Oxygen evolution reaction electrocatalyst and preparation method and application thereof

Abstract

The invention relates to the technical field of water electrolysis, in particular to an oxygen evolution reaction electrocatalyst and a preparation method and application thereof. The catalyst is formed by loading Ni3P and CoP bimetallic phosphide on a carbon nano tube, and is prepared by a chemical vapor deposition method and an electro-deposition method. The preparation method comprises the following steps: firstly, pretreating the carbon cloth through concentrated nitric acid / concentrated sulfuric acid mixed liquor; then growing carbon nanotubes on the carbon cloth in situ by adopting a chemical vapor deposition method; and finally, loading the bimetallic phosphide on the carbon nanotubes through an electrodeposition method to prepare the oxygen evolution reaction electrocatalyst material. The catalyst shows excellent oxygen evolution reaction performance and stability performance in a potassium hydroxide electrolyte solution, and the preparation method is simple, low in cost, safe to operate, suitable for a three-electrode system and wide in application prospect.

Description

Technical Field

[0001] The present invention relates to the technical field of catalysts, and particularly relates to an oxygen evolution reaction electrocatalyst, a preparation method thereof, and an application thereof. Background Art

[0002] Currently, we are facing a serious energy crisis and environmental pollution problems caused by the combustion of fossil fuels. In order to reduce the dependence on fossil fuels, it is urgent for us to find clean and efficient renewable energy sources. Among renewable energy sources such as wind energy, solar energy, tidal energy, and hydrogen energy, hydrogen has many advantages such as high energy density and clean combustion products, and is considered an ideal alternative renewable energy source to fossil energy. Currently, electrocatalytic water splitting is one of the most sustainable hydrogen production technologies. Electrocatalytic water splitting includes two half-reactions: hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). OER is a four-electron transfer process with slow reaction kinetics, so the overpotential is high, and it is usually the rate-determining step of electrocatalytic water splitting. So far, Ru / Ir-based materials are still the main oxygen evolution reaction electrocatalysts, which can minimize the energy barrier and provide satisfactory energy conversion. However, the low abundance, high cost, and poor durability of noble metal-based materials have greatly hindered their large-scale application.

[0003] The metal oxo / hydroxy species formed in situ during the OER process of transition metal phosphides are identified as active sites. However, the metal oxo / hydroxy species have poor conductivity, which will hinder charge transfer, thus greatly affecting the electrocatalytic activity of transition metal phosphides towards OER. Modifying metal phosphides with carbon materials is a common method to promote electron transfer and improve their stability. Carbon nanotubes (CNTs) have good conductivity, high thermal stability, and large specific surface area, and are ideal additive phases for electrode materials. On the one hand, CNTs can promote the electron transport of electrode materials; on the other hand, CNTs can increase the electrochemical surface area of electrode materials and expose more active sites. Modifying metal phosphides with carbon nanotubes can increase their conductivity and electrochemical active area.

[0004] Incorporating another metal element into a single metal phosphide to form a bimetallic catalyst system can also improve the catalytic performance of the single metal phosphide. This is because the formation of a bimetallic structure can change its surface charge density, increase electrochemical active sites, and enhance electrocatalytic activity. Based on this, researchers have tried to composite bimetallic phosphides with carbon nanotubes to prepare highly efficient OER electrocatalysts. However, how to prepare a bimetallic phosphide composite carbon nanotube electrocatalyst with excellent catalytic performance and stability through a simple, low-cost, and controllable synthesis method is still a technical problem to be solved urgently. Summary of the Invention

[0005] In view of this, the object of the present invention is to provide an oxygen evolution reaction electrocatalyst, a preparation method thereof and an application thereof, so as to solve the problems of strong noble metal dependence, insufficient carrier conductivity, low catalytic efficiency, poor stability and high preparation cost existing in the existing oxygen evolution reaction electrocatalysts.

[0006] Based on the above object, the present invention provides an oxygen evolution reaction electrocatalyst, which comprises carbon nanotubes and a bimetallic phosphide loaded on the carbon nanotubes, and the bimetallic phosphide is composed of Ni 3 P and CoP two phases.

