis can also occur, allowing initial bonds between carbon and silicon to form.

[0036] To produce the 3C-SiC microparticles, the precursor composition is heated at least until a pyrolysis temperature is reached. In other words, the precursor composition is pyrolyzed. The pyrolysis temperature can be in a range between 900 °C and 1200 °C, for example, between 1000 °C and 1100 °C. The heating of the precursor composition can be carried out in stages. Pyrolysis can, for example, be performed in a vacuum, e.g., at a pressure between 10 5 and 10' 3 bar, or in an argon atmosphere.

[0037] The process enables the production of a silicon carbide-containing material with a high degree of purity. This means, for example, that nearly pure silicon carbide can be produced. Preferably, the proportion of all residual impurities, e.g., N, P, B, Al, and other metals, can be below 1 ppm, and more preferably below 0.1 ppm. Optionally, the process can include the introduction of a reaction gas into the precursor composition. The term reaction gas refers to a gaseous composition that can chemically react with the precursor.

[0038] The introduction of the reaction gas can occur before, after, and / or simultaneously with the heating of the precursor composition. The reaction gas can further reduce the nitrogen content. For example, the reaction gas can be an argon-hydrogen mixture, where the hydrogen volume fraction in the argon-hydrogen mixture can be between 1% and 20%, for example, between 5% and 10%. Preferably, both argon and hydrogen can be used as high-purity gases, e.g., with a purity grade of 6.0 or 7.0, i.e., a purity of > 99.9999% or > 99.99999%. This can further improve the quality of the resulting silicon carbide.

[0039] The 3C-SiC microparticles, which can be used to split molecules, can be obtained in the form of a black powder as a direct product of the described pyrolysis of the precursor composition.

[0040] According to further design variants, the carbon surface can be formed by heat treatment in a vacuum.

[0041] In other words, graphitization and / or graphenization of the 3C-SiC surface can occur in a vacuum at, for example, a temperature in the range of 1200 °C to 1700 °C. Under such conditions, silicon desorbs from the surface of the SiC composite, resulting in a carbon surface.

[0042] Another aspect of the invention relates to an electrolysis cell for the solar splitting of molecules using 3C-SiC microparticles. The 3C-SiC microparticles can be arranged within the electrolysis cell. Within the electrolysis cell, the 3C-SiC microparticles can function as mini-electrodes and thereby, as explained above, effect the solar splitting of molecules, for example, water and / or CO2 molecules.

[0043] The doping can be, for example, boron, chromium, vanadium, and / or iron. The proportion of the dopant(s) or the doping density, e.g., the boron content, can be at least 10. 17 atoms per cm 3 3C-SiC microparticles, preferably between 10 18 and 10 20 atoms per cm 3 3C-SiC microparticles. The aspects and advantages of the 3C-SiC microparticles explained above also apply analogously to the proposed electrolysis cell, so reference is made to the above statements.

Claims

Patent claims:

1. Use of 3C-SiC microparticles with a carbon surface for solar cleavage of molecules.

2. Use according to claim 1, wherein the molecules are water molecules and / or CCh molecules.

3. Use according to any of the preceding claims, wherein the particle size of the 3C-SiC microparticles is between 1 pm and 150 pm.

4. Use according to any of the preceding claims, wherein the 3C-SiC microparticles are undoped.

5. Use according to any one of claims 1 to 3, wherein the 3C-SiC microparticles are doped.

6. Use according to claim 5, wherein the 3C-SiC microparticles are doped with a dopant selected from the group comprising boron, chromium, vanadium and iron.

7. Use according to any of the preceding claims, wherein the proportion of impurities in the 3C-SiC microparticles is less than 0.1 ppm, preferably less than 0.01 ppm.

8. Use according to any of the preceding claims, wherein the 3C-SiC microparticles are in powder form.

9. Use according to any of the preceding claims, wherein a co-catalyst is used.

10. Use according to any of the preceding claims, wherein the 3C-SiC microparticles are produced by heating a precursor composition.

11. Use according to claim 10, wherein the precursor composition is produced by mixing carbohydrates and nitrogen-free silicon dioxide.

12. Use according to claim 11, wherein the silicon dioxide is pyrogenic silicon dioxide and / or precipitated silicon dioxide.

13. Use according to any one of the preceding claims, wherein the The carbon surface is formed by heat treatment in a vacuum.

14. Electrolysis cell for solar splitting of molecules with 3C-SiC microparticles.

15. Electrolysis cell according to claim 14, wherein the particle size of the 3C-SiC microparticles is between 1 pm and 150 pm.

16. Electrolysis cell according to claim 14 or claim 15, wherein the 3C-SiC microparticles are undoped or doped.

17. Electrolysis cell according to any one of claims 14 to 16, wherein the proportion of impurities in the 3C-SiC microparticles is less than 0.1 ppm, preferably less than 0.01 ppm.

18. Electrolysis cell according to any one of claims 14 to 17, wherein the 3C-SiC Microparticles are present in powder form.

19. Electrolysis cell according to one of claims 14 to 18, wherein the electrolysis cell comprises a co-catalyst.