Preparation method and application of cathode material derived from ferrocene
阅读说明：本技术 源于二茂铁的阴极材料的制备方法及应用 (Preparation method and application of cathode material derived from ferrocene ) 是由 李洲鹏 汪述平 刘宾虹 于 2019-10-13 设计创作，主要内容包括：本发明涉及原位合成多孔催化剂,旨在提供一种源于二茂铁的阴极材料的制备方法及应用。包括：取二茂铁粉末加入环糊精饱和溶液,超声混合得到二茂铁环糊精包合物；将琼脂溶解于沸水后加入二茂铁环糊精包合物,搅拌溶解后加入氯化钠；搅拌溶解,得到凝胶体；挤出凝胶条在液氮中硬化后,冷冻干燥得到阴极材料前驱体；在氮气氛围碳化,冷却、洗涤、过滤再干燥,得到含铁的阴极材料。得到的含铁多孔碳具有比表面积大和大孔容的特点,具有更好的导电性。合成方法更为绿色,成本更低。采用二茂铁环糊精包合物为前驱分子,成功解决了二茂铁溶解性差的问题,有效提高阴极材料的催化性能,且成本低廉,有利于锂氧电池技术商业化。(The invention relates to an in-situ synthesis porous catalyst, and aims to provide a preparation method and application of a cathode material derived from ferrocene. The method comprises the following steps: adding ferrocene powder into a cyclodextrin saturated solution, and performing ultrasonic mixing to obtain a ferrocene cyclodextrin inclusion compound; dissolving agar in boiling water, adding the ferrocene cyclodextrin inclusion compound, stirring to dissolve, and adding sodium chloride; stirring and dissolving to obtain gel; after the extruded gel strip is hardened in liquid nitrogen, the gel strip is frozen and dried to obtain a cathode material precursor; carbonizing in nitrogen atmosphere, cooling, washing, filtering and drying to obtain the iron-containing cathode material. The obtained iron-containing porous carbon has the characteristics of large specific surface area and large pore volume, and has better conductivity. The synthesis method is more green and has lower cost. The ferrocene cyclodextrin inclusion compound is adopted as a precursor molecule, the problem of poor ferrocene solubility is successfully solved, the catalytic performance of the cathode material is effectively improved, the cost is low, and the commercialization of the lithium-oxygen battery technology is facilitated.)
1. A method for preparing a cathode material derived from ferrocene is characterized by comprising the following steps of:
(1) taking 1L of cyclodextrin saturated solution, adding 0.5-50 g of ferrocene powder, and ultrasonically vibrating and mixing for 30 minutes at 90 ℃ to enable ferrocene molecules to enter a cyclodextrin cavity; cooling to room temperature to obtain a ferrocene cyclodextrin inclusion compound;
(2) dissolving 10-20 g of agar in 1L of boiling water, adding 1-5 g of ferrocene cyclodextrin inclusion compound, stirring to dissolve, adding 10-20 g of sodium chloride, and stirring to dissolve for 1 h; cooling to 35 deg.C to obtain yellow gel-like material (gel); putting the gel into a strip extruder, and directly adding liquid nitrogen into the extruded gel strip; after the gel strip is hardened in liquid nitrogen, taking out the gel strip, transferring the gel strip to a freeze dryer for drying for 12 hours to obtain a cathode material precursor;
(3) heating the precursor of the cathode material to 400 ℃ under the protection of nitrogen atmosphere, and carbonizing for 2 hours at constant temperature; then heating to 900 ℃, and carbonizing for 2 hours at constant temperature to form a carbonized product; cooling to room temperature, washing with deionized water, and filtering at room temperature; and drying at the constant temperature of 110 ℃ for 4 hours to obtain the iron-containing cathode material.
2. A method according to claim 1, wherein the ferrocene is any one of: ferrocene sulfonic acid, carboxyferrocene, aldehyde ferrocene, hydroxy ferrocene, acyl ferrocene or amino ferrocene.
