Application of NiCo-MOF anodic oxidation catalyst for matching cathode CO2 reduction

Abstract

A method for preparing a NiCo-MOF anodic oxidation catalyst for matching cathode CO2 reduction is disclosed, belonging to the technical fields of electrocatalysis and fine chemical synthesis. Based on ion etching in a hydrothermal environment, a NiCo-MOF electrocatalyst with a uniform nanoflower-like structure was grown on the surface of substrates such as nickel foam. Benefiting from the modulating effect of cobalt on the electron cloud distribution of the active center nickel, this catalyst exhibits excellent catalytic activity for glycerol oxidation and glucose oxidation, achieving up to 400 mA cm⁻¹ at only 1.398 V. ‑2 The industrial-grade current density has a Tafel slope of only 42.3 mV dec. ‑1 The catalyst exhibited no performance degradation after 100 hours of stable cycling, with a Faradaic efficiency exceeding 90% for the main product, formic acid. This catalyst demonstrates a simple synthesis method, low cost, good catalytic activity and selectivity, and excellent stability, showing promising industrial application prospects in the co-production of formate using small-molecule electro-oxidation coupled with carbon dioxide electroreduction.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalysis and fine chemical synthesis, specifically relating to a method for preparing and applying a NiCo-MOF anodic oxidation catalyst for matching cathode CO2 reduction. Background Technology

[0002] In addition to reducing carbon emissions at the source, the development of green and low-carbon energy technologies and carbon utilization technologies has made the comprehensive utilization of carbon dioxide (CO2) a research hotspot and challenge, which is related to the overall layout of the country's ecological civilization construction and the overall economic and social development.

[0003] Despite the continuous development of the new energy industry, fossil fuels still dominate the global energy structure, and their use leads to massive CO2 emissions, contributing to the global greenhouse effect. Reducing atmospheric CO2 levels is therefore imperative to protect the ecological environment. Electrocatalytic CO2 reduction is one of the most promising technologies for addressing this dilemma. At room temperature, CO2 can be converted into high-value fuels and chemicals (such as formic acid, methanol, methane, and ethylene) using a range of new clean energy sources, including wind and solar power. With in-depth research, particularly the development of novel catalysts with high selectivity and stability, and the design of new electrolysis devices, electrocatalytic CO2 reduction technology shows promising industrial application prospects. However, in most current CO2 electroreduction processes, the oxygen evolution reaction (OER) occurs at the anode. This four-electron transfer process requires a high anode potential, significantly increasing energy consumption and generating secondary carbon emissions. More importantly, oxygen, as a product of the anode reaction, has relatively low economic value and is typically released directly into the atmosphere.

[0004] Developing novel anodic reactions with low onset and operating potentials offers a new approach to solving the high energy consumption problem of OER reactions. In catalyst research, nickel-based catalysts are the most widely used, such as metal hybrids represented by Ni₂P and NiS, and bimetallic oxides doped with high-valence metals such as Mo, Mn, and W. These catalysts have shown excellent catalytic activity and stability for small-molecule organic substrates. However, the synthesis processes of these catalysts often rely on high-temperature treatment to introduce heteroatoms, thus facing significant challenges in large-scale preparation and high energy consumption and costs. Therefore, developing novel methods for preparing nickel-based alcohol oxidation catalysts that achieve a balance of high activity, high selectivity, long lifetime, scalable preparation, and low cost is crucial for promoting the industrial application of fine chemical conversion coupled with electrocatalytic CO₂ reduction. Summary of the Invention

[0005] To overcome the shortcomings of existing technologies, this invention aims to provide a low-cost, scalable method for preparing electrocatalysts and apply it to the electrochemical oxidation of small organic molecules, with the goal of coupling cathodic electrocatalytic CO2 reduction to promote the development of low-cost carbon utilization technologies. This method is simple to operate, cost-effective, and energy-efficient, and exhibits excellent catalytic activity, product selectivity, and stability in the electrochemical oxidation of small-molecule alcohols. Small-molecule organic compounds such as methanol, ethanol, isopropanol, glycerol, 5-hydroxymethylfurfural, urea, hydrazine, and glucose can be used as reaction substrates for anodic oxidation reactions at a reaction potential below the OER (Organic Emission Reduction) potential.

