A composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide and a preparation method thereof

Abstract

The present invention discloses a composite-coated cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide and a method for preparing the same, belonging to the field of molten salt electrochemical carbonization technology. The composite-coated cathode comprises a three-layer structure: from the inside out, a stainless steel substrate layer, a chromium metal diffusion barrier layer, and a chromium carbide functional catalytic layer. When used in a molten salt electrolysis system for carbon dioxide, the composite-coated cathode effectively protects the cathode from oxidative corrosion and prevents metallic impurity contamination of the cathode carbon material.

Description

Technical Field

[0001] The present invention belongs to the technical field of molten salt electrochemical negative carbonization, and specifically relates to a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide and a preparation method thereof. Background Art

[0002] Molten salt electrolysis technology has attracted widespread attention for its ability to capture carbon dioxide at high throughput and convert it into high-value carbon materials. Molten salt electrolysis typically uses molten carbonate as the electrolyte and conductive metal materials as the cathode and anode. During electrolysis, carbon dioxide is reduced to elemental carbon at the cathode-metal interface, while oxygen is released at the anode. Reduction-active carbon atoms in the cathode react with certain metal elements to form intermetallic compounds, such as metal carbides with Ni, Fe, Mo, and W. These metal carbides are relatively fluffy, and when the carbon material is removed from the cathode, the molten salt and the carbon material are stripped away together, causing metallic impurities to contaminate the high-value carbon material. Furthermore, the operating temperature of molten salt electrolysis typically ranges from 350-800°C. Conventional metal-based cathode materials are susceptible to oxidative corrosion (even when electrolysis is stopped). Long-term oxidative corrosion significantly impacts the cathode's service life. Furthermore, the metal oxides formed by this corrosion dissolve to a certain extent and enter the molten salt electrolyte, where they co-deposit with carbon dioxide during electrolysis, causing metallic contamination. Therefore, the development of inert, corrosion-resistant cathodes is crucial for high-temperature molten salt CO2 electrolysis. Summary of the Invention

[0003] To address the aforementioned issues in the prior art, the present invention provides a composite-coated cathode for use in molten salt electrolysis of carbon dioxide to produce a carbon material system, and a method for preparing the same. When used in a molten salt electrolysis system, the composite-coated cathode effectively protects the cathode from oxidative corrosion and prevents metallic impurity contamination of the cathode carbon material.

[0004] To achieve the above object, the present invention provides the following technical solutions:

[0005] A composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide. The composite coating cathode consists of a three-layer structure, which includes, from the inside to the outside, a stainless steel substrate layer, a chromium metal diffusion barrier layer, and a chromium carbide functional catalyst layer.

[0006] Furthermore, the thickness of the stainless steel substrate layer is 1-20 mm, and the stainless steel substrate is 304 stainless steel.

[0007] Furthermore, the chromium metal diffusion barrier layer has a thickness of 1-100 μm.

[0008] Furthermore, the thickness of the chromium carbide functional catalytic layer is 2-30 μm.

[0009] The present invention also provides a method for preparing a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide, comprising the following steps:

[0010] The oxide film on the surface of the stainless steel substrate layer is removed by sandblasting, and the surface of the stainless steel substrate layer is electroplated with a chromium electroplating solution to obtain a chromium metal diffusion barrier layer, which is then placed in a molten ternary carbonate electrolyte system and electrolyzed using a molten salt electrochemical carburizing technology to obtain a chromium carbide functional catalytic layer on the surface of the chromium metal diffusion barrier layer. After the electrolytic carburizing is completed, the sample is slowly removed from the molten salt under argon protection, cooled to room temperature, and then washed with deionized water to obtain a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0011] Furthermore, the steps also include that the obtained chromium metal diffusion barrier layer needs to be pre-oxidized by air, and the surface of the chromium metal diffusion barrier layer is oxidized before being placed in a molten ternary carbonate electrolyte system.

[0012] Furthermore, the molten ternary carbonate electrolyte is composed of three inorganic salts: Li2CO3, Na2CO3 and K2CO3.

