Iron-doped cobaltosic oxide oxygen evolution catalyst as well as preparation method and application thereof

Abstract

The invention discloses an iron-doped cobaltosic oxide oxygen evolution catalyst as well as a preparation method and application thereof, belongs to the technical field of materials, and solves the problem of insufficient activity of cobaltosic oxide serving as an oxygen evolution catalyst in the prior art. The preparation method of the iron-doped cobaltosic oxide oxygen evolution catalyst comprises the following steps: mixing soluble cobalt salt and iron salt, urea, ammonium fluoride and water to form a mixed solution, putting the mixed solution into a reaction container, and performing hydrothermal reaction under an external weak magnetic field to obtain a precursor solution; washing and drying the precursor solution to obtain a ferrocobalt precursor; and calcining the ferrocobalt precursor to finally obtain the iron-doped cobaltosic oxide oxygen evolution catalyst. The iron-doped cobaltosic oxide oxygen evolution catalyst disclosed by the invention is high in catalyst activity.

Description

Technical Field

[0001] The invention relates to the technical field of materials, and in particular to an iron-doped cobalt tetroxide oxygen evolution catalyst and a preparation method and application thereof. Background Art

[0002] Hydrogen is currently recognized as one of the most promising renewable energy sources. Efficient and sustainable hydrogen production has become the most pressing issue in the hydrogen economy. As a green and clean hydrogen production method, water electrolysis can solve the current energy shortage problem.

[0003] The complex four-electron pathway of the oxygen evolution reaction (OER) at the anode leads to its slow kinetics, which severely limits the efficiency of the entire water electrolysis process. Currently, catalysts based on precious metal oxides (RuO2 and IrO2) have shown excellent catalytic activity, but their scarcity and high cost limit their development and application.

[0004] In recent years, cobalt tetroxide has attracted widespread attention due to its excellent electrochemical activity, high stability under alkaline conditions, and low price. However, since the kinetic rate during the redox reaction is usually limited by the conductivity of the catalyst and the diffusion of ions near the electrode surface, its catalytic performance is still far behind that of precious metal catalysts. Summary of the invention

[0005] In view of the above situation, the present invention aims to provide an iron-doped cobalt tetroxide oxygen evolution catalyst and a preparation method and application thereof, so as to solve the problem of insufficient activity of the existing cobalt tetroxide when used as an oxygen evolution catalyst.

[0006] The purpose of the present invention is mainly achieved through the following technical solutions:

[0007] In one aspect, the present invention provides a method for preparing an iron-doped cobalt tetroxide oxygen evolution catalyst, the preparation method comprising the following steps:

[0008] Step 1, mixing soluble cobalt salt and iron salt, urea, ammonium fluoride and water to form a mixed solution, placing the mixed solution in a reaction container, and performing a hydrothermal reaction under an external weak magnetic field to obtain a precursor solution;

[0009] Step 2, washing and drying the precursor solution to obtain a cobalt iron precursor;

[0010] Step 3: calcining the cobalt-iron precursor to finally obtain an iron-doped cobalt tetroxide oxygen evolution catalyst.

[0011] Furthermore, the cobalt salt in step 1 is any one of cobalt nitrate, cobalt chloride and cobalt sulfate.

[0012] Furthermore, the iron salt in step 1 is any one of ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride, ferrous sulfate and ferrous sulfate.

[0013] Furthermore, the molar ratio of the cobalt salt to the iron salt in step 1 is 10:1-5.

[0014] Furthermore, the concentration of the cobalt salt solution in step 1 is 0.02-0.1 mol / L.

[0015] Furthermore, the magnetic field strength of the weak magnetic field in step 1 is 5 to 50 mT.

[0016] Furthermore, the concentration of urea in step 1 is 0.1-0.5 mol / L.

[0017] Furthermore, the temperature of the hydrothermal reaction in step 1 is 100-150°C.

[0018] The present invention also provides an iron-doped cobalt oxide oxygen evolution catalyst, which is prepared by the above-mentioned preparation method.

[0019] The present invention also provides an electrode, the electrode comprises an oxygen evolution catalyst, and the oxygen evolution catalyst comprises the above-mentioned iron-doped cobalt tetroxide catalyst.

