A method for preparing a nickel hydroxide bipolar electrode for two-step water electrolysis

Abstract

The present invention discloses a method for preparing a nickel hydroxide bipolar electrode for two-step water electrolysis. The method comprises the following steps: firstly preparing an organic solution precursor, dissolving a binder in an organic solvent to form a precursor solution, adding the prepared active material Ni(OH)2 powder, conductive agent powder and cobalt oxide powder to the precursor solution, stirring, then adding a dispersant and continuing to stir to obtain a uniform and fluid slurry; then pouring the prepared slurry into a mold, immersing nickel foam in the mold, placing the mold in a vacuum drying oven for evacuation, and then drying the mold. The dried nickel foam is removed from the mold and rolled to obtain the nickel hydroxide bipolar electrode. The nickel hydroxide bipolar electrode prepared by the present invention has large capacity, high stability, and is easy to scale up for production. The electrode is applied to two-step water electrolysis and has important research value and broad application prospects in the field of water electrolysis and hydrogen production industry.

Description

Technical Field

[0001] The invention relates to a method for preparing a nickel hydroxide bipolar electrode for two-step water electrolysis, belonging to the technical field of water electrolysis. Background Art

[0002] Hydrogen is considered an ideal renewable energy source due to its pollution-free and environmentally friendly use process and extremely high energy density (120MJ / Kg). Among the many hydrogen production methods, electrocatalytic water splitting is considered an ideal hydrogen production method due to its abundant source of hydrogen raw materials and pollution-free production process.

[0003] Currently, water electrolysis processes are mainly divided into four types: alkaline electrolysis (Alkaline Electrolyzer), proton exchange membrane electrolysis (Proton Exchange Membrane Electrolyzer), anion exchange membrane electrolysis (Anion Exchange Membrane Electrolyzer), and solid oxide electrolysis (Soild Oxide Electrolyzer). Currently, all water electrolysis technologies face the problem of low energy conversion efficiency (50%-70%). In addition, during the water electrolysis process, since the cathode hydrogen evolution reaction (HER) and the anode oxygen evolution reaction (OER) are coupled in time and space, in order to separate the generated gases, all water electrolysis processes require the use of corresponding diaphragm materials to separate the cathode reaction chamber and the anode reaction chamber. In traditional water electrolysis processes, the inconsistent production rates of hydrogen and oxygen lead to inconsistent pressures between the two chambers, which causes mechanical damage to the diaphragm and reduces its service life. At the same time, some gas will pass through the diaphragm, requiring further purification of the gas for subsequent use, increasing production costs.

[0004] In order to effectively solve the limitations of the above-mentioned water electrolysis technology, two-step water electrolysis by spatially decoupling the HER and OER reactions can solve some of the problems. By using Ni(OH)2 as a bipolar electrode separator, the cathode chamber and the anode chamber are spatially isolated, so that the generated gases do not mix. In addition, the use of a diaphragm is avoided, allowing the water electrolysis to operate under high-pressure conditions. In the two-step water electrolysis, the nickel hydroxide bipolar electrode acts as a redox medium to replace the hydroxide transport, replacing the HER or OER reaction through its own redox conversion between Ni(OH)2 and Ni00H. When its charge or discharge reaches the upper limit of capacity, the polarity of the water electrolysis device needs to be reversed. Therefore, the capacity of the bipolar electrode is particularly important for actual industrial applications. The emergence of two-step water electrolysis has given the field of industrial water electrolysis a broader research prospect.