[0013] Furthermore, the electroplating parameters are: electroplating solution temperature is 30-70°C, chromic anhydride concentration is 10-200g / L, current density is 10-200mA / cm 2 , electroplating time 1-200min.

[0014] Furthermore, the electrolysis temperature is 450-800°C, and the current density is 1-100 mA / cm 2 , the electrolysis time is 5-300min.

[0015] The present invention also provides an application of a composite coating cathode for preparing a carbon material system by electrolyzing carbon dioxide in molten salt in the preparation of carbon materials.

[0016] Compared with the prior art, the present invention has the following beneficial effects:

[0017] 1) Compared with traditional metal / alloy cathodes, the composite coating cathode prepared by the present invention has the characteristics of high temperature resistance and corrosion resistance, and can still protect the cathode substrate from oxidation corrosion in the electrolysis gap, greatly improving the service life of the cathode;

[0018] 2) The composite coating cathode prepared by the present invention completely covers the alloy substrate with a dense chromium carbide functional catalytic layer, preventing the formation of fluffy metal carbides between the base metal elements and the activated carbon atoms, thereby solving the problem of metal impurity contamination of high-value carbon materials;

[0019] 3) The composite coating cathode prepared by the present invention has an interlayer comprising a chromium metal diffusion barrier layer, which effectively prevents the migration of base metal elements into the chromium carbide coating, thereby greatly improving the stability of the electrode;

[0020] 4) The composite coating cathode prepared by the present invention has a self-repairing function. When the surface chromium carbide functional catalytic layer is damaged, the intermediate chromium metal diffusion barrier layer reacts with the activated carbon atoms to generate chromium carbide, which repairs and fills the micro defects of the coating, thereby achieving self-repair of the coating. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] In order to more clearly illustrate the embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the embodiments. Obviously, the drawings described below are only some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0022] Figure 1 Schematic diagram of the composite coating cathode prepared in Example 1 of the present invention. DETAILED DESCRIPTION

[0023] Various exemplary embodiments of the present invention will now be described in detail. This detailed description should not be considered as limiting the present invention, but rather as a more detailed description of certain aspects, features, and embodiments of the present invention.

[0024] It should be understood that the terms described herein are intended only to describe particular embodiments and are not intended to limit the present invention. In addition, for numerical ranges herein, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each smaller range between any intermediate value within a stated value or stated range and any other stated value or intermediate value within the stated range is also encompassed by the present invention. The upper and lower limits of these smaller ranges may be independently included or excluded within the scope.

[0025] Unless otherwise indicated, all technical and scientific terms used herein have the same meaning as commonly understood by those skilled in the art. Although only preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein may also be used in the practice or testing of the present invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials associated with the documents. In the event of any conflict with any incorporated document, the contents of this specification shall prevail.

[0026] It will be apparent to those skilled in the art that various modifications and variations may be made to the specific embodiments of the present invention without departing from the scope or spirit of the invention. Other embodiments will be apparent to those skilled in the art from the present invention. The present description and examples are intended to be illustrative only.

[0027] The words “include,” “including,” “have,” “contain,” etc. used in this document are open-ended terms, meaning including but not limited to.

[0028] The raw materials used in the examples of the present invention were purchased from the Aladdin reagent platform, wherein the purity of Li2CO3, Na2CO3 and K2CO3 was 99.9%, the purity of chromic anhydride was 99.9%, and the Fe, Ni and Cr contents in 304 stainless steel were 70%, 11% and 19% respectively.