[0020] Compared with the prior art, the present invention can achieve at least one of the following beneficial effects:

[0021] a) In the preparation method of the iron-doped cobalt tetroxide oxygen evolution catalyst of the present invention, a weak magnetic field is used to regulate the electronic structure of the material surface, thereby improving the catalytic activity of the iron-doped cobalt tetroxide oxygen evolution catalyst. The preparation method is simple and pollution-free. No organic solvent is used in the preparation process, and large-scale production can be achieved.

[0022] b) The iron-doped cobalt tetroxide oxygen evolution catalyst of the present invention has a high oxygen vacancy content, for example, the oxygen vacancy content is greater than 26%; the iron-doped cobalt tetroxide oxygen evolution catalyst of the present invention has a low overpotential and high catalyst activity, and its application in the oxygen evolution reaction can improve the overall energy efficiency.

[0023] c) The electrode of the present invention includes an iron-doped cobalt tetroxide oxygen evolution catalyst, which can reduce the overpotential required to drive a specific current density and promote the occurrence of an electrochemical oxygen evolution reaction.

[0024] Other features and advantages of the present invention will be described in the following description, and part of them will become obvious from the description, or will be understood by practicing the present invention. The purpose and other advantages of the present invention can be realized and obtained by the contents particularly pointed out in the written description and the accompanying drawings. BRIEF DESCRIPTION OF THE DRAWINGS

[0025] The drawings are only for the purpose of illustrating particular embodiments and are not to be construed as limiting the invention.

[0026] Figure 1 The XRD pattern of the iron-doped cobalt tetroxide oxygen evolution catalyst prepared in Example 1;

[0027] Figure 2 This is the SEM-EDS analysis diagram of the iron-doped cobalt tetroxide oxygen evolution catalyst prepared in Example 1. DETAILED DESCRIPTION

[0028] Preferred embodiments of the present invention will be described in detail below in conjunction with the accompanying drawings, wherein the accompanying drawings constitute a part of the present invention and are used to explain the principles of the present invention together with the embodiments of the present invention.

[0029] The present invention provides a method for preparing an iron-doped cobalt tetroxide oxygen evolution catalyst, comprising the following steps:

[0030] Step 1, mixing soluble cobalt salt and iron salt, urea, ammonium fluoride and water to form a mixed solution, placing the mixed solution in a reaction container, and performing a hydrothermal reaction under an external weak magnetic field to obtain a precursor solution;

[0031] Step 2, washing and drying the precursor solution to obtain a cobalt iron precursor;

[0032] Step 3: oxidatively calcining the cobalt-iron precursor to finally obtain an iron-doped cobalt tetroxide oxygen evolution catalyst.

[0033] Specifically, in order to reduce impurity contamination, the water in the above step 1 is deionized water.

[0034] Specifically, the cobalt salt in the above step 1 is any one of cobalt nitrate, cobalt chloride and cobalt sulfate; the iron salt is any one of ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride, ferric sulfate and ferrous sulfate.

[0035] Specifically, considering that the cobalt-iron ratio is too high, the iron doping amount is small; the cobalt-iron ratio is low, other cobalt-iron oxides are easily produced, resulting in an inhomogeneous phase of the synthetic product. Therefore, the molar ratio of the cobalt salt and the iron salt in step 1 is controlled to be 10:(1-5), such as 10:1, 10:2, 10:3, 10:4, 10:5.

[0036] Specifically, in the above step 1, the concentration of the cobalt salt solution is 0.02-0.1 mol / L, such as 0.03 mol / L, 0.04 mol / L, 0.05 mol / L, 0.06 mol / L, 0.07 mol / L, 0.08 mol / L, or 0.09 mol / L, calculated as cobalt element.

[0037] Specifically, in the above step 1, the concentration of urea is 0.1-0.5 mol / L, such as 0.2 mol / L, 0.3 mol / L, 0.4 mol / L; the concentration of ammonium fluoride is 0.1-0.3 mol / L.

[0038] Specifically, in the above step 1, soluble cobalt salts and iron salts are mixed with water to form a solution, and then urea and ammonium fluoride are added, stirred until the solution is clear and homogeneous to obtain a mixed solution, and the mixed solution is placed in a reaction vessel, and a precursor solution is obtained after a hydrothermal reaction under an external weak magnetic field. Cobalt salts and iron salts are first mixed with water to form a solution, which can ensure that metal ions are evenly distributed in the solution; the addition of urea and ammonium fluoride may react with metal ions to form complexes, affecting the uniformity of the solution. Therefore, a uniform metal salt solution is first formed, and then urea and ammonium fluoride are gradually added, so that the uniformity of the solution can be better controlled.

