Device and method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation

Abstract

The invention relates to a device and a method for synthesizing ammonia through photoelectrochemical sodium-mediated nitrogen fixation. The photoelectrochemical sodium-mediated nitrogen fixation ammonia synthesis device comprises a photoelectric cathode, a counter electrode and an electrolyte, wherein the photoelectric cathode and the counter electrode are in contact with the electrolyte; the electrolyte comprises a sodium source, an organic solvent and an organic proton source. The photoelectrochemistry sodium-mediated nitrogen fixation ammonia synthesis device comprises a photoelectric cathode and a counter electrode, a sodium source, an organic solvent and an organic proton source are used as electrolyte solutions, photoelectrons, the sodium source and a proton source are provided for the nitrogen fixation process, a novel metal sodium mediator which is rich in reserves, proper in activity and low in reaction energy barrier is adopted, large-scale ammonia synthesis can be achieved, and the device is suitable for industrial production. Solar energy is intensively converted into ammonia which is easy to store and high in energy density, and efficient and low-energy-consumption ammonia synthesis is achieved.

Description

Technical Field

[0001] The present application relates to the field of photoelectrochemical technology, and in particular to a device and method for photoelectrochemical sodium-mediated nitrogen fixation to synthesize ammonia. Background Art

[0002] Ammonia (NH3) is one of the most important chemicals in agriculture, pharmaceuticals and other chemical industries. Due to its high energy density (1.5 times higher than liquid hydrogen) and carbon-free, NH3 is considered a promising energy carrier. The Haber-Bosch process (also known as the Haber process) is an industrial chemical method that produces ammonia by reacting nitrogen and hydrogen under high temperature, high pressure and catalyst conditions. The world's annual synthesis of NH3 through the Haber-Bosch process is about 200 million tons, consuming about 1.5% of the world's energy, and the carbon dioxide emitted by this process accounts for about 1.5% of the world's total emissions.

[0003] In order to overcome the energy and environmental challenges of the Haber-Bosch process, many studies have attempted to find a more sustainable NH3 synthesis technology under mild conditions. The photoelectrochemical nitrogen reduction reaction (PEC NRR) is a solar-driven pathway for the production of NH3 from N2 and proton sources. However, photoelectrocatalytic nitrogen fixation mainly uses a photoelectric reaction system with an aqueous solution as the electrolyte, and improves the nitrogen fixation efficiency by improving the photocathode. Currently, the photoelectrochemical nitrogen fixation to synthesize ammonia has the disadvantages of high energy consumption and low ammonia yield, which is still an important bottleneck hindering their practical application in sustainable NH3 synthesis.

[0004] Therefore, the traditional technology still needs to be improved. Summary of the invention

[0005] Based on this, the present application provides a device and method for photoelectrochemical sodium-mediated nitrogen fixation and ammonia synthesis with excellent reaction activity, high efficiency and low energy consumption. The specific scheme is as follows:

[0006] In a first aspect of the present application, a device for photoelectrochemical sodium-mediated nitrogen fixation to synthesize ammonia is provided, comprising a photocathode, a counter electrode and an electrolyte, wherein the photocathode and the counter electrode are in contact with the electrolyte;

[0007] The electrolyte includes a sodium source, an organic solvent and an organic proton source.

[0008] In one embodiment, the photocathode is a hierarchical photocathode, comprising a light absorber and a functional layer disposed on the surface of the light absorber;

[0009] The material of the functional layer includes at least one of tungsten trioxide, tungsten oxide containing oxygen defects, tungsten nitride and tungsten sulfide;

[0010] The light absorber is a semiconductor material;

[0011] Optionally, the material of the light absorber includes at least one of silicon, cuprous oxide, gallium phosphide, arsenic phosphide and gallium nitride.

[0012] In one embodiment, the electrolyte satisfies at least one of the following conditions (1) to (3):

[0013] (1) The sodium source includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium trifluoromethanesulfonate, sodium bis(fluorosulfonyl)imide, sodium chloride and sodium fluoride;

[0014] (2) the organic solvent comprises at least one of propylene carbonate, ethylene carbonate, dimethyl carbonate, methylpropyl carbonate, tetrahydrofuran, dimethyl phthalate, γ-butyrolactone, methyl formate and maleic anhydride;

[0015] (3) The organic proton source includes at least one of methanol, ethanol, propanol, trifluoroethanol, isopropanol, acetic acid and glycerol.

[0016] In one embodiment, the concentration of the sodium source in the electrolyte is 0.01 mol / L~2 mol / L.

[0017] In one embodiment, based on the total volume of the electrolyte, the volume proportion of the organic proton source is 3% to 20%.

