Co3O4@IrOx catalyst, its preparation method and application

Description

TECHNICAL FIELD

This invention relates to the field of catalyst materials for hydrogen production through water electrolysis, and more specifically to a Co₃O₄@IrO_x catalyst, its preparation method, and its application.

BACKGROUND

Electrolysis of water is the only zero-carbon emission method for hydrogen production, which can solve the problem of intermittent wind and solar power generation, enabling continuous and stable long-term energy storage. The oxygen evolution reaction in water electrolysis, due to its higher overpotential, is the main source of energy consumption. Therefore, synthesizing higher-performance catalytic materials to reduce the overpotential of the oxygen evolution reaction can significantly reduce the energy consumption of water electrolysis for hydrogen production, thereby improving the efficiency of industrial production.

Proton exchange membrane (PEM) electrolyzers are a commonly used type of electrolyzer in OER reactions, offering lower OER overpotentials. However, they typically rely on materials such as IrO₂ and RuO₂. The 5d7 electronic configuration of IrO provides a suitable d-band center position, balancing the adsorption strength of reaction intermediates *OH, *O, and *OOH, avoiding excessively strong or weak adsorption. Furthermore, IrO₂'s electronic structure allows for the stable high-valence Ir4⁺/Ir5⁺ states during the OER reaction, providing active centers. However, the reliance on precious metal materials, leading to high catalyst costs, remains a bottleneck in water electrolysis.

To address this challenge, reducing Ir dosage and improving catalytic efficiency are of significant research value. Current common methods, such as single-atom dispersion, have high matching requirements for Ir anchoring on supports like Co₃O₄ and N-C, and involve complex post-processing procedures such as acid washing and electrochemical activation. This can easily lead to cluster formation, resulting in a narrow process window and poor reproducibility. Alloying Ir with transition metals such as Co and Ni can improve atom utilization, but it also presents challenges such as separation difficulties and uncontrollable particle size. Non-precious metal doping suffers from low current density applicability, compositional inhomogeneity, and low long-cycle stability. Therefore, there is a lack of a simple, easy-to-implement, and scalable method for preparing Ir-doped oxygen evolution electrocatalysts.

SUMMARY

To address the above problems, this invention provides a Co₃O₄@IrO_x catalyst, its preparation method, and its applications. The Co₃O₄@IrO_x catalyst prepared by this invention exhibits excellent hydrogen production capacity through water electrolysis.

The first objective of this invention is to provide a method for preparing the Co₃O₄@IrO_x catalyst, comprising the following steps:

Using ZIF-67 as the core, a quaternary ammonium salt surfactant and an imidazole organic ligand are added, and a coordination reaction is carried out with a zinc source to prepare a ZIF-67@ZIF-8 core-shell material.

During the reaction, ZIF-67 serves as the core. The quaternary ammonium salt surfactant can control the morphology of the nanoparticles, modify the surface of ZIF-67, reduce its interfacial energy with the imidazole organic ligand, promote the uniform nucleation and growth of ZIF-8, and simultaneously alleviate the lattice mismatch problem between ZIF-67 and ZIF-8. The resulting ZIF-8 acts as the shell, forming the ZIF-67@ZIF-8 core-shell material.

ZIF-67@ZIF-8 electrode sheets were prepared by coating ZIF-67@ZIF-8 core-shell material onto carbon paper.

The ZIF-67@ZIF-8 electrode sheets were pyrolyzed at 300 ℃~400 ℃ in air. During pyrolysis, ZIF-67 collapsed into Co₃O₄, resulting in Co₃O₄@defect-type ZIF-8 electrode sheets. Specifically, during pyrolysis, ZIF-67 was unstable, and the dodecahedral structure collapsed first, being oxidized to Co₃O₄. The ZIF-8 core-shell material framework remained intact, while some Zn was oxidized, forming defects on the dodecahedral surface. The ZIF-67@ZIF-8 core-shell material was thus transformed into Co₃O₄@defect-type ZIF-8.

A standard three-electrode system was used, with a carbon rod as the counter electrode, an Hg/HgO electrode as the reference electrode, and a Co₃O₄@defective ZIF-8 electrode sheet as the working electrode. The Co₃O₄@defective ZIF-8 electrode sheet was subjected to pulsed potential etching in a potassium hydroxide solution. During the pulsed potential etching process, zinc ions on the surface of the defective ZIF-8 electrode were etched out into the solution, resulting in a Co₃O₄@vacancy-type ZIF-8 electrode sheet.

A standard three-electrode system was also used, with a carbon rod as the counter electrode, an Hg/HgO electrode as the reference electrode, and a Co₃O₄@defective ZIF-8 electrode sheet as the working electrode. Iridium was deposited onto the zinc ion vacancies of the Co₃O₄@vacancy-type ZIF-8 electrode sheet via electrochemical deposition in an iridium-containing potassium hydroxide solution, resulting in a Co₃O₄@Ir-deposited ZIF-8 electrode sheet, which is the Co₃O₄@IrO_x catalyst required in this invention.