[0005] However, the preparation process of nickel hydroxide bipolar electrodes currently used in two-step water electrolysis is in its infancy, and the preparation method is very immature. The capacity, stability and utilization rate of nickel hydroxide bipolar electrodes cannot meet industrial requirements. In order to solve these problems, researchers have made many attempts. At present, there is no universal method for the preparation of nickel hydroxide bipolar electrodes. Most preparation methods are still in the laboratory stage. The preparation of bipolar electrodes is even more limited to small-area and low-quality preparation. Preparation methods such as hydrothermal synthesis and electroplating deposition are often used, which are difficult to industrialize on a large scale. The capacity and stability of the prepared nickel hydroxide are difficult to meet industrial requirements. In order to realize the industrial application of two-step water electrolysis, we hope to prepare nickel hydroxide bipolar electrodes with high capacity, high stability and suitable redox potential. Currently, there are few literatures on the preparation of nickel hydroxide bipolar electrodes specifically for two-step water electrolysis. In 2019, Israeli scholar Dr. Hen Dotan proposed the use of electrochemical impregnation to prepare nickel hydroxide bipolar electrodes on nickel foam in his literature on electrochemical-thermal activation two-step water electrolysis. A layer of β-phase nickel hydroxide with a thickness of about 9 microns was obtained on the nickel foam substrate, and the redox window was adjusted by cobalt doping. However, the 50mAcm -2 The charge and discharge time at this current density is only 100 seconds, far from meeting industrial needs. Another method for preparing nickel hydroxide electrodes is to mix commercial nickel hydroxide with water and carboxymethyl cellulose to form a slurry, which is then filled into a nickel foam substrate. However, the electrodes prepared by this method have poor stability and extremely low utilization of active materials, far from meeting industrial needs.

[0006] The existing preparation methods of nickel hydroxide bipolar electrodes are mainly hydrothermal or electrochemical methods, and their main disadvantages are:

[0007] 1. The prepared nickel hydroxide material has poor stability and cannot withstand the conversion between Ni(OH)2 and NiOOH during the long-term redox process, resulting in shedding.

[0008] 2. The nickel hydroxide prepared on the substrate by chemical, electrochemical and other means has a low loading capacity, small capacity and short single cycle time, which results in the need for frequent electrode swapping in the two-step hydrolysis process, which is not conducive to industrial application.

[0009] 3. The preparation process is complex, costly, and difficult to scale up. Summary of the Invention

[0010] The main purpose of the present invention is to provide a two-step method for preparing a bipolar electrode for electrolysis of water by loading high-utilization nickel hydroxide on a nickel foam substrate, which is simple to operate and low in cost.

[0011] The purpose of the present invention is to propose a two-step method for preparing a Ni(OH)2 bipolar electrode for electrolyzing water. The method is characterized in that Ni(OH)2 powder, a conductive agent powder and an organic solvent are mechanically mixed to prepare a slurry with suitable viscosity and fluidity, and the slurry is loaded on nickel foam. Suitable binders and organic solvents are selected so that the Ni(OH)2 loaded on the surface of the nickel foam has good long-term stability, avoiding the problem of falling off during long-term use. At the same time, polyvinyl pyrrolidone is added as a dispersant to give the slurry suitable fluidity, thereby evenly loading the Ni(OH)2 on the surface of the nickel foam and improving its utilization rate.

[0012] A method for preparing a nickel hydroxide bipolar electrode for two-step water electrolysis comprises the following steps:

[0013] First, an organic solution precursor is prepared, and a binder is dissolved in an organic solvent to form a precursor solution. Active material Ni(OH)2 powder, conductive agent powder and cobalt oxide powder are added to the precursor solution and stirred. Then a dispersant is added and stirring is continued to obtain a uniform and fluid slurry. The prepared slurry is then poured into a mold, and the foamed nickel is immersed in the mold, placed in a vacuum drying oven for evacuation, and then dried. The dried foamed nickel is taken out of the mold and rolled to obtain a nickel hydroxide bipolar electrode.

[0014] Specifically, the present invention is implemented in the following manner: first, an organic solution precursor is prepared, a binder is dissolved in an organic solvent to form a precursor solution, and the prepared active material Ni(OH)2 powder and conductive agent powder and a certain amount of cobalt oxide powder are added to the precursor solution, mechanically stirred, and then a certain amount of dispersant is added and stirred continuously to obtain a uniform slurry with a certain fluidity. The prepared slurry is then poured into a designed mold, and the pre-treated nickel foam is immersed in the mold, placed in a vacuum drying oven for evacuation, and then dried. The dried nickel foam is removed from the mold and pressed to a suitable thickness on a roller press to obtain a nickel hydroxide bipolar electrode.

