Mg-ni bimetallic single-atom catalyst for electrocatalytic reduction of carbon dioxide to proportionally adjustable synthesis gas, preparation method and application thereof

Abstract

The application relates to a preparation method and application of a Mg-Ni bimetallic monatomic catalyst. The Mg-Ni bimetallic monatomic catalyst is prepared through the following steps: firstly, synthesizing a precursor containing glucose, dicyandiamide and a metal salt, and then one-step calcination. The Mg-Ni bimetallic monatomic catalyst is simple and easy to prepare. The Mg-Ni bimetallic monatomic catalyst is applied to electrocatalytic reduction of carbon dioxide, and CO2 can be reduced to synthesis gas with adjustable proportion in a water-containing electrolyte, the Mg-Ni bimetallic monatomic catalyst exhibits excellent catalytic activity, good stability, strong practical application, and has a good application prospect in the field of electrocatalytic reduction of carbon dioxide.

Description

Technical Field

[0001] This invention belongs to the field of electrocatalytic conversion technology, specifically relating to a novel magnesium-nickel (Mg-Ni) bimetallic single-atom catalyst for electrocatalytic reduction of carbon dioxide to carbon monoxide to produce syngas with adjustable proportions, its preparation method, and its application. Background Technology

[0002] While the rapid consumption of fossil fuels has driven human societal prosperity, it has also led to a continuous rise in atmospheric carbon dioxide concentrations. Excessive CO2 emissions cause a series of environmental problems, such as global warming, ocean acidification, and glacial melting. To alleviate the energy and environmental crisis caused by the rapid depletion of fossil fuels and to advance the goal of carbon neutrality, utilizing renewable energy for power generation and electrocatalytically reducing CO2 to convert it into carbon-based fuels and high-value-added chemicals is considered a forward-looking and effective strategy.

[0003] However, achieving rapid conversion and high selectivity of the CO2 electroreduction reaction (eCO2RR) remains challenging due to the stability and inertness of the CO2 molecule and the unavoidable competitive hydrogen evolution reaction (HER). Over the past few decades, numerous electrocatalysts for eCO2RR have been developed, with the core objective of activating CO2 and inhibiting HER. However, combining eCO2RR and HER to synthesize a mixture of carbon monoxide (CO) and hydrogen (H2) into syngas is an attractive strategy because syngas is a key feedstock for the production of various chemical products: for example, dimethyl ether has an H2 / CO ratio of 1:1, methanol has an H2 / CO ratio of 2:1, and fermented biofuels have an H2 / CO ratio of 1:1–1:3.33. Since HER is accelerated at high operating potentials, producing syngas in a controllable ratio over a wide potential range is a considerable challenge. Therefore, developing electrocatalysts capable of producing high syngas yields with suitable, controllable H2 / CO ratios is both necessary and urgent.

[0004] Single-atom catalysts have attracted widespread attention due to their uniform active sites, low cost, and high reactivity and atom utilization. The synergistic effect between atoms in bimetallic single-atom catalysts can be used to modulate catalytic activity and selectivity. Therefore, this invention is proposed. Summary of the Invention

[0005] To address the problems existing in the prior art, this invention provides a novel Mg-Ni bimetallic atom catalyst for the electrocatalytic reduction of carbon dioxide (CO2) to produce syngas with adjustable proportions. This catalyst exhibits excellent electrocatalytic CO2 reduction performance, with its MgNi5-NC catalyst achieving a precise H2 / CO ratio (approximately 1:2) (-0.6 ~ -0.9 V vs. RHE). In MgNi3-NC, a wider range of H2 / CO ratios satisfying downstream processes (0.95-4.34, including 1:1-2.4:1) can be obtained.

[0006] The present invention also provides a method for preparing the above-mentioned Mg-Ni bimetallic single-atom catalyst for electrocatalytic reduction of carbon dioxide to produce syngas with adjustable proportions and its application.