In a preferred embodiment of the present invention, during the pulsed potential etching process, short pulses at the cathode potential and anode potential are repeated. The cathode potential is -2V for 1-2 seconds; the anode potential is 2V for 1-2 seconds. This is repeated 150 times to form one activation cycle.

The activation cycle is repeated 3-4 times.

In a preferred embodiment of the present invention, during the electrochemical deposition process, short pulses at the cathode potential and anode potential are repeated. The cathode potential is -6V for 1-2 seconds; the anode potential is 6V for 1-2 seconds. This is repeated 500 times to form one activation cycle.

The activation cycle is repeated 5-10 times.

In a preferred embodiment of the present invention, the iridium concentration in the iridium-containing potassium hydroxide solution is 100 μmol/L.

In a preferred embodiment of the present invention, the pyrolysis time is 0.5 h. In a preferred embodiment of the present invention, the mass ratio of the quaternary ammonium salt surfactant to the imidazole organic ligand is 15:1135-1136.

The mass ratio of the quaternary ammonium salt surfactant to the zinc source is 15:72-73.

In a preferred embodiment of the present invention, the ratio of ZIF-67 to the quaternary ammonium salt surfactant is 6.9-7.4:15.

In another preferred embodiment of the present invention, the quaternary ammonium salt surfactant is hexadecyltrimethylammonium bromide, and the imidazole organic ligand is 2-methylimidazole.

A second objective of the present invention is to provide a Co₃O₄@IrO_x catalyst prepared by the above-described preparation method.

A third objective of the present invention is to provide the application of the above-described Co₃O₄@IrO_x catalyst in oxygen evolution electrocatalysis.

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

The Co₃O₄@Ir deposited ZIF-8 material prepared by the method of the present invention exhibits excellent catalytic performance in the oxygen evolution reaction, showing a low overpotential and excellent cycling stability, maintaining good redox performance and energy storage capacity during long-term electrochemical reactions exceeding 1500 h. Its performance is significantly superior to traditional noble metal catalysts and single metal oxides. This material achieves triple synergistic catalytic enhancement by anchoring atomically dispersed Ir active centers onto a ZIF-8-derived nitrogen-doped porous carbon framework and forming a strong coupling interface with Co₃O₄ nanoparticles. The synthesized ZIF-67@ZIF-8 inherently possesses a high specific surface area. Air calcination then forms Co₃O₄@defective ZIF-8, introducing defects to enhance conductivity. Further pulse activation etches Zn atoms from their intrinsic hexahedral organic framework, creating defects in the original 3D ordered porous structure without collapsing the hexahedral structure. This defective structure not only provides abundant active sites, significantly enhancing electron transfer efficiency, but also allows the Co₃O₄formed from the collapse of ZIF-67 to participate as active sites in the proton-electron transfer process of the oxygen evolution reaction (OER). By anchoring Ir atoms onto the ZIF-8-derived porous carbon matrix, the utilization rate of Ir is significantly improved, the dosage is reduced, the OER kinetics are accelerated, and the catalytic activity of the material is greatly enhanced. Furthermore, the preparation method of the present invention is simple to operate, has low equipment requirements, and the operation process is highly controllable.

A high-performance oxygen evolution reaction catalyst can be obtained with a relatively small amount of Ir doping required, which is very suitable for large-scale production and has significant value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a field emission scanning electron microscope (FESEM) image of ZIF-67@ZIF-8 synthesized in Example 1 of this invention.

FIG. 2 is a field emission scanning electron microscope (FESEM) image of ZIF-67@ZIF-8 synthesized in Example 2 of this invention.

FIG. 3 is a field emission scanning electron microscope (FESEM) image of ZIF-67@ZIF-8 synthesized in Example 3 of this invention.

FIG. 4 is a real energy filtered transmission electron microscope (REEEM) image of ZIF-67@ZIF-8 synthesized in Example 1 of this invention.

FIG. 5 is a field emission scanning electron microscope (FESEM) image of ZIF-67@ZIF-8 synthesized in Example 1 of this invention after calcination at 350°C in air.

FIG. 6 shows the original ZIF-67@ZIF-8 material synthesized in Example 1 of this invention and its powder X-ray diffraction patterns after electrochemical etching and electrochemical deposition, respectively.

FIG. 7 shows the X-ray photoelectron spectroscopy (XPS) spectra of cobalt in the Co₃O₄@defective ZIF-8, Co₃O₄@vacancy-type ZIF-8, and Co₃O₄@Ir-deposited ZIF-8 synthesized in Example 1 of this invention.

FIG. 8 shows the XPS spectra of zinc in the Co₃O₄@defective ZIF-8, Co₃O₄@vacancy-type ZIF-8, and Co₃O₄@Ir-deposited ZIF-8 synthesized in Example 1 of this invention.

FIG. 9 shows the XPS spectra of iridium in the Co₃O₄@defective ZIF-8, Co₃O₄@vacancy-type ZIF-8, and Co₃O₄@Ir-deposited ZIF-8 synthesized in Example 1 of this invention.

