Apparatus and method for photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia

Abstract

The present application relates to the technical field of photoelectrochemistry, and particularly relates to a device and a method for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis. The device comprises a photoelectrode, a counter electrode and an electrolyte, the photoelectrode and the counter electrode are in contact with the electrolyte; the electrolyte comprises a lithium source, an organic solvent and an organic proton source. The method is to use the above device, introduce nitrogen into the electrolyte, irradiate the photoelectrode of the device, and pass electricity, so as to realize lithium reduction, lithium nitride generation, reaction of lithium nitride and protons to synthesize ammonia. The ammonia synthesis rate of the method is high, the lithium source is recycled in the process, ammonia is synthesized by using nitrogen and hydrogen in the organic proton source, the driving energy is solar energy, the process is green, environmental protection and sustainable, and has practical amplification application prospect.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of photoelectrochemistry, in particular to a device and method for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis. BACKGROUND

[0002] Ammonia, as a basic chemical, is used in agricultural production and chemical synthesis. In addition, ammonia can also be used as energy storage, which has high hydrogen content (17.8%), high volumetric energy density (13.6 GJ / m 3 ), no carbon emission and easy transportation (boiling point of -33℃).

[0003] There are various methods for ammonia synthesis: Haber-Bosch method, ammonia synthesis at 150-350 atm and 350-550℃, which requires a large amount of fossil resources and emits harmful substances; biological nitrogen fixation, using nitrogen-fixing microorganisms to reduce atmospheric nitrogen to ammonia, which is environmentally friendly but has a long fermentation time and cannot achieve industrial production; photocatalytic nitrogen fixation, under the action of light and specific catalysts, nitrogen is directly reduced to ammonia, which requires high catalysts, and the current photocatalytic nitrogen fixation catalysts have low efficiency; photoelectrocatalytic nitrogen fixation, directly using solar energy to realize the reaction of nitrogen fixation and ammonia synthesis on the photoelectrode integrated with a light absorber and a cocatalyst. Photoelectrocatalytic nitrogen fixation mainly uses aqueous solution as the electrolyte of the photoelectrochemical reaction system, and improves the photo-cathode to improve the nitrogen fixation efficiency, but the ammonia yield and solar energy utilization rate are very low, which hinders the development and application of photoelectrochemical nitrogen fixation and ammonia synthesis. SUMMARY

[0004] Therefore, in view of the low ammonia yield and low solar energy utilization rate in the prior art, the present application aims to provide a device and method for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis, which not only improves the efficiency of photoelectrochemical nitrogen fixation, but also greatly improves the solar energy utilization rate.

[0005] The device for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis provided by the present application comprises a photo-cathode, a counter electrode and an electrolyte, wherein the photo-cathode and the counter electrode are in contact with the electrolyte.

[0006] The electrolyte comprises a lithium source, an organic solvent and an organic proton source.

[0007] In one embodiment, the photo-cathode is a hierarchical photo-cathode comprising a light absorber, a protective layer and a cocatalyst layer which are sequentially stacked on the surface of the light absorber.

[0008] The cocatalyst in the cocatalyst layer is one or more of palladium alloy, copper alloy, bismuth alloy, zinc alloy and manganese alloy.

[0009] The protective material in the protective layer is one or more of titanium oxide, aluminum oxide, silicon oxide, tungsten oxide and zirconium oxide;

[0010] The light absorber is a semiconductor material.

[0011] In one embodiment, the electrolyte comprises the following technical features:

[0012] The lithium source is one or more of lithium perchlorate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium hexafluorophosphate and lithium bisborate;

[0013] The organic solvent is one or more of propylene carbonate, ethylene carbonate, dimethyl carbonate, methyl propyl carbonate, tetrahydrofuran, dimethyl phthalate, gamma-butyrolactone, methyl formate and maleic anhydride;

[0014] The organic proton source is one or more of methanol, ethanol, trifluoroethanol, isopropanol, acetic acid and glycerol.

[0015] In one embodiment, the concentration of the lithium source in the electrolyte is 0.001-2 mol / L.