[0007] To solve the above-mentioned technical problems, the present invention adopts the following technical solution: A method for preparing a novel Mg-Ni bimetallic single-atom catalyst for electrocatalytic reduction of carbon dioxide to syngas with adjustable proportions (see schematic diagram of the synthesis). Figure 1 It includes the following steps: 1) Disperse glucose, dicyandiamide (DICY), aqueous solutions of magnesium salt and nickel salt in deionized water to form a mixed solution A; 2) Heat and stir the mixed solution A at 60-90°C to evaporate the solvent. After the solvent has evaporated completely, a solid mixture B is obtained. 3) Carbonize the solid mixture B by calcining it in a tube furnace under an inert atmosphere at 800-1100°C for 1-3 h (the heating rate can be 1-5°C / min) to obtain solid C, which is the Mg-Ni bimetallic single-atom catalyst.

[0008] Specifically, in step 1), the magnesium salt is MgCl2, and the nickel salt is NiCl2·6H2O; the molar ratio of magnesium salt to nickel salt is 40-200:1. Alternatively, the concentration of the aqueous solutions of the magnesium salt and nickel salt can be 0.0125 mol / L. -1 The amount of magnesium salt aqueous solution added can be 1 mL, 2 mL, 3 mL, etc., and the amount of nickel salt aqueous solution added can be 0.01 mL, 0.03 mL, 0.05 mL, etc. Taking the amount of magnesium salt aqueous solution added as 2 mL and the amount of nickel salt aqueous solution added as 0 mL, 0.01 mL, 0.03 mL, 0.05 mL as examples, they are respectively denoted as Mg-NC, MgNi1-NC, MgNi3-NC, and MgNi5-NC.

[0009] Furthermore, in step 1), the mass ratio of glucose to magnesium salt is 90-120:1, and the mass ratio of glucose to dicyandiamide is 1:15-30.

[0010] Specifically, in step 3), the inert atmosphere refers to argon or nitrogen, etc.

[0011] This invention provides a Mg-Ni bimetallic single-atom catalyst prepared by the above method.

[0012] This invention also provides the application of the above-mentioned Mg-Ni bimetallic single-atom catalyst in the electrocatalytic reduction of carbon dioxide (CO2) to produce syngas with adjustable proportions, and the catalyst can be used as a working electrode.

[0013] Specifically, the above application involves dispersing the Mg-Ni bimetallic single-atom catalyst in a solvent, adding Nafion solution as a binder, and ultrasonically dispersing to obtain a uniform electrode dispersion; then coating 100–200 μL of the electrode dispersion onto a 0.5 × 1 cm⁻¹ plate. 2 The working electrode is obtained by drying on carbon paper at room temperature; constant voltage electrolysis is then performed in a closed three-electrode system. In further preferred applications, the three-electrode system uses a saturated calomel electrode as the reference electrode and a platinum wire or platinum mesh electrode as the counter electrode, performing constant voltage electrolysis in a 0.1-0.5 M KHCO3 solution. The voltage used for constant voltage electrolysis is -0.6 V to -1.0 V (vs. RHE). An H-type electrolytic cell is used, with the working electrode and reference electrode at one end and the counter electrode at the other end.

[0014] Furthermore, the solvent is composed of an alcohol and deionized water in a volume ratio of 1:3-5; the alcohol includes methanol, ethanol, or isopropanol, etc. Preferably, in the electrode dispersion, the concentration of the Mg-Ni bimetallic single-atom catalyst is 2-5 mg / mL; the mass concentration of the Nafion solution is 4-6%, and the volume ratio of the electrode dispersion to the Nafion solution is 9-13:1.

[0015] Compared with the prior art, the beneficial effects of the present invention are as follows: 1) This invention is the first to propose using glucose as a carbon source and dicyandiamide as a nitrogen source to synthesize a Mg-Ni bimetallic single-atom catalyst by high-temperature calcination. This catalyst is applied to the electrocatalytic reduction of CO2 to produce syngas with adjustable proportions. Furthermore, the development of magnesium, a light metal in the main group, as a catalyst for the electrocatalytic reduction of CO2 to produce syngas with adjustable proportions is also the first of its kind, providing a new approach to the preparation of catalysts for the electrocatalytic reduction of CO2. 2) The catalyst of this invention exhibits excellent electrocatalytic reduction performance of CO2. Among the synthesized catalysts, MgNi5-NC can achieve a precise H2 / CO ratio (approximately 1:2) (-0.6 ~ -0.9 V vs. RHE); MgNi3-NC can achieve a wider range of H2 / CO ratios (0.95-4.34, including 1:1-2.4:1) for downstream processes. 3) In addition, the MgNi3-NC catalyst of the present invention did not show a significant decrease in electrocatalytic selectivity and activity during constant potential electrolysis at -0.7 V (V vs. RHE) for 20 h, and the electrocatalytic reduction of CO2 could maintain the production of syngas with an H2 / CO ratio close to 1, indicating good stability. 4) The synthesis method of this catalyst is simple and easy to implement. After calcination, the synthesized catalyst does not require post-treatment processes such as acid washing. It has the potential to be applied to industrial production and has good application prospects in the field of electrocatalytic reduction of carbon dioxide. Attached Figure Description