[0018] In one embodiment, the thickness of the functional layer is 20nm~200nm.

[0019] Another aspect of the present application provides a method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation, the method being carried out using the above-mentioned device for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation, comprising the following steps:

[0020] A reaction gas including nitrogen is introduced into the electrolyte, the photocathode is illuminated, and electricity is applied, at 20° C. to 100° C., sodium reduction, generation of sodium nitride, and synthesis of ammonia by sodium nitride and protons.

[0021] In one embodiment, the reaction gas further includes hydrogen, and the volume proportion of the hydrogen is 0.5% to 30% based on the total volume of the reaction gas.

[0022] In one embodiment, the reaction gas further includes an active gas, and the active gas includes one of carbon dioxide, oxygen, and sulfur dioxide.

[0023] In one embodiment, based on the total volume of the reaction gas, the volume of the active gas accounts for 1% to 40%.

[0024] The device for photoelectrochemical sodium-mediated nitrogen fixation and ammonia synthesis of the present application includes a photocathode and a counter electrode, and uses a sodium source, an organic solvent and an organic proton source as an electrolyte solution, which provides photoelectrons, a sodium source and a proton source for the nitrogen fixation process. It adopts a new type of metallic sodium mediator with abundant reserves, appropriate activity and low reaction energy barrier, and can realize large-scale synthesis of ammonia, concentrate solar energy into easy-to-store, high-energy-density ammonia, and realize efficient and low-energy-consumption synthesis of ammonia.

[0025] The photoelectrochemical sodium-mediated nitrogen fixation and ammonia synthesis method of the present invention realizes the Na + + photogenerated electrons → Na+N2→Na3N+3H + (or+CO2)→Na + + NH3, the rate of synthesizing ammonia is related to the circulation rate of Na, and under certain conditions, the rate of nitrogen fixation and ammonia synthesis can reach 10 -7 g / cm 2 / min, which is much higher than the rate of traditional photoelectric / electrochemical reduction of nitrogen to synthesize ammonia in aqueous solution (10 -10 g / cm 2 / min). In addition, the sodium mediation used in the photoelectrochemical sodium-mediated nitrogen fixation method for ammonia synthesis described in the present application has lower energy consumption and relatively more abundant sodium resources, higher stability, and the energy input is solar energy. Therefore, the reaction can realize the green, sustainable and practical application prospects of synthesizing ammonia. BRIEF DESCRIPTION OF THE DRAWINGS

[0026] Figure 1 This is a diagram showing the surface changes of the photocathode after being exposed to air in the photoelectrochemical sodium-mediated nitrogen fixation and ammonia synthesis reaction in Example 1;

[0027] Figure 2 For Si and WO in Example 1 x / Photoelectrochemical sodium-mediated nitrogen fixation results of Si-level photoelectrode;

[0028] Figure 3 The photoelectrochemical sodium-mediated ammonia synthesis results of different LiClO4 electrolyte concentrations in Example 2;

[0029] Figure 4 The photoelectrochemical sodium-mediated ammonia synthesis results of different proton source carriers in Example 3;

[0030] Figure 5 This is a graph showing the effect of the introduction of different CO2 concentrations on the photoelectrochemical sodium-mediated ammonia synthesis in Example 4;

[0031] Figure 6 This is a diagram showing the effect of reducing ammonia in nitrogen with different volume fractions of hydrogen in Example 5;

[0032] Figure 7 This is a graph showing the effect of different temperatures on the ammonia synthesis effect in Example 6. DETAILED DESCRIPTION

[0033] For ease of understanding of the present application, the present application will be described more fully below, and preferred embodiments of the present application are provided. However, the present application can be implemented in many different forms and is not limited to the embodiments described herein. On the contrary, the purpose of providing these embodiments is to make the understanding of the disclosure of the present application more thorough and comprehensive.

[0034] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as those commonly understood by those skilled in the art to which this application belongs. The terms used herein in the specification of this application are only for the purpose of describing specific embodiments and are not intended to limit this application.

[0035] The term "comprises", "comprising" or any other variant thereof is intended to cover non-exclusive inclusion, so that the process, method, article or equipment including a series of elements includes not only those elements, but also includes other elements not explicitly listed, or also includes elements inherent to such process, method, article or equipment. In the absence of more restrictions, the elements defined by the sentence "comprising one..." do not exclude the presence of other identical elements in the process, method, article or equipment including the elements. The indefinite articles "a kind of" and "one" before the elements or components of the present application are not restrictive to the quantity requirements (i.e., the number of occurrences) of the elements or components. Therefore, "one" or "a kind of" should be interpreted as including one or at least one, and the elements or components in the singular form also include the plural form, unless the number obviously refers only to the singular form. The meaning of "multiple" is at least two, such as two, three, etc., unless otherwise clearly and specifically defined.

