Photoelectric cell with silicon carbide electrode and production method for same

Abstract

The invention relates to a photoelectric cell with a silicon carbide electrode (4) for photocatalytic production of hydrogen and to a production method for same. The cell has, on one side of the silicon carbide electrode (4), a window (2) for letting in light (5) and, on the other side of the silicon carbide electrode (4), an aqueous electrolyte (10) and a counter electrode (6). The cell is electrolyte-free on the side of the silicon carbide electrode (4) facing the window. The silicon carbide electrode (4) is preferably produced by coating a substrate (3) with silicon carbide (4).

Description

[0001] Photoelectric cell with silicon carbide electrode and manufacturing method therefor

[0002] The invention relates to a photoelectric cell with a silicon carbide electrode for the photocatalytic production of hydrogen and a manufacturing process therefor.

[0003] For a long time, work has been carried out on the provision of hydrogen as an energy source, without a breakthrough towards widespread use having been achieved so far.

[0004] EP 3268512 B1 discloses a photoelectric cell for producing hydrogen by electrolytic splitting of water upon irradiation with sunlight (photocatalysis). The cell contains a SiC electrode in which sunlight generates charge carriers and an electric current flow that causes the electrolytic splitting of the water. The SiC electrode has a porous fiber structure. This structure is intended to provide a large surface area and is produced by transforming carbon fibers into silicon carbide using a process, for example, according to EP 2094622 B1.

[0005] Both EP 3268512 B1 and other known arrangements suffer from the disadvantage that the sunlight first has to pass through a glass plate several millimetres thick and then through a layer of water before it reaches the active material made of SiC. Glass and water absorb significant portions of the solar spectrum, which are then no longer available for photocatalysis. The photocatalysis for splitting water can be operated with photon energies starting at just over 2 eV, while conventional glass absorbs sunlight noticeably from around 3.5 eV. The usable part of the sunlight spectrum is therefore limited to the range from 2 to 3.5 eV. In addition, conventional glass reflects significant portions of the sunlight across the entire spectral range and is lost to the photocatalysis.

[0006] Furthermore, in the known manufacturing process, the carbon fibers of the electrodes are only superficially transformed into SiC. Carbon fibers with a thickness of, for example, 10 pm have only a 2-3 pm thin layer of SiC on their surface, in which only a portion of the incident light is photocatalytically absorbed. The core of the fibers is untransformed carbon and opaque. The portion of the light absorbed there merely heats up the electrode and is lost to the photocatalysis. The desired surface area enlargement through porosity of the fiber structure provides little relief here. The porosity is typically only about 50%.

[0007] A further disadvantage of the known electrodes is the generally high contact resistance within the electrode due to the fiber structure, which leads to significant electrical losses.

[0008] The invention is therefore based on the object of providing a technology which allows the photocatalytic production of hydrogen with better efficiency.

[0009] This object is achieved with the photoelectric cell and the manufacturing method specified in the appended claims. The invention uses a photoelectrode in the form of a layer of 3C-S1C or amorphous SiC. 3C-S1C (cubic silicon carbide), with a band gap of 2.36 eV, is suitable for efficiently generating charge carriers with sufficient energy to electrolytically split water simply by irradiation with sunlight, without the additional application of an electrical voltage. The spectrum of sunlight is efficiently used in this process. The same applies to amorphous SiC, which has a band gap of approximately 2.2 eV. 4H-S1C and 6H-S1C (hexagonal), on the other hand, have a band gap of 3.27 eV and 3.03 eV, respectively, and therefore only absorb shorter-wavelength light and use the spectrum of sunlight less efficiently.The layer can be a standalone plate or a coating of a SiC substrate, for example, a substrate made of metal, graphite, plastic, glass, or similar, with nano- or microcrystalline 3C-SiC or amorphous SiC. The SiC layer can be undoped, p-doped, for example, with Al or B, and / or co-doped, for example, with transition metal elements such as Fe, Cr, or V. The photoelectrode is an electrode with a surface facing the light. This surface can be coated with a co-catalyst, such as Pt or Pd.

[0010] Embodiments of the invention achieve an improvement in optical transmission or a reduction in reflection on the optical path to the electrode, an enlargement of the optically or electrochemically active electrode surface, an improvement in electrical conductivity and contacting of the electrode, and a simple scalability of the cell size ("up-scaling" to square meter-sized modules and larger). Embodiments of the invention are illustrated in the drawing. Therein shows:

[0011] Fig. 1 is a schematic representation of a photoelectric cell according to a first embodiment;

[0012] Fig. 2 is a more detailed representation of a photoelectric cell according to a second embodiment derived from the first, and

[0013] Fig. 3 is a schematic representation of a photoelectric cell according to a third embodiment.

[0014] Identical elements are provided with the same reference symbols in all figures and are not described again for each figure.