[0016] In one embodiment, the volume ratio of the organic proton source to the organic solvent in the electrolyte is 1:5-30.

[0017] The application also provides a method for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis, which uses the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device described above and comprises the following steps:

[0018] Nitrogen-containing reaction gas is introduced into the electrolyte, the photoelectrocathode is irradiated, and electricity is supplied to reduce lithium, generate lithium nitride, and synthesize ammonia by the reaction of lithium nitride and protons.

[0019] In one embodiment, the method further comprises:

[0020] The reaction gas further comprises active gas, and the active gas is any one of oxygen, carbon dioxide and sulfur dioxide.

[0021] In one embodiment, the molar ratio of the active gas to nitrogen is 1:80-99.

[0022] In one embodiment, the light intensity of the irradiation is 50-500 mW / cm 2 , and the wavelength of the irradiation is 380-1100 nm.

[0023] In one embodiment, the voltage of the electricity supply is greater than -3.05 V vs reversible hydrogen electrode (RHE), and the pressure of the reaction gas is maintained at 0.1-6 MPa during the introduction of the reaction gas.

[0024] Compared with the prior art, the application has the following beneficial effects:

[0025] The device for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis provided by the application provides photoelectrons, lithium sources and proton sources for the nitrogen fixation process by arranging a photoelectrocathode, a counter electrode and using lithium sources, organic solvents and organic proton sources as electrolyte solutions, and the lithium sources mediate the entire nitrogen fixation process, thereby improving the ammonia synthesis rate.

[0026] The method for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis provided by the application realizes fast Li + + photo-generated electrons → Li + N2 → Li3N + 3H + (or + O2) → Li + (or Li2O) + NH3, and under the condition of a photocurrent of 10 mA / cm 2 , the ammonia synthesis rate can reach 10 -7 g / cm 2 / min, which is much higher than the rate (10 -10 g / cm 2 / min) of the existing photoelectric or electrochemical reduction of nitrogen to synthesize ammonia in an aqueous solution. In the process, the lithium sources are recycled, ammonia is synthesized by using nitrogen and hydrogen in the organic proton source, the driving energy is solar energy, and the process is green, environmentally friendly and sustainable, and has application prospects for practical amplification. BRIEF DESCRIPTION OF DRAWINGS

[0027] Figure 1 The device for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis provided by the embodiment of the application is shown in the structural schematic diagram.

[0028] Figure 2 The photoelectrochemical lithium-mediated nitrogen fixation result diagram of the photoelectrocathode provided by the embodiment of the application is a TiO2 / Si and PdCu / TiO2 / Si hierarchical photoelectrocathode.

[0029] Figure 3 The photoelectrochemical lithium-mediated ammonia synthesis result diagram of different LiClO4 electrolyte concentrations provided by the embodiment of the application is shown.

[0030] Figure 4 The photoelectrochemical lithium-mediated ammonia synthesis result diagram of different volume ratios of organic solvents and organic proton sources provided by the embodiment of the application is shown.

[0031] Figure 5 The photoelectrochemical lithium-mediated ammonia synthesis result diagram under different nitrogen pressure conditions provided by the embodiment of the application is shown.

[0032] Figure 6 The photoelectrochemical lithium-mediated ammonia synthesis result diagram under different proton sources provided by the embodiment of the application is shown.

[0033] Figure 7 The influence of the introduction of activated gas on photoelectrochemical lithium-mediated synthesis of ammonia provided by the embodiment of the present application is shown in the following figure:

[0034] Figure 8 The results of photoelectrochemical lithium-mediated synthesis of ammonia under light of different wavelengths provided by the embodiment of the present application are shown in the following figure. DETAILED DESCRIPTION

[0035] In order to make the above objectives, features and advantages of the present application more apparent, specific embodiments of the present application will be described in detail below with reference to the accompanying drawings. In the following description, numerous specific details are set forth in order to provide a thorough understanding of the present application. However, the present application can be practiced in a number of different manners, which are different from those described herein, and it can be apparent that similar modifications can be made by those skilled in the art without departing from the scope of the present application, and therefore the present application is not limited to the specific embodiments disclosed below.