FIG. 10 shows the inductively coupled plasma (ICP) detection results for the Co₃O₄@defective ZIF-8 electrode etching solution, the Co₃O₄@Ir deposited ZIF-8 electrode deposition solution, and the Co₃O₄@Ir deposited ZIF-8 electrode.

FIG. 11 is a transmission electron microscope (TEM) image of the core-shell surface of the Co₃O₄@Ir deposited ZIF-8 synthesized in Example 1 of this invention at 0 nm defocus.

FIG. 12 is a TEM image of the core-shell interior of the Co₃O₄@Ir deposited ZIF-8 synthesized in Example 1 of this invention at 58 nm defocus.

FIG. 13 shows the electrochemically active surface area of the Co₃O₄@defective ZIF-8, Co₃O₄@vacancy-type ZIF-8, and Co₃O₄@Ir deposited ZIF-8 synthesized in Example 1 of this invention.

FIG. 14 is a linear sweep voltammetry curve of the Co₃O₄@defect type ZIF-8, Co₃O₄@vacancy type ZIF-8, Co₃O₄@Ir deposit type ZIF-8 synthesized in Example 1, the Co₃O₄@Ir deposit type ZIF-8-2 synthesized in Example 2, and the Co₃O₄@Ir deposit type ZIF-8-3 synthesized in Example 3.

FIG. 15 is a Tafel slope diagram of the Co₃O₄@defect type ZIF-8, Co₃O₄@vacancy type ZIF-8, Co₃O₄@Ir deposit type ZIF-8 synthesized in Example 1, the Co₃O₄@Ir deposit type ZIF-8-2 synthesized in Example 2, and the Co₃O₄@Ir deposit type ZIF-8-3 synthesized in Example 3.

FIG. 16 is a potentiostatic long-cycle test diagram of the Co₃O₄@Ir deposit type ZIF-8 synthesized in Example 1.

DETAILED DESCRIPTION OF EMBODIMENTS

The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only a part of the embodiments of the present invention, and not all of them. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

Under alkaline conditions, the equilibrium potential of the oxygen evolution reaction is approximately 0.401V relative to the reversible hydrogen electrode, while under acidic conditions, the equilibrium potential is approximately 1.23V relative to the reversible hydrogen electrode. Under alkaline conditions, the equilibrium potential of the oxygen evolution reaction is lower than that under acidic conditions, and it provides a high concentration of OH⁻ to directly participate in the rate-determining step of the oxygen evolution reaction, which is beneficial for the reaction to occur. However, in an alkaline environment, higher requirements are placed on the chemical stability of the catalyst, such as its resistance to oxidation and alkali corrosion. Furthermore, the alkaline oxygen evolution reaction involves OH⁻ adsorption or H₂O dissociation, which has a high energy barrier, requiring the catalyst to have high intrinsic activity. Therefore, this invention prepares a Co₃O₄@IrO_x catalyst with a high specific surface area. Defects are introduced through calcination, enriching the active sites. Furthermore, the atomically dispersed doping of Ir achieves economical and high-efficiency oxygen evolution catalysis.

In this invention, unless otherwise specified, all raw materials used are commercially available products in the art, and all operations are performed at room temperature.

Example 1

This example provides a method for preparing a Co3O4@IrO_x catalyst. The specific steps are as follows:

Step 1: Weigh 11.75 mg of cobalt nitrate hexahydrate and 27.5 mg of 2-methylimidazole into two beakers according to a molar ratio of 1:8.3. Add 1 ml of methanol to each beaker and sonicate for 10 min to fully dissolve and disperse. Then mix the two and transfer them to a magnetic stirrer at 500 rpm for 3 h. After the reaction time was complete, the mixed solution was centrifuged at 5000 rpm for 10 min to obtain 7.2 mg of ZIF-67. This ZIF-67 was ultrasonically dispersed in 3 ml of fresh methanol and labeled as solution one, for later use.

15 mg of cetyltrimethylammonium bromide was dissolved in 1 ml of methanol, and 1135.03 mg of 2-methylimidazole was dissolved in 17.5 ml of methanol. Both solutions were ultrasonicated for 10 min each, then mixed and magnetically stirred at 500 rpm for 20 min. This mixture was labeled as solution two, for later use.

72.5 mg of zinc nitrate hexahydrate was dissolved in 2.5 ml of methanol and ultrasonicated for 10 min. This solution was labeled as solution three.

Step 2: Solution one was placed in a 100 ml reaction vessel and ultrasonicated for 3 min. Solution two was then poured into the ultrasonicated solution one, and ultrasonication continued for 5 min. Solution three was then poured in, and ultrasonication continued for 5 min. The reaction vessel was then closed and allowed to stand at room temperature for 24 h. After the reaction, the product was separated by centrifugation at 5000 rpm for 3 minutes. The obtained product was washed with methanol. The product was then ultrasonically dispersed in a methanol solution for 5 minutes and centrifuged again at 5000 rpm for 3 minutes, repeated three times. The final separated product was then dried under vacuum at 80 ℃ for 12 hours. After drying, the sample was allowed to cool naturally to room temperature. The dried sample was then ground in an agate grinding mortar for 15 minutes to obtain ZIF-67@ZIF-8 core-shell material.

