Amorphous iron oxide, its preparation method and use

Abstract

The application belongs to the field of water oxygen evolution reaction catalysts, and discloses amorphous iron oxide as well as a preparation method and application thereof. The amorphous iron oxide is obtained by dispersing a nitrate or acetate of iron in a mixed solution of water and alcohol, and loading the iron on a foamed metal through a solvothermal reaction. The nitrate ion or acetate significantly promotes the amorphous process of the iron species under the solvothermal condition, forms amorphous iron oxide with a highly disordered structure and high defect density, and significantly improves the electrocatalytic performance of the alkaline OER reaction. Meanwhile, the preparation method is simple, the raw materials are cheap and easy to obtain, the performance is excellent, and the cost is low.

Description

Technical Field

[0001] This invention belongs to the field of catalysts for oxygen evolution reaction in water, specifically relating to an amorphous iron oxide, its preparation method, and its application. Background Technology

[0002] With the increasing depletion of fossil fuel resources and the resulting environmental pollution problems drawing widespread attention globally, finding sustainable energy solutions has become a crucial issue urgently needing to be addressed. Among various strategies, developing sustainable energy technologies is considered one of the most promising directions. Hydrogen energy, with its significant advantages such as high calorific value, water as the only reaction product, and no pollution, is widely regarded as an ideal energy carrier.

[0003] Among various hydrogen production technologies, water electrolysis is considered the most promising method for large-scale development due to its relatively simple process and high energy conversion efficiency. However, the electrocatalysts currently used in water electrolysis for hydrogen production are primarily precious metals. The limited natural reserves, high prices, and potential stability issues during long-term operation of precious metals result in high application costs. Therefore, developing novel electrocatalysts that simultaneously possess high catalytic activity, low production costs, and long-term operational stability has become a key requirement for promoting the large-scale development of the hydrogen energy industry.

[0004] Iron, due to its abundant reserves and low cost in the Earth's crust, has long been a focus of research. In recent years, with a deeper understanding of electrocatalytic reaction mechanisms, studies have revealed that iron not only serves as an ideal substitute for precious metal catalysts, but also exhibits excellent intrinsic catalytic activity in the oxygen evolution reaction, demonstrating enormous application potential and providing a solid foundation for industrial applications. However, to fully realize its performance advantages, a core challenge remains: it is necessary to deeply elucidate the atomic structure and electronic characteristics of its catalytic active center, and based on this, develop precise control methods to optimize and enhance catalytic activity and selectivity. Summary of the Invention

[0005] The purpose of this invention is to provide an amorphous iron oxide and its preparation method. The amorphous iron oxide is easily reconstructed to generate a highly active FeOOH phase during the oxygen evolution reaction, thereby significantly improving the electrocatalytic oxygen evolution activity. At the same time, the preparation method is simple to operate, the raw materials are inexpensive and readily available, and the product has a uniform morphology.

[0006] Another objective of this invention is to provide an application of amorphous iron oxide as a catalyst in the oxygen evolution reaction of water electrolysis.

[0007] The technical solution of the present invention is as follows:

[0008] This invention provides a method for preparing amorphous iron oxide, the method comprising the following steps:

[0009] 1) Disperse the iron salt in a solvent to form a homogeneous solution;

[0010] 2) Add the foamed metal to the homogeneous solution described in step 1) and carry out a solvothermal reaction. After the reaction is completed, wash and dry to obtain the amorphous iron oxide.

[0011] The iron salt mentioned in step 1) is one or more of iron nitrates or acetates.

[0012] The solvent mentioned in step 1) is a mixed solvent of water and alcohol, wherein the alcohol is one or more of methanol, ethanol or isopropanol, and the volume ratio of water to alcohol in the solvent is 1:0.1-10.

[0013] The mass concentration of iron salt in the homogeneous solution described in step 1) is 0.2-400 mg / mL.

[0014] Preferably, the mass concentration of iron salt in the homogeneous solution is 0.2-200 mg / mL.

[0015] The foam metal mentioned in step 2) is one or more of foam nickel, foam copper, or foam cobalt.

[0016] The size of the foam metal mentioned in step 2) is 0.5-4 cm × 0.5-6 cm.