[0016] Figure 1 This is a schematic diagram illustrating the synthesis of the Mg-Ni bimetallic single-atom catalyst of this invention; Figure 2 This is a Faraday efficiency diagram of the catalysts prepared in Examples 1(a), 2(b), 3(c) and 4(d) of the present invention for the catalytic reduction of carbon dioxide to syngas. Figure 3 This is a current density diagram of the electrocatalytic reduction of carbon dioxide to syngas by the catalysts prepared in Examples 1(a), 2(b), 3(c), and 4(d) of this invention; Figure 4 This is a proportion diagram of the electrocatalytic reduction of carbon dioxide to syngas prepared by the catalysts in Examples 1(a), 2(b), 3(c), and 4(d) of the present invention; Figure 5 This is a stability test diagram of Example 3 (MgNi3-NC) of the present invention; Figure 6 This refers to Example 3 of the present invention (MgNi3-NC), after electrolysis at -0.7 V for 20 h, the electrolyte... 1 H NMR spectrum; Figure 7 These are the XRD patterns of the catalysts prepared in Examples 1, 2, 3, and 4 of this invention; Figure 8These are X-ray photoelectron spectroscopy (XPS) structural characterization images of the catalysts prepared in Examples 1, 2, 3, and 4 of this invention, wherein a represents the full XPS spectrum of Mg-NC and MgNi3-NC; b represents the high-resolution N 1s spectrum of Mg-NC and MgNi3-NC; c represents the high-resolution Ni 2p spectrum of MgNi3-NC; d and e represent the high-resolution Mg1s and Mg2p spectra of Mg-NC and MgNi3-NC; f represents the full XPS spectrum of MgNi1-NC and MgNi5-NC; and g represents the high-resolution N 1s spectrum of MgNi1-NC and MgNi5-NC. Detailed Implementation

[0017] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to embodiments. The specific embodiments described herein are for illustrative purposes only and are not intended to limit the invention.

[0018] In the following examples, all raw materials used were commercially available products that could be purchased directly. For example, dicyandiamide was purchased from Shanghai Aladdin Biochemical Technology Co., Ltd.; Nafion solution (Nafion perfluorinated resin, 5% solution, containing 45% water) was purchased from Shanghai Dibai Biotechnology Co., Ltd.; and isopropanol was purchased from Tianjin Fuyu Fine Chemical Co., Ltd.

[0019] Room temperature refers to 25±5℃.

[0020] Example 1: Preparation of Mg-NC single-atom catalyst A method for preparing a Mg single-atom catalyst for electrocatalytic CO2 reduction to syngas includes the following steps: 1) Weigh 0.25 g of glucose, 5 g of dicyandiamide, and 2 mL of 0.0125 mol / L solution. -1 An aqueous solution of magnesium salt MgCl2 was added to 200 mL of deionized water and completely dissolved to form precursor mixed solution A. 2) Heat and stir the precursor mixture A at 80°C, evaporate the solvent deionized water to dryness, and after the solvent has evaporated, a white solid mixture B is obtained; 3) The white solid mixture B was calcined in a tube furnace under an inert argon atmosphere at 900℃ for 2 h (heating rate of 3℃ / min) to carbonize it, and then naturally cooled to room temperature to obtain solid C, which is the Mg single-atom catalyst, denoted as Mg-NC.

[0021] Example 2: Preparation of Mg-Ni bimetallic single-atom catalyst MgNi1-NC The specific experimental procedure is the same as in Example 1, except that in step 1), 0.01 mL of a 0.0125 mol / L solution is added. -1 An aqueous solution of the nickel salt NiCl2·6H2O. The resulting Mg-Ni bimetallic single-atom catalyst is denoted as MgNi1-NC.