Step 3: 0.3 mg of ZIF-67@ZIF-8 core-shell material was ultrasonically dissolved in 0.5 ml of ethanol and drop-coated onto ethanol-soaked, ultrasonically cleaned carbon paper (0.5 × 0.5 cm). After drying under an infrared lamp, ZIF-67@ZIF-8 electrode sheets were prepared.

The ZIF-67@ZIF-8 electrode sheets were placed in a quartz boat and then placed in a tube furnace for pyrolysis in air. The pyrolysis temperature was 350 ℃, the heating rate was 5 ℃·min⁻¹, and the electrode was kept at 350 ℃ for 0.5 hours before natural cooling to obtain a Co₃O₄@defective ZIF-8 electrode.

Step 4: Using a standard three-electrode system, a carbon rod was used as the counter electrode, and an Hg/HgO electrode as the reference electrode. The Co₃O₄@defective ZIF-8 electrode was used as the working electrode. The Co₃O₄@defective ZIF-8 working electrode was activated by pulsed potential in a 1 mol/L potassium hydroxide solution. During the pulsed potential activation process, the cathode potential was -2V for 1 s, and the anode potential was 2V for 1 s. This short pulse of cathode potential followed by the anode potential was repeated 150 times for 300 s. This activation process was repeated 4 times, with a total holding time of 1200 s for both oxidation and reduction activation. This process etched Zn²⁺ ions from the organic ligand on 2-methylimidazole, further creating defects in the organic framework, resulting in Co₃O₄@vacancy-type ZIF-8.

Step 5: Replace with a fresh 1M potassium hydroxide solution and add 100 μmol/L IrCl₄·H₂O. Perform electrochemical deposition using the same three-electrode system. The cathode potential is -6V for 1 s, and the anode potential is 6V for 1 s. Repeat this short pulse followed immediately by the anode potential 500 times for 1000 s. Repeat this deposition process 10 times. The total holding time for oxidation and reduction activation is 10000 s. This deposits Ir ions onto the vacancies in the Co₃O₄@vacancy-type ZIF-8 electrode, completing the electrochemical atomic exchange between Zn and Ir in the MOF material, resulting in a Co₃O₄@Ir deposited ZIF-8 electrode, i.e., the Co₃O₄@IrO_x catalyst.

Example 2

This example provides a method for preparing a Co₃O₄@IrO_x catalyst. The specific steps are as follows:

Step 1: Weigh 11.3 mg of cobalt nitrate hexahydrate and 28 mg of 2-methylimidazole into two beakers according to a molar ratio of 1:8.8. Add 1 ml of methanol to each beaker and sonicate for 10 min to fully dissolve and disperse. Then mix the two and transfer them to a magnetic stirrer at 500 rpm for 3 h. After the reaction time is complete, centrifuge the mixed solution at 5000 rpm for 10 min to obtain 6.93 mg of ZIF-67. Sonicate this solution in 3 ml of fresh methanol and label it as the first solution for later use.

Dissolve 15 mg of cetyltrimethylammonium bromide in 1 ml of methanol and 1135 mg of 2-methylimidazole in 17.5 ml of methanol. Sonicate each solution separately for 10 min, then mix them and stir magnetically at 500 rpm for 20 min. This mixture is labeled as the second solution and is ready for use.

Dissolve 72 mg of zinc nitrate hexahydrate in 2.5 ml of methanol by sonication for 10 min. This solution is labeled as the third solution.

Step 2: Take the first solution and place it in a 100 ml reaction vessel. Sonicate for 3 min, then pour the second solution into the sonicated first solution. Continue sonicating for 5 min, then pour in the third solution and sonicate for another 5 min. Close the reaction vessel and allow it to stand at room temperature for 26 h. After the reaction, separate the products by centrifugation at 5000 rpm for 3 min. Wash the obtained product with methanol. Sonicate the product for 5 min to disperse it in a methanol solution, then centrifuge at 5000 rpm for 3 min. Repeat this process three times. Finally, dry the separated product under vacuum at 80 °C for 12 h. After drying, allow the sample to cool naturally to room temperature. Remove the dried sample and grind it in an agate grinding bowl for 15 minutes to obtain ZIF-67@ZIF-8 core-shell material.

Step 3: Weigh 0.3 mg of the ZIF-67@ZIF-8 core-shell material and ultrasonically dissolve it in 0.5 ml of ethanol. Drop the solution onto ethanol-soaked carbon paper (0.5 × 0.5 cm) that has been ultrasonically cleaned. Dry the carbon paper under an infrared lamp to obtain ZIF-67@ZIF-8 electrode sheets.

Place the ZIF-67@ZIF-8 electrode sheets in a quartz boat and then in a tube furnace for pyrolysis in air. The pyrolysis temperature is 400 ℃, the heating rate is 5 ℃·min⁻¹, and the temperature is maintained at 400 ℃ for 0.5 hours before natural cooling to obtain Co₃O₄@defect-type ZIF-8-2 electrode sheets.