[0036] The weight of the relevant components mentioned in the specification of the examples of this application can not only refer to the specific content of each component, but also represent the proportional relationship between the weights of the components. Therefore, as long as the content of the relevant components is proportionally enlarged or reduced according to the specification of the examples of this application, it is within the scope disclosed in the specification of the examples of this application. Specifically, the weight described in the specification of the examples of this application can be mass units known in the chemical industry such as μg, mg, g, and kg.

[0037] In the present application, when it comes to numerical intervals (i.e., numerical ranges), unless otherwise specified, the optional numerical distribution is considered continuous within the above numerical interval, and includes the two numerical endpoints (i.e., the minimum and maximum values) of the numerical range, and each numerical value between the two numerical endpoints. Unless otherwise specified, when the numerical interval only refers to the integers within the numerical interval, it includes the two endpoint integers of the numerical range, and each integer between the two endpoints. In this article, it is equivalent to directly listing each integer, such as t is an integer selected from 1-10, indicating that t is any integer selected from the integer group consisting of 1, 2, 3, 4, 5, 6, 7, 8, 9 and 10. In addition, when multiple ranges are provided to describe features or characteristics, these ranges can be combined. In other words, unless otherwise specified, the ranges disclosed herein should be understood to include any and all sub-ranges included therein.

[0038] The temperature parameters in this application, unless otherwise specified, are allowed to be either constant temperature treatment or to vary within a certain temperature range. It should be understood that the constant temperature treatment allows the temperature to fluctuate within the accuracy range controlled by the instrument. Fluctuations within the range of ±5°C, ±4°C, ±3°C, ±2°C, and ±1°C are allowed.

[0039] As shown in the background technology, in traditional technology, there are three main working modes for the photoelectrochemical nitrogen reduction reaction process: the first is in aqueous solution, the photocathode provides active sites and photogenerated electrons, and N2 is activated and hydrogenated with protons in water to synthesize NH3; the second is to use ionic liquids as electrolytes, and N2 reacts with organic proton sources or H2 to generate NH3; the third is to use highly reducible metals (such as lithium (Li)) to activate N2 to form corresponding metal nitrides, which are more likely to accept protons and produce NH3 and metal ions. Compared with the first two working modes, Li as a medium can change the reaction path of NRR with a lower reaction energy barrier, thereby showing great advantages in NH3 yield and Faradaic efficiency (FE). However, lithium-mediated NRR still has some difficulties in industrial green NH3 synthesis. Due to the scarcity of lithium resources, it is difficult to meet the needs of lithium-mediated NRR and lithium-ion batteries on a large scale at the same time. In addition, the presence of metallic lithium has potential safety risks, and due to the strong activity of elements in nature, the use and storage conditions are relatively harsh. In addition, from an energy perspective, Li + The formation energy of reduction to metallic Li is very high (Li 0 / + The standard redox potential of hydrogen is -3.04 V compared to the standard hydrogen electrode (SHE), which requires a lot of energy to drive the reaction.

[0040] The applicant has found through a large number of creative experimental studies that sodium (Na) has similar physical and chemical properties to lithium (Li) in terms of high reduction ability, and its abundance in the earth's crust is 2.75%, which is more than 1,000 times that of lithium. + The standard redox potential of Na / Na is -2.71 V vs. SHE, which is also lower than the redox potential of Li (-3.04 V vs. SHE). Using a new type of metallic sodium mediator with abundant reserves, appropriate activity and low reaction energy barrier to replace lithium mediator can achieve large-scale synthesis of ammonia, concentrate solar energy into easy-to-store, high-energy-density ammonia, and achieve efficient and low-energy synthesis of ammonia.

[0041] Based on this, one embodiment of the present application provides a device for photoelectrochemical sodium-mediated nitrogen fixation to synthesize ammonia, the device comprising a photocathode, a counter electrode and an electrolyte, the photocathode and the counter electrode being in contact with the electrolyte; the electrolyte comprising a sodium source, an organic solvent and an organic proton source.

[0042] The device for photoelectrochemical sodium-mediated nitrogen fixation and ammonia synthesis of the present application includes a photocathode and a counter electrode, and uses a sodium source, an organic solvent and an organic proton source as an electrolyte solution, which provides photoelectrons, a sodium source and a proton source for the nitrogen fixation process. It adopts a new type of metallic sodium mediator with abundant reserves, appropriate activity and low reaction energy barrier, and can realize large-scale synthesis of ammonia, concentrate solar energy into easy-to-store, high-energy-density ammonia, and realize efficient and low-energy-consumption synthesis of ammonia.

