Self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen vacancy as well as preparation and application of self-supporting Co / CoO / Co3O4 catalytic electrode

Abstract

The invention discloses a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen vacancy as well as preparation and application of the self-supporting Co / CoO / Co3O4 catalytic electrode. The preparation method of the catalytic electrode comprises the following steps: step 1, dissolving Co (NO3) 3.6 H2O and dimethylimidazole in ethanol, uniformly mixing, adding a foamy copper substrate, putting into a drying oven, carrying out solvothermal reaction to generate a Co-ZIF precursor, and drying after the reaction is finished to obtain foamy copper containing the Co-ZIF precursor; and 2, putting the foamy copper containing the Co-ZIF precursor into a tubular furnace, introducing hydrogen-argon mixed gas, and carrying out reduction calcination at 300-500 DEG C to obtain the catalytic electrode. The invention provides application of the catalytic electrode as a cathode in a reaction of synthesizing ammonia through nitrate electroreduction. The process conditions are simple, the prepared catalytic electrode can effectively promote the electroreduction process of nitrate radicals, and high ammonia yield and high Faraday efficiency are achieved.

Description

Technical Field

[0001] The present invention belongs to the technical field of catalysts, and in particular relates to a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects, a preparation method thereof, and an application thereof in the electroreduction of nitrate to synthesize ammonia. Background Art

[0002] Ammonia (NH3) is a carbon-free energy source. In the context of the increasingly serious carbon dioxide emission problem, its status as an energy source is becoming increasingly important. As an ideal hydrogen energy carrier, ammonia has a high energy density of 4.3KWh / kg and is considered to be a green and clean energy source. The synthesis of ammonia is mainly produced by the Haber-Bosch process, which mixes hydrogen and nitrogen under high pressure and high temperature. It not only faces high costs, but also has a large amount of carbon emissions during its production process, which deviates from the current low-carbon theme. The electrochemical nitrate reduction to produce ammonia is expected to replace the Haber-Bosch process and become the mainstream ammonia production process due to its low pollution and low cost. However, the process is not yet mature and there are certain problems: 1. The accumulation of nitrite intermediates during the reaction leads to a low ammonia yield; 2. The competitive side reaction of cathode hydrogen production reduces the Faraday efficiency. 3. The low proton splitting ability of the copper-based catalyst itself leads to a slow reaction. Based on this, it is still necessary to further explore non-precious metal electrocatalysts with low cost and high reduction activity, and achieve high yield and high Faraday efficiency of nitrate reduction to ammonia. Summary of the invention

[0003] The present invention aims to prepare a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects, a preparation method thereof and an application thereof in the electroreduction of nitrate to synthesize ammonia. The catalytic electrode is used in the electroreduction of nitrate to synthesize ammonia in an alkaline solution. The coupling oxygen defect effect of Co species with different valence states can effectively overcome the existing problems of low yield and low Faraday efficiency.

[0004] The technical solution of the present invention is as follows:

[0005] In a first aspect, the present invention provides a method for preparing a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects, comprising the following steps:

[0006] Step 1: Dissolve Co(NO3)3·6H2O and dimethylimidazole in ethanol, mix well and add to the foam copper substrate, perform solvent thermal reaction in a hydrothermal reactor to generate a Co-ZIF precursor, wash and dry after the reaction to obtain a foam copper containing a Co-ZIF precursor;

[0007] Step 2: Place the copper foam containing the Co-ZIF precursor obtained in step 1 into a tubular furnace, introduce a hydrogen-argon mixture at 300-500°C for reduction calcination to obtain a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects.

[0008] Preferably, in step 1, the molar ratio of Co(NO3)3·6H2O to dimethylimidazole is 1:1-3, more preferably 1:1-1.5.

[0009] Preferably, in step 1, the solvent thermal reaction conditions are 100-150° C. for 1-6 hours, more preferably 120° C. for 2 hours.

[0010] Preferably, in step 1, the washing reagent is ethanol.

