Catalytic electrode for ammonia water electrolysis with improved durability by heat treatment and method for preparing the same

Abstract

Disclosed are a catalytic electrode for ammonia water electrolysis and a method for efficiently producing the same. For the catalytic electrode for ammonia water electrolysis, by introducing a heat treatment step in a specific temperature range after an electroplating process, the proportion of oxides and hydroxides in the catalytic electrode for ammonia water electrolysis is improved, thereby inhibiting the poisoning phenomenon caused by nitrogen oxides, improving durability, and the ammonia water electrolysis performance is excellent.

Description

Technical Field

[0001] This invention relates to a catalytic electrode for ammonia electrolysis and its preparation method, and more specifically, to a catalytic electrode for ammonia electrolysis with improved durability through heat treatment and its preparation method.

[0002] This application claims priority and benefit to Korean Patent Application No. 10-2023-0178841, filed on December 11, 2023, the entire contents of which are incorporated herein by reference. Background Technology

[0003] Ammonia is the nitrogen compound with the lowest oxidation state. It can be produced directly in various industrial processes or through the nitrogen compound cycle during the treatment of nitrate-based nitrogen compounds. Ammonia can be treated by degassing, biodegradation, chlorination, and electrochemical decomposition. Among these methods, electrochemical oxidation has recently attracted much attention due to its economic efficiency, rapid and convenient operation, and minimal generation of secondary waste.

[0004] Among the known technologies for producing hydrogen from ammonia, there are methods using ammonia as a raw material, namely the thermal decomposition of ammonia and the electrolytic decomposition of ammonia solution. In the thermal decomposition method, nitrogen and hydrogen are produced simultaneously, thus requiring the use of expensive palladium membranes to separate high-purity hydrogen, resulting in limitations in productivity and economics. Furthermore, research on the electrolytic decomposition of ammonia solution is currently limited, particularly regarding the development of electrodes suitable for ammonia solution electrolysis. Therefore, there is a need to develop electrodes effective for the electrolysis reaction of ammonia solution.

[0005] Furthermore, unlike water electrolysis, which theoretically requires 1.23V, ammonia electrolysis, requiring only 0.06V, is highly anticipated as a hydrogen fuel source. Among currently used catalysts, transition metal catalysts, aside from precious metals like platinum and iridium, suffer from very high overvoltages, hindering their widespread use. However, platinum-based catalysts are prohibitively expensive, making economic viability difficult to ensure.

[0006] In addition, the ammonia electrolysis reaction can be poisoned by nitrogen oxides, which are reactants or intermediates, causing the electrode activity to deactivate within minutes to hours, resulting in a decrease in the durability of the catalytic electrode.

[0007] Korean Patent Publication No. 10-2022-0068566 (May 26, 2022) relates to a hydrogen production apparatus using an ammonia solution, which prepares and uses an electrode deposited on a nickel metal foam, but does not mention a technique to improve the durability of the catalytic electrode through heat treatment.

[0008] In addition, Yejin Yang et al. (JOURNAL OF MATERIALS CHEMISTRY A, v.9, no.19, May2021, pp.11571-11579) disclosed a technique for using platinum electrodeposition on carbon paper as an ammonia oxidation catalyst via a potential cycling method, but similarly did not mention a technique for improving the durability of the catalytic electrode through heat treatment. Summary of the Invention

[0009] The present invention aims to provide a catalytic electrode for ammonia electrolysis and a method for effectively preparing the catalytic electrode, which can improve durability while maintaining the ammonia electrolysis activity.

[0010] To address the aforementioned issues, the present invention provides a method for preparing a catalytic electrode for ammonia electrolysis, the method comprising: step A, electroplating an active metal on the surface of a support to prepare a catalyst; and step B, heat-treating the catalyst.

[0011] Furthermore, the present invention provides a method for preparing a catalytic electrode for ammonia electrolysis, characterized in that the heat treatment temperature in step B is 50~250℃.

[0012] Furthermore, the present invention provides a method for preparing a catalytic electrode for ammonia electrolysis, characterized in that the support comprises one or more materials selected from nickel, iron, aluminum and carbon, and the active metal catalyst is selected from one or more materials selected from platinum, iridium, rhodium, palladium, ruthenium, iron, nickel, cobalt and manganese.

