Silicon photoelectric cathode and electrode system for reducing nitrate into ammonia through photoelectric catalysis

Abstract

The invention relates to a silicon photoelectric cathode and an electrode system for reducing nitrate into ammonia through photoelectrocatalysis, the silicon photoelectric cathode comprises a Si substrate, an n-type silicon heavily doped layer located on the Si substrate, a passivation protection layer arranged on the n-type silicon heavily doped layer, and a titanium dioxide catalyst layer arranged on the passivation protection layer. An electrolytic reaction system is assembled by taking a platinum electrode as an anode and adopting an acidic electrolyte, nitrate reduction synthesis of ammonia is realized at the cathode, and water is oxidized into oxygen at the anode. The photocatalysis method for producing ammonia through reduction of the nitrate radicals at the cathode has the advantages of low reaction system tank voltage, low energy consumption and the like, and meanwhile, denitrification of wastewater can be realized, and efficient ammonia production is realized.

Description

Technical Field

[0001] The present invention relates to the technical field of photocatalytic ammonia production, and in particular to a silicon photocathode and an electrode system for photocatalytically reducing nitrate to ammonia. Background Art

[0002] Ammonia (NH4 + ) is not only one of the most basic chemical raw materials for fertilizers and pharmaceuticals, but has also recently been recognized as an important energy carrier for the global hydrogen economy and carbon-free fuels. Although the demand for ammonia may continue to increase, current industrial ammonia synthesis still relies on the traditional energy-intensive Haber-Bosch reaction, which consumes 1-2% of the world's energy supply and causes 1% of global greenhouse gas emissions. In this regard, photoelectrochemical (PEC) ammonia synthesis is considered one of the most promising strategies to directly achieve solar-to-chemical energy conversion. However, compared with the N2 reduction reaction, the NO3 reduction reaction (NO3RR) is more practical for ammonia synthesis because NO3 (i.e., nitrate) has a much higher solubility in aqueous solution and the cleavage energy of the NO bond is much lower than that of the N≡N triple bond. In addition, NO3 is one of the most widely distributed pollutants from human activities and can accumulate and dissolve in water, leading to increased nitrate-concentrations, damaging aquatic ecosystems and threatening human health. Previous attempts have mainly focused on the reduction or denitrification of nitrate-, rather than the selective recovery of ammonia. Therefore, ammonia synthesized by NO3RR can produce useful chemicals or fuels through a sustainable route using only renewable solar energy, while incorporating the PEC NO3RR system into wastewater treatment processes can also achieve a circular economy and a balanced nitrogen cycle. Despite its promising prospects, achieving selective ammonia synthesis by PECNO3RR is challenging due to the lack of a powerful photocathode material that synergizes excellent light absorption ability, efficient photogenerated charge transfer, and selective surface catalytic reactions.

[0003] Therefore, how to find a more suitable reaction route to solve the above-mentioned technical problems existing in the above-mentioned NO3RR ammonia production reaction route has become one of the focuses of widespread attention of many front-line researchers in the industry. Summary of the invention

[0004] In view of the technical problems existing in the prior art, the primary purpose of the present invention is to provide a silicon photocathode and an electrode system for photocatalytic reduction of nitrate to ammonia. The silicon photocathode and the electrode system, under acidic conditions, realize nitrate reduction to synthesize ammonia at the cathode and water oxidation to oxygen at the anode, thereby realizing efficient synthesis of ammonia with low cell pressure and low energy consumption. The present invention provides at least the following technical solutions:

[0005] On one hand, the present invention provides a silicon photocathode, comprising a Si substrate, an n-type silicon heavily doped layer on the Si substrate, a passivation protection layer on the n-type silicon heavily doped layer, and a titanium dioxide catalyst layer on the passivation protection layer.

[0006] Preferably, the thickness of the titanium dioxide catalyst layer is 10 to 30 nm;

[0007] Preferably, the thickness of the titanium dioxide catalyst layer is 20 nm;

[0008] Preferably, the titanium dioxide catalyst layer is deposited on the passivation protection layer by an atomic layer deposition process.

[0009] Preferably, the Si substrate has a first surface and a second surface arranged opposite to each other, the n-type silicon heavily doped layer is located on the first surface, the p-type silicon heavily doped layer is arranged on the second surface, and the transparent conductive oxide layer is arranged on the p-type silicon heavily doped layer.