[0011] Preferably, in step 1, the drying conditions are: drying in a vacuum oven at 50-70°C.

[0012] Preferably, in step 2, the reduction calcination in the tubular furnace is carried out at 300-500° C. for 3 hours, more preferably at 400° C. for 3 hours.

[0013] Preferably, in step 2, the volume percentage of hydrogen in the hydrogen-argon mixed gas is 5-20%, more preferably 5%.

[0014] In a second aspect, the present invention provides a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects prepared according to the preparation method described in the first aspect.

[0015] In a third aspect, the present invention provides the use of the oxygen-deficient self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects described in the second aspect as a cathode in the reaction of nitrate electroreduction to synthesize ammonia.

[0016] As an embodiment, the reaction of synthesizing ammonia by electroreduction of nitrate is carried out in an H-type electrolytic cell, and the application is specifically as follows:

[0017] An H-type electrolyzer containing a proton membrane is used, a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects is used as the cathode, a platinum sheet is used as the anode, and a Hg / HgO electrode is used as the reference electrode. A KOH aqueous solution containing potassium nitrate is added to the cathode chamber as the cathode electrolyte, and a KOH aqueous solution is added to the anode chamber as the anode electrolyte. At room temperature and normal pressure, the cathode electrolyte is stirred and constant potential electrolysis is applied to obtain the product ammonia at the cathode.

[0018] Preferably, in the cathode electrolyte, the KOH concentration is 1-6M, and the potassium nitrate concentration is 10-1000mM; in the anode electrolyte, the KOH concentration is 1-6M.

[0019] As another embodiment, the reaction of synthesizing ammonia by electroreduction of nitrate is carried out in a flow electrolyzer, and the application is specifically as follows:

[0020] A flow electrolyzer separated by a proton membrane was used, a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects was directly used as the cathode, nickel foam was used as the anode, KOH aqueous solution was added to the anode chamber as the anolyte, and a KOH aqueous solution containing potassium nitrate was added to the cathode chamber as the catholyte. A constant current was applied for electrolysis to obtain the product ammonia at the cathode.

[0021] Preferably, in the cathode electrolyte, the KOH concentration is 1-6M, and the potassium nitrate concentration is 10-1000mM; in the anode electrolyte, the KOH concentration is 1-6M.

[0022] Compared with the prior art, the present invention has the following beneficial effects:

[0023] The process conditions provided by the present invention are simple, and copper foam is used as a copper source and a current collector material, thereby avoiding the use of a conductive agent and a binder, and ensuring the excellent conductivity and stability of the overall material; after calcination in a reducing atmosphere, Cu and Co in the copper foam form a copper-cobalt alloy phase; and a variety of Co species with different valence states and abundant oxygen defects are formed, benefiting from the collective effect of multiple components, effectively promoting the electrical reduction process of nitrate ions, and achieving a high yield of ammonia and a high Faraday efficiency. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 : Scanning electron microscope image (SEM) of Co-300°C prepared in Example 1;

[0025] Figure 2 : SEM of Co-400℃ prepared in Example 2;

[0026] Figure 3 : Transmission electron microscopy (TEM) of Co-400°C prepared in Example 2;

[0027] Figure 4 : SEM of Co-500℃ prepared in Example 3;

[0028] Figure 5 : X-ray diffraction (XRD) of three cobalt-based catalysts prepared in Examples 1-3

[0029] Figure 6 : Co 2p X-ray photoelectron spectroscopy (XPS) of three cobalt-based catalysts prepared in Examples 1-3;

[0030] Figure 7 : O1s XPS of three cobalt-based catalysts prepared in Examples 1-3;

[0031] Figure 8 :Linear sweep voltammetry (LSV)

[0032] Fig. 9 : Faraday efficiency and ammonia yield of Co-400°C prepared in Example 2 under constant potential electrolysis in a three-electrode system;

[0033] Fig.10 : Results of long-term electrolysis of Co-400℃ prepared in Example 2 in a two-electrode flow cell under constant current DETAILED DESCRIPTION

[0034] In order to facilitate the understanding of the present invention, the following will be described in detail with reference to the accompanying drawings. These embodiments are only used to illustrate the present invention but not to limit the scope of the present invention.