[0017] The solvothermal reaction in step 2) has a reaction temperature of 70-180 °C and a reaction time of 4-48 h.

[0018] This invention provides an amorphous iron oxide prepared using the above-described preparation method.

[0019] This invention provides an application of the aforementioned amorphous iron oxide as a catalyst in the oxygen evolution reaction of water electrolysis.

[0020] The preparation mechanism of amorphous iron oxide electrocatalysts is based on the hydrolysis and oxidation process of iron salt precursors under solvothermal reaction conditions. Specifically, in a mixed solvent of water and alcohol, the iron salt dissolves to form a homogeneous solution; the alcohol solvent, by regulating solvent polarity and viscosity, inhibits the ordered growth of iron oxide crystal nuclei, resulting in the formation of amorphous iron oxide without a long-range ordered structure; the foamed metal not only serves as a conductive substrate, but its three-dimensional porous structure also provides curvature fluctuations that alter the nucleation energy barrier, promoting the in-situ and firm growth of the amorphous thin layer on the substrate surface and preventing the detachment of active materials.

[0021] The nitrate ions in the iron salt provide a strong oxidizing environment in this system, ensuring the high valence state distribution of iron species. Furthermore, their weak coordination properties result in extremely rapid iron ion hydrolysis kinetics, generating a large number of supersaturated nuclei instantaneously under solvothermal conditions. This restricts the long-range ordered rearrangement of the crystal lattice, thus obtaining an amorphous structure and avoiding the transformation to α-Fe₂O₃ or γ-Fe₂O₃. The reaction process can be presumably described as: Fe 3+ + NO3 - + H2O / alcohol → [Fe(NO3) x (OH) y ] 3-x-y → A-FeO x The nitrate ions promote the formation of intermediate complexes, ensuring an increase in the degree of amorphization.

[0022] When iron acetate is used, the acetate ion acts as a bidentate ligand with Fe. 3+ Forming a stable chelate structure significantly reduces free Fe 3+ The concentration of acetate slows down the hydrolysis rate and prevents the rapid formation of ordered crystal nuclei. Under heating conditions, acetate ions partially dissociate to produce CH3COOH, forming a weak acid-weak base buffer system that maintains the reaction pH at 4-6. This prevents excessively high pH from causing rapid precipitation into crystalline ferric hydroxide or ferric oxide, while also preventing excessively low pH from inhibiting hydrolysis. This buffering effect is beneficial for a slow and uniform polymerization process, forming an amorphous network without long-range order. In addition, the adsorption of acetate ions on the particle surface generates a steric hindrance effect, inhibiting the directional growth of crystal faces, thereby guiding the product towards an amorphous form.

[0023] Compared with the prior art, the present invention has the following advantages:

[0024] 1. The solvothermal method requires only iron salt, water-alcohol mixed solvent and foamed metal matrix, without the need for complex equipment or high-temperature calcination. The reaction conditions are mild and easy to produce.

[0025] 2. The amorphous structure promotes rapid reconstruction into the FeOOH active phase, lowers the OER energy barrier, and enhances the electrocatalytic activity of the oxygen evolution reaction. In a 1.0 M KOH solution, the amorphous iron oxide described in this invention exhibits electrocatalytic activity at 200 mA·cm⁻¹. -2 The overpotential is between 150-300 mV vs. RHE, 1000 mA·cm -2 The overpotential is between 280-400 mV vs. RHE, which is significantly lower than that of ordinary iron-based catalysts;

[0026] 3. Using iron acetate or nitrate as raw materials to replace precious metals effectively reduces raw material costs and significantly improves catalytic performance through the synergistic effect of anions and iron ions in the synthesis process. Attached Figure Description

[0027] Figure 1 This is a scanning electron microscope (SEM) image of the amorphous iron oxide prepared in Example 1;

[0028] Figure 2 The XRD pattern of the amorphous iron oxide prepared in Example 1 is shown.

[0029] Figure 3 XRD pattern of iron oxide obtained in Comparative Example 1;

[0030] Figure 4 XRD pattern of iron oxide obtained in Comparative Example 2;

[0031] Figure 5 The LSV curve of the oxygen evolution reaction of amorphous iron oxide obtained in Example 1 is shown.