3. The method of claim 1, wherein the cyclodextrin is any one of: alpha-cyclodextrin, beta-cyclodextrin, gamma-cyclodextrin or delta-cyclodextrin; or cyclodextrin derivatives containing hydrophilic groups, the hydrophilic groups being amino, carboxyl, aldehyde or acyl groups.
4. The method according to claim 1, wherein the temperature rise rate in step (3) is: heating to 400 ℃ at the speed of 10 ℃/min; the temperature was raised to 900 ℃ at a rate of 50 ℃/min.
5. A method for preparing a cathode for a battery using the cathode material prepared by the method of claim 1, comprising the steps of: and (2) taking a perfluorinated sulfonic acid resin solution with the mass concentration of 5 wt% as a binder, preparing into slurry according to the mass ratio of the perfluorinated sulfonic acid resin solution to the cathode material of 3: 7, coating the slurry on hydrophobic carbon paper, air-drying, calcining for one hour at 150 ℃, and naturally cooling to obtain the cathode of the battery.
6. A method of making a lithium-oxygen battery using the battery cathode made by the method of claim 5, comprising the steps of: assembling a button cell in an argon atmosphere glove box: assembling the battery according to the sequence of the positive electrode shell with the array holes as the vent holes, the battery cathode, the diaphragm, the metal lithium sheet, the gasket, the elastic sheet and the lithium-oxygen battery negative electrode shell, wherein the concentration of the metal lithium sheet is 1MLiTFSI/TEGDME(Li[CF3SO2)2N]Tetraethylene glycol dimethyl ether) as an electrolyte.
Technical Field
The invention relates to a green preparation method of an in-situ synthesized porous catalyst, in particular to a preparation method of a high-performance lithium-oxygen battery cathode material, which is obtained by taking a cyclodextrin inclusion compound of ferrocene as a precursor molecule, agar as a carbon source and sodium chloride as a template, carrying out flash freeze forming, then carrying out freeze drying to obtain a precursor, and carbonizing.
Background
A lithium-air battery is a battery that uses lithium as a negative electrode and oxygen in the air as a positive electrode reactant. Lithium air batteries have a higher energy density than lithium ion batteries because their cathodes (predominantly porous carbon) are very light and oxygen is taken from the environment without being stored in the battery. Theoretically, since oxygen is not limited as a positive electrode reactant, the capacity of a lithium air battery depends only on the lithium electrode, and its specific energy is 5.21kWh/kg (including oxygen mass), or 11.4kWh/kg (excluding oxygen). Lithium is the lightest metal and has a higher specific energy than other metal-air batteries (lithium oxygen batteries).
The electrolyte of the organic electrolyte lithium-oxygen battery is an organic solvent, and positive and negative electrodes are separated by a diaphragm. In the discharging process of the battery, the lithium metal of the negative electrode loses electrons and turns into lithium ions, the lithium ions pass through the diaphragm to perform Oxygen Reduction Reaction (ORR) with Oxygen and electrons at the positive electrode, and the generated discharging product is Li2O2. Li of positive electrode during charging2O2Oxygen Evolution Reaction (OER) occurs to lose electrons and decompose into lithium ions and Oxygen, and the lithium ions return to the negative electrode and get electrons to become metallic lithium. The standard electrode electromotive force for the discharge reaction of this type of cell was 2.96V. The cell reaction is as follows:
during the charging and discharging processes of the lithium-oxygen battery, the negative electrode generates the reaction of losing electrons of the metal lithium and obtaining electrons of lithium ions, and the reactions are carried out on the surface of the metal lithium negative electrode; the positive electrode generates lithium ions, oxygen and electrons to generate lithium peroxide and the reverse reaction of the lithium ions, the oxygen and the electrons. The positive reaction is a more complex reaction, and is generally accepted as a two-electron transfer four-step reaction process, which is specifically as follows:
LiO2+Li++e-→Li2O2(4)
2LiO2→Li2O2+O2↑ (5)
the performance of a lithium-oxygen battery depends on the cathode material, and the catalytic activity of the cathode material towards ORR and OER determines the specific capacity and cycle life of the lithium-oxygen battery. The nitrogen-doped carbon material has good catalytic activity on ORR, and the reaction speed of ORR is remarkably improved due to the existence of heteroatom N or O on the carbon ring. The carbon nano-tube, the microporous carbon and the mesoporous carbon are subjected to N surface doping to form graphite nitrogen (graphite-N) and pyridine nitrogen (pyridine-N), and the catalytic activity on ORR is equivalent to that of a carbon-supported platinum catalyst of vendors. Some nitrogen-containing compounds such as phthalocyanine (Pc) and porphyrin are compounded with Co or Fe on atomic scale or nano scale to form graphite nitrogen and pyridine nitrogen, and form M-Nx (M is a transition metal element) which has a remarkable catalytic effect on ORR. The current research on low-cost cathode materials mainly focuses on transition metal cluster catalysts, transition metal-containing macrocyclic compound catalysts and metal carbides in the center; in addition, nitrides, sulfides, borides, silicides, and the like have been reported as oxygen reduction catalysts, but these catalysts have relatively poor performance and have been studied relatively rarely.
It is known that most of the oxygen inhaled by the lungs of mammals, except for a small amount dissolved in plasma, is bound to hemoglobin and carried by the blood circulation to the microvasculature of various tissues of the body, where it is released for cellular needs. Hemoglobin molecules are a very efficient molecular machine that regulates behavior through motion and small structural changes. The binding of oxygen and hemoglobin at the four sites is not synchronized. The first oxygen gas binds to hemoglobin, causing minor changes in the corresponding protein chains, which make them easier to bind to oxygen. This provides convenience for the functioning of hemoglobin. When blood flows through the oxygen-rich lungs, oxygen readily binds to the first subunit and then quickly fills the rest. Then as the blood circulates through the body, the oxygen concentration drops and the carbon dioxide concentration rises, in which case the hemoglobin releases oxygen, which changes shape as soon as one oxygen is removed, which causes the other three to be released quickly. In this way the hemoglobin is then pulmonary loaded with a maximum amount of oxygen and then transported to aerobic tissues. A hemoglobin molecule consists of a globin and four heme (also known as ferroprotoporphyrins). Each heme in turn consists of four pyrrole groups forming a ring, centered on a ferrous ion. This ferrous ion is the core carrying oxygen, otherwise known as the catalytic center. The working principle of the cathode material of the lithium-oxygen battery is close to the function of hemoglobin, so the research on the ferrous cathode material has great significance for improving the performance of the lithium-oxygen battery.
However, the conventional ferrous salt is adopted as the precursor molecule, and not only is the ferrous salt easily oxidized into ferric ions in the air, but also the ferric ions are easily reduced into zero-valent iron (metallic iron) in the carbonization process of the preparation of the cathode material of the lithium-oxygen battery, so that the catalytic activity is reduced, and therefore, the stability of the ferrous precursor molecule is fundamentally ensured in the preparation of the high-performance iron-containing cathode material.
Disclosure of Invention
The invention aims to solve the technical problem of overcoming the defects in the prior art and provides a method for preparing a ferrocene-derived in-situ synthesis lithium-oxygen battery cathode material.