[0007] Furthermore, the present invention also provides a preparation method of an oxygen evolution reaction electrocatalyst, which is characterized by comprising the following steps: Step 1: Put the carbon cloth into a mixed solution of concentrated nitric acid / sulfuric acid with a volume ratio of 1:3, reflux at 70 °C for 2 h, wash, and dry in vacuum to obtain pretreated carbon cloth; Step 2: Immerse the pretreated carbon cloth in an ethanol solution containing 0.1 M Ni(NO 3 ) 2 and 0.1 M PEG for 3 h, take it out and dry it in the air, then place it in a vacuum tube furnace, and heat it to 550 °C at a heating rate of 5 °C·min -1 in a hydrogen / argon mixed atmosphere with a volume ratio of 1:9. Then pump acetylene into the vacuum tube furnace, keep the furnace tube temperature at 550 °C for 1 h, and naturally cool to room temperature to obtain a carbon nanotube array grafted on the carbon cloth; Step 3: Use the carbon nanotube array grafted on the carbon cloth as the working electrode, a graphite rod electrode and a saturated calomel electrode as the counter electrode and the reference electrode respectively. Then dissolve 3 mmol nickel nitrate hexahydrate, 2 mmol cobalt nitrate hexahydrate, 5 mmol sodium hypophosphite and 1 mmol sodium acetate in 100 mL deionized water and adjust the pH to 7 with a sulfuric acid solution as the electrolyte solution. Perform electro-deposition at a potential of -0.8 V relative to the saturated calomel electrode for a deposition time of 10 - 30 min to obtain an electrode material loaded with the oxygen evolution reaction electrocatalyst.

[0008] Still further, the present invention also provides an application of an oxygen evolution reaction electrocatalyst, which is used in a three-electrode system.

[0009] Preferably, in the three-electrode system, the electrode material loaded with the oxygen evolution reaction electrocatalyst is used as the working electrode, mercury / mercuric oxide is used as the reference electrode, a graphite rod electrode is used as the counter electrode, and a potassium hydroxide solution is used as the electrolyte solution.

[0010] Preferably, the concentration of the potassium hydroxide solution is 1.00 mol·L -1 .

[0011] Advantages of the present invention: The bimetallic phosphide composite carbon nanotubes obtained in the present invention are excellent oxygen evolution reaction electrocatalysts, which can be obtained by chemical vapor deposition and electrodeposition methods. Moreover, the raw materials used in the experiment are simple and easy to obtain, with the advantages of simple and safe operation, short time consumption, low equipment requirements and low cost. In addition, the catalytic performance of this material is excellent and the stability is good.

[0012] The bimetallic phosphide composite carbon nanotubes obtained in the present invention can be used in a three-electrode system. Under a potassium hydroxide electrolyte solution, when the current density reaches 10 mA·cm -2 , the overpotential is only 320 mV, and the Tafel slope is 84.04 mV·dec -1 , and the stability is good. At the overpotential corresponding to the current density of 10 mA·cm -2 , the current density only decays by 23%.

[0013] In summary, the material of the present invention not only has the advantages of simple synthesis, low equipment requirements, low cost, short time consumption and convenient operation, but also can achieve a low overpotential and excellent stability when used in the oxygen evolution reaction of electrolyzed water. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] In order to more clearly illustrate the technical solutions in the present invention or the prior art, the following will briefly introduce the drawings required for use in the description of the embodiments or the prior art.

[0015] Figure 1 It is a synthesis diagram of the nickel-cobalt phosphide composite carbon nanotube electrode material provided in Example 2 of the present invention; Figure 2 It is a scanning electron microscope image of the Ni-Co-P / CNTs@CC-25 electrode provided in Example 2 of the present invention; Figure 3 It is an XRD spectrum diagram of the Ni-Co-P / CNTs@CC-25 electrode provided in Example 2 of the present invention; Figure 4 It is a double-layer capacitance curve diagram of the nickel-cobalt phosphide composite carbon nanotube electrode material provided in Example 2 of the present invention; Figure 5 It is a linear sweep curve and Tafel slope diagram of the nickel-cobalt phosphide composite carbon nanotube electrode material provided in Example 2 of the present invention; Figure 6 It is an impedance diagram of the nickel-cobalt phosphide composite carbon nanotube electrode material provided in Example 2 of the present invention; Figure 7Double-layer capacitance curves of the Ni-Co-P / CNTs@CC-25 electrode provided in Example 2 of the present invention and the Ni-P / CNTs@CC electrodes, Co-P / CNTs@CC electrodes, and Co-P / CNTs@CC electrodes provided in Comparative Examples 1-3; Figure 8 Linear sweep curves and Tafel slope diagrams of the Ni-Co-P / CNTs@CC-25 electrode provided in Example 2 of the present invention and the Ni-P / CNTs@CC electrodes, Co-P / CNTs@CC electrodes, and Co-P / CNTs@CC electrodes provided in Comparative Examples 1-3; Figure 9 Impedance diagrams of the Ni-Co-P / CNTs@CC-25 electrode provided in Example 2 of the present invention and the Ni-P / CNTs@CC electrodes, Co-P / CNTs@CC electrodes, and Co-P / CNTs@CC electrodes provided in Comparative Examples 1-3.