[0006] To achieve the above-mentioned objectives and solve the problems existing in the prior art, the present invention provides a method for preparing a NiCo-MOF anodic oxidation catalyst for matching cathode CO2 reduction, comprising the following steps:

[0007] Step 1: Immerse the substrate of commercially available nickel foam (NF) in a 1-3 mol / L HCl solution and sonicate for 30 min. Then rinse repeatedly with ethanol and deionized water to remove the oxide layer on the surface of the nickel foam. The nickel foam can be replaced with copper foam, iron foam, titanium foam, carbon fiber paper and carbon cloth, or no substrate can be used, which has almost no effect on the morphology of the catalyst.

[0008] Step 2: Dissolve 1 mmol Ni(NO3)3·6H2O and 0.5 mmol Co(NO3)3·6H2O in 15 mL of methanol as solution A, and dissolve 4 mmol 2-methylimidazole in 15 mL of methanol as solution B. After stirring for 20 min, quickly inject solution B into solution A and continue stirring for 5-10 min to form a homogeneous blue turbid solution. The ratio of nickel to cobalt is adjustable from 1:1 to 4:1. Depending on the organic substrate, the ratio of nickel and cobalt added to the precursor can be selectively controlled, which is key to the application of this catalyst to different substrates. According to research, different ratios of nickel and cobalt do not have a significant impact on morphology, but only on catalytic performance: compared to other ratios, a Ni to Co ratio of around 2:1 exhibits the best catalytic performance for the electro-oxidation of small molecule alcohols. In the aforementioned steps, the molar ratio of 2-methylimidazole to cobalt salt is adjustable from 6:1 to 10:1.

[0009] Step 3: Transfer the above mixed solution to a 50 mL polytetrafluoroethylene-lined stainless steel high-pressure hydrothermal reactor, add the pretreated nickel foam, and place it in an oven at 60-80°C for 6-12 hours. After cooling, wash the nickel foam with NiCo-MOF growth multiple times with deionized water and ethanol, and then dry it overnight at 60°C to obtain a self-supported NiCo-MOF electrocatalyst with uniform nanoflower-like structure.

[0010] Step 4: Using the electrocatalyst as the working electrode, Hg / HgO as the reference electrode, and a carbon rod as the counter electrode, perform CV activation in a standard three-electrode system. The voltage range is 0–0.5 V (vs. Hg / HgO), and the scan time is 10–20 min to complete the activation of NiCo-MOF and then directly use it for the electrocatalytic oxidation of small molecule alcohols (such as glycerol, methanol, glucose, etc.).

[0011] The presence of a substrate in this catalyst improves the uniformity of the nanoflower-like structure and, when used as a catalyst, avoids the use of binders, eliminating some charge transfer resistance, which is key to the catalyst exhibiting superior catalytic activity. The substrate can be nickel foam, or it can be replaced with copper foam, iron foam, titanium foam, carbon fiber paper, or carbon cloth, with almost no impact on the morphology of the catalyst.

[0012] Unlike conventional Ni-MOFs that use pyromellitic acid and 2-methylimidazole as precursors, this method utilizes the redox properties of Ni and Co to achieve Ni... 2+ Etching ZIF-67 and forming NiCo-MOF nanoflowers on its surface can also be extended to CuCo-MOF and RuCo-MOF, etc.

[0013] NiCo-MOF electrocatalysts exhibited excellent catalytic activity for novel anodic oxidation reactions, particularly methanol and glycerol, which could be oxidized to potassium formate as the final product under electrical drive, with a Faraday efficiency exceeding 90%, a yield close to 100%, and stability exceeding 100 h.