[0036] In addition, the terms "first", "second", etc. are only used for descriptive purposes and should not be construed as indicating or implying relative importance or implicitly indicating the number of the technical features indicated. Therefore, the features defined with "first", "second" can explicitly or implicitly include at least one of the features. In the description of the present application, the meaning of "a plurality of" is at least two, such as two, three, etc., unless otherwise specifically limited.

[0037] In the present application, unless otherwise specifically defined and limited, the terms "mounting", "connection", "connection", "fixing" and other terms should be understood in a broad sense, for example, it can be fixedly connected, or it can be detachably connected, or it can be integrated; it can be mechanically connected, or it can be electrically connected; it can be directly connected, or it can be indirectly connected through an intermediate medium; it can be the internal communication of two elements or the interaction relationship between two elements, unless otherwise specifically defined. For those skilled in the art, the specific meaning of the above terms in the present application can be understood according to the specific circumstances.

[0038] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as understood by those skilled in the art to which the present application belongs. The terms used in the specification of the present application herein are only for the purpose of describing specific embodiments and are not intended to limit the present application. The term "and / or" used herein includes any and all combinations of one or more related listed items.

[0039] The embodiment of the present application provides a device for photoelectrochemical lithium-mediated nitrogen fixation and synthesis of ammonia, and the structure is as shown in Figure 1 The device includes a photoelectrode 3, a counter electrode 2 and an electrolyte 1, and the photoelectrode 3 and the counter electrode 2 are in contact with the electrolyte 1.

[0040] The electrolyte comprises a lithium source, an organic solvent and an organic proton source.

[0041] It should be understood that a power supply is connected between the photocathode and the counter electrode for applying an external voltage.

[0042] In a specific example, the photocathode is a hierarchical photocathode comprising a light absorber, a protective layer and a cocatalyst layer; the cocatalyst in the cocatalyst layer is one or more of a palladium alloy, a copper alloy, a bismuth alloy, a zinc alloy and a manganese alloy; the protective material in the protective layer is one or more of titanium oxide, aluminum oxide, silicon oxide, tungsten oxide and zirconium oxide; and the light absorber is a semiconductor material, which can be silicon, cuprous oxide, gallium phosphide, arsenic phosphide, gallium nitride or the like. The photocathode has two main effects on the performance of nitrogen fixation and ammonia synthesis: one is the extraction of lithium ions and the deposition of metallic lithium; and the other is the formation of catalytically active protons, which promotes the decomposition of lithium nitride and the synthesis of ammonia.

[0043] In a specific example, the electrolyte comprises the following technical features:

[0044] The lithium source is one or more of lithium perchlorate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium hexafluorophosphate and lithium bisborate; the organic solvent is one or more of propylene carbonate, ethylene carbonate, dimethyl carbonate, methyl propyl carbonate, tetrahydrofuran, dimethyl phthalate, gamma-butyrolactone, methyl formate and maleic anhydride; and the organic proton source is one or more of methanol, ethanol, trifluoroethanol, isopropyl alcohol, acetic acid and glycerol. A proton source with strong proton-donating ability can accelerate the decomposition of lithium nitride and the synthesis of ammonia, and the proton source is preferably trifluoroethanol. It should be understood that the proton source can also be hydrogen or a hydrocarbon gas.

[0045] In a specific example, the concentration of the lithium source in the electrolyte is 0.001-2 mol / L, and is preferably 0.5 mol / L.

[0046] In a specific example, the volume ratio of the organic proton source to the organic solvent in the electrolyte is 1:5-30, and is preferably 1:19.

[0047] The application also provides a method for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis, characterized by using the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device described above, and comprising the following steps:

[0048] A reaction gas comprising nitrogen is introduced into the electrolyte in the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device, the photocathode of the device is irradiated, and electricity is supplied, so as to realize lithium reduction, the generation of lithium nitride, the reaction of lithium nitride with protons to synthesize ammonia.