[0016] Figure 10 Stability curve of the Ni-Co-P / CNTs@CC-25 electrode provided in Example 2 of the present invention. Detailed implementation manners

[0017] To make the objectives, technical solutions, and advantages of the present invention clearer and more understandable, the present invention will be further described in detail below in conjunction with specific embodiments.

[0018] Example 1: Carbon nanotubes were in-situ synthesized on carbon cloth by chemical vapor deposition (CVD).

[0019] The carbon cloth (WOS 1002, Taiwan Carbon Energy Technology Co., Ltd.) was put into a mixed solution of concentrated nitric acid / concentrated sulfuric acid (the mass fraction of concentrated nitric acid was 65%, and the mass fraction of concentrated sulfuric acid was 95%) (concentrated nitric acid: concentrated sulfuric acid = 1:3, v:v), refluxed at 70 °C for 2 h, then repeatedly ultrasonically washed with water and ethanol, and then placed in a vacuum drying oven and dried at 60 °C for 12 h to obtain pretreated carbon cloth; The pretreated carbon cloth was soaked in an ethanol solution containing 0.1 M Ni(NO 3 ) 2 and 0.1 M PEG for 3 h, taken out and dried in air, and then placed in a vacuum tube furnace, in a hydrogen / argon mixed atmosphere (H 2 :Ar = 1:9, v:v), at 5 °C·min -1Heat it to 550 °C at a heating rate, then pump 0.5 KPa of acetylene into a vacuum tube furnace. Keep the furnace tube temperature at 550 °C for 1 h, and after natural cooling to room temperature, carbon nanotube arrays (CNTs@CC) grafted on carbon cloth are obtained. The loading amount of CNTs is 0.5 mg·cm obtained from the mass difference of the carbon cloth before and after CVD. -2 。

[0020] Example 2: Prepare bimetallic phosphide nanospheres composite on carbon nanotubes by potentiostatic method.

[0021] Use a water bath to control the temperature at 30 °C. Take the CNTs@CC (1 cm×1 cm) prepared in Example 1 as the working electrode, a graphite rod electrode and a saturated calomel electrode (SCE) as the counter electrode and reference electrode respectively. Then dissolve 3 mmol of nickel nitrate hexahydrate, 2 mmol of cobalt nitrate hexahydrate, 5 mmol of sodium hypophosphite and 1 mmol of sodium acetate in 100 mL of deionized water and use 0.5 M H 2 SO 4 Adjust the pH to 7 as the electrolyte. Before electrodeposition, circulate nitrogen in the electrolyte for 30 min to remove the trapped air, and maintain the flow of nitrogen during electrodeposition. Perform electrodeposition at a potential of -0.8 V (vs. SCE), and the deposition times are 10, 15, 20, 25 and 30 min respectively to obtain nickel-cobalt phosphide composite carbon nanotube electrode materials. The prepared nickel-cobalt phosphide composite carbon nanotube electrode materials are denoted as Ni-Co-P / CNTs@CC-10, Ni-Co-P / CNTs@CC-15, Ni-Co-P / CNTs@CC-20, Ni-Co-P / CNTs@CC-25 and Ni-Co-P / CNTs@CC-30 respectively.