In order to solve the technical problem, the solution of the invention is as follows:
the preparation method of the cathode material derived from ferrocene comprises the following steps:
(1) taking 1L of cyclodextrin saturated solution, adding 0.5-50 g of ferrocene powder, and ultrasonically vibrating and mixing for 30 minutes at 90 ℃ to enable ferrocene molecules to enter a cyclodextrin cavity; cooling to room temperature to obtain a ferrocene cyclodextrin inclusion compound;
(2) dissolving 10-20 g of agar in 1L of boiling water, adding 1-5 g of ferrocene cyclodextrin inclusion compound, stirring to dissolve, adding 10-20 g of sodium chloride, and stirring to dissolve for 1 h; cooling to 35 deg.C to obtain yellow gel-like material (gel); putting the gel into a strip extruder, and directly adding liquid nitrogen into the extruded gel strip; after the gel strip is hardened in liquid nitrogen, taking out the gel strip, transferring the gel strip to a freeze dryer for drying for 12 hours to obtain a cathode material precursor;
(3) heating the precursor of the cathode material to 400 ℃ under the protection of nitrogen atmosphere, and carbonizing for 2 hours at constant temperature; then heating to 900 ℃, and carbonizing for 2 hours at constant temperature to form a carbonized product; cooling to room temperature, washing with deionized water, and filtering at room temperature; and drying at the constant temperature of 110 ℃ for 4 hours to obtain the iron-containing cathode material.
In the invention, the ferrocene is any one of the following: ferrocene sulfonic acid, carboxyferrocene, aldehyde ferrocene, hydroxy ferrocene, acyl ferrocene or amino ferrocene.
In the present invention, the cyclodextrin is any one of the following: alpha-cyclodextrin, beta-cyclodextrin, gamma-cyclodextrin or delta-cyclodextrin; or cyclodextrin derivatives containing hydrophilic groups, the hydrophilic groups being amino, carboxyl, aldehyde or acyl groups.
In the present invention, the temperature increase rate in the step (3) is: heating to 400 ℃ at the speed of 10 ℃/min; the temperature was raised to 900 ℃ at a rate of 50 ℃/min.
The invention further provides a method for preparing a battery cathode by using the cathode material prepared by the method, which comprises the following steps: and (2) taking a perfluorinated sulfonic acid resin solution with the mass concentration of 5 wt% as a binder, preparing into slurry according to the mass ratio of the perfluorinated sulfonic acid resin solution to the cathode material of 3: 7, coating the slurry on hydrophobic carbon paper, air-drying, calcining for one hour at 150 ℃, and naturally cooling to obtain the cathode of the battery.
The invention further provides a method for preparing a lithium-oxygen battery by using the battery cathode obtained by the method, which is characterized by comprising the following steps: assembling a button cell in an argon atmosphere glove box: according to the positive shell with the array small holes as the vent holes, the battery cathode, the diaphragm, the metal lithium sheet, the gasket, the elastic sheet and the lithium-oxygen battery cathodeSequential assembly of electrode casings cells using 1M concentration of LiTFSI/TEGDME (Li [ CF ] s3SO2)2N]Tetraethylene glycol dimethyl ether) as an electrolyte.
Description of the inventive principles:
ferrocene is an organic transition metal compound having aromatic properties. It is orange yellow powder at room temperature, and has camphor smell. The melting point is 172-174 ℃, the boiling point is 249 ℃, and the sublimation can be carried out at the temperature of more than 100 ℃; is easy to dissolve in organic solvents such as benzene, ether, gasoline, diesel oil, etc. The ferrocene and the derivatives thereof have no action with acid, alkali and ultraviolet, stable chemical property, no decomposition within 400 ℃, high thermal stability, chemical stability and radiation resistance, and wide application in industries such as industry, agriculture, medicine, aerospace, energy conservation, environmental protection and the like. But ferrocene is insoluble in water and is difficult to be used for directly constructing a catalytic center of the in-situ synthesized cathode material.
Because the ferrocene is difficult to dissolve in water, the uniformly distributed ferrocene cyclodextrin gel is difficult to form, the iron is easy to form metallic iron by partial polymerization after carbonization, and the oxygen reduction catalyst with excellent catalytic performance can not be obtained. The cavity at the inner side of the cyclodextrin molecule is hydrophobic, the ferrocene molecule tends to exist in the cavity of the cyclodextrin molecule to form a molecule inclusion compound, and the hydroxyl rich at the outer side of the cyclodextrin molecule has excellent hydrophilicity, so that the cyclodextrin molecule is favorable for dissolving in water. Moreover, the cyclodextrin and the agar belong to polysaccharide and have good compatibility, so the agar and the ferrocene cyclodextrin inclusion compound can form uniform gel.