[0043] In some embodiments, the photocathode is a hierarchical photocathode, including a light absorber and a functional layer disposed on a surface of the light absorber.

[0044] It can be understood that by using sunlight to excite the photocathode to generate electrons, the functional layer on the surface of the hierarchical photocathode can enrich sodium ions on the electrode surface and transport electrons to reduce them into metallic sodium, thereby obtaining metallic sodium at a potential lower than the sodium ion reduction site.

[0045] In some embodiments, the material of the functional layer includes at least one of tungsten trioxide, tungsten oxide containing oxygen defects, tungsten nitride and tungsten sulfide.

[0046] It is understood that the hierarchical photoelectrode can generate metallic sodium at a potential lower than the standard reduction potential of sodium ions. If it is necessary to speed up the generation speed and reduce the applied voltage, the composition and structure of the hierarchical photoelectrode can be selected preferentially. Furthermore, the photocathode is not limited to the functional layer / light absorber structure. It should be understood that a single-layer, double-layer or multi-layer photocathode can be selected as the reaction site.

[0047] In some embodiments, the light absorber is a semiconductor material.

[0048] Optionally, the material of the light absorber includes at least one of silicon, cuprous oxide, gallium phosphide, arsenic phosphide and gallium nitride.

[0049] In some embodiments, the sodium source includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium trifluoromethanesulfonate and sodium bis(fluorosulfonyl)imide.

[0050] In some embodiments, the organic solvent includes at least one of propylene carbonate, ethylene carbonate, dimethyl carbonate, methylpropyl carbonate, tetrahydrofuran, dimethyl phthalate, γ-butyrolactone, methyl formate and maleic anhydride.

[0051] In some embodiments, the organic proton source includes at least one of methanol, ethanol, trifluoroethanol, isopropanol, acetic acid and glycerol.

[0052] In a specific example, the organic proton source is methanol.

[0053] In some embodiments, the concentration of the sodium source in the electrolyte is 0.01 mol / L~2 mol / L, for example, 0.01 mol / L, 0.02 mol / L, 0.03 mol / L, 0.04 mol / L, 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.08 mol / L, 0.09 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L, 0.4 mol / L, 0.5 mol / L, 0.6 mol / L, 0.7 mol / L, 0.8 mol / L, 0.9 mol / L, 1 mol / L, 1.1 mol / L, 1.2 mol / L, 1.3 mol / L, 1.4 mol / L, 1.5 mol / L, 1.6 mol / L, 1.7 mol / L, 1.8 mol / L, 1.9 mol / L, 2 mol / L, or within the range formed by any two of the above values.

[0054] It can be understood that the amount of metallic sodium precipitated and the effect of nitrogen reduction to produce ammonia at a certain potential are related to the concentration of sodium salt.

[0055] In a specific example, the concentration of the sodium source in the electrolyte is 0.6 mol / L.

[0056] In some embodiments, based on the total volume of the electrolyte, the volume proportion of the organic proton source is 3% to 20%, for example, it can be 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, or within the range formed by any two of the above point values.

[0057] In a specific example, the volume proportion of the organic proton source is 2.6%.

[0058] In some embodiments, the functional layer is prepared by magnetron sputtering.

[0059] In some of the embodiments, the thickness of the functional layer is 20 nm to 200 nm, for example, it can be 20 nm, 30 nm, 40 nm, 50 nm, 60 nm, 70 nm, 80 nm, 90 nm, 100 nm, 110 nm, 120 nm, 130 nm, 140 nm, 150 nm, 160 nm, 170 nm, 180 nm, 190 nm, 200 nm, or within the range formed by any two of the above point values.

[0060] In some embodiments, the conditions of the magnetron sputtering are: the power source used for the magnetron sputtering is a DC power source or an AC power source, and the power density of the magnetron sputtering is 1-100 W / cm 2 , working pressure is 0.5~4 Pa, sputtering time is 0.5~60 min, and temperature is 20~300 °C.

[0061] One embodiment of the present application also provides a method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation, the method being carried out using the above-mentioned device for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation, comprising the following steps:

[0062] A reaction gas including nitrogen is introduced into the above electrolyte, the above photocathode is illuminated and powered, and at 20°C to 100°C, sodium reduction, sodium nitride generation, and sodium nitride and proton synthesis of ammonia occur.