[0013] Furthermore, the present invention provides a method for preparing a catalytic electrode for ammonia electrolysis, characterized in that the support contains nickel, the active metal catalyst is platinum, and through the heat treatment in step B, the ratio of nickel oxide (NiO) to nickel hydroxide (Ni(OH)2) in the catalytic electrode for ammonia electrolysis is 1:4 to 1:9.

[0014] To address the aforementioned issue, the present invention provides a catalytic electrode for ammonia electrolysis, which is prepared according to the method described above.

[0015] The present invention provides a catalytic electrode for ammonia electrolysis and a method for effectively preparing the catalytic electrode: For the catalytic electrode for ammonia electrolysis, by introducing a heat treatment step within a specific temperature range after the electroplating process, the ratio of nickel oxide and nickel hydroxide in the catalytic electrode for ammonia electrolysis is improved, thereby suppressing poisoning caused by nitrogen oxides, improving durability, and resulting in excellent ammonia electrolysis performance. Attached Figure Description

[0016] Figure 1This is a photograph showing the results of SEM analysis of the catalytic electrode for ammonia electrolysis prepared according to the examples and comparative examples in this invention.

[0017] Figure 2 This is a graph showing the results of measuring the ammonia oxidation characteristics of the catalytic electrode for ammonia electrolysis prepared according to the examples and comparative examples in this invention.

[0018] Figure 3 This is a graph showing the results of evaluating the electrochemical stability of the catalytic electrodes for ammonia electrolysis prepared according to the examples and comparative examples in this invention.

[0019] Figure 4 This is a graph showing the results of surface chemical species and chemical binding energy analysis of the catalytic electrode for ammonia electrolysis prepared according to Example 1 in this invention.

[0020] Figure 5 This is a graph showing the results of surface chemical species and chemical binding energy analysis of the catalytic electrode for ammonia electrolysis prepared according to the comparative example in this invention.

[0021] Figure 6 This is a graph showing the results of surface chemical species and chemical binding energy analysis of the catalytic electrode for ammonia electrolysis prepared according to Example 4 in this invention. Detailed Implementation

[0022] The present invention will now be described in detail through preferred embodiments. Prior to this, the terms or words used in this specification and claims should not be limited to their conventional or dictionary meanings, but should be interpreted as conforming to the meaning and concept of the technical idea of ​​the present invention, based on the principle that the inventors may appropriately define the concepts of terms in order to best illustrate their invention. Therefore, the structures of the embodiments described in this specification are merely one of the most preferred embodiments of the present invention and do not represent all the technical ideas of the present invention. It should be understood that various equivalents and modifications may exist to replace these embodiments at the time of filing this application.

[0023] This invention discloses a method for preparing a catalytic electrode for ammonia electrolysis, the method comprising: step A, electroplating an active metal on the surface of a support to prepare a catalyst; and step B, heat-treating the catalyst.

[0024] The support may contain at least one material selected from nickel, iron, aluminum, carbon, and nickel-based alloys. The support functions as a substrate for forming the active metal catalyst, and is preferably made of nickel.

[0025] The shape of the carrier described in this invention is not particularly limited and can be foam, plate, fabric, foil, or felt, preferably a metal foam carrier. When the carrier is a metal foam carrier, it can effectively improve the stability of the electrode, the catalyst loading, the fluidity of the electrolyte, and increase the surface area of ​​electrode catalyst particles such as platinum particles, thereby improving the electrode activity.

[0026] In one specific embodiment of the present invention, the metal foam support may be a nickel-based foam support. When the metal foam support is a nickel-based foam support, the active metal particles can be loaded more stably and exhibit the effective catalytic activity of the active metal particles.

[0027] In this invention, the active metal catalyst supported on the support is not particularly limited as long as it can function as an oxidation reaction catalyst for ammonia water electrolysis. For example, it can be a noble metal catalyst such as platinum, iridium, rhodium, palladium, ruthenium, etc., or a non-noble metal catalyst such as iron, nickel, cobalt, manganese, etc. From the perspective of promoting the adsorption of ammonia in the ammonia oxidation reaction to significantly improve the electrolysis efficiency of ammonia water solution, it is preferred to be a platinum catalyst.