[0039] Specifically, in the above step 1, the reaction container can be a reactor. Considering that too large or too small filling of the reactor is not conducive to sufficient mixing of the materials, the filling degree of the reactor is controlled to be 50-80%, for example, 55%, 60%, 65%, 70%, 75%, 80%.

[0040] Specifically, in the above step 1, the magnetic field can affect the electronic structure of the active center, promote the increase of the oxygen vacancy content, and ultimately lead to an increase in the rate of electrocatalytic oxygen evolution reaction. However, the magnetic field strength should not be too high. Too high a field strength is not conducive to the generation of oxygen vacancies in the synthetic product and affects the product performance. Therefore, the magnetic field strength of the weak magnetic field is controlled to be 5 to 50 mT, such as 10 mT, 15 mT, 20 mT, 25 mT, 30 mT, 35 mT, 40 mT, 45 mT.

[0041] Specifically, in the above step 1, if the hydrothermal reaction temperature is too high, the reaction rate will be too fast and the side reactions will increase; if the time is too long, it will lead to excessive growth of the product or secondary reaction, resulting in coarse grains and agglomeration; if the temperature is too low, the reaction rate will be too slow, the reaction will be insufficient, and the yield and quality of the product will be affected; if the time is too short, the grain growth will not be complete, resulting in low crystallinity of the product. Therefore, the temperature of the hydrothermal reaction is controlled to be 100-150°C, such as 110°C, 120°C, 130°C, 140°C; the time of the hydrothermal reaction is 3-6h, such as 3.5h, 4h, 4.5h, 5h, 5.5h.

[0042] Specifically, in the above step 2, if the drying temperature is too high, the surface of the material may dry too quickly, forming a hard shell, hindering the evaporation of internal moisture and affecting the uniformity of drying; if the drying time is too long, the material will be over-dried; if the temperature is too low, the drying rate will become very slow and the drying time will be too long; if the drying time is too short, the moisture inside the material may not be completely removed, resulting in incomplete drying. Therefore, the drying temperature is controlled to be 50-70°C, such as 50°C, 60°C, 70°C; the drying time is 2-4h, such as 2h, 3h, 4h.

[0043] Specifically, in the above step 3, if the calcination temperature is too low, the calcination will be incomplete, and if the temperature is too high, the structure will collapse and the original morphology will be destroyed. Therefore, the calcination temperature is controlled to be 300-500° C. and the calcination time is 2-4 hours.

[0044] The present invention also provides an iron-doped cobalt tetroxide oxygen evolution catalyst, which is prepared by the above-mentioned preparation method.

[0045] Specifically, the iron-doped cobalt tetroxide oxygen evolution catalyst has a high oxygen vacancy content, for example, the oxygen vacancy content is more than 27%; the iron-doped cobalt tetroxide oxygen evolution catalyst of the present invention has a low overpotential, which is less than 314mV, for example, 290-314mV.

[0046] The present invention also provides the use of the iron-doped cobalt tetroxide oxygen evolution catalyst in an electrode.

[0047] The present invention also provides an electrode, the electrode includes an oxygen evolution catalyst, and the oxygen evolution catalyst includes the above iron-doped cobalt tetroxide catalyst. The electrode of the present invention can reduce the overpotential required to drive a specific current density and promote the occurrence of electrochemical oxygen evolution reaction.

[0048] In the preparation method of the iron-doped cobalt tetroxide oxygen evolution catalyst of the present invention, a weak magnetic field is used to regulate the electronic structure of the material surface, thereby improving the catalytic activity of the iron-doped cobalt tetroxide oxygen evolution catalyst. The preparation method is simple and pollution-free, no organic solvent is used in the preparation process, and large-scale production can be achieved.

[0049] The iron-doped cobalt tetroxide oxygen evolution catalyst of the present invention has a high oxygen vacancy content, for example, the oxygen vacancy content is above 26%, for example, 26% to 35%; the iron-doped cobalt tetroxide oxygen evolution catalyst of the present invention has a low overpotential and high catalyst activity, and its application in oxygen evolution reaction can improve the overall energy efficiency.

[0050] The electrode of the present invention comprises an iron-doped cobalt tetroxide oxygen evolution catalyst, which can reduce the overpotential required for driving a specific current density and promote the occurrence of an electrochemical oxygen evolution reaction.