[0049] In the preparation process, the principle is as follows: under light, the hierarchical photoelectric cathode extracts lithium ions from the electrolyte, reduces the lithium ions to metal lithium by using photo-generated electrons, obtains metal lithium at a potential lower than the lithium ion reduction site; the metal lithium reacts with nitrogen to generate lithium nitride; then the lithium nitride can obtain protons from the proton source, decompose to synthesize ammonia and generate lithium ions; the generated lithium ions will participate in the lithium extraction step again to form a complete lithium cycle. That is, in the present application, the cycle conversion between lithium ions, metal lithium and lithium nitride plays the role of activation, fixation and hydrogenation of nitrogen. Metal lithium can react with nitrogen at room temperature, and has very strong activity. In the present application, the lithium cycle avoids the use of a large amount of lithium, can effectively reduce the cost and improve the safety, and has good application prospect.

[0050] The light source can be monochromatic light, composite light, simulated sunlight, natural light, etc., so as to distribute the use of solar energy and store solar energy in the form of chemical energy through photoelectrochemical method.

[0051] In a specific example, in order to decompose the lithium nitride, a small amount of active gas is mixed with nitrogen and introduced into the reaction system, which can promote the breakage of lithium-nitrogen bond and accelerate the hydrogenation of nitrogen and the recovery of lithium ions. Therefore, the method further comprises:

[0052] The reaction gas further comprises an active gas, and the active gas is any one of oxygen, carbon dioxide and sulfur dioxide.

[0053] In a specific example, the molar ratio of the active gas to nitrogen is 1:80-99.

[0054] In a specific example, the light intensity in the light irradiation process is 50-500 mW / cm 2 , and the light wavelength is 380-1100 nm.

[0055] In a specific example, the voltage of the power supply is greater than -3.05 V vs reversible hydrogen electrode (RHE), wherein -3.05 V vs RHE is equal to 0 V vs the reaction potential of lithium ion reduction to metal lithium (Li + / 0 In a specific example, the pressure of the reaction gas is maintained at 0.1-6 MPa during the process of introducing the reaction gas.

[0056] The following is further described with specific examples.

[0057] Example 1

[0058] Preparation of photoelectric cathode material

[0059] The surface of p-Si is textured by alkali etching method, and then n +p-Si as light absorber; clean and dry n + p-Si light absorber was placed in the deposition chamber for TiO2 layer deposition; the target material was high-purity metal Ti (99.995%), the sputtering gas was Ar and O2 mixed gas, the target-to-substrate distance was 10 cm, the deposition pressure was 2 Pa, and the sputtering power was in the range of 50-400 W, thereby obtaining a TiO2 protective layer with controllable crystal structure and film thickness, and obtaining a TiO2 protective layer / Si-based (TiO2 / Si) hierarchical photoelectrode. On the basis of the TiO2 / Si photoelectrode, metal Pd and Cu targets were co-sputtered, the sputtering gas was Ar, the deposition pressure was 2 Pa, and the sputtering power was in the range of 10-100 W, thereby obtaining a PdCu nanoparticle cocatalyst / TiO2 protective layer / Si-based (PdCu / TiO2 / Si) hierarchical photoelectrode.

[0060] According to the above method, n + p-Si as light absorber, W as target material, and Ar and O2 mixed gas as sputtering gas to form a WO3 protective layer, thereby obtaining a WO3 protective layer / Si-based (TiO2 / Si) hierarchical photoelectrode; then co-sputtering metal Cu and Bi targets with Ar as sputtering gas to form a CuBi nanoparticle cocatalyst / WO3 protective layer / Si-based (CuBi / WO3 / Si) hierarchical photoelectrode.

[0061] Cuprous oxide as semiconductor light absorber, W as target material, and Ar and O2 mixed gas as sputtering gas to form a WO3 protective layer, and on the basis of the above-mentioned WO3 protective layer / Cu2O-based (TiO2 / Cu2O) hierarchical photoelectrode, co-sputtering metal Zn and Mn targets with Ar as sputtering gas to obtain a ZnMn nanoparticle cocatalyst / WO3 protective layer / Cu2O-based (CuBi / WO3 / Cu2O) hierarchical photoelectrode.