[0022] Comparative Example 1: The difference between Comparative Example 1 and Example 2 is that cobalt nitrate hexahydrate is not added to the electrolyte; The specific steps are as follows: Use a water bath to control the temperature at 30 °C. Take the CNTs@CC (1 cm×1 cm) prepared in Example 1 as the working electrode, a graphite rod electrode and a saturated calomel electrode (SCE) as the counter electrode and reference electrode respectively. Then dissolve 3 mmol of nickel nitrate hexahydrate, 5 mmol of sodium hypophosphite and 1 mmol of sodium acetate in 100 mL of deionized water and use 0.5 M H 2 SO 4Adjust the pH to 7. As the electrolyte, before electrodeposition, circulate nitrogen in the electrolyte for 30 min to remove the trapped air, and maintain the flow of nitrogen during electrodeposition. Perform electrodeposition at a potential of -0.8 V (vs. SCE) for 25 min. The prepared electrode material is denoted as Ni-P / CNTs@CC.

[0023] Comparative Example 2: The difference between Comparative Example 2 and Example 2 is that nickel nitrate hexahydrate is not added to the electrolyte. The specific steps are as follows: Use a water bath to control the temperature at 30 °C. Take the CNTs@CC (1 cm×1 cm) prepared in Example 1 as the working electrode, a graphite rod electrode and a saturated calomel electrode (SCE) as the counter electrode and reference electrode respectively. Then dissolve 2 mmol of cobalt nitrate hexahydrate, 5 mmol of sodium hypophosphite and 1 mmol of sodium acetate in 100 mL of deionized water and use 0.5M H 2 SO 4 Adjust the pH to 7. As the electrolyte, before electrodeposition, circulate nitrogen in the electrolyte for 30 min to remove the trapped air, and maintain the flow of nitrogen during electrodeposition. Perform electrodeposition at a potential of -0.8 V (vs. SCE) for 25 min. The prepared electrode material is denoted as Co-P / CNTs@CC.

[0024] Comparative Example 3: The difference between Comparative Example 3 and Example 2 is that the pretreated carbon cloth in Example 1 is used as the working electrode. The specific steps are as follows: Use a water bath to control the temperature at 30 °C. Take the pretreated carbon cloth (1 cm×1 cm) prepared in Example 1 as the working electrode, a graphite rod electrode and a saturated calomel electrode (SCE) as the counter electrode and reference electrode respectively. Then dissolve 3 mmol of nickel nitrate hexahydrate, 2 mmol of cobalt nitrate hexahydrate, 5 mmol of sodium hypophosphite and 1 mmol of sodium acetate in 100 mL of deionized water and use 0.5 M H 2 SO 4 Adjust the pH to 7. As the electrolyte, before electrodeposition, circulate nitrogen in the electrolyte for 30 min to remove the trapped air, and maintain the flow of nitrogen during electrodeposition. Perform electrodeposition at a potential of -0.8 V (vs. SCE) for 25 min. The prepared electrode material is denoted as Ni-Co-P@CC.

[0025] Performance test: Perform morphological analysis on Ni-Co-P / CNTs@CC-25 using a scanning electron microscope. The results are as Figure 2 shown.

[0026] The X-ray diffractometer was used to analyze the phase of Ni-Co-P / CNTs@CC-25, and the results are as Figure 3 shown.

[0027] Figure 2 is the scanning electron microscopy image of Ni-Co-P / CNTs@CC-25. It can be seen from the figure that the electrode material has a unique cross-linked structure. The surface of the nickel-cobalt-phosphorus nanospheres is covered by cross-linked nanosheets. The curved nanosheets are disordered and cross-linked, forming a blooming flower-like structure. The blooming flower-like Ni-Co-P nanospheres are strung together to form a three-dimensional hierarchical structure, which can provide rich ion transfer channels and a large electrolyte / electrode contact area. Figure 3 is the spectrum of the nickel-cobalt phosphide composite carbon nanotube electrode material (Ni-Co-P / CNTs@CC-25), Figure 3 indicating that the synthesized sample Ni-Co-P / CNTs@CC-25 is mainly composed of Ni 3 P and CoP two phases, mostly amorphous, where Ni is the catalyst introduced in the CVD process.

[0028] Using a conventional three-electrode system, a mercury / mercuric oxide electrode was used as the reference electrode, a graphite rod electrode was used as the counter electrode, the electrode prepared in Example 2 was used as the working electrode, and the electrolyte solution was 1 mol·L -1 aqueous potassium hydroxide solution, and electrochemical tests were carried out. The potentials used in the electrochemical test results have been converted to the reversible hydrogen electrode potential (RHE).