When the ferrocene gel is extruded out of the gel strip through the strip extruder and directly put into liquid nitrogen, the gel strip quickly forms a surface shell layer to isolate the gel strip from the liquid nitrogen. The temperature of the gel in the shell is continuously reduced, when the cross-linked product of the agar and the ferrocene cyclodextrin inclusion compound and the sodium chloride crystal are separated out, the free water in the gel is quickly frozen, and the cross-linked product of the agar and the ferrocene cyclodextrin inclusion compound and the sodium chloride crystal are pushed to the boundary to form the micropore. The sodium chloride crystals in the gel are countless ice crystal seeds and are frozen and solidified instantly. During the subsequent vacuum freeze-drying process, the ice sublimes to form a cavity between the cross-linked product and the sodium chloride crystal, and the pore volume of the precursor is improved. In the subsequent calcining process, the temperature is raised to 400 ℃ for pre-carbonization, the framework is formed, ferrocene is adsorbed on the framework, the temperature is raised to 900 ℃ for complete carbonization, and a carbon thin wall is formed and a catalytic center is formed at the same time. The iron-containing porous carbon obtained by removing the NaCl template by using deionized water has good conductivity and catalytic activity.
Ferrocene is a ferrous conjugated complex containing two five-membered aromatic rings in a molecular structure to form a stable molecular structure, the ferrocene is not decomposed below 400 ℃, because of the hydrophobicity of the five-membered aromatic rings, the ferrocene tends to enter a cavity of a cyclodextrin molecule to form a molecular inclusion compound, the outer side of the cyclodextrin molecule is rich in hydroxyl, the cyclodextrin molecule has excellent hydrophilicity, and is beneficial to dissolving in water and forming hydrogen bonds with agar molecules, so that the cyclodextrin inclusion compound of the ferrocene can be uniformly distributed in agar gel.
The hydrophobic property of ferrocene also causes difficulty in forming the inclusion compound in the cyclodextrin water solution, and hydrophilic groups such as sulfonic acid group, carboxyl group, aldehyde group, hydroxyl group, acyl group and amino group are used for forming ferrocene sulfonic acid, carboxyl group ferrocene, aldehyde group ferrocene, ferrocene methanol, acyl ferrocene and amino ferrocene on five-membered aromatic ring by electrophilic substitution reaction, carboxylation reaction, condensation reaction, oxidation reaction, acylation reaction, amination reaction and the like, so that the hydrophilicity of ferrocene can be improved to a certain extent, and the formation of the cyclodextrin inclusion compound is facilitated.
Cyclodextrin is a general term for a series of cyclic oligosaccharides produced by amylose under the action of cyclodextrin glucosyltransferase produced by Bacillus, and generally contains 6 to 12D-glucopyranose units. Among them, the more studied and of great practical significance are molecules containing 6, 7, 8, 9 glucose units, called α -, β -, γ -and δ -cyclodextrins, respectively. Each D (+) -glucopyranose constituting the cyclodextrin molecule is in a chair-type conformation, and each glucose unit is bonded to a ring by a 1, 4-glycosidic bond. The cyclodextrin molecule has a slightly tapered hollow cylindrical three-dimensional annular structure, and in the hollow structure, the upper end (larger opening end) of the outer side is composed of secondary hydroxyl groups of C2 and C3, the lower end (smaller opening end) is composed of primary hydroxyl groups of C6, the cyclodextrin molecule has hydrophilicity, and a hydrophobic region is formed in the cavity due to the shielding effect of C-H bonds. Various organic compounds can be embedded into the hydrophobic cavity to form an inclusion compound, and the physical and chemical properties of the enveloped substance are changed; the cyclodextrin molecule can be crosslinked with a plurality of functional groups or the cyclodextrin is crosslinked on a polymer to carry out chemical modification or carry out polymerization by taking the cyclodextrin as a monomer. The larger the number of molecules of the cyclodextrin molecule containing glucose units, the larger the void volume of the cavity of the hydrophobic region, and the larger hydrophobic molecules can be contained.