[0010] Preferably, the passivation protection layer is made of a transparent conductive oxide material.

[0011] On the other hand, the present invention also provides an electrode system for photoelectrocatalytic reduction of nitrate to ammonia, which includes a photocathode, an anode, and an acidic electrolyte. The photocathode and the anode are placed in the acidic electrolyte, and the photocathode is the above-mentioned silicon photocathode.

[0012] Preferably, it further comprises an ion exchange membrane, wherein the cathode chamber is located on one side of the ion exchange membrane, the anode chamber is located on the other side of the ion exchange membrane, the photocathode is placed in the cathode chamber, and the anode is placed in the anode chamber.

[0013] Preferably, the electrolyte in the cathode chamber is an acidic potassium nitrate solution; the electrolyte in the anode chamber is an acidic aqueous solution or an acidic potassium nitrate solution;

[0014] Preferably, the acidic potassium nitrate solution is a mixed solution comprising sulfuric acid and potassium nitrate, and the molar ratio of sulfuric acid to potassium nitrate is (5-20):1;

[0015] Preferably, the acidic aqueous solution is a mixed solution of sulfuric acid and water, and the molar ratio of sulfuric acid to water is (1-10):100.

[0016] Preferably, the anode is a platinum electrode;

[0017] Preferably, the ion exchange membrane is a proton exchange membrane or a bipolar membrane.

[0018] Preferably, the driving voltage for the photoelectrocatalytic reduction of nitrate to ammonia is -0.4 to 0.2 V. RHE .

[0019] Preferably, a reference electrode is further included, and the reference electrode is placed in the cathode chamber.

[0020] Compared with the prior art, the present invention has at least the following advantages:

[0021] The present invention provides a method for + A silicon photocathode having a titanium dioxide catalyst layer disposed on the side of an amorphous silicon layer is disclosed. The photocathode is applied to photocatalytic production of ammonia by nitrate reduction, and a reaction route for producing ammonia by combining OER and NO3RR is obtained. Based on this, the present invention provides an electrode system for photocatalytic reduction of nitrate to ammonia. The electrode system provides a photocatalytic method for reducing nitrate to ammonia using a silicon photoelectrode modified with a titanium dioxide catalyst as a cathode, wherein nitrate reduction to synthesize ammonia is achieved at the cathode, and hydroxide oxidation to oxygen is achieved at the anode.

[0022] The present invention modifies titanium dioxide on the silicon electrode + The amorphous silicon layer side is used for the reduction of nitrate to synthesize ammonia at the cathode, thereby realizing efficient synthesis of ammonia with low cell voltage and low energy consumption. The silicon photoelectrode modified with titanium dioxide catalyst provided by the present invention has lower overpotential and higher ammonia Faraday efficiency than the reported silicon photoelectrode; compared with the catalyst containing precious metals, the silicon photocathode of the present invention has low cost and simple synthesis. The electrode system for photoelectrocatalytic reduction of nitrate to ammonia including the silicon photocathode modified with titanium dioxide catalyst provided by the present invention has the advantages of low cell voltage and low energy consumption of the reaction system compared with the prior art, and can also realize denitrification of wastewater and realize efficient ammonia production.

[0023] Using the photocatalytic ammonia production method provided by the present invention, in a three-electrode test of an embodiment of the present invention, at 0V RHE At a low overpotential of 1.5 Å, the silicon photoelectrode equipped with a titanium dioxide catalyst exhibited a high ammonia Faradaic efficiency of 89% and a high ammonia efficiency of 15 μmol / (h cm 2 ) with high ammonia yield and stability for up to 20 hours. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 A scanning electron microscope image of a silicon photocathode prepared according to an embodiment of the present invention;

[0025] Figure 2 An energy dispersive X-ray spectrum of a silicon photocathode prepared in one embodiment of the present invention;

[0026] Figure 3 This is an X-ray photoelectron spectrum of O1s of a silicon photocathode prepared according to an embodiment of the present invention;

[0027] Figure 4 The X-ray photoelectron spectrum of Ti 2p of the silicon photocathode prepared in one embodiment of the present invention;

[0028] Figure 5 A schematic diagram of the structure of an electrode system for photoelectrocatalytic reduction of nitrate to ammonia according to an embodiment of the present invention;

[0029] Figure 6 A comparison diagram of ammonia Faraday efficiency and yield of a silicon photocathode with a TiO2 catalyst layer of different thicknesses prepared in one embodiment of the present invention and a silicon electrode without a TiO2 catalyst layer in a three-electrode system;