[0032] Figure 6 The LSV curve of the oxygen evolution reaction of amorphous iron oxide obtained in Example 2 is shown.

[0033] Figure 7 The LSV curve of the oxygen evolution reaction of iron oxide obtained in Comparative Example 1 is shown.

[0034] Figure 8 The LSV curve of the oxygen evolution reaction of iron oxide obtained in Comparative Example 2 is shown. Detailed Implementation

[0035] To make the objectives, technical solutions, and advantages of the embodiments of the present invention clearer, the technical solutions of the embodiments of the present invention will be clearly and completely described below in conjunction with the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0036] Unless otherwise specified, all test materials and reagents used in the following examples are commercially available.

[0037] Example 1

[0038] A method for preparing amorphous iron oxide includes the following steps:

[0039] 1) Add 0.3 mmol Fe(NO3)3·9H2O to a mixed solution of 5.0 mL water and 10.0 mL ethanol, and stir to form a homogeneous solution;

[0040] 2) Add 1 cm × 2 cm nickel foam to the above solution and continue stirring for 10 min. Then transfer the entire mixture to a solvothermal reactor and react at 120 °C for 12 h. Cool and separate the reaction system. Wash the precipitate with water and then with alcohol. Dry it under vacuum at 40 °C to constant weight to obtain amorphous iron oxide, labeled as FeO. x -NO.

[0041] Example 2

[0042] A method for preparing amorphous iron oxide includes the following steps:

[0043] 1) Add 0.3 mmol Fe(CO2CH3)3·3H2O to a mixed solution of 7.5 mL water and 7.5 mL isopropanol, and stir to form a homogeneous solution;

[0044] 2) Add 1 cm × 2 cm nickel foam to the above solution and continue stirring for 10 min. Then transfer the entire mixture to a solvothermal reactor and react at 120 °C for 12 h. Cool and separate the reaction system. Wash the precipitate with water and then with alcohol. Dry it under vacuum at 40 °C to constant weight to obtain amorphous iron oxide, labeled as FeO. x -CO.

[0045] Example 3

[0046] A method for preparing amorphous iron oxide includes the following steps:

[0047] 1) Add 0.4 mmol Fe(CO2CH3)3·3H2O to a mixed solution of 15.0 mL water and 15.0 mL methanol, and stir to form a homogeneous solution;

[0048] 2) Add 2 cm × 3 cm nickel foam to the above solution and continue stirring for 10 min. Then transfer the whole solution to a solvothermal reactor and react at 140 °C for 12 h. Cool and separate the reaction system. Wash the precipitate with water and then with alcohol. Dry it under vacuum at 40 °C to constant weight to obtain amorphous iron oxide.

[0049] Example 4

[0050] A method for preparing amorphous iron oxide includes the following steps:

[0051] The difference between this embodiment and Embodiment 2 is that the reaction temperature of the solvothermal reaction in step 2) is changed to 140°C and the reaction time is changed to 8 hours. The rest is the same as in Embodiment 2.

[0052] Comparative Example 1

[0053] A method for preparing iron oxide includes the following steps:

[0054] 1) Add 0.3 mmol FeCl3·6H2O to a mixed solution of 7.5 mL water and 7.5 mL isopropanol, and stir to form a homogeneous solution;

[0055] 2) Add 0.5 cm × 1 cm nickel foam to the above solution and continue stirring for 10 min. Then transfer the entire mixture to a solvothermal reactor and react at 140 °C for 12 h. Cool and separate the reaction system. Wash the precipitate with water and then with alcohol. Dry it under vacuum at 40 °C to constant weight to obtain the iron oxide, labeled as FeO. x -Cl.

[0056] Comparative Example 2

[0057] A method for preparing iron oxide includes the following steps:

[0058] The amorphous iron oxide obtained in step 2) of Example 1 was calcined at 300 °C for 4 h in an argon atmosphere to obtain the iron oxide, labeled as FeO. x -NO-2.

[0059] Test Example 1

[0060] SEM characterization

[0061] The SEM characterization results of Example 1 are as follows: Figure 1 As shown, from Figure 1 It can be seen that iron oxide has a spherical particle structure with a diameter of about 10 nm to 400 nm.