Step 4: Using a standard three-electrode system, a carbon rod is used as the counter electrode, an Hg/HgO electrode as the reference electrode, and the previously prepared Co₃O₄@defective ZIF-8 electrode sheet is used as the working electrode. The Co₃O₄@defective ZIF-8 working electrode is activated by pulsed potential in a 1 mol/L potassium hydroxide solution. During the pulsed potential activation process, the cathode potential is -2V for 2 s, and the anode potential is 2V for 2 s. This short pulse of cathode potential followed by the anode potential is repeated 150 times for 600 s. This activation process is repeated 3 times, with a total holding time of 1800 s for both oxidation and reduction activation. This process etches Zn²⁺ ions from the organic ligand on 2-methylimidazole, further creating defects in the organic framework, resulting in Co₃O₄@vacancy-type ZIF-8.

Step 5: Replace the 1M potassium hydroxide solution with 100 μmol/L IrCl₄·H₂O, and perform electrochemical deposition in the same three-electrode system. The cathode potential was -6V for 2s, and the anode potential was 6V for 2s. A short pulse at the cathode potential followed immediately by the anode potential was repeated 50 times for 200s. This deposition process was repeated 8 times, with a total holding time of 1600s for both oxidation and reduction activation. This method deposited Ir ions onto the vacancies of the Co₃O₄@vacancy-type ZIF-8, completing the electrochemical atomic exchange between Zn and Ir in the MOF material, resulting in a Co₃O₄@Ir deposition-type ZIF-8-2 electrode, i.e., the Co₃O₄@IrO_x catalyst.

Example 3:

This example provides a method for preparing a Co₃O₄@IrO_x catalyst. The specific steps are as follows:

Step 1: Weigh 12mg of cobalt nitrate hexahydrate and 27.3mg of 2-methylimidazole into two beakers according to a molar ratio of 1:8.1. Add 1ml of methanol to each beaker and sonicate for 10min to fully dissolve and disperse. The two solutions were then mixed and transferred to a magnetic stirrer at 500 rpm for 3 hours. After the reaction time, the mixture was centrifuged at 5000 rpm for 10 minutes to obtain 7.36 mg of ZIF-67. This ZIF-67 was ultrasonically dispersed in 3 ml of fresh methanol and labeled as solution one, for later use.

15 mg of cetyltrimethylammonium bromide was dissolved in 1 ml of methanol, and 1136 mg of 2-methylimidazole was dissolved in 17.5 ml of methanol. Both solutions were ultrasonically dissolved for 10 minutes each, then mixed and magnetically stirred at 500 rpm for 20 minutes. This mixture was labeled as solution two, for later use.

73 mg of zinc nitrate hexahydrate was dissolved in 2.5 ml of methanol and ultrasonically dissolved for 10 minutes. This solution was labeled as solution three.

Step 2: Take the first solution into a 100ml reaction vessel, sonicate for 3 minutes, then pour the second solution into the sonicated first solution, continue sonicating for 5 minutes, pour in the third solution, and continue sonicating for 5 minutes. Then close the reaction vessel and let it stand at room temperature for 22 hours. After the reaction, separate the products by centrifugation at 5000 rpm for 3 minutes. Wash the obtained products with methanol. Sonicate the products for 5 minutes to disperse them in methanol solution, then centrifuge at 5000 rpm for 3 minutes, repeating three times. After final product separation, dry under vacuum at 80 ℃ for 12 hours. After drying, allow it to cool naturally to room temperature, remove the dried sample, and grind it in an agate grinding mortar for 15 minutes to obtain ZIF-67@ZIF-8 core-shell material.

Step 3: Weigh 0.3 mg of ZIF-67@ZIF-8 core-shell material and ultrasonically dissolve it in 0.5 ml of ethanol. The dissolved material is then drop-coated onto 0.5 × 0.5 cm carbon paper that has been ultrasonically cleaned with ethanol. After drying with an infrared lamp, a ZIF-67@ZIF-8 electrode sheet is obtained.

The ZIF-67@ZIF-8 electrode sheet is placed in a quartz boat and then in a tube furnace for pyrolysis in air. The pyrolysis temperature is 300 °C, the heating rate is 5 °C·min⁻¹, and the temperature is maintained at 300 °C for 0.5 hours before natural cooling to obtain a Co₃O₄@defect-type ZIF-8 electrode sheet.

Step 4: Using a standard three-electrode system, a carbon rod is used as the counter electrode, an Hg/HgO electrode as the reference electrode, and the previously prepared Co₃O₄@defective ZIF-8 electrode sheet is used as the working electrode. The Co₃O₄@defective ZIF-8 working electrode is activated by pulsed potential in a 1 mol/L potassium hydroxide solution. During the pulsed potential activation process, the cathode potential is -2V for 1 s, and the anode potential is 2V for 1 s. This short pulse of cathode potential followed by the anode potential is repeated 150 times for 300 s. This activation process is repeated 5 times, with a total holding time of 1500 s for both oxidation and reduction activation. This process etches Zn²⁺ ions from the organic ligand on 2-methylimidazole, further creating defects in the organic framework, resulting in Co₃O₄@vacancy-type ZIF-8.