[0014] The advantages of this invention are: the proposed electrocatalyst synthesis method can prepare NiCo-MOF catalysts with uniform nanoflower-like structures. The synthesis method is simple, with low raw material costs and low energy consumption, making it suitable for large-scale preparation. Furthermore, this catalyst exhibits excellent catalytic activity, selectivity, and stability for the electrooxidation of small molecule alcohol compounds. In particular, when the nickel to cobalt ratio is 2:1, a 400 mA cm⁻¹ anode potential of only 1.38 V (vs. RHE) is required to achieve this. -2The catalyst maintains stable performance for 100 hours at an industrial current density of 1.4V (vs. RHE) with almost no decrease in catalytic performance. This is attributed to the introduction of Ni providing active sites, while the presence of Co modulates the electron cloud distribution of Ni, enhancing its conversion from divalent to trivalent oxidation and thus exhibiting excellent catalytic performance. Most importantly, the catalyst demonstrates high selectivity for formic acid in the oxidation of glycerol, with formic acid as the main product exhibiting a Faradaic efficiency exceeding 90% and a selectivity approaching 100%. In a coupled electrocatalytic CO2 reduction system, not only can energy consumption be reduced, demonstrating energy-saving potential, but also high-value-added conversion of fine chemicals is achieved. Furthermore, the same product—potassium formate—is obtained at both the anode and cathode, avoiding the use of ion exchange membranes, further reducing the cost of electrocatalytic CO2 reduction and extending the operating cycle of the device.

[0015] In addition to exhibiting excellent catalytic activity for glycerol, NiCo-MOF catalysts are also suitable for the electro-oxidation of other small-molecule alcohols such as methanol and glucose. Furthermore, they can be extended to the oxidation of urea and hydrazine, providing a new low-cost route for the commercial development of coupled electrocatalytic CO2 reduction, with broad prospects. Attached Figure Description

[0016] Figure 1 This is a scanning electron microscope image of the NiCo-MOF electrocatalyst.

[0017] Figure 2 XRD pattern of NiCo-MOF electrocatalyst

[0018] Figure 3 This is a comparison of the polarization curves of glycerol oxidation and OER in Example 1.

[0019] Figure 4 This is a comparison of the polarization curves of glucose oxidation and OER in Example 2. Detailed Implementation

[0020] The present invention will be further described below with reference to the embodiments.

[0021] Example 1: Preparation of Catalyst

[0022] The preparation method of self-supporting NiCo-MOF electrocatalyst includes the following steps:

[0023] Step 1: Immerse the substrate, commercially available nickel foam (NF), in a 1 mol / L HCl solution and sonicate for 30 min. Then, rinse repeatedly with ethanol and deionized water to remove the oxide layer on the surface of the nickel foam. The nickel foam can be replaced with copper foam, iron foam, titanium foam, carbon fiber paper, or carbon cloth, or no substrate can be used, which has almost no impact on the morphology of the catalyst.

[0024] Step 2: Dissolve 1 mmol Ni(NO3)3·6H2O and 0.5 mmol Co(NO3)3·6H2O in 15 mL of methanol to obtain solution A, and dissolve 4 mmol 2-methylimidazole in 15 mL of methanol to obtain solution B. After stirring for 20 min, quickly inject solution B into solution A and continue stirring for 7 min to form a uniform blue turbid solution.

[0025] Step 3: Transfer the above mixed solution to a 50 mL polytetrafluoroethylene-lined stainless steel high-pressure hydrothermal reactor, add the pretreated nickel foam, and place it in an oven at 60°C for 12 hours. After cooling, wash the nickel foam with NiCo-MOF growth multiple times with deionized water and ethanol, and then dry it overnight at 60°C to obtain a self-supported NiCo-MOF electrocatalyst with uniform nanoflower-like structure.