[0035] If no specific conditions are specified in the examples of the present invention, the experiments were carried out under conventional conditions or conditions recommended by the manufacturer. If no manufacturer is specified for the reagents or instruments used, they are all conventional products that can be obtained by conventional technical means or purchased commercially.

[0036] Embodiment 1:

[0037] A preparation and application of a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects, wherein the preparation method comprises the following steps:

[0038] Add 2.4mmol Co(NO3)3·6H2O and 3mmol dimethylimidazole to 40mL ethanol, stir to dissolve, add a clean foam copper substrate, transfer to a 50mL hydrothermal kettle, heat to 120℃, and keep warm for 2h. After the reaction, take out the foam copper, wash it with ethanol, and dry it in a vacuum oven at 60℃. After it is completely dried, put it in a tubular furnace for reduction calcination, heat to 300℃ in a H2 / Ar (5 / 95%) atmosphere, and keep warm for 3 hours. After the reaction, a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects is obtained, which is recorded as Co-300℃.

[0039] Figure 1 The SEM image of Co-300℃ shows a multi-level nanostructure and a flower-like structure composed of nanoparticles.

[0040] Embodiment 2:

[0041] The preparation method is the same as that in Example 1, except that the calcination temperature in the tubular furnace is 400°C. Repeating the experimental process can obtain a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects, which is recorded as Co-400°C.

[0042] Figure 2 The SEM image of Co-400℃ shows a short and coarse fiber nanostructure. Figure 3 From the TEM image, it can be seen that Co-400℃ contains Co(002), CoO(220) and Co3O4(222) crystal planes.

[0043] Embodiment 3:

[0044] The preparation method is the same as that in Example 1, except that the calcination temperature in the tubular furnace is 500°C. Repeating the experimental process can obtain a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects, which is recorded as Co-500°C.

[0045] Figure 4 The SEM image of Co-500℃ shows a porous structure composed of multi-level nanoparticles. Figure 5 The following are the XRD patterns of the three catalysts. The three obvious diffraction peaks are all attributed to metallic cobalt (#15-0806). Figure 6 The Co 2p XPS spectra of the three catalysts show that Co 2+ and Co 3+ peak, Figure 7 These are the O1s XPS graphs of the three catalysts. It can be seen from the graph that all three catalysts contain abundant oxygen defects.

[0046] Embodiment 4:

[0047] The self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects obtained in Examples 1-3 was used for the reaction of nitrate electroreduction to synthesize ammonia. In an H-type cell, the anode chamber was 1M KOH, and the cathode chamber was a mixed solution of 1M KOH and 100mM KNO3. A platinum sheet was used as the anode, the cobalt-based catalyst rich in oxygen defects was directly used as the cathode, and Hg / HgO was used as the reference electrode. A linear voltammetric scan was performed, and the results were as follows: Figure 8 As shown in the figure, the dotted line is the case without nitrate substrate, and the electroreduction reaction of water mainly occurs; the actual line is the case with nitrate. As can be seen from the figure, as the potential becomes more negative, the current density increases, indicating that the electroreduction reaction of nitrate is carried out effectively, that is, all three catalysts have nitrate electroreduction activity; and at 0.3V to -0.3VRHE, the current density on the Co-400℃ catalyst is higher than that on the other two catalysts. Different electrolysis potentials of 0 to -0.7V vs. RHE were applied to the Co-400℃ catalytic electrode for electrolysis, and the results are shown in the figure. Fig. 9 As shown in Figure 2, as the potential becomes more negative, the ammonia yield increases, and the Faradaic efficiency first increases and then decreases. At -0.6 V vs. RHE, the Faradaic efficiency reaches 98%, and the ammonia yield is 2.4 mmol h -1 cm -2 .

