A Fe3O4 / BC composite electrocatalyst and its preparation method and application

Abstract

The present invention provides an Fe3O4 / BC composite electrocatalyst, a preparation method thereof and an application thereof, belonging to the technical field of preparation of electrocatalytic materials; in the present invention, biomass carbon with a rich mesoporous structure is first prepared by a calcination method, and then an Fe3O4 / BC composite electrocatalyst precursor is prepared by an evaporation method and annealed to obtain an Fe3O4 / BC composite electrocatalyst; the Fe3O4 / BC composite electrocatalyst uses mesoporous biochar as a substrate, and Fe3O4 nanoparticles are uniformly loaded on the biochar; the composite electrocatalyst can be applied to electrocatalytic reduction of nitrate to synthesize ammonia at room temperature.

Description

Technical Field

[0001] The present invention belongs to the technical field of electrocatalytic material preparation, and in particular relates to a Fe3O4 / BC composite electrocatalyst and a preparation method and application thereof. Background Art

[0002] Ammonia is an important chemical raw material, the main component of nitrogen fertilizer, and a green hydrogen-rich fuel in industry. At present, the Haber-Bosch process is mainly used to synthesize ammonia in industry. The innovation of the Haber-Bosch process mainly involves the irreversible reaction of hydrogen and nitrogen. This method uses hydrogen and nitrogen as raw materials, and can only achieve a relatively considerable ammonia production efficiency under high temperature and high pressure. However, this method consumes a lot of energy, which brings a huge burden to the global energy supply. A large amount of greenhouse gases will also be produced during the production process, posing a serious threat to the global environment. Inspired by microbial nitrogen fixation, the electrocatalytic reduction of nitrogen to ammonia has attracted the attention of more and more researchers. Although electrocatalytic nitrogen reduction technology has been widely used, the breaking of high-energy N≡N bonds in nitrogen molecules and the control of the accompanying hydrogen evolution reaction have always been difficult to overcome, which makes the efficiency and energy conversion rate of electrocatalytic nitrogen reduction low.

[0003] Nitrate (NO3 - ) electrocatalytic reduction has attracted much attention due to the low dissociation of the N=O bond in its reaction and the large amount of nitrate pollution faced in nature. Therefore, electrocatalytic nitrate synthesis of ammonia is a relatively promising strategy. The electrocatalytic nitrate synthesis of ammonia involves an eight-electron reaction pathway and requires a reaction potential of -1.20V (relative to the reversible hydrogen electrode), which significantly reduces the overall kinetic rate. The reduction of nitrate to nitrogen requires a reaction potential of -1.25V (relative to the reversible hydrogen electrode) and involves a five-electron reaction pathway, which inevitably reduces the Faraday efficiency and energy efficiency of ammonia synthesis. In addition, as a competing reaction, the hydrogen evolution reaction also consumes electron donors, which is not conducive to the reduction of nitrate to ammonia. Therefore, optimizing the reaction active sites of the catalyst, inhibiting the progress of the competing reaction, and promoting the adsorption and activation of nitrate are good strategies for designing suitable electrocatalysts for ammonia synthesis.

[0004] Iron-based catalysts are widely used in the electrocatalytic synthesis of ammonia. As a transition metal, Fe has an unfilled d electron orbital and can accept lone electron pairs. At the same time, its surface can adsorb nitrate ions, weaken the bond energy, and promote the electrocatalytic reduction of nitrate. As a new type of metal oxide electrocatalyst, Fe3O4 has many advantages such as relatively economical price, stable structure and properties, which makes it have broad application prospects. However, due to the insufficient electron transfer performance and reaction active sites of Fe3O4 catalyst, the efficiency of Fe3O4 catalyst in electrocatalytic reduction of nitrate is limited. Summary of the invention

[0005] In view of the shortcomings in the prior art, the present invention provides a Fe3O4 / BC composite electrocatalyst and a preparation method and application thereof; the present invention first prepares biomass charcoal with rich mesoporous structure by a calcination method, then prepares a Fe3O4 / BC composite electrocatalyst precursor by an evaporation method, and anneals the precursor to obtain a Fe3O4 / BC composite electrocatalyst; the Fe3O4 / BC composite electrocatalyst uses mesoporous biochar as a substrate, and uniformly loads Fe3O4 nanoparticles on the biochar; the composite electrocatalyst can be used for electrocatalytic nitrate reduction to synthesize ammonia at room temperature

[0006] The present invention achieves the above technical objectives through the following technical means.