[0062] Example 2

[0063] In this embodiment, the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device is an electrochemical reaction cell with a light window, the working electrode includes a hierarchical photoelectrode (PdCu / TiO2 / Si photoelectrode), and the counter electrode is a platinum wire. The electrolyte is a 1 mol / L lithium perchlorate (LiClO4) solution in propylene carbonate (PC)-ethanol (EtOH). The volume ratio of EtOH to PC is 1:10.

[0064] The preparation process of this embodiment is as follows: high-purity nitrogen gas is introduced into the electrolyte of the above-mentioned photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device, the flow rate is 5 sccm, the photoelectrode is irradiated at an intensity of 100 mW / cm 2and a certain external voltage (a plurality of voltage values are set, specifically including: 0.82V, 0.57V, 0.32V, 0.07V, -0.18V, -0.43V), under the light condition, the lithium ion is extracted from the electrolyte by the photocathode, the metal lithium is deposited on the surface of the photocathode, the metal lithium reacts with the nitrogen in the electrolyte to generate lithium nitride, and then protons are obtained to form ammonia products, while the lithium returns to the electrolyte in the form of lithium ions, and a lithium cycle is completed.

[0065] Example 3

[0066] This example is different from example 2 only in the photocathode, and the device composition and preparation process are exactly the same as those of example 2. The photocathode used in example 3 is a TiO2 / Si photocathode.

[0067] The ammonia yield and Faraday current efficiency of the different photocathodes in examples 2 and 3 for nitrogen fixation and ammonia synthesis under the same conditions were determined, and the results are shown in Table 1. Figure 2 As can be seen from Table 1, the different photocathodes and applied voltages have a significant influence on the behavior of nitrogen fixation and ammonia synthesis. For the TiO2 / Si photocathode, the ammonia yield and Faraday current efficiency are basically stable and remain unchanged under different voltage conditions. For the PdCu / TiO2 / Si photocathode, the ammonia yield increases with the decrease of voltage, and reaches the maximum value when the applied voltage is -0.18V. In the reaction of nitrogen fixation and ammonia synthesis, the photocathode has two main influences on the performance of nitrogen fixation and ammonia synthesis: one is the extraction of lithium ions and the deposition of metal lithium; the other is the formation of catalytic active protons, which promotes the decomposition of lithium nitride and the synthesis of ammonia.

[0068] Example 4

[0069] In this example, the device for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis is an electrochemical reaction cell with a light window. The working electrode includes a hierarchical photocathode (PdCu / TiO2 / Si photocathode), and the counter electrode is a platinum wire. The electrolyte is a lithium perchlorate (LiClO4) solution in propylene carbonate (PC)-ethanol (EtOH). The volume ratio of EtOH to PC is 1:10. In this example, the concentration of the electrolyte solution lithium source is controlled, and the concentration of LiClO4 is set to 0.01mol / L, 0.2mol / L, 0.5mol / L, 1.0mol / L, and 2.0mol / L.

[0070] The preparation process of this example is as follows: high-purity nitrogen gas is introduced into the electrolyte of the above-mentioned device for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis, the gas flow rate is 5sccm, the photocathode is under light intensity of 100mW / cm 2and external voltage is 0.07V, under the light condition, the photo-cathode extracts lithium ions from the electrolyte, deposits metal lithium on the surface of the photo-cathode, the metal lithium reacts with nitrogen in the electrolyte to generate lithium nitride, then obtains protons to form ammonia product, at the same time, lithium returns to the electrolyte in the form of lithium ions, completing a lithium cycle.

[0071] The influence of different lithium source concentrations on the photo-electrochemical lithium-mediated nitrogen fixation and ammonia synthesis performance in Example 4 was determined, and the specific results are shown in Figure 3 It can be seen that the concentration of LiClO4 does not always present a proportional relationship with the ammonia synthesis performance, and when the concentration of LiClO4 is about 0.5mol / L, the maximum value is reached, and when the concentration of LiClO4 is too high, it will lead to the deposition of metal lithium as the main factor affecting the photoelectric conversion behavior of the photo-cathode and further affecting the performance of ammonia synthesis.