[0022] Example 3: Preparation of Mg-Ni bimetallic single-atom catalyst MgNi3-NC The specific experimental procedure is the same as in Example 1, except that in step 1), 0.03 mL of a 0.0125 mol / L solution is added. -1 An aqueous solution of the nickel salt NiCl2·6H2O. The resulting Mg-Ni bimetallic single-atom catalyst is designated MgNi3-NC.

[0023] Example 4: Preparation of Mg-Ni bimetallic single-atom catalyst MgNi5-NC The specific experimental procedure is the same as in Example 1, except that in step 1), 0.05 mL of a 0.0125 mol / L solution is added. -1 An aqueous solution of the nickel salt NiCl2·6H2O. The resulting Mg-Ni bimetallic single-atom catalyst is designated MgNi5-NC.

[0024] Example 5 Accurately weigh 2 mg of the catalyst Mg-NC synthesized in Example 1, disperse it in a mixed solution of 200 μL of volatile solvent isopropanol and 700 μL of deionized water, then add 100 μL of Nafion solution (Shanghai Dibai Biotechnology Co., Ltd. Nafion perfluorinated resin, 5% solution, containing 45% water), and ultrasonically disperse for 30 min to obtain a homogeneous electrode dispersion. Take 150 μL of the above electrode dispersion and drop it onto a 0.5 × 1 cm plate. 2 The working electrode was prepared by drying the carbon paper (HCP020P, from HESEN) completely at room temperature.

[0025] Example 6 Accurately weigh 2 mg of the catalyst MgNi1-NC synthesized in Example 2 and disperse it in a mixed solution of 200 μL of volatile solvent isopropanol and 700 μL of deionized water. Then add 100 μL of Nafion solution (Shanghai Dibai Biotechnology Co., Ltd. Nafion perfluorinated resin, 5% solution, containing 45% water), and ultrasonically disperse for 30 min to obtain a homogeneous electrode dispersion. Take 150 μL of the above electrode dispersion and drop it onto a 0.5 × 1 cm plate. 2 The working electrode was prepared by drying the carbon paper (HCP020P, from HESEN) completely at room temperature.

[0026] Example 7 Accurately weigh 2 mg of the catalyst MgNi3-NC synthesized in Example 3, disperse it in a mixed solution of 200 μL of volatile solvent isopropanol and 700 μL of deionized water, then add 100 μL of Nafion solution (Shanghai Dibai Biotechnology Co., Ltd. Nafion perfluorinated resin, 5% solution, containing 45% water), and ultrasonically disperse for 30 min to obtain a homogeneous electrode dispersion. Take 150 μL of the above electrode dispersion and drop it onto a 0.5 × 1 cm plate. 2 The working electrode was prepared by drying the carbon paper (HCP020P, from HESEN) completely at room temperature.

[0027] Example 8 Accurately weigh 2 mg of the catalyst MgNi5-NC synthesized in Example 4, disperse it in a mixed solution of 200 μL of volatile solvent isopropanol and 700 μL of deionized water, then add 100 μL of Nafion solution (Shanghai Dibai Biotechnology Co., Ltd. Nafion perfluorinated resin, 5% solution, containing 45% water), and ultrasonically disperse for 30 min to obtain a homogeneous electrode dispersion. Take 150 μL of the above electrode dispersion and drop it onto a 0.5 × 1 cm plate. 2 The working electrode was prepared by drying the carbon paper (HCP020P, from HESEN) completely at room temperature.

[0028] Application Trial In a three-electrode system of an H-type electrolytic cell containing a 0.5 M KHCO3 aqueous solution, the electrodes prepared in Examples 5, 6, 7, and 8 were used as working electrodes, a saturated calomel electrode (SCCE) as a reference electrode, and a platinum mesh electrode as a counter electrode (working electrode and reference electrode at one end, counter electrode at the other end). Magnetic stirring was applied at a stirring speed of 500 rpm. -1 The electrolysis voltage range is -0.6 V to -1.0 V. The selectivity of the catalysts prepared in Examples 1, 2, 3, and 4 for catalytic reduction of CO2 to CO was tested.