[0029] The present invention proposes a composite coating cathode for molten salt electrolysis of carbon dioxide. The cathode consists of a stainless steel base layer, a chromium metal diffusion barrier layer, and a chromium carbide functional catalytic layer. Chromium carbide exhibits excellent high-temperature oxidation resistance, protecting the base metal from corrosion and extending the cathode's service life. Furthermore, chromium carbide itself is a carbide and will not further react with activated carbon atoms, thereby ensuring the purity of the carbon produced by cathode electrolysis. The densely grown chromium carbide layer, which strongly bonds to the base, does not separate from the carbon material during electrolytic preparation, preventing contamination of the carbon material by Cr metal impurities. Furthermore, the intermediate chromium metal diffusion barrier layer also plays an important role. Metallic elements such as Fe and Ni in the base alloy diffuse very slowly through the chromium metal intermediate layer, acting as a diffusion barrier to prevent the base metal from diffusing and migrating toward the coating interface. Furthermore, when the surface chromium carbide functional catalytic layer is damaged, the chromium metal diffusion barrier layer is exposed. The chromium metal then reacts with the activated carbon atoms generated at the cathode to form chromium carbide, which repairs micro-cracks in the coating, thereby achieving self-healing of the coating. Compared with conventional stainless steel cathodes, the composite coating cathode proposed in the present invention can effectively prevent the formation of metal carbides between activated carbon atoms and metal elements, and has excellent corrosion resistance, avoiding cathode corrosion caused by terminating electrolysis during use; moreover, the preparation method of the composite coating cathode is simple, and the raw materials are cheap, which is conducive to the industrial application of molten salt electrolysis to convert carbon dioxide technology.

[0030] The specific method is as follows:

[0031] A composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide. The composite coating cathode consists of a three-layer structure, which includes, from the inside to the outside, a stainless steel substrate layer, a chromium metal diffusion barrier layer, and a chromium carbide functional catalyst layer.

[0032] In some preferred embodiments, the thickness of the stainless steel substrate layer is 1-20 mm, and the stainless steel substrate is 304 stainless steel.

[0033] In some preferred embodiments, the chromium metal diffusion barrier layer has a thickness of 1-100 μm.

[0034] In some preferred embodiments, the thickness of the chromium carbide functional catalytic layer is 2-30 μm.

[0035] The present invention also provides a method for preparing a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide, comprising the following steps:

[0036] The oxide film on the surface of the stainless steel substrate layer is removed by sandblasting, and the surface of the stainless steel substrate layer is electroplated with a chromium electroplating solution to obtain a chromium metal diffusion barrier layer, which is then placed in a molten ternary carbonate electrolyte system and electrolyzed using a molten salt electrochemical carburizing technology to obtain a chromium carbide functional catalytic layer on the surface of the chromium metal diffusion barrier layer. After the electrolytic carburizing is completed, the sample is slowly removed from the molten salt under argon protection, cooled to room temperature, and then washed with deionized water to obtain a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0037] In some preferred embodiments, the step further includes pre-oxidizing the chromium metal diffusion barrier layer with air, oxidizing the surface of the chromium metal diffusion barrier layer before placing it in a molten ternary carbonate electrolyte system. Specifically, the pre-oxidation treatment involves placing the chromium-plated stainless steel in a muffle furnace and oxidizing it at 500-800°C for 1-20 hours.

[0038] In some preferred embodiments, the molten ternary carbonate electrolyte is composed of three inorganic salts: Li2CO3, Na2CO3, and K2CO3, with a molar ratio of Li2CO3, Na2CO3, and K2CO3 being 1:1:1.

[0039] In some preferred embodiments, the electroplating parameters are: the solution temperature in the electroplating solution is 50°C, the chromic acid anhydride concentration is 120g / L, and the current density is 100mA / cm 2 , the plating time depends on the thickness of the coating.

[0040] In some preferred embodiments, the electrolysis process adopts a constant current electrolysis method, the electrolysis temperature is 450-800 ° C, the current density is 1-100 mA / cm 2 , the electrolysis time is 5-300min.

[0041] The present invention also provides an application of a composite coating cathode for preparing a carbon material system by electrolyzing carbon dioxide in molten salt in the preparation of carbon materials.

[0042] In the following examples, the main chemical components of the 304 stainless steel plate used are: Fe, Ni, and Cr, with contents of 70%, 11%, and 19%, respectively.

[0043] In the following examples, the chromium electroplating solution used was chromic anhydride solution with a concentration of 120 g / L, an electroplating temperature of 50°C, and a current density of 100 mA / cm 2 .

[0044] Example 1

[0045] A 10mm thick 304 stainless steel plate was used as the substrate. The oxide film on the substrate surface was removed by sandblasting. The substrate was then electroplated in a chromium electroplating solution for 120 minutes to obtain a 100μm thick chromium metal diffusion barrier layer.