[0072] Example 5

[0073] In this example, the device for photo-electrochemical lithium-mediated nitrogen fixation and ammonia synthesis is an electrochemical reaction cell with a light window, the working electrode includes a hierarchical photo-cathode (CuBi / WO3 / Si photo-cathode), and the counter electrode is a platinum wire. The electrolyte is a lithium perchlorate (LiClO4) solution of tetrahydrofuran (THF)-trifluoroethanol (TFE), and the trifluoroethanol is the proton source. In this example, the volume ratio of the proton source and the organic solvent is controlled, and the volume ratio of trifluoroethanol and tetrahydrofuran is set to 1:5, 1:9, 1:19 and 1:30.

[0074] The preparation process of this example is as follows: high-purity nitrogen gas is introduced into the electrolyte of the above-mentioned photo-electrochemical lithium-mediated nitrogen fixation and ammonia synthesis device, the gas flow rate is 5sccm, the photo-cathode is under light intensity of 100mW / cm 2 and external voltage is 0.07V, under the light condition, the photo-cathode extracts lithium ions from the electrolyte, deposits metal lithium on the surface of the photo-cathode, the metal lithium reacts with nitrogen in the electrolyte to generate lithium nitride, then obtains protons to form ammonia product, at the same time, lithium returns to the electrolyte in the form of lithium ions, completing a lithium cycle.

[0075] The influence of different proton sources and organic solvent volume ratios on the photo-electrochemical lithium-mediated nitrogen fixation and ammonia synthesis in Example 5 was determined, and the results are shown in Figure 4 It can be seen that the volume ratio of the proton source and the organic solvent is an important factor affecting the ammonia yield, when the volume ratio of TFE / THF is too low, it greatly affects the content and concentration of protons, and when the volume ratio of TFE / THF is too high, it will lead to the deposition of hydrogen as the main factor affecting the performance of photo-electrochemical synthesis of ammonia.

[0076] Example 6

[0077] In this embodiment, the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device is an electrochemical reaction cell with a light window. The working electrode includes a hierarchical photoelectrode (PdCu / TiO2 / Si photoelectrode), and the counter electrode is a platinum wire. The electrolyte is a propylene carbonate (PC)-ethanol (EtOH) solution of lithium perchlorate (LiClO4). The volume ratio of EtOH to PC is 1:10. In this embodiment, the nitrogen pressure is regulated, and the nitrogen pressure is set to 4 MPa, 3 MPa, 2 MPa, and 1 MPa.

[0078] The preparation process of this embodiment is as follows: high-purity nitrogen is introduced into the electrolyte of the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device at a flow rate of 5 sccm. The photoelectrode is irradiated at an intensity of 100 mW / cm 2 and a certain external voltage (0.32 V, 0.07 V, and -0.18 V) is applied. Under light conditions, the photoelectrode extracts lithium ions from the electrolyte, deposits metallic lithium on the surface of the photoelectrode, and reacts with nitrogen in the electrolyte to form lithium nitride. Then protons are obtained to form ammonia products, while lithium returns to the electrolyte in the form of lithium ions, completing a lithium cycle.

[0079] The effect of different nitrogen pressures on the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis of Example 6 was determined, and the results are shown in Figure 5 It can be seen that the nitrogen pressure and the ammonia synthesis performance are positively related within a certain range. When the nitrogen pressure is at a relatively high value (3 MPa), further increasing the nitrogen pressure will have little effect on the ammonia synthesis performance.

[0080] Example 7

[0081] In this embodiment, the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device is an electrochemical reaction cell with a light window. The working electrode includes a hierarchical photoelectrode (Zn / WO3 / Cu2O photoelectrode), and the counter electrode is a platinum wire. The electrolyte is a propylene carbonate (PC)-organic proton source solution of 1 mol / L lithium perchlorate (LiClO4). The volume ratio of the organic proton source to PC is 1:10. In this embodiment, the type of proton source is regulated. The organic proton sources used in this embodiment include isopropyl alcohol (IPA), ethanol (EtOH), methanol (MeOH), and trifluoroethanol (TFE).