[0015] Furthermore, based on the total mass of the slurry, the total mass content of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder is 20 to 40 wt%. For example, based on the total mass of the slurry, the total mass content of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder is 20 wt%, 21 wt%, 22 wt%, 23 wt%, 24 wt%, 25 wt%, 26 wt%, 27 wt%, 28 wt%, 29 wt%, 30 wt%, 31 wt%, 32 wt%, 33 wt%, 34 wt%, 35 wt%, 36 wt%, 37 wt%, 38 wt%, 39 wt% or 40 wt%.

[0016] Furthermore, based on the total mass of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder, the mass content of the active material Ni(OH)2 is 70-80wt%, the mass content of the cobalt oxide powder is 0.1-20wt%, and the mass content of the conductive agent is 0.1-20wt%.

[0017] For example, based on the total mass of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder, the mass content of the active substance Ni(OH)2 is 70wt%, 71wt%, 72wt%, 73wt%, 74wt%, 75wt%, 76wt%, 77wt%, 78wt%, 79wt% or 80wt%.

[0018] For example, based on the total mass of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder, the mass content of the cobalt oxide powder is 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt% or 20wt%.

[0019] For example, based on the total mass of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder, the mass content of the conductive agent is 0.1wt%, 0.2wt%, 0.3wt%, 0.4wt%, 0.5wt%, 0.6wt%, 0.7wt%, 0.8wt%, 0.9wt%, 1wt%, 2wt%, 3wt%, 4wt%, 5wt%, 6wt%, 7wt%, 8wt%, 9wt%, 10wt%, 11wt%, 12wt%, 13wt%, 14wt%, 15wt%, 16wt%, 17wt%, 18wt%, 19wt% or 20wt%.

[0020] Furthermore, based on the total mass of the slurry, the mass content of the organic solvent is 50 to 70 wt%, for example, the mass content of the organic solvent is 50 wt%, 51 wt%, 52 wt%, 53 wt%, 54 wt%, 55 wt%, 56 wt%, 57 wt%, 58 wt%, 59 wt%, 60 wt%, 61 wt%, 62 wt%, 63 wt%, 64 wt%, 65 wt%, 66 wt%, 67 wt%, 68 wt%, 69 wt% or 70 wt%.

[0021] Furthermore, based on the total mass of the slurry, the mass content of the binder is 0.1 to 5 wt%, for example, the mass content of the binder is 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% or 5 wt%.

[0022] Furthermore, based on the total mass of the slurry, the mass content of the dispersant is 0.1 to 5 wt%, for example, the mass content of the dispersant is 0.1 wt%, 0.2 wt%, 0.3 wt%, 0.4 wt%, 0.5 wt%, 0.6 wt%, 0.7 wt%, 0.8 wt%, 0.9 wt%, 1 wt%, 2 wt%, 3 wt%, 4 wt% or 5 wt%.

[0023] Furthermore, the drying temperature is 30 to 100° C., and the drying time is 6 to 24 hours.

[0024] The organic solvent is N-methylpyrrolidone (NMP), the binder is polyvinylidene fluoride (PVDF), the dispersant is polyvinylpyrrolidone (PVP), the conductive agent is acetylene black powder, the thickness of the nickel foam is 5 mm, and the powders are all purchased directly from the commercial market without any treatment.

[0025] The total mass of the powder accounts for 36.4wt% of the final slurry mass, the active material Ni(OH)2 accounts for 80% of the total mass of the powder, the cobalt oxide doped powder accounts for 10wt% of the total mass of the powder, and the conductive agent added accounts for 10wt% of the total mass of the powder.