[0046] The stainless steel substrate containing a chromium diffusion barrier layer was pre-oxidized in air at high temperature (pre-oxidized in a muffle furnace at 600 °C for 3 h) and then placed as a cathode in a molten ternary carbonate system (Li2CO3:Na2CO3:K2CO3 molar ratio of 1:1:1) at 500 °C at a current of 70 mA / cm 2 Electrolysis was performed at a current density of 100 μm for 30 minutes to obtain a composite coating cathode with a base layer of 304 stainless steel, an intermediate layer of a chromium metal diffusion barrier, and a surface layer of a chromium carbide functional catalytic layer. The thickness of the chromium metal diffusion barrier layer was 80 μm, and the thickness of the chromium carbide functional catalytic layer was 25 μm. After the electrolytic carburization was completed, the sample was slowly removed from the molten salt under argon protection, cooled to room temperature, washed with deionized water, and dried to obtain a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0047] Figure 1 The three-layer composite structure of the composite coating cathode prepared in this embodiment shows that the surface chromium carbide grows densely and has a strong bonding force with the substrate.

[0048] The composite coating electrode was placed as a cathode in a ternary carbonate electrolyte at 550 °C and charged at 200 mA / cm 2 The carbon material was prepared by electrolysis at a current density of 10 hours, and the metal content of the carbon material stripped from the cathode was less than 5ppm. After 200 electrolysis cycles, a cumulative electrolysis time of 2000 hours, there was no corrosion on the cathode surface, and the metal content of the carbon material obtained from each electrolysis was consistently less than 5ppm.

[0049] Comparative Example 1

[0050] Same as Example 1, except that bare stainless steel is used as the cathode material, specifically:

[0051] A 10mm thick 304 stainless steel cathode was placed in molten ternary carbonate for electrolysis for 10 hours to prepare carbon material. After the electrolysis was completed, the carbon material collected by cathode stripping contained 120ppm of metal impurities such as Fe and Ni. After 200 electrolysis cycles, the cumulative electrolysis time reached 2000h. As the number of electrolysis cycles increased, obvious corrosion pits appeared on the cathode surface. As the number of electrolysis cycles increased, the content of metal elements in the electrolytic carbon always showed an upward trend. The metal element content of the carbon material obtained by the 200th electrolysis was higher than 500ppm.

[0052] Example 2

[0053] A 15mm thick 304 stainless steel plate was used as the substrate. The oxide film on the substrate surface was removed by sandblasting. The substrate was then electroplated in a chromium electroplating solution for 100 minutes to obtain a chromium metal diffusion barrier layer (chromium metal intermediate layer) with a thickness of 80μm.

[0054] The stainless steel substrate containing a chromium metal diffusion barrier layer was pre-oxidized in air at high temperature (pre-oxidized in a muffle furnace at 600 °C for 3 h) and then placed as a cathode in a molten ternary carbonate system (Li2CO3:Na2CO3:K2CO3 molar ratio of 1:1:1) at 500 °C at a current of 70 mA / cm 2 Electrolysis was carried out at a current density of 100 μm for 30 minutes to obtain a composite coating cathode with a base layer of 304 stainless steel, an intermediate layer of a chromium metal diffusion barrier, and a surface layer of a chromium carbide functional catalytic layer (chromium carbide surface layer). The thickness of the chromium metal diffusion barrier layer was 60 μm, and the thickness of the chromium carbide functional catalytic layer was 25 μm. The surface chromium carbide grew densely and had a strong bond with the base. After the electrolytic carburization was completed, the sample was slowly removed from the molten salt under argon protection, cooled to room temperature, and washed with deionized water. This yielded a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0055] The composite coating electrode was placed as a cathode in molten ternary carbonate at a temperature of 550 ° C and a current density of 100 mA / cm 2 Carbon materials were prepared by electrolysis under the conditions of 10 hours. The metal content of the carbon materials stripped from the cathode was less than 5ppm. After 200 electrolysis cycles, a cumulative electrolysis time of 1000 hours, there was no corrosion on the cathode surface. The metal content of the carbon materials obtained by each electrolysis was consistently less than 5ppm.