[0082] The preparation process of this embodiment is as follows: high-purity nitrogen is introduced into the electrolyte of the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device at a flow rate of 5 sccm. The photoelectrode is irradiated at an intensity of 100 mW / cm 2and an external voltage of 0.07 V, under light conditions, the photoelectrode extracts lithium ions from the electrolyte, deposits metal lithium on the surface of the photoelectrode, the metal lithium reacts with nitrogen in the electrolyte to form lithium nitride, then obtains protons to form ammonia products, and lithium returns to the electrolyte in the form of lithium ions, completing a lithium cycle.

[0083] The influence of different proton source conditions on photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia was determined in Example 7, and the results are shown in Figure 6 It can be seen that the proton-donating ability of the above-mentioned organic proton source is ranked as: TFE > MeOH > EtOH > IPA. The stronger the proton-donating ability of the proton source, the better the performance of photoelectrochemical nitrogen fixation to synthesize ammonia. This is because the strong proton-donating ability can facilitate the decomposition of lithium nitride to produce ammonia species.

[0084] Example 8

[0085] In this embodiment, the device for photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia is an electrochemical reaction cell with a light window, the working electrode includes a hierarchical photoelectrode (PdCu / TiO2 / Si photoelectrode), and the counter electrode is a platinum wire. The electrolyte is a 1 mol / L lithium perchlorate (LiClO4) solution in propylene carbonate (PC)-ethanol (EtOH). The volume ratio of EtOH to PC is 1:10.

[0086] The preparation process of this embodiment is as follows: nitrogen and oxygen mixed gas is introduced into the electrolyte of the above-mentioned device for photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia, the gas flow rate is 5 sccm, and the concentration of oxygen is specifically 0%, 1%, 3%, 5%, 7%, and 10%, the photoelectrode is under a light intensity of 100 mW / cm 2 and an external voltage of 0.07 V, under light conditions, the photoelectrode extracts lithium ions from the electrolyte, deposits metal lithium on the surface of the photoelectrode, the metal lithium reacts with nitrogen in the electrolyte to form lithium nitride, then obtains protons to form ammonia products, and lithium returns to the electrolyte in the form of lithium ions, completing a lithium cycle.

[0087] The influence of the proportion of active gas introduced in nitrogen on photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia was determined in Example 8, and the specific results are shown in Figure 7 It can be seen that when low-concentration active gas is introduced, the ammonia yield increases, and there is a maximum value between the concentration of oxygen and the ammonia synthesis yield. When the concentration of oxygen is too high, lithium oxide is formed, and nitrogen cannot be adsorbed and activated, losing the performance of nitrogen fixation to synthesize ammonia.

[0088] Example 9

[0089] In this embodiment, the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device is an electrochemical reaction cell with a light window, the working electrode includes a hierarchical photoelectrode (CuBi / WO3 / Si photoelectrode), and the counter electrode is a platinum wire. The electrolyte is a 0.5 mol / L lithium hexafluorophosphate solution in dimethyl carbonate-trifluoroethanol. The volume ratio of trifluoroethanol to dimethyl carbonate is 1:19.

[0090] The preparation process of this embodiment is as follows: high-purity nitrogen is introduced into the electrolyte of the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis device, the flow rate is 5 sccm, an external voltage of 0.07 V is applied, and monochromatic light (power is 100 mW / cm 2 ) of different wavelengths (420 nm, 500 nm, 550 nm, 620 nm, and 700 nm) is used to drive the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis process. Under light conditions, lithium ions are extracted from the electrolyte by the photoelectrode, lithium metal is deposited on the surface of the photoelectrode, lithium metal reacts with nitrogen in the electrolyte to form lithium nitride, then protons are obtained to form ammonia products, and lithium returns to the electrolyte in the form of lithium ions, completing a lithium cycle.