[0026] The amount of the organic solvent accounts for 60.2 wt % of the final slurry mass, the mass of the binder added to the organic solvent accounts for 3.3 wt % of the total slurry mass, and the mass of the dispersant added accounts for 0.3 wt % of the total slurry mass.

[0027] The thickness of the nickel foam is 1 to 6 mm, for example, the thickness of the nickel foam is 1, 2, 3, 4, 5 or 6 mm.

[0028] The temperature of the vacuum drying oven is 70° C., the drying time is 12 hours, the thickness of the nickel foam is selected to be 6 mm, and the thickness of the sheet pressed by the roller press is 2 mm.

[0029] Specifically, a two-step method for preparing a Ni(OH)2 bipolar electrode for electrolyzing water comprises the following steps:

[0030] 1) Select nickel foam metal of appropriate thickness and sequentially treat it with deionized water, concentrated hydrochloric acid and ethanol ultrasonically to prepare it as a substrate.

[0031] 2) A certain proportion of polyvinylidene fluoride was slowly added to an organic solvent of N-methylpyrrolidone (NMP) under magnetic stirring, and the mixture was magnetically stirred for 30 minutes to obtain a thick precursor solution.

[0032] 3) A certain proportion of nickel hydroxide, cobalt oxide, and acetylene black powder were added to the precursor solution, and then stirred with a vertical stirrer for 2 hours to obtain a black slurry. Subsequently, 0.3 wt% of polyvinyl pyrrolidone dispersant was added and stirring was continued for 10 minutes.

[0033] 4) The obtained black slurry was poured into a mold, and the treated nickel foam was immersed therein, followed by drying at 70° C. in a vacuum drying oven for 12 hours. The obtained black nickel hydroxide sheet was then pressed to a thickness of 2 mm in a roller press.

[0034] Through a large number of experimental verifications, it was found that in the scheme of the present invention, the mass ratio of powder to binder and organic solvent in the slurry has the greatest influence on the performance and stability of the prepared nickel hydroxide bipolar electrode. If the powder content is too high, it will affect the fluidity of the slurry and cannot be effectively loaded. If it is too low, the capacity of the prepared electrode will be small. In addition, the proportion of the binder has a greater influence on the stability of the material, and the dispersant improves the fluidity of the slurry, so that the slurry is more evenly loaded on the surface of the substrate, thereby improving the utilization rate of the active material (nickel hydroxide) in the prepared bipolar electrode. The average loading amount of the active material in the nickel hydroxide bipolar electrode prepared in the present invention is 200-300 mg per square centimeter.

[0035] The beneficial effects of the present invention are as follows:

[0036] The present invention improves the stability by adjusting the ratio between nickel hydroxide, cobalt oxide and acetylene black powders, the ratio between the total amount of powders and the solvent and binder; introducing organic solvent N-methylpyrrolidone (NMP) and binder polyvinylidene fluoride (PVDF); and introducing dispersant polyvinylpyrrolidone (PVP) to improve the utilization rate of active substances and the vacuum drying after loading, as well as the drying temperature and time. The prepared nickel hydroxide bipolar electrode has large capacity, high stability, and is easy to scale up for production. The electrode is applied to two-step water electrolysis and has important research value and broad application prospects in the field of water electrolysis hydrogen production industry.

[0037] 1) Compared with traditional chemical methods such as hydrothermal and electrodeposition, the preparation method of directly preparing the active material into a slurry and loading it on the substrate through a binder is simple, time-saving, and can be scaled up for production.

[0038] 2) Compared with the laboratory method for preparing nickel hydroxide, the nickel hydroxide bipolar electrode prepared in the present invention has a high loading capacity, large capacity, and a charge and discharge platform time of one hour, which can meet the needs of industrial two-step water electrolysis.

[0039] 3) Considering the working environment of the bipolar electrode (in potassium hydroxide aqueous solution), the introduction of organic solvent and binder makes the prepared bipolar electrode have excellent stability.