[0028] Furthermore, the precursor of the active metal catalyst is a compound containing active metal catalysts such as platinum, iridium, rhodium, palladium, ruthenium, iron, nickel, cobalt, and manganese. For example, chloroplatinic acid (H2PtCl6·xH2O) can be used as a precursor for platinum active metal electroplating.

[0029] Furthermore, the concentration of the precursor of the active metal catalyst contained in the solution of step A can be 0.5~20 mM, preferably 1~5 mM. If the concentration of the precursor of the active metal catalyst is lower than the above concentration range, the amount of deposited active metal catalyst will be insufficient, which may lead to a decrease in ammonia electrolysis activity; if the concentration of the precursor of the metal catalyst is higher than the above concentration range, excessive agglomeration of metal particles may occur, resulting in a decrease in mass activity.

[0030] Step A involves electroplating the active metal catalyst onto the surface of the support by applying a voltage to a solution containing the support, the precursor of the active metal catalyst, and the electrolyte. Even with a small amount of active metal catalyst, it can be uniformly deposited over a large surface area, thereby enabling excellent ammonia oxidation reactions to occur at more active sites.

[0031] The electrolyte is not particularly limited as long as it is a salt that enables the active metal catalyst to migrate from the precursor of the active metal catalyst and electrodeposit onto the support. For example, HClO4, H2SO4, HCl, NaCl, KCl, KOH, NaOH, etc. can be used. Considering the electrodeposition efficiency and the shape of the active metal, HClO4 is preferred.

[0032] The concentration of the electrolyte contained in the solution of step A can be 0.5~5000 mM, preferably 5~3000 mM, and more preferably 100~1000 mM. If the concentration of the electrolyte is lower than the above concentration range, the migration of the metal precursor in the electrolyte will not be smooth, which may result in insufficient amount of supported metal catalyst; if the concentration of the electrolyte is higher than the above concentration range, the oxidation of the metal support will be accelerated, which may result in a decrease in ammonia electrolysis activity.

[0033] In the electroplating process of step A, the voltage can be -1.0 to 1.0 V (relative to Ag / AgCl), preferably -0.5 to 0.8 V (relative to Ag / AgCl), more preferably -0.2 to 0.4 V (relative to Ag / AgCl); the electroplating time can be from 1 minute to 3 hours, preferably 5 minutes to 2 hours, more preferably 10 minutes to 1 hour. In particular, if the electroplating time is less than 1 minute, the amount of deposited active metal catalyst may be insufficient; if the electroplating time exceeds 3 hours, the amount of deposited active metal catalyst may be excessive, potentially reducing the economic efficiency of preparing the catalytic electrode for ammonia electrolysis.

[0034] In this invention, step B is a heat treatment step of the catalyst prepared in step A. Through this heat treatment, an oxide of the metal constituting the support within the catalytic electrode for ammonia electrolysis is formed, thereby preventing poisoning caused by nitrogen oxides generated during the ammonia electrolysis reaction and improving durability. Therefore, step B plays a role in improving the durability of the catalytic electrode for ammonia electrolysis prepared in this invention.

[0035] Furthermore, there are no particular limitations on the type of heat processor used in the heat treatment; a tube furnace can be used. Specifically, the treatment conditions of the heat processor can be set to 50–250°C for 5 minutes to 12 hours. From the perspective of maximizing the durability of the catalytic electrode for ammonia electrolysis, it is preferable to set the temperature to 70–230°C for 10 minutes to 6 hours, and more preferably to set it to 90–210°C for 30 minutes to 3 hours.

[0036] In addition, if the temperature of the thermal processor is below 50°C, the effect of improving the durability of the catalytic electrode may not be obvious; if the temperature of the thermal processor exceeds 250°C, the agglomeration of the active metal catalyst will occur, resulting in a decrease in the exposure of active sites, which may reduce the ammonia electrolysis activity.