[0007] The present invention first provides a Fe3O4 / BC composite electrocatalyst. The composite catalyst uses biochar with mesopores as a substrate and evenly loads Fe3O4 nanoparticles on the biochar. The composite catalyst uses biochar with mesopores as a substrate and evenly loads Fe3O4 nanoparticles on the biochar. The particle size range of the Fe3O4 nanoparticles is 40-130nm; the Fe3O4 nanoparticles are in an irregular block shape; the mass fraction of biochar in the composite catalyst is 53.82% to 57.40%.

[0008] The present invention also provides a method for preparing the Fe3O4 / BC composite electrocatalyst, which specifically comprises the following steps:

[0009] The corn stalk powder is mixed and ground with potassium bicarbonate to obtain a mixture, the mixture is calcined, and after the calcination, the product is washed and dried to obtain biomass charcoal BC;

[0010] The ferric chloride hexahydrate was dissolved in anhydrous ethanol, and the biomass carbon BC was added to perform ultrasonication and sufficient stirring, and heated in a water bath during the stirring process to obtain a Fe3O4 / BC composite electrocatalyst precursor;

[0011] The Fe3O4 / BC composite electrocatalyst precursor is annealed to obtain the Fe3O4 / BC electrocatalyst.

[0012] Preferably, the usage ratio of the corn straw powder, potassium bicarbonate, ferric chloride hexahydrate and ethanol is 0.5-2.0 g: 0.5-2.0 g: 0.135-0.54 g: 15-60 mL.

[0013] Preferably, the calcination time is 3 to 7 hours, the calcination temperature is 600 to 900° C., and the heating rate is 2 to 5° C. / min.

[0014] Preferably, the ultrasonic power is 100-300W, and the ultrasonic time is 0.5-1.0 hour.

[0015] Preferably, the sufficient stirring time is 6 to 12 hours, and the stirring rate is 500 rpm.

[0016] Preferably, the water bath is heated at a temperature of 55-65° C., and the water bath is heated until the anhydrous ethanol evaporates.

[0017] Preferably, the annealing temperature is 500° C. to 800° C., the heating rate is 2 to 5° C. / min, and the annealing time is 3 to 7 hours.

[0018] The present invention also provides the use of the Fe3O4 / BC composite electrocatalyst to electrocatalyze nitrate to synthesize ammonia at room temperature.

[0019] Compared with the prior art, the present invention has the following beneficial effects:

[0020] The biochar prepared by the present invention has the advantages of low cost, good electrical conductivity, the ability to ensure effective charge transfer during the electrochemical reaction, the ability to react synergistically with the catalyst, the provision of more active sites, and the improvement of catalytic activity. The present invention loads nanoparticles of Fe3O4 on biochar rich in mesoporous structure, and the conductivity of Fe3O4 is significantly enhanced after the composite, and the biochar rich in mesoporous structure enhances the adsorption capacity of Fe3O4, which is conducive to the electrocatalytic nitrate reduction synthesis of ammonia. The biochar substrate also makes the Fe3O4 nanoparticles evenly dispersed, so that the specific surface area of ​​the catalyst is larger, exposing more active sites, which is conducive to the nitrate reduction.