[0063] The above temperature range is "20℃~100℃", that is, the minimum and maximum values ​​in the range of 20℃~100℃, and every value between the minimum and maximum values. Specific examples include but are not limited to the point values ​​in the embodiments and the following point values: 20℃, 21℃, 22℃, 23℃, 24℃, 25℃, 26℃, 27℃, 28℃, 29℃, 30℃, 31℃, 32℃, 33℃, 34℃, 35℃, 36℃, 37℃, 38℃, 39℃, 40℃, 41℃, 42℃, 43℃, 44℃, 45℃, 46℃, 47℃, 48℃, 49℃, 50℃, 51℃, 52℃, 53℃, 54℃, 55℃, 56℃, 57℃, 58℃, 59℃, 60℃, 61℃ , 62℃, 63℃, 64℃, 65℃, 66℃, 67℃, 68℃, 69℃, 70℃, 71℃, 72℃, 73℃, 74℃, 75℃, 76℃, 77℃, 78℃, 79℃, 80℃, 81℃, 82℃, 83℃, 84℃, 85℃, 86℃, 87℃, 88℃, 89℃, 90℃, 91℃, 92℃, 93℃, 94℃, 95℃, 96℃, 97℃, 98℃, 99℃ or 100℃; or a range composed of any two of these values, as an example, including: 20℃ ~ 80℃.

[0064] In some embodiments, the reaction temperature is 40°C to 80°C.

[0065] It can be understood that controlling the reaction temperature within the above specific range can promote the formation of sodium nitride and further protonation to synthesize ammonia.

[0066] In some embodiments, the reaction gas further includes hydrogen. Based on the total volume of the reaction gas, the volume proportion of the hydrogen is 0.5% to 30%, for example, 0.5%, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, or within the range formed by any two of the above point values.

[0067] It can be understood that controlling the above-mentioned hydrogen within the above-mentioned volume ratio range can effectively promote the process of sodium nitride hydrogenation to produce ammonia.

[0068] In a specific example, the volume proportion of the hydrogen is 3%.

[0069] In some embodiments, the reaction gas further includes an active gas, and the active gas includes one of carbon dioxide, oxygen, and sulfur dioxide.

[0070] In a specific example, the active gas is carbon dioxide.

[0071] It can be understood that the introduction of active gas can promote the conversion of inactivated sodium on the electrode surface into sodium ions, accelerating the effective metal sodium-mediated nitrogen reduction to produce ammonia.

[0072] In some embodiments, based on the total volume of the reaction gas, the volume proportion of the active gas is 1% to 40%, for example, 1%, 2%, 3%, 4%, 5%, 6%, 7%, 8%, 9%, 10%, 11%, 12%, 13%, 14%, 15%, 16%, 17%, 18%, 19%, 20%, 21%, 22%, 23%, 24%, 25%, 26%, 27%, 28%, 29%, 30%, 31%, 32%, 33%, 34%, 35%, 36%, 37%, 38%, 39%, 40%, or within the range formed by any two of the above point values.

[0073] In some embodiments, the volume proportion of the above-mentioned active gas is 1%~10%.

[0074] In some embodiments, the light intensity of the illumination is 50 mW / cm 2 ~500 mW / cm 2 The wavelength of light is 380nm ~1100nm, preferably sunlight, and efficient ammonia synthesis can be achieved under low external voltage.

[0075] In the preparation process, the principle is as follows: under light, the photocathode is excited by light to generate photogenerated electrons in holes, and the electrons reduce the sodium ions accumulated on the surface of the photocathode to metallic sodium, so that metallic sodium is produced at a potential lower than the standard reduction potential of sodium ions; the metallic sodium contacts nitrogen to form a sodium-nitrogen compound, and the sodium-nitrogen compound further reacts with the protons transported to the photocathode by the proton source carrier to generate ammonia and sodium ions, wherein the precipitation of metallic sodium is related to the effect of nitrogen reduction to produce ammonia, but excessive sodium accumulation will increase the interface resistance of the photoelectrode, reduce the yield of nitrogen reduction to produce ammonia and the utilization rate of solar energy; in order to reduce the accumulation of inactive sodium on the surface of the photocathode, a trace amount of active gas is introduced to mix with the reaction nitrogen, and the active gas can destroy the inactive sodium on the electrode surface, so that the active metallic sodium on the electrode surface is exposed, and further forms a sodium-nitrogen compound with nitrogen, but excessive active gas will cause a large number of stable active gas and sodium compounds to form on the electrode surface, thereby reducing the active sites of metallic sodium, which is not conducive to the process of nitrogen fixation and ammonia synthesis. That is, the cyclic conversion between sodium ions, metallic sodium and sodium nitrogen compounds in this application plays the role of nitrogen activation, fixation and hydrogenation. This application can realize large-scale synthesis of ammonia, concentrate solar energy into easy-to-store, high-energy-density ammonia, and realize efficient and low-energy consumption synthesis of ammonia.