[0048] Embodiment 5:

[0049] The oxygen-deficient self-supporting Co / CoO / Co3O4 catalytic electrode obtained in Example 2 was used for the reaction of nitrate electroreduction to synthesize ammonia at industrial current density. In the two-electrode flow cell, the anode chamber was 3M KOH, and the cathode chamber was a mixed solution of 3M KOH and 1000mM KNO3. The nickel foam sheet was used as the anode, and Co-400℃ was directly used as the cathode. Different currents were applied for long-term electrolysis, and the results were as follows: Fig.10 As shown in Figure 2, the Faradaic efficiency is above 90% and the catalyst has excellent catalytic stability. -2 The ammonia yield was 3.7 mmol h -1 cm -2 ; at 1000mAcm -2 The ammonia yield was 7.6 mmol h - 1 cm -2 .

Claims

1. A method for preparing a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects, characterized in that: The preparation method comprises the following steps: Step 1: Dissolve Co(NO3)3·6H2O and dimethylimidazole in ethanol, mix well and add to the foam copper substrate, place in an oven for solvent thermal reaction to generate Co-ZIF precursor, and dry after the reaction to obtain foam copper containing Co-ZIF precursor; Step 2: Place the copper foam containing the Co-ZIF precursor obtained in step 1 into a tubular furnace, introduce a hydrogen-argon mixture at 300-500°C for reduction calcination to obtain a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects.

2. The preparation method according to claim 1, characterized in that: In step 1, the molar ratio of Co(NO3)3·6H2O to dimethylimidazole is 1:1.5-3.

3. The preparation method according to claim 1, characterized in that: In step 1, the solvent thermal reaction conditions are 100-150° C. for 1-6 hours, preferably 120° C. for 2 hours.

4. The preparation method according to claim 1, characterized in that: In step 2, the reduction calcination in a tubular furnace is carried out at 300-500° C. for 3 hours, preferably at 400° C. for 3 hours.

5. The preparation method according to claim 1, characterized in that: In step 2, the volume percentage of hydrogen in the hydrogen-argon mixed gas is 5-20%.

6. A self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects prepared according to the preparation method of any one of claims 1 to 5.

7. Use of the oxygen-deficient self-supporting Co / CoO / Co3O4 catalytic electrode as claimed in claim 6 as a cathode in the reaction of nitrate electroreduction to synthesize ammonia.

8. The use according to claim 7, characterized in that: The reaction of synthesizing ammonia by electroreduction of nitrate is carried out in an H-type electrolytic cell, and the application is specifically as follows: An H-type electrolyzer containing a proton membrane is used, a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects is used as the cathode, a platinum sheet is used as the anode, and a Hg / HgO electrode is used as the reference electrode. A KOH aqueous solution containing potassium nitrate is added to the cathode chamber as the cathode electrolyte, and a KOH aqueous solution is added to the anode chamber as the anode electrolyte. At room temperature and normal pressure, the cathode electrolyte is stirred and constant potential electrolysis is applied to obtain the product ammonia at the cathode.

9. The use according to claim 7, characterized in that: The reaction of synthesizing ammonia by electroreduction of nitrate is carried out in a flow electrolyzer, and the application is specifically as follows: A flow electrolyzer separated by a proton membrane was used, a self-supporting Co / CoO / Co3O4 catalytic electrode rich in oxygen defects was directly used as the cathode, nickel foam was used as the anode, KOH aqueous solution was added to the anode chamber as the anolyte, and a KOH aqueous solution containing potassium nitrate was added to the cathode chamber as the catholyte. A constant current was applied for electrolysis to obtain the product ammonia at the cathode.

10. The use according to claim 8 or 9, characterized in that: In the cathode electrolyte, the KOH concentration is 1-6M, and the potassium nitrate concentration is 10-1000mM; in the anode electrolyte, the KOH concentration is 1-6M.

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