[0021] The Fe3O4 / BC composite electrocatalyst prepared by the method of the present invention can effectively adsorb and activate nitrate, and can electrocatalyze nitrate reduction to synthesize ammonia at room temperature. The Fe3O4 / BC composite electrocatalyst prepared in the present invention has a high ammonia yield and Faraday efficiency in 1M KOH+0.1M KNO3 electrolyte. At -0.5V relative to the reversible hydrogen electrode potential, the ammonia yield can reach a maximum of 4384mmol h -1 g cat . -1 , with a Faraday efficiency of 93.4%. This is in contrast to the ammonia yield of existing catalysts such as Co3O4@NiO HNTs (6.93 mmol h -1 g cat -1 )、Co-NAs(2.6 mol g cat -1 h -1 )、Cu50Co50(960mmol g cat -1 h -1 )The catalyst prepared by the present invention has a higher ammonia yield. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 This is the SEM image of the Fe3O4 / BC composite electrocatalyst.

[0023] Figure 2 TEM image of Fe3O4 / BC composite electrocatalyst.

[0024] Figure 3 HRTEM image of Fe3O4 / BC composite electrocatalyst.

[0025] Figure 4 This is the EDS image of Fe3O4 / BC composite electrocatalyst.

[0026] Figure 5 Mapping element diagram of Fe3O4 / BC composite electrocatalyst.

[0027] Figure 6 This is the XRD pattern of Fe3O4 / BC composite electrocatalyst.

[0028] Figure 7 This is a comparison of the linear sweep voltammetry curves of Fe3O4 / BC composite electrocatalyst under different conditions.

[0029] Figure 8 Ammonia yield and Faraday efficiency diagram of Fe3O4 / BC composite electrocatalyst.

[0030] Fig. 9 Comparison of ammonia yield and Faradaic efficiency of Fe3O4 / BC composite electrocatalyst and empty carbon paper as working electrode under different conditions.

[0031] Fig.10 This is the XPS spectrum of Fe3O4 / BC composite electrocatalyst.

[0032] Fig.11 This is the Raman spectrum of the Fe3O4 / BC composite electrocatalyst. DETAILED DESCRIPTION

[0033] The present invention is further described below in conjunction with the accompanying drawings and specific embodiments, but the protection scope of the present invention is not limited thereto. The materials, reagents, etc. used in the following embodiments, unless otherwise specified, can be obtained from commercial sources.

[0034] Example 1: Preparation of Fe3O4 / BC composite electrocatalyst

[0035] 1.0g corn stalk powder was mixed with 1.0g potassium bicarbonate, fully ground and transferred to a porcelain boat, and then the mixture was placed in a tube furnace under an argon atmosphere and heated to 800℃ at a heating rate of 5℃ / min for 5 hours. After calcination, it was cooled to room temperature to obtain a black powder. The black powder was washed with deionized water to neutrality, then washed three times with anhydrous ethanol, and finally dried in a vacuum oven at 60℃ for 12 hours to obtain a biomass carbon (BC) rich in mesoporous structure.

[0036] 0.27 g of ferric chloride hexahydrate was dissolved in 30 mL of anhydrous ethanol, and then 0.10 g of BC was added thereto, and ultrasonic stirring was performed for 30 min to mix evenly, and the ultrasonic power was 300 W. During the stirring process, the mixture was heated in a water bath at 60° C. and evaporated to dryness, and the stirring speed was 500 rpm to obtain a Fe3O4 / BC composite electrocatalyst precursor.

[0037] 0.20 g of Fe3O4 / BC composite electrocatalyst precursor was heated to 500°C at a heating rate of 5°C / min in an argon atmosphere and annealed for 5 hours, and then cooled to room temperature to obtain a Fe3O4 / BC composite electrocatalyst. Since the annealing temperature was 500°C, it was recorded as Fe3O4 / BC-500 composite electrocatalyst.

[0038] Example 2: Preparation of Fe3O4 / BC composite electrocatalyst

[0039] 1.0g corn stalk powder was mixed with 1.0g potassium bicarbonate, fully ground and transferred to a porcelain boat, and then the mixture was placed in a tube furnace under an argon atmosphere and heated to 800℃ at a heating rate of 5℃ / min for 5 hours. After calcination, it was cooled to room temperature to obtain a black powder. The black powder was washed with deionized water to neutrality, then washed three times with anhydrous ethanol, and finally dried in a vacuum oven at 60℃ for 12 hours to obtain a biomass carbon (BC) rich in mesoporous structure.