[0040] 4) The addition of dispersant (PVP) greatly improves the fluidity of the slurry, thereby increasing the utilization rate of the active material (nickel hydroxide). BRIEF DESCRIPTION OF THE DRAWINGS

[0041] Figure 1 The nickel hydroxide bipolar electrode prepared in Example 1 is -2 The charge and discharge curve platform under the current density can reach 3600s and the capacity can reach 200mAh. -1 , loading capacity 206gcm -2 , the utilization rate of active material (nickel hydroxide) is 84%

[0042] Figure 2 The nickel hydroxide bipolar electrode prepared in Example 2 was -2 The 35-hour stable charge-discharge curve under the current density shows no significant changes in the redox potential and the charge-discharge platform capacity. DETAILED DESCRIPTION

[0043] The present invention will be described in detail below with reference to the accompanying drawings and specific embodiments. However, the following embodiments are intended only to explain the present invention, and the scope of protection of the present invention should include the entire contents of the claims. Moreover, through the description of the following embodiments, those skilled in the art can fully implement the entire contents of the claims of the present invention.

[0044] The organic solvent N-methylpyrrolidone (NMP) in the present invention may be replaced by other organic solvents such as N,N-dimethylformamide (DMF) to achieve the same effect, and the vacuum drying oven may be replaced by a blast drying oven to achieve the same effect.

[0045] Example 1: A 3 mm thick nickel foam was selected and ultrasonically cleaned with deionized water, 1 mol / L hydrochloric acid and ethanol respectively as a substrate. 3.3 g of polyvinylidene fluoride was added to 60.2 g of N-methylpyrrolidone organic solvent and mixed for 30 minutes using a magnetic stirrer. 29.12 g of nickel hydroxide powder, 3.64 g of cobalt oxide powder and 3.64 g of acetylene black powder were added to the above solution and stirred for 2 hours using a vertical mechanical stirrer. 0.1 g of polyvinyl pyrrolidone was then added and stirred for 10 minutes to obtain a black organic phase slurry. The slurry was then poured into a mold and the nickel foam was immersed therein. The mold was dried at 70°C in a vacuum drying oven for 12 hours. The resulting black block was then pressed into a sheet with a thickness of 2 mm.

[0046] The nickel hydroxide bipolar electrode prepared in the above embodiment has a -2 The charge and discharge curve platform under the current density can reach 3600s, and the discharge platform can reach 3600s at a load of 206gcm -2 Under the condition of charging and discharging capacity reaches 200mAh -1 The utilization rate of active substances reaches 84%, and the slurry capacity is increased by 3-4 times compared with the aqueous system.

[0047] Figure 1 The nickel hydroxide bipolar electrode prepared in the embodiment is -2 The charge and discharge curve platform under the current density is as follows: the discharge platform can reach 3600s.

[0048] Example 2: A 5 mm thick nickel foam was selected and ultrasonically cleaned with deionized water, 1 mol / L hydrochloric acid and ethanol respectively as a substrate. 3 g of polyvinylidene fluoride was added to 50 g of N-methylpyrrolidone organic solvent and mixed for 30 minutes using a magnetic stirrer. 24 g of nickel hydroxide powder, 3 g of cobalt oxide powder and 3 g of acetylene black powder were added to the above solution and stirred for 2 hours using a vertical mechanical stirrer. 0.1 g of polyvinyl pyrrolidone was then added and stirred for 10 minutes to obtain a black organic phase slurry. The slurry was then poured into a mold and the nickel foam was immersed therein. The mold was dried in a vacuum drying oven at 100°C for 12 hours. The resulting black block was then pressed into a sheet with a thickness of 2.5 mm.

[0049] The nickel hydroxide bipolar electrode prepared in the above embodiment has a loading capacity of 225 g cm -2 , at 50mAcm - 2. The cyclic charge and discharge can run stably for 35 hours at a current density of 2, and there is no obvious attenuation of the charge and discharge platform and voltage. However, the active material of the electrode prepared by the aqueous phase system slurry with a similar powder formula is obviously shed after running for about an hour, and the performance is significantly reduced.