[0015] The photoelectric cell shown in Figure 1 comprises a housing 1 with a light-permeable window 2. In the present embodiment, the window 2 comprises a transparent plate 3, in particular a glass plate 3. Alternatively, a transparent plastic plate can also be used.

[0016] Viewed from the direction of the incident sunlight 5, an electrode 4 made of 3C silicon carbide is located directly behind the transparent plate 3 inside the housing 1. The 3C-S1C is preferably nano- or microcrystalline. Alternatively, amorphous SiC can also be used. This SiC electrode 4 has the form of a non-porous or barely porous thin SiC layer 4 with a thickness in the range of 40 to 80 pm. Within this thickness, sunlight 5 entering through the window 2 is absorbed as best as possible. The SiC layer 4 can be applied to the transparent plate 3, which in this case serves as a transparent substrate 3.

[0017] On the other side of the SiC electrode 4 opposite the side with the window 2, here at a distance from the latter, there is arranged a counter electrode 6 which is a metal foam 6 with good electrical conductivity, in the present preferred embodiment nickel foam 6. The metal or nickel foam 6 has a thickness in the range of approximately 3 to 40 mm. The surface of the metal foam 6 can be provided with a catalyst which facilitates the splitting of water. Polyoxometalates are particularly suitable as catalysts, and on the nickel foam 6 in particular those made of nickel, cobalt and tungsten.

[0018] The nickel foam 6 is impregnated with an aqueous electrolyte 10, which also comes into contact with the SiC electrode 4 and in which the water contained therein is to be split into hydrogen and oxygen. The housing 1 has an inlet and an outlet (not shown in Figure 1) for circulating the electrolyte and water, respectively, through the nickel foam 6 in the housing 1.

[0019] An electrical conductor, in this case the electrically conductive or conductively coated housing 1, closes the circuit between the nickel foam 6 and the SiC electrode 4. A transparent conductive layer can be provided between the window 2 and the SiC electrode 4, which is electrically connected to this conductor and makes a large-area, low-resistance contact with the SiC electrode 4. The housing 1 has an outlet 8 for gaseous hydrogen and an outlet 9 for gaseous oxygen.

[0020] A proton-permeable membrane 7 (shown schematically) is provided between the SiC electrode 4 and the nickel foam 6. The membrane 7 separates the electrolyte-impregnated nickel foam 6 from the hydrogen outlet 8 and allows hydrogen to pass to the outlet 8, but not the aqueous electrolyte 10 itself and the oxygen. The gaseous oxygen outlet 9 is directly connected to the impregnated nickel foam 6.

[0021] The surface of the SiC electrode facing the electrolyte 10

[0022] 4 can be provided with a metal layer, for example made of solid metal and / or metal or nickel foam. Such a modification of the embodiment of Figure 1 is described further below in connection with Figure 3.

[0023] During operation, sunlight 5 passes through the window 2, here through the glass plate 3, illuminates the SiC electrode 4 and photoelectrically generates charge carriers therein, which are electrons or holes depending on the doping of the SiC electrode 4. Since the light

[0024] 5 can illuminate the electrolyte-free side of the SiC electrode 4 directly without passing through the electrolyte, it is only slightly attenuated on the way to the electrode 4. With the mentioned thickness of the SiC electrode 4 in the range of 40 to 80 pm, the penetration depth of the sunlight 5 is used to the maximum. Since the thin layer of the SiC electrode 4 is hardly or not at all porous, an effective absorption of the sunlight 5 and good conductivity for the charge carriers generated therein is achieved. With the above-mentioned doping and with sufficient purity of the SiC material, a sufficient charge carrier lifetime is achieved, so that the charge carriers generated in the SiC (depending on the doping, electrons or holes) can migrate to the electrolyte side of the SiC electrode 4, where the nickel foam 6 touches the SiC electrode 4 and supply the energy required for water splitting.With these charge carriers, hydrogen and oxygen are electrolytically generated from the water 10 .

[0025] The membrane 7 is permeable to protons, acts as a proton separator, and separates the hydrogen from the oxygen by allowing the hydrogen to diffuse to the outlet 8. The oxygen bubbles upward from the aqueous electrolyte 10 and exits the housing 1 through the outlet 9.

[0026] The portion of sunlight energy 5 not consumed in the electrolytic splitting of water, particularly the energy content of the infrared portion of the sunlight spectrum, leads to heating of the cell. This heat energy can be removed from the cell by circulating the aqueous electrolyte or water through the inlet and outlet (not shown in Figure 1) and used for other purposes.

[0027] Figure 2 shows in more detail a modification of the embodiment of Figure 1. The photoelectric cell is shown inclined so that the window 2 faces the obliquely incident solar radiation.