[0040] 0.27 g of ferric chloride hexahydrate was dissolved in 30 mL of anhydrous ethanol, and then 0.10 g of BC was added thereto, and ultrasonic stirring was performed for 30 min to mix evenly, and the ultrasonic power was 300 W. During the stirring process, the mixture was heated in a water bath at 60° C. and evaporated to dryness, and the stirring speed was 500 rpm to obtain a Fe3O4 / BC composite electrocatalyst precursor.

[0041] 0.20 g of Fe3O4 / BC composite electrocatalyst precursor was heated to 600°C at a heating rate of 5°C / min in an argon atmosphere and annealed for 5 hours, and then cooled to room temperature to obtain a Fe3O4 / BC composite electrocatalyst. Since the annealing temperature was 600°C, it was recorded as Fe3O4 / BC-600 composite electrocatalyst.

[0042] Example 3: Preparation of Fe3O4 / BC composite electrocatalyst

[0043] 1.0g corn stalk powder was mixed with 1.0g potassium bicarbonate, fully ground and transferred to a porcelain boat, and then the mixture was placed in a tube furnace under an argon atmosphere and heated to 800℃ at a heating rate of 5℃ / min for 5 hours. After calcination, it was cooled to room temperature to obtain a black powder. The black powder was washed with deionized water to neutrality, then washed three times with anhydrous ethanol, and finally dried in a vacuum oven at 60℃ for 12 hours to obtain a biomass carbon (BC) rich in mesoporous structure.

[0044] 0.27 g of ferric chloride hexahydrate was dissolved in 30 mL of anhydrous ethanol, and then 0.10 g of BC was added thereto, and ultrasonic stirring was performed for 30 min to mix evenly, and the ultrasonic power was 300 W. During the stirring process, the mixture was heated in a water bath at 60° C. and evaporated to dryness, and the stirring speed was 500 rpm to obtain a Fe3O4 / BC composite electrocatalyst precursor.

[0045] 0.20 g of Fe3O4 / BC composite electrocatalyst precursor was heated to 700°C at a heating rate of 5°C / min in an argon atmosphere and annealed for 5 hours, and then cooled to room temperature to obtain a Fe3O4 / BC composite electrocatalyst. Since the annealing temperature was 700°C, it was recorded as Fe3O4 / BC-700 composite electrocatalyst.

[0046] Figure 1 The SEM image of Fe3O4 / BC-700 composite electrocatalyst. Figure 2 TEM image of Fe3O4 / BC. Figure 1 and Figure 2 It can be seen that the Fe3O4 nanoparticles are dispersed on the biochar, and the Fe3O4 nanoparticles are in irregular blocks with a particle size of about 100 nm, proving that the catalyst was successfully synthesized.

[0047] Figure 3 This is the HRTEM image of the Fe3O4 / BC-700 composite electrocatalyst. Obvious lattice fringes can be seen in the image. The lattice spacing is 0.240nm, corresponding to the (2 2 2) crystal plane of Fe3O4, further proving that the catalyst synthesis was successful.

[0048] Figure 4 This is the energy dispersive x-ray spectrum (EDS) of the Fe3O4 / BC-700 composite electrocatalyst. As can be seen from the figure, through the location of each element, it can be seen that the three elements are present, which proves that the synthesis is successful.

[0049] Figure 5This is the mapping element map and the full mapping map of the Fe3O4 / BC-700 composite electrocatalyst. It can be seen from the figure that the presence of the three elements Fe, O and C further proves the successful synthesis of the catalyst.

[0050] Figure 6 This is the XRD spectrum of the Fe3O4 / BC-700 composite electrocatalyst. It can be seen from the figure that the XRD peak type of the sample obtained after calcination is most consistent with the PDF standard card of Fe3O4 (JCPDS:19-0629), proving that the catalyst was successfully synthesized.