[0030] Figure 7 Polarization curves of a silicon photocathode with a TiO2 catalyst layer of different thicknesses and a silicon electrode without a TiO2 catalyst layer in a three-electrode system prepared in one embodiment of the present invention;

[0031] Figure 8 The silicon photocathode with different thickness of TiO2 catalyst layer prepared in one embodiment of the present invention is subjected to different voltages (-0.1 to 0.2 V) in a three-electrode system. RHE ) Ammonia Faraday efficiency and yield diagram of GOR;

[0032] Fig. 9 The silicon photocathode with a TiO2 catalyst thickness of 20 nm prepared in one embodiment of the present invention is 0V under three electrodes. RHE It picture at the time. DETAILED DESCRIPTION

[0033] Next, the technical solutions in the embodiments of the present invention will be clearly and completely described in conjunction with the drawings of the present invention. The described embodiments are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, other embodiments obtained by ordinary technicians in this field without creative work are all within the scope of protection of the present invention. The experimental methods described in the following embodiments are all conventional methods unless otherwise specified; the reagents and materials, unless otherwise specified, can be obtained from public commercial channels.

[0034] Spatially relative terms such as "below," "beneath," "below," "above," "upper," etc. are used in this specification to explain the positioning of one element relative to a second element. These terms are intended to encompass different orientations of the device in addition to different orientations than those depicted in the figures.

[0035] In addition, the use of terms such as "first", "second", etc. to describe various elements, layers, regions, sections, etc. is not intended to be limiting. The use of "having", "containing", "including", "comprising", etc. is open-ended, indicating the presence of the stated elements or features, but does not exclude additional elements or features. Unless the context clearly states otherwise.

[0036] On one hand, the present invention provides a silicon photocathode, which uses an n-type Si substrate or a p-type Si substrate. In a preferred embodiment, an n-type Si substrate is used.

[0037] The n-type silicon heavily doped layer is disposed on the Si substrate. In one embodiment, a p-type silicon heavily doped layer is also disposed on the Si substrate. The n-type silicon heavily doped layer and the p-type silicon heavily doped layer constitute a heterojunction (HJT) structure. The n-type silicon heavily doped layer and the p-type silicon heavily doped layer are disposed on the upper and lower surfaces of the Si substrate, respectively. In a preferred embodiment, the n-type silicon heavily doped layer and the p-type silicon heavily doped layer are disposed on the upper and lower surfaces of the n-type Si substrate, respectively.

[0038] The n-type silicon heavily doped layer and the p-type silicon heavily doped layer may be crystalline silicon or amorphous silicon.

[0039] The selection of the Si substrate in the silicon photocathode of the present invention, as well as the arrangement of the n-type silicon heavily doped layer and / or the p-type silicon heavily doped layer can fully draw on the mature technology of existing silicon cells (such as PERC cells, TOPCon cells, HIT cells, IBC cells). Unlike the existing silicon cells, the silicon photocathode of the present invention is provided with a passivation protective layer on the side of the n-type silicon heavily doped layer, and a titanium dioxide catalyst layer is arranged on the passivation protective layer for photocatalytic production of ammonia by nitrate reduction.

[0040] The passivation protection layer is arranged on the n-type silicon heavily doped layer to protect and passivate the n-type silicon heavily doped layer. The passivation protection layer is made of transparent conductive oxide material. The titanium dioxide catalyst layer is deposited on the passivation protection layer. The thickness of the titanium dioxide catalyst layer is 10 to 30 nm. In a preferred embodiment, the thickness of the titanium dioxide catalyst layer is 20 nm. In a preferred embodiment, the titanium dioxide catalyst layer is deposited on the passivation protection layer using an atomic layer deposition process.

[0041] A transparent conductive oxide (TCO) layer is disposed on the heavily doped p-type silicon layer, and the transparent conductive oxide layer is, for example, indium tin oxide. A silver grid is disposed on the transparent conductive oxide layer as an electrode for external lead wires to complete circuit connection.