[0056] Comparative Example 2

[0057] Same as Example 2, except that no pre-oxidation treatment is performed, specifically:

[0058] A 15mm thick 304 stainless steel plate was used as the substrate. The oxide film on the substrate surface was removed by sandblasting. The substrate was then electroplated in a chromium electroplating solution for 100 minutes to obtain a chromium metal diffusion barrier layer (chromium metal intermediate layer) with a thickness of 80μm.

[0059] The stainless steel substrate containing the chromium metal diffusion barrier layer was placed as the cathode in a molten ternary carbonate system (Li2CO3:Na2CO3:K2CO3 molar ratio of 1:1:1) at 500 °C and the cathode was charged at 60 mA / cm 2 Electrolysis was carried out at a current density of 100 μm for 30 minutes to obtain a composite coating cathode with a base layer of 304 stainless steel, an intermediate layer of a chromium metal diffusion barrier, and a surface layer of a chromium carbide functional catalytic layer (chromium carbide surface layer). The thickness of the chromium metal diffusion barrier layer was 65 μm, and the thickness of the chromium carbide functional catalytic layer was 20 μm. After the electrolytic carburization was completed, the sample was slowly removed from the molten salt under argon protection, cooled to room temperature, and then washed with deionized water to obtain a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0060] Due to the lack of pre-oxidation process, the chromium carbide coating of the electrode has poor growth density and weak bonding with the substrate.

[0061] The composite coating electrode was placed as a cathode in molten ternary carbonate at a temperature of 550 ° C and a current density of 100 mA / cm 2 Carbon material was prepared by electrolysis under the conditions of 10 hours. The metal content of the carbon material stripped from the cathode was higher than 100ppm. After 200 electrolysis cycles, with a cumulative electrolysis time of 2000 hours, the chromium carbide layer on the cathode surface partially fell off. The metal content of the carbon material obtained by each electrolysis was consistently higher than 100ppm.

[0062] Example 3

[0063] A 15mm thick 304 stainless steel plate was used as the substrate. The oxide film on the substrate surface was removed by sandblasting. The substrate was then electroplated in a chromium electroplating solution for 110 minutes to obtain a 90μm thick chromium metal diffusion barrier layer (chromium metal intermediate layer).

[0064] The stainless steel substrate containing a chromium metal diffusion barrier layer was pre-oxidized in air at high temperature (pre-oxidized in a muffle furnace at 600 °C for 3 h) and then placed as a cathode in a molten ternary carbonate system (Li2CO3:Na2CO3:K2CO3 molar ratio of 1:1:1) at 550 °C at a current of 70 mA / cm 2Electrolysis was carried out at a current density of 100 μm for 30 minutes to obtain a composite coating cathode with a base layer of 304 stainless steel, an intermediate layer of a chromium metal diffusion barrier, and a surface layer of a chromium carbide functional catalytic layer (chromium carbide surface layer). The thickness of the chromium metal diffusion barrier layer was 70 μm, and the thickness of the chromium carbide functional catalytic layer was 25 μm. The surface chromium carbide grew densely and had a strong bond with the base. After the electrolytic carburization was completed, the sample was slowly removed from the molten salt under argon protection, cooled to room temperature, and washed with deionized water. This yielded a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0065] The composite coating electrode was placed as a cathode in molten ternary carbonate at a temperature of 550 ° C and a current density of 100 mA / cm 2 Carbon materials were prepared by electrolysis under the conditions of 10 hours. The metal content of the carbon materials stripped from the cathode was less than 5ppm. After 200 electrolysis cycles, with a cumulative electrolysis time of 2000 hours, there was no corrosion on the cathode surface, and the metal content of the carbon materials obtained from each electrolysis was consistently less than 5ppm.

[0066] Comparative Example 3

[0067] Same as Example 3, except that the electrolysis time is 0.1 h, and the thickness of the chromium carbide functional catalyst layer is adjusted as follows:

[0068] A 15mm thick 304 stainless steel plate was used as the substrate. The oxide film on the substrate surface was removed by sandblasting. The substrate was then electroplated in a chromium electroplating solution for 100 minutes to obtain a 90μm thick chromium metal diffusion barrier layer (chromium metal intermediate layer).

