Photocell with silicon carbide electrodes and method for manufacturing same

Abstract

A photocell having a silicon carbide electrode (4) for photocatalytic hydrogen production and a method for its manufacture are disclosed. The cell has a window (2) on one side of the silicon carbide electrode (4) for the incidence of light (5), and an aqueous electrolyte (10) and a counter electrode (6) on the other side of the silicon carbide electrode (4). The cell does not have an electrolyte on the side of the silicon carbide electrode (4) facing the window. The silicon carbide electrode (4) is preferably manufactured by coating a substrate (3) with silicon carbide (4).

Description

[Technical Field]

[0001] The present invention relates to a photocell having silicon carbide electrodes for photocatalytic hydrogen production and a method for manufacturing the same. [Background technology]

[0002] Research into supplying hydrogen as an energy source has been ongoing for a long time, but a breakthrough for widespread use has yet to be achieved.

[0003] EP 3268512 discloses a photocell for producing hydrogen by electrolysis of water when irradiated with sunlight (photocatalysis). The cell includes a SiC electrode, where sunlight generates charge carriers and an electric current that causes the electrolytic decomposition of water. The SiC electrode has a porous fibrous structure. This structure is intended to provide a large surface area and is produced, for example, by converting carbon fibers into silicon carbide by the method according to EP 2094622.

[0004] Both EP 3268512 and other known structures have the disadvantage that sunlight must first pass through a glass plate several millimeters thick and then through a layer of water before reaching the active SiC material. A significant portion of the solar spectrum is absorbed by the glass and water, making it unavailable for photocatalysis. While photocatalysts for water splitting can operate with photon energies of just over 2 eV, conventional glass significantly absorbs sunlight from about 3.5 eV onward. Therefore, the usable portion of the solar spectrum is limited to the range from about 2 eV to 3.5 eV. Furthermore, a significant fraction of sunlight across the entire spectral range is reflected by conventional glass and lost to the photocatalyst.

[0005] Furthermore, in known manufacturing methods, the carbon fibers of the electrode are converted to SiC only on their surface. For example, a 10 μm thick carbon fiber has only a thin layer of SiC, about 2–3 μm thick, on its surface, where only a portion of the incident light is effectively absorbed by the photocatalyst. The core of the fiber is unconverted carbon and opaque. The light absorbed there simply heats the electrode and is lost from the photocatalyst. The desired increase in surface area due to the porosity of the fiber structure is of little use here. The porosity is typically only around 50%.

[0006] Another disadvantage of known electrodes is that due to the fiber structure, the contact resistance within the electrode is essentially high, which causes considerable electrical losses. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] European Patent No. 3268512 [Patent Document 2] European Patent No. 2094622 Summary of the Invention [Problem to be solved by the invention]

[0008] Therefore, an object of the present invention is to provide a technology that enables hydrogen production using a photocatalyst to be carried out more efficiently. [Means for solving the problem]

[0009] The solution to this problem is achieved by the photocell and the manufacturing method specified in the appended claims.

[0010] The present invention uses a photoelectrode in the form of a layer of 3C-SiC or amorphous SiC. 3C-SiC (cubic silicon carbide), with a bandgap of 2.36 eV, is suitable for efficiently generating charge carriers with enough energy to electrolyze water simply by irradiating it with sunlight, without the need for additional voltage application. The solar spectrum is efficiently utilized. The same applies to amorphous SiC, with a bandgap of approximately 2.2 eV. On the other hand, 4H-SiC and 6H-SiC (hexagonal), with bandgaps of 3.27 eV and 3.03 eV, respectively, absorb only short-wavelength light and utilize the solar spectrum less efficiently. The layer can be a free-standing plate or a SiC coating on a substrate, for example, a substrate made of metal, graphite, plastic, glass, etc., coated with nanocrystalline or microcrystalline 3C-SiC or amorphous SiC. The SiC layer may be undoped, P-doped with, for example, Al or B, and / or co-doped with a transition metal element, for example, Fe, Cr, or V. A photoelectrode is an electrode with a surface facing the light. This surface may be coated with a co-catalyst, such as Pt or Pd.

[0011] Embodiments of the present invention provide improved light transmission or reduced reflection in the optical path to the electrodes, increased optically or electrochemically active electrode surface, improved electrical conductivity and adhesion of the electrodes, and easy scalability of cell size ("upscaling" to square meter sized modules and beyond). [Brief explanation of the drawings]

[0012] Embodiments of the invention are illustrated in the drawings in which: FIG. 1 shows a schematic diagram of a photocell according to a first embodiment; FIG. 2 shows a more detailed view of a photocell according to a second embodiment, which is derived from the first embodiment; FIG. 3 shows a schematic diagram of a photocell according to a third embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0013] Identical elements are labeled with the same reference numerals in all figures and are not described again from figure to figure.