[0042] In order to improve the film quality of the titanium dioxide catalyst layer, the titanium dioxide catalyst layer was deposited using the following ALD preparation process: titanium tetrakis dimethylamide was used as the titanium source and H2O was used as the oxygen source to grow the TiO2 layer. The growth rate was about 0.054nm / cycle, and the TiO2 film was grown in a 150℃ chamber environment, and finally a TiO2 catalyst layer was obtained on the first transparent conductive oxide layer. When the ALD cycles were 185, 370, and 555, the film thicknesses were 10nm, 20nm, and 30nm, respectively. Figures 1 to 4 is a performance test diagram of the TiO2 catalyst layer prepared in one embodiment of the present invention. Figures 1 to 4 The uniform distribution of Ti can be seen, the X-ray photoelectron spectroscopy of O 1s and Ti 2p indicates the generation of oxygen vacancies, and Si has good crystallinity.

[0043] In a preferred embodiment of the present invention, the silicon photocathode adopts a heterojunction structure, and an n-type single crystal Si substrate is selected. A constitutive amorphous silicon (a-si:H) layer is provided on one surface of the Si substrate, and a thin layer of amorphous silicon doped with hydrogen (a-si:H) is provided on the constitutive amorphous silicon layer as a p-type heavily doped layer, and a layer of amorphous silicon doped with boron (a-Si:B) is provided on the other surface of the Si substrate as an n-type heavily doped layer to form a heterojunction structure. Transparent conductive oxide layers are deposited on the p-type heavily doped layer and the n-type heavily doped layer, respectively, and then a titanium dioxide catalyst layer is deposited on the transparent conductive oxide layer on the n-type heavily doped layer using the above-mentioned atomic layer deposition process to form the silicon photocathode structure of the present invention.

[0044] The silicon photocathode with the above structure is used as the cathode to photocatalytically reduce nitrate to ammonia, and the anode is a platinum electrode to catalyze the oxygen evolution reaction. The above two electrodes are assembled into a two-electrode reaction system to achieve highly selective ammonia synthesis at low cell voltage.

[0045] Specifically, the silicon photoelectrode with the titanium dioxide catalyst layer is used as the cathode, and the platinum electrode is used as the anode to assemble an electrolytic reaction system. The electrolyte in the cathode chamber is 0.01M sulfuric acid and 0.1M potassium nitrate solution, and the electrolyte in the anode chamber is 0.01M sulfuric acid. The cathode and anode are separated by a proton exchange membrane, and the driving voltage is 0.3V. RHE .

[0046] In another embodiment, the silicon photocathode of the above structure is used as the cathode to photocatalyze the reduction of nitrate to ammonia, and the anode is a platinum electrode to catalyze the oxygen evolution reaction. The above two electrodes and a reference electrode are assembled into a three-electrode system to achieve highly selective ammonia synthesis at low cell voltage. Figure 5 As shown, an acidic electrolyte is used, which can absorb the generated ammonia well. The cathode and anode are separated by a proton exchange membrane to form an anode chamber and a cathode chamber. The electrolyte in the anode chamber is an acidic aqueous solution or an acidic mixed solution containing nitrate, and the electrolyte in the cathode chamber is an acidic solution containing nitrate. The acidic solution containing nitrate can be a mixed solution including sulfuric acid and potassium nitrate, or a solution including sulfuric acid and nitric acid, or a solution including sulfuric acid, nitric acid and potassium nitrate.

[0047] The photocatalytic activity and selectivity tests were performed using an electrochemical workstation. The specific steps are as follows:

[0048] The electrochemical performance was tested using a three-electrode system with an effective cathode area of ​​1 cm 2The acid electrolyte in the cathode chamber is a mixed solution of 0.01M sulfuric acid and 0.1M potassium nitrate solution, and the acid electrolyte in the anode chamber is a mixed solution of 0.01M sulfuric acid and 0.1M potassium nitrate solution. RHE The electrolysis was carried out at a constant potential for 1 h.

[0049] like Figure 6 As shown, at 0V RHE When the thickness of the TiO2 catalyst layer is 20 nm, the ammonia Faraday efficiency of the silicon photocathode can reach up to 89%, and the ammonia yield is 15 μmol / (h cm 2 ), the ammonia Faraday efficiency of the Si electrode without a TiO2 catalyst layer (the difference between the structure of the Si electrode and the silicon photocathode of the present invention is only that it does not have a TiO2 catalyst layer) can reach up to 29%, and the ammonia yield is 1.15 μmol / (h cm 2 ). The yield of the silicon photocathode modified with titanium dioxide is significantly improved, because titanium dioxide is a catalyst for producing ammonia with good catalytic performance. It can also be inferred that nitrate is more conducive to the conversion to ammonia on the silicon photoelectrode.