[0029] Cyclic voltammetry (CV) test: CV test was carried out under the conditions of a scanning rate of 20-100 mV·s -1 and a scanning range of -0.1-0 V (vs.Hg / HgO). The results are as Figure 4 shown.

[0030] Linear sweep voltammetry (LSV) test: LSV test was carried out in the voltage range of 0.173-0.973 V (vs.Hg / HgO) and a scanning rate of 5 mV·s -1 conditions. The results are as Figure 5 shown.

[0031] Electrochemical impedance spectroscopy (EIS) test: EIS test was carried out in the frequency range of 0.01-10 5 Hz at open circuit voltage. The results are as Figure 6 shown.

[0032] Analysis Figure 4, among the Ni-Co-P / CNTs@CC-x electrode materials with different electrodeposition times in Example 2 of the present invention, the electrochemical active area of the electrode material is the largest when the electrodeposition time is 25 min. Analysis Figure 5 , in Example 2 of the present invention, when the electrodeposition time is 25 min, the Ni-Co-P / CNTs@CC-25 electrode material obtained has the best electrochemical performance. At a current density of 10 mA·cm -2 , the overpotential of Ni-Co-P@CC-25 is only 320 mV, and the Tafel slope is 84.04 mV·dec -1 , and the performance is better than that of the Ni-Co-P / CNTs@CC-x electrode materials with other electrodeposition times. Analysis Figure 6 , in Example 2 of the present invention, when the electrodeposition time is 25 min, the charge transfer resistance of the Ni-Co-P / CNTs@CC-25 electrode material is the smallest, only 1.43 Ω, indicating that it has a faster electron transfer rate and higher efficiency in alkaline solution.

[0033] Using a conventional three-electrode system, with a mercury / mercuric oxide electrode as the reference electrode, a graphite rod electrode as the counter electrode, and then using the Ni-Co-P / CNTs@CC-25 electrode, Ni-P / CNTs@CC electrode, Co-P / CNTs@CC electrode, and Co-P / CNTs@CC electrode as the working electrodes respectively, and the electrolyte solution is 1 mol·L -1 aqueous potassium hydroxide solution for electrochemical testing. The potentials used in the electrochemical test results have been converted to reversible hydrogen electrode potentials (RHE).

[0034] Cyclic voltammetry (CV) test: CV test was carried out under the conditions of a scanning rate of 20 - 100 mV·s -1 and a scanning range of -0.1 - 0 V (vs. Hg / HgO). The results are as Figure 7 shown.

[0035] Linear sweep voltammetry (LSV) test: LSV test was carried out under the conditions of a voltage range of 0.173 - 0.973 V (vs. Hg / HgO) and a scanning rate of 5 mV·s -1 , and the results are as Figure 8 shown.

[0036] Electrochemical impedance spectroscopy (EIS) test: EIS test was carried out in the frequency range of 0.01 - 10 5 Hz at open circuit voltage, and the results are as Figure 9 shown.

[0037] Analysis Figure 7, among the Ni-Co-P / CNTs@CC-25 electrode with the best performance in Example 2 and the electrodes prepared in Comparative Examples 1-3 of the present invention, compared with the single-metal phosphide composite carbon nanotube materials, the double-metal phosphide Ni-Co-P / CNTs@CC-25 has a larger electrochemically active area. Analyzing Figure 8, comparing the Ni-Co-P / CNTs@CC-25 electrode with the best performance in Example 2 and the electrode materials prepared in Comparative Examples 1-3 of the present invention, the double-metal phosphide Ni-Co-P / CNTs@CC-25 has the most excellent electrochemical performance. At a current density of 10 mA·cm -2 , the overpotential of Ni-Co-P@CC-25 is only 320 mV, and the Tafel slope is 84.04 mV·dec -1 , and its performance is better than that of the single-metal phosphide electrode materials Ni-P / CNTs@CC (380 mV) and Co-P / CNTs@CC (371 mV), and is comparable to that of the commercial RuO 2 noble metal catalyst (308 mV), indicating that the synergistic effect between the double-metal phosphides is beneficial to improving their electrochemical performance. At the same time, Ni-Co-P@CC requires an overpotential of 383 mV, which is much higher than that of the Ni-Co-P / CNTs@CC-25 catalyst, indicating that the role of CNTs as a conductive skeleton can greatly improve the catalytic performance of the electrode material. Analyzing Figure 9, among the Ni-Co-P / CNTs@CC-25 electrode with the best performance in Example 2 and the electrode materials prepared in Comparative Examples 1-3 of the present invention, the Ni-Co-P / CNTs@CC-25 electrode material has the smallest charge transfer resistance, indicating that it has a faster electron transfer rate and higher efficiency in alkaline solution.