[0076] The above-mentioned photoelectrochemical sodium-mediated nitrogen fixation method for synthesizing ammonia has simple and easy steps, mild reaction conditions, and is easy to produce ammonia on a large scale.

[0077] The present application will be described below in conjunction with specific embodiments, but the present application is not limited to the following embodiments. It should be understood that the attached claims summarize the scope of the present application. Under the guidance of the concept of the present application, technical personnel in the field should realize that certain changes made to the various embodiments of the present application will be covered by the spirit and scope of the claims of the present application.

[0078] In order to make the purpose, technical solutions and advantages of the present application more concise and clear, the present application is described with the following specific embodiments, but the present application is by no means limited to these embodiments. The embodiments described below are only preferred embodiments of the present application and can be used to describe the present application, and cannot be understood as limiting the scope of the present application. It should be pointed out that any modifications, equivalent substitutions and improvements made within the spirit and principles of the present application should be included in the protection scope of the present application.

[0079] Example 1

[0080] In this embodiment, a photoelectrochemical sodium-mediated nitrogen fixation and ammonia synthesis device is an electrochemical reaction cell with a light window, the working electrode is a hierarchical photocathode, the counter electrode is a platinum wire, and the reference electrode is a silver / silver chloride electrode. The electrolyte is a 0.6 mol / L sodium perchlorate (NaClO4) propylene carbonate (PC) solution. Methanol (Ethanol) is a proton source carrier, and its content in the electrolyte is 2.6%. The nitrogen source for synthesizing ammonia is nitrogen gas, and the proton source is hydrogen gas, and the volume fraction of hydrogen gas in nitrogen gas is 3%.

[0081] Preparation of hierarchical photocathode: In this embodiment, the structure and composition of the hierarchical photocathode were adjusted accordingly during the preparation process, namely, a pure silicon-based (Si) photocathode and a tungsten oxide-loaded (WO3 / Si) photocathode on a Si substrate. The (WO3 / Si) photocathode was prepared using magnetron sputtering thin film deposition technology, and the detailed preparation method is as follows:

[0082] Firstly, an alkali etching method is used to form a nanostructure on the surface of p-Si, and then a high temperature diffusion method is used to prepare n + p-Si acts as a light absorber; n +The p-Si light absorber was placed in a deposition chamber for pre-vacuum treatment for 4 h, followed by deposition of a tungsten oxide layer (WO3); the target material was high-purity metallic tungsten (99.995%), the sputtering gas was a mixture of Ar and O2, the gas ratio was 97:3, the deposition pressure was 2 Pa, the sputtering power was in the range of 70-150 W, the deposition time was 60~300 s, and a WO3 film with controllable crystal structure and film thickness was obtained to prepare a (WO3 / Si) photocathode.

[0083] Photoelectrochemical sodium-mediated nitrogen fixation to ammonia:

[0084] (1) Extraction of sodium ions: The hierarchical photocathode was exposed to 1 simulated sunlight (100 mW / cm 2 ) and applying a certain external voltage (specifically 0.4~1 V vs. Na + / Na), sodium ions can be extracted from the electrolyte at 80°C.

[0085] (2) Sodium reduction: At 80°C, sodium ions are reduced to metallic sodium using photogenerated electrons, and metallic sodium (such as Figure 1 As shown in the figure, a white metallic sodium layer appears on the surface of the electrode just after the reaction is completed. After being exposed to the air for 5 minutes, it reacts with the air to transform into an aqueous solution of sodium hydroxide (NaOH). After being exposed to the air for 24 hours, it reacts with carbon dioxide and is air-dried to form solid sodium carbonate (Na2CO3).

[0086] (3) Formation of sodium nitride: At 80°C, the metallic sodium reacts with nitrogen in the electrolyte to form sodium nitrogen compounds.

[0087] (4) Synthesis of ammonia from sodium nitride and protons: At 80°C, the above sodium nitrogen compound reacts with protons to form ammonia products. At the same time, sodium returns to the electrolyte in the form of sodium ions, completing a sodium cycle.

[0088] Performance Test:

[0089] 1. The NH3 yield of ammonia synthesized by the photoelectrochemical sodium-mediated nitrogen fixation reaction in the above embodiment was tested, and the specific calculation formula is as follows:

[0090] ;

[0091] 2. The Faraday efficiency (FE) of the photoelectrochemical sodium-mediated nitrogen fixation reaction for ammonia synthesis in the above embodiment was tested, and the specific calculation formula is as follows:

[0092] ;

[0093] Among them, CNH3 represents the concentration of the product μg·mL -l; V (mL) represents the volume of the reaction electrolyte (6.5 mL). t (h) is the time for ammonia production. A (cm²) is the working area of ​​the photoelectrode. n is the number of electrons transferred (when nitrogen reduction produces ammonia, n=3), and F=96500 C·mol -l is the Faraday constant. M (g / mol) is the relative molecular mass of the product. Q (C) is the total amount of electricity applied.