[0069] The stainless steel substrate containing a chromium metal diffusion barrier layer was pre-oxidized in air at high temperature (pre-oxidized in a muffle furnace at 600 °C for 3 h) and then placed as a cathode in a molten ternary carbonate system (Li2CO3:Na2CO3:K2CO3 molar ratio of 1:1:1) at 500 °C at a current of 70 mA / cm 2 Electrolysis was carried out at a current density of 100 μm for 1 minute to obtain a composite coating cathode with a base layer of 304 stainless steel, an intermediate layer of a chromium metal diffusion barrier, and a surface layer of a chromium carbide functional catalytic layer (chromium carbide surface layer). The thickness of the chromium metal diffusion barrier layer was 89 μm, and the thickness of the chromium carbide functional catalytic layer was 0.5 μm. After the electrolytic carburization was completed, the sample was slowly removed from the molten salt under argon protection, cooled to room temperature, and washed with deionized water to obtain a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0070] The composite coating electrode was placed as a cathode in molten ternary carbonate at a temperature of 550 ° C and a current density of 100 mA / cm 2Carbon material was prepared by electrolysis for 10 hours under the conditions of , and the metal content of the carbon material peeled off from the cathode was higher than 70ppm. After 200 electrolysis cycles, with a cumulative electrolysis time of 2000 hours, the chromium carbide layer on the cathode surface fell off significantly, and the metallic chromium content in the carbon material produced by electrolysis reached 70ppm. This is because the initially prepared chromium carbide coating is too thin and easily consumed over a large area after multiple electrolysis-carbon separation processes. The subsequent chromium carbide produced in the chromium intermediate layer is relatively fluffy and easily peeled off along with the carbon material, causing metal contamination.

[0071] Example 4

[0072] A 13mm thick 304 stainless steel plate was used as the substrate. The oxide film on the substrate surface was removed by sandblasting. The substrate was then electroplated in a chromium electroplating solution for 90 minutes to obtain a chromium metal diffusion barrier layer (chromium metal intermediate layer) with a thickness of 85μm.

[0073] The stainless steel substrate containing a chromium diffusion barrier layer was pre-oxidized in air at high temperature (pre-oxidized in a muffle furnace at 650 °C for 2 h) and then placed as a cathode in a molten ternary carbonate system (Li2CO3:Na2CO3:K2CO3 molar ratio of 1:1:1) at 500 °C at a current of 70 mA / cm 2 Electrolysis was carried out at a current density of 100 μm for 30 minutes to obtain a composite coating cathode with a base layer of 304 stainless steel, an intermediate layer of a chromium metal diffusion barrier, and a surface layer of a chromium carbide functional catalytic layer (chromium carbide surface layer). The thickness of the chromium metal diffusion barrier layer was 60 μm, and the thickness of the chromium carbide functional catalytic layer was 25 μm. The surface chromium carbide grew densely and had a strong bond with the base. After the electrolytic carburization was completed, the sample was slowly removed from the molten salt under argon protection, cooled to room temperature, and washed with deionized water. This yielded a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0074] The composite coating electrode was placed as a cathode in molten ternary carbonate at a temperature of 500 °C and a current density of 200 mA / cm 2 Carbon materials were prepared by electrolysis under the conditions of 10 hours. The metal content of the carbon materials stripped from the cathode was less than 5ppm. After 200 electrolysis cycles, with a cumulative electrolysis time of 2000 hours, there was no corrosion on the cathode surface, and the metal content of the carbon materials obtained from each electrolysis was consistently less than 5ppm.

[0075] Comparative Example 4

[0076] Same as Example 4, except that the thickness of the chromium metal diffusion barrier layer is adjusted to 0.5 μm, specifically:

[0077] A 13mm thick 304 stainless steel plate was used as the substrate. The oxide film on the substrate surface was removed by sandblasting. The substrate was then electroplated in a chromium electroplating solution for 1 minute to obtain a 0.5μm thick chromium metal diffusion barrier layer (chromium metal intermediate layer).