Step 5: Replace the 1M potassium hydroxide solution with 100 μmol/L IrCl₄·H2O, and perform electrochemical deposition in the same three-electrode system. The cathode potential was -6V for 1s, and the anode potential was 6V for 1s. A short pulse at the cathode potential followed immediately by the anode potential was repeated 150 times for 300s. This deposition process was repeated 5 times, with a total holding time of 1500s for both oxidation and reduction activation. This method deposited Ir ions onto the vacancies of Co₃O₄@vacancy-type ZIF-8, completing the electrochemical atomic exchange between Zn and Ir in the MOF material, resulting in a Co₃O₄@Ir deposition-type ZIF-8-3 electrode, i.e., a Co₃O₄@IrO_x catalyst.

Comparative Example 1

This comparative example provides a method for preparing Co₃O₄@defective ZIF-8 material. The specific steps are as follows:

Step 1: Weigh 11.75 mg of cobalt nitrate hexahydrate and 27.5 mg of 2-methylimidazole into two beakers according to a molar ratio of 1:8.3. Add 1 ml of methanol to each beaker and sonicate for 10 min to fully dissolve and disperse. The two solutions were then mixed and transferred to a magnetic stirrer at 500 rpm for 3 hours. After the reaction time, the mixture was centrifuged at 5000 rpm for 10 minutes to obtain ZIF-67. This ZIF-67 was ultrasonically dispersed in 3 ml of fresh methanol and labeled as solution one for later use.

15 mg of cetyltrimethylammonium bromide was dissolved in 1 ml of methanol, and 1135.03 mg of 2-methylimidazole was dissolved in 17.5 ml of methanol. Both solutions were ultrasonically dissolved for 10 minutes each, then mixed and magnetically stirred at 500 rpm for 20 minutes. This mixture was labeled as solution two for later use.

72.5 mg of zinc nitrate hexahydrate was dissolved in 2.5 ml of methanol and ultrasonically dissolved for 10 minutes and this solution was labeled as the third solution.

Step 2: Take the first solution and place it in a 100 ml reaction vessel. Sonicate for 3 min, then pour the second solution into the sonicated first solution and continue sonicating for 5 min. Add the third solution and sonicate for another 5 min. Then close the reaction vessel and allow it to stand at room temperature for 24 h. After the reaction, separate the products by centrifugation at 5000 rpm for 3 min. Wash the obtained products with methanol. Sonicate the products for 5 min to disperse them in methanol solution, then centrifuge at 5000 rpm for 3 min, repeating this process three times. After final product separation, dry the product under vacuum at 80 ℃ for 12 h. After drying, allow it to cool naturally to room temperature, then remove the dried sample and grind it in an agate grinding mortar for 15 min to obtain ZIF-67@ZIF-8 core-shell material.

Step 3: Weigh 0.3 mg of ZIF-67@ZIF-8 core-shell material and ultrasonically dissolve it in 0.5 ml of ethanol. The dissolved material is then drop-coated onto 0.5 × 0.5 cm carbon paper that has been ultrasonically cleaned with ethanol. After drying with an infrared lamp, a ZIF-67@ZIF-8 electrode sheet is obtained.

The ZIF-67@ZIF-8 electrode sheet is placed in a quartz boat and then in a tube furnace for pyrolysis in air. The pyrolysis temperature is 350 °C, the heating rate is 5 °C·min⁻¹, and the temperature is maintained at 350 °C for 0.5 hours, followed by natural cooling to obtain a Co₃O₄@defective ZIF-8 electrode sheet.

A standard three-electrode system is used, with a carbon rod as the counter electrode, an Hg/HgO electrode as the reference electrode, and the aforementioned Co₃O₄@defective ZIF-8 electrode sheet as the working electrode. The performance of water electrolysis is tested using a 1 M potassium hydroxide solution.

FIG. 1 is a field emission scanning electron microscope (FESEM) image of ZIF-67@ZIF-8 synthesized in Example 1 of this invention. It shows that the material has a regular dodecahedral structure with an average particle size of 350 nm.

FIG. 2 is a FESEM image of ZIF-67@ZIF-8 synthesized in Example 2 of this invention. It shows that the material has a regular dodecahedral structure. Due to the reduced ratio of cobalt nitrate hexahydrate to dimethylimidazole, the product particle size also decreases to an average particle size of 170 nm.

FIG. 3 is a FESEM image of ZIF-67@ZIF-8 synthesized in Example 3 of this invention. It shows that the material has a regular dodecahedral structure. Due to the shortened reaction time, the product particle size also decreases to an average particle size of 150 nm.

FIG. 4 is a real energy filtered transmission electron microscope (REFEM) image of ZIF-67@ZIF-8 synthesized in Example 1 of this invention. It shows the coating of Co with Zn, indicating that the core-shell structure of ZIF-67@ZIF-8 was successfully constructed.