[0026] Example 2

[0027] Since the catalyst prepared in this invention has a self-supporting structure, there is no need to use Nafion solution as a binder. The nickel foam loaded with NiCo-MOF catalyst in Example 1 was cut into 1×1cm pieces. 2 The catalyst was used as the working electrode, with Hg / HgO as the reference electrode and a carbon rod as the counter electrode. The electrolyte consisted of a solution containing 0.3 mol / L glycerol and 1 mol / L KOH. Electrochemical performance tests were conducted in a standard three-electrode electrolytic cell using a Shanghai Chenhua CHI 760E workstation to verify the catalyst's activity. Specifically, argon was used as the circulating gas. Before the electrochemical tests, argon was introduced for 20 minutes to purge other gases from the electrolytic cell. Then, CV activation was performed using the electrochemical workstation, with the voltage set between 0 and 0.5 V (vs. Hg / HgO) and a scan rate of 10 mV / s. -1 The NiCo-MOF catalyst was activated by 40 cyclic scans, followed by polarization curve testing. The voltage range was -0.2 to 0.6 V, and the scan rate was 10 mV / s. -1 With iR compensation set to 85%, the polarization curve results are as follows: Figure 2 As shown, the Ni2Co1-MOF catalyst exhibits excellent catalytic activity for glycerol oxidation, achieving 10 mA cm⁻¹ oxidation at only 1.25 V, 1.305 V, 1.341 V, and 1.398 V. -2 100mA cm -2 200mA cm -2 and 400mA cm -2The current density is significantly better than that of the OER reaction, with the polarization potential reduced by up to 300 mV. Further Tafel slope results show that Ni₂Co₁-MOF has the lowest Tafel slope, at only 42.3 mV dec. -1 This indicates that Ni2Co1-MOF exhibits faster catalytic reaction kinetics. Furthermore, this invention compares electrochemical impedance and electrochemical active surface area, showing that the Ni2Co1-MOF catalyst possesses very low charge transfer resistance and a large electrochemical active surface area. This is partly due to the uniform nanoflower-like morphology, which provides abundant channels, facilitating electrolyte penetration and product transport, while also offering more active sites. The three-dimensional conductive self-supporting material structure avoids the use of binders, significantly reducing charge transfer resistance. Stability tests show that the Ni2Co1-MOF catalyst exhibits good durability for glycerol oxidation, with almost no change in catalytic activity during a continuous electrolysis test lasting up to 110 hours.

[0028] This invention utilizes proton nuclear magnetic resonance spectroscopy to perform qualitative and quantitative analysis of the electrolysis products. The results show that as the reaction proceeds, the concentration of glycerol gradually decreases, while the concentration of potassium formate gradually increases. Furthermore, potassium formate is the only liquid-phase product of glycerol oxidation, with a Faraday efficiency exceeding 90%, indicating that the Ni2Co1-MOF catalyst has good selectivity for formate.

[0029] Example 3

[0030] The preparation method of self-supporting NiCo-MOF electrocatalyst includes the following steps:

[0031] Step 1: Immerse commercial nickel foam (NF) as a substrate in a 1 mol / L HCl solution and sonicate for 30 min. Then rinse repeatedly with ethanol and deionized water to remove the oxide layer on the surface of the nickel foam.

[0032] Step 2: Dissolve 1.5 mmol Ni(NO3)3·6H2O and 0.5 mmol Co(NO3)3·6H2O in 15 mL of methanol to obtain solution A, and dissolve 4 mmol 2-methylimidazole in 15 mL of methanol to obtain solution B. After stirring for 20 min, quickly inject solution B into solution A and continue stirring for 10 min to form a uniform blue turbid solution.

[0033] Step 3: Transfer the above mixed solution to a 50 mL polytetrafluoroethylene-lined stainless steel high-pressure hydrothermal reactor, add the pretreated nickel foam, and place it in an oven at 60°C for 12 hours. After cooling, wash the nickel foam with NiCo-MOF growth multiple times with deionized water and ethanol, and then dry it overnight at 60°C to obtain a self-supported NiCo-MOF electrocatalyst with uniform nanoflower-like structure.