[0029] Figure 2 The Faraday efficiency (ad) diagrams of the catalysts prepared in Examples 1, 2, 3 and 4 of this invention for the catalytic reduction of carbon dioxide are given. Figure 3 The current density diagrams (ad) of the catalysts prepared in Examples 1, 2, 3 and 4 of this invention for the electrocatalytic reduction of carbon dioxide to CO and H2 in electrolyte are given. Figure 4 A proportion chart (ad) of the electrocatalytic reduction of carbon dioxide to syngas by the catalysts prepared in Examples 1, 2, 3 and 4 of this invention is provided. Figure 5Stability test results for Example 3 (MgNi3-NC) of the present invention are shown.

[0030] Faraday efficiency (FE) was evaluated by potentiostatic measurements to quantitatively compare the activity of catalysts prepared in various embodiments for producing syngas. For example... Figure 2 As shown, based on Mg-NC, FE CO The Faraday efficiency (FE) decreases with increasing potential. Conversely, the increase in FE for H2 indicates poor selectivity of Mg for CO. Although the FE values ​​for CO and H2 vary, the total Faraday efficiency of the catalysts prepared in Examples 1, 2, 3, and 4 for the catalytic reduction of carbon dioxide to syngas is close to 100%, meaning there are no other products, demonstrating that the catalysts prepared in Examples 1, 2, 3, and 4 all exhibit good selectivity. Figure 3 As shown, after Ni species participate in the formation of the bimetallic catalyst, FE CO The increase with increasing CO current density indicates that, compared to HER, FE... CO It exhibits stronger selectivity for CO reduction. With increasing Ni species abundance and current density, FE... CO And it is gradually increasing.

[0031] Besides effectively catalyzing CO2 reduction, controlling the H2 / CO ratio is also crucial for practical applications. Over the entire applied potential window of -0.6 to -1.0 V (relative to RHE), the H2 / CO ratio of MgNi1-NC varies from 1.26 to 8.46, as shown below. Figure 4 As shown in b, for MgNi3-NC and MgNi5-NC, the H2 / CO ratio decreases with increasing Ni doping concentration, resulting in the generation of more CO (…). Figure 4 c and Figure 4 d) The H2 / CO ratio for syngas production catalyzed by MgNi3-NC is approximately 1 at -0.6 to -0.7 V (relative to RHE), and is commonly used in the synthesis of dimethyl ether; at -0.9 V, the H2 / CO ratio reaches approximately 2, and it is widely used in methanol synthesis and the Fischer-Tropsch reaction. In the wide electrochemical window of -0.6 to -1.0 V (relative to RHE) for MgNi5-NC, the H2 / CO ratio is between 0.51 and 0.79. More importantly, they are close to the desired syngas ratio in practical applications.

[0032] In practical applications, catalyst stability is crucial. In this invention, the catalyst MgNi3-NC prepared in Example 3 was subjected to constant-potential electrolysis for 20 h at a potential of -0.7 V vs. RHE. The current density did not decrease significantly, and more importantly, the H2 / CO ratio remained consistently around 1, suitable for dimethyl ether production (see...). Figure 5This indicates that the single-atom catalyst has high stability.

[0033] Figure 6 The catalyst MgNi3-NC prepared in Example 3 of this invention was electrolyzed at -0.7 V for 20 h, and the electrolyte was then subjected to... 1 ¹H NMR analysis revealed no other liquid products, indicating that the Mg-Ni bimetallic single-atom catalyst prepared in this invention possesses excellent selectivity in the preparation of syngas.

[0034] Figure 7 XRD patterns of the catalysts prepared in Examples 1, 2, 3, and 4 of this invention are given. Only one broad peak of approximately 26° exists corresponding to the (002) plane of the graphene array. Furthermore, the broad peak indicates that the disordered stacking of graphene nanosheets along the C-axis results in a rich porous structure, and no diffraction peaks of metal nanoparticles are observed, indicating that the metal species in the catalyst structure are uniformly dispersed and have not agglomerated.