[0051] The technical solution of the present invention is further explained below in conjunction with specific embodiments. The described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0052] Example 1

[0053] This embodiment provides an iron-doped cobalt tetroxide oxygen evolution catalyst and a preparation method thereof, the preparation method comprising the following steps:

[0054] (1) Add 2 mmol of cobalt nitrate and 0.6 mmol of ferric nitrate to 40 mL of deionized water, then pour 10 mmol of urea and 5 mmol of ammonium fluoride into the mixed solution and stir until the solution is clear and homogeneous;

[0055] (2) The mixed solution was transferred to a 50 mL polytetrafluoroethylene-lined stainless steel autoclave, sealed, and reacted at 120°C for 3 h under a weak magnetic field (magnetic field strength of 40 mT);

[0056] (3) After the hydrothermal reaction, the autoclave was naturally cooled to room temperature; the powdered product was obtained by suction filtration, washed several times with deionized water, and dried at 60°C for 2 h;

[0057] (4) The dried powder was calcined at 400°C for 3 h in an air atmosphere at a heating rate of 5°C / min. The black powder sample finally obtained was an iron-doped cobalt tetroxide oxygen evolution catalyst.

[0058] This embodiment further provides an electrode, the electrode includes an oxygen evolution catalyst, and the oxygen evolution catalyst includes the above-mentioned iron-doped cobalt tetroxide catalyst.

[0059] In this example, the above electrode was also subjected to electrochemical testing: 3 mg of iron-doped cobalt tetroxide catalyst powder was dispersed in a mixed solvent of 985 μL of anhydrous ethanol, 200 μL of water and 40 μL of Nafion perfluorinated resin solution (5 wt%); the mixture was then ultrasonically treated for about 30 min to produce a uniform ink-like liquid;

[0060] The ink-like liquid was evenly coated on the nickel foam electrode. Each time the liquid was added, the nickel foam was kept as dry as possible. The obtained electrode was placed in a three-electrode system at a temperature of 25°C for electrochemical testing. The electrolyte used was a KOH solution with a concentration of 1 mol / L.

[0061] Example 2

[0062] This embodiment provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this embodiment is basically the same as that of Example 1, except that:

[0063] The amount of iron salt in (1) is 0.2 mmol and the temperature is 100°C;

[0064] (2) The magnetic field strength is 30 mT;

[0065] The calcination temperature in (3) is 500°C.

[0066] Example 3

[0067] This embodiment provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this embodiment is basically the same as that of Example 1, except that:

[0068] The amount of iron salt in (1) is 0.4 mmol, and the temperature is 130°C;

[0069] (2) The magnetic field strength is 20 mT and the duration is 5 h.

[0070] Example 4

[0071] This embodiment provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this embodiment is basically the same as that of Example 1, except that:

[0072] The cobalt salt is cobalt sulfate.

[0073] Example 5

[0074] This embodiment provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this embodiment is basically the same as that of Example 1, except that:

[0075] The iron salt is ferric chloride.

[0076] The inventor has conducted a large number of experimental studies during the research process, and now uses some solutions with poor performance as comparative examples.

[0077] Comparative Example 1

[0078] This comparative example provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this comparative example is basically the same as that of Example 1, except that:

[0079] In (2), no magnetic field is applied.

[0080] Comparative Example 2

[0081] This comparative example provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this comparative example is basically the same as that of Example 2, except that:

[0082] In (2), no magnetic field is applied.

[0083] Comparative Example 3

[0084] This comparative example provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this comparative example is basically the same as that of Example 3, except that:

[0085] In (2), no magnetic field is applied.

[0086] Comparative Example 4

[0087] This comparative example provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this comparative example is basically the same as that of Example 4, except that:

[0088] In (2), no magnetic field is applied.

[0089] Comparative Example 5

[0090] This comparative example provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this comparative example is basically the same as that of Example 1, except that:

[0091] The magnetic field strength in (2) is 200 mT.

[0092] Comparative Example 6

[0093] This comparative example provides an iron-doped cobalt tetraoxide oxygen evolution catalyst and a preparation method thereof. The preparation method of this comparative example is basically the same as that of Example 1, except that:

[0094] The magnetic field strength in (2) is 500 mT.