[0014] The photocell shown in Fig. 1 comprises a housing 1 having a window 2 through which light can pass. In this embodiment, the window 2 is provided with a transparent plate 3, in particular a glass plate 3. Alternatively, a transparent plastic plate can be used.

[0015] An electrode 4 made of 3C silicon carbide is positioned directly behind the transparent plate 3 within the housing 1, as viewed from the direction of the incident sunlight 5. The 3C-SiC is preferably nanocrystalline or microcrystalline. Alternatively, amorphous SiC can be used. This SiC electrode 4 takes the form of a thin, non-porous or barely porous (almost non-porous) SiC layer 4 with a thickness ranging from 40 μm to 80 μm. Within this thickness range, sunlight 5 entering through the window 2 is absorbed as well as possible. The SiC layer 4 can be coated on the transparent plate 3, which functions as a transparent substrate 3.

[0016] On the other side of the SiC electrode 4, i.e., the side opposite to the side where the window 2 is located, a counter electrode 6 is located, in this case at a position away from the SiC electrode. The counter electrode is a metal foam 6 having good electrical conductivity, which in this preferred embodiment is nickel foam 6. The metal or nickel foam 6 has a thickness in the range of approximately 3 mm to 40 mm. The surface of the metal foam 6 can be provided with a catalyst that promotes water decomposition. Polyoxometalates are particularly suitable as catalysts, and on the nickel foam 6, those made from nickel, cobalt, and tungsten are particularly suitable.

[0017] The nickel foam 6 is impregnated with an aqueous electrolyte 10, which also comes into contact with the SiC electrode 4, where the water contained therein is decomposed into hydrogen and oxygen. The housing 1 has an inlet and an outlet (not shown in Fig. 1) for circulating the electrolyte or water, respectively, through the nickel foam 6 within the housing 1.

[0018] An electrical conductor, in this case a conductive housing 1 or a 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 the electrical conductor and makes low resistance contact with the SiC electrode 4 over a large area.

[0019] The housing 1 has an outlet 8 for gaseous hydrogen and an outlet 9 for gaseous oxygen.

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

[0021] The surface of the SiC electrode 4 facing the electrolyte 10 may be provided with a metal layer, for example a solid metal and / or a metal layer of metal or nickel foam. This modification of the embodiment of FIG. 1 is further described below in connection with FIG. 3.

[0022] During operation, sunlight 5 passes through the window 2, in this case the glass plate 3, and illuminates the SiC electrode 4, photoelectrically generating charge carriers (electrons or holes, depending on the doping of the SiC electrode 4) therein. Because light 5 can directly illuminate the electrolyte-free side of the SiC electrode 4 without passing through the electrolyte, it is only slightly attenuated on its way to the electrode 4. As mentioned above, the thickness of the SiC electrode, ranging from 40 μm to 80 μm, maximizes the penetration depth of sunlight 5. The thin layer of the SiC electrode 4 has little or no porosity, providing efficient absorption of sunlight 5 and good electrical conductivity for the charge carriers generated there. The doping and sufficient purity of the SiC material ensure sufficient charge carrier lifetime to allow charge carriers (electrons or holes, depending on the doping) generated in the SiC to migrate to the electrolyte side of the SiC electrode 4 (where nickel foam 6 contacts the SiC electrode 4) and provide the energy required for water splitting. These charge carriers are used to electrolytically produce hydrogen and oxygen from water 10 .

[0023] Membrane 7 is proton permeable and acts as a proton separator, separating hydrogen from oxygen by allowing the hydrogen to diffuse to outlet 8. Oxygen bubbles upward from aqueous electrolyte 10 and exits housing 1 via outlet 9.

[0024] The portion of the energy of sunlight 5 that is not absorbed in the electrolysis of water, especially the energy content in the infrared part of the solar spectrum, leads to heating of the cell. This heat energy can be released from the cell and used for other purposes by circulating the aqueous electrolyte or water through the inlet and outlet (not shown in Fig. 1), respectively.

[0025] Figure 2 shows in more detail a variation of the embodiment of Figure 1. Here the photocell is shown tilted to face the window 2 towards obliquely incident solar radiation.