[0035] This invention further employs X-ray photoelectron spectroscopy (XPS) to elucidate the surface composition and chemical state, such as... Figure 8 As shown. XPS measurements of the full spectrum show the coexistence of Mg, Ni, N, and C, indicating successful doping of Mg and Ni species. Figure 8 a and 8f). In the N 1s high-resolution spectrum, the four deconvolution peaks at 398.0, 400.6, 401.2, and 403.3 eV are attributed to pyridine N, pyrrole N, graphitic N, and oxide N, respectively. Figure 8 b and 8g). Furthermore, the appearance of the porphyrin-like moiety at 399.7 eV is attributed to the interaction between Mg-N and Ni-N, indicating that Mg and Ni atoms are coordinated with N on the carbon support. In the Ni 2p spectrum of MgNi3-NC ( Figure 8 c), Ni 2p centered at 855.5 eV 3 / 2 The peak is below 855.7 eV and above 853.0 eV, which means that the chemical valence of Ni is between 0 and +2. And... Figure 8 In d and 8e, the Mg 1s and 2p spectra clearly show the presence of Mg in the +2 oxidation state in the catalyst. According to the binding energy data, there are no positions for elemental Mg (Mg 1s at 1303-1303.5 eV, Mg 2p at 49.3-49.7 eV) or elemental Ni (853.0 eV), indicating that Mg and Ni are distributed in atomic form in the nitrogen-doped carbon support.

[0036] In summary, this invention provides a simple and easy Mg-Ni bimetallic single-atom catalyst by synthesizing a precursor containing glucose, dicyandiamide, and a metal salt, followed by one-step calcination. The catalyst requires no post-treatment processes such as acid washing after calcination, exhibits excellent catalytic performance and good stability, and has the potential for industrial application, demonstrating promising prospects.

[0037] The above are merely preferred embodiments of the present invention, but the present invention is not limited to the specific implementation schemes described above. All equivalent changes and modifications made in accordance with the scope of the patent application of the present invention shall fall within the scope of the present invention.

Claims

1. A method for preparing a Mg-Ni bimetallic single-atom catalyst for electrocatalytic reduction of carbon dioxide to syngas with adjustable proportions, characterized in that, Includes the following steps: 1) Disperse glucose, dicyandiamide, aqueous solutions of magnesium and nickel salts in deionized water to form a mixed solution A; 2) Heat the mixed solution A to evaporate the solvent. After the solvent has evaporated completely, a solid mixture B is obtained. 3) Carbonize the solid mixture B by calcining it at 800-1100°C for 1-3 h under an inert atmosphere to obtain solid C, which is the Mg-Ni bimetallic single-atom catalyst.

2. The preparation method of the Mg-Ni bimetallic single-atom catalyst according to claim 1, characterized in that, In step 1), the magnesium salt is MgCl2 and the nickel salt is NiCl2·6H2O; the molar ratio of the magnesium salt to the nickel salt is 40-200:

1.

3. The preparation method of the Mg-Ni bimetallic single-atom catalyst according to claim 1, characterized in that, In step 1), the mass ratio of glucose to magnesium salt is 90-120:1, and the mass ratio of glucose to dicyandiamide is 1:15-30.

4. The preparation method of the Mg-Ni bimetallic single-atom catalyst according to claim 1, characterized in that, In step 3), the inert atmosphere refers to argon or nitrogen.

5. The Mg-Ni bimetallic single-atom catalyst prepared by any one of the methods described in claims 1 to 4.

6. The application of the Mg-Ni bimetallic single-atom catalyst of claim 5 in the electrocatalytic reduction of CO2 to produce syngas with adjustable proportion.

7. The application according to claim 6, characterized in that, The Mg-Ni bimetallic single-atom catalyst was dispersed in a solvent, and Nafion solution was added as a binder. The dispersion was ultrasonically dispersed to obtain a uniform electrode dispersion. The electrode dispersion was coated on carbon paper and dried at room temperature to obtain the working electrode. Constant voltage electrolysis was performed in a closed three-electrode system.

8. The application according to claim 7, characterized in that, The solvent is composed of an alcohol and deionized water in a volume ratio of 1:3-5; the alcohol includes methanol, ethanol or isopropanol.

9. The application according to claim 7, characterized in that, In the electrode dispersion, the concentration of the Mg-Ni bimetallic single-atom catalyst is 2-5 mg / mL; the mass concentration of the Nafion solution is 4-6%, and the volume ratio of the electrode dispersion to the Nafion solution is 9-13:

1.

10. The application according to claim 7, characterized in that, The three-electrode system uses a saturated calomel electrode as the reference electrode and a platinum wire or platinum mesh electrode as the counter electrode. Constant voltage electrolysis is performed in a 0.1-0.5 M KHCO3 solution, with the voltage used for constant voltage electrolysis ranging from -0.6 V to -1.0 V.

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