[0028] As in the example shown in Figure 1, the nickel foam

[0029] 6 is impregnated with the aqueous electrolyte 10. The nickel foam is thus located in a water bath. The water level is above the nickel foam 6. Figure 2 shows the water inlet 11 at the bottom of the housing 1 and the water outlet 12 at the top of the housing 1 at approximately the level of the water level, neither of which is shown in the more schematic Figure 1.

[0030] In this respect, the structure and operation of the cell are similar to the embodiment shown in Figure 1. However, the embodiment shown in Figure 2 differs from that shown in Figure 1 in the features of the membrane 7 and the oxygen outlet 9 described below.

[0031] 2, the membrane 7 is arranged in a gas space 13, 14 above the water level in the housing 1, where it divides the gas space into a part 13 with higher pressure on the water 10 side and a part 14 with lower pressure on the hydrogen gas outlet 8 side. The respective pressure is maintained during operation by the production of hydrogen gas and oxygen gas in the aqueous electrolyte 10 and by the work of a compressor (not shown) which draws the hydrogen from the outlet 8 and feeds it to a gas network or storage facility. The pressure difference promotes the passage of the hydrogen through the membrane 7 and thus the separation of hydrogen and oxygen.

[0032] In the embodiment of Figure 2, the oxygen gas outlet 9 is connected to an oxygen separator 15 located in the water outlet 12. The circulation of the aqueous electrolyte or water 10 through the outlet 12 thus serves not only to utilize the heat of the aqueous electrolyte 10 but also to extract the oxygen from the aqueous electrolyte 10.

[0033] Figure 3 shows a further modification of the embodiment of Figure 1, whereby the same modification can also be made to the embodiment of Figure 2. The embodiment of Figure 3 differs from those of Figures 1 and 2 in the following features.

[0034] The SiC electrode 4 (SiC layer 4) of the above-specified composition and thickness is applied here to a conductive substrate 36 made of graphite or metal, which is arranged between the SiC electrode 4 on the one hand and the aqueous electrolyte 10 and the nickel foam 6 on the other. The window 2 here does not comprise a thick transparent glass or plastic plate, but rather a thin transparent layer 33 made of a highly transparent plastic, e.g., a resin, Plexiglas, or similar, which seals the SiC electrode 4 against dust, contaminants, etc. Such highly transparent plastics exhibit a significantly higher transmission than glass in the spectral range in which the SiC electrode 4 is photosensitive, particularly in the UV range.

[0035] In this embodiment, the sunlight 5 strikes the SiC layer 4 without being absorbed by any thicker glass or plastic plate. The charge carriers generated therein pass through the conductive substrate 36 and are available for water splitting in the aqueous electrolyte 10 on the side of the substrate 36 opposite the SiC layer 4.A feature common to the embodiments is that the SiC electrode 4 is designed to be flat with two opposing main surfaces, one of which main surfaces (on the left in the figures, the "dry side" of the photoelectric cell - free of electrolyte) is provided with a window 2 through which sunlight 5 can fall onto the SiC electrode without passing through an aqueous electrolyte, and the other main surface of which (on the right in the figures, the "wet side" of the photoelectric cell) is electrically connected to an aqueous electrolyte 10 and subsequently to the counter electrode (the metal or nickel foam 6). In this way, the sunlight 5 can reach the SiC electrode 4 unaffected by the electrolyte and the charge carriers generated in the electrode 4 efficiently effect the photocatalysis of the aqueous electrolyte 10.

[0036] The SiC electrode 4 can be a stand-alone thin plate (wafer) laminated with the described components to form the photoelectric cell. However, the SiC electrode 4 is advantageously produced as a coating on a substrate, which is the transparent plate 3 serving as the transparent substrate 3, for example, made of glass or plastic, or the conductive substrate 36, for example, made of graphite or metal.

[0037] The process used to coat the substrate 3, 36 should be controllable such that the coating produces an electrode 4 made of substantially amorphous SiC or 3C-SiC (preferably nano- or microcrystalline), but not hexagonal SiC. This can be achieved by controlling (limiting) the temperature of the substrate 3, 36 during coating. A suitable coating process is the gas phase deposition of a 3C-SiC layer 4 or an amorphous SiC layer 4 on the substrate 3, 36 by exposing the substrate 3, 36 to a gas containing Si and C. The gas can be produced by heating a precursor containing Si and C, for example by heating a solid precursor which, for example, made from a mixture of pyrogenic silica and carbon black, to temperatures from about 1400 ° C, preferably about 1600 to 1900 ° C, or by heating a gaseous precursor, which is e.g.a mixture of tetrachlorosilane and a hydrocarbon gas, to temperatures of about 900 to 1300 ° C or more. The above-mentioned dopants can be added to the precursor and / or the gas.