[0094] The results show that the pure silicon-based (Si) photocathode and the tungsten oxide-loaded (WOx / Si) photocathode on the Si substrate have a significant effect on the photoelectrochemical sodium-mediated nitrogen reduction to ammonia synthesis. Figure 2 As shown. It can be seen that the photocathode has two main effects on the performance of nitrogen fixation and ammonia synthesis: one is the extraction of sodium ions and the precipitation of metallic sodium; the other is the adsorption and activation of protons to promote the decomposition of sodium nitrogen compounds and the synthesis of ammonia.

[0095] Example 2

[0096] The method of photoelectrochemical sodium-mediated nitrogen fixation to synthesize ammonia in Example 2 is basically the same as that in Example 1, except that the concentration of NaClO4 electrolyte is regulated from 0.2 to 1 mol / L.

[0097] The reaction apparatus and other reaction conditions used are consistent with those in Example 1. The photocathode is WO3 / Si and the preparation method used is the same as that in Example 1.

[0098] The effects of different NaClO4 electrolyte concentrations on the performance of photoelectrochemical sodium-mediated nitrogen fixation to ammonia were studied. The specific results are as follows: Figure 3 As shown. Comparative Example 1 shows that the NaClO4 electrolyte concentration is not always proportional to the ammonia synthesis performance. When the NaClO4 concentration is low (less than 0.6 mol / L), the sodium precipitation increases with the increase in concentration, and is accompanied by a significantly improved nitrogen reduction to ammonia production performance, indicating that the sodium precipitation is positively correlated with the nitrogen reduction to ammonia production performance within a certain sodium salt concentration range; however, as the sodium salt concentration increases further, the enhanced intermolecular attraction between the sodium ions and anions within the sodium salt itself due to the increase in concentration is not conducive to the precipitation of the sodium salt, resulting in a decrease in the nitrogen reduction effect.

[0099] Example 3

[0100] The method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation in Example 3 is basically the same as that in Example 1, except that methanol and isopropanol are selected as proton source carriers.

[0101] The reaction apparatus and other reaction conditions used are consistent with those in Example 1. The photocathode is WO3 / Si and the preparation method used is the same as that in Example 1.

[0102] The effects of different proton source carriers on the performance of photoelectrochemical sodium-mediated nitrogen fixation and ammonia synthesis were studied. The specific results are as follows: Figure 4 As shown. It can be seen that the photoelectrochemical ammonia synthesis effects presented by different proton source carriers are inconsistent, indicating that there are differences between the proton transport capabilities provided by different proton source carriers, and there is an optimal proton carrier.

[0103] Example 4

[0104] The method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation in Example 4 is basically the same as that in Example 1, except that the volume fraction of the active gas introduced is regulated to be 0, 10%, 17%, 29% and 38% respectively.

[0105] The reaction apparatus and other reaction conditions used are consistent with those in Example 1. The photocathode is WO3 / Si and the preparation method used is the same as that in Example 1.

[0106] The effects of introducing different proportions of active gases on the performance of photoelectrochemical sodium-mediated nitrogen fixation to synthesize ammonia were studied. The specific results are as follows: Figure 5 As shown. It can be seen that the ratio of active gas introduced is proportional to the ammonia synthesis performance within a certain range. When a certain ratio of active gas is introduced, the nitrogen reduction ammonia synthesis effect reaches the maximum value. When the active gas pressure is further increased, the ammonia synthesis performance begins to gradually decay. This is because the active gas can destroy the inactivated sodium on the electrode surface, exposing the active metallic sodium on the electrode surface, and further forming sodium-nitrogen compounds with nitrogen. However, excessive active gas will cause a large amount of stable active gas and sodium compounds to form on the electrode surface, which will lead to a reduction in the active sites of metallic sodium, which is not conducive to the process of nitrogen fixation and ammonia synthesis.

[0107] Example 5

[0108] The method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation in Example 5 is basically the same as that in Example 1, except that the hydrogen gas volume fraction is adjusted to 2%.

[0109] The reaction apparatus and other reaction conditions used are consistent with those in Example 1. The photocathode is WO3 / Si and the preparation method used is the same as that in Example 1.

[0110] Specific results such as Figure 6 As shown, it can be seen that the volume fraction of hydrogen in the system has a certain influence on the effect of synthetic ammonia, indicating that 3% hydrogen is the optimal concentration for providing protons.