[0050] Figure 2 The 35-hour stable charge-discharge curve of the nickel hydroxide bipolar electrode prepared in the implementation case at a current density of 50 mA cm-2.

[0051] Comparative Example 1: A 5 mm thick nickel foam was selected and ultrasonically cleaned with deionized water, 1 mol / L hydrochloric acid, and ethanol, respectively, as a substrate. Using a magnetic stirrer, 2 g of carboxymethyl cellulose and 1.02 g of polytetrafluoroethylene were each added to 100 g of deionized water and ultrasonically dissolved for 30 minutes before mixing. 46 g of nickel hydroxide powder, 7.6 g of cobalt oxide powder, and 7.6 g of acetylene black powder were added to the above solution and milled for 2 hours using a planetary ball mill to obtain a black aqueous slurry. The slurry was then poured into a mold, and the nickel foam was immersed in it. The mold was dried in a vacuum drying oven at 100°C for 12 hours, and the resulting black block was then pressed into a sheet with a thickness of 2.5 mm.

[0052] Subsequent tests found that the charge and discharge capacity and stability of the nickel hydroxide bipolar electrode prepared by the aqueous system slurry were less than half of those of the organic phase system.

[0053] The present invention does not describe in detail parts that belong to the common knowledge of those skilled in the art. The above-described embodiments are merely descriptions of preferred embodiments of the present invention. The preferred embodiments do not describe all details in detail, nor do they limit the invention to the specific embodiments described. Without departing from the spirit of the present invention, various modifications and improvements made by those skilled in the art to the technical solution of the present invention should fall within the scope of protection defined by the claims of the present invention.

Claims

1. A method for preparing a nickel hydroxide bipolar electrode for two-step water electrolysis, characterized in that: The steps include: First, an organic solution precursor is prepared, and a binder is dissolved in an organic solvent to form a precursor solution. Active material Ni(OH)2 powder, conductive agent powder, and cobalt oxide powder are added to the precursor solution and stirred. Then, a dispersant is added and stirring is continued to obtain a uniform and fluid slurry. The prepared slurry is then poured into a mold, and the nickel foam is immersed in the mold. The mold is placed in a vacuum drying oven for evacuation and then dried. The dried nickel foam is taken out of the mold and rolled to obtain a nickel hydroxide bipolar electrode. The organic solvent is N-methylpyrrolidone; the binder is polyvinylidene fluoride; and the dispersant is polyvinylpyrrolidone.

2. The method according to claim 1, characterized in that The conductive agent is acetylene black powder; the thickness of the nickel foam is 1 to 6 mm.

3. The method according to claim 1, characterized in that Based on the total mass of the slurry, the total mass content of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder is 20-40 wt%; Based on the total mass of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder, the mass content of the active material Ni(OH)2 is 70~80 wt%, the mass content of the cobalt oxide powder is 0.1~20 wt%, and the mass content of the conductive agent is 0.1~20 wt%.

4. The method according to claim 1, wherein Based on the total mass of the slurry, the mass content of the organic solvent is 50-70 wt %, the mass content of the binder is 0.1-5 wt %, and the mass content of the dispersant is 0.1-5 wt %.

5. The method according to claim 1, wherein Based on the total mass of the slurry, the total mass content of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder is 36.4wt%, the mass content of the organic solvent is 60.2wt%, the mass content of the binder is 3.3%, and the mass content of the dispersant is 0.1wt%; Based on the total mass of the Ni(OH)2 powder, the conductive agent powder and the cobalt oxide powder, the mass content of the active material Ni(OH)2 is 80%, the mass content of the cobalt oxide powder is 10wt%, and the mass content of the conductive agent is 10wt%.

6. The method according to claim 1, characterized in that The drying temperature is 30-100° C., and the drying time is 6-24 hours.

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