[0038] Using a conventional three-electrode system, with a mercury / mercuric oxide electrode as the reference electrode, a graphite rod electrode as the counter electrode, and then using the Ni-Co-P / CNTs@CC-25 electrode as the working electrode, and the electrolyte solution is 1 mol·L -1 aqueous potassium hydroxide solution, a stability test was carried out. At the overpotential corresponding to a current density of 10 mA·cm -2 , the stability of the Ni-Co-P / CNTs@CC-25 catalyst was evaluated by chronoamperometry. The stability test was carried out for 96 h, and the potential used in the test results has been converted to the reversible hydrogen electrode potential (RHE), and the results are as Figure 10 shown.

[0039] Analysis Figure 10, among which, after 96 h of stability test on Ni-Co-P / CNTs@CC-25 with the best performance in Example 2, the current density of the Ni-Co-P / CNTs@CC-25 electrode material only decreased by 23%, indicating that it has good oxygen evolution reaction stability in alkaline solution.

[0040] Those of ordinary skill in the art should understand that the discussion of any above embodiment is only exemplary and is not intended to imply that the scope of the present invention is limited to these examples; under the concept of the present invention, the technical features in the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations in different aspects of the present invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. An oxygen evolution reaction electrocatalyst, characterized in that: The invention comprises carbon nanotubes and bimetallic phosphides loaded on the carbon nanotubes, wherein the bimetallic phosphides are composed of two physical phases, Ni3P and CoP.

2. A method for preparing an oxygen evolution reaction electrocatalyst according to claim 1, characterized in that: The following steps are involved: Step 1: putting the carbon cloth into a mixture of concentrated nitric acid / concentrated sulfuric acid in a volume ratio of 1:3, reflux at 70° C. for 2 h, wash, and vacuum dry to obtain a pretreated carbon cloth; Step 2: Soak the pretreated carbon cloth in an ethanol solution containing 0.1 M Ni(NO3)2 and 0.1 M PEG for 3 h, take it out and dry it in air, then place it in a vacuum tube furnace and heat it at 5 °C min in a hydrogen / argon mixed atmosphere with a volume ratio of 1:

9. -1 The temperature was heated to 550°C at a heating rate of 100 °C, and acetylene was pumped into a vacuum tube furnace. The temperature of the furnace tube was kept at 550°C for 1 h. After naturally cooling to room temperature, a carbon nanotube array grafted on the carbon cloth was obtained. Step 3: The carbon nanotube array grafted on the carbon cloth was used as the working electrode, the graphite rod electrode and the saturated calomel electrode were used as the counter electrode and the reference electrode, respectively. 3 mmol nickel nitrate hexahydrate, 2 mmol cobalt nitrate hexahydrate, 5 mmol sodium hypophosphite and 1 mmol sodium acetate were dissolved in 100 mL deionized water and the pH was adjusted to 7 with sulfuric acid solution as the electrolyte. Electrodeposition was carried out at a potential of -0.8 V relative to the saturated calomel electrode for 10-30 min to obtain an electrode material loaded with an oxygen evolution reaction electrocatalyst.

3. An application of the oxygen evolution reaction electrocatalyst according to claim 1, characterized in that: For use in three-electrode systems.

4. The use of the oxygen evolution reaction electrocatalyst according to claim 3, characterized in that: The three-electrode system uses an electrode material loaded with an oxygen evolution reaction electrocatalyst as a working electrode, mercury / mercuric oxide as a reference electrode, a graphite rod electrode as a counter electrode, and a potassium hydroxide solution as an electrolyte solution.

5. The use of the oxygen evolution reaction electrocatalyst according to claim 4, characterized in that: The concentration of the potassium hydroxide solution is 1.00 mol·L -1 .

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