[0034] Example 4

[0035] The self-supported NiCo-MOF electrocatalyst from Example 3 was used for the oxidation of small molecule glucose. The electrolyte was a potassium hydroxide solution containing glucose with a glucose concentration of 0.5 mol / L. The performance of glucose electrooxidation was tested in a standard three-electrode electrolyzer using a Shanghai Chenhua CHI 760E workstation to verify the versatility of the catalyst.

[0036] Specifically, before the electrochemical test, argon gas was introduced for 20 minutes to purge other gases from the electrolytic cell. Then, CV activation was performed using an electrochemical workstation, with the voltage set between 0 and 0.5 V (vs. Hg / HgO) and the scan rate at 10 mV / s. -1 The NiCo-MOF catalyst was activated by 30 cyclic scans. The polarization curve parameters were set as follows: voltage range -0.2 to 0.6 V, scan rate 10 mV / s. -1 Setting iR compensation to 85% yields the following results: Figure 4 As shown, unlike glycerol oxidation, the optimal Ni to Co ratio for glucose oxidation is 3:1, and the Ni3Co1-MOF catalyst exhibits remarkable catalytic activity for glucose oxidation, achieving 10 mA cm⁻¹ oxidation at only 1.28 V, 1.35 V, and 1.42 V. -2 100mA cm -2 and 400mA cm -2 The current density is much higher, but for the OER reaction, electrode potentials of 1.557V, 1.667V and 1.78V are required, resulting in a reduction in polarization voltage of up to 300mV or more.

[0037] Further Tafel slope results show that Ni3Co1-MOF has a low Tafel slope of only 62.1 mVdec. -1 This indicates that Ni3Co1-MOF exhibits relatively fast catalytic reaction kinetics. Similar to the glycerol oxidation phase, the Ni3Co1-MOF catalyst also possesses very low charge transfer resistance and a large electrochemical active surface area. Its abundant pore structure exposes more active sites, which is beneficial for the contact between the reaction substrate and the catalyst, as well as the desorption of products.

[0038] Analysis of the electrolysis products by proton nuclear magnetic resonance spectroscopy showed that as the reaction proceeded, the glucose concentration gradually decreased, while the main product was gluconic acid, with a selectivity exceeding 90% and a Faraday efficiency approaching 90%. In the process of coupling with the cathode CO2 electroreduction, energy savings were achieved on the one hand, and the conversion of high-value-added products was realized on the other. In particular, gluconic acid, a naturally occurring organic acid with high added value, is widely used in food, chemical, and pharmaceutical fields, such as reducing steroid and some non-steroidal alcohols, and chemical anti-cancer agents. According to a 2004 report by the U.S. Department of Energy on the ten most valuable bio-based products produced using bio-methods, gluconic acid was considered "one of the most valuable biorefining products." Its high economic value further reduces the cost of electrocatalytic CO2 reduction, thereby contributing to the reduction of carbon emissions.

[0039] The influence of the Ni to Co ratio on the application in Examples 2 and 4 is analyzed in this invention. Specifically, the effect of the Ni to Co ratio on the material morphology and catalytic performance is analyzed. Ni serves as the active site for the anodic small molecule oxidation reaction, while the presence of Co plays a role in regulating the electron cloud distribution of Ni. Optimal electronic structure is necessary for the best catalytic performance. This invention found that when the Ni to Co ratio is 2:1, the catalytic activity is optimal, and the product selectivity is also the highest.

[0040] Example 5

[0041] The preparation method of self-supporting NiCo-MOF electrocatalyst includes the following steps:

[0042] Step 1: Immerse the substrate commercial nickel foam (NF) in a 1 mol / L HCl solution and sonicate for 30 min. Then rinse repeatedly with ethanol and deionized water to remove the oxide layer on the surface of the nickel foam.