Agar is a polysaccharide extracted from seaweed and is one of the most widely used seaweed gels in the world at present. It has wide application in food industry, medicine industry, daily chemical industry, biological engineering and other fields. Agar has coagulability and stability, can form complex with some substances, and can be used as thickener, coagulant, suspending agent, emulsifier, antistaling agent and stabilizer. Agar is a linear polysaccharide with the basic structure of long chains of alternating 1,3 linked β -D-galactose and 1,4 linked 3, 6-lacto-L-galactose. Agar dissolves in water, typically by heating to above 90 ℃, and forms a good semisolid gel when the temperature drops to 35-40 ℃, which is a major feature and basis for its many uses. Agarose dissolves in water, typically by heating to temperatures above 90 c, and forms a good semisolid gel when the temperature drops to 35-40 c, which is a major feature and basis for its many uses. The gelling of agar is caused by the presence of hydrogen bonds, and any factor that can disrupt hydrogen bonds can lead to the destruction of gelling. When the agar and the cyclodextrin are heated to 90 ℃, hydrogen bonds of the agar and the cyclodextrin are broken to dissolve to form a solution, when the temperature is reduced to 35-40 ℃, cyclodextrin molecules are dispersed in the agar gel, and the hydrogen bonds of the cyclodextrin molecules and the hydroxyl groups of the agar molecules are reestablished to form a uniform gel.
When the cyclodextrin molecules and the agar molecules form a gel in the NaCl solution, NaCl will also be uniformly distributed in the gel. When the gel is extruded out of the gel strip by the strip extruder and directly put into liquid nitrogen, the gel strip quickly forms a surface shell layer to isolate liquid drops from the liquid nitrogen. The temperature of the liquid in the shell is continuously reduced, and free water in the gel is quickly frozen while the agar-cyclodextrin mixture and the sodium chloride crystal are separated out. During the subsequent vacuum freeze-drying process, the ice sublimes, forming cavities. And in the subsequent calcining process, the temperature is raised to 400 ℃ for pre-carbonization, the framework is formed, the temperature is raised to 900 ℃ for complete carbonization to form porous carbon, NaCl is melted and discharged from the micropores, and the porous carbon floats on NaCl molten salt. And cooling to room temperature, and removing a small amount of NaCl remained in the porous carbon by using deionized water to obtain the porous carbon with good conductivity.
Compared with the prior art, the invention has the beneficial effects that:
1. the iron-containing porous carbon obtained by taking NaCl as the template has the characteristics of large specific surface area and large pore volume, because the NaCl is helpful for forming SP between C and C2Hybridization, compared with the traditional macroporous carbon material obtained by using a nano calcium carbonate template, the material has better conductivity.
2. Sodium chloride is used as a template, elution can be carried out by using water, and both the sodium chloride and the water can be recycled, but the macroporous carbon material obtained by using the nano calcium carbonate template needs to be subjected to template removal by using acid, so that the synthesis method of the cathode material is more green and has lower cost.
3. The ferrocene cyclodextrin inclusion compound is adopted as a precursor molecule, the problem of poor ferrocene solubility is successfully solved, the catalytic performance of the cathode material is effectively improved, the cost is low, and the commercialization of the lithium-oxygen battery technology is facilitated.
Drawings
Fig. 1 is a graph comparing the discharge performance of lithium-oxygen batteries with and without iron cathodes.
In the figure, 1-discharge curve of lithium-oxygen cell prepared with no iron cathode, 2-discharge curve of lithium-oxygen cell assembled with cathode obtained in example eleven; working temperature: 25 ℃, current density: 10mA/cm2The loading amount of cathode material is 0.5mg/cm2。
Detailed Description
The present invention is described in further detail below with reference to the attached drawings and specific embodiments, which enable those skilled in the art to more fully understand the present invention without limiting the invention in any way.