[0037] Furthermore, if the heat treatment time is less than 5 minutes, the effect of improving the durability of the catalytic electrode may not be significant; if the heat treatment time exceeds 12 hours, the metal oxide support present in the catalyst may deform, thereby leading to a decrease in ammonia electrolysis activity.

[0038] According to a specific example of the present invention, a method for preparing a catalytic electrode for ammonia electrolysis can be provided, characterized in that, in step A, the support comprises nickel, the active metal catalyst is platinum, and through heat treatment in step B, the ratio of nickel oxide (NiO) to nickel hydroxide (Ni(OH)2) in the catalytic electrode for ammonia electrolysis is 1:4 to 1:9.

[0039] Furthermore, if the ratio of nickel oxide (NiO) to nickel hydroxide (Ni(OH)2) deviates from the specified range, the hydrophilicity of the electrode surface may change. If the proportion of nickel oxide is high, the surface is in a low-hydrophilic state, hindering the adsorption of ammonia as a reactant and potentially reducing ammonia decomposition performance. Additionally, the high-temperature heat treatment process, while increasing the nickel oxide ratio, can also lead to the agglomeration of active metals, potentially reducing the durability of ammonia electrolysis. Conversely, a high proportion of nickel hydroxide promotes ammonia adsorption and improves ammonia decomposition performance, but excessive ammonia adsorption may reduce durability.

[0040] That is, from the perspective of maximizing the performance of ammonia decomposition and the durability of the catalytic electrode, the ratio of nickel oxide to nickel hydroxide in the catalytic electrode for ammonia electrolysis is preferably 1:4 to 1:9, and more preferably 1:5.6 to 1:9.

[0041] Furthermore, the present invention can provide a catalytic electrode for ammonia electrolysis prepared by the above method and with improved durability.

[0042] According to the present invention, by treating the product after electroplating the active metal catalyst on the support with heat treatment, and performing heat treatment at a specific temperature and time, the ratio of nickel oxide to nickel hydroxide in the catalytic electrode for ammonia electrolysis can be changed, and as the ratio of nickel oxide to nickel hydroxide changes, poisoning caused by nitrogen oxides can be prevented, thereby improving the durability of the catalytic electrode. These effects have been confirmed by the following experimental examples.

[0043] The present invention will now be described in more detail through specific embodiments and comparative examples.

[0044] Example 1

[0045] A nickel support (MTI Korea, nickel thickness 1600 μm, porosity 50~98%) with a transverse dimension of 1 cm and a longitudinal dimension of 1 cm was placed in a solution containing 2 mM chloroplatinic acid (H2PtCl6·xH2O) as an active metal catalyst precursor and 100 mM electrolyte HClO4. A voltage of -0.2~0.4 V (relative to Ag / AgCl) was applied for 50 minutes to deposit a platinum catalyst on the surface of the nickel support.

[0046] The electroplated product was washed five times with distilled water and dried. Then, it was heat-treated for 1 hour in an argon atmosphere using a heat processor (FU-PK-G4-L, purchased from Sanxing Energy, with a heating rate of 2.5℃ / min) set to 100℃ to prepare a catalytic electrode for ammonia electrolysis.

[0047] Example 2

[0048] Except that the heat treatment temperature in Example 1 was set to 200°C, the catalytic electrode for ammonia electrolysis was prepared using the same method as in Example 1.

[0049] Example 3

[0050] Except that the heat treatment temperature in Example 1 was set to 300°C, the catalytic electrode for ammonia electrolysis was prepared using the same method as in Example 1.

[0051] Example 4

[0052] Except that the heat treatment temperature in Example 1 was set to 400°C, the catalytic electrode for ammonia electrolysis was prepared using the same method as in Example 1.

[0053] Comparative example

[0054] Except for omitting the heat treatment described in Example 1, the catalytic electrode for ammonia electrolysis was prepared using the same method as in Example 1.

[0055] Experimental Example 1

[0056] To confirm the effect of heat treatment on the surface of the catalytic electrode for ammonia electrolysis in this invention, scanning electron microscopy (SEM) analysis was performed on the catalytic electrodes for ammonia electrolysis prepared according to the examples and comparative examples, and the results are shown below. Figure 1 middle.