[0091] The effect of monochromatic light of different wavelengths on the photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis of Example 9 is determined, and the specific results are shown in Figure 8 Therefore, visible light can facilitate the performance of photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis, which may be attributed to the fact that visible light facilitates the absorption of Si-based light absorbers, generating a larger photocurrent, and visible light can facilitate the localized plasmonic effect of Cu-based nanoparticles, promoting the activation and hydrogenation of nitrogen.

[0092] In summary, the present application can effectively improve the behavior of photoelectrochemical nitrogen reduction and ammonia synthesis, and can make the ammonia yield and energy conversion efficiency close to practical application, and has great potential to realize the conversion of solar energy to green chemical fertilizer / fuel at room temperature and normal pressure.

[0093] The technical features of the above-described embodiments can be combined in any manner. To make the description concise, not all possible combinations of the technical features in the above-described embodiments are described, but as long as the combinations of the technical features do not contradict, they should be considered within the scope of the present application.

[0094] The above-described embodiments only express several embodiments of the present application, and the description is more specific and detailed, but it should not be construed as limiting the scope of the patent. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of the present application. Therefore, the protection scope of the present patent should be based on the appended claims, and the description and drawings can be used to explain the content of the claims.

Claims

1. A method of photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia, characterized in that, The method is carried out using a photoelectrochemical lithium-mediated nitrogen fixation ammonia synthesis apparatus, which includes 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 lithium source, an organic solvent, and an organic proton source; the volume ratio of the organic proton source to the organic solvent in the electrolyte is 1:5 to 30. The photocathode is a hierarchical photocathode, which includes a light absorber and a protective layer and a co-catalytic layer sequentially stacked on the surface of the light absorber. The cocatalyst in the cocatalytic layer is one or more of palladium alloy, copper alloy, bismuth alloy, zinc alloy and manganese alloy; The protective material in the protective layer is one or more of titanium oxide, aluminum oxide, silicon oxide, tungsten oxide, and zirconium oxide; The method includes the following steps: A reaction gas including nitrogen is introduced into the electrolyte to illuminate the photocathode and then electrified, resulting in lithium reduction, lithium nitride formation, and the reaction of lithium nitride with protons to synthesize ammonia. The reactant gas also includes an active gas, which is any one of oxygen, carbon dioxide, and sulfur dioxide. The molar ratio of the active gas to nitrogen is 1:80 to 99.

2. The method of photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia according to claim 1, characterized in that, The light absorber is made of semiconductor material.

3. The method of photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia according to any one of claims 1-2, characterized in that, The lithium source is one or more of lithium perchlorate, lithium hexafluoroarsenate, lithium tetrafluoroborate, lithium hexafluorophosphate, and lithium diborate.

4. The method of photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia according to any one of claims 1-2, characterized in that, The organic solvent is one or more selected from propylene carbonate, ethylene carbonate, dimethyl carbonate, methyl propyl carbonate, tetrahydrofuran, dimethyl phthalate, γ-butyrolactone, methyl formate, and maleic anhydride.

5. The method of photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia according to any one of claims 1-2, wherein, The organic proton source is one or more of methanol, ethanol, trifluoroethanol, isopropanol, acetic acid, and glycerol.

6. The method of photoelectrochemical lithium-mediated nitrogen fixation to synthesize ammonia according to any one of claims 1-2, wherein, The concentration of lithium source in the electrolyte is 0.001–2 mol / L.

7. The method for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis according to any one of claims 1 to 2, characterized in that, The volume ratio of the organic proton source to the organic solvent in the electrolyte is 1:

19.

8. The method for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis according to claim 1, characterized in that, The light intensity of the light irradiation is 50-500 mW / cm 2 , and the wavelength of the light irradiation is 380-1100 nm.

9. The method for photoelectrochemical lithium-mediated nitrogen fixation and ammonia synthesis according to claim 1, characterized in that, The voltage applied is greater than -3.05V vs. the reversible hydrogen electrode; The pressure of the reactant gas is maintained at 0.1-6 MPa during the introduction of the reactant gas.

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