[0026] As with the embodiment of FIG. 1, nickel foam 6 is impregnated with aqueous electrolyte 10. The nickel foam is therefore bathed in water. The water level is above nickel foam 6. FIG. 2 shows a water inlet 11 at the bottom of housing 1 and a water outlet 12 at the top of housing 1, approximately at the same height as the water level, neither of which are shown in the more schematic FIG. 1.

[0027] In this respect, the structure and operation of the cell is similar to the embodiment of Figure 1. However, the embodiment of Figure 2 differs from the embodiment of Figure 1 in the characteristics of the membrane 7 and the outlet 9 for oxygen, as explained below.

[0028] In the embodiment of Fig. 2, the membrane 7 is located within the housing 1 in the gas spaces 13, 14 above the water level, dividing the gas spaces into a high-pressure section 13 on the water 10 side and a low-pressure section 14 on the hydrogen gas outlet 8 side. During operation, the respective pressures are maintained by the production of hydrogen and oxygen gases in the aqueous electrolyte 10 and the action of a compressor (not shown) that collects hydrogen from outlet 8 and delivers it to a gas distribution network or storage. The pressure difference encourages hydrogen to pass through the membrane 7, thereby facilitating the separation of hydrogen and oxygen.

[0029] In the embodiment of FIG. 2, the outlet 9 for oxygen gas is connected to an oxygen separator 15 arranged in the water outlet 12. Thus, the circulation of the aqueous electrolyte or water 10 through the outlet 12 serves not only to utilize the heat of the aqueous electrolyte 10 but also to extract oxygen from the aqueous electrolyte 10.

[0030] Figure 3 shows a further modification of the embodiment of Figure 1, which can also be modified in the embodiment of Figure 2. The embodiment of Figure 3 differs from the embodiments of Figures 1 and 2 in the following features.

[0031] The SiC electrode 4 (SiC layer 4) of the composition and thickness shown above 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 nickel foam 6 on the other hand. The window 2 here does not have a thick transparent plate of glass or plastic, but a thin transparent layer 33 made of highly transparent plastic, such as resin or Plexiglas, which seals the SiC electrode 4 against dust, impurities, etc. Such highly transparent plastic has significantly higher transmittance than glass in the spectral range to which the SiC electrode 4 responds, especially in the UV range.

[0032] In this embodiment, sunlight 5 strikes the SiC layer 4 rather than being absorbed by the thicker plate of glass or plastic. The charge carriers generated there pass through the conductive substrate 36 and are available for water splitting in the aqueous electrolyte 10 on the opposite side of the substrate 36 from the SiC layer 4.

[0033] These embodiments have in common that the SiC electrode 4 is designed as a plane having two opposing principal surfaces, one of which (the left side in the figure, the "dry side" of the photocell, no electrolyte) is provided with a window 2 so that sunlight 5 can reach the SiC electrode through this window 2 without passing through the aqueous electrolyte, and the other principal surface (the right side in the figure, the "wet side" of the photocell) is electrically connected over a large area to the aqueous electrolyte 10 and then to the counter electrode (metal or nickel foam 6). This allows sunlight 5 to reach the SiC electrode 4 without being blocked by the electrolyte, and charge carriers generated in the electrode 4 can efficiently cause photocatalysis of the aqueous electrolyte 10.

[0034] The SiC electrode 4 may be a free-standing thin plate (wafer) that is laminated with the aforementioned components to form a photovoltaic cell, although it is advantageous to create the SiC electrode 4 as a coating on a substrate (a transparent plate 3, e.g. made of glass or plastic, that serves as a transparent substrate 3, or a conductive substrate 36, e.g. made of graphite or metal).

[0035] The method used to coat the substrate 3, 36 must be controllable so that the coating produces an essentially amorphous SiC or 3C-SiC (preferably nanocrystalline or microcrystalline) electrode 4, but not hexagonal SiC. This can be achieved by controlling (limiting) the temperature of the substrate 3, 36 during the coating process.

[0036] A suitable coating method is vapor deposition of a 3C-SiC layer 4 or an amorphous SiC layer 4 on a substrate 3, 36 by exposing the substrate 3, 36 to a gas containing Si and C. The gas can be generated by heating a precursor containing Si and C, for example, a solid precursor made from a mixture of fumed silica and carbon black to a temperature of about 1400°C or higher, preferably about 1600°C to 1900°C, or a gaseous precursor containing, for example, a mixture of tetrachlorosilane and a hydrocarbon gas to a temperature of about 900°C to 1300°C or higher. The dopants mentioned above can be added to the precursor and / or the gas.