[0050] Figure 7 1 is a polarization curve diagram of a silicon photocathode with a TiO2 catalyst layer of different thicknesses and a silicon electrode without a TiO2 catalyst layer in a three-electrode system prepared in one embodiment of the present invention. Figure 7 As shown, the silicon electrode without a TiO2 catalyst layer has poor nitrate reduction activity; the silicon photoelectrode with a TiO2 catalyst layer deposited by atomic layer has obvious nitric acid reduction activity; among them, when the thickness of the TiO2 catalyst layer is 20nm, the performance of silicon photoelectric is the best.

[0051] like Figure 8 As shown, at 0V RHE When the thickness of the TiO2 catalyst layer is 20 nm, the ammonia Faraday efficiency of the silicon photocathode can reach up to 89%, and the ammonia yield is 15 μmol / (h cm 2 ), which is significantly higher than the performance at other voltages, demonstrating its good selectivity.

[0052] like Fig. 9 As shown in the figure, the TiO2-modified silicon photoelectrode has good stability and can RHE Work for more than 20 hours.

[0053] The above embodiments are preferred implementation modes of the present invention, but the implementation modes of the present invention are not limited to the above embodiments. Any other changes, modifications, substitutions, combinations, and simplifications that do not deviate from the spirit and principles of the present invention should be equivalent replacement methods and are included in the protection scope of the present invention.

Claims

1. A silicon photocathode, characterized in that: The invention comprises a Si substrate, an n-type silicon heavily doped layer located on the Si substrate, a passivation protection layer arranged on the n-type silicon heavily doped layer, and a titanium dioxide catalyst layer arranged on the passivation protection layer.

2. The silicon photocathode according to claim 1, characterized in that: The thickness of the titanium dioxide catalyst layer is 10 to 30 nm; Preferably, the thickness of the titanium dioxide catalyst layer is 20 nm.

3. The silicon photocathode according to claim 1 or 2, characterized in that: The Si substrate has a first surface and a second surface arranged opposite to each other. The n-type silicon heavily doped layer is located on the first surface, the p-type silicon heavily doped layer is arranged on the second surface, and a transparent conductive oxide layer is arranged on the p-type silicon heavily doped layer.

4. The silicon photocathode according to claim 3, characterized in that: The passivation protection layer is made of indium tin oxide, a transparent conductive oxide material; Preferably, the titanium dioxide catalyst layer is deposited on the passivation protection layer by an atomic layer deposition process.

5. An electrode system for photoelectrocatalytic reduction of nitrate to ammonia, comprising a photocathode, an anode, and an acidic electrolyte, wherein the photocathode and the anode are placed in the acidic electrolyte, and the photocathode is a silicon photocathode as described in any one of claims 1 to 4.

6. The electrode system according to claim 5, characterized in that It also includes an ion exchange membrane, wherein the cathode chamber is located on one side of the ion exchange membrane, the anode chamber is located on the other side of the ion exchange membrane, the photocathode is placed in the cathode chamber, and the anode is placed in the anode chamber.

7. The electrode system according to claim 6, characterized in that The electrolyte in the cathode chamber is an acidic solution containing nitrate; the electrolyte in the anode chamber is an acidic aqueous solution or an acidic solution containing nitrate; Preferably, the acidic nitrate-containing solution is a mixed solution of sulfuric acid and potassium nitrate or a solution of sulfuric acid, potassium nitrate and / or nitric acid, and the molar ratio of sulfuric acid to potassium nitrate is (5-20):1; Preferably, the acidic aqueous solution is a mixed solution of sulfuric acid and water, and the molar ratio of sulfuric acid to water is (1-10):

100.

8. The electrode system according to any one of claims 5 to 7, characterized in that The anode is a platinum electrode; Preferably, the ion exchange membrane is a proton exchange membrane or a bipolar membrane.

9. The electrode system according to claim 10, characterized in that The electrode system also includes a reference electrode, which is placed in the cathode chamber. The driving voltage of the electrode system for photocatalytic reduction of nitrate to ammonia is -0.4 to 0.2 V. RHE ; Preferably, the driving voltage of the electrode system for photocatalytic reduction of nitrate to ammonia is -0.2 to 0.2 V. RHE .

10. Use of the silicon photocathode according to any one of claims 1 to 4 or the electrode system according to any one of claims 5 to 9 in denitrification of wastewater.

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