[0051] Fig.10 This is the Raman spectrum of the Fe3O4 / BC-700 composite electrocatalyst. It can be seen from the figure that the -1 and 1600cm -1 The two obvious characteristic Raman peaks correspond to the Raman active D peak and G peak of biochar.

[0052] Fig.11 This is the XPS graph of the Fe3O4 / BC--700 composite electrocatalyst. It can be seen from the figure that Fe, O, and C all exist in the composite catalyst.

[0053] Example 4: Preparation of Fe3O4 / BC composite electrocatalyst

[0054] 1.0g corn stalk powder was mixed with 1.0g potassium bicarbonate, fully ground and transferred to a porcelain boat, and then the mixture was placed in a tube furnace under an argon atmosphere and heated to 800℃ at a heating rate of 5℃ / min for 5 hours. After calcination, it was cooled to room temperature to obtain a black powder. The black powder was washed with deionized water to neutrality, then washed three times with anhydrous ethanol, and finally dried in a vacuum oven at 60℃ for 12 hours to obtain a biomass carbon (BC) rich in mesoporous structure.

[0055] 0.27 g of ferric chloride hexahydrate was dissolved in 30 mL of anhydrous ethanol, and then 0.10 g of BC was added thereto, and ultrasonic stirring was performed for 30 min to mix evenly, and the ultrasonic power was 300 W. During the stirring process, the mixture was heated in a water bath at 60° C. and evaporated to dryness, and the stirring speed was 500 rpm to obtain a Fe3O4 / BC composite electrocatalyst precursor.

[0056] 0.20 g of Fe3O4 / BC composite electrocatalyst precursor was heated to 800°C at a heating rate of 5°C / min in an argon atmosphere and annealed for 5 hours, and then cooled to room temperature to obtain a Fe3O4 / BC composite electrocatalyst. Since the annealing temperature was 800°C, it was recorded as Fe3O4 / BC-800 composite electrocatalyst.

[0057] Example 5: Preparation of Fe3O4 / BC composite electrocatalyst

[0058] 0.5g corn stalk powder was mixed with 0.5g potassium bicarbonate, fully ground and transferred to a porcelain boat, and then the mixture was placed in a tube furnace under an argon atmosphere and heated to 800°C at a heating rate of 5°C / min for 5 hours. After calcination, it was cooled to room temperature to obtain a black powder. The black powder was washed with deionized water to neutrality, then washed three times with anhydrous ethanol, and finally dried in a vacuum oven at 60°C for 12 hours to obtain a biomass carbon (BC) rich in mesoporous structure.

[0059] 0.27 g of ferric chloride hexahydrate was dissolved in 30 mL of anhydrous ethanol, and then 0.05 g of BC was added thereto, and ultrasonic stirring was performed for 30 min to mix evenly, and the ultrasonic power was 300 W. During the stirring process, the mixture was heated in a water bath at 60° C. and evaporated to dryness, and the stirring speed was 500 rpm to obtain a Fe3O4 / BC composite electrocatalyst precursor.

[0060] 0.20 g of Fe3O4 / BC composite electrocatalyst precursor was heated to 700°C at a heating rate of 5°C / min in an argon atmosphere and annealed for 5 hours, and then cooled to room temperature to obtain a Fe3O4 / BC composite electrocatalyst. Since the annealing temperature was 800°C, it was recorded as Fe3O4 / BC-700-2 composite electrocatalyst.

[0061] Example 6: Preparation of Fe3O4 / BC composite electrocatalyst

[0062] 1.0g corn stalk powder was mixed with 1.0g potassium bicarbonate, fully ground and transferred to a porcelain boat, and then the mixture was placed in a tube furnace under an argon atmosphere and heated to 800℃ at a heating rate of 5℃ / min for 5 hours. After calcination, it was cooled to room temperature to obtain a black powder. The black powder was washed with deionized water to neutrality, then washed three times with anhydrous ethanol, and finally dried in a vacuum oven at 60℃ for 12 hours to obtain a biomass carbon (BC) rich in mesoporous structure.