FIG. 5 is a field emission scanning electron microscope (FESEM) image of the ZIF-67@ZIF-8 synthesized in Example 1 of this invention after calcination at 350 °C in air. It can be seen that it retains the polyhedral morphology of the MOF, with a slightly reduced particle size of 320 nm, and has become a hollow structure, i.e., a Co₃O₄@defective ZIF-8. Compared to the original ZIF-67@ZIF-8, the Co₃O₄@defective ZIF-8 introduces a large number of uncoordinated N and C sites as active sites due to defects introduced on the shell side and collapse on the core side, which is beneficial for subsequent Ir atom anchoring.

FIG. 6 shows the powder X-ray diffraction patterns of the ZIF-67@ZIF-8 raw material synthesized in Example 1 of this invention, and its powder X-ray diffraction patterns after electrochemical etching and electrochemical deposition, respectively. PDF#36-1451 and PDF#43-1003 correspond to the standard PDF cards for cubic cobalt oxide (Co₃O₄) and metallic cobalt, respectively. It can be seen that the characteristic peaks of ZIF-67@ZIF-8 after calcination are consistent with those of Co₃O₄, indicating that ZIF-67 has collapsed into Co₃O₄.

FIGS. 7-9 are, respectively, the X-ray photoelectron spectra of cobalt, zinc, and iridium in Co₃O₄@defective ZIF-8, Co₃O₄@vacancy-type ZIF-8, and Co₃O₄@Ir-deposited ZIF-8 synthesized in Example 1 of this invention.

FIG. 7 shows that Co can be detected in the Co₃O₄@defective ZIF-8, indicating that after calcination, the shell-side ZIF-8 of ZIF-67@ZIF-8 has formed defective pores or channels, exposing the core-side Co. It can also be seen that Co in the Co₃O₄@defective ZIF-8 has two characteristic peaks of the 2p orbital electrons unique to transition metals: 2p_3/2 and 2p_3/2. In X-ray photoelectron spectroscopy analysis, the 2p_3/2 peak of Co²⁺ is located near 780.5 eV, and the 2p_1/2 peak is located near 796.5 eV; while the 2p_3/2 peak of Co³⁺ appears near 779.0 eV, and the 2p_1/2 peak is located near 794.0 eV. Therefore, it is determined that ZIF-67@ZIF-8 transforms into Co₃O₄ after calcination.

In FIG. 8, Zn can be detected in the Co₃O₄@defect ZIF-8, with the binding energy range of Zn2p_3/2 from 1020 eV to 1025 eV and Zn2p_1/2 from 1045 eV to 1050 eV. However, the Co₃O₄@vacancy ZIF-8 surface lacks both Co and Zn, indicating that Zn has been etched away from the surface.

In FIG. 9, the Co₃O₄@Ir deposition ZIF-8 contains Ir⁴⁺ with binding energies of Ir4f_7/2 (61.5 eV to 63.5 eV) and Ir4f_5/2 (64.5 eV to 66.5 eV), and Ir³⁺ with binding energies of Ir4f_7/2 (63.5 eV to 65.0 eV) and Ir4f_5/2 (66.5 eV to 68.0 eV), collectively referred to as IrO_x, indicating successful Ir deposition.

FIG. 10 shows the inductively coupled plasma (ICP) detection of the Co₃O₄@defect-type ZIF-8 electrode etching solution, the Co₃O₄@Ir-deposited ZIF-8 electrode deposition solution, and the Co₃O₄@Ir-deposited ZIF-8 electrode. In the preparation of the original ZIF-67@ZIF-8, the masses of zinc nitrate hexahydrate and cobalt nitrate hexahydrate were 72.5 mg and 11.75 mg, respectively, and the molar ratio of Zn to Co was 6:1. Only Zn detached from the etching electrolyte and the Ir deposition electrolyte, while the Co₃O₄@Ir-deposited ZIF-8 electrode showed a significantly smaller distribution of Ir and Zn compared to Co, indicating successful Zn etching and Ir deposition with atomic exchange.

FIG. 11 is a TEM image of the core-shell surface of the Co₃O₄@Ir-deposited ZIF-8 synthesized in Example 1 of this invention at 0 nm defocus. It can be seen that Ir atomic clusters are distributed on the surface.

FIG. 12 is a TEM image of the core-shell interior of the Co₃O₄@Ir deposition-type ZIF-8 synthesized in Example 1 of this invention at a defocused wavelength of 58 nm. It can be seen that Ir atoms were successfully deposited in the product.

FIG. 13 shows the electrochemical active surface area diagrams of Co₃O₄@defective ZIF-8, Co₃O₄@vacancy ZIF-8, Co₃O₄@Ir deposited ZIF-8 synthesized in Example 1, Co₃O₄@Ir deposited ZIF-8-2 synthesized in Example 2, and Co₃O₄@Ir deposited ZIF-8-3 synthesized in Example 3. It can be seen that the electrochemical active surface area of Co₃O₄@defective ZIF-8 gradually increases from 1.28 mF·cm-2 to 8.41 mF·cm-2 and 49.27 mF·cm-2 after pulse creation of vacancies and Ir deposition. This indicates that during the electrochemical atomic exchange process, i.e. after electrochemical etching and deposition, the number of active sites exposed on the catalyst surface increases, the number of active sites participating in the oxygen evolution reaction increases, and the catalyst can withstand a larger current density, which is beneficial to the improvement of catalytic performance. The Co₃O₄@Ir deposited ZIF-8-2 and Co₃O₄@Ir deposited ZIF-8-3 obtained in Examples 2 and 3 have lower electrochemically active surface areas than the Co₃O₄@Ir deposited ZIF-8 in Example 1. This indicates that the improvement in electrochemical performance is greatly related to the reaction time, calcination temperature, and degree of Ir deposition, and the parameters used in Example 1 are optimal.