[0038] During deposition, a temperature gradient should be maintained in which the substrate 3, 36 has a lower temperature than the gas. For depositing an amorphous SiC layer 4, the substrate temperatures are in the range of 1100 to 1300 °C. For depositing a 3C-SiC layer 4, the substrate temperatures are in the range of 1400 to 1900 °C. For example, in order to deposit 3C-SiC, the substrate 3, 36 has a temperature of approximately 1500 °C and the Si- and C-containing gas has a temperature of approximately 1800 °C. A deposition process at these temperatures is particularly suitable for coating metal and in particular graphite and therefore for coating the conductive substrate 36.

[0039] Using the following methods, the SiC electrode 4 can be applied to the substrate 3, 36 as a thin layer 4 at lower temperatures, especially at room temperature, and thus particularly gently, without significantly altering or even damaging the substrate. These methods are therefore suitable not only for coating the conductive substrate 36 but also for coating the transparent substrate 3 made of glass or plastic:

[0040] Printing the SiC coating 4 on the substrate 3, 36 in the powder bed, which contains a powdered precursor made of pyrogenic silica and carbon black, by means of a laser beam in a 3D printing process at a relatively low laser power of, for example, only up to 20 or preferably 10 W for an IR laser or 15 W for a UV laser, wherein an array of several such lasers can also be used in parallel to accelerate the process; or

[0041] Printing the SiC coating 4 on the substrate 3, 36 in the powder bed containing a powdered precursor made of pyrogenic silica and carbon black, by means of microwave radiation which is excellently absorbed by such a precursor, this method being particularly suitable for coating a substrate 3 consisting of metal or a metal foil; or

[0042] Coating the substrate 3, 36 by means of cold plasma spraying, by adding said powdered precursor or a liquid suspension of the precursor to a cold plasma jet directed onto the substrate 3, 36; or

[0043] Flash lamp annealing of a precursor previously applied to the substrate 3, 36, which precursor contains a silicon source and a carbon source in liquid or powder form, for example the previously mentioned powdered precursor. When coating the transparent plate 3, no opaque layer should be formed which impedes sunlight 5, for example made of excess carbon, and when coating the conductive substrate 36, no insulating layer should be formed which impedes charge carrier transport to the nickel foam 6, for example made of silicon dioxide. These requirements can also be met with the specified temperature gradient and by controlling the composition of the precursor or of the Si- and C-containing gas, so that stoichiometric SiC is formed on the substrate 3, 36.

[0044] The above-mentioned embodiments can be supplemented and modified. For example, concentrators such as mirrors can be provided to focus the sunlight before it enters through the window 2 and to reduce the area required by the photoelectric cell. The window 2 can also simply be an opening in the housing 1 through which the SiC electrode 4 is exposed - without a transparent plate 3 being present. The thin transparent layer 33 also only needs to be present if the SiC electrode 4 is exposed to adverse environmental influences. The photoelectric cell is suitable not only for operation with sunlight but also with light from other sources. The individual features of a particular embodiment can be combined with the features of another embodiment.

Claims

Patent claims 1. A photoelectric cell for the photocatalytic generation of hydrogen, comprising: an electrode (4) containing silicon carbide and having a first and a second main surface opposite one another, a window (2) on the side of the first main surface of the electrode (4) for the incidence of light (5) from outside the cell onto the electrode (4), an aqueous electrolyte (10) on the side of the second main surface of the electrode (4), and a counter electrode (6) in contact with the aqueous electrolyte (10), the cell being electrolyte-free on the side of the first main surface of the electrode (4).

2. Photoelectric cell according to claim 1, wherein the electrode (4) is a silicon carbide layer with a thickness in the range of 40 to 80 pm.

3. Photoelectric cell according to claim 1 or 2, wherein the electrode (4) is a silicon carbide coating of a substrate (3, 36).

4. A photoelectric cell according to claim 3, wherein the window (2) comprises a transparent plate (3) and the substrate is the transparent plate (3).

5. A photoelectric cell according to claim 3, wherein the substrate is a conductive substrate (36) on the side of the second main surface of the electrode (4).

6. Photoelectric cell according to one of the preceding claims, wherein the counter electrode (6) comprises a metal foam which is impregnated with the aqueous electrolyte (10).

7. Photoelectric cell according to claim 6, wherein the metal foam is nickel foam (6).

8. Photoelectric cell according to one of the preceding claims with a proton-permeable membrane (7) and an outlet (8) for discharging hydrogen from the cell.

9. A method for producing an electrode (4) for a photoelectric cell according to any one of the preceding claims, wherein a substrate (3, 36) is exposed to a silicon and carbon-containing gas while maintaining the substrate at a lower temperature than the gas in order to deposit a layer (4) of silicon carbide on the substrate (3, 36).

10. Electrode (4) manufactured by a process according to Claim 9.

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