[0057] Reference Figure 1It was confirmed that in the case of the catalytic electrode for ammonia electrolysis without heat treatment (comparative example), the platinum nanoparticles on the surface exhibited a multilayer structure (see left photo); in the case of the catalytic electrode for ammonia electrolysis with heat treatment at 100~300°C (Examples 1 to 3), the platinum nanoparticles on the surface exhibited an uneven morphology (see middle photo); in the case of the catalytic electrode for ammonia electrolysis with heat treatment at 400°C (Example 4), the platinum nanoparticles on the surface dissolved in an aggregated morphology (see right photo).

[0058] In addition, the aggregation morphology of the platinum nanoparticles in Example 4 above is the result of the characteristic of attempting to deform into a morphology with minimal surface exposure in order to reduce the surface energy of the particles during high-temperature heat treatment above 400°C. It is expected that through this aggregation morphology, the area of ​​the platinum catalyst on the surface will be reduced, thereby reducing the catalytic activity of the catalytic electrode for ammonia electrolysis.

[0059] Experimental Example 2

[0060] To confirm the change in ammonia oxidation activity of the catalytic electrode for ammonia electrolysis caused by heat treatment in this invention, the ammonia oxidation characteristics of the catalytic electrodes for ammonia electrolysis prepared according to the examples and comparative examples were measured, and the results are shown below. Figure 2 All electrochemical measurements related to ammonia oxidation were performed using an electrochemical workstation (Zive, WonATech). Specifically, the electrochemical tests were conducted in a 1M NH4OH / 5M KOH electrolyte system within a three-electrode system in a membrane-free, sealed glass beaker (to prevent ammonia evaporation), using a Hg / HgO electrode (filled with 1.0M KOH) and a Pt mesh as the reference and counter electrodes, respectively. Cyclic voltammetry (CV) curves of the catalyst were obtained at a scan rate of 20 mV / s over a range of -0.15 V to 1.2 V relative to the reversible hydrogen electrode (RHE). Furthermore, under a hydrogen atmosphere using a Pt mesh as the electrode and Hg / HgO as the reference electrode, all potentials measured for Hg / HgO were converted to the RHE scale as shown in Equation 1 below.

[0061] [Mathematical Expression 1]

[0062] V RHE =V Hg / HgO +0.97 (experimental measurement)

[0063] Reference Figure 2 The catalytic performance of ammonia oxidation is as follows: the catalytic electrode for ammonia electrolysis prepared according to the comparative example has a performance of 520 mA / cm². 2 The catalytic electrode for ammonia electrolysis prepared according to Example 1 has a current rating of 500 mA / cm².2 The catalytic electrode for ammonia electrolysis prepared according to Example 2 has a strength of 550 mA / cm². 2 The catalytic electrode for ammonia electrolysis prepared according to Example 3 has a strength of 530 mA / cm². 2 The catalytic electrode for ammonia electrolysis prepared according to Example 4 has a strength of 230 mA / cm². 2 It was confirmed that the ammonia oxidation catalytic performance of Examples 1 to 3 was similar to that of the comparative example, and in the case of Example 4, the hydroxide catalytic performance was reduced by about 50% compared with the comparative example. It is speculated that this result of Example 4 is due to the reduced exposure of platinum particles as described in Test Example 1 above.

[0064] Experimental Example 3

[0065] To confirm the stability changes of the catalytic electrode for ammonia electrolysis caused by heat treatment in this invention, the electrochemical stability of the catalytic electrodes for ammonia electrolysis prepared according to the examples and comparative examples was evaluated under the following conditions, and the results are shown below. Figure 3 middle.

[0066] [Evaluation Criteria]

[0067] Use 1M NH4OH / 5M KOH electrolyte and stir at 200 rpm.

[0068] 50mA / cm at room temperature 2 The constant current test was used to measure the time required for water oxidation due to catalyst poisoning.