FIG. 14 shows the linear sweep voltammetry curves of Co₃O₄@defect type ZIF-8, Co₃O₄@vacancy type ZIF-8, and Co₃O₄@Ir deposit type ZIF-8 synthesized in Example 1, Co₃O₄@Ir deposit type ZIF-8-2 synthesized in Example 2, and Co₃O₄@Ir deposit type ZIF-8-3 synthesized in Example 3. It can be seen that after electrochemical etching and deposition, the overpotential of the catalyst at η=210mV decreases from 1.65V to approximately 1.45V, indicating that the driving energy required for the oxygen evolution reaction is reduced, and the reaction is more likely to occur. However, the kinetic performance of Co₃O₄@Ir deposit type ZIF-8-2 synthesized in Example 2 and Co₃O₄@Ir deposit type ZIF-8-3 synthesized in Example 3 is not as good as that of Co₃O₄@Ir deposit type ZIF-8 in Example 1, requiring a larger overpotential, which corresponds to the conclusion in FIG. 12.

FIG. 15 shows the Tafel slope diagrams of Co₃O₄@defective ZIF-8, Co₃O₄@vacancy ZIF-8, and Co₃O₄@Ir deposited ZIF-8 synthesized in Example 1, Co₃O₄@Ir deposited ZIF-8-2 synthesized in Example 2, and Co₃O₄@Ir deposited ZIF-8-3 synthesized in Example 3. It can be seen that during the electrochemical atomic exchange process, i.e., after electrochemical etching and deposition, the Tafel slope gradually decreases from 88.8 mV·dec⁻¹ to 65.6 mV·dec⁻¹. This indicates that the current density increases faster with increasing overpotential, resulting in a more favorable reaction kinetics. It also implies that the catalyst's ability to accelerate the oxygen evolution reaction rate is enhanced after electrochemical atomic exchange.

FIG. 16 shows the constant potential long-cycle test of the Co₃O₄@Ir deposited ZIF-8 synthesized in Example 1 of this invention. It can be seen that the Co₃O₄@Ir deposited ZIF-8 catalyst can maintain good performance during long-cycle operation for 1500 hours, making it more likely to be applied in practical electrocatalytic devices and possessing high industrial application value.

In the prior art, Liang et al., in their paper "Ir-Doped Bilayer Heterojunction Hollow Nanoboxes for Electrocatalytic Oxygen Evolution" published in *Inorganic Chemistry*, Volume 62, Issue 49, 2023, prepared Ir-doped ZIF-67@CoFe PBA hollow nanomaterials, abbreviated as Ir-ZIF-67@CoFe PBA. The Ir-ZIF-67@CoFe PBA material exhibits an overpotential of 269 mV at 10 mA·cm, which is higher than the 210 mV overpotential of the Co₃O₄@Ir deposited ZIF-8 in this invention.

In their 2021 paper "Fluorination of ZIF-67 framework templated Prussian blue analogue nano-box for efficient electrochemical oxygen evolution reaction," Gu et al. prepared ZIF-67 framework-templated Prussian blue analogue hollow nanomaterials. After calcination at 250°C, ZIF-67@CoFe-PBA-F-250 was obtained. This ZIF-67@CoFe-PBA-F-250 material exhibited an overpotential of 243 mV at 10 mA·cm. Although lower than commercially available IrO2 catalysts, it was still higher than the 210 mV overpotential of the Co₃O₄@Ir deposited ZIF-8 in this invention. This indicates that the Co₃O₄@Ir deposited ZIF-8 in this invention has better catalytic performance for oxygen evolution reactions.

Compared with existing technologies, this invention improves the atomic exchange method for Co₃O₄@defective ZIF-8. Unlike traditional atomic exchange methods such as solution impregnation, gas-phase permeation, and high-temperature solid-phase methods, which require high temperatures and long reaction times, this invention employs an electrochemical method to sequentially etch Zn²⁺ ions and deposit Ir elements. This method is simple, precise, and controllable, effectively controlling the degree of defect etching and Ir deposition in the ZIF-8 material, providing abundant active sites, thus facilitating electron transfer and improving the energy storage performance of ZIF-67@ZIF-8 as an oxygen evolution electrocatalyst. Furthermore, the material synthesis and etching methods are simple to operate, convenient to operate, suitable for large-scale production, and have significant green and economic benefits, making it widely applicable in the field of electrocatalysis.

Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of the invention. Obviously, those skilled in the art can make various modifications and variations to the invention without departing from its spirit and scope. Therefore, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention also intends to include these modifications and variations.