[0078] The stainless steel substrate containing the chromium diffusion barrier layer was pre-oxidized in air at high temperature (pre-oxidized in a muffle furnace at 650 °C for 2 h) and then placed as a cathode in a molten ternary carbonate system (Li2CO3:Na2CO3:K2CO3 molar ratio of 1:1:1) at 500 °C at a current of 50 mA / cm 2 Electrolysis was carried out at a current density of 100 nm for 10 minutes, resulting in a composite coating cathode with a base layer of 304 stainless steel and a surface layer of chromium carbide functional catalytic layer (chromium carbide surface layer). The chromium metal diffusion barrier layer was too thin and was completely converted to chromium carbide during pre-carbonization. The thickness of the chromium carbide functional catalytic layer was 1 μm. After the electrolytic carburization was completed, the sample was slowly removed from the molten salt under argon protection, cooled to room temperature, and washed with deionized water to obtain a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0079] The composite coating electrode was placed as a cathode in molten ternary carbonate at a temperature of 500 °C and a current density of 200 mA / cm 2 Under the conditions of 10 hours of electrolysis to prepare carbon materials, the metal content of the carbon materials peeled off from the cathode exceeded 50ppm. After 200 electrolysis cycles, a cumulative electrolysis time of 2000 hours, the chromium carbide on the cathode surface was consumed and detached. Without the chromium metal intermediate layer to continue to generate chromium carbide to repair the surface coating, the 304 stainless steel substrate was directly exposed, resulting in the metal impurity content of the carbon materials produced at the cathode reaching 100ppm.

[0080] Example 5

[0081] A 13mm thick 304 stainless steel plate was used as the substrate. The oxide film on the substrate surface was removed by sandblasting. The substrate was then electroplated in a chromium electroplating solution for 90 minutes to obtain a chromium metal diffusion barrier layer (chromium metal intermediate layer) with a thickness of 85μm.

[0082] The stainless steel substrate containing a chromium diffusion barrier layer was pre-oxidized in air at high temperature (pre-oxidized in a muffle furnace at 650 °C for 2 h) and then placed as a cathode in a molten ternary carbonate system (Li2CO3:Na2CO3:K2CO3 molar ratio of 1:1:1) at 800 °C at a current of 70 mA / cm 2The current density was lowered at 800°C for 50 minutes (the carburizing current density was lower at 800°C, so a longer carburizing time was required to prepare a chromium carbide layer of the same thickness). A composite coating cathode was obtained with a base layer of 304 stainless steel, an intermediate layer of a chromium metal diffusion barrier layer, and a surface layer of a chromium carbide functional catalytic layer (chromium carbide surface layer). The thickness of the chromium metal diffusion barrier layer was 60 μm, and the thickness of the chromium carbide functional catalytic layer was 25 μm. The surface chromium carbide grew densely and had a strong bonding force with the substrate. After the electrolytic carburizing was completed, the sample was slowly removed from the molten salt under argon protection, cooled to room temperature, and washed with deionized water to obtain a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0083] The composite coating electrode was placed as a cathode in molten ternary carbonate at a temperature of 500 °C and a current density of 200 mA / cm 2 Carbon materials were prepared by electrolysis under the conditions of 10 hours. The metal content of the carbon materials stripped from the cathode was less than 5ppm. After 200 electrolysis cycles, with a cumulative electrolysis time of 2000 hours, there was no corrosion on the cathode surface, and the metal content of the carbon materials obtained from each electrolysis was consistently less than 5ppm.

[0084] Example 6

[0085] A 13mm thick 304 stainless steel plate was used as the substrate. The oxide film on the substrate surface was removed by sandblasting. The substrate was then electroplated in a chromium electroplating solution for 90 minutes to obtain a chromium metal diffusion barrier layer (chromium metal intermediate layer) with a thickness of 85μm.