[0037] During deposition, a temperature gradient must be maintained such that the substrate 3, 36 has a lower temperature than the gas. For the deposition of an amorphous SiC layer 4, the substrate temperature ranges from 1100°C to 1300°C. For the deposition of a 3C-SiC layer 4, the substrate temperature ranges from 1400°C to 1900°C. For example, 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. These temperature deposition processes are particularly suitable for coating metals and, in particular, graphite, thus coating conductive substrates 36.

[0038] With the following methods, the SiC electrode 4 can be applied even at low temperatures, especially at room temperature, without significantly changing or even damaging the substrate 3, 36, and can therefore be applied particularly gently as a thin layer 4 onto the substrate 3, 36. These methods are therefore suitable not only for coating conductive substrates 36, but also for coating transparent substrates 3 made of glass or plastic. In the 3D printing method, a laser beam is used with relatively low laser power (e.g., 20 W or less, preferably 10 W or less for IR lasers, and 15 W or less for UV lasers) to print the SiC coating 4 onto the substrate 3, 36 in a powder bed containing a powdered precursor made of fumed silica and carbon black. Here, multiple such lasers may be used in parallel to accelerate the process. Alternatively, the SiC coating 4 can be printed onto a substrate 3, 36 in a powder bed containing a powdered precursor made of fumed silica and carbon black using microwave radiation, which is highly absorbed by such precursors. This method is also particularly suitable for coating substrates 3 made of metal or metal foil. Alternatively, the substrate 3, 36 is coated by cold plasma spraying, by adding the powdered precursor or a liquid suspension of the precursor to a cold plasma jet directed at the substrate 3, 36. Alternatively, a precursor previously applied to the substrate 3, 36 is flash lamp annealed, where the precursor includes a silicon source and a carbon source in liquid or powder form, such as the powder precursors described above.

[0039] The coating of the transparent plate 3 should not form an opaque layer, e.g., excess carbon, that blocks sunlight 5, and the coating of the conductive substrate 36 should not form an insulating layer, e.g., silicon dioxide, that prevents the transport of charge carriers to the nickel foam 6. These requirements can also be achieved by using the temperature gradient specified above and controlling the composition of the precursors or gases containing Si and C, so that stoichiometric SiC is formed on the substrates 3, 36.

[0040] The above-described embodiments can be supplemented and modified. For example, a concentrator such as a mirror can be provided to concentrate sunlight before it enters through the window 2, thereby reducing the area required by the photocell. The window 2 can also be an opening in the housing 1, without the transparent plate 3, through which the SiC electrode 4 is exposed. The thin transparent layer 33 only needs to be present if the SiC electrode 4 is exposed to adverse environmental influences. The photocell is suitable for operation not only with sunlight but also with light from other light sources. Individual features of each embodiment can be combined with features of other embodiments.

Claims

1. A photocell for photocatalytic hydrogen production. an electrode (4) comprising silicon carbide and having first and second major surfaces facing each other; a window (2) provided on the side of the first main surface of the electrode (4) for allowing light (5) to enter the electrode (4) from outside the cell; an aqueous electrolyte (10) on the second main surface side of the electrode (4); a counter electrode (6) in contact with the aqueous electrolyte (10); The cell is a photocell that does not contain an electrolyte on the first main surface side of the electrode (4).

2. 2. The photocell of claim 1, The photocell, wherein the electrode (4) is a silicon carbide layer having a thickness in the range of 40 μm to 80 μm.

3. 3. A photocell according to claim 1 or 2, A photocell in which the electrode (4) is a silicon carbide coating on a substrate (3, 36).

4. 4. The photocell of claim 3, The window (2) comprises a transparent plate (3), and the substrate is the transparent plate (3).

5. 4. The photocell of claim 3, The substrate is a conductive substrate (36) provided on the second main surface side of the electrode (4).

6. 6. A photocell according to any one of claims 1 to 5, The counter electrode (6) comprises a metal foam impregnated with the aqueous electrolyte (10).

7. 7. The photocell of claim 6, A photocell wherein said metal foam is nickel foam (6).

8. 8. A photocell according to any one of claims 1 to 7, A photovoltaic cell comprising a proton-permeable membrane (7) and an outlet (8) for discharging hydrogen from said cell.

9. A method for manufacturing an electrode (4) for a photovoltaic cell according to any one of claims 1 to 8, comprising the steps of: A manufacturing method comprising exposing a substrate (3, 36) to a gas containing silicon and carbon and maintaining said substrate at a temperature lower than said gas to deposit a layer (4) of silicon carbide on said substrate (3, 36).

10. 10. An electrode (4) manufactured by the method of claim 9.

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