[0062] XRD characterization

[0063] The XRD patterns of Example 1, Comparative Example 1, and Comparative Example 2 are as follows: Figure 2 , Figure 3 and Figure 4 As shown, from Figure 3 and Figure 4 It can be seen that 2θ has several obvious sharp peaks in the range of 30°-70°, indicating that the material has partially crystallized. Figure 2 It can be seen that 2θ has a broad diffuse scattering hump in the range of 30°-70° without sharp peaks, which is a typical characteristic of highly amorphous state.

[0064] Test Example 2

[0065] OER performance test

[0066] The iron oxide materials described in each embodiment and comparative example were cut to a size of 1.0 cm × 1.0 cm. The cut materials were immersed in a 1.0 M KOH solution, and the electrocatalytic process of the oxygen evolution reaction was recorded using an electrochemical workstation within a potential window of 1.20 - 1.60 V vs. RHE. The LSV curves of Examples 1 and 2 and Comparative Examples 1 and 2 are shown below. Figure 5 , 6 As shown in Figures 7 and 8, the current density is at 200 mA·cm⁻¹ -2 and 1000 mA·cm -2 The overpotentials below are shown in Table 1 below:

[0067] Table 1

[0068]

[0069] The above data demonstrate that amorphous iron oxide prepared using iron nitrate or acetate as precursors, due to its highly amorphous structure and high defect density, can rapidly reconstruct a highly active FeOOH phase during the oxygen evolution reaction (OER), thereby significantly reducing the overpotential and maintaining high-efficiency catalytic performance at high current densities. In contrast, the partial crystallization caused by the ferric chloride precursor and the relative reduction in amorphousness after additional sintering weaken the intrinsic active sites and reconstruction ability of the material, resulting in a significant increase in overpotential and failing to meet the industrial-grade high-current-density OER requirements. Furthermore, the additional heat treatment in Comparative Example 2 led to partial passivation of surface active sites, thereby increasing the overpotential, and the overall OER catalytic activity was inferior to that of the uncalcined example. These results further verify the key role of nitrate or acetate ions in promoting amorphization and enhancing catalytic activity.

[0070] It should be noted that the above embodiments are merely some preferred embodiments of the present invention, and not all embodiments. Obviously, based on the above embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0071] The above description of the embodiments is intended to enable those skilled in the art to understand and use the invention. It will be apparent to those skilled in the art that various modifications can be made to these embodiments, and the general principles described herein can be applied to other embodiments without inventive effort. Therefore, the present invention is not limited to the above embodiments, and any improvements and modifications made by those skilled in the art based on the disclosure of the present invention without departing from the scope of the invention should be within the protection scope of the present invention.

Claims

1. A method for preparing amorphous iron oxide, characterized in that, The preparation method includes the following steps: 1) Disperse the iron salt in a solvent to form a homogeneous solution; 2) Add the foamed metal to the homogeneous solution described in step 1) and carry out a solvothermal reaction. After the reaction is completed, wash and dry to obtain the amorphous iron oxide. The iron salt mentioned in step 1) is one or more of iron nitrates or acetates.

2. The preparation method according to claim 1, characterized in that, The solvent mentioned in step 1) is a mixed solvent of water and alcohol, wherein the alcohol is one or more of methanol, ethanol or isopropanol, and the volume ratio of water to alcohol in the solvent is 1:0.1-10.

3. The preparation method according to claim 1, characterized in that, The mass concentration of iron salt in the homogeneous solution described in step 1) is 0.2-400 mg / mL.

4. The preparation method according to claim 3, characterized in that, The mass concentration of iron salt in the homogeneous solution is 0.2-200 mg / mL.

5. The preparation method according to claim 1, characterized in that, The foam metal mentioned in step 2) is one or more of foam nickel, foam copper, or foam cobalt.

6. The preparation method according to claim 1 or 5, characterized in that, The size of the foam metal mentioned in step 2) is 0.5-4 cm × 0.5-6 cm.

7. The preparation method according to claim 1, characterized in that, The solvothermal reaction in step 2) has a reaction temperature of 70-180 °C and a reaction time of 4-48 h.

8. An amorphous iron oxide prepared by the preparation method as described in claim 1.

9. The application of the amorphous iron oxide as described in claim 8 as a catalyst in the oxygen evolution reaction of water electrolysis.

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