[0069] Reference Figure 3 The electrochemical stability of the catalytic electrodes is as follows: 14 hours for the catalytic electrode for ammonia electrolysis prepared according to the comparative example, 46 hours for the catalytic electrode for ammonia electrolysis prepared according to Example 1, 48 hours for the catalytic electrode for ammonia electrolysis prepared according to Example 2, 17 hours for the catalytic electrode for ammonia electrolysis prepared according to Example 3, and 20 minutes for the catalytic electrode for ammonia electrolysis prepared according to Example 4. In particular, it was confirmed that the electrochemical stability and durability of Examples 1 and 2 were more than twice that of the comparative examples.

[0070] Test Example 4

[0071] To confirm the factors contributing to the improved durability of the ammonia electrolysis catalytic electrode due to heat treatment in this invention, the changes in the oxidation numbers of platinum and nickel on the surfaces of the ammonia electrolysis catalytic electrode prepared according to Example 1, the ammonia electrolysis catalytic electrode prepared according to the comparative example, and the ammonia electrolysis catalytic electrode prepared according to Example 4 were analyzed by X-ray photoelectron spectroscopy, and the results are shown below. Figures 4 to 6The surface chemical species and chemical binding energies corresponding to the peak values ​​in each figure are shown in Tables 1 to 3 below.

[0072] [Table 1]

[0073]

[0074] [Table 2]

[0075]

[0076] [Table 3]

[0077]

[0078] Reference Figure 4 , Figure 5 As shown in Tables 1 and 2 above, compared with the catalytic electrode for ammonia electrolysis prepared according to the comparative example without heat treatment, a change was observed in the catalytic electrode for ammonia electrolysis prepared according to Example 1 with heat treatment, corresponding to the binding energy position of Ni 2p.

[0079] In addition, refer to Figure 6 As shown in Table 3 above, when heat treatment is performed at an excessively high temperature of 400℃, the peak area of ​​NiO species exceeds 20% in X-ray photoelectron analysis, indicating excessive growth. It is speculated that the Pt-Ni(OH)2 interaction maintained at low temperature weakens during high-temperature heat treatment, and the Ni(OH)2 species transforms into NiO species.

[0080] In other words, analysis based on X-ray photoelectron spectroscopy confirmed that when the heat treatment temperature exceeds 400℃, the active metal catalyst will agglomerate, resulting in a decrease in the exposure of active sites, which may reduce the ammonia electrolysis activity.

[0081] This means that as the heat treatment is carried out within a specific temperature range, the ratio of nickel oxide (NiO) to nickel hydroxide (Ni(OH)2) will change, and when nickel oxide is contained in the above-mentioned preferred ratio, the poisoning phenomenon caused by nitrogen oxides can be prevented. Therefore, it is expected that the durability of the catalytic electrode for ammonia electrolysis can be improved through the heat treatment in this invention.

[0082] The preferred embodiments of the present invention have been described in detail above. The description of the present invention is exemplary, and those skilled in the art should understand that it can be easily modified into other specific forms without changing the technical concept or essential features of the present invention.

[0083] Therefore, the scope of the invention should be defined by the claims rather than the foregoing detailed description, and all modifications or variations derived from the meaning, scope and equivalent concepts of the claims should be interpreted as being included within the scope of the invention.

Claims

1. A method for preparing a catalytic electrode for ammonia electrolysis, comprising: Step A involves electroplating an active metal onto the surface of a support to prepare a catalyst. as well as Step B involves heat-treating the catalyst.

2. The method for preparing the catalytic electrode for ammonia electrolysis according to claim 1, characterized in that, The heat treatment temperature in step B is 50~250℃.

3. The method for preparing the catalytic electrode for ammonia electrolysis according to claim 1, characterized in that, The carrier comprises one or more materials selected from nickel, iron, aluminum, and carbon. The active metal catalyst is selected from one or more of platinum, iridium, rhodium, palladium, ruthenium, iron, nickel, cobalt and manganese.

4. The method for preparing the catalytic electrode for ammonia electrolysis according to claim 1, characterized in that, The carrier contains nickel. The active metal catalyst is platinum. Through the heat treatment in step B, the ratio of nickel oxide (NiO) to nickel hydroxide (Ni(OH)2) in the support of the catalytic electrode for ammonia electrolysis is 1:4 to 1:

9.

5. A catalytic electrode for ammonia electrolysis, prepared according to any one of claims 1 to 4.

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