[0063] 0.135 g of ferric chloride hexahydrate was dissolved in 15 mL of anhydrous ethanol, and then 0.10 g of BC was added thereto, and ultrasonic stirring was performed for 30 min to mix evenly, and the ultrasonic power was 300 W. During the stirring process, the mixture was heated in a water bath at 60° C. and evaporated to dryness, and the stirring speed was 500 rpm to obtain a Fe3O4 / BC composite electrocatalyst precursor.

[0064] 0.20 g of Fe3O4 / BC composite electrocatalyst precursor was heated to 800°C at a heating rate of 5°C / min in an argon atmosphere and annealed for 5 hours, and then cooled to room temperature to obtain a Fe3O4 / BC composite electrocatalyst. Since the annealing temperature was 700°C, it was recorded as Fe3O4 / BC-700-3 composite electrocatalyst.

[0065] Example 7: Preparation of Fe3O4 / BC composite electrocatalyst

[0066] 1.0g corn stalk powder was mixed with 1.0g potassium bicarbonate, fully ground and transferred to a porcelain boat, and then the mixture was placed in a tube furnace under an argon atmosphere and heated to 800℃ at a heating rate of 5℃ / min for 5 hours. After calcination, it was cooled to room temperature to obtain a black powder. The black powder was washed with deionized water to neutrality, then washed three times with anhydrous ethanol, and finally dried in a vacuum oven at 60℃ for 12 hours to obtain a biomass carbon (BC) rich in mesoporous structure.

[0067] 0.27 g of ferric chloride hexahydrate was dissolved in 60 mL of anhydrous ethanol, and then 0.10 g of BC was added thereto, and ultrasonic stirring was performed for 30 min to mix evenly, and the ultrasonic power was 300 W. During the stirring process, the mixture was heated in a water bath at 60° C. and evaporated to dryness, and the stirring speed was 500 rpm to obtain a Fe3O4 / BC composite electrocatalyst precursor.

[0068] 0.20 g of Fe3O4 / BC composite electrocatalyst precursor was heated to 800°C at a heating rate of 5°C / min in an argon atmosphere and annealed for 5 hours, and then cooled to room temperature to obtain a Fe3O4 / BC composite electrocatalyst. Since the annealing temperature was 700°C, it was recorded as Fe3O4 / BC-700-4 composite electrocatalyst.

[0069] Example 8: Preparation of Fe3O4 / BC composite electrocatalyst

[0070] 2.0g corn stalk powder was mixed with 2.0g potassium bicarbonate, fully ground and transferred to a porcelain boat, and then the mixture was placed in a tube furnace under an argon atmosphere and heated to 800℃ at a heating rate of 5℃ / min for 5 hours. After calcination, it was cooled to room temperature to obtain a black powder. The black powder was washed with deionized water to neutrality, then washed three times with anhydrous ethanol, and finally dried in a vacuum oven at 60℃ for 12 hours to obtain a biomass carbon (BC) rich in mesoporous structure.

[0071] 0.54 g of ferric chloride hexahydrate was dissolved in 60 mL of anhydrous ethanol, and then 0.20 g of BC was added thereto, and ultrasonic stirring was performed for 30 min to mix evenly, and the ultrasonic power was 300 W. During the stirring process, the mixture was heated in a water bath at 60° C. and evaporated to dryness, and the stirring speed was 500 rpm to obtain a Fe3O4 / BC composite electrocatalyst precursor.

[0072] 0.20 g of Fe3O4 / BC composite electrocatalyst precursor was heated to 800°C at a heating rate of 5°C / min in an argon atmosphere and annealed for 5 hours, and then cooled to room temperature to obtain a Fe3O4 / BC composite electrocatalyst. Since the annealing temperature was 800°C, it was recorded as Fe3O4 / BC-700-5 composite electrocatalyst.