[0111] Example 6

[0112] The method of synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation in Example 6 is basically the same as that in Example 1, except that the reaction temperature is adjusted to 40°C.

[0113] The reaction apparatus and other reaction conditions used are consistent with those in Example 1. The photocathode is WO3 / Si and the preparation method used is the same as that in Example 1.

[0114] Specific results such as Figure 7 As shown. It can be seen that the reaction temperature has a certain influence on the effect of synthesizing ammonia. Compared with 40°C in Example 6, 80°C in Example 1 is a more suitable reaction temperature.

[0115] In summary, the present application adopts a new type of metallic sodium mediator with abundant reserves, appropriate activity and low reaction energy barrier. The sodium ions-metallic sodium-sodium nitrogen compounds are cyclically converted. The synthesis of ammonia only consumes proton sources and nitrogen. The driving energy is solar energy. The entire reaction is highly green and realizes efficient and low-energy synthesis of ammonia.

[0116] The technical features of the above-described embodiments may be arbitrarily combined. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described. However, as long as there is no contradiction in the combination of these technical features, they should be considered to be within the scope of this specification.

[0117] The above-described embodiments only express several implementation methods of the present application, and the descriptions thereof are relatively specific and detailed, but they cannot be construed as limiting the scope of the patent application. It should be pointed out that, for a person of ordinary skill in the art, several variations and improvements can be made without departing from the concept of the present application, and these all belong to the protection scope of the present application. Therefore, the protection scope of the patent application shall be subject to the attached claims.

Claims

1. A device for photoelectrochemical sodium-mediated nitrogen fixation to synthesize ammonia, characterized in that: comprising a photocathode, a counter electrode and an electrolyte, wherein the photocathode and the counter electrode are in contact with the electrolyte; The electrolyte includes a sodium source, an organic solvent and an organic proton source.

2. The device for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation according to claim 1, characterized in that: The photocathode is a hierarchical photocathode, comprising a light absorber and a functional layer disposed on the surface of the light absorber; The material of the functional layer includes at least one of tungsten trioxide, tungsten oxide containing oxygen defects, tungsten nitride and tungsten sulfide; The light absorber is a semiconductor material; Optionally, the material of the light absorber includes at least one of silicon, cuprous oxide, gallium phosphide, arsenic phosphide and gallium nitride.

3. The device for photoelectrochemical sodium-mediated nitrogen fixation and ammonia synthesis according to any one of claims 1 to 2, characterized in that: The electrolyte satisfies at least one of the following conditions (1) to (3): (1) The sodium source includes at least one of sodium hexafluorophosphate, sodium perchlorate, sodium trifluoromethanesulfonate, sodium bis(fluorosulfonyl)imide, sodium chloride and sodium fluoride; (2) the organic solvent comprises at least one of propylene carbonate, ethylene carbonate, dimethyl carbonate, methylpropyl carbonate, tetrahydrofuran, dimethyl phthalate, γ-butyrolactone, methyl formate and maleic anhydride; (3) The organic proton source includes at least one of methanol, ethanol, propanol, trifluoroethanol, isopropanol, acetic acid and glycerol.

4. The device for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation according to any one of claims 1 to 2, characterized in that: The concentration of the sodium source in the electrolyte is 0.01 mol / L~2 mol / L.

5. The device for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation as claimed in claim 4, characterized in that: Based on the total volume of the electrolyte, the volume proportion of the organic proton source is 3% to 20%.

6. The device for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation as claimed in claim 2, characterized in that: The thickness of the functional layer is 20nm~200nm.

7. A method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation, characterized in that: The method is carried out using the device for photoelectrochemical sodium-mediated nitrogen fixation and ammonia synthesis according to any one of claims 1 to 6, comprising the following steps: A reaction gas including nitrogen is introduced into the electrolyte, the photocathode is illuminated, and electricity is applied, at 20° C. to 100° C., sodium reduction, generation of sodium nitride, and synthesis of ammonia by sodium nitride and protons.

8. The method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation as claimed in claim 7, characterized in that: The reaction gas also includes hydrogen. Based on the total volume of the reaction gas, the volume proportion of the hydrogen is 0.5% to 30%.

9. The method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation according to any one of claims 7 to 8, characterized in that: The reaction gas further includes an active gas, and the active gas includes one of carbon dioxide, oxygen, and sulfur dioxide.

10. The method for synthesizing ammonia by photoelectrochemical sodium-mediated nitrogen fixation according to claim 9, characterized in that: Based on the total volume of the reaction gas, the volume proportion of the active gas is 1% to 40%.

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