[0086] The stainless steel substrate containing a chromium diffusion barrier layer was pre-oxidized in air at high temperature (pre-oxidized in a muffle furnace at 650 °C for 2 h) and then placed as a cathode in a molten ternary carbonate system (Li2CO3:Na2CO3:K2CO3 molar ratio of 1:1:1) at 500 °C at a current of 100 mA / cm 2 Electrolysis was carried out at a current density of 100 μm for 20 minutes to obtain a composite coating cathode with a base layer of 304 stainless steel, an intermediate layer of a chromium metal diffusion barrier, and a surface layer of a chromium carbide functional catalytic layer (chromium carbide surface layer). The thickness of the chromium metal diffusion barrier layer was 60 μm, and the thickness of the chromium carbide functional catalytic layer was 25 μm. The surface chromium carbide grew densely and had a strong bond with the base. After the electrolytic carburization was completed, the sample was slowly removed from the molten salt under argon protection, cooled to room temperature, and rinsed with deionized water to obtain a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

[0087] The composite coating electrode was placed as a cathode in molten ternary carbonate at a temperature of 500 °C and a current density of 200 mA / cm 2Carbon materials were prepared by electrolysis under the conditions of 10 hours. The metal content of the carbon materials stripped from the cathode was less than 5ppm. After 200 electrolysis cycles, with a cumulative electrolysis time of 2000 hours, there was no corrosion on the cathode surface, and the metal content of the carbon materials obtained from each electrolysis was consistently less than 5ppm.

[0088] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions and improvements made within the spirit and principles of the present invention should be included in the scope of protection of the present invention.

Claims

1. A composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide, characterized in that: The composite coating cathode is composed of a three-layer structure, which is composed of a stainless steel base layer, a chromium metal diffusion barrier layer, and a chromium carbide functional catalyst layer from the inside to the outside. The thickness of the chromium metal diffusion barrier layer is 1-100 μm; the thickness of the chromium carbide functional catalyst layer is 2-30 μm. The method for preparing a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide comprises the following steps: removing an oxide film on the surface of a stainless steel base layer by sandblasting, electroplating the surface of the stainless steel base layer with a chromium electroplating solution to obtain a chromium metal diffusion barrier layer, then placing the cathode in a molten ternary carbonate electrolyte system and electrolyzing the cathode using a molten salt electrochemical carburizing technique to obtain a chromium carbide functional catalytic layer on the surface of the chromium metal diffusion barrier layer; after the electrolytic carburizing is completed, slowly removing the sample from the molten salt under argon protection, cooling it to room temperature, and then washing it with deionized water to obtain the composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide.

2. The composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide according to claim 1, characterized in that: The thickness of the stainless steel substrate layer is 1-20 mm, and the stainless steel substrate is 304 stainless steel.

3. A method for preparing a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide according to any one of claims 1 to 2, characterized in that: The following steps are involved: The oxide film on the surface of the stainless steel substrate layer is removed, and the surface of the stainless steel substrate layer is electroplated with a hexavalent chromium electroplating solution to obtain a chromium metal diffusion barrier layer. The layer is then placed in a molten ternary carbonate electrolyte system and electrolyzed using a molten salt electrochemical carburizing technology to obtain a chromium carbide functional catalytic layer on the surface of the chromium metal diffusion barrier layer.

4. The method for preparing a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide according to claim 3, characterized in that: The steps also include that the obtained chromium metal diffusion barrier layer needs to be pre-oxidized by air and then placed in a molten ternary carbonate electrolyte system.

5. The method for preparing a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide according to claim 3 or 4, characterized in that: The molten ternary carbonate electrolyte consists of three inorganic salts: Li2CO3, Na2CO3 and K2CO3.

6. The method for preparing a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide according to claim 3 or 4, characterized in that: The electroplating parameters are: electroplating solution temperature is 30-70°C, current density is 10-200mA / cm 2 , electroplating time 1-200min.

7. The method for preparing a composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide according to claim 3 or 4, characterized in that: The electrolysis temperature is 450-800°C and the current density is 1-100 mA / cm 2 , the electrolysis time is 5-300min.

8. Use of the composite coating cathode for preparing a carbon material system by molten salt electrolysis of carbon dioxide according to any one of claims 1 to 2 in the preparation of carbon materials.

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