[0095] The iron-doped cobalt tetraoxide oxygen evolution catalysts obtained in Example 1 and Comparative Example 1 were analyzed. Figure 1 The XRD spectrum of the iron-doped cobalt tetroxide oxygen evolution catalyst prepared in Example 1 only shows the diffraction peak of Co3O4 crystals. Compared with Co3O4, the position of the diffraction peak of Example 1 is slightly shifted toward the lower 2θ direction, which is because the iron atoms replace the cobalt atoms. This shows that the prepared catalyst is relatively pure and basically does not contain other impurities. Figure 2 This is the SEM-EDS analysis diagram of the iron-doped cobalt tetroxide oxygen evolution catalyst prepared in Example 1. Figure 2 It can be seen that iron is uniformly incorporated into cobalt tetroxide.

[0096] The overpotentials of the embodiments and comparative examples are shown in Table 1.

[0097] Table 1 Overpotential of iron-doped cobalt tetraoxide oxygen evolution catalysts prepared in Examples and Comparative Examples

[0098] Overpotential / mV Oxygen vacancy content Example 1 290 35％ Example 2 303 32％ Example 3 308 31％ Example 4 314 26％ Example 5 310 28％ Comparative Example 1 334 20％ Comparative Example 2 343 Comparative Example 3 338 Comparative Example 4 330 Comparative Example 5 317 24％ Comparative Example 6 325

[0099] In the oxygen evolution reaction, the level of overpotential is directly related to the activity of the catalyst. A catalyst with good activity can provide a higher current density at a lower overpotential, thereby improving the overall energy efficiency. As can be seen from the data in Table 1, the present invention introduces a certain weak magnetic field during the preparation process of the iron-doped cobalt tetroxide oxygen evolution catalyst, which is beneficial to increase the oxygen vacancy content of the synthetic product, improve the catalyst activity, reduce the overpotential of the electrocatalytic oxygen evolution reaction, and promote the occurrence of the electrochemical oxygen evolution reaction. For example, the iron-doped cobalt tetroxide oxygen evolution catalyst of the present invention has a high oxygen vacancy content, for example, the oxygen vacancy content is more than 26%, for example, 26% to 35%; the iron-doped cobalt tetroxide oxygen evolution catalyst of the present invention has a low overpotential, a high catalyst activity, and an overpotential of less than 314mV, for example, 290 to 314mV.

[0100] The above description is only a preferred specific implementation manner of the present invention, but the protection scope of the present invention is not limited thereto. Any changes or substitutions that can be easily conceived by any technician familiar with the technical field within the technical scope disclosed by the present invention should be covered within the protection scope of the present invention.

Claims

1. A method for preparing an iron-doped cobalt tetraoxide oxygen evolution catalyst, characterized in that: The preparation method comprises the following steps: Step 1, mixing soluble cobalt salt and iron salt, urea, ammonium fluoride and water to form a mixed solution, placing the mixed solution in a reaction container, and performing a hydrothermal reaction under an external weak magnetic field to obtain a precursor solution; Step 2, washing and drying the precursor solution to obtain a cobalt iron precursor; Step 3: calcining the cobalt-iron precursor to finally obtain an iron-doped cobalt tetroxide oxygen evolution catalyst.

2. The preparation method according to claim 1, characterized in that: The cobalt salt in step 1 is any one of cobalt nitrate, cobalt chloride and cobalt sulfate.

3. The preparation method according to claim 1, characterized in that: The iron salt in step 1 is any one of ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride, ferrous sulfate and ferrous sulfate.

4. The preparation method according to claim 1, characterized in that: The molar ratio of the cobalt salt to the iron salt in step 1 is 10:1-5.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The concentration of the cobalt salt solution in step 1 is 0.02-0.1 mol / L.

6. The preparation method according to claim 1, characterized in that: The magnetic field strength of the weak magnetic field in step 1 is 5-50 mT.

7. The preparation method according to claim 1, characterized in that: The concentration of urea in step 1 is 0.1-0.5 mol / L.

8. The preparation method according to claim 1, characterized in that: The temperature of the hydrothermal reaction in step 1 is 100-150°C.

9. An iron-doped cobalt tetraoxide oxygen evolution catalyst, characterized in that: The iron-doped cobalt tetroxide oxygen evolution catalyst is prepared by the preparation method according to any one of claims 1 to 8.

10. An electrode, characterized in that: The electrode comprises an oxygen evolution catalyst, and the oxygen evolution catalyst comprises an iron-doped cobalt oxide catalyst prepared by the preparation method according to any one of claims 1 to 8 or an iron-doped cobalt oxide catalyst according to claim 9.

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