[0043] Step 2: Dissolve 1 mmol Ni(NO3)3·6H2O and 0.5 mmol Co(NO3)3·6H2O in 15 mL of methanol to obtain solution A, and dissolve 4 mmol 2-methylimidazole in 15 mL of methanol to obtain solution B. After stirring for 20 min, quickly inject solution B into solution A and continue stirring for 7 min to form a uniform blue turbid solution.

[0044] Step 3: Transfer the above mixed solution to a 50 mL polytetrafluoroethylene-lined stainless steel high-pressure hydrothermal reactor, add the pretreated nickel foam, and place it in an oven at 85°C for 12 hours. After cooling, wash the nickel foam with NiCo-MOF growth multiple times with deionized water and ethanol, and then dry it overnight at 60°C to obtain a self-supported NiCo-MOF electrocatalyst with uniform nanoflower-like structure.

[0045] Example 6

[0046] Except for the hydrothermal reaction in step 3, which is maintained at 85°C for 12 hours, the rest of the preparation method is the same as in Example 5.

[0047] In Examples 5 and 6, the other operational steps were the same as in Example 1. The main difference was the hydrothermal reaction temperature. In Example 5, the hydrothermal reaction temperature was 85°C, and in Example 6, it was 120°C. Neither of the prepared catalysts formed a uniform nanoflower morphology. Therefore, the hydrothermal temperature is a key parameter affecting catalyst performance in the catalyst preparation method. The upper limit of the temperature should not exceed 80°C, and the lower limit should not be lower than 40°C; otherwise, it is impossible to synthesize a nickel foam-supported nanoflower structure. The nanoflower structure facilitates the exposure of catalytic active sites and the penetration of the electrolyte, thus enabling excellent catalytic performance.

Claims

1. An application of a NiCo-MOF anodic oxidation catalyst for matching cathode CO2 reduction, characterized in that, The preparation method of the catalyst includes the following steps: (1) Immerse the substrate in a 1-3 mol / L HCl solution and sonicate, then rinse with ethanol and deionized water; (2) Dissolve nickel salt and cobalt salt in methanol as solution A, and dissolve 2-methylimidazole in methanol as solution B; After stirring, solution B is quickly injected into solution A and stirring is continued for 5 to 10 minutes to form a blue turbid mixed solution; The molar ratio of the nickel salt to the cobalt salt is 1:1 to 4:1; the molar ratio of the 2-methylimidazole to the cobalt salt is 6:1 to 10:

1. (3) Transfer the above mixed solution to a polytetrafluoroethylene-lined stainless steel high-pressure hydrothermal reactor, add the pretreated substrate, place it in an oven, and keep it at 60-80°C for 4-24 hours; after cooling, wash the nickel foam with NiCo-MOF grown with deionized water and ethanol, and dry it to obtain the self-supporting NiCo-MOF catalyst. Using NiCo-MOF catalyst as the working electrode, Hg / HgO as the reference electrode, and carbon rod as the counter electrode, CV activation was performed in a standard three-electrode system. The voltage range relative to the reference electrode was 0–0.5 V, and the scan lasted for 10–30 min to complete the activation of the NiCo-MOF catalyst, which can then be directly applied to the electrocatalytic oxidation of small molecule alcohols. The small molecule alcohol is glycerol, methanol, ethanol, isopropanol, or glucose; When the molar ratio of nickel salt to cobalt salt is 2:1, the catalyst is used for the electrocatalytic oxidation of glycerol; When the molar ratio of nickel salt to cobalt salt is 3:1, the catalyst is used for the electrocatalytic oxidation of glucose.

2. The application of the NiCo-MOF anodic oxidation catalyst for matching cathode CO2 reduction according to claim 1, characterized in that: The substrate is made of nickel foam, copper foam, iron foam, titanium foam, carbon fiber paper, or carbon cloth.

3. The application of the NiCo-MOF anodic oxidation catalyst for matching cathode CO2 reduction according to claim 2, characterized in that: The catalyst has a nanoflower-like structure.

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