[0073] Embodiment 9:

[0074] This example investigates the efficiency of catalytic ammonia production using different catalysts, and the steps are as follows:

[0075] Take 4 kinds of catalysts prepared in Examples 1 to 4 respectively, take 2.5 mg of each catalyst and dissolve it in 500 μL (V anhydrous ethanol: V deionized water = 3:1) and a mixture containing 20 μL of 5% Nafion solution by mass, and ultrasonicate at 300W for 1 hour. Take 50 μL of the ultrasonicated mixture and drop it on a carbon paper (1x1 cm) treated with 3M H2SO4, anhydrous ethanol and deionized water. 2 ), so that the catalyst loading is 0.25 mg cm -2 The carbon paper with catalyst was used as the working electrode, the calomel electrode was used as the reference electrode and the platinum sheet was used as the counter electrode in the H-type reaction cell (anion exchange membrane between the H-type reaction cells). Then the linear sweep voltammetry (LSV) test was carried out in 1MKOH+0.1MKNO3 solution with a scan rate of 50mV / s and a voltage window of -0.6V to -0.2V (relative to the reversible hydrogen electrode). The scan results are shown in Figure 7 shown.

[0076] Figure 7 The comparison of linear sweep voltammetry curves of Fe3O4 / BC composite electrocatalyst under different conditions. Figure 7 It can be seen that the Fe3O4 / BC-700 composite electrocatalyst has - The current density in the electrolyte is higher than that in the nitrate-free electrolyte, indicating that the Fe3O4 / BC-700 composite electrocatalyst is beneficial for the electrocatalytic synthesis of ammonia.

[0077] This example also tests the ammonia yield and Faraday efficiency of the four catalysts prepared in Examples 1 to 4 at a potential of -0.5 V relative to the reversible hydrogen electrode. The test method is:

[0078] Different voltages were applied to the four catalysts prepared in Examples 1 to 4 in 1M KOH+0.1M KNO3 electrolyte, respectively. Each test lasted for 1 hour. After the reaction, 2 mL of the electrolyte solution in the cathode cell was taken, and the ammonia concentration in the electrolyte was detected using Nessler reagent and the Faraday efficiency was calculated.

[0079] Figure 8 The ammonia yield and Faraday efficiency of Fe3O4 / BC composite electrocatalyst are shown in Figure 2. Figure 8 It can be seen that the Fe3O4 / BC-700 composite electrocatalyst has a high ammonia yield and Faraday efficiency in 1M KOH+0.1M KNO3 electrolyte. At -0.5V relative to the reversible hydrogen electrode potential, the ammonia yield can reach 4384mmol h -1 g cat . -1 , with a Faraday efficiency of 93.4%. This is in contrast to the ammonia yield of existing catalysts such as Co3O4@NiO HNTs (6.93 mmol h -1 g cat -1 )、Co-NAs(2.6 mol g cat -1 h -1 )、Cu50Co50(960mmol g cat -1 h -1 )The catalyst prepared by the present invention has a higher ammonia yield.

[0080] Embodiment 10:

[0081] The Fe3O4 / BC--700 composite electrocatalyst was used as the working electrode in 1M KOH electrolyte with and without nitrate at -0.5V relative to the reversible hydrogen electrode potential. Each test lasted for 1 hour. After the reaction, 2mL of the electrolyte solution from the cathode cell was taken, and the ammonia concentration in the electrolyte was detected using Nessler reagent and the Faraday efficiency was calculated.

[0082] Fig. 9 The comparison of ammonia yield and Faraday efficiency of Fe3O4 / BC composite electrocatalyst and empty carbon paper as working electrode under different conditions. Fig. 9 It can be seen that the ammonia yield of Fe3O4 / BC-700 catalyst in 1M KOH electrolyte containing nitrate is significantly higher than that in 1M KOH electrolyte without nitrate, and almost no ammonia is generated under nitrate-free electrolyte conditions, indicating that the nitrogen source of the catalytically produced ammonia comes from the N in the nitrate in the electrolyte rather than the interference of the catalyst itself.

[0083] Embodiment 11:

[0084] The Fe3O4 / BC-700 composite electrocatalyst was used as the working electrode, and the other group used empty carbon paper as the working electrode. The test was carried out for 1 hour in 1MKOH+0.1M KNO3 electrolyte at a potential of -0.5V relative to the reversible hydrogen electrode. After the reaction, 2mL of the electrolyte solution from the cathode cell was taken, and the ammonia concentration in the electrolyte was detected with Nessler reagent and the Faraday efficiency was calculated.

[0085] Fig. 9 The comparison of ammonia yield and Faraday efficiency of Fe3O4 / BC composite electrocatalyst and empty carbon paper as working electrode under different conditions. Fig. 9 It can be seen that the ammonia yield of Fe3O4 / BC-700 catalyst in 1M KOH+0.1M KNO3 electrolyte is significantly higher than that of empty carbon paper in 1M KOH+0.1M KNO3 electrolyte, and the ammonia yield of empty carbon paper in 1MKOH+0.1M KNO3 electrolyte is almost 0, indicating that the catalytic production of ammonia mainly depends on the Fe3O4 / BC-700 composite electrocatalyst.

[0086] The embodiments are preferred implementations of the present invention, but the present invention is not limited to the above-mentioned implementations. Any obvious improvements, substitutions or modifications that can be made by those skilled in the art without departing from the essential content of the present invention belong to the protection scope of the present invention.

Claims

1. A method for preparing a Fe3O4 / BC composite electrocatalyst, characterized in that: include: The corn stalk powder is mixed and ground with potassium bicarbonate to obtain a mixture, the mixture is calcined, and after the calcination, the product is washed and dried to obtain biomass charcoal BC; The hexahydrated ferric chloride was dissolved in anhydrous ethanol, and the biomass carbon BC was added to perform ultrasonication and sufficient stirring, and heated in a water bath during the stirring process to obtain a Fe3O4 / BC composite electrocatalyst precursor; The Fe3O4 / BC composite electrocatalyst precursor is annealed to obtain the Fe3O4 / BC electrocatalyst.

2. The method for preparing the Fe3O4 / BC composite electrocatalyst according to claim 1, characterized in that: The usage ratio of the corn stalk powder, potassium bicarbonate, ferric chloride hexahydrate and ethanol is 0.5-2.0 g: 0.5-2.0 g: 0.135-0.54 g: 15-60 mL.

3. The method for preparing the Fe3O4 / BC composite electrocatalyst according to claim 1, characterized in that: The calcination time is 3-7 hours, the calcination temperature is 600-900° C., and the heating rate is 2-5° C. / min.

4. The method for preparing the Fe3O4 / BC composite electrocatalyst according to claim 1, characterized in that: The ultrasonic power is 100-300W, and the ultrasonic time is 0.5-1.0 hour.

5. The method for preparing the Fe3O4 / BC composite electrocatalyst according to claim 1, characterized in that: The sufficient stirring time is 6 to 12 hours, and the stirring rate is 500 rpm.

6. The method for preparing the Fe3O4 / BC composite electrocatalyst according to claim 1, characterized in that: The water bath is heated at a temperature of 55-65° C. until the anhydrous ethanol evaporates.

7. The method for preparing the Fe3O4 / BC composite electrocatalyst according to claim 1, characterized in that: The annealing temperature is 500° C. to 800° C., the heating rate is 2 to 5° C. / min, and the annealing time is 3 to 7 hours.

8. The Fe3O4 / BC composite electrocatalyst prepared by the method according to any one of claims 1 to 7, characterized in that: The Fe3O4 / BC composite electrocatalyst uses mesoporous biochar as a substrate, and evenly loads Fe3O4 nanoparticles on the biochar. The particle size range of the Fe3O4 nanoparticles is 40-130nm; the Fe3O4 nanoparticles are in an irregular block shape; the mass fraction of biochar in the composite electrocatalyst is 53.82%~57.40%.

9. Use of the Fe3O4 / BC composite electrocatalyst according to claim 8 in electrocatalytic synthesis of ammonia from nitrate at room temperature.

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